Success usually resides in selecting the optimum combination of the right medium density fiberboard (MDF), the right powder coating and the right process. All three are critical to the production of high quality powder coated MDF parts. If that sounds familiar, it's because anyone who has been around finishing knows that it is the formula for any successful painting job.

The very nature of the substrate may, however, lead to other problems that show up after the parts are put in service. We refer to one such problem as post cure cracking (PCC). It has been observed that coating cracking can occur on the edges of powder coated MDF parts long after they are processed. Using feedback from customers, data from MDF suppliers and our own understanding of the coating chemistries involved, we designed a comprehensive study to understand the factors contributing to PCC. The results have enabled us to design coatings and coating processes that minimize the possibility of coating failure due to cracking.

MDF, Moisture and Dimensional Change

Degree of coating cure
Coating physical properties
MDF moisture content
Coating film thickness
Coating coverage or encapsulation of substrate
Environmental conditions of article storage
Substrate thermal history

MDF is a composite material composed of wood fibers and binder resin such as urea/formaldehyde that is fabricated into panels by heat pressing in the presence of moisture.2 Depending on ambient storage conditions of temperature and humidity, the moisture content of raw MDF typically ranges from about 4-8% by weight. Successful powder coating of MDF depends on moisture content. Normally, MDF is preheated prior to powder coating to drive internal board moisture to the surface of the panel and to activate the surface for more moisture pick up from the environment. The resulting surface moisture provides the electrical conductivity necessary to achieve electrostatic attraction of the powder. To some extent, powder may also adhere to the heated panel through physical impact and melting. If the initial MDF moisture content is low, electrostatic powder application becomes difficult due to lack of conductivity or poor path to ground. If initial moisture content is too high, MDF panel edges can crack prior to coating due to stresses encountered during preheat that exceed the internal bond strength of the composite panel. After coating, excessive moisture can also lead to out-gassing during cure thus yielding blistered or defective parts.

Powder coatings can be successfully applied to, and thermally cured on, MDF in a single-coat operation to provide decorative finishes for use in office furniture, cabinetry, shelving, point-of-purchase displays and other applications. Powder coating provides an attractive alternative finish to traditional laminates, liquid paints, paper and vinyl overlays. Of prime importance to coating performance over the useful life of a coated article is the relation between MDF dimensional changes and moisture content. As MDF is heated, it initially expands, yet begins contracting upon further heating as moisture content decreases. Shrinkage continues after heating, as the panel cools to ambient temperature. Usually, expansion and contraction are an order of magnitude greater through the thickness of the MDF panel compared to the length or width across the surface of the panel. The amount of moisture loss and shrinkage experienced by a MDF panel depends on the preheat and cure cycles used in the coating process in addition to the initial moisture content of the panel. For example, upon drying from an initial 5.7% moisture content to 0% moisture content, according to ASTM D 4442, the through thickness shrinkage of MDF-A was found to be 2.1% once cooled to ambient temperature. The natural tendency for dried or partially dried MDF is to re-equilibrate with its surroundings via moisture absorption and expansion. The degree of moisture absorption and expansion depends on ambient conditions of temperature and humidity. All of these factors influencing the dimensional stability of the substrate can have a dramatic affect on applied coating performance, in this case, coating edge cracking. For a coated MDF article, the rate of moisture absorption and expansion of the substrate depends on coating thickness, moisture permeability of the coating, and how well the substrate is encapsulated with coating.

The coating itself must also have enough mechanical integrity to resist cracking while under the influence of stresses induced by substrate dimensional changes. In most cases, coating toughness or resistance to cracking can be improved with adequate cure. Otherwise, other performance compromises may need to be made in order to improve coating toughness enough to resist cracking under any environmental conditions that the coated article may experience during its useful life.

Figure 1 / Back Film Thickness vs. Days-Until-Cracking for Non-Routed Panels

Experimental Details and Results

Coated MDF panels, completely encapsulated with coating or machined after coating to expose raw substrate on one face surface, were placed in an environmental chamber (Thermotron) at 80?F and 95% relative humidity. The test parts were routinely monitored for moisture weight gain, percent through thickness expansion and time to crack failure for a period of 13 months.

