In the traditional method of manufacturing powder coatings, dry ingredients are thoroughly blended and added to an extruder or heated batch-type mixer for the purpose of fluxing, dispersion, and creating a material as homogeneous as possible. For ease of this discussion, extruder processing will be discussed. It is in this melt stage that the dry pigments that have been incorporated must be uniformly and thoroughly dispersed.

Although some dispersion and color development of the pigment may occur during the dry-blending and mixing stages, the majority of the two processes, dispersion and color development, generally occurs in the extrusion stage, as this is where the majority of aggregates and agglomerates are separated into primary particles. If, as so often is the case, not all of the tinctorial strength of the pigment is developed, a decision as to whether to re-extrude must be made. This decision is made based upon cost of the pigment, the ability of the resin system to withstand additional heat, the effect on film properties, the ability to demonstrate a sufficient strength increase, and the physical ease or difficulty of performing this operation in production relative to the potential gain.

When a single pigment is involved, the decision is usually easier to make and often carried out after a short time of running, by testing the production material in the laboratory. The screw speed may then either be increased or decreased, depending upon the intended outcome. It is obvious that decreasing screw speed will increase residence time and may increase color development; it also reduces throughput and may have some effect on film appearance and performance.

An extreme example would be going from 400 RPM,* a typical operating condition that was arbitrarily chosen as 100% efficiency, down to 250 RPM, where a throughput reduction of 37.5% would occur. Corresponding reductions to 300 RPM and 350 RPM give reductions of 25% and 12.5%, respectively.

When we consider the energy to run the extruder at anything less than 100% (400 rpm) the associated energy increase is relative to the added time to complete the production. Similarly, all assigned overhead costs allocated to the operation of that piece of equipment will be increased, relative to the increased time needed due to running at less than optimum RPM.

As stated earlier, monochromatic (single pigment) cases are the easiest to use. However, with the exceptions of white and black, which, if not fully dispersed, will generally have more effect on application, flow and film properties than coloristic or economic properties, most colored powder coating is a mixture of two or more pigments. In this situation, white is being treated as a pigment that must be fully dispersed, due to its relative effect on the other pigments in the mixture. In this case, development of the TiO₂ pigment and the colored pigment may occur at different rates, depending upon the vehicle wetting characteristics, residence time, temperature, specific pigment and surface treatment.

In our laboratories, when predispersed colored pigment was added to a commercial white powder paint and then put through the extruder two times, the resulting color was lighter after the second extruder pass than after the first, indicating that the TiO₂ continues to develop tinting strength. When powder paint was made in the same fashion but using predispersed TiO₂ in combination with predispersed colored pigment there was no color shift between extruder passes one and two.

Even more difficult to control are custom colors made from blends of two or more colored pigments plus white. If any one of the pigments in the blend develops differently in the laboratory than during production, significant adjustments must be made in the laboratory, during which time production equipment is sitting idle, awaiting a revised, remixed dryblend to once again extrude.

Reviewing a recent series of formulas for RAL colors, developed by a major pigment company, the majority of the color matches required four pigments, quite a few required three or five pigments, and one required six individual pigments to be dispersed. Compound this problem by considering that both organic and inorganic pigments may be needed, not only for the match but also for opacity in thin films. It is not unusual for organic pigments to be supplied with a specification of +/- 5% for strength. Therefore, with a blend of several pigments to match a color, using a formula developed from prior lots of pigment, the variations caused by strength variation +/-5%, and differing color development rates can be significant and require numerous laboratory adjustments prior to a commercially acceptable match being achieved.

Powder coating manufacturers have lived with the difficulties of providing their coatings in an environment that is rapidly changing. The markets being served by powder are growing in number. Each market requires more and different colors to be available in both large and small quantities, and in different qualities, textures, and gloss levels. End users also require custom colors to be matched in a reasonable length of time; typically 2–4 weeks under the current way of doing things. To improve turnaround time on standard colors, many companies have taken the approach of increasing their inventories of stock colors. When combining the number of colors being sold and the chemistries being offered with the gloss levels required the number can be staggering, ranging from just under 1,000 to many thousands; a logistical nightmare and costly phenomenon.

The current era of powder coatings can be compared to liquid paint in the mid 1950s. Tinted paints were made in the factory and called ready mixed. Your choice in the paint store was made from among the 60 or so precolored paints available in several qualities and gloss levels. Today, however, the end user is accustomed to having available an almost unlimited number of colors in any quality, gloss level, and quantity. The consumer also expects same-day service and the ability to come back later and get the same color again and again.

The approach Elementis Specialties is developing is borrowed from liquid paint technology. The approach allows us to be able to reduce manufacturing time and inventory costs and increase throughput.

The company has designed the TINT-AYD “DPC” palette of colorants, which will be controlled, as its liquid system colorants are, to +/- 2% for strength; these are controlled gravimetrically; and, using a reflectance file generated for these colorants, will enable the company to provide formulas for commercial matches to RAL colors, Federal Standard 595 colors, and other series or custom colors wherever computer color-matching capability exists.

The colorants in this system are based upon a proprietary, universal vehicle that is compatible with all currently commercial powder coating chemistries. The pigments being incorporated have been chosen for their value as either interior or exterior quality. Having both grades of pigment allows the end use of the powder coating to dictate the correct pigment dispersions for greatest efficiency and value.

This color system will provide the powder coating manufacturer with the ability to achieve the following.

• Color all state-of-the-art powder coating resins.

  
  

• Complete color development in one extruder pass.

  
  

• Pre-blend only clear powder paint raw materials which eliminates lengthy and costly blender cleanups.

  
  

• Add pre-weighed, blended pre-dispersed pigments simultaneously with clear powder paint materials.

  
  

• Obtain commercial matches to custom colors reproducibly with predetermined formulas and minimum laboratory time.

  
  

• Minimize the need for large inventories of standard colors and reduce working capital.

  
  

• Minimize the response time to provide custom colors.

  
  

• Increase throughput by maximizing the efficiency of the pigment dispersion and color matching processes and in doing so reduce energy costs, fixed processing costs, and laboratory assessments per color.

  For more information on colorants, contact Elementis Specialties, Rheox Inc., Colorants & Additives, PO Boc 700; Hightstown, NJ 08520; phone 800/866.6800 or 609/443.2500; fax 609/443.2422; visit www.elementis-specialties.com; e-mail specialinfo.usa@elementis-na.com.
  
  

  Links

  • Elementis Specialties