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A study of the effect of the extrusion variables and screw design on the thermal and rheological characteristics of acetal and nylon 66.

INTRODUCTION

Emphasis today, in the extrusion field, is upon the production of better quality products at continuously increasing output rates. A primary goal is quality extrusion at a high rate. In order to produce quality product at a desired rate consistently, one needs to attain a high degree of extrusion stability and melt homogeneity to minimize polymer degradation and loss of properties usually experienced at higher rates.

Measurement and analysis of extruder stability and the importance of melt temperature and pressure uniformity in an extruder has been reported by Maddock (1), Marshall, et al. (2), Tadmor and Klein (3), and Rauwendaal (4). Accurate measurement and analysis of the melt temperature and melt pressure in the extruder barrel and the die is very critical in evaluating the performance of a screw (4).

Another important aspect of measuring the extrusion performance is to determine the breakdown of the polymer molecular weight and the loss of polymer properties. Recent work of Shah has shown how to determine an ideal melt temperature of a polymer for extrusion from the rheology data to minimize polymer breakdown (5). One can also use the rheology data of a polymer measured before and after the extrusion to correlate the effect of extrusion variables, melt temperature, and polymer degradation (6).

A research study has been undertaken to determine the effect of screw design, extrusion variables, and melt temperature and pressure on the thermal and rheological properties of several engineering plastics. An objective of this paper is to report the findings on the effect of a single stage metering screw and the melt temperature and pressure variations on the thermal and rheological properties of nylon 66 and acetal materials. It is also an objective of this paper to show how to determine the ideal melt temperature for each polymer from the rheology data, and correlate the effects of melt temperature and pressure variations in a metering screw on the rheological properties of nylon 66 and acetal resins.

EXPERIMENTAL

Materials used in this study are Zytel 42, an extrusion grade nylon 66, and Delrin II 150 SA, an extrusion grade polyacetal homopolymer, both from the DuPont company.

The Zytel and Delrin resins were characterized on a Perkin Elmer DSC to measure the melting behavior and melting point. The melt flow index (MFI) was measured using a Kayeness melt indexer. The Zytel 42 MFI was measured at 2160 g load at 288 [degrees] C, and the MFI of Delrin II 150 SA resin was measured at 5000 g load at 190 [degrees] C. The MFI was measured on the virgin resin in each case and on the extrudate as discussed later.

A detailed rheological characterization of acetal and nylon 66 resins was made on a Kayeness Galaxy V capillary rheometer. The nylon 66 resin was dried for 6 h at 95 [degrees] C before testing. The acetal resin was not dried. The materials were tested to obtain a full flow curve (rate sweep) to measure the shear dependence of melt viscosity, and the thermal stability (time sweep) was measured to determine the thermal degradation of each material for over 30 min of residence time at three temperatures. The testing procedure (5) and equipment description are covered elsewhere (7).

Extrusion experiments were conducted on a 63 mm, 24:1, Davis Standard extruder. A single stage metering screw with the following dimensions was used to extrude nylon 66 and acetal resins:

* 8 Diameter feed length, 8.4 mm deep

* 6 Diameter transition length

* 12 Diameter metering length, 2.8 mm deep

* Flight helix angle of 17.65 degrees, 63.5 mm flight lead.

The die used was a rod die (12.7 mm diameter) with a valve to control the pressure. A Dynisco pressure transducer was installed in the die to record the melt pressure. Three types of melt temperature sensors were used from Dynisco to measure the melt temperature variations in the die at various screw speeds during the extrusion of acetal and nylon 66 resins:

* A pneumatically activated high speed J thermocouple positioned as (1) flush mount, (2) 3.2 mm depth, and (3) 6.4 mm depth extended inside the melt stream.

* A flush mounted infrared pyrometer with fiber-optic bundle with very fast response time.

* A manual pyrometer to measure the melt temperature of the extrudate from the die.

The IR sensor was held stationary in the flush mount position while the J thermocouple probe was adjusted from the flush mount to 3.2 and 6.4 mm depth inside the melt stream to measure the melt temperature in the die. A high speed recorder was used to record all the melt temperature and pressure data during the extrusion experiments.

