New rheometer and Mooney technology.

New rheometer and Mooney technology

About 18 months ago Monsanto introduced prototypes of three totally new instrument designs - an oscillating disk rheometer (ODR 2000), a rotorless or moving-die rheometer (MDR 2000) and a new Mooney Viscometer (MV 2000). Performance improvements brought about by the new technology used in these new designs has been reported (ref. 1). This new technology focuses on improved temperature control in all three designs, improvements in torque measurement and signal processing in both rheometers, and easier calibration of the new Mooney. The new rheometers are less sensitive to operator variables (such as sample size and die open times) than the older model R100 and R100S rheometers produced by Monsanto. Cure time repeatability improved as well.

This article starts with a quick review of the new design features and then shows some reproducibility data from production built units. It contrasts sensitivity response of old and new designs and finally, presents some preliminary information on some of the new, non-cure-related data possible with the new designs. The target of the new designs is to decrease the variation that occurs when a sample is tested, whether it be a finished compound in the rheometer or a raw polymer in the Mooney. The variation that occurs is a combination of machine and material variation. The changes introduced in this new technology decrease machine variation. The new designs will satisfy the rubber industry's needs for improved quality and consistency. Improvements have occurred in terms of both electronic and mechanical composition.

New technology available in rheometers

Total instrument operation, including setting test temperatures, test times and torque ranges, and calculation of key data points of the ODR 2000 are controlled by a microprocessor located behind the keyboard. A dot matrix printer collects data automatically at the end of a test. Information, such as temperature and torque, is displayed in real time.

Temperature control, handled now by integrated software rather than separate temperature controllers, is much more accurate. Temperature accuracy in the new designs is [plus or minus] 0.2 [Deg.] C, compared to plus/minus0.5 [degrees] C available with the old technology. Part of the improvement in temperature accuracy is accomplished by programming probe resistivity values directly into the microprocessor.

Using a microprocessor allows us to read temperature in degrees Centigrade or degrees Fahrenheit, to read torque in inch-pounds or decinewton meters and to output data to a printer, recorder and computer simultaneously. The microprocessor also assists in calibration, in terms of a display prompting operator actions, as well as storing the torque standard (or torsion spring) value in memory.

Pushing a few keys allows the equilibrium value to be obtained after the torque standard is heated, without the need to "tweak" capacitors or resistors. The new torque standard has a higher value, to allow better referencing to compound torque values typically tested.

Several mechanical changes have occurred as well to improve system performance. These appear schematically in figure 1. Instead of using a torque arm strain gauge to measure force, a torque transducer is positioned immediately below the rotor. The bearings on which the rotor oscillates are away from the heated region of the sample, and out of the measurement system. A pneumatic cylinder imparts consistent clamping pressure to the rotor shaft.

The encoder system processes the signal measured by the transducer. It consists of a disc with 16 perforations. With each oscillation of the rotor, the disc also rotates, accurately measuring the displacement and corresponding torque value at 16 points. In the old R100S design, Hall effect switches measured the torque signal at maximum displacement only (2X per oscillation) compared to the 16 values now obtained. Finally, with the new design, all components are mounted to a rigid metal flatform to avoid cabinet deflection and to allow improved consistency in mounting components during manufacture.

The platen is smaller than the old design. A flat, circular heater concentrates the heat closer to the rotor. This efficient heating allows a more rapid thermal response when a cold sample is loaded into the rheometer.

Figure 2 shows the temperatur recovery of upper and lower dies during a test. Temperature recovery to setpoint is now 50-60 seconds compared to 4.5 to 6 minutes with the R100S. This faster, more consistent recovery leads to improved reproducibility performance, less operator influence on test results (ref. 1), and faster test times because the effective cure temperature is increased. The printouts also show the torque and cure time data that are calculated and printed automatically at the end of each test (not shown in figure). End of test temperature, for both upper and lower dies, is also automatically printed.

The rotorless design

The new rotorless or moving die rheometer (MDR 2000) has a microprocessor control instrument operation as with the ODR 2000. Digital display of temperature, torque, test time, etc., occurs at the keyboard as with the ODR. A dot matrix printer collects data at test end.

