Part Hardening in Use

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

Potential Causes

Soft Segment Crystallization
Soft segment crystallization is a phenomenon seen frequently with soft parts (<70 Shore A). The backbone in the polyol in these types of systems is generally a high molecular weight. With a very high molecular weight, the concentration of isocyanate and curative in the elastomer is very low, and the polyester forms concentrated domains in the elastomer. These concentrated domains then tend to solidify or crystallize, which increases the hardness of the elastomer.

Insufficient Cure
If a part is not sufficiently cured prior to testing, it will continue to increase in hardness as it cures.

Low Stoichiometry
With a low stoichiometry, the relatively fast prepolymer/curative reaction will occur and hardness will build in the prepolymer rather quickly. The subsequent high concentration of unreacted NCO groups will then react with each other or with moisture, resulting in increased elastomer cross-linking. This will gradually increase the hardness of the system.

Plasticizer Leaching
Plasticizers do not react in the system. Over time, they can leach out of the elastomer, leading to increased elastomer hardness.

Solutions

Choose a Mixed Backbone Prepolymer
Using a prepolymer with a mixed polyester backbone will disrupt the crystallinity of the elastomer enough to reduce soft segment crystallization. Prepolymers such as the Versathane® QM variety should be used for these systems.

To test for soft segment crystallization place a softened elastomer sample in an oven at 100°C for several hours. If the sample returns to its original hardness during heating, the cause of the hardening will most likely be soft segment crystallization. When this is the case, the part will generally harden again over time.

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.

Use Stoichiometry Above 90%
Using a higher stoichiometry will ensure that there are no unreacted NCO groups left after curing which could increase the hardness over time.

Ensure Plasticizer Compatibility
Not all plasticizers are compatible with every system. Benzoflex 988SG is the most universally compatible plasticizer. Be sure to use the SG grade of this product, which was designed specifically for polyurethane applications.