Developments in fuel hoses to meet changing environmental needs.
This article traces the development of fuel hoses to meet the demands of conservation and pollution reduction. The relevant legislation will be reviewed, as will the changes in fuel and materials precipitated by it. Specifications will be reviewed and the constructions which are, or may be required to meet them, will be discussed.
Reduction in toxic emissions
The introduction in North America in the 1970s of catalytic converters to reduce nitrogen oxides and carbon monoxide in exhaust gases resulted in the generation of considerable heat in the engine compartment.
The removal of lead from gasoline necessitated the addition of aromatic hydrocarbons or oxygenated fluids such as alcohols or methyl+butyl ether (MTBE) as anti-knock agents. This resulted in increased permeability of fuel through the elastomers used in hoses.
The fuel crises of the 1970s spawned the Corporate Average Fuel Economy (CAFE) regulations in the U.S., which led to reduction of size and weight of cars and a further increase in temperature in the crowded engine compartment. It also led to the development of fuel injection engines wherein a portion of the fuel is recirculated after having been exposed to air. Under these conditions, and in the absence of lead compounds, there is a tendency for the fuel to oxidize and form peroxides. The peroxidized gasoline can soften epichlorohydrin rubber (ECO) and harden nitrile robber (NBR). This led to a need to protect elastomers against peroxidized or "sour" gasoline or to use elastomers which were inherently resistant to it.
The need for fuel conservation brought about the use of `gasohol,' which could legally contain 10% of ethanol in North America.
Reduction in atmospheric pollution
Emission of hydrocarbons is measured by the Sealed Housing Evaporative Determination (SHED) test. The California Clean Air Act of 1988 called for at least 55% reduction in reactive hydrocarbon emissions from motor vehicles by the year 2000 (ref. 1). Eventually the permitted evaporative emissions from the entire vehicle will be only 2g per day. A major source of the hydrocarbons is permeation through fuel hose and leakage through joints in the fuel lines.
In September 1990, the California Air Resources Board adopted the world's most stringent emission standards and other U.S. states have followed its lead. The California regulations mandate that 2% of vehicles sold in the state in 1998 be zero emissions vehicles (ZEVs) and that at least 10% of passenger vehicles sold (about 200,000 cars) must be ZEVs by 2003. These must be propelled by non-combustion power sources such as electricity or fuel cells. Clearly, such vehicles have no fuel hoses. Meanwhile, tighter emission standards are set for all vehicles commencing in 1994.
Amendments to the U.S. Clean Air Act in 1990 mandate that winter gasoline in 41 U.S. cities must, after November 1992, contain 2.7% oxygen so as to meet the standard for carbon monoxide emission. Commencing 1995, gasoline must be reformulated in nine major U.S. cities which fail to meet ozone standards.
Fuels have always varied in composition depending on the source of crude and the process used by the refiner. The need to reformulate will complicate the situation still further.
Some of the results of the Auto/Oil Air Quality Improvement Research Program conducted by a consortium of North American automobile producers and oil refiners are discussed (ref. 2). Tests were conducted on a 1989 vehicle fleet and a fleet of older vehicles from 1983-1985. Reducing aromatics in the 1989 fleet reduced hydrocarbon emission by 6.3%, reduced CO emission by 13.6% and increased [NO.sub.x] by 2.1%. Reducing aromatics in the older fleet increased hydrocarbon emissions 13.8%, decreased CO emissions 3.3% and decreased [NO.sub.x] by 112%. Clearly the effects of reformulated gasolines will not be the same in all vehicles and the ideal composition will be elusive. Increasing MTBE from 0 to 15% lowered hydrocarbon and CO emissions in both fleets. It was more effective in the older fleet and reduced CO more than it reduced hydrocarbon emission. The effect of MTBE on [NO.sub.x] was not significant. Reduction in olefins from 20 to 5% increased hydrocarbon emission, reduced [NO.sub.x] and had no effect on CO emission. Reduction in distillation temperature from 180 degrees C to 140 degrees C decreased hydrocarbon emissions 21.6% in the 1989 fleet but this change increased [NO.sub.x] in that fleet and increased CO emissions in the 1983-1985 fleet. Reduction in sulfur content proved to be important. The reduction from 466 ppm to 49 ppm sulfur in gasoline reduced hydrocarbon emission 16%, carbon monoxide 13% and nitrogen oxides 9%.
