Hybrid calotropis gigantea fibre-reinforced epoxy composites with SiO2’s longer-term moisture absorbable and its impacts on mechanical and dynamic mechanical properties

Opportunities for the fabrication of plant fiber hybrids using thermoplastics and thermosets may be found in a variety of industries, including automobiles and agriculture. This can lessen reliance on crude oil, which contributes to a number of sustainability problems. In the current study, calotropis gigantea fiber (CGF) and nanosilicon dioxide (SiO2)-derived hybridized materials’ mechanical, dynamic mechanical, and water absorption properties were examined. Utilizing varying weight proportions of nanoSiO2 (0, 1.5, 3, and 4.5 wt%) and 30 wt% of CGF, we manufactured the composite using the hand lay-up method. The moisture absorption of the manufactured composites was measured during periods of 500, 1000, and 2000 h. For composite materials containing 1.5 wt% SiO2, the highest interlaminar shear strength (ILSS) failure point was 12.52 MPa for 500 h, which is 12.32% lower than the breaking strength for dried products (14.28 MPa). In comparison to the dry specimens, the bending strength of hybrids with 1.5% SiO2 that were immersed in water for 500, 1000, and 2000 h decreased by 2.56%, 5.21%, and 9.65%, respectively. The storage modulus of the damp hybrids with 3% and 4.5 wt% SiO2 was higher than that of the dry samples in terms of their dynamic mechanical properties. While the inclusion of nano-SiO2 significantly reduced water absorption and moisture diffusion, especially for hybrid materials with 4.5 weight percent SiO2, the water-absorption behaviour of hybrid natural fiber materials followed the Fickian law. With prolonged exposure time, the mechanical properties of the nanocomposite, both with and without nano-SiO2, such as ILSS and bending strength, declined. Due to the effective distribution of filler in the matrices, the samples with 4.5 weight percent SiO2 exhibited the smallest drop in strengths for both the flexural and interlaminar examinations, although all of them remained stronger than the CGF blends. The outcomes of the study point to potential applications in areas such as automobile manufacture, agriculture, construction, and general manufacturing.


