(139d) Cellulose Nanofibers from Recycled Pulp: Production, Characterization and Application to Reinforce Recycled Paper

Papermaking is a sustainable sector where paper recycling has been recognized of a great importance [1, 2]. In Europe, 54% of the paper industryâ??s raw material comes from recovered paper which corresponds to a paper recycling rate of 72% [3]. However, one of the main problems in the utilization of secondary fibers is to maintain the quality levels of the products that is continuously increasing following customer demands. On the other hand, paper demand has decreased due to both economic recession and replacement of paper by other information supports, causing cost pressures in the paper industry. Although natural and synthetic strength additives are commonly used in recycled paper, poor tensile strength is still the main source of customer complaints to paper manufacturers. In this context, new strategies to improve interfiber bonding must be explored and the use of cellulose nanofibers (CNF) are alternative additives to increase mechanical properties of paper with some advantages, such as their renewable nature, biodegradability and potential high availability.

CNF have gained increasing attention due to their high strength and stiffness combined with their low weight. These properties make CNF promising for applications in fields such as papermaking, composites, packaging, electronic devices, coatings, biomedicine and automotive. Among all of the applications, their use in the papermaking industry can improve paper quality and many studies have proved that the addition of CNF in mass to the pulp suspension increases the mechanical properties of paper [4]. CNF are currently produced from wood, mainly from eucalyptus one, but the reality is that the CNF market is still very limited [5]. The main drawbacks to use CNF in large volume industrial sectors, such us papermaking, is the cost of the product due to the high amount of energy consumption during CNF processing, the difficulties in producing uniform nanocellulosic particles, the dewatering difficulties and the problems to pump at high concentration [6]. A potential solution is to produce the CNF on-site from the mill pulp to avoid drying and transport costs, as well as dilution and dispersion problems.

Therefore this paper focuses on the on-site production of CNF from a cellulose source available at the recycled paper mill and its direct application to reinforce the recycled paper.

The production of CNF was optimized by studying different TEMPO-mediated oxidation levels before the homogenization mechanical process. The obtained CNF were characterized and several doses were added to the recycled pulp to evaluate their effect in terms of paper strength enhancement.

CNF were obtained from recycled newsprint pulp by TEMPO-mediated oxidation using 2.5, 5, 10 and 15 mmol of NaClO per each gram of pulp [7]. Once the pulp was oxidized, a cleaning process was performed through filtration steps using distillate water to reach a pH value around 7. Finally, homogenization at 600 bar was carried out in a PANDA PLUS 2000 laboratory homogenizer manufactured by GEA Niro Soavi (Italy).

Characterization of the oxidized cellulose pulp was performed by measuring the amount of carboxilate groups, as an indicator of the oxidation degree achieved after TEMPO oxidation, by conductimetric titration. CNFs were characterized by determination of the nanofibrillation yield, the cationic demand, the transmittance and the polymerization degree. Nanofibrillation yield was measured by centrifugation of a diluted CNF suspension (0.1% wt) at 4500 g for 30 min in order to isolate the nanofibrillated fraction in the supernatant from the non-fibrillated fraction deposited in the sediment. Cationic demand was measured by colloidal titration of the diluted suspension at 0.05%, with 0.001 N polyDADMAC, using a particle charge detector, Mütek PCD04, manufactured by BTG Instruments GmbH (Herrsching, Germany). Transmittance of the CNF suspensions diluted at 0.1% were measured between 400 and 800 nm of wavelength using a Cary 50Conc UV-Visible spectrophotometer manufactured by Varian Australia PTI LTD. Polymerization degree was calculated from the limiting viscosity number (intrinsic viscosity) of CNF suspensions, which was determined following the international standard ISO5351/1 with cupriethylendiamine as solvent [8].

Pulps were prepared through disintegration of 20 g of dry recovered paper in 2000 mL of water, by using a Messmer pulp disintegrator (Mavis Engineering Ltd, London). The recovered paper with the correspondent amount of CNF was left to soak at least 24 h before disintegration to favor swelling. A three-component retention system was added to the pulp (1.25 mg/g of coagulant, 0.75 mg/g cationic polyacrylamide as flocculant and 1.7 mg/g hydrated bentonite clay based on industrial recommendations). The pulp was used to prepare handsheets with basis weight of 60 g/m² in a normalized handsheet former Rapid-Köthen (ISO 5269/2, DIN 54 358). These handsheets were physically and mechanically characterized by using an AUTOLINE 300 from Lorentzen & Wettre (Stockholm, Sweden).

Results showed that cheaper CNF can be prepared on-site from recycled paper through chemical pretreatment followed by homogeneization treatment. The amount of carboxilic groups of the oxidized pulp increased linearly with the TEMPO-mediated oxidation level from 0.09 to 1.04 mmoles/ g o.d. pulp, for non-oxidized and 15 mmoles NaClO/g o.d. TEMPO-mediated oxidized pulp, respectively. Maximum CNF yield was achieved with 15 mmoles NaClO/g o.d. up to 77%. Cationic demand and transmittance also increased with TEMPO-oxidation level. The highest oxidized CNF achieved a 29% increase of tensile strength index by adding 3 wt% of CNF to the recycled pulp.

ACKNOWLEDGEMENTS

The authors wish to thank the Economy and Competitiveness Ministry of Spain for the support of the project NANOSOLPAPELREC with reference CTQ2013-48090-C2-1-R.

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