By: Kevin Biller
As part of our continuing series of powder coating technology nomenclature, in this installment we are presenting terms used by the people who create this unique coating—namely the formulators and manufacturing personnel.
Powder Coating Formulation Nomenclature
Additive—A material incorporated at nominally 3.0 percent or less into a powder coating formula. Additives have a functional purpose relating to the performance of the coating. Additives can be reactive or non-reactive. Reactive additives interact chemically with components in the formula or with environmental elements such as ultraviolet light, heat or ambient compounds (e.g., acid rain, moisture, etc.).
Non-reactive additives do not react chemically with the coating formula or the environment but influence physical properties such as melt point, viscosity and/or surface profile. Tinting pigments are typically used at low levels (i.e., < 3.0 percent) but are not considered additives.
Anti-gassing Agent—Volatiles can emanate from substrates via a few phenomenon. Some substrates are inherently porous such as cast magnesium or aluminum alloys.
These porous substrates evolve entrapped air and moisture during film formation and can result in pinholing or bubble defects. Other substrates such as galvanized steel evolve hydrogen gas as a reactant with acidic pretreatment chemicals. In addition some plastic substrates such as polyester composites can evolve low molecular weight polymer fragments that cause pinholing or bubble defects. These substrate-related volatile defects can sometimes be avoided by the incorporation of an anti-gassing agent. These materials are typically waxes that allow the coating film to expel the volatile then reflow or close the exit point in the film.
Antioxidant—A compound that reduces polymeric degradation caused by heat. The thermal degradation of a polymer can cause discoloration (typically yellowing) and a reduction in coating durability and mechanical performance. Antioxidants can be primary or secondary. Primary antioxidants can be used alone or with secondary antioxidants.
Binder—The resinous component of a powder formula that provides the film-formation of the coating. The binder is comprised of the resin, crosslinker and binder additives such as catalyst, flow agent, and degassing agent.
Catalyst—A compound that increases the rate of a chemical reaction without being consumed by the chemical reaction. Also described as an accelerator. Catalysts are used to lower the cure temperature and/or cure time of a thermosetting powder coating.
Colorant Pigment—A material comprised of insoluble particles that provide color and opacity. Pigments produce color by inherently absorbing certain wavelengths of visible light (ranging from approximately 380 to 730 nanometers). This absorption is observed then processed by the human eye and is interpreted by what humans describe as color.
Crosslinker—A relatively low molecular weight oligomer possessing two or more chemical groups capable of reaction with thermosetting polymers. Crosslinker is synonymous with “curing agent” and hardener.
Degassing Agent—During the film formation of a powder coating air is evolved during film formation. As powder particles melt and fuse interstitial air forms bubbles that exit the coating. This expulsion can cause “pinhole” defects in the coating surface. In addition volatiles of cure, moisture and low molecular weight polymer fragments can also be emitted from a powder coating during film formation. Degassing agents are incorporated into powder coating formulas to mitigate the defects caused by volatiles emitted during film formation. These materials influence the bubble formation of volatiles and enhance their escape from the coating thereby minimizing pinhole defects.
Dry-Blend Additive—A material incorporated into a finished powder coating to enhance handling properties. These products are used to improve fluidity and sometimes electrostatic performance of a powder. Also known as fluidization agent and anti-caking agent.
Dye—A colorant compound that is soluble in a medium such as a resin or solvent. Unlike pigments these materials dissolve and are transparent.
Extender Pigment—A material incorporated into a powder coating formula with the ideal purpose of lowering overall formula cost without affecting other performance features. Also known as extenders or fillers these materials are typically based on naturally occurring minerals or synthetic metal oxides.
Flow Agent—A material commonly used in powder coating formulations to minimize surface tension defects. Flow agents equalize the surface tension differential of binder components thus reducing the incidence of cratering. Flow agents can also enhance flow and leveling. The most common flow agents are high viscosity liquids that have been adsorbed onto amorphous silica particles.
Fluorescent Pigment—A pigment that emits light upon absorbing ambient light energy. The absorbed light is typically a shorter wavelength whereas the emission is typically longer wavelength light. These are also known as neon or Dayglo® pigments.
