Today's KNOWLEDGE Share: ELASTOMERS (THERMOPLASTIC vs THERMOSET)

 Today's KNOWLEDGE Share:

ELASTOMERS (THERMOPLASTIC vs THERMOSET)

There are times when a thermoplastic elastomer (TPE) cannot match the performance of a thermoset rubber compound, and rubber must be used in a demanding application.

 

I completed a material review in which my client was trying to replace a nitrile rubber (NBR) seal with a thermoplastic elastomer (TPE). They were looking for a material that would offer cost and manufacturing advantages over the thermoset NBR material. The screening tests paralleled the application environment, and included chemical exposure at elevated temperature under conditions of dynamic actuation of the seal.

 

Both thermoset rubber and TPEs are elastomers. As elastomers, both classes of materials exhibit some characteristic properties:

•  Substantial amorphous content

•  Subambient glass transition, which allows substantial segmental molecular motion

•  Relatively low hardness values, meaning they are relatively soft

•  Comparatively low modulus values, meaning that they are flexible

•  High degree of stretch and elongation at break

•     Compared with non-elastomeric materials, are elastic, meaning they exhibit good recovery from stress

 

While both thermoset rubber and TPEs are elastomers, their structure, and as an extension, their performance properties can be dramatically different. Thermoset rubber compounds have a cross-linked molecular structure, with covalent bonds joining the individual polymer chains, essentially forming a molecular network. Conversely, TPEs are thermoplastic in nature, and do not have covalent bonds that join individual polymer chains. Instead, the polymer chains are held together by weak intermolecular forces, such as hydrogen bonding and van der Waals forces.

 

Based upon the difference in molecular structure, the two material classes can exhibit desparate physical properties. It is somewhat difficult to characterize all TPE materials and contrast them against all thermoset rubber materials. There are numerous subcategories within each type of material, and each has its unique characteristics, as well as strengths and weaknesses. However, given the fundamental molecular structure, some differences can be highlighted. The cross-linked structure of thermoset rubber affords them superior performance properties in three general areas compared with TPEs:

 • Superior chemical resistance

 • Higher thermal stability

 • Greater stress recovery, resulting in better compression set - creep and stress relaxation properties

 

In this case, it was concluded through testing and review that a TPE material was not suitable based upon the test and application conditions - specifically the chemical resistance and the stress recovery at elevated temperature. It was recommended that the incumbent NBR rubber compound be maintained in the sealing application.

 

Source:Jeffrey A.Jansen/The Madison Group

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