Bio-composite materials: a short review of recent trends, mechanical and chemical properties, and applications

Recently, the attraction on the bio-composite (known as green composites) materials has significantly increased due to the potential of being substitute to conventional materials used in manufacturing industries. Biocomposite materials are produced with natural fibers or natural resins instead of synthesized fibres (carbon, glass, etc fibers) or resins (poly vinyl alcohol, epoxy, etc resins ). The bio-based fibers such as jute, sisal, flax, hemp, bamboo, hair, wool, silk etc., are obtained from plants or animals. Also, natural matrix materials such as natural rubber, polyester, etc., are produced from plants. The advantages of bio-composites such as the ease of disposal and being able to composted characteristics of bio-composites after the expiration date which is not generally possible with conventional synthetic materials, being renewable, sustainable have attracted many researcher. Furthermore, the comparable mechanical properties of bio-composites make feasible for application to many different products. This study reviews the, recent trends, mechanical and chemical properties, and application of bio-composites in recent years.

determined by many factors such as matrix material, fibre, fibre direction, fibre application type, production method, and etc. Matrix material is an important parameter for mechanical and chemical features of the composite material product and most of the matrix materials are petro-chemical based materials. Depletion of fossil sources is a critical issue for composites. Therefore, some researchers are studying on the natural resins which are obtained from plants in order to synthesize matrix material for composite-materials.
The increasing attraction and importance of the bio-composites increased the number of publications on that particular subject with a variety of different perspectives. The publications including books, articles and reviews show the importance of the subject. John Venkateshwaran and Elayaperumal (2010), reviewed bio-composite materials produced with natural fiber and matrices [1][2][3][4][5]. This paper includes the recent studies on the subject of bio-composites, focusing on the sources for natural fibers excluding animal (hair, silk, wool, etc) based fibers and matrices, production methods, mechanical and chemical properties of bio-composites and application of the bio-composite materials.

FIBERS
Fibers are the reinforcing agents of the composite materials and main part of the composite system that carries structural loads. Composite materials are mostly produced with synthetic reinforcing agents. Generally, carbon, aramid and glass fibers are used for composite material production. But, the sustainability and environmental issues need effective solutions for the mankind. The efforts on producing renewable and biodegradable materials for manufacturing industry are widening all around world.
Fibers can be divided into two main categories as natural and synthetic. Synthetic fibers are generally petro-chemical products. Natural fibers are obtained from plants or animals. Plant based fibers are composed of cellulose and animal fibers includes proteins (hair, silk, wool, etc.). Plant fibers consist of leaf, seed, bast, fruit, wood, grass, and stalk. In this study, plant fiber sources were reviewed and the results were given in following sections.

Plant Fibers (Lignocellulosic Fibers)
Plant fibers are promising reinforcement agents mainly composed of cellulose and, with the concerns of sustainability and renewability these fibers are getting more attention by the researchers and the industry.
Lignocellulosic fibers are formed basically from three chemical substances. These substances are cellulose (a-cellulose), hemicelluloses and lignin. Also, lignocellulosic fibers may contain other different substances depending on the plant, region, species, etc., such as waxes, pectin, inorganics, starch, protein, etc. Table 1 shows the chemical composition of the plant based natural fibers. Even though the all plant fibers are basically polymers of sugar, the chemical composition varies with many parameters such as climate, soil type, environmental factors, etc.

Cellulose
Cellulose is the main structural components of the natural fibers which consist of D-glucopyranose units joined each other with 1,4-b-D-glycosidic bonds (C1 and C4). Cellulose is hydrophilic and has around 10000 degree of polymerization which varies with fiber type. The hydrogen bonds provided by hydroxyl (OH) in the structure and also van der Waals forces makes cellulose molecules to be crystalline packed. This mainly determines the physical properties of celluloses. Besides these highly ordered regions, cellulose has less ordered amorphous regions. In the nature, two types of cellulose which are cellulose I α and I β exist [1,22]. Cellulose can be degraded with chemical applications, but also is resistant to oxidising and alkali agents and hydrolysis relatively.

Hemicellulose
Hemicellulose is the second major structural component of the fibers. It is not a type of cellulose and has more complex structure than natural cellulose. Hemicelluloses are composed of different sugar units, very hydrophilic, non-crystalline, and acts as the matrix for cellulose microfibrils. Hemi-celluloses have a degree of polymerization around 50-300. Hemicelluloses can be soluted in alkali and hydrolyzed in acids [28].

