[GreenKeys] plastic decay (was: 28 printing reperf hammer redux)

[Jones, Douglas W]

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From: GreenKeys [greenkeys-bounces at mailman.qth.net] on behalf of Geo. Hutchison [w7tty at centurylink.net]
Sent: Wednesday, February 11, 2015 7:21 PM
To: greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] 28 printing reperf hammer redux

My experience with the 173979 hammer heads is ... When the material that they were made of ages, it seems to retire whether or not it has had much use. Looks and works good, and then POOF!!! they are dust.
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In most cases I have encountered with NOS plastics, the key thing is, how
were they packed and stored.  Many plastics, if packed in anything that
excludes oxygen, will survive fairly well.  Foil, foil-backed paper, and even
varnished or waxed paper, if not torn, creased or cracked, will preserve
things very well.  Even heavy cardboard can do wonders.  Yes, oxygen
diffuses through it, but not very quickly.  The life of plastic parts inside may
be much longer than the life of parts left exposed to free air.

The one exception is old acrylic parts.  If stored in a closed container, any
depolymerization will release acetic acid, and this seems to accelerate
the depolymerizaiton.  This is called "vinegar disease" because of the smell
of acetic acid that is rather overwhelming when you open the sealed container
where this kind of decay is going on.  Acrylic parts need to be ventilated for
longevity.  The motion picture industry is very sensitive to this problem, as
once vinegar disease sets in on a reel of film, it can be very destructive,
and in a stack of reels of film, it can spread from reel to reel.  I had an
"infected" stack of microfiche once.  It was pretty awful.

Speaking of depolymerization, the PDP-8 I am restoring has flexprint
cables between the two halves of the backplane.  They are mylar ribbons
with etched copper conductors on one side, with polyurethane insulation.
The polyurethane is depolymerizing -- turning to an adhesive goo.  Any
suggestions?  Is there any chemical treatment to restore such?  Or
what is the appropriate cable to use to rebuild them?

                    Doug Jones
                    jones at cs.uiowa.edu