# Study of the Kinetics of Adhesive Bond Formation Using the Ultrasonic Method
The research conducted in the area of adhesive bonds also includes such areas as the identification of parameters that determine the quality of adhesive joints in terms of the surface preparation, temperature, and humidity [19]. Studies in this area have included not only the formation of bonds but also their degradation [20; 21; 22]. Adhesive bonds must be resistant to high humidity, oil contamination, and high temperatures, which is why the phenomena that occurred during the degradation of these bonds were studied. Considering the relative percentage, the reduction in failure stress in the epoxy is higher than in the polyurethane. With immersion in oil, the reduction is 20% for the polyurethane and 30.7% for the epoxy. With immersion in water, the reduction is 54.4% for epoxy and 23% for polyurethane. The failure stress with immersion in oil is higher than in water (for polyurethane and epoxy). At the end of 128 days of immersion and considering the relative percentage, the failure of a polyurethane adhesive joint with immersion in oil is 16% higher than in water [23]. There are also studies that have been conducted to develop a methodology to provide easy-to-peel adhesive bonds by combining the induction heating method with the use of thermally expandable particles (TEPs) [24]. In the paper [24], it was shown that it is possible to unstick bonded joints using commercial adhesive systems used in the automotive industry. The weight fraction of TEPs used and the temperature were found to be the major factors in determining the debondability of the joints. The mechanical properties (tensile lap-shear strength) depend on the TEP content, decreasing with increasing the TEP content. However, the joints retain an adequate amount of strength, i.e., more than 70% of the strength (approximately 15 Mpa) for Betamate(tm)2098 and 60% of the strength (approximately 14.5 Mpa) for the SikaForce(r)7888 adhesive, which can meet, depending on the application, the tensile lap-shear strength requirements of the automotive industry [24]. The research conducted also included surface preparation for bonding [25; 26; 27; 28]. In the study [28], the mechanical performance of adhesively bonded CFRP (carbon fiber-reinforced polymers) joints prepared with an ITRO treatment was investigated under static loading. It was found that ITRO treatment significantly increases the apparent shear strength of the CFRP single-lap joint specimens. High strength comparable to that of the sanded specimen was realized for 12 ITRO treatment passes. Damages in the form of matrix cracks and associated delaminations develop in the CFRP substrates during loading of the ITRO-treated specimens, and the specimens with high strength show the predominantly substrate failure mode. There are works related to the kinetics of adhesive bond formation, but they involved technological incidents. The work [29] included the effect of the thin-film contact oxidation process on the kinetics of joint formation and its strength. It has been proved that the peel strength at a fixed contact time is determined by the ratio of the gel-fraction in the polymer layer, cBel, which is a measure of the outcome of the oxidative crosslinking, and the content of extractable low-molecular products, elm, characterizing the effect of oxidative destruction. The effect of moisture in the air on the formation of the silicone sealant bond was also studied [30]. This study [30] has basically shown that the migration and diffusion processes that occur across a joint prepared with 1-RTV silicone sealant cause the formation of a gradient of the physical properties across the adhesive layer, from the outer to the inner regions. The consequence of this phenomenon is that properties like the modulus and hardness, as well as the adhesion strength against the substrates are not homogeneous.