Performance Evaluation of Natural Composites Made from Banyan and Cotton Fibers for Sustainable Thermal Insulation Applications

ABSTRACT Natural composites are gaining more attention due to the light weight and low cost characteristics. In the present investigation, an attempt has been made to fabricate the natural composites using natural fibers like Banyan and Cotton. Composites were made by compression molding technique and five types of composites were prepared, namely, B, BC(3:1), BC(1:1), BC(1:3) and C. Mechanical and thermal properties of the composites were tested. Results showed that Banyan composite exhibited the maximum tensile and flexural strength of 75 MPa and 113 MPa, respectively. This was attributed by the placing of high strength and stiffness Banyan fiber. Further, a maximum thermal resistance of 18.1 × 10−3 m2K/W was observed for Banyan composite. Moreover, the Banyan composite displayed the least coefficient of thermal expansion (CTE) of 1.81 × 10−5/°C which indicated the good dimensional stability of composite. Furthermore, a high thermal diffusivity of 8.79 × 10−7 m2/s was achieved for Banyan composites. Based on the experimental results, it is suggested that the Banyan composite could be a suitable candidate for sustainable thermal insulation applications.


Introduction
Natural composites are gaining more interest due to its environment-friendly nature, light weight, low cost, and good strength-to-weight ratio (Binu Jeyaraman, Jesuretnam, and Ramar 2020;Muthukrishnan et al. 2021) It finds applications in automobile industries, home appliances, and construction fields. In automobiles, dashboard, door panel, and windows are prepared by composite materials, which helps to replace part of the conventional material and assist in the improvement of economy of operation by reducing the weight (Ravishankar, Nayak, and Kader 2019). The other advantage of natural composites is thermal insulating ability as they possess low thermal conductivity.
Few works have been done on the thermal insulation study of natural composites. For example, Boonrawd et al. (2021) prepared silica aerogel/natural rubber composite and analyzed the thermal insulation performance. They reported that the composites showed the low thermal conductivity compared to neat rubber and was decreased from 0.081 W/mK to 0.051 W/mK due to the inclusion of silica aerogel. Further, authors reported that the composites displayed the low density than the neat rubber. Authors suggested that the composite could be applicable in thermal insulation purposes in the products and buildings due to its better heat retardant ability. Muthukumar et al. (2020) manufactured natural composite using jute, banana, and pineapple and investigated the thermal conductivity and thermal resistance of the prepared composites. Results showed that the thermal conductivity of the composite exhibited inverse relationship with thickness and the thermal resistance displayed a direct relationship with thickness. Further, authors reported that the composites made with 60% banana, 32% pineapple, and 8% jute exhibited the minimum thermal conductivity and maximum thermal resistance. Hassan et al. (2020) fabricated green composites using natural fiber wastes and investigated the properties such as thermal, mechanical, and acoustic. Experimental results revealed that high coefficient of thermal expansion were noted for cotton fiber composites. Further, authors reported that all the prepared composites i.e. cotton, sugarcane and coconut, withstood heat up to 300°C with minor mass loss. Finally, authors suggested that the manufactured composites could be used in making of automobile parts, building interiors and household furniture. Eschenhagen et al. (2019) fabricated composites using sunflower stalk and miscanthus and investigated the insulation properties of the composites. Authors reported that the thermal insulation ability of the composites belonged in the range of other materials and found to be more flexible without any breakage during the bending. Muthuraj et al. (2019) analyzed the sustainable thermal insulation properties of bio-composites made with wheat husk, rice husk, textile waste, and wood fiber. Results showed that all the prepared composites showed thermal stability up to 250°C. Further, the thermal conductivity of the composites was observed in the range of 0.08 W/mK to 0.14 W/mK. The densities of all the composites were noted in the range of 378 kg/m 3 to 488 kg/m 3 . Rice husk composite displayed the low density and low thermal conductivity. Authors suggested that the prepared composites could be used in the building interior applications.
Generally, the natural fibers like jute, sisal, bamboo, hemp, kenaf, abaca, flax and coir are commonly used in the preparation of composites for industrial applications in automotive and furniture industries. Further, the mechanical properties of aforementioned composites were comparable with the mechanical properties of composites made with synthetic fibers like carbon and glass (Kumar et al. 2019). However, the thermal stability of the composites remains an issue in the industrial applications of the composites. Hence, the searching for composites with better thermal performance is continuing and researchers are looking for new composite from different sources. By keeping this in mind, an attempt has been made to develop new hybrid composites using banyan (Ficus benghalensis) and cotton (Gossypium) fibers. Banyan fiber could be a good source for the production natural composites (Ganapathy et al. 2019(Ganapathy et al. , 2021. Hence, the aim of this present investigation is to make hybrid composites with different combinations of banyan and cotton fiber and to study its thermal performance.

