Paraloid B-72

Perry Hurt <phurt [at] ncmamail__dcr__state__nc__us> writes

>Eleonore Kissel <eleonore.kissel [at] conservationpreventive__com>
>writes
>
>>... does anyone have an idea why titanium white (and not, for
>>instance, zinc white) has been recommended in previous the National
>>Parks Services Conserv-O-Gram, at least one Conservation DistList
>>posting (Stephen Koob, 2002) as well as personal communications I
>>have had with colleagues, when making a white opaque
>>Paraload/Acryloid B-72 solution?
>
>It may be that titanium white is suggested as a white bulking
>material because of its chemical stability in most situations.  Zinc
>white is not recommended for resin based application because of its
>observed adverse affect on inpainting.  Over time zinc white
>fractures the resin matrix causing the inpainted area to appear much
>too light or white in color.

Both zinc white (ZnO) and titanium white (TiO2, especially anatase)
are photo-reactive pigments, i.e. they absorb UV (and some violet
light) and become chemically reactive.  Their photo-excited states
can lead to the generation of aggressive chemical species in the
presence of oxygen and humidity (e.g. hydroxyl and hydroperoxyl
radicals).  These species can then migrate into surrounding media
causing degradation in paper, resins, and paints.  For instance, see
V. Daniels article on paper discoloration due to zinc white
(Restaurator (1990) v 11, n 4, pp 236-243) or P. Whitmore's work on
both ZnO and TiO2 in acrylic inpainting medium (Cleaning Retouching
and Coatings (1990), IIC-London, pp144-149).  These reactions are
also covered in the appropriate volumes of the Artists' Pigments
series.

However, the TiO2 used in artists' materials is almost always coated
with a double layer of silica and alumina (you can see this when
examining the pigments with SEM).  It is my understanding that this
is less common in ZnO.  The Si and Al oxide protective layers
prevent the excited state pigment from affecting the nearby medium
due to the physical barrier provided by the inert coatings.  So, the
higher grade TiO2 pigments actually act to protect the medium by
absorbing the majority of the UV light and then safely dissipating
it as heat, thus preventing UV absorption in the acrylic coating
itself.  Spathis et al. report such results in films of B72 (Studies
(2003) v48, pp 57-64) and Whitmore and Bailie (vide infra) find
similar protection from crosslinking in nBMA coatings containing
some brands of TiO2.  These coated pigments are often used as well
in outdoor structural materials like vinyl and styrene siding in
which one can achieve incredible longevity with only minute
concentrations of quality titanium white pigment.  So, aside from
the practical aspects (like RI) in choosing TiO2 over ZnO, there may
well be good chemical reasons for doing so.

Dr. Gregory Dale Smith
Andrew W. Mellon Assistant Professor of Conservation Science
Buffalo State College
Art Conservation Department
1300 Elmwood Ave., RH#230
Buffalo NY 14222
716-878-4646 office
716-878-5025
Fax: 716-878-5039


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                  Conservation DistList Instance 19:29
                 Distributed: Tuesday, December 6, 2005
                       Message Id: cdl-19-29-004
                                  ***