Cure for Plexiglass / Lexan Crazing?

[cooper]

Sorry I still dont get it, maybe I am a bit simple but if you have two materials that are basicaly equal in their attributes but one has a much higher impact resitance, why wouldnt you go for that ? I would rather suffer with scuffing in my case and have the extra "water tightness" In most sailing boats portlights are for light more so than picture windows. What have I missed ? Is it to do with the "charter industry" ?

[MidLandOne]

The actual comment was that of the mechanical properties that were quoted some were similar but elongation was not - polycarbonate 100%, acrylic 5%. The comment was not that they were basically equal.

I have to talk in approximate terms as it is hard to source exact comparable figures for each mechanical property.

So, as said, ultimate tensile strength for both is roughly 10,000psi (it varies for both materials about that figure depending on grade and source) and at rupture polycarbonate will have permanently elongated about 100% and acrylic about 5%.

But the yield strength (the point at which it changes from elastic to non elastic strain ie permanent deformation) for polycarbonate is about 20% less than acrylic ie polycarbonate starts deforming in tension much earlier than acrylic does.

At yield polycarbonate elongation is about 6%. Acrylic at yield I could not find an elongation for but given it is only about 5% at rupture at yield it will be next to nothing.

So, for polycarbonate we have a material that yields ie permanently deforms considerably earlier than acrylic.

It is said that polycarbonate is considerably more flexible than acrylic. I have tried to find figures to validate that and best I can see the elastic modulus of polycarbonate is only about 3/4 that of acrylic so much more flexible.

For unnotched Izod impact test polycarbonate is essentially unbreakable while acrylic is breakable. For notched polycarbonate impact resistance is greatly reduced but still about twice that for UNNOTCHED acrylic. So for impact, polycarbonate is greatly superior. But contrary to what I think someone posted, notching does significantly reduce the impact strength of polycarbonate, albeit it still high even so.


Rounding those things up for acrylic and polycarbonate (but recognising that tempered glass is the superior material)-

A polycarbonate pane fixed in a window or hatch will be considerably more flexible than an acrylic one. This may produce sealing problems for the pane in the frame as the pane flexes. For small portlights (and perhaps very low profile but long strip type windows as may be found on the side of a small yacht's low coach roof, but these are basically a fixed portlight) this may not be significant because small size means small movement when flexing. The flexibility also means that for the likes of slide in washboards in a companionway one needs more thickness of polycarbonate than for acrylic.

A polycarbonate pane fixed in a window or hatch and stressed (say by solid water or someone jumping on it - which, by the way examples others have given such as motorcycle windshields do not face) will start to yield ie permanently deform, well before an acrylic one. By the time they reach ultimate tensile strength the polycarbonate one will have elongated about 100% and the acrylic one 5%. This is not entirely correct as we are actually talking about flat plates in frames not plate in tension, but it delivers the correct flavour. Repeated high stressing of acrylic may lead to light surface crazing though.

It is the common professional view that polycarbonate alone is softer than acrylic alone so its surface is more readily damaged. I do not have any quantitative data though as for engineering purposes they are both regarded as soft. This may be something to watch on dogged opening portlights if the dogs latch on the surface of the pane in the same manner as many acrylic types do.

I do not have any quantitative data but it seems to be the common professional view that polycarbonate is not as resistant to UV as acrylic and its resistance is unreliable even when treated.

Many of the disadvantages of polycarbonate could be overcome by going to thicknesses greater than required for acrylic. I do not know the current comparative cost between tempered glass and polycarbonate but suspect that it may be cost effective to go to glass given the cost of polycarbonate (tempered glass would likely be thinner).

Which leads to in the end tempered glass is the best choice without going to exotic laminates of glass with plastics, but more expensive than acrylic. The comment made by one that flopping rigging will break tempered glass is not so - it is what is used on large high quality yachts. It is very strong and resistant to impact in the correct thicknesses. Those needed thicknesses are not very great - large panes, much bigger than anything likely to be found on a moderate sized yacht for windows or hatches, in a 40 knot commercial vessel's front facing unframed ("glued" on) windows are around 10mm, they are required to stand impacts and driving into solid seas.

I sense only trouble and wrath from this .

