Sourcing managers and buyers have to select rubber materials that meet all of their application’s requirements. Elastomers have inherent physical properties, but these properties can be enhanced through compounding with other additives and ingredients. When compounding a rubber material, it is critical to understand a rubber’s physical properties and to understand how each property is measured.
Rubber has an initial level of hardness due to it’s polymer network however through compounding, this hardness can be modified. The rubber compound’s hardness can be measured in terms of durometer on a Shore A or Shore D scale. For softer rubbers, Shore A is used. Rubbers with a Shore A durometer of approximately 40 will be soft like a rubber band while a Shore A durometer of 90 will be hard like a skateboarding wheel or hockey puck.
Tensile strength is the amount of force required to stretch a rubber specimen apart until it breaks. Testing of tensile strength is typically done according to ASTM D412 standard and is usually a good indicator of how tough a rubber compound is.
Elongation is measured as a percentage increase in the original length of a rubber sample where a tensile stress is applied. Some rubber polymers stretch more than others. For example, natural rubber is capable of stretching over 700% of its original length before breaking. Alternatively, some harder rubber compounds with a higher filler content may rupture at less than 100% elongation.
Resilience refers to a rubber’s ability to return to its original shape and size after a temporary deformation. Resilience is important in dynamic seals which seal the flow of gas or liquids when closed or open. If your application requires weatherstripping, a rubber compound’s resilience is an important property to consider.
Compression set is the amount by which an elastomer fails to return to its original thickness after a compressive load is released. This is especially important for seals and o-rings as they age. Engineers need to keep compression set in mind so that their rubber seal will continue sealing even after continued use.
Tear resistance or tear strength describes an elastomer’s resistance to the growth of a nick or cut when tension is applied. Silicone rubber, while having many great properties, unfortunately has poor tear resistance in comparison to other rubber polymer types.
Abrasion resistance describes a rubber’s resistance to being worn away from rubbing or scraping. In industrial applications, abrasion-resistant rubber is used with conveyor belts and pumps. Material loss due to abrasion can be measured with various instruments according to testing standard ASTM D5963.
Specific gravity is similar to density however it is slightly different in that it is the ratio of the weight of rubber to the weight of an equal volume of water at a specified temperature. Using rubber compounds with a lower specific gravity allow for less mass required to mold a particular part, thereby lowering material cost relative to a higher specific gravity rubber.