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Nylon 12 in the automotive fuel system environment.

Nylon 12 in the Automotive Fuel System Environment

The development of pressurized fuel systems, the evolution of vehicle designs, and the increasing variety of fuels has created demanding environments for automotive fuel systems. Meeting the material demands of these environments has become a real challenge. This challenge, however, has been met with nylon 12 - the material meets environmental and flexibility requirements for automotive fuel systems. As a result, nylon 12 is the most widely used nonmetallic fuel line material. Almost half of the fuel line applications in Europe use extruded nylon tubes. If current trends continue, nylon will be the material of choice in more than half of the U.S.-produced fuel line business within the next 5 to 6 years.

The trend toward nylon 12 usage in fuel lines is being "fueled" by the many advantages it has over steel tube/coupled hose systems. Most auto companies experience a significant cost savings when they switch to a fuel line system utilizing nylon 12 tubes. (Part of those savings is realized in the assembly operation.) The properties of the resin solve many of the problems encountered in a "conventional" fuel system. Fuel systems utilizing nylon 12 were first commercialized prior to 1970; the resin has a proven capability for surviving fuel system environments.

Automotive fuel systems operate in perhaps more demanding environments than any other system in the vehicle. Besides the many different fuels the system must handle, there are roadway and other environmental chemicals encountered between the fuel tank and the engine, as well as a series of physical requirements that must be met. There are other vehicle systems exposed to higher loads and greater temperatures than the fuel system, but the combination of physical environment and chemical environment is probably unequaled.

For a thermoplastic fuel line, dimensional increases (primarily length change) can vary widely from swelling caused by different fuels. Nylon 12 is affected the least of all nylon resins. A plasticized nylon 12 grade exhibits even less swelling, but it does not eliminate the dimensional variations completely. In order to solve this problem, a method has been devised for producing nylon 12 tubing that exhibits no variation in length, even in fuels that are chemically aggressive (see "Modified Tubing," below).

Nylon 12 Resins Used for

Fuel Lines

Three basic types of resin are used for fuel lines: semirigid, semiflexible, and flexible. They differ primarily in level of flexibility, which is imparted by the addition of plasticizer (see Table 1).

Table : TABLE 1. Types of Nylon 12 Resin
 Used in Fuel Lines.
 Extraction of fuel
Resin grade content line usage
Semirigid [is less than or equal to] 3% Volkswagen
Semiflexible 7 [+ or -] 2% Ford
Flexible 14 [+ or -] 2% GM

The effect of the plasticizer on physical properties is most pronounced in the flexural modulus of the resin (see Fig. 1). Table 2 lists the physical properties of specific nylon 12 fuel line resins.

Besides the properties listed in Table 2, properties such as permeability, ease of handling and assembly, and overall dimensional stability are important. It is the balance of all properties that influences vehicle engineers in selecting nylon 12 resins with different flexibility levels for use in their particular fuel system. [Tabular Data Omitted]

Physical Environment

Fuel line tubing typically has a nominal outer diameter of 8 or 9.5 mm; wall thickness is usually 1 mm. Sizing the tube is based primarily on flow rates and presures required for engine operation. All fuel tubing discussed here has an 8-mm nominal diameter and a 1-mm wall, unless otherwise specified.

Temperature. The most limiting factor for the heat environment of a fuel system is the temperature of the fuel. For most engines to run properly, fuel temperature should not exceed 50 [degrees] C for any significant length of time. As fuel temperatures rise above 60 [degrees] C, the fuel is likely to boil and cannot be handled properly by most fuel systems.

The areas under the car through which the fuel lines are routed are made up of components that are exposed to extremely high temperatures (exhaust system and catalytic converter, for example). Fuel lines must be carefully routed to avoid areas of excessive heat.

The low end of the temperature environment is limited only by the weather. Most cold-start requirements for engines are at -30 [degrees] C. That temperature may be considered the lower operating limit because environmental temperatures lower than -30 [degrees] C are raised as fuel, warmed by the engine, circulates through the fuel system.

Nylon 12 tubing can be used continuously at 100 [degrees] C to 120 [degrees] C, with occasional peaks as high as 140 [degrees] C. Since fuel temperatures should not exceed 50 [degrees] C to 60 [degrees] C, a properly designed fuel system should not be subjected to temperatures at the upper limits of the material. The key factor again is routing; fuel lines should be located to avoid sources of excessive heat. The flexibility of nylon 12 facilitates such routing. When high temperatures are occasionally encountered, the flowing fuel in the lines can have a significant cooling effect.

Pressure. Operating pressure for a fuel system depends on the type of system it is:

Carburetted: 3 to 6 psi

Central fuel injection: 18 to 25 psi

Port fuel injection: 35 to 55 psi

Turbo-charged, port fuel injection: up

to 60 psi

In a "worst case" scenario, pressure can momentarily peak in excess of 100 psi (depending on the capability of the fuel pump).

