Differential scanning calorimetry (DSC) was used TO study the non-isothermal curing reaction of polyester /TGIC aluminum powder coatings, the curing reaction process of powder coatings was analyzed. Through DSC test analysis, it is determined that the curing temperature of the powder coating of the system is about 120℃, the better curing process parameter of electrostatic spraying is 200℃/20min, which provides an important reference for the determination of electrostatic spraying curing process control of aluminum powder coating.
Powder coating is a solvent - free solid powder environmental protection coating, with good decorative, protective high coating efficiency, widely used in the coating of metal surface, especially aluminum. Compared with conventional surface treatment methods such as anodic oxidation electrophoretic coating, powder coating is rich in color, environmental pollution energy consumption are significantly reduced, the mechanical properties of coating film are greatly improved. Therefore, electrostatic spraying of powder coating has become one of the main methods for surface treatment of aluminum profiles. Powder coating curing process, refers to the electrostatic adsorption on the surface of the workpiece under the action of gravity, powder coating layer, through high temperature curing processing to achieve a certain mechanical properties, appearance quality requirements of the coating process, is the aluminum electrostatic spraying products one of the key production process of quality control, the curing process conditions is good bad will directly affect the appearance mechanical properties of aluminum coating products. If the coating solidification is good, the coating coating will crack during extrusion assembly, then the use value of powder coating can be realized. This study by using differential scanning calorimetry of the commonly used polyester/TGIC powder coating of aluminum solidification process was analyzed, discussed the commonly used aluminum polyester/TGIC powder coating curing characteristics of the temperature, isothermal curing temperature, curing temperature on the influence of aluminum solidification process, for the electrostatic spraying curing process control provides an important reference basis, used in the process of actual production furnace temperature tracker testing to prove its correctness.
1.1 curing process
Differential scanning calorimetry (DSC) is widely used in the curing process of thermosetting powder coatings. The powder coating system of special polyester /TGIC (the mass ratio of polyester to TGIC is 0.93:0.07) for aluminum profiles was adopted, the DSC test was conducted at 10 ℃/min at constant temperature. The results are shown in figure 1.
The curing process of powder coating goes through the following several stages: 2 peaks appear in zone A zone B. Respectively with TGIC polyester resin glass transition melting process, the temperature of the top 1, 2, respectively 62. 97 ℃, 98. 8 ℃, area C for powder coating curing process, there are obvious exothermic peak, is 120-200 ℃ temperature area, the bottom of the peak three temperature is about 180 ℃, as the excellent curing powder coatings curing temperature, given that the addition of fillers, the production formula in the production of spraying curing temperature to 200 ℃ is suitable, fully guarantee the crosslinking curing reaction. In the melting extrusion process of powder coating, the extrusion temperature should exceed 120 ℃ to avoid the occurrence of curing reaction which leads to powder coating "dross".
1.2 prediction of curing characteristic temperature
The special polyester /TGIC powder coating system for aluminum profiles was adopted to conduct DSC testing at 5℃/min,10℃/min,15℃/min, 20 ℃/min respectively. The DSC curve is shown in FIG. 2.
With the increase of heating rate, the curing peak of heat flow-temperature curve drifts upwards, the initial curing temperature T0, curing temperature Tp post-treatment temperature Tf all move towards high temperature with the increase of heating rate. The heating rate plotted the characteristic values of curing reaction T0, Tp Tf fitted them linearly. The curves obtained when the heating rate was extrapolated to 0 were shown in FIG. 3.
Curing characteristics of the temperature (TO) initial curing temperature, curing temperature (TP) post-processing temperature (TF) 122 ℃ respectively, 150 ℃, 206 ℃, dynamic speed temperature control for the TO, TP characteristic value cannot be directly used for spraying curing process control temperature, because in powder coating formulation added a large amount of fillers, industrial production in the actual control of a much higher curing temperature, but it provides theory basis for the determination of curing process conditions.
1.3 low curing temperature
To test verify the starting temperature of aluminum electrostatic spraying curing process, DSC test in the following conditions: adopt aluminum with polyester/TGIC powder coating system, selection of curing temperature is 100 ℃, 110 ℃, 120 ℃, respectively in their temperatures down to 30 ℃ after 30 min, again with 10 ℃ / min heating temperature to 250 ℃, the change of heat flow rate - time curve as shown in figure 4.
100 ℃, 110 ℃ has fixed peak of DSC curve, 30 min after 120 ℃ melting temperature curing peak, in the process of the powder coating sample at 120 ℃, 30 min melting has occurred in the process of crosslinking curing reaction, so the aluminum powder electrostatic spraying curing process, to ensure that the material surface effect, the melting temperature, flow process as much as possible under curing temperature to keep for a period of time, in the process of powder coating production melt extrusion temperature must be lower than 120 ℃, can avoid the phenomenon of slagging.
1.4 determination of curing process temperature
The industrial production of electrostatic spraying of aluminum profile powder coating mostly adopts isothermal continuous curing process. The polyester /TGIC powder coating system used for aluminum profile was selected to conduct DSC test on the isothermal curing process under the conditions of 160℃, 180 ℃ 200℃, respectively.
With the same curing time, the curing heat flow rate of polyester /TGIC system gradually increases with the rise of temperature. After reaching a certain curing time, the heat flow rate tends to be stable. After 20 min, the heat flow rate hardly changes with time. Under different isothermal conditions, the heat flux decreases as the temperature increases. The shorter the time it takes to reach the same heat flux, the shorter the curing reaction time is. However, high temperature curing is easy to make the additives in the coating formula (such as deglow-agent benzoin) due to poor heat resistance cause coating film yellowing, low temperature curing time is long production efficiency is low, aluminum profile electrostatic spraying industrial production in the curing temperature time determined as 200℃/20min is more appropriate.
1.5 temperature control during production
The process of electrostatic spraying solidification of aluminum profile powder can be divided into three stages: heating, heat preservation cooling. The heating section is the melting leveling process of powder coating, the better surface effect is obtained. The curing cross-linking reaction of powder coating mainly occurs in the thermal insulation section. Certain temperature time guarantee the complete curing reaction of the coating to achieve the mechanical properties required by aluminum products. A aluminum coating factory production of furnace temperature control curve, using special polyester HT8043SF50 aluminum powder coating, Data tracker testing furnace temperature, preheating temperature profile of 8 min to curing temperature (200 ℃), heat preservation period is 200 ℃, 20 min to ensure curing, 27 min came after cooling, the whole curing process for 30 min, practice has proved its aluminum coating product appearance mechanical properties meet the requirements of production process.
The non-isothermal curing DSC method was used TO analyze the curing reaction process of polyester /TGIC powder coatings, the characteristic parameters of the system were determined by extrapolation of the temperature-heating rate diagram. The gel temperature (TO), curing temperature (TP) post-treatment temperature (TF) were 122 ℃, 150℃ 206℃, respectively.
DSC test was carried out in the isothermal curing process. The actual starting temperature of curing was about 120℃, the curing temperature time were controlled at 200℃ 20 min in the industrial production of electrostatic spraying of aluminum profiles.
The industrial production practice has proved that the solidification process of powder coating is detected by using the Data furnace temperature tracker, the profile is heated up for 8min in the pre-heating melting stage, the solidified insulation section is 200 ℃, cooled out after 20 min, the appearance decoration mechanical properties of the aluminum profile spraying products meet the production process requirements.