Carbon Black Agglomerates Research Articles

Acoustic Analysis of Composite Soft Materials IV. Evaluation of Compressibility of Bound Rubber in Carbon Black Filled SBR

A carbon black (CB) filled styrene-butadiene rubber (SBR) compound was investigated by acoustic techniques, scanning acoustic microscopy and longitudinal wave velocitometry.The CB agglomerates of larger than 5 µm dispersed in the compound mixed by two-roll mill were observed as black spots in acoustic micrographs. On the other hand, the CB agglomerates in the compound mixed by oil-pressure kneader were not observed in the acoustic micrograph, since the particle size of the agglomerates was less than 5 µm.The density and the longitudinal wave velocity of the compound were measured as a function of the weight percentage of the CB. The density and the velocity increased linearly with the content of the CB. The mass ratio of the bound rubber to the CB in the unvulcanized sample was determined by using toluene extraction and thermo gravimetric analysis. The partial specific adiabatic compressibility of the CB was estimated as (−0.5±0.5)×10−10 Pa−1 on the basis of the three states model. The adiabatic compressibility of the bound rubber was (2.2±0.5)×10−10 Pa−1, and it is half of that of the SBR matrix.

Effect of Temperature on Carbon-Black Agglomeration in Hydrocarbon Liquid with Adsorbed Dispersant

Suspensions of carbon black in oil, stabilized with adsorbed polyisobutylene succinimide (PIBSI) dispersant, are commonly used as model systems for investigating the soot-handling characteristics of motor oils. The structure of the carbon-black agglomerates changes dramatically with temperature; this results in a concomitant change in the suspension rheology. Linear and nonlinear rheological experiments indicate a large increase of the interparticle attractions as the temperature is raised. To elucidate the origin of this behavior, we investigate the effect of temperature on the stabilizing effect of the dispersant. Measurements of adsorption isotherms of the dispersant on carbon black indicate that there is little variation of the binding energy with temperature. Intrinsic viscosity measurements of PIBSI dispersants in solution clearly exhibit an inverse dependence of the dispersant chain dimension with temperature. These results suggest that the temperature-dependent changes in the chain conformation of the PIBSI dispersant are primarily responsible for the changes in the dispersion rheology, and we propose a simple model to account for these data.

Carbon Black Dispersion Measurement in Rubber Vulcanizates via Interferometric Microscopy

Abstract A new method for characterizing the carbon black dispersion in rubber compounds is introduced. This technique is based on interferometric microscopy (IFM) and utilizes the interference fringes between in-phase light beams reflected from the rubber sample and a smooth reference surface to measure the three-dimensional surface topography. The peaks and valleys present on the fresh-cut surface are representative of the carbon black agglomerates and are used to characterize the dispersion. A series of samples with different base rubbers and varying dispersion levels were created and characterized by both light microscopy and IFM. These results were used to generate a universal dispersion index based on the IFM data that correlates well with the LM dispersion index values. In addition, three-dimensional peak statistics were obtained from the IFM data and used to provide additional information about the carbon black agglomerate distribution. This data can be used for a more complete understanding of the compound behavior as a function of the carbon black dispersion and agglomerate distribution.

Filler Dispersion in Hyperbranched Polyisobutylene

Abstract The microdispersion of carbon black filler in linear and hyperbranched polyisobutylene (PIB) was assessed from dynamic mechanical and volume resistivity measurements. While no significant differences were observed in the carbon black concentration necessary for formation of a filler network, in comparison to the linear polymer, the highly branched PIB was found to have substantially more carbon black agglomeration. This occurs despite its higher viscosity, due to the relative inaccessibility of large portions of the molecule, as a result of the profuse treelike branching. The consequence is more extensive interaggregate interaction, and thus a larger Payne effect and greater mechanical hysteresis.

