The Effect of Alkalization Fiber on Mechanical, Microstructure, and Thermal Properties of Sugarcane Bagasse Fiber Reinforced PLA Biocomposite

1  Introduction

In today’s era, the use of plastic materials is necessary for various human activities, such as vehicle components, household appliances, interiors, and packaging [1–3]. However, plastics derived from petroleum (synthetic) have a negative impact on the environment due to their environmentally unfriendly nature [4]. One solution to replace synthetic plastic is the development of biocomposite materials.

Biocomposites are a combination of two or more materials, where one of the constituent elements is from nature. Generally, biocomposites are composed of a matrix as a binder and a filler as reinforcement [5]. In the last five years, PLA as a matrix in biocomposites has developed rapidly due to its environmentally friendly nature, abundant availability, and competitive mechanical properties compared to synthetic plastics [6,7]. However, pure PLA has disadvantages, such as low mechanical properties and poor thermal stability [8]. The presence of natural fibers is an alternative way to improve the weaknesses of pure PLA [9].

The addition of natural fibers into PLA has been reported by several previous researchers, such as hemp/flax [10], natural rubber/rice straw [11], and prosopis juliflora bark powder [12]. In their research, the solution casting method is a way to produce biocomposites. They reported that the addition of fibers into the matrix improves the mechanical and thermal properties of biocomposites. For example, PLA filled with 3% eucalyptus microfiber increases the tensile strength and thermal degradation point of the biocomposite [13]. However, the addition of natural fibers into the PLA matrix does not always improve the properties of the biocomposite. This occurs due to the poor surface interface bond between the matrix and the fibers. This phenomenon was observed in a study related to the PLA matrix and lemongrass filler [14].

The interfacial bond between the matrix and the fiber is the key to improving the properties of the biocomposite. Alkalization treatment with sodium hydroxide is one alternative to increase the surface adhesion between the matrix and reinforcement [15]. Several previous studies related to alkalization treatment on fibers have successfully improved the properties of biocomposites when combined with a PLA matrix. Previous reports related to the alkalization treatment on fibers and their biocomposites have been widely reported by several researchers [16,17]. Previous research reported that the preparation of betel leaf fiber using alkalization effectively increased the mechanical properties of PLA-based biocomposites. The comparison of these values was proven in PLA biocomposites from betel leaf fibers before and after alkalization with 3% filler loading, showing different values, namely 19.48 and 26.34 MPa, respectively. A similar case was also reported that alkalization treatment of sorghum bagasse increased interfacial bonding with PLA [18]. However, in previous studies, the fabrication process of the biocomposite used solution casting.

In this study, sugarcane bagasse fiber was subjected to alkalization and combined with the PLA matrix to become biocomposite filaments through extruder mixing. The resulting biocomposite filaments were used as 3D printing ink materials to make finished biocomposite products. Biocomposite products were tested for tensile, impact, flexural, microstructure, and thermal stability. To our knowledge, there has been no report on the study of PLA reinforced by alkalized sugarcane bagasse-based biocomposites that were processed by 3D printing.

2  Materials and Methods

2.1 Materials

This study used polylactic acid (PLA) in pellet form (density 1.25 g/cm3) produced by Indofilam, Indonesia as the matrix material. Sugarcane bagasse was obtained from traders around Jember, East Java, Indonesia. Bagasse fiber has a density value of 0.77 g/cm3 and a size of 177 microns.

2.2 Preparation of Sugarcane Bagasse Fiber (SBF)

Sugarcane bagasse was dried in the sun for one week to reduce its water content. The dried sugarcane bagasse was ground with an ice blender to obtain powdered fiber. Next, the sugarcane bagasse fiber (SBF) was alkalized using NaOH solution with concentrations of 0% (untreated), 4%, 6%, and 8%. The SBF was then neutralized using distilled water until the pH reached 7. The SBF was dried in an oven at 60°C for 3 h. Finally, the SBF was filtered using a mesh sieve to achieve a more uniform size of approximately 177 microns.

