Premixed pigments

Since the binder systems are solid at room temperature, they can be produced by the existing methods used for powder coafingsd Solid resins, pigments, photoinitiators, and other additives are premixed, then melted and dispersed in an extruder at 100 to 130°C (212 to 266°F). The molten blend is then squeezed into a thin ribbon between chilled rolls. This ribbon is further cooled to near room temperature on a water-cooled cooling belt. The cooled ribbon is broken first into flake and then ground into a fine powder ready for use. The process is illustrated in Figure 7.15. 

Because the binder systems are solid at room temperature, they can be produced by the existing methods used for powder coatings.32 Solid resins, pigments, photoinitiators and other additives are premixed, then melted and dispersed in an extruder... 

In the premix process, all additives are premixed in a mixer and then fed to the twin screw extruder at the primary intake stage via a volumetric feeder. A portion of the polymer is generally mixed with the pigments in powder form to achieve a better pre-distribution of the pigment (see Fig. 4.31). 

The pressing processes do not provide any dispersing action for the metallic pigments. The premixed components are only brought into the desired form and then sintered (melted). The homogenous appearance therefore depends totally on the preblend of all powdered components. 

Method of manufacture. Premix the vehicle in a clean vessel under HSD. Add the pigment and disperse to less than 12 pm, using the required solvent as required to maintain a suitable consistency. Add the letdown under an efficient stirrer. Adjust the viscosity with the final solvent. 

A premix is required for these mills. The mill paste normally has a consistency of 90-140 Kreb units. The mill base should be smooth and free from lumps, since the residence or dwell time in the mill is very short. The vehicle solids for the paste can be as low as 20% for low oil absorption pigments and up to as high as 75% vehicle solids for high absorption pigments. The capacity of the mill will vary with viscosity of the paste. 

Since the premix is a viscous composition, an intensive heavy duty mixer is required. The vehicle and then the pigment are added to the mixer. Sufficient vehicle to wet the entire pigment load must be used, and there must be enough vehicle so that the paste is fluid enough to charge the mill. The pigments must be thoroughly wet and free from lumps. 

Roller mills have certain advantages. They can handle viscous materials and produce high-quality dispersions of fine particle size these are necessary in specialty coatings such as inks. Difficult-to-disperse pigments are handled by these mills. However, these mills are costly because the mill base throughput is at best moderate and a premixed paste is required. While the roller mills are used usually for specialty coatings, other mills can handle easy-to-disperse pigments more efficiently. 

A relatively good quality premix is required for this mill, but all types of pigments can be used. It is difficult to clean because the paste can dry on the screen and the screen can become clogged with media. These mills disperse by shearing and by some impingement. 

No premix is needed in the Kady mill. The vehicle is fed into the tank, the motor is started, and the pigments are added at a relatively fast rate. The mill is allowed to run for about 15-45 min until the required degree of dispersion is attained. The dispersion is rapid, the mill is easily cleaned, but pigment throughput is usually low. Since the mill depends on Impact for dispersion, the more easily dispersed pigments are reserved for this mill. 

Some raw material suppliers offer precompounded, ready-to-use blends containing all the required additives. There are also on the market premixes of additives with PVC (e.g., Vinnolit K 301 is a premix of PVC with 15 wt% of antistatic). But most processors do precompounding using automatic in-house blending stations. Vestolit P 1982 K is another example of precompounded material. It is ready-to-use window compound complete with stabilizers, lubricants, fillers and pigments. 

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