Acrylic or Lexan for fixed windows?

[tombeau]

Our experience with original smoked lexan windows. The sun damaged them and they crazed and then cracked right in the middle. it took 20 years for the process to play out, but all 5 ports succumbed. In retrospect, i would have covered the windows with canvas when not using the boat to avoid/delay this result. Good luck with your project.

[Brewgyver]

Originally Posted by Pauls

(snip) Windows are glued onto modern skyscrapers. Window sealant thickness is commonly required to be 1/4" plus. Accomodating thermal expansion is why.

Also worth mentioning that Dow Corning 795 is the very sealant specified by architects for this purpose, in fact it's why Down Corning developed it.

[capt jgw]

Originally Posted by Brewgyver

Also worth mentioning that Dow Corning 795 is the very sealant specified by architects for this purpose, in fact it's why Down Corning developed it.

You hear a lot of people here say " no silicone on my boat, ever" but this is one application where it's the only thing that will work. I once tried to seal an acrylic deck hatch with 5200, it leaked worse after than before. Someone clued me in on the Dow 795 so I tried again with that, it never leaked another drop in the ten years I owned it after that.

[ShadyLady]

I fully agree with the previous comments regarding thermal expansion and sealant thickness.

In addition I would like to add that I favour an installation that does not involve drilling through the acrylic. After years of problems with cracks and sealant leaks, I replaced long externally mounted acrylic windows with three smaller sheets of acrylic (10mm I think), minimum 30mm hull overlap, encapsulated by through bolted teak caps top and bottom. 3mm minimum sealant thickness between hull and acrylic (use small spacers) and between acrylic and teak caps. The teak caps, as well as allowing the acrylic to 'float' without fasteners, also protect the sealant edges from UV and chemical damage and would also provide easy emergency attachment of storm boards.

So far no more problems with leaks or cracks after 20 years and several good wave tests.

[qwert]

I used lexan margard. It also has an anti graffiti coating. It occurred to me that glue might not stick to anti graffiti so I contacted Lexan. This caused a little consternation at HQ ......Eventually after several emails it was decided the best solution was to sand the area of the lexan to be used, clean it well with alcohol (doctors wipes,very cheap and convenient) and no touching after cleaning. Then glue with dow corning 795 It shouldn't drop below about 15deg c and needs about a week to harden properly. 795 is available from glazier's supplies.

There was also the problem of expansion.Obviously the seal has to be pretty thick or it will rupture in shear. My solution was to make several dozen small rubber 'lumps' 3mm x 3mm by using a centre pop on a sheet of 3mm rubber.By putting them in the sealant every half inch or so they kept the lexan 3mm away from the steel (metal boat) and formed an effective gasket which has not leaked.

[NPCampbell]

Originally Posted by Tupaia

In the past Acrylic tended to be quite brittle and craze easily. By selecting "cold cast" Acrylic, stresses created during the old extruding process are essentially eliminated. So there is very little to chose now between polycarbonate and acrylic.

Except for strength ... Polycarbonate has a tensile strength around 10 times that of acrylic, cast or extruded. An axe, hammer or pick will shatter acrylic. Poly is pretty much impervious to shattering.

Originally Posted by Tupaia

A word of caution about combining smaller windows into larger ones. Both materials expand and contract considerably due to thermal conditions the larger the pane the greater the expansion and hence they are much more difficult to seal.

Good point. To put this into perspective, a 1 meter window expands this amount for every degree Celsius:
Polycarbonate .07mm
Fiberglass .05mm
Fiberglass (parallel to weave) .015mm
Aluminum .025mm

So a temperature swing from freezing 0C to 55C (130F) would cause a 1 meter wide window to expand 3.85mm while the surrounding fiberglass only expands .83mm.

[sailcrazy]

following. We had our Lewmar overhead hatch "glass" replaced about 8 years ago with Lexan; the work was done in Phuket. Initially it looked great, but now (without suncovers) it is noticeably cloudy.

[Cheechako]

One thing about Lexan, it has rippling that poly doesn't have. Maybe there's a spec of Lexan that doesn't though?
Buy fiberglass sheet and use smaller windows?

[NPCampbell]

Originally Posted by Cheechako

One thing about Lexan, it has rippling that poly doesn't have. Maybe there's a spec of Lexan that doesn't though?
Buy fiberglass sheet and use smaller windows?

I don't think that ripple is exclusive to poly. I believe it's more a quality control issue with extruded plastics in general. I am guessing that ripple is non-existant with cast acrylics and poly.

[NPCampbell]

Originally Posted by sailcrazy

following. We had our Lewmar overhead hatch "glass" replaced about 8 years ago with Lexan; the work was done in Phuket. Initially it looked great, but now (without suncovers) it is noticeably cloudy.

There's a bunch of different coating materials for both poly and acrylic that are UV and scratch resistant. However, if it's an overhead hatch that you walk on I can't see a coating lasting 10 years+.

[mitiempo]

Originally Posted by sailcrazy

following. We had our Lewmar overhead hatch "glass" replaced about 8 years ago with Lexan; the work was done in Phuket. Initially it looked great, but now (without suncovers) it is noticeably cloudy.

All Lewmart hatches and ports use acrylic. All other manufacturers hatches as well with the exception being one series of Bomar hatch. The Bomar using polycarbonate has bars under the lens. The reason is that polycarbonate is flexible - without the bars stepping on the hatch can cause enough flex to break the seal.

As posted polycarbonats scratches and crazes easily. The other reason every manufacturer except one uses acrylic is longevity. I have seen acrylic ports 30 years old without any crazing or damage other than the odd mark from daily use.