Epoxy powder coating curing dynamics DSC study

China Coatings Online News Information
The curing dynamics of an epoxy powder coating in the dynamic heating process were studied at different heating rates, and the curing dynamic equations were studied using the Kissinger equation and the Crane equation. The active energy E, the pre-factor A and the reaction series n in the parameters are calculated, and the active energy E is verified by Oazwa method, and the results show that the curing reaction of this epoxy powder coating conforms to the n-order curing dynamic curing model. It is also verified that this equation can reflect the actual curing reaction process more truthfully, and can provide theoretical basis for determining the curing process.foreword
epoxy powder coating is a widely used thermosoated powder coating with excellent bonding, electrical insulation and corrosion protection. It is widely used in high-tech fields such as electrical, anti-corrosion pipes and bridges. Based on its excellent performance, the curing dynamic analysis of epoxy powder coating is helpful to better study its curing conditions and curing behavior, to establish a curing dynamic model by measuring the thermal effects in the curing process, and to predict the process of curing reaction in theory, so as to reduce the amount of experiments required to optimize the curing process.experimental part
1.1 raw
epoxy resin, a certain brand of epoxy resin. Curing agent, imported curing agent.
1.2 analysis instrument
the U.S. TAQ200 differential scanning thermal instrument.
1.3 sample preparation
powder amount of 10±1 mg, placed in the standard aluminum crucible, sealed.
1.4 test method
the temperature at a temperature of 5 degrees C/min, 10 degrees C/min, 15 degrees C/min, 20 degrees C/min, 25 degrees C/min, 30 degrees C/min to dynamically change the temperature of the resin system.
temperature control procedure is used to heat up from 25 to 80 degrees C, to cool down quickly to 25 degrees C, and then from 25 degrees C to 280 degrees C.
tests are performed in a nitrogen atmosphere with a nitrogen flow rate of 50 ml/min.results and discussion
2.1 curing process temperature prediction
experiments using different heating rate scanning of samples, the results are shown in Figure 1. The measured thermal peak characteristic temperature (i.e. starting temperature Ti, peak temperature TP, termination temperature Tf) is listed in Table 1.
shown in Figures 1 and 1, the characteristic temperature (starting temperature Ti, peak temperature TP, termination temperature Tf) in the curing reaction is closely related to the rate of warming. Heat peak characteristic temperature with the heating rate is linear relationship, with the increase of heating rate, the system's curing starting temperature and peak temperature are increased, moving to high temperature, curing temperature range is wider, because the heating rate increases, so that dH/dt increases, that is, the thermal effect per unit of time increases, but the system absorbs energy time is shorter, that is, the reaction lag is more, so the heating peak characteristic temperature will increase accordingly, the curing reaction heat peak will move to the high temperature.
the characteristic temperature of the curve heating peak varies significantly with the rate of warming. Curing reaction is generally carried out under constant temperature conditions, in order to β eliminate this effect, the curing process temperature is often determined by T-β extraterression, that is, through the characteristic temperature T and heating rate β At 0 o'clock, the values of the points on the ordinate axis can be approximated: Ti, 91.71, Tp, 142.41, and Tf, 167.75, respectively. The three can be approximated as epoxy powder coating gel temperature, curing temperature and post-treatment temperature. Data processing is shown in Figure 2.
2.2 Curing dynamics parameters calculation
the surface active energy E, refers to the front factor A, reaction series n, is the most important parameter of reaction curing dynamics, through some column methods to finally find the above three factors (E, A, n), determine the curing dynamics equation, through the equation to predict the curing process, which is also the most important purpose of curing dynamics analysis.
