ARTICLE pubs.acs.org/IECR
Periodic Switching of Monomer Additions for Controlling the Compositions and Microstructures of Segmented and Random Ethylene-Propylene Copolymers in Polypropylene in-Reactor Alloys Zhou Tian,† Xue-Ping Gu,† Gang-Liang Wu,† Lian-Fang Feng,*,† Zhi-Qiang Fan,† and Guo-Hua Hu*,‡,§ †
State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou 310027, P. R. China Laboratory of Reactions and Process Engineering, CNRS-Nancy University, ENSIC-INPL, 1 rue Grandville, BP 20451, 54001 Nancy, France § Institut Universitaire de France, Maison des Universites, 103 Boulevard Saint-Michel, 75005 Paris, France ‡
ABSTRACT: This work aims at shading fundamental insights into mechanisms that dictate the composition and microstructure of polypropylene/poly(ethylene-co-propylene) (PP/EPR) in-reactor alloys produced by the multizone circulating reactor (MZCR) technology, a novel one for polyolefin production. For this purpose, this technology is simulated by a batch process with periodic switching of monomer additions. An increase in the switch frequency leads to a decrease in the fraction of ethylene/propylene random (amorphous) copolymers denoted as EPR and an increase in the fraction and the length of PP segments of ethylene/ propylene segmented (partially crystalline) copolymers denoted as EPS. Concomitantly, the size of the dispersed phase domains (EPR) decreases, and its size distribution becomes more uniform as a result of the decrease in the fraction of the EPR and the increase in that of the EPS which acts as a compatibilizer for PP/EPR in-reactor blends. Mechanical properties of the PP/EPR inreactor blends are also discussed.
1. INTRODUCTION Sequential polymerization of propylene and then ethylene and propylene with an appropriate ZieglerNatta catalyst system produces in situ blends called high-impact polypropylenes (HIPP), polypropylene/poly(ethylene-co-propylene) (PP/EPR) in-reactor alloys, or impact polypropylene copolymers (IPC). PP/EPR in-reactor alloys exhibit an excellent balance between toughness and rigidity compared with homopolypropylene. It is believed that the superior properties of PP/EPR in-reactor alloys originate from their compositional heterogeneity and phase interactions among the components. It has been confirmed that PP/EPR inreactor alloys mainly consists of three types of polymer compoments: isotactic polypropylene (PP) as the matrix, ethylenepropylene random copolymers (EPR) as the dispersed phase, and semicrystalline ethylene-propylene segmented copolymers (EPS) with different sequence lengths of polyethylene (PE) and polypropylene (PP) segments as the compatibilizer.15 The fraction, molecular weight, composition, and sequence distribution of these components in the PP/EPR blends have key influence on their morphology and properties.611 These parameters are controlled by the polymerization process,12,13 namely, catalyst modification and process innovation. A notable recent breakthrough in polyolefin engineering technology is the development of the Spherizone process.1416 The core of this process is a multizone circulating reactor (MZCR), in which two different polymerization zones are created in a single gasphase reactor (see Figure 1). The reactor comprises two legs, through which the polymer growing particles keep circulating. The left leg (riser) is operated as a fast fluidized bed and in the right leg (downer) the growing particles descend as a plug flow. A gas barrier separates the two zones so that the hydrogen or comonomer r 2011 American Chemical Society
concentration in the two legs can be very different. Intimately mixed PP homopolymer with a controlled bimodal distribution or high performance PP in-reactor alloys can therefore be produced and can be suitable for a wider range of applications. Dong et al. were the first who reported on the synthesis, morphology, and mechanical properties of a series of PP/EPR in-reactor alloys obtained by a multistage sequential polymerization process resembling MZCR.17 They attributed the improvement in impact strength and flexural modulus of the alloy to changes in its phase morphology during the process. However, there was no detailed information on the composition, especially the amount and chain structure of EPR and EPS in PP/EPR in-reactor alloys. This work aims at studying the effect of periodic switching of monomer additions on the composition and structure of PP/EPR in-reactor alloys. The switch frequency was varied over a wide range. The composition of the alloys and the chain structure of each component were characterized by solvent fractionation and 13C NMR technique. The compositional heterogeneities of EP segmented copolymers were further studied by self-nucleation and annealing (SSA). Possible correlation between mechanical properties, molecular architectures, and phase structures was also discussed.
