Agglomerate Breakdown Research Articles

Control of Mullins stress softening in silicone elastomer composites by rational design of fumed silica fillers

The influence of fumed silica as reinforcement filler is investigated on the Mullins stress softening in composite silicone elastomers. Especially, the impact of surface area, loading level, surface chemistry, and structure modification of fumed silica is evaluated on the fatigue behavior of high consistency silicone rubber. It is observed that high loading level and large surface area of fumed silica increase the energy loss during initial loading cycles which is related to disruptions in filler-polymer interactions. Addition of a processing aid or hydrophobization of the silica surface decrease the brittleness and energy loss of the compounds. A close correlation between Shore A hardness and energy loss is found confirming the contribution of the filler-polymer interactions on Mullins stress-softening. At the same time, the permanent set remains almost independent of the surface hydrophobicity of the filler indicating a breakdown of the structured silica agglomerates during initial deformation. To improve the fatigue resistance of the silicone elastomer, a structure modified and hydrophobic silica, AEROSIL® R 8200, was used leading to the lowest permanent set even at high loading levels in combination with good mechanical reinforcement. The reduction in energy loss and permanent set is attributed to the combination of hydrophobic surface and low aggregate structure of the silica preventing agglomerate breakdown and chain disentanglements at the silica surface. This study provides fundamental understanding on the relation between filler-polymer interactions, filler morphology and Mullins stress softening for the rational design of future filler particles in reinforced elastomer composites with improved fatigue resistance.

Intrinsic properties of the matrix and interface of filler reinforced silicone rubber: An in situ Rheo-SANS and constitutive model study

The intrinsic properties of the silicone rubber matrix reinforced by filler silica have been investigated with in situ rheo-small-angle neutron scattering (SANS) and deuterated chain labeling. SANS results reveal that: (i) the average diameter of aggregates and the thickness of bound rubbers (BR) are about 81 nm and 5.6 nm, respectively. (ii) The aggregates size distribution of the filler network is statistically stable during deformation. (iii) Strain mainly causes the agglomerate breakdown/reconstruction and the rubber matrix deformation. Interestingly, the evolution of the aggregates covered with BRs has the similar tendency to the nonlinear strain-stress curve (SSC). However, the tendency of the BR evolution diverges from the SSC. Considering the hydrodynamic amplification effect of filler and the filler-rubber interaction, a modified constitutive model is established and utilized to correlate the macroscopic properties with the micro-structure evolution of silicone rubber. The results suggest that the reinforced rubber matrix might play the dominant role in sustaining the external loading with a hardening effect. While filler presents contradiction effects for reinforcing yet suffering from the agglomerate breakdown and the interface weakening.

Investigating the effect of surface modification on the dispersion process of polymer nanocomposites

Achieving controlled nanoparticle dispersion through melt processing has been challenging as processing-structure rules for polymer nanocomposites are still not well-defined. This work focuses on developing a quantitative understanding of the filler–matrix compatibility and melt mixing parameters on the dispersion of nanoparticles. Filler-matrix compatibility was varied by surface modification of silica nanoparticles. A twin screw extruder was used to prepare the nanocomposites and TEM imaging and image analysis were used to quantitively characterize the microstructure. It was found that matrix–filler compatibility strongly affected the method of agglomerate breakdown and dispersion. Under similar conditions, compatible systems tended to disperse via rupture of agglomerates while incompatible systems were found to disperse via erosion. A map was created to predict the dispersion mechanism as a function of processing conditions and system compatibility and systems from this study and literature were found to be in good agreement with the map.

