Handbook of Composites from Renewable Materials, Biodegradable Materials

By Vijay Kumar Thakur, Manju Kumari Thakur and Michael R. Kessler

The Handbook of Composites From Renewable Materials comprises a set of 8 individual volumes that brings an interdisciplinary perspective to accomplish a more detailed understanding of the interplay between the synthesis, structure, characterization, processing, applications and performance of these advanced materials. The handbook covers a multitude of natural polymers/ reinforcement/ fillers and biodegradable materials. Together, the 8 volumes total at least 5000 pages and offers a unique publication.

This 5th volume Handbook is solely focused on Biodegradable Materials. Some of the important topics include but not limited to: Rice husk and its composites; biodegradable composites based on thermoplastic starch and talc nanoparticles; recent progress in biocomposites of biodegradable polymer; microbial polyesters: production and market; biodegradable and bio absorbable materials for osteosynthesis applications; biodegradable polymers in tissue engineering; composites based on hydroxyapatite and biodegradable polylactide; biodegradable composites; development of membranes from bio-based materials and their applications; green biodegradable composites based on natural fibers; fully biodegradable all-cellulose composites; natural fiber composites with bio-derivative and/or degradable polymers; synthetic biodegradable polymers for bone tissue engineering; polysaccharides as green biodegradable platforms for building-up electroactive composite materials; biodegradable polymer blends and composites from seaweeds; biocomposites scaffolds derived from renewable resources for bone tissue repair ; pectin-based composites; recent advances in conductive composites based on biodegradable polymers for regenerative medicine applications; biosynthesis of PHAs and their biomedical applications; biodegradable soy protein isolate/poly (vinyl alcohol) packaging films and biodegradability of bio-based polymeric materials in natural environment.

• Language: English
• Publisher: Wiley
• Release date: Feb 28, 2017
• ISBN: 9781119224396

Vijay Kumar Thakur

Vijay Kumar Thakur is Permanent Faculty in the School of Aerospace, Transport and Manufacturing Engineering, Cranfield University, UK. Previously he was a Staff Scientist in the School of Mechanical and Materials Engineering at Washington State University, USA, Research Scientist in Temasek Laboratories at Nanyang Technological University Singapore and Visiting Research Fellow in the Department of Chemical and Materials Engineering at LHU Taiwan. He spent his postdoctoral study in Materials Science & Engineering at Iowa State University, USA. He has extensive expertise in the synthesis of polymers, nano materials, nanocomposites, biocomposites, graft copolymers, high performance capacitors and electrochromic materials. He sits on the editorial board of several SCI journals.

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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: [e-mail]

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., 2007).

1.2 Natural Fiber-Reinforced Polymer Composites

Composite materials consist of two or more ingredients in which one component acts as the matrix material and the other as the reinforcement (Pappu et al., 2015) and their overall properties depend on the individual characteristics of the polymer matrix and the reinforcement (Thakur et al., 2014a).

Although research on natural fiber-reinforced polymer composites (NFRCs) began in 1908 (John & Thomas, 2008), it has not received much attention until from about three decades ago (Westman et al., 2010). Nowadays, both the academic and industrial sectors are showing a significantly increased interest in the use of NFRCs due to their low cost, environmental friendliness, lightweight, biodegradable, and nonabrasive nature (Rozman et al., 2000). Moreover, they have high electrical resistance, good acoustic insulating properties, low energy consumption, less dermal and respiratory irritation, good chemical and corrosion resistance, and are safe to handle (Ticoalu et al., 2010; Taj et al., 2007; John & Thomas, 2008; Malkapuram et al., 2009; Ashori, 2008; Mavani et al., 2007).

With the reported advantages of NFRCs and the growing awareness on the depletion of petroleum-based resources as well as global environmental issues, the demand of NFRCs has predicted to grow 15–20% annually with a growth rate of 15–20% in automotive applications (Malkapuram et al., 2009), and 50% or more in building and construction applications. North America is known as the leading region of NFRC applications in the building and construction sectors with mainly wood fiber-based composites, whereas Europe is the leading region of NFRC applications in the automotive industries with mostly nonwood fiber-based composites (Lucintel, 2011). Of course, there are other NFRC applications emerging in the other regions of the globe. The earlier forecast for the NFRC market was with a compound annual growth rate (CAGR) of 10% to reach $3.8B by 2016 (Lucintel, 2011); interestingly, it has shown that RH is emerging as an alternative for wood fibers in the applications of the building and construction sectors.

