| High-density polyethylene (HDPE) pipe material, which owns a number of advantages, such as environment-friendly, wear resistant and environmental stress crack resistant etc., has been regarded seriously. The HDPE pipe material is mainly used for the gas and water transport with a gradual increase of market share. It is manufactured from ethylene/a-olefin copolymers using mostly two-stage polymerization processes. However, there are still lots of disadvantages, such as complex process, high operation requirements and high-energy consumption etc. until now. Moreover, these disadvantages seem particularly important with the background of energy-saving and environmental protection demands nowadays. Even though, the technology of the two-stage processes and the catalysts thereof are still monopolized by several developed countries. Hence, in order to fit the one-stage process with low cost and energy consumption, it is very important to develop novel catalysts for producing HDPE material with independent intellectual property right.Therefore, in this work, first the SiO2-supported silyl-chromate S-2catalyst used for producing HDPE in the industry was investigated. Several vital factors such as the different cocatalyst, the dosage of cocatalyst and1-hexene comonomer etc. were systematically investigated. And then the vanadium oxide and imido vanadium active components were chosen to be introduced into the S-2catalyst and obtained two series of novel SiO2-supported bimetallic catalyst named SiO2-supported silyl-chromate(Cr)/vanadium(V)-oxide (Cr-V) bimetallic catalyst and SiO2-supported silyl-chromate(Cr)/imido-vanadium(V)(Cr-imidoV) bimetallic catalyst. Taking the unique features of the vanadium active components to improve the molecular weight (MW), molecular weight distribution (MWD), short chain branches (SCBs) and short chain branches distribution (SCBD) of their product to further improve the purpose of the long-term mechanical properties of HDPE pipe materials. And the vital factors such as the different vanadium active components and the addition amount of vanadium active components, dosage of cocatalyst and1-hexene comonomer, polymerization temperature and hydrogen etc. which could influence the bimetallic catalysts ethylene homopolymerization and ethylene/1-hexene copolymerization behaviors and the microstructures of resultant polymers, were also systematically investigated.As for the polymerization kinetic study, the proper dosage of alkyl aluminium cocatalyst with the silyl-chromate S-2catalyst, vanadium oxide V catalyst and imido vanadium t-iV catalyst was Al/Cr or Al/V=20, the highest activity of S-2catalyst was975kgPE/molCr.h, and the proper dosage of alkyl aluminium cocatalyst with Cr/V bimetallic catalysts was Al/Cr=30. Triisobutylaluminium (TIBA) as the cocatalyst showed higher activity than the triethylaluminium (TEA) cocatalyst. In respect of Cr/V bimetallic catalysts, the mole ratio of Cr/V=1:1was the proper relative addition amount of vanadium active component. In bimetallic catalysts, comparing the a-iVCrl:1(1637kgPE/molCr.h) with the allyl ligand imido-vanadium and the VCrl:1(1221kgPE/molCr.h) with vanadium oxide, the t-iVCrl:1(2052kgPE/molCr.h) with the p-tolyl ligand imido-vanadium showed the highest activity, and compared with the traditional S-2catalyst, the activities of these novel catalysts increased by68%,25%and110%respectively. The XPS results confirmed the existence of synergistic effect between Cr and V species. Moreover, the addition of1-hexene comonomer and hydrogen could decrease the activity.In the aspect of the MW and MWD of the polymers, with the increasing dosage of cocatalyst, the high MW part of the product increased. The polyethylene obtained from S-2catalyst with TIBA cocatalyst showed more high MW part and for TEA cocatalyst could obtain the polyethylene with more low MW part, and the S-2catalyst with mix cocatalyst TIBA/TEA=1:1(mixl:1) could produce bimodal polyethylene. In addition, most polyethylene obtained from Cr/V bimetallic catalysts showed bimodal MWD. The increasing dosage of vanadium active component and the declining polymerization temperature could improve the high MW part of the product. On the contrary, with the addition of1-hexene comonomer and hydrogen could increase the low MW part.Finally,13C-NMR and TREF+SSA methods were using to character the SCBs and SCBD of HDPE samples. It was found that the HDPE obtained from S-2catalyst with TIBA cocatalyst showed the best SCBD than that with TEA cocatalyst. Comparing the SiO2-supported silyl-chromate S-2catalyst and SiO2-supported Cr-V bimetallic catalyst VCrl:1, it was found the copolymer obtained from VCrl:1bimetallic catalyst showed lower relative SCBs content in the low MW part and slightly higher relative SCBs content in the high MW part. Thus, it was suggested that the addition of vanadium active component in S-2catalyst was an effective method to increase1-hexene incorporation and improve the SCBD of the HDPE products. The13C-NMR results showed that the content of1-hexene incorporated in the copolymer obtained from t-iVCrl:1was0.53mol%, this value was further improved compared with VCrl:l (0.29mol%) and S-2(0.16mol%).In summary, in comparison with traditional S-2catalyst, the novel SiO2-supported Cr/V bimetallic catalysts showed more excellent comprehensive performance in several aspects such as polymerization activity and MW, MWD, SCBs and SCBD of their product etc. The research and development of these two series of novel SiO2-supported Cr/V bimetallic catalysts is an innovative attempt for the traditional industrial Cr-based catalyst. At present the Cr-V bimetallic catalyst and Cr-imidoV bimetallic catalyst are under further pilot-plant scale research by the Qilu Branch Co., SINOPEC and Daqing petrochemical research center, PetroChina, respectively. This research lays a solid foundation for further improvement of HDPE PE100pipe materials within one-stage polymerization process. |
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