Overview of capillary rheometry--Part 2.

Part 1 appeared in the January issue.

Comparison of capillary rheometry to rotorless shear rheometry (RPA)

Thirty years ago, capillary rheometry was the only high shear method available for measuring processability. Fifteen years ago the rubber process analyzer was introduced as a processability test alternative. Since then, hundreds of RPAs have been used throughout the world to measure processability. In many cases, the RPA has displaced the use of capillary rheometry as a processability test. In this section, we will discuss the capabilities of the RPA vs. the capillary rheometer for rubber processability measurement.

One of the functions of the RPA is to be used as a processability tester. The RPA molds the uncured rubber formulation in a sealed pressurized cavity and has a special direct drive motor that oscillates the lower die sinusoidally over a range of preprogrammed strains, frequencies and temperatures. RPA shear rate sweeps (through variations in frequency) are commonly performed at a fixed strain set between 2 to 14% strain to measure the drop in complex ([[eta].sup.*]) and real ([eta]') dynamic viscosity with increasing shear rate for rubber compounds. Also, sometimes very high strain sweeps are performed with the RPA. Other outputs from the RPA include: G' for storage (elastic) modulus, G" for loss (viscous) modulus, S' for elastic torque, and S" for viscous torque. An additional output, tan [delta], is derived from either dividing G" by G' or dividing S" by S'. This parameter is dimensionless, without units.

Since rubber compounds are non-Newtonian, their viscosity decreases with an increase in applied shear rate by the RPA. Typically, this decrease will be displayed as a power law drop. Different base elastomers impart different magnitudes of shear thinning behavior to a given compound. Therefore, they impart different slopes. So when the log of viscosity is plotted against the log of applied shear rate, a different linear slope results for different types of elastomers. Therefore, all of these non-Newtonian slopes from increasing shear rates for either the capillary rheometer or the RPA are derived from the power law formula given in equations 10 and 11.

y = K[[??].sup.-[alpha] (10)

Log y = -[alpha] log [??] + log K (11)

Where y = [[eta].sub.app] for capillary rheometer testing or [[eta].sup.*] for RPA testing; k = intercept; [alpha] = slope of shear thinning; and [gamma] = shear rate.

Correlation coefficients (r) for the statistical power law regressions for both the capillary rheometer and RPA data were calculated and reported in earlier work in 2000 (ref. 21). Also, an alternate method to compare non-Newtonian behavior is to simply calculate the percent drop in viscosity between a predesignated low shear rate and a pre-designated high shear rate.

The relationship between viscosity measurements from capillary rheometers vs. dynamic mechanical testers with sinusoidal deformation should also be addressed. In the 1950s, Cox and Merz published the following empirical relationship they found between capillary rheometer apparent viscosity (measured under conditions of steady state shear rate) and dynamic complex viscosity measured by sinusoidal deformations (and thus, a constantly changing shear rate) using a dynamic mechanical rheological tester (similar to the RPA). This relationship (ref. 22) is given below in equation 12.

[[eta].sub.app] ([gamma]) = [[eta].sup.*]([omega])[|[[omega] = [??]] (12)

Where [[eta].sub.app] is apparent (uncorrected) capillary viscosity at a steady shear rate of ([gamma]) (in [sec.sup.-1]); and [[eta].sup.*] is a dynamic complex viscosity measured at an oscillatory frequency of [omega] in radians per second).

Also, the same relationship is reported to work when comparing the corrected viscosity (true viscosity) from the capillary rheometer vs. the real dynamic viscosity [eta]' from a DMRT. This empirical relationship sometimes works quite well; however, it does not have a theoretical basis, therefore it may not always work (ref. 23).

Earlier work (ref. 24) reported a direct comparison of [[eta].sub.app] viscosity measurements from the capillary rheometer vs. the [[eta].sup.*] complex dynamic viscosity measurements from the RPA for compounds based on 20 different types of elastomers loaded with 25 parts by volume of N660 carbon black. These 20 elastomers included CSM, CR, FKM, CM, EPDM, NBR, HNBR, BIIR and ACM. This correlation is shown in figure 15.

[FIGURE 15 OMITTED]

As can be seen, the agreement (in Pascalo x Second units) is excellent, and the slope of the regression line is very close to unity. Thus, the agreement with the Cox-Merz rule is very good at this filler loading level.

