Computer model of extrusion process for AEMs.

AEM polymers (Vamac ethylene acrylic elastomers) have been commercially available for over 30 years. Cured compounds made from AEM polymers have a good balance of properties, including some of the following features (ref. 1):

* Heat resistance up to 175[degrees]C;

* good low-temperature properties down to -40[degrees]C;

* good resistance to transmission fluid and engine oils;

* good damping properties;

* low compression set values; and

* excellent performance in CSR testing.

Cured parts made from AEM polymers are used in automotive applications, including:

* Turbocharger hoses;

* fuel hose covers;

* transmission oil cooler hoses;

* seals and gaskets in transmission systems;

* seals and gaskets in engine systems; and

* torsional vibration dampers.

Most AEM grades are terpolymers made from ethylene, methyl acrylate and an acidic cure site monomer. They are cured with diamines in two stages, an initial press cure followed by a relatively long post-cure step. Some AEM polymers are dipolymers made from ethylene and methyl acrylate. The dipolymer compounds are typically cured with peroxide and do not need post curing.

Hoses--extrusion process

Hoses are a very important use for AEM compounds, and most are made by extrusion. A typical hose construction has an inside tube made from an AEM compound, a fiber reinforcement layer and a cover made from an AEM compound. In making such a construction, the tube and the cover compound are extruded. The design and operation of the extruder are key factors in making high-quality hoses.

Extrusion modeling of thermoplastics 

Extruders are used extensively to make a variety of products, including film, hose, wire, cable and other products. Most extruders are used for processing thermoplastic rather than thermoset materials such as rubber compounds. Most of the recent developments in extrusion technology have come from the world of thermoplastics.

Computer models for extruding thermoplastics have existed for some time, and they keep improving. A major advantage in modeling thermoplastics rather than thermosets is that no curing reaction occurs. During the cure of a thermoset rubber, the viscosity builds up quickly. When this occurs during the extrusion step from unwanted scorch, the increased viscosity leads to higher localized temperatures. The higher temperatures increase cure rate, which then leads to a higher viscosity. Modeling of an extrusion process in which a cure reaction occurs is very complicated.

Some of the inputs needed for computer modeling of thermoplastics include:

* Overall length and diameter of the extruder;

* screw design,

--channel depth, --distance between channels, --channel width;

* rheology curves (viscosity vs. shear rate) of the material,

--at several different temperatures typical for the process, --at shear rates typical for the extrusion processes;

* physical and thermal properties of the compound,

--density, --heat capacity, --thermal conductivity;

* screw speed (rpm); and

* barrel temperature profile.

Since AEM compounds contain curatives, they are not thermoplastics. For the computer simulations, the compound used did not contain curatives, and the AEM compound was treated as a thermoplastic. This was a key assumption to permit use of the thermoplastic model. The actual AEM compound with a curative is more difficult to process than one without curative.

The computer model used for this study was Flow 2000 from Compuplast. The model has many outputs, including:

* Production rate;

* melt temperature profile,

--average temperature along the length of the screw, --average temperature within the screw channels;

* pressure profile along the screw length;

* energy inputs and outputs; and

* residence time.

Rheology data

Two different AEM compounds were prepared in the lab; their ingredients are shown in table 1. The Mooney viscosity, ML (1+4) at 100[degrees]C, was about 40 MU for both compounds. A capillary rheometer was used to generate viscosity vs. shear rate curves at 80, 100 and 120[degrees]C. (The rheometer had a 30 to 1 L/D with a length of 30 mm and an aperture of 1 mm. The samples were preheated for six minutes before testing.)

The typical recommended extrusion temperatures for AEM compounds range from about 60 to about 90[degrees]C. The model uses the data to extrapolate to viscosity values at lower or higher temperatures and also at higher or lower shear rates. The rheology curve for the compound without a curative is shown in figure 1.