2.3 Preparation of Biocomposite Filament

PLA and SBF were mixed manually in a volume percentage of 98% PLA and 2% SBF. This composition was maintained in all biocomposites, and the difference was in the concentration of alkalization (Table 1). The biocomposite filament was prepared using a single-screw extruder (Wellzoom Type B, Shenzhen, China). The biocomposite mixture was fed into the extruder with parameters of 155°C, 25 rpm, and a nozzle diameter of 1.75 mm. Furthermore, the biocomposite filament was prepared as a 3D printing ink material.

2.4 Printing of Biocomposite Specimens

The finished biocomposite product was printed and adjusted to the ASTM D638 type 5 standard for tensile testing (63.50 mm × 9.53 mm × 3 mm) through a 3D printing machine (Creality Ender 3 V3, Shenzhen Creality 3D Technology Co., Ltd., Shenzhen, China). In addition, the biocomposite products were also printed according to ASTM E23 and ASTM D790-17 standards for impact testing and flexural testing, respectively. The printing parameters of the biocomposite by 3D printing include nozzle diameter 0.8 mm, nozzle temperature 200°C, layer temperature 60°C, raster angle 0°, layer height 0.2 mm, printing speed 50 mm/s, and bulk density 100%. The scheme of the biocomposite manufacturing process is illustrated in Fig. 1.

Figure 1: Schematic of preparation SBF, filament making, and specimen printing of biocomposite

2.5 Characterization

2.5.1 Tensile Tests

A 5 kN Tensilon Universal Testing Machine was used as the tensile test instrument. The tensile test specimens were 3D printed according to the ASTM D638 type V standard, and their thickness and width were measured at five points each in the gauge length region. The tensile test was conducted at a speed of 1 mm/min at room temperature.

2.5.2 Impact Tests

Impact test specimens were formed according to the ASTM E23-12C standard using 3D printing machine tools. Tests were conducted using an impact testing machine at room temperature.

2.5.3 Flexural Tests

All specimens were prepared in accordance with ASTM D790 standards and tested by the three-point flexural method. Flexural tests were conducted to determine the properties of all biocomposites using a flexural testing machine.

2.5.4 Morphological Observation

The fracture morphology of all biocomposites tested was observed using a digital microscope and a scanning electron microscope (SEM) Hitachi SU-3500 with a magnification of 5000× at 5 kV voltage. The morphological structure was identified at the fracture point after the tensile test. Observations were made at room temperature.

2.5.5 Thermogravimetric Analysis (TGA)

All samples were prepared with a weight of 5 mg to analyze the thermal stability. The test setup was carried out at 30–500°C with a heating rate of 10°C/min under nitrogen conditions.

2.5.6 Fourier Transform Infrared (FTIR)

The chemical functional groups present within the analyzed samples were characterized utilizing a Perkin-Elmer Frontier FTIR spectrometer. Each sample was meticulously sectioned into films measuring 1 × 1 cm. Spectral data were collected across the wavenumber range of 4000–1000 cm−1.

3  Results and Discussions

3.1 Tensile Strength

Fig. 2 shows the tensile results of PLA/SBF biocomposites with different alkalization treatments. The tensile strength of pure PLA increased from 16.06 to 34.59 MPa along with the increase in alkalization concentration to 6%. The increase occurred due to the well-formed interfacial bond between the fiber and PLA matrix. Good interfacial bonding between fiber and PLA matrix results in good compatibility and can increase tensile strength [19]. The role of alkalized fibers results in a cleaner and more reactive fiber surface, thus increasing the interaction between the fiber and the polymer matrix [20]. The results of this research are in accordance with previous research [19,20]. In SBF with 8% NaOH treatment, the tensile strength of the biocomposite decreased by 9.67 MPa compared to the 6% alkalization treatment on the fiber (Fig. 2a). This is because the high concentration of NaOH causes damage to the fiber [21]. Other factors contributing to the difference in tensile strength values are several defects such as cracks, debonding, and cavities in the matrix [22].

