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MATERIALS 47
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solutions (LiCl, RH = 10%; MgCl2, RH = 33%, NaNO2, RH where:
= 65%, NaCl, RH = 75%; ZnSO4, RH = 87%) providing the Cs1 capacitance of the empty capacitor [F]
six specified humidity levels (Table 1). Cs2 capacitance with the test specimen inserted [F]
Density was determined for each lamella based on oven- tg distance between the capacitor electrodes [m]
dry mass and conditioned volume before measurement. The td average thickness of the test specimen [m]
density was not experimentally controlled, but samples were D1 loss factor of the empty capacitor

taken to more closely mimic industrial conditions where it is D2 loss factor of the capacitor with the specimen.
not possible to experimentally control the density, but only the
texture of the element, which is, for aesthetic reasons, radial The relative dielectric constant was calculated indirectly
as were the samples used in this experiment. using Equation, while the dielectric loss factor was determined
After conditioning, specimens were sealed in polyethylene using Equation. Measurements were conducted at two sample
bags to prevent moisture exchange prior to measurement. The temperatures: 20 deg C and 90 deg C.
steamed group was prepared separately by exposing lamellas These two temperatures represent room temperature and
to saturated steam at 90 deg C for 90 min, after which they the typical temperature used when preheating wood elements
were cooled in sealed containers to avoid uncontrolled drying. before bending. A total of 26 electric field frequencies ranging

This ensured that the steaming treatment primarily affected the from 0.079 MHz to 25.1
softening of polymers without inducing large moisture gradients. MHz were applied which represent lower industrial microwave
and radio-frequency heating domain. Before the dielectric
Measuring of Dielectric Properties measurements at 90 deg C, the oak lamellas were wrapped
The dielectric properties were measured using an Agilent in aluminium foil and preheated to the target temperature in
4285A LCR metre and a 16451B Dielectric Test Fixture in a laboratory oven.
accordance with ASTM D150-22. This procedure ensured that the samples maintained their

Properties were measured at 20 deg C and 90 deg C, original moisture content and temperature during the test.
across six humidity levels and 26 frequencies. In this study, All measurements were performed with the fibre orientation
dielectric properties were measured with electric field frequencies aligned parallel to the electric field (longitudinal direction).
ranging from 0.079 MHz to 25.119 MHz. During testing, the Each frequency sweep was repeated twice to verify
lamella samples were positioned between two capacitor plates. repeatability; the mean values were used for statistical
Before measurement, the capacitance and dissipation
factor of the empty capacitor (C1, D1) were recorded at
each frequency. The specimen was then placed between the
electrodes, maintaining a constant spacing of 0.5 mm (verified www.freepik.com

by micrometre).
Capacitance and dissipation factor with the sample (C2,
D2) were measured under identical conditions. The relative
dielectric constant (ε′) and dielectric loss tangent (tan δ) were
calculated according to ASTM D150-22 as:
ε = 1/(1 − 1(1 − Cs1/Cs2) × tg/td)
Dt = D2 + εr × (D2 − D1) × (tg/td − 1)