The Effects of Extraction on Mechanical and Morphological Properties of Sisal Polyester Composite

Abstract

Natural fibers are replacing synthetic fibers and are used to develop different useful composite products due to their environmental advantages. To fabricate high-performance composites, high-quality natural fibers are essential. Fiber quality largely depends on the extraction method and subsequent treatment. In this study, fibers were extracted using both machine and manual methods, treated with 5% NaOH, and used at a 30:70 fiber-to-matrix volume ratio to fabricate composite laminates. Key properties such as tensile, flexural, and impact strength, water absorption, elemental composition, and morphological structure were analyzed. When comparing the untreated fiber composites, the machine-extracted samples exhibited a 6.7% increase in tensile strength and a 7.06% increase in flexural strength over those extracted manually. For treated fiber composites, the machine-extracted samples showed improvements in tensile, flexural, and impact strengths of 19.82%, 19.38%, and 26.59%, respectively, compared to those extracted manually. These enhancements indicate that machine extraction provides fibers with better structural integrity and consistency, contributing to stronger fiber–matrix bonding. The machine-extracted treated composites showed reduced water absorption and smaller fiber diameters, indicating that machine extraction was more effective in removing impurities from the fibers. Scanning electron microscopy (SEM) confirmed improved fiber–matrix interfacial bonding in the machine-extracted composites, which also exhibited better water resistance. This study highlights that fiber extraction and treatment significantly influence the mechanical, physical, and morphological properties of natural fiber composites, as verified through SEM, EDS, and universal testing machine (UTM) analysis.

1. Introduction

Natural fiber is environmentally friendly and can partially replace synthetic fiber in some areas of application. The properties of natural fibers substantially depend on their chemical composition and structure, which are related to the type of fiber, harvesting period, extraction methods, growing climate conditions, and modification techniques. Plant fibers contain larger quantities of cellulose, which contribute to greater strength and stiffness in laminate composites, making them suitable alternatives to synthetic fibers for various applications [1,2]. The most common plant fibers are bast fiber (i.e., a wood core surrounded by a stem), leaf fiber, grass fiber, and seed/fruit fiber [3].

The features of natural fibers depend on the preparation and processing of the fibers. One of the primary elements influencing the mechanical characteristics of natural fibers and their composites is the fiber extraction technique used. Decortication and retting techniques are the most widely used to extract fiber from plants [2]. Different types of plant fiber extraction methods are shown in Figure 1. The effects of different extraction methods on the mechanical properties of kenaf fiber have been investigated, and the results show that higher tensile strengths were achieved with water retting, decortication, and NaOH retting [4]. The results of physical and bio-extraction are nearly identical, except for a slight variation in the ideal period for bio-extraction [5]. Water retting is the extraction process in which the harvested parts of the plant are submerged in flowing or sluggish water for about 14 to 21 days. This method takes a long time and can cause significant water pollution [6,7].

The manual decortication technique is one of the most common techniques applied for extracting fiber from the pseudostem portion of plants [8]. The manually extracted fiber is more robust and glossy, but there is a greater loss of fiber as waste [9]. Two or three additional sheaths are rejected, and the remaining sheaths are splinted during the manual extraction procedures [10]. The manual extraction method requires a large force, time, and energy [11]. The decortication process has been automated to make the extraction process simple and fast, providing a high quantity and quality of fibers. The roller gap in machine decortication is established based on the fiber’s thickness to obtain high-quality fibers [12,13]. The tensile strength of Calotropis (Milkweed) steam bast fiber extracted by machine and manually was studied, and the result indicated that the manually extracted fibers had a tensile strength of 254.56 MPa, and the machine-extracted fibers had a tensile strength of 350.73 MPa [14].

