New Developments - Powder Coating of MDF

There is no doubt that the wood market (particularly engineered products such as MDF) are an exciting new market for powder coating.
A lot of articles have recently been written on the best way of entering this market, a lot of these reports are contradictory depending on the knowledge and bias of the author and their perceived opinion of what their particular market requires or indeed will pay for.
Eurotec is leading the field in the installation of low cure thermosetting systems having installed several lines in the UK and recently the first MDF powder coating line in New Zealand. The following is a report on our assessment of this exciting new field for powder coating.
Eurotec's expertise in installing high speed, automatic powder coating lines to successfully apply low cure thermoset powders to MDF has generated such global interest within the wood and furniture industry that the company has now established a separate R&D facility to further explore the application of powder to all heat sensitive products but in particular MDF and wood based materials.
The company is liaising with FIRA (Furniture Industry Research Association) based in Stevenage, Hertfordshire, UK to ensure that the coatings produced conform to the required standards.

Acre Products Ltd, Darlington U.K. 

What is MDF?

Medium Density Fibreboard is a wood composite material and is one of a variety of types of fibreboard. The typical wood proportion of MDF manufactured or supplied in Great Britain is 85-100% softwood, 0-15% hardwood, however the relative proportions of soft and hard wood varies according to the source of the wood, such that the MDF imported from countries where hardwoods predominate will have a relatively higher proportion of hardwood. The product is known for its versatility and is used predominantly in the furniture and construction industries.

Why the change to powder?

The traditional way to finish MDF has been to apply conventional wet spray, vacuum coating or covering with a laminate or membrane pressing. While these processes have been established for a number of years and are well proven systems they all have intrinsic drawbacks.

Wet Paint

If the coating is applied by spray gun there will always be the ongoing cost of the over sprayed paint, the maintenance of the spray booth and the disposal of the offending material. Even more concerning is that virtually all usable wet systems have the major problem of producing VOC's (Volatile Organic Compounds). Ever increasing environmental legislation means that users of wet systems continue to search for a replacement, environmentally friendly process such as powder coating.

Membrane Pressing

This process involves the application of glue to the MDF and the subsequent pressing of vinyl onto the surface. The application of vinyl can give an aesthetically pleasing, smooth finish. The major problem associated with this process is that it is a time consuming, relatively complicated process that does not lend itself to an efficient, high-speed production cycle. Sharp corners, acute angles and thick boards can result in stretching, bubbles and a high reject level while the process tends to restrict the shape and design of the product that can be satisfactorily manufactured.

MDF Specialised Coaters (Sheffield) U.K

 

Powder Coat

Powder is a dry film and solvents are not required the process therefore eliminates contravening the ever increasing legislation on reducing VOC emissions.
Over sprayed powder can be recovered and reused thus minimising waste and maximising powder usage.
The process enables complete flexibility in the design and shape of the board unlike the restraints that are encountered with membrane pressing.
Powder coating lends itself to automated, high-speed production techniques with low reject rates.

Low cure thermosets

Eurotec has over the past five years spent considerable time and resources working with powder manufacturers and customers to develop a complete coating system that allows them to satisfactorily apply powder to MDF. There are a number of prerequisites that are essential to ensuring that the end product produced is of the required standard:

Low Cure Thermosets - The Way Forward

The lines installed so far incorporate finely textured powders applied automatically at a line speed of 5 metres per minute with a reject rate of less than 2% of which 75 % can be successfully reworked giving a final reject rate of 0.5 %.

(MDF retains its temp for significant periods after curing therefore running at higher line speeds can cause a problem with loading and unloading and packing the product, this would need to be taken into consideration when designing a high speed plant).

The advantage of this low cure method is the relatively low cost of installing a plant compared to other options available and the cost of the powder which is in line with standard products.

In the past the question being asked was could smooth, low cure thermoset powders be developed to be applied to MDF board to give the quality of powder finish and the chemical and physical standards demanded by the large volume furniture manufacturers. We at Eurotec are confident the answer is a most definite ‘yes’, we have now developed coating techniques that give excellent results even on deeply routed doors.

The board manufacturers themselves now realise that the potential market for the powder coating of MDF is far in excess of the market for powder coating of metal products and urgently improving the density and consistency of their products as well as looking at the conductivity of the board to eliminate the need for preconditioning.

Alternatives

Preheat / Convection Cure

A system has been developed in the U.S.A. where the board is heated to high temperature in a standard convection oven and powder in excess of 200 microns (7.8mil) applied to the hot board where it is cured in a standard convection oven giving a smooth, aesthetically pleasing finish. We have found in our experience that to successfully apply powder electrostatically to MDF the moisture content of the board is critical (by heating the board to elevated temp before coating you are in effect drying out the board and therefore just applying the powder by preheat). We at Eurotec have major concerns about the long term effect to the coating and indeed to the board itself utilising this process. By curing MDF in a gas catalytic oven moisture is being generated as part of the process, we are believers in controlled heat being applied, maintaining a specific level of moisture content in the board during processing.

UV Cure

UV quickens curing speed but it should be noted that it is a two stage curing process. After a UV curable powder is applied an infrared / convection heat source is required to flow the powder prior to UV high intensity lamps activating coating polymerisation for crosslinking, this actually takes only seconds and the advantage of this process is that no heat is generated and the product can be handled immediately after processing. This allows for higher line speeds and productivity and lower energy requirements and space requirements.

The application to MDF of powder and subsequent curing by UV has however serious shortcomings to overcome.

Cost

The price of installing a UV line compared to a conventional low cure thermoset system is more than double the cost while the powders themselves are also extremely expensive.

Orange Peel

Reducing orange peel when using powders with low melt temperatures is necessary to produce a smooth powder finish.

3-D

The coating of three dimensional with complex of hidden areas is significantly more difficult than the coating of flat parts. the development of higher density board and the sanding of profiled parts can help to overcome this problem.
Pigmented colours present a challenge. This is particularly true with regards to yellow pigments, the choice of black pigment used in creating grey shades also needs to be carefully selected. Using the correct UV lamp can be critical when formulating and curing pigmented UV powders, there is a trade off between covering powder and curing properties in pigmented systems. The complexity of formulation and curing lamps is something to bear in mind when considering the cost of installing a UV type plant along with the cost of the UV curable powders and higher film build achieved.

Conclusions

There is a great deal to be learned in what is a process very much in its infancy with regard to heat sensitive substrates. Eurotec sees much of its future development involved in this market and intends to work hand in hand with the Furniture Research Institute, woodworking machinery manufacturers and the major powder manufacturers to accelerate the growth of powder coating in this exciting new field.