**4. Factors influencing the mechanical properties of NFPCs**

NFPCs are prepared by compounding polymer matrix (either pristine or blend) with natural plant fibers. The fibers can be single or hybrid, microcellulose or nanocellulose. The natural fibers are made up of different chemical constituents and are exposed to different physical and chemical treatments, therefore, the properties of the resulting composites varied widely. Factors influencing their eventual composite(s) include the following:

### **4.1 Fiber type**

Plant fibers are categorized based on the parts of the plant they are extracted from. Fibers can be extracted from the seeds, leaves or bast of the plants. Bast fiber is collected from the "inner bark" or the surrounding of the stem of certain dicotyledonous plants [61], like banana, flax, hemp, jute kenaf and ramie. These fibers have higher tensile strength and are mostly used in the packaging and paper industries [62, 63]. Sisal, pineapple and many others are extracted from the plant leaves while coir, cotton and abaca are examples of fibers extracted from plant seed. Generally, plant fibers give higher strength and stiffness; however their properties depend mainly on their structure and chemical composition. These invariably relate to the source of fibers, method of extraction, maturity, growing conditions, harvesting period, degree of retting and modification [64–67].

### **4.2 Fiber length, orientation and weight percentage loading**

NFPCs properties are affected by the length of the fibers used, their distribution, the percentage of the fiber volume or volume fraction and their orientation within the matrix. For polymer composites, stress is transferred by the matrix through the fibers both at the interface along the fiber length and at the ends of the fibers by

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shear [68, 69]. Hence, the degree of load conveyed from the matrix to the fibers is a function of: (i) fiber length, which is referred to as critical fiber length or aspect ratio, (ii) orientation of the fibers and their direction relative to each other. If the fiber orientation and direction is not in the line of the applied stress, failure is bound to occur easily. Unidirectional fiber composites tend to transmit external stress better, that is why hand laid fiber composites performs better mechanically [70, 71]. Depending on the fibers in the matrix's orientation and direction, we can obtain

three different types of reinforcement which include (i) longitudinally aligned fiber-filled composites, (ii) transversely aligned fiber-filled composites and (iii) randomly oriented short fiber composites [68, 72, 73]. While the longitudinally filled composites have low compression strength due to buckling of fibers and high tensile strength, the transversely filled composites on the other hand have low tensile strength. However, in the randomly oriented composites, it is far more difficult to predict the mechanical properties, due to the complexities of the distribution of load along the interface of the fiber and the matrix. Hence, considerable control over such elements as orientation dispersion and aspect ratio of the fibers, considerable improvement in the mechanical properties of the composites can be attained. In general, high performance NFPCs can be obtained mainly by using materials with high fiber content, hence the effect of fiber loading on the properties of the NFPCs is of great significance. Also it is noteworthy to mention that additional fiber content of the composites invariably causes to increased tensile properties [74, 75].

The effect of fiber-matrix adhesion cannot be over emphasized. A good number of researchers have reported their experimental results on the effect or importance of a good and strong fiber-matrix adhesion in the fabrication of composites with good mechanical properties [76, 77]. The type of bonds existing at the fiber-matrix interface greatly influences the mechanical properties of any fabricated composites. For a good transmission of stress from the matrix to the fiber to occur, the bond existing among the two components must be strong [64]. Due to the hydrophilic nature of the natural fibers, the interaction between the fibers and the hydrophobic polymer matrix is very weak [4, 78, 79]. Hence, the need to modify the fibers and introduce organic moiety that makes them more hydrophobic is necessary.

Fiber-matrix interface has been described as the reaction zone which plays a significant role in characterizing the composites mechanical properties [80]. A poor interaction between the two surfaces leads to poor transmission of load and therefore poor mechanical performance [81–83]. In addition, plant fibers need chemical modification for the distention or enlargement of the crystalline region, removal of surface impurities and elimination of hydrophilic hydroxyl groups for improvement

Polymer matrix could be either a thermoset or a thermoplastic, with varying preparation procedures and conditions, the performances of polymer matrix are affected quite differently. Thermoset are made in such a way that they develop good bonding with the fibers, especially during curing stage. However, the processes involved are time and energy consuming. Although in the case of tensile loading of the composites, the significance of matrix is evident, some researchers have reported good improvement with the same fiber when the matrices are changed. However, for compressive, in-plane shear and inter-laminar strength, they are

*DOI: http://dx.doi.org/10.5772/intechopen.85252*

**4.3 Fiber-matrix adhesion**

to some of its relative properties [68].

highly influenced by the type of matrix used [84].

