To even a casual observer, it is clear that laws governing the use of VOC-containing coatings are becoming increasingly more stringent. For example, in California, coatings in the category Essential Public Service Coatings (a group that would be expected to enjoy considerable leniency) are subject to the very restrictive schedule shown in Table 1.

Though California may be at the vanguard of environment-focused legislation, it is reasonable to expect that other states will eventually enact similar legislation. While the soundness of such legislation is a matter of debate, the outcome is not.1 For coatings manufacturers, future success depends upon the availability of products that have low levels of VOCs (i.e., low enough to comply with the laws where they are employed).

There is another even more important factor that motivates coatings manufacturers to offer products with low levels of VOCs - safety. Replacement of VOCs with water, or low-hazard solvents, results in a safer workplace for applicators. Moreover, the cost of managing VOCs is reduced.

This article describes a new low-VOC (133 g/L, excluding water and exempt solvents), non-toxic primer - Primgreen(r) 2. This product has been developed by ATOFINA Chemicals Inc. at its Research Center in King of Prussia, PA, for use with the Rilsan(r) Fine Powder line of powder coating products. Moreover, it can be used as a primer for liquid coatings, and other powder coating products.

Rilsan Fine Powder

We manufacture and sell a series of thermoplastic powder products, comprising primarily poly(11-aminoundecanoic acid) (i.e., Nylon-11), under the name Rilsan Fine Powder. The structure of Nylon-11 is shown in Figure 1. Among the many nylon polymers known, Nylon-11 stands out as the coating nylon.2 This is because its melting point (185 deg C/365 deg F) is low, hence convenient melting and easy processing; and its equilibrium level of water absorption is low (approximately 2% by weight).

Employed for the powder coating of metal articles, Rilsan powder products are produced in a variety of colors and formulations, to suit particular end-use applications. These coatings are unique in their ability to simultaneously provide excellent performance in the areas of corrosion prevention, wear/abrasion resistance, impact resistance and chemical resistance.

Because of this unique combination of abilities, these coatings are used worldwide in applications such as water handling equipment (e.g., pipes and valves), automotive components (e.g., spline columns and door rails), and wire goods (e.g., dishwasher baskets and shopping carts).

Examples of articles with these coatings are shown in Figure 2. In particular, for water handling equipment, a new American Water Works Association Standard that governs the use of Nylon-11 based coatings has just been issued: ANSI/AWWA C224-01: AWWA Standard for Two-Layer Nylon-11-Based Polyamide Coating System for Interior and Exterior of Steel Water Pipe, Connections, Fittings and Special Sections.3

These products are unique in another respect. They are derived from a renewable, vegetable source - castor beans. The monomer for Nylon-11, 11-aminoundecanoic acid, is produced through the processing of castor oil (which has a variety of other commercial uses, e.g., medicine and motor oil).

Primer Products

Powder coating products have either no, or very low, levels of VOCs. This is one of the features that makes them useful. In the case of Rilsan powder products, often a layer of primer must be placed on the surface of a metal article prior to application of powder and subsequent formation of coating by melting. The excellent performance properties of the coatings result from the presence of the hydrogen bonding attraction among amide groups in Nylon-11 chains. While this chemistry is very desirable, it also results in a high surface energy for molten Nylon-11. As a result, in the molten state, often Nylon-11 molecules self-aggregate, as opposed to wetting a metal surface. Such self-aggregation prevents formation of a good coating. Hence there is the need for a primer product. Moreover, use of a primer product maximizes the performance of the coatings, which is important for the most demanding applications, such as water industry and automotive industry.

Until now, primers for use with Rilsan powder products have been diepoxides dissolved in organic solvents, i.e., products with high levels of VOCs. Applied by spraying, dipping or brushing, the subsequent evaporation of solvents results in a thin layer (typically 2-10 microns) of diepoxide molecules on a metal surface. After application of the Rilsan powder product and subsequent melting, the diepoxide molecules bond Nylon-11 molecules to the metal surface. This process is shown schematically in Figure 3. Note that the thermal energy introduced to cause melting fosters this reaction. The thickness of the resultant coating is ordinarily 100-500 microns (4-20 mils).

We currently offer three solventborne primers: Rilprim(r) P, Rilprim(r) P23V40 and Rilprim(r) 204-A/104-B (also has a chromium-containing corrosion inhibitor). Each of these products has a level of VOCs of approximately 800 g/L (excluding water and exempt solvents). And although each product works very well, their levels of VOCs restrict their use in California now, and probably will restrict their use in the rest of the United States, eventually.

Primgreen 2

Since use of Rilsan powder products often is dependant upon use of a primer product, it was imperative to develop a primer product that can meet tight VOC regulations.

Based upon the history of successful use of diepoxide compounds in solventborne primer products, the most reasonable approach to developing a low-VOC primer product was to use waterborne diepoxide compounds. The most direct use of this concept would use water-soluble diepoxide compounds. However, this proved unfruitful. Preliminary experimental work did not provide encouraging results.

