Schematics of electrostatic powder-coating discharge and deposition. 

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... above analysis clearly demonstrates that application of hierarchical design concepts provides promising route to the design and fabrication of high-performance coatings, adhesives, composites and other materials. Wet paints, commonly used for decorative finishing of automotive plastic components exhibit very low transfer efficiency typically not exceeding 30-35%. Consequently they constitute a major source of solid waste in the form of sludge, and VOC’s, second only to the vehicles exhaust emission. Powder coatings, an attractive 100% solvent-free alternative technology achieve 95-98% material utilisation. However, they require electrostatic deposition on charged powder particles and hence are only used on inherently conductive metallic components in the automotive, marine and construction industries. Approximately 58 Mln passenger cars p/a manufactured in 2008-10 required 3.269 bln litres of automotive coatings p/a for painting. Globally, painting plants of auto-industry and refinishing shops release approx. 170-172,000 tones of VOCs (solvent-in-paint: ~3.5-4 kg/vehicle), and create ~96 Mln kg of hazardous waste. Until now, the key barriers for wide introduction of powder coating technology in plastics surface finishing are: non-conductive surfaces of plastics, inadequate adhesion to commodity plastics such as polypropylene blends, and frequently - the absence of low-temperature powder resins. This paper presents theoretical and practical aspects of novel technologies facilitating successful powder coating of automotive and commodity plastics, eg: (i) providing adequate surface conductivity of polymeric substrates, and (ii) coating adhesion. The principal driver for this initiative was our vision of a new transformational technology platform providing the globally first “zero-waste” coating technology for plastics and composites, eliminating all wastes (VOC’s, solid waste & water consumption) traditionally created when coating plastics with liquid paints. The principles of electrostatic powder-coating technology are illustrated in Figure 7. An ionised air zone is formed around the tip of corona discharge electrode at the outlet of electrostatic gun nozzle. This, in turn, imparts positive electrical charge on all particles of pigmented powder which consequently forms a fluidised particulate cloud in which all identically charged particles repel each other. A stream of pressurised air propells positively charged particles towards an electro-conductive substrate which is grounded and hence negatively charged. The design of electrostatic powder spray guns facilitates the control of key process parameters such as: (i) the magnitude of electrical charge imparted on powder particles; (ii) the powder flow- and deposition rate; (iii) the size and shape of spray pattern, and (iv) the spray density. As schematically illustrated in Figure 8(a), all powder particles, charged oppositely to the conductive substrate, are attracted to its surface on which a semi-continuous film of densely packed coating particles is self-assembled [see Figure 8(b)]. At the final stage of the process, the particles are melted by heat or surface-focused IR radiators [Figure 8(c)] to form a self-levelling fluid which cross-links at the appropriate temperature, or due to UV irradiation. Electrostatic powder coating facilities are equipped with overspray collectors to reclaim an over- sprayed powder which is then reused, resulting in ~95-98% coating transfer efficiency. Thermoplastic powders require heat to melt the powder and to facilitate its flow and self-levelling into a continuous film. Thermosetting powders additionally utilise heat energy to crosslink the coating polymer. Typically, the following methods are used in the curing of powder coatings: (i) convection heating, (ii) infrared energy (IR), (iii) a combination of heat and IR, and (iv) ultraviolet (UV) energy. In convection ovens, hot air is circulated around the powder coated parts, and the parts attain the pre- set oven temperature. Infra-red (IR) ovens emit radiation in the IR wavelength. Most of the irradiated IR energy is absorbed by the powder and by uppermost surface of the substrate immediately below the coating film. This, in turn, allows a relatively rapid powder coating flow and cure. Consequently, the coated component does need not be heated to the coating’s cure temperature. UV curing is commonly used with heat sensitive substrates. Specifically formulated UV powders flow at very low temperatures (110 - 120°C) and can be cured via UV radiation in approximately 20-30 ...