Mechanical Behavior of Epoxy Resin Polymers and Their Application in Civil Engineering

Epoxy resin is a thermosetting plastic that is widely used in many fields due to its excellent physical and mechanical properties, electrical insulation, adhesion and chemical corrosion resistance [1-4]. For example, it is used as a reinforcing agent for fiber composites, general adhesives, high performance coatings and packaging materials [5-9], and can even be used as a repair agent for concrete [10]. Current research on epoxy resin includes preparation, modification, compounding, physical and chemical properties and applications. Among them, the mechanical properties include high strength, stiffness and service temperature, but they are often brittle, resulting in poor fracture resistance, especially at high strain rates [11,12]. It is therefore particularly important to improve the toughness of epoxy resins while maintaining as much of the mechanical properties as possible.

Epoxy resin is a polymeric compound containing at least two epoxy groups in the molecule. When it reacts with a curing agent, it forms a thermosetting three-dimensional network structure [13,14]. According to the different groups in the molecule, epoxy resins can be divided into glycidyl esters, glycidyl ethers, glycidyl amines, alicyclic, epoxidized olefins and new epoxy resins [15]. These resins have excellent properties in various aspects. For example, high mechanical strength: the cured epoxy resin exhibits excellent tensile, compressive and flexural strength. Excellent bonding properties: commercial epoxy adhesives achieve optimum toughness by adding phase-separated thermoplastics, rubber particles or rigid inorganic particles to the matrix. Typically, the adhesive is cured at high temperature to increase its strength and activate chemical bonding at the substrate/adhesive interface [16-20]. Excellent electrical insulation properties: epoxy resins have high resistivity and breakdown voltage [21-26]. Good thermal stability: it has a high glass transition temperature (T_g). Chemical resistance: it is resistant to corrosion by most acids, alkalis, solvents and oils [27]. These properties make it close to life, widely used and extensively researched.

This paper mainly gathers information on the mechanical behavior of epoxy resin polymers and their applications in civil engineering. Firstly, the basic properties of epoxy resin and the excellent properties of epoxy resin composites cured with curing agents are reviewed. For example, modified epoxy resins with the addition of nanomaterials or other particles have improved strength and crack resistance. Secondly, the application of epoxy resin in civil engineering is presented. It can be used as a reinforced concrete and repair material and as an adhesive in construction. Finally, the shortcomings of epoxy resin and its limitations in civil engineering are discussed. There is a need for epoxy resin to retain its excellent mechanical properties while also having excellent properties in other areas, such as high toughness and heat resistance. At the same time, the 3D printing technology for the fabrication of Polylactic Acid (PLA) structures and its integration with epoxy resin composites is introduced.

Regarding epoxy resins, the most studied topic is material modification. For example, the incorporation of nanomaterials, rubber particles, rigid fillers or thermoplastic resins can effectively improve the toughness of lightweight epoxy resin systems [28-31]. Bernd et al. [32] provided information on the reinforcing effects of nanoparticles on the mechanical and fracture properties of epoxy resins, with particular emphasis on fracture and toughening mechanisms. The study found that the presence of nanoparticles in epoxy resins induces various fracture mechanisms, such as crack deflection, plastic deformation and crack pinning. Nanoparticles improve the stiffness, strength and toughness of epoxy resins without sacrificing thermomechanical properties, overcoming the shortcomings of traditional toughening agents. Subsequently, they added aluminium oxide nanoparticles to epoxy resins and showed that they were able to simultaneously improve the stiffness, impact energy and failure strain of epoxy resins at low filler contents (1-2% by the volume) [33]. The introduction of calcium silicate particles into the nanocomposite matrix containing the optimum number of nanoparticles (2% by the volume) can increase the flexural modulus to a higher level and further increase the wear resistance by three times. Gómez et al. [34] studied the effect of adding different types of nanoparticles (styrene-b-butadiene-b-polymethylmethacrylate triblock copolymers and carbon nanotubes) to epoxy resin systems on the compressive behavior of materials. The study found that carbon nanotubes significantly improved the compressive yield strength at high strain rates.

