Abstract

Lignin availability has increased significantly due to the commercialization of several processes for recovery and further development of alternatives for integration into Kraft pulp mills. Also, progress in lignin characterization, understanding of its chemistry as well as processing methods have resulted in the identification of novel lignin-based products and potential derivatives, which can serve as building block chemicals. However, all these have not led to the successful commercialization of lignin-based chemicals and materials. This is because most analyses and characterizations focus only on the technical suitability and quantify only the composition, functional groups present, size and morphology. Optical properties, such as the colour, which influences the uptake by users for diverse applications, are neither taken into consideration nor analysed. This paper investigates the quantification of lignin optical properties and how they can be influenced by process operating conditions. Lignin extraction conditions were also successfully correlated to the powder colour. About 120 lignin samples were collected and the variability of their colours quantified with the CIE L*a*b* colour space. In addition, a robust and reproducible colour measurement method was developed. This work lays the foundation for identifying chromophore molecules in lignin, as a step towards correlating the colour to the functional groups and the purity.

Highlights

  • IntroductionLignin is the most abundant aromatic polymer in nature since it constitutes typically 18–35 wt%

  • Lignin is the most abundant aromatic polymer in nature since it constitutes typically 18–35 wt%of lignocellulosic biomass, depending on the selected groups of species

  • To determine the parameters that can be varied in order to manipulate lignin colour during the precipitation and recovery process, a list comprising 22 variables was identified for the precipitation, washing and drying step 6

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Summary

Introduction

Lignin is the most abundant aromatic polymer in nature since it constitutes typically 18–35 wt%. Of lignocellulosic biomass, depending on the selected groups of species. It is spread around the outer surface of the cell wall in the middle lamella and is covalently linked with hemicelluloses. Lignin plays the crucial role of conducting water in addition to providing rigidity [1,2] and it is hydrophobic [3]. All biomass types exhibit differences (often significant) in their lignin content, the ratios of the monolignols and their interbonding patterns. Agricultural lignins are composed of the three monomers, while softwood are made up of only the

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