Polyurethane
From Freepedia
- This article is about polyurethane polymers. For information on mainframe processor units (PUs), please see IBM mainframe.
A polyurethane is any polymer consisting of a chain of organic units joined by urethane links. It is used in adhesives, elastomers, foams (both foam rubber and rigid materials), fibers, seals, gaskets, condoms, carpet underlayment, 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 proportion 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 has many and varied uses.
Polyurethane is 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 furniture 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.
Polyurethane foam is used extensively for interior components of automobiles where it is commonly found in seats, headrests, armrests, and instrument panels.
External Links
Polyurethanes are used to make automobile seats in a remarkable manner. The seat manufacturer has a mold for each seat model. The mold a closeable "clamshell" sort of structure that will allow quick casting of the seat. The inner surfaces of the mold have hundreds of small holes that all connect to a vacuum manifold. This creates a constant air flow from the core of the mold to the manifold. Then the assembly operator places a complete, fully-assembled seat cover in the mold and adjusts it so that the vacuum in the manifold is constantly pulling the seat cover snugly against the mold. In some operations, this effect is improved by adding a thin pliable plastic film as a backing to the fabric to help the vacuum work more effectively. When the seat cover is in place, the operator then places the metal frame of the seat into the mold and closes the mold. At this point the mold contains what could be visualized as a "hollow seat", a seat fabric held in the correct position by the vacuum manifold and containing a hollow space with the metal frame in place. The next step is to inject the polyurethane chemical mixture into the mold cavity. This is a two-part mixture that is metered exactly through a mixing head. Then the mold is held at a preset reaction temperature until the chemical mixture has foamed, filled the mold, and formed a stable soft foam. The time required is quite short. Then the mold is usually opened slightly for a minute or two for an additional cook time, then the seat is removed. The operator then trims the excess seat cover fabric and puts the finished seat onto a conveyor.
