INTRODUCTION TO URETHANE ELASTOMERS
Polyurethane (or "urethane") elastomers are one type of a large family of elastic polymers called rubber. There are 14 types in general use. All of them have been commercially successful, but they are all different in several ways. These charts provide a quick initial screening guide.
Thirteen of these elastic polymers are called conventional rubber. That means they are mixed, milled, and molded by techniques which have been in use by the rubber industry for the past 80 years. Polyurethanes are liquids and can be cast in low pressure molds.
URETHANES SHOULD NOT BE CONFUSED WITH PLASTICS
It is incorrect to refer to thermoset polyurethanes as plastic. Polyurethanes by definition are rubber. (We do process some thermoplastic urethanes by techniques used for plastics.) The general characteristics of rubber are:
· can be highly deformed without breaking
· has the ability to recover rapidly and repeatedly from deformation
· deformation is large in proportion to the original dimensions
· large deformations are produced at relatively low stress levels
· desired stress-strain properties can often be obtained by compounding
· stress-strain characteristics are non-linear, thus the material becomes stiffer with greater deflection and velocity of impact
· they are affected by the environment and conditions under which they are employed
URETHANE ADVANTAGES VERSUS METAL, PLASTIC, AND ORDINARY RUBBER
Urethanes have many advantages over metals, plastics and conventional rubbers:
| Urethane vs. Metal |
|
Lightweight Noise Reduction Abrasion Resistance Less Expansive Fabrication Corrosion Resistance Resilience Impact Resistance Flexibility Easily Moldable Non-Conductive Non-Sparking Often Lower Cost |
| Urethane vs. Plastics |
|
High Impact Resistance Elastic Memory Abrasion Resistance Noise Reduction Variable Coefficient of Friction Resilience Thick Section Molding Lower Cost Tooling Low Temperature Resistance Resistance to Cold Flow (or Compression Set) Radiation Resistance |
| Urethane vs. Rubber |
|
High Abrasion Resistance High Cut & Tear Resistance Superior Load Bearing Capacity Thick Section Molding Without a Cutting Gradient Colorability Oil Resistance Ozone Resistance Radiation Resistance Broader Hardness Range Castable Nature Lower Cost, Low Pressure Tooling |
LIKE ALL ENGINEERING MATERIALS, URETHANE HAS LIMITATIONS
1. Polyurethane rubber should not be used in dynamic applications above 200°F (93°C). When tested at 200°F (93°C) their properties are only half of those measured at 75°F (25°C). They heat age well however, and the effect of high temperatures up to 250°F (120°C) for weeks on physical properties is almost completely reversible when tested again at 75°F (25°C).
2. In most dynamic applications we recommend staying at temperatures below 160°F (70°C). The normal, high property working range is -40°F to 160°F (-40°C to 70°C). At 160°F (70°C) the properties of the elastomer begin to show a decline. The bond between urethane and metal weakens considerably above 160°F (70°C).
3. Urethanes exhibit high hysteresis and low thermal conductivity. They do not dissipate heat built up by dynamic action quickly. Avoiding heat build-up in an elastomeric part is a paramount consideration in design. In practice, this is usually done by controlling the amplitude of the deflection. For instance, using urethane elements in series allow large deflections (see design discussion).
4. Long term exposure to hot, humid environments should be avoided. Some urethanes are much more resistant than others to this environment. We can help you select the correct type.
5. Certain chemicals such as concentrated acids and polar solvents attack urethanes, and urethanes should not be put into continuous service in these environments. Refer to these tables.
THE DISTINCTION OF POLYURETHANE RUBBER
Polyurethane raw materials are liquid, which permits them to be pumped, metered, mixed and dispensed by machines under very precise control of temperature and ingredient proportions. They enter molds as a liquid at low pressure and are "cured" at the same elevated temperature as that which they are mixed. This unique characteristic allow us to mold very large urethane parts with thick cross-sections which are completely uniform throughout.
Selection guide to elastomers which generally qualify for use from -40°F to 160°F based on environmental and mechanical properties.
Useful temperature range of commercial elastomers.
Load bearing capacity of neoprene vs. polyurethane.
Typical elastomer characteristics.