(1) press sintering is the most primitive processing method of ultra-high molecular weight polyethylene (UHMW-PE). This method has low production efficiency and is easy to be oxidized and degraded. In order to improve production efficiency, direct electric heating can be used.
(2) Ultra-high-speed sintering process, using a blade mixer, the maximum rotating speed of the blades can reach 150m/s, so that the materials can be raised to the processing temperature in only a few seconds.
2. Extrusion molding
Extrusion molding equipment mainly includes plunger extruder, single screw extruder and twin screw extruder. Co-rotating twin-screw extruder is often used for twin-screw extrusion.
Plunger extruders were mostly used in the 1960s. In the mid-1970s, Japan, the United States and West Germany successively developed single-screw extrusion technology. Mitsui Petrochemical Company of Japan achieved the success of round rod extrusion technology as early as 1974. 1At the end of 1994, China developed φ 45 single-screw extruder for UHMW-PE, and 1997 achieved the success of φ 65 single-screw extrusion pipe industrial production line.
(3) injection molding
Mitsui Petrochemical Company of Japan developed the injection molding technology in 1974 and commercialized it in 1976, and then developed the reciprocating screw injection molding technology. 1985, Hoechst Company of the United States also realized the screw injection process of UHMW-PE. 1983 reformed the domestic XS-ZY- 125A injection machine, and successfully injected UHMW-PE roller for beer canning production line and shaft sleeve for water pump. 1985 also successfully injected medical artificial joint.
(4) blow molding
When processing ultra-high molecular weight polyethylene (UHMW-PE), when the material is extruded from the die, it will shrink to some extent due to elastic recovery, and almost no sagging will occur, thus creating favorable conditions for blow molding of hollow containers, especially large containers, such as oil tanks and vats. Ultra-high molecular weight polyethylene (UHMW-PE) can also obtain high-performance films with balanced longitudinal and transverse strength, thus solving the long-standing problem that HDPE films are inconsistent in longitudinal and transverse strength and prone to longitudinal damage. 1. Gel spinning
(1) development process
Preparation of high strength and high modulus polyethylene fiber by gel spinning-ultra-stretching technology is a new spinning method which appeared in the late 1970s. The Dutch DSM company first applied for a patent at 1979, and then the American Allied Company, the Japanese-Dutch joint venture Toyo Textile DSM Company and the Japanese Mitsui Company all realized industrial production. The Chemical Fiber Research Institute of China Textile University started the research of this project from 1985, and gradually formed its own technology to produce high-performance UHMW-PE fiber.
(2) Spinning technology
The spinning process of ultra-high molecular weight polyethylene (UHMW-PE) gel is briefly described as follows: UHMW-PE is dissolved in a suitable solvent to make a semi-dilute solution, which is extruded through a spinneret, and then the spinning solution is quenched with air or water to solidify into frozen collagen filaments. In the frozen collagen filament, almost all solvents are contained in it, so the unwinding state of UHMW-PE macromolecular chain is well maintained, and the drop of solution temperature leads to the formation of UHMW-PE folded chain sheet layer in frozen colloid. In this way, by superheating the frozen collagen filament, the macromolecular chain can be fully oriented and highly crystallized, and then the chain-folded macromolecules can be transformed into straight chains, thus making high-strength and high-modulus fibers.
(3) Application
Ultra-high molecular weight polyethylene (UHMW-PE) fiber is the third generation special fiber in the world, with the strength as high as 30.8cN/dtex, which is the highest specific strength among chemical fibers, and has good wear resistance, impact resistance, corrosion resistance and light resistance. Can be directly made into ropes, cables, fishing nets and various fabrics: bulletproof vests and clothes, cut-proof gloves, etc. Among them, the bulletproof effect of bulletproof vest is better than aramid fiber. Ultra-high molecular weight polyethylene (UHMW-PE) fibers have been woven into ropes with different denier to replace traditional steel wire ropes and synthetic fiber ropes. Ultra-high molecular weight polyethylene (UHMW-PE) fiber composites have been used as the protective shells of armored weapons, radars and helmets. Sporting goods are made into bowstring, sleigh and water skiing.
