PSI - Issue 45

Kentaro Yamauchi et al. / Procedia Structural Integrity 45 (2023) 125–131 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 4. The dependency of the adhesion strength on the egg white volume ratio.

epoxy resin and polymers increases with an increase in the surface free energy of the polymers. Except for the case of PET with γs = 41.6 mN/m, the adhesion strength decreases linearly with an increase in the egg white ratio. 4. Discussion 4.1. Deviation from the linear relationship in the low surface free energy region The adhesion strength in the region of low surface free energy region ranging from 2.3 to 15.3 mN/m was almost constant, as shown in Figs. 3(a), (b), (c), and (d). This indicates that the factor influencing the adhesion strength in the low surface free energy region is independent of the surface free energy. The surface free energy can be attributed to intermolecular forces such as the dispersion and dipole-hydrogen bonding forces reported by Owens and Wendt (1969). The constant value of the adhesion strength is not relevant to the intermolecular forces but to another factor, namely, the mechanical force caused by the anker effect, as reported by Brant et al. (1996). 4.2. The relationship between the adhesion strength and egg white volume ratio As can be seen from Fig. 4, the adhesion strength between the egg white infused epoxy resin and PET was peculiar. The values of the adhesion strength at 40 vol.% and 50 vol.% of egg white were larger than those in the other polymer cases. This led to a deviation from the linear relationship, as shown in Fig. 3 (e). This peculiarity is attributed to the difficulty associated with the uniform mixture of epoxy resin and egg white powder with a large egg white ratio ranging from 40 to 50 vol.%. 5. Conclusions By realizing low surface free energy coated adherends, a deviation from the linear relationship between the adhesive strength F and surface free energy γs in the low surface free energy region was observed for the first time. The following conclusions can be drawn from the results of this study. (1) The lower limit of the linear relationship between the adhesion strength and surface free energy of the adherend extended from the conventional value of 20 mN/m to 15.3 mN/m. (2) In the region of surface free energy less than 15.3 mN/m, the adhesion strength was constant, which represents a deviation from the linear relationship. The adhesion in the low surface free energy region can be attributed to factors such as mechanical forces (e.g., anker effect), and not to intermolecular forces. (3) A similar relationship was observed even when the epoxy adhesive was diluted by egg white up to 40 vol.%.