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Engineered PP impact copolymers in a single reactor as efficient method for determining their structure and properties

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Impact polypropylene (PP) copolymers (IPCs) are important materials for many commercial applications. These materials are usually synthetized through different methods involving two consecutive reactions in the same phase or in different phases. Here, a laboratory-scale synthesis method based on a sequential liquid- and gas-phase two-step process in a single reactor is developed. Propylene homopolymers and IPCs were synthesized with varying amounts of comonomers and hydrogen. The IPC materials obtained were fully characterized via analytical temperature rising elution fractionation (TREF), differential scanning calorimetry (DSC), ¹³C nuclear magnetic resonance (¹³C NMR), gel permeation chromatography with an infrared detector (GPC-IR5), Charpy impact, scanning electron microscopy (SEM), and cross-fractionation chromatography (CFC). The addition of only ethylene to the second step in the absence of hydrogen led to the creation of an ethylene-propylene (EP) copolymer with similar impact strength to that of a propylene homopolymer. The addition of hydrogen to the first step dramatically shortened the length of the PP chains and inhibited catalytic active centers that led to EP copolymer synthesis. This material exhibited very low molecular weight, low ethylene incorporation, and rubbery phases irregularly distributed along the isotactic polypropylene (iPP) matrix, resulting in the formation of an EP copolymer material with poor impact properties. IPCs synthesized without hydrogen and with a 50/50 (v/v) mixture of propylene/ethylene monomers in the second step enhance ethylene incorporation, facilitating adequate homogeneous and heterogeneous ethylene distribution and resulting in a high increment of amorphous ethylene-propylene-rubber (EPR) domains, which remarkably improves impact properties. Additionally, a criterion based on the ratio between EEE and EPE+PEP triads ranging between 1 and 2 was also established to predict the impact resistance of any heterophasic PP. Fractionation of the optimal sample provided a detailed understanding about the microstructure of this copolymer through the study of the molecular weight and composition of the fractions via GPC-IR, analytical TREF, and DSC measurements. Finally, the liquid–gas-phase two-step IPC material was compared, by means of SEM and CFC measurements, with synthesized IPC using liquid–liquid-phase two-step polymerization, and the results showed that the range of EP composition as well as ethylene distribution in the molar mass molecules of the IPCs was correlated to their mechanical behavior. This proves that crystalline families composed of high-molecular-weight EP copolymers in the liquid–gas-phase process can act as a compatibilizing agent between the iPP matrix and the elastomeric rubbery phase, allowing one to improve the impact resistance of the IPC, more so than that of IPCs obtained in the gas–gas and liquid–liquid phases. The results indicate that the synthesis of IPC resins in a single reactor is an efficient experimental method for fundamental research on IPCs.
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European Polymer Journal 157 (2021) 110642
Available online 8 July 2021
0014-3057/© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Engineered PP impact copolymers in a single reactor as efcient method for
determining their structure and properties
María Teresa Pastor-García , Inmaculada Su´
arez , María Teresa Exp´
osito , Baudilio Coto , Rafael
A. García-Mu˜
noz
*
ESCET, Universidad Rey Juan Carlos, 28933 M´
ostoles, Madrid, Spain
ARTICLE INFO
Keywords:
Impact polypropylene copolymers
IPCs synthesis
ZieglerNatta catalyst
Polyolenscharacterization
Impact resistance
ABSTRACT
Impact polypropylene (PP) copolymers (IPCs) are important materials for many commercial applications. These
materials are usually synthetized through different methods involving two consecutive reactions in the same
phase or in different phases. Here, a laboratory-scale synthesis method based on a sequential liquid- and gas-
phase two-step process in a single reactor is developed. Propylene homopolymers and IPCs were synthesized
with varying amounts of comonomers and hydrogen. The IPC materials obtained were fully characterized via
analytical temperature rising elution fractionation (TREF), differential scanning calorimetry (DSC),
13
C nuclear
magnetic resonance (
13
C NMR), gel permeation chromatography with an infrared detector (GPC-IR5), Charpy
impact, scanning electron microscopy (SEM), and cross-fractionation chromatography (CFC). The addition of
only ethylene to the second step in the absence of hydrogen led to the creation of an ethylene-propylene (EP)
copolymer with similar impact strength to that of a propylene homopolymer. The addition of hydrogen to the
rst step dramatically shortened the length of the PP chains and inhibited catalytic active centers that led to EP
copolymer synthesis. This material exhibited very low molecular weight, low ethylene incorporation, and
rubbery phases irregularly distributed along the isotactic polypropylene (iPP) matrix, resulting in the formation
of an EP copolymer material with poor impact properties. IPCs synthesized without hydrogen and with a 50/50
(v/v) mixture of propylene/ethylene monomers in the second step enhance ethylene incorporation, facilitating
adequate homogeneous and heterogeneous ethylene distribution and resulting in a high increment of amorphous
ethylene-propylene-rubber (EPR) domains, which remarkably improves impact properties. Additionally, a cri-
terion based on the ratio between EEE and EPE +PEP triads ranging between 1 and 2 was also established to
predict the impact resistance of any heterophasic PP. Fractionation of the optimal sample provided a detailed
understanding about the microstructure of this copolymer through the study of the molecular weight and
composition of the fractions via GPC-IR, analytical TREF, and DSC measurements. Finally, the liquidgas-phase
two-step IPC material was compared, by means of SEM and CFC measurements, with synthesized IPC using
liquidliquid-phase two-step polymerization, and the results showed that the range of EP composition as well as
ethylene distribution in the molar mass molecules of the IPCs was correlated to their mechanical behavior. This
proves that crystalline families composed of high-molecular-weight EP copolymers in the liquidgas-phase
process can act as a compatibilizing agent between the iPP matrix and the elastomeric rubbery phase, allowing
one to improve the impact resistance of the IPC, more so than that of IPCs obtained in the gasgas and liquid-
liquid phases. The results indicate that the synthesis of IPC resins in a single reactor is an efcient experimental
method for fundamental research on IPCs.
1. Introduction
The discovery of the ZieglerNatta (ZN) catalyst in the 1950s
signicantly impaired the production of polyolens incorporating
different chain microstructures and properties and since then has
continuously grown with the rapid development of catalyst technology
combined with polymerization innovation [1,2].
Polypropylene (PP) is one of the most important materials among
polyolens for three main reasons. First, PP holds remarkable properties
such as low density, high melting temperature, and chemical inertness
* Corresponding author.
E-mail address: rafael.garcia@urjc.es (R.A. García-Mu˜
noz).
Contents lists available at ScienceDirect
European Polymer Journal
journal homepage: www.elsevier.com/locate/europolj
https://doi.org/10.1016/j.eurpolymj.2021.110642
Received 24 April 2021; Received in revised form 3 July 2021; Accepted 6 July 2021
European Polymer Journal 157 (2021) 110642
2
with low cost, which make PP optimal for long-life applications. Second,
PP is a highly versatile material, meaning that diversity in structural
designs and mechanical properties is achievable. Third, different
morphological structures of PP are available via llers or reinforcing
agents and the blending of PP with other polymers that yield materials
with superior features [3,4]. However, isotactic polypropylene (iPP), as
is well known, has extremely poor low-temperature impact properties,
and therefore, several approaches to improve its toughness have been
implemented, such as blending with elastomers and copolymerizing
with
α
-olenic moieties [57]. Other methods are developed at the in-
dustrial scale, especially in situ copolymerization with ethylene by
means of sequential gas- and liquid-phase reactors in the production
line, which is proven to be extremely effective to obtain high-impact-
resistant PP, commonly referred to as impact PP copolymers (IPCs) or
high-impact PPs (HIPPs) [8].
