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Fuel permeation rates after changing fuel.

The fuel permeation rates of elastomers and plastics

continues to be a widely discussed topic in automotive engineering

circles. Two events resulting from the 1991 Clean Air Act

are keeping this subject at the forefront of engineers' and

materials scientists' minds. First, new enhanced evaporative

emissions limits are in the middle of their phase which

started in 1995 and will be 100% complete by 1999. These new

limits require that no vehicle emit more than 2.0 grams of

hydrocarbons in a diurnal test run by either the

Environmental Protection Agency (EPA) or the California Air Resources

Board (CARB) method. The EPA method is a two day

diurnal test cycled from 22 [degrees] C to 36 [degrees] C to 22 [degrees] C,

whereas the

CARB diurnal test is cycled from 18 [degrees] C to 40 [degrees] C to 18

[degrees] C.

Second, in addition to this "enhanced evap" requirement in

1998, the phase in for on board refueling vapor recovery

(ORVR) starts at a 40% level for passenger cars in 1998, and

by 2003 all cars and light trucks will be equipped with this

system. ORVR may require larger charcoal canisters plus

considerable new vapor-vent lines, all of which will add

additional sources for hydrocarbon emissions. In spite of this

enlarged vapor recovery equipment, the 2.0 gram

hydrocarbon evaporative per EPA and CARB methods must be

maintained both as the vehicle is built and after 100,000 miles

(160,000 kilometers) of service.

The requirement that the vehicle maintain its less than 2.0

gram hydrocarbon evaporative emission rate after 100M

miles of service leads to some interesting questions. The

permeation rates of elastomers and plastics to various fuels has

been the subject of previous work, including work done by

MacLachlan (ref. 1) in 1979, Goldsberry, Chilous and Will

(ref. 2) in 1991, Stahl and Steven (ref. 4) in 1992, Hopf, Reis

and Gray in 1994 (ref. 5) and Fuller and Stevens (ref. 6) in

1996. The fuel permeation rates of elastomers such as nitrile,

epichlorohydrin and fluoroelastomers of various fluorine

contents, as well as plastics such as nylon 12 and

fluoroplastics, are well documented in these works. However, one real

world element of fuel permeation that has not been discussed

widely is the situation that will be encountered after up to

70,000 miles when the EPA and CARB tests could be rerun.

In this "in use" case, it is quite possible, even probable, that a

vehicle will have been running for an extended time on a

"reformulated" or "oxygenated" gasoline. These gasolines

can contain alcohols and ethers in low percentages such as

10-15% methyl-t-butyl ether (MTBE), 10% ethanol, and

even blends of 5% methanol with 2.5% of a cosolvent such

as ethanol.

In the fuel permeation work referenced above, the

permeation rates were measured on one type of fuel. The potential

of switching or changing of the fuel part way through the test

has not been published. That changing or switching of the

fuel and the resulting patterns of fuel permeation behavior is

the focus of this article.

Experimental

Permeation testing was conducted by two weight loss

methods:

* Thwing Albert Cup Method -- ASTM E96-66

* Fuel hose testing -- modified GM9061 P "fill and plug

method."

It was desired to see if these two permeation test methods

would show similar trends when similar test specimens were

used. The Thwing Albert Cup method is well documented in

previous SAE work (refs. 1, 3 and 4). In this method, a cup is

filled with 100 ml of fuel, then a 76.2 mm diaphragm is

placed on top and secured metal flange ring. The exposed

surface area is 37 [cm.sup.2].

The fuel hose permeation testing was done by a modified

GM9061 P method. A longer length of hose was used for this

testing; in this case, a 91.4 cm length of hose having a

nominal ID of 7.9 mm. The hose was filled 75% by volume,

which was approximately 32 ml of fuel.

Both cup and hose testing were done in an oven @ 40 [degrees] C

to increase the severity of the test.

The materials tested included nitrile (NBR),

fluoroelastomers (FKM). and fluoroelastomer/fluoroplastic barrier

systems and are shown in table 1. The fluoroplastic barrier is

noted as FEP and had a thickness of 0.05 mm. The FKM cup

specimens were 0-76 mm thick, which is typical of the

thickness of FKM in FKM veneered fuel hose.

