Formulators Forum—Formulating Thermoset Powders for Wood-based Substrates

Powder coating heat sensitive substrates like wood and MDF continues to be an exciting development in the powder coating industry. But what are the differences in formulating for heat sensitive substrates compared with traditional substrates and what do chemists have to consider with this newer technology?

In some ways, formulating for wood-based substrates isn’t too different from traditional formulation methods. Simply put, we must think about substrates, end use, equipment, etc. Traditional thermosetting powders are usually applied to various metal substrates. Most commonly we see aluminum or steel, but other metals are coated as well.

Exactly the same scenario exists with heat sensitive substrates. Under the “wood-based products” label are hard wood, soft wood, HDF, MDF, etc. Just as there may be
different types or grades of steel, there are also different types and grades of HDF and MDF.

The porosity of different wood types comes into play. For example, hardwoods are much less porous than softwoods, and therefore they outgas less. The powder formulation needs to account for typical outgassing of the wood type in use. Wood also tends to move or expand and contract more than a metal substrate, which must be accounted for in formulation.

In addition, the preparation of a substrate can differ wildly, and can sometimes affect coating performance, which in turn affects formulation. When formulating for wood- based substrates, chemists need to know the type of wood and the ideal prep for that species. Will preparation involve sanding? Will a powder primer be applied, then sanded before a topcoat is applied? If so, avoiding certain types of waxes that can float to the top in the powder and cause adhesion issues is key. Likewise, some resin systems would be too hard for effective sanding, and at the other end of the spectrum, we don’t want the coating to become ‘gummy’ if sanding is a requirement.

Wood-based products typically have a moisture content, so formulating with this in mind is imperative—we’re going to need additives to account for degassing. However, moisture is important because powder coating wood-based substrates often involves preheating the part to draw moisture to the surface so the powder can be electrostatically applied. Therefore, we need some moisture present.

If the wood-based products being coated contain high tannins, there is a very high chance a primer will be needed— so now we have to think about it being a two-coat system.

When it comes to cure, let’s get the obvious out of the way first. Traditional substrates like aluminum and steel can handle high heat. Wood-based products? Not so much. They’re much more sensitive and therefore require lower oven and curing temperatures. Infrared (IR) cure tends to be the favored method for heat sensitive substrates as essentially it heats only the surface of the part, very quickly, thus allowing the powder to flow and cure quickly and at lower temperatures.

As with traditional coatings, the length of the cure cycle is important to the formulator. The chemist must ask how much time and heat will be available. This translates into how catalyzed the system needs to be. Chemists also pay a lot of attention to the flow of the powder and formulate with this in mind. This is extremely important as there isn’t as much heat during the curing process to assist with flow as with a traditional powder.

When it comes to coating performance, the product types or chemistries are perhaps a little more limited when compared with traditional thermosetting powder. This comes down to formulation requirements. For example, to get technical for a moment, polyurethane coatings cured with isocyanates blocked with E-caprolactam wouldn’t work at the low cure temperatures required for wood-based substrates because they don’t deblock until around 340 degrees Fahrenheit. That temperature is too high for most wood-based substrates.

The good news is that suppliers are listening and are providing resins with, among other things, much lower cure schedules. An alternative binder chemistry could include using an unsaturated polyester resin. Its advantage is an incredibly quick cure due to the radical reaction that occurs. That said, there is still a lot of choice out there and the end use of the product will inform the resin choice. Last, but not least, there are both clear and pigmented powder coatings available for wood-based substrates, so the chemist must also account for pigment selection and any additives needed to achieve those all-important special effects.

Yotzin Mason is lead PureClad chemist at IFS Coatings.