Munetake Satoh

Powder design for UV-attenuating agent with high transparency for visible light

To design high-performance ultraviolet (UV)-attenuating agent for dispersion type of functional paints and/or cosmetic powders, nano-meter size of zinc oxide (ZnO) powder was immobilized onto the surface of 4 μm square size of crystalline sheet (H-ilerite: synthetic polysilicic acid (PSA)). A high-speed elliptical-rotor-type powder mixer (HEM) was applied for exfoliating in a sheet one by one from the PSA agglomerates without breakage of the original shape and for arranging the ZnO in the mono-particle layer on the sheet during dispersing the self-coagulative ultra-fine ZnO. The critical operating conditions of the HEM were determined by an observation of the morphological change of PSA. Optical characteristics of the compounded powder (ZnO-PSA) were evaluated in the form of suspension dispersed into polymer medium. The UV-attenuating effect was confirmed in the UVA region (320–400 nm) and the heat treatment after immobilization was one of the effective methods to get higher transparency for visible light.

The effect of random positioning on the packing of particles adhering to the surface of a central particle

The number of (outer) particles, in a random-packed monolayer, adhering to the surface of a central particle has been evaluated by means of computer simulation. An approximate equation accounting for the influence of the central-to-outer particle size ratio is derived. This equation contains the average distance between two adjacent outer particles as a fitting parameter. It is found that the average distance between the centers of two contiguous outer particles is equal to λ = 4/3 times their diameter. The corresponding number of randomly packed particles equals 1/λ2 (= 0.56) times the maximum possible (hexagonal-packed) number of particles which can adhere to a given central particle.

Sound absorption measurements for evaluating dynamic physical properties of a powder bed

Abstract The normal incident sound absorption coefficient of a powder bed on a rigid plate has been measured in the frequency range of 200 Hz to 2000 Hz using an acoustic tube. Six different kinds of powder materials were used as samples; the mean particle diameter varied from 32.3 μm to 152.3 μm and the bed thickness varied from 10 mm to 40 mm. The sound absorption curves thus obtained had several distinct peaks; their frequency ratios were odd multiplications and the peak frequencies changed inversely with bed thickness. These peaks are attributable to the excitation of the normal vibration of the powder bed by sound wave incidence. Consequently, the sound absorption peak frequencies correspond to the natural frequencies of the powder bed. The measurements of vibration acceleration in the powder bed have clarified the fact that the primary sound absorption peak is related to the fundamental vibration mode, which is the one-end fixed longitudinal mode excited when one-fourth of the sound wavelength in the powder bed coincides with the bed thickness. These results show the possibility of estimating the dynamic physical properties of a powder bed, i.e. sound velocity and longitudinal elastic coefficient, from the primary sound absorption peak frequency of the powder bed. The measurements of the sound absorption coefficient by this method are hardly affected by powder vibration fluidization, etc., because they can be achieved in a non-contact state and in a short time, using a small vibrating force.

Dynamic measurements for the stiffness constant of a powder bed

Abstract A measuring apparatus was devised to determine the dynamic physical properties of a powder bed from the transfer function of vibration acceleration that results from the direct application of random vibration to the powder bed. By using this apparatus, the variation of the stiffness constant of a powder bed with various vibration forces and compression forces was measured. The stiffness constant gradually decreased when the vibration force exceeded a certain value. With respect to the change in stiffness constant with compression force, the following experimental equation was derived: k = SW n , where k is the stiffness constant of a powder bed, W the compression force applied to the powder bed, and S and n are empirical constants.

Powder coating in a rotary mixer with rocking motion

Abstract The mechanism of coating of a coarse powder by a fine adhesive powder in a rotary-type mixer with rocking motion was investigated. The fraction of powder that had been coated at any moment was measured continuously by an optical method and the rate equation was established. It was found that coating progresses as a second-order autocatalytic chemical reaction. However, the analogy was limited by the fact that not only the rate constant but also the ‘partial orders of reaction’ depend on the operating conditions. The shearing and friction forces generated by the rotational movement of the mixer turned out to exert a strong and positive effect on the rate of coating. On the contrary, the rocking motion proved to be useful only for relatively low rotation speeds. Rocking should only be used when the carrier particles are not well distributed within the bulk at the beginning of operation.

