Exotherm measurements

Figure 4 shows isothermal crystallization exotherms, measured by DSC, for different crystallization temperatures T ) for the pure polymer. It is observed that, as the crystallization temperature increases, the curves tend to get wider and smaller in amplitude, which means the material needs more time to fully crystallize. The integral under each peak as a function of time can be related with the overall degree of crystallinity, Fc, at each temperature. This calculation leads to the characteristic time tm, which is the time at which fifty percent of the total crystallization of the specimen has been achieved. For all samples, the half-crystallization time tn2 increases with the increase of crystallization temperature. However, caution should be taken on whether the comparison should be made at the same crystallization temperature or at the same degree of supercooling. 

In the flask were placed 10.0 g of the propargylic amine (see Chapter lIII-5, Exp. 1). The air in the flask was replaced with nitrogen and a solution of 0.01 mol of KO-tert.-Ci,H,3 in 10 g of THF (free from hydroperoxide) was added. The mixture was warmed at about 40 C. A weakly exothermic reaction was observed and the temperature rose to about 45°C. After 1-2 min the gel originally present, had disappeared almost completely and a brown solution had formed. The refractive index of the solution (note 1) was measured after intervals of about 2 min. After the... 

By allowing compounds to react in a calorime ter It IS possible to measure the heat evolved in an exothermic reaction or the heat absorbed in an en dothermic reaction Thousands of reactions have been studied to produce a rich library of thermo chemical data These data take the form of heats of reaction and correspond to the value of the enthalpy change AH° for a particular reaction of a particular substance... 

You have seen that measurements of heats of reaction such as heats of combustion can pro vide quantitative information concerning the relative stability of constitutional isomers (Section 2 18) and stereoisomers (Section 3 11) The box in Section 2 18 described how heats of reaction can be manipulated arithmetically to generate heats of formation (AH ) for many molecules The following material shows how two different sources of thermo chemical information heats of formation and bond dissociation energies (see Table 4 3) can reveal whether a particular reaction is exothermic or en dothermic and by how much... 

Polymerization. The polymerization of aziridines takes place ia the presence of catalytic amounts of acid at elevated temperatures. The molecular weight can be controlled by the monomer—catalyst ratio, the addition of amines as stoppers, or the use of bifimctional initiators. In order to prevent a vigorous reaction, the heat Hberated during the highly exothermic polymerization must be removed by various measures, ie, suitable dilution, controlled metering of the aziridine component, or external cooling after the reaction has started. 

Resoles. Like the novolak processes, a typical resole process consists of reaction, dehydration, and finishing. Phenol and formaldehyde solution are added all at once to the reactor at a molar ratio of formaldehyde to phenol of 1.2—3.0 1. Catalyst is added and the pH is checked and adjusted if necessary. The catalyst concentration can range from 1—5% for NaOH, 3—6% for Ba(OH)2, and 6—12% for hexa. A reaction temperature of 80—95°C is used with vacuum-reflux control. The high concentration of water and lower enthalpy compared to novolaks allows better exotherm control. In the reaction phase, the temperature is held at 80—90°C and vacuum-refluxing lasts from 1—3 h as determined in the development phase. SoHd resins and certain hquid resins are dehydrated as quickly as possible to prevent overreacting or gelation. The end point is found by manual determination of a specific hot-plate gel time, which decreases as the polymerization advances. Automation includes on-line viscosity measurement, gc, and gpc. 

The relative effectiveness of nucleating agents in a polymer can be determined by measuring recrystallization exotherms of samples molded at different temperatures (105). The effect of catalyst concentration and filler content has been determined on unsaturated polyesters by using dynamic thermal techniques (124). Effects of formulation change on the heat of mbber vulcanization can be determined by dsc pressurized cells may be needed to reduce volatilization during the cure process (125). 

