Polyoxymethylene (POM), also known as acetal resin, polyoxymethylene, and polyacetal, is a thermoplastic crystalline polymer, known as “super steel” or “saigang”.
Brief History of Research
Around 1955, DuPont Company of the United States obtained homopolymer of formaldehyde by polymerization of formaldehyde. Polyoxymethylene is easy to crystallize, with a crystallinity of more than 70%. The melting temperature of homopolyoxymethylene is about 180℃.
It is another engineering plastic with excellent comprehensive performance after polyamide. It has high mechanical properties such as strength, modulus, wear resistance, toughness, fatigue resistance and creep resistance. It also has excellent electrical insulation, solvent resistance and processability. It is one of the five general-purpose engineering plastics.
Acetal polymers, or polyoxymethylene, are formed by the polymerization of formaldehyde, and are often referred to as polyoxymethylene (POM). The preparation of polymers from formaldehyde was studied as early as the 1920s, but no thermally stable material was produced until the development of Delrin by DuPont in 1950. Homopolymers are made by anionic polymerization of very pure formaldehyde. The polymer formed is insoluble. It precipitates continuously as the polymerization proceeds. As the formaldehyde is separated, the acetal resin is pulled apart, and thermal degradation occurs. The thermal stability of the polymer can be improved by esterification of the terminal hydroxyl groups with acetic anhydride. Another way to improve thermal stability is copolymerization with a second monomer, such as ethylene oxide, and the polymer is prepared by cationic polymerization.
There are four mechanisms for thermal degradation of acetal resins. The first is heat or base catalyzed chain depolymerization; the result is the release of formaldehyde, and the end group cleavage of the polymer can reduce this tendency; the second is oxygen attack on the random position of the polymer, which also leads to depolymerization. The use of antioxidants can reduce the occurrence of this degradation mechanism, and copolymerization also helps to reduce this tendency; the third mechanism is the cleavage of the acetal resin chain by acid. The fourth degradation is thermal depolymerization when the temperature cliff exceeds 270°C. This is very important, and it warns the operator to keep the processing temperature below 270°C to avoid polymer degradation.
Acetal resin is highly crystalline, with a typical crystallinity of 75% and a melting point of 180°C. Compared with polyethylene (PE), the molecular chains are more densely packed due to the shorter C-O bonds, resulting in a higher melting point for the polymer. The high crystallinity gives acetal polymers good solvent resistance. The polymer is mainly linear, with a molecular weight between 20,000 and 110,000.
Acetal resin is a strong, hard thermoplastic with good fatigue and thermal stability. It has a low coefficient of friction and good heat resistance. Acetal resin is similar to nylon, but its fatigue resistance, creep resistance, hardness and water resistance are worse than nylon. However, the creep resistance of acetal resin is not as good as that of polycarbonate. As mentioned above, acetal resin has excellent solvent resistance. No organic solvent has been found that can dissolve acetal resin below 70°C; however, it can swell in some solvents. Acetal resin is sensitive to acids, bases and oxidants. Although the C-O bond is polar, it is balanced and much less polar than the carbonyl group in nylon. As a result, acetal resin has relatively low hygroscopicity. Small amounts of adsorbed moisture may cause swelling and dimensional changes, but will not cause polymer hydrolysis and degradation. The effect of moisture is much smaller than that of nylon polymers. Ultraviolet light can cause polymer degradation, which can be reduced by adding carbon black. Copolymers generally have properties similar to homopolymers, but the mechanical properties of homopolymers are slightly higher than those of copolymers. Its melting point is also higher, but its thermal stability and alkali resistance are inferior to copolymers. Both homopolymers and copolymers are filled with fillers (glass fiber, fluoropolymers, aramid fibers and other fillers) to make toughened and UV-stabilized grades. Acetal resins are blended with polyurethane elastomers to improve their toughness, and these materials are available commercially.
Acetal resins are available for injection molding, injection molding and extrusion. It is important not to overheat or severely overpressure during processing due to the generation of formaldehyde. The polymer should be cleaned before shutting down to avoid overheating during startup. Acetal resins should be stored in a dry place. The apparent viscosity of acetal resins is less dependent on shear stress and temperature than polyolefins, but its melt has low elasticity and low strength. Low melt strength is a problem when blow molding is used. For blow molding, copolymers with branched structures are more suitable. Crystallization is very fast and post-molding shrinkage can be completed within 48 hours after molding. It is difficult to produce transparent films due to rapid crystallization.
The market demand for acetal resin in the United States and Canada in 1997 was 368 million lb. Applications of acetal resin include: gears, rollers, pipe components, pump parts, fan blades, blown film aerosol containers, molded sprockets and chains, and it is often used to directly replace metal. Acetal resin is mainly used for injection molding, and secondarily for extruded sheets and rods. The low coefficient of friction of acetal resin makes it a good bearing.
