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1Polyolefins*
Olagoke Olabisi
* Based in part on the rst-edition chapters on polyolens.
CONTENTS
1.1 Introduction and Historical Background .................................................................................. 2
1.2 Catalyst Systems for Olen Polymerization ............................................................................. 3
1.2.1 Chromium-Based Catalysts .......................................................................................... 3
1.2.1.1 Phillips Catalysts ...........................................................................................3
1.2.1.2 Organochromium Catalysts ........................................................................... 4
1.2.2 Ziegler–Natta Catalyst Systems ....................................................................................5
1.2.3 SSC Systems ................................................................................................................. 9
1.2.3.1 Metallocene SSC ............................................................................................9
1.2.3.2 Post-Metallocene SSC ..................................................................................12
1.3 Production Technology ........................................................................................................... 13
1.3.1 Free Radical Polymerization Processes ......................................................................13
1.3.1.1 High-Pressure Autoclave Reactor Process ...................................................13
1.3.1.2 Tubular Reactor Process .............................................................................. 13
1.3.2 Polymerization Processes for Polyethylenes ...............................................................14
1.3.2.1 High-Pressure Processes ..............................................................................14
1.3.2.2 Low-Pressure Liquid Slurry Processes ........................................................15
1.3.2.3 Low- and Medium-Pressure Solution Processes ......................................... 16
1.3.2.4 Low-Pressure Gas-Phase Processes ............................................................. 17
1.3.3 Polymerization Processes for PP ................................................................................ 18
1.3.3.1 Low-Pressure Liquid Pool Slurry Phase Processes ..................................... 19
1.3.3.2 Low-Pressure Modular Gas-Phase Reactor Processes ................................ 19
1.3.4 Polymerization Processes for Other Polyolens .........................................................19
1.3.5 Process Technologies for SSCs ................................................................................... 19
1.4 Polyolen Structure–Property Relationships .........................................................................20
1.4.1 Polyethylenes ..............................................................................................................22
1.4.1.1 Branched LDPE ........................................................................................... 22
1.4.1.2 Linear LDPEs .............................................................................................. 23
1.4.1.3 High-Density Polyethylenes .........................................................................25
1.4.1.4 Ultrahigh-Molecular-Weight Polyethylenes .................................................26
1.4.2 Polypropylene .............................................................................................................26
1.4.3 Poly(butene-1) ............................................................................................................. 27
1.4.4 Poly(4-methylpentene-1) ............................................................................................. 28
1.4.5 Polyolen Elastomers .................................................................................................28
1.4.6 Polyolen Blends and Copolymers ............................................................................. 28
1.4.6.1 Polyolen Blends..........................................................................................28
1.4.6.2 Polyolen Copolymers ................................................................................. 30
1.4.7 Polyolens from SSCs ................................................................................................32
1.4.7.1 SSC Polyethylenes........................................................................................33
1.4.7.2 SSC PPs ........................................................................................................35
© 2016 by Taylor & Francis Group, LLC
2Handbook of Thermoplastics
1.1 INTRODUCTION AND HISTORICAL BACKGROUND
The rst known polyolen, polyethylene, was discovered by two research scientists at the Imperial
Chemical Industries (ICI) in 1933. See Ref. [1] polymerized ethylene using less than 0.2% oxygen
as a free radical polymerization initiator at 200°C and pressures of 0.1–0.3 GN/m2. The rst com-
mercial free radical polymerization plant, using peroxides and/or peroxyesters, was in operation
in September 1939. By the early 1940s, low-density polyethylene (LDPE) production was already
based on two high-pressure technologies, namely, autoclave reactor and tubular reactor, yielding
two signicantly different product streams, one for extrusion coatings and the other for lm produc-
tion. The ICI’s free radical polymerization process involves the following four reaction steps: initia-
tion, propagation, termination, and chain transfer. Chain transfer incorporates disproportionation,
hydrogen abstraction, scission reactions, and intermolecular as well as intramolecular hydrogen
transfer.
Although the original LDPE process did not involve the use of catalysts, its autoclave reactor and
tubular reactor technologies are still in use as they have been adapted for a variety of catalysts to make a
variety of polyolens. Generally, polyolens consist of LDPE, linear low-density polyethylene (LLDPE),
very-low-density polyethylene (VLDPE), ultra-low-density polyethylene (ULDPE), medium-density
polyethylene (MDPE), high-density polyethylene (HDPE), ethylene–octene copolymers, polypropylene
(PP), stereo-block PP, olen block copolymers, propylene–butane copolymers, propylene-based elas-
tomers, polyolen plastomers, poly(α-olen)s, and ethylene–propylene–ethylidenenorbornene(EPDM).
Polyolens are known for their low energy demand duringpolymerization and melt processing.
In the catalyzed production of polyolens, the most crucial differences are evident in the micro-
structure of the polymers resulting from each of the catalysts. Taking polyethylene as an example,
titanium-based catalyst normally yields narrow-molecular-weight linear polyethylene, whereas the
vanadium- and chromium-based catalysts yield intermediate-molecular-weight linear polyethylene
or broad-molecular-weight distributions. On the other hand, the modern metallocene and nonmetal-
locene single-site catalysts (SSCs) are able to produce narrow-molecular-weight linear polyethylene
with long chain branching.
Polyolens have established themselves among the most widely used commodity polymers dur-
ing the last eight decades. They now represent more than 50% of global production capacity for all
commodity plastics. The global plastics market was 1.5 million tons in 1945 and 245 million tons
in 2006 [2,3]. The global thermoplastics market, representing approximately 10% of the global
chemical industry [4,5], was about 90 million tons in 1995, 60% of which was accounted for by
polyolens [6]. In 2007, the global polyolen market consisted of 65 million tons for polyethylene
with 6% projected growth rate, 40 million tons for PP with 8% projected growth rate, and 5 million
tons for other olens polymers. It is noteworthy that the global consumption of plastics is 50% more
than steel consumption in volume. Today, the cost of polyolens is going down on account of the
natural gas boom from shale gas [7].
1.4.8 Poly(cycloolens) from SSCs ......................................................................................39
1.4.8.1 Poly(cycloolens) by Vinyl Polymerization.................................................39
1.4.8.2 Poly(cycloolens) by ROMP ........................................................................39
1.4.9 Cycloolen Copolymers from SSCs ...........................................................................40
1.5 Polyolen Composites and Nanocomposites .......................................................................... 41
1.5.1 Conventional Polyolen Composites .......................................................................... 41
1.5.2 Polyolen Nanocomposites .........................................................................................42
1.5.2.1 Nanocomposite Formation by Physical Methods ........................................ 42
1.5.2.2 Nanocomposite Formation by Chemical Reaction ...................................... 43
1.6 Processing Methods for Polyolens ........................................................................................ 44
References ........................................................................................................................................ 45
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3Polyolefins
Polyolens nd applications in a variety of commercial space including construction, agricul-
ture, transportation, appliances, electronics, and communication. Polyolens are mostly used in
packaging applications and use and throw products. Other applications include durables and con-
sumable products such as plastic pipes, wire and cable coatings, shoe soles, storage containers,
garbage bags, industrial packaging lms, food packaging lms, rigid food containers, paper coat-
ing, etc. Polyolens also nd applications in insulation, furniture, textiles, banknotes, and many
more. Exceptional properties are the driving forces in developing new polyolens, broadening
the property envelope and expanding their boundaries toward the areas traditionally occupied by
more sophisticated, expensive, and sometimes hazardous materials. Polyolens are economically
and ecologically attractive materials; they possess an extraordinary recycling capacity. Sometimes
referred to as solid crude oil, they offer advantages over other plastics in recycling. They may be
degraded by catalytic hydrogenation, cracked, or used as a source for incineration.
This chapter will focus on all polyolens including polyolen nanocomposites. In Section 1.2,
four types of polyolen catalysts will be discussed: (1) chromium-based catalysts; (2) Ziegler–Natta
catalysts; (3) metallocene SSC; and (4) postmetallocene SSC. All four categories are important for
polyethylenes, but the last three categories of catalysts are far more relevant for PPs.
1.2 CATALYST SYSTEMS FOR OLEFIN POLYMERIZATION
Much like the uncatalyzed free radical initiated polymerization, the ionic polymerization processes
of the transition metal halides and the transition metal oxide catalysts also involve initiation, propaga-
tion, and termination steps. The catalysts could be homogeneous or heterogeneous. A typical hetero-
geneous olen polymerization catalyst system may consist of (1) a support, (2) a surface-modifying
reductant, (3) a catalyst precursor, and (4) a cocatalyst that activates the catalyst. The order of addition
of components has an effect on the overall nature of the catalyst system. The support normally has
to be pretreated either by physical dehydroxylation (calcination), chemical dehydroxylation, thermal
degassing, or surface modication using a reductant [8]. The factors affecting the overall performance
of a supported catalyst include (1) dispersion of the catalyst precursor, (2) transformation character-
istics during support pretreatment, (3) interaction of the catalyst precursor with the support, (4) pos-
sible agglomeration of the catalyst precursor, and (5) catalyst impurities and poisoning. An important
element of catalyst design is the prevention of dangerous runaway reactions, particularly in gas-phase
polymerization where explosion could be especially devastating.
1.2.1 Chromium-Based Catalysts
The rst solution phase process for the production of linear HDPE, involving the use of transition
metal oxide catalysts at 100–250°C and pressures of 3–5 MN/m2 [9,10], was carried out in 1950–
1952. Independently, Standard Oil of Indiana and Phillips Petroleum Company used molybdenum
oxide and chromium oxide catalysts, respectively, to produce HDPE (after vaporizing the solvent).
By the 1960s, catalyst development efforts enabled the low-temperature production of linear HDPE
solid using a slurry phase reactor with an inert solvent. High-activity catalysts, developed by the
middle of the 1960’s nally enabled the introduction of gas-phase ethylene polymerization. Today,
several variants of these processes are in operation in different parts of the world using modern cat-
alysts. PP production followed a similar trend except that it almost always lags behind polyethylene.
1.2.1.1 Phillips Catalysts
The chromium oxide ethylene polymerization catalyst was a product of serendipity at the Phillips
Petroleum Company in 1950 when it was discovered by He, Lanning, Hogan, and Banks [10].
Chromium oxide was supposed to aid the conversion of renery stack gases into motor fuel; how-
ever, it converted the ethylene in the stack gases into polyethylenes. The chromium oxide catalyst
consists of a refractory support and an oxide of Cr(II), Cr(III), Cr(VI), or any inorganic chromium
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4Handbook of Thermoplastics
compound that could be calcined to chromium oxide, such as chromic nitrate, chromium sulfate,
ammonium chromate, chromium carbonate, chromyl chloride, and t-butyl chromate [11–14].
Since the early days, the Phillips catalyst has been modied using a variety of inorganic and
organic compounds including boron trichloride, boron phosphate, boron ester, isopropyl borate,
trimethyl borate, ammonium tetrauoroborate, ammonium hexauorosilicate, α,ω-aliphatic diene,
isoprene/nickel oxide, isoprene/nickel nitrate, isoprene/nickel acetate, isoprene/nickel chloride,
titanium/ tetraisopropyl titanate, magnesium ethoxide/tetraisopropyl titanate, dibutyl magnesium/
tetraisopropyl titanate, trialkyl dialkyl phosphate-titanate, or benzene. The supports that have been
used include silica, alumina (could be uorided or phosphated), silica-alumina, zirconia, zirconia-
silica cogel, thoria, germania, or mixtures thereof. Triisobutyl aluminum, 1,5-hexadiene/triethyl
aluminum, 1,7-octadieneand/triethyl aluminum, and triethyl borane have been used as cocatalysts
for chromium oxide catalysts [11–14].
The SiO2-supported chromium oxide catalyst could be activated at 750–1500°C in a stream of
nonreducing moisture-free gas containing oxygen or in vacuo at temperatures between 400°C and
900°C. Photoreduction with a mercury lamp light source, in the presence of carbon monoxide, was
also found to be just as effective, and it could be done at temperatures as low as 200°C. Generally,
activation with carbon monoxide, as with triethyl borane, reduces the oxidation state of chromium.
On the other hand, molybdenum oxide (or cobalt molybdate) on γ-alumina titania or zirconia sup-
port needs a reducing gas such as hydrogen or carbon monoxide.
The effects of metal oxide loading on reactivity and kinetic prole have been studied. Ethylene
polymerization is presumed to occur through the reaction of an adsorbed monomer with an adjacent
monomer or a polymer similarly adsorbed on the solid surface. In the case of Cr/AlPO4 catalyst, the
P/Al ratio has an effect on the catalyst activity as well as the tendency toward bimodality of the poly-
mer molecular-weight distribution. In general, however, chromium-based catalysts yield linear HDPE
with intermediate- or broad-molecular-weight distribution. Perhaps, the single outstanding issue with
chromium catalysts is the fact that the active site structures remain a matter of controversy until today.
1.2.1.2 Organochromium Catalysts
The organochromium family of catalysts, due to Union Carbide (UCC is now part of Dow
Chemical Company), consists of closed ring bis(cyclopentadienyl)chromate (chromocene) [15] and/
or bis(triphenylsilyl)chromate [16]. The latter is the reaction product of triphenylsilanol and chro-
mium trioxide, which when supported on silica-alumina is very active particularly for the gas-phase
ethylene polymerization process. The former, when supported on high-surface area silica, liber-
ates a cyclopentadienyl (Cp) group and polymerizes ethylene via a coordinated anionic mecha-
nism. Other organochromium catalysts with π-bonded ligands include open ring chromocene [17],
namely, dimethylpentadienyl chromate [Cr(DMPD)2], mixed open/closed ring chromocene, namely,
Cr(Cp)DMPD [18], biscumene [Cr(0)] [19], Cr(neopentyl)4, bismesitylene [Cr(O)] [20], as well as
those with σ-bonded ligands such as Cr4(trimethylsilyl)8, Cr(trimethylsilyl)4, chromium acetyl ace-
tonate, chromium acetate, chromium stearate, Cr(benzene)2, Cr(octate)3, as well as organophospho-
ryl chromium compounds. The supports normally used are silica, aluminophosphate, alumina, and
uorided and phosphated alumina.
The organochromium catalysts are not as thermally stable as the chromium oxide family of cata-
lysts, but a calcined chromium oxide catalyst could be modied by impregnation with organochrom ium
compound in order to form a highly active mixed Cr–Cr catalyst. This is exemplied by the modica-
tion of hexavalent t-butyl chromate with zero-valent organochromium dicumene Cr(0) as well as that
of divalent Cr(II) or hexavalent Cr(VI) oxide with the divalent alkyl chromium Cr4(trimethylsilyl)8
[19]. Similarly, organochromium esters could form bimetallic complexes with metal chlorides. The
hydrated chromium acetate [Cr(CH3COO)3·H2O]/magnesium chloride and anhydride chromium stea-
rate [Cr(C17H35COO)3]/magnesium chloride systems have been extensively investigated [21,22] with
AlEt2Cl, Al2Et3Cl3, AlEtCl2, or AlEt3 as the cocatalyst. The effects of chromium loading on reactivity
and kinetic prole have also been studied for organochromium catalysts [23].
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5Polyolefins
1.2.2 Ziegler–Natta Catalyst systems
The other type of ionic polymerization process for the production of linear HDPE became a reality
in 1953 when Karl Ziegler discovered [24] the rst-generation transition metal halide catalyst by
combining the salts or oxides of the periodic table groups IV-B and V-B metals with organometallic
aluminum alkyls cocatalyst. Guilio Natta’s major contribution [25] was the use of the Ziegler cata-
lyst, namely, TiCl4–AlEt3, for the isospecic polymerization of propylene in 1954, and the resulting
family of catalysts is collectively called the Ziegler–Natta catalysts. Stereoregularity is an impor-
tant practical property in the polymerization of vinyl monomers, CH2==CHR, which is capable of
yielding polymers that are atactic, characterized by a random arrangement of R; isotactic, charac-
terized by an arrangement of R uniformly on one side of the polymer backbone; and syndiotactic,
characterized by an arrangement of R on the alternate side of the polymer backbone plane.
The second-generation MgCl2 and/or donor-supported Ziegler–Natta catalyst system, which
were at least 100 times more active, led to the development of the low-pressure polymerization
processes for polyolens and synthetic elastomers. This revolutionary development resulted in the
simplied gas-phase low-pressure polymerization plant operation without the need for the removal
of residual trace catalyst from the polymer, making nonpelletized LLDPE, ULDPE, or VLDPE,
PP, and its copolymers. Simonazzi and Giannini [26] provided an impressive array of the accom-
plishments in the science, engineering, and technology of the Ziegler–Natta catalysis. Although the
review sought to highlight the signicant role of Montell (now called Basell), it provides an insight
into the worldwide efforts related to the simplication of the polyolen process technologies in
terms of economics, versatility, safety, and environmental efciency.
Basically, both the heterogeneous transition metal halide and oxide catalyst systems are character-
ized by the following common features: (1) a solid surface for monomer adsorption; (2) a transition
metal that is easily converted from one to the other of its several valence states; and (3) a propensity
for the formation of organometallic compound with another organometallic compound or a monomer.
However, stereospecic polymerization of butene-1 or propylene (small, nonpolar, volatile monomers)
requires the presence of a strong complexing active center adsorbed on a solid surface [27–30].
The Ziegler–Natta [24,25] catalyst system consists of two components, namely, the transition metal
compound customarily called the catalyst and the alkylaluminum compound customarily called the
cocatalyst. Some typical examples of these compounds are presented in Table 1.1. The reactions of
the various catalysts and cocatalysts have been studied extensively, and the product derived from the
reaction between, for example, TiCl4 and AlEt3 is known to consist of a partial colloidal mixture of
the titanium halides at various oxidation states [26]. No complex compound was found that includes
the two metal atoms such as titanium and aluminum [27–30]. The preferred Ziegler–Natta titanium
catalyst compounds are the high-surface-area violet crystalline forms of TiCl3, and the commercially
utilized titanium trichlorides are normally activated by hydrogen or by organometallic compounds
such as organoaluminum, organozinc, or organomagnesium compounds. Complete reduction of TiCl4
to TiCl3 could be accomplished with EtAlCl2 or Et2AlCl at 1:1 or 2:1 ratios, respectively [31–34].
The activity and yield of the catalyst largely depend on the nature of the cocatalyst (activator) and
on the catalyst/cocatalyst ratio. The effects of additional organic adjuncts attached to the aluminum
cocatalyst underscore the fact that the activity of a catalyst system depends strongly on the cocata-
lyst type [35–37]. Dual functional titanium catalysts and benzyl derivatives of titanium, which are
active in the absence of aluminum trialkyl, also exist [38–40].
Selected Ziegler–Natta catalysts, based on zirconium and vanadium, are as follows: (1) Zr(OC3H7)4
and Zr[OCH(CH2CH3)2]4; (2) Zr(OC4H9)2Cl2, Zr(OC6H13)2Cl2, and Zr(OC8H17)2Cl2; (3) VCl3 [40], VCl4
[41,42], and VCl3(THF)3 [42,43]; (4) VOCl3 [42,44–46], VO(OBu)3 [47], and VO(OC2H5)3; (5) vanadyl
acetate [48]; and (6) mixtures. Unlike titanium or the other transition metal catalysts, vanadium cata-
lysts need promoters such as chloroform [43,49], Freon-11 [42], dichloromethane or methylene dichloride
[41–43], trichlorouoromethane [42,43,49], 1,1,1-trichloroethane [42,49,50], hexachloropropane, hepta-
chloropropane, or octachloropropane [51]. Because of the structural and chemical homogeneity of its
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6Handbook of Thermoplastics
active center, the homogeneous vanadium-based catalysts are traditionally used for the production of
ethylene–propylene rubber (EPR) copolymer and ethylene–propylene–diene monomer (EPDM) terpoly-
mers. The preferred cocatalyst is halogenated aluminum alkyls, and the preferred promoters include
ethyl trichloroacetate, n-butyl perchlorocrotonate, and benzotrichloride. In the production of LLDPE,
silica-supported vanadium catalysts are particularly active in the presence of halocarbon promoters
resulting in a higher α-olen comonomer incorporation rate and better comonomer distribution along
the polymer chain. However, the vanadium-based catalysts are less capable of controlling the molecular-
weight distribution, yielding intermediate- or broad-molecular-weight distribution compared with those
based on titanium, zirconium, or hafnium. Calcium carbonate-mixed silica support could also be used
for the vanadium-based catalysts.
