Research progress on performance improvement and process optimization of thermally treated wood
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摘要: 热处理木材是一种环保无污染且绿色可持续材料。但木材经过热处理会导致其部分力学性能下降、表面润湿性能不足及容易发生光老化等问题。传统的热处理技术所需要的高温环境会产生大量能耗,导致生产成本较高。针对以上热处理木材存在的性能缺陷和生产工艺不足,从热处理木材性能改良和工艺优化两个角度,概述了国内外热处理木材的性能改良研究进展。具体内容涉及热处理木材力学性能改良、表面润湿性能改良、耐光老化性能改良和热处理工艺优化4个方面。分析了热处理木材相关性能改良方法的局限性和改进措施,提出了木材热处理改性应在结合改性机制和实际应用的基础上,对热处理造成的潜在木材性能劣化进行规避,以更好地体现其综合性能优势,提高附加值并拓展应用范围。Abstract: Thermally treated wood is an eco-friendly and sustainable material. After thermal treatment, some mechanical properties and surface wettability of wood reduces, and light aging is more likely to occur. In addition, the high temperature environment is required for the traditional technology during the thermal treatment process, which produces a lot of energy consumption, resulting in high production costs. In view of the performance defects of thermally treated wood and the shortcomings of production process, the research progress on performance improvement of thermally treated wood is summarized from the following two aspects: Performance improvement and process optimization. The enhancements on mechanical property, surface wettability, anti-weathering of thermally treated wood, and thermal treatment process optimization are discussed. Also, the limitation of themethods for properties improvement of thermally treated wood is analyzed, and the solutions are proposed. After that, this work suggest that it is better to combine the mechanism and practical application of thermally treated wood to avoid performance degradation caused by heat treatment, and to show the advantage of overall performance of thermally treated wood, which could increase the value-added utilization and expand the scope of application.
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图 3 (a)热处理木材的光降解机制;热处理木材的耐光老化性能改良方法:(b)有机紫外线吸收剂将光能转化为化学能和热能;(c)无机纳米粒子反射紫外线;(d)抗氧化剂破坏或者消除自由基
Figure 3. (a) Mechanism of photodegradation of heat-treated wood; Methods for improving light aging resistance of thermally treated wood: (b) Organic ultraviolet absorbers convert light energy into chemical energy and heat energy; (c) Inorganic nanoparticles reflect ultraviolet light; (d) Antioxidants destroy or eliminate free radicals
表 1 热处理对木材表面润湿性和漆膜附着力的影响
Table 1. Effect of heat treatment on surface wettability and adhesion of wood
Wood
speciesTreatment
conditionFilm adhesion grade Contact angle Ref. Waterborne wood coating Oil-based wood coating Diiodomethane Water Ethyl alcohol Poplar
woodControl 1 — 34.9° 71.6° — [37] 160℃, 2 h 1 37.6° 80.7° 180℃, 2 h 1 37.9° 79.5° 200℃, 2 h 3 41.1° 87.7° 220℃, 2 h 3 42.1° 90.3° Rubber
woodControl 1 1 — 62.1° — [38] 200℃, 2 h 2 1 107.1° Pometia
pinnataControl 1 — — 43.8° 22.9° [39] 200℃, 6 h 2 52.9° 46.9° Rubber
woodControl 1 2 — — [31] 200℃, 2 h 2 1 Notes: According to the national standard, the adhesion of the paint film can be divided into 6 grades. 1: A little coating falls off at the intersection of the incision, but the affected cross-cutting area is obviously not more than 5%; 2: A little coating falls off at the intersection of the incision, and the affected cross-cutting area is greater than 5%, but not greater than 15%; 3: There is a little coating falls of at the intersection of the incision, and/or partial or total peeling at different parts of the grid, and the affected cross-cutting area is significantly greater than 15%, but not greater than 35%. 表 2 等离子体中粒子能量和木材表面常见化学键的键能
Table 2. Particle energy in plasma and bond energy of chemical bonds on wood surfaces
Particle Particle energy/eV Chemical bond Bond energy/eV Chemical bond Bond energy/eV Electron 0-20 C—H 3.2-4.7 C=O 5.5 Metastable particle 0-2 C—C 2.6-5.2 C=C 3.3-7.5 Ion 0.03-0.05 C—O 0.95-3.0 C≡C 10 Photon 3-40 O—H 3.4-5.2 — — 表 3 常见热处理工艺条件
Table 3. Common conditions used during heat treatment
Process Source Medium Temperature/℃ Duration/h Drawback Ref. Thermowood Finland Water vapour 150-240 0.5-4 — [66] Plato Netherlands Water vapour 160-190 4-5 Complicated process and
long duration[67] Torrefaction France Water vapour (comes from the
evaporation of water in wood)200-240 — — [68] Retification France Nitrogen 210-240 — High requirements on
equipment accuracy[68] Hot oil treatment Germany Oil 180-220 2-4 Wood absorbs oil and waste
oil utilization and disposal difficulties[69] Smoke heating
treatmentJapan Smoke 80-200 — Waste gas [70] -
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