Table 1-cup and hose testing materials

Cup testing

1. NBR - 41% ACN

2. FKM - 66% fluorine

3. FKM - 68% fluorine

4. FKM - 70% fluorine

5. FKM/FEP laminate

Hose testing

1. FKM (66%F)/CSM/CSM(*)

2. FKM (68%F)/NBR/ECO(*)

3. FKM (70%F)/FEP/NBR/ECO(*)

4. FKM (70%F)/FEP/EAM/EAM(**)

5. FKM (68%F)/FEP/EAM/EAM(**)

(*) -- commercially made hose

(**) -- hose made on DuPont Dow prototype extrusion line

When evaluating via the cup or hose method, testing was

done for 336 hours at 40 [degrees] C. Weight losses were noted within

the first 24 hours, then every two to three days thereafter.

After 336 hours, the test specimens were broken down, fresh

fuel was placed inside, and the specimens returned for

another 336 hours at 40 [degrees] C. The testing sequence used was:

336 hrs. @ 40 [degrees] C [right arrow] Refill [right arrow] 336 hrs. @ 40

[degrees] C Fuel C [right arrow] Fuel C (control)

CE-10 [right arrow] Fuel C

CMTBE-15 [right arrow] Fuel C

The test sequence was designed to simulate an

oxygenated fuel such as 90/10 Fuel C/ethanol (CE-10), or 85/15 Fuel

C/MTBE (CMTBE-15) being used in a vehicle, then the

vehicle being refilled with a non-oxygenated fuel. The

question was, what would happen to the permeation rates of the

specimens that had been conditioned in the oxygenated fuel,

then switched to 100% Fuel C?

Results

The results of the tests are shown in figures 1 through 10.

The first five figures are for the cup testing, and figures 6

through 10 show the hose results.

The cup results with 100% Fuel C and CE-10 are shown

in figure 1.41% ACN nitriles are still used in filler neck

hose, so it was desired to document their performance. The

results show that at 40 [degrees] C, the 41% ACN-NBR had a peak

permeation rate of ~400 g/[m.sup.2]/day in 90% Fuel C/10%

ethanol. The results show a drop off in permeation rate after

48 hours. This drop, especially the drop in the oxygenated

fuel, may be in part attributed to the "speciating" of the fuel.

By speciating, it is meant that the fuel permeates as separate

species. In this example, Fuel C has isooctane, toluene and in

the case of E10, ethanol. Once the fuel composition changes

and becomes less aggressive (less ethanol and toluene), the

permeation rate drops. Another concern here is that the total

volume of the fuel is only 100 ml. and a large weight loss is

occurring during this 336 hours.

[FIGURE 1, GRAPH OMITTED]

The 41% ACN-NBR was not run in the second phase of

testing. The 76 mm diaphragms were swelled enough so that

they could not be reassembled without damaging the

diaphragm and affecting the seal of the test specimen.

Testing was discontinued on the NBR at this point.

Figure 2 looks at a FKM dipolymer of 66% fluorine in a

fuel hose compound that is not postcured. The results for all

three fuels is shown: Fuel C, CE10 and CMTBE-15. The

permeation rate of the Fuel C at ~10/[m.sup.2]/day and the

CMTBE-15 at ~50g/[m.sup.2]/day seem stable at 336 hours at 40 [degrees] C, but

the

permeation rate of the CE-10 peaks at 120 hours and drops.

Fuel speciating may be occurring here too.

[FIGURE 2, GRAPH OMITTED]

After 336 hours, the fuel in all three cups was changed to

Fuel C. It is interesting to note that the two FKM diaphragms

exposed to oxygenated fuels CE-10 and CMTBE-15

continue to show a higher permeation rate than the Fuel C control,

but both do drop in permeation rate. The diaphragm exposed

to CE-10 seems to move toward the Fuel C control faster

than does the CMTBE-15 diaphragm. This is curious because

the CE-10 showed a higher permeation rate than CMTBE-15

in oxygenated fuel.

