**3. Natural fibers reinforced composite**

Reinforced phase plays a very important role in determining the overall properties of the composite. Natural fibers can be broadly classified into 3 categories: Plant - based fibers, Animal - based fibers, Mineral - based fibers. One of the commonly used reinforcements is the mineral - based fibers, including carbon, fiberglass and Aramid [9].

#### **3.1 Carbon fibers**

Carbon fibers are unidirectional reinforcements. Due to this unique structural property of Carbon fibers, they can be structured in a way wherein the composite is stronger in a particular direction making it easier for the composite to bear heavy loads. The physical properties of carbon fiber can be modified by controlling different parameters such as the alignment of fiber, nature of the matrix, fiber-matrix volume fraction and the molding conditions.

#### **3.2 Glass fibers**

Glass is known to be extensively used as reinforcement in most of the polymeric matrix composites (PMCs). The main advantages observed during usage of glass

fibers include low cost of production, good chemical resistance, high tensile strength, and excellent insulating properties.

### **3.3 Kevlar fibers**

Kevlar fibers are known to belong to highly crystalline aramid (aromatic amide) fibers. These fibers have a very high ratio of tensile strength to weight and the lowest specific gravity among the currently used reinforcing fibers. Due to their superior mechanical properties, they tend to find major applications in marine and aerospace industries.

### **3.4 Boron fibers**

Boron fibers are especially known for having extremely high tensile modulus. Another prominent feature of these fibers is the buckling resistance that results in a high compressive strength for boron fiber reinforced composites (**Figures 3** and **4**).

Though the present composite industry majorly depends on synthetic reinforcement fibers, the use of natural fibers have been gaining attention in recent years for academic as well as industrial purposes. In the present time, many different plantbased natural fibers have been explored and researched upon to identify unique properties. Some of these are used in plastics as an reinforcement; some examples of which include hemp, kapok, jute straw, paper mulberry, oil palm empty fruit bunch, wood, wheat, barley, kenaf, rye, rice husk, cane (sugar and bamboo), flax, reeds, oats, sisal, grass, coir, pennywort, water hyacinth, raphia, ramie, pineapple leaf fiber, banana fiber and papyrus [5].

One of the on-demand natural fiber reinforced composite is the thermoplastic matrix containing reinforcements made of special wood fillers due to them being light in weight, possessing reasonable stiffness and strength. Natural fibers have shown immense potential to be used in replacement to non-renewable materials due to their low cost, promising thermoplastic properties, minimal to zero health hazards and can act as a solution for environmental pollution [10, 12]. Some plant proteins have also been used as reinforcements. One such example is the wheat gluten, which when plasticized have a unique ability to form a strong cohesive

**Figure 3.** *Different types of natural fibers used as reinforcement in polymer composites.*

*Eco-Friendly and Biodegradable Green Composites DOI: http://dx.doi.org/10.5772/intechopen.98687*

**Figure 4.** *Classification of various fibers of different origin and types with examples [10, 11].*

blend with high viscoelastic properties [13–15]. Due to these distinctive properties, wheat gluten has been tremendously used in the making of packaging materials and edible biodegradable films.

Composites based on biologically degradable polyester amide and different plant fibers like flax and cottons, have been investigated thoroughly. These composites generally possess good mechanical properties, such as high biodegradability and good water resistance. Kenaf is a biodegradable and environmentally friendly crop belonging to the hibiscus family (*Hibiscus cannabinus* L). In a particular research work, Aziz et al. manufactured composite made of a polyester resin reinforced with Kenaf fiber and further studied the mechanical properties of the composite. The properties displayed by the composite were highly satisfactory and fiber could be blownto a height of at least 10 meter. By looking at the present research scenario, natural fiber reinforced polymer composites have the capability to act as a substitute for scarce wood and wood based materials that have many structural applications in the future [16] (**Figures 3** and **4**).

