Model curing reaction

In order to quantify diffiisional effects on curing reactions, kinetic models are proposed in the literature [7,54,88,95,99,127-133]. Special techniques, such as dielectric permittivity, dielectric loss factor, ionic conductivity, and dipole relaxation time, are employed because spectroscopic techniques (e.g., FT i.r. or n.m.r.) are ineffective because of the insolubility of the reaction mixture at high conversions. A simple model, Equation 2.23, is presented by Chem and Poehlein [3], where a diffiisional factor,//, is introduced in the phenomenological equation, Equation 2.1. 

Kinetic models determine the minimum time required to cure the resin (i.e., guarantee sufficient physical and mechanical properties). They also determine the heat of reaction of the resin for use by heat transfer models and the degree of crosslinking for use in viscosity submodels. The exothermic cure reaction for the transformation of the epoxy resin to the cured matrix polymer can be expressed as ... 

Although the simple rate expressions, Eqs. (2-6) and (2-9), may serve as first approximations they are inadequate for the complete description of the kinetics of many epoxy resin curing reactions. Complex parallel or sequential reactions requiring more than one rate constant may be involved. For example these reactions are often auto-catalytic in nature and the rate may become diffusion-controlled as the viscosity of the system increases. If processes of differing heat of reaction are involved, then the deconvolution of the DSC data is difficult and may require information from other analytical techniques. Some approaches to the interpretation of data using more complex kinetic models are discussed in Chapter 4. 

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). 

Kannebley 30) and Sorokin et al. 31,32) obtained second-order rate constants for the relevant model bimolecular reactions whereas Arnold 19) and Doszlop et al.37) consider curing of epoxy resin with anhydride or reactions of monoepoxides with various proton-donor compounds to be first order (Table 1). Considering the mechanism of individual reactions which proceed during curing or in the non-catalyzed... 

DSC data is commonly fitted to semi-empirical models that accurately describe the curing reaction. Hence, the rate of cure can be described by the exotherm, Q, and the total heat released during the curing reaction, Qt, as... 

With the use of eqn. (2.32), it is now easy to take the DSC data and find the models that best describe the curing reaction. 

Many models exist that are used to predict the curing reaction during processing. All of them are of the form,... 

J.P. Hernandez-Ortiz and T.A. Osswald. A novel cure reaction model fitting technique based on dsc scans. J. Polym. Eng., 25(1) 23, 2005. 

Vulcanisation of EPDM with sulfur systems was studied by H-NMR using ethylidene norbornane (ENBH) as a model of ENB [62]. The use of ENBH was also effective to elucidate the curing reaction of EPDM with phenol-formaldehyde resin [63]. Similarly, halogenation reaction of HR was studied by H-NMR using 2,2,4,8,8-pentamethyl-4-nonene as a model [64],... 

The first attempt to relate changing dielectric properties to kinetic rate equations was by Kagan et al.S9), working with a series of anyhydride-cured epoxies. Building on Warfield s assumed correlation between d log (g)/dt and da/dt, where a is the extent of epoxide conversion, they assumed a proportionality between a and log (q), and modeled the reaction kinetics using the equation... 

Reactions with Model Compounds. To test whether carbohydrates were actually reacting with the phenolic resin, the reaction of methyl xyloside (III) and saligenin (V) under neutral conditions was studied. This reaction system was used as a model for the curing reaction. 

Fedtke, M., Acceleration mechanism in curing reactions involving model systems, Makrom. Kern. Makrom. Symp.. 7,153, 1987. 

Neither model is entirely satisfactory in fitting the experimental data. The complexity of epoxy curing reactions contributes to the discrepancies. Many different mechanisms have been proposed The diffusion controlled nature of the... 

A generalized kinetic model of cure is developed from the aspect of relaxation phenomena. The model not only can predict modulus and viscosity during the cure cycle under isothermal and non-isothermal cure conditions, but also takes into account filler effects on cure behavior. The increase of carbon black filler loading tends to accelerate the cure reaction and also broadens the relaxation spectrum. The presence of filler reduces the activation energy of viscous flow, but has little effect on the activation energy of the cure reaction. 

The model described in Eq.(l) not only can predict the cure behavior measured by standard curometers, but also can explain filler effects on the cure reaction. The model enables one to predict scorch time and cure time of elastomers at various filler loadings and cure temperatures. In the following discussion, this kinetic model of cure will be extended to explain and predict the modulus or viscosity of elastomers/thermosets during... 

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