The rheology curves for the compound with curative are shown in figure 2. During the test, the compound started to cure (scorch), so the results are not smooth lines. The test procedure used was to start and end at the same shear rate, 50 [sec.sup.-1]. The two values should be very close--unless the compound starts to cure. One major difference for the compound with a curative is that the highest viscosity was measured at the highest temperature, 120[degrees]C. This is in contrast to thermo plastics, which drop in viscosity as temperature increases.

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[FIGURE 2 OMITTED]

For comparison, the viscosity curves at 80[degrees]C and at 120[degrees]C with and without curatives are shown in figure 3. At 80[degrees]C, the two compounds have similar rheology profiles. However at 120[degrees]C there is a major difference in the rheology results.

The capillary rheometer test results at the three different temperatures point out the importance of avoiding excessively high temperatures. For this compound, these are above 100[degrees]C. This information is similar to the results from the Mooney scorch test, which is typically run at 121[degrees]C for compounds with this formulation's cure system.

Screw design

Several different screw designs are available for single screw extruders. This study compared results with a general purpose (GP) rubber screw and a vented screw (figure 4). The general purpose screw has relatively low shear rates and relatively high output. The vented screw has one region of high shear, located just before the vent. The high shear region helps to provide extra mixing to improve compound uniformity. It also provides a melt seal so that a vacuum can be drawn during venting. It is common, however, that processors do not use the venting feature of such screws.

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[FIGURE 4 OMITTED]

If the compound is well mixed prior to extrusion, then there is less incentive to use a vented screw. The vented screw does provide some insurance for compound uniformity in case mixing problems occur.

The screws recommended for AEM compounds are general purpose designs rather than vented or other mixing screws. The main reason for this recommendation is that the GP screws have no localized high shear region that can lead to high temperatures and possible scorch problems.

Computer modeling--GP screw compared to vented screw

The computer model was set up using the dimensions for a general purpose screw and a vented screw, along with the measured rheology information for the compound without curative. This is basically the best case scenario for limiting temperature buildup. The dimensions for the extruder were based on a 38 mm (1.5 in.) lab extruder with a 16 to 1 L/D equipped with either a general purpose or a vented screw. The temperature profile was set to 80[degrees]C along the length of the extruder, and the speed was set at 45 rpm.

Constant production rate--general purpose screw vs. vented screw

The first simulation case looked at process temperatures using the general-purpose screw. For the conditions mentioned above, the highest average temperature within the channel was 104[degrees]C, which occurred at the exit. The peak temperature within the channel at the exit was 116[degrees]C. This is a relatively high temperature considering the potential for scorch.

The second case looked at the vented screw, with speed increased to 70 rpm in order to obtain the same output (kg/hr.) as the general purpose screw. The highest average temperature for the case with the vented screw was 115[degrees]C, and the peak temperature was 132[degrees]C, which occurred in the high shear region. In actual practice, such high temperatures would probably cause major scorch problems. The results are summarized in table 2.

Constant RPM--general purpose screw vs. vented screw

The next simulation study looked at holding the screw speed constant at 70 rpm. The results are shown in table 3. There is a significant increase in temperature for both screws, but the material going through the vented screw will see a higher temperature for a longer time. Both screw types will have scorch issues with the high RPM, but the vented screw will present more significant scorch issues.

Residence time with the general purpose screw was shorter than with the vented design. One of the reasons was that the production rate was higher.

Use only general purpose screw--effect of production rates on temperature

The next simulation looked at the effect of production rate on process temperatures when using a general purpose screw. Screw speed was set at 25, 45 and 70 rpm, which led to production rates that increased from 9 kg/hour up to 20 kg/hour. The increased production rates produced large differences in the temperature profile, as shown in table 4.

The potential scorch issues at the low production rate are relatively minor. The average temperature is well under 100[degrees]C, and the peak temperature is close to 100[degrees]C. However, doubling the production rate produces a major change in the temperature profile and sharply increases chances for scorch problems. This compound contains no curative; if a curative were used, viscosity and temperatures could be significantly higher than those shown for the compound without curative.

[FIGURE 5 OMITTED]

Figure 5 shows temperature versus output for the general purpose screw.

