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Physical Properties of Rubber: A Beginner’s Guide
Nov
The physical properties of rubber are the measurable characteristics that determine how an elastomer performs in a part: hardness, tensile strength, elongation, tensile modulus, compression set, tear resistance, abrasion resistance, resilience, and specific gravity. Each is measured to an ASTM standard, and each can be tuned through compounding. Understanding them is the foundation of specifying the right material.
Sourcing managers, buyers, and design engineers all have to select rubber that meets every requirement of an application. Elastomers have inherent physical properties, but those properties can be enhanced or dialed back through compounding with fillers and additives. This guide explains each property in plain language and shows how it is measured, so you can ask for and receive the compound you actually need.
The Nine Properties at a Glance
| Property | What It Measures | Unit | Test Standard |
|---|---|---|---|
| Hardness (durometer) | Resistance to indentation | Shore A / Shore D | ASTM D2240 |
| Tensile strength | Force to stretch until it breaks | psi or MPa | ASTM D412 |
| Elongation | How far it stretches before breaking | % of original length | ASTM D412 |
| Tensile modulus | Force to reach a set elongation | psi or MPa | ASTM D412 |
| Compression set | Failure to recover after compression | % set | ASTM D395 |
| Tear resistance | Resistance to a nick or cut growing | lbf/in or kN/m | ASTM D624 |
| Abrasion resistance | Resistance to wear from rubbing | mm³ loss | ASTM D5963 |
| Resilience (rebound) | Energy returned after impact | % rebound | ASTM D2632 |
| Specific gravity | Density relative to water | g/cm³ | ASTM D297 |
Each Property Explained
Hardness (Durometer) ASTM D2240
Hardness is a rubber’s resistance to indentation, and it is the single most common criterion in the industry. Every elastomer has an inherent hardness from its polymer network, and that hardness is then modified through compounding and vulcanization. It is measured as durometer on a Shore scale. Shore A covers soft to medium-hard rubber: a compound around 40 Shore A feels like a rubber band, while 90 Shore A is as hard as a skateboard wheel or hockey puck. Shore D is used for very hard materials. For a full breakdown and a visual scale, see our durometer and material reference tools.
Tensile Strength ASTM D412
Tensile strength is the force required to stretch a rubber specimen until it breaks, reported in pounds per square inch or megapascals. It marks the point of failure under stretching, which makes it a key figure for designers assessing whether a part will survive its mechanical loads.
Elongation ASTM D412
Elongation is how far a specimen stretches before it breaks, expressed as a percentage of its original length. A part that must flex, stretch over a fitting, or absorb movement needs adequate elongation. It is measured in the same test as tensile strength.
Tensile Modulus ASTM D412
Tensile modulus is the stress needed to produce a specific elongation, for example the force to reach 100% elongation. It sounds like tensile strength but describes something different: stiffness at a given strain. Harder compounds usually have a higher modulus, making them more resistant to extrusion through gaps under pressure.
Compression Set ASTM D395
Compression set is the amount by which an elastomer fails to return to its original thickness after a compressive load is released, reported as a percentage. It is critical for seals, gaskets, and o-rings: a part with high compression set relaxes over time and stops sealing. Peroxide-cured compounds generally hold sealing force better than sulfur-cured ones, which is why cure system selection matters as much as polymer choice. This is one of the properties we tune through custom formulation.
Tear Resistance ASTM D624
Tear resistance, or tear strength, describes how well an elastomer resists a nick or cut growing when tension is applied. It matters for parts that are demolded, installed over sharp edges, or flexed repeatedly. Silicone, despite its many strengths, has relatively poor tear resistance compared with other polymers, which is a common trade-off to weigh during material selection.
Abrasion Resistance ASTM D5963
Abrasion resistance is a rubber’s ability to withstand being worn away by rubbing or scraping. It is essential for conveyor belts, pump components, wheels, and any part in sliding contact. Material loss is measured by volume under a controlled abrasive load.
Resilience (Rebound) ASTM D2632
Resilience is the percentage of energy a rubber returns after an impact, the opposite of damping. High-resilience compounds bounce back quickly and generate less internal heat under cyclic loading. Low-resilience compounds absorb energy, which is desirable in vibration isolators and dampers.
Specific Gravity ASTM D297
Specific gravity is the density of the compound relative to water. It affects part weight and, importantly, cost, since rubber is often priced and consumed by weight. A heavily filled compound has a higher specific gravity than a gum stock, so specific gravity is also a quick quality check that a batch matches its formulation.
Beyond the Basics: Environmental Properties
The nine properties above are mechanical. In real service, environmental resistance often decides material selection just as much. Temperature range, oil and chemical resistance, weathering, and ozone resistance vary widely by polymer and are the reason different rubbers exist. EPDM dominates outdoor and water service, nitrile handles oils, and fluoroelastomers survive aggressive chemicals and heat. To compare polymers on these environmental factors, use our Chemical Resistance and Material Selection Guide, or read individual rubber material pages for full property data on each elastomer.
Frequently Asked Questions
What are the main physical properties of rubber?
The main physical properties of rubber are hardness, tensile strength, elongation, tensile modulus, compression set, tear resistance, abrasion resistance, resilience, and specific gravity. Each is measured to an ASTM standard and can be adjusted through compounding.
How is rubber hardness measured?
Rubber hardness is measured as durometer on a Shore scale using ASTM D2240. Shore A covers soft to medium-hard rubber, where 40 Shore A feels like a rubber band and 90 Shore A is as hard as a hockey puck. Shore D is used for very hard materials.
What is compression set and why does it matter?
Compression set is the percentage by which rubber fails to return to its original thickness after a compressive load is released, measured to ASTM D395. It matters for seals, gaskets, and o-rings, because a compound with high compression set relaxes over time and stops sealing.
Can the physical properties of a rubber compound be changed?
Yes. Every elastomer has inherent properties, but compounding with fillers, plasticizers, and different cure systems tunes hardness, tensile strength, compression set, and more. This is why two compounds of the same polymer can perform very differently.
What is the difference between tensile strength and tensile modulus?
Tensile strength is the force needed to stretch rubber until it breaks. Tensile modulus is the force needed to reach a specific elongation, such as 100%. Strength describes the breaking point, while modulus describes stiffness at a given strain.
Specifying a Part? Let Us Formulate to Your Targets
Tell us the properties that matter for your application, hardness, compression set, temperature, chemical exposure, and our engineers will recommend a polymer and mix a compound to match. In-house compounding and testing from our Wixom, Michigan plant.
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