**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.

### **5.1 Surface modification**

*Renewable and Sustainable Composites*

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.

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

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

**4.5 Processing conditions**

**4.6 Presence of void**

**4.7 Thermal stability**

disparity in mechanical properties [94].

physical, chemical or biological means [97].

**118**

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 region is loose and allows penetration for possible modification.

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

**Figure 1.** *Schematic diagram showing the crystalline (a)/amorphous (b) regions in a fiber material [69].*

creates room for mechanical interlocking in composites [23, 110–112]. To improve chemical interactions in NFPCs which may lead to improved mechanical properties, more chemical treatments are required other than alkali treatment. Another advantage of alkali treatment is that it exposes the OH group on the cellulose chains which can again be utilized for further chemical modification in some cases and in others it can allow for chemical treatment to introduce functional groups onto the surfaces of the fibers. These functional groups can then be used for chemical interaction during the preparation of the composites [27, 113, 114]. Such chemical treatments include silane, sodium silicate, oxidization treatments with DCP, or KMnO4 treatments.

Biological methods involve the degradation by biological organism of all other components of the fiber except the cellulose. This is because the cellulose is highly crystalline and so cannot be easily degraded. Biological treatments are more environmentally friendly, produces more crystalline cellulose with better mechanical and thermal properties and it is cost effective. However, it is laborious and time consuming [115–121].

Physical methods of surface modification involve the use of plasma and corona treatment to change the fiber surface physically. This helps to create rough surfaces that are beneficial for mechanical interlocking [122–124]. However, plasma treatment can be used to introduce chemical functionality onto the fibers so that they can undergo chemical reaction with the polymer matrix [122–124]. Surface treatment introduces rigidity and stiffness to the fibers and by extension, the composites will be suitable for possible structural applications in industries such as building, aerospace marine, packaging, automobile, etc.

Liu et al. [125] reported the improved mechanical characteristics, resulting from alkali treatment on Indian grass fiber used as reinforcement. The tensile Young's modulus of grass fiber reinforced composites treated with 30 wt% alkali improved by 60%, while their impact and flexural strengths improved by 30 and 40% respectively when compared to the 30 wt% raw fiber refeinforced composites. The structural and morphological changes of alkali modified kenaf fibers modified using: 3, 6 and 9% NaOH solutions were reported by Edeerozey et al. [126]. From their SEM results, it was found that 6% and 9% NaOH treatment gave better results. However, the 9% NaOH treatment was considered to be too strong for the fibers, as it led to low tensile properties.

**121**

**5.3 Compatibilization**

*Improving the Mechanical Properties of Natural Fiber Composites for Structural and Biomedical…*

Polymer blends have been defined as the combination of two or more polymers,

prepared to enhance the properties of the products as well as to lessen the cost. Blends of polymer can simply be viewed as a polymer alloy. Therefore, blending is the mixing of two or more distinct polymers, in a way that gives a new material which has properties that are slightly different from the singular polymers involved. Polymer blending is a versatile procedure to obtain new polymer materials with characteristics far better than the individual polymers [127, 128]. It is a well-recognized option to solving emerging problem related to application requirements. The ability to combine existing polymers into new one with superior qualities that are commercially acceptable makes blending a better option [129]. There are large numbers of polymer blends reported in literature by researchers in academics and industry [130–132]. Polymer blending can give rise to miscible or phase separated blends. For miscible blends, mixing approaches a molecular dimensional scale and the properties of the new polymer blend are more like those of a single polymer. While phase separated blends still exhibit the different properties of the polymers involved [133–136]. Although, miscibility is extremely rare in blends, the advantage of miscible blends over phase separated blends is in the property profile, especially mechanical properties. Most phase separated blends exhibit inconsistency in their mechanical properties due to the poor adhesion at the interface of the blend phases. However, researchers have developed specific methods by which these phase separation problems can be alleviated [129]. Hence, polymer matrix can be designed to suit the required properties expected of the composites. Blending also helps to modify the matrix with specific chemical functionality that could be added deliberately to enable good interaction between the fibers and the blended matrix. Many researchers have shown that the performance properties of blends can be heightened significantly by reinforcing them with either synthetic or natural fibers [137–141]. However, bio-fiber reinforcements have gained ground in recent time

because of their numerous advantages over glass and carbon fibers [142].

be the preparing blends from specifically designated homopolymers [144].

