Laropal K-80

Dennis Baltuskonis <dab.conservation [at] sbcglobal__net> writes

>I have two important questions related to the commercial resin known
>as Laropal K-80 (formerly called Ketone-N resin).
>
>Laropal K-80 is a "homo-polymer", the product of a chemical
>condensation of cyclohexanone with itself (MSDS sheet).  It is/has
>been used as a final varnish for old master paintings and more
>importantly represents approximately 30% by weight of the adhesive
>formulation known as "BEVA-371".

This posting contains several statements and queries but seems to be
asking one simple question. The answer to that question requires
answering another question: Why was Laropal K-80 incorporated into
the formula and what does it do?

Perhaps the formulator is the best person to answer that and this
response is not a commentary on the worthiness or otherwise Beva
371. There is ample discussion of Beva in the literature. It was
designed with one task in mind--to provide an adhesive that is as
flexible as a stretched canvas, can be applied with minimal solvent
and removed (or at least a canvas peeled) with solvents or heat. To
achieve this the formulator chose EVA resins, which are, on the
whole, barely soluble in the solvents conservators use (as are some
of the tackifiers and other ingredients), but can be thermally
activated at low temperatures. By using aromatic solvents the
adhesive melange can be maintained as a gel and further diluted to
make workable liquid adhesives.

None of these design properties are enhanced by a brittle resin like
cyclohexanone but it does improve solubility options and can alone
be dissolved in the petroleum solvents variously called white spirit
and mineral turpentine. The Beva gel requires aromatics both for
dilution and (hopefully) removal at a later stage. Toluene is not
the conservator's best friend when spread over a broad surface.
There are other resins that can be dissolved in nonpolar solvents
such as the mixed alkanes while at the same time being more
flexible. Isobutyl methacrylates (IbM) have been regarded as a
suitable resin for conservation, manufactured under the names
Paraloid B67 and Elvacite 2045. These resins are soluble in pure
alkanes and are considerably more flexible than cyclohexanone, hence
possibly better suited to a flexible adhesive for linings etc.
Dispersed varieties of these acrylics have been used as lining
adhesives for some time.

IbM is less thermally active than Laropal but more stable in the
long term. Other acrylics such as the acrylate variants have very
low melting points and may reinstate the desired flexibility if
necessary but they are on the whole very susceptible to cross
linking. A resin with a glass transition temperature below ambient
is more prone to cross link than those of higher Tg. Methyl acrylate
is the softer component in Paraloid B72 and contrary to conservation
opinion, an enthusiastic cross linker once its Tg of 6 degrees is
exceeded.

Having pondered all of these factors when wanting an adhesive with
similar thermal properties to Beva, I have blended a highly flexible
EVA with IbM to achieve a partially resoluble and suitably tacky
adhesive.This has occasionally been used for permanent adhesion but
more often as part of a temporary facing system. In this case I did
not require strong adhesion with easy separation being a preferable
property.

So back to the question of the role of Laropal K-80, summarised as
solvation and moderate thermal activity--the answer is that there
are several alternatives but to bring these to a commercial
formulation requires a formulator with a similar dedication to the
creator of Beva. The profession is possibly less receptive to highly
complex resin blends now than it was 36 years ago.

Andrew Thorn


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                  Conservation DistList Instance 21:46
                  Distributed: Thursday, March 6, 2008
                       Message Id: cdl-21-46-004
                                  ***