European manufacturers were quick to appreciate the decorative possibilities of powder coating. In the more functional North American market, thicker powder coatings were used and decorative thermosetting powder resin systems were refined much more slowly than in Europe.
Application and material problems combined with a market resistant to turn away from high solids, solvent-based coatings resulted in a slow take-off of powder coating technology. This dependency upon high solids systems as a solution to the VOC problem or, more accurately, the perception of what high solids could accomplish, was a major reason that the growth of powder coatings in North America lagged behind that of Europe.
The major impetus for growth can be attributed to three factors.
1. Increasingly stringent environmental regulations forced many manufacturers to search for alternative finishing technologies to remain in compliance. As a virtually pollution-free finishing system, powder coating attracted a great deal of attention; powder coatings contain no solvents and, as such, emit zero volatile organic compounds (VOCs). Re-use of any over-spray powder almost eliminates waste from a powder coating operation. Exhaust air can safely be returned to the finishing room and less oven air is exhausted to the atmosphere.
2. Escalating energy costs created an argument for powder coating as did other economic advantages such as shorter turn around times, elimination of solvent recovery or clean up and a reduction of disposal costs.
3. The dramatic advances made in materials and equipment technology coupled with the quality of the highly durable finish that is produced without runs, drips and sags. Quality is now, in fact, as good as that achieved with conventional liquid spray operations and even better process economics are produced than with rival liquid coatings.
Competitive capital investment costs, high-operating efficiencies created by increased material utilization, reduced energy and labor costs and the elimination of solvent emissions reduce overall finishing costs. As a result finishers can produce high-quality products at a lower cost with increased profitability that comply with all current environmental regulations.
Competing systems that offer the user an equal opportunity to reduce VOC emissions are water-based, high solids solvent-based and UV-cured coatings. Powder coatings have replaced some electroplating and ceramic finishes. A limitation to the use of powder coatings is the substrate to be coated and the size of this item as limited by the size of the curing oven. Because of the need to use electrostatic spray the item being coated is most often metallic in nature. Research has shown, however, that even glass bottles can be coated if the bottle is first filled with a brine solution that will conduct electricity.
• Thin-layer powder coatings. Based on epoxy-polyester hybrids, these powder coatings provide applications in the range of 25 to 30 microns (1 to 1.2 mils) for colors with good hiding powder, currently suitable only for indoor applications.
• Low-temperature powder coatings. Powder coatings with very high reactivity have been developed to cure at temperatures as low as 250OP. Such low-curing powders make it possible to run higher line speeds, increasing production capacity. They also increase the number of substrates that can be powder coated, such as plastics; wood and heat-sensitive materials.
• Textured powder coatings. Powder coatings have been developed in a variety of textures ranging from a fine texture with low gloss, high abrasion and scratch resistance, to a rough texture useful for hiding the unevenness of substrates.
• Low-gloss powder coatings. Today's technology makes it possible to reduce gloss values without diminishing the flexibility, mechanical properties or appearance of powder coatings. Currently, it is possible to obtain gloss values down to three percent in pure epoxies. The lowest gloss in weather-resistant polyester systems is approximately 20%.
• Metallic powder coatings. A special process is now available in which aluminum flakes are blended into or with powder coatings and sprayed onto a part. Many of these metallic systems are suitable for outdoor applications. For superior exterior durability, a clear topcoat is often applied over the metallic base. Lately, efforts are concentrated on developing perfect matches for colors to meet the needs of the aluminum extrusion market. Another development is the replacement of metal flakes with pearlescent micas.
• Powder coatings with outstanding weatherability. Significant advances have been made in development of polyester and acrylic resin systems with excellent long-term weatherability to meet the extended warranties being offered by manufacturers.
• Clear powder coatings. Clear powder coatings have experienced dramatic improvements in the past several years in tendency to flow, clarity, and weather resistance. Based on polyester and acrylic resins, these clear powders set quality standards in automotive wheels, plumbing fixtures, furniture and hardware.
