Advanced mixing technologies of intermeshing mixer for silica-filled compounds.

The green tire technology has been introduced by Rauline (refs. 1 and 2) with the use of bifunctional organosilanes, particularly TESPT (bis-(triethoxy-silylpropyl)tetrasulfide) as coupling agent with silica, and has enabled a new compound technology in the tire industry in terms of a better wet traction, a reduced rolling resistance and an improved abrasion resistance. However, the application of silica instead of carbon black involves major problems due to the polar nature of silica, which leads to stronger filler-filler interactions and hydrophilic characteristics. Silica particles easily form agglomerates, and the non-polar polymer matrix makes the silica incompatible; therefore, the surface of silica has to be chemically covered with the appropriate coupling agent so as to form a hydrophobic layer around each silica particle.

There have been many efforts to improve the affinity of polymers to silica and the processing stability of the compounds. Traditionally, bifunctional silanes such as TESPT and TESPD (bis-(triethoxy-silylpropyl)disulfide) are widely used as coupling agents in silica-filled tread compounds. Highly dispersible silica with a treated surface of precipitated silica and modified polymers with effective end or functional groups have also been developed in order to reduce the processing difficulties during mixing. Furthermore, the recent requirement to lower emissions of carbon dioxide and VOCs (volatile organic compounds) led to the introduction of ethanol-free silanes as environmentally friendly products (refs. 3 and 4).

Internal mixers are the most commonly used mixing equipment for compounding in the rubber and tire industries. Tangential and intermeshing rotors are the two fundamental designs for internal mixers. The optimized rotor geometry and process parameters are always an area of interest because they are directly related to the efficiency of the mixing process and the quality of the compound produced. In general, the optimization of the distributive and dispersive mixing of various fillers and chemicals in internal mixers should be required to enhance the compound properties. There have been studies on the comparison of mixing characteristics between intermeshing and tangential mixers (refs. 5-10). The major advantage of intermeshing mixers is that their higher cooling efficiency provides better temperature control of the compound and a much higher energy input, while tangential mixers are characterized by fast feeding and incorporation of the material for short mixing cycles and multi-stage mixing.

For silica compounds, an additional important necessary aspect is the chemical reactions to complete the coverage of the silica surface with a coupling agent (refs. 11-13). The temperature control during mixing is becoming more and more important to control the silanization reaction and to reduce additional mixing stages for the production of silica compounds, in order to achieve the required properties. Therefore, intermeshing mixers are becoming more and more common, especially for silica compounds.

The present article describes the effects of mixing parameters on the properties of silica-filled compounds, and will focus on the important factors in order to enhance the efficiency of the silanization reaction. Accordingly, the mixing performance of the intermeshing mixer with optimized state of-the-art rotors will be discussed in comparison with the conventional intermeshing rotors in order to identify the efficiency of reactive mixing for silica-filled compounds.

Experimental

Materials

All the experiments were carried out using a tire tread composition, as shown in table 1. The compound formulation is based on a blend of S-SBR (24% styrene content, 39% vinyl content) and BR (95% cis 1,4 bonds). The silane used in this study was the TESPT, which is most widely used as a coupling agent for a "green tire" recipe.

Mixing procedure

In this study, we focused on the masterbatch mixing of silica-filled compounds. The masterbatch mixing procedure employed in the experiments is indicated in table 2. The cooling water was kept at a constant temperature of 40[degrees]C. The ram pressure was fixed at 0.7 MPa. The rotor speed and the mixers fill factor were 30-80 rpm and 65-75%, respectively, as variable parameters. The influences of the ram down mixing time and the additional ram sweep during the step 8 in table 2 were also investigated.

Mixing equipment

The mixing experiments were performed in a laboratory scale intermeshing mixer, MR-5E (5 liters) from Mitsubishi Heavy Industries (MHI). The mixer has been equipped with the conventional standard intermeshing rotors called E-type geometry. The configuration with this type of rotors has been employed for the reference trials.

