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Old 16-02-2007, 17:51   #16
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Laquer thinner disolves lexan, but not plexi .It just makes plexi craze later.
Plexi looks yellow on the edges. Lexan looks grey. A piece of plexy will shatter if you hit it with ahammer. Lexan will squash.Old lexan will be too foggy to see thru. Plexi will just be crazed.
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Old 14-03-2008, 09:16   #17
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I'm not sure about these applications where bendng is required but for areas where a flat piece is required, only cast acrylic should be considered. Cast acrylic is more expensive for good reasons. Extruded acrylic may be cheaper in the short term, but when you have to replace it 3 or even 4 times more often than cast acrylic, the disadvantages suddenly become apparent.

The costs associated with producing cast acrylic directly translates into a superior product. The extra strength and durability of a cast acrylic does not exclusively come from different resins or temperatures (though they play a critical role), but from the way that the sheet is allowed to cool and set up. Extruded products like Lexan are cooled rapidly, producing very little organization at the molecular level within the acrylic. Cast arcylic, on the other hand, cools much more slowly and naturally and allows for a kind of molecular lattice to set up which adds strength, durability and protection from the UV.

Extruded products like Lexan are very difficult to work with as others have noted. It tends to melt when cut and chip away when drilled. Cast acrylic, however, behaves a lot more like wood which makes it much easier to work with.

Lexan and other extruded acrylics may be similar but I am afraid that leaves out some important details that really should be considered by a responsible sailor.

Another problem with Lexan is that it just dosn't want to stick to sealants for any length of time. Over the span of a couple years, a kind of oily film will develop on the Lexan and compromise the seal. This of course is not to mention the vastly accelerated crazing process and inherent structural weaknesses.

If you do it right the first time you won't have to worry about it for another 15-20 years. Just my two-cents from someone with experience with both products.
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Old 14-03-2008, 09:38   #18
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Thanks Ben!
That's the type of expertise I've come to appreciate from some our our Commercial Vendors.
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Old 14-03-2008, 11:50   #19
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What is cast acrylic, plexiglass??? Is cast acrylic different from run of the mill acrylic??

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Old 14-03-2008, 11:59   #20
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"Extruded products like Lexan are very difficult to work with as others have noted. It tends to melt when cut and chip away when drilled."
Wait a minute now, you're mixing more than metaphors.

Lexan is NOT necessarily an extruded product. It can be extruded OR CAST, exactly like acrylics. If you are currently casting with acrylic resins, you can switch to polycarbonates for a cast polycarbonate end product.

As for problems with drilling and cutting--acrylic was famous for that before polycarbonates came on the market. Atoglas, formerly Rohm & Haas, own the Plexiglass trademark and for many years they have advised folks on how to work the material to avoid those problems. Again, this is nothing specific to polycarbonates. Plastics simply work differently than wood. Or metal, or masonry.

How about giving us some numbers comparing your cast acrylic, to a theoretical replacement made of cast Lexan or other polycarbonate? And by all means, including the difference in cost, which I would expect to be significant.
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Old 14-03-2008, 13:01   #21
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Whether or not Lexan is cast or extruded, (by far the most common method is extrusion), it is an inferior recipe. Cast acrylic (Acrylite, Plexiglass, etc. to name some brands) is designed to be used for outdoor uses.

"As for problems with drilling and cutting--acrylic was famous for that before polycarbonates came on the market. "

I'm not too sure I know what you means by this. We cut, drill, grind and do all kinds of machining with our cast acrylic with no problems whatsoever.

Some numbers... I am not a chemist nor do I have the adequate scientfic background to give you any specifics. However, we have done many tests using both Lexan brand CAST material and our standard cast Acrylite brand acrylic. The reason we carried out these extensive tests in the field about 10 years ago was becuase the Lexan was advertised as stronger and more UV resistant; however, the cost was higher. A+H's flagship was decked out in both Lexan and Acrylite hatches and ports. This included 20 5x12 ports, 4 large deck hatches and a very large companionway hatch. The Lexan fogged, crazed and lost its seal after less than two seasons. The hatches with the Acrylite are still in perfect condition, while the Lexan lenes were replaced with Acrylite.

