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High mechanical performance of 3-aminopropyl triethoxy silane/epoxy cured in a sandwich construction of 3D carbon felts foam and woven basalt fibers

High mechanical performance of 3-aminopropyl triethoxy silane/epoxy cured in a sandwich... 1IntroductionEpoxy resin offers good mechanical performance and corrosion resistance, among others, and is increasingly employed in aircraft, automobiles, and related sectors [1,2]. However, several characteristics of epoxy resin materials are disadvantageous, such as the inherent brittleness generated by the high density of cross-links formed during curing. This restricts their applicability in several areas [1,3–5]. Several studies have been conducted to improve the toughness of highly cross-linked epoxy resins. Nanoparticles, thermoplastics, and liquid crystalline elastomers are frequently used to toughen epoxy resins. Examples include graphene (Gr), multiwall carbon nanotubes (MWCNTs), poly (4,4-bis(6-hydroxyhexyloxy) biphenyl phenylsuccinate), polysulfone, and nanoclays [6–13]. Due to their smooth surface, high surface/volume ratio, and Van der Waals force, nanoparticles tend to cluster inside the resulting composite and interact poorly with the epoxy matrix, reducing the efficiency of load transfer at the interface [14–16]. In order to achieve high-performance mechanical qualities, the fillers must be matrix compatible [17]. As a result, various research studies on improving fiber/matrix connections have already been conducted. By changing the chemicals on the surface of the filler, modification techniques can improve the bonding between the fibers and the matrix [18–20].The limitations of nanofillers, such as poor dispersion and re-agglomeration, can be solved by a three-dimensional low-density sponge-like material having a very linked architecture, high strength, and flexibility [21]. Aerogels, sponges, and foams are examples of 3D graphene or carbon nanotube frameworks that are low in density while retaining the great qualities of 2D graphene or 1D carbon nanotubes. Highly porous carbon foams allow the polymer resin to penetrate their cellular structure, producing a composite with a consistent nanocarbon distribution throughout the matrix [22]. The nanocarbon foam’s linked network of nodes and branches provides smooth routes for stress transmission [23]. Compression and tensile strength for epoxy/3D graphene–polyaniline composites were observed to increase by 25.0 and 32.5%, respectively, compared to solid plain epoxy, as reported by Wang et al. [24]. Jia et al. [25] discovered that adding 0.1 wt% graphene foam to an epoxy increased the toughness by 78.0%, while adding 0.2 wt% graphene foam increased flexure modulus by 53.1% and strength by 38.1%. Anisotropy in electrical and mechanical characteristics was reported by Kim et al. Freeze-drying graphene oxide allowed for the aerogels to be injected with epoxy, resulting in a graphene aerogel/epoxy composite. When the crack spreads in a direction perpendicular to the symmetry of the graphene layers, the filler concentration of 1.4 wt% results in a 64.0% improvement in fracture toughness. There was a maximum difference of 113% between the fracture toughness of these composites in parallel and transverse alignment of the graphene sheets [26]. During the past 10 years, several studies have been conducted to produce carbon felt (CF) foam as a low-cost fiber material. CF foam is often employed as an electrode for environmental purposes and many other applications because of its outstanding mechanical properties, high conductivity, high porosity, cheap price, and because it has a superb 3D cross-linked network made of carbon fibers [27–29]. In general, felt is created by first processing needle punches and then drawing carbon. CF is created by graphitizing regenerated cellulose, and the operating temperature ranges between 1,200 and 1,600°C. However, other carbon-based materials have also been extensively used for composite applications, including vitreous nanocarbon, carbon aerogel, carbon fiber, and carbon bucky paper. In contrast to CFs, these materials do have certain drawbacks. For example, because other carbon-based materials are highly costly, they cannot be utilized to produce high-volume, low-cost composites. Xu et al. [27] prepared 3D CF foam/epoxy composites by impregnating epoxy resin into the 3D CF foam, in which the flexural strength of the composites rose from 96.0 to 125.0 MPa after adding CF foam into the epoxy matrix.Unfortunately, due to surface inertness and the absence of functional groups in 3D carbon foam, the interfacial interaction property between 3D interconnection filler and epoxy resin is weak. Because significant contact resistance exists at polymer composite interfaces, an effective interfacial bonding effect is required to increase the mechanical quality. The overall performance of composites could be significantly increased when the interface is effectively changed by introducing interfacial layers [30,31]. Furthermore, the use of these procedures is expensive, takes time, and complicates the method for composite synthesis. Silane coupling agents are widely used to boost the filler’s affinity for the epoxy matrix and to ensure that the filler is evenly distributed throughout the epoxy composite, which comprises at least two distinct functional groups. Coupling agents are frequently used to form a solid bond between the filler and the matrix. Therefore, the reinforcement performance is enhanced and the polymer matrix is preserved for a longer period of time due to load transfer from the weak polymer to the stronger filler [32–34]. However, the needed refluxing in the salinization process may harm the filler’s characteristics [35]. In addition, chemical reagents used in fiber surface modification might damage the environment with chemical pollutants.The remarkable mechanical capabilities of basalt fiber-reinforced epoxy composites have drawn the attention of scientists in the modern industrial sector [36–38]. Basalt fiber is produced by extruding molten basalt rock, which mostly consists of Si and Al oxides. Basalt fiber requires less energy to produce than nanofibers, glass fibers, and carbon fibers since its manufacturing method is simpler (melting and extrusion) and produces less carbon dioxide and other environmental waste. Basalt fiber composites are superior to glass fiber composites due to their superior combination of rigidity, strength, fracture toughness, and energy absorption. The tensile strength of a single basalt fiber may reach up to 2.8 GPa, and it also exhibits high chemical and thermal stability [36]. Basalt fibers hybridized with nanofillers can improve the mechanical properties of their epoxy composites [39–41]. Hosna and Azizi looked into the mechanical properties of woven basalt fiber (WBF)/epoxy composites with silane-modified nanozirconia (ZrO2) [42]. Flexural and tensile tests showed that, compared to a sample without nanozirconia, the strength of the material rose by 90.0 and 85.0%, respectively, with a dispersion of 3 wt% ZrO2. The flexural behaviors of modified carbon nanotube epoxy/basalt composites were examined by Kim et al. [43]. According to the findings, silane-treated CNT/epoxy/basalt composites have a flexural modulus and strength that is 10.0 and 14.0% greater, respectively, than those made with acidified CNTs. Nanofillers are known to form agglomerates within the polymer if not chemically modified properly. Through hybridization, it is important to make a composite system that takes advantage of the improved properties that come from mixing a synthetic, eco-friendly, cheap, and non-toxic filler with basalt fiber reinforcing. Table 1 highlights research on epoxy composites reinforced using basalt fibers.Table 1Overview of the research done on how WBFs improve the mechanical performance of epoxy compositesFiller typeProcessFlexural strength (MPa)Tensile strength (MPa)Ref.WBFHLC—202.0[49]CNT coated WBFHand lay-up240.4—[36]Basalt powder/WBFsHand lay-up—214.0[50]Nano-zirconia–graphene oxide/WBFsHand lay-up406.0—[51]Woven flax/WBFsHand layup88.7170.9[52]Graphene nanopellets/WBFsHand lay-up240.0273.9[53]3D interconnected graphene skeletonRTM175.0130.0[54]Woven glass fibers/WBFsRTM194.54225.9[55]TiC/WBFsVARTM215.2—[56]Silanized CNT/WBFsVARTM—232.0[57]Coir/Innegra/WBFsHot-pressing290.0170.0[58]Tourmaline/WBFVARTM266.1357.2[59]3D CF