Article pubs.acs.org/IECR
Synthesis of Polypropylene/Poly(ethylene-co-propylene) In-Reactor Alloys by Periodic Switching Polymerization ProcessEffects of Gas Phase Polymerization Time on Polymer Properties Reza Mehtarani,† Zhisheng Fu,† Zhiqiang Fan,*,† Songtao Tu,† and Lian-Fang Feng‡ †
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou 310027, China ABSTRACT: A series of polypropylene/poly(ethylene-co-propylene) in-reactor alloys was prepared by a periodic switching polymerization process (PSPP) in which the monomer feed was periodically switched between pure propylene and an ethylene/ propylene mixture using a commercial TiCl4/MgCl2/phthalate−Al(C2H5)3/DCPDMS Ziegler−Natta catalyst. In this work, the influence of the total time of gas phase PSPP (tgas) on the composition, morphology, and properties of the alloys were investigated. The rate of copolymer production was almost constant in the gas phase PSPP reaction for as long as 2 h, implying that the diffusion barrier does not significantly increase with tgas in the gas phase PSPP. By fractionating the PP/EPR alloys to three fractions, random ethylene/propylene copolymer (EPR), segmented ethylene/propylene copolymer (EPS), and isotactic polypropylene (iPP), it was found that the EPR and EPS content of the alloys increased with increasing tgas. The chain structure of the fractions also changed with an increase in tgas. An increase in tgas led to a decrease in the content of [PPP] and [EEE] triads in EPS fraction, but the influence of tgas on the sequence distributions of EPR fractions can be ignored. The average size of the dispersed phase domains or EPR domains increased first and then leveled off with increase in tgas. The number of EPR domains increased with an increase in tgas. Toughness of the alloy was improved by prolonging PSPP. The impact strength at low temperature (−20 °C) was more sensitive to the EPR content than that at room temperature. The flexural modulus was sensitive to the EPS content. The toughness−stiffness balance of the alloy is significantly influenced by both tgas and the switching frequency of PSPP. (riser and downer).3,14−17 Polypropylene/poly(ethylene-copropylene) (PP/EPR) in-reactor alloy can be produced in such a process. Because the reactions are quickly circulated between propylene homopolymerization in the downer and copolymerization of ethylene and propylene in the riser, the MZCR process provides a PP/EPR alloy with a smaller dimension of dispersed phase and thus an improved toughness−stiffness balance in comparison with the PP/EPR alloys prepared by the traditional two-step technology.14 PP/EPR alloy synthesized by multistage sequential polymerization that simulates the MZCR process also presented better properties than the alloys produced by the traditional two-step process.18,19 Special polymer chain structure and phase structure formed during the circulations between homopolymerization and copolymerization were found to be responsible for the properties modification.19−21 In our previous work,22−24 a periodic switching polymerization possess (PSPP) was applied to simulate PP/EPR synthesis in MZCR mode. In the PSPP polymerization, the supplying of monomers to the reactor is periodically switched between propylene and an ethylene/propylene mixture automatically. Thus, the reactions alternatively swing between homopolymerization of propylene and copolymerization of
