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Re: [BKARTS] PVA: beware?

During the 1960's, the W.J. Barrow Research Laboratory
conducted a series of hallmark experiments starting with
the causes of deterioration of book papers and continuing
with the general study of other factors causing the deterioration
of books.

In 1965 he published the results of his studies of PVA in
"Permanence/Durability of the Book-IV, Polyvinyl Acetate
(PVA) Adhesives for use in Lbrary Bookbinding."

In it, he described in meticulous, but intelligible detail,
the procedures and equipment he devised to test
six commercial PVAs and eleven special formulations,
all intended to be used in hand bookbinding.

He also discussed a procedure for testing the acidity
of aged, dried products.

Among his conclusions were that (quoting) :

- Calcium carbonate and calcium acetate were used
  to buffer the undesirably low pH which is common
  to nearly all PVA adhesives. Heat-aging tests indicated
  that these additions increased the lasting qualities of
  the unstable formulations but had almost no effect on
  those of very high stability. However, these additions
  have the advantage of producing producing a near-
  neutral adhesive, suitable for use with paper of good
  lasting qualities.

- Other tests indicated that the acidic volatile decompo-
  sition products (acetic acid) resulting from the aging
  of PVA adhesives escape quickly into the atmosphere
  and that it is consequently doubtful whether damage
  will result to books to which these adhesives are

- The increased viscosity of PVA adhesive required
  for certain binding operations can be achieved by
  the addition of methyl cellulose without endangering
  the stability of the adhesive. Since methyl cellulose
  is of value only during application and since it
  does not increase either the stability or strength
  of the adhesive, it should be used sparingly,
  especially in the areas (e.g., the spine) requiring
  high flexibility for long periods of time.

- While in the absence of more natural-aging experience
  of PVA adhesives than is now (sic.) available it is
  hazardous to equate heat-aging with natural-aging, yet
  a conservative con­jecture suggests one day of heat-aging
  at 100° C. as the equiva­lent of 5 years of natural-aging.
  Presuming this to be the case, the very stable adhesives
  identified by the study may be expect­ed to have a longevity
  of not less than 450 years, which should qualify them, with
  respect to the characteristic of stability, for library use.

In his discussion of the aging characteristics of PVA
adhesives, he noted four principal mechanisms:

- Loss of plasticizer - causes embrittlement - can be
  accelerated at high temperatures (a storage consideration)

- Oxidation - causes embrittlement - typically accelerated by
  tackifying resins (such as some rosin derivatives)

- Polymerization - breaks down and causes embrittlement -
  accelerated by high temperatures (a storage consideration)

- Hydrolysis - occurs at moderately alkaline pH (even 8.0) and
  in very acidic conditions, causing loss of strength - may be
  an environmental concern.

I also note that the materials Barrow noted for their
"high degree of stability" and "remarkable performance"
had pH measurements ranging from 4.48 to 6.48 all of
which increased with heat aging.

Alan Shalette 

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