The same comparison was made with compounds containing higher carbon black loadings of N650 at 55 parts by volume. At these higher filler loadings, the correlation was still very good (R = 0.94); however, the regression slope was 0.68, showing some deviation from the Cox-Merz rule (slope ~ 1.0) at this higher filler loading. This supports past studies that have also found very good correlation between the capillary rheometer apparent viscosity vs. the RPA complex dynamic viscosity at low and medium-high filler loadings, but some calculable deviations from the Cox-Merz rule at higher filler loadings (refs. 25-27). In some cases where selected polymers displayed noticeable slippage or "slip stick" from the capillary, rheometer testing, these results were deleted from the statistical regression analysis.

The shear thinning characteristics of the same selected polymers, loaded at 25 parts by volume with N660 carbon black, were measured by both the capillary rheometer and the RPA, and figure 16 shows the very good correlation between the shear thinning measurements.

[FIGURE 16 OMITTED]

Figure 17 compares the magnitude of shear thinning that each polymer imparted to the compound. It is interesting to note that in many cases, the percent drop in viscosity is about the same whether measured by the capillary rheometer or the RPA. The reason for this similarity is that in each case, the comparisons were made between the same shear rate conditions, i.e., 30 to 100 [sec-.sup.1] for the capillary rheometer, and 30 to 100 rad/sec for the RPA. In general, a word of caution is noted when trying to measure shear thinning characteristics with the capillary, rheometer vs. the RPA. The RPA can measure at very low shear rates, while the capillary rheometer does not normally measure in this very low shear rate region. To get the best correlation, one normally should not include very low shear rate measurements from the RPA, which are below the capillary rheometer's normal shear rate range. Including these very low frequency measurements is unnecessary and may introduce noise. Also, the capillary shear thinning calculations for CSM and CPE had to be left out of this comparison because a close examination of the capillary rheometer raw data confirmed that these polymers displayed "slippage" (ref. 28).

[FIGURE 17 OMITTED]

Figure 18 shows a good correlation between the shear thinning measurements from the RPA under high strain conditions vs. the conventional capillary rheometer measurements for the compound series containing 55 parts by volume of N650. This correlation could have been better: however, from examination of the capillary rheometer raw data, there was some "slip stick" and slippage that occurred with these loadings, even at 125[degrees]C.

[FIGURE 18 OMITTED]

Figure 19 compares again the shear thinning imparted by the different polymers to this series of compounds. Because of the significantly higher loading of carbon black in this comparison, the high strain test conditions on the RPA gave a definite advantage. Previous studies have found advantages in applying higher strains to the closed sealed cavity of the RPA in order to break up the carbon black aggregate network and/or gel and chain entanglements, which is exactly what happens to the rubber during factory processing (ref. 29).

[FIGURE 19 OMITTED]

It was observed that increasing loadings of filler imparted a greater shear thinning quality to the rubber compounds. The power law regression slopes ([alpha] in equation 10) were calculated for [[eta].sup.*] from RPA frequency sweeps of the raw subject rubbers, the rubbers mixed with 25 parts by volume of N660 carbon black, and the rubbers mixed with 55 parts by volume of N650 carbon black. This power law regression slope can be used as a shear-thinning index. Figure 20 illustrates the effects of increasing carbon black loading on shear thinning of the rubber compound. As can be observed for each polymer, its calculated |[alpha]| (the absolute value of the slope) increases from when it is tested in the raw state, to higher values with in creased loading of carbon black (ref. 30). This filler effect on shear thinning was also observed in earlier studies done in 1996 (ref. 31).

[FIGURE 20 OMITTED]

As we reported in 1996, carbon black and silica loading in a compound have a great impact on the shear thinning characteristics as measured either with a capillary rheometer or the RPA (ref. 31). This previous work presented shear thinning profiles from capillary rheometer measurements on compounds containing 10 phr to 130 phr of N330 carbon black, and contrasted to profiles of compounds with 10 phr to 130 phr of precipitated silica.

This whole set of experiments was repeated with the RPA, using the power law drop of the sinusoidally measured dynamic complex viscosity [[eta].sup.*], instead of the steady state flow measurements of the apparent viscosity [[eta].sub.app] measurements with the capillary rheometer. Figure 21 shows a good correlation ([R.sup.2] = 0.95) and the fairly good match among the power law slopes between RPA and capillary rheometer measurements (ref. 32).

