Alternative to xylene

Smadar Gabrieli <smadar.gabrieli [at] uwa__edu__au> writes

>Raymond A. Spiteri <raymond.a.spiteri [at] gov__mt> writews
>
>>We are seeking a less hazardous alternative to xylene to be used in
>>solvent mixtures and for dissolving acrylic resins like Paraloid B72
>>for eventual use in conservation treatments. Can anyone suggest, or
>>has used, a viable alternative. Has anyone used 'Clarify' as an
>>alternative to xylene, and if so, is it a viable alternative.
>
>I don't know of a substitute to xylene for all applications, but
>since you specify B72, it is miscible in Acetone and in Ethanol, or
>a mixture of the two. So is B67. Change the proportion according to
>the necessary working time. When making decision about which one to
>use, check also for change of colour - in different situations one
>or the other may cause change, while the other doesn't.

To pick up on the posts about solvents for Paraloid B72, there
are--of course--quite a lot of different solvents for this resin.
However, solvent selection depends on a whole host of factors:
chemical and physical properties, health hazards, environmental
effects, and many more; not to forget what might be safe to use with
any given object and all kinds of application-related factors. It
all depends on what you are trying to do with the resin. For sure,
acetone will dissolve B72 quite well (ethanol less so), but it
evaporates very fast.  While that might (just) be OK for B72 used as
an adhesive, that probably wouldn't be OK for most coating
applications (ie. lacquer or varnish), or penetrating consolidant.

In the context of finding safer alternatives to xylene, for use with
B72 as a retouching medium for easel paintings, over 20 years ago I
suggested the glycol ether 1-methoxypropan-2-ol CAS #107-98-2) as
one possibility.  This cannot be regarded as harmless, with European
occupational exposure limits (WELs in the UK) of 100ppm/ 375 mg per
m3; but it is appreciably less hazardous than xylene (UK WEL 50ppm/
220 mg per m3). And 1-methoxypropan-2-ol is a doubly-functional
oxygenated solvent, which might mean that it is too 'active' for use
in certain applications on certain objects on account of risk to the
original material.

There are, it seems, quite a few solvent products out there that
offer themselves as 'xylene substitutes'; these are often intended
for use in fields such as histology, where xylene has traditionally
been used for solubilizing lipids.  I don't know anything about the
product 'Clarify', but in the United States I am aware of a couple
of 'xylene substitute' solvents offered by CBG Technologies in Ohio:
Formula 83 and Formula 78H. (Formula 78H, which has the higher
boiling point is marketed actually as a substitute for higher
boiling aromatic solvents of the high-flash naphtha type, eg.
Shellsol A100, etc.).   I'm sure there are probably quite a few
other products of this type out there. Basically, these solvents are
naphthenic (ie., cycloparaffinic) hydrocarbon blends.  Shell North
America no longer offers any products of this type:  its solvents
Cypar 7 and Cypar9 were discontinued some years ago.  In Europe,
there is a potentially useful similar product from ExxonMobil in the
form of Nappar 10, but how one might get hold of small quantities of
that, I don't know.

Naphthenics are composed of saturated cyclic aliphatic hydrocarbon
compounds, not aromatic hydrocarbons, which accounts for their lower
'harmfulness'; but they are less polar/polarizable than aromatics,
so have lower solvent power.  While they have the greatest solvent
power of the wider group of aliphatic hydrocarbons (which includes
also n-paraffinic and iso-paraffinic types) such naphthenic solvents
are not active enough in themselves to dissolve Paraloid B72 (I have
in front of me some in B72 in a jar of 78H, and it has not
dissolved.).  In principle, though, it is possible to create solvent
mixtures of a non-polar solvent like these naphthenic hydrocarbons
(or any type of mineral spirits) and an oxygenated solvent (such as
one of the glycol ethers or ether esters) that would have sufficient
solvent power to dissolve B72.  However, I would not yet propose
trying that option until the research is done; one key factor is
balancing the evaporation rates of the components so that relatively
constant solvent composition is maintained throughout drying, in
order to prevent the resin precipitating out prematurely.  We are
presently in discussions with Dow Coatings Materials, makers of the
resin, on the specific topic of alternative solvents (and blends)
for Paraloid B72.

Alan Phenix


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                  Conservation DistList Instance 25:38
                 Distributed: Monday, February 20, 2012
                       Message Id: cdl-25-38-005
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