Figure 3 shows schematically the mechanical components of the system. The pneumatic clamping cylinder is not needed. The lower die itself oscillates. Instead of using a torque transducer in line with a rotor, reaction torque is measured from the upper platen. This means that only the force transmitted through the sample is measured. In this way, system friction is eliminated. This lack of friction, coupled with the signal processing system (the disk with 16 perforations mentioned earlier), allows some additional information to be obtaned on the MDR.

The simple die geometry, without a rotor present, provides the potential for future automation of the loading/ unloading process. For thermoset adhesive materials, Mylar can be used to sandwich the sample and keep the ideas clean. In addition, the thinner sample, without rotor, allows heat to be transferred more rapidly than with the ODR.

Figure 4 shows a typical cure curve as well as temperature recovery curve for the MDR. When a cold sample is loaded, the die temperature falls. The temperature recovers to set point here in 20-30 seconds, compared to 50-60 seconds with the ODR 2000. As with the ODR, faster temperature recovery leads to faster sample throughput. As with the ODR, the microprocessor does the data calculations at test end. In addition to cure data, a new value labeled S" is calculated which is not shown.

Viscous or out-of-phase modulus, S"

Figure 5 shows the strain applied to the MDR sample when it oscillates from -0.5 to +0.5 degree of arc. The resultant stress response, or S*, can be separated into its elastic S' and viscous S" components. This separation is made possible by the new signal processing system, by the elimination of friction in the torque measuring system and by use of proprietary mathematical correlation software programmed into the microprocessor. The elastic modulus S' is the curve that we traditionally use for cure characterization. It is in phase with the strain applied. The viscous modulus, S", is out of phase with the strain applied.

The ratio S" over S' is referred to as tangent delta. S', S" and tan delta are characteristics that describe the behavior of polymeric or viscoelastic materials. Thus, separation in this manner can give users valuable insight into the nature of the material being tested. The relative values of S', S" and tan delta are different for a material with high elasticity (like a rubber band) vs. high energy absorption (like a rubber shock absorber). The microprocessor plots S' and S" or S' and tan delta (see figures 6 and 7, respectively). The values mentioned earlier, S" at ML and S" at MH, refer to the viscous modulus at minimum torque, ML, and maximum torque, MH, respectively.

Figures 8 and 9 show the effects of anti-reverting cure systems on S' and S" curves obtained on the MDR (ref. 2). In figure 8, HTS stabilizes not only the plateau of the S" curve, relative to the conventional cure system where reversion is noted. That is, the S" curves provide information on the effects of materials not detectable in the cure curve alone. This point is reinforced in figure 9 where the same Duralink HTS data is shown compared to an EV (efficient vulcanizate) approach to reversion resistance. In figure 9, both cure systems stabilize the cure curve, the HTS stabilizes the S" curve as well. Behavior of the finished, fully cured compounds would be different in these cases. Note that whereas the cure curves alone in figure 9 would have indicated either curative approach to yield the same results, the S" curves provide new insight.

In figure 10, two different butadiene rubber cis contents are compared. In this case, the S" curves are quite different before the onset of cure. Therefore, different processing characteristics in the factory would be anticipated, whereas finished compound properties would be the same. Note that the conventional cure curves, S', do not separate or distinguish between the cis contents. Finally, figure 11 shows a comparison of three grades of carbon black in the same NR compound. The cure curves S' are very similar, the viscous curves S" are quite different. In this case the viscous response might indicate potential field problems with fully cured compounds if the wrong black is used. This type of problem is undetected through the cure curve alone.

To fully understand the implications of these preliminary data on curative, polymer and carbon black effects, users must correlate these results to factory and field experience. As more and more MDRs are put into use, these correlations will be developed by the end users.

Reproducibility of the new designs

Reproducibility is the ability to get the same test result on a standard material on multiple machines operated by different people. Improved machine reproducibility has been the target of the new rheometer designs.

Table 1 shows data from the ODR2000 and MDR2000 compared to the R100S. Shown are data on approximately 50 new machines of each type just before shipment to customer sites. The same SBR compound formulation was evaluated in all cases (see table 2 for formulations). This stock gives very consistent rheometer data on a standard machine when mill mixed under tightly controlled conditions. The equal-to-lower standard deviations in ML, MH and t50 responses indicate comparable reproducibility when the machines are new. The standard deviations were calculated from the average value obtained on each machine, each average represented five samples.

