Pearlescent pigments, such as fish scale powder, bismuth oxychloride, basic lead carbonate, lead hydrogen arsenate, lead hydrogen phosphite and mica pearlescent pigments, can show profound and crystal like colors under light. At present, mica pearlescent pigment is widely used. Mica titanium pearlescent pigment is a kind of flake inorganic pigment with high refractive index and glossiness. It is based on mica and coated with one or more transparent metal oxide films with high refractive index. Mica titanium pearlescent pigment is usually in the shape of thin sheet, which has a thin layer of high refractive index and plane structure. Under certain plastic forming conditions, the pigment sheets are distributed in parallel in the plastic carrier, producing pearlescent effect. Pearlescent effect is based on the interference of thin film to light. Synthetic pearlescent element is a very thin crystal particle, and it is basically transparent. After it is mixed with plastic, during the plastic forming process, with the flow of plastic, the chip particles tend to be parallel dispersed. The refractive index of transparent plastic is different from that of the wafer. The upper and lower interfaces of the wafer emit part of the incident light, while the crystal also transmits part of the incident light. When the projected part of the light meets the next wafer, it also produces refraction and reflection, and the incident light interferes on each wafer. This kind of multi-layer interference phenomenon makes the plastic with pearlescent pigment show profound and crystal pearl like color under the light.
Pearlescent pigments are widely used in plastic industry because of their light resistance, high temperature resistance (800 ℃), acid and alkali resistance, non-conductive, easy dispersion, color fastness and non migration, as well as safety and non toxicity. According to the different colors, pearlescent pigments are generally divided into silver white series, magic color series, golden light series, metal series, etc.
(1) Silver white series
According to the size of the lamellae, the effect of pearlescent pigment on the weight lifting of plastics is also different.
(2) Illusory color series
For the magic color series pearlescent powder, the interference color can be obtained by controlling the coating thickness of TiO? On the micro mica chip, which is not given by the general pigment. This effect varies with the viewing angle, and the color changes accordingly. The general reflected light of interference golden yellow is golden yellow, and the transmitted light is blue. Interference golden yellow can't be mixed with interference blue pearlescent pigment, otherwise the pearlescent luster will be reduced. The reflected light of interference blue is blue, and the transmitted light is golden yellow. Interference blue shall not be mixed with interference green pearlescent pigment, otherwise the pearlescent luster will be reduced. The reflected light of interference green is green and the transmitted light is red. Interference green shall not be mixed with interference red pearlescent pigment, otherwise the pearlescent luster will be reduced. The reflected light of interference violet is purplish red and the transmitted light is blue-green. Interference violet cannot be mixed with interference green pearlescent pigment, otherwise, pearlescent luster will be reduced.
All the pearlescent powder of the magic color series can be mixed with transparent organic pigments or silver white pearlescent pigments for color matching.
(3) Gold Collection
The gold series pearlescent powder is prepared by adding a layer of Fe2O3 to the coating controlling TiO? On the mica chip and mixing with the interference layer. Compared with metal pigments, gold series pearlescent powder has higher temperature resistance, which can withstand 800 ℃ without discoloration. Because the covering power is weaker than metal pigment, it can be mixed with transparent organic pigment, carbon black and ultramarine to achieve different effects.
As the gold series is coated with Fe2O3 on mica, it is necessary to pay attention to the damage caused by high shear stress in use. At the same time, if the system contains chloride or sulfur ions, it will lead to the formation of ferric chloride or iron sulfide, making the system black.
Pearlescent pigment is very ideal in PVC shoe material and film, plexiglass, acetate fiber and other plastics. In polypropylene, polyethylene, polystyrene and other plastics, the plastic parts produced by injection and extrusion also show obvious pearlescent effect. In the application of pearlescent pigment in plastic colouring, the following problems should be paid attention to.
① In order to get a good pearlescent color effect, the plastic itself must have a certain degree of transparency; in pearlescent coloring, we should also try to choose a variety of transparent additives including other colorants.
② In the process of mixing plastic and pearlescent pigment, the damage of pearlescent pigment wafer should be avoided as much as possible. If the plastic compound is mixed by internal mixer, pearlescent pigment should be added at the later stage of mixing.
③ Prevent the occurrence of flow marks between the intermediate materials and rationalize the structure of abrasive tools. The commonly used measures are: to design the gate of the mold larger or change the gate shape to wedge or flat; to design the material hole at the position where the material flow mark may occur. In the process of molding, the material temperature and mold temperature should be increased properly to reduce the viscosity of plastics.
④ In order to enhance the pearlescent effect, the mold finish should be improved.
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