LOW-VOC Non-Toxic Primer for Coatings
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 PowderWe 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).
Primer ProductsPowder 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.
Primgreen 2Since 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 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).
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.
- 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).
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).
Utility for Other Coating SystemsWhile 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
AcknowledgementsThanks 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.
References1. 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).