2. EXPERIMENTAL SECTION Preparation of Polypropylene/poly(ethylene-co-propylene) in-Reactor Alloys. The catalyst system used was a high activity Received: December 4, 2010 Accepted: April 6, 2011 Revised: April 1, 2011 Published: April 06, 2011 5992
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Industrial & Engineering Chemistry Research spherical ZieglerNatta catalyst, supplied by BRICI, SINOPEC (Beijing, China), with TiCl4 supported on MgCl2. Triethyl aluminum was used as a cocatalyst, and dicyclopentyl dimethoxy silane, a so-called D-donor, was used as an external electron donor. In all polymerization runs, the Al/Ti and Si/Ti molar ratios were kept constant at 100 and 5, respectively. The fraction of titanium of the catalyst was 2.7 wt %. n-Heptane was used as the solvent. The polymerization process consisted of three steps: prepolymerization of propylene in slurry, homopolymerization of propylene in slurry, and gas phase polymerization in a circular mode. 50 mL of n-heptane was charged to the reactor, followed by the aluminum alkyl/heptane and the D-donor/heptane mixtures sequentially. After that, the catalyst was injected together with 50 mL of heptane, and the prepolymerization reaction started. The propylene pressure in the prepolymerization stage was 1 atm, and the temperature was 20 °C. At the end of the prepolymerization (typically 20 min), the reactor was brought to the slurry propylene homopolymerization conditions, typically 60 °C and 0.6 MPa. The slurry propylene homopolymerization lasted 60 min. At the end of that step, propylene and solvent were removed by evacuation for 10 min, and a circular reaction mode began to proceed. Propylene of a constant pressure (0.4 MPa) was continuously fed to the reactor at 60 °C. After a given period of time I for the propylene
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homopolymerization, an ethylene/propylene mixture of a constant composition (propylene/ethylene molar ratio = 1.5) and a constant pressure (0.4 MPa) was continuously charged to the reactor at 60 °C. After a given period of time II for the ethylene/ propylene copolymerization, the polymerization process was switched back to the propylene homopolymerization under the same conditions as above for the propylene homopolmerization. After time I, it was switched back to the ethylene-propylene copolymerization under the same conditions as above for that reaction. The above procedure was repeated until the total polymerization time reached 80 min, and the ratio between times I and II was 3. Figure 2 shows the polymerization process, and Table 1 gathers the polymerization process parameters. It should be pointed out that the periodic operation mode used in this work is similar to that of the multistage sequential polymerization process employed by Dong et al.17 Nevertheless there are several differences between both studies on other aspects, especially from the process viewpoint. A very significant one lies in that Dong et al. used a semibatch feeding policy, whereas this work uses a continuous feeding policy. In other words, in their work the monomers were continuously fed to the reactor, but there was no output while in this work they are continuously fed to the reactor and the remaining nonpolymerized monomers continuously exit from the reactor. Moreover, switch frequency can be relatively high: a switch operation can be done in minutes. The purpose of the operation conditions of this work was to keep the composition of the gas phase constant and to minimize the compositional drifting phenomena. Solvent Fractionation. A polymer sample of approximately 1.5 g was fully dissolved in 200 mL of boiling n-octane and then precipitated by gradual cooling of the solution to room temperature overnight. The insoluble fraction was separated from the solution by centrifugation and filtration. The polymer dissolved in n-octane was recovered from the solution by rotating Table 1. Conditions of Periodic Switching Polymerization Process retention time in each polymerization cycle (min) switch sample frequency
Figure 1. Schematic representation of MZCR.