Figure 3, FKM (68% F). looks at a 68% fluorine FKM in

the same three fuels. As expected, the 68% fluorine FKM

shows an overall lower permeation rate than does the 66%

fluorine FKM with permeation rates of ~11 g/[m.sup.2]/day in Fuel

C, 50 g/[m.sup.2]/day in CE-10 and 34 g/[m.sup.2]/day in CMTBE-15

after 336 hours at 40 [degrees] C. The same drop in CE-10 permeation

rate is seen after 120 hours as was seen in figure 2, so fuel

speciating may be happening.

[FIGURE 3, GRAPH OMITTED]

After 336 hours, when all the cups are refilled with Fuel

C, a similar trend as is seen in figure 2 is noted. The CE-10

and CMTBE- 15 continue permeation at a higher rate than the

Fuel C, and it takes longer for the CMTBE-15 conditioned

diaphragm to return close to the Fuel C control than it does

for the CE-10 conditioned diaphragm.

Figure 4, FKM (70%F), charts the permeation rate of the

three fuels through a 70% fluorine FKM. As expected, the

70% fluorine FKM demonstrates an overall lower permeation

rate profile than does the 68% fluorine FKM in the first

336 hours of the test. After 336 hours at 40'C, permeation

rates of 6 g/[m.sup.2]/day for Fuel C, 25 g/[m.sup.2]/day for CE- 10 and 19

g/[m.sup.2]/day for CMTBE-15 are seen with the 70% fluorine

FKM.

[FIGURE 4, GRAPH OMITTED]

When the fuel in the cups is changed to 100% Fuel C in

the second 336 hour phase of the test, a similar pattern as was

seen in figures 2 and 3 is seen here in figure 4. Both the

CE-10 and 10 the CMTBE-15 conditioned diaphragms continue to

permeate at a higher rate than does the Fuel C control.

Gradually, the permeation rates of the CE-10 and

CMTBE-15 conditioned diaphragms come close to the Fuel C control

after 552 hours, but the CMTBE-15 conditioned diaphragm

takes longer to recover to the Fuel C control.

Figure 5 shows the permeation rate of the three fuels to a

laminate of 70% fluorine FKM with 0.05 mm FEP

fluoroplastic film.

[FIGURE 5, GRAPH OMITTED]

The results show a different permeation profile than is

seen with any of the fluoroelastomers. The permeation rates

were quite low, below 3 g/[m.sup.2]/day, under any conditions, and

different fuels or fuel changes did not to seem to significantly

affect the fuel permeation rate of these laminates.

A summary of the average permeation rates as determined

by the cup method is shown in figure 6. These data look at

the original permeation rate in Fuel C, excluding data before

48 hours to allow the cup to come to equilibrium, and

compares this permeation rate to the cups where the rubber

diaphragm was conditioned 336 hours @ 40 [degrees] C in Fuel C.

CE-10 and CMTBE-15. All perm testing on figure 6 was run

in 100% Fuel C, but the diaphragms that were preconditioned

in other fuels, especially the ones seeing oxygenated fuel,

showed higher average permeation rates. In this case, the

CMTBE-15 fuel seemed to have a greater effect than the

CE-10 fuel.

[FIGURE 6, GRAPH OMITTED]

Figure 7 shows the phase of testing that used hose tested

by the modified GM9061P method. The hose tested in figure

7 is based on a 66% fluorine FKM (~0.7 mm veneer) with a

CSM interlayer, a fabric braid and a EAM cover. The

oxygenated fuels CE-10 and CMTBE-15 seem to come to a peak

permeation rate faster than does the Fuel C control with max

permeation rates of 18 g/[m.sup.2]/day for Fuel C, 50 g/[m.sup.2]/day for

CE-10 and 25 /[m.sup.2]/day for the CMTBE-15. Fuel speciating

giving lower than expected results may once again be

occurring in this test, especially in the CE-10 fuel.

[FIGURE 7, GRAPH OMITTED]

After 336 hours, the fuel in all the hoses was changed to

100% Fuel C. A similar trend, as was noted in figure 2 (FKM

66%F diaphragm), is noted with the CE-10 and CMTBE-15

conditioned hose permeating at a higher rate than the Fuel C

control, although the magnitude of the difference is not as

great. In this case, the CE-10 conditioned hose does not

closely approach the permeation rate of the Fuel C control

until 672 hours into the test.

