White glue is arguably the most benign and safest adhesive available. Sold as wood, carpenter, craft, or school glue (and possibly others), these white glues all share common chemistry.
A water emulsion of polyvinyl acetate — PVAc or PVA — is a useful glue. PVA is a sticky polymer, providing good adhesion. The opaque white liquid entails small drops of PVA suspended in water. There is no cure chemistry — just a loss of water that leaves a polymer film.
More specifically, the free-radical polymerization of vinyl acetate in water, or emulsion polymerization, yields a milky liquid. Small PVA particles, insoluble in water, are suspended in the emulsion. The emulsion is unstable and, upon water loss, the PVA particles merge into a polymer film. So, the glue is set by the loss of water and coalescence of the polymer.
PVA is a disordered, highly branched, non-crystalline thermoplastic. It’s inexpensive and typically produced at an extensively large scale. PVA has good resistance to UV and oxidation. Water can be used for wet glue clean-up and mechanical removal is ideal for dried glue.
The application as an emulsion is beneficial. The viscosity of the emulsion is far lower than that of the pure polymer, a solid at room temperature. The emulsion easily flows into porous surfaces where the bulk polymer would not work well. There are no volatile organic solvents released, only water.
PVA adhesives bond well to several porous substrates though it’s not an ideal choice for bonding non-porous surfaces. On a porous surface, PVA glue diffuses into the surface and coalescences as the water evaporates or soaks into the surface. Setting times are comparatively rapid at room temperature.
The set resins are light in color and often transparent, resulting in a nearly invisible glue line. Some surfaces, such as paper, can swell and wrinkle due to the water content of the glue. PVA is not particularly good at gap-filling. It bonds best when surfaces are narrowly separated.
This adhesive becomes brittle at low temperatures, which can lead to failure. It can soften at higher temperatures or in moist and humid environments, resulting in a weak or failed bond. Interior applications are best given these limitations.
Cold flow, the tendency for the glue to creep or yield under sustained load, can be a problem. When used as wood glue, warping of wood due to moisture content can stress joints to failure. PVA adhesives do not bond particularly well to dried PVA. Failed joints must be cleaned of old glue before attempting repairs.
Formulation changes and the addition of cross-linking agents can improve the initial tack and enhance bonding performance. PVA is polymerized with small amounts of vinyl alcohol, creating terminal hydroxyl groups on the polymer. These hydroxyl groups offer sites for cross-linking. Cross-linking agents can further improve mechanical strength and moisture resistance, making bonds more stable at higher temperatures.
PVA-based wood and craft glues are affordable, safe choices for bonding porous materials. They have limitations, particularly for industrial assemblies, but they have their place.