Presidential Green Chemistry Challenge: 2001 Designing Greener Chemicals Award
Yttrium as a Lead Substitute in Cationic Electrodeposition Coatings
Innovation and Benefits: PPG Industries developed a novel metal primer that uses yttrium instead of lead to resist corrosion in automobiles. The metal yttrium is far less toxic to human health and the environment than is lead and is more effective in preventing corrosion. PPG's primer should eliminate one million pounds of lead from automobile manufacture over the next few years. In addition, this primer does not require chromium- or nickel-based pretreatments, potentially eliminating the use of 25,000 pounds of chromium and 50,000 pounds of nickel each year.
Summary of Technology: PPG Industries introduced the first cationic electrodeposition primer to the automotive industry in 1976. During the succeeding years, this coating technology became very widely used in the industry such that today essentially all automobiles are given a primer coat using the chemistry and processing methods developed by PPG. The major benefits of this technology are corrosion resistance, high transfer efficiency (low waste), reliable automated application, and very low organic emissions. Unfortunately, the high corrosion resistance property of electrocoat has always been dependent on the presence of small amounts of lead salts or lead pigments in the product. As regulatory pressure on lead increased and consumer demand for improved corrosion resistance grew, lead was regularly exempted from regulation in electrocoat because there were no cost-effective substitutes. This is especially important in moderately priced cars and trucks where the high cost of using 100 percent zinc-coated (galvanized) steel could not be tolerated. Lead is very effective for protecting cold-rolled steel, which is still a common material of construction in automobiles.
For more than 20 years, PPG and other paint companies have sought a substitute for lead in this application. This search led to PPG's discovery that yttrium can replace lead in cationic electrocoat without any sacrifice in corrosion performance. Yttrium is a common element in the environment, being widely distributed in low concentrations throughout the earth's crust and more plentiful in the earth’s crust than lead and silver. Although yttrium is much less studied than lead, the available data on yttrium indicate orders of magnitude lower hazard. As a dust hazard, yttrium is 100 times safer than lead at typical levels of use.
Numerous other benefits are realized when yttrium is used in electrocoat applications. Yttrium is twice as effective as lead on a weight basis, allowing the formulation of commercial coatings that contain half the yttrium by weight relative to lead in comparably performing lead-containing products. In addition, it has been found that as yttrium is deposited in an electrocoat film, it deposits as the hydroxide. The hydroxide is converted to yttrium oxide during normal baking of the electrocoat. The oxide is extraordinarily nontoxic by ingestion as indicated by the LD50 of over 10 grams per kilogram in rats, which is in stark contrast to lead. The ubiquitous nature of yttrium in the environment and the insoluble ceramic-like nature of the oxide combine to make it an unlikely cause of future environmental or health problems.
An environmental side benefit of yttrium is its performance over low-nickel and chrome-free metal pretreatments. In automotive production, a metal pretreatment is always applied to the body prior to electrocoat, which is designed to assist in adhesion and corrosion performance. This process generates significant quantities of chromium- and nickel-containing waste and, like lead, is also a concern to recyclers of the finished vehicle. By using yttrium in the electrocoat step, chrome can be completely eliminated using standard chrome-free rinses and low-nickel or possibly nickel-free pretreatments, both of which are commercially available today. This should be possible without concern of compromising long-term vehicle corrosion performance. For PPG pretreatment customers, this should result in the elimination of up to 25,000 pounds of chrome and 50,000 pounds of nickel annually from PPG products. As PPG customers implement yttrium over the next several years, approximately one million pounds of lead (as lead metal) will be removed from the electrocoat applications of PPG automotive customers.
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