Epoxy polymers

Abstract

The current usa of non-isothermal DTA or DSC for the investigation of polymers is based upon assumptions which allow a number of conclusions supported only by data drawn from the DTA or DSC traces of prepolymer samples.A more adequate procedure involving the analysis of the DTA or DSC traces of several samples previously cured to different polymerization degrees under isothermal conditions is proposed.A comparison of the results obtained with the two procedures is reported for two composite materials.Calorimetric investigations on polymers are currently carried out under non-isothermal conditions, viz. through DTA or DSC at a given heating rate.This kind of approach, although experimentally simple and rapid, must be considered with some caution, as it can result in misleading conclusions when the chemical process investigated occurs with a complex mechanism, as in the polymerization and reticulation of resins.The present work reports some typical examples of the misuse of non-isothermal DTA and DSC data and suggests how this kind of investigation should be employed to characterize the kinetics of a polymerization process.A direct comparison of the results obtained with the misleading and the correct approaches is also reported. General considerations Typical misuseMany authors and qualified firms report the kinetic parameters of curing processes, drawn from the evaluation of point derivatives, dQ/dT, along the profile of the exothermic peak recorded in the DTA or DSC scan for the so called prepolymer.This procedure is based on the following assumptions [1 ]:the heat Q delivered at a given time in the DSC scan is proportional to the curing degree (~ reached at that time;

Differential scanning calorimetry applied to cross-linking of a filled epoxy resin: Accuracy of the Borchardt and Daniels equation for describing the curing process kinetics

“…Last but not least, a clear relationship must be established between the extent of cure measured on a dynamic DSC curve and the degree of polymerization well defined by Schiraldi and Baldini [11].…”

Investigation of thermally activated or photoinitiated polymerization by DSC

“…The typical reaction conditions used by the preparative organic chemist correspond to the isothermal mode rather than to the dynamic one. However polymers obtained at constant or rising temperature are not necessarily identical, as concluded by Baldini [1] from his theoretical considerations and practical results. To get an unambiguous dynamic baseline, separated from device-typical curvature and changes in the heat capacity and heat conductivity of the reacting mixture, the often made connection of the initial and the final level of specific heat by a straight line seems to be oversimplified.…”

DTA traces of epoxy resins and composites

“…They are characterized by an epoxy matrix, which is the continuous "primary phase", and by a reinforcing or filling agent, discretely dispersed throughout the matrix, which mainly serves to improve the mechanical properties.Accordingly, the inert filling agent is generally assumed not to have a significant effect on the polymerization process of the epoxy matrix.The present study, undertaken to verify this assumption, shows that the filling agent can in fact affect the kinetics of polymerization of the epoxy host.Four CIBA materials, commonly employed as composites in the aerospace industry and here designated as A, B, C and D, respectively, were investigated by means of nonisothermal DTA, which allows monitoring of the curve process, provided that the experimental results are properly evaluated [1][2][3]. Although the phenomenological reaction order remains unchanged, always being equal to one, and although no significant difference may be observed for the kinetic constant, relevant effects have been observed for the trend of the polymerization degree vs. the time of cure.…”

“…Four CIBA materials, commonly employed as composites in the aerospace industry and here designated as A, B, C and D, respectively, were investigated by means of nonisothermal DTA, which allows monitoring of the curve process, provided that the experimental results are properly evaluated [1][2][3].…”

“…Samples of about 40 mg were previously cured at 373 K within sealed DTA silver pans before the DTA scan [1][2][3]. …”