propylene
ethylene-propylene
homopolymerization
copolymerization 20
A
1
60
B
4
15
5
C
8
7.5
2.5
D
20
3
1
Figure 2. Schematic representation of the periodic switching polymerization process. 5993
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Industrial & Engineering Chemistry Research evaporation. Both the insoluble and soluble fractions were vacuum-dried. This procedure separated the amorphous EPR from the PP/EPR in-reactor alloys. The above n-octane insoluble fraction was further fractionated by n-heptane into n-heptane soluble fraction (semicrystalline EPS) and insoluble fraction (IPP) using a modified Kumagawa extractor.18 The n-heptane soluble fraction was obtained by rotating evaporation, and the insoluble one was recovered from the extractor. They were vacuum-dried and weighted. The above solvent fractionation procedure allowed obtaining the three typical components of PP/EPR in-reactor alloys: EPR, EPS. and IPP. Nuclear Magnetic Resonance (NMR). 13C NMR spectra were recorded on a Varian Mercury 300-plus spectrometer at a resonance frequency of 75 MHz. o-Dichlorobenzene-d4 was used as a solvent, and the concentration of the polymer in the solution was 10 wt.%. The spectra were recorded at 120 °C with hexamethyldisiloxane as an internal chemical shift reference. Cr(acac)3 was used to reduce the relaxation time of carbon atoms, and the delay time was set as 3 s. The pulse angle was 90° and typically 6000 transients were collected. Thermal Analysis and Thermal Fractionation. A TA-Q200 DSC apparatus was used for measuring the thermal properties of PP/EPR in-reactor alloys. About 46 mg of the sample was sealed in an aluminum pan. The accuracy of the temperature measured was (0.05 °C. All the experiments were carried out under nitrogen. For normal melting and crystallization analyses, the measurements were performed in the following manner: samples were heated to 200 °C and kept at that temperature for 5 min in order to erase their previous thermal history. Subsequently, they were cooled down to 25 °C at a rate of 10 °C/min and then heated up to 200 °C at a rate of 10 °C/min. The crystallization thermograms were recorded during the first cooling scan, while the melting temperature and fusion enthalpy of the samples were determined during the second heating scan. Successive self-nucleation and annealing (SSA) was performed according to the following procedure: samples were first held at 200 °C for 5 min and then cooled to 25 °C at a rate of 10 °C/min to create an initial “standard” thermal history. They were heated to a prescribed first self-seeding temperature (Ts) at a rate of 10 °C/min and was held at that temperature for 5 min. This step resulted in partial melting and annealing of unmelted crystals, while some of the melted species might isothermally crystallize. Crystallization after the self-nucleation was achieved by subsequent cooling of the samples to 25 °C at a rate of 10 °C/ min. The first Ts was set at 170 °C, the fraction window 5 °C and the annealing time 5 min. The scanning rate used during the thermal conditioning steps was 10 °C/min. The temperature for the thermal fractionation ranged from 170 to 60 °C. After the completion of the thermal fractionation process, the samples were heated from 25 to 200 at 10 °C/min, and the corresponding endothermic curves were recorded. Scanning Electron Microscopy (SEM). The morphology and state of dispersion of the EPR phase domains in the PP matrix were investigated using a Hitachi s-4800 field emission scanning electron microscope. The SEM samples were prepared as follows: Specimens of the polymer were prepared as described in Section Measurement of mechanical properties and were fractured in liquid nitrogen. The fractured surfaces were etched by toluene under ultrasonic for 5 min at different temperatures and sputtered with platinum and finally subjected to SEM
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Figure 3. Effect of the switch frequency on the fractions of the PP/EPR in-reactor alloys. Samples A, C, and D were fractionated twice, and the corresponding results are plotted accordingly.
observation. An operating voltage of 5 kV and magnification of 5000 were adopted for the observation. The morphology of the fracture surfaces of the impact test specimens were also observed by SEM. The samples for impact testing were prepared as described in Section Measurement of mechanical properties. Measurement of Mechanical Properties. Polymer pellets were compressed-molded using a 80 10 4 mm mold at 180 °C and 17.5 MPa for 5 min. Specimens were slowly cooled down to room temperature. The notched Izod impact strength of the specimens was measured on a Ceast impact strength tester according to ISO 180. Their flexural modulus was measured on a Zwick/Roell Z020 electronic tester following ISO 178. For a given polymer sample, at least five specimens were measured and the average value used.
