Comparison of tension wood and normal wood for oxidative nanofibrillation and network characteristics

Simon Jonasson,Oisik Das,Anne Bünder,Kristiina Oksman,Totte Niittylä

doi:10.1007/s10570-020-03556-1

Abstract

Cellulose nanofibrils (CNFs) are top-down nanomaterials obtainable from abundant lignocelluloses. Despite recent advances in processing technologies, the effects of variations in the lignocellulose structure and composition on CNF isolation and properties are poorly understood. In this study, we compared the isolation of CNFs from tension wood (TW) and normal wood (NW) from Populus tremula (aspen). The TW has a higher cellulose content, native cellulose fibrils with a larger crystalline diameter, and less lignin than the NW, making it an interesting material for CNF isolation. The wood powders were oxidized directly by 2,2,6,6-tetramethylpiperidin-1-oxyl, and the morphology and mechanical behaviors of the nanofibril suspensions and networks were characterized. The TW was more difficult to fibrillate by both chemical and mechanical means. Larger nanofibrils (5–10 nm) composed of 1.2 nm structures were present in the TW CNFs, whereas the NW samples contained more of thin (1.6 nm) structures, which also comprised 77% of the solid yield compared to the 33% for TW. This difference was reflected in the TW CNF networks as decreased transmittance (15% vs. 50%), higher degree of crystallinity (85.9% vs. 78.0%), doubled toughness (11 MJ/m3) and higher elongation at break (12%) compared to NW. The difference was ascribed to greater preservation of the hierarchical, more crystalline microfibril structure, combined with a more cellulose-rich network (84% vs. 70%). This knowledge of the processing, structure, and properties of CNFs can facilitate the breeding and design of wood feedstocks to meet the increasing demand for nanoscale renewable materials.Graphic abstract

Highlights

Cellulose nanofibrils (CNFs) are high-aspect-ratio fibers isolated from the native cellulose microfibrils that comprise the cell walls of all plants
TEMPO oxidation of wood samples The NaOH consumption versus time during the oxidation of tension wood (TW) and normal wood (NW) is shown in Fig. 1a; the rate and amount consumed for both types of wood were initially similar
TW and NW from P. tremula were subjected to experimental nanofibrillation by mechanical, chemical, and chemical–mechanical processing, which was mediated by direct TEMPO oxidation of the milled wood

Summary

Introduction

Cellulose nanofibrils (CNFs) are high-aspect-ratio fibers isolated from the native cellulose microfibrils that comprise the cell walls of all plants. Wood from managed forests and tree plantations is the largest source of cellulosic biomass, and potentially of CNFs, worldwide. The isolation of CNFs from woody feedstock has attracted increasing attention. Efficient chemical pretreatment enables mechanical disintegration of wood pulps into CNFs. In particular, 2,2,6,6tetramethylpiperidin-1-oxyl (TEMPO) has proven effective in loosening the cell walls for enhanced fibrillation (Saito et al 2007). Increased efforts have been made to exploit the nanoscale nature of the fibers for various applications, including high-volume/ low cost components and high-tech applications (Dufresne 2017). The perceived environmental drawbacks of established pretreatments (Jiang et al 2017) have driven the development of greener processing methods and materials, for example, deep-eutectic solvents (Li et al 2017), ionic liquids (Li et al 2012), and electro-mediated (Isogai et al 2011) or laccasemediated TEMPO oxidation (Jiang et al 2017)

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