In order to identity the efficiency of the silanization reaction, the different rotor concepts would be compared for the mixing of silica-filled compounds. For these experiments, the state-of-the-art intermeshing rotors have been installed in the MR-5E. These intermeshing rotors are called EX7-type, which MHI has developed, having the higher distributive mixing and cooling principle. The basic concept of this rotor is to enhance especially the distributive mixing efficiency and the temperature control capability with the optimized wing arrangement and profiles.

Characterization for the coupling reaction 

The characteristics of uncured compounds were determined by measuring the Mooney viscosity, ML(1+4) at 100[degrees]C, of the compounds after the masterbatch mixing process. The Mooney viscosity provides a first index of dispersion of the filler and the degree of silanization. To more accurately evaluate characteristics of the chemical reaction, dynamic mechanical theological properties were obtained using an RPA 2000 rubber process analyzer from Alpha Technologies. The incorporation of filler particles into a polymeric system causes an increase in the elastic storage modulus, G', at low strain amplitudes, but G' decreases as the strain is increased, commonly referred to as the Payne effect (ref. 14). This nonlinear behavior was measured at 100[degrees]C with a frequency of 0.5 Hz and variable strain.

Results and discussion

The payne effect on the silanization reaction

It is generally understood that the silanization reaction consists of two reaction mechanisms (ref. 15). The primary step is the reaction where the silane is bound to the silanol groups on the silica surface by condensation and hydrolysis. The secondary reaction is a condensation reaction between adjacent silane molecules on the silica surface creating a hydrophobic shell around the silica particle. This silane/silica reaction significantly reduces filler/filler interactions due to the reduction of hydrogen bonds between the silica particles.

In order to clarify the Payne effect on the silanization reaction, the influence of the silanization time was investigated for an MR-5E mixer with conventional E-type rotors. The silanization time, which was indicated as the mixing time of step 8 in table 2, was employed as a variable parameter of 2, 4 and 7 minutes, respectively. Figure 1 demonstrates the results of the Payne effect as a function of the silanization time. As a reference, the test result performed for the masterbatch compound without TESPT is also shown in figure 1. It illustrates that the compound modulus at low strain decreases with increasing silanization time. For the compound without TESPT, significantly higher G' values were obtained. A coupling agent is essential to reduce filler/filler interactions and has a strong effect on the dispersion of silica. The measurement of the shear modulus at low strain can be used as an index of the degree of silanization progress.

Influence of the dump temperature for compounds"

The Mooney viscosity gives a first estimation of the filler/ filler interactions in a rubber compound. Figure 2 shows the effect of the dump temperature on the Mooney viscosity, ML(I+4) at 100[degrees]C after the masterbatch compounding. It indicates that the viscosity decreases steadily until a dump temperature of 145[degrees]C; however, it gradually increases at dump temperatures above 150[degrees]C. The latter increase of the Mooney viscosity indicates the premature silane/rubber reaction that is caused by too high mixing temperatures.

In order to study the interaction between silane and silica, or that between silane and rubber, the G' at low strain (1%) and at high strain (350%) were measured. The results of those measurements are given in figures 3 and 4, respectively. The filler/filler interaction can be illustrated at low strain amplitudes, and the filler/polymer network can be highlighted at higher strain values because the filler network is broken. In figure 3, the G' at the low strain value of 1% can be seen to dramatically decrease, and it remains at a low level, even at dump temperatures of 150[degrees]C or higher. The degree of silanization reaction is improved with increasing the mixing temperature. On the contrary, the increase in G' at high strain of 350% (figure 4) is observed at dump temperatures above 150[degrees]C, indicating the scorch problems between the coupling agent and the rubber matrix. These results obtained in this study correspond to the vigorous work reported elsewhere (refs. 11-13) using a Brabender Plasti-Corder lab internal mixer.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Mixing temperatures below 140[degrees]C result in inefficient reaction of the silanization. On the other hand, the silanization temperatures above 150[degrees]C will initiate undesired silane/rubber reactions. Traditional silane coupling agent TESPT has a very narrow temperature window for mixing, due to its polysulfidic nature, which can cause premature vulcanization during mixing. In the present experiments, the optimal silanization temperature is given at a temperature level around 145[degrees]C.

