CONSOLIDATION OF FRAGILE WOOD WITH LOW VISCOSITY ALIPHATIC EPOXY RESINS

Wooden objects, such as sculptures and ethnographical artifacts, attacked by the action of wood-boring insects and fungi have been treated traditionally with a great variety of materials [1]. This is relevant to the consolidation of affected parts as well as to the prevention of further decay. One of the oldest known techniques is the consolidation with hot animal glue solutions. This which was developed in the beginning of the century, does have some advantages. Hot animal glue solutions can be applied in rather high concentrations up to 40-50%, yet they have a low viscosity of about 5-7 cP, which theoretically favours consolidation as well as penetration. Unfortunately, however, there are many drawbacks to this method. The glue itself is very susceptible to attack from fungi and insects and the penetration is insufficient because of premature gelling of the solution on' cooling to 35--45°C. Besides, stresses may arise through swelling and shrinkage of the wood due to the action of water. Results with different kinds of wax were also unsatisfactory. Mineral, vegetable, and animal types of wax (microcrystalline, camauba and beeswax) may considerably discolour the surface of the object and will only penetrate superficially. Subsequent glueing or grounding of wax-treated parts also becomes impossible, and the treatment in baths with molten wax at 70-100°C may severely damage polychromy and gilt-work. Another unsuccessful method is the treatment with drying ails, such as linseed or-tung oil, possibly mixed with natural resins. This method fails because of insufficient curing of these materials due to a lack of oxygen, needed for the reaction, in the interior of the object. The' consolidation method which is usually employed is the iInpregnatton with solutions of natural or synthetic resins. In this respect the natural resins such as rosin, dammar, mastic and shellac indeed have certain advantages due to their low molecular weight allowing high concentrations at low viscosity [2]. Synthetic polymers, which are mostly applied for this purpose, show much'higher viscosities at comparable concentrations. In general, however, ~olutions are not ideal for consolidation. In the first place most solvents are swelling agents for paint and may eventually damage polychromy and gilt-work. Another property which may affect the results is the enrichment of polymer at the surface of the impregnated object after treatment, either due to evapor4tion of solvent and subsequent transport of more solution to the surface, or to i'nitial chromatographic processes. This effect can be offset by slowing down the evaporation through wrapping the object in plastic foil, in this way allowing a transport of polymer back into the object by diffusio~ [3]. On biggero!,jects, however, this cannot be done. As stated earlier, the synthetic polymers can only be employed in a diluted form (10-20%) to minimize the viscosity. This means, however, that mairily solvent is introduced, making a repeated treatment necessary to get sufficient consolidation. The evaporation of solvent may also be harmful to the object because of shrinkage stresses which it may produce. Working with large quantities of organic solvent may moreover present health and fire risks. Finally the chemically or radiatiort(4] curing synthetic polymers, such as epoxy, polyester, acrylic and polyJlrethane resins should be mentioned. Advantages of these materials are that they can be applied without solvents, that they have a negligible shrinkage on curing and that they are versatile with regard to curing time and properties 'of the finished product. The high viscosity of these materials, however, limits their application for the impregnation of porous materia\s. Dilution of polyesters or acrylic systems with solvents or solvent-like monomers such as styrene or Ill:ethyl methacrylate introduces the usual adverse effects mentioned above, especially with regard topaint layers. The same holds true for epoxy resins. Common commercial epoxy resin (bisphenol A diglycidyl ether):