poly(ethylene-co-propylene

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Synthesis of Polypropylene/poly(ethylene-co-propylene) In-Reactor Alloys by Periodic Switching Polymerization Process: Dynamic Change of Gas-Phase Monomer Composition and Its Influences on Polymer Structure and Properties Reza Mehtarani,† Zhi-Sheng Fu,† Song-Tao Tu,† Zhi-Qiang Fan,*,† Zhou Tian,‡ and Lian-Fang Feng‡ †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ‡ State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: Using a MgCl2-supported Ziegler−Natta catalyst, a series of polypropylene/poly(ethylene-co-propylene) (PP/ EPR) in-reactor alloys with high EPR content (>20 wt %) were prepared by a periodic switching polymerization process (PSPP) in which the monomer feed was periodically switched between pure propylene and an ethylene/propylene mixture. The change of gas-phase monomer composition with time in the PSPP process was investigated. Transition periods were identified when switching the monomer feed from propylene to ethylene/propylene mixture (or vice versa). Because the gas-phase ethylene content in the transition periods was lower than that of the monomer mixture feed, copolymer formed in these periods has lower ethylene content than those formed in the steady periods of the copolymerization stage. As a result, the transition periods led to formation of more segmented ethylene/propylene copolymer (EPS) and decreased its ethylene content. When the switching frequency was altered in an adequate range, the content of EPS in the alloy was increased by increasing the switching frequency, and dispersion of the copolymer phase in the PP matrix became more uniform. The toughness−stiffness balance of PP/EPR alloy synthesized by PSPP process with adequate switching frequency was better than the alloy synthesized by a conventional sequential polymerization process with only one copolymerization stage. When the switching frequency exceeded a certain limit, the alloy’s EPS content was decreased and its toughness−stiffness balance tended to be worse.

1. INTRODUCTION With continuous progress in controlling catalyst and polymer particle morphology and tailoring mechanical properties of the products, industrial production of polypropylene/poly(ethylene-co-propylene) (PP/EPR) has been growing rapidly since the 1980s.1,2 PP/EPR in-reactor alloy is mainly synthesized by a sequential polymerization process, in which propylene homopolymerization with a spherical TiCl4/MgCl2based Ziegler−Natta catalyst is followed by gas-phase copolymerization of ethylene and propylene.3−9 The product is a heterophasic polymer with polypropylene (PP) as the matrix phase and ethylene−propylene copolymer (EPR) as the dispersed phase.10−13 It has been experimentally proven that in PP/EPR alloys, there are at least three major types of polymer components: crystalline polypropylene (iPP), random ethylene−propylene copolymer (EPR), and segmented ethylene− propylene copolymer (EPS).6,14−19 The character of Ziegler− Natta catalysts to produce both EPR and EPS in the copolymerization stage is advantageous, because the segmented copolymer in these alloys acts as a compatibilizer between the PP matrix and the ethylene−propylene random copolymer phase, and thereby significantly enhances the mechanical and physical properties of the alloys.4,20−23 Recently, a new manufacturing process, called the Spherizone process, based on multizone circulating reactor (MZCR), was developed by LyondellBasell. 24−26 In comparison with mechanical blends of PP/EPR and traditional multistep technologies such as the Spheripol process, the MZCR provides © 2013 American Chemical Society

more-uniform polymer alloy with a smaller dimension of EPR phase (high homogeneity). This target is achieved by quickly circulating the growing polymeric granules between two different gas-phase reaction zones: a reaction zone containing pure propylene (downer) and a reaction zone where a mixture of ethylene and propylene is fed (riser).2,12,13 In an ideal condition of the MZCR process, the concentrations of the monomers around a growing polymer particle change instantaneously when the polymer particles are circulated from one zone to another. However, when the size of the spherical polymer/catalyst particles (the reactor granules) is large enough, the time needed for the monomers to diffuse from the bulk of the gas phase into the central zone of a particle cannot be neglected. The complete consumption of monomers dissolved in the polymer particle also takes time. Therefore, the actual switching of monomers inside the particle will lag behind the switching of monomers in the gas phase. In our previous work,11 a bench-scale multistage sequential polymerization process (MSSP) was used to simulate PP/EPR synthesis in MZCR. In that process, according to a semibatch feeding policy, at the end of each polymerization stage, the remaining monomer in the autoclave was removed by evacuation to 5 mmHg for 3 min, and a different monomer Received: Revised: Accepted: Published: 9775

