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<p><font size="2" face="Times New Roman">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.</font><br>
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<font size="2" face="Times New Roman">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.</font><br>
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<font size="2" face="Times New Roman">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. </font><br>
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<font size="2" face="Times New Roman">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. </font><br>
<br>
<font size="2" face="Times New Roman">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.</font><br>
<br>
<font size="2" face="Times New Roman">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, </font><a href="http://www.conservation-us.org/_data/n_0001/resources/live/02_jan_aicnews.pdf"><font size="2" color="#0000FF" face="Times New Roman"><u>http://www.conservation-us.org/_data/n_0001/resources/live/02_jan_aicnews.pdf</u></font></a><font size="2" face="Times New Roman"> and a very outdated article, Ethafoam and Other Polyethylene Foams in Conservation, on CoOL at </font><a href="http://cool.conservation-us.org/byauth/williams/foam.html"><font size="2" color="#0000FF" face="Times New Roman"><u>http://cool.conservation-us.org/byauth/williams/foam.html</u></font></a><font size="2" face="Times New Roman">.</font><br>
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<font size="2" face="Times New Roman">R. Scott Williams</font><br>
<font size="2" face="Times New Roman">Senior Conservation Scientist (Chemist), Canadian Conservation Institute</font><br>
<font size="2" face="Times New Roman">1030 Innes Road, 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__=0ABBF0DDDFC3F2A28f9e8a93df9386@pch.gc.ca" border="0" alt="Inactive hide details for "Hawks, Catharine" ---2012-08-01 07:21:34 PM---Did Kohls note what was used as the blowing agent? I'v"><font size="2" color="#424282" face="sans-serif">"Hawks, Catharine" ---2012-08-01 07:21:34 PM---Did Kohls note what was used as the blowing agent? I've seen many polyethylene foams that were blown</font><br>
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<font size="1" color="#5F5F5F" face="sans-serif">From: </font><font size="1" face="sans-serif">"Hawks, Catharine" <HawksC@si.edu></font><br>
<font size="1" color="#5F5F5F" face="sans-serif">To: </font><font size="1" face="sans-serif">Paul Callomon <callomon@ansp.org>, "NH-COLL listserv (nhcoll-l@mailman.yale.edu)" <nhcoll-l@mailman.yale.edu></font><br>
<font size="1" color="#5F5F5F" face="sans-serif">Cc: </font><font size="1" face="sans-serif">"Scott.Williams@pch.gc.ca" <Scott.Williams@pch.gc.ca></font><br>
<font size="1" color="#5F5F5F" face="sans-serif">Date: </font><font size="1" face="sans-serif">2012-08-01 07:21 PM</font><br>
<font size="1" color="#5F5F5F" face="sans-serif">Subject: </font><font size="1" face="sans-serif">RE: Update on jar lid liner issue</font><br>
<hr width="100%" size="2" align="left" noshade style="color:#8091A5; "><br>
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<tt><font size="2">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.<br>
<br>
Scott, might this contribute to the deterioration of the liners in Paul's collection?<br>
<br>
Cathy<br>
Catharine Hawks<br>
Conservator<br>
National Museum of Natural History, MRC 106<br>
Research & Collections, NHB 394<br>
Smithsonian Institution<br>
PO Box 37012<br>
Washington, DC 20013-7012<br>
Office 202.633.0835<br>
SI Cell 202.701.8458<br>
CH Cell 703.200.4370<br>
hawksc@si.edu<</font></tt><tt><font size="2"><a href="mailto:hawksc@si.edu">mailto:hawksc@si.edu</a></font></tt><tt><font size="2">><br>
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<br>
________________________________<br>
From: nhcoll-l-bounces@mailman.yale.edu [nhcoll-l-bounces@mailman.yale.edu] On Behalf Of Paul Callomon [callomon@ansp.org]<br>
Sent: Wednesday, August 01, 2012 1:33 PM<br>
To: NH-COLL listserv (nhcoll-l@mailman.yale.edu)<br>
Subject: [Nhcoll-l] Update on jar lid liner issue<br>
<br>
Colleagues,<br>
<br>
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.<br>
<br>
<br>
- 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.<br>
<br>
- 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.<br>
<br>
- 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.<br>
<br>
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.<br>
<br>
<br>
Paul Callomon<br>
Collections Manager in Malacology, Invertebrate Paleontology and General Invertebrates<br>
The Academy of Natural Sciences of Drexel University<br>
1900 Benjamin Franklin Parkway<br>
Philadelphia, PA 19103<br>
callomon@ansp.org<br>
Tel. 215-405-5096<br>
ansp.org<br>
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