Emerging and developing technology. (Powder Coatings Corner).
Unless cure temperatures of powder can be substantially lowered, liquid technologies will continue to dominate in areas where the coating is field-applied and thermal curing is not feasible, such as industrial maintenance coating and coating heat-sensitive substrates such as wood, plastics, glass, paper and composites. Low-temperature-cured powder coating has moved from being a technical curiosity to emerge as a viable technology poised to have significant impact on the major facets of the organic coating market and the coating industry as a whole. Advances in new low-temperature powder coating chemistries will further open up uncharted coating applications and market segments for broader powder application and gain market share beyond the seemingly stagnated nine percent share of the industrial finishing market it currently enjoys. The emergence of practical, low-temperature cure powders and their further development to provide better performing coatings and continuous cost savings to customers will not only follow powder coatings' acclaimed tradition of prominent quality performance and environmental excellence, but also contribute to energy management standards and resource sustainability.
Cure Regimes and Feasible Technologies
Reduction of cure temperatures has been pursued by the powder coating industry consistently over the years. Indeed, some progress has been made: in the decorative market, powders curing below 400[degress]F were once considered low temperature cure. Product definitions of low temperature powders now vary from supplier to supplier, but on the average low temperature cure powders today must cure at 325[degrees]F and below. Low temperature cure coating powders could be classified into two categories and temperature cure regimes. There are two underlying technologies for the two categories: powder formulations with cure schedules between 325[degrees]F and 200[degrees]F are defined as low temperature thermal cure; UV-cure formulations are also typically cured in this range. Even within this conventional low cure range, some performance and appearance properties are sacrificed, and there are no robust technical solutions that give differentiated products. With current technology, UV-cured powder technology seems more realistic and feasible for lower temperature cure below 200[degrees]F.
Temperature and viscosity profiles show how fluid powder coating formulations become and for what time duration. Fluidity and its duration time is determined by factors such as temperature and its rate of rise, molecular weight build-up, kinetics of cross linking, types of additives (e.g. pigment) and their volume concentrations. The interplay between rheology and reactivity in these formulations determine the smoothness or lack thereof (e.g., orange peel) of the final films formed.
In conventional thermal curing of powder coating, the melting/flowing/ leveling process (cycle) has, invariably, some degree of overlap with the curing process (cycle) even with powders formulated to cure at fairly high temperatures. This overlap is mainly responsible for the orange peel and the lack of great smoothness synonymous with the technology compared to liquid coating. With current lower temperature curing powders, this phenomenon is exacerbated because the cycles are compressed within a narrower process window to give a bigger overlap. The greater the overlap between the melt/flow/leveling and cure cycles the worse the orange peel and smoothness of the final film. In the ideal case, the two cycles are distinctly separated, so that the powder melts, flows and levels to form a smooth film before the onset of a reaction to cure. When the cure reaction is accelerated, the duration of low viscosity is shortened. UV and EB curing provide examples of this ideal separated cycles case: powders melt, flow and level thermally without any cross linking and hardening and then a reaction to harden the film is brought about using UV or EB. Delay of the cure reaction, keeps a low viscosity for a longer time.
In new and emerging technologies, advances driven by new government regulations, cost reduction, higher productivity, better products, new materials and new ideas are being made rapidly in the key coating technologies, including waterborne, high solids, electrodeposition and autodeposition. The pace at which low temperature cure technologies develop and how well they address technology gaps, cost reduction and other customers' needs will determine which new markets are captured and how many gains are made to advance the market share of powder coatings.
Nikoi Annan is market manager with Morton Powder Coatings, a Rohm and Haas Company.
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|Date:||Oct 1, 2002|
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