Why Won’t RTV Silicone Adhesive Cure?

 

"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.

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4 thoughts on “Why Won’t RTV Silicone Adhesive Cure?

  1. Kyle Rhodes Post author

    RTV silicones are generally referred to as acetoxy cure systems or condensation cure systems. These systems use moisture to react with the silicone polymer and are designed to work under 40-60% relative humidity conditions. During the summer time, if the humidity conditions increase to 70-80%, it will actually slow down the curing rate. While this sounds counter-intuitive, too much moisture causes the silicone polymer to become chemically confused with so many hydroxide (-OH portion of H2O water molecule) that it takes longer to cure – like walking into a restaurant and having too many choices to pick from, versus ordering Happy Meal #2. With the right amount of hydroxide in the system, the silicone polymer reacts with the water, releases a little bit of acetic acid (that vinegar smell), and cures quickly. It can take 1-2 days to cure a thick bead during the fall or spring and up to 5 days to cure a thick bead during summer (too much moisture) and winter (not enough moisture). Controlling the production environment will help stabilize your process

    To avoid this effect of humidity, try a 2-part addition cure silicone. They will still cure at room temperature (RTV = Room Temperature Vulcanization) and take away any dependence on moisture. Or better yet, try DYMAX 9440 light-curable silicone sealant.

  2. C.Ising

    The comment, high moisture will slow down curing time, IS WRONG!
    Read data sheets and you will learn, that silicone adhesives require a high humidity to fully cure!

  3. Irene Boutin

    Hello,
    Depending on what chemistry you are working with the effects of moisture will vary. For example, when working with cyanoacrylate adhesives a high moisture environment will work much better for this chemistry. Any chemistries with a secondary moisture cure will also do much better with high humidity environment. Acrylated urethane based chemistry will be more forgiving with regards to moisture environments. UV cationic epoxies systems can be sensitive to high humidity environments and this could cause a slowdown in cure.

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