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Split-feed compounding of highly-filled polymers.

Split-Feed Compounding of Highly Filled Polymers

Twin-screw extruders are well established in the compounding of highly filled polymers, where 20% to 90% filler rates are common. Their main advantage over conventional single-screw extruders is their capability of positive conveying and of establishing full melt in short, high shear areas. The mixing action in twin screws is much more intense, resulting in homogeneous quality at stable extrusion rates. However, even with a co-rotating, intermeshing twin-screw extruder, melting a preblend of fine filler powder and polymer resin can be a challenging task. The fine powder insulates the pellets from the heat of the barrel wall, and, by acting as a lubricant between the solid pellets, inhibits the introduction of friction. There is also high abrasion when melting finally occurs under compression.

The compounding process can be improved significantly by separating the melting step from the filler-mixing step. This is achieved by moving the feed location for fillers downstream to a barrel section where homogeneous polymer melt is found. In this article, typical compounding operations are presented, and the effect of filler loading on process conditions is discussed. As a result of these studies, an optimum compounding line is suggested that should provide minimal barrel wear, low specific energy input, and excellent dispersion.

Split-Feed Compounding

Use of the split-feed compounding technique generally results in the following process improvements: * Higher rates per revolution. * Lower product temperatures, as specific energy is reduced. * Elimination of secondary agglomerates from compacted filler (which is experienced during pellet melting). * Reduced wear of barrel and screw.

Three methods of downstream feeding of fillers have been developed:

1. Fillers are dropped from the auger of the feeder into the extruder barrel. This methods offers a very simple solution and may function adequately for easy flowing, nonfluidizing powders or glass fibers. The shape of the barrel opening can be selected based on the viscosity of the melt.

2. Fillers are added via a vertical feed screw into the extruder barrel. This method was developed to ensure continuous flow by agitating the filler flow with the auger screw, thereby preventing bridging or settling out at the hopper walls. The vertical feeder has a certain capability of generating pressure, which helps to clean the extruder barrel opening and to push melt and agglomerates back into the extruder. The vertical feeder, however, has its limitations when feeding fine particle size powders, which have a tendency to entrap air and become fluidized by the action of the vertical fall and the agitator. In addition, hot air or volatiles can rise from the process and interfere with the flow of the filler.

3. The deficiencies of the vertical feeder initiated the development of the twin-screw side feeder, the latest advancement in the feeding of very fine powder grades to a twin-screw compounder. In combination with the modular screw and barrel setup of the main extruder,it allows optimization of the compounding process for a wide range of fillers and pigments. Consisting of a short, twin-screw extruder with feed hopper and agitator, it is rigidly connected to the side of a special extruder barrel section (Fig.1). At the same location, the extruder barrel is vented at the top to permit volatiles and entrapped air to escape without interfering with the feed stream.

Process Example

Fine particle size talc is used for special polypropylene and polyethylene compounds, as a nucleating agent for film and fibers, and to improve impact properties and fire retardancy. Thus talc is no longer used only as a cost-saving filler but also to improve product quality.

A talc powder's bulk density of 200 g/l can easily change to 50 g/l through agitation and various feed actions. However, after proper compounding, it will end up at the specific gravity of talc, 2500 to 2800 g/l. This means that the talc stream going into the extruder contains 98% air and 2% solids.

A side feeder in combination with the proper extruders setup, as shown in Fig. 2, forms a compounding system that separates the process tasks into the following steps: melting of base resin; conveying of filler powders from the metering feeder to the extruder barrel; separating air from solid particles and removing air from the process; mixing and wetting out of the filler; vacuum venting of moisture and volatiles; and discharge and pelletizing.

Effect of Filler Geometry

Fillers can be categorized as 1-dimensional, such as glass and carbon fibers; 2-dimensional, such as mica platelets and calcium carbonate; or 3-dimensional, or spherical, such as talc and kaolin. It is important to evaluate the different behavior of the three filler types on the twin-screw extruder for the compounding process.

