Reproducible Evaluation of DSC Measurements: Glass Transition Followed by Chemical Reaction - METTLER TOLEDO

Reproducible Evaluation of DSC Measurements: Glass Transition Followed by Chemical Reaction

Introduction 

The reproducible quantitative determination of heats of reaction plays an important role both in quality assurance and in research and development. This necessitates both a careful measurement and a reliable evaluation procedure. In practice, it is often difficult to choose an optimum baseline when determining peak areas. This is particularly the case when different relatively small or broad thermal effects follow one another. This study presents a method for the reproducible evaluation of such DSC curves using the postcuring of an epoxy resin as an example.

 

Postcuring 

At temperatures below the glass transition temperature the epoxy resin is in glassy state and behaves like a solid. Diffusion of the reactants is restricted. Almost no curing reaction can therefore take place. At the glass transition the diffusion improves suddenly by several degrees of magnitude. The sample becomes liquid and the chemical reaction begins.

 

An example of this is shown in Figure 1. In this case, it is the postcuring reaction of an epoxy-amine system (DGEBA: diglycidylether of bisphenol A; DDM: diamino diphenylmethane). Samples of the mixture were cured at 100 °C for different periods of time, shock-cooled, and then heated at 10 K/min. In the DSC curves, the glass transition and the postcuring reaction are immediately apparent. The reaction begins as soon as the glass transition occurs and shows a relatively broad exothermic peak. With longer reaction times the glass transition shifts to higher temperatures and the exothermic postcuring reaction becomes smaller. In particular, with reaction times of more than 50 minutes, an endothermic peak due to enthalpy relaxation is superimposed on the glass transition. In such cases, it is then difficult to perform a reproducible evaluation of the glass transition and reaction peak.

 

Evaluation Procedure 

Richardson has described how the baseline of the reaction peak can be determined with the help of the curve of the fully cured material [1]. On the basis of these considerations, a practicable evaluation algorithm for the evaluation of the peak area and the glass transition is presented below.

Summary

A reproducible evaluation routine is proposed for evaluating a glass transition and a subsequent reaction when the two events lie close to each other. For this evaluation, the curve of the fully reacted material (second measurement) is necessary. The evaluation is performed on the normalized heat flow curve in three steps:

  • Construction of a line fitted to the heat flow of the fully cured material above the glass transition. 
  • Subtraction of the line from the measurement curve. 
  • Integration of the subtracted curve using a horizontal baseline. 

To evaluate the glass transition, a tangent is drawn above the glass transition that coincides with the baseline of the peak integration.

Reproducible Evaluation of DSC Measurements: Glass Transition followed by Chemical Reaction | Thermal Analysis Application No. UC 146 | Application published in METTLER TOLEDO Thermal Analysis UserCom 14