November 21, 2012 May 10, 2013 June 23, 2013 June 24, 2013 dx.doi.org/10.1021/ie3032179 | Ind. Eng. Chem. Res. 2013, 52, 9775−9782

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feed was introduced to the autoclave to launch the next stage of reaction. That process was complicated and could not be operated automatically. Therefore, in the subsequent work, PP/ EPR was synthesized in a process called the “Periodic Switching Polymerization Process” (or PSPP).10,11 In the PSPP process, a continuous feeding policy was adopted. The streams of propylene and ethylene/propylene mixture gas were alternatively fed to the autoclave and the nonpolymerized monomers continuously exited from the autoclave to let the monomer pass through the reactor at a nearly constant rate. Switching between the homopolymerization and copolymerization was realized by expelling the existing monomer with another stream of monomer. It is clear that the PSPP process is more convenient to operate than the MSSP process. In the previous works,10,11 we have found that increasing the frequency of switching between the homo- and copolymerization reactions in PSPP process cause evident decrease in the content of EPR and increase in the content of EPS. The strength of this effect can be weakened by reducing the mean residence time of monomer gas in the reactor.11 This can be explained by partial mixing of the monomer stream entering the reactor with the monomer gas already existing in the reactor, which forms in the reactor monomer mixture with ethylene content lower than the targeted value and thus copolymer with lower ethylene content. For this reason, the mechanical properties, especially the toughness and stiffness of the PP/ EPR alloys synthesized in PSPP process, did not show evident improvement, compared to the alloys synthesized in a conventional two-stage sequential polymerization process.10 In this work, we attempt to directly determine the changes of monomer composition in the gas phase of PSPP process. Based on the experimental data, the PSPP process parameters are modified in order to improve the properties of PP/EPR alloys. Evident improvements in the toughness−stiffness balance of the PP/EPR alloys have been achieved. The phase morphology of the alloys has also been investigated.

Figure 1. Schematic representation of transition of monomer gas in the PSPP reactor.

ethylene content in ethylene/propylene mixtures, initially, a working curve was made. Many ethylene/propylene mixtures with predetermined composition were prepared and stored in 10-mL Schlenk flasks with a Teflon stopcock. Then they were measured using a gas chromatograph apparatus (Shimadzu, Model GC-8AF). The following working curve for the relationship between the molar ratio of ethylene to propylene in the mixture (C2/C3) and the ratio of the peak areas of ethylene and propylene (AC2/AC3) was obtained: ⎛A ⎞ C2 C = 1.3967⎜⎜ 2 ⎟⎟ + 0.0067 C3 A ⎝ C3 ⎠

(1)

where the ratios were determined using gas chromatography (GC). Measurement of monomer gas transitions in the hollow reactor without catalyst and polymer particles was made as follows: an autoclave (volume = 0.8 L, inner diameter = 50 mm) was first heated to 70 °C and charged with propylene of 0.4 MPa at a flow rate of 4.8 SLM for 6 min. [The term “SLM” denotes standard liters per minute.] The propylene flow then was switched to an ethylene/propylene (C2:C3 = 0.667) mixture of 0.4 MPa, which was continuously charged into the autoclave at a flow rate of 4.8 SLM for 3 min (mean residence time = 1.13 min). During this process, monomers were discharged from the outlet on the autoclave. To investigate the change of C2/C3 ratio with time during the switching between propylene and ethylene/propylene mixture, a series of gas samples were taken at the outlet of the autoclave at different time from the switching point and analyzed by GC. According to eq 1, the C2/C3 value of each sample was calculated. Subsequently, as a comparison, the monomer gas transition of a real PSPP process was determined in the same autoclave with 60 mg Ziegler−Natta catalyst and Al/Ti = 100. Dicyclopentyldimethoxysilane (D-donor, Si/Ti = 5) was used as an external electron donor. The entire polymerization process has three steps, as mentioned in the previous work.11 In the first step, propylene prepolymerization was conducted at 0.1 MPa and 20 °C for 20 min in 50 mL n-heptane. In the second step, propylene homopolymerization was carried out at 0.6 MPa and 70 °C for 30 min. In the third step, gas-phase propylene homopolymerization for 20 min and ethylene/propylene (C2:C3 = 0.667) copolymerization for 10 min were performed alternatively in PSPP mode at 0.4 MPa and 70 °C. The changes