Note the relative average values of ML, MH and t50 in table 1 as well. ML is somewhat lower on the ODR vs. R100S due to removal of bearing friction from the measurement; the ML on the MDR is much lower because of its different die design and even lower inherent friction. MH is somewhat higher on the ODR vs. R100S due to increased mechanical rigidity, MH on the MDR is lower due to its different die design. Finally, the t50 cure time gets progressively faster in moving from the R100S to ODR to MDR due to faster temperature recovery rates.

Table 3 shows rheometer reproducibility data under a somewhat different set of circumstances. In this case, the same SBR compound was used to compare 14 machines of varying vintages (1968-1985) located at multiple plants of a single customer. Note first the very poor reproducibility performance (indicated by both high standard deviations and coefficients of variation, CV%) for the machines before service. This service included both maintenance and operator training. Note next the significant improvement in reproducibility after service (much lower standard dev. and CV%). Finally, note the performance of the new ODR2000 on the same stock. Although differences here are caused by both age and design factors, improved reproducibility with the ODR2000 should remain as the instrument ages, because the new electronic and mechanical designs are more stable.

Figure 12 shows the relative cure curves for the SBR C/O compound. The figure shows the curves for 0.5 [degree] as well as 1 [degree] arc on the MDR. Although the MH values on the MDR are significantly lower, the ability of the instrument to detect material variations is not sacrificed. The difference which occurs in torque depends on the stock and its response to the strain applied. Cure times will always be shorter on the new designs, with the difference becoming greater, the higher the temperature.

Table : Table 3-rheometer reproducibility (field machines) (SBR C/O, 3 [degrees] arc, 177 [degrees] C., 14 machines each)

ML (dNm) MH (dNm) T50 (min)

R100/S Pre-service

Average            10.96    68.91      4.67
Std. dev.           1.25     4.07      .400
CV %               11.4       5.9       8.6

After service

Average            10.15    71.58      4.74
Std. dev.            .24     2.11      .167
CV %                 2.4      2.9       3.5

ODR 2000*

Average             9.22     74.56     3.67
Std. dev.            .12       .98     .018
CV %                 1.3       1.3      0.5

(*) New machines, no long term service

Sensitivity of the ODR, MDR and R100S

In order to assess sensitivity, two different compounds were mixed with variable loadings of oil, carbon black and curatives. The formulations, one based on natural rubber and one based on EPDM, appear in table 2. The typical cure values ML, MH, [t.sub.s] 2, t50 and t90 were measured on all three machines at both 177[Deg.] C and 195[Deg.] C for all formulations. Three samples of each stock were tested under each test condition.

Two methods were used to assess sensitivity of the new rheometer designs compared to the R100S. The first method consisted of an analysis of % change of a cure property when a compounding ingredient changed. The % change was calculated by subtracting the average test result at the low ingredient level from the average result at the high ingredient level and dividing this difference by the average result at the low level. [Mathematical Expression Omitted] where [Mathematical Expression Ommitted] is the average of 3 test results of ML, MH, [t.sub.s] 2, t50, t90 at [Mathematical Expression Omitted], the high level of ingredient, or at [Mathematical Expression Omitted], the low level of ingredient.

The results for the NR compound at 177 [degrees] and 195 [degrees] C showed the % change in minimum torque, for example, ranged from -10.5 to -12.3% when the oil level was increased from 3 to 7 phr in the formulation. Sensitivity, as measured by % change in the test result, is similar in all three instrument types.

The second method to assess sensitivity comes from analytical chemistry. This method determines the minimum detectable level (MDL) of an ingredient. In this case, the phr change of an ingredient is divided by the absolute value of the difference in the average test results. This is then multiplied by the pooled standard deviation of test results. Remember in this experiment that three samples of each stock were run so the op below represents a pooled value for 6 samples only. [Mathematical Expression Omitted]

The minimum detectable level values (MDLs) are very similar in all three instruments. All three instruments are more sensitive to accelerator level changes (lower MDLs) than they are to carbon black and oil changes, but overall differences between instrument types are insignificant.

The results for the EPDM stocks tested also showed that sensitivity (based on % change or MDL values) is equal for all three instrument types.

