Paper-based packaging materials are generally assembled using adhesives formulated with oil-based polymers. These adhesives make the recyclability of the materials more complex and may be the source of material contamination by mineral oil. In view of developing an adhesive-free process, the potential of ultrasonic compression was investigated in this study. 100% lignocellulosic papers were assembled using an ultrasonic welder dedicated to thermoplastic polymers. For papers containing lignin, the measured peeling strengths were equivalent to those achieved by hot-melt gluing, provided that the water content of papers was well adjusted. At the interface between bonded papers, the fiber network was dense and rather continuous. SEM examinations, 3D X-ray microtomography images, and temperature measurements suggested that the development of adhesion originated to a large extent from a thermoplastic welding mechanism: wood fiber polymers passed their glass transition temperatures, creeped and formed a matrix that coated fibers. Thus, ultrasonic welding appears as an efficient adhesive-free technique for assembling papers that are used in a broad range of packaging applications.

Today’s environmental concerns are pressuring industries to substitute paper-based materials in place of plastics in many sectors including packaging. However, assembling papers and paperboards using environmentally friendly solutions remains a technological challenge. In this context, ultrasonic (US) welding is an alternative to adhesives. In this work, the potential of US welding to assemble folding boxboards was investigated. Folding boxboards are commonly coated to enhance printability. This coating is generally composed of mineral pigments (85 to 90%) and polymer binders (10 to 12%). This study evaluated whether the presence of the coating layer allows the assembly of paperboards by US welding. Results indicated that welding coated folding boxboards is possible provided that coating weight and binder content are high enough. The mechanical performances of the welded boards met the requirements of most packaging applications. Adhesion in the welded joint resulted from a combination of thermoplastic (melting and flowing of the binder) and thermoset (degradation reactions) effects. However, it was not possible to assemble coated folding boxboards without degrading the welding zone. While the materials and process need to be optimized, this work represents a big step forward toward the adhesive-free assembling of paper-based materials.

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