Some of the changes made in reformulation will not affect the elastomers used in fuel systems, but some changes will have major effects. Reduction in aromatics, including the mandated maximum benzene content of 1% should result in reduced fuel permeation through elastomers while increase in oxygenates may increase permeation through elastomers and, in the case of ethanol, definitely will do so.
It is predicted that the supply of oxygenates will be insufficient to meet demand and less reformulated gasoline will be used than the regulators desire (ref. 3). Nevertheless, the changes are coming and all vehicles must be ready to use reformulated gasolines since they will be used throughout the U.S.
The other alternative fuels are more vehicle specific. These include M 100 (100% methanol) for buses, M 85 (85% methanol -- 15% by volume gasoline) for specially equipped automobiles, flex-fuel for vehicles equipped to utilize methanol and gasoline blended in any ratio, natural gas, electricity and hydrogen.
The effect of alcohol based fuels on fuel system elastomers will be discussed in the following section.
As the fuel situation becomes more complex, the list of test fuels continues to grow. Until about 10 years ago, only ASTM Fuel B and Fuel C were used in testing parts for automotive fuel systems. The list now includes ASTM Fuel B, 30% toluene/70% iso-octane, representing a normal gasoline without additives; ASTM Fuel C, 50% toluene/50% iso-octane, representing a gasoline with high aromatic content; ASTM Fuel D, 40% toluene/60% iso-octane, representing a medium high aromatic content, used mainly in Europe; M 25, 75% ASTM Fuel C/25% methanol representing a "worst case" gasoline containing methanol as oxygenate; FAM fuel, 50% toluene/30% iso-octane/15% di-isobutylene/5% ethanol, a European test fuel representing an aggressive gasoline; and M 15, 85% FAM fuel/15% methanol, a European test fuel representing a "worst case" gasoline containing methanol as oxygenate.
Fuel lines in early automobiles were entirely of copper tubing which was long lived and certainly posed no problems ensuing from permeability. The problem with metal tubing was its inflexibility and consequent noise transmission and the need to be bent around comers. As fuel resistant elastomers became available, hose was introduced into parts of the fuel system to impart the desired flexibility. Now that low permeability has become important, there is a trend toward use of more metal lines or lines of plastic material such as nylon. As the systems become more rigid, the noise transmission and flexibility problems are re-emerging.
NBR tube/CR cover
A hose with an NBR tube and a chloroprene (CR) cover was standard in the automotive industry for both fuel line and fuel filler neck until the mid 1970s and still is used if "sour" gasoline resistance and permeability are not a problem.
The typical hose tube until the mid 1970s was of a 32-34% acrylonitrile NBR with black and clay loadings (ref. 4). It resisted aging at 100 degrees C, as did the CR cover.
Improvements in temperature resistance
Extraction of an antiozonant from the CR cover by unleaded fuel with higher aromatic content lead to premature failure in service (ref. 5). As a result, chlorosulfonated polyethylene (CSM), chlorinated polyethylene (CM) and ECO are now the polymers of choice for fuel hose jackets. In the late 1970s, ECO also came into use in fuel hose tubes due to its low fuel permeability and heat resistance.
NBR had often been described as a '100 degrees C rubber,' but it could, in fact, meet fuel hose tube specifications calling for resistance to cracking on being straightened after being bent for 720 hours at 125 degrees C (ref. 6). This can be achieved in a low sulfur-sulfur donor cure system containing zinc and magnesium oxides and silica as the principle filler, with selected antioxidants and plasticizers. The advantage of using a polymer-bound antioxidant and also the advantage of using NBR with 39-45% acrylonitrile to reduce fuel permeability were noted.
