The deformation processing of wood can effectively promote its use in various applications, such as furniture, building, construction, automotive, and other daily necessities. Some conventional methods of wood deformation include compression and bending processing using cell-wall deformations (Sandberg et al. 2012). These methods produce simple two-dimensional products while retaining the cell arrangement and structure of the wood. On the other hand, reducing the element size of wood can efficaciously enable the formation of more complex products. Wood-plastic composites (WPCs), which are a mixture of wood powder and plastic, are widely used as building materials (Spear et al. 2015). However, WPCs require an input of energy to miniaturize the wood, which results in the destruction of the original cell structure, such that the products do not retain the wood texture. Furthermore, in conventional processing, the element size of wood and the complexity of the product shape have a contradictory relationship. To date, the formation of a complex shape from solid-state wood has seen little success.
Recently, a new technique, wood flow forming (WFF), has been developed, which can form complex shapes with solid-state wood because it takes advantage of plastic flow deformation (Abe et al. 2020; Abe et al. 2021; Miki et al. 2014-1; Miki et al 2014-2; Miki et al. 2017; Seki et al. 2016; Yamashita et al. 2009). The wood is compressed in a heated mold, which enables it to plastically flow owing to the shear sliding between wood cells to produce a final shaped product (Miki et al. 2017). By applying the traditional plastic forming techniques used for metal and plastic materials, wood products can be efficiently produced in a short time. These techniques can maintain the original cell structure of wood, providing a unique cell-derived texture to the product (Miki et al. 2014-1). However, to improve the deformability of wood during the forming process and the durability of the products, it is necessary to modify the wood by pretreatment before forming. The cell wall and compound middle lamella (CML) are modified by impregnating the wood with resin monomers (Miki et al. 2014-2; Seki et al. 2016) and/or chemical modification (Abe et al. 2020; Abe et al. 2021). WFF has great potential for various applications; however, it requires high temperatures (< 100°C) and pressures (< 50 MPa), making it energy-intensive and less productive. To solve these problems, this study focused on lignin, which acts as an adhesive and binds the polysaccharides (cellulose and hemicellulose) and cells together. In addition to the CML containing the highest amount of lignin, it is where plastic flow originates; therefore, lignin has a significant effect on plastic flow.
Delignification is a process that can remove lignin without breaking the cellular structure of the wood, and it has primarily been studied for pulping wood. However, in recent years, delignified wood (DW) (Kumar et al. 2021) has attracted attention as a functional material for use in transparent wood (Li et al. 2016; Li et al. 2017; Li et al. 2020), high-strength structural materials (Frey et al. 2019; Jakob et al. 2020; Song et al. 2018), high-performance thermal insulators (Li et al. 2018), and thermal energy storage materials (Montanari et al. 2019). Deformation processing that takes advantage of the flexibility of DW has also been developed (Khakalo et al. 2020; Frey et al. 2018; Frey et al. 2019). Frey et al. (2019) reported that a completely delignified veneer in a water-swollen state exhibited significant deformability in the fiber direction. However, the effect of delignification on the plastic flow deformation associated WFF has not been reported.
The thermal softening properties of water-swollen wood depend on the glass transition of lignin (Kojiro et al. 2008; Nakajima et al. 2009). When the molecular mass of lignin in wood decreases, its glass transition temperature also decreases, in turn significantly softening the wood (Nakajima et al. 2009). Therefore, it is expected that the changes in lignin due to delignification will promote the plastic flow deformation of wood and reduce the production energy required for WFF.
The objective of this study was to clarify the effect of lignin on the plastic flow deformation of wood. DW and untreated-wood (UW) samples with different molecular masses and amounts of lignin were prepared by subjecting them to delignification and varying the delignification time. Free compression testing was used to evaluate the deformability of the samples based on the initial resistance to plastic flow and final stretchability. The samples were characterized by attenuated total reflection infrared (ATR-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and dynamic viscoelastic measurements. The effects of the chemical structure and glass transition temperature of lignin on the plastic flow deformation of wood are also discussed.