PC window bonding - manufacturer vs Sika

[nfbr]

Plaskolite Tuffak AR (polycarbonate) bonding instructions are very different to Sika bonding instructions. In fact plaskolite lists sika as not compatible.
They seem to recommend silicones (several models / brands) rather than polyurethanes.

It feels weird to use silicone on a boat window

Comments?

[Chotu]

Originally Posted by nfbr

Plaskolite Tuffak AR (polycarbonate) bonding instructions are very different to Sika bonding instructions. In fact plaskolite lists sika as not compatible.
They seem to recommend silicones (several models / brands) rather than polyurethanes.

It feels weird to use silicone on a boat window

Comments?

I have way too much experience with this.

silicone will actually work. Proper window glazing silicone. However, depending on how big the piece is, the thermal expansion coefficient will cause it to rip free. How big is the piece we are talking about? Because that means everything.

Mine are so large that they could only be contained by physical window frames. No amount of bonding by any material whatsoever worked.

[nfbr]

Originally Posted by Chotu

I have way too much experience with this.

silicone will actually work. Proper window glazing silicone. However, depending on how big the piece is, the thermal expansion coefficient will cause it to rip free. How big is the piece we are talking about? Because that means everything.

Mine are so large that they could only be contained by physical window frames. No amount of bonding by any material whatsoever worked.

4ft x 2ft approx (6 of)
Replacing 30yo current 1/4” PC
Curved (salon windows on 61ft cat)
Also has a mechanical exterior frame strip for strength & UV

[Chotu]

Originally Posted by nfbr

4ft x 2ft approx (6 of)
Replacing 30yo current 1/4” PC
Curved (salon windows on 61ft cat)
Also has a mechanical exterior frame strip for strength & UV

those are actually borderline huge. The 4 foot length will have a pretty good expansion and contraction.

if multiple 4 foot sections are butt joined to each other to make a continuous sort of window, it will be difficult to keep cracks from forming at the joint where they touch. unless you have a physical window frame piece that goes down that spot.

I have a joint like that where an 8ft window meets a 4ft window. I left something like 3/4 of an inch or half an inch of sealant in between the two. It didn’t work. The sealant (Dow 795) still cracked. But on the other side where I used a physical frame in the same location, no problems.

whatever you do you will need to float these windows quite a bit above the mounting surface and fill in behind them like that.

The idea is to have very thick beads of sealant. It sounds like you are just sealing these right? The frames would hold them in otherwise?

but the idea is that the sealant bead needs to be thick so it can be flexible. If it’s thin and the polycarbonate moves too much it will tear. But if it’s thick, as you can picture in your head, it will just bend and follow the polycarbonate as it moves.

Dow 795 is a really popular product for this purpose on the forum. but, silicone does work just as well. People just don’t like to clean up after silicone once they have applied it. It’s a little more difficult to get off later. That's why it's avoided.

It would be helpful to see how this all goes together on your boat. If there are any pictures or drawings or anything. The problem spots will be very evident looking at them.

but answering the main question, yes. Silicone can work for this. People just don’t like it. I actually have some larger sheets than you held on with silicone (and no frames from experimenting. These are out of the sunlight however. But they have held on for about six years now.

[nfbr]

Pics below

[Chotu]

Oh, you're in great shape with those frames and the fact that the windows don't butt join each other. Very nice.

yes, glazing silicone will work. However people prefer Dow 795. Just make sure to make the bead as thick as you possibly can so that there’s less chance of the bead being pulled apart by thermal expansion. That’s it. Yours is much more simple than mine.

[chris mac]

We have large curved salon windows on our cat, 42' tpi. They are 1/2" acrylic, roughly 4.5' by 3'
When I had new ones built I installed with Dow 795. I used roughly 3/8" thick bonding layer with about a 1/2" surrounding beads as a gasket.
Acrylic and othe plastics expand and contract at much nigher rates than fiberglass. So your adhesive must have high bonding properties and very high flexibility, so it stays attached to both surfaces as they grow and shrink.
For a reference point, I installed them may of 2021. We have definately had some aggressive weather and sails since and no leaks. And I can be sure of that because it is pouring rain here in eluthera today

[wholybee]

Dow 795 is a window glazing silicone used for installing windows in skyscrapers with no other mechanical fasteners. Any silicone should work, but 795 is made for it, and pretty cheap. A whole lot cheaper than sika. As mentioned, you would need a large bead if the window is large. 1/4" thick at a minimum, maybe as much as 3/8".

My hatches, which are much smaller, everything else failed until I switched to 795.

Also note that windows have been falling out of skyscrapers onto the streets in SF recently.