Deterioration of polyurethane sculptural relief panels

Karen Potje <kpotje [at] cca__qc__ca> writes

>At the CCA we have recently acquired 14 relief panels by Jim Dine
>that were made between 1976 and 1984 to decorate the walls of a
>hotel.
>...
Subject: Deterioration of polyurethane sculptural relief panels
>...  The motifs appear to be life-size so perhaps they were
>cast from moulds made from the real things.  According to the
>previous owners the panels are made of polyurethane.  (I hope to
>have the plastic analysed to see if that description is accurate.)
>They are painted white but, as cracks are now opening up in them, we
>can see the ochre-coloured plastic interior.  On each panel the
>moulded plastic components are distributed across the surface of an
>approximately 3/8 inch thick layer of the same plastic which is, in
>turn, laid down on a 1/2 inch thick wood or plywood panel, also
>painted white.  I don't know if there are any other materials hidden
>by the paint layer.

I have some experience with polyurethane/wood composites, but I have
never encountered anything such as you are describing in the
Conservation DistList. A thought occurred to me that is probably off
the wall but that I thought would be worth passing along anyway--are
you certain that it is the polyurethane component that is unstable?
If the polyurethane is on a wooden board-type of panel then it might
just be possible that dimensional changes (swelling due to moisture
absorption) in the underlying wood are creating sufficient stresses
to create the cracks you are observing. (Plywood is more
dimensionally stable than a piece of wood board and I wouldn't
expect wood dimensional changes to be causing your problem). Are the
cracks primarily running in one direction, or is the polyurethane
separating mostly along 2 opposite sides and not along all 4 sides?
This might support my guess that you have a wood moisture problem
and not a polyurethane stability problem, as wood shrinks and swells
unevenly according to the grain direction.

Frankly I am somewhat doubtful that this is the explanation for your
problem--as the conditions in the warehouse likely fluctuated I
agree that it would seem likely that cracks ought to have been
evident at the earlier examination (even if they had opened and
closed again by the time of the first examination I think that
pre-existing cracks should have been noticeable). Perhaps the
warehouse conditions were normally drier than your current storage
conditions, or perhaps the wrapping job for storage was so good that
moisture flow into and out of the packing was greatly inhibited and
the panels were dryer than they are now when they were packed? If
the polyurethane is not mounted on plywood and if the cracking seems
similar to my guess-description in the paragraph above, however, it
might be worthwhile attempting to lower the RH to shrink the
underlying panel slightly, and hopefully this might decrease the
stresses that could be creating the cracks you are observing.

I'd like to hear back from you if you have any further information,
or if anything I've written sounds even partially applicable.

Terry Conners, Ph.D.
Extension Specialist in Forest Products
Department of Forestry
202 Thomas Poe Cooper Building
University of Kentucky
Lexington, KY 40546-0073
859-257-2463
Fax: 859-323-1031


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                  Conservation DistList Instance 15:49
                Distributed: Saturday, January 12, 2002
                       Message Id: cdl-15-49-002
                                  ***