Closer analysis of these results also showed two unusual results which are consistent from machine to machine. First, the MH values went down and cure times went up when Santocure level was increased from 1.35 to 1.65 phr, a very unusual result indeed. This effect occurred on all machines at both temperatures. This is believed to have been caused by a weigh error or by improper labeling of the mixed stock.

The second unusual effect was dramatic shifts in cure time results moving from [t.sub.s] 2 to t90 for the black variable as well as the Thiurad variable. Large % change occurred for [t.sub.s] 2 and t50, but not for t90 in the black variable. A dip (zero % change) occurred for t50 in the Thiurad variable. This second unusual effect is believed to be caused by slippage of the highly black/oil loaded EPDM compound. The unusual effect appeared on all three instrument types. Table 4 shows the average t90 values for all compounds used. Note the improvement in cure efficiency that comes from the faster temperature recovery of the ODR and MDR relative to the R100S. The efficiency number in the table represents the relative number of samples that can be tested in a given instrument in a given time based on the ratios of t90 cure times.

In practice, it is the time to reach maximum plateau cure that is used to establish sample test time, but the relative t90 values still give an indication of the cure efficiencies to be gained by the new designs. In general, the higher the test temperature, the greater the increase in efficiency. Also, a material with a distinct maximum torque (like NR) shows a greater efficiency gain than a material (like EPDM) with a less distinguishable maximum torque.

In addition to cure efficiency gains at a given temperature, the new temperature control system allows much faster temperature changes. The R100S was changed through use of pre-set buttons, which are expected to give [+ or -] 0.5 [degree] C temperature accuracy compared to the [+ or -] 0.2 [degree] C accuracy of the new designs.

Table : Table 4-cure efficiency of new designs

                            NR Compound   EPDM compound
        Average t90                      Average t90
        @ 177 [degrees] C   Efficiency   @ 177 [degrees] C   Efficiency
MDR     2.365               156          3.881               133
ODR     3.103               119          5.00                103
R100S   3.681               100          5.15                100
        Average t90                      Average t90
        @ 195 [degrees] C   Efficiency   @ 195 [degrees] C   Efficiency
MDR      .923               203          1.481               172
ODR     1.566               119          2.18                117
R100S   1.871               100          2.55                100

New Mooney technology

The technology available in the new ODR 2000 and MDR 2000 also extends to a new Mooney viscometer design called the MV2000. Extending the new technology to the MV 2000 means that the same microprocessor sets test temperatures, test and preheat times and viscosity ranges, and calculates key data points for viscosity or scorch tests.

The keyboard is similar in its overall operation to that on the rheometer. Temperature accuracy in the new MV 2000 is [+ or -] 0.2 [degrees] C, compared to [+ or -] 0.5 [degrees] C in the M1500 model Mooney. The temperature recovery at the die is faster, but overall sample response is controlled to produce the same Mooney viscosity value on NBS butyl as with the old design. The MV2000 calibration is fully automated. At the front is a balance beam or weigh platform mechanism for calibrating the instrument. A counter balanced weigh platform is located behind the motor. Press the calibration button and two weights lower automatically. The system then self calibrates to a 100 Mooney unit reference value. This eliminates the need to manually hang a heavy weight over a pulley as with the M1500.

In addition to automatic calibration, the MV2000 has been designed for easy cleaning of the rotor shaft area. If material slips past the o-ring, this material can be easily cleaned out by dropping the motor assembly (which is now attached by a 3-pin coupling), removing a horizontal rotor height screw and then cleaning the shaft with a brass brush. The whole operation takes less than 10 minutes.

The bearings have been moved out of the heated platen region. This should improve or prolong bearing life.

The die and rotor system is the same as that of the M1500, so that ASTM D1646 is still satisfied. The rotor shaft length has increased to assure proper contact of the shaft with the sheave.

Mooney stress relaxation

Figure 13 shows a Mooney viscosity curve from the MV2000. The microprocessor automatically calculates the 4 or 8 minute value at test end. Also note the stress decay or stress relaxation curve. After the viscosity has been determined with the rotor turning at 2 cpm, the rotor stops and the relaxation of stress is recorded automatically for whatever time period we choose.