Several methods exist for the preparation of the varieties of supported Ziegler–Natta catalysts.
Some of these are impregnation, milling, comilling [32], or solution methods. Cocrystallization using
TABLE 1.1
Examples of Two-Component Ziegler–Natta Catalyst Systems
Transition Metal Salt Organometallic Compounds
TiCl4Et3Al
Zr(OC3H7)4Et3Al
Zr[OCHEt2]4Et3Al
VCl3Et2AlCl
V(acac)3aEt2AlCl
Cr(acac)3aEt2AlCl
CoCl2 2 pyridine Et2AlCl
Zr(OC3H7)4Et2AlCl
Zr[OCHEt2]4Et2AlCl
TiCl4BuLi
TiCl3Bu2Mg
Cp2bTiCl2EtAlCl2
TiCl4, VCl3, or TiCl3Et2AlCl
TiCl4, VCl3, or TiCl3(i-C4H9)xAly (C5H10)zc
TiCl4, VCl3, or TiCl3Et3Al2Cl3
TiCl4, VCl3, or TiCl3Et3Al
TiCl4, VCl3, or TiCl3(i-C4H9)3Al
TiCl4, VCl3, or TiCl3(i-C4H9)2AlH
DEAC
Isoprenyl
EASCd
TEAL
TIBAL
DIBAL-
Source: Simonazzi, T. and U. Giannini, Gazz. Chim. Ital. 124: 533, 1994; SRI International,
Polyolen Markets and Resin Characteristics, Vol. 2, Project No. 3948, SRI, Menlo
Park, CA, 1983; SRI International, Polyolens Production and Conversion Economics,
Vol. 3, Project No. 3948, SRI, Menlo Park, CA, 1983; SRI International, Polyolen
Production Technology, Vol. 4, Project No. 3948, SRI, Menlo Park, CA, 1983.
a acac, acetylacetonate anion.
b Cp2, cyclopentadienyl.
c Where z ≈ 2x, made by reacting TIBAL or DIBAL-H with isoprene.
d Ethyl aluminum sesquichloride; Zr(OC3H7)4 and Zr[OCH(CH2CH3)2]4 react with Et3Al and
Et2AlCl.
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7Polyolefins
low-valency transition metal carbonyls [52] such as Mn2(CO)10, Mn(CO)5Cl, V(CO)6, and Fe(CO)8
results in solid solutions, such as FeCl2·2TiCl3 and MnCl2·2TiCl3, which are known to be quite
active. Dialkyl magnesium compounds have also been used as reducing agents including the fol-
lowing: dimethyl magnesium, diethyl magnesium, di-n-butyl magnesium, n-butyl-s-butyl magnesium,
ethyl-n-butyl magnesium, ethyl-n-hexyl magnesium, dihexyl magnesium, and butyloctyl magnesium
[32,33,36,52]. Metal chloride reducing agents, such as SiCl4 [50] and BCl3 [37,53], have also been used.
Generally, the most active catalyst is based on titanium, and the high-activity, high-yield
MgCl2-supported titanium chloride catalyst is produced either by dry comilling of MgCl2 and
titanium halides or by cocondensing MgCl2 vapor with the vaporized toluene/TiCl4 or heptane/
TiCl4 or diisopropylbenzene/TiCl4 substrates [35] or by solution. The solubility of MgCl2 in the
electron donor solvent, such as tetrahydrofuran (THF), increases in the presence of the reduc-
ing Lewis acid such as aluminum chloride, ethyl aluminum, and boron trichloride. This is a
good technique for activating the magnesium halide-based titanium or vanadium catalysts [53].
However, the catalyst reactivity and stereospecicity of the MgCl2-supported titanium chloride is
related to the structure of α-TiCl3, γ-TiCl3, and δ-TiCl3 vis-à-vis that of the MgCl2 support [26].
The crystalline layer structure of the violet TiCl3 is similar to that of MgCl2, and dry comilling
of the two results in favorable epitaxial placement of the active dimeric titanium chloride on
the (100) lateral planes of MgCl2 exposing a larger number of stereospecic sites, and hence the
increased propagation rate. While the lateral (100) surfaces are known to be stereospecic, the
(110) planes are known to be aspecic.
In addition, the chemical nature and porosity of MgCl2 are said to play more effective roles than
the specic surface area [34,54]. Indeed, complexes containing titanium and magnesium bonded by
double-chloride bridges have been observed, exposing the titanium atoms on the catalyst surface where
they are more accessible [26]. Silica, silica–alumina, modied or unmodied, as well as MgO supports
have been used with mixed results [31–36,53,55–57]. Catalyst modiers such as NdCl3, BaCl2, ZnCl2,
ZnEt2, and Grignard reagents (C6H5MgCl) have been used. Magnesium alkoxide modiers that have
been used include magnesium methoxide and magnesium ethoxide [31,32,55,58–61]. For the MgCl2/
TiX4/Al(iBu)3 system, the nonchloride ligands impart decreased activities, although the resulting poly-
olens might have improved properties [62]. With nonchloride ligands, the activity of the titanium-based
catalysts increases with decreasing electron-releasing capability of the ligand [63] in the following order:
Ti(OC6H5)4 > Ti(O(CH2)3CH3)4 > Ti(N(C2H5)2)4. This is further illustrated by another study where the
catalyst activity is in accordance with the following order [64]: TiCl4 > TiCl2(OBu)2 > TiCl(OBu)3.
The high-yield, high-stereospecicity MgCl2 donor-supported titanium chloride catalyst was rst
developed by using a Lewis base modier like the esters of aromatic monocarboxylic or phthalic
acids or alkylphthalate as the internal donor, which is added in the preparation of supported cata-
lysts. This fourth-generation Ziegler–Natta catalyst system, rst developed in the 1980s, is still
extensively used in polyolen manufacturing. Other internal donors, developed beyond the year
2000, include 1,3-dione, isocyanate, 1,3-diether, malonic ester, succinate, 1,3-diol ester, glutaric
acid ester, diamine, 1,4-diol, 1,5-diol ester, phthalate esters, and cycloalkyl esters.
Further developments led to the use of an additional modier as the external donor such as bifunc-
tional Lewis base, which is added during the olen polymerization process. The bifunctional Lewis
base is essentially a cocatalyst, such as polyalkoxysilane, along with the aluminum trialkyl, which is a
strong Lewis acid. The use of the internal and external donors led to the possibility of controlling the
morphology of the catalyst granules. The catalyst morphology (size, shape, and porosity) is important,
and there are signicant differences in the polymerization rate pattern between granular versus pow-
dered catalyst or between spherical powder versus granular powder [37,65,66]. Indeed, the morphology
of the resulting polymer particle replicates that of the catalyst, which essentially acts as a template for
the polymer growth, justifying the trend toward spherical catalyst particles [26,65,66]. The polymer
particle size distribution is also similar to that of the catalyst. This similarity has made it possible to con-
trol the polymer granule size, its porosity, and its stereoregularity. Additionally, the polymer molecular-
weight distribution could be similarly controlled by changing the structure of the external donor.
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8Handbook of Thermoplastics
The organic silane external donors that have generated most interest in terms of stereospecic-
ity include dicyclopentyl dimethoxy silane (DCPDMS), diisopropyl dimethoxy silane (DIPDMS),
methyl cyclohexyl dimethoxy silane (CHMDMS), and diisobutyl dimethoxy silane (DIBDMS), in
that order. The excellent variety of internal and external donors in use consolidated the position of
the fourth-generation Ziegler–Natta catalyst system as one of the most widely used in polyolen
manufacturing.
A further advance, made in 1989 by Himont, made it possible to use a single donor, such as
1,3 diethers, eliminating the need for the external donor. The development was considered to be
the fth-generation Ziegler–Natta catalyst systems. The mechanism of the Lewis base donors is
presumed to be related to the selective poisoning and/or modication of the aspecic catalyst sites
through complexation with the base. The 1,3 diether is characterized by the desired oxygen–oxygen
distance, which is crucial for chelating to the tetracoordinate magnesium atoms located on the (110)
aspecic plane of MgCl2.
This line of catalyst development made it possible to exploit the living polymerization capa-
bility of each catalyst granule whereby each granule contains a living polymer whose growth
could be spontaneously continued by the addition of the same or different sets of monomers
resulting in block or multimonomer copolymers. Thus, not only does the technology facilitate
the synthesis of homopolymers whose molecular weight ranges from very low to extremely high,
but the technology also facilitates the in situ manufacture of heterophase polyolen copolymers,
blends, and alloys, incorporating nonolenic comonomers with each catalyst granule serving as
a self-contained reactor. The traditional and high-yield MgCl2-supported Ziegler–Natta catalyst
systems for PP polymerization appear in Table 1.2 and those for polyethylenes appear in Table
1.3. Catalyst preparation effort sometimes results in process innovation exemplied by Borealis
development of an emulsion process. Focused effort by BASF and SINOPEC BRICI also yielded
unique Ziegler–Natta catalyst systems. Supported or unsupported Ziegler–Natta catalyst systems
are also used in olen oligomerization [67].
Ziegler–Natta catalyst research effort continues with multiple purposes. The rst is focused on
catalyst morphology and particle size distribution with the goal of controlling the resulting polyole-
n particle size distribution, ne content, and bulk density. The second is to increase the catalyst
activity while decreasing its surface area and porosity. The third is devoted to catalyst improvement
TABLE 1.2
Traditional and High-Yield MgCl2-Supported Ziegler–Natta Catalyst Systems for Propylene
Polymerization
Catalyst System Activity [kg PP/(mol Ti) MPa h] lsotactic Indexa (%)
TiCl3–Et2AlCl 76 90–95
MgCl2-supported TiCl4–Et3Al 9000 30–50
MgCl2-supported TiCl4/LB1b–Et3Al 7000 50–60
MgCl2-supported TiCl4/LB1b–Et3Al/LB2c6000 92–95
MgCl2-supported TiCl4/LB3d–Et3Al/LB4e15,000 98–99
MgCl2-supported TiCl4/LB5f–Et3Al 20,000 97–99
Source: Simonazzi, T. and U. Giannini, Gazz. Chim. Ital. 124: 533, 1994.
a Weight percent of polymer insoluble in boiling n-heptane.
b LB1, ethyl benzoate.
c LB2, methyl 4-methylbenzoate.
d LB3, diisobutyl phthalate.
e LB4, dicyclopentyldimethoxysilane.
f LB5, 2,2-diisobutyl-1,3-dimethoxypropane.
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9Polyolefins
in terms of stereospecicity. It was such an effort that resulted in evolutionary catalysts, which
enabled the formation of diversied molecular structures during polymerization. Such catalysts
enabled reproducible molecular and morphological structures for homopolymers, copolymers, and
blends, and the control of molecular weight, molecular-weight distribution, monomeric units’ con-
guration, chain conguration, and size and shape of polymer particles. These were the periodic
table groups IV catalysts [68–70] now christened metallocene SSC systems.
1.2.3 ssC systems
The key distinctions between Ziegler–Natta catalyst and SSC relate to the following characteristics.
Ziegler–Natta catalysts possess multiple reaction sites, use simple aluminum alkyls as cocatalysts,
use internal and external electron donors, and produce polyolens with broad-molecular-weight dis-
tribution, nonuniform chain lengths, high bulk density, and high soluble content. On the other hand,
SSCs are single sited, use alkyl aluminoxane and bulky anions as cocatalysts, use no internal or
external electron donors, and yield polyolens with narrow-molecular-weight distribution, uniform
chain lengths, low bulk density, and low soluble content. SSCs are characterized by higher activity,
outstanding ability to incorporate sterically demanding comonomers, and the ability to vary the
comonomer distribution (alternating, random, or block) over the entire polymer chain backbone.
From an ecological viewpoint, the total ash arising from the incineration of SSC polymer is only a
fraction of what is produced by an equivalent Ziegler–Natta polymer.
1.2.3.1 Metallocene SSC
The typical chemical structure of a Group IV metallocene catalyst is characterized by a well-dened
organometallic molecular complex constrained mostly by tetrahedral geometry. Metallocene SSC
consists of a transition metal atom that is sterically hindered in that it is sandwiched between
π-carbocyclic ancillary ligands such as Cp, uorenyl, indenyl, or other substituted structures, and
it is sometimes referred to as a half-sandwich titanium amide system. When the two π-carbocyclic
ancillary ligands on either side of the transition metal are unbridged, the metallocene is nonste-
reorigid and is characterized by C2ν symmetry. When the two ligands are bridged, the metallocene
is stereorigid, and it is called ansa-metallocene, which could be characterized by C1, C2, or Cs
symmetry. Although the π-ligands are most common, halides and σ-homoleptic hydrocarbyls are
the other two classes of ligands of the Group IV metallocene SSCs. In addition, the bridging moi-
eties on ansa-metallocenes have electronic and steric implications. Variations within ligands and/or
bridging moieties result in variations of catalyst stability, catalytic activity, kinetic prole, polymer
stereoregularity, monomer/comonomer incorporation capability, molecular-weight characteristics,
and polymer microstructure.
The rst homogeneous metallocene catalyst was discovered in 1957 by Natta et al. [25], who
replaced the chloride ligand of the Ziegler–Natta transition metal catalyst with bis(cyclopentadienyl)
titanium compounds together with aluminum alkyls for ethylene polymerization. A breakthrough
TABLE 1.3
Traditional and High-Yield MgCl2-Supported Ziegler–Natta Catalyst
Systems for Ethylene Polymerization
Catalyst System Activity [kg polym/(mol Ti) MPa h]
TiCl3–Et2AlCl 320
TiCl3Et3Al 710
MgCl2-supported TiCl4–Et3Al 17,000
Source: Simonazzi, T. and U. Giannini, Gazz. Chim. Ital. 124: 533, 1994.
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10 Handbook of Thermoplastics
occurred when Kaminsky and his coworkers noticed that the addition of water to trialkyl aluminum
in a molar ratio of 1:1 signicantly improved the catalyst activity [68]. Thus, by the 1980s and 1990s,
homogeneous catalysts based on group IVA metallocenes and an aluminoxane, especially methyl-
aluminoxane (MAO), as cocatalysts gained widespread industrial and scientic interest [68–75].
The catalyst components, as well as the active species generated from them, are soluble in hydro-
carbons; thus they were originally referred to as homogeneous Ziegler–Natta catalysts but later
called post Ziegler–Natta catalysts. These are the periodic table groups IV [68–70] metallocene
SSC systems based primarily on three key transition metals, namely, zirconium (Zr), titanium (Ti),
and hafnium (Hf), and are now referred to as SSC systems.
1.2.3.1.1 MAO Cocatalyst
MAO cocatalyst is a mixture of oligomers formed by the controlled reaction of trimethylaluminum
and water under elimination of methane. Its structure was a riddle until Barron [76–78] and Sinn
[79] discovered that association/dissociation phenomena, condensation and cleavage by trimethyl-
aluminum, characterize the dynamic behavior of MAO in solution. MAO is an amorphous, pyro-
phoric solid that is soluble in toluene but insoluble in hexane. The average number of aluminum
units in the cluster of MAO varies between 10 and 20; higher-molecular-weight MAO compound
is essentially insoluble. MAO is oligomeric and its degree of oligomerization, n, strongly inu-
ences the metallocene catalyst activity. Depending on the metallocene type and composition, the
most effective range for MAO is 3 < n < 50. The catalyst stability, catalytic activity, kinetic prole,
polymer stereoregularity, monomer/comonomer incorporation capability, molecular-weight charac-
teristics, and polymer microstructure are affected not only by the amount of MAO but also by the
metallocene/MAO ratios. In large-scale commercial processes, heterogenization has enabled the
reduction of the amount of metallocene/MAO ratios such that the ratio of aluminum to transition
metal is low. Silica, silica–alumina, resinuous materials, and mixtures have been used as catalyst
supports.
MAO is crucial in the formation of the metallocenium active species. The generation of the active
species involves alkylation of the metallocene by the MAO and abstraction of a methyl group from
the dimethyl metallocene. The resulting active species is a 14-valence electron cationic alkylmetallo-
cenium ion [80] formed by dissociation of the metallocene–aluminoxane complex. Because the cata-
lyst adjunct, [MAO Me]– anion, is essentially noncoordinating or weakly coordinating, the incoming
olen monomer coordinates instead with the cationic alkylmetallocenium ion, via the metal–carbon
bond, into the previous monomer. Thus, the growing chain migrates through the 4-center transitional
state regenerating the vacant coordination site for the next incoming monomer.
The Lewis acid catalyst adjunct, the [MAO Me]– anion, may be replaced by another Lewis acid
such as Me2AIF [15], [B(C6H5)4]– [16], [C2B9H12]2– [81], [B(C6F5)3] [82], [Ph3C][B(C6F5)4] [83,84],
[R2R′NH][B(C6F5)4] [85], or [Ph3C][B(C6F4(SiR3)] [86], in combination with trialkylaluminum
(TMA) compounds as alkylating agent and dichlorometallocenes or simply in combination with
dialkylmetallocenes. In most of these substituted systems, two problems arise. Because of the
highly unsaturated character of the active species, a scavenger is needed, usually TMA. Secondly,
almost all the Lewis acids interact somewhat with the active species and may not be regarded as
noncoordinating [87]. The only systems that seem to fulll the requirements as good cocatalysts are
those based on [Ph3C][B(C6F5)4], [R3NH][B(C6F5)4], or [Ph3C][B(C6F4(SiR3)].
1.2.3.1.2 Importance of Metallocene SSC Symmetry on Tacticity
Owing to the fact that the polymer chain migrates during insertion, the symmetry of the metal-
locene SSC is of fundamental importance to the tacticity of the polymer produced. When a chiral
metallocene active center is combined with a prochiral monomer such as propylene or hexane-1
monomer, a diastereotopic transition state is formed. This makes possible two sets of activation
energies for monomer insertion. Stereospecicity may arise either from the chiral β-carbon atom at
the terminal monomer unit of the growing chain (chain end control) or from the chiral catalyst site
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11Polyolefins
(enantiomorphic site control). Therefore, the microstructure of the polymer produced depends on
the mechanism of stereocontrol as well as on the nature of the metallocene used.
The rst chiral-bridged zirconocene synthesized in 1984 by Brintzinger and used as an isospecic
polymerization catalyst by Kaminsky was racemic ethylenebis(4,5,6,7-tetrahydro-1-indenyl) zirconium
dichloride [88–90]. Ewen [91] showed that the analogous ethylenebis(1-indenyl)titanium dichloride
(amixture of the meso form and racemate) produces a mixture of isotactic PP (iPP) and atactic
PP(aPP). The chiral titanocene as well as the zirconocene were shown to work by enantiomorphic
site control. With titanocene, the achiral mesostructure causes the formation of atactic polymer. Also,
achiral metallocenes like Cp2ZrCl2 or [Me2Si(Flu)2]ZrCl2 produce aPP. The polymerization is not
stereo- but regioselective due to the bent structure of the tetrahedral active complex favoring 1, 2
insertions. The nonbridged metallocene catalyst (2-Ph-Ind)2ZrCl2 is a mixture of the meso form and
racemate. The racemate produces iPP, whereas the meso form produces aPP.
