Formulators Forum—It Only Takes a Little: The Role of Flow Control Additives in Powder Coatings
Posted on Tuesday, January 24, 2023
In today’s world, with an eye on environmental awareness and the drive towards increased sustainability, powder coatings are an ideal solution for coating applications.
The formulation of a finished coating that meets predefined criteria is a science. Additives are a critical piece of the puzzle for meeting these requirements. The function they serve in both liquid and powder coatings are nearly identical; liquid coatings are heavily reliant on the solvent/coalescing agent for participation in film formation; powder coatings are solely dependent on the system viscosity and flow control agent (FCA). The role of the FCA is vital to the final aesthetics and performance yet is only a minimal component of the formulated paint, often at levels as low as tenths of a percent. Within this role, flow and leveling agents reduce or eliminate surface defects affecting appearance and performance.
Flow and Leveling: Powder coatings undergo a three- stage process during cure: the particles melt; flow out; and gel, crosslink, and cure.
Flow promoters consist primarily of an active acrylic resin that is dispersed onto either silica powder or a polyester resin (masterbatch). Selecting silica as a carrier allows for greater content concentration and a higher activity level by limiting the non-active carrier component. Alternatively, to compensate for protrusions that result from very thin films or the FCA not releasing from the silica, masterbatches may be preferred. These also allow for the use of either carboxylated or hydroxylated resins, allowing for the carrier resin to react with the hardener.
Surface smoothness is controlled by the surface tension during the melt phase; as the additive acts as a flow promoter, it migrates to both the coating/air and coating/substrate surface interfaces, uniformly wetting the substrate surface and resulting in a smoother, more consistent appearance after crosslinking. The combination of polymer melt viscosity and the FCA work together to promote uniform substrate surface wetting and smooth coating flow. Simply, the flow of the system is the driving force for the elimination of surface defects.
Common Defects: Craters are formed by two different mechanisms: a) material flowing from areas of low surface tension to areas of higher surface tension, resulting in improper wetting of the substrate, or b) contamination of the substrate or film with a low surface tension material. A low interfacial tension to contact angle is required, so that the paint will have a surface tension equal to or lower than the substrate, minimizing the formation of surface tension gradients.
Thermal gradients, which induce flow, can also be a cause of defects. Viscosity and surface tension both decrease as temperatures increase, causing material to flow from areas that are hotter to ones which are cooler, resulting in defects such as increasing orange peel and picture framing. Pinholes are caused by either a disruption in the gel state that prevents the powder from achieving a uniform finish, or when there is a retarded flow of volatiles to the surface of the coating, including adsorbed gases, cured volatiles, or entrapped air. Surface defects may also arise from unaccounted compositional changes which may occur at any stage of the manufacturing process.
Defect Control: Opposing levels of surface tension is the mechanism by which defects are controlled or prevented; this is where the role of the FCA comes into play. Virtually all FCAs act by modifying the surface tension of the coating. Common technologies include surfactants, fluorinated alkyl esters, silicones, and solvents which are extremely surface active, and polyacrylates which are less active. Polyacrylates work by promoting the development of even surface tension across the surface of the film. The FCA migrates to both the substrate/paint interface and to the air/coating interface. The overall high surface area of the film promotes leveling.
Flow Control Function and Design: Flow control additives can be designed for quite different and very specific purposes. The development of the active portion of the additive is dependent on the primary variables of monomer(s) type, molecular weight, functionality, and viscosity, to achieve any of a lengthy list of predefined attributes. Another design criterion is carrier selection to optimize adsorption related to activity level and handling, as well as desorption for processing, surface defects, and appearance. Determining the preferred percentage of the active component which is correlated to the total percent of the flow additive in the final formulation is also a part of the design process.
This summary highlights the key role that flow control additives play in the performance of a powder coating. The most important thing is to realize that although the volume of the additive is but a small percentage of the formulation, the impact is significant. There is an old saying from Turkish playwright and novelist Mehmet Murat Ildan that applies to this topic—“If you can see the little things and the small details around you as big things, you’re enlightened”—such is the role of the flow control additive.
Dr. Beth Ann Pearson is global director marketing and business development at Estron Chemical.