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Processing polyolefins on single-screw extruders.

Processing Polyolefins on Single-Screw Extruders

The molecular weight distribution of the resin has an important effect on the output rate and pressure development in the extruder, thereby influencing productivity and product uniformity.

Most thermoplastic products are fabricated from pellets or granules of resin that are melted and extruded in a single-screw extruder or in a single-screw plasticating unit of an injection molding machine. Desired product properties are achieved by selecting a resin having specific attributes, such as particular molecular weight (MW), molecular weight distribution (MWD), and additive levels. The process stability, production rate, and energy consumption of the extrusion system strongly depend upon these resin characteristics.

For example, the first attempts at processing narrow MWD linear low-density polyethylene (LLDPE) into film in the early 1980s frequently led to major equipment modifications because of the material's higher torque requirements. Drives were re-geared to increase the available torque. Screws were modified or replaced to decrease the specific energy transmission to the material, which resulted in excessive melt temperatures, causing output and stability problems.

Major considerations in the design of an extrusion system and the screw configuration are output rate, resin type, resin molecular weight [or melt flow rate (MFR), melt index (MI), intrinsic viscosity, etc.], feed composition, melt temperature, and pressure requirement. If the system is required to process various grades, the design is compromised in order to accommodate the differences in feed materials.

If the extrusion system is designed specifically for a well-defined resin grade, its performance for a different grade (due to supplier change or new product requirements) is often difficult to predict. A main reason is our lack of detailed understanding of the effect of resin parameters on extruder performance. This article describes the effect of one important parameter--resin MWD--on the performance and design of single-screw extruders.

Polyolefin MWD

The MWD of polyolefins has an important effect on polymer performance. Narrow MWD favors applications requiring low die swell, low melt strength, and better optical properties. Narrow MWD provides higher impact strength in injection molded parts because of greater anisotrophy, and lower elongation and higher tenacity in fiber applications because of improved uniformity.

Broad MWD, on the other hand, favors applications requiring increased melt strength or melt elasticity. The high molecular weight "tail" also provides the possibility of higher tenacity and orientability in these applications. In extrusion coating, MWD "tailoring" of polyethylene and polypropylene has long been practiced to provide a balance of performance in the process, including the ability to draw down at high line speeds with minimal edge "neck in."

Polyolefin MWD is determined by the catalyst employed, polymerization conditions, and post-reactor finishing (pelletizing) steps. Different catalyst systems produce different ranges of MW and MWD resins.

Experimental

Commercial narrow and broad MWD pairs, with the same MFR or MI, of polypropylene (PP), LLDPE, and high-density polyethylene (HDPE) were studied. The PPs are used in sheet extrusion and thermoforming. The broad MWD LLDPE is used in profile extrusions for drip irrigation, and the narrow MWD LLDPE is used in blown and cast stretch-wrap film. Both HDPEs are used in blowmolded detergent container applications.

Resin MWs, MWDs, and melting behaviors, as characterized by differential scanning calorimetry (DSC), are shown in Table 1. Slightly varying but similar melting behaviors were observed for the pairs of materials.

Melt viscosities were determined by capillary rheometry. The difference between the broad and narrow MWD PPs, shown in Fig. 1, is typical of all the polymers studied. The melt viscosity of the broader MWD resin always showed greater shear rate sensitivity. Note that the viscosities of the two polymers are equal only at low shear rate, where MFR is determined. This difference in rheological behavior is the key to understanding the difference between the extrusion behaviors of broad and narrow MWD resins.

Used in this study were 4.5-in, 2.5-in, and 3/4-in 24:1 L/D extruders. The 4.5-in extruder was well instrumented and had a low shear barrier screw with a Maddock mixing section located at 2D from the end of the screw. The three screw configurations used in the 2.5-in extruder are given in Table 2. The 3/4-in extruder had a conventional metering screw with accurate torque measurement capability. The two larger extruders had pressure taps along the barrel that enabled comparisons of the pressures generated by the different screws and materials.

Downstream resistance was kept constant by use of the same die for all materials run in each extruder. The observed head pressure differences are therefore the result of material response and not the mechanical configuration. The same temperature profile was also used for each MWD pair. No attempt was made to optimize the performance of individual resins.

Output Rate

In all cases, outputs ranked in this order: LLDPE > HDPE > PP; also, the broad MWD resin of the same MI or MFR had a lower output rate than the narrow MWD grade. Output rates for the 4.5-in and the 2.5-in (BS-1 screw) extruders are shown in Figs. 2 and 3, respectively. Similar results were obtained with the 3/4-in extruder, except that the differences were much smaller than shown for the larger extruders.