IPCs combine a crystalline iPP matrix (produced in the rst one or
two reactors) with embedded particles of ethylene-propylene-rubber
(EPR) and polyethylene segments (incorporated in the sequentially
following reactors) dening impact and low-temperature resistance [9].
Therefore, IPCs are alloys of PP and ethylene-propylene (EP) copolymers
of varying ethylene content with an array of block lengths [10]. The
amorphous copolymer fraction is responsible for increasing the impact
strength of the homopolymer matrix formed in the rst reactor [11].
Toughening efciency has been associated with rubber content,
morphological structure, phase composition, particle shape, particle
size, size distribution, viscosity, and rubbermatrix compatibility
[1218]. These materials have high impact resistance and have appli-
cations in the consumer and automotive industries [1922].
On the industrial scale, the Spheripol process offers licenses a simple
and economical method of producing a wide range of PP products of the
highest quality. This product range can be expanded easily. These plants
offer on a single polymerization line the widest range of homopolymers,
random copolymers, and terpolymers as well as heterophasic impact
copolymers covering all PP applications elds [23]. In the Spheripol
process (Basell), two loops are used in a series to narrow the residence
time distribution of the catalyst particles. The liquid propylene/polymer
suspension from the rst stage is ashed to gas/solid conditions prior to
entering the second stage. Then the particles are transferred to a
continuous gas-phase uidized bed reactor where the elastomeric phase
is produced within the iPP. The properties of a HIPP mainly depend on
particle morphology, which, in turn, depends on both the catalyst and
the process conditions [24].
Laboratory-scale hybrid technologies have been developed to study
the inuence of synthesis variables on material properties. Polymers are
synthesized through a combination of both types of reactors in a two-
stage reaction process, homopolymerization in liquid propylene, and,
in the second stage, successive gas-phase EP copolymerization in a
stirred-bed reactor [25]. These materials have also been synthetized in
only one reactor using a multistage subsequential polymerization pro-
cess, in slurry in the rst stage and with n-heptane and EP gas in the
second stage, with discharge during the process to keep the composition
constant [26,27]. Thus, slurry and gas are used for synthesizing IPC
materials [28,29].
The main objective of this study is to develop an alternative approach
for engineering IPC materials in only one reactor for laboratory-scale
research to emphasize the importance of experimental variables in IPC
synthesis and their inuence on IPC structure and properties. This
method is based on a sequential liquid- and gas-phase two-step process
but in a single reactor using a ZN catalyst. Process variables such as the
addition of hydrogen, the amount of ethylene and propylene, and the
pressure and time of monomer additions have been studied, and the
morphological characteristics of IPC materials were compared with
those obtained in the liquidliquid-phase [30] and gasgas-phase pro-
cesses [31]. Mechanical performance was obtained from notched
Charpy impact strength. Thermal properties in the solution and in the
solid state were determined via temperature rising elution fractionation
(TREF) and differential scanning calorimetry (DSC). Two-dimensional
distribution interrelating molar mass and chemical composition to ac-
quire full bivariate distribution was determined via cross-fractionation
chromatography (CFC). The molecular microstructure was acquired
via liquid-state
13
C nuclear magnetic resonance (
13
C NMR) and gel
permeation chromatography (GPC-IR5). Finally, the morphology and
distribution of the EPR phase in the iPP matrix were determined via
scanning electron microscopy (SEM) for the required comprehensive
characterization of these complex materials.
2. Experimental method
2.1. Materials
Propylene, ethylene, and hydrogen were supplied by Praxair SA
(99.99%) and the solvent nheptane by Scharlab SA (99%). All the
polymers were synthetized using a standard TiCl
4
/MgCl
2
ZN catalyst
containing 2.5 wt% Ti, triethylaluminum was used as a cocatalyst and
scavenger (TEA 1 M in nheptane, supplied by Witco), and cyclo-
hexylmethyldimethoxysilane (Cdonor, Wacker Química Ib´
erica SA)
was used as an external donor [32]. Sodium chloride used for every
experiment in the gas phase was supplied by Scharlab SA.
2.2. Synthesis procedure
A schematic drawing of the experimental setup is shown in Fig. 1a.
In this procedure, IPCs have been synthesized in two sequential
liquid- and gas-phase steps using one 2 L autoclave stirred reactor. A
jacketed steel vessel allowed for heating the reaction volume with a
water bath by a thermostat (Julabo), and a coil placed inside the reactor
allowed for refrigeration by cool water. This system is used to control the
temperature at 70 C for all the runs. Monomers were fed through
calibrated gas owmeters supplied by Bronkhorst Hi-Tec. To accomplish
this sequential reaction, a helical stirrer (previously described) [31] and
an extraction system are placed for emptying the reactor through an
outlet at the bottom of the reactor (Fig. 1a) with a mesh to prevent the
movement of the polymer out of the reactor. A cooling system to allow
internal refrigeration was implemented and previously described [31].
The polymerization procedure requires two different steps, as shown
in Fig. 1b: (1) propylene homopolymerization (pre-polymerization and
main polymerization) and (2) EP copolymerization. In this work, pro-
pylene homopolymers and IPCs were synthesized at varying amounts
and times of addition of monomers, pressures, and hydrogen. Propylene,
ethylene, and n-heptane were deoxygenated and dried through columns
containing a R-3/15 BASF catalyst, alumina, and 3 Å molecular sieves
before being fed to the polymerization reactor.
The solvent (400 mL of n-heptane) was introduced into the reactor
and was saturated with propylene at pre-polymerization conditions
described in a previous work [30]. This stage was maintained for 17
min. Then the temperature was raised to 70 C, and the desired amount
of hydrogen was added according to Fig. 1b as the start of the main
polymerization stage. The PP matrix synthesis stage in the liquid phase
was maintained for 60 min, and the propylene monomer was fed to keep
the pressure at 8 bars. The solvent was carefully removed before the next
gas-phase step to avoid the loss of synthesized PP and without stopping
the reaction and opening the reactor to prevent catalyst deactivation. A
solvent extraction time of 15 min was set up to generate reproducible
results and for comparison purposes. Solvent extraction occurred at the
bottom of the reactor by means of pressure difference, while stirring was
maintained to avoid polymer deposition. Inert sodium chloride (60 g)
was used for every experiment to prevent the polymer particles from
sticking to one another and to the reactor wall during the gas-phase
stage. Sodium chloride also improved the heat transfer from the react-
ing particles to the reactor wall [33,34], was calcined at 150 C for 24 h,
and was introduced in the reactor with overpressure of N
2
.
After the rst step, the reactor was vented several times with N
2
, and
M.T. Pastor-García et al.
European Polymer Journal 157 (2021) 110642
3
stirring was maintained to ensure the complete removal of unreacted
propylene, hydrogen, and residual solvent. Then an ethylene monomer
or a 50/50 (v/v) mixture of propylene/ethylene monomers was fed at 8
bars for 4 min, as shown in Fig. 1b. After the polymerization reaction,
the polymers were washed with water to dissolve the sodium chloride,
and they were recovered via ltration and dried.
2.3. Characterization of samples
The samples were characterized using different techniques. GPC-IR5
(7890A Polymer Char) was used to determine the average molecular
weights (M
w
and M
n
), molecular weight distributions (MWDs), and
polydispersity indices using three columns (PLgel Olexis of Agilent). The
conditions for these analyses were a temperature of 160 C, a ow rate of
1.0 mLmin
1
, and 1,2-dichlorobenzene as a solvent. This GPC instru-
ment also records the content of methyl groups as CH
3
/1000 total car-
bons or as a function of molar mass.
