Polyurethane and PVA (polyvinyl acetate) glues are both polymer-based adhesives and are both used in many different applications, including woodworking. They have very different properties, however. Both can bond diverse materials, but PVA glue only works well on porous surfaces, whereas polyurethane glue can be used to join smooth, non-porous surfaces.
Polyurethane Glue Characteristics
Polyurethane glue comes as a thick, dark liquid that foams on contact with water. Once the foam has dried, it becomes hard and waterproof, bonding surfaces together firmly. Common brands of polyurethane glue include Gorilla Glue and Elmer's Ultimate Glue.
PVA Glue Characteristics
PVA glues are viscous liquids that may be yellow or white, with the yellow form being somewhat stronger than the white. PVA glue is water-based. It sets best at room temperature and needs good air circulation, as it dries by evaporation. To form a really strong join, PVA glue may need pressure from a weight or clamp while it dries. Some brands of PVA glue include Elmer's Carpenter's Wood Glue and Unibond PVA.
Polyurethane Glue Advantages
Polyurethane glue's foaming characteristic gives it superior gap-filling properties, as it expands to fill any available space. This makes it a popular choice for uneven surfaces or loose joints. Unlike PVA glue, it bonds well with smooth surfaces such as glass, some plastics and rubber. Once the plastic foam has cured, it becomes hard, durable and waterproof.
Polyurethane Glue Disadvantages
Polyurethane glue gives off strong fumes. For this reason it's best used in a well-ventilated area. For the glue to foam and cure properly, surfaces to be glued must be moistened first. The glue generally escapes the joint as the foam expands. This creates messy leakage that must be cleaned up, either by wiping with acetone before the glue dries or by scraping and sanding afterwards. Despite its reputation for superior strength, polyurethane glues have underperformed in some trials.
PVA Glue Advantages
PVA glue is relatively non-toxic, making it a popular choice for children's crafts (although it should not be swallowed). It dries fairly fast and forms strong, flexible bonds on paper, textiles and other porous materials. Although typically not waterproof, all PVA glues are water-resistant. Used properly, yellow PVA adhesives are very effective wood glues, outperforming polyurethane glue and other adhesives under certain circumstances. It is significantly cheaper than polyurethane glue.
PVA Glue Disadvantages
Although a few waterproof PVA glues have begun to appear on the market, most PVA glues do not perform well in wet or very humid conditions, making them unsuitable for outdoor use. PVA glues can't stick to smooth surfaces like glass, glazed pottery or smooth plastics. While it can be bulked up with materials such as sawdust, PVA glue doesn't have the space-filling properties of polyurethane glue.