Introduction to Polymer Materials — Polyoxymethylene

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Introduction to Polymer Materials — Polyoxymethylene Polyoxymethylene, also known as polyoxymethylene, has the English name polyoxymethylene (abbreviated as POM). The crystallinity resulting from POM’s regular molecular structure gives it excellent physical and mechanical properties, earning it the nickname \"metal plastic\". 　　 POM is a milky, opaque, crystalline linear thermoplastic resin with a high elastic modulus, great stiffness and hardness; its specific strength and specific stiffness are close to those of metals. It also has a low friction coefficient, is wear-resistant, and exhibits good dimensional stability. POM is commonly used as a building material to replace non-ferrous metals such as copper, zinc, tin, and lead, and it is widely applied in industries such as industrial machinery, automobiles, electronics and electrical appliances, daily necessities, pipes and fittings, and precision instruments. The main drawback of POM is its poor flame retardancy; it burns slowly when exposed to fire, and even the addition of flame retardants does not yield satisfactory results. 　　 In addition to homopolymerized POM, it is often copolymerized with ethylene oxide in order to prevent the degradation of POM at higher processing temperatures. Homopolymerous polyoxymethylene has a high degree of crystallinity, and its mechanical strength and rigidity are better than those of copolymerous polyoxymethylene. Copolymerous polyoxymethylene has a lower melting point, and its thermal stability, flow properties, and processability are superior to those of homopolymerous polyoxymethylene. Ultra-high molecular weight polyethylene (UHMW-PE) is a new type of thermoplastic engineering plastic. Its molecular structure is identical to that of ordinary polyethylene; the molecular weight of ordinary polyethylene is generally between 40,000 and 120,000, whereas that of UHMW-PE can reach 1 million to 4 million. As the molecular weight increases significantly, certain properties of the resin undergo dramatic changes, such as improved wear resistance ； It has high impact resistance, which remains high even at low temperatures; it also exhibits good self-lubricating properties. UHMW-PE can and does replace materials such as carbon steel, stainless steel, and bronze, and is used in industries such as textiles, papermaking, food processing machinery, transportation, ceramics, and coal mining. Currently, the global annual production capacity for ultra-high molecular weight polyethylene is 80,000 tons. China’s annual production capacity for UHMW‑PE is 10,000 tons. Molding Processing Techniques and Processes 　　 Due to the poor fluidity of UHMW‑PE and its high viscosity in the molten state, it is difficult to process using conventional methods. Compression sintering is the earliest processing method for UHMW-PE; it involves placing UHMW-Pe powder in a mold, applying pressure to form a green part with certain strength and density, and then sintering it at a specified temperature. Extrusion molding involves processing UHMW-PE using a plunger extruder, and can be regarded as a continuous compression sintering process. The reciprocating motion of the piston generates high extrusion pressure, but the UHMW-PE within the barrel does not plasticize well, resulting in low production efficiency and difficulty in manufacturing larger products. Nippon Sanpei Petrochemical Company developed the injection molding process in 1974 and brought it into industrial use in 1976. During injection molding, the material flows in a jet-like manner under high pressure, which facilitates filling of the mold and helps maintain the stability of the product’s dimensions. International Development Status 　　 Production Situation The production and application of UHMW-PE in the world have a history of over 30 years. Over the past 10 years, with the continuous advancement of processing technologies, its production and consumption have been on the rise: the consumption volume was 50,000 tons in 1989, while market sales reached 60,000 tons in 1995. Montel is a major global producer of UHMW-pE; it has a factory in North America with an annual production capacity of 16,000 tons. In 1997, its annual production capacity was increased from 16,000 tons to 27,000 tons. Currently, Montel holds a 47% market share in North America, while its sales volume in other regions is not significant. The company’s current focus on research and development is on modifying its existing products. The production scale of UHMW-PE at the Dutch company DMS and the Japanese company Mitsui is relatively small. In addition to producing standard grades, special grades are also available (such as injection molding grades, fiber grades, and ultra-fine UHMW-PE). Demand situation: In 1997, the international market demand for UHMW-PE was approximately 60,000 tons, of which North America accounted for 56% and Western Europe for 34%. Compared to the well-developed markets in North America and Western Europe, the consumption of UHMW-pE in Asia is very low, accounting for only 10%. Among Asian countries, Japan has the highest annual consumption, at around 2,000 tons. Domestic development 　　 Research and development of ultra-high molecular weight polyethylene began in China in the late 1970s. The research institutions at that time included Shanghai Gaoqiao Chemical Plant, Anhui Chemical Research Institute, Guangzhou Plastic Factory, etc., but due to insufficient technology, the molecular weight of the resin could only reach around 1.5 million. Beijing Aditive Factory No. 2 produces ultra-high molecular weight resins using a low-pressure polyethylene plant with an annual production capacity of 8,000 tons; the output has been increasing year by year, reaching 10,000 tons per year at present. This company focuses its research and development on improving product quality. At present, the intrinsic quality of its products (such as molecular weight, bulk density, tensile strength, elongation at break, etc.) can reach the level of Mitsubishi’s 240M and 340M grades in Japan, as well as GUR432 grade from Hestel in Germany. 　　 In 1996, China’s total demand for UHMW-PE was around 2,000 tons, with supply mainly coming from Beijing Adhesive No. 2 Factory. The products were primarily used in industries such as textiles, mechanical equipment, papermaking, food processing, and transportation. Among them, sheets account for 60%, profiles account for 25%, and filtration components account for 15%. By application, 60% is used in villages, 15% in the paper industry, 15% in filters, and 10% for other purposes. In recent years, significant advancements have been made in the processing technology of UHMW-PE. It has evolved from initial sintering and compression molding to extrusion molding using specialized equipment, and its fields of application have also continued to expand. UHMW-PE has been widely used in fields such as textiles, papermaking, packaging, transportation, machinery, chemicals, mining, petroleum, agriculture, construction, electrical engineering, food, medicine, and sports. 　　 