Preparation Of Low Entanglement Ultra-High Molecular Weight Polyethylene By Ziegler-Natta Catalysts Modified With Functionalized Additives

UHMWPE is an engineering plastic with excellent performance and has important applications in medical,defense,and other fields,contributing to many scientific and technological advances.However,due to the existence of a large number of physical entanglement between and within the ultra-long polyethylene chains,which greatly restricts the movement and relaxation behavior of polyethylene chains,increases its melt viscosity,leads to processing difficulties,and the mechanical properties are also affected by chain entanglement,which is only 1/3 of the theoretical value.The chain crystallization rate is greater than the chain growth rate to inhibit the occurrence of chain entanglement,or increase the spacing of polyethylene chains to reduce the probability of chain entanglement.Currently,the production of low entanglement UHMWPE mostly adopts low polymerization reaction temperature and very dilute catalyst concentration to increase the chain crystallization rate and decrease the chain growth rate,thus inhibiting the entanglement between the chain segments,but the lower temperature leads to lower catalytic activity,which seriously limits the application of such processes.Therefore,reducing the degree of entanglement of primary UHMWPE at the catalyst design level has become a hot research topic.In this thesis,the mechanism of action of modifier-modified catalysts was proposed by studying the molecular structure and active center state of the modified catalysts,starting from the modification of commercial Ziegler-Natta catalysts with functionalized hybrid additives(modifiers).Based on the above-mentioned modified catalysts,low entanglement UHMWPE was prepared in a 1 L reactor with high activity,and the effects of polymerization medium as well as polymerization temperature and time on the molecular weight and entanglement state of the products were investigated.The paper further investigated the relationship between the re-entanglement behavior and mechanical properties of the low entanglement UHMWPE sintered materials,and systematically studied the rheological behavior of UHMWPE gels.The main findings of the paper are as follows.(1)The modified catalyst particles are well shaped and have a typical mesoporous structure.During the modification process,the Si-O-Si and Si-OH groups on the functionalization additive coordinated with some TiCl4 on the catalyst and carried Ti into the solvent and finally lost with the filtrate.It was shown that the active centers in the modified catalysts became dispersed because the modifier molecules "snatched"some of the metal centers Ti from the catalysts by coordination with TiCl4,which suppressed the bimetallic deactivation effect and produced more effective active centers Ti(Ⅲ),and when reacting with alkyl aluminum,the modified catalysts When reacting with alkyl aluminum,the modified catalyst completed the activation process faster and the reduction of the active center was deeper,which was beneficial to catalyze the ethylene polymerization.(2)Based on the loaded Ziegler-Natta catalyst modified with the above functionalization additive,the ethylene polymerization behavior was investigated,and it was found that the ethylene catalytic activity of the modified catalyst was higher than that of the original catalyst,and the produced UHMWPE products had a lower primary ecological entanglement density with a minimum reduction of 47%,indicating that the modifier produced more effective It also increases the distance between polyethylene growth chains,thus reducing the probability of chain entanglement and resulting in a lower entanglement density of the polymerized product.In high viscosity solvent B,the catalytic lifetime of the modified catalyst was up to 12 hr,and the molecular weight of the polymerization product increased from 6 to 10 million g/mol as the polymerization reaction temperature decreased and the reaction time increased,while the initial ecological entanglement density of UHMWPE was lower,indicating that longer ethylene polymerization contributed to the formation of a polymer with high molecular weight and low entanglement.The low entanglement property on higher molecular weight UHMWPE was successfully achieved,which provides a basis for the preparation of high modulus and high strength UHM WPE products.(3)High temperature tensile tests and rheological analysis show that the higher the molecular weight of low entanglement UHMWPE,the faster the re-entanglement rate of its sintered material is established,and Dis-PE-1000,which has a low degree of initial ecological entanglement but the highest molecular weight,has the highest reentanglement network density,giving its sintered material the most excellent tensile properties,much higher than commercial UHMWPE PE-1000.The low entanglement properties of UHMWPE remain in the gel,contributing to the strength and tensile multiplicity of the spun fiber.It is shown that UHMWPE gels at high temperature first undergo chain explosion and chain entanglement establishment in the high concentration region generated by the melting of polyethylene particles,while accompanied by shearing,the macromolecular chains undergo diffusion to the lower concentration region and gradually unentangle;as time progresses,the rate of unentanglement begins to be greater than the rate of entanglement establishment,and the elastic modulus begins to decrease.The rheological characterization of the preannealed gels demonstrates that the entanglement density of the fully unentangled polymer gels is extremely low and no longer increases with time.