1 Introduction
The main function of the coating, on the one hand, is to make the coating products have a good decorative and landscaping effect; on the other hand, it is to improve the coating products rust, corrosion, and protection and other functions. Powder coating is no exception, the same for these two purposes. Powder coating of decoration and beautification, a very important factor is the appearance of the film and color issues. Generally speaking, the color of the coated product is required to be consistent with the color plate or color card provided by the user, and the difference of the color should be within the range of the color difference of the coating film allowed by the user. Especially between different batches of coated products sprayed with different batches of powder coatings; with the same batch of powder coatings, coating different materials, different shapes, different sizes and thicknesses of the workpiece between the color difference of the film, will affect the final assembly of the product film color difference, and ultimately affect the quality of the coated products.
From the powder coating production experience and powder coating plant coating experience shows that, in order to ensure that the powder coating product color difference, to meet the user's requirements, can not rely on powder coating manufacturer or powder coating plant on a party can be resolved, must be produced and the use of powder coatings on both sides of the close cooperation in order to meet the requirements of the color difference of the coating product. Should be from the powder coating formula design and selection of raw materials; powder coating manufacturing process of strict control; powder coating process of strict control of the three aspects *** with the cooperation in order to overcome the problem of color difference in coating products. The following experience from the production practice, the above three issues to talk about our views.
2 Powder coating formula design and selection of raw materials
Powder coating formula design and selection of raw materials is to influence the coating film color stability of the coating products inherent fundamental factors. If this pass is not grasped, then the powder coating of the congenital deficiencies, with the measures to make up for the latter is very difficult to solve, or even impossible to make up for. Therefore, this problem is put in the first place to consider, and then consider the powder coating manufacturing and coating problems that need to be solved.
2.1 Powder Coating Formulation Design
The formulation design of powder coatings has an important significance on the stability of the color of the powder coating film.
(1) First of all, in the formulation design of powder coatings, because the choice of pigment varieties play a decisive role in the stability of the color of the coating film, so we must take into account the high baking temperature of powder coatings, most of the current baking temperature of powder coatings in the 180 ℃ ~ 200 ℃ characteristics of the heat-resistant temperature of the pigment must meet this requirement. For the pigments used in weathering products, the requirements of light resistance and weathering resistance should also be taken into consideration, and it is best for the pigments to reach the requirements of light resistance grade 7 to 8 (grade 8 is the best) and weathering resistance grade 4 to 5 (grade 5 is the best). For indoor products, it is also necessary to take into account that when the painted products are put in the warehouse with sunshine, or temporarily put in the outdoor factory before placing, it will produce the problem of color change of the coating film to affect the color difference, therefore, the light resistance of the pigment can not be too bad.
Secondly, some components in the formula, such as resins, curing agents and additives, etc., poor thermal stability, in the baking and curing process will also make the film color instability, such as epoxy and polyester epoxy powder coatings commonly used in epoxy resins, 2-methylimidazole, ring squinting multicarboxylic acid matting agent in the light-colored powder coatings in the baking and curing of the coating film is prone to yellowing. In order to solve this problem, in the formulation design to change the resin varieties, add anti-yellowing agent, or change to non-yellowing accelerator, or non-yellowing matting resin and other methods to solve the problem of unstable film color difference.
(2) in the powder coating formula design, but also consider the issue of powder coating covering power, should be done to achieve the film thickness to meet the user requirements, the film's covering power to meet the substrate completely covered, so that the color of the substrate will not affect the film color difference. Because the covering power of the powder coating is not good, the thickness of the coating film will affect the color difference of the coating film, so in the design of the formula to take into account, when the thickness of the coating film to achieve the coating thickness, the covering power of the coating film should be good, so that you can avoid the poor covering power of the coating film produced by different thickness of the film of the color difference between the workpiece problem.
2.2 Selection of raw materials
Most of the raw materials used in powder coatings are colorful, and the stability of the quality of these raw materials will affect the stability of the color of the final coated products, among which pigments have the greatest influence.
From the current use of powder coatings in the pigment products, domestic pigments account for a large proportion, but due to a variety of reasons, the same product in different manufacturers, a manufacturer of the same product between different batches of pigment products, the color difference is large, if the production is not in time for the color work, the color of the film on the product to bring about a great impact. With different origins and models of rutile titanium dioxide, respectively, with hydroxyalkyl amide (HAA) and TGIC curing polyester powder coating film color difference of the impact of the experimental results are shown in Table 1. the results show that different manufacturers of titanium dioxide in the white of the same kind of powder coatings in the color difference between some of the film color difference is not big, and some of the difference is big; for the same kind of titanium dioxide in the HAA curing and TGIC curing polyester powder coating in the film color difference is also obvious. There is also a significant difference in film color difference, indicating that the curing agent varieties also have an impact on the film color difference.