foam/WBFsCast-in-place363.3325.0This workNote: HLC, hand layup followed by compression molding; RTM, resin transfer molding; VARTM, vacuum-assisted resin transfer molding.Sandwich composites are a practical way to reduce the cost of a high-performance composite material while also increasing ultimate strain and impact properties [44]. Sandwich structures are a type of laminated composite. It is made up of two thin face sheets bonded together with a thick lightweight core [45]. It is possible to mix many fiber types in a way that reduces or eliminates the drawbacks of each. High-stiffness, high-cost fibers like woven fibers make up one kind of hybrid fiber composite, while low-modulus, low-cost fibers make up the other [46]. The stiffness and energy absorption capabilities are provided by the high modulus fiber, while the low-stiffness fiber increases the composite’s resistance to damage and keeps the cost of the raw materials manageable. The impact load might cause sandwich composites to break catastrophically. The impact damage resistance of sandwich composites influences structural reliability and is critically dependent on the core. The most prevalent core materials are honeycomb or foam-based, although various unique core structures are also being developed to improve the mechanical and impact characteristics of sandwich composites [46,47]. However, short fibers placed with uniform dispersion inside epoxy composites can provide high mechanical qualities at a cheap cost. Therefore, a suitable product solution is the introduction of foam made of short fibers as a core material and woven fibers as faces for composites, which can lower their cost and improve their mechanical performance. Basalt and flax fiber hybrids were created in an epoxy matrix by Fiore et al. [48]. Flax fibers were utilized inside, while basalt fibers were employed on the outside. The testing results showed that the addition of four outside basalt layers increased the flexural strength. Flexural strength increased in flax-basalt specimens from 63.0 to 107.0 MPa. The rise in flexural strength is due to the ability of basalt fibers to handle bending pressures and better stress transmission from the matrix. This makes the strength properties better.The main motivation for this research is using a cast-in-place process for fabricating a sandwich of epoxy composite reinforced by 3D CF foam as a core and WBFs as a face. In addition, the study investigated how 3-aminopropyl triethoxy silane mixed with the resin influenced the sandwich of epoxy composites’ mechanical performance. To the author’s knowledge, the modified epoxy by 3-aminopropyl triethoxy and reinforced by CF foam and WBFs made by the cast-in-place process has not been documented in the literature. So, it was decided that a proposal for a composite system was needed to improve the interface between the epoxy and fibers by making sandwich composites of 3D fiber foam and woven fiber-reinforced epoxy composites using modified epoxy resin.2Experimental2.1MaterialsBoth the diethanolamine curing agent and bisphenol A diglycidyl ether epoxy resin (E55) were purchased from Dongguan Qiancheng plasticizing materials Co., Ltd in Guangdong, China. Pure CF foam, 5 mm in thickness, was supplied by SGL Carbon Se Co., Ltd. The density of CF is 1.79 g/cm3, and the diameter of the felt is 23 μm. Basalt fiber plain wave mats of density 2.8 g/cm2, filament diameter of 13 μm, and tensile strength of 4,840 MPa were supplied by Basalt Fiber Tech Co., Ltd. 3-Aminopropyl triethoxy silane (KH550) and nitric acid were purchased from Chengdu Kelong Chemicals Factory Co. Ltd in Chengdu, China.2.2Treatment of the CF’s surfaceTo improve the interface contact between the CF and the silanized epoxy resin at the core of the sandwich composites, the 3D CF foam was oxidized by soaking it in a diluted HNO3 aqueous solution (2 M) for 1 h at 60°C.2.3Sandwich composites preparationTypically, the curing agent (9 mL), epoxy resin (100 g), and 3-aminopropyl triethoxysilane (0, 3, 6, and 9 wt% of epoxy resin) were initially combined together while being mechanically stirred. After that, a cast-in-place method was used to imbue the 5 mm thick 3D CF foam with the epoxy and silane coupling agent combination [60,61], as seen in Figure 1. The WBFs were then incorporated as a face sheet in five layers with a thickness of about 0.5 mm. The resin-impregnated WBF sheets were positioned above and below the core (3D CF foam). The sandwich composite was then sealed with a vacuum bag, and air pressure was used to remove the air bubbles within the sandwich and keep the core and faces together. The fiber volume fraction of 3D CFs/epoxy (core) and basalt/epoxy (face) was approximately 31 and 45%, respectively. The 3D CF/WBF epoxy composite sandwich fabrication method is shown in Figure 1.Figure 1A schematic of the method used to create a 3D CF foam core sandwiched between a face sheets made of laminated WBFs.2.4Characterization and testingField-emission scanning electron microscopy (FE-SEM) was used to examine the surface morphology of sandwich composites and the morphology of 3D CFs (JEOL JSM-7800F, Japan).The CF distribution in epoxy was measured using an optical microscope (LEICA, Wetzlar, Germany).The Fourier transform infrared (FTIR) spectrometer was used to analyze the chemical composition of the pure and acidified CF (FTIR-7600, Lambda, Australia).The structural integrity of CF was analyzed by taking Raman spectra using a Raman spectrometer (inVia, Renshaw, London, UK) and a 532 nm laser excitation before and after the surface modification.Dynamic mechanical analysis (242 E Artemis, Netzsch, Germany) in the bending mode was used to estimate the glass transition temperature and storage modulus of epoxy composites sandwich. The temperature range and heating rate were set to 30–180°C and 10°C/min, respectively.The flexural and tensile characteristics were evaluated using universal testing equipment (WDW-300, Beijing, China) following the GB1040-92 and GB1449-2005. The XJU impact testing machine was used for the impact test, following the ISO 179.3Results and discussion3.1Core morphology/structure analysisTaking into account the unique structure of the 3D network of CFs, epoxy was incorporated into 3D CFs to manufacture the core of sandwich composites based on epoxy composites as performing reinforcement using a cast-in-place process. A cast-in-place technique was developed and used to directly cast epoxy resin into porous 3D CFs, resulting in a bicontinuous composite with excellent mechanical properties. When seen via the lens of the camera, the optical photograph of the original three-dimensional skeleton of CF foam revealed that the three-dimensional CFs have remarkable flexibility properties (Figure 2a). CF is composed of networks that are both regular and highly linked (Figure 2a), which provides 3D CFs with excellent flexibility. 3D CFs fibers create regular and densely linked networks, allowing 3D CFs to be pretty flexible. The excellent incorporation of superior flexibility would aid in improving the flexibility of composites.Figure 2Optical photo, morphology, and chemical structure of the core based on 3D CFs: (a) optical image of highly flexible 3D CFs, (b) scanning electron micrographs of 3D CFs, (c) 3D CFs/epoxy composite, (d) FTIR spectrum of unmodified and modified 3D CFs treated with acid, and (e) Raman spectra of 3D CF.FE-SEM was used to examine the morphology of pristine CF foam. The 3D CF foam has a regular and uniform shape as well as a surface structure that is spongy (Figure 2b). The dispersion of CF within the epoxy was examined using optical microscopy. Figure 2c shows that the 3D CF/epoxy composite (core) has a large number of dark lines (fibers) that cover a large area compared to the bright patches (epoxy). This shows that the CFs are evenly distributed in the epoxy because the fibers are arranged in a regular way within the foam.Figure 2d compares the FTIR spectra of pristine carbon foam (P-3D CFs) and acidified carbon foam (A-3D CFs) to identify the chemical groups of 3D CFs caused by surface treatments. The distinctive peak (3,418 cm−1) in the pristine CF spectrum corresponds to the hydroxyl group O–H vibration and the H–O–H bending, respectively. The C–H vibration in CH2 is responsible for the peaks at 2,973 cm−1. The carbon backbone of a fiber has a C–C peak at 1,637 cm−1. The strength of the O–H and H–O–H bands shifted from 3,418 to 3,438 cm−1 when the CF was