Polyurethane is frequently referred to as the "most versatile polymer." It is the hard rigid resin used to make the bumper beams of cars and the soft foam armrests on your chair. It can be found in refrigerator insulation, in-line skate wheels, and shoe soles. It houses electronics, coats, golf balls, and forms the elastic waistbands of undergarments. It is the diversity of the materials used to synthesize polyurethanes that give them their "versatility." Because of this versatility, polyurethanes are useful in four major types of products: foams, fibers, coatings and elastomers.
Polyurethane is the reaction product of polyisocyanates and polyols. This reaction was first carried out in 1937 by Otto Bayer in Leverkusen, Germany.1 Commercial polyurethanes were first available to the marketplace in 1943.2 The resulting polymer has "hard" and "soft" segments along the polymer chain. The isocyanate portion of the molecule makes up the "hard" segment while the polyol portion (which can be polyester or polyether-based) makes up the "soft" segment. The bulk physical properties of the polymer are determined by the balance of hard and soft segments. Polyurethanes with large hard segments are strong, rigid plastics. Polyurethanes with large soft segments are flexible, rubbery materials. The soft polyurethanes belong to the family of macromolecules called elastomers. These are substances that exhibit a high degree of rubber-like elasticity similar to latex properties. Polyurethane elastomers are noted for extremely good abrasion resistance and toughness combined with elasticity. Because of the wide range of commercial polyisocyanates and polyols in the market today, polyurethanes can be tailored to meet many end-use requirements.
There are many ways to synthesize and process polyurethanes. These can be categorized as foams (flexible, rigid and integral skin), elastomers, coatings and fibers.3 Flexible foams, developed in 1952, are widely used in the furniture and bedding industries. The use of flexible urethane foams for cushions for furniture and automobiles displaced natural rubber foam in these applications because of improved strength, lower density, and easier fabrication. The polyurethane is also formulated to offer durability, comfort, good humidity transfer and physiological compatibility. Rigid foams, developed in the early 1940s, are used as insulation in prefabricated construction parts, refrigerators, and packaging. Integral skin foams are unique because they have a continuous, nonporous surface over a foamed body. They are used in armrests, steering wheels, and shoe soles, to name a few. For these applications, polyurethane is formulated for good elasticity, high abrasion resistance, and shock absorbency. Polyurethane coatings (lacquers, water-based dispersions) are used in paints, wood finishing, floor protection, and the textile, paper and leather industries. Polyurethane elastomeric fibers comprise about 85% by weight of spandex. These elastane fibers have elastic properties with high extensibility and are replacing rubber because of their ability to produce finer fibers, dyeability, and chemical resistance.
In recent years, polyurethanes have entered the medical arena. Both hard, rigid polyurethane resins and soft, elastomeric polyurethane resins have found uses in the healthcare industry. Some medical applications include light-weight casts, artificial kidney components, and catheters. According to the Polyuvethane Handbook by Gunter Oertel, "..,polyurethanes appear to be the most promising class of polymers in in-vivo applications. Reasons for this are their high mechanical strength, flexibility, fatigue resistance, and tissue compatibility."
As concern over latex allergy heightens, more and more healthcare workers are turning to synthetic gloves. Nitrile, polychloroprene, block copolymer, and vinyl gloves are among the choices available in the marketplace today. Glove manufacturers are searching for the material that best mimics latex in barrier properties, strength, fit, feel, and comfort. Because of its proven track record for versatility, polyurethane is a good choice for a synthetic surgical glove. Water dispersible polyurethane elastomers are now being coagulant-dipped to manufacture medical gloves, along with polyurethane elastomer solvent-dipped industrial gloves. For these special applications, the question of polyurethane barrier properties versus other synthetic materials needs to be addressed. No studies have been published examining the passage of bloodborne pathogens through a polyurethane membrane. Recently, Nelson Laboratories4 completed a study of synthetic medical gloves including polyurethane, evaluating the barrier properties using the bacteriophage
. The size of bacteriophage is similar to Hepatitis B, Hepatitis C and HIV viruses. Polyurethane gloves were found to be comparable to other synthetic (nitrile, polychloroprene, etc.) gloves sold to the healthcare industry. Polyurethane does not contain latex proteins, and does not require chemical crosslinking or accelerators. Therefore, polyurethane addresses both Type I and Type IV allergic reactions. Polyurethane medical gloves do not need to be chlorinated to render them powder-free. Unlike natural rubber latex, which has a very tacky surface if unpowdered, polyurethane has very little tack, but an acceptable grip. Eliminating the chlorination process eliminates the potential for discoloration, degradation, and slippery effects often seen with chlorinated natural rubber latex gloves. The specific polyurethane chosen for medical gloves has good tensile strength and puncture resistance, while remaining soft enough to offer excellent fit, feel, and comfort. The medical community now has a new alternative to natural rubber latex medical gloves. As polyurethane chemistry continues to advance, this versatile polymer will undoubtedly find its way into new and diverse industries.
REFERENCES
- Oertel, Gunter: Polyurethane Handbook 2nd Edition. Hanser, New York 1993.
- Seymour, R.B. and Carraher, Jr., C.E.: Polymer Chemistry 2nd Edition Marcel Dekker, Inc., New York 1988.
- Gillis, H.R.: Polyurethanes: Diversity of uses fuels rapid market growth in foams, films, and parts. Modern Plastics Encyclopedia '98.
- Report on file, Ansell Perry, Massillon, Ohio 1998.
