Cure for Plexiglass / Lexan Crazing?

[MidLandOne]

Originally Posted by Celestialsailor

I am surprised someone would say that Acrylic is the same strength as PolyC. I use to make bullet resistant windshield for the police. The other claim I found odd was that the "scratches in Lexan(polyC) can precipitate failure".

Polycarbonate gets what many confuse as being "strength" from its elasticity and flexibility, not from ultimate tensile strength. It is resistant to shock loads, almost to the extent of being unbreakable, due to those characteristics giving it resistance to high mechanical energies - but that not due to its tensile strength in which it does not have any unique advantage over, for example, acrylic.

In fact it suffers the disadvantage in many applications that by the time one reaches its ultimate tensile strength it will have elongated (ie permanently deformed) somewhere between 70-100%. So it behaves like strong gum. It is the characteristics of flexibility and elongation under stress that are reasons why it is not a good material for hatches and windows (nor especially for companionway washboards). It is an excellent material for many other things, especially where resistance to shock is required.

Another poster refers to aircraft transparencies and mentions that those are acrylic, and that is typically so, wheras one would assume that polycarbonate would be the better choice if it really were better for windows. On large aircraft the cockpit transparencies may be special glasses often laminated with a plastic such as PCB but as far as I am aware acrylic is the material used for smaller aircraft. It may be that polycarbonate is used in some smaller transparencies, but if so it would be in limited manner where its disadvantages are offset by its advantages and be used in conjunction with another material such as acrylic (passenger cabin transparencies are normally acrylic but it may be in some instances that polycarbonate is used as a secondary material, I don't know - as you've probably noticed these transparencies are made up of multiple panes).

Scratches on polycarbonate, as on most (all?) other materials create stress raisers which can precipitate failure. Given its flexibility and elasticity I suspect, but have not investigated, that its mode of failure would be different to that of a more brittle material such as acrylic, high carbon steel, etc but have a mode of failure it surely will - probably exaggerated elongation at the point of the scratch.

EDIT: Meant to mention that as far as I am aware, if polycarbonate is used in bullet resistant windows in vehicles it is used in a laminate with glass. The glass ofsetting the structural deficiencies of the polycarbonate and the polycarbonate giving the resistance to high mechanical energies.

[cooper]

and that is the whole point...If all things being equal and size for size, acrylic and polc are the same strength then given equal fastenings, the elongation absorbs energy and creates a much safer material. If the force causes deflection, then a rigid material will fail. Lets be blunt, I think it fair to say that there are two causes of failure, under normal use. One : Point impact, somthing hitting it, Ie a block, stray object etc. OR two a surface load, Ie a large volume of water, a person or similar large area impacting it. In a crappy situation a point inpact becomes more likely. (going forward to stop a madly flogging sail). If at total deflection both materials have the same strength given equal size, why would you go for rigid? To quote the Japanese saying (and also car crash testers) "The strong branch that moves with the wind will survive longer than the ungiving"...Further 100 % is a right angle to its fixed plane. I would love a plastics chemist to step in here. The question I have is "If you had a equaly well fixed piece of the two materials at say 300 mm square, and you subjected them to point of failure stress from high impact water, which would fail first ?

[MidLandOne]

Originally Posted by cooper

and that is the whole point...If all things being equal and size for size, acrylic and polc are the same strength then given equal fastenings, the elongation absorbs energy and creates a much safer material. If the force causes deflection, then a rigid material will fail.

Sorry, but you clearly do not understand. You should also take on board that your claim that if a rigid material deflects it will fail is totally wrong.

I have mentioned the problems in earlier posts and will be just tiresome for me to prolong the agony. I suggest you reread them and also ask yourself why if Lexan is so effective and "strong" it is not widely used for windows and hatches (in fact I have never seen it used for windows, as opposed to portlights, in anything other than an amateur built boat) not only in boats but also in other applications such as aircraft.