1. INTRODUCTION Polypropylene (PP) has become one of the most widely applied synthetic materials today thanks to its satisfactory combination of mechanical properties, green nature, and low cost. In the last decades, many new polymerization processes have been developed to further improve the properties of PP, including synthesis of polypropylene/poly(ethylene-co-propylene) in-reactor alloy (also called PP impact copolymer) by multistep sequential polymerization. Actually, more than ten million tons of PP impact copolymer is produced each year as toughened plastics or elastomers. In the multistep sequential polymerization process, each polymer particle is considered as a mini reactor in which polymerization occurs, so it is also called “reactor granule technology” (RGT).1,2 According to the RGT concept, it is possible to synthesize a series of heterophasic polymers in which different amounts of olefin copolymer with designed composition, crystallinity, and molecular weight are uniformly distributed within the PP matrix.3,4 To produce a PP impact copolymer with designed properties, it is necessary to know the relationship between the polymerization conditions and copolymer content as well as the chain structure. There are several literature reports on such relationships in the synthesis of PP impact copolymer by a homopolymerization−copolymerization two-step process.5−13 Recently, a new manufacturing process based on a multizone circulating reactor (MZCR) was developed by LyondellBasell Co. In the MZCR method, the growing polymeric granules are continuously circulated between two gas phase reaction zones © 2013 American Chemical Society
Received: Revised: Accepted: Published: 13556
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Table 1. Polymerization Conditions and Composition of PP/EPR In-Reactor Alloysa sample
tgasb (min)
tcyclec (min)
SFd (times)
yield (kg polymer/g catalyst)
EPRe (wt %)
EPSf (wt %)
iPPg (wt %)
P20EP10-30 P20EP10-60 P20EP10-120 P10EP5-30 P10EP5-120 P5EP2.5-60 P5EP2.5-120
30 60 120 30 120 60 120
30 30 30 15 15 7.5 7.5
1 2 4 2 8 8 16
1.35 1.61 2.17 1.32 2.09 1.67 2.21
17.6 18.0 24.0 17.9 26.2 18.7 24.3
5.3 6.7 10.6 6.8 13.2 7.9 11.8
77.1 75.3 65.4 75.3 60.6 73.4 63.9
a Al/Ti = 100, Si/Ti = 5. Prepolymerization: P = 0.1 MPa, T = 20 °C, t = 20 min. Slurry polymerization: P = 0.6 MPa, T = 70 °C, t = 30 min. PSPP: P = 0.4 MPa, T = 70 °C. btgas = duration of gas phase PSPP. ctcycle = duration of a PSPP cycle. dSF = switching frequency, or number of cycles. eEPR = random ethylene/propylene copolymer. fEPS = segmented ethylene/propylene copolymer. giPP = isotactic polypropylene.
ethylene and propylene. In a previous work,24 we directly determined the dynamic change of the gas phase monomer composition in PSPP. It was experimentally confirmed that there was a limitation in increasing the switching frequency. When the switching frequency exceeded the limitation, the toughness−stiffness balance of the PP/EPR alloy tended to be worse. It has been proven that the toughness−stiffness balance and phase morphology of the PP/EPR alloys can be improved by increasing the switching frequency within the limitation while fixing the total duration of PSPP polymerization. However, it is not clear how the structure and properties of the alloy would be affected by the total duration of PSPP polymerization. In the production of PP/EPR in-reactor alloys, the duration of polymerization reaction, or the reactor mean residence time, is one of the key parameters to control the polymer structure and properties. In the present work, a series of PP/EPR in-reactor alloys were synthesized by PSPP polymerization with different durations of gas phase polymerization (30, 60, and 120 min). The correlations between the duration of PSPP polymerization and the alloy’s chemical composition, chain structure, molecular weight, mechanical properties, and phase morphology have been studied. The aim of this work is to find the principles for optimizing the PP/EPR alloy synthesized in MZCR mode.
