*4.6. Flexural Testing*

Figure 7a,b shows the effect of treatment on flexural strength and modulus of nonhybrid and hybrid SPF/GF/PLA composites. The bending results analysis confirmed the flexural strength of the composite. In the S5 hybrid composite, the alkaline treatment of SPF enhanced the flexural strength. Comparing S3, S5, and S8 hybrid composite having the same wt % (SPF/GF/PLA-15/15/70), the S5 (alkaline treated) hybrid composite showed maximum flexural strength of 27.3 MPa among all the non-hybrid or hybrid composites. The incorporation of alkaline treated 15 wt % of SPF led to improved flexural strength of the S5 hybrid composite. After the alkaline treatment, about 17% of flexural strength was increased compared with the S3 (untreated) hybrid composite. The increasing of flexural strength after the alkaline treatment was due to partial elimination of hemicellulose,

wax, as well as disruption —OH bonding on the fiber, which ensured a better adhesion bonding between the PLA matrix and SPF, whereas the S3 hybrid composite showed low flexural strength caused by poor interfacial bonding between the PLA matrix and SP fiber. This finding was comparable with the SPF/GF-reinforced polypropylene investigated by Mukhtar et al. [18]. Another research highlighted that the fiber treatment plays an important role for flexural strength value. The alkaline treatment of fiber can reduce the cell wall thickening, leading to improved adhesion between fiber and matrix [3]. Similar research reported the effect of alkaline treatment on mechanical properties for roselle/SPF reinforced TPU hybrid composites [20]. Atiqah et al. [3] reported that the flexural strength of SPF hybrid composites also depends on types of surface treatment. The contrast results of alkaline treatment with BC treatment for SPF/GF/PLA hybrid composites are shown. The value of flexural strengths for all three hybrid composites treated with BC were less than the untreated ones.

For non-hybrid S1 composite (only 30 wt % untreated SPF and 70 wt % PLA), the flexural strength was 26.3 MPa, which was supported by the study of Sherwani et al. [40] for flexural analysis of different ratio SPF/PLA composite. The analysis showed that the hybridization of GF did not significantly affect the flexural strength of composites. In the case of S2, it decreased by means of bending ability decrease on the addition of GF. Comparing with untreated SPF/GF, it was clear that the hybridization of the composite resulted in a good flexural modulus of 1811 MPa (S3 hybrid composite), while the flexural moduli for non-hybrid S1 and S2 composite were very low of 1317 MPa and 1564 MPa, respectively. From the comparison of the flexural modulus after alkaline and BC treatment for same wt % hybrid composites (i.e., S5 with S8), it was observed that after BC treatment, the flexural modulus was increased from 1336 MPa to 1536 MPa. BC treatment reduces the diameter of SPF as well as eliminate the lignin and wax layer on fiber. The compatibility between fiber and matrix increased due to the benzene rings' availability in the benzoyl group attached to the fibers.

Safri et al. [28] also reported the same reason for an increment of flexural modulus after BC treatment of SPF/GF/epoxy hybrid composites. On comparing S6 and S9 having the same wt % ratios (20/10/70) SPF/GF/PLA hybrid composites, almost the same flexural modulus about 1450 MPa was observed in all composites. This value is maximum among alkaline treated SPF, which was proved by Radzi et al. [20] that the alkaline treatment of SPF would improve flexural modulus due to better wetting and good interfacial bonding between matrix and the treated fiber. S7 exhibited 1847 MPa, the highest value of flexural modulus. This was due to various changes at the surface of SPF after BC treated that increased the adhesion between treated fiber and matrix and mechanical interlocking by distributing several small voids on the fiber surface and creating extra fiber interpenetration to the interface. A similar reason was reported for studying the effect of 6% alkaline treatment of SPF for roselle/SPF reinforced TPU hybrid composites [20].

Safri et al. [28] also reported the same reason for an increment of flexural modulus after BC treatment of SPF/GF/epoxy hybrid composites. On comparing S6 and S9 having the same wt.% ratios (20/10/70) SPF/GF/PLA hybrid composites, almost the same flexural modulus about 1450MPa was observed in all composites. This value is maximum among alkaline treated SPF, which was proved by Radzi et al. [20] that the alkaline treatment of SPF would improve flexural modulus because of better wetting and good interfacial bonding between matrix and the treated fibre. S7 exhibited 1847MPa, the highest value of flexural modulus. This was due to various changes at the surface of SPF after BC treated that increased the adhesion between treated fibre and matrix and mechanical interlocking by distributing several small voids on the fibre surface and creating extra fibre interpenetration to the interface. A similar reason was reported for studying the effect of 6% alkaline treatment of SPF for roselle/SPF reinforced TPU hybrid composites [20].

**Figure 7.** (**a**) Flexural strength versus samples of non-hybrid and hybrid SPF/GF/PLA composite. (**b**) Flexural modulus versus samples of non-hybrid and hybrid SPF/GF/PLA composite. \* Values with different letters in the figures are significantly different (*p* < 0.05).