3. RESULTS AND DISCUSSION Composition and Chain Structure. As stated above, PP/EPR in reactor alloys were fractionated by n-octane and n-heptane into three fractions. Figure 3 shows the weight percentages of those soluble fractions in samples A, B, C, and D. The soluble fractions F1 and F2 accounted for about 25 wt % of the PP/EPR in-reactor alloys. According to ref 18, the n-octane soluble fraction F1 corresponds to the amorphous EPR and the n-octane insoluble fraction a mixture of semicrystalline EPS and IPP. The n-heptane soluble fraction (F2) is mainly composed of EPS and likely some PP chains with stereodefects and the n-heptane insoluble fraction (F3) mainly IPP. It is interesting to note the effect of the periodic switching of monomer additions. F1 decreased with increasing switch frequency whereas F2 increased. On the other hand, F3 remained almost constant. These results indicate that the periodic switching did not affect the sum of F1 and F2 but the ratio between them. In other words, the periodic switching did not affect the fraction of the isotactic polypropylene but those of the EPR and EPS. Figure 4 shows the 13C NMR spectra of the fractions in sample A. From the spectra, F1 was indeed mainly an EPR, F2 was mainly composed of EPS and also likely some low molecular weight isotactic polypropylene, and F3 corresponded to PP homopolymer. 5994
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Table 2 gathers the sequences of the EPR and EPS in samples A, B, C, and D. For all samples, the values of the sequence
Figure 4. 13C NMR spectra of sample A fractions obtained by solvent fractionation.
Table 2. Composition and Sequence Distribution of EP Copolymers Obtained by 13C NMR sample A EPR
a
EPS
sample B EPR
EPS
sample C EPR
EPS
sample D EPR
EPS
E (%)
44.0
59.3
40.4
50.2
34.8
33.6
40.4
28.1
EE
21.6
53.9
20.1
42.0
16.3
26.3
20.3
20.0
EP
46.0
15.7
39.8
13.7
40.4
16.4
38.4
17.2
PP
32.5
30.4
40.2
44.2
43.2
57.3
41.3
62.7
EEE
10.6
50.3
12.2
39.2
9.3
23.6
13.1
17.2
EEP PEP
21.9 11.5
7.4 1.7
15.7 12.4
5.7 5.3
14.0 11.4
5.4 4.7
14.4 12.9
5.6 5.3
EPE
9.9
6.7
8.0
4.0
7.7
3.0
7.7
3.0
PPE
27.2
7.2
22.9
3.1
28.5
12.1
21.0
12.2
PPP
18.9
26.7
28.7
42.7
29.0
51.2
30.8
56.6
rE 3 rPa
1.3
26.6
2.0
39.5
1.7
22.3
2.3
16.9
nEb
1.9
7.6
2.0
7.3
1.7
4.1
2.1
3.3
nPc
2.4
5.2
3.0
7.3
3.2
8.1
3.1
8.3
Product of the reactivity ratios. b Number average sequence length of ethylene unit. c Number average sequence length of propylene unit.
