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Old 16-01-2020, 14:02   #1
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Help! What Sealant is Better than Dow 795 for polycarbonate??

Can anyone suggest a sealant better than Dow 795? I have large poly carbonate windows that move about an eighth of an inch each day due to heat. They have VHB tape holding them in place. That is working. But they also need to be sealed around the edges. And everything is cracking. Doubt 795 is failing. What is a more soft and supple type of sealant? One that will move with the window.
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Old 16-01-2020, 14:12   #2
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Note: It's sticking to the boat and the window just great. It's just tearing apart.
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Old 16-01-2020, 14:17   #3
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Maybe try butyl tape....... great stuff. Not an adhesive but stays supple. Maybe use butyl tape then a sealant on top. Maine sails website sells the stuff.... great product.
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Old 16-01-2020, 14:19   #4
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Or honestly, if no suggestions, F this. Does anyone have a good supplier for glass for a boat? Should it be tempered or "safety glass?" Looking for tinted/mirrored as well. Ideally a Florida supplier.
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Old 16-01-2020, 14:21   #5
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

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Maybe try butyl tape....... great stuff. Not an adhesive but stays supple. Maybe use butyl tape then a sealant on top. Maine sails website sells the stuff.... great product.
Thanks for the input. That's a compression type of seal. It works when bolts squeeze it.

These windows (if you can call them that) are held in place like skyscraper windows using 3M VHB tape. They have an open edge that needs sealing up. Just to keep water from flowing. There are no fasteners.

I need some kind of very flexible goop to bridge the gap between the edges of the window and the boat. (VHB tape is more toward the center of the window). It should stay wet ideally, so it can move. The Dow 795 was working great for a couple days until it cured fully. Then it cracked. When it was still wet and flexible it was perfect.

Some kind of tape would work too if they made such a thing.
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Old 16-01-2020, 14:26   #6
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Try 3M 06580 Marine.

In Tallahassee I use Whiddon Glass. They were able to supply me two large ports, of irregular shape roughly 35x25 inches, two layers of 1/4" tempered with safety laminate between. I don't think you could stump them.

http://whiddonglass.com/
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Old 16-01-2020, 14:26   #7
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

A vital factor for your windows is the thickness of the adhesive - ie the bondline thickness. A thin bondline cannot accommodate the large amount of movement your windows experience. The thicker the bondline the lower the stress in the adhesive. The thicker layer has a much easier time accommodating the movement.

Thick - which means at least 1/8", and 1/4" is better - bondlines are done by installing the windows with small spacers to maintain that bondline while the adhesive cures.
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Old 16-01-2020, 14:27   #8
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

I'd assume this is similar adhesive to what they use when installing windshields. 3M makes:

3M (08609) Window-Weld Super Fast Urethane Black Cartridge - 10.5 fl oz.
3M (08693) Auto Glass Urethane Windshield Adhesive Cartridge - 10.5 fl. oz.

I would think either of these should work. If it's sticking both surfaces and not each other, it sounds like it hasn't cured. I would not expect movement in a window and would assume either an incorrect installation product or a faulty installation product.
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Old 16-01-2020, 14:36   #9
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

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I'd assume this is similar adhesive to what they use when installing windshields. 3M makes:

3M (08609) Window-Weld Super Fast Urethane Black Cartridge - 10.5 fl oz.
3M (08693) Auto Glass Urethane Windshield Adhesive Cartridge - 10.5 fl. oz.

I would think either of these should work. If it's sticking both surfaces and not each other, it sounds like it hasn't cured. I would not expect movement in a window and would assume either an incorrect installation product or a faulty installation product.
Thank you, but please. These are 48" polycarbonate so they move 1/8" every day. That's how it goes.

That movement is breaking the sealant.

Dow 795 isn't the wrong product if you research and there is no "installed wrong" possible. It's installed and fixed in place just fine.

The glazing is failing due to movement, which windshields don't have.
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Old 16-01-2020, 14:40   #10
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

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A vital factor for your windows is the thickness of the adhesive - ie the bondline thickness. A thin bondline cannot accommodate the large amount of movement your windows experience. The thicker the bondline the lower the stress in the adhesive. The thicker layer has a much easier time accommodating the movement.

