Section 3 Applications

#### **Chapter 5**

## Epoxy Resin Adhesives: Modification and Applications

*Jun Zhang, Hai Luo, Xiaojian Zhou and Bowen Liu*

#### **Abstract**

Epoxy resin adhesives (ERAs) as easily prepared thermosetting adhesives have been extensively employed in building construction, electrical appliance manufacturing, automobile manufacturing and wood industry because of their excellent mechanical properties, water resistance, low cost, long service life and strong bonding properties. This chapter aims to introduce the synthesis, properties and development of ERAs and to illustrate how defects in their curing properties, thermal properties, brittleness and flammability affect their global development. Furthermore, this study introduces the modification of ERAs according to these defects and their development in main application fields. Lastly, the limitations and prospects of ERAs in future applications are also discussed.

**Keywords:** epoxy resin, adhesives, mechanical properties, curing properties, modification

#### **1. Introduction**

Epoxy resin adhesive (ERA) is the general term for polymers with two or more epoxy groups in the molecule. It is widely used in industries and is an important thermosetting resin adhesive [1, 2]. ERA is a thermosetting adhesive with strong adhesion, high cohesion, low shrinkage, low cost and low creep rate. It can be used for several materials, such as metal, cement and wood; thus, it is referred to as a 'universal and strong' glue [3]. It has a history of more than 70 years. The molecular end of epoxy resin is connected with epoxy groups. During curing, hydroxyl and ether bonds are formed, and the structure contains benzene or heterocyclic rings. Due to the presence of epoxy groups, hydroxyl groups, ether bonds, ester groups and other polar groups, it has a strong bonding effect on many substances other than non-polar polymers [4]. After the epoxy-based adhesive is cured, it forms a complex three-dimensional (3D) network structure with strong cohesion. The epoxy-based adhesive hardly generates low-molecular products during curing, has a small linear expansion coefficient, stable dimensions, small internal stress and better bonding strength. Epoxy-based adhesives meet the requirements of structural adhesives, but they also have some shortcomings. Because the curing process of epoxy-based adhesives needs a higher temperature, and it contains many rigid groups, such as a benzene or heterocyclic ring, the flexibility of the molecular chain is minimal [5].

Besides, after cross-linking to form a network structure, the deformability is further weakened, showing strong brittleness, which results in low bonding strength, poor impact strength, delamination and easy cracking resulting of epoxy-based adhesives [3]. Meanwhile, its flame retardancy is poor. As a structural adhesive, it is expected to cure quickly, have higher heat resistance and flame retardancy. Therefore, epoxybased adhesives must be modified to expand its scope of application. This study mainly introduces the curing, heat resistance, toughening and flame-retardant modification of epoxy-based adhesives and their application in different fields.

#### **2. Modification of ERAs**

#### **2.1 Curing modification of epoxy adhesives by curing agent**

As a thermosetting adhesive, ERA must be cured at high temperatures. However, for industrial applications, it must be cured at room temperature [6, 7], so the importance of developing curing agents for epoxy-based adhesive cured at room temperature is self-evident. Especially, room-temperature fast-curing epoxy adhesives can be used in aerospace and marine engineering applications, as well as in traditional manufacturing and daily life [8, 9], because of their fast-curing speed, high strength and strong durability [10]. With the continuous development of curing agents, roomtemperature fast-curing ERAs as chemical products have become indispensable in the manufacturing industry [11]. The room-temperature curing of ERAs is an energysaving curing method. The curing process is simple, and it is suitable for various curing situations that do not require heating.

#### *2.1.1 Classification of curing agent for ERAs*

According to the curing temperature, epoxy-based adhesive curing agents can be divided into amines, acid anhydrides, synthetic resins and latent curing agents by different chemical components. Among them, amine-curing agents are often used in ambient curing at room temperature [12]. Amine-curing agents are the earliest room-temperature curing agent used. It adheres excellently to most adherents. However, amine-curing agents have high volatility and toxicity and have strong water and carbon dioxide absorption abilities. The cured surface is prone to whitening and blistering [13]. Among the amine-curing agents, there are mainly polyamides, aliphatic amines, alicyclic amines. In industrial applications, curing agents, such as aliphatic amines, polyamides and alicyclic amines, are often used [14].

#### *2.1.1.1 Polyamide-curing agent*

As one commonly used curing agent, polyamide accounts for more than 30% of the total epoxy resin-curing agents. It is mainly made of dimers or unsaturated fatty acids and polyamine as raw materials and forms amide bonds through dehydration condensation [15]. Among them, the dimerised fatty acid polyamide can overcome the shortcomings of epoxy-based adhesives' fragility and has low toxicity, good workability and high-paint film adhesion [16]. Modifying the polyamide-curing agent can effectively improve the properties of epoxy resins. For example, Bryan et al. [17] used polyamide and phthalic anhydride as the curing agent of epoxy resin to improve the

curing rate at room temperature. Gholipour et al. [18] improved the thermal properties of epoxy resins by preparing polyamidoamine (PAMAM) dendrimer-curing agents grafted with graphene oxide.

#### *2.1.1.2 Aliphatic amine-curing agent*

The amount of aliphatic amine-curing agents in various curing agents is second only to polyamide because most are liquid and have good miscibility with epoxy resin. Epoxy resin can be cured at room temperature. Modifying the aliphatic amine-curing agent can effectively improve the mechanical properties of epoxy resin. For example, Patel et al. [19] brominated unsaturated castor oil, which was the main raw material, and reacted the resulting material with excess aliphatic diamines, such as ethylenediamine, 1,3-propanediamine and 1,6-hexanediamine, to obtain an amino-functionalised castoroil-curing agent to improve the mechanical strength of epoxy resins. Wan et al. [20] synthesised a novel low-volatility star aliphatic polyamine with extremely high-NH2 functional groups as the curing agent of bisphenol A diglycidyl ether epoxy resin. The novel curing agent has a high reaction activity, and the reaction has autocatalytic properties. Additionally, compared with linear propylene diamine, it can significantly increase the crosslinking density and glass transition temperature (Tg) of the cured epoxy resin.

#### *2.1.1.3 Alicyclic amine-curing agent*

Alicyclic amines are amine compounds containing alicyclic rings. The alicyclic amine-curing agent has many spatial conformations and good flexibility. Most alicyclic amines are low-viscosity liquids with long pot life and excellent chroma and gloss. Alicyclic polyamine compounds are widely used as curing agents for epoxy-resin adhesives and other structure adhesives because the molecular structure contains alicyclic rings (five-membered or six-membered rings) with higher stiffness and better stability. Xu et al. [21] used alicyclic polyamines and acrylonitrile to synthesise the curing agent to improve the bonding strength of epoxy-resin-based adhesives.

#### **2.2 Heat-resistant modification of ERAs**

ERAs can withstand high temperatures up to 175°C and are compatible with all common reinforcement materials. A higher-temperature-resistant ERA can be essentially applied in many fields. In addition to having high-temperature-resistant properties, it can also show strong properties in many aspects, such as high-temperature-resistant epoxy. It has excellent mechanical properties, relatively outstanding strength and has good corrosion resistance and insulation properties [22, 23]. Therefore, research on high-temperature-resistant ERA is extremely necessary. There are two main measures in implementing the modification of high-temperature ERA. The first measure entails introducing new structures into the epoxy resin itself to improve its high-temperature properties. The second measure is blending or co-polymerisation to modify the hightemperature epoxy resin.

#### *2.2.1 ERA itself introduces a new structure to improve its high-temperature resistance properties*

The modification treatment of high-temperature-resistant ERA is mainly performed to promote the structure of ERA to be changed to a certain extent. It is more common to improve the high-temperature resistance effect by introducing new structures. This method for introducing a new structure through the epoxy resin also involves many types of processing in actual implementation. For example, the effective use of multifunctional structures can promote the formation of ring structures [24]. The functionality of the epoxy-resin structure increases, which can promote the stability and cross-linking density of the corresponding structure and finally effectively enhance the high-temperature resistance of the epoxy resin. Furthermore, introducing rigid groups that have a good high-temperature resistance effect can promote the epoxy resin to show excellent performance, such as benzene ring, fused ring and biphenyl are some of the more commonly used rigid groups [25]. Moreover, Bismaleimide and epoxy resin may form an interpenetrating network or two-phase system during the polymerisation process, which improves the toughness and heat resistance of the epoxy resin [26].

Luo et al. [27] modified bisphenol A epoxy resin with bismaleimide and 4,4′-diaminodiphenylsulfone to produce a two-component high-strength bismaleimide modified epoxy-based adhesive with high cross-linking. The viscosity of the adhesive gradually decreases as temperature increases and can maintain good mechanical properties and storage stability. Cheng et al. [28] used 2,7-dihydroxynaphthalene and epi-chlorohydrin as raw materials to synthesise an epoxyresin-based adhesive containing a naphthalene ring structure. Also, Yang et al. [29] used 1-naphthol and dicyclopentadiene as the main raw materials to synthesise an ERA containing naphthalene ring and dicyclopentadiene structure. The results show that the ERA has a higher heat resistance than the bisphenol A epoxy resin.

