Extraction and Characterization of Natural CASCABELA Thevetia Bast Fibers: A Potential Candidate as Reinforcement in Epoxy Composites

ABSTRACT

The aim of this study is to explore the potential of CASCABELA thevetia (CT) bast fibers as reinforcement in polymer composites. For this purpose, the extracted fibers were chemically treated with various chemicals such as sodium hydroxide, potassium permanganate, sodium chlorite, and benzoyl chloride. After surface modifications, its physical and mechanical properties, chemical composition, and structure were studied. The X-ray diffraction (XRD) results revealed an increase in crystallinity index of the fibers as compared to untreated fibers with benzoyl chloride treated fibers showing the optimum result which agrees with the results obtained in the mechanical tests. Composites were fabricated by taking various wt.% of benzoyl chloride treated CT fibers in an epoxy matrix (i.e. 5, 10, 15, 20, 25, and 30 wt%) and their mechanical and wettability tests were carried out. It was observed that composites with 20 wt% of fiber loading show the highest mechanical properties (52.79 MPa tensile strength, 2.45 GPa Young’s modulus, 71.72 MPa flexural strength). All the fabricated composites showed contact angle less than 90°, which is associated with composite hydrophilic surface properties. These composites can be utilized in lightweight structural material and in automobile industries owing to its better properties.

摘要

本研究的目的是探索CASCABELA thevetia（CT）韧皮纤维在聚合物复合材料中作为增强材料的潜力. 为此，提取的纤维用各种化学物质进行化学处理，如氢氧化钠、高锰酸钾、亚氯酸钠和苯甲酰氯. 对其表面改性后的物理力学性能、化学成分和结构进行了研究. X射线衍射（XRD）结果显示，与具有苯甲酰氯处理的纤维的未处理纤维相比，纤维的结晶度指数增加，显示出与机械测试中获得的结果一致的最佳结果. 采用环氧基中不同重量%的苯甲酰氯处理的CT纤维（即5、10、15、20、25和30 wt%）制备复合材料，并对其机械性能和润湿性进行了测试. 观察到，纤维负载量为20 wt%的复合材料显示出最高的机械性能（52.79MPa的拉伸强度、2.45GPa的杨氏模量、71.72MPa的弯曲强度）. 所有制备的复合材料都显示出小于90°的接触角，这与复合材料的亲水表面性质有关. 这些复合材料由于具有更好的性能，可用于轻质结构材料和汽车工业.

KEYWORDS:

• CASCABELA thevetia
• chemical treatment
• chemical composition
• mechanical properties
• thermal stability
• wettability

KEYWORDS:

• 化学处理
• 化学成分
• 机械性能
• 热稳定性
• 润湿性

Introduction

Global warming has increased environmental concerns which have effectively led various organizations and entities to focus on some alternatives to synthetics and plastics. The mechanical properties of polymers are inadequate for many structural purposes; their strength and stiffness are lower than those of ceramics and metals (Santhosh et al. 2014). As such, they are often reinforced with strong but expensive synthetic fibers or particles to achieve improved mechanical properties and widen their structural applications (Thiruchitrambalam et al. 2010). The potential of natural fibers are abundant, environmentally friendly, and inexpensive with their best properties is very attractive to be developed into synthetic or inorganic fiber replacement products in composites materials (Raghunathan et al. 2022; Ravikumar et al. 2022; Sumesh et al. 2022). However, composites made with synthetic fibers are very much hazardous and pose severe environmental pollution as they are not recyclable. This problem has forced researchers to focus on plant-based natural fibers (NFs) for fabricating polymer-based composites. The major problem relating to the use of natural fibers in fiber reinforced polymer (FRP) composites is their moisture-absorption property. This is mainly due to the hydrophilic nature of natural fibers, which is incompatible with hydrophobic polymer matrices leading to poor bonding and ineffective stress transfer throughout the interface of the composites. To overcome the above problems, NFs are often chemically treated to modify the surface properties (Bledzki and Gassan 1999; Nayak and Mohanty 2018a). Chemical surface treatment of NFs cleans the fibers by removing pectin, waxes, proteins, natural oils, and other impurities; it also increases their surface roughness which enhances the fiber-matrix bonding. Priyadarshini et al. (Priyadharshini et al. 2023) studied the physiochemical, thermal stability, and surface properties of WALTHERIA indica L. stem fiber to explore its suitability as a reinforcing agent in polymer composites as an alternative to synthetic fiber.

