Particle Number Stability

In continuous polymerizations, the main problem is the dynamic stability of the reactor. The stability problems have various different aspects the thermal stability as introduced above (see Section 11.2.4), the concentration stability, the particle number stability, and the viscosity stability. Even under isothermal conditions these problems may lead to multiplicity or oscillatory behavior. It is worth emphasizing the fact that stability and safety are in no case synonymous a reactor may be unsafe even if working at a stable working point, or conversely it may be run safely at an instable working point. But knowledge of stability limits of the reactor is essential for the design of a safe process. 

In a continuous stirred tank reactor (CSTR) where a homogeneous polymerization is being performed, the mass balance or the performance equation can be written in a very simple way [Eq. (33)]. 

The particle size stability is a special form of the dynamic concentration stability [52]. If the emulsifier is present at a concentration above the critical concentration, new particles will be created. Thus the area growth rate, and therefore the emulsifier consumption, are increased. Then the emulsifier supply by the feed may be in- 

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Surfactants increase particle number and decrease particle size as their concentration in the initial reaction charge is increased. However, one can use delayed addition of surfactant after nucleation is complete to improve particle stability, without affecting the particle number, size, and size distribution. 

Emulsifier is not a necessary component for emulsion polymerization if ihe following conditions are satisfied The particles are formed by homogeneous nucleation mechanism, and the particles are stabilized by factor(s) olher than emulsifier. As to the latter, the sulfate end group that is the residue of persulfate initiator serves for stabilization of dispersion via interparticle electrorepulsive force (20). When the stabilization mechanism works well, a small number of particles grow during polymerization without aggregation, keeping the size distribution narrow. Finally stable, monodisperse, anionic particles are obtained. 

Buonanno G, Morawska L, Stabile L, Viola A (2010) Exposure to particle number, surface area and PM concentrations in pizzerias. Atmos Environ 44 3963-3969... 

Batch miniemulsion polymerization of MMA using PMMA as the costabilizer was carried out with SLS as the surfactant and KPS as the initiator. Solids content was kept at -30%. A low surfactant level was used with the miniemulsions to ensure droplet nucleation. The initiator concentration of the polymer-stabilized miniemulsion polymerizations was varied from 0.0005 to 0.02 Mjq, based on the total water content. An aqueous phase retarder, (sodium nitrite) or an oil-phase inhibitor (diphenylpicrylhydrazol [DPPH]), was added to both the miniemulsions and the macro emulsions prior to initiation. Particle numbers and rates of polymerization for both systems were determined. 

An enhanced robustness can benefit a process in a number of ways. Since the polymer-stabilized miniemulsions are less susceptible to disturbances, their polymerization is less hkely to be affected by operator error, fluctuations in feed stream concentrations and residual contaminants in the reaction vessel. Many monomers contain species that can act as inhibitors or retarders as a result of monomer production, storage, or processing. These contaminants also cause batch-to-batch variability in particle number in macroemulsions. Therefore, miniemulsion polymerization may be an alternative to seeded polymerization as a way of maintaining robust control of particle number. 

Latex stability will be determined by the combined effect of two factors the probability of collision between particles and the fraction of the encounters between particles which lead to permanent contact. Tha first factor, the collision frequency, will increase with increasing particle size and particle number. It will also increase with increasing shear rate. The influence of various test conditions on the second factor ought to be discussed on the basis of the DLVO theory of colloid stability. 

The nudeation stage constitutes the so-called Interval I in an emulsion polymerization, the initial perind in which the particle number is changing. In Intervals II and III the paftide number is believed to he essentially constant. Nudeation of new particles may in some cases also take place during Intervals II and III. This phenomenon is often referred to as secondary nudeation and may be encountered in systems with poor stability (coagulation) or with changing composition (continuous and semi-continuous polymerizations). The present chapter will attempt to treat all mechanisms that may lead to formation of polymer partides, in whatever stage of the polymerization they take place. 

Light scattering coulter counter Particle size distribution, volume average particle diameter, number average particle diameter Stability of packed bed, hydrodynamic column properties, column performance... 

At the end of the precipitation reaction, the solid particles must be colloidally stable if a uniform particle-size distribution is to be observed. A question important to final uniformity is the particle size when this stability is achieved. The particles will always feel the long-range van der Waals attractive interactions. Interactions of an electrostatic or solvation origin can give rise to a repulsive barrier that can provide kinetic stabilization. At the end of the reaction, particles precipitated from TEOS and titanium alkoxides have final particle number densities, N , of 1016—1018 m-3. These particles are suspended in a solvent with an ionic strength of approximately 10-4 M and have surface potentials of 10-35 mV. Our studies indicate that the particles also feel a short-range repulsive interaction that we have modeled as a solvation interaction with decay... 

Figure 19. Three-dimensional film thickness—stability diagram of stratifying horizontal microscopic film in the presence of 19-nm silica particles number of particle layers versus particle concentration and film diameter.

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