Figure 8 shows the permeation data for the hose based on

68% fluorine FKM (~0.7 mm veneer), with a NBR

interlayer, fabric braid and a ECO cover. The peak permeation rate

after 336 hours at 40 [degrees] C was 16 g/[m.sup.2]/day for Fuel C, 40

g/[m.sup.2]/day for CE-10 and 22 /[m.sup.2/day for CMTBE-15. Fuel

speciating in the CE-10 fuel once again is a concern based on

the shape of the curve for this fuel. One other observation is,

that while this 68% fluorine FKM veneer hose does show

slightly lower permeation rates than is seen with the 66%

fluorine FKM veneered hose in figure 7, the magnitude of the

difference is not as great as is seen between the 66%F and

68%F FKM diaphragms in figures 2 and 3. It is speculated

that the differences in the interlayer and cover compounds on

the hose may be having an effect here. It was noted by

Edmonson and Balzer (ref. 3), that hoses made with a CSM

interlayer and cover had lower fuel-methanol permeation

than did hoses made with a ECO interlayer and cover when

the same FKM veneer was used. We may be seeing a similar

trend here with ethanol and MTBE blended fuels rather than

methanol blends.

[FIGURE 8, GRAPH OMITTED]

After 336 hours, all the hoses were refilled with 100%

Fuel C, and the results show that the CE-10 and CMTBE-15

conditioned hoses continued to permeate at a higher rate than

the Fuel C control, but in this case, the CE-10 conditioned

hose did recover toward the Fuel C control faster, reaching

close to the Fuel C control permeation rate by 528 hours.

Figure 9 looks at a barrier hose made of 70@/( fluorine

FKM/FEP barrier/NBR interlayer/fabric braid/ECO cover. A

very different pattern is seen here compared to the hoses in

figures 7 and 8. The permeation rates are quite low and the

data seem to have no distinct peak. Based on the data curve.

it seems that fuel speciating is not so much of an issue here.

The permeation rates after 336 hours at 40'C of this hose is

1.5 g/[m.sup.2]/day in Fuel C. 2.0 g/[m.sup.2]/day in CE-10 and 1.6

g/[m.sup.2]/day in CMTBE-15.

[FIGURE 9, GRAPH OMITTED]

After 336 hours, the fuel in these hoses was changed to

100% Fuel C. In this case, no change in permeation rate for

any of the conditioned hoses was seen. The permeation rate

was still quite low with a rate varying between 1.5 to 3.5

g/[m.sup.2]/day throughout the entire 672 hour test.

Figure 10 looks at a hose made in a prototyping line. The

hose has a 0.4 mm veneer of 70% fluorine FKM, a 0.05 mm

FEP barrier, an interlayer and cover of 150 [degrees] C heat resistant

EAM and an aramid fiber braid. The results of the first 336

hours of permeation testing look quite similar to what was

seen on figure 9; the permeation rates are very low and no

distinct peaks in permeation are seen. After 336 hours, the

permeation rate for this hose was 1.6 g/[m.sup.2]/day in Fuel C, 1.9

g/[m.sup.2/day in CE-10 and 1.4 g/[m.sup.2]/day in CMTBE-15.

[FIGURE 10, GRAPH OMITTED]

After changing out to 100% Fuel C in all the hoses, no

significant change in permeation rate was seen through 672

hours at 40 [degrees] C.

In figure 11, data are seen on another prototype hose

made by DuPont Dow Elastomers. This hose uses exactly the

same construction as the hose in figure 10, except the 0.4 mm

FKM veneer is based on a 68% fluorine FKM. The

permeation rates are quite similar to those seen in figures 9 and 10,

with results for this hose of 2.0 g/[m.sup.2]/day in Fuel C, 3.0

g/[m.sup.2]/day in CE-10 and 1.8 g/[m.sup.2]/day in CMTBE-15 after the

336 hours at 40 [degrees] C phase of the test was completed.

[FIGURE 11, GRAPH OMITTED]

Once again, all the hoses were drained and refilled with

100% Fuel C. The permeation rates remained quite low and

did not vary much through the entire 672 hours of the test.

