Langmuir 1995,11, 3601-3602
Influence of Silane Addition on the Electrorheological Behavior of Calcium Carbonate-Polypropylene Oil Suspensions
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0. Quadrat,*,' P. Bradna,' L. Titkova,* J. Dybal,' and P. Sthag Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6,Czech Republic, Institute of Petrochemical Synthesis, Academy of Sciences of Russia, 117912 Moscow B-71,Russia, and Faculty of Technology, Technical University, 760 00 Zlin, Czech Republic Received July 11, 1994. I n Final Form: May 22, 1995
Introduction The electrorheological (ER) behavior of suspensions of various materials in a nonconducting continuous phase has been the object of many studies in the past critically evaluated in r e v i e ~ s . l -This ~ phenomenon, represented by a reversible increase in non-Newtonianviscosity and, at higher particle concentration, by yield stress appearance, results from a dramatic change in suspension structure upon the application of an electric field due to formation of fibers or columns of polarized particles spanning the electrode gap. One of the main factors influencing the effect is the particle polarizability (ionic or electronic),which depends on particle structure and on the fluid particle interface. Hence a surface modification of particles may considerably affect the ER effect of the system. It has been reported4that silane addition to the calcium carbonate particles dispersed in polypropylene oil influences interactions between the suspension particles and continuous phase and consequently their rheological properties. This paper shows that such a modification may significantly change also their ER efficiency.
Experimental Section Materials. Calcium carbonate with an average particle size of 4 pm and a specific surface of about 2 m2/ghaving a narrow particle size distribution4 was prepared by the precipitation of calcium hydroxide with carbon dioxide in a heating plant. The total impurity content was less than 0.2wt %, water content maximum 0.1wt %. Calcium carbonate was dried to a constant weight a t 110 "C and kept over phosphorus pentoxide. Vinyltrimethoxysilane (CH2=CHSi(OCH3)3) (silane A) was made by UCC (Austria); chlorotrimethylsilane (ClSi(CHd3) (silane B) was a product of Fluka. Polypropylene oil (Polypropyloil K, SlovnaR, Slovakia) was a mixture of branched hydrocarbons CS-C~O, viscosity about 240 mPa. Viscometry. ER measurements in the range of shear rates 10.7-831 s-1 was carried out using a coaxial cylinder rotational viscometer, Rheotest 2 (type RV, Priifgerate Werk Medingen, Dresden, Germany), modified in the Institute of Macromolecular Chemistry for ER experiments. The rotatinginner cylinder, 38.8 mm in diameter, and the outer cylinder separated by a gap of 0.6 mm were connected to a dc power supply, E = 0.4-2 kV (high-voltage source, NBZ 411,Tesla, Czech Republic), which corresponded to the voltage gradient from 0.66to 3.33 kV/mm. Permittivity Measurement. The permittivities measured at 40 kc (4275A multifrequency LCR meter, Hewlett-Packard) were 5.4for silane A,9.5 for silane B, and 2.0 for polypropylene oil. +Academyof Sciences of the Czech Republic. Academy of Sciences of Russia. 9 Technical University. (1)Deinega, Yu.F.; Vinogradov, G. V. Rheol. Acta 1984,23,636. (2)Jordan, T.C.; Shaw, M. T. Trans. Electr. Insul. 1989,24, 849. (3)Block, H.;Kelly, J. 0.; Bin, A.; Watson, T. Langmuir 1990,6,6. (4) Modlitbovd, I. Ph.D. Thesis. 1994, Res. Inst. Macromol. Chem.
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Bmo, Czech Republic.
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Figure 1. Dependence of the apparent viscosity 7 on shear rate y : calcium carbonate suspension without silane addition. The voltage gradient (kV/mm) are (0)0, (0)0.66,(0)1.33,(@) 2.00,(e)2.66,(e) 3.25. Measurementof IR Spectra. IR spectra were measured on a B d e r spectrometer, IFS-55. Sample Preparation. Suspensions containing 10 wt % of calcium carbonate and 0, 0.5, 1, 1.5,and 2 w t % of silane were used in the study. After mixing, the samples were stabilized by one day's standing.
Results and Discussion The ER behavior of the suspension ofnontreated calcium carbonate particles is illustrated in Figure 1. It is clear that both apparent viscosity and non-Newtonianbehavior were strongly enhancedby increasingthe voltage gradient. However, a yield stress was not observed due to the low particle concentration. At higher shear rates, the ER effect diminished and viscosity values did not depend on electric field. The effect of silane addition on the ER behavior depended on the chemical composition of this modifier. (a) In the case of silane A, practically the same ER behavior as for nontreated sample was found. (b)On the other hand, when silane B was used, the ER effect increased several times and reached a limit at a 1 wt % silane concentration. This behavior is well demonstrated by a plot of relative apparent viscosity rlr]o vs shear rate (Figure 2). Here r] is the apparent viscosity of suspension at the voltage gradient applied, and 70 is the viscosity obtained without applied voltage at the same shear rate. According to the theory, the ER effect rises with increasing permittivity of suspension particles or with decreasing permittivity of the continuous phase. Hence, the presence of silanes in the oil should cause only suppression of the effect as both silanes increase the permittivity of polypropylene oil. However, the experimental results discussed above indicate that the different effects arisingfrom the use of different silanes must result from the specific influence of the silane molecules on the polarizability of particles due to their different chemical structures. This assumption is confirmed by the analysis of the integrated intensity of the 840 cm-' band in the IR spectra of supernatant obtained after sedimentation of
0743-746319512411-3601$09.00/0 0 1995 American Chemical Society
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3602 Langmuir, Vol. 11, No. 9, 1995 3
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Figure 2. Dependence of the relative apparent viscosit, r j d ~ o on the shear rate y: (a)calcium carbonate suspensionwithout silane addition; (b) 0.5 wt % of silane B added; (c) 1wt % of silane B added. Points denoted as in Figure 1.
suspension particles by centrifugation (ultracentrifuge, Beckmann L8-55,15000 rpm, 2 h). For 1wt % of silane
in the oil it was found that all of silane A remains dissolved in the continuous phase. In this case the polarizability of the particles did not change, and their ER effect was the same (or slightly smaller due to increasing oil permittivity) as that of a simple calcium carbonate suspension. With silane B, however, about 10wt % of the total amount was bound on the surface of particles. Due to the high dipole moment of chlorotrimethylsilane molecules, the polarizability of particles increased and caused the stronger ER effect. Taking into account that the specific surface of carbonate particles is about 2 m2/g, a simple calculation shows that at a silane concentration of 1wt % the particles are covered with a 0.5 nm thick layer, which corresponds to the length of one silane molecule perpendicularly bonded to the particle surface and confirms that the particle surface is fully occupiedby silane molecules. Conclusion The ER effect of suspensions containing particles with nonsemiconducting character is usually conditioned by hydrophilic additives activating the particle surface and causing the particle polarizability. Mainly water is often used5for activationofboth inorganic and organicdispersed phases. Surfactants and other polar liquids as additives were proposed,"6 too. Our results demonstrate that the presence of water is not always necessary, and even dispersionof dry particles of nonconducting material such as calcium carbonatedisplays a distinct ER behavior which may well be increased by a suitable modification of the particle surface by silane sorption. Amore detailed study of these systems is in progress. LA9405408 (5) Uejima, H. Jpn. J. Appl. Phys. 1972, 11, 319. (6) Chertkova,0.A.; Petrshik, G. G.;Trapeznikov,A. A. Colloid.J . USSR 1902,44,68.
