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A polyurethane is any polymer consisting of a chain of organic units joined by urethane links.
It can be made in a variety of textures and hardnesses by varying the particular monomers used and adding other substances. It is used in adhesives, elastomers, foams, fibers, seals, gaskets, condoms, carpet underlayment, and hard parts.
The largest use of polyurethane is foam rubber. Commonly to make foam rubber, water is mixed with a liquid polyurethane and chemically reacts with it. During the reaction Carbon dioxide is released, creating bubbles in the liquid, which then hardens to form a solid foam. This reaction forms a urea linkage -NC(O)N-, between the polyurethane molecules.
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 body temperature than at room temperature.
It is now also used in making tires. Modern roller blading and skate boarding 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.
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