DSC (Mettler-Toledo 822e) equipped with a liquid nitrogen sub-
ambient accessory was used for thermal analysis. Melting and crystal-
lization thermograms were recorded by heating the samples in three
steps: from 100 C to 250 C, followed by cooling from 250 C to
100 C and reheating again to 250 C at rates of 10 Cmin
1
. The
Fig. 1. (a) Scheme of polymerization reactor, and (b) scheme of reaction process in two sequential liquid- and gas-phase steps.
M.T. Pastor-García et al.
European Polymer Journal 157 (2021) 110642
4
melting point and crystallization degree values were taken from the
second heating scan. The equilibrium melting enthalpy of a PP innite
crystal was taken as 482 J g
1
.
The monomer composition and triads distribution values were
determined via
13
C NMR. The samples were solved in TCB and d
6
-ortho-
dichlorobenzene and analyzed at 100 C on a Bruker DRX 500 spec-
trometer operating at 75.4 MHz, and an assignment of chemical shift
was carried out according to Randall [35] and Kakugo [36].
The powder samples were compression-moulded to a 4 mm plaque
using a hydraulic press at 180 C and a nominal pressure of 50 bars, with
a cooling rate of 15 C/min. Charpy specimens of 4 ×10 ×80 mm
(thickness ×width ×length) were machined from the plaque. A V-notch
of 45±1and a root radius of 0.25 ±0.05 mm was made via sawing
with a razor. The Ceast Charpy impact tester allows one to obtain impact
measurements, which were performed at 23 C to determine the fracture
mechanics values as resistance according to UNE-EN ISO179-1:01/
A1:05. The impact speed was always 2.9 ms
1
.
The analytical TREF experiments were carried out in a CRYSTAF-
TREF 200+(PolymerChar, Spain) equipped with ve separate crystal-
lization vessels for the sequential or simultaneous analysis of ve
different polymeric samples. In addition, 80.0 ±0.5 mg of the polymer
were dissolved in TCB with 300 ppm of antioxidant 2,6-di-tert-butyl-4-
methylphenol (BHT) at 160 C for 60 min at a concentration of 4 gmL
1
.
CFC was performed in a fully automated Polymer Char instrument.
Two-dimensional distribution relating to molar mass and composition
distribution are obtained. The PP samples are placed (40 mg) into
disposable vials, which are brought to the autosampler tray. The in-
strument automatically lls the vials with o-DCB and dissolves them at
temperatures of 140 C for 90 min. The polymer solution (2 mg/mL) is
loaded into the TREF column, where it is crystallized by cooling down
the TREF oven from 140 C to 30 C at 0.5 C/min and subsequently
eluting the fractions in stepwise temperature increments, from 30 C to
140 C, toward the GPC columns. The instrument fractionates polymers
according to crystallinity in 28 fractions, which are continuously
injected toward online GPC columns where a second round of frac-
tionation, this time according to molar mass, is performed.
The iPP
l
-4EP
g
sample was divided into three fractions (F1, F2, and
F3) using a preparative fractionation PREP mc2 instrument (Polymer
Char) in TREF mode. The polymer (1.5 g) was dissolved in 100 mL of
xylene at 130 C under stirring. The mixture was cooled to 95 C, kept at
this temperature for 45 min, and then cooled gradually to 50 C at
0.1 Cmin
1
. The rst fraction (F1) was obtained at 50 C. Then the
temperature was raised in different steps at 100 C and 130 C to gather
the rest of the fractions. A volume of 200 mL of each fraction was
collected in a beaker. These fractions were further precipitated in
acetone, ltrated, and dried under vacuum.
The morphology of polymer particles and dispersion of the EPR
phase in the iPP matrix were studied using an environmental scanning
electron microscope (XL 30 ESEM, Phillips). The SEM samples to analyze
EPR dispersion were prepared by cutting them in liquid nitrogen by
ultramicrotome, and their surfaces were dipped into and etched by n-
heptane under ultrasonic at 60 C for 60 min. An operating voltage of
22.0 kV was used to observe the surfaces of the samples after coating
them with gold. The counts of EPR particles were determined using
image analysis software (Digital Micrograph version 3.6.5).
3. Results
Table 1 compiles the synthesized resin samples. The homopolymer
was named as an iPP with a subscript liquid phase (l). Three different
IPC resins were synthesized in this work: an IPC material with only an
ethylene monomer (E) incorporated in the second step for 4 min (iPP
l
-
4E
g
); an IPC resin with a mixture of ethylene and propylene (EP) (50%
each) also incorporated in the second step for 4 min (iPP
l
-4EP
g
); and an
IPC copolymer similar to iPP
l
-4EP
g
but with the addition of hydrogen for
1 min at the rst step (iPP
l
-1.0-4EP
g
). Also, an IPC resin, iPP
l
-1E
l
, pre-
viously synthesized in the liquidliquid phase in two steps, was used for
comparison. Fig. 1b displays the experimental conditions for the syn-
thesis of all the materials. Likewise, Figs. 2, 3, 4, and 5 show the GPC,
TREF, DSC, and SEM results, respectively, of the PP homopolymer and
synthesized IPC resins.
3.1. Effect of ethylene addition on IPCs
PP homopolymerization to obtain iPP
l
was performed in the liquid
phase. This involved polymerization for 60 min at 8 bars and 70 C. To
generate IPC materials, the second-step polymerization process was
performed by adding an ethylene comonomer to the reactor for 4 min at
the conditions displayed in Fig. 1b to generate EPR elastomeric domains
embedded in the iPP matrix. Table 1 shows the activity expressed as
g
polym
g
1cat
min
1
and the characterization results of the propylene
homopolymerization (sample iPP
l
) and IPC resin (sample iPP
l
-4E
g
and
iPP
l
-1.0-4EP
g
). The activity values in the second gas-phase step are lower
than those obtained in the liquid phase excepting the polymerization in
the presence of hydrogen, in agreement with the previous results
[30,31].
A
13
C NMR analysis of the sample iPP
l
-4E
g
(presented in Table 1)
determined a total ethylene molar percentage of 4.5 mol%, with a het-
erogeneous distribution, quantied by EEE triad mol%, of 3.9% and a
Table 1
GPC, DSC and
13
C NMR results of PP homopolymer and IPCs materials.
Sample Productivity
g
polym
g-
1
catmin-
1
M
w
(kg/
mol)
PI T
m
(C) T
g
(C)
Crystallinity
(
α
, %)
MFI (g/
10 min)
Charpy impact
(kJm
2
)
P (%
mol)
E (%
mol)
EPE +
PEP (%
mol)
EEE (%
mol)
iPP
l
36.7 1122.1 6.9 163.0 5.5 46 0.35 8.0 ±0.6 100 0.0 0.0 0.0
iPP
l
-4E
g
11.7 981.7 5.2 162.0 5.5 54 0.15 7.6 ±0.9 95.5 4.5 0.1 3.9
iPP
l
-4EP
g
23.4 806.7 6.5 115.9/
162.8
56.3 25 0.05 13.8 ±1.7 78.7 21.2 6.3 12.2
iPP
l
-1.0-
4EP
g
42.8 153.4 9.5 119.3/
160.5
9.4 57 56.10 1.8 ±0.7 95.5 4.5 1.8 2.6
Fig. 2. Molecular weight and methyl group distributions determined from GPC
of PP homopolymer and IPC resins.