Analysis of collision energy of bead media in a high-speed elliptical-rotor-type powder mixer using the discrete element method

In a high-speed elliptical-rotor-type powder mixer (HEM), to which zirconia beads are added to aid stress transmission, the energy generated during the bead-to-bead, bead-to-rotor and bead-to-vessel wall collisions has been analyzed. The motion of beads in the HEM was expressed by a simplified quasi-two-dimensional (2D) model adopting the discrete element method (DEM), so that actual physical properties can be used in the model calculation and the calculation time is reduced. Comparison between the calculated and experimental results has proved that the quasi-2D model can adequately express the motion of beads within the practical range of application. By using the collision energy obtained from the model calculation, the local temperature rise on the contact surface of a bead in a collision was estimated surprisingly to exceed several thousand degrees Kelvin under severe operating conditions. It has been found that the net energy applied to the particles is several percent of the increased kinetic energy of beads in the bead-to-rotor collision.

Estimation of net energy applied to powders in a newly designed bead mill

Abstract We have developed a new method for estimating experimentally the net energy applied to powders in a particle-compounding process with a high-speed elliptical-rotor-type powder mixer (HEM). Spherical copper particles, of which the stress–strain relation is known, are used as the standard material to obtain the net applied energy. Simplified models for the changes of the size distribution and shape of copper particles with plastic deformation have been proposed. The amount of deformation of a copper particle is determined from the diameters of projected images of particles before and after deformation. The net energy applied to the copper particles is estimated as the total of the compressive energy of individual particle. The net applied energy in single compression by the elliptical rotor is proportional to the square of the rotational speed of the rotor; the energy in single compression depends on the kinetic energy of rotational motion of the rotor. The method for estimating the net applied energy is effective in the analysis of the energy transmission from a media mill to powders and in the determination of optimal operating condition in a particle-compounding process.

Determination of optimum operating conditions based on energy requirements for particle coating in a dry process

This paper describes the method for determining the optimum operating conditions of equipment on dry coating processes based on the energy requirement for immobilizing the fine materials on the surface of coarser particles. In the model system consisting of the spherical copper particles (host particles) and the submicron-sized fine alumina particles (guest particles), the guests are immobilized by embedding onto the surface of the host with a high-speed elliptical-rotor-type powder mixer (HEM) to which Zirconia beads are added. The embedding process is approximated by the intruding process of the diamond indenter at the measurement of Vickers hardness of copper plate, and the energy required for embedding is estimated. We have found that both the allowable deformation of copper particles in a single compression by the rotor and the depth of embedding of alumina particles are important parameters in the determination of operating conditions (the rotor speed and the operation time). When the allowable deformation is set to be approximately 0.5% (corresponding to the strain in the intermediate region between the elastic and plastic ones of copper particle), the shape and size distribution of copper particles can be maintained throughout the coating process. Furthermore, the surface condition of coated particles can be controlled quantitatively by the depth of embedding.

Kinetics of fines transfer among carriers in powder coating

Abstract A discrete model for the description of the exchange of fines between carriers in the powder coating process is presented. The model is based on a population balance relating the distribution of fines within the carriers at a given time to that existing after a small time-interval, within which each carrier suffers on the average just one collision. The evolution of this distribution as coating proceeds is simulated, and it is found that the temporal variation tendency of the fraction of carriers which are coated by a number of fines equal to the average number of fines per carrier present in the mixture is in accordance with the second-order auto-catalytic chemical reaction analogy previously proposed. The quality of coating (degree of homogeneity) is defined, and its dependency on parameters such as fines concentration, carrier-to-fine size ratio and transfer probability (a parameter of the model) is discussed.

Mechanical-dry coating of wax onto copper powder by ball milling

Mechanical dry coating process has been applied to formation of the oxidation protecting film of polymer wax onto spherical copper particles (median diameter of 69.1 μm) using a conventional ball milling. The wax used here showed poor adhesiveness to the metal surface and low plasticity at room temperature due to relatively hard and high melting point (446 K). In order to determine the necessary mechanical force for adhering between the metal and the wax without any deformation of metal particles, the coating experiments were carried out with process variables as processing time, wax content and combination of different kinds of wax. The degree of surface coverage of wax film related to the operating conditions was evaluated by means of the dissolution experiment with acid solution in the optical cell. It has been shown that the addition of the soft wax as a sealant material for filling the cavity of the hard wax was effective to stabilize the coating state and to enhance the degree of coverage.