Resistance to peroxidized gasoline
ECO vulcanizates soften in peroxidized (or sour) gasoline while NBR vulcanizates harden (ref. 7). Fluoroelastomers (FKM) change very little on exposure to peroxidized gasoline because they are not subject to oxidative degradation. This, together with the low permeability of FKM to fuels, accounts for the growth of FKM based fuel hose. Hydrogenated nitrile rubber (HNBR) has been used in fuel lines in Japan because it is more resistant to peroxidized gasoline than is NBR. Since both FKM and HNBR are expensive, efforts have been made to improve the resistance of ECO and NBR to peroxidized gasoline.
Researchers found that an ECO containing 5% allyl content had improved resistance to peroxidized gasoline, presumably because it was subjected to a combination of degradation and crosslinking (ref. 8). The optimum crosslinking system contained triazine trithiol, magnesium oxide and calcium carbonate. Earlier 1,2 di-(3,5 -t- butyl-4 hydroxy cinnamoyl) hydrazine had been found to improve the resistance of ECO vulcanizates to peroxidized gasoline. Its unique effect appears to involve chelation of trace metal ions (ref. 9).
The initial specifications for peroxidized gasoline resistant NBR were able to be met with appropriately compounded regular (ref. 10), or bound antioxidant type NBR (ref. 12). It was also found that zinc oxide activated NBR in a heat resistant compound exhibited greatly improved resistance to 14 days' exposure to peroxidized gasoline when 0.5 phr 1,2 di(3,5 -t- butyl-4 hydroxy cinnamoyl) hydrazine was present in the compound (ref. 16). Resistance to peroxidized gasoline was also found to improve with increasing acrylonitrile content, at least up to 40% acrylonitrile.
Measurement of fuel permeability
Cup method - With this method for measuring the fuel permeability of hose tube materials, thirty (30) [cm.sup.3] of fuel is placed in an aluminum chamber and a 58 mm diameter disc of vulcanized material is placed over the top, held in place by a brass ring and secured by a threaded locking ring (ref. 10). The assembly is weighed, inverted for 30 minutes to ensure that no leakage has occurred and then kept inverted in a controlled environment and weighed every 24 hours until constant weight loss is observed. The advantage of this method is that it does not require the facility to produce hose or tubing. The disadvantage is that the composition of the fuel will change with time since its components will not permeate the rubber at the same rates.
Plugged hose method - This method has been commonly used in industry (ref. 13). A length of hose or tubing, usually 300 mm, is plugged at each end and contains a specified volume of fuel. This assembly is weighed every 24 hours and the weight loss used to calculate the permeation rate in g/[m.sup.2]/24h. Since the volume of fuel used in this test is relatively small, the volume decreases appreciably and its composition changes as the test proceeds.
Reservoir method - This was introduced to overcome the faults of the previous methods (ref. 13). In tiffs case, one end of the hose is attached to a reservoir of fuel and normally extends horizontally from the reservoir's base so as to be kept full of fuel. After each weighing, the assembly is inverted to drain the fuel and then it is refilled. This ensures a much more constant fuel composition. This method is now incorporated in the SAE J30 tests for fuel hoses.
Fuel permeability and fuel swell - Permeation and swelling of NBR vulcanizates by Fuel C or a 85/15 Fuel C/methanol blend was reduced by increasing acrylonitrile or blending with PVC (ref. 14). Permeability, but not swelling, could also be reduced by replacing black with a platy filler such as talc. A 50% acrylonitrile NBR had lower permeability than a 70:30 blend of 34% acrylonitrile NBR with PVC. The penalty of high acrylonitrile is reduced low temperature flexibility. However, this should not be a problem in practice since gasoline is an excellent plasticizer and hoses are normally wet with fuel.
