Optimising wood chemical modification with lactic acid oligomers by screening of processing conditions and chemical additives

Oligomer systems based on lactic acid (OLA), were impregnated in wood and polymerised in-situ by heat treatment to improve the properties of the treated wood especially regarding anti-swelling efficiency (ASE), hygroscopicity, water leaching resistance and durability. This article relates the optimisation of the heat treatment conditions and the consideration of selected chemicals for improvement of the treatment efficiency. Maleic anhydride has been considered as a potential additive for oligomers fixation to wood polymers and condensed tannins of Mimosa have been considered as potential additives for improvement of the biological resistance. In both cases, the additives have been mixed into the oligomers or reacted with the oligomers prior to impregnation. The heating step parameters (especially temperature) have a substantial influence on the modified wood properties. The global wood hygroscopic behaviour has been substantially improved by all treatments, with equilibrium moisture content decreased by half for ca. 70% ASE* values for the best performing treatments. The increase in temperature decreases significantly the leaching rate, down to values <5% after extended leaching periods. The treatment provides a significant biological resistance to wood, as shown by weight loss <0.6% after 6-week accelerated fungal exposure, due to the combined action of the oligomers impregnation and the heat treatment. The same global performances are observed with M-OLA, with an improvement for the persistence in the wood which is globally better for M-OLA than OLA, and confirms the relative efficiency of this approach. Treatments with added tannins are also promising as the tannins, known for their hydrophilic nature, have however no damaging effect on treated wood hygroscopic behaviour.