Water-based coating of metal substrate coatings.

Abstract: Powder coatings are used in many fields for their unique environmental protection and excellent coating effect, but the commonly used electrostitive spraying process produces a lot of dust and higher maintenance costs. In this paper, a water-based coating process of powder coating is mainly studied, focusing on the influence of different desticants and different baking conditions in the process of water-based coating. The results showed that when the water powder ratio was (0.65 to 0.75):1, the coating test plate with a 1.5% added amount of silicone deblister was dried at 60 degrees C and 3min at 10 degrees C. C/min heating rate rose to 180 degrees C, curing 15min, the resulting coating performance in line with HG/T2006-2006 requirements, and static spraying effect is basically the same

. The water-based coating method of powder coating has a short process, simple equipment, convenient operation and no dust pollution, and is an environmentally friendly and effective coating method.
0 Foreword
Powder coating is mainly divided into thermal coating process and cold coating process, including air spraying, fluidized bed immersion, electrostitive powder coating, flame spraying and so on. Among them, thermal coating mainly to fluidized bed immersion method, cold coating process mainly to electrostitectrectectectrectrectrectration spraying. Although each of these two conventional coating methods has advantages, there are some defects. The preheat temperature of the fluidized bed immersion method work piece determines the coating quality, the material and shape of the work piece should be considered when the coating is immersed, and the melting point and demarcation point of the resin should be considered, the labor intensity of the worker's operation at high temperature, the working environment is bad, etc. Static spraying method is easy to produce Faraday effect so that the coating film is not uniform, the paint selectively makes the coating efficiency is not high, high-voltage static electricity safety guarantee, dust makes the working environment of workers poor.
use of water-based coatings has resulted in significant reductions in solvents, significantly reducing emissions of volatile organic compounds during production and use. Water-based coatings with easy-to-source and easy-to-purify water as a solvent to disperse film substances, not only can reduce costs, but also can reduce the fire risk of the production process and dust concentration, improve the safety factor of the working environment of workers. At the same time, water-based coating methods are flexible, can be sprayed, brushed, painted and other ways, easy to achieve automated coating.
in view of this, in view of the coating defects of commonly used powder coatings, it is proposed to mix the original powder coating with the appropriate amount of water and trace additives to explore a new water-based coating process system.
1 Experimental Part
Experimental Method
1.1 Water-based Coating Method
Coating Preparation: Weighing a certain amount of mixed powder coating into a paint container, using a bener to claim a certain amount of water (water powder determined according to the summary of the experiment) Mass ratio (0.65 to 0.75):1, then add the system's total mass of 1.5% silicone emulsion desulbant and 0.06% sodium hexaphosphate, stirred to make a mixture. Pour part of the mixture into a well-called powder coating, stir at 400r/min, and then slowly add the remaining mixture to the mixing coating.
metal substation pre-treatment: the size of 100mm ×200mm×0.3mm of the horse mouth iron sheet with 500 purposes of water sanding paper, and then ultrasonic cleaning 1min;
coating plate: the pre-treated horse mouth iron plate fixed to the coating machine, the coating preparation device placed on the coating position, add about 4g of prepared paint, adjust the coating machine coating speed of 10mm/s, preparation coating. The thickness control after coating curing is 70 to 80 m.
test plate curing: (1) one-step direct curing method, through the water-based coating process prepared coating directly placed in the hot drum air drying box, 180 degrees C curing 10 to 20min after hanging to room temperature to be measured; Curing method, the prepared coating before 60 to 90 degrees C environment drying 3 to 6min, placed in a hot blowing air drying box, to 10 degrees C/min heat up to 180 degrees C, curing 10 to 20min after hanging to room temperature to be measured.
1.2 electrostitive spraying method
pre-treat the soleplate according to the 1.2.1 method. Adjust the static voltage of the equipment, powder flow rate pressure and other parameters, and then adjust the powder chamber to reach the appropriate temperature and humidity. Under the high-pressure electrostitive field, the powder gun is connected to the negative pole, and the bottom plate ground (positive) forms a circuit, so that an electric field is formed between the base plate and the spray gun, and finally spraying begins.
1.3 Performance Test
1.3.1 Adhesion
Reference to the national standard GB/T9286-1998 "Color paint and varnish paint film scratch test", the use of lacquer film lacquer adhesion test.