Conclusions after modeling and capillary rheometer tests

* Capillary rheometer tests suggest that temperatures above 100[degrees]C can cause scorch problems for compounds using the standard AEM cure package.

* At the same production rate, computer simulations show that the temperatures are significantly lower for a general purpose screw compared to a vented screw. This is in part due to the lower rpm of the GP screw.

* At the same screw speed, computer simulations show that the production rate for a general purpose screw is higher than a vented screw, and that the predicted temperatures are lower.

* Doubling the production rate can lead to a significant increase in temperature, even for a general purpose screw. A compound that does not have scorch issues at one production rate may have significant scorch issues at a higher production rate.

Extrusion trials with general purpose and vented screw

Extrusion trials were run to investigate the conclusions suggested by the computer modeling. Some of the factors varied in the trials were:

* Screw type--general purpose vs. vented;

* temperature profile--relatively low to relatively high;

* compound type--relatively low viscosity, low scorch compound vs. relatively high viscosity, high scorch compound; and

* screw speed four different levels, from low to high.

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The extrusion trials were run on a 63 mm (2.5 inch) Davis-Standard extruder with a 20 to 1 L/D. This was not the extruder on which the computer model was based. In the extrusion trials, a 9.5 mm (3/8 inch) tube with a nominal wall thickness of 1.5 mm (60 mils) was produced. A crosshead die was used; this was not included in the model. There was a significant temperature increase across the die.

A schematic of the process is shown in figure 6. It includes the temperature profiles used for the relatively low-temperature and relatively high-temperature settings. It also shows the location where the temperature of the extrudate was taken as it exited the crosshead die.

Two different compounds were used in the trial (table 5). The first was based on AEM GXF and a cure package designed for good flex properties, with the result that the compound is relatively non-scorchy. The second was based on AEM HVG (higher viscosity) with a standard cure package. The AEM HVG compound uses the same formulation used for the computer modeling work described above, except that it uses AEM HVG rather than AEM G (compound viscosity went from 40 to 65 MU). The AEM HVG compound was more scorchy than the AEM GXF compound because of the polymer structure and the more aggressive cure package. Neither compound contained plasticizer.

Thirty-two different extrusion states were run (two screw types x two compounds x two temperatures x four screw speeds). The following measurements were taken for each state:

* Production rate in kg/hour;

* temperature of the extrudate as it left the crosshead die;

* the temperature profile along the extruder--set point and actual;

* extruder pressures; and

* theology data on compounds after they exited from the extruder,

--Mooney viscosity, --MDR, --Mooney scorch.

The temperature measurements in the extruder and die were measured at the wall of the extruder. Actual melt temperature was not directly measured; it is probably higher than the measured wall temperatures, especially in high shear regions. Some of the highlights of the data analysis are discussed in the next sections.

Comparison of output vs. screw speed with two different screws

According to the computer model, the output from a general-purpose screw should be higher than for a vented screw running at the same screw speed. The next graph (figure 7) shows this indeed was the case. This was true for all of the side-by-side comparisons, but the graph below compares the two screw designs in processing:

* Low viscosity compound (AEM GXF) with relatively low scorch; and

* high process temperatures, 85[degrees]C at die.

[FIGURE 7 OMITTED]

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For this case, at the highest rpm, the output of the GP screw is 28% higher. Another way of stating this is that at high production rates, the vented screw needs 30% higher rpm to get the same output as the general purpose screw. The lab results for this part of the study match the predictions of the computer simulation. Similar trends were seen for the high viscosity compound at the high process temperature, as well as the two compounds at the lower process temperatures. The output for the general purpose screw was always higher than that of the vented screw.

Temperature profile for general purpose screw as production rate increases

The computer simulation predicts that the temperature of the material in the extruder should increase as the production rates increase, even if temperature set points are held constant. This is predicted for either the GP screw or the vented screw.

Figure 8 shows the temperature profile for the case of:

* Low viscosity compound (AEM GXF) with relatively low scorch;

* high process temperatures, 85[degrees]C at die; and

* GP screw.

