Polyoxymethylene plastic (POM)

Polyoxymethylene refers to a class of polymers containing methylene oxide repeating structural units in the macromolecular chain, the scientific name is "polymethylene oxide", and the English abbreviation is POM. POM is the third largest general-purpose engineering plastic.
POM can be divided into two types: "homopolymer POM" and "copolymer POM" according to different structures. Due to the different structures, there are certain differences in the properties of the two POMs, such as the density, crystallinity and mechanical properties of homopolymer POM are slightly higher, while the thermal, chemical and processability of copolymer POM is better. Copolymer POM is more widely used than homopolymer POM.
The outstanding properties of POM are: good mechanical properties and rigidity, close to metal materials, it is an ideal material to replace copper, cast zinc, aluminum and other metal materials, excellent fatigue resistance and creep resistance, good wear resistance, self-lubrication and friction, and UHMWPE, PA, F4 together called the four major wear-resistant plastic materials, high thermal and chemical stability, excellent electrical insulation.
The disadvantages of POM are high density, poor acid and flame resistance, large and unstable post-shrinkage, poor dimensional stability, and low weather resistance.
POM is widely used in the fields of electronic appliances, machinery, automobiles, instrumentation, construction and daily necessities. In Japan, 40% is used for electrical and electronic appliances, 27% is used for automobiles, 45% is used in the United States, 17.5% is used for automobiles, and 39% is used for automobiles in Western Europe.
1. The performance of polyoxymethylene plastic
(1) General performance The appearance of POM is light yellow or white translucent or opaque powder or granular, and the surface of the product is smooth and shiny, hard and dense, similar to ivory. Molding shrinkage is as high as 3.5%. It is flammable, its oxygen index is only 14~16, the upper end of the flame is yellow, the lower end is blue, the melt drips, and there is a pungent formaldehyde smell and fishy smell. POM has low air permeability, only a fraction of that of PE.
(2) Mechanical properties POM has excellent mechanical properties, with a specific strength of 50.5Mpa and a specific stiffness of 2650Mpa, which is very close to metal. The mechanical properties of POM change little with temperature, and the copolymerization ratio is slightly larger than that of homopolymerization.
The impact strength of POM is higher, but the conventional impact is not as good as that of ABS and PC, and POM is sensitive to notching, which can reduce the impact strength by 90%.
The fatigue strength of POM is very prominent, after the action of 104 alternating load, the fatigue strength can reach 35MPa, while PA and PC are only 28MPa. The creep resistance of POM is similar to that of PA, which is only 2.3% at 20°C, 21MPa, and 3000h, and is less affected by temperature.
POM has a small coefficient of friction, good wear resistance (POM>PA66>PA6>PC>ABS>HPVC>PS), a large ultimate PV value, good self-lubrication, and is suitable for the production of friction products such as gears and bearings.
(3) Thermal performance The long-term heat resistance of POM is not high, but it can withstand 160 °C in the short term, and the short-term heat resistance of homopolymer POM is more than 10 °C higher than that of copolymerization, but the long-term heat resistance of copolymer POM is about 10 °C higher than that of homopolymerization.
(4) Electrical properties POM has good electrical insulation, almost not affected by temperature and humidity, dielectric constant and dielectric loss tangent change little in a wide range of temperature, humidity and frequency, excellent arc resistance, and can be maintained at high temperature. The dielectric strength of POM is related to the thickness, which is 82.7 Kv/mm at 0.127 mm thickness and 23.6 Kv/mm at 1.88 mm thickness.
(5) Environmental performance POM is not resistant to strong acids and oxidants, and has certain stability to dilute acids and weak acids. POM has good solvent resistance, can resist hydrocarbons, alcohols, aldehydes, ethers, gasoline, lubricating oil and weak alkalis, etc., and can maintain considerable stable performance at high temperatures. The weather resistance of POM is not good, and the mechanical properties of POM will be reduced under the action of ultraviolet light for a long time, and the surface will be pulverized and cracked.
2. Molding and processing of polyoxymethylene plastics
(1) Processing characteristics
The rheology of POM melt is non-Newtonian, and the viscosity of the melt is not sensitive to temperature.
POM has a large crystallinity, a narrow melting range, and a large molding shrinkage (up to 3.5%). For injection molding thick products, it is necessary to pay attention to pressure holding and material replenishment, so as not to cause too large shrinkage holes and be scrapped.
POM has poor thermal stability, and decomposition can be caused by too high a temperature or too long a time. Especially when the temperature exceeds 250 °C, the decomposition rate will be accelerated, and the formaldehyde gas that strongly irritates the eyes will be overflowed, and in severe cases, the product will produce bubbles or discoloration, and even cause explosion. Therefore, the processing temperature and residence time must be strictly controlled, and antioxidants and cyandiamide formaldehyde absorbers must be added.
POM has a fast condensation rate, and the product is prone to surface defects such as wrinkles, markings and weld marks. This can be solved by increasing the injection speed and increasing the mold temperature.
POM products are prone to internal stress, and the post-shrinkage is also large, so post-treatment is required. The conditions for post-processing are: thickness
Below 6mm, temperature 100°C, time 0.25~1h, thickness above 6mm, temperature 120~130°C, 4~6h.
POM does not absorb much water, but drying can improve the gloss of the surface of the product. The drying conditions are: temperature 110~120 °C, time 3~5h.
(2) POM can be processed by injection molding, extrusion, blow molding and overmolding, and injection molding is the main process.
3. Modified varieties of polyoxymethylene plastics
(1) Enhance POM
The main reinforcing materials are glass fiber, glass beads and carbon fiber, and glass fiber is the most commonly used. The enhanced mechanical properties can be increased by 2~3 times, and the heat deflection temperature can be increased by more than 50°C.
(2) High lubrication POM
F4, graphite, molybdenum disulfide, lubricating oil and low molecular weight PE can be added to POM to improve its lubricating properties. For example, adding 5 parts of F4 to POM can reduce the coefficient of friction by 60% and increase the wear resistance by 1~2 times, while adding liquid lubricating oil to POM can greatly improve the wear resistance and the ultimate PV value. The wear resistance of POM with 5% oil is increased by 72%, and the ultimate PV value can reach 3.9MPa•m/s, (pure POM is 0.213MPa•m/s), which is 3~20 times that of other engineering plastics.
4. Application range of polyoxymethylene plastics
(1) Machinery industry can be used to manufacture gears, bearings, pulleys, cams, pulleys, pump bodies, housings, valves, faucets and pipe joints by using the characteristics of POM strength, wear resistance, fatigue resistance, high impact strength and good self-lubrication.
(2) Automobile industry uses the advantages of high specific strength to replace metal copper, aluminum, zinc, etc. in vehicles, and is used as water tank valves, radiator covers, fans, control levers, switches, gearbox shells and bearing brackets.
(3) Electronics/electrical appliances are used for power tool shells, switch handles, as well as accessories for televisions, computers, fax machines, timer parts, tape recorders, video recorder tape holders, etc., by virtue of its advantages of high dielectric strength, small tangent value of dielectric loss, and good arc resistance.
(4) Other second-generation zipper materials, water tanks, washbasins and toys, etc.