I should note that at the time (about 1997) we purchased the most expensive, UV resistant Lexan on the market. The Lexan costed more than our standard Acrylite, however we would have already replaced it 3 three times. So, not only was the initial cost higher, but the trouble of replacing the Lexan 3 times would be ridiculus.

For the record, I'm the type of person who needs to feel something in their hands and see how it works. Running a bunch of numbers off is pretty meaningless compared to hard won experience. The simple fact is that it comes down to what is better, side by side.

I should also note that cost was not an issue for us. We wanted the best product on the market, period. It just turned out that Acrylite worked much better.

I just did a little non-professional googling and found that Lexan is more suited for applications where great impact strength is required. Motorcyle helmets are made with Lexan for instance. Windows, on the other hand require different properties.

Also, forum rules clearly state that I cannot give my costs or prices. For that information, send me a private message and I can give specific quotes on specific sizes of hatches/ports.
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Old 14-03-2008, 16:22   #22
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If you do it right the first time you won't have to worry about it for another 15-20 years
This is fascinating info Ben. Just to clarify (no pun intended), a cast arcrilyc window will last 15-20 years in the sun without crazing? Sounds like just the stuff I am looking for. My cabin sides will have a slight radius. Bent glass is too expensive and there's no way I'm going to replace 12 60cm x 60cm lexan windows every 5 years.
I have a friend that has acrylic portlights in his boat that are 17 years old and look like glass. He says this is because they are "annealed". Could this be the cast acrylic you are talking about?

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Old 14-03-2008, 19:23   #23
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Annealing is a procss of heating an acrylic to about 180degees to give it more rigid strength and durabilty. This can be done with either cast or extruded acrylic, however the process is more effective with the cast variety. This is an expensive process for both types because the entire sheet must be heated and cooled perfectly evenly.
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Old 14-03-2008, 23:15   #24
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Thank you, Ben. I fully agree that real world experience is the best measure, but I've found that within their limits (what is the saying, figures don't lie but liars figure?) numbers can be a good way to predict what will happen in the real world. I'm surprised that the Lexan showed UV failure (assuming that's what it was) so quickly.

I don't think forum rules prohibit you from posting the MSRP of any product along with a comparison of how some change would affect that, unless I've been here shilling for you somehow. Or, that they could be in any way interpreted as preventing you from saying something like "using Lexan would triple our costs for the glazing", which would give a comparison point without mentioning the price of the complete hatch at all. However, I do not argue that point with you. Perhaps one of the moderators would be kind enough to give use the forum's official comment on that.

"I'm not too sure I know what you means by this [problems]. We cut, drill, grind and do all kinds of machining with our cast acrylic with no problems whatsoever. " What I mean seemed clear to me.[g] Very simply, that as you say there are no problems with machining these materials--if someone knows how to machine them. Reports of "clogging the blade" etcetera usually just mean the wrong blade, wrong saw, wrong speed, are at fault, not the material. Hit a nail into balsa wood, it pops in real easy. Hit a nail into a nice piece of oak--and the wood splits, or the nail needs two more whacks. That doesn't mean there's something "wrong" with oak, it just means that it needs to be handled differently from other "same"[sic] wood materials.
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Old 15-03-2008, 00:13   #25
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Originally Posted by Benjamaphone View Post
I just did a little non-professional googling and found that Lexan is more suited for applications where great impact strength is required. Motorcyle helmets are made with Lexan for instance. Windows, on the other hand require different properties.
I would disagree, and this is probably why hatches usually have Lexan vs ports/windows have acrylic.

e.g. On my vessel recently I replaced the 25+ year old forward hatch Lexan and the side acrylic cabin windows. The 3/8" thick side windows literality broke in half trying to get them out. While the 1/4" thick, hazed over and badly scratched hatch lens, I practically bent it in half getting it out.

Hatches and some forward windows take a beating at sea. Personally, I would rather have a fogged over lens then one that gave out in a storm.


Sure Lexan may need replacing more often, but if your off shore what's your priority. It all comes down to where they're placed and the intended use of the vessel. My forward hatch is 30" square (inside) and if that were to break out it would leave a gapping hole for water to pour into.

A little tip for trying to get sealant to seal to Lexan is to tape off the clear area and lightly sand blast, or even hand sand, the area around the edges for the sealant to adhere to. It works for acrylic too!

