Technology Interchange: Hybrid Powder Coating
Posted on Wednesday, May 11, 2016
By: Kevin Biller
Hybrid powder coatings are ubiquitous. The Powder Coating Institute estimates the North American market for hybrid powders to be about 25 percent of powders consumed. Globally the numbers are much higher as experts estimate the use of hybrid powder coatings to be between 50 and 60 percent depending upon geographic region. But do powder users really know what they’re getting when they purchase a hybrid powder coating? The following will give you a better understanding.
Hybrids Make Their Debut
Hybrid powder coatings first hit the coatings scene in the mid-1970s. At the time powder chemistries were limited to epoxies and an array of thermoplastics. Polyesters and polyurethanes were emerging at this time as well. The majority of powder coatings are thermosetting in nature and therefore the binder system undergoes a chemical reaction during the curing stage (in the oven). Typically powder binders are comprised of a base resin (e.g., epoxy, polyester, acrylic) that is reacted with a low molecular weight curing agent. The base resin usually makes up 75 to 95 percent of the binder with the remainder comprised of the curing agent and additives. Resins are relatively low in cost whereas curing agents are more specialized compounds that are higher in price.
Hybrid powders were conceived by combining two base resins into a thermoset curing scheme. Epoxies typically cured with highly reactive curing agents were instead matched with a specially designed carboxyl functional polyester resin. The overall cost of the binder system was significantly less than a traditionally cured thermoset and the coating performance was pretty decent. The overall performance was somewhere between very hard and chemically resistant epoxy and a softer, very flexible polyester.
This type of chemistry fills a large niche in the coating market. Because of the epoxy content hybrids are relegated to indoor applications but the addition of a significant polyester content gave this chemistry very good color stability in the curing process. The color of epoxies can shift dramatically due to variable oven conditions and part size and density. Hybrid’s low cost, stable color and relatively good film performance made them a natural choice for the a multitude of indoor applications.
Getting Inside a Hybrid
The most common hybrid powder coating consists of a blend of carboxyl polyester and epoxy resins. Depending upon the formula the ratio of polyester to epoxy ranges from 50:50 to 70:30. Why, you wonder, would a chemist vary this ratio? There are a number of factors that influence this choice.
Although resin prices ebb and flow depending on supply dynamics, polyesters have been historically lower in cost than epoxies. Hence higher polyester content equates with a lower overall raw material cost. Because polyester resins are inherently softer the higher content produces a somewhat softer coating surface. In addition lowering the epoxy content reduces chemical resistance and hardness.
Polyester resins are generally more viscous than epoxies and therefore higher content exhibits somewhat more orange peel.
Another interesting property of hybrid powder coatings is that they can be formulated for contact with food and beverage products. CFR (Code of Federal Regulations) 175.300 specifies which components of a coating are acceptable for incidental contact with food and beverages. The epoxy and polyester resins used in some hybrid formulations comply with this regulation and therefore allow these hybrids to be used on parts used for refrigerators and food handling equipment. Not all hybrids comply so it is wise to consult with your powder coating supplier if you have this requirement.
Hybrids typically require a bake of around 15 minutes at 375°F (metal temperature) however recent advances in materials have allowed chemists to formulate hybrids capable of cure around 280 to 300°F. Bake times vary but this does open the door for energy savings and the possibility of coating some non-metallic substrates.
Hybrid Powder Uses
The market for hybrids is rather broad but is limited to indoor applications in most cases. Some of the biggest consumers of hybrid products are the metal office furniture, shelving, appliance and automotive industries. Each has its own specific requirements and specifications.
The metal office industry requires semi and low gloss finishes that are scratch and impact resistant and that can resist an array of cleaning agents. Color and gloss stability is critical to provide consistency between units in a large order as well as units purchased long after the first furniture was installed. Powder formulators carefully choose the right resins and use special additives to ensure oven bake stability.
Historically the appliance industry has used mainly polyurethane powders for their excellent hardness and chemical resistance. Since the 1990s they have adopted hybrids as a lower cost, but still acceptable alternative. These coatings are found in on laundry equipment and small appliances and must possess detergent, fabric softener and bleach resistance as well durability under very humid conditions.
The shelving industry is diverse as it encompasses wire racking for refrigerators and closets and general purpose metal shelving and pallet racking. Refrigerator coatings usually have to comply with FDA requirements for food contact and also resist food staining and degradation from common cleaning materials. Closet and general purpose shelving require highly abrasion resistant finishes with good impact resistance.
The automotive industry uses hybrids for under hood parts and as a primer-surfacer. The under hood parts require good aesthetics and resistance to oil, gasoline and sometimes brake fluid. Hybrid powder primer surfacers are an intermediate coat between electrocoat primers and liquid topcoats. They have been around for a couple decades and need to be extremely smooth, corrosion resistant and have excellent intercoat adhesion with electrocoats and liquid topcoats. Excellent adhesion allows these coatings to be highly chip resistant and durable.
Hybrids are also used for wide array of general purpose applications such as electrical cabinets, business equipment, and internal machinery components such as brackets and fasteners. In all cases the high performance and attractive economics of this everyday powder coating make it an excellent choice for finishers far and wide.
Kevin Biller is technical editor of Powder Coated Tough magazine and president of The Powder Coating Research Group. He can be reached via email at firstname.lastname@example.org.