Paraformaldehyde, also known as polyoxymethylene, is the condensation product of formaldehyde with a typical chain length of 8 - 100 units. Long chain-length (high molecular weight) paraformaldehyde is used as a thermoplastic and is known known as polyoxymethylene plastic (POM, Delrin, polyformaldehyde). Paraformaldehyde commonly has a slight odor of formaldehyde due to decomposition. Paraformaldehyde is a poly-acetal. Paraformaldehyde is used as fumigant, disinfectant, and fungicide, and for the preparation of pure formaldehyde.
Paraformaldehyde forms slowly in formaldehyde solutions (formaline) as a white precipitate, especially if stored in the cold. Formaline actually contains very little monomeric formaldehyde most of it forms short chains of polyformaldehyde. A few percent of methanol is often added as a stabilizer to limit the extent of polymerization.
Paraformaldehyde can be depolymerized to formaldehyde gas by dry heating and to formaldehyde solution by water in the presence of alkali or heat. The very pure formaldehyde solutions obtained in this way are used as a fixative for microscopy and histology.
Paraformaldehyde is flammable as it decomposes in the heat into the flammable gas formaldehyde.
As a formaldehyde releasing agent, paraformaldehyde is a suspected carcinogen. Its acute oral LD50 in rats is 592 mg/kg.