Non-stick surfaces for applications requiring long-time contact with fluids

The aim of this doctoral thesis is to design and develop polymeric surfaces that repel liquids of different nature even in conditions of total and long-term contact with the fluid, as well as to understand the structural parameters that determine non-stick properties. The objective is to develop a method to generate these surfaces that is easy, fast and scalable at an industrial level. In this doctoral thesis, the obtaining of superhydrophobic, omniphobic and slippery surfaces is explored, using Nature as a source of inspiration for their design.
The non-adherence in surfaces is a useful feature to meet technological challenges in many sectors, since it can provide properties such as self-cleaning or low friction, prevent corrosion or surface degradation, ice accumulation, the proliferation of bacteria and biofouling in general. The development of non-stick properties on a surface implies the generation of roughness on a micrometric scale and the use of low surface tension materials. Polymers are presented in this doctoral thesis as materials whose characteristics are favorable for developing roughness generation methodologies in a sustainable, versatile and scalable way. A top-down roughness generation method was developed in two steps: swelling-coagulation, which consists of immersing the polymeric film in a solvent that swells the polymer and subsequent immersion in a coagulant that freezes the swollen state. It is an easy, fast and scalable method with which the inherent roughness of the material was obtained. With the roughness generation method, polymeric surfaces structured at the microscale and hierarchical at the micro and nanoscale were obtained, with properties ranging from hydrophobicity to superhydrophobicity.
The polymeric surfaces were prepared from Poly(vinylidene fluoride), PVDF, selected for its low surface tension, and blends of PVDF and Poly(methyl methacrylate) PMMA, to reduce the crystallinity of PVDF and to be able to use milder time and temperature conditions in roughness generation method.
The roughness generation method was extended to two swelling-coagulation systems based on the pair of solvents acetone-ethanol and dimethylformamide-ethanol. In addition, it was extrapolated to PVDF/PMMA blends with content up to of 50% of PVDF. In this study, 18 surfaces were obtained by varying the chemical nature of the substrate, by using different polymer blends, and the rough structure, by using different swelling-coagulation treatment conditions. With these surfaces, the topographic requirements necessary to obtain different wettability behaviors were studied, proving the need for hierarchical structures to obtain superhydrophobic properties. In addition, the viability of this method was demonstrated with an industrial PVDF part. These surfaces were used as a rough substrate for the preparation of omniphobic surfaces by modification of the surface chemistry and liquid-infused slippery surfaces by infiltration of an oily lubricant.
Surfaces with superhydrophobic, omniphobic, and slippery properties were evaluated for use as materials in medical devices. The liquid-infused surfaces were optimal for use in medical devices where the adhesion of blood and consequent formation of clots, as well as the accumulation of superresistant bacteria, are avoided.
The non-stick properties of the surfaces were studied in total and prolonged contact with water. Superhydrophobic and omniphobic properties were evaluated in total immersion of the surface. In immersion, a plastron or air cushion is created on the surface that gives rise to a silver shine on it known as a mirror effect. The quality and durability of this mirror was evaluated and related to the topography and surface chemistry. Slippery properties of liquid-infused surfaces were studied in total immersion under a turbulent regime, evaluating the quality of the lubricating layer over time. In addition, a method was developed to increase the stability of the lubricant on the surfaces by cross-linking. Increasing lubricant retention through optimal surface design and lubricant cross-linking proved to be an effective strategy to extend the useful life of these systems.
Finally, surfaces with hydrophobic and superhydrophobic properties were prepared using Poly(ethylene terephthalate), PET. PET is the most widely used polymer in packaging industry. The non-stick properties in the packaging favor self-cleaning and reduce costs in the stages prior to recycling. To develop superhydrophobic PET surfaces, a solvent-induced crystallization method was used for roughness generation followed by a surface modification with a fluorinated silane.