Thermosets derive their usefulness from the ability to start as a low viscosity resin formulation and through a curing process via well-defined chemical pathways transform into a highly crosslinked network. The final properties of the crosslinked networks such as the glass transition temperature (Tg) and the moduli can be tailored via the choice of monomers and curing agents (i.e., the resin formulation). Thermosets with high Tg are used extensively in advanced composite applications due to their high temperature stability.
Thermosets during curing:
- Low initial viscosities enable many types of processing
- Polymer chains are crosslinked by well-defined chemical reactions
- Thermoset crosslinked networks are stable on heating (major advantage in electronic applications)
The curing process is a critical part of the manufacturing process and having a means to measure the degree of cure during processing would be very useful. The objective of this paper is to demonstrate how dielectric spectroscopy can be effectively utilized to monitor the build-up of the crosslinked network during curing. Unfortunately, dielectric methods have not achieved widespread adoption as a cure monitor during thermoset processing. The main reason in my opinion is that dielectric spectroscopy appears to be overly complicated, uses terminology not easily understood, and the dielectric response can be complicated to analyze. The purpose of this series of posts is to “demystify” or present in a simple, understandable manner how the dielectric response can be correlated with the physical properties such as Tg and viscosity during thermoset curing. Once the relationship between the dielectric response and the physical changes during crosslinking are established, the utility of the dielectric method as a manufacturing process monitor will become obvious.
Dielectric Measurements
This section will provide a basic introduction to how dielectric measurements are conducted. I present a “black box” approach where the sample is shown between two electrodes and the apparatus applies a time varying voltage and measures the time-varying current (or in the case of the microdielectrometer, the time-varying charge) as shown in Figure 1.
Figure 1. Black box approach to dielectric measurements.
The resulting time-varying current i(t) has a phase lag, and the dielectric permittivity and loss factor are calculated using the phase lag (ϕ):
Commercial instruments are very good at measuring the phase angle and the software performs calculations, so we are going to focus on how to use and interpret dielectric data.
Dielectric Terminology
In the previous section, the basics of the dielectric measurements were presented. The focus of this paper is to “demystify” dielectric measurements. With that said, dielectric terminology can be a bit confusing so we will take time here to set the stage for future sections.
Dielectric Permittivity:
- represents the polarization of the medium
- typically called the “dielectric constant” but for curing systems the dielectric “constant” changes as a function of the cure state and temperature
Dielectric Loss Factor arises from two sources:
- Energy loss associated with time-dependent dipolar relaxations
- Bulk (or ionic) conduction
For most curing thermosets, for example epoxies, they have polar groups that give rise to dipoles that can respond to the applied voltage. A good example would be the hydroxyl groups formed during amine-epoxy curing. Additionally, most thermosets are not completely pure, that is they have some residual ions from the synthetic process used to make them. In epoxies, there is residual chlorine and sodium from the process used to put the epoxy functionality on bisphenol A. As we will see, these residual ions (even at low levels) can be very useful probes for the dielectric measurements.
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