acidified by NHO3. Also, clear peaks at 2,932/2,849–1,463 cm−1 (C–H), 1,738 cm−1 (C═O), 11,625 cm−1 (C═C), and 1,120 cm−1 (C–O) show that the surface of the modified CF has more oxygen-containing carboxyl groups.To evaluate the changes in graphitization caused by the treatment procedures, Raman spectroscopy was used. In Figure 2e, two significant Raman peaks are seen in both the P-3D CFs and the A-3D CFs. The Raman peaks of 1,345/1,249 and 1,595/1,599 cm−1 correspond to graphite’s sp3 carbon networks (D-band) and sp2 carbon networks (G-band), respectively [29,62]. The ID/IG ratio was found by dividing the intensity of the D band by the intensity of the G band [9]. Figure 2e shows that the ID/IG ratio barely changed after the surface treatments, which shows that the acid oxidation method had little effect on the graphite integrity of the CFs.3.2Structure characterization of modified epoxy matrix by 3-aminopropyl triethoxy silaneIn this work, the 3-aminopropyl triethoxy silane was directly mixed into the epoxy solution to produce an epoxy matrix modification. Silane coupling agents of 3-aminopropyl triethoxy include alkoxysilane groups that may respond to the network structure of fibers, which consists of Si–OH or OH, forming a new bond of Si–O–Si–, resulting in the most regular silicon-oxide network structure, likely to result in enhanced interfacial interaction between fibers and epoxy, and forming a chemical link between the coupling agent molecules (NH2) and the epoxy group in the resin [63,64]. The mechanism of the 3-aminopropyl triethoxy silane reaction with epoxy is illustrated in Figure 3a. Figure 3b displays the FTIR spectrum of an untreated and treated epoxy resin. (–NH) stretching is attributed to the peak at 3,373 cm−1. The peak at 2,970 cm−1 is asymmetric (–CH3) bands, whereas the peak at 2,849 is symmetric stretching modes (–CH2). Additionally, the absorption of epoxide groups is shown by the peak at 1,025 and 910 cm−1. The substituted C═C group in epoxy resin is what causes the absorption bands at 1,604 and 828 cm−1. The spectra of silane-modified epoxy resin differed from that of pure epoxy resin. The –NH peak is broadened and moved to 3,428 cm−1 coming from amine stretching vibrations. Furthermore, a new peak is appearing at 1,610 cm−1, which is characteristic of primary amines and which develop via a chemical bond between the coupling agent molecules (NH2) and the epoxy group in the resin. The silane coupling agent’s amine group initiates a nucleophilic attack on the epoxy ring, resulting in a “ring-opening reaction” and the production of a new hydroxyl group [65]. Peak intensification at 1,036 and 1,100 cm−1 caused by the creation of C–O–Si and O–Si–O linkages in the silane-treated epoxy resin [66–68]. Methoxy groups in the 3-aminopropyl triethoxy silane may produce silanols during the reactions, and hydroxyl groups on the epoxy–fiber surfaces may result in the development of oxygen linkages. Several publications have described an amino-silane coupling mechanism in fiber/epoxy composites [69,70].Figure 3(a) Reaction formula among epoxy resin, 3-aminopropyl triethoxy silane, and fiber and (b) FTIR spectra of 3-aminopropyltriethoxy silane/epoxy after-curing.3.3Mechanical performance of the epoxy sandwich composite constructed from WBFs as a face sheet on 3D CF coreThe epoxy composite sandwich of 3D CF/WBFs was made using a cast-in-place process, and its mechanical strength was evaluated using flexural, tensile, and impact strength tests. The flexural performance of an epoxy-based sandwich composite entirely reinforced with 3D CFs foam as the core and strengthened with WBF face-sheets is shown in Figure 4. Compared to the pure epoxy (91 MPa), the unmodified epoxy resin by KH550 and impregnated in sandwich composites of 3D CFs/WBFs (0 wt% KH550–epoxy/A-3D CFs/WBFs) showed a higher increase in flexural strength of 298.1%. Also, Figure 4a shows that adding KH550 to epoxy resin reduces the flexural strength for A-3D CFs/WBF epoxy composite sandwiches. Flexural strength is reduced by 2.5, 6.0, and 17.3% when the KH550 content of the epoxy is 3, 6, and 9 wt%, respectively. The flexural strain of pure epoxy, untreated epoxy/A-3D CFs/WBFs sandwich composites, and treated epoxy/A-3D CFs/WBFs sandwich composites by KH550 is shown in Figure 4b. It is found that the flexural strain of the epoxy composite sandwich is significantly improved by the addition of KH550 with epoxy resin within A-3D CFs/WBFs. Flexural deformation increased from 5.0% for 0 wt% KH550–epoxy/A-3D CFs/WBF sandwich composites to 6.5, 7.6, and 8.2% for sandwich composites modified by 3, 6, and 9 wt% KH550, respectively.Figure 4Flexural and tensile performance of the 3D CFs felt/WBFs epoxy composite sandwich modified by 3-aminopropyl triethoxy silane: (a) flexural strength, (b) flexural strain, (c) tensile strength, (d) tensile strain, (e) typical stress–strain curves for flexures, and (f) typical tensile stress–strain curves.Figure 4c demonstrates that the tensile strength of the sandwich composite made of 0 wt% KH550–epoxy/A-3D CFs/WBFs was 400% higher than that of unmodified epoxy (65 MPa). When compared to unmodified epoxy composite sandwich (0 wt% KH550 epoxy/3D CFs/WBFs sandwich composites) (325.0 MPa), the tensile strength decreases to 300.3, 255.0, and 200.6 MPa and decreases by 7.5, 21.5, and 38.2%, respectively, when the concentration of KH550 within the epoxy matrix is 3, 6, and 9 wt%. As the amount of KH550 within the epoxy matrix of sandwich composites is 9 wt% (9 wt% KH550–epoxy/A-3D CFs/WBFs), the biggest gains in tensile strain are 61.0 and 41.2%, respectively, when compared to pure epoxy and unmodified sandwich composites (0 wt% KH550–epoxy/A-3D CFs/WBF). Salinized epoxy matrix of 3DCFs/A-WBFs epoxy composites sandwich contribute much more to the tensile strain than sandwich composites without silane coupling agent. The typical flexural and tensile stress–strain curves of the 3D CFs felt/WBFs epoxy composite sandwich prepared by the cast-in-place process are shown in Figure 4e and f. These curves show that the deformation performance of the epoxy composite sandwich made from 3D CFs and WBFs was improved by the addition of the KH550 fibers with epoxy resin.Interestingly, silane coupling agents increase the flexural strain of epoxy composites. The flexural strain of sandwich composites treated with long-chain silane agents was highest in comparison to those without any treatment to the epoxy matrix. The increased deformation performance was induced by the significant physical connection between the silane molecules and the CF and basalt fibers, which was created by the long CH2 chains for the coupling agents and functioned as mechanically connecting sites against crack spreading. Long and flexible CH2 chains may intimately interact with epoxy, allowing silane agent molecules to infiltrate deeply into the epoxy matrix and chemically stimulate the Si group of silane molecules to bond with the epoxy. Furthermore, the polysiloxane (Si–O–Si) in the 3-aminopropyl triethoxy silane has good elasticity, as seen by the increasing number of polysiloxane (Si–O–Si) elements introduced into the epoxy matrix, enhancing composite elasticity.Impact strength is a very important measure of how tough a material is. The impact strength of a fiber-reinforced polymer is closely linked to the interaction between the fibers and the matrix as well as the properties of the matrix and the fibers. When a sudden force is applied to the composites, the impact energy is dispersed via a mixture of fiber pullouts, fiber breakage, and polymer deformation [58]. The results of charpy flatwise impact tests on various specimens of 3D CF/WBFs epoxy composite sandwich manufactured by casting modified epoxy resin with varied KH550 content within the 3D CF/WBFs sandwich are shown in Figure 5. The impact strength is raised from 90 kJ/m2 for the untreated 3D CF foam/WBFs epoxy composite sandwich (0 wt% KH550) to 125, 135, and 120 kJ/mm2 for the silane with concentration (3, 6, and 9 wt%) infused with epoxy resin inside the 3D CF foam/WBFs composites sandwich, respectively. The adherence of fibers to the matrix was enhanced by combining a silane coupling agent with an epoxy resin. Because of the greater interfacial bonding between the filler and epoxy matrix, the higher impact strength value was reflected in the energy absorpted capacity of the manufactured epoxy composite sandwich [58,71]. Furthermore, increasing