Abstract
Reinforcing fibers, nanofillers, matrix materials, and manufacturing techniques all have a role in the mechanical characteristics of hybrid composites. MWCNTs-reinforced E-glass/Kevlar/epoxy composites are appropriate fillers for structural applications. The impact of different concentrations of MWCNT fillers (0.4%, 0.8%, and 1.2% wt) on the mechanical characteristics of hybrid composites has been studied. Tensile and bending strength, as well as hardness, were measured in compression-molded composites. The effects of compression pressure, mold temperature, and applied pressure on hybrid (0.8% MWCNT) were investigated. When it came to composite tensile and bending strength, compression pressure was the most important factor, closely succeeded by mold temperature and pressure period. Compression molding were optimized, resulting in a tensile strength of 183 MPa, a bending strength of 158.3 MPa, and a hardness value of 23.8 HV.
1. Introduction
Composites are taking the place of traditional materials due to their low cost and high productivity in manufacturing and processing [1]. Polymeric materials are superior to nonpolymeric materials in a wide range of applications, including structural, automotive, electrical, marine, biomedical, and chemical parts [2, 3]. For the above-mentioned uses, polymer composites made with synthetic or else natural fibers were widely used. Composite structures can benefit from the superior stiffness and strength of synthetic fibers (such as basalt, Kevlar, glass, and carbon) when used for load-bearing purposes [4–6]. Glass-fiber reinforcement has outstanding mechanical and thermal qualities that are ideal for aeronautical use. To make ballistic armor, bulletproof, and other various protective materials, polymer composites are made using matrix material [7, 8]. Using epoxy resin as the matrix material for hybrid composite parts has been shown to increase their tensile, flexural, and impact strength significantly. Epoxy matrix also reduces composite part density by 3.12 percent, allowing for the production of lighter components [9–11]. Sui for aerospace applications, GFEC assessed for adhesive bonding strength exhibits high adhesion. Polymer composites with improved strength and hardness can be made by combining synthetic fibers through epoxy materials and its manufacturing methods [12]. There is an information in Table 1 about the materials and characteristics of polymer matrix composites (PMCs). With significant qualities (physical and mechanical as well as tribological) in epoxy composites, they were best suited for use in many different applications. Polymer matrix composites (PMCs) are made up of fibers that enhance the composite’s structural integrity and provide protection from corrosion and heat resistance [13–16].
Figure 1 shows different materials for composite preparations. Epoxy resins have great strength making more useful in aircraft, automobile, and marine applications, despite the large variety of matrix/binding elements utilized [17]. Hydraulic cylinder weight was reduced by 96%, excavator engine hood wear resistance was improved, and engine frame weight was reduced, while stiffness and strength were improved. These techniques include spray-up, resin transfer molding (RTM), and other methods such as vacuum infusion or vacuum-based RTM, compression molding, pultrusion, and filament winding [18, 19]. Compression or hot-pressing procedures require less tool than other processes, but each has advantages and disadvantages. It is possible to produce compound shapes with outstanding dimensional solidity, repeatability, mechanical properties, and spark resistance using the compression molding technique. Compression molding processes for epoxy-based FRPC need to be studied further, according to the aforementioned literature study [20–22].
To make high-strength composite compounds for the automobile and aircraft industries, compression molding is the preferred method. Compression molding was used to create carbon-fiber epoxy composites with improved mechanical properties [23, 24]. Nonwoven mats made of natural fibers and polylactic acid were molded at high temperatures using a compression molding progress. After improving the molding conditions, woven flax/PLA polymer composites made via compression molding had higher impact strength (temperature, pressure, and time). Sheet-molded CFR composites have even more strength and stiffness when exposed to compression molding techniques. Polypropylene materials were examined by [25] for the influence of compression molding temperatures on their viscosity. Viscosity and melt flow velocity of PP are reversed when molding temperature is incorrect. Compression molding factors like as mold temperature, pressure, and duration are critical to the success or failure of thermoset items made using this process [26]. In composite parts, compression molding is an excellent approach for improving qualities that may be found in the preceding literature when the molding parameters are properly adjusted.