The extrusion experiments were performed using the shallow metering screw described above. Nylon 66 and acetal resins were extruded at various screw speeds. When the steady state conditions were reached, the measurements of melt temperature, pressure, and output rates were recorded. Samples of extrudate from the die were collected from both materials for rheological analysis of the polymer properties.

RESULTS AND DISCUSSION

Engineering plastics such as nylon 66 (Zytel 42) and acetal (Delrin II 150 SA) are commercially extruded in a wide variety of shapes including profile, strip, tubing, and other complex shapes such as pipe and rod. Unlike injection molding, the residence time in extrusion could be fairly long ranging from less than 5 min to more than 45 min for complex shapes (5). When exposed to excessive shear and temperature for prolonged period of residence time, a polymer may undergo thermal and shear degradation at high output rates resulting in a dramatic loss of properties (8).

In order to optimize the extrusion process to attain desired output rates and product quality one needs to consider the following issues very seriously:

* What is the ideal melt temperature of a polymer for extrusion?

* Thermal and rheological characteristics of a polymer

* Selection and evaluation of a screw design

* Measurement of melt temperature and melt pressure variations and analysis of the data

* Rheological analysis of the virgin resin and the extrudate to determine the loss of polymer properties.

In the present study, a careful consideration to each of the above factors was given. A recent paper by Shah shows how one can determine the ideal melt temperature, rate of degradation, and critical processing parameters of a polymer from the melt rheology data (5). We have used the new method and some data in this study from the rheology work reported by Shah (5).

Table 1 shows the results of DSC melt point for nylon 66 and acetal resins. Also shown in Table 1 is the melt flow index data for each material. Usefulness of this data will be discussed later.

Figure 1 shows the melt stability of nylon 66 measured on a Kayeness capillary rheometer at 288, 302, and 316 [degrees] C respectively. The time sweep (thermal run) was measured at a total dwell time of 40 min to determine the effect of time and temperature on the melt viscosity and the rate of degradation. Even though actual residence time for certain extrusion process may be less than 10 min, the residence time distribution due to the laminar flow of the melt near the wall could be very long (8). The results of Fig. 1 show that even at 288 [degrees] C which is only 35 [degrees] C above the melting point, nylon 66 shows a significant thermal degradation which becomes more severe at a higher temperature. According to a new concept introduced in an earlier paper, one can calculate the actual rate of degradation of a polymer by taking the ratio of the initial and final melt viscosity from the time sweep data (5). Figure 2 shows the plot of the rate of degradation of nylon 66 as a function of temperature. As seen from the results of Fig. 2, rate of degradation of nylon is 6.6 at 288 [degrees] C and it increases to 20 at 316 [degrees] C. These results agree with the results of Shah (5). Based on this study, ideal melt temperature for extruding nylon 66 is less than 288 [degrees] C. Based on the previously reported data, ideal melt temperature for nylon extrusion is 271 to 280 [degrees] C (5). In other words, if we could extrude nylon 66 at a melt temperature range of 271 to 280 [degrees] C where the degradation rate is less than 2.5, minimum loss of properties should result.
Table 1. Thermal and Rheological Properties Nylon 66 and Acetal Homopolymer.

Material Property Nylon 66 Acetal

DSC Melt Point, [degrees] C 254 176
At 10 [degrees] C/min [degrees] F (489) (349)
Melt flow index 7.8 2.1
Zero shear viscosity 300 3900
Pa.s


Figure 3 shows a typical flow curve measured on the Kayeness rheometer at 288 [degrees] C. Nylon 66 shows Newtonian flow behavior at low shear rates with zero shear viscosity of 300 Pa.s. The nylon resin shows non-Newtonian flow at higher shear rates indicating a strong shear thinning behavior.

Figure 4 shows a plot of thermal stability of acetal (Delrin II 150 SA) at three temperatures. Note that at 191 [degrees] C, acetal shows no decrease in melt viscosity up to 30 min. At a higher temperature acetal shows some decrease in melt viscosity indicating thermal instability at higher temperatures. Based on the results of this study, ideal processing melt temperature for acetal is 191 to 204 [degrees] C. Figure 5 shows the flow curve for acetal with Newtonian flow at low shear rates and non-Newtonian flow above 50 reciprocal seconds shear rates. The zero shear viscosity of acetal is 3900 Pa.s which is approximately thirteen-fold higher than nylon 66. In other words the melt viscosity of acetal and nylon 66 is vastly different and one can expect greater viscous heating from acetal extrusion if the same screw design is used for both materials.

