Study on the adherence and superhydrophobicity of polypropylene composite coating on aluminum alloy substrate

A composite superhydrophobic coating was tape casted on the surface of aluminum alloy specimens which is composed of maleic anhydride grafted polypropylene (PP-g-MAH) and polypropylene (PP). The aluminum alloy specimens were pretreated by silane coupling agent KH550 to increase the adherence between aluminum alloy specimens and PP-g-MAH. The pretreatment surfaces were characterized by the Fourier transform infrared spectroscopy (FTIR). The adherence and superhydrophobicity of the composite coating were also tested by QFZ-II adhero meter and OCA-20 optical contact angle meter, respectively. Finally, the morphology of the composite coating was also investigated by Scanning Electron Microscopy (SEM). The results indicate that KH550 is mainly responsible for the increase of adherence between aluminum alloy specimens and PP-g-MAH due to the formation of chemical bond Si-O-Al and polysiloxane coating. As for the superhydrophobicity, the PP coating covers the polar group maleic anhydride which restricts the decrease of superhydrophobicity. At the same time, the concentration of PP-g-MAH and PP solution has an important effect on the hierarchical structure and superhydrophobicity of the composite coating. When the concentration is 5 g L-1, the coating has a fine structure with an unsatisfying hydrophobicity. When the concentration is more than 17 g L-1, the composite coating has a prominent hierarchical structure and its superhydrophobicity keeps at relative stable level. However, when the concentration is more than 50 g L-1, the solution has a weak spreadability and the coating can not easily flow flatly. Sometimes the coating will be cracked.