1.3.2 hardness
the hardness performance of the coating is detected according to GB/T6739-2006 "Color paint and varnish pencil method to determine the hardness of the paint film".
1.3.3 impact resistance
the impact resistance of the coating is detected according to GB/T1732-1993 Paint Film Impact Resistance Determination.
1.3.4 Bend-resistant
detection of bending resistance according to GB/T6742-2007 Color Paint and Varnish Bending Test (Cylindrical Shaft).
1.3.5 Gloss
Based on GB/T9754-2007 "Color paint and varnish without metallic pigments, color paint film 20 degrees, 60 degrees and 85 degrees mirror gloss determination", using a 60 degree gloss meter to test the paint film gloss.
2 results and discussion of the effects of
2.1 desticants in the water-coated coating process
2.1.1 dosage of desulbants
the amount of desobbists has a significant impact on the surface performance of the coating. In the same coating formula according to the total quality of the coating system to add different amounts of silicone emulsion desmodant, curing under the step temperature system, the series of coating surface comparison as shown in Figure 1.
As can be seen from Figure 1, there are obvious pinholes, bubbles and scratches on the coating surface without desolation agent, because the non-defying agent coating is mixed with water to form bubbles, these large unresolve bubbles are driven by the coating preparer to produce scratches, the coating inside the small bubbles will eventually form air holes. With the increase in the amount of silicone emulsion defystant from 1.2% to 1.7%, the bubbles were eliminated, and the coating table was slippery and flat, with no obvious pores. In order to have a stable effect, the amount of deboulant addition was selected to be 1.5%.
2.1.2 Desmoulator Type Impact
Silicone emulsion desmodant, triglyceride phosphate, mineral oil desmoist were selected for comparative experiments, control other experimental conditions unchanged, the addition volume is 1.5%, grinding mixing uniformly, the resulting coating is applied with a coating machine on the thickness of 0.3mm of the horse mouth iron plate, solidified at the step temperature, as shown in Figure 2.
as can be seen from Figure 2, the coating using silicone emulsion desmoulizer is smooth on the surface, no scratches, no bubbles, and the test board surface using triglycerides phosphate and mineral oil desmoist has significant scratches caused by bubble drag. This is because triglycerides phosphate and mineral oil deblisters are incompatible with water, resulting in not being well dispersed in the coating system, and therefore can not fully play its deblistering effect, so the subsequent experiments in this paper choose silicone emulsion desulbant.
when the coating with desmoulizer is further cured directly, a large number of pores appear in the coating due to the instantaneous rapid evaporation of the residual moisture of the coating. Therefore, the thermal re-analysis is used to observe the water change and reaction temperature range of the coating during the heating process. According to the curve to establish the step temperature curing system, the step temperature curing process mainly includes evaporation time, curing temperature, curing time, using a single-factor method, to study the impact of various factors on coating performance.
2.2.1 Thermal Weight Analysis
Figure 3 shows the TG and DTG curves of the coating
as can be known from Figure 3, the quality of the coating in the temperature range of 40 to 90 degrees C changes greatly, so it is considered that the rapid evaporation of water in the 40 to 90 degrees C coating, at 60 degrees C mass peak maximum, you can choose 60 degrees C as the evaporation temperature point. When the temperature is in the 120 to 200 degrees C range, there is again a large mass change, from 120 degrees C this temperature point began to lose mass, indicating that there is a low boiling point of matter waved out, in the 120 to 200 degrees C range to choose an optimal baking temperature point, baking a certain time.
2.2.2 Effect of evaporation time
Control other variables unchanged, the coated test plate is placed in a hot blower drying box at a temperature of 60 degrees C, evaporating 3min, 6min, 9min, respectively, and then heating up to 180C curing 15min, and compared with the coating cured directly at 180 OC without evaporation, as shown in Figure 4.
As can be known from Figure 4, the surface of the coating cured after evaporation is relatively flat, while the surface of the coating without evaporation direct curing has a large number of pores, and the surface is uneven, mainly due to a short period of time temperature rise, moisture evaporated from the inside of the coating to the surface, resulting in a large number of pores, affecting the apparent. Without affecting the coating's surface, it is advisable to consider energy savings and stable selection of evaporation of 3min.
coating samples evaporate by 3min at 60C, equivalent to continued evaporation in the process of rising to 90C (10C/min) in the evaporation interval, and are calculated to evaporate by 3min at 60C and then up to 90C, equivalent to a total evaporation of 6min. In order to further accurate evaporation time range, choose not to stay in the evaporation interval, the sample plate is placed at 40 degrees C, 50 degrees C, 60 degrees C, 70 degrees C evaporation temperature point at 10 degrees C/min heating rate directly heated to 180 degrees C curing, its effect on the appearance of the coating as shown in Figure 5.