2. EXPERIMENTAL SECTION All preparations and manipulations were carried out under dry oxygen-free nitrogen using standard Schlenk techniques and a glovebox for air-sensitive substances. Determination of Gas-Phase Monomer Composition in an Autoclave. The apparatus of the PSPP process used in this work has been described in the previous work.11 As shown in Figure 1, the monomer continuously flows into the reactor through the inlet located at the bottom of the autoclave, and flows out of the reactor through the outlet on the top plate of the autoclave. The height/diameter ratio of the autoclave is 6. The temperature in the autoclave was controlled to an assigned level in two ways: (1) hot water at the assigned temperature was circulated through the jacket of the autoclave with a thermostatic circulator; and (2) the monomer stream was preheated to the assigned temperature before flowing into the autoclave. To ensure ideal mixing of the particles and the minimum temperature fluctuation in the autoclave, a helix-type stirrer has been used, and the stirring speed was 200 rpm. When the monomer stream is switched from propylene to an ethylene/propylene mixture (or vice versa), the composition of monomer stream from the outlet will continuously change with time. To trace this transition of monomer composition, monomer gas at the outlet was sampled every 0.5−1 min and the ethylene content of the samples was measured by gas chromatography (GC). To make quantitative measurement of 9776

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Figure 2. Gas-phase composition of the monomers at the outlet of the reactor in the absence of catalyst. Temperature = 70 °C, pressure = 0.4 MPa, gas flow rate = 4.8 SLM. In period “P”, pure propylene is supplied to the reactor; in period “E + P”, an ethylene/propylene mixture is supplied to the reactor.

of C2/C3 of the outlet gas in the third step were determined. In all of these experiments, a backpressure regulator connected in the pipe of the outlet was used to adjust pressure in the autoclave. This can ensure constant pressure (0.4 ± 0.001 MPa) in the autoclave during the switching of monomers. Synthesis of Polypropylene/Poly(ethylene-co-propylene) Alloy. The apparatus and polymerization setup for synthesizing PP/EPR in-reactor alloy are the same as mentioned in our previous work.10,11 In our previous work,10−12 PP/EPR alloys with an EPR content of 20 wt %) and the influence of EPR content on the properties of PP/ EPR alloys, the time of propylene homopolymerization in the second step was reduced to 30 min and the ratio between the time of propylene homopolymerization and the time of ethylene/propylene copolymerization in gas-phase mode (the third step) was set at 2:1. The synthesis of the alloys consisted of three steps: propylene prepolymerization at 0.1 MPa, propylene homopolymerization at 0.6 MPa, and alternative gas-phase homopolymerization and ethylene/propylene copolymerization in a PSPP mode for a designated time (60 or 120 min). The operation conditions are the same as described above. A TiCl4/ MgCl2/phthalate type commercial catalyst (DQ catalyst) supplied by SINOPEC (Beijing, PRC) was used, which has 2.7 wt % of Ti. Triethylaluminum (TEA, Albermarle Co.) was used as a co-catalyst after dilution in n-heptane. Dicyclophenyldimethoxysilane (Shandong Lujing Chemical Co.) was used as an external electron donor. Fractionation of Polymer Alloys. Approximately 1.5 g of the PP/EPR in-reactor alloy was completely dissolved in 200 mL of boiling n-octane and then precipitated by a gradual cooling of the solution to room temperature overnight. The insoluble part was separated from the solution by centrifuging and named as C8-insol. The part dissolved in n-octane was recovered from the solution by rotation evaporation and named as C8-sol. The C8-insol part then was further extracted in a Kumagawa extractor by boiling n-heptane for 12 h. Thus, it was further divided into two fractions, namely, a n-heptane soluble fraction (C7-sol) and a n-heptane insoluble fraction (C7-insol). According to our previous work,10 C8-sol, C7-sol, and C7-insol are composed of random ethylene/propylene copolymer