Lignin
Lignin is the hydrophobic, insoluble in the water, amorphous aromatic polymer resulting from the oxidative combinatorial coupling of 4-hydroxyphenylpropanoids which has strong intramolecular bonds. Lignin is especially presented in vascular plants and acts as filler between cellulose hemicellulose and pectin structures. Lignin provides stiffness to cellulose and hemicellulose structures. Figure 1 depicts the chemical structure of the cellulose, hemicellulose and lignin molecules [1,22,28].
Some of the most important fiber sources and the recent studies were reviewed and the important mechanical properties such as tensile strength, elastic modulus, and elongation at break were given in Table 2.

Bagasse
Bagasse also known as sugarcane bagasse is the waste (by-product) of sugar production. While paper is produced from bagasse in some Latin American countries, also bagasse can be used to produce animal feed, furfural, and biodegradable composite materials [29].

Bamboo
Bamboo which is mostly grown in Asia is a member of grass family and there are above 1400 types. Bamboo stem is hard and woody. The bamboo fibers are produced from pulp of bamboo plants. It has wide application area such as food, paper, textile, furniture, construction, and in most of the woody goods. Bamboo fibers can provide fine mechanical strength, stiffness, low density and high modulus [30,31].

Corn
Corn is one of the major plants used in food industry all around the world. The high amounts of corn production and processing ends up with different by-products such as corn husk and corncob. The fibrous structure of these by-products makes corn promising candidate for bio-composite production [32].

Wood
Woods are categorized according to their botanical properties. The angiosperm woods are called hard and gymnosperm woods are soft. The main anatomical difference is the enclosement of the seeds in the ovary of the flower. Both hardwoods and softwoods can be used as reinforcement material or filler [22,33].

Hard Wood
Hardwoods contain vessels and are porous and most of the tropical woods are hardwoods. Alder red Ash, Oregon, Aspen, Birch, paper Cottonwood, Oak, Beech, and Maple are the examples of the hardwoods.

Soft Wood
Softwoods do not contain vessels and are non-porous. Cedar, Western Red, Cedar, Larch, and Pine are the examples of softwoods.

Coir
Coir is a by-product of different coconut products. Coir contains a high amount of lignin (41-45 %) compared to other natural fibers [11]. The lignin is generally extracted with a chemical or organic solvent. Annually, over 50 billion coconuts are harvested and a very small amount of coconut husk is recycled (around 15%) [34]. Thus, coir husk fibers have a great potential for bio-composite reinforcement material production.

Kapok
Kapok fibers are produced from natively tropical tree kapok Bio-composite materials: a short review of recent trends, mechanical and chemical properties, and applications.
which can grow up to 50 meters. Kapok products have different applications like pillow filling from kapok cotton and oil extraction from its seeds. The high amounts of kapok harvesting produce a potential for recycling the wastes and by-products of the kapok [35,36].

Oil Palm
Oil palm is an important crop mostly harvested in Malaysia and Indonesia and generally used for palm oil extraction and paper production. The great amount of oil palm production also ends up with a high amount of biomass including empty fruit bunches, fronds and trunks [37].

Jute
Jute which has highest production volume for fibers after cotton all around the world is mostly cultivated for its fiber. It mainly grows in India and Bangladesh [38]. Also, China, Nepal, Brazil and Thailand are the some other countries that cultivate jute fiber. Despite the disadvantages of the jute fiber such as being highly brittle and relatively lower tensile strength it has a broad range of application due to fine texture and heat resistance [39] .

Flax
Flax is one of the known oldest textile raw material and cultivated for centuries. Linen fabric is produced from flax plant and due to its high strength has a wide application area. Flax fibers are used for production of tents, canvas, towel, sails furnishing fabrics, household textiles etc [40].

Hemp
Hemp is another important crop used in fiber reinforcement and the history of hemp usage is very old. Even though the bad reputation about the hemp (narcotic issues), industrial hemp can't be used as narcotic since it produces 9-tetrahydrocannabinol (THC) less than 0.2% [41]. Hemp is an annual crop which has significant growth rate and suitable for temperature climates [2]. .

Kenaf
Kenaf has a high potential to be the reinforcement material for bio-composite production due to its growth rate, and similar structure to jute and cotton which are the mostly used crop for fiber production [42]. Kenaf is an annual crop and its origin is Africa. It is cultivated mostly in Africa, Bangladesh, India, China and Malaysia [43].

Kudzu
Kudzu is an invasive crop native to southern regions of Japan and China. It is mostly used for erosion control. It is used for clothing and basketry for centuries. It has a similar morphology to flax and hemp [17] and thus it has potential to be the reinforcement material for bio-composite production.

Nettle
Nettle also known as stinging nettle is a perennial crop which has similar properties to flax. There are several species of the nettle crop.

