Compounding Close-Up

A radically different approach to in-line compounding of rigid PVC can replace a conventional batch mixer with a smaller, continuous, high-intensity mixing system. It requires dramatically less space, energy, and processing time. It greatly reduces capital cost for equipment and plant construction by eliminating the need for a mezzanine. And it also greatly reduces the heat history of the resin. It simplifies material handling, allows greater formulating flexibility, saves resin on color changes, and may need only one operator instead of two. It also lets processors add capacity easily, without the high fixed cost of batch mixers.

The newly commercial powder blending system, called Novablend, was developed jointly by Colortronic GmbH in Germany and Waeschle S.r.L. in Italy. Waeschle builds the turbine mixer in Italy, and Colortronic supplies the gravimetric metering/feeding auxiliaries. Colortronic Inc., Runnemede, N.J., will market the system in the NAFTA region. Colortronic says the new mixing approach has an economic advantage for small to intermediate-sized plants with throughputs starting at 5 million lb/yr of compound. It will show the Novablend system for the first time at NPE 2000 in Chicago next month.

 

In and out

The new Novablend system consists of a standard Colortronic eight-component gravimetric blender and liquid doser. They meter ingredients into a specially designed horizontal mixing screw, also developed by Colortronic. Then the material enters the top of the mixing turbine, where it remains for a few seconds in a constantly flowing turbulent stream, and then exits the bottom directly to an extruder or storage vessel. Residence time in the turbine is controlled by a restriction ring.

The mixing turbine is a vertical cylinder 20 in. high and 7-16 in. diameter. The 7-in. size, which holds about 1 liter of material while running, processes 3000 lb/hr. The 10-in. size runs at 6000 lb/hr, the 14-in. size at 8000 lb/hr, and the 16-in. at 11,000 lb/hr.

It mixes with an impeller consisting of two to six interchangeable disc-shaped blades on a vertical shaft. The discs rotate between fixed cone-shaped rings around the inner wall of the chamber. Liquids are fed through nozzles in the cone-shaped rings. Peripheral speed of the impeller discs is similar to the speed of conventional mixer blades--over 7 ft/sec. Discs can be configured differently for different compounds and are easily changed to create different shear gaps between discs and rings.

The unit opens like a clamshell for easy access. Color changes reportedly take 10 minutes instead of several hours for a conventional batch mixer. Energy use is only 10% of that required by a batch mixer. The mixing turbine has a 20-hp motor vs. the 200-hp used by normal PVC mixers. Novablend consumes 8-22 kwh per operating hour, compared with 60-95 kwh for a batch mixer with comparable throughput, says Colortronic.

 

 

Rewriting the rules

The Novablend process flouts a lot of conventional compounding wisdom. "Processors' mindset is that all PVC has to go through hot/cold batch mixing first," says Marcel Rohr, president and CEO of Colortronic. Typical high-intensity batch mixers generate enough frictional heat to raise the batch temperature to 200-240 F. This temperature is thought necessary to permit absorption of liquid stabilizers into the PVC particles.

"The myth is that this heat is necessary to the mixing process," Colortronic's Rohr notes. "But you don't need that much heat. It wastes energy because you need to remove it again."

The Novablend system uses high-intensity mixing but without prolonged heat. The mixing turbine raises the PVC powder temperature to only 125-135 F, which is reportedly sufficient to absorb the small fraction of liquid stabilizers used in rigid PVC. Even flexible PVC compounds, which contain a far larger proportion of liquid plasticizer (25-45%), have been made with the mixing turbine. The liquids are heated to 350 F and injected with atomizing nozzles into the turbine chamber, where they are immediately absorbed by the resin powder.

 

Radical, but proven

Radical as this approach seems, it's based on proven technology that has been used commercially to make rigid and flexible PVC dryblend for over 12 years. The mixing turbine was originally developed by Buss AG in Switzerland in the mid 1960s to make oil-and-water emulsions for pharmaceuticals and was later used as a premixer in front of the Buss Kneader to mix silicones, flexible PVC, and other compounds.

In the mid 1980s, Buss tested the system in Italy for in-line compounding and extrusion into rigid PVC window profiles. Since then, Buss has installed turbine mixers to make dryblend on 10 PVC pelletizing lines and two calendering lines, mostly in Europe. But one of the largest applications is an eight-year-old captive compounding line in the U.S. that premixes 9000 lb/hr of dryblend for extruding flexible PVC pellets. In all these cases, the Waeschle turbine mixer supplies a Buss Kneader. (Waeschle and Buss both belong to the Georg Fischer group.) Buss America Inc., Bloomingdale, Ill., continues to offer systems combining the turbine mixer and Buss Kneader but does not market the turbine mixer on its own.

Colortronic's new Novablend system emerged in response to improved accuracy of blending systems. "Feeding and dosing technology 15 years ago wasn't accurate enough," says Martin Skrabak, general sales manager for Waeschle,which has U.S. offices in Bloomingdale, Ill. "Since the process uses continuous dosing and weighing, you have to have a minimum fluctuation of less than a few grams of material."

The first successful trial of the Novablend system was in making dryblend for rigid PVC profiles at Crane Plastics in Columbus, Ohio, two years ago. The first commercial system will be installed in late April at a U.S. rigid PVC sheet plant, where it will run 4000 lb/hr.