Inspection issues for UHMWPE polymer polyethylene materials

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This post was last edited by liaifeng on 2018-7-28 at 07:54. UHMWPE (Ultra-High Molecular Weight Polyethylene) is a thermoplastic engineering plastic with excellent overall properties; its molecules have a linear structure, and its molecular weight exceeds 1.5 million. It has a low density, as well as excellent wear resistance, self-lubricating properties, impact resistance, corrosion resistance, and more. The extremely high molecular weight of UHMWPE (the molecular weight of HDPE is usually only 20,000 to 300,000) endows it with excellent performance characteristics. It is a thermoplastic engineering plastic that offers good value for money and superior properties; it essentially combines the advantages of various plastics, possessing comprehensive characteristics such as wear resistance, impact resistance, self-lubrication, corrosion resistance, ability to absorb impact energy, low-temperature tolerance, hygiene and non-toxicity, resistance to sticking, low water absorption, and a low density – properties that are unmatched by ordinary polyethylene and other engineering plastics. In fact, there is currently no single polymer material that possesses so many excellent properties. Wear resistance: UHMWPE has the highest wear resistance among plastics, even exceeding that of some metals. Compared with other engineering plastics, UHMWPE’s sand abrasion index is only 1/5 that of PA66, and 1/10 that of HEPE and PVC ； Compared to metals, it is 1/7 of carbon steel and 1/27 of brass. The wear resistance is so high that it is difficult to measure it using conventional plastic wear testing methods; therefore, a special mortar wear testing apparatus was designed. The wear resistance of UHMWPE is proportional to its molecular weight; the higher the molecular weight, the better its wear resistance. Impact resistance: The impact strength of UHMWPE is among the highest among all engineering plastics. It is approximately twice that of impact-resistant PC, five times that of ABS, and more than 10 times that of POM and PBTP. Its impact resistance is so high that it is difficult to cause it to fracture using conventional impact testing methods. Its impact strength increases with increasing molecular weight, reaching a maximum at a molecular weight of 1.5 million; thereafter, it gradually decreases as the molecular weight continues to rise. It is worth noting that it can also maintain excellent impact strength in liquid nitrogen at -195°C, a property that other plastics do not possess. Furthermore, its surface hardness is higher under repeated impacts. Self-lubricating property: UHMWPE has an extremely low coefficient of friction (0.05–0.11), thus exhibiting excellent self-lubricating properties. Table 1 shows a comparison of the friction coefficients between UHMWPE and other engineering plastics. As can be seen from Table 1, the dynamic friction coefficient of UHMWPE is half that of PA66 and POM under water-lubricated conditions, and it ranks second only to polytetrafluoroethylene (PTFE), which has the best self-lubricating properties among plastics, under unlubricated conditions ； When it operates in a sliding or rotating manner, its lubricity is even better than that of steel and brass when lubricated. Therefore, in the field of tribology, UHMWPE is regarded as a friction material with an extremely favorable cost/performance ratio. Friction coefficient Name Self-lubricating Water Lubricating oil Lubrication UHMWPE 0.10–0.22 0.05–0.10 0.05–0.08 PTFE 0.04–0.25 0.04–0.08 0.04–0.05 PA66 0.15–0.40 0.14–0.19 0.06–0.11 POM 0.15–0.35 0.10–0.20 0.05–0.10 Table 1 Comparison of friction coefficients between UHMWPE and other engineering plastics Chemical resistance UHMWPE exhibits excellent chemical resistance; aside from strongly oxidizing acids, it can withstand various corrosive media (acids, bases, salts) as well as organic media (except aromatic solvents) within certain temperature and concentration ranges. When immersed in 80 organic solvents at 20°C and 80°C for 30 days, no abnormal phenomena were observed on its surface, and its other physical properties remained almost unchanged. UHMWPE possesses excellent impact energy absorption properties; its impact energy absorption value is the highest among all plastics. As a result, it has good noise damping capabilities and offers outstanding sound reduction effects. Low-temperature resistance: UHMWPE possesses excellent low-temperature resistance and remains ductile at liquid helium temperatures (-169°C), making it suitable for use as low-temperature components in the nuclear industry. Hygienic and non-toxic: UHMWPE is hygienic and non-toxic; it fully complies with the standards set by the Japan Sanitary Association. It is also approved by the U.S. Food and Drug Administration and the U.S. Department of Agriculture, making it suitable for use in contact with food and pharmaceuticals. The adsorption capacity of the non-stick UHMWPE surface is extremely low; its resistance to sticking is second only to PTFE, which is the most non-stick material among plastics. As a result, the surface of such products does not stick easily to other materials. Low water absorption: UHMWPE has a very low water absorption rate ； It is generally