Table 1 Effect of different sizes of titanium dioxide on the color difference of the coating film of white HAA and TGIC cured polyester powder coatings
No.
1
2
3
4
5
6
7
Place of production
Zhenjiang
Ishihara
Japan
The color difference of the film of the curing agent is also affected.
Ishihara, Japan
Chongqing
DuPont, USA
DuPont, USA
Imported
Gansu
Model
R940
R930
R244
R902
R706
R822
R215
Polyester/HAA color difference △E
2.34
2.56
2.74
2.97
4.17
4.39
3.60
Polyester/TGIC color difference △E
1.23
1.14
1.50
2.83
2.50
2.51
1.60
In addition, in the formulation of the amount of resin and filler (especially the natural products) of the color change, although not as much as the impact of pigment, but there are varieties of coating film Color difference brings obvious influence. The effects of precipitated barium sulfate, calcium carbonate and kaolin from different manufacturers on the color difference of the coating film in green TGIC-cured polyester powder coatings are listed in Table 2
Table 2 Effects of different varieties and specifications of fillers on the color difference of the coating film of TGIC-cured polyester powder coatings
No.
1
2
3
4
5
6
7
8
Place of Origin
Nanfeng, Shanxi
Xinji, Hebei
Junfeng, Hunan
Xinji, Hebei
Changzhou
Fujian
Henan Gongyi
Jiangsu
Product Name
Precipitated Barium Sulfate
Precipitated Barium Sulfate
Precipitated Barium Sulfate
Heavy Calcium Carbonate
Light Calcium Carbonate
Kaolin
Kaolinite
Kaolinite
Color Difference △E
12.76
12.86
p> 12.65
12.27
12.64
14.05
12.43
-
Gloss
87.0
90.8
89.0
78.0
63.5
66.4
69.3
32.4
From the results of Table 2, it shows that the color difference between precipitated barium sulfate is very small, and the color difference between calcium carbonate is not big, but the color difference between kaolin is very big, and the color difference of coating film is also very big when kaolin is compared with precipitated barium sulfate and calcium carbonate, which shows that the color difference between different fillers cannot be ignored.
In order to avoid the influence of raw materials on color difference, when purchasing raw materials, we should try to purchase the same batch of pigments such as titanium dioxide, etc.; for the pigments with less usage, we should try to purchase the same batch of pigments as much as possible, so as to reduce the number of times of adjusting the color in the production, which is conducive to the stability of the color of the coating film.
If the resin and filler manufacturers to re-select, or the original manufacturers of different batches of products between the color of the color has a significant color difference, in time to raw materials on the color difference of the coating film to do some of the necessary tests and inspections on the formula for the necessary adjustments.
3 Powder Coating Manufacturing Process
In the powder coating manufacturing process, the powder coating film color has an impact on the factors, one is the equipment of the clearing problem; the second is the control of raw materials mixing process; the third is the control of melt extrusion mixing process and other issues.
3.1 Clearance of equipment
In the powder coating manufacturing, the main equipment includes ① high-speed mixer; ② melt extrusion mixer and cooling crushing equipment; ③ micro-fine crushing and grading sieve equipment. When the powder coating color varieties are different at the same time, be sure to carefully clear these equipment, especially the color of the big difference between the varieties should be thoroughly clear, to prevent the former varieties of color contamination of the latter varieties of color. The conditions of the unit, the best according to the deep, medium, light three colors on the production equipment classification after the arrangement of production, so that you can avoid clearing the machine is not complete and brought about by the coating film color difference problem.
3.2 Influence of raw material pre-mixing process
In the manufacturing process of powder coatings, high-speed mixer is the commonly used raw material pre-mixing equipment. The mixing time of the high-speed mixer has an obvious effect on the dispersion of pigments, and this effect makes the color of the coating film of powder coatings with different mixing times have obvious differences, especially the poorly dispersed pigments, such as carbon black, phthalocyanine blue, phthalocyanine green and other pigments are more obvious. Therefore, when manufacturing powder coatings, for the same product, must be fixed mixing time for mixing. Because the mixing effect of different capacity mixing equipment also has a difference, so for the same product, try to fix the production equipment, and must fix the mixing time for production.