Compatibilization is described as the addition of a chemical substance to an immiscible or phase separated blend that help increase their stability. Compatibilizers are also referred to as coupling agents. They react at the interface of the blend to stabilize the phases. That is, they help to improve the compatibility

According to Linares et al. [143], polymer blends have a recognized potential to produce high performance materials, however the polymer combination must be carefully selected. Müller-Buschbaum et al. [144] showed that blend composition has great influence on surface topography which is just one of the many factors that may affect the properties of the blend. Therefore, in the preparation of any blend, the selection of complementary polymers that will give the right kind of material hybrid with the required properties is the most important step. For a biodegradable polymer that will be green in all its ramifications, a blend of biopolymer material with a biodegradable counterpart is highly desirable. Among these, are: polycaprolactone, polybutylene succinate, etc. The use of blend started some decades ago [145], however, in the present day, the understanding of miscibility has undergone several changes [146]. The broad range of application of polymer materials requires varying properties according to the specific application area. With homopolymers, the range usually calls for special surface treatment or in some cases new polymer synthesis for each and every application. However, an economical alternative could

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

**5.2 Blending**

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

### **5.2 Blending**

*Renewable and Sustainable Composites*

creates room for mechanical interlocking in composites [23, 110–112]. To improve chemical interactions in NFPCs which may lead to improved mechanical properties, more chemical treatments are required other than alkali treatment. Another advantage of alkali treatment is that it exposes the OH group on the cellulose chains which can again be utilized for further chemical modification in some cases and in others it can allow for chemical treatment to introduce functional groups onto the surfaces of the fibers. These functional groups can then be used for chemical interaction during the preparation of the composites [27, 113, 114]. Such chemical treatments include silane, sodium silicate, oxidization treatments with DCP, or KMnO4 treatments. Biological methods involve the degradation by biological organism of all other components of the fiber except the cellulose. This is because the cellulose is highly crystalline and so cannot be easily degraded. Biological treatments are more environmentally friendly, produces more crystalline cellulose with better mechanical and thermal properties and it is cost effective. However, it is laborious and time

*Schematic diagram showing the crystalline (a)/amorphous (b) regions in a fiber material [69].*

Physical methods of surface modification involve the use of plasma and corona treatment to change the fiber surface physically. This helps to create rough surfaces that are beneficial for mechanical interlocking [122–124]. However, plasma treatment can be used to introduce chemical functionality onto the fibers so that they can undergo chemical reaction with the polymer matrix [122–124]. Surface treatment introduces rigidity and stiffness to the fibers and by extension, the composites will be suitable for possible structural applications in industries such as building,

Liu et al. [125] reported the improved mechanical characteristics, resulting from alkali treatment on Indian grass fiber used as reinforcement. The tensile Young's modulus of grass fiber reinforced composites treated with 30 wt% alkali improved by 60%, while their impact and flexural strengths improved by 30 and 40% respectively when compared to the 30 wt% raw fiber refeinforced composites. The structural and morphological changes of alkali modified kenaf fibers modified using: 3, 6 and 9% NaOH solutions were reported by Edeerozey et al. [126]. From their SEM results, it was found that 6% and 9% NaOH treatment gave better results. However, the 9% NaOH treatment

was considered to be too strong for the fibers, as it led to low tensile properties.

**120**

consuming [115–121].

**Figure 1.**

aerospace marine, packaging, automobile, etc.

