Improvement of wood properties by impregnation with liquefied spruce bark based resins
X Duret, E Fredon, E Masson, P Gérardin
In this paper, biobased phenolic thermosetting resins are designed to be used as wood stabilization treatment. The originality of the study stands in substituting part of phenol by spruce (picea abies) bark: this is achieved by a liquefaction process in phenol in the presence of sulfuric acid as catalyst. Next, thermosetting resins are synthetized from liquefied bark by condensation on formaldehyde and further used for impregnation in beech blocks and curing. Two kinds of spruce barks -unmodified and pretreated ones- are used as starting material and characterized. Concerning pretreatment, a hydrolysis has been applied to spruce bark in order to enhance phenolic part. Two liquefaction processes are compared: the first one a simple heating under reflux in presence of phenol. A second one consists in heating at 200°C in closed reactor in phenol/ethanol/water solvent (20/50/30). For the different systems attempted, liquefaction yields are determined and analyzed regarding FT-IR analysis. 91% yield was obtained for liquefied spruce bark under reflux, but the highest bark to solvent ratio (50%) was obtained for process lead in closed reactor. Reactivity of liquefied bark resins is found to be better than control phenolic-formaldehyde resin. Impregnations in beech (Fagus sylvatica) wood blocks are performed using different dried matter content (10%, 20% and 30%). After curing, weight percent gain (WPG), leaching rate, anti-swelling efficiency (ASE) and resistance to decay of different systems are compared. It is shown that liquefied bark based resins confers to wood significant dimensional stabilization and resistance to decay against brown rot and white rot fungi. Concerning the impact of the different processes attempted such as hydrolysis pretreatment or liquefaction at 200°C in closed reactor, final properties of treated wood are decreased compared to liquefied bark resins.