Polyamide 6 grades are claimed to meet new gas-permeation rules: manufacturers of gas-powered vehicles and power tools are under pressure to develop fuel-delivery systems and even portable fuel containers that meet stringent federal and state hydrocarbon-emission standards

Plastics Engineering, May, 2008 by Patrick A. Toensmeier

In North America, the U.S. Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) both have mandated low permeation rates for the fuel tanks of almost every device powered by a gasoline engine. In Europe, fluorination, a key technique for permeation control in fuel tanks, is being phased out because of environmental and worker-safety concerns.

While automotive fuel-tank emissions are regulated around the world, the current round of restrictions in the U.S. are aimed at small off-road engines (SORE), small recreational vehicles like mopeds, motor scooters, all-terrain vehicles, and wave runners, and farm equipment like tractors.

The EPA, for example, has mandated that fuel tanks of small recreational vehicles and motorcycles (225 cc and less) achieve permeability levels of no more than 1.5 g/[m.sup.2]/24 hr at ambient temperature. The fuel tanks on mopeds and large motorcycles may not allow permeation to exceed 2.5 g/[m.sup.2]/24 hr.

CARB plans to apply the 1.5-g standard to most SORE fuel tanks, including lawn mowers, weed trimmers, power washers, and chain saws, in January 2012. CARB has also set permeation levels of 0.4 g/gal/24 hr on portable fuel containers, and will lower this level to 0.3 g next year.

As a result, OEMs are evaluating materials that are formulated to meet tougher permeation standards. Most of these reduce hydrocarbon emissions through blends of resins, use of additives, and in some cases with compounds of nanocomposites. Examples from various resin suppliers and compounders include blends of polyamide 11 and metallocene polyethylene; polycarbonate and polybutylene terephthalate; and polyamide 6 and nanoday.

Polyamide 6

One company, however, has developed grades of polyamide 6 that it claims provide efficient and cost-effective options for barrier resistance in monolayer and multilayer structures. Rhodia Engineering Plastics says the PA materials, part of the Technyl product line, control emissions to below emerging EPA and CARB permeation standards. They are easy to process by blow molding or injection molding, the company says, and in the case of multilayer structures like automotive fuel tanks, they provide savings of around 10% per unit by replacing ethylene vinyl alcohol (EVOH) as the barrier layer.

The grades for monolayer applications also eliminate the need for fluorination, which is expected to come under increased regulatory pressure in North America.

The newest commercial PA 6 grade offering permeation resistance is Technyl C 536XT, says Gary Kachin, market development manager. The first monolayer applications went into full production in Europe in 2007--a motorcycle gas tank and a tank for a moped made for the U.S. The grades for multilayer applications are still in testing and will probably take several years to qualify since they are for automotive. Kachin notes that Rhodia hasn't formally commercialized any of these new materials.

"We're positioning these products for automotive and nonautomotive markets," Kachin says. He reports that Rhodia is working with a major global manufacturer of automotive fuel tanks to qualify one of the PA 6 grades as a barrier layer, but adds, "It's going to take a long time to replace EVOH in automotive." (Kachin declines to identify the company.)

Nevertheless, he says that using the PA 6 in place of EVOH would cost about 10% less per tank at an equivalent barrier thickness and reduce the layers in the structure. A seven-layer fuel tank, comprising inner and outer layers of high-density polyethylene, two layers of HDPE recyclate, a core layer of EVOH, and two tie-layers to bond the EVOH to the recyclate, would be reduced to five layers with the PA 6. In addition to permeation benefits, the PA 6 does not need tie-layers to bond to HDPE or recyclate. Eliminating the EVOH and two ties yields the 10%-per-unit savings. Blow molding would also be easier, Kachin claims, since the PA 6 has a wider processing window than EVOH.

With commercial applications developing for the monolayer PA 6 grades, some performance data are available. Kachin says that the fuel tanks on the European-made moped and motorcycle achieve permeability of 0.05 to 0.75 g/[m.sup.2]/24 hr--well below the EPA levels. A fuel tank for an ATV that has been approved for sale in the U.S., but is not yet in production, achieved permeation levels as low as 0.05 g/[m.sup.2]/24 hr.

Kachin notes that some of these results stem from tank thickness--the moped and motorcycle tanks are 2.5 mm thick, while the ATV tank is almost 5 mm thick. But overall, the barrier properties of the PA 6 grades appear to outperform those of most competitive materials.

Citing confidentiality agreements, Kachin declines to identify the end-users.

Rhodia has not released many details on how the PA 6 materials are made or what accounts for their improved barrier properties. Kachin says that the permeation levels are achieved at the molecular level. "It's still polyamide 6, but the polymerization process is significantly different than in standard polyamide."