Plastic coating traps indoor air pollutants.

Plastic coating traps indoor air pollutants

A Canadian chemist has developed a plastic coating to filter several toxic pollutants, including formaldehyde and acidic gases, from indoor air. Air filters coated with the plastic might one day help furnaces, box fans and air conditioners remove some of the indoor air contaminants responsible for so-called sick-building syndrome (SN: 9/23/89, p.206), suggests Hyman D. Gesser of the University of Manitoba in Winnipeg. In preliminary tests, the newly patented coating absorbed susceptible pollutants for at least one month -- the recommended life of most furnace filters.

Polyethyleneimine (PEI) is a water-soluble plastic used in applications ranging from adhesion and disinfection to carbon dioxide absorption. It comprises three types of amines--subunits derived from ammonia. These primary, secondary and tertiary amines are distinguished by whether one, two or all three of the hydrogens in ammonia have been replaced by a carbon-based molecular fragment.

Because amines bind aldehydes, Gesser reasoned that PEI might trap formaldehyde. Indeed, his first tests confirmed that PEI's primary and secondary amines would absorb the pollutant from air recirculating through a filter coated with the plastic. However, as soon as all molecules on the coating's surface reacted with formaldehyde -- within 10 days or so -- its pollutant-trapping activity ceased.

The problem, Gesser discovered, was that the originally gooey PEI coating quickly dried and hardened, anchoring its molecules fairly firmly. So he added glycerol to soften the plastic. This allowed PEI molecules to remain mobile so that those on the surface could trade places with others underneath, effectively replenishing the surface until nearly every molecule had reacted. In the April ENVIRONMENTAL SCIENCE AND TECHNOLOGY, he and Shali Fu report filtering out roughly 96 percent of the formaldehyde in air (at concentrations of 2, 5 and 10 parts per million) by drawing the air through a PEI/glycerol-coated filter at 500 milliliters per minute.

Since PEI is alkaline, Gesser also tested the coating against three acidic gases: sulfur dioxide, nitrogen dioxide and hydrogen sulfide. Again, it chemically neutralized 98 percent of them. Here, however, the tertiary amines proved not only active but also the most potent agents. Some of the acid also dissolved into the glycerol, becoming trapped there.

"An abatement technology like this is useful," comments George Semeniuk, who works in EPA's indoor chemical control division in Washington, D.C. However, he notes, materials that give off indoor gases will sometimes increase their emission rates to compensate, at least temporarily, for a lowering in their air concentrations. That's one reason why EPA's program to reduce indoor formaldehyde focuses on limiting emissions from their sources, such as pressed wood, Semeniuk says.

For a "few dollars" in material costs, Gesser says, "it looks like we can now develop a [filter] coating to work for up to six months." He is about to begin testing prototypes in homes insulated with ureaformaldehyde foam or containing new particle board and other building materials that initially emit high levels of formaldehyde.