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UV- Curable Powder Coating A new paint technology has been developed that is likely to change the way that many products are manufactured in the future. UV-curable powder coating combines most of the advantages of conventional powder coating, together with the advantages of radiation- curable coatings. The result is a coating process with exceptionally low environmental impact and which can be used on temperature-sensitive substrates. For many manufacturers it opens up the possibility of coating ready-assembled articles in-house instead of using outside paint shops. Coating Importance The coating on the outside of a product is usually of vital importance. A good product can be ruined by a poor coating; just as much as a poor product can be improved - at least in appearance - by a good coating. It is not just a matter of applying an attractive coat of paint onto an article. Attention must be paid to the physical properties required of the coating, such as substrate adhesion, resistance to abrasion, corrosion and weathering. Mechanical properties, such as the ability to withstand bending, and impact can be important, as well as chemical resistance towards attack by water, solvents, oils, hydraulic fluids etc. In short, the coating enables an article to do the job it was designed to do and does not just look good. This has its price, of course, particularly in the automotive industry, where the costs of applying the different surface coatings make up approximately one quarter of the total manufacturing cost of a vehicle. For general manufacturing industry, this figure is much less, but nevertheless a factor for careful consideration. Choice of the appropriate paint technology is of great importance too. On the one hand, environmental legislation on the toxicity of coatings, emission of solvents and waste water treatment is becoming increasingly severe. On the other hand, market forces dictate both fast fabrication times and a minimisation of expenditure. Against this background, the new technology of uv-curable powder coating has been developed, which should help some manufacturers find an attractive solution to their coating problems. How UV-Curable Powder Works The article to be coated is first hung or placed onto a conveyor band and the uv-curable powder is then sprayed electrostatically onto the object to give sufficient coverage in a single pass. So far this process is the same as in conventional powder coatings. The coated article then passes out of the spray booth and into an oven, where it is then briefly Figure 1. Principle of UV-Curable Powder Coatings. heated under IR lamps, just enough to melt the powder IR heating (Figure 1). Here temperatures in the range 90 - 120°C are sufficient. The powder then flows together to form a smooth IR heating surface film and, while still hot, powder applied is passed under a uv-lamp. The to substrate, IR heating uv-light triggers a heating starts photopolymerisation reaction in the molten resin, which UV irradiation powder melts toughens the film. On emer- flow starts gence from the cure unit, the painted object is fully hardened, flow completed but may need a few minutes to cool down. The whole process, coating cures including spray application, takes at most 5 minutes. The chemistry of the system is a little more complex. The resins used to make the powders are Figure 2. Schematic Diagramme of Cure Reaction. typically unsaturated polyester resins, often in combination with UV irradiation other unsaturated compounds. Key ingredients of uv-powders are photoinitiators, which absorb the uv-light to form highly reactive free-radicals and which in turn initiate the crosslinking of the resin (Figure 2). Flow unsaturated resin promoters are also included to + photoinitiator assist the levelling of the coating in the molten state. Pigments photoinitiator are added to give the required breaks down to form free colour shade and hiding power radicals (ability to cover white or black resin substrates crosslinks without change of colour). This is only possible, to the extent that the absorption of light by the photoinitiator is not unduly interfered with. Powder Application The first stage in the powder application process is the spray application of the powder. As with all coating processes, it is important that the surface to be coated is clean and free of oils and here uv- powder is no exception. Adhesion of the cured uv-powder to metals, such as steel, is much improved if the surface has been phosphated prior to coating. Conductive materials present no problem for the electrostatic powder application as long as Figure 3. Principle of Powder Coating Guns care is taken to ensure that the object to be coated is adequately earthed. Electrically non-conducting substrates, Tribo gun Corona gun however, are more difficult to coat and best results are usually achieved with a triboelectric powder gun rather than the corona guns more commonly Corona wire PTFE barrel Polypropylene used in the powder coating barrel industry (Figure 3). Powder / air feed line With particularly non- conductive materials, build up of Friction of powder against PTFE Charged ions from corona attach a powder film is difficult and barrel causes powder charging to powder paint particles warming the substrate before application of the powder can help. It is also possible to charge the surface of the object to be coated with a charge of opposite polarity to that of the powder which is to be applied. Care must be given not to apply too much powder to the substrate, as penetration of light to the bottom of the coating must be sufficient to ensure good through-cure. Failure to do this will result in a cured coating surface that “floats” on an uncured layer of resin and hence loss of the mechanical properties of the coating. This is an important difference to conventional powder coatings, which are mostly not affected by excessive film build-up. Although it is certainly easier to coat and cure flat stock with uv- powder, 3-D objects can be Figure 4. Layout of a UV-Powder Coating Line. painted with this technology if care is given to the design of the installation. It is necessary to hang up/ take optional cleaning, control both the residence time down station pre-treatment and blow dry and the intensity of the infra-red lamps (using pyrometers), so as to ensure that sufficient heat is given to the object to melt the powder in all parts without scorching (Figure 4). Combination IR-convection uv cure ovens are usually the best choice. To cure the powder, it is IR-convection powder spray important that the lamps be so oven application positioned, that all parts of the coated surface are adequately exposed. It may also be advantageous if the object is rotated during passage under the uv-lamps. The use of reflectors and careful positioning and direction of the lamps helps to ensure