Dielectric measurement of cure for one-part epoxy adhesive [5]
Dielectrometry [4] is the most commercialised of all the cure monitoring methods, and is purportedly the most comprehensive at detecting all important stages during cure:
The technique uses electrodes in contact with the resin to monitor the evolution of the intrinsic electrical properties of the material, occurring due to chemical and physical changes during cure (viscosity and Tg). The basic principle of operation is the measurement of changes in the voltage and current between a pair of electrodes [4]. The application of a sinusoidal voltage to a pair of electrodes creates a localised electric field. This induces ion motion and dipole rotation within the resin generating a sinusoidal current. These motions are hindered by viscous drag, resulting in a phase difference between the applied voltage and the stimulated current. This charge redistribution and reorientation only occurs whilst the charged species are sufficiently mobile to respond within the timescale/frequency of the exciting field. By measuring the current and associated voltage at regular intervals and several frequencies as cure progresses (as wide a range as possible within 10–4—1011 Hz), the changing conductance and capacitance of the resin can be determined. The applied voltage is kept low in order to minimise the possibility of electrochemical reactions.
Dielectric sensors can be either implanted or reusable, and positioned for surface or internal measurements. In addition, they need protection from shorting against any conductive components within the material (e.g. metallic particulate fillers), but must still be in intimate contact with the resin. Usually a thin layer of glass fibre fabric, polyimide film or thin porous PTFE peel ply is sufficient to protect electrodes from shorting due to contact with conductive fillers. The distorting effect of conductive fillers on the electric field and subsequent results has not been fully investigated and can itself cause problems in interpretation. Alignment and separation of the electrodes must be tightly controlled for valid results. The impedance frequency sweep time must be short enough that the curing system does not change appreciably during the measurement and can be considered instantaneous. Dielectric sensors may act as flaws in the cured product, and therefore re-usable sensors built into mould tool walls are preferred.
Electrical resistance of an adhesive can be used as a measure of the state-of-cure, although this is most applicable to conductive adhesives, such as those used in microelectronics that have silver, or other metallic filler particles. The technique works by measuring the current flowing through the adhesive and observing the changes in resistance as the adhesive cures. There is a marked decrease in resistance as the adhesive cures, associated with the formation of the cross-linked network and stabilisation of the filler particle locations, so that current can flow more easily. Since the technique requires a conductive adhesive, it is only applicable in a limited range of industrial applications and is not suitable for development as a general cure-monitoring method.
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