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Various studies on wood adhesives filled with conductive fillers for future application to structural monitoring showed a piezoresistive (resistance change with strain) response of the adhesive bond lines that is measurable under direct current. The results also showed a relatively high signal noise with low sensitivity. Using impedance spectroscopy as a measurement technique, the improvements in frequency-dependent piezoresistivity over DC (Direct Current) resistography of multifunctional bonded wood were studied. Beech specimens were bonded by one-component polyurethane prepolymer (1C-PUR) filled with carbon black and tested under shear load. The quality of the piezoresistive properties was described by calculating the signal-to-noise ratio (SNR) of the measured signal. A setup-specific frequency band with optimized SNR between 100 kHz and 1 MHz could be derived from the measurements. Several frequencies showed a signal with higher quality resulting in a higher SNR. Regardless of the variations in impedance spectra for all specimens, this frequency band provided several frequencies with improved signal quality. These frequencies give a more reliable signal with lower noise compared to the signal from DC resistography.
Multifunctional adhesives out of a polymer matrix and electrically conductive fillers are known to typically decrease in bond strength with increasing filler content, while the electrical resistance drops extensively at the percolation threshold. Three experiments have focused on different aspects of the production of electrically conductive adhesive wood bonds, experiment I with a variation of the adhesive components, experiment II is varying the process parameters in the bonding process, and experiment III is investigating a thermal postcuring effect. Tensile shear strength (EN 302–1, requirement for structural wood bonding dry condition (A1 treatment)) and electrical resistography under direct current (DC measurements) of the bondline was used to identify major influences on tensile shear strength and the formation of the electrically conductive network. The results show high differences due to the adhesive polymer selection, the filler type and some of the process parameters. As an example, it was revealed that the bond strength of multifunctional adhesive wood joints can be increased by integrating increasing contents of carbon nanotubes in polyurethane prepolymers, if the polymers do not contain fibrous fillers before the dispersion.
Electrically conductive fillers enhance the functionality of adhesive bondlines by inducing a strain-dependent electrical resistance. Applied as sensor elements in engineered timber, these multifunctional adhesive bondlines are influenced by a lot of process parameters, including the dispersion process, manufacturing parameters and postcuring conditions. Additionally, the sensitivity of these sensor elements is counteracted by instabilities of the piezoresistivity. A setup and analysing method of the piezoresistive response have been developed, which includes sensitivity as well as instability effects. With this setup, the quality of the piezoresistive bondline has been assessed by three experiments, varying 10 influencing factors. The piezoresistive response was investigated in compression perpendicular to the bondline. It was shown that the electrical sensitivity in compression and stability of the piezoresistive bondline can be influenced by polymer formulation, filler type, filler content as well as the press temperature, glue spread, dispersion technique and thermal postcuring.