Study of the Curing Behavior of a Trifunctional Epoxy Resin

This article describes how the curing behavior of a highly crosslinked epoxy resin system was investigated using conventional DSC and TOPEM® measurements. The results showed that the curing behavior is much easier to study by TOPEM® than by conventional DSC. Furthermore, the glass transition temperature of the completely cured material can only be determined by TOPEM®.

Introduction

Epoxy resins are thermosetting polymers that are used for many different applications, for example, as adhesives, surface coatings, matrix materials for fiber-reinforced composites or for polymer-layered silicate nanocomposites (PLS).

One of the most frequently used epoxy resins for these applications is the bifunctional epoxy diglycidyl ether of bisphenol- A (DGEBA). However, many high-performance applications, for example in the aerospace industry, put greater demands on the properties of the resin and require very highly crosslinked composites that are stable up to temperatures of 250 °C.

One resin that is suitable for producing such materials is the trifunctional epoxy triglycidyl p-aminophenol (TGAP). The resin is cured with a diamine.

The resulting rigid, three-dimensional crosslinked polymer has a higher net work density, a correspondingly high glass transition temperature, and better thermal stability compared with DGEBA. Conventional differential scanning calorimetry (DSC) is a technique widely used to investigate the curing behavior of crosslinking systems. Both isothermal and dynamic methods are employed.

Nevertheless, the technique has certain disadvantages when used to characterize the curing reaction of TGAP with a diamine:

• Due to the large reaction enthalpy involved in the curing reaction, the curing of TGAP is usually performed in several isothermal steps at temperatures below the final glass transition temperature. In this process, the material vitrifies during each individual curing step. To perform the multistep curing process as efficiently as possible, the vitrification time (the time taken by the material to vitrify at a certain temperature) has to be known. However, the determination of the vitrification time by conventional DSC requires considerable experimental time and work.
• The glass transition of the fully cured TGAP/DDS thermoset cannot be measured by conventional DSC. The network density is so high that the change in heat capacity at the final glass transition is too small to detect. The glass transition temperatures given in the literature for this system are therefore usually determined by DMA [1, 2].

In this article, we will show how these difficulties can be elegantly solved using TOPEM®.

Experimental details

Sample

TGAP epoxy resin (Araldit MY0510) and 4,4-diamino diphenyl sulphone, DDS (Aradur 976-1) as the curing agent from Huntsman Advanced Materials were used without further purification...

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Conclusions

If the isothermal curing behavior is investigated using conventional DSC, the measured isothermal DSC curves show no indication of possible vitrification.

Vitrification during isothermal curing can only be identified by measuring the glass transition of the cured material in the change in the specific heat capacity signal.

Estimation of the vitrification time by conventional DSC is only possible with considerable experimental time and effort: several isothermal and dynamic measurements have to be performed each time with new samples. In this case, the glass transition of the completely cured TGAP/DDS system could not be determined by conventional DSC.

The vitrification time can therefore be directly determined. TOPEM® also al lows the glass transition temperature of measured.

In this method, one takes advantage of the fact that if the heating rate is sufficiently low, complete curing of the resin takes place in the glassy state; when the curing reaction is finished, the measured glass transition corresponds to a good approximation to the glass transition temperature of the completely cured system.

If TOPEM® is not available, the glass transition of a completely cured TGAP/ DDS system can only be determined by DMA.

TOPEM® measurements therefore allow one to understand both the isothermal and the dynamic curing behavior of a material and to determine the glass transition temperature of the completely cured material.

Study of the Curing Behavior of a Trifunctional Epoxy Resin | Thermal Analysis Application No. UC373 | Application published in METTLER TOLEDO Thermal Analysis UserCom 37