Medium viscosity

The problems of monomer recovery, reaction medium viscosity, and control of reaction heat are effectively dealt with by the process design of Montedison Fibre (53). This process produces polymer of exceptionally high density, so although the polymer is stiU swollen with monomer, the medium viscosity remains low because the amount of monomer absorbed in the porous areas of the polymer particles is greatly reduced. The process is carried out in a CSTR with a residence time, such that the product k jd x. Q is greater than or equal to 1. is the initiator decomposition rate constant. This condition controls the autocatalytic nature of the reaction because the catalyst and residence time combination assures that the catalyst is almost totally expended in the reactor. 

The flow properties of sodium alginate solutions depend on concentration. A 2.5% medium viscosity sodium alginate solution is pseudoplastic, especially at the higher shear rates in the range of 10—10,000/s. 

Starting cellulose, prepared by deacetylation of commercial, medium viscosity cellulose acetate (40.4% acetyl content). 

The idea of adding smaller and smaller particles to fill in the interstices left by the larger particles can be continued. The viscosity of a multimodal suspension may be predicted from unimodal data based on the premise that the viscosity of the mixture of smaller fractions is the medium viscosity for the next largest fraction. This is an effective medium theory and basically assumes that the smaller particles act as a medium toward the larger particles. This was assuming at least an order of magnitude difference in size between successive fractions [26]. Thus, the viscosity of the ith component is ... 

According to cq. 1, the term/should take into account all side reactions that lead to loss of initiator or initiator-derived radicals. These include cage reaction of the initiator-derived radicals (3.2.8), primary radical termination (3.2.9) and transfer to initiator (3.2.10). The relative importance of these processes depends on monomer concentration, medium viscosity and many other factors. Thus/is not a constant and typically decreases with conversion (see 3.3.1.1.3 and 3.3.2.1.3). 

Thus, the size and the reactivity of the initiator-derived radicals and the medium viscosity (or microviscosity) are important factors in determining the initiator efficiency. Thus, the extent of the cage reaction is likely to increase with... 

In a concentrated solution, characterized by an effective medium viscosity r e "Hs, the hydrodynamic field decays much faster due to the shielding effect of the encountered polymer segments ... 

According to the transition state theory, the pre-exponential factor A is related to the frequency at which the reactants arrange into an adequate configuration for reaction to occur. For an homolytic bond scission, A is the vibrational frequency of the reacting bond along the reaction coordinates, which is of the order of 1013 to 1014 s 1. In reaction theory, this frequency is diffusion dependent, and therefore, should be inversely proportional to the medium viscosity. Also, since the applied stress deforms the valence geometry and changes the force constants, it is expected... 

From the Arrhenius form of Eq. (70) it is intuitively expected that the rate constant for chain scission kc should increase exponentially with the temperature as with any thermal activation process. It is practically impossible to change the experimental temperature without affecting at the same time the medium viscosity. The measured scission rate is necessarily the result of these two combined effects to single out the role of temperature, kc must be corrected for the variation in solvent viscosity according to some known relationship, established either empirically or theoretically. 

From the weak dependence of ef on the surrounding medium viscosity, it was proposed that the activation energy for bond scission proceeds from the intramolecular friction between polymer segments rather than from the polymer-solvent interactions. Instead of the bulk viscosity, the rate of chain scission is now related to the internal viscosity of the molecular coil which is strain rate dependent and could reach a much higher value than r s during a fast transient deformation (Eqs. 17 and 18). This representation is similar to the large loops internal viscosity model proposed by de Gennes [38]. It fails, however, to predict the independence of the scission yield on solvent quality (if this proves to be correct). 

Many high-pressure reactions are done neat, but if a solvent is used, the influence of pressure on that solvent is important. The melting point generally increases at elevated pressures, which influences the viscosity of the medium (viscosity of liquids increases approximately two times per kilobar increase in pressure). Controlling the rate of diffusion of reactants in the medium is also important. In most reactions, pressure is applied (5-20kbar) at room temperature, and then the temperature is increased until reaction takes place. 

Evaporator type Very viscous > 1000 Medium viscosity < 1000 max Low viscosity < 100 Foaming Scaling or fouling Crystals produced Solids in suspension ... 

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