By subjecting the resin to an oscillating electric field, dielectric properties such as the mobility of the ions (ion conductivity) and the ability of the dipoles to re-orient can be measured from the electrical response.
Essentially the dielectric response can be divided into 2 parts – the in phase component, which relates to the storage part of the material, and the out of phase response (the loss ) which is a measure of ion viscosity These are sometimes expressed as the real and imaginary components of the electrical signal. The phase angle tan d is also used as a measure of the viscous component of the response, as is done with dynamic mechanical analysis (DMA). However, compared with DMA or rheology, it is possible to monitor the final stages of curing much more closely, well beyond the point at which the material is effectively behaving as a high modulus solid. Scanning over a wide range of frequencies is also possible.
The measurement can be made by placing opposing electrodes on opposite faces of the sample, or by using single sided closely spaced comb electrodes which cause the electric field to penetrate only a limited distance into the surface, and are ideal for paints or adhesives. Either permanent or disposable electrodes can be employed, allowing the technique to be used in the laboratory or on production items, with the capacity to monitor curing in mould or in an oven and therefore to optimise the in mould time for each individual part.
Because every moulding will be subject to small variations in resin composition, temperature and humidity, the result is much more efficient utilisation of the curing time. The cost savings which flow from mould use optimisation, and avoidance of distortion due to premature demoulding, can often pay for the DEA system in a very short time.
There are versions of the DEA with single or up to 10 channels for both DEA signal and temperature, and they can be used even for high speed (e.g. UV curing of paints and adhesives) or conventional thermosetting resins.
