Cooling impact on rubber mixing.

Heat generation in rubber subjected to mixing in an internal mixer has always been a problem. In general, rubber mixes are poor conductors of heat, and heat generated in the rubber compound due to mixing is not easily conducted away, thus causing heat build up.

Very little is reported on the aspect of rubber mixing relevant to heat generation. For instance, a number of references only state the tact that rubber will warm up when subjected to shear rate/shear stress during processing. This lack of data is surprising in view of the importance of the heating and cooling cycles, which always occur in the production of rubber compounds.

Knowing the total energy input in a mixing cycle for a known batch weight, it is easy to calculate the specific energy input (kWh/kg), and this is likely to be more relevant to assessing the state of mix, as well as compounding costs. However, where more accurate measurements are to be made it is necessary to allow for energy dissipated as heat and energy losses. This means that it should not be assumed that all the net energy has necessarily contributed to mixing. The work reported here represents an attempt to provide some useful calculations relating to the components of the energy balance for a rubber mixing cycle. The calculations also offer a very effective means of assessing the energy absorbed by the mixed material, energy removed by the cooling system and energy losses; thereby the net power consumption related to, say, a multi-stage or a single-stage mixing cycle.

Due to the continued increase in the utilization of silica, especially in the tire manufacturing industry, questions have been raised about the ability of the conventional multi-stage mixing method to provide an effective and favorable means of processing silica-filled compounds. In recognition of this, an essential part of this work reported here is to review briefly the work currently being undertaken by Farrel so that reactive mixing, involving silica-filled compounds, can be achieved in a reasonable length of time. The reaction between the coupling agent and silica was carried out in the mixer using a specially modified mixing process, namely steady-state, where the reaction time was varied between two, three and four minutes. This article explains why the use of an Intermix mixer, due to its efficient cooling, can provide an efficient means of handling the silica-filled compounds.

Importance of cooling

The most difficult task in rubber mixing for achieving effective mixing is maintaining a high Mooney viscosity level in the mix so that high shear forces can be applied. Such difficulties arise because of the relationship between viscosity ([eta]) and temperature (T):

(1) [eta] = A exp (B/T)

where A and B are constants and

(2) [tau] = [eta] * [[gamma].sup.n]

where [tau] is shear stress, [gamma] is shear rate (which is a measure of the rate at which the rubber is deformed) and n is power law index (for many rubbers, n = 0.2 to 0.3). Equation 1 indicates that upon increasing the temperature, the viscosity will drop. Equation 2 indicates that upon increasing the temperature and thus, by virtue of the temperature dependence of viscosity, the shear stress that can be applied during mixing will therefore be reduced to the point where insufficient dispersive mixing can occur. It follows that for effective mixing, the temperature of a given mix should be kept low and must not exceed a particular level during mixing. But the low thermal conductivity of rubber, and the fact that a considerable proportion of the power required for mixing is being transformed into heat, in general, make this requirement of low temperature difficult to achieve. This seems true despite the considerable emphasis that has been placed upon cooling efficiency in the present generation internal mixers. Figure 1 shows the dependency of compound Mooney viscosity on testing temperature.

Surface-to-volume ratio

It is also important to realize that heat transfer in an internal mixer is proportional to the surface area in contact with the mixed material. In view of this, the need to examine the ratio of the cold surface area to the net volume of the machine is of paramount importance in relation to temperature build-up. With increased machine size, the ratio of surface area to machine volume is decreased. In consequence, the rate of temperature build-up in larger machines will be higher than in smaller machines, and obviously cooling capacity will decrease. Figure 2 shows the calculated ratio of the surface area to the net volume for the different sizes of the Intermix. Figure 2 also indicates that temperature effect in the larger machine may mean that the total shear stress generated in the mixing chamber is below the value calculated from machine geometry. It follows, therefore, that in addition to the genuine response of the compound to shear deformation in the internal mixer, the deviation of the shear stress from the calculated value can be attributed to the shear heating effect (ref. 1).