Textile machinery: Textile machinery is one of the fields where UHMW-PE was applied relatively early. Currently, abroad, there are on average around 30 UHMW-PE components used in each textile machine, such as shuttle injectors, shuttle beaters, gears, connectors, yarn clearing rods, buffer blocks, eccentric blocks, rod bushings, and swinging rear beams. By using ultra-high molecular weight polyethylene skin ties in place of cowhide ties, the number of impact cycles increased from 1 million to 5–6 million. Papermaking machinery is the main field for UHMW-PE. UHMW‑PE has a low friction coefficient and is wear-resistant. Therefore, using it for the cover plates and wiper blades of the water tanks on paper machines, as well as for compaction components and joints, can extend the lifespan of the polyester mesh on paper machines, save costs associated with replacing it, and reduce losses caused by downtime. Furthermore, UHMW‑PE can also be used to manufacture components for papermaking machinery such as sealing shafts, idler wheels, scrapers, and filters. Currently, the amount of UHMW-PE required by the paper industry accounts for 10% of its total consumption. 　　 Packaging and storage of UHMW-PE offers excellent chemical stability and water resistance, making it suitable as a lining material for various solution storage containers as well as for large-scale packaging. In particular, UHMW-PE boasts advantages such as being non-toxic and water-resistant, making it suitable as a material in direct contact with food. It can be used to manufacture components for automatic food bottling packaging lines, helping to prevent bottle breakage, reduce noise, and improve work efficiency. Furthermore, UHMW-PE can be used to manufacture linings for hoppers, silos, and chutes used to store powdered materials such as coal, cement, lime, mineral powder, salt, and grains. Thanks to its excellent self-lubricating properties and non-stick characteristics, it prevents these powdered materials from sticking to the storage and transportation equipment, thus ensuring stable transportation. Using UHMW-PE to manufacture conveyance pipes for coal yards and grain processing plants increases their service life by 10 to 50 times compared to metal materials, while also **reducing costs and improving conveying efficiency. 　　 In agricultural and construction machinery, UHMW-PE boasts excellent self-lubricating properties; dirt does not stick to its surface. Additionally, it has high wear resistance. Using it to manufacture components for agricultural and construction machinery can significantly improve work efficiency and reduce energy consumption. Using it to manufacture gears for agricultural harvesters reduces costs by 1/3 compared to steel products, while extending their service life by more than twice. 　　 In addition, UHMW-PE can also be used in medicine for orthopedic components and artificial joints; it is an ideal medical polymer material. In terms of performance, artificial hip joints made from UHMW-PE exhibit superior wear resistance and safety compared to PTFE. UHMW-PE possesses excellent low-temperature resistance, and can be used in various freezing machines. It serves as a low-temperature resistant component in the nuclear industry as well as a shielding panel in nuclear power plants. UHMW-PE has good electrical insulation properties and can be used to manufacture separators (membranes) for acidic lead-acid batteries. UHMW-PE fibers have a strength 10 times that of steel wires and 2 times that of carbon fibers. Their tensile strength ranges from 150 to 350 kg/square centimeter, while their tensile modulus is between 8000 and 10,000 kg/square centimeter. They have a wide range of applications in both defense and civilian fields (such as in the manufacture of bulletproof vests, airplane seats, and ropes for maritime and fishing use). Thanks to its self-lubricating properties, wear resistance, and cold resistance, it can be used to manufacture skates and snowboards. Parts and components for sports equipment such as water sports gear. 　　 Ultra-high molecular weight polyethylene possesses a wear resistance that is unmatched by other engineering plastics. Properties such as impact resistance, chemical resistance, and self-lubrication give it unique advantages for use in various fields of the national economy, especially in applications related to wear-resistant conveyors, equipment linings, and various mechanical components. The development and application of such products hold great prospects, and there is growing interest in exploring new areas of application, such as in the automotive industry. 　　 However, due to its high melt viscosity and the difficulty in processing it, similar to the methods used for processing polytetrafluoroethylene, such as sintering and plug extrusion, have been primarily employed for its processing both domestically and internationally. These methods result in low production efficiency, which in turn limits the widespread use of this product. To this end, it is possible to take into account China’s national conditions, introduce advanced foreign experiences and technologies, and develop processing equipment and techniques suitable for the needs of deeply processed products based on the actual market demand for such products in China. Furthermore, to meet the demands of industrial product diversity, the current development trend is to generalize special engineering plastics. To obtain products with high added value, it is necessary to explore new markets and expand into fields with high levels of production technology in order to meet societal demands. Methyl methacrylate (PMMA) is a transparent plastic, also known as plexiglass. It is not as fragile as glass, and its light-transmitting properties are better than those of glass; as the thickness of glass increases, its transparency decreases, whereas PMMA can remain transparent even at a thickness of 33 cm. This property is very useful – for example, aquarium display cases need to withstand several tons of pressure. If glass is used, very thick glass is required, which results in reduced transparency, while thick PMMA can solve this problem. The largest display case in the world is found at the Monterey Bay Aquarium in California; it uses a piece of PMMA that is 6.6 meters long, 5.5 meters high, and 33 cm thick. PMMA is also used in acrylic pigments. Lubricants or hydraulic fluids become viscous at low temperatures, which can affect the operation of machinery. Adding a small amount of PMMA can prevent this problem, allowing the fluids to remain functional even at low temperatures of up to 100°C. PMMA is produced through the free-radical polymerization of methyl methacrylate. Polycarbonate (PC), like PMMA, is transparent and can be used for explosion-proof glass, but it is more expensive than PMMA.