The acrylonitrile level of the base polymer has the largest effect on peak permeation rate (ref. 15). With acrylonitrile levels at or below 40%, increasing PVC content reduces peak permeation rates, but above 40% acrylonitrile PVC is detrimental.
Data comparing fuel and peroxidized gasoline resistance of compounds based on NBR with 45 and 50% acrylonitrile and a 70/30 NBR/PVC blend based on a 40% acrylonitrile NBR were examined (ref. 16). The NBR/PVC compound had lower swell and better physical property retention than the NBR compounds. Furthermore, 45% acrylonitrile NBR was generally better than that containing 50% acrylonitrile. NBR/PVC blends have been used in fuel systems in Japan and Europe for many years and are now being used in fuel filler necks in North America for reduced fuel permeability.
When the permeation resistance of FKM, ECO and NBR vulcanizates was compared to highly aromatic fuel (Fuel C) and standard gasoline Indolene HO III, equilibrium permeation rates for FKM were under 5% of those of any other elastomer tested and 0.6% of that of the nitrile used in fuel hose at the time (ref. 17). FKM is now extensively used in fuel hose tubes because of its low fuel permeability.
Veneer constructions - Since FKM is relatively expensive, it has become commonplace for hose tube to be of dual construction with a veneer of FKM in contact with the fuel and an undertube of ECO, CSM or NBR. The least expensive construction would be that with an NBR undertube, but good adhesion between FKM and NBR is not readily obtained without surface treatment. It was found that direct adhesion was possible if calcium hydroxide was added to the NBR compound, the level of organic phosphonium accelerator was increased in the FKM and the NBR was functionalized with a diethylamino group (ref. 18). Patented technology (ref. 20) shows the NBR compound again contained calcium oxide and also contained two parts of a polysulfidic silane (ref. 15). The NBR was peroxide cured. The peel strength of the bond between the NBR compound and an FKM compound was 3.3 kN/m.
Nylon has low fuel permeability and is used in fuel line but, as stated earlier, nylon fuel line transmits noise and has to be pre-formed because of its low flexibility. Using the cup test and NBR vulcanizate discs coated with melted nylon, it was demonstrated that a veneer of nylon backed with NBR should provide low fuel permeability (ref. 21). More than 10 years later, such constructions are beginning to emerge in fuel hose.
Resistance to permeation by M 85 and M 100 -- If the distribution system were developed for vehicles to use M 85 or M 100 exclusively, the materials for fuel line hose would not be a problem. Dunn (ref. 16) showed that a bromobutyl rubber (BIIR) compound was extremely resistant to swell and permeation by M 85 and M 100. A 40% acrylonitrile NBR was almost as good and would provide resistance to fuel containing higher levels of gasoline should they ever be introduced accidentally. HNBR also showed very low swell but somewhat higher permeability. It could be used if there was likelihood of peroxides being present in the M 85.
EPDM has been successfully used in 100% ethanol fuel systems in Brazil and natural rubber is also satisfactory for use in a 100% alcohol fuel system (ref. 4).
Resistance to flex fuel - Selection of hose tube material to resist methanol/gasoline blends of all proportions presents a major challenge. Addition of ethanol or methanol to fuel increased the permeation rate through an NBR vulcanizate and the effects were most marked at 10-20% alcohol, with methanol having the greater effect (ref. 22). The low swell in pure alcohol was attributed to hydrogen bonding reducing the polanty of the alcohol. Dilution with fuel was believed to reduce this bonding and promote affinity between polar groups in the alcohol and the NBR.
