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Abstract

In this work, we reported the development of a mini-reactor experimental setup for synthesizing of polypropylene with heterogeneous Ziegler–Natta catalysts in gas-phase. Use of pro-activated 4th generation of Ziegler–Natta catalyst and preheated monomer feed enabled the polymerization reaction to be carried out at constant temperature. Evaluation of monomer consumption with high precision (0.01 bar pressure drop) allowed the detection of polymerization yield at low reaction rates. In this regard, polymerization yield, particle morphology and catalyst fragmentation were studied, as well. The results of melt microscopy showed that catalyst fragmentation was developed during the reaction, and was not restricted to the initial rupture of catalyst particles. The rate determination showed a peak during the polymerization (not necessarily at the initial stage). The results showed that depending on the reaction condition, this peak could be either a consequence of a major catalyst fragmentation or overheating. Low reaction yield, large fragments of catalyst and agglomeration of particles were considered as evidence of particle overheating and polymer local melting. As we imposed the results of melt microscopy for the polymerization conditions, a layer-by-layer fragmentation of the catalyst was found to be the main fragmentation process, at least at the beginning of the polymerization reaction.
Vol.:(0123456789)
1 3
Iran Polym J
DOI 10.1007/s13726-017-0573-6
ORIGINAL PAPER
Gas‑phase polymerization ofpropylene atlow reaction rates:
aprecise look atcatalyst fragmentation
MehrsaEmami1· MahmoudParvazinia1· HosseinAbedini1
Received: 9 March 2017 / Accepted: 14 October 2017
© Iran Polymer and Petrochemical Institute 2017
Introduction
Understanding of the mechanism of polymer growth has
been, and still is, one of the most important fields of hetero-
geneous olefin polymerization studies. Many efforts have
been made by research groups to give insight into the polym-
erization kinetics, catalyst morphology, polymer properties
and mass transfer interdependency [13].
Catalyst fragmentation during polymerization affects
polymerization activity of the catalyst as well as proper-
ties of the produced polymer (particle density, porosity,
etc.) [46]. The morphology of the final polymer particles
depends on the mechanical and structural properties of both
the catalyst support and the polymer formed at the critical
very early stage of the polymerization [79].
The first observation relevant to the mechanism of growth
of polyolefins on heterogeneous catalysts dates back to the
1970s. Bulls and Higgins with the aim of microscopic
images showed fragments of the catalyst particle dispersed
within the expanding polymer [10]. Beside model predic-
tions [1113], many experimental investigations were car-
ried out by researchers on catalyst fragmentation.
Microreactors have been recently employed for study-
ing catalytic olefin polymerization reactions [14]. The use
of microreactor devices makes it possible for researchers
to perform reactions with good control over mixing, pres-
sure, flow rate, residence time, mass and heat transfer, which
resulted in enhanced reproducibility [15]. Different reactor
and configurations have been designed to study very specific
aspects of the reactions process.
Based on the use of microreactors coupled with visual
and/or infrared (IR) microscopy, many insitu or online study
methods have been developed to investigate particle growth.
With the aim of online microscopy, particle growth kinet-
ics during polymerization had been followed and profitable
Abstract In this work, we reported the development of a
mini-reactor experimental setup for synthesizing of polypro-
pylene with heterogeneous Ziegler–Natta catalysts in gas-
phase. Use of pro-activated 4th generation of Ziegler–Natta
catalyst and preheated monomer feed enabled the polym-
erization reaction to be carried out at constant temperature.
Evaluation of monomer consumption with high precision
(0.01bar pressure drop) allowed the detection of polymeri-
zation yield at low reaction rates. In this regard, polymeri-
zation yield, particle morphology and catalyst fragmenta-
tion were studied, as well. The results of melt microscopy
showed that catalyst fragmentation was developed during
the reaction, and was not restricted to the initial rupture of
catalyst particles. The rate determination showed a peak dur-
ing the polymerization (not necessarily at the initial stage).