Marichelvam et al. (Marichelvam et al. 2021) fabricated novel palm sheath and sugarcane bagasse fiber-based hybrid composites for automotive applications. The most used lignocellulosic fibers are kenaf (Ayadi et al. 2016; Liu et al. 2018), jute (Erdogan et al. 2016), sisal (Belaadi et al. 2014) and especially hemp (Kabir et al. 2013), luffa (Behera, Dehury, and Thaware 2019) and flax (Belopukhov et al. 2017).

Several studies have been conducted which deal with the chemical treatments of natural fibers in order to not only modify the interphase but also produce morphological changes on the fiber surfaces (Aziz and Ansell 2004; Nayak and Mohanty 2018a). As far as CASCABELA thevetia bast fiber is concerned, no work has been carried out to study the effect of surface modification on its properties as it is a newly discovered natural fiber as far as the authors’ knowledge is concerned.

Among all natural fibers, CASCABELA thevetia appears to be a promising reinforcing material because it is inexpensive and abundantly available. It is also known as “THEVETIA peruviana” in Mexico and Central America and widely cultivated as an ornamental. CT fibers contain 43.18% of cellulose, 32.18% of hemi-cellulose, 19.62% of lignin, and 5.20% of others which include waxes and oily substances. Owa et al. fabricated composites using THEVETIA peruviana oil extracted from their seeds reinforced with sisal fibers and studied their various properties for packaging applications (Owa et al. 2021). Synthesis and characterization of yellow oleander seed oil-based alkyd resin was studied by Moni et al. for fabricating light-weight materials for automobile industries (Moni et al. 2014). Rao et al. (Rao, Singh, and Ramulu 2022) characterized various properties of CAREYA arborea fibers to find its potential use as reinforcement for lightweight polymer biodegradable composites. Reddy et al. studied the mechanical and thermal properties of alkali treated CORDIA-dichotoma epoxy composites (Reddy et al. 2020). Preparation and characterization of activated carbon from THEVETIA peruviana for the removal of dyes from textile waste water was studied by Baseri, Palanisamy, and Sivakumar (Baseri, Palanisamy, and Sivakumar 2012). The utilization of CT seeds for biodiesel production has been well reported in the literature (Betiku and Ajala 2014; Deka and Basumatary 2011). Usman and coauthors studied the properties of CT plant shell powder as a possible filler in polymer composites for fabricating lightweight composites (Usman, Momohjimoh, and Adeniyi 2021). Very little work has been carried out taking this CASCABELA thevetia plant. Among the few literatures available it is observed that all the work has been performed taking the seed of the plant. The fiber from the stem of the plant shown in Figure 1 is still unexplored.

Figure 1. Cascabela thevetia (a) plant (b) stem (c) extracted fibers.

The aim of this research is to explore the properties of the novel CASCABELA thevetia bast fiber and study the effect of chemical modification of fibers on its various properties to ascertain the productive use of those fibers in FRP composites. Composites were fabricated with varying wt% of CT fibers in epoxy resin using hand lay-up method. The fabricated composites were allowed for various material properties testing, such as tensile strength, Young’s modulus, flexural strength, Impact strength, and contact angle measurement, and the results were compared.

Materials and methods

Material preparation

CASCABELA thevetia stems of 30 cm length were collected from nearby local areas. These stems have been thoroughly washed with freshwater and then dried in sunlight for 2–3 days. The fibers from stems have been extracted after soaking in water for 5 days by a retting process. The extracted fibers have been oven-dried at 70°C for 24 h to remove the moisture present, if any. The fibers at this stage are designated as untreated (raw) CT fibers (Figure 1). Chemicals such as sodium hydroxide, potassium permanganate, sodium chlorite, and benzoyl chloride of analytical grades have been used without any further purification. All solutions are made in double distilled water.

Methods

Surface modification of CT fibers

Chemical treatment helps in removing the hemicellulose, lignin, wax, and oils that surround the external surface of the fiber, leading to an increase in the surface roughness which improves the fiber/matrix bonding. Various chemical treatments on untreated CT fibers were carried out as follows:

Alkali treatment: The untreated CT fibers have been treated in 5% sodium hydroxide (NaOH) solution for 1 h at a temperature of 80°C. Then, the fibers have been cooled to room temperature and washed in running water to remove any trace of sodium hydroxide solution adhered on the surface of the fibers so that the pH level of the fiber is approximately 7 (neutral). Then, they have been dried in an oven at a temperature of 60°C for 24 h. The CT fibers obtained at this stage have been designated as alkaline pretreated fibers.

