Gels from Borate-Crosslinked Partially Hydrolyzed Poly(vinyl acetate)s: Characterization of Physical and Chemical Properties and Applications in Art Conservation
Angelova, Lora Vihrova
Weiss, Richard G, Ph.D.
The formation of gel-like dispersions from borate-crosslinked, partially hydrolyzed poly(vinyl acetate)s (xPVAc, where x is the percent of hydrolysis) and their application for treatment of art is explored. It is shown that gels can be prepared in organic liquid/water mixtures with organic portions up to 75 wt %; gels prepared from poly(vinyl alcohol) can form only when the organic fraction is less than 25 wt %. Variables including the degree of hydrolysis, molecular weight, crosslink density, and the type of crosslinking agent (especially borax), as well as the proportion of organic liquid, are examined. Melting range and rheological studies of the materials show that the organic liquids confer higher thermal stability and longer relaxation times to the gel networks. These effects are explored using 11B NMR spectroscopy, differential scanning calorimetry, and small angle neutron scattering. It is shown that addition of an organic liquid causes an increase in the number of crosslinks in the gel network and agglomeration of the polymer chains into bundles. Modifications of the gels which are relevant to their application in art conservation, such as addition of chelating agents and short-chain glycol ethers, are explored. An alternative crosslinker, trimethyl borate, is demonstrated to form water-sensitive organogels.xPVAc-borate gels are shown to be appropriate for conservation treatments in several case studies. The gels can be prepared with solvents used by conservators, including ethanol, 1-propanol, 2-propanol, and acetone. Their rheological characteristics allow the gels to conform to the topography of complex artwork surfaces and to be removed with ease by being lifted from the surface. It is demonstrated that no detectable residual polymer or crosslinker is deposited on the treated surface if the softened varnish or coating is removed after treatment with the gel. Examination of the lateral and orthogonal spread of solvent in the gels shows that the liquid is constrained within the area on which the gel is applied and that diffusion into sublayers can be controlled. Studies of the materials removed from a varnished oil-painting suggest that the gels act by softening the varnish layer and facilitate its removal by a swab moistened with a mild solvent.
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Hydrogels and Organogels from Partially Hydrolyzed Poly(vinyl acetates) and Boron-based Crosslinkers: Development, Characterization, and Application to the Conservation of Art Duncan, Teresa (Georgetown University, 2017)Soft, peelable hydrogels and organogels formed from partially hydrolyzed poly(vinyl acetate)s and different crosslinkers have been developed, characterized, and evaluated for cleaning treatments on works of art. Partially ...