MPa, and homopolymerization of propylene was carried out at 70 °C for 30 min. Finally, propylene homopolymerization and ethylene/propylene (C2:C3 = 0.667) copolymerization were performed alternatively in the gas phase in PSPP mode at 0.4 MPa and 70 °C for a designated time (30, 60, or 120 min). During this process, the nonpolymerized monomers were discharged from the outlet of the autoclave. The experimental conditions and main results are listed in Table 1. The samples in Table 1 have been named according to the duration of homo- and copolymerization in each PSPP polymerization cycle and the total duration of PSPP polymerization (tgas). For instance, the sample P20EP10-120 was prepared by 4 cycles of PSPP polymerization in which each cycle composed of 20 min homopolymerization and 10 min copolymerization. Thus, tgas of this sample was 120 min, and the switch frequency was 4. Fractionation of Polymer Alloys. A two-step process was used to fractionate each alloy sample into three fractions: the fraction soluble in n-octane at room temperature (C8-sol), the fraction soluble in boiling n-heptane (C7-sol), and the fraction insoluble in boiling n-heptane (C7-insol). Detailed descriptions on the fractionation operations have been presented in our previous work.24 According to our previous works,22 C8-sol, C7-sol and C7-insol are random ethylene/propylene copolymer (EPR), segmented ethylene/propylene copolymer (EPS) and isotactic polypropylene (iPP), respectively. Measurement of Molecular Weight. Molecular weight and molecular weight distribution of the fractions were measured by gel permeation chromatography (GPC) at 150 °C on a PL 220 GPC (Polymer Laboratories, Ltd.) instrument equipped with three PL mixed-B columns (500−107). 1,2,4Trichlorobenzene was used as a solvent and mobile phase in the GPC at a flow rate of 1 mL/min. This solvent contained about 125 ppm BHT (butylated hydroxytoluene) as antioxidant. 13 C NMR Measurement. 13C NMR was recorded on a Varian Mercury 300-Plus spectrometer at 75 MHz at 120 °C. oDichlorobenzene-d4 was used as a 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 an 80 × 10 × 4 mm mold at 180 °C and 17.5 MPa for 15 min. Specimens were slowly
2. EXPERIMENTAL SECTION All experiments with air-sensitive substances were carried out under dry oxygen−free nitrogen using standard Schlenk techniques. 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.24 A series of polypropylene/poly(ethylene-co-propylene) in-reactor alloys (PP/EPR) were prepared by a three-step experiment including prepolymerization, propylene slurry polymerization, and gas phase polymerization in PSPP mode. A detailed description of the polymerization process was presented earlier.24 The catalyst used was a high yield spherical Ziegler−Natta catalyst, TiCl4/MgCl2/phthalate, supplied by SINOPAC (Beijing, China). Ti content of the catalyst was 2.7 wt % triethylaluminum (TEA, Albermarle Company) was used as cocatalyst. Dicyclopentyldimethoxysilane (D-donor) was used as an external electron donor. Al/Ti and Si/Ti molar ratios used in all the polymerization runs were kept at 100 and 5, respectively. After introducing n-heptane, TEA, and D-donor into the reactor, the prepolymerization stage was started by injection of the catalyst and continued at 0.1 MPa and 20 °C for 20 min. Then propylene was added to the reactor at 0.6 13557
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cooled to room temperature. The notched Izod impact strength of the specimens was measured on a Ceast impact strength tester according to ASTM D256 at 20 °C or −20 °C. The flexural modulus of samples was measured according to ASTM D790. For each sample, seven parallel measurements were carried out and the average values were reported. Morphology Investigation. The morphology and dispersion of the EPR phase in the PP matrix were investigated by scanning electron microscope (Hitachi s4800). The SEM samples were prepared as follows: specimens of the polymer were prepared as described in the above section Measurement of Mechanical Properties and were fractured in liquid nitrogen. The fractured surfaces were etched by xylene under ultrasonic at 50 °C for 5 min, sputtered with platinum, 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 Effects of PSPP Reaction Time on Copolymerization Rate. The most effective way to control EPR content of the PP/EPR in-reactor alloys includes either increasing the monomer pressure or increasing the duration of gas phase ethylene/propylene copolymerization. In this work, the duration of propylene homopolymerization in the second stage was fixed at 30 min, and the ratio between the durations of homopolymerization and copolymerization in the PSPP stage (the third step) was fixed at 2:1. Under such conditions, the EPR content of the alloy can reach 24 wt % or more when the duration of PSPP stage is long enough. PP/EPR in-reactor alloys with such a high EPR content can show toughness much higher than the PP homopolymer.8 By changing either the total PSPP reaction time or the length of each homopolymerization−copolymerization cycle, a series of PP/EPR alloys has been prepared. Table 1 lists the experimental conditions, polymer yield, and fractionation results of three groups of samples. For the samples P20EP1030, P20EP10-60, and P20EP10-120, each PSPP cycle has 20 min of homopolymerization and 10 min of copolymerization, whereas the duration of PSPP stage is prolonged from 30 to 120 min. For the samples P10EP5-30 and P10EP5-120, each PSPP cycle is composed of 10 min of homopolymerization and 5 min of copolymerization. For the samples P5EP2.5-60 and P5EP2.5-120, each PSPP cycle is composed of 5 min homopolymerization and 2.5 min copolymerization. It is clear that the third group of samples has the fastest circulation rate in the PSPP stage. Each polymer sample was fractionated into three fractions as described in the experimental section: EPR, EPS, and iPP. For gas phase polymerization in an industrial reactor operating in a continuous reaction mode, it is of key importance to prevent the polymer granules from sticking to each other or even clumping. Figure 1 shows the morphology of polymer granules of a P10EP5-120 sample that was produced by 120 min gas phase polymerization and 8 cycles in PSPP mode. It is seen that most of the granules are spherical and have a smooth outer surface, so they are not sticky. The other samples also present similar particle morphology. Such excellent morphology of a polymer powder is beneficial for industrial application. As shown in Table 1, the yield of PP/EPR alloy increased with the duration of PSPP stage, and the slope of the increase was roughly the same for the three groups of experiments with different tcycle. On the basis of the data in Table 1, the yield of
Figure 1. Picture of the PP/EPR in-reactor alloy granules produced with 120 min gas phase PSPP polymerization (sample P10EP5-120).
