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)