Introduction
Natural fibers, or cellulosic fibers, offer several advantages as reinforcements in polymer mixtures compared to synthetic fibers due to their ready availability, cost-effectiveness, recyclability, and environmental friendliness [1].Various plant components, including leaves, stems, bark, roots, pods, flower petals, fruits, fruiting bunches, sorghum, and nutshells, have been employed for extracting cellulose fibers [2].These fibers can be used as organic reinforcements or filler materials in polymeric composites for applications in the construction, packaging, and automotive industries.Natural fibers have gained popularity as reinforcements in polymeric composites over the past decade due to their numerous advantages over synthetic filaments, including abundance, sustainable resource status, recyclability, biodegradability, low density, excellent mechanical properties, lightweight nature, and affordability [3,4].As a result of these benefits, natural fibers are now extensively used in construction and building projects, automotive components, and various commercial applications.However, it has been demonstrated that natural fibers are highly sensitive to moisture and often incompatible with most polymers.This sensitivity arises from their hydrophilic nature, causing them to readily absorb water when submerged or exposed to high humidity environments [5].When natural fibers are used as reinforcements, the absorption of water can lead to expansion and the creation of microcracks within the polymer matrix, which can significantly impact the overall properties of the composite.Therefore, given their widespread use across various scientific fields, it is crucial to evaluate the moisture absorption characteristics of natural fiber hybrids.Numerous studies have highlighted the susceptibility of natural fiber-reinforced polymer (NFRP) materials to humid conditions [6].Researchers have concluded that prolonged exposure to harsh environmental conditions can lead to a reduction in the mechanical properties of composites.The type of fibers used for reinforcement, the amount of filler added, and the choice of matrix material all influence the extent of strength reduction.The movement of water particles through microcracks in the polymeric matrix plays a crucial role in determining the water dispersion characteristics of NFRP materials [7].Other processes involve the penetration of water into the microcracks within the polymeric matrix due to fiber expansion and the capillary transport of water particles through the interface between the fibers and the polymer matrix.It is worth emphasizing that the expansion of natural fibers can limit their effectiveness as internal reinforcements for polymers in outdoor applications, as it weakens the fiber-matrix interface, affecting the long-term performance of such materials in humid conditions.Therefore, understanding the dispersion characteristics of NFRP composites is essential [8,9].
The degree of porosity and internal composition of plant fibers, among other factors, plays a role in how effectively plant fibers absorb moisture.On the other hand, the presence of cellulose in vegetable fibers hinders water absorption and increases the hydrophilicity of the fiber structure [10,11].The variety of calotropis known as the 'crowned blossom' is native to Southeast Asia and tropical Africa.This is a popular and rapidly-growing flowering shrub identified by its odorless purple blooms and thick, rectangular leaves that thrive in challenging environments [12,13].India has utilized the calotropis gigantea shrub for centuries as a traditional medicine due to its recognition as a valuable medicinal herb.The fibers extracted from the stem of the calotropis gigantea plant are collected for use in various products.Additionally, the resilient fibers produced by the CGF plant species can be used to create ropes, carpets, fishing lines, and sewing threads [14].The fibers extracted from the stems of the calotropis gigantea (CGF) shrub were described and documented.The typical liquid regeneration process using a sodium hydroxide solution proved effective in extracting cellulose fibers from the fibrous outer layer of the CGF plant [15].Vegetative fiber derived from the calotropis gigantea species exhibits appropriate buoyancy, hydrophobic-oleophilic properties, significant absorbent capacity, and efficient oil-water separation when used as a petroleum-absorbing material in the oil separation process [16,17].The higher concentration of cellulose in CGF strands contributes to their strength, while the latter characteristic influences their rigidity.Both factors affect the strength, durability of the fiber, and the performance of the materials produced.Hydroxyl groups in cellulose interact with water molecules, forming hydrogen bonds.As CGF are hydrophilic, they are incompatible with hydrophobic polymers [18].Even after treatment with both chemical and physical methods, such as an alkaline peroxide solution and silane pretreatment, these inherent incompatibilities persist in natural fiber-reinforced polymer (NFRP) materials.Research into improved methods is necessary to enhance the bond between plant fibers and the polymer matrix, as well as to increase the materials' water-absorption capacity [19][20][21].
Nanoparticles are recognized as significant and promising filler materials for enhancing the physical and mechanical properties of matrix polymers.There are currently numerous research efforts in the fields of technology and natural sciences aimed at gaining a better understanding of the specific characteristics of materials with nanometric dimensions, which can result in extensive surface areas exceeding 1000 m 2 g −1 [22,23].Nanofillers have the potential to significantly enhance or modify various properties of the materials they are incorporated into, such as fire resistance, optics, mechanical strength, electromagnetic behavior, and thermal conductivity, often in combination with conventional additives.Typically, nanofillers make up 1% to 10% (by mass) of polymer matrix materials.Examples of nanofillers used in nanocomposites include nanoclays, TiO 2 , nano-oxides, SiO 2 , carbon nanotubes, Al 2 O 3 , and organic nanofillers [24].In some cases, the introduction of nanoparticles has been found to significantly reduce liquid absorption propensity and increase thickness expansion through swelling.Another interesting study investigated the impact of compatibilization and the introduction of organoclay on the physical and thermal properties of hardwood floor/HDPE hybrid materials [25].Researchers observed that the Young's modulus of microcrystalline cellulose-reinforced ethylenepropylene (EP) polymer increased significantly, ranging from 1.06 to 1.31 GPa, when nanoclay and microcrystalline cellulose were combined at a 5% weight ratio.Additionally, the effects of nanoclay particles and exothermic chemical foaming agents (CFA) such as azodicarbonamide (AZD) on the morphological, mechanical, and physical properties of nanocomposites made from wheat stalk fibers (WSF) with HDPE were investigated [26].The water absorption propensity and thickness swelling characteristics of HDPE/WSF hybrids were found to increase with the addition of AZD and decrease with the addition of nanoclay to the substrate.The impact resistance was also reduced with the addition of nanoclay and CFA.
Furthermore, Kushwaha et al [27] discovered that bamboo fiber-reinforced epoxy materials containing carbon nanotubes exhibited high mechanical strength and low moisture absorption characteristics after 1600 h of exposure to a liquid environment.These properties were found to be 5.6%, 3.2%, 6.1%, 28%, 80%, and 10.52% better than those of materials without carbon nanotubes.Carbon nanotubes enhance bonding at contact points, improving mechanical properties and reducing water permeability, resulting in highperformance hybrids.The addition of nanofillers was also found to enhance the interfacial area by forming strong covalent bonds with the plastic matrix, thereby attempting to restrict the movement of dissolved water within the interaction region.
The mechanical stability of NFRP hybrids in dry settings is considered satisfactory, yet there is limited research on how these materials behave in prolonged wet environments.Therefore, further research is necessary to fully understand how SiO 2 nanoparticles impact the effectiveness of natural fiber composites in humid conditions, as this is crucial for ensuring their safety and reliability in outdoor applications.To enhance the use of naturally occurring calotropis gigantea fiber composites in long-term humid environments and to grasp the key factors affecting moisture diffusion behavior, the present study investigated the effects of silicon dioxide (SiO 2 ) nanoparticles, a topic not previously addressed in the literature.This research examined how calotropis gigantea natural fiber blends with varying SiO 2 concentrations by weight (0%, 1.5%, 3%, and 4.5%) behaved when submerged in water for 500, 1000, and 2000 h.The impact of these factors on bending and inter-laminar (ILSS) characteristics was assessed both before and after exposure to these conditions.A deeper understanding of the properties of CGF-natural hybrids containing SiO 2 nanoparticles under moist conditions is essential for their widespread adoption and application in outdoor mechanical and structural engineering projects.This research goes beyond standard composite materials by resolving the issue of long-term moisture absorption, which is a major problem in a variety of applications.This study sheds light on the way such hybrid composites can preserve structural integrity and durability in real-world settings by investigating the interaction between the absorption of moisture and mechanical and dynamic mechanical characteristics.This study stands out because of its unique material combination and emphasis on long-term moisture stability.It has the potential to influence industries that demand durable, water-resistant substances, including automobile construction and agriculture, in an environmentally conscious and sustainable way.

Experimental works 2.1. Materials
The laminates were made using a mixture of epoxy polymer (521 grade) and curing agent (marine assistance grade).According to the vendor's advice, the resin-to-hardener weight proportion was 4:1 when using the material.Deeshitha Chemicals, Madurai, Tamil Nadu, India, provided the silicon dioxide nanoparticles, which had a standard surface area of 200 m 2 g −1 .Organic Fibre Industries, Salam, Tamil Nadu, India, provided the bidirectional calotropis gigantea fibres with an average thickness of 0.25 mm.The use of Calotropis gigantea fiber (CGF) and nano SiO 2 (nanosilicon dioxide) in the investigation contributes significantly to improving the mechanical properties of composites via CGF's natural fiber reassurance, solving the critical issue of absorbing moisture over time, and employing nano SiO 2 as a potent filler that enhances both mechanical strength and moisture resistance.This choice supports environmental objectives by decreasing reliance on fossil fuels and exploring sustainable alternatives, providing valuable insights and content for a variety of industrial uses, while simultaneously encouraging a more environmentally conscious strategy for material creation and application.

Chemical pre-treatment
The non-cellulosic and other contaminants that remained on the dehydrated CGF surfaces were subsequently eliminated by submerging the fibres in a 5-weight percent sodium hydroxide (NaOH) liquid for 4 h at ambient temperature.To neutralise the treated fibres, they were submerged in the dissolved Hydrochloric acid (HCl) liquid for 2 min.The fibres were then dried and cleaned in distilled water in preparation for more research.Figure 1 shows the raw and chemically processed CGF [28].