The effects of different extraction methods for kenaf fiber were studied and the results showed that the extraction methods affect the strength of the composite [4]. The mechanical, chemical, and physical properties of untreated and 5% and 10% NaOH-treated fibers were characterized, and the results showed that the 5% NaOH-treated fibers have better properties [15]. The chemical, physical, thermal, and morphological characterization of untreated and 5 wt% NaOH-treated salago and sisal fiber was performed, and the results showed that the 5 wt% NaOH improves the fiber’s mechanical properties [16,17].

Moisture absorption is the main cause of poor fiber–matrix adhesion, a reduction in the interfacial bonding between the fiber and matrix, and a decline in the composite’s mechanical qualities. Natural fibers with high cellulose concentrations let the composite absorb more water [18]. Increased fiber swelling due to water absorption creates the forces that lead to the composite’s failure [19]. Different studies in the literature [17,20] have shown that fiber extraction and treatment techniques are important factors in enhancing the mechanical and water-absorbing qualities of natural composites. Treated sugar fiber has a smaller diameter than untreated fiber. Treating the fiber might lower the amount of amorphous material, such as hemicellulose and lignin. Treated fiber has superior mechanical qualities compared to untreated fiber. Fibers treated with a 5% alkali solution for sugar fiber showed the greatest increase in mechanical properties [21,22].

The three most popular techniques for creating thermosetting polymer composites are pultrusion, resin transfer molding, and hand lay-up. The simplest method of producing composites is the hand lay-up approach, which features an easy-to-maintain mold, lower mold costs, simplicity, lower processing costs, and the ability to create intricate designs. One of the technique’s minor drawbacks is that it is labor-intensive and time-consuming to process [23,24,25].

The quality of the fibers extracted using various techniques was examined, and mechanical decorticators for banana fibers were fashioned [10,26]. The quality of the fibers and the mechanical decortication of pineapple leaf fiber have been described in the literature [27,28]. A raspador machine, hand scraping, or microbiological retting can all be used to extract sisal fibers. By shattering and splitting the leaf throughout the process, sisal fiber can be mechanically extracted using a “periquita” machine [29,30,31]. The natural fiber from stem, leaf, and seed extraction procedures is primarily extracted by machine decortication and manual extraction (a hand scraper), as indicated in

Table 1. Different types of fiber with extraction techniques.

Types of Fibers	Extraction Techniques	Reference
Sisal	Retting, hand scraper, machine decortication	[32,33]
Flax	Water retting, dew retting, machine decortication, ultrasonic treatment	[34]
Abaca	Hand scraper, machine decortication	[9]
Banana	Hand scraper and machine decortication	[5]
Coir	Hand scraper, beating on stone, retting, decortication machine	[35]
Remie	Hand scraper, decortication husk	[36]
Bamboo	Mechanical extraction and steam explosion	[37,38]
Pineapple leaf	Mechanical milling, machine decortication, and hand scraping	[39,40]
Ventricosum	Manual decortication	[41]
Hemp	Dew and water retting	[42]

.

The mechanical performance of fiber–polymer composites is often constrained by inadequate adhesion at the fiber–matrix interface, primarily due to the inherent hydrophilicity of natural fibers and the hydrophobicity of most polymer matrices. From the reviewed literature, it is evident that fiber extraction methods and chemical treatments are among the most critical parameters influencing composite performance. The extraction process determines the initial irregularity, surface roughness, and residual non-cellulosic components of the fiber, all of which directly affect interfacial bonding with the polymer matrix. Similarly, chemical treatments modify the fiber’s surface roughness and remove impurities such as lignin and hemicellulose that improve its compatibility with hydrophobic polymers. In this study, the combined effects of extraction and chemical treatment on the mechanical and water absorption characteristics of sisal–polyester composites are studied, which is an area that, to the best of our knowledge, has not been covered in prior published research.

2. Materials and Methods

2.1. Materials

Manually and machine-extracted sisal fibers were used. An unsaturated polyester matrix purchased from the local market was used. For fiber treatment, NaOH was used. For fiber and composite morphological properties, optical microscopy, electron dispersive spectroscopy (EDS), and scanning electron microscopy (SEM) were used. For mechanical properties, a UTM and Charpy impact tester were used.