**4.4 Choice of polymer matrix**

*Improving the Mechanical Properties of Natural Fiber Composites for Structural and Biomedical… DOI: http://dx.doi.org/10.5772/intechopen.85252*

shear [68, 69]. Hence, the degree of load conveyed from the matrix to the fibers is a function of: (i) fiber length, which is referred to as critical fiber length or aspect ratio, (ii) orientation of the fibers and their direction relative to each other. If the fiber orientation and direction is not in the line of the applied stress, failure is bound to occur easily. Unidirectional fiber composites tend to transmit external stress better, that is why hand laid fiber composites performs better mechanically [70, 71].

Depending on the fibers in the matrix's orientation and direction, we can obtain three different types of reinforcement which include (i) longitudinally aligned fiber-filled composites, (ii) transversely aligned fiber-filled composites and (iii) randomly oriented short fiber composites [68, 72, 73]. While the longitudinally filled composites have low compression strength due to buckling of fibers and high tensile strength, the transversely filled composites on the other hand have low tensile strength. However, in the randomly oriented composites, it is far more difficult to predict the mechanical properties, due to the complexities of the distribution of load along the interface of the fiber and the matrix. Hence, considerable control over such elements as orientation dispersion and aspect ratio of the fibers, considerable improvement in the mechanical properties of the composites can be attained.

In general, high performance NFPCs can be obtained mainly by using materials with high fiber content, hence the effect of fiber loading on the properties of the NFPCs is of great significance. Also it is noteworthy to mention that additional fiber content of the composites invariably causes to increased tensile properties [74, 75].

### **4.3 Fiber-matrix adhesion**

*Renewable and Sustainable Composites*

properties of composites being the major challenge.

composite(s) include the following:

**4.1 Fiber type**

**4. Factors influencing the mechanical properties of NFPCs**

harvesting period, degree of retting and modification [64–67].

**4.2 Fiber length, orientation and weight percentage loading**

NFPCs are prepared by compounding polymer matrix (either pristine or blend)

Plant fibers are categorized based on the parts of the plant they are extracted from. Fibers can be extracted from the seeds, leaves or bast of the plants. Bast fiber is collected from the "inner bark" or the surrounding of the stem of certain dicotyledonous plants [61], like banana, flax, hemp, jute kenaf and ramie. These fibers have higher tensile strength and are mostly used in the packaging and paper industries [62, 63]. Sisal, pineapple and many others are extracted from the plant leaves while coir, cotton and abaca are examples of fibers extracted from plant seed. Generally, plant fibers give higher strength and stiffness; however their properties depend mainly on their structure and chemical composition. These invariably relate to the source of fibers, method of extraction, maturity, growing conditions,

NFPCs properties are affected by the length of the fibers used, their distribution, the percentage of the fiber volume or volume fraction and their orientation within the matrix. For polymer composites, stress is transferred by the matrix through the fibers both at the interface along the fiber length and at the ends of the fibers by

with natural plant fibers. The fibers can be single or hybrid, microcellulose or nanocellulose. The natural fibers are made up of different chemical constituents and are exposed to different physical and chemical treatments, therefore, the properties of the resulting composites varied widely. Factors influencing their eventual

Wood fibers/plastic composites have been used in large quantities for applications in window and door frames, decks, docks and molded panel components [49–51]. Natural fiber composites have been used to replace asbestos in the building industry, because of their health related issues [52]. The European Union policy tagged "end of life vehicle (ELV)" regulations, promulgated in 2003 and amended in 2005 and 2010, projected reduction of the final waste to be disposed at the end of life of vehicles to 5% by the year 2015. In the stated regulations, 85% of material used in manufacture of the vehicle must be recoverable through reuse or through recycling mechanically [49, 53]. This has generally increased interest and widespread use of NFPCs worldwide. It is noteworthy that this policy was promoted basically because of environmental and social concerns and not necessarily because of economic or technological reasons. NFPCs durability and the availability of technology has allowed for large and complex shaped manufacturing of NFPCs, making them appealing in the automobile industries [54–56]. In Brazil, automobile industries consume, on average, 10–12.7 kg of natural fiber reinforcement per vehicle. These are circulated through the vehicle, such as rear door liners, front doors, boot liners, parcel shelves, sun roof interior shields and headrests [54]. Although NFPCs have gained tremendous interest in the industries, their applications are not unconnected to their environmental sustainability, low cost and renewability [57–60]. According to a review by Kiruthika [61], the challenge of replacing synthetic fibers completely in widespread applications is far from being overcome, with the improvement to the mechanical

