Mechanical fabrication of high-strength and redispersible wood nanofibers from unbleached groundwood pulp

Cellulose, Jul 2017

In the past, the direct production of lignin-containing nanofibers from wood materials has been very limited, and nanoscale fibers (nanocelluloses) have been mainly isolated from chemically delignified, bleached cellulose pulp. In this study, we have introduced a newly adapted, heat-intensified disc nanogrinding process for the enhanced nanofibrillation of wood nanofibers (WNF) with a high lignin content (27.4 wt%). The WNF produced this way have many unique and intriguing properties in their naturally occurring form, for example, being able to be dispersed in ethanol and having ethanol solution viscosities higher than water solution viscosities. When WNF nanopapers were formed with ethanol, the properties of the nanofibers were recoverable without a notable decrease in the viscosity or mechanical strength after redispersing them in water. The preservation of lignin in the WNF was noticed as an increase in the water contact angles (89°), the rapid removal of water in the fabrication of the nanopapers, and the enhanced strength of the nanopapers when subjected to high pressure and heat. The nanopapers fabricated from the WNF were mechanically stable, having an elastic modulus of 6.2 GPa, a maximum stress of 103.4 MPa, and a maximum strain of 3.5%. Throughout the study, characteristics of the WNF were compared to those of the delignified and bleached reference cellulose nanofibers. We envision that the exciting characteristics of the WNF and their lower cost of production compared to that of bleached cellulose nanofibers may offer new opportunities for nanocellulose and biocomposite research.

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Mechanical fabrication of high-strength and redispersible wood nanofibers from unbleached groundwood pulp

Mechanical fabrication of high-strength and redispersible wood nanofibers from unbleached groundwood pulp 0 R. Sliz Optoelectronics and Measurement Techniques Unit, University of Oulu , P.O Box 4500, 90014 Oulu , Finland 1 M. Visanko (&) J. A. Sirvio ̈ P. Piltonen H. Liimatainen M. Illikainen Fibre and Particle Engineering Research Unit, University of Oulu , P.O. Box 4300, 90014 Oulu , Finland In the past, the direct production of lignincontaining nanofibers from wood materials has been very limited, and nanoscale fibers (nanocelluloses) have been mainly isolated from chemically delignified, bleached cellulose pulp. In this study, we have introduced a newly adapted, heat-intensified disc nanogrinding process for the enhanced nanofibrillation of wood nanofibers (WNF) with a high lignin content (27.4 wt%). The WNF produced this way have many unique and intriguing properties in their naturally occurring form, for example, being able to be dispersed in ethanol and having ethanol solution viscosities higher than water solution viscosities. When WNF nanopapers were formed with ethanol, the properties of the nanofibers were recoverable without a notable decrease in the viscosity or mechanical strength after redispersing them in water. The preservation of lignin in the WNF was noticed as an increase in the water contact angles (89 ), the rapid removal of water in the fabrication of the nanopapers, and the enhanced strength of the nanopapers when subjected to high pressure and heat. The nanopapers fabricated from the WNF were mechanically stable, having an elastic modulus of 6.2 GPa, a maximum stress of 103.4 MPa, and a maximum strain of 3.5%. Throughout the study, characteristics of the WNF were compared to those of the delignified and bleached reference cellulose nanofibers. We envision that the exciting characteristics of the WNF and their lower cost of production compared to that of bleached cellulose nanofibers may offer new opportunities for nanocellulose and biocomposite research. Nanogrinding; Nanofiber; Lignin; Viscosity; Nanopaper; Redispersion Introduction Lignocellulosic raw materials from plants (Bhatnagar and Sain 2005) , trees (Herrick et al. 1983) , and waste materials (Nair and Yan 2015a; Tarre´s et al. 2017) have been studied to explore their suitability for nanocellulose production. The properties of fabricated nanofibers depend on the method of processing and the fiber origin. In plants, cellulose nanofibers tend to be more loosely bound, due to their structural and chemical composition (Valadez-Gonzalez et al. 1999) , than in more organized wood structures having a high lignin content. Thus, the liberation of nanofibers from plant sources requires less chemical and mechanical processing, whereas wood-derived nanofibers (WNF) are mainly isolated solely from bleached, chemically delignified cellulose pulp. However, lignin-containing mechanical pulp fibers (e.g., thermomechanical pulp, or groundwood pulp; GWP) are attractive raw materials for nanofiber production at a price 50% lower (Arppe 2001) than that of chemical pulps and with a manufacturing yield of around 85–95% (Sixta 2006) . For this purpose, alternative processing methods are required to enable the breakage of the strong lignin matrix holds the wood fibers together. In addition, the inclusion of lignin components within the liberated nanofibers can result in a novel nanomaterial with chemical, mechanical, and surface properties different from traditionally fabricated cellulose nanofibers. This approach can also advance the discovery of novel applications without the need for the surface chemical functionalization of the nanofibers (Habibi 2014) , leading to a reduction in environmental stress and costs in the production process. Above all, the conservation of lignin is of interest because it increases the hydrophobicity of the nanofibers, which could increase their use in multiple applications, for example, flotation (Laitinen et al. 2016) , oil–water stabilization (Visanko et al. 2014a; Ojala et al. 2016) , and biocomposites (Herzele et al. 2016; Winter et al. 2017) . The most effective methods for cellulose nanofiber production have so far been based on the use of chemical (Saito et al. 2007; Liimatainen et al. 2012, 2013b) or solvent (Selka¨la¨ et al. 2016) pretreatments for the surface functionalization of bleached pulp fibers. Chemical functionalization involves many drawbacks compared to the use of fibers in their native state, considering the high cost of chemicals, their toxicity, and the difficulties of both recycling (Kuutti et al. 2016) and regenerating them (Liimatainen et al. 2013a) . Chemical treatments also tend to alter the properties of the resulting cellulose nanofibers, for example, by reducing the degree of polymerization (Lavoine et al. 2012) and drastically decreasing the thermal degradation threshold to around 200 C (Fukuzumi et al. 2009; Eyholzer et al. 2010) , which limits t (...truncated)


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Miikka Visanko, Juho Antti Sirviö, Petteri Piltonen, Rafal Sliz, Henrikki Liimatainen, Mirja Illikainen. Mechanical fabrication of high-strength and redispersible wood nanofibers from unbleached groundwood pulp, Cellulose, 2017, pp. 1-15, DOI: 10.1007/s10570-017-1406-7