M.T. Pastor-García et al.
European Polymer Journal 157 (2021) 110642
5
homogeneous distribution, PEP +EPE mol%, of around 0.1%. This
result indicates the formation of long ethylene sequences with the
capability to crystallize and an almost negligible presence of elastomeric
EPR chains. The lower molecular weight of iPP
l
-4E
g
compared with that
of iPP
l
(Table 1 and Fig. 2) and the variation of the content of methyl
branches with molar mass shown in Fig. 2 suggest that a propylene
comonomer is incorporated into the lower molar mass molecules and an
ethylene comonomer is added in the higher molar mass chains during
the gas-phase step. Additionally, in the MWD of the iPP
l
-4E
g
resin, a
shoulder of lower molar mass is observed that can be associated with
those chains composed mainly of propylene since the methyl branch
content (Fig. 2) at this low molecular weight is around 400 CH
3
/1000C,
which matches well with that of the iPP
l
homopolymer. As a conse-
quence, the crystallinity degree becomes higher than that of the iPP
l
sample (Table 1).
The TREF thermogram (Fig. 3) shows a low soluble fraction (5.6 wt
%) at 28 C, slightly higher than that of iPP
l
[30], proving that only iPP
glass transition is observed in DSC. The limitation of DSC implies that
the thermal features of minority components at low concentrations (i.e.,
EPR elastomeric domains) cannot be observed. At around 95 C, a small
peak is displayed that could be matched with chains containing EP co-
polymers with long ethylene sequences, mentioned above, with the
capability to crystallize because of the high temperature, in agreement
with DSC measurement, in which a slight endotherm at around 122 C is
also identied in the thermogram related to these sequences, and
matching well with the low ethylene content seen via NMR analysis.
Finally, an intense peak at about 122 C may be unequivocally assigned
to iPP chains with high molecular weight, synthesized in the liquid-
phase step, since it is also observed in the iPP
l
homopolymer in TREF
and DSC analysis.
SEM measurements were performed to determine the morphology of
the IPC resins. Fig. 5a and b show SEM images of the surface of the IPC
resin, iPP
l
-4E
g
, after etching. The amorphous components, present as
dispersed domains and cavities embedded into the iPP matrix, are
negligible or rarely observed in this sample. Finallyand in agreement
with the NMR, GPC, TREF, DSC, and SEM resultsthe resistance to
impact of the iPP
l
-4E
g
sample measured by the Charpy strength test does
not improve with respect to the PP homopolymer.
3.2. Effect of hydrogen addition on IPC
In this section, different modications of the sample iPP
l
-4E
g
have
been proposedrst, the addition of hydrogen for controlling the mo-
lecular weight; and second, the feeding of a 50/50 (v/v) mixture of
propylene/ethylene monomers. The addition of hydrogen to the reactor
during the rst stage was xed at a pressure of 1.0 bars, and the
ethylene/propylene comonomers were fed to the same reactor during
the second step, as depicted in Fig. 1b. The reaction time for this step
was xed at 4 min [30,31].
The ethylene content incorporation is rather limited (4.5 mol%)
because the addition of hydrogen during synthesis increases the iPP
homopolymerization at the expense of catalytic site deactivation for
ethylene copolymerization [30]. However, the ethylene units are un-
evenly distributed with respect to the iPP
l
-4E
g
sample. Thus, 1.8 mol% is
dispersed across the matrix as random ethylene/propylene chains (EPE
+PEP triads), and the rest (2.6 mol%) was inserted heterogeneously
(EEE triad), as reected in Table 1. As might be expected, the molecular
weight for iPP
l
-1.0-4EP
g
remarkably decreased (Table 1), and as with the
sample iPP
l
-4E
g
, the ethylene units were also incorporated in the long
molecules (Fig. 2).
The TREF thermogram of iPP
l
-1.0-4EP
g
(Fig. 3) exhibits a similar
prole to that of the iPP matrix synthesized with hydrogen in the liquid-
phase stage [30,37]. This highlights the higher intensity of the shoulder
near 110 C, assigned to the presence of short PP chains as a conse-
quence of the addition of hydrogen, which acts as a chain transfer agent.
The melting endotherm of iPP
l
-1.0-4EP
g
, displayed in Fig. 4, is
similar to that of iPP
l
-4E
g
, in which a dominating peak of around 160 C
is assigned to melt the iPP crystallites, along with a small peak of around
119 C given the limited amount of incorporated long ethylene se-
quences (EEE triads). Glass transition displacement is observed at a
lower temperature, with regard to iPP
l
-4E
g
resin, which may be due to
the lower PP chainslength. The glass transition temperature associated
with the EPR fraction (EPE +PEP triad) is practically undetectable, as is
the case in TREF measurement, in which a rise in the soluble fraction
area is not perceivable. Furthermore, the exhibited high crystallinity,
very close to the iPP
l
-4E
g
value (around 55%), corroborates the low
ethylene incorporation.
Fig. 5e and 5f provide SEM micrographs of iPP
l
-1.0-4EP
g
. The images
show the presence of a large number of spherical voids surrounding the
amorphous EPR fraction with extremely regular size distribution. These
domains are also randomly distributed throughout the iPP matrix.
Despite these satisfactory morphological results and the presence of an
EPR component embedded into the iPP matrix, low molecular weight
impedes this resin from being ranked as an adequate IPC material. As
expected, this sample shows a remarkably low value of the Charpy
impact strength test (1.79 ±0.67 kJm
2
).
Fig. 3. Compositional distribution determined by TREF of PP homopolymer
and IPC resins.
Fig. 4. Melting thermograms obtained from the second heating scanning
determined by DSC of PP homopolymer and IPC resins.
M.T. Pastor-García et al.
European Polymer Journal 157 (2021) 110642
6
3.3. Synthesis of IPC in hydrogen absence
Experimental conditions at the rst and second steps clearly affect
the molecular, thermal, and mechanical properties of the synthesized
material, as the above results have demonstrated(i) hydrogen addition
led to a material with low molecular weight; (ii) 100 v/v% ethylene fed
as the only monomer neither improved its incorporation into the poly-
mer nor produced proper homogeneous distribution, so basically, no
EPR domains were formed; and (iii) the addition of a 50/50 (v/v%)
propylene/ethylene enhanced ethylene incorporation embedded in the
iPP matrix with adequate homogeneous and heterogeneous ethylene
distribution and, as a consequence, increased the presence of numerous
amorphous EPR domains, which is a prerequisite condition to enhance
impact properties and stiffness.
Accordingly, a new polymerization sequence is examined in
hydrogen absence at the rst step, and a 50/50 propylene-ethylene
comonomer mixture is added at the second stage, in accordance with
the procedure shown in Fig. 1b, to obtain the sample iPP
l
-4EP
g
. The lack
of hydrogen generates a resin with an MWD not too wide and located at
high MW values, as shown in Fig. 2 and Table 1. The variation of the
content of methyl branches with molar mass is also depicted in Fig. 2. It
is worth noting that the iPP
l
-4EP
g
material presents lower content of
methyl groups of the synthesized IPC resins. This can be attributed to the
fact that higher ethylene content is incorporated in the PP matrix.
Furthermore, the ethylene comonomer is preferentially incorporated in
the higher molar mass molecules (>500 kgmol
1
), which agrees well
with the abrupt decrease of methyl branch content as MW grows, while
the maximum of 250 CH
3
/1,000C remains constant for lower molecular
weights.
