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Comparison of mixing breakdown profiles for compounds based on different elastomers.

During the rubber mixing process for a rubber compound, the base elastomer is being masticated while starting to incorporate typically carbon black and other ingredients. As this process continues, the carbon black agglomerates are deagglomerated and dispersed as primary aggregates, while the base raw elastomer(s) is (are) simultaneously masticated and "broken" down, usually through some degree of depolymerization (refs. 1 and 2).

In this study, we measured the rheological changes that occur under moderate and severe strain conditions with the RPA for a variety of raw elastomers in order to understand more about the degree of "breakdown" or mastication that these different rubbers are experiencing.

Also, we constructed simple model recipes of these selected raw elastomers with N330 carbon black and studied the rheological effects on these experimental compounds from controlled amounts of applied work history during internal mixing in the laboratory.

Experimental

Table 1 shows the 17 different raw elastomers which were selected for this study.

As can be seen, the selected polymers include a very wide selection of both tire and non-tire rubbers that are commonly used. This selection includes NR, SBR, BR, IIR, CR, Q, FKM, BIIR, IR, NBR, EPDM, CSM, CM, ACM, AEM and HNBR. These raw elastomers were tested as such through time sweeps with the Alpha Technologies RPA 2000 rubber process analyzer, as well as with the Alpha Technologies Mooney viscometer MV2000. In addition, these raw elastomers were also mixed with N330 carbon black in a carefully controlled manner with a laboratory BR Banbury mixer which was stopped periodically during the mixing cycle to take small sample aliquots between 3 and 7.5 minutes of mixing time. Since the specific gravity for these elastomers was different, all comparisons were made on an equal volume basis so that each batch would always contain exactly 35% by volume carbon black.

Mastication and breakdown of raw elastomers during mixing RPA time sweeps were performed at different strains at moderate frequencies in order to try to relate to the mastication and breakdown profiles of the different raw elastomers studied. The RPA has a distinct advantage over other types of DMAs in being able to apply a relatively high strain, such as [+ or -] 400 or even [+ or -] 1,000% strain at a moderate frequency such as 0.5 or 1.0 Hz.

For moderate severity conditions, all raw elastomers discussed were tested on the RPA in a time sweep at 400% strain, 1 Hz and 100[degrees]C for 20 minutes each. Also, more severe strain conditions were applied where a series of five five-minute time sweep subtests at 100[degrees]C and 0.5 Hz were performed at 10%, 50%, 100%, 500% and 1,000% strain.

Measuring carbon black incorporation and deagglomeration This part of the study involved measuring Theologically the effects of carbon black incorporation and deagglomeration at different stages of the internal mixing process.

In this design of experiment, each raw elastomer compound was mixed with exactly 35% by volume of N330 carbon black.

During the internal mixing process, 20 g aliquot samples were taken from the mixer after 3, 3.5,4.5, 5.5, 6.5 and 7.5 minutes, as sampling intervals. Each of these samples was tested on the RPA by the new ASTM D8059 Standard for the Payne effect. For this study, a time test was applied at 0.07% strain, 1 Hz and 100[degrees]C for two minutes before the Payne effect strain sweep was applied. This strain sweep was applied at 1 Hz and 100[degrees]C, starting with [+ or -]0.07% strain, followed by [+ or -]0.1,0.14,0.2,0.28,0.35,0.5,0.7,1.0,1.4,2.0, 2.8,3.5,5.0,7.0,10.0,14,20,28,35,50,70,100 and 140%.

Discussion

Breakdown of raw elastomers

From the RPA time test at [+ or -] 400% strain, 1 Hz, 100[degrees]C for 20 minutes, the following changes in the complex modulus G*

As can be seen, the severity of this strain history was not high enough to measure a significant drop in complex shear modulus G*. However, figure 2 shows a much larger drop in the elastic modulus G' from the same strain history applied during the 400% strain time sweep.

However, an even higher strain history was applied, as discussed in the experimental section, which caused an even higher percent drop in the G' elastic modulus. This was a result of the final five-minute time sweep at [+ or -] 1,000% strain at 0.5 Hz and 100[degrees]C, as shown in figure 3.

