Cellulose Nano Whiskers as a Reinforcing Filler in Polyurethanes

Abstract

Bio-based polymers and biocomposites are a relatively new and growing market in light of recent societal concerns including dwindling petroluem reserves, environmental and endof-live disposal issues (Mohanty et al., 2005; Vijay, 2009). Polymers derived from plants, especially those from non-food resources, are gaining the attention of governments, industries and institutes, primarily due to their environmental compatibility, superior physical properties and low stable market prices which are becoming competitive with petroleum-derived polymers. The three major chemical components of biomass, cellulose, hemicellulose and lignin are utilized in diverse fields, such as biofuels, particularly bioethanol and green diesel, biomaterials, including conventional composites and novel nanocomposites, and other value-added chemicals. Among them, cellulose is the most abundant biopolymer in the world with a total annual biomass production of about 1.5 × 1012 tons (Klemm et al., 2005). It has led to a large body of research due to its renewable nature, wide availability, non-food agricultural based economy, low density, high specific strength and modulus, high aspect ratio and reactive surface (Samir et al., 2005). Cellulose is a polydispersed linear polymer of β-(1,4)-D-glucose. A cellulose fiber is composed of bundles of microfibrils where the cellulose chains are stabilized laterally by inter and intramolecular hydrogen bonding. Microfibrils are comprised of elementary fibrils where monocrystalline domains are linked by amorphous domains. Generally, monocrystallite cellulose has been reported with length ranges from 100 to 300 nm and diameter between 5 and 20 nm. In other words, cellulose monocrystallite has a high aspect ratio of 20-60 (Helbert et al., 1996; Eichhorn et al., 2001; Mathew & Dufresne, 2002; Morin & Dufresne, 2002; Samir et al., 2004). Table 1 summarizes the degree of crystallinity and the lateral dimension of elementary fibrils from several cellulose samples measured by X-ray diffraction (XRD). Tensile strength and modulus of native cellulose crystallites are approximately 10000 MPa and 150 MPa, respectively (Kamel, 2007). Under certain process conditions, transverse cleavage of the cellulose happens primarily in the amorphous zone of the fiber and releases needle-like monocrystals referred to as cellulose nano whiskers. Whisker dimensions depend on both the origin of the cellulose and reaction conditions employed. In general, wood and cotton cellulose nano whiskers have a smaller length and cross section compared to those derived from tunicate, bacterial and algae (Hanley et al., 1992; Terech et al., 1999;