To achieve thorough dry mixing of the resin, pigment and additives, a typical line may incorporate two high-intensity pre-blending mixers and a low-intensity final mixer. High-speed, revolving blades in the mixers evenly distribute small amounts of pigments and additives and aid in the dispersion of resin. The final mixer accepts product from the pre-blending mixers along with raw materials that have not passed through pre-blending. This mixer contains tumble and chopper blades. Tumbling action is initiated for several minutes, followed by the start-up of the motors that drive the chopper blades.
Dry-mixed material from the weigh hopper enters the feed port on the extruder. The screws drive the melt mix into the chamber, raising the material to its melt temperature. The melt temperature varies widely dependent on the particular resins used in the formulation but is typically controlled between 250-300ºF. About halfway through the heated chamber the dry mix melts, to become what is termed a "melt mix." The screws drive the melt mix through the heated chamber to the extruder exit. Churning of the melt mix by the screws is designed to ensure a homogeneous dispersion of all constituents of the dry mix. Residence time during this simple melting and dispersion process can be as short as 10 to 20 seconds.
The temperature of the melt mix and its time in the extruder are critical. If the temperature gets too high or the material exceeds its time requirements, the formulation may begin to cross-link and produce gelled particles that have cured prematurely.
Melt mix exiting the extruder falls into the nip of counter-rotating, water-cooled, steel rollers. The rollers flatten the melt mix into a ribbon from 1-2 mm thick and as wide as the rollers (about 4ft.). After the material goes through the roller, a belt carries it to the flaker, which breaks the material into small flakes. These flakes are then conveyed into a mill hopper from where they are fed into the body of a hammer or air mill. A hammer mill consists of a revolving wheel containing hammers that smash the flakes and a revolving separator wheel to keep the powder in the grinding area until it is reduced to a specific particle size, dependent upon specifications.
Powder particles exiting the mill's separator wheel enter a product collector that separates the powder from the air stream. The product collector consists of a set of cloth bags through which the air passes. The powder collects on the outside and drops into a hopper. From the hopper the material is fed into a rotary air lock and then into a sifter or classifier. The sifter contains screens of between 40 and 200 mesh to remove large particles. Powder, usually averaging from 15 to 60 microns, passes through the screens of the sifter, falls through a set of magnets that collect metal particles and is then packed off for shipping.
Particle size distribution of the finished powder is extremely important and will dictate the powder's end use. Particle size is controlled in the mill using the following variables: time in the mill (the longer the time, the finer the particle) and airflow (the faster the airflow, the coarser the particle. Airflow is usually between 1000 and 2000 cfm); and the RPM of the hammer wheel and separator wheel (the faster the RPM, the finer the particles).
Over-sized material falling from the sifter into a collector is usually reworked. If excessive under-sized particles are encountered in a batch, they can sometimes be put through the extruder a second time, depending on whether or not the particular formula can withstand additional heat.
Ferro pioneered a novel process for making powder coatings known as VAMP (Vedoc Advanced Manufacturing Process) that is a supercritical fluid process utilizing carbon dioxide. Under high pressure the carbon dioxide acts as a processing fluid for the effective dispersion of materials in the powder coating formulation. The powders to be mixed are loaded into a pressure vessel equipped with a mixing blade. Carbon dioxide is then charged to the vessel until a supercritical state is reached, fluidizing the raw materials. Once mixed the batch is discharged through special sized nozzles, converting the mixture to a fine powder that requires minimal processing prior to packaging. The process eliminates the mesh-mix extrusion stage and significantly reduces the processing temperature providing increased formulation latitude.
Powder coatings may be defined as 100% solids coatings applied as a dry powder and formed into a film with heat. Two types of powder coating are used, thermoset and thermoplastic. Prior to 1980, thermoplastic dominated the market, especially PVC, applied by fluidized bed technology. Since then, thermoset resins, applied by electrostatic coating, have grown rapidly with demand reaching 317 million pounds in 1998. Thermoset resins were expected to reach a usage of 465 million pounds annually by 2003, fueled by their use in automotive primers and clear coats coupled with their projected use on non- metallic surfaces such as wood and plastic as current research achieves success with these substrates.
Powder systems are available in a wide range of colors and textures displaying versatility in both decorative and functional applications.
Thermoplastic resins include such polymers as polyamides, polycarbonates, PVC, PVA, ABS, polyethylene, polystyrene and various acrylics. Thermoset resins encompass such polymers as epoxy, polyurethane, polyester and epoxy/polyester hybrids, to name the major resins.
