Ultrasonic and thermo-kinetic characterization of curing epoxy resin

This study combines cure kinetics modelling and thermal and ultrasonic cure monitoring to characterize the cure state of a complex commercial modified epoxy thermosetting system of industrial importance containing two epoxies, diethylene triamine hardener, external catalyst, aliphatic reactive diluent, and mica. Both catalyst and reactive diluent in the formulation of two epoxy resin mixture keep this complex system odd from others and to some extent a new one to report cure kinetics to the best of our knowledge. The cure was monitored using differential scanning calorimetry (DSC) and broadband ultrasonic techniques over a group of isothermal cure temperatures within corresponding acceptable time scales. The sensitivities of both techniques to the chemical, physical, and mechanical changes associated with each part of the cure was discussed comprehensively and critically together with an inspection of the similarities between them coupled with qualitative and quantitative correlations. An in depth details analysis of the chemical cure kinetics of the investigated system was presented utilizing the model free iso-conversional method coupled with the light of physics of advanced kinetics research. The modelling of the calorimetric cure kinetics of the epoxy system under study was developed utilizing the empirical approach of fitting of the experimental data to various kinetic models. The best fit model which best possibly describe the non-typical autocatalytic cure behaviour of the resin system and predicts the reaction course was evaluated and analyzed in details. Utilization of the maximum attained conversion at a specific curing temperature enables this model to most closely simulate the curing reaction under both chemical controlled and diffusion controlled conditions with almost a reasonable degree of satisfaction over the entire range of conversion and temperature studied without the a priori need of a glass transition temperature model. The non-conventional autocatalytic effect and prediction of the trimolecular catalysis mechanism of the curing reaction was found to be manifested in temperature dependence of reaction orders, which was elucidated and justified. In comparison to other epoxy resins without reactive diluents, the analysis of our data shows that most possibly, the reactive diluent increased the maximum value of calorimetric conversion and reaction rate, reduced the viscosity, while the values of activation energy and process parameters remained within the typical values of epoxy formulations and the crosslink density was unaffected. The performance of each particular model tested was discussed along with their comparisons. Implementing diffusion factor in conventional models some useful information associated with the diffusion controlled kinetics related to our data were explored. The cure kinetics was also analyzed from both kinetic and thermodynamic viewpoint in the context of Horie model. This approach we employed, is, to some extent uncommon, can contribute towards a new way of characterization and the critical understanding of the cure reaction from the microkinetic standpoint providing information of the effect of reactive diluent on kinetics, regarding reaction pathways, kinetic homogeneity I inhomogeneity associated with reaction phase and the properties of the end product which are important to monitor and ultimately control the cure to attain desired properties in the end material. A TTT diagram of the cure process of this system was also constructed. The ultrasonic compression wave velocity was demonstrated to be the most interesting and potential parameter for monitoring and characterizing the cure process at all stages which provided with the information of degree of mechanical property development and can detect gelation and vitrification that occur during cure. Therefore, ultrasonic velocity measurement could be exploited for non-destructive on-line process control in an industrial environment. It was demonstrated that ultrasonic compression wave velocity can be used as a predictor of calorimetric conversion measurements and thus can be used to track chemical reaction online which is of potential importance for cure monitoring. Though system specific, the methodology we utilized, at least in part, constitutes a novel way of quantifying the degree of cure of a commercial epoxy thermoset network from ultrasonic longitudinal velocity measurements which is interesting and promising. It was found that the DSC is much more sensitive to changes occurring at the early stages of the cure but is relatively insensitive to the changes occurring at the latter stage. Ultrasonic compression wave velocity shows a better sensitivity at the end of the cure. It was also demonstrated that ultrasonic compression wave attenuation, real and imaginary parts of compression modulus, ultrasonic loss tangent and associated central relaxation time, also provided information of the material state and the cure process as well. The end of cure ultrasonic data, in general, provide a convenient assessment of final product quality