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New insights into the mixing process.

The concept of using highly dispersible silica as the sole reinforcing filler, together with a silane coupling agent, for the tread compounds of low rolling resistance tires was first patented (ref. 1) in 1991. Since then, it has become well established. These compounds have a number of problems associated with them:

* Energy intensive, multiple-stage mixing cycles are required to achieve processable compound.

* Close time and temperature control are required during mixing to achieve silica to silane coupling and to avoid silane degradation (refs. 2 and 3).

* The compounds tend to have high viscosities that become even higher with time leading to difficult processing.

* The compounds tend to have short scorch times.

* The silica to silane coupling reaction is believed to be hindered by a number of normal compound additives (ref. 4), some of which might be used to improve mixing or process-ability.

While investigating this possible hindering of the silica to silane coupling reaction by an aliphatic zinc soap, we discovered some aspects of the mixing process that we believe have not been reported before. In addition, we have, in collaboration with Alpha Technologies, shown that viscoelastic testing of the uncured compounds can yield useful information regarding filler dispersion, silica to silane coupling and silane degradation.

Experimental

Compounds

The compounds are shown in table 1. Compound 1 is a control based on the formulation in the published patent for energy efficient tread compounds. Compounds 2 and 3 have the stearic acid replaced by an aliphatic zinc soap (Struktol A50P). In compound 2, the zinc soap was added during masterbatch mixing and in compound 3, it was added during the remill stage. Compounds 4 and 5 are the same as the control but the level of stearic acid is increased.
Table 1 - compounds

 1 2 3 4 5
 Control Zn soap Zn soap 2 phr 3 phr
 phr phr phr phr phr

Buna VSL 4020-1 103.0 103.0 103.0 103.0 103.0
Buna CB 10 25.0 25.0 25.0 25.0 25.0
Ultrasil 3370 GR 80.0 80.0 80.0 80.0 80.0
Silane X50S 12.5 12.5 12.5 12.5 12.5
HA oil 5.0 5.0 5.0 5.0 5.0
Zinc oxide 2.5 2.5 2.5 2.5 2.5
Stearic acid 1.0 - - 2.0 3.0
Aliphatic zinc soap - 4.0 4.0 - -
6ppd 2.0 2.0 2.0 2.0 2.0
Wax 1.50 1.50 1.50 1.50 1.50
Sulfur 1.40 1.40 1.40 1.40 1.40
CBS 1.70 1.70 1.70 1.70 1.70
DPG 2.00 2.00 2.00 2.00 2.00


Mixing

All the compounds were mixed in a Werner and Pfleiderer GK 1.5, 1.5 liter laboratory internal mixer. Actual compound dump temperatures were measured immediately after dumping using a needle pyrometer.

The batches were left to stand at room temperature for one day between each mixing stage.

Stage 1

Compounds 1, 2 and 3 were mixed using cycles controlled by time. Both indicated temperature and energy input were continuously measured. The mixing cycles are shown in tables 2, 3 and 4.
Table 2 - masterbatch mix cycle

Time, min Operation

 0 Add polymers plus silane
 1/2 Add zinc oxide, 3/4 silica and stearic acid or
 zinc soap (if used)
 3 1/2 Add rest of silica, oil, 6 ppd and wax
 4 Sweep down
 4 1/2 Dump


Rotor speed: 65 rpm, start and circulating water temperature: 65 [degrees] C
Table 3 - remill mix cycle

Time, min. Operation

 0 Add masterbatch and zinc soap (if used)
 3 1/2 Dump


Rotor speed: 65 rpm, start and circulating water temperature: 65 [degrees] C
Table 4 - final mix cycle

Time, min. Operation

 0 Add remill masterbatch plus curatives
1 max. Dump at max. 105 [degrees] C or after one minute


Rotor speed: 65 rpm, start and circulating water temperature: 50 [degrees] C

Stage 2

Compounds 1,4 and 5 were mixed also using the same time controlled mixing procedures as shown in tables 2, 3 and 4.