As I have said, in most countries people can use whatever they wish for non coded/class vessels. You are welcome to do so for your own vessel if you are not convinced so go to it.

[David M]

I got rid of some of my plastic windows and replaced them with laminated glass, not tempered..(big difference). Never use tempered glass on a boat..it has little comparative resistance to impact and suddenly you will have a gaping hole with thousands of pieces of glass.

I no longer have to deal with scratches or crazing..and the price difference was more than worth it. Plastic eventually looks like crap no matter what type you use or how well you cover it.

You don't see plastic windows on the bridges of ships. Those are subject to impact, bigtime.

You can get laminated glass in the thickness you need.

[MidLandOne]

I agree entirely with the use of glass being the preferred option, but I should add that if laminated glass is used then it should be laminated tempered glass.

All codes/class rules I know of prohibit both plain glass and plain laminated glass for windows and portlights (don't recall for deadlights as have never used those).

Again, very similarly to the beliefs about Lexan , many assume that laminated is stronger than plain glass whereas it is not stronger. Plain laminated glass is actually generally weaker than the same thickness of plain sheet due to part of the thickness being taken up with the plastic interlayer. Also, again contrary to popular belief, if it is stove right in into pieces then one is confronted with very sharp shards of glass.

Also, again contrary to popular belief, tempered glass is much stronger than plain laminated glass its only disadvantage being susceptability to sharp pointed impacts (see just like acrylic compared with polycarbonate ) and cost.

Generally single ply tempered glass is used for marine glass but laminated tempered is also less frequently so. Standards for marine glass exist in all western countries and when the glass is cut and tempered for the window the standard mark should be fused in one corner (similar to the way the fused standards mark is fused into the corner of automobile glass). My own yacht uses tempered glass in the hard dodger and the marks are so fused into each pane of that.

That all being said plain or plain laminated glass is sometimes found on very old small commercial vessels built when standards did not exist for marine glass. I was once involved in researching, including discussions with a major glass maker, what to do about this problem as these windows (usually front facing wheelhouse ones) were often stove in, sometimes with the loss of the vessel. But it turned out that the only reliable way to test for tempering was to break the window which was obviously counterproductive for those vessels where the glass turned out to be tempered .

Another indicator is the air blast marks one can see on the glass through a polaroid filter (like when look at tempered auto glass with polaroid sun glasses on) but is not a reliable indicator of the extent of the tempering.

In newer small commercial vessels I know that plain glass (single sheet or laminated) sometimes gets into them as replacement glass, but that will not happen if the statutory or class inspector/surveyor checks for the fused standard mark or certification as to source as he should do but sometimes doesn't, especially if he is not told that the replacement has occurred (as he should be ).

In the end any glass can fail. Recently an around 130 foot fast ferry I had some involvement with the design of, and which was in class and used tempered glass, nose dived into a big wave at speed when it got away surfing down a wave. The big front viewing windows in the passenger compartment were stove in and water swept right through - fortunately it had no passengers on it at the time.

Again, for pleasure vessels one can in most countries use what one likes. If one is unconvinced and prefers plain glass or plain laminated, or Lexan then go to it .

[scotte]

How do the relative merits/strengths of materials change when windows are on a curved surface (for example wraparound windows)? I'm curious how this changes things.

[delmarrey]

The problem I see with glass is a flopping piece of rigging in the wind smacking the glass. My front hatch would be busted out on the first sail along with the sides possibly being hit by the genoa clew.

No thank you!

[Celestialsailor]

"EDIT: Meant to mention that as far as I am aware, if polycarbonate is used in bullet resistant windows in vehicles it is used in a laminate with glass. The glass ofsetting the structural deficiencies of the polycarbonate and the polycarbonate giving the resistance to high mechanical energies".[/quote]

Not sure were you got this fact from so I have attached a Youtube to show you the type of windscreens I made. Sorry, they were not lamed with glass.

