Powder coating of wood plastic composites

Powder coating of wood plastic composites

F O C U S It is not, however, too late to reverse the trend but the old image can never return. The current crisis will remove the unprofitable small ...

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F O C U S It is not, however, too late to reverse the trend but the old image can never return. The current crisis will remove the unprofitable small companies during the next twelve months. Liquid coatings will decline further for all types from solvent containing materials to 100% reactive components have an inherent fault. Wastage is too high a cost element and recovery is impossible. New technologies must be introduced to open new market opportunities and the old technology phased out. This implies a concerted effort on the part of coatings producers to apply pressure to end users to upgrade the application methods to gain full benefit from the new technologies. Emphasis should be placed on the real values of one coat application, lower energy costs due to a combination of faster processing times and reduced energy demands for curing, little or no waste of coating, with products capable of application to all substrates. There will need to be compromise between the supplier and customer to achieve the aims of modernisation and sustainable profitability. The coatings technology that is most capable of achieving these aims is powder coating but this is not the time for complacency and new concepts must replace the old. This publication will continue to keep readers up to date with the latest technical trends and open the gate to a new future. Sid Harris

TECHNICAL Powder coating of wood plastic composites Problems associated with the application of powder coatings to wood composites were reviewed in an article abstracted in the January issue of Focus on Powder Coatings. Further

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investigations carried out at another technical institution in Dresden were the subject of a paper presented at the PRA’s 6th Woodcoatings Congress. Powder coatings offer the ideal solution to end use applications requiring long-term weather protection as a one-coat, environmentally friendly and decorative finish. Potentially, high volume, market opportunities include the protection of windowsill profiles and façade elements formed from wood plastic composites based on blends of wood fibres in a plastic matrix. Progress in lowering the curing temperature of thermoset powder coatings and the introduction of radiation curable powder coatings has proved to be successful for coating MDF board. However, it has not been possible to apply the same techniques to WPC profiles due to their different physical characteristics. An examination of the electrical properties of the WPC samples compared to those of MDF confirmed that powder coatability was only possible if the WPC samples show an electrical volume resistance of <1010Ω or if the surface is sufficiently electrostatically conductive. Six different types of powder coating, which are commercially available for coating MDF were assessed for application to WPC. Two of these products were based on epoxy/polyester hybrid systems and therefore unsuitable for exterior exposure. The thermal curing systems with exterior durability qualities included acrylic and uretdione crosslinked powders requiring stoving temperatures of at least 140°C for a minimum time of 15 minutes. UV curable powders designed for exterior exposure required only two minutes IR heating of the substrate to 120°C to allow the powder to flow into a smooth continuous film followed by a few seconds of UV exposure to cure the coating.

C OAT I N G S When applying powder to WPC substrates the surface tension of the substrate must be higher than that of the powder system in order to obtain good wetting and high adhesion of the cured powder to the substrate. The low polarity of the thermoplastic binders in the WPC formulation makes it necessary to activate the surface of the samples by use of flame or plasma treatment and the application of adhesion promoters. Flame treatment involves briefly exposing the surface to the flame of an oxygen-enriched gas. Activated molecules generated within the flame partly attach themselves to the surface molecules, breaking up the long molecular chains in the surface, forming a cracked profile that assists the polar groups in the powder to bond to the substrate. An excess of 10 to 15% oxygen is required in the flame for the process to be effective. In plasma treatment, a highly ionized and activated gas is generated with the capability to effect chemical and physical reactions on the material surface. The excited gas molecules, electrons, ions and molecular fragments activate the surface and increase the surface tension to improve wetting properties. This may be accomplished by plasma treatment at atmospheric air pressures. Plasma treatment was found to be more effective in most of the trials although the benefit only lasted for two days when using either treatment. While the surface of the coating is now pretreated to assist wetting during the powder flow period, it is still necessary to modify the conductivity of the material to ensure that the powder can be applied electrostatically and it is also important that the substrate is earthed so that the charges built up during application can be safely discharged. Earthing can be achieved by several methods: contact earthing; electrostatic

