36.2 Chemistry and research- keeping a close watch on vinyl based resins

15/11/13

Despite the entry of synthetic resins such as aliphatic, urea-aldehyde, polyoxazole, that are characterized by high levels of stability and reversibility, vinyl based resins are still widely used in restoration because they gave birth to a multitude of adhesives and varnishes and consolidants that keep on excelling for specific applications. Their strength is the adhesive property, in fact Beva 371 is the most famous product. Nevertheless we cannot leave out of account white glue (Vinavil) for woody features and polyvinyl alcohol used as fixative for paint films of murals.

Besides, we should mention minor resins such as: K40 and K60 adhesives for pottery or ceramics repair; Mowilith for fabric; Mowital B60HH for bonding and consolidating ceramics; the others products based on Beva (Gel, D-8-S and related Film grades); and finally Peoval 33 as a good additive for hydraulic lime based grouts (Bollettino CTS 25.3).
What 
mostly concerns is the fact these products could interact with the pigments of the painting layers. Metallic cations may coordinate with hydroxyl groups inside polyvinyl alcohol and also in PVA. Especially along the aging process, acetate groups of PVC, because of hydrolysis, leave the main chain and generate acetic acid and hydroxyl. The coordination bond between hydroxyl groups and metallic cations of the pigments reduces solubility and irreversibility as a consequence.

These interactions have been studied by Virgilio Vecchio, a student of Materials Science for Restoration at the University of Florence. His findings were presented during the master degree dissertation[1] under the supervision of the scientist Antonella Salvini.
The research was focused on four vinyl based compounds available on the market. Among these, three were chosen because their important connection with the preservation of the artistic heritage and they are as follow: 
Mowital 60HH (PVB vinyl butyral polymer) resin for archeological restoration for bonding and consolidating ceramics; Mowiol 30-92 (PVOH polyvinyl alcohol) protective varnish for metals and consolidant for paint film; BEVA 371 (made with different components, mainly based on two ethylene vinyl acetate co-polymers of different vinyl acetate contents) heat-seal adhesive paste for the lining of paintings.

Finally they chose a water dispersion of ethyl vinyl acetate in order to combine the properties of aqueous adhesives with those ones of EVA resins such as BEVA 371. They prepared four different coatings made of pigments dispersed in a binder or just the binder. In such a way they could study the interactions between those compounds and the pigments and check any change in chemical and optical properties, paying attention to any solubility decrease. They selected classical binders: rabbit-skin glue; linseed oil; ammonium caseinate and cherry tree 
gum. They chose metallic ions (two or three valency) based pigments, the most commonly used in the ancient paintings. The aging treatment was carried out for 300 hrs under Solar Box Xeno 3000 according to UNI 10925 and after cutting off UV radiations below 315nm.

The colorimetric measures demonstrated that keeping aged samples indoor and exposing them to the light has no effects on the color changing on the vinyl coating.
FTIR analysis 
revealed that artificial aging does not produce relevant variations on the samples.
Finally, proton nuclear magnetic resonance 1H-NMR did not detect any significant effect of aging treatment on change in the solubility of vinyl resins. The solubility is more likely affected by the interactions with pigments and binders especially for PVOH and for PVB to a smaller extent.

The most significant results about PVOH are reported below:

Binder                    Pigment         PVOH solubility %          PVB solubility %
    -                            -                       100                               100
Cherry tree gum          -                       49                                  79
                             Gypsum                60                                  99

                            'Robbia'red              71

                             Azurite                  74
Rabbit skin glue           -                      69                                  91
                             Gypsum                57                                  98

                            'Robbia'red              65

                             Azurite                  68

The cherry tree gum decreases the solubility of PVOH (49%) and PVB (79%) much more than how pigments do. They noticed a moderate effect on samples prepared with rabbit skin glue (respectively 69% and 91%). When samples are made of gypsum, the solubility of PVOH drops to 57% due to the coordination bonds of Ca²+.
Despite PVB is more reversible than PVOH, when aqueous dispersion EVA is adopted, there is no decay in the solubility and that report makes us confident about future application on restoration projects. On the other side, 1H-NMR analysis was not able to corroborate the reversibility of the Gustav Berger's patent (does it need that?), because its complex formula.

These findings turn into interesting premises if we consider the possibility to study the treated samples for a long time (some years). Keep in mind the specimen were subjected to a short term investigation because of the master degree deadline.


References
1-Vecchio V; "L’influenza dei pigmenti sul comportamento di alcune resine viniliche utilizzate nel restauro”, Degree Thesis, University of Florence, Materials Science for Restoration study-program, 2013.

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