Materials
In this research, the materials such as banyan fiber, cotton fiber, epoxy resin, and hardener were used to fabricate the composites. Bisphenol-A based epoxy (LY556) with medium viscosity was used to fabricate the composites. The epoxy was pale yellow in color and used without any modifications. Further, the hardener (HY951) was used along with the epoxy for curing purpose, which results the thermo set cross-linked structure. The properties of epoxy resin are shown in Table 1. Fibers were supplied by Nano Wings Private Limited, Telangana, India. The properties of fibers are detailed in Table 2.

Fiber preparation
The banyan and cotton fibers were directly purchased from the supplier Nano Wings Private Limited, Telangana, India. Before making composite, the fibers were involved into alkalization process which modify the fiber surface as rough by combining the -OH of fiber with NaOH as below.
During alkalization, the fibers were treated with 5% NaOH solution to remove the wax, sand particles, and impurities from the fiber surface. The chemical treatment was done by immersing the fibers in the solution for four hours. After treatment, it was cleaned with distilled water and dried in sun light for one hour. Dried fibers were cut into the length of 10 mm.

Composite preparation
Composite was manufactured by compression molding method. A mold of size 200 mm × 200 mm × 3 mm was used. Fibers were eventually distributed in the mold without voids. Epoxy and hardener were mixed in the ratio of 10:1 (Sundararaju Perinbakannan, Karuppusamy, and Ramar 2021) and stirred well to achieve the uniform mixture. Prepared mixture was poured in the mold evenly over the fiber. The details of composite structure are shown in Table 3. A Teflon sheet was placed over the composites to allow in the compression molding. Composites were compressed in the hydraulic compression molding machine to the pressure of 100 bar at 80°C for 30 minutes and then kept at room temperature for curing purpose. Finally, the composites were sliced into required dimensions for experimental.  (Petrović et al. 2013

Density and porosity
Porosity (%) was calculated using the following equation.
where ρ T -theoretical density, ρ E -experimental density Theoretical density was determined using the following equation.
where W R -reinforcement weight fraction, ρ R -reinforcement density, W M -matrix weight fraction and ρ M -matrix density. Experimental density was determined by water immersion method using Archimedes principle (Nampoothiri et al. 2020).

Testing
FTIR spectrometer of Perkin Elmer R×1 was used to analyze the fibers in the spectrum range of 4000-500 cm −1 with the step of 4 cm −1 . Tensile test was performed in universal testing machine (Make: Instron 3369) according to the standard ASTM D3039 (Specimen size: 250 mm × 25 mm × 3 mm) and the cross-head speed was maintained as 5 mm/min. Flexural test was conducted in universal testing machine according to the standard ASTM D 7264 (Specimen size: 154 mm × 13 mm × 4 mm). Five samples were tested for each reading and the average value was considered for the analysis. The thermal properties like thermal conductivity, thermal resistance, and thermal diffusivity were measured using TCi thermal conductivity analyzer (C-therm). The linear coefficient of thermal expansion was determined using dilatometer (TA instruments). Samples were heated from 40°C to 120°C and the heating rate was maintained as 5 °C/min. Figure 1 shows the FTIR results of banyan and cotton fibers. The banyan fiber showed the broad peak at the 3280 cm −1 due to the presence of cellulose (Porras, Maranon, and Ashcroft 2015). This peak appeared owing to the OH stretching of hydrogen bond. Further, another peak was noted at 2909 cm −1 due to CH stretching which appeared because of the presence of α-cellulose (Maache et al. 2017). The spectrum observed at 1613 cm −1 belonged to O-H bending owing to the presence of hemicelluloses. Further, a small peak was observed around 1328 cm −1 which belonged to the bending vibration of C-O groups. The peak at 1010 cm −1 appeared due to C-OH vibration because of the presence of lignin (Senthamaraikannan et al. 2016). The cotton fiber showed the absorption peak at 3286 cm −1 belonged to OH stretching which confirmed the presence of cellulose (Tomczak, Satyanarayana, and Sydenstricker 2007). The spectrum observed at 2897 cm −1 belonged to the C-H stretching vibration because of the presence of cellulose and hemicelluloses (Satyanarayana, Guimarães, and Wypych 2007). The peak appeared at 1314 cm −1 was aroused due to the C-O groups of bending vibration. The peak appeared at 1029 cm −1 belonged to CO stretching vibration (Romanzini et al. 2012).