#### *2.2.2 Modification of high-temperature-resistant ERA by blending or co-polymerisation*

The blending and co-polymerisation methods can effectively and mainly help select an ideal material and ERA for effective combination to ensure greater hightemperature-resistant properties. Combined with the specific application and implementation of these methods, the requirements for specific blended or co-polymerised materials are relatively strict [6]. For example, the appropriate use of heat-resistant polymers, nano-materials and silicones can achieve ideal modifications. The treatment effect improves the heat resistance of the epoxy resin; besides, it can also guarantee its toughness or strength to be ideally optimised. Zhang et al. [30] modified ordinary bisphenol A epoxy resin with organic silicon active intermediates, added nitrile-40 and nano-TiO2 active fillers to toughen and strengthen the resin. The results showed that the modified ERA that could be cured at room temperature, used for a long time at 250°C and can withstand 300°C for a short period has been developed. Hu et al. [31] used polymethyltriethoxysilane (PTS) to react with a synthetic phosphorus-containing silane coupling agent in a certain ratio to modify the bisphenol A epoxy resin. The modified ERA retained its tensile strength. However, the Tg, high-temperature thermal stability, impact strength and limiting oxygen index (LOI) were all improved. Ramirez et al. [32] combined epoxycyclohexyldimethylsilyl with the curing agent 4,4′-(1,3-phenylene diisopropylidene) diphenylamine after coordination. Due to the dispersion of the heat-resistant epoxycyclohexyldimethylsilyl in the ERA, the silicon oxide compound formed during the thermal decomposition process deposited on the surface of the unburned polymer, partially forming a protective layer, slowing down the heat transfer to a certain extent and inhibiting the flammability as the gas volatilises, thereby preventing the mixture of flammable gas and oxygen.

#### **2.3 Toughening modification of ERAs**

ERAs are cost-effective and have simple moulding and processing methods, low chemical shrinkage after curing, good chemical stability, excellent mechanical properties and good bonding properties [33]. However, due to several epoxy groups, the cured structure has a high chemical cross-link density, low-molecular chain flexibility and high internal stress, resulting in greater brittleness, poor impact resistance and fatigue resistance of the ERA. It limits its application and development in some high-tech fields that require high durability and reliability.

ERA has good compatibility with rubber and other elastomers. After the rubber is dissolved in the uncured epoxy-resin matrix, the ERA undergoes a curing reaction, separates from the rubber and is dispersed in the resin to form a 'sea island' structure, thereby improving the toughness of the epoxy resin [34]. The rubber molecules containing no reactive groups cannot react with epoxy resin and will precipitate out during curing, which has a toughening effect. However, if excessively added, it weakens the adhesion of the bonding interface. Therefore, rubber molecules with active groups are generally used to modify and toughen epoxy resins. Carboxyl-terminated liquid nitrile rubber (CTBN) and amino-terminated liquid nitrile rubber (ATBN) have been widely mixed with epoxy resins to improve their toughness. For example, Wang et al. [35] used CTBN and ERA to prepare a structural adhesive with high shear and excellent peel strength. Meanwhile, to adapt to the application in different fields and improve the toughness of the cured ERA, flexible segments are often introduced into the curing agent to control its physical and chemical properties. Lou et al. [36] used dendritic polyester polyol as the branching unit and toughening segment and imidazole-terminated diisocyanate as a functional group to synthesise a functional toughening-curing agent to improve the toughness of ERAs. In the initial curing stage, several secondary hydroxyl groups react with the isocyanate groups to form a dendritic epoxy structure, and the bisphenol A epoxy molecule acts as a long-chain polyol to react with the dendritic epoxy structure. Simultaneously, the epoxy group opens a ring to form a secondary hydroxyl group and continuously reacts with the isocyanate group in the cross-linking structure. Furthermore, NH- existing in the carbamate reacts with the epoxy group, thereby obtaining an epoxy resin-curing cross-linking system with a 3D dendritic cross-linking structure. Thus, the toughness of epoxy resin has been greatly improved. Meanwhile, Zheng et al. [37] studied the toughening effect of nano-SiO2 on cycloaliphatic epoxy systems. They used nanosilica to improve the toughness of cycloaliphatic ERA. The coupling agent γ-glycid oxypropyltrimethoxysilane (KH-560) was used to modify the surface properties of SiO2. The results show that adding nano-SiO2 effectively improves the toughness and thermal stability of the cycloaliphatic ERA.

#### **2.4 Flame-retardant modification of ERA**

ERAs have been widely used due to their excellent properties [38]. However, the conventional ERA is formed from reacting bisphenol A and epi-chlorohydrin [39]. It is flammable when cured, which could cause a high fire risk when ERAs are used in certain applications, such as printed manufacture, furniture, aircraft and train interiors [40]. Serious consequences could occur due to the high release rate of heat and smoke accompanied by the combustion of epoxy resins. Therefore, it is very important to enhance the flame retardancy of ERA for expanding their application in this field. Many studies have reported improving the flame retardancy of epoxy resins via structural modification or adding various flame retardants [41, 42]. Structural modification introduces the elements with flame-retardant functions into the molecular structure of ERAs. The representative of structural modification is brominated ERA, which is the reaction product of epi-chlorohydrin and brominated bisphenol A, such as tetrabromo diphenylolpropane. The brominated ERA has an outstanding flame ignition resistance, whereas the bromine content is ~18–20% in the finished adhesive. When the product is thermally decomposed at the temperature generated in the fire, it will release acid halide gas, which protects the product from fire. These halide gases act as extinguishers to significantly increase the ignition temperature of the cured ERA. The addition of flame retardants in ERA shows good properties, such as simple processing, low cost, wide source of raw materials and obvious flameretardant effect. It is one of the most popular strategies for flame-retardant modification of ERA. Flame retardants could be an integral part of the ERA by reacting chemically with the polymers or simply mixed with the ERA without any reaction. All kinds of flame retardants work by acting chemically and/or physically either in the vapour phase and/or condensed phase to interfere with the combustion process during heating, pyrolysis, ignition or flame spread [43]. The types of flame retardants and their operating characteristics are described as follows [38]: (1) char formers: usually, phosphorus compounds, which remove the carbon fuel source and provide an insulation layer against the fire's heat. (2) Heat absorbers: usually metal hydrates, such as aluminium trihydrate (ATH) or magnesium hydroxide, which remove heat by evaporating the water in their structure. (3) Flame quenchers: usually, bromineor chlorine-based halogen systems that interfere with the reactions in a flame. (4) synergists: Usually, antimony compounds, which enhance the performance of the flame quencher. The flame-retarding action of ERA could be divided into physical and chemical actions. Physical action includes cooling, barrier action via the formed protective layer and fuel dilution. For the cooling action, flame retardants absorb the heat when they decompose, and the endothermic decomposition may consume the released heat from the combustion of ERA, then the burning adhesive is cooled. Generally, most inorganic-hydrated compounds, such as aluminium and magnesium hydroxides, may play a role via this mode. For the barrier action, the decomposition products of some flame retardants shield the surface of the adhesive and form a protective layer that may act as a barrier to resist oxygen and the produced heat. Consequently, the burning process is difficult to sustain. For fuel dilution, some flame retardants may release water vapour, carbon dioxide, or other inert gases, thereby decreasing the concentration of free radicals and combustible gases in the burning adhesive. Chemical action includes gas-phase and condensed-phase reactions. The gas-phase reaction mechanism is generally regarded as the interruption of the chain reaction of the ERA structural system during burning. The flame retardant that provides the flame-retarding action via the gas-phase reaction action may capture free radicals to decrease the concentration of free radicals than the combustion threshold and then prevent or delay burning, in which halogen-containing flame retardants are the most representative. During burning, the halogen-containing flame retardants release the hydrogen halide, which may react with the free radicals formed during burning to inhibit the combustion of substrates. For the condensed-phase reaction, the flame retardant that provides the flame-retarding action via the condensed-phase reaction may promote the formation of a carbonised or vitreous layer by crosslinking, aromatising, catalytic dehydration of polymers or reacting with the ERA. In this flame-retardant mode, intumescent flame retardants may form an intumescent char layer by some chemical reactions during burning, and generally, the formed char

#### *Epoxy Resin Adhesives: Modification and Applications DOI: http://dx.doi.org/10.5772/intechopen.101971*

layer may promote the barrier action and improve the flame retardancy of the ERA. Furthermore, some flame retardants can accelerate the rupture of the chains of the ERA, and several droplets are produced under this condition. Then, a large amount of heat may be taken away when these droplets move away from the burning zone [44]. Flame retardants can be classified into several families, including halogen-based compounds, phosphorus-based compounds, silicon-based compounds, nano-composites and metal-based compounds. Among them, halogen-based and phosphorus-based flame retardants are widely used.

