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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 efcient 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
Ziegler–Natta catalyst
Polyolens’ characterization
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 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 efcient experimental
method for fundamental research on IPCs.
1. Introduction
The discovery of the Ziegler–Natta (Z–N) catalyst in the 1950s
signicantly impaired the production of polyolens 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
polyolens 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
-olenic moieties [5–7]. 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) dening 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 efciency has been associated with rubber content,
morphological structure, phase composition, particle shape, particle
size, size distribution, viscosity, and rubber–matrix compatibility
[12–18]. These materials have high impact resistance and have appli-
cations in the consumer and automotive industries [19–22].
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 inuence 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 inuence 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 Z–N 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 liquid–liquid-phase [30] and gas–gas-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 n–heptane by Scharlab SA (99%). All the
polymers were synthetized using a standard TiCl
4
/MgCl
2
Z–N catalyst
containing 2.5 wt% Ti, triethylaluminum was used as a cocatalyst and
scavenger (TEA 1 M in n–heptane, supplied by Witco), and cyclo-
hexylmethyldimethoxysilane (C–donor, 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 mL⋅min
−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 ◦C⋅min
−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 innite
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◦±1◦and 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 m⋅s
−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 g⋅mL
−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 ◦C⋅min
−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 liquid–liquid 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, quantied 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
cat⋅min-
1
M
w
(kg/
mol)
PI T
m
(◦C) T
g
(◦C)
Crystallinity
(
α
, %)
MFI (g/
10 min)
Charpy impact
(kJ⋅m
−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 identied 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. Finally—and in agreement
with the NMR, GPC, TREF, DSC, and SEM results—the 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 modications 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 reected 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
prole 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 chains’ length. 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 kJ⋅m
−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 kg⋅mol
−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) conrms the
remarkable increase of ethylene content incorporation (21.2 mol%) into
the matrix. Moreover, ethylene is more heterogeneously than randomly
distributed—12.2 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
–1.0–4EP
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 dened 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 conrms 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
toughness–stiffness balance of IPC in-reactor alloys in situ. All these
results explain the excellent value of Charpy impact resistance of 13.83
kJ⋅m
−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 proles 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
weight—around 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, conrming 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 conrm its heterogeneous arrangement,
composed of propylene segments and propylene-ethylene chains,
together with long ethylene segments, as reected 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 EPs’ segments, 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 Gas–Gas-, Liquid–Liquid-, and Liquid–Gas-
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 steps—liquid–liquid [30], gas–gas [31], and liquid–gas—this
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 kJ⋅m
−2
, was synthesized in sequential gas–gas-
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 kJ⋅m
−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 kJ⋅m
−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 reected in simi-
larities and differences among these three copolymers. However,
considering that the impact resistance of PP copolymers synthesized in
the gas–gas phase is three times lower than that of IPC copolymers
synthesized in the liquid–liquid and liquid–gas 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 magnication 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 kJ⋅m
−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 kJ⋅m
−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 ◦C–125 ◦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 ◦C–90 ◦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 inuence 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 ◦C–90 ◦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 (liquid–liquid, gas–gas, and liq-
uid–gas) and in the absence of hydrogen, considering impact resistance,
the values of impact strength of the IPC copolymers synthesized in the
gas–gas phase were three times lower than those of the IPC copolymers
synthesized in the liquid–liquid and liquid–gas phases, respectively.
The use of the CFC (bivariate TREF-GPC distribution) technique
allowed a comparison of the liquid–liquid- and liquid–gas-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 liquid–gas-phase IPC material showed
generally lower molecular weight than the liquid–liquid-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 efcient 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 inuence
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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0
2
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