The extrusion experiments were conducted on a 63.5 mm Davis Standard extruder with the metering screw described earlier. Nylon and acetal resins were extruded at several screw speeds and the melt temperature and pressure variations were recorded at steady state as described earlier. The barrel profiles used for each resin is shown in Table 2. Extrudate samples were collected for measuring the MFI of the extrudate.

Figure 6 shows the output rate characteristics of the metering screw for Delrin (acetal) and Zytel (nylon 66). As screw speeds are increased for higher output rates, the pressure stability usually deteriorates as seen from the data in Fig. 8. Note in Fig. 8 that the extrusion pressure stability was fair for nylon (2 to 2.5% variation), while the acetal exhibited good stability at low rpm (1.5% variation at 20 rpm) but poor stability at higher speeds (4 to 5% variation at 40 rpm).

Results of Fig. 7 show the melt temperature in the mid-stream measured by the thermocouple at 6.4 mm depth. As shown in Table 2, the ideal processing melt temperature for acetal is 191 [degrees] C and 271 to 280 [degrees] C for nylon. However, the results of Fig. 7 show melt temperature in excess of 11 [degrees] C and 8 [degrees] C above the ideal melt temperature for acetal and nylon respectively at 25 rpm. These results lead us to conclude that the shallow metering screw is not satisfactory even at 25 rpm to extrude acetal and nylon at the desired melt temperature. At higher screw speed, the mid-stream melt temperature in this study is 19 [degrees] C higher for acetal and 21 [degrees] C higher for nylon than the desired ideal melt temperature. The shallow metering screw produces excess viscous heat as well as pressure instability which may result in polymer degradation, loss of properties, discoloration, and poor product quality. Furthermore, one will have to operate this screw at very low screw speed ([is less than] 30 rpm) and still be on the border line of quality.
Table 2. Thermal, Rheological, and Extrusion Parameters Nylon 66 and Acetal
Homopolymer.

Parameter Nylon 66 Acetal

DSC peak melt point, [degrees] C. 254 176
Zero shear melt viscosity
 Pa.s 300 3900
Ideal melt temperature
 from rheology data, [degrees] C 271-280 191
Extrusion heat profile [degrees] C
 Barrel Zone 1 299 204
 Zone 2 288 204
 Zone 3 282 190
 Zone 4 282 190
 Die 282 190
Melt temperature
 measured at 6.2 mm
 depth in the die
 with J thermocouple
 25 rpm 288 201
 50 rpm 297 204
 75 rpm 301 210
Melt Flow Index
Virgin resin 7.8 2.1
Extrudate at 50 RPM 10.7 6.4
Percent increase MFI +36 +197


Results of Figs. 9 and 10 show temperature rise for acetal and nylon at different screw speeds. This data was measured by a flush mounted IR probe and a pneumatic probe thermocouple that was traversed from the flush mount to 3.2 and 6.4 mm depth in the melt stream. Note that the temperature gradient is larger for nylon than for acetal. It is interesting to note that the flush mounted IR probe provides sensitivity comparable to the actuated thermocouple extended between 3.2 and 6.4 mm depth in the melt stream. The flush mounted type J thermocouple, however, did not record the true melt temperature rise due to the wall conduction effect.

In order to measure the effect of the screw design, shear rates, and the melt temperature and pressure variations on the polymer, the extrudate samples of acetal and nylon were tested for MFI. As shown in Table 2, the MFI of acetal increased from 2.15 (for virgin resin) to 6.39 for the extrudate, a 197% increase. The MFI of nylon 66 increased from 7.85 (for virgin resin) to 10.70, a 36% increase. The MFI has been used traditionally to measure the molecular weight of a polymer in an approximate way according to Kohan (9). To better understand this, consider that the extrusion grade acetal, Delrin 150, has a reported MFI of 1.5 and the injection molding grade acetal, Delrin 500, has a reported MFI of 5.0 according to the DuPont literature. Based on the result of this study, we started our extrusion experiment with an extrusion grade acetal resin with MFI of 2.15 but ended up with an injection molding grade polymer with MFI of 6.39 due to the degradation of acetal resin during extrusion.

