:: Literature ::

Rotor Stator yields uniform dispersion in ¼ the time
(Reprinted from Chemical Processing, July 1987)

SCOTT WAGMAN, President, Scott Paint Company
ALAN E. HODEL, Associate Editor

Problem: Scott Paint Company is a small paint manufacturer, located in Sarasota, FL, specializing in both water and solvent based, high quality coatings, including gloss and semigloss enamels. A typical
production batch is 500 gal. Dispersers are used to manufacture the company's products. Making quality paint with a disperser requires a special tank configuration and much time. Growing sales made the company want to increase capacity.

Normally the company would have increased capacity by configuring a suitable tank (such as a conical type tank) and then adding a high speed disperser with a regular sawtooth blade. The plant
could expect to produce a batch of quality paint in 2 hours with such equipment.

For years, Scott Paint had been working very closely with a disperser manufacturer that has a development laboratory and manufacturing plant located about 35 miles away in St. Petersburg. The disperser manufacturer wanted to put a rotor stator type machine in Scott Paint's plant. Scott's close proximity would enable the manufacturer's development engineers to stay in touch and work with the machine.

The disperser manufacturer suspected that a rotor stator type machine would be more efficient in enamel production than a unit with the conventional sawtooth blade. The paint company had no experience with rotor stator technology. The disperser manufacturer offered to sell the machine, put on the rotor stator attachment and, if the company didn't like it, reconfigure the machine back to a regular disperser.

Solution: The rotor stator equipment promised to fulfill the plant's needs for a higher speed, higher volume enamel making machine. The long-term relationship that existed between Scott Paint Company and the disperser manufacturer helped convince the paint company personnel to be receptive to trying a rotor stator.

A variable speed, rotor stator unit was installed. It has four shafts bolted to the bottom of the housing to hold the stator in place. The rotor is mounted on a center disperser shaft that ordinarily would hold the saw-toothed blade. The machine can be disassembled and turned into a fixed shaft disperser with standard configuration by removing eight bolts, if necessary.

Variable speed makes adjustments to mill base and make up viscosities easy to accomplish. A slower speed is used for the mill base premix. When all the pigments are in, the speed is increased to that designated for the batch. Following dispersion, the machine is slowed down for final dilution. Variable speed also gives more flexibility on splashing and lets the operator have control over air entrapment.

The rotor stator head can be moved up and down and the whole machine can be rotated 360'. The installation serves two open-top tanks. The rotor stator is kept about a foot off the walls of the conical tank. The rotor is belt driven at the top through a variable speed mechanism. The machine is bonded to the tanks to eliminate static electricity.

Standard materials used in paint batches, like TiO2, talc, CaCO, can be bought with very controlled particle size The particle size of the raw materials used with the rotor stator machine is the same as that used with the sawtooth blade disperser. The plant did not have to make a significant reformulation. This was a major factor in the decision to use the rotor stator.

A batch is dispersed, let down to final viscosity, given gloss adjustments and then, in order not to waste time, the rotor stator head is lifted, cleaned and moved to the alternate @. While one tank is filling shipping containers, paint is being made in the next. Most manufacturers would have two to three tanks around a disperser. The same operations apply to solvent or water based paints.

The production personnel were surprised because the rotor stator does not make loud noise or give the appearance that it is doing anything special. Its physical vortex action is not pronounced. The dispersion is virtually completed by the time the bagged pigments are in, the sides of the tank are scraped clean, and the height of the blades is adjusted. It takes only 10-15 min to make a paint dispersion with the rotor stator as opposed to more than 45 minutes with the conventional sawtooth blade disperser.

Results:
The immediate benefits of the rotor stator are

• Speed - The machine is producing faster grinds
• Quality of grind - Plant production personnel think they're getting much better total dispersion.
• Much less air entrapment - The rotor stator is not whipping air into the paint like the sawtooth blade disperser did.

If there is any negative, it is that the machine is a little more difficult to clean. There are five shafts to clean as opposed to one shaft with a blade on it.

It is not clear whether or not the unit actually reduces the particle size in the paint dispersion. However, the machine has superior dispersing capabilities. With a semigloss or gloss enamel, there is no noticeable roughness. In addition, the rotor stator saves time. The plant is currently achieving a 7 hegman in 10-15 minutes. Previously, with a 30 hp high speed disperser in a 500 gal tank, dispersion time was 45 min to achieve about the same reading on the hegman scale.

The hegman is a paint scale for measuring extremely fine particle dispersion. A hegman gauge is a stainless steel device that consists of a blade which is drawn across a flat, precision ground surface. The hegman reading is determined as particles start to streak. A reading of 8 is almost perfect and virtually unreachable with a disperser. A reading of 6.5-7 is considered excellent.

This installation is the first time that the plant has configured conical tanks for dispersing. Although the use of a conical shaped tank is an asset to any disperser, plant personnel feel the rotor stator machine would also work well in a round bottom tank.

The biggest hindrance to the acceptance of the rotor stator machine is that there are a number of paint people that will have difficulty believing it will work. It does not generate the kind of physical action that a disperser blade does. It runs quietly.

The prototype unit in use at Scott Paint is made of carbon steel. A potential rust problem exists. The plant circumvents the problem when a water based paint is run by cleaning it quickly and putting it into a solvent based batch. A subsequent unit will be made in stainless steel. Its cost will be 50%-100% more than a sawtooth disperser.

After more than six months of service, the rotor stator machine has required no more maintenance than a standard unit. The machine has been extremely reliable.

Model 30hp VHS500 disperser with rotor stator dispersion head was manufactured by Schold Machine Company, Southern Division, 10590 Oak Street N.E., St. Petersburg, FL 33716.