*3.10. Statistical Analysis*

SPSS software was used to perform an analysis of variance (ANOVA) on the obtained experimental results. 'Duncan's test was employed to conduct a mean comparison at a 0.05 level of significance (*p* ≤ 0.05).

#### **4. Results and Discussion**

#### *4.1. Density*

Figure 2 illustrates the density (*ρcomp*. *exp*.) value measurement for non-hybrid and hybrid SPF/GF/PLA composites. The density result showed that the density was improved by adding GF to the SP/PLA composites from 1.21 to 1.32 gm/cm<sup>3</sup> . Comparing S1, –S3 composites, the density of S3 exhibited the highest value of 1.32 gm/cm<sup>3</sup> , concluding that the hybridization of SPF/PLA composite increases the density of the composite. A similar finding was also revealed from a previous work done by Afzaluddin et al. [36] that density increment resulted from the addition of glass fiber into SPF/GF/TPU hybrid composites. This was due to the fact that GF has a higher density than SPF. The densities of untreated S3 and treated S5, S8 SP/GF hybrid composites were 1.32, 1.31, and 1.19 gm/cm<sup>3</sup> . The density slightly decreased after treatment of SPF. In previous studies by Atiqah et al. [38] and Merlini et al. [50], density reductions of treated fibers were also recorded. Comparing alkaline treated S4–S6, and BC treated S7–S9 hybrid composites, density results showed that the density decreased slightly as the SPF content was increased, irrespective of whether the SPF has been alkaline or BC treated. As SPF wt % increased, the hydrophilic nature of the SPF made it more difficult to develop the composite properly. This led to voids in the hybrid composites, decreasing the composite density. A similar finding was supported by Safri et al. [51], where the density was measured for SPF/GF/epoxy hybrid composites.

**Figure 2.** Composite experimental density *ρcomp*. *exp*. vs. samples for non-hybrid and hybrid SPF/GF/PLA composites. \* Values with different letters in the figures are significantly different (*p* < 0.05).

#### *4.2. Moisture and Void Contents*

Table 4 shows moisture contents, theoretical/experimental composites density, and void contents of non-hybrid and hybrid SPF/GF/PLA composites. In general, the experimental and theoretical densities differed from each other due to a significant influence of voids and pores in the composite towards the behavior of the composite [40]. It is clear from Table 4 that, with the help of glass hybridization, the percentage of voids decreased, as confirmed by SEM images in the morphological investigation. The finding was also supported by the results from work by Radzi et al. [20]. The S4–S6 (alkaline treated) hybrid composites exhibited lower %voids than S7–S9 (benzoyl treated) hybrid composites. This was due to the good compatibility of SPF and GF with the PLA matrix. According to Jawaid et al. [36], voids formation is due to the incompatibility of natural fiber with matrix to displace all the trapped air which is entrained during fabrication of hybrid composite and incomplete wetting out of the fibers by the matrix. This research also reported that after Brabender mixing, the composite samples need to be placed in an electric oven for 24 h at 60 ◦C before the hot press. Otherwise, more voids are visible due to their moist content.

**Table 4.** Moisture contents, densities, and void contents of non-hybrid and hybrid SPF/GF/PLA composites.


\* Values with different letters in the same column are significantly different (*p* < 0.05).

#### *4.3. Water Absorption Analysis*

Figure 3 shows the effect of treatments on the values of water absorption (*WA*) of non-hybrid and hybrid SP/GF/PLA composites. The higher SPF content (S1 composite) resulted in a higher *WA* value. The S2 (GF/PLA) composite was the lowest *WA* compared to other non-hybrid or hybrid composites. Among the SPF/GF/PLA hybrid composites, S5 and S8 composites had the lowest *WA* values as both composites exhibited the same 15 wt % SPF. S5 composite was alkaline treated, indicating that alkali hydroxyl (—OH) groups inside the molecules were broken down and when reacted with H2O molecules and left the fiber structure. The other reactive molecules produce fiber-cell-O-Na groups in the molecular cellulose chains. This reduced hydrophilic hydroxyl groups and increased the water absorption resistance of the fiber. A considerable number of hemicelluloses, lignin, pectin, wax, and oil were also extracted after alkaline treatment [25], while BC treated S8 composite indicated that the fibers were treated with BC after pre-alkaline treatment. The —OH fiber groups were substituted by the benzoyl group and bonded to the backbone of the cellulose. This resulted in the increased hydrophobicity of the fiber and enhanced matrix adhesion. The following is the order of the decreasing value of hybrid and non-hybrid SPF/GF/PLA composites' water absorption S2< S8< S5< S7< S9< S6< S4< S3< S1. After the 6th day, no change was observed in *WA*. For hybrid composite, the higher loading of natural fibers was considered to have more *WA*, whereas the higher GF loading was related to lower *WA*. This was also consistent with the past work of Afzaluddin et al. [36], who found that water resistance increased with the addition of GF in the SPF/TPU composites.