Furthermore, the experimental results were compared with the compressive yield stress model of various composites to verify the rationality of the model. Nie et al. [35] prepared a new flame retardant derived by metal organic framework and blended it with epoxy resin matrix. The results show that the modified epoxy resin has better mechanical properties and wear resistance, and its tensile strength, flexural modulus, hardness, dispersibility, thermal and chemical stability are improved. Li et al. [36] investigated the effect of nanomaterials (such as functionalized nano-SiO2 and carbon nanotubes) on the thermo-mechanical properties of epoxy resin adhesives, where the glass transition temperature and mechanical properties of the modified epoxy resins were characterized. The results show that the high degree of cross-linking limits the molecular chain movement and improves the strength of the epoxy resin adhesive, and the epoxy resin adhesive doped with nanomaterials has better thermo-mechanical properties. Notably, the performance of the epoxy resin adhesive doped with a single nanomaterial is lower than that with functionalized nano-SiO2 and carbon nanotubes, indicating that the nanoparticle enhancer has an opposite influence on the product. Furthermore, Umberto et al. [37] proposed an improved numerical model based on the cohesive zone method and the embedded truss model, in which the concrete reinforced with nano-reinforced fabric was simulated. The results showed that the bond strength, peak load and final deflection of the nano-reinforced concrete beam were significantly improved. It can be seen that the modification of the epoxy resin is not limited to the resin, but also includes the preparation of the reinforced concrete material by using the modified epoxy resin.

Uncured epoxy resin has poor mechanical, chemical and thermal resistance [38], which limits the use of epoxy resin composites in the manufacturing of some high temperature resistant packaging materials. By reacting linear epoxy resin with a suitable curing agent, a three-dimensional cross-linked thermoset structure can be obtained [13,14]. This is ideal for mechanical and thermal properties, and can bring high modulus, fracture strength and excellent adhesion to many industrial applications, but the higher cross-linking density of the epoxy resin will lead to a decrease in fracture toughness, thereby limiting its application. Many researchers have concluded that high cross-link density reduces the fracture toughness of the original epoxy resin. This is due to the internal stresses generated during the curing process of the epoxy resin. In high cross-link density epoxy resin, the crack initiation resistance is very low and the void growth caused by plastic deformation is limited [39,40]. Ahmetli et al. [41] investigated the effect of polymer structure and amount on the physical, mechanical and thermal properties of epoxy resins, where the epoxy resins were modified with bio-based and styrene-based polymers. The results showed that the surface hardness, tensile strength, elongation and maximum load stress of the composites were improved compared to epoxy resin. Hwang et al. [42] synthesized a low molecular weight redistributed Dicyclopentadiene-Containing Phenylene Oxide (DCPD-PPO), in which a series of DCPD-PPO/ epoxy resins were prepared by varying the weight ratio of DCPDPPO and epoxy resin. The comparison results show that the cured DCPD-PPO/epoxy resin has lower hygroscopicity, lower dielectric constant, lower dissipation factor and better CTE than other epoxy resins, while maintaining good thermal performance [43]. It can be seen that doping epoxy resins can significantly improve their mechanical properties, which opens up possibilities for some specific engineering scenarios, but it is worth noting that this will lead to a decrease in other properties, so balancing the levels of different properties is an important issue.

Mechanical behavior is generally described using mathematical models to understand elastic, plastic, visco-elastic and visco-plastic phenomena. In terms of pure epoxy resin, Zhu et al. [44] conducted quasi-static tensile and compression studies and pointed out its viscoelastic-plastic mechanical behavior, where a classical constitutive model was established. Under room temperature conditions, the modulus and yield strength of epoxy resin in tension and compression increase as the strain rate increases, and the compression results of epoxy resin at different temperatures show that its strength and modulus decrease with increasing temperature [43], as shown in Figure 1(a-b). Zou et al. [45] studied the dynamic compressive mechanical properties of epoxy resin under high strain rate conditions and obtained dynamic constitutive relationships including strain rate effect and temperature effect. Based on the obtained constitutive model, the mechanical behavior of epoxy resin at different strain rates and temperatures was calculated and compared with the experimental results, as shown in Figure 2. Epoxy resin has a typical strength differential effect; namely tensile and compressive strength are different. There are many studies on its constitutive model, such as compressive nonlinearity, tensile nonlinearity, viscoelasticity, visco-plasticity, elasto-plasticity and so on [46-50]. The constitutive equation is mainly constructed based on the Maxwell and Kelvin models [51], such as the ZWT and Voigt models, and another equation may be based on phenomenology. However, different microscopic molecules determine the mechanical behavior of epoxy resins, and different epoxy resins have different constitutive models, which should be noted. At the same time, the mechanical behavior of epoxy resins is also different in different application scenarios, such as extreme cold, extreme heat and humid heat. Therefore, it is important to consider actual engineering scenarios to perform material performance testing and constitutive relationship modeling.

Figure 1:Stress-strain curves [43]: (a) Tensile behavior at different strain rates and room temperature, (b) Tensile behavior at three temperatures and 0.001s^-1.

