Exotic coestrusions: produce low-permeation fuel lines for cars.

Produce Low-Permeation Fuel Lines for Cars

Increasing barrier requirements for automotive fuel lines are spurring development of a variety of exotic combinations of materials in multi-layer tubing structures. Under pressure of new EPA regulations, monolayer structures of nylon 12 or rubber have given way to two- to six-layer combinations of materials such as ETFE fluoropolymer with nylon, PTFE with nylon, or constructions using PVdF, PBT, PPS, or elastomers. One processor coextrudes two layers of different PTFE paste formulations in a ram extrusion process. "It's up to the automobile makers to decide how to meet the EPA standards, and they have taken a number of approaches," says Ed Gray, manager of vehicle systems at Huls America, a supplier of resins for fuel lines.

The EPA's standard on evaporative emissions for unburned hydrocarbons will fall from 10 g/sq m/24 hr down to 2 g/sq m/24 hr for 1998-model vehicles and perhaps for existing models that are redesigned in '98. In addition, it is said that fuel lines on these cars must maintain this degree of permeation resistance for at least a 10-year life - even if, as is expected, fuel formulations evolve toward higher percentages of more aggressively permeating methanol.

The diverse line-up of materials showing up in fuel-line systems today reflects differing approaches to meeting performance requirements cost-effectively. This evolution began about five years ago. Since then proprietary processes and tooling technologies, as well as new resin packages from materials suppliers, have been developed to address the permeation challenge. Fluoropolymers are generally agreed to provide the best fuel-barrier protection, though to be cost-effective, they typically make up less than 20% of a multi-layer structure that on average is 1 mm thick.

Alternative barrier materials are less expensive but require a thicker layer. For example, four-layer constructions developed by Huls America use either PBT at 25% of the tubing thickness or PVdF (a more costly fluoropolymer) at 20%. Some manufacturers believe that the more expensive fluoropolymers will stand up better against more aggressive fuels that are expected in the future.

Some unusual techniques have been developed to achieve adhesion between incompatible layers of fluoropolymer and nylon or PBT. High-pressure fuel lines (those running between the tank pump and engine) also use carbon black for conductivity to prevent static burial-up.

PROPRIETARY PROCESSES

Manufacturers are secretive about their proprietary solutions to these processing and performance challenges. Most fuel-line makers say they designed their own die heads. Companies such as Davis-Standard, Genca, and Harrel supply extrusion equipment to the fuel-line makers but will not publicly discuss details of the hardware involved. Information on material choices is more freely available from resin suppliers such as Huls, DuPont, and Elf Atochem, which can propose complete fuel-line structures.

Huls, for instance, offers a ready-made package of resins for multi-layer tubing. Its 2000 series package contains a modified nylon 12, PVdF, and an optional modified conductive nylon 12 layer. No tie layer is required, the company says. Huls also offers the 500 series package that uses a modified PBT as the barrier with modified nylon 12. Huls' 2001 package is designed for three-layer tubing of nylon 12, PVdF, and a Huls-designed adhesive.

Processors concede that making sophisticated coextrusions imposes a penalty on productivity. "With multiple layers, you produce at a 25-30% slower rate versus monolayer tubing. The material also costs 25-30% more, so the extrusion set-up is more critical, and downtime has to be reduced because you are working with a $1-million line instead of a $50,000 monolayer line," says Scott Maly, general manager of fuel-line producer Eagle-Picher Fluid Systems Inc. in Brighton, Mich.

BETTER BONDS

Good interlayer adhesion at high extrusion speeds is the next challenge for fuel-line makers. An innovative technique uses sequential crosshead extrusion with in-line plasma surface treatment of the first layer to promote bonding to the next layer that is extruded on top of the first.

A proponent of sequential extrusion with a plasma bond is Pilot Industries Inc., Dexter, Mich., which is generally acknowledged to be the first domestic firm to deliver a multi-layer, low-permeation fuel line in 1994. The firm's P-CAP (Pilot Conductive Anti-Permeation) product uses DuPont's Tefzel ETFE as the barrier layer and a modified nylon 12 as an economical chemical-resistant layer. "Tefzel has the lowest permeation of any material being used," says Edward Krause, R&D manager at Pilot.

The company joins the incompatible layers using a patented plasma-arc treating system based on equipment from Enercon Industries. Proprietary know-how includes determining what energy level is needed to activate the modified nylon 12 layer for effective adhesion.

Other fuel-line makers utilize simultaneous coextrusion with a tie layer to bond incompatible materials. For example, Elf Atochem says its Adheflon PVdF-based tie-layer resin can achieve a coextruded bond between its Kynar PVdF and Rilsan nylon 12.

