Properties and application conditions of polyoxymethylene

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Polyformaldehyde: http://imgsrc.baidu.com/baike/abpic/item/246cca2adfa57388033bf682.jpg Polyformaldehyde (English: polyformaldehyde) is a thermoplastic crystalline polymer. Known as “supersteel,” it is also called polyoxymethylene. Its structure is such that the English abbreviation is POM. Polymers obtained by the polymerization of formaldehyde usually have a low degree of polymerization and are prone to depolymerization upon heating. Around 1955, DuPont Company produced the homopolymer of formaldehyde by polymerizing formaldehyde. Polyoxymethylene crystallizes easily, with a crystallinity of over 70%. The melting temperature of homopolymerized formaldehyde is around 180°C. 　　The scientific name for polyoxymethylene is polyoxymethylenepolymer, abbreviated as POM. 　　Polyoxymethylene is a linear polymer without side chains, featuring high density and high crystallinity, as well as excellent overall properties. 　　Polyoxymethylene is a hard and dense material with a smooth, shiny surface; it is pale yellow or white in color, and can be used over extended periods at temperatures ranging from -40 to 100°C. Its wear resistance and self-lubricating properties are also superior to those of the vast majority of engineering plastics, in addition to having good oil resistance and peroxide resistance. It is very sensitive to acids, strong alkalis, and ultraviolet radiation from moonlight. 　　Polyoxymethylene has a tensile strength of 70 MPa, low water absorption, dimensional stability, and a glossy finish – all properties that are superior to those of nylon. It is a highly crystalline resin and the toughest among thermoplastic resins. It possesses high heat resistance, high bending strength, and high fatigue resistance, as well as excellent wear resistance and electrical properties. 　　Properties of polyoxymethylene:
Property values:
Density: 1.43
Melting point: 175°C
Tensile strength (yield): 70 MPa
Elongation at yield: 15%
(Elongation at break): 15%
Impact strength (notched): 108 KJ/m2
(Impact strength with notch): 7.6 KJ/m2
The synthesis of homopolymerized polyoxymethylene generally involves the condensation polymerization of an aqueous solution of formaldehyde in the presence of acid. a-Polyoxymethylene with a degree of polymerization of 100 or more is obtained, and then it is heated to decompose into formaldehyde gas. After purification and dehydration, the monomer is usually purified using a partial pre-polymerization method, and then polymerization is carried out in a dry solvent containing a small amount of initiator. The presence of water causes the molecular weight to decrease significantly. Initiators can be Lewis acids or bases, etc. However, tertiary amines are mostly used for anionic addition polymerization, with the reaction proceeding as follows: The end groups of polyoxymethylene are hemiacetals (—CH2OH), and at temperatures above 100°C, these end groups tend to break apart; therefore, end-group treatment is generally required to stabilize them. After stabilization treatment, it can withstand temperatures up to 230 ℃. Polyformaldehyde can be processed at temperatures of 170–200 °C, such as by injection, extrusion, blow molding, etc. It is mainly used as an engineering plastic in applications such as automobiles and mechanical components. www.seafar.cn 　　Typical applications of POM polyoxymethylene: POM has a very low friction coefficient and excellent geometric stability, making it particularly suitable for manufacturing gears and bearings. Due to its high-temperature resistance, it is also used in piping components (pipe valves, pump housings), lawn equipment, and more. 　　Injection molding process conditions: Drying treatment: If the material is stored in a dry environment, drying treatment is usually not required. 　　Melting temperature: 190~230 for homopolymer materials℃ ； The copolymer material is at 190~210°C. 　　Mold temperature: 80~105°C. A higher mold temperature can be used to reduce the shrinkage rate after molding. 　　Injection pressure: 700~1200 bar Injection speed: moderate to high injection speed. 　　Flow channels and gates: Any type of gate can be used. If a tunnel gate is used, it is best to use a shorter type. For homopolymer materials, it is recommended to use a hot nozzle runner. For copolymer materials, both internal and external hot runners can be used. 　　Chemical and physical properties of POM polyoxymethylene: POM is a tough and elastic material that retains excellent creep resistance, geometric stability, and impact resistance even at low temperatures. POM includes both homopolymer and copolymer materials. Homopolymer materials possess excellent ductile strength and fatigue resistance, but are difficult to process. Copolymer materials have excellent thermal stability and chemical stability, and are easy to process. Both homopolymer materials and copolymer materials are crystalline materials and do not absorb moisture easily. The high degree of crystallinity of POM results in a fairly high shrinkage rate, which can reach up to 2%–3.5%. Different enhanced materials have different shrinkage rates. 　　Polyoxymethylene (POM) is an engineering plastic with excellent properties; it is known abroad as a material that can replace steel or even surpass it in performance. POM possesses metal-like hardness, strength, and rigidity; it exhibits excellent self-lubricating properties over a wide range of temperatures and humidity levels, good fatigue resistance, and elasticity. In addition, it also has good chemical resistance. At a cost lower than that of many other engineering plastics, POM is replacing metals in various markets where they have traditionally been used; it is used to manufacture numerous components in place of zinc, brass, aluminum, and steel. Since its introduction, POM has been widely applied in fields such as electronics and electrical equipment, machinery, instruments, consumer goods, automobiles, building materials, and agriculture. In many new application areas such as medical technology and sports equipment, pom also shows good growth trends. 　　