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Re: [BKARTS] PVA, internally plasticized



Perhaps the term "internally plasticized" is poorly chosen since I would
expect the alternative ("externally plasticized"???) would have no
effect on the structure of the dried glue. If the plasticizer isn't *in*
the vinyl resin I don't see how it could have any effect.

I think that all PVA is plasticized to varying extents; padding glue is
essentially a PVA plasticized to the point that it is soft an rubbery.
The plasticizers are added to the vinyl resin to control the flexibility
and strength of the dried glue. The problem is that many plasticizers
are volatile and gradually diffuse out of the hardened plastic (the
dried glue is essentially a film of vinyl). This is one thing that
contributes to that "new car smell" and why not all plastic containers
are food-safe.

The loss of plasticizers is one reason why plastic objects tend to
become brittle as they age. The other major reason is UV damage. Both
these effects contribute to your shower curtain example, the diffusion
being aided by the large amount of surface area.

As mentioned in another posting, the dried film does not have a pH per
se (nor for that matter do dry paper, thread, or leather), but it does
have an effect on the pH of any moisture that contacts it. Furthermore,
because of the release of gases during aging (not only plasticizers, but
also emulsifiers and stabilizers from the original liquid and breakdown
products of the vinyl itself) these gases can affect the pH of moisture
at a distance. Thus the glue used in a sealed (or semi-sealed) display
case can also be a cause for concern.
-Kevin Martin
 the Papertrail 



> -----Original Message-----
> From: Book_Arts-L [mailto:BOOK_ARTS-L@xxxxxxxxxxxxxxxx] On Behalf Of
Nancy Nitzberg
> An issue which I don't believe has yet been mentioned in regard to the

> quality of PVAs is that of "plasticizers."   
> 
> While this information may be somewhat dated, it was the case that the
PVAs
> sold by conservation suppliers were "internally plasticized," meaning
that 
> their flexible qualities were molecularly locked in.   

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