The results showed that depending on the reaction condition,
this peak could be either a consequence of a major cata-
lyst fragmentation or overheating. Low reaction yield, large
fragments of catalyst and agglomeration of particles were
considered as evidence of particle overheating and polymer
local melting. As we imposed the results of melt microscopy
for the polymerization conditions, a layer-by-layer fragmen-
tation of the catalyst was found to be the main fragmenta-
tion process, at least at the beginning of the polymerization
reaction.
Keywords Mini-reactor· Propylene· Polymerization·
Gas-phase· Fragmentation
* Mahmoud Parvazinia
M.Parvazinia@ippi.ac.ir
1 Iran Polymer andPetrochemical Institute, P.O.
Box14185/458, Tehran, Iran
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results were obtained [16, 17]. Oleshko etal. [18] employed
insitu video-microscopy (combination of a microreactor and
a video camera) to follow the polymer particle growth dur-
ing polymerization of olefins with supported catalysts. Pater
etal. [19] developed a method for online observation of pol-
ymer particle growth, using optical microscopy and infrared
imaging. They measured surface temperatures of growing
particles and estimated particle temperature as a function
of time with infrared imaging method. Abboud etal. also
employed insitu video microscopy connected to a 100-mL
mini-reactor, to follow the particle growth during polym-
erization of olefins with supported catalysts [2022]. May-
rhofer and Paulik used a microreactor equipped with a video
microscope to follow the growth of polyethyleneparticles
during polymerization. They conducted gas-phase polym-
erizations at conditions close to the industrial set points.
The rate of polymerization was calculated from the obtained
particle growth curves [23]. Video-microscopy methods are
limited by the resolution of microscope and infrared imaging
techniques. The advantage of online techniques is that the
polymerization and analysis are concurrent.
A number of off-line or ex situ techniques have been
developed to study particle growth during polymerization,
as well. In off-line methods, product is recovered and ana-
lysed after reaction. Different specially designed reactors
have been reported in the literature to follow heterogeneous
catalyst fragmentation, particle expansion, and polymeriza-
tion kinetic. Stopped-flow and quenched-flow methods are
two techniques applied for morphological and kinetic studies
in slurry polymerizations [24, 25]. In these methods, two dif-
ferent streams of reactants are contacted inside a mixing ele-
ment, reacted for a defined time and then fed into a quench-
ing vessel [26, 27]. Di Martino etal. [28, 29] conducted
slurry polymerizations of ethylene on a MgCl2-supported
Ziegler–Natta catalyst at very short times (40ms) with a
quenched-flow apparatus. Their investigations provided
detailed information on the kinetic, morphology and poly-
mer properties at the very early stages of polymerization.
Silva etal. [30] developed the short-stop technique in
2005. The major advantage of this technique is its potential
to perform the polymerization reaction at conditions similar
to the industrial conditions in terms of temperature, pressure
andflow rate. In short-stop technique, gas-phase polymeri-
zation was carried out continuously in a fixed bed reactor.
The particles of catalyst were dispersed in an inert seed-
bed and monomer flows over the particles. The reaction is
halted with the flow of carbon dioxide. Using stopped-flow
method, it was possible to study the evolution of the particle
morphology during the first instants of the polymerization
reaction [31, 32]. Some research works were developed by
reactor optimization and calorimetric method to follow heat
transfer, polymerization kinetics and evolution of catalyst
temperature during the first moments of reaction [33, 34].
McKenna etal. [35] have shown that it was not a trivial
task to run experiments that allow one to obtain reliable
data on the kinetic and evolution of material properties and
particle morphology during the early stages of the catalytic
olefin polymerization. The main difficulties come from the
sensitivity of the catalysts to impurities, the small size of
the catalyst particles in comparisonwith very rapid reac-
tions that lead to heat and mass transfer limitations. For all
of these reasons, it is difficult to build an experimental setup
capable of studying all phenomena at once.
