White flammable crystalline powder with formaldehyde smell. It dissolves slowly in cold water and quickly in hot water. The solubility in water at 20℃ is 0.24g/ 100cm3H2O. Insoluble in ethanol and ether. Soluble in caustic soda and potassium solution.
General performance
Polyoxymethylene pom is a hard and dense material with smooth and shiny surface, pale yellow or white, and its thin-walled part is translucent. Combustion characteristics are easy to burn and continue to burn after leaving the fire. The upper end of the flame is yellow, and the lower end is blue, with melting and dripping phenomena, strong irritating formaldehyde smell and fishy smell. Polyformaldehyde is white powder, generally opaque, with good coloring, specific gravity1.41-1.43g/cm3, molding shrinkage 1.2-3.0%, molding temperature 170-200℃, and drying. The long-term heat resistance of POM is not high, but it can reach 160℃ in a short time. The short-term heat resistance of homopolymer POM is higher than that of * * * POM 10℃, but the long-term heat resistance of * * POM is about 10℃ higher than that of homopolymer POM. It can be used for a long time in the temperature range of -40℃ ~ 100℃. POM is easy to decompose, the decomposition temperature is 280℃, and irritating and corrosive gases are produced during decomposition. Therefore, die steel should be made of corrosion-resistant materials.
mechanical character
Polyoxymethylene has high strength and stiffness, good elasticity and good wear resistance. Its mechanical properties are excellent, the specific strength can reach 50.5MPa, and the specific stiffness can reach 2650MPa, which is very close to metal. The mechanical properties of POM have little change with temperature, and the change of * * * poly POM is slightly greater than that of homopolymer POM. The impact strength of POM is high, but the conventional impact is not as good as ABS and PC. POM is sensitive to notch, which can reduce the impact strength by as much as 90%. The fatigue strength of POM is very prominent. After 10 alternating load, the fatigue strength can reach 35MPa, while the fatigue strength of Pa and PC is only 28MPa. The creep property of POM is similar to that of PA, and the creep rate is only 2.3% at 20℃, 265±438±0 MPa and 3000h, which is less affected by temperature. POM has low friction coefficient and good wear resistance (POM >；; PA66 & gtPA6 & gtABS & gtHPVC & gt； PS & gtPC), with large limit PV value and good self-lubrication. POM products are easy to produce screaming noise under grinding and high load.
behaviour of electricity
POM has good electrical insulation and is almost unaffected by temperature and humidity. In a wide range of temperature, humidity and frequency, the dielectric constant and dielectric loss change little. Excellent arc resistance, can be maintained at high temperature. The dielectric strength of POM is related to the thickness, which is 82.7kV/mm when the thickness is 0. 1.27mm and 23.6kV/mm when the thickness is1.88 mm. ..
Environmental performance
POM is not resistant to strong acids and oxidants, but it is stable to olefins and weak acids. POM has good solvent resistance, resistance to hydrocarbons, alcohols, aldehydes, ethers, gasoline, lubricating oil and weak alkali, and can maintain considerable chemical stability at high temperature. Small water absorption and good dimensional stability.
POM has poor weather resistance. Under the action of ultraviolet rays for a long time, the mechanical properties decreased and the surface cracked.
Formability
Crystallized materials have narrow melting range, fast melting and solidification, moderate fluidity and low hygroscopicity. They can be crystallized when the material temperature is slightly lower than the melting temperature and can be dried without treatment.
production process
Different production processes will produce different kinds of paraformaldehyde and paraformaldehyde.
Homopolyoxymethylene
To make paraformaldehyde, we must first make anhydrous formaldehyde. The main method is firstly to generate formaldehyde condensate (diethoxymethane, CH2(OC2H5)2) by the reaction of hydrated formaldehyde (dimethyl glycol, HCH(OH)2) with ethanol, then dehydrate the mixture of methylal and water by extraction or vacuum distillation, and then release the formaldehyde in methylal by heating it. At this time, formaldehyde began to polymerize under the catalysis of anions, and then it was capped with acetic anhydride to obtain stable paraformaldehyde.
