Weeping polyurethane elastomer sculpture

James Martin <james.s.martin [at] att__net> writes

>This posting seeks information about the deterioration and treatment
>of cast elastomeric polyurethane sculptures.
>
>I am investigating a cast elastomeric polyurethane sculpture.  A
>portion of the sculpture is converting from an elastic solid to a
>viscous liquid. ...

Cast diisocyanate polyurethanes are susceptible to hydrolysis in
water and humid atmospheres. The susceptibility varies widely
according to the composition of the polyurethane, with polyether
polyurethanes being much more resistant than polyester types.
However, polyether polyurethanes will hydrolyze under certain
conditions. I participated in a study carried out at a National Lab,
which tested the useful life two different cast polyether
polyurethanes at varied temperatures and relative humidities. The
results showed a large reduction in useful life for every 10 percent
increase in relative humidity or 10 deg. C increase in temperature
of exposure. At longer times in the more aggressive conditions the
polyurethanes under test swelled and lost all strength.

Thus you should consider that hydrolysis of the polyurethane is a
possible cause of the statue's "melting". This explanation is
consistent the process proceeding from the outside-in, and with your
observation that the the elastic solid softens and slowly dissolves
on prolonged immersion in the viscous liquid. It is also consistent
with your FTIR results. (In those results the detection of an
aromatic component is probably due to the polyurethane curative,
MOCA or other, rather than a plasticizer. Plasticizers are not
normally needed or used in cast polyurethane formulations.)


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                  Conservation DistList Instance 22:60
                 Distributed: Saturday, April 18, 2009
                       Message Id: cdl-22-60-007
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