The effect of this treatment is to disrupt the hydrogen bonding in the network structure, thereby increasing the surface roughness. Further, it removes certain amount of hemicellulose, lignin, wax, and oils from the fiber surface. Removal of these cementing substances increases the amount of cellulose exposed on the fiber surface

Permanganate treatment: The alkaline pretreated fibers have been soaked in potassium permanganate solution maintaining a concentration of 0.003% in acetone for 1 min. Similar to the previous treatment, the resulting fibers have been rinsed with distilled water and oven dried at a temperature of 80°C for about 24 h.

Potassium permanganate contains permanganate (MnO⁻⁴) group which after treatment leads to the formation of cellulose radical through MnO⁻³ ion formation. These highly reactive Mn3+ ions are responsible for initiating graft copolymerization as a result of which the hydrophilic tendency of the fibers reduces.

Sodium chlorite treatment: The alkaline pretreated fibers have been treated with sodium chlorite solution (Sodium chlorite: water = 1:25) at 70°C for 2 h. Then, the fibers have been washed with distilled water and oven dried as discussed earlier.

Benzoylation treatment: During benzoylation treatment alkali pre-treatment is used. At this stage, extractable materials such as lignin, waxes, and oil covering materials are removed, and more reactive hydroxyl (OH) groups are exposed on the fiber surface. Then the fibers are treated with benzyl chloride. OH groups of the fiber are further replaced by benzoyl groups and attached to the cellulose backbone. This results in a more hydrophobic nature of the fiber and improves adhesion with the matrix.

The alkaline pretreated CT fibers have been treated with 10% NaOH and benzyl chloride solution for 15 min. The resulting fibers have been dipped in ethanol for 1 h to remove benzoyl chloride sticky to the fiber surface. Then, the fibers have been washed with distilled water and put in an oven maintaining 80°C temperature for 24 h for drying purpose.

Density of CT fibers

The apparent density of both untreated and treated CT fibers has been measured in accordance with the BS EN 325:1993 standard. For density measurement, the diameter of the fiber has been measured with the help of a sliding caliper having an accuracy of ±0.02 mm. The weight of 20 samples from each treated and untreated fibers has been measured using an analytical balance with an accuracy of 0.001 g, and the average value has been recorded for further analysis.

Durability studies

Durability studies on untreated and treated fibers were carried out by immersing the fibers in three different mediums, namely, (i) in clean and freshwater (pH = 7.5) (ii) in a deci-normal (0.1 N) solution of sodium hydroxide (NaOH) maintained at pH = 13 (iii) saturated lime solution [Ca(OH)2] maintained at pH = 14. Airtight containers were used to immerse the fibers of length 15–20 cm in the above mediums. The fibers were immersed in the container continuously for a period of 60 days. After the completion of the immersion period, the fiber was taken out, washed properly with water, and dried at room temperature ($29\pm 2$°C). Then, the mechanical properties in terms of tensile strength were determined and compared with their original values.

Chemical analysis of CT fibers

The chemical compositions of CT fibers such as cellulose, hemicellulose, lignin, wax, and ash contents were assessed by typical testing procedures. The proportion of ash substance of CT fibers was estimated as per the ASTM E1755–01 standard, and the moisture content of CT fibers was analyzed using an electronic moisture analyzer (Sartorious, model MA45) (Saravanakumar et al. 2014). Wax substance was calculated by the Conrad method (Senthamaraikannan et al. 2016).

Fabrication of composites

Composites were made using the hand lay-up process. A mold with dimensions of 180 mm × 60 mm × 6 mm was designed to cast the composite sheets. The epoxy resin was prepared by adding HY-951 hardener with a proportion of 10:1 ratio. To guarantee proper mixing, varying weight percentages of benzoylation treated CT fibers (i.e. 5, 10, 15, 20, 25, and 30 wt%) with a length of 2 mm were mixed with epoxy and properly stirred with a mechanical stirrer. The epoxy and fiber solution was then poured into the created mold and covered with a plate. After 24 h of drying at room temperature, the composite samples were removed and cured in an oven at 80°C for 3 h.