Influence of mechanical integrity during pneumatic conveying on the bulk handling and rehydration properties of agglomerated dairy powders

Agglomerated powders are susceptible to breakdown on handling, most notably, during powder conveying. In this study, three agglomerated dairy powders (whey protein concentrate powder, WPC; fat-filled milk powder, FFMP and infant formula powder, IF) were conveyed through a custom-fabricated dispersion rig to understand the effects of agglomerate breakdown on dairy powder handling and application. All samples displayed significant breakdown on dispersion, evidenced by reducing particle size and increasing bulk density. The resulting flowability of these powders was impaired (flow index: WPC: 9.3 to 5.1, FFMP: 5.7 to 4.9 and IF: 16 to 10) via increased particle-particle interactions. The initial stages of rehydration were impeded by agglomerate breakage (42.9–47.0% wettability reduction and 7.22–16.4% dispersibility reduction), while powder solubility remained relatively unchanged. This study provides insights into the alterations of agglomerated dairy powder properties on agglomerate breakdown, while identifying the effects these alterations have on the functional properties of these powders.

Mechanical properties of individual MgAl2O4 agglomerates and their effects on densification

The presence of agglomerates during nanopowder sintering can be problematic and can limit achievable final densities. Typically, the practical solution is to use high pressures to overcome agglomerate breakdown strengths to reach higher packing fractions. The strength of agglomerates is often difficult to determine and makes processing parameters challenging to optimize. In this work, we used in situ transmission electron microscopy nanoindentation experiments to assess the mechanical properties of individual MgAl2O4 agglomerates under constant indenter head displacement rates. Electron microscopy revealed highly porous agglomerates with pores on both the micron and nanometric length scales. Individual agglomerate strength, at fracture, was calculated from compression tests with deformation behavior correlating well with previously reported modeling results. Macroscopic powder properties were also investigated using green-pressed pellets consolidated at pressures up to 910MPa. The unexpectedly high strength is indicative of the role agglomerates play in MgAl2O4 nanopowder densification.

Adaptive Extrusion Force Control of Freeze-Form Extrusion Fabrication Processes

Freeze-form extrusion fabrication is an additive manufacturing process that extrudes high solids loading aqueous ceramic pastes in a layer-by-layer fashion below the paste freezing temperature for component fabrication. Due to effects, such as the air bubble release, agglomerate breakdown, and change in paste properties during extrusion as a result of liquid phase migration, the extrusion force is difficult to control. In this paper, an adaptive controller is proposed to regulate the extrusion force. Recursive least-squares is used to estimate the extrusion force model parameters during fabrication and a low-order control scheme capable of tracking general reference trajectories is designed and implemented to regulate the extrusion process. The controller is implemented for sinusoidal reference trajectories and the results demonstrate excellent tracking performance of the adaptive extrusion force controller. Several parts were fabricated with the adaptive extrusion force controller. These results illustrate the need for extrusion force control and that variable reference extrusion force profiles are required to fabricate complex features.

Aqueous-based extrusion of high solids loading ceramic pastes: Process modeling and control

A novel fabrication method known as aqueous-based extrusion fabrication (ABEF) has been developed for the manufacture of ceramic-based components in an environmentally friendly fashion. The method is based on the extrusion of ceramic pastes using water as the medium. This paper describes a machine developed for the extrusion of high solids loading ceramic pastes, investigates the dynamic behavior of the extrusion process, and develops and implements a feedback controller to regulate the extrusion force. The results demonstrate that aqueous-based ceramic extrusion processes have a very slow dynamic response, as compared to other non-compressible fluids such as water. A substantial amount of variation exists in aqueous-based ceramic extrusion processes, most notably during the transient period, and a constant ram velocity may either produce a relatively constant steady-state extrusion force or it may cause the extrusion force to steadily increase until the ram motor skips. It is demonstrated that the use of feedback control reduces the effects of process disturbances (i.e., agglomerate breakdown and air bubble release) and variations in the process as the material in the reservoir decreases, substantially decreases the time to obtain a desired extrusion force level, and dramatically improves part quality.