In addition,
13
C NMR liquid-state analysis (Table 1) conrms the
remarkable increase of ethylene content incorporation (21.2 mol%) into
the matrix. Moreover, ethylene is more heterogeneously than randomly
distributed12.2 mol% vs. 6.3 mol%, respectively. By comparing these
values with those obtained for other HIPPs with different molecular
weights, one observes a ratio between EEE and EPE +PEP triads be-
tween 1.0 and 2.0 [30,37,38,39]. All the tested materials that meet this
criterion present adequate resistance to impact as required for IPC ma-
terials. Accordingly, Table 1 shows that the sample iPP
l
-4EP
g
has the
Fig. 5. SEM micrographs of IPC resins: (a) iPP
l
-4E
g
; (b) iPP
l
-4EP
g
; and (c) iPP
l
1.04EP
g
.
M.T. Pastor-García et al.
European Polymer Journal 157 (2021) 110642
7
best impact resistance measured by the Charpy test.
Fig. 3 displays the chemical composition distribution (CCD) of iPP
l
-
4EP
g
, determined from TREF measurements. In this case, four different
regions can be dened with the variation of crystallization tempera-
tures. The peak at 28 C is the soluble fraction that can be assigned to
two contributions: the atactic fraction of the PP matrix and the EPR
domains from the elastomeric phase. This result is supported by the
difference in the peak intensity observed with respect to the analysis of
the PP homopolymer resin, iPP
l
, synthesized in the liquid-phase stage.
The soluble fraction of homopolymer iPP
l
, assigned to the atactic region,
depicts 2.3 wt%, while the contribution of this soluble fraction in the
iPP
l
-4EP
g
copolymer represents a 35.4 wt%, and therefore, most parts
can be ascribed to the amorphous elastomeric domains (EPE +PEP
triads). Unlike the other IPC resins, this EPR elastomeric contribution is
also observed in the DSC melting thermogram (Fig. 4) as a glass tran-
sition, located at 56.3 C, which conrms its formation. In the second
region, in the range between 30 C and 90 C, a broad signal corresponds
to crystallizable sequences with different levels of comonomer distri-
bution as well as ethylene, propylene, and/or EP segmented copolymers.
In the third region, at 95 C, a small peak appears that could be assigned
to relatively long ethylene sequences with the capability to crystallize,
matching well with the
13
C NMR results shown in Table 1 and with the
hump at 119.3 C detected via DSC. In the fourth region, the presence of
an intense peak at about 122 C is assigned to iPP crystallites, in
agreement with the intense melting signal at 162.8 C provided via DSC.
Fig. 5 shows the SEM micrographs of the etching cut surface of the
iPP
l
-4EP
g
sample. Dark areas are assigned to elastomeric EPR, present as
homogeneous domains dispersed along the iPP matrix. The number of
cavities is remarkably higher than those of the other IPC samples studied
in this work given the higher ethylene content, particularly the random
EP segments. These EPR domains present a predominantly circular
morphology with diameters from 1 to 3 µm, which leads to a decrease in
the gap of the cavities, enabling enhanced compatibility by lowering the
interfacial tension of the dispersed phase against the matrix [40,41].
This indicates that random sequences might act as exible molecules
that make different components in this resin compatible, increasing
mechanical resistance and proving that this is a key factor in the
toughnessstiffness balance of IPC in-reactor alloys in situ. All these
results explain the excellent value of Charpy impact resistance of 13.83
kJm
2
shown by the sample iPP
l
-4EP
g
, very similar to those obtained
for commercial high molar mass PP impact copolymers.
3.4. Fractionation of iPP
l
-4EP
g
To obtain more detailed information on the microstructure of iPP
l
-
4EP
g
, preparative TREF was performed to fractionate the individual
components. Therefore, this IPC copolymer was divided into three
fractions: F1, F2, and F3, at temperatures of 50 C, 100 C, and 130 C,
respectively, based on the different regions observed in the corre-
sponding analytical TREF thermogram curve of the raw iPP
l
-4EP
g
shown
in Fig. 3. The weight fraction distributions collected for these fractions
are registered in Table 2. The different fractions were also analyzed via
GPC-IR5, analytical TREF, and DSC.
Fig. 6a reveals the TREF proles of the three fractions of iPP
l
-4EP
g
. As
expected, peaks associated with crystalline regions appeared at higher
temperatures with the increase of the fractionation temperature. F1
represents the majority of the soluble component, which includes the
atactic PP and the elastomeric EPR phase. F2 showed an almost negli-
gible multimodal broad signal assigned mainly to intermediate crystal-
lizable EP copolymers. Moreover, this fraction contains a part of the
soluble fraction as noticed by a peak at 30 C. Finally, F3 shows bimodal
compositional distribution. A peak at 115 C clearly matches the iPP
chains with higher molecular weight, and a peak at around 110 C
corresponds to the co-crystallization phenomenon of a minority iPP
component with lower molecular weight.
Fig. 6b shows the second melting DSC curves for the fractions of the
sample iPP
l
-4EP
g
. In F1, one can only possible observe one glass tran-
sition temperature at 51.9 C, which is associated with the EPR
incorporated into the iPP matrix. F2 displays several melting peaks; the
most intense are placed at 120.3 C, 153.1 C, and 160.2 C. The peak at
120.3 C could be assigned to EP segments with long crystallizable
ethylene sequences, and the peaks at 153.1 C and 160.2 C, which may
be related to each other via recrystallization [42], are associated with
crystallizable propylene sequences with either molecular weight or
isotactic index lower than the iPP matrix. F3 presents a sharp and pre-
dominant peak at 161.7 C assigned to the iPP matrix with higher mo-
lecular weight.
Fig. 6c exhibits the MWD of the three fractions of the iPP
l
-4EP
g
resin.
The fractions of the iPP
l
-4EP
g
copolymer have similar MWD shapes,
distinguishing a fraction of high molecular weight associated with the
PP matrix and the presence of an EPR fraction with high molecular mass
that clearly provides remarkable impact on the strength and toughness
of this resin [30,31,39,43,44,45].
Fig. 6c also shows the methyl group distribution (associated with the
propylene comonomer) for the different fractions of iPP
l
-4EP
g
. Refer-
encing the maximum number of methyl branches for each molecular
weightaround 400 CH
3
/1000C, as shown in Fig. 2 for iPP
l
one can
determine the methyl content distribution for each fraction [31]. In F3,
the methyl group number remains constant through the MWD being
equivalent to the aforementioned value of the PP matrix; as a conse-
quence, this fraction is unambiguously composed of large iPP chains
associated with the PP matrix. F1, associated with a soluble fraction and
composed mainly of an EP random copolymer (EPR), contains approx-
imately 180 CH
3
/1000C, the half-content with respect to the reference
value, conrming the amorphous elastomeric EPR phase. It must be
pointed out that methyl groups decrease smoothly from low to high
molar masses. Finally, in F2, the methyl group distribution experiments
a sharp decline as the molecular weight increases, from 95% to 20% of
methyl groups, which conrm its heterogeneous arrangement,
composed of propylene segments and propylene-ethylene chains,
together with long ethylene segments, as reected in the results ob-
tained via TREF and DSC.
The crystalline EP segments could perform as compatibilizing agents
between the two main phases of amorphous EPR and crystalline iPP
[38,46]. This fact, coupled with the remarkably high molecular weight
presented by the EPssegments, allows one to improve the impact
strength of this material over the iPP
l
resin, leading to comparable
values for commercial IPCs with similar molecular weight [47].
Table 2
Characterization results of fractions of iPP
l
-1EP
g
material.
