The isothermal curing of a diepoxide‐cycloaliphatic diamine resin by temperature modulated differential scanning calorimetry

Abstract

ABSTRACT:The thermal properties and the isothermal cure of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with a diamine based on 4,4Ј-diamino-3,3Ј-dimethyldicyclohexylmethane (3DCM) were analyzed by differential scanning calorimetry (DSC) and temperature modulated DSC (TMDSC). The quasi-isothermal TMDSC scans were performed at curing temperatures between 40 and 140°C for different periods of time, and the modulation conditions were amplitude of 0.5K and a period of 60 s. The heating rates used on the DSC scans were between 2.5 and 20 Kmin Ϫ1. The heat of curing measured nonisothermally increases when the heating rate decreases. An average value of 440 Jg Ϫ1 was estimated. The glass transition of the unreacted system measured by DSC was Ϫ40.4°C. The final glass transition temperature of the resin depends on the heating rate of cure and postcure conditions. Values between 143°C (measured by DSC) and 154°C (measured on the total heat flow by TMDSC) were obtained after isothermal curing and postcure at 10 and 1 Kmin Ϫ1 , respectively. The vitrification was analyzed by the modulus of the complex heat capacity ͉C* p ͉, which decay gives the interval and the time of vitrification. These properties were used to build the time-temperature-transformation cure diagram. The intensity of the vitrification was measured by the change in ͉C* p ͉, which decreases quasi-linearly with the curing temperature. The phase angle of the heat flow shows a peak in the vitrification region, which agrees with the vitrification time determined by ͉C* p ͉. . The analysis of the diffusion controlled step is studied using a mobility factor determined from the normalised variation of ͉C* p ͉ during the vitrification. The simulated overall reaction rate, which is obtained from the product of the chemical kinetics and the mobility factor, agrees with the experimental reaction rate.

Vitrification, devitrification, and dielectric relaxations during the non‐isothermal curing of diepoxy‐cycloaliphatic diamine

“…According to other authors,5–7, 9, 21, 28 TMDSC measurements have proven to be a suitable technique for studying the vitrification of thermosets. In particular, in the DGEBA–3DCM system, the measurement of the dynamic T g , which is frequency‐dependent, obtained by the C p * signal at a modulation period of 60 s (equivalent to a measuring frequency of 16.7 mHz), gives a value for the vitrification time (in isothermal measurements) that is very close to that measured by conventional DSC 28. This property is common to other epoxy‐amine systems that follow a step reaction, but it cannot be generalized to other types of reacting systems, such as the chain reaction that occurs in epoxy‐anhydride systems 29.…”

Calorimetric and Rheological Measurements of Three Commercial Thermosetting Prepreg Epoxies

“…Other studies that investigated epoxy resins found monotonic decreases as well; however, the highest scan rate investigated in these studies was 20 C/min [14,15]. Presumably, the reason for the dependence of the enthalpy on the scan rate is the time lag associated with faster scan rates, which in turn causes faster hardening of the resin and prevents full reaction.…”

The Use of Matlab in the Study of the Glass Transition and Vitrification in Polymers

“…By TMDSC, therefore, it is possible to identify the vitrification time directly from the isothermal cure curve, and it is conventionally taken to be at the mid-point of the sigmoid. This approach has been adopted successfully by a number of workers (Alig et al, 1999;Lange et al, 2000;Montserrat & Cima, 1999;Montserrat & Martin, 2002;Van Assche et al, 1998). Nevertheless, it raises the question of how this measurement of the vitrification time, which depends on the frequency of modulation, can be reconciled with that obtained by the lengthy conventional DSC procedure, which does not depend on the frequency.…”