Epoxy resins thermosetting for mechanical engineering

1 Introduction

Epoxy resins are classified as thermoset polymers that have unique characteristics when manufacturing, such as low pressure required to make products, very small cure shrinkage, and low residual stresses. They can be used in a wide temperature range with the selection of the right curing agent to adjust the level of cross-links. In the marketplace, epoxy resins are available in the liquid form with low viscosity and powders (solids) [1]. Generally, concerning both the manufacturing characteristics and the performance of products, epoxy resins are widely applied as structural adhesives, surface coatings, engineering composites, and electrical insulation. Substitution of tools made from metal, wood, and other materials with epoxy resin has been proven to increase efficiency and save production costs and accelerate processes in the industries. Epoxy is also widely used as a binder in paints to improve the resistance of painted materials from corrosion [2].

There are two epoxy resins families, namely, (1) aromatic epoxy saturated ring and also called aliphatic and (2) nonaromatic saturated ring epoxy and also called cycloaliphatic. The presence of aromatic rings on aliphatic epoxy improves the resistance of the epoxy to ultraviolet rays and usually is used for outdoor applications. The aliphatic epoxy family comes from the monomer diglycidylether of bisphenol A (DGEBA), while the cycloaliphatic epoxy family (CAE) comes from the 3,4-epoxycyclohexyl-3,4-epoxycyclohexana carboxylate [3]. For applications as matrices of composite materials, conventional difunctional epoxy is used. However, some high-performance and critical defense applications require the use of epoxy with higher functionality, tri or tetrafunctional epoxy, which is called multifunctional epoxies [4].

For producing fiber-reinforced polymer composites, especially carbon fiber reinforcement (CFRP), only certain types of epoxy resins can be properly applied as matrices for carbon fiber (CF). The existence of a large number of epoxy resins and curing agents requires that a material engineer must be careful in choosing the type of epoxy for sizing, prepreging, and molding purposes. Table 1 presents the common types of epoxy resins and curing agents that are used for research and commercial.

This article elaborates fiew types of epoxy resins that are applied frequently in mechanical engineering for creating a fiber-reinforced polymer composite with thermoset matrix. The effects of curing agent on the properties of cured epoxy resin is a significant topic, and it will be described in this article as well. Furthermore, the most urgent problem of using epoxy resins in the form of composites is how to recycle them or recover the reinforcing fibers used. Technologically, it is discussed in this article in the section on decomposition and degradation of epoxy resins.

Mechanical engineers has to know the ins and outs of thermoset epoxy resin because of almost entirely of the fiber-reinforced composite materials for construction use epoxy resin as a matrix. Mechanical engineers must know in detail the properties of the epoxy resins with respect to their mixing with the curing agents in various ratios, the shear strength of their interface with the fiber reinforcement, their behavior in environmental exposure and the strategies to decompose or recycle them. Regarding the preparation of epoxy resin raw material, the detailed mechanism of the stoichiometric reaction that occurs along with the accompanying energy is part of the object of discussion, which is the responsibility of the chemical engineer. Generally, both engineers will work together when trying to find a solution to the impact of the use epoxy resins on humans and environments. Currently, many methods and technologies for recycling fiber-reinforced epoxy resin composites are under development with a focus on recycling reinforcing fibers in new composite products.

2 Epoxy resin types and synthesizing/manufacturing

The most dominant epoxy resin commercially made by the synthesis reaction of a compound consists of at least two active hydrogen atoms and epichlorohydrin followed by a dehydrohalogenation process. The compounds in question can be derived from polyphenolic compounds, mono and diamines, amino phenols, heterocyclic imides and amides, aliphatic diols, and polyols and dimetric fatty acids. Epoxy resins derived from epichlorohydrine are termed glycidyl-based resins. Alternatively, epoxy resins derived from aliphatic epoxidized compounds or cycloaliphatic dienes are prepared by direct epoxidation of cycloolifin compounds by parasetic acids [17]. Referring to Table 1, the most common types of resin used in CF composite fabrication are briefly explained in the following description.

2.1 Epoxy resins of bisphenol A and bisphenol F

DGEBA epoxy resin thermoset is constructed through combining reaction between epichloro-hydrin and bisphenol A with involving standard catalyst such as NaOH, as illustrated in Figure 1. The epoxy has been estimated to cover 75% of the volume of industrial and home needs [17]. The properties of DGEBA epoxy resin depend strongly on the length of the polymer chains. Low-molecular-weight (MW) long polymer chains epoxy resins tend to be in the liquid state, and high MW epoxy resins may be in the form of jelly or solid. DGEBA oligomers usually contain a number of certain hydroxyl groups that play an important role as catalysts in the kinetics of the curing process. In addition, two oxirane (ethylene oxide – C₂H₄O) function groups enable to create epoxy with a three-dimensional structure. The oxirane is highly reactive to nucleophilic compounds such as amines; thus, the highest cross-linked level of DGEBA epoxy resin is achieved through the addition of aliphatic or aromatic diamines [18].

Figure 1

Formation of DGEBA from bisphenol A and epichlorohydrine [19].