Using Cs-symmetrical metallocenes, syndiotacticity is due to enantiotopic vacancies formed by
the chain migratory insertion [92–94] during polymerization. If, however, the π-carbocyclic ancil-
lary ligand used in the catalyst has substituents at key positions, isospecic polymerization of pro-
chiral monomer units could occur owing to steric hindrance. Chiral C2-symmetric metallocenes,
like bridged bis(indenyl) compounds, possess homotopic coordination sites, which favor identical
orientation of the approaching prochiral monomer resulting in isospecic polymerization. For achi-
ral C2ν symmetric metallocenes, the polymer tacticity is determined by the chain-end controlled
mechanism, and atactic polyolens are formed because the conguration of the asymmetric center
of the last-inserted prochiral monomer unit occasionally changes. Hemi-isotactic polymerization of
prochiral monomer units normally results from the use of C1-symmetrical metallocene SSC. That
is, every other methyl group has isotactic placement; the remaining methyl groups are randomly
placed.
1.2.3.1.3 Significance of Metallocene SSC in Polyolefins
The metallocene SSC has the capability of producing polyolens with terminal unsaturation, which
are used for building functionalities in the polymer. Considering only the olens, the metallocene
SSC possesses an extraordinary versatility for polymerizing a variety of monomers that include
simple olens, diolens, cyclic olens, and cyclic diolens. Higher-molecular-weight polyolens are
enhanced with Group IV bis(2-R-indenyl) ansa-metallocenes [73]. Metallocenes living homopoly-
merization and copolymerization capability is exceptional, and the number of copolymer composi-
tional permutations made possible is large. Mixed metallocene SSC made possible the development
of new types of polymer blends (PBs) that are made in situ. These mixed metallocenes differ in
terms of transition metals, C1, C2, C2ν, or Cs symmetry, π-ligands, halides ligands, σ-homoleptic
hydrocarbyl ligands, bridging moieties, Lewis acid catalyst modiers, or scavengers.
The rst commercial utilization of Kaminsky-type metallocene SSC, a conventional biscyclo-
pentadienyl catalyst, was in 1991 by Exxon (now ExxonMobil) using high-pressure autoclaves and
gas-phase reactors (Exxpol technology) to produce EXACT® polymers. This was followed in 1992
by Dow using an ansa-cyclopentadienylamido constrained geometry SST to produce AFFINITYTM
polyolen plastomers, ENGAGETM polyolen elastomers (POEs), ELITETM enhanced polyethylene,
and NORDELTM-IP EPDM. Later developments included the slurry process INSITETM technology
as well as the gas-phase technologies for LLDPE and HDPE.
Commercial production of syndiotactic PP (sPP) utilized a silica-supported metallocene SSC.
By 2003, ExxonMobil introduced propylene–ethylene copolymers VistamaxxTM, and Mitsui intro-
duced propylene–butylene copolymers Tafmer® XM both based on metallocene SSC. Attention
was also focused on the process of catalyst support preparation that enabled signicant reduction
of the MAO cocatalyst content while increasing the metallocene catalyst activity. One such method
involved spray-drying of clay and SiO2 reactant to form microspherical particles with high poros-
ity, which was used as a catalyst activator during pre-polymerization and a catalyst support during
polymerization processes. This is exemplied by IOLA, which was developed by W. R. Grace & Co.
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12 Handbook of Thermoplastics
1.2.3.2 Post-Metallocene SSC
Group IV metallocenes are well-dened organometallic molecular complexes characterized mostly
by tetrahedral geometry. It is the precise catalyst structure that facilitated incremental improve-
ments in science and technology that eventually lead to the development of the second-generation
postmetallocene SSC systems [95]. These new-generation SSC systems are based on a variety of
transition metals including Group IV and other ligands in addition to the conning Cp-based ancil-
lary ligands generally used with Group IV–metallocene SSC systems.
Aside from the tetrahedral geometry that mostly characterizes the metallocene SSC, the new-
generation SSC systems are characterized by a variety of coordination geometries including square
planar, trigonal bipyramidal, octahedral, as well as tetrahedral geometries. While metallocene SSC
enables signicant control of polyolen composition distribution and long-chain branching, the
postmetallocene SSCs, in addition, enable signicant control of polyolen purity, tacticity distribu-
tion, and unique comonomer incorporation. The key impact of postmetallocene SSC technology
is in enabling the effective control of stereo-/regio-defects on polymer backbone during polyolen
polymerization as well as comonomer blockiness for olen block copolymers.
The novel well-dened structures of the postmetallocene SSC enable unusual polymerization reac-
tions including (1) polymerization with exceedingly fast monomer insertion [96]; (2) extensive branch
formation through chain walking [97]; (3) extensive syndiospecic propylene polymerization using
C2-symmetric catalyst complexes [98]; (4) chain-end functionalization with vinyl groups and alumi-
num alkyls [7]; (5) copolymerization with vinyl functionalized polar olens [97,99]; (6) polymer-
ization of ultrahigh-molecular-weight polymers [100] with α-diimine Ni and Pd complexes bearing
sterically demanding ortho-substituted N-aryl groups [97]; and (7) adaptable olen living polymer-
ization mechanisms [101]. Indeed, an emerging concept posits that the unusual living polymerization
mechanisms are occasioned by noncovalent attractive, rather than steric, intramolecular interactions in
postmetallocene SSCs [102]. The unique features of postmetallocene SSC have resulted in exceptional
polymers that were hardly possible with previous chromium-based catalysts, Ziegler–Natta catalysts,
or metallocene SSC systems.
Postmetallocene catalyst systems include Group III to Group XIII catalysts. The ligands include
(1)Cp and other carbon-donor ligands; (2) chelating amides and related ligands; (3) chelating alkox-
ides, aryloxides, and related ligands; (4) chelating phosphorus-based ligands; (5) non-Cp-based amide,
amine, and phenoxy ligands; (6) neutral bis(imino)pyridine; (7) neutral α-diimine, neutral nitrogen-
based, and related ligands; (8) anionic ligands; and (9) monoanionic ligands [95]. Recent developments
have demonstrated that, when uorine-containing ancillary ligands are used, noncovalent interactions
control polymerization reactions rather than steric inuences [102]. That is, postmetallocene SSCs are
predominantly based on alkyl or aryl ligands containing nitrogen, oxygen, sulfur, or phosphorus atoms
and are usually devoid of Cp, indenyl, or uorine-based ligands. A variety of metal–ligand combinations
are available involving such metals as nickel, iron, copper, zinc, and titanium. In spite of the tremendous
success of late transition metal systems, an overriding metal–ligand combination rule remains elusive.
1.2.3.2.1 Postmetallocene SSC Polyolefins
Dow Chemical Company was the rst to commercialize postmetallocene SSC polyolens with
the use of metal–ligand combinations of zirconium or hafnium with pyridyl amine-based ligands.
Under the trade name of VERSIFY, propylene–ethylene plastomers and elastomers are character-
ized by narrow-molecular-weight and unique composition distribution. The second postmetallo-
cene SSC polyolens were the INFUSETM ethylene–octene block copolymers. These were produced
using chain shuttling technology, whereby two independent postmetallocene catalyst systems with
differing monomer selectivity characteristics are involved along with a diethyl zinc chain shuttling
agent [103]. Perhaps the best known postmetallocene SSC systems used for the production of PP are
(1) a diimine nickel/palladium system, (2) a pyridine diimine iron/cobalt system, and (3) a salicyl-
aldiminato titanium/zirconium system. The industrial impact of high-performance postmetallocene
SSCs is yet to be fully realized.
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13Polyolefins
1.3 PRODUCTION TECHNOLOGY
The rst commercial polyolen process was introduced by ICI [1,104] in 1939. Since then, large
capacity innovative technologies, with process optimizations, have continued to fuel the expansion
of polyolen market worldwide. The contemporary commercial olen polymerization processes
include (1) loop reactors (Spheripol, Borstar, Phillips, ExxonMobil, Sinopec ST, Hypol-II), (2) auto-
clave reactors (Hypol-I, Exxon Sumitomo, Rexene), (3) uidized bed reactors (Unipol, Sumitomo,
Catalloy), and (4) stirred bed reactors (Innovene, Horizone, Novolen). Other industrial participants
include Chevron Phillips, Hostalen, LyondellBasell (Lupotech), and Dow (Unipol).
As a result of the competitive nature of polyolen business, there have been a number of expen-
sive patent litigations exemplied by DuPont (LLDPE), Montecatini/Basell (PE), Exxon, Phillips,
Mobil and Dow (SSC catalysts), and British Petroleum and UCC (now part of Dow Chemical
Company) for the polyolen process. The historical perspectives of polyolen process develop-
ments are discussed next.
1.3.1 Free radiCal PolymeriZatioN ProCesses
Branched LDPE was rst commercialized [1] in 1939 by ICI using its high-pressure technology,
based on tubular or stirred autoclave reactors at temperatures of 200–300°C and pressures of 0.1–
0.3 GN/m2. Although the high-pressure technology was developed for free radical polymerization,
subsequent developments of tubular or stirred autoclave reactors have enabled the use of chromium-
based catalysts, Ziegler–Natta catalysts, metallocene SSCs, and postmetallocene SSCs.
1.3.1.1 High-Pressure Autoclave Reactor Process
The high-pressure autoclave reactor process operates at pressures of about 0.2 GN/m2 at temperatures
of 150–315°C. The commercial high-pressure autoclave technology available for the production of
branched LDPE generally uses oxygen or peroxide initiators. They were originally proprietary to the
following companies: ICI, Cities Services/ICI, CdF Chimie, Dow Chemical, DuPont, El Paso, Gulf,
National Distillers/ICI, and Sumitomo. The LDPE is produced at 15–20% conversion, and the den-
sity range falls within 0.915–0.925 gm/cm3. A telogen, such as butane, is normally added to the feed
stream for molecular-weight control. The LDPEs are generally used for injection molding, extrusion
coating, heavy-duty lm, and wire and cable covering. The autoclave reactor is also used in the pro-
duction of ethylene–vinyl acetate (EVA) copolymers.
The control of the LDPE density, chain branching, crystallinity, molecular weight, and its dis-
tribution require a judicious adjustment of pressure, temperature prole, initiator type and con-
centration, and telogen type and concentration. For example, low temperatures are used for the
production of injection molding grades requiring broad-molecular-weight distribution. Uniform
high temperature and high initiator concentration are used for the production of polymers with
narrow-molecular-weight distribution. Long-chain branching (LCB) is favored by high temperature,
low pressure, and high initiator concentration. An increase in temperature also leads to an increase
in chain transfer reaction (decrease in molecular weight), whereas an increase in pressure leads to an
increase in chain growth reaction resulting in an increase in density. Initiator and telogen type and
concentration could affect the LDPE chain length by a factor of 2–4. Density, which depends on the
extent of chain branching vis-à-vis the polymer molecular weight, is also sensitive to temperature
and pressure to a much lesser extent.
1.3.1.2 Tubular Reactor Process
The ow charts for the tubular and autoclave reactor processes are similar except for the reactor section
where the heavy-walled reactor tubing replaces the autoclave. The reactor tubing is 1000–2000 m long
and has 25–50 mm internal diameter, and the process operates at pressures of about 0.3 GN/m2 with
temperatures ranging between 170°C and 330°C. The commercial high-pressure tubular technology
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14 Handbook of Thermoplastics
available for the production of branched LDPE generally uses oxygen or peroxide initiators. They were
originally proprietary to the following companies: ICI, ANIC, Arco, ATO Chimie, BASF, Exxon, El
Paso, Imhausen, Distillers/ICI, Stemicarbon, Sumitomo, UCC (now part of Dow Chemical Company),
and VEB-Leuna Werke.
Molten LDPE is produced at a conversion of 25–35%, which is higher than that of the autoclave
processes principally because of the easier method of heat removal through cooling jackets. As with
the autoclave reactor, polymer density, chain branching, crystallinity, and molecular weight and its
distribution are controlled during the polymerization process. A telogen is normally added to the
feed stream for molecular-weight control, and the density of the resulting LDPE ranges between
0.918 and 0.93 g/cm3. The polymers nd applications in tough, stiff clarity lm and packaging, as
industrial liners, heavy-duty bags, shrink lm, lamination lm, wire, and cable. The tubular reactor
is also used in the production of EVA co- and terpolymers characterized by high-environmental-
stress cracking resistance useful in frozen food packaging and other high-clarity and high-gloss
applications. Certain specic operating conditions of the tubular reactor process could be related to
the following structure–property characteristics:
• The higher operating pressure results in higher reaction propagation rate, higher density,
lower degree of branches, higher molecular weight, and stiffer polymers.
• The plug ow character leads to elongated as opposed to the near-spherical structure of the
polymer made in the autoclave.
• The variability of the temperatures and pressures somewhat results in the production of
polymers characterized by a relatively wide-molecular-weight distribution.
1.3.2 PolymeriZatioN ProCesses For PolyethyleNes
The advent of the Phillips and Ziegler catalysts gave birth to the low-pressure olen polymer-
ization processes operating at pressures of about 2–10 MN/m2 and temperatures of about 100°C.
In most commercial polymerization processes, the supported catalyst system is conveyed to the
reactor either in the form of a powdery solid or a Bingham uid composition [105,106]. While
pressure is a key variable for density control in the high-pressure branched LDPE polymeriza-
tion, the amount and type of α-olen comonomer in the feed composition is the key density-
controlling variable for the low-pressure LLDPE processes. In addition, the average molecular
weight of the polymer is responsive to the polymerization temperature, whereas the molecular-
weight distribution is responsive to the catalyst system. Hydrogen is used as a molecular-weight
modier.
1.3.2.1 High-Pressure Processes
Although the high-pressure autoclave and tubular processes were developed for branched LDPE,
catalyst developments resulted in several adapted high-pressure technologies for linear MDPE, lin-
ear HDPE, and LLDPE. The available high-pressure technologies, with their corresponding catalyst
systems, were proprietary to the following companies [104,107–109]:
1.3.2.1.1 High-Pressure Autoclave Reactor
• ARCO technology using a Ziegler catalyst system, produces LLDPE.
• Bayer technology using a silyl ester catalyst system, produces linear MDPE.
• Dow Chemical technology using a Ziegler catalyst system, produces linear MDPE, HDPE,
and LLDPE.
• CdF Chimie technology using a Ziegler catalyst system, produces linear MDPE, HDPE,
and LLDPE.
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15Polyolefins
1.3.2.1.2 High-pressure tubular reactor
• ATO Chimie technology using a Ziegler catalyst system, produces LLDPE.
• Dow Chemical technology using a Ziegler catalyst system, produces linear MDPE, linear
HDPE, and LLDPE.
• El Paso/Montedison technology using a Ziegler catalyst system, produces LLDPE.
• Imhausen technology using a Ziegler catalyst system, produces linear MDPE, linear
HDPE, and LLDPE.
• Mitsubishi Petrochemical technology using a Ziegler catalyst system, produces linear
MDPE, linear HDPE, and LLDPE.
1.3.2.2 Low-Pressure Liquid Slurry Processes
The rst low-pressure linear HDPE slurry technology to utilize the Ziegler catalyst was com-
mercialized in 1955 by Hoechst. The process is based on a stirred-tank reactor containing a
heavy hydrocarbon diluent with a continuous ethylene feed. The reactor is operated at a pressure
of 0.8MN/m2, a temperature of 85°C, and an average residence time of 2.7 h. The concentration
of the solid polymer particles in the reactor discharge stream ranges between 13 and 45wt%,
depending on the particular polymer grade, and the overall ethylene conversion is generally
98wt% [104,107–110]. The Ziegler catalyst system consists of a reaction product of magnesium
tetrachloride, titanium tetrachloride, and titanium tetraisopropylate, with aluminum isopropylate
as the cocatalyst.
The Mitsubishi continuous stirred-tank reactor process, utilizing either a chromium oxide
or titanium–vanadium catalyst system, operates at a pressure of 3.5 MN/m2, a temperature of
80–90°C, and an average residence time of 2 h with an overall ethylene conversion of 95 wt%.
The Montedison stirred-tank reactor technology is essentially similar to the Mitsubishi process
[107–109]. Another stirred-tank reactor technology was developed by R.G.C. Jenkins and Company
[111], and a multiple cascade reactor technology was developed by BP Chemicals Ltd. [112].
The liquid pool slurry process technology for PP, developed by El Paso Polyolens Company,
was also used for linear polyethylene provided that isobutane or propane is the liquid hydrocarbon
diluent for the ethylene monomer feed and with appropriately modied catalyst [104,107–109]. It
is based on a jacket reactor with a transition metal catalyst system that was proprietary to Mitsui
Petrochemical/Montedison. It operates at a pressure of 2.6 MN/m2, temperature of 60°C, and an
average residence time of 1–2 h. Polymerization takes place in the presence of the diluent, and the
average spherical particle size of the polymer formed is about 1200 μm. The concentration of the
solid polymer particles in the product stream is 30–43 wt%.
The rst low-pressure linear polyethylene slurry loop reactor technology, using supported
chromium oxide catalyst in a light hydrocarbon diluent, was commercialized in 1961 by Phillips
Petroleum Company [11]. Isobutane was used as the diluent in a continuous-path double-loop
reactor operating at a pressure of 3.5 MN/m2, a temperature range of 85–110°C, and an average
residence time of 1.5 h. A modied chromium/titanium catalyst system enabled the production of
a broad range of polymer products, which, as with the original Phillips catalyst, do not have to be
de-ashed but are stabilized and pelletized. The polymer is formed inside the pores of the catalyst
that it eventually pulverizes, distributing the microscopic catalyst particles uniformly. The con-
centration of the solid polymer particles in the reactor discharge stream ranged between 18 and
50 wt%, depending on the particular polymer grade. The overall ethylene conversion is generally
about 98 wt%. Another process, developed by Phillips Petroleum Company [113], used chromium-
based catalyst on high silica–titania cogel support with a cocatalyst of triethyl borane or diethyl
aluminum ethoxide. The polymers produced are characterized by a range of density between 0.915
and 0.965 g/cm3.
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16 Handbook of Thermoplastics
The Solvay heavy hydrocarbon diluent slurry technology, based also on loop reactors, used
a supported Ziegler–Natta catalyst system consisting normally of magnesium ethylate, titanium
butylate, and butyl aluminum dichloride, with aluminum as the cocatalyst. The hydrocarbon diluent
was n-hexane, and the continuous-path double-loop reactor operated at a pressure of 3 MN/m2, a
temperature of 85°C, and an average residence time of 2.5 h. The concentration of the solid polymer
particles in the reactor discharge stream ranged between 16 and 50 wt%, depending on the particu-
lar polymer grade. The overall ethylene conversion is about 97 wt%. The liquid slurry technology
available for the production of linear polyethylenes and the corresponding catalyst systems were
proprietary to the following companies [107–110].
1.3.2.2.1 Slurry Phase Heavy-Diluent Stirred-Tank Reactor
• Amoco technology using a proprietary catalyst system
• Asahi Chemical technology using a Ziegler catalyst system
• Hercules technology using a Ziegler catalyst system
• Exxon technology using a Ziegler catalyst system
• Chisso technology using a Ti/Mg/V catalyst system
• Dow Chemical technology using a Ziegler catalyst system
• DuPont technology using a Ziegler catalyst system
• Hoechst technology using a Ziegler catalyst system
• ICI technology using a Ziegler catalyst system
• Hüls technology using a Ziegler catalyst system
• Idemitsu technology using a Ziegler catalyst system
• Mitsubishi Chemical technology using Ti, Cr, V, and Ti/V catalyst systems
• Mitsubishi Petrochemical technology using a Ziegler catalyst system
• Mitsui Petrochemical technology using a Ziegler catalyst system
• Montedison technology using a Ziegler catalyst system
• Shell technology using a Ziegler catalyst system
• Sumitomo technology using a Ziegler catalyst system
• Stamicarbon technology using a Ziegler catalyst system
1.3.2.2.2 Slurry Phase Light-Diluent Stirred-Tank Reactor
• Sumitomo technology using a Ziegler catalyst system
• Mitsui Toatsu technology using a Ziegler catalyst system
1.3.2.2.3 Slurry Phase Light-Diluent Loop Reactor
• Chemplex technology using Cr/Sn/Al/Ti catalyst systems
• Solvay technology using a Ziegler catalyst system
• Phillips technology using Ziegler Cr/Ti, Cr/P, Cr, Ti/Mg, V, and Ti/V catalyst systems
1.3.2.2.4 Slurry Phase Heavy-Diluent Loop Reactor
• National Distillers/ICI technology using a Ziegler catalyst system
• Solvay technology using a Ziegler catalyst system
1.3.2.2.5 Slurry Phase Liquid Pool Reactor
• El Paso technology using a Ziegler catalyst system
• Montell (now called Basell) technology using Ti/Mg Ziegler–Natta catalyst systems
• Montedison technology using a Ziegler catalyst system
1.3.2.3 Low- and Medium-Pressure Solution Processes
Solution polymerization processes predate, but were superseded, by the slurry processes [107–110].
