A TRANSLUCENT WAX-RESIN FILL MATERIAL FOR THE COMPENSATION OF LOSSES IN OBJECTS
SUSANNE GÄNSICKE, & JOHN W. HIRX
2 COMPONENTS OF THE FILL MATERIAL AND THEIR PROPERTIES
Polyvinyl acetate resins are used variously by conservators as adhesives and varnishes (Horie 1987). PVAC AYAC is the primary binder and adhesive in the fill mixture. It is clear and photochemically relatively stable, and it has a molecular weight of 13,000 (Union Carbide Corp. 1982, 1989). Its primary industrial application is as a chewing gum elastomer (Reid 1994). It is soluble in a variety of organic solvents and has well-documented reversible characteristics that have been reviewed (Feller et al. 1978). The softening point of PVAC AYAC is 160°F (71°C), and it must be heated above 450°F (230°C) before it will begin to char.
This resin, like the other PVAC resins, does not have a specific melting point; rather, it softens as the temperature rises until it eventually becomes a viscous flow. This characteristic is based on the fact that PVAC resins are blends rather than a specific molecule with a specific melting point (Reid 1994). Their glass transition temperature (Tg), which has been defined as “the temperature at which a material changes from a solid, glassy state to a softer rubbery state” (Schilling 1989, 110), is about 61°F (16°C).
The ethylene acrylic acid copolymers are materials of low molecular weight (2000–2500) that are used as additives in adhesives and coatings and in hot-melt adhesives (Domine and Schaufelberger 1977). They are also emulsifiable in water-based systems (Huff 1995). The softening points are below 221°F (105°C) for A-C 540 and 203°F (95°C) for A-C 580 (Allied Signal Corp. 1993). These resins have some solubility in acetone; and there is no free acid present in the EAA copolymers. Their acrylic acid component gives these waxes increased toughness and better adhesion to polar substrates than PVAC resins. To the wax-resin they lend both translucency and toughness.
At elevated temperatures polymers are susceptible to thermo-oxidative degradation; hot-melt adhesives are particularly prone to this degradation due to the method of their application. The addition of an antioxidant prevents yellowing of the wax-resin at high temperatures, either while melting or on the spatula tip.
There are two basic classes of stabilizers: primary or chain-terminating antioxidants and secondary hydroperoxide-decomposing antioxidants (Ciba Geigy Corp. 1990, 1993; Earhart et al. 1994). Primary antioxidants have also been described as radical scavengers because they react with peroxy and alkoxy radicals to form phenoxy radicals, which are relatively stable (de la Rie 1988a, 1988b). The Irganox 1076, a primary hindered phenolic antioxidant, was chosen for this formula because its melting point of 50–55°C is below the formulating temperature of the blend. Irganox 1035, another primary hindered phenolic antioxidant, has also been used in this formula. It has a sulfur bridge that provides peroxide decomposition capability in addition to standard antioxidant functionality. It is also a slightly larger molecule than Irganox 1076 and has a melting point of 145–54°F (63–68°C); but de la Rie (1988a) cautioned that the use of sulfur compounds, such as Irganox 1035, in metal ion-containing materials might lead to darkening.
As Burke (1983) observed, the ratio of components can be altered according to the desired qualities of the fill material. He increased the PVAC AYAC content to harden the blend and to increase gloss, melting point, and viscosity. He noticed that increasing the EAA content increases the waxiness of the blend but also decreases the cold flow while simultaneously decreasing the melt viscosity. A-C 580 is softer and more adhesive than the A-C 540; varying the 540/580 proportion, therefore, affects those properties. Burke also experimented with adding an ultraviolet (UV) absorber, which appeared to prevent yellowing, but the UV absorber (Uvinul D-49) he used was strongly colored and imparted a slight tint of its own.