Study of the influence of nejayote and other additives on the cohesive strength and electric properties of carbon black agglomerates

AbstractAn investigation was carried out on the influence of nejayote and other additives (water, potassium lignosulfonate, and furfuryl alcohol), on the cohesive strength and electrical properties of carbon black agglomerates. The thermal stability of carbon black agglomerates with potassium lignosulfonate and nejayote was similar. The influence of nejayote and potassium lignosulfonate on the cohesive strength of carbon black agglomerates was also similar but the electrical properties were different. The carbon black agglomerates with nejayote have a lower electrical conductivity. The influence of water on the cohesive strength of carbon black agglomerates was not significant but it was important in their electrical conductivity. Nejayote, a waste by‐product from the corn‐processing industry, appears as a potential agglutinant in the carbon black pelletization process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3965–3972, 2003

Experimental and numerical study of the rotation and the erosion of fillers suspended in viscoelastic fluids under simple shear flow

When a porous agglomerate immersed in a fluid is submitted to a shear flow, hydrodynamic stresses acting on its surface may cause a size reduction if they exceed the cohesive stress of the agglomerate. The aggregates forming the agglomerate are slowly removed from the agglomerate surface. Such a behaviour is known when the suspending fluid is Newtonian but unknown if the fluid is viscoelastic. By using rheo-optical tools, model fluids, carbon black agglomerates and particles of various shapes, we found that the particles had a rotational motion around the vorticity axis with a period which is independent on shape (flat particles not considered), but which is exponentially increasing with the elasticity of the medium expressed by the Weissenberg number (We). Spherical particles are always rotating for We up to 2.6 (largest investigated We in this study) but elongated particles stop rotating for We>0.9 while orienting along the flow direction. Erosion is strongly reduced by elasticity. Since finite element numerical simulation shows that elasticity increases the local stress around a particle, the origin of the erosion reduction is interpreted as an increase of cohesiveness of the porous agglomerate due to the infiltration of a viscoelastic fluid.

Investigation of the dispersion of carbon black agglomerates of various sizes in simple-shear flows

The dispersion of spherical carbon black agglomerates suspended in polydimethyl siloxane liquid and subjected to simple shear flows has been studied in a cone-and-plate shearing device. Sets of dispersion experiments were carried out for agglomerates of various size and packing density. For agglomerates of equal density and under conditions of equivalent cone rotation rates, the dispersion rates of small agglomerates were smaller than those observed for larger agglomerates. Since the agglomerates occupy a significant fraction of the flow domain, the magnitude and distribution of shear stress acting on the agglomerate at a fixed cone rotation rate depends on the ratio of agglomerate size relative to the size of the gap between the cone and plate. To investigate whether this effect could cause the observed variation in dispersion behavior, we performed three-dimensional simulations of the flow fields within the cone-and-plate device and calculated the resulting stress fields acting on spherical agglomerates. These results helped guide additional experiments in which the peak stress acting on agglomerates of various sizes was matched. However, even under matched stress conditions, the dispersion kinetics was found to vary according to the agglomerate size. In addition, the dispersion kinetics for identical sized agglomerates was found to depend on their processing history. Both of these results lead to the conclusion that some other effect, likely the infiltration of the processing fluid within the agglomerate structure, also influences the dispersion behavior.

Mesostructure, electron paramagnetic resonance, and magnetic properties of polymer carbon black composites

