5. What is the meaning of solvent resistance of pigments?
In the production of coatings, we must disperse the pigments uniformly and stably in most of the organic base materials (composed of resins and solvents), which means that the pigments must be surrounded by organic solvents.
In addition, after most paints are colored by pigments, they will inevitably come into contact with organic solvents (detergents, gasoline and lubricating oil, etc.) during their effective service life. Therefore, this requires that the pigment should be as insoluble in organic solvents as possible.
If it cannot be insoluble, we should realize that in various organic solvents, there is a limit to the addition of pigments. If this tolerance is exceeded, staining caused by pigments dissolved in solvents will occur.
The solvent resistance of pigments is essentially the ability of pigments to resist the dissolution of solvents and cause staining. Inorganic pigments (determined by their own chemical structure characteristics) and some organic synthetic pigments with complex structures generally have good solvent resistance.
However, some lower grade organic pigments and surface treated pigments have poor solvent resistance. Solvents used to determine the solvent resistance of pigments include water, rosin, toluene, xylene, methyl ethyl ketone, ethanol, ethyl acetate, diethylene glycol and trichloroethylene.
6. What is the difference between light resistance and weather resistance of pigments?
Many coatings that use pigments (or dyes) as colorants need to keep the inherent color stable in application. We define the lightfastness of a pigment as a quality technical indicator of the pigment's resistance to sunlight.
Of the components of sunlight, the most destructive component to the lightfastness of pigments is ultraviolet (UV). When we discuss the light fastness of pigments, we only evaluate the quality technical indicators of the pigment's ability to withstand the light environment in the external environment.
In fact, it is difficult for us to make an accurate definition of the weather state. From a certain point of view, the lightfastness index of pigments that excludes other external environmental factors may help us give a meaningful and reproducible objective evaluation of the field stability of the coating.
The weather resistance index of pigments is affected by various external environmental factors, including sunlight exposure, high-energy ultraviolet radiation, temperature, humidity and erosion of various impurities in the atmosphere.
Pigment weather resistance indicators can be measured through outdoor exposure experiments or indoors through artificial atmospheric aging equipment to simulate the field environment.
7. What can oil absorption tell us?
Wetting is a very important link in the whole dispersion process. The quality of the wetting effect largely depends on the degree of affinity between the dispersion medium and the surface morphology of the pigment, and the spatial interaction between the molecular morphology of the dispersion medium and the structure of the pigment agglomerates.
Simply put, the oil absorption is actually the minimum amount of oil required for the oil to infiltrate the surface of the pigment particles and fill the gaps between the particles. The specific quantitative method means that the minimum amount of pure linseed oil that can be absorbed per 100 grams of pigment is the oil absorption of the pigment.
Please note that the absorption referred to here refers to the addition of refined linseed oil with a burette while manually blending with a spatula, and finally the mixture of pigment and linseed oil reaches a thick paste state.
For example, the oil absorption of 30g/100g means that 30 parts of oil mixed into 100 parts of the pigment to be tested by the above method will reach the thick paste state required by the experiment.
The oil absorption reflects the specific surface area of a particular pigment to some extent, the lower the specific surface area, the lower the oil absorption and the better the pigment wettability, and vice versa.
8. What measures can I use to improve the hiding power of the paint system?
For most coating applications, hiding power is the basic and main performance requirement. This is fully reflected in yellow paint, because yellow pigments have poor light absorption ability, and the hiding power can only be achieved by light scattering, which is why the industry has always believed that bright organic yellow pigments have poor hiding power.
Therefore, when formulators can only choose a single pigment, they often choose chrome yellow with stronger scattering effect and higher hiding power (the refractive index of inorganic pigments is about 2.5) and give up organic yellow pigments (the refractive index of organic pigments is about 2.5). The reason is that the ratio is about 1.6).
Of course, where the pigments can be compounded, the formulator can give the organic pigments stronger hiding power and tinting power by adding high-hiding inorganic pigments (titanium dioxide, iron oxide pigments).
Carbon black absorbs almost all of the light to make up for the lack of relative absorption and poor hiding power of organic pigments with poor scattering ability. But it must be emphasized that the fewer pigments in the formula, the better the color saturation, and the addition of inorganic pigments that absorb sunlight must be within the allowable range of the formula.
9. What does the paint hiding power index indicate?
When light passes through a transparent medium, it can pass through without any change, and then be reflected on the surface of the substrate. When light encounters an opaque medium, it cannot penetrate but can only be absorbed or reflected. When discussing the optical properties of pigments, we cannot simply describe them as transparent or opaque.
Hiding power refers to the ability of a pigment to cover the background color of an object's surface when it is uniformly applied to the surface of the object in a specific coating system.
The color paint achieves the hiding power index through two ways. These two ways refer to the absorption and scattering of light. For example, black pigments absorb light of any wavelength and have strong hiding power.
Various color-developing pigments achieve hiding power by selectively absorbing light of different wavelengths; white pigments do not absorb any light, and mainly rely on strong scattering to reflect hiding power.