the concentration of silane coupling agents to 9 wt% results in a considerable decrease in impact strength to 120 kJ/m2. The reason for this is that excess of silane coupling agents might cause a weak boundary layer due to physical adsorption at high silane coupling agent concentrations. The excess KH550 concentration within the epoxy matrix could decrease the cohesion between the fibers and epoxy matrix. So, as the silane coupling agent concentration goes up, the mechanical properties and characteristics of the composites go down [72].Figure 5Impact strength of the 3D CFs felt/WBFs epoxy composite sandwich strengthened by the modified epoxy matrix by 3-aminopropyl triethoxy silane (KH550).3.4Flexural performance and dynamic mechanical analysis of a 3D CF/epoxy composite designed as the core of a sandwichThe core of the sandwich composite, in comparison to the face sheets, is recognized to play a vital role in increasing energy absorption capacity in a sandwich structure. Table 2 depicts the influence of epoxy combined with different 3-aminopropyl triethoxy silane (KH550) contents on the flexural performance of the core of a sandwich composite reinforced with 3D CF foam. Flexural strength was increased by 105.5% in the 0 wt% KH550–epoxy/A-3D CFs composites compared to pure epoxy (91.0 MPa) (Table 2), and by 70.3, 58.2, and 49.5% in the 3D epoxy/A-3D CFs composites modified by combining epoxy resin with 3, 6, and 9 wt% KH550. When the epoxy solution was mixed with 3, 6, or 9 wt% KH550, the flexural strain of 3D CF foam reinforced epoxy improved by 25.8, 41.9, and 64.5%, respectively, when compared to pure epoxy.Table 2Flexural performance and DMA characteristics of core sandwich compositeMaterialsTg (°C)Storage modulus (MPa)Flexural strength (MPa)Flexural strain (%)Epoxy1481,73491.0 ± 93.1 ± 0.40 wt% KH550–epoxy/A-3D CFs 1503,822187 ± 53.5 ± 0.73 wt% KH550–epoxy/A-3D CFs 1433,408155 ± 83.9 ± 0.56 wt% KH550–epoxy/A-3D CFs 1372,923144 ± 64.4 ± 0.29 wt% KH550–epoxy/A-3D CFs 1292,737136 ± 95.1 ± 0.4The mobility of the epoxy chain within the core of 3D CFs/epoxy composite sandwich was investigated using dynamic mechanical analysis (DMA) in the bending mode in order to determine the effect of KH550 combined with epoxy resin on the performance of the core. The presence of KH550 can change the flexibility of a sandwich material, but the benefits received are dependent on its inclusion at the core. When they are introduced at the level of a core, for example, a core with high flexibility would increase the toughness and impact performance of sandwich [73]. Figure 6a shows the thermo-mechanical characteristics of the sandwich core, which was created by casting an epoxy resin blended with a variable concentration of KH550 into 3D CF foam. At room temperature, the flexural modulus and storage modulus indicate the rigidity of viscoelastic materials [74]. Figure 6 shows that the core prepared by casting pure epoxy within the acidified 3D CF foam (0 wt% KH550–epoxy/A-3D CFs) has a larger storage modulus (124.0% higher) than the pure epoxy. The mechanical interlocking brought on by the inclusion of CF foam, which reduces epoxy chain mobility, is responsible for the increase in storage modulus. The use of a silane coupling agent within the epoxy improves the epoxy chain mobility. As a consequence, the viscoelastic samples’ stiffness gradually decreases as the KH550 concentration rises, which causes a dip in the storage modulus curve. As a result, the stiffness of the viscoelastic samples steadily reduces as the KH550 concentration increases, causing the storage modulus curve to decline. Furthermore, Figure 6b and Table 2 show that increasing the concentration of KH550 in the epoxy liquid reduced the glass transition temperature of 3D CFs foam/epoxy systems from 150 to 129°C. By replacing some stiff phenyl chains with flexible siloxane molecules (–Si–O–Si–), the mobility of the epoxy chain was increased, resulting in a lower storage modulus and a lower glass transition temperature for the core of a sandwich composite (epoxy/A-3D CFs). This is also one of the explanations for why adding more KH550 to sandwich composites causes more deformation, as demonstrated in Table 2.Figure 6(a) Storage modulus and (b) loss factor (tan delta) of the representative samples as indicated in the graphs.3.5The analysis of strengthen mechanismsSEM micrographs of the cracked surface of sandwich composites were taken after flexural testing to examine the fiber/matrix bonding in sandwich composites reinforced with WBFs and CF foam. Figure 7 shows the considerable changes in fracture surfaces caused by the change in fiber–epoxy–KH550 adhesion. Figure 7a and c illustrates the surface of fracture of the core and face of unmodified sandwich composite (without mixing epoxy with KH550). It can be seen that no epoxy adhered to the fiber surfaces of carbon fibers (Figure 7a) and basalt fibers (Figure 7c), and the epoxy between fibers was divided up. These characteristics indicate that the carbon fiber and untreated epoxy matrix do not adhere to one another. Furthermore, the fracture surfaces of 3, 6, and 9 wt% KH50-modified sandwich composites are shown in Figure 7b (core) and Figure 7d (face). The epoxy matrix is continuous between carbon fibers (Figure 7b) and basalt fibers (Figure 7d), and the fiber is clearly strongly bonded with the epoxy matrix. Furthermore, the epoxy matrix did not exhibit any micro-cracks where the carbon fibers and basalt fibers met. Based on these results, it can be said that KH550-modified sandwich epoxy composites reinforced with carbon fibers or basalt fibers have better fiber/matrix adhesion than both unmodified and KH550-modified epoxy composites. Furthermore, the fracture surfaces of epoxy/A-3D CFs composites that were strengthened with different concentrations of KH550 within the epoxy matrix were examined by SEM (Figure 8). Presenting the KH550 at 6 wt% enhances the cohesion between the fibers and epoxy matrix. Nonetheless, the 9 wt% KH550 demonstrates a little drop in bonding between the fibers and matrix. This adds more evidence to the idea that too much silane in composites makes them less resistant to breaking.Figure 7SEM images of fracture surfaces of epoxy composites sandwich (epoxy/3D A-CFs foam/WBFs): (a) unmodified core of sandwich composite (0 wt% KH550–epoxy/3D A-CFs), (b) modified core of sandwich composite (6 wt% KH550–epoxy/3D A-CFs), (c) unmodified face of sandwich composite (epoxy/WBFs), and (d) modified face of sandwich composite (6 wt% KH550–epoxy/WBFs).Figure 8SEM images of the KH550–epoxy/A-3D CFs composites: (a) 0 wt% KH550–epoxy/A-3D CFs, (b) 3 wt% KH550–epoxy/A-3D CFs, (c) 6 wt% KH550–epoxy/A-3D CFs, and (d) 9 wt% KH550–epoxy/A-3D CFs.4ConclusionThis work developed a low-cost epoxy composite sandwich with high mechanical properties by preparing WBFs/epoxy composite as a face combined with a core of CF/epoxy. The cast-in-place method worked well for making epoxy sandwich composites. 3-Aminopropyl triethoxy silane was added to the epoxy liquid to improve the interaction between the epoxy matrix and the fibers, which made the composites tougher. The three-dimensional pore structure of 3D CFs is highly interconnected, providing interlocking sites for the epoxy to impregnate and form a bicontinuous network. As compared to the pure epoxy, the flexural and tensile strengths of the 3D CFs/WBFs epoxy composite sandwich were improved by 298.1 and 353.8%, respectively, while the impact strength was increased to 135 kJ/m2. As a result of the increased stiffness brought about by the addition of 3D CF, DMA investigation of epoxy and A-3D CFs samples revealed maximum increases in glass transition temperature (Tg) and storage modulus of 150°C and 3,822 MPa, respectively. Furthermore, due to enhanced interfacial conditions and the replacement of certain rigid phenyl chains with flexible siloxane molecules (Si–O–Si), the flexural strain of epoxy/A-3D CFs composite with additional 3-aminopropyl triethoxy silane (KH550) was improved by 64.5%. This article is about a good way to make high-performance structural materials using 3D fibers and basalt fibers, as well as a way to make materials that are cheap and good for the environment. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nanotechnology Reviews de Gruyter

High mechanical performance of 3-aminopropyl triethoxy silane/epoxy cured in a sandwich construction of 3D carbon felts foam and woven basalt fibers

