Nano-scale modification of paint surfaces for contamination resistance

Currently, soft polyester-melamine thin films used for exterior coatings have been known to show discolouration on outdoor exposure in tropical regions during periods of high temperature and high humidity. This discolouration is believed to be due to airborne and rain-borne carbonaceous and silicaceous matter depositing and adhering to the surface of the paint. For aesthetic reasons it is desirable to prevent this contamination from building up and to make these surfaces self-cleaning. This phenomenon of pre-painted steel panel discolouration in areas of high temperature and high humidity, also known as tropical discoloration as it is most prevalent in tropical areas such as Malaysia, Thailand, Indonesia and Singapore, is particularly noticeable on external roofing and panelling of lighter colours. One objective of this study was to determine the nature of the dirt particles that contaminate the surfaces of pre-painted metallic panels in tropical climates causing tropical discoloration. This will add further insight into various techniques used to prevent the discolouration from occurring via prevention of the contaminating species from depositing and embedding in the surface of the coating. Investigating the nature of the dirt particles will increase the understanding of their chemistry and morphology in order to best determine how to prevent the particles from becoming adhered to the painted surface. The research presented here shows the particulate contamination found to build up on surfaces in Singapore to be of carbonaceous and silicaceous nature and of 1-10 µm in size. Contamination resistance in pre-painted steel panelling can be increased by subjecting the painted surface to oxygen plasma treatment. The oxygen plasma affects the surface characteristics in several ways that have been shown to be beneficial to improving the contamination resistance of the painted surface. By a process of oxidation, the oxygen plasma etches and roughens the surface of the coating in combination to producing a nanometre scale SiOx ‘crust’ like hard surface on the paint making it more difficult for the contamination to adhere to the surface, and hence easier to remove by simply washing the contamination away with water. The addition of a surface segregating, siloxane containing molecule to the paint formulation has been shown to improve the effectiveness of the oxygen plasma as a greater concentration of silicon is present at the surface to be oxidised to form the SiOx surface crust. The use of carbon/water slurry tests simulated the build up of contamination on the paint surfaces and ease of removal of the contamination from the modified surfaces by simply washing the contaminated surfaces with water. Particle adhesion was also measured by use of an in-house built force-rig, for measuring the pull-off force of micron sized particles adhered to the paint surfaces. These results were correlated to show that a polyester-melamine paint surface containing a small percentage of a caprolactone-dimethylsiloxane block copolymer additive which has undergone oxygen, surface plasma treatment, shows greater dirt shedding ability than an unmodified polyester-melamine paint surface. The incorporation of coated silica microspheres and coated silica nanoparticles was attempted in the hope of also creating a hard, rough surface to prevent the adhesion of contamination to the polyester-melamine via a single coating process. The silica microspheres were found to aggregate within the surface coating and hence did not produce the regularly roughened surface that was desired. The silica nanoparticles were found to not diffuse to the coating-air interface but were found to create hard contamination resistant coatings when applied to the final coated surface as a secondary coating process