Organic Pigments for Powder Coatings
Organic pigments used in powder coatings should be high performance organic
pigments，which are used as a supplement to inorganic pigments，complementing the limitation of inorganic pigments in terms of color brightness and hue ranges.
Many application properties of organic pigments are not only determined by the chemical structure，but also the particle size and distribution，surface polarity，
crystal types and crystallinity，as well as determined by their surface modification.
This paper described the performance of organic pigments from the aspects of physical form，crystal types and chemical structure characteristics of organic pigments，and indicated the performance requirements of organic pigments used in powder coatings and some commonly used organic pigments in powder coatings，, as well as the characteristics of these pigments, were given.
The problems that are likely to occur in the use of organic pigments in powder coatings were summarized to understand the causes and mechanisms of their occurrence，benefiting the users in the selection of suitable high-performance organic pigments for various colors.
The usage properties of pigments include light and weather resistance, coverage/transparency, coloring strength, heat resistance, chemical resistance, gloss, rheology, and dispersibility. Organic pigments are widely used in many industrial fields for their bright color, high coloring strength, and complete color spectrum. As with other industrial
Like other industrial technologies, with the expansion of application fields and the development of industrial technologies, higher requirements are put forward for organic pigment products, such as higher grade products, higher durability, heat resistance, solvent resistance, and migration resistance of coatings. Organic pigments are generally insoluble in the medium used and are available as particles or crystalline aggregates.
Therefore, many application properties of organic pigments are not only determined by the molecular chemical structure of organic pigments but also depend largely on the surface modification of organic pigment particles and the influence of crystalline shape on the properties, such as particle size, distribution, shape (particle surface polarity), crystal type and crystallinity.
1 Performance requirements of organic pigments for powder coatings
Powder coating is a new type of solvent-free 100% solid coating, whose production process includes premixing, extruding, slicing, and grinding, which has its special characteristics compared with a liquid coating. For the pigment, there is no solvent in its dispersion process and there is no wetting step like in liquid coatings, so the powder coating has very high requirements on the performance of the pigment, which should have the following characteristics.
(1) Good pigment dispersion, not easy to agglomerate: pigment particles have a great impact on the extrusion process of powder coatings because powder coatings are pre-mixed plus the extruder mixing process, there is no solvent-based coatings fine grinding process, so agglomerated pigments and pigments that are not easy to disperse are difficult to pre-mix, it will lead to uneven extrusion and mixing, resulting in color difference and failure to meet the uniformity of the coating requirements. Normally, the ideal dispersion particle size of pigment in powder coating is 0.2~0.7 μm.
(2) Extrusion resistance: powder coating production process of pigment dispersion, all rely on premixing and extrusion, especially the extrusion process determines the degree of pigment dispersion. Whether single-screw extrusion or twin-screw extrusion, the temperature range is 90~110 ℃, when the pigment must be in the extruder, together with other materials, in the high temperature, high pressure, and molten mixing environment. Most organic pigments cannot withstand this process. This process is somewhat similar to plastics processing, so many organic pigments for powder coatings originate from the plastics industry.
(3) Covering power and coloring power should be strong: the coating formed after powder coating needs to form a decorative or functional protective effect on the substrate, so the covering power of the coating color is very critical. It must cover the substrate, which has certain requirements on the covering power of organic pigments, in addition, once the covering power of organic pigments is not good, the number of pigments added will increase, which will inevitably lead to a decrease in the leveling of the coating, a decrease in mechanical properties and an increase in cost.
(4) The oil absorption is moderate: the best oil absorption is not more than 60 g/(100 g), if the oil absorption of organic pigment is especially large, the fluidity will be affected and its usage will be limited during the extrusion process. Compared with inorganic pigment, the amount of organic pigment added is much more, and the leveling performance will be reduced, so the oil absorption of organic pigment needs to be concerned.
(5) Good thermal stability: when powder coating is cured, the baking temperature range is 160~220 ℃. Therefore, the heat resistance of pigment should be better than 20 ℃. Therefore, the heat resistance of the pigment should be better than 20 ℃. At the same time, organic pigments cannot react with other raw materials in the formula. The organic pigment cannot react with other raw materials in the formula.
(6) Color bleeding resistance, non-frosting, precipitation, and lamination: for some thicker structural parts, powder coating should be used. When powder coating is used for some thicker structural parts, it is easy to frost and sink. This is because the relative molecular mass of organic pigment is small or the structure is not stable. This is due to the small relative molecular mass of organic pigments or unstable structure, which makes it difficult to dissipate heat on the coating surface when the workpiece is thick. Therefore, powder coating has high requirements on the relative molecular mass of organic pigments. Therefore, powder coatings have high requirements on the relative molecular quality of organic pigments and try to choose organic pigments containing hydrogen bonds in the structure. (7) Toxicity
(7) Toxicity: In the process of powder coating, certain aromatic amines and PCB may exist as intermediates or by-products in the pigments. It is necessary to limit the content of such substances in the pigment.
