Of primary particle

The quantity dnjdt is essentially the rate of disappearance of primary particles and hence the rate of decrease in the total number of particles. Integration gives... 

Since polymer swelling is poor and the aqueous solubiUty of acrylonitrile is relatively high, the tendency for radical capture is limited. Consequentiy, the rate of particle nucleation is high throughout the course of the polymerization, and particle growth occurs predominantiy by a process of agglomeration of primary particles. Unlike emulsion particles of a readily swollen polymer, such as polystyrene, the acrylonitrile aqueous dispersion polymer particles are massive agglomerates of primary particles which are approximately 100 nm in diameter. 

For both suspension and mass polymerisations at less than 2% conversion, PVC precipitates from its monomer as stable primary particles, slightly below 1-p.m dia (4,10—12). These primary particles are stabilised by a negative chloride charge (4,13). Above 2% conversion, these primary particles agglomerate. Sectioning the PVC grains of either suspension or mass resins readily shows the skins primary particles at 1-p.m dia, and agglomerates of primary particles at 3—10-pm dia (4,7,8,14). 

Fig. 1. Effect of primary particle size in a 1% carbon black dispersion on rate of change in reflectance. O = min = 20 min 3 = 180 min ...

Dispersion polymer obtained by coagulation of a dispersion. It consists of agglomerates with an average diameter of 450 pm made up of primary particles 0.1 pm in diameter. 

Figure 8.25 Monte Carlo simulation of distribution of primary particle residence times (oo size) within MSMPR precipitated agglomerates of 5 and 20 crystals (Hostomsky and Jones, 1993a)...

This step is termed nucleation and leads to the formation of primary particles. The nuclei, the smallest particles formed in the nucleation step, are considered to be formed by aggregation of growing polymer chains precipitating from solution as they exceed their critical chain length. 

Particle Formation, Electron microscopy and optical microscopy are the diagnostic tools most often used to study particle formation and growth in precipitation polymerizations (7 8). However, in typical polymerizations of this type, the particle formation is normally completed in a few seconds or tens of seconds after the start of the reaction (9 ), and the physical processes which are involved are difficult to measure in a real time manner. As a result, the actual particle formation mechanism is open to a variety of interpretations and the results could fit more than one theoretical model. Barrett and Thomas (10) have presented an excellent review of the four physical processes involved in the particle formation oligomer growth in the diluent oligomer precipitation to form particle nuclei capture of oligomers by particle nuclei, and coalescence or agglomeration of primary particles. 

For the analysis of primary particles it is possible to calculate the spherical diameter for a particle from Rg described above as P = (5/3) Pg or 2.6 Rg. It is also possible to calculate diameter for a particle through the volume/surface ratio, which is called the Sauter mean... 

The most challenging part of rubber mixing is the dispersion of the filler The filler agglomerates have to be broken into smaller particles, the aggregates, but not completely to the level of primary particles. An optimal particle size distribution has to be achieved in order to obtain the best properties of the final rubber product [14]. 

The American Society for Testing Materials proposes as a practical working unit the term ultimate working unit as a primary or individual particle or a group of primary particles that retains its structure throughout a dispersion process and subsequent applications. 

Powders dispersed in liquids consist of agglomerates—a collection of aggregates—which in turn are composed of primary particles. Agglomerates... 

According to the packing geometry, the systems present different porosity and specific surface. The final characteristics of the dried gel are determined by the physicochemical conditions at every step of the preparation the size of primary particles at the moment of aggregate, the concentration of particles in solution, the pH, salt concentration, temperature, and time of aging or other treatment in the wet state, mechanical forces present during drying, the temperature, pH, pressure, salt... 

Figure 9.17 The basic schemes of (a) bidisperse (biporous) porous solid structure 1, nonporous primary particles, 2, aggregates of primary particles (secondary particles), 3, porous solid (granule, grain, pallet, etc.) (b) a bed of granules in a catalytic reactor 4.

The lower the value of D the more open the packing. Suppose we now imagine a dispersion which has been driven to instability by, say, changing temperature. We can visualise sites for the nucleation of floes occurring randomly throughout the whole volume of the dispersion. The total number of primary particles remains unchanged so we can determine the volume fraction of floes ... 

Thielmann, F., Naderi, M., Ansari, M. A., Stepanek, F. The effect of primary particle surface energy on agglomeration rate in fluidised bed granulation. Powder Technol., 181, 2008, 160-168.M. J. Valazza, G. G. Wada. Creating a successful partnership with a contract manufacturer. Pharmaceutical Technology Europe, 13(5), 2001, 26-34. 

Fig. 9.4. Simple illustration of an instantaneous collision cascade generated as a result of primary particle impact in desorption/ionization mass spectrometry. Adapted from Ref. [24] by permission. John Wiley Sons, 1995.

The mass median diameter (MMD) is the most common descriptor of primary particle size and may be determined by sieving or centrifugal sedimentation. The volume median diameter, as determined by laser diffraction, may be used as an approximation of MMD, provided that the particle density is known and does not vary with size, and that the particle shape is near spherical. The MMD of a powder can be used as a predictor of aerodynamic diameter by Eq. (1),... 

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