This article presents a new two-frequency method for determining the center frequency of a resonant microwave sensor for monitoring the curing reactions of thermosetting and thermoplastic resins and adhesives. The results show that non-invasive, continuous and continuous monitoring of the dielectric properties of polymers can be realized, which will allow to control the adequate evolution of the curing process according to known initial conditions. Examples of experimental measurements during the cure reaction of polyurethane adhesives were selected to illustrate the effect of the method. In comparison with the known methods, this method allows to increase the signal-to-noise ratio of measurements and their accuracy by performing the measurement procedure for the maximum signal and to the region of the minimal noises of the envelope detector. As a method development, in the conclusion, a method of variation of the difference frequency and microwave sensors in forbidden bands was proposed, which is due to the obtained results only from the measurement of the central frequency and the existing need to make measurements in real time and in a complicated interference situation, taking into account the changing quality factor and the reflection coefficient of the resonance sensor. The article did not focus on the simplicity and cost of the implemented device. However, the obviousness of this allows us to use the system as a means of control both for the production of polymers and for their laboratory research.

Keywords: microwave technologies, thermosetting and thermoplastic polymers, microwave measurements, resonance sensor, two-frequency symmetric probing, beat envelope, central frequency, dielectric constant, degree of curing

In this paper, we present a new two-frequency method for determining the resonant frequency, transmission bandwidth, reflection coefficient, and quality factor of a resonant microwave sensor for monitoring the curing reactions of thermosetting and thermoplastic resins and adhesives. The method was named - the method of variation of the difference frequency. The results show that non-invasive, continuous and continuous monitoring of the dielectric properties of polymers can be realized, which makes it possible to control the adequate evolution of the process of their curing by known initial conditions. The questions of the realization of the method are considered in the case of variations in the difference and mean frequencies of the probing oscillation, as well as the option for fixed sounding, provided that all four components of the two two-frequency oscillations are in the spectral band of the microwave sensor. In addition, an estimate is given for determining the error in measuring the resonance frequency when probing different zones of the spectral characteristics of the sensors from 0.1 to 0.707 of the maximum reflection value. The universality of the method makes it possible to use a measurement system based on it as a means of monitoring both the production of polymers and for laboratory research

Keywords: Microwave technology, thermosetting and thermoplastic polymers, microwave measurements, resonance sensor, difference frequency variation method, beat envelope, resonant frequency, Q-factor, bandwidth, reflection coefficient, dielectric constant, degree of