[Nhcoll-l] Update on jar lid liner issue

Thu Aug 2 11:18:18 EDT 2012

There are many factors that could cause one brand of foam to degrade while
others do not, including blowing agents, stabilizer additives, polymer
properties (tensile strength, elasticity) and foam properties (compression
set, degree of crosslinking, cell size, presence of integral skin).  The
description of the Kohls foam as having a different blowing agent may imply
different degree of crosslinking.  I think it is more common to use
chemical blowing agents, not physical blowing agents, for crosslinked foam.
This could make a big difference to longevity.  Not all crosslinked
polyethylene foams are chemically blown.  Volara and Plastazote are
crosslinked polyethylene foams acceptable for conservation applications
that are physically blown.  I do not know whether Kohls or TriSeal liners
have uncrosslinked or crosslinked foam.

There are two types of blowing agents – physical and chemical.  Physical
blowing agents are gases like nitrogen and carbon dioxide or low boiling
solvent like pentane and isopentane or in the old days CFCs (now no longer
used by anyone), which are dissolved in the molten plastic in a high
pressure chamber in an upstream part of the manufacturing process.  This
pressurized molten plastic containing blowing agent is ejected (extruded)
through a die.  The downstream side of the die is at atmospheric pressure,
so the pressurized gas boils out of the molten mass creating gas bubbles in
the polymer matrix, just like carbon dioxide bubbles out of champagne
bottle when the cork is popped (or in my case when the beer cap is
removed).  The molten plastic freezes to trap the bubbles and now you have
a plastic foam. This foam contains only the plastic (with required
additives) and the gas.

Chemical blowing agents are solids which are dissolved in the molten
plastic.  The solids have the property of decomposing when heated to
produce gas degradation products (plus residual solid degradation products)
such as azodicarbonamide, or they are combinations of chemicals such as
sodium bicarbonate and citric acid which release gas when they react (plus
residual solid reaction products).  The solids react and gases are produced
and dissolved in the plastic in the hot high pressure chamber then the
plastic is extruded and foam is produced as described above.

The main difference between the two types of foam is that chemically blown
foam has blowing agent degradation and reaction products left behind in the
foam but physically blown foams do not.  These can affect long term
stability and conservation suitability of the chemically blown foams.  In
general I recommend that chemically blown foams be avoided because of
potential problems related to these residual degradation products.

Generally the blowing gas originally present diffuses out of the foam and
is replaced by air that diffuses in.  The grade of plastic used must be
carefully selected or a carefully selected suite of additives must be added
to the plastic to control diffusion and permeation rates (permeation
control agents) so that the rate of diffusion of the blowing gas out of the
foam is the same as the rate of diffusion of air into the foam.  Otherwise
the foam will collapse if the blowing gas diffuses out more quickly.  It is
a complicated and carefully balanced process.  It is one of the reasons why
manufacturers do not arbitrarily change the composition of their products.
I think it is one of the reasons why there were problems with degradation
of some PE foams in the 1990s.  Manufacturers were forced to abandon CFCs
and some did not quite figure out the correct new combinations of grade of
plastic and suite of additives for using new blowing agents like pentane
and isopentane, with the result that some foams degraded.  Eventually they
corrected this (or went out of business), so now that problem no longer
exists.

A more detailed discussion polyethylene foam, including blowing agents and
other additives, is in my article Polyolefin Foams, AIC News 2002 Vol 27-1
January starting on page 26,
http://www.conservation-us.org/_data/n_0001/resources/live/02_jan_aicnews.pdf
 and a very outdated article, Ethafoam and Other Polyethylene Foams in
Conservation, on CoOL at
http://cool.conservation-us.org/byauth/williams/foam.html.

R. Scott Williams
Senior Conservation Scientist (Chemist), Canadian Conservation Institute
1030 Innes Road, Ottawa, Ontario, Canada K1A 0M5
tel: (613) 998-3721
fax: (613) 998-4721
email: scott.williams at pch.gc.ca

From:	"Hawks, Catharine" <HawksC at si.edu>
To:	Paul Callomon <callomon at ansp.org>, "NH-COLL listserv
            (nhcoll-l at mailman.yale.edu)" <nhcoll-l at mailman.yale.edu>
Cc:	"Scott.Williams at pch.gc.ca" <Scott.Williams at pch.gc.ca>
Date:	2012-08-01 07:21 PM
Subject:	RE: Update on jar lid liner issue

Did Kohls note what was used as the blowing agent? I've seen many
polyethylene foams that were blown with gases other than nitrogen that have
discolored (yellowed) quite rapidly.

Scott, might this contribute to the deterioration of the liners in Paul's
collection?

Cathy
Catharine Hawks
Conservator
National Museum of Natural History, MRC 106
Research & Collections, NHB 394
Smithsonian Institution
PO Box 37012
Washington, DC 20013-7012
Office 202.633.0835
SI Cell 202.701.8458
CH Cell 703.200.4370
hawksc at si.edu<mailto:hawksc at si.edu>

________________________________
From: nhcoll-l-bounces at mailman.yale.edu [nhcoll-l-bounces at mailman.yale.edu]
On Behalf Of Paul Callomon [callomon at ansp.org]
Sent: Wednesday, August 01, 2012 1:33 PM
To: NH-COLL listserv (nhcoll-l at mailman.yale.edu)
Subject: [Nhcoll-l] Update on jar lid liner issue

Colleagues,

Realizing the possible implications for us all of age-related failure of
jar lid liners, I have been doing some survey work. The results are
encouraging.

-          The supplier of the liners, O. Berk (KOLS Containers in those
days) have confirmed that the liner in question is a generic version of the
Tekniplex F217 liner that differs from the brand-name version only in the
blowing agent that is used to foam the center core. The outer skins are
solid low-density polyethylene, and the center is foamed LDPE.

-          I retrieved and tested several other jars using the same lid
that were put into service in the same or following year (2000-01). The
lids thus came either from the same batch as the failed one, or from
another batch that was purchased shortly after. I carried out a simple
visual examination followed by a pliability test. For the latter, I lifted
the edge of the liner (it is glued to the inside of the lid) in two places
using a dental hook and folded a flap over until it touched the surface of
the liner (folded double). When it had returned, I inspected the surface.
The material showed some wrinkling from this treatment, as is normal, but
no cracking or crazing. This is the same thing that happens with a brand
new liner, and although the wrinkling remains to a certain extent, this
does not seem to compromise sealing.

-          By contrast, the failed liner snapped when folded over, and its
surface is covered with a combination of fractal (branching) crazing as
well as the cell-like crazing one sees in old ceramic glazes. The material
has thus clearly undergone a major chemical change, and a colleague has
generously offered to run tests to try and diagnose this.

In conclusion, it seems that this might have been a single incident. The
next step is to trace the history of the contents (13 separate specimen
lots in glass tubes) and see what treatments were used on them that might
have caused this problem.

Paul Callomon
Collections Manager in Malacology, Invertebrate Paleontology and General
Invertebrates
The Academy of Natural Sciences of Drexel University
1900 Benjamin Franklin Parkway
Philadelphia, PA  19103
callomon at ansp.org
Tel. 215-405-5096
ansp.org
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