The initial temperature set point for barrel zone 4 was 75[degrees]C, and the initial temperature at low production was 75[degrees]C. The initial set point for die temperature was 85[degrees]C and the initial temperature at low production was 85[degrees]C. As the production rate increased, there was a large increase in temperature. The extrudate temperature at the high production rates increased to 111[degrees]C, significantly higher than the initial 85[degrees]C. Such a high temperature at the die could lead to scorch problems.

The extruder has cooling facilities to maintain the temperature profile. At low production rates, the cooling capability of the extruder was able to maintain the set point. At the high production rates, the cooling capacity may have been exceeded and this could have contributed to the temperature increase at the wall of the extruder.

[FIGURE 9 OMITTED]

(The computer model assumes that there is enough cooling capacity to maintain the temperature at the wall of the extruder. That means that as the production rate increases, so does the cooling. Even with the higher cooling in the model, there is still a significant increase in the predicted melt temperature as the rates increase.)

Comparison of GP and vented screw low scorch vs. high scorch conditions

This study generated a significant amount of data for comparing the two types of screws. One way to explain the data is to look at some of the extremes to compare the two screws. One extreme is a low viscosity, low-scorch compound running at low temperature (relatively low scorch processing conditions); the other extreme is a high viscosity, high scorch compound running at high temperature (relatively high scorch processing conditions).

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

Relatively low scorch conditions

Figures 9 and 10 show the results for the relatively low-scorch conditions, low process temperature with the low-scorch, low-viscosity compound. Figure 9 shows the extrudate temperature for the two types of screws at different production rates. The set point temperature for the die was set at 65[degrees]C and the initial temperatures were about 75[degrees]C; so there was an increase in temperature across the crosshead die, even at low production rates. At high production rates (42 kg/hr.), the extrudate temperature for the vented screw was 18[degrees]C higher than for the GP screw extrudate. The absolute temperature of the vented screw extrudate was only 103[degrees]C, so scorch under these conditions may not be an issue.

As the extruded tubes exited the die, they were collected and then tested for rheology properties, including Mooney viscosity, Mooney scorch and MDR. The trends for all three tests were similar. Figure 10 shows the Mooney viscosity of the extrudate after it exited the extruder.

The initial viscosity of the compound was 53 [ML(1+4) @ 100[degrees]C]. At low production rates there was a slight drop in the Mooney viscosity. (Such drops in MV can be seen if the compounds are milled for increasingly longer times.) As the production rate increased, there was a slight increase in viscosity, but probably not significant enough to cause processing problems.

Relatively high scorch conditions

The other extreme is a high viscosity, high scorch compound processed at high temperatures. This is the case with the most scorch concerns.

Figure 11 shows extrudate temperature versus output for this scenario. The die temperature set point was 85[degrees]C, and the measured temperature at the low production rates was close to this. As production rates increased, temperature rose significantly. At high production rates (40 kg/hr.), the temperature in the extruder with the vented screw was 17[degrees]C higher than the one with the GP screw. The absolute temperature with the vented screw was 118[degrees]C, a relatively high temperature that could create scorching.

[FIGURE 12 OMITTED]

Figure 12 shows the Mooney viscosity versus output for the high viscosity, high temperature scenario. At relatively low production rates, viscosity increases slightly (about 20%). But at a production rate of about 40 kg/hour, viscosity increases by 230% for the vented screw and by 160% for the GP screw. When the production rate was raised to 55 kg/hour, viscosity using the GP screw almost doubled. (The vented screw could not reach the higher production rate.) Scorch issues could arise with both screws with this compound at high production rates, and processing problems will be more severe with the vented screw.

The computer simulation showed that the vented screw would have a higher temperature than a GP screw at the same output. The rheology results show how important it is to keep the temperatures low.

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Comparison of low scorch vs. high scorch compound

The computer model was not used to compare relatively high scorch and low scorch compound. The model assumed that there was no scorch reaction. This was a key assumption that allowed the use of a thermoplastic model.

