Learn about the nine primary physical properties of a rubber compound: hardness, tensile strength, modulus, elongation, tear resistance, abrasion resistance, compression set resistance, resilience and specific gravity. This presentations offers detailed information on each property, why it is important and how it is measured.

1. The Physical Properties of Rubber
Rick Hudson

2. 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 & Dashpot
The 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.

3. 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.

4. 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 Hardness
As 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.

5. 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 Hardness
Processed 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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