**1. Introduction**

Due to environmentally friendly customers' desires to save the earth, recently there has been a growing interest in using renewable resources and biodegradable products. For the development of polymer composites, many major industries have focused on using natural fibers. This is owing to the benefits offered by natural fibers (e.g., they are cost-effective, dense, easy to obtain, environmentally safe, non-toxic, durable, reusable, biodegradable, abrasion-resistant, have high strength and modulus, and are simple to process) [1,2]. Natural fibers' biodegradability makes them ideal for reinforcement in polymer composites [3,4]. Both natural and synthetic fibers can be used to create hybrid composites. This combination demonstrates outstanding structural and mechanical properties [5,6]. Recently, sugar palm has been considered by many studies as a desirable natural fiber due to its easy cellulose separation from other components [7]. Sugar palm (*Arenga pinnata Wurmb. Merr*) is found abundantly in Malaysia, Indonesia, India, and Thailand [8]. SPF has the advantages of having low density, biodegradable, lack of toxicity, low cost, non-abrasive nature, and long-lasting [9,10].

In the world of biodegradability, efforts in commercializing PLA polymers have been taking place since the last decade. PLA is a thermoplastic bio-based product obtained from fermentation processes of natural agricultural raw materials (starch and sugar) [11,12]. PLA is recyclable, readily decomposable, has cheap manufacturing costs, and is commercially available [13]. These features enable PLA to replace petroleum-based polymers for several

**Citation:** Sherwani, S.F.K.; Zainudin, E.S.; Sapuan, S.M.; Leman, Z.; Khalina, A. Physical, Mechanical, and Morphological Properties of Treated Sugar Palm/Glass Reinforced Poly(Lactic Acid) Hybrid Composites. *Polymers* **2021**, *13*, 3620. https:// doi.org/10.3390/polym13213620

Academic Editors: Andrea Sorrentino, Domenico Acierno and Antonella Patti

Received: 22 September 2021 Accepted: 18 October 2021 Published: 20 October 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

biomedical, textile, plastic, 3D printing materials, and packaging applications [14,15]. However, some drawbacks of PLA include its high price and water sensitivity, low crystallinity rate, and fragility [16]. Many researchers aim towards overcoming these limitations, commonly by mixing PLA with natural fibers [17].

Moisture absorption is a major problem with natural fibers that negatively affects the strong interfacial bonding between a fiber and a matrix. This problem can be solved by fiber pre-treatment, which eliminates lignin and other related materials, improving the interfacial bonding between the fiber and the matrix [18,19]. Treating the lignocellulosic fiber for improved adhesiveness and good stress transfer from the matrix to the rigid fiber will increase the performance of different hybrid composites. Since hydrophilic fiber does not tightly bond with a hydrophobic polymer, weak fiber-matrix bonding degrades composite strength [8,18]. Treatment of the fiber is needed to enhance bonding between the fiber and the matrix. After the treatment, the fiber has reduced moisture absorption, increased bonding, and, most significantly, both enhanced physical and mechanical properties. Alkaline treatment is one of the most effective chemical treatments for fiber surface modification [18–20]. An alkaline treatment procedure has proven to be a successful treatment for extracting waxy substances and impurities [20,21]. As a result, the fiber's surface becomes rough, allowing for greater adhesion with the polymer. This treatment affects the fiber in two ways. Firstly, hydrogen bonding in the network structure is disrupted, enhancing the surface roughness. Secondly, lignin, wax, and oils are eliminated from the surface, enhancing the exposure of cellulose to the fiber surface. This results in increasing the number of possible reaction sites [22]. However, alkaline treatments have the added problem of fiber degradation at high concentrations, which may be addressed with a moderate chemical treatment such as benzoyl chloride.

