Amine-epoxy curing reactions mechanism

Epoxies can be synthesized fiom halohydrins and also fiom alkenes. Epoxies react readily with active hydrogen compounds such as alcohols or amines and are frequently cured with diamitKxliphenyisulfone. Most epoxy curing reactions occur by a cationic mechanism. 

The second important feature of the reaction mechanism of the epoxy compound curing under the action of amines (primary, secondary and tertiary) and their mixtures consists in formation of various hetero-, auto-, inter- and intramolecular donor-acceptor complexes between the components of the reaction system — the starting substances and reaction products. Consideration of this complex formation can adequately explain the reaction kinetics. 

The cure reactions, the viscosity-time-temperature profile, the processing conditions, the resultant epoxy chemical and physical structure, and the mechanical response of a C-fiber/TGDDM-DDS cured epoxy composite are modified by the presence of a BF3-amine complex catalyst within the prepreg. These factors also will be modified... 

In general the amine-epoxy resin curing reactions show complex kinetics typified by an initial acceleration due to autocatalysis, while the later post-gelation stages may exhibit retardation as the mechanism becomes diffusion-controlled. However some workers 72 80) have found that over a limited range of conversion the kinetic data may be described by the simple models of Eq. (2-6) or (2-9). 

After the amines, acid anhydrides constitute the next most commonly used reagents for curing epoxy monomers. The epoxy-acid reaction proceeds through a stepwise mechanism (Sec. 2.2.4) while the reaction of epoxides with cyclic anhydrides, initiated by Lewis bases, proceeds through a chain-wise polymerization, comprising initiation, propagation, and termination or chain transfer steps. Some of the postulated reactions are shown in Table 2.25 (Matejka et al., 1985b Mauri et al., 1997). 

Chemical clusters can be obtained also with two monomers, when two reaction mechanisms are in competition, favoring formation of regions of higher and lower crosslink densities. This situation is more complex and more difficult to control. It is certainly the case for dicyanodiamide (Dicy)-cured epoxies with this hardener an accelerator is always used and a competition between step (epoxy-amine addition) and chain (epoxy homopolymerization) occurs (Chapter 2), leading to inhomogeneous networks. 

The polyaddition reaction is the most commonly used type of reaction for the cure of epoxy resins. The curing agents used in this type of reaction have an active hydrogen compound, and they include amines, amides, and mercaptans. With this reaction mechanism, the most important curing agents for adhesives are primary and secondary amines containing at least three active hydrogen atoms and various di- or polyfunctional carboxylic acids and their anhydrides. 

Another possible preassembly reaction mechanism has been noted with regard to amine cured epoxy resins.10 A variability and reduction in the rate of conversion of epoxy groups in DGEBA epoxy resin cured at room temperature with diethylene triamine (DETA) was noticed. This is due to a side reaction of the amine with air, resulting in bicarbonate formation. As a result, the adhesive strength decreased drastically when the uncured epoxy amine was exposed to ambient air for a significant period of time. 

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. 

Another curing mechanism is operative in the curing reaction initiated by a polymerization catalyst for epoxy ring opening, where anionic catalysts such as tertiary amines 1S-l9) and imidazols 20 21> and cationic catalysts such as amine complexes of... 

Resin Types and Structure Typical Bisphenol Epoxy Resins Reactions of Epoxides and Curing Mechanisms Addition Reactions of Amines... 

The reaction mechanism of liquid dimer polyamides and fatty amido amines with epoxy resins has been studied by Peerman et al. (27). who employed infrared spectroscopic analysis to determine reaction rates. They showed that the terminal epoxy content of a blend of amino-containing polyamide and epoxy resin disappeared more rapidly at 150 °C than does the epoxy content of blends of epoxy resin with triethylenetetramine or tris[(dimethylamino)-raethy1Jphenol. Both of these compounds are well-known for their fast cure at ambient temperatures. Correspondingly, the liquid polyamide or fatty amido amines-epoxy combinations cure slower than the other two systems at ambient conditions. 

One of the key parameters defining the mechanical properties of the cured epoxy is the crosslink density. In a diepoxy fully cured with a diamine, if additional effects such as side reactions are neglected, each primary amine group is expected to react with two epoxide groups. Branched crosslinks result from this situation. With the chemical functionalities /a = 4 and /e = 2 of the diamine and the diepoxy molecules, respectively, the amine/epoxy mixing ratio, r, is given by Eq. (1), where and Ng denote the respective numbers of moles [10]. 

Theoretical treatment of this polymerization is difficult because of the presence of both primary and secondary amine reactions as well as tertiary amine catalyzed epoxy homopolymerization. To obtain kinetic and viscosity correlations, empirical methods were utilized. Various techniques that fully or partially characterize such a system by experimental means are described in the literature ( - ). These methods Include measuring cure by differential scanning calorimetry, infra-red spectrometry, vlsco-metry, and by monitoring electrical properties. The presence of multiple reaction mechanisms with different activation energies and reaction orders (10) makes accurate characterizations difficult, but such complexities should be quantified. A dual Arrhenius expression was adopted here for that purpose. 

Several epoxy formulations are cured by both step-growth and chain-growth polymerizations occurring sequentially or in parallel. For example, BF3 complexes or tertiary amines may be added as catalysts of an amine-epoxy reaction, leading to different reaction mechanisms taking place whose relative significance depends on the cure temperature (or thermal cycle) and the initial stoichiometry. The structure and properties of the resulting polymer networks depend on the relative contribution of both mechanisms. 

Tertiary amines are commonly referred to as catalytic curing agents since they induce the direct linkage of epoxy groups to one another. The reaction mechanism is believed to be as follows ... 

The reaction mechanism indicates that the epoxy concentration decreases, and this is observed in the spectra as the decrease of the band centered at 4530 cm-i and also of the weak overtone of terminal CH2 at 6060 cm-i. The primary amine combination band decreases too ( = 4900 cm-i), and once it is exhausted it can be observed that there are still epoxy groups in the reaction media, which will react with the previously formed secondary amines up to vitrification or until the reaction is completed. The band correponding to O-H overtones ( 7000 cm-i) also increases during curing as a consequence of the oxirane ringopening, although this band is not suitable for quantification because of the low signal/noise ratio. The behavior of the band located at 6500 cm-i is more complex in this... 

Theoretical study on mechanisms of the epoxy-amine curing reaction. Macromolecules, VoL40, No.l2, (2007), pp.4370-4377, ISSN0024-9297. 

Epoxy-amine systems follow an addition step-growth polymerisation mechanism. The two principal reactions of primary and secondary amines with epoxy oligomers are shown in Reaction scheme 1 [30]. These reactions are catalysed by acids, phenols and alcohols (e.g. impurities in commercial epoxy resins). The presence of water causes a tremendous acceleration, but does not alter the network structure. The hydroxyl groups formed by the amine-epoxy addition steps are also active catalysts, so that the curing reaction usually shows an accelerating effect in its early stage (autocatalysis). 

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