Plant - based fibers, also called vegetable fibers, are classified into different types based on their origin [17]. The characteristics of plant fibers majorly depend on certain factors such as the type of plant used, the area where it is grown, the plant's botanical age, and the protocol of extraction used. One such example is coir which is known to be a tough and hard fiber with multicellular layers, with the central portion called as "lacuna". Another familiar example is Sisal leaf fibers which are observed to have a high mechanical strength. Pineapple leaf fiber extracts are soft and are rich in cellulose. Oil palm fibers, having a similar cellular structure to coir, are hard and tough. Cellulose molecules make the major constituents in most of these plant fibers. The hydroxyls groups present in the basic unit of cellulose have the ability to form intra-molecular hydrogen bonds where the bonding is

within the macromolecule, or intermolecular hydrogen bonding between two different cellulose macromolecules and or form hydrogen bonds with hydroxyl groups present in the atmosphere [18, 19]. It can be observed that all plant fibers have a high hydrophilicity, with their amount of retained moisture reaching about 8–13% [20]. Though cellulose is present in a huge quantity inside a plant, they also contain other natural substances such as lignin. The major role of lignin is to act as a cementing or bonding material between the cells of plant fibers. The content of lignin fibers influences a plant's structure, its morphology and its properties.

An important property of vegetable fiber influencing its reinforcement properties is the degree of polymerization (DP). The fibers differ drastically from each other due to the presence of cellulose molecules with differing DPs. Most of these fibers generally consist of a mixture of a base polymer homolog with the configuration (C6H10O5)n. The plant fiber known to illustrate the highest DP among other plant fibers is Bast fibers, with values nearing 10,000. In the olden times, these fibers found tremendous applications as packaging materials such as gunny bags and sacks, for making ropes, as a geo-textile material, for making twines and cords, and as carpet-backing [21, 22]. The most common bast fiber found in Cannabis sativa plants is Hemp, which is a lingo cellulosic fiber, repeatedly used as reinforcement in biodegradable composites. It is used in the making of various items such as shoes, toys and clothing, due to its non-toxicity, biodegradability and its ease for recycling [23].

#### **3.5 Advantages of natural fiber reinforced composite**

As mentioned earlier, natural fibers being used as reinforcement in composite materials tend to display good mechanical properties, thereby gaining a lot of attention in recent years. They are known to be fully biodegradable, renewable, environmentally friendly, cheap, available in abundance and have a low density [24]. Plant fibers are observed to be considerably light in weight when compared with mineral-based fibers. Due to their organic origins, plant fibers tend to possess high biodegradability which contributes to maintaining a healthy balance in the ecosystem. Due to the considerable low cost and high performance, these fibers can be termed as economically superior to other counterparts and can find huge applications in the industries. When natural fiber-reinforced plastics are combusted at the end of their life cycle, the amount of CO2 released from the combustion process is equal in amount assimilated during their growth [25]. Plant fibers have very low abrasivity which makes the recycling process of the composite materials much easier. Plastics reinforced with natural fibers, when used along with biodegradable polymers as matrix material, are considered to be the most environmentally friendly compounds because of their ability to be completely decomposed at the end of their life cycle [26, 27]. Natural fiber composites are used as a substitute to glass in non-structural applications. One such example is the automotive components that were initially manufactured with glass are now replaced with natural fiber reinforced composites [11]. Most of the plant fibers, when used in the unmodified form inside a composite, tend to produce unsatisfactory mechanical characteristics. To avoid this problem, the surface of plant fibers are treated with certain chemicals or compatibilizing agents prior to the fabrication of composite. The properties of these plant fibers can be further improved either by physical treatments such as corona treatment and cold plasma treatment, or by chemical treatments using peroxide, sodium hydroxide permanganate, isocyanates and maleic anhydride organosilanes [28]. In terms of mechanical properties of natural fibers, the values are quite low when compared to glass fibers or other mineral based fibers. When it comes to specific properties, especially stiffness, natural fibers values are near to

those of glass fibers [29, 30]. Natural fibers have shown high tensile strength and stiffness values. Tensile strength within composites are majorly governed by the reinforcement used. Hence, natural fibers along with matrices can prove to produce the desired mechanical properties needed for the specific application [6, 9].
