Crystallization processes viscosity

There has been considerable interest recently in an alternative type of ABA triblock structure, where the end blocks could form crystalline domains, by crystallization, rather than amorphous domains by phase separation. It was felt that, since such a crystallization process need not depend on the incompatibility between the blocks, it should be possible to have a homogeneous melt, which should exhibit a much lower viscosity, and hence much easier processing, than the heterogeneous media of the conventional triblock copolymers. Furthermore, thermoplastic... 

On the other hand, the presence of the salt, LiPEe, assists the occurrence of supercooling by increasing the solution viscosity and by depressing the liquidus temperature. At practical concentrations of LiPFe ( 1.0 M), even the solidus temperature can be circumvented, since there is no crystallization process observed for LiPFe/EC/EMC solution down to —120 °C, while the glass transition occurs at —103 °C. In such concentrated solutions, even the presence of MCMB cannot initiate crystallization, and the supercooling is completely suppressed at the cooling rate of 10 °C/min. 

Ciystallization from solution is an important separation and purification process in a wide variety of industries. These range from basic materials such as sucrose, sodium chloride and fertilizer chemicals to pharmaceuticals, catalysts and specialty chemicals. The major purpose of crystallization processes is the production of a pure product. In practice however, a number of additional product specifications are often made. They may include such properties as the ciystd size distribution (or average size), bulk density, filterability, slurry viscosity, and dry solids flow properties. These properties depend on the crystal size distribution and crystal shape. The goal of crystallization research therefore, is to develop theories and techniques to allow control of purity, size distribution and shape of crystals. 

The limited results obtained are not inconsistent with a 1.25 dependence on supersaturation as proposed by Shiau and Berglund (10). The dependence on solvent composition and temperature appears complex and is probably related to the solution viscosity which varies considerably over this range. It thus appears that mass transfer plays a significant part in the crystallization process. Further work needs to be done to clarify these effects. 

Results for Commercial Operations The content of a-form was up to 99% and average size of the crystal was about 24-35 jum. The formation of 3-form crystal In commercial operation Induced considerable Increase of the viscosity of the suspension. The features of the semi-batch cooling crystallization process are as follows. Even if crystallization temperature is considerably lowered in order to avoid the formation of 3-form crystal, and also even if the feed solution is highly concentrated at high temperature above -SSSK, obtained crystal size is large enough to separate the solvent by centrifuge. 

This study showed that the overall crystallization processes for mor-denite, zeolite X, and zeolite A were similar. However, the physical properties of the crystallizing system determine the rate-limiting step for a particular zeolite synthesis. In the case of mordenite in which both the viscosity of the batch composition and the morphology of seed crystals were varied, it was observed that diffusion in the liquid phase was the ratedetermining step. For zeolite X the actual growth rate on the crystal-liquid interface was the rate-limiting factor as shown by identical conversion rates for the seeded and unseeded systems. For zeolite A in the system chosen, both processes influenced the conversion rate. 

Dextrans first came under close examination about a hundred years ago. The high viscosity of these slimes, as they were called, caused much trouble in the beet-sugar industries the slimes blocked filters and interfered with the crystallization process. Scheibler assigned the correct empirical formula... 

When compared with current liquids, crystallization processes in glass-forming melts are slowed down considerably by the high viscosity,which is the main reason for their ready undercooling to the glassy state. The rate of crystal growth is so low that within relatively short periods of time, glass fails to crystallize perceptibly even in cases when the melt contains nuclei or a phase boundary which may assume the function of the former. 

A further consideration with regard to the use of DSC for the measurement of 7 / is that as the temperature is lowered and the viscosity of the system increases, the crystallization process becomes kinetically hindered, leading to lower 7 g and... 

Finally, shear viscosity is strongly affected by the clay in the blends, especially at high PEN contents. A lubricating effect rather than a filler effect reveals the possibility that the clay is not well dispersed in the polymer blend, and migration of particles in the flow to the wall region can explain the observed reduction in shear viscosity. When MMT clay is mixed with crystallizable polymers such as polyesters, some processing problems arise because the crystallization process is modifled by nucleation effects induced by the nanoparticles. Moreover, these particles also influence the kinetics of transesteriflcation between PET and PEN, besides other factors such as the reaction time and extruder processing temperature. In Reference 72, a quaternary alkyl ammonium compound (Cl8) and MAH were used to modify the surface properties of the clay... 

The effect of crystallization on the measured viscosity, which is not equal to the true viscosity of the melt, depends upon the details of the crystallization process. If a sample crystallizes via growth of crystals from the surface, the sample is essentially encapsulated in a shell of crystals which prevent deformation, and the viscosity appears to... 

Although the polymer matrix does not take part in charge carrier transport which, at least in the first approximation, proceeds along the conductive paths formed by CT complex crystallites, it plays an important role in obtaining the conductive systems. The polymer used must fulfill quite specific conditions for the crystallization process to occur quickly in a relatively high viscosity medium. 

As the filaments are spun above the critical speeds, that is, above the stresses that induce crystallization, their temperature profile is also likely to change significantly. At the region of the neck where stress-induced crystallization takes place, the temperature of the filament rises due to the heat liberated by the crystallization process. This may reduce the apparent viscosity by a small amount. However, since the stress-induced crystallization locks the structure of the spun material, further deformation does not take place and the orientation of the crystalline region is not lost even at this higher temperature. However, the orientation in the amorphous region may be lost to some extent. 

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