MDF, Environmental legislation prompts a new direction.


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Environmental MDF moisture content. MDF has a natural water content of typically 5 percent to 7 percent. The new system developed uses this as a means to make the board conductive. By quickly raising the surface temperature of the MDF to 200�F during a total dwell time of up to 2 minutes in a catalytic preheat oven, the board becomes conductive and remains so for up to 5 minutes after leaving the oven. Board temperature at application is 120�F to 150�F, well below the temperature at which the powder would fuse to the MDF. This method of applying the powder to the MDF is contrary to the convection process. In that process, the preheat lasts for some 15 minutes, virtually drying the board and raising the temperature to some 275�F at which point the powder will fuse to the board. By forcing the board to become conductive, however, the powder is easily attracted to the MDF at much lower temperatures, ensuring a high degree of transfer efficiency. The attraction causes powder to wrap around the board perimeter in a similar fashion to metal. Powder cure. Once the powder is evenly deposited on the board surface and around the edges, the board enters into a catalytic cure oven (Figure 2). Curing is complete within 5 minutes. The catalytic heaters within the oven are arranged in such a pattern as to drive the temperature of the powder on the edges through to cure as quickly as possible. This action has a number of benefits, the primary one being to seal off the edge to any out gassing that tends to happen along the edges of the board. By concentrating the IR towards these edges, the face of the board easily absorbs the required IR to flow and cure these flat areas. MDF has a natural tendency to expand when the relative humidity increases, especially at low ambient temperatures. Some MDF expands more than others. That�s why the powder on the edges has to be well-cured to reach the full physical properties of the coating. For a line running at 10 feet per minute, the typical oven length is 45 feet to 50 feet. Throughout the oven, some 50 heaters operate on 40 zone controls. Understanding the heating profile through temperature logging is critical to achieving good cures. A sample board has small lightweight thermocouples glued to the four edges and the flat top and bottom surfaces. This board is periodically sent down the oven to compare oven settings for peak temperature and duration. Environmental legislation prompts a new direction If the process is stalled in North America, the reverse has been the case in Europe. The wood industry in Europe has fallen under the tough new environmental legislation of the Kyoto Protocol1. This in turn has forced solvent-based wood and MDF finishers to look for alternatives to bring them into compliance. Seven such companies throughout Europe have adopted powder coating processes to produce MDF components ranging from kitchen cabinet doors, point-of-purchase displays, and office furniture. (See Figure 1.) Planning for this potential use of powder coatings started some 3 years ago when a leading powder coatings producer collaborated with a catalytic oven manufacturer. A new approach and understanding of the variables that had apparently held back the anticipated growth potential was required to kick-start the use of powder coatings for MDF in North America and Europe. Early decisions had to be made as to which process would produce a forgiving and economical system. UV powders, while having some distinct benefits, were eliminated on the grounds of high powder cost, issues with long-term reliability and operating costs for the UV generators, and issues with color matching. This left thermal powders as the way forward. The debate then centered on which type of heat to use for both the preheat of the MDF and the cure of the powder. Historically, the convection process required a high cost in ovens and long dwell times of more than 15 minutes in the preheat oven and a further 8-plus minutes for the cure of the powder. MDF is inherently a bad conductor, hence heat transfer via conduction is poor, requiring long conveyor tracks for the heating dwell times and, consequently, long cooling times. Infrared (IR) is known as a faster method of heating the surface of flat panels compared with convection, especially if the panel is a poor conductor. Understanding these differences in the heating dynamics of IR and the effects this has on the MDF and the powder coating would be key to designing a new process that would reduce the overall exposure of heat that is characteristic of convection. The basic difference in the two heat sources is that convection heats objects via the conduction of heat from hot, high-velocity air to the MDF substrate, and IR heats the surface by radiation. The convection process promoted in the US required that the MDF be at such a high temperature that the powder fused to the hot board. This high temperature is inherently bad for the board because it causes stress and damage to the glues that hold the board together while driving the majority of the moisture from the board, especially at the extremities of the part. If the process is stalled in North America, the reverse has been the case in Europe. The wood industry in Europe has fallen under the tough new environmental legislation of the Kyoto Protocol1. This in turn has forced solvent-based wood and MDF finishers to look for alternatives to bring them into compliance. Seven such companies throughout Europe have adopted powder coating processes to produce MDF components ranging from kitchen cabinet doors, point-of-purchase displays, and office furniture. The Kyoto Protocol to the United Nations Framework Convention on Climate Change is an amendment to the international treaty on climate change, assigning mandatory emission limitations for the reduction of greenhouse gas emissions to the signatory nations. The objective is the "stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system." As of December 2006, a total of 169 countries and other governmental entities have ratified the agreement (representing over 61.6 percent of emissions from Annex I countries). Notable exceptions include the US and Australia. Other countries, like India and China, which have ratified the protocol, are not required to reduce carbon emissions under the present agreement. There is still some debate about the usefulness of the protocol, and there have been some cost-benefit studies performed. From [www.wikipedia.org]
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