Experimental evidence of the interface/interphase formation between powder coating and composite material

• “In mold powder coating process” is an efficient way to coat composite materials.
• Low pre-curing time of powder coatings induces a high interphase thickness.
• Interphase properties were studied by means of SEM, FT-IR, Raman and DSC analyses.
• High contribution of chemical bonding implies excellent adhesion properties.

The present study focuses on the physicochemical characterization of the interface between an epoxy/polyester-based powder coating and an epoxy/thermoplastic-based composite material. An in-mold process has been used for the powder coating deposition on carbon/glass fibers reinforced composite substrates, and different methods have been performed to characterize the interface. We evidence here the strong dependence of the structural, chemical and thermal behaviors of the interface on the cure time/temperature conditions at which the powder coating was crosslinked. At a low crosslinking rate of coating (∼48%), experimental results reveal the development of a large heterogeneous organic/organic interphase between coating and substrate. However, thin interphases have been detected when the crosslinking rate goes beyond 69%. Besides, a phase segregation of thermoplastic additive within the composite matrix was identified in the formation of this interphase. Energy-dispersive X-ray spectroscopy (EDS) as well as FTIR/Raman experiments enabled us to put in evidence the diffusion process of the thermoplastic additive toward the coating. From thermal analysis, glass transition temperature Tg for both components was observed, which confirm the proposed mechanism. This study highlights the importance of the thermal processes on the complex competition between the interdiffusion between two epoxy matrix and the existence of thermoplastic toughening agent at the interface of powder coating and composite material.

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