For halogen-containing flame retardants, the flammability of ERA can be greatly reduced by incorporating a halogen into the molecule. The best known are the halogen-containing ERAs based on chlorinated, brominated and fluorinated bisphenol A. They often comprise blends of two or more epoxy-resin systems, one of which is a halogenated resin, and the other of which perhaps contains a halogenated curing agent, such as chloric anhydride. These halogen-containing ERAs have been developed over decades and are still used widely due to the obvious advantages of low cost, processability, miscibility and low reduction in physical/mechanical features of the flame-retardant systems. Halogen-containing flame retardants function by liberating acid halide gases as the product thermally breaks down at the high temperatures incurred in a fire. These halide gases act as extinguishers to significantly increase the ignition temperature of the cured ERA. The mechanism of these flame retardants is the release of hydrogen halides (HCl and HBr) during the thermal decomposition of the ERA. The chemical reaction during burning is a free radical chain reaction, and the continuous growth of free radicals is important for maintaining the burning process for the ERA. Several chemical halide intermediates form during the burning of ERAs. These halide species are carried into the flame front of the burning polymer where they inhibit key free radical reactions of combustion. This inhibition results in flames becoming unsteady and extinguishing and lowers the release of heat overall [10, 11]. Generally, alicyclic or aliphatic halogen-containing flame retardants are more efficient than aromatic halogen compounds. Alicyclic or aliphatic halogencontaining flame retardants burn at low temperatures for most polyolefins because of lower carbon-halogen bond energies and easier halogen release [45]. Beach et al. [46] synthesised brominated polybutadiene-polystyrene (BrPBPS) flame retardant from styrene-butadiene-styrene triblock architecture by bromination. The BrPBPS flame retardant contains similar aliphatic bromine as in hexabromocyclododecane, but with a higher-molecular-weight structure. It provides similar flame-retardant activity as hexabromocyclododecane in polystyrene blends, where both release HBr to provide the gas-phase activity. Both also provide enhanced ERA degradation as another major pathway for condensed flame-retardant activity. Jiang et al. [47] added BrPBPS into epoxy asphalt adhesive to enhance its flame resistance. Meanwhile, the Tg of the epoxy asphalt adhesive was notably enhanced with the inclusion of BrPBPS. Wu et al. [48] synthesised liquid-oxygen-compatible bromine-containing ERA by the polycondensation of tetrabromobisphenol A and epoxy resins. The bromine element was introduced into the ERA to improve the liquid oxygen compatibility and enhance flame retardancy. The results showed that limiting oxygen index increased drastically when the bromine content was increased from 0% to 21.20%.

Phosphorus-containing flame retardants are identified as one of the most promising halogen-free flame retardants [49, 50] since they possess excellent properties, such as low-smoke emission, low toxicity, form a stable carbonised layer after burning effectively [44, 46, 51, 52] and are environmentally friendly [35]. For preparing organophosphorus epoxides-based adhesive, three general methods were employed.

First, the condensation of 1-chloro-2,3-epoxypropane and organophosphorus compounds containing two or more hydroxyl groups. Second, the Michaelis-Arbuzov reaction of phosphites with 1-halogeno-2,3-epoxypropanes. Finally, the epoxidation of tertiary phosphine oxides by peroxy acids [53]. Phosphorus-containing flame retardants can be generally classified into three categories: (1) simple reactive phosphate monomers; (2) linear polyphosphazenes; (3) aromatic cyclic phosphazenes. They may be integrated into the ERA chains through co-polymerisation, homo-polymerisation, surface modification or blending; simple inorganic or organic additives are excluded [25, 53–55]. During the burning of ERAs, most of the current phosphorus-containing flame retardants may act simultaneously in the condensed and gaseous phases [56]. In the condensed phase, phosphorous-containing flame retardants can make the amount of carbonaceous residue or char, which acts as the thermal insulation, and a barrier of oxygen to transfer to the burning adhesive increase. Afterwards, a carbonised layer is formed. The carbonised layer prevents further pyrolysis of the corresponding ERA [57]. In the gaseous phase, some phosphorus-based additive flame retardants may produce several free radicals during the thermal decomposition process, and they may react with the free radicals which are generated from the ERA. Then, the free-radicalssupported combustion of polymers might be stopped due to the lack of fuel [58–60]. Wazarkar et al. [61] synthesised phosphorus–sulphur-containing di and tetra functional carboxyl curing agents and used them in preparing high-performance ERA and coating. The anticorrosive and flame-retardant properties of the adhesives and coatings were improved as the concentration of the flame-retardant-curing agents increased, and they exhibited excellent mechanical and chemical properties and thermal stability. Ma et al. [62] synthesised a phosphorus-containing bio-based ERA from itaconic acid and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide. As the matrix, its cured epoxy network with methyl hexahydrophthalic anhydride as the curing agent showed comparable Tg and mechanical properties to diglycidyl ether in a bisphenol A system, as well as good flame retardancy with UL94 V-0 grade during a vertical burning test.

#### **3. Application of ERAs**

ERA is often used mainly because it has the advantages of low-temperature curing, good bonding performance and improved engineering efficiency. ERAs are usually used for metal bonding, concrete bonding and wood adhesive.

#### **3.1 Application of ERA for metals**

Several studies have been conducted on the use of ERAs for metal bonding. In a previous study, polyurethane 1,2-polyethylene oxide was first cured onto the metal surface and then used 1,1-polyoxyethylene ether. The adhesive adheres the polyurethane resin material to the metal [63]. Subsequently, another study prepared a novel high-temperature curing epoxy adhesive using polysulfone as a raw material. Studies have shown that the tackifier resin accelerates the bonding of ERA and steel and promotes the bonding and vulcanisation of fluorine rubber [64]. Uehara et al. [65] prepared a monolithic ERA layer with a porous structure on the surface of a stainless steel (SUS) plate through a polymerisation-induced phase separation process, as a mediator for bonding SUS and various thermoplastic resin plates. The research results show that the bond strength of the apparent bond area between SUS and ERA is two to three times higher than those for direct metal-resin bonding.

#### **3.2 Application of ERA for concrete bonding**

Concrete is the most widely used worldwide building material. Traditional cement is used as a binding material for concrete. Cement concrete also has some shortcomings, such as low tensile and flexural strength, high porosity, low durability and abrasion resistance and longer solidification time. The mechanism of epoxy concrete is such that ERA forms a 3D structure through cross-linking in the combination of masonry mortar and concrete [66]. Afterwards, the ERA particles are dispersed into the system. Then, a part of the adhesive particles settle on the surface of the aggregate particles and participate in the cross-linking reaction. Finally, the ERA particles form a cured network structure, and the aggregates are bonded in the cured epoxy network structure. In ERA/mortar/concrete composites, the adhesive network forms a bridge between aggregates, so that the epoxy-based composites have higher mechanical properties and impermeability [67]. Also, it was found that the resin bond concrete has great advantages in the manufacture of machine tool beds. Kim et al. [68] studied ERA as the matrix material of resin concrete. The reinforced aggregate of resin concrete comprises pebbles and sand. The results showed that when the mass fraction of resin in the resin concrete is 7.5%, the thermal expansion coefficient of the resin concrete is the same as that of cast iron. In this case, the specific heat of resin concrete is 63% larger than that of cast iron. Beutel et al. [69] added epoxy resin-based adhesive concrete and coarse aggregate to the mixture and found that adding the aggregate did not affect the strength of the mixture. Additionally, the specimens exhibited much higher tensile and flexural strength than ordinary concrete, and the compressive strength was similar to ordinary high-strength concrete.

#### **3.3 Application of ERA for wood bonding**

Lei et al. [70] modified soy protein isolate with a surfactant grafted with maleic anhydride and blended with epoxy resin to prepare wood adhesives with higher bonding strength and good water resistance. Zhang et al. [71–73] prepared wood adhesive with good water and heat resistance by the co-polycondensation of tannin or lignin or starch with furfuryl alcohol and blending with epoxy resin. After bonding with wood, the adhesive was cured at high temperature under pressing to prepare a wood-based panel. After testing, the material showed good shear strength.

#### **4. Conclusions**

Although research activities on modifying ERAs have greatly progressed, several problems still need urgent resolution. For example, most of the toughening methods of ERAs are at the expense of the rigidity and strength of the modified product, and it is difficult to increase the toughness and strength of the ERA simultaneously. The ERA modified with rubber or nano-particles, due to the large specific surface area of nano-particles or rubber, is very easy to agglomerate. Therefore, how to uniformly disperse it in the ERA system to obtain a reinforced and toughened high-performance ERA remains an important research topic. Therefore, the future development direction of ERAs should be towards low-temperature fast curing, high performance, green environmental protection and multifunctional development. ERAs have developed in a more stable, safe and scientific direction.

#### **Acknowledgements**

This work was supported by The Yunnan Provincial Natural Science Foundation (Grant No. 202101AT070038, 2018FG001095), and the Yunnan Provincial Youth top talent project (YNWR-QNBJ-2020-166) and Youth talent support project and Middle-age Reserve Talents of Academic and Technical Leaders (2019HB026) and the 111 project (D21027).

#### **Conflict of interest**

The authors declared that they have no conflicts of interest.