Sample Fraction wt-fraction
a
(wt-%) T
mb
(C) T
gb
(C) Crystallinity
b
(%) T
ec
(C)
iPP
l
-4EP
g
F1 24.1 51.9 30
F2 16.3 120.3/153.1/160.2 52.0/-6.9 45.5 30/72.1/78.2/94.7/104.5
F3 59.6 161.7 18.7 56.1 110/115
a
Weight fraction for the fractions collected from preparative-TREF.
b
From second heating scanning.
c
Elution temperature obtained by TREF.
M.T. Pastor-García et al.
European Polymer Journal 157 (2021) 110642
8
3.5. Comparison between GasGas-, LiquidLiquid-, and LiquidGas-
Phase Two-Step IPCs (iPP
g
-1EPR
g
, PP
l
-1E
l
, and iPP
l
-4EP
g
)
Beyond the individual molecular characterization and properties of
the PP copolymers synthesized in only one reactor following two
different stepsliquidliquid [30], gasgas [31], and liquidgasthis
section summarizes and compares the main features of the PP co-
polymers that can be considered as IPCs.
The sample iPP
g
-1EPR
g
, with an ethylene content of 19.6 mol% and
Charpy impact of 4.5 kJm
2
, was synthesized in sequential gasgas-
phase two steps, feeding in the second phase ethylene/propylene
monomers in a ratio of 50/50 for 1 min. The sample PP
l
-1E
l
(4.8 mol%
ethylene; 13.56 kJm
2
) was synthesized in two sequential liquid-
liquid-phase steps, adding in the second stage ethylene monomer for 1
min. The sample iPP
l
-4EP
g
(21.2 mol% ethylene; 13.83 kJm
2
) has been
obtained in two sequential liquid- and gas-phase steps, adding a 50/50
(v/v) mixture of propylene/ethylene monomers for 4 min in the second
phase. Such differences in synthesis procedures are reected in simi-
larities and differences among these three copolymers. However,
considering that the impact resistance of PP copolymers synthesized in
the gasgas phase is three times lower than that of IPC copolymers
synthesized in the liquidliquid and liquidgas phases, respectively, far
from the value required to be considered as an IPC resin, the compara-
tive study has focused on the last two IPC copolymers that have similar
impact resistance.
The copolymer particle morphology has been observed via SEM
(Fig. 7). The micrographs show that both resins are composed of parti-
cles with a spherical morphology that, in some cases, are interconnected
to form agglomerates, especially in iPP
l
-4EP
g
. The average size of the
particles has been estimated by cross-sectional area using imaging
analysis to get an average area of 0.37 mm
2
and 0.21 mm
2
for PP
l
-1E
l
and iPP
l
-4EP
g
, respectively. This difference in size is because elastomeric
component (EPR) diffusion in the liquid-phase stage is the most favored,
so average particle size is also larger. Higher magnication images of the
particle surface show the presence of brils with a gummy appearance
attributed to the elastomeric phase since these were not observed on the
surface of the iPP homopolymer particle [29]. These brils are more
numerous in iPP
l
-4EP
g
.
Fig. 8 shows the SEM micrographs of the etching cut surface of both
copolymers. In these images, the dark cavities are assigned to elasto-
meric EPR, and these areas are homogeneously dispersed along the iPP
matrix in both samples, which results in enhanced compatibility be-
tween the two main phases of amorphous EPR and crystalline iPP, re-
ected in the similar high-impact-resistant values of 13.6 kJm
2
and
Fig. 6. (a) Normalized crystallinity distributions obtained by TREF, (b) melting thermograms, and (c) molecular weight distributions and number of methyl groups of
iPP
l
-4EP
g
fractions.
Fig. 7. SEM micrographs of particles corresponding to PP
l
-1E
l
and iPP
l
-4EP
g.
M.T. Pastor-García et al.
European Polymer Journal 157 (2021) 110642
9
13.8 kJm
2
present in both samples.
To this day, separate information on the molar mass distribution and
CCD of IPC resins has been obtained via SEC and TREF. To provide more
information about the molecular structure for a more comprehensive
characterization of these complex IPC materials, two-dimensional dis-
tribution interrelating molar mass and chemical composition is deter-
mined by means of CFC [48,49]. Fig. 9a and 9b show the bivariate
distribution of the samples PP
l
-1E
l
and iPP
l
-4EP
g
. In general, both plots
show two well-differentiated contributions, the amorphous soluble
fraction at temperatures below 30 C, and the crystalline fractions in a
broad range of temperatures above 40 C.
From the compositional point of view, both samples are quite similar
in those regions eluted in the 90 C 100 C and 100 C125 C ranges,
which are associated with chains based on long ethylene molecules or
long propylene chains, respectively. However, between 75 C and 90 C,
the tridimensional surface plot of sample iPP
l
-4EP
g
reveals areas of
clearly distinct positions in the molar mass/chemical composition plane
shifted to high molar mass molecules, mainly at 85 C90 C, compared
with those observed for sample PP
l
-1E
l
, which can be associated with
various copolymers composed by extended ethylene and propylene
Fig. 8. SEM micrographs of cut surfaces corresponding to PP
l
-1E
l
and iPP
l
-4EP
g
.
Fig. 9. a) 3D-CFC and b) 2D-contour of samples PP
l
-1E
l
and iPP
l
-4EP
g
.
M.T. Pastor-García et al.
European Polymer Journal 157 (2021) 110642
10
copolymer (EP) sequences.
Fig. 10 displays the CFC bidimensional plot from the molar mass
point for both samples. In general, all the iPP
l
-1E
l
fractions show a
higher molecular weight than the iPP
l
-4EP
g
ones except for those frac-
tions eluted in the range from 75 C to 90 C whose molar masses are
lower (at 75 C) than those of the iPP
l
-4EP
g
fractions.
Bearing in mind that the molar mass of whole iPP
l
-4EP
g
is lower than
that of whole iPP
l
-1E
l
, one would expect that its impact resistance should
also be lower, in agreement with the inuence of the molecular weight
on the impact strength, but rather, the opposite is the case. A possible
explanation for this nding could be the great compatibilizing effect
between the iPP matrix and the rubber phase supplied by the high-
molecular-weight EP segments eluted at 85 C90 C [42], together
with the increased overall ethylene content in the material (21% mol)
and the right balance between EEE and EPE +PEP heterogeneous and
homogeneous segment distributions.
4. Conclusions
For the synthesis of IPC in two sequential steps in different phases
(liquid and gas) in a single reactor, changes have been made in the
process synthesis. When the feeding process at the second step consisted
solely of ethylene, the impact resistance of the polymer was similar to
that of the PP homopolymer. When hydrogen was added during the
reaction, in the rst step, the molecular weight of the copolymer ob-
tained was too low, and consequently, its impact resistance was also too
low. Finally, an IPC material with impact resistance like that of com-
mercial PP impact copolymers with a good distribution of the elasto-
meric phase was achieved in a single reactor following a liquid- and gas-
phase sequential process in hydrogen absence and the addition of a 50/
50 (v/v) mixture of propylene/ethylene monomers. The IPC so obtained
exhibited a good distribution of the elastomeric EPR phase in the PP
matrix in size and number.
It was also determined that the ratio between EEE and EPE +PEP
triads ranging from 1.0 and 2.0 is a good criterion to predict the proper
impact resistance of any heterophasic PP copolymer in-reactor alloy.
Preparative TREF fractionation and subsequent fraction character-
ization via DSC and GPC-IR5 as well as SEM characterization of the
surface of the IPCs proved that the adequate ratio between the contin-
uous iPP phase and the elastomeric domains of EPR dispersed phases is
the key to provide a good balance between toughness and stiffness.
