Abrasive control factors for mass finishing systems

Understanding the performance characteristics of various media

The FABRICATOR June 2001
May 30, 2001
By: A.F. Kenton

The article circulates around the different media styles for mass finishing systems, discussing the cost, weight, and ability of each style. The styles discussed are divided into several areas: Random versus preform shapes; ceramic versus plastic; spherical shapes versus angular shapes versus hybrids; and dry finishing options versus wet options.

Figure 1:
Literally hundreds of media compositions, sizes, and shapes are available.

With the advent of the Clean Air Act in the early 1970s, a new playing field for the coating industry began evolving. Simply providing better service and possibly better economics with the established technology no longer could be the basis for long-term growth, but exciting new possibilities were emerging.

New products and new technologies had to be developed. The first Environmental Protection Agency (EPA) goal was 3.5 pounds of volatile organic compounds (VOC) per gallon of coating. The industry had to learn how to lower the VOC content of conventional products, which was frequently at levels above 6 pounds per gallon, to meet this goal.

To achieve this goal, the coating industry first developed lower molecular-weight resins. This met the requirements for high-speed production lines because it allowed the paint formulator to increase the solids content of the paint without increasing the viscosity. That is, some of the VOC was replaced by resin. This type of product was called "high solids."

Another approach was to use solid resin instead of liquid resin, thereby making a powder coating instead of a liquid paint. Powder coatings have near-zero VOC, but they carry their own disadvantages, such as difficulty in controlling color between batches, the need for higher curing temperatures, and higher capital investment cost.

A third approach was to replace much of the organic solvent with water. While technically more demanding than the high-solids or powder approaches, the waterborne route seemed advantageous in terms of health, safety, and cost of the manufacturing and application equipment.

Water-reducible Products

One technology considered was resin emulsification in water, which did not prove to be very successful. A more successful approach has been to make the resin soluble in water. For example, an alkyd resin can be made with carboxyl groups, and it becomes water-dispersible when neutralized with a volatile amine.

In this approach, water-soluble cross-linking agents were used to cure the alkyd. The VOC, using this approach, was about 3.5 pounds per gallon. That VOC came from the amine and from cosolvents, alcohols, and glycol ethers, which were required to maintain viscosity, stability, and satisfactory application characteristics.

A line of products based on this approach was introduced in 1974. During the early years of the product, technical support remained with the high-selling conventional solvent-based products, but a strong effort was made to divert more resources to water-reducible research and development. As field experience and confirmation of accelerated test results accumulated, the risk of more strongly promoting water-reducible products diminished.

These water-reducible products were growing quickly, and VOC levels in the 3.5 range were still acceptable. However, demand for lower VOC was growing.

Alkyd-acrylic Blends

While 3.0 VOC may have satisfied the EPA's VOC regulation for a specific type of paint, some users needed lower-VOC products to satisfy the total emissions requirements set for particular sites. In other cases, new regulations dictated the lower VOC requirements.

An option for meeting these needs was developed in 1985 using another approach to waterborne resins: latex polymerization. In latex polymerization, the resin is made in a water medium, and although the resin is insoluble in water, it becomes dispersed as tiny particles in the water as it is being formed.

It was found that certain acrylic latexes could be blended with alkyds in water, again along with a water-soluble melamine cross-linking agent. With this composition, VOC levels as low as 2.5 pounds per gallon were able to be achieved.

A downside compared with the water-reducible products was that these alkyd-acrylic blends could not achieve full gloss. For flat and semigloss products, however, VOC even lower than 2.5 were achievable because flattening agents are zero-VOC raw materials.

Technical risks came with introducing waterborne alkyd and alkyd-acrylic, because technical improvements needed to be made, such as better corrosion resistance and still lower VOC content. In addition, the solventborne products were still less expensive than the waterborne ones. Even so, the development of waterborne products paved the way for users to meet the further regulations that would develop.

The Next Step

On March 16, 1996, a new set of VOC requirements laid down by the Illinois EPA became effective. The new regulations were complicated, covering a multitude of coating types and coating applications.

For most general-purpose OEM bake enamels, the Illinois requirement was hardly more restrictive than the federal EPA regulation that was already in effect at that time. However, a special Illinois regulation called for a 2.30 VOC maximum for coatings used for metal office equipment.

One manufacturer of folding chairs, the kind that are used in stadiums, arenas, churches, and schools, used mostly high-gloss coatings. These high-gloss coatings could not be made at 2.30 VOC without being cost-prohibitive using technology that was available in the early 1990s. It turned out that the Illinois EPA was regarding such folding chairs as office equipment, although the manufacturer did not regard the chairs as such.

Just six months before that March 16 deadline, no satisfactory 2.30-VOC coatings were available in an acceptable price range. The folding chair manufacturer was getting nervous.

Fortunately, a solution was developed a few weeks before the deadline. The trouble was that no long-term exposure test results were available for these newly developed coatings, which were expected to experience exposure to sunlight, rain, and beer.Based on bench testing and chemical reasoning, there were high hopes that the new product would perform, but bench tests, and even a priori chemical reasoning, can be misleading. Prudence would demand a better empirical demonstration of outdoor durability than was available in early March of 1996.

The story had a happy ending, though. The chair manufacturer began using the new coating just before the deadline even without any long-term durability testing, and eventually it was demonstrated that the coating did have acceptable durability (based largely on the real-life data collected on the durability of the coating used on actual chairs).

Pushing the Limit Below 2.0

In the late 1990s, manufacturers were under pressure to decrease their total VOC emissions, and often they would take any VOC decrease that could be provided. For example, if one of their colors could be formulated so that the VOC would be 2.20 or 2.15 without sacrificing coating properties, they were happy to pay more for that color because it could help them decrease their overall emissions.

For some colors, the VOC could be decreased to about 1.8 or 1.9 using the same technology that had allowed the development of high-gloss 2.30-VOC paints, without sacrificing properties that the manufacturers needed and without raising the cost of the paint to a prohibitive level.

These efforts to decrease the VOC even further faced a technical limitation: The paints were being made using color dispersions that themselves had VOCs of 3 or higher. Zero-VOC color dispersions had been commercially available, but none of them could be used to make coatings with the combination of hardness and durability necessary for most OEM applications.

In the last two years, very low-VOC color dispersions have been developed for OEM applications that can be used in baked enamels to produce coatings with acceptable properties and reasonable cost. As of June 2000, field tests were being conducted with products using these very low-VOC dispersions combined with the same technology that allowed the development of high-gloss 2.30-VOC paints.

The first tests were with products having a VOC of less than 0.3 pound per gallon without water, although testing has now begun on products having VOC of less than 0.05 pound per gallon.

Reaching for the Goal

If federal and state regulations allow VOC appreciably higher than 1.5 pounds per gallon, why would any company be interested in developing very low-VOC paints? One main reason is that such a product might allow a fabricator to use higher-VOC products in some applications, because the very low-VOC product would help reduce overall emissions.

Some risks still lie ahead. Unknown technical problems might arise; for instance, certain pigmentations required to formulate a complete color selection may fail. Sometimes, unexpected and unfavorable results may be encountered with a new system. Nevertheless, confidence is high that the goal of developing a near-zero-VOC paint will eventually be reached.

A.F. Kenton

Contributing Writer

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The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.

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