Printed sensors on paper and wood — The frugal way of in-line detection to characterize the crosslinking behaviour of water-based glues

Engineered wood, a high-tech material of the 21st century demonstrates the revival of timber for the construction of green urban eco-buildings. However, the testing of wood-based materials is still based on destructive methods known for centuries. In-situ monitoring of curing, cross-linking, and aging is essential for the optimisation of glues, resins and coatings. For press moulding, impedance spectroscopy is the only practical way for in-line detection of the crosslinking. Flat sensors that can be integrated into glue joints are known, but the substrate material of these sensors weakens the glue joint as they do not interact with the commonly used wood glues. Sensors printed on porous paper and wood can solve this problem. We show that although almost every tested paper is printable but not all of them are suitable for the detection of cross-linking analysis, since paper additives can catalyse or inhibit the cross-linking of glues or resins. Printed impedance sensors on pH-neutral porous papers allow an unaltered in-situ analysis of the cross-linking in complex glue joints. The porous papers allow the glue or resin to penetrate the paper and bridge it. Integrated sensors can provide valuable information about the state (aging, moisture) of bonding throughout the life cycle of the engineered wood. With the help of paper-based sensors, wood can be equipped integratively, network-compatible and communicative. Damage in the glue joints can be detected on time. With this innovative sensor technology, wood is not only an ingenious material, but also an intelligent one.