Pigments for architectural coatings are divided into three types according to their uses, namely coloring pigments, extender pigments and pearlescent pigments.
White pigments in coloring pigments are:
1. Titanium dioxide. is a stable compound. Slightly soluble in alkali, insoluble in water and acid, good heat resistance. The commonly used ones are divided into rutile type and anatase type, both of which belong to the tetragonal crystal shape.
( 1 ) Rutile type. Optical properties such as gloss and hiding power are good; color reduction power ( compared ) is 100 ; relative density is 4.2-4.3 ; refractive index is 2.74 , which is higher than ordinary pigments; oil absorption is 20-22 %; High hardness and dielectric constant; superior to anatase in terms of weather resistance and powder resistance. Rutile titanium dioxide absorbs slightly more visible spectrum near the blue end than anatase, so the whiteness of rutile is better than that of anatase.
( 2 ) sharp titanium type. The relative density is 3.8-3.9 ; the refractive index is 2.52 , which is higher than that of ordinary pigments; the oil absorption is 23-25 %.
2. Lithopone, also known as lithopone. The hiding power is only equivalent to 20-25 % of titanium dioxide; it is chemically inert and has excellent alkali resistance; it is not acid-resistant and darkens in sunlight . Coloring pigments include inorganic color pigments such as iron oxide yellow and iron oxide red. It is not easy to fade and can protect the polymer material from UV damage.
Extender pigments are different from general achromatic pigments and coloring pigments. They are not as good as them in terms of color, tinting power, hiding power etc., but they can improve some properties or eliminate some disadvantages of coatings in coating applications, such as adding color paint systems. It can improve the construction performance of the coating, improve the suspension of the pigment and prevent sagging, improve the water resistance, abrasion resistance and temperature resistance of the paint film, and reduce the cost of the coating. Commonly used are:
1. Calcium carbonate. Divided into natural product heavy calcium and synthetic light calcium.
( 1 ) Heavy calcium. The purity is low; the particle size is 1.5 ~ 12µm , relatively large; the relative density is 2.71 . It is widely used in primers and putties. It is a good joint material, which can not only deposit on the substrate, but also be compatible with the paint to increase the strength of the paint.
( 2 ) Light calcium. High purity; average particle size < 3µm ; oil absorption 28 % to 58 %. It is not suitable to be used with non-alkali-resistant pigments, but it can be used as a buffer in latex paint. Adding it to the finish paint can make flat paint and semi-gloss paint.
2. Precipitate barium sulfate. Purity ≥ 97 %; high whiteness and fine texture; small and uniform particles; inert substance, acid- resistant , alkali-resistant, light-resistant, heat- resistant ; density 4.3-4.5g /cm 3 ; refractive index is the largest among extender pigments, 1.63-1.65 , oil absorption as low as 10 % to 15 %. In architectural coatings, it has good filling performance , good leveling, good impermeability, and increases the hardness and wear resistance of the paint film. It is easy to grind, easy to mix with other pigments and paints, and used for primers.
3. Fumed silica. Extremely high purity; extremely fine particles, with an average particle size of 0.012µm ; light weight. The refractive index is 1.45 ; the specific surface can reach 50-350m 2 / g, and the oil absorption is correspondingly very high, reaching 280 % ; properties : strong chemical stability. It exhibits thickening and thixotropy in liquid medium, and generally adding 1 % to 4 % of fumed silica can obtain suitable thixotropy. Can not play a flat .
4. Silicates, including talcum powder and calcined kaolin.
( 1 ) Talc powder. Density 2.7 ~ 2.8g/cm 3 ; oil absorption 30 % ~ 50 %, refractive index 1.54 ~ 1.59 ; there are two types of flaky and fibrous talcum powder, which is more water-resistant and moisture-proof to paint films than fibrous talc powder favorable. Talc powder is not easy to sink in the paint, and can suspend other pigments and prevent the paint from sagging. It can absorb stretching stress in the paint film and avoid cracks and voids . It is suitable for exterior wall paint. But it is easy to pulverize and should be used together with other pigments.
( 2 ) Kaolin. Density 2.58-2.63g /cm 3 ; Refractive index 1.56 ; Oil absorption 32 % -55 % ; Particle size 0.5-3.5µm
. Used in primers to improve suspension, prevent pigment settling, and enhance film hardness.
5. Sericite. Light weight, scaly structure; fineness 800 mesh to 1000 mesh; good UV shielding function, inert, good acid and alkali resistance, excellent lubricity, fluidity, and wear resistance. Increase the fullness, improve the water permeability of the coating film, contribute to the frost resistance of the coating film, improve the adhesion of , make it bond with the base layer more firmly, and easily disperse after adding the coating, which can improve the suspension of the coating and the coating film Good scrub resistance.
In recent years , it has been found that fine extender pigments can improve the covering ability . The particle size range of titanium dioxide that can exert the maximum covering efficiency should be 0.2-0.4μm .
Pearlescent pigments. Pearlescent pigments are used to obtain pearl luster, iridescent effect and metallic luster . It reflects and partially transmits the incident light, and the light reflects and transmits on the multi-layer sheet to produce a deep pearl luster. The mica titanium series is used in the experiment.
(1) General form: transparent thin flaky crystals, the flaky structure is arranged in parallel ; particle size: 5 ~
100µm .
(2) Mica titanium: particle size: 5 ~ 140µm ; thickness: 0.1 ~ 1µm . One is silver, large particles can obtain metallic shimmering luster; small particles can obtain silky soft luster. Another category is the coloring category. Mica titanium series pearlescent pigments have good and are non-toxic. When using them, care should be taken not to mix them with interference color systems (reflection and transmission colors will disappear) .