### **Author details**

Jun Zhang1 \*, Hai Luo2 , Xiaojian Zhou1 and Bowen Liu1

1 Yunnan Provincial Key Laboratory of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming, China

2 East China Woody Fragrance and Flavor Engineering Research Center of National Forestry and Grassland Administration, College of Forestry, Jiangxi Agricultural University, Nanchang, China

\*Address all correspondence to: zj8101274@163.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **References**

[1] Shahrokhinia A, Biswas P, Reuther JF. Orthogonal synthesis and modification of polymer materials. Journal of Polymer Science. 2021;**59**(16):1748-1786

[2] Song S, Yang S, Li C, et al. Preparation of nanometer conductive powder cul and study on its performances. Journal of Donghua University, Natural Science. 2004;**30**(5):97-101

[3] Zhang YL. Epoxy Adhesive. Beijing: Chemical Industry Press; 2017. pp. 1-6

[4] Yu S, Li X, Zou M, et al. Effect of the aromatic amine curing agent structure on properties of epoxy resin syntactic foams. ACS Omega. 2020;**5**(36):23268-23275

[5] Cao J, Duan H, Zou J, et al. A biobased phosphorus-containing co-curing agent towards excellent flame retardance and mechanical properties of epoxy resin. Polymer Degradation and Stability. 2021;**187**:109548

[6] Huang S, Kong X, Xiong Y, et al. An overview of dynamic covalent bonds in polymer material and their applications. European Polymer Journal. 2020;**141**(80):110094

[7] Levchik SV, Weil ED. Thermal decomposition, combustion and flame-retardancy of epoxy resins—A review of the recent literature. Polymer International. 2010;**53**(12):1901-1929

[8] Hodgkin JH, Simon GP, Varley RJ. Thermoplastic toughening of epoxy resins: A critical review. Polymers for Advanced Technologies. 2015;**9**(1):3-10

[9] Mai G. Failure mechanisms in toughened epoxy resins—A review. Composites Science and Technology. 1988;**31**(3):179-223

[10] Dong W, Yao SS, Wang S. Reform and practice of production practice for our college polymer material and engineering major. Polymer Bulletin. 2016;**12**:66-70

[11] Shao KC. Research on the toughening modification of epoxy resin adhesive. Rubber Technology and Equipment (Plastics). 2016;**42**:89-90

[12] Zhao JL, Zhang GC, Li HQ, et al. Research progress in toughening epoxy resin. Plastics. 2002;**31**(3):40-44

[13] Pouladvand AR, Mortezaei M, Fattahi H, et al. A novel custom-tailored epoxy prepreg formulation based on epoxy-amine dual-curable systems. Composites Part A: Applied Science and Manufacturing. 2020;**132**:105852

[14] Yin H, Li B, Liu YT, et al. Synthesis and properties of polyamide curing agent for epoxy resin. China Adhesives. 2016;**25**(1):9-12

[15] Chen L, Zhu BY, Zhang HQ, et al. Research progress of waterborne polyamide curing agent. Shanghai Paint. 2021;**59**(1):34-39

[16] Stemmelen M, Lapinte V, Habas JP, et al. Plant oil-based epoxy resins from fatty diamines and epoxidized vegetable oil. European Polymer Journal. 2015; **68**:536-545

[17] Bryan EH et al. Water absorption isotherms for cured bisphenol A type epoxy resins: Effects of isotherm temperature and resin hardener. Journal of Applied Polymer Science. 1984;**29**(6):2021-2038

[18] Gholipour-Mahmoudalilou M, Roghani-Mamaqani H, Azimi R, et al. Preparation of hyperbranched poly (amidoamine)-grafted graphene nanolayers as a composite and curing agent for epoxy resin. Applied Surface Science. 2018;**428**(15):1061-1069

[19] Patel BP, Patel HS, Patel SR. Modified castor oil as an epoxy resin curing agent. Journal of Chemistry. 2012;**1**(1):11-16

[20] Wan J, Bu ZY, Xu CJ, et al. Preparation, curing kinetics, and properties of a novel low-volatile starlike aliphatic-polyamine curing agent for epoxy resins. Chemical Engineering Journal. 2011;**171**(1):357-367

[21] Xu YH, Yue CY, Wang WJ. Synthesis and properties of modified alicyclic amine curing agent. Adhesion. 2007;**28**(5):17-18

[22] Yin H, Jin H, Wang C, et al. Thermal, damping, and mechanical properties of thermosetting epoxy-modified asphalts. Journal of Thermal Analysis & Calorimetry. 2014;**115**:1073-1080

[23] Hao B, Liu Y, Yu X, et al. Synthesis, polymerization kinetics and thermal properties of benzoxazine resin containing ortho -maleimide functionality. Macromolecular Research. 2021;**29**(1):24-32

[24] Galià M, Serra A, Mantecón A, et al. Curing reaction of diglycidylesters containing alicyclic imide structures with anhydrides and amines as hardeners. Journal of Applied Polymer Science. 2010;**57**(4):413-420

[25] Liu D, Zhao WJ, Wu F, et al. The effect of curing agent molecular structure on the friction and corrosion properties of epoxy resin coatings. Automotive and Motorcycle Tribological Materials Advanced Technology and Application Promotion Association. 2015;**472**:85-91

[26] Freitas A. Polymer degradation and stability. Polymer Science. 2006;**87**(87):171-181

[27] Luo ZW, Yu XH, Chen Q, et al. Development of bismaleimide modified epoxy adhesive. Adhesion. 2015;**36**(12):56-59

[28] Cheng J, Yan HQ, Fu SY, et al. Preparation and thermal properties of epoxy resin containing naphthalene ring. Journal of Fujian College of Forestry. 2011;**31**(3):281-284

[29] Yang MS, Ya L, Li LK, et al. Preparation and properties of special epoxy resin containing dicyclopentadiene and naphthalene ring. Modern Plastics Processing and Application. 2010;**22**(4):5-8

[30] Zhang B, Liu WQ. Organic silicon modified epoxy resin. New Chemical Materials. 2001;**29**(8):5

[31] Hu CH, Liu WQ, Wang ZF, et al. Study on heat resistance, toughness and flame retardancy of silicone-phosphorus hybrid modified epoxy resin. China Plastics. 2009;**23**(10):73-77

[32] Ramirez FA, Carlsson LA, Acha BA. Evaluation of water degradation of vinylester and epoxy matrix composites by single fiber and composite tests. Journal of Materials Science. 2008;**43**(15):5230-5242

[33] Dong LL, Li L. Application research progress of epoxy resin in electronic industry. Fine and Specialty Chemicals. 2013;**21**(08):35-38

[34] Gui D, Gao X, Hao J, et al. Preparation and characterization of liquid crystalline polyurethane-imide modified epoxy resin composites. Polymer Engineering & Science. 2014;**54**(7):1704-1711

*Epoxy Resin Adhesives: Modification and Applications DOI: http://dx.doi.org/10.5772/intechopen.101971*

[35] Wang HX, Zheng YQ, Hou YB, et al. Development of one-component epoxy resin structural adhesive with high shear and high peel strength. China Adhesives. 2008;**17**(8):19-12

[36] Lou CH, Liu XJ, et al. Functional dendritic curing agent for epoxy resin: Processing, mechanical performance and curing/toughening mechanism. Composites Part B: Engineering. 2018;**136**:20-27

[37] Zheng YP, Ning RC. Study on properties of nano-SiO2 epoxy resin composites. FRP/Composite Materials. 2001;**1**(2):34-36

[38] Chen L, Wang YZ. A review on flame retardant technology in China. Part 1: Development of flame retardants. Polymers for Advanced Technologies. 2010;**21**(1):1-26

[39] Qian L, Qiu Y, Wang J, et al. Highperformance flame retardancy by char-cage hindering and free radical quenching effects in epoxy thermosets. Polymer. 2015;**68**:262-269

[40] Tang S, Wachtendorf V, Klack P, et al. Enhanced flame-retardant effect of a montmorillonite/phosphaphenanthrene compound in an epoxy thermoset. RSC Advances. 2017;**7**(2):720-728

[41] Katsoulis C, Kandola BK, Myler P, et al. Post-fire flexural performance of epoxy-nanocomposite matrix glass fibre composites containing conventional flame retardants. Composites Part A-Applied Science and Manufacturing. 2012;**43**(8):1389-1399

[42] Petrie EM. Epoxy Adhesive Formulations. New York: McGraw-Hill; 2006

[43] Le W, Deng C, Zhao ZY, et al. Flame retardation of natural rubber: Strategy and recent progress. Polymers. 2020;**12**(2):429

[44] Morgan AB. The future of flame retardant polymers—Unmet needs and likely new approaches. Polymer Reviews. 2019;**59**(1):25-54

[45] Liu Q, Wang D, Li Z, et al. Recent developments in the flame-retardant system of epoxy resin. Materials. 2020;**13**(9):2145

[46] Beach MW, Hull JW, King BA, et al. Development of a new class of brominated polymeric flame retardants based on copolymers of styrene and polybutadiene. Polymer Degradation and Stability. 2017;**135**:99-110

[47] Jiang Y, Han X, Gong J, et al. Laboratory investigation of epoxy asphalt binder modified by brominated SBS. Construction and Building Materials. 2019;**228**:116733

[48] Wu Z, Li S, Liu M, et al. Study on liquid oxygen compatibility of bromine-containing epoxy resins for the application in liquid oxygen tank. Polymers for Advanced Technologies. 2016;**27**(1):98-108

[49] Cheng J, Wang J, Yang S, et al. Benzimidazolyl-substituted cyclotriphosphazene derivative as latent flame-retardant curing agent for onecomponent epoxy resin system with excellent comprehensive performance. Composites Part B-Engineering. 2019;**177**:107440

[50] Wu Q, Lv J, Qu B. Preparation and characterization of microcapsulated red phosphorus and its flameretardant mechanism in halogen-free flame retardant polyolefins. Polymer International. 2003;**52**(8):1326-1331

[51] Horold S. Phosphorus flame retardants in thermoset resins.

