Effect of coir fiber and inorganic filler hybridization on Innegra fiber-reinforced epoxy polymer composites: physical and mechanical properties

The present investigation compares the results of different fillers in terms of the physical, mechanical, and thermal characteristics of epoxy polymers. These epoxy hybrid composites were produced using a mechanical stirring-assisted wet lay-up method with coir microparticles, fly ash, titanium carbide (TiC) nanoparticles, and Innegra fabrics by mechanical stirring with a stirring rod. The tensile, flexural, and interlaminar shear characteristics of the fabricated epoxy hybrid composites were determined using a universal testing machine. Reinforcement with fly ash and TiC nanoparticles offers the most remarkable improvement in tensile, flexural, and impact strength, at approximately 2.84, 1.65, and 9.19 times compared with pure epoxy polymer. Differential scanning calorimetry and thermogravimetric analysis showed that the epoxy hybrid composites had enhanced thermal stability. The homogeneity of filler dispersion in the epoxy polymer was observed by scanning electron microscopy.


Introduction
Epoxy-based resins are broadly utilized in polymer laminates, automobiles, aerospace applications, adhesives, paint fabrication, surface coating methodology, and other engineering materials (May and Tanaka 1973;Rs 1979;Ramesh et al. 2020;Vinod et al. 2020;Yorseng et al. 2019;Arpitha et al. 2017;Mohit and Selvan 2020a, b;Mohit and Selvan 2018). The good performance of epoxy composites in these applications is due to their higher corrosion and chemical resistance, higher mechanical and thermal characteristics and lower shrinkage, which can be seen under specific environmental conditions (Adams and Gannon 1986Gannon ,2020Gannon ,2021Yashas et al. 2019;Sanjay et al. 2019). Cured epoxy polymer exhibits a 3D (three-dimensional) extremely cross-linked chain scheme. The characteristics of epoxy polymers are strained in different applications because of a higher range of cross-linking density, which is influenced by the intrinsic brittle behaviour of components and lower toughness. Various investigations have been performed to enhance the toughness and other characteristics of epoxy polymers, such as the thermal stability, dynamic mechanical properties and crack resistance, by adding different types of nanoparticles to a polymer (Duraibabu et al. 2014;Mohit et al. 2020;Jagadeesh et al. 2020;Dinesh et al. 2020;Vinay et al. 2020;Ganapathy et al. 2019;Abhisek et al. 2018;Ganesan et al. 2020). The uniform dispersion of nanoclusters inside a polymer possesses two primary nanolaminate fabrication issues (Sun et al. 2015; Mohit and Selvan 2020a, b;Mohit and Selvan 2019a, b, c). The dispersion and cluster size of nanoparticles in polymer resin shows confident development in pure polymer fundamental characteristics with stronger interfacial bonding within the molecular networks and fillers (Radoman et al. 2014;Bal 2010;Jenish et al. 2020;Setty et al. 2020a, b;Setty et al. 2020a, b). Particle accumulation occurred due to nonuniform dispersion and van der Waals forces in an epoxy polymer, which reduced the material properties. There are many methods used to prevent these accumulations in epoxy polymers for the fabrication of nanolaminates, including manual stirring, melt mixing, solution mixing (Becker et al. 1996;Rong et al. 2001), direct addition to chemical route techniques (Kumar et al. 2017;Kang et al. 2001), acoustic cavitation (Eskin 2001), ultrasound vibrations (Wang et al. 2001), ultrasonic waves (Kumar et al. 2016;Xu et al. 2004), and ultrasonic irradiation (Xia and Wang 2003;Zunjarrao and Singh 2006;Halder et al. 2012a, b;Tsekmes et al. 2015).
The reinforcement of inorganic fillers such as carbon-based fillers, calcium carbonate, silica, calcium sulfate, fly ash, and ceramic fillers in polymeric resin-based laminates can decrease the drawback of plant fibers and improve the characteristics of the laminates (Abdel-Gawad et al. 2018;Yuan et al. 2016;Cui et al. 2017;Khaleghi et al. 2017). Among these fillers, fly ash is a reinforcing agent that can adequately enhance the thermal resistance, impact, and modulus of polymers, owing to its other advantageous properties, such as chemical corrosion resistance, lower cost, higher mechanical properties, availability, higher mechanical strength, and more necessary usage of industrialization (Dahalan et al. 2018;Liu and Zhong 2014). Fly ash is the primary solid waste released from the pulverized coal combustion process in thermal power plants. It is now the best individual resource of solid waste management (Ding et al. 2017;Temuujin et al. 2019;Wang et al. 2014). The consistency, surface