2.2. Fiber Extraction and Treatment Methods

The steps of fiber extraction are demonstrated in Figure 2, Figure 3 and Figure 4, in which manual extraction is shown in Figure 2a–d, machine extraction is shown in Figure 3a–d, and treatment is shown in Figure 4a–c. After extraction, fresh water is used to wash and dry fibers outdoors. The sisal fibers were treated with 5% NaOH for 2 h. The fibers were then repeatedly cleaned with fresh water, rinsed with distilled water to remove any remaining NaOH, and allowed to dry for 24 h at room temperature.

2.3. Composite’s Fabrication Methods

The composite was prepared using a 300 × 300 × 5 (mm) mold using hand lay-up techniques. The composite sample was made from manually and machine-extracted sisal fibers treated with 5% NaOH, with a volume ratio of 30:70 polyester fibers, as shown in Figure 5a,b.

2.4. Mechanical Testing Methods

According to ASTM standards, the test specimens were produced and put through various mechanical tests (such as tensile and flexural tests) and moisture absorption tests. For every sample set, five specimens were tested, and the average value was taken into account for analysis.

2.4.1. Tensile Strength

Tensile testing is used to identify the force required to break the test specimen and the extent to which the specimen stretches or elongates to that breaking point. The specimen dimension used for the tensile tests is 250 × 25 × 5 (mm) (length × width × thickness) as per ASTM D3039 standards [43]. The test was run on a universal test machine (Bairoe, Shanghai, China, universal test apparatus).

2.4.2. Flexural Strength

Flexural strength testing determines a material’s capacity to withstand deformation under load. The test specimens’ cross-section is rectangular because they are either cut from molded pieces or directly manufactured. The four-point bend test is used to encourage inter-laminar shear failure. The specimens had dimensions of 127 × 13 × 5 (mm) according to ASTM D790, and the test was carried out on a UTM (WP 310 universal material tester, GUNT, Barsbüttel, Germany) [44].

2.4.3. Impact Strength

An impact test measures a material’s resistance to impact loads from a swinging pendulum on a specimen until it fractures. Following ASTM D256 standard, the impact strength was assessed using a Charpy impact tester (Ceast S.P.A, Torino, Italy) [45], and the specimens’ dimensions were 65 × 13 × 5 (mm).

2.4.4. Water Absorption

The ASTM D570 [46] standard was followed during the water absorption test. The water intake was investigated by submerging the specimens in room-temperature water. Periodically, the samples were removed, weighed right away, and then their surfaces were cleaned with a dry cloth. A precise 4-digit balance was then used to weigh the samples, allowing for the determination of the amount of water absorbed. Every sample was dried in an oven until its weight remained consistent, and then it was submerged in water once more. The formula found in Equation (1) was used to calculate the percentage of water absorption.

$$WA (\%)={\displaystyle \frac{m_2-m_1}{m_1}}$$

(1)

where $WA$ = water absorption$m_1$, $\mathrm{a}\mathrm{n}\mathrm{d}$ $m_2$ are the weights of the composite sample before and after soaking it in water.

2.4.5. Composite’s Morphology

A scanning electron microscope (SEM) is an electron microscope that produces images of a sample by scanning it with a focused beam of electrons. The composite fracture surface was characterized using Gemini SUPRA 35VP (Carl Zeiss, Jena, Germany), which was capable of energy dispersive spectroscopy (EDS). After the flexural test, the fractured surfaces of the composite samples, both those machine- and manually extracted, and the treated and untreated samples were characterized. Elemental compositions of both machine- and manually extracted treated and untreated fibers were characterized using energy dispersive spectroscopy (EDS). After being divided into tiny splices, a single fiber sample was put on a microscopy slide and seen through a microscope on a screen. For both manually and machine-removed sisal fibers, the diameter and morphological structure were noted.