random copolymer (EPR), segmented copolymer (EPS), and iPP was calculated, and their changes with the corresponding copolymerization or homopolymerization time in the PSPP stage are shown in Figures 2 and 3. It can be seen that the
Figure 2. Change of copolymer yield with total copolymerization time in gas phase PSPP.
increasing of EPR and EPS yields in the first 10 min of copolymerization was much faster than that in the later copolymerization stage. This phenomenon can be explained by
Figure 3. Change of iPP yield with homopolymerization time in gas phase PSPP. 13558
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fractions of the three samples synthesized with tcycle = 30 min. It is seen that the influence of tgas on the sequence distributions of EPR fractions can be ignored (Figure 4a). In the EPS fractions, the [EEE] and [PPP] triads strongly dominate the triad sequences because they are blocky copolymer chains with long ethylene and propylene sequences. Prolonging tgas led to a decrease in the fraction of the [PPP] and [EEE] triads (Figure 4b). It means that the properties of active centers produce EPS change with time. Table 2 shows the composition and sequence distribution of EPR and EPS fractions of the samples in Table 1. Ethylene
increasing the diffusion barrier with time in the initial stage of copolymerization. At the switching point from propylene slurry polymerization to gas phase copolymerization of the first PSPP cycle, the diffusion barrier in the PP granules was the lowest because there was no copolymer that fills the tiny pores in the granule. This will ensure high a copolymerization rate. After several minutes of copolymerization, the formed copolymer may fill in the pores, leading to a decrease in the copolymerization rate. In our previous work, it has been found that the reaction rate decays continuously for more than one hour in a continuous gas phase copolymerization experiment.9 In Figure 2, the copolymerization rate decays to a stationary level after 10 min, implying that the diffusion barrier in the polymer granules does not increase after 10 min. This might be ascribed to the presence of more homopolymerization periods in the later stages of the PSPP reaction. These homopolymerization periods can regenerate tiny pores in the PP/EPR granules, thus counteracting the effects of increasing the diffusion barrier by the copolymer. It can be seen that reducing tcycle hardly influences the copolymerization rate after the first 10 min. It is interesting to see that the rate of gas phase propylene polymerization decays slightly even in the later stage of PSPP reactions (see Figure 3). This may mean that the active centers producing iPP are more unstable than those active centers capable of producing EPR and EPS. Kissin has found that the rate of propylene polymerization decays much faster than the rate of ethylene polymerization when the same MgCl2supported catalyst is used.25 Chain Structure and Molecular Weight of the Alloys. The chain structure of the EPR and EPS fractions of the alloy samples in Table 1 has been characterized by 13C NMR. Figure 4 shows the triad sequence distribution of EPR and EPS
Table 2. Composition and Sequence Distribution of EPR and EPS Fractions Determined by 13C NMR fraction
sample
Ea (%)
[EEE]
[PPP]
nEb
npc
EPR fraction
P20EP10-30 P20EP10-60 P20EP10-120 P10EP5-30 P10EP5-120 P5EP2.5-60 P5EP2.5-120 P20EP10-30 P20EP10-60 P20EP10-120 P10EP5-30 P10EP5-120 P5EP2.5-60 P5EP2.5-120
45.8 41.8 44.5 47.1 43.7 41.8 38.8 58.4 50.7 55.0 55.3 47.7 49.1 43.6
13.1 12.0 13.7 16.9 14.4 12.2 10.4 47.3 41.7 41.2 46.8 39.0 37.8 37.6
20.4 18.5 19.3 17.6 22.0 20.5 27.1 40.4 36.8 31.6 36.4 34.6 36.5 30.4
1.9 1.8 2.0 2.2 2.1 1.9 1.9 9.5 7.0 6.0 9.8 5.7 5.6 5.0
2.6 2.6 2.5 2.4 2.6 2.6 3.0 7.1 6.8 4.9 7.9 6.3 5.8 5.2
EPS fraction
a Ethylene content of the fraction. bNumber average sequence length of ethylene unit. cNumber average sequence length of propylene unit.