Physical and chemical properties
Polyoxymethylene is a linear polymer with no side chains, high density, and high crystallinity, and has excellent comprehensive properties.
Polyoxymethylene is a hard and dense material with a smooth and shiny surface, light yellow or white, and can be used for a long time in the temperature range of -40-100°C. Its wear resistance and self-lubrication are also superior to most engineering plastics, and it has good oil resistance and peroxide resistance. It is not resistant to acid, strong alkali and sunlight ultraviolet radiation.
The tensile strength of polyoxymethylene is up to 70MPa, it has low water absorption, stable dimensions and gloss, all of which are better than nylon. Polyoxymethylene is a highly crystalline resin and the toughest among thermoplastic resins. It has high heat resistance, bending strength, fatigue resistance, and excellent wear resistance and electrical properties.
Application Areas
Polyoxymethylene (POM) is an engineering plastic with excellent performance, known as “steel” and “super steel”. POM has the hardness, strength and rigidity similar to metals, and has good self-lubrication, good fatigue resistance and elasticity in a wide range of temperature and humidity. In addition, it has good chemical resistance. With a lower cost than many other engineering plastics, POM is replacing some markets traditionally occupied by metals, such as replacing zinc, brass, aluminum and steel to make many parts. Since its introduction, POM has been widely used in electronics, machinery, instrumentation, daily light industry, automobiles, building materials, agriculture and other fields. In many new fields of application, such as medical technology and sports equipment, POM has also shown a good growth trend.
It is widely used in manufacturing various sliding and rotating mechanical parts, making various gears, levers, pulleys, sprockets, and is particularly suitable for bearings, hot water valves, precision metering valves, chain links and rollers of conveyors, flow meters, automobile internal and external handles, cranks and other window rotating machinery, oil pump bearing seats and impeller gas switch valves, electronic switch parts, fasteners, terminal mirror masks, electric fan parts, heating plates, instrument buttons; bearings for audio and video tapes; various pipelines and agricultural sprinkler systems as well as valves, nozzles, faucets, and bathtub parts; switch keyboards, buttons, audio and video tape reels; temperature control timers; power tools, gardening tool parts; in addition, it can be used as surfboards, sailboats and various sled parts, watch micro gears, frame accessories for sports equipment and backpacks with various buckles, fasteners, lighters, zippers, buckles; pacemakers in medical devices; artificial heart valves, apical vertebrae, prostheses, etc.
Used in chemical synthesis in the chemical industry, pharmaceutical industry, and synthesis using anhydrous formaldehyde as raw material.
Structure
The molecule of polyoxymethylene is a high-density, high-crystalline linear polymer without side chains. Since the bond length of CO bond is shorter than CC bond, the packing density of polyoxymethylene chain axis is large. Compared with polyethylene, polyoxymethylene has short carbon-oxygen bond, high cohesive energy density and high density.
According to the different chemical structures in its molecular chain, it can be divided into homopolyoxymethylene and copolymer polyoxymethylene. The important difference between the two is that homopolyoxymethylene has high density, crystallinity and melting point, but poor thermal stability, narrow processing temperature range (about 10°C), and slightly low acid and alkali stability; while copolymer polyoxymethylene has low density, crystallinity, melting point and strength, but good thermal stability, not easy to decompose, wide processing temperature range (about 50°C), and good acid and alkali stability. It is an engineering plastic with excellent comprehensive properties. It has good physical, mechanical and chemical properties, especially excellent friction resistance. Commonly known as Saigang or Duogang, it is the third largest general-purpose plastic. It is suitable for making wear-reducing and wear-resistant parts, transmission parts, as well as chemical, instrumentation and other parts.
The polyoxymethylene molecular chain is highly flexible and has a highly regular chain structure, so it has a high degree of crystallinity and a strong crystallization ability. The crystallinity of homopolyoxymethylene is 75%~85%, and that of copolymer is 70%~75%. Even with rapid quenching, the crystallinity can reach more than 65%. Completely amorphous polyoxymethylene can only be obtained at -100℃.
High density and high crystallinity are the main reasons why polyoxymethylene has excellent performance, such as high hardness and high modulus, good dimensional stability, outstanding fatigue resistance, and not easily corroded by chemical media. Although the CO bond in the polyoxymethylene molecular chain has a certain polarity, the high density and high crystallinity constrain the movement of the dipole moment, so that it still has good electrical insulation and dielectric properties.
The end groups of polyoxymethylene contain hemiacetal structures. When heated to about 100°C, it can gradually depolymerize from the hemiacetal at its end groups, so its heat resistance is low. When heated to about 170°C, an auto-oxidation reaction can occur at any point in the molecular chain to release formaldehyde. Formaldehyde will be oxidized to formic acid in the presence of high temperature and oxygen. Formic acid has an automatic accelerating catalytic effect on the degradation reaction of polyoxymethylene. Therefore, heat stabilizers, antioxidants, formaldehyde absorbers, etc. are often added to homopolyoxymethylene resins to meet the needs of forming and processing. Since the copolymer formaldehyde molecular chain contains a certain amount of CC bonds, it can prevent the oxidative degradation of the polyoxymethylene molecular chain, so the copolymer formaldehyde has much better thermal stability than homopolyoxymethylene. However, whether it is homopolyoxymethylene or copolymer formaldehyde, full attention should be paid to its shortcomings of poor thermal stability and thermal oxygen stability during processing and application.
