The Sintering Kinetics Of UHMWPE And Preparation Of High-performance Sintered Materials

Ultra-high molecular weight polyethylene(UHMWPE)is one of the high-end polyethylene materials,which has excellent wear resistance,impact toughness,corrosion resistance,chemical resistance and self-lubricating properties.However,for commercial UHMWPE,the melt viscosity is very large due to the large number of entanglements in the inter-and intramolecular chains,which causes the poor processability.As a result,the process of sintering inspired from metal powders has been developed for the synthesis of UHMWPE commodity with a desirable shape.Reducing the initial chain entangled state and particle size of nascent UHMWPE were two main methods to improve the processability and mechanical properties of sintering materials,but the contribution of initially entangled state and particle size to the sintering kinetics during sintering process was still unclear,which greatly limited the development of high-performance polyethylene products.This thesis aimed to study the structure-property relationship between particle size as well as initially entangled state of nascent UHMWPE and mechanical properties of sintered materials.By studying the influence of UHMWPE nascent particle size on sintering kinetics and chain-reentanglement behavior under different entangled states,the influence of particle size and chain entangled state on sintering behavior and mechanical properties of UHMWPE sintered materials were systematically explored,and it has been clarified that less entangled state and small particle size are the key to the preparation of high-performance UHMWPE products.On this basis,the mass transfer and dissolution process of ethylene were adjusted by changing the polymerization medium to reduce the chain growth rate during the polymerization process.The regulation of nascent particle size and initial entangled state of UHMWPE is realized under different catalytic systems,and a high-performance UHMWPE sintered material was finally obtained.The main work and research results are as follows:(1)A series of highly entangled UHMWPE samples with different particle sizes were prepared with high activity using a Ziegler-Natta catalyst,while the influence of particle size on the primary structure of nascent UHMWPE,chain-reentanglement sintering behavior,and mechanical properties of sintering materials were studied in detail.It showed that the chain diffusion and reentanglement behavior in highly entangled UHMWPE was greatly restricted by a large number of physical entangled points and stable entanglement regions,and sideways motions of chain segments were more difficult to occur at the sintering temperature,which caused a slower chain reentanglement in the particle interface region and greatly affected the performances of sintering materials.However,small particles can strengthen the chain explosion and promote chain diffusion as well as reentanglement in the particle interface area due to the larger sintered specific area,and thereby improve the tensile properties of the sintered material.A quantitative analysis method based on the coordinate system of the particle size and the build-up rate of reentanglement was proposed for the first time to quantify the chain reentanglement process.It was found that for the highly entangled UHMWPE,the build-up rate of reentanglement has a negative linear relationship with the average particle size(D50),while the chain reentanglement behavior and the mechanical properties of the sintered material had an obvious time dependence.In addition,the nascent UHMWPE with a wide particle size distribution had poor mechanical properties due to the slow sintering kinetics,which emphasizes the necessity of sieving for highly entangled UHMWPE.(2)A series of less entangled UHMWPE samples with different particle sizes were prepared with low activity using a Ziegler-Natta catalyst,while the influence of particle size on the primary structure of nascent UHMWPE,chain-reentanglement sintering behavior,and mechanical properties of sintering materials were studied in detail.It showed that the chain diffusion and reentanglement behavior in highly entangled UHMWPE was not constrained by the topological structure of chain entanglement points because the initial chain-entanglement degree in less entangled UHMWPE was significantly reduced.As a result,the samples of different particle sizes can reach a similar thermal stability state in a short time through rapid chain reentanglement,showing similar tensile strength for all samples.Besides,the chain reentanglement was faster for fine particles with larger sintered specific area,which can approach a thermally stable state within 5 minutes and show outstanding toughness.Therefore,due to the fast sintered kinetics,the linear relationship of less entangled UHMWPE between the build-up rate of reentanglement and average particle size(D50)was completely opposite to that of the highly entangled UHMWPE,and the chain reentanglement behavior of UHMWPE and mechanical properties of the sintered material were not depended on the sintering time in a long-time range(15-60 min).It was further emphasized that less entanglement was the key to the preparation of high-performance UHMWPE products,while the sintered material of the wide particle size distribution UHMWPE which were not sieved exhibited the characteristics of high performance with balance of strength and toughness.(3)On the basis of clarifying the structure-property relationship between the particle size as well as the initially entangled state of nascent UHMWPE and the mechanical properties of the sintered material,the polymerization medium was changed to control the mass transfer and dissolution process of ethylene in order to prepare high-performance UHMWPE products with less entanglement and small particle size.The influence of the polymerization medium on the polymerization behavior of ethylene,the particle size of nascent UHMWPE,initially entangled state and the mechanical properties of sintered materials were explored.It showed that a mixture of the high boiling point solvent and low boiling point solvents with different proportions can be used as the polymerization medium.As the volume fraction of low boiling point solvents decreases,the solubility and liquid phase mass transfer coefficient of ethylene gradually decrease,which greatly affected the kinetics of ethylene slurry polymerization.By adjusting the volume fraction of the low boiling point solvent in the polymerization medium,it was possible to achieve coordinated control of the particle size of nascent UHMWPE and the initially entangled state.Therefore,UHMWPE with smaller particle size and lower entangled state can be prepared under the conventional Ziegler-Natta catalyst and the POSS modified Ziegler-Natta catalyst(POSS-Cat)respectively.In the POSS-Cat catalytic system and polymerization medium of 100%high boiling point solvent,the prepared UHMWPE had a very low initial entangled state due to the blocked active center and low chain growth rate,and the sintered material exhibited much more excellent tensile strength(38.1 MPa)and impact strength(109.9 kJ/m2),achieving the preparation of the high-performance UHMWPE sintered materials.