Written by Glenn Campbell, President Composites Association of New Zealand
If you have produced a part using the vacuum infusion process, you may have noticed what appear to be air bubbles in the finished composite part. These bubbles are likely not just air from a leak in your bag but are vaporised gas from the catalyst you used.
Styrene monomer boils at a partial vacuum of 747.1 mmHg, so it is not the cause (762 mmHg is equal to full vacuum). If you have used MEKP to cure the resin, you need to know that MEKP boils at a partial vacuum of 589.5 mmHg. If you use a catalyst based on Cumene Hydro-peroxide, it boils at a partial vacuum of 759.9 mmHg, meaning it too cannot be the cause.
Therefore, one can conclude that the use of MEKP significantly influences the onset of boiling. The grade of MEKP will also determine the quantity of volatilization, depending on the Hydrogen Peroxide content contained in the grade of MEKP that you use. Hydrogen Peroxide, when it reacts with the promoter system in the resin, breaks down and emits oxygen.
If we assume a standard 1.5% w/w addition of MEKP to the resin, our actual reactive component content of the MEKP is close to 0.51% weight percentage. Using the ideal gas law, the aforementioned quantity of MEKP will yield a potential volume of about 2.63 litres of evolved gas based on one kilogram of resin (15 grams of MEKP).
. This implies that while volatilization may not be visibly evident during the early stages of infusion, it will likely become more prevalent just before the gel time of the resin when the viscosity of the resin is very high.
Using a catalyst based on Cumene Hydro-peroxide is a much better option than using MEKP when infusing. Of course, there are other reasons why Cumene systems should be used, but that is another story.