N. Altmann

Understanding vitrification during cure of epoxy resins using dynamic scanning calorimetry and rheological techniques

The gelation and vitrification transitions during cure of an epoxy-amine system are examined using rheological, modulated differential scanning calorimetry (DSC) and FTIR techniques. The results from dynamic mechanical analysis show that gelation is observed before vitrification at all temperatures where it can be rheologically defined. By comparing different rheological criteria for vitrification, it is seen that the vitrification transition is a gradual process that extends over a large part of cure at all temperatures where it occurs. Results from modulated DSC measurements show that the calorimetric vitrification times are longer than the vitrification times obtained from rheological measurements at cure temperatures above 100°C, but that at lower temperatures calorimetric vitrification occurs before, or at the same time as, rheological vitrification. Theoretical gelation times, estimated from FTIR conversion data, were found to be consistently shorter than the observed gelation times. Theoretical vitrification times agreed well with the observed times. The magnitude of the vitrification transition, expressed either as amount of change in heat capacity or as maximum value of loss tangent, was found to decrease approximately linearly with increasing cure temperature.

The effects of silica fillers on the gelation and vitrification of highly filled epoxy-amine thermosets

Highly filled thermosets are used in applications such as integrated circuit (IC) packaging. However, a detailed understanding of the effects of the fillers on the macroscopic cure properties is limited by the complex cure of such systems. This work systematically quantifies the effects of filler content on the kinetics, gelation and vitrification of a model silica-filled epoxy/amine system in order to begin to understand the role of the filler in IC packaging cure. At high cure temperatures (100°C and above) there appears to be no effect of fillers on cure kinetics and gelation and vitrification times. However, a decrease in the gelation and vitrification times and increase the reaction rate is seen with increasing filler content at low cure temperatures (60-90°C). An explanation for these results is given in terms of catalysation of the epoxy amine reaction by hydrogen donor species present on the silica surface and interfacial effects.

Toward a new universal model for polymer rheology based on group interactions

Recent work [1-2] has developed a a dynamic monte carlo percolation grid simulation which can successfully predict the linear viscoelastic response of thermosets materials during the whole isothermal cure, including the power-law relaxation at gelation. The model is based on extension of the group interaction model [3] to incorporate connectivity and branching effects. This paper will discuss the usefulness of this viscoelastic model in describing thermoset polymer viscoelasticity and gelation behaviour and include new interpretations for network development from gelation through to vitrification for thermoset systems. Additionally the extension of this model to predictions of the viscoelasticity of branched thermoplastic polymer systems (hyperbranched polymers, long chain branched polymers and polydisperse polymers) will then be discussed with surprising results [including the successful prediction of viscosity dependence on molecular weight shifting from a power of 1.0 to 3.4, as seen experimentally for many thermoplastic systems]. In this way we hope to describe the potential of this energetic approach for developing a new universal model for polymer viscoelasticity.