less than 0.01%, only 1% of that of PA66, so drying is usually not necessary before molding. Density Table 2 shows a comparison of the densities of UHMWPE and other engineering plastics. As can be seen from Table 2, the density of UHMWPE is lower than that of all other engineering plastics; it is generally 56% lower than that of PTFE, 33% lower than that of POM, and 30% lower than that of PBTP. Therefore, products made from UHMWPE are extremely lightweight. Name, Relative Density, Density of UHMWPE Compared to It/%: PTFE – 2.12, 56; POM – 1.41, 33; PBTP – 1.31, 30; PC – 1.20, 22; PA – 1.02–1.14, 8–18; UHMWPE – 0.94. Table 2: Comparison of the densities of UHMWPE with other engineering plastics. Tensile Strength: Due to its structural characteristics that are ideal for tensile loading, UHMWPE possesses an exceptionally high tensile strength. Fiber with extremely high elastic modulus and strength can be produced using gel spinning methods, with a tensile strength of up to 3–3.5 GPa and an elastic modulus of up to 100–125 GPa ； The specific strength of this fiber is the highest among all fibers that are currently available on the market; it is 4 times greater than that of carbon fiber, 10 times greater than that of steel wire, and 50% greater than that of aramid fibers. Other properties: UHMWPE also possesses excellent electrical insulation properties, superior resistance to environmental stress cracking compared to HDPE, as well as better fatigue resistance and resistance to r-rays than HDPE. Production method: Ultra-high molecular weight polyethylene can be produced using the methods employed for regular high-density polyethylene, by adjusting the process conditions to control the molecular weight. The production methods include the Ziegler method, the Solvay method, and the solution method. Ziegler method: The Ziegler method is a low-pressure polymerization method. It uses titanium tetroxide and alkyl aluminum compounds such as G-ethylaluminium or diethylaluminium chloride as catalysts, with heptane or gasoline as the solvent, and carries out polymerization at 60–90°C under normal pressure. Reactor devices generally use batch reactors or vertical tower reactors. When using a tower reactor, multi-layered stirring propellers should be installed at the bottom of the tower. The production process of the Ziegler method is as follows: First, the catalyst suspended in a solvent and the ethylene feedstock are fed into a reactor for polymerization. Since polymerization is an exothermic reaction, it must be cooled to the specified reaction temperature. Next, the products of the polymerization reaction are transferred to a catalyst decomposition tank, where methanol is added to decompose the catalyst. Finally, after filtration and drying, UHMWPE products with an average molecular weight of 1 million to 5 million are obtained. The Solvay process is a new production method that combines the ring reactor used in the Phillips process with a Ziegler catalyst that uses magnesium-containing compounds as a carrier. The Solvay process uses titanium tetrachloride as a catalyst, and alkyl aluminum compounds and similar substances as activators; the catalyst carrier is usually MgCl2 or MgClOH that has been calcined to obtain a large pore size. The advantage of this method is that the catalyst is adsorbed onto a carrier with a large specific surface area, allowing the crystal structure to unfold; as a result, most of the transition metal atoms act as polymerization active centers, and the catalyst efficiency can reach 50–600 kg per 4Tio. The polymerization reaction takes place in a closed-loop reaction tube equipped with a cold water jacket on the outside; inside, a high-speed turbine stirrer ensures the complexation of the catalyst and its mixing with the introduced ethylene and comonomers used to adjust the density. The reaction temperature is 80–90°C, the ethylene partial pressure is 1.0 MPa, and the reaction time is 2–3 hours. The resulting slurry is subjected to centrifugal filtration; the filter cake is then stripped with steam to remove residual solvents, and after that it is dried using air flow, thereby yielding UHMWPE with an average molecular weight of 1.5 million to 6 million. The solution method uses Cr2O3 as a catalyst and Al2O3-SiO2 as a carrier, with C6 alkane as the solvent. The polymerization reaction takes place at a temperature of 170–180°C above the melting point of the polymer, under a pressure range of 1.96–2.94 MPa. Ethylene and a small amount of the comonomer hexene are used as raw materials; during the polymerization, both ethylene and polyethylene are dissolved in the solvent to create a homogeneous reaction system. After the polymerization is complete, ethylene is removed by flash evaporation, followed by centrifugal separation, concentration, and cooling to precipitate the polymer. Using this method, UHMWPE with an average molecular weight of over 1 million can be produced. Application Editing: Currently, UHMWPE is widely used in fields such as textiles, papermaking, packaging, transportation, printing, the chemical industry, mining, petroleum, construction, electrical engineering, food processing, healthcare, and sports. It is also beginning to find application in areas such as conventional weapons, ships, and automobiles. In the future, it will also be