Synthesis and Experimental Thermal Adsorption Characteristics of Epoxy Hybrid Composite for Energy Storage Applications

Polymer-based matrix hybrid composites meet their demand in various engineering applications and food industries due to their excellent mechanical, thermal, corrosion, and biodegradable performance. The polymer-based hybrid composites have been a better choice for high thermal insulation at low cost. This experiment attempted to find the thermal adsorption characteristics, heat deflection temperature, linear thermal expansion, and thermal conductivity of epoxy hybrid composites, which contained four different layers of Kevlar and basalt fiber fabricated via a low-cost conventional hand mold layup technique. This experiment revealed that the effect of basalt/Kevlar fiber on epoxy increased thermal performance. The results noted that the hybrid composite consists of less Kevlar fiber with the maximum basalt fiber of sample 4, showed excellent thermal adsorption effect on weight loss limited at 70.98%, and a better heat deflection temperature and 11.78 × 10 − 6 per °C linear thermal expansion were obtained. Sample 3 exhibited a maximum thermal conductivity of 0.251 W/mK. However, the thermal adsorption of hybrid composite has been limited by more basalt fiber, leading to a 1 wt%/°C decomposition rate.


Introduction
Great potential with lightweight polymer matrix composites is considered the best alternative for replacing conventional plastic material in several engineering, food, and medical industries. Polymer matrix composites are invented by polymer resin bonded with secondary phase material like fibers. It has been produced quickly using conventional techniques at a reasonable cost. Epoxy resin is mainly used for polymer matrix fabrication to obtain a high feature of mechanical, thermal, and chemical resistance [1,2]. The polymer composite consists of various laminate fibers and dramatically impacts engineering manufacturing. It engages more percentage of the total production volume in various automotive applications [3]. The advanced grouping of natural and synthetic fiber composites is a better substitution for conventional composites due to their enhanced compatibility with resin [4][5][6]. More research investigation is done relevant to polymer matrix composite with various combinations of natural and synthetic fibers. The Kevlar/ basalt fiber combinations play a vital role in various industrial applications due to their enhanced thermal characteristics compared to other combinations [7][8][9]. The sandwichconfigured Kevlar-flax-epoxy hybrid composite found superior impact and tensile strength [10].
Similarly, the epoxy/basalt fiber composite laminate with various graphene nanopellets (GNP) showed increased tensile, flexural, and impact strength of the composite [11]. Recently, Kevlar has been grouped with aramid fiber, resulting in low density, good thermal behavior, high thermal stability, and high strength. It is used to replace steel material in various applications, like bicycle frames and racing cars [12]. The mixed vinyl ester composite grouping with flax/basalt fiber showed higher tensile strength with limited brittleness and stiffness behavior [13]. The thermal conductivity of hemp-reinforced polymer composite is studied and experimentally measured and compared with that of the poly matrix. It showed good relation between theoretical to actual [14,15]. The granite/basalt fiber/sandstone-reinforced polymer composite found better thermal stability at 25°C and 1000°C [16]. The E-glass-reinforced epoxy composite was developed via a conventional method to find increased thermal behavior [17]. The impact strength of glass fiberreinforced polymer laminates shows a better resistive force on high-impact load [18]. The Kevlar-basalt fiber-mapping polymer composite with three layers thick showed better mechanical strength. It is impacted with 40-60 m/sec velocity, increasing the bonding strength [19]. The thermal conductivity of composite varies for the following reasons: fiber orientation, base material, number of layers, and method of composite fabrication [20]. Incorporating fiber content in large volumes can increase thermal characteristics and provide good mechanical strength [21]. The thermal gravimetric analysis of the glass/carbon/Kevlar/polymer matrix hybrid composite showed minimum weight loss during hightemperature evaluation [22]. The following factors may affect the thermal characteristics of the composite fiber: length, diameter, mixing ratio, and defects [23]. The main aim of the experimental work is to evaluate the thermal adsorption characteristics of epoxy composite hybridization with Kevlar/basalt fiber via the conventional hand layup technique. The thermal behavior, like thermal conductivity, heat deflection temperature, and linear thermal expansion of composite, is also evaluated by different thermal conditions.  [7][8][9]. The mechanical and thermal properties of both fibers are listed in Table 1.

