Activated Conductive Layer for Powder Coating on Wood

By Chhiu-Tsu Lin

A metallic substrate is a conductor and the electrostatic application of powder coating can adhere quite easily. Wood, such as medium- and low-density fiberboard (MDF and LDF), are low conductivity substrates, which reduce dramatically applica­tion efficiency and inhibit a successful powder coating practice. For a powder coating processed at high temperature, metallic substrates can take the heat treatment, but wood would exude its moisture content and lose its structural value.

In this article, we formulate an aqueous conductive emulsion of organic-inorganic hybrid that contains sol-gel coupling agents and a conductive nanomaterials dispersion. A single-coat appli­cation of this conductive emulsion on nonconductive substrates, followed by air-dry or a thermal drying at 65-80 °C for 1 min, gives a thin (,v 1-5 µm), uniform, dense, and hard (2H-5H pencil hardness) conductive film that adheres extremely well (5B, ASTM D 3359) on a part’s surface.

The activated conductive emulsion film on non­conductive substrates offers excellent conditions for the applications of powder coating on MDF and LDF panels. The formulation chemistry of conductive emulsion will be described.

The quality and product performance of powder coated panels were assessed visu­ally and by optical microscope. The results indicate that the conductive emulsion coat­ing has a remarkable ability to attract coating powder to otherwise insufficiently con­ductive surfaces. On LDF and MDF panels, it can eliminate preheating for initial powder attraction. It is functional at coat weights below those that are typically meas­ured by the paint and powder coating industries.

At these coat weights there is little to no surface roughening by fiber raising, which avoids a sanding step. Also, air-drying is feasible except at high relative humidity. It may ensure that there is sufficient pow­der on well-smoothed sawn and routed panel ends to negate the need for a filling primer.

Furthermore, it can ensure good coverage of low-charge powders, which gives formulators a great deal more latitude. When applied to other low- or non-conductive materials, comparable benefits and few drawbacks can be anticipated.