Figure 2: Tensile properties of all biocomposites tested: (a) Tensile strength; (b) Elongation at break

The addition of alkalization concentration to SBF increases the elongation at break (Fig. 2b), where pure PLA initially has an elongation of 3.23%, increasing to 13.12% (6% alkalized fiber) [23,24]. This phenomenon indicates good interfacial bonding of fiber and PLA, which results in increased ductility properties. However, in the 8% alkalization treatment, there was a decrease in elongation. This is due to excessive depolymerization of cellulose bonds [21,25].

3.2 Impact Strength

The impact test results can be seen in Fig. 3, where the impact strength of pure PLA was achieved at 25.63 kJ/m2. Increasing the concentration of alkalization showed an increasing trend in the biocomposite, namely 27.81, 32.87, and 45.03 kJ/m2 for fiber alkalization treatments of 0% (untreated), 4%, and 6%, respectively. The high impact strength is caused by good fiber and matrix interfacial bonding [26]. Increased impact strength occurs due to the role of fibres that undergo alkalization treatment. This results in a more reactive surface due to the reduction of lignin and hemicellulose layers, allowing the fibres to bond well with the PLA matrix [27]. This phenomenon is similar to previous studies [27,28]. These results are also in line with tensile strength.

Figure 3: Impact strength of biocomposite before and after alkalization treatment with different concentrations

However, a decrease in impact strength occurred in fibers with an 8% alkalization treatment, resulting in 31.91 kJ/m2. This occurs because the amount of sediment residue increases due to the increasing alkalization treatment on the fiber [26,29]. In addition, impact strength is also influenced by building direction, infill percentage, and layer height [30].

3.3 Flexural Strength

Fig. 4 shows the flexural strength of PLA/SBF biocomposites with different alkalization treatments. The flexural strength value of pure PLA is 24.97 MPa then increases to 32.99 MPa in PLA/SBF biocomposites (without alkalization treatment). This proves that the addition of SBF as a reinforcement increases the flexural strength of PLA. This phenomenon is similar to previous studies on PLA and hemp fiber biocomposites [31].

Figure 4: Flexural properties of all biocomposites tested: (a) Flexural strength; (b) Flexural modulus

The highest flexural strength value is found in the biocomposite with alkalization treatment on SBF of 6% at 45.62 MPa. The increase in value after alkalization is due to the release of lignin, hemicellulose, and wax content in the fibers, resulting in a more responsive surface when bonded to the matrix [32]. This is also supported by other studies that utilize coconut and pineapple fibers in the PLA matrix, which report similar results [33]. However, the flexural strength value decreased after fiber alkalization treatment by 8%. This decrease is due to the fibers being damaged from the high NaOH content. This damage results in a decrease in the mechanical properties of the biocomposite [27,34].

The flexural modulus produced the same upward trend as the flexural strength. The highest flexural modulus value was at the 6% alkalization treatment of the fibers, at 6808.12 MPa. Although the increase is not significantly different, it proves that the alkalization treatment of the fibers increases the elastic modulus of PLA [35]. Similar research reported an increase in the elastic modulus value with the addition of sisal, kenaf, and jute fibers in biocomposites [35].

The flexural modulus decreased at the highest alkalization variation of 8%, which was 5903.02 MPa, because there was porosity, indicating that the bond between the matrix and fiber was not good. This event can occur due to the influence of the alkalization treatment. The properties of the fibers produced after the alkalization treatment vary depending on the parameters used, such as processing temperature, solution concentration, and soaking time [32,36,37].

3.4 Fracture Morphology by SEM

Fig. 5a–e shows the results of the microstructure test on the fracture of the samples: pure PLA, PLA/SBF-A0, PLA/SBF-A4, PLA/SBF-A6, and PLA/SBF-A8. The surface of the pure PLA sample (Fig. 5a) looks more even and smoother [38,39]. This result confirms the morphology test with SEM and is similar to previous studies [40,41].

