关键词: 塑性变形, 永久固定, 残余应力, 应力释放, 湿热处理

Abstract:

Permanent fixation of wood plastic deformation has been one of the key scientific and technologic issues for wood modifications such as densification and bending. This paper reviews plastic deformation fixation via hygro-thermal treatments from the perspectives of mechanisms for plastic deformation formation and set recovery. Recent research on fixation of solid wood plastic deformation, especially the advances and challenges in wood plastic deformation fixation by hygro-thermal techniques and the associated mechanisms, are overviewed, with the objective of providing a reference for breaking through the bottleneck of hygro-thermal treatment techniques scale-up to industrial level for wood deformation fixation. Hygro-thermal treatment is the main technique to fix plastic deformation by releasing wood interior stress. Depending on the treatment method, hygro-thermal treatments are categorized into heat treatment under atmospheric pressure and heated treatment under pressurized conditions. Currently, hygrothermal treatment requires the heating temperature to reach to over 180 ℃ for the permanent fixation of wood plastic deformation. Thus, to accelerate the heat transfer rate and heat wood center to 180 ℃ can effectively shorten the time required for wood plastic deformation fixation. Research on permanent fixation of wood plastic deformation is generally carried out by hygrothermal treatment in forms of heat treatment and saturated steam treatment, and was explained by the relationships between wood internal stress release and chemical changes. Three mechanisms for hygro-thermal treatment based permanent fixation of plastic deformation have been widely acknowledged. The first one is stabilized structures caused by the intercrossing of matrix molecules, the second one refers to molecular chains scission caused relaxation of stress in microfibril and matrix, the third is that polymerization of hydrophilic wood cell wall components especially hemicellulose and the resultant retardancy of the hydrophilic wood cell wall components to re-softening. However, research on hygro-thermal treatments for wood plastic deformation fixation is actually limited to two extreme conditions: one is the heat treatment under dry conditions, by applying elevated heating temperatures and extended heating time for internal stress release; the other is the heat treatment under almost water saturated conditions, by employing saturated steam at high temperatures under high pressure, to release wood interior stress. Neither of these involves processing variables/parameters of heat and mass transfer affecting wood plastic deformation fixation and wood properties, causing a contradiction between permanent fixation of wood deformation and wood properties deterioration. Even though permanent fixation of wood fixation has been theoretically realized by hygro-thermal treatment, it has not been completely applied in industry. The main reason for this is the immaturity of theories and technology system supporting hygro-thermal treatment based permanent fixation of wood deformation. To solve the application issues in hygro-thermal treatment based permanent fixation of wood deformation, more attention should be paid to how to make the hygro-thermal treatment conditions applicable rather the conditions themselves. Thus, research on hygro-thermal treatment for permanent fixation of wood deformation from the perspectives of functions of water and heat, oxidation and their interaction, as well as wood interior stress relaxation during hygro-thermal treatment should be carried out.

Key words: plastic deformation, permanent fixation, residual stress, stress release, hygro-thermal treatment