Figure 2:Yield stress versus temperature for different strain rates [45,50].

Construction materials are often required to have high stiffness, high strength and good corrosion resistance. Epoxy resin, as a matrix, can be combined with reinforcing materials such as glass and carbon fibers to produce high performance composites [52,53]. These composite materials have the characteristics of high strength, high modulus, high temperature resistance and corrosion resistance, and can be used in civil engineering to reinforce and repair concrete structures. Through the bonding and reinforcing effect of epoxy resin, the load-bearing capacity and durability of concrete structures can be effectively improved [54]. In addition, epoxy resin can also be used for waterproofing, anti-corrosion and wear-resistant treatment of civil engineering structures such as bridges, roads and tunnels [55,56]. Kocaman & Ahmetli [10] conducted a comparative study of the mechanical properties of neat epoxy resins, such as tensile strength and hardness, in order to investigate the effect of curing agents on epoxy resins. The results showed that the tensile strength and elastic modulus of the neat epoxy system were higher than those of the modified epoxy resins, which was in contrast to the elongation at break. Furthermore, a comprehensive analysis of mechanical properties, water absorption and resistance to acidic corrosive media was performed, which is important for pavement materials and pavement maintenance.

Cement particles have a good ability to penetrate textile bundles compared to polymers, which improves the textile/ cement interface bond compared to unimpregnated reinforcing materials [55,56]. Zhang et al. [57] investigated the flexural spalling failure of epoxy resin polymer reinforced concrete and analyzed the flexural response and flexural spalling failure mechanism in detail. The results showed that the flexural spalling failure of epoxy resin-impregnated basalt fabric-reinforced concrete reduced the load bearing capacity in the post-crack stage, thereby affecting the overall flexural performance. Ye et al. [58] prepared functionalized molecules (tetraaniline grafted polyhedral oligomeric silsesquioxane/graphene, POSS-TA/G) to form functional epoxy composite coatings. Compared to pure graphene, POSS-TA-G showed strong hydrophobicity, excellent dispersibility, full electroactivity and low conductivity. In long-term immersion tests, the addition of appropriate amounts of graphene, POSS-TA and POSS-TA-G can significantly improve the corrosion resistance of epoxy resin coatings. This study is particularly important for pavements and building surfaces that are often exposed to acidic rainwater. Quadflieg et al. [59] introduced and discussed the characterization results of coated warp-knitted and weft-knitted reinforcement fabrics and cementitious composites based on them. The results show that among the different impregnating polymers (potassium silicate, carboxylated styrene-butadiene rubber and epoxy resin), the impregnating polymer with the highest flexural strength is epoxy resin. Zhang et al. [60] researched the bonding properties of polymer-impregnated basalt fabrics to fine-grained concrete using pull-out tests. The test results showed that due to the high viscosity and good permeability of epoxy resin, the bond strength of epoxy resin-impregnated fabrics was 238% and 592% higher than that of acrylic emulsion and polyvinyl chlorideimpregnated fabrics, respectively.

Messori et al. [61] evaluated the mechanical properties in uniaxial traction of rectangular samples and three-point bending of laminated clay bricks and showed that the post-peak behavior of the stress-strain curve was strongly influenced by the coating thickness. In fact, the thin coating group showed plastic behavior while the thick coating group showed softening. Dvorkin et al. [54] claimed that carbon fiber fabrics have strong adhesion to the cement matrix, forming a finely distributed crack network, where they discussed the crack pattern, crack width and filament bonding, and the results showed that the coated carbon bundles can improve the tensile properties of the textile cement-based matrix. It can be seen that epoxy resin can be applied to building materials such as cement concrete and asphalt concrete to improve the material properties and make them meet the needs under various construction requirements. Taking the uniaxial tensile test on rectangular samples as an example, the functionalized fabric used in concrete has been coated with epoxy resin [61], as shown in Figure 3(a). Figure 3(b) shows the typical failure modes that occurred in the glass fiber-epoxy resin groups, including fabric breakage, debonding, fiber pull-out, etc. The experimental results show the affinity of epoxy resin for concrete and the reinforcing effect of the functionalized fabric prepared with it on concrete.

Figure 3:Coated fabric after cross-linking [61]: (a) Glass fiber-epoxy resin fabric, (b) Failure mode.

