BEVA 371

This is in response to Chantal Bernicky's inquiry concerning the
yellowing of Beva 371. Beva 371 was included in the Canadian
Conservation Institute (CCI) testing program and yellowing was
studied. It was found that visible discoloration (degree of
yellowing = 0.10) was perceived at about 6 months of light aging
(700-800 lux, 1905W/lm) and at about 10 years of dark aging.
Extrapolation of the curves suggests that strong discoloration
(degree of yellowing = 0.25) might occur within 80 to 120 years.
These results are for a 0.1 mm thick film. One can see with the
naked eye that the light aged samples are markedly yellow. One must
remember that the thinner the film the less the yellowing is visible
because yellowing is proportional to the thickness of the film. So
depending on the application and if the film is thin enough, this
yellowing may or may not be a problem.

There has been some discussion as to why this yellowing occurs. Some
say it might be the grade of toluene used to make up the Beva. A
lower quality solvent giving more yellowing than a higher quality
solvent. However, we have a bag of Beva 371 pellets (age circa 1986)
that was given to us by the National Archives of Canada and some of
the ingredients are markedly yellow even before any solvent has been
added, leading us to think that it may not be a solvent quality
issue at all. Others have told me that the manufacturer has solved
this problem and Beva no longer yellows....this we have not
confirmed. Unfortunately, the mechanism for the Beva yellowing
phenomenon has not been studied by CCI.

It would seem to me that use of Beva 371 would be application
dependent. Applications where very thin films of Beva are required
might be acceptable, especially if the bond will not be exposed to
light. Applications where very thick films of Beva might be exposed
to light would not be recommended.

I hope this is useful. If anyone would like to discuss this or any
adhesive question further, please do not hesitate to contact me.

Sincerely,
Jane Down
Senior Conservation Scientist,
Canadian Conservation Institute
613-998-3721
Fax: 613-998-4721


                                  ***
                  Conservation DistList Instance 14:41
                 Distributed: Monday, January 29, 2001
                       Message Id: cdl-14-41-001
                                  ***