Density measurements
Utilising a pycnometer and recognised standards for fluid toluene, density was measured.To remove any moisture contained in the gathered unprocessed and processed CGF, they were placed in dehydrators that were each stuffed with Silica gel and left there for roughly four days.The fibres were cut into 5 mm lengths and placed in a pycnometer to conduct a density experiment.The equation that follows (1) was used to determine the densities of CGF that were either raw or processed.Experimental calculations using established methods were used to determine the chemical structure found in organic cellulosic fibres like lignin, cellulose, etc Kurshner and Hoffer's technique was implemented to measure the amount of cellulose [29].
where m 1 and m 2 are the masses of an empty pycnometer filled with CGF in kg, and m 3 and m 4 are the masses of the pycnometer filled with toluene and CGF with toluene.

Composite fabrication
The making of fibre-reinforced epoxy-composite materials is seen in figure 2. The 300 mm length and 300 mm breadth of bidirectional CGF filaments have been eliminated.A conventional panel thickness of 3 mm was produced using six layers of material.As instructed by the supplier, moisture was removed from the CGF layers before applying the epoxy resin.This was done to prevent humidity from affecting the wettability and binding among the polymer and fibres.Employing the manual layup method, the epoxy material was blended for five minutes before being applied to the fibre layers until saturating.The materials containing a 30 wt% of fibre proportion had been subsequently built by stacking the CGF layers on top of one another.When soaking the yarn, a steel roller was employed to make sure the epoxy was applied uniformly and to eliminate air pockets.When applying the hardener, an epoxy resin was combined with different weight proportions of SiO 2 nanoparticles (1.5%, 3%, and 4.5%) to create the mixed hybrids.It is important to note that a 4.5% SiO 2 percentage was chosen since a greater SiO 2 concentration was required in the high viscosity epoxy solution.As a result, severe stirring of the ingredients using a conventional shearing blender resulted in above 4.5% SiO 2 ,  which considerably increases the formation of aggregations.Experiments and previous studies have also revealed that mechanical properties do not significantly improve at 4.5%.According to the provider of SiO 2 , a homogenous mixture was produced by using an electrical shearing blender for five minutes.The sheets were soaked before being placed into a hoover bag that was airtightly closed using yellow sealing adhesive.After that, a steady pressure equal to 100 kPa was used, and the specimens were then allowed to dry for 24 h.The produced plates were then demoulded and post-cured for 4 h at 90 °C, per the vendor's advice.Figure 2 shows the CGF/SiO 2 based hybrid composites fabrication process.
2.5.Material characterization 2.5.1.X -ray diffraction (XRD) analysis On a Bruker diffractometer, an XRD study was carried out employing Cu-K as the target origin and irradiation with a spectrum of 1.5406 A°.In order to capture diffractograms within 10°and 80°at an intermediate dimension of 0.05°at an operational electrical current and voltage of 20 mA and 40 kV, respectively, the pulverised CGF specimen was selected.Employing the subsequent equation (2), the crystallisation index (CI) for the CGF was calculated from the XRD patterns.The XRD patterns also confirm the presence of nanoSiO 2 .
Where C.I is the crystalline index, I 002 and I 001 are the low and high crystalline peaks

Mechanical characterization
The flexural and ILSS, regulated hybrids mechanical characteristics were examined based on the ASTM standard of D-790 and D2344.Both the experiments were carried out with a variable span-to-depth ratios utilising the 10 kN UTM equipment in a three-point test configuration at an average loading speed of 1.5 mm min −1 .The study's proposed span-to-depth ratio was 16 for the bending test specimens, which had dimensions of 80 × 16 × 3 mm and underwent testing across a 64 mm supporting length.At the same time the ILSS examination, was carried out employing testing samples that were 24 × 16 × 3 mm with a supporting length of 20 mm, resulting in a span-to-depth proportion of 5 [30].All the mechanical, dynamic mechanical testing and microstructural analysis were carried out in SRM university, Chennai, Tamil Nadu, India.

Dynamic mechanical properties
According to earlier research, the glass transition temperature of both wet and dry specimens was determined employing a DMA apparatus (Q800 type of thermal measurement equipment), used to analyse the thermomechanical characteristics of the polymers.DMA specimens for testing have been sliced using 50 × 8 × 3 mm measurements.DMA experiments have been carried out utilising multi-frequency strain modes and an increased temperature rate of 10 °C min −1 to measure temperatures over ambient to 150 °C while fastening samples in a dual-cantilever setup.

Moisture absorption characterization
According to ASTM D570, CGF blends and hybrid nanocomposites had their water uptake and diffusion coefficients evaluated.In order to assure that moisture enters the material only via its top and bottom substrates, every specimen was initially weighed in a dry state after having a small layer of resin applied to the edges of each one.This method was also used in prior research.Once submerged in water from the faucet for 500, 1000, and 2000 h, the specimens were removed.The specimens were removed from the water source after 24 h, dried using newspaper, and then promptly weighted with an electronic scaled weight meter that had a precision of 0.001 mg.
After the samples were weighed, they were promptly submerged in water once again.Throughout 84 days of continuous submersion in liquid, the weighing procedure was carried out at regular intervals.The variation in weight among the both wet and dry specimens was then used to compute the amount of water digestion, and equation (3) was used to get the amount of moisture percentage.
Where in W0 and Wt represent the weights of the dry and wet specimens at time t.On the other hand, it was assumed that the diffusion of moisture in mixtures complies with Fickian diffusion behaviours, and the formula (4) was utilised for calculating the coefficient of diffusion (D) [31].
Were h-Thickness of the specimen, K -Initial slope of the moisture curve of M(t) against t 1/2 M m -Maximum increase in weight of the specimen

Microstructural analysis
The material damage-related characteristics, fracturing surface, and fibre-matrix interaction of both dry and wet specimens with and without SiO 2 following lengthy soaking in water and testing at ambient temperature were examined using the scanning electron microscope (SEM).While meticulously assembling each specimen of the examined materials with dimensions that were 10 mm by 10 mm, the SiO 2 concentration inside the blended matrices was also determined by SEM.