DGEBF is manufactured similar to DGEBA, but the bisphenol F group is used to substitute the bisphenol A group. The bisphenol F compound is obtained by the reaction of phenol (C₆H₆O) in excessive amounts of formaldehyde (CH₂O), as shown in Figure 2 [20].

Figure 2

Formation reaction of bisphenol F from formaldehyde and phenol [20].

Bisphenol F has a lower viscosity and is slightly higher functional than bisphenol A. DGEBF does not adversely affect the mechanical properties of cured thermosets, has similar chemical reactivity as liquid bisphenol A epoxy resins (DGEBA), has a good established track record as a crystallization inhibitor for liquid DGEBA epoxy resins, and greatly reduces the viscosity of liquid DGEBA epoxy resins. Alternatives to DGEBF are continually being sought for use with bisphenol A-based epoxy resin composition to provide improved crystallization resistance while maintaining or improving other performance attributes at an affordable price [21]. The application of bisphenol F is often found in constructions that require high chemical resistance, such as in tank and pipe systems, floors, coatings, varnishes, and adhesives.

2.2 Cycloaliphatic epoxy resins

Cycloaliphatic epoxy resin (3′,4′-epoxycyclohexymethyl 3,4 epoxycyclohexane carboxilate [CAE]) is well known in the market by the name of GPE-221 and is obtained by reaction between 3′ cyclohexenylmethyl 3-cyclohexenecarboxylate (cyclo-olefin) and parasitic acid. Figure 3 shows the chemical structure of CAE. This epoxy resin is recognized by the characteristic of the absence of a saturated aromatic ring. CAE is generally in a liquid form with low viscosity, and thus, it is widely used for sizing or coating a fiber because of its good wetting ability even on oily surfaces [13]. CAE has an aliphatic “backbone,” and its molecular structure is completely saturated. Anhydrides that form with heating or UV light is usually used for the CAE curing process. The molecular structure of CAE shows high resistance to ultraviolet ray and excellent thermal stability, and hence, CAE is used to make components for outdoor and high-temperature exposure applications [22]. Another utilization of this epoxy resin is as an additive to improve the characteristic of the bisphenol A epoxy resin [23].

Figure 3

Formation reaction of GPE-221 from cycloolefin and peracetic acid [24].

2.3 TGDDM epoxy resins

TGDDM belongs to the multifunctional epoxy resin group that has a higher cross-linked density than bisphenol A epoxy and its thermal and chemical resistance properties are better. A tetrafunctional epoxy resin based on a reaction between 4,4-diaminodiphenyl methane (DDM) and epichlorohydrin has been widely applied in the fabrication of fiber-reinforced composites [25,26]. The reaction of these two compounds produced an epoxy of 4,4′-TGDDM, as shown in Figure 4. The synthesis of TGDDM resin requires approximately 6 h, of which 5 h is used for binding chloride via NaOH to produce chloride salt, which then becomes precipitated with the help of toluene. A multifunctional epoxy resin with a diaminodiphenyl base allows the methane group to be replaced by a group such as ether, ester, or sulfur [27].

Figure 4

Formation reaction of TGDDM from DDM and epichlorohydrine [26].

2.4 Novolac epoxy resins

Novolac resins are made by condensation of phenol and formaldehyde with an acid catalyst followed by the results of condensation with epichlorohydrin [28]. The increase in MW of novolac resins has an effect on an increase in resin functionality that can be achieved by changing the ratio of phenol and formaldehyde. Generally, the ratio ranges between 1.49 and 1.72. The reaction of novolac resin formation, as in Figure 5, must occur at 160°C for 2–4 h to ensure the formation of novolac resin and releasing water vapor as an excess of the product [29]. The epoxide group in novolac resins contributes to high cross-link densities, and so novolac resins have thermal and chemical properties that are ideal for both composite adhesives and matrices [30].

Figure 5

Synthesis reaction of novolac epoxy resins [28].

3 Curing agents for epoxy resins

Epoxy resin polymers form a solid, infusible and insoluble three-dimensional cross-linked network through a curing process. The curing process of epoxy resins requires additional substances called hardeners or curing agents to be able to create cross links. The curing agent affects the viscosity and reactivity of epoxy resins and determines the type of chemical bond and the level of cross links formed. In general, the epoxy crystal structure is affected by the curing process, and the parameter is classified as amorphous, not homogeneous, structure with high cross-link density [31]. Epoxy resins can be cured by amine, thiol, and alcohol compounds [32], and some of them are presented in Table 1. Based on the chemical composition, the epoxy curing agent is further divided into amines, anhydrides, alkalis, and catalysts [30]. The first two curing agents are widely used for constructing a composite system.

3.1 Amine curing agents

Amine compounds are the type of curing agent that is most widely used for the formation of epoxy resin thermoset. Amine compounds are classified into three categories based on their characteristic of nucleophilic reactivity, namely, aliphatic, cycloaliphatic, and aromatic. The density of the cured cross link network of epoxy resins can be carefully designed by selecting the type of epoxy monomer and amines hardener such that the stoichiometry equilibrium state is met. High functional epoxy resins with low MW will produce high cross-linked network after curing with amine compounds. Figure 6 shows the chemical structure of four curing agents of amine types.

Figure 6

Chemical structure of several curing agents of amines [32].