Stage 3

Compound 1 was mixed four times, mixes 1 to 4, this time using mixing cycles controlled by energy input. Both time and indicated temperature were continuously measured. The mixing cycles are shown in tables 5 to 9.
Table 5 - masterbatch mix cycle for mixes 1 to 3

Time, Operation Energy,
min. W. hours

0 Add polymers 0
1/2 Add zinc oxide, 3/4 silica, 3/4 silane and 50?
 stearic acid
? Add rest of silica, rest of silane, oil, 6 ppd 550
 and wax
? Sweep down 700
? Dump at 750 W. hours. 750


Rotor speed: mix 1, 65 rpm; mix 2, 55 rpm; mix 3, 45 rpm. Start and circulating water temperature: 65 [degrees] C
Table 6 - masterbatch mix cycle for mix 4

Time, Operation Energy,
min. W. hours

0 Add polymers 0
1/2 Add zinc oxide, 3/4 silica, 314 silane and 50?
 stearic acid
? Add mst of silica, mst of silane, oil, 6 ppd 300
 and wax
? Rotor speed from 65 rpm to 45 rpm 600
? Sweep down 750
? Dump at 750 W. hours 800


Rotor speed: 65/45 rpm, start and circulating water temperature: 65 [degrees] C
Table 7 - remill mix cycle for mixes 1 to 3

Time, Operation Energy,
min W. hours

0 Add masterbatch 0
? Dump at 650 W. hours 650


Rotor speed: mix 1,65 rpm; mix 2, 55 rpm; mix 3, 45 rpm. Start and circulating water temperature: 65 [degrees] C
Table 8 - remill mix cycle for mixes 4

Time, Operation Energy,
min. W. hours

0 Add masterbatch 0
? Rotor speed from 65 rpm to 45 rpm 400
? Dump at 750 W. hours 750


Rotor speed: 65/45 rpm, start and circulating water temperature: 65 [degrees] C
Table 9 - final mix cycle for mixes 1 to 4

Time, Operation
min.

0 Add remilled masterbatch plus curatives
1 max. Dump at max. 105 [degrees] C or after one minute


Rotor speed: 65 rpm, start and circulating water temperature: 50 [degrees] C

Stage 4

Compound 1 was mixed four more times, mixes 5 to 8, again using mixing cycles controlled by energy input, but with the total energy input increased by about 25%. Both time and indicated temperature were continuously measured. The mixing cycles are shown in tables 10 to 14.
Table 10 - masterbatch mix cycle for mixes 5 to 7

Time, Operation Energy,
min. W. hours

0 Add polymers 0
1/2 Add zinc oxide, 3/4 silica, 3/4 silane and 50?
 stearic acid
? Add rest of silica, rest of silane, oil, 6 ppd 300
 and wax
? Sweep down 750
? Dump at 1,000 W. hours 1,000


Rotor speed: mix 5, 65 rpm; mix 6, 55 rpm; mix 7, 45 rpm. Start and circulating water temperature: 65 [degrees] C
Table 11 - masterbatch mix cycle for mix 8

Time, Operation Energy,
min. W. hours

0 Add polymers 0
1/2 Add zinc oxide, 3/4 silica, 3/4 silane and 50?
 stearic acid
? Add rest of silica, rest of silane, oil, 6 ppd 300
 and wax
? Rotor speed from 65 rpm to 45 rpm 600
? Sweep down 750
? Dump at 1,000 W. hours. 1,000


Rotor speed: 65/45 rpm, start and circulating water temperature: 65 [degrees] C
Table 12 - remill mix cycle for mix 5 to 7

Time, Operation Energy,
min. W. hours

0 Add masterbatch 0
? Dump at 750 W. hours 750


Rotor speed: mix 5, 65 rpm, mix 6, 55 rpm, mix 7, 45 rpm. Start and circulating water temperature: 65 [degrees] C
Table 13 - remill mix cycle for mix 8

Time, Operation Energy,
min. W. hours

0 Add masterbatch 0
? Rotor speed from 65 rpm to 45 rpm 400
? Dump at 750 W. hours 750


Rotor speed: 65/45 rpm, start and circulating water temperature: 65 [degrees] C
Table 14 - final mix cycle for mix 5 to 8

Time, Operation
min.