distributions were much closer for F1 than for F2, implying that the n-octane soluble fractions were all EPR. Moreover, an increase in the periodic switching frequency led to an increase in the fraction of the PPP triad and the average length of polypropylene segments (nP) of the EPS and a decrease in the fraction of its EEE triad and the average length of polyethylene segments (nE). However, the molar fractions of ethylene in the whole PP in-reactor alloys were 11.2, 9.3, 9.1, and 8.8% for samples A, B, C, and D, respectively (also determined by 13C NMR). In other words, a higher switch frequency led to a higher fraction of EPS in the PP/EPR in-reactor alloy and a longer length of PP segments in the EPS. Thermal Behavior of Each Fraction. Figure 5 shows the thermal properties of the EPR. As expected, its DSC endothermic and exothermic curves did not show obvious peaks between 20 and 200 °C, confirming that this component consisted of amorphous EPR (see Figure 5a). Figure 5b shows the glass transition temperatures of the EPR in all samples. They were 46.4, 39.9, 40.2, and 39.5 °C for EPR-A, EPR-B, EPR-C, and EPR-D, respectively. In other words, an increase in the periodic switching frequency led to an increase in the glass transition of the amorphous EPR. Figure 6 shows the DSC curves of the EPS. Several peaks appeared in the melting and crystallization curves, indicating that the n-heptane soluble fractions contained crystalline components. As the area of the DSC curves is normalized by the total sample mass, the mass fractions of the crystalline PE segments in the EPS can be represented by the areas of the melting peaks. From Figure 6a, the area under the melting curves of the PE components followed the order: EPS-A > EPS-B > EPS-C . EPS-D, indicating that the fraction of the crystalline PE segments followed the order: EPS-A > EPS-B > EPS-C . EPS-D. This is consistent with the results of the NMR spectra: their molar contents of [EEE] triad were 50.3, 39.2, 23.6, and 17.2%, respectively (see Table 2). Moreover, while the melting temperatures of those four EPS fractions were very close, their crystallization temperatures and rates differed remarkably. The EPS-D crystallized at a much lower temperature and more slowly than EPS-A, EPS-B, and EPS-C. Figure 7 shows the DSC curves of the IPP. Surprisingly, the PP components also showed several peaks. IPP-A and IPP-C exhibited three distinct melting peaks located at about 151, 165, and 170 °C, respectively. This indicates that the misplacement of methyl moiety varied in frequency and number in the polypropylene chain.
Figure 5. DSC thermograms of the EP random copolymers: (a) 20200 °C and (b) low temperature domain. 5995
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Figure 6. DSC melting (a) and crystallization (b) curves of the EP segmented copolymers.
Figure 7. DSC melting (a) and crystallization (b) curves of the isotactic polypropylenes.
normalized differential area under each endothermic peak is proportional to the number of lamellae that melt within the prescribed temperature interval, the lamellae thickness and crystalline methylene sequence length (CMSL) and their distributions can thus be calculated. According to Keating’s method,19 the CMSL of a fraction can be calculated by
Figure 8. DSC endotherms of the EP segmented copolymers after the SSA treatment.
SSA Treatment of EPS. SSA is basically a thermal fractionation method based on the sequential application of self-nucleation and annealing on a polymer sample. It may yield information on the distributions of short chain branching and lamellar thickness. Figure 8 shows the SSA results on the EPS. Multiple melting peaks were observed for each annealing step. The peaks below 130 °C represent the crystalline PE segments in the EPS and those above that temperature correspond to polypropylene with regio-defects. Each endotherm represents a population of crystals with almost the same thermodynamic stability. As the
lnðXÞ ¼ 0:331 þ 133:5=Tm
ð1Þ
CMSL ¼ 0:2534X=ð1 XÞ
ð2Þ
where X is the molar fraction of ethylene monomer, and Tm is the melting point for each fraction obtained from SSA. _ _ The statistical arithmetic mean L n, weighted mean L w, and the polydispersity index I of CMSL of ethylene-based copolymers can be calculated by Ln ¼
n1 L1 þ n2 L2 þ n3 L3 þ , 3 3 3 , þ nj Lj ¼ n1 þ n2 þ n3 þ , 3 3 3 , þ nj
∑fj Lj
ð3Þ
Lw ¼
n1 L21 þ n2 L22 þ n3 L23 þ , 3 3 3 , þ nj L2j ¼ n1 L1 þ n2 L2 þ n3 L3 þ , 3 3 3 , þ nj Lj
∑fjL2j ∑ fj L j
ð4Þ
I ¼
Lw Ln
ð5Þ
where nj is the normalized peak area of fraction j, and Lj is the corresponding CMSL or lamellae thickness. The relationship 5996
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between the melting temperature T and lamellae thickness l of ethylene based copolymers20 l¼
2σTm0 ΔHv ðTm0 Tm Þ
ð6Þ
where Tm0 is the equilibrium melting temperature of an infinitely thick lamella (418 K), σ is the lamellar surface free energy (70 103 J/m2), and ΔHv is the enthalpy of fusion for infinitely thick lamellae (288 106 J/m3). Table 3 shows the methylene sequence length and lamellae thickness of the EPS in all samples. For all the four samples, the values of the polydispersity index I of the CMSL were larger than those of the polydispersity index of the lamellae thickness. This fact may be explained by two aspects.9 First, some of the chain with region-defects might be excluded from the crystals in the crystallization fractionation process. Second, some of the longer methylene sequences could be long enough to fold in the same crystal. Moreover, the CMSL or lamellae thickness decreased with increasing switch frequency, confirming that an increase in the switch frequency led to a decrease in the ethylene sequence length. This is in agreement with the results of 13C NMR.