The fiber treatment with BC decreases the hydrophilic nature of natural fibers and strengthens fiber attachment to the matrix, improving the bio-composites' strength [23,24]. Alkali pre-treatment is used during the benzoylation process. When further treated with BC, these alkaline pretreated fibers lead to the —OH groups of the cellulose fibers overtaken by benzoyl groups and render them hydrophobic [22]. Extractable materials such as waxes, lignin, and oil-coating materials are removed at this stage, exposing more volatile hydroxyl groups on the fiber surface. The fiber's —OH groups are substituted by the benzoyl groups, which binds to the cellulose backbone [25]. Prabhu et al. [24] addressed that the benzoylation of fiber improved adhesion to the fiber-matrix, significantly improved composite strength and decreased water absorption. The treatment improved the wettability of *Impomea pes-caprae* fiber and epoxy composites, resulting in stronger bonding and increased overall composite strength [26]. Palmyra palm-leaf stalk fiber composites were also treated with BC, which improved tensile strength and modulus by 60% [27]. The BC treatment strengthened fiber and matrix adhesion, improved strength, and reduced the water absorption character of the whole composite by reducing SPF's hydrophilic nature and strengthening the blend with epoxy resin matrix [25,28]. Currently, Mohd Izwan et al. [29] investigated SPF benzoylation treatment and found that the highest tensile strength was achieved after 15 min of soaking time, indicating good SPF properties for use as reinforcement in composites.

Several studies were conducted on the hybridization of natural–natural fibers, natural– synthetic fibers and synthetic–synthetic fibers in a single matrix, showing that hybrid composites have been promising to enhance mechanical properties [30–33]. However, in hybrid composites, fiber loading is typically limited to a maximum of 50% [34]. Atiqah et al. [35] studied the mechanical properties of kenaf-glass reinforced unsaturated polyester hybrid composites. They found that combining these fibers improved the tensile, flexural, and impact properties. Afzaluddin et al. [36] developed SPF/GF reinforced TPU hybrid composites and studied their physical, tensile, flexural, and impact properties. They reported that tensile and impact properties of the hybrid composites were increased with the increase of the content of SPF (%) compared to GF reinforced composites. Still, the flexural properties were improved when the content of GF was increased. Atiqah et al. [37]

studied the effects of silane and alkaline treatment of fiber on the tensile, flexural, and impact properties of SPF/GF/TPU hybrid composites and found that treated SPF can increase the mechanical properties of hybrid composites. In another study, Atiqah et al. [38] reported that physical properties could be improved by alkaline treatment of SPF in fabricating the SPF/GF/TPU hybrid composites. Recently, Radzi et al. [20] determined the physical and mechanical properties of roselle/SPF reinforced thermoplastic polyurethane hybrid composites and revealed that the alkaline treatment on RF/SPF hybrid composite has improved the mechanical properties of hybrid composite and proven the RF/SPF composite suitability for making automotive parts.

To the best of our knowledge, no study on treated SPF/GF/PLA hybrid composites has been reported in the literature. Novel combinations of treated SPF/GF are selected as effective reinforcements PLA hybrid composite due to their positive effects on improving physical, mechanical, and morphological properties. Due to the presence of SPF and PLA bio-degradable plastic manufactured from plant-based resources such as corn starch or sugarcane, this hybrid composite is environmentally friendly. The hybrid composites had a constant SPF/GFs weight fraction of 30%. Sherwani et al. [39] demonstrated that a 70:30 (PLA/SPF) ratio exhibits excellent mechanical properties, especially tensile and flexural properties. Hence, a 70% PLA to 30% fiber ratio is considered in this research. The effect of alkaline and benzoyl chloride treatment of SPFs on PLA were also determined by Sherwani et al. [40] and reported that the optimum percentage is 6% alkaline and the best soak duration is 15 min for enhancing the mechanical properties of SPF/PLA composites. As a result, a 6% alkaline and 15-min benzoyl chloride treatment is considered here. The aim of this paper is to evaluate the physical, tensile, flexural, and impact properties as well as the post-tensile and flexural morphological tests, aiming to propose the best natural/green hybrid composite formulation for engineering material which can be used for the applications of automotive (especially motorcycle) components manufacturing. With the help of this research, an environmentally friendly biodegradable material may be developed for possible application in the manufacture of motorcycle components.

