Bonding aluminum to polycarbonate: primer?

[cghubbell]

I need to bond new sheets of polycarbonate (Makrolon) into the old aluminum frames from my boat. I'm planning to use Dow 795 structural silicone. My ports have no mechanical fastening. That is, the Dow 795 will need to hold the sheet in place. I have seen mention in some places about using a primer, but I can't seem to find any guidance on when you do or do not need it.

Does anyone know whether the Dow 795 will sufficiently bond the aluminum and polycarbonate without a primer? And, whether or not the bonding surfaces need to be lightly abraded to aid in the bond?

Disclaimer: I am aware of the lexan vs. polycarbonate conversation. I need to use what I have, and I'm fine with replacing it in a few years if a problem develops. Just need to make the best of on-hand materials.

Thanks in advance,
Chris

[cghubbell]

To clarify, the aluminum frames are mechanically fastened to the cabin, but the makrolon "floats" in the aluminum frame, held in place only by the silicone.

[sailingfool]

These guys have some instructions
Which Sealants To Use

If you remind me who made the hatch, I may be able to find the link/address for a US distributor of replacement lens and related sealants. Back when I rebedded my CS 36T hatches with a silicone kit with primer I bought from the distributor it all worked great.

[cghubbell]

Originally Posted by sailingfool

These guys have some instructions
Which Sealants To Use

Yes, this is where I picked up the cue to use Dow 795. It doesn't answer the question about primers though...

Originally Posted by sailingfool

If you remind me who made the hatch, I may be able to find the link/address for a US distributor of replacement lens and related sealants. Back when I rebedded my CS 36T hatches with a silicone kit with primer I bought from the distributor it all worked great.

It's a standard CS27 port light. No idea who made it; there's no label to be found. In fact, I can't even find a replacement in a dimension close enough, or I would go that route. It's approximately 48" long by 5-1/8" cut out, so totally non-standard, unfortunately.

If I can't bond this stuff into place, my fall back is awful in terms of both time and money. Laminating glass into the center of that cut-out to use smaller ports is the only way I can see to replace what I have. I'm so sick of glass work at this point that I'll need therapy if that happens. I'm on the tail end of a year-long refit at the moment and my mental health is pretty volitile

[casioqv]

West G/flex epoxy has very strong adhesion to both aluminum and polycarbonate:

WEST SYSTEM adhesion data

[LookingForCruiser]

Epoxy will not flex enough between aluminum and polycarbonate. They expand/contract at different rates, the epoxy will crack and it will leak.

You need something that flexes. 795 will flex. I don't know that it's the best choice or not for this project, but it's a better idea than epoxy.

I will be bedding my acrylic windows without primer, using 795. I believe that is safe to do so, but time will tell.

Make sure all mating surfaces are clean clean clean, use alcohol to clean up the edges of the aluminum & the polycarbonate (assuming alcohol is safe on polycarbonate), any oils from your fingers, etc, will cause the 795 to not bond properly and it will leak.

[sailingfool]

Originally Posted by cghubbell

.... No idea who made it; there's no label to be found. In fact, I can't even find a replacement in a dimension close enough, or I would go that route. It's approximately 48" long by 5-1/8" cut out, so totally non-standard, unfortunately.

Try asking the CSOA and i bet you'll get the answer.

[casioqv]

Originally Posted by LookingForCruiser

Epoxy will not flex enough between aluminum and polycarbonate. They expand/contract at different rates, the epoxy will crack and it will leak.

Are you sure? The g/flex is marketed as an extremely flexible epoxy- for bonding plastics. But I guess it's better to use a tried and true method than to try and engineer something new.

[cghubbell]

Originally Posted by sailingfool

Try asking the CSOA and i bet you'll get the answer.

I'm on the CSOA list as well. They haven't made these windows since the days of wooden canoes. I've reviewed the dimensions from every port manufacturer I could find and nothing is even remotely close. I spend weeks over the winter researching this one.

[cghubbell]

I received a frustrating response from Dow when I asked them for guidance. Because I used the word "marine" in my query, they refused to provide guidance, saying that they do not certify any polycarbonate for marine use. So, not even Dow is willing to answer the question about whether or not a primer is required when bonding aluminum to polycarbonate.

Meanwhile, I know for a fact that polycarbonate is used in architectural scenarios bonder to aluminum frames. Same UV situation as a boat, much higher winds hitting buildings...

Ok, the polycarbonate will fail sooner than lexan. I get it, and it's acceptable to me. Meanwhile, how do I bond it? Frustrating. I guess I'll just clean it as best I can and use the Dow 795 I have, hoping for the best.