Who knows how silicone works?

Anyone know how Silicone (the adhesive/caulk) cures?  I mean chemically?

I used some 100% silicone to assemble some plexi in a wooden frame.  Instructions say tack-free in 30 minutes.  Well, 3 hours later it's still just as goopy as when it came out of the tube.  I also noticed in hindsight that it didn't smell acetic like silicone does.  I don't think the tube is that old, maybe a couple months.

I can get new silicone, but my concern is that I've now coated the wood with basically silicone grease and I won't get anything to stick to it.

Will new silicone "activate" the uncured stuff in the pores of the wood and make a good bond?  Is there something I can add to this (whatever evaporated out of the old stuff) to activate it?

Just for the sake of argument I'm assuming you didn't use Part A of a 2-part silicone?

Is there a SDS sheet for this?  Can you post it?  Might help figure out what the activating agent is.

Sorry for the screenshot, phone crashed on me

Sorry last post. Point being if you are using it long term and it needs to be weather proof you may want a different sealer/adhesive

Most home silicone is acetic acid cure. The vinegar smell tells you that it is activated. I am assuming that is what you used.

A: 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 summertime, 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.

Lobsterpennies said:

Sorry for the screenshot, phone crashed on me

87 tabs, dude. 

And I thought I had excess with 30 open tabs on my laptop.

One-part silicones cure by reacting with moisture from the air.  The reaction with water also releases acetic acid which leads to the smell.  I'm not sure what your assembly looks like but it needs to have some exposure to air so it can get the  moisture it needs.  If there is only a very fine line exposed to the air then it will take longer for the required moisture to diffuse into the silicone and react.  There isn't anything you can add (except water) to speed things up.  If you're really into chemistry, there is a summary of the chemistry here.

So, update.  20 hours later it's still the exact same.  It's just basically grease.  Obviously no good.  It's not a moisture or time issue, it's bad silicone.

The longer story is that I'm building a jukebox prop for a play at the theater and this is the "glass" front.  It's not a critical thing, I'm just using it as clear glue.  Basically what I did was assembled a 29" square wood frame using 1x2, then routed a rabbit to inlay the plexi.  I squirted a 1/4" bead in the rabbit and dropped the plexi in.  After pressing in the plexi and squeegee-ing the fillet, I doubt there is anywhere with more than 1/16" thickness of silicone

My concern is that I may have to re-fab all the pieces I did (which was relatively significant) because now the pores are soaked with silicone grease.  I'm hoping that new silicone will activate the old stuff?  He says hopefully?

The product I used was GE 100% silicone, clear.

Simple solution to that.  Squeeze some of the crappy stuff onto something, wipe it off, squeeze some new good stuff on and see how it cures.

Except you probably threw the old stuff out already, right?

One way to find out if new silicone will adhere..

I wish I knew the answer about re-activating. I don't - but I know the stuff will go bad on the shelf. What was the date on that tube?

Unfortunately you may have to re-make the box because typically only more silicone will stick to silicone. 

Can you get some glazing points and use those to hold it in or will they show

OHSCrifle said:

One way to find out if new silicone will adhere..

I wish I knew the answer about re-activating. I don't - but I know the stuff will go bad on the shelf. What was the date on that tube?

Unfortunately you may have to re-make the box because typically only more silicone will stick to silicone. 

Date on the tube says use by 12/21.  I realize it's after 12/21, but I've never had silicone do this.  Usually it's either good or it's solid.  I've never had non-viable liquid stuff before.

Lobsterpennies said:

Can you get some glazing points and use those to hold it in or will they show

I cut the rabbit to keep the plexi flush with the top.  I could cut a deeper rabbit and use points but one of the reasons I wanted a full perimeter of adhesive is because it's a working jukebox prop... it has speakers and an amp.  I'm afraid it would rattle terribly without the adhesive.

Curtis73 (Forum Supporter) said:

OHSCrifle said:

One way to find out if new silicone will adhere..

I wish I knew the answer about re-activating. I don't - but I know the stuff will go bad on the shelf. What was the date on that tube?

Unfortunately you may have to re-make the box because typically only more silicone will stick to silicone. 

Date on the tube says use by 12/21.  I realize it's after 12/21, but I've never had silicone do this.  Usually it's either good or it's solid.  I've never had non-viable liquid stuff before.

That's weird. Wonder if it was overheated or frozen or something. Sucks man. Sorry. 

Just an idea in case adhering it doesn't work. 

Take rabbit to half inch deep.

1/4 self adhesive foam weather stripping on the Plexi, this is assuming the Plexi will clean up from the silicone 

Compress until you can get points in. 

Alternatively foam both sides and sandwich it on the back side of frame with wood strips around perimeter.

The one section of plexi is easy to reproduce to get fresh wood.  It's just a square frame.  The other two pieces that hold plexi are some 5/4" MDF that took rather considerable time getting a curved rabbit just right for the plexi.  I can redo it, just reeeaaallly don't want to.

Tomorrow I'll take some fresh silicone and put it on a piece of the old greasy stuff.  I'll also smear some of the old stuff on some wood, wipe it off, and put some new on it to see if it sticks.  I don't have high hopes

I can't help with the silicone issue, but I can help (in my own special way) by pedantically telling everyone it's "rabbet", not "rabbit".

Something, something, something, fake boobs, something, something.

Duke said:

I can't help with the silicone issue, but I can help (in my own special way) by pedantically telling everyone it's "rabbet", not "rabbit".

 

Today I learned....

After hopelessly trying to clean out as much of the greasy mess in the rabbet (thanks Duke), I think I'll just re-build the easy frame part.  The Plexi will be fine, I just wiped off the stuff and cleaned it up with acetone.  I did put some fresh silicone on the greasy wood and also smeared a glob of the old stuff on a board and then smeared some new stuff on top.  Curious to see if the new kicks the cure on the old.  If it does, I'm in business.  If it doesn't... looks like I'm cutting new MDF which pisses me off.

Using scraper tools like picks and screw drivers and small wire brushes and lots of paper towels, you should be able to clean up the wood using acetone or lacquer thinner. Mineral spirits for the plexiglass. 

Tape off everything you can.

I got it as clean as I could and rebuilt the easier frame.  No amount of acetone or mechanical means will get the greasy stuff out of the MDF.  It has soaked in 1/8" and the whole area is dark like it sat in olive oil overnight.  What I did today was drill about fifty 1/16" holes in the rabbet of the MDF and mixed up some epoxy.  I basically made a keyed-in epoxy layer that hopefully will be OK.  Like I said, it's not critical or structural, but nothing is more embarrassing than firing up a functioning practical stage prop and the face falls off of it in the middle of a performance.

I should have it finished tomorrow.  Pics then.