Tuesday, November 23, 2010

Why I Hate 1-Part Silicones (and Urethanes and...)

Personally, I really don't have much against the 1-part systems, it's just that I've seen other people use them willy-nilly because they don't like mixing, and then they get in trouble. Properly used, a 1-part, be it sealant, adhesive or whatever, can be great.

But one problem that I've seen repeatedly across countless industries [1] is people thinking that because these are moisture-cured products, as long as they have high humidity in the assembly area, everything will be fine. Seriously, I've heard people say "We're in East Texas where it is more humid than all-get-out. There must be something wrong with the adhesive since it isn't curing. Ship it back!"

The problem with this line of thought is that yes, moisture is needed to aid the cure [2], but there is that other end of the reaction to worry about called the products, or in this case, the byproducts. The water displaces some other molecule, often acetic acid or ethanol, but no matter what it is, that byproduct has to get out and disappear. And if you are putting the silicone into an enclosure of some sort, or even a partial enclosure, all that acetic acid may not be able to find its way out and that causes problems. If instead it is used in thin layers, everything should be fine.

Trapped inside, it can form hollow pockets or end up at the surface where they cause adhesion problems. And thick sections like this also can take a long time to cure just because of the diffusivity issues. [3] I've seen all this too many times. Please, next time you reach for a 1-part, make sure that you've thought it all through.

[1] Medical devices, construction and telecommunication to name a few. Your industry is certainly on the list, I just haven't seen it yet.

[2] So you see, a 1-part silicone is actually a 2-part silicone; it's just that the other part is present in the atmosphere.

[3] Remember that with Fickian diffusion, time scales with the thickness squared.

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