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

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 13C-NMR spectra, two-dimensional heteronuclear single quantum coherence (2D-HSQC) spectra, 31P-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.