Result and discussions
3.1.Fiber characterization 3.1.1.Physical properties Natural cellulosic fibres were used to strengthen composites made from polymers in a variety of industries, including automotive, aerospace, wrapping, manufacturing, and home usage.The fibre density and proportions in the matrices, which can vary depending on the composition of the element and its uses, determine the durability of fibre-reinforced composites.For a variety of uses, the design of lightweight parts must take fibre density into consideration.Once the alkaline processing was finished, the density of the CGF increased from 451 kg m −3 to 472 kg m −3 .In table 1, different natural fibres have been compared according to their density results.It was observed that CGF had a greater density than Phoenix pusilla leaves.According to density measurements, the CGF are better suited to generate eco-friendly and compact polymeric composites.

Chemical properties
The findings of the chemical compositional examination for each of the raw and processed fibres are reported in table 1.According to the data, processed CGF had more cellulose than raw fibre.In addition, the other amorphous components contained in the processed fibre, like hemicellulose, lignin, and wax content, were dramatically reduced.The processed fibres have less hemicellulose than the untreated fibres.The treated fibres' lower wax content improves the mechanical interaction between the cellulosic fibres and the polymeric substrate.The pretreated CGF also showed a reduction in water content, which serves to strengthen the connection between the fibre and resin.The amount of ash increased from 2.36 to 3.57 percent, indicating that amorphous materials such as hemicelluloses and lignin were excluded from the alkali-treated CGF.

XRD analysis
Figure 3 displays the XRD diffractogram for both raw and processed CGF.Both the raw and chemically modified samples had two distinct peaks.The amorphous state is represented by the initial peak of both raw and chemically treated fibre at 16.2 °and 16.5 °, respectively.For raw and processed fibre, accordingly, the primary spikes for the crystallised stage, which is a component of cellulose, are at 23.1 °and 24 °.The raw CGF Crystallinity Index (CI) rating is 37.26%, while for processed CGF, it is 40.96%.It is seen that the crystallised peaks have risen somewhat after the alkaline processing.The elimination of noncellulosic constituents, including the components hemicellulose, lignin, and pectin, that impact the overabundance of amorphous composition and also contribute to enhanced thermal and mechanical features, is the main cause of the increase in the CI level.The CI value of CGF is substantially lower than fibres such as cylindrical snake plant (59.63%),Sisal plant leaves (55.12%), but nearly identical to alkaline processed coatbuttons (41.08%), and higher than Dhaman (9.21%).Table 2 compares the crystalline indices of CGF with those of various natural fibres.Figure 4 shows the XRD pattern of nanoSiO 2 with low and high concentrations like 1.5 wt% and 4.5 wt%.The XRD configuration for 1.5 wt% of SiO 2 was 22.36°, and when more SiO 2 was added (from 1.5 weight percent to 4.5 weight percent), the structure was altered from 26.32, 35.10, 41.20, and 54.31°.With a rise in the materials' SiO 2 concentration, this peak's intensity falls.This confirms the hydroxyl section of epoxy compounds could interact with SiO 2 additives.By interfering with the orderly packaging of polymeric chains through both the steric impact and the hydrogen link among resin and additives, the integration of fillers in the epoxy structure decreased the crystalline structure of the epoxy-composite materials.As a consequence,   hydroxyl bonds became longer, making the crystal structure stiff and causing the degree of crystallisation to decline.The degree of the XRD pattern was altered by adding nanofillers to epoxy matrices.Research found that the XRD structure of the nanocomposite altered where the peak positions were located in accordance with the inclusion of the additives, indicating that the 2Ѳ-degree reduction was reduced with the filling action of fillers or that the peaks on the XRD vanished [38].

Mechanical properties 3.2.1. ILSS properties
The inter-laminar shear strength (ILSS) for both dry and wet hybrid materials is shown in figure 5. Regarding moist samples with various concentrations of SiO 2 , the same ILSS activity can be seen despite the exposure time.
The maximum ILSS failure point for moist materials containing 1.5 wt% SiO 2 was 12.52 MPa for 500 h; that's 12.32% less than the failure point for dried materials (14.28 MPa).When the matrix has adequate filler dispersion, the identical process is shown in the following bending strength: Nevertheless, for samples in 3 wt% SiO 2 , the wet ILSS strength was reduced the most, falling by 14.25%, 17.65%, and 19.01%for 500, 1000, and 2000 h, respectively, compared to the dry ILSS strength.This decrease was due to the plasticizing impact at the CGF/epoxy matrix link brought on by greater water digestion, which was shown to occur at lower Tg values (figure 11).As the epoxy material gets more viscous when the SiO 2 content is increased, this impact may be amplified.As a result, the beginning of fractures and their progression are caused by a higher level of interface stress, which was also found by Kong et al [39].This finding may be due to irregular dispersal of SiO 2 while blending, leading to a dramatically decreased effective area of the material at 3 wt% SiO 2 addition.Nanoparticles naturally exhibit an important propensity to aggregate; this is related to the high interaction among the components, according to Ponnusammy et al [40].This accumulation lowers the nanoparticles' functional aspect ratio, therefore lessening their contact with the matrix's surface and lowering adherence at the fibre/ matrix interaction.Another variable that lowers the durability of those samples is CGF swelling, which, as was previously described, results in small cracks at the contact point of the specimens and raises the water concentration [41].These justifications are supported by the SEM findings in figure 14.Nevertheless, elevating the percentage of weight of the SiO 2 to 4.5 wt% in the epoxy backing resulted in a decrease in the quantity of liquid that could pass across the interface area of the hybrid materials, which may be attributed to the barrier mechanisms created by those tiny particles and a little drop in ILSS strength.Due to the degradation of the CGF/epoxy matrix interface adhesive brought on by the moisture-induced plasticity impact, the ILSS durability of the wet specimens, independent of the time they were exposed and nanofiller concentration, remains lower than that of the dried hybrids for the purposes of comparison.The poor mechanical characteristics of organic fibre composite materials containing nanofillers and submerged in water were additionally related by Alaaeddin et al [42].to the development of hydrogen bonds caused by chemical relationships between water atoms and cellulose's hydroxyl bonds, which reduces the adhesion between the surfaces of the CGF within the matrices.Figure 6 shows the ILSS preservation findings demonstrate that the inclusion of SiO 2 may dramatically affect the maintained strength, which was 77%, 75%, and 72% for 1.5 wt% of nano SiO 2 , 76%, 75%, and 73% for 3 wt% of SiO 2 , and 78%, 77%, and 74% for 4.5 wt% of SiO 2 after 500, 1000, and 2000 h, correspondingly.Because of the minimal quantity of water absorbed, the greatest increase in ILSS preservation was shown for samples containing 4.5% in all circumstances.SEM images also illustrates how samples without the inclusion of SiO 2 failed as a result of ILSS within the fibre layers across the boundary caused by instability at the fibre/matrix contact.Nevertheless, the SiO 2 -containing samples exhibit identical failure behaviours both before and after being exposed to water.Irrespective of the SiO 2 weight proportion, the hybrid materials showed tensile damage instead of inter-laminar shearing mode, this was caused by the more robust fibre/matrix interaction created by the addition of nanomaterials.That could also result from the use of SiO 2 , which makes the epoxy matrix harder and more durable than natural materials like CGF [43].