The number of hydrogen atoms in an amine molecule determines the functionality of amines. The primary amine group that has two hydrogens bound to nitrogen will react with two epoxy groups, while the secondary amine will just react with one epoxy group. The tertiary amine group that has no active hydrogen atom will not react with epoxy groups, but it will act as a catalyst that can speed up the curing process [33].

The advantage of aliphatic amine is that it can cure epoxy resins at room temperature, so it does not require additional energy. It is very beneficial for coating and adhesion work in complex structures. Other amine curing agents require heating in the curing process, which is sometimes difficult or impossible for certain structures and require additional energy. However, aliphatic amines require high temperature in the post-curing stage to get a perfect curing reaction [34]. The main limitation of epoxy resins with a curing agent using the aliphatic amine is that it cannot form a cross-link network system at glass temperature (T _g) above 120°C. For anticipating this weakness, an aromatic amine curing agent is used. Although aromatic amines require temperatures of 250–300°C during the curing process [35], the thermosetting epoxy resins are also able to withstand high temperatures and hence they are mostly used as a matrix system in structural composites.

3.2 Anhydride curing agents

An epoxy-anhydride thermoset system generally shows low viscosity and long pot life, low exothermic heat reaction, and very small shrinkage when cured at high temperatures. The curing process occurs slowly at 200°C and is usually catalyzed with a Lewis base or acid or tertiary amines or acids compounds. Catalyst concentration needs to be carefully calculated based on the type of anhydride curing agent for obtaining epoxy resin that is resistant at high temperatures. Practically, curing results of an epoxy-anhydride system can produce epoxy thermoset that exhibits excellent thermal, mechanical, and electrical properties by mixing 1 of part epoxy with 0.85 part of anhydride [36]. Commercially, there are several types of anhydride curing agents with varying chemical structures, as shown in Figure 7. All anhydrides are hygroscopic to moisture, so they need to be carefully coded to the environment before and during the curing process.

Figure 7

Chemical structure of anhydride epoxy resins [37].

4 Epoxy resin state before curing

Commercial polymers are not mostly complete as a pure single homogeneous material with a structure as stated on the name plate. In epoxy resins, the products sometimes consist of small amounts of isomers, oligomers, and other elements or compounds. Generally, epoxy resins are characterized by epoxy content, viscosity, color, density, hydrolysable chloride, and volatile elements. In addition, MW, MW distribution, oligomer composition, functional groups, and impurities of epoxy resins are calculated through measurements of gel permeation chromatography, high-performance liquid chromatography, and other analytical procedures such as nuclear magnetic resonance and infrared spectroscopy. The resin components that are in the form of α-glycol and chlorine have been known to affect the reactivity and formulation of the resin that depends on its interaction with the resin composition such as a basic catalyst (tertiary amine) and/or amine curing agent. Knowing the type and level of chlorine can be used as a guide in regulating formulations to obtain reactivity and ideal flow [24].

The epoxy content is the most common measurement analysis performed on epoxy resin. The epoxy content is expressed as an epoxide equivalent weight (EEW), namely, the number of grams of resin containing 1 g equivalent of the epoxy group. EEW is an initial requirement for creating thermoset epoxy to predict the number of stoichiometrically balanced cross links. The methods for measuring epoxy content usually involve reactions with halogenic acid to open the epoxy ring and produce halohydrin [38].

The concentration of secondary hydroxyl alcoholic group in epoxy resin is very important to be known for characterization of the thermoset epoxy structure. Hydroxyl groups can potentially become reactive to curing agents or hardeners, and thus, their concentration determines the epoxy/hardener stoichiometric equilibrium. This secondary alcohol compounds can also act as a catalyst for the reaction between the hardener and the epoxy group. Other hydroxyl groups are α-glycol, which is formed from the hydrolysis of the epoxy group and phenolic hydroxyl that is generated due to the imperfection reaction of phenol when epoxy resins are produced from bisphenol A with high concentrations. The α-glycol concentration can be determined by the periodic acid method or lithium aluminum hydride, which only reacts with active hydrogen atoms [39]. Because α-glycol usually appears in small amounts, it is necessary to be careful in measuring an accurate result. Phenolic hydroxyl is obtained frequently in low amounts and concentrations. This concentration can be determined by acetylation with acetic anhydrides in a solution of pyridinium chloride. The epoxy group will react with two acid groups, while the hydroxyl group will be converted to an ester [40].

Epoxy resins with high chloride content are able to have a lower thermal stability, especially when they are cured with a hardener of amine. Meanwhile, flame retardancy has commonly become a special requirement that thermoset epoxy resin must have, and hence, the epoxy resin manufacturer or designer must be able to formulate compositions that are satisfactory for thermal applications. In bisphenol A epoxy resins, the presence of chloride has a destructive effect on the electrical properties when they are applied as semi-conductor coating compounds [41]. The color and reactivity of the resin can also be bad due to the presence of chloride. Active chloride may block reactions of epoxy resins with low base catalysts (such as tertiary amines). When organic chloride bonds appear in epoxy resins, they reduce the functionality of the epoxy resin, and resin cross-link networks become weak.