[FIGURE 21 OMITTED]

Also, figures 22 and 23 show how well the Cox-Merz rule works for the SBR model compounds loaded with 10, 30 and 50 phr of N330 carbon black and precipitated hydrated silica, respectively. As noted by these figures, the viscosity measurements from the RPA as complex dynamic viscosity [[eta].sup.*] in kPa-sec, units compare fairly well to measurements from the capillary rheometer as apparent viscosity [[eta].sub.app] (also in kPa-sec. units) when these values are plotted, respectively, against shear rate as rad/sec, for the RPA and shear rate as [sec..sup.-1] units for capillary rheometer on the same scale. Both capillary rheometer and RPA viscosity data points fit on the same line (ref. 31).

[FIGURES 22-23 OMITTED]

The RPA viscosity data lined up well with the capillary rheometer data when the filler loadings were at 50 phr or lower. However, very high loadings of either N330 carbon black or precipitated hydrated silica, well above 50 phr, did not necessarily line up well (ref. 31).

Lastly, a comparison is made between the ability of the capillary rheometer and the RPA to predict die swell in the factory for rubber compounds.

Figure 24 shows a correlation that was measured in the 1980s between capillary rheometry die swell measurements and the actual factory die swell measurements on the same rubber compounds (ref. 33).

[FIGURE 24 OMITTED]

Starting in the 1990s, the RPA has also been used to predict die swell in the factory. Figure 25 illustrates the inverse correlation between the RPA uncured tan [delta] and die swell measured in the plant (refs. 34 and 35). The RPA has advantages over the capillary rheometer in accuracy and speed. The RPA tan [delta] measurement has much better test repeatability than the die swell measurement off the capillary rheometer extrudate. Also, the RPA measurement takes far less time to make than the die swell off the capillary rheometer.

[FIGURE 25 OMITTED]

Advantages/disadvantages of capillary rheometry vs. RPA

From these comparisons between the performance of the capillary rheometer vs. the RPA, the following observations are made.

* Melt fracture and extrudate appearance problems can be directly observed with the capillary rheometer.

* The capillary rheometer can directly measure rheology at shear rates greater than 1,000 [s.sup.-1], and extrapolations are not necessary.

* The capillary rheometer actually measures rheology under steady state flow, very similar to what occurs in the factory with extrusion, calendering and injection molding. While the RPA can correlate well to changes in viscosity for rubber compounds containing very high loadings of fully reinforcing fillers, the capillary rheometer is more accurate in measuring the true viscosity value in this high range of loading, provided there is no wall slippage. Also, the capillary rheometer's steady state flow allows direct measurement of the effects from changes in L/D.

* The RPA/PPA uncured G' and tan [delta] values (used for predicting factory die swell variation) have much better repeatability and reproducibility than the capillary die swell measurements.

* The RPA/PPA can be configured to run much shorter processing tests that consistently keep up with the mixing process. This enables it to be a very effective "on-line" instrument in a manufacturing operation.

* The RPA/PPA is a much more versatile instrument. It functions not only as a processability tester, but also as a raw polymer tester, an advanced curemeter and a dynamic mechanical rheological tester (for after-cure dynamic property measurements).

Applications of the capillary rheometer for problem solving in production

In this section, we will discuss how the capillary rheometer has been used to solve rubber compounding processing problems and aid the compounder in developing better compounds.

Most capillary rheometer tests are set up to apply multiple shear rates to the compound during testing, as demonstrated in figure 26.

[FIGURE 26 OMITTED]

John Sezna with Monsanto Instruments published a large volume of rubber applications work from capillary rheometer testing in the 1980s.

Mixing

Sixteen batches of an SBR compound from a rubber manufacturer were tested with the capillary rheometer. This manufacturer had Mooney viscosity data that did not show significant differences among these stocks; however, they did not extrude in the plant in the same manner. The capillary rheometer was able to distinguish batches with regard to factory performance (ref. 36).

The variations in shear stress and viscosity values recorded by the capillary rheometer matched related variations in the extruder outputs. These variations were caused by different states-of-mix from the internal mixer.

Calendering

Three calendering stocks were also tested with the capillary rheometer. These rubber stocks had similar Mooney viscosity values and were supposed to process the same on the calender. Unfortunately, they did not.

High shear measurements from the capillary rheometer showed great differences among these stocks. In figure 27, the capillary rheometer barrel pressure indicates the flow resistance of the rubber stocks.

