Traditionally powder coatings have been used as a finish for metal-based parts, generally in a factory setting. Powder is applied electrostatically with automatic and/or manual spray guns, and the ware is placed in an oven until the parts are heated to the recommended conditions (time and part temperature) required to cure the powder coating.
The development of low-temperature-cure (LTC) and ultralow-temperature-cure (ULC) powder coatings has opened the door to a vast array of non-traditional substrates that can be finished with powder coatings, the most common of which is medium density fiber board (MDF) (“wood”); kitchen cabinetry, ready-to-assemble (RTA) furniture, shelving, etc. Both ULC powders and UV-curable powder coatings are used to finish MDF components.
For the detailed explanation of LTC and ULC powder coating, check out this technical paper in Coatings World by Kevin Biller, President, ChemQuest Powder Coating Research: Low-Temperature-Cure Powder Coating Technology Concepts
Powder coating wood, plastic and glass is a little different from powder coating metal. Here is an explanation of the differences.
Several powder coat manufacturers have worked extensively on developing systems for powder coating wood, especially MDF (medium density fiberboard), for use in furniture and shelving. One concern is the retained moisture in the wood.
If you have the time and patience, you can preheat the wood, spray hot (250°F or so), cure at an elevated temperature (say 350°). The moisture in the wood will blister out and show as blow holes in the coating. You could then sand the coating and repeat the operation. The second coat should come out okay. If not, repeat.
With the relatively LTC and ULC powder coatings used for wood substrates, a typical convection oven won’t work well, nor will a batch/manual style system, as the process needs more control.
The most important aspect of the whole process is heating the powder up to a melt/flow point quickly.
For a typical wood powder coating process, follow these four steps:
- One minute of infrared (IR) preheat to bring moisture to the surface of the board
- Powder application within 2-4 minutes of preheating
- 5-7 minutes of IR cure
- 15-20 minutes of cool down
The same questions apply to the powder coating of plastic. Since plastics tend to be non-conductive, the electrostatic charge will not be transferred to ground, and the powder will tend to not stick to the plastic unless it is preheated. The softening point of the plastic will be the constraint on doing this.
If it is an engineering plastic, it may take a 300°+ preheat and post heat. If it is a commercial molded product, it probably won’t. I spoke with a coater a few days ago who is coating cast pewter parts. The pewter tends to melt at the standard 400° oven temp. Since the part was dense and solid, I guessed that it would not do any flexing after being coated, and the coater could get by on less than a full cure. He will try coating at 275° to 300°. The only down side is poor impact resistance, but he said that should not be a problem.
So, wood and plastic can be coated, but not with a standard procedure and cure schedule.
Glass should not be a problem as long as the coater can get the powder on the part. One coater I know said he would put a “grounding rod” down the center of glass lamp bases to enhance electrostatic attraction. Others routinely spray hot to get the powder to adhere to the glass, then do a conventional cure. It may take two coats for complete coverage. After the first coat is applied and melted (not cured), it is easier to get the second coat to stick to the first.
*See this technical paper in Coatings World by Kevin Biller, President, ChemQuest Powder Coating Research:
Low-Temperature-Cure Powder Coating Technology Concepts
Special thanks to:
David Collander, Coating Solutions Co.
“Ask the Experts”, Powder Coated Tough, Sponsored by Gema
