Melting and crystallisation processes

Not only can the melting points of the constituent polymers be measured accurately, but the fact is that the increased sensitivity obtained at higher rates allows the melting process to be observed easily, whereas at low rates httle would be seen. 

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In thermodynamic language it is said that a reaction will occur if there is a decrease in the free energy, i.e. AF is negative. Since at the melting point melting and crystallising processes are balanced AF is zero and the expression may be written... 

Typical applications that are ideal for TG-DSC are temperature stability, decomposition behaviour, drying and firing processes, transition and reaction temperatures, melting and crystallisation processes. Redfern [290] has reviewed single sample simultaneous thermal analysis, i.e. TG-DSC and TG-DTA smdies of polymers, and has reported TG-DSC of an uncured polyimide resin in which a more accurate determination of the quantitative measurement... 

These two processes—solidification of a melt and crystallisation from a solvent, are the most important cases in which solid solutions appear. 

Diallqrlimidazolium halides have also been used to study the dynamics of processes occurring during melting and crystallisation, including one very recent study [49] of [C4inim]Br. Calorimetry showed that a 3 mg sample of [C4mim]Br required 150 min to... 

Several 1,3-dialkylimidazolium halides have been intensely structurally investigated, not only as a consequence of polymorphism but also as subjects of studies of the dynamics of processes occurring during melting and crystallisation and associated butyl group conformer lability. These aspects have been introduced in Section 11.2. 

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. 

A method was described for measuring the heat and rate of crystallisation following partial melting. The method uses a specific sequence of temperatures with a differential scanning calorimeter, and the melting and recrystallisation processes were confirmed by optical observations. The method was applied to poly(butylene terephthalate). The melting temperatures obtained from recrystallisation were used in the Hoffman-Weeks equation to deduce 236 C as the equilibrium melting temperature for PBTP. 22 refs. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

In the Sulser-MWB process the naphthalene fractions produced by the crystallisation process are stored in tanks and fed alternately into the crystalliser. The crystalliser contains around 1100 cooling tubes of 25-mm diameter, through which the naphthalene fraction passes downward in turbulent flow and pardy crystallises out on the tube walls. The residual melt is recycled and pumped into a storage tank at the end of the crystallisation process. The crystals that have been deposited on the tube walls are then pardy melted for further purification. Following the removal of the drained Hquid, the purified naphthalene is melted. Four to six crystallisation stages are required to obtain refined naphthalene with a crystallisation point of 80°C, depending on the quaHty of the feedstock. The yield is typically between 88 and 94%, depending on the concentration of the feedstock fraction. 

When polymerised the polymer is crystalline but has a surprisingly low reported melting point (T ) of 257°C. The ratio T /T of 0.91 (in terms of K) is uniquely high. Because of the small difference in Tg and there is little time for crystallisation to occur on cooling from the melt and processed polymer is usually amorphous. However, if molecular movements are facilitated by raising the temperature or by the presence of solvents, crystallisation can occur. 

Although crystals can be grown from the liquid phase—either a solution or a melt—and also from the vapour phase, a degree of supersaturation, which depends on the characteristics of the system, is essential in all cases for crystal formation or growth to take place. Some solutes are readily deposited from a cooled solution whereas others crystallise only after removal of solvent. The addition of a substance to a system in order to alter equilibrium conditions is often used in precipitation processes where supersaturation is sometimes achieved by chemical reaction between two or more substances and one of the reaction products is precipitated. 

A melt is a liquid or a liquid mixture at a temperature near its freezing point and melt crystallisation is the process of separating the components of a liquid mixture by cooling until crystallised solid is deposited from the liquid phase. Where the crystallisation process is used to separate, or partially separate, the components, the composition of the crystallised solid will differ from that of the liquid mixture from which it is deposited. The ease or difficulty of separating one component from a multi-component mixture by crystallisation may be represented by a phase diagram as shown in Figures 15.4 and 15.5, both of which depict binary systems — the former depicts a eutectic, and the latter a continuous series of solid solutions. These two systems behave quite differently on freezing since a eutectic system can deposit a pure component, whereas a solid solution can only deposit a mixture of components. 

A single-stage crystallisation process may not always achieve the required product purity and further separation, melting, washing, or refining may be required. Two approaches are used ... 

AIChESymp. Ser. (a) 65 (1969) no. 95, Crystallization from solutions and melts (b) 67 (1971) no. 110, Factors affecting size distribution (c) 68 (1972) no. 121, Crystallization from solutions Nucleation phenomena in growing crystal systems (d) 72 (1976) no. 153, Analysis and design of crystallisation processes (e) 76 (1980) no. 193, Design, control and analysis of crystallisation processes (f) 78 (1982) no. 215, Nucleation, growth and impurity effects in crystallisation process engineering (g) 80 (1984) no. 240, Advances in crystallisation from solutions. 

Diphenyl tellurium dibromide, (C6H5)2TeBr2, occurs when diphenyl telluride reacts with bromine,3 or when the bromine is replaced by hydrobromic acid and the process conducted in the manner described for the corresponding dichloride.4 The dibromide may be crystallised from chloroform, melts at 203 5° C., and when heated to 280° C. the bromine still remains attached to the tellurium atom. It forms crystals belonging to the tetragonal system and a second modification which is triclinic and gives the following values 5... 

A simple apparatus for steam distillation is shown in Fig. 2.102. Flask A contains the liquid to be steam distilled it is fitted with the splash-head B which prevents the carry-over of the contents of the flask A into the receiver. To carry out a steam distillation, the solution (or mixture of the solid with a little water) is placed in the flask A, and the apparatus is completely assembled. Steam is passed into flask A, which is itself heated by means of a flame to prevent too rapid an accumulation of water. If the substance crystallises in the condenser and tends to choke it, the water should be run out of the condenser for a few minutes until the solid material has been melted and carried by the steam into the receiver the water should then be cautiously readmitted to the hot condenser. It is best to use a condenser of the double surface type if the rate at which the steam distillation is carried out is rapid if necessary two such condensers connected in series may be used since in most steam distillations best results are obtained when the process of distillation is carried out rapidly. The passage of steam is continued until no appreciable amount of water-insoluble material is... 

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