Effects of thermal modification conditions on chemical composition and decay resistance of Pinus sylvestris

  Objective  The aim of this study was to investigate the effects of temperature and pressure conditions on the chemical composition changes and decay resistance of Scots pine (Pinus sylvestris), as well as reveal the response mechanism among thermal conditions, chemical composition changes and wood decay resistance.

  Method  The sapwood was modified at different temperatures (150, 180 and 210 ℃) under different pressures: atmospheric pressure (AP) or high pressure (HP), while wood mass loss rate and chemical composition including extractives, lignin, holocellulose, α-cellulose and hemicellulose mass fraction were analyzed. The modified wood was decayed by Gloeophyllum trabeum for different durations. The chemical composition changes of modified wood during decay were analyzed and the microscopic morphology was characterized by field emission scanning electron microscope (FE-SEM).

  Result  As the thermal modification temperature increased, the mass loss rate of Scots pine was higher. The mass fraction of extractives and lignin increased after thermal modification, whereas the holocellulose, α-cellulose and hemicellulose decreased. At the same temperature, the changes of mass loss rate and chemical composition in HP condition were more significant compared with those in AP condition. After 12 weeks of decay, the decay resistance grades of the 180 ℃ HP thermal modified wood reached the level Ⅱ (durable), and the mass loss rate was 18.8%. Meanwhile the mass loss rate of the 210 ℃ HP modified wood was 8.4% after 12 weeks of decay and the grade reached level Ⅰ (highly durable). There was no obvious change in decay resistance for 150 and 180 ℃ AP thermally modified wood and 150 ℃ HP thermally modified wood. In addition, the holocellulose, α-cellulose and hemicellulose mass fractions constantly decrease with decay, while the relative amount of lignin keeps growing. The lignin mass fraction of the thermal modified wood in HP condition at 180 and 210 ℃ and in AP at 210 ℃ did not change significantly, however, the degradation of holocellulose and α-cellulose distinctly slowed down.

  Conclusion  Different temperatures and pressures during thermal modification have varied effects on the chemical composition and decay resistance of wood. The thermal modification conditions with higher temperature and pressure would increase the degradation of hemicellulose and α-cellulose during the thermal modification. Meanwhile, the holocellulose mass fraction reduces greatly and the mass fraction of extractives and lignin increases sharply. The decrease of holocellulose after thermal modification leads to the reduction of wood degradation degree and degradation rate becomes slower by brown rot fungi. At the same time, the increase of the percent of lignin and extractives has negative effects on the further degradation of cell wall components by fungi. For the above reasons, the decay resistance of thermally modified wood is improved.