Despite the promising forecast, NFRCs do have some inherent issues which need to be addressed properly before their full potential can be realized for widespread industrial applications in various sectors. Along with the nature of the fibers, the properties of the resulting composites are also influenced by the type of polymer matrix used and the amount and dimensions of the fiber. One of the critical issues is the weak adhesion and poor interfacial bond strength between natural fibers and the matrix (Lee et al., 2004; Hristov et al., 2004), and formation of aggregates during their processing (Taj et al., 2007; Ashori, 2008), leading to inferior mechanical properties. Natural fibers are polar and hydrophilic in nature and polymer matrix is nonpolar and hydrophobic, which form the heterogeneous systems for NFRCs. Surface tension as well as polarity of matrix and the fibers should be the same in order to have good interfacial adhesion in NFRCs (Mwaikambo & Ansell, 2002), and a suitable adhesion is required between the matrix and the filling material in order to improve mechanical properties of the composites (Yang et al., 2004).

Due to the hydrophilicity of natural fibers, NFRCs could absorb water when used in moist conditions which leads to the poor compatibility between fibers and hydrophobic polymer matrices (Yang et al., 2006a). The water absorption (WA) is due to the hydrogen bond developed between the hydroxyl groups (OH) in the natural fibers and water molecules present in the environment. Therefore, it is essential to prevent such moisture infiltration so that swelling and/or permanent damage can be avoided for effective usage of these cellulosic composites in wet conditions (Ishak et al., 2001).

In order to enhance the performance of NFRCs, the compatibility between hydrophilic fibers and hydrophobic polymers can also be improved by using coupling agents and/or surface modification of fibers. These measures can create efficient interfacial bond strength between the fibers and the polymer matrices so that the effective load transfer can be achieved when NFRCs are subjected to loading during applications.

Coupling agents, who have both the hydrophilic and hydrophobic properties necessary to bond well with the fiber and the polymer matrix, make polymers more reactive toward the surface of the natural fiber (Panthapulakkal et al., 2005b; Ershad-Langroudi et al., 2008; Ahmad Fuad et al., 1993; Stark & Rowlands, 2003; Toro et al., 2005; Park et al., 2004; Sombatsompop et al., 2005). They chemically link with the hydrophilic fiber on one side and the hydrophobic polymer chain on the other to facilitate the wetting of polymer surrounding the fibers. The interfacial region between the fiber and the matrix has two types of interaction, i.e., primary and secondary bonding represented by covalent bonding and hydrogen bonding, respectively (Rozman et al., 2005a; Rozman et al., 2003).

1.3 Rice Husk and its Composites

Rice is a source of primary food for the majority of the population worldwide. Around 20 wt% of paddy received is husk which is separated from the rice grain during milling process (Chand et al., 2010); therefore, rice husk (RH) is abundantly available in significant quantity. The annual production of rice in 2012 was approximately 718 million tons according to the Food and Agriculture Organization of the United Nations (FAO, 2012). RH is biodegradable, abundant, cost effective, lightweight, easily available, reduces the density of the finished product, has no residues or toxic by-products, is environmentally friendly, low density, and recyclable (Yang et al., 2004; Yang et al., 2006a; Ibrahim and Kuek, 2011; Rahman et al., 2010a; Premalal et al., 2002).

RH is mainly used as fuel, fertilizer in agriculture, landfill, and animal bedding (Kim & Eom, 2001; Park et al., 2003; Mano, 2002), but the majority of RH is burnt for disposal because of its resistance to decomposition in the ground, and its difficulty to digest and low nutritional value for animals (Piva et al., 2004). In the past few years, researchers have looked into the possibility of using RH, which is mostly an unwanted material, for making composite materials (Razavi-Nouri et al., 2006).

Similar to other natural fibers, RH has cellulose, hemicelluloses and lignin as its main constituents (George and Ghose, 1983), noticeably it also contains significant amount of silica (20 wt%), which is present on its outer surface in the form of silicon–cellulose membrane (Yoshida, 1962). RH has a cellulose content (35 wt%) similar to that of wood (Martí-Ferrer et al., 2006; Rosa et al., 2009b) but has lower contents of lignin (20 wt%) and hemicellulose (25 wt%) than those found in most other natural fibers including wood. Since the thermal degradation of RH occurs due to the degradation of hemicellulose and lignin (Kim et al., 2004), a similar mechanism to that of wood, the lower content of lignin and hemicellulose allows RH-filled polymers to be processed at higher temperatures as compared to wood polymer composites. While wood has thermal stability issues at temperatures over just 200 °C, RH degrades and decomposes at temperatures around 250 °C which enables the manufacturing of RH composites to be performed at higher temperatures up to 250 °C (Martí-Ferrer et al., 2006) without concern of losing material properties.

