*4.7. Morphological Investigations*

The morphological analysis was conducted from two different perspectives: tensile and flexural testings of the fractured cross-sectional area, as shown in Figure 8. The analyses were carried out from 100× magnification to 300× magnification for both non-hybrid and hybrid SPF/GF/PLA composites from S1 to S9. In SEM morphology, S1 (untreated/nonhybrid) composites showed that the SPF demonstrated pull-outs, voids, were also visible that indicated poor compatibility with the PLA matrix. Due to the untreated SPF, some wax and lignin contents were visible, and these SEM images were identical to date palm leaf (DPL) images studied by Swain et al. [54], showing the untreated DPL's lignin and wax contents. Non-hybrid S2 composite revealed that GF was stretched when the tensile load was applied and fewer voids were observed, proving good tensile strength and modulus compared with S1 and S3 composites. Atiqah et al. [36] also mentioned that when the tensile load is applied to SPF/GF/TPU composites, GF stretching occurred. SEM images of the S3 hybrid composite revealed that SPF breakage began when analysing the flexural fracture surface, whereas the other two flexural fracture surfaces of S1 and S2 did not reveal any broken fibres. As a result, the flexural modulus of S3 hybrid composite was the highest among S1, S2, and S3 hybrid composites. SEM figures generally showed a good bonding between alkaline and BC treated fibres and PLA matrix compared with untreated SPF. S4, S5, and S6 (alkaline treated) hybrid composites possessed a rough surface than other hybrid composites that contributed to the enhancement of interfacial bonding between SPF and PLA matrix. The rough surface after alkaline treatment of SPF was because of the removal of hemicellulose, lignin, and waxy layer, where this part was visible on SEM images. Because of this reason, S4, S5, and S6 tensile and flexural fracture surfaces showed broken SPF, which indicated the increments in the tensile and flexural strengths after alkaline treatment. This phenomenon might be due to the rougher surface of SPF, the increased bonding strength between SPF and PLA matrix, and few visible voids on the surface. Breakage of the SPF indicated more energy dissipation when tensile or flexural loads were applied. SPF did not pull out, as proven by the breakage images of SPF in all three alkalines treated S4, S5, and S6 hybrid composites. Similar results were reported by Radzi et al. [20] for alkaline surface-treated roselle fibre/SPF hybrid composite that demonstrated better adhesion between treated fibre and matrix. The tensile and flexural properties of the hybrid composites were also enhanced. S7, S8, and S9 hybrid composites showed the big gaps/voids and low bonding strength between SPF and PLA matrix after BC treatment. SEM images also presented visible fibres break and dislocation, as well as voids/gaps, as aforementioned above. The weak interfacial adhesion caused the pull-outs in SPF. The fractured surface observed for S7, S8, and S9 (BC treated) hybrid composites showed broken/breakage SPF attributed to the increase of both flexural and tensile moduli of the hybrid composites. In previous studies, morphological analysis of BC treated fibre also reported that the wettability of treated fibre with matrix was increased [26,56].

**Figure 8.** *Cont*.

**Figure 8.** *Cont*.

**Figure 8.** Morphological investigations for fractured surface analysis of (**a**) tensile and (**b**) flexural test. The SEM images clearly defined the presence of voids, breakage of SPF, GF, and PLA matrix.

SEM images revealed that composites of untreated SPF or BC treated had poor interfacial adhesion, as reported by fiber pull-outs and the presence of holes/voids/gaps. On the other side, alkaline treated S4, S5, and S6 hybrid composites were distinguished by fibers breakage that showed strong adhesion between SPF and PLA matrix. This morphological analysis showed that alkaline and BC treatment was able to modify the SPF surface for good adhesion, as reported in other studies [20,51]

#### *4.8. Impact Testing*

Table 5 shows tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength values of non-hybrid and hybrid SPF/GF/PLA composites. Impact strength is used to calculate the dissipation of total energy before ultimate fracture. Figure 9 shows the effect of treatment on the impact strength of non-hybrid and hybrid SPF/GF/PLA composites. Therefore, on hybridization, as wt % of GFs content was increased, the failure mechanism was GF fracture, not GF pull-out, due to the brittle nature of GF. Due to this reason, the composite can withstand a high-speed impact load at higher wt % GFs content. In general, the energy absorption mechanism was not in role, only the energy dissipated in frictional sliding of one fiber with the other due to the interaction of fibers. Moreover, impact strength was increased after hybridization, which increased the stress capabilities. The impact strength of S1 (untreated) composite was 2.09 kJ/m<sup>2</sup> that