expanded to fields such as aerospace and nuclear energy. Applications requiring high wear resistance and impact resistance: 1.1 Textile machinery. Textile machinery is the earliest field in which UHMWPE has been used; as early as 1958, several companies began using UHMWPE to manufacture parts for textile machinery. For example, it was used to replace the leather components in looms, which are made from the most durable material available – crocodile skin. These UHMWPE-based components can withstand vibrations and impacts at a rate of 40–180 times per minute, and their lifespan is 5–6 times longer than that of leather components. Currently, abroad, there are on average around 30 UHMWPE components used per weaving loom, such as impact and wear-resistant parts like shuttle throwers, shuttle rods, gears, couplers, pick-up rods, buffer blocks, rod bushings, and swinging back beams. 1.2 Papermaking machinery: Papermaking machinery is the second industrial sector in which UHMWPE is used. Around 1960, UHMWPE anti-wear strips were installed for the first time on the conveyor system of a lumber shaver, providing effective protection for the bottom plates and chains; after 5 years of use, almost no wear was observed. It is estimated that the lifespan of wear-resistant strips is more than twice that of chains. Wear-resistant materials such as UHMWPE, stainless steel, maple wood, cast polyurethane, and laminated phenolic plastics are all used as covers for water suction tanks. Importantly, UHMWPE covers exert less resistance on the stainless steel mesh in the water suction tank compared to other materials, thereby extending the service life of the expensive stainless steel mesh. Today, UHMWPE used in the papermaking industry accounts for 10% of the total amount; it is employed to manufacture components such as wiper blades, water tank covers, compaction elements, and joints in papermaking machines ； Furthermore, UHMWPE can also be used to manufacture components such as sealing shafts, idler wheels, scrapers, and filters for papermaking machinery. 1.3 Packaging machinery: The high wear resistance, low friction coefficient, and non-stick properties of UHMWPE make it more suitable for use in packaging machinery than certain metals and other plastics; it can replace phosphor bronze and fluoroplastics. Replacing modified fluoroplastics with UHMWPE for making guides, slider seats of conveying devices, fixing plates, etc., not only **reduces equipment investment costs but also extends the service life by 10 to 50 times. In a liquid detergent production line, the timing gear wheels were replaced with UHMWPE instead of the original laminated thermoset plastics, which solved the problems of wear and tear, scratches on the bottles, and softening that used to occur. This reduced the costs associated with spare parts. Timing screws made of UHMWPE are now also used as standard components in many bottling lines. A major Western brewery used UHMWPE to create a belt-type bottle conveyor that is about 1.6 km long; this conveyor is several times more durable than those made of phosphor bronze. On the conveyor belt of an automobile assembly plant abroad, chain-driven sprockets move carts across a steel plate conveyor; due to the friction between the chain and the steel plates, the chain is very prone to damage. Replacing steel plates with UHMWPE sheets reduces wear** and also decreases power consumption. 1.4 General Machinery: Due to its excellent wear resistance and impact resistance, UHMWPE is widely used in the machinery manufacturing industry; it can be used to produce various mechanical components such as gears, cams, impellers, rollers, pulleys, bearings, bushings, shaft sleeves, shafts, gaskets, seals, elastic couplings, screws, and more. Applications emphasizing self-lubrication and non-stick properties 2.1, Material storage and transportation: UHMWPE can be used to manufacture linings for hoppers, silos, and chutes used for storing powdered materials such as coal, cement, lime, mineral powder, salt, and grains. Thanks to its excellent self-lubricating and non-stick properties, it prevents these powdered materials from sticking to the storage and transportation equipment, thus ensuring stable conveying. Previously, hoppers for coal, ore powder, grain, and similar materials were lined with stainless steel for packaging and transportation. Now, UHMWPE lining is used instead. In long-term use, this reduces heat generation. The hoppers used by Baoshan Iron & Steel to transport ore are lined with UHMWPE; their service life is 10 to 50 times longer than that of those lined with metal materials previously used. Using UHMWPE to make hopper linings simplifies construction and reduces costs; therefore, the feed hoppers used for transporting coal in large power plants are also lined with UHMWPE. The coal chutes in coal mines are made of UHMWPE, which can reduce the inclination angle and thereby increase the extraction volume. It can transport 400,000 tons of coal without the need to replace the material chute, and its service life is twice that of chutes lined with magnesium alloy. UHMWPE is used in solid transport pipelines for fluidized sand and similar applications; compared to steel pipes, its service life is 18 times longer and its cost is 1/25 of that, while compared to nylon pipes, its service life is 3 times longer and its cost is 1/8 of that. During transportation, the internal resistance in the pipe is 25% lower than that of metal pipes, **improving transportation efficiency. When traditional metal materials are used for components such as chutes, buckets, and the lining of ore cars, the material will freeze to the metal in cold and humid weather; however, this does not happen with UHMWPE, thereby **reducing unloading time**. After lining the dump funnel of the bulk carrier with a layer of UHMWPE sheet, the average unloading time was reduced from 16–20 hours to 8 hours. 