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Old 27-10-2009, 20:01   #1
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Acrylic and / or Polycarbonate ? Confused !

Well, our windows and hatches have seen there better day. It's time to replace but what is the better solution? The hatches were Lewmar so I know they are acrylic. But the problem comes into play are the saloon windows which are 44" tall by 38" wide being the largest. They are walked upon and will see the most wear and tear (two kids). What would be the best solution for theses windows? Is 1/2" able to support and will polycarbonate flex too much and break the seal? There are no fasteners to hold the windows place. Or just use OEM parts.

What acrylics brands do you like? Cast?

Thank You
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Old 27-10-2009, 20:08   #2
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Cast acrylic are the expensive ones. They last. My ports are from 1991 and still crystal clear. You can find several threads on ports. You can surface mount them with the right adhesive.
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Old 27-10-2009, 21:01   #3
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Does cast acrylic make that much of a difference over extruded?
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Old 27-10-2009, 21:24   #4
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Quote:
Does cast acrylic make that much of a difference over extruded?
A lot.
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Old 27-10-2009, 21:54   #5
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MITSUBISHI RAYON CO., LTD
I have used Non Mitsubishi branded that have been equally as good.
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Old 28-10-2009, 00:01   #6
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Polycarbonate is stronger initially but is more susceptible to UV degradation and will need to be replaced sooner.
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Old 28-10-2009, 04:56   #7
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Thank you, that is what I needed.
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Old 28-10-2009, 05:04   #8
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Polycarbonate

Polycarbonate is super strong... you can hit it with a seriously big hammer and it will not break. Unfortunately if you wipe the salt crystals off it with a dry cloth it will get scratched.
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Old 28-10-2009, 06:40   #9
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I think the key is when you say "They are walked upon and will see the most wear and tear (two kids)" With that kind of use I would go with polycarbonate, or better known as Lexan. Goes by other names as well such as Marrolon. It is more expensive but will not crack, yes it will craze but not lose strength because of that. There are different grades of this stuff some more resistant to scratching and UV than others, Some with better optics. I would look at the Marrolon SL, or Lexan XL, and SGC. Might pay to google this and see whats available and go from there. I think Piedmont plastics has some good info if I recall. They are in your neck of the woods to keep shipping cost down. Plexiglass or acrylic while cheaper and having better UV properties will not withstand the abuse as well and will crack all the way through causing leaks. If you are not walking on it, it is the better choice for like small side windows, but for hatches you are walking on I would go with the polycarbonate.

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Old 28-10-2009, 07:22   #10
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Old 28-10-2009, 07:27   #11
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I replaced all my side windows earlier this year with new polycarbonate. Some of the original polycarbonate windows were replaced by the previous owner about 5 years ago with acrylic. On removal the acrylic windows were brittle and broke into large chunks whilst trying to remove them. All the polycarbonate windows were removed intact. They were still supple and flexible after 18 years and still very strong. I'm glad I choose the polycarbonate. Also the old windows were screwed, and sika-flexed in. They almost ALL leaked thru the screw surrounds. The old Sikaflex was also too thin. When screwing the windows in most of the sealant was squeezed out. I felt a means of spacing the glazing from the coach house would most likely have helped.

We have now installed the new windows with 3M double sided tape (instant holding and spacing all in one) AND Dow Corning 961 on both sides of the tape. The sealant is over 1/8” thick (the thickness of the tape) and can expand 50% without difficulty. The result is a well bonded and watertight window… at least for now we’ll see what it’s like in 10 years!

The best suggestion from me is to be well informed as to the technique for fixing the glazing and be pedantic regarding the detail with the task. Also the technique for placing the glazing onto the coach house is a MUST get it right first time process which we eventually found was helped enormously by having 2” cubes attached to the inside of the window forcing the window into exactly the right place as it was pushed against the coach house. This reduced the stress levels during application and gave excellent results. I liked the sleek look of a single long window but our daily temperature range can be scary so the window was split into four larger panels and three smaller sections on each side. The expansion joint is 3/8" wide and the 6 joints per side eliminate the expansion problem.

The photo is of the first side we did. The second was much better. I'll redo these one day when I get bored and want something to do. That won’t happen for some time though.

Cheers,

Fabian
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Old 28-10-2009, 07:46   #12
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Quote:
Originally Posted by sailvayu View Post
polycarbonate... yes it will craze but not lose strength because of that.