Flexural strength and its modulus
In both dry and wet environments, the following figures 7 and 8 displays the bending strength and strength preservation of hybridised CGF-reinforced epoxy material with various nanoSiO 2 proportions.The inclusion of SiO 2 improved the bending properties of CGF materials under dry circumstances.The SiO 2 at the point of contact among the epoxy binder as well as the cellulose fibres provides a superior stress transmission system, which accounts for the improvements.Another straightforward reason is that SiO 2 's large surface area allows for  efficient chemical interactions involving matrix resin, boosting its durability.It ought to be noticed that the mean bending strength values rose, while the mixtures recorded the greatest increase (1.5 wt%), which can be attributed to the nanomaterials' excellent distribution.According to the previous research's [44][45][46], the enhancement in the mechanical characteristics of the fibre-based material with nano-filler is due to an improvement in the bonding of the fibre/matrix connect following nano-filler combinations, which results in an efficient transfer of load from an epoxy matrix to the filaments and increases its mechanical characteristics.Nevertheless, incorporating more SiO 2 into the epoxy matrix than 1.5 wt% leads to a decrease in bending strength because the SiO 2 nanoparticles clump together as a consequence of the epoxy resin's increased viscosity, as seen by the SEM images.As a result, several studies have supported the same tendency of increased viscosity with a substantial amount of nano SiO 2 [39].The glass fibre-reinforced epoxy composites containing nano-TiO 2 particles exhibited a similar pattern of bending strength data that revealed a decrease in bending strength because of the aggregate impact, as seen by Natrayan et al [47].Figure 7 shows how water that was absorbed decreased the bending strength of hybrid materials.Additionally, the bending strength declines with exposure time.The decline in flexibility is linked to a rise in moisture absorption percentage, which leads to the formation of the epoxy matrix with fibre morphology and deterioration of the filaments' interfacial adherence to the polymer substrate.
The flexibility of each specimen with various SiO 2 proportions was reduced after submersion into water compared with their dry endurance, irrespective of the period of exposure.This finding suggests that water intake and dissemination have an impact on the fibre-matrix interconnected characteristics, which control flexural durability.The bending strength of hybrids containing 1.5% SiO 2 that were submerged in water for 500, 1000, and 2000 h fell by 2.56%, 5.21%, and 9.65%, respectively, compared with the drying hybrid endurance, although it was greater than the moist durability of the remaining samples, as shown in figure 4. The uniform dispersion of SiO 2 nanoparticles can account for this phenomenon.Increased wet toughness is the consequence of a uniform distribution of SiO 2 nanoparticles across the matrix's structure, which minimises the amount of space that water atoms may occupy and prevents them from diffusing at the composite boundary by occupying the microchannels.The preservation of bending strength at various soaking times demonstrates that adding SiO 2 strengthened the fibre/matrix interaction and decreased the absorption of water, increasing the durability of the hybrid materials [48].Nevertheless, irrespective of the length of exposure, the mixtures containing about 3 wt% SiO 2 had smaller bending strength compared to those with 1.5% and 4.5% because of greater water intake brought on by a greater amount of voids created in the altered matrices throughout the manufacturing procedure as well as the existence of small holes at the specimen's interface, as seen in the SEM (figure 15(b)).Because of a more gradual process of absorbing water, the wet specimens containing 4.5% SiO 2 had somewhat less strength than the dry ones.This discovery could be explained by the fibre expansion brought on by absorbing moisture and exerting stress on the matrix around it [49].
This swelling impact filled the space between each filament and the backing material, leading to greater fibre adherence to the matrices and a modest decrease in bending strength.The beneficial impact of fibre expansion on the immediate bending and tensile characteristics of bio-epoxy materials supplemented with flax fibres was also noted by Muoz and Garca-Manrique [50].Another reason for this could be the extra decrease in empty space in the matrix that is caused by so many nanosized SiO 2 particles.Figure 8 shows how the amount of nanofillers affects the restoration of flexible strength.In comparison to samples without SiO 2 , the preservation of flexural integrity is usually enhanced by boosting the quantity of SiO 2 in all situations.The biggest increase in flexibility preservation was achieved by hybrid materials containing 4.5 wt% SiO 2 , which preserved 79%, 78%, and 75% of the dried strength for 500, 1000, and 2000 h, respectively.As the SiO 2 nanoparticles cover the matrix's empty spaces, water uptake inside the combined matrices is decreased, leading to this enhancement.
Figures 9 and 10 displays the bending modulus and its retention of SiO 2 -reinforced CGF-reinforced hybrid epoxy materials at various proportions of weight (0%, 1.5%, 3%, and 4.5 wt%).Due to the elevated levels elasticity of SiO 2 nanoparticles in a dry setting, a predicted rise in the flexural modulus accompanied by higher SiO 2 was seen.Additionally, the inclusion of SiO 2 particles are sturdier than the epoxy resin.Furthermore, the integration of SiO2 particles enhances the durability of the epoxy resin, simultaneously bolstering the matrix's  stiffness while rendering it somewhat more brittle.No matter how much SiO 2 was used, a decrease in the bending modulus was seen for wet materials with longer periods of exposure.The quantity of absorbing moisture that led to plasticity at the boundary within the CGF and the epoxy framework and caused the fall in wet modulus may prove attributed to this phenomenon.The particles of water that enter the mixture matrix work as a plasticizer to compromise the interaction at the filaments' interaction with the artificial matrices, lowering a matrix's elasticity in the process [44].The samples containing 4.5 wt% of SiO 2 had the highest amounts of flexibility modulus at 6.01, 5.78, and 5.1 GPa for 500, 1000, and 2000 h, respectively.These outcomes are correspondingly 36.21%,40.54%, and 43.65% greater than those of CGF based composites.The inclusion of more nanofiller fragments, resulting in greater the density of the material (1.63 g cm −3 for 1.5 wt%, 1.95 g cm −3 for 3 wt% and 2.28 g cm −3 for 4.5 wt% of nano SiO 2 filler), is thought to have been the cause of the samples with 4.5% SiO 2 lower humidity uptake.
However, regardless of the duration of exposure, wet-modulus samples with 3 wt% SiO 2 exhibited a significant reduction in bending modulus.This has been linked to greater absorption of water, resulting in to the swelling of the CGF.Due to the tiny fractures that the enlargement at the materials' interfaces created, humidity was taken in across the material itself.it was lowering the modulus.The blended composite's interfaces continue to deteriorate when exposure periods are extended up to 2000 h.The expansion led to the formation of tiny cracks at the composite interface (figure 15(b)), allowing moisture to penetrate the entire matrix and consequently lowering the flexural modulus.As the exposure time extended to 2000 h, we observed additional degradation at the hybrid composite interface.Specifically, the inclusion of 3 wt% SiO 2 contributed to an even greater loss of strength, as indicated by the modulus retention depicted in figure 10.The occurrence of small fractures in the epoxy matrix as well as fissures in the filament and substrate as a result from fibre expansion owing to absorbing moisture, that lowered the bonding at the interaction, are solely to blame for the poor modulus preservation of moist specimens containing 3 wt% SiO 2 .For 500, 1000, and 2000 h, accordingly, the bending modulus of the wet samples containing 4.5% SiO 2 fell by 6.21%, 7.02%, and 9.14% compared with the dried samples.