Characterization of CT fibers

Thermogravimetric analysis (TGA)

The thermal stability of the CT fibers is characterized using a thermogravimetric analyzer (TA instrument, Q50 V20.13 Build 39). About 10 mg of CT fiber is exposed to atmospheric air with a heating rate of 10°C min⁻¹ from room temperature to 500°C.

Fourier transform infrared (FTIR) spectroscopy

FTIR spectra of untreated and chemically treated CT fibers have been recorded in a Nicolet 6700 spectrophotometer in the form of KBr Pellets. The test apparatus is equipped with Germanium Attenuated Total Reflectance (ATR). About 200 scans have been collected for each measurement over the spectral range 4000–500 cm⁻¹ with a resolution of 4 cm⁻¹.

X-ray diffraction (XRD)

The effect of chemical treatment on crystallinity of CT fibers for both treated and untreated has been investigated using a Mini Flex diffractometer (shimadzu, XRD-7000). The graph of the samples has been obtained at 40 kV and 150 mA in reflection mode, with a step of 0.05° and 1 s of checking time under CuKα radiation. The crystalline structure of cellulose-I of CT fiber has been measured by the crystallinity index (CI) from the empirical method proposed by Segal et al. (Segal et al. 1959) as per the following equation.

(1) $\mathrm{C}.\mathrm{I}\mathrm{\%}=\frac{{\mathrm{I}}_{200}-{\mathrm{I}}_{\mathrm{a}\mathrm{m}}}{{\mathrm{I}}_{200}}\times 100$(1)

Where I₀₀₂ is the maximum intensity of the (0 0 2) crystalline peak and I_am is the minimum intensity of the amorphous material between $(10\bar{1})$and (0 0 2) peaks.

Mechanical properties

The mechanical properties of CT fibers (treated and untreated) and the fabricated composites were evaluated and discussed. The tensile strength was evaluated as per ASTM D3379–75. To find out the single fiber tensile strength of CT fiber, a tab was prepared from a thick cardboard paper to hold the fiber in the machine. A slot was cut in the middle of the tab having length equal to the gage length (50 mm) of the specimen. A single filament of CT fiber was pasted by gluing both the ends (about 20 mm) of the fiber in the slot. The specimen to be tested was mounted on INSTRON 3382 machine with a 1 N load cell while maintaining a crosshead of speed 0.2 mm/min. From each treated and untreated fiber bundles, a total of 20 filaments were tested one after another and the average of the readings were recorded. Similarly, the composites were tensile tested with 5 kN load cell using INSTRON (5500 R) universal testing equipment according to D3039–76 ASTM standard. The experiments were carried out at 28°C (room temperature) with a crosshead speed of 2 mm/min and a 50 mm extensometer to measure displacement. The flexural tests were performed on a similar machine, with a three-point bending strategy and a crosshead speed of 1 mm/min in accordance with ASTM D 790–99. Five samples from each fiber stacking were tested to determine the average data. The dimension of the samples for tensile test was 153 mm × 12.7 mm × 4 mm, whereas for flexural test, the dimension was 100 mm × 12.7 mm × 4 mm. Figure 2 shows fabricated composites for tensile test.

Figure 2. Fabricated CT/epoxy composite.

Contact angle measurement test

The wettability of the composite surfaces is determined using the contact angle measuring technique. The liquid component’s capacity to maintain the interaction when it comes into contact with the solid surface is defined by its wetting property. It is found that the contact angle is an attractive method to observe the behavior of the liquid on a composite surface. This is used to find the hydrophobicity of the composite surfaces. The specimens were tested as per ASTM D-7334. The dimensions of the specimens are taken as 80 mm length, 20 mm width, and 5 mm thickness. A total of five samples were tested for each composite. In this method, a water droplet is placed on the composite surface with the help of a glass pipe. The volume of the water droplet is kept constant for all the composites. The experimental setup used for contact angle measurement is shown in Figure 3, and it consists of an image processor, a moveable holder, camera, optical lenses, test sample holder, and a composite placed on a holder rest.

Figure 3. Experimental setup used for contact angle measurement.

Surface morphology

Microscopic examinations were performed using a HITACHI SU3500 scanning electron microscope (SEM). All specimens are sputtered with a 10 nm layer of gold and mounted on aluminum holders using double-sided electrically conductive carbon adhesive tabs prior to SEM observations.

