Since January 2012, fuel tanks fitted to new small off-road spark-ignition engines must comply with permeation requirements set out in the Environmental Protection Agency (EPA) regulation 40 CFR 1060. Permeation emissions from the tanks may not exceed 1.5 g/m2/day in tests carried out at 28°C, or 2.5 g/m2/day at 40°C. The barrier performance of the tanks cannot degrade over time, and it must retain this emission performance for the life of the equipment.
The EPA regulation 40 CFR 1060 is part of a complete set of regulations issued in 2008 on the exhaust and evaporative emissions of fuel tanks for small engines used in handheld power equipment, riding mowers, walk-behind mowers, generators, non-fixed marine tanks, ATVs, utility vehicles, and personal watercraft.
Traditionally, Small Off-Road Engine (SORE) fuel tanks have been blow molded in high-density polyethylene (HDPE). However, HDPE on its own does not meet the new stringent permeation regulations. Typical HDPE tanks emit 25 to 30 g/m2/day, more than 15 times the allowable limit.
So tank manufacturers have been looking for new solutions. It goes without saying that they would prefer a solution that costs as close to the original one as possible. Several solutions are currently available on the market, including post-treatment of HDPE tanks with fluorination, blends of HDPE with polyamide nanocomposites, EVOH (ethylene vinyl alcohol) barrier layers in a multi-layer co-extrusion with HDPE, and so-called monolayer systems. In this last category, solutions of polyamide 6 (PA6) and also of acetal resin are now available. All these alternative solutions differ from each other in various ways, not only in their barrier properties and costs, but also in process stability, quality control, and impact resistance of the tanks.
Processing stability, quality control, and cost
It is clear that the simpler the process is, the less costly it is likely to be and the easier it will also be to maintain quality control. At the same time, consideration needs to be given to the state of the technology. EVOH barrier layer co-extrusion is, for example, well-established in the production of large fuel tanks for automobiles. Not surprisingly, it is also seen as a possible barrier solution for SORE fuel tanks. However, the two markets are quite distinct, and multi-layer constructions might be considered too costly for SORE applications, where production volumes are lower. Capital costs are high, and recycling excess material back into the process is only possible as an extra layer in the co-extrusion process.
Fluorination has been in use for even longer, but the extreme toxicity and corrosive nature of the gaseous fluorine used in the process requires the use of dedicated, offsite fluorination centers. This means that molded tanks need to be shipped to the center, fluorinated, and then returned to the molder for tank fittings before finally being sent to the OEM. This extra handling is not only costly, but also requires significant coordination and timing to assure OEMs receive treated tanks in time for their production needs.
As a batch process, the use of nanotechnology additives to HDPE for evaporative emission remediation has the advantages of being able to use blow-molded HDPE as the primary material and the cost of use is attractive. Effectively dispersing the needed nano layers in the HDPE requires very high attention to additive amounts and extreme control of the processing temperatures. Temperatures that are either too high or too low can result in tanks that behave like neat HDPE—very high permeation characteristics. Again, this property is almost impossible to assess in a production process.
Fluorination may require more rigorous ongoing quality testing than continuous ones. Fluorinated HDPE has also been shown to lose its barrier performance during normal use of the equipment.
Use of nanotechnology certainly appears very interesting, but it is still little used on a commercial level, and the barrier properties it achieves are more susceptible to process conditions than is the case with other monolayer solutions. As a result, particularly close attention needs to be paid to process and quality control.
PA6 shows promise
Of all commercially available solutions to reach the EPA permeation requirements for SORE fuel tanks, monomaterial PA6 technology scores the highest when it comes to the combination of permeability, process stability, mechanical properties, and costs. PA6 has an inherently high barrier to hydrocarbons; permeation rates do vary between specific grades, but at least one commercial available solution, uniquely developed specifically for SORE applications, has a permeation rate that is less than 20% of the EPA limit.
Perhaps the most significant characteristic of Akulon Fuel Lock (DSM Engineering Plastics’ trade name for its emission remediation PA6 material) is consistently low permeation rates for ethanol-containing fuels. Since the material is inherently permeation resistant, it is insensitive to changes in processing. The permeation rate has been shown to be well below the regulatory limits, even for wall thicknesses that are half of current HDPE tanks.
In addition, the material is relatively straightforward to process, so tank producers can expect to obtain a reliable and durable barrier that shows the same performance over many years. The ability to use 100% regrind without compromising in processing stability and material performance results in an even further decrease in cost. Impact resistance tests show good results even at -40°C (-40°F).
PA6 also has the advantage that, with no modifications to hardware, it can run on equipment originally intended to process HDPE. Its mold shrink values are very similar to those of HDPE, and polyamide compounds for fuel tanks have been successfully processed using injection molds, blow molding equipment, and hot plate and vibration welding equipment that were originally used for HDPE.
Production of monomaterial polyamide 6 tanks is also recycling-friendly; 100% production regrind has the same melt viscosity as the virgin material. The dynamic melt rheology and sagging performance are the same for the virgin and the 100% regrind material.
Various solutions are now commercially available that meet EPA permeation requirements for SORE fuel tanks. Each has its own merits, but overall, latest monomaterial PA6 technology scores the best when it comes to the combination of barrier, mechanical properties, long-term reliability, process stability, and system cost. With specially developed compounds, processors have the opportunity to produce thin wall tanks that still meet the EPA and California Air Resources Board (CARB) evaporative emission regulations.
Karen Scholz, PhD, Research & Technology Manager Europe for Consumer Goods at DSM, and Mark Schierson, MSc, Business Development Manager at DSM, wrote this article for SAE Off-Highway Engineering.