Understanding the Mechanism of Self-Bonding of Bamboo Binderless Boards: Investigating the Structural Changes of Lignin Macromolecule during the Molding Pressing Process

Abstract

Binderless boards were produced from bamboo particles through a molding pressing process. The boards were investigated for the chemical and structural changes of lignin and their mechanical strengths to evaluate the mechanism of self-bonding. The structural transformations of milled wood lignin (MWL) obtained from raw bamboo and molding pressed bamboo boards were investigated by Fourier transform infrared (FT-IR), gel permeation chromatography (GPC), quantitative 13 C-NMR spectra, two-dimensional heteronuclear single quantum coherence (2D-HSQC) spectra, 31 P-NMR, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) techniques. Both molding press temperature and time affected the distribution and abundance of typical lignin linkages (β-O-4′, β-β′, and β-5′), as well as the S/G ratios of the lignin, demethoxylation, and the contents of attached molecules (PCE). In addition, a decrease in aliphatic OH and an increase in phenolic hydroxyl groups occurred in lignin as molding pressing proceeded. The optimal internal bonding strength (0.98 MPa) of the bamboo binderless board was obtained under the condition of 180 ºC for 20 min. Although the lignin obtained under this condition was structurally similar to the raw MWL, the decreased molecular weight, increased phenolic hydroxyl groups, and observable glass-transition temperature (thermal softening of lignin) provide some evidence to explain the acceptable internal bonding strength.

Effects of Heating Temperature on the Properties of Bio-Board Manufactured by Using Soybean Straw

“…Heating temperature seems to be the most influential parameter of lignin changes, subsequently affecting the properties of binderless board. A similar conclusion has been reported by Hashim et al [20], Salvadó et al [21], and Wang et al [22].…”

The role of lignin in wood working processes using elevated temperatures: an abbreviated literature survey

“…In fiberboard production, during thermomechanical pulping, woodchips are converted to lignin-covered fibers by shearing wood fibers along the lignin-rich middle lamellae (Felby et al 1997;Kharazipour et al 1998). Goring (1971), Yelle (2001), van Dam et al (2004, Bouajila et al (2005), Okuda et al (2006a, b), Halvarsson et al (2009), Wang et al (2017) and Laine et al (2019) suggested that lignin has an important role in the production of binderless boards due to the softening of lignin at elevated temperatures and under pressure, fibers with lignin-rich surfaces fuse together as the softened lignin molecules flow from one fiber surface to another one, and possibly form covalent bonds too. If the water content of the raw material is above or near the saturation point, the T g of the cell wall polymers is relatively low.…”

Constructive applications of totora (Schoenoplectus californicus) in binderless boards

“…Acid-insoluble lignin content increased by about 5% for both tissues. Although the pressing temperature at 150°C and a moisture content of 2% used to make the studied boards may have allowed lignin plasticization to occur (Bouajila et al, 2006;Poletto, 2017;Tejado, Peña, Labidi, Echeverria, & Mondragon, 2007), this temperature is lower than the temperature reported for lignin degradation which starts at around 180°C (Lamaming et al, 2013;Özgenç, Durmaz, Boyaci, & Eksi-Kocak, 2017;Wang et al, 2016). And considering that although during the hot-pressing process higher temperatures than the hot plates are likely to occur due to high pressures and vapor generation, the venting step after the first 30 s of pressing to allow the discharge of initial vapor may have prevented an excessive temperature increase in the mat core (Myron, 1977).…”

“…Studies have shown that during the dry-forming process of binderless boards the initial moisture content is an important factor and its internal distribution and effects depend highly on the material used (Hidayat, Keijsers, Prijanto, Van Dam, & Heeres, 2014;Uitterhaegen et al, 2017). Although some initial moisture content may promote the glass transition of biopolymers such as lignin during the hot-pressing, which could undergo plasticization and enhance the particles' self-bonding at lower temperatures (Bouajila et al, 2005;Rouilly, Orliac, Silvestre, & Rigal, 2001;Wang et al, 2016), the rapid formation of vapor during the hot-pressing process may also promote the formation of micro-fissures and blisters inside the boards that affect their mechanical properties (Evon, Vinet, Labonne, & Rigal, 2015;Uitterhaegen et al, 2017). With these considerations previous tests were conducted to define the optimal initial moisture content.…”

“…And considering that although during the hot-pressing process higher temperatures than the hot plates are likely to occur due to high pressures and vapor generation, the venting step after the first 30 s of pressing to allow the discharge of initial vapor may have prevented an excessive temperature increase in the mat core (Myron, 1977). Therefore, the apparent increase of lignin content may be due to the degradation of other substances such as extractives and hemicelluloses, which are prone to react at lower temperatures (Boonstra & Tjeerdsma, 2006;Wang et al, 2016). However, lignin plasticization may have promoted its mobility and adhesion with other compounds during the hot-pressing process (Bouajila et al, 2005).…”