Quote:
AGENCY APPROVALS OR LISTING : FDA, USP
BASE RESINS TRADE NAMES : LexanŽ

MECHANICAL PROPERTIES:
Specific gravity (ASTM D 792) : 1.20
Tensile strength, Ultimate (ASTM D 638) : 9,000 p.s.i.
Elongation at break (ASTM D 638) : 130%
Tensile modulus (ASTM D 638) : 3.1x10~5 p.s.i.
Rockwell hardness (ASTM D 785) : R118
Impact strength (73° F) (ASTM D 256) (notched) : 17.0 ft-lb/inch
Flexural strength (ASTM D 790) : 14,200 p.s.i.
Flexural modulus (ASTM D 790) : 3.4x 10~5 p.s.i.
Wear factor against steel 40 psi 50fpm : 2500x10~10
Coefficient of friction 40psi 50fpm : 0.38 Dynamic

THERMAL PROPERTIES:
Melting point : 310° F
Heat deflection at 66 psi (ASTM D 648) : 285° F
Heat deflection at 264 psi (ASTM D 648) : 270° F
Maximum serving temperature for short term : 275° F
Maximum serving temperature for long term : 240° F
Thermal conductivity (ASTM C 177) : 1.35 Btu-inch/hr-ft~2- ° F
Specific heat : 0.30 Btu/lb- ° F
Coefficient of linear thermal expansion (ASTM D 696) : 3.7x10~5
Applicable temperature range for thermal expansion : 0-200° F

ELECTRICAL PROPERTIES:
Dielectric constant at 60Hz (ASTM D 150) (73° F, 50% RH) : 3.2
Dissipation factor at 60Hz (ASTM D 150) (73° F) : 0.001
Volume resistivity (ASTM D 257) : 10~17 ohm-cm
Dielectric strength (ASTM D 149) : 380 v/MIL

MISCELLANEOUS:
Water absorption - 24 hours (ASTM D 570) : 0.15%
Water absorption - saturation (ASTM D 570) : 0.35%
Density (ASTM D 792) : 0.0434 lb/inch~3
Flammability (UL 94) : V-2
Weathering Resistance : Limited resistance (UV Sensitive)
Quote:
AC-350™ Acrylic (Bending Grade)
Typical Physical Properties (Typical but not guaranteed values for 0.25 inch material)
property test method units AC-350 Acrylic
PHYSICAL
Specific Gravity ASTM D-792 -- 1.19
Pencil Hardness ASTM D-3363 Hardness Scale 3H
MECHANICAL
Tensile Strength--Ultimate ASTM D-638 psi 10,000
Tensile Strength--Elongation ASTM D-638 % 4.2
Tensile Modulus ASTM D-638 psi 400,000
Flexural Strength ASTM D-790 psi 16,500
Flexural Modulus ASTM D-790 psi 475,000
Compressive Strength ASTM D-695 psi 18,000
Izod Impact Strength (milled notch) ASTM D-256 ft-lb/inch of notch 0.4
THERMAL
Deflection Temperature (264 psi load) ASTM D-648 oF 210
Vicat Softening Point ASTM D-1525 of 239
Maximum Continuous Service Temperature -- of 180
Coefficient of Thermal Expansion ASTM D-696 in/in/of 3.5 x 10-5
Coefficient of Thermal Conductivity Cenco-Fitch BTU•in/hr•ft2•oF 1.3
FLAMMABILITY
Horizontal Burn (Flame Spread) ASTM D-635 inch/min 1.1
UL 94 Rating UL 94 UL Classification HB
OPTICAL
3mm Transparent Clear Transmittance--Total ASTM D-1003 % 80
Haze ASTM D-1003 % Less than 3.0
POLYCARBONATE PLASTIC

I'll continue to use Lexan for oncoming waves!
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Old 15-03-2008, 04:00   #26
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Here is a web site that sells sheet material. They have a lot of different types including some of the mil spec products, bullet proof and other exotic materials with prices.