a good cure without waste of energy. Where UV-Powder fits in… Conventional Powder Coating Currently a manufacturer has to choose which of the available paint technologies is the most suitable for the object that requires finishing. For metal articles that can withstand a bake cycle of ca. 20 minutes at temperatures in the range 160 - 200°C, conventional powder coating is usually the technology of choice. The powder-coated article then passes into an oven where the coating successively melts, then flows together to form a smooth, pinhole-free film and finally crosslinks to form a tough, durable surface. On emergence from the oven the painted articles first have to cool before they can be removed from the hangers. The process is environmentally friendly, as the powder overspray can be mixed with fresh powder and resprayed and also because there are no solvent emissions. The thermal energy requirements of the ovens are, however, considerable and the floor space requirements can be prohibitive. Unfortunately only metal objects can withstand stoving in the ovens. UV-Curable Liquid Coatings Liquid radiation-curable coatings are usually free of solvents. They can be curtain coated or sprayed using the same equipment as solvent paints, but are cured more or less instantaneously by passage under a uv-lamp. Traditionally these lacquers find use mainly in the furniture industry and in specialised clear coatings, such as on compact disks and as gloss coatings on packaging and printed papers. Until recently uv-curable lacquers were restricted to clearcoats, but with new advances in the chemistry of photoinitiators, coloured uv-lacquers can now be made. The very low energy requirement to cure the paints and the low content of volatiles make this a clean technology. It is also well suited to use on heat-sensitive materials. The principle drawback, however, is the problems involved in handling the sticky lacquers and the reuse of overspray. Solvent Coatings To give a durable finish to articles such as electric motors, pumps and gearboxes etc. that contain heat-sensitive components, like seals, wiring and electrical insulation is not straightforward. One solution is to send off the fully-assembled article to a paint shop to be spray painted with a reactive two-part polyurethane paint. An equally unattractive alternative is to paint each component individually, for example with a conventional powder coating, and then assemble the finished product. Although polyurethane paints have excellent and proven durability, the problems of treating the contaminated water in the spray booths and the restrictions on the emission of solvent vapours greatly increases coating costs. For small to medium sized manufacturers, such paint jobs are generally handled externally and hence cause delay. Benefits of UV-Curable Powder With UV-powder, it is necessary to apply enough heat only to melt the powder and achieve Figure 5. Advantages of UV-Curable Powder Coating flow. The thermal exposure is much less than in conventional powder coatings, where prolonged heating at higher = Combined advantages of …... temperatures is necessary to bring about the cure process. UV-Curing Powder Coating This enables uv-powder to be used on a wide variety of • very fast • dry handling substrates such as medium density fibreboard (MDF), • low energy demand • ok on heat sensitive + • recyclable overspray • easily automated substrates • almost no emissions paper, cardboard, plastics, • low floor space • thick coatings in one pass leather and wood, in addition to requirements • textured surfaces possible pre-assembled objects contain- ing heat sensitive materials. For uv-powder, all the advantages of conventional powder coating combine with the advantages of uv-curing technology, as summarised in the Figure 5. For the manufacturer, this means the possibility to install a small coating line, that occupies little floor space and to have in-house control of the coating process. The paint application step can be integrated into the production process, so that immediately after assembly the products can be painted and shipped out the same day . Limitations of UV-curable Powder At present uv-powder can be made in a variety of finishes and textures and in most colours. Although uv-light is needed for the cure process, black is not the problem that most people would expect it to be. Yellow shades are difficult to cure and are, at present state of the art, best avoided. The production of fully flat matt finishes has also not as yet been demonstrated. With regard to the curing line, it is important that the curing equipment be installed in such a way that objects of different geometries can be cured, so as to ensure full flexibility of production. Here creative solutions must be found if the technology is to realise its full potential in the manufacturing industry. Economics It is certainly difficult to put a value on a coating process that requires only minutes and which can be integrated into a production line. Whereas the powder application equipment is today available at competitive prices and from a number of manufacturers; the investment costs in an IR-UV line can be expected to depend on the sizes and shapes and the diversity of the articles to be coated. In the coming months more knowledge can be expected to emerge from pioneer users of uv-powder. First Users The first line for coating uv-powder was installed at the Baldor Electric Company at Westville, Oaklahoma, USA and is used for coating electric motors. The radiators for some Dodge trucks are now also being produced with a uv-powder coating in the USA. In Europe, the first uv-powder line is now running in Wales at a factory that produces tables and stands for TV and HiFi equipment. Here the ability of uv-powder to give exactly the same finish on both MDF and metal is considered to be an advantage. Further installations are planned in applications in which uv-powder is to be applied to MDF panels and mouldings. For any company seriously considering the installation of a Figure 6. Teamwork Requirement for an Integrated uv-powder line, there is one factor of overriding importance. Solution for Coating with UV-Curable Powder. It is essential that all the suppliers of equipment and Powder powder - even down to the Formulator suppliers of the powder raw • Resin supplier(s) materials - are willing to work • Photoinitiator supplier • Pigment supplier(s) together and pool their knowledge in order to get the total package to work properly (see Figure 6). The supplier Powder End User / commitment must also extend Application Customer UV-Cure Unit to guaranteeing a full back-up Equipment Constructor service for future trouble- Supplier • UV-Lamp shooting. Teamwork, more than • Substrate supplier supplier(s) any other aspect, is likely to decide the future success of uv- powder coating.
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