Chemistry Reference
In-Depth Information
By extrapolating the nonisothermal results to infinitely slow heating rates, i.e., zero
heating rate, the dependence of cure kinetics on heating rate could be eliminated [2].
Since zero heating rate is equivalent to isothermal conditions, the calculated curing
temperature at zero heating rate can be used as a reference for the selection of curing
temperatures in the study of isothermal curing [3, 4]. The temperature range within
which cure reactions occurred at zero heating rate was between 119 and 149 °C
(
Table 1.1
). This temperature range coincided with the curing temperature range of
conventional epoxies.
Figure 1.3b
shows the progress of reaction conversion with
temperature at different heating rates. The S-shaped curves of the conversion (a)
versus
temperature indicate that the cure reaction was autocatalytic [5]. The slope
reached a maximum in the range of low to medium conversions. This result indicated
that the reaction intermediates accelerated the cure reaction. At higher conversions,
the linearity is lost due to the decrease in reaction rate.
Figure 1.3c
shows the cure
rate as a function of curing temperature at different heating rates. The maximum
reaction rate occurred around the peak exothermic temperature and increased with
heating rate. This result was because the curing temperature shifted to higher levels
as the heating rate increased, and the cure reactions were accelerated with increase in
curing temperature. Since there was no crosslinked network structure formed in this
model reaction system, the slowdown of the reaction in the high conversion region
was probably due to the decrease in the reactant concentrations. The enthalpy (DH) of
the cure reaction generally increased with heating rate up to 10 °C/min, then showed
a significant decrease at 20 °C/min. The dependence of cure reaction enthalpy on
heating rate is supposed to be of a chemical nature [3]. The curing of epoxy involves
a sequence of elementary reactions, and these elementary steps and reaction pathway
are temperature-dependent. Because the curing occurred at higher temperatures at
the heating rate of 20 °C/min, the relatively low enthalpy was probably related to
the different reaction pathways involved at the higher cure temperature.
a
20 ºC/min
10 ºC/min
5 ºC/min
2.5 ºC/min
80
100
120
140
160
180
200
220
240
Temperature (°C)