[FIGURE 27 OMITTED]

The problem relating to the calendering could only be seen at the higher shear rates. High shear rate measurements are better for predicting calendering performance (ref. 37).

Extruding

A rubber manufacturer needed a method that would assure that his stock received enough work history during mixing so as to not have appearance problems from surface roughness after extrusion. This concern could have a major economic impact on his cost of operation, since the extruded stock went directly into a continuous vulcanization unit. Roughness would cause an increase in scrap costs. Figure 28 shows how capillary

high shear testing could detect bad stocks before they were placed into the extruder (ref. 33, 38).

[FIGURE 28 OMITTED]

The photograph provided earlier in figure 11 (see part 1) shows the rough appearance of the "F" capillary extrudate vs. the appearance of "G" and "H" from this study.

Injection molding

For a processing aid to work for an injection molding process, it must lower the shear stress at high shear rates.

In this study, three variations of a given rubber compound were studied with the capillary rheometer. The first compound contained no processing aid or wax. The second compound contained wax. The third compound contained a very special processing aid. From the low shear rate measurements with a Mooney viscometer, it appeared that the wax was almost as good as the processing aid for lowering viscosity. However, when we examined the complete capillary rheometer barrel pressure curve, we could see that the wax was not very effective at reducing the shear stress at very high shear rates, in the range that is important in injection molding operations (refs. 39 and 40). This is demonstrated in figure 29.

[FIGURE 29 OMITTED]

Conclusions

Capillary rheometry still possesses unique capabilities for service in the rubber industry.

Capillary rbeometers can compliment the capabilities of other newer instruments to allow a laboratory to have a full range of capabilities to meet today's new challenges.

References

(21.) J. Dick, "Comparison of shear thinning behavior of different elastomers using capillary and rotorless shear rheometry, " paper no. 50 presented at the Spring Rubber Div., ACS, Meeting, 2000.

(22.) W.P. Cox and E.H. Merz, J. of Polym. Sci., 28, 619 (1958).

(23.) EP. Khanna, "Dynamic melt rheology. 1: Re-examining dynamic viscosity in relationship to the steady shear flow viscosity," Polymer Engineering and Science, March, 1991, vol. 31, no. 6, p. 440.

(24.) ibid ref. 21.

(25.) J. Dick and H. Pawlowski, "Applications of the rubber process analyzer in characterizing the effects of silica on uncured and cured compound properties," ITEC '96 Select, Rubber and Plastics News, Sept., 1997.

(26.) J. Dick, "Comparison of shear thinning behavior using capillary and rotorless shear rheometry, " Rubber World, January 2002, p. 23.

(27.) J. Dick, H. Pawlowski and J. Moore, "Viscous heating and reinforcement effects of different fillers using the rubber process analyzer," paper no. 7, Rubber Division, ACS, Spring meeting, 1999.

(28.) ibid ref. 26.

(29.) J. Dick and H. Pawlowski, "Rubber characterization by applied strain variations using the rubber process analyzer," Rubber World, January 1995.

(30.) ibid ref 26.

(31.) J. Dick and H. Pawlowski, "Applications of the rubber process analyzer in characterizing the effects of silica on uncured and cured compound properties, "paper no. 34 presented at the Rubber Div., ACS, Spring meeting, 1996.

(32.) ibid ref. 25.

(33.) J. Sezna, "Processability testing of extrusion compounds," Elastomerics, Aug. 1984, p. 35,

(34.) H. Burhin, "Proactive extrusion control through die swell prediction, by compound viscoelastic measurements and neural network software modeling, "paper at IRC, Prague, 2002.

(35.) J. Dick, "New developments for the RPA, PPA and APA, " slide presentation at TLARGI Meeting, February 2006.

(36.) J. Sezna and P. DiMauro, "synthetic polymer processability testing-mixing," Rubber World, November 1983.

(37.) Monsanto MPT Brochure, 1987.

(38.) J. Sezna and M. Saake, "The use of the MPT in predicting processability," paper no. 9, Fall ACS Rubber Division meeting, 1985.

(39.) J. Sezna and P. DiMauro, "Processability testing of injection molding rubber compounds, "Rubber Chem. and Tech., vol. 57, no. 4, p. 826, Sep.-Oct. 1984.

(40.) J. Sezna, "Rubber testing for injection molding," paper no. 3 presented at the Rubber Division, ACS, Spring meeting, 1992.

by John S. Dick, Alpha Technologies ([e-mail])