chemical reaction, and mineralogy of fly ash have essential influences on its utilization (Yao et al. 2015;Hower et al. 2017), such as industrial wastewater treatment, catalyst carriers for the modification of poisonous gases, raw materials for plastics, adsorbents, silica aerogels, soil conditioners in agriculture, and construction materials as an admixture (Asl et al. 2018(Asl et al. , 2019Cai et al. 2020). Fly ash consisting of 47% silica epoxy-based laminates showed higher flexural and tensile characteristics up to 3 wt% reinforcement and further decreased these properties. Fly ash-based epoxy composites decrease comprehensive cost production with higher mechanical characteristics. Polyethylene/banana laminates improved the hardness, flexural, and tensile characteristics up to 37, 38, and 17%, respectively, with the addition of fly ash (Satapathy and Kothapalli 2017). Ajorloo et al. (2021) studied the effect of recycled polypropylene and fly ash constituents in polypropylene-based composites for automotive applications. It has been shown that there is an enhancement in the elongation at break, whereas other mechanical properties are reduced due to microstructural mutation and inappropriate interactions between polymeric chains and fly ash particulates. Sathishkumar et al. (2021) examined the influence of lignite fly ash on the mechanical properties of banana fiber-reinforced epoxy composites. They observed that 5 wt% lignite fly ash had a higher mechanical strength and exhibited minimal fiber pullouts and voids, as examined by SEM. Sumesh et al. (2021) investigated the effect of banana, coir, and pineapple fly ash as reinforcements to epoxy polymers for their mechanical and morphological characteristics. It was observed that the filler adhered between the matrix/fiber and improved the mechanical characteristics. Similarly, the impact and flexural characteristics improved by 21.77% and 22.11%, respectively, with the addition of these fly ash fillers, which explains the excellent bonding nature by utilizing filler particles. Baheti and Wang (2021) studied the ohmic heating and mechanical stability of carbon fabric/green epoxy composites after adding milled and unmilled fly ash particles. The composites reinforced with milled fly ash exhibited higher mechanical stability than unmilled fly ash laminates, which signified faster heat transportation features from milled fly ash particles to prevent matrix melting around carbon fabrics. The minimal loss in mechanical characteristics was observed because of the increase in the accumulation of fly ash particles under ohmic heating. In our previous investigation, the impact of TiC nanoparticles and fly ash on mechanical characteristics such as flexural strength, interlaminar shear strength, tensile strength, shore D hardness, impact, and thermal stability were measured. The incorporation of TiC nanoparticles, fly ash, and coir fiber in the epoxy improved the mechanical and thermal properties of composites, which can be ascribed to the uniform distribution between the epoxy and fillers (Kavya et al. 2021). Mohit et al. (2021a, b) studied the effect of coir fibers and TiC nanoparticles as reinforcements under both bio-and synthetic epoxy composites to evaluate the mechanical and thermal characteristics. The results showed that the addition of TiC nanoparticles enhanced the tensile strength by 4.99%, flexural strength by 115.05-124 MPa, and thermal stability by up to 402.71°C.
In the literature, it has been observed that limited investigations have been conducted to manufacture hybrid composites comprised of coir fiber and epoxy resin with fly ash and TiC as filler materials. Titanium carbide (TiC) nanoparticles are known for their higher thermal stability, higher surface area, and excellent mechanical characteristics, making them more prominent inferior aspirant fillers (Mallakpour and Khadem 2015;Omrani et al. 2009;Arshad et al. 2021;Mohit et al. 2021a, b). In the present study, an experiment was conducted to fabricate composites combined with Innegra fibers, fly ash, and TiC fillers comprised of coir fibers in epoxy polymers. The effects of fly ash and TiC particles in coir fiber-Innegra fiber-epoxy composites on morphological, mechanical, and thermal characteristics were measured by tensile, impact, interlaminar shear strength, flexural, hardness, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) testing. The present investigation exhibits the influence of fly ash and TiC nanoparticles as a reinforcement material to reduce the fractures, voids and deformations in the polymeric chain and how the mechanical and thermal characteristics of coir fiber-Innegra fabrics of epoxy polymer composites are influenced.

