Chemically bonded surfaces get active

A chemical technique that briefly makes inert surfaces highly reactive can be used to cover substrates with an ‘incompatible’ coating. The thin coating layer can dramatically alter colour or adhesion – or incorporate novel activity such as disinfection – without adversely affecting the mechanical properties of the substrate. Because no adhesives or heat bonding are used, the risk of delamination and consequent surface damage is lowered. The technology is just out of the laboratory, but has recently received £550,000 to help commercialise it. Products from mobile phones and to hospital equipment could all be employing it very soon. It is the brainchild of Mark Moloney and Jon-Paul Griffiths, who work in the University of Oxford’s Department of Chemistry but have started a spin-out company called Oxford Advanced Surfaces to exploit the technology. "Academically, I have been working on it since 1990," says Dr Moloney. Most commonly used polymers are inert and difficult to modify chemically. It is possible to produce chemically modified polymers, but this means changing the chemistry and risks compromising the mechanical or chemical properties of the bulk material. As a result, conventional plastics are often painted or coated, but many polymers are difficult to coat, or will not bond reliably to other materials. Various surface treatments have been developed to improve the adhesion of paints, adhesives and coatings, but as users of plastic products will often be aware, such coatings often do not adhere well. This technology is based on a family of highly reactive chemical species, which can be generated by heat or light treatment of a stable precursor using conventional, industrial technologies such as ovens and ultraviolet lamps. These chemical species can form strong chemical bonds with organic polymers and materials that would not normally react with anything else. The technology can be be applied to provide an irreversible link between a wide range of coatings and substrates. "It solves problems related to the surface modifications of polymers,” says Dr Moloney. “It creates irreversible links to the surface by a simple and direct chemical approach, which we hope is going to be very attractive to customers." Examples of substrates that have been successfully coated include polypropylene, alumina, nylon beads, aramid fibre (‘Kevlar’), polystyrene, PTFE (‘Teflon’), silica, glass, diamond and diamond-like carbon. The process can attach surface layers that change the effective pH, improve resistance to aggressive chemicals or light, change surface tension or surface electric charge, or exhibit chromophoric, photochromic or thermochomic effects. It is also possible to change ultraviolet or infrared absorbance, or exhibit fluorescence or luminescence. One interesting target application area is to produce plastic products with biocidal surfaces. Numerous medical and hygiene applications are possible. The team has developed the technology to allow such biocidal surfaces to be made regenerable using hydrogen peroxide solution. Coatings can be added with ion exchange properties for chemical processing applications. An important application is in the area of manufacturing polymer laminates where sheet to sheet bonding can be improved. Possible applications include laptop computer screens and printed circuit boards for mobile phones and other electronic consumer durables. The manufacturing is fairly simple and conventional, applying chemicals by dipping or spraying followed by treatment in an oven or by an ultra-violet lamp to generate the reactive chemical species. The number of potential applications is enormous. In the medical field, contact lenses and respiratory masks could be made biocidal to resist possible infection. Stents and other temporary or permanent medical implants could be given biocompatible surfaces to help bone or tissue bond to them. Infrared lenses for military and security surveillance systems could be inexpensively coated to protect them from abrasion damage and help keep them clean. Road signs could be made inexpensively, with light-interacting surfaces to improve viewing at night. At the same time, car door handles could be made to glow in the dark without having to manufacture the whole component from such materials. Products for use outdoors could be made anti-fungal and anti-pest, and of less technical but great commercial importance, logos, decorative finishes and security tags and marks could be applied to plastic products that will not rub off. "The research is going very well,” Dr Moloney concludes. “We have already been inundated with enquiries and potential applications, so are very optimistic for the future of the technology." Oxford Advanced Surfaces Pointers * Smart surfaces may be chemically bonded to a range of substrates * The mechanism is by the temporary formation of reactive chemical species, which will strongly and permanently attach themselves to most polymer and many inorganic surfaces * Application is by dipping or spraying followed by treatment in an oven or by ultra violet light