"We use a lot of RTV adhesives. Every lot is tested by QA. Last week we found out that in one of our components the adhesive did not cure at all. The RTV is one component and it adheres metal to ceramic. It has holes for air to penetrate. Even after opening the bond and exposing it for one week it still did not cure. What could be the reason for not curing?"
RTV silicone adhesives rely on moisture and humidity in the air to cure properly. Generally the conditions have to be 40-60% RH, but can extend down to 20%, and up to 70% in certain cases. The moisture in the air reacts with the stabilizer in the RTV, and once the stabilizer is removed, the adhesive can cure fully. In a very high-humidity environment, the humidity in the air can saturate the surface of the RTV, and effectively seal it off, limiting the penetration of the humidity to deeper levels. If you have a thick bond line or cross section of material, it may take longer than one week to cure fully. The silicone manufacturers generally set a 5-7 day cure schedule for RTV’s before they can test the physical properties in a thick slab of material, and that’s with the condition of 40-60%. If you have a 70% RH condition during the summer time, it may take longer, or disrupt the cure enough to appear gummy or semi-cured. Acidic surfaces may also cause problems with the cure mechanism.
Another avenue to explore: Was this failure linked to just one lot of material? And was it 100% failure for this lot, or 1% failure of one tube within the lot? These answers can lead the manufacture of the RTV to help determine the root cause of the failure.