### **2. Materials and Methods**

#### *2.1. Materials*

Sugar palm fiber (length of fiber up to 1.19 m, average diameter 0.5 mm, the density of raw SPF 1.2–1.3 gm/cm<sup>3</sup> , and tensile strength 15.5 MPa [41]) were purchased from Kampung Kuala Jempol, Negeri Sembilan, Malaysia. The poly(lactic acid) (NatureWork 2003D) (density 1.25 gm/cm<sup>3</sup> at 21.5 ◦C, yield tensile strength of 52 MPa, and melting point of 170 ◦C), BC with reagent plus 99%, ethanol, and E-glass fiber (properties of E-glass is shown in Table 1) were delivered by Mecha Solve Engineering, Petaling Jaya, Selangor, Malaysia. Sodium hydroxide (NaOH) pellet was delivered by Evergreen Engineering and Services, Taman Semenyih Sentral, Selangor, Malaysia.


**Table 1.** Some typical properties of E-glass fiber [42].

The electrical resistivity of E-glasses at room temperature is exceptionally high. E-glass compositions by weight percentage are SiO2-60; Al2O3-9; MgO-4; CaO-27.

#### *2.2. Preparation of Sugar Palm Fiber* to a length of 10 mm to 15 mm using a crusher. The fiber was then cleaned several times

*2.2. Preparation of Sugar Palm Fiber* 

A crusher machine was used for crushing a bundle of SPF. The dry SPF was graded to a length of 10 mm to 15 mm using a crusher. The fiber was then cleaned several times with water to remove impurities. The SPF was left outdoor for 24 h before being dried in an air-circulating oven at 60 ◦C. with water to remove impurities. The SPF was left outdoor for 24 h before being dried in an air-circulating oven at 60 °C. *2.3. Chemical Treatments* 

The electrical resistivity of E-glasses at room temperature is exceptionally high. E-

A crusher machine was used for crushing a bundle of SPF. The dry SPF was graded

glass compositions by weight percentage are SiO2-60; Al2O3-9; MgO-4; CaO-27.

#### *2.3. Chemical Treatments* 2.3.1. Alkaline Treatment Alkaline treatment of natural fibers was used to eliminate surface impurities as well

**Cellulose** 

character of fiber.

#### 2.3.1. Alkaline Treatment as hemicelluloses within the fibers [20]. In our study, 50 g of SPFs was soaked in a 6% *w/v*

Alkaline treatment of natural fibers was used to eliminate surface impurities as well as hemicelluloses within the fibers [20]. In our study, 50 g of SPFs was soaked in a 6% *w/v* alkaline solution of 1000 mL for 1 h at 25 ◦C. After that, they were immersed with an acetic acid solution till a neutral pH value was obtained prior to washing with distilled water and oven-drying for 24 h at 60 ◦C. The dried SPF was placed into zipper plastic storage bags. Table 2 shows the chemical composition of SPF and alkaline treated SPF. alkaline solution of 1000 mL for 1 h at 25 °C. After that, they were immersed with an acetic acid solution till a neutral pH value was obtained prior to washing with distilled water and oven-drying for 24 h at 60 °C. The dried SPF was placed into zipper plastic storage bags. Table 2 shows the chemical composition of SPF and alkaline treated SPF. **Table 2.** Composition of alkaline treated SPF.



*Polymers* **2021**, *13*, 3620 4 of 26

#### 2.3.2. Benzoyl Chloride Treatment 2.3.2. Benzoyl Chloride Treatment 50 g SPF was immersed in 18% NaOH solution for 30 min, then washed SPF twice

50 g SPF was immersed in 18% NaOH solution for 30 min, then washed SPF twice with tap water. The SPF was suspended in a 10% NaOH solution and vigorously agitated for 15 min in 50 mL BC. Once again, SPF was washed in water, filtered, and dried at 25 ◦C. SPFs is normally immersed in ethanol for an hour before being washed, filtered, and dried in a 60 ◦C oven for 24 h [28,29]. The reaction between the cellulosic —OH groups of fiber and BC is given in Scheme 1. with tap water. The SPF was suspended in a 10% NaOH solution and vigorously agitated for 15 min in 50 mL BC. Once again, SPF was washed in water, filtered, and dried at 25 °C. SPFs is normally immersed in ethanol for an hour before being washed, filtered, and dried in a 60 °C oven for 24 h [28,29]. The reaction between the cellulosic —OH groups of fiber and BC is given in Scheme 1.

