Crystallization rapid

Solubility and Solvent Resistance. The majority of polycarbonates are prepared in methylene chloride solution. Chloroform, i7j -l,2-dichloroethylene, yy -tetrachloroethane, and methylene chloride are the preferred solvents for polycarbonates. The polymer is soluble in chlorobenzene or o-dichlorobenzene when warm, but crystallization may occur at lower temperatures. Methylene chloride is most commonly used because of the high solubiUty of the polymer (350 g/L at 25°C), and because this solvent has low flammabiUty and toxicity. Nonhalogenated solvents include tetrahydrofuran, dioxane, pyridine, and cresols. Hydrocarbons (qv) and aUphatic alcohols, esters (see Esters, organic), or ketones (qv) do not dissolve polycarbonates. Acetone (qv) promotes rapid crystallization of the normally amorphous polymer, and causes catastrophic failure of stressed polycarbonate parts. 

Small concentrations of vinylcarboxyhc acids, eg, acryhc acid, methacrylic acid, or itaconic acid, are sometimes included to enhance adhesion of the polymer to the substrate. The abihty to crystalline and the extent of crystallization are reduced with increa sing concentration of the comonomers some commercial polymers do not crystalline. The most common lacquer resins are terpolymers of VDC—methyl methacrylate—acrylonitrile (162,163). The VDC level and the methyl methacrylate—acrylonitrile ratio are adjusted for the best balance of solubihty and permeabihty. These polymers exhibit a unique combination of high solubihty, low permeabihty, and rapid crystallization (164). 

The crystalliza tion resistance of vulcaniza tes can be measured by following hardness or compression set at low temperature over a period of time. The stress in a compression set test accelerates crystallization. Often the curve of compression set with time has an S shape, exhibiting a period of nucleation followed by rapid crystallization (Fig. 3). The mercaptan modified homopolymer, Du Pont Type W, is the fastest crystallizing, a sulfur modified homopolymer, GN, somewhat slower, and a sulfur modified low 2,3-dichlorobutadiene copolymer, GRT, and a mercaptan modified high dichlorobutadiene copolymer, WRT, are the slowest. The test is often mn near the temperature of maximum crystallization rate of —12° C (99). Crystallization is accelerated by polyester plasticizers and delayed with hydrocarbon oil plasticizers. Blending with hydrocarbon diene mbbers may retard crystallization and improve low temperature britdeness (100). 

The measured growth rates are illustrated by the circles in Fig. 7. The interface velocity is plotted versus the interface temperature T. The value of T is always greater than Tq because of the release of the latent heat at the interface. Dimensionless units for T and the velocity are used here. The maximum velocity corresponds to 80m /s for argon. The most surprising aspect is the rapid crystallization at low temperatures. Most materials exhibit sharply reduced rates at low temperatures, as expected for an activated growth process. That is, the kinetics can be represented as the product of an Arrhenius factor F(T) and a term that accounts for the net production of crystalline material as a result of the atoms ordering and disordering at the interface,... 

Raman LAM, SAXS and TEM have been used to obtain the dependence of crystallite thickness with molecular weight under rapid crystallization as well as... 

From the initial state, chains which are close to the growth front are pulled in and rapidly crystallized, adding one layer to it. Chains which are far from the attractive influence of the growth front undergo homogeneous nucleation. 

The crystallization of turanose was first reported by Pacsu and the writer24 as follows In 1918 one of us (H.) found an abundant supply of the rare melezitose in a certain kind of honeydew honey and from it he prepared a small quantity of sirupy turanose in the hope of crystallizing it. Other samples of turanose sirup were prepared subsequently from this stock of melezitose by other workers in the same laboratory. Recently it was observed by D. H. Brauns that one of these sirups, the exact history of which is not now known, had crystallized after standing many years. By the use of these crystals to nucleate turanose sirups which we have lately prepared from melezitose, it has been possible to obtain a rapid crystallization of the sugar in abundant quantities. To this quotation the writer can now add the information, kindly supplied recently by Mr. C. F. Walton, Jr., that Mr. Walton prepared the other samples of turanose sirup. ... 