Electron paramagnetic resonance (EPR) has now become firmly established as one of the methods of choice for analyzing the carbon network over a range of different volume fraction of the carbon black in the composite, i.e., below and above the respective conduction threshold concentration. In the present article, two types of carbon blacks, having very different primary structures, surface areas, and percolation thresholds, were used; Raven 7000 (of high surface area and high percolation threshold volume fraction) and Y50A (of low surface area and low percolation threshold volume fraction). A semiquantitative image analysis of the microstructure from transmission electron microscopy reveals information about the spatial distribution of the carbon aggregates and agglomerates inside the composite. We observe that the apparent surface of agglomerates increases significantly with increasing carbon black content for the two types of blacks investigated. Adsorbed oxygen on the carbon black cristallites and dynamic coalescence under mixing conditions can be responsible for the broadening of the dispersed phase surface distribution. The interagglomerate distance in two samples of concentrations f<fc and f≅fc of Raven 7000 are nearly identical indicating that the dc condition threshold can therefore be almost entirely attributed to the coalescence of smaller aggregates. Line shape simulation showed that the changes in the absorption EPR spectra, at temperatures between 105 and 300 K, of the composite samples containing Raven 7000 can be described by a linear superposition of two distinct Lorentzian (one broad and the other narrow) resonance lines and a single (narrow) Lorentzian resonance line for composite samples containing Y50A. The spins giving rise to the EPR signal reside in the carbon black particles. In Raven 7000, the significant difference in linewidth between the two signals demonstrates a different environment where the restriction of the motion of the paramagnetic centers varies. The narrower line was assigned to spin probes with high mobility (carbon black aggregates) and the broad one to probes with restricted mobility incorporated in carbon black agglomerates. In Y50A, only the sites with high mobility were detected. When the temperature is increased the data demonstrate that the EPR signal intensity, which is the double integral in arbitrary units divided by the mass of the carbon black contained in the sample, decreases slowly in the temperature range 105–300 K. The various phenomena observed are attributed mainly to the aggregates and agglomerates structure in the composite samples. The temperature dependence of the paramagnetic susceptibility deduced from the EPR integrated intensity is discussed in terms of Adriaanse et al.’s model [L. J. Adriaanse, J. A. Reedijk, P. A. A. Teunissen, H. B. Brom, M. A. J. Michels, and J. C. M. Brokken-Zijp, Phys. Rev. Lett. 78, 1755 (1997)]. The magnetic susceptibility of the composite samples is also measured with a superconducting quantum interference device magnetometer, operating at an applied magnetic field of 0.5 T, from 2 K to room temperature. The observed temperature dependence of the spin susceptibility is discussed and suggests that morphology heterogeneity is of overwhelming importance to understand the magnetic properties of these materials.

In situ study of the dynamics of erosion of carbon black agglomerates

With the help of a counter-rotating transparent rheometer equipped with an optical microscope, the erosion of single, commercial carbon black agglomerates suspended in a polyisobutylene fluid was accurately recorded in situ. We observed that clouds of very small aggregates were leaving the agglomerate at 45 and 225° in the shear gradient–flow direction plane, where the stress is maximum. The distribution of the arrached aggregates is more efficient when the carbon black agglomerate is more asymmetric. A spherical agglomerate of critical radius R0 will reach a radius Rt at time t following R − R = at, a being a constant. This implies that the rate at which the number of aggregates is leaving the agglomerate is a constant, independent of the size of the agglomerate. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1627–1629, 2001

Influence of additives on the dielectric strength of high-density polyethylene

In this study, we present the results of the influence of chemical additives (antioxidant and UV stabilizer) and pigments (titanium dioxide and carbon black) on the short-term dielectric breakdown test of high-density polyethylene (HDPE). These additives and pigments are commonly added to polyolefins, which are used as insulating material for medium voltage cables. The incorporation was performed in a single screw extruder and thin films specimens were obtained by hot compression from extruded materials. For the dielectric breakdown test, an automated system has been used. A voltage ramp of 500 V/s was applied to specimens immersed in a silicon oil bath at room temperature. The degree of crystallinity and chemical modification of the formulations were evaluated by X-ray diffraction and Fourier transform infrared (FTIR), respectively. The dielectric breakdown results have been analyzed by a Weibull distribution. The shape and scale parameters of this distribution have been obtained by a graphic and maximum likelihood method. These results showed that the carbon black is the component that affects the dielectric strength, that the /spl beta/ shape parameter from the graphic method can be used to evaluate additive mixing conditions, and that the weakest point for formation of the rupture channel is on the carbon black agglomerate.

