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Hasse

Cracks in the window and doors

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Just a quick edit- I was included in the 1800 people made redundant at Origin.I received DCM phone call this morning, it wasnt really a surprise as the industry is going through downsizing. Its not the first time I have been made redundant, it happened in the construction industry as well.Origin was a great company to work for and I will miss the close freindships made over the last 3.5 years I worked for them.

 

But nobody is owed a living so I guesd I will go back to plumbing swimming pools.

Sorry to hear that Dazza. All the best for the future and hope you pick up work soon.

 

 

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Sorry to hear that Dazza. All the best for the future and hope you pick up work soon.

Thanks mate, its all good, I will just go back to pool plumbing. Actually I have to ring my old boss this morning. He heard that I was made redundant and I got a message yesterday to ring him this morning. They have a big project coming up at Pacific Fair shopping centre. Probably water features to build.

 

 

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An interesting video supporting the views of those of us who like to use flat-wrapped polycarbonate windshields, regardless of their pitfalls (can't/difficult to buff out scratches, sensitivity to rapid cooling when curved etc). I was quite astounded to see how well it resisted a shotgun blast too - at the 3min mark.

 

 

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Yes "Head in the Clouds",

 

Agree, polycarbonate is the obvious choice even with petrol spilled all over the screen - provided the sheet is flat and unstressed.

 

Unfortunately, windows and doors in the Savannah are bent and under stress and do not love petrol.

 

 

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Greetings all...

 

I've been following this thread with some interest and thought that the article attached might help explain some of the issues we encounter with Lexan under stress.

 

I can't confirm or refute anything the author says, so leave it to you to use - or not use - any information provided.

 

Fly safely,

 

Dan

 

Lexan vs Plexiglas.doc

 

Lexan vs Plexiglas.doc

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Yes "Head in the Clouds",Agree, polycarbonate is the obvious choice even with petrol spilled all over the screen - provided the sheet is flat and unstressed.

 

Unfortunately, windows and doors in the Savannah are bent and under stress and do not love petrol.

Yes, but you may have mis-interpreted my meaning when I said "flat-wrapped". The Savannah is what we would call a flat-wrapped windshield, meaning it's a flat sheet wrapped around the front, and in many aircraft it's also wrapped over the top of the cabin. So although it starts as a flat sheet it is actually curved in two directions when installed. This does mean it is under constant stress, so as you said, unfortunately it will crack if fuel is spilled on it. At least it's cheap to replace, and excellent protection if you have a bird-strike, or similar.

 

Greetings all...I've been following this thread with some interest and thought that the article attached might help explain some of the issues we encounter with Lexan under stress.

 

I can't confirm or refute anything the author says, so leave it to you to use - or not use - any information provided.

 

Fly safely,

 

Dan

That's an article I wrote. Not sure where I posted it, probably on HBA. I note that the video above disproves one of the things I said in the write-up, about the shot-gun. When we tested it we used No.8 shot IIRC from about 5m away and firing at a flat sheet, perpendicular to the impact. That blew a fairly large hole in the sheet. The video above is probably a better representation though, because it shows an angled windshield more like the aircraft we fly. However, I am surprised that the curvature didn't seem to make it more prone to shattering from impact. Good stuff that polycarbonate.

 

 

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Sorry, but I can't leave this "crack business" to rest. Where I live, I have several pilot friends who claim to have spillt lots of standard aircraft petrol on lexan doors and windows with no or only little signs of cracks. Why the difference from my experience? The only difference as I see it is that they have to use the lead free aircraft fuel (called 91/96 here in Sweden) that lack aromatics like benzene and toluene, which in contrast are always present in car petrol. Therefore, I made this small test comparing the effect of 91/96 aircraft fuel and car petrol (mogas) on Lexan (bent with rough edges). I also added a piece of Lexan where the edges were protected by polyurethane tape (same as propeller tape). The results was pretty obvious - fuel without aromatics does not crack the Lexan. Also, polyurethane tape appears to give a considerable protection to the effect of mogas.

 

I will show the video as soon as I learn how to./Hasse

 

 

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Sorry, but I can't leave this "crack business" to rest. Where I live, I have several pilot friends who claim to have spillt lots of standard aircraft petrol on lexan doors and windows with no or only little signs of cracks.

The cracking occurs because the lexan is such a good thermal insulator and the evaporating fuel chills the tensioned face of the curved sheet causing it to shrink even more which causes the cracking. I am thinking your friends put fuel on un stressed sheets or else the ambient temperature was much lower than in Australia so the temperature reduction of the evaporation became ineffective.

 

 

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Sorry, but I can't leave this "crack business" to rest. Where I live, I have several pilot friends who claim to have spillt lots of standard aircraft petrol on lexan doors and windows with no or only little signs of cracks. Why the difference from my experience? The only difference as I see it is that they have to use the lead free aircraft fuel (called 91/96 here in Sweden) that lack aromatics like benzene and toluene, which in contrast are always present in car petrol. Therefore, I made this small test comparing the effect of 91/96 aircraft fuel and car petrol (mogas) on Lexan (bent with rough edges). I also added a piece of Lexan where the edges were protected by polyurethane tape (same as propeller tape). The results was pretty obvious - fuel without aromatics does not crack the Lexan. Also, polyurethane tape appears to give a considerable protection to the effect of mogas.I will show the video as soon as I learn how to./Hasse

I'll look forward to seeing the video of your test.

 

I'm inclined to agree with rankamateur's comment above. It's all about the rate of evaporative cooling, and has nothing to do with the chemistry of the liquid being splashed on the plastic. If the fuel with aromatics evaporates faster in the ambient conditions being experienced at the time, then the damage to the screen will be worse, not because of the aromatics attacking the plastic, but because of the faster evaporation rate. Note - it's not how cold it gets eventually, but how quickly it gets cold.

