Interphase viscosity

The material parameter 0 in Eq. 7.124 governs the NDB behavior. It was shown that its value is inversely proportional to the thickness of the interphase, Al, and its viscosity, rii terphase (Bousmina et al. 1999). Theoretically, the same molecular mechanism should be responsible for both factors, viz., better miscibility, better interdiffusion, thus higher Al and tiimerphase- However, the low molecular weight components of the blend, that are forced by the thermodynamics to diffuse to the interphase, may not change much the former parameter, but drastically reduce the latter. For immiscible blends, Al is small, typically 2-6 nm. Thus 0 is large, and interlayer slip takes place. For compatibUized blends, the macromolecules of the two phases interact and interlace, which increases both factors thus, the slip effects are negligible. Measured or calculated values of the interphase viscosity are listed in Table 7.10. 

Static mixing of immiscible Hquids can provide exceUent enhancement of the interphase area for increasing mass-transfer rate. The drop size distribution is relatively narrow compared to agitated tanks. Three forces are known to influence the formation of drops in a static mixer shear stress, surface tension, and viscous stress in the dispersed phase. Dimensional analysis shows that the drop size of the dispersed phase is controUed by the Weber number. The average drop size, in a Kenics mixer is a function of Weber number We = df /a, and the ratio of dispersed to continuous-phase viscosities (Eig. 32). 

Rheological methods of measuring the interphase thickness have become very popular in science [50, 62-71]. Usually they use the viscosity versus concentration relationships in the form proposed by Einstein for the purpose [62-66], The factor K0 in Einstein s equation typical of particles of a given shape is evaluated from measurements of dispersion of the filler in question in a low-molecular liquid [61, 62], e.g., in transformer oil [61], Then the viscosity of a suspension of the same filler in a polymer melt or solution is determined, the value of Keff is obtained, and the adsorbed layer thickness is calculated by this formula [61,63,64] ... 

Topographical features comprise a significant portion of the interphase. In general, the epoxy matrix will conform to the topographical features of the substrate down to the molecular dimensions of the resin molecule. Since most epoxies are applied as a liquid of moderate to low viscosity, intimate contact between epoxy and substrate is achieved. Two aspects of the topographical features of the substrate must be considered as to their effect on the interphase structure of the epoxy. 

Similar dependences for describing the viscosity of emulsions on the basis of the volume averaging method were derived by Mellema and Willemse [60]. They took into account the contribution to the effective viscosity of the interphase tension and obtained a general expression, different from (52) ... 

LIQUID RESISTANCE TO INTERPHASE MASS TRANSFER. Liquid viscosity, gas solubility in absorbers, and relative volatility in rectification columns are important factors in determining the liquid resistance to interphase mass transfer. Increase in liquid viscosity, decrease in gas solubility for absorbers, and increase in relative volatility for rectification columns cause an increase in the liquid resistance to interphase mass transfer and a resultant reduction in plate efficiency. The ratio of the liquid rate to the gas rate influences the relative importance of the liquid resistance to interphase mass transfer. An increase in the ratio of liquid rate to gas rate reduces the importance of the liquid resistance and can cause an increase in the plate efficiency. 

Reducing the bed length while keeping the space velocity the same will reduce the fluid velocity proportionally. This will affect the fluid dynamics and its related aspects such as pressure drop, hold-ups in case of multiphase flow, interphase mass and heat transfer and dispersion. Table II shows the large variation in fluid velocity and Reynolds number in reactors of different size. The dimensionless Reynolds number (Re = u dp p /rj, where u is the superficial fluid velocity, dp the particle diameter, p the fluid density and t] the dynamic viscosity) generally characterizes the hydrodynamic situation. 

The rate of interphase mass transfer is affected by the physical and chemical characteristics of the system and the mechanical features of the equipment. The former include viscosities and densities of the phases, interfacial surface properties, diffusion coefficients, and chemical reaction coefficients. The latter include, for example, the type and diameter of the impeller, vessel geometry, the flow rate of each phase, and the rotational speed of the impeller. 

Han and Kim (14) have observed a similar phenomenon in a completely different system consisting of polypropylene blended with polystyrene. The conclusion of that work was that the polystyrene and polypropylene have no interaction at the interface and do not form an interphase. Han and coworkers (14) have studied other systems which display minima in the viscosity versus composition graphs and have concluded that there is little chemical interaction between the two phases. 

Investigations of polymer blends has developed an increased understanding of interphase organization. In blends two interfaces exists the interface between two matrix types and distribution of filler and its interfaces with this matrices. The interphase of carbon black in blends of natural rubber and EPDM depends on the character of carbon black (surface groups available for interaction), the viscosity,... 

Shang et al. (1995) show that the work of adhesion between a silica filler surface and a polymer matrix is directly related to the dynamic viscosity and moduli. Additionally, at lower frequencies there is a greater influence of the work of adhesion. The influence is shown to be described well by an effective increase in interphase thickness due to the increase in the work of adhesion, such that polymer chains are effectively immobilized around the filler, and the friction between the immobilized layer and the polymer then governs the dynamic rheology. It was noted that the immobilized layer could be reduced in extent at higher frequencies. 

---