Microwave sensor system for continuous monitoring of adhesive curing processes

Abstract

A microwave sensor system has been developed for monitoring adhesives curing processes. The system provides continuous, real-time information about the curing progress without interfering with the reaction. An open-coaxial resonator is used as the sensor head, and measurements of its resonance frequency and quality factor are performed during cure to follow the reaction progress. Additionally, the system provides other interesting parameters such as reaction rate or cure time. The adhesive dielectric properties can be also computed off-line, which gives additional information about the process. The results given by the system correlate very well to conventional measurement techniques such as Differential Scanning Calorimetry, combining accuracy and rate with simplicity and an affordable cost.

Non-invasive and continuous monitoring of the sol–gel phase transition of supramolecular gels using a fast (open-ended coaxial) microwave sensor

“…Fig. 23 Fig . The system comprises a microwave sensor, a microwave transmitter and receiver (from 1.5 to 2.5 GHz) and a control unit to provide real-time information about the gelation progress without interfering with the reaction.…”

“…(D) Magnitude representation of the resonance measurement of a sol-gel sample. 21,22 For example, a microwave system designed for adhesive cure monitoring has been previously described by some of us 23 where in situ dielectric measurements correlate very well with conventional measurement techniques such as DSC, combining accuracy and rate with simplicity and an affordable cost. At least 30 mm of the test tubes used for the measurements (40 Â 7 mm, H Â D) should be filled with the material for accurate data acquisition.…”

“…20,21 Changes in dielectric parameters can be related to critical points in different material processes, such as cure reaction onset, gelation, end-of-cure, build-up of the glass-transition temperature, etc. 21,22 For example, a microwave system designed for adhesive cure monitoring has been previously described by some of us 23 where in situ dielectric measurements correlate very well with conventional measurement techniques such as DSC, combining accuracy and rate with simplicity and an affordable cost.…”

“…24,25 When the low-intensity electromagnetic waves penetrate into the material, its molecules tend to orient with the (applied) external field and the material gains certain polarization, reflecting the back part of the microwave signal from the sensor. This reflected signal is measured continuously to determine the resonance frequency and quality factor of the sensor 23 during gelation to monitor the transition process. Fig.…”

“…We have reported elsewhere the fundamental details of the microwave system with a different sensor head. 23 Fig. 2 shows the library of known gelators that we prepared (ESI †) to test the ability of the microwave sensor to monitor the sol-gel transition of physical gels.…”