Results and discussion

Chemical analysis of the fibers

The chemical compositions of CT fibers were analyzed and are listed in Table 1. Plant’s age, place of growth, soil conditions, extraction environments, and techniques used to identify the chemical compositions are the factors that affect chemical analysis (Batra 1985). The chemical composition of untreated CT fibers consists of cellulose (45.72 wt%), hemicellulose (32.27%), and lignin (18.08 wt%), and the remaining 5.20% is attributed to other components (e.g. pectin, wax, protein, oil, and ash). Changes in cellulose, hemicellulose, and lignin contents of NFs subjected to chemical treatments have been well documented in the literature (Soleimani et al. 2018; Vardhini et al. 2016). A closer look at the data in Table 1 shows benzoylation of treated fibers reveals the highest increase in cellulose (22.9%), 23.4% and 17.4% decrease in hemicelluloses and lignin, respectively, relative to the untreated and other treated fibers. Thus, a better fiber-matrix bonding is expected from benzoylation treated CT fibers as reinforcement.

Table 1. Chemical composition of untreated and treated CASCABELA thevetia (CT) fibers.

Fibers	Cellulose (%)	Hemicellulose (%)	Lignin (%)	Others (%)
Untreated	45.72	32.27	18.08	5.20
Alkaline treated	52.12	27.58	13.43	4.87
Sodium chlorite treated	51.70	26.65	12.41	4.24
Permanganate treated	56.50	26.12	12.84	4.54
Benzoyl chloride treated	59.30	24.62	11.95	4.13

Density of the fibers

The apparent densities of CT fibers have been measured as per the procedure described earlier and found to be 989 kg/m³, 1021 kg/m³, 1033 kg/m³, 1078 kg/m³, and 1098 kg/m³ for untreated, alkali-treated, sodium chlorite-treated, permanganate treated and benzoylation-treated CT fibers. A slight positive increase was observed in the densities of the treated fibers in comparison to untreated fiber. This increase is primarily due to the densification of the treated fibers cell wall (Aziz and Ansell 2004). The obtained densities were significantly less than those of other biofibers such as ARECA catechu (1245 kg/m³) (Nayak and Mohanty 2018a, 2018b), CYPERUS pangorei (1102 kg/m³) (Mayandi et al. 2016), and ACACIA leucophloea (1385 kg/m³) (Arthanarieswaran, Kumaravel, and Saravanakumar 2015). The low density of CT fibers may be helpful in fabricating lightweight composite structures.

Thermogravimetric analysis (TGA)

TGA of untreated and various chemically treated fibers was performed and presented in Figure 4. It is observed that the degradation starts at around 30°C for all the fibers. This is due to entrapped moist particles, which get evaporated during the heating process (Sahoo et al. 2019). The initial decomposition temperature (IDT) of untreated and all treated fibers have been found to be in the ranges of 250–270°C showing a weight loss of 8–15%. The lowest weight loss is observed in the case of benzyl chloride-treated fibers, while the highest is observed in the case of untreated fibers. The final decomposition temperature (FDT) of untreated fiber is 385°C (62% weight loss), whereas the FDT of all the treated fibers falls in the range of 390–450°C with a weight loss of 48–55%. Hence, the TGA results confirm that benzyl chloride-treated CT fibers possess better resistance to thermal degradation as compared to other treated and untreated fibers.

Figure 4. TGA of treated and untreated fibers.

X-ray diffraction (XRD)

The X-ray diffraction patterns of untreated and chemically treated CT fibers have been shown in Figure 5 and the CI in Table 2.

Figure 5. XRD of treated and untreated fibers.

Table 2. Crystallinity Index (CI%).

Material	I(002)	I(am)	C.I %
Untreated	162	61	62.34
Alkali treated	135	48	64.44
Sodium Chlorite treated	110	32	70.90
Permanganate treated	105	29	72.38
Benzoylation treated	115	30	73.91

From the data presented in table, it can be seen that the CI of all treated fibers is superior to the untreated fiber. The CI gives a quantitative measure of the orientation of cellulose crystals in the fibers with respect to the fiber axis. This could be due to the removal of waxy materials such as lignin, pectins, cementing materials, and hemicelluloses, which lead the inter fibrillar regions to be less dense and less rigid and better packing of cellulose chains. The diffractograms of all the samples show the presence of reflections at 2θ = 16.9º and 23.9º, which represents the crystalline pattern of cellulose I, indicating that the crystalline structure of the cellulose has not been altered in different treatments performed in this work. Increased crystallinity of treated fibers clearly shows the removal of lignin and hemicelluloses. An increase of 15.65% in CI is observed in the case of benzoylation-treated fiber in comparison to all other fibers due to the removal of most of the lignin content.