Osoactive energy E is an important parameter to measure the activity of curing reaction of curing system, its size determines the degree of difficulty of curing reaction, curing system can only be carried out if it obtains energy greater than the surface activator energy, curing reaction can be carried out; The more collisions there are between active molecules, the easier the reaction will be, the more intense the reaction will be, and the faster the reaction speed will be; the reaction series n is a macro-symptom of the complexity of the reaction, determined by the type of chemical reaction in the reaction process and the interaction between the reactions, and the process of curing reaction step can be roughly estimated the structure of the curing reaction.
experimental data processing is shown in Table 2.
(1) for reseeding energy E
Kissinger is the most commonly used method at present, assuming that epoxy curing conforms to the n-order curing dynamics model, i.e., the curing dynamics equation:
Kissinger method expression is expressed as:
(2) reaction series n
after the active energy E, the reaction series n is determined using the Crane formula, i.e.:

<2> (3) refers to the pre-factor A
refers to the pre-factor A can be derived from the Kissinger method approximate formula:
(4) Ozawa method verifies
the above hypothesis of epoxy powder curing dynamics, can be used another dynamics "three-factor" method, namely Ozawa method. Compared with other methods, the advantage is that it avoids the experimental error caused by the different assumptions of the reaction function, so it can be used to test the active energy values found by other methods. The Ozawa formula can be expressed as:
the resulting dynamic parameters are listed in Table 3.
can be derived from the above curing reaction rate dynamics expression:
oscic active energy E-63.797kJ-mol-1 is small, the reaction is easy to carry out.
reaction series n s 0.8977 non-integer, indicating that the epoxy powder curing reaction is a complex reaction process.
rate increases with the increase of temperature, and with the increase of curing degree, i.e. the curing time, the curing reaction rate decreases, which can be explained as the decrease of the relative concentration of curing system reactants. The curing temperature and curing time compete with each other and affect the curing reaction rate together.
2.3 Application of curing dynamics equations
2.3.1 curing degree mathematical model
curing degree (or conversion rate)α, the degree of macro-reaction curing reaction can be determined, thus determining the performance of curing coating film. The mathematical model of curing degree obtained by using the above-mentioned curing dynamic equation is also one of the methods of indicating curing degree. By integraling formula 6 obtained above, the dynamic model of curing reaction curing degree of this epoxy powder coating is obtained, namely: formula
(7) reflects the relationship between curing degree α and curing temperature T and curing time t, through which the relationship between curing α and curing time t can be obtained under any temperature T of this epoxy powder, so as to predict the actual curing reaction process. Table 4 is the "curing degree-time" data at different temperatures, as shown in Figure 6 according to the "equal temperature curing degree-time" curve fitted by Table 4. It can be seen from the figure that in order to achieve the same curing degree, two ways can be used to extend the reaction time at low temperature and increase the reaction temperature.
2.3.2 Verification
For this epoxy powder, a cure degree of α>99% is actually required, and if the curing temperature is set at 230 degrees C, the theoretical time to reach this curing degree is at least 45s or more. In practice, factors such as the substrate, shape, thickness, heat transmission performance and the difference between the set temperature of the oven and the oven and the actual furnace temperature are taken into account in the actual operation. In addition, taking into account the appropriate increase in curing time, can improve some of the performance of the coating film, but not too long, otherwise it will cause the coating aging, poor performance and unnecessary heat loss. Taken together, the curing process is set at 230 degrees C/90s. According to this process to make a coating film, its curing degree is verified, the results can be seen in Table 5.
the actual results are basically consistent with the theoretical prediction, which shows that the curing reaction dynamic equation obtained by dynamic test can reflect the actual curing reaction process of the system and provide a strong basis for determining the curing process.Conclusion
3.1 The curing reaction dynamic equation obtained from this dynamic test of epoxy powder is:
3.2 The curing reaction stage n of this epoxy powder is 0.8977, indicating that the curing reaction is a complex reaction.
3.3, the results show that the curing reaction dynamics equation obtained by dynamic test can reflect the actual curing reaction process of the system.
the theoretical analysis and practical application of the above-mentioned curing reaction dynamic equations, which is of great significance to the construction of epoxy powder.