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<DIV>Hi Scott,</DIV>
<DIV> </DIV>
<DIV>Many thanks for the very useful information.</DIV>
<DIV> </DIV>
<DIV>Cathy</DIV>
<DIV> </DIV>
<DIV><FONT lang=0 size=2 face=Arial FAMILY="SANSSERIF" PTSIZE="10">Catharine
Hawks<BR>Conservator, NMNH<BR>(h) 703.876.9176<BR>(o) 202.633.0835<BR>mobile
703.200.4370<BR>mobile 202.701.8458<BR>
<DIV> </DIV>
<DIV>
<DIV>In a message dated 8/2/2012 11:20:22 A.M. Eastern Daylight Time,
Scott.Williams@pch.gc.ca writes:</DIV>
<BLOCKQUOTE style="BORDER-LEFT: blue 2px solid; PADDING-LEFT: 5px; MARGIN-LEFT: 5px"><FONT style="BACKGROUND-COLOR: transparent" color=#000000 size=2 face=Arial>
<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><BR><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><BR><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><BR><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 title=http://www.conservation-us.org/_data/n_0001/resources/live/02_jan_aicnews.pdf href="http://www.conservation-us.org/_data/n_0001/resources/live/02_jan_aicnews.pdf"><FONT color=#0000ff size=2 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 title=http://cool.conservation-us.org/byauth/williams/foam.html href="http://cool.conservation-us.org/byauth/williams/foam.html"><FONT color=#0000ff size=2 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><BR><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 SRC="cid:X.MA1.1343948034@aol.com" 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" width=16 height=16 DATASIZE="105" ID="MA1.1343948034" ><FONT color=#424282 size=2 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><BR><FONT color=#5f5f5f size=1 face=sans-serif>From: </FONT><FONT size=1 face=sans-serif>"Hawks, Catharine"
<HawksC@si.edu></FONT><BR><FONT color=#5f5f5f size=1 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 color=#5f5f5f size=1 face=sans-serif>Cc: </FONT><FONT size=1 face=sans-serif>"Scott.Williams@pch.gc.ca"
<Scott.Williams@pch.gc.ca></FONT><BR><FONT color=#5f5f5f size=1 face=sans-serif>Date: </FONT><FONT size=1 face=sans-serif>2012-08-01 07:21
PM</FONT><BR><FONT color=#5f5f5f size=1 face=sans-serif>Subject: </FONT><FONT size=1 face=sans-serif>RE: Update on jar lid liner issue</FONT><BR>
<HR style="COLOR: #8091a5" align=left SIZE=2 width="100%" noShade>
<BR><BR><BR><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 title=mailto:hawksc@si.edu href="mailto:hawksc@si.edu">mailto:hawksc@si.edu</A></FONT></TT><TT><FONT size=2>><BR><BR><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>Follow us: Facebook<</FONT></TT><TT><FONT size=2><A title=http://www.facebook.com/AcademyofNaturalSciences href="http://www.facebook.com/AcademyofNaturalSciences">http://www.facebook.com/AcademyofNaturalSciences</A></FONT></TT><TT><FONT size=2>> | Twitter<</FONT></TT><TT><FONT size=2><A title=http://twitter.com/#!/AcadNatSci href="http://twitter.com/#!/AcadNatSci">http://twitter.com/#!/AcadNatSci</A></FONT></TT><TT><FONT size=2>><BR>Join us as we celebrate the Academy’s 200th anniversary with a
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