The loss of molecular weight during extrusion as indicated by a dramatic increase in the MFI values of acetal and nylon can be attributed to the following factors:

1. The shallow metering screw generated melt temperature in excess of 10 to 20 [degrees] C above the desired ideal melt temperature at 25 to 50 rpm. Based on our results of the degradation rates of these materials, higher melt temperature is a strong contributor to the loss of properties.

2. Zero shear viscosity of acetal (3900 Pa.s) is 13-fold higher than that of nylon 66 (300 Pa.s). Shallow meter screw with 3:1 compression generates higher viscous heat for a polymer with high viscosity (4).

3. The metering screw exhibited greater pressure instability for acetal than for nylon. A combination of shear and viscous heat resulted in more severe breakdown of acetal MFI compared to that of nylon 66.

Based on the results of this study, the shallow meter screw is not satisfactory for extruding acetal or nylon materials to attain high output rates with good quality product. Many processors often use one metering screw to extrude many polymers and end up sacrificing output and quality.

The results of this study show that the ideal melt temperature and the rate of degradation measured from the fundamental rheological study is an extremely powerful tool to optimize extrusion process. The measurement and analysis of pressure and temperature variations provide a good insight into the extrusion rheology as discussed above. The rheological characterization of the virgin resin and the extrudate provides valuable information to make an objective assessment of the extrusion process and the loss of polymer properties during extrusion. One can use the MFI measurements as a quality control tool to monitor and control the production rates and the product quality.

CONCLUSIONS

Based on the results of this study, the following conclusions are made:

1. The measurement and analysis of the rheological and thermal properties are shown to be very useful to determine the ideal melt temperature for extrusion and the rate of degradation of acetal and nylon.

2. The metering screw used generated a high degree of pressure instability and viscous heat resulting in temperatures 10 to 20 [degrees] C higher than the desired ideal melt temperature even at low screw speed. Based on these findings, this metering screw is not considered satisfactory for these materials. Additional work is planned to evaluate a new barrier screw design.

3. Use of the pressure transducer, and the IR as well as the traversable thermocouple to measure the melt temperature in the die resulted in very useful information to evaluate the performance of the screw design. The flush mounted IR probe showed the sensitivity comparable to the J thermocouple immersed in the melt stream between 3.2 and 6.4 mm depth. However, the flush mounted J thermocouple was not sensitive to record true melt temperature rise due to the wall conduction effects. For accurate measurement of melt temperature, one should use an infrared sensor or a traversable thermocouple immersed at least 6.4 mm in the melt stream. Our next paper will discuss the latest developments in the infrared and other sensors for more accurate measurement of melt temperature since it is the most important variable to monitor and control.

4. The results show that in evaluating the performance of a screw, one needs to study the rheology of the polymer at each stage of extrusion. A dramatic increase in the MFI of acetal (197%) and nylon (36%) extrudates substantiates a severe loss of molecular weight of acetal and some loss of molecular weight of nylon. The rheological analysis of the extrudate and the virgin resin can be used as a simple and effective quality control tool to monitor and control the process and product quality.

ACKNOWLEDGMENTS

We wish to thank Norton Wheeler of Davis Standard for his assistance in conducting the extrusion experiments and for his valuable input to this study.

REFERENCES

1. B. H. Maddock, SPE J., December 1964.

2. D. Marshall, I. Klein, and R. Uhl, SPE J., April 1964.

3. Z. Tadmor and I. Klein, Engineering Principles of Plasticating Extrusion, Van Nostrand, New York (1970).

4. C. Rauwendaal, Polymer Extrasion, Hanser, New York (1986).

5. P. L. Shah, SPE ANTEC Tech. Papers, 38, 937 (1992).

6. P. Shah, "Practical Melt Rheology," SPE seminar.

7. J. Reilly, Kayeness Inc., Morgantown, Pa.

8. P. Shah, Extrusion of Engineering Plastics, Rheo-Plast Associates, Wyoming, Pa. 19610.

9. M. Kohan, Nylon Plastics, John Wiley, New York (1973).
COPYRIGHT 1994 Society of Plastics Engineers, Inc.
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Author:Shah, Pravin L.; Steward, Edward; Yazbak, Gene
Publication:Polymer Engineering and Science
Date:Aug 15, 1994
Words:3160
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