Occasionally, the hardness of an elastomer will be acceptable at the time of manufacture, but later, during storage or field use, it will soften considerably. What causes this?

There are three main causes for this softening—contact with solvents, exposure to excessively high temperatures and incorrect prepolymer choice. In addition, incorrect stoichiometry can also adversely affect a system.

Contact with Solvents: Continued contact with most petrochemical-based solvents can cause many polyether-based elastomers to soften significantly. Water, which is also a solvent, can have a softening effect on most polyester elastomers after sustained contact. To avoid these problems, we recommend designing the application area so that elastomers do not come in contact with any solvents.

Exposure to Excessively High Temperatures: Continued exposure to elevated temperatures can cause the urethane bonds within an elastomer to break down, leading to part softening. To minimize overall heat exposure, elastomers should be insulated from the heat source. In addition, the temperature level should be monitored. A typical maximum high temperature for continuous use for common elastomers is 180-200°F, though this maximum temperature varies with formulation, application and environment.

Incorrect Prepolymer Choice: Polyether and polyester-based elastomers react very differently to solvents. It is important to use the most appropriate prepolymer backbone for the application. Generally, polyether prepolymers should be chosen for use with aqueous environments, and polyesters for use with most organic solvents. It has been documented that polyester-based elastomers can soften considerably when stored at elevated temperatures under high humidity.

For further information about common elastomer processing and performance problems, go to our on-line Trouble Shooting Guide, accessible from the navigation at the left of this page.