Materials
In the present investigation, Epotec YD-535 LV synthetic epoxy and Epotec TH-725 hardener were used as polymer matrices and procured from Aditya Birla Chemicals Ltd., Thailand. Coconut fibers purchased from Tongmongkol Coconut fiber, Prachuapkhirikhan, Thailand, were applied as reinforcement material for the manufacturing of synthetic epoxy laminates. Two types of inorganic fillers were utilized in this investigation: TiC (10 nm particle size) was supplied by Zhuzhou Sanyinghe International Trade Co. Ltd., China, and fly ash was collected from an industrial chimney at the Chennai Thermal Power Plant and applied as another filler reinforcement to the matrix. The chemical composition of fly ash is given in Table 1. A novel Innegra fiber was purchased from Innegra Technologies, South Carolina, United States in the form of a woven mat and applied as a reinforcement material to the synthetic epoxy polymer.

Fabrication of synthetic epoxy hybrid composite
The purchased coconut fibers were cleaned with fresh water to eliminate the dirt particles and dried in sunlight for three days to remove the moisture content. The cleaned coconut fibers were sectioned into small pieces and transferred to a universal milling machine to produce microfillers from coconut fibers subjected to sieving and separated with a 250 lm filter. The synthetic hardener, epoxy polymer, and reinforcement fillers were weighed accurately in different ratios with the help of a weighing balance. The total volume fraction of combined fibers and fillers is 7 vol% of the epoxy resin, which is the same in every sample. The details for the fabrication of the epoxy hybrid composite are shown in Table 2. Mechanical stirring was applied to achieve a homogeneous distribution of fillers in the epoxy polymer. The combined mixture was rapidly transferred to the die mould to avoid premature curing and discharge for a slower curing process. The fabricated sample was removed from the die mould and shifted to the furnace for post-curing at 80 ± 2°C to decrease the sample's residual moisture.

Characterization of fabricated epoxy hybrid composite
Fourier Transform Infrared (FTIR) spectrum FTIR spectra analysis was conducted with a sample comprised of a KBr (potassium bromide) filler from a Perkin Elmer Spectrum 2 FTIR spectrometer. The epoxy hybrid composite wavenumber ranged between 4000 and 400 cm -1 with a resolution of 1 cm -1 under 32 scans per minute.

Water absorption capacity
According to the ASTM D 570 standard, the water absorption capacity was determined for epoxy hybrid laminates. The specimen was dried in a heating furnace for a specific period at 80 ± 2°C, and the initial mass of the specimen was determined. The fabricated epoxy hybrid laminates were then immersed in fresh water at 24 ± 2°C for 3 months. After immersion, specimens were removed, excess water was removed with a cotton cloth, and the final weight was measured. The variation between the final and initial mass was determined, and the amount of water absorption was estimated as per the standard.

Contact angle
The contact angle test was performed in 15 LJ data physics. Optical contact angle (OCA) under 26 ± 2°C and 1 lL of deionized water were poured on the surface of epoxy hybrid laminate with the help of a syringe. The left and right contact angles were determined, and for each specimen, the mean value of five parts was recorded as an outcome.