Reinforcing epoxy composites with micro- or nanofillers and nanofibers has been a major focus of current research publications, which have improved structural, mechanical, thermal, tribological, and functional qualities [27]. For better performance in engineering materials, fine particles and fillers have a higher aspect ratio and surface area per unit volume than microparticles. A strong interfacial bond between the nanofillers and the fibers is critical to transmitting load from the matrix to the fiber. Epoxy composite filler materials including carbon nanostructures can also improve thermal characteristics. The incorporation of MWCNT to nickel-cobalt materials increases supercapacitor characteristics of electrochemical [28]. MWCNTs are reusable. An increase in compressive strength and reduced shrinkage was achieved by using MWCNTs as nanoreinforcements in hybrid epoxy composites (MWCNT/silica fume cement). The compressive properties of hybrid polymer composites are also improved when MWCNTs are included [29]. Furthermore, it is necessary to do further research into the mechanical and tribological properties of carbon nanotube-based hybrid composite materials.
Despite the fact that the available literature contains a plethora of studies examining the mechanical performance of fiber-reinforced epoxy composites [30, 31], hybrid nanocomposites research, on the other hand, is still mostly underreported to this day. Thus, our aim is to study the influence of factors on mechanical properties of compression-molded polymer composites completely in this work.
2. Materials and Methods
2.1. Materials
Fiber-reinforced plastic has a high level of strength due to the transmission of loads from the matrix to the fibers. Interlaminar bonding between matrix and fiber shear strength are all factors in this phenomena. With over 10 million tons of global yearly output and over 95% of all fiber reinforcement coming from glass fiber, the composite sector relies heavily on this lightweight material with an excellent performance to cost ratio. The primary reinforcing materials in this investigation were 250 gsm (g/m2) woven E-glass fabrics with a diameter of 12 mm. The following elements are found in E-glass fiber: There are 52-56 percent SiO2, 12-16 percent Al2O3, 16-25 percent CaO, and 8-13 percent B2O3 in that mix, for a total of 52-56%. One of the principal reinforcing fibers in composite laminates is Kevlar fiber, which also goes by the name PPTA aramid fiber because of its strong rigid molecular structure. 90% pure MWCNTs (multiwall carbon nanotubes) were employed as a nanofiller in hybrid composites. They have a vast scope of current and emerging uses, including superior structural composites, conductive polymers, battery cathodes, solar arrays, battery packs, nanoelectronics, semiconductors, and power storage. MWCNTs were manufactured by chemical vapor deposition and have the following dimensions: outer diameter of 10-15 nm, interior diameter of 2-6 nm, and length of 1-10 mm.
2.2. Experimental Details and Methodology
Surface contaminants on the MWCNTs were first removed by rinsing them in deionized water. It took three hours to ultrasonically dissolve nanotubes in epoxy resin, which have an average diameter of 10 to 15 nm and an average length of 1-10 mm. A magnetic stirrer was used to mix the epoxy and MCWNTs with K-6 Hardener, ensuring that the nanoparticles were dispersed. All three MWCNT weight-percentage hybrid composites are created with the following: glass, Kevlar, and epoxy. Table 1 shows the physical and mechanical characteristics of filler and fibers materials. Table 2 shows epoxy matrix properties. Agglomerations which impair mechanical characteristics occur when carbon nanotubes make up more than one weight percent of a composite matrix. However, the properties of MWCNTs with varying weight proportions were found to vary after showing lab tests and consulting literature. It is therefore being attempted to establish the ideal percentage of MWCNT in hybrid composites to achieve bending, tensile, and hardness.
Layers of composites are assembled. It was covered with a nanofiller-reinforced matrix on every layer of bi-directional fabric. For automotive and industrial applications, compression molding can produce high-strength sheet molding parts. Figure 2 shows a schematic representation of the compression molding process experimental set-up. As a result of the introduction of heat to the vice, compression molding is sometimes known as a “hot press.” Compression molding’s experimental set-up appears to be small, with the supporting structure resting on a firm base. Hydraulics can move up and down the press thanks to the press’s four slide pillars. Electricity is used to heat the bottom or lower platen to a predetermined temperature of 120°C. Figure 3 depicts the key phases in compression molding method for producing composite products. In order to gain a deeper understanding of composite properties, existing literature led us to select the most important factors. Table 3 shows the production of composites with various compositions.
