[Nhcoll-l] Potential problem with older PE jar lid liners

Tue Jul 31 17:25:32 EDT 2012

I preface these comments with the caveat that I have not investigated the
problems associated with the design of leak free bottle caps.  These
comments are related to the materials mentioned in the query, not the
bottle or cap design.

The query refers to polyethylene foam.  There are a great many different
grades of polyethylene plastic, and polyethylene foam products, available
from many different manufacturers.  Also, it is possible that a specific
product, or a specific batch of a product may show problems, not shown by
others.  Therefore, the following comments must be of a general nature.

Closed cell uncrosslinked or crosslinked polyethylene foams such as
Ethafoam and Volara and closed cell foams made of other plastics rely for
their compressive strength on the air or other gas filled bubbles or cells
in the foam. As the foam is compressed, such as when an object is placed on
a foam pad or a foam gasket is squeezed when a bottle cap is tightened, the
bubbles deform and the plastic in the cell walls stretches.  The ease of
deformation and stretching of the plastic determines the compressive
strength.  The long term stability of compressive properties of the foam
depends on the long term stability of chemical and physical properties of
the plastic, such as elasticity and gas permeability.  In the case of
polyethylene, oxidation decreases elasticity and increases brittleness and
susceptibility to fracture.  Compression of oxidized polyethylene foam is
more likely to cause fracture of embrittled cell walls than compression of
more flexible unoxidized polyethylene foam.  Fracture leads to loss of gas
and collapse of the foam structure.  In the case of a gasket like a bottle
cap liner, this means loss of contact with the joint walls and leakage.

When a foam is compressed the pressure of the gas inside the cells
increases.  Since permeability through a plastic membrane is proportional
to the pressure difference across that membrane, the rate of permeation of
the gas from the cell increases.  Since the pressure in the closed volume
of the cell is proportion to the amount of gas in the cell the pressure in
the cell decreases as the gas leaves.  In the case of a gasket like a
bottle cap liner, this means that the pressure of the foam liner against
the joint walls decreases and leakage may occur.  Ultimately all the gas
could be squeezed out until all cells have collapsed and the foam liner is
no longer a foam but just a pile of sheets of polyethylene film from the
cell walls.  Furniture stored on foam shelf liners often shows completely
collapsed foam under the feet.  The phenomenon is called compression set.

Polyethylene, like all plastics, contains additives to improve or stabilize
long term behaviour.  Polyethylene without antioxidant stabilizers will
oxidize quickly in the atmosphere.  Additives can be extracted from
polyethylene with organic solvents.  Ethanol is commonly used for this
purpose in the analytical laboratory.  It is likely that ethanol in the
alcohol collections will extract the additives from the polyethylene
liners, leaving the polyethylene foam susceptible to oxidation, which
invariably results in embrittlement and fracture.  If oxidizing conditions
exist inside the vials of the alcohol collection, then oxidation of the
polyethylene foam line is likely with subsequent failure as described. Do
oxidizing conditions exist in the vials?

Ethanol is known to be an environmental stress cracking agent.  Bottle cap
liners are physically stressed in the zone of compression at the joint
between cap and vial rim and therefore may be even more susceptible to
environmental stress cracking.  This may show up as cracks located at the
transition zone between uncompressed foam exposed to ethanol in the vial
and the compressed foam between the vial rim and the cap which is not
directly exposed to the ethanol.

Because the plastic matrix in a foam makes up such a small fraction of the
volume of the foam compared to a solid piece of plastic of the same
thickness (maybe less than 5% of the volume), a small amount of change in
physical properties such as brittleness, of the plastic in a foam can have
very much greater effect on the foam product than on the solid product.  A
small increase in brittleness, sufficient to cause fracture of thin cell
walls with loss of gas, and fracture pillars between cells, might lead to
collapse of the foam structure but would be unnoticeable in a solid.

Open cell foam is very different from closed cell foam. There are no
discrete gas filled bubbles or cells completely enclosed by plastic. The
membrane or window between adjacent cells is open or missing, and only the
pillars where three or more cells meet remains. Gas is free to move from
cell to cell throughout the entire volume of the foam. Uncompressed open
cell foam is not a gas barrier.  Completely compressed open cell foam where
the entire gas volume is compressed to nothing may be a gas barrier.
Compressive strength does not depend on compression of gas in cells, but
only on the elastic properties of the plastic that constitutes the pillars
of the foam.  In this sense an open cell foam is something between a closed
cell foam and a solid.  Compressible and elastic open cell foams are made
with compressible and elastic plastics.  Polyethylene is not such a
plastic.

Based on the arguments above, use of any type of closed cell foam as a
gasket may be inappropriate and open cell foam is probably not be better.
Gaskets made from solid elastic plastics that are impermeable to ethanol
may be better choices.  Another alternative may be to avoid gaskets and
seek a cap design that has a structure that makes a leak free seal, such
as, for example, a V-shaped groove into which a V-shape protrusion inserts.
I do not know if such caps are available.  Perhaps chemical supply houses
which sell organic solvents in bottles and sample bottles for liquid
specimens or beverage companies that sell pressurized carbonated beverages
have developed solutions that may be adaptable, keeping in mind that
beverage applications are for relatively short shelf life.

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:	Paul Callomon <callomon at ansp.org>
To:	"NH-COLL listserv (nhcoll-l at mailman.yale.edu)"
            <nhcoll-l at mailman.yale.edu>
Date:	2012-07-30 03:56 PM
Subject:	[Nhcoll-l] Potential problem with older PE jar lid liners
Sent by:	nhcoll-l-bounces at mailman.yale.edu

Colleagues,

I have today found a severely deteriorated jar lid liner in our alcohol
collection. The liner is made from polyethylene foam. It was sold to us in
1999 and installed in 2000. The material has failed completely, with
crazing and cracking of the surface and tearing along the compression face.
The foam has become weak and powdery in places. The jar was filled with 70%
ethanol.

The manufacturers have suggested that the use of this liner for long-term
storage of alcohol is responsible for the deterioration. If this is true,
the implications for our collection – we have about 20,000 bottles that use
these liners – are potentially serious.

Does anyone have experience with this problem? Are Teflon liners a better
alternative?

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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