Performance Numbers
Strength: 70MPa (yield)
Elongation: 15% (yield), 15% (break)
Impact strength: 108 kJ/m² (unnotched), 7.6kJ/m² (notched)
The synthesis of homopolyformaldehyde is generally carried out by condensation polymerization of an aqueous solution of formaldehyde in the presence of an acid. The a-polyformaldehyde with a degree of polymerization of more than 100 is obtained, and then it is heated and decomposed into formaldehyde gas. After refining and dehydration, the monomer is usually purified by partial prepolymerization, and then introduced into a dry solvent containing a small amount of initiator for polymerization. Because of the presence of water, the molecular weight is significantly reduced. The initiator can be Lewis acid or base. However, most of them use tertiary amines for anionic addition polymerization, and the reaction is as follows: The end group of polyformaldehyde is hemiacetal (-CH2OH). When the temperature is higher than 100°C, the end group is easy to break, and generally needs to be stabilized by end group treatment. After stabilization treatment, it can be heat-resistant to 230°C. Polyformaldehyde can be processed at a temperature of 170-200°C, such as injection, extrusion, blow molding, etc. It is mainly used as engineering plastics for automobiles, mechanical parts, etc.
Characteristic
POM is a tough and elastic material that has good creep resistance, geometric stability and impact resistance even at low temperatures. POM has both homopolymer and copolymer materials. Homopolymer materials have good ductility and fatigue strength, but are not easy to process. Copolymer materials have good thermal stability, chemical stability and are easy to process. Both homopolymer and copolymer materials are crystalline materials and do not absorb moisture easily. The high degree of crystallinity of POM leads to a relatively high shrinkage rate, which can be as high as 2%~3.5%. There are different shrinkage rates for different reinforced materials.
Parameter
| Density | g/cm³ | 1.39-1.43 |
| Water absorption | % | 0.2 |
| Continuous use temperature | ℃ | -50~105 |
| Yield tensile strength | MPa | 63 |
| Yield tensile strain | % | 10 |
| Ultimate tensile strain | % | 31 |
| Notched impact toughness | Kj/㎡ | 6 |
| Rockwell hardness | 135 | |
| Shore Hardness | 85 | |
| Elastic modulus | MPa | 2600 |
| Softening temperature | ℃ | 150 |
| Heat Deflection Temperature HDT | ℃ | 155 |
| Thermal expansion coefficient | 1.1 | |
| Thermal conductivity | W/(m·K) | 031 |
| Friction coefficient | 0.35 |
Its comprehensive performance is: high fatigue strength; good wear resistance; low water absorption; high surface hardness and good rigidity; good dimensional stability and high dimensional accuracy of the product; and good sliding properties.
POM Environmental Performance
POM is not resistant to strong alkali and oxidants, but has a certain stability to olefinic acid and weak acid. POM has good solvent resistance, and is resistant to hydrocarbons, alcohols, aldehydes, ethers, gasoline, lubricating oils and weak alkalis, etc., and can maintain considerable chemical stability at high temperatures. It has low water absorption and good dimensional stability.
Conductive modification
Adding conductive carbon black is a common method for making conductive POM. The so-called conductive carbon black refers to a type of carbon black with a smaller particle size, a larger surface area and more lock-like structures.
Carbon black is generally made of various organic hydrocarbons by incomplete combustion or thermal decomposition. It is an insoluble and infusible microspherical particle. In addition to lone pairs of electrons and aromatic rings, its surface also has polar functional groups such as quinone carbonyl and phenolic hydroxyl. The amount of conductive carbon black added is generally 0.5%-20%. If the conductivity of carbon black is good, the surface resistivity or volume resistivity of POM can be reduced to the order of 1×10 2. However, due to the effect of polar functional groups on the surface of carbon black, the thermal stability of POM is often reduced, which in turn causes a decrease in physical and mechanical properties. To overcome this shortcoming, the method of using conductive carbon black and hydrophilic polymer compounds (such as PEG) together can be adopted to reduce the amount of carbon black used. It is also possible to use a heat stabilizer mainly composed of formaldehyde scavengers to improve the thermal stability of the system.
In comparison, the use of carbon fiber can not only greatly improve the various properties of POM (including self-lubrication), but also achieve good antistatic properties. For example, when 20% of carbon fiber with good conductivity is added, the surface resistivity and volume resistivity of POM can reach the order of 1×10^2.