RH as reinforcement/filler in polymer-based composite materials has proven to be a good option, provided there is good compatibility between RH and base polymer matrix (Chand et al., 2010). Like other plant-based natural fibers, RH is hydrophilic and its use with hydrophobic thermoplastics results in poor compatibility and adhesion between the counter parts (Panthapulakkal et al., 2005a; Dhakal et al., 2007; Sain & Kokta, 1993; Lai et al., 2003; Kazayawoko et al., 1999; Sain et al., 1993; Li & Matuana, 2003). One of the reasons for poor adhesion is the presence of silica, which is present in the form of a silicon–cellulose membrane on the outer surface of RH (Vasishth, 1974). Removal of silica and other surface impurities can result in a better adhesion between the fiber and the matrix and in turn improve properties of composites (Sain & Panthapulakkal, 2006). Fiber matrix adhesion can also be improved by introducing coupling agents (Panthapulakkal et al., 2005a; Dhakal et al., 2007; Kazayawoko et al., 1999, Lai et al., 2003; Sain et al., 1993; Sain & Kokta, 1993).

RH is also more resistant to WA and fungal decomposition because it contains 20 wt% amorphous silica in combination with 30 wt% of a phenyl propanoid structural polymer called lignin (Rahman et al., 2010b). As mentioned earlier, common NFRCs have a major disadvantage of WA mainly due to diffusion or infiltration (Czél & Kanyok, 2007). In the case of RH, the percentage of cellulose is very low and the waxes contained also make it comparatively less prone to water uptake.

Composites made from RH have better dimensional stability under moist conditions, good termite resistance, and high resistance to biological attack as compared to wood-based materials (Kim et al., 2007). These RH composites have reasonable strength and stiffness, no residues or toxic by-products when burnt, are recyclable, and low CO2 emissions when compared with inorganic-filler-reinforced polymer composites (Kim et al., 2007; Yang et al., 2006a; Razavi-Nouri et al., 2006; Kim et al., 2005).

Flammability is another problem faced by natural fiber composites. Synthetic polymers are petroleum based and are highly flammable. Various flame-retardant materials such as halogen and phosphorus-based compounds can be used with polymers to improve flame retardancy, but these flame retardants have a negative impact on the environment and raise health concerns as well (Zhao et al., 2009). RH could prove to be a good flame-retardant material in composites as it contains silica as one of the main constituents. Silica is mainly responsible for the improved flame retardancy by providing thermal shielding and diffusion barrier effects during the combustion process (Zhao et al., 2009; Arora et al., 2012).

RH has been used both in thermoplastics and thermosets. The following subsections discuss a wide range of research undertaken in the area of RH composites. The main focus of discussion is the type of polymers and manufacturing processes involved in the manufacture of RH composites. The effects of RH loading and coupling agents on mechanical, physical and functional properties of RH composites are also discussed.

1.3.1 Polymers Used in the Manufacturing of RH Composites

Over the past two decades, although both thermoplastics and thermosets have been used as matrices in manufacturing of RH composites, yet thermoplastic polymers have been the primary candidate for RH composites. Among the commonly available thermoplastic resins, PE and PP of different densities (i.e., low, medium, and high) have been used the most. On one hand, PE is primarily used as an exterior building component. Low-density polyethylene (LDPE) has properties such as fluidity, flexibility, transparency, and a glossy surface and has been used mainly as a food packing material in the forms of sheet and film; whereas, high-density polyethylene (HDPE) has toughness, stiffness, solvent resistance, and electrical insulation and is mainly used as an insulating material for electric wire and for producing various types of containers (Yang et al., 2007b). The manufacturing of composites with RH as reinforcement and PE (low, medium, and high densities) as polymer matrix has been carried out by quite a number of researchers (Yang et al., 2007b; Kim et al., 2004; Panthapulakkal et al., 2005b; Ghofrani et al., 2012; Yang et al., 2006a; Rahman et al., 2010a; Panthapulakkal et al., 2005a; Rahman et al., 2010b; Zhao et al., 2009; Khalf & Ward, 2010; Najafi & Khademi-Eslam, 2011; Fávaro et al., 2010; Syafri et al., 2011; Rahman et al., 2011; Bilal et al., 2014a-c).

