Rubber Field Info

Rubber Field Info

Compression Set

Compression Set (CS):

The Compression Set (CS) refers to the permanent deformation that remains in soft materials, like elastomers, even after the removal of an applied force. When elastomers are compressed, they may not fully regain their original thickness over time. Compression Set Testing evaluates the material’s capability to recover its original thickness following the application of a compression force at a specific deflection and temperature.

Effect of Fillers on Compression Set:

Compression set, a significant property of elastomers, is influenced by the choice of filler materials. Extensive research has been conducted to investigate the impact of silica on silicone rubber vulcanizates. The results indicate that an increase in the surface area of silica corresponds to an increase in compression set. This phenomenon can be attributed to the increased abundance of functional groups on the surface of silica, which possess the capability to potentially interact with siloxane. The resulting interaction between the filler and the matrix plays a crucial role in reducing the compression set. Moreover, higher loading levels of silica in natural rubber and larger particle sizes of silica are associated with higher compression set values in rubber composites.

Fillers like precipitated calcium carbonate, whiting, and calcinated clay, which exhibit limited interaction with the matrix, result in significantly lower compression set values. On the other hand, as the interaction between the filler and the matrix increases, the compression set also increases. This observation holds true for various grades of silica as well.


Compression Set Measurement:
IIn practical terms, a material that demonstrates a compression set of zero fully reverts to its original thickness, while a material with a compression set of 100% fails to regain any of its initial thickness.
ASTM D395 provides two testing methods, namely Compression Set A and Compression Set B, for assessing the compression set characteristics of a material.

Test Method A

Compression Set Under Constant Force in Air applies a 1.8kN force to the specimen for a set time and temperature.

Test Method B

Compression Set Under Constant Deflection in Air applies a force necessary to the specimen to compress it to 25% of its original height for a set time and temperature.

Compression sets A and B are defined as the percentage of original specimen thickness after it has been left in normal conditions for 30 minutes.

O-ring compression set testing is performed using the similar ASTM D1414 B Method.

Calculating Compression Set

Calculating the compression set for a material is accomplished using formulae that are specific to each of the test methods.

Test Method A

CA, the compression set A, is given by CA = [(to – ti) / to] * 100 where to is the original specimen thickness, and ti is the specimen thickness after testing.

Test Method B

CB, the compression set B, is given by CB = [(to – ti) / (to – tn)] * 100 where to is the original specimen thickness, ti is the specimen thickness after testing, and tn is the spacer thickness or the specimen thickness during the test.


Testing compression = (t0 – ti) = 0.25cm – 0.177cm = 0.073cm (29% compression)

Permanent compression = (t0 – tn) = 0.221cm – 0.177cm = 0.044cm

CB = {(0.25 – 0.221)/(0.25 – 0.177)}/100 = 39.72%

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