Plastic Sheets (PLASTIC SHEET) - Order Online

I'm not sure if this is a bargain source or not but they seem to sell a lot of different products so you can see the relative costs and properties. The aircraft grade plastic is about $1500 for a 4ft x 6 ft sheet. Some of the others run even higher.
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Old 15-03-2008, 04:31   #27
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I would disagree, and this is probably why hatches usually have Lexan vs ports/windows have acrylic.
Well actually neither usually have Lexan in them - and furthermore acrylic is by far the most common material in pleasure vessel hatches and ports and Lexan should be avoided in windows and hatches and never used at all if those are other than small.

Lexan is no stronger than acrylic but withstands shock much better - but it gets its ability to withstand shock from its elasticity and deformation (look at your own figures, by the time it fails it will have deformed by around 130%!!!, whereas acrylic will hardly have deformed at all. Your figures do not give elongation at failure for acrylic but is typically around 5%). Deformation is an obvious problem if you want hatch and window integrity where the larger sizes mean the deformation dimension is greater (this is especially important for companionway hatch boards only fixed in slots on 2 sides, yet I see amateurs wanting to use it) - acrylic is the superior material.

Interestingly, the other week I came across a draft Canadian standard for small commercial vessels and they now explicity restrict the maximum size of any window in which Lexan is used.

What Benjamaphone says is entirely correct, but what does he know he only makes them ?

I have been all through this in another thread and still the old Lexan is a super material myth comes up for this application. But what do I know, I have only been resposible for managing the design appraisals and surveys of a fleet of over 2,000 commercial vessels and the design and build of a number of high quaility vessels? So I am another that knows nothing .

I can understand that some of those who have fitted Lexan will die in the trenches defending it sheltered behind their bullet proof Lexan shields but perhaps some others will at least take note and research it very carefully before using it themselves.
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Old 15-03-2008, 09:41   #28
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Midlandone,

Is there a formula that can be used to calculate the thickness of acrylic necesary for a given surface area in cabin windows that will be exposed to breaking waves?

Thanks, Mike
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Old 15-03-2008, 10:20   #29
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What Benjamaphone says is entirely correct, but what does he know he only makes them ?
...
But what do I know, I have only been resposible for managing the design appraisals and surveys of a fleet of over 2,000 commercial vessels and the design and build of a number of high quaility vessels? So I am another that knows nothing .
MidLandOne,

This is the difference between trusting experience and general consensus. As we both know, there are many variables that determine the overall properties of a material. If any one of these properties is not put into context with the rest, you get an inaccurate picture of how that material will behave. At least for me, this is often the problem with trying to define a material/method in such exact, scientific terms. There is always the chance of leaving something out and getting it wrong. This is why I have always trusted the hard won experiences that have been achieved through 40 years in the marine industry. By using my experience, and the experience of others, it is possible to cut through the numbers and find what actually works.

I am glad that you have the technical knowledge to back up what we have discovered over the years. I know that we both just want to use the best materials and methods for our boats. That's the only agenda and I hope some of our experiences can help others on this board as they have surely helped us.
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Old 15-03-2008, 10:35   #30
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Well actually neither usually have Lexan in them - and furthermore acrylic is by far the most common material in pleasure vessel hatches and ports and Lexan should be avoided in windows and hatches and never used at all if those are other than small.
Pompanette LLC. (see the word Lexan)

Quote:
Lexan is no stronger than acrylic but withstands shock much better - but it gets its ability to withstand shock from its elasticity and deformation (look at your own figures, by the time it fails it will have deformed by around 130%!!!, whereas acrylic will hardly have deformed at all. Your figures do not give elongation at failure for acrylic but is typically around 5%).
Obviously, you didn't read the lines in orange! And these are not MY figures but of the plastics industries! Izod Impact Strength Testing of Plastics

Quote:
Deformation is an obvious problem if you want hatch and window integrity where the larger sizes mean the deformation dimension is greater (this is especially important for companionway hatch boards only fixed in slots on 2 sides, yet I see amateurs wanting to use it) - acrylic is the superior material.
And that's why Lexan is more expensive???

Quote:
Interestingly, the other week I came across a draft Canadian standard for small commercial vessels and they now explicity restrict the maximum size of any window in which Lexan is used.
What does that mean?

Quote:
What Benjamaphone says is entirely correct, but what does he know he only makes them ?
And so does Pompanette/Bomar!!!!! (see link above)

Some people are frugal, and others are cheap!
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