**116**

The effect of fiber-matrix adhesion cannot be over emphasized. A good number of researchers have reported their experimental results on the effect or importance of a good and strong fiber-matrix adhesion in the fabrication of composites with good mechanical properties [76, 77]. The type of bonds existing at the fiber-matrix interface greatly influences the mechanical properties of any fabricated composites. For a good transmission of stress from the matrix to the fiber to occur, the bond existing among the two components must be strong [64]. Due to the hydrophilic nature of the natural fibers, the interaction between the fibers and the hydrophobic polymer matrix is very weak [4, 78, 79]. Hence, the need to modify the fibers and introduce organic moiety that makes them more hydrophobic is necessary.

Fiber-matrix interface has been described as the reaction zone which plays a significant role in characterizing the composites mechanical properties [80]. A poor interaction between the two surfaces leads to poor transmission of load and therefore poor mechanical performance [81–83]. In addition, plant fibers need chemical modification for the distention or enlargement of the crystalline region, removal of surface impurities and elimination of hydrophilic hydroxyl groups for improvement to some of its relative properties [68].

### **4.4 Choice of polymer matrix**

Polymer matrix could be either a thermoset or a thermoplastic, with varying preparation procedures and conditions, the performances of polymer matrix are affected quite differently. Thermoset are made in such a way that they develop good bonding with the fibers, especially during curing stage. However, the processes involved are time and energy consuming. Although in the case of tensile loading of the composites, the significance of matrix is evident, some researchers have reported good improvement with the same fiber when the matrices are changed. However, for compressive, in-plane shear and inter-laminar strength, they are highly influenced by the type of matrix used [84].

### **4.5 Processing conditions**

The properties of NFPCS have been shown to vary from one processing technique to another [85–89]. Common techniques for the preparation of NFPCs include injection, extrusion, compression and resin transfer molding. These techniques use different processing conditions or parameters even when the materials being processed are the same. Changes in factors such as mixing speed, pressure and temperature can change the properties of the final product with any slight change [90]. For example, the preparation of sisal fiber polyester composites by employing both the compression molding and resin transfer molding technique (RTM) gave products with varying mechanical properties. The products of the RTM gave a composite with higher Young's modulus, tensile strength and flexural strength than the product of the compression molding [90]. Vacuum molding technique is one the simplest manufacturing methods for plastic materials [91]. It is suitably adapted for molding a required shape from a plastic sheet material. In this molding technique, a plastic sheet is heated up to its molding temperature using electric heat; it is then transferred to a molded shape. To obtain the shape, a vacuum is created between the mold and the sheet. Vacuum molding is an inexpensive method when compared to other molding methods. It has numerous application including, aircraft, skin tight packing, disposable tray and caps. It is a low cost methods already being employed in many areas of endeavors as mentioned. A lot of research work is being carried out on how to improve the vacuum forming method, for instance, vacuum-assisted resin transfer (VARI), also known as vacuum infusion process (VIP) which was reported by [92] to be considered as an attractive method for the production of NFPCs low cost and good performance. It uses low-cost one-sided tooling and injects low-viscosity resin into dry fiber that was performed under low pressure. The method was found to be economically suitable for the manufacturing of large composite structures, such as boat hull, wind turbine blade and aircraft structures with low or high volumes of production.

However for vacuum forming method the cycle time of production is still much affect by the cool rate and time, amount of pressure applied and the fiber content [93]. These factors can greatly affect the mechanical properties the final product.

### **4.6 Presence of void**

The introduction of fibers into the matrix during processing is accompanied with the introduction of air and other volatile substances. These substances which are mostly trapped in the fibers may form voids in the composites after processing and curing along the individual fibers. This can negatively affect the composites mechanical properties. In addition, the rate of cooling during processing can also result to the formation of voids [73]. When the void content is too high, it leads to greater affinity for water diffusion, lowering fatigue resistances and increased disparity in mechanical properties [94].