content of the EPR fraction slightly decreased with tgas. The average sequence length of the ethylene unit (nE) and the polypropylene unit (nP) of the EPR fraction was almost constant, but in the EPS fraction, both of them (nE and nP) decreased with increasing tgas. The ethylene content of the EPS fraction also decreased with tgas. From these phenomena, it can be said that the active centers producing EPS chains undergo certain changes in the course of polymerization. As a result, the blockiness of the EPS fraction decreased with increasing tgas. It should be considered that there are two factors that influence the absorption of ethylene and propylene in PP particles, diffusivity and solubility. The decreases of ethylene content of both the EPS and EPR fractions with tgas can be explained by the different solubility of propylene and ethylene in the polymer granule. According to the report of Sliepcevich et al., the diffusivity of ethylene in PP particles 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.26 Because the solubility of propylene in polypropylene is much higher than that of ethylene, the propylene to ethylene concentration ratio ([P]/[E]) inside the granule will be higher than that in the gas phase. With the polymerization proceeds, polymer particle size increases and leads to even higher [P]/[E] inside the granule. As a result, copolymer formed in the later stage of gas phase polymerization will have lower ethylene content than that formed in the initial stage. As mentioned in our previous work, in PSPP polymerization, 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
Figure 4. Triad sequence distribution of EPR and EPS fractions of three samples prepared in different times (tgas). 13559
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polydispersity index of EPR and EPS fractions was much larger than that of the iPP fraction, their differences in weight-average molecular weight are smaller. It is also seen that molecular weight of the EPR and EPS fractions slightly increased with an increase in tgas; however, the molecular weight of iPP is almost constant. This phenomenon is similar to the change of molecular weight of PP with polymerization time reported in literature.27 Phase Morphology of the Alloys. The phase morphology of the alloys can be observed in SEM pictures of the cryofractured surface of the samples etched by xylene at 50 °C (see Figures 6 and 7). In these pictures, many tiny cavities can be seen, which should have been left by the phase rich in EPR and EPS that is dissolved in xylene in the etching treatment.8,9 These SEM pictures were interpreted based on image analysis. The number density and size distribution of the cavities were determined by counting the number and measuring the areas of cavities observed in the SEM photograph, and the results are presented in Table 4. The
ethylene content remains almost constant can be considered to be the steady period of the copolymerization stage. Much of the EPS fraction is produced at the transition period.24 As we proved in our previous study, the difference between [P]/[E] inside and outside the granule in the transition periods is larger than that of the steady period. As a result, prolonging the duration of PSPP has a stronger influence on the ethylene content of the EPS fraction than on the EPR fraction. In this work, the influence of tgas on molecular weight and polydispersity index (PDI) of the fractions of alloys (EPR, EPS, and iPP) was studied, and the results were listed in Table 3. Table 3. Molecular Weight and Polydispersity Index of EPR, EPS, and iPP Fractions fraction
