PolyurethanePU

The highly reactive chemical group that combines with polyols to form polyurethane, and with atmospheric moisture in one-part moisture-curing products. Isocyanates such as MDI and TDI are respiratory sensitisers, so spraying and heating polyurethane products are subject to strict workplace exposure controls.

A curing mechanism in which a one-part adhesive, sealant or coating reacts with atmospheric humidity, hardening from the exposed surface inwards. One-part polyurethanes, silicones and silane-modified polymers cure this way; cure through thick sections is slow and depends on humidity and temperature.

A flexible material applied to joints and interfaces to exclude water, air or other media while accommodating movement between the parts, in contrast to an adhesive whose primary job is to transfer load. Silicone, polyurethane, polysulphide and silane-modified polymer chemistries dominate construction and industrial sealing.

An adhesive designed to carry significant service loads as part of an engineered joint, transferring stress between substrates over the whole bonded area rather than at discrete fasteners. Epoxies, polyurethanes and methacrylates are the main chemistries, used in applications from aircraft assemblies to wind turbine blade bonding.