Microfibrillated lignocellulose (MFLC) and nanopaper films from unbleached kraft softwood pulp

Erfan Oliaei,Fredrik Berthold,Pär A Lindén,Qiong Wu,Lars Berglund,Tom Lindström

doi:10.1007/s10570-019-02934-8

Abstract

Microfibrillated cellulose (MFC) is an important industrial nanocellulose product and material component. New MFC grades can widen the materials property range and improve product tailoring. Microfibrillated lignocellulose (MFLC) is investigated, with the hypothesis that there is an optimum in lignin content of unbleached wood pulp fibre with respect to nanofibril yield. A series of kraft fibres with falling Kappa numbers (lower lignin content) was prepared. Fibres were beaten and fibrillated into MFLC by high-pressure microfluidization. Nano-sized fractions of fibrils were separated using centrifugation. Lignin content and carbohydrate analysis, total charge, FE-SEM, TEM microscopy and suspension rheology characterization were carried out. Fibres with Kappa number 65 (11% lignin) combined high lignin content with ease of fibrillation. This confirms an optimum in nanofibril yield as a function of lignin content, and mechanisms are discussed. MFLC from these fibres contained a 40–60 wt% fraction of nano-sized fibrils with widths in the range of 2.5–70 nm. Despite the large size distribution, data for modulus and tensile strength of MFLC films with 11% lignin were as high as 14 GPa and 240 MPa. MFLC films showed improved water contact angle of 84–88°, compared to neat MFC films (< 50°). All MFLC films showed substantial optical transmittance, and the fraction of haze scattering strongly correlated with defect content in the form of coarse fibrils.Graphic abstract

Highlights

In the late 1970s, cellulose fibres were for the first time fibrillated in a process where pulp fibres were passed through a homogeniser (Herrick et al 1983; Turbak et al 1983)
The present Microfibrillated lignocellulose (MFLC) is of high technical relevance due to the use of unbleached kraft fibres readily available in industry
Optimum conditions were sought so that the resulting fibres could be readily disintegrated into microfibrillated lignocellulose (MFLC)

Summary

Introduction

In the late 1970s, cellulose fibres were for the first time fibrillated in a process where pulp fibres were passed through a homogeniser (Herrick et al 1983; Turbak et al 1983). Since these pioneering efforts, microfibrillated cellulose (MFC) has recently been developed into industrial products. Fibrillation of wood cellulose fibres is energy-intensive but can be facilitated by pre-treatment such as induction of electrostatically charged groups, mechanical treatment or mild acid or enzymatic treatment (Klemm et al 2018; Moon et al 2011). Charged groups can be introduced to fibril surfaces in the wood pulp fibre cell wall during pulping and bleaching operations or later by carbonyl or carboxyl groups through e.g. TEMPO (2,2,6,6tetramethylpiperidinyloxyl) catalyzed oxidation (Atalla and Isogai 2010; Isogai et al 2011) or through e.g. carboxymethylation (Klemm et al 2018; Wagberg et al 2008), or cationic functionalities (Olszewska Eronen et al 2011)

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Results

Conclusion