Melting-point, pressure effect

It has been observed that the effectiveness of antioxidants, as measured by OIT at high temperatures, may differ as a function of temperature. OIT test temperatures should preferably be close to actual use temperature. At ambient pressures, significant oxidation is often not detected until the polymer is above the melting point. At these temperatures, essential ingredients in the polymer formulation can be lost. Also, tests performed above the melting point cannot be extrapolated reliably to temperatures below the melting point. Pressure DSC suppresses volatilisation of additives and degradation by-products, an event which thermally competes with the oxidation exotherm. Moreover, the... 

Effect of impurities upon the melting point. Let us take a specific example and examine the effect of the addition of a small quantity of naphthalene to an equilibrium mixture of pure solid and liquid a-naphthol at the temperature of the true melting point (95 5°) at atmospheric pressure. 

Evaporation Retardants. Small molecule solvents that make up the most effective paint removers also have high vapor pressure and evaporate easily, sometimes before the remover has time to penetrate the finish. Low vapor pressure cosolvents are added to help reduce evaporation. The best approach has been to add a low melting point paraffin wax (mp = 46-57° C) to the paint remover formulation. When evaporation occurs the solvent is chilled and the wax is shocked-out forming a film on the surface of the remover that acts as a barrier to evaporation (5,6). The addition of certain esters enhances the effectiveness of the wax film. It is important not to break the wax film with excessive bmshing or scraping until the remover has penetrated and lifted the finish from the substrate. Likewise, it is important that the remover be used at warm temperatures, since at cool temperatures the wax film may not form, or if it does it will be brittle and fracture. Rapid evaporation occurs when the wax film is absent or broken. 

Nylon-11. Nylon-11 [25035-04-5] made by the polycondensation of 11-aminoundecanoic acid [2432-99-7] was first prepared by Carothers in 1935 but was first produced commercially in 1955 in France under the trade name Kilsan (167) Kilsan is a registered trademark of Elf Atochem Company. The polymer is prepared in a continuous process using phosphoric or hypophosphoric acid as a catalyst under inert atmosphere at ambient pressure. The total extractable content is low (0.5%) compared to nylon-6 (168). The polymer is hydrophobic, with a low melt point (T = 190° C), and has excellent electrical insulating properties. The effect of formic acid on the swelling behavior of nylon-11 has been studied (169), and such a treatment is claimed to produce a hard elastic fiber (170). 

Physical characteristics Molecular weight Vapour density Specific gravity Melting point Boiling point Solubility/miscibility with water Viscosity Particle size size distribution Eoaming/emulsification characteristics Critical temperature/pressure Expansion coefficient Surface tension Joule-Thompson effect Caking properties... 

Although the effect of pressure on melting point is very small, its direction is still important To decide whether the melting point will be increased or decreased by compression, a simple principle is applied. An increase in pressure favors the formation of the more dense phase. 

The liquid is the more dense phase (Figure 9.7b). The liquid-solid line is inclined to the left, toward the y-axis. An increase in pressure favors the formation of liquid that is, the melting point is decreased by raising the pressure. Water is one of the few substances that behave this way ice is less dense than liquid water. The effect is exaggerated for emphasis in Figure 9.7b. Actually, an increase in pressure of 134 atm is required to lower the melting point of ice by 1°C. 

Effect of pressure on the melting point of a solid, (a) When ihe solid is die more dense phase, an increase in pressure converts liquid to solid the melting point increases, (b) If the liquid is the more dense phase, an increase in pressure converts solid to liquid and the melting point decreases. 

Melting point The temperature at which the solid and liquid phases of a substance are in equilibrium with each other, 13 effects of pressure on, 235 liquids, 235 low, 235 metals, 245... 

The effect of change of pressure on the melting-point of a substance can be predicted qualitatively in the following manner ... 

Bunsen (1850), Hopkins (1854), Batelli (1887), and de Yissier (1892) also made experiments on the effect of pressure on the melting-point of bodies of the wax-type. The latter found for acetic acid ... 

PBTP was depolymerised in excess methanol under high temperature (473-523 K) and high pressure (4-14 MPa) conditions. Depolymerisation was earried out at 483 K and 4-12 MPa, and at 513 K and 6-14 MPa. The temperature had a great effect on the depolymerisation rate, but the reaction pressure did not. Under the former eonditions, depolymerisation took over 80 min, but only about 20 min under the latter eonditions. The se results showed that the supereritieal state of methanol was not a key faetor in the depolymerisation reaction. A kinetic study of the reaetion at 473-523 K and 12 MPa showed that the deeomposition rate eonstant of PBTP increased dramatieally when the reaetion temperature was higher than the melting point of PBTP (500 K). This indicated that partial miseibility of the molten PBTP and methanol was an important faetor for the short-time depolymerisation. 9 refs. 

Air or water cooled mercury discharge lamps find many uses, one of the more obvious of which is the study of photochemical reactions. These lamps are usually made of vitreous silica because of its low thermal expansion, high melting point and its transparency to ultraviolet radiation. Their operating pressure has a profound effect on the spectral distribution of the radiation produced and therefore it is important to consider the requirements in the design of such lamps. 

QSPRs in which solubilities and vapor pressures are correlated against molecular structure are done exclusively using the liquid state property. This avoids the complication introduced by the effect of fugacity ratio or melting point on the solid state property. 

As was discussed earlier in Section 1.2.8 a complication arises in that two of these properties (solubility and vapor pressure) are dependent on whether the solute is in the liquid or solid state. Solid solutes have lower solubilities and vapor pressures than they would have if they had been liquids. The ratio of the (actual) solid to the (hypothetical supercooled) liquid solubility or vapor pressure is termed the fugacity ratio F and can be estimated from the melting point and the entropy of fusion. This correction eliminates the effect of melting point, which depends on the stability of the solid crystalline phase, which in turn is a function of molecular symmetry and other factors. For solid solutes, the correct property to plot is the calculated or extrapolated supercooled liquid solubility. This is calculated in this handbook using where possible a measured entropy of fusion, or in the absence of such data the Walden s Rule relationship suggested by Yalkowsky (1979) which implies an entropy of fusion of 56 J/mol-K or 13.5 cal/mol-K (e.u.)... 

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