In the present work, a mini-reactor experimental setup
was developed to study the catalyst fragmentation and
polymerization yield. Monomer consumption evaluation at
precision of 0.01 bar in gas-phase propylene polymerization
was enabled. The rate of reaction was calculated from early
stages of polymerization using instance monomer consump-
tion approach. The yield of polymerization and catalyst frag-
mentation were studied. Catalyst fragmentation mechanism
and progress were investigated by molten particle micro-
scopic imaging. The results showed that uniform fragmen-
tation of catalyst particles to smaller sub-particles on the
bulk of polymer/catalyst particles played a crucial role in
the yield of reaction.
Experimental
Materials
The Ziegler–Natta type catalyst (industrial grade) supported
on MgCl2 with 2.4wt% of Ti was utilized. Polymerization
grade propylene (99.99%), triethyl-aluminium(TEAL),
cyclohexyl methyl dimethoxysilane (CMMS) and dicyclo-
penthyl dimethoxysilane were provided by Marun Petro-
chemical Co., Iranas external electron donors. Additionally,
distilled n-hexane (polymerization grade, Pars Cylinder Co.,
Iran) dried overnight in the presence of sodium wire and
molecular sieves, were used as the solvent.
Polymerization set‑up
Gas-phase reactions were carried out in a homemade mini-
reactor shown in Fig.1. The reactor was a stainless steel
horizontal cylinder with a volume of 2.8cm3. The reactor
was equipped with two solenoid valves controlled by a logic
controller equipped with homemade software for the feed.
The reactor pressure was monitored with a pressure transmit-
ter. Propylene was purified with three commercial purifiers
and preheated passing through the heating unit, as shown in
Fig.1. The heating unit was a shell and tube heat exchanger
connected to heating bath, with a 4 m length tube to ensure
that the gas temperature reaches the desired temperature.
A heating bath was used to maintain the temperature of the
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reaction medium at the desired value. A K-type thermocou-
ple was used to monitor the inlet gas temperature.
The reactor was charged by a pro-activated catalyst in a
dry box to avoid its contamination and then fixed to the gas
line. Afterwards, the reactor was plunged into a heat jacket
(connected to heating bath) to ensure constant reaction tem-
perature during the reaction. Pressure and temperature of the
monomer feed were controlled, temporally. Polymerizations
at different temperatures and pressures with commercial Zie-
gler–Natta catalyst were enabled in this mini-reactor. Mon-
omer consumption evaluation at high precision (0.01bar)
from the beginning of polymerization was implemented. The
rate of reaction was calculated during polymerization using
instance monomer consumption.
Polymerization procedure
Before polymerizations, the catalyst was activated with
triethyl-aluminum (TEAL) and an external electron donor
and dried. At first, solution of TEAL in hexane was allowed
to react with electron donorfor about 5min and then the
mixture was added to the catalyst (Al/Ti molar ratio was 88).
After about 10min, the catalyst was washed with hexane and
dried. After drying, a small amount of catalyst was weighed
into the reactor and it was closed. All these steps were car-
ried out in the dry box. To be able to control the reaction
temperature and pressure, it was necessary to use catalysts
loadings of less than 10mg in the reactor. After weighing,
the reactor was brought out of the glove box and connected
to the propylene line in mini-reactor setup. Polymerizations
with different conditions were carried out using the same
activated batch of the catalyst. Polymerization time ranged
from 1s to 2h, at the end of the pre-set polymerization time,
the reactor was simultaneously vented and fed with CO2.
Polymerization temperature was ranged from 40 to 70°C
and the monomer pressure from 2 to 10bar. Activities of
the catalysts samples were calculated by weighing the final
particles after the polymerization.
Characterization
SEM
Morphology of the produced polymer particles was observed
with scanning electron microscopy (SEM). Particle mor-
phology micrographs were observed using a VEGA II
microscope (Tescan, Czech Republic) operating at acceler-
ating voltages of 10 or 15kV (depending on each individual
case). As the catalyst we used (and especially its morphol-
ogy) is highly sensitive to moisture, it is essential to carry
out SEM observations under inert conditions.