### **4.7 Thermal stability**

The importance of the stability in the preparation of the composites cannot be over emphasized as it affects the mechanical properties considerably. The different components of plant fibers are sensitive to different range of temperatures, i.e., hemicellulose, cellulose, pectin [95, 96]. Most fibers start degrading thermally at 220°C, thereby limiting the composites thermally [68]. In recent reports, the thermal stability of these fibers were greatly improved by removing maximally the lignin, hemicellulose and other alkaline soluble substances in the fiber through physical, chemical or biological means [97].

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In general, the above mentioned factors do not influence the mechanical properties of NFPCs individually or in isolation. Rather, in the fabrication of a composite, the cumulative effect of two or more of these factors may be responsible for the composites mechanical failures. Jain et al. evaluated the effect of inter-fiber interactions of different class of reinforced polymer composites on the mechanical properties and the relation to stress field [98]. The research findings highlighted the important role of microstructural arrangement in the determination of the final response of the composites. They reportedly concluded that the local fiber matrix arrangement and their neighborhood density are highly influenced and sensitive to

**5. Methods for improving mechanical properties of NFPCs**

There are different methods which can be employed for mechanical properties improvement of NFPCs. These methods are not rigid formulas that once applied will result in massive enhancements, rather improving mechanical properties of NFPCs is an active area which is ongoing. Mechanical improvement in one area of application may not necessarily yield the same result in another area of application. For example, improving stiffness of a material might be good for the construction and building industries but may not be required or worthwhile for biomedical applications, as it might prefer improvement in flexibility and toughness. Therefore all possible options will be provided; it is left for the researchers to select as appropriate. Furthermore, these methods can be combined to produce synergistic effect.

There are many literatures and reviews on the benefits of surface modification of natural fibers to the enhancement of fibers mechanical properties and by extension, the composites [99–101]. The mechanical properties of plant fibers depend greatly on the chemical structure, chemical composition and the structural arrangement of cellular fibrils [101]. Other factors such as climatic conditions, age, extraction procedures, growth condition and time of harvest also influence the mechanical properties of natural fibers. All these affect the percentage composition of cellulose in the fibers. Furthermore, the hemicelluloses and lignin are less thermally stable compared to cellulose, modification is one way of reducing the percentage content of hemicelluloses and lignin or even eliminating them completely. Graphical sample of cellulose structure which consists of amorphous (untreated) and crystalline (treated) regions is shown in **Figure 1** [69]. There exist strong intra-molecular hydrogen bonds with large molecules in the crystalline regions of the cellulose ensuring the crystalline region is very compact and this makes it difficult for chemical penetration. On the other hand, the amorphous

Surface modification of plant fibers involves the treatment given to the plant fibers

in order to increase its cellulose content, improve its interaction with the polymer matrix and also improve their mechanical, thermal and dimensional stability properties. Surface modification can be physical, chemical or biological [11, 21, 79, 102, 103]. Chemical modification involves chemical reaction with the fiber components, thereby making them soluble so that they can be removed by repeated washing. Alkali treatment also known as mercerization involve the use of alkali solution to dissolve all soluble contents of the fibers including wax, oil, pectin, lignin and some part of hemicelluloses [11, 104–107]. Alkali treatment makes the fibers surface rougher and reduce the fibers to fibrils [108, 109]. It improves the aspect ratio and

region is loose and allows penetration for possible modification.

*DOI: http://dx.doi.org/10.5772/intechopen.85252*

the stress and overall strain energy.

**5.1 Surface modification**

*Improving the Mechanical Properties of Natural Fiber Composites for Structural and Biomedical… DOI: http://dx.doi.org/10.5772/intechopen.85252*

In general, the above mentioned factors do not influence the mechanical properties of NFPCs individually or in isolation. Rather, in the fabrication of a composite, the cumulative effect of two or more of these factors may be responsible for the composites mechanical failures. Jain et al. evaluated the effect of inter-fiber interactions of different class of reinforced polymer composites on the mechanical properties and the relation to stress field [98]. The research findings highlighted the important role of microstructural arrangement in the determination of the final response of the composites. They reportedly concluded that the local fiber matrix arrangement and their neighborhood density are highly influenced and sensitive to the stress and overall strain energy.
