The Synthesis Of Weakly Entangled UHMWPE And Its In-situ Blending With HDPE

Ultra-high molecular weight polyethylene(UHMWPE)is one of the high-end polyethylene materials,and has good impact resistance,corrosion resistance,friction resistance,and self-lubricating properties.Up to now,in the commercial UHMWPE resin particles,there exist a large number of physical entanglement points between and within the UHMWPE chains,which limits the creep and relaxation behavior of long-chain polyethylene.A mass of chain entanglements of UHMWPE greatly increase the melt viscosity,crystal defects,and the processing difficulty.As a result,the mechanical properties are only 1/3 of the theoretical value.In addition,entanglements also limit the microscope blending behavior,orientation behavior,and simultaneous enhancement effect(strength,stiffness,and toughness)of UHMWPE with other polyolefin materials.Therefore,reducing the chain entanglements can not only lower the processing difficulty,but also increase the amount of UHMWPE mixed in the blend,leading to the further enhancement of mechanical properties.In this paper,from the perspective of the separation of the active sites of heterogeneous catalysts,POSS blockers were supported on the Ziegler-Natta catalysts surfaces(hereinafter abbreviated as ZN catalyst)to achieve the weakly entangled UHMWPE resins at high temperature and high activity.The method for studying the entanglement formation mechanism of the heterogeneous catalytic system was proposed.The sequential mixing of weakly entangled UHMWPE and high-density polyethylene(HDPE)was achieved in a single reactor.The effects of blending amount and entanglement density of UHMWPE in the UHMWPE/HDPE blends on the orientation structures and mechanical properties were studied.As a result,the high-performance HDPE with simultaneous improvement of strength,stiffness,and toughness was achieved.Ti/V bimetallic catalyst was supported on the POSS modified support to produce the branched UHMWPE and the linear weakly entangled UHMWPE.The influence of UHMWPE's branched structure on the blend orientation and relaxation behavior was further studied,and the mechanical properties of HDPE were further improved.Based on the results of laboratory pilot studies,the 10 L and 300 L industrial scale-up tests of ZN catalyst modified with POSS were successfully completed and high-performance HDPE products were successfully developed.The main work and research results are as follows:(1)The preparation of POSS modified ZN catalyst and entanglement formation mechanism of the synthesized nascent polyethylene.By studying the interaction between POSS,MgCl2,and TiCl4,it was found that POSS molecules can coordinate with MgCl2 and TiCl4 molecules,and MgCl2 molecules are more likely to interact with hydroxyl groups on POSS molecules to form stable nano-aggregates.The active sites and corresponding catalytic activity of the catalyst under different chemical microenvironments were explored.It was found that the TiCl4 active sites loaded on the POSS/MgCl2 nano-aggregates had very low activity,and the catalytic activity was mainly concentrated on the TiCl4 molecule loaded on ?-MgCl2.This indicates the nano-aggregates do not have polymerization activity,can separate the active sites,and inhibit the bimetallic deactivation phenomena to improve the activity.For the first time,the polymerization activity and initial elastic modulus were used as coordinate systems to quantitatively characterize the entanglement formation mechanism of heterogeneous catalyst.UHMWPE's initial elastic modulus G'(t=0)shows a linear single-factor relationship with catalytic activity at different polymerization time and[Al]/[Ti]ratios;the initial elastic modulus G'(t-0)shows a multi-factor dependent behavior(power function relationship)with catalytic activity at different polymerization temperatures;the nearly exponentially decrease of initial elastic modulus G'(t=0)with higher activity of more POSS/MgCl2 isolators.POSS/MgCl2 nano-aggregate separators can effectively compensate for the increase of entanglement caused by the increase in polymerization temperature,so that the influence of the polymerization temperature on the entanglement is significantly reduced,thereby producing weakly entangled UHMWPE with high activity at high temperature.In addition,by studying the entanglement density of nascent polyethylene,it was found the residual ash of catalyst has strong effect on the chain entanglement establishment.The small amount of ash uniformly dispersed in the polyethylene product has a strong interaction with the polyethylene segments,which can significantly promote the chains crystallization and inhibit the entanglement.