2.2 Agricultural and construction machinery: UHMWPE boasts excellent self-lubricating properties; mud can slide easily on its surface without sticking. Additionally, it has good impact resistance and wear resistance, making it suitable for use in manufacturing components for agricultural and construction machinery. Items such as the sliding plates of harvesters, the outer lining plates of plows, the scrapers of soil removal machines, the inner linings of excavator buckets, and the inner linings of dump truck compartments can significantly improve work efficiency and reduce energy consumption. 2.3 Stationery items: Taking advantage of UHMWPE’s self-lubricating properties, wear resistance, and cold resistance, it can be used in the bottoms of skating blades and snowboards; in Europe and the United States, UHMWPE is primarily used for manufacturing such products ； By covering the ski slope with UHMWPE (with a layer thickness of 20 mm), the maximum speed of the skates can reach 110 km/h. The UHMWPE skating rink manufactured by Mitsui Petrochemical Industries of Japan was opened to the public in Kagoshima Prefecture in 1975; its construction cost was one-fourth that of a regular skating rink. For applications where corrosion resistance and water resistance are important, UHMWPE boasts excellent chemical stability and water resistance; it can be used as lining material for various solution storage devices as well as in large packaging containers such as floats, catch basins, gasoline tanks, pesticide containers, and water drainage containers for solar energy systems. This is also one of the fields where UHMWPE is currently most widely used. UHMWPE containers possess excellent impact resistance to drops. If the container is filled with 20 kg of water and dropped from a great height onto an iron plate 10 mm thick, the breaking height for a conventional polyethylene container is only 9 M, while that for a UHMWPE container can exceed 15 M. Applications emphasizing hygiene and non-toxicity 4.1, Food and beverage industry: UHMWPE possesses the non-toxicity, water resistance, and non-stick properties required by the food industry. Designated as a material that can be used in direct contact with food. Can be used in conveying lines for beer, soft drinks, seasonings, etc. When transporting items, it can prevent breakage of bottles and the like, reduce noise, minimize wear and tear on conveyors, screws, etc., and decrease power consumption. It can also be used to manufacture components for equipment used in the production of meat, milk, confectionery, pickles, and baked goods. 4.2 Medical Use: UHMWPE possesses excellent physiological inertness and adaptability. Approved by the U.S. Food and Drug Administration and the Department of Agriculture, it can be used in medical applications where it comes into contact with the human body. It is used in clinical settings such as for heart valves, orthopedic components, artificial joints, and contraceptive implants, making it an ideal polymer material for medical use. Artificial hip and knee joints composed of a glenoid made of UHMWPE and a metal femur exhibit superior wear resistance and safety compared to PTFE. To date, hundreds of thousands of people worldwide have undergone replacement with such artificial joints. Applications of other properties 5.1 Low-temperature resistance applications: UHMWPE possesses excellent low-temperature resistance, making it suitable for use in various freezing machines. It can also serve as components resistant to low temperatures in the nuclear industry, and is an ideal insulating material in the field of low-temperature superconductivity. UHMWPE provides excellent radiation shielding, making it suitable for use as shielding panels in nuclear power plants. 5.2 Electrical Insulation Properties: UHMWPE possesses excellent electrical insulation properties, particularly a low dielectric loss tangent value. It can be used to manufacture plating tanks, rollers, as well as insulators and insulating brackets that operate in the high-frequency and ultra-high-frequency ranges. It is also suitable for use in cable conduits, circuit breakers, cable terminals, and other electrical devices. Weaknesses: Compared to other engineering plastics, UHMWPE has lower heat resistance, stiffness, and hardness, but these properties can be improved through methods such as \"filling\" and \"cross-linking\" ； In terms of heat resistance, the melting point of UHMWPE (136°C) is roughly the same as that of ordinary polyethylene. However, due to its high molecular weight and high melt viscosity, it is difficult to process. Here’s the question: How can the properties and molecular weight of polyethylene materials be tested? What are the testing standards?