As opposed to the slurry processes, polymerization took place in a solvent at high pressure and
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17Polyolefins
above the melting point of the polymer. The polymer formed dissolves in the solvent resulting in
a homogeneous single-phase liquid product stream that is subsequently devolatilized. The major
advantage of the solution processes is the wide operating temperature range limited only by the
polymer solubility and the polymer degradation temperatures. This enables a more efcient control
of the polymer molecular-weight distribution. The need for the energy-intensive solvent vaporiza-
tion is its major disadvantage. Improved solution processes were used by Dow, DuPont of Canada,
Mitsui, Phillips, Stamicarbon/DSM, and Eastman Kodak.
The DuPont solution process technology, based on a stirred-tank reactor system, used soluble
Ziegler–Natta catalyst system such as titanium or vanadium tetrachloride with triisobutyl aluminum
as the cocatalyst in a solvent such as cyclohexane. It is an adiabatic process operating at a pressure of
10 MN/m2, a temperature of 200°C, and an average residence time of 2 min with an overall ethylene
conversion of about 88 wt%. Another process due to DuPont [112] operates at a temperature range of
105–320°C and a pressure of 1.7 MN/m2. The Stamicarbon technology is an adiabatic low-pressure
stirred-reactor process operating at a pressure of 3 MN/m2, temperatures of 130–175°C, and an
average residence time of 5 min with an overall ethylene conversion of 95 wt%. The Dow technol-
ogy is a cooled low-pressure twin stirred-reactor process based on a conventional soluble Ziegler
catalyst. The reactors operate at a pressure range of about 1.9–2.6 MN/m2, a temperature of 160°C,
and an overall residence time of 30 min with an overall ethylene conversion of 94 wt%. The solution
phase technology available for the production of linear polyethylenes and the corresponding catalyst
systems were proprietary to the following companies [107–110].
1.3.2.3.1 Solution Phase Medium-Pressure Adiabatic Reactor
• Amoco technology using a Mo oxide catalyst system
• DuPont technology using a Ziegler catalyst system
• Mitsui Petrochemical technology using a Mo oxide catalyst system
• Eastman technology using a Ziegler catalyst system
• Phillips technology using Cr and Cr/Ti catalyst systems
• Stamicarbon technology using a Ziegler catalyst system
1.3.2.3.2 Solution Phase Low-Pressure Cooled Reactor
• Dow technology using a Ziegler catalyst system
1.3.2.4 Low-Pressure Gas-Phase Processes
The gas-phase, uidized bed ethylene polymerization technology, rst described by UCC (now part
of Dow Chemical Company) in 1957, was commercialized in 1968. The UCC low-pressure Unipol
process, introduced in 1975, was based on silica-supported titanium-modied chromium oxide cata-
lysts [114–116], although high-activity Mg–Ti catalysts and a variety of process modications were
later developed by UCC [116–120] and British Petroleum [121–124]. The reactor was operated at a
pressure of 2 MN/m2, a uidized bed temperature of 75–100°C, depending on the particular polymer
grade, and an average residence time of 3–5 h with an overall ethylene conversion of 97 wt%. The
reactor is operated at 85–100°C for HDPE or 75–100°C for LLDPE. The polyethylene product range
is broad, and the particle size could be as high as 15–20 times the size of the original catalyst particle.
The product withdrawal rate is used in ensuring a constant uidized bed volume. UCC licensed the
Unipol process in the United States, Japan, Europe, Australia, and Saudi Arabia. It remains one of the
single largest technologies for LLDPE production. The process is characterized by simplicity, reaction
uniformity, and suitability for large-scale production.
The BP Chimie/Napthachimie process is similar to the Unipol technology except for the use
of a proprietary highly active titanium/alkylaluminum catalyst system. It is operated at a pressure
range of 0.5–3.3 MN/m2 and a uidized bed temperature of 60–100°C. The vertical continuous
stirred (mechanical) bed gas-phase polyolen technology developed by BASF came on stream in
1976 as Novolen process [107–110,125]. It is operated at a pressure of 3.4 MN/m2, a temperature
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18 Handbook of Thermoplastics
of 100–110°C, depending on the particular polymer grade, and an average residence time of 4 h
with either a magnesium-supported two-component Ziegler catalyst or a silica-supported modied
chromium oxide catalyst. The horizontal continuous stirred-bed gas-phase polyolen technology,
developed by Amoco [126–128], is based on a compartmentalized cylindrical vessel stirred by a
series of axially mounted longitudinal paddles. It is operated at a pressure of 2 MN/m2, a tempera-
ture of 82–88°C, depending on the particular polymer grade, and an average residence time of 4.3 h
with either a magnesium-supported two-component Ziegler catalyst or a silica-supported Phillips
catalyst.
The Spherilene gas-phase technology developed by Montell (now called Basell) [6,26] is a hybrid
process for the production of linear polyethylenes, in a spherical granular shape, whose molecular
weight ranges from very low to very high. The process consists of a loop reactor followed by a
uidized bed gas-phase reactor, and it is equipped with a stripping unit so effective that the nal
polymer contains no monomer residue. It is characterized by high productivity, rapid and low-cost
changeover of polymer grades, and excellent product quality.
The gas-phase technology available for the production of linear polyethylenes and the corre-
sponding catalyst systems were proprietary to the following companies [107–110].
1.3.2.4.1 Gas-Phase Fluidized Bed Reactor
• UCC technology using proprietary Cr, Cr/Ti, and Ti/Mg catalyst systems
• Cities Services/ICI technology using a proprietary Ziegler catalyst system
• ICI technology using a proprietary Ziegler catalyst system
• Mitsubishi Petrochemical technology using a proprietary Ziegler catalyst system
• Mitsui Petrochemical technology using a proprietary Ziegler catalyst system
• Sumitomo technology using a Ziegler catalyst system
• Nippon Oil technology using a proprietary Ziegler catalyst system
• Shell technology using a proprietary Ziegler catalyst system
• BP Chimie/Napthachimie technology using a proprietary Ziegler catalyst system
1.3.2.4.2 Gas-Phase Horizontal Stirred-Bed Reactor
• Amoco-Chisso technology using proprietary Ti/Mg and Cr catalyst systems
• Amoco technology using a proprietary Ti/Mg catalyst system
1.3.2.4.3 Gas-Phase Vertical Stirred-Bed Reactor
• BASF technology using proprietary Cr/Ti/Mg/Sn/Al catalyst systems
• Amoco-Chisso technology using proprietary Ti/Mg and Cr catalyst systems
1.3.2.4.4 Modular Gas-Phase Fluidized Bed Reactor
• Montell (now called Basell) technology using a proprietary Ti/Mg catalyst system
1.3.2.4.5 Hybrid Slurry-Phase Loop Reactor/Gas-Phase Fluidized Bed Reactor
• Montell (now called Basell) technology using proprietary Ti/Mg catalyst systems
The olens gas-phase uidized bed technology used presently can be traced to the earlier pro-
cesses, namely, Unipol, Sumitomo, and Catalloy.
1.3.3 PolymeriZatioN ProCesses For PP
Practically all of the above processes have been used in the production of PP. The special process tech-
nologies originally developed for PP and propylene copolymers included variants of the liquid pool
slurry process; the Eastman Kodak solution process; the Novolen gas-phase process jointly developed
by BASF, ICI, and Quantum; as well as the Catalloy modular gas-phase process of Montell (now
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19Polyolefins
called Basell). Contemporary and emerging catalyst developments have resulted in adapted PP process
technologies. The autoclave reactor technology used presently for liquid bulk PP processes is due to
Exxon Sumitomo, Rexene, and Hypol-I. The loop reactor technology used presently for liquid bulk
PP processes is due to Spheripol, Borstar, Phillips, Exxon Mobil, ST (Sinopec), and Hypol-II. The
gas-phase uidized bed PP technologies used presently are due to Unipol, Sumitomo, and Catalloy.
1.3.3.1 Low-Pressure Liquid Pool Slurry Phase Processes
The liquid pool slurry process technology for PP, developed by El Paso Polyolens Company
[109,110], uses liquid propylene as the diluent as well as the feed. It is based on a jacket reactor with
a highly active, highly stereospecic transition metal Ziegler catalyst system that is proprietary to
Mitsui Petrochemical/Montedison. It operates at a pressure of 2.6 MN/m2, a temperature of 60°C,
and an average residence time of 1.3 h, and the concentration of the solid polymer particles in the
product stream is 43 wt%. The removal of aPP or catalyst residue is not required because of the high
productivity and stereospecicity of the catalyst.
The Spheripol process, developed by Montell (now called Basell) [6,26], is based on a propri-
etary high-yield, high-stereospecicity donor/MgCl2-supported titanium-based Ziegler–Natta cata-
lyst system. The process operates without wastewater, with no solid waste, with minimal steam
requirement, and with signicantly low power requirement. A signicant percentage of the global
production of PP is based on the Spheripol as well as the Spherizone process, developed in 2003
by Montell (now called Basell). The Spherizone process consists of a multizone circulating reactor
(MZCR) with two regions, namely, Riser and Downcomer. The key advantage of Spherizone, com-
pared with the multireactor technology, is the much shorter residence time that enables uniformity
of polymer particles [129–131].
1.3.3.2 Low-Pressure Modular Gas-Phase Reactor Processes
The Catalloy gas-phase technology, developed by Montell (now called Basell) [6,26,129–131], con-
sists of three mutually independent gas-phase reactors in series and is also known as reactor par-
ticle technology. It is based on a high-yield, highly stereospecic, high-surface-area catalyst system
whose morphology ranges from a dense spherical shape to a sponge-like structure. The process
operates with no by-product, no liquid waste, and no solid waste. It is capable of producing (1) PP
with high polydispersity and high stereoregularity, (2) random copolymers containing up to 15 wt%
comonomers, and (3) multiphase alloys containing up to 70 wt% multimonomer copolymers.
1.3.4 PolymeriZatioN ProCesses For other PolyoleFiNs
Ultrahigh-molecular-weight polyethylene (UHMWPE) is produced in the slurry-phase heavy-
diluent stirred-tank reactor by Hercules and Hoechst based on the proprietary Ziegler catalyst
system. Polybutene-1 (PB-1) and copolymers are made in a slurry-phase light-diluent stirred-tank
reactor by Hüls and in a solution-phase medium-pressure adiabatic reactor by Hüls and Shell
using the Ziegler catalyst system. In addition, PB-1 is made by Mitsui Petrochemical, which
also manufactures poly(4-methylpentene-1) based on a stereospecic Ziegler–Natta catalyst. The
tubular high-pressure process is used in the production of EVA co- and terpolymers.
1.3.5 ProCess teChNologies For ssCs
Contemporary and emerging catalyst developments have resulted in several adapted technologies
for producing SSC polymers. New technologies with improved exibility were developed based
on new catalyst design. The processes, earlier developed for large-volume polyolens, have been
adapted to enable the exploitation of the unique advantages of contemporary and emerging SSC
developments. Thus, different process technologies ranging from solution-, slurry-, and gas-phase
processes have found applications using a variety of SSCs.
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20 Handbook of Thermoplastics
1.4 POLYOLEFIN STRUCTURE–PROPERTY RELATIONSHIPS
The choice of a catalyst, polymerization process, and the appropriate control of reactor conditions
determine the polyolen molecular weight, molecular-weight distribution, density, and other prop-
erties [107–109]. From the free radical initiated noncatalyzed polymerization technology of the
1930s, to Ziegler–Natta and chromium catalyst technologies of the 1950s, to the metallocene SSC
catalyst technologies of the 1990s, and the postmetallocene SSC catalyst technologies of the pres-
ent, polyolens continue to evolve capturing signicant market share in applications that were not
previously accessible. To underscore the impact of SSC in the polyolen industry, Section 1.4.7 is
devoted to the discussion of polyolens from SSCs.
A classication of polyolens and their corresponding densities appears in Table 1.4. A list of pos-
sible polyolen structural characteristics and properties is presented in Table 1.5. Density is generally
a reection of polyolen linearity. The higher the density, the higher the following polymer character-
istics and performance properties: (1) chain linearity, (2) stiffness, (3) tensile strength, (4) tear strength,
(5) softening temperature, and (6) brittleness [101]. On the other hand, polyolen failure properties
such as impact strength, exural strength, and environmental stress crack resistance (ESCR) decrease
as the polyolen density increases. A simplied outline of polyolen catalyst/technology–structure–
property interdependence [26] is presented in Figure 1.1.
Polyolens are used in several different applications primarily because of their wide range
of resin characteristics and end-product properties. These properties, particularly processability,
physical, and mechanical properties, are highly dependent on the average molecular weights and
molecular-weight distribution. For example, injection molding requires resins with low melt viscos-
ity and elasticity, whereas blow molding requires resins characterized by high molecular weights,
molecular-weight distribution, and high melt viscosity and elasticity. The molecular-weight charac-
teristics are also the determinants of molded part warpage, propensity for failure, and ESCR, albeit
not in the same direction. For example, as the high-molecular-weight species in polyolen resin
increase, the positive salutary effects on the ESCR properties would be counterbalanced by the
negative effects of part warpage, surface roughness, and opacity.
The use temperatures of the different polyolens are limited by the glass transition temper-
ature (Tg) at the low end and the crystalline melting point (Tm) at the upper end. However, the
degree of mechanical property retention is better related to either the heat deection temperatures
(HDT or DTUL) or the Vicat softening points. Of all polyolens, only the poly(4-methylpentene-1)
TABLE 1.4
Classification and Density of Polyolefins
Polyethylene Type
Macromolecular
Classification Density Range (g/cm3)
LDPE Homopolymer 0.910–0.925
MDPE Homopolymer 0.926–0.940
LLDPE Copolymer 0.910–0.940
VLDPE Copolymer 0.890–0.915
HDPE Copolymer 0.941–0.959
HDPE Homopolymer 0.960 and higher
HMWPE Homopolymer 0.947–0.955
UHMWPE Homopolymer 0.940
Polypropylene Homopolymer 0.904–0.906
Ethylene–propylene copolymer Copolymer 0.904–0.907
PB-1 Homopolymer 0.910
Poly(4-methyl pentene-1) Homopolymer 0.830
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21Polyolefins
TABLE 1.5
Possible Polyolefin Structural Characteristics and Properties
Structural Characteristics Physical Properties Mechanical Properties
Molecular weights Density Tensile strength
Molecular-weight Average particle size Impact strength (tensile, Izod, dart)
Distribution Particle density Flexural strength
Polydispersity Surface texture, gloss Tear strength
SCB Film clarity, haze Tensile strength, at yield
LCB Thermal expansion Ultimate tensile strength
Unsaturation Specic heat Tensile modulus
Crystallinity Thermal conductivity Flexural modulus
Morphology Electrical conductivity Stiffness, rigidity
Stereoregularity Glass transition temperature, TgToughness, ductility
Randomness Melting temperature, TmBrittleness
Chain linearity Crystallization temperature, TcNotch sensitivity
Processing Properties Softening temperature Elongation at yield
Shear viscosity Heat distortion temperature Elongation at break
Extensional viscosity Permeability (gas, water vapor) Orientation factor
Intrinsic viscosity Degradation Properties Creep resistance
Formability, malleability Thermal degradation Strain hardening
Melt ow (index, rate) Oxidative degradation Resilience
Melt strength Photodegradation Hardness
P-V-T relation Biodegradation Compressive strength
Shear degradation Shear yield strength
Environmental stress cracking Shear ultimate strength
Puncture resistance
Lubricity
Abrasion resistance
Catalyst
Technology
Molecular
weight
Mol. weight
distribution
Stereospecificity
Processability
Stiffness
Heat distortion
temperature
Weldability
Randomness
Transparency
Impact resistanceMorphology
Softness
FIGURE 1.1 Catalyst/technology–structure–property relationship of polyolens.
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22 Handbook of Thermoplastics
copolymer has a Vicat softening point (179°C), which is higher than that of PP (145–150°C), and
a melting point of 240°C compared to 165–179°C for PP. The specic heat of polyolens is in the
following order polyethylenes > PP > PB-1, but PP has about the lowest thermal conductivity and
thermal expansion coefcient. Although polyolens generally have good ESCR (with PP being bet-
ter than PE), all polyolens require a degree of stabilization against all forms of degradation.
1.4.1 PolyethyleNes
The density of conventional polyethylenes, commercially manufactured with the Ziegler–Natta or
chromium catalyst technologies, ranges between 0.890 and 0.980 g/cm3. The density of polymeth-
ylene is 0.980 g/cm3 and that of the amorphous phase of polyethylene is 0.850 g/cm3. The density
of conventional polyethylene depends on its relative content of LCB and short-chain branching
(SCB) [132]. The degree and type of chain branching strongly inuence the molecular-weight
distribution, degree of crystallinity, lamellar morphology, density, and rheology of polyethyl-
enes. The LCB may vary from 20 to thousands of carbons, and the SCB may have 5–10 carbons
depending on whether it is copolymerized with α-olen. SCB could be attached either to the main
polymer backbone or to the LCB.
SCB is expressed as methyl groups per 1000 carbon atoms and is measured by infrared absorption
techniques. Empirical relations exist for comparing the degree of LCB of different polymers [133].
LCB is normally irrelevant for polymer properties below the melting point; however, it is relevant in
determining the rheological and processing properties, which are highly dependent on the molecular-
weight distribution. LDPE has LCB and, consequently, has much broader-molecular-weight distri-
butions [132] compared to the other polyolens. Because of their relatively broad-molecular-weight
distributions, polyethylenes have a broad melting range.
An industrially important empirical ow parameter for linear polyethylenes is the melt ow
index. A melt index is inuenced by molecular weight, its distribution, chain branching, branch
length, and many other molecular structural parameters. In spite of the absence of a denitive
correlation, there is a reasonable consensus that a melt ow index is directly related to polyolen
part clarity and mold shrinkage, but inversely related to melt viscosity, impact strength, notch sen-
sitivity, creep resistance, heat resistance, toughness, melt strength, and average molecular weight.
The apparent relationship is utilized in many ways including the down-gauging of molded parts,
extruded lms, and proles. Nonetheless, it has been shown that the molecular weights of two
LDPEs having similar melt indexes but different LCBs could differ by a factor of 2. That is, a melt
index is a poor indicator of the molecular structure for LDPE. The comparative processing and per-
formance property proles of LDPE, HDPE, and LLDPE appear in Table 1.6 [134].
All polyethylenes are non-Newtonian, but LDPE is less so than HDPE, LLDPE, PP, and PB-1,
in that order, and could therefore be more easily compounded with colorants and additives. Its melt
viscosity is far more temperature-sensitive and shear-sensitive, and it has an enhanced extensional
viscosity. On the other hand, the more non-Newtonian polyolens have a more plug-like velocity
prole with a corresponding higher pumping efciency. Their viscosities are less temperature- or
shear-sensitive, and they have considerably less extensional viscosities.
1.4.1.1 Branched LDPE
Branched LDPE is not closely packed as it contains a substantial amount of LCB; it looks much
like a tree. It has a density of 0.910–0.930 g/cm3, 45–60% crystallinity, and respectable exibility.
