Synthesis Of Multicomponent Polyolefin Materials With Polyethylene And PolypropyleneMatrix By Sequential Polymerization

Multizone circulating reactor (MZCR) technology developed by LyondellBasell enables the catalyst/polymer particles to be quickly switched between different monomers within a single reactor, so that the crystalline and elastomer components of polyolefin alloys can be adequately mixed in a smaller scale, which thus improves the performance of materials and reduces the risk of reactor fouling. An experiment method called periodic switching polymerization process (PSPP) was used to imitate the MZCR technology, in which different monomers were periodically switched within a single reactor in the gas-phase polymerization. Currently, PP/EPR in-reactor alloys synthesized in the laboratory still stay at the level of impact polypropylene copolymer (IPC) since the elastomer content is limited seriously. The PP/EPR in-reactor alloys synthesized by PSPP exhibit a good balance between toughness and rigidity. Besides, it can prevent the particles from sticking to each other as often occurs during long time of continuous copolymerization. Based on the above background, this work is mainly dedicated to preparation of novel PE/EPR in-reactor alloys composed of crystalline ethylene homo- or copolymer as the matrix and ethylene-propylene random copolymer as the dispersed phase using the PSPP method, and the characteristics of such PSPP experiments was studied. Meanwhile, the influences of polymerization conditions on the polymer structure and properties, as well as the optimization of polymerization conditions and process parameters were also studied with the ultimate purpose of preparing a PE/EPR alloy-type thermoplastic elastomer. In addition, high melt flow rate (MFR) PP and PP/EPR in-reactor alloys with MFR of ～30 g/10 min and linear low-density polyethylene (LLDPE) with broad molecular weight distribution (MWD) were prepared by sequential polymerization process. The relationship between polymer structure and properties and polymerization conditions were also studied.High MFR polypropylene (HF-PP) and polypropylene/poly(ethylene-co-propylene) in-reactor alloys (HF-PP/EPR) were synthesized by propylene slurry polymerization and slurry-gas phase sequential polymerization, respectively, using two industrial catalysts G and Y with cyclohexylmethyldimethoxysilane (CHMDMS) or dicyclopentyldimethoxysilane (DCPDMS) as an external donor (De). By changing the volume of hydrogen added to the different catalytic systems, MFRs of HF-PP and HF-PP/EPR samples were all tailored to 30 g/10 min. The effects of H2 and De on composition, molecular weight and MWD, chain structure, thermal behavior, mechanical properties and phase morphology of HF-PP and HF-PP/EPR were studied in detail, and catalytic behaviors of the two catalysts combined with different external donors were also compared. It was found that the sensitivity to hydrogen of system containing DCPDMS was worse than that containing CHMDMS. To adjust the MFR to around 30 g/lOmin, more than doubled amount of H2 was needed when DCPDMS was used as the external donor. More hydrogen was required during the propylene homopolymerization stage in the preparation of the alloy, because the molecular chain of ethylene-propylene copolymer was longer than that of PP homopolymer. Using CHMDMS as De leads to more ethylene-propylene copolymer and a better rigidity-toughness balance, while the copolymer formed in the presence of DCPDMS has higher molecular weight. Catalyst Y exhibits a lower hydrogen response and stereospecificity but can produce the alloy with higher EPR content, while catalyst G provided polymers with higher molecular weight. Using catalyst Y combined with DCPDMS can produce HF-PP with higher flexural modulus and strength, while catalyst G combined with CHMDMS can produce HF-PP/EPR with a better rigidity-toughness balance and a high toughening efficiency.A series of polyethylene/poly(ethylene-co-propylene) (PE/EPR) in-reactor alloys were synthesized by PSPP using an industrial spherical Ziegler-Natta catalyst. The whole process included ethylene slurry polymerization, ethylene-propylene copolymerization in gas-phase for 30 min, followed by periodically switched ethylene-propylene copolymerization and ethylene homopolymerization in gas phase (PSPP). In a PSPP cycle, monomer pressure of the two reaction stages maintains the same while their sequence and duration can be adjusted. The characteristics of PSPP were investigated by observing the relations between the reaction conditions and alloy properties. In order to control the particle morpholog by increasing mass transfer resistance in the particle,30 min of continuous ethylene-propylene copolymerization was first conducted at the early stage of the gas-phase polymerization. All the PE/EPR alloys were fractionated into three parts by two-step solvent extraction, and the chain structure of each fraction was characterized. It was found that the n-octane soluble fraction (C8-sol) and boiling n-heptane insoluble fraction (C7-insol) were ethylene-propylene random copolymer (EPR) and PE, respectively. The boiling n-heptane soluble fraction (C7-sol) was mainly composed of ethylene-propylene segmental copolymer (EPS) and small amount of low molecular weight PE. As the switching frequency (SF) was increased from 1 to 30, EPR content first decreased then rose to the highest, while EPS and EPS/EPR ratio showed an increasing trend. When the SF increased to 30, compositions of the two mixtures cannot transform completely during the switching process. Ethylene