The next approach then was to evaluate water-dispersed diepoxide compounds, i.e., diepoxide emulsions. An advantage is that these are readily available on a commercial scale for use in waterborne epoxy products. This approach proved very fruitful, and a low-VOC, non-toxic primer was developed - Primgreen 2. The approximate structure of the diepoxide molecules that comprise the diepoxide emulsion particles in the primer is shown in Figure 4.

Primgreen 2 has two parts: Parts A and B. Parts A and B are mixed as needed. Particulars of Primgreen 2 are given in Table 2.

Primgreen 2 is applied exactly like a solventborne primer - by spraying, dipping or brushing. Drying at room temperature requires 15-30 minutes, compared to 2-5 minutes for a solventborne primer. The drying rate for Primgreen 2 is slower because of the fact that it comprises approximately 90% water. Of course, the drying rate can be increased by heating.

After Primgreen 2 application and drying, emulsified diepoxide particles are resident on a metal surface. It is thought that during the thermal processing used to melt Rilsan powder products, said particles coalesce and make available diepoxide molecules to perform priming action (see Figure 5).

Primgreen 2 dries more slowly than solventborne primers, but there are advantages to such slower drying. When a solventborne primer is applied to a large metal article, areas treated first can dry prior to completion. Without careful inspection, already primed areas can be re-primed. This results in areas with excess primer. Such areas are susceptible to topcoat blistering. This issue is unlikely to occur with Primgreen 2. Moreover, because of the reduced drying rate, any excess primer applied simply runs off the article. This has the net effect of helping to ensure uniformity of primer thickness.

It should be noted that the performance of a waterborne primer product is more sensitive to uncleanliness on a metal surface than that of a solventborne primer product. This is because with a solventborne primer product, residual oils/greases can be lifted by the solvent, and the diepoxide molecules can still access the metal surface. In contrast, a waterborne primer product will not wet areas with residual oils/greases, and thus no diepoxide molecules can access the metal surface. Thus, when using Primgreen 2, metal surfaces must be suitably clean.

The performance of Primgreen 2 was evaluated with the two most common types of powder coating methods: electrostatic spray and fluidized bed. Figures 6-9 show performance as a function of time resident in a salt fog (ASTM B 117). For each method, comparison was made versus solventborne primer products. The two performance parameters considered were:

Encroachment: A measure of lift-off/loosening/delamination of Rilsan powder coating from a metal surface. Lower values are better, reflecting a better ability of the primer to resist detachment of the coating. This value is measured in distance from a line inscribed in the coating.

Adhesion: A measure of attachment strength of Rilsan powder coating to a metal surface. Higher values are better, reflecting a better ability of the primer to maintain attachment of the coating. This value is measured in arbitrary units (1-4).

The Rilsan powder product used in each case was a natural product - i.e., one that comprises essentially 100% Nylon-11, with no pigments/additives. The benefit of such is that clear coatings are formed, so issues like encroachment can be inspected visually.

It can be seen from Figures 6-9 that Primgreen 2 performs as well or better than the most commonly used solventborne products, Rilprim P and Rilprim P23V40. However, performance is not quite as good as that of Rilprim 204-A/104-B - a high VOC product that has a chromium-containing corrosion inhibitor. Rilprim 204-A/104-B is a more specialized primer product, reserved for the most demanding applications (e.g., service in hot water).

Thus, Primgreen 2 has been shown to be an effective product for the majority of applications requiring the use of a primer. Moreover, the response of numerous customers to Primgreen 2 has been favorable from the standpoints of performance, safety and ease of use.

Utility for Other Coating Systems

While use has been evaluated primarily as regarding Nylon-11 based powder coating products, Primgreen 2 would show general utility as a primer for the attachment of coatings and resins to metal surfaces (by powder, liquid, or melt process methods), where the coating/resin has a functional group that reacts with an epoxide. Preliminary work has shown the utility of Primgreen 2 for liquid and powder coatings comprising primarily blends of poly(vinylidene fluoride) and acrylic acid polymers. A patent application covering all aspects of use of Primgreen 2 has been made.4

Acknowledgements

Thanks are extended to E. Morrissey (ATOFINA Chemicals Inc.) for support with experimental work and P.L. Drooks (Metropolitan Water District of Southern California, La Verne, CA) for advice regarding VOC regulations.

For more information on primers, contact Atofina Chemicals Inc., 2000 Market St., Philadelphia, PA 19103; phone 800/596.2750; or visit www.atofinachemicals.com/techpol.

References

1. Maty, J. In Search of Science. PCI. 90-91 (August 2001).
2. Kohan, M., Ed. Nylon Plastics Handbook. Hanser/Gardner Publications Inc., Cincinnati (513-527-8977), 576-581 (1995).
3. Available from American Water Works Association (303-794-7711): www.awwa.org
4. McAndrew, T.P. Aqueous Based Primer Systems for Attaching Resin Systems to Metals. U.S. Patent Application (filed 10/4/01).