Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!news.larc.nasa.gov!grissom.larc.nasa.gov!kludge
From: kludge@grissom.larc.nasa.gov ( Scott Dorsey)
Newsgroups: sci.electronics
Subject: Re: What is the life-time of magnetic tape?????
Message-ID: <1991Feb5.144610.11714@news.larc.nasa.gov>
Date: 5 Feb 91 14:46:10 GMT
References: <1991Feb4.100546@axion.bt.co.uk>
Sender: news@news.larc.nasa.gov (USENET Network News)
Reply-To: kludge@grissom.larc.nasa.gov      ( Scott Dorsey)
Organization: NASA Langley Research Center
Lines: 24

In article <1991Feb4.100546@axion.bt.co.uk> bcollins@axion.bt.co.uk (bill collins) writes:
>The article mentions, in particular, the use of polyurethane as a binder
>on tapes (the "glue" that holds the magnetic iron oxide onto the polyester
>film base). The polyurethane absorbs water, and releases an acid which 
>produces gummy residues on the tape. The tape then sticks and judders in 
>the tape recorder.

There are a few tapes which have had binder problems, but the vast majority
of tape problems from that era are due to backcoating lubrication.  Whale-oil
derivatives were very popular for tape backcoating, and when it was no longer
available, petroleum derivatives began to be used.  Many of the petroleum 
based lubricants break down and cause the backcoating to become a slimy scum
which sticks to the heads and capstan, tears off the oxide coating, and 
generally makes a mess of things.  Heating the tape to stabilize it for one
playing is possible, as are silicone treatments and use of various solvents.
A lot of master tapes and instrumentation materials were lost.  Acetate tapes
which are not usually backcoated don't have this problem (although they do
tend to become very brittle with age).

In any event, I have tapes (actually paper with oxide coating) from 1938 which
are in playable condition, and other tapes that are ten years old (recorded
on early seventies Scotch 208) which are not possible to salvage.
--scott
   (who has altogether too many tapes in his house)