Cross-sections of the catalyst/polymer particles were
studied with high-resolution scanning electron microscope
as well. For preparation of the cross-sections, the samples
were mixed with very light epoxy resin in dry box. The
resin/sample mixtures were cured with the aim of light. The
cross-sections were cut with a microtome (Leica RM2165)
and used for SEM observations as soon as possible to pre-
serve the paticlesstructure. Before SEM measurements the
samples were coated with gold (2nm coating). The coatings
were used to prevent charging of the samples. In the case of
short polymerization times, catalyst/polymer samples were
sealed in an inert vial in the glove box, and just before SEM
observation, they were fixed on a tape and quickly moved
to the chamber of the SEM to prevent their contact with air
and moisture.
Melt microscopy method
Melt microscopy of polymer particles was first employed to
study the fragmentation of catalyst particles during olefin
polymerization by Abboud [22]. In our experiments we used
melt microscopy method to investigate catalyst fragments in
one particle. One particle was placed on a glass plate and
fixed by a metal ring on the microscope platform. Then, the
sample heated to 175°C (above the polypropylene melting
point) and temperature is held constant for 10min. Pictures
were taken from the molten staged sample. The resulting
polymer melt is transparent and allows observation of solid
fragments inside its phase.
Particle size measurement
To measure the size of particles, a thin layer of particles
was spread on a microscopeslide. Image of about 200
particles was taken with the aim of acamera. Diameter
Fig. 1 Polymerization set-up with the mini-reactor: C3 monomer, N2
inert gas, PG pressure gauge, PH gas pre-heat, PR pressure regulator,
R reactor, J jacket, PT pressure transmitter, V storage vessel
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of each particle was measured individually, then, mean
particle size was calculated for each sample.
Results anddiscussion
The objectives of this work were to investigate the rate
and yield of polymerization, catalyst fragmentation and
morphology of the catalyst particles at relatively low reac-
tion rates. Most of polymerization experiments were car-
ried out at constant pressure (semi-batch), it means that,
the reactor pressure was kept constant with a tolerance
of 0.2bar. For instance, if the reactor pressure was set
to 6bar, solenoid valve acted after the pressure falls to
5.8bar and fed propylene to the reactor to recover the set
point at 6.0bar. The monomer was fed with the precision
of±0.01bar. Temperature and pressure data acquisition
were performed in the course of reaction. Table1 shows
reaction conditions used in the experiments.
Overall polymer yield was measured by weighing the
catalyst used and final polymer samples. Also for each
monomer injection pulse, time interval and pressure
change were measured and catalyst productivity was cal-
culated. Therefore, a graph of the rate of polymerization
versus time was plotted.
Table 1 Reaction conditions
used in the experiments
* C donor: cyclohexyl methyl dimethoxysilane (CMMS)
** D donor: dicyclopenthyldimethoxysilane
Run Cat (mg) Trec (°C) N2 (bar) P (bar) Ext. donor t (min) Yield (mg
pol/mg cat)
D (mean)
(µm)
E3 6.6 70 1 8 (batch) C*25s 1.5
H5 5 70 1 8 (batch) C 42 10.2
H6 5.1 70 1 6 C*60 24.5
E1 6.9 70 1 8 C 3 5.1 97
G4 2.6 70 1 8 C 24 27
G7 2.2 70 1 8 C 25 36.1 141
G3 2 70 1 8 C 73 84.15 187
F2 4.6 70 1 10 C 60 17.9
NC6-2 5.2 70 1 8 D** 3 11.6 131
NC6-1 4 70 1 8 D 7 21.7 175
NC6-4 4.3 70 1 6 D 3 10.3 162
NC6-3 3.4 70 1 6 D 7 20.5
NC4-4 3.3 40 1 8 D 12 30 130
NC4-5 3 40 1 4 D 12 2.8 110
NC5-1 5.5 70 1 2 D 12 4.1 119
NC5-2 4.2 70 1 6 D 12 19.8 181
NC5-3 4.5 70 1 4 D 12 17 200
NC5-4 3.1 70 1 8 D 12 26 215
Fig. 2 Rate profile for experiments carried out at 70°C and different
pressure of: a 8bar and b 6bar
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Reproducibility
To ensure that the results were reproducible, experiments
were repeated twice, at least. Figure2 shows the result
obtained under the same polymerization conditions at dif-
ferent reaction times (runs NC6-1 and NC6-2; NC6-3 and
NC6-4 in Table1). As can be seen in Fig.2, rate profiles
match well with each other. Acceptable results were taken
at different polymerization pressures.
