Vinyl Record

Sticky Tape Syndrome

The binder in a new tape is a crosslinked network of polymer chains.  Over time moisture, which has been absorbed by the tape from the air, reacts with the polymer chains causing them to break.  (Specifically water reacts with, and opens up, the polyester links in the binder polymer).  This results  in the production of low molecular weight chain segments.  These tend to make the binder `gummy’ and can migrate to the surface of the tape causing the `sticky tape’ phenomenon.  As more of the initial polymer network degrades it will loose its integrity, be less durable and wear more quickly.

The binders typically used in tape formulations are polyester-polyurethane based.  These materials are subject to hydrolysis in humid environments.  The polyester linkages in the polymer will react with the water in the environment and polymer chain scission will occur reducing the integrity of the crosslinked binder system.  This results in the sticky tape syndrome characterised by high tape friction and low coating integrity.  The high friction can result in tape seizing and a general inability of the tape to be transported in the drive.

The majority of affected tapes are of AMPEX 406 and 456 stock manufactured between 1975 and 1984.  During those years polyurethane supplied to the company was evaluated with a viscosity check.  Unfortunately batches with a wide mix of very long and very short strings can average out to the same viscosity as those with mostly medium strings.  Therefore a number of tape batches were manufactured with undesirably high levels of short urethane strings.  Once started the breakdown process can render tapes unplayable in anywhere from two to fifteen years.

Tapes can be rendered playable again by baking in a laboratory oven at 50C (120F) for several hours.  The tapes should be allowed to cool slowly.  Ideally they should be left in the switched off oven with the door closed.  After treatment tapes should remain playable for up to 30 days.  This process can be repeated several times.  One problem of baking is the increase of print through.  This can to some extent be minimised by baking the tapes tail-out.  Baking at higher temperatures will reduce the treatment time but considerably increases the level of print-through.

This treatment does not result in a reversal of the binder hydrolysis and a reconstruction of the tape binder coating.  Much longer treatment times would be required for a reversal of the hydrolysis reaction.  Heat treatment merely provides a temporary clean-up of the tape surface and firms up the tape top coat allowing the tapes to be played and copied to another medium.

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