Preheating your filler makes compounding easier.

Faster mixing, lower energy consumption, reduced equipment wear, and improved product quality in filled thermoplastics can all result from a simple process modification: preheating the filler before mixing it with resin. So says Norman Shorr, a registered professional engineer who is co-owner of a patent on the process and is interested in licensing it to manufacturers. (U.S. Patent #4,980,390 was issued December 25, 1990 and recently revalidated.)

Shorr notes that conventional "cold" filler addition to the mixing cycle has several disadvantages. One is difficulty in maintaining consistent mixing cycles or production rates during seasonal temperature and humidity changes, especially if the filler is stored outdoors. During the winter months, unheated fillers (which may have greater levels of condensed and adsorbed surface moisture than in summer) can require as much as 15-20% longer mixing times, Shorr says. Furthermore, since mixing generally relies on frictional shear heating to accomplish plastication and dispersion, excessive wear can result on compounding equipment blades, screws, barrels, and walls. High friction can also break down the aspect ratio of the filler, degrading the properties of the finished compound.

PREHEAT SAVES TIME, ENERGY

Shorr notes that fillers can be pre-heated by a number of means. For instance, a belt conveyor could transport the filler past a source of radio-frequency (RF) or infrared energy. Alternatively, filler can be fed by a hollow-flighted auger that has internal fluid heating.

Whatever the means used, Shorr says the input of significant amounts of thermal energy (via the filler) at the very onset of the mixing process can greatly speed melting--achieving nearly immediate plasticity and providing a quick reduction in mass viscosity. This reduces the need for further energy input via intensive shear friction, and in turn reduces mixing time, equipment wear, and filler degradation.

Faster mixing is also achieved by evaporating surface moisture, which can act as a barrier to wetout of the filler by the polymer. In addition, resin to which preheated filler has been added becomes tacky almost immediately, which helps it "grab hold" of light, fluffy, fine particles such as carbon black, which otherwise can be quite difficult to compound.

Results of various tests show that pre-heating reduces mixing times by 15-30%, with an average reduction greater than 20%, Shorr says.

The example shown in the accompanying schematic is representative data for processing polypropylene and talc filler in a Banbury-type high-intensity mixer. The upper curve shows a batch with unheated resin and filler. The lower curve shows filler pre-heated to 250-350 F and resin at 74 F.

Note that conventional mixing of unheated filler requires a fairly high and constant electrical power draw during the first two-thirds of the mixing cycle. That portion of the curve represents grinding of solid materials and a very viscous mass against metal, which tends to cause equipment wear.

By contrast, note the much lower power draw for the batch with pre-heated filler. Region "A" between the two curves denotes the energy savings from lower-viscosity mixing.

The preheated batch also achieves a final mix about 30% sooner. The area under the curve in region "B" marks the energy savings from this factor. Total energy savings equal "A" plus "B." That savings more than offsets the energy expenditure for preheating, Shorr claims.

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