Polymerizations atdifferent pressures
A number of experiments were carried out at different mon-
omer pressures (runs NC5-1, NC5-2, NC5-3 and NC5-4 in
Table1). Figure3 shows rate profiles obtained from polym-
erizations performed at 8, 6, 4 and 2bar. It is clear in Fig.3
that the rate of polymerization was strongly affected by
monomer pressure. The higher the monomer pressure, the
higher was the rate of polymerization. Also it can be seen
from Fig.3 that, the rate of polymerization was a decay
type, which is in agreement with the results of other research
works [32, 36, 37]. This can be the result of the intrinsic
instability of the active centres [3840] and mass transfer
limitations [4143]. Note that in these experiments, Al/Ti
ratio was 80 (not at the optimum value) thus, productivity
of the experiments shown in Fig.3 was lower than those in
Fig.2.
Catalyst morphology
SEM micrographs of the initial catalyst are presented in
Fig.4 which show that the initial catalyst particles had
fairly rough surface and were consisted of uniform spheri-
cal particles. The catalyst particles had cracks as large as
1μm. These cracks were probably caused by sampling of
the commercial catalyst and the treatment (activation and
drying) we imposed on it to obtain dry activated catalyst.
Mean catalyst diameter (40μm) was calculated from SEM
micrographs.
Effect ofpolymerization time onparticle morphology
To study the effect of polymerization time, polymeriza-
tion reactions were performed at different polymerization
time (runs E3, E1, G7 and G3 in Table1). The morpho-
logical inspection of the surface and bulk of the growing
polymer particles are shown in Fig.5. For samples shown
in Fig.5a–c, polymerization was carried out for 25s and
only one pulse of the monomer was injected into the reac-
tor. The polymer obtained from this polymerization was
3.5mg (less than the weight of the catalyst used: 6.6mg
represented in Table1). Comparing the surface of the
Fig. 3 Polymerization rate profiles obtained from experiments per-
formed at 70°C and different pressure and time
Fig. 4 SEM micrographs of the initial catalyst: a single particle, b its subparticles and c surface cracks
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particle after 25s of polymerization (Fig.5a, b) with that
of the catalyst (Fig.4) shows that the surface of the par-
ticle became smoother at initial stages of polymerization,
which means that the produced polymer had encapsulated
the micro grains of the catalyst and caused its surface to
be smooth. This is in agreement with the observations of
Di Martino etal. [28, 29].
The external primarycracks of the catalyst particles were
not filled with the produced polymer. In the cross section
micrograph of the particle (Fig.5c) some non-reacted cata-
lyst moieties can be seen in the centre of the particle. For
polymer sample shown in Fig.5d–f polymerization was car-
ried out for 3min up to the yield of 5.1mg/mg catalyst.
Comparison of the particle in Fig.5a with the one shown in
Fig.5d, e, reveals that the surface of the latter is rougher and
a lot of pores were formed, caused the particle porosity to be
raised. Cross section micrograph of this particle confirmed
the existence of polymer and large pores and deep cracks
in the whole particle (Fig.5f). This type of fragmentation
(layer-by-layer) is in agreement with the studies of Tanike
etal. [25] who conducted slurry propylene polymerization
using a spherical Mg(OEt)2-based Ziegler–Natta catalyst
with stopped-flow technique. Their results showed that poly-
mer species initially filled macrospores mainly located in the
middle layer of the catalysts, then caused the fragmentation
of the middle layer, and finally provoked the fragmentation
of the particle core in a stepwise manner [25]. Further stud-
ies on the mechanism of catalyst fragmentation were carried
out by molten polymer microscopy observations.
Referring to Fig.5g–l (particles obtained at 25 and
73min of polymerization) the surface of particles were
smooth although a lot of holes and cracks were maintained
in the particle. This can be due to the fact that polymer spe-
cies tend to surround macro pores of the catalyst, instead
of cracks.
