Exotherm curve

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...

Exotherm curve Temperature vs. time curves during a curing cycle. Peak exotherm is the point of highest temperature of a plastic during the cure. 

Figure 1. Exotherm curves for exposure of mixtures of 10 weight percent MM and diacrylate monomers (200 ppm hydroquinone inhibitor) to medium pressure mercury lamp through Pyrex filter (I = 19.4 mW cm 2).

Figure 3. Idealized temperature-time exothermic curve.

The addition of DK F-10 and P-170 reduced the value of by 17 and 10%, respectively, in comparison to the fat sample without the emulsifier. S-170 prolonged by 15% in comparison to the control fat sample. It seems that the first peak in the exothermic curve represents the a-form crystal, which transformed to the P-form as... 

For DSC measurements, the area under an exothermal curve corresponds to the heat of crystallization, AH. The fraction of crystals / at any time t during the crystallization process is given by Equation 1 ... 

Representative exotherm curves are shown in Figure 1 where four types of catalytic activity are observed ... 

Notes of the author In many cases of determination of the OIT, the exotherm curve does not have a well-defined onset point. This particular question was studied by the ASTM D20 (plastics) committee. It was noted that the steepest linear slope (see Figs. 15.19 and 15.20) sometimes varies. An initial increase in the slope can be followed by a small decrease in the slope, and then followed by an abrupt increase leading to the steepest slope of the run. In that case it was unclear, if to follow the ASTM procedure instructions, which slope shall be used to determine the OIT—the first steepest point before the slope decrease, or the second steepest point, which is the steepest point of the run Besides, the ASTM procedure says that a run can be stopped 2 min after the steepest point of the exotherm has been displayed. However, because the steepest point can come later than expected, it was unclear whether to follow the ASTM procedure, whether to wait until the exotherm reaches a plateau, or whether to consider only the first steep portion of the plot ... 

Effects of resin purity are observed in the thermal polymerization of the TGMDA samples and in the curing behavior of Characteristic exotherm curves are generated... 

The baseline at the end of the reaction was extrapolated to determine the total area under the exotherm curve and hence the isothermal heat of cure q.. Dynamic DSC analyses were run at a heating rate of 2°C/min. f)e8ween 30 and 200°C on each isothermally cured sample to obtain the residual heat of reaction... 

Bischoff [102] showed that the generalized modulus concept of Eq. 3.6.b-8 can be extended by substituting the right hand side of Eq. 3.7.a-14 or Eq. 3.7.a-14a as the rate form. This then asymptotically unified all the isothermal and endothermic curves, and the suitable portions of the exothermic curves, but still would not permit prediction of the maxima or stability aspects. 

The breadth of the exothermic curve (195 °C to 225 °C) indicates the temperature range over which the decomposition of azo occurs. The shape of the peak indicates the uniformity of the decomposition. In addition, the size of the peak, i.e., the area under the exothermic curve, is a quantitative measure of the amount of blowing agent that has decomposed in the sample. This integrated area under the exothermic peak is referred to as heat of decomposition, AH. ... 

Normalised DSC curves as a function of azodicarbondimide (azo) blowing agent concentration are shown in Figure 9.9, which clearly shows that the magnitude of the exothermal curve (i.e., its area) increases with the azo concentration. 

Figure 5.6 shows the DSC curves for a methacrylic acid sample radiated for 30, 60 and 90 s. It can be seen from the curves that an exothermic reaction occurs during irradiation and then the reaction slows gradually when the radiation stops, resulting in an exothermic curve due to the after-polymerization. 

By pulsing H2S four times with 16 ml of the titrant for 13 min, the Ni/AljOj catalyst lost its activity. The exothermic curve decreased rapidly, the CH4 peak disappeared and the CO peak maintained its original height on the gas chromatogram. 

The practical consequences of an auto-acceleration effect on exotherm and conversion are illustrated by the exotherm curves in Figure 9.1. The urethane methacrylate/methyl methacrylate resin that shows auto-acceleration at all oligomer/monomer ratios is compared with an unsaturated polyester/styrene resin, which only shows an autoacceleration (or gel effect ) at high alkyd/styrene ratios. The urethane methacrylate oligomer copolymerised with styrene shows virtually the same exotherm behaviour as the unsaturated polyester for equivalent styrene/oligomer unsaturation ratios. 

The transformed volume fraction, x, during the isothermal process at a particular temperature, T, can then be determined by measuring the area under the exothermic curve. It is assumed that the volume fraction transformed, x, up to any time, t, is proportional to the fractional area of the exothermic peak or the integrated enthalpy. Therefore, in the isothermal DSC scans, the transformed volume fraction, Xc(t,T), up to any time t is... 

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