0 Add remilled masterbatch plus curatives
1 max. Dump at max 105 [degrees] C or after one minute


Rotor speed: 65 rpm, start and circulating water temperature: 65 [degrees] C

Testing

Viscosity and scorch were measured using a Mooney MV2000 viscometer. Curing characteristics were measured using an MDR Rheometer 2000. Both instruments are from Alpha Technologies. Viscoelastic properties of the uncured compounds were measured using a rubber process analyzer (RPA 2000) by Alpha Technologies.

Garvey die extrusion trials were made using a Troester GS 30/K10D 30 mm diameter cold feed extruder, run at 70 rpm. This extruder has an L/D ratio of 10:1.

Cured physical properties were measured on samples cured to [t'.sub.c](100) at 160 [degrees] C using an Instron tensometer.

Dynamic heat build-up was measured on samples cured to [t'.sub.c](100) at 160 [degrees] C using a Doli Goodrich flexometer, Model Flex 380.006. The test conditions were: Pre-load 1 Mpa; stroke 6.35 mm; start temperature 50 [degrees] C; frequency 30 Hz; and test duration 30 minutes.

Viscoelastic properties on samples cured to [t'.sub.c](100) at 160 [degrees] C were measured using an MTS 831.50 servohydraulic tester. Viscoelastic properties were measured in shear at a frequency of 10 Hz at 20 [degrees] C, using discs nominally 20 mm in diameter and 3 mm thick. The shear was made in the direction of material flow during processing. The tests were displacement sweeps of 17 to 20 points logarithmically arranged from about 0.5% DSA to 20% DSA. They were conducted in duplicate and repeat tests carded out when differences of primary properties greater than 5% were encountered.

Results

The masterbatch mixing energy versus time graphs for the stage 1 compounds are shown in figure 1. At this masterbatch stage, the compound labeled zinc soap in the remill has neither stearic acid nor zinc soap in it. The difference between the curve for this compound and the other two is clear. Without either stearic acid or the zinc soap, the mixing energy falls off sharply after the first power peak. This indicates that the mix is having difficulty in maintaining a cohesive mass and in achieving effective filler incorporation. When the final addition of filler is made, the compound without either stearic acid or the zinc soap does not even show a clear first power peak.

It is also interesting to note that none of the compounds exhibits a second power peak (indication of full wetting out of the filler by the polymer) during masterbatch mixing.

Despite a diligent search of the literature, we have been unable to find any reference to this strong effect of stearic acid on the mixing of silica reinforced compounds, nor to the absence of a second power peak when mixing the masterbatch of silica filled compounds.

The remill mixing energy versus time graphs for the stage 1 compounds are shown in figure 2. These curves clearly show that the second power peak for these silica reinforced compounds occurs during the early part of the remill. The most pronounced second power peak is shown by the compound having the zinc soap added during the remill. The curve for this mix then goes on to exhibit a steeper slope for the dispersive mixing region after the second power peak. This improved mixing behavior during the remill stage should help to offset the poor mixing behavior this compound showed during its masterbatch stage.

[Figure 2 ILLUSTRATION OMITTED]

The integrated mixing energy data and the compound dump temperatures, measured with a needle pyrometer immediately after dumping, for these compounds are shown in table 15. The integrated mixing energy data could have easily been misinterpreted had the energy versus time graphs not been taken into consideration. Because of the low energy input after the first power peak of the masterbatch and the sharp fall off of the energy input during the dispersive mixing section of the remill, for the compound with the zinc soap added in the remill, this compound requires the least total energy to mix.
Table 15 - stage 1 mixing data

 1 2 3
 Control Zn soap Zn soap
 phr in M/B in remill
 phr phr

Stearic acid 1.0 - -
Aliphatic zinc soap - 4.0 4.0
 Mixing energy

Masterbatch, GJ/[m.sup.3] 1.940 1.783 1.552
Remill, GJ/[m.sup.3] 2.180 2.042 1.980
Final mix, GJ/[m.sup.3] 0.666 0.592 0.584
Total, GJ/[m.sup.3] 4.641 4.281 3.984

 Needle pyrometer dump temperature

Masterbatch, [degrees] C 161 165 151
Remill, [degrees] C 183 189 184
Final mix, [degrees] C 135 141 139


The dump temperatures for the remill and final mix are rather higher than would normally be used in production, but they do not appear to have adversely affected the compound properties. This is shown by the Mooney viscosities, the Mooney scorch times and the extrusion data, shown in table 16, and the rheometer cure curves, shown in figure 3.