Mechanical characteristics
Mechanical characteristics are essential for the utilization of composite materials. The specimens (width 9 length 9 thickness: 15 mm 9 100 mm 9 3 mm) for the tensile experiments were examined as per the ASTM D 3039 standard. The specimens  Fig. 1. The Shore D hardness of the fabricated synthetic epoxy hybrid composites was determined by an indenter-based hardness measurement machine. The indenter was pressed on the sample's surface up to the bottom portion of the indenter in full contact with the composite. The dial of the hardness indenter signifies the Shore D hardness of the epoxy hybrid composites. The experiments were carried out under ten different positions, and mean hardness outcomes were determined. All experiments were performed under 60% relative humidity and 24 ± 2°C room temperature.

Fractographic evaluation
The microstructures were observed with a scanning electron microscope (FEI 450 Quanta) with a 15 kV accelerating voltage to examine how different reinforcements influence the polymer structure.

Thermal characteristics
Differential scanning calorimeter (DSC) graphs from 20 to 300°C were created using a Mettler Toledo DSC 3?/700/2167 machine in a nitrogen atmosphere under a heating rate of 10°C/min. Thermogravimetric analysis (TGA) was performed in a Mettler Toledo TGA 2 TGA/DSC 3?HT/1600 machine to examine the epoxy hybrid thermal stability composites. The specimens were heated between 30 and 700°C under a nitrogen atmosphere with a heating rate of 15°C/ min. The mass loss was determined, and the residual char content was measured from 600°C with the help of a derivative thermogram (DTG).

Results and discussion
Effect on FTIR spectra Figure 2 exhibits the FTIR spectra of the pure epoxy polymers and their respective laminates based on the filler and Innegra fabric reinforcement to examine the effect of inorganic fillers on epoxy functional groups. The pure epoxy polymer shows a clear carboxyl peak at 1654 cm -1 corresponding to C-O stretch functional groups. Other peaks at 1490 cm -1 and 1596 cm -1 are ascribed to a benzene ring. Two constrained intensity peaks between 930 and 770 cm -1 and the peak at 1226 cm -1 relate to the asymmetric and symmetric stretching peaks of R-O-R, respectively (Niu et al. 2014(Niu et al. , 2021. The effect of various inorganic filler constituents on the epoxy polymer molecular chain primarily followed the peak at 630 cm -1 . The vibration types in this region are obscured and imbricating, and the features are terrible. The FTIR spectrum signifies that epoxy polymer and laminate specimens exhibit the evident characteristic peaks of epoxy polymer. The chemical scheme of epoxy polymers is not considerably modified by the incorporation of different fillers. In the FTIR spectra of the CFIPE and CFTIPE laminates, two other characteristic peaks can be observed at 1430 cm -1 and 880 cm -1 , which can be assigned to the asymmetric and out-of-plane bending vibrations of the CO 3 2functional groups in fly ash (Jena and Sahoo 2019). Hence, it can be proven that calcium carbonate is the principal constituent of fly ash. Hydrogen bonds are efficiently produced between the hydroxyl regions. When this occurs, the hydroxyl stretch vibration shows red movement in the infrared spectra. The production of hydrogen bonds between the hydroxyl functional groups affects this principle, known as the effect of hydrogen bonding (Finch 1970).
From the infrared spectrum, the hydroxyl stretch vibrations of CIPE, CFIPE, CTIPE, and CFTIPE composites were 3430 cm -1 , 3431 cm -1 , 3440 cm -1 , and 3445 cm -1 , respectively. As shown in CIPE, CFIPE, CTIPE, and CFTIPE, the incorporation of fillers resulted in the production of more hydrogen bonds between coir fiber, epoxy, fly ash, TiC, and Innegra fabric, signifying an increase in crosslinks among the various constituents (Zhang et al. 2018). This outcome is the production of a close compact system; hence, enhanced mechanical characteristics can be described in these laminates (Deshmukh et al. 2021). The hydroxyl stretch vibration of the CISE composite was lower than that of CFIPE, CTIPE, and CFTIPE due to the presence of inorganic fillers (Wu et al. 2020). Furthermore, this can produce a new hydrogen bond with the polymer as an essential Fig. 1 Coir fiber// Innegra fiber// TiC/fly ash synthetic epoxy hybrid composites: a before tensile test, b after tensile test, c before flexural test, d after flexural test, e before short-beam interlaminar shear strength test, f after short-beam interlaminar shear strength test, g before impact test, and h after impact test constituent of fly ash, TiC, and Innegra fabric, which may also induce hydroxyl functional groups to create hydrogen bonds with the polymer. Furthermore, the crystallinity index more actively influenced the mechanical characteristics than the hydroxyl stretch vibration, signifying that crystallinity was the primary parameter that impacted the mechanical aspects of the polymer composites (Ji et al. 2021;Sun et al. 2019).
Effect on physical properties Figure 3a shows that after 90 days, water absorption attains 2.48% (CTIPE), 2.01 (PE), 1.83% (CFIPE), 1.44% (CIPE), and 1.42% (CFTIPE). The CTIPE composite consumed more water than the other combinations due to the greater pore structure. A similar outcome was observed by Ajdary et al. (2020), who concluded that greater pores lean towards higher water absorption under capillary action. Furthermore, the particular kind of filler did not noticeably modify the quantity of water consumed. This can be assigned to all the specimens being porous and consuming more water under capillary action. However, polymers reinforce most hydrophilic functional groups, and CFTIPE, CIPE, and CFIPE are more hydrophobic than polymers (Deshmukh et al. 2021;Ajdary et al. 2020;Zhang et al. 2017b, a). The particles were uniformly distributed in the laminates, improving their durability with water. The hydroxyl functional groups of CIPE and CFTIPE influenced them to combine strongly with the laminate, resulting in this marginally decreasing the quantity of water consumed from the CIPE and CFTIPE laminates. The water contact angle is an  In 5 min, the PE, CFIPE, CFTIPE, and CIPE composites were near the hydrophobic region, whereas the CTIPE was hydrophilic. The fillers enhanced the water resistance and almost attained the influence of waterproofing (Sun et al. 2019). The contact angle of the laminates decreased with the addition of fillers, which is consistent with the published literature (Wu et al. 2020). In this context, the dense architecture affected by the increment of crosslinking and the capillary action of pores of various diameters also result in dissimilar water contact angles (Deshmukh et al. 2021;Ajdary et al. 2020).