[clausont]

Originally Posted by Maddog

On my boat I'm planning to ditch the beautiful bronze deadlights and use as thick of lexan as deemed necessary backed up with an aluminum backing plate. Shooting for strong enough that it adds stiffness to the cabintop. Took keep it clear I'm thinking a piece of 1/4 inch plexi on the outside as a sacrificial layer. Lexan = bedded in, plexi just bolted up.

Search for the thread on "Sexy Windows". That is the method I am going to use next spring.

I have just replaced the windows in my Cal 29. I am no expert by any means and this is my first attempt ever at replacing boat windows.
I used the technique that Maddog refers to:

http://www.cruisersforum.com/forums/...dows-4526.html

by Bob Norson.
Time will tell how well they work in the long run, but I am extremely pleased with the way they turned out. They are made of Smoked Lexan.
Right? Wrong? Indifferent? I don't know - there are so many opinions. But for the use I intend for this boat, I believe that they will do quite well.
This is not the greatest picture as I am still working on the boat:
http://www.cruisersforum.com/gallery/showimage.php?i=3879&c=2

[Keegan]

Originally Posted by MidLandOne

I sense only trouble and wrath from this .

Nice unbiased report. You must be an expert

Keegan

[Kanani]

Originally Posted by clausont

I have just replaced the windows in my Cal 29. I am no expert by any means and this is my first attempt ever at replacing boat windows.
I used the technique that Maddog refers to:

http://www.cruisersforum.com/forums/...dows-4526.html

by Bob Norson.
Time will tell how well they work in the long run, but I am extremely pleased with the way they turned out. They are made of Smoked Lexan.
Right? Wrong? Indifferent? I don't know - there are so many opinions. But for the use I intend for this boat, I believe that they will do quite well.
This is not the greatest picture as I am still working on the boat:
http://www.cruisersforum.com/gallery/showimage.php?i=3879&c=2

Those windows will do quite well (if properly sealed). The weak link on that set-up is the mounting for the windows. However, that is a boat design and size issue.

It would be highly rare for you to get hit hard enough to cause any window damage but if you did take enough water on the window, it would knock the window completely through the deck-house (no matter what it is made of) however, extremely unlikely.

[clausont]

Kanani - That was my exact thought when I was installing those windows. I believe that the cabin sides would fail equally or before the windows do. I believe that they are properly sealed according to the research I had done prior to installing them.
Thanks for the comment - I appreciate the input.

[MidLandOne]

Originally Posted by clausont

I have just replaced the windows in my Cal 29...

...They are made of Smoked Lexan.
Right? Wrong? Indifferent? I don't know - there are so many opinions. But for the use I intend for this boat, I believe that they will do quite well.

I would class those by their small size as being fixed portlights rather than windows - one can check any hatch supplier's web site eg Lewmar, to see what I mean between windows and portlights (maybe that is where some others have been confused?). Given their small size (small height), meaning little opportunity for flexing, I doubt that you would have much problem with them from that (as another has said).

My personal choice would have been acrylic though .

[clausont]

MidLandOne - You are probably correct, I am not very smart on the difference between the two. (Note to self: Learn more! )
Acrylic would have been cheaper to build and install, but I decided to use Lexan anyway.
At any rate, If I do this job again at some point, I have learned a lot by doing this one.
Soon we will have this boat in the water

[hellosailor]

""scratches in Lexan(polyC) can presipitate failure". "
GE, DuPont, Rohm & Haus, everyone who make acrylic and polycarbonates as either resins or "structured" (i.e. pre-cast sheet) forms, will tell you the same thing. Crazing is a structural failure starting in the material, whether it is acrylic or polyc. Crazing can come from improper installation (which causes stress crazing) or from excessive UV exposure (and there are hundreds of varying grades of material, with and without UV resistance ratings or UV protective layers) or from exposure to common chemicals.

Used any wax on the boat? Odds are it has a naphtha solvent base. Get it on the plexi, or polyc, and congratulations you have started failure modes. Neither of those plastic families should be exposed to petrochemicals of any kind.

It is all well documented, and the different source makers all pretty much agree in their recommendations and precautions. Polycarbonate ranges from roughly 10x to 100x stronger than acrylic, in the same thickness. If you want a deck hatch or portlight that can withstand the extreme impact of green water breaking off a 50' wave, or your boat pitchpoling, you will want polycarbonate and you will gladly pay 4x more than acrylic costs. And then pay 4x more again, to get the highest UV-resistance and scratch resistance in the material. You'll find the same thing in acrylic--that you pay more for UV and scratch resistance. You'll just wind up with a sheet 10x thicker to begin with, and still only a fraction of the strength.

