Fiber Inclusions-Based Epoxy Composites and Their Applications

Abstract

Because of their low cost, lightweight, easy production methods, and design flexibility, polymer-based composites are widely employed in a wide range of applications. Because of its high specific strength, superior mechanical characteristics, super adhesiveness, heat and solvent resistance, and so on, epoxy polymer or polyepoxide represent a significant majority of matrix composites. As a result, fiber fillers-reinforced epoxy resin composites have been investigated for a variety of applications, including high-tech in the ballistic, aircraft, automobile, construction, and sports sectors. In this chapter, the manufacturing procedures of fiber-reinforced epoxy composites have been described. Different categories of fiber are used as fillers in an epoxy matrix and their morphology is discussed as a function of the obtained properties.

1. Introduction

In recent decades, our societies have been confronted with climatic disturbances and resource use, leading to the degradation of ecosystems. In order to combat these threatening changes, the international community is committed to finding new ways of producing and creating value, including light-weighting structures and the valorization of lignocellulosic biomass as possible solutions towards sustainable innovation [1, 2, 3]. Indeed, light-weighting implies a reduction in production energy, raw materials produced and materials to be managed at the end of life [4, 5]. The reduction in mass also leads to a reduction in the energy consumption of means of transport and their emissions of pollutants. For these reasons, sandwich structures are increasingly used instead of monolithic structures in various applications, thanks to their lightness, their mechanical performance in bending and their thermal, vibratory, and acoustic features.

Because of their good features, they may be found in vital industries such as aeronautics, automotive, sports, marine, and construction. These properties include high mechanical strength and stiffness, high-impact resistance, low weight, corrosion resistance, and low maintenance costs [6]. Traditionally, composite materials are reinforced with synthetic fibers such as glass, carbon, aramid, or ceramic fibers. These fibers are used because of their strength, stiffness, low moisture absorption, and good compatibility with polymer resins. Glass fibers are the most commonly used because of their low cost, ease of production, and specific mechanical characteristics.

The epoxy matrix combined with rigid fiber allows for the creation of building materials with high stiffness and strength. Given the variety of technical and material options, developing a composite material necessitates taking into account the chemical and physical interactions between all components [7]. As a result, the effects of production processes, fiber reinforcement type, and reactive or nonreactive modifiers on the characteristics of epoxy composites remain intriguing study issues. Many different types of synthetic and natural fibers are used to strengthen the epoxy matrix, including glass, carbon, basalt, aramid, ramie, hemp, jute, and flax [8].

In this chapter, a detailed description of epoxy polymer was represented. Moreover, various fiber types such as glass, carbon, and plant materials. In addition, some fabrication procedures of epoxy reinforced fiber composite are reported. Furthermore, a representation of some applications was described as well as the coming challenges.

2. Thermosetting organic matrix: Epoxy resin

Epoxy resin is a thermosetting polymer. It comprises two parts: an epoxy base catalyst and an amine-containing hardener (-NH₂ or -NH). During cross-linking, each hydrogen atom in the amine group opens the epoxy ring and produces a polymer chain (Figure 1) [9]. The glass transition temperature denoted T_𝑔, increases with the rate of crosslinking. Thanks to its 3D polymeric structure and high phase change temperature, epoxy phase change temperature, epoxy achieves good mechanical and thermal properties [10].

Figure 1.

Main chemical reactions taking place during the curing of an epoxy resin.

Table 1 shows the advantages and disadvantages of epoxy thermoset resin. Compared to thermoplastic resins, epoxy resins are more brittle on impact due to their susceptibility to cracking. According to Vieille et al. [12], the impact response of thermoset matrix composites has some weak points:

• For the same impact energy, the delamination area due to impact is higher compared to thermoplastic matrix composites;

• For impact energy of 25 J, the epoxy reinforced structure is perforated while the maximum displacement of the thermoplastic composite structure is around 11.4 mm;

• For the same level of impact energy, the ratio of dissipated energy to impact energy of the epoxy matrix composite is higher than the ratio obtained for the thermoplastic matrix composite. The risk of perforation of the structure increases with the increase of this ratio.

Thermosetting epoxy resin
Benefits	Disadvantages
Mechanical properties superior to those of polymers of the same family (tensile, compression, aging, etc.);	Preservation at low temperature (for prepregs or single-component resin);
Good temperature resistance from 150–190°C approximately;	Long curing time;
Excellent chemical resistance;	High cost (about 5 times more than polyester resin);
Low molding shrinkage (0.5 to 1%);	Need to take precautions during implementation;
Good wettability of the reinforcements;	Sensitivity to cracking.
Bonne adhérence aux matériaux métalliques.	—

Table 1.

