Multi-voided particle

Abis and co-workers [32,33] reported on a multi-technique characterization of immiscible blends between sPS and several polyolefins, prepared in a mixer at 285 °C and thermo-compressed at the same temperature. SEM performed on the cryo-fractured surface of the blends shows a gross phase separation, while the presence of voids due to the easy pull-off of particles from cryo-fractured surfaces points to a lack of adhesion between components (Figure 20.4a). The particles of polyolefins present in 80 20 wt% blends have dimensions decreasing in the following order PP LLDPE > HDPE > EPR > PIB. 

PMMA particles with hollow structures were synthesised by water-in-oil-in-water emulsion polymerisation. Sorbitan monooleate was used as a primary surfactant and sodium lauryl sulphate and Glucopen (a polypeptide derivative) were used as secondary surfactants. Urethane acrylate, with a hard segment in the molecular backbone, a long soft segment in the middle and vinyl groups at both ends was used as a reactive viscosity enhancer. Only a few particles contained a void in the polymer phase at low concentrations of urethane acrylate, but as the concentration of urethane acrylate increased, so did the number of particles containing the void. This was because urethane acrylate increased the viscosity of the monomer mixture and helped to form the stable emulsion droplets. At concentrations of urethane acrylate above 7 wt%, multi-hollow structured particles were produced. The mechanism of formation of the hollow particles was discussed. 7 refs. 

This is based on the same concept as the free surface model Ro represents the outer radius of the concentric shell within which one particle is responsible. Ro is related to the void fraction defined from the multi-particle arrangement. 

For the mass transfer coefficient in a stagnant fluid system, 5A,ug = 2 is used in the case of mass transfer from a single particle. In multi-particle systems such as packed beds, however, Shmg takes a different value for the reason noted in Chapter S, and is given as a function of void fraction in the packed bed, c, as shown in Fig. 5.11 (Suzuki,... 

Kinetics models of gas-solid non-catalytic reaction include uniform conversion model (UCN), multiple fine particle model (GPM), crack core model (CCM), phase-change model (PCM), change void model (CVM), thermal decomposition model (TDM), shrinking core model with multi-step reactions, and multi-step reaction model of formation porous structure in reaction etc. Among these models, the shrinking core model (SCM) is the most important and most widely used. For conversion of solid it is also the most simple and practical model. Commonly it is suitable for experimental data. However, it can only be used in some reactions of many solid reactions. A more complex model must be used in other cases. 

Core-shell emulsion polymerisation. VanderholF et al. (1991) prepared particles consisting of a core of a copolymer of methacryUc acid and methyl methacrylate and a shell of crosslinked material. After neutralisation with NH4OH, the core material collapses and the particles contain voids of between 130 and 760 run. A similar approach was applied by Okubo and Ichikawa (1994) where the particles were produced by an emulsion-free terpolymerisation of styrene, butyl acrylate and methacrylic acid. The effect of pH, temperature and time of acid treatment on the multi-hoUow structure formed were investigated. 

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