Optimum pressure for degassing epoxy resins 2

[esteess]

Is there any recommended vacuum pressure for best performance degassing of epoxy resins?
What pressure is too much and why?
What is minimum pressure?

 

[Compositepro]

Simple questions. The answers are not so simple.

Degassing is distinct from devolatilization. Degassing is done to remove gas bubbles suspended in the resin. Devolatilization removes volatiles, which are liquid components that have a vapor pressure. Air is a gas and water and acetone are volatiles. The resin itself can be volatile (styrene, for example).

Gasses pretty much behave as described in the ideal gas law, i.e. if you reduce the absolute pressure by a factor of two, or double the absolute temperature, then the bubble expands by a factor of two. Volatiles stay as liquids until the ambient pressure falls below below the vapor pressure, which is when boiling starts. If there are gasses present the gas will contain a partial pressure of volatile equal to the vapor pressure. Vapor pressure of any material increases exponentially with temperature.

To answer your question. The best way to degas is to pull vacuum until the resin starts to boil. The boiling volatiles will almost instantly strip all gasses from your resin. In Chemical Engineering the process is called stripping. When you release the vacuum all bubbles should almost instantly disappear. Your resin is now degassed.

If there are no volatiles in the resin then it will not boil. If there are very little volatiles in the resin you may not be able to get to a low enough pressure to start boiling. You can buy degassing aids which are solvents like heptane to help. Just add a few drops to the resin surface before degassing. If you do not get the resin to boil the air bubbles will expand and then sit on the resin as a stable foam. When vacuum is released, the foam will shrink but it will remain as a scum on the surface.

The scum or floating air bubbles are not necessarily a problem if you draw resin from the bottom of the container rather than pouring off the top, which can itself reintroduce air bubbles.

Sometimes devolalilzation is also needed but people erroneously call it degassing. At high cure temperatures volatiles can boil during cure. To devolatilize resin you want an absolute pressure of only a few mmHg (Torr). Stirring is important because the pressure increases with depth in the resin and also evaporation only occurs at the exposed surface.

If you are unclear on any of the terms like vapor pressure or partial pressure, now is the time to do a little background reading because you must understand these concepts to understand the answer to your questions. There are also many additional complexities due to dealing with a dynamic process where concentrations are changing with time and location.