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Publisher
de Gruyter
Copyright
© 2023 the author(s), published by De Gruyter
ISSN
2191-9097
eISSN
2191-9097
DOI
10.1515/ntrev-2022-0519
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Abstract

1IntroductionEpoxy resin offers good mechanical performance and corrosion resistance, among others, and is increasingly employed in aircraft, automobiles, and related sectors [1,2]. However, several characteristics of epoxy resin materials are disadvantageous, such as the inherent brittleness generated by the high density of cross-links formed during curing. This restricts their applicability in several areas [1,3–5]. Several studies have been conducted to improve the toughness of highly cross-linked epoxy resins. Nanoparticles, thermoplastics, and liquid crystalline elastomers are frequently used to toughen epoxy resins. Examples include graphene (Gr), multiwall carbon nanotubes (MWCNTs), poly (4,4-bis(6-hydroxyhexyloxy) biphenyl phenylsuccinate), polysulfone, and nanoclays [6–13]. Due to their smooth surface, high surface/volume ratio, and Van der Waals force, nanoparticles tend to cluster inside the resulting composite and interact poorly with the epoxy matrix, reducing the efficiency of load transfer at the interface [14–16]. In order to achieve high-performance mechanical qualities, the fillers must be matrix compatible [17]. As a result, various research studies on improving fiber/matrix connections have already been conducted. By changing the chemicals on the surface of the filler, modification techniques can improve the bonding between the fibers and the matrix [18–20].The limitations of nanofillers, such as poor dispersion and re-agglomeration, can be solved by a three-dimensional low-density sponge-like material having a very linked architecture, high strength, and flexibility [21]. Aerogels, sponges, and foams are examples of 3D graphene or carbon nanotube frameworks that are low in density while retaining the great qualities of 2D graphene or 1D carbon nanotubes. Highly porous carbon foams allow the polymer resin to penetrate their cellular structure, producing a composite with a consistent nanocarbon distribution throughout the matrix [22]. The nanocarbon foam’s linked network of nodes and branches provides smooth routes for stress transmission [23]. Compression and tensile strength for epoxy/3D graphene–polyaniline composites were observed to increase by 25.0 and 32.5%, respectively, compared to solid plain epoxy, as reported by Wang et al. [24]. Jia et al. [25] discovered that adding 0.1 wt% graphene foam to an epoxy increased the toughness by 78.0%, while adding 0.2 wt% graphene foam increased flexure modulus by 53.1% and strength by 38.1%. Anisotropy in electrical and mechanical characteristics was reported by Kim et al. Freeze-drying graphene oxide allowed for the aerogels to be injected with epoxy, resulting in a graphene aerogel/epoxy composite. When the crack spreads in a direction perpendicular to the symmetry of the graphene layers, the filler concentration of 1.4 wt% results in a 64.0% improvement in fracture toughness. There was a maximum difference of 113% between the fracture toughness of these composites in parallel and transverse alignment of the graphene sheets [26]. During the past 10 years, several studies have been conducted to produce carbon felt (CF) foam as a low-cost fiber material. CF foam is often employed as an electrode for environmental purposes and many other applications because of its outstanding mechanical properties, high conductivity, high porosity, cheap price, and because it has a superb 3D cross-linked network made of carbon fibers [27–29]. In general, felt is created by first processing needle punches and then drawing carbon. CF is created by graphitizing regenerated cellulose, and the operating temperature ranges between 1,200 and 1,600°C. However, other carbon-based materials have also been extensively used for composite applications, including vitreous nanocarbon, carbon aerogel, carbon fiber, and carbon bucky paper. In contrast to CFs, these materials do have certain drawbacks. For example, because other carbon-based materials are highly costly, they cannot be utilized to produce high-volume, low-cost composites. Xu et al. [27] prepared 3D CF foam/epoxy composites by impregnating epoxy resin into the 3D CF foam, in which the flexural strength of the composites rose from 96.0 to 125.0 MPa after adding CF foam into the epoxy matrix.Unfortunately, due to surface inertness and the absence of functional groups in 3D carbon foam, the interfacial interaction property between 3D interconnection filler and epoxy resin is weak. Because significant contact resistance exists at polymer composite interfaces, an effective interfacial bonding effect is required to increase the mechanical quality. The overall performance of composites could be significantly increased when the interface is effectively changed by introducing interfacial layers [30,31]. Furthermore, the use of these procedures is expensive, takes time, and complicates the method for composite synthesis. Silane coupling agents are widely used to boost the filler’s affinity for the epoxy matrix and to ensure that the filler is evenly distributed throughout the epoxy composite, which comprises at least two distinct functional groups. Coupling agents are frequently used to form a solid bond between the filler and the matrix. Therefore, the reinforcement performance is enhanced and the polymer matrix is preserved for a longer period of time due to load transfer from the weak polymer to the stronger filler [32–34]. However, the needed refluxing in the salinization process may harm the filler’s characteristics [35]. In addition, chemical reagents used in fiber surface modification might damage the environment with chemical pollutants.The remarkable mechanical capabilities of basalt fiber-reinforced epoxy composites have drawn the attention of scientists in the modern industrial sector [36–38]. Basalt fiber is produced by extruding molten basalt rock, which mostly consists of Si and Al oxides. Basalt fiber requires less energy to produce than nanofibers, glass fibers, and carbon fibers since its manufacturing method is simpler (melting and extrusion) and produces less carbon dioxide and other environmental waste. Basalt fiber composites are superior to glass fiber composites due to their superior combination of rigidity, strength, fracture toughness, and energy absorption. The tensile strength of a single basalt fiber may reach up to 2.8 GPa, and it also exhibits high chemical and thermal stability [36]. Basalt fibers hybridized with nanofillers can improve the mechanical properties of their epoxy composites [39–41]. Hosna and Azizi looked into the mechanical properties of woven basalt fiber (WBF)/epoxy composites with silane-modified nanozirconia (ZrO2) [42]. Flexural and tensile tests showed that, compared to a sample without nanozirconia, the strength of the material rose by 90.0 and 85.0%, respectively, with a dispersion of 3 wt% ZrO2. The flexural behaviors of modified carbon nanotube epoxy/basalt composites were examined by Kim et al. [43]. According to the findings, silane-treated CNT/epoxy/basalt composites have a flexural modulus and strength that is 10.0 and 14.0% greater, respectively, than those made with acidified CNTs. Nanofillers are known to form agglomerates within the polymer if not chemically modified properly. Through hybridization, it is important to make a composite system that takes advantage of the improved properties that come from mixing a synthetic, eco-friendly, cheap, and non-toxic filler with basalt fiber reinforcing. Table 1 highlights research on epoxy composites reinforced using basalt fibers.Table 1Overview of the research done on how WBFs improve the mechanical performance of epoxy compositesFiller typeProcessFlexural strength (MPa)Tensile strength (MPa)Ref.WBFHLC—202.0[49]CNT coated WBFHand lay-up240.4—[36]Basalt powder/WBFsHand lay-up—214.0[50]Nano-zirconia–graphene oxide/WBFsHand lay-up406.0—[51]Woven flax/WBFsHand layup88.7170.9[52]Graphene nanopellets/WBFsHand lay-up240.0273.9[53]3D interconnected graphene skeletonRTM175.0130.0[54]Woven glass fibers/WBFsRTM194.54225.9[55]TiC/WBFsVARTM215.2—[56]Silanized CNT/WBFsVARTM—232.0[57]Coir/Innegra/WBFsHot-pressing290.0170.0[58]Tourmaline/WBFVARTM266.1357.2[59]3D CF foam/WBFsCast-in-place363.3325.0This workNote: HLC, hand layup followed by compression molding; RTM, resin transfer molding; VARTM, vacuum-assisted resin transfer molding.Sandwich composites are a practical way to reduce the cost of a high-performance composite material while also increasing ultimate strain and impact properties [44]. Sandwich structures are a type of laminated composite. It is made up of two thin face sheets bonded together with a thick lightweight core [45]. It is possible to mix many fiber types in a way that reduces or eliminates the drawbacks of each. High-stiffness, high-cost fibers like woven fibers make up one kind of hybrid fiber composite, while low-modulus, low-cost fibers make up the other [46]. The stiffness and energy absorption capabilities are provided by the high modulus fiber, while the low-stiffness fiber increases the composite’s resistance to damage and keeps the cost of the raw materials manageable. The impact load might cause sandwich composites to break catastrophically. The impact damage resistance of sandwich composites influences structural reliability and is critically dependent on the core. The most prevalent core materials are honeycomb or foam-based, although various unique core structures are also being developed to improve the mechanical and impact characteristics of sandwich composites [46,47]. However, short fibers placed with uniform dispersion inside epoxy composites can provide high mechanical qualities at a cheap cost. Therefore, a suitable product solution is the introduction of foam made of short fibers as a core material and woven fibers as faces for composites, which can lower their cost and improve their mechanical performance. Basalt and flax fiber hybrids were created in an epoxy matrix by Fiore et al. [48]. Flax fibers were utilized inside, while basalt fibers were employed on the outside. The testing results showed that the addition of four outside basalt layers increased the flexural strength. Flexural strength increased in flax-basalt specimens from 63.0 to 107.0 MPa. The rise in flexural strength is due to the ability of basalt fibers to handle bending pressures and better stress transmission from the matrix. This makes the strength properties better.The main motivation for this research is using a cast-in-place process for fabricating a sandwich of epoxy composite reinforced by 3D CF foam as a core and WBFs as a face. In addition, the study investigated how 3-aminopropyl triethoxy silane mixed with the resin influenced the sandwich of epoxy composites’ mechanical performance. To the author’s knowledge, the modified epoxy by 3-aminopropyl triethoxy and reinforced by CF foam and WBFs made by the cast-in-place process has not been documented in the literature. So, it was decided that a proposal for a composite system was needed to improve the interface between the epoxy and fibers by making sandwich composites of 3D fiber foam and woven fiber-reinforced epoxy composites using modified epoxy resin.2Experimental2.1MaterialsBoth the diethanolamine curing agent and bisphenol A diglycidyl ether epoxy resin (E55) were purchased from Dongguan Qiancheng plasticizing materials Co., Ltd in Guangdong, China. Pure CF foam, 5 mm in thickness, was supplied by SGL Carbon Se Co., Ltd. The density of CF is 1.79 g/cm3, and the diameter of the felt is 23 μm. Basalt fiber plain wave mats of density 2.8 g/cm2, filament diameter of 13 μm, and tensile strength of 4,840 MPa were supplied by Basalt Fiber Tech Co., Ltd. 3-Aminopropyl triethoxy silane (KH550) and nitric acid were purchased from Chengdu Kelong Chemicals Factory Co. Ltd in Chengdu, China.2.2Treatment of the CF’s surfaceTo improve the interface contact between the CF and the silanized epoxy resin at the core of the sandwich composites, the 3D CF foam was oxidized by soaking it in a diluted HNO3 aqueous solution (2 M) for 1 h at 60°C.2.3Sandwich composites preparationTypically, the curing agent (9 mL), epoxy resin (100 g), and 3-aminopropyl triethoxysilane (0, 3, 6, and 9 wt% of epoxy resin) were initially combined together while being mechanically stirred. After that, a cast-in-place method was used to imbue the 5 mm thick 3D CF foam with the epoxy and silane coupling agent combination [60,61], as seen in Figure 1. The WBFs were then incorporated as a face sheet in five layers with a thickness of about 0.5 mm. The resin-impregnated WBF sheets were positioned above and below the core (3D CF foam). The sandwich composite was then sealed with a vacuum bag, and air pressure was used to remove the air bubbles within the sandwich and keep the core and faces together. The fiber volume fraction of 3D CFs/epoxy (core) and basalt/epoxy (face) was approximately 31 and 45%, respectively. The 3D CF/WBF epoxy composite sandwich fabrication method is shown in Figure 1.Figure 1A schematic of the method used to create a 3D CF foam core sandwiched between a face sheets made of laminated WBFs.2.4Characterization and testingField-emission scanning electron microscopy (FE-SEM) was used to examine the surface morphology of sandwich composites and the morphology of 3D CFs (JEOL JSM-7800F, Japan).The CF distribution in epoxy was measured using an optical microscope (LEICA, Wetzlar, Germany).The Fourier transform infrared (FTIR) spectrometer was used to analyze the chemical composition of the pure and acidified CF (FTIR-7600, Lambda, Australia).The structural integrity of CF was analyzed by taking Raman spectra using a Raman spectrometer (inVia, Renshaw, London, UK) and a 532 nm laser excitation before and after the surface modification.Dynamic mechanical analysis (242 E Artemis, Netzsch, Germany) in the bending mode was used to estimate the glass transition temperature and storage modulus of epoxy composites sandwich. The temperature range and heating rate were set to 30–180°C and 10°C/min, respectively.The flexural and tensile characteristics were evaluated using universal testing equipment (WDW-300, Beijing, China) following the GB1040-92 and GB1449-2005. The XJU impact testing machine was used for the impact test, following the ISO 179.3Results and discussion3.1Core morphology/structure analysisTaking into account the unique structure of the 3D network of CFs, epoxy was incorporated into 3D CFs to manufacture the core of sandwich composites based on epoxy composites as performing reinforcement using a cast-in-place process. A cast-in-place technique was developed and used to directly cast epoxy resin into porous 3D CFs, resulting in a bicontinuous composite with excellent mechanical properties. When seen via the lens of the camera, the optical photograph of the original three-dimensional skeleton of CF foam revealed that the three-dimensional CFs have remarkable flexibility properties (Figure 2a). CF is composed of networks that are both regular and highly linked (Figure 2a), which provides 3D CFs with excellent flexibility. 3D CFs fibers create regular and densely linked networks, allowing 3D CFs to be pretty flexible. The excellent incorporation of superior flexibility would aid in improving the flexibility of composites.Figure 2Optical photo, morphology, and chemical structure of the core based on 3D CFs: (a) optical image of highly flexible 3D CFs, (b) scanning electron micrographs of 3D CFs, (c) 3D CFs/epoxy composite, (d) FTIR spectrum of unmodified and modified 3D CFs treated with acid, and (e) Raman spectra of 3D CF.FE-SEM was used to examine the morphology of pristine CF foam. The 3D CF foam has a regular and uniform shape as well as a surface structure that is spongy (Figure 2b). The dispersion of CF within the epoxy was examined using optical microscopy. Figure 2c shows that the 3D CF/epoxy composite (core) has a large number of dark lines (fibers) that cover a large area compared to the bright patches (epoxy). This shows that the CFs are evenly distributed in the epoxy because the fibers are arranged in a regular way within the foam.Figure 2d compares the FTIR spectra of pristine carbon foam (P-3D CFs) and acidified carbon foam (A-3D CFs) to identify the chemical groups of 3D CFs caused by surface treatments. The distinctive peak (3,418 cm−1) in the pristine CF spectrum corresponds to the hydroxyl group O–H vibration and the H–O–H bending, respectively. The C–H vibration in CH2 is responsible for the peaks at 2,973 cm−1. The carbon backbone of a fiber has a C–C peak at 1,637 cm−1. The strength of the O–H and H–O–H bands shifted from 3,418 to 3,438 cm−1 when the CF was acidified by NHO3. Also, clear