5 Curing phenomenon of epoxy resins

As a polymer material, epoxy resins are described as a long chain of continuous carbon–carbon bonds, leaving two valence bonds that are important for binding hydrogen or other relatively small parts of hydrocarbons. Figure 8a–c show the linear polymer configuration scheme without cross links such as mostly found in thermoplastics structures. Other types of chains form networks as a result of chemical interactions between linear polymer chains or the buildup of monomer resin reactants that have a three-dimensional net configuration, such as the schemes shown in Figure 8d and e. The interaction process is called the cross-linked process. It is the main distinguishing element of thermoset resin materials with other types of polymer materials. The cross-linking process can take place with heat energy input, and some of them occur at room temperature (25°C) through the epoxy resin curing mechanism.

Figure 8

(a–c) Schemes of linear configuration polymers, (d) lightly cross-linked network polymer, and (e) highly cross-linked network polymer [42].

The epoxy curing process is an important factor influencing the quality and performance of the epoxy resin. The mechanism of the epoxy resin curing phase can be traced by following the time temperature transformation (TTT) diagram, as shown in Figure 9. In the TTT diagram, the time for gelation and vitrification is plotted as a function of isothermal curing temperature. Gelation and vitrification are two macroscopic phenomena that are encountered as a consequence of chemical reactions changing the state of the fluid to become solid in the thermoset process. At the molecular level, gelation is related to the beginning of molecular branching generation from the very high end of MW formation. The gelation process is accompanied by a drastic increase in viscosity and a decrease in the diffusional process of the condensed phase and the processability of the material. The molecular network structure in the gelation phase becomes an elastomer at a certain temperature if the inter-point segment of the network is flexible. If the segment does not move due to subsequent chemical reactions or due to cooling, the structure will turn gray into a glassy or vitrified condition. Thus, the vitrification process usually follows gelation as a consequence of the increasing MW and the subsequent cross-linked process, which causes a decrease in the degree of freedom of the structural tissue. Vitrification occurs during isothermal curing when the glass transition from the reactants reaches the curing temperature. The vitrification process is identified by slowing down chemical reactions [43].

Figure 9

TTT diagram of isothermal curing process for the thermoset system [44].

Based on the TTT diagram, the S-shaped vitrification curve and gelation curve divide the TTT diagram into four phases of the thermoset curing process, namely, liquid, gelled rubber, ungelled glass, and gelled glass. T _g o is the glass transition temperature of the unreacted resin mixture, T _g ∼ is the glass transition temperature of a fully curing resin, and Gel.T _g is the intersection point between vitrification and gelation curves. In the initial stages of curing before gelation or vitrification, the kinetic epoxy curing reaction can be controlled. When vitrification occurs, the reaction takes place by diffusion, and the rate is lower than in the liquid phase. Increasing the cross link in the glass phase leads to diminishing the rate of reaction and even may stop the reaction. In the region between gelation and vitrification (rubber region), reactions can occur from kinetic to diffusion. The competition of these reactions causes the minimum vitrification temperature seen in the TTT diagram between T _g ∼ and Gel.T _g. When the curing temperature is just increased, the reaction rate will increase and the vitrification time will become slow, and thus, the diffusion reaction starts to defeat the progression of the kinetic reaction rate. Finally, domination of diffusion reactions in the rubber area brings down all reaction rates so that an increase in vitrification time is seen. At T _g ∼, the reaction does not occur completely. As the curing process takes place, the viscosity of the epoxy system increases as a result of an increase in MW. The reaction becomes diffused and eventually stops when the epoxy is vitrified [45]. After the reaction has stopped, the curing process is replaced by post-curing by elevating the temperature to obtain maximum curing and improvement of the epoxy character. Post-curing is only effective at temperatures above T _g ∼. However, it must be noted that at a temperature slightly above T _g ∼ and when time is sufficient, degradation of the epoxy system’s cross-link network will occur. Thus, controlling temperature and curing time must be done carefully because of the potential for “over-curing.”

One important application of the TTT diagram is to manage the curing temperature (T _c) and the heating rate. When T _c is too low, vitrification can occur before gelation and subsequent reactions may not be completed. This condition results in the incomplete structural network and degrading performance of epoxy resin. This phenomenon is generally related to the process of curing at room temperature or curing with radiation [46]. Furthermore, the relationship between reactant mixing and the Gel.T _g point must also be noted. Epoxy resins and curing agents must be thoroughly mixed before the Gel.T _g point because of the rapid addition of viscosity to the Gel.T _g point will inhibit the mixing of reactants and produces inhomogeneity in structure and morphology and defects in curing results [47].

Special properties of epoxy resins for coatings and composites are largely determined by the curing and quenching processes. It is related to a phenomenon known as internal stress or residual stress and physical aging of epoxy resins [48]. Internal stress arises mainly because of the reduced capacity of the epoxy resin cross link to expand or contract at the same rate to the coated material or substrate. This case is triggered by a mismatch of thermal expansion coefficient between the substrate (such as metal, glass, fiber, and ceramics) and the cross-linked epoxy during the nonisothermal curing process and/or curing shrinkage due to solvent loss. This influence contributes to the failure of adhesion, which often results in metal coating and large-dimensional composite components, especially when T _g of epoxy cross-link approaches T _c. Many efforts have been made to overcome this phenomenon by focusing on understanding the mechanism of stress generation and minimizing these stresses by modifying the curing and post-curing cycles. One practical way is to set the final curing temperature above the glass transition temperature [49]. The thermoset system phase diagram needs to be analyzed in each curing condition to avoid the incompleteness and error of the curing process. The curing process under different external isothermal conditions, such as constant heating rate, adiabatic, and mold wall temperature, are indicated by trajectories in the TTT diagram [50].