Reference 1 describes how the original shear stress data of the NR compound were corrected for the effect of shear heating using the following equation:

(3) ln [[tau].sub.c] = ln [[tau].sub.m] + b [DELTA]T

where [[tau].sub.c] is the corrected shear stress, [[tau].sub.m] is the measured shear stress, b is the coefficient of viscosity and [DELTA]T is the temperature rise due to shear heating. Figure 3 shows the flow curves before and after the correction and indicates that the corrected curve gives a better representation of NR compound flow characteristics over the shear rate range of 4 to 100 [s.sup.-1].

[FIGURE 3 OMITTED]

Cooling efficiency

It is clear, then, that one of the important factors in rubber mixing when assessing the merits of a mixer is the cooling efficiency of the temperature control system. For the temperature of the mix to be accurately controlled, obviously the cooling capacity in the mixer must be efficient enough to counteract heating of the material resulting from the mixing action.

Many internal mixers are therefore equipped with water cooling, which can also be electrically heated, to all parts of the internal surfaces of the mixer that come into contact with the rubber during the compounding process. More recently, much care has also been taken in arranging efficient cooling passages in the several cooling zones of the mixer by incorporating the following features, for instance, into the design:

* Cooling passages to the rotors;

* water cooled rotor end wear plates;

* drilled chamber bodies:

* cooling to the plunger; and

* cooling to the drop door.

As an example, the water flow rates needed in the above regions for a K2A-Mark-5 mixer (49 liters) are given in table 1.

Energy balance

The logical extension of the cooling efficiency issue discussed above is to determine the level of energy taken away by the cooling water during a rubber mixing cycle. When thermal equilibrium is reached in an internal mixer, the energy balance can be simplified to:

(4) E1 - E2 = E3 + E4

where El is energy input by the motor. This can be accurately determined using a watt-hour meter. E2 is electrical loss in the motor and mechanical loss in the drive system (usually about 10% of E1). E3 is energy required to raise the temperature of the batch and is given by:

(4a) E3 = [m.sub.1] x specific heat (compound) x [DELTA][T.sub.1]

Where [m.sub.1] is batch weight in kg and [DELTA][T.sub.1] ([degrees]C) = T2 - T1 (rubber temperature at discharge - initial temperature).

E4 is energy taken away by the cooling water and is given by:

(4b) E4 = [m.sub.2] x specific heat (water) x [DELTA][T.sub.2]

Where [m.sub.2] (kg) is mass of flowing water (= flow rate x density of water x cycle time) and [DELTA][T.sub.2] ([degrees]C) = T2 -T1 (water outlet temperature - inlet temperature).

A K2A Mark-5 (chamber net volume capacity 49 liters), which gives reasonably comparable performance to that in considerably larger scale production mixers, was used for calculating the components of the energy-balance equation for a rubber mixing cycle.

According to equation 4a, E3 = 9,878,592 J (2.74 kWh) from inserting values of [m.sub.1] = 36.8 kg, specific heat for rubber compound = 2,237 J/kg[degrees]C and [DELTA][T.sub.1] = 120[degrees]C.

Similarly according to equation 4b, E4 = 2.01 kWh from: [m.sub.2] = mass of flowing water in each zone (table 1), cycle time = 195 s, water density = 1 g/[cm.sup.3], specific heat for water = 4,200 J/kg[degrees]C and [DELTA][T.sub.2] ([degrees]C) for each zone. On this basis and at these conditions, the components of equation 4 are then: E1 is energy input by the motor = 5.33 kWh; E2 is electrical losses = 0.58 kWh: E3 is energy required to increase the temperature of the batch = 2.74 kWh; and E4 is energy removed by the cooling water = 2.01 kWh.

It can be seen from the above example that about 38% of E1 was removed by cooling (water temperature setting; 30[degrees]C/ 35[degrees]C) during the mixing cycle, and about 51% of E1 was utilized to raise the batch temperature from 24[degrees]C to 144[degrees]C.

