Civil Engineering Reference
In-Depth Information
Differential thermal analysis can provide insight into the nature of
some of the reactions. For example, cure temperatures for thermosetting
resins can be determined by DTA. As the formulation is heated, it will reach
a temperature at which the reaction is spontaneous and self-sustaining.
Similarly, the degree of cure can be estimated from the heat effect observed
on heating a partially-cured resin.
[2]
The evaluation of adhesion energies
would require interpretation of differences in curves arising from the
degradation of the bond between the coating and the substrate surface. It is
well known that additional energy due to strain, to mechanical history, or
to thermal history can affect thermal decomposition.
[2]
Failure of coatings due to thermal degradation is a concern because
their applications may involve service at elevated temperature. It is necessary
to have analytical methods that establish a product's useful life expectancy
at temperatures encountered in the field. Thermogravimetric analysis is an
analytical tool that provides information on degradation processes and the
kinetics involved at a given temperature. TG data obtained at different
temperatures can be used to construct master curves by relating them to a
single reference temperature. This can be done through a single Arrhenius
factor that accounts for changes in the time scale. The method of building the
master curve is called
Time-Temperature Superposition
(TTS). The infor-
mation obtained helps in predicting service-life at elevated temperatures.
Neag and co-workers
[22]
reported the application of TG to the service-life
prediction of coatings. They studied coating formulations with different
levels of additive. The coating preparations were aged at 140°, 160°, 180°,
and 200°C in a forced-air oven. Formulations were checked daily for the
onset of change in color, and film blackening was reported as the end of the
degradation. Formulations were studied by TG under isothermal and non-
isothermal conditions. The isothermal conditions consisted of scans at 100°,
110°, 120°, and 130°C whereas the non-isothermal ones consisted of scans
performed at 1°, 2°, 5°, and 10°C/min from room temperature to 600°C
under a gas purge of 50 mL/min. The TG/DTG curves (Fig. 12) obtained at
1°C/min show the failure and several other features of the degradation of the
formulation.
The Time-Temperature Superposition (TTS) method
[23]
for moni-
toring cure behavior of coatings was adapted to predict the rate of degrada-
tion between 60° and 93°C and to obtain the degree of degradation curves.
From the isothermal data, the endpoints between 100° and 130°C of the
DTG curve in Fig. 12 can identify the onset and endpoint of the “cata-
strophic” failure process. By using Flynn's, et al.,
[24]
and Toop's
[25]
meth-
ods, the activation energy and estimated life-time of the material from the
