Also termed polyurethanes, urethanes are a group of plastic materials based on polyether or polyester resin. The chemistry involved is the reaction of a diisocyanate with a hydroxyl-terminated polyester or polyether to form a higher-molecular-weight prepolymer, which in turn is chain-extended by adding difunctional compounds containing active hydrogens, such as water, glycols, diamines, or amino alcohols. The urethanes are block polymers capable of being formed by a literally indeterminate number of combinations of these compounds. The urethanes have excellent tensile strength and elongation, good ozone resistance, and good abrasion resistance. Combinations of hardness and elasticity unobtainable with other systems are possible in urethanes, ranging from Shore hardnesses of 15 to 30 on the "A" scale (printing rolls, potting compounds) through the 60 to 90 A scale for most industrial or mechanical goods applications, to the 70 to 85 Shore "D" scale. Urethanes are fairly resistant to many chemicals such as aliphatic solvents, alcohols, ether, certain fuels, and oils. They are attacked by hot water, polar solvents, and concentrated acids and bases.
Urethane Foams
Urethane foams are made by adding a compound that produces carbon dioxide or by reaction of a diisocyanate with a compound containing active hydrogen. Foams can be classified somewhat according to modulus as flexible, semiflexible or semirigid, and rigid. No sharp lines of demarcation have been set on these different classes as the gradation from the flexibles to the rigids is continuous. Densities of flexible foams range from about 16 kg/m3 at the lightest to 64 to 80 kg/m3 depending on the end use. Applications of flexible foams range from comfort cushioning of all types, e.g., mattresses, pillows, sofa seats, backs, and arms, automobile topper pads, and rug underlay, to clothing interliners for warmth at light weight.
Flexible Types
The techniques of manufacture of flexible ure-thane foam vary widely, from intermittent hand mixing to continuous machine operation, from prepolymer to one-shot techniques, from slab-forming to molding, from stuffing to foamed-in-place.
Future applications envision the flexible foam not as a substitute for latex rubber foam or cotton, but as a new material of construction allowing for design of furniture, for example, that is essentially all foam with a simple cloth cover and a very simple metal-supporting framework.
Rigid Types
Densities from about 24 to 800 kg/m3 on the semirigid side have been produced with corresponding compression strengths again for particular end uses ranging from insulation to fully supporting structural members. The usefulness of the urethane system has been in the foam-in-place principle using a host of containing wall materials.
Applications in the more rigid foam field have been thermal insulation of all types (low-temperature refrigeration ranging from liquid nitrogen temperatures up to the freezing point of water and high temperature insulation of steam pipes, oil lines, etc.); shock absorption such as packaging, crash pads, etc., where the higher hysteresis values produce either a better one-time high impact "crash" use or, more often, lower amplitude but higher frequency container end use; filtration (air, oil, etc., where a large surface-to-volume ratio is needed with a simple technique to produce a reusable filter to allow for its initially higher cost factor); structural (building applications of all kinds combining a good thermal as well as structural behavior, filling of building voids, and curtain walls are some basic applications); flotation (boats, buoys, and every other imaginable object afloat represents some possible application of urethane foams); and, finally, general-purpose applications that include all other uses such as decorative applications.
Rigid foams can be produced using a simple spray technique and a number of machines are sold on the market for this technique. Time-consuming layup of foam is eliminated using this method. Insulation of walls, tanks, etc. are applications in use today. With the use of low-vapor-pressure isocyanates such as MDI (4,4′-diphenylmethane diisocyanate), the potential irritant hazard during spraying is greatly lowered. Self-adhesion of the sprayed foam is a valuable asset of this type of system.
Urethane foams offer advantages over many of the better-known foams such as latex foam rubber, polystyrene, and polyethylene, with the combination of excellent properties and lower installed costs. Depending on the application, a lower foam density can be used with similar load-bearing properties, also one having an extremely low thermal conductivity can be fabricated. The oil resistance, high-temperature resistance, good high-tensile properties, good permanence properties, resistance to mildew, resistance to flammability, and so on are in general the types of properties that, combined with foamed-in-place technology, put urethane foam far ahead of competitive materials.
Other Urethanes
Thermoplastic polyurethanes (TPU) include two basic types: esters and ethers. Esters are tougher, but hydrolyze and degrade when soaked in water. There also are TPUs based on polycaprolactone, which while technically being esters, have better resistance to hydrolysis. TPUs are used when a combination of toughness, flex resistance, weatherability, and low-temperature properties are needed. These materials can be injection-molded, blow-molded, and extruded as profiles, sheet, and film. Further, TPUs are blended with other plastic resins, including polyvinyl chloride, ABS, acetal, SAN, and polycarbonate.
Urethane elastomers are made with various isocyanates, the principal ones being TDI (tolylene diisocyanate) and MDI (4,4′-diphe-nylmethane diisocyanate), reacting with linear polyols of the polyester and polyether families. Various chain extenders, such as glycols, water, diamines, or aminoalcohols, are used in either a prepolymer or a one-shot type of system to form the long-chain polymer.
