Vitrification during cure

The three curves correspond to cure below, near and above Tg o, and it may be seen that only in the first case is there a clear step-change in heat capacity. The processing of the resin above Tgco (150°C) results in complete chemical reaction since there is no vitrification during cure. 

These results clearly show that the presence of CRL is linked to the occurrence of a glassy state, and that the increase in CRL signal correlates closely with the onset of vitrification during cure. The findings thus confirm that this technique has the potential of being used as an in-situ sensor for vitrification during cure. 

The frequency-dependent increase in storage modulus to 1 GPa and the frequency-dependent loss peaks signify vitrification during cure. Vitrification, as expected, is frequency-dependent since it is the cure-induced glass transition. Gelation, however, is an event that is independent of frequency. It is important to note that the shear sandwich experiment is useful primarily for B-staged resins whose viscosity is sufficient so that the resin does not flow out of the gap between the parallel-plate shear sandwich fixtures. 

No. 15, 2000, p.5949-55 UNDERSTANDEVG VITRIFICATION DURING CURE OF EPOXY RESINS USING DYNAMIC SCANNING CALORIMETRY AND RHEOLOGICAL TECHNIQUES Lange J Altmann N Kelly C T Halley P J Queensland,University... 

An interesting method of eliminating the undercure caused by vitrification when using microwave radiation is to modify the formulation, including the use of polar thermoplastic that phase-separates during cure (Chapter 8). The thermoplastic material can convert microwave energy into heat, which enables the thermosetting polymer to devitrify and reach full cure. 

In Section 4, we have examined, from a fundamental point of view, how temperature and cure affect the dielectric properties of thermosetting resins. The principal conclusions of that study were (1) that conductivity (or its reciprocal, resistivity) is perhaps the most useful overall probe of cure state, (2) that dipolar relaxations are associated with the glass transition (i.e., with vitrification), (3) that correlations between viscosity and both resistivity and dipole relaxation time are expected early in cure, but will disappear as gelation is approached, and (4) that the relaxed permittivity follows chemical changes during cure but is cumbersome to use quantitatively. 

Fig. 29. Change of the specific volume (Vsp) of a system during cure. Tjure < T < T UIC O Vitrification points at Tjute and TPure (conversion The start of cure ...

Figure 3.5 shows an isothermal DSC curve for the curing of TGDDM with the aromatic amine DDS (de Bakker et al, 1993b). Also shown is the NIR analysis of the instantaneous rate of reaction of epoxide. The difference arises due to the change in heat capacity on gelation and vitrification that affects the DSC baseline during cure, as discussed below. 

The time for half-decrease in Cp is shown and, in a separate experiment, the resin at this point was found to have a Tg of 80 C, confirming that the change in Cp arises predominantly due to vitrification of the resin during cure. If cure occurs at a temperature above then the change in Cp is small. Figure 3.7 shows the change in Cp as a function of the time of cure of an epoxy resin with an aromatic amine at three different tempemtures in a MDSC (Lange et al, 2000). 

Both for the bulk and the layers, curing is not hampered by vitrification during network formation because the Tg values are lower than the Tc (180 °C) at... 

This chapter discusses the intenelation between mechanical properties, molecular mobility and chemical reactivity of curing epoxy-amine thermosets, illustrated by examples of how the charge recombination luminescence (CRL), heat-capacity and rate constants of chemical reactions are influenced by gelation and vitrification during isothermal cure. A comparison of dynamic mechanical, CRL and modulated temperature DSC data shows that vitrification is accompanied by an increase in CRL and a decrease in heat-capacity, and that the heat-capacity and CRL continue to change after the viscoelastic properties have levelled out. It is also shown how the rate constant of an intermolecular secondary amine reaction, measured by near infirared spectroscopy, is sensitive to gelation, whereas the intramolecular rate constant instead is sensitive to vitrification. 

The changes in reaction rates of different species measured by FTNIR can thus be utilised as "molecular probes" of gelation and vitrification, providing simultaneous information on the progress of the chemical reaction and the transitions encountered during cure. However, the technique requires significant data analysis and is therefore not readily adapted to on-line monitoring. 

The reaction mechanism indicates that the epoxy concentration decreases, and this is observed in the spectra as the decrease of the band centered at 4530 cm-i and also of the weak overtone of terminal CH2 at 6060 cm-i. The primary amine combination band decreases too ( = 4900 cm-i), and once it is exhausted it can be observed that there are still epoxy groups in the reaction media, which will react with the previously formed secondary amines up to vitrification or until the reaction is completed. The band correponding to O-H overtones ( 7000 cm-i) also increases during curing as a consequence of the oxirane ringopening, although this band is not suitable for quantification because of the low signal/noise ratio. The behavior of the band located at 6500 cm-i is more complex in this... 

---