Modification of polypropylene surfaces with micropits and hierarchical micropits/nanodepressions

Micropit surfaces and hierarchically structured micropit/nanodepression surfaces were fabricated in polypropylene by injection moulding. The first step in the process was the microstructuring of aluminium foils with a micro-working robot to obtain a pattern of micropits. Dimensions of the micropits could be controlled by adjusting the working parameters of the robot. The microstructured faces of some of the foils were anodized to aluminium oxide giving a surface in which both micropits and the smooth areas between were covered with nanopores. Mould inserts for the injection moulding were obtained by cold mounting of an epoxy resin mixture on the foils. After the epoxy resin had hardened, the foils were etched away. Dimensions of the micropillars and micropillars/nanobumps on the epoxy related to the dimensions of the structures on the foils. Finally, structures were replicated in polypropylene as micropits and hierarchical micropits/nanodepressions by injection moulding. The dimensions of these hollow structures corresponded well with those of the epoxy pillars and the original micropits.

Static and dynamic contact angles on the micropit-structured polypropylene surfaces were elevated from the slightly hydrophobic values of smooth polypropylene. With hierarchical micropit/nanodepression structures, the contact angles approached the superhydrophobic limit of 150°. The work demonstrates an inexpensive and reproducible technique to fabricate hollow structures of various dimensions and scales on polypropylene and modify the surface properties of the polymer.