Re: Plexiglas or whatever

[Dick Morrisson <rmorrisson@unidial.com>]

Gary,
I did a little searching after reading your post and would like to contribute the
following to the thread.

"LEXAN" is a registered trademark of the General Electric Company.  It is a
polycarbonate resin available in a wide variety of grades and formulations.  I
counted 21 in the literature I have.  These formulations are designed to maximize
properties such as dielectric strength, modulus, moldability, flame retardancy,
and high heat capabilities.  The tensile strength for the various LEXAN
formulations seems to stay around 8,000-9,600psi for most grades but can go as
high as 23,000psi when enhanced with 40% glass fiber.  Compressive strengths are
around 12,500-14,000psi.  There are different figures for the Tensile, Flexural,
and Compressive Modulus' but they are all around 325kpsi to 345kpsi (for non glass
re-inforced).

In the ASME-PVHO Safety Standard the term "acrylic" is used for "cast polymethyl
methacrylate plastic" and is the only material they allow.  The minimum tensile
strength is 9,000psi, minimum compressive strength is 15,000psi, and the minumum
modulus (tensile and compressive) is 400kpsi.

According to these numbers, normal general purpose "LEXAN" is actually weaker than
acrylic and would not pass the minimums for the standard!  The "250 times" figure
your friend quoted probably has to do with the impact resistance compared to
glass, not the strength!  In one catalog I have they state lexan is about 250
times as impact resistant as glass, 30 times more than plexiglass.  Again, this is
"impact resistance", not strength!  If your friend does not know specifically what
he has then he is diving in a window test chamber!  Please let your friend know
about these figures so he doesn't end up on the "accident data site".

Sincerely,
Dick Morrisson


protek@shreve.net wrote:

> Ray,
>     I think you are right on all counts.  Lexan is a trade name for a
> certain polycarbonate.  They are related to acrylic plastic.  It is used
> for bullet proof windows.  I know of people using it for viewports, but
> here is my reasoning why not to use it;  I know of no studies of this
> material used in viewport designs.  It seems logical, but I just don't know
> of any solid figures on it.  It is a lot more durable than acrylic and it
> is far more shatter resistant.  I have heard though that it bends more
> under stress when used in sub windows.  My friend Harold from Longview
> Texas used it in his sub.  He has taken it down to 100 feet before and his
> windows are slightly larger than mine.  He used 0.5 inch thick Lexan.
> Personally, I think this is too thin for any window at 100 feet.  He
> reasoned that Lexan is "250 times stronger" than plexiglass so it did not
> need to be thicker.  I don't agree, but he has made 100+ dives, some to
> better than 100 feet.  I can see why people want to use it for sub windows.
>  However, the acrylic figures are there and it is tried and proven.
>
> Gary
>
> At 11:17 AM 3/31/99 -0800, you wrote:
> >Hi,
> >
> >>From what I can remember is:
> >
> >1. Lexan is a trade name for one manufacture's polycarbonate.
> >
> >2. Polycarbonates are simular to acrylic but not the same thing.
> >   Polycarbonates are tougher but more brittle than acrylic. Occasionally
> >   a piece of polycarbonate being cut would catch in a saw and kick back
> >   causing some nasty cuts. Vicious stuff to work with even with carbide
> tipped
> >   blades.
> >
> >3. Because polycarbonate is tougher it is the material used for bullet
> >   proof (resistant really) glass.
> >
> >4. Can polycarbonates for used for view ports? I don't know. I would consult
> >   the viewport book.
> >
> >Regards,
> >Ray
> >
> >> Okay, here's part  what I didn't know.
> >>
> >> >David,
> >> >    The reason I use, and suggest the use of, acrylic plastics for windows
> >> >can easily be summed up. ....The main contender for replacement is
> >> >polycarbonate (Lexan)....
> >>
> >> ...I just always thought of all this stuff as the same -- "Clear Plastic"
> >> -- And the different names being trademarks. Duh. I'm not even sure, but I
> >> think I recall the guy at the window place saying the stuff he was putting
> >> in my windowframes was "lexan." So... this is apparently polycarbonate
> >> which is not what you're using. Is there a common brand name of acrylic
> >> thrown around just as carelessly? Is "plexi-glass" (further bastardized by
> >> the careless "nucular" crowd into "flexi-glass") something else entirely?
> >>
> >> If what I've got in my trailer is polycarbonate (or, to be on topic, if I
> >> use a spare hunk of it in an ambient sub or glass-bottomed boat)... can I
> >> polish it in a similar way?
> >>
> >>
> >> ---------
> >> David
> >> buchner@wcta.net
> >> http://customer.wcta.net/buchner
> >> Osage MN USA
> >>
> >>
> >
> >
> >