On the other hand, PP is one of the most widely used packaging materials (George et al., 2007). It is also commonly used in the automotive industry and recently has been studied for use as building profiles (Razavi-Nouri et al., 2006). Similar to PE, composites manufactured with PP (low, medium, and high densities) and RH has also been widely researched (Kim et al., 2007; Kim et al., 2004; Ashori & Nourbakhsh, 2009; Yang et al., 2004; Yang et al., 2006a,b; Ishak et al., 2001; Ershad-Langroudi et al., 2008; Premalal et al., 2002; Razavi-Nouri et al., 2006; Rosa et al., 2009a,b; Czél & Kanyok, 2007; Santiagoo et al., 2011; El Sayed et al., 2012; Aminullah et al., 2010; Yang et al., 2007a; He et al., 2011).

Apart from PE and PP, phenol formaldehyde (PF) (Bhatnagar, 1994; Ndazi et al., 2007), polyurethane (PU) (Sheriff et al., 2009; Rozman et al., 2003), polyester (Rozman et al., 2005a; Ahmad et al., 2007, Rozman et al., 2005b), polymer lactic acid (PLA) (Yussuf et al., 2010, Hua et al., 2011), polyvinylchloride (PVC) (Chand et al., 2010), polyvinyl alcohol (PVA) (Arora et al., 2012), polystyrene (Rozman et al., 2000), urea formaldehyde (UF) (Bakar & Muhammed, 2011), and epoxy (Ibrahim & Kuek, 2011) have also been used to manufacture composites with RH.

Injection molding, compression molding, extrusion, and hot press are the most commonly used techniques to manufacture RH-reinforced composite materials. The manufacturing of composites with different manufacturing processes using RH is shown in Table 1.4.

Table 1.4 Manufacturing processes used in the manufacture of RH composites.

1.3.2 Effects of RH Loading on the Properties of RH Composites

RH has been used with different percentages for the manufacturing of composites, as shown in Table 1.5. The effect of RH loading on mechanical, physical, and functional properties has been widely investigated.

Table 1.5 Different percentages by weight of RH in composites.

The various mechanical properties of RH composites studied are tensile strength and modulus, flexural strength and modulus, impact strength, percentage elongation, energy at break, and hardness. For tensile strength, it has been reported to reduce generally as RH loading is increased (Ahmad et al., 2007; Aminullah et al., 2010; Arora et al., 2012; El Sayed et al., 2012; He et al., 2011; Ishak et al., 2001; Kim et al., 2007; Premalal et al., 2002; Rosa et al., 2009b; Rozman et al., 2005b; Santiagoo et al., 2011; Yang et al., 2006a; Yang et al., 2007a; Yang et al., 2004; Yang et al., 2007b). However, some researchers did find that the increase of RH loading to around 50 wt% could result in the increase of composite tensile strengths (Bilal et al., 2014b; Bilal et al., 2014a; Czél & Kanyok, 2007; Rozman et al., 2003). Other results have also been reported that tensile strength could be increased with RH loading of up to 15 wt% (Zurina et al., 2004), 25 wt% (Khalf & Ward, 2010, Rahman et al., 2010a) and 40 wt% (Tong et al., 2014). Additionally, natural fibers (including RH) cannot be easily dispersed in thermoplastic polymers because of the strong intermolecular hydrogen bonding and tend to agglomerate during processing (compounding) with the matrix polymer. This often leads to an inferior strength of the final products (Aziz et al., 2005, Rozman et al., 1998).

The critical fiber length is an important aspect which dictates the stress transfer between fibers and the matrix which in turn determines the strength of the composites (Wang et al., 1991). Critical fiber length is the maximum embedded fiber length for a fiber to be pulled out from the matrix without rupture. Fibers must have this critical length or the minimum required length, to strengthen a material to their potential. If the fibers have length equal to or greater than the critical fiber length, they carry maximum possible applied load before they fail and transfer load to the matrix. The fibers can not carry the maximum possible applied load before their failure, if they have a length shorter than the critical fiber length.

The same trend applies to flexural strength where a decrease in the flexural strength generally occurs with an increase in RH loading (Aminullah et al., 2010; Rozman et al., 2005b). However, the increase in flexural strength was also reported with an increase of RH loading up to 30 wt% (He et al., 2011), 35 wt% (Rahman et al., 2010a), 50 wt% (Bilal et al., 2014b), and 54 wt% (Rozman et al., 2003).

The modulus (tensile and flexural) enhancement is normally dependent on that of the reinforcements. The tensile and flexural moduli of RH composites have been seen to increase with the increase of RH loading (Ahmad et al., 2007; Aminullah et al., 2010; Bilal et al., 2014b; Bilal et al., 2014a; Czél & Kanyok, 2007; El Sayed et al., 2012; He et al., 2011; Ishak et al., 2001; Khalf & Ward, 2010; Premalal et al., 2002; Rahman et al., 2010a; Razavi-Nouri et al., 2006; Rosa et al., 2009b; Rozman et al., 2003; Santiagoo et al., 2011). The best tensile and flexural moduli were reported to be achieved at RH loading of around 50 wt% (Aminullah et al., 2010; Bilal et al., 2014a,b; Rozman et al., 2003).

