Part Softening in Use

Occasionally, the hardness of an elastomer will be acceptable at the time of manufacture, but it will have softened considerably by a later date. This softening may have occurred during storage or field usage.

Potential Causes

Contact with Solvents
Continued contact with solvents will generally soften a polyurethane elastomer. Most petrochemical-based solvents cause significant softening of most polyether-based elastomers. In addition, water also has a great softening effect on most polyester elastomers after significant contact. It has even been documented that polyester-based elastomers which have been stored at elevated warehouse temperatures with high humidity have softened considerably.

Exposure to Excessively High Temperatures
Continued exposure to elevated temperatures will begin to break down the urethane bonds within the elastomer, which will cause softening. A typical recommended high temperature for continuous use of common elastomers is 180-200°F. This upper use temperature varies with formulation and application.

Incorrect Stoichiometry

Solutions 

Shield Elastomer from Solvent
In some cases, it is possible to engineer the process such that the elastomer does not come into contact with the solvents.

Insulate the Elastomer From the Heat Source
In some applications, it is possible to insulate the elastomer from the heat source to minimize its exposure to heat.

Choose the Best Prepolymer
Polyether and polyester-based elastomers react very differently to solvents. Be sure to choose the correct backbone for the application. Generally, choose polyethers for water environments and polyesters for most organic solvents.

Calculate the Amount of Curative Based on the NCO of Each Lot of Prepolymer
Many processors will use an average percent NCO for determining the amount of curative to use with a prepolymer. However, with some prepolymers, this can amount to a stoichiometry error of up to 5%, which can affect properties significantly.

Each drum of prepolymer has a specific percent NCO. It is important to use the percent NCO of the particular prepolymer lot that you are using. The amount of curative to use can then be determined with the following equation: