Recrystallization processes

From a comparison of the evolution of hardness of all samples during isochronal armealing it can be concluded that for high deformation in the disordered state and deformation in the ordered state, recovery and recrystallization is prevented up to T, in the sample deformed to 40% reduction in the disordered state recovery and recrystallization processes seem to start as soon as atomic mobility is enabled (260°C). 

Irreversible transformations, from more disordered toward more ordered modifications of a given form have been, for instance, observed for the a form of i-PP [39,40,43,44] (see Sect. 2.4), as well as for the most common form of i-PS [74], In these cases the transitions occur by recrystallization processes in the respective melting regions. 

Stale bread tastes and feels dry, even though the moisture content is actually the same as in fresh bread. The recrystallization of the starch in bread is what makes it go stale. To combat this, fats and oils are added to bread to form a complex with the starch in its gelled form, slowing down the recrystallization process and keeping the starch in its flexible gel form. 

The solution to this problem has been to isolate the lactide and to polymerize this directly using a tin(ii) 2-(ethyl)hexanoate catalyst at temperatures between 140 and 160 °C. By controlling the amounts of water and lactic acid in the polymerization reactor the molecular weight of the polymer can be controlled. Since lactic acid exists as d and L-optical isomers, three lactides are produced, d, l and meso (Scheme 6.11). The properties of the final polymer do not depend simply on the molecular weight but vary significantly with the optical ratios of the lactides used. In order to get specific polymers for medical use the crude lactide mix is extensively recrystallized, to remove the meso isomer leaving the required D, L mix. This recrystallization process results in considerable waste, with only a small fraction of the lactide produced being used in the final polymerization step. Hence PLA has been too costly to use as a commodity polymer. 

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 simultaneous presence of the CaC03-reflections and of the CaO-reflections over a wide temperature range, and the persistence of certain calcite reflections point to topotaxy. This behaviour was generally observed during decomposition and recrystallization processes. The growth of the CaO crystallites with increasing temperature can be also seen in Fig. 61 (decrease of line broadening in the diffraction pattern). 

With regards to the conversion coated substrate, Wittel (25) observed that at temperatures greater than 140°C, tetrahydrate zinc phosphates lose part of their water of hydration. It is likely that the water of hydration liberated in the phosphate recrystallization process has a negative effect on the adhesion of the polymer matrix to the B40 panel. 

Rocks that contain talc, or the chemically and structurally similar minerals mentioned previously, are usually the result of alteration and recrystallization of rock formations that contained magnesian silicate minerals. Steati-zation or serpentinization are the terms given the processes that create layered (sheet) silicates from chain or other tetrahedral arrangements adopted by silicate minerals (see Fig. 2.1). The recrystallization process is expedited by temperature and pressure, and especially through the action of hydrothermal solutions. 

Siekmann B. and Westesen K., Thermoanalysis of the recrystallization process of melt-homogenized glyceride nanoparticles, Colloids Surf. B, 3, 159, 1994. 

SNPE, France, produced reduced sensitivity RDX (RS-RDX) by the Woolwich synthesis by employing a proprietary recrystallization process. This RS-RDX displayed reduced sensitivity to shock initiation. Subsequently, some other manufacturers also claimed to produce some form of RDX that exhibits reduced sensitivity to shock compared with the conventional RDX produced by the Bachmann process. EURENCO has also developed a process to mass-manufacture a variety of low sensitive Hexogen (RDX), called I-RDX. 

The section on crystallization comprises zeolite synthesis, kinetics and mechanism of formation, stability relationships, recrystallization processes as well as the genesis of natural zeolites. Recent advances in this field have been surveyed, and some new perspectives have been outlined in the review by E. M. Flanigen. Most of the studies in this field are still empirical because of the complexity of the systems involved. Considerable progress has been made, however, towards a better understanding of the processes and mechanisms governing zeolite crystallization. It is not unreasonable to expect that conditions for synthesizing new zeolite structure types can eventually be predicted. 

The particle-size and size-distribution of solid materials produced in industrial processes are not usually those desired for subsequent use of these materials and, as a result comminution and recrystallization operations are carried out. Well known processes for particle size redistribution are crushing and grinding (which for some compounds are carried out at cryogenic temperatures), air micronization, sublimation, and recrystallization from solution. There are several practical problems associated with the above-mentioned processes. Some substances are unstable under conventional milling conditions, and in recrystallization processes the product is contaminated with solvent, and waste solvent streams are produced. Applying supercritical fluids may overcome the drawbacks of conventional processes. 

The overall recrystallization process, consisting of the initial nucleation and the subsequent growth of nuclei, can be modelled by the basic equations describing these respective processes. The rate of formation of nuclei as developed is ... 

In Tables 9.9-3 to 9.9-5 a summary of the recrystallization process of pharmaceuticals and bio-polymers, and co-precipitation by supercritical anti-solvent is presented. In Figures 9.9-1 and 9.9-2 examples of very long needle crystal and nano-particles are reported. 

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