(EPR), segmented ethylene/propylene copolymer (EPS), and isotactic polypropylene (iPP), respectively. 13 C NMR Measurement. 13C NMR analysis of the fractions of polymer alloy was made on a Varian Mercury 300-Plus spectrometer at 75 MHz at 120 °C. o-Dicholorobenzene-d4 was used as solvent, and the concentration of the polymer solution was 10 wt %. Hexamethyldisiloxane was used as an internal reference. Chromium(III) acetylacetonate (2−3 mg) was added to each sample to shorten the relaxation time and ensure the quantitative results. Broadband decoupling with a pulse delay of 3 s was employed. Typically, 5000 transients were collected. Measurement of Mechanical Properties. Polymer granules were melted and mixed at 170 °C on a Thermo Haake Rheomix in the presence of 0.5 wt % antioxidant and then compression-molded using a 80 mm × 10 mm × 4 mm mold at 180 °C and 17.5 MPa for 15 min. Specimens were slowly cooled to room temperature. The notched Izod impact strength of the specimens was measured on a Ceast impact strength tester, according to ASTM Standard D256 at 20 °C and −20 °C. The flexural modulus of samples was measured according to ASTM Standard D790. For each sample, seven parallel measurements were carried out and the average values were reported. Morphology Investigation. The morphology and state of dispersion of the EPR phase in the alloy sample were investigated by scanning electron microscopy (SEM) (Hitachi, Model S4800). The SEM samples were prepared as follows. Specimens of the polymer were prepared as described in the previous subsection (“Measurement of Mechanical Properties”) and were fractured in liquid nitrogen. The fractured surfaces were etched by xylene under ultrasonic treatment at 50 °C for 5 min, sputtered with gold, and finally subjected to SEM observation. An operating voltage of 5 kV and magnification of 5000× were adopted for the observation.

3. RESULTS AND DISCUSSION Change of Monomer Composition with Time in the Autoclave. As shown in Figure 1, when the monomer continuously flows into the cylindrical autoclave from the inlet and leaves the autoclave from the outlet in a PSPP process, a sudden switching of the monomer supply from propylene to ethylene/propylene mixture (or vice versa) will cause a transition zone that moves upward in the reactor, in which 9777

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Figure 3. Gas-phase composition of the monomers at the outlet of the reactor in the presence of catalyst. Temperature = 70 °C, pressure = 0.4 MPa, gas flow rate = 10 SLM.

and the change of gas-phase monomer composition with time in the presence of catalyst was determined. The results of the first two cycles of the PSPP step are shown in Figure 3. In this experiment, the duration of supplying the ethylene/propylene mixture in a cycle was increased to 10 min. In Figure 3, we can see that only ∼1 min is required for the C2/C3 value to reach the level of the supplied feed after switching from propylene to the ethylene/propylene mixture, and ∼1.5 min is needed for the C2/C3 value to drop to 0 after switching back to propylene. Such short transition periods are in accord with the short mean residence time of the gas phase (0.6 min) at a monomer feed rate of 10 SLM.11 Because the duration of the transition periods is much shorter than the 9 min steady period with C2/C3 value > 0.6, it is expected that their influence on polymer structure will be limited. It is seen that the C2/C3 value does not keep at a constant level after the transition from propylene to an ethylene/ propylene mixture. Initially, the C2/C3 value went up to a level higher than the feed ratio (0.667) and then went down to a minimum value after ∼5 min from the switching point. The C2/ C3 value then rose again and gradually reached 0.667. Such phenomena seem strange at first glance, since the diffusivity of ethylene in PP particles is larger than that of propylene.27 Therefore, solubility of the monomers in the PP particle should also be taken into consideration to explain the extra rise of C2/ C3 value in the copolymerization periods. As reported by Sliepcevich et al.,27 the diffusivity of ethylene in PP particle is only 1.1−2.0 times of that of propylene, but the solubility of propylene in PP is 3.4 times greater than that of ethylene. This means that, when both diffusivity and solubility influence the amount of absorbed monomer in the PP particles, it is possible that propylene can be absorbed more than ethylene. When the rate of monomer consumption by the active centers is not too high, the amount of absorbed propylene and ethylene in the particles might be closer to the saturation level, not the level of serious starvation. If this situation happens in the experiment of Figure 3, the molar ratio of absorbed propylene to absorbed ethylene will be larger than the monomer feed ratio (1.5:1), leaving more ethylene in the gas phase. As a result, the C2/C3 value at the outlet will exceed 0.667. Since the reactivity of ethylene is much higher than that of propylene, the absorbed ethylene is consumed by the catalyst faster than propylene. This will cause reduction of the gas-phase C2/C3 value in the later stage. When a dynamic balance between absorption and polymerization is established, the value of C2/C3 will gradually return to the feed ratio (0.667).

