Re: [PSUBS-MAILIST] polycarbonate vs acrylic pros cons and testing$

[Rob Ossian <rob_neptune@yahoo.com>]

"Panasonic TH-37PWD"


I also looked heavily at Polycarbonates (primarily
Lexan, but a few others as well) when researching my
sub's design but quickly found that this material was
not a good choice for a viewport. Very disapointing,
as I had a source lined up for a brand new f16 canopy
for less than $1500 that I had hoped to be able to
modify into a shallow water (<250 feet) 1 atm.
 
The problem with polycarbonate is in the molecular
structure of the material and how it reacts when
stress is put on it.  Polycarbonates react to
stress/pressure by flowing away from the stress point,
which is why they are bullet resistant and used in
helicopter/jet canopies.  When an object travelling at
high velocity hits the poly, the molecules of the
material react much like the surface of a jello cube,
shifting the energy away from the point of impact in
order to mainstain integrity.  Conversely, the
molecular structure of acrylic does not share this
force across itself, as it is a more crystaline
structure.  In the same experiment, acrylic would keep
the force centered on the point of impact, causing the
sample to buckle/fail/chip/fracture.

In terms of submersible viewports, there is a
tremendous amount of compressive pressure placed upon
the viewports and this causes polycarbonates to deform
as that pressure increases.  This means that the
reliability of Lexan as a viewport material would be
extremely low at depth, and entirely dependent on
variables unqie to each dive (specific depth,
temperature variations, age, wildcards, etc).  

Meanwhile the behavior of Acrylic/Plexiglass under
pressure is well known/tested to be capable (when
properly manufactured) of maintaining it's cohesive
structure until it reaches a fairly easily predictable
fail-point.  At that fail point, the crystalline
structure of the acrylic will shatter like glass, and
then the port itself will likely fail
catastrophically.  (<--Think about that word)

I am very frustrated at the apparent high cost of
Acrylic/Plexiglass, but at present I am not aware of
any other alternative materials that can be used for
viewports with any equally high degree of safety or
confidence.  If anyone knows of any, please share!
 
Hope this info is of some help,
Rob
 
--- skrewgun007@aim.com wrote:

>  Thank you most helpful. I do need to apologize for
> the fact that I accidentally dropped my sub testing
> under Franks poetic outlook. I read it and used it
> to send out my message without renaming the subject.
> does anyone else know of any poly test that have
> been done or have anything else. ohh yea, I asked my
> local glass man how much 1.5" acrylic ran and he
> told me $24 a square foot. I don't know if he was
> right since he was guessing. what is a good rate on
> this material and where can I find 2" thick stock
> for my front view port? any on line suppliers. what
> do you mean to expensive. I figured you guys where
> made of money LOL just kidding.
>   
>  -----Original Message-----
>  From: empiricus@telus.net
>  To: personal_submersibles@psubs.org
>  Sent: Thu, 5 Oct 2006 2:15 AM
>  Subject: Re: [PSUBS-MAILIST]sinking....
>  
>   Hi there - I brought just that topic up a few
> months ago about polycarbonate vs. acrylic. Phil
> Nuytten was kind enough to give his two cents. I'm
> paraphrasing here.   First: Acrylic has been
> researched to the nth degree and is very predictable
> because of that research. The numbers are out there.
>   Second: It is more brittle, however, and in an
> ambient wet or dry sub my choice would be the
> polycarbonate simply because of collision, either
> with a falling anchor or a bump into a reef.  
> Third: Poly under pressure will creep apparently.
> That makes it unpredictable. Which way is it going
> to flow, etc. Bad news.   Forth: Poly is really
> really really expensive.   Fifth: Poly is stronger
> but scratches easier and is harder to get the
> scratches out (I think I've got this one right)  
> Sixth: They may have different UV light properties
> that exclude using one or the other depending on
> your mission statemtent.   Seventh: Failure with
> acrylic is often catastrophic, meaning it's sudden.
> I!
>   would imagine incipient creep with poly would give
> some indication of failure, especially with strain
> gauges attached.   Some pros, some cons. Hope this
> helps.   Rick L Vancouver    
>    
>
________________________________________________________________________
> Check Out the new free AIM(R) Mail -- 2 GB of
> storage and industry-leading spam and email virus
> protection.
> 


------------------------------------------------------------------
http://www.ThePirateKing.com
Everything you ever wanted to know about Pirates, Privateers, Explorers, Buccaneers, Sailing, Shipwrecks and Nautical Archaeology.



************************************************************************
************************************************************************
************************************************************************
The personal submersibles mailing list complies with the US Federal
CAN-SPAM Act of 2003.  Your email address appears in our database
because either you, or someone you know, requested you receive messages
from our organization.

If you want to be removed from this mailing list simply click on the
link below or send a blank email message to:
	removeme-personal_submersibles@psubs.org

Removal of your email address from this mailing list occurs by an
automated process and should be complete within five minutes of
our server receiving your request.

PSUBS.ORG
PO Box 311
Weare, NH  03281
603-529-1100
************************************************************************
************************************************************************
************************************************************************