All three were processed at a 40% filler level in nylon 6/6 at a rate of 700 kg/hr, using the same screw speed on the same equipment setup. Figure 3 shows that processing of the 1-D glass fibers consumed 30% more energy than the mixing of the 3-D kaolin, with the 3-D mica in between.

In the case of glass fibers, the additional energy was used to destroy or break down the fibers. The small-particle-size kaolin experienced no particle-size reduction; mixing was accomplished by distributive action rather than dispersive (particle reduction) action. Because glass fiber breakage should, of course, be kept to a minimum, the mixing length for glass fiber compounding is generally 50% shorter and less intensive than for kaolin.

Effect of Filler Particle Size

and Ratio

Talc of various bulk densities (and particle sizes) was compounded with polypropylene (PP) on a ZE90-A twin-screw extruder using the side feeder. The typical performance data given in Fig. 4 show that the output rate is a linear function of the bulk density of the talc. For example, at the 40%-talc ratio, the rate is 1600 lbs/hr at a 200-g/l bulk density, and 2700 lbs/hr at a 450-g/l bulk density under identical operating conditions. Increasing the talc ratio makes the mixing process more difficult and decreases the output rate.

Effect of Resin Viscosity

Calcium carbonate with a bulk density of 340 g/l was compounded with 15-MFI, 0.8-MFI and 0.3-MFI PP. Figure 5 shows that for a given screw speed the lower-viscosity melt accepts a higher filler loading, as wet-out is easier. For the 0.8-MFI and 0.3-MFI high-viscosity grades, a maximum filler rate of approximately 300 kg/hr was reached; a further increase in screw speed would not affect the conveying and mixing capacity. The residence time in the mixing section is too short, and therefore, the twin-screw extruder would not accept any more filler. For the 15-MFI grade, the filler rate was increased to 650-kg/hr CaC[O.sub.3] without reaching the mixing limit of the twin-screw extruder. In this case, the limiting factor was the conveying capacity of the side feeder.

Effect of Filler Level on

Product Temperature

During compounding, the melt temperature should stay low enough to avoid degradation of the base resin. However, mixing at the highest possible product temperature is favorable because the reduced viscosity makes wet-out easier.

At higher filler levels, temperatures would be expected to drop because of the cooling effect of the downstream addition of the fillers. This, however, is not experienced, as shown in Fig.6. The Melt temperatures increase from 260 [Degrees] C to 280 [Degrees] C when the talc ratio is increased from 25% to 50% for the 8-MFI PP. For the tougher 0.9-MFI PP grade, product temperatures are higher, increasing from 300 [Degrees] C to 312 [Degrees] C with the same increase in talc ratio.

Higher filler ratios are achieved in general, at higher screw speeds, which increase the energy input. Melt temperatures and specific energy input are linked together. Figure 7 shows that a 10% to 20% increase in specific energy is experienced when the talc level is increased from 25% to 50%.

Effect of Filler Level

on Motor Load

Talc/2.3-MFI PP compounds were prepared on a 96 -mm twin-screw extruder operating at a constant 330 rpm. The talc stream was kept at 500 kg/hr, the maximum rate of the 60-mm twin-screw side feeder. The talc ratio in the compound was increase from 40% to 70% by simply adjusting the polymer feed. At 40% talc, the total rate was 1250 kg/hr. At 70% talc, the total rate was 714 kg/hr, of which only 214 kg/hr is polymer. For the mixing tasks, this means that only one third of the polymer used at the 40% level is available to wet-out the same amount of filler!

Figure 8 shows that the motor load dropped nearly linearly with increasing talc ratio. Most of the energy input is used for polymer melting. Mixing of talc is distributive and does not consume much energy. At this point, it appears that the filler limit is approximately 70% talc. Higher talc ratios were not accepted by the extruder. They caused fluidization in the atmospheric vents or in the side feeder.