(8) Light and weather fastness: lightfastness reflects the degree of pigment darkening and fading under the light. The degree of darkening and fading under the light, from serious to slight is divided into 1~8 grades, 8 grades indicate the best light resistance. 8 level shows the best light resistance. When powder coating is used, generally choose 7~8 grade is the best. The weather resistance is a more comprehensive reflection of the pigment. Weathering resistance reflects the fastness of pigments under natural outdoor conditions more comprehensively. It is divided into 1~5 grades from the worst to the best. Depending on the amount and color used, the best choice for outdoor coatings is 4~4. The best choice for outdoor coatings is grade 4~5. The performance of powder coatings is The xenon lamp aging test is generally required to pass 800 h, the light retention rate is >50%, and the lightfastness and weathering resistance of the color is >50%. The light and weathering resistance of the material mainly depends on the chemical structure, which is related to the crystal type, particle size, and surface treatment. The lightfastness and weathering resistance of the pigment depends mainly on the chemical structure and is also related to the crystal type, particle size, and surface treatment. If the pigment is used for color mixing, it is also necessary to The light and weather resistance of organic pigment after washing out.
Chemical reagent resistance, acid resistance, and alkali resistance: some organic pigments have reactive groups in their chemical structure. For example, C.I. pigment yellow 151 contains a carboxyl group, so the alkali resistance is not good. For example, C.I. Pigment Yellow 151 contains a carboxyl group, so the alkali resistance is not good. Therefore, when choosing the color, we also need to pay attention to and consider the use of the final product. For example, C. I. Pigment Yellow 151 contains carboxyl groups, which are not good for alkali resistance.
2 Effects of organic pigment physical form, crystalline form, and chemical structure on the performance of powder coatings
2. 1 Physical form of organic pigments
The particle size distribution of organic pigments
The particle size distribution influences the color vividness and color intensity. For organic pigments with the same average particle size, the narrower the particle size distribution, the higher the vividness and color intensity. Therefore, the decrease of pigment particle size and the concentration of particle size distribution will change the color phase, increase the transparency, decrease the covering power, decrease the light fastness, increase the color intensity, decrease the crystalline stability, decrease the solvent resistance and decrease the dispersibility.
2. 2 Crystalline properties of organic pigments
Pigment molecules have a strong formation of highly ordered crystalline arrangement of the pouring phase, the main driving force of forming this crystal is intermolecular hydrogen bonding, π-π interaction and van der Waals forces are also factors that lead to the stability of the crystal lattice. The microstructure of organic pigments is with the same chemical structure, polycrystalline type, and different molecular arrangements, which is the homogeneous heterocrystalline phenomenon. For example, the synthesis and post-processing process of phthalocyanine pigments (such as grinding and dispersion, kneading, solvent treatment, acid dissolution and acid expansion, additives, additives, etc.) occur changes in the crystal structure, which lead to the pigment showing different crystalline characteristics and directly affect the application performance of the pigment, such as, color light, color intensity, thermal stability, etc.
2. 3 Chemical structure characteristics of organic pigments
Looking at the chemical structure characteristics of high-performance organic pigments, it can be found that heterocyclic derivatives have better molecular planarity and molecular symmetry. heterocyclic derivatives have good molecular planarity and molecular symmetry and high color intensity, and they can form intra- and intermolecular hydrogen bonds, metal complexes, or increase the relative molecular mass to improve the durability and migration resistance of pigments. They can form intra- and intermolecular hydrogen bonds, metal complexes or increase the relative molecular mass to improve the durability, migration resistance, and thermal stability of pigments. Early The early varieties of monoazo pigments can form quinone hydrazone molecules due to the presence of the nitro group in the azo group in the molecule. The presence of the nitro group in the molecule can form a quinone hydrazone type intramolecular hydrogen bond, which helps to improve the light fastness, weather resistance, and heat stability. The improvement of light fastness, weathering resistance, and heat resistance stability. Nowadays, the commonly used benzimidazole The molecular structure of benzimidazolone, quinacridone, pyrroloxypyrrolidone, isoindoline, etc. The presence of amino and carbonyl groups in the molecular structure increases the durability and solvent-resistance of the pigments. At the same time, the physical morphology and crystalline characteristics of organic pigments also affect the color phase, color phase, coverage, dispersion, and migration resistance of organic pigments. The physical morphology and crystalline characteristics of organic pigments also have a great influence on the color phase, color phase, coverage, dispersion, migration resistance, solvent resistance, and weather fastness of organic pigments. The physical morphology and crystalline properties of organic pigments also have a great influence on the color phase, color phase, coverage, dispersion, migration resistance, solvent resistance, and weather fastness.
With the rapid development of the powder coating industry, the selection of organic pigments for powder coatings is becoming more and more targeted. Organic pigments are becoming more and more targeted and the standard is getting higher and higher. If you don't know the chemical composition and performance of various pigments, you will be confused in the face of thousands of If you don't know the chemical composition and related properties of various pigments, you will be at a loss in the face of thousands of pigment types. The correct use of organic pigments can provide color matching and management convenience for the powder coating industry with small batches and many colors. The correct use of organic pigments can facilitate the color matching and management of small batches and large colors in the powder coating industry.