The use of TGIC (triglycidyl isocyanurate) has suffered a rapid decline in recent years as the material was classified as a "Category 2 mutagen." Hydroxyalkylamide (HAA), tetramethoxymethyl glycoluril (TMMGU) and glycidyl methacrylate (GMA) have been developed as TGIC replacements. Hydroxyl terminated polyesters are used to formulate polyester urethanes.
Epoxy thermoset resins were used almost exclusively during the early years of thermosetting powder coatings and continue to be used today for a range of applications. As other resin systems are refined for outdoor durability, especially the epoxy/polyester hybrids, simple epoxy resins are losing some dominance in the market. They will, however, remain as a high volume thermoset resin owing to their versatility, especially in the decorative marketplace.
Typical errors in formulation that have to be corrected by the producer include incorrect binder to catalyst ratio, which results in poor flow (this can take place either too slow or too fast) or too high a pigmentation causing major flow and surface defects such as pin-holing and orange peel.
Improper pre-mixing can also result in an inhomogeneous mix with the attendant defects this brings. Additionally control at the extrusion and pulverization stages is critical to ensure thorough compounding and the correct particle size distribution. All the above parameters are processing situations that can be controlled and corrected by the producer.
Table 1 gives an inter-comparison of the resin systems properties in addition to their end use applications.
However, there are also more conventional organic pigments which meet the above criteria, but which are used primarily in inks and plastics, that can also be considered for powder coatings applications. It may be difficult to understand why pigments such as Red Lake C, PR 53:1, or Lithol Rubine, PR 57:1, find use in such demanding applications but their use is well documented and they obviously have a niche. Most likely these pigments will find an outlet in areas such as coated electrical components that are intended for interior use, often within fuse boxes that never see the light of day. Other pigments are being explored in light of their success in the plastics industry, such as Pigment Yellow 62, a pigment that should have application in powder in view of its economy and heat fastness properties.
In order for an organic pigment to be used in powder coatings the following criteria must be satisfied:
Heat stability. This property is very much influenced by the resin being used in the powder coating system. Therefore, the heat stability data obtained in an oven-dry enamel system (solvent-based paints) is only of limited use. If the particular pigment does not have sufficient heat stability, it usually will shift shade when observed with subsequent increase in curing temperature.
Bleed resistance: The bleeding of organic pigments means that a color film develops on the coating surface due to the solubility of the pigment in solvent/binder at increased temperatures. This surface film is easily removed by wiping and the phenomenon is known as "crocking." The bleeding is normally observed immediately after the powder coating is cured through the oven and reappears once it is wiped off.
Plate out: The phenomenon of plate out is observed only in powder coatings and is not a phenomenon in solvent-based coatings. In plastic applications plate out is a condition occurrence. Similar to bleeding, plate out appears as a film on the coating surface, but once wiped off, the film does not reappear. The plate out phenomena is very much influenced by the binder/hardener system; the plate out can be different due to different reactivity of the hardener. The other additives, such as those used in polymerization of the resin, also have an influence on plate out.
Color stability: The application of organic pigments in powder coatings is very much dependent on their color stability in the system. Insufficient stability leads to change in shade and strength of the pigment, also influenced by stoving temperature of the coating. The color variation is primarily due to a chemical reaction between the pigment and the hardener used and is not due to inadequate thermal stability of pigment. The binders containing amidine-compounds especially are known to cause this reaction. The colon variation can also be initiated by reaction between pigment and the additives and catalysts containing amines.
The reaction between pigments and other components of powder coatings function are not clearly understood. The determination of the suitability of pigments in a specific powder coating system based on chemical constitution alone is therefore not possible. It is recommended that a given organic pigment be tested in the specific formulation to determine application suitability.
Flow (rheology): Organic pigments influence the flow of powder coatings because of their relatively large surface area. In general the organic pigment content of a powder coatings formulation should not be more than four percent, and for pigments with a high surface area, it should not be more than three percent. Organic pigments with a small surface area and large particle size distribution that show better flow and opacity, do not exhibit the same effect in powder coatings as in conventional liquid paint systems. The performance of organic pigments, however, can be improved by adding suitable additives called flow modifiers.