LDPE is characterized by a broad-molecular-weight distribution with a polydispersity index as high
as 20. It has a broad molecular weight ranging between 17,000 and 30,000, but skewed toward
the higher-molecular-weight components. Although its melting temperature is normally centered
around 110°C, its actual range depends on the degree and randomness of branching [135].
LDPE, made in a highly back-mixed autoclave reactor, tends to branch equally in all directions
giving rise to somewhat spherical molecules. On the other hand, commercial products of a tubular
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23Polyolefins
reactor contain fewer and longer branches on the main backbone. LDPE also contains SCB con-
sisting of ethyl and butyl groups whose frequency distribution is narrow. For a given melt index,
higher LCB implies higher mechanical properties, broader-molecular-weight distribution, higher
ow resistance, and enhanced possibility of entanglement with concomitant effects on performance
properties of the nished products. The degree of LCB, rather than a melt index, is the indicator of
LDPE molecular structure and properties. It is because of the enhanced melt strength and elastic-
ity imparted by the high degree of LCB that LDPE is a preferred material for blown lm, shrink
lm, and extrusion coatings. The negative aspect of its high tolerance for extension is that its melt
strength could be exceeded during blown lm processing unless adequate allowance is made in die
design.
The individual crystallite dimensions in LDPE lms are about 10–30 nm; an agglomerate
of the crystallites results in a spherulite whose dimension is larger than the wavelength of light.
Consequently, higher lm clarity is obtained if the formation of aggregates is minimized and the
spherulite size is reduced. The translucence of LDPE-blown lm is due partly to the lm surface
unevenness and partly to the presence of spherulites whose size is a function of the degree of
branching and the material thermal history. Because of this, melt elasticity, viscosity, temperature,
and their effects on surface texture are, in the nal analysis, the primary determinants of lm clar-
ity in commercial LDPE. The morphology of LDPE solid depends on the relative magnitude of the
rates of crystallite nucleation and spherulite growth, both of which are signicant [136].
1.4.1.2 Linear LDPEs
Conventional linear LDPE is a copolymer of ethylene and a linear α-olen comonomer such as
propylene, butene-1, pentene-1, hexene-1, heptene-1, octene-1, decene-1, tetradecene-1, or methyl-
4-pentene-1 [133,134,136–138]. Commercially available conventional LLDPE, made with conven-
tional catalyst technology, normally contains 8–12% of butene-1, hexene-1, or octene-1, and it has
TABLE 1.6
Processing and Performance Properties of LDPE, HDPE, and LLDPE
Property LDPE HDPE
LLDPE Relative
to LDPE
LLDPE Relative
to HDPE
Tensile strength (MN/m2) 6.9–15.9 21.4–38 Higher Lower
Elongation (%) 90–650 50–800 Higher Higher
Impact strength (J/12.7 mm) No break 1.02–8.15 Better Similar
Environmental stress cracking resistance – – Better Same
Heat distortion temperature (°C) 40–50 60–82 15°C higher Lower
Stiffness (4.5 MN/m2) 1.18–2.42 5.53–10.4 Higher Lower
Warpage Less Similar
Processability Excellent Good Easier
Haze (%) 40 – Worse Better
Gloss (45° %) 83 – Worse Better
Clarity Near transparent
to opaque Translucent
to opaque Worse Better
Melt strength – – Lower Lower
Softening point range (°C) Permeability
(mlcm−2 g−1 mil−1 cm)
Hg
−1
at 25°C × 10−8 85–87 120–130 Narrower Narrower
(a) H2O vapor 420 55 Better Worse
(b) CO260 13 Better Worse
Source: Mukherjee, A. K. et al., Popular Plastics: 15 (October), 1985.
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24 Handbook of Thermoplastics
a nonrandom broad comonomer distribution with density ranges between 0.900 and 0.945 g/cm3.
With SSC technology, the polyolen produced is VLDPE, which is structurally similar to LLDPE
[139]. VLDPE has a density between 0.890 and 0.915 g/cm3. There is an inverse relationship between
comonomer content and LLDPE density and between the molecular weight and density. The SCBs
of the low-molecular-weight fractions are two to four times those for the high-molecular-weight
fractions. This heterogeneity is catalyst-specic, and it depends on the production technology used
[140]. A homogeneous LLDPE with random and narrow comonomer sequence distribution as well
as narrow-molecular-weight distribution is characterized by lower density, lower melting point,
higher impact strength, signicantly lower lm haze, and improved lm properties in the machine
and transverse directions. This type of LLDPE essentially belongs in Section 1.4.7.
In spite of the shortcomings of conventional LLDPE, it still has much improved properties over
LDPE of the same density, molecular weight, and molecular-weight distribution. The primary
advantages of LLDPE, arising from its backbone linearity and the presence of SCB, are as follows:
higher tensile strength, impact strength, toughness, stiffness, lm gloss, puncture resistance, tear
strength, ESCR, and permeability of water vapor and carbon dioxide. Up to a point, the extent of
the property improvement is directly related to the amount and chain length of the oligomeric como-
nomer [132,141]. For equally the same reasons, its optical properties, melt strength, and rheological
properties are not necessarily optimum. For the same density, LLDPE is about 5% more crystalline
and has a melting point that is about 14°C higher than that of LDPE [142].
The α-olen comonomer forms the SCBs on the linear ethylene chain backbone of LLDPE dis-
rupting the crystallization process and limiting the crystalline size structure, thus increasing the
percentage of the amorphous phase at the expense of the crystalline phase and lowering the density
[107,136]. In conventional LLDPE, the SCB frequency distribution is uneven, and, combined with
the highly linear character of the polymer, the nonuniformity increases the relative magnitude of
the rate of crystallite nucleation and that of spherulite growth resulting in higher haze values. The
haziness of LLDPE-blown lm is due partly to the lm surface unevenness and partly to the pres-
ence of spherulites whose size is a function of the nonuniformity of SCB and the material thermal
history. Higher density LLDPE has lower SCB and higher nonuniform frequency distribution with
a corresponding larger spherulite and higher haze. Because of this, melt elasticity, viscosity, tem-
perature, and their effects on surface texture are not as important in determining lm clarity as they
are for LDPE.
LLDPE is more non-Newtonian than LDPE, but its higher melt viscosity is not as temperature-
sensitive or as shear-sensitive as LDPE. Because of its higher non-Newtonian character, its veloc-
ity prole in a channel is more plug-like with a corresponding higher pumping efciency. But the
absence of LCB and the narrower polydispersity imply a lower melt strength and a lower shear
thinning behavior, which, when combined with the higher viscosity, makes LLDPE more prone
to melt fracture given the higher pressure and screw torque required for commercial extrusion.
Because of the absence of LCB and the narrower polydispersity, LLDPE melt strain-hardens: it
is soft in extension but stiff in shear. It therefore has to be processed at a higher melt temperature
with a wider die gap to permit the use of a lower pressure and a higher drawdown ratio in blown
lms [107]. For the same melt index and density, LLDPE-cast lm properties are superior to those
of LDPE; consequently, a higher melt index LLDPE could easily yield a lower lm thickness and a
higher productivity.
Generally, LLDPE has penetrated the market normally dominated by other polyethylenes prin-
cipally because of the high optical quality of its lm and its corresponding performance properties
even at rather low lm thickness. The best LLDPE would be characterized by uniform intermolecu-
lar- and intramolecular comonomer SCB distribution with a variety of comonomers such as betene-1,
hexene-1, octene-1, and 4-methylpentene-1. The super random copolymer of Dow Chemical, made
in the proprietary solution process, is based on octene-1, and it has a high compositional uniformity
and lm performance. On the other hand, the LLDPEs produced in the gas-phase process using
supported titanium chloride catalysts are characterized by nonuniform composition distribution
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25Polyolefins
and high hexane extractables, and so are the LLDPEs made with the high-pressure retrotted LDPE
processes based on butene-1. The estimated annual growth rate for LLDPE is 7–8%.
1.4.1.3 High-Density Polyethylenes
HDPE is closely packed with a density of 0.960–0.980 g/cm3, a crystallinity as high as 95%, and a
melting point as high as 138.5°C, with the highest values corresponding to those of polymethylene.
Commercially available conventional HDPE normally contains 1–3 wt% of butene-1, hexene-1, or
octene-1. It has a few SCBs with a polydispersity index in the range 5–15, although its molecular-
weight distribution is generally sharper than that of a corresponding LDPE. The situation is of course
different for the HDPE produced with SSC technology discussed in Section 1.4.7. The homopolymer
MDPE has a density ranging between 0.926 and 0.940. Quenched HDPE could have a density as low as
0.945 g/cm3 because of the reduced crystallinity [135,136]. However, the morphology of HDPE solid is
determined by the relative magnitude of the rates of crystallite nucleation and spherulite growth, both
of which are so high that it is nearly impossible to quench HDPE fast enough to obtain spherulites that
are small enough to give high optical quality. Film clarity could only be improved through the use of
heterogeneous nucleating agents.
Nonetheless, the longer the molecular weight of HDPE is, the more the crystallization process
is inhibited, limiting the crystalline size structure and increasing the percentage of the amorphous
phase at the expense of the crystalline phase resulting in reduced density. The increased amorphous
phase enhances the impact properties and lowers the yield properties, hardness, as well as stiffness
up to a point. As the molecular weight of HDPE increases, the ultimate tensile properties and elon-
gation increase up to the point where chain entanglement becomes important. There is an inverse
relationship between the melt index and the molecular weight, but the intrinsic viscosity is related
to the viscosity average molecular weight as expressed by the Mark–Houwink equation. Generally,
HDPE molecular weight has an effect on the following properties: density, tensile strength, tensile
elongation, impact strength, toughness, stiffness, and permeation characteristics. Up to a point,
the HDPE molecular weight is inversely related to its hardness and exural stiffness, whereas it is
directly related to Vicat softening point and Izod impact strength. On balance, however, it is the
molecular-weight distribution that determines the end-product physical, mechanical, rheological,
and processing properties.
HDPE is non-Newtonian and its velocity prole in a channel is plug-like with a high pumping
efciency; this is truer for the polymer with the narrow-molecular-weight distribution. The melt
viscosity of HDPE with a narrow-molecular-weight distribution is not particularly temperature-
sensitive or shear-sensitive, making it more amenable to higher drawdowns when used as bers and
monolaments. The HDPE with a broader-molecular-weight distribution has a higher melt index,
higher melt strength, and an enhanced shear thinning ability, which facilitate processing in blow
molding of large containers, extrusion of parts with large cross-sectional areas, and blow extrusion
of thin crisp lms used as replacements for paper. On the other hand, for a given melt index, the
HDPE with a narrower-molecular-weight distribution has higher impact strength. Generally, HDPE
has excellent low-temperature exibility, and its brittleness temperature is almost independent of its
melt index. The high-molecular-weight HDPE [weight average molecular weight (Mw) of 3–4 × 105]
with fractional melt indices is used in the manufacture of carrier bags, garbage bags, drums, and
pipes, whereas the medium-molecular-weight varieties (Mw ≈ 2.5 × 105) nd applications as wrap-
ping paper, grease-proof paper for meat, and ower packaging.
The specialty-grade HDPE remains the high-molecular-weight HDPE with the best performance–
processability balance for blow molding and extrusion applications such as thin lm, pipes, and
heavy drums. A judicious choice of catalyst and polymerization process enables the production of
HDPE with the best combination of structural molecular parameters vis-à-vis the required prop-
erties. For this premium grade of HDPE, the stirred-tank slurry processes are best suited. The
commodity large-volume HDPE is dominated by the cost-effective gas-phase processes that have
essentially surpassed the most licensed light diluent slurry loop reactor process. HDPE includes
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26 Handbook of Thermoplastics
the narrow-molecular-weight injection molding grades as well as the medium-molecular-weight
extrusion grade for lms and pipes. The Unipol gas-phase process is able to produce bimodal
HDPE grades, and the polymerization technology from Mitsui Petrochemical produces what
has been called “superpolyethylene.” Of all the available processes, the new Spherilene process
developed by Montell (now called Basell) is the most versatile as it is able to produce HDPE with
molecular weights ranging from very low to very high [6,26]. It could therefore supply both the
specialty and the commodity market.
1.4.1.4 Ultrahigh-Molecular-Weight Polyethylenes
The UHMWPE [142] with Mw in excess of 3.1 × 106 and a density of 0.94 g/cm3 is structurally similar
to HDPE with folded zigzag crystalline conformations having fold spacings between 10 and 50 nm. The
chemical properties, electrical properties, tensile impact strength, and elastic modulus are similar except
that UHMWPE is characterized by exceptional self-lubricating properties and low-temperature perfor-
mance properties, and its abrasion resistance is superior to that of abrasion-resistant steel. It normally
decomposes before it melts, and some of its processing techniques mimic those of powder metallurgy.
The UHMWPE, prepared under exceedingly high pressures between 0.2 and 0.7 GN, christened
RT UHMWPE by DuPont, is tough, malleable, permanently deformable without breaking, and pro-
cessable by solid-phase extrusion or other solid phase-forming technique [142]. Its impact strength
and elastic modulus are comparable to those of polycarbonate. The RT UHMWPE has extended
chain crystal structure fold spacings of about 1000 nm compared to about 250 nm for the extended
chain crystal structure of conventional HDPE, produced under high-pressure crystallization, which
is brittle at ordinary temperatures. The RT UHMWPE nds application as machined parts for
prosthetic devices, small gears, automotive parts (that ordinarily would have required ABS or poly-
acetal), and many other applications requiring its outstanding combination of properties. Like other
polyethylenes, its upper use temperature is below 100°C.
1.4.2 PolyProPyleNe
There are basically three forms of PP homopolymers: crystalline, amorphous, and elastomeric
[6,26,107,143,144]. The SCB of PP interferes with its polymer crystallization, limiting the size and
formation of its crystallites, increasing the proportion of amorphous polymer, and decreasing the
density. The syndiotactic crystalline form has a calculated density of 0.90 g/cm3. The isotactic
crystalline form has a measured density of 0.90–0.91 g/cm3, a degree of crystallinity 60–80%, Tm
of 165–179°C, and Vicat softening point of 145–150°C, and it is capable of signicant supercooling.
Aside from being dependent on the method of measurement, the measured crystallinity depends on
the molecular weight, the molecular-weight distribution, crystallization conditions, and annealing.
The atactic amorphous PP is soft and gummy. It has a measured density of 0.86–0.89 g/cm3, its
mechanical properties are signicantly lower than those of the crystalline forms, and it nds appli-
cation primarily as hot melt adhesives and bitumen binders.
The crystalline form of PP depends on the relative magnitude of the rates of crystallite nucleation and
spherulite growth, both of which are determined by the cooling rate as well as the relative nearness of
the crystallization temperature (Tc) to the crystalline melting point (Tm). When Tc is slightly below Tm and
the cooling rate is slow, spherulite growth predominates and the resulting material is highly crystalline.
For this material, the higher the crystallinity, the higher the density, tensile yield strength, and elastic
modulus, and conversely, the lower the impact strength, toughness, and elongation. With high cooling
rates, Tc decreases fast, nucleation becomes predominant, smaller spherulites are formed, and the degree
of crystallinity is diminished. For this material, impact strength, toughness, and elongation are high,
but density, tensile yield strength, and elastic modulus are low. If the cooling rate is fast enough, small-
enough spherulites could be formed to obtain high optical quality [145]. Film clarity could be further
improved through the use of heterogeneous nucleating agents [146], which would also enhance the crys-
tallinity, density, tensile yield strength, and elastic modulus.
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27Polyolefins
The weight-average molecular weight of PPs strongly inuences the physical, mechanical, rheo-
logical, and processing characteristics. PP melt is non-Newtonian at high shear rates but approaches
Newtonian behavior when the shear rates are very low. Its shear rate dependence is a function of
the polydispersity index. PP with broad-molecular-weight characteristics has a better processabil-
ity, particularly for injection molding and extrusion applications, where PP competes with amor-
phous polymers such as impact modied polystyrene and polycarbonates. Because of its crystalline
nature, the signicant shrinkage of PP is a disadvantage relative to its amorphous competition, but
its high Tm and the concomitant high heat distortion temperature compensate adequately. Broad-
molecular-weight distribution PP could also be obtained by blending PP of different polydispersity
indices, and those with good-enough melt strength nd applications in thermoforming, pipe extru-
sion, blown lm, and blow molding.
There is an inverse relationship between the melt ow index and the molecular weight, but
the intrinsic viscosity is related to the viscosity average molecular weight through the Mark–
Houwink equation. Commercial PP could have a narrow or a broad range of polydispersity index
between 5 and 15. The narrow polydispersity index PPs with a high melt ow index are used
in injection molding of thin-walled containers, cast lm production, and ber spinning. They
are characterized by high strength, toughness, ductility, elasticity, resilience, and orientation.
Oriented PP lms nd applications in food packaging, graphics, and solar control; the lm tapes
are used as primary carpet backing fabric, sacks, ropes, twine, and nets for seedlings [147]. PPs
with controlled rheology and narrow polydispersity index (3.0) are obtained by the controlled
chemical or thermal degradation of high molecular grades (visbreaking). This grade of PP could
also be produced by the Montell (now called Basell) technology using donor-assisted Ti/Mg cata-
lyst systems [6,26].
Other PPs made with conventional catalyst technologies include (1) high-modulus homopolymers,
random and block copolymers for solid-state pressure-forming applications requiring superior clarity,
stiffness, impact strength, and heat distortion; and (2) high-melt-strength PPs with enhanced elonga-
tion viscosity for application as extrusion coating, foaming, blow molding, melt-phase thermoforming,
and a host of other applications that previously eluded PPs [6,26].
1.4.3 Poly(ButeNe-1)
There are primarily three crystalline polymorphs of PB-1 classied as follows [148–153]: (1) the
hexagonal or rhombohedral polymorph form I (Tm = 136°C) and form I' (Tm = 98–103°C); (2) tetrag-
onal form II (Tm = 124°C); and (3) orthorhombic form III (Tm = 106°C). Although each polymorph
has different properties with form I having the highest density, crystallinity, hardness, rigidity, stiff-
ness, and tensile yield strength, the commercially available PB-1 is 98–99.5% isotactic and 50–55%
crystalline, and its weight-average molecular weight is 2.5–6.5 × 105.
Structurally, PB-1 is similar to PP except that it has ethyl chain branches rather than methyl.
The commercially available PB-1 exists rst in the form II polymorph structure, which has 11
monomer units in 3 turns or 40 monomer units in 11 turns of its helical molecular structure, before
it transforms irreversibly to the form I polymorph with 3 monomer units per turn in its helix. The
phase transformation occurs within seconds, days, or weeks depending on the prevailing pressure,
temperature, state of strain, molecular weight, tacticity, catalyst residue, nucleating agents, additive
content, and comonomer composition and distribution. The addition of about 10% of PP has been
known to shorten the transformation half-time by as much as 70%.
The physical properties of PB-1 cut across all other polyolens, and its general characteristics
are a cross between polyethylenes and PP. Its Tg is −25°C, which is in between that of polyethylenes
(−60°C) and PP (≈0°C); its Tm is much like that of HDPE, i.e., 124–136°C, and it has a measured
density of 0.91 g/cm3, which is essentially identical to that of PP. Its tensile yield strength is about
the same as that of MDPE; its exibility is much like that of LDPE having a density of 0.92 g/ cm3,
and its high-temperature mechanical property retention is much like that of HDPE.
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28 Handbook of Thermoplastics
The non-Newtonian character of PB-1 is much like that of LDPE, and it could therefore be easily
compounded with colorants, additives, and high ller loading. The relative magnitude of its specic
heat compares as follows: polyethylenes > PP > PB-1. The tear strength of its lm is about six times
that of LDPE and about three times that of LLDPE [154,155]. Compared to PP and PEs, PB-1 has good
electrical properties, moisture barrier properties, an excellent ESCR, as well as an outstanding abrasion,
impact, and creep resistance. While polyethylenes and PPs do not strain-harden, PB-1 strain-hardens as
its chain orientation and the corresponding material state of stress are a strong function of strain.
