Polytetrafluoroethylene Films in Rigid Polyurethane Foams’ Dielectric Permittivity Measurements with a One-Side Access Capacitive Sensor

Abstract

As a non-metallic composite material, widely applied in industry, rigid polyurethane (PUR) foams require knowledge of their dielectric properties. In experimental determination of PUR foams’ dielectric properties protection of one-side capacitive sensor’s active area from adverse effects caused by the PUR foams’ test objects has to be ensured. In the given study, the impact of polytetrafluoroethylene (PTFE) films, thickness 0.20 mm and 0.04 mm, in covering or simulated coating the active area of one-side access capacitive sensor’ electrodes on the experimentally determined true dielectric permittivity spectra of rigid PUR foams is estimated. Penetration depth of the low frequency excitation field into PTFE and PUR foams is determined experimentally. Experiments are made in order to evaluate the difference between measurements on single PUR foams’ samples and on complex samples “PUR foams + PTFE film” with two calibration modes. A modification factor and a small modification criterion are defined and values of modifications are estimated in numerical calculations. Conclusions about possible practical applications of PTFE films in dielectric permittivity measurements of rigid PUR foams with one-side access capacitive sensor are made.

Recent Advances in Flexible Sensors and Their Applications

“…When force is applied to a capacitive sensor, the distance or the area of the plates changes, and the capacitance changes accordingly. Employing this principle, wearable sensors can be made using nanomaterials that measure capacitive strain [73][74][75][76][77][78][79][80]. A novel capacitive strain sensor using CNTs demonstrates good durability with a strain range between 1% and 300% [81].…”

The Impact of Atmospheric Parameters on the Dielectric Permittivity Values of SikaBlock®-M150 and Other Rigid Polyurethane Foams Measured with a Capacitive One-Side Access Sensor

“…The foams are also used as encapsulants for electronic components to mitigate harsh thermal and mechanical environments, as well as to provide electrical insulation [ 6 ]. The foams exhibit a low dielectric interference and their dielectric permittivity (permittivity) is nearly non-dispersive; therefore, a good dielectric performance can be ensured in a wide frequency range [ 7 , 8 , 9 ]. SikaBlock ® -M150 PU foams are dense, easily workable, and industrially manufactured (Sika JSC, Baar, Switzerland) with a fine structure, exhibiting a low dust formation when milled.…”

“…Knowledge on the permittivity of rigid PU foams is necessary in NDE, in practical applications, related to the electromagnetic field, as well as air- and space-craft applications, etc. [ 7 , 8 ]. The method had wider application with developing capacitive one-side access (COSA) sensors [ 11 , 12 , 13 , 14 , 15 ].…”

“…COSA sensors are widely used in non-destructive testing, proximity/displacement measurements, material characterization, non-metallic material thickness determination, intelligent human interfacing, security systems, etc. [ 7 , 8 , 12 , 15 ].…”

“…The major technical difficulties, connected to the PU foams themselves, lie in low permittivity values. At excitation frequencies of 10 Hz–0.33 MHz, the permittivity of (a) SikaBlock ® -M150 PU foams is measured as 1.23–1.27 and the permittivity of (b) rigid PU foams (with a density of 33 kg/m 3 ) is measured as 1.050–1.048; i.e., only ~5% higher than the permittivity of vacuum [ 7 , 8 ].…”