**Figure 3.** % Water absorption (*WA*) versus samples for non-hybrid and hybrid SPF/GF/PLA composites.

#### *4.4. Thickness Swelling*

Figure 4 presents the effect of treatments on the values of thickness swelling (*TS*) of non-hybrid and hybrid SP/GF/PLA composites. With the increase in *WA*, the thickness swellings of all the composites were increased. The higher the percentage of GF, the lower the *TS* value, while the higher the SPF content, the higher the *TS* value. This was due to the fact that GF possessed water resistance ability while SPF has hydrophilic nature [52]. The highest *TS* value was shown by untreated S1 (SPF/PLA) composite, having maximum content of SPF. In the S2 composite, no *TS* value was observed since it contained only

GF and PLA. Among the hybrid composite, the maximum *TS* value was observed in the S3 (untreated SPF/GF/PLA) hybrid composite. The thickness swelling was due to the prolonged immersion duration, where more water molecules were bonded to the hydrogen bonds of fiber. This *TS* value was randomly decreased after alkaline treatment due to the reduction in the micropores and collapsing capillaries, as well as the removal of wax and impurities at the fiber surface after the treatment. This predicted reduced water retention, as indicated by lessening the amount of water absorbed by the fiber. This reduced the *TS* value of the hybrid composite. Due to this cause, as the alkaline treated SPF content was increased, the *TS* value decreased. A similar finding was reported by N.B.M. Hafidz et al. [53] after alkaline treatment of palm oil fiber composites as well as kenaf fiber composites.

**Figure 4.** % Thickness swelling (*TS*) versus samples for non-hybrid and hybrid SPF/GF/PLA composites.

The reduction of *TS* was also observed after BC treatment of SPF for hybrid composite. S8 composite showed the lowest *TS* value among all hybrid composite. For the S9 composite, as the benzoyl treated SPF content was increased to 20 wt %, the *TS* value increased due to the disruption of lignin and polysaccharides during treatment that enhanced cellulose concentration. The cellulose chemical structure is composed of hydroxyl that is accessible to water. This directly stimulated the *WA*, thus increased the *TS* [51].

#### *4.5. Tensile Testing*

Figure 5a,b shows the effect of treatment on tensile strength and modulus of nonhybrid and hybrid SPF/GF/PLA composites. Hybrid composites were used for various weight percentages of SPFs and GFs. Hybrid composites had a combined weight percentage concentration of fibers (SPF/GF) fixed at 30%, whereas PLA was at 70%. Figure 5a confirmed that by incorporating the GFs, the tensile strength of the PLA composite was

increased significantly. From Table 1, the tensile strength of GF was more than SPF, while the tensile strengths of S1 (SPF only) and S2 (GF only) reinforced PLA composite were 16.0 MPa and 23.7 MPa, which indicated that the tensile strength on the addition of GFs was better than untreated SPF. It was observed that the addition of alkali-treated SPF increased the tensile strength of the hybrid SPF/GF reinforced PLA composites. The highest tensile strength of 26.3 MPa was shown by the S6 composite among all hybrid composites. According to Afzaluddin et al. [36], the tensile strength of SPF/GF reinforced TPU hybrid composites could be increased by adding SPF. Among S4–S6 (alkaline treated) hybrid composites, S6 exhibited a maximum tensile strength of 26.3 MPa, while S5 and S4 demonstrated only 14 MPa and 18.7 MPa, respectively. This proves that the interfacial bonding between SPF and PLA matrix had improved after alkaline treatment. S6 hybrid composite exhibited maximum tensile strength that might be due to GF (10% loaded); therefore, SPF can effectively transfer the load from the GF on this particular composition [30]. When alkaline treated SPF/GF/PLA hybrid composites were compared with benzoyl treated SPF/GF/PLA hybrid composites, it is clearly shown in Figure 5a that the alkaline treatment was the best treatment approach used for the improvement of tensile strength. This was due to the improvement in the interface bonding by giving rise to additional sites of mechanical interlocking, facilitating the interpretation of fiber-matrix at the interface. The alkaline treatment of fiber increased the binding properties of the surface by eliminating natural and artificial impurities, which created rough surface topography [30]. This finding was also supported by the morphological investigation of this research.