**Scheme 1.** The reaction between the cellulosic —OH groups of fiber and BC. Alkaline pretreated fibers, when further treated with BC, the OH groups of the cellulose fibers are replaced by benzoyl groups which reduced the hydrophilic **Scheme 1.** The reaction between the cellulosic —OH groups of fiber and BC. Alkaline pretreated fibers, when further treated with BC, the OH groups of the cellulose fibers are replaced by benzoyl groups which reduced the hydrophilic characterof fiber.

#### *2.4. Fabrication of SPF/GF Reinforced PLA Hybrid Composites*

The melting compounding and hot press moulding methods were used to prepare the SPF/GF reinforced PLA hybrid composites. The 10–15 mm sugar palm fiber, 12.5 mm chopped E glass fiber, and PLA pellets were dried at a temperature of 60 ◦C in electric ovens for 48 h. Nine sets of SPF/GF composites (30/0, 20/10, 15/15, 10/20, and 0/30) wt % reinforced PLA were developed as seen in Table 3. Based on past research [34,36,44], such a ratio was considered. In a Brabender Plastograph (co-rotating twin-screw extruder), untreated/treated SPF and chopped E-glass fibers reinforced PLA were mixed for 10 min at 160 ◦C with a rate of 50 rpm to ensure consistent mixing. These samples were then crushed using a crushing unit. After Brabender mixing, to reduce voids or gaps, the composite samples must be placed in an electric oven for 24 h at 60 ◦C before the hot press.

A compression moulding Techno Vation machine model 40 tons was used for hot-press moulding. These samples were pre-heated for 7 min at 170 ◦C before being completely pressed for 6 min. There were three vent cycles to remove voids in the composites. During the final cycle, the cold-pressed time was 6 min at 25 ◦C. Figure 1 describes the detailed methodology of this research. *Polymers* **2021**, *13*, 3620 5 of 26 *2.4. Fabrication of SPF/GF Reinforced PLA Hybrid Composites*  The melting compounding and hot press moulding methods were used to prepare


**Table 3.** Formulation of non-hybrid and hybrid composites. the SPF/GF reinforced PLA hybrid composites. The 10–15 mm sugar palm fiber, 12.5 mm

BC—Benzoyl Chloride.

**Figure 1.** Detailed description of the methodology flow diagram. **Figure 1.** Detailed description of the methodology flow diagram.

#### **Table 3.** Formulation of non-hybrid and hybrid composites. **3. Characteristics of SPF/PLA/GF Hybrid Composites**

#### *3.1. Density*

BC—Benzoyl Chloride.

**No. of Samples Matrix Reinforcement PLA (wt %) SPF GF (wt %) Treatment (wt %)** S1 70 - 30 0 Non-hybrid and hybrid composites' experimental densities were measured using the Mettler Toledo XS205 electronic densitometer as per the ASTM D792 standard [45]. Equation (1) was used to evaluate the theoretical density of the composite,

S2 70 - 0 30

S6 70 6% NaOH 20 10 S7 70 15 min BC 10 20 S8 70 15 min BC 15 15 S9 70 15 min BC 20 10

$$\text{Theoretical density } \rho\_{cl} = \frac{1}{\left(\frac{w\_{sf}}{\rho\_{sf}}\right) + \left(\frac{w\_{sf}}{\rho\_{\ddagger}}\right) + \left(\frac{w\_m}{\rho\_m}\right)}\tag{1}$$

where *ws f* , *wg f* , and *w<sup>m</sup>* represent the weight fraction of the SPFs, glass fibers, and matrix, respectively, while *ρs f* , *ρg f* , and *ρ<sup>m</sup>* denote the density of the SPF, glass fibers, and matrix, respectively. Subsequently, the volume fraction of voids in the percentage of the composites is calculated by using Equation (2) [18].

$$\text{Volume fraction of void } V\_{void} = \frac{\rho\_{ct} - \rho\_{comp.\ exp.}}{\rho\_{ct}} \times 100\tag{2}$$

*ρcomp*. *exp*.—Composite experimental density.

