## 3 Results and Discussion
### Density Profile  
The graph below (Figure 6) shows coniferous and deciduous plywood density profiles with different fillers. In the case of spirulina and coniferous wood, the glue had a higher density after solidification than the reference glue, which could mean that the glue is more brittle and, thus, less durable. In the case of birch plywood, less penetration of the veneer structure by the adhesive mass was observed. This may be related to the higher density of birch wood. The strength of the bond is affected by the wood's density. While denser woods tend to form stronger bonds, effective penetration and adhesion rely on the adhesive matching the wood's density [57]. In the case of the coconut flour filler, it can be seen that the peaks of the adhesive with the alternate filler in the joint are very similar and even slightly higher compared to the reference peaks. This is a good result, as it indicates the high substitution potential of rye flour with coconut flour. In the example of plywood with bamboo meal filler, no significant differences were observed in the joint compared to the adhesive with reference filler. Only in the case of the variant of 20 pbw can it be noted that the joint penetrates the wood to a lesser extent, which may be due to the high water absorption of bamboo meal, so that the adhesive, when applied to the plywood, took a very dense, rather than liquid, form. As a substitute for conventional filler, lupine flour gave density profiles similar to the reference adhesive, confirming the potential for using this filler. A similar trend was also observed for birch plywood.  
Figure 5: The water absorption of alternative flours and rye flour as reference filler.  
Similar density profiles were obtained for chestnut flour. The graphs were identical to those of the alternative sample, as with lupine flour [47]. Moreover, the adhesive layer thickness is critical in bond quality, and the filler regulates this factor [58; 59]. Research shows that fillers can alter the viscosity of adhesives. For instance, nanocellulose increases viscosity, improving bond quality but potentially complicating application if not controlled [60; 61]. Similarly, different fillers cause adhesives to behave in various ways,  
Figure 6: Density profiles of plywood containing different fillers (**a**,**c**,**e**,**g**—pine wood; **b**,**d**,**f**,**f**,**h**—birch wood).  
influencing their performance and ease of application. The viscosity of the adhesive is significantly influenced by the type of filler used. For example, incorporating rye flour into urea-formaldehyde resins increases viscosity, which can impact the uniformity of the adhesive layer [62; 63]. This effect has led to the search for alternative fillers with similar properties to maintain the desired adhesive consistency and performance. The thickness of the adhesive bond is significantly affected by the particle size of the filler. Smaller particles generally promote a more uniform and denser adhesive coating, improving adhesion performance [64; 65]. For example, adhesives containing finer fillers tend to form a more consistent layer, enhancing bonding properties. This demonstrates that the selection of filler particle size is crucial in optimizing adhesive effectiveness [64]. In conclusion, these variables influence the density profile of the studied plywood and impact other important parameters, such as shear strength. The interaction between the adhesive composition, filler type, particle size, and wood density can significantly affect the mechanical properties of the plywood, including its resistance to stress and its overall durability. Understanding these relationships is crucial for optimizing adhesive formulations and improving the performance of plywood in various applications.