Thick - which means at least 1/8", and 1/4" is better - bondlines are done by installing the windows with small spacers to maintain that bondline while the adhesive cures.
This is probably the way to fix it. BUT... The VHB tape is about 1/8" thick so it holds the rest of the window at about a 1/8" distance from the surface.

If I go and increase that to 1/4"at the edges, to make a bigger bond line for the Dow795, I'll introduce a peeling tension permanently in the window which will work at the VHB tape every day. Probably it'll win and pop the windows off. At least that's what I'm thinking.

What would your thoughts be on that.
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Old 16-01-2020, 14:43   #11
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

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Try 3M 06580 Marine.

In Tallahassee I use Whiddon Glass. They were able to supply me two large ports, of irregular shape roughly 35x25 inches, two layers of 1/4" tempered with safety laminate between. I don't think you could stump them.

Whiddon Glass | Tallahassee | Tallahassee's First Family In Glass
Thanks for the referral. I'll look into this too.
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Old 16-01-2020, 14:45   #12
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

The spec for Dow 795 says 50% elongation. The spec for 3M 4000UV says 350% elongation. So maybe that would work?

Boat windows are usually polycarbonate, or acrylic. I am not sure about glass. Is it strong enough?

Also, please note, that tinted windows will get hotter than clear which would increase your problem.
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Old 16-01-2020, 14:49   #13
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

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The spec for Dow 795 says 50% elongation. The spec for 3M 4000UV says 350% elongation. So maybe that would work?

Boat windows are polycarbonate, or acrylic. I most certainly would not use glass. Also, please note, that tinted windows will get hotter than clear which would increase your problem.
Yep. I'm hitting close to 100% elongation. That's the problem. Good catch.

Also a good point about the tints (and paint I used to frame the windows to hide the bonding like cars do) being very hot.

I'll take a look at 3M 4000 UV and make sure it's compatible with polycarbonate and give it a shot.
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Old 16-01-2020, 14:50   #14
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

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Thank you, but please. These are 48" polycarbonate so they move 1/8" every day. That's how it goes.

That movement is breaking the sealant.

Dow 795 isn't the wrong product if you research and there is no "installed wrong" possible. It's installed and fixed in place just fine.

The glazing is failing due to movement, which windshields don't have.
LOL!!!!!

I apologize for the confusion. Please allow me to clarify. I never said it was installed 'wrong'.

Quote:
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I would not expect movement in a window and would assume either an incorrect installation product or a faulty installation product.
My assumption wold be that the adhesive used is either defective or not suitable for the installation. If you are expecting the window to move and not expecting the adhesive/sealant to fail (and it is STILL failing) then I would have to assume that either of these is then the truth.

Either the adhesive used can't handle the job, or should and is defective. I'm not sure what was used, but suggested a few products which look like they might work.

The snarky sarcasm when all I'm trying to do is help is kind of childish. Best of luck with your problem.
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Old 16-01-2020, 15:03   #15
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Re: Help! What Sealant is Better than Dow 795 for polycarbonate??

Quote:
Originally Posted by Pauls View Post
A vital factor for your windows is the thickness of the adhesive - ie the bondline thickness. A thin bondline cannot accommodate the large amount of movement your windows experience. The thicker the bondline the lower the stress in the adhesive. The thicker layer has a much easier time accommodating the movement.

Thick - which means at least 1/8", and 1/4" is better - bondlines are done by installing the windows with small spacers to maintain that bondline while the adhesive cures.
What he said. Dow recommends a minimum sealant depth of 6mm (1/4") for glazing applications to allow sufficient movement with thermal differentials. Usually this would be accomplished by using a thick VHB, by having a raised rim for the VHB, or by having a depressed channel for the sealant.

If you go with a very elastic sealant you can get away with a thinner bead, but you will likely lose the adhesive properties. If the VHB is sufficient.... Polyurethane generally offers the best elongation, but is also generally incompatible with polycarbonate. Polysulfides also offer good elongation, but soften polycarbonate. Which mostly leaves you with stretchier, but not as strong silicones. Look for the one with the greatest elongation.
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