6 Curing reaction of epoxy resins

Conversion of epoxy resins from a liquid state into solid and hard thermosets can occur through several cross-linking mechanisms. Epoxies can be catalytic homo-polymerization or form heteropolymers through compounding reactions with functional epoxide groups or curing agents [51]. Epoxy homo-polymerization is generally initiated by tertiary amines, imidazole, and ammonium salts involving complex reactions. This reaction produces epoxy resin characters that are practically undesirable, namely, (a) the reaction rate is low and (b) the main structure of the chain is short. This short chain is found in DGEBA homopolymerization contributing to a low glass transition temperature (T _g is about 100°C). This epoxy system is not often found in commercial applications. Some research is carried out to find the catalysts that meet the technical and economic requirements in the epoxy resin homo-polymerization process. One of them is the dimethyl-aminopyrine catalyst. This catalyst is able to increase the polymerization rate and extend the main chain of DGEBA resin indicated by high cross-link density and its glass transition temperature of 160°C [52]. Modification of epoxy homopolymerization can also be done by presenting a multiwalled carbon nanotube (MWCNT) within tertiary amines. Research showed that MWCNT is able to speed up the process of curing epoxy resins up to two times. Nano carbon is also bonded very well in epoxy resins and is distributed in both single and bundle forms. Consequently, the distribution and infiltration of nano carbon can increase the glass transition temperature, which is proportional to the nano carbon concentration in epoxy [53].

Unlike heteropolymer epoxy resin systems consisting a number of expensive, toxic, and volatile curing agents, homopolymerization epoxy curing systems just use a small amount of catalyst to substitute the function of the curing agent. The selection of catalyst type and amount determines dominantly the final properties of the epoxy resin. For example, the addition of 5 wt% of epoxide-terminated hyperbranched polyether (EHBPE) to the DGEBA epoxy resin was able to simultaneously improve the tensile strength of the epoxy by 47%, toughness of 19%, and a glass transition temperature of 173°C. The mechanism of homopolymerization and a possible structure of the epoxy resin are shown in Figure 10 [54].

Figure 10

Homopolymerization of DGEBA with EHBPE catalyst and epoxy structure prediction after the completed curing process. Homopolymerization has always left an unreacted epoxy group [54].

Almost all of thermoset epoxy resins are made by the heteropolymerization mechanism through very complicated reactions with curing agents. For the purposes of CFRP composite matrices, epoxy resins are often combined with other additive ingredients for matching with the composite property requirements. For example, the composite CFRP prepreg FIBREDUX 913C was made by Ciba Geigy Co., which is used for Boeing aircraft components. This material was constructed from CF surrounded by an epoxy resin matrix, which was composed of a mixture of tetrafunctional epoxy resin (TGDDM), low MW resin (DGEBA), hardener of dicyanodiamide (DICY), and DDS, and a mixture of dichlorophenyl dimethylurea, polysulphone hard catalyst-based bisphenol A and polysulphone type polyarylether [55]. This compound requires a very complex polymerization reaction. However, the complexity of heteropolymerization of epoxy resins can be simplified by taking into account the principal constituent components, namely, polymer resins, curing agents, and catalysts. Curing agents sometimes also function as thinners (co-reactive diluent) to improve the epoxy resin flowability [56].

In epoxy monomers, epoxy or oxirane groups have three carbon-bonded rings that are ready to undergo ring-opening reactions with a number of curing agents or hardener. Every monomer there is a reactive epoxy portion at the end of each molecule. Other parts of the epoxy monomer are shown in Figure 11. Commercial DGEBA epoxy resin monomers have an average MW of 340 g/mol, and some modification of these monomers results in a MW of 380 g/mol (Epicote 828) [58]. The hydroxyl group can help for adhesion, wetting the surface during the application or curing process, and also having a function as an additional reactive part in the nucleophile reaction. These curing and nucleophile reactions contribute greatly to the formation of thermoset chains and cross links.

Figure 11

Parts or situses of a compound constructing epoxy DGEBA monomer [57].

In mechanical engineering applications, epoxy resins of DGEBA base are the most dominantly used both in state of pure and modification in MW through variations of the ratio of epichlorohydrin to bisphenol A [59]. When DGEBA resin is added by a hardener containing primary amine, this hardener reacts to an epoxide ring to form a secondary amine and a hydroxyl group. Then, the secondary amine undergoes further reactions with other epoxide groups producing additional hydroxyl groups and making tertiary amines. This reaction continues until all active groups of hardener and/or epoxy resins have completely reacted and reached the phase of complete vitrification. An ideal schematic diagram depicting epoxy resin curing using amine hardener is presented in Figure 12a.

Figure 12

Networks scheme generated by reaction between epoxy DGEBA and (a) polyetheramine hardener and (b) anhydride hardener [60].