sample
tgasa (min)
Mnb/104
Mwc/104
PDId
EPR fraction
P20EP10 P20EP10 P20EP10 P10EP5 P10EP5 P5EP2.5 P5EP2.5 P20EP10 P20EP10 P20EP10 P10EP5 P10EP5 P5EP2.5 P5EP2.5 P20EP10 P20EP10 P20EP10 P10EP5 P10EP5 P5EP2.5 P5EP2.5
30 60 120 30 120 60 120 30 60 120 30 120 60 120 30 60 120 30 120 60 120
2.3 2.5 2.9 1.6 2.5 2.1 3.0 1.7 1.7 2.3 2.0 3.0 2.6 3.0 8.8 7.6 7.1 9.4 7.8 10.0 8.6
12.9 14.8 21.0 12.2 21.3 20.3 24.4 10.1 11.4 17.3 12.4 20.7 17.1 18.9 38.7 34.0 38.6 47.8 43.5 45.8 46.2
5.6 6.0 7.2 7.7 8.4 9.7 8.1 6.1 6.7 7.4 6.3 6.8 6.6 6.3 4.4 4.5 5.4 5.1 5.6 4.6 5.4
EPS fraction
iPP fraction
Table 4. Number Density and Size Distribution of the EPR Phase sample
tgas (min)
densitya (μm−2)
Anb (μm2)
Aa/Anc
P20EP10-30 P20EP10-60 P20EP10-120 P10EP5-30 P10EP5-120 P5EP2.5-60 P52.EP5-120
30 60 120 30 120 60 120
0.50 0.59 0.66 0.97 1.30 0.79 0.99
0.153 0.225 0.235 0.079 0.132 0.170 0.174
2.15 2.99 2.30 2.13 2.27 2.66 2.12
a
Number of cavity per square micrometer counted in the SEM photograph. bNumber-average cavity area calculated according to the following equation: An = Σ(NiAi)/ΣNi, 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 = Σ(NiAi2)/Σ(NiAi).
a tgas = the total time of gas phase polymerization. bMn = numberaverage molecular weight. cMw = weight-average molecular weight. d PDI = polydispersity index.
SEM pictures and data in the table demonstrate that the phase morphology of the in-reactor alloys is strongly related to the gas phase polymerization time. The increase in tgas causes increases in the number density and number-average cavity area (average dimensions of the dispersed phase domain). Figure 6 shows the SEM pictures of the polymer samples produced with the same tcycle but different tgas (30, 60, and 120 min). In the first hour of gas phase polymerization (P20EP1030 vs P20EP10-60), it can be seen that the average size of the cavities (Aa) increased greatly with tgas (an increase of 47%), whereas the number of cavities changed much less (18%). Further prolonging the gas phase polymerization almost did not increase the average size of cavities but increased the number of cavities by about 12% (P20EP10-60 vs P20EP10-120). Similar results can be observed in samples P5EP2.5-60 and P5EP2.5120, which were prepared with different tcycle. Here, the number of cavities increased by 25%, but the average size of the cavities almost kept constant (Figure 7). It is interesting to compare the phase morphology of samples P20EP10-120, P10EP5-120, and P5EP2.5-120, which have similar EPR content (24 − 26%). The average size of EPR domains decreases in the order of P20EP10-120 > P5EP2.5-120 > P10EP5-120, and the number of EPR domains decreases in the order of P10EP5-120 > P5EP2.5-120 > P20EP10-120. As a
Figure 5 shows the relation between the molecular weight of the fractions and tgas. As shown in Figure 5, the weight average molecular weight (Mw) of the EPR and EPS fractions are much lower than that of the iPP fraction. However, because the
Figure 5. Change of molecular weight of the polymer fractions with tgas (tcycle = 30 min). 13560
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Figure 6. SEM pictures of cryofractured surface of PP/EPR in-reactor alloys produced at different tgas (tcycle = 30 min).