In the melt, PB-1 has excellent melt strength, and it nds applications in pipe extrusion for hot
water pipes, blown lm, laboratory/medical ware, food/meat packaging, agricultural packaging,
compression wraps, and hot-ll containers. The blown lm is characterized by high strength, tough-
ness, ductility, elasticity, resilience, and orientation potential.
1.4.4 Poly(4-methylPeNteNe-1)
Poly(4-methylpentene-1) is principally isotactic. It has a tetragonal crystalline form whose helical
molecular conformation has seven monomer units in two turns of its helix. It has a high Tm (235–
250°C) and a low density of about 0.83 g/cm3—the lowest density of all polyolens. Its helical confor-
mation is presumed to be retained even within the amorphous phase. Consequently, its crystalline and
amorphous phases have a similar density, and this is responsible for its exceptional transparency with
a light transmission value of 90–93% [156]. Its monomer, unlike that of other polyolens, is not a by-
product of steam-cracking operations; it results from propylene dimerization. The monomer confers
on poly(4-methylpentene-1) a crowded side chain branching, which in turn confers a rigid backbone
to the stereoregular variety. The commercial poly(4-methylpentene-1) copolymer variety, with a trade
name of TPX, has the best high-temperature mechanical property retention of all of the polyolens
(up to 205°C) [157]. For this reason, it is capable of hundreds of sterilization cycles with high-pressure
steam (150°C) in laboratory and medical applications. Because of the bulky and crowded side groups,
poly(4-methylpentene-1) has a signicant free volume that is responsible for its high water vapor per-
meability, high gas permeability, high oxygen uptake, and a high susceptibility to photooxidation.
The physical properties of poly(4-methylpentene-1) are generally similar to those of PP: their tensile
strength, exural modulus, hardness, and impact strength are similar. However, poly(4-methylpentene-1)
is transparent while PP is translucent at best. Poly(4-methylpentene-1) is thixotropic and could easily be
compounded with colorants, additives, and llers. Its melt viscosity is far more temperature-sensitive
than that of all the other polyolens and could be processed in injection molding, blow molding, and
extrusion. Its market includes laboratory/medical ware, lighting, automotive, appliances, electronics, and
electrical parts. It has superior electrical properties.
1.4.5 PolyoleFiN elastomers
Commercially available POEs include the following: (1) poly(transisoprene); (2) poly(chloroprene);
(3) poly(1,2 butadiene); (4) poly(styrene-co-butadiene); (5) nitrile rubber; (6) butyl rubber; (7) EPR;
(8) EPDM rubber; and others. The estimated global consumption of polyolen elastomers in 1995
is 8 million tons [6]. The subject of thermoplastic elastomers is very broad and it is the basis of
several books. Because of this, only EPR and EPDM will be discussed briey along with propylene
copolymers in Section 1.4.6.2.2.
1.4.6 PolyoleFiN BleNds aNd CoPolymers
1.4.6.1 Polyolefin Blends
PBs are mixtures of structurally different homopolymers, copolymers, terpolymers, and the like.
The copolymers, terpolymers, etc., may be random, alternating, graft, block, star-like, or comb-like,
as long as the constituent materials exist at the polymeric level. The raison d'être of PBs is the cost/
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29Polyolefins
performance ratio. An expensive polymer, whose property spectrum is much higher than is needed
for a new application, may be blended with an appropriate inexpensive polymer whose property
spectrum is such that the resulting polyblend has an attractive cost/performance ratio. Thus, the
standard of performance demanded by the new application is satised by a mixture of commer-
cially available polymers without the need to develop a new polymer or to invest in a new plant. The
situation is even more attractive if, by a judicious choice of compatibilizers, a signicant degree of
synergy, rather than additivity, is achieved in the polyblends and a new material is essentially cre-
ated [158–161].
PBs could be classied in terms of the technology of manufacture. For polyolens, mechanical
PB is probably the most relevant and is made by melt-blending the polymers either on an open roll,
an extruder, or any other suitable intensive mixer at a processing temperature well above the Tm.
Depending on the state of thermal stability of the polymers being mixed, the high-intensity process-
ing shear could initiate degradation, resulting in free radicals that could cause polymeric reactions
and possible cross-linking of the constituent polymers. This may or may not be desirable.
PBs could also be classied in terms of the polymer–polymer phase behavior. Most of the com-
mercially important polyolen PBs presented in Table 1.7 are multiphase and would be characterized
TABLE 1.7
Commercially Important Polyolefin Blends
HMWPP/PP
PP/LDPE
PP/HDPE
PP/EPDM
PP/PIB/LDPE
PP/EPDM/HDPE
PP-EP/HDPE
PP/EPR or EBR/LDPE or EVA
HDPE/EPR or EBR/LDPE or EVA
PB-1/EPR or EBR/LDPE or EVA
LDPE/LLDPE
LDPE/LLDPE/EVA
HMW HDPE/HDPE
HMW MDPE/MDPE
HMW HDPE/LLDPE
PB-1/PP
PB-1/HDPE
Polyolens/S-EB-S/engineering thermoplastics
PP/S-EB-S/PBT
PP/S-EB-S/polyamides
Polyolens/S-EP-S/engineering thermoplastics
Polyolen/S-EP-S/polyphenylene ether
Polyolen/S-EP-S/thermoplastic polyurethane
Source: SRI International, Polyolen Markets and Resin Characteristics, Vol. 2, Project No. 3948, SRI, Menlo Park, CA,
1983; SRI International, Polyolens Production and Conversion Economics, Vol. 3, Project No. 3948, SRI, Menlo
Park, CA, 1983; SRI International, Polyolen Production Technology, Vol. 4, Project No. 3948, SRI, Menlo Park,
CA, 1983; Mai, K. and J. Xu, Toughening of thermoplastics. In Handbook of Thermoplastics, ed. O. Olabisi,
Marcel Dekker, New York, 1997; Chang, F. C., Compatibilized thermoplastics blends. In Handbook of
Thermoplastics, ed. O. Olabisi, Marcel Dekker, New York, 1997; Hudson, R. L., Polypropylene blend composi-
tions, U.S. Patent 4,296,022, 1981.
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30 Handbook of Thermoplastics
by low ductility and elongation [107–109,161–164]. This is why the blends must be compatibilized to
ensure a very effective interfacial stress transfer between the constituent polymers. Depending on the
morphology of the mixture, the resulting PB could, in some cases, be referred to as polyolen alloys.
Generally, a block copolymer could operate as a dispersing agent (emulsier), reducing the inter-
facial tension and the particle size of the dispersed phase. It could signicantly contribute to the
dispersion of the constituent polymers and improve elongation, tensile strength, and impact strength
of the PB. In effect, an A-co-B nonreactive compatibilizer could increase the compatibility of a
normally immiscible mixture of A and B homopolymers. For example, the impact properties of the
blends of poly(ethylene-b-propylene)/HDPE, poly(ethylene-b-propylene) EP (rubber), and PP/EP
(rubber) are enhanced because the ethylene or the propylene components of the copolymer provide
the needed compatibilization effects. Pendant reactive chemical groups, namely, the A–X reac-
tive compatibilizers and ionomer-reactive compatibilizers, have also been used to enhance poly-
mer compatibility. PB-1 is compatible with PP and HDPE, and the PB-1/PP lms nd applications
where the required toughness, creep, ESCR, memory, and heat sealability properties would have
precluded the use of either PP or PB-1. HDPE enhances the heat distortion temperature and the Tm
of PB-1 [153].
1.4.6.2 Polyolefin Copolymers
Inasmuch as most polyolen blends are incompatible, copolymerization is one method of creating new
commercially viable polyolen materials such as random, block, and graft copolymers from the wide
range of olen monomers. LLDPE is a copolymer of ethylene and a-olens. Other commercially impor-
tant polyolen copolymers are as follows: (1) those based on ethylene such as EVA copolymer; (2) those
based on propylene such as ethylene–propylene (EP) copolymer; (3) those based on butene-1 such as
copolymers of butene-1 with ethylene, propylene, pentene-1, 3-methylbutene-1, 4-methylpentene-1, and/
or octene-1; and (4) those based on 4-methylpentene-1 such as the copolymers of 4-methylpentene-1 with
pentene-1 and hexene-1.
1.4.6.2.1 EVA Copolymers
Ethylene-vinyl acetate (EVA) copolymers a random statistical copolymer that normally contains up
to 40% vinyl acetate, is usually made in a free radical process at pressures higher than 0.1 GN/m2.
The vinyl acetate–ethylene (VAE) copolymers containing above 40% and up to 100 wt% vinyl
acetate are made either in a medium-pressure solution-phase process or in a low-pressure emul-
sion polymerization process. It is the EVA rather than the VAE that is generally predominant and
is considered a variant of LDPE with lower crystallinity but higher density. When the vinyl acetate
content approaches 50 wt%, the crystallinity decreases to zero, as the polymer density approaches
1.0 g/cm3. The property advantages of EVA could be appreciated by comparing the stretch wrap
lm, containing 9–12 wt% vinyl acetate, with LDPE or plasticized PVC. The EVA has superior
stretchability, puncture resistance, retained wrap force, and tear resistance—all of the properties
that are sine qua non for stretch-wrap lms [163,164]. EVAs containing 2–40% vinyl acetate nd
applications as high-clarity lms for packaging, heavy-duty stretch-wrap lms, agricultural lms,
injection molding, prole extrusion, shoe soles, and hot melt adhesives.
1.4.6.2.2 PP Copolymers
The conventional poly(ethylene-co-propylene) or EP copolymer is available primarily in two forms,
namely, random or block copolymers. The raison d’être for EP copolymer is to increase the low-
temperature impact properties as well as the amorphous phase of the crystalline PP. Random EP
copolymers have good melt strength and are used in thermoforming and for blow molding of bottles
requiring high clarity. The thin lm of the single-phase random copolymer is transparent as the
material has a signicantly reduced crystallinity. The random copolymerization of propylene with
as much as 20 wt% α-olens such as decene-1 or longer yields a tough polymer whose density is
0.896 g/cm3 with signicantly improved low-temperature properties and polymer rheology.
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31Polyolefins
On the other hand, the lm of the block copolymer is essentially opaque, reecting the presence
of the two types of crystallite that render the EP block copolymer multiphase. In comparison with
a PP homopolymer, an EP block copolymer exhibits lower density, lower brittleness temperature,
higher impact strength, higher toughness, higher elongation, lower notch sensitivity, plus the char-
acteristic hinge break mode of the PP homopolymer [6,26,107,143,144]. The ESCR of an EP block
copolymer is outstanding, and the effect of the weight-average molecular weight is similar to that of
the PP homopolymer. It nds applications where its property prole makes it superior to PP.
The MgCl2 catalyst technology was the rst to produce, in one reactor, a tough, high-impact,
multiphase PP alloy containing a blend of iPP homopolymer as the continuous phase and EP rub-
ber as a uniformly dispersed phase [6,26]. Ordinarily, this sort of material would have had to be
made by blending the component polymers in an intensive mixer such as an extruder. In addition,
random copolymers containing up to 15% comonomers as well as multiphase alloys containing
up to 70% multimonomer are produced with the same catalyst technology. The ability to combine
olenic and nonolenic monomers has made it possible to produce polymers that combine the
desirable properties of olens with the desirable characteristics of amorphous engineering ther-
moplastics [6,26].
EPR copolymer containing 25–70 wt% propylene is amorphous and elastomeric. When made in
the presence of a small amount of nonconjugated diene, such as 1,4-hexadiene, dicyclopentadiene, or
5-ethylidene-2-norbornene, EPDM rubber is obtained. Because the only remaining double bond is
pendant to the backbone, EPDM, like natural rubber, is less sensitive to oxygen and ozone. EPDM is
characterized by excellent low-temperature performance properties, good dielectric properties, and
excellent resistance to chemicals. It is commercially produced in solution and suspension processes.
Its unique structural molecular parameters include the possible presence of random/alternating/block
comonomer distributions, variable block sequence lengths, a measure of tail-to-tail enchainment, and
absence or presence of crystallinity and of gels. There are observable relationships between these
structural parameters and the characteristic elastomeric performance properties such as cure ability,
processability, wear resistance, elasticity, and mechanical properties.
1.4.6.2.3 Poly(butene-1) Copolymers
Copolymers [148] of butene-1 with ethylene, propylene, pentene-1, 3-methylbutene-1, 4-methylpentene-1,
and octene-1 have been investigated. Of these, 3-methylbutene-1, 4-methylpentene-1, and octene-1 are
easily incorporated into the PB-1 lattice structure, and they signicantly retard the phase transformation
of polymorph form II to that of polymorph form I. On the contrary, ethylene and propylene are not well
incorporated into the PB-1 lattice structure, and, even at very low quantities, they signicantly accelerate
the phase transformation to such an extent that the form I polymorph precipitates directly from the melt.
Although poly(ethylene-co-butene-1) with more than 20 mol% ethylene comonomer is virtually amor-
phous with a Tg less than −40°C, the random copolymer, poly(propylene-co-pentene-1) retains a small
measure of crystallinity, and its 50:50 block copolymer actually contains both the PP crystalline struc-
ture and the form I structure of PB-1 with the respective melting points. The poly(1-pentene-co-butene-1)
exists in the form I polymorph with up to 84 mol% pentene-1 comonomer. The injection molding pro-
cessability of PB-1 is enhanced by the incorporation of ethylene or propylene comonomer, and, at very
low comonomer concentrations, propylene has been known to increase the isotacticity of the polymer.
1.4.6.2.4 Poly(4-methylpentene-1) Copolymers
The commercially available variety of poly(4-methylpentene-1), manufactured by Mitsui Petrochemical,
is christened TPX and contains pentene-1 or hexene-1 comonomer. It is more crystalline than the homo-
polymer with a melting range of 230–240°C, a Vicat softening temperature of 179°C, and a heat dis-
tortion temperature of 100°C. The corresponding values for the homopolymer are 100°C, 245°C, and
58°C, respectively [156,157]. TPX has higher elongation, impact, and softening temperature than the
homopolymer; it has a higher melting and softening temperature than PP. Like its homopolymer, its
lm is exceptionally transparent, whereas the PP lm is translucent at best. It could be processed in
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32 Handbook of Thermoplastics
injection molding, blow molding, and extrusion processes. Its major applications are identical to those of
the homopolymer; it is also used in polyolen composites.
1.4.7 PolyoleFiNs From ssCs
The development and commercialization of polyolens from SSCs began in the early 1990s
[102,165–167]. As shown in Table 1.8, the commercialization of metallocene-based products in
1995 included ethylene/α-olen copolymers and iPP. SSC polyolens are generally characterized
by lower density and other unique features that improve processability, physical, and mechanical
performance. Such features include narrow-molecular-weight distributions, long-chain branching,
narrow composition distributions, and blocky structure. Compared to earlier generation technolo-
gies, SSC polyolens are relatively simple and the catalysts are more predictable, enabling the
designing of tailor-made polyolen products through the use of molecular architecture approach
and economic models.
SSC systems enable the synthesis of modern polyolens with tailor-made microstructures, mor-
phology control, and controlled isotactic/syndiotactic, hemi-isotactic, and isotactic–atactic prop-
erties. PBs and block copolymers are also generated within the process of polymerization. SSC
polyolen manufacturing has now attained a high level of technological optimization resulting in
low energy consumption processes and enhanced polymer properties. The global market size of
SSC polymers is modest, but they nd high-performance applications in food and specialty pack-
aging, wire and cable applications (low-voltage exible cable insulation/high-voltage jacketing),
hoses, tubes, weather stripping, gaskets, foams, sealants, carpet ber, elastic ber and lm, belts,
footwear, as well as apparel applications such as swimwear, wrinkle-free shirts, woven fabrics, den-
ims, and other durable products. In addition, thermoplastic polyolens (TPOs), produced using SSC
systems, have replaced engineering plastics in interior and exterior automotive applications such as
bumper fascia, claddings, air dams, and instrument panels on account of superior cost/performance
scenario.
TABLE 1.8
Commercialization of Metallocene-Based Products in 1995
Company Capacity (kt/y) Location
Ethylene/α-olefin copolymers
Dow 112 USA
56 Spain
Exxon 100 USA
15 USA
Mitsubishi Yuka 100 Japan
Nippon Petrochemical 100 Japan
Ube Industries 20 Japan
Total 453
Isotactic polypropylene
BASF 12 Germany
Chisso 20 Japan
Exxon 100 USA
Hoechst 100 Germany
Mitsui 75 Japan
Total 307
Source: Catalyst Consultants.
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33Polyolefins
1.4.7.1 SSC Polyethylenes
Metallocenes have been used most successfully in the production of LLDPEs since the production
technology was commercially operational in 1991. LLDPE is a copolymer of ethylene and α-olens.
While the copolymerization of ethylene and α-olens using conventional Ziegler–Natta catalysts
results in an isotactic arrangement of the α-olen blocks, with SSCs, the polymer stereochemistry
varies over the whole range of possible microstructures involving syndiotactic to atactic to isotactic.
Table 1.9 presents the comparison of catalyst systems in terms of polymer microstructures produced.
Spherilene process was introduced by Himont whose successor company was Montell and the
last successor company is Basell. The process was a low-cost gas-phase technology for polyethylene
using a spherical MgCl2-supported catalyst to control the polymer morphology on a level similar to
that of PP gas-phase processes [131]. The catalyst and cocatalyst are reacted in a prepolymerization
loop in a diluent slurry to achieve morphology and mileage control. The polyethylene produced has
a spherical morphology and a high bulk density. In addition, a combination of two gas-phase reac-
tors in series allows the synthesis of PBs within the production process. The two gas-phase reactors,
separated by a hook loop, are used for the production of the polymer, which is stabilized and bagged
downstream. Pelletization is optional but not necessary due to the spherical form of the polymer.
The polyethylene products, as well as the blends, have properties that are similar to the competitive
polymers produced by different technologies.
Another gas-phase technology that improved upon the capacity of older plants was introduced
by Exxon [168]. It exploits the following advantages of SSC catalysts: (1) good comonomer incor-
poration, (2) homogeneous composition, and (3) high activity of the copolymerization. In gas-phase
polymerization, the reactor capacity is mainly determined by the heat removal capability of the
gas stream circulated. Thus, capacity is dependent on the recycle gas composition and the physical
properties of the reactant to determine the following: (1) gas-phase density (the higher the density
becomes, the more gas is circulated), (2) gas dew point (control is necessary to keep the uid-
ized bed free of liquid), and (3) reactor temperature (higher temperature allows higher dew points,
increasing the heat removal capacity). Hence, by injecting the reactants as condensed phase, it is
possible to utilize their heat of vaporization to increase the plant capacity. By controlling the density
of the uidized bed, it is possible to increase the amount of liquid above 10 wt% per cycle without
the formation of hot spots, lumps, or instabilities of the reactor-supercondensed mode technology.
The capacity of the plant, as well as the product range, may be improved further by using appropri-
ate postmetallocene SSC catalysts.
