Particle primary

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... 

A = /W//Wp, P is impact parameter and Tq is the distance of closest approach (apsis) of the collision pair. The transformations from the CM coordinates (scattering angle y) to the laboratory coordinates with the scattering angle 0 for the primary particle and (]) for the recoiled surface atoms Is given by... 

Figure Bl.24.3. Layout of a scattering experiment. Only primary particles that are scattered within the solid angle O spaimed by the solid state detector are counted.

In discussions of the surface properties of solids having a large specific surface, it is convenient to distinguish between the external and the internal surface. The walls of pores such as those denoted by heavy lines in Fig. 1.8 and 1.11 clearly comprise an internal surface and equally obviously the surface indicated by lightly drawn lines is external in nature. In many cases, however, the distinction is not so clear, for the surfaces of the primary particles themselves suffer from imperfections in the forms of cracks and fissures those that penetrate deeply into the interior will contribute to the internal surface, whereas the superficial cracks and indentations will make up part of the external surface. The line of demarcation between the two kinds of surface necessarily has to be drawn in an arbitrary way, but the external surface may perhaps be taken to include all the prominences and all of those cracks which are wider than they are deep.,The internal surface will... 

Both (i) and (ii) necessitate recourse to a model of pore shape. By far the commonest, chosen on grounds of simplicity, is the cylinder but the slit model is being increasingly used where the primary particles are plate-like, and the model where the pore is the cavity between touching spheres is beginning to receive attention. 

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. 

The carbon black in semiconductive shields is composed of complex aggregates (clusters) that are grape-like stmctures of very small primary particles in the 10 to 70 nanometer size range (see Carbon, carbon black). The optimum concentration of carbon black is a compromise between conductivity and processibiUty and can vary from about 30 to 60 parts per hundred of polymer (phr) depending on the black. If the black concentration is higher than 60 phr for most blacks, the compound is no longer easily extmded into a thin continuous layer on the cable and its physical properties are sacrificed. Ionic contaminants in carbon black may produce tree channels in the insulation close to the conductor shield. 

Transparent red iron oxide is composed mainly of hematite, a-Ee202, having primary particles about 10 nm. It is prepared by a precipitation reaction from a dilute solution of an iron salt at a temperature around 30°C, foUowed by a complete oxidation in the presence of some seeding additives,... 

Because of demands for improved fuel consumption through reduced rolling resistance, a new series of carbon blacks referred to as LH, ie, N300 with this innovation would be N300 LH. Basically this series of blacks has a wider size range in both the primary particles and primary aggregates in addition to a more chemically active surface area. 

Amorphous sihca exists also ia a variety of forms that are composed of small particles, possibly aggregated. Commonly encountered products iaclude sihca sols, sihca gels, precipitated sihca, and pyrogenic sihca (9,73). These products differ ia their modes of manufacture and the way ia which the primary particles aggregate (Fig. 8). Amorphous sihcas are characterhed by small ultimate particle si2e and high specific surface area. Their surfaces may be substantially anhydrous or may contain silanol, —SiOH, groups. These sihcas are frequentiy viewed as condensation polymers of sihcic acid, Si(OH)4. 

In the suspension polymerization of PVC, droplets of monomer 30—150 p.m in diameter are dispersed in water by agitation. A thin membrane is formed at the water—monomer interface by dispersants such as poly(vinyl alcohol) or methyl cellulose. This membrane, isolated by dissolving the PVC in tetrahydrofuran and measured at 0.01—0.02-p.m thick, has been found to be a graft copolymer of polyvinyl chloride and poly(vinyl alcohol) (4,5). Early in the polymerization, particles of PVC deposit onto the membrane from both the monomer and the water sides, forming a skin 0.5—5-p.m thick that can be observed on grains sectioned after polymerization (4,6). Primary particles, 1 p.m in diameter, deposit onto the membrane from the monomer side (Pig. 1), whereas water-phase polymer, 0.1 p.m in diameter, deposits onto the skin from the water side of the membrane (Pig. 2) (4). These domain-sized water-phase particles may be one source of the observed domain stmcture (7). 

Fig. 1. A grain of (a) suspension PVC and (b) its cross-section showing the skin and primary particles.

Mass-polymerized PVC also has a skin of compacted PVC primary particles very similar in thickness and appearance to the suspension-polymerized PVC skin, compared in Figure 3. However, mass PVC does not contain the thin-block copolymer membrane (7). 

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). 

These primary particles also contain smaller internal stmctures. Electron microscopy reveals a domain stmcture at about 0.1-p.m dia (8,15,16). The origin and consequences of this stmcture is not weU understood. PVC polymerized in the water phase and deposited on the skin may be the source of some of the domain-sized stmctures. Also, domain-sized flow units may be generated by certain unusual and severe processing conditions, such as high temperature melting at 205°C followed by lower temperature mechanical work at 140—150°C (17), which break down the primary particles further. 

Fig. 7. Model for PVC fusion, accounting for molecular weight effects and processing temperature effects (a) unfused PVC primary particles (b) partially melted PVC primary particles (c) partially melted then recrysta11i2ed high molecular weight PVC, showing strong three-dimensional stmcture and (d) partially melted then recrysta11i2ed low molecular weight PVC, showing weak three-dimensional stmcture.

Characterization. The proper characterization of coUoids depends on the purposes for which the information is sought because the total description would be an enormous task (27). The foUowiag physical traits are among those to be considered size, shape, and morphology of the primary particles surface area number and size distribution of pores degree of crystallinity and polycrystaUinity defect concentration nature of internal and surface stresses and state of agglomeration (27). Chemical and phase composition are needed for complete characterization, including data on the purity of the bulk phase and the nature and quaHty of adsorbed surface films or impurities. 

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 ...

Agglomeration a process where precipitation particles grow by coUision with other particles. Pigment agglomerates can be broken into smaller primary particles with the aid of mechanical shear. 

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