the ethylene content is lower than that of the supplied ethylene/propylene mixture. For example, when the ethylene/ propylene mixture (C2:C3 = 0.667) replaces propylene, if the flow rate is large enough, three zones will be formed in the reactor at a certain instant: zone I is pure propylene to be replaced, zone II is a ethylene/propylene mixture with C2:C3 < 0.667 formed by mixing of propylene with the incoming monomer mixture, and zone III is the incoming monomer mixture with C2:C3 = 0.667. The lifetime of zone II will depend on the flow rate or the mean residence time (τ). When τ is larger, the time needed to complete the monomer switching will be longer, and more copolymer will be formed by the ethylene/propylene mixture with C2:C3 < 0.667. This will influence the chain structure and properties of the alloy. The change of gas-phase monomer composition with time in the absence of the catalyst was recorded and shown in Figure 2. It is seen that, after switching the propylene flow to an ethylene/propylene (C2/C3 = 0.667) mixture, the ethylene content of the monomer stream at the outlet of the autoclave increased to the maximum within ∼2 min, and then kept constant for ∼1 min. It should be noted that the maximum ethylene/propylene ratio (∼0.65) is still lower than that of the monomer feed. This small difference may be caused by insufficient mixing of the flowing gases in the hollow reactor, because there are no moving polymer particles that can deliver the mixing force of the stirrer to the gases. The three switching cycles in Figure 2 showed slightly different C2/C3 vs time profiles with each other, meaning that the flow field of monomer gases was unstable. When the monomer stream was switched from an ethylene/propylene mixture to propylene, it also took more than 2 min for propylene to completely replace the ethylene/propylene mixture. The periods where ethylene content changes with time can be named as transition periods between the homopolymerization and copolymerization stages, and the periods where the ethylene content remains almost constant can be considered to be the steady period of the copolymerization stage. It is clear that if PSPP polymerization was carried out under the conditions of Figure 2, within a homopolymerization−copolymerization cycle, copolymerization would occur in monomer mixture of C2/C3 ≥ 0.65 for 1−2 min, while more than 2 min of copolymerization would occur in a monomer mixture with a ratio of C2/C3 < 0.65. In this case, much copolymer with low ethylene content will be formed, which should influence the properties of the alloy. To reduce the influence of the transition periods on the polymer structure, the gas flow rate was increased to 10 SLM, 9778

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Table 1. Polymerization Conditions and Composition of the PP/EPR In-Reactor Alloys sample

duration, tgasa (min)

switching frequency, SFb

yield (kg polymer/g Ti)

EPRc (wt %)

EPSd (wt %)

EPR + EPS (wt %)

EPS/EPR

P20EP10−60 P5EP2.5−60 P80EP40−120 P20EP10−120 P10EP5−120 P5EP2.5−120

60 60 120 120 120 120

2 8 1 4 8 16

61.2 61.7 80.5 80.5 77.2 82.0

18.0 18.7 35.5 24.0 26.2 24.3

6.7 7.9 8.2 10.6 13.2 11.8

24.7 26.6 43.7 35.6 39.4 36.1

0.37 0.42 0.23 0.44 0.50 0.49

a Duration of gas-phase PSPP polymerization. bNumber of switches from homopolymerization to copolymerization in the PSPP step. cEPR = random ethylene/propylene copolymer. dEPS = segmented ethylene/propylene copolymer.