[Figure 3 ILLUSTRATION OMITTED]

The rheometer cure curves for both compounds containing the zinc soap exhibit much lower torque development than the compound containing stearic acid. At first we believed this was due to die slippage caused by surface lubrication from the soap. However, we were able to show that this was not the case. Rheometer cure curves were run, using the RPA 2000, with different angles of oscillation, from very low strain to very high strain. While the actual torque values varied, the torque development for the compounds containing the soap relative to that for the compound containing stearic acid remained the same.

It must be remembered that the rheometer measures the viscoelastic property, elastic torque. This will only be proportional to crosslink density if all other aspects of the compound are equal. If there are differences in the type or loading of filler or the dispersion of the same filler, then this property will be affected. We believe that the lower rheometer torque values given by both the compounds containing the zinc soap indicate superior filler dispersion.

The beneficial influence of the zinc soap on filler dispersion and on both Mooney viscosity and Mooney scorch time is also clearly shown and is independent of the addition point.

From the extrusion trial data of these compounds, shown in table 16, both compounds containing the zinc soap show a higher output rate and reduced back pressure. Again, the effect of the soap is independent of its point of addition.
Table 16 - stage 1 compound processability data

 1 2 3
 Control Zn soap Zn soap
 phr in M/B in remill
 phr phr

Stearic acid 1.0 - -
Aliphatic zinc soap - 4.0 4.0

 Mooney viscosity at 100 [degrees] C

Masterbatch, MS 1+4 74 83 84
Remill, MS 1+4 76 84 72
Final mix, MS 1+4 57 52 48
Final mix, ML 1+4 104.3 87.4 81.3

 Mooney scorch at 135 [degrees] C

[t.sub.5], min. 6.6 14.1 13.9

 Garvey die extrusion, screw speed 70 rpm

Extrusion rate, g/min. 204 227 228
Extrusion pressure, bar 76 53 49


The cured modulus data, shown in table 17, shows that the compounds containing the zinc soap have slightly lower modulus values than the compound containing stearic acid. However, the ratio of M300 to M100, which is sometimes used as an indication of the strength of the silane coupling, is higher for the compounds containing the zinc soap. It is possible that this property is also affected by filler dispersion, as well as by silane coupling.
Table 17 - stage 1 cured modulus and heat build-up

 1 2 3
 Control Zn soap Zn soap
 phr in M/B in remill
 phr phr

Stearic acid 1.0 - -
Aliphatic zinc soap - 4.0 4.0

Cured modulus, cure: [t'.sub.c](100) @ 160 [degrees] C

M100, MPa 3.2 2.6 2.8
M200, MPa 8.7 7.6 8.1
M300, MPa 15.8 14.3 15.2
Ratio M300:M100 4.94 5.50 5.43

 Goodrich heat build up
Pre-load 1 MPa, stroke 6.35 mm, start temp. 50 [degrees] C,
 time 30 min.

HBU, [degrees] C 122.5 128.6 125.3


As mentioned in the introduction, this first stage was intended to investigate the potential for a zinc soap to interfere with the silane coupling reaction. To evaluate this effect, we carded out viscoelastic dynamic tests on cured samples. The curves for elastic and viscous modulus versus strain and tangent [Delta] versus energy input are shown in figures 4, 5 and 6.

[Figures 4-6 ILLUSTRATION OMITTED]

The two compounds containing the zinc soap show very similar properties, regardless of the addition point of the soap. The elastic and viscous moduli of these compounds are considerably lower than those of the compound containing stearic acid. This is also probably due to improved filler dispersion. The compounds containing the zinc soap also show lower tangent [Delta] values than the compound containing stearic acid, but here the difference is not so large as with the moduli values.

The fact that there are practically no differences between the viscoelastic dynamic properties of the compound where the zinc soap was added during masterbatch mixing and the compound where it was added during the remill stage indicates that there is no interference of the silane reaction by the zinc soap.

