Pearlescent Pigments in Powder Coating Systems
2017-07-24

Powder coatings are gaining popularity in European, U.S. and Asia-Pacific markets because they do not emit VOCs, and they are efficient and durable. Recent innovations in powder coating technology are opening new markets as more applications become attainable. Powder coatings are available in high and low gloss, metallic or satin finishes and an unlimited range of color.

Special-effect pigments can create unique and exciting visual effects in powder coatings. They are compatible in virtually all resin systems, and many are available for durable exterior and industrial applications.

The Benefits of Powder Coatings

New alternative - Powder coatings do not release VOCs during the curing process. This benefit makes these coatings viable alternatives to the more traditional solvent- and waterborne coatings.

Efficient - Powder coatings are highly efficient because its components are 100% solid, and little or no waste is produced. There is no need for solvent-capturing systems. If necessary, a spray booth can be installed in a relatively small area. In addition, liquid manufacturing systems usually can be changed easily into powder systems, because the conveyer and curing equipment are interchangeable. While a capital expenditure is necessary to replace the liquid delivery system, it often quickly pays for itself through improved efficiency.

Durable - Powder coatings are more highly resistant to acid rain and ultraviolet rays from the sun than liquid coatings. They withstand weather, abuse and the rigors of everyday use, and are chip- and scratch-resistant. They are already being used effectively on products that demand durability, such as architectural building panels and window extrusions, office furniture, outdoor furniture, appliances, children's toys, bicycles, automotive rims, and motorcycles. 

Special-Effect Pigments in Powder Coatings

The use of special-effect pigments in powder coatings is also gaining popularity. Because of their ease of application and safety benefits, as well as their decorative appeal, effect pigments are gaining market share over metallics in the powder-coating industry.

The largest market for effect pigments is architectural coatings, which consist of aluminum panels and profiles. Gray metallic shades are the most popular colors for these applications, and can be achieved by combining black or gray powder coatings with exterior white pearlescent pigments. Mica-based effect pigments are a cost-effective and more durable alternative to aluminum pigments. Metallic pigments require an additional topcoat of clear resin to add durability and reduce tarnishing; mica-based effect pigments do not, because they are chemically inert.

Incorporation

Effect pigments cannot be incorporated into powder resins using the traditional melt-mix procedure through an extruder, followed by pulverization and classification. This process would cause the pigments to fragment, resulting in little or no luster. Alternatively, the dryblending or bonding processes may be used.

The dryblending process was developed specifically to handle fragile pigments. It involves mixing the finished, ground and classified powder coating with the effect pigment using a gentle or low-shear mixer such as a P/K tumbler or mechanical mixer (Henshel or Littleford/Day at low rpm and short cycle). The homogeneous pearl/powder blend is then ready for powder coating.

In the bonding process, the powder resin is heated slightly above its softening point and mixed with the effect pigment. The pigment adheres to the tacky resin and is considered a single unit. Independent companies will bond effect pigments for a processing fee. 

Usage Levels

Dryblended effect pigments are typically used at loadings of 3-5% and mixed with the powder coating to produce a metallic effect. Bonded materials can be used at a higher concentration. Some applicators use effect pigments at very low concentrations (e.g., 0.25% to 1.0%) to act as a toner. The overall usage level depends on the desired finish.

Effect pigments can be combined with transparent colorants to form various colors. Opaque colorants and minerals should be avoided because they mask the benefit of effect pigments. Most commercially available powder coatings are not designed to be transparent bases and are therefore not optimally suitable for dryblending with effect pigments. In these cases, large-particle-size pigments will yield the greatest effect. If possible and economical, a two-stage process provides the brightest effect appearance: 1) an opaque color is sprayed first and partially cured; and 2) the effect pigment is dispersed in a clear powder and fully cured. The transparency of effect pigments creates a sense of depth in the coating. 

Application

While powder coating is the fastest growing finishing process in the coatings industry, it is important to understand some of the current limitations. This section outlines the most crucial technical difficulties associated with using effect pigments in powder coating systems so that powder coaters can be prepared to address them. Our researchers are also developing products that will further improve performance of applications in the powder coating system.

Currently, there are two types of application systems: corona and tribo-charging. Both require effect pigments to be bonded for the application process to work most effectively in a powder-coating system. The corona application process is used most often. Tribo-charging generally is used on a very limited basis and reserved for complicated geometric-shaped parts.

In corona applications, in order for the powder coating to transfer to the grounded part, it must first pass through an electrostatic field where it accepts a negative charge and then adheres to the positively grounded part. Although the powder coating's dryblend mixture appears uniform, mica-based effect pigments have inert properties that affect their ability to accept a charge. This parameter is critical to successful powder coating applications.

Because mica-based effect pigments do not readily carry a charge, they can easily separate from the rest of the "charged" coating during their path to the part. The difference in chargeability results in a decrease in the transfer efficiency of the pearl pigment. In other words, during spray-out a portion of the uncharged pearl separates from the powder on the way to the part and falls into the collection system, which could lead to multiple problems. The most obvious result would be an uneven or mottled coating. Uncharged pearl could also collect on the deflector tip, accumulate to a large clump and then spit onto the panel. If this occurred, the concentration of the pearlescent pigment on the panel would fluctuate continuously, making it impossible to produce a consistent finish on the part in day-to-day operations.

Additionally, if the composition of the powder fluctuates in the application process the oversprayed powder cannot be recycled, since the pigment concentration is different from that of the virgin powder. Instead, coaters must spray to waste, which greatly reduces the economic benefit of using a powder coating. However, effect pigments that are bonded act as the resin does, which reduces the aforementioned shortcomings and will be recyclable or reusable.

In the corona process, it is important to monitor the charge or kilovolt (kV) read-out on the control unit. In most situations, the voltage should be no greater than 80kV for optimum spray conditions. Higher voltages could cause a problem called back ionization. This condition occurs during electrostatic application of powder where an excessive build-up of charged powder particles limits the amount of powder that can be deposited on the substrate. The additional powder sprayed to the panel is repelled from the part, taking with it previously coated powder and forming craters.

Researchers continue to refine the efficiency of today's application methods. As an example, the powder slurries used for automotive clearcoat applications appear to be promising. In this system, the powder resin is dispersed in water. The resulting finish, which is achieved by increasing flow and leveling characteristics of the powder system, resembles the smoothness of a liquid coating.

The Relationship of Particle Size to Performance

The fluidization properties of a powder-coating formulation influence the application and coating results. When introducing pigment to the composition, the degree of fluidizing may change depending on the flow characteristics of the pigment. Larger-particle-size effect pigments typically are not agglomerated and move easily when swirled in a container. This is a good indication of how the pigment will influence the overall fluidization and transfer of the powder through the hopper and the delivery system. Effect pigments with larger particle sizes typically work better in corona applications.

Smaller or micro-size pigments are naturally clumpy in the bulk powder and the platelets appear "stacked" when viewed microscopically. This leads to reduced fluidization and powder transfer efficiency through the delivery system. Poor fluidity of the powder coating leads to surging of powder through the gun, which results in uneven coating as local clumps of powder are integrated over the panel. Over-fluidizing, conversely, causes the finer-particle-size material to separate from the bulk powder and be evacuated through the breather tube, changing the final appearance of the part.


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