Influence of the rotor geometries

In general, it can be difficult to keep low mixing temperatures in internal mixers because the high viscosity of compounds generates a rapid temperature increase during mixing. The mixing temperature is dependent upon the rotor geometries and processing conditions. Mixing tests were carried out in order to compare the influence of the rotor geometries on mixing temperature. A geometrical advantage of an intermeshing mixer is that a constant mixing temperature is achieved due to the high mixing and cooling efficiency (refs. 9 and 16).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Figure 5 shows the equilibrium temperatures after mixing of masterbatch compounds, and compares the EX7 rotor to the standard E-type rotor. The mixing temperature for masterbatches is increased with increasing the rotor speed. Comparing the two types of rotors, the mixing temperature of the EX7 rotor is 15% lower than that of the E-type rotor at the same rotor speed. This indicates that the cooling effect of the EX7 rotor was significantly enhanced compared to that of the conventional intermeshing rotor.

Comparison of reactive mixing efficiency on silanization time

Figure 6 demonstrates the results of the silanization efficiency for the different rotor geometries under standard conditions of the fill factor of 65% and rotor speed of 60 rpm. The result shows that the Mooney viscosity and G' at low strain of 1% tend to decrease with increasing the silanization/mixing time, while the G' at high strain of 350% remains constant. There is little difference between the two rotor geometries on the properties of compounds at the same processing conditions. The specific energy input (referred to as the unit work in figures) is, however, indicative of less power consumption for the intermeshing mixer with the EX7 rotor to obtain the required properties. It is considered that the efficient mixing capability in internal mixers is a very important factor for the reactive mixing of compounds.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Influence of the sweep action during mixing

It is known that one of the important factors of the silanization reaction is the removal of ethanol in the compound, as well as in the mixer (ref. 10). In order to evaporate the reaction products, such as ethanol and water, including a sweep action (open ram and air injection) during mixing is expected to improve the efficiency on the silanization reaction.

The influence of the sweep action during the silanization step was investigated. As shown in figure 7, a slight improvement with the sweep action was observed for the EX7 rotor system, whereas no change was observed for the standard Etype rotor. It is anticipated that the influence of the sweep action could not appear distinctly because the mixer used in the present experiment is a laboratory scale, so that the removal of ethanol is insufficient.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Comparison of reactive mixing efficiency on rotor speed

The effect of rotor speed was studied in the experiments with standard conditions of the fill factor of 65%. Figure 8 shows the influence of different rotor speeds in comparison with rotor geometries. It is obvious that the more the rotor speed increases, the more the dump temperature rises.

In the case of the E-rotor, the Mooney viscosity decreases steadily up to a rotor speed of 50 rpm, after which it gradually increases again. The G' at low strain of 1% indicates that the silanization reaction becomes more effective with increasing the rotor speed; however, the G' at high strain of 350% shows the premature scorch at rotor speeds above 60 rpm. The increase of Mooney viscosity at higher rotor speed is due to the undesired silane/rubber reaction during mixing.

On the other hand, the Mooney viscosity steadily decreases with increasing the rotor speed in the case of the EX7-rotor. Although the poor silanization was observed when the mixing temperature was at or below 120[degrees]C, the efficiency of the silanization reaction (which is indicated by G' at 1% strain) is dramatically improved as a function of the rotor speed. Besides, the G' at 350% strain shows that no premature scorch can be seen, even at the high rotor speed of 80 rpm with the EX7-rotor.

The results of the experiments illustrate that the efficiency of the silanization reaction for silica compounds will be strongly influenced by the mixing and cooling performance of an internal mixer. A better temperature control gives a more effective silanization reaction. In addition, the high rotor speed promotes a better surface renewal for the compound in a mixer, which is also effective in the reactive mixing of compounds.

