# Alternative Fiber-Based Paperboard Adhesion Evaluation with T- and Y-Peel Testing
## 3 Results and Discussion  
### Paperboard Samples Characterization  
Based on earlier research mentioned in the introductory part, the paperboard surface properties can influence the ability of adhesion and consequently the peel adhesion strength. In Figure 2, one can observe the measurement results for porosity (air method) and roughness expressed in mL/min. The largest roughness was measured for the black locust (BL) sample, with an average value of 3984 mL/min, while the lowest value was measured for the Japanese knotveed (JK) samples, with 1200 mL/min. The porosity values show a difference between the SD sample made with sawdust filler and the black locust sample, which indicates that the filler material left open the pore network. So the JK samplehad the lowest porosity as well as roughness while the BL sample had low porosity and very high roughness. As we used non-coated papers and no classic coating was used, the fillers were mixed in the bulk material before paper formation on the moving web. Interrupting the bonding between fibers, fillers in general tend to decrease the strength properties of papers. They also can increase the voids in the paper structure. The sawdust paper had the largest share of fillers, 25%, which had typical values of 0.2 to 2 mm, while the CaCO\({}_{3}\) fillers typically range from 0.5 to 3 \(\upmu\)m. As the calcium carbonate fillers have smaller sizes and content ratio to mass, this would create smaller intermittent voids and, thus, lower the porosity and roughness compared to larger size and higher sawdust filler content. During the paper production, the larger size sawdust particles have also larger possibilities of mechanical retention (entanglement with the fibers) and, thus, forming a denser surface and bulk structure. With air leak measurement, this applies according to the existing literature as the sawdust paper has the highest air porosity (Figure 2). In contrast to porosity, the z-directional tensile strength gave slightly different trends in material properties. Z-directional tensile strength is a measure of the internal bond strength of paper or paperboard composites. It is particularly useful for measuring the internal bond strength of single-ply sheets such as the alternative fiber paperboards used in this research, which do not contain additional coatings. The results presented in Figure 3 show that the paperboard from black locust (BL) had the highest z-tensile strength with an average value of 436 kPa, and the lowest value was achieved for the sample made with sawdust (SD) filler. These differences indicate that during the fiber bonding and formation, the higher fiber content (less filler) increased the z-tensile strength, while the lower amount of fibers lowered the value for the SD sample (larger particles and larger air voids--lowering the number of hydrogen bonds between cellulose fibers and lowering overall the mechanical strength of the paper). Furthermore, the BL sample had somewhat longer fibers, which in general increase the tensile stiffness and strength (the z-tensile values were the highest for the high-fiber-content papers with the largest longer fiber ratio). These results also indicate a lower elastic region during the mechanical testing that can induce quicker ply cracking if forces during the peel testing are straining the vertical paperboard composition. Figure 2: Porosity and roughness measurement of the used alternative paperboards: JK—Japanese knotweed, SD—sawdust, and BL—black locust samples. The surface interaction of the adhesive and paperboards was tested with contact angle measurements which are presented in Figure 4. The contact angle was nearly constant for all samples in the first 60 s, which is an extended time (in industrial converting of fibrous materials) where the adhesive/water and fiber matrix interact with each other. Due to high grammage and some surface sizing, the contact angle for all tested paperboards is high.