Crystallization, rate

Other crystallization parameters have been determined for some of the polymers. The dependence of the melting temperature on the crystallization temperature for the orthorhombic form of POX (T = 323K) and both monoclinic (T = 348K) and orthorhombic (T = 329K) modifications of PDMOX has been determined (284). The enthalpy of fusion, Aff, for the same polymers has been determined by the polymer diluent method and by calorimetry at different levels of crystallinity (284). for POX was found to be 150.9 J/g (36.1 cal/g) for the dimethyl derivative, it ranged from 85.6 to 107.0 J/g (20.5—25.6 cal/g). Numerous crystal stmcture studies have been made (285—292). Isothermal crystallization rates of POX from the melt have been determined from 19 to —50 C (293,294). Similar studies have been made for PDMOX from 22 to 44°C (295,296). 

Crystallization. Acidified aluminum sulfate solutions can be supercooled 10 °C or more below the saturation point. However, once nucleation begins, the crystallization rate is rapid and the supersaturated solution sets up. The onset of nucleation in a gentiy stirred supersaturated solution is marked by the appearance of silky, curling streamers of microscopic nuclei resulting from orientation effects of hydraulic currents on the thin, platelike crystals. Without agitation, nucleation in an acidified solution, in glass tubes, can yield extended crystalline membranes of such thinness to exhibit colors resulting from optical interference. 

A sodium salts of resin acids 37.5 12.0 50 medium strength cured films having a slow crystallization rate adhesives, dipped goods, saturants, coatings, bonded batts... 

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). 

Fig. 3. Crystallization rate of polychloroprene vulcanizates under compressive stress (99).

Carboxylated polymers such as AF use similar but not identical compounds. The higher strength, especially hot bond strength, is due to the interaction of the carboxyl groups on the polymer chain with the metal oxides. The crystallization rate of AF is low and does not contribute to bond strength. Manufacture of adhesive compounds from AF is more demanding than manufacture of those from AD. 

The properties of the solvent-bome CR adhesives depend on the molecular weight, degree of branching and rate of crystallization of the polymer. The ability of polychloroprene adhesives to crystallize is unique as compared to other elastomers. The higher the crystallization rate, the faster the adhesive strength development. 

Polyester diols are often combined with polyether diols to provide green strength through crystallization or elevated r . Most prevalent and least expensive is hexamethylene diol adipate (HDA) with a Tm of about 60°C. A variety of polyesters are available with various levels of crystallinity — from wax-like to amorphous — and crystallization rate, and with values ranging well below 0°C to above room temperature. Polybutadiene diols are the most expensive and most hydrophobic. They provide low surface tension and thus good wet out of non-polar surfaces. 

There are 16 grades of DuPont neoprene alone. They vary in crystallization rate and potential, viscosity, molecular weight, additive content, and other properties ([216], pp. 284-306). Selection of the right materials requires sophisticated knowledge of both the neoprenes and the phenolic additives. Guggenberger provides a good overview of this situation and some basic formulation information ([216], pp. 284-306). She also provides a prototype formula for a heat-resistant contact cement as shown in the Table 18 ([216], source Table 10, p. 293). 

Tanimoto, A.K., Kobayashi, K. and Fujita, S., 1964. Overall crystallization rate of copper sulfate pentahydrate in an agitated vessel. International Chemical Engineering, 4(1), 153. 

J. Broughton, G. Gilmer, K. Jackson. Crystallization rates of a Lennard Jones fluid. Phys Rev Lett 49 1496, 1982. 

The homologous polybibenzoate with a central oxygen atom in the spacer, PDTMB, exhibits a liquid-crystal state at room temperature, owing to its low crystallization rate [15]. This fact is one of the main differences between P7MB and PDTMB, and is reflected on the mechanical properties of both polybibenzoates. The initial... 

As usual, this can be due both to thermodynamic and kinetic reasons. In fact, the presence of comonomeric units increases, in general, the energy content of all the crystalline forms, but, since the extent of increase may be different, it may destabilize some chain conformation or some kind of packing more than other ones. On the other hand, the influence of the comonomeric units on the polymorphic behavior of a polymer can be due to a change in the crystallization rates of the various forms. 

By using diamide units, melt phasing can be avoided and at the same time the crystallization rate is rapid. Surprisingly, diese segmented copolymers widi diamide units not only crystallize quickly but also have a high fracture strain... 

Crystalline polyimide powders, 304 Crystalline transition temperature. See Melting temperature (Tm) Crystallization rate, for processing semicrystalline polymers, 44 CTE. See Coefficient of thermal expansion (CTE)... 

Fujiwara et al. studied the precipitation phase boundary diagrams of the sodium salts of a-sulfonated myristic and palmitic acid methyl esters in the presence of calcium ions [61]. The time dependency of the precipitation showed that the calcium salts have an extremely slow crystallization rate at room temperatures. This is the reason for the good hardness tolerance of the a-sulfonated fatty acid methyl esters. 

Steady-state crystallization rates were measured for a range of temperatures below the melting point by Broughton et al (13). A face-centered cubic (100) crystal-melt interface was equilibrated in a box elongated in the... 

This is the process of the crystallization of one or more of the constituents of glass. Generally a glass is thermodynamically unstable with respect to these crystals, but at ordinary temperatures the crystallization rate is quite negligible. Crystallization may occur when the glass is worked at high temperature. The crystals which appear in a supercooled melt are not necessarily those of the stable solid phase at the temperatiure concerned for example, cristobalite can appear at temperatures for which tridymite is the stable crystalline... 

A The crystallization rate must exceed the rate of solvation. 

Eventually however, a crust of increasing thickness and density degins to appear on the surface, which, along with blocking of pores due to salt deposition, hinders evaporation of the liquid. Consequently, with increasing salt content the strength attains its maximum and then drops as the crystallization rate slows down. 

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