Properties of acetal resins

The principal features of acetal resins leading to commercial application may be summarised as follows  

Property Asm test Acetal homopolymer Acetal copolymer Unit 

Although in many respects acetal resins are similar to the nylons, they may be considered to be superior to them in their fatigue endurance, creep resistance, stiffness and water resistance. The nylons (except under dry conditions) are superior in impact toughness and abrasion resistance. 

Some mechanical and thermal properties of acetal polymers are listed in Table 19.2. The value quoted are those supplied by the manufacturers. 

It must, however, be stressed that for design purposes such data have little value. Like the nylons, which are also widely used for load-bearing light engineering applications, the poly acetals exhibit a small but finite creep under load. It is thus necessary to consider mechanical properties under those main headings. 

---

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

Electrical properties of acetal resin are collected in Table 3. The dielectric constant is constant over the temperature range of most interest (—40 to 50°C). Table 3. Electrical Properties of Acetal Resins... 

The polyoxymethylenes are presently widely used in different areas. Approximatively one-third of the market is represented by homopolymers and two-thirds by copolymers. Homopolymers are produced by anionic polymerization of formaldehyde using amines, alkoxides, and other types of anionic initiators. The details of these polymerizations will not be discussed in this paper, although some of their properties will be compared to those of copolymers which are obtained by cationic copolymerization of trioxane with cyclic ethers or cyclic esters. Comprehensive reviews on general aspects of synthesis and properties of acetal resins are available [158-162],... 

---