Phenol and alkenes react quite exothermically. The reaction between 1 mole of phenol and 1 mole of isobutylene to yield 1 mole of / -Z fZ-butylphenol PTBP Hberates approximately 79.8 kj /mol (19.1 kcal/mol) (24). In an adiabatic system, this reaction, if started at 40°C, would result in a reaction product at about 250°C. Temperatures above 200°C are considered unacceptably high in the reactor so design measures are employed to keep the temperature down. 

Organic Solids A few organic compounds decompose before melting, mostly nitrogen compounds azides, diazo compounds, and nitramines. The processes are exothermic, classed as explosions, and may follow an autocatalytic law. Temperature ranges of decomposition are mostly 100 to 200°C (212 to 392°F). Only spotty results have been obtained, with no coherent pattern. The decomposition of malonic acid has been measured for both the solid and the supercooled liquid. The first-order specific rates at 126.3°C (259.3°F) were 0.00025/min for solid and 0.00207 for liquid, a ratio of 8 at II0.8°C (23I.4°F), the values were 0.000021 and 0.00047, a ratio of 39. The decomposition of oxalic acid (m.p. I89°C) obeyed a zero-order law at 130 to I70°C (266 to 338°F). 

Temperature gradient normal to flow. In exothermic reactions, the heat generation rate is q=(-AHr)r. This must be removed to maintain steady-state. For endothermic reactions this much heat must be added. Here the equations deal with exothermic reactions as examples. A criterion can be derived for the temperature difference needed for heat transfer from the catalyst particles to the reacting, flowing fluid. For this, inside heat balance can be measured (Berty 1974) directly, with Pt resistance thermometers. Since this is expensive and complicated, here again the heat generation rate is calculated from the rate of reaction that is derived from the outside material balance, and multiplied by the heat of reaction. 

The reaction mixture is heated and allowed to reflux, under atmospheric pressure at about 100°C. At this stage valve A is open and valve B is closed. Because the reaction is strongly exothermic initially it may be necessary to use cooling water in the jacket at this stage. The condensation reaction will take a number of hours, e.g. 2-4 hours, since under the acidic conditions the formation of phenol-alcohols is rather slow. When the resin separates from the aqueous phase and the resin reaches the requisite degree of condensation, as indicated by refractive index measurements, the valves are changed over (i.e. valve A is closed and valve B opened) and water present is distilled off. 

Figure 25.9. Typical exotherm curves for polyester resin cured with 1% benzoyl peroxide over a range of bath temperatures. (Test tubes of 19 mm dia are filled to height of 8 cm with a mixture of resin plus peroxide. The tubes are immersed in a glycerin bath to the level of the resin surface. Temperature is measured with a thermocouple needle whose point is half-way down the resin...

In the ARC (Figure 12-9), the sample of approximately 5 g or 4 ml is placed in a one-inch diameter metal sphere (bomb) and situated in a heated oven under adiabatic conditions. Tliese conditions are achieved by heating the chamber surrounding the bomb to the same temperature as the bomb. The thermocouple attached to the sample bomb is used to measure the sample temperature. A heat-wait-search mode of operation is used to detect an exotherm. If the temperature of the bomb increases due to an exotherm, the temperature of the surrounding chamber increases accordingly. The rate of temperature increase (selfheat rate) and bomb pressure are also tracked. Adiabatic conditions of the sample and the bomb are both maintained for self-heat rates up to 10°C/min. If the self-heat rate exceeds a predetermined value ( 0.02°C/min), an exotherm is registered. Figure 12-10 shows the temperature versus time curve of a reaction sample in the ARC test. 

The exotherm starts at 150°C and ends at a final temperature of 272°C, resulting in an adiahatie temperature inerease of 122°C. This value is the uneoiTeeted measured temperature inerease for both sample and vessel (homh). The eorreetion involves multiplieation hy the (t)-faetor. For example, if the value of the ([t-faetor from an experiment is 1.68, then the eoireeted adiahatie temperature is 205°C. 

In scale-up, runaway exothermic chemical reactions can be prevented by taking appropriate safety measures. The onset or critical temperature for a runaway reaction depends on the rate of heat generation and the rate of cooling, which are closely linked to the dimensions of the vessel. 

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