In addition, epoxy resin can also be used in 3D printing to assist in the design and application of civil engineering structures. For example, the PLA Bouligand structure and its integration with epoxy resin composites were manufactured using Fused Deposition Modeling (FDM) 3D printing technology to study the effects of material filling angle and interlayer orientation on the performance of epoxy composites [62, 63]. It was found that the fracture toughness of the Bouligand structure with an interlayer grid angle of 15° was significantly improved by 176.53% compared to the control epoxy resin group, providing insight for 3D printed engineering structures [64-66], such as concrete casting molds and some replacement tools. Cao et al. [67] developed a 3D printing method for reinforced concrete, as shown in Figure 4(a), where they inserted steel fibers into 3D printed concrete beams and coated them with epoxy resin to improve bending and interfacial properties. Salazar et al. [68] used 3D printing technology to manufacture polymer lattices, and then filled them with concrete to obtain lattice-reinforced concrete. They conducted mechanical property tests and found that the lattice-reinforced concrete had better crack resistance, with the fracture interface shown in Figure 4(b). Lee et al. [69] proposed a channel system for assembling modular blocks using 3D printing and epoxy resin injection. The results of uniaxial tensile and three-point bending tests on four different channel types showed that the assembled channel system had better durability compared to the jointless model, enriching the application of 3D printers in civil engineering. Tosto et al. [70] prepared two resin systems by mechanical mixing and developed an epoxy-acrylate mixture for liquid crystal display 3D printing technology. By analyzing the material data under light and heat curing conditions, the best material formulation with excellent printability and overall performance was obtained. The dual curing analysis of epoxy resin polymers was carried out to obtain a variety of polymer formulations that can promote the development of high quality and reliable 3D printed objects and provide a printing material with thermal performance evaluation in civil engineering applications. Furthermore, Xu et al. [71] also carried out similar work to that of lattice-reinforced concrete. It can be seen that the application of polymer 3D printing technology in the field of civil engineering still needs to be continuously studied.

Figure 4:Application of 3D printing polymer in civil engineering: (a) Epoxy resin-steel fiber reinforced concrete [67], (b) Polymer lattice reinforced concrete [68].

With the application of epoxy resin in various fields, the problems of high thermal expansion coefficient, inherent brittleness and easy cracking of conductive adhesives in general epoxy resins have become increasingly apparent. These problems seriously affect the structural stability and service reliability of packaging devices using epoxy resin as an adhesive or coating material [70,71-74]. For example, epoxy resin ages under ultraviolet light, resulting in performance degradation, making it unsuitable for long-term outdoor use. At the same time, epoxy resin materials are brittle and have low impact strength. In order to give epoxy resins higher temperature and impact resistance, research has focused on modifying them by designing their molecular structure and synthesizing new epoxy resins. For example, by introducing groups such as biphenyl, naphthalene, sulfone and fluorine into the epoxy resin skeleton, the moisture resistance and heat resistance of the cured product can be significantly improved. In addition, the hydrophobic and non-polar dicyclopentadiene group has attracted much attention due to its excellent moisture resistance and thermal stability. This method has been successfully applied in the research and development of conductive adhesives, with remarkable results [75,76]. Through molecular modification, high-temperature resistant and high-toughness epoxy resins can be produced in the future. By introducing new high-temperature resistant groups, such as aromatic heterocycles, silicon-containing groups, etc., the heat resistance of epoxy resins can be improved, and the cross-linking density can be optimized to improve toughness. It is also possible to combine molecular simulation with experimental research to precisely control the molecular structure from the nanoscale to the microscale, and achieve multi-scale structural regulation to optimize mechanical and thermal properties. It is worth noting that although the concept of molecular or atomic manufacturing has been proposed, it is limited to small-scale production. How to produce large quantities of such chemical materials, which require precise control of molecular or even atomic structures, will be a point that needs to be broken through for a long time to come.

The above studies have each carried out doping modification and synthesis of new epoxy resins from the perspective of molecular structure design, but all of them have only improved single properties such as impact strength or thermal properties of epoxy resins. However, the overall properties such as mechanical properties and thermal properties have not been improved as a whole. Therefore, improving the mechanical and thermal properties at the same time is still a major challenge [77]. To improve the performance of epoxy resins, researchers have proposed methods to toughen and modify epoxy resins, such as adding rubber elastomers [78] and using materials with soft molecular structures [79]. However, these methods have shortcomings: while the toughness of epoxy resin is improved, other properties such as heat resistance are usually compromised. In recent years, other modification methods have been proposed, including chemical copolymerization, thermosensitive liquid crystal polymer structure toughening, nanomaterial modification [80-82], fiber modification, etc. These are only improvements to the single performance of the epoxy resin, probably because the added nanogroups only a single performance. If bifunctional or multifunctional groups can be introduced, new functional groups with high mechanical properties, thermal stability and anti-ultraviolet aging performance can be developed, such as epoxy resins with imide structures, which will provide a basis for the development of high-temperature resistant walls, hightemperature resistant coatings, crack-resistant adhesive materials, high-temperature resistant electronic packaging products, etc. However, how to achieve the introduction of multifunctional groups without losing the excellent mechanical properties of epoxy resin itself, and at the same time, these multifunctional groups must have good compatibility with epoxy resin, which requires a lot of experimental research to summarize.

