The Physical Properties of Rubber

Rick Hudson

Visco-ElasticityVisco-Elasticity

This is because an elastomer goes through

both a viscous phase and an elastic phase.

An elastomer is a visco-elastic material.

Spring & DashpotThe visco-elastic behavior of elastomers can be simulated using a spring coupled with a dashpot (damper). The spring illustrates the elastic phase, and the dashpot exemplifies the viscous phase.

Elastomeric materials have inherent, or original, physical properties.

Physical PropertiesPhysical Properties

Key Physical Properties:

• Hardness • Tensile Strength • Tensile Modulus • Elongation • Tear Resistance • Abrasion Resistance • Compression Set • Resilience • Specific Gravity

Original properties are often enhanced through compounding.

Hardness is the measure of a rubber material’s relative resistance to an indentor point on a testing device.

HardnessHardness

An elastomer’s hardness is more accurately thought of as two related properties: inherent hardness and processed hardness.

Inherent HardnessAs a result of chemical structure, each elastomer has its own inherent hardness.This inherent hardness can be modified (and is typically supplemented) via compounding and vulcanization.

Hardness in molded rubber articles (processed hardness) is a factor of cross-link density (and the amount of fillers).

The more cross-linking a given material undergoes during vulcanization, the harder the final molded part will be.

Processed HardnessProcessed hardness is one of the most common criteria in the rubber industry.There are two primary types of hardness tests: Shore durometer and International Rubber Hardness Degrees (IRHD).

HardnessHardness

Shore gauges are the most common, so the words “Shore” and “durometer” are virtually synonymous.

There are a number of different durometer scales, but two are most often used for measuring rubber hardness. Shore A durometers gauge soft to medium-hard rubber. Shore D durometers are more accurate on samples harder than 90 Shore A.

HardnessHardness

The other widely-used test measures material hardness in International Rubber Hardness Degrees (IRHD).

IRHD is more common in other parts of the world than in the United State. Some testing instruments generate both IRHD and durometer readings.

HardnessHardness

Tensile strength is the amount of force required to break a rubber specimen.

Tensile strength is typically noted in either pounds per square inch (psi) or megapascals (MPa).

Testing

Per ASTM D 412, tensile strength is generally tested using a molded dumbbell pulled between the grips of a tensile tester.

The sample is pulled at a rate of 20 inches per minute until it breaks. The force exerted at the time of rupture divided by the initial cross-sectional area of the dumbell is reported as the sample’s tensile strength.

Tensile StrengthTensile Strength

Tensile modulus is the force in psi (stress) required to produce a certain elongation (strain) in a rubber sample.

This elongation might be 50%, 100%, or even 300%, though 100% is the most widely used figure for testing and comparison purposes.

Generally speaking, the harder a compound, the higher its modulus. Compounds with a higher modulus are more resilient and more resistant to extrusion.

Tensile ModulusTensile Modulus

Elongation is the percentage increase (strain) in the original length of a specimen produced by a tensile force (stress) applied to the specimen.

Ultimate Elongation

Ultimate elongation is the elongation at the moment the specimen breaks.

Some materials can stretch more than others. Natural rubber may stretch up to 700% before breaking; fluoroelastomers typically rupture at about 300%.

ElongationElongation

Tear resistance (or tear strength) is resistance to the growth of a nick or cut in a vulcanized rubber specimen when tension is applied.

Tear resistance is typically noted in kilonewtons per meter (kN/m) or pound force per inch (lbf/in.).

Tear resistance is an important consideration, both as the finished article is removed from the mold and as it performs in actual service.

Tear ResistanceTear Resistance

Abrasion resistance is the resistance of a rubber compound to wearing away when in dynamic contact with an abrasive surface.Abrasion resistance is measured as a loss percentage based on original weight. Abrasion resistance can be gauged with a variety of test instruments, including an NBS Abrader, a Pico Abrader, or a Taber Abrader.

NBS Abrader Pico Abrader Taber Abrader

Abrasion ResistanceAbrasion Resistance

Compression set is the amount, expressed as a percentage of deflection, by which a rubber specimen does not return to its original thickness following release of a compressive load.

Compression set is the end result of a progressive stress relaxation, which is the steady decline in sealing force that results when an elastomer is compressed over a period of time. In terms of seal life, stress relaxation is like dying, and compression set is like death.

Compression SetCompression Set

Also known as rebound, resilience refers to a compound’s ability to regain its original size and shape following temporary deformation.

Compounding can enhance this property, but it can also hurt resilience, which is largely an inherent property.

As a general rule, resilience is most critical in dynamic seals.

ResilienceResilience

Specific gravity is the ratio of the weight of a given substance to the weight of an equal volume of water at a specified temperature.

Specific gravity is often used to identify rubber compounds.

Specific GravitySpecific Gravity

ASTM D2000 & SAE J200 are standards that provide a method for specifying rubber compounds via a simple line-call out.

The American Society for Testing and Materials (ASTM) and the Society of Automotive Engineers (SAE) devised these standards. J200 finds it widest use in the auto industry; D 2000 is more common among rubber manufacturers.

ASTM D2000 & SAE J200ASTM D2000 & SAE J200

A typical line call-out contains:

• Document name with revision year (ASTM D 2000-95)

• Letter “M” if units are metric; no “M” means English units

• Grade Number to define added test requirements as defined in Table 6 of the D 2000 document (in our example, Grade 2)

ASTM D2000 & SAE J200ASTM D2000 & SAE J200

A typical line call-out contains:

ASTM D2000 & SAE J200ASTM D2000 & SAE J200

strength of not more than 30%, elongation of not more than -50%, and hardness of not more than +/- 15 points after heat aging for 70 hours at one of the temperatures shown in the table at right

• Type, based on changes in tensile

• In our example, the Type is B, so the test temperature is 100 C

A typical line call-out contains:

• Class, based on the material’s resistance to swelling in IRM 903 Oil

• Sample is immersed for 70 hours at same temperature as previously determined for Type

• Swell is calculated, then compared to limits in the table at right

• In our example, the material is class G, indicating a maximum swell of 40%

ASTM D2000 & SAE J200ASTM D2000 & SAE J200

A typical line call-out contains:

• The next three digits (“714”) specify hardness and tensile strength

• First digit…7 for 70 +/- 5

• Second and third digits…14 for 14 MPa

ASTM D2000 & SAE J200ASTM D2000 & SAE J200

A typical line call-out contains:

• Additional suffix requirements

• Letters are shown in table at right

• Numbers indicate test method and temperature as shown in Tables 4 and 5 of the D 2000 document

ASTM D2000 & SAE J200ASTM D2000 & SAE J200

A material test report shows the performance of a cured rubber compound when subjected to a variety of standardized ASTM tests.

Test reports typically reflect original physical properties and the individual tests to which a sample has been subjected.

In each area, both the specification (requirement) and the actual performance are listed.

Test ReportTest Report