It is different from ordinary pigment because of its processing characteristics. In order to obtain excellent pearlescent effect, the following conditions should be paid attention to: 1. The transparency of the colored plastic resin is better (or high brightness); 2. The pigment with good transparency should be mixed with the pearlescent pigment as far as possible; 3. The damage of the pearlescent pigment sheet in the processing process should be reduced as far as possible; 4. The pearlescent pigment is fully dispersed and the pigment sheet is parallel to the plastic surface orientation. In addition to the above conditions, the characteristics and processing methods of plastic resin will also affect the gloss performance of the products.
The amount of pearlescent pigment in plastics is generally 0.5% - 2%. For thin film or thin-walled products, the amount should be larger, generally 4% - 5%.
Colouring of plastic resin with pearlescent pigment
There are many ways to color thermoplastic resin. Among them, pearlescent masterbatch is better. Direct coloring is simple, but the quality is difficult to guarantee.
Direct coloring of resin powder
When the powder is colored, the pigment only needs to be directly added into the kneader to mix evenly. There are two types of kneaders: rotary type and high-speed type. When using high-speed kneader, ordinary pigment and other additives should be added to the resin powder to mix evenly, and pearlescent powder should be added before the end of mixing, because the lamellar structure of pearlescent pigment is easy to be damaged under the action of high shear force, which affects the pearlescent effect.
Direct coloring of resin pellets
Pearlescent pigment can not be directly added to the resin particles, but must be fixed on the surface of the resin particles with liquid coupling agent to produce a physical or chemical combination. These coupling agents include various softeners and refined mineral oils. In addition, the wettability of the interface was changed by adding emulsifier or high boiling solvent. Generally, the dosage of coupling agent is 0.2-0.5% of the resin weight. If the pearlescent pigment needs to be more evenly dispersed in the resin, 1% low molecular weight polyethylene powder can be added to the mixture of pigment and resin.
When the amount of pigment added is less than 2%, the excess pigment can only be scattered because there is no space for adsorption on the resin surface.
Liquid curing and mixing
Pearlescent pigment can also be mixed with solvent, auxiliary agent and organic pigment to make liquid pigment, namely pearlescent slurry. In case of pulping with organic pigment, the organic pigment shall be ground to the specified fineness in advance, and then mixed with pearlescent pigment. Attention should be paid to the precipitation of pearlescent pigment in the pearlescent slurry. Appropriate anti sedimentation additives should be used and the mixture should be evenly stirred before application to ensure the uniformity of concentration of pearlescent pigment in the slurry.
Pearlescent masterbatch coloring
The color masterbatch method is the best one. Ordinary coarse and fine resin powder can be used to prepare color masterbatch. Because the addition of pearlescent powder will reduce the melting index Shu, the similar polymer with higher melting index Shu and stronger fluidity and lower molecular weight than the colored resin is generally used as the pigment carrier. There are two methods to prepare Color Masterbatch: continuous and intermittent. Generally, the continuous production of twin-screw extruder is used in industrial production.
Precautions for preparation of pearlescent masterbatch: 1. In the mixing process, pearlescent pigment should avoid the action of high shear force for a long time, so as not to damage the lamellar structure of pearlescent pigment and affect the pearlescent effect of the product. 2. Increase the back pressure to improve the mixing effect of screw and fully disperse pearlescent pigment. 3. Increase the temperature in the possible range, reduce the viscosity of the melt, and reduce the structural damage of pearlescent pigment. 4. Mold finish, mold gate design and construction adjustment are also very important, which will lead to disordered arrangement of pearlescent pigments and affect the pearlescent color of products.