Pre-treatment with sodium silicate, sodium hydroxide, ionic liquids or methacrylate resin to reduce the set-recovery and increase the hardness of surface-densified Scots pine

Abstract: The hardness of the outer regions of solid wood can be improved by surface densification, and this opens up new fields of application for low-density species. So far, surface densification relies on time- and energy-consuming batch processes, and this means that the potential advantages over more expensive hardwood species or non-renewable materials are reduced. Using fossil-based plastics or applying wood densification processes with a high energy consumption has adverse effects on the environment. In a previous study, it was shown that the surface of wood can be densified by a continuous high-speed process, adopting a roller pressing approach. The desired density profiles could be obtained at process speeds of up to 80 m min-1, but an equally simple and fast method to eliminate the moisture-induced set-recovery of the densified wood cells is still required. For this reason, the goal of the present study was to evaluate the effect on the set-recovery and hardness of surface-densified Scots pine after a fast pre-treatment with solutions of sodium silicate, sodium hydroxide, methacrylate resin, and ionic liquids. The Scots pine specimens were pre-treated by applying the chemical treatment and impregnation agents to the wood surface with a paper towel, before the specimens were densified. For each type of treatment, 15 specimens were densified in a hot press. The set-recovery was measured after two wet-dry cycles, and 30 Brinell hardness measurements were carried out on each group of specimens. In general, the effect of the treatments on the set-recovery was rather low. Ionic liquid solutions appear to work as a strong plasticiser and the treatment led to a reduction in set-recovery by 25%. The treatments with sodium silicate, ionic liquids and methacrylate resin led to a greater hardness than in untreated and densified specimens. Further experiments are needed to improve the depth of penetration of the treatment solutions into the wood surface, as this was identified as one of the main causes of the rather weak effects.