Melt Temperature

The broad MWD resin consistently achieved a lower melt temperature than its narrow MWD counterpart. This is significant in many extrusion applications of low MI/MFR resins. The results obtained on the 4.5-in extruder (Fig. 4) show that LLDPE had the greatest difference in melt temperature between the broad and narrow MWD grades. This is consistent with the shear viscosity differences between the broad and narrow MWD grades found for the three resins. The melt temperatures of the broad MWD and narrow MWD grades of PP differed by 10 [degrees] F to 12 [degrees] F, while the HDPE grades (not shown in Fig. 4) had only very small melt temperature differences of < 5 [degrees] F. Again, similar results, only with much smaller differences, were obtained on the 3/4-in extruder.

Extruder Torque

As would be expected from the output and melt temperature results, the broad MWD grade has a lower torque for screw rotation than the narrow MWD grade of the same MI/MFR. The results for the 4.5-in extruder, shown in Fig. 5, are similar to those obtained on the 2.5-in and 3/4-in extruders, with the latter having much smaller differences between the broad and narrow MWD grades. For high MW resins, such as the low MI/MFR grades used in these experiments, the lower extruder torque may be very important. It enables an existing extruder with low torque capability to run the resin successfully without being limited by the maximum available motor amperage.

Pressure Profile

The differences in output rate can be visualized by examining the pressure profile along the extruder. The profiles at 50 rpm for the 4.5-in and 2.5-in extruders, shown in Figs. 6 and 7, respectively, indicate that the pressure levels for the narrow MWD grades are greater than for the broad MWD grades. The pressure levels for each material type are also ranked in the same order as the output rates, that is, LLDPE > HDPE > PP.

The lower head pressure for the broad MWD resin is the result of the lower output and lower melt viscosity at the high shear rate corresponding to flow through the die. However, a higher output rate would be expected at lower die pressure (because of greater shear sensitivity) if the solid melting, conveying, and pumping of the two grades were the same. This result clearly demonstrates that the differences between the broad and narrow MWD grades result from one or a combination of the three basic extrusion processes.

Screw Design

These experiments were conducted to gain some insight into the effect of screw design on the extrusion behavior differences of broad MWD resins vs. narrow MWD resins. Results are tabulated in Table 3 in terms of the output ratio--the ratio of the narrow MWD grade to the broad MWD grade at each extrusion condition. The screws used in these experiments are described in Table 2. The BS screw used in the 4.5-in extruder is similar to the BS-2 barrier screw.

The output differences between the broad and narrow MWD grades are different for the three 2.5-in screws evaluated. The screw design dependency is expected if we envision that the melting characteristic of the screw is dependent on both the resin and the screw geometry. The rate differences observed for each screw design are fairly constant over a wide range of output rates and screw speeds. The simple metering screw, which shows the smallest differences between the broad and narrow MWD grades, is worthy of further investigation.

Conclusions

For each polymer of the same MI or MFR, the broad MWD grade has the advantage of lower melt temperature and lower torque, while the narrow MWD grade has the higher output rate. A quantitative account of these differences in extrusion behavior requires a detailed consideration of the nature and degree of the MWD differences and their impact on the rheological properties of the polymers and on the solid conveying, melting, and metering characteristics of the extruder. These and other factors are being studied and will be reported separately. [Tabular Data 1 and 2 Omitted]

PHOTO : FIGURE 1. All broad MWD grades had the greater shear sensitivity. Shown are 1.2-MFR PPs.

PHOTO : FIGURES 2 and 3. In all cases, the narrow MWD grade had the higher output rate. Data are for 4.5-in (left) and 2.5-in, BS-1 screw extruder (right).

PHOTO : FIGURES 4 and 5. The broad MWD grades had lower melt temperatures (left) and lower torques (right). Data are for 4.5-in extruder.

PHOTO : FIGURES 6 and 7. In accord with previous results, the broad MWD grades had the lower pressure profiles. Data are for 4.5-in (left) and 2.5-in, BS-1 screw extruder (right) at 50 rpm.

R.E. Christensen and C.Y. Cheng Exxon Chemical Company Baytown, Texas
COPYRIGHT 1991 Society of Plastics Engineers, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Author:Christensen, R.E.; Cheng, C.Y.
Publication:Plastics Engineering
Date:Jun 1, 1991
Words:1660
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