Simulations With an

Internal Mixer

General pretesting of new filler types and resin formulations on an internal mixer can help predict maximum filler loading, mixing time, and torque requirements. After the results from a series of different compounds on the twin-screw extruder are confirmed, the torque curves from an internal mixer, such as a Haake or Brabender, can be used for setting up the mixing length on the twin-screw extruder.

On the typical torque-rheometer curve shown in Fig. 9, a 3.4-Nm torque peak is observed after loading the 40% mica/PP blend in the mixing bowl. During the 8-min fusion time, the torque increases to 6.8 Nm, at which point the melting process if fully established. Figure 10 shows that reducing the mica level to 20% cuts the fusion time in half, but at 50% mica, the fusion time is 14.5 min. This indicates that mixing time is increasing disproportionally at higher filler levels.

The second torque peak on Fig.9 can be informative about the energy input of the product. In Fig. 11, the glass fibers are shown to use 40% higher torque than either mica or talc in compounding 40% filler/15-MFI PP powder blends. This correlates well with the mixing experiments on the twin-screw extruder (Fig.3), where glass fibers used the highest energy input. However, the fusion time for the glass fiber is the shortest (2.4 min versus 13.5 min for talc; see Fig. 12) . This also reflects our mixing results on twin-screw extruders.

Finally, Fig. 13 shows fusion time decreasing linearly with increasing rotor speed of the internal mixer for a 40% talc/PP powder blend. The same effect is found in continuous compounding tests on the twin-screw extruder. Therefore, the experiments on the internal mixer can be used to simulate and predict mixing performance on the twin-screw extruder, especially after databases for resins and filler grades have been established for both systems.

Conclusions

The twin-screw side feeder offers the most advanced technique for mixing fine fillers into a twin-screw compounding process. The system operates at a higher stability level than conventional feeding techniques and yields compounds of a consistent quality. Bridging in the feed zone is eliminated. Fluidizing of the filler, experienced with vertical feeding devices, is greatly reduced because the entrapped air can escape through a separate vent near the side feed location.

From a series of compounding experiments, the following key parameters for the basic mixing process have been established: * Three-dimensional fillers require less energy input for compounding than 1- or 2-dimensional fillers. * Performance rates are dramatically increased when larger particle sizes of the filler are used. * The mixing process is more difficult for high-viscosity resins, and therefore, they require more mixing length in the extruder. * The mixing step consumes a low amount of energy compared with the melting or pressurization steps.

Compounding of fine fillers is accomplished by distributive mixing rather than dispersive mixing. The twin-screw extruder should be set up with the correct screw geometries for this mixing task.

PHOTO : FIGURE 1. Downstream filler addition with the ZSFE side-feed extruder.

PHOTO : FIGURE 2. Twin-screw extruder setup for compounding of fine fillers.

PHOTO : FIGURE 3. Different shaped fillers require different compounding energies.

PHOTO : FIGURE 4. Output rate is a linear function of the talc bulk density.

PHOTO : FIGURE 5. Higher viscosity grades accept lower maximum filler rates.

PHOTO : FIGURE 6. Melt temperatures increase with increasing talc ratios.

PHOTO : FIGURE 7. Higher talc ratios require higher screw speeds, which incsrease energy consumption.

PHOTO : FIGURE 8. Motor load at constant screw speed drops at higher talc ratios because there is less polymer to melt.

PHOTO : FIGURE 9. Typical torque-rheometer curves from an internal mixer experiment.

PHOTO : FIGURE 10. Internal mixer torques times increase nonlinearly with mica ratio.

PHOTO : FIGURE 11. Internal mixer torques mirror twin-screw extruder energies.

PHOTO : FIGURE 12. Internal mixer fusion times for various fillers.

PHOTO : FIGURE 13. Internal mixer fusion times decrease at higher rotor speeds.
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Author:Mack, Martin H.
Publication:Plastics Engineering
Date:Aug 1, 1990
Words:2159
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