[sdiy] D-70 red goo of death - not epoxy, maybe polyurethane

[Robin Whittle]

Hi Mike,

Thanks for pointing to this thread from 2008:

   http://www.soundonsound.com/forum/showflat.php?Cat=&Number=623176

The writers there consistently refer to "epoxy", but I have never heard
of epoxy breaking down.  Epoxy is more expensive than polyurethane.  I
strongly suspect this adhesive is polyurethane, which can break down
after 10 or 20 years or so.  Please see my earlier response with
"-polyurethane?" in the subject line.

Dave G Dixon wrote:

> The stuff you are describing sounds like some kind of thermoplastic
> polymer.  Epoxies are crosslinked polymers which are untouched by
> most solvents (acetone, etc).  However, they don't generally soften
> and drip.

He also wrote:

> It seems to me that anything that could melt could be dissolved (but
> I'm not really sure).

This is not necessarily the case, for instance ordinary polythene
(polyethylene) as used in plastic bags, food containers etc. melts but
is unaffected by any solvent, as far as I know.

The material is unlikely to be a plain thermoplastic, in that the way it
was made to flow for initial application is unlikely to be heat, since
it was used in plastic keys, which would be made of ABS.

There are many polyurethane potting compounds which would work fine as
adhesives in this application - until they turn to crap.  They are made
by mixing two parts, after which there is a certain pot-life before the
material stiffens and solidifies.  This is not an adhesive which relies
on a solvent evaporating.  That wouldn't work between the weights and
the insides of the keys.  Also, many solvents would upset the ABS which
the keys are surely made from.  So polyurethane would be an apparently
suitable approach, very widely used in many industrial applications -
until you see or read about how it may turn to goo a decade or two later.

While polyurethane is probably easy, inexpensive and effective at first,
I think it would be better to use contact adhesive for the key weights.
 As far as I know that never breaks down.  However, the solvents might
be a problem for the ABS, and it could take a long time, such as days or
weeks for the solvents to evaporate from the deeper recesses of the
keys.  Epoxy might be a good choice, but I am not sure how well it bites
into, or adheres to, ABS.  It would become rigidly hard, and so be
subject to separation with heating and cooling, vibration and flexing of
the keys.  My sense of polyurethane and contact adhesive is that both
are inherently sticky and more flexible than epoxy, so I think they will
adhere, without biting into (partially dissolving) the ABS.

The writers in the SOS thread found various techniques of dealing with
this horrible stuff:

 1 - Freezing it with butane (gas or liquid) - best done outside, being
     careful of static electricity, fires, explosions etc.!  This would
     make it stiff enough to peel away from whatever it was bonded to.

     Dry ice might be better, though it is not a liquid.  Maybe ice
     water might be helpful.

 2 - Hot air - makes it less viscous and therefore likely to drop the
     weights out of the keys.  However I think this involves a serious
     risk of melting the plastic keys unless only moderate warmth is
     required.

 3 - Dilute (in water) caustic soda (NaOH).  I am not sure what
     concentration they used, but this apparently softens the material
     after some time.


 - Robin   http://www.firstpr.com.au/rwi/dfish/