From programming a series of time sweeps at [+ or -] 10, 50, 100, 500, and finally 1,000% strain (all at 0.5 Hz and 100[degrees]C), and measuring the percent change in the G' from two to five minutes at [+ or -] 1,000% strain, a somewhat better representation of raw polymer breakdown during actual mixing is achieved. For example, there is a large breakdown in uncured elasticity for natural rubber and G type polychloroprene, which was expected. On the other hand, there is not very much breakdown on fluoro-elastomer or regular butyl rubber, which was also expected.

Incorporation and deagglomeration of carbon black The Payne effect is an effective way of studying deagglomeration of fully reinforcing carbon black during the rubber compound mixing process. It should be an effective way of relating to carbon black aggregate-aggregate attraction versus the carbon black aggregate attraction of the specific rubber hydrocarbon medium (ref. 3).

Different rubber hydrocarbon media have different affinities for the carbon black filler. For example, it is well known that in various blends of different types of rubber polymer, one domain of rubber (at the microscopic level), will have a greater attraction (or affinity) for the available carbon black than the other rubber present in the blend. Usually, different rubbers in a blend are not perfectly soluble with each other, so they will commonly establish continuous and discontinuous phases (refs. 4 and 5). Therefore, typically the available carbon black is attracted more to either the continuous or the discontinuous rubber phase. The typical carbon black affinity for different elastomers is shown below (refs. 6-9).

BR > SBR > CR > NBR > NR > EPDM > IIR

So carbon black is much more attracted to the BR phase (polybutadiene rubber) than the IIR (butyl rubber) phase.

From the Payne effect RPA measurements, the SBR test results show a great distinction in the Payne effect curves based on the time intervals being applied during the mixing shown in figure 4.

The affinity of carbon black to SBR 1500 is high. So during the mixing process, the carbon black readily deagglomerates in four other time sweeps at 10, 50, 100 and 500% the SBR medium because of this attraction between SBR and carbon black.

In figure 5, the Payne effect curves at different stages of carbon black mixing in WRT polychloroprene are also shown.

Polychloroprene does not have quite the affinity for carbon black that SBR displays. On the other hand, it does have a significant affinity for carbon black, as shown in figure 5.

Natural rubber has a somewhat similar chemical structure to that of polychloroprene. According to figure 6, NR has some affinity for carbon black, but not quite as strong as polychloroprene, as shown in figure 5.

Here one can see that the Payne effect curves are a little closer together. Natural rubber, of course, has a good affinity for carbon black; however, carbon black's attraction for SBR is greater. On the other hand, carbon black's affinity for EPDM is much less, as shown in figure 7.

All the Payne effect curves appear very similar because of the relatively low affinity of EPDM for carbon black. This point is reinforced more by figure 8, which shows the resulting Payne effects when regular butyl rubber (IIR) is used.

True to the relative carbon black affinity comparison given in figure 8, regular butyl rubber has a relatively poor affinity for carbon black. Additional work history and mixing time does not necessarily improve the carbon black dispersion or promote greater deagglomeration in this butyl rubber matrix.

Other comparisons were also made. Figure 9 shows the results of this mixing study using nitrile rubber (NBR) as the base rubber.

These Payne effect comparisons are very similar to those seen with SBR in figure 4. So NBR is behaving somewhat similar to SBR in accepting carbon black during mixing.

Figure 10 contrasts HNBR with the NBR shown in figure 9. This particular HNBR shows a little less separation in the Payne effect curves than NBR shows, which might be expected.

Figure 11 shows the Payne effect profiles for synthetic natural rubber (IR). Perhaps figure 11 shows this synthetic polyisoprene rubber to be a little more receptive to carbon black than the natural rubber shown in figure 6. This might be because of the greater gel and protein content of the NR.

Figure 12 shows the Payne effect mixing profiles for bromobutyl rubber (BIIR).

