Help! What Sealant is Better than Dow 795 for polycarbonate??

[Chotu]

Originally Posted by Dsanduril

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.

Looks like I’ll give this another go. I do have the thickest VHB tape made. Because previous VHB tape failed due to not being thick enough.

Unfortunately, I do not have a recessed for the sealant. That would make it perfect. However, I don’t know that I can grind into the laminate and create that type of channel. It’s pretty much a flat surface I’m working with here. I will check for the longest elongation. Probably that 3m 4000 UV.

[slug]

Originally Posted by Chotu

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.

Tape is a poor choice . When using tape , clamping pressure is needed when making the frame to window bond

Best to use tube sealant such as sika flex ..then follow the directions

Google Sika and they have a complete tutorial for bedding marine windows..glass and plastic

To create the correct adhesive bead thickness, bond line ... Use clear rubber
Dots around the perimeter of the frame

[Chotu]

A picture is worth 1000 words....

This is the top of the window. Near the roof. Not the side or the bottom.

Sorry, the picture is completely upside down.

[Chotu]

Here’s another one for good measure. The first window was glazed about a week ago. The second one pictured here in this post was glaze a couple days ago. Temperatures were high today. Lots of direct sun. 87° or so. Plus of course it heats up like crazy since it’s black.

[Chotu]

These beads are concave in shape. Would it be better to have a triangular bead created by just using a flat scraper along it rather than a round object? They seem to be giving way right where they join the window. They aren’t visibly giving way between the window and the hull. As far as I know. But I don’t want water getting in compromising the VHB tape.

Even if they separated a little, if there was sort of a triangular flat surface to it rather than concave, and it could direct the water past the gap, onto the window, I would have something working.

[Dsanduril]

One problem might be time. Dow 795 takes 7-14 days to cure and 14-21 days to achieve full adhesion and tensile strength. Might be you need to shade your installation for a couple of weeks. They don't publish (that I know of) a curve of strength vs. time, so no idea whether it reaches 90% of that the first day, or much slower strength buildup.

If you select an alternate pay careful attention to the time required to develop full strength.

[Dsanduril]

It looks like the bead is outside the window? Not under the window? If that is the case think about what happens at the bottom of the bead where it is fixed to the deck/hull/cabin. You're asking the portion at the deck to move the full 1/8" with no cross-section to distribute the stretch.

With the VHB installations I have done the VHB is inset ~1/2" from the edge of the glazing material. Then the sealant is squirted under the window to fill that gap. When the window moves the whole bottom section stays fixed in place to the hull and the whole top of the bead moves with the window. The elongation is spread across the whole bead thickness and cross-section. If you bed around the outside you're asking the stuff at deck level to be fixed in one place and also to be 1/8" away at the same time connected to the shrunk window. With no cross-section across which it can stretch.

[Edit] - because a picture.... Here's the bead Sika recommends. You can't do it quite this way when adding after VHB, but it gives the general idea to get proper elongation.

[Sailorman Ed]

Chotu, while I haven't yet tried this on PC, I don't see any reason Big Stretch ( 500% elongation ) will not work. Lowes carries white, Amazon has various colors, I am using it sealing Corian to a large acrylic glass block window facing south.
Ed

[Chotu]

Originally Posted by Dsanduril

It looks like the bead is outside the window? Not under the window? If that is the case think about what happens at the bottom of the bead where it is fixed to the deck/hull/cabin. You're asking the portion at the deck to move the full 1/8" with no cross-section to distribute the stretch.

With the VHB installations I have done the VHB is inset ~1/2" from the edge of the glazing material. Then the sealant is squirted under the window to fill that gap. When the window moves the whole bottom section stays fixed in place to the hull and the whole top of the bead moves with the window. The elongation is spread across the whole bead thickness and cross-section. If you bed around the outside you're asking the stuff at deck level to be fixed in one place and also to be 1/8" away at the same time connected to the shrunk window. With no cross-section across which it can stretch.

[Edit] - because a picture.... Here's the bead Sika recommends. You can't do it quite this way when adding after VHB, but it gives the general idea to get proper elongation.

Attachment 207053

I went through the Sika flex tutorials and they all have a rebate. I don't have a rebate (installed wrong as the other poster said? Lol)

So given I don't have a rebate, it is exactly like the diagram you showed in your last post looks identical to the window edge in my case. It looks identical to the right side of the image.

I filled it under the window as shown, had plenty oozing out of the top, then used a round object to create a nice looking bead. The bead is cracking at the edge where it meets the polycarbonate but is most likely intact below that where the window and hull are 1/8" apart.

[s/v Jedi]

You can simply use a router to put a rabbet into the backside perimeter of the window. If the current gap is 1/8”, add a 1/8” rabbet and you get 1/4” without changing anything with the vhb tape.

The sealant should only go into the rabbet, not on the outside. That outside edge needs to be painted black though, so that light can’t enter there. (It will travel through the material much like through an optical fiber, creating UV damage very quick.

I think many of the people replying here don’t realize the function of vhb tape and that it is used here. I have some I use out in the open where it’s hit by tropical UV every day and rain whenever it rains. Still good after two years.

[Matt Johnson]

I had a similar issue with Dow 795 when bedding windows in Florida's sunshine. The fix for us was to completely shade the windows for at least a week to prevent them from expanding while the silicone cured. Like yours, they were also held in with 3m VHB 4991 tape, but they're smaller dimensions, so didn't have the expansion you're experiencing.

Good luck,


Matt

[Compass790]

Don't know if it's available in the USA but a product called seal once is very stretchy. I think 350% but thats from memory so not guaranteed. Its clear but can't remember what its temp rating is. Worked on our windows but it's not as hot in NZ.

[Lake-Effect]

Originally Posted by Chotu

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.

What sort of sealant did the factory use, and how long did that hold up, before you had to re-do it?


I've never liked just exposed sealant, with no cover or other mechanical support. Could some sort of plastic trim be put over the joints?

[Blue Claw]

Originally Posted by Chotu

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.

Try geocel 2300 caulk its used in the construction industry to bond and waterproof dissimilar materials. such as vinyl siding to wood or metal roof flashing to brick. it stays very rubbery.It is available in clear or white. Butyl caulk is even more flexible but tends to remain sticky attracting dust and dirt.

[mdenize]

Hi Chotu

Glued on windows can be a real pain but I can share my experience which is with an Ali boat that only had glued on windows (no other fixings)

My Mclay has glass windscreens and tinted poly side windows and hatch glazing.

All are installed the same way which is mostly very successful (except in extreme storm conditions when the boat flexes just too much for the curved windows which I bolted as well to retain the curves.

The system they use is an etch primer on the window and the boat (3m I think) which as well as prepping the surface stops UV getting to the sealant,

followed by a Black double sided sticky foam packer tape (at least 6mm thick maybe 8 and about 10mm Wide). This locates the window and stops the sealant squeezing through to the inside so it looks really neat but the main function is so the sealant has real thickness and give it lots of time to cure.

Then the product that does the sealing is a black windscreen sealant ( I think 3m) that sticks like Sh#$ to blanket, The way to avoid a mess like you seem to have is to mask the window and boat (wide tape about 5mm away from the outline of the window to allow a nice cove then go to town with the sealant gun and wipe away the excess with a wet plastic coated finger, you end up with a nice cove that sheds water very well. Frankly it is hard to believe it will keep a 1m2 window installed in 80 Knots but it does!

If you want the exact product detail I can try to find out for you.

Hope this helps at least a bit!