The Chain End Distributions and Crosslink Characteristics in Black-Filled Rubbers

Abstract A black-filled rubber compound consists of two phases: the free polymer phase, where no particle exists, and the carbon black agglomerate phase, where highly concentrated particles are bound by a small amount of the polymer—so called bound rubber. The Charlesby—Pinner virtual linear number-average molecular weight Mn1 of the polymer in each phase is determined for black-filled compounds to obtain information about the chain end distribution in the compounds. The nominal crosslink density of the bound rubber is also measured by means of the swelling measurement of the “carbon gel” to characterize the crosslink variation in the vulcanizates. The results indicate that the chain end density is much higher in the agglomerate phase than in the free polymer phase due to enhanced chain scission during mixing. The enhancement of scission is considered due to the free radical crosslinking which imposes restriction to chain slippage in the flow field. This together with the previous findings suggests the features of the phase construction in the filled vulcanizates: tightly crosslinked free polymer phase with fewer chain ends, and loosely crosslinked agglomerate phase with more chain ends but, with rather suppressed chain mobility due to the dense nano-scale particles. The features match well with the energy dissipating and the toughened nature of the filled vulcanizates.

Finite Element Analysis of Mixing in Partially Filled Twin Blade Internal Mixers

Abstract Twin blade internal mixers are the main devices used in rubber processing industry to mix natural and synthetic rubbers with carbon black and other compounding ingredients. To facilitate mixing, these internal mixers are always partially filled. Consequently, the flow regime that becomes established inside the mixer is a multiple random free surface flow with moving external boundaries. The stress field associated with this flow regime is usually very uneven. In addition, the flow stream is continuously divided and rejoined by the action of the counter rotating blades. The combination of these complex effects results in the break down, dispersion and distribution of carbon black agglomerates within the rubber matrix. The efficiency of the entire process mainly depends on the geometry of the mixer blades and the operating conditions. Predictive computer modelling offers a very convenient method for the quantitative analysis of mixing process and can be used to design more efficient internal mixers. However, the successful modelling of a complex process such as rubber mixing requires the development and use of sophisticated mathematical algorithms that can take into account its main characteristics. In this respect, a major difficulty is the imposition of the transient boundary conditions in a continuously varying flow domain. In the present paper we describe a robust method which can very effectively resolve this problem. This method is based on the combination of Lagrangian and Eulerian approaches for the modelling of moving boundary flows. We have used this scheme to simulate flow and mixing in the cross-sectional plane of the blades of a tangential rotor internal mixer.

Observation and analysis of the infiltration of polymer liquids into carbon black agglomerates

The infiltration of polymeric liquids into carbon black agglomerates has been studied using buoyancy measurements. Both the rate of infiltration and the degree of saturation decrease with increasing matrix viscosity. The influence of interfacial chemistry was demonstrated by comparing the enhanced infiltration and saturation that occurs with polybutadiene and ethylene propylene copolymers in comparison to polydimethyl siloxane liquids. However, the differences attributable to interfacial chemistry are masked when highly viscous liquids are used. The influence of the agglomerate pore structure on infiltration and saturation characteristics was also examined. In the case of low-structure carbon black, the infiltration and saturation characteristics showed a sensitivity to packing density while the same was not true for agglomerates of the high structure carbon black.