Finally, from the comparison of the IPCs synthesized in only one
reactor following two different steps (liquidliquid, gasgas, and liq-
uidgas) and in the absence of hydrogen, considering impact resistance,
the values of impact strength of the IPC copolymers synthesized in the
gasgas phase were three times lower than those of the IPC copolymers
synthesized in the liquidliquid and liquidgas phases, respectively.
The use of the CFC (bivariate TREF-GPC distribution) technique
allowed a comparison of the liquidliquid- and liquidgas-phase IPC
copolymers and highlighted that although both materials presented
similar CCD, they presented remarkable differences in terms of molar
mass distribution. Thus, while the liquidgas-phase IPC material showed
generally lower molecular weight than the liquidliquid-phase IPC, the
presence of a region of crystalline families eluted between 85 C and
90 C composed of high molecular weight and attributed to EP co-
polymers suggests that this can act as a compatibilizing agent between
the iPP matrix and the rubber phase, allowing one to improve its impact
resistance and equaling, even slightly exceeding, that of the liquid-
liquid-phase IPC copolymer. Considering these results, the synthesis of
IPC resins in a single reactor is an efcient experimental method for
research on PP impact copolymers.
Funding
This research received no external funding.
CRediT authorship contribution statement
María Teresa Pastor-García: Investigation. Inmaculada Su´
arez:
Investigation, Writing - review & editing. María Teresa Exp´
osito:
Investigation, Writing - original draft. Baudilio Coto: Supervision,
Writing - review & editing. Rafael A. García-Mu˜
noz: Supervision,
Conceptualization, Writing - review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgments
The authors are kindly acknowledged to Polymer Technology Lab-
oratory (LATEP) from Rey Juan Carlos University for the ethylene-
propylene copolymers characterization.
Data availability
The raw/processed data required to reproduce these ndings cannot
be shared at this time as the data also forms part of an ongoing study.
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4.5
5.0
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6.0
6.5
7.0
0
2
4
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Fraction weight
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M.T. Pastor-García et al.
... Thus, as shown in Table 4, the content of the soluble in PP-1 was very low. However, as EPR was introduced, it would produce a steric resistance to the arrangement of PP chain in crystal, and result in some polymer chains of PP existing as the amorphous form [12]. Because this amorphous PP could be extracted by solvent, the content of the soluble would increase. ...
Article
Full-text available
Four samples, including homopolymerized PP (PP-1), random impact copolymerized PP (PP-2), random impact copolymerized PP of ethylene-propylene (PP-3), and random impact copolymerized PP of ethylene-propylene-butylene (PP-4), which were prepared by 75 KG Spheripol process pilot plant using ZN104M as catalyst, were adopted to study the structure and performance, i.e., the influence of the different polypropylene molecular chain structures on the crystallization behavior of random impact copolymer polypropylene, and the changes in mechanical and optical performance due to the different aggregation structures of random impact copolymer polypropylene, and following results were achieved. Firstly, when ethylene-propylene rubber (EPR) and copolymerization monomers of ethylene and butylene were added in turn, the regularity of PP molecular chains decreased in different degree with the order of PP-1 > PP-2 > PP-3 > PP-4, and which further led to the same pattern for the crystallization peak temperature and the crystallinity. Secondly, half-crystallization time (T1/2) of the same cooling rate and the crystallization activation energy increased with the addition of EPR, ethylene and butylene. Crystallization activation energies were calculated to be 12.05 kJ mol⁻¹, 12.09 kJ mol⁻¹, 12.38 kJ mol⁻¹ and 12.64 kJ mol⁻¹ for PP-1, PP-2, PP-3 and PP-4, respectively. Last but the most importantly, the addition of EPR, ethylene and butylene would enhance the impact strength, but decrease the transmittance, whereas the haze changed little. Based on the theory between structure and performance, the reason that caused above results were analyzed. This work provided some guidance for the development of high-performance polypropylene used in identical fields.
Article
Impact polypropylene copolymer (IPC) is utilized in various commercial products worldwide due to its excellent physical properties and low production costs. Introducing polyethylene(PE) into IPC is known to improve the impact properties with little rigidity loss, but little is known about the influence of the multiphase interfacial interaction on these properties. In this work, we investigate this matter through fabricating a series of in-reactor alloys with multi-stage polymerization. Morphology observations revealed that introducing PE into IPC in-situ can promote the phase structure without the external force. Dynamic mechanical analysis results revealed that the introduction of well-dispersed PE in-situ could strengthen the interface interaction between PP matrix and EPR dispersed phase, which is conducive to the dissipation of impact energy. Atomic force microscopy-infrared results suggested that the in-situ introduction of PE promotes the enrichment of the ethylene-propylene block copolymers (EbP) to the multiphase interface. This work offered a new insight into the rational design of the next generation IPC with improved properties.
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High-impact copolymers of propylene and ethylene are complex materials with several components and at least two different phases, a crystalline polypropylene (PP) matrix and a predominantly amorphous ethylene–propylene copolymer (EPC) normally present as disperse particles. Next to overall composition analysis and morphology studies, fractionation into crystalline and amorphous components followed by further analysis of those components is the most important characterization technique for this technically very relevant class of copolymers. This process, involving separation into xylene cold-soluble (XCS) and insoluble fractions and follow-up determination of intrinsic viscosities (IV) as well as ethylene (C2) content is, however, tedious and time-consuming. A far more rapid approach is presented here through the separation of the amorphous and crystalline fractions of PP in 1,2,4-trichlorobenzene (TCB) using the crystallization extraction technique and the Crystex QC instrument. As the standard methods and Crystex technique are using different solvents (xylene and decalin versus TCB), correlations need to be established in order to be able to compare the results. Based on nine structurally very different polymers, respective correlation curves were developed and predictions for 95% confidence intervals were calculated statistically. In addition, these correlation curves were validated by testing over 100 different PP samples covering a broad range of all tested characteristics. The results show agreement and comparability between the traditional test methods techniques and the Crystex technique inside statistically calculated prediction intervals at 95% over a broad range of all tested characteristics (XCS, C2 and IV).
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High impact polymer composites are generally composed of a matrix polymer and an impact modifier. The effect of modifier structure and properties on the balance between the toughness and rigidity of high impact polymer composites has been reported [1, 2]. As the main component, the structure and properties of the matrix polymer can certainly affect this balance. This balance has been assessed using both theory and experiment. A relation between the critical interparticle distance IDc and the brittle strength of the matrix polymer σb, i.e. IDc ∼ σb23, was identified from both experiment and theory. Based on the relation between σband molecular weight, i.e. σb=A−B/M¯n, the effect of the matrix polymer molecular weight can be thereby studied quantitatively. It is clear that the higherM¯n, the higher σb, and the larger IDc is. Larger IDc means lower content of rubber or elastomer modifier is needed to induce the brittle-ductile transition, leading to less loss for the rigidity of impact polymer composites.