Dynamic mechanical properties
After being submerged in liquid for 2000 h, the dynamic mechanical examination profiles of hybrid materials with various SiO 2 loadings are shown in figure 11.The specimen's glass transition temperature (Tg) was determined to correspond to the point in temperature at which the storage modulus reached its maximum value at that time.Reduced Tg readings are mostly due to the polymerization of the matrix made up of epoxy brought on by the retention of water, which often serves as a surfactant.Similar to this, Rifai et al [48].found that basalt fibre-reinforced polymeric bars had reduced glassy transitional rates caused by the plasticity impact.Additionally, the upward trajectory of the Tg values in the present investigation is consistent with those of previous studies that have been described in the literature [51,52].On the other hand, figure 11 demonstrates that the storage modulus of the moist hybrids containing 3% and 4.5 wt% SiO 2 was greater compared to that of the dry specimens.According to Sekar et al [53], this phenomenon's rationale lies in the improved alignment of molecular chains in wet samples due to moisture presence.Consequently, the enhanced chain alignment leads to superior shear resistance, and the wet specimens exhibited increased storage energy compared to the dry ones.Furthermore, the inclusion of nanofillers in the study bolstered the molecular chains, leading to a noticeable improvement in the storage modulus.
This research also demonstrated how the addition of nanofillers improved the structure of the molecule chains, which clearly improved the storage modulus.It is advised that more research be done to examine this issue.The weakened interactions among epoxy resin with the SiO 2 fillers could be utilised to clarify how plasticity affects Tg.This indicates that the weakened filler/matrix interactions lower the glass transition temperatures [54].As the filler material concentration rises, the fillers aggregate as a result of the rise in viscosity.This weakens the connection among the SiO 2 nanoparticles and creates micropores that let water droplets pass among them.Minimal Van der Waals forces link to the additive agglomeration in the present situation [52].As seen in the results obtained with the addition of 3 wt% and 4.5 wt% SiO 2 water soaking, that may lead to an impairment of the bonding capability of the hybrid materials and a considerable decrease in Tg levels.