content in the EPS fraction tended to increase with increase in SF, which is ascribed to the formation of EPS with longer ethylene sequences. The PE/EPR produced at a SF of 30 showed the best toughness, while that produced at a SF of 6 exhibited the best rigidity-toughness balance. Furthermore, PE/EPR prepared by PSPP showed excellent low-temperature toughness.The influences of gas composition and switching sequence in a PSPP cycle and the amount of hydrogen as chain transfer agent on the structure and properties of PE/EPR were further studied in this work. Increase in propylene content in the comonomers lead to the formation of EPS with higher propylene content, stronger blockiness and longer propylene sequences, which is unfavorable to the compatibility between EPR dispersed phase and PE matrix. So the mechanical properties of the alloy became worse. In the case of similar EPR content, changing the switching sequence in a PSPP cycle can realize a uniform distribution of EPR phase in the PE matrix, which can improve the tensile properties of PE/EPR.In order to further improve the properties of PE/EPR alloys to make it similar to an elastomer material, the PSPP operation parameters were modified to conduct repeated switches between two ethylene-propylene copolymerizations with different monomer compositions. Meanwhile, the long continuous gas-phase copolymerization stage before the PSPP operation was canceled. Polymerization activity was excessively improved as the propylene content in gas tank T2 increased. When it reaches and exceeds 30 mol%, the PE/EPR alloys exhibited good particle morphology and contain high EPR content. Compared with the two-stage sequential polymerization process, PSPP process can significantly improve the tensile properties of PE/EPR alloys. The proportion of gas-phase copolymerization product can be improved by a higher monomer pressure, but the extent of EPR improvement is also related to the monomers composition and copolymerization duration. The highest proportion of gas-phase copolymerization product in PE/EPR is more than 70 wt%, and EPR content is 55.6 wt%. This sample (S18) has lower modulus, and no yielding appears in the tensile curve, which is in accord with the characteristics of reactor-type olefinic thermoplastic elastomer (RTPO). However, its tensile strength is not high enough without vulcanization.Three RTPO (PP/EPR) prepared at different conditions, along with a commercial EPDM were respectively applied to toughen isotactic PP by melt-mixing. It was found that the impact strength of PP/PE/EPR blends gradually increased, while the flexural strength and flexural modulus decreased with the content of toughening agent. Sample S18 showed the best rigidity-toughness balance. Toughening effect of the PE/EPR alloys is significantly better than EPDM. EPR-PE phase disperses in PP matrix in the form of a shell-core structure, which plays a toughening and strengthening effect on the matrix. Meanwhile, the segmental copolymers in PE/EPR enhance the compatibilities of EPR with PE and EPR with PP. The more EPR content is, the thicker and more perfect shell will be formed. This may lead to a better dispersion of the particles, and significantly improves the mechanical properties of PP matrix. In conclusion, PE/EPR RTPO has broad application prospects in plastics toughening. Besides, granulated RTPO product can be obtained directly in the polymerization process, which eliminates the granulation process and save much energy as compared with the EPDM production. However, the RTPO product still contains high content of crystalline components, resulting a weaker elastomer characteristic.Slurry-gas phase two-stage sequential polymerization process was applied to prepare linear low-density polyethylene (LLDPE) with a broad molecular weight distribution by using BCSII industrial catalyst. Ethylene/1-hexene slurry copolymerization at lower 1-hexene concentration (0.4 mol/L) was first conducted, followed by ethylene/1-hexene gas phase copolymerization at higher hexene initial concentration (200 mmol/L in gas phase). Ethylene/1-hexene copolymer product with lower and higher hexene content was respectively synthesized in the two different stages. Exploratory studies on the characteristics of ethylene/1-hexene slurry and gas-phase copolymerization were conducted in this work. The effects of 1-hexene initial concentration, catalyst modifier, cocatalyst, polymerization temperature and monomer pressure on the chemical composition distribution (CCD), melting point and melting enthalpy, molecular weight and MWD, tensile properties of two-stage copolymers were systematically studied. Fraction distribution of gas-phase product and its hexene content were also estimated. It was found that the proportion of gas-phase product gradually decreased but its hexene content increased with an increase of 1-hexene initial concentration. When the 1-hexene concentration reached 200 mmol/L, the total hexene content seemed to be stable, which is related to particles absorption on the liquid hexene. The addition of catalyst modifier resulted in reduction in the activity of the product and deterioration in the mechanical properties, but has little effect on the 1-hexene content. Using TEA/TiBA mixture as cocatalyst, the product has a relative higher activity and hexene content, narrower CCD and a broad MWD. The suitable temperature and pressure in gas-phase polymerization were 70℃ and 0.4 MPa. The two-stage copolymer prepared at 70℃ and 0.4 MPa has higher polymerization activity and 1-hexene content, a relative uniform CCD and excellent tensile properties.