Polymer Degradation and Stability. 1999;**64**(3):427-431

[52] May CA. Epoxy Resins: Chemistry and Technology. New York: Taylor & Francis Group; 1988

[53] Troitzsch J. Flame retardant polymers current status and future trends. Makromolekulare Chemie. Macromolecular Symposia. 1993;**74**(1):125-135

[54] Rad ER, Vahabi H, Anda A, et al. Bio-epoxy resins with inherent flame retardancy. Progress in Organic Coatings. 2019;**135**:608-612

[55] Velencoso MM, Battig A, Markwart JC, et al. Molecular firefighting-how modern phosphorus chemistry can help solve the challenge of flame retardancy. Angewandte Chemie. 2018;**57**(33):10450-10467

[56] Veen I, Boer JD. Phosphorus flame retardants: Properties, production, environmental occurrence, toxicity and analysis. Chemosphere. 2012;**88**(10):1119-1153

[57] Lligadas G, Ronda JC, Galla M, et al. Synthesis and properties of thermosetting polymers from a phosphorous-containing fatty acid derivative. Journal of Polymer Science Part A: Polymer Chemistry. 2006;**44**(19):5630-5644

[58] Dong LP, Deng C, Li RM, et al. Poly (piperazinyl phosphamide): A novel highly-efficient charring agent for an EVA/APP intumescent flame retardant system. RSC Advances. 2016;**6**(36):30436-30444

[59] Yang AH, Deng C, Chen H, et al. A novel Schiff-base polyphosphate ester: Highly-efficient flame retardant for polyurethane elastomer. Polymer Degradation and Stability. 2017;**144**:70-82

[60] Yang J, Zhao Y, Li M, et al. A review of a class of emerging contaminants: The classification, distribution, intensity of consumption, synthesis routes, environmental effects and expectation of pollution abatement to organophosphate flame retardants (OPFRs). International Journal of Molecular Sciences. 2019;**20**(12):2874

[61] Wazarkar K, Sabnis A. Synergistic effect of P S and crosslink density on performance properties of epoxy coatings cured with cardanol based multifunctional carboxyl curing agents. Reactive & Functional Polymers. 2018;**128**:74-83

[62] Ma S, Liu X, Jiang Y, et al. Synthesis and properties of phosphorus-containing bio-based epoxy resin from itaconic acid. Science China Chemistry. 2014;**57**(3):379-388

[63] Lee MC, Ho TH, Wang CS. Synthesis of tetrafunctional epoxy resins and their modification with polydimethylsiloxane for electronic application. Journal of Applied Polymer Science. 1996; **62**(1):217-225

[64] Lin ST, Huang SK. Thermal degradation study of siloxane-DGEBA epoxy copolymers. European Polymer Journal. 1997;**33**(3):365-373

[65] Uehara F, Matsumoto A. Metal-resin bonding mediated by epoxy monolith layer. Applied Adhesion Science. 2016;**4**(1):18

[66] Zhao C, Zhang G, Zhao L. Effect of curing agent and temperature on the rheological behavior of epoxy resin systems. Molecules. 2012;**17**(7):8587-8594

[67] Jiao C, Dong J, Zhang C, et al. Synthesis and properties of a phosphate *Epoxy Resin Adhesives: Modification and Applications DOI: http://dx.doi.org/10.5772/intechopen.101971*

ester as curing agent in an epoxy resin system. Iranian Polymer Journal. 2014;**23**(8):591-598

[68] Kim HS, Park KY, Dai GL. A study on the epoxy resin concrete for the ultra-precision machine tool bed. Journal of Materials Processing Technology. 1995;**48**(1-4):649-655

[69] Beutel A. Optimal Mix Design for Epoxy Resin Polymer Concrete. Australia: University of Southern Queensland; 2015

[70] Lei W, Yang T, Wang KJ, et al. Preparation and characterization of epoxy resin modified soybean-based wood adhesive. Soybean Science. 2010;**29**(1):118-120

[71] Zhang J, Liu BW, Zhou YX. Preparation of a starch-based adhesive cross-linked with furfural, furfuryl alcohol and epoxy resin. International Journal of Adhesion and Adhesives. 2021;**110**:102958

[72] Zhang J, Xi XD, Liang JK. Tanninbased adhesive cross-linked by furfuryl alcohol-glyoxal and epoxy resins. International Journal of Adhesion and Adhesives. 2019;**94**:47-52

[73] Zhang J, Wang WL, Zhou XJ. Ligninbased adhesive crosslinked by furfuryl alcohol–glyoxal and epoxy resins. Nordic Pulp & Paper Research Journal. 2019;**34**(2):228-238

#### **Chapter 6**

## Epoxy Composites for Radiation Shielding

*Hayriye Hale Aygün*

#### **Abstract**

Due to the increase in use of radiation energy in many industrial applications, radiation shielding has become a crucial topic in order to diminish its hazardous effects. Radiation shields can be of various weights depending on the materials from which they are produced and the area in which they are used. In this sense, polymer composites have taken attention by researchers because it is aimed to obtain shields with good processability, sufficient flexibility, low weight, and subsequent performance properties. Epoxy resin is one of the mostly used synthetic polymers as a matrix element in composite material production due to its improving characteristics by means of electrical insulation, chemical resistance, service life, bonding characteristic, and mechanical properties. Besides, epoxies have intermediate radiation shielding characteristics as well. By loading epoxy matrix with fibers and/or fillers having different radiation absorption rates or mechanical resistance properties, multifunctional shields can be produced to serve in numerous applications. This chapter focuses on radiation shielding efficiency of fiber-reinforced epoxy composites and the role of fillers and fiber-based materials on manufacturing of functional radiation shields.

**Keywords:** composite, epoxy resin, fiber, filler, radiation, shielding

#### **1. Introduction**

For protecting humans and the environment from the hazardous effects of radiation, various forms of shields have been used in different fields in which radiation has been utilized. Shielding materials manufactured from lead, stainless steel, and concrete are heavy and rigid structures and also not resistant to corrosion. These structures have been generally used as blocks for shielding against radiation and have not sufficient flexibility and comfort properties in order to be used in shielding garments (**Figure 1**). Therefore, researchers have been focused on the manufacturing of advanced materials with good shielding capability, lightweight, high modulus, and mechanical properties. At this point, composite materials have taken the attention of researchers due to manufacturing of a unique material from different materials with dissimilar characteristics.

Epoxy polymer is one of the most important thermoset polymers used in composite manufacturing due to its good wetting ability, low cure shrinkage, excellent chemical corrosion resistance, good dimensional stability, high tensile, fatigue, and compression strength. By courtesy of its use in composites, it contributes to

#### **Figure 1.** *Types and penetration depths of radiation.*

the properties of the whole composite by means of good mechanical strength, high stiffness, excellent chemical resistance, flame retardancy, and high electrical strength [1, 2]. Solid epoxy polymer is the output material obtained by reaction of curing agent and liquid resin. There are various types of epoxy-based liquid resin because the numbers of epoxide groups on its starting material can be variable. Diglycidyl ether of bisphenol A (DGEBA) has two epoxide groups in its structure, which is the most common starting material used in the manufacturing of epoxybased liquid resin. For solidification, liquid resin is treated with small amounts of reactive curing agent and then a tridimensional network occurs as a result of crosslinking. The use of different types of starting materials and reactive curing agents results in various types of solid epoxy polymers with different characteristics. Thereby, the properties of epoxy resin are given a range of values as it is seen in **Table 1**. In case of being cured, the system exhibits brittle characteristic due to crosslinking mentioned above, and this case causes incomparable decreases in their relevant mechanical properties, especially in impact strength [3]. In addition to crosslinking occurred by reaction between the curing agent and epoxy resin, some additional structural changes are observed in case of irradiation of epoxybased system. The color of epoxy resin alters from transparent to yellow and resin can be even degraded depending on exposing dose and starting material used for manufacturing epoxy resin. Even so, epoxies are addressed as assuring matrix elements with high radiation stability for composite manufacturing [4]. In order to

*Epoxy Composites for Radiation Shielding DOI: http://dx.doi.org/10.5772/intechopen.104117*


#### **Table 1.**

*Typical properties of epoxy resin at room temperature.*

limit brittle characteristics and develop mechanical performance of epoxy-based systems, epoxy resins should be reinforced with flexible materials.