Water absorption behaviour
For CGF/epoxy hybridised materials with varying amounts of SiO 2 by the total weight of the matrices at ambient temperature, figure 12 illustrates the association among the proportion of weight increase and the square root of the water immersion period.It has been demonstrated that such samples' retention of moisture processes adheres to Fickian law, that might be characterised as linear during the water uptake curve's beginning stage before slowing down till it hits the point of saturation over a considerable amount of time.The findings demonstrate that introducing SiO 2 nanoparticles to the epoxy framework causes hybridised CGF-strengthened epoxy materials to absorb less water than samples without SiO 2 for all soaking times.The existence of SiO 2 nanoparticles, that serve as shields towards water in the epoxy matrices, is credited with this.Additionally, the results demonstrated that when the concentration of SiO 2 nanoparticles rises, both the beginning water rate of absorption and the highest possible moisture values decreased.This is because SiO 2 efficient dimension ratio produced a tortuous route and increased the adhesive intensity at the contact, causing the tortuousness route.The decrease in water diffusion parameters, as seen in figures 13(a) and (b), supports this.The findings of this research's inquiry into water absorption characteristics are consistent with those published by Ashok et al [55] for luffa/epoxy composites containing graphene.Nevertheless, specimens containing 3 weight percent SiO 2 absorbed less water than 1.5 wt % of SiO 2 sample.This is because, as seen by the SEM picture in figure 15(b), more voids were created in the changed matrices in the earlier (1.5 wt.% of SiO 2 ) specimens.Multiple variables can be attributed to the greater water absorption seen in the 4.5 wt% SiO 2 -based composites compared to their 1.5 and 3 wt% counterparts.For starters, the higher filler content in the 4.5 wt% composites provide more sites for water adsorption and may result in a more porous structure that retains more moisture.The aggregation of SiO 2 nanoparticles occurs as a result of the creation of porous structures induced by collecting SiO 2 nanoparticles.Furthermore, if the SiO 2 nanoparticles utilized have hydrophilic characteristics, they may bind and retain water molecules, hence increasing water absorption.Water ingress might be facilitated by interactions at the nanoparticle-matrix interface, which are affected by nanoparticle concentration.Additionally, the obtained results demonstrate the existence of tiny fissures at the fibre-matrix contact, which helps water seep into the majority of the matrices throughout the submersion phase.Due to the water-repellent character of CGF, that's related to the hydroxyl groups, CGF/epoxy composite experiments exhibit poor resistance to water for three separate soaking periods.This additionally clarifies why the unaltered specimens' saturation points were greater than those of the hybrid's specimens.When examining CGF ability to absorb water, this occurrence may be understood.Since CGF has a significant amount of cellulose (about 60.25%), the resulting material absorbs a lot of water, causing the fibre to inflate.The epoxy matrix mixture develops microscopic fissures as a result of the CGF swelling.In order to allow the water particles to enter the material's surface via those micro-channels, the micro-cracks served as an effective capillaries action.
As shown from the SEM (figure 13), this eventually leads to swelling tension, which degrades the characteristics of the organic fibre-based hybrid by detaching the CGF from its epoxy resin substrate.An additional factor that affects the efficiency of the composite fibre/matrix interaction and weakens their binding is the plasticity brought on by absorbed water.Although CGF/epoxy materials have a propensity to absorb a lot of liquid, SiO 2 nanoparticles appear to be able to counteract this propensity by removing gaps and plugging tiny channels, slowing down the movement of water molecules into the matrix that makes up the hybrid.Because of the hydrophobic nature of SiO 2 nanoparticles, hybrid mixtures exhibit reduced consumption of water compared to composites that do not include SiO 2 .By making the outermost layer of the fibre harder, the tiny particles connected to its surface serve as exterior safeguards that help lower the rate of absorbed water and boost the bonding of the fibre/resin matrix contact.The inclusion of 4.5 wt% of SiO 2 nanoparticles reduced the liquid permeability and transpiration rate of CGF hybrids by 36.54% and 48.51%, respectively, during the shortest possible immersion in water duration (500 h).Conversely, after 2000 h of immersion, those specimens' water diffusion coefficient and absorbed moisture contents were considerably higher than those of the baseline specimens, increasing by 33.65% and 49.52%, respectively.It has been demonstrated that the inner plasticity of the matrices, as indicated by the decrease in Tg discovered by dynamic mechanical evaluation (DMA), displayed in figures 11(a) to (d), is responsible for rising moisture levels and diffusion coefficients with greater exposure duration.However, by using Silicon nanoparticles, the water diffusion rate and absorption of humidity concentration may be decreased for every given period of water saturation.3.5.Microstructural analysis Figure 14(a) and (b), which depict the surface microstructure of the raw and chemically modified CGF, respectively, were obtained from the SEM analysis.The raw fibres' surface displayed the contaminants.Additionally, raw fibre was seen to have a rough and irregular texture.Hemicellulose, lignin, and other contaminants that have been coated over the fibre are mostly to blame for this.The CGF were alkaline processed to remove contaminants and improve their interfacial interaction with polymer matrix structures.Due to the removal of non-cellulosic materials found on the fibre surface after the alkaline treatment, the fibre texture exhibits a smooth surface and a uniform size.Alkalization of fibres partially enhanced the CGF's surface appearance [29].
SEM images of the morphology of hybrid materials in both dry and moist states at ambient temperature are shown in figures 15(a)-(e).The inner structure of the hybrid materials as it relates to the distribution of SiO 2 particles in their matrix can be seen in these photos (up to 500 times magnified).With 1.5% SiO 2 nanoparticles added to the epoxy framework in dry composite materials, the fibre-matrix interface was enhanced irrespective of exposure period, leading to additional fibre breakage instead of fibre pull-out (figure 15(a)).Nevertheless, increasing the amount of SiO 2 causes aggregation due to the epoxy resin's higher viscosity and agglomeration due to the large surface area and van der Waals reactions among nanomaterials.The number of vacuum forms in the composite hybrids then rises as a result of all this (figure 15(b)).Because of the fibre expansion brought on by the absorption of water, which puts stress on the underlying matrices, no fibre pull-out can be seen in samples with a 4.5 wt% of SiO 2 content.Additionally, the successful proportion of SiO 2 boosted the resin's adherence to the fibre notwithstanding the presence of filler aggregation (figure 15(c)).This additionally clarifies how those samples absorb the least humidity.
As shown in figure 15(d), the wet samples with 1.5% SiO 2 had excellent bonding strengths among the CGF fibre/filler and the matrix of resin after being submerged in liquid.This was due to the evenly distributed distribution of SiO 2 particles, which decreased the number of vacancies in the matrices.Figure 15(e) demonstrates that the separation among the CGF fibre and epoxy matrix caused by the plasticity of the epoxy contributed to the decrease in fibre/matrix interaction and matrix breakdown in the moist hybrids at 3 wt%.This can be a sign of a thin bond between the fibre and matrix at the point of contact [56].

