High speed disperser

Rubber-mill chips are dissolved similarly to resins, to provide color concentrates. Dough mixer and chip concentrates must be diluted with solvent and other vehicles to make finished inks. Media milling is becoming a method of choice in both flexo and gravure ink manufacturing. Other high speed dispersing units, such as the Morehouse, Cowles, Kady, and others, are also used. 

Technical data. Fundamentals of High Speed Dispersion, Myers Engineering Inc., BeU, Calif. 

For the purpose of paint formulation the most important units of equipment are the laboratory ball mill, bead mills and high speed dispersers. The most common, the ball mill, consists of a cylindrical porcelain vessel a little more than half filled with steel, porcelain balls or pebbles. Pigment, together with sufficient binder and solvent to make a free-flowing mix, is loaded into the mill until it is approximately two-thirds full. The mill is then closed and fixed into a device whereby it is made to rotate about its major axis. Normally, a period of about 16 hours is required for thorough dispersion of the pigment, whereupon the mill-base is emptied out and blended with the remainder of the ingredients. 

The appropriate amount of biocide carrier composition needed to yield a specified dry film concentration of biocide was added to a fixed quantity of waterborne acrylic paint and premixed using a high-speed disperser. This dispersion was then transferred to a Silverson-type mixer to obtain a finer dispersion. At this stage the influence of addition of silica or zeolite on the physical/mechanical properties of the paint film was assessed, though only minor changes in properties were noted, which could be eliminated by appropriate adjustments to the paint formulation. 

The emulsification process is simple but must be carefully controlled. Epoxy resin is loaded into a high-speed disperser, and the surfactant is added. A defoamer is generally added to prevent excessive aeration, and high-shear mixing is employed. Water is then slowly added to the mixture. The system at this stage has the epoxy resin as the continuous phase and the water as the dispersed phase. As the water addition continues, the ratio of the dispersed phase to the continuous phase increases until a phase inversion occurs. The inversion occurs at about 65 percent volume ratio of dispersed to continuous phase and is accompanied by a rapid reduction in viscosity. Water addition is then continued until the desired solids concentration is achieved. Additional additives and modifiers can be incorporated into the formulation at this stage. 

The basic high-speed dispersing impeller blade is a flat disk with teeth or similar serration around the diameter, as shown in Fig. 17.1. On some models, disks may be without teeth, or several may be stacked up with small gaps between disks so that material between the disks is sheared as the impeller turns. Sometimes two shafts are used in the same mixing chamber their disks may interlock and counterrotate, thereby enhancing shear. 

FIGURE 17.1 Typical blades for high-shear, high-speed dispersers. 

High speed dispersion vertical mixers which are fitted with a variable speed drive and a toothed disc at the end of the vertical shaft. This type suits the lower viscosity plastisols used in wall and floor coverings. 

Hochberg, S. Bosse D, "Agitator Ball Mill - Sand Grinding Process, U.S. Patent 2 855 156 Nov. 1958 Bosse, D. "Pigmented Systems and Application of the Sand Grinder" Purdue Univ. Short Course March 1959. Daniel, F. "Determination of Mill Base Composition for High Speed Dispersers" Jour, of Paint Technology... 

Method of manufacture. Load the vehicle into a clean vessel. Add pigments slowly under high speed dispersion (HSD). Disperse to 40 to 50 pm. Add the remainder in order while stirring. Adjust viscosity and pH. 

Testing procedure. Water is added to a blender, followed by ammonium hydroxide to adjust pH to the required level. Carbon black is added in 30 s with low speed mixing. Dispersing agent is added in portions from a burette under high speed dispersion. The endpoint is determined as 5 min of uninterrupted fluidity under high speed mixing. The total time of determination should not exceed 10 min. Standard methods. The method is described in Cabot Corporation Technical Report S-131. 

Conical mixers are also known as Nauta mixers (Fig. 18-53). Material placed in the conical bin is lifted by the rotation of the helical screw, which in turn is rotated around the wall of the cone. The lifting actions of the screw combined with motion around the cone provide bulk mixing for flowable dry powders, paste materials, and even viscous fluids. The specific energy input is relatively small, and the large volume of the mixers can even provide storage capacity The mixers may have multiple screws, tapered screws, and high-speed dispersers for different applications. At constant speed, both the mixing time and power scale up with the square root of volume. Sizes from 0.1 to 20 m3 (3.3 to 700 ft3) are available. 

High-Speed Dispersers Horizontal Mill Kinetic Dispersion Mills Other Mills... 

High-Speed Dispersers. The high-speed disperser consists of a disk that rotates at high speed in the center of a vertical tank (Figure 8). The disk, mounted on a shaft, rotates at a speed of 4500-5500 ft/min. Although there are stationary shaft dispersers, most high-speed dispersers are designed so that the shaft can go up or down the shaft in some cases can be moved sideways. The disk is driven either by a variable-speed or a two-speed motor. 

The disk (impeller) is the key feature of a high-speed disperser. The original impeller was a sawtooth disk (Figure 9) it and variations of the sawtooth disk are still the most popular. The teeth are formed by bending the edges of the disk alternately up and down. The slant and height of the teeth can vary. 

Figure 8. Variable high-speed disperser. Courtesy Jaygo, Inc.

Figure 9. Type of impeller for high-speed disperser. Courtesy Morehouse-Cowles, Division of Morehouse Industries, Inc.

Still another method for determining the mill base for the highspeed disperser is the wet point/flow. It was developed in 1946 for ball and pebble mills, but since then it has been successfully used on the sand mill and on the high-speed disperser. The method developed by Daniel is quite simple, but some experience is needed to obtain the correct interpretation (18). 

Myers, C. K. "Bud" "Fundamentals of High Speed Dispersion" Myers Engineering, Inc. Bell, Calif. 1976 p. 11. 

Paint mills are available in different sizes to meet production requirements for high viscosity. (See Table XIV) Other mixers available for high speed processing of sealants include the SIGMA blade type supplied by Hockmeyer Corp. or the high speed Meyers mixer. High speed dispersers are generally limited to compounds up to 600 poises in viscosity. 

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