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Subject: Acrylic adhesives

Acrylic adhesives

From: Geoffrey I. Brown <gibrown>
Date: Monday, April 3, 1995
Thanks to Jane Down for finally solving the question of the resin
content of 3M's #4475 adhesive.  In 1972, when this material first
surfaced as part of an adhesive research project I was doing, most U.S.
manufacturers were refusing to give specific information about their
proprietary products.  Of course, that predated the advent of MSDS's and
the mandating legislation.  Since there was no choice then but to accept
the informal information supplied by a 3M Technical Rep, extended
empirical testing was used to augment the paucity of information.

Knowing the resins explains a number of unanswered questions such as the
superior bond strength relative to acrylics, the more rapid resolubility
in acetone, the higher working temperature, and (partially) the peculiar
color characteristics.

This product appears to be one of those materials whose total is greater
(or different) than the sum of its parts.  I also question polyurethanes
as they have a history of long-term insolubility and instability,
although their bonding to problematic substrates can be excellent.
PVA's have a long history of use in conservation and their main
drawbacks are excessive hygroscopicity which cause them to become tacky
in higher humidities, limited operating temperatures, and occasional
tendency to yellow.  PVC's tend to be rare in the adhesives used in
conservation, although the solid resin has proved to be quite stable in
many adverse environments; it shares the sensitivity of most resins to
UV light, however.  Most concerns about PVC seem to relate to the
possibility of free vinyl chloride monomer (which is quite toxic) or
break-down of the polymer to release chloride ions.  The potential for
these trace materials is, I believe, fairly low and the amount of such
trace materials would also be very low unless the adhesive were
subjected to very unusual and extreme conditions that would be extremely
deleterious to the objects bonded with 4475.

In this formulation, I would worry more about the polyurethane component
than the PVC.  The overall characteristic of 4475, however, is of an
"enhanced" and stabilized poly vinyl acetate.  Again, I wish to point
out that I now have 20+ years of experience with this product, including
having to reverse bonds done many years before.  Discoloration does not
seem to be a problem once the adhesive has dried, although I rarely
leave visible adhesive at joints.  The last batch that I purchased which
is now about 2-1/2 years old (the lab was closed for two years for a
building-wide renovation) has not colored in the tube, indicating an
improvement in manufacture/ingredients, perhaps.

I continue to recommend this adhesive for a variety of uses, although it
must be used with care, knowledge and judgement, the same as any other
product used in conservation.  I refuse to accept that *any* product is
"all good" or "all bad", but each may have benefits and bonafide uses
and each will also have drawbacks and risks.  We know, in fact, very
little about the long-term behavior of most of the materials we use--to
a great extent we trust our gut-feelings and hope that the limited
amount of research that has been done on these products has some
long-term relevance.  Remember soluble nylon?  It was, indeed, too good
to be true. Even the 50+ years of experience we have with the acrylics
does not qualify as long-term for those of us who deal daily with
adhesives that are hundreds of years old.

3M #4475 has some unique uses, among the more common ones: Bonding
alabaster, marble, thick glass (very limited strength), steatite
(soapstone), Velcro hook tape to wood battens for support of textiles.
It's only fair as a porcelain adhesive (similar to glass) but excellent
on most porous ceramics.  It's excellent on wood but don't forget about
the effect of the solvent on surface finishes.  4474 is definitely not a
gap-filler adhesive.  Let's not forget some of the more practical
problems too, such as wet-or-dry abrasive paper to blocks or repair of
shoes (how often have you had a staff-member arrive at the lab with a
broken sandal or detached sole and ask for "something to get me home"?)

A number of list subscribers have messaged an inquiry about where to get
4475.  Most adhesive dealers who represent 3M can get it.  I have been
successful finding it through AIN Plastics, although most branches don't
carry it in tubes.  I believe that my last order came from the Chicago
branch via Ain of Michigan.  I assure you all that you want to buy
*only* the 5 oz. tubes and not pints or quarts.  This stuff must be
controlled, both for mess and solvent evaporation.  The last price was
about $7.50 U.S., but it should be used fairly sparingly, so a tube will
give you good mileage (is there an equivalent term for kilometers?).

I recommend good ventilation and perhaps a local fume exhaust or fan
that keeps the MEK vapor away from your breathing zone.  If you work
cleanly, you shouldn't have too much trouble with skin contact.  Gloves
are problematic because 4475 will stick well to most types.

With most objects, I apply a thin layer to one side of the joint and
quickly join both parts before the adhesive can skin over.  Exert the
maximum pressure that is safe to squeeze out excess.  In most cases, it
is better to let the squeezed-out beads dry as-is and then scrape them
off with a fingernail or similar tool when dry.  You can use a swab to
"roll-up" the surplus, but try not to spread it around to minimize
clean-up and potential abrasion to the surface.  Acetone is an excellent
reversing solvent--soaking is best to remove totally from deep joints,
but surface smears and shallow joints reverse easily with swabs
moistened with acetone.  If you are bonding Velcro to battens, use it as
recommended on the label: coat each side, allow to "flash" dry and press
together.

I will be interested to know your results and opinions if you try this
material.

    **** Moderator's comments:   Geoff wrote back later to add:

Since the discussion now includes safety issues, we should all be aware
that all polymers contain some fraction of unreacted monomer (single
molecules of the compound).  All the resins we use are polymers but many
we use in the monomer form, such as epoxies, 2-part polyesters, and
several acrylates.  Although the fully-reacted polymer may be innocuous,
virtually all of these monomers, whether as residual unreacted material
or the pre-cured resins, are toxic or irritating to some degree.  Most
of the monomers are in liquid form and produce noxious vapors.  The
various acrylates can be quite toxic and can give rise to acute
sensitivities. Sensitivity to epoxies is also extremely well documented.

We work with an array of materials that have high potential for
toxicity.  Whether the acrylics, epoxies, instant-glues, polyvinyl
chloride or whatever, we should be aware of the potential risks to
ourselves and the objects we treat.

Geoffrey Brown
Curator of Conservation
Kelsey Museum
University of Michigan
313-747-0439

                                  ***
                  Conservation DistList Instance 8:82
                   Distributed: Sunday, April 9, 1995
                        Message Id: cdl-8-82-002
                                  ***
Received on Monday, 3 April, 1995

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