Generally, a decrease in the impact strength (notched, un-notched, and Charpy) has been reported with the increase of RH loading (Ahmad et al., 2007; Aminullah et al., 2010; Bilal et al., 2014b; Bilal et al., 2014a; Czél & Kanyok, 2007; Premalal et al., 2002; Rahman et al., 2011; Razavi-Nouri et al., 2006; Yang et al., 2006a; Yang et al., 2004; Yang et al., 2006b; Zurina et al., 2004). But, like the tensile and flexural strengths, the increase of the impact strength with RH loading up to 20 wt% (Zuhaira et al., 2013), 35 wt% (Rahman et al., 2010a), and 50 wt% (Rozman et al., 2003) has also been reported in the literature.

Both percentage elongation and energy at break decrease with an increase in the percentage of RH in composites (Ahmad et al., 2007; Aminullah et al., 2010; Arora et al., 2012; Czél & Kanyok, 2007; El Sayed et al., 2012; Ishak et al., 2001; Premalal et al., 2002; Razavi-Nouri et al., 2006; Rosa et al., 2009b; Rozman et al., 2003). The hardness of RH composites increases with an increase in RH loading (El Sayed et al., 2012).

The physical properties, including WA and thickness swelling (TS), of RH composites have also been studied. WA and TS are measured to check the dimensional stability of the composites as these two are the main factors used to qualify the medium-density fiberboards’ (MDFs) dimensional stability according to established international standards (Ali et al., 2014). Moisture absorption in composites is governed by three different mechanisms. The first involves diffusion of water molecules inside the micro gaps between polymer chains. The second involves capillary transport of moisture into the gaps and flaws at the interfaces between fiber and the matrix. This is a result of poor wetting and impregnation during the manufacturing stage. The third is the transport of micro-cracks in the matrix arising from the swelling of fibers. Cellulose in natural fibers is mainly responsible for water up take. When the fiber content is increased in the composites, the number of free hydroxyl (OH) groups from the fiber increases and, hence, the WA increases, because free OH groups come in contact with water and form hydrogen bonding, which results in weight gain in the composites (Lin et al., 2002). The TS is an important property that represents the stability performance of composites. Generally, the swelling rates for polymer matrix composites are low during the initial stages of moisture absorption. In addition, any pores or voids that are present after fabrication will help to accommodate some of the fiber swelling. The swelling of fibers places stress on the surrounding matrix and leads to micro-cracking, which would eventually cause composites to fail catastrophically (Adhikary et al., 2008).

Like all other natural fibers, the WA and TS of RH composites increase with increase in RH loading (Ahmad et al., 2007; He et al., 2011; Hua et al., 2011; Rahman et al., 2010a; Rosa et al., 2009b; Rozman et al., 2003; Rozman et al., 2005b; Santiagoo et al., 2011; Yang et al., 2006a; Zurina et al., 2004). The WA and TS of RH composites have been compared with wood flour composites, MDFs, and particle boards, and the results show that WA and TS were the least in the case of RH composites (Najafi & Khademi-Eslam, 2011).

The main functional property reviewed in this article for RH composites is their flammability characteristics. Studies on the flammability properties of RH composites indicate that they have better flame retardancy compared to virgin polymers (Arora et al., 2012; Bilal et al., 2014c; Zhao et al., 2009), and that composites with a high content of RH show good flame retardancy (Zhao et al., 2009). The reason for the positive outcome in flammability is mainly due to the organic components within RH decompose during combustion and leave inorganic silica as the main constituent of the residue when RH composites are burnt. The accumulation of silica results in the formation of silica ash layer acts as a shield and barrier to fire and heat (Hshieh, 1998). The presence of silica ash layer is the main contributing factor in lowering the heat release rate (HRR) of the composites with high percentage of RH loading. The formation of silica char acts as a barrier to obstruct the access of oxygen to unburnt material, insulate, and protect the inner-layers and subsurfaces of the composites and reduce the heat and mass transfer between the flame and the samples (Samal & Sahoo, 2009).

