Issue Date: July 21, 2008
Solvent Users Look To Replace NMP
ONCE TOUTED as benign, the solvent N-methyl-2-pyrrolidone (NMP) is under scrutiny because of concerns over its potential health effects. Although manufacturers say NMP is safe to use when handled properly, these health concerns have opened opportunities for alternative solvents and processes that make do without NMP.
Paint makers and other solvent users regarded NMP as something of a wonder chemical during the 1980s and '90s, when they used it to create environmentally friendly polyurethane coatings, paint strippers, and agricultural chemical formulations.
But NMP has increasingly attracted attention as environmental regulators, first in California and more recently in the European Union, have sought to exercise control over the solvent in markets where it poses an inhalation hazard. The impact on one group of users was evident at last month's American Coatings Show in Charlotte, N.C., where polyurethane material suppliers were offering customers alternatives to NMP-containing coatings.
"NMP is a great solvent for waterborne polyurethanes," says Peter Schmitt, aqueous technology platform leader at Bayer MaterialScience, a major urethanes manufacturer. It enables smooth handling in the dispersion production process and remains in the polymer as it dries, thereby helping to form a crack-free film, he explains.
However, in 2001, California listed NMP as a reproductive toxicant, and in 2003, the state set workplace exposure limits, Schmitt notes. Also in 2003, the European Commission Working Group on the Classification & Labeling of Dangerous Substances labeled NMP dangerous to children in utero. It has since recommended limits on worker exposure to NMP.
In response, Schmitt says, "we've been reformulating our polyurethane product line for several years." In the polymer preparation process, Bayer is replacing NMP with acetone but removing the acetone before it ships the product to paint makers. Bayer makes just about all its polyurethane coatings materials in the U.S. and Germany with the acetone process, Schmitt notes, and the same will soon be true at its facilities in China.
NMP is a desirable paint solvent because it doesn't contribute high levels of volatile organic compounds, has a low level of toxicity, and has low flammability, says Glenn H. Petschke, urethanes technical manager for Reichhold. But his firm has also adopted the acetone process and produces NMP-free polyurethanes that either don't require a solvent besides water or use an alternative such as dipropylene glycol dimethyl ether, made by both Dow Chemical and Clariant. Petschke would like to find a simple drop-in replacement for NMP, but he says one is not available.
BASF, LyondellBasell Industries, and International Specialty Products (ISP), which together make up the Washington, D.C.-based N-Methylpyrrolidone Producers Group, are not eager to talk about their plans for NMP in the coatings industry. BASF will only state that it sees no major impact on its NMP business because of regulatory concerns. LyondellBasell referred C&EN to the NMP Producers Group, but the group's spokeswoman did not return C&EN's calls.
Carlos Restrepo, vice president of industrial chemicals for ISP, says that because "the polyurethane coatings and paint-stripping markets represent only a small portion of the overall market, any potential impact is not significant to ISP." He adds that the NMP Producers Group is trying to evaluate the impact of any new regulations.
Two years ago, the group evaluated NMP in personal care products and suggested that manufacturers of such products consult NMP makers before using the solvent. The group did so because it anticipated a ban on NMP in cosmetics in the European Union.
WORLD PRODUCTION capacity for NMP was 226 million lb per year in 2006, according to Sean Davis, an analyst at SRI Consulting, publisher of the Chemical Economics Handbook. Accounting for about 20% of all uses for NMP, the coatings market, which includes waterborne polyurethanes, is the largest one affected by regulatory concerns, Davis notes. However, the coatings category includes uses where workers are not exposed to NMP, such as the application of electric motor wire enamels, an industry source says.
Davis says worker exposure to NMP cannot always be easily controlled in applications such as paint stripping (8%) and agriculture (5%), where NMP is used as a solvent in the formulation of insecticides, fungicides, and herbicides. Additional applications for NMP include recovery of hydrocarbons in petrochemical processing and cleaning of microelectronic components.
Alternatives to NMP for agricultural use include dimethylamide, an oleochemical solvent made by Cognis, Davis adds. Potential substitutes for paint stripping, he suggests, include ethyl lactate and benzyl alcohol.
NMP makers may yet come up with a drop-in replacement for the polyurethane coatings market. Even if they don't, users still have options ranging from the less-than-perfect substitutes to process modifications that do not require NMP at all. But one thing that's clear is that regulators are gunning for NMP, and users will have to adjust their strategies accordingly.
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