An X-ray micro-computed tomography study of agglomerate breakdown during the extrusion of ceramic pastes

By using X-ray micro-computed tomography it was possible to determine the extent of agglomerate breakdown during the mixing and extrusion of ceramic pastes. The technique allowed a significant improvement over previous attempts to visually monitor agglomerate breakdown which have been hampered by the lack of a suitable technique with which to obtain fast and reliable three-dimensional data regarding the volume and size distribution of agglomerates. The extent of agglomerate breakdown in a model agglomerate/matrix system was compared after processing by z-blade mixing followed by ram extrusion and by a co-rotating twin-screw mixing/extrusion process at various operating parameters; the latter process was found to be more effective at breaking down the agglomerates under all processing conditions.

On‐Line Monitoring of Ceramic Paste Extrusion

Fluctuations in pressure signals that are measured at a die during the extrusion of α‐alumina‐based ceramic pastes have been shown to contain information about the homogeneity of the paste that is fed into the extruder and phenomena that occur during the extrusion process. Standard error and fractal‐analysis techniques give quantitative measures that have shown good correlation with agglomerate breakdown and variation in solid‐liquid mixing. Gross extrudate surface fracture could be detected via frequency analysis. The potential use of these techniques for the feedback control of extruders is discussed.

Breakdown of agglomerates in ideal pastes during extrusion

By separating the matrix from the agglomerate it was possible to determine the nature of agglomerate breakdown during extrusion within a known geometry. Previous attempts at observing agglomerate breakdown have been unsuccessful at relating the properties of the paste to the properties of the agglomerate due to the indistinguishability of the paste and the agglomerates. By using well-characterized materials, it was found that the breakdown probabilities of the agglomerates during extrusion were independent of the position of the agglomerate. This indicated that as the shear strain rate was known to be highly position dependent, the agglomerate breakdown was shear strain rate independent. Following breakdown, the distributive mixing was observed to be position dependent.

Acoustic studies of the effect of SiC particle reinforcement on the compaction of alumina powder

A number of acoustic techniques namely, ultrasonic (US), acousto-ultrasonic (AU) and acoustic emission (AE), were employed to study the effect of reinforcement in the form of SiC particles on the compaction of spray-dried alumina powder. The US and AU techniques were shown to be capable of monitoring the processes occurring during loading and unloading, whereas AE could be used, in certain cases, to monitor the relative movement of particles in a powder compact during stress relaxation. The results were consistent with three stages of compaction, namely rearrangement of agglomerates, agglomerate breakdown and elastic deformation. The two pressure break-points between the stages i.e. P 1 and P 2 the pressures corresponding to the onset of breakdown of the agglomerates and to the change from permanent to elastic deformation, respectively, were determined by a number of different methods as a function of SiC content (10–40 vol%), type (equiaxed, angular or platelet) and size.

Distinct element simulation of impact breakage of lactose agglomerates

Traditional theoretical and experimental investigations of the mechanical behavior of particulate solids are restricted by the limited quantitative information about what actually happens inside particulate assemblies. This paper presents computer simulation results of the breakage of lactose agglomerates due to impact on a target plate using distinct element analysis. The agglomerates of interest here are generally weak and easy to disintegrate as no binder other than weak surface forces is holding the primary particles together. Particle interaction laws in the simulation code are based on theoretical contact mechanics, where adhesive interface energy determines the bond strength between individual particles of the assembly. Experimental investigations have been conducted to validate the computer simulation results, using a simple air-eductor where particles are accelerated to the required velocity by an air flow and impacted against a rigid target plate. Computer graphics of the simulation results of agglomerate breakdown are compared with the images obtained by high speed video recording of the impact events. A good agreement has been found between the simulation results and experimental measurements. Dynamic features and loading compliance of weak agglomerates are found to be distinctly different from those of high strength agglomerates and solid particles.

