Crystallisation temperature process

The number of inputs which are available for controlling crystallisation processes is limited. Possible Inputs for a continuous evaporative crystallisation process are, crystalliser temperature, residence time and rate of evaporation. These Inputs affect the crystal size distribution (CSD) through overall changes in the nucleatlon rate, the number of new crystals per unit time, and the growth rate, the increase in linear size per unit time, and therefore do not discriminate directly with respect to size. Moreover, it has been observed that, for a 970 litre continuous crystalliser, the effect of the residence time and the production rate is limited. Size classification, on the other hand, does allow direct manipulation of the CSD. 

Polyethylene crystals change shape with time at temperatures between the crystallisation temperature and the final melting point [42]. The process is commonly referred to as crystal thickening because the major effect is that the crystals increase their dimensions along the c axis. These changes may occur without any change in crystal volume, i.e. crystal thickening is associated with a lateral contraction of the crystals (Fig. 9). A second option is that... 

Crystallisation of most polymers is accompanied by the separation of different molecular species, a process referred to as molecular fractionation. Bank and Krimm [147] provided the first direct evidence of molecular fractionation in polyethylene. The first extensive study performed by Wunderlich and Mehta [148] indicated that, at each crystallisation temperature, there exists a critical molar mass (MCIjt) such that the molecules of molar mass greater than Mcrjt, are able to crystallise at this temperature, whereas... 

In the second step the temperature of the synthesis mixture is raised to the crystallisation temperature (generally between 100 and 200°C). During this process continued dissociation of silicate and aluminate oligomers occurs, resulting in an increasing concentration of monomeric silicate and aluminate species in the solution. 

In general, a direct comparison of different alloys cannot be made and might, if at all, only be allowed under restricting conditions. One of these restrictions is that stable crystalline compounds be absent. Consequently, the crystalline state should be phase segregated and the crystallization process itself should be diffusion controlled. As already mentioned, this is fulfilled for the alloys considered here at Z x 1.8 e/a. In Fig. 5.13, the crystallisation temperatures TK of (Au, Ag, Cu)-Sn alloys, already published elsewhere [5.55, 57], are redrawn. 

Luo, Cui and Li (1999) addressed the problem of temperature sensitivity of IBAD of ACPs and their subsequent crystallisation forming hydroxyapatite. Post-depositional annealing temperatures were decreased to 400 °C. The crystallisation of calcium phosphate coating is a hydroxyl ion diffusion-controlled process, thought to be the mechanism responsible for the decrease of the crystallisation temperature. The detailed study of the crystallisation process of calcium phosphate coatings shows that the crystallinity of the hydroxyapatite coating can be well controlled by adjusting the post-heat-treatment time. 

The equilibrium melting temperature for the or form of i-PP is still uncertain values in the range between 185.2°C and 208.2°C are reported [13]. Whichever of these two extreme values might be the right one, the rather big difference between the melting temperature and the crystallisation temperature means that the crystalline phase of PP is very sensitive for its thermal history i.e. for annealing processes. 

Growth rate of PET spherulites, of different molecular weights vs. crystallisation temperature (From Polymer Engineering, PT6I4, Unit 8 Advanced Processing, Open University Press, Milton Keynes, 1985). 

In the course of the work it was found that PE 15000 is characterised by the highest degree of crystallinity and by the highest crystallisation temperature, as compared with other PE samples studied in the course of this work. The non-reversing component of the crystallisation process depends strongly on... 

Polymer applications of DSC are numerous and concern the determination of Tm (ASTM E 794), Tg (ASTM E 1326-03, ISO/FDIS 11357-2), specific heat capacity of a material (ASTM E 1269, ASTM D 4816), crystallisation temperature upon cooling (ASTM E 794), transition temperatures (ASTM D 3418, ASTM D 4419, ASTM D 4591), purity of a material [79,80], contamination outgassing (ASTM E 1559), reaction rates, sample composition, reaction kinetic constants (ASTM E 698), reaction mechanisms, thermal stability (ASTM E 537), minimum processing temperatures, heat of fusion and crystallisation (ASTM D 3417), heat of crystallisation (ASTM E 793), additive effects on a material, quality control of raw materials [25], discrimination between materials, detection of polymorphism [81], characterisation of thermally and UV cured materials (cure state, degree of cure) (ASTM D 2471, ASTM D 5028), oxidative stability testing, QIT (ASTM D 3895, ASTM D 3012, ASTM E 1858-03), etc. 

Traditionally the analytical chemist has provided support to an industrial process line by supplying information about the chemical composition of raw materials, intermediates and end-products. However, chemical composition information may not always fultil the needs of the process engineer, who is responsible for quality management and quality assurance. The quality specifications of a product frequently use parameters other than chemical composition and the relationship between chemical composition and product quality specifications is often obscure. In a marketplace in which products are accepted on the basis of performance specifications, there is an increasing interest in on-line analytical techniques that can predict polymer product performance beyond melt index, YI, melting and crystallisation temperature. 

Li and co-workers [63] studied the crystallisation and melting behaviour of poly(]3-hydroxybutyrate (P-HB)-co-P-hydroxyvalerate (P-HV)) and a blend of poly(P-HB-co-P-HV)/polypropylene carbonate (30/70 w/w) using DSC and FT-IR spectroscopy. Transesterification occurred between poly(P-HB-co-P-HV) and polypropylene carbonate during the melt blending process. During crystallisation from the melt, the crystallisation temperature of the blend decreased by 8 °C compared with that of neat poly(P-HB-co-P-HV) and the melting temperature decreased by 4 °C. This indicated that the presence of polypropylene carbonate reduced the perfection of the poly(P-HB-co-P-HV) crystals, inhibited by the crystallisation of poly(p-HB-co-P-HV) and weakened its crystallisation ability. The equilibrium melting temperatures of... 

Tg of 60 °C and a crystallisation temperature (TJ of 100-105 °C, and a decomposition temperature (Tj) of 185-190 °C. It is brittle and undergoes unzipping to lactide when thermally processed. Copolymerisation with D-lactic acid and other hydroxyacids to obtain polyesters with a lower T and thus better thermal processing characteristics has been the common approach to obtain useful PLA. Stereo copolymers of L-lactic acid and D-lactic acid have lower crystallinity and T than the homopolymer of L-lactic acid and the polyester properties vary with the optical purity with the 50/50 DL polylactic acid (PDLLA) having no crystallinity at all. Variation of the optical purity is the most commonly used means to produce PLA of different property. 

A typical study by Simon and Suverkrup [ 170] compared techniques used to characterise the crystallisation behaviour of fatty acid bases. In addition to DSC, isothermal DSC, NMR, oscillation rheometry and thermorheography were used. The crystallisation of hard fats was a biphasic process [170]. The formation of crystal nuclei was controlled by the partial glyceride content the degree of super-cooling determined the formation of crystal nuclei the final solidification level depended on the crystallisation temperature. Similar studies [171] showed that the recrystallisation rates and the degree of supercooling before crystallisation occurred depended on the molecular weight of PEG. 

---