What Is It?
Think of polyurethanes and you think of soft, stretchy, materials, and fabrics (Lycra or Spandex), but they can also be leathery or rigid. Like PVC, polyurethanes have thermoplastic, elastomeric, and thermosetting grades. They are easily foamed; some 40% of all PU is made into foam by mixing it with a blowing agent. The foams can be open- or closed-cell, microcellular, or filter grades. PU is a versatile material.
PU foams are cheap, easy to shape, and have good structural performance and resistance to hydrocarbons.
- Most foamable PUs are thermosets, so they are shaped by casting rather than heat-molding, giving a high surface finish and the potential for intricate shapes.
- In solid form PUs can be produced as sheet or bulk shapes.
- For load-bearing applications such as power-transmission belts and conveyer belts tpPUs are reinforced with nylon or aramid fibers, giving flexibility with high strength.
tpPUs can have a wide range of hardness, softening point, and water absorption. They are processed in the same way as nylon, but are considerably more expensive. tppu fibers are hard, wiry, and have a low softening point compared to nylon; they have been used as bristles on brushes. elpu fibers are much more common – they are used in clothing and flexible products under the trade-name of Spandex or Lycra. elpu foams are used for mattresses, seating of furniture, and packaging; more rigid foams appear as crash protection in cars, and, in low density form (95% gas) as insulation in refrigerators and freezers. These flexible resins are good in laminate systems where damping is required. elpu is amorphous, tppu is crystalline; elpu is commonly cast or drawn, tppu is commonly injection molded or extruded. Polyurethane elastomers have exceptional strength (up to 48 mpa) and abrasion resistance, low compres- sion set, and good fuel resistance. They have useful properties from −55 C to 90 C. pu foam is usually processed by reaction-injection molding: the resin and hardener are mixed and injected into a mold where they react and set. pu can be bonded with polyurethane, nitrile, neoprene, epoxies, and cyanacrylates adhesives. It has good resistance to hydrocarbons, degrades in many solvents, and is slow burning in fire.
Cushioning and seating, packaging, running shoe soles, tires, wheels, fuel hoses, gears, bearings, wheels, car bumpers, adhesives, fabric-coatings for inflatables, transmission belts, diaphragms, coatings that are resistant to dry-cleaning, furniture, thermal insulation in refrigerators and freezers; as elastomers: truck wheels, shoe heals, bumpers, conveyor belts, and metal forming dyes.
pu is synthesized from diisocyanate and a polyester or polyether. The diisocyanate is toxic, requiring precautions during production. pu itself is inert and non toxic. The flammability of pu foam, and the use of cfcs as blowing agents in the foaming process were, at one time, a cause for concern. New flame retardants now mean that pu foams meet current fire safety standards, and cfcs have been replaced by CO2 and hfcs which do not have a damaging effect on the ozone layer.
Thermoplastic pus can be recycled (thermosetting pus cannot), and when all useful life is over, incinerated to recover heat. Legislation for return of packaging and disposal problems may disadvantage pu.
Almost all polyurethanes are co-polymers of linked polyester, alcohols, and isocyanate groups. Depending on the mix, polyurethanes can be soft and elastic (Lycra, Spandex) or nearly rigid (track-shoe soles, floor tiles), making pu one of the most versatile of polymers.