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Research progress on modification of UHMWPE

Jun 28, 2019

Physical modification: The so-called physical modification refers to the mechanical blending of resin with one or more other materials to achieve certain special requirements, such as reducing the melt viscosity of UHMWPE, shortening the processing time, etc. Change the molecular configuration, but can give the material new properties. At present, the commonly used physical modification methods mainly include low melting point, ground viscosity resin blending modification, flow agent modification, liquid crystal polymer in-situ composite modification, and filler blending composite modification. It is the most effective, easiest and most practical way to improve the flow of UHMWPE melt. Among them, the special special compound flow modifier MS2 of Beijing University of Chemical Technology has realized continuous extrusion on the specially developed UHMWPE single-screw extruder, and the performance of the products has not changed much, and the effect is good. The industrial production has been realized, and the UHMWPE used is sticky. The average molecular weight is reached, and the added composite flow modifier is generally less than 5%, wherein the processing temperature (100~240 ° C) is significantly reduced, the screw rotation speed can reach 35 r / min, and the various specifications of the pipe and the bar can be smoothly extruded. Qingdao University of Science and Technology has studied the processing properties of UHMWPE by using calcium stearate (CaSt2) and internal and external lubricants. The results show that CaSt2 can significantly improve the processing properties of UHMWPE without causing tensile strength and impact strength. The reduction of the internal and external lubricants is second to the modification effect of the system, and the modification effect with the internal lubricant is the worst. Tsinghua University used aluminum oxide, manganese dioxide, carbon black and glass microbeads as fillers to improve the hardness, heat distortion temperature and wear resistance of UHMWPE. It was found that an appropriate amount of glass microspheres can improve the wear resistance of UHMWPE by about 40%, and when the notched impact strength is maintained at 70%, the heat distortion temperature can be increased by 30 to 40 ° C, but the addition of the above fillers leads to the notched impact strength. As the filler content increases, the notched impact strength of carbon black-filled UHMWPE decreases rapidly, and the glass beads decrease slowly. At the same time, it was found that the surface resistivity of the system reached Q&#8226;m when 4% of expanded graphite was added to UHMWPE. When modified with 4 antistatic agent and a synergist with 3% content, the surface resistivity was less than Q& #8226;m (meet the requirements). The Institute of Chemistry of the Chinese Academy of Sciences has studied the modification of UHMWPE with nano-scale layered silicate. Since the bonding force between the layers of layered silicate is relatively weak, the friction coefficient is small, and the relative sliding between the layers can be used to improve UHMWPE. The fluidity of the melt, thereby improving its processing properties, and the internal structure of the sheet is tight and the rigidity is high, and the performance of UHMWPE is enhanced in a two-dimensional direction. <BR>&nbsp;&nbsp;&nbsp; 3.2 Chemical modification: Chemical modification mainly includes two methods: chemical crosslinking modification and radiation crosslinking modification. <BR>&nbsp;&nbsp;&nbsp; 3.2.1 Chemical cross-linking modification is to change the molecular structure or molecular morphology of the resin to obtain new properties of the resin. This method can not only change the properties of a resin, but also produce a new resin material. By cross-linking, the crystallinity of UHMWPE decreases, and the toughness that is masked is manifested. It can be divided into two methods: peroxide crosslinking and coupling agent crosslinking. UHMWPE has a bulk structure after oxide crosslinking but is not completely crosslinked, so it has thermoplasticity and excellent hardness, toughness and resistance to stress cracking. Tsinghua University uses DCP as a cross-linking agent to cross-link modified ultra-high molecular weight polyethylene. The dosage of DCP is generally controlled within 1%. When the amount of DCP is 0.25%, the impact strength can be Increase by 48%. As the amount of DCP increases, the heat distortion temperature also increases. The coupling agent in the cross-linking of the coupling agent mainly has two types of silane coupling agents, a vinyl siloxane and a propylene siloxane. The molding process of silane cross-linked UHMWPE first decomposes the peroxide into a highly chemically active radical. These free radicals take hydrogen atoms in the polymer molecule to change the polymer backbone into active radicals, and then react with the silane. After the branch reaction, the grafted UHMWPE undergoes hydrolysis and condensation under the action of water and a silanol condensation catalyst to form a crosslink bond to obtain a silane crosslinked UHMWPE. <BR>3.2.2 Radiation cross-linking modification is the direct irradiation of UHMWPE products by electron beam or -ray to crosslink the molecules, under the action of a certain dose of electron rays or other rays, in the ultra-high molecular weight polyethylene molecule. A part of the main chain or the side chain is cut by the ray to generate a certain amount of radicals which are combined with each other. A crosslinked chain is formed inside the ultrahigh molecular weight polyethylene to achieve the purpose of crosslinking modification. The radiation cross-linking reaction mainly occurs on the surface of the polymer, and does not affect its internal structure and properties. After a certain dose of irradiation, the physical properties such as creep, oil immersion and hardness of UHMWPE are improved to some extent. Irradiation of artificial UHMWPE joints with radiation, cross-linking while disinfecting, enhances the hardness and hydrophilicity of the artificial joints, and improves creep resistance, thereby extending its service life. In addition, the use of radiation to graft UHMWPE with polytetrafluoroethylene (PTFE) can also improve the wear and creep behavior of UHMWPE. <BR>&nbsp;&nbsp;&nbsp; 3.3 Polymer Filling Modification: Polymerization filling process is a new type of polymerization method in polymer synthesis, which is to treat the surface of the particles to form an active center after the filler is processed. The olefin monomer such as ethylene and propylene is polymerized on the surface of the filler particles to form a resin which closely encapsulates the particles, and finally a composite material having unique properties is obtained. In addition to the properties of the blended composite material, it also has its own characteristics, that is, it does not need to melt the polyethylene resin, can maintain the shape of the filler, prepare a powdery or fibrous composite material, and is not composed of the filler and the resin. The ratio of the filler can be arbitrarily set, and the obtained composite material is uniform, which makes the tensile strength and impact strength of the composite material have little difference with UHMWPE, and the hardness and bending strength of the composite material, especially The flexural modulus is much higher than that of pure UHMWPE, and the thermodynamic properties of the composite are also improved. By adding hydrogen or other chain transfer agents to the polymerization system, the molecular weight of UHMWPE can be controlled to make the resin easier to process, such as crystalline alumina, silica, water insoluble silicate, calcium carbonate, basic sodium aluminum carbonate. Hydrocarbon-based wollastonite and calcium phosphate can be made into high-modulus homogeneously packed UHMWPE composites; UHMWPE composites synthesized by using diatomaceous earth and kaolin as fillers have better comprehensive performance than blended materials. <BR>&nbsp;&nbsp;&nbsp; 3.4UHMWPE self-reinforcing modification: UHMWPE fiber is added to the UHMWPE matrix. Because the matrix and fiber have the same chemical properties, the chemical compatibility is good, and the interface of the two components is strong. , a composite material with excellent mechanical properties can be obtained. The addition of UHMWPE fiber can greatly improve the tensile strength, modulus, impact strength and creep resistance of UHMWPE. Compared to pure UHMWPE, the addition of 60% UHMWPE fiber to UHMWPE increases the maximum stress and modulus by 160% and 60%, respectively. This self-reinforced UHMWPE material is especially suitable for biomedical weight-bearing applications, integral replacement of artificial joints, etc. The low volume wear rate of this material can increase its service life.