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F O C U S charge compensation by a counter-polar DV corona; or electrostatic charge compensation by AV ionizers applied at the rear of the object. In the laboratory trials the latter method was found to be the most reasonable earthing method. Transfer efficiency is high and small edge coating is best achieved by this method. Since WPC samples could not be coated by direct application of powder to give adequate adhesion, the use of a primer or intermediate liquid layer was investigated. It was found that better wetting and adhesion was achieved by applying an adhesion promoting intermediate layer. Further measures were taken to minimize evaporation during the melting and curing of thermoset powder coatings. The panels were tempered in a convection oven at an air temperature of 130-140°C for over 30 minutes. Acceptable powder coated surfaces were obtained by spraying the powder onto the heated substrate and an increase in the time taken in the tempering oven also improved adhesion. Tempering times in excess of 120 minutes at 130-140°C considerably improved the quality of the powder coated surfaces. Good coating results achieved by hot coating were only observed when the tempering period lasted more than thirty minutes and the hot surface was coated immediately after tempering. It seems from the results obtained that there are two options for coating the WPC materials: predrying, pretreatment by flame or plasma, conductive adhesion promoter and hot coating; or, predrying, conductive adhesion promoter, hot coating. The best results were achieved by the following procedure: predrying, 140-145°C > 40 minutes, convection oven; coating at approx. 110-120°C substrate temperature; melting/curing 130150°C in IR/convection oven ranging from 3 minutes for

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thermoplastic powder (UV) to 35 minutes for acrylic thermoset powder. The beneficial effect of hot coating suggests that the industrial process for coating WPC should begin immediately after the extrusion of the WPC material. The test arrangement shows hot application of powder by corona and tribo guns onto untreated, flamed and conductively primed façade profiles, which were at 120°C and dry, immediately after leaving the extruder with flow and curing completed thermally or by UV curing. This technique subsequently showed that after one year of outdoor weathering exposure, no adverse effects were observed. Paper entitled “Powder Coating of Wood Plastic Composites” by Rico Emmier and associates at the Institute for Holztechnologie, Dresden presented at PRA’s 6th Woodcoatings Congress, in Amsterdam, The Netherlands 14-15th Oct 2008. Copies of the papers are available in CD format from website: http://www.pra-world.com

The survival of powder coatings Cynthia Challener is a US based consultant who communicates “Letters from America” in the Polymers Paint and Colour Journal. In a recent report she considers the economic downturn and the need for new powder coating technology. Although this report was written before the traumatic economic realities of November and December it is, nevertheless, a useful account of many of the factors which have contributed to the 2009 global industrial depression. Powder producers in North America have seen the economy decline over the past three years to a rate of 1.5% year-on-year growth. The market is mature and there is less movement to convert from liquid to powder. Higher energy costs and raw material prices are a problem, and additionally the move of

C OAT I N G S manufacturing to lower cost regions is beginning to have a negative impact, with powder production over capacity eventually leading to further rationalisation within the industry. The growth rate of 1.5% compares to volume growth rates of 3.5% in South America and 4% in Europe. In 2007 the North American powder coating segment was valued at US$890 M but accounting for only 15% of the global market valued at US$5.75 bn. Value growth to 2012 is predicted to increase at a compound annual rate of 3.5%. The largest outlet for powder coatings is in general metal applications, which accounts for 55% of sales. This sector is growing at about 4% annually. Appliance and metal auto parts account for 28% of sales with annual growth rates of 2 and 3% respectively. According to one market survey group, the demand for powder coatings is largely driven by the global industrial GDP, with powder outpacing liquid due to continued conversion of liquid. However, this liquid conversion rate has diminished considerably in the last decade with powder growth rates converging on liquid growth rates. In North America, the number of new installations has declined dramatically. In 2000, there were 570 new installations sold, compared to 238 in 2007. The initial driver for powder coatings was the need to reduce VOC’s in paint formulations but increasing competition from alternative environmental technologies such as 100% solids and UV/EB curable coatings have penetrated the powder market to the extent of 5% but this is still growing at twice the rate of powder coatings. The VOC driver is no longer a major factor in the switch from liquid to powder coatings in developed regions such as North America. Nevertheless, the powder option does offer value in terms

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