Technology Interchange: Looking Good! Powder Coating Nomenclature—Appearance Terminology

Today's nomenclature lesson is all about looking good. Powder coatings not only need to provide a durable, high quality finish but also have to conform to the aesthetic expectations of the finisher and their customer. The following are terms regularly used in our industry to describe the appearance of a powder coating finish.

Film Thickness - The thickness of a cured powder coating usually measured in mils (thousands of an inch) in North America or microns (10-6 meters) everywhere else. Some people incorrectly describe it as “DFT”, dry film thickness, which is a throwback to wet paint. Liquid paint folks have to differentiate between “wet” applied paint and the final “dry” film thickness. We don’t have that complication in powder.

Instruments abound for film thickness measurement. Ferrous substrates (i.e. steel) use magnetic technology to gauge film thickness whereas non-ferrous surfaces use an eddy current technique. Both methods can be incorporated into a single instrument. Uncured powder can be measured using an ultrasonic method and correlated to cured thickness through an algorithm. Ultrasonic techniques are also capable of measuring powder on non-metallic substrates.

Gloss - The reflection of specular light as measured from a symmetrical angle from the perpendicular. Gloss is typically measured at 20°, 60° and sometimes 85° angles from the perpendicular. It is reported in “gloss units” and not in percent gloss as is commonly mistaken. Gloss units are derived from the specular reflectance of a surface compared to that of a highly polished black glass standard. The gloss range of the surface usually dictates which angle the measurement is taken. See Table 1. for details.

Interestingly some powder coated surfaces possess a gloss of over 100 units. These include some silver metallics and also clearcoats over bright substrates. This occurs because the reference point is black glass and these coatings appear much brighter.

Dead Matte - A finish possessing a gloss of < 1.0 gloss units (60° measurement) and < 3.0 gloss units (85° measurement).

Matte finish - A finish that exhibits a gloss of 1.0 to 10.0 gloss units (60° measurement)

Low Gloss - A finish that exhibits a gloss of 10 to 30 gloss units (60° measurement).

Semi-Gloss - A finish that exhibits a gloss range of 30 to 70 gloss units (60° measurement).

MediumGloss - A finish that exhibits a gloss range of 70 to 100 gloss units (60° measurement)

High Gloss - Nominally a finish that exhibits a gloss range over 100 gloss units (20° measurement)

Haze - The scattering of light due to microtexture of a surface or non-clarity in an unpigmented coating.

This can also be referred to as surface haze, a condition which resembles waxiness. A number of specific instruments have been developed that quantify haziness. This is especially critical for automotive topcoats.

Orange Peel - A description of surface that resembles the topography of the rind of citrus fruit.

Distinctness of Image (DOI) - Refers to the crispness or distinctness of a reflected image. Initially this was assessed visually by projecting a series of different sized images onto a coated surface. The observer accomplished this by using a “glow box” that passed focused light through a slide containing varying images onto the coating surface. The observer would choose the image that displayed recognizable distinctness. DOI measurement has since evolved into an instrumental technique (see ASTM D-5767) Instrumental assessment eliminates operator influence and is therefore more precise and reproducible.

DOI is affected by both long (i.e., orange peel) and short (i.e., micro-texture) surface wavelengths. Both micro-texture and orange peel reduce DOI.

Surface Profile - The characteristic topography of a coating. Surface profile, also known as goniometry, was originally measured physically by traversing a needle across the surface. More recent techniques use optical methodology to describe surface profile including wavelength, amplitude and frequency.

Protrussion - A non-melting particle that projects through the surface of a powder coating film. Also known as bits or seeds.

Crater - A concentric depression in the coating surface. The edge of a crater processes a gradual radius as opposed to a sharply defined hole. Craters are typically caused by contaminants such as oils, lubricants or dissimilar powder coatings

Pinhole - Clean, sharply defined cavity in powder coating surfaces caused by the expulsion of volatiles such as gas, moisture or reaction byproducts. The volatiles can emanate from the substrate such as voids in cast metal or the coating itself

Color - The visual perception of the reflected light off a surface. The visible spectrum wavelengths range from about 420 nanometers (violet) to 700 nanometers (red). Color is described by hue (intrinsic color), saturation (or chroma) and value (brightness). Color is typically measured with an instrument surprisingly called a colorimeter. This instrument measures the reflected visible light spectrum and converts it into:

L - lightness-darkness range: 0 to 10
a - red – green range: approximately +50 to -5
b - yellow-blue range: approximately +50 to -5

Color can be measured using different light sources that simulate typical environmental conditions such as sunlight, fluorescent light and metal halide light. The spectral output of the light source is an important consideration as most colors appear differently under different light conditions.

Color differences are usually quantified by calculating a composite difference of L, a, b values. This value is known as ΔE and is derived from this equation:

Coating users specify the tolerance for color difference usually with a maximum ΔE value, Some may also specify separate ΔL, Δa and Δb maximums

Metamerism - A color that visually changes under different light sources

Using this common language of appearance characteristics should help you in your understanding of the aesthetic requirements of a powder coating finish. Everyone from the coating formulator to finishing engineer and even the ultimate consumer should have the vocabulary to express their expectations for appearance. Hopefully this glossary will ensure that everyone will be looking good.

Kevin Biller is technical editor of Powder Coated Tough and the president of The Powder Coating Research Group. He can be reached at kevinbiller@yahoo.com.