The Silver Institute-Silver News

Over 100 million ounces annually of silver could be used in biocides for the wood preservation industry and for marine antifouling coatings if further testing shows them to be technically and financially superior to current chemical-based biocides that are falling out of favor — some are being banned — because of their toxicity.

The recent banning of tributyl-tin compounds in coatings for marine vessels, and the prospective banning of chromated copper arsenate for preserved wood products, opens up huge potential markets for silver-based substitutes which are not harmful to people and animals. This potential market is 20 times larger than current markets for silver based biocides – mainly for water purification — estimated at 6 million ounces (See Feb/March 2001 Silver News).

Although for many years manufacturers of preserved wood products — such as railroad ties, utility poles, decking, benches and playground equipment — have been able to use copper arsenate in conjunction with other chemicals as a preservative, recent legislative initiatives may put an end to this practice. Recent studies by the University of Miami show that arsenic tends to accumulate in the soil under these structures and can be harmful to people. Children with bare feet are particularly susceptible to absorbing levels of arsenic that could cause cancer. There is also concern that the arsenic could enter aquifers from which drinking water is supplied. In addition, retired railroad ties and utility poles containing arsenic compounds present another problem; they can not be recycled or safely incinerated. Florida has already placed a moratorium on the purchase of arsenic-containing wood products for state parks and marinas, and the legislature is considering a ban on all purchases by consumers. California, Minnesota and several European countries are also considering similar bans.

If silver were to replace the arsenic in these wood products, about 80 million ounces would be consumed in the United States, based on 1997 production figures. An additional 50 million ounces would be consumed in other countries. If other preservatives such as creosote were to be banned — several European countries are considering such a ban — even more silver would be consumed.

In the area of marine antifouling paints — which prevent the growth of marine organisms on a marine structure or ship — a United Nations agency ban on using tributyl-tin compounds in marine coatings will take effect in January pending ratification by 25 countries representing 25 percent of the world’s merchant shipping tonnage. Although the International Maritime Organization does not have jurisdiction over navy and coast guard vessels, many countries, including the United States, are expected to comply with the restrictions for their vessels.

The tributyl-tin compounds are being banned because they injure marine life. Alternatives are being evaluated, including those based on copper, zinc and organic compounds. Any substitute must not harm marine animals or plants and must last for at least five years. Also, they must be compatible with resins used in coatings on vessels. Silver appears to fulfill all the requirements for a marine paint biocide.

Overall sales of biocides for marine antifouling products are estimated to be $50 million annually with the average cost of the coating to be $44 per gallon. If silver were to replace tin completely, about 30 million ounces of silver would be consumed in these paints alone.

Research programs to test the technical and economic feasibility of using silver in wood preservation should be undertaken. If silver can be shown to take the place of arsenic and other toxic, chemical-based wood preservatives, it could rival photographic uses in the amount of silver consumed annually, over 200 million ounces. If silver were to be used in marine antifouling paints, even greater consumption would be realized.