As shown in Figure 3, this phenomenon exactly appeared again in the second cycle. However, the strength of the first peak in the second cycle (peak 3) is higher than that of the first peak in the first cycle (peak 1). It could be due to the growth of the polymer particles with the polymerization time. As a result, more propylene could be absorbed by the bigger polymer particles. Decay of the polymerization rate with time, a common feature of propylene polymerization with supported catalysts,28,29 may also play a role. At the later stage of polymerization, lower consumption rate of ethylene and propylene in the particle will further reduce the degree of monomer concentration unsaturation in the particles, leading to even higher C3/C2 ratio in the particles. According to these experimental results, we expect that there is a limitation on the switching frequency or the length of each polymerization period for an effective PSPP process. When the length of a copolymerization period is too short, even if the flow of monomer gas is fast enough to make very short transition periods, there will not be enough time for ethylene to diffuse into the polymer particle and reach a suitable C2/C3 ratio for synthesizing copolymer with designated composition. This type of limitation will become more serious when the polymer particles are larger. Synthesis of Polypropylene/Poly(ethylene-co-propylene) In-Reactor Alloys. At the same polymerization temperature and monomer pressure as Figure 1, a series of PP/EPR alloys were synthesized with different duration of the PSPP step and different monomer switching frequencies. The results of polymerization reaction and polymer fractionation are summarized in Table 1. The samples of Table 1 were all synthesized in a three-step process: propylene prepolymerization for 20 min, propylene homopolymerization at 0.6 MPa and 70 °C for 30 min, and gas-phase PSPP polymerization for 60 or 120 min. Each polymer sample was fractionated into three fractions, as described in the Experimental Section: EPR, EPS and iPP. In all six samples of Table 1, the total copolymerization time is half of the gas-phase homopolymerization time in the PSPP step. For the samples P20EP10−60 and P5EP2.5−60, the total copolymerization time was 20 min, and for the rest four samples, the total copolymerization time was 40 min. It is seen that longer copolymerization time leads to an increased total copolymer content (EPR + EPS). This is mainly caused by increased ratio of total copolymerization time to total polymerization time by prolonging the PSPP step. For the samples P20EP10−60 and P5EP2.5−60, their copolymer yield (calculated by multiplying the total yield by the copolymer content) was ∼16 kg/g Ti, but for P20EP10−120 and P5EP2.5−120 that were synthesized with double the total copolymerization time at the same switching frequency, the copolymer yield was increased to only ∼29 kg/g Ti, which is

lower than the expected 32 kg/g Ti. The slight decrease in copolymerization activity with time means that the catalyst experienced moderate decay. When the length of PSPP step was fixed but the switching frequency (SF) was increased from 1 to 16, an evident decrease in the content of random copolymer and increase in the content of segmented copolymer was observed. Similar phenomena have been observed in our previous works.10,11 Increase in the EPS content can be explained by the presence of multiple transition periods in the PSPP mode. Because the gasphase C2/C3 ratio in the transition periods is lower than the feed ratio, copolymerization in these periods produces a copolymer with higher propylene content. Such a copolymer may have high crystallinity and cannot be dissolved in alkane solvents at room temperature. A more puzzling phenomenon is the sharp decrease in EPR content when the switching frequency was increased from 1 to only 4. Sample P80EP40− 120 has an EPR yield of 28.6 kg/g Ti, in contrast to only 19.3 kg/g Ti of sample P20EP10−120. Such a difference cannot be ascribed to the presence of more transition periods in the latter case. Considering that the copolymerization ran continuously for 40 min in sample P80EP40−120, and the rate of copolymerization is much faster than propylene homopolymerization, accumulation of heat in the polymer particle will be serious as the copolymerization continues for a long time. Such an accumulation of heat can happen even if the gas-phase temperature remains constant, because the heat conductivity of the polymer particle is rather low. This will lead to a marked temperature rise in the central part of each particle, and, thus, faster copolymerization. However, in synthesizing sample P20EP10−120, fast copolymerization was allowed to run for only 10 min, and the reaction was then switched to 20 min of slower propylene polymerization. This may greatly ease heat accumulation in the particles, leading to a lower copolymerization rate. Further reduction in the length of copolymerization period from 10 min to 5 or 2.5 min (compare sample P20EP10−120 with samples P10EP5−120 and P5EP2.5−120) exerted almost no effects on the EPR content, but the EPS content was slightly increased. Ethylene content of the EPR and EPS fractions of four alloy samples was measured by 13C NMR spectroscopy (Table 2). The ethylene content of the EPR fraction was not influenced Table 2. Ethylene Content of EPR and EPS Fractions Determined by 13C NMR Ethylene Content (%)

