Observation of Carbon Black Agglomerate Dispersion in. Simple Shear Flows. S. P. RWEI and I. MANAS-ZLOCZOWER. Department of Macromolecular Science.
Observation of Carbon Black Agglomerate Dispersion in Simple Shear Flows S. P. RWEI and I. MANAS-ZLOCZOWER
Department of Macromolecular Science Case Western Reserve University Cleveland. Ohio 44106 and D. L. FEKE Department of Chemical Engineering Case Western Reserve University Cleveland, Ohio 44106 Experiments aimed at studying the mechanisms of agglomerate breakup due to the application of a simple shear flow field were performed in a cone and plate transparent device. Spherical compacts of carbon black (diameters 1-2 mm) in a range of different porosites were used in the experiments. Two distinct breakup mechanisms, denoted as “rupture” and “erosion”, were observed. The critical stress for erosion was found to be smaller than that for rupture. Once erosion starts, it continues for very long times. Rupture occurs shortly after reaching a critical stress and concludes abruptly. For this analysis of rupture, the dimensionless group a = (V.Y/K’@~), which is the ratio of applied stress to cohesive strength, was found to be a significant parameter for determining the final particle size distribution. The size analysis of fragments produced by shearing pellets for 1 minute showed a lognormal distribution function.
BACKGROUND
arbon black is one of the most important reinforcing fillers for many polymers, particularly for rubbers. The relationship between the number of dispersed particles and the mechanical properties of the system has been the subject of many studies (1, 2). However, the dispersion mechanism itself has not been studied as extensively. Most existing mixing models consider the rupture of the agglomerates to be the most important step in the overall dispersion process. Bolen and Colwell (3)were the first to propose that rupture occurs when internal stresses induced by viscous drag on the agglomerate exceed a certain threshold value. Following their lead, several authors (4-8) extended the analysis of agglomerate rupture and developed various models for the mixing process. Shiga and Furuta (9)proposed what they referred to as a n “onion peeling” mechanism of dispersion, but they did not develop the idea to the form of a predictive model. They suggested that the dominant mechanism of dispersion is the scraping of the individual constituent particles off the surface of the agglomerate. This scraping process was attributed to motion of the matrix relative to the surface of the
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POLYMER ENGINEERING AND SCIENCE, JUNE 1990, Vol. 30, No. 12
agglomerate. The authors based their conclusion on observed shifts in the particle size distribution due to shearing. The goal of the research presented in this paper is to elucidate the dispersion mechanisms of carbon black agglomerates in a polymer matrix by directly observing the breakup of particles in a controlled flow field. The effect of shear rates on the particle size distribution of agglomerates was also examined. The ultimate goal of these investigations is to model the dispersion mechanisms in terms of system variables such as fluid viscosity and particle solid fraction. EXPERIMENTAL PROCEDURE
Three types of compacted carbon black were used. One type of pellet was made by compressing aggregates of fluffy carbon black into 2 mm diameter spheres. The other two types of pellets were 2 mm clusters selected from samples of either fluffy carbon black or pelletized carbon black provided by Cabot Corporation. The density of each pellet was measured by pycnometry. Shearing experiments were performed in a rotating cone-and-plate device (shown in Fig. 1 ) having a 4” cone angle. By controlling the rotation rate of the 701
S. P. R w e i , I. Manas-Zloczower, and D. L. F e k e A:
motor
B: s p e e d c o n t r o l l e r C: f i x t u r e D: c o n e
E: t r a n s p a r e n t g l a s s p: m i r r o r
G: c a m e r a or v i d e o c a m e r a
$*: 40
n
+,,=---constant *O
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Fig. 1 . Schematic of the visualization cone and plate device for observation of the particle breakup under shearing.
cone, shear rates up to 90 s-' could be produced. Various grades of poly(dimethy1 siloxane) (PDMS), having kinematic viscosities ranging from 30,000 to 600,000 cS, were used as suspending media for the carbon black pellets. Particle breakup was observed by reflection in a mirror placed under the transparent plate. A Panasonic PV420 video camera was used to record the breakup phenomena. In most experiments, the carbon black pellets were sheared immediately after they were placed into the suspending fluid. However, to study the effects of fluid penetration into the cluster on the breakup process, some experiments were performed on carbon black pellets that had been soaked in PDMS for one week before shearing. Following shearing of the pellets for 1 minute, a size analysis was performed on the resulting fragments. Here, the suspensions in PDMS were taken and diluted (3:97) with toluene, and were allowed to remain undisturbed for 5-9 h. In this time, fragments larger than 1 pm settle out, while submicron fragments remain suspended. The sediment and supernatant were separately analyzed. The weight fraction of solids in the supernatant (assumed to be aggregates of carbon black) was analyzed by visible light absorbance. The sediments were observed by optical microscopy. Photographs of representative microscopic images were analyzed using a Zeiss Videoplan I1 image analysis system. To contrast the breaking strength of carbon black pellets under hydrodynamic stress to the mechanical properties of dry carbon black, the cohesive strength of carbon black agglomerates was also measured in a tensile strength apparatus (Fig. 2). The operation of this device is described in detail elsewhere (10).
RESULTS AND DISCUSSION Breakup Mechanisms Two distinct breakup mechanisms, denoted a s "rupture" and "erosion", were observed. The rupture 702
process is characterized by a n abrupt breakage of the carbon black pellet into a small number of relatively large pieces. The erosion process, on the other hand, initiates more gradually and a t a lower applied shear stress. Erosion is characterized by the detachment of small fragments (presumably aggregates) from the outer surface of the core pellet, and tends to continue for extended periods of time. A sequence of photographs to illustrate the dispersion process is presented in Fig. 3. At low shear rates, the carbon black pellets remain intact (see Fig. 3a). A typical erosion process outcome is shown in Fig. 3b. The black ring tracing the pellet trajectory indicates that some small agglomerates (
