## 3 Results and Discussion  
### Preliminary Study  
In a preliminary study, 5%, 10%, 15%, and 20% UF resin was replaced with soy flour to gain a better estimation of the most effective combination to decrease formaldehyde emission. Results of the preliminary study demonstrated a clear decreasing trend in formaldehyde emission (FE) as soy flour content increased (Figure 4). This is in line with the data reported in the literature. [PERSON] et al. [22] reported that the partial replacement of urea-formaldehyde adhesive with soy flour, particularly with a substitution rate of 15%, significantly reduced the formaldehyde emission. A similar observation was also made by [PERSON] et al. [23] who applied five types of flours (rye, hemp, coconut, rice, and pumpkin) as fillers with urea-formaldehyde resin in plywood manufacture. All soy-treated panels  
Figure 3: Linear diagram of shear-strength test specimen according to European standard specifications EN 314-1: 2004.  
showed a significant decrease in formaldehyde emission in comparison to control panels. The highest and lowest formaldehyde emissions were observed in the control panels (130.1 mg/kg) and panels with 20% soy flour (88.6 mg/kg), respectively. No significant difference was observed between formaldehyde emissions in panels produced with 10%, 15%, and 20% soy flour, although the FE trend was a decreasing one as soy flour content increased to 20%. Two factors potentially contributed to this behavior. The first one may have been the great dependence of formaldehyde emission on the resin content. A lower content of formaldehyde in the adhesive mixture causes a lower level of formaldehyde emission from the panel. In fact, a higher substitution of UF resin with soy flour results in a lower formaldehyde emission. Another factor is the reaction of free formaldehyde with amino groups present in the soy flour. [PERSON] et al. [37] found that the use of soy protein as a natural formaldehyde scavenger in wood particleboard production can contribute to a decrease in the formaldehyde content of particleboard panels, without significantly affecting the properties of the panels  
The shear strength of all panels produced in the preliminary phase of the study was also measured to find out the effects of the addition of soy flour on at least one mechanical property. Results showed that the shear strength values of all treatments were more than the standard limit of 1 MPa, even the control panels (1.55 MPa). The highest shear strength was observed in panels with 15% soy flour content (2.1 MPa); that is, soy flour resulted in a 34% increase in shear strength. The soy flour content of 5% did not have a significant effect on the shear strength of plywood panels (Figure 5). A further increase in soy flour content to 20% resulted in a decrease in shear strength (1.8 MPa), although it was still higher than that of the control specimens. Results of the delamination tests showed that all panels successfully passed the test (Table 2). This indicated that substitution of soy flour for UF resin (for soy flour contents lower than 20%) cannot significantly affect the delamination property of plywood panels. This finding is in accordance with a previously reported study, in which it was found that the partial replacement of UF adhesive with soy flour, particularly at substitution rates of 10% and 15%, did not significantly affect the shear strength of plywood under both dry and wet conditions [22]. Cluster analysis using the two properties of formaldehyde emissions and shear-strength values in the preliminary phase demonstrated similar clustering of the control panel with panels containing 5% soy flour (Figure 6). This indicated that 5% soy flour is too low to significantly influence the studied properties of plywood panels. The other three panels were distinctly clustered away from control panels.