What Are The Differences Between Organic Pigments, Dyes And Inorganic Pigments
The difference between organic pigments, dyes and inorganic pigments:
1. Organic pigments and dyes:
The difference between organic pigments and dyes is that they have no affinity with the object to be colored. Only organic pigments can be attached to the surface of the object through adhesives or film-forming substances, or mixed inside the object to color the object. The intermediates, production equipment and synthesis process required for its production are similar to those of dye production, so organic pigments are often organized in the dye industry.
2. Organic pigments and inorganic pigments:
Compared with general inorganic pigments, organic pigments usually have higher tinting power. The particles are easy to grind and disperse, not easy to precipitate, and the colors are brighter, but they have poor light resistance, heat resistance, and weather resistance.
Organic pigments are commonly used for coloring materials such as inks, coatings, rubber products, plastic products, cultural and educational supplies, and building materials.
Inorganic pigments are light fast, heat resistant, weather resistant, solvent resistant, and have strong hiding power, but the chromatogram is not very complete, the coloring power is low, the color is poor, and some metal salts and oxides are highly toxic.
4. Mechanism of action:
The basic optical properties and pigment properties of inorganic pigments are mainly determined by the following three aspects:
①The difference in refractive index between the pigment and the dispersion medium;
②Light absorbed by the solid (including impurities in the solid);
③ Particle size and particle size distribution. The particle size and particle size distribution can be improved by surface treatment.
In the pigment production process, no matter how fine the pigment powder is ground, there will always be some aggregated and flocculated particles. In the process of transportation and storage, the pigment will further flocculate into large particles due to extrusion and moisture. The finer the pigment, the larger the surface area, and the higher surface energy, making it easier to flocculate together. If treated with an appropriate surfactant, these large flocculated particles can be easily dispersed during use. The main dispersion mechanism is as follows:
The dispersion of inorganic pigment powder in liquid mainly passes through the following three stages: ① Wetting of the powder, the liquid not only wets the surface of the powder, but also replaces the air and moisture between the powder particles;
②After passing the wet powder and displacing the air and moisture between the particles, the flocs and aggregates in the pigment powder are destroyed;
③ The wetted and destroyed flocs and aggregate powder maintain a stable dispersion state in the liquid. In other words, dispersion is a process of wetting-dispersing-keeping the dispersion stable. In general, inorganic pigments are rarely dried before use. In addition to air, the surface of the pigment also absorbs a water film. The amount of water usually adsorbed on the surface of the pigment is equivalent to the amount of water required for the monomolecular film formed on the solid surface. For example, the surface area of TiO 2 per gram is 10 m 2, the thickness of the water molecule adsorption layer is 10 × 10 -10 m, and the amount of water required by the monomolecular film is about 0.3% of the weight of the pigment, so the water content in the pigment also affects its dispersion performance One of the main factors. Whether the solid is wet or not can be judged according to its contact angle. A contact angle of 0° means complete wetting, and the liquid is completely spread on the surface of the solid; a contact angle of 180° means completely non-wetting, and the liquid is attached to the surface in the form of water droplets. Solid surface.
Whether a solid can be well wetted in a liquid can be judged by not only the contact angle but also the heat of wetting. Generally, hydrophilic powders (such as TiO 2) have a high heat of wetting in polar liquids. The heat of wetting in polar liquids is small, while the heat of wetting in hydrophobic powders in polar and non-polar liquids is roughly constant.
The sedimentation speed and sedimentation volume of the solid powder in the liquid can also be used to judge the degree of wetness (see Table 1). A polar solid like TiO 2 has a small sedimentation volume in a polar solution. The smaller polar solution is larger; the non-polar solid powder generally has a larger sedimentation volume. After treatment by adding a surfactant, the surfactant molecules are strongly oriented and adsorbed on the surface of the solid, which helps to reduce the surface tension of the liquid and improve its wetting and dispersing properties.
2 Electric repulsion (ξ potential)
The dispersion and dispersion stability of inorganic pigments in aqueous solutions are mainly determined by their electrical repulsion in water, that is, the magnitude of zeta potential.
Electric repulsion is the use of charge repulsion to maintain dispersion stability.
Surfactants can ionize a large number of negatively charged (or positively charged) ions in the aqueous solution, and firmly adsorb on the surface of the pigment particles, so that these particles have the same charge, and other ions with opposite charges diffuse freely into the liquid medium. Around it, a diffusion layer of charged ions (electric double layer) is formed. The potential difference between the two layers of ions from the solid surface to the farthest point of the diffusion layer (where the opposite charge is 0) is called the zeta potential. The electrostatic repulsion between particles comes from this, these particles with the same charge repel each other once they come into contact, thus maintaining the stability of the dispersion system, which is the famous D.L.V.O. theory.
In the case of electric repulsion, the surfactant must have high ionization performance, usually anionic surfactants and some inorganic dielectrics, such as: tripotassium polyphosphate, potassium pyrophosphate, sodium polyphosphate, alkyl aryl sulfonate Sodium, sodium methylene naphthalene sulfonate, sodium polycarboxylate, etc.
3 steric hindrance effect (or entropy effect)
When the pigment is dispersed in a non-aqueous medium, the possibility of the above-mentioned ionic reaction is greatly eliminated. The non-ionic surfactant does not ionize in water. In this case, the effect of the surfactant is called steric hindrance or Entropy effect. Because the surfactant can be oriented to adsorb on the surface of the pigment particles to form a monomolecular adsorption layer, this oriented buffer layer can prevent the particles from agglomerating, thereby maintaining the stability of the dispersion system (also known as protective colloid or micelle).
As the surfactant concentration increases, the entropy of the surfactant molecule group on the pigment surface will decrease and the movement will be restricted. The closer and more compressed the pigment particles are, the entropy will further decrease, which is beneficial to the stability of the dispersion system.