Understanding the effect of combined thermal treatment and phenol–formaldehyde resin impregnation on the compressive stress of wood

Thermal modification is widely applied to improve moisture dynamics of wood, however often decreasing the mechanical strength. It is therefore required to enhance the mechanical strength of thermally modified timber (TMT), for example by impregnation with adhesives. Specimens, cut from Douglas-fir, were thermally modified (TM) either after phenol formaldehyde (PF) resin impregnation (IM-TM) or before (TM-IM). The microstructural and chemical properties were investigated with SEM and FTIR. Compressive stress, as one of the important mechanical properties, was measured using a universal testing machine, while strain distribution was recorded with digital image correlation (DIC). The results show that the compressive stress of TM specimens can be enhanced significantly by PF resin impregnation. Compressive stress differences between IM-TM and TM-IM specimens are small despite of the larger amount of resin in TM-IM specimens. Thermal modification decomposes part of the PF resin in the cell lumens and promotes chemical reaction between the PF resin and wood. PF resin improves the stiffness and ductility of the wood cell wall, resulting in smaller strain and homogenous distribution thereof. These factors lead to high compressive stress of IM-TM and TM-IM specimens. Although PF resin impregnation contributes to narrowing strain accumulation in earlywood of TM specimens, control specimens have the smallest strain ratio between earlywood and latewood. The findings of this study are helpful for optimizing the cost effective thermal-impregnation technology of producing TMT with improved compressive stress.