Figure 5: Fracture morphology by SEM: (a) PLA; (b) PLA/SBF-A0 (untreated); (c) PLA/SBF-A4; (d) PLA/SBF-A6; (e) PLA/SBF-A8

Fig. 5b (PLA/SBF without alkalization treatment) shows the failure of a matrix rich in fiber pull-out on the fracture surface. This matrix richness occurs because the amount of fiber used is insufficient and the fiber distribution in the composite is uneven. These results are in line with previous studies related to hybrid composites of hemp and sisal fibers [42]. Fig. 5c shows the fracture microstructure of the PLA biocomposite with sugarcane bagasse fiber that has undergone 4% alkalization treatment. From the figure, it can be seen that there is a shrinkage of the matrix (matrix-covered fibers) which indicates that the alkalization-treated fibers have a good bond with the matrix, thus requiring a greater force to pull the fibers out of the matrix [43]. However, with 4% NaOH treatment, there are still areas of smooth fracture surface. This indicates that there are still some areas in the sample that are dominated by the matrix without fibers, although there are not many of them. Meanwhile, Fig. 5d (SBF treatment with 6% alkalization) shows that PLA and SBF have a good bond that is dominant throughout the fracture surface. This is supported by the increased number of OH groups on the fiber surface after the alkalization process, so that the contact area with the matrix becomes larger [44]. Fig. 5e shows the rough fracture surface of the sample with voids, and agglomeration. Interestingly, there are no visible fibers pulled out on the fracture, even though the surface is predominantly rough. This is due to the brittle fibers resulting from the high alkalization treatment concentration. High alkalization concentrations can corrode fibers resulting in structural damage [45]. These findings confirm the cause of the decrease in mechanical properties that occurred.

3.5 Thermal Stability

Fig. 6 shows the graph of TGA test results of PLA/SBF biocomposites. Zone 1 (95–105°C) shows minor mass reduction (1–5 wt%), and mass degradation is caused by the evaporation of moisture content in the biocomposite material [46]. This is evidenced by the decrease in mass percentage in the biocomposite material. In zone 2 (270–350°C), mass degradation will occur in all variations due to the loss of the main constituent components of biocomposites. The main constituent components are PLA [47] and SBF. There is an improvement in properties due to the addition of SBF to PLA. The initial temperature of structural degradation of PLA increased after alkalization SBF rather than PLA alone. PLA/SBF-A6 is the best in thermal stability as it can withstand a temperature of 345.92°C before undergoing significant mass degradation due to the evaporation of the biocomposite content from the polymer matrix and the inner structure of the fiber. In this case, the mass reduction between zone 2 to zone 3 reaches 85 wt%. The mass reduction from the thermal stability testing process is presented in Table 2.

Figure 6: Thermal stability of PLA and its biocomposites

Alkalization treatment improves the thermal stability of natural fibers by increasing the fiber surface content [48]. Another influencing factor is that lignin has higher thermal stability than cellulose because it acts as a barrier to fiber degradation due to environmental influences [49]. The thermal stability of PLA/SBF-A6 is also supported by good bending strength values due to the lignin content that is still present in the fiber. This causes the resulting biocomposite to be strong and have the best thermal stability. However, there are limitations to its properties, such as lack of elasticity and flexibility based on its elastic modulus and bending strain values. In zone 3 (360–390°C), significant mass degradation has occurred. This occurs because the main constituent components of the biocomposite have evaporated in the form of PLA matrix and cellulose structure in SBF, leaving residual substances such as charcoal and residue of 1–20 wt% of the total weight. The residues from the thermal degradation process are presented in Table 2. This finding is supported by previous studies [50–53].

