Influence of polymer/filler composition and processing on the properties of multifunctional adhesive wood bonds from polyurethane prepolymers II: electrical sensitivity in compression

Improving the usability of piezoresistive bond lines in wood by using impedance measurements

“…A drift in the measured signal can be seen and has also been reported in several studies (Abu-Abdeen et al 2016;Kang et al 2006;Wang and Ding 2010;Winkler et al 2020b;Zhao et al 2013). As the specimens are composites of two viscoelastic materials-the polymer-based bond line and the wood itself-it is possible that the drift is generated by the time-dependent material creep or strain hysteresis of the polymer, the wood or both components at the same time (Abu-Abdeen et al 2016;Hunt 1999).…”

“…Electrically conductive filler (Ketjenblack EC-300 J, Akzo Nobel Functional Chemicals B.V., Arnhem, Netherlands/Agglomerate size before dispersion 50 to 200 μm, primary aggregate size before dispersion 100 to 1000 nm, primary particle size of 30 to 50 nm according to data sheet) was dispersed into a one-component polyurethane prepolymer (Laboratory-Sample 1506-PV, Jowat, Detmold, Germany) by using a dissolver (Dispermat CV-SIP, VMA-Getzmann GMBH, Reichshof, Germany). A filler content of 4 wt% (≙ 25.35 vol%) was chosen, positioned at the upper limit of the percolation threshold area (2 wt% to 4 wt%) for this polymer-filler combination (Winkler et al 2020a(Winkler et al , 2020b to reliably reach a usable sensitivity and at the same time a high reproducibility (Sanli et al 2017). Based on the findings from Winkler et al (2020b) that signal quality rises with rising amount of conductive adhesive, an amount of 640 ± 19.3 g/m 2 of electrically conductive adhesive was applied to two pairs of lamellas using a glass scraper.…”

“…A filler content of 4 wt% (≙ 25.35 vol%) was chosen, positioned at the upper limit of the percolation threshold area (2 wt% to 4 wt%) for this polymer-filler combination (Winkler et al 2020a(Winkler et al , 2020b to reliably reach a usable sensitivity and at the same time a high reproducibility (Sanli et al 2017). Based on the findings from Winkler et al (2020b) that signal quality rises with rising amount of conductive adhesive, an amount of 640 ± 19.3 g/m 2 of electrically conductive adhesive was applied to two pairs of lamellas using a glass scraper. Afterward, the two bonded beech lamellas were pressed in a laboratory press next to each other under a pressure of 0.9 MPa for 240 min at room temperature and were then stored in standard climate (20 °C/65% RH) for approximately 14 days.…”

“…For simplification, the stress is labelled compression shear in the whole paper. The swelling shear load was used in earlier studies (Winkler et al 2020b) with the intention to analyze the repeatability of the piezoresistive response. The swelling load rate of 0.1 Hz, compromising between a timeefficient testing cycle and a real load situation, ranks below the typically applied dynamic load rate, but above the quasi-static load.…”

Piezoresistive bond lines for timber construction monitoring—experimental scale-up

“…In previous publications by the working group, this piezoresistive effect has been shown for small samples of adhesively bonded wood specimens from beech wood (Fagus sylvatica L.) under compression stress (Winkler et al 2020b), under tensile and compression stresses in beams (Winkler and Schwarz 2016) as well as under pull-shear stress (Winkler and Schwarz 2016).…”