In the real world, however, scorch reactions occur in rubber compounds. Figures 13 and 14 show what happens when comparing a low scorch and high scorch compound. Using the general purpose screw and the high temperature profile, we compared the relatively high-viscosity, high scorch compound to the relatively low viscosity, low scorch compound.

Figure 13 shows zone 4 and extrudate temperature versus output. The temperatures of the two compounds are relatively close for the different production rates.

Figure 14 shows the Mooney viscosity of the extrudate at different production rates. There is a major difference between the two compounds. The relatively low scorch compound only has a slight increase in viscosity (11%) while the relatively high scorch compound basically doubled in viscosity at the high production rates.

Conclusions from lab tests

The conclusions from the lab tests are listed below, along with comparisons to the predictions from the modeling work.

* At the same screw rpm, the production rate for a general purpose screw is higher than that of a vented screw (as predicted by the extrusion model).

* At high production rates, temperatures in the extruder and die are significantly lower for a general purpose screw than for a vented screw. At low production rates, the temperature profile is similar (as predicted by the model).

* Increasing the production rate can lead to a significant increase in temperature, even for a general-purpose screw (as predicted by the model). A compound that does not have scorch issues at a relatively low production rate may have significant scorch at a higher rate, even with a general purpose screw.

* At the same process conditions (temperature profile, rates and screw type), a low scorch compound will exhibit a smaller increase in viscosity than a higher scorch compound. This is not a surprise, but it was not predicted by the model, which assumed that no scorch reaction occurred.

Scorch becomes a greater concern as extruder temperatures increase. The combination of high temperatures, high shear rates (vented screw) and high production rates can cause major problems with scorch.

This article is based on a paper presented at a meeting of the Rubber Division, ACS (www.rubber.org).

References

(1.) http://www2.dupont.com/Vamac/en_US/index.html--Background information on AEM polymers.

by E. McBride and C.S. Grant, DuPont Performance Elastomers, and B.A. Morris, DuPont Company (www.dupontelastomers.com/Products/Vamac/)

Table 1--compounds for rheology testing

                             Compound with        Compound
                               no curative   with curative

AEM G                                 100             100

N550 carbon black                      50              50

Stearic acid (release)                1.5             1.5

Alkyl phosphate (release)             1.0             1.0

Octadecyl amine (release)             0.5             0.5

Hindered amine AO                     2.0             2.0

HMDC curative                          --             1.5

DOTG accelerator                      4.0             4.0

Total phr                             159           160.5

Table 2--production held constant at 15 kg/hour

                                        GP screw    Vented screw

RPM                                            45             70

Average temperature, [degrees]C               104            115

Peak temperature, [degrees]C                  116            132

Table 3--rpm held constant at 70 rpm

                                        GP screw    Vented screw

Production rate, kg/hr.                        20             15

Average temperature, [degrees]C               110            115

Peak temperature, [degrees]C                  128            132

Average residence time, sec.                   48             70

Time for 90% of material to                    67            101
  go through, sec.

Table 4--vary production rate from 9 to 20 kg/hour with GP screw

RPM                                 25     45     70

Production, kg/hour                9.1   15.1   20.0

Average temperature, [degrees]C     91    104    110

Peak temperature, [degrees]C        99    116    128

Table 5--compounds used for extrusion trials

                                      Compound based   Compound based
                                          on AEM GXF       on AEM HVG

AEM GXF                                          100

AEM HVG                                                           100

N550 carbon black                                 52               50

Stearic acid (release)                           1.5              1.5

Alkyl phosphate (release)                          1                1

Octadecyl amine (release)                          0             0.50

Hindered amine AO                                  2                2

HMDC curative                                    1.1              1.5

DPG accelerator                                    2                0

DOTG accelerator                                   2                4

Total phr                                      161.6            160.5

Mooney viscosity,
  ML(1+4)/100[degrees]C                           53               65

Mooney scorch at 121[degrees]C,
  T5, minutes                                   10.7              6.0