Performance indicators of pigment pastes and their impact on coatings
1、The color of pigment
The color of pigment is due to the selective absorption of different wavelengths of light in visible light, which is affected by the physical properties such as crystalline shape, particle size and dispersion performance, for example, the color of iron oxide red, with the increase of its particle size from orange-red to purple-red; the color of pigment is also affected by the light irradiated on it, for example, in the dark, the pigment does not show any color, in the strong For example, in darkness, the pigment does not show any color, in strong light the color appears brighter than in dark light, and the same pigment under different light sources (such as sunlight, incandescent light, fluorescence, etc.) can also show different colors.
2、Coloring power
The coloring power of pigment refers to the ability to show the color shades after mixing one kind of pigment with another kind of 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. So 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 degree, the stronger the coloring power.
3、Light resistance and weather resistance
The color of the pigment will change to different degrees under the effect of light. Inorganic pigments will gradually darken under the sunlight for a long time, and some pigments will be chalked under the action of ultraviolet rays 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. Inorganic pigments such as iron oxide are generally good in light resistance, and the color retention of the corresponding color paint is good; organic pigments are relatively worse than inorganic pigments, and the price is much higher. The higher the light resistance of organic pigment, the higher the price, and the price and weathering color retention will be very different for each level of light resistance, and the price will be more than 4 times different for light resistance ≤6 and weathering resistance ≤3. Therefore, the price of exterior wall coating, especially organic color coating, is one price per penny.
4、Covering power
The pigment in the coating film can cover the surface of the object to be decorated, 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 the degree of dispersion and other factors. The hiding power of a pigment depends not only on the ability of the coating to reflect light, but also on its ability to absorb the light that hits the surface of the coating. For example, carbon black can completely absorb the light shining on it, and thus it has a very high hiding power. Opaque color pigments also depend on their selective absorption of light. 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 is 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 paint is high, the painted area is high and the project cost is low.
Covering power of common pigments
Pigment Covering power
Titanium white 50g/m2
Ryder powder 120g/m2
Iron oxide yellow 60g/m2
Iron oxide red 60g/m2
5、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, 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.
6、Fineness
The fineness of the color paste is not the finer the better, because the pigment like phthalocyanine blue and phthalocyanine green is small molecule pigment, the fineness is too small, the particle size difference is large, the dispersion is poor, the compatibility with the paint is not good, the cost of color mixing increases, and also lead to floating color flowering.
7、Acid and alkali resistance
The acid and alkali resistance of the pigment is also an important performance indicator for its use in architectural coatings. For example, iron blue or chrome yellow will decompose when it meets alkali, and calcium grey powder and calcium carbonate will react when they meet acid, so attention should be paid to the choice when using them. If you choose low price, non-professional manufacturers of emulsion paint yellow, blue, etc., because the price is very low or do not know its pigment performance, emulsion paint will soon have fading chalking alkali phenomenon, so customers choose paint and emulsion paint coloring must pay attention to the choice.
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