Nonisothermal cure

Arrhenius form, to obtain an expression which can predict viscosity-time behavior at other temperatures. If temperature is allowed to be a function of time (nonisothermal cure), Eq. 3 results, due to Roller.(J )... 

The parameters of the model are the activation energies for viscous flow (AE ) and reaction rate ( Ej ) their respective pre-exponential erms. This equation provides a predictive, analytical expression with which one can model nonisothermal cure using any appropriate time-temperature function [T(t)] one chooses, appropriate to the curing process of interest. For example, the baking of a coated substrate in an oven may be modeled by a relaxation-type heating function, with time constant (t) to take account of the thermal inertia of the substrate. (Eq. 4)... 

The curing of a thermoset epoxy resin can be expressed in terms of a time-temperature-transformation (TTT) diagram (Fig. 10) (149,150). Later, a CTP (cure-temperature-property) diagram was proposed as a modification of the TTT diagram (151). For nonisothermal cure, the conversion-temperature-transformation (CTT) diagram has been shown to be quite useful (152). In the TTT diagram, the time to gellation and vitrification is plotted as a function of... 

For nonisothermal cure experiments it is not necessary to perform the experiment in heat-only conditions. However, for curing experiments, it is advisable to use larger modulation amplitudes with low underlying heating rates, to improve the accuracy of the heat capacity measurement. The amplitude of the temperature modulation should not exceed a certain value in order to limit its effect on the cure kinetics. Typical modulation amplitudes range within 0.1-1 °C. 

Figure 2.114. Nonisothermal cure at 1 °C/min of a stoichiometric DGEBA + MDA mixture (a) nonreversing (NR) heat flow (b) glass transition as a function of temperature calculated from the total heat flow signal (c) heat capacity (reversing signal) for the first (1 ) and second (2 ) heating (l°C/60s modulation period) [reprinted from Swier et al. (2004) with permission of John Wiley Sons, Inc.].

An example relating some of the features of nonisothermal cure discussed here is provided by Patel et al. (1989). These authors describe the dynamic mechanical behavior at constant heating rate of an epoxidized Novolac-anhydride system both uncured and at various stages of cure. The different degrees of cure were imparted by curing isothermaUy at the temperatures indicated on each curve. Samples coated on a wire-mesh support were char-... 

Bmardic, M. Ivankovic, H. Ivankovic, and H. J. Mercer. Isothermal and nonisothermal cure kinetics of an epoxy/poly(oxypropylene)diamine/octadecyl-ammonium modified montmorillonite system. Journal of Applied Polymer Science, 100 (2006), 1765 1771. 

An improvement in the model predictability is expected by allowing parameters D and E in Eq. 23.130 to change with temperature during nonisothermal cure. Furthermore, both of these parameters also show some variation with the volume fraction (shear rate Ay/At). A modified comprehensive model was thus proposed as follows (Zhou and Sancaktar 2008) ... 

DSC scans of nonisothermal curing processes of (a) nano-Al203/epoxy, (b) nano-Si3N4/epoxy, and (c) nano-SiC/epoxy composites at a heating rate of 7.5°C/min. 

Nonisothermal curing behavior of epoxy and its composites at a heating rate of 2 C/min (reprinted from Macromol. Mater. Eng., vol. 287, M. Q. Zhang et a/., Improvement of the tribological performance of epoxy by the addition of irradiation grafted nano-inorganic particles, p. 111,2002, with permission from John Wiley Sons, Inc.). 

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