Most elastomers are severely affected by mixtures of gasoline and methanol and, except in the case of FKM, the effects of ethanol are similar although slightly less severe (ref. 23). Vulcanizates of 16 elastomers in all were tested and it was found that several of them swelled less than those based on NBR and retained tensile strength better after aging in alcohol-gasoline mixtures. The better performers included FKM, fluorosilicone (FVMQ), polysulfide elastomer, F-70A polyether, polyepichlorohydrin homopolymer and polyester urethane. The maximum swell occurred at alcohol concentrations between 0 and 25% for all elastomers except FKM for which maximum swell was in 100% methanol and ECO for which maximum swell was in 40% methanol.
Later work showed that the methanol/Indolene ratio which produced the greatest swell in a given elastomer was that which had solubility parameters most similar to that of the elastomers (ref. 24). The only exception was FKM. The reason given for the high swell of FKM by pure methanol was the fact that it had a hydrogen bonded structure and, consequently, a lower solubility parameter than might be expected.
Balz (ref. 25) examined the effect of alcohol/Fuel C blends on hoses meeting SAE specifications in comparison with Fuel C. Ten percent (10%) ethanol extended fuel was found to increase the permeation rate through SAE 30 R7 hose approximately 25%, while 15% ethanol extension increased it 63%. With SAE 30 R8 hose, although the Fuel C permeation was appreciably less than 30 R7 hose, 10% ethanol extension increased fuel permeation rate 151% and 15% ethanol extension increased it 342%. SAE J30 R9 type hose was found to have much greater resistance to permeation, regardless of fuel type.
Methanol-Fuel C blends at ratios of 25:75 and 80:20 had serious negative effects on the physical properties of FKM containing 66% fluorine, a fluorosilicone elastomer or blends of the two (ref. 26). Fuel C alone had little negative effect on physical properties. The methanol-Fuel C blends only moderately affected the physical properties of FKM containing higher levels of fluorine.
Comparing the fuel permeation rates of several FKM veneer construction fuel hoses in gasoline/methanol blends, the use of a 68% fluorine FKM was recommended rather than a 66% fluorine FKM (ref. 27). The material used in the undertube and coverstock is also important.
Additional work showed that higher fluorine content provided higher swelling resistance (ref. 28). The effect of methanol-gasoline exposure on physical properties was generally harder to predict because equilibrium is generally not attained and exposure results in permeation, network dilation and extraction of minor components which results in network contraction. Overall the main effect of fuel was considered to be "plasticization." It was recommended that greater attention be paid to the kinetic3 of permeation.
The permeation rates of NBR, HNBR and FVMQ were compared to those of FKM, fluoroplastics and Nylon 12 using the cup method (ref. 25). The tests were conducted for 21 to 28 days and the average and peak permeation rates were recorded. The materials with the lowest permeation rates at all methanol levels were the fluoroplastics. Among the FKM grades, a 70% fluorine type gave the lowest permeation rate of all elastomers tested and a 68% fluorine type gave the next lowest.
A 66% fluorine FKM was somewhat lower in permeability than Nylon 12 to Fuel C, Fuel C/ethanol and 85/15 Fuel C/methanol, while Nylon 12 was better with 15/85 Fuel C/methanol (M 85). After norrealization for thickness, the results for FVMQ were generally similar to those for a 44% acrylonitrile HNBR. These elastomers showed much greater permeation rates than any of the other materials, except a 33% acrylonitrile NBR, which was the poorest performer in all fuels.
Several papers (refs. 30-33) have been published recently advocating the use of FVMQ in methanol-fuel systems because of their good low temperature flexibility and retention of physical properties over a broad range of temperature. None of these papers contain any permeability data.
Certain elastomers (presumably FKM) do not reach equilibrium quickly in methanol-fuel blends. Therefore, testing of swell and physical property changes is recommended after three months' fuel exposure (ref. 38). The properties of elastomers were compared after three months' exposure at in various fuel blends (ref. 32) . The physical property changes of one FVMQ were generally similar to those of the FKM elastomer with which it was compared. The FKM exhibited lower swell than the FVMQ at methanol/Fuel C ratios below 50 and higher swell at methanol/Fuel C ratios above 50%. It was noted that "fluorosilicones cannot be judged as a class." If one formulation passes or fails in a given fuel, it does not follow that other formulations will show similar behavior. It was also noted that FVMQ is resistant to peroxidized gasoline and that methyl or ethyl-t-butyl ether (MTBE and ETBE) had less effect on the properties of swollen FVMQ than methanol at concentrations up to 20%.