Advantages and disadvantages of epoxy resin [11].

Moreover, a high brittleness of this family of resins is also the cause of the pseudoplastic behavior of the composite. Upon impact, the opening of intralaminar and interlaminar cracks is triggered. At the same time, epoxy debris forms and blocks the closure of the cracks after impact, which is impact, which is unfavorable to the impact resistance of the composite [13].

3. Fiber materials and types

Composite materials are categorized based on their content, which includes the base material (matrix) and the filler material. A matrix or binder material is the basic material that binds or retains the filler material in structures, whereas filler material is present in the form of sheets, pieces, particles, fibers, or whiskers of natural or synthetic material. Composite based fibers are categorized into three major groups based on their structure, as shown in Figure 2.

Figure 2.

Composites structure types.

3.1 Epoxy resins reinforced with glass fibers

Fiber or particulate inclusions in epoxy matrix with different types and shapes are studied by many researchers to characterize their mechanical, electrical, thermal and so on properties [14, 15, 16, 17]. Glass fibers are the most often used synthetic fibers because of their high strength and durability, thermal stability, impact resistance, chemical, friction, and wear qualities. However, machining glass fiber-reinforced polymers (GFRPs) using traditional machining techniques is generally slow, difficult, and results in lower tool life [18]. They are easily made from raw material, which is readily available in an almost limitless supply. There are numerous types of GFs that are often utilized in GFRP composites, depending on the raw materials used and their quantities in fabrication (see Figure 3). GFs also have the disadvantage of being disposed of at the end of their useful life [19]. Glass fiber reinforced polymer composites were created using various production technologies and are widely employed in a variety of applications [20]. Because of their superior mechanical qualities, glass fiber reinforced composites have received more attention in recent years. Glass fibers have excellent features such as high strength, flexibility, stiffness, durability, and so on. The characteristics of GFRP composites improved when the amount of glass fiber was increased. The mechanical and thermal properties of different polymer composites reinforced with glass fiber when exposed to mechanical stress are been listed in the following Table 2.

Figure 3.

Glass classification.

Fiber glass type	Manufacturing procedure	Composite structure type	Reference
E-Glass fiber	Hand lay-up technique	—	[21]
E-Glass fiber	Hand lay-up	Randomly oriented	[22]
Glass Fabrics	hand layup vacuum bagging	—	[23]
C-Glass fiber	Hand lay-up	Randomly oriented	[24]
E-Glass fiber	Open layup molding	—	[25]
D-Glass fiber	Hand lay-up techniques	Randomly oriented	[26]

Table 2.

Example of composites-based epoxy reinforced glass fibers.

3.2 Epoxy resins reinforced with carbon fibers

Carbon fibers were first used in 1880 by T. Edison as a filament in lamps. From 1960 onwards, research was directed towards the development of high modulus and high strength carbon fibers. The carbon fibers are more required in applications that need more stiffness. Carbon fiber-reinforced polymer (CFRP) composites have extensive uses in aircraft, automotive, sports, and a variety of other sectors. In the literature, many other fillers type such as particulate and fiber fillers [27, 28]. In general, carbon fibers can be categorized by their mechanical properties, manufacturing methods, application field, precursor, fiber materials, final heat treatment temperature, and their function.

3.3 Epoxy resins reinforced with plant fibers

Nowadays, industrial businesses are concentrating on providing environmentally friendly products, and the globe is moving towards sustainable development. Because of their biodegradability, natural fibers are employed in the production of such eco-friendly products. The key causes influencing the rising use of natural fiber-reinforced composites are increased awareness of concerns such as pollution, waste of raw materials and energy, and depletion of petroleum reserves (FRC). Due to their lightness, mechanical performance, ability to integrate functions, physical–chemical resistance, and ease of processing, composite materials have made considerable progress in terms of volume and have dominated practically all sectors. Such as wood fiber which are transformed using the steam explosion process and are treated at various steam pressures. Because of the increased explosion pressure, the fiber’s affinity for water, mechanical characteristics, and dissolving ability in caustic solution diminish after the steam explosion [29].

4. Fabrication methods

The preferred procedure is determined mostly by the resin used, the length of the fibers, the required qualities of the composite material, and the production run and rate (Figure 4) [30].

• Contact molding: It is a procedure for short series. On a waxed mold, layers of catalyzed resin and layers of cloth are alternatively deposited. After applying the resin to the reinforcement with a brush, it is debulked with a roller. This process is continued until the appropriate number of layers has been attained. This method is easy and economical, and it allows for the fabrication of pieces of any form and size with a nice surface look on the mold side. The component produced by this approach is heavily reliant on the molder’s competence.