increased to 2.70 kJ/m<sup>2</sup> after the hybridization of GF for S3 hybrid composite. Non-hybrid S2 composite showed good impact strength of 3.07 kJ/m<sup>2</sup> . S4 (alkaline treated SPF) hybrid composite exhibited the maximum impact strength value of 3.22 kJ/m<sup>2</sup> due to the removals of hemicellulose, lignin, and pectin, wax generation of moisture resistance, and the creation of rough fiber surface after alkaline treatment, which improved the adhesion between treated fiber and matrix. Comparing S4–S6 hybrid composites showed a decrease in impact strength as the wt % of glass content decreased. The impact strength directly depended upon the toughness of the entire composite. Fibers play a crucial role in impact resistance, where the combined effect of both fibers improved their impact strength. Uma Devi et al. [57] reported that as the percentage of GF increased, the impact strength increased for short pineapple fiber/GF/polyester hybrid composites.

**Table 5.** Tensile, flexural, and impact properties of non-hybrid and hybrid SPF/GF/PLA composites.


\* Values with different letters in the same column are significantly different (*p* < 0.05). SP30 defined for Untreated SPF (30%)/PLA (70%). SA defined for 6% conc. of NaOH treated SPF (30%)/PLA (70%). SB defined for 15 min soaking BC treated SPF (30%)/PLA (70%).

**Figure 9.** Impact strength versus samples of non-hybrid and hybrid SPF/GF/PLA composite. \* Values with different letters in the figures are significantly different (*p* < 0.05).

Comparing the same wt % hybrid composites, i.e., S3, S5, and S8, S5 (alkaline treated) hybrid composite showed the highest impact value of 3.10 kJ/m<sup>2</sup> followed by 2.78 kJ/m<sup>2</sup> for S8 (BC treated) whereas S3 (untreated) exhibited the lowest impact strength value of 2.7 kJ/m<sup>2</sup> . A similar improved impact strength value was reported by Atiqah et al. [3] after alkaline treatment of SPF for SPF/GF/TPU hybrid composites. After S4, the value of impact strength decreased as the SPF content was increased. With an increase in SPF content, the interspaces and stress concentration shoot up, which acted as crack propagation. The same trend of decreasing impact strength value with increasing fiber content was reported by Swain et al. [53] for date palm leaf/GF reinforced hybrid composite.

BC treated SPF also improved the impact strength from 2.70 kJ/m<sup>2</sup> for S3 (untreated) hybrid composite to 2.80 kJ/m<sup>2</sup> for S7 (BC treated) hybrid composite. This might be due to good interlocking between treated fiber and matrix, which allowed maximum energy absorption and stopped the crack propagation, enhancing the impact properties [58,59]. Thiruchitrambalam et al. [27] reported a 12% impact strength increment after BC treatment for palmyra palm leaf stalk fiber-polyester composites. Swain et al. [60] also revealed that after BC treatment of jute fiber, the impact strength increased for developing jute/epoxy composites. The lowest value of impact strength of 1.97 kJ/m<sup>2</sup> was shown by the S9 hybrid composite that might be due to the insufficient resistance to pull out fiber during impact fracture. Fracture of a matrix, fiber/matrix debonding, and fiber pull-out are three main causes for impact failure [60].

#### *4.9. Fourier Transform Infrared (FTIR)*

Fourier transform infrared (FTIR) spectroscopy is often used to verify the correct mixture of matrix-fiber ratio due to the interplay between components in polymer composites is complex [61–68]. Figure 10 shows the FTIR analysis for untreated, alkaline, BC treated of non-hybrid, and hybrid SPF/GF/PLA composites. From the figure, it is clear that all hybrid composite showed almost similar patterns. This analysis was used to determine the effect of alkaline and BC treatment on SPF and the chemical bonding nature between SPF, GF, and PLA. FTIR of untreated and treated SPF/GF/PLA composites revealed changes in the associated functional groups. Spectrum helps us to determine the presence of lignin, cellulose, and hemicellulose in (C-H rocking vibrations), 1180 cm−<sup>1</sup> cellulose (C-O-C asymmetric valence vibration, 1316 cm−<sup>1</sup> cellulose (C-H<sup>2</sup> rocking vibration), 1370 cm−<sup>1</sup> cellulose (C-H<sup>2</sup> deformation vibration), 1424 cm−<sup>1</sup> cellulose, 1227 cm−<sup>1</sup> lignin (C-C plus C-O plus C=O stretch) [69,70].

