Rheokinetic changes

It should be emphasized that in all these cases, combined or superimposed phenomena must be dealt with, viz. for stage IV, fluiddynamics, kinetics of polymerization, and rheokinetic changes caused by chemical reactions for stage V, polymerization kinetics, crystallization kinetics and heat transfer effects a thermomechanical problem in combination with crystallization kinetics. Construction of a mathematical model requires simultaneous solution of a set of equations in order to describe these related phenomena. 

The main rheokinetic problem consists of the determination of the time dependence of the degree of conversion (3(t) and the correlation between (3(t) and the changes in the rheological properties of the reactants. One possible kinetic function (3(t) was discussed in Section 2.2. In considering the rheological properties of the reactive medium, we can neglect non-Newtonian... 

The manufacture of products by reactive molding results in the superposition of interrelated chemical and physical phenomena. These include polymerization, crystallization, vitrification, heat transfer, rheokinetic effects, changes in the physical properties and volume of a material injected into a mold. It is quite natural that special experimental methods are required to study and control the complex processes which take place in molds. 

Figure 4.29. Comparison of velocity profiles (a), degrees of conversion (b) and MWD-H (c) for calculations carried out for a rheokinetic model with changes in of the velocity profile along the reactor (solid lines) and for an unchanging Poiseuille velocity profile (dashed lines). Values of the parameters in calculations Da/Da = 0.2 [A] = 0.7 mol%.

Successful solution of these technical problems depends on the initial viscosity of the material in the sprayer, the surface tension of the material, changes in the rheokinetic properties during the process, sprayer design, and jet configuration. 

Industrial equipment use for reactive processing is limited by the permissible injection pressure. The model being discussed can also be used to estimate the pressure drop necessary to fill the mold. The pressure changes during the filling process, and the time dependence of the pressure is determined by the rheokinetic properties of the reactive mixture and the dimensions... 

When the time changes are characterized by rheological methods the apparent autocatalytic shape of (t) may be generated by a non-linear dependence of the rheokinetic parameter on / . For instance, it can be shown that, if p is the storage modulus G, G is proportional to the concentration of elastically active... 

Rheokinetic analysis shows that rheokinetic equations su ested to describe changes in viscosity and elastic modulus reflect the characteristics of this process. Therefore, the nature of the viscosity increase up to the gel point is defined not only by a variation of the molecular structure of the material but sometimes also by the effect of formation of a new phase (microgel) from particles of a cured product. 

It should be noted that all investigations of flow stability of polymerizing liquids are few in number and have been carried out up till now only for unidimensional problems. The problem of stability of steady rheokinetic two-dimensional flows to local hydrodynamic perturbations has not been discussed in the literature yet. Obviously the problem can be solved (the solution is difficult from the technical point of view), for example, by numerical methods solving the problem on unsteady development of the flow of polymerizing mass directly after a forced local change of the profile of the flow velocity. 

To optimize the pre-reaction a detailed understanding of the polymerization kinetics and resulting changes of material properties is necessary. As it is mentioned before, the chain growing will increase the viscosity of the polymer. A novel approach to combine the rheology with kinetics is called rheokinetics and is the subject of discussion in the following second chapter. 

The influence of viscosity on atomization of polymer solutions is extensively discussed in the previous chapter. But the atomization of reactive solutions means changing molecular weights, polymer concentrations, and changes in the viscosity as well. To investigate the kinetics of viscosity during polymerization, the rheokinetics is applied. 

The theoretical optimizing of this pre-reaction nozzle is done with the combination of the reaction kinetics and the rheokinetics, discussed in the previous chapters. The material balance over the nozzle is shown in (20.39). It calculates the change of conversion X of the monomer within the nozzle depended on the dimensionless length z/L and the overall reaction rate R, compare (20.22). 

But it is also reconunended to do further investigations on the rheokinetics. Different monomers or solvents have to be analyzed in detail. Another interesting investigation is the analysis of the changing viscosity during drying. Additional information can support drying models of drops that consist of suspensions or emulsions. 

The change of rheological properties of reaction systems is determined by two main factors kinetic features and rheological properties of initial and forming components. To establish the conditions of gel formation during reaction, both the kinetics of reaction and the rheokinetics were studied [119,319]. Simultaneous semi-IPNs produced from cross-linked PU... 

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