Date: 2020-10-15
introduction
Powder coating, as a new type of environmentally friendly product developed due to its pollution-free, resource-saving, and high-efficiency characteristics, has a history of more than 50 years of development and application in China. In recent years, with the driving force of national policies on environmentally friendly coatings, the rapid development of powder coatings has been further promoted. The application end has also put forward higher requirements for powder coatings, such as improving the clarity of wood grain transfer printing, increasing the hardness of coatings, enhancing the corrosion resistance, aging resistance, boiling resistance, impact resistance, and so on.
Powder coating engineers often focus on adjusting resins and additives when improving coating performance, while neglecting the assistance of fillers. Research has shown that the use of special pigments is also essential in the formulation process of functional powder coatings. If selected properly, it can endow powder coatings with special effects.
Research on the application of commonly used fillers in powder coatings shows that silicon micro powder has the best comprehensive performance, followed by mica powder. The comprehensive performance of talc powder and barium sulfate decreases in order. Among the tested fillers, calcium carbonate has the worst comprehensive performance. At present, silicon micro powder has been widely used in functional powder coatings such as heavy-duty anti-corrosion, high temperature resistance, insulation, and flame retardancy. However, it is rarely applied on a large scale in building material powder coatings, which have a large market share. This is mainly because although silicon micro powder (SiO2) is a non-metallic material with high hardness, low thermal conductivity, high temperature resistance, insulation, and stable chemical properties, most of the silicon micro powder on the market is obtained through low-cost physical methods, with a Mohs hardness of 7. This cannot be ignored for the wear and tear of processing equipment such as powder coating extruders and mills, which will increase the overall cost of their large-scale application in the field of building material powder coatings and not be worth the loss.
Composite inorganic powder is a filler used for copper-clad laminates, which has now widely replaced ordinary silicon micro powder in the production and processing of copper-clad laminates. The application of composite inorganic powder benefits from its special processing technology, which reduces the Mohs hardness and avoids the problems of increased hardness of copper-clad laminates caused by ordinary silicon micro powder, accelerated tool wear, white edge phenomenon, and significantly increased production costs in subsequent cutting processes. The product has excellent physical and chemical properties such as high whiteness, ultra purity, excellent particle size control, low electrical conductivity, strong weather resistance, high resistance (acid, alkali, salt spray, etc.), low coefficient of expansion, and high dielectric constant.
Given the functional application of silicon micro powder and the experience of composite inorganic powder in the field of copper-clad laminates, this study conducted preliminary exploratory experiments on the use of composite inorganic powder in powder coatings based on its existing experimental conditions.
1、 Experimental section
1.1 Raw materials and instruments
The main raw materials include carboxyl terminated polyester resin, TGIC (trichloroisocyanurate), rutile titanium dioxide, precipitated barium sulfate, leveling agent, dispersant, deaerator, matting agent, and other commonly used materials for outdoor powder coatings.
The experimental equipment includes laboratory twin-screw extruders, crushers, electrostatic powder spraying equipment, constant temperature ovens, etc. The instruments used include laser particle size analyzer, electronic balance, pencil hardness tester, gloss meter, paint film impactor, etc.
1.2 Preparation of Composite Inorganic Powder
In the first stage, raw materials are weighed according to their mass percentage composition, with priority given to purified mineral powders such as quartz, alumina, calcite, fluorite, etc. After mixing evenly, they are added to a high-temperature tank furnace for melting. The flame temperature of the tank furnace is 1500~1560 ℃, and the molten liquid flows out through the flow channel for water quenching, draining and drying
In the second stage, the dried block material is ground using dry grinding technology, and then finely classified into ultrafine powder with the target particle size. The raw materials that pass the inspection in the first stage will be put into a ball mill for grinding. This grinding process can continuously feed and discharge materials, or can input several quality raw materials at once and continuously grind for several times before discharging; When discharging, the particle size needs to be controlled by a micro powder classifier. Coarse products are returned to the mill for further grinding, while fine products are the final product. Dry grinding requires strict control of the moisture content of the material, so that the product is no longer dry.
1.3 Preparation of Powder Coatings and Coatings
Using the same processing method as conventional powder coatings, according to the experimental formula, the composite inorganic powder is physically mixed with resin, additives, pigments, and other fillers, and then extruded using a twin-screw extruder. The temperature in zone I is 90 ℃, and the temperature in zone II is 110 ℃. The flakes are crushed by a crusher and sieved through a 180 mesh screen to obtain the finished product. Spray the finished powder coating onto 25mm× using an electrostatic spray gun; Place the sample on a 16mm steel plate and bake it in a constant temperature oven at 200 ℃ for 10 minutes. After removing the steel plate, cool it to room temperature and test the basic properties of the coating.