Catalyst fragmentation mechanism bymelt microscopy
Since the discovery of Ziegler–Natta catalysts for olefin
polymerization, many research groups investigated cata-
lyst fragmentation and particle growth. Two catalyst frag-
mentation patterns were reported as ‘‘layer-by-layer’’ and
“rapid fragmentation”. The ‘‘layer-by-layer’’ fragmenta-
tion means the catalyst particle breakage starts from the
external surface to the centre of the particle until the
whole catalyst particle is fragmented [6, 7, 25]. Pater and
Weickert proposed a different particle growth mechanism.
They showed a rapid fragmentation of the catalyst into a
large number of small sub-particles at the beginning of
the polymerization and the fragments initially were well
distributed throughout the particle [44].
To study the catalyst fragmentation, a number of polym-
erization experiments were carried out at different condi-
tions. Molten polymer microscopy studies were performed
to follow the evolution of catalyst fragmentation during
polymerization. Beside it, the catalyst fragmentation and
reaction yield were compared and studied.
Figure6 shows molten polymer microscopy of two dif-
ferent particles obtained at 25s and 42min of polymeri-
zation with productivity at 1.5 and 10.2mg pol/mg cat,
respectively (runs E3 and H5 in Table1). Polymerization
in these two experiments were carried out batch wise,
means only one pulse of monomer was injected to the
reactor. Pressure drop was due to monomer consumption.
Figure6 shows, as polymerization proceeds (10.2mgpol/
mg cat in comparison with 1.5mgpol/mg cat), the size
of catalyst fragments became smaller as shown in Fig.6b
(about 10μm) confirming development of fragmentation
of the catalyst carrier as polymerization yield increases.
Figure6a clearly shows that the fragmentation was initi-
ated from outer parts of the particle confirming a layer-by-
layer fragmentation at the beginning.
Fig. 5 SEM micrographs of a single particle obtained from polym-
erization at 8bar pressure and 70°C: a, b surface and c its cross sec-
tion at t=25s and yield=1.5mg/mg cat; d, e surface and f its cross
section at t=3 min and yield=5.1 mg/mg cat; g, h surface and i its
cross section at t=25min and yield=37mg/mgcat; j, k surface and
l its cross section at t=73min and yield=85.4mg/mgcat
Fig. 6 Optical microscope images of melted polypropylene par-
ticles at 175 °C with catalyst fragments (solid dots) inside them: a
yield=1.5mg/mg cat and b yield=10.2mg/mg cat, polymerization
performed at 70°C and initiated from 8bar pressure (100× magnifi-
cation)
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Surface and cross section SEM micrographs of these par-
ticles are shown in Fig.7. Figure7a shows that the surface of
the particle is smooth. Figure7b shows formation of fibres
between catalyst segments. With progress of the reaction
(Fig.7d, e) fibres of polymer are replaced by the bulk of
polymer, caused the catalyst carrier to be fragmented. In the
cross section image of the particle obtained at 25s of polym-
erization (Fig.7c) pieces of unreacted catalyst exist near the
centre of the particle. In Fig.7f, cross section micrograph
of the particle obtained at 42min of polymerization shows
polymer was formed in the internal part of the particle. Fig-
ure7f shows large pores in the inner parts of the particle.
This may be due to mass transfer limitations into the inner
parts of the particle, which caused higher level of polymeri-
zation at the outer parts of the particle. The outer parts grow
more than inner parts and therefore, the cracks and empty
volume inside the particle were developed.
Yields andcatalyst fragmentation
Evolutions of fragmentation of different particles obtained
from different polymerization yields were investigated.
Figure8 shows polymerization kinetic profile and micro-
graphs of molten particles (runs G4, G7 and G3 in Table1).