2. Lubrication extrusion (injection)
Lubricating extrusion (injection) molding technology is to form a lubricating layer between the extruded (injected) material and the die wall, thus reducing the shear rate difference between different points of the material, reducing the deformation of the product, and at the same time improving the extrusion (injection) speed of high-viscosity polymers at low temperature and low energy consumption. There are two main methods to produce lubricating layer: self-lubrication and * * * lubrication.
(1) self-lubricating extrusion (injection)
Self-lubricating extrusion (injection) of ultra-high molecular weight polyethylene (UHMW-PE) is to add a proper amount of external lubricant to it, so as to reduce the friction and shear between polymer molecules and metal mold wall, and improve the uniformity of material flow, demoulding effect and extrusion quality. Lubricants for external use mainly include low molecular weight resins such as higher fatty acids, complex fats, silicone resins and paraffin wax. Before extrusion (injection) processing, the lubricant is first mixed into the material together with other processing AIDS. During production, the lubricant in the material oozes out to form a lubricating layer, thus realizing self-lubricating extrusion (injection).
It is reported in patent that 70 parts of paraffin oil, 30 parts of ultra-high molecular weight polyethylene (UHMW-PE) and 1 part of oxygen-phase silica (highly dispersed silica gel) can be mixed and granulated, and smooth extrusion (injection) can be realized at 190℃.
(2)*** Lubrication extrusion (injection)
There are two kinds of lubricating extrusion (injection) of UHMW-PE. Firstly, the lubricant is pressed into the mold by gap method to form a lubricating layer between the inner surface of the mold cavity and the molten material; The second is to mix with low viscosity resin to make it a part of the product.
For example, in the production of ultra-high molecular weight polyethylene (UHMW-PE) sheet, the appearance quality of the product is obviously improved by conveying SH200 silicone oil into the die cavity as a lubricant, especially due to the small extrusion deformation, the tensile strength is increased. Ultra-high molecular weight polyethylene (UHMW-PE) is filled with glass beads, glass fibers, mica, talcum powder, silica, alumina, molybdenum disulfide and carbon black, which can improve the surface hardness, stiffness, creep, bending strength and thermal deformation temperature. After treatment with coupling agent, the effect is more obvious. For example, filling glass beads can increase the thermal deformation temperature by 30℃.
Glass beads, glass fibers, mica and talcum powder can improve hardness, rigidity and temperature resistance; Molybdenum disulfide, silicone oil and special wax can reduce friction coefficient, thus further improving self-lubricity; Carbon black or metal powder can improve antistatic property, conductivity and heat transfer. However, the impact strength of the modified filler decreased slightly. If the content is controlled within 40%, the impact strength of UHMW polyethylene is still very high. Ultra-high molecular weight polyethylene (UHMW-PE) resin has a long molecular chain, which is easily destroyed by shear force or degraded by heat. Therefore, it is necessary to reduce the processing temperature, shorten the processing time and reduce the shear.
In order to solve the processing problem of ultra-high molecular weight polyethylene (UHMW-PE), in addition to the special design of the ordinary molding machine, the resin formula can be improved: mixed with other resins or added with flow modifiers, so that it can be molded on ordinary extruders and injection molding machines, which is the lubrication extrusion (injection) introduced in 2.2.2. * * * Mixing method is the most effective, simple and practical method to improve the fluidity of UHMW-PE melt. This technology is more common in patent literature. * * * The second component for mixing mainly refers to resins with low melting point and low viscosity, such as LDPE, HDPE, PP and polyester. Among them, medium molecular weight PE (molecular weight 400,000 ~ 600,000) and low molecular weight PE (molecular weight
(1) mixed with medium and low molecular weight PE * * *
Mixing ultra-high molecular weight polyethylene (UHMW-PE) with low molecular weight LDPE (molecular weight 1 0,000 ~ 20,000, preferably 5000 ~12,000) * * can significantly improve its processability, but at the same time it will reduce its mechanical properties such as tensile strength and bending elasticity. HDPE can also significantly improve the processing fluidity of ultra-high molecular weight polyethylene (UHMW-PE), but it will also cause the decline of impact strength, friction resistance and other properties. In order to keep the mechanical properties of UHMW-PE * * mixed system at a high level, an effective compensation method is to add PE nucleating agent, such as benzoic acid, benzoate, stearate and adipate. The mechanical properties can be strengthened by increasing the crystallinity of PE and homogenizing the spherulite size, thus effectively preventing the decline of mechanical properties. The patent points out that adding a small amount of fine nucleating agent wollastonite (particle size 5-50 nm, specific surface area 100 m2/g-400 m2/g) to the UHMW-PE/HDPE * * mixed system can well compensate for the decline of mechanical properties.
