Glass particles

The dissolution of soluble sihcates is of considerable commercial importance. Its rate depends on the glass ratio, sohds concentration, temperature, pressure, and glass particle size. Commercially, glasses are dissolved in either batch atmospheric or pressure dissolvers or continuous atmospheric processes. Dissolution of sodium sihcate glass proceeds through a two-step mechanism that involves ion exchange (qv) and network breakdown (18). 

Cheapest of all are the particulate composites. Aggregate plus cement gives concrete, and the composite is cheaper (per unit volume) than the cement itself. Polymers can be filled with sand, silica flour, or glass particles, increasing the stiffness and wear-resistance, and often reducing the price. And one particulate composite, tungsten-carbide particles in cobalt (known as "cemented carbide" or "hard metal"), is the basis of the heavy-duty cutting tool industry. 

Fig. 1. Log-log plot of the contact radius as a function of particle radius for soda-lime glass particles on polyurethane (from ref. [56]).

Rimai et al. [57] determined the power-law dependence of the contact radius on the substrate s Young s modulus for another quintessential JKR system that of a soda-lime glass particles on polyurethane substrates. They reported that the contact radius varied as with iua calculated to be 0.12 J/m. The results... 

Lest one be lulled into a false sense that, assuming that the JKR theory properly describes particle adhesion within its regime, DeMejo et al. [56] also reported that, for soda-lime glass particles with radii less than about 5 p.m, the contact radius varied, not as the predicted but, rather, as Similar results were reported for other systems including polystyrene spheres on polyurethane [58], as shown in Fig. 2, and for glass particles having radii between about 1 and 100 p,m on a highly compliant, plasticized polyurethane substrate [59] as illustrated in Fig. 3. 

Fig. 3. Plot of the a for glass particles as a function of on a highly compliant polyurethane substrate (from ref [59]).

An example of a Maugis-Pollock system is polystyrene particles having radii between about 1 and 6 p.m on a polished silicon substrate, as studied by Rimai et al. [64]. As shown in Fig. 4, the contact radius was found to vary as the square root of the particle radius. Similar results were reported for crosslinked polystyrene spheres on Si02/silicon substrates [65] and micrometer-size glass particles on silicon substrates [66]. 

A glass particle settles under the action of gravity in a liquid. Obtain a dimensionless grouping of the variables involved. The falling velocity is found to be proportional to the square of the particle diameter when the other variables are constant. What will be the effect of doubling the viscosity of the liquid ... 

It may be expected that the variables expected to influence the terminal velocity of a glass particle settling in a liquid, uq, are ... 

The deposition procedure described earlier allows one to obtain protein films chemically bound to the activated surface of spherical glass particles. Subsequent compression of preformed protein monolayer with these particles permitted to coverage of the particle area that initially has not come in contact with the monolayer, as schematically shown in Figure 14. Even if such a procedure does not initially result in deposition of strictly one monolayer, this fact does not seem to be critical, because only the monolayer chemically attached to the surface remains after washing. 

Assuming that the initial sodium concentration of glass particle with a radius R is C, and its surface sodium concentration Cj is zero at t > 0, dimensionless terms can be written as... 

Figure 5.14 The microstructure of the set cement is clearly revealed by Nomarski reflectance optical microscopy. Glass particles are distinguished from the matrix by the presence of etched circular areas at the site of the phase-separated droplets (Barry, Clinton Wilson, 1979).

Figure 5.15 More detail than seen in Fig. 5.14 is obtained in a scanning electron image. The reacted glass particles are covered by a distinct reaction layer of silica gel (Barry, Clinton Wilson, 1979).

The latter interpretation of data is more in accord with the recent Al and Si NMR findings of Ellison Warrens (1987), who found that the structure of an appreciable fraction of the glass changed under acid attack with some loss of aluminium including all in fivefold coordination (see Section 5.9.2). Thus, acid attack was not entirely confined to the surface layer of a glass particle. If this is so then silicic acid as well as ions must migrate from the body of the particle and it is reasonable to suppose that silicic acid deposits as siliceous gel at the particle-matrix interface. 

Crisp, Lewis Wilson (1980) found that these same three species were released, in greater amounts but in roughly the same proportions, under acid attack. The association of silica with fluoride suggests, perhaps, that it is principally the glass particles that are attacked rather than the matrix... 

Crisp, Lewis Wilson (1980) made a chemical study of the erosion of a glass polyalkenoate cement under acid attack. They found that the chief species eluted were sodium and fluoride ions and silicic acid suggesting that attack occurred mainly on the glass particles rather than on the matrix. 

The nature of the setting reaction was finally elucidated by Wilson et al. (1970a), who established that formation of an aluminium phosphate gel was responsible although siliceous gel was also formed it merely coated the partly reacted glass particles. 

In the subsequent hardening phase, precipitation and hydration continue. The set cement consists, essentially, of partly-reacted glass particles embedded in an aluminium phosphate gel. The morphology of the filler particles is one where a glass core is sheathed by silica gel. 

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