Flexible urethane fibers, used for flexible garments, are more durable than ordinary rubber fibers or filaments, and are 30% lighter in weight. They are resistant to oils and to washing chemicals, and also have the advantage that they are white in color. Spandex fibers are stretchable fibers produced from a fiber-forming substance in which a long chain of synthetic molecules are composed of a segmented poly-urethane. Stretch before break of these fibers is from 520 to 610%, compared to 760% for rubber. Recovery is not as good as in rubber. Span-dex is white and dyeable. Resistance to chemicals is good but it is degraded by hypochlorides.
There are six basic types of polyurethane coatings, or urethane coatings, as defined by the American Society for Testing and Materials (ASTM), Specification D16. Types 1, 2, 3, and 6 have long storage life and are formulated to cure by oxidation, by reaction with atmospheric moisture, or by heat. Types 4 and 5 are catalyst-cured and are used as coatings on leather and rubber and as fast-curing industrial product finishes. Urethane coatings have good weathering characteristics as well as high resistance to stains, water, and abrasion.
Fabrication
Urethane elastomers can be further characterized by the method of fabrication of the final article. Three principal types of fabrication are possible: (1) casting technique where a liquid prepolymer or a liquid mixture of all initial components (one-shot) is cast into the final mold, allowed to "set" and harden, and is then removed for final cure; (2) millable gum technique where conventional rubber methods and equipment are used to mill the gum, add fillers, color, etc., and/or banbury, extrude, calender, and compression mold the final shaped item; (3) thermoplastic processing techniques where the resin can be calendered, extruded, and injection- or blow-molded on conventional plastic machinery in final form (an important benefit here is that scrap can be reground and reused in fabricating other parts).
The choice of the proper method of fabrication largely depends on the economics of the process, because the properties of the final product may be about the same regardless of the method of fabrication. If a few large-volume items are needed, casting these into a single mold is usually more economical. However, if many thousands of small, intricate pieces are needed, usually injection molding is the preferred, more economical method of fabrication.
Uses
Applications of urethane elastomers have been developed where high abrasion resistance, good oil resistance, and good load-bearing capacity are of value, as in solid tires and wheels, especially of industrial trucks, the shoe industry, drive and belting applications, printing rolls, gasketing in oil, etc. Other applications include vibration dampening; for example, in hammer heads, air hammer handles, shock absorption underlays for heavy machinery, etc.; low coefficient of friction with the addition of molybdenum disulfide for self-lubricating uses as ball and socket joints, thrust bearings, leaf spring slide blocks, etc. In the electrical industry, cable jacketing and potting compounds are developing as important uses. Various systems of ure-thane elastomers with specific fillers have been developed into an important class of caulks and sealants, which is just beginning to take hold in applications such as concrete road-expansion joints, building caulking, and so on, in direct competition with such older materials as the polysulfides but at a much lower price and superior properties.
A host of other applications varies from adhesive bonding of fibers of all kinds to rocket fuel binders of the more exotic variety, which are becoming so important in the U.S. national defense picture. Therefore, it is imperative that design engineers understand fully the material they are using and how they intend to utilize it in the final piece of equipment. For example, one recommendation is to limit the use of ure-thanes to below 82°C in water for continuous exposures. Dry uses can go somewhat higher, e.g., to 107°C for certain systems. In oil, exposures can be up to 121 C. Disregard of such limitations can result in failures, but the design engineer can eliminate these by the proper choice of material. On the other hand, the design engineer should choose the urethanes for their virtues, such as hardness and elasticity, where other materials such as natural and other synthetic rubbers may fail.
Properties
The urethanes have excellent tensile strengths and elongation, good ozone resistance, and good abrasion resistance. Knowledge of these properties is mandatory for good engineering design.
The greater load-bearing capacity of ure-thanes as compared to other elastomers is noteworthy, for it leads to smaller, less costly, lower-weight parts in equivalent applications. Tear strength is extremely high, which may be important in particular applications along with the very high tensile strengths. The high abrasion resistance has made possible driving parts for which no other materials could compete. However, in every such dynamic application, the engineer must design the part to allow for the higher hysteresis losses in the urethane. Whereas in some applications such as dampening, the higher hysteresis works to advantage, in others hysteresis will lead to part failure if the upper temperature limit is thereby exceeded. Redesign of the part (thinner walls, etc.) to allow for greater dissipation of the heat generated will permit the part to operate successfully. This has proved to be the case many times.
Urethane elastomers generally have good low-temperature properties. The same hysteresis effect works in reverse here so that a part in dynamic use at temperatures as low as -51 °C, while stiff in static exposure, immediately generates enough heat in dynamic use to pass through its second-order transition and does not show any brittleness but becomes elastic and usable. By proper choice of the polyester or polyether molecular backbone, lower use temperatures (as low as -62°C) have been formulated in urethane elastomers.
In addition to good mechanical properties, urethanes have good electrical properties, which suggest a number of applications. Oxygen, ozone, and corona resistances of this system are generally excellent.
