Influence of macro- and microscopic paper structures on the seam strength in ultrasonic bonding of fiber-based materials

The joint strength that develops in ultrasonic bonding of fiber-based materials is governed by multiple macro- and microscopic structural parameters whose interactions complicate the isolation of individual effects. In this study, paper made from cellulose-containing natural fibers was examined with respect to three structural factors: fiber orientation, paper side, and refining degree (°SR). Fiber orientation proved to be the dominant parameter. Samples with fibers aligned along the vibration direction exhibited significantly higher joint strengths than those with fibers oriented perpendicularly. The effect of paper side varied depending on fiber type and basis weight, indicating additional influences from factors such as porosity, surface roughness, and fiber distribution. Consequently, no universal relationship between paper side and joint strength was established. The refining degree showed no distinct influence within the typical range (°SR 24 to 36); however, at an exceptionally high value (°SR 80), a clear effect was observed. The impact of refining degree was found to depend strongly on basis weight: at 170 g/m², increased fibrillation enhanced bonding due to larger specific surface area, while at 80 g/m², shortened fibers and fewer contact points reduced joint strength. These findings highlight the need for a differentiated evaluation of structural parameters in optimizing ultrasonic bonding of fiber-based materials.