• Vacuum molding: This is a method for producing medium-sized series. Following the placement of the reinforcements and resin on a waxed hard mold, a waterproof membrane is applied to the whole structure. A vacuum established between the mold and the membrane by a vacuum pump allows the resin to be distributed and debulked. The suction is kept up until the resin is completely dry.

• Low-pressure liquid resin injection molding: This method is also known as RTM (Resin Transfer Molding). It entails inserting the reinforcements into a hard, waxed, two-sided mold. The catalyzed resin is then pumped into the mold at pressures ranging from 0.1 to 0.4 MPa.

• Filament winding: This automated method is intended for the high-pressure molding of high-performance innovative parts like tubes and fluid storage tanks. The method entails dipping rovings in a bath of catalyzed resin before wrapping them around a mandrel. There are three forms of winding based on the speed of movement of the roving in proportion to the angle and speed of rotation of the mandrel: circumferential winding, polar winding, and helical winding.

• Autoclave: Bag molding is another name for autoclave molding. It entails compacting the reinforcement and resin on a stiff mold, then passing it through an elastic, flexible membrane to form a tight bag with the tooling. The mold is put in a confined chamber with a few megapascals of internal pressure. This pressure is given to the membrane by a fluid (air, water, nitrogen, or steam), which aids in resin polymerization.

• Simultaneous spray molding: This method evolved from the contact molding procedure. Molding is done by spraying chopped fibers combined with catalyzed resin onto a waxed mold at the same time. A roller is then used to condense the sprayed layer and eliminate bubbles. This technology allows for the low-cost production of medium and large parts as well as basic forms. However, because this form of molding solely employs chopped fibers, the pieces produced have poor mechanical qualities.

Figure 4.

Different procedures of fabrication for polymer composites-based fibers.

According to studies, the constraints of each technique and the production parameters employed during composite processing might induce undesired internal flaws into the material, such as bubbles or cavities, poor or rich areas, delamination, shrinkage, and so on [30]. As a result, these flaws can compromise the mechanical characteristics of composite materials. Several investigations on carbon and glass reinforced composites have been conducted. Liu et al. investigated the influence of autoclave pressure cycling on the porosity of a [0/90]3 s carbon/epoxy cross-linked composite. In comparison to tensile strength and modulus, they demonstrated a considerable sensitivity to porosity in the interlaminar shear strength and flexural parameters of the composites. They discovered that when the porosity inside the composites is less than 4%, the interlaminar shear strength reduces by roughly 8% for every 1% increase in porosity [31]. Gu et al. investigated void formation by transforming hygroscopic water absorbed by glass and carbon fibers, as well as trapped air, into vapor bubbles as a result of the temperature rise during the thermocompression process [6]. Compared to synthetic fibers, the problems associated with the processing of plant fiber composites are more complicated due to the particular characteristics of this type of fiber. The use of plant fibers for resin reinforcement necessitates careful consideration of the production conditions. The essential criteria for regulating the thermal deterioration of the fibers are the process temperature and time. To reduce viscosity, the hot-molding temperature must be higher than the melting point of the resin, and the time must be long enough to allow the molten resin to permeate the fibers, assuring good adhesion between the reinforcement and the matrix. In conflict with these needs, the melting temperature and time should be as low as possible to slow down the thermal deformations that occur and cause fissures and permanent damage to the fibers, as well as the pectin breakdown, which begins at 180°C [32, 33].

5. Applications

E-glass fibers are commonly used as reinforcements in shipbuilding, while carbon fibers saturated with epoxy resin are commonly used in aeronautics. The use of synthetic fibers in composites is supported by their high chemical resistance, compatibility with most impregnation resins, and mechanical and thermal performance. However, the usage of this sort of reinforcement is no longer adequate: On the one hand, their comparatively large density penalizes them; on the other hand, they endanger the health and the environment. Since the 1980s, these environmental problems have become a major concern for our society and the media. And since then, the industrial optimization of eco-composites is booming thanks to their high specific mechanical properties. The limits of applications are constantly being pushed back through the development of fiber preforms and the adaptation of processing methods.

5.1 Automobile

Automobile body sections, such as engine hoods, dashboards, and storage tanks, are made using natural fiber reinforcements such as flax, hemp, jute, sisal, and ramie. The VARTM manufacturing technology was used for these composite constructions, and its liability was tested through structural testing and impact stress analysis. As a consequence, the material’s weight was reduced while its stability and strength were improved. The increase in safety characteristics was tested using the head impact criterion (HIC), and it was discovered that composite constructions with natural fiber reinforcements are appropriate for automotive body sections [34, 35, 36]. Figure 5 depicts the external body elements of a Volkswagen x11 crazy carbon fiber replica.