Figure 4 illustrates the way in which the selection of the circulating water temperature settings in the temperature control unit (TCU) affects the energy balance for a final mix cycle carried out in a laboratory mixer (net volume, 1.8 liters). The significance of this figure is the realization that the proper selection of water temperature in the different zones of the mixer, as long as the desired quality in the product is achieved, will minimize the energy input requirements to mix.

[FIGURE 4 OMITTED]

Practical examples

It may be of interest to conclude the cooling efficiency section by discussing two examples of successful temperature control in the Intermix. In the first, the conversion of a two-stage to a single pass process, and in the second example, the reactive mixing, involving a silica-filled compound.

Single pass

Due to its design and efficient cooling, the Intermix equipped with the NR5 rotor design is capable of imparting a high level of energy into the compound, and thus permitting the conversion of most two pass compounds to a single pass operation. As an example, in one comparison a K2A Intermix mixed three different compounds in a single pass mixing process and then in a two-stage mixing process where a masterbatch and final mix were used to produce the products (ref. 2). The total mixing time to produce the product in either a two-stage mixing process or a single pass mixing process is shown in figure 5. The specific energy input used to produce the product in a single pass mixing process was calculated and compared to that of the two-stage mixing procedure, as clearly shown in figure 6 for the NR compound. The reduced energy usage to produce the product can alone be translated into a significant manufacturing cost saving. In this analysis, product quality was equivalent for both single and multiple pass mixing.

[FIGURE 5 OMITTED]

Silica mixing

The outstanding heat transfer of the cooling system in the Intermix can also be exploited in the formulations that involve reactive mixing at elevated temperatures. Reactive mixing involving silica filled compounds, in which reaction of silica with silane is carried out in the mixer, has become of increasing importance in recent years, both for economic and ecological reasons.

The continued increase in the utilization of silica in passenger tire treads due to improved rolling resistance and the benefits produced in lower fuel consumption has raised some questions regarding conventional mixing; particularly the practical constraint of coping with the multi-stage mixing approach.

When processing a rubber compound containing high levels of silica and the required silane, special attention has to be paid to the thermal reactions of silane with silica and with the rubber. The silane-silica reaction that must be achieved in the mixer is highly sensitive to the mixing parameters, time and temperature. Increasing the temperature can shorten the silane-silica reaction time, but this rise must be limited to below 160[degrees]C to avoid the second reaction of silane with polymer, leading to premature vulcanization. The silane-silica reaction is best achieved between 140-160[degrees]C, whereas with temperatures above 160[degrees]C, the silane-polymer reaction becomes active (ref. 3).

With silica technology, the rubber compound passes three to four times in the mixer. This multi-pass process technology is needed for two reasons. Firstly, for obtaining easy processing levels of Mooney viscosity in the compound, a good degree of silica silanization must be achieved. Also, the filler must attain optimum dispersion in the rubber matrix. Secondly, in a large mixer, especially with limited heat transfer capability, the batch cannot be maintained at a given temperature. Instead, in each pass, the batch is mixed at an elevated temperature for a short time and dropped prematurely, hence avoiding the onset of the thermal reaction of silane with polymer.

By way of analogy, the good temperature control of the Intermix permits the machine to operate in a specially developed cycle so that efficient mixing involving silica-filled products can be achieved in a reasonable length of time.

For the sake of simplicity, the developed cycle is referred to in the next section as a reactive mixing cycle or a steady state mixing cycle. Additionally, assessment of the silanization reaction (silica-silane) was obtained from Mooney measurements. Questions may be raised about the ability of the conventional Mooney test to provide a satisfactory assessment of the level of silica silanization. The basis for this questioning is that a more accurate measurement of the silanization reaction possibly can be obtained by quantifying the strength and amount of filler-filler interaction in the mixed compound (refs. 4 and 5). However, the results obtained from this study have indicated a fair correlation between the level of silica silanization reaction and Mooney viscosity measurements.