Polytetrafluoroethylene (PTFE) and a copolymer of tetrafluoroethylene and a perfluoroalkoxy monomer (PFA) were found to be superior to ethylene tetrafluoroethylene (ETFE), FKM or Nylon 12 in resistance to methanol containing fuels, with Nylon 12 exhibiting the poorest resistance overall (ref. 34). The fluoroplastics all exhibited much lower permeability to a 20/80 methanol/Fuel C blend than Nylon 12 or Nylon 12,12 (ref. 35).
The physical properties of acetal copolymer, PBT polyester, LCP (liquid crystal polymer), nylon 6,6 and polyphenylene sulfide (PPS) were compared after exposure to Fuel C, peroxidized fuel and methanol fuel blends (ref. 36). The choice of resin depends on the environment to be experienced. Acetal copolymer was recommended for applications up to 60 degrees C and PPS for higher temperature (121 degrees C) applications. LCP was recommended for dimensional stability at lower temperatures. Other thermoplastic materials such as EVOH (ethylene vinyl alcohol copolymer), PBT (polybutylene terephthalate) and PVDF (poly vinylidene fluoride) also have potentially good barrier properties.
There is no doubt that crystalline thermoplastics provide excellent resistance to aggressive fuels but their high cost and inflexibility will make it imperative to use barrier or veneer constructions in which a thin layer of thermoplastic is backed by a layer of elastomer.
Fuel hose specifications
Evolution of fuel hose specifications
Fuel hose specifications, by and large, have evolved over the past 20 years to meet the many environmental needs or the impact of these needs. Unfortunately, there is little consistency between the various automobile manufacturers, either domestic or global, in setting these specifications. This is largely due to the differences in under the hood conditions in the automobiles and regional differences in fuels. For hose fabricators, a universal fuel hose specification that would meet all automotive requirements would be ideal. This is only a utopian dream. We will continue to have a multitude of specifications, meeting the same end use requirements. The closest to this ideal situation is the drive by the Society of Automotive Engineers (SAE) or the International Standard Organization for Standardization (ISO) to set various universal standards. There is also a desire expressed by many hose producers to set a hose performance specification, rather than a materials based specification. This may eventually be realized.
The changes in fuel hose specifications in the past 20 years have been driven by higher under the hood temperatures, longer vehicle warranty periods, changes in fuels and lower fuel permeation needs.
Fuel line hoses
These changes are well illustrated by the changes in the fuel line hose specifications in North America by the big three auto companies and the SAE J30R specifications. Twenty years ago, an NBR based tube and a CR based cover were adequate to meet the Fuel B or Fuel C resistance, 100 degrees C service temperature and a 250 g/[m.sup.2]/24h fuel permeation requirement. Later on, the service temperature was raised to 125 degrees C. At the same time, sour gasoline resistance was added to the specifications. In order to meet these requirements, a higher quality NBR tube material and CSM or CM cover material was required. In the early to mid-'80s, longer vehicle warranty periods and reduced fuel permeation requirements necessitated changes in specifications to use FKM as the tube materials of choice. The heat aging period was raised from 720 h @ 125 degrees C to 1,000 h @ 125 degrees C and the permeation reduced to 25g/[m.sup.2]2/24h.
