Chapter 1
Rice Husk and its Composites: Effects of Rice Husk Loading, Size, Coupling Agents, and Surface Treatment on Composites' Mechanical, Physical, and Functional Properties

A. Bilal, R.J.T. Lin* and K. Jayaraman

Centre for Advanced Composite Materials, Department of Mechanical Engineering, University of Auckland, Auckland, New Zealand

*Corresponding author: rj.lin@auckland.ac.nz

Abstract

Among the many natural fibers used as reinforcements/fillers in the manufacture of natural fiber composite materials, rice husk (RH) has not been attracting the deserved attention despite its significant annual yield of tens of million tons due to the huge worldwide rice-consuming population. This chapter presents an introduction to natural fibers and their composites with an emphasis on RH and its use in the manufacture of composite materials. A thorough review has been carried out on the manufacturing of RH composites with various polymers and manufacturing processes. The effects of RH loading, size, surface treatment, and the use of coupling agents on mechanical, physical, and functional properties of RH composites have been discussed in detail. Although RH has also been used in the form of ash in manufacturing different composites, this chapter only focuses on RH used in its natural form and its resulting composites.

Keywords: Rice husk, coupling agents, surface treatment, composites manufacturing, mechanical, physical and functional properties

1.1 Introduction

By definition, natural fibers are fibers which are not artificial or manmade (Ticoalu et al., 2010). Natural fibers can be plant based such as wood, sisal, flax, hemp, jute, kenaf, and ramie or animal based, e.g., wool, avian feather, and silk or mineral based such as basalt and asbestos. They have been used as reinforcements with a variety of materials for over 3000 years (Taj et al., 2007) and have demonstrated immense potential to replace synthetic fibers, such as glass and carbon fibers, because of their ecofriendly and biodegradable characteristics.

There is a large variation in the properties of natural fibers, which is affected by several factors such as fiber's place of growth, cultivation conditions, growth time, moisture content, and form (yarn, woven, twine, chopped, and felt) (O'Donnell et al., 2004; Ochi, 2008; Pickering et al., 2007). Table 1.1 shows various plant-based natural fibers and their regions or countries of origin.

Table 1.1 Fibers and their origin (Taj et al., 2007; Kim et al., 2007).

The mechanical and physical properties of natural fibers are greatly affected by their chemical composition and structure (Taj et al., 2007). The majority of plant-based natural fibers have cellulose, hemicellulose, and lignin as their main constituents along with pectin and waxes (John & Thomas, 2008). The reinforcing ability of natural fibers depends on cellulose and its crystallinity (Bledzki & Gassan, 1999, John & Thomas, 2008), whereas biodegradation, micro-absorption, and thermal degradation of natural fibers depend on hemicelluloses (Taj et al., 2007), which is hydrophilic in nature (John & Thomas, 2008). On the other hand, lignin which is hydrophobic in nature plays a critical role in protecting the cellulose/hemicellulose from severe environmental conditions such as water (Thakur & Thakur, 2014), and is thermally stable but prone to UV degradation (Olesen & Plackett, 1999); pectin gives plants flexibility, while waxes consist of various types of alcohols (John & Thomas, 2008). Each of these constituents of natural fibers plays an important role in determining the overall properties of natural fibrous materials (Thakur et al., 2014b).

These fibers are chemically active and decompose thermo-chemically between 150 °C and 500 °C (cellulose between 275 °C and 350 °C; hemicellulose mainly between 150 °C and 350 °C; and lignin between 250 and 500 °C) (Kim et al., 2004).

The relative percentages of cellulose, hemicellulose, and lignin vary for different fibers (John & Thomas, 2008). Table 1.2 shows the chemical composition of some natural fibers.

Table 1.2 Chemical composition of some natural fibers (Malkapuram et al., 2009).

Generally, an increase in the cellulose content increases tensile strength and Young's modulus of fibers, whereas stiffness also depends on the micro-fibrillar angle. Fibers are rigid, inflexible, and have high tensile strength if the micro-fibrils have an orientation parallel to the fiber axis. If the micro-fibrils are oriented in a direction spiral to the fiber axis, the fibers are more ductile (John & Thomas, 2008). This variation of material properties does cause some concerns about the use of such materials in the more advanced and critical applications such as composite components for automobiles, infrastructure, aeronautical, and aerospace industries.

Agricultural wastes such as RH, wheat straw, rice straw, and corn stalks also come under the category of natural fibers. Researchers are now increasingly looking toward these by-products for manufacturing composite materials (Panthapulakkal et al., 2005b; Nourbakhsh & Ashori, 2010; Ghofrani et al., 2012). The use of these agricultural by-products provides a great opportunity to start a natural fiber industry in those countries which have little or no wood resources (Ashori & Nourbakhsh, 2009). The chemical components and contents of these materials are similar to those of wood and they can be used in the form of fibers or particles (Yang et al., 2004; Yang et al., 2006b). With the comparatively large quantity of agro-wastes from annual crops, Table 1.3, there is a potential that wood can be substituted by these alternative materials (Ashori & Nourbakhsh, 2009). These agro-residues are normally used as animal feed or household fuel and a large proportion is burned for disposal, which adds to environmental pollution (Ashori & Nourbakhsh, 2009). These agricultural waste fibers can be formed into chips or particles similar to wood (Yang et al., 2003), and their exploration and utilization will contribute to rural agricultural-based economies in a positive way (Sain & Panthapulakkal, 2006).

Table 1.3 Annual production of natural fibers and sources (Taj et al.,...