Epoxy Curing Mechanisms

This transformation or conversion is accomplished by the addition of a chemically active compound known as a curing agent or catalyst. Depending on the particular details of the epoxy formulation, curing may be accomplished at room temperature, with the application of external heat, or with the application of an external source of energy other than heat such as ultraviolet (uv) or electron beam (EB) energy. 

Two primary types of epoxy curing reactions are discussed in this section  

Both polyaddition and homopolymerization reactions can result in increased molecular weight and crosslinking. Both types of reaction occur without the formation of by-products. The curing reactions are exothermic, and the rates of reaction increase with temperature. 

Since the epoxy resin cures primarily by a ring-opening mechanism, it exhibits a smaller degree of cure shrinkage than other thermosetting resins. In these reaction processes, the epoxy group may react in one of two different ways anionically and cationically. Both are of importance in epoxy resin chemistry. In the anionic mechanism, the epoxy group may be opened in various fashions to produce an anion, as shown in Fig. 2.10. 

The anion is an activated species capable of further reaction. In the cationic mechanism, the epoxy group may be opened by active hydrogen to produce a new chemical bond and a hydroxyl group. This reaction may proceed in a number of different ways. 

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Cure Rate Studies. The literature on curing of epoxies indicated two ways to quantitatively assess the difference in cure rate of various curing agents. Fava has reported on cure mechanism studies using differential scanning calorimetry and Dannenberg of Shell has reported on epoxy-cure mechanism studies by near infrared spectroscopy3 (NIR). 

Of the commercially available EB-curable adhesives [9-12], the resins fall within one of two categories based on their curing mechanisms. The majority of EB-curable resins are based on (meth)acrylate-functionalized oligomers involving a free-radical curing mechanism. The second category is the epoxy resins that cure by a cationic mechanism. 

An analogous mechanism should also produce polymers on irradiation of epoxies. Crivello s recent mechanistic suggestions [29] are consistent with the mechanisms given above. One can conclude that radiation-induced polymerization of epoxies can proceed via several mechanisms. However, further work is needed to determine the relative contributions of the different mechanisms, which might vary from one epoxy to another. As part of the Interfacial Properties of Electron Beam Cured Composites CRADA [37], an in-depth study of the curing mechanism for the cationic-initiated epoxy polymerization is being undertaken. 

Materials and additives that are chemically basic in nature have a detrimental effect on the curing of cationic-initiated epoxy systems. These substances can either stop the curing mechanism completely or produce under-cured polymers. Therefore such additives as amines or imides that are known to be adhesion promoters cannot be used in the EB-curable epoxy adhesive formulations. 

Epoxy ester Epoxy esters are a type of alkyd where a high molecular weight resin is reacted with alkyd resin. The curing mechanism remains primarily through the oil-oxidation reaction and their properties are in no way similar to the chemically reacted epoxies. They have similar properties to alkyds although with improved chemical resistance but inferior appearance. They form a reasonably hard, oil-resistant coating, which can sometimes be suitable for machinery enamels, but are primarily for interior use, since they tend to chalk rapidly on exteriors. Their best use is for chemical or water resistance where circumstances dictate that finishes that are more superior cannot be used. 

The curing mechanism shown below demonstrates the behaviour of one small polyamine molecule with four epoxy resin molecules. Similar reactions will occur at the other end of the epoxy resin molecules. 

Kamon, T., Furukawa, H. Curing Mechanisms and Mechanical Properties of Cured Epoxy Resins. Vol. 80, pp. 173 — 202. 

Literature search shows that epoxy-based nanocomposites have been prepared by many researchers [34-38]. Becker et al. have prepared nanocomposites based on various high-functionahty epoxies. The mechanical, thermal, and morphological properties were also investigated thoroughly [39 3]. The cure characteristics, effects of various compatibilizers, thermodynamic properties, and preparation methods [16,17,44 9] have also been reported. ENR contains a reactive epoxy group. ENR-organoclay nanocomposites were investigated by Teh et al. [50-52]. 

Compatibility. Owing to the high reactivity of the isocyanate group, polyurethane propellants require a more sophisticated processing technique than the rather foolproof, carboxy-terminated polybutadiene aziri-dine and/or epoxy-cured propellant systems. Processing is even more complicated if bonding agents (see below) are present, which are used to bolster mechanical properties in practically any modern propellant. 

So, the curing mechanism of epoxy oligomers with amines is rather complicated apd, to a first approximation, may be described by Scheme (8) taking into consideration all the donor-acceptor interactions of the starting reagents with the reaction products and with each other... 

Elucidation of the Mechanism of Epoxy Curing by Model Reactions... 

Curing Mechanism and Mechanical Properties of Cured Epoxy Resins... 

The results of DSC studies on the anhydride cure of epoxy resins are summarised in Table 2. These studies have confirmed that the cure mechanism is complex. The early stages show autocatalytic features while the later stages are complicated by the effects of diffusion control. Intermediate stages of cure can show an approximation to overall kinetic orders of 1 or 2. In general the isothermal DSC data are easier to... 

Fundamental reaction mechanisms of epoxy cure have been fully investigated in the literature with the work of Lee and Neville being the reference of choice [2]. Recent studies focusing on fundamental reactions of epoxy and various hardeners have been performed using NMR and NIR/FTIR methods in the works of many researchers [86-95]. Mechanisms and kinetics of various epoxy systems has been evaluated and tabulated by Mijovic et al. and others [96, 97]. 

The early reaction mechanism of DICY with epoxy resin consists of the epoxy reaction with all four hydrogen atoms on DICY and the epoxy-to-epoxy reaction that is catalyzed by the tertiary amines. The final curing mechanism is between hydroxyl groups in the partly cured resins and DICY cyano groups. This results in the disappearance of the cyano groups to form amino groups. This step is also catalyzed by tertiary amines. 

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