Exothermic curing reaction

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 ... 

Fig. 17 Schematic representation of the temperature gradient occurring during the exothermic curing reaction. Owing to the comparatively high thermal conductivity of the continuous metallic filler (e.g. wires consisting of copper), exothermic reaction heat Q is conducted away from the interface. By changing locally the thermal conditions of the curing reaction, the temperature gradient T(N) may influence the resulting network structure D (N) which in turn defines the elastic properties of the epoxy, e.g. its elastic modulus E(N). jw denotes the heat current density...

Apart for exothermic curing reactions, another thermal effect has to be considered, due to the interaction of ionizing radiation with matter. The temperature profile depends on the balance among (on one hand) the rate of heat production, due both to curing reactions and radiation absorption, and (on the other hand) the heat released, in unit of time, by the system toward the environment. Taking constant the geometry of the reacting system, the heat released toward the environment is constant, while the heat production increases with the pulse frequency. 

Heat evolves from the exothermic curing reaction as soon as the temperature is high enough this heat is responsible for another increase in temperature of the rubber. 

The process of cure in the sample set in the calorimeter is similar to that taking place in a mold. The rubber should be heated to a temperature at which the exothermal cure reaction starts, provoking an increase in temperature through the rubber, as well as a heat flux on the rubber surfaces. It should be especially mentioned that the surface must be kept in contact with the sensible part of the calorimeter. We will consider the case of each of the two types of calorimeters. 

It is apparent that plasticizers are mostly used in epoxy resins to reduce viscosity. At the same time they increase the mobility of reacting components and help in dissipation of thermal energy formed from exothermic curing reactions. 

Although the temperature rise due to exothermic curing reaction and mold heating is not significant in RTM for typical resin systems (typically 100°C above the room temperature, or less), this may shorten the mold life especially if the mold is nonmetaUic (such as composites). 

New epoxies with very low thermal expansion coefficient have been developed for fabricating composite molds which significantly reduce the shrinkage due to mold heating/coohng and due to the exothermic curing reaction. 

Second, the exothermic curing reaction releases heat and alters the isothermal conditions as a result, it is essential that the test samples have a large surface-to-volume ratio to ensure heat transfer. 

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