The consumption of recovered paper has been in continuous growth during the past decades. This development has been boosted by technological progress due to the environmental awareness, saving energy, population growth and the shortage of wood supply. Furthermore, the demand of paper has decreased due to both the economic recession and the replacement of paper by other information supports and the good price competitiveness of recycled fiber decreases the production cost of the papermaking process.
Recovery rates have been increasing in North America for over a decade. In 2013 more than 63% of the paper and paperboard consumed in the U.S. was recovered. Nowadays, old corrugated containers (OCC) are the most significant category of recycled papers, based on the use (19.8 million tons in 2013) and recovery rate (88.5%) of this raw material.
However, by increasing the number of cycles in the fiber recycling, the mechanical properties of paper and board is gradually reduced which is mainly due to the decrease in the fiber bonding strength through the hornification phenomenon. In addition, the substitution of cellulose fibers by fillers in the paper stock due to their lower cost and their ability to improve optical properties in the final product also affect the mechanical properties of the paper. Two problems are usually associated with the use of the fillers. First, filler particles added to fibers suspended in water are not easily retained in the forming sheet, because there are often too small to be entrapped mechanically and because filler particles and fibers are negatively charged, so they repel each other. Second, filler particles interfere with the fiber-fiber bonding; therefore the linting problems in the offset printing can increase and furthermore other mechanical properties as tensile strength of the filled paper decrease. Synthetic retention systems such as cationic polyacrylamide (CPAM) and dual systems based in CPAM and microparticles are generally used to increase the interaction between fillers and fibers, leading to improved retention. However, the use of those retention systems can further contribute to decrease the mechanical properties of the product. Consequently, the overcoming of the second problem associated to the use of fillers is not easy.
On the other hand, customers are demanding products with better mechanical properties and quality. Consequently, during the last decades, many researches are focused on the feasibility of using different natural and/or synthetic polymers to improve the strength of paper and board made from recycled fibers; however, poor tensile strength and linting are still the main source of customer complaints to paper manufacturers. Therefore, new ways to improve the strength of recycled paper products should be explored to meet the industry’s demand.
In this context, the use of nanofibrillated cellulose (CNF) has been studied to improve filler-fiber interaction and interfiber bonding and thus increasing tensile strength and reducing linting and dusting during printing process, since the mechanism of linting is generally a combined effect on the ink film splitting forces and the inter fiber bonding energy of the paper surface; at the same time that drainage and retention are improved.
The use of CNF presents some very interesting properties for papermaking such as a large specific surface area and high aspect ratio; moreover, it enhances tensile strength and reduces the porosity of the final paper sheet. Therefore, CNF is potential candidate for high filler-loaded papers and board as they are able to compensate for strength loss caused by the filler itself. However, the interaction of CNF and the filler during sheet forming is not yet well understood.
In this work, CNF was obtained from two different sources of cellulose: eucalyptus bleached pulp (E), and pine bleached pulp (P). Nanofibrillated material was obtained by TEMPO mediated oxidation, using 0.1 g NaBr, 0.016 g TEMPO and 5 mmol NaClO per each gram of pulp. The oxidation was performed at pH 10 by continuous addition of NaOH 0.5 M. Once the pulp was oxidized, a cleaning step was performed through filtration cycles using water. Finally, five steps of homogenization at 600 bar were applied to the oxidized pulp (1% consistency) in a laboratory homogenizer PANDA PLUS 2000 de GEA Niro Soavy (Parma, Italy). The yield in nanofiber material was higher than 95% in both CNF from the eucalyptus pulp (CNF-E) and CNF from pine pulp (CNF-P) and in both cases the transmittance at 800 nm was higher than 90%. The cationic demand and the polymerization degree of the CNF-E were the double than those for CNF-P.
Different dosages of additives and CNF (CNF-E and CNF-P) were individually added to the recycled furnish and the handsheet properties in terms of mechanical properties were investigated. In addition, new combinations were examined to find the optimal procedure for improving paper and paperboard properties. Moreover, the interaction between fillers and CNF was studied in order to determine their effects on drainage time, retention and strength properties of the handsheets. The results showed CNFs to have a potential to enhance the mechanical properties of the recycled paper and board they could be combined with traditional strength aids and with fillers for improving the quality of the final paper. Furthermore, the retention and drainage process were not significantly affected providing the dose of the retention aids is adapted to the presence of CNF and fillers.
In conclusion, the use of nanocellulose can open the door to the increase in the filler content of the paper and to the production of high quality recycled paper products, reducing the claims from customers to the manufacturers related to sheet failure and/or linting, both caused by poor interfiber bonding.
The authors wish to thank the Economy and Competitiveness Ministry of Spain for the support of the projects with references CTQ2012-36868-C02-01 and CTQ2013-48090-C2-1-R.
See more of this Group/Topical: Forest and Plant Bioproducts Division - See also ICE