Polyvinyl acetate

With regard to the question about PVA emulsions (and yes, I mean
polyvinyl acetate, which some refer to as PVAc to avoid confusion
with polyvinyl alcohol which I call PVOH but others call PVA,
yikes!), the expression "internally plasticised" might refer to a
co-polymer, nowadays usually an EVA (or polyethylene/vinyl acetate).

Look at it this way:  the PVA molecule is a bit like a comb, with a
polyethylene backbone and acetate pendant groups as the teeth. These
acetate groups, being somewhat polar, tend to attract each other,
making the structure (and hence the resin) sorta stiff.  In the old
days (before the beatles) it was necessary to add a viscous solvent
to the resin that would get in between the teeth and "plasticise"
the stuff.  Unfortunately, the union was not permanent and the
mixture tended to migrate apart.  Then somebody smart figured out
that if they made the PVA resin with a bunch of teeth missing here
and there (some parts polyethylene and some parts polyvinyl
acetate), the plasticiser would not be necessary and the resin would
be permanently flexible.  Hence an ethylene vinyl acetate copolymer,
or EVA, which is what the last three letters of Beva stand for, and
in emulsion form is found as Elvax 1874, CM-Bond M-4, and a slew of
others--internally plasticised, permanently flexible white glues.

As for their "archival-ness", it depends on what you mean.
Emulsions have always been less soluble after drying than their
straight resin relatives (something to do with polymerization in
aqueous conditions?).  Compare B-72 with Rhoplex AC-34, or compare
AYAA with Jade 403, or compare Elvax 150 (the EVA resin in Beva)
with CM-Bond M-4.  Does this mean they are less stable over time?
Hard to say considering these none of these materials have been
around more than a few decades.  You want permanently flexible, do
Yoga.

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                  Conservation DistList Instance 11:45
                 Distributed: Monday, November 17, 1997
                       Message Id: cdl-11-45-002
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