热改性工艺对欧洲赤松化学成分变化及耐腐性的影响

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

【目的】为了探究热改性温度和压力条件对欧洲赤松(Pinus sylvestris)化学成分变化及耐腐性的影响规律,进而揭示热改性工艺、化学成分变化和木材耐腐性之间的响应机制。【方法】在不同温度(150、180、210℃)、加压或常压条件下对欧洲赤松边材进行热改性,分析热改性前后抽提物、木质素、综纤维素、α-纤维素和半纤维素质量分数的变化。以密黏褶菌为试验菌种,研究热改性木材在腐朽过程中化学成分的变化,并采用场发射扫描电子显微镜(FE-SEM)对其微观形貌进行表征。【结果】热改性温度越高,欧洲赤松质量损失率越高;热改性过程中抽提物和木质素质量分数上升,综纤维素、α-纤维素和半纤维素质量分数降低;在同一温度下,加压条件比常压条件下热改性质量损失率和化学成分变化更为显著。与常压热改性相比,加压热改性材耐腐性较好,且温度越高,耐腐性越强。加压180℃腐朽12周后质量损失率为18.8%,耐腐等级为Ⅱ级(耐腐);加压210℃腐朽12周后质量损失率为8.4%,耐腐等级为Ⅰ级(强耐腐)。腐朽过程中,耐腐性能无明显变化的常压150℃、加压150℃和常压180℃热改性材与对照组的化学成分变化趋势相似,随着腐朽时间的延长,综纤维素、α-纤维素和半纤维素质量分数持续下降,木质素质量分数持续上升。常压210℃、加压180℃和加压210℃热改性材的木质素质量分数变化不明显,综纤维素和α-纤维素的降解速度明显变慢。【结论】热改性过程中不同的温度和压力对木材的化学成分变化和耐腐性产生不同程度的影响。温度较高且加压的热改性条件会增加热改性过程中半纤维素和α-纤维素的降解,综纤维素质量分数大幅下降,抽提物、木质素质量分数大幅上升。热改性后综纤维素的减少使得褐腐真菌对木材的降解程度降低,降解速度变慢,木质素、抽提物质量分数的上升又对真菌进一步降解细胞壁成分具有一定的抑制作用,从而提升热改性材的耐腐性。

[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 levelⅡ(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 levelⅠ(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 andα-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 andα-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.