Polyurethanes or Urethanes are terms that are used interchangeably for a family of elastomers that were invented as ‘artificial rubbers’ during World War II. Polyurethane elastomers are a unique breed of engineering materials that combine the advantages of rigid plastics, metals and ceramics with the flexibility and elasticity of rubber.
Polyurethanes are available as Thermosets or Thermoplastics:Thermosets are a blend of a resin and a curative, that yield high-performance engineering properties that are not obtained by any other process. The molding process is similar to epoxies, Bakelite, etc., wherein a resin and a curative are blended to form a chemical cross-linking, and once this reaction takes place, the mix has to be poured into a mold cavity before it cures. The main advantages of this molding process are the ability to mold large or complex shaped parts with fairly inexpensive molds, and the ability to obtain a variety of physical properties and combination of properties that can be optimized for a given application. These are commonly known as Cast Polyurethanes.
Thermoplastic polyurethanes are available in pellet shapes, similar to other plastic materials and can be re-melted and molded by injection molding or extrusion. The size of the molded part or the cross section of the extrusion is limited.
Cast Polyurethanes: Cast Polyurethanes are a versatile family of elastomers that can be molded to optimize a combination of the following properties, depending on the application:
- Excellent resilience (high or low )
- Extra toughness and durability
- Very high load bearing capacity
- High tear strength and cut resistance
- Low compression set
- Very good impact resistance
- Outstanding abrasion resistance
- Good machinability
- Oil, solvent, water and ozone resistant.
- Non-marking
- Mold, mildew, fungus resistance
- Flex fatigue resistance
- Good electrical properties
- Friction (high or low)
Mechanical Properties: Cast Polyurethanes have an extraordinary combination of many of the above properties in the same material, unlike any other engineering material. The molder has the ability to vary the ratio between the resin and the curative to optimize some of these properties (and some times it may be at the cost of some other properties). So, a properly engineered compound can outlast an identical hardness part by a factor of several times.
Types of Polyurethanes: Cast Polyurethanes based on TDI, MDI, PPDI, polyethers, polyesters, polycaprolactones etc as building blocks, offer a wide variety of application possibilities and advantages.
Polyethers perform well in dynamic applications and in hydrolytic environments. They are extensively used in applications such as die-forming pads, press brake dies, die springs, fork lift tires, roller coaster wheels, rolls for steel and paper mills.
Polyesters have superior cut, tear and abrasion resistance as well as better resistance to oil and solvents. Typical applications include hydraulic seals, scraper blades, rolls for graphic arts, pipeline pigs, chute liners, etc.