Article
It is different from one phase impact modifier such as ethylene-propylene rubber (EPR), ethylene-propylene-diene monomer copolymer (EPDM), polyolefin elastomer (POE) et al., that the core-shell rubber particles constructed by a hard core and a soft shell is a typical two phases impact modifier. They have been successfully and widely introduced to toughen polymers by melt blending or alloying in reactor. However, an important but unclear question is how to obtain the high impact thermal plastic polymer composites with less rigidity loss by optimizing the structure and the properties of the core and shell phases. To answer this question, we develop the model from one phase modifier to two phases modifier and make a quantitative calculation in this study and obtain the following results: (1) In order to obtain the high impact polymer composites with low rigidity loss, the modulus of the core should be as higher and the modulus of shell should be as lower as they can; (2) Comparing to one phase impact modifier, core-shell rubber particles toughened polymer composites can have less rigidity loss. The lowest modulus loss for the high impact PP can decrease from 26.1% for one phase modifier, to 13.5% for the core-shell modifier with PE core, and to 5.4% for the core-shell modifier with PP core; (3) The impurity, i.e. the rubber shell contains homo PP or PE and the core contains EPR, leads to the increase of the rigidity loss for the high impact PP alloys in reactor.
Article
Me2Si(2-Me-4-pbind)2ZrCl2 supported on SiO2/MgCl2 binary support was prepared for the preparation of heterophasic copolymer of polypropylene. The bi-support underwent surface treatment with various alkyl aluminum compounds such as trimethylaluminum (TMA), triethylaluminum (TEAL), and triisobutylaluminum (TIBA) before supporting the metallocene catalyst for 3 or 24 hours and were used for homopolymerization. It was notable that the generated SiO2/MgCl2 bi-support had lower surface area, pore volume and size as compared to the conventional SiO2. Impact polypropylene copolymers (IPCs) were obtained using two-step polymerization in one reactor with the presence of metallocene catalyst supported on SiO2. Propylene was polymerized in the reactor to produce the iPP matrix followed by polymerization of ethylene resulting to heterophasic material. It is apparent that the molecular weight of the polymer increased with longer PE polymerization time and as the polymerization time was more than 40 min, PP peak appeared near 147.9–149.2 °C, and a new peak emerged at 116.9–119.9 °C which could be attributed to the melting temperature of iPP crystallites and a less intense peak to either chains of ethylene-propylene copolymers. SEM images also confirmed that spherical PE particles were deeply embedded in the crystalline PP matrix and a large amount was produced as the polymerization time of the second stage ethylene polymerization was increased.
Article
For highly impact polypropylene (HIPP), a good balance between stiffness and toughness has attracted much attention from both industrial and academic research. Herein, two HIPP resins (A and B) with different ethylene contents and impact resistances especially at low temperatures are investigated. Their chain microstructure is studied by combining preparative temperature rising elution fractionation (P-TREF) with multiple characterization techniques such as high-temperature gel permeation chromatography (HT-GPC), Fourier transform infrared (FTIR) spectroscopy, ¹³C nuclear magnetic resonance (¹³C NMR), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), rheometer, and thermal fractionation. Both samples are mainly composed of four portions: amorphous ethylene−propylene random copolymer (EPR), ethylene−propylene (EP) segmented and blocky copolymer, and isotactic polypropylene homopolymer. Sample A contains ~30 wt% EPR fractions eluted at 35 °C, ~9 wt% EP segmented and blocky copolymers eluted at 80–100 °C, and ~19 wt% highly isotactic polypropylene fractions eluted at 125–140 °C, which are higher than the contents of corresponding fractions in sample B. Moreover, most fractions of sample A have higher molecular weights than the corresponding fractions of sample B. The contribution of different components to stiffness and toughness is also discussed. The excellent impact resistance of sample A is demonstrated from the perspective of chain microstructure.
Article
Toughening mechanism of in-situ core-shell dispersed particle in polymer blends was re-examined by using different polymers as cores. Three kinds of polyethylene including high density polyethylene (HDPE), medium density polyethylene (MDPE) and low density polyethylene (LDPE) were adopted to prepare polypropylene/ethylene-propylene rubber/polyethylene (PP/EPR/PE) blends, and all of them presented simialr morphological characteristics of core-shell dispersed particles with similar size in which PE acts as the core. However, PP/EPR/HDPE and PP/EPR/MDPE presented excellent toughness while poor toughening effect appeared in PP/EPR/LDPE. The impact section morphology showed that shear yielding occurred not only in the matrix for PP/EPR/HDPE and PP/EPR/MDPE but also in the HDPE and MDPE cores. The results of crystallization enthalpy loss indicated that the interfacial strength between EPR and LDPE is much weaker than those of EPR with other PEs. By introducing LDPE into PP/EPR/HDPE blends, shear yielding in core disappeared and the impact strength significant declined, which was ascribed to weakening of core-shell interfacial strength caused by LDPE. Finally, a complete toughening mechanism of equivalent rubber content effect for polymer core-shell particles was proposed. Besides enhanced percolation of stress volumes by introduction of crystallizable core, an efficient shear yield of the core and the matrix based on good core-shell interfacial strength under impact is also critical for effective toughening.
Article
Based on Takayanagi's two-phase model and the brittle-ductile transition equations, we established a relation between the composite modulus E c and related parameters including modifier modulus E d , modifier content φ modifier particle size d and interparticle distance ID, which determined the stiffness and toughness of the composite toughened with one phase modifier particle. From this relation, we can quantitatively study how the composite modulus depends on E d , φ d, and ID at the critical brittle-ductile transition point, and thereby can give the theoretic guidance for designing the composite with balanced toughness and rigidity by optimizing these parameters. The results show that lower critical interparticle distance (ID c ) leads to a great loss of the stiffness for rubber or elastomer toughened polymer composites. This is a main reason that the high impact polypropylene (HIPP) composites with high stiffness is more difficult to be obtained.
Article
Impact polypropylene copolymers (IPC) are materials important for many commercial applications. These materials are usually synthetized by different methods involving two consecutive reactions: in liquid, gas or liquid-gas phase. This work presents a method of synthesis in a laboratory scale based on two sequential steps in only one reactor in gas phase. Variables such as H2 addition, reaction time, and composition during the second step were studied, and their influence on the formation of propylene-ethylene copolymer materials with different properties was analyzed. The IPC materials so obtained were characterized by analytical temperature rising elution fractionation (TREF), calorimetric methods (DSC), 13C nuclear magnetic resonance (13C NMR), gel permeation chromatography with an infrared detector (GPC-IR5), Charpy impact, and scanning electron microscopy (SEM). The isotactic polypropylene matrix (iPP) obtained in the first step of the gas-phase process displays spherical morphology and lower mean particle size than those obtained in the liquid-phase process, even though the polymer grains experimented a certain tendency to agglomerate. In particular, hydrogen addition caused a significant decline in the catalyst productivity and dramatically shortened the length of the propylene homopolymer chains. Similarly, the presence of hydrogen on the synthesis of ethylene-propylene copolymers was demonstrated to lead to materials with very low molecular weight, low ethylene incorporation, and rubbery phases irregularly distributed along the iPP matrix and therefore with poor impact properties. On the other hand, ethylene-propylene copolymers synthesized without hydrogen yielded a suitable combination of molecular weights and molecular weight distribution that can contribute to good polymer processing and were proven to incorporate adequate amounts of ethylene mainly randomly distributed into the iPP matrix. SEM measurements revealed that the amorphous rubbery phase was homogeneously dispersed in the iPP matrix, and the Charpy test allowed to rank these materials as IPC. Finally, fractionation of IPC materials by preparative TREF provided information about the microstructure formation. Subsequent studying of molecular weights and composition of the fractions by GPC-IR, analytical TREF, and DSC measurements based on the ethylene-propylene composition and ethylene distribution in the molar mass molecules was shown to enable the design of IPC materials. Additionally, the optimal IPC material was compared with the best IPC using liquid-phase polymerization, and the results showed that the range of ethylene-propylene composition, as well as the ethylene distribution in IPC molar mass molecules, correlated with IPC mechanical behavior.