Fuel Resistant Plastics
Mike Braeckel, Dwight Smith, Joseph G. Tajar and John Yourtee
Celanese
8040 Dixie Highway
Florence, KY 41042 USA
(Excerpts appeared in Advanced Materials & Processes - August 2000 - pages 37ff )
Limits on emissions and the need to simplify production
have led manufacturers to replace steel with plastic in
many fuel components, from gas caps to fuel rails.
This fuel module is made of Celanese Celcon® acetal
copolymer because of its superior performance in
aggressive fuels.
Without plastics, automakers would have been hard
pressed to develop fuel systems that simultaneously
withstand aggressive fuels, reduce vehicle weight, aid
impact resistance, and enable complex geometries.
Furthermore, given the complexity of fuel systems,
changes in one area often affect other areas. This is the
case when hot fuel from the engine compartment is
returned to the tank. Plastic was introduced for fuel tank
construction to improve chemical resistance and impact,
and to enable tighter auto layouts versus steel tank
designs. However, fuel temperatures rose sharply higher
in the plastic tanks, because plastic is a good insulator,
and because tighter layouts reduce the air flow that
dissipates heat. As a result, fuel temperatures in tanks may
reach 65°C (150°F) in plastic tanks, versus about 40°C
(105°F) in steel tanks, and can rise to 120°C (250°F) or
more in the engine compartment. The higher temperatures also make fuel more reactive and require more
stable plastics.
Environmental laws have also made fuel more reactive.
The Clean Air Act of 1990 called for reformulated gasoline,
which led to the addition of the oxygenate methyl tertiary
butyl ether (MTBE). Because this chemical can cause
plastics to swell, automakers either adjusted tolerances to
compensate for expansion, or found MTBEstable plastic
grades. However, recent MTBE health and groundwater-contamination concerns make it likely MTBE may be
replaced by other oxygenated materials such as ethanol.
Unfortunately, many of these also are aggressive toward
plastics.
Environmental initiatives have affected plastics in autos in
other ways for the past few decades. For example, limits
on evaporative emissions that began in the 1970s, have
tightened since then until in the 1990s they drove
changes in polymers for fuel caps, valves, charcoal
canisters, seals, and other parts (Table 1 and 2). Such
limits continue to tighten. The California Clean Air Board’s
Low Emission Vehicle II (LEV II) standard, scheduled for
2003, will reduce vehicle evaporative emissions from 2
g/day to 0.5 g/day, and make dimensional stability and
permeability more crucial issues. In Europe, EURO 2000,
which will take effect in 2005, also will severely limit
evaporative emissions and highlight similar issues.
Table 1 • Properties of Fuel System Plastics
Material
Property
Acetal
Dimensional stability, chemical
resistance, and low fuel permeability
Nylon 6/6
Good impact and other mechanical
properties
High temperature
nylon (HTN)
Easier processing and better dimensional
stability, chemical resistance and impact
than nylon 6/6, but not as good as PPS
Nylon 12
Extruded multilayer parts to provide
barrier properties, impact resistance and
low permeability
Aliphatic polyketone
Impact and thermal resistances that fall
between acetal and nylon
Polyphenylene
sulfide
Excellent high temperature and
chemical resistance, as well as excellent
dimensional stability, erosion resistance
and low permeability
High density
polyethylene (HDPE)
Good economics, as well as impact
resistance and chemical resistance to
road salts
Polybutylene
terephthalate (PBT)
Dimensional stability, but should be
used where temperature resistance and
permeability are not important
Page | 1
Fuel Resistant Plastics (continued)
Table 2 • Typical Material Applications
Component
Polymers
Fuel rails
PPS*, nylon 6/6* and HTN*
Fuel lines
Nylon 12
Fuel tanks
HDPE*
Canisters
Nylon 6/6, PBT*
Quick connects
Acetal, nylon 6/6, PPS
Fuel filter housings
Acetal, nylon 6/6
Fuel pump components
Acetal, PPS, and PBT
Fuel sending unit flange
Acetal, PBT
Fuel caps
Acetal, nylon 6/6
Inlet housing (fuel cap receiver)
Acetal
Fuel filler pipes
Nylon 6/6, HDPE
Throttle bodies
PPS, HTN, PBT
Valves: Fuel rollover valves
Acetal, PBT
Fuel fill limit valves
Acetal, PPS
ORVR valves
Acetal, PBT
Inlet check valve filler pipe
Acetal
Properties Needed For Fuel Applications
Plastics in fuel systems must perform at a consistently
high level under demanding conditions for the life of a
car. Several chemical, physical, mechanical and thermal
properties are important for plastics to survive long-term
fuel contact.
Chemical resistance relates to a change in chemical
structure or composition, which generally causes a
loss in performance.
