Thermo-physical and fire properties of sustainable bio composites: Experimental and computational analysis

ABSTRACT Bio-composites were prepared by incorporating broom grass, fishtail palm, sansevieria fibers as reinforcement in unsaturated polyester resin using hand lay-up method. The thermo physical and fire properties of bio-composites has been examined by varying the fiber content (0 to 39 wt. %) and temperature (30–120°C). The thermal insulation and heat storage capability of samples was assessed using guarded heat flow meter and differential scanning calorimeter. The experimental results reveal that, as the weight fraction of fiber increased, the insulation capability of composites increased, whereas it decreased with the temperature. The response of specific heat and thermal diffusivity of composites with temperature was analyzed. Temperature distribution and heat transfer through the composite materials was computed using ANSYS fluent solver. The rate of heat transfer through the fishtail palm fiber composite is 14.02% lesser than glass fiber composite. Cone calorimeter was used to measure fire behavior of the samples and the results reveal that the broom grass fiber composite material possesses better fire resistance characteristics against fire hazard. Morphological study of composites was performed with the help of Scanning Electron Microscopy (SEM) for the visualization of homogeneity of surfaces. Developed bio-composites are the potential materials to replace petrochemical-based products.

fiber composites, the stringent rules imposed by several governments across the globe, the demand for green composite materials attracts the focus of the researchers to develop new materials. The straw and reed hybrid composite and sea grass fiber-based composites are the new eco-friendly materials for thermal insulation applications (Aliaksandr, Florindo, and Boltrushevich 2022;Mayer et al. 2022). Rajesh, Ratna Prasad, and Gupta (2018) studied structural behavior of elephant grass, sisal, and jute fiber-based materials and summarized that the elephant grass composite exhibited better mechanical properties than other two materials. Flexural and impact strength of alkali-treated short jute fiberreinforced composite was greater than the untreated composites (Rajesh, Ratna Prasad, and Gupta 2016). Sabarinathan et al. (2020) studied the structural behavior of fishtail palm fiber and summarized that this material is the promising material for structural applications. The mechanical properties of flax, date palm, and kenaf fibers were investigated and reported that specific strength of natural fibers was higher than glass fiber (Al-Sulaiman 2002;Malik, Ahmad, and Gunister 2021;van den Oever, Bos, and van Kemenade 2000). Ramlee et al. (2019) and Ramlee, Naveen, and Jawaid (2021) studied the thermal insulation capacity of oil palm empty fruit bunch and sugarcane bagasse fibers and concluded that safe, renewable, economic, and sustainable energy saving materials are the alternatives to wood. Abu-Jdayil et al. (2019) have noticed that the conductivity varied with temperature (0 to 60℃) from 0.166 to 0.17 W/ m/K for date pit-polyester composite and 0.366 to 0.456 W/m/K for banana-epoxy composite. Takagi et al. (2007) adopted the hot wire method to measure thermal conductivity of bamboo-Poly lactic acid (PLA) composite and concluded that thermal conductivity of bamboo-PLA composite is smaller than glass/carbon fiber-reinforced composites. Kim et al. (2006) measured the thermal conductivity of hemp, kenaf, flax, and sisal fiber/polypropylene composites. The experimental data indicate that the thermal conductivity of composites is in the range of 0.05-0.07 W/m/K at 48.5% fiber content. Kim, Dutta, and Bhattacharyya (2018) reported that the flammability and thermal decomposition of the composites is a prerequisite to choose suitable composites. High flammability of bio-fiber composites is the main disadvantage for various applications. Lee, Salit, and Hassan (2014) studied the fire behavior of kenaf/PLA material and concluded that the heat release rates considerably decreased with kenaf fiber content and emphasized that fire properties are significant factors for the composites in terms of safety. Earlier researchers Ducoulombier and Lafhaj (2017) and Lamrani et al. (2017) studied the fire behavior and smoke generation of bio-composites by varying the fiber content. The bonding between hydrophobic biopolymers and hydrophilic bio-fibers and low fire retardant are the drawbacks of green composites (Akampumuza et al. 2017). Vahedi et al. (2022) are analyzed the thermo physical properties and char formation characteristics of sandwich bridge decks at 850℃. Thermo physical properties depend on the combustion, direction of heat flow and density and concluded that the charring rate in the direction of reinforcement is 1.5 times higher than transverse direction. Flammability is one of the significant fire properties of material and that often limit the application of the composites (Kozłowski and Władyka-Przybylak 2008). The HRR decreased with the addition of kenaf and wool with polypropylene (Bhattacharyya et al. 2015). Natural fibers mainly consist of cellulose, lignin, and other substances. Cellulose is the main component of fiber and it offers good strength, stiffness, and structural stability (Fiore, Scalici, and Valenza 2014). The properties of natural fibers are summarized in the Table 1. It was noticed that the cellulose content of fibers considered in this study is in the range of 70 to 80% higher than the other natural fibers (Pratim Das and Chaudhary (2021). The natural fibers with high lignin content exhibit good fire-resistance properties (Fatima and Mohanty 2011). Thermal conductivity of natural fiber-reinforced composites depends on the pore structure, density, humidity, and temperature (Koru, Buyukkaya, and Kan 2022). Muthu Kumar et al. (2021) developed the needle punched nonwoven fabrics from textile waste for thermal insulation applications and noticed that thermal insulation of composite was increased with the porosity of nonwovens. The thermal conductivity of fibers aligned in longitudinal and transverse direction was investigated and reported that the fibers placed in transverse direction exhibited better insulation behavior (Ioannou, Hodzic, and Gitman 2017).
Several investigators studied the structural behavior of the natural fiber-reinforced composites. Nevertheless, the limited literature is available on thermophysical and fire behavior of bio-fiber composites. A comprehensive study on thermophysical and fire properties of bio-fiber composites does not exist in the literature to select suitable bio-composite material for different applications. Therefore, in this research work, comprehensive study on thermophysical and fire behavior of broom grass, fishtail palm, sansevieria fiber-reinforced composites was carried out and verified the potential use for thermal insulation applications.

