Influence of Fiber Volume and Fiber Length on Thermal and Flexural Properties of a Hybrid Natural Polymer Composite Prepared with Banana Stem, Pineapple Leaf, and S-Glass

Department of Mechanical Engineering, Bannari Amman Institute of Technology, Sathyamangalam 638401, Tamil Nadu, India Department of Mechanical Engineering, College of Engineering, Jazan University, Jazan, Saudi Arabia Department of Mechanical Engineering, KPR Institute of Engineering and Technology, Coimbatore 641407, Tamil Nadu, India Department of Mechanical Engineering, M. Kumarasamy College of Engineering, Karur 639113, Tamil Nadu, India Department of Mechanical Engineering, Gokaraju Rangaraju Institute of Engineering and Technology, Hyderabad 500090, Telangana, India Department of Mechanical Engineering, Faculty of Manufacturing, Institute of Technolog, Hawassa University, Hawassa, Ethiopia


Introduction
Due to the demand for lower dense material and green environment, fiber-reinforced composites (FRCs) have drawn more awareness towards the alternatives to metalreinforced composites. Natural fibers are effectively utilized in polymer matrices as reinforcement [1]. Fillers such as particles or filaments are created with polymers to get items with the most needed mechanical, thermal, and electrical properties. e characteristics of natural composites are primarily subject to their particular fiber characteristics [2].
ere are different variables, which influence the properties of the microstructural boundaries, such as fiber radius, length of fiber, fiber spread, fiber direction, loading weight of the fiber, and production method [3]. Natural fibers have more limitations, such as hydrophilic behavior, which leads to decreased adhesive properties [4]. In addition, natural fibers have more attraction to water particles from moisture, and they are affected easily by ultraviolet rays, so more variations in their thermal and mechanical properties occur [5]. Hence, to limit these issues, researchers are focusing on natural-fiber-strengthened composites with polymer matrices. e addition of natural fibers to polymers is referred to as hybrid composites. It overcomes the limitations and enhances the hindering properties of the natural and the polymer composites [6,7]. Gowda et al. [8] worked on various types of natural fibers, i.e., silk, bamboo, kapok, coconut fiber, which were reinforced with polymer matrices and reported that the addition of natural fiber enhances the mechanical and thermal characteristics of the polymer matrices by 6-10%. Similarly, Abdul Karim et al. worked on various natural fibers, such as hemp, coir, and jute, and studied the properties of the mechanical and thermal characteristics of hybrid natural polymer composites and enhanced its properties to 4-8% [9]. Sheng and Gimbun found that an increment in the fiber volume fraction increases the flexural strength linearly from 4% to 6% up to 0.4 fiber fraction [10]. Jain et al. varied the fiber fraction from 0.1 to 0.3 and found that there is increment in tensile strength and flexural strength by 3-8% [11]. Chollakup et al. used the different fibers and found that PALF and banana fiber provides better mechanical and thermal properties [12]. Shih et al. conducted a study and discovered that the tensile and flexural strengths of PALF and banana fibers are approximately 80% and 50% higher, respectively, than the other fibers [13]. Asim et al. deliberated different fiber loadings with PALF and banana fibers resulting in flexural strength increase with an upsurge in fiber loading [14].
Nur et al. [15] investigated the influence of the fiber length and found that an increment in the PALF length intensifies the tensile strength and flexural strength of the composite by 8-12%. Aji et al. varied the fiber length and found that an increase in the fiber length increases the tensile strength and flexural strength by 8-12% [16]. Luo and Netravali varied the fiber length of the natural composites and found that there were significant effects on the thermal and mechanical properties [17]. Chollakup et al. discovered that the increment in the fiber length provides positive effects on the flexural and thermal properties up to a certain fiber length of 30 cm, after which it shows a counter-effect [18]. Lopattananon et al. varied the fiber length and found that the fiber length increases the loss of mass during the analysis of thermal characteristics [19]. e fiber loading and fiber length discover the drastic changes in the characteristics of the composites, and it creates both positive and negative effects [13,20,21]. e improvement was noticed in tensile strength, hardness, and strain modulus with an upsurge in the fiber volume, fiber length, and fiber type, respectively. e fiber fraction, fiber length, fiber type, and fiber treatment process are the significant elements influencing the properties of the composites [22][23][24]. Hence, it is a significant role being a researcher to study the effects of various compositions of natural fibers, their treatment methods, fiber length, and fiber volume on the mechanical and thermal properties [25,26].
Natural fiber composites retain better thermomechanical and electrical properties if the interfacial bonds between the matrix and fiber are strong enough [27][28][29]. As of now, there is less data available on natural fiber reinforcement with S-glass polymer matrices; especially, there is no optimized data about the combination, which provides a better result of thermomechanical properties. e novelty of this work is that it pays special attention to determining the optimum mechanical and thermal characteristics of the polymer hybrid composites with various fiber loadings and different fiber lengths of banana stem, PALF, and S-glass.