 

In a couple of months I'll be fitting the windshield to the plane I'm building so I'll have some polycarbonate offcuts I can use in some new experiments to demonstrate this. It's very relevant to me too, because my fuel filler is in the lower right hand corner of the windshield - the main tank is behind the instrument panel and it's a taildragger, so that's the only practical place to have the filler cap, like a Wittman Tailwind see picture below. Having the filler there means that fuel will often get splashed on the windshield and it doesn't worry the Tailwind, not will it worry mine, because the windshields are flat, so they're not under stress when installed. Actually the Tailwind's windshield is very slightly curved, but evidently not enough to cause stress-cracking problems.

 

954168717_Wittman_W-10_Tailwind._(3729290613)(Custom).jpg.85a2522d0697d1746819a2972dd33ac7.jpg

 

 

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Ok rankamateur and Heads in the cloud,

 

I'm not familiar with the chemico/physical basis of the development of cracks in the Lexan, polycarbonate. However, let's say that you are right (which I doubt) and that it all depends on the rate of evaporation. If so, than Mogas (with aromatics)evaporates at such a rate that it creates cracks while aviation fuel 91/96 (without aromatics) does not. This is not very likely as the two fuels have approximately the same vapour pressure. If you distribute small aliquots of the two fuels over a surface you will find that they evaporate at approximately the same rate. If the cracks in lexan is due to evaporation this cannot be the whole truth. There has to be some sort of initial dearrangement of the polycarbonate (caused by the aromatics) that is especially pronounced in areas under stress prior to the temperature effect caused by evaporation. Personally, I wouldn't be surprised if mogas would induce cracks without any evaporation. Something to test?

 

Anyway, here are the links to my small tests. Please excuse my English and poor directing (not all Swedes are related to Ingmar Bergman). You should be able to open the links by coping to Google./Hasse

 

 

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Hi Hasse, don't worry, your English is better than many Aussies ... so you can be Ingmar Hasse from now on!

 

It's good to see your tests but you need more 'controls' to really draw any conclusions.

 

Total vapour pressure won't tell you much, since both of those fuels are compounds and one is a mixture of several compounds. If you were to accept my version of the 'rate of vaporisation' being the issue here, then it would be the rate of the vaporisation of the fastest of the components of the mixture, that is relevant. So, if your mixture 'A' contained ethanol, for example, that ethanol would probably be the fastest to vaporise even though the rest of the fluid is still liquid and has not yet evaporated, and hence the test piece subjected to the fluid containing that alcohol would be cooled much more rapidly than the other piece.

 

However, if you are convinced that the polycarbonate is actually being chemically attacked by one or several of the components in the fluids, then the control test is very simple, immerse some flat Lexan in that fluid for as long as you like, and take it in and out several times, allowing the fluids to evaporate each time, and see what damage that fluid does to the Lexan. If there is no damage, as I contend will be the case, then the fluid does not attack the polycarbonate chemically, and the damage we see on windshields has only to do with the internal stresses in the sheet material when it is curved. If the polycarbonate material is not chemically attacked by the fluid then it is being attacked by something else - and I think you will find it is to do with very rapid heat loss on the outer surface.

 

Further tests you can conduct - warm your test piece gently in an oven or with a hair-dryer before curving it and subjecting it to mixture no.2 (aircraft fuel). Try another test with an ambient temperature test piece and aircraft fuel but this time have a fan blowing on the experiment to increase the rate of vaporisation (and hence rate of cooling).

 

Try dipping your hand in each of those fuels, when you take them out does one feel colder than the other as it evaporates? Swap hands and do it again to be sure ...

 

 

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Yes "Head in the cloud"

 

there are many more test that can be done but not by me. I am sufficiently convinced by my test results and will change from car petrol and start using, as often I can, 91/96 aircraft fuel which is ready accessible in Sweden. Btw, neither ethanol nor iso-propanol gave any cracks in my tests.

 

 

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This thread continues to be a valuable study for those using polycarbonate windscreens.

 

Googling "lexan" provided a Wikipedia listing of the physical and chemical properties of "polycarbonate" which may suggest Hasse's experiments and conclusions are very astute. For example, the chemical resistance of polycarbonate is listed as "poor" to concentrated acids, aeromatic hydrocarbons, halogens and ketones; "good - fair" to greases and oils; and "good - poor" to alkalis and hydrogenated hydrocarbons (whatever that means). Chemical resistance to Alcohol is shown as "good".

 

Wikipedia also lists the thermal properties of polycarbonate including the coefficient of linear expansion and thermal conductivity figures. Perhaps a reader with a working understanding of these indexes could suggest whether or not they contribute anything further to this discussion.

 

 

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https://www.google.com/search?sourceid=navclient&hl=en-GB&ie=UTF-8&rlz=1T4GUEA_enAE573AE573&q=SABIC+LEXAN+solid+uncoated+and+coated+sheet+technical+manual

 

This manual from SABIC (the makers of LEXAN) gives detailed do's and dont's regarding the installation and cleaning of Lexan. In particular, on page 24:

 

 

"Benzene, gasoline, acetone, carbon tetrachloride or butyl cellosove should never be used on Lexan* sheet."

 

 

 

 

...while on page 19:

 

"Table 14: Recommended Solvent Cleaners"

 

 

 

 

 

White Spirit various

 

 

Petroleum Ether (BP65˚) various

 

 

Hexane various

 

 

Heptane various

 

 

I think Hasse is on the right track.... (Interesting results with Aviation Fuel)

 

Dan

 

 

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