Although widely recognized as one of the most versatile families of elastomeric materials, many engineers forget to consider these urethane elastomers when designing with conventional rubber and plastic products. The combination of outstanding properties within this family allows polyurethanes to be considered as a design bridge between rubber and plastics. Properly formulated, these materials can range in hardness from very soft (10 Shore A) to that of a bowling ball (85 Shore D), and exhibit properties that can surpass many rubber materials in areas of abrasion and impact resistance, solvent and ozone resistance, resilience and compression set.
Property Variability and Potential Range of Applications
Polyurethanes in use today are remarkably different than those introduced in the mid 20th century. The expansion of the range of application possibilities and the utilization of the exceptional combination of available properties staggers the imagination of the development researcher.
Polyurethanes are called such due to the repeating urethane groups in the backbone of the polymers. Due to other groups that can appear in the structure and formulating choices of hydrogen bearing materials that react with diisocyanates (urea, ester, ether, allophanate, biuret, amine, and others), many of these polymers show little resemblance but are still classified as polyurethanes. For simplification, they are all commonly referred to as urethanes, recognizing that once completely polymerized, the polymer is formed.
The combination of chemical and engineering properties that can be made available with urethane structures include:
- Hardness: 10 Shore A to 85 Shore D
- Load-bearing capacity and excellent compression set
- Resilience: Backbone choice allows for resilience changes which can then remain constant as service temperature varies.
- Tear Resistance: Results generated can be measured and based on tensile factors of the polymer (die c). Or as split tears that really give a more realistic evaluation of cut growth resistance. As formulating stoichiometry is changed, improved tear strength can result.
- Abrasion Resistance: Two types are available; sliding and impingement. Chutes, hoses, and rebound applications require impingement resistance. While the frictional forces in certain applications (like rollers) require exceptional sliding abrasion resistance.
- Electrical properties can be excellent insulators with high dielectric strength and resistivity and low dielectric constant. Electrical properties can be moderate to highly conductive when static dissipation is desired.
- Great bond to many metals, elastomers and plastics, and select primers protect metallic inserts from attack by chemical of varying pH readings and temperature.
- The choice of backbone, isocyanate, crosslinkers, and other additives can improve the chemical resistance of cured products. Ozone resistance is exceptional. And when required, hydrolytic resistance can easily be developed.
Potential Range of Applications
Casting, coating, spraying, with thermoplastic or rubber handling equipment, can prepare urethanes. It can be a foam, an elastomer, a fiber, a coating – so where can we use these materials? Find out next week when we continue to talk about polyurethanes applications and end-uses.