As the summer months begin and humidity increases, a unique set of processing issues may arise. This is when it's more important than ever to keep all raw materials as dry as possible. Prepolymers can react with water in the atmosphere, leading to adverse processing behavior. Many curatives, plasticizers, fillers and pigments can also absorb water.

Processors often comment that their material is difficult to degas or its pot life has shortened. These difficulties are frequently due to prepolymer exposure to moisture. Exposure to a combination of water and heat causes a prepolymer to start reacting in the drum, and by the time it's processed, chemical cross-links have formed, leading to increased viscosity. CO₂ gas also forms in this reaction, and increased foaming may be noted. Because the cure process has already started, you'll see a decrease in pot life, but the demold time will probably be longer. In addition, if the part isn't pigmented, it may appear more translucent. And, the hardness of the final part may be several points lower.

Many liquid curatives, or those that melt below 100°C, as well as plasticizers, adhesives, primers and pigment carriers are hygroscopic, meaning they pick up moisture from the air. They appear normal when processed, but when demolded, bubbles can be seen in the finished part. These bubbles are from the CO₂ gas that forms when the NCO in the prepolymer reacts with water. When this happens, the final part is often softer than expected.

Molds, hubs and inserts should also be kept well above 100°C, since moisture may condense on the surface if the metal cools below the boiling point of water. Moisture can cause voids to form on the surface or at the bond line. The bubbles that form are typically visible in clusters and appear as if they were moving to the top of the mold.

The best protection from moisture contamination is the use of dry nitrogen. Each time a material is opened and resealed, nitrogen padding should be applied. Whenever possible, liquid curatives and additives should be degassed before addition to the prepolymer, and solid additives should be heated above 100°C.

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