Determination of the modes of action of a cationic surfactant for property development in silica-filled natural rubber compounds

Work has been carried out to separate the contributions of a diamine salt of fatty acid [termed a multifunctional additive (MFA)] of a silica-filled natural rubber (NR) compound to property development by (1) improved filler dispersion from elastomer-filler interface, (2) mastication of the NR, (3) modification of the NR-silica interface, and [4] cross-link density effect. Results have shown that there is a substantial increase in mechanical properties at equivalent filler dispersion level due to the effect of MFA on cross-link density. Between 0 and 0.3 phr of MFA, the enhancement of the mechanical properties is due to improvement in silica dispersion. The addition of MFA accelerates filler disagglomeration and results in reduction of the effective volume fraction of filler, as agglomerate breakdown releases immobilized rubber. When the level of MFA is more than 0.3 phr, the cross-link density becomes the dominant factor for mechanical property enhancement. However, after the optimum amount of MFA is exceeded, the properties start to deteriorate owing to weak interfacial interaction between adjacent layers of MFA molecules.

Agglomerate breakdown in fine alumina powder by multiple extrusion

Repeated extrusion through dies of various diameters and die entry angles was used to determine the rate of agglomerate breakdown in a paste consisting of a fine alumina powder, carbon black and a binder of hydroxyl propyl methyl cellulose in water. It was found that three extrusion passes were enough to break up all but ∼0.4% of the agglomerates. Dies with orifices of approximately 1 mm diameter and die entry angles of 45 to 90‡ (where the elongational strain and the deformation rates were highest) were the most efficient for disrupting and dispersing agglomerates and distributing the moisture evenly. This process of deagglomeration was studied by monitoring the load required to extrude and moisture distribution during five repeated extrusion passes of each test paste. The density, agglomerate area fraction and agglomerate circularity of dried extradates were quantified and plotted.

The measurement of mixture homogeneity and dissolution to predict the degree of drug agglomerate breakdown achieved through powder mixing.

Interactive mixing of agglomerates of small, cohesive particles with coarse carrier particles facilitate the deaggregation of agglomerates. In this study dispersion of agglomerates of microfine furosemide particles by such a mixing process was followed by measuring changes in the content uniformity and area under the dissolution curve. Interactive mixtures between agglomerates of different sized furosemide particles and coarse sodium chloride particles were prepared using different mixers, mixing times and mixer speeds. The dissolution rate of the drug from and content uniformity of the mixtures were measured, and degrees of dispersion were calculated. These degrees of dispersion were compared to the dispersion values obtained from the decrease in agglomerate size after mixing. An increase in mixing time led to an increase in dispersion. An initial fast deagglomeration, indicated by an increase in dissolution, increase in content uniformity and a decrease in particle size, was followed by substantially slower deaggregation of remaining agglomerates and smaller aggregates. For all mixtures studied the degree of dispersion estimated from dissolution measurements, when compared to equivalent content uniformity measurements, agreed closely with the degree of dispersion as indicated by the decrease in particle size. The use of the area under the dissolution curve to predict agglomerate breakdown proved useful and may find application in situations where it is impossible to follow directly deagglomeration through particle size measurements.

The assessment of dispersion of fine ceramic powders for injection moulding and related processes