Effect on mechanical properties
Tensile properties Figure 4a presents the stress-strain graphs of the epoxy hybrid composites reinforced with different fillers. The influence of other filler additions on the tensile strength and modulus of epoxy hybrid composites was measured and is presented in Fig. 4b. Figure 4b shows that the tensile modulus and strength of the CIPE, CFIPE, CTIPE, and CFTIPE composites improved by 2.89, 1.71, 2.52, and 2.84 times, respectively, when compared with the pure epoxy sample. The improvement in the strength and modulus of the epoxy hybrid composites was based on the dispersion of fillers in the polymer and bonding between the matrix and particles. The uniform distribution of particles in the complete polymer acted as a specific characteristic to enhance the tensile characteristics of the laminate. The testing process efficiently damages the cluster size of the filler and disperses the filler uniformly incomplete polymer. The CFIPE laminate presented lower tensile properties because of the cluster size increment and density of the filler. The tensile characteristics of the CTIPE and CFTIPE composites were reduced compared with those of the CIPE composite. The reduction in tensile characteristics under this reinforcement can be explained by an incrementing susceptibility of accumulation, which tends to lower energy dissipation in the scheme under the deformation of viscoelasticity. Due to the increment in accumulation size and weight, the movement between molecules and chains increases, which tends to the end of the elastic region and reduces the tensile characteristics. The larger cluster size of the filler also decreases the bonding of the filler-polymer and offers a more straightforward method to increase the initial damage during tensile loading.
Other investigators have observed similar results: the improvement in strength is attributed to the zirconia filler having much better stability than the epoxy polymer and to the strong bonding between the polymer and filler that allows suitable load dispersion in hybrid laminar regions (Ma et al. 2018;Kumar et al. 2021). The decrease in strength is ascribed to the appearance of a higher number of clusters. Furthermore, a homogeneous cluster-free distribution and strong bond interaction of optimum filler reinforcement with polymers permit part of the load transfer, leading to the epoxy composites' considerable tensile characteristics (Haldar et al. 2012a, b).