3. Results and Discussion

Fiber Structure Microscopy

The untreated fiber extracted by machine is depicted in Figure 6a, followed by the untreated fiber extracted manually (Figure 6b), the treated fiber extracted by machine (Figure 6c), and the treated fiber extracted manually (Figure 6d). Comparing the manually extracted and machine-extracted fibers, the former has more amorphous material, which affects the fiber qualities and composite strength. This indicates that when the fiber is treated with NaOH, the majority of the lignin content is eliminated, which alters the fiber’s shape, structure, and characteristics. Because the treated fiber contains fewer impurities, the fiber–matrix interfacial bonds are enhanced, improving the mechanical and water absorption properties of the composite.

The measured diameter of the fiber is shown in

Table 2. Fiber diameter.

Types of Extraction	Fibers	Diameter (µm)
Machine extraction	Untreated	411.4 ± 0.025
	5% NaOH	298.7 ± 0.121
Manual extraction	Untreated	464.3 ± 0.045
	5% NaOH	309.8 ± 0.123

, which indicates that the sisal fibers treated with 5% NaOH and machine-extracted fibers had a smaller fiber diameter due to the removal of impurities from the inner and outer fiber surface.

As shown in Figure 7a, the machine-extracted fiber composites have a greater tensile strength than the manually extracted fiber composites. A similar result was observed for the machine-extracted treated fiber composite and the manually extracted treated fiber composite. Machine-extracted fiber composites had a greater tensile strength, as the fibers have fewer impurities and a rough surface. Similarly, the NaOH treatment improves the tensile strength of the composite of both manually and machine-extracted fibers. The fiber that was extracted by machine and chemically treated was the best among all the types of composites. The tensile strength of the machine-extracted untreated fiber was improved by 6.7% and that of the treated fiber was improved by 19.82% compared to those of the untreated and treated manually extracted fiber composites.

The flexural strength of the sisal polyester composite with manually (treated and untreated) and machine- (treated and untreated) extracted fiber composites can be observed in Figure 7b. It was observed that the machine-extracted fiber-based composites have a greater flexural strength than all the other composites. Like the tensile strength, the treated types using both extraction methods improved the composite flexural strength. The flexural strength of the machine-extracted untreated fiber composite was improved by 7.06% and that of the treated composite was improved by 19.38% compared to the untreated and manually extracted fiber composites. Treatments that increase interfacial adhesion have decreased fiber defragmentation from the matrix, improving the mechanical characteristics of composites made of natural fibers [37,47]. It is also known from previous studies [38,48] that the flexural properties of natural fiber composites are influenced by the fiber’s extraction and adhesion levels.

The amount of energy that a composite material dissipates before ultimately failing is measured by its impact resistance. Matrix fracture, fiber/matrix deboning, and fiber fracture in composite materials are the reasons behind impact failure. When the stress surpasses the fiber strength, a fracture happens. The broken fibers can be extracted from the matrix, causing energy loss. The effect of extraction and treatment on the sisal composite is depicted in Figure 8a. The impact strength of the machine-extracted untreated fiber was improved by 26.59% compared to the untreated manually extracted fiber composite. The treated impact results of the machine-extracted and manually extracted fibers are nearly identical.

Figure 8b displays the results of the water absorption characteristic test. The machine-extracted untreated and treated fiber polyester composites improved by 27.05% and 29.62%, respectively, compared to the manually extracted untreated and treated fiber polyester composites. This is due to the removal of non-cellulose components like lignin, which helps improve the fiber matrix interfacial bonds. The water absorption characteristic of the natural fiber was improved by the treatment.

As shown in Figure 9, nearly all the composites exhibited similar water absorption behaviors during the first 24 h, with an increase from 24 h to 144 h and leveling off after 168 h. Compared to the other samples, the machine-extracted, alkali-treated fiber composites absorbed water at a slower rate.

