## 3 Results and Discussion
### Shear Strength  
The shear strength results are shown in Figure 3. Analyzing the shear strength results, it can be found that the highest values for coniferous plywood were recorded for plywood with coconut flour filler (1.70 N mm\({}^{-2}\) at 15 pbw) and plywood with lupine flour filler (1.69 N mm\({}^{-2}\) at 20 pbw). The weakest results, consistent with previous studies, were observed in the variant with spirulina filler. The bonding mass with spirulina has the highest curing time (Table 2), which can be influenced by the highest gelatinization temperature from all tested fillers, as given in Table 3. In general, the highest gelatinization temperature of spirulina starch should positively affect the mechanical and physical properties of the bonding line in plywood. A higher gelatinization temperature means the starch requires more heat to transition to a gel state, allowing for more substantial and stable bonds when the plywood is pressed. This leads to a more durable bonding line, contributing to improved mechanical properties such as tensile strength, shear strength, and resistance to delamination. Additionally, the heat resistance of starch with a high gelatinization temperature ensures that the bond remains intact even under varying temperatures. This is particularly beneficial for plywood exposed to environmental conditions, including heat and moisture, as the bond's stability is maintained over time. The starch's improved resistance to moisture also helps prevent bond failure in humid conditions, enhancing the plywood's durability. Moreover, the higher gelatinization temperature enables better control over the curing process, ensuring a more uniform distribution of the adhesive and a consistent bond quality. Overall, spirulina starch with a significantly higher gelatinization temperature should improve the performance of plywood by providing stronger, more heat- and moisture-resistant bonds, making it a valuable material for plywood applications. Since it has not been found in our research, it can be concluded that the pressing temperature has probably not been sufficient to reach the proper gelatinization with all the benefits described above. For burch plywood, the reference variant exhibited the highest value (2.73 N mm\({}^{-2}\)). The best results among the modified variants were for plywood with lupine flour filler (2.45 N mm\({}^{-2}\) at 10 pbw) and plywood with coconut flour filler (2.37 N mm\({}^{-2}\) at 15 pbw). The only statistically significant differences in mean shear strength for burch plywood were found between REF plywood and the remaining samples. For pine plywood, such differences were found between REF plywood and 20 pbw lupine plywood compared to 10 and 15 pbw spirulina samples. The examples of the birch specimens with different fillers after the shear strength test are presented in Figure 4.  
Figure 3: Shear strength values for different filler content. Coffee grounds, a form of food waste, have also been tested as a filler [3]. In the experiments, the adhesive mass contained coffee grounds in proportions of 1, 5, 10, and 20 pbw. The highest shear strength values were observed at 1 pbw and 20 pbw. This outcome illustrates another case where, despite the high potential of the raw material, the results are suboptimal. Coffee beans' naturally high oil content likely contributes to the reduced performance parameters. Spirulina, like coffee, has low water absorption capacity, which can also translate into impaired performance in shear strength. Wood dust, like flour, is an organic material known for its water-absorbing properties, making it suitable as a filler in three-layer plywood technology. Research [39] investigated various filler proportions of 1, 5, 10, and 20 pbw. The shear strength increased with higher filler content, reaching a maximum of 1.39 N mm\({}^{-2}\) at 20 pbw for native wood dust. Similar values were achieved with spirulina at the highest filler content, bamboo flour at 15 and 20 pbw, and coconut flour at 10 and 20 pbw.