The better comonomer incorporation allows lower comonomer concentrations, and the better
response to molecular-weight regulation by hydrogen leads to a decrease in hydrogen concentration
by a factor of 100. Thus, a more condensing agent may be added, improving the polymer density as
well as the heat removal capacity. The narrow-molecular-weight distribution and the uniform compo-
sition of the copolymer allow higher temperatures because of the absence of low-molecular-weight,
high-comonomer-content fraction that could otherwise become sticky at elevated temperatures. It is
possible to increase the capacity of a plant by a factor of up to 2 with this technology and at the same
TABLE 1.9
Catalyst Systems and the Resulting Polyolefin Microstructures
Property Ti Catalyst V Catalysts Metallocene
Molecular-weight distribution Narrow 4–6 Medium to broad Very narrow (Mw /Mn = 2)
α-Olen incorporation Moderate High Low to very high
Composition distribution Heterogeneous Homogeneous Homogeneous
α-Olen blocks Isotactic Atactic to isotactic Syndiotactic to atactic to isotactic
Polymer unsaturation Low Very low 0.1–1 vinyl group per chain
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34 Handbook of Thermoplastics
time broaden the product envelope signicantly by incorporating additional comonomer and utilizing
comonomers like hexene and octene for the gas-phase processes. Other development by the Westlake
Company enabled efcient control of comonomer distribution. Through adaptation of Sclairtech tech-
nology by NOVA Chemicals, bimodal SSC LLDPEs became a reality using dual reactors.
Generally, polyethylenes produced by metallocene catalysts are characterized by narrow-
molecular-weight distributions (Mw/Mn = 2–2.5) reecting the single-site character of the catalyst.
They are highly linear and usually contain a vinyl end group in each polymer chain. The molecu-
lar weight may be controlled by the choice of the SSC, by the experimental conditions (monomer/
SSC ratio, temperature), or by addition of small amounts of hydrogen. Zirconium metallocene
catalysts are more active than their hafnium or titanium analogs. Indeed, hafnocenes have lower
numbers of active species, and rapid decomposition decreases the activity of titanium metallocene
systems [169]. The more effective incorporation of comonomers results in lower consumption of
comonomer and better recovery efciency. The combination of two SSCs enables the possibility
of combining the toughness of high-molecular-weight polymers with the processability of lower-
molecular-weight polymers.
The uniformity of active species in SSC enables the uniformity of the physical properties of poly-
olens. Advantages over conventional Ziegler–Natta catalysts include (1) higher activity, (2) uniform
comonomer incorporation, (3) narrow-molecular-weight distribution, (4) high ability to incorporate
sterically demanding comonomers, and (5) the possibility of controlling comonomer distribution in
the polymer chain from alternating or random or blocky structures through a judicious selection of
SSCs. The higher activity is reected in the reduced amount of ash and metal found after incineration
of the polymer. With conventional catalysts, the amount of ash and metal is much higher (30 ppm Al,
7–8 ppm Ti, 330 ppm total ash) than a metallocene catalyst (<20 ppm Al, <0.5 ppm Zr, <20 ppm total
ash) [170], making SSC quite attractive from an ecological view point.
The absence of low-molecular-weight high-comonomer-content molecules in LLDPE results in
low extractables, enabling applications in food packaging and medical sectors. In the conventional
LLDPE, the high-molecular-weight low-comonomer-content fraction has a crystallization tempera-
ture that is signicantly higher than that of the fractions containing more comonomer units, and these
latter chains form the nuclei for crystallization. This heterogeneous nucleation leads to the formation
of thick lamellae with minimum tie molecules. On the contrary, the nucleation in the copolymers pro-
duced by SSC is almost homogeneous, due to the narrow comonomer distribution, resulting in thinner
lamellae and a signicant number of tie molecules [171–173]. As a result of the homogeneous nucle-
ation, the melting point of SSC LLDPE, which decreases as the density decreases, is lower than that
of the conventional LLDPE. The lower melting point allows lower heat seal temperatures, whereas the
increased number of tie molecules enhances the dart impact strength. Another advantage of homoge-
neous composition in SSC polyethylenes is the higher clarity of the products. In conventional LLDPE,
the crystallites formed by the low-monomer-content chain scatter light and give rise to haze.
The major drawback of the SSC LLDPE is its viscous nature occasioned by the absence of LCB
and low-molecular-weight fraction. This may be overcome by (1) using mixtures of SSCs to produce
bimodal distributed polymers, (2) blending of copolymers of different molecular-weight distribu-
tions, or (3) using SSC that allows a desired amount of LCB.
For the copolymerization of ethylene and higher α-olens such as octene-1, Dow [174] and
Exxon introduced half-sandwich metallocenes, namely, the constrained geometry catalysts. Using
the constrained geometry catalysts, the incorporation of octene, plus the vinyl-terminated polymers
formed during the polymerization, leads to a product containing a limited amount of long-chain
branches. LCB in this case differs from LCB in LLDPE for the following reasons: (1) the long-chain
branches are fewer and longer; (2) the level of LCB is controlled by the catalyst used; (3) the long-
chain branches are almost linear [175]. Without affecting the polymer properties, the low level of
LCB signicantly improves processability to a level that is superior to that of conventional PE in
spite of the narrow-molecular-weight distribution. The polymers extrude about 2.5 times faster than
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35Polyolefins
conventional LLDPEs, and their melt tensions, which are correlatable to melt strength and bubble
stability, are about twice as high. This is important for lm blowing applications.
Copolymers that contain up to 20 wt% α-olen, christened plastomers, are exible thermoplas-
tics that match the property proles of conventional LLDPEs and VLDPEs at much lower densities.
Copolymers containing more than 20 wt% of comonomers are called polyolen elastomers (POEs).
Their exibility, clarity, and tensile strength enable them to replace other competitive exible thermo-
plastics such as polyvinyl chloride (PVC), EVA, EMA, styrene block copolymers, as well as conven-
tional EPR and EPDM [176].
1.4.7.2 SSC PPs
The fact that LLDPE production was ahead of that of PP may be attributed to the difculties that SSC
catalysts had in the beginning in competing with high mileage Ziegler–Natta catalysts. Nonetheless,
the properties of SSC PP in terms of isospecicity, molecular mass, as well as tailor-made micro-
structure represent a signicant advantage for SSC PP. Highly stereoselective metallocene SSCs
produce highly crystalline and stiff PPs. These polymers exhibit a stiffness that is 25–30% above
that of conventional PP, essentially equivalent to reinforced PPs [177]. Packages made from these
PPs have reduced wall thicknesses, are easier to recycle, and show enhanced impact strength, heat
resistance, lower density, and lifetime stability.
While iPP could be made in appreciable amount by both Ziegler–Natta and SSC, sPP can only
be made in appreciable amount by SSC. The tailoring of the polymer microstructure by the choice
of appropriate SSC has enhanced the market penetration of iPP, sPP, as well as the high-molecular-
weight aPP. The term stereospecicity does not refer to the extractable aPP, which contributes to the
fact that conventional iPP always has a melting point of 160–165°C.
1.4.7.2.1 Isotactic PP
The properties and the melting point of iPPs prepared by SSCs are determined by the amount of irreg-
ularities (stereo- and regio-errors) randomly distributed along the polymer chain. The most important
feature of SSC iPP is the low amount of extractables, which makes it possible for the PPs to be used
for food wrapping and other applications even at cooking temperature. Depending on their substitution
pattern, SSC PPs have melting points between 132°C and 165°C. This is presented in Table 1.10, which
illustrates a broad range of product properties obtained in propylene polymerization experiments with
various metallocene/MAO catalysts. The table compares the catalyst productivity, PP molecular
weight, melting point, and isotacticity.
Metallocene SSC, supported on 1,3,2,4-dimethyl benzylidene sorbitol (DMBS), an effective nucle-
ating agent, is used in producing iPP grades with granular morphology and ne spherulites [178]. The
state-of-the-art metallocenes are supported on silica treated with MAO [179,180]. They operate at
low Al/Zr ratios and are used in gas-phase processes [181]. With these free-oating catalyst powders,
polymer particle morphology control is possible and could match conventional systems with particle
diameters of several hundred millimeters with narrow particle size distribution and bulk densities
above 0.45 g/cm3. Table 1.11 presents a comparison of iPPs prepared by metallocene SSC and high-
mileage MgCl2-supported Ziegler–Natta catalyst systems in a bulk polymerization at 70°C.
The low melting points obtained with some SSCs even at high pentad isotacticities are occa-
sioned by the 2,1 and 1,3 mis-insertions [182,183]. Low-melting-point polymers with conventional
catalysts are obtained by copolymerization of propylene with small amounts of ethylene. Table 1.12
compares the properties of low-melting-point Ziegler–Natta propylene copolymers, metallocene
propylene copolymers, and homopolymers. The data illustrate the enhanced stiffness and transpar-
ency of the polymers produced using metallocene SSC.
The molecular-weight distribution of SSC iPP (Mw/Mn = 2–2.5) is lower than that of the conven-
tional grade obtained by peroxide degradation (Mw/Mn = 3–4). Its processing performance by thin-
wall molding or ber spinning is good. Compared with the conventional iPP grades, metallocene
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36 Handbook of Thermoplastics
products possess enhanced mechanical strength, which could be further improved by tailoring the
molecular-weight distribution.
Metallocenes iPP waxes are used as pigment dispersants, toner, or lacquer surfaces [184]. With
an appropriate choice of metallocene, it is possible to prepare iPP with molecular weights ranging
between 10,000 and 70,000 g/mol and with melting points ranging between 140°C and 160°C. With
conventional catalysts, hydrogen or polymer degradation is used to control the molecular weight
while the melting point is controlled by the addition of a comonomer. Table 1.13 compares iPP
waxes prepared by metallocene SSC to waxes produced by Ziegler–Natta catalysts using molecular-
weight regulation by hydrogen, visbreaking of Ziegler–Natta random EP copolymers, or through
TABLE 1.10
Metallocene Range of Catalyst Systems and the Resulting Polypropylene Isotacticitya
Metallocene
Productivity
[kg PP/(mmol Zr*h)]
Mw × 10–3
(g/mol) m.p. (°C)
Isotacticity
(% mmmm)
[En(Ind)2]ZrCl2188 24 132 78.5
[Me2Si(Ind)2]ZrCl2190 36 137 81.7
[Me2Si(IndH4)2]ZrCl248 24 141 84.5
[Me2Si(2Me–Ind)2]ZrCl299 195 145 88.5
[Me2Si(2Me–4iPr–Ind)2]ZrCl2245 213 150 88.6
[Me2Si(2,4Me2–Cp)2]ZrCl297 31 149 89.2
[Me2Si(2Me–4tBu–Cp)2]ZrCl210 19 155 94.3
[Me2Si(2Me–4,5BenzInd)2]ZrCl2403 330 146 88.7
[Me2Si(2Me–4Ph–Ind)2]ZrCl2755 729 157 95.2
[Me2Ge(2Me–4Ph–Ind)2]ZrCl2750 1135 158 –
[Me2Si(2Me–4Naph–Ind)2]ZrCl2875 920 161 99.1
Source: Antberg, M. et al., Makromol. Chem. Macromol. Symp. 48/49: 333, 1991; Spaleck, W. et al., Organometallics 13:
954, 1994.
a Conditions: Bulk polymerization in liquid propylene at 70°C, Al/Zr ratio 15,000.
TABLE 1.11
Comparison of Isotactic Polypropylenes by SSC and Ziegler–Natta Catalysts at 70°C
Property (I) (II) (III) (IV)
Melting point (°C) 139 151 160 162
Mw /Mn2.2 2.3 2.5 5.8
Tensile modulus (N/mm2) 1060 1440 1620 1190
Hardness (N/mm2) 59 78 86 76
Impact resistance Izod (mJ/mm2) 128 86 100 103
Light transmission (% 1 mm plate) 56 44 35 34
Melt ow rate (°/min) 2 2 2 2
Source: Reprinted from Catalyst Design for Tailor-Made Polyolens, K. Soga and M. Terano (eds.), Shiomura, T. Kohno,
M. Inoue, N. et al., Syndiotactic polypropylene, p. 327, Copyright 1994, with permission from Elsevier and
Kodansha Ltd., Tokyo.
Note: MAO catalyst systems with Al/Zr = 15,000: (I) [En(IndH4)2]ZrCl2; (II) [Me2Si(4,5Benzind)2]ZrCl2; (III)
[Me2Si(4,6iPrInd)2]ZrCl2; Ziegler–Natta catalyst system: (IV) MgCl2 supported TiCl4–Et3Al.
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37Polyolefins
Ziegler–Natta propylene homopolymerization. SSCs offer several property combinations not acces-
sible with conventional systems. For example, the vinyl end groups of the metallocene products are
utilized for functionalization, whereas with conventional catalysts, only saturated end groups are
formed due to the high amount of hydrogen used for molecular-weight regulation. With conventional
catalysts, the reactor normally has to be run under non-optimal conditions (high temperature, high
hydrogen pressure, and lower productivity) to obtain the desired product. There is also the problem
of a decreased output rate, which is necessitated by the difculty of condensing the hydrogen/
TABLE 1.12
Comparison of Low-Melting-Point Ziegler–Natta Ethylene–Propylene Copolymers,
Metallocene Ethylene–Propylene Copolymers, and Propylene Homopolymers
Property
Ziegler–Natta
Copolymer
Metallocene
Copolymer
Metallocene
Homopolymer
Melting point (°C) 141 140 142
Tensile modulus (N/mm2) 620 940 1120
Hardness (N/mm2) 41 59 65
Impact resistance Izod (mJ/mm2 ) 23.1 11.3 7.3
Light transmission (% 1 mm plate) 57 65 48
Extractables (% hexane, 69°C) 7.9 1.1 0.7
Source: Soga, K. et al., Makromol. Chem. Rapid Commun. 8: 305, 1987.
TABLE 1.13
Isotactic Polypropylene Waxes Prepared by Metallocene SSC I and II, Compared to Waxes
Produced by Ziegler–Natta Catalyst Using Methods III, IV, and V
Property (I) (II) (III) (IV) (V)
Mw × 10–3 (g/mol) 68 50 40 44 36
Mw /Mn1.8 2.0 3.8 2.2 1.9
C2/C4 (%) – – – 4.0/2.4 –
m.p. (°C) 163 133 159 133 155
Crystallinity (%) 69 50 60 30 59
Isotacticity (mmmm %) 96 85 91 80 91
Double bonds/chain 0.5–1 1 0 4 5
Mis-insertions/1000°C 1.7 1.3 4.7 1.3 19 C2n.d.
0.2 2.1 0.3 2.1 0.3 2.1 7 C4
Melt viscosity (200°C) (mm2/s) 2800 2521 900 1684 1040
Hardness (bar) 2000 874 1800 423 1870
Yellowness index 0.5 0.8 1 5–6 1–2
Dropping point (°C) 168 147 160 149 169
Congealing temp. (°C) n.d. 113 117 102 124
Source: Reprinted from Catalyst Design for Tailor-Made Polyolens, K. Soga and M. Terano (eds.), Shiomura, T. Kohno,
M. Inoue, N. et al., Syndiotactic polypropylene, p. 327, Copyright 1994, with permission from Elsevier and
Kodansha Ltd., Tokyo.
Note: (I) [Me2Si(2Me–4tBuCp)2]ZrCl2/MAO; (II) [Me2Si(IndH4)2]ZrCl2/MAO; (III) Ziegler–Natta propylene polymer-
ization with molecular-weight regulation by hydrogen; (IV) Visbreaking of Ziegler–Natta random ethylene–propyl-
ene copolymers; and (V) Ziegler–Natta propylene homopolymers.
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38 Handbook of Thermoplastics
propylene feed with its large amount of hydrogen coupled with the low heat removal capacity of
these mixtures. If run at optimum conditions, polymer degradation with expensive peroxides must
be used to control the molecular weight. Metallocenes avoid these difculties and thereby enable
process simplication.
1.4.7.2.2 Syndiotactic PP
Using Cs-symmetrical metallocenes, the production of sPP is possible due to enantiotopic vacan-
cies formed by the chain migratory insertion [92–94] during polymerization. As illustrated in
Table 1.14, sPPs, produced by SSC metallocene catalysts, show a higher level of irregularities
than iPP. With the same degree of tacticity, the syndiotactic polymer exhibits a lower melting
point, lower density, lower crystallinity, and lower crystallization rate [185,186]. The smaller
crystal size in sPP enables a higher clarity of the material but is also responsible for its inferior
gas barrier properties, limiting its utility in food packaging applications. On the other hand, its
strong resistance to radiation allows medical applications. Other advantages of sPP are the higher
viscous and elastic modulus at high shear rates and its outstanding impact strength, which disap-
pears at low temperatures. The combination of exibility, clarity, and tensile strength as well as
the low heat seal temperatures enables sPPs to replace PVC, EVA, and LLDPE in lms, foils, and
extruder products [175].
1.4.7.2.3 Reactor Blends and Copolymers of Stereospecific PP
In the 1980s, reactor granules of spherical morphology were rst introduced with processes such
as the Unipol with conventional Ziegler–Natta catalysts. These products do not need to be melt-
extruded [187,188]. The advent of SSC catalysts facilitated the use of two reactors in series, for the
production of reactor blends of PP, by transferring the propylene homopolymer-containing active
catalyst to a second reactor where an EPR phase is produced. Combination of the products of differ-
ent composition in the two reactors allows the synthesis of a broad range of products with medium
to “super-high” impact strength. By replacing the PP homopolymer of these blends with a random
copolymer matrix, the exibility and tensile strength at low temperatures are enhanced although the
melting point and stiffness are decreased. Blending of these copolymers with LLDPE results in soft
PPs (less than 100 MPa tensile-modulus) and enables sPPs to replace thermoplastic elastomers like
EVA, plasticized PVC, and SEBS [189].
The excellent performance of SSC copolymers offers improvements in impact properties. Among
the wide variety of properties of impact copolymers, it is safe to say that the stiffness of the material
is determined by the matrix material, whereas the impact resistance depends largely on the elasto-
meric EPR phase. While conventional catalysts have some inhomogeneities in the EPR phase due to
crystalline ethylene-rich sequences, the more homogeneous comonomer distribution obtained with
TABLE 1.14
Comparison of Isotactic and Syndiotactic Polypropylenes
oftheSame Stereoregularities Prepared by Metallocene Catalysts
Property [Me2Si(Ind)2]ZrCl2[Me2C(Flu)(CP)]ZrCl2
Tacticity Iso Syndio
mmmm%/rrrr% 83.1 83.6
Niso/nsyn 33 25
m.p. (°C) 138.4 133.2
Crystallinity DSC (%) 41.6 27.2
MFI 230/5 16.4 21.1
Density 0.899 0.885
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39Polyolefins
metallocene catalysts results in a totally amorphous EPR phase [190]. For applications demanding
broader-molecular-weight distributions, two or more SSCs may be combined to give a tailor-made
molecular-weight distribution.
1.4.8 Poly(CyClooleFiNs) From ssCs
Strained cyclic olens like cyclobutene, cyclopentene, and norbornene can be used as monomers
and comonomers in a wide variety of polymers. Generally, they can be polymerized by double-bond
opening (vinyl polymerization) or by ring-opening metathesis polymerization (ROMP).
1.4.8.1 Poly(cycloolefins) by Vinyl Polymerization
Homopolymerization of cyclic olens by double-bond opening is achieved by several transition
metal catalysts, such as palladium catalysts as well as metallocene catalysts. The polymers feature
two chiral centers per monomer unit and therefore are ditactic. While polymers produced by achiral
palladium catalysts seem to be atactic using chiral metallocene catalysts, highly tactic, crystalline
materials could be produced featuring extraordinarily high melting points (in some cases above the
decomposition temperature) and extreme chemical resistance.
The microstructures of these polymers have been investigated using oligomers as models.
Norbornene was shown to polymerize via cis-exo insertion [191,192], whereas in the case of cyclo-
pentene, quite unusual cis- and trans-1,3 insertions are observed [192–195].
