Natural fibers have caught the attention of many because they are renewable and abundantly available resources. They have been considered as an alternative material to replace the inorganic fillers and fibers due to serious environmental issues. Natural fibers are naturally occurring polymers in our environment which appear pervasively in grasses, leaves, stalks of plants or even animals. In the composite industry, they are usually referred to as plant fibers and further categorized into wood or non-wood sources. They are also referred to as lignocellulose fibers since lignin and cellulose are the main components in their structure.
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Besides natural fibers, agro-wastes such as rice husks, wastes from rubber plants, cocoa cultivation, sugar cane cultivation and oil palm cultivation have also been considered. In Malaysia, the oil palm industry is the biggest biomass producer. It was estimated a total of 17 million tons of empty fruit bunches waste were produced annually. The wastes from palm oil (excess fiber, empty fruit bunches and shell) have been utilized on-site to provide energy for the mill and electricity exports to the grid. For low-pressure systems with an assumed conversion rate of 2.5 kg of palm oil waste per kW·h, potentially GW·h could be generated [ 1 ]. The Malaysian government strongly promotes the uses of palm diesel as a replacement for fossil fuel. The biofuel policy framework has been drafted by the government to encourage the use of biofuels.
Apart from being used as sources of energy, the empty fruit bunches have found alternative uses such as fiberboard in furniture making, eventually reducing the waste at palm oil mills. In the making of nanocomposites, previous studies have also shown that nanocellulose could be extracted from agro-wastes and used as nano-reinforcement in various polymer matrices [ 2 ]. Promoting the usage of agro-wastes in composites may help to solve part of the global agriculture refuse problem. Huge loads of wastes could be converted into affordable value-added products which allow them to be more valuable for wider applications. Besides, they are renewable, cheap, completely or partially recyclable, and biodegradable. In automotive industries, the variety of bio-based automotive parts currently in production is astonishing. For example, DaimlerChrysler is the biggest proponent with up to 50 components in its European vehicles being produced from bio-based materials.
The properties of natural fibers are closely related to the nature of cellulose and its crystallinity properties. For example, fibers with higher cellulose content possess impressive specific mechanical properties but also tend to be more flammable than those with higher hemicellulose content. The mechanical properties of some common fibers are listed in Table 1 . Fibers with higher hemicellulose content tend to absorb more moisture and char formation is generally better with fibers that have higher lignin content as they experience degradation at relatively lower temperatures [ 3 4 ]. For reinforcement purposes, cellulose is extracted from the natural fibers and used for the production of composites due to its hierarchical structure and semicrystalline nature [ 5 ].
In spite of their advantages, the major challenge of natural fibers is the difficulty manufacturing them into the desired form or film because they cannot be melted or dissolved in a common solvent due to the strong intermolecular hydrogen bonding, high degree of polymerization, and high crystallinity degree [ 6 ]. In conjunction with the completely green environmental policy, it is preferable to add the natural fibers in biodegradable polymers such as polyvinyl alcohol (PVA) to produce eco-sustainable composites.
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Table 1. Mechanical properties of some natural fibers [Mechanical properties of some natural fibers [ 7 ].
Table 1. Mechanical properties of some natural fibers [7]. Natural fibersDensity (g/cm3)Young&#;s modulus (GPa)Tensile strength (MPa)Elongation at break (%)Flax1..5&#;&#;2,&#;4Ramie1.5&#;1.&#;&#;1,.2&#;3.8Hemp1.&#;&#;.6Jute1.&#;&#;.5&#;1.8Sisal1.45&#;1.59&#;&#;&#;7Coconut1.154&#;&#;&#;40Cotton1.5&#;1.65.5&#;12.&#;&#;8Kenaf1.214&#;&#;.6Bamboo0.6&#;1.111&#;&#;230-PVA is a common and well-known polymer that possesses salient features such as water solubility, ease-of-use, film-forming property, and biodegradability. The global production of PVA is around 650,000 tons per year [ 8 ]. Polyvinyl alcohol (PVA) has been widely used for the preparation of blends and composites with several natural, renewable polymers like chitosan, nanocellulose, starch or lignocellulosic fillers. It is important that the development in composite manufacturing technology to achieves ecological sustainability while striving to meet consumers&#; needs. The harmful effects of our disposable consumer lifestyle on our environment are well known and well documented and as a result, the government is increasingly introducing legislation to control the effects on the environment of the materials used in the manufacture of many of our everyday products. The composites materials should be able to recycled, reused, reprocessed or biodegradable, to minimize its impact to ecosystem. At the same time, the supply of the materials should be sustainable and renewable. Plant based fiber is sustainable in its supply and it is biodegradable.
PVA is one of the most promising examples of biodegradable matrix polymers used in mulch films. These polymers have good potential as biodegradable matrices in environmental friendly composites, in comparison to carbon fibers composites or any non-biodegradable, recyclable fillers. PVA is widely used in agricultural mulch film or biodegradable packaging. For many innovative and environmentally conscious manufacturers, composite consists of PVA, a biopolymer, with natural fibers, that will further improve PVA biodegradability and physical properties, is choice of eco-sustainable materials.
Unlike most of the polymers, PVA cannot undergo polymerization from its own monomer, but through poly(vinyl acetate) (PVAc) due to the instability of vinyl monomer. Hence, PVA can only be obtained through a saponification process from PVAc or alcoholysis by reacting PVAc with methanol [ 9 10 ]. PVA can be hydrolyzed from PVAc into two different grades, fully or partially hydrolyzed based on their applications. The degree of hydrolysis indicates the number of residual acetate groups that are present in the polymer in which saponification or alcoholysis has not taken place [ 11 ]. The degree of hydrolysis will eventually affect the properties of PVA including its solubility [ 12 ].
The PVA/natural fiber composites have been well developed and documented. Although much work has been carried out on such a broad topic, the information is scattered in nature. This paper reviews PVA/natural fibers composites and their nanocomposites. This review is prepared based on the present research state of natural fibers from macro to nanoscale. The properties, processing techniques, biodegradability, applications of the composites, and future works will also be discussed. It is hoped that the researchers can continue to explore the new potentialities for such superior composites and increases their applications as a way to achieve long term environmental sustainability.
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