Vitrification and Devitrification during the Non-Isothermal Cure of a Thermoset Studied by TOPEM

Introduction

When an epoxy resin cures, the resin molecules react and form a highly crosslinked network. The physical properties of the material change drastically: before the curing reaction, the resin is liquid whereas after the curing process it is a highly crosslinked solid. In the initial stages of the curing reaction, the rate of cure is controlled by the kinetics of the chemical reaction. As the reaction proceeds and the degree of crosslinking increases, the glass transition temperature (T_g) of the system continuously increases from its initial value.

If the epoxy system is heated at a rate that is sufficiently high, the momentary sample temperature will always be higher than the glass transition temperature of the system. At the end of such an experiment, the system is completely cured (the degree of cure, α, that was initially 0, is now 1) and the glass transition temperature reaches its highest value (T_g∞).

On the other hand, if the system is heated at a low heating rate, at some point, the momentary glass transition temperature of the system can reach the momentary sample temperature. In this case, the system changes to a glassy state (vitrifies) as a result of the chemical reaction. From this point onward, the rate of the curing reaction is no longer limited by the kinetics of the reaction but by the mobility of the molecules involved in the reaction. Following vitrification, the molecules lose their mobility almost completely; the curing reaction slows abruptly and practically comes to a standstill. The same process occurs in isothermal curing if the system is cured at a temperature below T_g∞ [2, 3]. However, in non-isothermal curing (in contrast to isothermal curing) vitrification is followed by devitrification. This occurs when the sample temperature once again becomes higher than the glass transition temperature. In this case, the molecules regain their mobility and the reaction can proceed to completion.

In the past, the non-isothermal curing of resins has been measured by temperature-modulated DSC at a single frequency [4-6]. The frequency behavior of vitrification and devitrification has been studied in detail by dielectric relaxation spectrometry [7] or mechanical spectrometry (e.g. by torsional braid analysis [8]).

The frequency dependence of vitrification and devitrification can also be measured by conventional temperature-modulated DSC (ADSC).

This is, however, very time-consuming because a new experiment has to be performed for each frequency, usually at a very low heating rate and with sample and blank runs. Furthermore, each experiment needs a fresh mixture of the resin and the crosslinking agent because the samples are reactive systems and cannot be stored. Each new mixing process inevitably leads to small variations in the composition of samples, which in turn contributes to experimental uncertainty.

In this article, we show how the frequency behavior of vitrification and devitrification in non-isothermal curing processes can be investigated by TOPEM^®. Knowledge of the frequency behavior of vitrification and devitrification allows the temperature dependence of the relaxation time to be determined. This is of fundamental interest for understanding the glass transition. 

Experimental Details 

Sample Preparation 

The epoxy resin used was a diglycidylether of bisphenol-A (DGEBA), Epon 828 from Shell Chemicals. The hardener (crosslinking agent) was a polyoxypropylene diamine, Jeffamine D-230 from the Huntsman Corporation. Stoichiometric mixtures of resin and hardener were prepared and samples of suitable mass (typically about 25 mg) were weighed into 40-uL aluminum crucibles.

Conclusions

The vitrification and devitrification of systems curing under non-isothermal conditions can easily be studied using TOPEM^®. In contrast to conventional temperature-modulated techniques (ADSC, IsoStep), the frequency dependence of the vitrification process can be investigated by TOPEM^® in one single measurement. The entire frequency range of the DSC can be used. A “Continuous Heating Transformation” (CHT) diagram can be constructed by performing just a few measurements at different heating rates. This diagram summarizes the dependence of vitrification and devitrification on heating rate, frequency and time. In the example discussed here, a CHT diagram was constructed from five measurements of the DGEBA (resin) and polyoxypropylene diamine (hardener) system.

^{Vitrification and Devitrification During the Non-Isothermal Cure of a Thermoset Studied by TOPEM ®  | Thermal Analysis Application No. UC 302 | Application published in METTLER TOLEDO Thermal Analysis UserCom 30}