Modification of ammonium lignosulfonate by phenolation for use in phenolic resins

Abstract

The structural modification of softwood ammonium lignosulfonate by phenolation was studied. A central factorial design was applied to determine the influence of reaction conditions (temperature, time and lignosulfonate content) on the properties of phenolation products. The reaction was monitored through the concentration of free phenol determined by gas chromatography (GC). The characterization of phenolation products was accomplished by gel permeation chromatography (GPC), fourier transform infrared (FTIR) and ¹H nuclear magnetic resonance (¹H NMR). Empirical models were developed to predict phenol conversion and adduct formation according to operating conditions. The optimum phenolation conditions, attained by means of a response surface method, were found to be 120 °C; 160 min and 30% lignosulfonate content.

Introduction

The production of phenol–formaldehyde (PF) resins has not undergone significant changes for decades. However, the oscillations in crude oil price have generated a great interest in development of alternatives to oil-derived binders. Thus, naturally occurring raw materials such as lignin and furfuryl alcohol offer interesting possibilities as substitutes for phenol (Gardziella et al., 2000). The idea of using lignin materials in phenolic resins is not new. Kratzl et al. (1962) studied the possibility of utilizing lignosulfonates in phenol-formaldehyde resins. Sakakibara (1974) employed concentrated sulphite liquor and heated it with phenol and formaldehyde. The resin obtained was used in the manufacture of boil-resistant plywood. Allan et al. (1989) and Ysbrandy et al. (1992) employed two lignin based compounds as co-reactants for phenolic laminating resins.

On the basis of its structure, lignin may be considered a type of phenolic condensate in the broadest sense of the term, making it logical to use it in the production of phenolic resins. Lignin represents a widely occurring basic substance that can exhibit various structures depending on the vegetal species it is derived from, but in contrast to other biopolymers it exhibits no regular structure. Moreover, the structural changes suffered by lignin during the pulping process can exert a great influence on its characteristics.

Technical lignins can be obtained by different pulping processes but only lignosulfonates are available in great quantities. Among them, ammonium lignosulfonates (LAS) have been reported to be the most adequate to formulate PF resins (Allan et al., 1989; Peng, 1994). Lignosulfonates are a type of technical lignin whose utilization in polymer synthesis is still insufficiently developed. There are certain properties of lignosulfonates which make them a potential substitute for phenol in PF resins. For example, like phenol–formaldehyde resins, they contain large molecules that allow quick gelation. There are, however, certain limitations, such as the insufficient number of reactive sites in lignosulfonate molecules. In the early studies on this subject non-modified lignosulfonates were incorporated into phenolic resins, but the current trend is to modify the lignosulfonate chemical structure in order to increase the reactivity towards formaldehyde (Falkehag, 1975; Allan et al., 1989; Alonso et al., 2001). Of all modification treatments, those that increase reactive sites in lignin, such as methylation and phenolation, are the most effective for formulation of PF resins (Nada et al., 1987; Peng, 1994; Sellers et al., 1994).

In the phenolation reaction, the first step involves the protonation of the benzyl hydroxyl group, followed by dehydration at the α-carbon, to give a carbonium ion. The phenol molecule undergoes an electrophilic attack by carbonium ion giving rise to a phenol condensation product (adduct). After the incorporation of the ortho- or para-phenyl substituent to the α-hydroxyl groups of the propane side chains of lignin the adduct fragmentation takes place (Kratzl et al., 1962; El-Saied et al., 1984; Nada et al., 1987; Ysbrandy et al., 1992; Vázquez et al., 1997). These steps result in a decrease of molecular weight (Calvé et al., 1988), which favours the incorporation of phenolation products to the resin. Some side reactions can also occur. Thus, depending on the reaction conditions, the carbonium ion can react with a lignosulfonate molecule leading to a self-condensation product (Lindberg et al., 1989).

Up to date few works have been reported on the modification of lignosulfonates by phenolation, in spite of its great interest for the possible application in the formulation of novolac resins (Mathur, 1982; Ysbrandy et al., 1992; Vázquez et al., 1997). These materials are used in large quantities to produce molding compounds, textile felts, foundry, refractories, abrasives and friction linings (Shimatani and Yoshihiro, 1995; Gardziella et al., 2000). Since both synthesis of novolac resins and lignin phenolation are carried out in acidic medium, phenolation is an interesting way of modifying lignosulfonates. Besides, unreacted phenol would not need to be removed from the phenolated product. It would be incorporated into novolac formulation mixture (Allan et al., 1989).

In this work, the modification of lignosulfonates by phenolation was studied in order to obtain a suitable material for the formulation of PF resins (novolac). LAS was selected as starting material because this lignin derivative is soluble in liquid phenol. Besides, carbohydrates contained in LAS react with phenol under acidic conditions and contribute to phenolation (Mudde, 1980; Allan et al., 1989). The extent of the reaction was followed through phenol consumption, determined by gas chromatography, and the structural changes of lignosulfonate were analyzed by FTIR, ¹H NMR spectroscopy and gel permeation chromatography (GPC).