We would not normally recommend this aliphatic zinc soap for silica reinforced compounds, because a zinc and potassium soap that is more polar usually gives better compound processing properties, particularly for extrusion (ref. 5). However, we believe that this more polar soap has a much greater attraction for the silica surface and that it will compete with the silane for the silica surface if it is added during masterbatch mixing. Where processing problems are encountered within the mixing process, it may be necessary to add a process additive during the masterbatch mix cycle, hence the need for this investigation.

A further demonstration of the remarkable effect that stearic acid has on the masterbatch mixing of this silica reinforced compound is shown by the mixing energy versus time curves for the stage 2 compounds. These are shown in figure 7. Here, the levels of stearic acid are 1, 2 and 3 phr. The rate of energy rise to the first power peak is strongly affected by the stearic acid level. The mixing cycles were the same as for the previous mixes. There is still no second power peak achieved during masterbatch mixing at any of the levels of stearic acid used.

[Figure 7 ILLUSTRATION OMITTED]

The mixing energy versus time curves for the remill cycles of these compounds are shown in figure 8. Again, the second power peak occurs during the early part of the remill cycle. As the stearic acid level rises, the second power peak becomes less defined. There were virtually no differences between the total mixing energies, the dump temperatures and the Mooney viscosities of these compounds.

[Figure 8 ILLUSTRATION OMITTED]

The Mooney viscosities and scorch times, together with some basic data from an extrusion trial, are shown in table 18. The masterbatch viscosities are reduced with increasing stearic acid levels, but the final mixes show little variation with a tendency for higher viscosities at high stearic level. The Mooney scorch values show an increase with stearic acid level.
Table 18 - stage 2 compound processability data

 1 2 3
 Control Zn soap Zn soap
 phr in M/B in remill
 phr phr

Stearic acid 1.0 2.0 3.0

 Mooney viscosity at 100 [degrees] C

Masterbatch, MS 1+4 95.4 91.2 84.4
Final mix, MS 1+4 51.1 48.8 55.6
Final mix, ML 1+4 93.5 90.9 104.6

 Mooney scorch at 135 [degrees] C

[t.sub.5], min. 9.2 12.3

 Garvey die extrusion, screw speed 70 rpm

Extrusion rate, g/min 193.5 220.2 214.9
Extrusion pressure, bar 88 69 72


The extrusion trial data, which are comparable with the data shown for the stage 1 compounds, show that increasing the stearic acid level confers some improvement, but it is only about half that given by the zinc soap. All the improvement occurs with the change from 1 phr to 2 phr. No further improvement is given by increasing the stearic acid level to 3 phr.

The rheometer cure curves, show that they all reach the same level of torque and show very similar cure rates, but that the cure initiation time increases with stearic acid level.

In stage 3, compound 1 was mixed four times using mixing cycles controlled by energy, but with different rotor speeds. Mix 1 at 65 rpm, mix 2 at 55 rpm, mix 3 at 45 rpm and mix 4 were a combination of 65 rpm and 45 rpm. This combination of rotor speeds was used to try to extend the mixing time to allow the silane reaction to proceed further without the temperature rising to a level at which silane degradation might occur.

The masterbatch mixing energy versus time graphs for the stage 3 compounds are shown in figure 9. The influence of the reducing rotor speed from mix 1 to mix 3 is clear and as expected, as is the effect of changing the rotor speed from 65 rpm to 45 rpm during the cycle of mix 4.

[Figure 9 ILLUSTRATION OMITTED]

The indicated temperature versus time curves clearly show that reducing rotor speed reduces the rate of temperature build-up, as expected. It is also shown that changing from 65 rpm to 45 rpm during the cycle effectively halts the temperature rise.

For the remill mixing, the energy versus time graphs for these mixes are shown in figure 10 and the indicated temperature versus time graphs are shown in figure 11. The second power peak for all these mixes again occurs during the early part of the remill. The second power peak of the 45 rpm mix is less clearly defined than at the higher rotor speeds. There was also less difference between the energy levels of the mixes at 65 rpm and 55 rpm than there was between those at 55 rpm and 45 rpm. As with the masterbatch mixing, reducing rotor speed reduces the rate of temperature build-up and changing from 65 rpm to 45 rpm during the cycle effectively halts the temperature rise.

