BIOREFINERY Nordic Pulp & Paper Research Journal Vol 28 no.2/2013 182 Nanofibrillated cellulose for enhancement of strength in high-density paper structures Houssine Sehaqui, Qi Zhou, and Lars A. Berglund KEYWORDS: Paper, Nanofibrillated cellulose, wet strength, xyloglucan, tensile properties SUMMARY: In order to enhance dry and wet strength properties of paper, handsheets were made of wood pulp fibers and nanofibrillated cellulose (NFC). 10% NFC was mixed with wood pulp fibers (90%) subjected to different number of beating revolutions. Effects from xyloglucan (XG) hemicellulose addition were also studied. High density paper handsheets from these mixtures were prepared using a laboratory handsheet former. Strength properties were measured and densities of the materials estimated. Scanning electron microscopy was used to observe paper sheet surfaces. NFC significantly enhances strength for the paper handsheets both at 50% relative humidity and in the wet state so that NFC addition may be an alternative to mechanical beating. The main reason for property improvements is increased density of the final material. Tensile energy absorption improved strongly through favorable fiber-fiber interaction. NFC or NFC/XG addition combined with some mechanical beating may decrease energy needs compared with beating only. ADDRESSES OF THE AUTHORS: Houssine Sehaqui (houssine@kth.se), Lars A. Berglund (blund@kth.se), Qi Zhou (qi@kth.se): Department of Fibre and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Paper is a fibrous material typically based on cellulosic fibers derived from wood, rags or grasses, where the fibrous network is held together by fiber/fiber joints (Torgnysdotter et al. 2007). In order for paper products to compete in packaging, it is of interest to improve their properties and functions. Paper strength is one of the most important physical properties, e.g. the resistance to burst stress for paper bags, resistance to rupture by tensile stress for printing paper, tearing resistance toughness of packaging papers. Traditional methods, such as mechanical beating (Dasgupta 1994), the use of additives including fillers and sizing agents (Roberts 1996), paper surface modification, or the combination of these methods are widely used to improve critical paper characteristics such as strength, density, surface smoothness, light scattering properties, water and grease resistivity. In order to produce paper with improved properties from cellulosic fibers, the approach would involve two steps. The first step is to specifically tailor the fiber surface properties. Typical examples are the use of high molecular mass polyelectrolytes multilayers for fiber surface engineering (Wågberg et al. 2002), the adsorption of xyloglucan onto cellulosic fibers for better sheet formation (Zhou et al. 2006) and mechanical properties (Christiernin et al. 2003), and covalent grafting of chemical groups to fiber surfaces to improve paper strength (Laine et al. 2003). For increased performance, the second step could involve assembly of tailored fibers into structural materials with highly oriented or hierarchically organized structures. In wood for instance, the hierarchical structure improves mechanical performance as individual cells subjected to an impact absorb the energy of impact by breaking away from the surrounding cells. However, this second step has not been widely explored. Nanofibrillated cellulose is nanoscale in diameter, and has high aspect ratio obtained through disintegration of plant fibers (Turbak et al. 1983, Henriksson et al. 2007, Pääkko et al. 2007, Saito et al. 2007). NFC is an attractive material building block for the preparation of nanopaper (Henriksson et al. 2008, Sehaqui et al. 2010) light-weight aerogels (Pääkko et al. 2008, Sehaqui et al. 2011d) and foams (Svagan et al. 2008, Sehaqui et al. 2010b), and for reinforcement of polymer matrices (Bruce et al. 2005, Sehaqui et al. 2011e). Nanopaper refers to paper-like fibrous networks made from NFC which show new “paper” properties such as optical transparency and low thermal expansion (Nogi et al. 2009, Sehaqui et al. 2010a), dramatically improved mechanical properties (Henriksson et al. 2008, Sehaqui et al. 2010), good barrier properties to oxygen particularly at moderate humidity atmospheres (Liu et al. 2011, Syverud et al. 2009), and possibility for functionalization (Nyström et al. 2009, Olsson et al. 2010, Boujemaoui et al. 2012). The small size of the NFC requires utilization of submicron pore-size membranes during preparation of nanopaper by filtration. NFC as a reinforcing additive for paper is of strong industrial interest as it is a non- petroleum-based additive with potential in large scale production. The addition of NFC to pulp fibers may lead to a hierarchical structure due to the presence of fibrous materials of different length scales, NFC at fiber-fiber bonds may give a more favorable type of failure process so that the material becomes more tough (Sehaqui, Allais et al. 2011b). Chemical similarity with pulp fibers may result in good interaction with wood pulp fibers. NFC prepared from kraft pulp by homogenization or grinding methods has been used as a strength enhancer in handsheets made of thermomechanical pulp (TMP) (Eriksen et al. 2008). The increase in tensile index of paper handsheets was 7% to 21% after incorporation of NFC. In our previous work, composites of NFC and beaten wood pulp fibers were prepared by vacuum filtration and drying using a submicron membrane to prevent NFC loss during filtration. The composites thus prepared comprised NFC adsorbed onto fibers and also excess NFC filling the large pores between the wood fibers. Strong increases in strength and toughness were observed even at low content of NFC (2-10%) (Sehaqui, Allais et al. 2011b).