Dyes are perhaps the most common additives used in the plastics industry, and it is the variety of colors available that makes products attractive to customers. Although dyes are primarily used for beauty, they also have other benefits, such as the ability to absorb ultraviolet light by soot. Dyes are used in a variety of polymers. The four most common types are pre-painted compounds, paint sprays, dry paints and liquid paints. Pre-painted compounds are ready for final processing. This type of pigment is combined with the polymer in a certain ratio before the polymer enters the extruder. This is the easiest way to color polymers because it does not require additional materials or equipment other than an extruder. In addition, the color uniformity is usually excellent. However, this method is expensive because it requires a step for the initial composition of these compounds. Dyes are often granules that contain a high percentage of pigments that are combined with a suitable polymer carrier. These granules are then added to other materials in a certain ratio to achieve a certain color. Dyes are often used in about 4% of the total materials and require minimal activity and equipment compared to when pre-painted compounds are used. If the material in the extruder is mixed properly, this is one of the most common methods for painting plastics. Dry paints are colored powders that are used without a polymer carrier. Although these materials can be used with a variety of polymers, they are somewhat difficult to work with. Very fine particles of these materials are scattered in the air like dust and cause pollution. Also, due to their low volumetric density, they may clump at the feed intake area and not enter the extruder evenly. Liquid paints contain liquid-based pigments. The use of liquid paints has solved some of the problems of working with dry paints, however, a pump must be used to transfer them accurately. Because the concentration of pigments in liquid paints is high, their consumption is low (about 1%). Although these are four methods of transferring dyes to polymers, there are basically two types of base dyes associated with these methods: dye and pigment (insoluble dyes). Pigments produce very high quality paints (bright appearance) on polymers. These materials dissolve in the polymer and are therefore easily distributed. Pigments are limited in terms of temperature stability, so pigments appropriate to the temperature range of the polymer should be used. Pigments are mainly used to dye transparent products. Pigments are tiny colored particles that usually need to be completely distributed to create a uniform color. Pigments are based on organic or inorganic compounds, but are generally more thermally stable than pigments and also have better properties (coating strength), but their luminosity is not as good as that of pigments. Until now, most pigments were based on heavy metals such as lead and cadmium, but environmental considerations have led to the production of a variety of heavy metal-free pigments.