2、 Results and Discussion
2.1 Technical indicators of composite inorganic powder and its feasibility as a universal filler
The common component of silicon micro powder on the market is SiO2, which is a white powdery solid. The composite inorganic powder described in this study is processed by refining several minerals, compounding them, high-temperature melting, quenching them into amorphous materials, and then finely grinding and grading them. In terms of chemical composition, the SiO2 content in ordinary silicon micro powder is greater than 99.0%, while the SiO2 content in the composite inorganic powder described in this study is between 60% and 70%. The decrease in SiO2 content effectively reduces the Mohs hardness of the powder.
The physicochemical properties of the composite inorganic powder are shown in Table 1. From Table 1, it can be seen that the powder has the characteristics of high whiteness, high purity, excellent particle size control, low conductivity, low impurity content, and good control of magnetic impurities. As mentioned above, the composite inorganic powder is different from the silicon micro powder circulating on the market, with a Mohs hardness ranging from 5 to 6; The common silicon micro powder on the market is made of natural quartz (SiO2) or fused silica (natural quartz is amorphous SiO2 after high temperature melting and cooling) through crushing, ball milling (or vibration, airflow milling), flotation, acid cleaning purification, high-purity water treatment and other processes, with Mohs hardness of 7; The Mohs hardness of barium sulfate commonly used in powder coatings is 3-4. From the comparison of the Mohs hardness of the three types of powders mentioned above, the Mohs hardness of the composite inorganic powder is between that of silicon micro powder and barium sulfate, so its wear on powder coating production equipment is also between the two. Considering the comprehensive cost-effectiveness of coating performance improvement and equipment damage, this powder can be used as a universal filler in powder coatings.
2.2 Application of Composite Inorganic Powder in Powder Coatings
In specific experiments, we compared the composite inorganic powder (referred to as CM powder) with barium sulfate and preliminarily tested its application characteristics in three fields: super weather resistant powder coating system, transparent powder coating system, and heat transfer powder coating system. We also briefly introduced the application results in other fields.
2.2.1 Application in the field of super weather resistant extinction
Because the addition of CM powder reduces the gloss of the coating, the barium sulfate solution is combined with physical matting agent DT3329-1, using DSM's P6600 weather resistant resin as the main resin. Outdoor matting powder coating is formulated according to the formula in Table 2 and sprayed onto the board. Table 2 lists the coating test formula and routine testing indicators.
In the conventional super weather resistant formula system, replacing 20% barium sulfate with CM powder can achieve a paint film hardness of 2H. However, the scheme that uses all barium sulfate only has a paint film hardness of F, which means that CM powder can significantly improve the pencil hardness of the paint film. This is mainly due to the high Mohs hardness of the CM powder itself, which increases the hardness of the paint film; On the other hand, considering the influence of inorganic powder particle size on paint film gloss, research has found that when the particle size D50 of CM powder is controlled at 12μ m. D99 is controlled at 34μ When adding 20% CM powder, the gloss of the paint film can be significantly reduced. The test results show that its extinction performance can reach the same effect as adding 1.5% DT3329-1 extinction agent. The extinction characteristics come from precision machining equipment and strict grading control.
At present, the evaluation methods for the weather resistance of powder coatings mainly include UV aging, xenon lamp aging, and natural exposure testing. Research has shown that the loss of gloss in polyester powder coatings is mainly caused by ultraviolet radiation. In order to save testing time, this study used a UV-C lamp for accelerated aging testing. Compared with UVA (315-400nm) and UVB (280-315nm), UVC has a shorter wavelength (100-280nm), higher photon energy, and greater destructive power to the resin.
After adding CM powder to the sample, QUVC still maintains a light retention rate of>95% after 25 hours of aging; The sample without adding CM powder and using DT3329-1 extinction had a light retention rate of<95% after 10 hours of irradiation,<85% after 15 hours of irradiation, and<80% after 25 hours of irradiation. The coating with 20% CM powder has better light retention in UV testing, with a loss of light rate of less than 5% throughout the entire testing cycle, which is significantly better than the combination of barium sulfate and matting agent. The self-made composite inorganic powder CM helps outdoor products achieve better weather resistance and can assist the super weather resistant resin system powder coating in achieving higher weather resistance requirements.
