Kraft lignin behavior during reaction in an alkaline medium

Abstract

The reaction of kraft lignin in an alkaline medium was studied using a microreactor set. Chemical changes in reacted kraft lignins that include ash content, Klason lignin, acid-soluble lignin and sugars were studied. Structural characteristics of these lignins were also determined in terms of phenolic-hydroxyl, aliphatic-hydroxyl, methoxyl, M_w, M_n and polydispersity. The techniques employed were HPLC, UV spectroscopy, FTIR spectroscopy, proton nuclear magnetic resonance spectroscopy and organic gel permeation chromatography (GPC).

The effects of temperature and reaction time on lignin properties were studied using response surface methodology. The reaction temperature ranged from 116 to 180 °C and the reaction time ranged from 18 to 103 min. The obtained response surfaces show that both factors affected lignin properties within these ranges. The phenolic and aliphatic-hydroxyl content and the number of active sites increased when the treatment severity was increased. Weight-average molecular weight (M_w), number-average molecular weight (M_n) and solid-yield percentage decreased when the treatment severity was increased.

Introduction

With the exception of cellulose, no other lignocellulosic renewable resource is more abundant than lignin. Lignin is an amorphous, polyphenolic material arising from the copolymerization of three phenylpropanoid monomers: coniferyl, sinapyl, and p-coumaryl alcohols. These structures are linked by a multitude of interunit bonds that include several types of ether (α-O-4, β-O-4, 4-O-5) and carbon–carbon linkages. Lignin is a highly branched, three-dimensional polymer with a wide variety of functional groups providing active centers for chemical and biological interactions. In wood, the lignin content generally ranges from 19 to 35% [1]. It is extracted by several pulping techniques and as a by-product of the ethanol production process. It is inexpensively and available in large quantities.

Technical lignins are divided into two categories [2]. The first comprises sulfur-containing lignins, including lignosulfonates and kraft lignins, which are produced in huge quantities. Lignosulfonates are found in large quantities (around 1 million tonnes of solids per year), and kraft lignins are found in more moderate quantities (around 100,000 tonnes of solids per year) [2]. Conventional lignins that are used industrially are mainly obtained from softwoods. The second category comprises sulfur-free lignins obtained from many different processes. Most of these, soda lignins, organosolv lignins, steam-explosion lignins, bioethanol lignins, and oxygen-delignification lignins, are not yet produced commercially. Of these, only the soda lignins have the short-term potential for industrial availability. Hydrolysis lignins offer important new opportunities, such as the production of bioethanol to replace fossil fuels for transportation. They may come from woods or non-woods. Almost all of the lignins extracted from lignocellulosic materials in the pulp and paper (P&P) industry are burned to generate energy and recover chemicals. Only 1–2% of the lignins produced in the P&P industry are used commercially. Lignins can be used in a wide range of products, including materials for automotive brakes, wood-panel products, phenolic resins, biodispersants, polyurethane foams, epoxy resins for printed circuit boards, and surfactants [2], [3], [4]. In our laboratory, kraft lignin has been successfully converted into activated carbon and used in the production of fibreboard [5], [6]. Derivatives of other technical lignins such as adhesives, especially lignin–phenol–formaldehyde resins, are also being developed.

Kraft lignins are clearly underutilized, as nearly all lignins produced in the P&P industry are burned to generate energy and recover chemicals. However, industrial applications are only possible if lignin’s added value is enhanced and its quality and properties are standardized. Researchers currently face several problems that could be overcome, such as the low purity, heterogeneity, odour and colour of lignin-based products and the absence of reliable analytical methods for characterizing lignin [2]. Lignins must have acceptable purity and the desired chemical and physical properties if they are to be used for any industrial applications. One attractive area for commercial lignin applications is wood adhesives, for instance phenolic resins.

Lignin is more readily available, less toxic and less expensive than phenol. Using lignin to replace phenol in phenol–formaldehyde resin, where the price is subjected to fluctuations in oil prices and supply is generally lower than demand, is considered a potentially attractive application from the economic and environmental points of view. Moreover, this polymer is obtained from renewable resources, can be used without previous treatment and has a similar chemical structure to that of phenol–formaldehyde resins [7].

There is a considerable mass of literature on partially replacing the phenol in phenolic resins with lignin [7], [8], [9], [10], [11]. Because of their low phenolic-hydroxyl content, high ring substitution and steric hindrance, the reactivity of industrial lignins is much lower than that of phenolic resins [12]. Only a limited amount of industrial lignin can therefore be used as a direct replacement for phenol in the formulation of phenolic resins without losing adhesive properties [9], [10], [11]. However, a higher level of replacement can be achieved if modified lignins are used. Modification enhances the reactivity of lignins by increasing their functionality via demethylation, phenolation and methylolation. These modification techniques have been widely studied [13], [14]. Several studies have focused on producing high-functionality (i.e., high-reactivity) lignin in the pulping process, which is desirable for most applications, by optimizing the operation conditions to produce both a good pulp and a high-reactivity lignin [15]. These studies have confirmed that although very severe conditions produce high-functionality lignin, they also lead to a loss in pulp quality. These conditions therefore cannot be used because pulp is the main product of the process. Alkaline depolymerization of lignin has recently been proposed as an easier process that obtains lignin with properties suitable for use in adhesives for particleboards production [16].

The hydrolysis of lignin in an alkaline medium degrades lignin molecules so that new phenolic-hydroxyl groups can be generated. Under proper conditions, this technique yields reactive degradation products that can be used in the condensation reaction that forms phenolic resins. Another advantage of this technique is that the hydrolyzed products can be used directly in the synthesis of phenolic resins.

A lignin’s suitability for manufacturing of lignin–phenol–formaldehyde resins depends on the sources of wood and on the intensity of the delignification process, which determines whether it has enough free phenolic-hydroxyl groups and whether the ortho and para positions of its phenyl rings are blocked by methoxyl groups or aliphatic side chains [9].

In this study, the behavior of kraft lignin was observed during its reaction in an alkaline medium. The optimum reaction conditions for producing lignin with an acceptable purity and properties suitable for adhesives production were determined. The structural changes and purity of all kraft lignins were studied using FTIR spectroscopy, proton nuclear magnetic resonance (¹H-NMR) spectroscopy, ultraviolet (UV) spectroscopy and gel permeation chromatography (GPC).