How to choose pigments for coatings from the seven performance indicators of pigments?
There are various pigments for coatings, how to choose a certain pigment for coatings? Here are seven performance indicators of pigments to talk about.
First, the color of pigment.
The color of pigment is due to the selective absorption of different wavelengths of light in visible light, and the color of pigment is also affected by the physical properties such as crystalline shape, particle size and dispersion performance. The color of the pigment is also affected by the light shining on it, such as in the dark, the pigment does not show any color, the color in the strong light is brighter than the one in the dark light, the same pigment under different light sources (such as sunlight, incandescent light, fluorescence, etc.) can also show different colors.
Second, coloring power.
The coloring power of pigment refers to the ability of a pigment to show the color shade after mixing with another pigment. Tune the same color, the stronger the coloring power, the less the amount of color paste, the lower the degree of water resistance of the color paste coating and the impact of the coating film performance is smaller. The same color, the quality of the products of different manufacturers will be very different. The strength of pigment coloring power not only depends on its nature, but also has a certain relationship with its dispersion degree. The greater the dispersion of the pigment, the stronger the coloring power.
Third, covering power.
The pigment in the coating film can cover the surface of the object to be coated, so that the substrate is no longer covered through the coating film and the ability to reveal. The strength of the pigment covering power mainly depends on the refractive index, light absorption ability, crystal structure, and dispersion degree and other factors, and also depends on its ability to absorb the light irradiated on the surface of the coating. For example, carbon black can completely absorb the light irradiated on it, so its covering power is very strong. The strength of coverage of opaque color pigments also depends on their selective absorption of light.
When the pigment is dispersed evenly in the base material, the particle size is small and the specific surface area is increased, thus the covering power is also increased. However, if the size of the pigment particle is equal to half of the wavelength of light, the light will pass through the particle without refraction, and the particle will be transparent.
The higher the crystallinity of the pigment, the stronger its covering power. The covering power of mixed pigment can not be calculated by the law of addition according to the covering power of each component of the mixture, in fact, the covering power of most mixed pigments is larger than the calculated value. Therefore, mixing pigments and fillers in a suitable proportion will not affect their covering power and will help to reduce the cost. If the covering power of the paint is high, the painting area is high and the project cost is low.
Fourth, dispersibility and adaptability.
The dispersibility of pigment refers to the difficulty of dispersion of pigment particles in the coating base material and its dispersion state after dispersion, which is influenced by the pigment performance, preparation method, particle size and particle size distribution. The dispersion of pigment has obvious influence on the strength of covering power and coloring power of pigment, and also has influence on the physical and chemical properties of coating film.
The problem of pigment adaptability, which is especially important for emulsion architectural coatings. Due to the different types of pigments, the role of pigments will also show a certain degree of difference, and this tendency is more obvious for organic pigments. If the pigment is poorly dispersed in the paint and poorly matched with the paint, the paint will have potential flocculation or even fading.
V. Light resistance weathering resistance.
The color of the pigment will change to different degrees under the action of light. The color of the pigment will gradually darken under the sunlight for a long time, and some pigments will be chalked under the action of the ultraviolet ray in the sunlight. Exterior wall paint should use pigment with good light and weather resistance, generally the light resistance is more than 7~8 grade, 8 grade is the best, the weather resistance is more than 4~5 grade, 5 grade is the best. UV absorbers, light stabilizers and other additives can enhance the weather resistance of some organic pigments to a certain extent.
The fineness of the color paste is not the finer the better, because like phthalocyanine blue, phthalocyanine green pigment itself is a small molecule pigment, fineness is too small, the particle size difference is large, poor dispersion, and paint compatibility is not good, color mixing cost increases, and also lead to floating color flowering.
Seven, acid and alkali resistance.
The acid resistance and alkali resistance of pigment is also an important performance index for its use in architectural coatings. Some pigments are not acid and alkali resistant, so they can not be used in acidic or alkaline paints, and the paints made are not suitable for acidic or alkaline environment.
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