Nanofibrillated cellulose (CNF) from eucalyptus sawdust as a dry strength agent of unrefined eucalyptus handsheets

Highlights

• Eucalyptus sawdust was fractionated to remove hemicelluloses (91%) and lignin (99%).

• CNF was produced from the cellulosic fraction by TEMPO oxidation and homogenization.

• The specific surface and diameter of CNF were 60 m² g⁻¹ and 41.0 nm, respectively.

• Tensile Index of handsheets increased 95% by addition of 9% CNF (27 to 55 N m g⁻¹).

• The increase in Tensile Index was equivalent to 1600 rpm of PFI refining.

Abstract

Nanofibrillated cellulose has been obtained from the cellulosic fraction of eucalyptus sawdust. The fractionation process involved the partial removal of hemicelluloses and lignin. CNF was obtained using TEMPO oxidation with NaOCl in basic medium followed by mechanical homogenization. The obtained CNF was subsequently used as a dry strength agent on unbleached unrefined eucalyptus pulp. The addition of 3, 6 and 9 wt.% of CNF increased lineally the tensile index of handsheets to about 55 N m g⁻¹ at 35°SR, compatible with papermachine runnability. The other mechanical properties also increased substantially, and porosity decreased moderately. The estimated specific surface and average diameter of these CNF were 60 m² g⁻¹, and of 41.0 nm, respectively. The addition of 9 wt.% of CNF produced an increase in mechanical strength, equivalent to that produced by PFI refining at 1600 revolutions.

Introduction

Biomass is the main biopolymer source for sustainable and renewable uses in chemical, material and fuel bio-products. The demand for bio-based products with equal or better properties than conventional petroleum-based derived products has generated great market opportunities. Nanotechnology is an emerging area of science and technology that has revolutionized the field of materials. The application of nanotechnology will exploit the potential of the forest and pulp and paper industries as a platform for bio-products.

The forest processing industry generates globally around 180 million of cubic metric of lignocellulosic residues (FAOSTAT, 2014) such as sawdust, bark and other wood waste. The excess global sawdust potential after summit raw material demand of the forest industry was estimated of 75 million of cubic metric. More than 67% of global sawdust potential is located in Brazil, Canada and China (Heinimö, Pakarinen, Ojanen, & Kässi, 2007). Eucalyptus and pine sawdust are important byproducts of the primary processing of wood in Argentina, Brazil and Chile which so far have not found a successful exploitation. In some cases these residues represent a problem and its treatment consumes resources of management, treatment and disposal. In this context, governments have established environmental regulations about its disposal in order to promote their use and evaluate its potential as a renewable resource. Usually, sawdust is used to obtain low value added products as pellets and briquettes (bioenergy), charcoal (adsorbent), filler (composites) and wood panels (furniture and construction materials).

An advantage of sawdust is that it does not require further mechanical treatment for size reduction. Its main disadvantage is the heterogeneity of the material, because sawmills process woods from different origins, which generates a mixture of sawdust in which species and age are difficult to know.

By chemical pulping processes, biomass is fractionated to obtain a pulp rich in cellulose, lignin and other chemical components. This general principle can be used to fractionate sawdust, separating all components by sequential processes, thus producing high value products that can be used in different applications to improve the profit, following the bio-refinery concept. Although sawdust has been tested to make pulp for paper, product quality is not good. However, the possibility of producing CNF with this resource appears as an excellent chance of valorization of this waste. In this form, wood can be fully exploited and traditional pulping can be combined with new technologies, conversion infrastructure, and technical knowhow, to develop new bio-based and high value added products.

The autohydrolysis of lignocellulosic materials in aqueous medium at elevated temperature in a pressure reactor is one of the most effective treatments for hemicelluloses solubilization (Garrote et al., 2003, Sasaki et al., 2003). The partial solubilization of hemicelluloses (70–90%) to oligosaccharides and monosaccharides takes place, which can be used for different purposes (Teramoto, Tanaka, Lee, & Endo, 2008). Hardwood and grass species are more suitable for the autohydrolysis treatment than conifers, due to their higher content of acetyl groups which provide an increase of catalyst concentration in the reaction medium. On the other hand, lignin can be removed by alternative pulping processes such as kraft, soda-anthraquinone or organosolv pulping to obtain sulfur-free lignin.