ITT Automotive, Auburn Hills, Mich., coextrudes three different three-layer fuel-line constructions, each with a different barrier material. A proprietary compound provides adhesion between the conductive and barrier layer. The first construction uses PBT as the barrier layer sandwiched between two nylon 12 layers to provide up to 80 g/sq m/24 hr permeation protection. A nylon 12/PVdF/nylon 12 construction has 40 g/sq m/24 hr permeation, while a three-layer construction with ETFE as the barrier provides up to 20 g/sq m/24 hr. The company can run all three constructions on the same three-extruder set-up with a change of the proprietary die head, says George Szabo, product engineering manager of the Fluid Engineering Div.

Bundy Corp., Warren, Mich., is setting up its first multi-layer fuel-line process in the U.S. The firm will transfer extrusion technology from its Technoflow Tube Systems operations in Kassel, Germany, to a plant in Marysville, Mich., where monolayer structures are made now, says Dan Collins, product development manager. Bundy produces a four-layer construction of conductive and non-conductive layers of nylon 12, a layer of PVdF, and then another nylon 12 layer. (The entire material system comes from Huls.) Bundy also uses EVOH as the barrier in a five-layer construction, primarily for vapor lines rather than high-pressure fuel lines.

PILOT INDUSTRIES' COMPARISON OF PERMEATION RATES FOR BARRIER
MATERIALS

                                                Permeation Rate
Material                                      (g/[m.sup.3]/24 hr)

ETFE Tefzel                                              15
PTFE Teflon                                              20
THV                                                      30
PVDF (homopolymer)                                       30
PVDF (copolymer)                                         75
PBT                                                      75
PBT (flexible)                                          105
FKM                                                  75-225
EVOH                                                    225
Nylon 12                                                450

Test conditions: 75% Fuel C + 25% Methanol, 60 [degrees] C, 2 bar
pressure, recirculation method per SAE 1737

Eagle-Picher Fluid Systems uses a Huls resin package to coextrude a four-layer fuel line with PVdF as the barrier layer. "The barrier typically makes up about 10% of the construction," says Scott Maly. Eagle-Picher's European operation makes a multi-layer EFTE barrier fuel line with a jacket of Santoprene olefinic TPE for abrasion resistance and flame retardancy. Advanced Elastomer Systems, the maker of Santoprene, says it has a new TPE halogen-free Santoprene (designated 251-80W232) that can serve as a flame-retardant covering for nylon,based fuel lines. It's developing a conductive Santoprene as the outer layer for furl lines.

The move toward multi-layer arrangements has drawn two U.K.-based global fuel-line suppliers to set up shop here. McKechnie Vehicle Components, Troy, Mich., a supplier of three- to five-layer systems using PVdF, PBT, and THV barriers, is planning to have its first fuel-line plant running in 1998. It will use a five-layer coex turnkey system from Betol Machinery Ltd. of the U.K. Siebe Fluid Systems is also said to be establishing operations here.

Some coextrusions don't require a tie layer because they utilize similar materials - for example, a carbon-filled nylon conductive layer adjacent to an unfilled nylon barrier layer. DuPont and other suppliers are working on new techniques to bond incompatible resins, such as DuPont's Teflon PFA to nylons or elastomers. Meanwhile, Dyneon says its melt-processable THV fluoropolymer can be chemically crosslinked in a patented process to achieve a good bond with rubber. THV can be used as the innermost layer in contact with the fuel, says Dennis Hull, global sales and marketing manager.

Some resin suppliers are looking to modify resins so that no tie layer or surface treatment is required to achieve bonding. Ube Industries  is developing a new alloy of ETFE fluoropolymer and nylon 12. The material is undergoing confidential trials but may be available by year's end.

UNIQUE APPROACHES

An unusual technique comes from the Epic Technical Group of Echlin Automotive, Auburn Hills, Mich., which supplies a two-layer PTFE fuel line that's created in a ram extrusion process called M-Bond. The equipment is made by Markel Corp., Norristown, Pa., for which Echlin acts as the exclusive agent. M-Bond involves simultaneous ram extrusion of two PTFE pastes - a conductive inner layer and non-conductive outer one. No delamination occurs between layers because they are of the same resin. The finn uses a proprietary process to defluorinate the outer surface of the PTFE barrier layer to prepare it for adhesion to an outside layer of another resin specified by the customer. William Paris, director of engineering, claims M-Bond produces the least permeable structure on the market.

Epic is testing the feasibility of two barrier layers vs. a thick single layer. Dual barriers may perform better, yet it's a problem to maintain their concentricity during extrusion. Echlin also offers a four-layer PVdF-based coextrusion using a Huls resin package.

Another approach that eliminates bonding comes from Teleflex Fluid Systems, Suffield, Conn. The firm makes Fluor-Comp fuel hose from a monolayer PTFE tube over which is braided a yarn. of fiberglass mingled with DuPont's Kevlar aramid fiber. Kevlar doubles flame resistance, improves cut-through strength, and eliminates need for a fuel-line shield. Fluor-Comp is used for tracks, although it also appears in one passenger vehicle, Ford's Lincoln Mark VIII.

DuPont is also developing elastomer tubing overwrapped with extruded Teflon FEP film.