High strength-to-weight ratio is one of the advantages of fiber-reinforced composites. By increasing interactions between fiber and epoxy matrix, the resistance of the whole composite to many destructive forces is improved. The improvement can be successfully achieved by the incorporation of elastomeric/thermoplastic phases or by adding organic/inorganic particles into epoxy resins [5–7]. Gojny et al. dispersed carbon-based nanoparticles in epoxy resins and reported that fracture toughness of produced composites effectively improved at low nanoparticle concentration as well as stiffness [8]. In another study, carbon-based nanoparticles lead to the development of flexural strength and modulus of the epoxy composite [9]. There are numerous researches about the use of inorganic and organic particles in reinforcement of epoxies and improvement of composite properties in different ways [10–20].

#### **2. Polymers and fillers used for radiation shielding**

Polymers have been intensively used for fabricating radiation shields due to their lightness, low cost, and elasticity [21, 22]. However, polymers behave differently when they are irradiated. Under different radiation sources with variable frequency rays, crosslinking, chain scission or degradation may be observed in a polymer chain [23]. The behaviors of some irradiated polymers and classifications according to their radiation resistance are given in **Table 2**.

In order to delay the degradation of polymeric structures and diminish the hazardous effect of radiation on polymers, fillers are added to the structure during manufacturing process. The radiation shielding efficiency of a filler depends on its atomic number and the atomic structure of filler is a crucial factor in order to fabricate functional structures from polymer and filler for intended end-use. Fillers with high atomic number are generally used for gamma radiation shielding (**Table 3**). However, the use of fillers with low atomic number is preferable for neutron radiation shielding. When considering that there are generally low atomic number elements in a polymer chain, compatibility of polymer/filler combination has a significant effect on radiation shielding efficiency of fabricated composite material [25]. Many researches have been performed on the use of polymers [26–30] and fillers [31–33] for the manufacturing of radiation shields.


#### **Table 2.**

*Reactions and resistance of polymers against radiation [23, 24].*


**Table 3.**

*Comparison of some elements used in radiation shielding [34].*

#### **3. Filler loaded epoxy composites for radiation shielding**

There are many attempts for improving the shielding characteristics of epoxy or other polymer matrix with elemental particles. The researchers observed effects of particle loading via numerous trials by means of minimizing particle size, doping matrix with different particle concentration in weight, exposing composite specimens

#### *Epoxy Composites for Radiation Shielding DOI: http://dx.doi.org/10.5772/intechopen.104117*

to different radiation sources, analyzing microstructural changes under different radiation doses, and testing failure mechanism of composites. Radiation shielding properties of epoxy-based composite panels were tested by Al-Sarray et al. Epoxy resin was loaded with different barite concentrations of 0−50 wt %, and the linear attenuation coefficient of fabricated composites was tested by Co60 and Cs137 radioactive sources. Radiation shielding efficiency improved with the increase in barite concentration [35]. Ergin et al. compared the effects of lead oxide and barium oxide on the radiation shielding performance of epoxy composites. They reported that gamma radiation performance handled by lead oxide/epoxy composites could be obtained by the addition of barium oxide but the weight percentage of barium oxide must be two times more than lead oxide addition. Besides, addition of barium oxide at 40 wt % in epoxy resin exhibited better radiation shielding performance than gadolinium oxide/ epoxy composites, concrete, and steel [36]. Li et al. dispersed micro- and nano-gadolinium oxide particles into epoxy matrix and evaluated both mechanical properties and radiation shielding characteristics of fabricated composites. Gadolinium oxide addition enhanced shielding characteristics due to dominating photoelectric effect of the gadolinium element. Nano-gadolinium oxide/epoxy composites exhibited better X-ray and gamma shielding characteristics and had similar flexural strength but higher flexural modulus with those of micro-gadolinium oxide/epoxy composites [37]. More et al. determined radiation shielding parameters of metal chloride/epoxy composites. Doping epoxy resin with higher weight ratios of metal chloride lead to increase in attenuation parameters of epoxy composite and test results were comparable with those of pure lead metal [38].

Mechanical properties, structural characteristics, and gamma shielding efficiency of epoxy composites were reported by Alavian et al. Epoxy resin was doped with inorganic nanoparticles such as lead, zinc, zinc oxide, titanium, and titanium oxide. Increasing nanoparticle loading enhanced shielding efficiency. 25 wt% Pb/epoxy nanocomposites showed better shielding properties but low mechanical strength than those of their counterparts [39]. Degradation of epoxy composite by high-frequency rays was investigated by Saiyad et al. They fabricated three different composite materials by loading graphite, boron nitride, and lead into epoxy resin. They irradiated epoxy composites by Am-Be neutron sources. They reported that linear absorption coefficients of composites were strictly dependent on the dispersion of filler and the highest shielding performance was observed in graphite/epoxy composites [40]. Aldhuhaibat et al. examined gamma radiation shielding performance of pure epoxy resin and epoxybased nanocomposites with aluminum oxide or ferrium oxide nanopowder at different concentrations of 10−15 wt %. Specimens were irradiated by Cs137(1.05 kBq with single gamma-ray emission energy of 0.662 MeV) and Co60 (74 kBq with two gamma-ray emission energies of 1.173 and 1.333 MeV) radioactive sources and linear attenuation coefficients of specimens were measured by NaI detector. They claimed that epoxy nanocomposites were potential gamma radiation shields with improved characteristics [41]. Another study was performed by Azman et al. Nano-sized tungsten oxide/epoxy composites had higher attenuation properties at 22–35 kV X-ray tube voltages used in mammography and radiography units. The particle size of tungsten was found as negligible by means of transmitted beam intensity at 40−120 kV tube voltages [42]. Cheng et al. studied the radiation degradation mechanism of tungsten/epoxy composites. They tested composites specimens under different activities of Co60 sources. Loading with tungsten improved shielding characteristic of composites. However, measurements showed that an increase in radiation dose caused a decrease, then a slight increase and a sharp decrease in thermal and mechanical properties of composites [43].

#### **4. Modification of epoxy resin with fibers/fillers for radiation shielding**

In recent years, studies performed on radiation shields have focused on developing failure mechanisms and decreasing the weight of shield for supplying personal comfort. For this goal, researchers offered dissimilar alternative shields by studying various polymeric matrices with different fillers. Kim observed the effect of particle size and dispersibility of tungsten particles on radiation shielding performance of samples. For this aim, three types of tungsten-loaded HDPE shields were manufactured with identical thickness and sizes by doping nanoparticles, microparticles, and their mixture. It was claimed that sufficient protection was handled against low dose exposure notwithstanding particle size. But nanoparticle loaded HDPE sheets were more resistant to high-energy radiation. The shielding sheets produced with a mixture of different particle sizes of tungsten showed similar shielding performance as microparticle tungsten-loaded sheets [44]. Manufacturing and radiation shielding properties of nano gadolinium oxide/PMMA composites were searched by Shreef and Abdulzahara. Composite shields were fabricated with varying concentrations of gadolinium oxide (10−40 wt %). The thickness of composites was measured and shielding performance was tested with Co60 and Cs137 radiation sources. Test results showed that increasing nanoparticle concentration lead to an increase in thickness and improvement in attenuation coefficient but a decrease in half-value layer values of epoxy composites [45]. Zheng et al. fabricated S-glass fabric/epoxy composites with a ratio of 1:1. Composites were irradiated by Co60 source and the effects of irradiation on properties of composites were compared. They claimed that gammaray irradiation caused negligible damage on S-glass fiber and possible degradation on epoxy resin. By increasing the exposing dose of gamma radiation, the color of composite altered from yellow to brown and tensile strength of composite reduced gradually. However, composite preserved its thermal and dimensional stability and exhibited excellent thermal conductivity after irradiation [46]. Li et al. tried to produce a novel radiation shielding composite with high mechanical strength. For this aim, they fabricated erbium oxide-loaded basalt fiber/epoxy composites by prepreg autoclave process and test shielding performance of composites by exposing them to X and gamma rays. They claimed that basalt fiber/erbium oxide/epoxy composites had high mass attenuation coefficient than aluminum at low photon energies ranging from 31 keV to 80 keV [47]. The fracture toughness of carbon fabric/ epoxy composites was investigated by Phong et al. They produced micro/nano-sized bamboo fibrils and dispersed these fibrils into epoxy matrix. They reported that matrix cracking was delayed, crack growing was reduced, and fracture toughness of composites was improved [48]. Haque et al. reinforced epoxy matrix with layered nano silicate particles at very low concentrations (1 wt %) and claimed that flexural strength, toughness, decomposition temperature, and interlaminar shear strength of S2-glass/epoxy composites were improved due to enhanced fiber/matrix adhesion and reduced residual stresses [49]. Recycled PET fibers were used to mimic marble material. Nguyen et al. modified calcium carbonate particles with stearic acid in order to enhance compatibility between epoxy resin and calcium carbonate. They fabricated composites by positioning a single layer of recycled PET fiber mat in the core and by coating the front and backside of PET mat with epoxy/calcium carbonate mixture. They concluded that flexural properties, impact resistance, and thermal stability of epoxy composites were improved [50]. Saleem et al. presented an empirical approach and compared radiation shielding of lead nanoparticles loaded epoxy composites with glass or carbon fiber. The results showed that lead nanoparticles improved shielding

#### *Epoxy Composites for Radiation Shielding DOI: http://dx.doi.org/10.5772/intechopen.104117*

characteristics and lead to an increase in mass attenuation coefficients of composites. Mass attenuation coefficients were 0.2145 cm2 /g and 0.2152 cm2 /g for carbon and glass fiber reinforced epoxy composites at lead nanoparticle concentration of 50 wt%. But half-value length of epoxy composite with glass fiber was reported as 1.431 cm, which was lower than epoxy composites with carbon fiber (1.756 cm) [51].