[billr]

The issue of the Coefficient of Thermal Expansion (CTE)of polycarbonate can be an additional problem if you use large pieces of it. It has a very high CTE compared to metals, and to keep it sealed requires some fairly complicated seals. The seal has to accomodate the dynamic range of the polycarbonate movement (relative to the frame) over the expected temperature range.

[hellosailor]

"The seal has to accomodate the dynamic range "
And again, every maker/supplier has technical data sheets and often architectural drawings showing eactly how to seal it properly--and how to seal it improperly, as well. You can't just screw the stuff in and expect it to work well. You CAN easily set up seals that will work for a long time.

Then there's also 3M's new VNC? VBC? double-sided tape, designed to hold large sheets of architectural glazing to metal and FRP substrates, with enough flex to hold it properly for a very long time with no leakage. It is being used on both skyscrapers and ships, it is new but information can be found on it.

[clausont]

Originally Posted by hellosailor

"The seal has to accomodate the dynamic range "
And again, every maker/supplier has technical data sheets and often architectural drawings showing eactly how to seal it properly--and how to seal it improperly, as well. You can't just screw the stuff in and expect it to work well. You CAN easily set up seals that will work for a long time.

Then there's also 3M's new VNC? VBC? double-sided tape, designed to hold large sheets of architectural glazing to metal and FRP substrates, with enough flex to hold it properly for a very long time with no leakage. It is being used on both skyscrapers and ships, it is new but information can be found on it.

Here are a couple of pictures of the new windows (or portlights) that I installed in our Cal 29. They are made out of Lexan. I did use the 3M tape. It has an incredible grip. I was amazed at how strong the grip was. If you make a mistake in your placement, forget pulling it off to reposition it. It's not going to happen.
http://www.cruisersforum.com/gallery/showimage.php?i=3904&c=2

http://www.cruisersforum.com/gallery/showimage.php?i=3903&c=2

http://www.cruisersforum.com/gallery/showimage.php?i=3902&c=2

I am quite certain that others will be able to do a better cosmetic job of sealing the windows with the Dow Corning 795 than I was, but I have never tried anything like this before. Overall, I am pretty happy with the way they have turned out. As far as expansion, they have already been through 100+ (F) and below freezing with no ill effects. I finished these windows later in the summer this past summer.

[Benjamaphone]

Originally Posted by MidLandOne

For hatches you will find that builders such as Freeman, Lewmar, Maxwell, Atkins & Hoyle, etc all use acrylic for hatches (or in the case of Freeman, choice of acrylic or tempered glass, but not polycarbonate, and in Bomar's case mostly acrylic).

Just for the record, there are different grades of acrylic that should be taken into account with this discussion. There are cast acrylics and extruded acrylics. By far, cast acrylic is stronger and more resistant to UV damage. Of course, the production costs of cast acrylic is more than extruded but the finished product is by far superior. Atkins & Hoyle exclusively uses cast acrylic on all products while Lewmar and Bomar, for example, use extruded acrylic.

[mdelannoy]

Hello Cruiser-Captains,

I work for an acrylic processor which also does ship windows.

We generally use thermoforming to bend tunnels for aquaria. This is done by heating acrylic up to thermoforming temperature (between 100 - 200 degrees C) until it is fully heated. As acrylic is a terrific insulator this can take up to a day. If will then be bend and slowly cooled according to schedule.

I live on a boat myself, which is about 30 years old. The Plexiglas windows are still original. Crazing seems to occur at the areas that have been bend in the shipyard. The shipyard might not have used enough oven time. Therefore crazing has occured.
I figure that you can anneal (find annealing temperature here)
by putting the whole sheet in the oven. This might remove some if not all crazing.

I've not tried it out myself - so good luck and share your results.

[Roy M]

Whatever conclusions you folks arrive at, I have been using acrylic for thirty years. I just installed my third set, not because they failed, but because they were old, the color had changed, and I wanted new portlights. They were slightly crazed, still very transparent, and never broke, even with blocks that gave way and hit them. They were half the weight, 10% of the cost, and far easier to replace than laminated glass. I never considered Lexan because of the "clouding" it takes after prolonged exposure to UV. Here's a pic of flame polishing those port edges.