Powder Coating Basics
Continued improvement in formulations, applications equipment and process economics have established powder coatings as one of today's preferred finishing technologies. Powder coating is the fastest growing segment of the coatings industry because it is an environmentally acceptable method that provides top quality performance for what is probably the lowest applied film cost per square foot of any coating type.
The selection of certain epoxy resins and curing agents suitable for the powder coating process started in the US in the early sixties, when the powder coating technique gained industrial acceptance. At this point, the Dow Chemical Company and its subsidiaries, both in the US and in Europe, engaged in development projects for specialty resins and curing agents to fit powder coatings only. While the US market concentrated more on protective powder coatings, the European needs were more in the direction of decorative systems. However, in recent years, a lot of emphasis has been placed on decorative powder coatings in the US, where the market is rapidly expanding.
Powder coatings are 100 percent solid material and are free from solvents. They are generally applied by means of electrostatic spray equipment where the powder particles are charged and, therefore, attracted to the grounded objects to be coated.
The coated objects then go into a high-temperature oven (usually 140oC to 200oC), where the powder particles melt, flow and react chemically to form into a smooth finish.
Decorative powder coatings are typically found on camping gear, hospital equipment, toys, appliances, automotive parts, office and kitchen furniture, etc.
Protective powder coatings are typically found in the area of heavy corrosion protection of steel pipelines, steel reinforcing bars, motor windings and electronic encapsulation.
Some of the most significant advantages or powder coatings, when compared to solvent-based coatings are:
In 1989, an estimated 250,000 metric tons of powder coatings were applied worldwide. This amounted to about 5% of all industrial finishes applied in 1989. Of all industrial coatings, the use of powders has increased most rapidly. A formulated powder coating is a homogeneous mixture of resins, curing agent, filler, pigment and flow modifier. Once applied, these free-flowing powders are fused by heat to form a continuous film. Bisphenol A-based epoxy resins and novolac-modified epoxy resins are used in both decorative and protective powder coatings.
Decorative Powder Coatings
Decorative epoxy powder coatings provide excellent product aesthetics, high performance protection and attractive economics. Characterized by outstanding toughness, corrosion resistance, flexibility and adhesion, decorative powder coatings based on epoxy resins also come in a variety of finished: from low to high gloss, and from smooth to complex specialty textures.
Epoxy resins can also be effectively combined with other functionalized polymers, such as polyesters. Epoxy/polyester hybrid systems generally offer good film gloss and impact, with excellent color retention and yellowing resistance on over-bake. Epoxy hybrids are attractive alternatives where coating hardness and solvent resistance are not critical performance requirements.
Generally, cured decorative powder coatings range in thickness from 1 mil to 6 mils and are used on metals, glass and plastic. Applications include toys, recreational equipment, business and farm equipment, office furniture, appliances, garden and power tools and automotive parts, such as oil filters, shocks and engine blocks.
Protective Powder Coatings
Protective epoxy powder coatings provide long-term protection from corrosion for metal substrates. Epoxy powder coatings continue to be the accepted standard of performance in protective applications.
Fillers, Pigments and Additives
Depending upon the kind of service a powder coating must provide, its composition varies considerably. Pigments must be carefully selected if the powder coating is applied to toys, but may be of secondary importance in a a pipe coating system. Masterbatches of liquid additives, such as flow agents, may be suppressed in one system, whereas they can be an integrated part of the specialty epoxy resin or curing agent in another.
The combination of resin, curing agent and flow control agent is usually referred to as the binder part of an epoxy powder system. All other components refer to the filler part of the system.
Measuring Performance - Powder and Cured Film Testing
Powder coating formulations must remain free-flowing and latent (unreactive) during transport and storage, particularly during the summer months. Protective powder coatings are generally faster in reactivity than decorative coatings and extra care should be taken to transport and store these products under cool, dry conditions to maximize shelf-life and performance and to minimize caking or blocking. Refrigeration may be beneficial for some powder formulations. When appropriate, the powder formulator should specify refrigeration.
Reactivity (Gel Time)
Gel time indicates the relative reactivity of a powder coating formulation at a specific temperature, usually expressed as the number of seconds to gelation. Using a hot plate, gel points are indicated when stroking with a stick no longer provides a polymer thread. Accordingly, this test is commonly referred to as "stroke cure gel time".
The reactivity of a protective powder coating is usually measured at 450oF (232oC) and can be adjusted by the level of accelerator in the formulation for each specific application. Exterior protective powder coatings are usually separated into two categories: small- and large-diameter pipe coatings. These two classifications often overlap and depend on the application factors such as pipe temperature, line speed, powder spray rate, desired film thickness, time before quench and the location of pipe rollers.
