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Testing Of Silicone Rubber
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Testing Of Silicone Rubber

Silicone rubber is tested according to standard ASTM methods with few exceptions. The following section summarizes mechanical and process related testing for silicone rubber. Figures 14 – 16 show the cure rate dependence of rheology and compression set for injection molded automotive gasketing compounds and demonstrate the relationship between state of cure and optimum properties.

Tensile Testing
Tear Strength
 Cold Temperature retraction
 Processing Test Equipment
 Cure Profile
 Compression Stress Relaxation
Tensile Testing, Die C


Tensile:Force to break sample

Modulus:Force at a given elongation

Elongation:Distance in percent that sample stretches

Durometer Shore A:A measure of hardness by indentation on 3 plied dumbbells. Silicone Rubber 20-85 Shore A



Tensile Strength = Force = Force
    Area   .25 x thickness  


Tensile Strength, Elongation, Modulus Chart

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Tear Strength

Die B: Measures tear propagation in pounds. Pull on tensile testing equipment.

Die C: Measures initiation in pounds. Pull on tensile testing equipment


William Plastometer: Measures deformation of silicone rubber or compound to determine molecular weight on structuring (hydrogen bonding of filler).

Penetration: Measure of viscosity or molecular weight using a foot as shown in a polymer.


Cold Temperature Retraction
Comparison of the TR-10 cold temperature retraction of typical silicone automotive sealing compounds with those of ethylene acrylic elastomers and clearly demonstrates the superiority of silicones under extreme cold temperature conditions, not uncommon to many parts of the country.

TR-10 Cold Temperature Retraction
For Silicone And VAMAC Ethlene Acrylic Elastomer

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Processing Test Equipment

Mooney Viscometer (ASTM D-1646)
The Mooney is a shearing disk viscometer which is used for measuring the viscosity of rubber compounds. A steel rotor disk, centrally embedded in the uncured rubber specimen, is caused to rotate at low speed within the confines of a tight cavity. The greater the resistance of the rubber to shear, the higher the viscosity.

The value obtained after a specified time is called “The Mooney”.

Williams Plastometer (ASTM D-926)
The Williams consists of two parallel steel plates, the lower of which is stationary, and the upper free to descend under a standard weight. A 2 cc cylindrical shaped sample of uncured compound is placed between the plates, and the upper plate is released. At the end of three minutes, the thickness of the resulting squashed cylinder is measured in mm, multiplied by 100, and recorded as the Williams Plasticity value.

This empirical measurement is often a useful tool in differentiating “good” from “bad” material for a particular piece of rubber processing equipment such as an extruder.


Cure Profile
  • Oscillating Disc Rheometer (graph)
  • Cure Profile vs. Cure Temperature (graph)
  • Temperature Dependence of Cure (graph)
  • Compression Set vs. Cure Time and Temperature (graph)

Oscillating Disk Rheometer
Description…Oscillation of a biconical disk embedded in the rubber specimen confined in a heated square cavity exerts a sinusoidal shear strain on the specimen. The force (torque) needed to oscillate the disk is directly proportional to the stiffness (shear modulus) of the specimen. As the specimen cures, modulus increases, and torque is recorded as a function of time yielding the following characteristic curve:

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  1. Preheat
  2. Initial Torque
  3. Minimum Torque
  4. Structure
  5. Scorch Time
  6. 90% Cure
  7. Maximum Torque
Cure Profile vs Cure Temperature
Cure Profile vs Cure Temperature Graph -- Figure 14
Durometer Gasketing Compound
Monsanto R100 Rheometer

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Temperature Dependence Of Cure

Temperature Dependence Of Cure Graph -- Figure 15
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Compression Set vs Cure Time and Temperature
Compression Set vs Cure Time and Temperature Graph-- Figure 16

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Compression Stress Relaxation
A relatively new test which measures the actual force on a compressed elastomer as a function of time and environmental exposure is compression stress relaxation. The standard test specimen is a 0.75” OD flat O-ring which is compressed between parallel steel plates. The initial compressive force is measured, and the test jig with specimen in place is then subjected to environmental aging such as thermal cycling or oil immersion. The compressive force is then measured again at several time intervals such that a stress decay curve is obtained. This test more closely resembles the actual sealing environment of a static gasket and is now called out on several OEM specifications. Figure 17 shows the test jig, and Figure 18 compares the ethylene acrylic elastomer as a function of immersion time in IRM903 oil at 150°C.



Test Jig For Compression Stress Relaxation -- Figure 17

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An actual jig for measuring sealing force -- The two upper plates are numbers 4 and 5, the lower plate is number 6, and the test specimen is number 8. Number 7 is an adjusting screw for initial compression of the test specimen. The entire jig fits in a test instrument equipped with a load cell. The test instrument has a U-shaped adapter to apply a force to the load arms, number 3.Loss of electrical continuity when the sealing force is marginally exceeded is measured between the underside of number 1 and the top side of number 2.



Compression Stress Relaxation Graph -- Figure 18
Compression Stress Relaxation Graph
Silicone Rubber And Ethylene Acrylic Elastomer
IRM903 Oil At 150°C

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