1.4.8.2 Poly(cycloolefins) by ROMP
The concept of catalyzed metathesis polymerization of cyclic olen parallels the catalyzed double-
bond opening ethylene polymerization occasioned by the 1953 Karl Ziegler’s discovery [24] of
the rst-generation transition metal halide catalyst with an organometallic aluminum alkyl cocat-
alyst, which earned Ziegler and Natta the 1963 Nobel Prize in Chemistry. It was, however, Guilio
Natta, in 1966, that successfully undertook the polymerization of cycloheptene, cyclooctene, and
cyclododecene using a catalyst consisting of a combination of tungsten hexachloride with either
triethylaluminum or diethylaluminum chloride. Thus, the ring-opening polymerization of cyclic
alkenes to polyalkenemers was born, even though the underlying carbene catalyzed metathesis
mechanism was not understood at the time. Several generations of ROMP catalysts were later
developed leading to the revelation of carbene catalyzed metathesis mechanism and the award of
the 2005 Nobel Prize in Chemistry to Dr. Yves Chauvin at the Institut Français du Pétrole, Rueil-
Malmaison, France; Professor Robert H. Grubbs, California Institute of Technology, Pasadena,
California; and Professor Richard R. Schrock, Massachusetts Institute of Technology, Cambridge,
Massachusetts [196].
Catalyzed ROMP of cyclic olens is driven by the ring strain in monomers such as cyclobu-
tene, cyclopentene, dicyclopentadiene, and norbornene. First, a transition metal (ruthenium, molyb-
denum, tungsten) carbene catalyst complex is formed. The carbene catalyst complex attacks the
double bond in the monomer ring structure forming a highly strained metallocyclic intermediate
that opens up, yielding a double-bonded unit with the transition metal as well as a terminal double
bond. This constitutes the beginning of living ring-opening catalyzed metathesis polymerization.
Variations within ligands, bi- and tricyclic rings, and ring substituting moieties and/or solvents
result in variations of catalyst stability, catalytic activity, kinetic prole, catalyst regio- and stereo-
selectivity, monomer/comonomer incorporation capability, and polymer molecular-weight charac-
teristics and microstructure.
Important industrial ROMP polymers include Vestanamer, a trans-polyoctenamer (from cyclooc-
tene), polynorbornene (Norsorex), and polydicyclopentadiene (PDCPD, a side reaction of norborn-
ene polymerization). These polymers could also be functionalized for further reaction with a variety
of nonpolar monomers or oligomers.
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40 Handbook of Thermoplastics
1.4.9 CyClooleFiN CoPolymers From ssCs
The homopolymers of cycloolens like norbornene or tetracyclododecene are barely processable
due to their high glass transition temperatures and their insolubility in common organic solvents.
Functionalization enables copolymerizing of ROMP polyalkenemers with a variety of monomers or
oligomers resulting in copolymers with impressive mechanical, thermal, rheological, and crystal-
lization properties. Also, the metallocyclic intermediates enable the production of block copolymers
by changing feed compositions.
Cycloolen copolymers (COCs) of cyclic olens with ethylene or α-olens represent a new class
of thermoplastic amorphous polyolens [197–199]. Early attempts to produce such copolymers were
made using a heterogeneous two-component Ziegler–Natta catalyst system, namely, TiCl4/AlEt2Cl
[24,25]. In the 1980s, vanadium catalysts were used for the copolymerization, but real progress was
made by utilizing metallocene SSC. Metallocenes are about 10 times more active than vanadium
systems, and, by judicious choice of the metallocenes, the comonomer distribution is varied from
statistical to alternating. Statistical copolymers are amorphous if more than 15 mol% of cycloolen
is incorporated in the polymer chain. The glass transition temperature can be varied over a wide
range by appropriate choice of the cycloolen, with the right amount of the cycloolen incorporated
in the polymer backbone.
As for the ethylene/norbornene copolymerization, it is possible to produce copolymers with
molecular-weight distributions of Mw/Mn = 1.1–1.4 by controlling the polymerization conditions
[200]. This pseudo-living polymerization enables the production of block copolymers by changing
the feed composition. Statistical copolymers are transparent due to their amorphous character; they
are colorless and show a high optical anisotropy. Because of their high carbon/hydrogen ratio, these
polymers have a high refractive index of 1.53 as illustrated in Table 1.15 for an ethylene/norbornene
copolymer at 50 mol% incorporation. Their stability against hydrolysis and chemical degradation
in combination with their stiffness and very good processability makes them potential materials for
optical applications in compact disks, lenses, and optical bers [201].
The ethylene/norbornene alternating copolymer has a glass transition temperature of 130°C and
a melting point of 295°C. Thermoplastic processing is therefore possible at 300–330°C. Its melting
point as well as its crystallinity may be inuenced by the choice of the metallocene and polymeriza-
tion conditions. Compared to the statistical copolymers, the alternating structures are characterized
TABLE 1.15
Properties of aRandomEthyleneNorborneneCopolymer
Containing 52 mol% of Norbornene
Mechanical Properties
Density (g/cm3) 1.02
Glass transition temperature (°C) 150
Tensile modulus, ISO 527 (MPa) 3100
Tensile strength, ISO 527 (MPa) 66
Elongation at break, ISO 527 (%) 2–3
Optical Properties
Clarity White, clear
Anisotropy Very low
Refractive index 1.53
Source: Land, H.-T. and D. Niederberg, Kunststoffe/plast Europe 85(8): 13, 1995; Land, H.-T.,
New cyclic-olen copolymers from metallocenes. In Proceedings of the International
Congress on Metallocene Polymers Metallocenes ’95, 217, Brussels Schotland
Business Research Inc., 1995.
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41Polyolefins
by better heat resistance and are unaffected by nonpolar solvents. The diameter of the crystallites
is about 0.05–1 mm; thus, these copolymers are transparent. Similar alternating structures could be
obtained by the ring-opening polymerization of multicyclic polyolens followed by hydrogenation
of the unsaturated polymer.
1.5 POLYOLEFIN COMPOSITES AND NANOCOMPOSITES
The birth of the composite, as a material containing polymers plus particulate llers and ber rein-
forcements, could be traced to the rst truly synthetic plastic, a phenolic resin, christened Bakelite,
after Dr. Leo Hendrick Baekeland [202] who founded the Bakelite Corporation, which later became
the Union Carbide Corporation (UCC). The pressure vessel used to commercialize Bakelite, called
the Bakelizer, was designated the rst National Historic Chemical Landmark in November 1993 and
is housed at the Smithsonian Institution’s National Museum of American History in Washington,
DC. In the 1910s, the addition of wood our to Bakelite gave rise to the rst synthetic composite. By
the 1950s, synthetic composite had included the glass ber-reinforced plastics, and this proceeded
the era, in the late 1960s, of intense interest in composites beginning with the development of the
high-performance carbon and aramid bers.
1.5.1 CoNveNtioNal PolyoleFiN ComPosites
Fibers that could be used in polyolen composites include acicular particulates, alumina, aramid,
boron, ceramic, glass, carbon, polyolens, modern rigid rod polymer bers, and knitted reinforce-
ments. The desired features of bers are as follows: high aspect ratio, chemical stability, thermal
stability, low cost, low health hazard, minimum grain size, minimum porosity, minimum surface
aws, minimum surface roughness, high specic strength, high specic stiffness, and high tough-
ness. No single ber is characterized by the best combination of all of these properties, and a sig-
nicant body of knowledge exists pertaining to the dependence of the polyolens–ber composite
performance on the interplay among the individual ber component, matrix, interface, nature of
damage, and failure mechanisms. The key applications of polymer–ber composites are in simple
primary structures, and macromechanics has been used to elucidate the paramount issues dictating
the design requirements for polymer–ber composites in structural applications [203].
Particulate llers that have been used as polyolen additives or extenders are as follows: wood
powder, glass spheres, hollow silicates, calcium silicates (wollastonites, CaSiO3), silica minerals
(diatomaceous earth, kaolin clays), magnesium silicates (talc, mica, asbestos), calcium sulfate whis-
kers, barium sulfate, calcium carbonate, and carbon black. Fillers or bers are used either to lower
the cost or to improve the physical properties of polymeric materials. In general, however, bers
have the strongest effects on polyolen properties followed by plate-like and particulate llers, in
that order.
Flexural modulus, tensile modulus, stiffness, abrasion resistance, antistatic behavior, and heat
distortion temperature are almost always improved, and the coefcient of thermal expansion
is considerably reduced by the addition of bers or llers. The same cannot be said for impact
strength (toughness) and elongation to break (ductility) unless an appropriate interfacial agent
is used. Toughness and ductility are low in lled polymers because of strain magnication, as
rigid inclusions constitute stress concentration aws and all of the strains are imposed on the
diminished quantity of ductile matrix. This is why polyolen composites are weak and brittle
if the interfacial agent is not effective. Coupling agents improve adhesion and thereby enhance
the composite toughness with little or no increase in ductility, resulting in a strong but brittle
material. On the contrary, a decoupling agent decreases adhesion but facilitates microcavitation,
which signicantly increases ductility with little or no increase in toughness, resulting in a ductile
but weak material.
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42 Handbook of Thermoplastics
The most desired interfacial agent is that which signicantly increases both toughness and ductil-
ity through enhanced particle-to-matrix adhesion and the formation of a discernible tough interface.
This implies diminished interface stress concentration and uniform microcavitation so as to obtain
an overall reduced stress even if a moderate triaxial state of stress exists throughout the composite.
Synergism is an ultimate goal. The search for such reinforcement promoters [204,205] constitutes
a dynamic ongoing research area in industry, universities, and research institutes. So far, there is a
loose consensus that shear banding, crazing, and/or microcavitation are the essential mechanisms
for toughness and ductility, as they are largely responsible for creating desired new surfaces that are
capable of absorbing large amounts of energy during deformation.
In preparing polyolen composites, the interfacial agents are rst deposited on the llers and/
or bers before the melt compounding and/or interfacial reaction is effected. The interfacial agents
that have been used with polyolens include organosilanes, organotitanates, other organometal-
lic compounds, cross-linking agents, and polymerization catalysts. Composite materials could be
processed in one or more of the following: autoclave molding, lament winding, hand lay, injection
molding, and pultrusion. The various aspects of the methods are well documented.
Under the inuence of adverse environmental agents, the ultimate properties of polyolen com-
posites are susceptible to degradation. Such an adverse environment could be something as innocu-
ous as moisture. The effect of adsorbed moisture is to degrade the matrix-dependent properties with
a resultant effect on the load-bearing performance. In a less friendly environment, plasticization and
possibly environmental stress cracking could occur.
1.5.2 PolyoleFiN NaNoComPosites
Polyolen nanocomposites are multiphase systems, with ller particle dimensions in the range of
1–100 nm, consisting principally of polyethylene, PP, copolymers, or blends intimately mixed with
nanoscale llers. Relevant nanoparticles are nanoclays, nanotubes, nanobers, and nanopowders;
specic examples include carbon nanobers, carbon nanotubes, graphite (graphene), nanometals,
nanometal oxides, and nanoscale inorganic llers and bers. The types of clay used include mainly
montmorillonite (MMT), hectorite, saponite, as well as organoclays. The performance of polyolen
nanocomposites depends on the morphology of the nanocomposites. For clay nanocomposites, the
comprehensive properties depend strongly on the degree to which deagglomeration, dispersion,
intercalation, and exfoliation of clay platelets are achieved within the polyolen matrix.
Polyolen nanocomposites with deagglomerated and well-dispersed nanoclay, but no expansion
of the interlayer spacing of the nanoclay platelets, are normally characterized as immiscible [206]. If
the intensity of mixing is such that the interlayer spacing of the platelets is expanded but the charac-
teristic diffraction peak of the nanoclay remains observable (with clay loading > 2%), the polyolen
nanocomposites are normally characterized as intercalated. If, on the other hand, the deagglomer-
ated nanoclay is well dispersed and well intercalated, and the interlayer spacing of theplatelets is
so expanded that the characteristic diffraction peak of the nanoclay is not observable, the resulting
polyolen nanocomposites are characterized as exfoliated, provided that the clay loading is greater
than 2%. As an example, the interlayer spacing of MMT, in a highly exfoliated polyolen/MMT
nanocomposite, is higher than 3.27 nm [207–212]. Polyolen nanocomposite formation process
could be roughly delineated into physical method and chemical reaction process.
1.5.2.1 Nanocomposite Formation by Physical Methods
Physical process may include solution method, latex method, and melt processing. Because of its
capacity to achieve signicant exfoliation of clay platelets within the polymer matrix, the commer-
cial physical method of choice is melt processing involving formulation compounding and fabrica-
tion. In this regard, exfoliation depends on the nanocomposite loading, the surfactant used to form
the organoclay, the effectiveness of nanoller surface treatment, the positive interaction or afnity
of polyolen–ller surfaces, the polyolen melt ow, and the intensity of the dispersive energy of
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43Polyolefins
the processing method. Aside from the degree of exfoliation, appropriate orientation of nanoplatelet/
nanober is also affected by the type of processing used. In general, twin screw extrusion com-
pounding is the commercial physical method of choice for producing polyolen nanocomposites.
With this method, polyolen nanocomposites have penetrated the following markets: automo-
bile, packaging, and re-retardant. Nanoplatelet/nanober ability to reduce ammability as well
as the maximum heat release rate during combustion [208] reduces the amount of ame-retardant
additives that need to be incorporated. Indeed, TPO nanocomposites have been replacing conven-
tional composites in automotive applications [209] since the rst thermoplastic polyamide nano-
composites [209] were commercially introduced as timing belt cover in 1991 by Toyota Motor.
For polyolen nanocomposites in injection or extrusion blow molding, melt strength, rather than
melt viscosity, is the key controlling resin characteristic. Consequently, nanoclay-lled TPO nano-
composites, used as a matrix, have found applications in lm-blowing and other blow-molding
operations on account of enhanced melt strength. Polyolen nanocomposites are also being used as
a matrix for conventional llers in special applications requiring tailor-made property sets where
the melt ow index of TPO nanocomposites is appropriately adjusted to be identical to that of the
original polyolen matrix used in such applications.
Ordinarily, polyethylenes and PPs, particularly sPP, possess inferior gas barrier properties, com-
pared to PB-1, limiting their applications in food packaging. However, polyolen nanocomposites
have enhanced gas barrier properties on account of the tortuous path created by the nanollers.
Because of this, the barrier properties of these polyolens are being enhanced with the addition
of exfoliated nanoclay platelets having an appropriate aspect ratio to alter the diffusion path of
penetrant molecules. Because of the excellent surface nish characteristics of nanocomposites, the
addition of the exfoliated nanoclay platelets does not impair the smoothness and transparency of the
resultant polyolen thin lm. Consequently, single-site polyolen nanocomposites, with enhanced
stiffness, tensile strength, gas barrier properties, and tensile and dynamic storage modulus, are
assuming a preeminent role in food-packaging applications.
1.5.2.2 Nanocomposite Formation by Chemical Reaction
Nanocomposite chemical reaction process is the in situ polymerization or copolymerization of ole-
ns with nanoclay. This formation process could involve the use of any of three types of nanoclay
(MMT, hectorite, or saponite) and any of four types of catalysts, namely, Ziegler–Natta catalysts,
metallocene catalysts, nonmetallocene catalysts, and late transition metal catalysts [210]. Beginning
with immobilizing the precatalysts onto the clay, the nanocomposite chemical reaction formation
method ends with olen polymerization within the interlayer of the nanoclay platelets. The layered
structure of the polymerizing system has an effect on the chain transfer and the termination steps,
thereby affecting the molecular weight and other microstructure of the polymers or copolymers.
The heat of olen polymerization and propagation has positive effects on the intercalation and
exfoliation of nanoclay in the polyolen matrix. By doing so, the nanoclay interlayer spacing is so
expanded that complete exfoliation of nanoclay is accomplished [211]. The nanoclay serves as a
catalyst support or an adjunct, which remains with the nal nanocomposite. For polyethylene nano-
composites, both the crystalline and the amorphous phases of the matrix are known to be affected
with concomitant effect on the γ, β, and α transitions [212]. Increasing amount of nanoller is
known to produce increasing percentage of crystallinity and higher melting temperature, suggesting
a heightened heterogeneous nucleating activity of the polymer in the presence of nanollers.
Generally, completely exfoliated polyolen nanocomposites are characterized by excellent sur-
face nish and high modulus-to-weight, strength-to-weight, and surface-to-volume ratios, which
could result in weight reduction of up to 40% compared to conventional polyolen composites.
With chemical reaction formation process, polyolen nanocomposites having less than 5 wt%
nanoclay are characterized by complete exfoliation. Such nanocomposites are generally superior
to those formed through physical methods particularly for impact properties, elongation at rup-
ture, and tensile and exural properties. They possess high heat distortion temperature, enhanced
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44 Handbook of Thermoplastics
glass transition temperature, high on-set decomposition temperature, outstanding mechanical
properties, excellent gas barrier properties, respectable ame-retardant properties, and effec-
tive dyeability. Other properties affected by the higher degree of exfoliation include crystallinity,
improved thermal oxidative stability, reduced thermal expansion coefcient, improved melt ow,
and enhanced hardness.
Selecting the most appropriate formation method is the key to realizing the variety of property
advantages inherent in polyolen nanocomposites. Regardless of the method of formation, the com-
prehensive properties of polyolen nanocomposites are superior to those of conventional polyolen
composites reinforced with microsized conventional glass ber and/or other llers.
1.6 PROCESSING METHODS FOR POLYOLEFINS
The key processing methods for polyolens are injection molding, compression molding, rotational
molding, blow molding, structural foam molding [213], structural web molding [214,215], extru-
sion, blown lm extrusion, and cast lm extrusion [132]. Aside from the independent processing
variables of time, temperature, and pressure, the choice of an appropriate processing machine for
a given polyolen product is critical. Available machines include single screw extruder, twin screw
extruder, gear pumps, Buss-kneaders, Readco mixers, and Farrel continuous mixers. Each of these
machines nds appropriate applications in compounding polyolens with particulate llers and/or
ber reinforcements to produce composites or nanocomposites.
In injection molding, the polyolen granules or pellets are placed in a hopper that continu-
ously feeds the heated barrel of an extruder. The polymer is melted and the molten material is
injected, under high pressure, into a relatively cold mold where the material solidies replicating
the shape of the mold cavity. It is essential for the melt viscosity to be sufciently low to ensure
that the mold cavity is lled in a minimum possible cycle time. Injection molding is a cyclic
process.
In rotational molding (rotomolding or rotoforming), nely ground polyolen powders are heated
inside a rotating mold where the polymer melts and uniformly coats the inner surface of the mold.
The mold is cooled in a special chamber just prior to part removal. The process is used for the pro-
duction of large complex polyolen parts such as large containers, storage tanks, water tanks, and
portable sanitary facilities. Rotational molding is also a cyclic process.
The injection or extrusion blow molding makes hollow parts through the formation of a parison
that is expanded, with a gas, against a mold cavity. Smaller containers (<1 L) are produced with
injection blow molding, whereas extrusion blow molding is suitable for larger containers and for
containers with handles. Melt strength, rather than melt viscosity, is the key controlling resin char-
acteristic, and large containers require high-molecular-weight polyolens with broad-molecular-
weight distribution that are easier to process and less likely to exhibit parison sag. Blow molding is
a cyclic process.
In extrusion forming, polyolen granules or pellets are placed into a hopper that continuously
feeds the heated barrel of an extruder. The polymer is plasticated and melted, and the molten mate-
rial is pumped through a die of roughly the same shape as the nal product such as sheets, pipes,
lms, and wire-and-cable coatings. The extruded product is drawn by some type of takeoff equip-
ment, sized, and cooled until solidied. Extrusion is a continuous process.
In blown lm extrusion, molten polyolen is extruded through a circular die whereby the die
mandrel introduces an internal air pressure that expands the extruded tube from 1.5 to 2.5 times the
die diameter. Melt strength, rather than melt viscosity, is the key controlling resin characteristic; die
swell and melt fracture are undesirable. Blown lm extrusion is a continuous process.
In cast lm extrusion process, molten polyolen is extruded, as a thin sheet on a mirror-surfaced
chill roll, through a large die whose size is equal to the width of the lm to be cast. The extruded
thin sheet is then drawn down by other rolls. Cast lm extrusion is a continuous process. Cast lms
are used as diaper liners, pallet stretch wrap, household cling wrap, and overwrap.
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45Polyolefins
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