FTIR results of fibers
Density and porosity of the Banyan-Cotton composites were determined, and the results are detailed in Figure 2. It was seen from the results that the Banyan composite showed the lower density and cotton composite displayed the higher density. This was attributed by the density characteristics of corresponding fiber in the composites. Hybrid composites showed the middling values among the Banyan and Cotton composites due to the hybrid effect of fibers. Further, the difference between the theoretical and experimental densities exhibited the presence of voids in the composites. The maximum void percentage or porosity (approximately 5%) was noted for Banyan composites and rest of the materials displayed lower values. The difference in porosity was aroused between the banyan and cotton composites due to the different impregnation porosity of the materials during the manufacturing because of the intrinsic properties of fibers. Hence, it was concluded from this analysis that the composite fabrication was well executed with minimum amounts of voids percentage.

Tensile strength
Tensile strength of Banyan-Cotton composites was calculated and the results are shown in Figure 3. Results indicated that the Banyan composites displayed the high tensile strength of 75 MPa and the Cotton composites showed the low tensile strength of 62 MPa in this research. Owing to the high tensile strength of Banyan fiber, greater tensile strength was achieved in Banyan composites compared to that of Cotton composite. During tensile loading, the Banyan fiber involved in more load transfer that resulted in high tensile strength. Further, the hybrid composite B:C(3:1) exhibited the tensile strength of 70.7 MPa which is nearer to the strength of Banyan composites. Due to the hybrid effect of both Banyan and Cotton fibers, the B:C(3:1) composites experienced better stress transfer and showed significant improvement in tensile strength. Further, the presence of more amounts of Banyan fiber with high strength assisted to enhance the tensile strength of B:C(3:1) composite. In case of B:C(1:3), the improvement in tensile strength was not notable compared to Banyan and B:C(3:1) composites owing to the presence of more amounts of low strength Cotton fibers. Nampoothiri et al. (2020)) prepared Indian almond-Kenaf composites and studied the mechanical properties. Report showed that the K/I/K composite exhibited the high tensile strength owing to the placing of more amounts of high strength Kenaf fibers in the composites. This result is good agreement with the results of present study.

Flexural strength
Flexural strength of Banyan-Cotton composites was calculated and the results are shown in Figure 4. It was seen from the plot that the Banyan composite displayed the high flexural strength of 113 MPa and the Cotton composite exhibited the low flexural strength of 91 MPa. Owing to the high stiffness of Banyan fiber, the Banyan composite showed greater strength, whereas low stiffness Cotton was responsible for the low strength of Cotton composites. When the bending load was applied, the Banyan fiber assisted in more stress transfer that helped to improve the flexural strength of the composites. However, due to the low strength of Cotton fiber, the composite experienced earlier failure which resulted in low flexural strength. Further, the hybrid composite B:C(3:1) displayed the greater flexural strength of 106 MPa compared to other hybrid composites such as B:C(1:1) and B:C (1:3). This could be due to the placing of more amounts of Banyan fiber compared to Cotton fiber. Sundararaju Perinbakannan, Karuppusamy, and Ramar (2021) manufactured the hybrid composites using Indian almond and Banana fiber and investigated the mechanical properties. They reported that the high stiffness Indian almond fiber increased the flexural strength of the composites compared to Banana fiber with low stiffness. This result is well in accordance with the results of present study.

Thermal conductivity
Thermal conductivity of the Banyan-Cotton composites was determined and the results are shown in Figure 5. The Cotton composites exhibited the higher thermal conductivity and the Banyan composite displayed the lower thermal conductivity. Thermal conductivity of the composites depends on the density and porosity. High density material shows higher thermal conductivity and high porosity material shows low thermal conductivity (Hassan et al. 2020). In this research, a low thermal conductivity of 0.11 W/mK was noted for Banyan composite due to the presence of more porosity (5.12%) which was entrapped with more amounts of air. As the air was the bad conductor of heat, the  Banyan composites exhibited low thermal conductivity. In case of Cotton composite, a high thermal conductivity of 0.51 W/mK was observed owing to the low porosity (3.42%). Further, the experimental density of 0.973 g/cc and 1.214 g/cc was seen for Banyan and Cotton composites respectively, which assisted in low and high thermal conductivity for the aforementioned composites. Moreover, the hybrid composites showed the intermittent values between the Banyan and Cotton composites due to the hybrid effect of fibers.