Obviously, halogenation has a large effect on the compatibility of BIIR with carbon black. Figure 8 showed practically no effect on the Payne effect from increasing mixing time. With bromination, there is a significant amount of separation.

Figure 13 shows the results of the mixing study with ethylene acrylic elastomer (AEM).

As can be seen, there is some separation of these Payne effect curves, at least initially. However, at higher strains these curves overlap more.

Figure 14 shows the Payne effect curves when chlorinated polyethylene (CM) is used as the base rubber.

As can be seen, CPE imparts strain softening with carbon black very similar to that of EPDM in figure 7.

Carbon black is usually only used in silicone rubber (Q) as a colorant. It is not normally used in silicone rubber as a reinforcing agent (silica is used instead). Figure 15 shows the nature of the strain softening effects from this series of experiments with silicone.

Figure 15 shows a small Payne effect at very low strains, followed by a merger of G' at the higher strains. Figure 16 shows an unusual G' modulus profile for a specific polyacrylate rubber (ACM).

What is unusual about this Payne effect profile is that it is going in the opposite direction with increasing mixing time. The other profiles presented more or less show the G' modulus decreasing with increasing mixing time. This is expected because greater work history from longer mixing times causes the apparent modulus G' to decrease because of the destruction of the filler network, which makes the low strain G' drop in value. However, with this proprietary ACM elastomer, it is moving in the opposite direction, probably due to some sort of chemical side reaction.

Lastly, fully reinforcing carbon blacks are not used that often in fluoroelastomer (FKM) compounds. However, figure 17 also shows some unusual results.

These results from the FKM mixing study appear to be somewhat erratic. As mentioned earlier, N330 carbon black is not used that often in FKM. Even though the results are somewhat erratic, there is only a small spread of the Payne effect curves with increased mixing time, which is not surprising.

Conclusions

The RPA Payne effect test (ASTM D8059) is quite effective in providing an objective method for evaluating the "affinity" of different raw elastomers for fully reinforcing carbon black during the mixing process. The extended dynamic range (EDR) on the RPA 2000 was quite effective at measuring subtle differences in G' at very low strains. An RPA time sweep at 1,000% applied strain is somewhat effective at comparing the raw elastomer "breakdown" during the compound mixing process.

This article is based on a paper presented at the 190th Technical Meeting of the Rubber Division, ACS, October 2016.

by John S. Dick and Edward Norton, Alpha Technologies

References

(1.) Wesley Wampler, Chapter 6, Carbon Black, Rubber Compounding, Chemistry and Applications (edited by Brendan Rodgers), CRC Press, Boca Raton, FL, 2016.

(2.) Steve Laube, Steve Monthey and Meng-Jiao Wang, Chapter 12, Compounding with Carbon Black and Oil, Rubber Technology Compounding and Testing for Performance (edited by J. Dick), Hanser, 2009.

(3.) J. Dick and H. Pawlowski, "Applications of a new dynamic mechanical rheological tester in measuring carbon black and oil effects on rubber compound properties," J. of Elastomers and Plastics, Vol. 27, January 1995.

(4.) J. Dick and H. Pawlowski, "Applications for the rubber process analyzer, part 1, " Rubber and Plastics News, April 26, 1993.

(5.) J. Dick and H. Pawlowski, "Applications for the rubber process analyzer, part 2, " Rubber and Plastics News, May 10, 1993.

(6.) E.S. Castner, "Where's the filler? Morphology control for improved dynamic and mechanical properties, " Paper No. 13 presented at the Fall Meeting of the Rubber Division, ACS, October 5-6, 2004.

(7.) W. Hess, C. Herd and P. Vegvari, "Characterization of immiscible elastomer blends," Rubber Chemistry and Technology, July-August, 1993, Vol. 66, p. 329.

(8.) E. McDonel, K. Baranwal and J. Andries, Polymer Blends, Vol. 2, Chapter 19, Elastomer Blends in Tires, Academic Press, 1978, p. 282.

(9.) J. Dick, How to Improve Rubber Compounds, 1,800 Experimental Ideas for Problem Solving, Second Edition, Hanser Publications, 2014, p. 16.