Synthesis of Hybrid
Composite. The epoxy resin and hardener mix at a ratio of 15 : 1 through manual stir action. The dimensions of 300 mm × 100 mm × 2 prepare the Kevlar   Figure 3(a), and their developed composites are illustrated in Figure 3(b). The prepared KF/BF/BF/KF composite mat was kept in a hydraulic compression machine configured with a 1-ton capacity. The composite was compressed with an applied compressive force of 1 KN maintained for the next 1 hr. It helps increase adhesive properties [8,9]. Finally, the developed composites are shaped as per test standards. The decomposition rate of the hybrid composite was measured by its composition, pyrolysis, and material volatility. The composite's thermal adsorption on mass loss decomposition percentage was examined from 25°C to 670°C at a 27°C/min heat flow rate. Normal airflow may lead to material decomposition during inert atmospheric conditions, whereas increased temperature may prevent this [23][24][25]. The various stacking sequences of KF and BF layer results showed lower weight loss (less than 40%) at higher temperatures (more than 240°C). The thermal adsorption   Table 2 depicts the weight loss of a hybrid composite with various stacking sequences, which results in superior thermal adsorption properties. It is observed from Figure 4 and the corresponding Table 2 that the percentage of weight loss due to high-temperature decomposition is 58% related to sequences of hybrid composites. The thermal reactions were observed during the evaluation of thermal adsorption behavior, and the effect of KF/BF on hybrid composite showed an exothermic effect of heat able to decompose the structure. However, stacking different sequences could resist the decomposition rate during high temperatures. It forms V and U curves related to the exothermic effects of Kevlar and basalt fiber. Samples 1 and 2 found as U and V exothermic curves represent the effect of exothermic during the higher temperature of decomposition, and their percentages are mentioned in Table 2. The decomposition rate of sample 4 was limited by 1% as compared to sample 1. Figure 5 illustrates the effect of KF/BF on the heat deflection temperature of an epoxy hybrid composite evaluated by different stacking positions of KF/BF, as shown in Figure 3(a). It is observed from Figure 5 that the heat deflection of epoxy hybrid composite samples 1-4 found a variation value with increased thermal conductivity. The higher heat deflection temperature of 105:74 ± 1:56°C/Kg was identified in sample 1 with its thermal conductivity range of 0.198 W/mK. While compared to sample 1, the heat deflection temperatures of samples 2, 3, and 4 were noted as the decreased value of 66:91 ± 1:29°C/ Kg, 71:18 ± 1:72°C/Kg, and 69:23 ± 1:57, respectively. However, the basalt fiber-faced epoxy hybrid composite has a low heat deflection temperature compared to the Kevlar fiber-faced epoxy hybrid composite. The basalt fiber laminates with epoxy resin have performed extreme thermal stability behavior [11]. Sample 2 was noted by a minor heat deflection temperature of 66:91 ± 1:29°C/Kg. It was due to the effect of basalt fiber effectively bonded with epoxy material by an applied compressive load of 1 KN. The basalt fiber exhibited good mechanical and thermal characteristics [24]. The heat  deflection evaluation of the composite helps find the thermal stability of the composite [26]. Moreover, the coated ceramic material has to lead to good mechanical and thermal charac-teristics [27]. Both sample 2 and sample showed the least heat deflection temperature. So these samples may promote good insulation for energy storage applications.

Coefficient of Linear Thermal
Expansion. The coefficient of thermal expansion of various stacking sequences of a developed KF/BF hybrid composite bonded with epoxy was evaluated by the relations between heat supplied and temperature difference. Figure 6 presents the coefficient of thermal expansion of hybrid composites (samples 1, 2, 3, and 4). It is evidenced in Figure 6 that the composite, which contained a multilayer of KF and BF, shows a good coefficient of thermal expansion compared to a similar fiber-fabric hybrid composite. The coefficient of thermal expansion of samples 1 and 2 is found to be 11:42 × 10 −6 per°C and 10:71 × 10 −6 per°C. Heat transformation is separated by a multilayer fabric mat composite with different sequences. The multifabric arrangement with 0°and 90°orientations makes a perfect heat transfer during the isotropic process [28]. Sample 4 has an excellent thermal expansion coefficient of 11:78 × 10 −6 per°C, and effective heat transfer was found between the fabric mats via an epoxy matrix. Kevlar and basalt fiber contribute to thermal expansion with limited effective binding performance. Sample 4 represents that the coefficient of thermal expansion of hybrid composite with basalt fiber-faced epoxy hybrid composites (BF/KF/KF/BF) reduces the effect of maximum heat transformation. Figure 7 depicts the effect of KF/ BF on the thermal conductivity of epoxy hybrid composites with different stacking sequences. It is noted from Figure 7 that the hybrid composite with similar layers like KF/KF/ KF/KF and BF/BF/BF/BF epoxy hybrid composite samples 1 and 2 showed the most negligible thermal conductivity of 0:198 ± 0:18 and 0:196 ± 0:11 W/mK, respectively. At the same time, the multilayer KF/BF combinations of samples 3 and 4 of the epoxy hybrid composite were found to have good thermal conductivity compared to samples 1 and 2. The composite grouping with KF/BF/BF/KF (sample 3) showed a higher thermal conductivity of 0.251 W/mK. The thermal conductivity of sample 3 was 26.6% higher than that of sample 1. It was due to the effective basalt fiber layer occupied between Kevlar fibers. However, the thermal conductivity of epoxy composites was enriched by Kevlar fiber bonded with epoxy with a BF interlayer. The Kevlar fiber has good thermal conductivity [14,15,17].

Thermal Conductivity.
The stacking thermal gravimetric curve of Kevlar and basalt fibers has a low thermal conductivity compared to other multilayer KF/BF fiber hybrid composites. The Kevlar fiber facilitates good heat transfer between the layers more than once. However, the basalt fiber leads to enhanced thermal conductivity in composites.

Conclusions
The effect of Kevlar and basalt fiber on thermal adsorption, heat deflection temperature, coefficient of linear thermal expansion, and thermal conductivity of epoxy hybrid composite was studied experimentally, and their results were compared to obtain optimum properties. A low-cost conventional hand layup technique successfully fabricated the epoxy hybrid composite. Samples 3 and 4 of multilayer (KF/BF with epoxy) composites performed good thermal performance compared to the monolayer (KF and BF with epoxy) samples 1 and 2. Sample 4 (BF/KF/KF/BF) had an excellent thermal adsorption effect and a reduced mass loss during decomposition of 70.98%. Similarly, the heat deflection temperature and coefficient of linear thermal expansion are offered with basalt fiber at 69.23°C/Kg and 11:78 × 10 −6 per°C. The multilayer sample 3 epoxy hybrid composite found good thermal conductivity, with a value of 0.251 W/ mk, and has improved by 26.6% compared to sample 1. However, the basalt fiber-faced sample 4 epoxy hybrid composite observed good thermal adsorption behavior and limited mass loss with controlled decomposition with reduced heat deflection temperature acting as better insulation for heat energy storage applications in power plant sectors.

Data Availability
All the data required are available within the manuscript.