Introduction to Polyurethane
Polyurethane (or "urethane") elastomers are one type of a large family of elastic polymers called rubber. There are 14 types of rubber in general use today. All of them have been commercially successful, but they are all different in several ways.
Technically, It is incorrect to refer to polyurethanes as plastic. Polyurethanes by definition are rubber. The general characteristics of rubber are:
The outstanding abrasion resistance of urethane elastomers makes it an excellent material in hundreds of applications where severe wear presents a serious problem such as ore mining, and sand & gravel processing.
Polyurethane has many advantages over metals, plastics and conventional rubbers. The following chart compares the various materials.
While polyurethane elastomers are usually noted for their mechanical properties, they also possess excellent resistance to many environmental influences. Polyurethane benefits from advanced technology, which allows superior stability in oils and solvents as compared to conventional hydro-carbon rubber materials.
Like all engineering materials, urethanes have limitations. 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).
Polyurethane (or "urethane") elastomers are one type of a large family of elastic polymers called rubber. There are 14 types of rubber in general use today. All of them have been commercially successful, but they are all different in several ways.
Thirteen of these elastic polymers are known as conventional rubber. That means they are mixed, milled, and molded by techniques which have been in use by the rubber industry for nearly 100 years. Polyurethane is known as synthetic rubber. Its raw materials are liquid and enter molds at low pressure and are generally "cured" at the same temperature as that which they are mixed.
Polyurethanes are not plastics
Technically, It is incorrect to refer to polyurethanes as plastic. Polyurethanes by definition are rubber. 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.
- The material becomes stiffer with greater deflection and velocity of impact.
- They are affected by the environment and conditions under which they are employed.
Abrasion resistance of polyurethane
The outstanding abrasion resistance of urethane elastomers makes it an excellent material in hundreds of applications where severe wear presents a serious problem such as ore mining, and sand & gravel processing.
Field applications have proven polyurethanes provide a definite wear advantage due to its superior abrasion resistant properties. In working service, molded Polyurethane has outworn rubbers, plastics, steel, and even ceramics - often by a factor of 10 to 1.
The table below provides a comparison of the abrasion resistance of polyurethane versus various rubbers and plastics.
Polyurethane vs. metals, plastics & rubbers
Polyurethane has many advantages over metals, plastics and conventional rubbers. The following chart compares the various materials.
| Urethane vs. Metal | Urethane vs. Plastic | Urethane vs. Rubber |
|---|---|---|
|
|
|
Environmental properties of polyurethane
While polyurethane elastomers are usually noted for their mechanical properties, they also possess excellent resistance to many environmental influences. Polyurethane benefits from advanced technology, which allows superior stability in oils and solvents as compared to conventional hydro-carbon rubber materials.
| Environmental Resistance Polyurethane Elastomers vs. Conventional Rubbers |
|||||
|---|---|---|---|---|---|
| Environment | Polyester Elastomers | Polyether Elastomers | Natural Rubber | Neoprene Rubber | Butadiene Rubber |
| Heat | G | F | F | G | G |
| Cold | G | G | E | G | G |
| Weather | E | E | P | G | F |
| Ozone Resistance | E | E | P | F | P |
| ASTM No.1 Oil | E | F | P | G | P |
| ASTM No.3 Oil | E | P | G | P | P |
| Aliphatic Solvents e.g., Heptane |
E | F | P | G | P |
| Chlorinated Solvents e.g., Trichloroethane |
F-G | P | G | P | P |
| Aromatic Solvents e.g., Toulene |
F | P | P | F | P |
| Dilute Acids e.g., 5% HNO |
P-F | F | G | G | F-G |
| Dilute Alkalies e.g., 5% NaOH |
P-F | F | G | G | F-G |
E=excellent G=good F=fair P=poor
Limitations of Polyurethane
Like all engineering materials, urethanes have limitations. 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).
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).
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.
Long term exposure to hot, humid environments should be avoided. Some urethanes are much more resistant than others to this type of environment.
Certain chemicals such as concentrated acids and polar solvents attack urethanes, and urethanes should not be put into continuous service in these environments.