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Introduction:
Epoxy resins are a family of thermosetting materials widely used as adhesives, coatings and matrices in polymer composites because of the low viscosity of the formulations, good insulating properties of the final material even at high temperatures and good chemical and thermal resistance (May, 1988). Epoxy thermosets can be described as 3D polymer networks formed by the chemical reaction between monomers ("curing"). This 3D covalent network structure determines the properties of thermosetting polymers: unlike thermoplastics, this kind of polymers does not melt, and once the network has been formed the material cannot be reprocessed. Maybe one of the main advantages of epoxy thermosets is that the starting monomers have low viscosity so that complex geometries can be easily shaped and fixed after curing the monomers. Thus the formation of the network via chemical reaction is a key aspect in this kind of materials.
Epoxy formulations usually include more than one component, although there are different crosslinking mechanisms involving either chemical reaction between one single type of monomer (homopolymerization) or two kinds of monomers with different functional groups. In both cases, a common constituent is always found: the epoxy monomer. The main feature of the epoxy monomer is the oxirane functional group, which is a three member ring formed between two carbon atoms and an oxygen, as shown in Figure 1. This atomic arrangement shows enhanced reactivity when compared with common ethers because of its high strain. Due to the different electronegativity of carbon and oxygen, the carbon atoms of the ring are electrophilic. Thus epoxies can undergo ring opening reactions towards nucleophiles. The polarity of the oxirane ring makes possible detection by IR spectroscopy.
There are mainly two families of epoxies: the glycidyl epoxies and non-glycidyl epoxies (also called aliphatic or cycloaliphatic epoxy resins). The absence of aromatic rings in aliphatic epoxies makes them UV resistant and suitable for outdoor applications and also reduces viscosity. The most common epoxy monomers of each family are diglycidylether of bisphenol A (known as DGEBA) and 3,4-Epoxycyclohexyl-3’4’-epoxycyclohexane carboxylate (ECC) respectively and their structures are given in Figure 2 (a, b). Cycloaliphatic resins are usually found in the form of pure chemicals with a definite molecular mass. But DGEBA-based resins are synthesized via the addition of epichlorohydrine and bisphenol A so oligomers with a relatively narrow distribution of polymerization degrees are obtained instead; their chemical structure is presented in Figure 2 (c) where n is typically 0.2. DGEBA oligomers typically contain a certain amount of hydroxyl groups, that play an important catalytic role in the kinetics of the curing process, providing a higher viscosity which is dependent on n. In addition, all of them have at least two oxirane functional groups, so they can finally lead to the 3D network. The nature and functionality of the epoxy monomer will determine its reactivity as well as the properties and performance of the final material.
Despite of having the same main functional group, the reactivity of both families of epoxies is completely different as a consequence of the structure of the molecules. It is worthy to note that the linkage between the aromatic ring and the oxygen (ether) in DGEBA has a strong electron-withdrawing effect that makes the oxirane group highly reactive towards nucleophilic compounds (like amines), unlike the cyclohexyl group in aliphatic epoxies which is reactive towards Lewis acids like anhydrides (Mark, 2004). Additionally, a protecting effect of axial and equatorial protons of the cyclohexyl ring against nucleophilic attack has been proposed as an explanation of the characteristic low reactivity of the oxirane ring in these aliphatic epoxies (Soucek et al., 1998). This way, the best performance and the highest crosslinking degree for DGEBA-based resins is achieved when cured via an addition mechanism with diamines (either aliphatic or aromatic), whilst cycloaliphatic epoxies are commonly cured with anhydrides (Barabanova et al., 2008; Chen et al., 2002; Tao et al., 2007; Wang et al., 2003) or homopolymerized via a cationic mechanism induced by UV radiation (Crivello, 1995; Crivello & Fan,1991; Crivello & Liu, 2000; Hartwig et al., 2003; Wang & Neckers. 2001; Yagci & Reetz, 1998).
The chemical reactivity of the epoxies enables using a wide variety of molecules as curing agents depending on the process and required properties. The commonly used curing agents for epoxies include amines, polyamines, polyamides, phenolic resins, anhydrides, isocyanates and polymercaptans. The choice of both the resin and the hardener depends on the application, the process selected, and the properties desired. It is worthy to note that the reaction mechanism, the curing kinetics and the glass transition temperature (Tg) of the final material are also dependent on the molecular structure of the hardener. As it has been previously mentioned, amines are the best performance curing agents for diglycidylether- type epoxies. Aliphatic diamines like m-xylylenediamine or 1,2-trans-cyclohexyldiamine can be used for curing from room to moderate temperatures (Paz-Abuin, 1997a, 1997b, 1998), although the glass transition temperature of the material is also moderate. For high Tg materials aromatic amines, like 4,4 ́- methylen- bis (3- chloro- 2,6- diethylaniline) or 4,4 ́- diaminodiphenyl sulphone (Blanco et al., 2004; Girard-Reydet et al., 1999; Marieta et al., 2003; Siddhamalli, 2000a) are used, although high curing temperatures are needed.
