<html><body>
<p><font size="2" face="Times New Roman">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. </font><br>
<br>
<font size="2" face="Times New Roman">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.</font><br>
<br>
<font size="2" face="Times New Roman">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.</font><br>
<br>
<font size="2" face="Times New Roman">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.</font><br>
<br>
<font size="2" face="Times New Roman">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?</font><br>
<br>
<font size="2" face="Times New Roman">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.</font><br>
<br>
<font size="2" face="Times New Roman">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. </font><br>
<br>
<font size="2" face="Times New Roman">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. </font><br>
<br>
<font size="2" face="Times New Roman">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.</font><br>
<br>
<font size="2" face="Times New Roman">R. Scott Williams</font><br>
<font size="2" face="Times New Roman">Senior Conservation Scientist (Chemist)</font><br>
<font size="2" face="Times New Roman">Canadian Conservation Institute</font><br>
<font size="2" face="Times New Roman">1030 Innes Road</font><br>
<font size="2" face="Times New Roman">Ottawa, Ontario, Canada K1A 0M5</font><br>
<font size="2" face="Times New Roman">tel: (613) 998-3721</font><br>
<font size="2" face="Times New Roman">fax: (613) 998-4721</font><br>
<font size="2" face="Times New Roman">email: scott.williams@pch.gc.ca</font><br>
<br>
<img width="16" height="16" src="cid:1__=0ABBF0DFDFE7E7068f9e8a93df9386@pch.gc.ca" border="0" alt="Inactive hide details for Paul Callomon ---2012-07-30 03:56:58 PM---Colleagues, I have today found a severely deteriorated jar "><font size="2" color="#424282" face="sans-serif">Paul Callomon ---2012-07-30 03:56:58 PM---Colleagues, I have today found a severely deteriorated jar lid liner in our alcohol collection. The</font><br>
<br>
<font size="1" color="#5F5F5F" face="sans-serif">From: </font><font size="1" face="sans-serif">Paul Callomon <callomon@ansp.org></font><br>
<font size="1" color="#5F5F5F" face="sans-serif">To: </font><font size="1" face="sans-serif">"NH-COLL listserv (nhcoll-l@mailman.yale.edu)" <nhcoll-l@mailman.yale.edu></font><br>
<font size="1" color="#5F5F5F" face="sans-serif">Date: </font><font size="1" face="sans-serif">2012-07-30 03:56 PM</font><br>
<font size="1" color="#5F5F5F" face="sans-serif">Subject: </font><font size="1" face="sans-serif">[Nhcoll-l] Potential problem with older PE jar lid liners</font><br>
<font size="1" color="#5F5F5F" face="sans-serif">Sent by: </font><font size="1" face="sans-serif">nhcoll-l-bounces@mailman.yale.edu</font><br>
<hr width="100%" size="2" align="left" noshade style="color:#8091A5; "><br>
<br>
<br>
<font size="2" face="Calibri">Colleagues,</font><br>
<font size="2" face="Calibri"> </font><br>
<font size="2" face="Calibri">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.</font><br>
<font size="2" face="Calibri"> </font><br>
<font size="2" face="Calibri">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. </font><br>
<font size="2" face="Calibri"> </font><br>
<font size="2" face="Calibri">Does anyone have experience with this problem? Are Teflon liners a better alternative? </font><br>
<font size="2" face="Calibri"> </font><br>
<font size="3" face="Times New Roman"><b>Paul Callomon</b></font><br>
<font size="3" face="Times New Roman">Collections Manager in Malacology, Invertebrate Paleontology and General Invertebrates</font><br>
<font size="3" face="Times New Roman">The Academy of Natural Sciences of Drexel University</font><br>
<font size="3" face="Times New Roman">1900 Benjamin Franklin Parkway</font><br>
<font size="3" face="Times New Roman">Philadelphia, PA 19103</font><br>
<font size="3" face="Times New Roman">callomon@ansp.org</font><br>
<font size="3" face="Times New Roman">Tel. 215-405-5096</font><br>
<font size="3" face="Times New Roman">ansp.org</font><br>
<font size="3" face="Times New Roman">Follow us:</font><font size="3" color="#1F497D" face="Times New Roman"> </font><a href="http://www.facebook.com/AcademyofNaturalSciences"><font size="3" color="#0000FF" face="Times New Roman"><u>Facebook</u></font></a><font size="3" color="#1F497D" face="Times New Roman"> | </font><a href="http://twitter.com/#!/AcadNatSci"><font size="3" color="#0000FF" face="Times New Roman"><u>Twitter</u></font></a><br>
<font size="3" face="Times New Roman"><b>Join us as we celebrate the Academy’s 200th anniversary with a year of exciting events, special programs, and our bicentennial exhibit, </b></font><font size="3" face="Times New Roman"><b><i>The Academy at 200: The Nature of Discovery</i></b></font><font size="3" face="Times New Roman"><b>. </b></font><a href="http://www.ansp.org/"><font size="3" color="#0000FF" face="Times New Roman"><b><u>www.ansp.org</u></b></font></a><br>
<font size="2" face="Calibri"> </font><br>
<font size="2" face="Calibri"> </font><tt><font size="2">_______________________________________________<br>
Nhcoll-l mailing list<br>
Nhcoll-l@mailman.yale.edu<br>
</font></tt><tt><font size="2"><a href="http://mailman.yale.edu/mailman/listinfo/nhcoll-l">http://mailman.yale.edu/mailman/listinfo/nhcoll-l</a></font></tt><tt><font size="2"><br>
</font></tt><br>
</body></html>