nothing sticks to silicone

You may know how this goes …  you use silicone caulk in the shower and it needs to go down in one pass.  There’s only one chance to get it right because nothing bonds to cured silicone, including silicone.  If you don’t like what you squeezed down, you will have to redo the caulk, the old bead has to come off completely.  If the bead is rough and ugly, it will stay that way – it cannot be sanded or smoothed, filled over or retouched.  While still uncured and sticky, you can smooth it with a finger.

There are innumerable consumer and industrial products using silicone because of  its many unique and amazing properties.  To name a few,  bathtub caulk, hot pads, kitchen gadgets , breast implants, gummy pen-bumpers.  Silicone can take on colors from black, white, boring clear to hot orange and bright yellow.  For two things in my universe, it has no equal: durable, flexible, chemically inert tubing, and silicone insulation for wire (in this application unrivaled for its inert and stubborn durability, extreme heat resistance, and noodle-like pliability – to me, its highest purpose).  Then there’s it’s original application: as a heat shield for your space capsule.

Silicone is strange stuff.  Like many products of industry, while it is seemingly everywhere,  it is difficult to get information about silicone. Like the formula for Coca Cola, silicone seems to be a trade secret.  Apparently, facts don’t stick to it either.

I read a thread on The Candlepower forums  – someone was trying to glue a silicone sheet to make a light diffuser.  A former engineer who back in the 70’s  working in the space program and who was assigned to find a way to bond silicone – commented.  Here’s what he said …

Sorry bub, silicone sticks to nothing but other silicone, and that not very well.
You best-bet glue is silicone caulk…the type which smells like vinegar (acetic acid) while curing.

Once cured, silicone won’t melt, just burns, so soldering irons, etc don’t work. That’s why it was used as the ablative heat shield material by NASA, back in the day. Even then they used a silicone impregnated wire mesh so they could attach it to the spacecraft without adhesives.

While working as an engineer back in the 70’s I inherited a project to find an adhesive that would stick to Dow Corning Sylgard silicone (similar to your diffuser material). The previous eng’r had tried for a year to make anything stick to the silicone. I worked on it, on and off for two years, then handed the project off to a new-guy engineer, who worked for another year on it. We never found a good adhesive for it, and Dow Corning said it was impossible. The acetic-acid/moisture curing silicones (like GE RTV112, and RTV108) worked the best, but not well enough for the industry”


2 Responses to “nothing sticks to silicone”

  1. I discovered acetic-acid silicone in the 70s, used it for a time to fix surfboard dings. It was fast and easy. Unfortunately it didn’t work because silicone does not bind to surfboard resin. It’s deceptive because when it’s not cured, it is stickier than anything – you’d think it ought to work. The ding may hold for a short time, but water will suck past the silicone patch causing as the repair to fail. I’ve seen new stuff now to fix small dings – resin/catalyst in a tube, and UV cure resin, that does work.

  2. F.Gordon Corner Says:

    so silicone is a useless sealant

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