It is difficult to simultaneously improve the strength and toughness of epoxy resins by controlling the amount of curing agent, as the various physical and chemical properties of the material are simultaneously affected by the modified material, demonstrating the limitations of modification. A recent study showed that by adding a specific chemical agent to induce phase separation of epoxy resins, resulting in a large number of hydrogen bonds, this new method can improve both strength toughness and variable stiffness, thus providing a new idea and design direction for the preparation of high-strength and high-toughness epoxy composites [83]. In addition, by constructing a three-intersection network in the epoxy resin, it can maintain the heat resistance of the epoxy resin and at the same time have a rapid thermal response recovery ability, and even compensate for the strength loss caused by the addition of reinforcing agents [84]. Inducing the formation of a large number of hydrogen bonds can optimize the performance of epoxy resins and develop dynamic hydrogen bond networks that can change with temperature or external stimuli. At the same time, the design of other dynamic covalent bond networks can have the same effect. For example, reversible covalent bonds (such as disulfide bonds and Diels-Alder reaction bonds) can be used to give epoxy resins the ability to self-heal and be recyclable, while optimizing their mechanical and thermal properties. Chemical or physical modification is easy to perform, but obtaining high performance products is difficult, making it challenging to elucidate the modification mechanism and control performance changes. The inherent molecular structure of resin makes it highly flammable and produces a lot of smoke, which will cause environmental pollution under conditions such as incineration and fire. Therefore, how to reduce flammability and pollution is a problem. The current practice is to add flame retardant particles for modification, and starting from the chemical composition and molecular structure of the material may be a method worth exploring.

Among these, the study of modifying epoxy resin with fibers has become the current research focus, as fiber-reinforced plastics are the most advanced materials. Future research on epoxy resins can focus on improving the toughness of epoxy resins while improving or maintaining their own mechanical properties, such as their fracture resistance when used as a building material, and their heat and corrosion resistance when used as a building surface coating material or road repair adhesive. Therefore, changing the chemical molecular structure is the first step, finding the application occasion is the second step, and the later problems include recycling, repair and reuse. It is worth mentioning that the development of degradable epoxy resin based on more advanced biotechnology is an interesting issue that will greatly improve the sustainability of composites. The currently popular smart materials have active response capabilities. If this technology can be combined with epoxy resin, so that epoxy resin can spontaneously make some movements when stimulated by external stimuli, this will greatly expand the application scenarios of epoxy resin. It must be said that artificial intelligence is developing rapidly. If it can be used to optimize the molecular structure and computational properties of epoxy resin, this will greatly shorten the development process of new materials and accelerate the pace of the materials revolution.

Epoxy resin is widely used due to its excellent physical and mechanical properties, chemical corrosion resistance and so on. However, the inherent brittleness of the material makes it easy to crack or form cracks when used as a coating or road paving material, which causes erosion inside the building structure. To address this problem, doping modification and synthesis of new epoxy resins have been carried out from the perspective of advanced molecular structure design [85,86]. In civil engineering, epoxy resin can effectively improve the load-bearing capacity and durability of concrete structures. Compared with pure epoxy resin, the surface hardness, tensile strength, elongation and maximum load stress of reinforced composite materials prepared with it as a matrix can be improved [87]. This provides an opportunity for concrete reinforcement using the polymer-based fiber fabric. However, it is not enough to improve only the individual properties of the epoxy resin such as impact strength or thermal properties. Therefore, the improvement of comprehensive properties such as mechanical properties and thermal properties of epoxy resin, while improving the mechanical properties, heat resistance and crack resistance of epoxy resin, is the future research direction of epoxy resin and its application in civil engineering. On the other hand, epoxy resins and their fabrics used to strengthen building materials such as concrete can greatly improve their service life, reduce the loss of buildings such as bridges and roads, and thus reduce their maintenance costs, which is relatively important for global sustainable development and is also a valuable direction of research [88].

The authors gratefully acknowledged the financial supports from the Key Research and Development Program of Heilongjiang Province (grant number: 2023ZX07D03, corresponding to H. Liu).

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