A method for the assessment of dispersion of fine ceramic powders in an organic vehicle for use in injection moulding and related processes is described. The procedure combines the quantitative evaluation of maximum shear stress in twin-roll mixing with the examination of agglomerate breakdown by scanning electron microscopy using the back-scattered electron image technique. The effect of shear stress, mixing time and the strength of agglomerates on the dispersive mixing process was investigated. Two-spray-dried ZrO 2 powders and one TiO 2 powder were examined by mixing with a polystyrene vehicle. The results suggest that the maximum shear stress during mixing should be five to 10 times greater than the agglomerate strength in order to disperse the powder in an organic vehicle. Mixing time was found to be less important. The microscopical technique can be used on its own to examine the state of dispersion of powders in organic materials after dilution in the mixing device. Eine Methode zur Einschätzung der Dispersion von feinen Keramikpulvern in einem organischen Träger wird beschrieben. Diese Dispersionen werden beim Spritzgießen und ähnlichen Verfahren verwendet. Das Verfahren verbindet die quantitative Berechnung der maximalen Scherspannung beim Zwillingswalzen, Mahlen und Mischen mit der elektronenmikroskopischen Untersuchung des Zerbrechens des Agglomerats unter Verwendung von rückgestreuten Elektronen. Der Einfluß der Scherspannung, der Mischzeit und der Festigkeit des Agglomerats auf den dispersiven Mischprozeß wurde untersucht. Zwei sprühgetrocknete ZrO 2 -Pulver und ein TiO 2 -Pulver wurden mit einem Polystyrenträger gemischt und untersucht. Aus den Ergebnissen läßt sich schließen, daß die maximale Scherspannung während des Mischens mindestens 5–10 mal größer sein sollte als die Festigkeit des Agglomerats, um das Pulver im organischen Träger zu verteilen. Es hat sich gezeigt, daß die Mischzeit keinen großen Einfluß hat. Die Mikroskopietechnik kann auch alleine verwendet werden, um den Dispersionszustand des Pulvers in einem organischen Material nach der Verdünnung in einem Mischer zu untersuchen. On décrit une méthode permettant d'évaluer la dispersion de poudres céramiques techniques dans un milieu organique, destinée au moulage par injection et aux procédés connexes. La procédure utilisée combine l'évaluation quantitative de la contrainte de cisaillement maximale, effectuée dans un broyeur-mélangeur à rouleaux, avec l'examen de la rupture des agglomérats par microscopie électronique à balayage, en utilisant la technique des images d'electrons rétrodiffusés. On a examiné l'effet de la contrainte de cisaillement, de la durée du mélange et la résistance des agglomérats sur le procédé de mélange dispersif. Des poudres de ZrO 2 séchées par atomisation et une poudre de TiO 2 ont fait l'objet d'un examen lors du mélange dans un milieu polystyrène. Les résultats suggèrent que la contrainte de cisaillement maximale durant le mélange devrait être au moins 5–10 fois plus grande que la résistance des agglomérats pour pouvoir disperser la poudre dans un milieu organique. La durée de broyage est moins importante. La technique microscopique peut être utilisée pour elle-même afin d'examiner l'état de dispersion des poudres dans des matériaux inorganiques après dilution dans l'appareil de mélange.

APPLICATIONS OF DEM TO PROCESS ENGINEERING PROBLEMS

Although computer simulated experimentation using the distinct element method (DEM) was originally developed as a tool for examining geomechanical problems, the dynamic nature of the methodology used lends itself more readily to many other areas of scientific and industrial interest. One such area is that of process engineering in which large volumes of particulate materials have to be handled in the context of flow problems (e.g. chutes, hoppers, and pipes). In addition, these particulate materials are often in the form of powders which themselves are agglomerations of much smaller sized particles. Processes such as agglomeration and agglomerate breakdown, either by attrition or comminution, are also amenable to investigation by the DEM.

Milling and Suspension Behavior of A12O3 in Methanol and Methyl Isobutyl Ketone

Milling and suspension behavior of Al2O3 were studied in two suspending liquids, i.e. methanol (MeOH) and methyl isobutyl ketone (MIBK). Properties were investigated for various mixed solvent ratios. Ball-milling studies were carried out on powders containing “hard” agglomerates. Agglomerate breakdown was monitored by rheology, sedimentation density, and particle size measurements and by direct microscope observations. Particle size reduction kinetics were observed to decrease at high MIBK/MeOH ratios due to increased flocculation in these suspensions. Suspension behavior of deagglomerated (by ball-milling 20 h) Al2O3 was also investigated. Rheology, sedimentation density, and sedimentation “particle” size distribution measurements demonstrated that increased flocculation occurred at high MIBK/MeOH ratios. Electrophoresis measurements indicated that this behavior was due to decreased electrostatic repulsion between particles. Suspension stability (against flocculation) was analyzed using DLVO theory.