Preparation of composites
Broom grass, fishtail palm, and sansevieria fibers were extracted using water retting method (Mohan Rao and Mohana Rao 2017). Extracted fibers were dried under sun light for a period of two days for the removal of moisture content. The preparation of the composites and tests carried out in this study is shown in Figure 1. Cobalt (1.5%) and Methyl Ethyl Ketone Peroxide (1.5%) are added to resin as accelerator and catalyst in the preparation of the matrix. Composite samples are prepared using Hand lay-up method. Resin is used as matrix and fibers are used as reinforcement in the preparation of composites. Releasing agent is applied on the surface of the mold to avoid sticking of samples to the mold. The mold is filled with the alternate layer of matrix and fibers. Pressure is applied on the filled mold using the rollers for the removal of entrapped air and is allowed to cure for a period of 24 h under the compressive load of 50 kN. The composite is taken out from the mold and cut it into the test samples as per ASTM standard size of 100 mm × 100 mm × 10 mm for testing the fire properties and a circular disc of 50 mm diameter with 10 mm thickness for the measurement of thermal conductivity. The prepared fibers and samples are shown in Figure 2.

Measurement of thermophysical properties
Thermal conductivity (W/m/K) of the samples was measured according to the procedure adopted in ASTM E1530-99 standards. Thermal conductivity meter (DTC 300 model: TA instruments) shown in Figure 3 is used for the measurement of thermal conductivity of composite samples under steady-state condition. The pressure applied on the specimen is 0.7 bar to maintain the contact between the sample and the guard surface. The thermal conductivity was measured in the temperature range of 30 to 120℃. Equations (1)-(3) are used for the calculation of thermal conductivity. In these equations, q is the heat flux (W/m 2 ), L is the thickness of the sample (m), and R is the resistance of sample (m 2 K/W). A differential scanning calorimeter (DSC) is used to test the specific heat of the composites. The density of the samples was measured using picnometer. The diffusivity of the composites was calculated using Equation (4).