Properly extruded nylon 12 fuel line tubing has a room-temperature-burst that ranges from 800 psi to over 1400 psi, depending on the level of plasticizer in the nylon 12 material that is used. This pressure level relates directly to the hoop stress that the wall of the tubing can withstand. The level of hoop stress at burst declines as the temperature of the tube is raised.

The utilization factor - defined as a comparison of the minimum hoop stress (according to DIN 73 378, the German nylon automotive tubing specification) at 23 [degrees] C to the hoop stress at higher temperatures - supplies information on how much hoop stress can be utilized at higher temperatures. DIN 73 378 specifies for the semirigid and flexible nylon 12 grades the same utilization factor, as shown in Fig. 2.

Even at extremely high temperatures, the tubing is still capable of withstanding over 25% of its room temperature hoop stress levels. This equates to about 2 to 3 times the highest pressure generally achievable in the fuel system. Tubing with a slightly thicker wall would significantly raise that factor of safety.

Impact and abrasion. Impacts from stones and debris can occur on the fuel line as the vehicle is moving. These impacts are more difficult to withstand when temperatures are low. There is no widely accepted standard or measured levels of energy of actual under-vehicle impacts that are used in developing fuel lines. The industry, however, does use two different test methods for measuring impact resistance. Tests are performed at room temperature and at -40 [degrees] C. The falling dart test (SAE J844 - used mostly in the U.S.) and the pendulum test (DIN 73 378 - used mostly in Europe) are used for nylon tubing. Similar to temperature concerns, routing is a key factor in this area. Fuel lines can be positioned where impacts are much less likely to occur.

Although nylon 12 has good abrasion resistance, direct contact with moving components, sharp edges, or rough surfaces should be avoided. If routing cannot eliminate concerns in this area, then a protective cover or other means of shielding the tube may be necessary.

Vibration. Vibration is another factor in the environment of actual vehicle operation that nylon 12 fuel line tubing must be able to withstand. As with impact, there is no widely accepted standard or measured level of vibration (amplitude and frequency) that is used in fuel line developments. To illustrate the capability of this tubing to withstand vibration, it was subjected to vibrations at -40 [degrees] C, according to a test procedure described in SAE J1131 (performance requirements of hose assemblies). The test requires sine wave oscillations with an amplitude of [+ or -] 6.35 mm at a frequency of 10 Hz, a temperature of -40 [degrees] C, and a constant tubing pressure (of 145 psi) during the test. Nylon 12 did not show any crazing or breakage after 8 million cycles.

Handling. The handling of a nylon 12 fuel tubing is dependent on the stiffness of the tube and its resistance to kinking. Both of these are dependent on wall thickness, tubing diameter, and flexibility of the resin. Fuel system designers must choose the combination of those factors to obtain results that are best for their vehicle.

In general, fuel lines are all about the same diameter and wall thickness, so flexibility of the resin plays the largest role in ease of handling. Fuel tubes are preformed into bends when more rigid grades are used or if chosen routing requires tight curves. Preforming can be avoided only if flexible nylons are used and/or the required bends of routing are gentle curves. With proper choice of routing bends and the inherent kink resistance of the nylon resin, kinking is not a problem during assembly unless the tubes are mishandled.

Connections. Nylon 12 fuel lines are usually attached to another part of the fuel system by insertion of a barbed fitting or metal tube with upsets into the tubing. Design of the inserted component is the key factor in determining the quality of the connection. Nylon 12 contributes to the quality of the connection by its resistance to creep and its retention properties. Nylon 12 resins can maintain a good grip on the inserted component over the long term. When a nylon 12 tube is elongated, its diameter decreases. This means that if a force is applied that is in the direction of pulling the tubing off, the tubing elongates and applies a greater gripping force on the inserted member.

Chemical Environment

From the standpoint of the chemicals that can occur both inside and outside the fuel system, nylon 12 is one of the few materials that can withstand all the chemicals it encounters.

Under the vehicle. Most of the fuel system is located under the vehicle. Thus any influences of the environment that are encountered by the underside of the vehicle must be withstood by the fuel system. Such environmental influences include water, greases and oils, undercoating chemicals, road salts, and other roadway chemicals as well as spilled fuels and other automotive liquids.

The most severe chemical that can be encountered under the vehicle is the product of the interaction of road salts and galvanized steel surfaces in the presence of some moisture. When the chlorides and zinc react, the result is zinc chloride, which can cause stress cracking in many plastic resins. Since failure can occur in some resins in less than 1 min, zinc chloride does not have to be in strong concentrations to be the source of many difficulties.

Water is the most commonly encountered substance under the vehicle. Nylon 12 is the fuel tubing resin that is least affected by water (see Fig. 3).