As can be known from Figure 5, when the evaporation temperature is 70 degrees C, the surface of the cured coating has a large number of clear pores and uneven, because the moisture of the coating from room temperature to 70 to 90 degrees C environment evaporation, the temperature rise leads to rapid evaporation of water, resulting in pores, and ultimately affect the coating. In the evaporation interval, the evaporation time from 40 degrees C, 50 degrees C, 60 degrees C to 90 degrees C, respectively, is 5min, 4min, 3min, and then heated up to the reaction interval of 180 degrees C curing. Through the above experiments, it can be determined that the evaporation time range is 3 to 6min, and the final effective evaporation time is selected to stay at 60 oC for 3min.
2.2.3 curing temperature
the predicted coating curing temperature point can be set in the 120 to 200 degrees C range by the thermal weight analysis of Figure 3. The coating after evaporation of 3min at 60 degrees C was heated to 120 degrees C, 140 degrees C, 160 degrees C, 180 degrees C and 200 degrees C, respectively, and the optimal curing temperature was determined by adhesion. The results are shown in Table 1.
can be seen from Table 1, when the curing temperature at 180 degrees C and 200 degrees C when the coating adhesion is up to standard, and the curing temperature in the 120 to 160 degrees C range, although the surface is flat and smooth, but the adhesion does not meet the GB/T9286-1998 requirements. When the curing time is the same, when the curing temperature can not reach the coating fully cross-link reaction temperature, the reaction is not sufficient, there will be poor adhesion, poor coating performance. Taking into account energy consumption and cost, the curing temperature is selected at 180 degrees C.
effect of the curing time of 2.2.4
60 degrees C evaporation 3min, curing temperature 180 oC, fixed other amount unchanged, change the curing time is 5min, 10min, 13min, 15min, 17min, 20min, 25min. After curing, the surface of the coating was observed and the coating-related properties were detected, compared with the requirements of HG/T2006-2006, and the results were found in Table 2.
can be known from Table 2, with the increase of curing time, adhesion first increases and then decreases, curing time is 13min, 15min, 17min, 20min, adhesion qualified, it is not easy to judge the best curing time. However, only 15min and 17min hardness is known to pass through the hardness variation curve. When the curing temperature reaches the temperature required for crosslinking, the curing time determines whether the coating meets the performance target requirements. If the time is not enough, affect the physical mechanical properties of the coating, if the curing time is too long, it is easy to cause the coating adhesion, hardness and other mechanical performance decline. According to the above experiments, the curing time at 15min and 17min coating performance is qualified, 15min is the best. The
project is known from Table 2, as the curing time increases, the gloss, impact resistance and bending resistance of the coating show a tendency to rise and fall first, only when the curing time is 15min and 17min when the coating performance is qualified, proving that when the curing temperature reaches the crosslink reaction temperature, the curing time determines the mechanical properties of the coating.
2.3 Water-based coating and electrostitive spraying coating performance comparison
The water-based coating process conditions are: 60 degrees C evaporation of 3min, at a rate of 10 degrees C/min directly heated to 180 degrees C curing 15min to obtain water-based coating coating, with static coating coating performance as shown in Table 3.
by table 3 can be known, 2 kinds of coating process coating performance is basically no difference, all meet the national standard requirements.
3 Conclusion
water-based coating of powder coatings can reduce dust concentration in the working environment. At the same time, in the experiment, it was determined that the bubbles produced during the mixing process of powder and water were removed by using silicone emulsion deblister, and the effect was stable when the amount of silicone emulsion desulbant was 1.5% of the system. Using the step temperature curing system to cure the coating, to avoid the direct curing of the coating caused by bubbles at the same time, the coating surface, adhesion, hardness and other properties can be comparable with electrostectrulation spray coating, all performance is up to standard. However, there are still shortcomings in water-like coating, such as the thickness of the coating is not easy to control, still need further study.
Wen/Xie Wensheng, Song Huiping, Cheng Fangqin, director of
the Institute of Resources and Environmental Engineering, Shanxi
.