Principle
Differential scanning calorimetry is a thermal analysis technique that studies the physical transformation and chemical reactions of materials by measuring the heat flow difference between a sample and an inert reference under programmed temperature control conditions with temperature or time. For thermosetting materials such as epoxy resin, the curing reaction that occurs during the heating process is an exothermic process, and the differential scanning calorimeter can accurately record the heat flow curve of the reaction, thereby providing core data for reaction kinetics analysis.
Theoretical basis
Curing kinetics is designed to describe the relationship between reaction conversion rate and temperature, time. The following basic kinetic equations are usually employed:
dα/dt = k(T) f(α)
where α is the reaction conversion rate, t is the time, T is the absolute temperature, and k(T) is the reaction rate constant, following the Arrhenius equation: k(T) = A exp(-Ea/RT), A is the pre-index factor, Eais the apparent activation energy, and R is the gas constant. f(α) is the function of the reaction mechanism. These kinetic parameters can be solved by differential scanning calorimetry experimental data.
Experimental methods
Typical kinetic analysis experiments use dynamic heating mode. Uncured epoxy samples were scanned at several different constant heating rates (e.g., 5, 10, 15, 20 K/min) to obtain a series of cured exothermic peaks. Peak temperature T at β per warming ratepData α conversion rate is the basis for subsequent calculations.
Common analytical methods include Kissinger, Ozawa, and Crane. The Kissinger method calculates the activation energy by changing the peak temperature at different heating rates, and its formula is:
ln(β/Tp2) = ln(AR/Ea) - Ea/RTp
Ln(β/Tp2) to 1/TpThe plot can be obtained by using the slope of the straight linea。 The Ozawa method provides an integral method for supplementary validation. Crane's law is often used to estimate the reaction series n.
Results and discussion
By using the above methods, key parameters describing the curing reaction of epoxy resin can be obtained. These parameters are instructive for understanding curing behavior and optimizing process conditions (e.g., temperature, time). For example, higher activation energy means that the reaction is more sensitive to temperature and requires more precise temperature control. Epoxy systems with different formulations, such as with different curing agents, exhibit vastly different kinetic parameters, reflecting differences in their reaction mechanisms.
The following table shows an example of a hypothetical epoxy system kinetic analysis:
| Heating Rate (K/min) | Peak temperature Tp (K) |
| 5 | 412.5 |
| 10 | 425.8 |
| 15 | 434.2 |
| 20 | 440.7 |
| Apparent activation energy Ea (Kissinger method) | 65.3 kJ/mol |
| Reaction series n (estimated) | ~0.9 |
It should be noted that a single kinetic model may not fully describe the complex curing process. The use of equal conversion rate methods (such as the Friedman method) can verify whether the activation energy is constant at different conversion rate stages, so as to judge the complexity of the reaction.
Conclusion
Differential scanning calorimetry is an effective tool for studying the curing kinetics of epoxy resins. By designing a reasonable dynamic heating experiment and using a variety of kinetic analysis methods, the key parameters such as apparent activation energy, pre-index factor and reaction series of the reaction can be reliably obtained. These studies provide important theoretical basis and data support for in-depth understanding of epoxy resin curing mechanism, predicting curing behavior, and guiding actual production processes.
References
Kissinger, H. E. Reaction Kinetics in Differential Thermal Analysis. Analytical Chemistry, 1957.
Ozawa, T. A New Method of Analyzing Thermogravimetric Data. Bulletin of the Chemical Society of Japan, 1965.
Prime, R. B. Thermosets. In Thermal Characterization of Polymeric Materials; Turi, E. A., Ed.; Academic Press: New York, 1997.