Modifications of lignocellulosic fibers by Ar plasma treatments in comparison with biological treatments

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Abstract

Cold Ar plasma treatments can be used to modify the structure of lignocellulosic fibers for a variety of applications. In order to better understand the effects of such treatments, the type and amount of radicals formed on lignocellulosic fibers obtained from chemical pulp and chemothermomechanical pulp were evaluated by Electron Paramagnetic Resonance (EPR) spectroscopy and the chemical modifications induced on the same fibers by plasma treatment were assessed by Nuclear Magnetic Resonance (13C-NMR) spectroscopy and Gel Permeation Chromatography (GPC) analysis. The obtained results were compared with those obtained after enzymatic treatments of lignocellulosic fibers reported in literature.

Introduction

An increasing concern for the environment has given impetus to research on lignocellulosic fibers for total or partial substitution of petroleum-based synthetic fibers, which are neither renewable nor biodegradable [1]. Lignocellulosic fibers are composite materials of lignin, cellulose, hemicellulose and extractives, in which lignin, with hemicellulose, is situated as filler between the highly ordered cellulose microfibrilles [2]. In the field of packaging applications, materials with high barrier and mechanical properties are generally required and lignocellulosic fibers can achieve these properties by modifying the chemical and physical properties of their surface. In order to obtain this result, the radicalisation of lignocellulosic fibers by enzymatic, chemical and physical means has been extensively investigated during the last twenty years [3]. Among these, the most promising technologies are non-equilibrium plasma and enzymatic treatments with laccase [4], since they can provide high modifications through the formation of high amount of radicals with low environmental impact. A laccase treatment of thermomechanical wood fibers has been shown to increase fiber bonding through the activation of lignin surface [5], while Young evaluated the radical formation by EPR and surface modification by ESCA on juta lignocellulosic fibers after plasma treatment [6]. In order to better understand the effects of plasma treatment on lignocellulosic fibers, in the present paper the type and amount of radicals formed after radiofrequency (RF) non-equilibrium plasma treatment were studied by electron paramagnetic resonance (EPR) spectroscopy. Moreover, the generation of radicals in fibers was related to structural changes of lignin units detected by 13C-nuclear magnetic resonance (13C-NMR) spectroscopy and gel permeation chromatography (GPC). The investigated fibers were obtained from chemothermo-mechanical pulp (CTMP), and from chemical pulp (kraft). The obtained results are compared with those obtained after radicalisation of fibers with laccase [7], [8], [9].

Section snippets

Wood fibers

Two commercial lignin-rich fibers obtained after chemothermo-mechanical treatment (unbleached softwood CTMP) and chemical treatment (unbleached softwood kraft) were examined. The amounts of lignin in pulps were evaluated by Klason methods [10] and came back with results of 27.3% for CTMP and 10.5% for kraft. The fibers were frozen after pulp and conditioned to the ambient environment just before usage.

Radicalisation of fibers

Fibers were treated in a cold Ar plasma reactor which has been previously described [11], [12].

Radicalisation of fibers

In order to determine the type and amount of radicals formed by plasma treatment, ctmp and kraft fibers were characterised before treatment by epr spectroscopy. On these fibers, the signals typical of phenoxy radicals (g=2.004 and ΔHpp=9 g) were observed [17]. These signals have absolute intensities of 1016 and 2×1016 spin/g for ctmp and kraft, respectively, and probably are photochemically induced and/or caused by thermal, mechanical or chemical treatment during pulp production.

After EPR

Conclusions

For both CTMP and kraft fibers, the cold Ar plasma treatment promotes the formation of a relevant amount of phenoxy radicals. However, the amount of radicals detected by EPR after treatment on kraft fibers is higher than that observed on CTMP fibers.

In both cases, the chemical structure of lignin is heavily modified by plasma treatment. The amount of phenoxy groups decreases in both cases, but for CTMP fibers the decrease is much higher than for kraft ones. This result suggests that radicals

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