Curing endotherm

Figure 7. Differential scanning calorimeter traces of 87- and 88-series Halthanes show the soft segment glass transition and a curing endotherm at about —70° and 100°C, respectively.

It is disappointing that DSC exotherms, presumably caused by further crosslinking reactions, interfered with the detection of LC phase transitions in the cured enamels. These exotherms were broad, and recurred when the samples were cooled and reheated. This behavior may be attributable to slow consumption of -OH groups in the domains or to slow self-condensation of HMMM (21,22). If these reactions occur to any appreciable extent, their exotherms are likely to obscure the relatively weak endotherms produced by the low concentration of the LC phase. 

This particular DSC has the added feature of an endotherm that is indicated by a valley just prior to the exotherm. The data is for a condensation-curing phenolic, and the endotherm is caused by evaporation of reaction by-products. This evaporation is confirmed by the third trace in Figure 15.2. The TGA shows a rapid weight loss beginning at about the same time as the endotherm in the DSC. 

Other fibers. The other major class of synthetic fibers, the polyacrylonitriles (orlon, acrilon, etc.) like the cellulosics (rayon, cotton) show no thermal activity up to 300 °C. Above these temperatures degradation of sample accompanies any characteristic transitions or curing exotherms. To minimize this effect, the samples are run in an inert environment such as N2, as seen in figure 16. Under these conditions reproducible characteristic endotherms were obtained for identifying wool, cotton and rayon. In roughly the same temperature region,... 

The heat peaks of the nonreversing and reversing MDSC traces can be associated with nonreversible reactions such as a chemical reaction or cure and reversible reactions such as plasticizing processes in PU resin. Pad samples exposed to all tested media showed nonreversing heat peaks between 70 and 100 °C. Therefore, nonreversible chemical reactions are responsible for the pad softening. Endothermic irreversible heats reached their maximum value after approximately 180 h of exposure, as shown in Fig. 2.15. This suggests that chemical reactions that lead to pad softening are complete after approximately 180 h of exposure. 

A differential scanning calorimeter (DSC 1-B, Perkin-Elmer Corporation) was used to determine the extent of cure 10-mg to 20-mg specimens were tested at a scanning rate of 10°C/min. An exothermic peak on the thermograph indicates the heat of reaction whereas an endothermic peak in the amorphous polymer indicates the presence of residual stresses or the occurrence of a transition such as the glass transition. The presence of an exothermic peak in the DSC-scan of a pre-cured sample is an indication of incomplete curing. 

Figure 16.15 Hypothetical DTA thermal curve for a semicrystalline pol3mier with the ahihty to cross-hnk. The plot shows the baseline shift that occurs at the glass transition temperature, Tg, exothermic peaks for crystallization and cross-linking (or curing), an exothermic peak (offscale) for oxidative decomposition, and an endothermic peak for melting of the pol3mier. A similar thermal plot would be obtained by DSC analysis. (Courtesy of TA Instruments, New Castle, DE, www.tainst.com.)...

Specific DSC test methods are described in ISO 11357 and IPC TM-650 2.4.25C. By measuring the isothermal heat of reaction, whether exothermic or endothermic, the degree of cure is represented by the ratio of the energy at time t to the total energy. 

Three endothermic peaks occur when the cured paste samples are heated. These peaks are a result of evaporation of the bonded water from the paste, which is incorporated in different structures. The sharp peak at 330 °C is related to melting of the metallic lead in the paste. When the plates are soaked in H2SO4 solution of 1.05 rel. den., the peak C disappears and the energy of the endothermic peak A increases, i.e. this type of hydration increases. The height of peak B diminishes slightly. 

During the process of curing, adhesives, as well as other polymeric materials, undergo physical and chemical changes due to absorption of heat (endotherm) and evolution of heat (exotherm). 

---