(2)Sequential blending of weakly entangled UHMWPE/HDPE.Based on a systematic study of the catalytic capabilities(copolymerization performance,temperature sensitivity,and hydrogen sensitivity)of the above-mentioned catalyst,weakly entangled UHMWPE was prepared in a single reactor,and then hydrogen was added to produce HDPE around the original active sites to achieve UHMWPE and HDPE sequential mixing blends.The effects of UHMWPE content and entanglement density on the mechanical properties of the blends were studied.The results show that the increase of weakly entangled UHMWPE content is essential for enhancing the mechanical properties.The UHMWPE content in the weakly entangled Dis-UH/HD blend can be as high as 30 wt%.The tensile strength,Young's modulus,and impact strength of the Dis-UH/HD-30 blend can reach 52.4 MPa(+97.7%),604.2 MPa(+43.6%),and 74.4 kJ/m2(+675%).This enhancement effect is mainly attributed to the large number of shish-kebab crystalline structures exist in the injection molded samples.The difference in the effect of linear UHMWPE with different entanglement density on the mechanical properties of UH/HD blends is mainly reflected in the impact strength.The weakly entangled UHMWPE chains can not only form shish-kebab crystals during the injection process,but also co-crystallizes with the HDPE chains and acts as tie molecules to connect the kebab lamellas in the shish-kebab structures and partially oriented stacked lamellas.(3)The enhanced orientation behavior of UHMWPE/HDPE blends.Due to the strong molecular chain motion behavior and fast diffusion speed of weakly entangled UHMWPE,the orientation structure formed under the shear flow field during the injection molding process will relax,and the mechanical properties of the blend do not reach the expected performance.Therefore,it is proposed that a small number of branched UHMWPE chains blended in-situ with the weakly entangled UHMWPE to achieve the enhancemend orientation behavior.In this chapter,the support modified with POSS was first loaded with VOCl3,and the ethylene polymerization capabilities(copolymerization performance,temperature sensitivity,hydrogen sensitivity,and POSS separation effect)of the catalyst were systematically studied.It was found that the copolymerization ability of the V active sites was high.Subsequently,combined with the research results of the supported TiCl4 catalyst,a POSS modified Ti/V bimetallic catalyst was designed to introduce branched UHMWPE around the weakly entangled UHMWPE.The effects of short-chain branch contents of weakly entangled UHMWPE on the mechanical properties and compatibility of UH/HD blends were discussed.The results show that the weakly entangled UHMWPE with a small number of short-branched chains can inhibit the relaxation behavior of the shish crystals during the injection molding process,and retain more shish crystals and shish-kebab strings in the injection spline.As a result,the mechanical properties(strength/stiffness/toughness)of the injection splines can be significantly enhanced.The tensile strength,Young's modulus,and impact strength of the B10-Dis-UH/HD-10 blend can reach 72.4 MPa,704.9 MPa,and 87.0 kJ/m2.(4)The industrial scale-up of POSS modified ZN catalyst.According to the step-by-step scale-up process,industrial small-scale scale-up was first carried out in a 10 L autoclave.The results show that the properties(catalytic activity,bulk density,and mechanical properties)of the prepared weakly entangled UHMWPE/HDPE blend are consistent with the results of the 1 L test in laboratory.The weakly entangled UHMWPE/HDPE reactor blend synthesized in the pilot scale of 300 L has a significant increase in the bulk density of the product,reaching 0.35-0.38 g/cm3,and the catalytic activity reaches 35000 g PE/g cat.Parameters like bulk density,density,and melt index all meet the requirements of PE 100 tube materials,and the product's strength,stiffness,and toughness are better than PE 100 tube materials.The 300 L pilot test results show that the polymerization activity and kinetic characteristics of the POSS modified ZN catalyst both meet the needs of industrial production and the product has excellent mechanical properties.So,the catalyst shows great potential for further industrial application development.