- 23 October 2006 -

Choosing the right elastomer for the right application

In order to increase the mean time between failure (MTBF), improve safety and reduce leaks, system and maintenance engineers must match a material's performance and corresponding cost to process conditions. To ensure that the best suitable product is chosen for an application, engineers must either have a working knowledge of elastomer characteristics or have a resource readily available. William M Stahl from DuPont Performance Elastomers investigates the options available and suggests an easy evaluation process to help you make the right choice.

Elastomers, both natural and synthetic, have common characteristics including elastic recovery after stress, flexibility, extrusion resistance and relative impermeability to gases and liquids. There are over 20 classes of elastomers and each class has its own unique properties and performance that can be modified by the inclusion of other ingredients.

Elastomers are usually formulated by compounders to present specific engineering properties for an application. Vulcanization converts the thermoplastic compound into the desired thermoset shape. Crosslinks between (or linking) the polymer chains impart strength and elasticity to the sealing product. With the wide choice of materials available, it is important for seal specifiers to consult with the seal manufacturer to determine the appropriateness of particular materials for an application

Relative heat and oil resistance
SAE J200 (ASTM D2000) describes 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 for characterizing and positioning elastomers, it does not address chemical and thermal resistance in hostile environments encountered in chemical processes. To predict the degree 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.

Chemical compatibility
Generally, elastomers are rated with respect to compatibility with various media including fluids, weather, ozone, etc. Usually, elastomers must perform in more than one medium, so it is important to understand all the elements to which the elastomer will be exposed.

Elastomers in a fluid environment may absorb the liquid and swell, react chemically with the fluid causing changes in 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 other ingredients that the compounder has used to make the part.

Most chemical resistance guides correlate the percentage of swelling with part performance in a certain chemical. Generally, below 5-10% swell is considered excellent. Although swelling is a key criterion for assessing part compatibility and one that is generally accepted as a standard for compatibility, other properties should be measured as well.

Thermal properties
The mechanical properties of an elastomer will generally change after prolonged exposure to high temperatures. Natural rubber, for example, will become gummy whereas polychloro-prene 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 temperature. Relatively 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 long 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 medium has been evaluated and documented. In addition, tests should be performed under conditions that closely mirror the actual process conditions.

Mechanical properties
There are many mechanical properties that influence elastomer performance including compression set, tensile strength, elongation, hardness and abrasion resistance. These properties help determine the durability of elastomer parts in particular applications. Most of these properties can be modified by the part manufacturer through the compounding process. Compounders can alter the ingredients and/or the cure system to impart the required characteristics.

Other properties
Depending on the application, other properties may be important. For example, if the pump will be used in extreme conditions, then weathering should be considered. Below are other properties to consider in specific applications.

Weathering - deterioration in physical properties can occur when elastomers are exposed to weather. Cracking, peeling, chalking, colour changes and other surface defects that ultimately may lead to failure in surface, can manifest this. The most important cause of deterioration is ozone. Sunlight, oxygen, moisture and temperature also affect 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, vapour, or gas can pass through an elastomeric film or laminate. Permeability is an important factor in many elastomer applications, including gaskets, seals and diaphragms.

www.dupontelastomers.com

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