* * * Polyformaldehyde
To make * * * polyoxymethylene, formaldehyde must be converted into trioxane (especially 1, 3,5-trioxane, also known as trioxane).
merits and demerits
superiority
1, with high mechanical strength and rigidity; 2. The fatigue strength is the highest; 3. Good environmental resistance and organic solvent resistance;
4. Strong resistance to repeated shocks; 5. Wide working temperature range (-40℃ ~120℃); 6. Good electrical performance;
7. Good toughness; 8. It has its own lubricity and good wear resistance; 9. Excellent dimensional stability.
disadvantaged
Strong acid corrosion, poor weather resistance, poor adhesion, close thermal decomposition softening temperature, and small oxygen limiting index. [2]?
use
Paraformaldehyde is a solid formaldehyde with high formaldehyde content, which is solid and granular and convenient for storage and transportation. It can be converted into formaldehyde vapor at higher temperature, which is easy to replace high concentration formaldehyde to participate in various reactions, and is beneficial to chemical synthesis in chemical industry, pharmacy and other industrial fields, especially the synthesis using anhydrous formaldehyde as raw material. There are mainly the following pesticides (1): the synthesis of acetochlor, butachlor and glyphosate; (2) Coating: synthetic high-grade automobile paint; (3) Resin: synthetic urea-formaldehyde resin, phenolic resin, polyacetal resin, melamine resin, ion exchange resin and other adhesives; (4) papermaking: synthetic paper sheet reinforcing agent;
(5) casting: turning sand and removing film to synthesize casting glue; (6) Aquaculture: fumigation disinfectant. (7) Organic raw materials: used to prepare pentaerythritol, trimethylolpropane, glycerol, acrylic acid, methyl acrylate, methacrylic acid, N- hydroxymethylacrylamide, alkylphenol, methyl vinyl ketone, etc. (8) Others: medicine and disinfection. [3]?
Development Status of China
Polyoxymethylene (pom) is replacing some markets traditionally occupied by metals at a lower cost than many other engineering plastics, such as replacing zinc, brass, aluminum and steel to manufacture many parts. Since its inception, POM has been widely used in electronic and electrical, machinery, instrumentation, daily light industry, automobiles, building materials, agriculture and other fields. In many new fields, such as medical technology and sports equipment, POM also shows a good growth trend.
1, application consumption continues to grow.
POM is used for sliding and rolling mechanical parts with strict requirements on lubricity, wear resistance, rigidity and dimensional stability, and its performance is particularly superior, so it is mainly used in industrial machinery, automobiles, electronics and electricity, pipe fittings and irrigation supplies. The polyformaldehyde market in China is growing rapidly. In 2002, the apparent consumption of polyoxymethylene market in China was 654.38+0.3657 million tons, and the average annual growth rate of polyoxymethylene market in 2000-2002 was 654.38+0.7%. It is estimated that the apparent consumption of polyoxymethylene market in China will be1680,000 tons in 2005, and the average annual growth rate of polyoxymethylene market will reach 10.3% in 2000-2005. By 20 10, the apparent consumption of polyoxymethylene market in China will increase to197,000 tons, and the average annual growth rate of polyoxymethylene market will reach 3.2% from 2005 to 20 10.
2. The technology gap cannot be ignored.
Although the market demand of polyoxymethylene in China is rising, the production scale, output, variety and quality of polyoxymethylene in China can not meet the market demand because of its relatively late development. Compared with foreign advanced level, there are still some problems in the production of polyoxymethylene in China, such as high unit consumption of raw materials, small factory scale, unstable quality and few varieties and brands.
3. Capacity growth is not enough to meet the demand.
The capacity and output of polyoxymethylene industry in China have been low for many years, and it still cannot meet the market demand. In 2002, there were only three pom production enterprises in China, of which only Yuntianhua Group's pom production device was 1 10,000 tons (the production capacity has been expanded to 20,000 tons/year). In 2002, the production capacity of POM in China was 654.38+280,000 tons/year, and the output was about 1.8 million tons. From 1966 to 2002, the average annual growth rate of pom production capacity in China was 16.7%, and the average annual growth rate of output was 18.8%. It is imperative for domestic pom production enterprises to expand their production capacity. It is estimated that there will be six POM manufacturers in China in 2005, with a total production capacity of 65438+300,000 tons/year. (Not feasible, it is suggested to delete) It is estimated that China's pom production capacity will be 6.5438+0.9 million tons/year in 2065.438+00, and the average annual growth rate of pom production capacity will reach 7.9% in 2005-2065.438+00.
The reasons why the growth of POM production capacity in China can't meet the market demand are as follows: the POM market in other countries is growing rapidly, while the production base of POM in China is relatively weak; In addition, polyformaldehyde is a capital and technology-intensive material chemical product. China's huge market has attracted great attention from foreign countries. Other countries have been trying to occupy the China market with their own products, and are unwilling to transfer technology, which makes the technical level of pom in China slow to improve and cannot meet the needs of users. Furthermore, for a long time, China's economic system and enterprise management mechanism did not conform to the laws of market economy, and enterprises could not get enough capital investment in time, which restricted the development of polyoxymethylene production.