It is important to remember that the thermoplastic nature of TPEs allows them to be processed, but also limits their use in high temperature applications. The continuous use temperature is the highest temperature at which the TPE will retain its elasticity and continue to function in an application.
Various types of TPEs exhibit a wide range of continuous use temperatures, and understanding the operating temperature of an application is the crucial first step in selecting the best TPE for that application. The continuous use temperature is expressed in Fahrenheit or Celsius.
Under continuous use temperature, various types of thermoplastic elastomers exhibit a wide range of chemical and fluid resistance. Each type of TPE has its own strengths and weaknesses when exposed to organic solvents, oils, acids, alkalis, or any other chemical or fluid.
To make a good material selection, it is important to know what types of chemicals or fluids the TPE may be exposed to. Chemical resistance or fluid resistance is typically expressed in terms of weight change, volume change, or specific performance changes caused by controlled exposure to a specific substance.
Hardness (usually measured in Shore A or D) may be the most commonly used property for differentiating between TPEs, but may also have the least correlation with a given application.
For example, two different TPEs may have the same hardness, but completely different bending moduli.
Tensile strength: As the name suggests, this property measures the tear resistance of a material. This is an important consideration for any application where a material may experience some form of damage, which could result in a type of tear strain. Tensile strength is measured in "pli" (pounds per inch) or "N/mm" (Newtons per millimeter).
Compression performance: The response of compression is important for sealing and gasket applications. Two measures of material performance under compression load include:
Compression permanent deformation: The ability of a material to return to its original shape after being subjected to a defined compression strain or load;
Compression stress relaxation: The ability of a material to continue to push back compression load over time;
Both are the results of "cold flow" or solid TPE plastic deformation, and both are influenced by time, temperature, and exposure to chemicals or fluids. Compression permanent deformation is expressed as a percentage of the unrecovered compression, while stress relaxation is expressed as a percentage or absolute level of the initial force disappeared.
Peeling strength: Peeling strength is a measure of how well TPEs adhere to rigid substrates.
While there is no agreed-upon testing standard for thermoplastic elastomers, the various methods used by TPE suppliers come from accepted testing methods for thermoset rubber and adhesives.
Adhesive failure: This failure occurs at the interface between TPE and rigid materials. Most connection failures are a combination of adhesive and cohesive failures; 100% bonding failure indicates that all TPEs have been torn off the substrate.
TPE materials often tear. This failure is not always a sign of poor performance. In fact, if the material has high tear strength, it may indicate that it has very high bonding strength.
Bonding failure: This failure occurs at the interface between TPE and rigid materials. Most connection failures are a combination of adhesive and cohesive failures; 100% bonding failure indicates that all thermoplastic elastomers (TPE) have been torn off the substrate.