In Fig.8a, a secondary peak was appeared at 25min of
polymerization. Micrographs of molten particle are shown
in Fig.8b–d. Figure8b belongs to the sample obtained at
21min of polymerization and Fig.8c shows the sample at
25min of that. By comparing these two figures it is clear
that the number of catalyst fragments are higher. This is an
evidence of catalyst fragmentation at about 25min after the
beginning of the reaction. It can be concluded from Fig.8
that the number of fragments increased as reaction pro-
ceeded and the size of fragments was decreased. In Fig.8b,
appearance of fragments with the size more than 10μm
confirmed incomplete catalyst fragmentation. Gradually,
with the progress of reaction, the fragments were broken
and smaller fragments were appeared.
Fig. 7 SEM micrographs of a single particle obtained from polymerization initiated from8bar pressure and 70°C: a, b surface and c cross sec-
tion at t=25s; d, e surface and f cross section at t=42min
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With respect to the melt microscopy images and com-
paring them before and after the peak time, it seems that
the fragmentation is the main reason for enhanced rate and
appearance of the peak. Other works, beside catalyst frag-
mentation, stated overheating and over reduction of Ti+3 to
Ti+2 [40] and formation of cracks [45] as possible reason
for appearance of the peak. SEM micrographs in Fig.6 at
the same experimental conditions do not confirm overheat-
ing. As Fig.6 shows there is no particle agglomeration or
polymer melting inside the particle. Based on the experi-
mental conditions, the peak can be occurred by overheating,
whichis explained in the next section.
Experiments indifferent polymerization pressure
Furthermore, a number of experiments were carried out at
mild conditions at 40°C to study the progress of catalyst
fragmentation. Figure9 illustrates catalyst fragmentation for
particles obtained from experiments carried out at 40°C
and different pressures (runs NC4-4 and NC4-5 in Table1).
Figure9 shows at low reaction yields (2.8mgpol/mg cat)
fragmentation is incomplete and a broad range of fragments
size from 3 to 10μm was existed in the particle (Fig.9a, b).
When polymerization was carried out at 8-bar pressure, the
reaction yield reached to 30mg pol/mg cat and fragment
sizes reduced to less than 2μm (Fig.9c, d).
Also fragmentation of two particles obtained from polym-
erizations at 70°C and different pressures (runs NC5-1 and
NC5-4in Table1) was investigated in Fig.10. Figure10
shows that at low polymerization yields (4.1mg pol/mg cat)
fragmentation was incomplete and fragments with different
sizes up to 10μm with a broad size range can be seen inside
the particle. While, in the particles in Fig.10c, d fragmen-
tation was more uniform and size of fragments reduced to
2μm.
Our results confirmed the observations of Noristi etal.
who performed slurry polymerization of propylene with sup-
ported Ziegler–Natta catalyst at different polymer yields.
They suggested that if the site distribution in the catalyst
was uniform and the polymerization conditions were mild,
the polymer growth started uniformly throughout the cata-
lyst particles, which then undergoes an even and progres-
sive fragmentation into very fine particles homogeneously
dispersed in the polymer matrix [46]. Figures9 and 10 also
show a progressive development of fragmentation inside the
catalyst particle. This shows that the fragmentation is not
limited to the initial stage of the polymerization as stated by
Pater etal. [44], previously.
To confirm our observations, fragmentation of two
particles obtained from different polymerization pres-
suresare compared in Fig.11 (runs H6 and F2 in Table1).
In Fig.11,by increasing the pressure from 6 to 10 bar a
non-uniform fragmentation of the catalyst can be observed.
Table1 showsat higher pressure of 10bar, the yield was
Fig. 8 a Rate profile of polymerization initiated at 8 bar pressure
and 70 °C and optical microscope images of melted particles after:
b 21min and yield=27mg/mg cat; c 25min and yield=36mg/mg
cat; d 73min and yield=84mg/mg cat
Iran Polym J
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decreased in comparison with 6bar pressure. This shows the
vital role of catalyst fragmentation on polymerization yield.
