(542a) Effect of Cellulosic Nanofibrils on Strength and Structural Properties of Paper

Cellulose in its various nano-sized structural forms is an interesting new material with a potential to be a component in a variety of high performance materials. Nano-cellulosic structures (2-20 nm width and a few µm length), referred to by various names such as nanocrystals, whiskers, nanowhiskers, microfibrillated cellulose, microfibril aggregates or nanofibers can be obtained from various sources such as wood fibers, cotton, potato tuber cells, and fruits (Fauze et al. 2011; Weinshuai et al. 2011; Samir et al. 2005; Eichhorn et al., 2010; Siró & Plackett, 2010). As cellulose is abundantly available, biocompatible, biodegradable, and sustainable, numerous new research projects are undergoing on production/isolation of nanofibrils from many different natural resources of cellulose and their applications in the field of reinforced plastics, biomedical implants, and electromechanical devices etc (Kalia et al. 2011, Duchemin 2008).

In the present study, nanofibrillated cellulose fibers derived from wood and bagasse pulps were added in different doses to the unrefined and partly refined pulps. The paper sheets made from the mixtures in a laboratory sheet former were evaluated for various properties. Preliminary results show that paper sheets made from unrefined pulp and cellulose nanofibrils have tensile strength values nearly at the same levels as the conventionally refined pulp sheets, but with a higher tear strength, bulk, and opacity.

 

Background

Paper is a network of cellulosic fibers obtained largely from woody and non-woody plants. The fibers in the network are bonded with each other through hydrogen bonding between the cellulose molecules present in the fibers. A fibrous plant cell has a complex ultra-structure consisting of a cavity (lumen) surrounded by a layered cell wall.  A cell wall layer itself consists of cellulose fibrils embedded in hemicelluloses, lignin, and extractives. The layers of a cell wall differ in alignment of fibrils with respect to the fiber axis, thickness, and chemical composition. The cellulose fibrils, in turn, are made up of microfibrils (or nanofibrils) that are essentially linear cellulose crystals consisting of cellulose chains laterally packed together through intermolecular hydrogen bonding.

The mechanical properties of the papers commercially available today are far less than the estimated mechanical properties of the nanofibrils or their networks as these nanofibrils from inside the cell wall have limited contribution in fiber-fiber bonding. In practice, the inter fiber bonding is promoted by mechanical beating (refining) of the fibers that breaks down the fiber structure and exposes cellulose fibrils and nanofibrils for greater participation in fiber bonding. Extended refining of the fibers leads to the desired increased wet and dry tensile strength of the resulting paper, but has many adverse effects such as fiber shortening (reduced tear strength), lower bulk (reduced opacity and bending stiffness), and increased dimensional stability.

Considering that the cellulose nanofibrils have very high specific surface area, high tensile strength and stiffness, and ability to form networks, it is believed that these can act as strength-aids in paper by promoting fiber bonding without the extensive refining of the pulp and its consequent reduction in tear strength, bulk, and drainability on the paper machine.

Methodology and results

In the present study, cellulose fibers of commercial bleached paper pulps of three different raw materials (softwood, hardwood, and sugarcane bagasse) have been refined in a PFI mill to the extent of nanofibrillation. Laboratory handsheets prepared from mixtures of the nanofibrillated fibers with unrefined and partly refined fibers were found to have favourable combinations of tensile strength, tear strength, and bulk. Various property combinations can be achieved by varying the proportions of refined and unrefined fibers.

References

1. Duchemin, B.,(2008)  “Structure, property and processing relationships of all-cellulose composites”, PhD thesis, Mechanical Engineering University of Canterbury, Christchurch, New Zealand

2. Eichhorn, S., Dufresne, A., Aranguren, M., Marcovich, N., Capadona, J., Rowan, S., et al. (2010). “Review: Current international research into cellulose nanofibres and nanocomposites”, Journal of Materials Science, 45(1) 1–33.

3. Fauze A. Aouada, M_arcia R. de Moura, William J. Orts, and Luiz H. C. Mattoso, (2011). “Preparation and Characterization of Novel Micro- and Nanocomposite Hydrogels Containing Cellulosic Fibrils”, J. Agric. Food Chem. (59) 9433–9442

4. Kalia, S., Dufresne, A., Cherian, B., Kaith, B. S., Av´erous, L., Njuguna, J. and Nassiopoulos, E., (2011). “Cellulose-Based Bio- and Nanocomposites: A Review”, International Journal of Polymer Science, doi:10.1155/2011/837875

5. Samir, A., Alloin, M. A. S., F., and Dufresne, A. (2005). “Review of recent research into cellulosic whiskers, their properties and their application in nanocomposite field”, Biomacromolecules, 6(2) 612–626.

6. Siro, I., & Plackett, D. (2010). “Microfibrillated cellulose and new nanocomposite materials: A review”,Cellulose, 17, 459–494.

7. Wenshuai, C., Haipeng, Y., Yixing, L. Peng, C. Mingxin, Z., and Yunfei, H., (2011). “Individualization of cellulose nanofibers from wood using high-intensity ultrasonication combined with chemical pretreatments”, Carbohydrate Polymers (83) 1804–1811