Thermal resistance
Thermal resistance of Banyan-Cotton composites was calculated and the results are detailed in Figure 6. Thermal resistance is the ability of material to prevent the heat transfer which is related with the following equation (Hassan et al. 2020).
where R is the thermal resistance; h is the thickness and K is the thermal conductivity According to the equation, thermal resistance is the inverse of thermal conductivity as thickness is constant. From the Figure 6, it was noted that the Banyan composite showed the greater thermal resistance compared to other composites and the Cotton composite displayed the low thermal conductivity. Due to the lower thermal conductivity of Banyan composite as discussed earlier, the material exhibited the high thermal resistance. Similarly, the Cotton composite displayed low thermal resistance owing to the high thermal conductivity. In this research, a maximum thermal resistance of 18.1 × 10 −3 m 2 K/W was achieved for Banyan composite and minimum thermal resistance of 3.9 × 10 −3 m 2 K/W was obtained for Cotton composites. In case of hybrid composites, B:C(3:1) exhibited the higher thermal resistance compared to B:C(1:3) due to the low thermal conductivity as noted previously.

Thermal diffusivity
Thermal diffusivity of Banyan-Cotton composites was calculated and the results are detailed in Figure 7. The heat spread ability of the material is represented by the thermal diffusivity. As can be seen from Figure 6, the Banyan composite displayed higher thermal diffusivity and the Cotton composite exhibited the lower thermal diffusivity. Generally, the thermal diffusivity of the material depends on its chemical composition. Material with rich in cell members like cellulose and lignin shows the greater thermal diffusivity. As can be seen from Table 1, the Banyan fiber showed the higher hemicelluloses and lignin content that enhanced the thermal diffusivity of the composites. Similarly, the Cotton composite showed low thermal diffusivity due to the presence of low amounts of hemicelluloses and lignin. In this research, a maximum thermal diffusivity of 8.79 × 10 −7 m 2 /s was observed for Banyan composites. Further, hybrid composites showed the middling values between the Banyan and Cotton composites due to the hybrid effect of fibers.

Linear coefficient of thermal expansion (CTE)
Linear coefficient of thermal expansion of Banyan-Cotton composites was calculated and the results are detailed in Figure 8. CTE is used to understand the dimensional changes of the material with  respect to heat. Higher CTE represents the higher expansion of material under heat. Similarly, the low CTE indicates the low expansion of material. In this research, Cotton composite showed the higher CTE compared to all other composites and the Banyan composite showed the low CTE. Generally, the CTE in interconnected with the cellulose content of the material. As cellulose is the less stable cell member compared to hemicelluloses and lignin, the material with high amounts of cellulose shows the high CTE. Owing to the high cellulose contents observed in Cotton composites, it exhibited higher CTE. Further, the Banyan composite displayed low CTE due to the presence of less amounts of cellulose. In this investigation, a maximum CTE value of 3.40 × 10 −5 /°C was noted for Cotton composites and minimum CTE of 1.81 × 10 −5 /°C was observed for Banyan composite.

Morphological analysis
Morphological analysis was done on the tensile fractured samples of Banyan and Cotton composites. SEM images taken at the fractured samples are shown in Figure 9. It was seen from the images that the Banyan composite displayed the resin rich with short pullout of the fibers. This indicated that the bonding strength was good and involved in more load transfer. The SEM images of Cotton composite exhibited more number of broken fibers. Further, the pullout of fiber was large compared to Banyan composite. This could be due to the weak bonding strength compared to Banyan composite. This resulted in less amounts of load transfer during the tensile loading and the fibers were broken earlier. Hence, the morphological analysis confirmed the experimental results discussed previously in this research.

Conclusion
Natural composite was made from natural fibers such as Banyan and Cotton. Mechanical and thermal properties were analyzed for the sustainable thermal insulation applications. The following points were deduced from the present research.
• Banyan composite displayed the maximum tensile strength of 75 MPa and flexural strength of 113 MPa. This was attributed by the presence of high strength and high stiffness Banyan fiber compared to Cotton fiber. • Banyan composite showed the least thermal conductivity of 0.11 W/mK due to the presence of 5.12% porosity, which was filled with air and acted as thermal insulator. • Further, a high thermal resistance of 18.1 × 10 −3 m 2 K/W was observed for Banyan composites. • Banyan composite exhibited a greater thermal diffusivity of 8.79 × 10 −7 m 2 /s compared to all other composites. • A high CTE of 3.40 × 10 −5 /°C was obtained for Cotton composites and low CTE of 1.81 × 10 −5 /°C was achieved for Banyan composite. • It was concluded from this current research that Banyan composite could be applicable in sustainable thermal insulation applications as showed better thermal and mechanical properties.