The decay of the plastics might have caused the other decay (tape, screws and anodized metal door) but I'm pretty sure some outside mechanism would still be needed to cause the plastics to start decaying. The plastics affected were the double guide unit on one side of the cassette, the single guide unit on the other side, the door lock mechanism on the side with the single guide and the red record button on the back of the cassette. Each unit is a different shape and certainly came from different manufacturing batches so some contaminant in a manufacturing batch is pretty well ruled out. Also, millions of these pieces were distributed and I haven't heard of this happening elsewhere.
From my extensive disaster recovery work, I would say the damage looks more like heat damage than liquid damage but I believe the damage is chemically induced rather than by exposure to fire.
My best guess, at this point, is some airborne contaminant that reacts with one type or group of plastic and not some others. The plastics that were affected were all fairly "soft" plastics whereas the unaffected cassette shell, slip pads and pressure tabs are made from much harder plastics. For instance, methyl chloroform is used to decontaminate tape because it does not react with the polymers in the tape but care must be taken as it will melt some hard plastic cassettes and plastic reels. This instance seems to be the opposite. The tape and other "soft" plastics were damaged but the hard plastics showed no effect.
It would likely help a lot to know exactly what kinds of plastics were used for the different parts of the cassettes. Unfortunately, since the manufacturers are all gone, this data might be hard to obtain now.
SPECS BROS., LLC
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Audio and video restoration and re-mastering since 1983
From: Association for Recorded Sound Discussion List [mailto:[log in to unmask]] On Behalf Of Richard L. Hess
Sent: Tuesday, September 11, 2018 3:10 PM
To: [log in to unmask]
Subject: Re: [ARSCLIST] very strange/unique decay found on u-matic
Is it possible that the plastic parts that decayed were the cause of the problem? I had a toolbox where several tools were horribly corroded and the only culprit I could find was what used to be a screwdriver handle that had totally decayed.
This was about 25 years ago and I don't remember which screwdriver it was (you know how you inherit these things from your grandparents <sigh>).
So if these parts broke down and outgassed, they could self-destruct and also damage/corrode adjacent parts.
What did John Glenn say about sitting on top of everything made by the lowest bidder?
Richard L. Hess email: [log in to unmask]
Aurora, Ontario, Canada 647 479 2800
Quality tape transfers -- even from hard-to-play tapes.