Light Stabilizer—A compound that slows down polymeric degradation from exposure to UV light. These compounds are typically used in conjunction with UVAs.
Masterbatch—A mixture of a functional material and a carrier. There are two broad categories of masterbatches used in powder coatings: tint masterbatches and additive masterbatches. Tint masterbatches enable a formulator to incorporate small amounts of tinting pigments into a powder coating without having to weigh miniscule quantities. Additives masterbatches are typically comprised of a liquid additive adsorbed onto a dry carrier (e.g., silica) to enable the powder coating producer to incorporate a dry powder rather than a liquid into the formula.
Oligomer—A compound consisting of a small number of monomers or chemical constituents.
Optical Brightener—A chemical that absorbs light energy in the UV and violet areas and emits specifically in the blue wavelength range. These can be used to create bright, clean blue shade white colors.
Photoluminescent Pigment—A pigment that slowly emits light over a period of time after absorbing ambient light energy. This is known as phosfluoresence and the materials are sometimes described as “glow-in-the-dark” pigments.
Plasticizer—A material that affects a binder by intertwining between polymer molecules. In thermoplastic powder coatings plasticizers make rigid polymers softer and more flexible. In thermosetting powders plasticizers depress melt point and reduce melt viscosity.
Polymer—A compound comprised of molecular chains of monomers. Polymer is a generic term that refers to classes of compounds based on their constituent monomers. Hence an acrylic polymer is comprised of a molecular chain of acrylic monomers. Polyesters are comprised of a molecular chain of glycols and diacids (these form ester bonds). Thermosetting polymers possess reactive chemical groups that can participate in crosslinking. Thermoplastic polymers do not chemically react and therefore can melt and remelt upon heating above the polymer’s melt point.
Resin—A specific product based on a polymer type. Resins are produced by chemical companies and given specific tradenames and product codes.
Tinting Pigment—A colorant pigment incorporated at a low concentration to shift a color.
UV Absorber—A compound that absorbs UV energy and converts it into a low level of heat. UVAs are used to reduce the polymeric degradation associated with exposure to sunlight.
Wax—A compound that is a plastic solid at ambient conditions that forms a low viscosity fluid at elevated temperatures. Waxes are naturally occurring or can be synthesized chemically. It is common for a wax product to contain a blend or alloy of two or more constituent waxes. Waxes are used to enhance surface properties and sometimes to reduce gloss.
Powder Coating Manufacturing Terms
Classification—The process that segregates and/or removes specific particles in a powder coating. Classification by cyclones eliminates very fine particles (i.e. < 10 microns) whereas sieving segregates large particles from smaller particles.
Extrudate—The material after extrusion and prior to pulverization.
Extrusion—The process of melt-mixing premixed powder coating materials. Also known as compounding this process melts and mixes the resinous components of a formula and introduces shear to the inorganic and non-melting materials. Both distributive mixing (resins and additives) and the dispersion of particulates (pigments and extenders) occurs during extrusion.
Flaking—The process of breaking extrudate into chips that can be handled by the pulverization process. Also known as kibbling.
Grinding—The process of reducing the size of extruded flakes to a particle size distribution suitable for application of the powder coating. Also known as pulverization or milling.
Premix—The process of blending powder coating raw materials to create a homogeneous dry material to feed to an extruder for compounding. “Premixing” refers to the process, “premix” describes the blended composition.
Sieving—Passing a powder coating through a mesh screen to segregate large particles from smaller ones. Also known as sifting or screening. Sieves can be rotary or flat-deck design.
Now you have the vocabulary to contemplate the manner in which powder coatings are conceived and produced. The formulator has thousands of components to choose from to create the perfect formulation. The manufacturing engineer and operators have a multitude of designs and conditions to consistently produce the exact product to meet the finisher’s application and curing process.
Kevin Biller is technical editor of Powder Coated Tough magazine and president of The Powder Coating Research Group. He can be reached at kevinbiller@yahoo.com.