Intense Pigment
The basic material of pearlescent pigments is natural mica. Mica is ground into fine powder of different particle sizes, then coated with titanium dioxide to form different thicknesses of coating. After being treated at a temperature above 700 degrees Celsius and special processing, pearlescent pigments are formed, which are semi-transparent, very stable, and almost chemically inert. They are not affected by temperature during application. Their lamellar structure will not precipitate, frost or melt in the resin. Pearlescent pigments are non-toxic and do not contain metal components. They rely entirely on chemical principles to form different colors. By controlling the particle size of mica and the thickness of the coating, a series of pearlescent pigments of different particle size ranges, white, interference, metallic, and gold color series can be obtained.
Pearlescent pigments are semi-transparent, and their optical properties must be taken into account and utilized in the application process to achieve a good pearlescent effect. In recent decades, the plastic industry has developed rapidly in the world. It also occupies an important position in China’s national economy. With the improvement and development of plastic molding and processing methods, plastic products are becoming more and more diverse. The application of pearlescent pigments in plastics is also becoming more and more popular. From artificial leather, wallpaper, plastic packaging film, cosmetics, shampoo packaging, household appliance plastic parts, to artificial pearls, plastic buttons, and other plastic products, pearlescent pigments are widely used to increase their attractiveness. Pearlescent pigments can be used in almost all thermoplastic plastics and some thermosetting plastics. The concentration of pearlescent pigments used in plastics depends on different application conditions. However, generally 1-2% (by weight of resin) can achieve satisfactory pearlescent effect, and for some thin film products, a pearlescent pigment concentration of up to 4-8% needs to be used. In addition, some plastics with low transparency also require higher concentrations of pearlescent pigments or larger particles to achieve the desired effect.
Materials selection
Since pearlescent pigments are semi-transparent and need to rely on external light reflection to achieve a good pearlescent effect, transparent materials should be used as much as possible when selecting plastics. The following plastics are commonly used, and their common feature is that they are relatively transparent. After adding pearlescent pigments, light can pass through the surface plastic and reach the surface of the pearlescent pigment chip to cause reflection and refraction. HDPE, LDPE, PP, EVA, PMMA, PET, PC, PVC, AK, EPT, and PU are commonly used plastics with good transparency. Plastics with low or no transparency, such as GPPS, HIPS, ABS, etc., have less obvious pearlescent effects, but this does not mean that these plastics cannot use pearlescent pigments. It just means that a larger amount of pearlescent pigments needs to be used.
When it comes to selecting mixed pigments, pearlescent pigments cannot be mixed with conventional opaque pigments such as chromium, cadmium pigments, and titanium dioxide. Ordinary pearlescent pigments can be used in transparent and translucent plastic resins, and using pearlescent pigments can bring a charming color visual effect. Pearlescent pigments are widely used in plastic products such as cosmetic containers, various packaging, toys, decorative materials, various films, and so on.
Regarding the application of pearlescent pigments in plastics, there are some considerations due to the unique processing characteristics of pearlescent pigments. Generally, to achieve the best pearlescent effect, the following conditions should be noted: 1. The transparency (or high glossiness) of the colored plastic resin should be good; 2. Transparent pigments should be used as much as possible in combination with pearlescent pigments; 3. The damage to the pearlescent pigment flakes during the processing should be minimized; 4. The pearlescent pigments should be fully dispersed and oriented parallel to the plastic surface. In addition to the above conditions, the properties of the plastic resin and the processing method will also affect the gloss performance of the final product.
The dosage of pearlescent pigments in plastic is generally 0.5%-2%, and the dosage should be correspondingly larger for films or thin-walled products, usually around 4%-5%.
There are many methods for coloring thermoplastic resins, among which pearlescent masterbatch is better. Direct coloring is simple but difficult to ensure quality.
Direct coloring of resin powder:
When coloring the powder, the pigment can be directly added to the kneading machine and mixed evenly. There are two types of kneading machines, namely, the rotary type and the high-speed type. When using a high-speed kneading machine, ordinary pigments and other various additives should be mixed evenly with the resin powder. Before the mixing is about to end, add the pearl powder. This is because the lamellar crystal structure of pearl pigments is easily destroyed under high shear force, which affects the pearl effect.
Direct coloring of resin pellets:
Pearl pigments cannot be directly added to resin pellets, but must be fixed on the surface of the resin particles using a liquid coupling agent to produce a physical or chemical bond. Such coupling agents include various softeners and refined mineral oils. In addition, adding emulsifiers or high-boiling solvents can improve the wetting properties of the interface. Generally, the amount of coupling agent added is 0.2-0.5% of the amount of resin. If a more uniform dispersion of the pearl pigment in the resin is required, 1% of low molecular weight polyethylene powder can be added to the mixture of pigment and resin.
The amount of colorant added by this method should not exceed 2%. If the amount exceedsthis, there is no space on the surface of the resin to continue adsorbing, and the excess pigment can only scatter.
Liquid-solid curing mixing:
Pearl pigments can also be pre-mixed with solvents, additives, and organic pigments to form a liquid pigment, namely pearl paste. When making the paste with organic pigments, the organic pigments should be ground to a specified fineness before being mixed with the pearl pigment. The pearlescent paste should pay attention to the sedimentation of the pearlescent pigment and use appropriate anti-settling agents. Stir well before use to ensure the uniformity of the pearlescent pigment concentration in the paste.
Pearlescent masterbatch coloring:
The masterbatch method produces better coloring effects. Ordinary resin powder can be used to prepare the masterbatch. Since the addition of pearlescent powder will reduce the melt index, a carrier with a lower molecular weight than the resin being colored, but with a higher melt index and flowability, is generally used. There are two methods for preparing masterbatches: continuous and intermittent. Generally, industrial production uses twin-screw extruders for continuous production.
Notes for preparing pearlescent masterbatch:
During the mixing process, the pearlescent pigment should avoid being subjected to high shear forces for a long time to avoid damaging the crystal structure of the pearlescent pigment and affecting the product’s pearlescent effect.
Increase the back pressure to improve the mixing effect of the screw and fully disperse the pearlescent pigment.
Increase the temperature as much as possible within a reasonable range to reduce the viscosity of the molten material and reduce damage to the structure of the pearlescent pigment.
The cleanliness of the mold, the design of the mold gate, and the adjustment of construction are also very important and can cause the pearlescent pigment to be arranged disorderly, affecting the pearlescent color of the product.