Fourier transform infrared spectroscopy

The extent of chemical changes associated with the use of alkaline and other acid treatments in CT fibers was analyzed using FTIR spectroscopy. Typical absorbance spectra obtained from this study are presented in Figure 6 for untreated and treated fibers, with the absorbance peaks of interest clearly marked. Hydrogen bond (O – H) stretching is generally observed around the absorption band of 3500 cm⁻¹. In the present investigation, this band is available at around 3450 cm⁻¹ for untreated CT fiber. However, for the treated fibers, this band is found to be shifted toward right (3428 cm⁻¹) with variation of intensity after chemical treatment. This indicates that reduced hydrogen bonding in cellulosic hydroxyl groups results in reduced hydrophilic nature, exposing the – OH groups to interact with the polymer matrix during reinforcement (Kabir et al. 2012). The intensity of the absorption peak at approximately 2900 cm⁻¹ in the untreated fiber, which is attributed to C – H and CH₂ stretching in hemicellulose, cellulose, and lignin (Taha, Steuernagel, and Ziegmann 2007; Tibolla, Pelissari, and Menegalli 2014) did not change significantly with chemical treatment as that at 3450 cm⁻¹. The peak at 1750 cm⁻¹ which is attributed to C=O stretching of the acetyl groups of hemicelluloses in untreated fibers vanishes in all treated fibers, confirming the removal of hemicelluloses during the treatments. The peak around 1236 cm⁻¹ in the untreated fiber, attributed to the C – O–C stretching in cellulose and lignin (Moran et al. 2008), reduced after chemical treatments. An increase in the intensity of the prominent absorption peak at 1028 cm⁻¹, attributed to C – O deformation in primary alcohols (Chen et al. 2010), C – H deformation in guaiacyl with C – O deformation in the primary alcohol in hardwood lignin (Kubo and Kadla 2005) and C – O, C – O–C and C=C stretching in cellulose, hemicelluloses, and lignin occurred after chemical treatments (Xu et al. 2013).

Figure 6. FTIR of treated and untreated fibers.

Effect of chemical treatment on tensile strength

The tensile strength of untreated and different chemically treated CT fibers has been measured and presented in Table 3. From the table, it is observed that the tensile strength of all the treated fibers is found to increase in comparison to the untreated one. In comparison with all treated fibers, benzyl chloride treated fibers show the highest strength (255.36 MPa) which is better than the tensile strength obtained for acrylic acid treated date palm fibers, i.e. 70.27 MPa (Mohanty et al. 2014) and benzyl chloride treated areca sheath fibers, i.e. 115.48 MPa (Nayak and Mohanty 2018b). This is due to the fact that, OH groups of the fiber are replaced by benzoyl groups and they get attached to the cellulose backbone. This results in a more hydrophobic nature of the fiber and improves adhesion with the matrix. Moreover, mechanical properties of natural fibers depend strongly on their cellulose content (highest in the case of benzoylation treated fibers) since it provides strength and stability to the fibers.

Table 3. Tensile strength of fibers after 60 days of continuous immersion.

			Tensile strength (MPa) (60 days of immersion)
Sl no.	Fiber type	Tensile strength (MPa)	Fresh water	Sodium hydroxide	Saturated lime
1	Untreated	212.50 ± 3.91	69.54 ± 1.04	89.91 ± 1.30	44.32 ± 0.69
2	Alkali treated	228.45 ± 4.10	75.10 ± 1.27	79.25 ± 1.35	49.10 ± 0.81
3	Sodium chlorite	215.74 ± 3.23	73.28 ± 1.09	88.19 ± 1.52	60.81 ± 1.02
4	Benzoylation	255.36 ± 4.85	118.32 ± 2.07	122.28 ± 2.29	96.03 ± 1.77
5	Permanganate	238.22 ± 4.98	98.72 ± 1.62	100.09 ± 1.88	89.92 ± 1.54

Note: The values presented correspond to average of 20 test values for each treatment.

