A polyurethane is any polymer consisting of a chain of organic units joined by urethane links. It is used in adhesives, elastomers, foams, fibers, seals, gaskets, condoms, carpet underlayment, foam rubber , and hard parts.
Production
Polyurethane can be made in a variety of textures and hardnesses by varying the particular monomers used and adding other substances.
Softer polyurethane can be made by adding flexible Polyethylene glycol segments between urethane links. This strategy is used to make spandex fibers, as well as foam rubber. Careful control of viscoelastic properties can lead to memory foam, which is much softer at skin temperature than at room temperature.
Polyurethane foam (including foam rubber) can be produced by adding a small amount of water to one of the liquid precursors of polyurethane before they are mixed together. This modifies the polymerization reaction, causing carbon dioxide to be released as the material cures. Gas is generated throughout the liquid, creating relatively uniform bubbles which then harden to form a solid foam as polymerization progresses. The small poportion of reactions affected by the water result in urea linkages -NC(O)N-, rather than urethane linkages, so that the resulting material should technically be called poly(urethane-co-urea).
Uses
Polyurethane is now also used in making tires. Modern roller blading and skateboarding became economical only with the introduction of tough, abrasion resistant polyurethane parts.
Another common usage is found in carpentry or woodworking. A polyurethane sealant is frequently employed as a finishing coat to protect or seal wood. This use results in a hard, inflexible coat that is popular for protecting floors, but considered by some to be unsuitable for finishing antiques or other detailed pieces. Polyurethane varnish tends to de-laminate if subjected to heat or shock, leaving transparent or white patches. Because it doesn't penetrate into the wood, polyurethane also lacks the lustre of other treatments.
The precursors of expanding polyurethane foam are available in many forms, for use in insulation, sound deadening, floatation, packing material, and even cast-in-place upholstery padding. Since they adhere to most surfaces and automatically fill voids, they have become quite popular in these applications.
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