Machine Design


Extremely wide variations in forms and in physical and mechanical properties are available in polyurethanes. Grades can range in density from 0.5 lb/ft3 in cellular form, to over 70 lb/ft3 in solid form, and in hardness from rigid solids at 85 Shore D to soft, elastomeric compounds.

Polyurethane polymers, produced by the reaction of polyisocyanates with polyester or polyether-based resins can be either thermoplastic or thermosetting. They have outstanding flex life, cut resistance, and abrasion resistance. Some formulations are as much as 20 times more resistant to abrasion than metals.

The noncellular grades - millable gums and viscous, castable, liquid urethanes are elastomeric thermoset types, processed by conventional rubber methods. These are discussed in the chapter, Thermoset rubber. Grades processed by thermoplastic methods are covered in the chapter, Thermoplastic elastomers. This chapter discusses the cellular polyurethane materials.

Polyurethane foams are thermoset materials that can be made soft and flexible or firm and rigid at equivalent densities. These foams, made from either polyester or polyether-type compounds, are strong, even at low density, and have good chemical resistance. Polyether-based foams havi greater hydrolysis resistance, are easier to process, and cost less. Polyester-based foams have higher mechanical propertie.% better oil resistance, and more uniform Cill structure. Both types can be rayed, sP' molded, foamed in place, or furnished'ir sheets cut from slab stock - buns, 30 to-A in. high, to 80 in. wide, and to 2013 ft 10 ft

A low-pressure molding process reaction-injection molding (RIM) is us most exclusively to produce ureth~ane some weighing as much as 100 lb. In the process, two or more highly reactive liquid systems are injected with high-pressure impingement mixing into a closed mold at low pressure, where they react to form a fin~ ished polymer. Depending on formulation, the polymer can be a rigid, integral-skin, microcellular urethane foam with a flexural tnodulus of over 100,000 psi, a soft, flexible elastomer with a flexural modulus as low as 7,000 psi, or a rigid structural foam having a density of 30 lb/ft3. Cycle time is short; parts can be demolded in less than a minute.

Reinforcement in the form of milled glass fiber, glass flake, or mineral filler increases the stiffness, thermal properties, and dimensional stability of RIM parts. Maximum glass content in reinforced reaction-injection molding (RRIM) is about 25% - a limit determined by the increased viscosity with increasing glass. Natural color of unpigmented RIM urethane parts is tan.

Flexible foams: Glass-transition temperatures (the temperature at which an elastomeric material becomes stiff and brittle) of flexible foams is well below room temperature. The foams can be pigmented to any color but, regardless of pigmentation, they yellow when exposed to air and light. Some types of flexible foams are excellent liquid-absorbing inedia, and can hold up to 40 times their weight of water.

Polyether-type foams are not affected by h igb -temperature aging, either wet or dry, but UV exposure produces brittleness and reduces properties. In use, these foams are always covered with a fabric or other material.

Most solvents and corrosive solutions decrease tear resistance and tensile strength and cause swelling of flexible foams. Swelling is not permanent, however, if the solvent is removed and the foam dried. However, the foams can be destroyed by strong oxidizing agents and hydrolyzed in strong acids or bases. Generally, the polyether foams are more resistant to hydrolytic degradation: the polyester foams are more resistant to oxidative attack.

Applications for polyester flexible urethane foam include gasketing, air filters, sound-absorbing elements, and clothing interliners (laminated to a textile material). The polyether types are used in automobile and recreational -vehicle seats, carpet underlay, furniture upholstering, bedding, and packaging.

Rigid foams: Bases for rigid foams are polymers having glass-transition temperatures higher than room temperature. The cells of rigid foam are about the same size and uniformity as those of flexible foam, but rigid foams usually consist of 90% closed cells. For this reason, water absorption is low. Compressing the foam beyond its elastic limit damages the cellular structure.

Rigid foams are blown with either carbon dioxide or fluorocarbons. Gas generated by vaporizatioq_of fluorocarbons, entrapped in the closed cells, gives the foam a very low thermal conductivity of 0.11 to 0.14 Btuin./hr-ft°- °F. Conductivity increases with age, however, to a constant value of aboul 0. 16. Conductivity Of C02-blown foam starts at about 0.22 Btu-in./hr-ft'-'F.

Properties of rigid foams vary with density and formulation. Compressive strength of a 2 lb/ft' foam is 30 to 40 psi for a polyether type and 25 to 40 for a polyester type (parallel to the direction of foam rise). The values increase in a 12 lb/ft° foam to 560 and 420 psi. Strengths are lower, by about 50%, in the direction perpendicular to foam rise.

Rigid urethane foams are used for thermal insulation of refrigerators, refrigerated trucks and railroad cars, cold-storage warehouses, and process tanks because of their low conductivity and high strength-toweight ratio. Other applications include flotation devices, encapsulation, structural and decorative furniture components, and sheathing and roof insulation for buildings.

Integral-skin foam: Urethane foams that are formed with integral skins range from soft and flexible types to impact-absorbing grades and rigid foams used in structural parts. Color can be added, but since the foams yellow on aging, black is most practical for the surface color. If other colors are required, coatings are recommended. The tough, high-density, integral skin is formed against the mold surface and the low-density core is produced by a blowing agent - usually a fluorocarbon.

Elastomeric foams of this type are used in automotive bumper and fascia systems and, most recently (reinforced with milledglass fibers), in fenders and other exterior body panels. The semirigid types are used in athletic protective gear, in automotive crash-protection areas, horn buttons, sun visors, and arm rests. Applications for the rigid structural foams include housings for computer systems, chair shells, furniture drawers, and sports equipment.

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