Generally, at 450oF (232oC), small diameter systems have gel times of about 5 - 12 seconds, with complete cure in 30 - 90 seconds. Larger diameter formulations have gel times of about 16 - 45 seconds, with complete cure in 60 - 120 seconds. Rebar powder coatings require gel times of about 5 - 15 seconds, with a preference towards the shorter gel times to increase rebar through-put. Interior pipe powders vary in gel time depending on the application, but are generally longer than exterior pipe formulations and often require post curing.
Inclined plate pill flow determinations are used to indicate relative powder flow-out when heated. Pressed pellets, or "pills", of the powder formulation are placed on a preheated glass plate or panel, tilted to a 65o angle and allowed to flow and gel. Flow distances are measured in millimeters (mm). As examples, inclined plate pill flows at 350oF (177oC), rebar powders tend to be very short (less than 20mm measured from the top of the original 12mm pill), exterior pipe powders moderate (about 30 - 40 mm), interior pipe powders relatively longer (above 40mm) and for decorative coatings, about 30 - 80mm.
Cured Film Testing
Testing of cured powder coating films varies somewhat with application and the type of system, either protective or decorative, and are beyond the scope of this discussion.
Like most other paint systems, powder coatings are easily contaminated by the substrates to which they are applied. Thus, removing all scale and rust and greasy residues in important.
A very well-established pretreatment for steel and other metallic substrates includes a vapor decreasing operation using chlorinated solvents. Alkaline rinsing followed by phosphatizing or treatment with chromates produces surfaces suitable for the highest quality coatings. Etching processes usually improve corrosive resistance of epoxy powder coatings.
If the application process allows for preheating of the objects, the preferred cleaning method is usually shot/grit blasting followed by blasting. Sand blasting is also possible, but removing all fine dust is essential. Shot/grit blasting is typically used for industrial pipe coating processes.
Primers and Adhesion Promoters
Powder coatings based on D.E.R. epoxy resins generally do not require primers (basecoats) or other adhesion promoters on metallic substrates. Like many paint systems, standard epoxy powders have good adhesion to steel and aluminum, but marginal adhesion to copper. Therefore, the curing agent, dicyandiamide, is added to promote adhesion for copper. While thermoplastic powder coatings usually require a primer for applications to metallic substrates, epoxy powders adhere well to unprimed metals.
Application by Fluidized Bed
The fluidized bed method was the first powder coating application technique used commercially and is still in use today. The fluidized bed consists of an open-topped tank with a bed of porous material a few inches from the bottom. The powder coating is put into the tank and is kept in a state of continued fluidization by air that is blown through the porous bed.
By monitoring both air flow and particle size, the powder is prevented from leaving the coating tank and dusting into the air. Preheated objects are dipped into the fluidized powder, which is deposited by melt-sintering. Once the desired film thickness is obtained, the objects are removed from the powder than and sent to a post-cure oven where the cross-linking reaction between resin and curing agents takes place.
The fluidized bed method is practical for applications that:
However, the fluidized bed method has the following disadvantages:
Application by Electrostatic Spraying
This application technique eliminates most of the disadvantages of the fluidized bed. At the present time, corona electrostatic spraying is the most frequently used application method for powder coatings with very high standards in terms of both automation and yield. In this process, the powder is deposited in a reservoir quite similar to the fluidized bed tank just described. Gentle stirring prevents caking and ensures an even powder flow. Then, the powder is conveyed to an electrostatic spray head that may be built into a handheld gun or may have many other shapes. As the powder leaves the spray orifice, it is electrostatically charged. The electrodes are connected to a high-tension generator. The powder is then projected towards the electrically grounded object, where it adheres because of its electrostatic charge. The resulting film thickness is primarily a function of the object's total charge, which in turn is a function of the voltage applied by the high-tension generator and the conductivity of the powder. Once coated, the objects are transferred into ovens for curing.
Recently developed triboelectric guns utilize static charge generated by the friction of powder moving through plastic tubes. In contrast to corona charging, powders applied by tribo guns appear to be less sensitive to the well know Faraday Cage effect. For this reason, tribo guns may provide an advantage when coating complex shapes and/or deep recesses. Potential drawbacks to using tribo charging are the lower application rate of powder delivered, the variability of charging based on powder composition and particle size, and the change in application characteristics with time. (Note: As with all processes, individual requirements for a specific operation should be carefully considered when selecting equipment.)
Electrostatic coating offers these advantages: minimum heat exposure, improved film thickness control, and nearly 100% yield because overspray can be easily collected, filtered and reintroduced. Electrostatic coating is also adaptable to sophisticated automation or robotics.
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