The Influence of Matrix Viscosity and Interfacial Properties on the Dispersion Kinetics of Carbon Black Agglomerates

Abstract The dispersion of carbon black agglomerates suspended in polydimethyl siloxane (PDMS) or polybutadiene (PBD) liquids has been studied. Agglomerates comprised of either a low-structure or a high-structure carbon-black were subjected to simple shear flow. Two characteristic length scales that affect the dispersion process are identified. One length scale (Lp) is a measure of the ease with which fluid can flow through the porous agglomerate structure. The second (δ) is a measure of the depth to which processing fluid has soaked into the agglomerate. Values of Lp were found to be independent of types or viscosity of the polymer but were sensitive to the structure and packing density within agglomerates. The parameter δ, which varies with time of immersion in the processing fluid, was found to be smaller at comparable immersion times for matrix fluids with high viscosity. Shearing experiments showed two kinetic regimes (fast and very slow dispersion). For each type of matrix liquid, there exists a critical packing density; agglomerates having a packing density lower than this critical value exhibit only fast erosion, but those with packing density higher than this critical value show dispersion kinetics that starts as fast but shifts to slow after a certain period of shearing. The critical packing density was higher in PBD than in PDMS for agglomerates of the low-structure carbon-black but was not significantly different for agglomerates of the high-structure carbon-black. The critical packing density was found to shift to higher values if higher viscosity PDMS was used. The dispersion kinetic regime for the different polymers liquids could be correlated with the value of δ/Lp. If δ/Lp remained less than a critical ratio, only fast erosion was observed. Measurement of fragment size distributions support the notion that fragments are produced primarily from the periphery of the agglomerate in the slow kinetic regime, but can be larger (up to the size of δ) for the fast kinetic regime.

The influence of state-of-mix on the extrudate swell of a carbon black-filled styrene-butadiene rubber compound

The factors which govern the extrudate swell of a styrene–butadiene rubber compound filled with 30 phr of N330 carbon black at various states-of-mix were investigated. The state-of-mix is quantified by an effective filler volume fraction, based on an estimate of the amount of rubber immobilized in the carbon black agglomerates. The swell has been found to be dominated by recoverable strain and relaxation time, which are both controlled by the effective filler volume fraction. In contrast, shear rate, wall slip, and the rubber–carbon black network have not been found to have a significant effect on the extrudate swell. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:305–315, 1997

Influence of matrix infiltration on the dispersion kinetics of carbon black agglomerates

The dispersion of carbon black agglomerates suspended in poly(dimethylsiloxane) liquids and subjected to simple shear flow has been studied. Two characteristic length scales that affect the dispersion process are identified and estimated from hydrodynamic drag coefficients measured in sedimentation experiments. One length scale, Lp, is a measure of the ease with which fluid can flow through the porous agglomerate structure and is directly related to its packing density and the morphology of the constituent particles. The second, δ, is a measure of the depth to which processing fluid has soaked into the agglomerate. Values of Lp were found to be relatively large for high structure carbon blacks and for low density agglomerates. The parameter δ varies with time of immersion in the processing fluid. Shearing experiments showed two kinetic regimes for the rate of dispersion. Analysis showed that fast dispersion occurred when Lp/δ exceeds a critical value and, conversely, relatively slow erosion occurred when Lp/δ is below a characteristic threshold value. For agglomerates having a density higher than a critical value, the kinetic regime shifts from fast to slow as the infiltration depth exceeds Lp after some shearing time. However, for agglomerates with a density lower than this value, erosion rates exceeded infiltration rates, Lp/δ remained greater than the critical ratio, and only fast erosion was observed.

Effects of a multifunctional additive on bound rubber in carbon black and silica filled natural rubbers

The effects of a multifunctional additive (MFA), i.e. a diamine salt of a carboxylic acid, on the bound rubber in carbon black and silica filled natural rubber compounds in o-xylene, have been studied. For the carbon black filled compound, the bound rubber initially decreases with the MFA concentration and then does not change significantly above an MFA concentration which approximately corresponds to the monolayer surface coverage of the carbon black. This phenomenon has been explained in terms of the release of immobilised rubber within the carbon black agglomerates. Bound rubber measured at high temperatures indicated that the MFA did not improve the chemical interaction between the carbon black and the rubber in an unvulcanised rubber mix. The MFA has no pronounced effects on the bound rubber for silica. This may be caused by the combined effects of improved filler dispersion and silica-rubber interfacial interaction. The ozone oxidation of the carbon black significantly increases its surface oxygen concentration but has little effect on its specific surface area. The oxidation has been found to decrease the bound rubber.