1.3.3 Effects of RH Size on the Properties of Composites

Apart from the RH loadings, the resulted properties of RH composites can also be affected by the RH particle sizes. Research has been undertaken on RH composites with various sizes of RH particles to reveal the influence of RH particle size on mechanical, physical and thermal properties of the manufactured composites. There are actually two conflicting results obtained from these researches. On one hand, some found the larger the RH particle size, the better the resulting composite properties. The effect of RH size on the impact strength and thermal stability of the manufactured composites with four different sizes of RH (250–500, 125–250, 63–125, and less than 63 µm) showed that the largest particle size was most appropriate for composites since it resulted in the best impact strength as well as thermal stability (Rahman et al., 2010b). Moreover, those large size particles could be dispersed better/more uniformly within the composites. In another similar study (Nordyana et al., 2013), three different sizes of RH (≤60, <60–≤80, <80–≤100 µm) were used to study the effect of RH size on mechanical properties of composites. The results showed that the composites with the largest RH size exhibited best tensile properties and larger energy at break. Additionally, raw and ground RH were used to manufacture composites, and the results showed that the grinding of RH does not improve mechanical properties of composites (Ndazi et al., 2007). The grinding of RH results in reduction of residual expansion of the compressed material which minimizes chances of fiber matrix delamination or de-bonding (Ndazi et al., 2007). Grinding results in poor fiber–matrix distribution as well as poor wetting of the fibers (Vasishth, 1974). On the other hand, researchers have also found the smaller the RH particle, the better the resulting properties for composites. The mechanical properties of RH composites were studied with RH sizes of 150–180 and 180–250 µm (Rozman et al., 2003). The results revealed that composites with smaller size of RH presented higher strength due to the greater surface area for interaction as compared to larger particles. Furthermore, RH fiber sizes of 250–500, 180–500, and 38–150 µm were used to manufacture composites, and their mechanical and dimensional properties were investigated (Rozman et al., 2005b). The results showed that composites with smaller filler size displayed higher tensile, flexural and impact properties. Similarly, dimensional stability (WA and TS) in composites with small fiber size was better than those with large fiber size. However, an odd set of results was also obtained. Three different sizes of RH (198, 245, and 350 µm) were used in the manufacture of composites and their mechanical, physical, and thermal properties were evaluated (He et al., 2011). The results revealed that the composites with size of 245 µm exhibited the best properties.

1.4 Effects of Coupling Agents on the Properties of RH Composites

Researchers have studied the effects of coupling agents in RH-filled composite materials. RH-filled composites with and without coupling agents have been manufactured and studied. Generally, the results showed that use of coupling agents improves the mechanical as well as physical properties of RH-filled composites. A brief summary of the research reviewed is given below.

The results using 1, 3, and 5 wt% of maleic anhydride-grafted polypropylene (MAPP) as the coupling agent in RH/PP composites showed that tensile properties and interfacial bonding improved with the addition of coupling agent, but notched and un-notched Izod impact strength remained almost the same (Yang et al., 2007a). It was concluded that 3 wt% of coupling agent was most effective. Another set of results using MAPP showed reduced WA, smoother polymer/particle contact surfaces and improved tensile strength for the RH/PLA composites (Hua et al., 2011). In a similar study, it was found that the use of a coupling agent (MAPP) reduced water uptake by up to 20% and improved mechanical performance as well (Rosa et al., 2009b). It was shown that a coupling agent to RH ratio of 0.03 produces optimum results and that a further increase in coupling agent lowered tensile strength. Similarly, the effect of coupling agents (MAPP and MAPE) on WA and TS were studied and it was concluded that the incorporation of coupling agents has a positive impact on TS and WA (Yang et al., 2006a). Scanning electron microscopy (SEM) micrographs indicated that use of coupling agents improves interfacial adhesion in composites (Ershad-Langroudi et al., 2008). A similar study also recommends the use of coupling agents in order to improve interfacial bonding (Kim et al., 2004).

Maleic anhydride polyethylene (MAPE) was used as a coupling agent at 0, 3, and 6 wt% to manufacture RH/PE composites, and the results showed that incorporation of MAPE improved both the mechanical and physical properties, and that the best properties were achieved with 6 wt% of MAPE (Ghofrani et al., 2012).

RH/PP composites were manufactured using MAPP at 0 and 3 wt% (El Sayed et al., 2012). The results showed that the presence of the coupling agent enhanced the interfacial bonding between RH and PP. In a similar study, MAPP was used as a coupling agent at 0, 1, 2, and 3 wt% in RH/PP composites. It was concluded that the overall performance of the composites improved with the addition of the coupling agent and the optimum percentage of MAPP was observed to be 1 wt% (Aminullah et al., 2010). A further increase in the amount of MAPP did not result in any significant improvement to the overall performance of the composites. Similar studies also concluded that the use of MAPP as coupling agent results in better distribution of RH in composites and improved both the mechanical and physical properties of the composites (Razavi-Nouri et al., 2006; Yang et al., 2006b; Zurina et al., 2004).