6. Light- and weatherfastness: The light and weather fastness of organic pigments in conventional coatings can be used as a guideline for selecting pigments for powder coatings provided that there is no chemical reaction between the pigment and other ingredients in the formulation. The chemical reaction between the binder and pigment can lead to reduction in light and weather fastness of the pigment. While establishing the fastness properties of the powder coatings, the fastness of the binder should also be examined. The chart below shows organic pigments used within powder coatings.
Once a pigment has been approved for use in a powder coating application, as having the required end use properties such as light fastness, durability and heat fastness the next problem that faces the user is that of dispersibility or ease of incorporation into the powder product. Production of a powder coating using a single or twin-screw extruder may not result in maximum color strength development or consistency in color development due to the very nature of the premixing, extrusion, flaking, grinding and mixing operations. This is especially true when light tint or grays are being produced where the actual usage of colored pigment may be less than one percent of the total weight of powder produced.
Use of twin screw or single screw extruders with a low length/diameter ratio will allow relatively high throughput rates during the production of clear powder coatings, but can only be run at slower rates when producing a pigmented, colored powder. Additionally, it is not unusual for as much as 25% of the production of colored powder to have to be reprocessed back through the extruder in order to more fully disperse the color and achieve reproducible color matches.
Commercial pigment preparations are usually dispersions of high performance, organic pigments in a suitable resin having very low glass transition temperature (usually around 130 oF), offered as a colored, low-dusting, standardized, dry, pulverized powder. They are suitable for exterior and interior powder coating applications and all the pigments offered must have the required heat stability compatible with the curing process in addition to the chemical stability with respect to the resins and hardener catalysts used in commercial powder coating systems.
Each of the available colored pigments must be optimally pre-dispersed in the carrier resin, ensuring that the peak color strength, inherent in the primary particle of the pigment, is fully realized. As a result of the dispersion process, which begins with the organic pigment in a water wet or "press cake" form, the pigment particle is preserved in it's "primary" form as an optimum particle size that enables the creation of thin, powder coated films with a high gloss. The finished product is free flowing and almost dust-less, the only dust present as a result of fines generated during shipping and handling.
The resin used as the carrier medium must have a wide-ranging compatibility with a multitude of commercial binder systems used in the production of finished powder coatings. In some cases the resin contains carboxylic groups and is reactive in polyester/ TGIC as well as in hybrid formulations (polyester/epoxy) and should allow the formulator to substitute part for part of the binder. In others the resin must be non-reactive, as a for instance a flow modifier resin with lower melting point. The carrier chosen has no influence on the properties of the finished, powder applied film, such as gloss, solvent, light and heat fastness, durability, film integrity or mechanical properties.
The physical and coloristic properties of the pigment dispersions will assist in improving the general quality of a powder coating formulation, while also enhancing the production throughput efficiency, in the following key areas:
• Better rheological properties during extrusion resulting in higher throughput.
• The use of a fully dispersed pigment preparation results in consistency of shade, reduced batch processing times, higher color strength per equivalent pound of pigment and a finished product that can be used in both metallic and thin film powder coatings.
• Pre-dispersions are usually standardized dry, low-dusting preparations that offer advantages during both the handling and premixing stages.
• Both pigment and carrier used in preparations must have high fastness properties that allow use in both exterior and interior end use applications.
• The carrier used in the preparation is either reactive, allowing formation of stable films with TGIC and epoxies or non-reactive, which makes it compatible with them and also with polyurethanes.
The main application advantages offered by pigment pre- dispersions are:
1. Reduction of any coloristic batch to batch variation that may be a negative feature of powder coating production due to the fact that the pigment is optimally pre-dispersed in the carrier resin.
Additionally, weighing errors are reduced since a larger bulk of the pigment preparation is required to achieve the same overall pigmentation when compared to dry color. This is especially of value when tinting white with a very small amount of color to produce very low tint levels and such colors as grey where incorporation of a granular or powdered carbon black is notoriously difficult. AA more consistent color development is thus obtained, even in the non-ideal conditions of manufacture using a single screw extruder with a low length/diameter ratio. This is a major advantage achieved only by using these uniquely prepared pigment dispersions.
