Abstract: [Objective]The present research is for further understanding the structure characteristics and thermal properties of soluble lignin and insoluble lignin of moso bamboo (Phyllostachys edulis) by low alkaline charge kraft pulping to provide theoretical reference to the application of lignin from bamboo.[Method]The moso bamboo was pretreated by kraft pulping with effective alkalinity of 12%. To isolate the soluble lignins, black liquor was acidified to precipitate black liquor lignin, namely kraft lignin (KL). Enzymatic hydrolysis was performed on the solid pulp to obtain residual insoluble lignin in pulp, namely enzymatic hydrolysis lignin (EHL). Organic solvent purification was utilized on the lignin isolates to ensure the absence of unbound carbohydrates. The molecular weights of KL and EHL were determined by gel permeation chromatography (GPC). ³¹P nuclear magnetic resonance (NMR) and the combination of ¹³C and 2D-HSQC NMR were performed to quantify the amount and variety of hydroxyl functional groups and inter-lignin linkages respectively. Thermal properties of KL and EHL were measured by thermogravimetric analysis (TGA) to determine the thermostability and differential scanning calorimetry (DSC) for estimating glass transition temperature.[Result]The approximate values of molecular weights of KL and EHL obtained by GPC were 5 414 g·mol^-1 and 7 673 g·mol^-1 respectively. Hydroxyl quantification from ³¹P NMR indicated a greater amount of aliphatic hydroxyl in EHL (4.87/100 C₉) than in KL (3.13/100 C₉). As for phenolic hydroxyl, an opposite trend was shown, 1.58/100 C₉ for EHL and 3.09/100 C₉ for KL. Quantification of inter-lignin linkages through the combination of 2D-HSQC and ¹³C NMR spectra showed the inter-lignin linkages in EHL were 30.85/100 C₉ for β-O-4, 7.43/100 C₉ for β-β, 2.40/100 C₉ for β-5, and 0.19/100 C₉ for β-1. KL, bearing less inter-lignin linkages, was found to contain 11.75/100 C₉ of β-O-4, 2.35/100 C₉ of β-β, 0.71/100 C₉ of β-5, and 0.09/100 C₉ of β-1. The thermal property measurement under heating showed that the greatest weight losses with EHL and KL occurred at 361℃ and 339℃ respectively, and the glass transition temperatures were 162℃ and 135℃ respectively.[Conclusion] Low alkaline charge kraft pulping of moso bamboo yields liquor-soluble lignin which has lower molecular weight and less inter-lignin linkage than the residual lignin in pulp. Phenolic hydroxyl groups have the greater functionality in the black liquor lignin, than aliphatic hydroxyl groups in the residual lignin in pulp. The black liquor lignin has greater advantage for the preparation of the lignin-polymer thermoplastic material than the residual lignin in the pulped bamboo, due to lower thermostability and glass transition temperature.

Key words: kraft pulping, lignin, molecular weight, lignin linkages, functional group, thermal property