9779

fraction

P80EP40−120

P20EP10−120

P10EP5−120

P5EP2.5−120

EPR EPS

41.1 58.8

44.5 55.0

43.7 47.7

38.8 43.6

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than that in sample P10EP5−120. So it could be concluded that the morphology of the in-reactor alloy was strongly related to the switching frequency. As the switching frequency increased, the size of the dispersed phase domain (mainly EPR) decreased and the dispersion of the rubber phase became more and more uniform.10,12 The relationship between the size of the dispersed EPR phases and the switching frequency can also be found in samples synthesized with shorter total duration of gas-phase polymerization. When the total duration of gas-phase polymerization was reduced to 60 min (see samples P20EP10−60 and P5EP2.5−60), as the switching frequency increased, an increase in the number of cavities and a decrease in cavity size can also be observed (see Table 4). Mechanical Properties of the Polymer Alloys. To investigate the mechanical properties of PP/EPR in-reactor alloys, the notched Charpy impact strength (at 20 °C and −20 °C) and flexural modulus (at 20 °C) of the samples was measured. The results are summarized in Table 5. As shown in Table 5, among the four samples prepared with tgas = 120 min, since sample EP40P80−120 contains much more EPR, it is inappropriate to compare it with the other three samples. Comparing P20EP10−120 with P10EP5−120, as the switching frequency increased, the impact strength at 20 and −20 °C increased. However, further increase in switching frequency (P5EP2.5−120) lead to a slight decrease in impact strength. This change might be related with the changes in copolymer structure caused by the increase in the transition periods. Although the alloys synthesized under PSPP mode (P20EP10−120, P10EP5−120, and P5EP2.5−120) have smaller low-temperature impact strength than the sample P80EP40− 120, their flexural modulus are all larger than P80EP40−120. Comparing sample P10EP5−120 with P80EP40−120, we can find that they have similar impact strength at both room temperature and −20 °C, but the former has larger flexural modulus than the latter. This means that the toughness− stiffness balance of PP/EPR alloy can be improved by applying the PSPP process with adequate switching frequency.

much by reducing the length of copolymerization period. Only sample P5EP2.5−120, which has the shortest copolymerization period, showed a slightly lower ethylene content in its EPR fraction, probably attributable to the high percentage of the transition periods. The evident decrease in the ethylene content of the EPS fraction with increasing switching frequency clearly supports our consideration that the copolymer formed in the transition periods has lower ethylene content. The molecular weight and polydispersity index (Mw/Mn) of the fractions of four alloy samples were measured by gel permeation chromatography (GPC) and are listed in Table 3. It is seen that increasing the switching frequency only slightly changed the molecular weight and polydispersity index of the three alloy sample fractions. Table 3. Molecular Weight and Polydispersity Index of the Polymer Fractions molecular weight (× 10−4) weightaverage molecular weight, Mw

