Small Fuel Tanks: New Emissions Rules Spur Hunt for Barrier Solutions
Coex blow molding has the inside track, but makers of small gas tanks and jerry cans are also looking at fluorination and other monolayer systems. Rotomolders, too, are considering multilayer alternatives.
New and stricter fuel-permeation regulations are pushing processors to consider a range of barrier technologies for portable fuel containers (jerry cans) and small-engine fuel tanks. Monolayer HDPE containers and tanks won’t meet new fuel vapor emissions standards from the California Air Resources Board (CARB), which are becoming de facto standards for the whole country.
Among the process alternatives under consideration are multilayer coextrusion blow molding (similar to that used in automotive fuel tanks), fluorination surface treatment, and monolayer blow molding with a special barrier additive. For lower volume applications, rotomolders are also being forced to consider various multilayer approaches.
States follow California
While California has taken the lead role in emissions control, manufacturers fully expect that other states will soon follow with similar legislation. In addition, California’s requirements generally dictate the standard for products distributed nationwide.
For portable fuel containers, CARB’s new regulations limit vapor emissions to 0.4 g/gal/day, effective June 2007. That’s a 33% reduction from the current limit of 0.6 g/gal/day. By 2009, the law will be toughened even further, with emissions limited to 0.3 g/gal/day.
Small off-road engine (SORE) fuel tanks for weed trimmers, leaf blowers, lawn mowers, tractors, and generators previously had no fuel-emission requirements. In January 2007, the limit mandated by CARB will be 2.5 g/m2/day for engines with displacements less than 225 cc. In January 2008, the same limits will affect engines with displacements greater than 225 cc. By January 2012, the fuel permeation limit will be tightened even further to 1.5 g/m2/day.
However, California’s SORE regulations has hit a roadblock because of a 2004 Congressional amendment that pre-empts state rules and instead calls for EPA to propose a federal law.
Meanwhile, the U.S. EPA already has enacted fuel-permeation regulations for recreational vehicles and motorcycle fuel tanks that limit emissions to 1.5 g/m2/day. This will take effect for the 2008 model year. Both CARB and EPA are said to be writing new permeation requirements for marine fuel tanks.
Further adoption of the CARB regulations for portable fuel containers is just a matter of time, according to the Ozone Transport Commission (OTC), a Washington, D.C.-based governmental organization that represents 12 eastern states and the District of Columbia. Adoption on a state-by-state basis is expected in 2008-2009.
Multilayer: tried & true
Multilayer extrusion blow molding is a familiar process used to produce automotive fuel tanks for more than a decade. Not surprisingly, it has gained an early following as a barrier solution for smaller tanks and containers.
Coextruded six-layer HDPE/EVOH portable fuel containers are being manufactured by leading producers such as Blitz USA, Inc., Miami, Okla., and Midwest Can Co. in Melrose Park, Ill. Both companies are using Bekum’s continuous-extrusion model BM705D to make jerry cans. Blitz is molding up to 5-gal sizes and Midwest Can makes up to 2-gal containers.
They are using a standard Bekum shuttle machine that is tailored for portable fuel containers, according to Bekum general sales manager Gary Carr. The machine is enhanced with an expanded extruder base, coextrusion head, and special controls. A special pinch-off design ensures weld-seam integrity and drop-impact resistance.
Bekum sees similar opportunities in small-engine fuel tanks but the market is “trickier,” Carr says, because of varied tank sizes and smaller annual volumes. Bekum has moved proactively by building a six-layer development line that will be installed this month at its Michigan headquarters. The machine, a model BA14 coex, produces small fuel tanks and containers up to 5 gal. The BA series has larger platens than BM machines in order to mold larger parts. The BA14 will be shown at NPE in Chicago this June.
FGH Systems has sold several continuous-extrusion coex machines to jerry-can makers for HDPE/EVOH containers up to 5 gal. These Uniloy B&W UMS 35D horizontal-shuttle units come in single- and double-station versions. Cycle times for jerry cans are 40 to 50 sec. The 38.5-ton machine accommodates products with multiple handles and threads and performs all trimming and finishing at the machine.
Fuel-systems supplier Walbro Engine Management in Tucson, Ariz., is entering the small-engine fuel-tank market with a six-layer, continuous-extrusion coex machine from MBK Blowmolding Machinery. The single-head, dual-clamp machine, called the TBA220D, molds HDPE/EVOH tanks up to 20 liters for weed trimmers, lawn mowers, and leaf blowers. For these small tanks, a recessed pinch-off is used to prevent a protrusion above the tank’s surface. Walbro has programs with three lawn and garden manufacturers and is also exploring multilayer tanks for all-terrain vehicles.