Ramie
Ramie also known as Chinese nettle is a perennial crop that grows in warm climates. The treatment of ramie fibers is expensive due to gummy resins in the structure. Ramie is generally used for clothing but recently it has been started to use in automotive industry. Ramie fibers have long length and lower density than glass fibers [44].

Roselle
Roselle is a crop that is cultivated for its jute-like fibers mostly in tropical regions. Its origin is assumed as Africa. It is an annual crop and can grow up to 2.5 m [45].

Abaca
Abaca is one the of most important leaf fibers. Abaca is mostly cultivated in where its origin is Philippines. It is related with banana family. Abaca fibers have good mechanical properties like high strength and flexibility [46].

Banana
Banana is a tropical plant mostly cultivated for its nutritious fruit. Its tree bark have good mechanical properties which can be used as reinforcement agent instead of synthetic ones [47].

Henequen
Henequen plant natively grows in Guatemala and Mexico. It is cultivated for its fibers in order to made ropes mostly. It has similar properties to sisal and also called Cuban or Yucatan sisal [48].

Pineapple
The pineapple which origin is Brazil now spreaded to all tropical regions. It has a very high amount of production and thus it provides a great potential for fiber production as by-product.

Sisal
Sisal is native to Mexico and mostly cultivated in Brazil. It has wide range of application area. The sisal fibers possess good mechanical properties and thus it is used in bio-composite material production [49].

Cotton
Cotton is one of the most important crops that used all over the world. It has variety of uses. But due to its relatively low mechanical properties it rarely used as reinforcement agent. It is generally used as secondary fiber in green hybrid-composites.

Kapok
Kapok has similar properties to cotton and mostly cultivated in Malaysia. Kapok husk which is the by-product of the kapok production can be used as the fiber production raw material [36].
Using bio-fibers has some cons as well as pros. The main drawbacks of bio-fibers are the high moisture absorption, relatively lower structural strength and poor adhesion with matrix material. Thus, many researchers studied on morphological properties and the treatments for modifications of bio fibers [6,14,16,24,44,46,. Also many articles have been published that reviews the treatment methods and the effects of these treatments on mechanical, chemical and morphological properties of bio-fibers and bio-composites [55,[95][96][97]. The main goal for all treatment types (physical, chemical or physico-chemical) is to improve the adhesion characteristics of the fibers and by this way to improve the mechanical properties of the bio-composites. The most common physical methods are simple mechanical methods (stretching, calendaring, rolling), solvent extraction, electric discharge (corona, plasma, ionized air), thermal treatments. Alkaline, coupling (silane, acylation, graft copolymerization), bleaching (reduction, oxidation), enzyme, peroxide treatments are the chemical methods for modification of the fibers. Also there are some methods that combine the physical and chemical methods and called pyhsico-chemical

BIO-COMPOSITES MECHANICAL PROPERTIES
While the fibers carry the structural loads of the composite parts matrix material keeps the structure in solid phase that forms the shape and the appearance of the product. Today, most of the products produced from composite material include non-renewable matrices of petroleum based chemicals. But, there are numerous studies that investigate the usage of renewable and sustainable matrices for composite material production. The most commonly used thermoplastic composites for bio-composite material production are polystyrene (PS), polyethylene (PE), polypropylene (PP), and polyvinylchloride (PVC). Epoxy, vinyl esters, polyester and phenol formaldehyde are the thermoset resins which are used to produce composite materials reinforced with natural fibers [2]. Mechanical properties of the final material depend on many different parameters. The main parameters are the fiber and matrix material properties and the compatibility of matrix and the fiber bundle. Mechanical properties of different matrices and fibers were studied by many researchers and a brief summary of the studies were given Table 3.

CONCLUSIONS
This paper is a brief review study of the recent investigations on mechanical and chemical properties and applications of bio-fibers and bio-composites. The study showed that bio-composites are promising materials for the future applications in many industrial sectors especially in automotive sector which is the leading sector of the industrial and technological development. The remarkable advantages such as the renewability and sustainability and further being bio-degradable characteristics of bio-composite materials make bio-composites the "future materials". Also studies show that, the issues of bio-composites such as moisture absorption and low adhesion characteristics are need to be solved in order to replace conventional materials or synthetic composites. Reinforcing synthetic matrix with natural fibers could be a temporary solution for the progress. But, it is mandatory to use both natural fibers and natural resins to produce goods from completely recyclable, bio-degradable, "green" materials. The increasing awareness of the people and the legislations will enforce the usage of bio-composite materials instead of conventional materials to ensure the sustainability.
[7] Yang, Z., Li  Bio-composite materials: a short review of recent trends, mechanical and chemical properties, and applications.