[Figures 10-11 ILLUSTRATION OMITTED]

The integrated mixing energy data and the pyrometer dump temperatures are shown in table 19. As expected, all the compounds have very similar total mix energies because the mixes were controlled by energy. The data show that reducing the rotor speed solves the problem of excessively high dump temperatures, particularly for the remill. The two speed mixing process also gave satisfactory dump temperatures.
Table 19 - stage 3 mixing data

 Compound 1 Compound 1
 mix 7 mix 2
 65 rpm 55 rpm

 Mixing energy

Masterbatch, GJ/[m.sup.3] 1.925 1.900
Remill, GJ/[m.sup.3] 1.650 1.660
Final mix, GJ/[m.sup.3] 0.658 0.627
Total, GJ/[m.sup.3] 4.123 4.076

 Needle pyrometer dump temperature

Masterbatch, [degrees] C 155 145
Remill, [degrees] C 180 161
Final mix, [degrees] C 136 138

 Compound 1 Compound 1
 mix 3 mix 4
 45 rpm 65/45 rpm

Masterbatch, GJ/[m.sup.3] 1.893 2.043
Remill, GJ/[m.sup.3] 1.650 1.862
Final mix, GJ/[m.sup.3] 0.634 0.592
Total, GJ/[m.sup.3] 4.067 4.373

Masterbatch, [degrees] C 146 145
Remill, [degrees] C 146 160
Final mix, [degrees] C 137 136


The rheometer cure curves for these mixes show that the lowest torque development was given by the mix at 65 rpm and the one mixed at 65/45 rpm. The mix at 55 rpm showed a torque development about 20% higher and the mix at 45 rpm another 20% higher. These results probably indicate that reducing the rotor speed of the mixer has reduced the filler dispersion achieved, even though the mixes were mixed to equal energy input. The mix starting with 65 rpm and changing to 45 rpm appears to have equal dispersion to the mix run completely at 65 rpm.

The Mooney viscosities and scorch times of these mixes are shown in table 20. The viscosities do not show big differences, but there is a tendency for the viscosity to fall as the rotor speed during mixing is reduced. The mix with the dual rotor speed gave the lowest viscosity. These results probably indicate that as the temperatures reduce with lower rotor speeds, so less silane degradation occurs. For the dual rotor speed mix, this is complemented by a better filler dispersion. The Mooney scorch values show that the mix carded out at 65 rpm had a scorch time about 20% lower than the other three mixes. This is also indicative of silane degradation having occurred in the mix run at 65 rpm.
Table 20 - stage 3 compound processability data

 Compound 1 Compound 1
 mix 1 mix 2
 65 rpm 55 rpm

 Mooney viscosity at 100 [degrees] C

Masterbatch, MS 1+4 89.7 87.8
Remill, MS 1+4 68.7 65.4
Final mix, MS 1+4 51.4 48.4
Final mix, ML 1+4 94.0 90.0

 Mooney scorch at 135 [degrees] C

[t.sub.5], min. 7.5 10.6

 Compound 1 Compound 1
 mix 3 mix 4
 45 rpm 65/45 rpm

Masterbatch, MS 1+4 84.6 84.5
Remill, MS 1+4 68.1 60.5
Final mix, MS 1+4 48.0 46.9
Final mix, ML 1+4 89.0 85.8

[t.sub.5], min. 10.3 10.5


The cured moduli values, shown in table 21, show that all the single rotor speed mixes have similar modulus values at all elongations and that these are higher than those of the dual rotor speed mix. However, the ratio of the M300 to the M100 is higher for the dual rotor speed mix.
Table 21 - stage 3 cured modulus

 Compound 1 Compound 1 Compound 1 Compound 1
 mix 1 mix 2 mix 3 mix 4
 65 rpm 55 rpm 45 rpm 65/45 rpm

 Cured modulus. Cure: [t'.sub.c] (100) @ 160 [degrees] C

M100, MPa 3.3 3.5 3.7 2.8
M200, MPa 8.7 8.9 9.2 7.1
M300, MPa 15.8 15.9 16.0 14.5
Ratio M300:M100 4.79 4.54 4.32 5.18


Viscoelastic tests were carried out at 100 [degrees] C and 0.1 Hz on uncured samples of these mixes by Alpha Technologies using a rubber process analyzer.