Other blow molders of six-layer, HDPE/EVOH small fuel tanks include Agri-Industrial, Fairfield, Iowa, and AcroTech, Watertown, S.D. Both use Kautex KBS series continuous-extrusion machines.
Monolayer options revived
For processing simplicity and minimal equipment and tooling cost, manufacturers would prefer a monolayer structure for these fuel containers and tanks. One choice is fluorination post-treatment of molded monolayer HDPE articles. While many observers question the economics of fluorination, Fluoro-Seal International claims the process is cost-competitive because it avoids the capital cost of a coex machine and the expense of a barrier resin. Fluoro-Seal is the only North American firm that provides post-molding fluorination services. While many sources believe the post-treatment process is more expensive due to the cost of shipping tanks back and forth to a treatment facility, Fluoro-Seal has eliminated that cost for two of its customers by setting up on-site treatment at their plants.
For fluorination, molded tanks or containers are typically placed in a sealed reactor and exposed to a measured amount of fluorine gas under controlled conditions. Fluorine bonds chemically to the HDPE outer surface. The reaction forms a thin fluorinated surface layer with heightened chemical stability and hydrocarbon barrier protection. Fluoro-Seal has reached agreements with makers of jerry cans up to 5 gal and fuel tanks for lawn mowers and weed trimmers.
Blending a barrier resin or additive with monolayer HDPE eliminates the need for post-treatment. The main option of this sort is DuPont’s Selar RB, which is said to be enjoying a revival of interest for fuel tanks and containers. This modified nylon with a proprietary compatibilizer is blended with HDPE and molded on standard monolayer extrusion blow equipment. The result is a laminar microstructure in which the barrier resin forms stacks of large overlapping platelets or discontinuous layers within the container wall, which limit permeation by creating a “tortuous path” for molecules attempting to diffuse through the plastic.
Although the process is proven, a DuPont spokesman admits that “you have to learn to process the material correctly.” Precise temperature control and some hardware modifications such as a special screw are required.
Scepter Corp., a blow molder in Scarborough, Ont., is using Selar RB in its monolayer HDPE EcoGas cans of up to 6 gal. Selar RB has not yet found use in small-engine fuel tanks but DuPont is pursuing such applications.
Another monolayer solution is proposed by GE Plastics. Its Xenoy PC/PBT alloy is a chemically resistant, high-impact, uv-stable material that reportedly can meet CARB standards for small-engine fuel tanks. GE is currently emphasizing injection molding of its Xenoy 6620U into two tank halves that would be welded together. GE is also developing blow molding grades to meet CARB requirements.
The rotomolding option
Stricter emissions standards have also affected manufacturers of small-engine tanks rotomolded of monolayer HDPE. These tanks for motorcycles, tractors, and recreational vehicles are generally bigger and more complex than small blow molded tanks, and the production volumes are smaller. To address the CARB regulations, Arkema has developed a two-layer rotomolded barrier system called PetroSeal. Because of its effective barrier performance, users of this approach are said to be exempted by CARB from costly fuel-permeation testing.
Arkema’s system has a barrier layer of Rilsan nylon 11 inside a shell of metallocene MDPE from sister company Total Petrochemicals in Houston. The nylon layer is deposited over the MDPE layer by means of an insulated chamber, or drop box. A 4-mm-thick tank typically has 1 mm of nylon and 3 mm of PE.
Arkema also has developed a “one-shot” rotomolding process that enables both materials to be introduced into the mold simultaneously without the need for a drop box. The process makes use of the fact that the MDPE melts and fuses first, at around 120 C (250 F), while the nylon melts at about 185 C (365 F). PetroSeal technology is currently undergoing field testing.
Lower cost, higher quality and productivity, and the potential for lightweighting beyond anything yet seen are benefits claimed for a new continuous compression molding process for making PET bottle preforms.
With single-serve containers raising shelf-life demands, packagers are seeking the barrier with the best cost-performance for PET bottles. Multilayer seems to have the upper hand, but monolayer, coating, and oxygen-scavenger technologies have all won niches.
Today’s industrial blow molding machines are highly efficient and predictable and generally can be relied on to produce sophisticated parts from the first shot.