Effect of exposure conditions on tensile strength of chemically treated fibers

The decomposition of fibers has a tremendous effect on the tensile strength of various fibers. The tensile strength of untreated and various treated fibers was compared with the tensile strength of fibers after exposure in various mediums and presented in Table 3. A substantial reduction in the tensile strength of fibers is observed in all the mediums, irrespective of the type of exposure. This may be attributed to the chemical breakdown of the fibers due to reaction with water and other solutions. The above chemical dissolution is responsible for the loss in strength of the fibers and their efficiency as reinforcement. Maximum loss in strength is found in the case of saturated lime for all fibers; however, benzoylation treated fibers are found to retain higher percentages of their initial strength than all other fibers, after the specified period of exposure in the various mediums. It is also observed that after 60 days of continuous exposure to saturated lime, untreated, and alkali – treated fibers were the most affected while retaining only 20.8% and 21.4% of the original strength. Benzoylation treated fiber retains about 37.7% (saturated lime), 47.8% (sodium hydroxide), and 46.33% (Fresh water) of its original strength, which is much better than other fibers in the same exposure medium.

This is because the benzyl chloride treatment enhances the amount of cellulose exposed on the fiber surface which results in the lowest degradation of fiber in various mediums.

Mechanical properties of fabricated composites

Various mechanical properties of the treated composites were tested, and the results are presented in Table 4. From the data, it was noticed that all the properties have increased with the increase in filler loading till it reaches a maximum value then further decreased. 20 wt% of fiber loading composites revealed the highest properties as compared to other fabricated composites. This is because the CT fibers and matrix are well interfaced and firmly bonded. The values obtained at 20 wt% of fiber loading are 52.79 MPa of tensile strength, 2.45 GPa Young’s modulus, 71.72 MPa flexural strength, and 5.97 kJ/m² Impact strength. The data obtained are far better than the values obtained in CALOTROPIS gigantea bast fiber/epoxy composites fabricated by (Sahu et al. 2022) (i.e. 42.54 MPa tensile strength and 1.85 GPa young’s modulus) and date palm/epoxy (42.07 MPa) (Dehury et al. 2021). As 20 wt% of treated CT fibers have shown the optimum fiber loading, a control sample was prepared taking 20 wt% of untreated CT fibers for comparison purposes. The maximum tensile strength of untreated fiber composite was found to be 37.42 MPa. The enhancement of tensile strength for benzoylation treated CT/epoxy composite was due to elimination of unwanted materials like lignin, wax, and oily substances from the fiber surface and reduces the presence of hydroxyl group that causes poor interfacial bonding between matrix and reinforcement (Ramesh et al. 2021).

Table 4. Mechanical properties of untreated and treated CT/epoxy composites at different fiber loadings.

Composites	Tensile strength (MPa)	Young’s Modulus (GPa)	Flexural Strength (MPa)	Elongation at break (%)	Impact Strength (kJ/m^2)
Neat Epoxy	18.25 ± 0.5	0.386 ± 0.26	19.81 ± 0.86	1.78 ± 0.09	3.82 ± 0.21
Untreated
20 wt%	37.42 ± 0.49	2.01 ± 0.06	42.01 ± 0.91	2.19 ± 0.10	5.34 ± 0.28
Treated
5 wt% 10 wt% 15 wt% 20 wt% 25 wt%	28.11 ± 0.19	1.49 ± 0.03	44.13 ± 0.81	1.77 ± 0.13	4.76 ± 0.82
	35.22.87	1.98 ± 0.03	54.38 ± 0.92	2.12 ± 0.15	4.92 ± 0.71
	43.52.91	2.28 ± 0.02	66.29 ± 0.89	2.39 ± 0.12	5.31 ± 0.70
	52.79.78	2.45 ± 0.05	71.72 ± 0.67	2.58 ± 0.13	5.97 ± 0.84
	48.19.71	2.11 ± 0.03	62.11 ± 0.68	2.05 ± 0.12	5.19 ± 0.87
30 wt%	33.21 ± 0.17	1.78 ± 0.04	53.02 ± 0.73	1.78 ± 0.13	5.10 ± 0.79

Contact angle measurement

The wettability properties and hydrophilic nature of the composites were calculated using the contact angle measurement analysis. The contact angle was measured with the image processing apparatus. All the values are measured (recorded) in a dark place and for each composite five samples were tested. The contact angle of the composites was measured and with deviation in each value presented in Figure 7.

