Re: [PSUBS-MAILIST] acrylic vs polycarbonate

[Phil Nuytten <pnuytten@compuserve.com>]

Hi, Rick:
Polycarbonates  (Macrilon,Lexan,etc.,) are not used for ports or pressure
hulls because of their cold-flow characteristics. This material deforms and
flows away from an imposed loadimg - whether sustained or sudden shock.
It's this cold -flow ability that allows them to absorb massive energy from
shock-loads without disintegration ( remember the tv ad " Like space-age
plastics that bullets bounce off . . .". 
         Acrylics (plexiglas,perspex, etc.,) are rigid structures that,
like glassine materials, fail suddenly and catastrophically, but 'hold the
fort' until the moment of failure, Acrylic G (plexiglas) has high
resistance to compressive loads but is relatively weak in tension -
polycarbonate is the opposite. Since the loads that we typically see in a
one-at submersible are compressive, polycarbonate is a poor candidate. If
you have some transparent plastic and aren't sure which it is, look for the
conchoidal fracture (spall) if you put a spot load on an edge or corner. If
it spalls like glass, it's acrylic or other rigid plastic - if it just
gouges, it's polycarbonate or other soft plastic.
Regards
Phil Nuytten 




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