Polyurethane adhesive

This family record covers the familiar one-part, moisture-cure polyurethane adhesive, often sold as expanding wood or construction glue. Moisture reacts with isocyanate-ended material and builds the polyurethane network. The reaction releases carbon dioxide, so the adhesive foams and expands. Lightly dampening one surface can start cure on dry materials, but soaking the joint adds too much water and encourages coarse foam. The expansion is not useful structural gap fill. Foamed adhesive contains voids and is weaker than a thin, well-clamped bond line.

Use it where the parts are reasonably close fitting and at least one surface can supply moisture. It bonds many porous materials, including wood, stone and some foams, and can join them to suitable metals, ceramics or rigid plastics. It is useful outdoors only when the exact product has the required water rating and the joint is protected from sunlight where necessary. Clamp firmly while it expands or the joint can be pushed apart. Full cure is usually much slower than the first skin. Polyurethane adhesives also include non-foaming construction grades and two-part industrial systems, so flexibility, service temperature and chemical resistance vary too widely for one family figure. Do not assume that a product called polyurethane will bond oily rubber, PE, PP or every coated metal.

Close-fitting exterior wood joints with a suitable rated product; bonding wood to stone, ceramic, foam or suitable metal; repairs where a little moisture is available and firm clamping is possible; porous building materials; laminating rigid insulation or panels when the product explicitly lists the materials.

Clean away dust, oil, wax, loose coating and old foam. Dry-fit the joint and arrange clamps before opening the bottle. On dry porous material, lightly dampen one surface. Do not saturate it and do not put water into the bottle. Apply a thin, even layer because the adhesive expands. Assemble promptly and clamp firmly for the stated time. Protect adjacent faces from squeeze-out. Test plastics, coated metals and dense tropical hardwoods. Sand or scrape cured squeeze-out rather than trying to wash it away.