Coalescence of particles

From the data listed in Table 13.5 it can be seen that the Sauter mean diameter of the dried product, di2, is larger than that of the wet precipitate obtained under the same reaction conditions by about 10%, or by 0.15 pm. An obvious fact is that no matter whether at the bottom of the dryer or in the cyclone or in the bag filter, the recovery of the finer particles must be lower than that of the larger particles. These differences between the recoveries of particles of different sizes must lead to an increased mean diameter of the product. If this fact is taken into account, the sizes of the particles can be considered to be stable enough during the final treatment of the precipitate, without coalescence of particles occurring. 

The results of the spray drying experiment of the wet precipitate show that the particle sizes of particles in the product produced in the present investigation are stable, and no coalescence of particles during the final treatment of the reacted wet-precipitate is observed. 

Products comprising hydrophilic polymers dissolved in water are well-known and used widely as adhesives but are of little general significance for bonding plastics. The present chapter is concerned only with products based on polymer dispersions, which consist of small discrete particles of diameter about one micron (1 pm, or 10-3 mm) suspended in a continuous water phase. In most instances a protective colloid is present at the interface between the particles of polymer and the water and this helps to stabilize the dispersion and prevent premature coalescence of particles. Dispersions such as these are known as oil-in-water types. With them, the molar mass of the polymer species comprising the dispersed particles does not affect the viscosity and so polymers of high molecular weight can be applied in this way. 

There are difficulties involving coalescence of particles, however, so a variety of additives (protective colloids, etc,) are used to stabilize the droplets. The beads or monomer droplets in such a suspension polymerization (sometimes also called bead or pearl polymerization) are usually abont 5 mm in diameter and require the mechanical energy of stirring to maintain their integrity. If the stirring is stopped, a gross phase separation into two layers occurs. 

An aging treatment results in a partial coalescence of particles and a strengthening of the network occurs. At the neck joining the particles there is a negative radius of curvature. Thus, local solubility at the neck is less than near the particle surface. Therefore, transport and deposition of silica occur preferentially to the neck region and neck thickening results. This results in a strengthening of the particulate network, Fig.7.5 (Zarzycki etal, 1982). 

Prevention of the coalescence of the sticky, partially polymerized particles is a major problem in suspension polymerizations, and proper selection of stabilizing agents is important. Two kinds of additives are used to hinder coalescence of particles in suspension polymerizations. These are platelet-like mineral particles that concentrate at the organic-water interface, like Ca3(P04)2, and/or macromolecu-lar species that are soluble in water and insoluble in the particular organic phase. Poly(vinyl alcohol) and starch products are examples of the latter type. 

The differences in the polymerization kinetics and colloidal behavior of polymerization systems based on monomers of different polarity may be illustrated (Bakaeva et al., 1966 Yeliseyeva and Bakaeva, 1968) by the polymerization of the model monomers, methyl acrylate and butyl methacrylate, at various concentrations of sodium alkylsulfonate (C,5H3 S03Na). The fact that the solubility of the monomers in water differs by two orders of magnitude (5.2 and 0,08/ , respectively) was used as a criterion of polarity. An additional advantage to comparing these two monomers is that their polymers have rather close glass transition temperatures which is important for coalescence of particles at later stages of polymerization. 

Polymer latex particles play a major role in coatings and paint industry. The size distributions in multicomponent formulations as well as the drying of paints and the coalescence of particles into a continuous protective film are topics that have been frequently investigated by AFM approaches. AFM provides direct access to the visualization down to the individual particle level and, as discussed in Sect. 4.3 in Chap. 4, to the assessment of the mechanical properties. 

Most practitioners deflne the flow behavior of polymers based on the melt flow index however, this property is not entirely relevant to the rotational molding process because it is essentially a shear-free and pressure-free process. The use of zero-shear viscosity has been proposed as a better way to assess the coalescence behavior of resins. Resins with lower zero-shear viscosity coalesce at a faster rate and can thus be processed using a shorter molding cycle.The coalescence of individual powder particles is initiated as the particles stick and melt onto the mold surface or melt front. As the melt deposition process continues, pockets of air remain trapped between partially fused particles and lead to the formation of bubbles. In the rotational molding process, the coalescence of particles occurs at a temperature range close to the melting point of the material thus, from a processing standpoint, low values of zero-shear viscosity at low temperatures (i.e., close to the temperature at which the particles adhere to the mold surface) are preferable. 

As a result of the mechanical action of mixing tools, turbulent or high intensity mixers do create fast moving, aerated, particulate matter systems. Therefore, interparticle collision and coalescence take place in a very similar fashion to that in suspended solids agglomerators. The main difference between the two methods is that in mixers particle movement is caused by mechanical forces while in suspended solids agglomerators drag forces induced by a flow of gas are the principal reason for movement of the bed of particulate matter, coalescence of particles, and agglomeration. 

Intuitively, bubble coalescence is related to bubble collisions. The collisions are caused by the existence of spatial velocity difference among the particles themselves. However, not all collisions necessarily lead to coalescence. Thus modeling bubble coalescence on these scales means modeling of bubble collision and coalescence probability (efficiency) mechanisms. The pioneering work on coalescence of particles to form successively larger particles was carried out by Smoluchowski [109, 110]. 

Figure 5.3 Simplified 2D-representation of (c) Gel shrinkage during drying, (d-f) Further the sol-gel process, (a) Colloidal particles shrinkage during sintering accompanied by with diameters around 4nm form a gel net- (complete) pore closure. (Adapted from Her work, (b) Coalescence of particles into chains. (1986).)...

Microscopic flaws oxmicroporosity axQ voids that are created by poor coalescence of particles. 

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