Thermal characterization of composites
The conductivity of three materials at different weight fraction fiber is shown in Figure 4. It was noticed that, as the fiber content increases, the insulation capability of composite increased. This increase is due to the porosity of fiber. The thermal conductivity of air (0.026 W/m/K) is lower than the natural fibers. The presence of air in the pores of fiber increases with the content of fiber, therefore conductivity of the composites is decreased (Liu et al. 2012;Mia et al. 2017).
The experimental values of thermal conductivity of fishtail palm, sansevieria, and broom grass fiber composites are 0.163 W/m/K, 0.183 W/m/K, and 0.196 W/m/K, respectively, at weight fraction of fiber 0.39. The insulation capability of fishtail palm fiber composite is 16.8% higher than broom grass fiber composite. Fishtail palm composite could be considered as energy saving material in automotive and building industries. At weight fraction of fiber 0.39, the response of conductivity against temperature is depicted in Figure 5. The response of fishtail palm fiber composite is very less against temperature (300C to 1200C) and more for broom grass-based material. The conductivity of fishtail palm (0.176 W/m/K) is lowest and broom grass (0.198 W/m/K) is highest at 1200C. Thermal conductivity of sansevieria fiber composite is less sensitive to temperature compared to the other composites. The thermal response of three materials is depicted in Figure 6.
Material stores large amount of heat at higher value of Cp. The heat absorbed by the materials increased against temperature due to the molecules vibration. The intensity of vibrations was increased against temperature (Qiu et al. 2019(Qiu et al. , 2020(Qiu et al. , 2021. Among the three materials tested, Cp of broom grass material is maximum at room temperature and high temperature. Hence, it is recommended for heat storage applications. Specific heat of broom grass composite is 40.65%, 18.5% greater than sansevieria and fishtail palm fiber-based materials. The specific heat of broom fiber composite at 300C, 600C, 900C, and 1200C are 1493, 1521, 1586, and 1639 J/kgK, respectively, and it is increased with the temperature. Broom grass composite is better material compared to other two composites for energy storage applications due to its higher specific heat capacity. Thermal diffusivity is a measure of the rate of heat penetration into the material with respect to change in temperature and it is a significant parameter. The effect of temperature on thermal diffusivity is shown in Figure 7. Thermal diffusivity of fishtail palm fiber composite is lowest (0.86 × 10 −7 m 2 /s) compared to other composites. Hence, it is recommended to select the fishtail palm composite material for heating, ventilation, and air-conditioning applications. Thermal properties of three materials are given in Table 2. The density of the materials considered in this study is in the range of 1095 to 1312 kg/m 3 . Hence, these materials are suitable, where high specific strength is important. The comparative study reveals that the fishtail palm fiber material is the promising material to replace synthetic-based materials for thermal insulation applications.

Computational analysis on composites
A rectangular plate of size 1.82 × 0.91 × 0.02 m was created in ANSYS workbench and hexahedral mesh generated in plate with 56,645 elements. The plate dimensions and mesh generated is shown in Figure 8. ANSYS Fluent solver was used for computational analysis. The measured density, k, and C p of the composites were used as an input data in computational analysis. Glass and banana fiber-reinforced composite material properties have been taken from open literature and wood material properties were taken from the fluent data base. Energy equation was enabled in solver. Convective boundary condition was defined for inner surface of the plate exposed to conditioned air at 25℃. Natural convective heat transfer coefficient considered is 6 W/m 2 K for analysis. Wall boundary condition was defined for outer surface of the plate and assumed that this wall is at a temperature of 45℃. The remaining surfaces of the plate are considered as adiabatic wall for simulation. The contours of temperature on plate are shown in Figure 9 and the computed temperature at the inner surface of the plate is of 37℃. Total surface heat flux through the plate was computed from the software for each material for the defined boundary conditions and the data are given in Table 3.
Among all the composites, the total surface heat flux is relatively lowest for fishtail palm composite. This analogy emphasizes that fishtail palm composite thermal insulation was good and the experimental data are conveying the same. Temperature distribution along the plate thickness for different fiber-reinforced composites and wood is plotted in Figure 10.
The wall temperature on conditioned air side is lowest for fishtail palm fiber composite and lower temperature was observed across the thickness of plate compared with other composites. The     temperature distribution of the composites studied is very close to the wood. Therefore, these materials can be used in place of wood for energy-saving applications. The surface heat flux of glass material and fishtail composite is 78.85 W/m 2 and 69.1 W/m 2, respectively. Therefore, insulation capability of the tested composites is better than the glass fiber composite and the synthetic composites can be replaced by the eco-friendly developed composites in this study. The developed fibers are inexpensive and light weight. The density of the composites is in the range of 1095 to 1312 kg/m 3 whereas the density of glass fiber composite is 1760 kg/m 3 . The cost of the glass fiber is 1-2 $/kg but the fibers used for the development of composites are extracted from waste plants and the preparation cost for the fibers is in the range of 0.13 to 0.2 $.