Influence of Aggregate Structure and Matrix Infiltration on the Dispersion Behavior of Carbon Black Agglomerates

Abstract The dispersion of carbon black agglomerates in poly(dimethyl siloxane) (PDMS) has been studied experimentally. Both the structure of carbon black aggregates comprising the agglomerate and the presence of the matrix within the agglomerate were found to affect the mode of dispersion, critical stress conditions, and the kinetics of the dispersion process. Agglomerates of high structure carbon black are generally more difficult to disperse than agglomerates of low structure carbon black at the same agglomerate density. Depending on the degree of saturation of the agglomerate by PDMS, the dispersion process may be either enhanced or retarded compared to the dry state.

Reduction in Viscosity of an SBR Compound Caused by Mastication and Disagglomeration during Mixing

Abstract It would be advantageous to determine the causes of changes in rheological behavior of a compound during mixing, so that mixing times and conditions could be optimized to give the desired processing behavior and properties of the final product. The causes of changes in viscosity during mixing of a compound containing only elastomer and carbon black were investigated by examining concurrent changes in viscosity of the gum, density of the compound and degree of carbon-black disagglomeration. The reduction in viscosity occurring during mixing could be attributed to mastication of the elastomer and disagglomeration of the carbon black. The effect of mastication could be removed by calculating values of relative viscosity, using the viscosity of the gum treated in a similar way to the compound and with an allowance being made for strain-rate amplification. The decrease in relative viscosity of the compound with increased mixing was attributed to a reduction in effective volume fraction of filler caused by immobilized rubber being released as carbon black agglomerates were broken down. The dibutyl phthalate absorption (DBPA) value of the carbon black gave a good indication of the amount of immobilized rubber present in an agglomerate.

Combustion of CWF agglomerates from pulverized or micronized bituminous coal, carbon black, and diesel soot

Fundamental studies were conducted to explore the role of coal grind size on the combustion characteristics of coal-water fuel (CWF) agglomerates. The agglomerates were prepared from bituminous coals of two different grind sizes: pulverized (40 μm mean size), and micronized and beneficiated (4 μm mean size) as well as blends thereof. Experiments involved single predried agglomerates, free-falling in a drop-tube laminar-flow furnace at high heating rates. Combustion took place in oxygen partial pressures of 10% or 21% at a furnace gas temperature of 1450 K. Additional experiments were performed with predried agglomerates from water slurries of carbon black or diesel soot (both of particulate size and aggregate sizes in the order of 0.050 and 0.5 μm, respectively). All agglomerates were in the size range of 150–900 μm and were either plain or impregnated with calcium magnesium acetate (CMA). Upon heating and devolatilization, the bituminous coal agglomerates studied were found to melt, mildly swell, and form cenospheric structures. Individual pulverized coal particles also formed small cenospheres themselves, superimposed on large agglomerate-derived cenospheres. Combustion of CWF agglomerates, in the size range examined, occurred with distinct volatile and char combustion phases with the latter burning in a diffusion-controlled mode. The combustion behavior of pulverized and micronized coal agglomerates of the same size was strikingly similar. The presence of dissolved calcium magnesium acetate (CMA) accelerated setting of the slurries and, upon water evaporation it suppressed melting and cenosphere formation of the coal particles and agglomerates alike. Also, CMA mildly influenced the agglomerate combustion behavior, causing both larger volatile flames and char temperatures that peaked at the first half of the burnout period and decreased thereafter, but did not substantially affect the char burnout time. Carbon black and diesel soot agglomerates did not form cenospheres and the latter burned a little hotter and faster but, basically, their combustion was similar to that of CWF chars. Bulk fragmentation was consistently observed only in thecase of CMA-impregnated carbon black agglomerates.