Flash polymerization of silicone oils using gamma radiation for conserving waterlogged wood

Abstract

A series of experiments were designed to develop a collection of irradiation parameters for polymerizing a variety of molecular weights of polymer siloxanes for the purpose of bulking and conserving waterlogged archaeological organic materials. Waterlogged wooden tongue depressors (Betula spp) were bulked with five different silicone oils of varying molecular weights, denoted as PS340, PS341, PS343, SFD-1, and an equal mix of SFD-1 and SFD-5. The lightest weight oil being PS340 and the heaviest being the SFD-I /SFD-5 mix. These bulked samples were exposed to gamma radiation emitted from a nuclear research reactor and received gamma doses ranging from 30 Gy to 228 Gy with dose rates ranging from 0.6 Gy/min to 5.1 Gy/min. Following irradiation, thin cross sections of each tongue depressor were analyzed and photographed using a photomicroscope. The results of the irradiation indicated that the heavier molecular weights of siloxanes fully bulked the wood cells. For these fully bulked samples, large radiation doses induced polymerization in the oil to such an extent the cells swelled and became extremely distorted. However, for the same fully bulked samples, small radiation doses produced little to moderate polymerization of the oil in the wood cells. For the tongue depressors bulked with the lighter weight siloxanes, low radiation doses produced very little polymerization, whereas the higher doses induced substantially more polymerization within the wood cells but not to the point of distortion. With these lighter weight siloxanes, the wood cells were not as fully bulked nor looked as natural as those cells bulked with the heavier weight oils. The best combination of molecular weight and radiation dose occurred with the samples bulked in PS343 and irradiated with gamma-rays to a dose of 150 Gy.