In addition to the amine curing agent, epoxy resin systems with anhydride hardener are also widely used commercially. Because the anhydride group cannot react directly with the epoxy group, the anhydride ring first initiates a reaction by binding to the hydroxyl (OH) group existing in the system to form a monoester containing a carboxylate group. Furthermore, the carboxylate group reacts with an epoxy group to produce an ester and hydroxyl bond (called the esterification reaction), as shown in Figure 12b. Post-curing in this system does not have much effect on the final structure of the epoxy resins [61]. The epoxy group can also react with the OH group in the system forming ether bonds (the reaction is called etherification), and if the OH group comes from a resin backbone, a homopolymerization reaction will occur. Esterification and homopolymerization reactions are likely to occur with an increase of the curing temperature, either with an amine curing agent or anhydride.

The reaction leading to network formation during the DGEBA resin curing process depends on the curing agent used, which will impact the level of epoxy resin reactivity. The illustration in Figure 13 schematically explains the network structure produced by the reaction shown in Figure 12 when DGEBA resin is cured using an amine or anhydride hardener. The DGEBA resin molecule is represented by reactive groups when it reacts with an amines hardener is just considered to have only two groups of epoxide reactive terminals. However, in the case of the curing anhydride system, besides the two terminal epoxide groups, there are OH groups that react in the anhydride initiation process. This OH group is found in resin backbones, which ideally have one OH group in the DGEBA molecule. Amines and anhydrides are schematically represented as tetrafunctional (four hydrogen in amines) and bifunctional (two terminal anhydrides) [60].

Figure 13

Reactivity diagram of epoxy resin DGEBA within (a) amine curing system and (b) anhydride curing system [60].

Recently, a number of nonlinear multifunctional epoxy resins play an important role in the research and the application of epoxy resin material. An example is an epoxy resin glycidyl amide, namely, TGDDM. Comparing to conventional bisphenol resins, TGDDM has a lower density, better flowability and processability, and higher cross link density after curing. TGDDM has also begun to be used in aircraft applications, electronic industries, and other high-tech fields [62]. However, to obtain superior properties and characteristics of TGDDM epoxy resin, the curing process is still needed to convert these epoxy resins from monomers and/or oligomers into macromolecules with high three-dimensional cross link networks through the selection of suitable curing agents and optimal conditions of the curing process [63]. Hardener in TGDDM epoxy resin system plays a crucial role for determining the mechanism and condition of curing reaction, pot life, cross-link network structure, as well as practical application and end-use properties. One curing agent that is not yet widely known but has accordance with TGDDM characters is DICY. This hardener is a solid powder with limited solubility within epoxy resin at room temperature. It outplays as a prepolymer with excellent process capability and stability at room temperature. For this reason, DICY is widely used as a thermally latent curing agent for applications such as laminate, prepreg, coating, adhesion, and composite matrices [64].

7 Degradation and decomposition of composites with epoxy resins matrices

Thermoset epoxy resins have superior thermal and mechanical properties as well as dimensional stability due to chemical cross-link networks. These resins are widely used in applications for coatings, adhesives, composites, cashing electronic products, and others. In applications of high-performance coatings, adhesives, rubber blends, light-emitting diode lamps, and solar cell protectors, they are also made from epoxy resins with permanent cross-link networks via covalent bonding [65]. Due to the existence of the covalent cross-link bonding network, the epoxy resin cannot be reformed and reprocessed by heat or by any solvents [66]. Thus, after the end of life, the thermoset epoxy resin polymers are very difficult to be recycled because shortly after curing, these polymers cannot be reprinted or remolded and decomposed under mild reaction conditions [67]. However, concerning environmental responsibility and sustainability, the topic of recycling and reusing of thermoset epoxy materials has become very important and interesting. The need for recycling is also driven by the rapid advancements in electronic technology, where most of the chasing uses thermoset epoxy materials that cause the life of these polymers to be shorter. For example, cell phones are used for no more than 18 months and computer usage ranges within 3 years duration [68], even though the lifespan of a thermoset can reach 30 years. This condition will accelerate the volume of thermoset epoxy waste that needs to be handled wisely recycling. In addition, the demand for reinforcement fibers reclamation from composites recycled, which reduces production costs, tends to grow continuously. For example, CF is an expensive composite reinforcement for CFRP production. Recycling activities have the potential to reuse CF as a cheap composite reinforcement for applications that do not require high strength structure, and they guarantee the availability of CF in the market [69].

Research on composite recycling technology with matrices of thermoset epoxy resins is currently focused on mechanical, thermal, and chemical methods. In the mechanical recycling process, composites are ground or crushed into particles with lengths of 10–50 mm. The thermal recycling process can burn the epoxy resin matrices, so that it can recover the reinforcing fibers. Chemical recycling uses solvents to depolymerize or decompose composite matrices. Some recycling technologies have been applied on an industrial scale. For example, Filon Ltd in the United Kingdom uses a grinding machine to recycle glass fiber reinforced composites, ELG Carbon Fiber Ltd applies the pyrolysis process, and Adherent Technologies Inc. in America applies a wet chemical process to break down thermoset resins to obtain a composite reinforcing fiber [70]. In Germany, SGL carbon uses a solvolysis process to decompose the epoxy resin matrices and reclaim CF that is then reused for the roof and back seat of a BMW car [71]. Each recycling method has advantages and disadvantages. Mechanical recycling is suitable for reclaiming glass fiber from composites because this fiber has a potential to be damaged during the thermo-chemical process, whereas CF can be recovered effectively using thermal and chemical processes [72]. The following texts will describe the degradation and/or decomposition of epoxy resins in CF composites through thermal and chemical processes.

