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Dielectric Characterization of the Curing Process of Polymeric Resins

According to the current state of the art it is not possible, or at least extremely difficult, to use mechanical, thermal and electrical methods to monitor the reaction process during the curing of polymer systems. In recent years dielectric measurements have increasingly been used to monitor the curing process of polymer systems. Using a fixed frequency, the mobility of the various charge carriers is generally calculated as a function of the curing time, which enables the viscosity change undergone by resins during the curing process to be described. However, until now no suitable evaluation methods were available which would have allowed dielectric measurement processes to be applied during production as "on-line" quality control methods.

The aim of these investigations was to develop a model enabling the reaction process during curing to be described on the basis of dielectric measurement data. Based on the the established time (frequency) - temperature superposition law, a time (frequency) - cross-linking time superposition law is postulated. The validity of this model in describing the reaction process of curable polymers is demonstrated experimentally for a variety of different resin systems. The measurement method needed to be changed in order to carry out these investigations. In each case, an entire frequency spectrum of the dielectric dissipation factor was measured to enable a meaningful evaluation. The evaluation method is widely applicable and easy to implement in practice.

Where the dielectric measurement is concerned, the details of the measurement technology used, as well as the set-up and application area of the various sensors, are introduced. Here it is shown that the frequency-dependent measurement of the dielectric dissipation factor as a function of the curing time (in a "classic" plate capacitor set-up) provides the data needed for the evaluation most rapidly and reliably. The calculated time - cross-linking superposition function a(t,t0) can be accurately described using an exponential approach. It can be shown that, as is the case with calorimetric measurements, the exponent of this function can be interpreted as a reaction time constant. For systems that react in two stages, the reaction process can also be described with reasonable accuracy using a broadened approach - the sum of two exponential functions. The two exponents provide the reaction time constants for the individual reaction steps.

Where the reaction time constants are determined for different reaction temperatures, the Arrhenius relationship can be used to calculate the activation energies of the reaction steps. The calculated data agree well with data presented in the literature. To further investigate the time - reaction time superposition law, simulation calculations are carried out which confirm the fundamental assumptions concerning the dielectric behavior of a reacting polymer system. These simulations can also be used to model the time and frequency-dependent dielectric measurements carried out on the epoxy resin systems. For resin systems showing very rapid reactions, the measurement method and evaluation procedure are modified and then implemented by a processing company in near-real-life conditions, using the example of polyurethane, (reaction time of <10 s). Here, too, the fundamental assumptions concerning the dielectric behavior of reacting resin systems are confirmed, and the suitability of the measurement and evaluation methodology for online quality control can be demonstrated.

In principle the dielectric measurement method can be used for all polymerization processes, i.e. for the synthesis, vulcanization and curing of reaction systems, as from a qualitative perspective the same physical and chemical principles apply in each case.

Lesen Sie die deutsche Zusammenfassung auf Kunststoffe.de
Author
 Peter Elsner

Peter Elsner
Institut für Kunststoffkunde und Kunststoffprüfung
Universität Stuttgart

Information

Free keywords: Polymers, resins, crosslinking, curing, dielectric measurements, reaction kinetics, activation energy
Institute / chair: Institut für Kunststoffkunde und Kunststoffprüfung der Universität Stuttgart
Language: German
Technical consultant for expert services: Prof. Dr. rer. nat. G. Busse (Betreuer), Prof. Dr. rer. nat. R. Wienecke
Publication year: 1992
Provider: Wissenschaftlicher Arbeitskreis Kunststofftechnik (WAK) / Kunststoffe.de

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