Materials.
Banana stems and pineapple leaves were taken as fiber materials, and their fiber lengths were chosen as 20 mm, 30 mm, and 40 mm. e S-glass fabric was selected as synthetic fiber with 600 gsm, and its size was fixed as 30 cm square. Epoxy resins were used as resins. e properties of materials are given in Table 1. e pineapple leaf was chemically treated with 6% sodium hydroxide, and it was immersed in it for 3 hours [30]. e banana leaf was also treated with the same method, and its immersion time was 2 hours. en, it was dried in the furnace for 24 hours at 60°C, and it was weighted for the required volume as per equation (1) [19]. Naturally, untreated fibers tend to absorb moisture, which can cause delamination between the fibers and the polymer matrix and seriously affect the strength of the resulting composite material [31]. is is because natural fibers are hydrophilic, so their reinforcing effect can be minimized. Second, the hydrophobicity of the polyurethane matrix in conjunction with the hydrophilic fibers will lead to poor adhesion, phase separation, and limited stress load transfer in the composite foam [32]. Additionally, the presence of certain organic compounds (such as wax and pectin) found on the surface of these fibers can sometimes act as a barrier to destroy the effective interfacial adhesion between the filler and the polymeric matrix [33], where V f is the volume fraction of fiber (%), W m and W f are the mass of the matrix and fiber, respectively, and ρ m and ρ f refer to the density of the matrix and fiber, respectively. e S-glass was kept in the middle layer to avoid stress concentration on a single point [34]. Hence, stresses were transferred to the natural fiber of both the bottom and topsided fibers. As per the ASTM: D790 standard, flexural assessments were carried out.

Methods.
e different sets of hybrid composites were made by the hot compression method as shown in Figure 1. e specimens prepared with different fiber volume fraction and fiber length are labelled as shown in Table 2.
A morphological investigation was accomplished to analyze the presence of interfacial attachment between the fiber and epoxy resin. Sample tests were covered with a skinny layer of gold, proceeded to scan to build the thermal and electrical conductivity, and to forestall electrostatic charging during test assessment. e pictures were examined to research the proper circulation of the natural fibers in the synthetic polymer matrix and their character of connection with one another. e flexural tests were accompanied to find the flexural strength and flexural modulus. e flexural tests were piloted based on the ASTM standard D790 [35]. e specimen was supported by a support span, and the load was applied to the center by the loading nose, resulting in three-point bending at a set rate. e support span, loading speed, and maximum deflection for the test were the parameters for this test. As per the recommended   procedure, the ASTM D790 test was accomplished till the point of failure of the specimen. e thermogravimetric analyzer was used to record the variation in the thermal characteristics of the manmade hybrid composites based on their weight loss at different temperatures [36]. It measures the rate of change of material weight for different temperature profiles based on the effects of dehydration, decomposition, and oxidation. e dynamic mechanical investigation was carried out while examining the viscoelastic characteristics at various temperature profiles [37].
is investigation included the application of an oscillatory sprain at various temperatures.

