PHASE SELECTIVE DISTRIBUTION OF CARBON BLACK IN BINARY RUBBER BLENDS

Transcription

1 HASE SEECTIVE DISTRIBUTION OF CARBON BACK IN BINARY RUBBER BENDS S. Ilisch, H.H. e, H.-J. Radusch Martin uther University Halle-Wittenberg, Center of Engineering Sciences, D-699 Halle (Saale), Germany The phase selective carbon black localization plays an important role regarding the performance of rubber composites based on rubber blends. Since 22 we use the online measured electrical conductance for the description of dispersion and distribution processes during the mixing process of carbon black filled rubber mixtures /1-4/. Recently, we used this method also for the qualitative analysis of the morphology development of binary blends during the mixing process /5/. With our new developed method, analyzing the carbon black-rubber-gel by thermogravimetric analysis (TGA) in correlation with differential thermo analysis (DTA) it is possible, to describe the wetting behavior of the polymer chains to the carbon black and the phase selective carbon black localization during the mixing process. The determined carbon black localization corresponds very well with the given carbon black localization in blends of the masterbatches, and with results from the online conductance measurements. With this method it is possible to characterize the free carbon black portion and the portion of carbon black in the particular blend phases depending on mixing time. In the present work we have investigated the effects of the material and technological parameters on the kinetic of CB distribution in / blends by the use of the newly developed TGA/DTA method. Introduction For the characterization of the phase selective filler localization of the finished mixtures or vulcanizates direct methods exist on the basis of microscopy and also indirect methods, like rheological and different mechanical-physical tests (Differential Scanning Calorimetry (DSC) /6/, Dynamic Mechanical Thermal Analysis (DMTA) /7, 8/ or analysis of electrical conductivity). In cases, where the carbon black content in the compounds is high, like for technical applications, the blend phases are small like the sizes of the aggregates, and the degree of unsaturation of the rubbers is nearly uniform, direct microscopic methods fail. In addition, there exists no methods for the quantitative estimation of the filler localization for very small sample volumes, taken out from the compounding process. To get a deeper insight into the mixing process and especially to the wetting behavior, we have developed a new method by TGA/DTA for the calculation of the phase specific localization of the filler in rubber blends. With our method of the online measured electrical conductance, a qualitative analysis of the carbon black distribution is able. Two of the possible cases for the run of the electrical conductance are shown in Figure 1. Case 1: Uniform carbon black distribution in both rubber phases Case 2: Non-uniform carbon black distribution

2 Conductance G, ms Rubber A Blend A/B 5/5 Rubber B Conductance G, ms Rubber E Blend E/F 5/5 Rubber F Figure 1: Hypothetical run of the electrical conductance of binary rubber blends (blue) in correlation with the morphology; red and black line: the online measured electrical conductance of the single compounds (-red, -black), carbon black content in all compounds 5 phr (above the percolation threshould) Experimental The used rubbers are solution styren-butadiene rubber (S-) with 21 % styrene and 63 % vinyl content and natural rubber () SMR 1. The used carbon black was N 22. All compounds were produced in a laboratory mixer (Rheomix 61 p, Thermo Electron) according Table 1 with an initial temperature of 5 C, rotor speed 5 rpm and a fill factor of,7. Table 1: Recipe of the single mixtures and the one-step blend Ingredients, phr -mixture -mixture One-step blend S Carbon black Zinc oxide Stearic acid Sulphur Accelerator As basis for compounds with defined localization of carbon black in the - and -phases, single masterbatches with, 25, 4, 5, 6, 75 and 1 phr N 22 were produced and blended in a second compounding step (Table 2). Table 2: Sample names of the two-step blends Sample name CB in -hase, phr CB in -hase, phr S_N1 5 S25_N75 12,5 37,5 S4_N6 2 3 S5_N S6_N4 3 2 S75_N25 37,5 12,5 S1_N 5 Used methods for the analysis of the morphology development during the mixing process were online measured electrical conductance, characterization of the macro dispersion of the carbon black by optical microscopy, solubility

3 measurements of the raw mixtures versus the mixing time, analysis of the carbon black-rubber-gel of single and binary mixtures by TGA/ DTA. Furthermore the mechanical-physical properties of the vulcanizates were tested. Results and Discussion Investigations of the real compounds show, that the online measured electrical conductance curve of the 5/5 /-blend lays between the curves of the single - and -blends, shown in Figure 2. Thus one can assume a uniform localization of the carbon black in both blend phases. 18 Conductance G, ms Blend Figure 2: Online measured electrical conductance for single - and compounds and the one-step blend /=5/5, all compounds filled with 5 phr N 22 For the two-step blends, only in three cases (S25_N75, S4_N6, S5_N5) electrical conductance occurs during the mixing process. This shows the non-uniform localization of the carbon black in the phases of this binary blends (Figure 3). 12 Conductance G, ms S_N1 S25_N75 S4_N6 S5_N5 S6_N4 S75_N25 S_N1 Figure 3: Online measured electrical conductance for the second mixing step of the two step blends (sample names according Table 2)

