Epoxy resin curing mechanism

Studies of the particle—epoxy interface and particle composition have been helphil in understanding the mbber-particle formation in epoxy resins (306). Based on extensive dynamic mechanical studies of epoxy resin cure, a mechanism was proposed for the development of a heterophase morphology in mbber-modifted epoxy resins (307). Other functionalized mbbers, such as amine-terminated butadiene—acrylonitrile copolymers (308) and -butyl acrylate—acryhc acid copolymers (309), have been used for toughening epoxy resins. 

Different behaviors and mechanisms were clearly recognized between these resins. Epoxy resin cured with amine showed no degradation during immersion because of its stable crosslinks. Epoxy resin cured with anhydride showed the uniform corrosion with the softening and dissolution of the surface and also behaved similar to the oxidation corrosion of the metal at high temperature obeying linear law. 

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). 

DSC is increasingly being applied to the study of epoxy resin cure in combination with other analytical methods such as nuclear magnetic resonance and Fourier transform infra-red spectroscopy, chromatographic methods, and dynamic mechanical or dielectric studies. It is probably as part of such combined investigations that DSC can be used most effectively in basic research, and in quality control and assessment. 

Levchik, S. V., Camino, G., Luda, M. P., Costa, L., Muller, G., and Costes, B., Epoxy resins cured with aminophenymethylphosphine oxide Mechanism of thermal decomposition, Polym. Degrad. Stab., 1998, 60, 169-183. 

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 viscosity increase in a epoxy resin-curing agent system could result in poor wetting of the substrate surface, resulting in suboptimal adhesion. Several reaction mechanisms can also occur to an epoxy adhesive once it is mixed and applied to a substrate but before the substrates are mated. These mechanisms can result in a weak boundary layer, which will prevent optimal wetting and reduce the strength of the adhesive. 

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. 

Zukas, W. X., Schneider, N. S., MacKnight, W. J. Dielectric and dynamic mechanical monitoring of epoxy resin cure, Polym. Prepr., 25 (2), 205 (1984)... 

Structures and Dynamic Mechanical Properties of Epoxy Resins Cured... 

The curing mechanism of epoxy resins with carboxylic hydrazides has not been sufficiently revealed so far. However, Kamon 36) has shown that one hydrazide group reacts with two epoxy groups. This result suggests that the structure of the epoxy resin cured with hydrazide is presumed to be nearly the same as that shown in Fig. 1 a, where the — NC segment is substituted by the -CONHN segment. 

Table 6. Dynamic mechanical properties for epoxy resin cured with acid anhydrides...

The dynamic mechanical properties of three epoxy resins cured with diphenyl-methane diisocyanate (MDI) are shown in Fig. 15. Since these resins consist of many bulky cyclic structures, Tg (a-dispersion) is above 200 °C and the transition region is wide. In the DEN 431 —L-MDI resin, Tg is higher than 300 °C. The resin is expected to be highly heat-resistant51). 

Figure 5.3. Illustration of a hardening mechanism responsible for epoxy resin curing.

The fact that a viscosity increase after phase segregation (for t > tp) is connected with such mechanism is evidenced by the results of gel chromatographic (GPC) analysis of solfi action in the network formation process of low-molecular siloxane rubbers (Fig. 15). As the reaction proceeds the molecular mass of the sol fraction decreases and so does its viscosity. However, network formation of a number of epoxy resins cured with amines or other curing agents conform the homogeneous model without microgel formation [88 a]. 

Thermoplastics are sometimes added to epoxy resins. Thermoplastic-modified epoxy resins [43,44] based on tri- and difunctional epoxy resins cured with DDS and blended with polyethersulfone form the basis for the matrix material in a composite used for the Boeing 777 aircraft. The incorporation of the thermoplastic helps the processing characteristics and also improves the mechanical properties, notably the toughness. The thermoplastic is able to phase separate from the epoxy phase and acts as a reinforcement for the epoxy matrix, enhancing its high temperature properties. The maximum use temperatures of all these resins will typically be 30 to 50 degrees lower than the cited Eg, assuming the same cure schedule. 

D Almeida [40] has reported interlaminar shear strength (ILSS) results for 65v/o Kevlar 49 fibres in DER 383 epoxy resin cured with 27phr DEH 50 aromatic polyamine with a void content below 0.5%. The ILSS values decreased as a function of immersion time, with specimens immersed in distilled water declining faster than those in saline solution, which is absorbed more slowly. When the results were normalized to water content the results were coincident, suggesting that the degradation mechanism was the same in both cases (Fig. 7.7). 

OlPar Park, S.-J., Seo, M.-K., Lee, J.-R., Lee, D.-R. Studies on epoxy resins cured by cationic latent thermal catalysts The effect of the catalysts on the thermal, rheological, and mechanical properties. J. Polym. Sci. Part A - Polym. Chem. 39 (2001) 187-195. 

TABLE 3.14 The Effects of Addition of SKD-KTRA and SKN-3KTR Rubbers upon the Mechanical Properties of Epoxy Oiigomers Based on UP-652 Epoxy Resin Cured by UP-0639... 

Study of the morphological features of ERC indicate precipitation in the course of the epoxy resin cure of particles of the discrete phase of the rubber, the dispersion of which in the epoxy polymer matrix has a considerable effect on the mechanical properties of the latter. If the rubber particles are Imge enough, they can be detected by optical microscopy, but the most important results are obtained by electron microscopy. 

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