Traditionally thought of as either springy foam products (such as cleaning sponges and furniture filling etc) or dense finished plastic-type hard rubber such as durable elastomeric wheels and tyres (think shopping trolley or skateboard wheels), polyurethane is also used extensively in woodworking for bonding wood to wood and also used for gluing metal, ceramics, stone, glass and most plastics. Polyurethane adhesives have been the ‘go to’ material for many trades for many years and for good reason, they are a solid choice for tasks that require a strong but flexible final bond, we sell 3 distinctly different types of polyurethane adhesives, expanding foam-types that come in pressurised tin cans for extrusion by either spray nozzle or special foam gun, liquified polyurethanes in squeezable tubs (used mainly for joinery) and mastic-type cartridges, gunned out of a standard mastic gun onto a substrate for bonding onto another or existing substrate. Here’s the chemistry techy stuff on wikipedia:
Polyurethanes are in the class of compounds called reaction polymers, which include epoxies, unsaturated polyesters, and phenolics. Polyurethanes are produced by reacting an isocyanate containing two or more isocyanate groups per molecule (R−(N=C=O)n) with a polyol containing on average two or more hydroxyl groups per molecule (R′−(OH)n) in the presence of a catalyst or by activation with ultraviolet light.
The properties of a polyurethane are greatly influenced by the types of isocyanates and polyols used to make it. Long, flexible segments, contributed by the polyol, give soft, elastic polymer. High amounts of crosslinking give tough or rigid polymers. Long chains and low crosslinking give a polymer that is very stretchy, short chains with lots of crosslinks produce a hard polymer while long chains and intermediate crosslinking give a polymer useful for making foam. The crosslinking present in polyurethanes means that the polymer consists of a three-dimensional network and molecular weight is very high. In some respects a piece of polyurethane can be regarded as one giant molecule. One consequence of this is that typical polyurethanes do not soften or melt when they are heated; they are thermosetting polymers.