Conclusions

Experimental studies of the mixing efficiency for typical silica-filled tread compounds have been carried out using an internal mixer with different intermeshing rotors. The influence of various factors on the efficiency of the silanization reaction was obtained by measuring the Payne effect. For silica-filled compounds with the traditional coupling agent TESPT, the mixing operating window for temperatures of 140[degrees]C to 150[degrees]C is required to accelerate the silanization reaction and to prevent the premature scorch problem.

The comparison of the state-of-the-art intermeshing rotor system with the conventional one showed that the efficiency of the silanization reaction is enhanced not only by a better temperature control, but but also by a better surface renewal for the compound. Better cooling capabilities for an internal mixer allow higher rotor speeds; therefore, more fresh surfaces are created and the evaporation of the reaction products is promoted. This advantage brings an improvement of the efficiency of the silanization reaction, as well as that of the distributive and dispersive mixing.

This article is based on a paper presented at a meeting of the Rubber Division, ACS (www.rubber.org).

References

(1.) R. Rauline (Michelin), Europe Patent EP0501227 (1991).

(2.) R. Rauline (Michelin), U.S. Patent 5,227,425 (1993).

(3.) A. Chaves, ITEC 2006, paper 28B, Akron, OH, (Sept. 2006).

(4.) O. Klockmann, ITEC 2006, paper 29B, Akron, OH, (Sept. 2006).

(5.) W.M. Wiedmann and H.M. Schmid, Rubber Chem. Technol., 55, p. 363 (1982).

(6.) P.S. Kim and J.L. White, Rubber Chem. Technol., 67, p. 880 (1994).

(7.) P.S. Kim and J.L. White, Kautsch. Gummi Kunstst., 49, p. 10 (1996).

(8.) C. Koolhiranand J.L. White, J. Appl. Polym. Sci., 78, p. 1,551 (2000).

(9.) T. Moribe, Nippon Gomu Kyokaishi, vol.74, No.2, p. 70 (2001).

(10.) W. Dierkes, J.W.M. Noordermeer, K.-U. Kelting and A. Limper, Rubber World, 229, 6, p. 33 (March 2004).

(11.) L.A.E.M. Reuvekamp, J. W. ten Brinke, P.J. van Swaaij, and J.W.M. Noordermeer, Rubber Chem. Technol. 75, p. 187 (2002).

(12.) J.W.M. Noordermeer, L.A.E.M. Reuvekamp, J. W. ten Brinke and P.J. van Swaaij, Rubber Chem. Technol. 76, p. 12 (2003).

(13.) J.W.M. Noordermeer, International Rubber Conference; IRC 2005 Yokohama, paper 28-G9-I-01 (2005).

(14.) A.R. Payne, Rubber Chem. Technol, 39, p. 365 (1966).

(15.) U. Goerl, A. Hunsche, A. Mueller and H.G. Koban, Rubber Chem. Technol., 70, p. 608 (1997).

(16.) T. Moribe, PPS-20 Annual Meeting of Polymer Processing Society, Technical paper #352, Akron (June 20004).

by Takashi Moribe, Mitsubishi Heavy Industries

http://www.mhi.co.jp/indexe.html

Table 1--silica
compound

Component        Phr

S-SBR             75
B R               25
Silica            80
TESPT              7
Carbon black
  (N330)           1
Aromatic oil      25
ZnO              2.5
Stearic acid     1.0
6-PPD            2.0
Sulfur           1.4
CBS              1.7
DPG              2.0
Total          223.6

Table 2--mixing procedure

Step         Time    Action
       (min.:sec.)

 1           0:00    Open ram; add polymers
 2           0:30    Close ram; ram down mixing for one minute
 3           1:30    Open ram; add 1/2 silica, 1/2 silane, 1/2 oil,
                     ZnO, stearic acid
 4           2:00    Close ram; ram down mixing for one minute
 5           3:00    Open ram; add 1/2 silica, 1/2 silane, 1/2 oil
 6           3:30    Close ram; ram down mixing for one minute
 7           4:30    Open ram; sweep 30 seconds
 8           5:00    Close ram; ram down mixing for two minutes
                     (variable)
 9           7:00    Dump