At moderate temperatures, nylon 12 is, without appreciable swelling, resistant to aliphatic hydrocarbons (including conventional fuel), lubricants, and diesel fuels. More swelling (but still within reasonable tolerances) occurs in aromatic hydrocarbons and alcohols. This means that the typical greases, oils, and usual roadway chemicals do not have a harmful effect on the tubing.

Parts made of lower-number nylons are destroyed in some solvents (such as alcohols and aliphatic, aromatic, and chlorinated hydrocarbons) if they are under stress and dry. In a moisture-saturated state, they are more resistant, i.e., nylon 12 remains unaffected in either state. In some inorganic reagents, like zinc chloride, stressed nylon 6/6 and nylon 6 parts are severely affected.

Table 3 illustrates the results of a laboratory test in which stressed plaques of nylon 6 and nylon 12 were placed in contact with a hot-dipped galvanized steel surface. The plaques were then subjected to salt spray for six weeks.

Table : TABLE 3. Stress Crack Resistance

in Salt Spray Test.(a)

Length of Percent of parts exposure showing stress cracking
 Nylon 6 Nylon 12
7 days 0 0
14 days 0 0
21 days 20% 0
35 days 100% 0
42 days 100% 0

(a)Molded parts under stress (bent) in contact with zinc-plated steel subjected to 5% mineral salt solution spray at 35 [degrees] C.

This stress-cracking phenomenon is illustrated in a short-term test with injection molded plaques that were bent and immersed in a 50% zinc chloride solution (see Table 4).

Table : TABLE 4. Stress Crack Resistance.(a)

Length of time

before stress cracking
Temp. Semirigid
of test Nylon 6 & 6/6 nylon 12
20 [degrees] C <1 min >2000 hr
70 [degrees] C several sec >500 hr

(a)Molded parts under stress in 50% zinc chloride solution.

Inside the fuel system. The fuels that can be encountered in today's vehicles vary widely. All systems must be able to operate with these fuels; they cannot be designed for just some of them. The fuels are: regular fuel (many types), unleaded and premium fuel, methanol (0% to 100% along with other hydrocarbon fuels), ethanol (0% to 100% along with other hydrocarbon fuels), and auto-oxidized fuel (also known as "sour gas").

Reactions with fuels fall into four general categories.

1. Resistance to change of physical properties.

2. Chemical resistance (stress cracking, embrittlement, etc.).

3. Permeation resistance.

4. Resistance to dimensional changes.

Any material used in the fuel system must perform satisfactorily in each of these areas with all types of fuels.

The fuels typically encountered in today's vehicles (such as regular, premium, no-lead, and diesel fuels) do not pose any difficulty to nylon 12. When some of the more "unusual fuels" (like gasohol - a blend of fuel with ethanol or methanol) are encountered, the detrimental effects can be more pronounced. However, with nylon 12, only a dimensional change is noted; the integrity of the fuel system is maintained.

In today's highly refined fuel systems, previously unencountered problems have developed. One such occurrence is autooxidized fuel or "sour gas" - the name applied to fuel that contains hydroperoxide. Hydroperoxides can arise from oxidation processes in fuel-injected vehicles where there is continuous fuel recirculation. The hydroperoxide decomposes to form free radicals, which attack and degrade polymers and elastomers. Commonly used rubber materials such as NBR or ECO are susceptible to sour gas. Nylon 12 grades of fuel line resin and NBR were tested in ASTM fuel B containing tertiary butyl hydroperoxides. Tensile elongation was measured after different exposure times. The exposure to the sour gas was carried out at a temperature of 60 [degrees] C. The results are shown in Fig. 4.

Besides withstanding all the chemical action that can occur in the presence of fuel, the material used for fuel lines must be able to adequately "contain" the fuel as well. This means the rate of permeation of fuel through the wall of the tubing must be kept to a minimum.

Rate of permeation is greatly affected by temperature. Even at high temperatures (50 [degrees] C, for example), the permeation of fuel through a nylon 12 fuel line is very low (see Table 5).

Table : TABLE 5. Fuel Permeability of Nylon

 Grams/m Grams/[m.sup.2]
Resin grade per day per day
Semirigid 0.02 0.7
Semiflexible 0.1 4
Flexible 0.2 8

(a)Temperature: 50 [degrees] C; fuel: unleaded gasoline with 12.5% aromatic hydrocarbons.

Interaction with various fuels affects both physical properties and dimensions of fuel lines. For nylon 12, the physical properties change, but not enough to significantly affect its ability to function as a fuel system material. The properties change somewhat as a result of the plasticizing effect of the absorbed fuel.

Dimensional stability. All nylons swell in automotive fuel, the result being dimensional changes. This is particularly prevalent in fuels containing methanol. Nylons with a higher number of carbonamide groups (such as nylon 6) swell more than those with a lower number (such as nylon 12). Research has found that fuels containing 15% methanol have the greatest effect on nylons. Figure 5 shows the change in length of different homopolymeric nylon tubings exposed to methanol fuel.

