Elastomers and Rubber Compounds Information

High-strength silicone mold making rubber via Ellsworth AdhesivesElastomers and rubber compounds are natural or synthetic polymers with a high degree of flexibility and viscoelasticity (elasticity). The products described on this page take the form of raw, unformed compounds used for molding and fabrication. Elastomers are usually thermosets that require vulcanization, but also include various thermoplastics.

Types

There are several different material grades of elastomer and rubber materials which include thermoplastics, thermosets, and composites.

Thermoplastics are polymers that turn to liquid when heated and turn solid when cooled. They can be repeatedly remelted and remolded, allowing parts and scraps to be reprocessed. In most cases they are also very recyclable. 

Thermosets or thermosetting plastics are polymer materials that have been irreversibly cured. They are generally stronger than thermoplastics due to polymer cross-linking and are better suited for high-temperature applications (below their decomposition points). They tend to be more brittle than thermoplastics and many cannot be recycled due to irreversibility. 

Composite compounds are types of resins made from two or more constituent materials with significantly different physical or chemical properties. One of these constituents is generally a strengthening phase, reinforcement fibers, toughening phase, or other specialty fillers that provide unique properties. 

Chemical Systems

The chemical system for an elastomer determines its thermal, mechanical, electrical, and optical properties. Listings of numerous elastomer and rubber material chemical systems are available on the Engineering360 SpecSearch Database. Some of the most common include butyl, silicone, polyurethane, and isoprene.

Butyl is a synthetic rubber that provides resistance to water, steam, alkalis, and oxygenated solvents. Butyl has low gas permeation and is capable of providing high-energy absorption (dampening) and good hot tear strength. The suggested operating temperature for butyl is -75° to 250° F. It commonly copolymers with isoprene and is used as the inner lining of tires, basketballs, and other things that hold air.

Silicone is a polymer containing silicon. Silicone rubbers are widely used to create a variety of products from automotive applications to clothing to electronics. They are non-reactive, stable, and resistant to extreme environments from -55°C to +300°C while still maintaining its useful properties. Silicone may be part of a two-part polymer system to enhance properties or lower the cost.

Polyurethane (PUR) resins provide excellent flexibility, impact resistance, and durability. Polyurethanes are formed through the reaction of an isocyanate component with polyols or other active hydroxyl group compounds. Polyurethanes require a catalyst, heat, or air evaporation to initiate and complete curing. It is used to form foams, electrical potting compounds, adhesives, surface coatings, sealants, hard plastics, and synthetic fibers.

Isoprene is an organic compound that makes up the polymer of natural rubber. It is commonly added in small amounts as a copolymer of synthetic rubbers such as polyisobutylene to enhance properties for specific applications.

Specifications

Properties of an elastomer or rubber are determined primarily by the material grade and chemical system of the product. Rubber is characterized especially by tensile strength (break), tensile modulus, and elongation.

Tensile strength at break is the maximum amount of stress required to fail or break the material under tension loading test conditions. Tensile tests are typically performed according to test procedure standards such as:

  • ASTM D-638 -  Standard test method for tensile properties of plastics
  • ISO 527-1 -  Plastics - determination of tensile properties - part 1: general principles
  • ASTM D-1708 -  Standard test method for tensile properties of plastics by use of microtensile specimens
  • ASTM D-2289  - Plastics at high strain rates
  • ASTM D-882 - Thin plastic sheets
  • Other OEM proprietary standards

Young's modulus or the modulus of elasticity is a material constant that indicates the variation in strain produced under an applied tensile load. Elastomers and rubber materials with a higher modulus of elasticity have higher stiffness or rigidity. Elongation is the percent amount of deformation occurring during a tensile test or other mechanical test.

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