Control of particle size

Aerosol products are hermetically sealed, ensuring that the contents caimot leak, spill, or be contaminated. The packages can be considered to be tamper-proof. They deUver the product in an efficient manner generating Httie waste, often to sites of difficult access. By control of particle size, spray pattern, and volume deUvered per second, the product can be appHed directiy without contact by the user. For example, use of aerosol pesticides can minimize user exposure and aerosol first-aid products can soothe without applying painful pressure to a wound. Spray contact lens solutions can be appHed directiy and aerosol lubricants (qv) can be used on machinery in operation. Some preparations, such as stable foams, can only be packaged as aerosols. 

However, in the case of mini- and microemulsions, processing methods reduce the size of the monomer droplets close to the size of the micelle, leading to significant particle nucleation in the monomer droplets (17). Intense agitation, cosurfactant, and dilution are used to reduce monomer droplet size. Additives like cetyl alcohol are used to retard the diffusion of monomer from the droplets to the micelles, in order to further promote monomer droplet nucleation (18). The benefits of miniemulsions include faster reaction rates (19), improved shear stabiHty, and the control of particle size distributions to produce high soHds latices (20). 

Stricter control of particle size distribution improves powder handling (qv) and transport through the system, and appHcation efficiency. It also reduces blinding of the final filter (69). Additives have been developed which improve the triboelectric charging characteristics (70). 

To achieve the desired cast density for Octol of 1.8g/cc it is necessary that the ratio of HMX TNT be 3 1. However, at this ratio the apparent viscosity, or efflux, is strongly dependent on the polymorphic variety of HMX used and on its particle size distribution. In the initial pilot production of Octol (Ref 3) it was found that for the desired efflux of < 15 sec, 60—70% of the solid HMX must consist of the beta-polymorph having particle diameters in the range of 500—800 microns. Such precise control of particle size was not possible at that time and early Octol casts were made at approximately 50 secs efflux. The economical production of Octol with a satisfactorily short efflux time continues to present a problem in loading shells with this expl (Refs 4, 11 29)... 

ME technique is of special interest in the preparation of catalytically active materials, as the control of particle size and monodispersity are very important for structure-sensitive reactions, like hydrogenations [15]. Metal... 

Nyutu, E.K., Chen, C., Sithambaram, S., Crisostomo, V.M.B. and Suib, S.L. (2008) Systematic control of particle size in rapid open-vessel microwave synthesis of K-OMS-2 nanofibers. Journal of Physical Chemistry C, 112, 6786-6793. 

A review of preparative methods for metal sols (colloidal metal particles) suspended in solution is given. The problems involved with the preparation and stabilization of non-aqueous metal colloidal particles are noted. A new method is described for preparing non-aqueous metal sols based on the clustering of solvated metal atoms (from metal vaporization) in cold organic solvents. Gold-acetone colloidal solutions are discussed in detail, especially their preparation, control of particle size (2-9 nm), electrophoresis measurements, electron microscopy, GC-MS, resistivity, and related studies. Particle stabilization involves both electrostatic and steric mechanisms and these are discussed in comparison with aqueous systems. 

The control of particle size by concentration indicates that particle growth is a kinetic phenomenon. It is unlikely that particle growth is reversible once a Au-Au bond is formed it would not break under these experimental conditions. In a dilute solution of atoms, the frequency of encounters would be lower. As the gold atom-solvent matrix warms, the atoms and subsequent metal particles become mobile. It is the number of encounters that occur before particle stabilization that is important. If metal concentration is high the frequency of encounters is higher and the particles become bigger. 

Brown, K.R., Walter, D.G., and Natan, M.J., Seeding of colloidal Au nanoparticles solutions.2. Improved control of particle size and shape, Chem. Mater., 12, 306, 2000. 

Testing of hydrocyclones for control of particle size in the feed streams to both agent and energetics electrochemical cells. 

The major disadvantages of colloidal catalysts studied so far can be attributed to problems in controlling the metal colloid formation (control of particle size, particle size distribution, structure of metal colloids) and stabilization of the prepared particles, which are not yet completely solved. But it is exactly the stability of the nanoparticles, that is decisive for long-term usage during catalytic processes. Moreover for catalytic application, it is extremely important to preserve the large surface of such colloidal systems. 

Both experimental and theoretical work has demonstrated that growth rate dispersion exists, and has a measurable effect on the CSD in both batch and continuous crystallization processes. Further understanding of this phenomenon on a fundamental level will be required to develop methods to make use of or control growth rate dispersion and make it a tool in control of particle size and shape. 

To date, it has been difficult to produce monodisperse spherical powders from metal alkoxides with low valence, such as Al and Fe, because the hydrolysis and condensation of metal alkoxides in alcohol are too rapid to allow control of particle size, size distribution, and morphology. Many previous studies reported (28-30) that alkoxide-... 

It has been established that manipulation of MC optical properties can be effected in LB films through control of particle size or of the chemical nature of species in the film. 

This section, discusses the mechanism of dispersion polymerization, role of components, control of particle size, and the application to new systems by tracing its history. 

Many reports have been published on the control of particle size by changing the composition of the solvent. The result obtained with the AAm/MAc/MB system is shown in Figure 11.2.2 (19). The particle size seems to increase almost linearly with solubility parameter in the range of 11 to 13. In the case of hydrophobic monomer... 

Jobbins 26) of the New Jersey Agricultural Experiment Station, has suggested the need for a list of adjuvants and the quantities to be used in oil sprays to be applied from aircraft that will permit control of particle size on the ground under various temperature conditions. He would also like to include in oil specifications some reference to their natural spreading coefficients when they are applied on clear water. 

Suspension polymerizations are among the most convenient laboratory procedures as well as plant procedures for the preparation of polymers. The advantages of this method include wide applicability (it may be used with most water-insoluble or partially water-soluble monomers), rapid reaction, ease of temperature control, ease of preparing copolymers, ease of handling the final product, and control of particle size. 

Globulation is the formation of droplets of solution, slurry, or melt followed by solidification by prilling, spray drying, or fluidized bed operation. Control of particle size is best achieved in fluidized beds. 

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