3.6 Chemical Functional Groups Change

Fig. 7 shows the FTIR spectra of the biocomposite. The peak detected between 3500–3200 cm−1 indicates stretching vibrations of hydroxyl (OH) groups in the cellulose structure [54]. The peak at 2800–3000 cm−1 can be associated with stretching vibrations of the CH group [55]. The peaks at 1700 and 1650 cm−1 indicate stretching of the carboxyl and acetyl carbonyl groups of hemicellulose, respectively [56].

Figure 7: Chemical functional group changes of PLA and its biocomposite

The peak intensity decreases around 1750 to 1250 cm−1 (Fig. 7), which indicates the stretching of CO and C=O in hemicellulose, respectively. This phenomenon indicates a reduction in the hemicellulose content of the filler fiber (SBF) in the PLA matrix. Meanwhile, the peak around the range of 1150–1030 cm−1 indicates the removal of lignin content due to the influence of chemical treatment as shown by the stretching of C-O and C-H vibrations [57–59]. These results are similar to those discussed in previous studies where stretching of carboxylic acids and esters from hemicellulose occurs at a wave peak of around 1739 cm−1 (untreated) and disappears in treated fibers. On the other hand, the decrease in the C-O stretching peak indicates that fibers treated with alkali can retain cellulose that is purer from impurities such as lignin [60,61].

4  Conclusions

The addition of alkalization treatment on SBF increased tensile, impact, and flexural strength, with the highest values observed in the 6% NaOH treatment. This is due to the good bonding of the fiber and matrix interface. Meanwhile, 8% NaOH on SBF decreased the mechanical properties of the biocomposite because of fiber damage and excessive residue buildup on the surface. The morphological structure of the pure PLA biocomposite fracture shows a smooth and flat (homogeneous) surface. PLA/SBF biocomposites without alkalization treatment exhibited debonding between fibers and the matrix, as well as matrix-rich and fiber pull-out failures on the fracture surface. Alkalization-treated fibers led to the formation of a compact structure due to their even dispersion, supported by a wider contact area with the matrix, as their surface contained more cellulose OH groups. FTIR analysis also confirmed the reduction of hemicellulose due to alkalization treatment characterized by a decrease in peak intensity around 1750 and 1250 cm−1. On the other hand, the removal of lignin was characterized by a change in the peak between 1030–1150 cm−1. The best thermal stability was also achieved by PLA/SBF-A6, which was able to withstand temperatures up to 345.92°C according to TGA analysis. With the mechanical and thermal properties of PLA/SBF biocomposites, they have the potential to be good materials in various applications such as automotive and household components.

Acknowledgement: We thanks to Institute for Research and Community Services (LPPM), Universitas Jember, Indonesia for the funding with the International Research Collaboration Scheme. In addition, we also thank to Abduh for providing a place for discussion.

Funding Statement: This research was funded and supported by the Institute of Research and Community Service (LPPM), Universitas Jember, for International Research Collaboration Scheme with project number: 3565/UN25.3.1/LT/2023.

Author Contributions: Conceptualization, data curation, project administration, funding acquisition, and supervision: Mochamad Asrofi. Writing—original draft: Mochamad Asrofi, Muhammad Oktaviano Putra Hastu, Muhammad Luthfi Al Anshori, Feyza Igra Harda Putra. Research assistant and investigation in laboratory: Mochamad Asrofi, Muhammad Oktaviano Putra Hastu, Muhammad Luthfi Al Anshori, Feyza Igra Harda Putra, Revvan Rifada Pradiza, Haris Setyawan, Muhammad Yusuf, and Mhd Siswanto. Formal analysis, data validation, and writing review and editing: R.A. Ilyas and Muhammad Asyraf Muhammad Rizal. Supervision and validation: Salit Mohd Sapuan. Formal analysis: Mohd Nor Faiz Norrrahim and Victor Feizal Knight. All authors reviewed the results and approved the final version of the manuscript.

Availability of Data and Materials: The detailed data obtained through this study and presented in this article can be requested from the corresponding author.

Ethics Approval: Not applicable.

Conflicts of Interest: The authors declare no conflicts of interest to report regarding the present study.

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