As expected, moisture uptake results in expansion of the MDF, with through-thickness expansion being an order of magnitude greater than expansion across the substrate surface. The through-thickness substrate expansion induced stress on the coating edges that eventually led to cracking failure of the test coating. At the time of failure, substrate expansion forces exceeded the tensile strength and elongation properties of the test coating. Coating cracking did not occur on the substrate faces due to the lower degree of expansion upon moisture absorption. Complete encapsulation of the substrate with coating provided a moisture barrier, thus slowing expansion and prolonging the time until cracking. Machining through the coating to expose the substrate directly to moisture resulted in rapid substrate expansion and coating edge failure. Increasing the coating film thickness on fully encapsulated parts improved the moisture barrier thereby reducing the substrate expansion rate and prolonging the time until edge cracking. Coatings that were under-cured cracked sooner than coatings that were fully cured due to lack of complete physical property development. The time until coating cracking increased as the initial (prior to coating) MDF moisture content increased.

Figure 2 / Days Until Cracking vs. Initial Board Moisture Content for Routed and Non-Routed Boards

Data Analysis

In these studies, the effects of eight variables on days until post-cure cracking were investigated. In order of magnitude, the estimated effects and percent contribution to the total variance are shown in Table 2.

The most significant variables are the first four: routing, back film thickness, type of MDF and degree of cure. The moisture level and the interaction between routing and moisture are somewhat less significant. The remaining variables (heat and face film thickness) do not have much effect on days until cracking.

Exposing raw MDF through routing is clearly the most important variable determining the time until cracking. Routed boards crack much sooner than non-routed boards; on average, non-routed boards last 194 days before cracking; routed boards crack after 32 days (These averages are calculated after adjusting for the other effects). For all non-routed boards, the average percent moisture weight gain at the time of cracking was found to be 5.4% and the average percent through-thickness expansion of the MDF was 3.3%. This degree of expansion of the boards, under the harsh humidity testing conditions of this experiment (95% RH), exceeds the amount of contraction that the MDF could experience from moisture loss during any preheat or cure oven conditions.

Back film thickness is next most important. The thicker the film on the back of the part, the longer it can withstand cracking, due to the improved moisture barrier that slows the rate of moisture uptake and board expansion. Each additional mil of coating adds more than 9 days to the expected time before cracking (see Figure 1).

Figure 3 / Actual vs. Predicted Days-Until-Cracking for Routed and Non-Routed Boards

The type of board also determines the amount of time before cracking. In this study, MDF-A boards lasted an average 167 days, compared to 113 days for MDF-B boards, calculated after adjusting for the other effects.

Complete cure is also important. Fully cured coatings average 163 days before cracking; under-cured coatings average 117 days. (Estimated after adjusting for the other effects.)

The moisture effect is small, and depends on whether or not the boards are routed. Routed boards crack a bit earlier when their initial moisture content is high; non-routed boards crack a bit later with initial high moisture content (see Figure 2).

The statistical model based on routing, back film thickness, cure, board, moisture, and the interaction between moisture and routing, explains more than 80% of the variation in days-until-cracking for all coated boards whether routed or non-routed. There are no outliers in the data. The fit of actual to predicted days-until-cracking is shown in Figure 3.

The data and subsequent analysis thus indicate that PCC of powder coated MDF is the result of substrate expansion due to moisture absorption. Edge cracking of the coating can occur when substrate expansion forces exceed the tensile strength and elongation characteristics of a particular coating system. It should be noted that, when properly cured, panels fully encapsulated with 5-10 mils of coating on all sides resist post cure cracking for nearly one year of continuous exposure to the extremely harsh conditions of 95% humidity at 80 deg F.

Recommendations

Fully encapsulate substrate with coating to slow the rate of moisture uptake.

Apply coating as thick as possible to slow the rate of moisture uptake.

Ensure adequate cure of powder coating on all areas of the part in order to develop maximum physical properties.

Adjust oven preheat and cure cycle to minimize moisture loss from MDF substrate.

Store/use coated articles in environments of controlled lower humidity.

For more information on powder coating MDF, contact Mike Favreau, Morton Powder Coatings, No. 5 Commerce Drive, Reading PA 19607; phone 610/775.6703; fax 610/775.6732; e-mail mfaverau@rohmhaas.com.

References

2 Maloney, Thomas. Modern Particleboard & Dry-Process Fiberboard Manufacturing, Miller Freeman Inc. 1993.

3 ASTM D 4442-92. "Direct Moisture Content Measurement of Wood and Wood-Base Materials."