Flexural Test.
e condition of the samples before and after the flexural test is depicted in Figure 2. ree random samples from each fiber composition are presented in Figure 2(b) for better understanding. Flexural test results are presented in Figure 3. e flexural strength of the hybrid composite is nonlinear. It showed an increasing trend from 30% to 40%, and after 50% volume fraction, it started to decrease. e flexural strength of the S-glass/PALF hybrid composite was 18% higher than the S-glass/banana composite [23]. e maximum flexural strength of the SGP40 composite recorded at 20 cm, 30 cm, and 40 cm fiber lengths was 97.5 MPa, 103.6 MPa, and 112.3 MPa, respectively. As the fiber length of the hybrid composite upsurges, the flexural strength also increases linearly. e percentage strain of the composites decreases with the increase in the volume fraction of the glass fibers. Since the glass fiber is extremely brittle, the flexural strength of the composite increases with an increase in the fiber volume. e maximum flexural strength of the SGB40 composite recorded at 20 cm, 30 cm, and 40 cm fiber lengths were 94.5 MPa, 99.8 MPa, and 104.5 MPa, respectively. e increase in the fiber length of the SGB40 hybrid composite increases the mechanical flexural strength linearly. e greatest flexural strength of the SGBP composite recorded at 20 cm, 30 cm, and 40 cm fiber lengths were 62.5 MPa, 64.6 MPa, and 66.3 MPa, respectively. As the fiber length of the SGBP40 hybrid composite increases, the flexural strength also increases linearly. e same kinds of results were obtained and reported by Shen and Gimbun [28]. e reason for this phenomenon was characterized to the insufficient resin. Natural fibers are normally hydrophobic in nature, and they absorb more resin. Hence, insufficient resin led to a decrement in the flexural strength of the polymer hybrid composites, when the fiber volume was increased after 50%. e after-effects of the insufficient resin increased the brittleness, and hence, it started to fail permanently. e flexural modulus results followed same as that of the flexural strength results, which are shown in Figure 4. It shows an increasing trend from 30% to 40%, and at 50% volume fraction, it started to decline. e stiffness of the hybrid composite would be better if the value of the flexural modulus is high. e flexural modulus of the SGP 40 has found to be superior (8.56 MPa) at 40 cm fiber length. e flexural modulus of the SGBP 40 has been obtained as 2.76 MPa at 40 cm fiber length, which is the lowest value ever. Hence, the determined results accredited that the SGP 40 would absorb more forces than the other hybrid composites [27]. e flexural modulus exhibited a decrement with the reduction in the fiber length due to high strain in the material.

ermal Test.
e thermogravimetric analysis was carried out between 30°C and 600°C at 10°C/min rate, and its results are presented in Figures 5(a)-5(c). e major degradation for SGB4030 occurred at the temperature between 288°C and 451°C. e peak degradation occurred at 381°C temperature. e degradation of the SGP4030 occurred at the temperature between 258°C and 426°C, and the peak happened at 342°C. e reason for the same is attributed to a different phase of degradation. e first phase of the weight loss occurred from 30°C to 100°C, and it may be due to the moisture evaporation.
e subsequent phase occurred between 150°C and 430°C, and it was attributed to the process of decomposition. e next decomposition was hemicellulose decomposition, which started at 250°C and ended at 326°C. In addition, the cellulose decomposition was started. e thermal stability of the cellulose was accredited to their strong chemical structure. Furthermore, lignin decomposition started at 30°C and extended till 600°C. It can be understood that lignin has shown more thermal stability, and hence, it provided the toughness to the natural fibers [38]. e banana and S-glass hybrid composites possess higher thermal degradation temperature, and their mass loss was higher than the PALF and S-glass hybrid composites due to their higher fiber weight [38,39]. Hence, SGB 3020, SBP3020, and SGBP 20 have shown lower thermal degradation temperatures. Figure 5 depicts that a decrease in the fiber length would cause a decrease in the mass loss of the hybrid composite. It can be understood that a low rate of decomposition of fiber would be the decline in the mass loss of the hybrid fiber composite.

Storage Modulus Test.
e dynamic mechanical analyzer (V4.5A TA Model) was used to measure the thermal storage modulus. It is preferred because of its comparative effectiveness in measuring the storage modulus [40,41]. e hybrid composites were heated from 30°C to 200°C, and their results were plotted as shown in Figures 6(a)-6(c). e results evidenced that owing to the less stiffness of the fiber, a slight variation in the storage modulus for the hybrid polymer composites happened. Furthermore, the storage modulus diminished with temperature for both the natural composites. Hence, the higher fiber volume caused to increase the stiffness of the hybrid composites. However, it demonstrated the waning of stiffness with the increase in temperature [42]. At the PALF fiber proportion of 40%, the least flexural modulus was recorded. It was observed that, if the fiber volume decreases, the storage modulus also decreases due to the mobility of molecules in natural fiber composites.
e storage modulus decreased with the decrease in temperature of the glass fiber, and it was compensated with the reinforcement of banana fiber and PALF fiber. Because the thermal expansion coefficients of epoxy resin, banana fiber, and PALF fiber are all positive, it became the reason for the changes in mechanical and thermal properties. However, in the absence of residual stress, it has become responsible for a drop in the shear modulus for the increment in the fiber volume and fiber length.

