What are the physical properties of rubber?
These are the physical properties of rubber.
The following sections provide a high-level look at each physical property. Future articles in this series will examine each property in greater detail.
Elastomers have an inherent hardness because of their chemical structure. This hardness can be modified, and the processed hardness then measured in terms of durometer (duro) on a Shore scale. For soft to medium-hard rubber, Shore A is used. At 40 duro, solid rubber profiles have the consistency of pencil erasers. At 90 duro, they’re hard like hockey pucks. Which hardness do you need?
Tensile strength (TS) is the amount of force that’s required to pull a rubber specimen apart until it breaks. Known also as ultimate tensile strength (UTS), TS is measured in either pounds per square inch (psi) or megapascals according to ASTM D412. For technical buyers and part designers, tensile strength matters because it represents a rubber’s point of failure caused by stretching.
Tensile modulus (TM) sounds similar to tensile strength, but these two properties are not the same. TM is the force or stress that’s required to produce an elongation percentage or strain in a rubber sample. In general, harder rubber has a higher tensile modulus. Such rubber is more resilient, but also more resistant to extrusion, a process for manufacturing stock materials used in custom fabrication.
Elongation is the percentage increase (strain) in the original length of a rubber sample where a tensile force (stress) is applied. Some elastomers stretch more than others. For example, natural rubber (NR) may stretch up to 700% before reaching its ultimate elongation, the moment the NR breaks. By contrast, fluoroelastomers may rupture at 300% elongation. How much do you need your rubber parts to stretch?
Resilience or rebound refers to a rubber’s ability to regain its original shape and size after a temporary deformation, such as contact with a metal surface. Resilience is especially important in dynamic seals, components that create a barrier between moving and stationary surfaces. If your application requires weatherstripping between a door and a door frame, the compound’s resilience is important to consider.
Compression set is the amount by which an elastomer fails to return to its original thickness after a compressive load is released. When a rubber seal is compressed repeatedly over time, progressive stress relaxation occurs. In terms of the seal’s life, stress relaxation is like dying. Compression set is like death itself – the end result of a steady decline in sealing force. How long do you need your seal to last?
Tear resistance describes an elastomer’s resistance to the growth of a nick or cut when tension is applied. Also known as tear strength, this physical property is measured in either pound force per inch (lbf/in) or kilonewtons per meter (kN/m). If you need edge trim that will contact rough metal edges or sharp objects, consider tear resistance during compound selection.
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 that move coal and in pumps that handle slurries. Material loss due to abrasion can be measured with various instruments according to tests such as ASTM D394.
Specific gravity is the ratio of the weight of a material to the weight of an equal volume of water at a specified temperature. For chemists, it provides a way to identify compounds. For technical buyers and part designers, it’s important to know that rubber with a low specific gravity offers more square inches per pound of stock. Rubber with a higher specific gravity provides advantages in molding consistency.
Do you have questions about the physical properties of rubber? Contact Elasto Proxy for more information, and enjoy the short video below that accompanies this article.
Today, there are multiple natural and synthetic rubber products in production. In this section, we explore some of the artificially produced rubber products available.
Silicone rubber is used in applications that require good hygiene and resistance to high temperatures, such as food preparation. Manufacturers make it from a combination of silicone and oxygen, mixing in various additives to improve its properties. For example, some manufacturers add fluorine to silicone rubber to make it more resistant to chemicals. Others add vinyl for better vulcanisation or phenyl for higher flexibility at lower temperatures.
Butyl rubber production involves combining isoprene with isobutylene. Combining the two makes the rubber unsaturated and suitable for vulcanisation.
Ethylene propylene diene monomer rubber is a class M rubber made of saturated polyethylene chain elastomers. Mostly, people use it for garden hose tubing, pond liners, belts, electrical insulation, solar panel heat collectors, rubber bands, and speaker cone surrounds.
Nitrile-type rubber is made using an emulsion polymerization process from butadiene and acrylonitrile compounds. Acrylonitrile is a highly reactive substance. When mixed with butadiene, it forces a vulcanisation reaction, producing oil-resistant nitrile as a result.
Polybutadiene rubber or just butadiene rubber is a type of rubber made from solution polymerization of high molecular weight monomer 1,3-butadiene. Like the other rubber products in this list, polybutadiene rubber monomers come from petroleum processing. Manufacturers use this rubber-like substance directly in rubber products as well as plastics and some fuels.
Styrene butadiene rubber (SBR) is a combination of butadiene rubber and styrene, usually in a 3:1 ratio of the former to the latter. Adding styrene gives the rubber superior abrasion and crack resistance compared to standard formulations.
Methyl rubber is a rubber substance made by Germany in World War I via the polymerisation of dimethyl butadiene. It is rare today.
Lastly, neoprene is a type of rubber made of hydrochloric acid, acetylene and chloroprene. It is made by using chloroprene to bind molecules together into chips that are then melted with foaming agents and carbon pigments. Further heating causes the rubber to expand and take on its final form.
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