The use of the cellulosic fraction to produce micro and nanofibrillated cellulose (MFC or CNF, respectively) grew strongly in recent years due to their unique properties, renewable nature and large availability. CNF and MFC are fibrils or aggregates of cellulose with a diameter between 5 and 60 nm and several microns in length. CNF is obtained from the delamination of the cellulosic fibers by mechanical treatment of chemical or enzymatically pretreated fibers (González et al., 2012). The processes used affect the structure, properties, and price of CNFs, therefore the great challenge of research studies is to find a process to produce large amounts of cellulose nanofibers at low-cost (low amount of energy, inexpensive reagents and efficient enzymes). This process should keep the properties of cellulose and generate nanofibers with uniform size distribution (length and diameter). CNF is produced by pumping the pretreated fiber slurry (consistency 1–2%) in a high pressure homogenizer where the fibers are forced to collide with a valve and an impact ring. This procedure generates shear forces and a pressure drop, causing fiber delamination and the release of the microfibrils from the fiber wall layers. The fibrous slurry acquires the appearance and characteristics of a gel after several passes through the homogenizer. Micro-fibrillation may be facilitated by TEMPO oxidation in water (Nakagaito & Yano, 2004), which consists of a selective surface catalytic oxidation of the primary hydroxyls of cellulose at the C6 position to carboxylate groups by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) (Saito and Isogai, 2004, Saito et al., 2007, Saito et al., 2006). This chemical pretreatment is effective in the cellulose oxidation and also reduces the energy consumption in the subsequent mechanical processing for MFC and CNF production. Recently, this methodology has been used to facilitate the mechanical treatment of cellulosic fibers (Isogai, 2013, Isogai et al., 2011, Puangsin et al., 2013).

Since the advent of nanotechnology, CNF has been used for nanopaper manufacture and many authors have also studied the effect of nanoparticles (mostly, MFC) on conventional paper. CNF applied directly in the paper sheet showed the enhancement of the mechanical strength and the improvement of the filler retention in the wet-end. This was attributed to the increment of the number of hydrogen bonds fibril-fibers (Kajanto & Kosonen, 2012). Similar to cellulosic fines, CNF increases the network strength due to its high surface area and flexibility. CNF has several advantages as reinforcing element: small diameter, high aspect ratio, biocompatibility, functionalization ability, and high strength and modulus. These characteristics become attractive to CNF as dry strength additive for paper. The use of CNF as reinforcement improves the permeability and tensile index of papers (Eriksen, 2008, González et al., 2012, Mörseburg and Chinga-Carrasco, 2009, Nakagaito and Yano, 2004, Taipale et al., 2010). The increase of paper strength with CNF addition can be compared with strength gain after a slight refining (González et al., 2012). Also it has been shown that CNF addition increases the strength of canola waste chemithermomechanical pulp in fluting and linerboard manufacture for packing papers (González, Alcalá, Arbat, Vilaseca, & Mutjè, 2013). The addition of CNF to an unrefined pulp produce handsheets with higher tensile strength than standard handsheets (Bardet & Bras, 2014) allowing significant reduction of grammage and energy saving (refining or drying process). Nevertheless, CNF have different chemical natures and morphologies depending on the source and process, so results are variable (Bardet & Bras, 2014). However, the addition of CNF as dry strength additive in the papermaking furnish decreases the drainage capacity of the slurry (González et al., 2012, González et al., 2014), limiting the amount of CNF that could be added.

The aim of this study was to evaluate the effect of the addition of CNF obtained from the cellulosic fraction of eucalyptus sawdust on the mechanical properties of paper made from unrefined unbleached eucalyptus pulp.