4 Conductance G (t=const.), ms CB in -hase, phr Figure 4: Electrical conductance at the end of the second mixing step of the twostep-blends (1 min) via the given carbon black content in the -phase The electrical conductance in dependence on the CB content in the -phase is the highest, if the carbon black localization in the phases of the blends is nearly uniform (Figure 4). Similar results we got for mechanical properties like stress, strain at break, E-modulus and loss factor at C. To reach a deeper insight into the development during the mixing process, we generated a new method for the charcterization of the wetting behavior of the rubber to the carbon black. Therefor we analyzed by means of solubility measurements of the raw mixtures in different solvents the carbon black-rubber-gel, the soluble rubber and the bounded rubber layer at the surface of the carbon black (Figure 5). Bounded rubber at the surface of the carbon black () m2 cr m1 = Equation 1 m 2 Bounded rubber layer at the surface of the carbon black m 1 Σ (mass CB, bounded rubber, soluble rubber) m2 Σ (mass CB, bounded rubber) Soluble rubber Carbon black c R mass content carbon black (%) Figure 5: Calculation of the bounded rubber layer at the surface of the carbon black ( ) The bounded rubber layer at the surface of the carbon black ( ) according Equation 1 is characteristic for the wetting behavior of the different rubber types, like it is shown in Figures 6 and 7. Figure 6 shows the differences between the wetting behavior of and. The wets the carbon black surface much more faster than the and reaches rather a higher plateau. The plateau values for and, and are characteristic values for the two rubbers. Till high CB contents they do not depend on the carbon black content in the mixture (Figure 7).

5 ,6,5 =,492,4,3,2 =,415 Figure 6: Development of the layer in single - and -compounds (5 phr N 22) in dependence on the mixing time,1, ,6,5 =,492,4 =,415,3,2 Figure 7: ayer in single - and -compounds in dependence on the carbon black content,1, Carbon black R, phr / ) In binary mixtures we assume, that the layer is the sum from the fractions of and according Equation 2. B / ) = + ( Equation 2 The fractions of and rubber in the layer bounded on the carbon black surface of the binary blend, we calculate by means of the analysis of the carbon black-rubber-gel by TGA/DTA. Figure 8 shows such a run. By means of Equation 3 and 4 we calculate the fractions of carbon black in the phases of the binary blend.

6 1 ) =,234 8 Mass m, % 6 4 ) =,194 m/ T Figure 8: TGA/DTA run of the carbon black-rubber-gel of the binary /-blend Temperatur T, C R R ) ) = ) ) Equation 3 R B = R ) + R ) + R B f Equation 4 ) R is the wetted carbon black content in the blend, R B f (t) black content in the blend and ) R the wetted fraction, B R the whole carbon the free carbon black content in the blend in dependence on the mixing time. Calculated R ), phr Given R ), phr Figure 9: Comparison between the given carbon black localization in the -phase of the binary blend and the analyzed carbon black localization by TGA/DTA

7 Figure 9 shows the good correlation between the given and the analyzed carbon black localization in the -phase of ) the binary blend. The deviations for the samples with non-uniform localization ( R =, 12,5 and 5 phr) result from carbon black transfer during the second mixing step of the two-step compounds. Conclusion As result of the correlation between the online measured electrical conductance and the new developed method for the quantitative estimation of the filler localization by means of TGA/DTA of the filler-rubber-gel, we obtained deeper findings regarding the morphology development during the mixing process of carbon black filled single and binary rubber compounds. Via the analysis of the carbon black-rubber-gel, the influence of the wetting behavior of the polymers to the carbon black dispersion and distribution in /-systems was shown. With the TGA/DTA-method it was possible, to show a phase transfer of the carbon black from to during the mixing process of the blend. During our investigations, the dependence of the online measured electrical conductance and the mechanical-physical properties on the carbon black localization was analyzed. Acknowledgements The authors wish to thank the Federal Ministry of Education and Research (BMBF) of Germany and the German Research Foundation (DFG) for the financial support of this work. References 1 H.H. e, S. Ilisch, B. Jakob, H.-J. Radusch: Online Characterization of the effect of the mixing parameter on carbon black dispersion in rubber compounds using electrical conductivity, Rubber Chem. Technol. 24, 77, H.H. e, I. rodanova, S. Ilisch, H.-J. Radusch: Online electrical conductivity as a measure to characterize the carbon black dispersion in oil containing rubber compounds with different polarity of the rubber, Rubber Chem. Technol. 24, 77, H.H. e, M. Tiwari, S. Ilisch, H.-J. Radusch: Effect of molecular structure on carbon black dispersion in rubber compounds, Kautsch. Gummi Kunstst. (25), 58, H.H. e, M. Tiwari, S. Ilisch, H.-J. Radusch: Development of an online method for characterization of the homogeneity of rubber compounds filled with non-conductive carbon black, Rubber Chem. Technol. 26, 79, H.H. e, Z. Qamer, S. Ilisch, H.-J. Radusch: Carbon black distribution in the compounds of rubber blends monitored by online measured electrical conductance, Rubber Chem. Technol. 79 (26) M. Klüppel, R.H. Schuster, J. Schaper: Carbon black distribution in rubber blends: a dynamic-mechanical analysis, Rubber Chem. Technol. 1999, 72, S. Maiti, S.K. De, A.K. Bhowmick: Quantitative estimation of filler distribution in immiscible rubber blends by mechanical damping studies, Rubber Chem. Technol. 1992, 65, A.K. Sircar, T.G. amond: Carbon Black Transfer in Blends of Cis-oly(Butadiene) With Other Elastomers, Rubber Chem. Technol. 1973, 46,