peaks at 2,932/2,849–1,463 cm−1 (C–H), 1,738 cm−1 (C═O), 11,625 cm−1 (C═C), and 1,120 cm−1 (C–O) show that the surface of the modified CF has more oxygen-containing carboxyl groups.To evaluate the changes in graphitization caused by the treatment procedures, Raman spectroscopy was used. In Figure 2e, two significant Raman peaks are seen in both the P-3D CFs and the A-3D CFs. The Raman peaks of 1,345/1,249 and 1,595/1,599 cm−1 correspond to graphite’s sp3 carbon networks (D-band) and sp2 carbon networks (G-band), respectively [29,62]. The ID/IG ratio was found by dividing the intensity of the D band by the intensity of the G band [9]. Figure 2e shows that the ID/IG ratio barely changed after the surface treatments, which shows that the acid oxidation method had little effect on the graphite integrity of the CFs.3.2Structure characterization of modified epoxy matrix by 3-aminopropyl triethoxy silaneIn this work, the 3-aminopropyl triethoxy silane was directly mixed into the epoxy solution to produce an epoxy matrix modification. Silane coupling agents of 3-aminopropyl triethoxy include alkoxysilane groups that may respond to the network structure of fibers, which consists of Si–OH or OH, forming a new bond of Si–O–Si–, resulting in the most regular silicon-oxide network structure, likely to result in enhanced interfacial interaction between fibers and epoxy, and forming a chemical link between the coupling agent molecules (NH2) and the epoxy group in the resin [63,64]. The mechanism of the 3-aminopropyl triethoxy silane reaction with epoxy is illustrated in Figure 3a. Figure 3b displays the FTIR spectrum of an untreated and treated epoxy resin. (–NH) stretching is attributed to the peak at 3,373 cm−1. The peak at 2,970 cm−1 is asymmetric (–CH3) bands, whereas the peak at 2,849 is symmetric stretching modes (–CH2). Additionally, the absorption of epoxide groups is shown by the peak at 1,025 and 910 cm−1. The substituted C═C group in epoxy resin is what causes the absorption bands at 1,604 and 828 cm−1. The spectra of silane-modified epoxy resin differed from that of pure epoxy resin. The –NH peak is broadened and moved to 3,428 cm−1 coming from amine stretching vibrations. Furthermore, a new peak is appearing at 1,610 cm−1, which is characteristic of primary amines and which develop via a chemical bond between the coupling agent molecules (NH2) and the epoxy group in the resin. The silane coupling agent’s amine group initiates a nucleophilic attack on the epoxy ring, resulting in a “ring-opening reaction” and the production of a new hydroxyl group [65]. Peak intensification at 1,036 and 1,100 cm−1 caused by the creation of C–O–Si and O–Si–O linkages in the silane-treated epoxy resin [66–68]. Methoxy groups in the 3-aminopropyl triethoxy silane may produce silanols during the reactions, and hydroxyl groups on the epoxy–fiber surfaces may result in the development of oxygen linkages. Several publications have described an amino-silane coupling mechanism in fiber/epoxy composites [69,70].Figure 3(a) Reaction formula among epoxy resin, 3-aminopropyl triethoxy silane, and fiber and (b) FTIR spectra of 3-aminopropyltriethoxy silane/epoxy after-curing.3.3Mechanical performance of the epoxy sandwich composite constructed from WBFs as a face sheet on 3D CF coreThe epoxy composite sandwich of 3D CF/WBFs was made using a cast-in-place process, and its mechanical strength was evaluated using flexural, tensile, and impact strength tests. The flexural performance of an epoxy-based sandwich composite entirely reinforced with 3D CFs foam as the core and strengthened with WBF face-sheets is shown in Figure 4. Compared to the pure epoxy (91 MPa), the unmodified epoxy resin by KH550 and impregnated in sandwich composites of 3D CFs/WBFs (0 wt% KH550–epoxy/A-3D CFs/WBFs) showed a higher increase in flexural strength of 298.1%. Also, Figure 4a shows that adding KH550 to epoxy resin reduces the flexural strength for A-3D CFs/WBF epoxy composite sandwiches. Flexural strength is reduced by 2.5, 6.0, and 17.3% when the KH550 content of the epoxy is 3, 6, and 9 wt%, respectively. The flexural strain of pure epoxy, untreated epoxy/A-3D CFs/WBFs sandwich composites, and treated epoxy/A-3D CFs/WBFs sandwich composites by KH550 is shown in Figure 4b. It is found that the flexural strain of the epoxy composite sandwich is significantly improved by the addition of KH550 with epoxy resin within A-3D CFs/WBFs. Flexural deformation increased from 5.0% for 0 wt% KH550–epoxy/A-3D CFs/WBF sandwich composites to 6.5, 7.6, and 8.2% for sandwich composites modified by 3, 6, and 9 wt% KH550, respectively.Figure 4Flexural and tensile performance of the 3D CFs felt/WBFs epoxy composite sandwich modified by 3-aminopropyl triethoxy silane: (a) flexural strength, (b) flexural strain, (c) tensile strength, (d) tensile strain, (e) typical stress–strain curves for flexures, and (f) typical tensile stress–strain curves.Figure 4c demonstrates that the tensile strength of the sandwich composite made of 0 wt% KH550–epoxy/A-3D CFs/WBFs was 400% higher than that of unmodified epoxy (65 MPa). When compared to unmodified epoxy composite sandwich (0 wt% KH550 epoxy/3D CFs/WBFs sandwich composites) (325.0 MPa), the tensile strength decreases to 300.3, 255.0, and 200.6 MPa and decreases by 7.5, 21.5, and 38.2%, respectively, when the concentration of KH550 within the epoxy matrix is 3, 6, and 9 wt%. As the amount of KH550 within the epoxy matrix of sandwich composites is 9 wt% (9 wt% KH550–epoxy/A-3D CFs/WBFs), the biggest gains in tensile strain are 61.0 and 41.2%, respectively, when compared to pure epoxy and unmodified sandwich composites (0 wt% KH550–epoxy/A-3D CFs/WBF). Salinized epoxy matrix of 3DCFs/A-WBFs epoxy composites sandwich contribute much more to the tensile strain than sandwich composites without silane coupling agent. The typical flexural and tensile stress–strain curves of the 3D CFs felt/WBFs epoxy composite sandwich prepared by the cast-in-place process are shown in Figure 4e and f. These curves show that the deformation performance of the epoxy composite sandwich made from 3D CFs and WBFs was improved by the addition of the KH550 fibers with epoxy resin.Interestingly, silane coupling agents increase the flexural strain of epoxy composites. The flexural strain of sandwich composites treated with long-chain silane agents was highest in comparison to those without any treatment to the epoxy matrix. The increased deformation performance was induced by the significant physical connection between the silane molecules and the CF and basalt fibers, which was created by the long CH2 chains for the coupling agents and functioned as mechanically connecting sites against crack spreading. Long and flexible CH2 chains may intimately interact with epoxy, allowing silane agent molecules to infiltrate deeply into the epoxy matrix and chemically stimulate the Si group of silane molecules to bond with the epoxy. Furthermore, the polysiloxane (Si–O–Si) in the 3-aminopropyl triethoxy silane has good elasticity, as seen by the increasing number of polysiloxane (Si–O–Si) elements introduced into the epoxy matrix, enhancing composite elasticity.Impact strength is a very important measure of how tough a material is. The impact strength of a fiber-reinforced polymer is closely linked to the interaction between the fibers and the matrix as well as the properties of the matrix and the fibers. When a sudden force is applied to the composites, the impact energy is dispersed via a mixture of fiber pullouts, fiber breakage, and polymer deformation [58]. The results of charpy flatwise impact tests on various specimens of 3D CF/WBFs epoxy composite sandwich manufactured by casting modified epoxy resin with varied KH550 content within the 3D CF/WBFs sandwich are shown in Figure 5. The impact strength is raised from 90 kJ/m2 for the untreated 3D CF foam/WBFs epoxy composite sandwich (0 wt% KH550) to 125, 135, and 120 kJ/mm2 for the silane with concentration (3, 6, and 9 wt%) infused with epoxy resin inside the 3D CF foam/WBFs composites sandwich, respectively. The adherence of fibers to the matrix was enhanced by combining a silane coupling agent with an epoxy resin. Because of the greater interfacial bonding between the filler and epoxy matrix, the higher impact strength value was reflected in the energy absorpted capacity of the manufactured epoxy composite sandwich [58,71]. Furthermore, increasing the concentration of