Effects of radiation on neat epoxy resin and carbon fiber/epoxy composites were studied by Hoffman and Skidmore (2009). Front and back surfaces of plain woven carbon fabrics were treated with epoxy/hardener mixture (2:1). Prepared composites were exposed to mechanical and thermal testing and also analyzed by means of microstructural properties and radiation characteristics. After being irradiated, there were no remarkable changes in mechanical resistance of composites but significant variations were observed in thermal properties, spectroscopic analysis, and hardness value of neat epoxy samples as a result of gamma radiation [52]. Zhong et al. examined the cosmic radiation shielding properties of hot-pressed UHMWPE/nano-epoxy composites and they concluded that epoxy composites with the combination of continuous fibers such as UHMWPE and/or graphite nanofibers found as multifunctional hybrid systems by means of good structural properties, cost-effectiveness, and high radiation shielding performance [53]. In another study, UHMWPE/epoxy composites were fabricated by Mani et al. and test results showed that composites containing gadolinium and boron nanoparticles had good neutron shielding performance [54]. Condruz et al. suggested coating carbon/epoxy composites with different types of functional materials such as tantalum foil, babbitt and Monel for protecting hazardous effects of proton radiation. They impregnated 2 × 2 twill woven carbon fabric into epoxy resin and then coated polymeric substrates with zinc, Babbitt or zinc/ Monel particles by thermal spray technique. They concluded that the coating process reduced penetration depth of ion beam and produced composites were lightweight shields for proton radiation [55]. The effects of hybridization on mechanical, thermal and radiation shielding efficiency of composites were also examined and reported by Zagaoui et al. They blended epoxy resin (90 wt%) with benzoxazine resin (10 wt%) and reinforced bicomponent matrix with silane-treated glass and basalt fibers. Hybridization of different types of resins developed mechanical and thermal properties and excellent shielding characteristic was gained by integrating hybrid fibers into bicomponent matrix system [56].

#### **5. Conclusion**

Heavy concretes, lead plates, and stainless steel blocks or plaques are known as conventional radiation shielding materials but they are heavy and not suitable for individual protective equipment. Polymers are functional lightweight materials but do not supply adequate protection on their own. Thereby advanced radiation shields should be fabricated by the composition of polymer-based materials and substances with high radiation shielding activity. At this point, material selection has crucial importance on the efficiency of protection by which radiation source it is irradiated.

The destructive effect of radiation on the material is related to the type of radiation source, exposing dose rate, exposure period, radiation absorption rate of material and strength of interbonding forces between components if a composite material is used. By taking into consideration the advantages introduced with composite manufacturing, the destructive effect of radiation can be limited by a combination of materials with high attenuation rates. At this point, the shielding efficiency of

composite material depends on how components in a composite behave in case of irradiation. Epoxies, the mostly used matrix elements in composite manufacturing, exhibit physical changes such as color transition and low shrinkage percentage and mechanical changes such as a decrease in flexural and impact strength due to crosslinking when they are irradiated. Despite these changes, they are known as reliable materials for being used as matrix elements in radiation shielding. Undesired physical and mechanical changes observed in irradiated epoxies tried to be eliminated by fiber and/or filler loading for handling effective radiation shields with long life. Fiber addition into epoxy matrix causes an increase in hardness, fracture toughness, impact resistance, flexural strength and modulus, and also consistency in dimensional stability and thermal properties with respect to neat epoxy. However, loading fillers into epoxy matrix outputs composite material with inconsistent mechanical and thermal properties, especially in heterogenic filler dispersion and inappropriate particle size. Modification of epoxy with fillers having high radiation absorption rate develops radiation shielding efficiency of the whole composite but not mechanical or thermal characteristics for long-term use of composite. Thereby epoxy-based composites, which are designed to be used for radiation shields, should contain both fillers and fibers. Epoxy-based radiation shields serve as effective protectors in the case of reinforcing with fiber-based structures and fillers having high radiation absorption rate and photoelectric properties.

Fiber and filler reinforced epoxy composites are functional engineering materials and compete with some conventional radiation shields by means of strength and modulus properties per unit weight. The functionality is improved by proper fiber/ matrix combination, high interfacial bonding between these constituents, functional additive/filler loading, modification of fiber surface with an appropriate sizing agent, well-dispersed nano-sized filler addition, and suitable manufacturing technique. In this way, epoxy composite serves as a unique shielding material for which goal it is fabricated and in which field it is intended to be used. Moreover, there is a need to figure out the best alternative to be used in medical diagnostics and nuclear industry.

### **Author details**

Hayriye Hale Aygün University of Kahramanmaras Sutcu Imam, K. Maras, Turkey

\*Address all correspondence to: hhalesolak@hotmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **References**

[1] Ratna D. Handbook of Thermoset Resins. UK: iSmithers; 2009. p. 410

[2] Aygün B. Epoxy based metal and metal oxide doped new composite neutron and gamma radiation moderator material. Erzincan University Journal of Science and Technology. 2019;**12**(3):1442-1453

[3] Mallick PK. Fiber Reinforced Composites: Materials, Manufacturing and Design. New York: CRC Press; 2007. p. 638

[4] Akbari R, Beheshty MH, Shervin M. Toughening of dicyandiamide-cured DGEBA based epoxy resins by CTBN liquid rubber. Iranian Polymer Journal. 2013;**22**:313-324

[5] Özdemir T, Usanmaz A. Degradation of poly(bisphenol- a-epichlorohydrin) by gamma irradiation. Radiation Physics and Chemistry. 2008;**77**:799-805

[6] Njuguna J, Pielichowski K, Alcock JR. Epoxy-based fibre reinforced nanocomposites. Advanced Engineering Materials. 2007;**9**(10):835-847

[7] Dodiuk H, Kenig S, Blinsky I, Dotan A, Buchman A. Nanotailoring of epoxy adhesives by polyhedraloligomeric-sil-sesquioxanes (POOS). International Journal of Adhesion and Adhesives. 2005;**25**:211-218

[8] Gojny FH, Wichmann MHG, Köpke U, Fiedler B, Schulte K. Carbon nanotube reinforced epoxy composites: Enhanced stiffness and fracture toughness at low nanotube content. Composites Science and Technology. 2004;**64**:2363-2371

[9] Kong J, Ning R, Tang Y. Study on modification of epoxy resins with

acrylate liquid rubber containing pendant epoxy groups. Journal of Materials Science. 2006;**41**:1639-1641

[10] Zhao DL, Qiao RH, Wang CZ, Shen ZM. Microstructure and mechanical property of carbon nanotube and continuous carbon fiber reinforced epoxy resin matrix composites. Advanced Materials Research. 2006;**11**:517-520

[11] Chisholm N, Mahfuz H, Rangari VK, Ashfaq A, Jeelani S. Fabrication and mechanical characterization of carbon/ SiC-epoxy nanocomposites. Composite Structures. 2005;**67**(1):115-124

[12] Schmidt H. New type of noncrystalline solids between inorganic and organic materials. Journal of Non-Crystalline Solids. 1985;**73**:681-691

[13] Wang K, Chen L, Wu J, Toh ML, He C, Yee AF. Epoxy nanocomposites with highly exfoliated clay: Mechanical properties and fracture mechanisms. Macromolecules. 2005;**38**(3):788-800

[14] Mark JE. Ceramic reinforced polymers and polymer modified ceramics. Polymer Engineering and Science. 1996;**36**(24):2905-2920

[15] Gilbert EN, Hayes BS, Seferis JC. Variable density composite systems constructed by metal particle modified prepregs. Journal of Composite Materials. 2002;**36**(17):2045-2060

[16] Timmerman JF, Hayes BS, Seferis JC. Nanoclay reinforced effects on the cryogenic microcracking of carbon fiber/ epoxy composites. Composites Science and Technology. 2002;**62**(9):1249-1258

[17] Brunner AJ, Necola A, Rees M, Gasser P, Kornmann X, Thomann R, et al. The influence of silicate-based nanofiller on the fracture toughness of epoxy resin. Engineering Fracture Mechanics. 2006;**73**(16):2336-2345

[18] Mohan RV, Kelkar AD, Akinyede O. Vartm processing and characterization of composite laminates from epoxy resins dispersed with alumina particles. In: Proceedings of 50th International SAMPE Symposium and Exhibition; 1-5 May 2005; USA. Long Beach: SAMPE; 2005. pp. 2425-2431

[19] Kornmann X, Rees M, Thomann Y, Necola A, Barbezat M, Thomann R. Epoxy-layered silicate nanocomposites as matrix in glass fibre-reinforced composites. Composites Science and Technology. 2005;**65**(14):2259-2268

