1.4.1.10 Sisal/Roselle and Sisal/Silk FRP Hybrid Composites
Sisal (Agave sisalana), from the Agavaceae family, is a hard‐fiber plant with wide cultivation in the tropical countries of Africa, America, and Asia, though it has its origin in Mexico and Central America. Their fibers are strong and tough, and extracted from sisal plant leaves. Sisal fibers are widely utilized in composites and plastic/paper industries. The nativity of roselle (Hibiscus sabdariffa) can be traced to West Africa. It is a species of Hibiscus, whose plant is naturally abundant and majorly used for fruits and bast fibers. Roselle fibers have extensive applications in the textile industry and in composites, because they exhibit greater mechanical behaviors in comparison with some other naturally occurring fibers, such as jute and kenaf.
Moreover, Athijayamani et al. [42] used a fiber ratio of 1 : 1 for the sisal and roselle fibers and investigated, under wet conditions, how absorption of moisture affected the mechanical responses of short hybrid sisal/roselle FRP unsaturated polyester composites. Using different fiber contents and lengths, their results showed an improvement (increase) in the flexural and tensile strengths of the hybrid composites of sisal/roselle fibers at increased lengths and contents of fibers and under dry condition. For the wet condition, the strengths (tensile and flexural) were significantly reduced, while inverse proportionality between the impact strength and fiber length and content was observed for both conditions (wet and dry), as depicted in Table 1.8. In addition, Noorunnisa Khanam et al. [43] also carried out a study on sisal/silk fiber ratio of 1 : 1 and prepared a polyester‐based hybrid composite to evaluate various fiber lengths. Their results showed higher mechanical (tensile, flexural, and compressive) responses from the composite sample with fiber length of 20 mm than that of 10 and 30 mm counterpart hybrid composites (Table 1.8). Also, the results obtained after fiber modification depicted that the same mechanical behaviors of the alkali‐treated hybrid fiber composites improved significantly.
1.5 Other Related Properties that Are Dependent on Mechanical Properties
There are other properties of biocomposites that are relevant for material analysis. Some of these are dynamic mechanical properties, thermal and water absorption behaviors, as well as tribological properties. Only tribological and thermal behaviors are subsequently discussed.
1.5.1 Tribological Behavior
Tribological behavior refers to friction‐related properties of the materials. The frictional coefficient of hybrid sisal/glass fiber (GF)‐reinforced epoxy composites was measured by Ashok Kumar et al. [44], using different sliding speeds of 0.2, 2.0, and 4.0 mm/s, under a constant load of 10 N. At an atmospheric temperature of 22 °C and relative humidity of 45%, both alkali‐treated samples of fiber composites and untreated ones were tested. The graph of frictional coefficient against fiber length (Figure 1.5) revealed that the frictional coefficient was lower, up to 2 cm fiber length. However, the frictional coefficient increased with an increasing composite fiber length. Moreover, fiber length addition led to a decrease in the frictional coefficient when sliding speeds were higher. The treated fiber of the reinforced composites yielded an optimum improvement at 2 cm in comparison with the untreated samples.
Figure 1.5 Frictional coefficients of (a) treated and (b) untreated sisal/glass FRP hybrid composites as a function of fiber length, after 50 cycles.
Source: Ashok Kumar et al. [44]. © 2010, SAGE Publications.
Biswas and Xess [45] studied the behavior of short bamboo/E‐GF‐reinforced epoxy hybrid composites with respect to erosion wear, using different compositions by weight as thus: 65 wt.% of epoxy, 22.5 wt.% of bamboo fiber, 22.5 wt.% of GF; 70 wt.% of epoxy, 15 wt.% of bamboo fiber, 15 wt.% of GF; and 75 wt.% of epoxy, 7.5 wt.% of bamboo fiber, 7.5 wt.% of GF, as well as 100 wt.% epoxy. The graph of the result of erosion rate against impact velocity showed that the 15 wt.% bamboo/GF FRP composites possessed the lowest rate of erosion in comparison with the other composites.
1.5.2 Thermal Behavior
The thermal property is concerned with the response of hybrid FRP biocomposites to heat variation. Boopalan et al. [46] worked on hybrid raw jute/banana fiber‐reinforced epoxy composites with regard to their thermal analysis by varying the fiber weight ratio. They used ratios of 100/0, 75/25, 50/50, 25/75, and 0/100, while varying the temperature with the use of thermogravimetric analysis (TGA) and heat deflection temperature (HDT) analysis. With the TGA, the curve depicted that the 50/50 jute/banana FRP epoxy hybrid composite demonstrated greater thermal stability. There was a shift in the temperature during degradation from a value of 200 °C to a higher value of 380 °C. For the HDT, thermal property was sustained in the 50% jute with 50% banana FRP epoxy hybrid composite at the highest temperature of 90 °C in comparison with other composite samples.
Also, thermal properties of OPEFB/woven jute FRP epoxy hybrid composites were investigated by Jawaid et al. [47], using various temperatures. It was reported in their work that there was an increase in thermal stability when woven jute fibers were added to the EFB composite in its pure state. That is, hybridizing OPEFB with the woven jute fiber caused the thermal stability to be higher as compared with OPEFB fiber. The temperature of degradation was also reported to have shifted from a value of 292 °C to a higher value of 457 °C, leaving 12.1% char residue.
1.6 Progress and Future Outlooks of Mechanical Behaviors of Natural FRP Hybrid Composites
Application of various hybrid natural FRP composites is increasing with their innovative designs and developments through optimized manufacturing techniques. The advent of automation and robots in manufacturing of hybrid natural FRP composite materials has been improving their properties. This will increase in the next century, with synergy of sophisticated processes and techniques. Mechanical
