The role of accessible hydroxyl groups in reversible and irreversible EMC changes by thermal wood modification

Heat treatment (HT) is a well-known means to reduce the room-temperature equilibrium wood moisture content (EMC) at a specified relative humidity (RH). Already at slightly elevated temperature above room temperature, EMC is decreased in proportion to the exposed temperature. At higher elevated temperature, where wood chemical changes become activated, the EMC is profoundly reduced, mainly caused by the loss of accessible hydroxyl groups (OHacc) in the wood matrix. However, earlier experiments have shown that the obtained EMC reduction after HT can be partly reversible, depending on the ability of the wood matrix polymers to fully mechanically relax during HT. We present an experimental study on the OHacc content and the associated EMC decrease at 93%RH by a relaxation inhibiting dry-HT versus a relaxation enabling wet-HT. The results of this study quantitatively support a molecular explanation of this phenomenon, in which wood moisture is principally bound by OHacc-pairs, instead of solitary OHacc. The irreversible part of EMC reduction is assigned to the thermochemical removal of OHacc from the wood matrix. The reversible part (EMC recovery), is attributed to a regrouping of solitary OHacc into pairs, thereby restoring moisture sorption sites. Each solitary OHacc, may either be the remaining half of an OHacc-pair after thermal destruction of its partner OHacc, or originate from thermally disrupted OHacc-pairs without OHacc removal. The OHacc-pair recovery mechanism explains the involvement of wood polymer relaxation, necessary to translocate OHacc in the wood matrix.