Paraloid B-72 and Tinuvin

Carolina Araya <c.araya [at] centrocrea__org> writes

>In the Conservation Center I am working (CREA), we are discussing
>the use of stabilizers as Tinuvin 292, 1130 and 327
> ...
>As a Scientific Laboratory Director I would like to know if it is
>possible the use of a top coat of Paraloid B-72 with some of the
>Tinuvin additives to avoid the deterioration and yellowing of the
>Dammar layer already applied. What would be your opinion and advice?

The following is a precis of an e-mail response sent to a previous
Cons DistList inquiry regarding UV barrier top coats for paintings:

Paraloid B-72 as a varnish top coat will not protect a dammar film
from yellowing and deterioration.   Tinuvin 292 is not a UV absorber
but a polymer stabilizer will be effective slowing deterioration of
dammar only when UV exposure of the film is controlled (note
research published by Dr. Rene de la Rie).  The Tinuvin 1130 is a UV
absorber and can be added to a Paraloid B-72 top coat to form a UV
barrier film.  Because this product is a solution of the absorber in
13% polyethylene glycol, I recommend the concentration to be
approximately 3.5% to 4% of the weight of the liquid additive to the
weight of the B-72 resin.  I have, however, a few reservations
regarding the 1130. The spectra indicate that this absorber does not
fully protect in the long wavelength UV region, thus it is not the
most efficient absorber to use.  I would recommend using Tinuvin 327
(powder form with no PEG added to the product).  Measurements of the
transmission/absorption spectra indicate that Tinuvin 327 cuts off
the UV more efficiently from about 370 to 400nm than the Tinuvin
1130, thus in samples tested with both additives in acrylic top
coats, there was less yellowing and physical deterioration of the
dammar when Tinuvin 327 was used.  You could use the Tinuvin 292 in
the dammar varnish coat as an added measure of protection.  (There
is no reason to add it to the B-72 top coat.)

The Tinuvin 327 additive must be added to the B-72 solution.  The
recommended amount is 3% of the weight of the resin.  The top- coat
must not be applied until all of the residual solvent has evaporated
from the dammar varnish coat, especially if you are using mineral
spirits or turpentine.  The off-gassing solvent can cause the
acrylic top coat to become milky in appearance.  This can be avoided
by using the same solvent for both the dammar and the B-72. When the
dammar is dry, the UV barrier top coat must then be spray-applied in
successive thin coats.  The painting can be examined under UV and
when it appears to be dark, i.e., no fluorescence is present, the
top coat is probably an adequate thickness.  I usually add 3-4 thin
coats and then examine the painting under UV light with the lamps
placed at a standard viewing distance of about 1-2 metres from the
painting surface.

After the top coat is dry, the surface should appear dark under UV.
The surface can be viewed by increasing the intensity of UV viewing
lamps (shorten the distance to the surface) or by lengthen the
exposure time of UV photography up to a 6-12 minute exposure, to
show re-touches, etc.

James Bourdeau
Conservator
Fine Arts and Architectural Services
Canadian Conservation Institute


                                  ***
                  Conservation DistList Instance 15:24
                Distributed: Tuesday, September 18, 2001
                       Message Id: cdl-15-24-003
                                  ***