The effect of different coupling agents on the mechanical, thermal and morphological properties of RH composites manufactured with poly(propylene-co-ethylene) as the matrix has also been studied (de Carvalho et al., 2011). Four different coupling agents of 10 wt% were used – MAPE, MAPP, PP grafted with acid comonomer (CAPP), and HDPE grafted with acid comonomer (CAPE). The results showed that the coupling agents improved interfacial bonding and that the mechanical properties of composites having MAPP and CAPP were better than those having MAPE and CAPE. Thermal stability of the composites was also improved with the addition of coupling agents.

A study of the effects of different types of MAPP on the interfacial adhesion of RH/PP composites revealed that MAPP-treated composites with sufficient molecular weight and maleic anhydride (MA) graft percentage showed improved mechanical and thermal properties compared to coupling agents with low molecular weights (Kim et al., 2007).

MAPE was varied from 1 to 6 wt% and the mechanical and flammability properties of the composites were studied (Bilal et al., 2014b,c). It was concluded that the incorporation of MAPE improved the tensile, flexural, and Charpy impact strength of the composites. On the other hand, MAPE did not improve the fire retardancy of the composites. The maximum strengths were achieved with 4.2 wt% of MAPE for tensile strength, 2.9 wt% of MAPE for flexural strength, and 5.3 wt% of MAPE for Charpy impact strength.

MAPP in 0, 5, 10, and 15 wt% was used as a coupling agent in RH/PP composites with 40 wt% of RH content to study the mechanical properties of the composites (Czél & Kanyok, 2007). It was concluded that 5 wt% of MAPP was best for 40 wt% of RH in RH/PP composites. Other studies also concluded that coupling agents improve the mechanical, physical, thermal, and functional properties (weathering performance and dielectric properties) of RH composites (Khalf & Ward, 2010; Rahman et al., 2010b; Rahman et al., 2011). The performance of MAPE and MAPP in RH/PE composites was investigated and it was shown that MAPE performed better in PE composite systems compared to MAPP due to the better compatibility of MAPE with PE (Yang et al., 2007b). Similar results have been reported where the incorporation of MAPE had a positive effect on mechanical as well as physical properties (Najafi & Khademi-Eslam, 2011; Rosa et al., 2009a).

1.4.1 Effects of Surface Treatment of RH on the Properties of RH Composites

Another commonly used technique to enhance reinforcement/matrix adhesion in NFRCs is through surface treatment of the reinforcement to change either the surface chemistry or morphology. With this intention, RH has been subjected to different treatments hoping to enhance the properties of the resulting composites. These surface treatments aim to improve interfacial bonding between RH and the matrices by removal of surface impurities such as waxes, silica, and carboxylic compounds (which blocks reactive chemical groups) and carbonyl groups. This could potentially result in an improvement in mechanical and physical properties.

RH was treated with ϒ-amiopropyltrimethoxysilane in a mixture of water and ethanol, and the manufactured composites with treated and untreated RH were compared in terms of mechanical and physical properties (Santiagoo et al., 2011). Both mechanical and physical properties improved with the treatment of RH in composites as compared to those of untreated RH composites. The hardness of the composites with untreated, steam-treated and chemically treated (with 10 wt% of NaOH) RH was compared (Sheriff et al., 2009). The results showed that the surface treatment of RH with steam resulted in better hardness of the composites followed by untreated and chemically treated RH composites. Moreover, steam-treated RH composites showed high thermal stability and better interfacial adhesion. In a similar study (Ndazi et al., 2007), RH was treated with steam and/or NaOH. Steam treatment was done between 100 and 140 °C, whereas chemical treatment of RH was done in 2%, 4%, 6%, and 8% (w/v) solutions of NaOH. The modification of RH with both steam and NaOH (up to 4% NaOH) improved mechanical properties with better fiber–matrix interaction. The results of chemical treatment of RH with NaOH (2–8%) only concluded that treatment of RH with more than 4% NaOH causes chemical degradation which results in decrease in thermal stability of RH (Ndazi et al., 2008). Mechanical properties as well as WA of untreated RH and chemically treated RH with 5 wt% of NaOH were compared (Ahmad et al., 2007), and it was concluded that chemical treatment of RH with 5 wt% of NaOH yielded better mechanical properties and WA than untreated RH composites. In another study, RH was first treated by 10% HCl which was followed by further treatment of RH by 2% NaOH (El Sayed et al., 2012). The results showed that acid/alkali-treated RH improved the modulus and the hardness of the composites. In another study (Fávaro et al., 2010), RH was mercerized by immersion in 10% NaOH. After that RH was acetylated by immersion in pure acetic acid followed by immersion in acetic anhydride acidified with 0.1% sulfuric acid. Finally, the samples were rinsed with water until pH ~7.0 was reached. It was found that the treatment resulted in enhancement of mechanical properties and a better interfacial adhesion between RH and the polymer. RH surface was treated with alkali pre-treatment (NaOH solution 5%w/v), and further modification was done by coating three batches of RH with liquid-epoxidized natural rubber (LNER) at three levels, 5, 10, and 20 wt% LNER solution in toluene, respectively (Syafri et al., 2011). Modification of RH with 5% NaOH followed by 10% LENR resulted in optimum tensile stress, modulus, and impact strength as compared to those of untreated RH, NaOH-only treated and LENR-only treated RH composites. SEM micrographs indicated that the surface of RH changed after the treatment process. Benzene diazonium salt in different media of different pH (i.e., alkali, acidic, and neutral media) was used to treat RH (Rahman et al., 2010a). The mechanical properties of RH composites treated with alkaline media increased evidently as compared to acidic, neutral, and untreated composites.