silane coupling agents to 9 wt% results in a considerable decrease in impact strength to 120 kJ/m2. The reason for this is that excess of silane coupling agents might cause a weak boundary layer due to physical adsorption at high silane coupling agent concentrations. The excess KH550 concentration within the epoxy matrix could decrease the cohesion between the fibers and epoxy matrix. So, as the silane coupling agent concentration goes up, the mechanical properties and characteristics of the composites go down [72].Figure 5Impact strength of the 3D CFs felt/WBFs epoxy composite sandwich strengthened by the modified epoxy matrix by 3-aminopropyl triethoxy silane (KH550).3.4Flexural performance and dynamic mechanical analysis of a 3D CF/epoxy composite designed as the core of a sandwichThe core of the sandwich composite, in comparison to the face sheets, is recognized to play a vital role in increasing energy absorption capacity in a sandwich structure. Table 2 depicts the influence of epoxy combined with different 3-aminopropyl triethoxy silane (KH550) contents on the flexural performance of the core of a sandwich composite reinforced with 3D CF foam. Flexural strength was increased by 105.5% in the 0 wt% KH550–epoxy/A-3D CFs composites compared to pure epoxy (91.0 MPa) (Table 2), and by 70.3, 58.2, and 49.5% in the 3D epoxy/A-3D CFs composites modified by combining epoxy resin with 3, 6, and 9 wt% KH550. When the epoxy solution was mixed with 3, 6, or 9 wt% KH550, the flexural strain of 3D CF foam reinforced epoxy improved by 25.8, 41.9, and 64.5%, respectively, when compared to pure epoxy.Table 2Flexural performance and DMA characteristics of core sandwich compositeMaterialsTg (°C)Storage modulus (MPa)Flexural strength (MPa)Flexural strain (%)Epoxy1481,73491.0 ± 93.1 ± 0.40 wt% KH550–epoxy/A-3D CFs 1503,822187 ± 53.5 ± 0.73 wt% KH550–epoxy/A-3D CFs 1433,408155 ± 83.9 ± 0.56 wt% KH550–epoxy/A-3D CFs 1372,923144 ± 64.4 ± 0.29 wt% KH550–epoxy/A-3D CFs 1292,737136 ± 95.1 ± 0.4The mobility of the epoxy chain within the core of 3D CFs/epoxy composite sandwich was investigated using dynamic mechanical analysis (DMA) in the bending mode in order to determine the effect of KH550 combined with epoxy resin on the performance of the core. The presence of KH550 can change the flexibility of a sandwich material, but the benefits received are dependent on its inclusion at the core. When they are introduced at the level of a core, for example, a core with high flexibility would increase the toughness and impact performance of sandwich [73]. Figure 6a shows the thermo-mechanical characteristics of the sandwich core, which was created by casting an epoxy resin blended with a variable concentration of KH550 into 3D CF foam. At room temperature, the flexural modulus and storage modulus indicate the rigidity of viscoelastic materials [74]. Figure 6 shows that the core prepared by casting pure epoxy within the acidified 3D CF foam (0 wt% KH550–epoxy/A-3D CFs) has a larger storage modulus (124.0% higher) than the pure epoxy. The mechanical interlocking brought on by the inclusion of CF foam, which reduces epoxy chain mobility, is responsible for the increase in storage modulus. The use of a silane coupling agent within the epoxy improves the epoxy chain mobility. As a consequence, the viscoelastic samples’ stiffness gradually decreases as the KH550 concentration rises, which causes a dip in the storage modulus curve. As a result, the stiffness of the viscoelastic samples steadily reduces as the KH550 concentration increases, causing the storage modulus curve to decline. Furthermore, Figure 6b and Table 2 show that increasing the concentration of KH550 in the epoxy liquid reduced the glass transition temperature of 3D CFs foam/epoxy systems from 150 to 129°C. By replacing some stiff phenyl chains with flexible siloxane molecules (–Si–O–Si–), the mobility of the epoxy chain was increased, resulting in a lower storage modulus and a lower glass transition temperature for the core of a sandwich composite (epoxy/A-3D CFs). This is also one of the explanations for why adding more KH550 to sandwich composites causes more deformation, as demonstrated in Table 2.Figure 6(a) Storage modulus and (b) loss factor (tan delta) of the representative samples as indicated in the graphs.3.5The analysis of strengthen mechanismsSEM micrographs of the cracked surface of sandwich composites were taken after flexural testing to examine the fiber/matrix bonding in sandwich composites reinforced with WBFs and CF foam. Figure 7 shows the considerable changes in fracture surfaces caused by the change in fiber–epoxy–KH550 adhesion. Figure 7a and c illustrates the surface of fracture of the core and face of unmodified sandwich composite (without mixing epoxy with KH550). It can be seen that no epoxy adhered to the fiber surfaces of carbon fibers (Figure 7a) and basalt fibers (Figure 7c), and the epoxy between fibers was divided up. These characteristics indicate that the carbon fiber and untreated epoxy matrix do not adhere to one another. Furthermore, the fracture surfaces of 3, 6, and 9 wt% KH50-modified sandwich composites are shown in Figure 7b (core) and Figure 7d (face). The epoxy matrix is continuous between carbon fibers (Figure 7b) and basalt fibers (Figure 7d), and the fiber is clearly strongly bonded with the epoxy matrix. Furthermore, the epoxy matrix did not exhibit any micro-cracks where the carbon fibers and basalt fibers met. Based on these results, it can be said that KH550-modified sandwich epoxy composites reinforced with carbon fibers or basalt fibers have better fiber/matrix adhesion than both unmodified and KH550-modified epoxy composites. Furthermore, the fracture surfaces of epoxy/A-3D CFs composites that were strengthened with different concentrations of KH550 within the epoxy matrix were examined by SEM (Figure 8). Presenting the KH550 at 6 wt% enhances the cohesion between the fibers and epoxy matrix. Nonetheless, the 9 wt% KH550 demonstrates a little drop in bonding between the fibers and matrix. This adds more evidence to the idea that too much silane in composites makes them less resistant to breaking.Figure 7SEM images of fracture surfaces of epoxy composites sandwich (epoxy/3D A-CFs foam/WBFs): (a) unmodified core of sandwich composite (0 wt% KH550–epoxy/3D A-CFs), (b) modified core of sandwich composite (6 wt% KH550–epoxy/3D A-CFs), (c) unmodified face of sandwich composite (epoxy/WBFs), and (d) modified face of sandwich composite (6 wt% KH550–epoxy/WBFs).Figure 8SEM images of the KH550–epoxy/A-3D CFs composites: (a) 0 wt% KH550–epoxy/A-3D CFs, (b) 3 wt% KH550–epoxy/A-3D CFs, (c) 6 wt% KH550–epoxy/A-3D CFs, and (d) 9 wt% KH550–epoxy/A-3D CFs.4ConclusionThis work developed a low-cost epoxy composite sandwich with high mechanical properties by preparing WBFs/epoxy composite as a face combined with a core of CF/epoxy. The cast-in-place method worked well for making epoxy sandwich composites. 3-Aminopropyl triethoxy silane was added to the epoxy liquid to improve the interaction between the epoxy matrix and the fibers, which made the composites tougher. The three-dimensional pore structure of 3D CFs is highly interconnected, providing interlocking sites for the epoxy to impregnate and form a bicontinuous network. As compared to the pure epoxy, the flexural and tensile strengths of the 3D CFs/WBFs epoxy composite sandwich were improved by 298.1 and 353.8%, respectively, while the impact strength was increased to 135 kJ/m2. As a result of the increased stiffness brought about by the addition of 3D CF, DMA investigation of epoxy and A-3D CFs samples revealed maximum increases in glass transition temperature (Tg) and storage modulus of 150°C and 3,822 MPa, respectively. Furthermore, due to enhanced interfacial conditions and the replacement of certain rigid phenyl chains with flexible siloxane molecules (Si–O–Si), the flexural strain of epoxy/A-3D CFs composite with additional 3-aminopropyl triethoxy silane (KH550) was improved by 64.5%. This article is about a good way to make high-performance structural materials using 3D fibers and basalt fibers, as well as a way to make materials that are cheap and good for the environment.

Journal

Nanotechnology Reviews – de Gruyter

Published: Jan 1, 2023

Keywords: sandwich epoxy composite; 3D carbon felt; woven basalt fiber; cast-in-place process; 3-aminopropyl triethoxy silane

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