Caption: Figure 1--percent change in G* from eight minutes to 20 minutes from RPA time sweep at 400% strain, 1 Hz, 100[degrees]C.

Caption: Figure 2--percent change in G' from eight minutes to 20 minutes at 400% strain, 1 Hz, 100[degrees]C

Caption: Figure 3--% change in G' from two to five minutes at 1,000% strain, 0.5 Hz, 100[degrees]C following tour other time sweeps at 10, 50, 100 and 500%

Caption: Figure 4--effects of increasing mixing time on measured Payne effect curves for internal mixing of SBR 1500 and N330 carbon black

Caption: Figure 5--effects of increased mixing time on Payne effect curves for internal mixing of WRT polychloroprene and N330 carbon black

Caption: Figure 6--effects of increased mixing time on measured Payne effect curves for internal mixing of natural rubber SIR 20 and N330 carbon black

Caption: Figure 7--effects of increased mixing time on measured Payne effect curves for internal mixing of EPDM (Nordel IP5565) and N330 carbon black

Caption: Figure 8--effects of increased mixing time on measured Payne effect curves for I internal mixing of butyl 268 (IIR) and N330 carbon black

Caption: Figure 9--effects of increased mixing time on measured Payne effect curves for internal mixing of NBR (DN 2850) and N330 carbon black

Caption: Figure 10--effects of increased mixing time on measured Payne effect curves for internal mixing of HNBR (Zetpol 2010) and N330 carbon black

Caption: Figure 11--effects of increased mixing time on measured Payne effect curves for internal mixing of synthetic natural rubber (Natsyn 2200) and N330 carbon black

Caption: Figure 12--effects of increased mixing time R on measured Payne effect curves for 1 internal mixing of bromobutyl 2244 rubber fl (BIIR) and N330 carbon black

Caption: Figure 13--effects of increased mixing time on measured Payne effect curves for internal mixing of Vamac G (AEM) and N330 carbon black

Caption: Figure 14--effects of increased mixing time on measured Payne effect curves for internal mixing of CPE 4235 (CM) and N330 carbon black

Caption: Figure 15--effects of increased mixing time on measured Payne effect curves for internal mixing of silicone RBB 2001-65 and N330 carbon black

Caption: Figure 16--effects of increased mixing time on measured Payne effect curves for internal mixing of Hytemp AR12 (ACM) and N330 carbon black

Caption: Figure 17--effects of increased mixing time on measured Payne effect curves for internal mixing of Viton B600 (FKM) and N330 carbon black
Table 1--raw elastomers selected for this study

Name of rubber                ASTM             Trade    Specific
                      abbreviation              name    gravity

Styrene butadiene              SBR          SBR 1500       0.94
  rubber

Chlorinated                     CM          CPE 4235       1.13
polyethylene
rubber

Silicone Rubber                  Q          Silicone        1.2
  (contains silica)                      RBB 2001-65
Fluoroelastomer                FKM        Viton B600       1.82
Chlorosulfonated               CSM      Hypalon 3570        1.2
  polyethylene
Hydrogenated NBR              HNBR       Zetpol 2010       0.95
Polychloroprene                 CR               GRT       1.23
Polychloroprene                 CR               WRT       1.24
Bromobutyl rubber             BIIR   Bromobutyl 2244       0.93
Ethylene-acrylic               AEM           Vamac G       1.03
  elastomer
Polyacrylate                   ACM       Hytemp AR12       1.10
  elastomer
Ethylene propylene            EPDM     Nordel IP5565       0.87
  diene rubber
Synthetic natural               IR       Natsyn 2200       0.91
  rubber
1,4 cis                         BR        Budene 220       0.92
  polybutadiene
  rubber
Acrylonitrile                  NBR       NBR DN 2850       0.97
   butadiene
  rubber
Butyl rubber                   IIR         Butyl 268       0.92
Natural rubber                  NR            SIR 20       0.92
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Author:Dick, John S.; Norton, Edward
Publication:Rubber World
Date:Mar 1, 2017
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