The effects of chemically modifying RH with glycidyl methacrylate (GMA) on the mechanical and physical properties of RH composites were investigated (Rozman et al., 2000). The treatment of RH led to better interfacial bonding, which resulted in better mechanical properties as well as dimensional stability of the composites as compared to composites with untreated RH. RH was treated with three different chemicals, GMA, MA, and succinic anhydride (SAH). The results showed that treatment of RH with GMA displayed the best mechanical properties for its resulting composites, whereas the dimensional stability of composites with both GMA- and MAH-treated RH was the best (Rozman et al., 2005a).

Mechanical and physical properties of composites with untreated, esterified (using 2% MA in xylene), and acetylated (using 50% acetic acid) RH were investigated, and results revealed that esterification improved dispersion of RH in the composites, whereas acetylation reduced WA (Zurina et al., 2004).

RH surface was treated using MAH and benzylperoxide (BP) and the tribological properties of the composites were evaluated (Chand et al., 2010). It was concluded that wear resistance of RH composited can be increased by surface treatment of RH.

1.4.2 Potential Applications of RH Composites

The potential for making panels from RH-based composites is very promising for specific end usage since these composites would be low cost and have high aesthetic value compared to those of wood composites (Bakar & Muhammed, 2011). The composites made from RH may be used for building interiors, wood decks and food packaging (Yang et al., 2006a). RH has good weathering resistance in comparison with wood and has the potential to be used as a durable eco-friendly furniture material (Rahman et al., 2011). Composites made from RH can also be used for other diverse applications such as parquet, shoe heels and covering of basketball, volley ball and hand ball playgrounds (Sheriff et al., 2009) as well as in wet environment (Najafi & Khademi-Eslam, 2011). These composites have the potential to be used as construction materials, furniture and many plastic products in a variety of future industrial applications (Kim, 2009). They can also be used for insulation purposes (Khalf & Ward, 2010). RH can be applied to prepare low cost, thermally stable, and low flammable composites (Arora et al., 2012), which are suitable for application inside buildings due to good flame-retardant properties to replace wood-based materials (Bhatnagar, 1994; Zhao et al., 2009).

1.5 Summary

RH has a great potential as a reinforcement/filler in the manufacture of composite materials. RH is abundant, low cost, and easily available. RH has higher thermal stability, less WA, and better fire retardancy in comparison with other natural fibers including wood fiber. Therefore, use of RH in composites will ensure a more useful exploitation of this material unlike other agro-wastes of being disposed without value recovery. This will not only make less dependency on the use of wood in composites which is already causing deforestation, but also ensure low-cost products and will benefit the society in a positive way.

RH has been used especially with PE and PP to manufacture composites due to their ease of processing. The compatibility and interfacial bond strength between RH and the matrix can be improved by surface treatment and/or by incorporation of coupling agents. Surface treatment and compatibilization improve fiber–matrix interface resulting in improved mechanical, physical, and other properties. The use of coupling agents should be preferred over surface treatment in high-volume applications, because surface treatment may not be an option for industrial/commercial scale production. The RH-based composites possess a great potential to replace wood and wood-based composites in the application areas of construction, furniture, and automobile interiors.

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