3.3 Influence of melt extrusion mixing process
In the manufacture of powder coatings, melt extrusion mixing process is a key role in the mixing and dispersing of materials in the formulation of the process, and the same for the dispersion of pigments play a decisive role. Test results show that the extrusion temperature, screw speed and charging speed all have an effect on the gloss, color and texture of the coating film, of which the screw speed has a greater impact. Therefore, for products with strict requirements on film color, they must be produced under the same process conditions, so as to avoid the color difference between batches as much as possible. If there are conditions, it is better to fix a production line to produce the same product.
4 The impact of powder coating coating process
When the powder coating product quality problems, coating manufacturers often think of the first is the quality of powder coating problems, which is also normal. In fact, powder coatings and coating have an impact on the quality of powder coating products, including powder coatings on the quality of the coating products accounted for fifty percent; powder coating process on the quality of the coating products accounted for fifty percent of the impact of the quality of the products, which accounted for twenty-five percent of the pre-processing, powder spraying and baking curing situation accounted for twenty-five percent. Therefore, the powder coating process on the color difference of the coating film also has an important impact that can not be ignored, specifically the degree of clearing equipment, powder coating effect, baking temperature and time control on the stability of the color difference of the coating film have a certain impact.
4.1 Coating equipment clear machine
In the powder coating, according to user requirements, to often change the color of the coated products, then accordingly on the powder coating varieties or colors are also replaced. When the replacement of powder coating product color color difference is large, the powder spraying system cleaning is not clean, then the old powder on the pollution of the new powder, the coating film color of the coated workpiece interference, affecting the film chromatic aberration. Therefore, when replacing the paint color varieties, be sure to clean the powder spraying room, spray gun, powder pipe, powder supply tank and other powder spraying system, to prevent the old powder on the pollution of the new powder, to avoid the impact on the quality of the coated products. If there are conditions, for products with large color contrast, it is best to paint in different powder spraying rooms or different painting lines, which is conducive to cleaning equipment, is conducive to improving productivity, is conducive to ensuring the quality of the painted product, can meet the requirements of the film color difference.
4.2 Influence of powder coating process
Powder coating process is to ensure that the powder coating film thickness of the key processes, if the film thickness does not reach a certain thickness, it is difficult to achieve sufficient film coverage, then it is easy to bring about the color of the coated products color difference problem. Therefore, in the powder coating, on the one hand, we must ensure that the amount of powder so that the coating film has enough coverage film thickness; on the other hand, the film thickness of each part of the coating film thickness uniformity, especially between the different parts of the film thickness, so as to ensure that the assembly of the color difference between the painted product requirements.
4.3 Effect of baking temperature
The baking temperature of powder coating has a very obvious effect on the color difference of the coating film. Because the powder coating with organic pigments and some inorganic pigments of the heat-resistant temperature has a certain limit, especially indoor powder coating composition, some resins, curing agents and additives of the heat-resistant temperature is not high, so when the baking temperature exceeds the temperature range, it is difficult to achieve the film color difference requirements. Therefore, must control the baking temperature, must ensure that the powder coating formula design range, if more than this temperature limit, the color of the coating film is also easy to exceed the specified color difference range.
4.4 Effect of baking time
The baking time of powder coatings also has a significant effect on the color difference of the powder coating film. This is because there are certain limitations on the heat resistance of the resins, curing agents, organic pigments and additives used in powder coating formulations. Heat resistance includes not only the heat resistance temperature, but also the retention time at that temperature. If the baking and curing time of the powder coating is too long, due to the oxidation of air or thermal decomposition of certain components in the powder coating composition of some of the components in the powder coating chemical changes, and ultimately make the coating film yellowish or due to certain side reactions to make the coating film discoloration, or even the coating film mechanical properties decline, and at the same time, produce and the color difference of the standard color palette problem. Therefore, the control of baking time is as important as the control of baking temperature.
5 Conclusion
The reasons for the color difference of powder coating film are manifold, first of all, the choice of raw materials in the design of powder coating formula, especially the choice of pigment varieties is unreasonable, while the film in the normal thickness of the film there is not enough hiding power? Secondly, is the powder coating manufacturing process, raw material pre-mixing process and melt extrusion mixing process is strictly controlled? Then is the powder coating process, whether the film thickness, baking temperature and time are also very important factors? And also in the powder coating manufacturing and coating process whether the equipment is carefully cleaned is also a problem that can not be ignored. Therefore, it is necessary to do the above things to ensure that the color difference of the coating film of the coated products.
News Audit: China Coating & Chemical Network