Figure 12 shows a summary of all hose average

permeation data much as figure 6 did for the cup data. This chart

looks at the hoses that were originally exposed to Fuel C

(data after 48 hrs. up to 336 hrs.) and compares them to the

hoses that were conditioned in Fuel C, CE-10 and

CMTBE-15, then rerun in 100% Fuel C. The results here are

interesting in that in this case the hose preconditioned in CE-10

seems to permeate at the highest rate with the CMTBE-15

having a lesser effect. This is different than figure 6 where

the CMTBE-15 had the greater effect. This difference

between hose and diaphragm may be due to the outerlayers

of the hose being a better barrier to CMBTE-15 than CE-10.

This hose effect was discussed in figure 8, and was noted by

Edmonson and Balzer (ref. 3).

[FIGURE 12, GRAPH OMITTED]

The trend noted in figure 12 supports what was seen in

figure 6 in that the hoses preconditioned with oxygenated

fuel show higher Fuel C permeation rates than hoses conditioned

with Fuel C only. These data support the theory that

just because the fuel handling components on a car pass an

initial SHED hydrocarbons emission test in a

non-oxygenated test fuel, the same components may not pass after

extended field service in oxygenated fuel in spite of the fact that the

test on the used vehicle will be run by draining the

oxygenated fuel out of the car and refueling with the non-oxygenated

test fuel. An extra degree of permeation resistance robustness

may be needed to insure SHED compliance after 160,000

kilometers of field service.

Conclusions

Based on the cup permeation data and the hose permeation

data, the following conclusions can be made:

1. When a FKM diaphragm, simulating a veneer in fuel

hose, having fluorine contents of 66, 68 and 70% is

conditioned in oxygenated fuel, then switched to a non-oxygenated

fuel like Fuel C, it:

* Permeates like the oxygenated fuel is still in the hose

for 48-72 hours.

* Gradually comes back toward the non-oxygenated

fuel permeation rate. This may take 336 hours or more to

complete.

* CMTBE-15 preconditioned FKM maintains a higher

permeation rate after being switched back to Fuel C than do

E10 conditioned specimens.

2. Hose data indicate that preconditioning in oxygenated

fuels, then switching back to Fuel C, does increase

permeation rate as compared to hose which has only seen Fuel C,

but in this case the CE-10 seems to have a larger effect than

the CMTBE-15.

3. Barrier hoses and laminates using the FKM/FEP

fluoroplastic design:

* Show significantly lower permeation rates than 70%

fluorine FKM, and regular FKM veneered hose.

* Are not significantly affected in permeation rate by

preconditioning in oxygenated fuel then switching back to

regular Fuel C.

It is further noted that fuel speciating may be occurring in

these types of "set volume of fuel" permeation tests even

with low permeating materials such as fluoroelastomers.

These phenomena had not been seen in fluoroelastomers

tested at 23 [degrees] C in previous work, but the test condition of 40

[degrees] C is

significantly more rigorous.

Further studies on changing fuel with a sufficient volume

of fuel available would be recommended. The new

recirculating fuel test method J 1737 would be an excellent way to

accomplish this.

References

[1.] J.D. Maclachlan -- SAE Paper 790657, "Automotive fuel

permeation resistance -- a comparison of elastomeric

materials, June 11-15, 1979.

[2.] Goldsberry, Chillous and Will -- SAE Paper 910104,

"Fluoropolymer resins: Permeation of automotive fuels,"

Feb. 25-March 1, 1991.

[4.] W.M. Stahl and RD. Stevens -- SAE Paper 920163,

"Fuel-alcohol permeation rates of fluoroelastomers, fluoroplastics

and other fuel resistant materials," February 24-28, 1992.

[5.] Hopf, Reis and E. Gray, SAE Paper 940165,

"Development of multilayered thermoplastic fuel lines with

improved barrier properties," Feb. 28 -- March 3,1994.

[6.] R.E. Fuller and R.D. Stevens -- SAE Paper 960140,

"Unique low permeation elastomeric laminates for fuel

hose," February 26-29, 1996.

[7.] D. Kulp, M. Masten and M. Harrigan -- Detroit Rubber

Group Paper, Oct. 1995, Ford Report On Fuel Systems

Overview.
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Author:Fuller, Robert E.
Publication:Rubber World
Date:Feb 1, 1998
Words:3573
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