2.3. Testing of Composite Samples
The ASTM D790 standards were used to prepare and test tensile composite specimens for tensile and flexural strengths. An AG-X Plus machine was used for the tests. Each tensile and flexural test was performed five times, and the average results were recorded. For testing, 5 samples of size 300 × 300 × 3 mm are taken from composites at random. A microhardness tester was used to quantify the hardness of the composite specimens in accordance with the ASTM E 384. The examinations were performed with a 100 g load and a dwell period of 15 seconds. The mechanical characteristics of the produced composites were tested. Two distinct thicknesses of composites, 3 mm and 5 mm, are created and post-cured at room temperature for 10 hours to meet the pre-determined dimensions. ASTM standards were used to characterize the mechanical of the prototypes.
3. Results and Discussion
Experiments conducted in this part demonstrated how MWCNTs, compression molding parameters, and other variables affect hardness, tensile, and bending strength. Compression molding operating parameters, as well as the degree to which each of these process factors affected mechanical performance, were identified using single-factor studies on compression temperature, pressure holding time and pressure.
3.1. Effect of Multiwall Carbon Nanotubes on the Composite Samples
With the use of compression molding, composite samples (neat and hybrid) can be created. Table 4 shows the mean TS of five samples under each testing condition when MWCNT amounts were changed. The tensile characteristics of MWCNT-reinforced composites were found to be superior to those of the unreinforced composites. Adding MWCNTs (nanofillers) prevents crack formation and propagation, allowing the composite to absorb the maximum amount of load until it cracks. There was a drop in tensile strength after adding 0.8 weight percent of MWCNTs to the clean composite, which may be the result of nanofiller agglomeration in the matrix.
Composite samples with 0.8 percent weight of MWCNTs had the highest tensile strengths. This may be because of consistent distribution of MWCNTs between the epoxy matrix and the fiber surface. The gaps in composites increase when MWCNTs nanoparticles exceed the critical weight limit (i.e., 1.2 wt. percent). As a result of the epoxy resin having a higher viscosity, trapped gases or bubbles and volatile impurities may be held back from dissolving and contaminating the finished product. Since the MWCNTs in the epoxy matrix are spread evenly, the fiber and epoxy have better interfacial interaction and interlocking. When comparing a neat composite to a hybrid composite, the hybrid composite has a greater surface area enclosed in stress versus strain area curves, and it is shown in Figure 4. In the MWCNT composites, the MWCNTs serve as crack arresters and bridge the cracks, resulting in a toughening effect.
The hardness variations in composite samples are shown in Table 5. At 1.2 wt. percent of MWCNT, agglomeration causes nanocomposites to be more prone to porosity. Nanotubes in nanocomposites experience axial compression, bending, and buckling as the indenter slides. The following features of MWCNTs contribute to their improved interfacial bonding: One-dimensional nanostructure with a high aspect ratio and a C-C covalent bond directed on axis of CNT improved strength of the material. In nanocomposite, a material’s robust network structure provides higher hardness and strength. In contrast, inadequate dispersion can lead to agglomeration and void formation in composites when MWCNTs are used in concentrations greater than 0.8%. Consequently, the microhardness of these composites is greater than the microhardness of their neat counterparts.