polydispersity index, Mw/Mn

sample

fraction

numberaverage molecular weight, Mn

P80EP40−120 P80EP40−120 P80EP40−120

EPR EPS PP

3.2 2.9 8.4

20.0 14.4 45.3

6.2 4.9 5.4

P20EP10−120 P20EP10−120 P20EP10−120

EPR EPS PP

2.9 2.3 7.1

21.0 17.3 38.6

7.2 7.4 5.4

P10EP5−120 P10EP5−120 P10EP5−120

EPR EPS PP

2.5 3.1 7.8

21.3 20.7 43.5

8.4 6.8 5.6

P5EP2.5−120 P5EP2.5−120 P5EP2.5−120

EPR EPS PP

3.0 3.0 8.6

24.4 18.9 46.2

8.1 6.3 5.4

4. CONCLUSIONS In this work, a periodic switching polymerization process was applied to synthesize PP/EPR in-reactor alloys, and the influence of switching frequency of monomer feed on the gas-phase monomer composition and then on the composition and properties of the alloys were investigated. Transition periods were identified when switching the monomer stream from propylene to ethylene/propylene mixture or vice versa. Because the gas-phase ethylene content in the transition periods was lower than that of the monomer mixture feed, copolymer formed in these periods has lower ethylene content than those formed in the steady periods of the copolymerization stage. As a result, the presence of transition periods leads to the formation of more segmented ethylene/propylene copolymer and decreases the ethylene content of EPS fraction. When the switching frequency was altered in an adequate range to limit the effects of the transition periods, the content of EPS in the alloy can be increased by increasing the switching frequency, and dispersion of the copolymer phase in the PP matrix became more uniform. The toughness−stiffness balance of PP/EPR alloy can be improved by applying the PSPP process with adequate switching frequency. However, when the switching frequency exceeded a certain limit, the alloy’s EPS content tended to decrease.

Phase Morphology of Polymer Alloys. Figure 4 shows SEM photographs of the cryogenically fractured surface of the alloy sample strips etched by xylene. In the pictures, a biphasic structure can be clearly seen. The dispersion phases are shown as tiny cavities left by EPR-rich domains etched out by xylene. The number density and size distribution of the cavities was determined by counting the number and measuring the areas of cavities observed in the SEM photograph with the help of an image analysis software, and the results are listed in Table 4. As shown in Figure 4, in sample P80EP40−120, because of the high content of EPR, many EPR particles merged with each other. In sample P20EP10−120, fewer EPR particles merged with each other. The larger polydispersity (Aa/An) of cavities in sample P80EP40−120 than the others may be caused by the particle merging. It can also be seen in Table 4 that the number of cavities evidently increases, while the average size of cavities (Aa) decreases with increasing switching frequency. It means that the EPR phases can be separated into smaller domains in the PSPP mode, where the copolymerization reaction is frequently interrupted by the homopolymerization periods. However, when the switching frequency was further raised to 16 (sample P5EP2.5−120), the number of cavities decreased slightly. This may be partly ascribed to the fact that the copolymer content in sample P5EP2.5−120 was slightly lower 9780

dx.doi.org/10.1021/ie3032179 | Ind. Eng. Chem. Res. 2013, 52, 9775−9782

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Article

Figure 4. SEM photographs of the fracture surfaces of PP/EPR alloys produced with a total gas-phase polymerization stage of 120 and 60 min at different switching frequencies.

Table 4. Number Density and Size Distribution of the EPR Phases property

P80EP40−120

P20EP10−120

P10EP5−120

P52.EP5−120

P20EP10−60

P5EP2.5−60

densitya (μm−2) Anb (μm2) Aa/Anc

0.69 0.330 2.37

0.66 0.235 2.30

1.3 0.132 2.27

0.99 0.174 2.12

0.59 0.225 2.99

0.79 0.170 2.66

a

Number of cavity per square micrometer counted in the SEM photograph. bNumber-average cavity area calculated according to the following equation: An = ∑(NiAi)/∑Ni, where Ni and Ai are number and area of cavities of a certain size, respectively. cPolydispersity of the cavity area, where Aa is the area-average cavity area calculated according to the following equation: Aa = ∑(NiAi2)/∑(NiAi).

Table 5. Mechanical Properties of the PP/EPR In-Reactor Alloys

Notes

The authors declare no competing financial interest.

■

impact strength (kJ/m2)

sample

switching frequency, SF

P20EP10−60 P5EP2.5−60 P80EP40−120 P20EP10−120 P10EP5−120 P5EP2.5−120

2 8 1 4 8 16

■

at 20 °C at −20 °C 39.5 53.2 55.6 52.5 60.6 58.3

3.2 6.3 52.8 37.0 48.9 41.6

ACKNOWLEDGMENTS Support by the Major State Basic Research Programs (Grant No. 2011CB606001) and the National High-tech R&D Program of China (Grant No. 2012AA040305) is gratefully acknowledged.

flexural modulus at 20 °C (MPa) 760 627 450 670 500 596

■

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