[20] Ragosta G, Abbate M, Musto P, Scarinzi G, Mascia L. Epoxysilica particulate nanocomposites: Chamical interactions, reinforcement and fracture toughness. Polymer. 2005;**46**(23):10506-10516

[21] Kaphle A, Umapathi NPNA, Daima H. Nanomaterials for agriculture, food and environment: Applications, toxicitysss and regulation. Environmental Chemistry Letters. 2018;**16**:43-58

[22] More CV, Alsayed Z, Badaw MS, Thabet AA, Pawar PP. Polymeric composite materials for radiation shielding: A review. Environmental Chemistry Letters. 2021;**19**:2057-2090

[23] Ivanov VS. Radiation Chemistry of Polymers. Wakefield: VSP Publishing; 1992. p. 321

[24] Aygün HH. Production and Characterization of Shielding Surfaces with Bismuth Containing Polymeric Materials against X-Ray Radiation [PhD Thesis]. Kahramanmaras: University of Kahramanmaras Sutcu Imam; 2018

[25] Kaçal M, Akman F, Sayyed M. Evaluation of gamma-ray and neutron attenuation properties of some polymers. Nuclear Engineering and Technology. 2019;**51**(3):818-824

[26] Kilicoglu O, Kara U, Inanc I. The impact of polymer additive for N95 masks on gamma-ray attenuation properties. Materials Chemistry and Physics. 2021;**260**(16):124093

[27] Mirji R, Lobo B. Computation of the mass attenuation coeffcient of polymeric materials at specifc gamma photon energies. Radiation Physics and Chemistry. 2017;**135**:32-44

[28] Sayyed MI. Investigation of shielding parameters for smart polymers. Chinese Journal of Physics. 2016;**54**(3):408-415

[29] Bhosale RR, More CV, Gaikwad DK, Pawar PP, Rode MN. Radiation shielding and gamma ray attenuation properties of some polymers. Nuclear Technology and Radiation Protection. 2017;**32**(3):288-293

[30] Mann K, Rani A, Heer M. Shielding behaviors of some polymer and plastic materials for gamma-rays. Radiation Physics and Chemistry. 2015;**106**:247-254

[31] Kaçal MR, Dilsiz K, Akman F, Polat H. Analysis of radiation attenuation properties for polyester/Li2WO4 composites. Radiation Physics and Chemistry. 2021;**179**:109257

[32] Lanina S, Kaminskaya N, Benyaev N, Suslova V, Grigorevskaya M. On possible use of inorganic fillers and matrix polymers in radiation shielding materials. Biomedical Engineering. 2013;**46**(6):228-231

[33] Engelmann HJ. Material for shielding from radiation; 2009. WO2009097833A1

*Epoxy Composites for Radiation Shielding DOI: http://dx.doi.org/10.5772/intechopen.104117*

[34] McCaffrey JP, Shen H, Downtown B, Mainegra-Hing E. Radiation attenuation by lead and nonlead materials used in radiation shielding garments. Medical Physics. 2007;**34**(2):530-537

[35] Al-Sarray E, Günoğlu K, Evcin A, Bezir NÇ. Radiation shielding properties of some composite panel. Acta Physica Polonica A. 2017;**132**:490-492

[36] Ergin Y, Karabul Y, Guven Ozdemir Z, Kılıç M. Experimental comparison of PbO and BaO addition effect on gamma ray shielding performance of epoxy polymer. European Journal of Science and Technology. 2019;**16**:256-266

[37] Li R, Gu Y, Wang Y, Yang Z, Li M, Zhang Z. Effect of particle size on gamma radiation shielding property of gadolinium oxide dispersed epoxy resin matrix composite. Materials Research Express. 2017;**4**(3):035035

[38] More CV, Pawar PP, Badawi MS, Thabet AA. Extensive theoretical study of gamma-ray shielding parameters using epoxy resin-metal chloride mixtures. Nuclear Technology and Radiation Protection. 2020;**35**(2):138-149

[39] Alavian H, Samie A, Tavakoli-Anbaran H. Experimental and Monte Carlo investigations of gamma ray transmission and buildup factors for inorganic nanoparticle/epoxy composites. Radiation Physics and Chemistry. 2020;**174**:108960

[40] Saiyad DM, Devashrayee N, Mewada R. Study the efect of dispersion of filler in polymer composite for radiation shielding. Polymer Composites. 2014;**35**(7):1263-1266

[41] Al-Dhuhaibat M, Salman M, Jubier N, Salim A. Improved gamma radiation shielding traits of epoxy

composites: Evaluation of mass attenuation coefficient, effective atomic and electron number. Radiation Physics and Chemistry. 2020;**179**:109183

[42] Azman NN, Siddiqui S, Hart R, Low IM. Effect of particle size, filler loadings and X-ray tube voltage on the transmitted X-ray transmission in tungsten oxide-epoxy composites. Applied Radiation and Isotopes. 2013;**71**(1):62-67

[43] Chang L, Zhang Y, Liu Y, Fang J, Luan W, Yang X, et al. Preparation and characterization of tungsten/epoxy composites for γ-rays radiation shielding. Nuclear Instruments and Methods Physics Research Section B: Beam Interactions with Materials and Atoms. 2015;**356-357**:88-93

[44] Kim SC. Analysis of shielding performance of radiation shielding materials according to particle size and clustering effects. Applied Sciences. 2021;**11**:4010

[45] Sheerif AM, Abdulzahara NA. Manufacture of shielding for attenuation ionization ray by preparation of nano-gadolinium oxide with PMMA. Neuroquantology: An Interdiciplineray Journal of Neuroscience and Quantum Physics. 2021;**19**(8):66-78

[46] Zheng LF, Wang LN, Wang ZZ, Wang L. Effects of γ-ray irradiation on the fatigue strength, thermal conductivities and thermal stabilities of the glass fibres/epoxy resins composites. Acta Metallurgica Sinica. 2018;**1**(31):105-112

[47] Li R, Gu Y, Zhang G, Yang Z, Li M, Zang Z. Radiation shielding property of structural polymer composite: Continuous basalt fiber reinforced epoxy matrix composite containing

erbium oxide. Composites Science and Technology. 2017;**143**:67-74

[48] Phong NT, Gabr MH, Okubo K, Chuong B, Fujii T. Enhancement of mechanical properties of carbon fabric/ epoxy composites using micro/nanosized bamboo fibrils. Materials and Design. 2013;**47**:624-632

[49] Haque A, Shamsuzzoha M, Hussain F, Dean D. S2-glass/ epoxy polymer nanocomposites: Manufacturing, structures, thermal and mechanical properties. Journal of Composite Materials. 2003;**37**(20):1821-1837

[50] Nguyen M, Vu T, Nguyen T, Nguyen T, Ha Thuc N, Bui QB, et al. Synergistic infuences of stearic acid coating and recycled PET microfbers on the enhanced properties of composite materials. Materials. 2020;**13**(6):1461

[51] Saleem RAA, Abdelal N, Alsabbagh A, Al-Jarrah M, Al-Jawarneh F. Radiation shielding of fiber reinforced polymer composites incorporating lead nanoparticles: An empirical approach. Polymers. 2021;**13**(21):3699

[52] Hoffman AN, Skidmore TE. Radiation effects on epoxy carbon fiber composite. Journal of Nuclear Materials. 2009;**392**(2):371-378

[53] Zhong W, Sui G, Jana S, Miller J. Cosmic radiation shielding tests for UHMWPE fiber/nano-epoxy composites. Composites Science and Technology. 2009;**69**(13):2093-2097

[54] Mani V, Prasad N, Kelkar A. Ultra high molecular weight polyethylene (UHMWPE) fiber epoxy composite hybridized with nanoparticles of gadolinium and boron for radiation shielding. In: Proceedings of SPIE Planetary Defense and Space

Environment Applications; 22 September 2016; USA. California: SPIE; 2016. pp. 1-10

[55] Condruz MR, Puscasu C, Voicu LR, Vintila IS, Paraschiv A, Mirea DA. Fiber reinforced composite materials for proton radiation shielding. Materiale Plastic. 2018;**55**(1):5-8

[56] Zegaoui A, Derradji M, Medjahed A, Ghouti HA, Cai W, Liu WB, et al. Exploring the hybrid effects of short glass/basalt fibers on the mechanical, thermal and gamma radiation shielding properties of DCBA/BA-a resin composites. Polymer Plastics Technology and Materials. 2020;**59**(3):311-322

*Edited by Samson Jerold Samuel Chelladurai, Ramesh Arthanari and M.R.Meera*

Epoxy-based composites are used in automotive and aerospace applications because of their high strength-to-weight ratio, high stiffness-to-weight ratio, and good resistance to wear and corrosion. This book presents research on epoxy-based composites and their applications. It explains methods of preparing and testing these composites, including the hand lay-up technique, compression molding, and others. This book is useful for industrialists, undergraduate and postgraduate students, research scholars, and scientists.

Published in London, UK © 2022 IntechOpen © Jekaterina Voronina / iStock

Epoxy-Based Composites

Epoxy-Based Composites

*Edited by Samson Jerold Samuel Chelladurai,* 

*Ramesh Arthanari and M.R.Meera*