Experimental

A silica filled passenger tire tread containing 80 parts phr of precipitated silica and 14 parts phr of silane X50S was mixed in two-stage, three-stage and four-stage procedures. For the purpose of this study, the formulation selected is given in table 2. Details of the conventional mixing stages and mixing parameters are given in table 3. An attempt to simulate similar quality and similar level of Mooney reduction as in a multistage mixing process was also made by using a specially modified mixing process, namely reactive mixing. Details of the mixing stages and mixing parameters for the reactive mixing process are given in table 4.

Conventional multi-stage mixing

Indications of processability at each stage of the multi-stage mixing processes were obtained from Mooney viscosity measurements.

In the two-stage mixing process, the first-stage (MB) viscosity was high, being 108 (ML1+4,125[degrees]C), and thus not adequately milled.

To decrease the level of Mooney viscosity from the first masterbatch (MB1), a three-stage mixing procedure was tested where 1/3 of the filler (silica) was added in the second masterbatch (MB2). The mix was prepared in a three-stage mixing process comprising MB1. MB2 and final. This mixing procedure, which follows the general industrial practice, produced nevertheless 72 Mooney in the final mix, which lies near the upper limit of the easy processing range. It is a good compromise if very low viscosity is not necessary.

Furthermore, the level of Mooney viscosity of the compound can be reduced further if the precipitated silica is better dispersed. In view of this, in the third stage (MB3) of the four-stage mixing procedure, the compound was re-milled for 70 seconds, and the results obtained gave better filler dispersion, as well as better Mooney reduction (table 3).

Reactive mixing

Thermal efficiency of the Intermix and the ability to run it in a steady-state mixing mode have always been strong pluses for the rubber compounds involving reactive mixing. Under the steady-state mixing condition, the mixer power requirement and the batch temperature have both reached constant values for certain batch size and rotor speed, as shown in figure 7. This was done by dividing the mixing process into two parts. In the first part of the mixing cycle the batch was mixed efficiently at high speeds to reach a good level of dispersion. In the second part, rotor speed was reduced and held at a level where the heat dissipation and heat flow to the metal surfaces of the mixer were in equilibrium.

[FIGURE 7 OMITTED]

A two-stage mixing procedure was employed throughout this study and was used as a basis. The rubber, silica, silane coupling agent and other ingredients were incorporated in the first-stage (MB). Alter allowing the masterbatch (MB) to cool for about three hours, sulfur and accelerators were then added and mixed with the MB in the K2A mixer.

Since the silanization of the silica is a chemical reaction, a sufficiently high temperature and a reaction time of several minutes are needed to complete this reaction. In this study, the reaction was carried out in the mixer at 150[degrees]C in a specially modified mixing process, namely steady-state, where the reaction time was varied between two, three and four minutes.

Basically, the investigation comprised a comparison of the extent of Mooney reduction, as well as the level of filler dispersion under three sets of conditions:

* reactive mixing at 150[degrees]C for 120 seconds:

* reactive mixing at 150[degrees]C for 180 seconds; and

* reactive mixing at 150[degrees]C for 240 seconds.

Table 4 shows that there is a good correlation between the degree of silanization process and the level of Mooney reduction in the masterbatch, and consequently the final mix. This article suggests that alter three minutes, the modification reaction between the silica surface and silane is completed in the Intermix to a high level, hence reaching optimal Mooney reduction. It also demonstrates that with the improved cooling in the Intermix and the ability to hold the temperature constant for three minutes (or longer) during the modification process, mixing stages can be reduced from three or tour stages to only two stages while maintaining similar Mooney viscosity (figure 8). Our data also show that filler dispersion was at its optimum, and those results will be reported at a later date.

Conclusions

This article has set out to demonstrate the importance of the cooling system in internal mixers, particularly in the Intermix. In this respect, two examples were discussed. In the first, the conversion of a two-stage mixing process to a single-pass process was discussed. The results obtained indicated that, because of the increased cooling capacity in the NR5 rotors, the conversion from a multi-stage to a single-pass mixing procedure was successfully achieved. In the second example, reactive mixing in the Intermix involving silica filled compounds was evaluated. A good level of correlation between the degree of silanization process and the level of Mooney reduction in the masterbatch and consequently in the final mix was accomplished. The reactive mixing approach used in this work has demonstrated performance advantages over the conventional multi-stage mixing method in many aspects such as better throughput, improved Mooney viscosity and with even greater efficiency.

