Polyformaldehyde

[—CH₂O—]_n, a linear polymer of formaldehyde; a white, horny crystalline substance with molecular weight 40,000–120,000, density 1.43 g/cm³ at 20°C, melting point 164°-180°C, and crystallinity 60–85 percent. Polyformaldehyde is one of the most rigid structural thermoplasts, with high mechanical strength and resistance to fatigue, wear, and moisture. It is used in a stressed state at temperatures from – 40” to 90°–20°C. Polyformaldehyde is resistant to nearly all neutral solvents and alkalies but decomposes under the action of mineral acids; it is flammable. Nonstabilized polyformaldehyde undergoes complete decomposition to formaldehyde upon heating to temperatures above 200°C. Polyformaldehyde is physiologically harmless.

Polyformaldehyde is produced industrially by polymerization of an anhydrous monomer or its cyclic trimer trioxane. During polyformaldehyde synthesis, 2–4 percent of a comonomer (for example, ethylene oxide or 1,3-dioxolane) is added to the reaction mixture to increase the thermostability of the product; other properties of these copolymers and polyformaldehyde are analogous. Polyformaldehyde is processed by injection molding and extrusion and is used mainly in the manufacture of various machine parts and, to a lesser extent, in fiber manufacture. It is produced under the trade names SFD and STD (USSR); Delrin, Celcon, and Polyfaid (USA); and Hostaform S (Federal Republic of Germany). World production of polyformaldehyde in 1972 was 120,000 tons.