Is it time to drop surface damaging pre-treatments in favour of excimer?
The point of adhesion between a base material and the applied hard coating is of critical importance. Since last year’s LOPEC, Ushio has gained the attention of printed electronics manufacturers, and it is fast becoming the primary producer and supplier of a revolutionary surface science solution, excimer technology. Prominent pre-treatment techniques, such as corona and atmos- pheric plasma, rely on a significant energy transfer applied to the material surface. The proliferation of electrical irradiation within industrial manufacturing is a credit to its ability to imbue a surface with desirable properties, for example, a significant improvement in contact angle and wettability. The downside of direct plasma methods appears in the form of a subtle, yet destructive, molecular bombardment. As a direct result of this relatively brutal treatment released directly upon the surface, several hard-to-catch side effects can materialise. Along with the significant risk of catastrophically frying any microelectronics contained within proximity to the treated material, the high-energy molecular impact of these treatments will also permanently damage sensitive materials, such as nanocomposite foils. While the damage is invisible to the naked eye, viewing the treated surface through an electron microscope will unveil less than desirable surface adhesion conditions. The moon-like surface reveals a lack of homogeneity, which is required to truly ensure long-term coating durability. The irradiated surface is still ‘activated’, but this comes at a great cost to the success of the intended treatment with the considerably diminished surface integrity. Additionally, the static discharge present in many surface treatments has been known to attract dust to a surface from several metres away. If the dust is still present on the substrate surface when ink, paint, resin, or varnish are applied, there is a danger of the deployed substance wetting and bonding to the dust rather than the surface. Subsequently, any hydrophilic properties imparted to the surface are lost when the unwanted debris inevitably falls away, taking the deployed substance along with it. Excimer is a solution which confines the high-energy discharge within a modular lamp and releases only low-temperature, high-energy electrons to treat the surface. Its uses are manifold: surface activation, modification cleaning and pretreatment for hard coatings, UV curing, matting, curing nanocomposite coatings, disinfection (as an ozone-producing component of the low-energy SteriLux medical sterilisation device), pre-treatment for coating of composite materials, metal pattern wiring (PCB), photo bondingTM (optical, adhesive-free adhesion between identical or non-identical substrates. How does this technology work? The external application of high-energy electrons into the sealed quartz glass chamber generates an intense plasma field, also known as dielectric barrier discharge. The generated plasma contains high-energy electrons and can be immediately extinguished. The plasma induces the atomic excitement of the noble gas inside the lamp which leads to the formation of excited dimers. Excited dimers occur when high-energy orbital atoms form excited molecular pairs; in the case of Ushio‘s xenon (Xe) excimer lamps, high-energy Xe atoms form excited Xe2 molecules. The final VUV emission stage of the reaction is completed when the excimer molecules dissociate and return to their original ground state within a matter of nanoseconds, resulting in the emission of the excimer-specific spectrum. These VUV light particles travel through the air, partly absorbed by oxygen to create ozone on the substrate, where they crack the molecular bonds of the surface and create even more excited oxygen atoms. The cracked molecules are partly removed, creating a chemically medicated top layer of the surface and removing local organic substances. This effect is distributed along the length of the lamp, which can emit VUV light at a wavelength from 170 nm to 350 nm, within the range not dangerous for humans, and can be customised and adapted to customer needs to be chosen for the specific qualities achieved at each individual wavelength during manufacture. The most commonly used excimer wavelength is 172 nm which has the energy of 7.23eV and is powerful enough to directly crack all major bonds of organic molecules, except for bonds between carbon and oxygen (C=O) and some inorganic oxides. VUV light also enjoys a rather shallow penetration depth which leaves a thin polymerised film on the upper layer of UV-curable coatings. This effect leaves deeper layers unaffected, yet the resulting shrinkage leaves behind a homogeneously matted finish and brings scratch resistance to composite coatings. This technology is beginning to have a significant impact on the automotive and aerospace industries, it has eliminated the need for heavy adhesives and allows electronics to be integrated into already existing composite, polymer, glass, and even metal components. Ushio envisions the future of excimer-treated printed electronics in super-flat heating elements printed directly onto the inside of car seats and panels; external vehicular LEDs embedded into film coatings which follow the curve of any surface on which they are to be applied; the hydrophilisation of polymer substrates to prepare them for the application of metal printed circuit patterning will allow electronics to be part of any supporting structure rather than contained within it as a separate component. This technology will be presented at the company‘s booth at LOPEC as well as in the company‘s Excimer Innovation Laboratory, where customers will soon be invited to experiment with excimer for themselves.
Ushio Europe B.V., NL -1438 BD Oude Meer, www.ushio.eu, Hall B0, Booth 209