Polyurethane

From Free net encyclopedia

A polyurethane is any polymer consisting of a chain of organic units joined by urethane links. It is widely used in flexible and rigid foams, durable elastomers and high performance adhesives and sealants, fibers, seals, gaskets, condoms, carpet underlayment, and hard plastic parts.

1 Production
2 Uses
3 References
4 External links

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Production

The main polyurethane producing reaction is between a diisocyanate (aromatic and aliphatic types are available) and a polyol, typically a polyethylene glycol or polyester polyol, in the presence of catalysts and materials for controlling the cell structure, (surfactants) in the case of foams. Polyurethane can be made in a variety of densities and hardnesses by varying the type of monomer(s) used and adding other substances to modify their characteristics, notably density, or enhance their performance. Other additives can be used to improve the fire performance, stability in difficult chemical environments and other properties of the polyurethane products.

Though the properties of the polyurethane are determined mainly by the choice of polyol, the diisocyanate exerts some influence. The cure rate is influenced by the functional group reactivity and the number of isocyanate groups, the functionality. The mechanical properties are influenced by the functionality and the molecular shape. The choice of diisocyanate also affects the stability of the polyurethane upon exposure to light. Polyurethanes made with aromatic diisocyanates yellow with exposure to light, whereas those made with aliphatic diisocyanates are stable.Template:Ref

Softer, elastic and more flexible polyurethanes result when linear difunctional polyethylene glycol segments, commonly called polyether polyols, are used to create the urethane links. This strategy is used to make spandex elastomeric fibers and soft rubber parts, as well as foam rubber. More rigid products result if polyfunctional polyols are used, as these create a three-dimensional cross-linked structure which, again, can be in the form of a low-density foam.

An even more rigid foam can be made with the use of specialty trimerization catalysts which create cyclic structures within the foam matrix, giving a harder, more thermally stable structure, designated as polyisocyanurate foams. Such properties are desired in rigid foam products used in the construction sector.

Polyurethane foam (including foam rubber) is usually made by adding small amounts of volatile materials, so-called blowing agents, to the reaction mixture. These can be simple volatile chemicals such as acetone or methylene chloride, or more sophisticated fluorocarbons which yield important performance characteristics, primarily thermal insulation.

Another common route to produce foams is the addition of water to one of the liquid precursors of polyurethane before they are mixed together. This reacts with a portion of the isocyanate, generating carbon dioxide throughout the liquid, creating relatively uniform bubbles which then harden to form a solid foam as polymerization progresses.

The presence of water means that a small proportion of reactions result in urea linkages —NC(=O)N—, rather than urethane linkages, so that the resulting material should technically be called poly(urethane-co-urea).

Careful control of viscoelastic properties — by modifying the catalysts and polyols used —can lead to memory foam, which is much softer at skin temperature than at room temperature.

There are then two main foam variants: one in which most of the foam bubbles (cells) remain closed, and the gas(es) remains trapped, the other being systems which have mostly open cells, resulting after a critical stage in the foam-making process (if cells did not form, or became open too soon, foam would not be created). This is a vitally important process: if the flexible foams have closed cells, their softness is severely compromised, they become pneumatic in feel, rather than soft; so, generally speaking, flexible foams are required to be open-celled.

The opposite is the case with most rigid foams. Here, retention of the cell gas is desired since this gas (especially the fluorocarbons referred to above) gives the foams their key characteristic: high thermal insulation performance.

A third foam variant, called microcellular foam, yields the tough elastomeric materials typically experienced in the coverings of car steering wheels and other interior automotive components.

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Uses

Polyurethane products have many uses. Over three quarters of the global consumption of polyurethane products is in the form of foams, with flexible and rigid types being roughly equal in market size. In both cases, the foam is usually behind other materials: flexible foams are behind upholstery fabrics in commercial and domestic furniture; rigid foams are inside the metal and plastic walls of most refrigerators and freezers, or behind paper, metals and other surface materials in the case of thermal insulation panels in the construction sector.

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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Varnish

Polyurethane materials are used in coatings and varnishes used in furniture manufacture, carpentry or woodworking. A polyurethane varnish 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.

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Glue

Polyurethane is used as an adhesive, especially as a woodworking glue. Its main advantage over more traditional wood glues is its water resistance. It was introduced in the general North American market in the 1990s as Gorilla Glue and Excel, but has been used much longer in Europe.

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Wheels

Polyurethane is also used in making solid tires. Modern roller blading and skateboarding became economical only with the introduction of tough, abrasion-resistant polyurethane parts. Other constructions have been developed for pneumatic tires, and microcellular foam variants are widely used in tires on wheelchairs, bicycles and other such uses. These latter foam types are also widely encountered in car steering wheels and other interior and exterior automotive parts, including bumpers and fenders.

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Furniture

Polyurethane is also used in furniture manufacture for casting soft edges around table tops and panel that are stylish, very durable and prevent injury. These are used in school tables, hospital and bank furniture as well as shop counters and displays.

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Automobile Seats

Flexible and semi-flexible polyurethane foams are used extensively for interior components of automobiles, in seats, headrests, armrests, roof liners and instrument panels.

Polyurethanes are used to make automobile seats in a remarkable manner. The seat manufacturer has a mold for each seat model. The mold is a closeable "clamshell" sort of structure that will allow quick casting of the seat cushion, so-called molded flexible foam, which is then upholstered after removal from the mold.

It is possible to combine these two steps, so-called in-situ, foam-in-fabric or direct moulding. In this case, 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. The assembly operator first places a complete, fully-assembled seat cover in the mold and adjusts it so that the vacuum in the manifold pulls the seat cover snugly against the mold surface. 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 about two to three minutes, depending on the size of the seat and the precise formulation and operating conditions. Then the mold is usually opened slightly for a minute or two for an additional cure time, before the fully upholstered seat is removed. The operator then trims any excess seat cover fabric and puts the finished seat onto a conveyor.

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Houses, Sculptures, & Decorations

The walls and ceiling (not just the insulation) of the futuristic Xanadu House were built out of polyurethane foam. Domed ceilings and other odd shapes are easier to make with foam than with wood. Foam was used to build oddly-shaped buildings, statues, and decorations in the Seuss Landing section of the Islands of Adventure theme park.

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Surfboards

Some surfboards are made with a solid polyurethane core

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Condoms

Several types of condoms are made out of polyurethane, including the Trojan Supra and Durex Avanti. These condoms are ideal for users sensitive or allergic to traditional latex condoms, and have been tested to provide the same level of protection from STDs and pregnancy.

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