# Effectiveness of Bonding Steel Elements with Polyester-Coated Paint
## 4 Discussion  
A comparison of strength results for all types of adhesive joints is also shown in Figure 7.  
Through analyzing the above figure and all of the shear strength results for the adhesive joints of the steel elements made using two types of adhesive, it can be concluded that the adhesive joints made with the polyurethane adhesive show lower strength shear strength values for any type of adhesive joint. The highest strength value belonged to an adhesive joint made with the epoxy adhesive. A zinc primer was applied to the surface of these elements, as was a paint coating in the form of powder paint. The average strength of these adhesives joints is equal to 20.33 MPa. This value significantly exceeds the research results of other adhesive joints. The lowest shear strength is shown by the adhesive joints of the reference sheet samples made using polyurethane adhesive. The strength of this type of adhesive joints is 12% of the strength of the adhesive joints of the specimens with a primer and coating (made with epoxy adhesive). In the case of the adhesive joints made with the polyurethane adhesive, the importance of using a paint coating on the surface of steel elements is noted. A significant impact of the presence of a paint coating on the strength of steel sheet joints was observed. In addition, the zinc primer increases this strength compared to the paint coating by about 32%. However, in the case of the adhesive joints made with epoxy adhesive, the presence of a paint coating slightly reduces the strength of the adhesive joints (by about 6%), and the use of a zinc primer and paint coating causes a more than two-fold reduction in the strength of the considered adhesive joints. In both cases, the highest adhesive joint strength value was achieved by the joints in which the parts to be joined had a paint coating with a pre-applied primer. Through comparing these results to the measured contact angle values, it was noted that, in this case (joining elements with a paint coating with a primer applied earlier), the lowest contact angle value  
Figure 7: Comparison of shear strength of adhesive joints. (38.72\({}^{\circ}\)) was obtained (among the tested surfaces of the samples), and these samples are characterized by having the best wettability. Based on the obtained results, it was noted that the presence of a paint coating increases the strength of adhesive joints, although it depends on the type of adhesive used. The adhesive bonds formed between the particles of the surface layer of the coating and the particles of the adhesive are probably different because adhesives with different chemical bases were used. This affects the subsequent adhesive strength of the adhesive joint. The Pearson correlation coefficient r (X,Y) was calculated using the value of the contact angle of the surfaces of the three types of adherends (Table 8) and the shear strength of the adhesive joints (Figure 6). Through analyzing the Pearson's correlation coefficient r values (X,Y), it could be seen that the correlation coefficient (r) between the contact angle and the strength of the adhesive joints made with the epoxy adhesive was \(-0.47\), indicating a negative correlation between the compared values. In the case of comparing the value of the contact angle and the strength of adhesive joints made with the polyurethane adhesive, the correlation coefficient is \(-0.91\), which also provides a negative but strong correlation between the compared values, and the greater the absolute value of the correlation coefficient, the stronger the linear relationship between the variables. These values mean that, as the value of the contact angle decreases (increasing the wettability), the strength of the adhesive joints increases. By relating the obtained contact angle values (Table 8) of the three types of adherent surface to the assessment of wettability, determined on the basis of Table 5 and [47], it was decided that all three types of samples can be considered to have surfaces with high wettability.