3.2. Influence of Compression Molding Parameters
The mechanical performance of the compression-molded hybrid composite (0.8MWCNTGKEC) is assessed at a pressure time of 10 minutes and a compression pressure of 20 MPa. Optimized molding temperature settings of 80°C yield the best mechanical qualities, and it is shown in Figure 5. Fiber impregnation is reduced when the epoxy matrix does not fully melt or flow at low mold temperatures. There will be excessive flow viscosity and inadequate saturation if the heating temperature is too low during the molding process because the resin cannot fully melt or flow; if heating temperature is too high, the resin will be degraded and its mechanical performance will be reduced. When a thermoset matrix material has hardened, it can never come back to its original state. This may be due to the formation of cross-links, which are three-dimensional molecule chains. In order for the matrix to be so strong and thermally robust, the curing temperature must be increased. Fibers begin to disorder and epoxy matrix starts to destroy above the critical set mold temperature (>80°C), resulting in brittleness and decreased strength. The findings are in line with previous studies. Using experimental input-output data, the impact of mold temperature was evaluated. The tensile and bending strengths of composites are determined to be 139.87 and 169.88 MPa, respectively, when compression temperatures changed among their corresponding ranges. In hybrid composites, tensile and bending strength increase by 21.46% and 21.26%.
3.3. Compression Pressure Has an Effect on Tensile and Bending Strength
Hybrid composites are manufactured using pre-set compression molding conditions (0.8MWCNTGKEC) and the effect of compression pressure on mechanical properties.
Figure 6 demonstrates that the tensile and bending strength increases about to 22 MPa at pressure and then stabilizes. To maximize the strength of hybrid composites, compression pressure of 20 MPa should be applied. As the resin impregnation increases, porosity decreases, as does the resin flow and spreading across the fiber surfaces. Fiber structure may be damaged if compression is applied beyond 20 MPa, resulting in lower mechanical strength. With regard to part removal, a high level of compression pressure makes it difficult to separate solidified parts from their mold cavities, notwithstanding shrinkage as a result of cooling and an adequate die design for ease of removal. Compression pressure is examined by altering the pressure within and between the two ranges they occupy. When it comes to composite materials, the low and high tensile strengths are 140 and 178.23 MPa, respectively, while the bending strengths range from 99.23 to 142.32 MPa. In terms of tensile and bending strengths, the hybrid composite caused in a 27.3 percent and 43.42 percent increase in compression pressure, respectively.
3.4. The Tensile and Bending Strength of a Material as a Function of the Amount of Pressure Applied over Time
The mechanical properties of composites are shown in Figure 7 as a function of the amount of pressure applied over time. In order to maintain a consistent compression pressure and mold temperature of 80°C, the experiments must be performed under constant conditions. When pressure is applied for a longer period of time, mechanical strength increases substantially for the first 15 minutes, but after that point, there are no discernible improvements. The fabricated composites may have some voids or porosity if a low compression pressure holding time is used.
Hybrid composites were subjected to a variety of pressure applied times between 5 and 25 minutes. The tensile strength ranges from 165 MPa to 175 MPa, while the bending strength ranges from 120.12 MPa to 136.69 MPa. The localized heat is a mix of the two. When MWCNTs are included in the composite, they enable strong interaction among matrix fibers, resulting in enhanced load transfer as an outcome of the larger aspect ratio and surface area of nanofiller.
4. Conclusions
The mechanical and tribological characteristics of MWCNT reinforced with different fibers and epoxy composites were investigated in this work under the impact of compression molding parameters. The current study discovered that MWCNT reinforced with glass/Kevlar/epoxy composites resulted in composites that were stronger and tougher. 0.8 percent MWCNTGKEC outperformed 0.4% and 1.2% nanofiller additions to a pure composite, respectively. All compression molding factors were shown to have a considerable impact on composite mechanical strengths and hardness. Compression pressure has the biggest influence on composite tensile and bending strength, followed by mold temperature and pressure application duration. In 0.8% CNTGKEC, the hybrid composite had higher tensile strengths of 183 MPa, bending strengths of 158.3 MPa, and hardness of 23.8 HV.
Data Availability
The data used to support the findings of this study are included within the article. Further data or information is available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Acknowledgments
The authors appreciate the supports from Seenu Atoll School, Maldives, for providing help during the research and preparation of the manuscript. The authors thank to Sathyabama Institute of Science and Technology, Lendi Institute of Engineering and Technology, King Saud University, University of Manchester, and REVA University for providing assistance in completing work. This work was funded by the Researchers Supporting Project number (RSP-2021/265) King Saud University, Riyadh, Saudi Arabia.