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Subject: Foams


From: Doug Nishimura <dwnpph>
Date: Thursday, September 5, 1991
I recall that there was an engineer at the Canadian Conservation
Institute working on the problems of packing (for shipment) and foams.
One interesting fact that came out of the study was that they found open
cell foams to be more effective for cushioning than the more
"traditional(?)" closed cell foams.  The closed foams cushion by virtue
of the compressibility of the air cell gas while the open cell foams
cushion by virtue of polymer elasticity.  The open celled foams tend to
act like pistons while the closed cell foams often suffer from rebound
(or bounce-back).  The engineer who did the work on foam packing was
Paul Marcon. Physically blown foams are generally safer than chemically
blown foams because of residual chemicals that may be left. One example
is that azodicarbonamide produces N2 gas but leaves other dicarbonamides
as residues.  There is also concern about some of the thermal blowing
agents that are sulfur compounds.  However the following foams are
generally considered to be safe:

Ethafoam  phys blown with pentane (and replaced with air)
Sentinel  phys blown with butane (and replaced with air)
Minicel  chem Xlinked, chem. blown
Volara   radiation Xlinked, chem. blown
Plastazote chem Xlinked (?), phys blown (N2)
Nalgene (Clean Sheets I think), rad Xlinked (?), chem blown

Plastazote has apparently caused slow tarnishing of metal coupons and
may have a sulfur containing blowing agent.

Xlinking tends to make the foam more rigid, increases the softening
point, makes it more tear resistant, less soluble and often less
abrasive. (Volara, for example, has much smaller bubbles.)

Scott Williams at CCI also brought up a point about cut faces on foams.
Several of the foams used in conservation are sold by the manufacturers
in 2" thick sheets, which the distributor may cut into thinner slices.
As I recall, the cut faces tended to be less abrasive and less apt to
snag things than the manufactured face.

Regarding actual bubble wrap, they found that object left too long in
pure polyethylene bubble wrap caused the bubbles to flatten.  (This is
the problem of vapor porosity of polyethylene.)  If objects must remain
wrapped in bubble wrap for long periods, CCI suggested PVDC coated PE
bubble wrap.  The interior side (ie inside the bubbles) has been coated
with PVDC to prevent the air from diffusing out.  PVDC is a little more
stable than PVC and since it is inside the bubbles, it hopefully not as
much of a problem as PVC.

BTW I hope you didn't get the idea that polyethylenes are bad. They are
inherently good, safe materials, although like all materials, you may
get bad batches or a grade not to your specification.  You could run
into the same problem with polyester too.   Also, much of the work done
by Paul Marcon involved the used of an inner and outer box.  Among the
problems to be solved was whether it was better to pad the contents of
the inner box and then anchor the inner box in the outer box or to
anchor the contents of the inner box and put the padding around the
inner box.  The latter case is the same as handling a traditional glass
plate box (wood with grooves cut out for the plates), where you might
want to protect the box and plates. However, even if the methods are not
directly applicable to your problem, I think that the information would
be useful.

Oh yes, from a chemical point of view, Scott Williams has not found any
reason to question the foams mentioned (with the exception of the
Plastizote.) We (IPI) haven't really tested any of the foams. Certainly,
they are much harder to evaluate in terms of additive exudation.  This
property we currently measure optically for films, but foams don't tend
to be very transparent :-).

                  Conservation DistList Instance 5:19
                 Distributed: Sunday, September 8, 1991
                        Message Id: cdl-5-19-007
Received on Thursday, 5 September, 1991

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