Effects of mechanical fibrillation time by disk grinding on the properties of cellulose nanofibrils

Abstract

Cellulose nanofibrils (CNF) were successfully produced from a bleach kraft eucalyptus pulp by a supermasscolloider. Effects of grinding time on structure and properties of CNF and the corresponding CNF films were investigated. Grinding time was important to increase the optical transparency of CNF suspensions. The degree of polymerization (DP) and crystallinity index (CrI) of CNF decreased linearly with the increase in CNF suspension transparency. This suggests optical transparency of a CNF suspension can be used to characterize the degree of fibrillation. Specific tensile strength and Young’s modulus of the CNF films made of CNF suspension with only 0.5 h grinding were increased approximately 30% and 200%, respectively, compared with conventional handsheets prepared by valley beating to 300 Canadian Standard Freeness (CSF). Energy input was only 1.38 kWh/kg for 0.5 h grinding. Grinding beyond 0.5 h produced negligible improvement in specific tensile and specific modulus. Opacity of CNF films decreased rapidly during the first 1.5 h of fibrillation and then plateaued. Application: Disk milling time affects the morphology of cellulose nanofibrils as well as the optical and mechanical properties of film made of the resultant fibrils. Cellulose nanomaterials, such as cellulose nanofibrils (CNF) derived from renewable lignocelluloses, have attracted great interest recently. Lignocelluloses are available in nature in great abundance. Cellulose nanofibrils have been used for producing a range of functional materials including films, membranes, aerogels, scaffolds, and hybrid composites [1-4] and have the potential to replace a variety of materials derived from nonrenewable petroleum. Mechanical fibrillation remains the most common approach to produce CNF from lignocelluloses. Microgrinding has the potential for large-scale CNF production and has been widely used [5-9]. Microgrinding leads to a series of dramatic changes in fibers, such as internal fibrillation, external fibrillation, and fiber shortening. Continued fibrillation resulted in fragmentation of cell wall and produced microand nanofibrils [10]. The dominant factors that dictate nanocellulose material strength are the fibril length and fiber bonding. The orientation of bonds between nanoparticles is an important factor in tuning the Young’s modulus [11]. Increased grinding often results in increased bonding as a result of the fine materials produced that substantially increase fibril surface area. On the other hand, increased grinding time can also result in short fibrils simply because of mechanical actions. There is a tradeoff between increasing bonding and reducing fibril length with extended grinding; in other words, an optimal grinding time exists for producing CNF for polymer reinforcement. Unfortunately, such an understanding has not been well documented. The objective of the present study is to investigate the effects of mechanical fibrillation time on the properties of resultant CNF films. MATERIAL AND METHODS Dry lap of a bleached kraft eucalyptus pulp from Fibria (Aracruz, Brazil) was the same pulp used in previous studies [10,12–14], with major chemical composition of 78.1% ± 1.0% glucan, 15.3% ± 0.6% xylan, and 0.7% ± 0.1% Klason lignin. The dry lap was soaked in distilled water for 24 h before disintegration in a laboratory disintegrator. Mechanical fibrillation For the experiment, 140 g of o.d. bleached kraft eucalyptus pulp was fibrillated at 2 wt% consistency using a supermasscolloider (model MKZA6-2, Masuko Sangyo; Kawaguchi, Japan) at 1500 rpm as described previously [10]. Approximately 130 g was beaten in a Valley beater (Valley Laboratory Equipment, Voith; Appleton, WI, USA) to approximately 300 mL Canadian Standard Freeness (CSF) as a control sample. Fractionation of CNF by centrifuge An attempt was made to fractionate large networked CNF from small ones by centrifuge. Cellulose nanofibrils solution from 11 h of fibrillation was diluted to 0.2 wt% and continuously stirred for 1 h and then centrifuged at 1000 rpm for 15 JUNE 2016 | VOL. 15 NO. 6 | TAPPI JOURNAL 419 CELLULOSE NANOFIBRILS 1. Transmission electron microscopy images of cellulose nanofibrils (CNF) samples at different fibrillation times: (a) 0.5 h, (b) 3 h, (c) 7 h, and (d) 11 h; scale bar = 500 nm. min in a Sorvall Superspeed RC2-B 5.75-in. rotator (Ivan Sorvall; Norwalk, CT, USA). The supernatant liquid was carefully removed with a pipette from the precipitation layer. Preparation and testing of CNF film Solutions of CNF were diluted to 0.1 wt% and mixed for 4 h using a magnetic stir. A 9-in. vacuum filtration system with a 0.45-μm Durapore membrane (Millipore; Billerica, MA, USA) was used to prepare CNF film. Wet films together with blotting paper were pressed at 206 and 345 kPa for 3 min and then dried in a copper dry ring. Film opacity, basis weight, and thickness were measured according to TAPPI T519 om-06 “Diffuse opacity of paper (d/0 paper backing),” T410 om-08 “Grammage of paper and paperboard (weight per unit area),” and T411 om-10 “Thickness (caliper) of paper, paperboard, and combined board,” respectively. Strain-stress testing was determined using an Instron 5865 Advanced Mechanical Testing System (Instron; Norwood, MA, USA). Conventional handsheets as control were also made using the Valley-beaten pulp (control sample) and tested using the same procedure.