Polyvinyl alcohol

Lore & Background

Polyvinyl alcohol is not prepared by polymerization of its monomer, vinyl alcohol, because that monomer is thermodynamically unstable and tautomerizes to acetaldehyde. Instead, PVA is produced by hydrolysis of polyvinyl acetate, typically via base-catalysed transesterification with ethanol. The properties of the resulting polymer depend on the degree of transesterification. Its microstructure consists mainly of 1,3-diol linkages, with a few percent of 1,2-diols depending on the polymerization conditions of the vinyl ester precursor. PVA has excellent film-forming, emulsifying, and adhesive properties, and is resistant to oil, grease, and solvents. It exhibits high tensile strength and flexibility, as well as high oxygen and aroma barrier properties, though these are humidity-dependent: absorbed water acts as a plasticiser, reducing tensile strength but increasing elongation and tear strength. Without an externally added crosslinking agent, PVA solution can be gelled through repeated freezing-thawing, yielding highly strong, ultrapure, biocompatible hydrogels used for vascular stents, cartilages, and contact lenses. Polyvinyl acetals are prepared by treating PVA with aldehydes; butyraldehyde and formaldehyde yield polyvinyl butyral (PVB) and polyvinyl formal (PVF), respectively. Preparation of PVB is the largest use for PVA in the US and Western Europe. PVA is also used as an aid in suspension polymerizations and in photographic film.

Reader's Guide

Polyvinyl alcohol is significant for its combination of water solubility, biocompatibility, and mechanical properties, enabling applications ranging from household sponges to medical implants. Its use as a protective colloid in PVAc dispersions dominates in China, while in Japan it is primarily used for Vinylon fiber, which is also manufactured in North Korea for self-sufficiency because no oil is required. In additive manufacturing, PVA-based polymers are used for 3D printed oral dosage forms with modified drug-release characteristics. Research has explored PVA for cartilage, orthopedic applications, and vascular grafts. Its biodegradability varies with molecular weight; highly water-soluble grades used in detergents can be readily biodegradable under OECD screening conditions. Orally administered PVA has very low acute toxicity (LD50 15-20 g/kg), is poorly absorbed from the gastrointestinal tract, does not accumulate in the body, and is not mutagenic or clastogenic. Despite the common acronym PVA, it more generally refers to polyvinyl acetate, a wood adhesive and sealer.

Did You Know?

• Polyvinyl alcohol is not made by polymerizing its monomer because vinyl alcohol is thermodynamically unstable and tautomerizes to acetaldehyde.
• Without an external crosslinking agent, PVA solution can be gelled through repeated freezing-thawing to produce biocompatible hydrogels used for vascular stents and contact lenses.
• The largest use of polyvinyl alcohol in the US and Western Europe is the preparation of polyvinyl butyral (PVB).