7.1 Thermolysis decomposition of composites with epoxy resins matrices

The thermal stability of an epoxy resin depends on the monomer structure, the curing agent structure, and the density of the cross linked. The thermal stability of aromatic epoxy resins is generally higher than that of aliphatic resins although the density of aromatic epoxy cross linked may be lower. Aliphatic or aromatic epoxy copolymerization with self-cured novolac, which is cured by amines, results in higher cross-link density and is able to improve the thermal stability of epoxy resins. Conversely, too much curing agent composition in novolac resins will result in a decrease of thermal stability so that the amines in the novolac epoxy resin system may not exceed 15 wt%. Although amines are widely used in epoxy resin systems, ether bonding shows a better performance in terms of thermal stability. The better thermal stability of epoxy resins results in a higher “thermolysis resistance” [73].

The thermolysis method basically applies thermal energy, which is able to break the polymer bonds and cross-link networks so that the polymer compound decomposes into atoms or its constituent elements. Pyrolysis and fluidized bed (FB) processes are the most commonly applied. Pyrolysis is decomposed polymers at high temperatures in the range of 300–800°C in the absence of oxygen. Static bed pyrolysis reactors are generally used for decomposing of thermoset resin matrices in fiber-reinforced composites, as schematically shown in Figure 14. The reactor consists of a certain volume of chemical tubes heated by an electric or gas oven. Removable stainless-steel crucible as a composite container is placed in the middle of a chemical tube. Four cold-trap bottles are positioned at the bottom of the chemical tube to maximize the collection of condensable products. The first bottle is cooled with cold water, and others are cooled with ice. The last bottle is given an additional glass wool to catch the product in the form of oil mist. Dreschel bottles with deionized water are placed after the last condenser glass trap to remove water-soluble gases. During the pyrolysis process, nitrogen gas is flowed into the reactor to expel the pyrolysis gas from the reactor’s hot zone to prevent secondary reactions from pyrolysis vapor and help in quantifying the pyrolysis gases products. Nitrogen gas is generally preheated to a temperature of 180°C before being flowed into the reactor [74]. The mechanism of decomposition of CFRP composites via thermolysis is illustrated using SEM photographs and graphs of thermogravimetric test results (TGA), as illustrated in Figure 15.

Figure 14

Pyrolysis reactor scheme [74].

Figure 15

Pyrolysis decomposition mechanism of CFRP composite. (a) SEM images of decomposition steps and (b) TGA testing graph [75].

When polymer matrices are pyrolyzed, it is transformed into smaller molecules at a temperature of 300°C in the oven. This micro molecule is able to evaporate from composite materials and can be used as an energy source because it has a high caloric value. A number of basic studies have been carried out regarding the pyrolysis process on thermoset resin materials. The thermal degradation behavior of DGEBA and tetraglycidyl methylene dianyline epoxy resins is strongly influenced by the curing agent, amine concentration, and nucleoplicity of nitrogen atoms. The presence of sulfur in DDS can improve the thermal stability of epoxy resins so that it takes longer or higher temperature during degradation or decomposition [76]. The composition of the thermoset resin matrix in the composite will determine the parameters of the pyrolysis process, including the replenishment of an additive in the form of a modifier. Pyrolysis of composite sheet molding compound waste and a mixture of various materials from automotive waste containing an elastomer modifier can produce hydrocarbon liquids that have the potential to be used as fuel. In addition, the pyrolysis gas produced by thermoset material contains hydrogen, methane, and other hydrocarbon gases, which have high calorific value and the potential to be used as an energy source for a continuous pyrolysis refinery system [77].

