Organic Pigments for Weatherable PE [PE]

Heavy metal colorants have long been used in plastics applications for the simple reasons that they are heat stable and durable. In the coloration of polyethylene for weatherable applications, for example, the cadmium and lead chromate yellows have had a substantial market. With environmental concerns of the last fifteen years, however, more and more merit has been found in the organic, heavy metal free (HMF) alternatives to these pigments. Recently, a ruling by the Third Circuit Court of Appeals prompted the United States Occupational Safety and Health Organization (OSHA) to impose new rules for tighter workplace restrictions for hexavalent chromium. This new rule, first published in February of 2006 and requiring compliance by most companies by November of 2006, has prompted a closer examination of the organic alternatives to lead and cadmium in the most weatherable and durable applications, with a special challenge to formulate using good value in use alternatives. High performance organic pigments are proving most useful here. The relevant ASTM standard requires that when color is applied to identify gas service, a yellow color is to be used. This paper will present data comparing some HMF yellow pigments against their heavy metal counterparts in this application, including accelerated weathering (fade) and physical property retention after long-term exposure. Particular attention is paid to the requirements with respect to tensile retention and percent elongation. Results show that the high performance organic pigments examined compared favorably with the heavy metal controls. Coloration (chroma, hue, and lightness) of the pipe- grade polyethylene with the organics was equivalent to the heavy metal controls before exposure. Color change of the organics in stabilized polyethylene (either HALS and/or UV absorbers) during accelerated exposure was measured for as long as 8000 hours with selected trials achieving a total color change DE* of less than 10 units. Tensile and elongation results among the various organic and inorganic colored trials were equivalent seeming not to depend upon coloration. Performance in hydrostatic burst pressure is underway, as is fouryear outdoor real-time exposure.