Figure 7. Contact angle of various fabricated composites.

In general, the highest contact angle is recorded on rough composite surfaces, and the lowest is recorded on smooth composite surfaces. If the contact angle is less than 90°, then the composite has a hydrophilic nature. If it is more than 90°, then the composite has hydrophobic nature. It was found that all fabricated composites showed a contact angle less than 90°, which indicates that all composites have hydrophilic nature. From Figure 7, it was observed that the composites exhibited a contact angle between 58° and 70°. In addition, it was also observed that the composites fabricated with 20 wt% of CT fibers shows smooth surface, whereas 30 wt% CT fibers exhibit rougher surfaces as compared with other composites.

Surface morphology

Figure 8 shows the effect of chemical treatment on the microstructure of untreated and various treated CT fibers. In Figure 8a, a smooth surface is observed due to the availability of lignin and waxy materials that make the fiber hydrophilic in nature, thereby helping to soak up more water. The surfaces of the chemically treated fibers (Figure 8b-e) are much cleaner, rougher, and more porous than those of untreated fiber, which is an indication that these chemicals remove wax, protein, pectin, oil, and other impurities from the surfaces of the fibers. It was reported that the use of chemical treatment led to the removal of noncellulosic components and also resulted in changes in both surface chemistry and thermal properties of NFs (Gonzalez et al. 1999). The removal of these non-cellulosic materials as well as the increased surface roughness are expected to promote strong bonding between the fibers and polymer matrices when used in composite manufacturing (Kabir et al. 2012).

Figure 8. SEM of untreated and treated fibers.

Conclusion

The effects of various chemical treatments on CASCABELA thevetia bast fibers were investigated using different material characterization techniques. The following conclusions were drawn within the scope of this study.

• Chemical treatment with benzyl chloride solution changed morphological, thermal, and chemical properties of CT fibers. Both cellulose content and crystallinity index of CT fibers increased with treatment.

• A slight increase in densities of the treated fibers has been observed in comparison to untreated fiber.

• It is found that the tensile strength of treated fibers and particularly benzoylation treated CT fiber is better in comparison to the untreated one. This may be due to cellulose crystallinity with an increased CI as confirmed by XRD.

• The treated fibers reveal better resistance to thermal degradation at elevated temperatures as compared to untreated one, with benzoylation treated CT fibers showing overall less weight loss.

• Compared to the overall properties of CT fibers, it has been observed that benzoylation treated CT fibers give better performance in comparison to other treatments.

• Benzoylation treated CT fiber reinforced with epoxy composites was successfully fabricated with various weight percentages of fiber loadings. 20 wt% was viewed as ideal fiber loading as it shows the most elevated mechanical properties (i.e., 52.79 MPa of tensile strength, 2.45 GPa of Young’s modulus, 71.72 MPa of flexural strength) as contrasted with other fabricated untreated and treated composites.

• From contact angle measurement analysis, all the composites showed a contact angle at less than 90°, which means composites exhibit hydrophilic surface properties. There is no such deviation in the contact angle results among all composites.

• Hence, the experimental results prove that CASCABELA thevetia fiber reinforced epoxy composites can be a good potential candidate for development of lightweight materials especially for automobile industries for manufacturing weight reduction vehicles to enhance their fuel efficiency.

Highlights

• The CASCABELA thevetia (CT) fibers were successfully extracted from the bast of the stem by a retting process.

• The fibers were undergone through various chemical treatments which enhanced the exposure of cellulose on the fiber surface.

• Benzoylation treated fibers revealed better mechanical properties as compared to untreated and other treated CT fibers.

• Composites were successfully fabricated reinforcing benzoylation treated CT fibers in an epoxy matrix. 20 wt% of fiber loading reveals the highest mechanical properties as well as the lowest contact angle, indicating a smoother surface.

• All the composites exhibited contact angles lower than 90° which is associated with composite hydrophilic surface properties.

• CASCABELA thevetia (CT) fibers can be a potential candidate for reinforcement in fabrication of polymeric composites.

Disclosure statement

No potential conflict of interest was reported by the author(s).