Curing of a Large Body of Epoxy Resin

see also: Thermal Simulations, Algorithm of Thermal Simulations, Application Zoalene

During the curing of epoxy resin a large amount of heat is generated: approximately 350 J/g. Under adiabatic conditions and a thermal capacity Cp of 2 J/gK the heat generation results in a temperature jump of 175¡C. Because the decomposition of epoxy resins starts at 230¡C, the start temperature (for the adiabatic case) must be lower than 55¡C.

In the following sample, the curing kinetics are determined for a composite containing epoxy resin and a filler. On the basis of kinetics analysis and of caloric data as heat capacity, heat conductivity over the course of temperature is calculated for specific conditions, especially the thermal coupling at ambient temperature.
The goal of this work is the check of maximum temperature which is achieved for the specific conditions.

Kinetic analysis of DSC measurements

Kinetic parameters of the best model
lg A1/s^-1:10.69
React.ord 1:1.36
lg A2/(kJ/mol):6.04
React.ord 2:0.91
lg A3/(kJ/mol):8.82
lg Kcat 3:0.70
FollReact. 1:4.50E-02
FollReact. 2:0.776
Area 1 ..4/(J/g):-287.0


Simulation of self-heating

On the basis of results of kinetic analysis and conditions of reactor, the simulation is performed.

Conditions of simulation
Reactor type:cylinder
Transfer Coeff/(W/cm^2K):1.36E-3
Start temperature/¡C56
Heat conductivity/(W/cmK): 0.0025


Conditions of DSC measurements
Instrument:NETZSCH DSC 204 Phoenix©
Heating rates/(K/min):1, 2,5, 5, 10
Sample mass/mg:4 .. 5
Crucible:Aluminium, pierced
Using a triple-step model, a useful fit-quality is achieved

Temperature vs. time at different distances from the center.

The self-heating starts very slowly. In the center the curing reaction is finished first. Now the heat is transferred more to the cold border. Because the curing reaction at the border starts from a higher temperature value, here the maximum temperature is located.
In order to achieve a full curing after a time of 12 hrs the ambient temperature is increased to 140 ¡C.

Temperature vs. time at different distances from the center.

3D-plot of self-heating

In the 3D plot the general behavior is more clearly depicted. The maximum near the border and the general decrease of temperature after full curing is visible.
This picture demonstrates the problems which are combined with the curing reaction of a large body: the behavior in the center is very close to the behavior of an adiabatic system. The jump in temperature is approximately Delta T = Heat/Cp. Unexpectedly, the critical position is near the border.

3D-plot of self-heating: temperature as function of time and distance from center
Temperature profile of cylinder with curing epoxy at 11.hrs.
Temperature profiles of cylinder with curing epoxy at times from 10.5 to 16 hours