Crystallization rates, temperatures below

This phenomenon will clearly be more important at low temperatures, a regime of great current interest in atmospheric chemistry in connection with the Antarctic "ozone hole" where temperatures as low as 160 K are encountered in the lower stratosphere in winter. It is exceptionally difficult to study many important reactions in the laboratory at such temperatures because of the low vapour pressure of the species involved. Indeed, an important factor in the chemistry of the Antarctic lower stratosphere may well be die condensation of nitric acid on ice crystals at temperatures below 180 K which could result in heterogeneous reactions which would again be very hard to study in the laboratory. Furthermore, our understanding of the temperature dependence of the rates of simple bimolecular reactions with small activation energies is not such that we could confidently extrapolate measurements to temperatures down to, say, 250 K as far as 160 K. Past experience of shorter extrapolations to 220 K have exposed the risks involved. 

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. 

Into a stirred, cooled (10°-15°C) solution of 26.2 grams (0.1 mol) of 2-amino-5-chlorobenzo-phenone (3-oxime in 150 ml of dioxane were introduced in small portions 12.4 grams (0.11 mol) of chloracetyl chloride and an equivalent amount of 3 N sodium hydroxide. The chlor acetyl chloride and sodium hydroxide were introduced alternately at such a rate so as to keep the temperature below 15°C and the mixture neutral or slightly alkaline. The reaction was completed after 30 minutes. The mixture was slightly acidified with hydrochloric acid, diluted with water and extracted with ether. The ether extract was dried and concentrated in vacuo. Upon the addition of ether to the oily residue, the product, 2-chloroacetamido-5-chlorobenzophenone (3-oxime, crystallized in colorless prisms melting at 161°-162°C. 

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... 

Temperature has a complex effect on crystallization rate. Initially, as the temperature falls below the equilibrium melting temperature, the crystallization rate increases because nucleation is favored. However, as the temperature continues to fall, the polymer s viscosity increases, which hampers crystallization. As a rule of thumb, a polymer crystallizes fastest at a temperature approximately mid-way between its glass transition temperature and its equilibrium melting temperature. 

The effect of physical aging on the crystallization state and water vapor sorption behavior of amorphous non-solvated trehalose was studied [91]. It was found that annealing the amorphous substance at temperatures below the glass transition temperature caused nucleation in the sample that served to decrease the onset temperature of crystallization upon subsequent heating. Physical aging caused a decrease in the rate and extent of water vapor adsorption at low relative humidities, but water sorption could serve to remove the effects of physical aging due to a volume expansion that took place in conjunction with the adsorption process. 

In a i-l. three-necked flask fitted with a fetirrer and thermometer are placed 93 g. (0.5 mole) of dodecanol (Note 1) and 158 g. (2 moles) of pyridine. The flask is surrounded by a water bath sufficiently cold to lower the temperature of the mixture to io°. At this temperature 105 g. (0.55 mole) of />-toluenesul-fonyl chloride is added in portions over a twenty- to thirty-minute period, or at such a rate that the temperature does not exceed 20° at any time. The mixture is then stirred for three hours at a temperature below 20°, after which it is diluted with 300 cc. of hydrochloric acid (sp.gr. 1.19) in 11. of ice water. The ester which crystallizes is collected on a chilled Buchner funnel and sucked as dry as possible. The solid is transferred to a 600-cc. beaker, 250--300 cc. of methyl alcohol is added, and the mixture is warmed on the steam bath until the ester melts. It is then cooled in a freezing mixture while being stirred continuously the ester separates in a fairly fine state. It is then collected on a chilled funnel and allowed to dry in the air, preferably at a temperature, below 20°. The yield of ester is 152-156 g. (88-90 per cent of the theoretical amount based upon the dodecanol used). It melts at 20-250 (Note 2) and is sufficiently pure for most purposes. 

During crystallization, the bulk of the moisture and AA are removed from the pellets. In the case of moisture, this is critical before the pellets are heated to SSP temperatures above 180 °C. Moisture present at higher temperatures can lead to hydrolysis and a drop in IV, which leads to a reduction in the SSP reaction rate later in the process, as shown in Figure 4.15. The IV drop has been shown to increase significantly at temperatures over 200 °C [15]. Even at crystallization temperatures below 180 °C, a small IV drop of <0.01, depending on the initial moisture content, can be expected [86],... 

Diffusive crystal growth at a fixed temperature would not result in a constant crystal growth rate (see below). However, under some specific conditions, such as continuous slow cooling, or in the presence of convection with diffusion across the boundary layer, time-independent growth rate may be achieved. Similarly, time-independent dissolution rate may also be achieved. 

Sometimes, reversion tendencies cannot be overcome by the addition of higher treat rates of a wax crystal modifier. Under these circumstances, only dilution by low-viscosity products or the constant addition of heat will keep the oil fluid at temperatures below its base pour point. 

The low temperature ( 140°C) anionic ring opening polymerization is further complicated by the crystallinity in nylon 6. Magill [66] has reported that the temperature for maximum crystallization rate in nylon 6 is about 140-145°C. The nucleation rate is low above 145°C, whereas viscous effects hinder crystal growth below this temperature. As a result, at about 140-145°C, heterogeneous reaction conditions can be encountered (as we have seen in our studies) if there is simultaneous polymerization of caprolactam and crystallization of the nylon 6 formed. 

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