Era of elastomers

Imagine a material that combines the toughness and loadbearing capacity of metal, the abrasion resistance of ceramics and the resilience of rubber. Add several processing and performance options for economy and efficiency and you've got polyurethane elastomers, extremely versatile materials which can be subdivided into five groups:

• Millable gums, which can be processed on standardrubber processing equipment;

• Cast elastomers, both solid and microcellular cast systems, both heat- and room-temperature curable;

• Fibers of the type known widely as Spandex, usually produced by solution dry spinning of polyurethane polymers;

• Thermoplastic urethanes or TPU's, solid polymers injection moldable on conventional plastic processing equipment, and;

• RIM and RRIM elastomers, low density microcellular elastomers processed on reaction injection molding equipment.

The history of polyurethane elastomers began more then 150 years ago and continues today as new application ideas spur growth and diversification, while ongoing technical development works to make those ideas commercial realities.

The first 90 years of this history could aptly be called the Era of Discovery. It began in 1849, when Wurtz first isolated an isocyanate molecule, and takes us through 1937, when Otto Bayer first discovered the diisocyanate polymerization reaction. One goal of this early research was to duplicate or surpass the properties of nylon.

Through the 40s, 50s and 60s, a few key urethane producers built upon this fundamental understanding of the urethane reaction to develop process technology, products and markets for elastomers. This stage can appropriately be called the Era of Development Highlights included:

• Introduction of Adiprene B isocyanate cured millable gums by Du Pont in 1954;

• Introduction of Adiprene castable systems by Du Pont as a rubber replacement in 1958, and;

• Introduction of Spandex under the name "Fiber K" by Du Pont in 1958.

By the late 60s polyurethane elastomers were entering a period of accelerated growth and development.

This, then, is the Era of Response, when a fast-changing world created new market niches for elastomers. At the beginning of this era, in 1970, U.S. consumption of non-cellular polyurethane was about 56 million pounds in four major categories - millable gums, liquid castables, thermoplastic elastomers and fibers.

Now automotive applications may represent the major end use by volume. But they by no means reflect the broad application diversity of elastomers, which by now have found their way into uses from helicopter covers to artificial hearts, from cattle tags to running tracks.

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Phenolic, non-staining antioxidants

Phenolic antioxidants – a historical background

At the early beginning of manufacturing rubber goods, the producers were faced with rapid deterioration. It resulted in either hardening, brittleness or in sickness of the rubber goods. Around 1910 the first type of antioxidants were discovered, mainly naturally existing materials like creosote, asphalt and coal tar pitch. These were used until roughly 1925 when phenol, cresol, hydroquinone and aniline were used as antioxidants. Between 1925 and 1940 a big number of patents on antioxidants were issued, most of them based on secondary aromatic amine derivatives.

It was not until after 1940 when an increasing worldwide production of synthetic rubber, and consequently the development of tires based on synthetic rubber, called for a more demanding type of antioxidant, not only protecting the rubber goods and tires against oxygen and heat, but also against the severe cracking caused by ozone. The result was the development of a new class of antioxidants, p-phenylen-diamine derivates, which was soon after called antiozonant, due to their ability to protect rubber goods against cracking.

After 1945 the existing large synthetic rubber production had to be diverted to civilian use. The known and common used antioxidants and antiozonants were up to this stage staining types of chemicals. With the new approach to civilian use, there was a demand for non-staining antioxidants because synthetic rubber for civilian use included natural, white or light colored rubber goods, for example:

• sports rubber goods;

• surgical rubber goods;

• latex products, including foam;

• white side-walls;

• footwear;

• sheetings;

• flooring tiles;

• sponge rubber;

only to name a few. With the development of non staining antioxidants, it was possible to protect rubber against deterioration, caused by oxygen, heat, light and certain metals like copper and manganese, and still maintain the, natural colour of the rubber.

Antioxidants economical background

Antioxidants are rubber chemicals added to the rubber in the range of 1 to 2 %. Although the price of these antioxidants is ranging from $5 to $10/kg, the cost of protecting the rubber is only a few cents. It is estimated that the lifetime of rubber goods is three to five times longer when protected with antioxidants and one can get some ideas on the indirect value of antioxidants. Adding to this indirect value, costs of failure like idle machinery, break down of cars, blown tires and so on, then it is easy to understand that the costs of antioxidants becomes insignificant compared to the value of the improved rubber goods.

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