Comparison.
e present work is compared with the previous research work, which was piloted by Mohd Hanafee Zin and Nurazzi Norizan [43] for the 40% fiber volume and 30 cm fiber length of banana, PALF-reinforced polymer matrix as presented in Table 3. ey used E-glass fiber with banana stem and PALF. It is obvious that SGB4030 exhibited a 14.89 percent more flexural strength, 6.85 percent higher flexural strength, 3.95 percent higher loss modulus, 1.77 percent higher storage modulus than [43]. S-glass is a highperformance glass fiber that differs from E-glass fiber principally in terms of silica content. Silica, aluminium, and magnesium oxides are abundantly found in S-glass fiber, and    Advances in Materials Science and Engineering 5 hence, S-glass fibers are superior in terms of mechanical and thermal properties compared to E-glass fibers.

Optimization.
Hybrid Taguchi Grey relational analysis (HTGRA) was used to find the optimum output parameters from the different input parameters as shown in Table 4. Initially, L27 design of experiments were selected with different combination of fiber type, fiber load, and fiber length. e test specimens were prepared as per the L27 Design of Experiments (Doe), and mechanical and thermal characteristics tests were performed as described in the earlier sections. e output responses (data from Table 5), i.e., flexural strength, flexural modulus, and storage modulus were first converted into a signal to noise ratio based on the following first equation, and the percentage of mass loss was determined by the next equation [44,45]: where "S/N" refers to signal to noise ratio, and "yi" refers to the output response of specific sequence. en, the data from the S/N ratios were preprocessed using the following equations:  Advances in Materials Science and Engineering 7 Larger the better (xi) * (k) � Smaller the better (xi) where xi (o) (k) is an output response value at original sequence, and maxxi (o) (k) is the largest value of sequence, and minxi (o) (k) is the lowest value of that responses. Equation (4) was used for flexural strength, flexural modulus, and storage modulus as these values should be as large as possible. In addition, equation (5) was used for finding the percentage of mass loss responses as these values should be as small as possible [46,47]. en, the deviation sequence (€i (k)) was found using the following equations (ddistinguished coefficient (€) was chosen as 0.5): Finally, grey relation grade (GRG) was calculated using equation (7) [48,49]. GRG was given as input to the software Minitab 19, and then, the optimized parameter plot, regression equation, and R 2 values were found, R 2 value was found to be 95.34%, and the predicted R 2 value was 89.10%. Hence, the model was found to be valid. Before calculating the R 2 values, the empirical correlations were created. At a 95% confidence level, all the coefficients were evaluated for significance. Using the Student's t-test, insignificant coefficients were removed without impacting the correctness of the empirical connections. In order to establish the final empirical relationships, the significant coefficients were taken into account. For all replies, the final developed empirical connections with processing components in the coded form are given in the following regression equation : where X 1 , X 2 , and X 3 refer to the different levels of fiber combinations, Y 1 , Y 2 , and Y 3 are different fiber volume   levels, and Z 1 , Z 2 , and Z 3 are different levels of fiber length. Based on the GRG values, the main effects and the interaction effects of the input parameters were determined, and the best parameter settings were identified from the main effects plot of grey relational grade response as follows ( Figure 7): S-glass PALF fiber combination, 40% fiber load, and 40 cm fiber length. According to the interaction effect plot as shown in Figure 8, there is no interaction effect between the input parameters even though S-glass PALF fiber combination, 40% fiber load, and 40 cm fiber length combination provides optimum mechanical and thermal properties. e HTGRA technique proposed a set of optimal input parameters, and the tests were carried out once again to validate the results. It yielded the GRG value of 0.796, and when compared it to the average values of the experimental results, GRG was raised by 0.015 percent during the confirmation test.

Conclusions
is work deals with the influence of fiber volume and length on mechanical and thermal properties of the polymer hybrid composite. e following inferences are drawn from the critical investigation: (1) e increment in the fiber volume and fiber length induces the enhancement in the flexural and thermal characteristics of the hybrid composites. After reaching 40% of the fiber volume, the mentioned characteristics started to decline. Furthermore, an increment in the fiber length always has positive effects on the mechanical and thermal properties. Similarly, the flexural strength of SGP4030 was 6.85% higher than the SEB4030. (3) e assessment of the thermal properties revealed that SGB4020 and SGP4020 exhibited a low mass loss, and SGB4030 presented the higher storage modulus. e storage modulus of SGB4030 was perceived as 3.95% higher than the SEB4030. Likewise, the storage modulus of SGP4030 was 1.77% higher than the SEB4030. (4) Hybrid Taguchi GRA techniques suggested that PALF-reinforced S-glass fiber with a 40% fiber volume and 40 cm fiber length provides the optimum mechanical and thermal properties compared to other combinations.

Advances in Materials Science and Engineering
Data Availability e data used to support the findings of this study are included within the article.