Phase Morphology. Figure 9 shows the SEM micrographs of cryo-fractured surfaces of samples A, B, C, and D. They all exhibited clear phase separation. The dimensions of the dispersed phase domains which likely corresponded to the EPR were about 0.5 to 1.5 μm. However, there were two obvious differences among those four samples. On the one hand, the fraction of the EPR was the highest in sample A. This is in agreement with the results in Figure 3. On the other hand, the size of the dispersed phase domains decreased with increasing switch frequency. This is because an increase in the switch frequency not only led to a decrease in the fraction of the EPR
Table 3. Parameters of Polydispersity of EPS methylene sequence length
lamellae thickness
sample
Ln (nm)
Lw (nm)
I
Ln (nm)
Lw (nm)
I
EPS-A
10.1
12.3
1.22
5.3
5.9
1.11
EPS-B
10.4
12.2
1.18
5.4
5.9
1.08
EPS-C
9.8
12.1
1.23
5.2
5.8
1.11
EPS-D
9.3
11.6
1.24
5.0
5.6
1.12
Figure 10. Effect of the switch frequency on the size of the EPR dispersed phase domains. The dashed curve is to guide eyes. The average contents in EPR and EPS based in Figure 3 are also plotted. Solid symbols: size of the dispersed phase; open rectangular symbols: EPR content; open circular symbols: EPS content.
Figure 9. SEM micrographs of the fracture surfaces of the PP/EPR in-reactor alloys etched by toluene at 20 °C. 5997
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Table 4. Mechanical Properties of PP/EPR in-Reactor Alloys polymer sample
switch frequency
EPRa content%
EPSb content%
IPP content%
crystallinityc %
impact strength
flexual modulus
(kJ/m2), 23 °C
(Mpa)
A
1
18.6
6.9
74.5
44.7
32.3 ( 3.4
784 ( 41.6
B C
4 8
18.1 15.2
7.3 9.0
74.6 75.8
45.8 46.8
25.3 ( 1.7 17.2 ( 1.5
845 ( 22.0 855 ( 24.8
D
20
13.9
10.0
76.1
47.6
25.4 ( 1.7
909 ( 30.0
a
The average content of EPR were calculated from the results in Figure 3. b The average contents of the EPS were calculated from the results in Figure 3. c The crystallinity of mechanical testing specimens was determined by DSC, Xc = ΔHm/ΔHm0, ΔHm0 = 165 J/g.21 The data during the first heating scan were used for calculation.
