Vitrification and further structural relaxation in the isothermal curing of an epoxy resin

Abstract

SYNOPSISIsothermal curing of an epoxy resin based on diglycidyl ether of bisphenol A, using a hardener derived from phthalic anhydride, has been performed a t curing temperatures between 30 and 130°C. Samples were cured isothermally at various intervals of time and analyzed by differential scanning calorimetry (DSC ) , the glass transition temperature Tg, and the conversion degree being determined by the residual enthalpy technique. The vitrification phenomenon and a further structural relaxation process, occurring at curing temperatures ( T,) lower than the maximum Tg ( 109"C), at which Tg equalizes T,, have been studied at curing temperatures between 30 and 100°C. The structural relaxation process is analyzed by the endothermic peak that appears superposed on Tg in dynamic DSC scans.The area of this peak ( Q ) is a measure of the recovery enthalpy, and thus of the extent of the relaxation process. This process begins at higher curing times ( t , ) when T, decreases because the vitrification of the system starts later. Both the enthalpy recovery ( Q ) and the temperature of the endothermic peak ( T,) increase with the annealing time ( t o ) , calculated as the difference between t, and the time in which vitrification occurs, and tend to have a limiting value due to the fact that the system loses mobility when the free volume decreases during its asymptotic approach toward the metastable equilibrium state. Furthermore, the dependence of Q and T,,, on t. at different T, shows that the relaxation process in partially cured resins depends on the conversion degree of the system and consequently on the crosslinking density of the network.

Vitrification during the Isothermal Cure of Thermosets: Comparison of Theoretical Simulations with Temperature‐Modulated DSC and Dielectric Analysis

“…The chemical and diffusion rate constants can be combined into overall rate constants, k tot (1,2) , by means of the Rabinowitch equation: [19] ( )…”

“…The equivalence of T g and T c was the standard method of investigating vitrification by DSC before the introduction of TMDSC. [1] The rate of cure dramatically slows down at vitrification, being dominated now by the diffusion rate constant, but does not stop completely. The cure continues, albeit rather slowly, and when T g once again reaches the continuously increasing cure temperature, devitrification takes place.…”

“…Since the vitrification time determined by conventional DSC makes use of a glass transition temperature determined on heating after, typically, previously cooling freely, [ 1] we can use the correspondence between cooling rate and frequency [21,22] to assign a value of 1/60 s -1 for the reference frequency. The effect of various values of the activation energy, E f , is investigated in the simulation.…”

“…On the other hand, if the curing process takes place at a sufficiently slow heating rate, the T g of the reacting system can reach the instantaneous cure temperature, whereupon the system changes to a glassy state and vitrifies, analogous to the case of isothermal curing at sufficiently low cure temperatures. [1,2] The subsequent process of devitrification occurs when the continually increasing cure temperature, T c , again exceeds the T g of the vitrified system. The vitrification and devitrification processes which occur during non-isothermal cure have been studied by temperature modulated differential scanning calorimetry (TMDSC) [3][4][5][6] and other dynamic techniques such as dielectric relaxation [7] and torsional braid analysis.…”

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

“…A plot of T g as a function of cure time then allows the vitrification time, t v , to be found as the cure time for which T g = T c . This procedure has been used in the past (Montserrat, 1992), but it is impractical on account of the length of time involved in making numerous repeated isothermal cure experiments. It is in this respect that TMDSC offers an important advantage: the reversing or complex heat capacity signal is responsive to the changes that take place when the system vitrifies, in just the same way as these signals respond to a glass transition on cooling, and hence at the time of vitrification in isothermal cure the complex heat capacity shows a sigmoidal change from a liquid-like to a glassy value.…”