Pigment particle size refers to the shape and size of pigment particles. Particle size is the measure of particle size, and pigment samples are composed of tens of thousands of particles, and the sizes of particles are different from each other. The size needs to be described by particle size distribution. The so-called particle size distribution usually refers to the proportion of particles of various sizes in the powder sample to the total number of particles (such as the particle size distribution diagram of dry powder), which is generally measured by a laser particle size distribution analyzer.
The size and shape of pigment particles will affect its hiding power, tinting strength, shade and fastness to resistance, etc. The reflection effect of the pigment on light is related to the difference in the refractive index between itself and the surrounding medium. The greater the difference in the refractive index, the stronger the reflection effect and the higher the hiding power. Within a certain range, as the particle size decreases, the hiding power of the pigment increases, while the particles become smaller, the specific surface area increases , and the coloring strength also increases. However, when the particles are too fine, light diffraction will occur, and the hiding power will be reduced instead. Therefore, the size of the particles should be controlled within an appropriate range. The distribution of pigment particles also affects the shade of the pigment. Usually, the particles are coarse , the particle size distribution is wide, and the shade is dark; otherwise, the shade is bright. The particle size distribution will also affect the light resistance, weather resistance, fastness .
The size of the pigment particle size is an important factor affecting its transparency. To make the application medium appear opaque, in addition to requiring a significant difference in the refractive index between the dispersion medium and the pigment particles, it is also related to the light scattering effect of the pigment particles . When the particle size of the pigment is half of the light wavelength, that is, when the particle diameter of the pigment is 0.2-0.5 um, the light scattering ability is the strongest, which can lead to opacity with high hiding power; and when the average pigment dispersion When the particle size is smaller than this value, if the particle size is 0.015-0.025 um , it will be transparent and the coloring power of the pigment will be high.
The fading process of pigments under light is a gas-solid heterogeneous reaction. The reaction rate is mainly related to the chemical structure, but also related to the size of the pigment particles, that is, its specific surface area. Usually, the specific surface area of fine particles is large, and its weather fastness is relatively high. Difference.
The particle size of the pigment particle state not only affects the coloring strength, transparency, shade, weather fastness, solvent resistance , but also has a certain impact on the gloss of the colored coating film. Of course, it also has a relationship with the gloss of the application medium. important relationship. Experiments have shown that when the pigment particles are spherical and the particle size is in the range of 0.01-1µm, a smoother surface coating film will be formed, and it is related to the shape of the pigment particles, especially the length/length of the needle-shaped or rod-shaped particles. The magnitude of the aspect ratio value plays an important role.
Tests have shown that there is no direct relationship between gloss and the average particle size of each pigment; but the same needle-like pigment after wet grinding reduces the length of the particles and shows a fairly high gloss.