S3, S5, and S8 had the same composition of 15/15/70 wt % SPF/GF/PLA. The only difference they possessed was the untreated or treated SPF. The analysis showed that the tensile strength of S5 was improved by 27% compared to the S3 hybrid composite. The tensile strength of the alkaline treated SPF/GF/PLA was higher than untreated hybrid composites due to the strong interfacial bonding between treated fiber with PLA matrix. A similar study was carried out by Atiqah et al. [3], that the surface treatment of fiber improved the tensile properties of SP/GF reinforced TPU hybrid composites. The minimum value of tensile strength was 9.3 MPa, which was recorded for S8. The inclusion of BC treated SPF in SPF/GF/PLA hybrid composites caused a remarkable decrease in tensile strength. One of the investigations by Swain et al. [54] demonstrated that the tensile strength only decreased when there was weak bonding between fiber and the matrix.

In general, the stiffness property of the SPF/GF reinforced PLA hybrid composites was also determined. Material stiffness property is mainly indicated by tensile modulus. In general, the value of tensile modulus was increased with an increase in wt % of GFs. This statement was valid for both untreated as well as treated SPF/GF reinforced PLA hybrid composite. The tensile moduli for BC treated SPF/GF/PLA hybrid composites were increased by the GF percentage, i.e., for 10, 15, and 20 wt % GF in S9, S8, and S7 composites, the value of tensile moduli were 500, 505, and 607 MPa.

Comparing untreated SPF of S1–S3 composites indicated increasing percentage of GFs improved the tensile modulus of composites. The highest tensile modulus was shown by S2, having 30 wt % of GFs as depicts in Figure 5b. This is since the tensile modulus and strength of GF was higher than SPF. The increment of tensile modulus denoted the improvement of load-bearing capacity.

**Figure 5.** (**a**) Tensile strength versus samples of non-hybrid and hybrid SPF/GF/PLA composite. (**b**) Tensile 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).

On comparing the alkaline treated SPF/GF/PLA composites, the analysis showed that the S6 hybrid composite exhibited the maximum tensile modulus of 561 MPa. The S6 hybrid composite had a maximum of 20 wt % of SPF, indicating improvement in the interfacial bonding between SPF and PLA matrix after the alkaline treatment of SPF that consequently resulted in the improved tensile modulus [20]. In addition, after alkaline treatment, rougher topography might cause the qualitative interface between fiber and matrix due to the removal of waxy and impurity substances. A similar increment of tensile modulus after alkaline was revealed by Mukhtar et al. [18] for SPF/GF-reinforced polypropylene composites. Comparing the alkaline, BC treated, and untreated SPF/GF/PLA hybrid composites, it might be noted that BC treatment increased the tensile modulus of the same wt % composite, i.e., S3, S5, and S8. Among these three composites having 15/15/70 wt % (SPF/GF/PLA), the highest value of tensile modulus 505 MPa was shown by S8, followed by 435 MPa and 423 MPa, for S5 and S3 composites, respectively. Previous studies [29,51,55] indicated that the treatment of fibers with BC enhanced the mechanical contact surface with the matrix, which enhanced the interfacial bonding between fiber and matrix and increased stress transfer and tensile modulus of the entire composite. The maximum tensile modulus was shown by the S7 (BC treated) hybrid composite. The tensile properties in terms of the tensile stress–strain curve of various composites is shown in Figure 6.

**Figure 6.** Tensile stress (MPa) versus % tensile strain for various composites.