The presence of plasticizer affects the total amount of swelling that occurs. While unplasticized nylon 12 tubing elongates up to 4%, the plasticized versions elongate less (about 3% for the low plasticized and about 1% for high plasticized nylon 12).

The change in length is measured when tubing is attached vertically on a board and filled with fuel. Fuel lost to vaporization was continuously replaced from a storage vessel, and the fuel was renewed every day or every three days at weekends. The elongation of tubing was recorded and converted to graphs showing the change in length in tubing versus the exposure time.

In Figs. 6 through 8, the linear change in length of tubing is plotted versus exposure time. Nylon 12 tubing was exposed to ethanol (Fig. 6); fuel containing 15% methanol (Fig. 7); and unleaded premium (aromatics, aliphatics, 5% methanol + tertiary butyl alcohol, +2.6% methyl-tertiary-butylether) (Fig. 8).

Homopolymeric and low plasticized nylon 12 continually increase in length up to an equilibrium. Tubings of nylon 12 with high levels of plasticizer elongate to a maximum and then contract after a few days. This change in length is caused by the interaction of fuel and plasticizer.

Modified Tubing

If a fuel system designer wants to utilize the physical properties of a semirigid nylon 12, but does not want to experience the dimensional changes that can occur with certain fuels, there is an alternative. Huls developed a patented process to manufacture tubing that does not elongate when exposed to fuel. The so-called "modified" tubing balances the elongation (caused by the fuel) with a shrinking process.

The processing principle is to conventionally extrude tubing in a slightly larger diameter; subsequently, the tubing is sized in a second sizing disc to the final dimensions. This second sizing adds an "extra stretch" to the tubing. As the tube absorbs fuel, it stress relieves itself. This causes the tube to shrink since the "extra stretch" is an induced stress. This shrinkage balances length increases that are caused by swelling of the fuel lines. Figures 9 and 10 show the change in length of modified tubing as compared with conventional tubing.

Even in very aggressive fuel compositions, modified nylon 12 tubing does not elongate. Tubing made of semirigid nylon 12 is particularly applicable for manufacturing modified tubing. The modification can be tailored specifically to the resin so the elongation brought about by fuel is fully compensated for and no change in length occurs.

With plasticized grades, the variable change in length due to the interaction of fuel with plasticizer has to be considered. An initial elongation cannot be prevented. After attaining a maximum length, the tubing then shrinks and reaches its original length.

Modified tubing exhibits properties that are comparable to those of conventionally produced tubing. To illustrate this point, three of the most critical properties of a fuel line are shown in Table 6. [Tabular Data Omitted]

Properly manufactured tubing is dimensionally stable in dry heat up to 50 [degrees] C (necessary for the storage of the tubing). Drying out and shrinkage, which can cause a shortening of the tubing, occur only during a long-lasting idle time. Therefore, this somewhat detrimental property should not be exaggerated.

Modified tubings can also be preformed; however, only the area of the bend should be heated up, since this area loses the property to balance the elongation when exposed to fuel.

Other Applications

Liquid lines are the most common usage of nylon 12 in the fuel system, but they are not the only suitable application of the resin. Auto companies are closely examining every part of the fuel system with an eye toward possible plastic usage, including filler neck and housing, fuel tank and integral valves, fuel sending units, fuel rail, fuel filter bodies, connection systems, fastening systems, and on-board vapor recovery systems. Nylon 12 is one of the leading resin candidates for most of these applications.

PHOTO : FIGURE 1. Nylon 12 extrusion grades, flexural modulus versus extraction content.

PHOTO : FIGURE 2. Utilization factor of nylon 12 tubing as a function of temperature.

PHOTO : FIGURE 3. Moisture absorption of nylon resins as a function of time.

PHOTO : FIGURE 4. Sour fuel resistance of nylon 12 compared with standard NBR.

PHOTO : FIGURE 5. Change in length of different nylon tubing in fuel containing 15% methanol.

PHOTO : FIGURE 6. Change in length of nylon 12 tubing exposed to ethanol.

PHOTO : FIGURE 7 (right). Change in length of nylon 12 tubing exposed to fuel containing 15% methanol.

PHOTO : FIGURE 8. Change in length of nylon 12 tubing exposed to unleaded premium fuel.

PHOTO : FIGURE 9 (right). Change in length of modified and conventional nylon 12 tubing in unleaded premium fuel.

PHOTO : FIGURE 10. Change in length of modified and conventional nylon 12 tubing in fuel containing 15% methanol.
COPYRIGHT 1989 Society of Plastics Engineers, Inc.
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Gray, Edward K.; Hopf, Gerhard
Publication:Plastics Engineering
Date:Dec 1, 1989
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