Kynar and gentler streets.

Kynar and gentler streets

As gridlock grows, reliable traffic information becomes more valuable in helping drivers and traffic managers to make routing and signaling decisions. An unusual plastic film that generates electrical signals when run over by vehicles is well suited to gather such information, says Peter F. Radice, a research scientist with the Pennwalt Corp. in Valley Forge, Pa.

Materials that respond to mechanical stresses with a voltage difference across their bulk are said to be piezoelectric. The novel plastic, sold commercially as Kynar, is based on polyvinylidene fluoride (PVDF), a polymer made of repeating molecular units composed of a pair each of carbon, fluorine and hydrogen atoms. Normally PVDF has randomly winding molecular carbon chain-links and is not piezoelectric, but its formability and inertness make it useful in chemically harsh environments and for outdoor protective coatings.

To transform PVDF into Kynar piezo film, Pennwalt's chemical engineers first stretch the hot polymer as it pushes through a sheet-shaping device. This aligns the PVDF's carbon chains into parallel, zigzagging strips and planes. Next, the engineers deposit a metal coat such as their proprietary "silver ink" on each side of the stretched PVDF. Finally, they place the metal-polymer sandwich in a strong electric field. This makes the molecular units swivel so that their hydrogen atoms point in one direction and their electron-loving fluorine atoms point in another. The result is an electrically polarized polymeric film that generates a voltage difference across its faces when stretched or compressed. When traffic managers and researchers attach leads to the metallized areas, mechanical stress in the polymer results in small but easily measurable electrical currents with voltages proportional to the amount of stress.

Traffic-sensing devices made with Kynar would be useful for, among other things, counting vehicles, measuring speeds and weighing trucks in motion. More specifically, city transportation workers might use the data to ease their urban gridlock by changing the timing in traffic lights and thereby altering traffic flow. The devices -- piezoelectric tape and cable -- are either a strip or cable of metallized Kynar embedded in a roughly 2-inch-wide rubber belt that can stretch over one or more lanes depending on its length. Unlike existing multi-lane sensors, the Kynar devices can easily keep separate accounts on individual lanes. For permanent traffic monitoring, the sensors are placed in aluminum housings, which in turn can rest in grooves carved into road surfaces, Radice says.