The EDS spectrum of the sisal fibers was characterized and is shown in Figure 10. The manually extracted untreated and treated sisal fibers are shown in Figure 10a,b, and the machine-extracted untreated and treated sisal fibers are shown in Figure 10c and Figure 10d, respectively. The SEM image clearly shows the structure of the sisal fibers. The related EDS spectrum results are shown for the machine-extracted and treated sisal fiber sample, supporting the detection of six chemical elements. In addition to SEM images, the chemical elements of the fiber were characterized by EDS, which showed that carbon, oxygen, magnesium, silicon, aluminum, and calcium (C/O/Mg/Si/Al/Ca) were observed in all samples of the fibers. C and O are the major elements; Mg, Si, Al, and Ca are trace minerals or contaminant elements in the sisal fiber.

In natural fibers, lignin contains 60–65% C, 28–35% O, and 5–6% H, while hemicellulose contains 48–50% C, 44–46% O, and 5.8–6.8% H. Lignin is carbon-rich due to its aromatic structure, while hemicellulose is oxygen-rich due to the abundance of hydroxyl [49,50].

Table 3. Elemental composition of the fibers.

Element	Weight Percentages (wt%)
	Machine Treated	Machine Untreated	Manually Treated	Manual Untreated
C	39.91	52.19	55.22	65.27
O	55.97	42.67	41.74	34.07
Al	0.92	-	0.15	0.01
Mg	1.45	-	0.72	-
Ca	1.48	1.54	1.04	-
Si	0.52	1.9	0.27

presents the quantitative elemental analysis of the fibers based on weight percentages. The results show that manually extracted, untreated fibers have a higher percentage of carbon, indicating a higher lignin content, whereas machine-extracted, treated fibers have a lower carbon percentage, reflecting reduced lignin content. This reduction in lignin alters the fiber’s surface chemistry, improving its compatibility and adhesion with hydrophobic polymer matrices.

Physical modifications such as the creation of rough surfaces by alkali treatment led to good fiber matrix adherence, which enhanced the sisal properties of both composites. Figure 11a–d display the SEM images of the sisal polyester composites’ tensile fractured surfaces for both the machine-extracted and manually extracted fibers. SEM was used to assess the developed composites’ ruptured surfaces during tensile testing. In comparison to other composites, we find that the machine-extracted treated fiber composite exhibits better surface morphology.

When comparing machine-extracted fiber types to manually extracted fiber composites, a satisfactory fiber matrix was found. Figure 11b shows that the alkali-treated fiber composite has a good fiber matrix interfacial surface from the manually extracted fibers. This is because alkali helps to increase the surface roughness of natural fibers by eliminating the amorphous particles. As seen in Figure 11d, the treated fiber composite also exhibits a superior fiber matrix interfacial bond in the treated machine extraction composites.

4. Conclusions

This article presents an investigation into the effects of extraction and treatment on the mechanical and morphological properties of sisal polyester composites. The main findings of this research are as follows.

• The microscopy images of the treated, machine-extracted fibers showed clearer surfaces and smaller diameters compared to those of the untreated, manually extracted fibers;
• The treated sisal fiber composites exhibited lower moisture absorption than the untreated fiber composites for both extraction methods;
• The machine-extracted fibers exhibited a rougher surface and fewer impurities compared to the manually extracted fibers, indicating a potential advantage in composite reinforcement applications;
• The machine-extracted fibers show a greater mechanical (tensile and flexural) strength than the manually extracted fiber composite;
• The manually extracted, untreated composites exhibited greater water absorption compared to the machine-extracted, treated composites, demonstrating the effectiveness of the treatment and extraction method in reducing moisture uptake;
• The SEM analysis demonstrated a superior fiber–matrix interface in the machine-extracted, treated fiber composites compared to the other types, in line with the mechanical and water absorption test results. This enhancement is attributed to the removal of surface impurities and the development of a rougher fiber surface through the machine extraction and treatment, which together facilitate stronger interfacial bonding and mitigate moisture-induced degradation.