Heat Treatment of Industrial Alkaline Lignin and its Potential Application as an Adhesive for Green Wood–Lignin Composites

Abstract

An industrial alkaline lignin preparation and poplar particles were mixed and hot pressed under different conditions. The alkaline lignin and the lignin isolated from the poplar particles were thoroughly investigated by quantitative nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and elemental analysis techniques. For the first time, it was found that the content of β-O-4′ linkages decreased accompanying with the formation of β-β′, β-5′, and β-1′ linkages at mild heat treatment temperatures (130–170 °C). However, it should be noted that most of the β-O-4′, β-β′, β-5′, and β-1′ linkages nearly disappeared at a higher temperature (180 °C). Cross-linking reactions were predominant during the hot-pressing process as the molecular weight of lignin increased at elevated temperature. Owing to the self-bonding between lignin fragments during the hot-pressing process, a green poplar wood–lignin composite was successfully prepared with poplar particles and a small amount of alkaline lignin (∼20%, w/w). Internal bond strength with 0.47 MPa was surprisingly achieved under the conditions of pressing temperature 160 °C, pressure 5 MPa. An in-depth understanding of the concerted reactions between fragmentation and cross-linking reaction in lignin during hot pressing was beneficial to a better development of self-bonding green wood–lignin composites in future.

Effect of Heat Treatment on the Chemical Structure and Thermal Properties of Softwood-Derived Glycol Lignin

“…(Scheme 1). The visual color change from pale brown to dark brown observed with increasing temperature suggests that thermochemical reactions, such as depolymerization, side chain cleavage, and repolymerization through condensation, may occur during heat treatment [23][24][25][26], even though only a trace amount of weight loss (less than 1 wt%) was measured after heat treatment. Scheme 1.…”

“…A continuous supply with constant qualities as a common feedstock is another goal for potential industrial implementations of GL-based material production. It is well known that heat treatment alters the chemical structure of lignin as well as its molecular weight due to thermal decomposition and structural rearrangement through a series of chemical reactions, depending on the treatment temperature and duration [23][24][25][26]. In this study, three types of PEG400-substituted GL (GL400) samples obtained from the test plant were subjected to heat treatment in a constant-temperature drying oven at temperatures ranging from 100 to 220 • C for 1 h. To obtain an in-depth understanding of the physicochemical properties of heat-treated GL400 samples, ultraviolet (UV) and Klason lignin analyses, size exclusion chromatography (SEC), attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy, thioacidolysis, 2D nuclear magnetic resonance (NMR), thermomechanical analysis (TMA), and thermogravimetric analysis (TGA) were performed to determine the lignin content, average molecular mass, functional group distribution, frequency of ether linkages and intermonomeric linkages, and thermal transition temperatures.…”

Multiscale Modification of Populus cathayana by Alkali Lignin Combined with Heat Treatment

“…However, the values of LI 1263 , LI 1427 , and LI 1507 of AL 180 °C were lower than those of AL, with an opposite trend for LI 1632 , which belongs to the aromatic conjugated C=O in lignin. This may be due to the degradation of alkali lignin after heat treatment (Figure 4), followed by the formation of aromatic organics [25,26].…”

Development of sustainable bio-adhesives for engineered wood panels – A Review

“…Recently, for the rst time, Prof. Sun's research group reported the self-crosslinking of industrial alkali lignins for green wood-lignin composite fabrication via heat-treatment. 109 Commercial tannin-formaldehyde applications already exist and several reviews on the use of tannin as a wood adhesive can be found in literature. 89,110 Currently, tannins are not available in large enough quantities to compete with synthetic adhesives in most parts of the world.…”