Observation and Control of Single-Component Adhesion Interphase of Polyamide 66 through Confocal Raman Microspectroscopy

Manufacturing using adhesion technology has attracted much attention. Examples of adhesion include the lay-up of carbon fiber reinforced thermoplastic prepregs and the lamination of food packaging. In single-component adhesion systems, the analysis of the boundary region poses challenges because of the absence of chemical and physical discrimination at the adhesion interphase. Polyamide 66, one of the typical engineering plastics, is widely accepted as a structural material in automobiles and packaging films. Therefore, finer control of adhesion with polyamide 66 is crucial for advancing adhesion manufacturing. In this work, we focused and investigated the interphase of a single-component adhesion system with polyamide 66. For the analyses of single-component polyamide 66 laminates, an adhesion system with nondeuterated and deuterated polyamides was utilized, and their interphase structures were evaluated by confocal Raman microspectroscopy. The interphase region of the adhesion specimens was able to be characterized and evaluated, revealing an expansion to a thickness of several micrometers. The interphase thickness was increased with thermal annealing postlamination, whereas no thickness increase was observed in adhered specimens using the polyamide 66 substrates through thermal crystallization before lamination. The formation of the interphase region can be attributed to the crystal growing and lamella interlocking in the boundary region. Moreover, the larger interphase thickness was strongly associated with an increase in adhesion fracture toughness. These results suggested that the adhesion properties of crystalline substrates were decided by crystallization behavior and the thermal annealing process, even when using the same component adhesion systems.