Designing “green” plasticizers unique approach to come up with new plasticizers that mimic DEHP’s structure but lack its toxic effects on common soil microorganisms, says Milan Maric, a chemical engineer at McGill. Drawing on the knowledge of a reproductive toxicologist, an environISTOCKPHOTO
Recent events, such as the listing of the plastic bisphenol A (BPA) as a toxic substance by the Canadian government, have led to consumer worries over the safety of plastics. Yet alternatives for many plastics abound, and researchers are hot on the trail of a “green” substitute for the most widely used phthalate plasticizer, di(2-ethylhexyl) phthalate (DEHP). Because of DEHP’s versatility, high performance, and low cost, 90% of it is used in flexible polyvinyl chloride (PVC) products, from wall coverings and shower curtains to wire-cable coatings and medical devices. PVC accounts for 95% of plastic consumption, says Mark Rossi of Clean Production Action, an advocacy organization. Designed to lubricate the long chains of PVC polymers, DEHP is not chemically bound to the plastic and readily migrates out of PVC products. Human exposure to DEHP, estimated to be an average of 1-30 micrograms per kilogram of body weight per day, has been increasing over the past few decades, according to the U.S. National Toxicology Program (NTP). In a draft report released in April, NTP noted that it has “some concern” over the safety of BPA. Rodent studies have pegged DEHP and its metabolites as reproductive toxicants and carcinogenic compounds, but information on the human-health impacts is still limited. “The more progressive suppliers of health care products are developing alternatives to DEHP and PVC,” says Pam Eliason, with the Massachusetts Toxic Use Reduction Institute, a nonprofit. David Yopak with Teknor Apex Co., a plastics manufacturer, says that many alternatives are on the market, but none has achieved the high quality of the phthalate esters, such as DEHP. One research team at McGill University (Canada) has used a
Researchers are hot on the trail of alternatives to DEHP. The plasticizer is found in a wide array of consumer products from shower curtains to medical devices.
mental engineer, and a chemist, the group has devised an approach that uses reverse engineering. The researchers have turned convention on its head, by first identifying which chemical structures cause problems for microorganisms and then engineering substances that do not have the problematic components. Some phthalate plasticizers can kill soil and aquatic organisms when they degrade into relatively persistent toxic metabolites, explains David Cooper, a chemist at McGill. To avoid this, the team is striving to design a parent compound that degrades completely and quickly to CO2 and water. Soil microorganisms are wellknown for their ability to break the
5838 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / August 15, 2008
bonds of naturally occurring compounds. Yet the scientists suspected that some functional groups on phthalate plasticizers could evade microbial breakdown. The researchers created molecules identical to DEHP, except that the new molecules were missing one or more functional groups such as aromatic rings, ester bonds, ether bonds, hydroxyl groups, or alkyl branches. When the researchers allowed soil microorganisms to degrade the novel compounds, they discovered that breakdown resistance is caused by ether groups as well as by esters paired with either alkyl branches or hydroxyl groups. The scientists handed the readily degradable alternatives over to colleague Bernard Robaire, a reproductive toxicologist at McGill, who has developed a human cell culture system that screens compounds for hormonal and endocrine-disrupting effects. Compounds that pass Robaire’s screen will be tested for mechanical properties such as flexibility and strength in plastics formulations, Maric says. The McGill scientists are looking at several promising alternatives to DEHP that they plan to unveil later this summer, he adds. A second group, at Teknor Apex, has teamed up with Dan Schmidt, a materials scientist at the University of Massachusetts Lowell, to develop novel PVC compounds that are free of phthalates and lead. Schmidt began with epoxidized linseed oil (ELO). But the epoxidized vegetable-oil plasticizers have a reputation for breaking down in the presence of water and separating from PVC, resulting in a very sticky surface and substantially degraded properties, he says. Schmidt hypothesizes that the addition of naturally occurring nanoclay, known as montmorillonite or bentonite, to ELO will prevent hydrolytic breakdown. “We have to prove it before we can really push this as commercially relevant,” Schmidt says. —JANET PELLEY
10.1021/es801828p
2008 American Chemical Society
Published on Web 07/08/2008
