Very Flexible Sealant (not adhesive) For Polycarbonate?

[daydreamer]

I believe the design of the joint is more important than the particular sealant used.

Post #55 shows a diagram of a proper 2-sided joint. Letting the bottom of that joint stick to anything turns it into the dreaded 3-sided joint, which will tear itself apart with any significant expansion. Blocker tape stops the third side from sticking, which would ruin the joint. My guess is that 3-sided joints are the cause of most failures of catamaran window repairs.

I think user Thumbs Up has given good advice and good links.

Note that while using one large piece of plastic for the windows instead of two smaller ones basically halves the number of joints, at the same time it doubles the expansion you must contend with on the remaining joints. And too much expansion is the problem you claim to have, not too many joints.

I used two beads of 795 to re-bed the big 3x4ft windows on my Privilege 42 catamaran. One bead is on the face of the window for adhesion, with double-stick foam tape on eithe side of the caulk to maintain width and thickness of the joint at the preferred 3-to-1 ratio, while also acting as a bond breaker against the second bead. The second bead is on the edge as a weather seal, with breaker tape behind the joint to keep it 2-sided. It's only been four months, but it all still looking good so far, even on the south side against the dock, which is in full south Florida sun.

[daydreamer]

Originally Posted by Chotu

Attached, please find some pics of what Dow 795 does when used as a sealant with large pieces of polycarbonate.

Is there a better, more flexible and resilient sealing material I can use just to seal the gaps? Not looking to take any structural load with the sealant. Just looking for something that will not crack as things expand and contract. Something that stays stuck on the polycarbonate and the hull but gives really nicely in between the two.

Thanks!

If I'm interpreting the photo correctly, I'm looking at a vertical seam between two adjoining windows. If I'm interpreting the text correctly, that vertical seem is attached to both windows and to the hull. If so, that would make the vertical seam a 3-sided joint. Such joints are notorious for tearing themselves apart due to thermal expansion and contraction.

If this joint is as I described, the windows will expand and contract, however because the sealant is attached to the hull, it is not free to compress or stretch when the windows move. Something has to give! Apparently the bond to the hull was stronger than the bond to the windows, and the weaker bond gave way. I believe both sides of the seam separated because both windows moved. If it were just lack of elongation of the sealant, only one side would have come apart, as once that side was loose there would be no force to pull the other side loose. It was the bond to the hull that pulled the sealant away from both windows.

I believe the solution for this joint is to use bond breaker tape behind the seam. Stick the tape to the hull so that the sealant is attached only to the windows. That way, as the windows expand and contract, the sealant can freely slide over the tape.

For Dow 795, the ideal thickness of the sealant is between 1/2 and 1/3 the width of the joint, but at least 1/4 inch. Thicker is not better! How wide is the gap? How thick are the windows? If my guess of 1/2" x 1/2" is close, the joint is square and weaker than it would be if it were half as thick. You can buy bond breaker tape made of foam to help fill the joint and keep the joint from being too thick.

I used SEA Foam brand Closed Cell Bond Breaker Tape from Sealant Engineering (https://www.sealantengineering.com/p.../bond-breaker/), but they don't seem to stock it at the moment.

[Chotu]

Originally Posted by daydreamer

If I'm interpreting the photo correctly, I'm looking at a vertical seam between two adjoining windows. If I'm interpreting the text correctly, that vertical seem is attached to both windows and to the hull. If so, that would make the vertical seam a 3-sided joint. Such joints are notorious for tearing themselves apart due to thermal expansion and contraction.

If this joint is as I described, the windows will expand and contract, however because the sealant is attached to the hull, it is not free to compress or stretch when the windows move. Something has to give! Apparently the bond to the hull was stronger than the bond to the windows, and the weaker bond gave way. I believe both sides of the seam separated because both windows moved. If it were just lack of elongation of the sealant, only one side would have come apart, as once that side was loose there would be no force to pull the other side loose. It was the bond to the hull that pulled the sealant away from both windows.

I believe the solution for this joint is to use bond breaker tape behind the seam. Stick the tape to the hull so that the sealant is attached only to the windows. That way, as the windows expand and contract, the sealant can freely slide over the tape.