2. At equal pigmentation these preparations will give higher color strength than an equivalent weight of conventional dry pigment. Because of this higher strength, it is then possible to reduce the amount of organic pigment required to match a predetermined shade and strength. This also means better rheological properties during the critical stage as the powder coating flows, before the onset of gelation which leads to an improved surface appearance, resulting from the even flow and coverage of the powder in its molten stage. A higher gloss in deep shades is a possible additional advantage. In matt finishes, where gloss is not important, the better pigment incorporation will allow economies by the use of lower levels of pigmentation as the desired color is achieved by the use of less actual color when the pigment preparation is compared to dry toner.
For the manufacture of pigment preparations, the press cake is instead charged to an alternative form of dispersing equipment, combining the capabilities of mixing and dispersing, a form of equipment able to handle very high viscous blends, where the resin is added and mixed under high shear at a temperature high enough to melt the incorporating resin. Since both the resin and the pigment are hydrophobic the water associated with the pigment is "flushed" from the pigment surface in favor of the polyester resin. By a series of mechanical processing techniques the water associated with the pigment is totally displaced from the pigment's surface only to be immediately replaced by the polyester resin. Thus, an intimate dispersion of primary pigment particles encapsulated with the carrier resin is produced. The determining factor that sets the pigment content of each of the different pigment types is the rheological behavior experienced during the milling/flushing operation.
Too high a pigment loading usually results in extremely high shear being developed within the sigma blade mixer with subsequent problems also being experienced with the finished pigment preparation.
Pigment dispersions are incorporated into the powder coating formulation in the same way as a dry pigment, by being premixed together with the resins and other ingredients of the powder coating formula and subsequently fed into the extruder. Although not eliminating any operation in the powder coating process, as in their corresponding versions for solvent or waterborne finishes, the use of the pigment preparation still ensures both quick and homogeneous incorporation of the color into the powder coating system. During extrusion the carrier resin of the pigment preparation melts together with the other components, distributing the pigment and taking advantage of full color development.
It is therefore important to always keep in consideration that the melting temperature interval of the pigment dispersions has been designed to suit a wide range of binder systems, combining the low melting point of the binder and optimal pigment concentration for each product, to fall between somewhere between 212-230°F (100 -110°C).
The critical stage in the process is therefore that of the incorporation and extrusion process, in that the pigment preparation should melt and flow as early as possible after its introduction into the barrel of the extruder. The formulator does not have to rely upon the extruder developing the pigment strength when a pre-dispersed pigment is being used, but rather that the pigment preparation is being thoroughly mixed during the brief residence time in the extruder. Once the extrusion process has been completed the manufacture of the finished powder coating takes place as normal with the dispersion going through the required cooling, flaking, grinding, classification and standardization stages. Graph 1 illustrates a comparison between a dry organic pigment and the corresponding pigment pre-dispersion, dispersed at different throughput ratios in a laboratory single screw extruder, in a white reduction 1:10 with TiO2. The dotted line indicates the normal extrusion residence time and the expected color strength realized for each product.
In this example, the quinacridone pigment is Sunfast Red 122 228-0066, and the corresponding dispersion Predisol Magenta 122 PRP-3441. Both were dispersed at 212°F (100°C), by one time extrusion pass, using different motor speeds to vary the residence time in the extruder. The graph shows that the dry pigment has a slow developing dispersion pattern while the pigment preparation disperses faster to give a more homogeneous and stronger product.
The formulator can assess the efficiency of pigment incorporation by trying to develop additional strength and color from the finished powder coating. Under normal process efficiencies, the full color will be obtained by one pass through the extruder and no hue or strength change should be evident, when additional energy is applied to the finished powder coating.
Comparison of a powder coating made using an equivalent dry pigment, will show that such a formulation will almost always develop additional color strength or show a change of hue, as energy is applied to the powder by such a piece of equipment as a two roll mill or laboratory extruder. The use of pigment preparations allows manufacturing benefits such as improved throughput, increased color consistency and considerably easier quality control, leading to better particle distribution, which allows thinner films.
Maximiliano G. Meyer is coatings marketing manager with Sun Chemical Corp. in Wavre, Belgium.