Fire properties of composites
Fire behavior of bio composites was presented in Table 4. TTI is an important fire property because it defines how quickly combustion of composite will occur when exposed to a heat source. As per TTI results, the broom grass and fishtail palm fiber composites begin to catch fire slower than sansevieria. The pHRR of the broom grass, fishtail palm, and sansevieria fiber-reinforced composites was reduced by 36.4%, 35.6%, and 25.7% than pure resin. FIGRA is the ratio of pHRR to time to reach pHRR. Higher magnitude of FIGRA of material indicates that it easily catches the fire. FPI is the ratio of TTI to pHRR. Higher values of FPI indicate that the fire risk of composites is less. Broom grass composite performance is 37.8% higher than sansevieria composite. The broom fiber material MAHRE magnitude is less. EHC is less for broom fiber material (19.92 MJ/kg). SEA reflects the quantity production of smoke during the combustion process. The reduction of SEA of sansevieria fiber composite is 7.8% less than the matrix, and it produces less quantity of smoke. The comparative study reveals that fishtailbased material is appropriate fire retarding material. More amount of smoke generated during the process of combustion lead to create breathing issues to the human beings. TSR is lowest (12964 m 2 / m 2 ) for broom grass fiber composite. Broom grass fiber composite emits minimum CO (0.0328 kg/kg). The broom grass-based material is the better material for fire safety.

Morphology study
The structure of the samples is examined using a VEGA3-TESCAN SEM with 15 kV of accelerating voltage. A layer of gold coating was applied on a specimen in vacuum chamber to increase image resolution and to avoid an electrostatic charge. The measured range of diameter and length of the fibers are given in Table 1. The structures of the fishtail palm, sansevieria, and broom grass composites are illustrated in Figure 11. It was observed that the fibers are properly distributed and homogenized in the matrix. More pores are observed in fishtail palm compared with other two composites. The presence of these pores enhances the thermal insulation of the composite materials.  Figure 11. SEM images of composite (a,b) Fishtail palm 500 µm, 20 µm, (c) Sansevieria 500 µm, (d) Broom grass 500 µm.

Conclusions
Energy saving, biodegradable, eco-friendly, cheap, lightweight, sustainable broom grass, fishtail palm, and sansevieria fiber-reinforced composites were developed. The effect of fiber content and temperature on thermo physical properties of the composites was studied and fire characteristics of the composites were elucidated. This systematic study reveals that the thermal insulation of broom grass, fishtail palm, and sansevieria fiber-based materials is improved with the weight percentage of fiber. At maximum volume fraction, thermal conductivity of fishtail composite is lowest (0.0163 W/m/ K). All three developed composites possess good thermal insulation characteristics and are suitable for energy-saving applications. The broom fiber-based composite is the best fire retardant material compared to other two materials. The composites studied in this research are suitable for industrial applications to save energy loss. The heat transfer through the composite plate made up of fishtail palm, broom grass, and sansevieria fiber reinforced composites is lower than glass fiber composite and close to the wood. This quantitative assessment builds up confidence to use these composites in place of wood and glass fiber composite.