Continuous growth in application demand 　　POM is used in sliding and rolling mechanical components where high requirements are placed on lubricity, wear resistance, stiffness, and dimensional stability; its excellent performance makes it particularly suitable for use in industrial machinery, automobiles, electronics and electrical equipment, plumbing fittings, and irrigation products. In recent years, China’s POM market has experienced rapid growth. In 2002, the apparent consumption volume of POM in China was 136,570 tons, and the average annual growth rate of apparent consumption in this market from 1990 to 2002 was 11.7%. It is estimated that China’s apparent consumption of Pom in 2005 will be 168,000 tons, with an average annual growth rate for apparent consumption in the Pom market from 2000 to 2005 reaching 10.3%. By 2010, the apparent consumption volume of Pom in our country is expected to rise to 197,000 tons, with an average annual growth rate for the Pom market’s apparent consumption volume between 2005 and 2010 reaching 3.2%. 　　The main factors contributing to the rapid development of the Pom market in our country are: the overall growth of the national economy, which leads to an expanding application market for Pom, one of the five major engineering plastics, with increasing consumption levels ； In our country, the raw materials used for Pom production are inexpensive and readily available, resulting in substantial profits from Pom production and trading ； New breakthroughs are continually emerging in the fields of pom applications, such as the research and development of modified materials. Given the steady growth in market demand in our country, the price of Pom is expected to continue rising at a steady pace in the near future. 　　The technological gap cannot be ignored. Although the market demand for Pom in our country is on the rise, due to the relatively late start of research and development related to Pom in our country, the domestic production scale, output, as well as the variety and quality of Pom fail to meet market demands. Compared with the advanced levels abroad, China’s pom production still faces issues such as high raw material consumption per unit, small plant scale, unstable quality, and a limited range of product grades. For example, DuPont’s consumption of formaldehyde per ton of homopolymerized Pom is 2.56 tons, with some reports indicating 3.08 tons; in contrast, the formaldehyde consumption per ton of Pom produced in China is 5–6 tons. Although this figure has decreased as a result of improvements, the gap remains significant. 　　For many years, the production capacity and output of the Pom industry in our country have remained low, and these figures still fail to meet market demand. In 2002, there were only 3 Pom manufacturers in China, and among them, only Yuntianhua Group’s Pom production facility had a capacity of 10,000 tons per year (which has since been expanded to 20,000 tons per year). In 2002, China’s Pom production capacity was 12,800 tons per year, with a production volume of around 10,000 tons. From 1966 to 2002, the average annual growth rate of China’s Pom production capacity was 16.7%, while the average annual growth rate of output was 18.8%. It is evident that it is necessary to expand the production capacity of domestic Pom manufacturers. It is estimated that by 2005, there will be 6 POM manufacturers in China, with a total production capacity of 130,000 tons per year ； It is estimated that China’s Pom production capacity will be 190,000 tons per year in 2010, with an average annual growth rate of 7.9% for the period from 2005 to 2010. 　　The reason why the increase in China’s Pom production capacity is not sufficient to meet market demand is that the domestic Pom market is growing rapidly, while China’s foundation for Pom production is relatively weak ； In addition, PM is a capital- and technology-intensive chemical product in the materials sector. China’s huge market has attracted the attention of foreign companies, which have always tried to gain a foothold in this market with their own products. They are reluctant to transfer technology, which has resulted in slow progress in improving the technical level of domestically produced PM, failing to meet the needs of consumers ； Furthermore, for a long time, China’s economic system and corporate management mechanisms have not conformed to the laws of a market economy; as a result, enterprises have been unable to obtain sufficient funding in a timely manner, which has hindered the development of Pom production. 　　Imbalance in imports and exports Although China’s Pom production has increased, it still cannot meet the demands of the domestic market, resulting in the need to import large quantities of Pom each year. The main countries and regions from which China imports Pom are Japan, the United States, South Korea, and Taiwan Province of China, among others. In 2002, China’s import volume of Pom was 135,570 tons, and the average annual growth rate of Pom imports from 1995 to 2002 was 26.3%. Our country also engages in limited export trade of Pom on an annual basis; in 2002, 9,000 tons of Pom were exported. 　　The development of high-tech industries and emerging sectors in our country has led to a significant increase in the demand for engineering plastics. As an engineering plastic suitable for various industries, pom has promising prospects for development. It is understood that there are several ongoing Pom projects in China at present; for example, the Pom project of DuPont-Asahi Kasei (Zhangjiagang) has already begun construction, while the planned Pom project at Shanghai Lanxing Chemical New Materials Factory is also in active development. Given the situation of planned Pom projects in our country, it will be the trend for the development of the Pom industry there to introduce capital and advanced technical equipment to build Pom plants on a scale that is economically viable, in locations that possess the necessary raw material resources and technical foundations.