Deterioration of polyurethane sculptural relief panels

I have corresponded with Scott Williams from CCI, who specializes in
the deterioration of plastics.  Scott has come up with a theory
about the deterioration of the Jim Dine panels.  He specifies that
until he knows the composition of the panels, though, this is just
conjecture.  I will send Scott samples for analysis.  Also, I am
hoping Scott will visit CCA in a few months--a follow-up visit to a
couple of days he spent here in November analysing plastics in other
collections--so he will see the panels himself.

With Scott's permission I present his theory, as patched together
from a couple of e-mail messages:

    There are only a few plastics that can be moulded around objects
    in a studio--polyester, epoxy, polyurethane, acrylic.  None of
    these are known to be responsive to moisture fluctuations, so it
    is unlikely that it is changes of moisture content in the
    plastic that is causing problems.  Nor do I think it is a
    question of expired life expectancy.  It is more likely that
    something happened during the move from Chicago to Montreal.

    Perhaps transport loosened the bond between plywood base/back
    and plastic to a sufficient extent that dimensional changes in
    the wood due to moisture changes are now sufficient to break
    plywood/plastic bond, allowing warping of the plastic which was
    previously constrained by adhesion to the plywood, leading to
    cracking throughout. (Scott was assuming the plastic was
    adhered, not mechanically fastened, to the plywood.  I need to
    look closely at the panels to try to determine how the layers
    are bonded, but when I told him that at one corner of one panel
    I see what appears to be the head of a nail, painted over, he
    replied that "the same kind of sudden cracking and warping could
    just as easily occur if attachment at a nail was broken as if an
    adhesive bond had released.

    In either case there would be a loss of attachment and a loss of
    constraint in the plane of the plywood, which could result in
    damage as I suggest.) If the plastic developed stresses over
    time but was still firmly adhered to the plywood, it would
    remain flat and not show any signs of the stress.  If the bond
    between plastic and plywood broke, then the plastic would warp
    or crack in response to the stress. This would in turn lead to
    new stresses and/or cracks in all other layers.

    Your measurements indicate no further damage from when you did
    your initial measurements.  This is as I would expect.  The
    whole thing snapped all at once, relieving stress by cracking
    and warping.  No stresses remained to cause more cracking and
    warping.

    I think the damage you have observed is physical damage, perhaps
    caused by chemical changes in the plastic since it was made, but
    not by chemical changes since it was shipped.  Therefore, you do
    not need to worry about arresting chemical change.  You must
    worry about stabilizing the physical environment. You want to
    avoid temperature and RH fluctuations in the object, especially
    in the plywood, not so much in the plastic.  The plastic is
    probably quite insensitive to RH and temperature.  Your
    environmental controls provide stable

    RH and temp so you are ok there.  Do not wrap them to seal them
    up. This will provide no benefit and may result in them stewing
    in their own juices.

    I think the best thing you can do now is not disturb them. (In a
    subsequent note Scott stressed the importance of identifying the
    plastic, though, because it is possible that something may turn
    up that suggests things are changing more quickly than he
    suspects.) If they are flexible, you will need to figure out how
    to move them without flexing the plastic/plywood bond.

    This validity of this explanation rests on determining whether
    the plastic/plywood bond has failed.  If you are going to do
    anything I would try to resolve this issue.  Look for wood
    attached to detached pieces of plastic and warping where thin
    sections of the plastics meet the wood.

    Scott Williams


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                  Conservation DistList Instance 15:49
                Distributed: Saturday, January 12, 2002
                       Message Id: cdl-15-49-003
                                  ***