Tensile fracture
The SEM micrograph of the tensile experimental samples is shown in Fig. 5. The fracture microstructure of the epoxy polymer sample examines the crack and brittle behaviour, as observed from previously published literature (Mohit et al. 2021a, b). Figure 4a shows that the CIPE sample affirms a portion of inadequate filler dispersed from the epoxy polymer and matrix fragmentation. Extensive matrix damage and failure of a filler-matrix interface are also apparent. SEM micrographs of CFIPE signify that a portion of the Innegra fiber prevented and constrained microcracks. The addition of fly ash decreases the bulging of Innegra fiber bundles and attempts at damage at an initial point of reinforcing by prohibiting crack generation (Fig. 4b).
The SEM micrograph of CTIPE shows the isolated epoxy/TiC/coir fiber cluster and the Innegra fibers forecast to the outer part. Debonding of the Innegra fiber-epoxy/TiC/coir fiber is found in certain portions because of the failure of interfacial bonding (Fig. 4c).
Furthermore, compared with the neat epoxy and CIPE, in this condition, almost all the Innegra fibers imply attachment to the epoxy/coir fiber even after disintegration. This signifies better bonding between the Innegra fabric and epoxy/coir microparticles. SEM micrographs of the CFTIPE composite observed at the fractured part represent disintegration, whereas the Innegra fabric-fly ash-TiC-epoxy bonding does not isolate in certain parts even after the damage, which signifies the higher adhesive load between them. Filler accumulation, Innegra fabric damage, debonding, and inadequate filler-polymer adhesion portions are also observed in the SEM micrograph of the CFTIPE tensile experimental sample (Fig. 4d) (Rao et al. 2021).

Flexural properties
The flexural stress-strain graphs are shown in Fig. 4c. The flexural experiment outcome in Fig. 4d shows that the incorporation of TiC, fly ash or both particles affected the flexural modulus and strength. The epoxy polymer resin reinforced with coir microparticles, fly ash, and Innegra fibers shows the highest flexural strength, showing a 127.22% enhancement over the neat polymer. The fly ash filler possesses a greater specific surface area than the TiC nanoparticles. Thus, when comprised with polymeric networks, the fly ash particles can produce stronger bonding, and a flexural load can be shifted through these inorganic filler networks to other enclaves. Even though one network can be damaged under external force, others can still hold the laminate structures, and the whole sample is not compromised. It can also be concluded from the outcome that TiC incorporation is considered to Interlaminar shear strength in MPa   Fig. 4 Mechanical properties of coir fiber/ Innegra fiber/TiC/fly ash synthetic epoxy hybrid composites: a tensile stressstrain, b tensile properties, c flexural stress-strain, d flexural properties, e ILSS stress-strain, and f ILSS enhance the modulus, potentially because of uniform fillers in the epoxy polymer that rigidly support the nearby Innegra fabric layers. The versatility of the inorganic filler in improving the flexural strength decreases beyond the crucial addition of fly ash because of the accumulation of inorganic fillers (Shivamurthy et al. 2013;Taurozzi et al. 2012). This filler accumulation is affected by an increase in fillerfiller interactions during the mechanical stirring process (Sudheer et al. 2013). Filler accumulation would potentially increase the polymer-filler interruption viscosity and affect bad wetting reactions between them (Sudheer et al. 2013). Hence, the lower enhancement in the flexural characteristics was endorsed for CFTIPE, which signifies a 65% improvement compared to a neat epoxy polymer. CFIPE has a greater flexural modulus than the CTIPE composite (Zhang et al. 2017b, a;Chaurasia et al. 2019). Hence, the highest flexural modulus was endorsed for the CTIPE composite, which signifies a 102.88% improvement compared to a pure epoxy polymer. The flexural modulus also enhanced TiC nanoparticle reports and epoxy polymer physical networking (Chaurasia et al. 2019). Furthermore, CFTIPE and CIPE exhibited a lower flexural modulus than CFIPE and CTIPE since filler accumulation was considered, which reduced the reinforcement efficacy and decreased the efficient contact area within the polymer and filler particles (Rao et al. 2021).