More recently, Fuel C/ethanol (90/10) and Fuel C/methanol (85/15) have been used in North America as test fuels. However, the more aggressive M15 (FAM/methanol 85/15) has been in use in Europe for some time. The fuel line hose specifications, in North America and Japan, are heading toward similar targets for fuel resistance, temperature resistance and fuel permeation. These would also necessitate the use of higher fluorine containing FKM as the tube material for now. However, more stringent fuel permeation requirements in the U.S. and the use of alternative fuels may require new materials or innovative hose constructions. ETFE/nylon tubes are already specified by GM for the 1995 model year J-car (under GM 213M).
In Europe, the situation in fuel line hose specifications is less clear. The specifications for some of the luxury cars or export models to the U.S. are similar to the North American specifications requiring FKM as the tube material. A number of fuel line specifications are written around the performance of NBR, NBR/PVC or ECO as tube material.
Fuel filler hose
There is somewhat greater consistency between the big three U.S. auto companies in the materials used for fuel filler hoses, but differences in test fuels and permeation resistance requirements still exist. The general trend is similar to the fuel line hoses, where permeation resistance requirements are becoming more stringent. General Motors has already introduced a draft specification (GM 6289) to reduce permeation towards meeting requirements of California's emissions standards. This calls for an FKM veneered hose with NBR backing with CSM or CM cover; and Ford has a specification (Ford WSA-M96D33-A2) based on 70% fluorine containing FKM veneer with ECO backing and cover. A large diameter curved hose poses some challenges to the hose fabricator.
The materials and constructions are somewhat different in Japan and in Europe. The hose performance requirements are easily met by NBR/PVC blends. High ACN based NBR/PVC blends are used extensively in Japan, in hoses which are mostly injection molded with no textile reinforcement. It is, however, doubtful if any significant advantages in fuel resistance or fuel permeation resistance are achieved above 40% ACN content of the NBR base polymer in the NBR/PVC blend. In Europe, both injection molded and reinforced hoses are used with NBR/PVC. There is also an increasing trend in Europe to use 40% ACN based NBR/PVC.
Conventional hoses Fuel hoses have been made for many years both with textile reinforcement and without reinforcement. Most of the effort from hose manufacturers has involved reduction of costs through improved productivity and reduced materials costs. Some of the improved productivity has been achieved through faster production rates by changing methods of reinforcement. Spiral reinforcement of hoses not only increases the rate of production, it also provides a means of continuous manufacture of long length hoses. In the case of non-reinforced mandrel cured hoses, the manufacturing process has shifted to automated injection molding. The changes in manufacturing processes have had little or no impact on the environmental needs of the finished part. In some cases, a heavier gauge fuel tube can be used to reduce fuel permeation. This, however, can be costly and it increases the weight of the component.
Both metal tubing and thermoplastic tubing have been used as fuel lines for many years. As noted earlier, these materials do not provide the flexibility required by the design engineers and they have a tendency to transmit noise. In the case of thermoplastic materials, special attention to fittings and fittings design are required to avoid leakage. Polyamide has been used by Ford as a fuel line material. The noise transmission and potential safety problems associated with thermoplastic materials have been overcome by using an extruded rubber sleeve over the plastic tubing. Materials such as polyamide and PTFE provide exceptionally low fuel permeation combined with good heat aging resistance. Pilot Industries has developed a co-extruded thermoplastic tubing with ETFE and polyamide under the tradename, P-Cap, to meet the low permeation requirements of fuel-alcohol blends.
Reinforced hoses have also been made with polyamide tube, textile reinforcement and a thermoset rubber cover. Such a hose was commercially introduced by Parker Hannifin (Parflex 80) for air conditioning application. This type of construction did not gain any significant interest in fuel hoses due to noise transmission, rigidity and difficulty with inserting fittings or leakage at fittings.
