Dielectric Permittivity of Rigid Rapeseed Oil Polyol Polyurethane Biofoams and Petrochemical Foams at Low Frequencies

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

“…Rigid closed-cell petrochemical PUR foams were made in laboratory conditions in a range of apparent core density 80 kg/m 3 ≤ ρ ≤ 850 kg/m 3 , in blocks, in (a) open, free-rise moulds (25 cm × 25 cm × 20 cm) and (b) closed cylindrical polypropylene moulds (height = 8.0 cm and inner diameter = 10.0 cm) according to the technology and formulations given in [ 5 ]. Responding to the needs of bioeconomy, biopolyol was synthesized from Latvia-grown rapeseed oil by the trans-esterification method with triethanolamine (molar ratios 1 M:2.5 M and 1 M:2.9 M), in an environmentally friendly process, without emission of harmful substances, at temperatures T = 175 °C ± 5 °C and rigid closed-cell polyurethane biofoams were made in a range of apparent core density 80 kg/m 3 ≤ ρ ≤ 450 kg/m 3 .…”

“…The value of PTFE’s permittivity ε ≈ 2.1 coincides with that of rigid PUR foams of density 600 kg/m 3 that corresponds to the middle of PUR foams’ full range of densities: at 31 kg/m 3 permittivity ≈ 1.065; at 550 kg/m 3 ε ≈ 2.1 and for monolithic polyurethane, density 1280 kg/m 3 , permittivity ≈ 3.3 (1 kHz) [ 5 ]. PTFE is nonreactive, it reduces mechanical friction and wear (PTFE’s static frictional coefficient ≈ 0.04), therefore, a longer service time of the protective coating can be expected.…”

“…Several values of dielectric permittivity (permittivity) are reported in an experimental investigation of rigid PUR foams with a capacitor at frequencies 100 Hz and 10 MHz according to ASTM D1673 [ 4 ]. In [ 5 ] a one-side access (OSA) capacitive sensor was used to determine permittivity at low frequencies 10 Hz–0.33 MHz for (1) petrochemically-rigid PUR foams, density 32–539 kg/m 3 : 1.059 ≤ ε ≤ 2.067 and for (2) monolithic petrochemical-origin polyurethane, density 1280 kg/m 3 : 3.47 ≥ ε ≥ 3.31. To characterise the dielectric dispersion, the dropping factor was introduced and calculated for the investigated PUR foams of densities up to 400–450 kg/m 3 as F ≤ 5.0% and F ≈ 6.0% for monolithic petrochemical polyurethane.…”

“…PUR foams release gasses from foaming process [ 1 , 2 ] that may cause oxidation or other chemical processes in the metal of the capacitive sensor’s active area (e.g., brass or copper). PUR foams can carry considerable static electricity (caused by exploitation and processing conditions) that damages the electronic system of sensors [ 5 ]. The daylight degraded and mechanically-processed surfaces of PUR foams’ drop plastic micro-particles [ 1 , 2 ], leading to mechanical contamination of the sensor’s active area, thus the accuracy of the permittivity measurements is reduced.…”

“…An analysis of scientific information sources revealed a lack of investigations and reliable experimental data on OSA capacitive sensor’s protective coverage and coating in permittivity measurements of cellular plastics, including rigid PUR foams. The major technical difficulties, connected to the plastics foams themselves, lie in extremely low permittivity values; e.g., rigid PUR foams of density 31 kg/m 3 exhibit permittivity ≈ 1.065 (1 kHz), that is only ≈ 6% higher than the permittivity of vacuum [ 5 ].…”