Conclusion
The bending and ILSS capabilities of hybrid CGF reinforced epoxy composites were examined in this study in relation to moisture.These hybrid composites were produced and submerged in water for 500, 1000, and 2000 h, respectively, at ambient temperature, containing nano-SiO 2 contents of 0, 1.5, 3, and 4.5% by weight of the epoxy material.The subsequent evaluations and experimental findings can lead to the following conclusions.
• Calotropis gigantea filaments were physiochemically characterised and found to have low density, favourable chemical substance, medium crystallisation index, acceptable mechanical capabilities, and comparatively low surface roughness.These findings also demonstrated that the hemicellulose, lignin, and moisture content contained in the CGF were decreased by the NaOH processing.By using the XRD technique, the crystallisation indices of the alkali-treated CGF were enhanced.
• The XRD configuration for 1.5 wt% of SiO 2 was 22.36 °, and when more SiO 2 was added (from 1.5 weight percent to 4.5 weight percent), the structure was altered from 21.72 to 22.89 °.With a rise in the materials' SiO 2 concentration, this peak's intensity falls.This confirms the hydroxyl section of epoxy compounds could interact with SiO 2 additives.
• ILSS preservation findings demonstrate that the inclusion of SiO 2 may dramatically affect the maintained strength, which was 77%, 75%, and 72% for 1.5 wt% of nano SiO 2 , 76%, 75%, and 73% for 3 wt% of SiO 2 , and 78%, 77%, and 74% for 4.5 wt% of SiO 2 after 500, 1000, and 2000 h, correspondingly.Because of the minimal quantity of water absorbed, the greatest increase in ILSS preservation was shown for samples containing 4.5% in all circumstances.
• The biggest increase in flexibility preservation was achieved by hybrid materials containing 4.5 wt% SiO 2 , which preserved 79%, 78%, and 75% of the dried strength for 500, 1000, and 2000 h, respectively.As the SiO 2 nanoparticles cover the matrix's empty spaces, water uptake inside the combined matrices is decreased, leading to this enhancement.
• DMA results demonstrates that the storage modulus of the moist hybrids containing 3% and 4.5 wt% SiO 2 was greater compared to that of the dry specimen.More potential energy chains in the moist samples exhibited greater reserves of energy than fewer potential activity shackles, indicating that high-oriented strands are more resilient to shear.
The overall conclusions from this investigation emphasised the advantages of nanoSiO 2 in reducing the water absorption of CGF/epoxy mixtures and improving their strength retention, allowing this kind of organic fibre composite to be used in moist situations.Nevertheless, lengthier tests may be done to examine the water intake from hybrid CGF/epoxy mixtures beyond the exposure duration taken into account in this work.

Future scope of the research
Several possibilities emerge from the results of our study in the context of future investigations.To begin, more research is required to investigate the long-term effectiveness and resilience of hybridized calotropis gigantea fiber (CGF)-reinforced epoxy-composite materials with varying nanoSiO 2 concentrations under diverse environmental conditions.It could be beneficial to evaluate how these substances function over time and in different industries, including automobiles, construction, and agriculture, to assure their dependability and applicability.Such blended substances offer an environmentally friendly option for building durable products and equipment with better moisture resistance by mixing plant fibers with nanosilicon dioxide.It can minimize dependency on crude oil and improve sustainability while retaining strength in the automobile sector.They may be utilized in agriculture to make strong and water-resistant equipment, contributing to more sustainable agricultural operations.Furthermore, in the construction industry, these materials may lead to the creation of eco-friendly building materials with improved durability and moisture resistance, thereby boosting sustainability in construction projects.Furthermore, they offer diverse potential in a variety of production industries that demand robust, moisture-resistant materials, providing eco-friendly alternatives to standard plastics.
Future research can also explore more of the processes that influence the appropriate filler concentration, interfacial bonding, and dynamic mechanical behaviour reported in our work.In addition, studies might be directed toward improving the manufacturing process and investigating alternative fibres made from plants and fillers for composite materials with improved characteristics.Our findings can help future researchers comprehend the link between filler content, absorption of moisture, and mechanical characteristics in composite materials.The findings may be used as a starting point for accelerating the creation of ecologically sound and resilient components in a variety of sectors, leading to future eco-friendly and robust solutions.

Figure 4 .
Figure 4. XRD pattern of nanoSiO 2 inclusion with different proportions in CGF-based composites.

Figure 5 .
Figure 5. Interlaminar shear strength of CGF/nano SiO 2 based hybrid composites with different filler weight Proportions.

Figure 6 .
Figure 6.Interlaminar shear strength retentions of CGF/nano SiO 2 based hybrid composites with different filler weight Proportions.

Figure 7 .
Figure 7. Bending strength of CGF/nano SiO 2 based hybrid composites with different filler weight Proportions.

Figure 8 .
Figure 8. Bending strength retentions of CGF/nano SiO 2 based hybrid composites with different filler weight Proportions.

Figure 9 .
Figure 9. Bending modulus of CGF/nano SiO 2 based hybrid composites with different filler weight Proportions.

Figure 10 .
Figure 10.Bending modulus retentions of CGF/nano SiO 2 based hybrid composites with different filler weight Proportions.

Figure 12 .
Figure 12.Moisture absorption characteristics of different weight proportion of nano SiO 2 /CGF based hybrid composites.

Table 1 .
Physical and chemical properties of CCF with other natural fibres.

Table 2 .
Crystalline Index of CCF with other natural fibers