The reason for reduced yield at higher pressure can
be explained by monomer diffusion limitation inside the
particle, causing low reaction progress. At 10bar pressure,
the reaction rate was increased by increasing monomer con-
centration. Polymerization first happens at the surface of
the catalyst particle. The high level of the bulk of polymer
Fig. 9 Optical microscope images of melted polypropylene particles at 175°C with catalyst fragments (solid dots) inside them: polymerization
at 40°C and pressure of a, b 4bar and c, d 8bar after 12min (100× magnification)
Fig. 10 Optical microscope images of melted polypropylene particles at 175°C with catalyst fragments (solid dots) inside them: polymerization
at 70°C and pressure of a, b 2bar and c, d 8bar after 12min (100× magnification)
Iran Polym J
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covers the surface of the catalyst particle, leads to reducing
the monomer transport into the particle. This causes lower
yield and it is the reason of observing the larger fragments of
the catalyst particle in the melt microscopy images. In fact,
the larger fragments are the reason of lower reaction yield.
It is possible that overheating happens if any peak is
observed at this condition. To confirm it, rate profile data
and SEM micrographs are shown in Fig.12 (run F2 in
Table1). As can be seen in Fig.12 a, b, agglomeration of
particles is the evidence of particle overheating and poly-
mer local melting. The polymer layer reduced the particle
heat transfer and overheating can occur that at first increased
the reaction rate, and then by deactivation of the catalyst, it
would be decreased. If Fig.12c is comparedwith Fig.8a,
it can be seen that, at the first moments of the reaction the
primary peak was not appeared, but there is one peak after
about 18min. The SEM micrographs of particles relevent
toFig.8a,have been presented in Fig.5. Theyshow no evi-
dence of agglomeration or polymer melt inside the catalyst
particle. But SEM micrographs of Fig.12 show particles
agglomeration, confirming particle overheating.
Fig. 11 Optical microscope images of melted polypropylene parti-
cles at 175°C with catalyst fragments (solid dots) inside them: polym-
erization at 70°C and pressure of a 6bar andb 10bar after 60 min
(100× magnification)
Fig. 12 a, b SEM micrographs of polypropylene particles and c rate
profile of polymerization at 10bar pressure and 70°C
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Conclusion
In this work, gas-phase polymerization of propylene was
performed in a homemademinireactor. The study focuses
on the reaction yield and catalyst fragmentation. Melt
microscopy method was implementedto study the cata-
lyst fragmentation. It was observed that the fragmentation
was started by a layer-by-layer mechanism. The size of the
catalyst fragments inside theparticle showed continues
development of catalyst fragmentation inside the polymer/
catalyst particles and it was not limited to an initial rupture
of the catalyst particle. Comparison of the reaction yield
with melt microscopy showed the crucial role of catalyst
fragmentation in polymerization yield. More fragmenta-
tion and smaller fragment pieces resulted in higher reac-
tion yield.
Acknowledgements The authors would like to express their special
thanks to Mr. F. Hormozi for his assistance in set-up construction and
Mrs. F. Khosravi and Mrs. L.Tolami for microscopic imaging. Fur-
thermore, the authors wish to thank Marun Petrochemical Co. (MPC),
Iran, for supplying catalyst.
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Four R1R2Si(OMe)2 type compounds were added as an external electron donor (De) in propylene polymerization with TiCl4/Di/MgCl2 type supported Ziegler–Natta catalysts (Di = internal donor). Each polypropylene (PP) sample was fractionated into three parts (atactic, medium-isotactic and isotactic PP), and the number of active centers ([C*]/[Ti]) in each PP fraction was counted using 2-thiophenecarbonyl chloride as the quenching and tagging agent. The gradual decrease of [C*]/[Ti] with De/Ti ratio is ascribed to competitive and reversible coordination of De on either central Ti of the active center or Mg adjacent to the central Ti. The former coordination leads to deactivation of C*, and the latter one leads to still living C*. The chain propagation rate constant (kp) of the active centers producing atactic, medium-isotactic and isotactic PP change with De/Ti in different ways. Only the kp of active centers producing isotactic PP was evidently increased by De. Enhancement in isotacticity of PP product is found to be a combined result of both deactivation of active centers by De and selective activation of the active centers that produce isotactic PP. Changing the alkyl groups of R1R2Si(OMe)2 leads to an altered balance between the deactivation and activation effects of De.