Prepolymer Difficult to Degas

All prepolymers should be degassed prior to being mixed with the curative, and if time permits, the prepolymer/curative mixture should be degassed again prior to being poured into the mold. This will help prevent potential problems caused by small amounts of nitrogen which may have become entrained in the prepolymer during production and drum filling.

Potential Causes

High Prepolymer Viscosity
It is more difficult to degas a high-viscosity prepolymer than a low-viscosity prepolymer. High prepolymer viscosity can be a function of the specific prepolymer in use, or it may be because the prepolymer has been degraded. See the High Prepolymer Viscosity Problem for further details on the causes of high viscosity.

Insufficient Vacuum
If degassing seems to be progressing at a slow rate for an extended period of time, the vacuum system may be faulty. With insufficient vacuum, the pressure may not be low enough to allow entrained air to escape. Insufficient vacuum may be caused by a weak or worn vacuum pump or a leak in the vacuum lines.

No Degassing Agent
Degassing aids are additives which lower the surface tension of the prepolymer, thereby allowing for easier degassing. Without the use of a degassing aid, degassing problems could be noted.

Solutions

Check the Prepolymer Temperature and Heat History
The higher the prepolymer temperature, the easier it is to degas. Ensure that the prepolymer is at the correct processing temperature when degassing. Also check that the prepolymer has not been previously overheated, which will result in increased viscosity. See the High Prepolymer Viscosity Problem for further details on high prepolymer viscosity.

Check the Gauge on the Vacuum Pump
In order to properly degas raw materials, the vacuum gauge on the degassing system should read at least 27 inches mercury (Hg). With less vacuum than this, it will take an excessively long time to degas.

Add a Degassing Agent
Several commercial degassing aids are available from various suppliers. Two degassing aids used commonly are SAG-47 from Witco, and X-Air from Ciba-Geigy. Use levels are typically recommended at 0.2 parts per hundred parts prepolymer.