Figure 11. SEM micrographs of the fracture surfaces of the impact test specimens.
but also an increase in the fraction of the EPS which acted as a compatibilizer between PP and EPR. Both factors were favorable for a decrease in the EPR domain size in the PP matrix. The diameter of the dispersed phase domains was calculated using a semiautomatic image analysis method. It was characterized by the volume average particle diameter, dv, defined by dv ¼
∑ni di4 ∑ni di3
ð7Þ
For each sample, the fracture surfaces were etched by toluene either at 20, 50, or 80 °C. At least 500 particles were counted for statistically meaningful values of dv. From Figure 10, upon increasing the switch frequency the size of the dispersed phase decreased from about 1.1 μm to about 0.7 μm. This is because the EPR content decreased while the EPS content increased; both
were in favor of the decrease in the size of the EPR phase domains. Mechanical Properties. Table 4 compares the mechanical properties of the four PP/EPR in-reactor alloys obtained in this work. It is interesting to note that the impact strength first decreased with increasing switch frequency and then went up again. This could be explained based on Figure 10. An increase in the switching frequency led to a decrease in the EPR content and an increase in the EPS content. The former obviously disfavored the impact strength, whereas the latter favored it by decreasing the size of the ERP domains and enhancing the interfacial adhesion between the PP and EPR phases. Therefore there was a trade-off between those two factors. It is noted that an increase in switch frequency also led to an increase in the flexual modulus. This can be easily explained by the fact that the EPR content decreased. However, the above results are slightly 5998
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Industrial & Engineering Chemistry Research different from those of Dong et al.17 They showed improvement both in impact strength and flexural modulus of PP/EPR inreactor alloys with an increase in switch frequency. This may be because the recipes of the catalyst system and operation conditions used in this work were different from theirs, as stated in the Experimental Section. Figure 11 shows the morphology of the impact fractured surfaces of PP/EPR in-reactor alloys. It is well-known that the impact fractured surface pattern of a material can be related to its impact property. The fractured surfaces of sample A exhibited a ductile and coarse pattern, reflecting good toughness. In contrast, those of sample C were very smooth, indicating low impact strength. The EPR content of sample D was the lowest, and its impact fractured surface pattern was very different from those of the other samples. There were strips or wrinkles, and they might have been brought about by yield deformation which can consume massive energy when impact testing. This confirms the good impact strength in spite of its low EPR content.
4. CONCLUSIONS In this work, the multizone circulating reactor (MZCR) technology, a novel one for polyolefin production, is simulated by a batch process with periodic switching of monomer additions in order to understand mechanisms that dictate the composition and microstructures of polypropylene/poly(ethylene-co-propylene) (PP/EPR) in-reactor alloys. The effects of switch frequency on the composition and structure of the alloys are studied by solvent fractionation, NMR, DSC, and SEM techniques. It is found that the PP/EPR in-reactor alloys are composed of EP random copolymer (EPR), EP segmented copolymer (EPS), and PP homopolymer. The content of the EPS is increased, whereas that of the EPR decreased with increasing switch frequency or faster circulation between the homopolymerization of propylene and copolymerization of ethylene and propylene. As a result, the size of the EPR dispersed phase domains decreased and the interfacial adhesion between EPR and PP improved. The periodic operation provides an efficient way to control the structure and properties of EPR/PP in reactor alloys. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (L.-F.F.);
[email protected] (G.-H.H.).
’ ACKNOWLEDGMENT The authors thank the National Basic Research Program of China (Grant no. 2011CB606001) and the State Key Laboratory of Chemical Engineering for the Special Funds for Open Research Projects (SKL-ChE-08D03). ’ REFERENCES (1) Mirabella, F. M. Impact Polypropylene Copolymer: Fractionation and Structural Characterization. Polymer 1993, 34, 1729. (2) Fan, Z. Q.; Zhang, Y. Q.; Xu, J. T.; Wang, H. T.; Feng, L. X. Structure and Properties of Polypropylene/Poly(ethylene-co-propylene) In-situ Blends Synthesized by Spherical Ziegler-Natta Catalyst. Polymer 2001, 42, 5559. (3) Fu, Z. S.; Fan, Z. Q; Zhang, Y. Q.; Feng, L. X. Structure and Morphology of Polypropylene/poly(ethylene-co-propylene) In Situ
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