Yellowing of polyethylene bags

Laura H. Nightengale <lnightengale [at] mail__utexas__edu> writes

>Many of our artifacts are curated in a climate-controlled room in
>very tight cabinets. Within the cab's many of the individual
>artifacts are enclosed in zip-top 4 mil thick polyethylene bags.
>Over the years we have noticed that some of the bags and the paper
>tags inside them have turned a yellow/green, especially, but not
>limited to, where the artifacts are touching them. This process can
>take as little as 4 months.

I came across a paper once that discussed the origin of yellow
markings on white fabrics whilst in storage. As I remember, the
yellowing on the fabric was especially apparent around holes in the
plastic bags. It took a few studies and some years to discover the
cause: Apparently, a common additive to the polyethylene called BHT
(a common anti-oxidant, also I believe found in corn flakes- go
figure) was migrating to the surface of the plastic over time, and
being deposited on the fabric. While on the fabric, it was reacting
with nitrogen compounds in the air (pollution coming through the
breaks in the bag), and breaking the long BHT molecules into shorter
molecules. The shorter molecules were yellow in colour.

The only other things I remember are that the yellowing process
could be reversed by placing the objects into bright sunlight, and
that the paper might have been written by a chemist at Dupont.

Wish I could remember more, but this might be related to the problem
you describe. The larger problem we all face is that we don't
usually know what additives are in most plastic films. Laura
Nightengale does not mention the provenance of the bags. I strongly
suggest that anyone using plastic bags for long term storage be
certain they know who made the plastic, and what additives are in
it.

Jerry Shiner
Keepsafe Systems
Supplies and Solutions for Microclimate and Oxygen-free storage
800-683-4696
Fax: 416-703-5991
Object and Textile Conservation Services Ltd.


                                  ***
                  Conservation DistList Instance 15:65
                  Distributed: Tuesday, March 26, 2002
                       Message Id: cdl-15-65-008
                                  ***