Interlaminar shear strength
The general shear stress versus strain variation presented in Fig. 4e is remarkably distinct for every laminate. From the curve, it can be concluded that the CIPE composite can withstand a force 1.98 times higher than that of the pure epoxy polymer, whereas CFIPE bears a force 28% larger than that of the pure polymer. Additionally, CFIPE shows a greater shear strength than pure epoxy polymer, as observed in Fig. 4e. Five specimens were tested in every combination to calculate the shear strength value. The mean and standard errors of every combination are also shown in Fig. 4f. The higher value of interlaminar shear strength for CIPE composites is obtained due to the addition of Innegra fabric, which improves the interlayer roughness of the surface compared with CFIPE, CTIPE, and CFTIPE composites. As the expansion of inorganic filler expands the crucial value, the epoxy resin network's movement may be harmed. This can influence bad wetting with the fiber and potentially reduce the shear strength value (Rao et al. 2021). The resin networks around the surface of the particle are entirely dissimilar from the extent polymer.

Impact strength and hardness
The impact energy and strength of the epoxy polymer composites can be examined in Fig. 6a. Compared with the CFIPE or CTIPE composite, the impact strength of the CFTIPE composite was reduced. It has already been discussed in different studies that incorporating hybrid inorganic fillers in polymer resin decreases the impact strength (Zhou et al. 2019;Zhang and Qi 2014). A higher concentration of incorporated fillers was more significant than the decrease in the impact strength. Furthermore, reinforcing TiC or fly ash in the coir-Innegra fiber-epoxy composite considerably enhanced the impact strength for the CFIPE and CTIPE specimens compared to CFTIPE by 7.52% and 18.4%, respectively. This probable increase in the impact strength was because of the epoxy matrix's toughening with fly ash and TiC particles. Additionally, CTIPE exhibited a higher impact strength of 46.59 kJ/m 2 , slightly higher than CFIPE. Hence, the positive influence of TiC nanoparticle addition on impact strength is apparent. The hybridization exhibited a possible effect on impact strength assignable to the enhanced interfacial adhesion and good toughening features transmitted by the appearance of TiC nanoparticles, coir microparticles, and Innegra fabric in the epoxy matrix. Similarly, Khandelwal and Rhee (2020) also examined and expressed that inorganic filler influences the basalt fabric polymer laminate interface. Within the hybrid laminates, the effect of reducing filler concentration was more evident than the incrementing filler-to-filler interactions (Satapathy and Kothapali 2018). The shore-D hardness of coir-Innegra fiber epoxy hybrid composites is presented in Fig. 6b. The outcome shows that the reinforcement of fillers enhances the hardness of the fabricated CIPE, CFIPE, CTIPE, and CFTIPE composites. As a result of the addition of retrieved filler, the hardness enhancement was reported to be higher for CFTIPE laminates than for CIPE, CFIPE, and CTIPE laminates. Furthermore, a considerable enhancement in hardness was evident for CFTIPE laminates. Remarkably, the enhancement was up to 4% for the CFTIPE composite when compared to other combinations. The enhancement in the hardness of a laminate is based on fly ash and TiC nanoparticles (Agarwal et al. 2014). The interparticle void of the fillers in the epoxy polymer is considerably reduced with fly ash and TiC nanoparticles. This outcome results in a higher quantity of load-bearing regions being obtained in the composites. A similar result was also reported in glass fabric epoxy composites reinforced with fire alumina (Sabarinathan et al. 2020).