Figure 7. SEM pictures of the cryofractured surface of two series of PP/EPR in-reactor alloys produced with a shorter tcycle and different tgas (tcycle = 15 min for P10EP5-30 and P10EP5-120, tcycle = 7.5 min for P5EP2.5-60 and P5EP2.5-120).
the PP domains formed in the short cycle might not be big enough to prevent the EPR domains from merging with each other, and bigger but fewer EPR domains will be observed. Mechanical Properties of the Alloys. To investigate the influence of the tgas and composition on the mechanical properties of PP/EPR in-reactor alloys, the notched Charpy impact strength (at 20 and −20 °C) and flexural modulus (at 20 °C) of the samples were measured. The experimental data was presented in Table 5. In our previous works, we improved the impact strength of the PP/EPR in-reactor alloys to about 1318 and 33 kJ/m2 21 by applying PSPP in alloy synthesis. In the current work, the impact strength of alloys is further improved by applying PSPP
general rule, the average size of the cavities decreased and the number of the cavities increased with increasing the switching frequency of the PSPP stage. There is a nearly direct correlation between the total switching number (or total cycles) and the number of EPR domains. This phenomenon can be explained by the polymer granule growth model proposed in our previous work.18 When a short period of copolymerization is followed by a short period of propylene polymerization, the small copolymer domain may be separated from the copolymer domains formed in the subsequent cycles by the polypropylene domain. However, though shorter tcycle is beneficial to increasing the number of EPR domains, there is a limitation to this effect. When the tcycle is too short (e.g., tcycle = 7.5 min), 13561
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experimental data to exactly confirm the relationship, especially in the range of EPR content of 19 − 23 wt %.
Table 5. Mechanical Properties of the PP/EPR In-Reactor Alloys
sample
tgasa
(min)
impact strength at 20 °C (kJ/m2)
impact strength at −20 °C (kJ/m2)
flexural modulus (20 °C, MPa)
EPRb (wt %)
EPSc (wt %)
P20EP10-30 P20EP10-60 P20EP10-120 P10EP5-30 P10EP5-120 P5EP2.5-60 P5EP2.5-120
30 60 120 30 120 60 120
34.5 39.5 52.5 46.7 60.6 53.2 58.3
− 3.2 37.0 4.2 48.9 6.3 41.6
− 760 670 690 500 627 596
17.6 18.0 24.0 17.9 26.2 18.7 24.3
5.3 6.7 10.6 6.8 13.2 7.9 11.8
a
tgas = duration of gas phase PSPP. bEPR = random ethylene/ propylene copolymer. cEPS = segmented ethylene/propylene copolymer.
Figure 8. Correlation between impact strength at −20 °C with EPR content of the alloys. These samples were prepared at different tgas and tcycle.
with adequate switching frequency. As shown in Table 4, the impact strength of the alloy with 18.7 wt % EPR increased to 53.2 kJ/m2 (P5EP2.5-60) and to 60.6 kJ/m2 for the alloy with 26.2 wt % EPR (P10EP5-120) at 20 °C. By introducing more EPR and EPS in the alloys that were synthesized with tgas = 120 min, the impact strength at low temperature (−20 °C) was increased by more than 7 times. Generally, as shown in Table 5, in each series with a fixed tcycle, as tgas increased, both EPR and EPS contents increased. As a result, the impact strength of these PP/EPR in-reactor alloys increased. However, their flexural modulus decreased. This indicated that the impact strength was proportional to the content of EPR and EPS, but the flexural modulus was proportional to the content of iPP. It is interesting to note the effect of the EPR content on the difference between impact strength at 20 and −20 °C. For each sample, the difference decreased with an increase in the percent of EPR in the samples. For example, in P5EP2.5-60 with an EPR content of 18.7%, the difference between its impact strength at 20 and −20 °C is about 46.9 kJ/m2, but for sample P10EP5-120 with an EPR content of 26.2%, this difference is lowered to only 11.7 kJ/m2. When tgas was less than 60 min, the EPR content in all of the samples was about 18 wt %. However, as tgas was increased to 120 min, besides the evident increase in the EPR content, the EPS content also increased. As a result, the impact strength of these samples increased not only at 20 °C but also at −20 °C. Because EPS acted as a compatibilizer between EPR and isotactic polypropylene, high EPS content led to improved compatibility between EPR and isotactic polypropylene. When tgas was increased to 120 min, EPR content in the alloys was higher than 24 wt %. Compared with samples P10EP5-30 and P5EP2.5-60, although the EPR content in those samples prepared with tgas =120 min was much higher, the increse of impact strength at 20 °C was very limited. Namely, once the EPR content was higher than 18 wt %, the influence of EPR content on impact strength at 20 °C could be ignored. However, the impact strength at −20 °C of those samples prepared with tgas =120 min was much higher than that of sample EP5P10-30 and EP2.5P5-60. For example, the impact strength at −20 °C of sample EP2.5P5-120 was about 7 times that of sample EP2.5P5-60. It is interesting to find that the impact strength at −20 °C was roughly proportional to the EPR content (see Figure 8). However, it needs more
