Correlation Between the Molecular-Level Behavior of Polyurethane on Oily Surfaces and Adhesive Strength

Adhesive bonding is commonly used in various industrial fields. Among the various types of adhesives, polyurethane adhesives have unique properties, such as room-temperature curing, flexibility, and heat insulation, making them indispensable materials in the current automotive and aerospace industries. In these industries, adherends coated with mineral oil or press oil on their surfaces to prevent corrosion are often required, and bonding without degreasing is preferred. Hence, understanding the mechanism of surface adhesion in the presence of oil is crucial. This study aimed to understand the molecular behavior of oil at adherend interfaces and its impact on adhesion. The correlation between the behavior of silicone oil at polyurethane interfaces and adhesion strength was investigated using vibrational sum frequency generation (SFG) spectroscopy, an interface-specific vibrational spectroscopic technique. When polyurethane is cured at room temperature, the silicone oil present at the interface is absorbed into the bulk and disappears from the interface. After being absorbed into the polyurethane during room-temperature curing, the silicone oil remained near the interfacial region, and when the polyurethane was annealed to promote polymerization, it reappeared at the interface, resulting in a significant decrease in adhesion strength. These observations of the behavior of silicone oil at the polyurethane adhesive interface can be explained by the relationships between the solubility of silicone oil, the raw compounds of polyurethane, and polyurethane and provide significant insights into the reliability of adhesion on oily surfaces. They will also contribute to the design of curing behavior for the development of polyurethane adhesives with high adhesion strength to oil-covered adherend surfaces.