#### *3.2. Moisture Content*

All of the non-hybrid and hybrid composites were tested for moisture content analysis. In an oven, the composites were heated to 100 ◦C for 24 h. To calculate the moisture content, the weights of the composites were determined before (*Mbh*, gram) and after (*Mah*, gram) being placed into the oven. The following Equation (3) was used:

$$\text{Moisture content} \left(\%\right) = \frac{M\_{bh} - M\_{ah}}{M\_{bh}} \times 100\tag{3}$$

#### *3.3. Water Absorption Test*

Non-hybrid and hybrid composites were tested for water absorption (*WA*) using the ASTM D570 standard [46]. From the composite plate, a rectangular geometry sample measuring 10 mm × 10 mm × 3 mm was removed. The weight of the composite was calculated as an initial mean before it was immersed in water, *Wiw*, and *Wfw* as a final stage mean after a day of immersion in water at about 25 ◦C. The test was conducted for eight days. The following Equation (4) was used for calculating water absorption:

$$\text{Water Absorption (\%)} = \frac{\left(W\_{fw} - W\_{iw}\right)}{W\_{iw}} \times 100\tag{4}$$

#### *3.4. Thickness Swelling*

The thickness swelling (*TS*) of all non-hybrid and hybrid composites with measurements of 10 mm × 10 mm × 3 mm was evaluated. Using a digital vernier caliper, the thickness was calculated as *T*1*<sup>b</sup>* (before) and *T*2*<sup>a</sup>* (after) they were immersed in water [20]. The *TS* reading was taken for eight days and was determined using Equation (5).

$$\text{Thickness Swelling (\%)} = \frac{T\_{2a} - T\_{1b}}{T\_{1b}} \times 100\tag{5}$$

#### *3.5. Tensile Test*

An Instron3366 universal testing machine (UTM, University Ave Norwood, Norwood, MA, USA) was used to conduct a tensile test in accordance with ASTM Standard D638- 10 [47]. The gauge length of the non-hybrid and hybrid composites was 80 mm, and the crosshead velocity was 2 mm/min, using a 5 KN load cell. Five samples measuring 150 mm × 25 mm × 3 mm were tested. The average of the five samples provided the final result.

#### *3.6. Flexural Test*

Using an Instron 3365 dual column tabletop UTM with a span length of 50 mm and a crosshead speed of 12 mm/min, the flexural properties of non-hybrid and hybrid composites were tested according to the ASTM D790 standard [48]. Five composite samples measuring 127 mm × 12.7 mm × 3 mm were obtained from the composite plate. The average of the five samples provided the final result.

### *3.7. Impact Test*

ASTM D256 (2010) [49] Standard Izod impact test samples measuring 65 mm × 15 mm × 3 mm were taken out of the non-hybrid and hybrid composite plates. Rayran RR/IMT/178 Izod impact testers were used for the impact study. For each sample, the average of the three readings was calculated for the final result of the impact test. Five identical samples were rigidly placed in a vertical position with each type of composite and struck in the center of the instruments with a pendulum with a force of 10 J. The impact had 2.75 J energy and a velocity of 3.46 m/s.

#### *3.8. Morphological Investigations*

SEM (Coxem-EM-30AX+) was used to examine broken surfaces of tensile and flexural samples for morphological analysis. Scanning electron microscopy (SEM) with a working distance of 14.7 mm, a 58 A emission current, and a 20.0 kV acceleration voltage was used. The samples were coated with a fine gold film for electrical conductivity, improving the image resolution appreciably.

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

The FTIR spectra were used to examine the presence of functional groups existing in non-hybrid and hybrid composites. The samples were segregated into 10 mm × 10 mm × 3 mm squares and then analyzed with a power FTIR spectrometer with attenuated total reflectance (ATR) (Nexus Analytics-Isio 713601, Petaling Jaya, Selangor Darul Ehsan, Malaysia). The wavenumber spectrum for the spectra was 4000 cm−<sup>1</sup> to 500 cm−<sup>1</sup> , and the measurements were taken with a resolution of 4.0 cm−<sup>1</sup> and 16 scans per sample.
