Research progress in isolation, main physical and mechanical properties of wood lignin

Abstract

Lignin is an important component of wood cell wall, and its moisture absorption characteristics, thermal characteristics, and mechanical characteristics influence each other at the micro-macro scale of wood, and play a decisive role in its high-value application. In this paper, the research progress of wood lignin was reviewed from five aspects: molecular structure, separation methods, moisture absorption characteristics, thermal characteristics and mechanical characteristics. Wood lignin is a highly heterogeneous and irregular three-dimensional network polymer structure. Compared with in-situ lignin, the lignin separated by different separation methods has different degrees of depolymerization condensation, which leads to differences in molecular structure, hygroscopicity, thermal properties and mechanical properties. Wood lignin has an approximate S-shaped isothermal adsorption curve and there is a hysteresis phenomenon of moisture absorption. The equilibrium moisture content is below 20%(mass fraction), and the BET and GAB theories can be used to qualitatively describe and quantitatively analyze the adsorption capacity of monolayer water molecules. Wood lignin has high polymer plasticity at low temperatures with a glass transition temperature of 90-160 ℃, and has a thermosetting property and thermal decomposition at high temperature, with α-O-4, β-O-4, carbon-carbon linkages of aliphatic hydrocarbons, 5-5 and 4-O-5 linkages breaking successively, and the activation energy of pseudo in-situ lignin is 82-150 kJ/mol. The mechanical properties of lignin are isotropic, and the elastic modulus is decreased with the increase of moisture content, with the elastic modulus of 2.8-9.0 GPa and the shear modulus of 1.1-2.3 GPa, but the research scope is limited to the elastic stage. The green and efficient separation method of in-situ lignin, the molecular structure sequence of lignin and the elastic-plastic mechanical properties of lignin need to be further studied.

Keywords

• wood lignin
• molecular structure
• thermal characteristic
• hygroscopic property
• mechanical property