Carbopol

Ritianne Psaila <gotharty [at] yahoo__com> writes

>I am trying to use gels made of Carbopol (Ultrez10) for cleaning a
>wall painting. However, I noticed that sometimes the gel tends to
>become liquid after a few seconds from its application and is
>readily absorbed  by the substrate.

I can't say that I have worked with Carbopol poly(acrylic acid) gels
much, but they seem to be fairly similar to the polyacrylate
thickeners used in acrylic paints (Acrysol type).  These also
thicken in alkaline conditions.  The higher viscosity is due to
polymer expansion from electrostatic repulsion of now deprotonated
acrylic acid groups.  All those negative charges are trying to get
as separated as possible, creating a greatly expanded polymer
volume.  These can be susceptible to divalent cations like Ca+2
which associate with the negatively charged acrylic acid groups
causing the gel to collapse into insoluble salts--it's the same
association process in hard water regions responsible for soap scum.

However, the gelling ability of the polymer is also directly related
to the pH of the solution: essentially you are relying on there
being lots of deprotonated acrylic acid groups.  Your range of pH
5-8 would suggest that you are just on the verge of making the
acrylic acid groups deprotonate.  Such a wide range also suggests
that you might be using the pH to adjust the thickness instead of
using varying polymer concentrations to do that.  When the solution
is applied to the painting, a near neutral pH may be adjusting to a
lower value (more acidic) and thus protonating the acrylic acid
groups that are responsible for the higher viscosity and thereby
causing the gel to collapse into a running liquid.

If your surface can tolerate it, try using a higher pH (or at least
stay consistently on the high end of your pH range) and then adjust
the amount of Carbopol to achieve the viscosity you want.

Gregory Dale Smith, Ph.D.
Andrew W. Mellon Assistant Professor of Conservation Science
Art Conservation Department
Rockwell Hall #230
1300 Elmwood Avenue
Buffalo NY 14222
716-878-4646
Fax: 716-878-5039


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                  Conservation DistList Instance 20:24
                 Distributed: Friday, November 3, 2006
                       Message Id: cdl-20-24-002
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