Melt-down, crystals

The air cell stabilizing effect of agglomerated fat globules, promoted by emulsifiers and the ice-crystal-growth-controlling effect of hydrocoiloid stabilize the foam structure of ice cream to a great extent. This is evident by melt down analysis (see section 5.2) of ice cream exposed to heat shock. 

When the bulk of the crystals of 68 (Table 13) in a test tube was irradiated at 15°C under an argon atmosphere, intramolecular [2 + 2] cyclization proceeded effectively without melting down, and two diastereomeric oxetanes, 69 and 70, were obtained in 95 and 5% yield, respectively. The enantiomeric purity of the main product 69 was determined as > 99%. When 68 was irradiated after dissolving in THF at various temperatures, optically active oxetanes were isolated below — 20°C, whereas the racemic oxetanes were naturally obtained from the photolysis above 0°C in THF. The photolysis in THF at — 60°C gave 87% ee of 69 and 62% ee of 70, in 76 and 24% chemical yields, respectively. The memory of... 

Recent calculations of the authors (37) have shown that about 16 displacements will be produced in the slowing down process in molecular crystals, and that the volume raised above the melting temperature includes about 4000 atoms. A similar volume for the hot zone can be obtained from a consideration of the latent heat of fusion. For a recoil energy of 300 ev, and a bond energy of 4 ev, about 240 ev will be left for melting the crystal. Taking a latent heat of fusion of 0.1 ev, this corresponds to a final molten zone of about 2400 molecules. 

Discuss physical properties of isolated product, e.g., compressibility of crystals, melt-down in dryer. 

Considerable supercoolings are realized in small liquid drops. Water drops from 500 to 20 pm in diameter in oil were located on the junction of a differential thermocouple. Every drop was melted down and crystallized several tens of times. Measurements at the same temperature were made on 5-10 drops similar in size. The distribution of crystallization events of isolated drops was studied in repeated experiments under isothermal conditions and continuous supercooling. ... 

The crystals have to be separated from the remaining melt (mother liquor) to achieve the intended purification. In case of layer crystallization, this is done by draining the remaining melt, collecting it separately, and melting down the crystal layer afterward (Fig. 8.2-11). In case of suspension crystallization the solid-liquid separation is done either by conventional filtration or by a sedimentation apparatus, with or without support of centrifugal forces. Another device repeatedly discussed in the context of solid-liquid separation is the wash column (Arkenbout 1995). 

Figure 7.12 SEM image of products of ferrosilicon combustion in nitrogen, (a) Solidified drop of iron-silicon melt with crystals of silicon nitride and (b) upper half of the same drop with "flowing down" crystals of silicon nitride forming a large crystal (in the background).

Different procedures are used for co-crystallization or stereo-complexation in bulk from melt (l) crystallization at a fixed temperature directly from the melt (2) crystallization at a fixed temperature quenching from the melt (3) crystallization during cooling down from the melt and (4) crystallization during heating of a quenched melt/ °... 

Figure 2.43. A full DSC curve of quenched (amorphous) poly(ethylene terephthalate) recorded at a heating rate of 10°C/min the glass transition (at 80°C) is followed by cold crystallization and melting of crystals formed in nonisothermal cold crystallization. The baseline during the transition was drawn to show that the crystallization process lasts to the beginning of melting. Endotherm is down. (Menczel, tmpnbhshed results.)...

The plot in Fig. 2.11 shows the actual crystal size measurements for a variety of temperatures. The plots show a constant growth rate for every temperature. The actual data are listed in the bottom table of Fig. 2.11. The linear increase in crystal size observed in the present example is lost as soon as transport processes become rate-determining. Transport control is observed when crystal growth is so fast that for solution crystallization, as an example, the solvent can not diffuse away from the crystal surface fast enough to maintain constant concentration. In the case of melts, the heat of crystallization may build up on fast crystallization and raise the temperature, and thus slow down crystallization temporarily. One can spot transport control by the dendritic, snowflake-like crystal morphology it produces. 

Crystallized PET does not exhibit a glass transition any more and would not become really pliable until the crystals start softening on approaching the melt region. Hence, crystallized preform cannot be blown into bottles unless special means are used. To slow down crystallization, the polymer is usually modified into a copolymer or by additives. Copolymers are formed by using diacids, glycols, or cyclohexane dimethanol (CHDM) [8], In all cases, retardation of crystallization opens a window for molding amorphous preforms. The crystallinity level in preforms is usually less than 4%. 

The melting and crystallization behaviors were studied in a Mettler TA 3000 DSC. The non-isothermal crystallization was performed as follows heating at 20°C/min up to 200°C, 3 min. of dwelling time, cooling down at cooling rates variable from 30 to l°C/min. 

In its ultimate form, the problem of controlled continuous pulling boils down to the production of a single crystal that has a transverse cross section that is unchanging with height, and grows with a constant axial rate. (In the first approximation, we shall consider that the melt should crystallize only on the growing crystal evaporation of substances from the free surface of the melt can be... 

Method 1. Arrange the flask containing the reaction mixture for steam distillation as in Fig. II, 40, 1. Proceed with the steam distillation until crystals of p-dibromobenzene appear in the condenser. Change the receiver and continue with the distillation until all the p-dibromobenzeiie has passed over from time to time run out the water from the condenser so that the crystals melt and run down into the receiver. Reject the residue in the flask. Transfer the first distillate to a separatory funnel, wash it with a httle water, and dry the lower layer with a little anhydrous magnesium sulphate or anhydrous calcium chloride filter. Distil slowly from a small distilling flask use a wire gauze or an air bath (Fig. II, 5, 3). Collect the fraction which passes over at 150-170° pour the residue (R), while it is still hot, into a small beaker or porcelain basin for the isolation of p-dibromobenzene. Redistil the fraction of b.p. 150-170° and collect the bromobenzene at 154-157° (3). The yield is 60 g. 

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