Hi,

In our experience of making hatches over the course of 40 years, after trying out every imaginable material for lenses, the best choice by far is cast acrylic. Unfortunately, the less expensive extruded products like Lexan are very sensitive to UV. The crazing that you see are actual fractures inside the material. The idea that this doesn't affect the strength of the sheet is wrong. Also, the strength of a cast acrylic over the entire surface area is much greater than the point-impact strength of Lexan. Walking on a sheet of 1/2" cast acrylic is completely safe and recommended by the manufacturers up to and above large 40"x40" hatches.

So, cast acrylic provides a better seal to eliminate leaks over the life of the sheet, it is stronger against broadly applied pressure like feet or a crashing wave, is far more resistant to fogging and crazing and is more scratch resistant. For us the choice clear (forgive the pun..), cast acrylic is the best material for hatch and portlight lenses.

--Benjamin Atkins
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Old 28-10-2009, 10:27   #13
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I may be wrong but I have to respectfully disagree with you. After 35 years building and repairing boats and replacing many hatch lens yours included I have to say the acrylics do not hold up. When they fail they crack all the way thru. That is why they use polycarbonate for ballistic glass not acrylic. Now of course there could be some new materials I am not aware of. When you say "Also, the strength of a cast acrylic over the entire surface area is much greater than the point-impact strength of Lexan." you are really comparing 2 different things. Glass can be very strong overall but has little point impact strength. So does the acrylic have the same point impact as polycarbonate? My experience is it does not drop a winch handle on it and it will crack and when it cracks it cracks all the way thru unlike polycarbonate that will just chip. Admittedly a crazed sheet of polycarbonate will not have the ultimate fail strength as a new sheet but it will still be quite serviceable unlike a acrylic that is cracked.

Real life experience has taught me to use the polycarbonate. Now I also know things are changing all the time but I tend to wait and see real world proof of new product claims.

You also say "So, cast acrylic provides a better seal to eliminate leaks over the life of the sheet," I am curious about this statement. What do you mean by this? Not trying to be difficult I am really curious and want to learn.

If like you say the cast acrylic is truly better I just want to be convinced of this. I am more than willing to change my mind but I am an old fart and take a bit of convincing. You as a hatch manufacture could educate us all but I as a builder, repairer, and user will be the devils advocate and expect convincing lol.

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Old 28-10-2009, 13:22   #14
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Most of the time on a boat you don't worry about point impacts as much as the more regular broadly applied impacts from waves or people walking on it. And while cast acrylic isn't as strong against point impacts as lexan, it is much better than glass. So what I'm saying is that cast acrylic is stronger than glass for point impacts and stronger than lexan for broad pressure. Its the perfect median.

Remember, we're talking about cell cast acrylic sheet (ie. Acrylite brand).

By saying that it provides a better seal, this is what I mean; as the temperature rises and falls with the sun and seasons, materials obviously expand and contract. The thing with lexan is that expands and contracts much more than cast acrylic. As this happens, stress is put on the seal which eventually breaks. Cast acrylic will hold its seal for much longer than polycarbonate, an obvious advantage for marine applications.

The yellowing, crazing and scratching is much more likely to happen with polycarbonates as well. Many non-leaking hatches come into my repair shop to be reglazed simply because the customer wants a clear view. This shouldn't be dismissed.

When we choose a material for marine applications we aren't looking for "ballistic glass". We want something that will provide good adhesion, excellent weatherability, and high strength (both impact and broad pressure). For all three (excluding impact strength) cell cast acrylic is by far and above better than polycarbonate.

Police car windshields may use lexan, but aircraft windows use cast acrylic. Two different applications, and marine use is much closer to the weathering/stresses found in the air than at the shooting range...

In any case, I appreciate the opportunity to explain why we use cell cast acrylic and would welcome any further discussion on the subject.

--Benjamin Atkins
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Old 28-10-2009, 14:32   #15
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Yea.... the windows in my Lagoon 42 were in good shape except they had mini cracks/crazing everywhere... Whatever they use in aircraft windows has GOT to be the best.... They are in a harsh unprotected UV environment at 35000 ft, withstand a frozen chicken shot out of a cannon in testing and are subjected to dust particles at 400-500 MPH...... and they can be repolished/restored!
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