Influence Of The Particle Size Of Organic Pigments

Pigment orange 34 manufacturers say that in the production process of organic pigments and some of the applications of organic pigments, printing inks, coatings, and coloring of plastics, organic pigments are used to be made into micro-fine particles. Usually, after the grinding process, the particle size of organic pigments will be below a few micrometers and the distribution of the diameter particles is more concentrated, and there are no particles with excessive diameter particles. The resulting dispersion has good dispersion and stability over a long period of time. The particle size, distribution state, surface state of the particles and physical properties of the organic pigments significantly affect the application properties of the organic pigments such as chromaticity, brightness, color, tinting power, light resistance and solvent resistance.

In view of the fact that organic pigments are colored in the dispersion state of fine particles, extensive and in-depth research has been carried out in recent years on the dispersion state of pigment crystal particles in different media. According to the theory of pigment modeling, the centerpiece of organic pigment morphology is the study of the unit structure of different pigments: the size and distribution morphology of particles such as microcrystals, crystals, aggregates, agglomerates, flocs, etc., as well as the evaluation of the dispersion system with different characteristics.

Well-crystallized organic pigment particles can be divided into the following categories:

Microcrystalline: it is not in the form of smaller isolated particulate units, is part of a crystal, and can exhibit complete X-ray diffraction properties.

Crystals: It is a crystalline particle consisting of several closely arranged microcrystals, which can be cubic, rod-like, or brick-like.

Aggregate: a dense particle further composed of several crystal units which can be connected in a two-dimensional way by parallel sides or at an angle.

Coalescence: It is a loose connection between crystals without certain rules. It is usually a one-dimensional combination in point form, with a certain angle, and after dispersion, it can be separated again to become the basic structural units constituting the condensate group.
5 Flocculent: It refers to the agglomerate which is easily re-dispersed in the process of placing after the agglomerate and crystals are dispersed.

The size of pigment particles is directly related to their optical properties. Usually the reflection of light by pigment particles is related to the difference of refractive index between the crystal particles and the surrounding medium. The larger the difference in refractive index, the more obvious the reflection effect on light and the more its covering power and brightness increase. For example, when chalk is dispersed in a linseed oil medium, the refractive indices of the two are 1.6 and 1.5. because the difference is so small, it cannot be used as a white pigment.

The particle size of organic pigments also significantly affects the pigment’s wettability, lightfastness and solvent resistance. The particle size of organic pigment is closely related to its nature, which affects the color strength, color light, transparency, weather resistance, solvent resistance and gloss of organic pigment. The following is the introduction of Shanghai Fine Face Chemical.

Coloring strength and color light
The reduction of the average particle size of organic pigments can lead to an increase in their coloring strength, i.e. the coloring power depends largely on the dispersion level of the pigment. The distribution of organic pigments with different particle sizes and shapes in the medium has different conditions for light absorption and reflection, thus affecting the color light presented by the pigments in the medium. The intensity of coloration is not only directly dependent on the particle size, but is also related to the shape of the particles. Thinner or elongated particles usually absorb light better and exhibit a higher molar extinction coefficient. On the contrary, a thick layer of aggregated particles is not favorable for light absorption.

Transparency
The particle size of an organic pigment is an important factor affecting its transparency. There is a curvilinear function between the particle size of an organic pigment and its hiding power. When the particle size of the pigment is large, the covering power is not large, but with the decrease of the particle size of the pigment and increase. When the pigment particle size reaches a certain critical range, the maximum covering power can be obtained, and then as the particle size decreases, the covering power decreases. When the particle diameter is less than half of the light wavelength, the diffraction of light occurs and the transparency increases greatly. Therefore, the particle size of organic pigment can be adjusted to adjust the requirement of covering power or transparency of the pigment.

Light fastness, weather fastness and solvent fastness
Weathering fastness is one of the important properties in the durability of pigments, and it is also the basic application performance of high-grade organic pigments. The fading process of pigment under light belongs to gas-solid non-homogeneous reaction, and its reaction speed is mainly related to the chemical structure, but also related to the size of pigment particles, that is, related to its specific surface area. The smaller the diameter of organic pigment particles, the larger its specific surface area, the more light energy is absorbed, the greater the destructive effect of redox substances such as water vapor in the air, the greater the fading or discoloration of organic pigment color occurs rapidly, and the light resistance and weathering resistance are worse. On the contrary, increasing the diameter of particles and enhancing the ability to resist the above damage can improve the light resistance and weather resistance of organic pigments. In addition, pigments used for coloring paints and plastics should have excellent solvent resistance and migration resistance, which is related to the molecular structure of the pigment, the type of substituent group and the polarity of the surface of the particles, and the size of the particle size of the pigment will also affect its solvent resistance.

Glossiness
The particle state and particle size of organic pigment not only affect the coloring strength, transparency, color light, fastness to weathering and solvent resistance, but also affect the glossiness of the coloring film, of course, it also has an important relationship with the film glossiness of the application medium. 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, which is related to the shape of the pigment particles, especially the aspect ratio of needle or rod particles plays an important role.