Fuel hoses are in use where the tube is a composite containing a thin veneer of FKM backed by a lower cost fuel resistant elastomer. The FKM layer provides an excellent barrier against fuel permeation, good fuel resistance and outstanding heat resistance. While the backing material, usually CSM, ECO or NBR, provides a means of reducing cost, it also provides some degree of insurance against any leakage through the thin veneer. Such constructions have only been possible, to date, with mandrel extruded smaller diameter fuel line hoses. The large diameter fuel filler hoses pose considerable manufacturing challenges. Direct adhesion between the FKM layer, without the use of a solvent based adhesive system, and backing material is not easy to achieve, although it can be done, as described earlier. Continuous dual extrusion of the veneer and backing material also presents production challenges.
The veneer concept can also be used with thermoplastic materials which have good resistance to fuels and heat resistance. This concept is already in use with air conditioning hoses, where the thin veneer of polyamide is backed with a conventional elastomer compound based on butyl rubber. Thermoplastic veneer materials such as polyamides still present potential fitting problems and noise transmission, but provide a cost effective means of achieving low permeation hoses.
The concept of using multi-layer packaging films to achieve a combination of moisture barrier, air barrier and other desirable characteristics has been used in the plastics industry. A novel approach to hose design, based on this concept, was first presented by Anchor Swan, a division of Dayco. A fully commercial hose using this barrier concept was introduced by Goodyear under the name Galaxy. This construction makes use of a thin layer of polyamide or a thermoplastic barrier material which is sandwiched between layers of conventional rubber. This type of hose is already in use in air conditioning systems. While this type of hose may have overcome some of the problems - such as noise transmission and potential to leak at fittings - associated with a themoplastic veneer hose, there is a potential to blister between the inner tube and the barrier. Unless exceptionally good adhesion between the inner tube and the barrier material is achieved, the fuel (or fluid) can permeate through the inner tube and build up as blisters at the interface.
This concept presents an opportunity and a challenge to the hose fabricator to develop a cost effective fuel hose to meet the low permeation requirements of the auto industry. Some of the potential inner layer and the barrier materials have been discussed earlier. The primary challenges are direct bonding between the barrier and the inner layer without the use of an applied adhesive; selection of the appropriate materials with the correct balance of performance properties; and a cost effective manufacturing process. This type of hose may also require costly "quick connect" fittings. The challenge is even greater for the large diameter curved fuel filler hose. Despite the problems, the barrier hose concept remains a viable means of making a cost effective fuel hose in the long term.
In the short term, high fluorine containing FKM veneer with a lower cost backing material such as CSM, ECO or NBR looks to be the most practical. High ACN (40-45%) NBR looks attractive as a backing material to meet the cost pressures from the automotive manufacturers (as in GM 6289). The automotive companies are likely to set performance based specifications rather than material specifications. This would encourage the hose fabricators to come up with novel cost effective hose constructions. The surface coatings industry may provide techniques and ideas on surface treatment of a conventional hose to improve permeation resistance. Meanwhile, metal tubing with shorter lengths of flexible hoses may resolve some of the industry needs.
The demands for conservation and pollution reduction will dictate the direction of fuel hose developments. Conservation of fuels and the reduction of pollutants in exhaust gases has led to the use of oxygenated fuels. The oxygenated fuels also have a pronounced effect on the permeation resistance of many conventional elastomeric materials used in fuel hose construction. The materials and methods of hose constructions are in a state of flux due to these demands for conservation and pollution reduction.
The short term trend will be toward the use of more metal or thermoplastic tubing and shorter lengths of flexible hose. The hose will tend to be composed of a veneer of high fluorine content FKM backed by less expensive elastomers. Alternatively, use may be made of a barrier construction composed of a thin polyamide, EVOH, PVDF, PTFE, ETFE, PBT or other thermoplastic barrier between two layers of lower cost elastomers.
The development of performance based specifications will encourage fabricators to seek novel constructions, such as hose surface treatment, to reduce permeability. The major threat is that zero emission vehicles, not powered by liquid fuels, will become widespread. Such vehicles do not need fuel lines.
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[Tabular Data Omitted]
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|Article Type:||Cover Story|
|Date:||Mar 1, 1994|
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