In the range, 1300 cm−<sup>1</sup> to 1160 cm−<sup>1</sup> belonged to the C-C group (lignin in-ring stretch mode). The difference in peak heights of the untreated (S1, S3) and alkaline treated (S4–S6) hybrid composites was observed that resulted from the reduced amount of lignin and hemicelluloses in SPF after alkaline treatment. Significant changes at peaks 1180 cm−<sup>1</sup> were observed, showing C-O in alkaline treated fiber concerning primary alcohol stretching, peak reduction compared to untreated fiber. Other studies have also confirmed this related disappearance of lignin and hemicelluloses after alkaline treatment that improved the adhesion between the fiber and the matrix [3,19,71]. The sharp peak at 1745 cm−<sup>1</sup> for S1 composite was observed that was associated with the presence of hemicellulose C=O stretching vibration [71]. The intensity changes at the 1756 cm−<sup>1</sup> peak showed the esterification among the —OH groups of SP fiber and -COOH terminal groups of PLA. A small peak in all nine samples at 2995 cm−<sup>1</sup> was ascribed with the frequency of the O-H group [72,73]. This intensity was also decreased after treatment of SPF. After 6 wt % alkaline treatment of SPF, the —OH groups were substituted with –ONa groups. According to Bachtiar [71], at 4% alkaline treatment, the nature of SPF changes to hydrophilic since the cellulose I changed to Cellulose II.

The peak at 1450 cm−<sup>1</sup> revealed C-C stretching in the aromatic ring, and the peak at 1719 cm−<sup>1</sup> indicated the C=O stretching of the benzoyl carbonyl group in the benzoylated fiber. As the benzoyl group reacted with the —OH group of SPF, the hydrophilic character decreased by reducing the hydroxyl group, which was indicated at 2995 cm−<sup>1</sup> . A

similar FTIR result was also reported by Salisu et al. [58] after benzoylation of sisal fiber unsaturated polyester-reinforced composites.

**Figure 10.** FTIR spectrum detailed data analysis of non-hybrid and hybrid SPF/GF/PLA composites.

#### **5. Conclusions**

The physical, mechanical, and morphological properties of treated SPF/GF reinforced PLA hybrid composites were investigated. This novel SPF/GF reinforced PLA hybrid composites exhibited lower densities after alkaline treatment of SPF, improved water absorption, and thickness swelling after both SPF treatments. It was observed that the incorporation of alkaline treated SPF/GF reinforced PLA matrix increased the tensile and flexural strengths. Both alkaline treated S6 and S5 hybrid composites exhibited the highest tensile strength of 26.3 MPa and flexural strength of 27.3 MPa. It was also found that BC treated SPF/GF reinforced PLA hybrid composites exhibited the highest tensile and flexural moduli. S7 hybrid composite recorded the highest tensile and flexural moduli of 607 MPa and 1847 MPa, respectively. Nevertheless, the incorporation of alkaline treated S4 hybrid composite showed the highest value of impact strength of 3.22 kJ/m<sup>2</sup> . This value was reduced as the SPF content was increased. The morphological investigation revealed that alkaline treatment of SPF possessed better interfacial adhesion between SPF and PLA matrix. FTIR results also showed that after alkaline treatment, the adhesion between fiber and matrix was improved. This combination of alkaline and BC treated SPF/GF reinforced PLA hybrid composite resulted in good physical and mechanical properties. Therefore, this composite can be proposed for the fabrication of automotive components. Future research will primarily focus on replacing Acrylonitrile butadiene styrene (ABS) plastic motorcycle battery housing parts with these hybrid composites.

**Author Contributions:** Conceptualization, S.F.K.S., E.S.Z. and S.M.S.; methodology, S.M.S.; software, S.F.K.S.; validation, S.F.K.S. and E.S.Z.; formal analysis, S.F.K.S.; investigation, S.F.K.S. and S.M.S.; resources, S.F.K.S. and S.M.S.; data curation, S.F.K.S. and S.M.S.; writing—original draft preparation, S.F.K.S.; writing—review and editing, E.S.Z. and S.M.S.; visualization, S.F.K.S., E.S.Z. and S.M.S.; supervision, Z.L., A.K., E.S.Z. and S.M.S.; project administration, E.S.Z.; funding acquisition, E.S.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Universiti Putra Malaysia for the financial support through the Grant Putra Berimpak UPM.RMC.800–3/3/1/GPB/2020/9694500 (vote number 9694500).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author, upon reasonable request.

**Acknowledgments:** The authors are gratefully acknowledged to Universiti Putra Malaysia (UPM) for funding this research.

**Conflicts of Interest:** The authors declare no conflict of interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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