**4.1 Merits and demerits of natural fibres**

The inherent properties of the natural fibres of plants origin are important in developing the bio-composites. Natural fibres comprise of cellulose, hemicelluloses, lignin, waxes and tannins etc. The percentage of the cellulose, hemicelluloses, lignin etc., length and width of the fibres, varies with the source and processing of fibres. Further, natural fibres possess low density (1.25–1.50 g/cm3 ), sufficient mechanical properties, sustainability, recyclability, biodegradability, availability and low-priced in comparison to synthetic fibres such as glass and carbon fibres [51, 52]. Intriguingly, these properties do not meet the requirements of biocomposites. The natural fibres are used for increasing the mechanical strength as reinforcement material in composites [53]. The synthetic matrix and natural fibres are not compatible to each other leading to poor mechanical properties properties. Further, fibres have also water absorption capacity of cellulose due the presence of numerous hydroxyl groups [54–56].

Natural fibres are used as reinforcement materials in composites. However, due to their susceptibility to moisture [56] mechanical properties of polymeric composites have a strong impact on the interface adhesion between the fiber and the polymer matrix [54]. The natural fibres are rich of cellulose, hemicelluloses, lignin, pectins, waxes and tannins etc, all of which are composed of hydroxyl groups. Thus, there are major challenges of suitability between the matrix and fiber that weakens interface region between matrices and natural fibres [55]. Generally, outer surface of the composite materials absorb water and decreases gradually into the bulk of the matrix. High water absorption capacity of the composite materials leads to decline in their mechanical strength and pressure on nearby structures due to absorption of water pertaining to the hygroscopic nature of the fibres and


#### **Table 1.**

*Plant fibres and properties [36–45].*

subsequently can cause warping, buckling, bigger possibility of their microbial inhabitation, freeze, and unfreeze leading to destruction of mechanical characteristics of composite materials. Therefore, fibres are required to improve these limitations by physical and chemical modifications [57].

#### **4.2 Alteration of properties of natural fibres**

The compatibility of the natural plant fibres with the synthetic matrix is the main and foremost concern of developing bio-composites due to the different nature and properties of these two materials. The various methods have been studied and reviewed in the past in order to increase the functionality and compatibility of natural fibres. Fibres compatibility with the matrix and mechanical strength thereof may be increased by physical and chemical modification of the fibres.


#### *Opportunity of Non-Wood Forest Products in Biocomposites DOI: http://dx.doi.org/10.5772/intechopen.97825*

#### **Table 2.**

*Chemistries of important plant fibers [43, 46–50].*


#### **Table 3.**

*Plant based fibres, source and utilization in biocomposites.*

#### *4.2.1 Physical modification*

The surface properties may be increased by physical treatments of the fibres. However, during extraction process, the journey of fibres to a final destination also involves the multi stepping process leading to stress and physical changes in the inherent properties of fibres. During the extraction process there are some fibres which involve simple process of extraction for example *Agave sislana* fibres. In most of the cases, the processing of plant material containing raw material for biocomposites as fibres encompass the physical and mild chemical treatments leading to change in original properties of fibres. Therefore, extraction of fibres is also an important factor in considering the evaluation of properties of bio-composites. There are various processes developed and optimized for extraction of fibres and well documented.

The physical treatments of the isolated fibres change the structural and surface properties of the reinforcing fibres without altering the properties and disintegration of fibres. The physical treatments influence the mechanical properties resulting in proper bonding to the matrix and affects interfacial adhesion. The commonly used method for plant-based fibres is corona and cold plasma treatment, however other physical methods are also successfully used for surface activation such as thermotreatment [58, 59]; calandering [60], stretching [61] and hybrid yarns [62]. The corona treatment provides oxidation of the fibres, which changes the surface energy of the fibres and increases the number of aldehyde groups [63–65]. The corona and cold plasma treatment are called electric discharge methods and mostly used to activate cellulose fibres leading to increase in mechanical strength [63, 64, 66].

#### *4.2.1.1 Corona treatment*

Corona treatment is employed for treatment of fibres to increase the morphological and mechanical properties of lignocellulosic fibres resulting in an improvement of the interfacial compatibility between matrix and fillers. Homogeneity of composite materials, adhesion properties and mechanical properties (tensile strength, Young modulii) increase to a certain level (10–30%) with corona treatment [67, 68]. Recently, *Aloe vera* fibres [69] were treated with corona discharge during different time intervals and it was observed that rough surface morphology and degradation of fibres occurred due to etching mechanism caused by corona treatment.

#### *4.2.1.2 Plasma treatment*

Plasma treatment is an environmentally friendly green electric discharge method for treatment of fibres [70–72] and provides changes in surface energy, increase of the roughness and micro-cleaning of the treated fibres. The process causes surface crosslinking and can introduce reactive groups. Mostly low plasma treatment is being carried out in presence of gases to alter the surface properties of fibres. The base material is treated under atmospheric plasma glow discharge for various periods of time using helium, helium/nitrogen, and helium/acetylene, argon, oxygen, air etc. gas. The significance lies in the fact that sometimes desired properties obtained in seconds. Intriguingly, changes in surface roughness, tip-surface adhesion, and surface chemistry of the fibres and flexural strength, flexural modulus, and interlaminar shear stress, storage modulus and glass transition temperature increased significantly. The treatment is successfully employed to alter the surface properties of natural fibres used in composites as reinforcing material. The adhesion between sisal fibres and polypropylene matrix [73] increase the interfacial adhesion between flax fibre and matrix polyethylene and unsaturated polyester [74, 75]; improvement

in mechanical properties of ramie fibres [76, 77] polypropylene composites, increase in flexural strength and tensile strength of the composite prepared from jute fibre [78–81] were obtained by employing plasma treatments.
