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ABSTRACT
Although adhesive-adherend interfaces play an important role in improving adhesive joint strength, their observation is a challenging task. If the adhesive and adherend can be clearly separated, the interface can be observed directly. Because this was fortunately achieved with the combination of a second-generation acrylic adhesive and several plastics, the differences in the formation of sea-island structures at interfaces was verified as related to adhesive strength. Phase mapping, stiffness measurements, and thermal analysis were conducted with atomic force microscopy (AFM) to investigate the physical properties of the sea-island structures. The internal adhesive structure observed by AFM was in good agreement with the transmission electron microscopy analyses. The structure of the adhesive-adherend interface that showed a good bonding strength was similar to the internal structure, forming a large acrylic-rich island and rubber-rich sea. In the case of plastics with low surface energies, however, the interface formed a distinctive structure with small acrylic islands in a rubber sea. The difference in the formation of the sea-island structure is considered to be related to the wettability of the adhesive to the adherends, and it was revealed that a stronger interface corresponds to a structure more closely resembling that of the inside.
Acknowledgements
The authors are deeply grateful to Dr. Shin Horiuchi of National Institute of Advanced Industrial Science and Technology and our colleagues of Suzukakedai Materials Analysis Division in Tokyo Institute of Technology for their support to specimen preparation, TEM observation and AFM analysis. Local thermal analysis using nano-TA was conducted with the support of Nihon Thermal Consulting Co. Ltd, which is also greatly acknowledged.