Vitrification during the Isothermal Cure of a Thermoset Studied by TOPEM

Vitrification occurs during the isothermal cure of a crosslinking system if the cure temperature is below the glass transition temperature of the fully cured resin. The process of vitrification was studied using TOPEM^® [1], a new temperature-modulated DSC technique.

The results are compared with those obtained from conventional temperature-modulated DSC (ADSC). The comparison showed that TOPEM^® offers several important advantages.

Introduction

When an epoxy resin reacts with a hardener (crosslinking agent), the system changes from a viscous liquid (with an initial degree of conversion, α = 0) to a highly crosslinked molecular network (with a final degree conversion, α ≤ 1).

The reaction rate or rate of cure is initially controlled by chemical kinetics. The unreacted mixture has a glass transition temperature, T_g0. As the curing reaction proceeds, the glass transition temperature, T_g, of the system increases parallel to the increase in the degree of crosslinking. If the curing temperature is sufficiently high, the crosslinking reaction proceeds to its limit, α = 1, and the final glass transition temperature, T_g∞, is that of the fully cured thermoset.

On the other hand, if the curing process takes place at a temperature below T_g∞, at some point the system changes to a glassy state and vitrifies. In fact, vitrification occurs when T_g reaches the isothermal cure temperature, Tc. Under these circumstances, molecular mobility is greatly reduced and the reaction becomes diffusion controlled. The reaction rate slows and the degree of conversion, α, reaches a final value that is less than 1. The time at which the reaction kinetics change from being mainly chemically controlled to diffusion controlled is known as the vitrification time. Previously, the study of the vitrification by conventional DSC was a very timeconsuming process [2, 3]. It required a series of isothermal experiments with different reaction times. The samples were then cooled and the glass transition temperature of the partially cured systems measured in a heating run in order to determine the cure time when T_g = T_c for any given cure temperature

More recently, temperature modulated DSC (TMDSC) techniques such as alternating DSC (ADSC) have simplified such experiments [4, 5]. Using these techniques, one can obtain the curve of the total heat flow curve, equivalent to that given by conventional DSC, together with the so-called complex heat capacity curve c_p * . The c_p * curve is obtained from the measured heat flow generated by the low-amplitude temperature modulation. When the system vitrifies, c_p * changes rather abruptly from a value corresponding to the liquid state to a lower value corresponding to the glassy state. This process is referred to as dynamic vitrification because it is frequency dependent. 

For the epoxy-amine systems used here, it has been shown that the dynamic vitrification time determined for periods of approximately 30 to 60 s corresponds well with that obtained by conventional DSC [6]. In the same experiment, using TMDSC it is thus possible to follow both the overall heat of curing (as in conventional DSC) and the process of dynamic vitrification. The investigation of the frequency dependence of the vitrification process is therefore possible with this method. However, it requires a series of individual TMDSC experiments together with the corresponding blank runs for each of the frequencies of interest. This is once again time consuming, especially if the reaction mixture begins to react at room temperature. 

In such cases, it is necessary to prepare a fresh mixture of resin and hardener for each frequency measured. There is then the possibility that the composition of each mixture is slightly different, which in turn increases the overall experimental error. A multi-frequency technique is advantageous because it avoids this problem and reduces the experimental time involved. The TOPEM^® technique does precisely this.

The purpose of the present work was to compare studies of the vitrification process made by ADSC and by TOPEM^® for the isothermal cure of an epoxy-diamine system. 

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 were weighed into aluminum crucibles. For the TOPEM^® measurements, samples of approximately 20 mg were used. 

Conclusions

TOPEM^® offers significant advantages for the study of vitrification processes during the isothermal cure of thermosetting resins compared with other temperature-modulated DSC techniques:

• The dynamic vitrification time can be determined in a selected frequency range in just one single experiment. This saves much time and reduces experimental error because only one sample is required. 
  
• Higher frequencies can be attained using TOPEM^®. 
  

The better reproducibility and the availability of higher frequencies leads to a new result namely that the dependence of the dynamic vitrification time is not a linear function of the logarithmic frequency. The upward turn in the corresponding curve had not been previously observed but is expected on theoretical grounds.

_{Vitrification during the Isothermal Cure of a Thermoset Studied by TOPEM^® | Thermal Analysis Application No. UC 294 | Application published in METTLER TOLEDO Thermal Analysis UserCom 29}