Freezing-in temperature

Figure 10-1. The molar volume of an atactic poly(styrene) with Mn = 20 000 g/mol as a function of temperature. Te, softening point Tc, glass transition temperature 7>, freezing-in temperature (after G. Rehage).

The viscosity at the freezing temperature was found to be about 10 Pa s and independent of the substance in the earliest measurements of glass transitions. An isoviscous process was considered to characterize this freezing-in process. Today, the freezing-in temperature is considered to be... 

Keep in mind environmental effects on the temperature of eyewash fluid. In extreme cold environments, look for products with heated options. Depending on your application, consider freeze-rated portable stations, which keep fluid from freezing in temperatures as low as -32°F, or those with heated accessories that maintain a tepid temperature in environments as cold as -40°F. In locations with excessive heat, look for a unit with... 

Gefrierpunktsemiedrigung freezing-in temperature Tf Einfriertemperatur frequency Frequenz (FLertz) ... 

The freeze in temperature due to kinetic limitations during sample cooling is shown as shaded area. (Adapted from Ref [75].)... 

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

Dowtherm LE is a mixture of diphenyl oxide and methylated biphenyl for use in Hquid-phase systems. The low crystal point and low viscosity obviate protection from freezing at temperatures down to —30°C. 

To avoid maintenance problems, the location of pressure measurement devices must be carefully considered to protect against vibration, freezing, corrosion, temperature, overpressure, etc. For example, in the case of a hard-to-handle fluid, an inert gas is sometimes used to isolate the sensing device from direct contact with the fluid. 

Thermodynamic efficiency is hurt by the large ATbetween the temperatures of melting and freezing. In an analogy to distillation, the high a comes at the expense of a big spread in reboiler and condenser temperature. Erom a theoretical standpoint, this penalty is smallest when freezing a high concentration (ca 90%) material. 

Potassium hydroxide is the principal electrolyte of choice for the above batteries because of its compatibiUty with the various electrodes, good conductivity, and low freezing point temperature. Potassium hydroxide is a white crystalline substance having a mol wt = 56.10 density = 2.044 g/mL, and mp = 360° C (see Potassium compounds). It is hygroscopic and very soluble in water. The most conductive aqueous solution at 25 °C is at 27% KOH, but the conductivity characteristics are relatively flat over a broad range of concentrations. 

Surface temperatures may range from below freezing in the case of freeze dryers to above 800 K in the case of indirect dryers heated by combustion products. 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

The rather rigid molecules and high setting temperatures are conducive to molecules freezing in an oriented position with consequent high frozen-in stresses. 

Some typical moulding data for a range of plastics are given in Table 5.1. Note that the de-moulding temperature will be generally about 30°C below the Freeze-off temperature in order to ensure that the moulding is sufficiently solid for handling. 

Repeat the previous question for the situation in which the mould temperature is 60°C and the freeze-off temperature is 128°C. What difference would it make if it had been assumed that the material was Newtonian with a viscosity of 1.2 x 10 Ns/m. ... 

The freezing point, temperature of maximum density, osmotic pressure and specific heat for seawater of various salinities are given in Table 21.23. 

As methyl alcohol freezes in the neighborhood of — 95°C, measurements at a temperature — 70°C would be comparable with water at room temperature at 25°C methyl alcohol is very unlike water, having a compressibility more than 2.5 times as large. Ethyl alcohol and liquid ammonia are likewise unsuitable for comparison with wat er. 

Acetone, cooled by passing through a copper coil placed in a dry ice-acetone bath may be circulated through the condenser. If a temperature lower than —40° is used, the distillate will freeze in the condenser... 

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