Polymer blends have been defined as the combination of two or more polymers, prepared to enhance the properties of the products as well as to lessen the cost. Blends of polymer can simply be viewed as a polymer alloy. Therefore, blending is the mixing of two or more distinct polymers, in a way that gives a new material which has properties that are slightly different from the singular polymers involved. Polymer blending is a versatile procedure to obtain new polymer materials with characteristics far better than the individual polymers [127, 128]. It is a well-recognized option to solving emerging problem related to application requirements.

The ability to combine existing polymers into new one with superior qualities that are commercially acceptable makes blending a better option [129]. There are large numbers of polymer blends reported in literature by researchers in academics and industry [130–132]. Polymer blending can give rise to miscible or phase separated blends. For miscible blends, mixing approaches a molecular dimensional scale and the properties of the new polymer blend are more like those of a single polymer. While phase separated blends still exhibit the different properties of the polymers involved [133–136]. Although, miscibility is extremely rare in blends, the advantage of miscible blends over phase separated blends is in the property profile, especially mechanical properties. Most phase separated blends exhibit inconsistency in their mechanical properties due to the poor adhesion at the interface of the blend phases. However, researchers have developed specific methods by which these phase separation problems can be alleviated [129]. Hence, polymer matrix can be designed to suit the required properties expected of the composites. Blending also helps to modify the matrix with specific chemical functionality that could be added deliberately to enable good interaction between the fibers and the blended matrix. Many researchers have shown that the performance properties of blends can be heightened significantly by reinforcing them with either synthetic or natural fibers [137–141]. However, bio-fiber reinforcements have gained ground in recent time because of their numerous advantages over glass and carbon fibers [142].

According to Linares et al. [143], polymer blends have a recognized potential to produce high performance materials, however the polymer combination must be carefully selected. Müller-Buschbaum et al. [144] showed that blend composition has great influence on surface topography which is just one of the many factors that may affect the properties of the blend. Therefore, in the preparation of any blend, the selection of complementary polymers that will give the right kind of material hybrid with the required properties is the most important step. For a biodegradable polymer that will be green in all its ramifications, a blend of biopolymer material with a biodegradable counterpart is highly desirable. Among these, are: polycaprolactone, polybutylene succinate, etc. The use of blend started some decades ago [145], however, in the present day, the understanding of miscibility has undergone several changes [146]. The broad range of application of polymer materials requires varying properties according to the specific application area. With homopolymers, the range usually calls for special surface treatment or in some cases new polymer synthesis for each and every application. However, an economical alternative could be the preparing blends from specifically designated homopolymers [144].

### **5.3 Compatibilization**

Compatibilization is described as the addition of a chemical substance to an immiscible or phase separated blend that help increase their stability. Compatibilizers are also referred to as coupling agents. They react at the interface of the blend to stabilize the phases. That is, they help to improve the compatibility between the two phases and increase miscibility [146]. The high interfacial tension caused by coalescence phase separated blends can be reduced by the compatibilizers, allowing a continuous flow of externally applied stress from the matrix to the fibers [130]. Compatibilization can be done by (i) introduction of specific interacting groups, (ii) in-situ polymerization grafting, (iii) addition of a ternary polymeric component, (iv) addition of block co-polymers of the blend polymers, (v) interpenetrating networks of crosslinked system and (vi) using reactive compatibilization methods [147–150]. For instance, polyhydoxyether of bisphenol A (Phenoxy: PHE) a non-reactive compatibilizer has been noted in some reports to have provided improved interfacial adhesion between immiscible and marginally compatible polymer blends [129]. Specifically, the addition of PHE to some polymers blends led to the blends yielding improved dispersion of the polymers within the blend, gave uniform injection molded surfaces and considerably increased the notched impact strength of the blends (polysulfone (PDF)/ABS; PSF/PA, PMMA/ PA6 and PHE/PBT) [129, 151, 152]. Also reactive compatibilizers can be design to react with the fiber when introduced during processing. This method will involve modifying the fiber surfaces prior to the composite preparation. If successful, this method has the capability to improve interfacial interaction and subsequently improve mechanical properties.
