Low Hardness

Hardness measurements are considered statistically the same if they are ±3 points from the average of several measurements. If the hardness is outside this range, then there could be a hardness variation concern. For further information on elastomer hardness testing, see Hardness Test Procedure.

Potential Causes

Overheated Prepolymer
Use of an overheated prepolymer will have the same ultimate effect on the elastomer as processing at an incorrect stoichiometry. You will be able to tell that a prepolymer has become overheated because it will have a lower percent NCO than what is indicated on the prepolymer container.

Incomplete Cure
Post-cure is used to relieve internal stresses and to allow proper alignment of the polymer chains.

Incorrect Stoichiometry
With gross errors in stoichiometry, the hardness of the elastomer may decrease.

Inadequate Mixing
Inadequate mixing will leave areas in the elastomer that are rich in prepolymer or curative, which will lead to incorrect stoichiometry, and in turn, will result in problems with part physical properties.

Solutions

Minimize the Heat History
Follow the manufacturer's guidelines for exposure of the prepolymer and curative to heat.

Follow the Supplier Curing Guidelines
Prepolymer suppliers have standard recommendations for the curing of their materials. A typical guideline is to cure the prepolymer for 16 hours at 100°C, though please verify this recommendation for each system. Some prepolymer/curative combinations require different conditions.

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:


Ensure Sufficient Mixing of All Components
Use of a pigment will help highlight areas where materials are mixed inadequately. See Swirls/Streaks in Parts for further detail.