Mechanical Thermal Electrical Chemical Properties of Polyoxymethylene (POM)

The basic properties of polyoxymethylene (POM)

Paraformaldehyde is a linear polymer with high melting point, high density, crystalline, containing -CH2 -O- links in the main chain of the molecule without side chains, with milky white or light yellow appearance. It can be divided into two categories: one is a copolymer of paraformaldehyde and a small amount of dioxane, called copolyformaldehyde; the other is a homopolymer of formaldehyde or paraformaldehyde, called homopolymer of paraformaldehyde. Although there are differences in the structure of the two types of paraformaldehyde, but the proportion of C-C bonds in the molecular chain of copolyformaldehyde is very small (3% to 5%), therefore, the performance of the two types of paraformaldehyde is basically similar, they have similar characteristics.

Properties data of polyoxymethylene (POM)

2. Mechanical properties

Paraformaldehyde is a highly crystalline polymer, with high modulus of elasticity, high hardness and stiffness, good toughness, can withstand repeated impacts, in repeated impact loads can maintain high impact strength, and strength value by the impact of temperature change is small, can be in -40 ~ 100 ℃ for long-term use.

The crystallinity of paraformaldehyde is more than 70%, thus it has excellent fatigue resistance. It is the most superior fatigue resistance among thermoplastic materials, and is especially suitable for gear products subject to repeated external forces and parts under constant vibration.

The creep resistance of polyformaldehyde is similar to that of polyamide and other engineering plastics, and its creep value changes less with temperature, even at higher temperatures, the creep resistance is still good. At 23 ℃, 21MPa load, after 3000h creep value is only 2.3%.

The bonding energy of polyformaldehyde is large, and the cohesive energy of molecules is high, so the wear resistance is good. The friction factor and wear amount of polyformaldehyde are small, and the limit PV value is large, so it is suitable for long-term sliding friction parts. And its self-lubricating properties are more oil-free environment or prone to early oil breakage of the working environment under the choice of friction material, provides a unique value, polyformaldehyde as a friction material of a new choice to enter the various fields.

3. Thermal Properties

Polyformaldehyde has a high heat distortion temperature of 124°C for homopolyformaldehyde and 110°C for copolyformaldehyde. The heat distortion temperature of paraformaldehyde is higher than that of copolyformaldehyde, but the thermal stability of paraformaldehyde is lower than that of copolyformaldehyde. Generally, the long-term use temperature of paraformaldehyde is about 100℃. The main thermal property data of polyformaldehyde are shown in Table 1-1. Polyformaldehyde will produce a certain degree of moisture and heat aging in hot water, and its service life in hot water is lower than in hot air.

4. Electrical properties

Acetal has good electrical properties, dielectric loss is small, the breakdown voltage is high, the insulation resistance is not low, and the dielectric constant is not much affected by the water absorption, in the frequency of 102 ~ 105Hz and 20 ~ 100 ℃ temperature range, the dielectric constant of acetaldehyde is maintained at the level of 3.1 ~ 3.9. The dielectric loss angle tangent has the same situation: when the water absorption rate increases from 0.2% to 0.8%, its dielectric loss angle tangent value increases only about 0.003. The electrical properties of paraformaldehyde are shown in Table 1-1.The high frequency electrical properties of paraformaldehyde are not very good. As the temperature increases, the dielectric constant and dielectric loss angle tangent increases dramatically. Therefore, in the high-frequency electronics industry, especially the ultra-high-frequency electronics industry should be noted when using.

The breakdown voltage of paraformaldehyde is relatively high, and its resistance to arc leakage is very superior. For the dry arc and dust and fog test, does not produce traces of leakage and carbonization. 5.

5. Chemical resistance

The chemical resistance of paraformaldehyde resin is shown in Table 1-2. The basic structure of paraformaldehyde determines that it has no room temperature solvent. Below or near the melting point of the resin, it is almost impossible to find any solvent, and only individual substances such as perfluoroacetone can form a very dilute solution. Therefore, in all engineering plastics in the polyformaldehyde resistance to organic solvents and oil resistance is very outstanding. Especially in the high temperature conditions have quite good erosion resistance, and the size and mechanical strength of the change is not big.

The polyformaldehyde resin has good resistance to dilute acids, but for strong acids, especially sulfuric acid, hydrochloric acid, nitric acid, sulfurous acid, nitrous acid, etc., stress cracking will occur.

Due to the esterification of blocked end homoformaldehyde will be hydrolyzed by alkali off the acid end group, followed by the sequence of formaldehyde chain off, so the alkali resistance of co-polyformaldehyde is significantly better than homoformaldehyde. Generally, it is safe to use homopolyformaldehyde only in alkali solution with pH value below 10.

The absorption ability of engineering plastics to water can often lead to dimensional changes in products, while the dimensional changes generated by the absorption of water in paraformaldehyde are extremely small and do not pose a problem for practical applications.

6. Application of polyoxymethylene (POM)

In the machinery industry, paraformaldehyde is widely used in the manufacture of gears, rollers, cams, bearings, springs, bolts, nuts, as well as a variety of pumps, shells, impellers, etc. The gears and couplings made of paraformaldehyde are commonly used as general-purpose power transmission function structural parts. Modified paraformaldehyde used in the manufacture of bushings, gears, sliders and other wear-resistant parts, the metal wear is small, reducing the amount of lubricating oil, enhance the service life of the parts, and therefore can be a wide range of alternatives to copper, zinc and other metals to produce bearings, gears, tie rods and so on. Modified polyformaldehyde friction factor is very small, very strong rigidity, very suitable for the manufacture of automotive pumps, carburetor parts, fuel lines, power valves, universal coupling bearings, cranks, handles, instrument panels, automotive window lifter devices, electric switches, seat belt buckles and so on. In electronic appliances, paraformaldehyde is used in the manufacture of various electric tool parts, such as electric wrench shell, switch handle, etc., as well as household appliances in the parts; in the construction field for the manufacture of window frames, washbasins, water tanks, doors and windows, pulleys and so on.