Some pyrolytic carbon residues are usually produced by the pyrolysis process in the nitrogen environment, and these residues are bonded to the surface of the composite reinforcing fiber. The existence of this residue is the weakness of the pyrolysis process because it can degrade the mechanical and electrical properties of reclaimed fibers and potentially aggravate fiber-matrices adhesiveness. The residual quantity is very dependent on the pyrolysis process parameters, such as the oven environment, temperature, heating rate, and others. Control of the parameter values in the pyrolysis reactor is very important to obtain the results of perfect polymer decomposition and obtain clean reinforcing fibers [78]. Pyrolysis of CFRP composites at high temperatures in the air environment is able to remove carbon residues but present oxidation and reduce the strength of CF. Oxygen concentration is a major factor of oxygen content on the surface of CFs, while epoxy resin decomposition produces hydrogen, carbon monoxide, and methane and liquid products such as bisphenol A and amines [79]. In addition, steam pyrolysis is performed at a maximum temperature of 600°C and atmospheric pressure. Using superheated steam as an oxidant, the epoxy resin matrices in CFRP composites can be converted to lower MW hydrocarbon compounds, CO gas, and CO₂ without damaging CF properties. At temperatures of 600–800°C, the steam pyrolysis process only takes 60 min to decompose the thermoset resin [80]. Vacuum pyrolysis, which is applied for recycling automotive shredder residues, is carried out at a temperature of around 500°C and a pressure below the atmosphere that is between 1 and 5 kPa. This method is capable of producing 27.7% liquid oil and 6.6% hydrocarbon gas, which can all be converted into fuels or their mixtures [81]. The heating rate also influences the performance of epoxy resin decomposition. Decomposition research in the nitrogen environment with heating rates of 2, 5, 10, and 20°C/min showed a shift in the epoxy resin degradation zone toward higher temperatures, and the Arrhenius conversion rate was reduced when the heating rate was increased. At a heating rate of 2°C/min, epoxy degradation began at 258°C and ended at 458°C with a residue of 17.9 wt%. Meanwhile, the heating rate of 20°C/min shows that epoxy begins to degrade at 279°C and ends at 590°C with a residue of 12.1 wt% [82].

Experimental studies related to the pyrolysis process generally are efforts to obtain the pyrolysis process parameters to get maximum epoxy resin decomposition results and reclaim reinforcing fibers without any suffers. Optimization has been conducted for achieving a maximum pyrolysis output. Environmental considerations and process costs are taken into account in modeling research and pyrolysis process optimization. For example, CFRP composite pyrolysis steam with temperature parameters, isothermal residue time, and steam flow rate were optimized to decide optimal parameters using the Taguchi method. Each parameter was made on two levels to determine the strongest influence of these parameters on decomposition epoxy resins rate and mechanical properties of CF produced. Normally, two factors are considered in the Taguchi method, namely, control factors and noise factors. Variance analysis and standard least-square linear regression were used to analyze experimental results. The optimization results show that the rate of thermoset matrices decomposition correlates directly with the steam/sample ratio, heating temperature, and the presence of steam at high temperatures during the final stages of the pyrolysis process [83]. The decomposition reaction scheme on a hydrogen fuel cell composite was developed using the shuffled complex evolution (SCE) method to obtain a set of optimal reaction parameters. The predicted value by the SCE optimization method shows good compatibility with experimental data; thus, it is potentially applicable in practice [84].

Decomposition of the thermoset resin with a FB occurs by flowing hot air through silica sand bed to composite waste. Figure 16 shows the schematic device and the sequence process of FB. Typically, fine silica sand with a particle size of 0.85 mm is used as a bed, which is then converted to a FB by a stream of hot air between 0.4 and 1.0 m/s. Process temperature range is usually from 450 to 550°C. Composite waste pieces decompose into fibers and gases in the freeboard reactor due to rapid ignition and friction and matrices decomposition carried out in the air stream. Furthermore, fibers and gases are separated using a cyclone separator. Mesh filters are usually placed at the bottom of the cyclone separator to separate the fiber length or to separate contaminants carried in the air stream [85]. Gasses from epoxy resin decomposition undergo combustion to oxidize other secondary products. This process is suitable for expired composite components since contents such as rivets, bolts, and other fittings can be collected in the reactor and the reinforcing fibers of the composite can be recycled [86].

Figure 16

Simple schematic of FB CFRP waste recycling reactor [87].

8 Future challenges for mechanical engineers

Advancement in material technology has led to the creation of sturdy and lightweight materials. In the automotive and aerospace industries, this demand has been going on for a long time to reduce the weight of vehicles, which has implications for reducing fuel consumption. The trend in electric vehicles development also requires high-performance smart materials. Fiber-reinforced composites with epoxy matrices have a great opportunity to meet these needs and demands. Mechanical engineers must respond to this situation by preparing a composite composition that is reliable and tested. In other applications, such as in building construction, fire retardance, corrosion or weather resistant, robotic, and others materials also require innovation by mechanical engineers to be creative in solving every challenge.

Conversely, the impact of epoxy resin mass production in the form of composite waste also requires qualified handling so as not to continue into a complex global case. Any form of waste can be actually reused as long as the treatment is accurate and precise. Epoxy in composites can theoretically be recycled into fuel or reused as composite matrices. The biggest obstacle at this time is that there is no technology that can recycle epoxy resin in a short time and low cost. Pyrolysis offers the fast recycling process but still requires high cost for investment and operation. Meanwhile, solvolysis can be relied on in decomposing epoxy resins at a low cost but requires a long processing time. Innovative and creative efforts are still needed to obtain recycled parameters that are technically and business acceptable.

9 Conclusion

The availability of various types of epoxy resins provides a wide opportunity for engineers to design composite materials that are most suitable for their applications. The advantages of epoxy resin as a matrix in composite materials can be achieved by selecting the appropriate type and the dose ratio of the curing agent. Fiber-reinforced epoxy resin composites, in particular CF, are increasingly in demand and have the potential to create waste problems after the end of their use. The right strategy in the recycling process of the pyrolysis or solvolysis method has the potential to solve this problem by reclaiming CF and reusing processed epoxy resin. Some existing research and recycling technology can be a reference for developing more constructive ideas in solving this waste problem.