Pressure melt crystallization

Transformations in the Solid State. From a practical standpoint, the most important soHd-state transformation of PB involves the irreversible conversion of its metastable form II developed during melt crystallization into the stable form I. This transformation is affected by the polymer molecular weight and tacticity as well as by temperature, pressure, mechanical stress, and the presence of impurities and additives (38,39). At room temperature, half-times of the transformation range between 4 and 45 h with an average half-time of 22—25 h (39). The process can be significantly accelerated by annealing articles made of PB at temperatures below 90°C, by ultrasonic or y-ray irradiation, and by utilizing various additives. Conversion of... 

Of these five methods all but pressure-swing distillation can also be used to separate low volatiUty mixtures and all but reactive distillation are discussed herein. It is also possible to combine distillation and other separation techniques such as Hquid—Hquid extraction (see Extraction, liquid-liquid), adsorption (qv), melt crystallization (qv), or pervaporation to complete the separation of azeotropic mixtures. 

A solution of 192 g of 1 -phenethyl-4-hydroxy-4-aminomethyl piperidine in BOO cc of diethyi-carbonate is heated for 1 /i hours to refiux at about B0°C in the presence of sodium methylate (prepared for immediate use from 2 g of sodium). After this time, the ethyl alcohol formed during the reaction is slowly distilled while the maximum temperature is reached. The excess ethyl carbonate is distilled under reduced pressure. A crystallized residue Is then obtained, which is stirred with 400 cc of water and 400 cc of ether. The solution is filtered and 125 g (77.6%) of practically pure product melting at 232°C to 233°C, are obtained. 

Vill. EFFECT OF MOLECULAR WEIGHT ON THE MELTING-CRYSTALLIZATION OF PE UNDER HIGH PRESSURE... 

By means of In-situ observation of crystal growth under very high pressure, a defect healing process was studied with p-xylene crystals In p-, m-xylene mixture. Crystals, partially melted by decrease In pressure, were repressurized to grow and to heal their melted surfaces. It was found that the half-melted crystals rapidly grow and recover their rectangular growth shape within a few minutes. 

A fundamental pressure procedure used in this experiment is shown In Figure 2. First, pressure was applied to the sample to crystallize some portion of it. After a few seconds, pressure was decreased rather rapidly so as to allow most of the crystals to melt, but some crystals still remained. Soon after, the pressure was Increased again to prevent further melting, and adjusted to the pressure Po, where the remaining crystals did not grow or melt further. It was maintained there for five or ten minutes. After the temperature stabilized, a slight additional supersaturation pressure P was applied. And then, the growth behaviour of the partially melted crystal surfaces was observed continuously. 

Steel-gray crystalline brittle metal hexagonal crystal system atomic volume 13.09 cc/g atom three allotropes are known namely, the a-metaUic form, a black amorphous vitreous solid known as P-arsenic, and also a yellow aUotrope. A few other allotropes may also exist but are not confirmed. Sublimes at 613°C when heated at normal atmospheric pressure melts at 817°C at 28 atm density 5.72 g/cc (P-metallic form) and 4.70 g/cm (p-amor-phous form) hardness 3.5 Mohs electrical resistivity (ohm-cm at 20°C) 33.3xlCh (B—metallic polycrystalline form) and 107 (p—amorphous form) insoluble in water. 

On a nucleation rate versus pressure diagram (Figure 4-2c), melt nucleation rate below the crystal-melt equilibrium pressure and crystal nucleation above the pressure are roughly S3mimetric. In Equation 4-9, only AG would vary with pressure or concentration. Hence, both melt nucleation rate and crystal nucleation rate increase monotonically with departure from equilibrium. There is no peak nucleation rate. 

The 2,3-dimethoxybenzaldehyde employed was a dark-colored product supplied by the Monsanto Chemical Company or Eastman Kodak Company. It was purified by distilling at reduced pressure and crystallizing from methanol, after which it was colorless and melted at 52 54°. 

The melting point of pentaerythritol corresponding with the above mentioned specification lies within 250 and 260°C (pure pentaerythritol melts at 261-262°Q. To produce it an aqueous solution of acetaldehyde is treated with formaldehyde in the presence of milk of lime, the temperature being gradually increased from 15 to 55°C. When the reaction is complete, the whole is treated with sulphuric acid sufficient to precipitate the lime as calcium sulphate. This is filtered off, and the solution concentrated under reduced pressure. The crystallized product is centrifuged and then recrystallized from water. The yield is about 62% (calculated with respect to acetaldehyde) or 51% if related to formaldehyde. 

Gogolewski, S. and Pennings, A. J. Crystallization of polyamides under elevated pressure 5 Pressure-induced crystallization from the melt and annealing of folded chain crystals of nylon-11, poly(aminoundecaneamide) under pressure, Polymer 18, 660 (1977) Stamhuis, J. E. and Pennings, A. J. Crystallization of polyamides under elevated pressure 6. Pressure-induced crystallization from the melt and annealing of folded chain crystals of nylon-12, polylaurolactam under pressure. Polymer 18, 667 (1977)... 

A major complication in the analysis of convection and segregation in melt crystal growth is the need for simultaneous calculation of the melt-crystal interface shape with the temperature, velocity, and pressure fields. For low growth rates, for which the assumption of local thermal equilibrium is valid, the shape of the solidification interface dDbI is given by the shape of the liquidus curve Tm(c) for the binary phase diagram ... 

Figure 13. Cell dimensions plotted against the lattice distortion parameter of polyethylene (40) ((O) melt crystallized, A (9) melt crystallized, B (Is) single crystal ( ) cast film high pressure crystallized, C)...

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