YouTube - Polycarbonate vs Acrylic Plastic

Here is another sight with actual scientific numbers.
Please note Tensile Strength:- Acrylic 10,000PSI Poly 10,5ooPSI
Impact strength:- Acrylic .9NI Poly 13.0NI
Elongation:- Acrylic 5% Poly 100%
Comparison Table for Plastics.


As far as crazing. I have never seen poly craze. that's not saying it dosn't of course. I have seen it get a little foggy over time...enjoy

[jayncee]

On our last boat we had Strataglas--what a nuisance maintaining. They want you to devote your life taking care of it. We are buying a new boat and considering Rainier. Anyone have any comments??

[Keegan]

Originally Posted by MidLandOne

Sorry, but you clearly do not understand. You should also take on board that your claim that if a rigid material deflects it will fail is totally wrong.

I have mentioned the problems in earlier posts and will be just tiresome for me to prolong the agony. I suggest you reread them and also ask yourself why if Lexan is so effective and "strong" it is not widely used for windows and hatches (in fact I have never seen it used for windows, as opposed to portlights, in anything other than an amateur built boat) not only in boats but also in other applications such as aircraft.

As I have said, in most countries people can use whatever they wish for non coded/class vessels. You are welcome to do so for your own vessel if you are not convinced so go to it.

What makes you the expert on Lexan? You have presented no credentials, no factually supported statements and have made statements that are hardly substantiated by anything other than you. Your opinion is certainly valid but to tell people they are all wrong or novices for using Lexan would imply that you are an expert on the subject.

The fact is lexan is used in many boat window applications with success. Also, just because it is not used in airplane windows regularly does not in any way indicate that it is not worthy of being used as a boat window.

FACT: Harley Davidson uses LEXAN for many of their windsheilds, but I guess they are "amatuers"

Check this website out under "ocean going BOATS" Boat Windows
More "amatuers" I presume.

Oh, and this one, look under LEXAN polycarbonate: Plastic and Plexiglas
More "amatuers"...

Lookie here, a book excerpt regarding how great LEXAN is for BOAT WINDOWS Fitting Out Your Boat: In Fiberglass ... - Google Book Search "amatuers" are taking over the planet...

Gemini Catamarans use LEXAN WINDOWS Sailing Magazine : Boat Test

OUTREMER Catamarans USE LEXAN WINDOWS, but what do the French know anyway...

and of course many more that I dont have time to find and post.

If you want to appear as an expert on the subject then I suggest you present the facts that show people that you are.

'Nuff said..

Keegan

[Celestialsailor]

Originally Posted by Keegan

What makes you the expert on Lexan? You have presented no credentials, no factually supported statements and have made statements that are hardly substantiated by anything other than you. Your opinion is certainly valid but to tell people they are all wrong or novices for using Lexan would imply that you are an expert on the subject.

The fact is lexan is used in many boat window applications with success. Also, just because it is not used in airplane windows regularly does not in any way indicate that it is not worthy of being used as a boat window.

FACT: Harley Davidson uses LEXAN for many of their windsheilds, but I guess they are "amatuers"

Check this website out under "ocean going BOATS" Boat Windows
More "amatuers" I presume.

Oh, and this one, look under LEXAN polycarbonate: Plastic and Plexiglas
More "amatuers"...

Lookie here, a book excerpt regarding how great LEXAN is for BOAT WINDOWS Fitting Out Your Boat: In Fiberglass ... - Google Book Search "amatuers" are taking over the planet...

Gemini Catamarans use LEXAN WINDOWS Sailing Magazine : Boat Test

OUTREMER Catamarans USE LEXAN WINDOWS, but what do the French know anyway...

and of course many more that I dont have time to find and post.

If you want to appear as an expert on the subject then I suggest you present the facts that show people that you are.

'Nuff said..

Keegan

Guess I'm not the only one questioning MidLandOne.