For Dow 795, the ideal thickness of the sealant is between 1/2 and 1/3 the width of the joint, but at least 1/4 inch. Thicker is not better! How wide is the gap? How thick are the windows? If my guess of 1/2" x 1/2" is close, the joint is square and weaker than it would be if it were half as thick. You can buy bond breaker tape made of foam to help fill the joint and keep the joint from being too thick.

I used SEA Foam brand Closed Cell Bond Breaker Tape from Sealant Engineering (https://www.sealantengineering.com/p.../bond-breaker/), but they don't seem to stock it at the moment.

You nailed it! Only thing different from your description is that the bond was a touch more than 1/4” deep. But everything else is correct, including the solution.

Now, what about the rest of the cracks? There are cracks around every window in the sun.

Here is a picture of one of the front windows to the left in this pic. To the right, the bowed out one is the side window. Ignore the bowed out one momentarily. This front window is 1/4” polycarbonate with VHB tape floating it between 1/8” and 1/4” off the hull. The adhesive bond where 795 is acting like an adhesive below the polycarbonate isn’t broken. The sealant bond where 795 is acting like a sealant and making things look pretty where the window meets the hull is broken.

What about that joint is causing it to crack. Every single one around the entire boat is cracked like that if in the sun.

[thruska]

Originally Posted by Chotu

I think I should clear up some terminology.

Adhesive: Holds things in place by bonding to them. Intended to act as a structural component. It’s what holds skyscraper windows in place.

Bedding compound: Acts as a sealant in conjunction with traditional hardware. Basically is a soft gasket material you use traditional hardware to tighten against to make a seal and keep water out.

Sealant: Something you use seal cracks and gaps to keep water from dripping into these cracks and gaps.

Many products are more than one of these at a time. Dow 795 for instance. It’s an adhesive sealant. Holds your piece in place AND keeps water out (if it doesn’t fail like it does for me).

Sealant/bedding compound. Silicone. It can be used for bedding and sealing.

I am looking for a very good sealing product that is applied over gaps and cracks to prevent water from getting into gaps and cracks. I don’t need a bedding compound (butyl) and I don’t need a structural adhesive (3m 5200)

I need a sealant to close gaps and cracks to keep water out.

The window is held in place by tracks now so it can expand and contract at will and not fall off when Dow 795 and VHB tape fails.

I have a crack at the edge of the window toward the bow to fill. I have a transition from the hull to the upper aluminum track to fill. I’m filling them to keep water from entering them.

The leading edge of the window toward the bow needs something incredibly flexible since it will be doing the usual thermal expansion thing that killed Dow 795 and VHB tape.

The windows do NOT have fasteners. They are held in place by upper and lower tracks.

As you say, some products are adhesive sealants. These days such products also overlap industry usage. For instance as said skyscrapers use very robust multipurpose products. Likewise the auto industry uses adhesive sealant instead of welding, very aggressive adhesion. The rv industry and automotive industry apply windows without gaskets or tracks and they survive 100 mph testing.
I have seen marine applications using this technology.
Sikaflex is one product line that has proven results .
I believe the chemical term used on some of these products is polysulfides ?

[dnight]

Along with sailing my own personal 41 footer, I also work as a Captain on superyachts. We replaced our poly windows on the sailing boat 3 years ago and asked the construction shipyards what they use. Even though the tempered glass does not have nearly the expansion as poly, they use what they consider the best stuff so there are no warranty issues. That stuff is Bostik Simpson ISR 70-03. Some of the yard workers also had sailing boats and that's what they all swore by. Been only on 3 years now, but nothing to complain about so far. Windows are about 20 inches wide by about 6 feet long, 4 of them, all poly. A lot of movement in the sun and no cracking. It is an adhesive sealant, and is used in car windscreens, highrise windows, and the superyacht industry. Worked great for us.
https://www.bostik.com/globalassets/...s_15122017.pdf

[RKsailsolo]

Originally Posted by srbaum

Chotu,
The best product for that job is Dow Corning 795. I purchase mine through a local industrial supply and it is about $7 for a regular caulk gun size tube.
Here is some information that I have put together on this sort of issue.
Best regards,
Steve Baum
Homeport of Portsmouth, Virginia, USA

Attachment 227302

Love to learn from your experience but can't get your link to work. Can you repost?