The obvious benefits, such as saving in specific energy input and less material handling, are of significant economic impact to the rubber compound manufacturers who are using or are willing to use the single pass approach and reactive mixing technology.

The combination of the heat transfer measurement with the analysis of the energy balance components has provided some useful calculations relating to the level of energy absorbed by product, energy removed by cooling and energy losses. In the example given, it was calculated that the compound absorbed 51% of the input energy, 38% was removed by the cooling system and 11% was losses. The recognition of these parameters is important not only to avoid misusing the energy input and result in cost saving, but as a key step in understanding the concept of rubber mixing in internal mixers.

Table 1--water flow rate in different zones
of a K2A (49L)

Zone    Controlling         Flow rate, 1/min.

1       Front rotors and          28.9
          back rotor              26.1
2       Front wear-plate          15.0
        Back wear-plate           15.0
3       Front body                31.6
        Back body                 24.9
4       Ram weight                21.4
5       Drop door top             11.0


Table 2-tire tread
compound

Ingredients        PHR

SBR                115
BR                  16
Silica              80
Silane X50S         14
Zinc oxide           3
Stearic acid         2
Other chemicals      8

Table 3--conventional process; two-, three- and
four-stage mixing

Mixing stages          Mix-        Dump       Specific        ML
                       ing        temp.        energy        (1+4)
                       time    ([degrees]C)    input     125[degrees]C
                      (sec.)                  (kWh/kg)

2-stage
Masterbatch (MB)                   150         0.158          108
Final                  240         105         0.053           73
Total                   80                     0.211
                       320
3-stage
Masterbatch 1 (MB1)                145         0.161           69
MB2                    225         150         0.118          100
Final                  150         105         0.053           72
Total                   80                     0.332
                       455
4-stage
Masterbatch 1 (MB1)                145         0.151           70
MB2                    220         145         0.117          101
Re-milled MB3          145         145         0.084           86
Final                   70         105         0.051           63
Total                   90                     0.403

Table 4--reactive mixing; two-stage mixing

Mixing stages      Mixing       Dump       Specific        ML
                    time       temp.        energy        (1+4)
                   (sec.)   ([degrees]C)    input     125[degrees]C
                                           (kWh/kg)

2-stage, reactive mixing @ 150[degrees]C for 120s
Masterbatch (MB)    340         150         0.231          93
Final                90         105         0.055          65
Total               430                     0.286
2-stage, reactive mixing @ 150[degrees]C for 180s
Masterbatch (MB)    400         150         0.271          88
Final                90         105         0.055          63
Total               490                     0.326
2-stage, reactive mixing @ 150[degrees]C for 240s
Masterbatch (MB)    450         150         0.297          87
Final                90         105         0.055          62
Total               540                     0.352

Figure 2--surface-to-volume ratio vs. Intermix size

Intermix size

1 = K1 (5.5L)
2 = K2 (20L)
3 = K2A (49L)
4 = K4 (91L)
5 = K5 (143L)
6 = K6 (206L)
7 = K6A (257L)
8 = K7 (306L)
9 = K8 (484L)

Note: Table made from line graph.

References

(1.) S.N. Ghafouri and P.K. Freakley, Polymer Testing, vol. 11, p. 101 (1992).

(2.) F.J. Borzenski, Asia Rubtech. Expo., New Delhi, India (November 2002).

(3.) S. Wolff, Tire Sci. Technol., vol. 15, p. 276 (1987).

(4.) S. Wolff; Tyretech, Basel (1993).

(5.) L.A.E.M. Reuvekamp, W. Dierkes and J.W.M. Noordermeer, Tire Technol. International (March 2003).