Chemical changes of raw materials and manufactured binderless boards during hot pressing: Lignin isolation and characterization

Abstract

Thermomechanical pulp (TMP) is used for fiber production in binderless boards industries. Milled wood lignin (MWL) and enzymatic mild acidolysis lignin (EMAL) isolated from raw material and from binderless boards (BB) were comparatively analyzed to investigate the effects of chemical changes on the bonding performance in BB. The results showed that acid-insoluble lignin of the BB were increased during the sodium silicate solution pretreatment after hot-pressing. The lignin fractions obtained were characterized by gel permeation chromatography (GPC), Fourier transform infrared (FT-IR) spectroscopy, and 1H-13C correlation heteronuclear single-quantum coherence (HSQC) nuclear magnetic resonance (NMR) spectroscopy. Results showed that 31.1% of EMAL (based on Klason lignin) with low molecular weight (Mw=1630 g/mol) was isolated from the BB. The increased total phenolic OH groups (3.97 mmol/g) of EMAL from sodium silicate solution pretreated BB indicated that there was degradation of lignin and cleavage of lignin-carbohydrate linkages during hot-pressing. In addition, the content of β-O-4' aryl ether linkages of EMAL from the BB increased to 69.2%, which was higher than that of the untreated sample (60.1%). It was found that S units (syringyl-like lignin structures) were preferentially condensed by hot pressing over G (guaiacyl-like lignin structures) units, and the S/G ratio increased after the hot-pressing process.