(2)*** mixed form
Although the chemical structure of ultra-high molecular weight polyethylene (UHMW-PE) is similar to that of other kinds of PE, it is difficult for its mixture to form a uniform shape under the general melting and mixing equipment and conditions, which may be related to the great difference in viscosity between the components. Ultra-high molecular weight polyethylene (UHMW-PE)/LDPE * * mixture obtained by ordinary single screw mixing, the two components crystallize separately, and cannot form * * * crystals. UHMW-PE is basically dispersed in LDPE matrix in the form of filler. After melting for a long time and mixing with a twin-roll extruder, the interaction between the two components was strengthened and the properties were further improved, but the morphology of * * * crystal could not be formed.
Vadhar found that when a two-step * * * mixing method is adopted, that is, ultra-high molecular weight polyethylene (UHMW-PE) is melted at a high temperature, and then LLDPE is added at a lower temperature for * * * mixing, a * * * mixture forming * * crystals can be obtained. Vadher also obtained the * * * mixture of UHMW-PE/LLDPE which can form * * crystals by solution mixing method.
(3) the mechanical strength of the mixture
For ultra-high molecular weight polyethylene (UHMW-PE)/PE system without nucleating agent, large spherulites will be formed during cooling, and there are obvious interfaces between spherulites, where there are internal stresses caused by different molecular chain arrangements, which leads to cracks. Therefore, compared with the matrix polymer, the tensile strength of * * * mixture tends to decrease. When impacted by external force, the crack will develop rapidly along the spherulite interface, leading to the final crushing, which will lead to the decline of impact strength. Flow improvers promote the unwinding of long-chain molecules, play a lubricating role among macromolecules, and change the energy transfer between macromolecular chains, thus making segment replacement easier and improving the fluidity of polymers.
The flow improvers used for ultra-high molecular weight polyethylene (UHMW-PE) mainly refer to aliphatic hydrocarbons and their derivatives. Among them, aliphatic hydrocarbons include: normal alkanes with more than 22 carbon atoms and lower alkane mixtures with them as the main components; Paraffin wax obtained by petroleum cracking and refining. Its derivatives refer to functional groups with aliphatic hydrocarbon groups and 1 or above (preferably 1 or 2) carboxyl groups, hydroxyl groups, ester groups, carbonyl groups, nitroformyl groups, sulfhydryl groups, etc. ; Fatty acids, fatty alcohols, fatty esters, fatty aldehydes, fatty ketones, fatty amides, fatty thiols, etc. The carbon number is more than 8 (preferably12-50) and the molecular weight is130-2000 (preferably 200-800). For example, fatty acids include decanoic acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid and the like.
An effective suspending agent (MS2) was prepared in China. Adding a small amount (0.6% ~ 0.8%) can significantly improve the fluidity of ultra-high molecular weight polyethylene (UHMW-PE), reduce its melting point as high as 65438 00℃, and can be injection molded on an ordinary injection molding machine with only a slight decrease in tensile strength.
In addition, UHMW-PE modified by styrene and its derivatives can not only improve the processability and make the products easy to extrude, but also maintain the excellent friction resistance and chemical corrosion resistance of UHMW-PE. 1, 1- diphenylacetylene, styrene derivatives and tetralin can all make UHMW-PE have excellent processability, high impact strength and wear resistance.