Extension of the basic Mooney viscosity test to include relaxation can generate information on both raw polymer and compounded green stock rheological behavior. The measured stress relaxation data obeys the power law

ML = [Kt.sup.-a] where ML is the decaying Mooney stress, t is the relaxation time and K and a are characteristic parameters of the relaxation curve. K is a measure of stiffness of the rubber and is proportional to Mooney viscosity ML 1+4 or ML 1+8. The value "a" is a measure of speed of relaxation, which is a combination of viscous and elastic response of the material (the slope of the log ML vs. log time plot). Parameter "a" correlates with compound processing behavior (milling, extrusion, die swell) and raw polymer molecular weight distribution (refs. 3 and 4).

Reproducibility of the MV 2000

Table 5 shows reproducibility data for the new Mooney design. The standard deviation of the NBS butyl largely reflects the variation of the material itself. The machine variation is believed to be a small part of this number.

Table : Table 5-Mooney reproducibility (new machines)

                 Scorch                       Viscosity
         SBR @ 150 [degrees] C   NBS butyl @ 100 [degrees] C
Average     9.06        12.11        67.14*
Std. dev.    .84          .94          .33

MV 2000

Average     8.21        11.36        68.90*
Std. dev.    .25          .22          .36

(*) Different lots of NBS butyl used, with ref. values of 67.5 and 68.9 respectively. Data represent 50 machines of each type.

The real improvement in reproducibility appears in the significantly lower standard deviations of the SBR C/O t5 and t35 scorch times. The temperature recovery and temperature accuracy on the new MV2000 can both be better controlled than on the older Mooney design.

Test efficiency should improve as reflected in table 6 The gain here comes from the shorter time it takes to change from one test temperature to another.

Table : Table 6-response of Mooney viscometers (minutes)

Temperature change MV 2000 M-1500

100 to 121 [degrees] C (212 to 250 [degrees] F) 2.7 34.0

121 to 100 [degrees] C (250 to 212 [degrees] F) 13.9 26.0

Room to 100 [degrees] C (Room to 212 [degrees] F) 7.6 68.0

The enhanced module

An additional keyboard, along with an additional microprocessor and additional software, will form what is called the enhanced module of all three designs. The enhancement allows sample identification in alpha-numeric form. In addition, when the identity is input, the machine can automatically configure itself for up to 10 conditions (of temperature, test time and torque range).

Whereas the "standard" instruments previously discussed will automatically calculate key ASTM data points at test end, the enhancement will allow calculation of a wide range of programmable data points, for instance torque at a given cure time and corresponding cure rate. For example, we could select t70 (time to 70% cure), [M.sub.t] 70 (torque at t70) and [M.sub.t] 70 (derivative of torque at time or cure rate). For a reverting natural rubber cure, a reversion index can also be calculated. The S" curve on the MDR can be analyzed in the same way and the Mooney stress relaxation constants, K and a, can be calculated.

In the "standard" designs, information can be sent to a printer, recorder or computer, in a one-way fashion. With the enhancement, two way communication is available, that is, information could be sent from a remote computer to the instrument to establish test conditions or load configurations.

The enhanced module will, in the future, operate with a printer-plotter to allow multiple curve traces on a single chart. The plotter would also allow a curve to be re-ranged, that is, the torque range could be redefined in the event that a wrong range were selected and the curve went "offscale." Time of day and date appear on the test printout with the enhanced module.

Summary

New technology provides better reproducibility of rheometer and Mooney test results, as well as more complete characterization of compound properties. In addition to the normal cure properties available through use of an oscillating disk curemeter, the new rotorless (moving die) design allows viscoelastic properties of materials to be obtained simultaneously. Several examples of using viscous modulus to differentiate between compounds have been shown. Mooney stress relaxation, used to better characterize polymer properties, has also been discussed. Cure response sensitivity of both rotorless and rotored designs have been compared to data obtained with traditional instruments. Finally, an enhanced module which allows system flexibility has been presented.

References

[1.] J.A. Sezna, P.J. DiMauro and H. Pawlowski, "Technology advances improve cure testing," Rubber & Plastics News, April 18, 1988. [2.] J. de Rudder and J.P. Etienne, "New dimensions in quality control using viscoelastic measurements," DKT Nurnberg, July, 1988. [3.] J.L. Leblanc, "Stress relaxation related to rubber processability," European Rubber Journal, February, 1980. [4.] R. Koopman and H. Kramer, "Improvement of standard rheological tests for better material characterization," ASTM Symposium, Toronto, June, 1982.