Natural Fibre Reinforced Polymer Composites: A Review on Dynamic Mechanical Properties

To overcome the environmental imbalance, interest of researchers and scientist has been grown in proper consumption of natural fibres opposite to synthetic fibres due to their advantages such as low cost and density, huge availability, environmental friendliness, renewability, biodegradability, high specific strength, high stiffness and easy processing [1-7]. Nevertheless, these fibres also face some demerits such as higher moisture absorption, poor thermal stability, lower interfacial bonding and lower impact strength [8-14]. NFRPCs are being used in various applications such as packaging, building and construction, electrical and electronics, house wares, marine, aerospace and so far [15-17].

Performance of NFRPCs is required to investigate not only under static stress but also under cyclic stress prior to their applications as above mentioned using some more advanced analysis; DMA is one of them. DMA is useful in analysis of dynamic mechanical properties of the NFRPCs as a function of frequency, temperature, time, stress, and atmosphere. Dynamic mechanical properties of NFRPCs mainly depend upon weight/volume percentages, shape and size, and stacking sequences of fibres. In addition to this, hybridization of fibres also influences the dynamic mechanical properties. Moreover, dynamic mechanical properties of NFRPCs are also affected by chemical treatments of fibres and incorporation of nano fillers.
Recently, dynamic mechanical analysis of NFRPCs has been increasingly increased, and several researchers had reported studies on the same. The present study shows the gentle effort to compile literatures available on dynamic mechanical analysis of NFRPCs.

Dynamic mechanical analysis
DMA technique is useful in characterising the NFRPCs in terms of morphology, viscoelastic behaviour, cross linking density, dynamic fragility, storage modulus, loss modulus, glass transition temperature, stress relaxation modulus and effective constant of reinforcement. In addition, DMA is also useful to characterize an important parameter damping as a function of frequency, temperature, time, stress, atmosphere or a combination of these parameters. Moreover, DMA is also very useful in civil infrastructure systems because dynamic loading conditions are frequently stumbling due to sound, earthquakes, winds, ocean waves and live loads. Vibration damping parameters shows prime importance for structural applications in order to enhance the reliability, performance, buildings comfort and in the alleviation of bridges hazards [18].
DMA characterize the mechanical responses of a material by monitoring dynamic property changes as a function of frequency, temperature or time. In DMA, an oscillating force is applied on sample and the sinusoidal stress and strain curves are recorded as a function of time. A typical such response is shown in Figure  1.
The modulus from DMA is not exactly same as the Young's modulus from stress-strain diagram. The slope of initial linear region of stress-strain diagram is Young's modulus but in DMA, Complex modulus ( )

Storage modulus
It is a real part of complex modulus; defined as amount of the maximum energy stored by material during one cycle of oscillation [19][20]. It also gives an estimate of temperaturedependant stiffness behaviour and load-bearing capability of the polymer composites.

Loss modulus
The imaginary part of complex modulus is named as loss modulus; defined as amount of energy dissipated in form of heat by materials during one cycle of sinusoidal load [19,20]. It represents the viscous response of the polymer composites. The peak of loss modulus curve for polymer composites is known as dynamic glass transition temperature.

Damping
The damping property of the polymer materials is the ratio of loss modulus and storage modulus. It is related to degree of molecular mobility in polymer composites [19,20]. The higher value of Tan δ is characterized by high non-elastic behaviour while the low value of Tan δ exhibit a high elastic behaviour of the material.

Glass transition temperature
The glass transition temperature is the temperature range where state of polymer composites changes from a glassy (hard, rigid) to rubbery (flexible, yielding). The higher value of g T shows the higher thermal stability of polymer composites.
Polymer composite loses its properties above g T which results into breaking of cross linking between molecules as gradually increase of further temperature. The glass transition temperature can be obtained from: (i) Peak of tan delta curve.
(ii) Peak of loss modulus curve.

Dynamic mechanical analysis of NFRPCs
Dynamic mechanical analysis has become a far used technique to determine the interfacial characteristics of NFRPCs. Several researchers had reported studies on dynamic mechanical analysis of NFRPCs. Dynamic mechanical analysis of banana fibre reinforced polyester composites was studied by Pothan & Oommen [21]; reported that the composite with 40 vol.% fibre content had the maximum value of storage modulus, and lower value of loss modulus and damping parameters. In case of short sisal fibres reinforced polystyrene composites, storage modulus decreased upon increasing the temperature and the glass transition temperature of composite shifted towards lower temperature as compared to neat polystyrene [22]. Gupta & Srivastava [23] presented study on dynamic mechanical properties of unidirectional jute fibre [23] and sisal fibres [24] reinforced epoxy composite. The viscoelastic behavior of jute fibre reinforced high density polyethylene was studied by Mohanty et al. [25]. It was observed that the storage modulus was found to increase with increase in fibre content up to 30%. Shinoj et al. [26] investigated the dynamic mechanical properties of oil palm fibre linear low density polyethylene biocomposite in terms of storage modulus, loss modulus and damping parameter.

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increased with increase in fibres content whereas the values of Tan δ peak decreased.
Several studies were made to investigate the effect of hybridization on dynamic mechanical properties of NFRPCs. Gupta & Srivastava [27] investigated the dynamic mechanical properties of hybrid jute/sisal fibre reinforced epoxy composite in terms of storage modulus, loss modulus and damping, and found that each hybrid composite had a better dynamic mechanical performance than pure jute and sisal composite, which shows a positive effect of hybridization. The dynamic mechanical analysis of hybrid banana/sisal fibre reinforced polyester composites was carried out by Idicula et al. [28]. The composite with 40 vol.% fibre content had the maximum storage modulus and peak width of Tan δ but minimum height of damping. In case of DMA of hybrid composites, high strength glass fibres were frequently added with various natural fibres: sisal [29][30][31], ramie [32], bamboo [33], curaua [34][35], banana [36], pine apple leaf [37] and oil palm [38]. It was observed that dynamic mechanical properties of natural fibres were improved due to incorporation of glass fibres. The reason behind this fact may be due to a strong restriction imposed by the glass fibre to the polymer matrix, which allows a better stress transfer and hence improved dynamic mechanical properties.
Several studies were also made to investigate effect of surface modification of fibres by chemical treatment in order to increase adhesion between fibres and matrix, and thereby improvement in dynamic mechanical properties of NFRPCs. Shanmugam & Thiruchitrambalam [39] proposed studies on effect of alkali treatment on dynamic mechanical properties of hybrid palmyra palm leak stalk/jute fibre reinforced polyester composite. It was reported that alkali treatment of fibres had improved dynamic mechanical properties. Influence of concentrations of alkali treatment on dynamic mechanical properties of hemp/polyester composite was reported by Gupta & Gond [40]. The results suggested that the alkali treated hemp composites had a better storage modulus and glass transition temperature than untreated composites. The dynamic mechanical properties of vinylesterresin-matrix composites reinforced with alkali treated jute fibres were found to improved [41].

Conclusion
In this study, several studies on dynamic mechanical properties of NFRPCs have been presented, and main points are concluded as follows:

1.
DMA is very useful technique to measure phase transitions of NFRPCs under influence of wide range of frequencies and temperature.

2.
DMA is very helpful to accurately measure the storage modulus, loss modulus, damping and glass transition temperature of NFRPCs.

3.
Dynamic mechanical properties were greatly influenced by fibre's contents, shape and size, and stacking sequences.

4.
Variations in frequencies, chemical treatment of fibres and hybridization also affected the dynamic mechanical properties of NFRPCs.