# Alternative Fiber-Based Paperboard Adhesion Evaluation with T- and Y-Peel Testing
## 3 Results and Discussion
### Adhesion Strength Testing  
In Figure 6, the average maximum force of the peel adhesion strength for all three paperboards is presented. The trends are similar for both Y- and T-peel testing, where  
Figure 5: The liquid (water + IPA) penetration dynamics analysis: JK—Japanese knotowed, SD—sawdust, and BL—black locust samples. the SD sample had the largest maximum value for the Y-peel (23.8 N) and somewhat lower T-peel value of 11.8 N. While for the T-peel testing setup, there was no significant difference between the BL and JK samples (9.9 and 9.3, respectively), this difference was more pronounced in the Y-peel setup with values of 12.4 N for the JK sample and 16.9 N for the BL sample. This indicates that the setup can cause some differences in the maximum value due to the testing setup and force strains and angles used on the fiber materials, where the Y-peel value gives higher results as the tails of the samples are already under mechanical stress due to their fixation method. The average strain mechanism of the samples for the two testing setups can be seen in Figures 7 and 8. The force-elongation curves have to be separated into their elastic and dissipative (inelastic opening) parts, where the crack propagation begins and ends in the dissipative part of the force-elongation regime, and the fracture development depends on how the crack propagates after its initiation [1]. Figure 6: The maximum force value for peel adhesion strength (Y- and T-peel testing setup): JK—Japanese knotowed, SD—sawdust, and BL—black locust samples. Figure 7: Y-peel force elongation results for all three paperboard samples: JK—Japanese knotowed, SD—sawdust, and BL—black locust samples. All three samples had the so-called mode M3 failure curve described in [PERSON] et al. (2018), on the other hand, they do not break completely within the specified maximum 3 mm displacement during testing. Several peaks are not uncommon, and they have a significant proportion of dissipative descending energy. The force is constant during the final part of the force-elongation curves. In fracture mode M3, the failure starts with small microcracks in the ascending part of the curve; the microcracks appear between the fiber matrix. In the descending part, a major crack has developed and this crack leads to final failure with fibers and the complete top part being torn off from the lower part of the paperboards without pulling the adhesive part. The strain curve for the Y-peel is shorter as the fracture happens often at the fixed part of the sample holder due to the low cohesion force of the basic paper materials in comparison with the cohesion force of the adhesive and fiber matrix. It has long been known that the tendency of a paper to delaminate in peeling can be very sensitive to the peeling direction, apparently reflecting whether the fiber ends are orientated up or down in the z-direction, which in turn is a function of the drag-to-rush ratio ([PERSON] et al., 2016). Comparing the Y- and T-peel adhesion strength test methods, the adhesive joint during the T-peel test is not fixed (is free to move), while during the Y-peel test the adhesive joint is fixed. During the Y-peel test, the sample is clamped with three points and the tester pulls the upper adhesive joint with constant speed. The sample stretches and, finally, the fracture of the sample happens. This is also the reason why the testing length (x-axis of Figure 7) is just a few millimeters. To visually check the differences in adhesion and cohesion failures of the adhesive joints, scanning electron microscopy images were prepared. They are presented in Figure 9.  
From the SEM images, it can be observed that the three paperboards had visually similar fiber tears at the beginning of the samples, where larger irregularities are observed for the T-peel testing. All three samples had larger border variations in the line and surface of the adhesive joint starting line.