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- Conductive adhesives; adhesives for wood; adhesives with nanoparticles; polyurethane; wood; civil engineering (1)
- Conductive adhesives; adhesives for wood; piezoresistivity; signal stability; adhesives with nanoparticles; civil engineering (1)
- Furnier; Zugfestigkeit; Elastizitätsmodul; Bruchdehnung; Buche; Berechnung; Simulation (1)
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Several laboratory studies and experiments have demonstrated the usability of polymer films filled with electrically conductive filler as piezoresistive material. Applied to adhesives, the glue lines of wood products can achieve multifunctional—thus bonding and piezoresistive/strain sensing—properties. Based on critical load areas in timber constructions, upscaled test setups for simplified load situations were designed, especially with regard to a stress-free electrical contact. In a second step, another upscaling was done to small glulam beams. Based on an experimental test sequence, the piezoresistive reactions as well as the behaviour until failure were analysed. The results show in all cases that a piezoresistive reaction of the multifunctionally bonded specimens was measurable, giving a difference in the extent of relative change. Additionally, measured phenomena like inverse piezoresistive reactions, electrical resistance drift and the absence of a piezoresistive reaction were discussed, based on additional strain analysis by digital image correlation. A model of macroscopic and microscopic strains influencing the piezoresistive reaction of the electrically conductive bond line in wood was used to explain all experimental results. Finally, a first scale-up of piezoresistive bond lines from laboratory samples to glulam beams was possible and successful.
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.
Materialwissenschaftliche Analyse des Abbindeverhaltens von Holzklebstoffen am Beispiel von 1K-PUR
(2020)
Das Abbinden von Holzklebstoffen wird bisher anhand von Parametern bewertet, die kaum eine prozesstechnische Optimierung ermöglichen. Mithilfe von Oszillationsmessungen und einem an dem Klebeprozess angepassten Versuchsaufbau kann dieser jedoch detailliert erfasst werden. Am Beispiel durchgeführter Timesweep-Messungen kann der Klebeprozess in charakteristische Bereiche molekularer und struktureller Abläufe eingeteilt werden. Diese Methode wird anhand eines Polyurethan-Prepolymers zum Fügen von Holzwerkstoffen dargestellt und analysiert.
In dieser Publikation wird der Frage nachgegangen, ob mit Klebstoff beschichtete Furniere gegenüber nativen Furnieren geeigneter sind, um Kennwerte zur Berechnung und Simulation von Sperrhölzern zu erhalten. Der Frage wurde mittels experimentellen Versuchen, analytischer Berechnung und Simulation nachgegangen und die Ergebnisse anhand von Buchenfurnier, teils beschichtet mit Phenol-Resorzin-Formaldehyd-Klebstoffen, dikutiert. Die Ergebnisse zeigen, dass beschichtete Furniere eine bessere Übereinstimmung mit BErechnungsansätzen und Simulationen von Sperrhölzern als native Furniere bieten und auch einem vereinfachten Ansatz eines Laminats aus Furnieren mit separaten Klebstoffschichten überlegen sind.