Figure 5.

Volkswagen xl1 carbon fiber body pieces, adapted from [36] under a creative commons license. (a) the 45 kg of natural fibers in a Mercedes S-class. (b) Car door panel made of natural fibers. (c) Spare wheel cover made of natural fibers.

The automobile sector, in particular, has shown a genuine commitment to economic and environmental concerns by using natural fibers in different non-structural components (dashboards, door panels, spare wheel covers, etc.) with the goals of lowering mass, fuel consumption, and emissions (Figure 5).

5.2 Aerospace

Fiber-reinforced epoxy composites manifest the properties required for aircraft interior panels, such as resistance to heat and flame and disposal of materials. Fiber-reinforced epoxy composite shows a variety of applications in the aerospace industry due to its superior mechanical properties and lightweight structure. Conductive fibers in the layer of fiber composite structure eliminate the requirement of separate wires for transceivers of communication devices. High stiffness with a lower coefficient of thermal expansion is achieved when P100 graphite fibers diffused in 6061 aluminum matrix composite material are employed to the high gain antenna of the Hubble space telescope [37].

For example, the wing of the plane is a composite material, the fiber is carbon fiber and the resin is epoxy. The manufacturing technique involves resin infusion: all the reinforcing fibers are dried, shaped and then the resin is infused into the reinforcement. The choice of polymer matrix must both ensure good performance for the finished wing after curing and also maintain a well-tuned reactivity, not too high to allow the wing to be infused, which can take several hours, but enough to allow the reactions to take place effectively.

5.3 Marine

Components and structures functioning in the marine environment are subjected to significant stresses caused by wind, waves, and tides. Furthermore, they must endure hostile and harsh environmental conditions throughout their lives, including being placed in the splash zone if not submerged in seawater. The use of polymer composites in maritime systems has been the subject of much research in recent decades, showing the potential benefits of replacing various components such as ship hulls, propeller blades, wind, and tidal turbine blades, to name a few [38]. For example, in the offshore construction (seawater piping, stairways and walkways, firewater piping, grating, fire and blast walls, cables and ropes, storage vessels, and so on), valves and strainers, fans and blowers, propeller vanes, gear cases, condenser shells, and so on.

And more other applications, which we will not be able to represent all of them such as:

• Civil engineering includes the construction of new advanced structures (roofs, plate and shell elements, linear elements, pipes and tanks, folded structures, and so on) as well as the rehabilitation of existing metallic and concrete structures such as buildings, bridges, pipelines, masonry construction, and so on.

• Sporting goods: Golf club shafts, tennis rackets, bicycle frames, fishing rods, and so forth.

• Electrical and electronic components include power line insulators, fiber optics tensile members, lighting poles, and so on.

• Chemical Industries: stacked bottles for fire departments, composite containers for substances, mountain climbing, ducts and stacks, subterranean storage tanks, and so on.

• Medical field: Orthopedic medicine, prosthetic devices, and imaging (MRI).

6. Challenges

Understanding the significant material characteristics of fiber/epoxy constituents, as well as the fundamental structures and availability of production technologies, is required for the use of fiber/epoxy composites in a range of applications.

Furthermore, the manufacturing technique used has an effect on the ultimate qualities of the material. The cost of materials is influenced by production volume—the bigger the volume of production, the lower the cost of materials. In the instance of the car industry, increasing production volume increases the risk of investing in raw materials while building manufacturing set-up based on production rate and cycle time. In addition, the product’s design complexity increases the cycle time, decreasing the manufacturing pace.

7. Conclusions

On fiber-reinforced composite manufacturing, current progress, novel advancements, and future research prospects are summarized and presented. However, the ongoing demand for composite constructions necessitates a large consumption of environmentally hazardous components. Certain fibers (for example, carbon fibers) utilized to improve qualities in numerous sectors are a significant hindrance to recycling at the end of the composites’ life. As a result, the current environmental crisis, which has reached a tipping point, necessitates immediate and objective action to cut greenhouse gas emissions. As a result, obtaining advanced composites from renewable energy resources would be the best ecological answer. Furthermore, future research areas might focus on recycling current composites into high-value alternative goods. Furthermore, new innovative methods for post-consumer waste treatment must be developed. Additionally, new sophisticated technologies for post-consumer waste treatment must be developed, or existing FRP composite production technologies must be improved.

Conflict of interest

The authors declare no conflict of interest.