1. Introduction

Organic pigments are a class of organic compounds with bright colors and excellent coloring properties, which are highly favored for their high coloring power, bright color, complete chromatography, and low relative density. However, in terms of heat resistance, weather resistance, and covering power, they are usually slightly lacking. From a chemical structure perspective, organic pigments are mainly divided into azo, phthalocyanine, and heterocyclic groups. According to the strength of fastness, organic pigments can be divided into classic organic pigments and high-performance organic pigments. Classic organic pigments mainly include most azo pigments, while high-performance organic pigments include benzimidazolone pigments, azo condensation pigments, quinacridones, pyrrolo pyrrolidinediones (DPP), anthraquinone heterocycles, etc.

2、 Organic pigment crystal form

The molecular structure and physical characteristics (particle size and distribution, crystal configuration, etc.) of organic pigments determine their color and application properties. The coloring intensity and color brightness are related to molecular structure, particle size, and distribution; The stability of crystal form, pigment color, and crystal structure are related.

Many organic pigments, like other crystalline substances, exhibit the phenomenon of homogeneous polycrystals, where the same molecule has the ability to form crystals with different structures in different environments. The geometric features of a crystal are called crystal forms. Due to different molecular arrangements in the crystal lattice, multiple crystal forms can be formed. The X-ray diffraction patterns of each crystal form have their own characteristics, which can be used to distinguish them.

The color of organic pigments is actually the color of crystalline particles, not just the color of pigment molecules, so it cannot be represented by visible light absorption spectra, but should be represented by the reflection curve of visible spectra. Pigments with the same molecular structure, due to their different crystal forms, interplanar spacing, and arrangement, have different absorption and reflection of light, resulting in different colors.

For example, the common pigment Red 254 also has two crystal forms, as shown in Figure 1. The alpha crystal form is a moderately pale blue light red and thermally stable, while the beta crystal form is yellow light red. The common crystal form of commercial pigment Red 254 is the alpha crystal form. Svetlana N. Ivashevskayad conducted in-depth research on the crystal structure of the beta crystal form of pigment Red 254.

Figure 1. Two crystal forms of pigment red 254 [3] (α, β)

3、 Application of X-ray Diffraction Method in Crystal Form Analysis of Organic Pigments

X-ray diffraction is an important method for studying the crystal structure of organic pigments, and its applications are as follows:

1) Qualitative identification of homogeneous and heterogeneous crystals: Typical crystal types such as α - type CuPc and β - type CuPc are identified.

2) Determination of the respective contents of different crystal forms (mixed crystals): When there are two or more different crystal forms in the organic pigment sample, and it is a mechanical mixture of different crystal form components, that is, it does not form a solid solution or eutectic, X-ray diffraction can be used to determine the corresponding percentage contents of each crystal form.

3) Determining the particle size of microcrystals in crystal samples: The grain size can be calculated using the Scherrer formula. If the pigment crystal surface is treated with a solvent and the crystal grows, the relationship between pigment grain size, color light, and physical properties can be studied.

4) Control the quality of pigment products: Organic pigment products can be qualitatively identified to detect the singularity of product crystal form, that is, to determine the relative proportion of other crystal form products, or to detect impurities introduced during the production process (such as inorganic fillers, certain raw materials, etc.). If a small amount of filler, such as barium sulfate, is added to phthalocyanine blue products, the characteristic peaks of the filler can be displayed in their X-ray powder diffraction pattern.

5) Comparative test of crystallinity of pigment samples: The crystallinity of pigments also affects their color characteristics. For example, when α - type CuPc is treated with solvents, the crystallinity increases and the diffraction peaks become more prominent. However, when the crystallinity of pigments is low or close to amorphous, no characteristic diffraction peaks appear. The lower the crystallinity, the lower its light resistance and firmness.

4、 Experimental section

This experiment used the FRINGE EV desktop X-ray diffractometer from Suzhou Langsheng Scientific Instrument Co., Ltd. to detect organic pigments provided by a certain company.

(1) Sample Display

Figure 2. Powder sample of pigment red 254 (PR254)

Before kneading (marked as PR254); After kneading (referred to as PR254 kneading)

Pinching method is one of the commonly used methods for pigment modification. Kneading machine is a necessary machine for modifying pigments using pinching method. Its working principle is to apply energy to the water containing pigments through mechanical stirring in the presence of organic solvents, causing changes in the crystal lattice or surface shape of the pigments, or removing water from the pigments (squeezing water phase change).

Pinching method is a technique in which coarse pigments and salts are mixed in the presence of a solvent, and the salt is squeezed by stirring and kneading force to grind the pigment. At the same time, the pigment is grown in the solvent. The pigment crystals produced by pinching method are regular and symmetrical, and have better application properties such as light resistance, heat resistance, weather resistance, and migration resistance.

(2) Analysis conditions

(3) Result and Conclusion

Figure 3. Diffraction pattern and qualitative analysis of PR254

Figure 4. Diffraction pattern and qualitative analysis of PR254 after kneading

Figure 5. Stacked diffraction patterns of PR254 before and after kneading

Table 1. Analysis results of pigment red 254 before and after kneading

(4) Analysis results

1. Figures 3 and 4 show the diffraction patterns and qualitative results of pigment red 254. It can be seen that these two organic pigment samples have a good degree of crystallization, and their diffraction patterns match the standard PDF card 00-055-1984 in the CrystalX software's organic pigment database, belonging to the alpha crystal form. The chemical formula is C18H10Cl2N2O2, and the diffraction peak at 2 θ=28.24 ° is a strong peak.

2. Figure 5 shows the superimposed diffraction patterns of pigment red 254 before and after kneading treatment. It can be seen that there is a significant difference in diffraction peak intensity at 2 θ=28.24 °, and the diffraction intensity after kneading is significantly higher than before kneading. However, other diffraction peaks in the range of 3-60 ° can basically overlap, indicating a small difference in peak intensity. Meanwhile, Table 3 presents the fitted peak areas of the diffraction peaks of pigment red 254 before and after kneading in the angle range of 22.5-34 °, as well as the grain size in the crystal plane direction at 2 θ of 28.24 °. After kneading, the pigment red 254 crystals continued to grow, with better crystallinity and larger grain size.

V. Conclusion

By using the desktop X-ray diffractometer FRINGE EV from Langsheng Science to collect diffraction patterns of organic pigments, the differences in crystal state and grain size of pigment Red 254 before and after kneading can be analyzed. This can provide strong data support and guarantee for the rapid adjustment of organic pigment research and development, production processes, and ensuring efficient production.