It is also interesting to compare the flexural modulus of alloys with similar EPR content. The three samples P20EP10-60, P10EP25-30, and P5EP2.5-60 have roughly the same EPR content of 18%, but their flexural modulus decreased in the order of P20EP10-60 > P10EP5-30 > P5EP2.5-60. Such difference in flexural modulus might be caused by the different EPS content in these samples. The EPS content increased in the order of P20EP10-60 < P10EP5-30 < P5EP2.5-60, but the total increment was only 1.2%. It means that the flexural modulus of the alloy is very sensitive to the EPS content. The three samples synthesized with tgas = 120 min (P20EP10-120, P10EP5-120, and P5EP2.5-120) presented a similar trend of EPS content on flexural modulus. The effect of the EPS fraction on flexural modulus may be explained by its high compatibility with the iPP matrix of the alloy. Because the flexural modulus of the materials largely depends on the crystalline iPP phase, changes in the structure of this phase by a small amount of the EPS fraction will lead to marked changes in the mechanical properties of the iPP phase. Therefore, it may be beneficial to limit the EPS content at a reasonable level in order to improve the toughness−stiffness balance of PP/EPR in-reactor alloy. Summarizing the results presented in this work, it can be said that prolonging the gas phase polymerization time by increasing the number of PSPP cycles exerts significant effects on the chain structure, phase structure, and mechanical properties of PP/EPR alloys. A suitable combination of tcycle with tgas may help improve the toughness−stiffness balance of the materials.
4. CONCLUSIONS It has been shown experimentally that the gas phase polymerization time (tgas) has significant influences on composition, chain structure, mechanical properties, and phase morphology of polypropylene/poly(ethylene-co-propylene) in-reactor alloys prepared by a periodic switching polymerization process (PSPP). By fractionating the PP/EPR alloys into three fractions, ethylene/propylene copolymer (EPR), ethylene/propylene segmented copolymer (EPS), and polypropylene (iPP), it was found that the EPR and EPS content of the alloys increased with increasing tgas. The chain structure of the fractions also changed with increase in tgas. An increase in tgas led to a decrease in the content of [PPP] and [EEE] triads in EPS fraction, but the influence of tgas on the sequence distributions of EPR fractions can be ignored. The 13562
dx.doi.org/10.1021/ie400378j | Ind. Eng. Chem. Res. 2013, 52, 13556−13563
Industrial & Engineering Chemistry Research
Article
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ethylene content of the EPS and EPR fractions decreased with increase in tgas. The molecular weight and PDI of the alloy’s fractions increased with increase in tgas, especially in the second hour of gas phase PSPP reaction. The rate of copolymer production was almost constant in the gas phase PSPP reaction for as long as 2 h, implying that the diffusion barrier does not significantly increase with tgas in the gas phase PSPP reaction. The phase morphology of the alloys is strongly related to tgas. In the first hour of gas phase polymerization, the average size and the number of EPR domains increase with increase in tgas. With a further increase in tgas, the size of the EPR domains stays almost constant, while their number increases. Toughness of the alloy was improved by prolonging gas phase polymerization in PSPP mode. The impact strength at low temperature (−20 °C) was more sensitive to the EPR content than the impact strength at room temperature. The flexural modulus was sensitive to the EPS content. The toughness−stiffness balance of the alloy is significantly influenced by both tgas and tcycle.
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AUTHOR INFORMATION
Corresponding Author
*Z.-Q. Fan. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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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) are gratefully acknowledged.
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REFERENCES
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