Generation of superhydrophobic surfaces for automotive and marine applications

There exists a broad range of automotive and marine coatings which cater for aesthetic considerations as well as protecting against a number of potentially deleterious environmental factors. Polyurethanes (PU) paint coatings are particularly widely used in this application. It is also possible to optimise the aesthetic and protective properties of these coatings yet further through the application of an additional paint protection scheme (PPS). Such PPS layers are often siloxane based thin films which are commonly applied due to the particularly demanding performance requirements in this automotive and marine sectors.A range of experimental methods and chemical analysis techniques have been used to study and establish the formulation and baseline performance characteristics of one such, commercially available siloxane based PPS.Orbitrap, gas chromatography (GCMS) and tandem mass (MS/MS) spectroscopy techniques have shown the siloxane based PPS polymers within the present study to contain amino, alkoxy and hydroxyl functionalities, as well as methyl pendants. Atomic force microscopy (AFM), glossmeter measurements, spectrophotometry, water contact angle analysis (WCA) and X-ray photoelectron spectroscopy (XPS) have been used to characterise the effect of the PPS on a polyurethane paint coating. Results show that the existing system deposits: ~20 at.% of a Si-O based coating. This alters the surface wettability, from water contact angles and surface free energy (SFE) values of ~65° and ~56 mN/m, to ~90° and ~31 mN/n respectively. In addition, this coating increases surface gloss from ~64° to ~75° at 60° measurement angle; and decreases surfaces roughness (Ra) from ~60 to ~30 nm.Attempts to modify and enhance the wettability, gloss and stiffness of this existing coating, using a number of polymers and fillers were generally unsuccessful. However, an increase in surfaces stiffness of ~38% was observed through the use of 0.25 wt.% carboxylic functionalised nanodiamond.In recognition of the difficulty in augmenting the existing siloxane PPS coating, an entirely new coating and matrix was developed using an oxygen/argon pretreatment and a fluoroalkyl silane (FAS) coating, with sole emphasis on two critical performance characteristics: superhydrophobicity and anti-biofouling.It was found that the oxygen/argon plasma treatment increased both the surface roughness (Ra) and SFE of the PU paint coating from approximately ~60 to ~320 nm, and, ~52 to ~80 mN/m respectively. It was also found that the plasma process created a multiscale roughened texture through the process of differential ablation between the PU polymer and the barium sulphate solid content present in the paint as an extender, and other additives. In addition, the process also imparted favourable polar groups into the PU surface from the ionised and radical oxygen species in the plasma.When the FAS coating was subsequently applied to the PU without prior plasma treatment, there was a significant increase in water contact angles. This parameter increased from approximately 60° on untreated PU to around 130° with FAS applied. In this case, the SFE decreased to ~7.5 mN/m and showed 42.0 at.% fluorine present as indicated by XPS.However, subsequently applying the FAS polymer after plasma pretreatment takes advantage of the known synergistic relationship that exists between surface roughness and low surface free energy coatings. The two processes combined to create superhydrophobicity with a surface that exhibited water contact angles up to 153.1°. With this optimised process, the apparent SFE was reduced to 0.84 mN/m with a more highly fluorinated surface present. In this case 47.2 at.% surface fluorine was observed by XPS, with a coating thickness of ~26nm. The coating has also been shown to exhibit excellent resistance to a salt spray environment compared to the siloxane based coating, as well as provide a an improvement in the PU’s anti-biofouling capability against staphylococcus epidermidis.In addition to the changes in SFE, the plasma treatments also alters levels of surfaces gloss and colour. After exposure to 600s of plasma gloss levels are shown to reduce from ~50 to ~21 (at 60°), with a small but significant increase in the lightness and yellowness of the surface.