Elastic modulus versus strain curves are shown in figure 12. Applying the concept developed by Coran and Donnet (ref. 6) to this work, the elastic modulus values at low strain show that the mix run at 65 rpm and the dual speed mix show equal and the best filler dispersion. The work by Coran and Donnet was with carbon black and it can not be certain that the interpretation of the data can be the same with silane treated silica reinforced compounds (ref. 7). Indeed, the lower values of elastic modulus may also indicate better silane coupling. The poorest filler dispersion is shown by the compound run at 45 rpm.

[Figure 12 ILLUSTRATION OMITTED]

The elastic modulus at high strain should be indicative of silane degradation, but the resolution of this property at high strain is poor. However, if the elastic torque is plotted against strain, the resolution at high strain is improved. Here we can see that the mix run at 65 rpm gave the most silane degradation, followed by the mix run at 55 rpm. The least silane degradation is shown by both the mix run at 45 rpm and the dual rotor speed mix.

In stage 4, compound 1 was mixed another four times, again using mixing cycles controlled by energy and with the same rotor speeds as used in stage 3, but with the total energy input increased by 30%. The masterbatch mixing energy versus time graphs for these mixes are shown in figure 13. Even with the total masterbatch energy input increased to 2.5 GJ/[m.sup.3] there was still no second power peak during the masterbatch mixing. The remill mixing energy versus time curves, shown in figure 14, again show the second power peak occurring during the early part of the remill. This would seem to indicate that time is also required for the polymer to fully wet out the surface of the silica filler rather than mixing work alone.

[Figures 13-14 ILLUSTRATION OMITTED]

Conclusions

* Stearic acid has a surprisingly strong influence on the mixing efficiency with silica filler.

* An aliphatic zinc soap used in place of stearic acid in a silica reinforced SSBR/BR tire tread compound confers improved processing properties.

* The addition of this soap during the masterbatch stage does not appear to adversely affect the silane coupling reaction.

* The addition of this soap gives improved filler dispersion.

* The second power peak of this silica reinforced compound does not occur during the masterbatch cycle, even when it is considerably extended.

* The second power peak of this silica reinforced compound always occurs during the early part of the remill.

* Mixing with high rotor speeds gives better filler dispersion, but makes temperature control more difficult.

* Reducing the rotor speed for the entire mix cycle results in poor filler dispersion.

* Starting both the masterbatch and remill mix cycles with high rotor speeds, then reducing the rotor speed to maintain temperature, gives a good combination of filler dispersion, silane coupling and reduces silane degradation.

* Viscoelastic properties carried out on uncured compound can give valuable information on filler dispersion, the silane coupling reaction and silane degradation.

References

(1.) Patent application EP 0501 227, Michelin, R. Rauline, February 25, 1991.

(2.) Degussa Information for the Rubber Industry. "Compounding of Si69-stocks." Degussa AG.

(3.) U. Gorl and A. Hunsche, "Advanced investigation into the silica/silane reaction system," presented at the 150th meeting of the Rubber Division, ACS. Louisville, Kentucky. Oct. 8-11, 1996. Paper No. 76.

(4.) S. Wolff, Rubber Chem. Technol. 55,967 (1982).

(5.) H. Bartels, M. Hensel, K-H. Menting, R. Schuster and H. Umland, "Progress in downline processing of solution SBR high silica tread compounds for 'green tires' - influence of processing promoters," presented at IRC Manchester, UK. June 17-21, 1996.

(6.) A.Y. Coran and J.P. Donnet, Rubber Chem. Technol. 65, 1016(1992).

(7.) J.S. Dick and H. Pawlowski, "Applications of the rubber process analyzer in characterizing the effects of silica on uncured and cured compound properties," presented at the meeting of the Rubber Division, ACS, Montreal, Canada. May 4-8, 1996. Paper No. 34.3
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Comment:New insights into the mixing process.
Author:Hensel, M.
Publication:Rubber World
Geographic Code:1USA
Date:Jul 1, 1999
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