[MidLandOne]

Originally Posted by Celestialsailor

Here is another sight with actual scientific numbers.
Please note Tensile Strength:- Acrylic 10,000PSI Poly 10,5ooPSI
Impact strength:- Acrylic .9NI Poly 13.0NI
Elongation:- Acrylic 5% Poly 100%

Thanks for posting those mechanical characteristics, they corroborate exactly what I have been saying. Similar tensile strength, elongation of polycarbonate 100% versus Acrylic 5% and polycarbonate has high resistance to impact (that due to its elasticity and flexibility).

I normally find I don't need to make anything of my credentials but as they have been challenged, some of my experience includes:- Chief Executive Officer of a company providing regulatory surveys, management systems and other services for customer fleets comprising over 3,000 commercial vessels from very small up to around 150 foot (including the likes of charter sail boats and power boats), occasional overview of quality of build or of refit of super yachts for an international financier (for vessels he is exposed to in his loan book), advice regarding the operation and design of high quality, high performance commercial vessels (mainly high powered water jet powered fast ferries, 80 - 130 foot approx) and especially with managing the design of those vessels through their classification society design appraisal process, the project management of the build and entering into service of those vessels, management of inspectors and non exclusive surveyors representing classification societies and the flag administrations of a number of countries, etc, etc. That experience has been international. I have also been associated with pleasure vessels (sail and power) for over 50 years, but I am the first to concede that experience in the pleasure world does not necessarily indicate merit as ones only customer is oneself and errors are ones own .

As I have said, for pleasure vessels one can use whatever one likes - if one wants to use polycarbonate or plain or laminated glass for windows and hatches and one is confident that one understands ones particular application of them then go for it. They may work for you and they may not, I have just tried to point out the issues with them and how the regulatory/class world regards them.

I can only relate how the professional marine world I have worked in uses or does not use materials and why. Maybe some will place some consideration on what I have said and maybe not. Of course most (whose credentials I have no knowledge of ) are saying quite clearly that they discard it. So, I don't think there is really anything else I can add in this forum, and after all I'd rather be doing this for pay than for angst . Adieu.

[Pblais]

An experienced and respectable cruising sailor recommends carefully using a heat gun to "erase" crazing from plexiglass & lexan hatches.

A heat gun won't do it. While a heat method could work I doubt you can do it without removing the lexan from the frame.

You'll find with three grades of polish you can remove the crazing. West marine has a three set of products you can use. You start from coarse to very fine. As noted above at a 2000 grit it starts to look pretty good but you may want to go all the way to a 2500 for better optical clarity. Wet sand paper comes in 1000 grit and I've never seen any finer sand paper. You may want to use a machine applicator if the crazing is deep. Aqua buff 2000 is a great water based polish and works well with a machine since you just keep spraying with water as it starts to dry out. I restored the shine on some old Alwgrip with it after we wet sanded the old boat name off.

A 2000 grit does not remove much material very quickly. Doing larger hatches by hand isn't an easy process. Once you get it looking good - make a sunbrella cover for it. UV is the real enemy.

On our last boat we had Strataglas--what a nuisance maintaining. They want you to devote your life taking care of it.

Using anything on Strataglas other than the product made for it by the Strataglas company or plain water is a bad idea. Once you do that you void any warranty and probably do damage to it. This has nothing what so ever to do with the thread topic.

[CharlieCobra]

I used to refinish aircraft and would take the crazing out by wet sanding with 1500/2000 grit and then buff it with Liquid Ebony. The trick was to not get too much pressure in any one area or you'd have a distortion. I did not use this technique on pressurized aircraft. You also had to keep the surface wet or you'd burn the Plexiglas.

[GordMay]

Handbook of Polycarbonate Science and Technology
Edited By Donald G. LeGrand, John T. Bendler

Handbook of Polycarbonate Science ... - Google Book Search

Goto “Crazing & Cracking” page 119:
Handbook of Polycarbonate Science ... - Google Book Search