The main reason for this is that CM powder has higher purity, good chemical stability, and no volatile metal elements under the action of ultraviolet light, making it more stable to ultraviolet light. In addition, it forms a denser and more uniform microstructure of the paint film to assist in improving weather resistance. In the above comparative tests, the weather resistance was significantly improved, and another reason is that CM powder has a certain matting effect on the paint film due to its fine particle size control. Adding 20% CM powder can matting up to 55 ° C; The scheme of using only matte barium requires the addition of a physical matte to achieve this gloss, and the addition of a physical matte compound of organic zinc salts and wax powder also causes a decrease in the coating's light retention rate during UV testing.
2.2.2 Application in Transparent Powder Coatings
Transparent powder coating is mainly used for light protection of automotive, motorcycle parts, metal crafts, etc. The formula of transparent powder coating usually does not add fillers such as barium sulfate, as their addition will affect the transparency of the coating film. To ensure the high transparency and stability of the coating, the refractive index of the filler used in the formula should be as close or consistent as possible with the refractive index of the resin. Currently, the refractive index of most synthetic resins is 1.43-1.48. Usually, the refractive index of inorganic non-metallic materials is above 1.5, and to control their refractive index between 1.43 and 1.48, special formula design must be carried out for inorganic non-metallic materials to reduce their refractive index.
A comparative test was conducted using CM composite inorganic powder produced by Jinyi Company and barium sulfate as fillers. In terms of surface transparency, the addition of 20% CM powder resulted in the best transparency, with a clear and visible metal substrate and a slightly blue phase; The sample corresponding to 20% barium sulfate has the worst transparency, poor visibility of the metal substrate, and a yellow turbid coating; When 10% CM and 10% barium sulfate are used together, the transparency of the corresponding sample is centered. In terms of gloss, the coating with 20% CM powder has the highest gloss, mainly because the high transparency of the coating has little effect on the reduction of incident light, and the reflection light of the metal substrate and the surface of the coating overlap; The sample corresponding to 20% barium sulfate has the worst gloss, mainly because barium sulfate itself has a certain coloring effect, and the paint film is slightly yellow. There is no superposition effect of reflected light from the metal substrate during the light reflection process.
From the above discussion, it can be seen that CM powder can be used as a filler to prepare transparent powder coatings due to its similar refractive index to resin. The amount of addition needs to be explored according to the actual situation. The good transparency of CM powder is due to its special melting process and formula design during production. If one of the two is not suitable, the refractive index of the prepared composite inorganic non-metallic powder will deviate significantly from the target value.
2.2.3 Application in Wood Grain Transfer Printing Powder Coatings
Thermal transfer printing, also known as sublimation, is a special process in which various patterns are printed on high-precision paper using transfer ink, and heated to a set temperature in a short period of time using thermal transfer equipment, so that the patterns on the transfer paper are transferred to materials such as aluminum profile coatings. The heat transfer powder coating does not require the base powder to have good covering power, so no pigment was added to the tested basic formula in Table 4. The fillers used were CM powder and commonly used matte barium.
When barium sulfate is used as a filler, due to its poor transparency and low hardness, adhesion between the coating surface and ink occurs, resulting in messy transfer lines and poor pattern clarity. The transfer coating with CM powder as a filler has high hardness, no ink residue on the coating surface, and is easy to tear off after transfer printing, with high clarity.
The main factors affecting the transfer printing effect are the hardness and density of the coating, which are determined by the crosslinking density brought by the acid value, average functionality, and curing degree of the polyester resin, as well as the hardness of the filler itself. In polyester resin systems with the same acid value, the hardness of CM powder itself is high, and the hardness of powder coating is also high, resulting in better final transfer printing effect. It can be clearly seen from Figure 4 that the pattern transferred by the composite inorganic filler is clearer. This indicates that the selection of fillers cannot be ignored in the formulation design of high-definition wood grain transfer powder coatings. The composite inorganic filler described in this study is not necessarily more is better, and its addition at 40% will have a negative impact on leveling. The specific amount of addition needs to be adjusted based on the actual product.
3、 Conclusion
By designing special formulas and specific melting processes for various inorganic non-metallic materials, composite inorganic powder materials with reduced Mohs hardness and refractive index close to resin were obtained. Using this material as a filler for powder coating, the test results show that the super weather resistant powder coating prepared from this composite inorganic powder is more stable to ultraviolet light, can form a denser and more uniform microstructure, and assist in improving the weather resistance of the coating; Due to its refractive index being close to that of resin, it can be used as a filler to prepare transparent powder coatings; The transfer coating prepared using this series of powders as fillers has high hardness, easy to tear paper after transfer printing, and high clarity. In summary, the CM series composite molten inorganic powder is a functional filler with excellent comprehensive performance.
The above is all the content shared by Yongji Industrial, an indoor powder coating manufacturer.