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Subject: Plastics


From: Doug Nishimura <dwnpph>
Date: Thursday, May 2, 1991
I read a request about PE plastic bags from some earlier posting and
though I would comment.

I was hoping that I could find another article that had the formula for
water vapor permeability half lives.  (Given an inside and outside RH
how long does it take the inside to reach halfway between its initial
RH and the outside RH).  Anyway, I can't find the article. 

I have other calculations mucked with however.

I would recommend that anyone using plastic enclosures or bags, know
their suppliers.  Many suppliers use various fabricators, who in turn
use different raw polymer suppliers.  In our experience, this could make
a big difference to how "good" the plastic is.

Use inherently safe plastics only.  These include things like
polyethylene, polypropylene, and polyester.  All of the plastics, even
inherently good ones, have potential problems.

Polyethylene: Use a high grade polymer.  Things like garbage bags often
contain scraps in the melt from other products (producing bags of
uncertain content).  Potential problems include loss of additives.
These additives may come out as a liquid or film.  Specifically, we have
seen slide pages with clear liquid coming out that softens the gelatin
enough to make it come right off of the film support.  Waxy films
usually seem to be slip agents used in manufacturer.  These are designed
to slowly come out of the plastic to leave a micro-thin layer of the
plastic.  However, in some cases, the content of slip agent is high
enough that too much comes out.  The substance is likely inert to most
objects (the wax usually being a long chain amine), but who wants wax on
their coins, stamps, photographs, etc.?

Be aware that polyethylene is not really a good water vapor barrier.
Even in articles on cold storage of color, Henry Wilhelm does not
recommend the use of PE freezer bags.  I did some calculations on two
zip lock bags we have in the lab.  One is a thin bag, holding 1 gallon.
The other is smaller - 1 quart, but with a thicker PE.  Several
assumptions were made:

a) The zip lock seal is perfect.
b) The bag was packed at 70 F, 50% RH.
c) The conditions outside the bag changed to 70 F, 100% RH
   (assuming raining, flood, etc.)
d) the calculation only deals with water VAPOR. (During the rain/flood
   the bag stayed on a protected, high shelf.)
e) The air pressure stayed at a constant 760 mm Hg.

A few more points:

A) The large bag is 0.00394 cm thick
   The small bag is 0.0060  cm thick

B) The total surface of the large bag is 1517.4 sq. cm.
   The total surface of the small bag is 729.8 sq. cm.

The initial permeability of the small bag is between 0.0028g/hr and
0.0074 g/hr.  This may not seem like much, but at 70 F it is enough
water to saturate between 148 and 392 milliliters of dry air. The total
bag volume is only about a liter and I would assume that it has a
reasonably small air volume remaining after filling. (For non SI people,
that translates to between 5/8 cup to 13.3 oz. (a little over a beer
bottle full.))  The larger bag is much worse since it is made from a
thinner PE and has a larger surface area.  The initial permeability of
the larger bag is between 0.0089 g/hr and 0.0233 g/hr.  At 70 F this
will saturate between a half and one full liter of dry air.  Of course
the permeability of the polyethylene decreases as the difference in
vapor pressure on the two sides of the PE film decreases, but none the
less, PE is not a great vapor barrier.  Tables of data from DuPont give
values of 0.3370 g/24 hours and 1.0622 g/24 hours for the small and
large bags respectively (if we assume that the same plastic was tested.)
These convert to 0.014 and 0.0443 grams per hour respectively.  Their
test was run in accordance with ASTM standard E96-63T.  In real life, I
know people with food dryers who expected dried food in zip lock bags to
stay dried.  They were quite mistaken and ended packing desiccant in
large bags and putting the smaller food bags inside.

Polypropylene:  These are very much like polyethylenes in terms of
recommendations and problems.  In this case, we have found that cast
films seem to have more problems associated with them than biaxially
oriented films.  It's possible that the difference in manufacturing
method allows the biaxially oriented product to get away with less
additives. Both CCI and DuPont show PP to have about 1/4 the water vapor
permeability of PE (water vapor has a harder time getting through PP.)

Cellulose Acetate:  Once considered to be a safe material, this is now
being questioned.  Certainly under good storage conditions, this product
is still quite good.  However, the sleeves I have seen are about 15%
plasticizer which may be a problem.  In addition, under poor storage
conditions, this plastic starts to hydrolyze and produce acetic acid
fumes.  The PLASTICIZER may also hydrolyze producing high acidity
analogs of phosphoric acid and phosphoric salts plus phenol.  The acidic
degradation products may catalyze the degradation of safety film or
other polymeric materials as well as attack paper (and other acid
sensitive objects).  The chairman of ANSI committee IT9 (Permanence and
Physical Properties of Imaging Media) is seriously considering balloting
the removal of cellulose acetate from the list of acceptable plastics.
Cellulose acetate has about 43 times the water vapor permeability of PE.

Polyester:  Generally a good material.  It is expensive however and
comes in more grades than is imaginable.  Be careful about the type of
polyester you use.  Tuck Taylor once mentioned that the stamp people
were excited about a heat-sealable polyester (Mylar M).  The problem was
that this polyester has been coated with PVDC (polyvinyladine chloride)
- a chlorinated polymer that is more stable than PVC but..... Polyester
also can have abrasive edges when simply cut, and it has rather high
static properties.  Avoid antistatic coated plastics. These plastics
have been coated with a humectant that produces a localized area of high
humidity around the bag.  Uncoated polyester has about the same water
vapor permeability as PE.

Tyvek:  Tyvek is a non-spun PE that therefore should be an inherently
good plastic.  Unlike regular PE films, however, Tyvek is made in upteen
ways for different applications.  Unfortunately there is no conservation
grade.  The distributors I have talked to who carry "archival" supplies
have all specified "uncoated, medical grade".  Likely, this meets the
needs of the field.  In our lab experience, under fairly aggressive
(abusive) handling, the Tyvek may produce a white powder when abraded
against photographic test materials.  Unfortunately, Tyvek is very hard
to write on without some kind of surface treatment and I don't know how
safe any of these treatments are. As yet, I have not heard of any
complaints about Tyvek enclosures.

                  Conservation DistList Instance 4:57
                   Distributed: Saturday, May 4, 1991
                        Message Id: cdl-4-57-006
Received on Thursday, 2 May, 1991

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