Turanose Phenylosazone. A mixture of 4 g. of turanose, 2 ec. of water, and 1 co. of phenylhydrazine was warmed on the steam-bath until solution was complete. To the cooled solution was added 3.5 cc. of phenylhydrazine and 4 cc. of glacial acetic acid, and the mixture returned to the steam-bath for one hour. At the expiration of this time, 40 cc. of warm 60% alcohol was added and, upon cooling, a rapid crystallization of the osazone occurred. The osazone was recovered by filtration and washed with absolute alcohol followed by ether to yield 4.2 g. (69%) of lemon-yellow needles. The osazone is soluble in hot water and separates on cooling as jelly-like particles, but water is not a satisfactory solvent for its purification. It was recrystallized from 15 parts of 95% alcohol with good recovery, as needles which melted with decomposition at 200-205° and rotated [ ]d +24.5° - +33.0° (24 hours, constant value c, 0.82) in a mixture of 4 parts of pyridine, by volume, and 6 parts of absolute ethyl alcohol. In methyl cellosolve (ethylene glycol monomethyl ether) solution it rotated C< 3d" + 44.3°— + 48.5° (24 hours, constant value c, 0.80). 

Ethyl 2-(D-amiino-tetrahydroxybutyl)-5-methyl-4-furoate (5.5 g.) is mixed with 80 ml. of dry benzene and 20 ml. of glacial acetic acid, and cooled in ice plus water. While stirring and cooling, 182 g. of lead tetraacetate (purity, 99.7%)62 is added during about sixty minutes stirring is continued until all the oxidant has been consumed. The lead dioxide is then removed by filtration, and the benzene solution is extracted twice with water.58 The benzene layer is dried with calcium chloride and the solvent is evaporated under diminished pressure, giving an oily residue which rapidly crystallizes in colorless plates yield, 3.6 g. (quantitative). The product is purified by recrystallization from dilute acetic acid or by steam distillation m.p., 57°. 

When the methyl red is crystallized from toluene, it sometimes separates in the form of bright-red lumps, probably on account of too rapid crystallization. Under these conditions it is advisable to crystallize again, using a somewhat larger amount of toluene. 

Crystallization of PCT is relatively rapid, but because of its higher Tg (90 °C) the maximum rate of crystallization occurs at a higher temperature than is typical of other crystalline polymers such as PET (Tg at about 70 °C) or PBT (Tg at about 35 °C). Figure 7.2 compares the crystallization half-times of PET and PCT from both the glass and the melt (data were obtained via DSC measurements). The effect of the higher Tg on the temperature of maximum crystallization rate (i.e. minimum half-time) is most clearly seen in the data from the melt. The basic rapid crystallization rate of PCT allows it to be used as a high-performance injection molding material. 

PCT forms the basis of a family of reinforced, crystalline plastics for injection molding. As mentioned above, the high melting point of the polymer is a key property, as this results in high heat deflection temperatures (HDTs) in glass-fiber-reinforced formulations. Good toughness, flow into the mold, and rapid crystallization are also important in these applications. 

Although PBT has many desirable properties, its rapid crystallization almost always renders it opaque. While transparent PET products are quite common, PBT crystallization can rarely be quenched to the extent necessary for the formation of transparent articles. Thin films of PBT are translucent, allowing some light to penetrate, but are almost never clear. 

PTT is a rapidly crystallizing polymer. A melt-processed PTT tends to crystallize with a crystallinity of between about 15 and 30 wt%. It is therefore more difficult to dissolve in solvents commonly used for amorphous PET. Stronger solvents, such as hexafluoroisopropanol (HFIPA) or a 1 1 mixture of trifluo-roacetic acid and methylene chloride are typically used to dissolve PTT. However, HFIPA is a very expensive solvent for routine IV measurements, and methylene chloride is too volatile to maintain in a 1 1 mixture with trifluoroacetic acid at elevated temperatures or in prolonged storage. With care, a 60/40 mixture of phenol/tetrachloroethane can be used satisfactorily for IV measurement when it is heated to 110 °C to ensure complete dissolution of PTT [37],... 

Poly(butylene terephthalate) (PBT) is a semicrystalline, thermoplastic polyester which is completely analogous to PET except that it has a longer, more flexible butylene chain linkage which imparts a rapid crystallization rate, thus making PBT well suited to injection moulding processes. This polyester is used widely for electrical and electronic components due to its high temperature resistance and good electrical properties (Chapter 8). 

Kadam S.S. Vissers J.A.W. Forgione M. Geertman R.M. Daudey P.J. Stankiewicz A.I. Kramer H.J.M. Rapid crystallization process development strategy from lab to industrial scale with PAT tools in skid configuration. Organic Process Research and Development, 2012), 16 (5), 769-780. 

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