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Higher loads bring processing advantages

Article-Higher loads bring processing advantages

Reinforcing agents provide physical properties that can’t be achieved in resins alone, so these highly-filled compounds continue to hold fascination for plastics processors, who envision a double bonus of lower material costs coupled with improved overall dispersion of fillers, and thus better properties in finished parts, notes Stanley Broadhead, senior sales engineer at extruder manufacturer Farrel (Ansonia, CT).

Oliver Eitel, business segment manager for engineering plastics at compound extruder manufacturer Coperion (ex-Werner & Pfleiderer; Stuttgart, Germany), also sees increased interest and demand for such highly-filled compounds. He says the motivation today to use, for example, highly-filled calcium carbonate (CaCO3) compounds is that the particle size distribution can be chosen, plus the filler is non-toxic, food safe, offers good organoleptic properties, and has a spherical shape and low hardness, resulting in low wear and tear on processing equipment. High-filler content naturally results in lower resin costs, but also provides a powder-free shop floor.

Eitel says such fillers increase the modulus of elasticity of the matrix material, improve anti-block behavior, result in less shrinkage during cooling, and provide higher thermal conductivity compared to the polymer alone. This last feature results in quicker temperature equalization and faster processing speed.

He also says a 50% CaCO3 content in injection molding applications can result in a shorter cooling cycle by half, and 20% filler content in blown film extrusion can give a 20% higher haul-off speed. Farrel’s Broadhead says both injection molders and film extrusion processors generally opt for highly filled concentrates, with loadings up to 85%, to get both desired opacity and lower unit costs.

Traditionally, highly filled polyolefin compounds have been produced by repeated loading via several feed positions on the compounding extruder, followed by mixing up to three or more times. This, says Eitel, is done because CaCO3, with a bulk density of 600-800kg/m3, tends to be sticky and build bridges, often creating headaches for an operator. To overcome such problems, agitators are often used, hoppers are designed with special geometry to improve material drop, or flexible walls can be incorporated that can shake up the filler to break up bridging.

Coperion says experience has shown, in its latest line design for such applications, that during refilling of loss-in-weight feeders using slide, butterfly, or rotary valves, the valve itself needs to be installed closer to the feeder hopper so that it can react quickly in shutting off. This is necessary to avoid overfilling. The company has developed a new layout for its ZSK compounder by mounting the CaCO3 feeder directly above the unit’s co-rotating twin-screw side feeder, ZSB. This setup, says Eitel, results in the need for only two feeders and two refill hoppers for the CaCO3. It also provides a longer mixing section for the filler, better dispersion, and higher output. With the increase by percentage of the compound/weight of filler, the discharge pressure in bar is increased while the specific energy input measured in kWh/kg is reduced.

Farrel takes a different approach to compounding highly-filled concentrates. Its FCM (Farrel Continuous Mixer) operates on the principle that the material is exposed to a variable mixing intensity in the form of shear stress and strain developed between the rotor tips and chamber wall, as well as melt blending by the kneading action between the rotors. “The measure of dispersion through either visual examination or filter pressure values are as good or better than those achieved with twin-screw extruders,” says Broadhead. “Besides we get an added benefit of lower energy costs.” [email protected]

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