[RKsailsolo]

Originally Posted by Chotu

Thank you very much. I’ll check these out.

Dow 995 spec calls out similar substrates only. FRP and poly carbonate have different expansion rates. This is the issue with chotu application, exacerbated by a large sheet of poly carbonate. ( wide expansion)

Skyscraper window is glass, not plastic.

[RKsailsolo]

Originally Posted by CarlF

I really don’t know anything that’s as good as 795 for frameless plexi - and I’ve tried just about everything over the years. I’ve seen 795 fail for three reasons::

1) The caulk bead was too thin for the thermal expansion. 1/4” is usually enough and the OP’s picture looks plenty thick

2) The caulk was put in very thick on a very hot day. On hot days I’ve been told it’s better to apply half thickness and then 1/2 the next day

3) The tube was past the expiration date printed on the tube (which is usually less than six months when I bought mine). You have to be really careful to get fresh tubes.

Here is the 795 technical manual
https://www.dow.com/documents/en-us/...df?iframe=true

Reason four, inadequate surface prep ( contamination)
Reason five,, premature fixturing removal

[RKsailsolo]

Originally Posted by Chotu

Ok, ok. Fair point. No way to know the size of the bead from the pics.

So yes. That particular pic shows a 1/2” bead attempting to seal the spot where 2 windows are butting up to each other. Just like you see in a storefront on land. Like at an Apple store.

There were 2 modes of failure:

1). The 795 under the windows that held them to the boat

2) areas where I attempted to use 795 as a sealant like you are pointing out here.

This thread is asking for a *sealant* that will work in the situation above. 795 doesn’t work as an adhesive OR a sealant on these windows.

I’ve solved the problem of the adhesive. I don’t need one anymore. The windows are mechanically fastened with tracks.

Now I just need a sealant for cracks like the one you are pointing out 795 is no good for. Which I agree with.

Back to the point of the thread: Does anyone know of the best quality polycarbonate compatible SEALANT (not adhesive) that can put up with huge thermal expansion and stay attached to the window and boat, filling the cracks to keep water out?

Starting to think most don’t.

Although there are a few good things to try in the thread.

Good clarification. 795 may work for your modified application ( only need edge sealing ). At least its not now needed for adhesion. Your time spent on doing edge sealing is substantial less.

[Chotu]

I think I’m going with glass and 795 as an adhesive for just my problematic windows (the side ones).

[RKsailsolo]

Thanks for this informative discussion chotu. I learned a ton.

[Chotu]

Originally Posted by RKsailsolo

Thanks for this informative discussion chotu. I learned a ton.

Same here!

Thanks to everyone involved. Glazing is really tricky stuff

[Chenega]

Originally Posted by srbaum

Chotu,

The best product for that job is Dow Corning 795. I purchase mine through a local industrial supply and it is about $7 for a regular caulk gun size tube.

Here is some information that I have put together on this sort of issue.

Best regards,

Steve Baum

Homeport of Portsmouth, Virginia, USA



Attachment 227302

[emoji106][emoji106] - agree

[Tupaia]

It seems that most of Chuto's and others issues are with the edge seals and not the attachment of the windows themselves.

So is the issue purely cosmetic or are there leaks also?

Something that has not been mentioned is that because sealant/adhesives cure in the way they do I was advised that you should apply the sealant to both surfaces and then bring the together. You can spread the goo evenly on both and it does not require compression to form the joint. This method is preferable rather than applying beads to one surface and then putting them together.

[Chotu]

Originally Posted by Tupaia

It seems that most of Chuto's and others issues are with the edge seals and not the attachment of the windows themselves.

So is the issue purely cosmetic or are there leaks also?

Something that has not been mentioned is that because sealant/adhesives cure in the way they do I was advised that you should apply the sealant to both surfaces and then bring the together. You can spread the goo evenly on both and it does not require compression to form the joint. This method is preferable rather than applying beads to one surface and then putting them together.

It’s different for different parts of the boat for me.

The east side that goes from 60F to over 100F in an hour each morning has the windows popping right off. That’s the port side.

The starboard side that warms up more gradually and doesn’t get direct sun until afternoon just has the surface cracks in the 795.

The mostly vertical forward windows have little to no cracks in the 795.

The aft doors and windows do not receive direct sunlight and have no problems.