Exothermic crystallization process

Chlorinated polymers/Copolyester-aniides Recent studies (5) of blends of chlorinated polyeAylenes with caprolactam(LA)-caprolactone(LO) copolymers have been able to establish a correlation between miscibiUty and chemical structure within the framework of a binary interaction model. In some of the blends, both components have the ability to crystallize. When one or both of the components can crystallize, the situation becomes rather more complicated. Miscible, cystallizable blends may also undergo segregation as a result of the crystallization with the formation of two separate amorphous phases. Accordingly, it is preferable to investigate thermal properties of vitrified blends. Subsequent thermal analysis also produces exothermic crystallization processes that can obscure transitions and interfere with determination of phase behavior. In these instances T-m.d.s.c has the ability to separate the individual processes and establish phase behavior. 

Measurements of thermal analysis are conducted for the purpose of evaluating the physical and chemical changes that may take place in a heated sample. This requires that the operator interpret the observed events in a thermogram in terms of plausible reaction processes. The reactions normally monitored can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

Exothermic events, such as crystallization processes (or recrystallization processes) are characterized by their enthalpies of crystallization (AHc). This is depicted as the integrated area bounded by the interpolated baseline and the intersections with the curve. The onset is calculated as the intersection between the baseline and a tangent line drawn on the front slope of the curve. Endothermic events, such as the melting transition in Fig. 4.9, are characterized by their enthalpies of fusion (AHj), and are integrated in a similar manner as an exothermic event. The result is expressed as an enthalpy value (AH) with units of J/g and is the physical expression of the crystal lattice energy needed to break down the unit cell forming the crystal. 

The monazite sand is heated with sulfuric acid at about 120 to 170°C. An exothermic reaction ensues raising the temperature to above 200°C. Samarium and other rare earths are converted to their water-soluble sulfates. The residue is extracted with water and the solution is treated with sodium pyrophosphate to precipitate thorium. After removing thorium, the solution is treated with sodium sulfate to precipitate rare earths as their double sulfates, that is, rare earth sulfates-sodium sulfate. The double sulfates are heated with sodium hydroxide to convert them into rare earth hydroxides. The hydroxides are treated with hydrochloric or nitric acid to solubihze all rare earths except cerium. The insoluble cerium(IV) hydroxide is filtered. Lanthanum and other rare earths are then separated by fractional crystallization after converting them to double salts with ammonium or magnesium nitrate. The samarium—europium fraction is converted to acetates and reduced with sodium amalgam to low valence states. The reduced metals are extracted with dilute acid. As mentioned above, this fractional crystallization process is very tedious, time-consuming, and currently rare earths are separated by relatively easier methods based on ion exchange and solvent extraction. 

Also, after determining the temporal distribution of crystallization peaks in a complete isothermal experiment, the melting thermogram of a particular polymorph can be determined. First, the sample is remelted completely, and held to erase its thermal memory. Then, the isothermal crystallization process is repeated, but only up to the time at which the polymorph of interest has completely solidified (exothermic peak fully formed). A heating scan is then performed immediately (Kawamura, 1980, 1981). 

The crystallization process is a highly exothermic transformation (or glow phenomenon ) that normally occurs at remarkably low temperatures (350-400°C) this... 

For DSC measurements, the area under an exothermal curve corresponds to the heat of crystallization, AH. The fraction of crystals / at any time t during the crystallization process is given by Equation 1 ... 

The DTA measurements reveal a further - somewhat smeared - exothermic peak in the temperature range from 1390 to 1510 °C. Obviously, this peak arises from processes of re-crystallization. By XRD a drastic increase of the crystallite sizes is observable in the same temperature region. Thus, when the ACC is consumed by the crystallization process, a new mechanism starts to be responsible for the development of the microstructure of the final SiC ceramics. A further increase of the crystallite size becomes possible only by recrystallization, i.e. Ostwald ripening. The activation energy for such processes determined by the Kissinger equation in the way mentioned above was found to be significantly lower Ea, recr = 581 20 KJ moP. ... 

Polymers without flexible spacer groups. The DSC curves of t he polymer 3 indicated the existence of a liquid crystalline glassy state at room temperature. The polymer was found to be smectic. Two melting peaks were observed in the temperature range between 300 and 310 deg. C. These peaks are not as easily observed as in the case of the polymers discussed above, since the decomposition takes place in the same temperature range. The occurrence of exothermic peaks on cooling nevertheless indicates that reversible melting and crystallization processes take place. 

The crystallization exothermic trace is directly measurable by DSC method when the crystallization temperature chosen is between 250°C and 258°C, which is several degrees higher than the onset temperature. By following Avrami treatment, the relative crystallinity versus time is calculated. Then, the Avrami parameter n is obtained from log (—In (1 — 0)) versus logC curves as shown in Figure 3.13. The average n value is around 2.6, which is much higher than the n value for the crystallization process observed at 260 and 262°C. 

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