Thermal stability
The synergy between fillers, Innegra fabric, and epoxy resin is investigated by DSC examination. To examine the glass transition temperature, DSC investigations are conducted with an optimal heating rate of 10°C/ min. In Fig. 7a, heat flow with increasing temperature in epoxy hybrid laminates is exhibited. The glass transition temperature of fabricated epoxy hybrid composites is estimated from the extrapolation of the endothermic peak. The glass transition temperature measured from DSC curves is highest for the CTIPE composite, and it entails better interaction within the fillers, Innegra fabric, and epoxy matrix. The CFTIPE composite exhibited a slight decrease with the addition of fly ash because of the lower stability of hydrogen bonding at elevated temperatures (Lei et al. 2016;Wang et al. 2016). The glass transition temperature also exhibited an improvement for the CTIPE composite at approximately 100°C when compared with the pure epoxy polymer because of coir fibers (glass transition temperature of 120°C) (Stelte et al. 2018) and titanium carbide (melting point of 1900°C) (Wolfe et al. 2014). The reason may also be assigned to the uniform dispersion and bonding of coir fiber and TiC nanoparticles with the matrix, which restricts molecular chain movement (Ashok et al. 2020).
The thermal stability curves of pure and different filler/Innegra fabric-reinforced epoxy hybrid composites were examined by TGA and DTG (Fig. 7b, c). The enduring weight percentage of pure epoxy and their respective laminates as a function of temperature with a heating rate of 15°C/min was explained. The epoxy hybrid composites that withstand a higher temperature range at a given weight percentage are more thermally stable than other combinations. The thermal characteristics of pure epoxy and their respective hybrid laminates were determined in a temperature range of 160-600°C. This temperature range was considered because the deviation of weight exhibited additional influence due to appreciable oxidation and water absorption after 600°C and before 160°C (Ogasawara et al. 2011). CFIPE epoxy hybrid laminates  (Table 3). The initial weight loss of epoxy hybrid composites is critical to interpreting their thermal stability. Furthermore, epoxy resins have a comparably higher cross-linking focus, which establishes a comparably higher degradation temperature (Niu et al. 2014;Chatterjee and Islam 2008). The improvement in the thermal stability of the CFIPE composite was because of the strong interaction of the polymeric network, fillers, and Innegra fabrics. The reduction in thermal stability with TiC fillers may be because of the higher cluster size deviation of fillers in an epoxy polymer. The greater cluster size with nonhomogeneous dispersion in the epoxy polymer may negatively impact the interfacial bonding of filler-polymer and decrease the thermal stability of laminates. The carbon residue yield of the epoxy hybrid composite was considerably enhanced in the CFIPE composite (Table 3). The endorsed carbon residue could be assigned to further cross-linking between the polymer, Innegra fabric, and fillers (Ma et al. 2018).

Conclusion
The developed epoxy hybrid composites were fabricated using the mechanical-stirring assisted wet lay-up method. The tensile, flexural, interlaminar shear, impact strength, resistance to water absorption, glass transition temperature, and thermal stability of epoxy hybrid composites were improved compared with those of neat epoxy polymers. The impact strength has the lowest and highest impact strength of 46.59 kJ/m 2 and 32.03 kJ/m 2 by CTIPE and CIPE composites, respectively, which signified a possible hybridization influence transmitted from the TiC nanoparticles on the epoxy polymer. The stronger interfacial adhesion of fly ash, TiC nanofiller, and Innegra fabrics with the epoxy matrix, as observed from the FTIR examination, permitted efficient load transfer using the interphases. The uniform distribution of filler coir microparticles, fly ash, TiC nanoparticles, and Innegra fabrics in the epoxy polymer was proven from SEM micrographs. The CFIPE composite showed the most remarkable improvement in onset and end-set temperatures, up to 344.66°C and 416.72°C, respectively. The glass transition temperature also exhibited an improvement for the CTISE composite at approximately 100°C when compared with the pure epoxy polymer because of coir fibers (glass transition temperature of 120°C) and titanium carbide (melting point of 1900°C). The reason may also be assigned to the uniform dispersion and bonding of coir fiber and TiC nanoparticles with the matrix, which restricts molecular chain movement. The cross-linking density and thermal stability of the CFIPE, CTIPE, and CFTIPE composites also exhibited considerable improvement compared with pure epoxy polymer.