Properties of Elastomers

SAE J200 (ASTM D2000) is a classification system for specifying rubber products for automotive use which has been adopted by other industries as a general guideline for relative performance. The following chart positions elastomers by their resistance to heat aging and swelling in oil. Although this system is a valuable tool in characterizing and positioning elastomers, it does not address the chemical and thermal resistance in hostile environments encountered in chemical processes. Due to the chemical attack on the backbone and or crosslinks of the rubber part, one must rely on field experience, laboratory testing and guidance from material experts.

Elastomers are rated for their compatibility to a variety of media including fluids, weather, ozone, etc. Usually, elastomers must perform in more than one media so it is important to understand all the elements that the elastomer will be exposed. In chemical processing environments, it is important to understand the elastomers resistance to a variety of fluids.

Elastomers in a fluid environment may absorb the liquid and swell, react chemically with the fluid and change the polymer structure, or solubles may be extracted from the elastomer causing an actual decrease in volume. In some cases, it is not the polymer itself which is adsorbing or desorbing, but the other ingredients that the compounder has used to make the part. For this reason, it is important to contact the part manufacturer in critical applications.

Most chemical resistance guides (including this one), report the percentage of swelling to indicated part performance in a certain chemical, with below 5 to 10 percent swelling considered excellent. Although swelling is a key criterion for part compatibility and one that is generally accepted as a standard for compatibility, other properties should be measured as well. A knowledgeable supplier or part manufacturer can provide additional guidance.

The mechanical properties of an elastomer will generally change after prolonged exposure to high temperatures. Natural rubber, for example, will become gummy compared to neoprene which slowly hardens. The extent to which hardening or softening is undesirable will depend upon the service required. The rate at which properties of an elastomer change increases logarithmically with the temperature. Relative small changes in temperature may, therefore, cause large differences in the degree of deterioration.
Heat aging tests are usually based on 70-hour exposure in thermally controlled hot air. While this may correlate well with longer exposure periods, it may not correspond to higher temperature in other chemical media. In critical applications, engineers should consult with the part manufacturers to see if long-term exposure in a particular media have been evaluated and documented. In addition, tests should be performed under conditions which closely mirror the actual process conditions.

	Normal Recommended Temperature Range
	Possible with Special Types or Formulations
	Extended Temperature Range for Short Term Only

There are several mechanical properties that will help determine the serviceability of elastomer parts in particular applications, most of these properties can be manipulated by the part manufacturer through the compounding process. Compounders can alter the ingredients and/or cure system to impart the required characteristics.

There are many mechanical properties that influence an elastomers performance including compression set, tensile strength, elongation, hardness and abrasion resistance. In determining specification requirements, some of these properties (or other properties) may be significant.

Compression set - Rubbers deform under load and rarely return completely to their original form. The difference between the initial and final dimensions is known as compression set or permanent set. The main difficulty with the interpretation of compression set is that the testing time is so short in relation to the time in service. Gaskets for water mains, for example must retain their sealing properties for decades, whereas the tests are done for hours. Of equal or greater importance is how the elastomer behaves while it is under conditions of constant stress or strain for long periods.

Tensile strength - Tensile strength is the maximum tensile stress reached in stretching a test piece, usually a flat dumbbell shape, to its breaking point. Tensile tests can also be made after exposure to heat, chemicals, etc. Retention of tensile after exposure is much more important than the values after exposure.
Most rubbers that have a tensile strength below 7 MPa are usually rather poor in most mechanical properties and those above 21 MPa have good mechanical properties. However, those elastomers that fall in between those numbers usually have adequate mechanical properties. In addition, rubber components are rarely loaded in tension above 1 MPa, so elastomers with a higher tensile strength rarely reach the degree of their ultimate strength.

Elongation - Elongation or strain, is the extension between bench marks produced by a tensile force applied to the test piece and is expressed as a percentage of the original distance between the marks. Elongation at break or ultimate elongation, is the elongation at rupture.

Hardness - Hardness is measured by the depth of indentation on a ball and the results are converted to International Rubber Hardness Degrees (IRHD). This scale ranges from 0 (infinitely soft) to 100 (infinitely hard). The Shore durometer is a typical hand instrument that approximates to IRHD degrees. Because of the limitations of the testing instruments, hardness is usually within -/+5.

Abrasion resistance - In many applications resistance to wear is one of the most difficult to analyze and measure. Wear is usually considered in terms of abrasion, which is defined as the loss of material that results from mechanical action on a rubber surface. Abrasion resistance is complicated and depends on many things, resilience, stiffness, thermal stability, resistance to cutting and tearing, etc. Because these variables cannot be reproduced accurately in the field, abrasion tests are not recommended for specifications.

Depending on the application, other properties may be required. If the elastomer is used in exteriors, then weathering should be considered. For wire and cable applications, electrical properties should be considered. Below are a few other properties that may be considered in specific applications.

Electrical - Elastomers are used extensively in electrical applications because they provide an excellent combination of flexibility and electrical properties. Electrical testing is a complex and specialized subject. An understanding of the basic principles is necessary when specifying elastomers in electrical applications.

Weathering - Deterioration in physical properties can occur when elastomers are exposed to weather. This can be cracking, peeling, chalking, color changes and other surface defects that ultimately may lead to failure in surface. The most important cause of deterioration is ozone. Sunlight, oxygen, moisture and temperature also effect elastomers. Most defects can be avoided through proper compounding. Synthetic elastomers are inherently more resistant than natural rubber.

Permeability - Permeability is a measure of the ease with which a liquid, vapor, or gas can pass through an elastomeric film or laminate. Permeability is an important factor in many applications of elastomers. Linings for reservoirs, flexible fuel tanks, gaskets and seals, diaphragms, etc are some applications where permeability must be kept within reasonable limits.