Dimensional stability is an essential property because
many fuel-system parts have tight tolerances. This
factor is also important where different plastics or a
plastic and a metal meet, since their different dimensional stabilities can affect seals.
Mechanical properties include impact, tensile, and
compressive strength, as well as elongation and
strength at break. Parts must be designed to retain
sufficient integrity to do their jobs over the life of a
vehicle.
Coefficient of thermal expansion is one of a number of
temperature-related effects that include chemical
reaction rates and mechanical property changes.
Permeability is now under intensive study in light of
the LEV II standards. The industry is evolving standard
tests and measurements.
Extended Auto Fuel Study
As auto design life reaches 150,000 mi. or 15 years, it is
essential to know how fuel system plastics withstand
prolonged exposure to the new generation of morereac-
Page | 2
tive fuels. Unfortunately, tests on plastics for fuel contact
are typically limited: some are run for just 48 hours, and
others rarely exceed 500 hours (three weeks).
Recognizing the need for more extensive design verification, Celanese tested combinations of nine fuel blends
and seven plastics at two temperatures (65° and 121°C)
for more than 5000 hours each (Table 3). The lower
temperature simulated fuel tank conditions, and the
higher one simulated conditions under the hood. Initial
results from this program, which lasted two years in total,
support the continued use of most plastics now found in
auto fuel applications. The study also indicated that one
material (acetal homopolymer) might not be suitable for
extended life with some aggressive fuels.
Table 3 • Celanese Extended Fuel Study
Test Criteria
Independent Test Lab
EG&G Automotive Research
(now Perkin-Elmer Automotive
Research)
Test protocol
SAE J1748
Temperature
65°C and 121°C
Time
5000 hours
Factors measured
Dimensional stability, Weight
change, Tensile strength and
elongation, Tensile modulus, ISO
notched Charpy
Plastics Tested
65°C Test
121°C Test
Acetal copolymer
Polyphenylene sulfide
Acetal homopolymer
Nylon 6/6
Polybutylene terephthalate
High temperature nylons (two
tested)
Aliphatic polyketone
Fuels tested (65°C)
CMO
Fuel C (50% isooctane and toluene)
CAP
Fuel C + aggressive water + peroxide (sour gas)
CM15A
85% Fuel C + 15% methanol + aggressive water
CM25A
75% Fuel C + 25% methanol + aggressive water
CM85A
15% Fuel C + 85% methanol + aggressive water
CE22A
78% Fuel C + 22% ethanol + aggressive water
CE85A
15% Fuel C + 85% ethanol + aggressive water
TF1
GM TF1 (equivalent to IE10)
TF2
GM TF2 (equivalent to IM5E2)
Fuels Tested (121°C)*
CM15A
85% Fuel C + 15% methanol + aggressive water
CE22A
78% Fuel C + 22% ethanol + aggressive water
CE85A
15% Fuel C + 85% ethanol + aggressive water
TF1
GM TF1 (equivalent to IE10)
* Fuels recipes match those in SAE J1681.
Fuel Resistant Plastics (continued)
The wide range of methanol and ethanol concentrations
evaluated reflects the practice by some gasoline distributors of cutting gasoline with less-expensive alcohol. The
effects of this can vary. For example, fuel blends with
methanol are most aggressive in the 20% range.
Although the study began well before the July 1999
release date for SAE J1681, which sets standard fuels for
screening plastics and elastomers, it anticipated many of
the recipes contained in J1681. The study also followed
SAE J1748 protocols for testing and evaluating components in fuel tests, and evaluated mechanical and physical
properties. Dimensional stability results showed that the
resins tested generally swelled by about 1% to 3% during
the first seven to 21 days of exposure, and changed
relatively little after that.
One exception to this pattern was acetal homopolymer
(Figure 1), which swelled during the first seven days of
exposure to CM15A (85% Fuel C and 15% aggressive
methanol), held almost steady for the next 35 days, and
then shrank steadily over the next 182 days until the
study ended. The slope of the curve implies that shrinkage would continue unabated after this. The weight
change data (Figure 2) followed the same pattern,
suggesting that the fuel reacted with the homopolymer
and altered its composition. The study also found that
acetal copolymer had significantly less swelling and
weight change than aliphatic polyketone in oxygenated
and peroxidecontaining fuels.
3.0
Dimensional Change (%)
The study also looked at many current and potential
future fuel blends. Nine fuels were tested: three methanol
blends; three ethanol blends, including TF1; TF2, an
ethanol/ methanol blend; and C and CAP fuels with
aggressive water (i.e., water that contains highly reactive
ions such as chloride) and peroxide. (Fuel that contains
peroxide, a refining impurity that makes fuel more
reactive, is often called sour gas.)
Figure 1 • Dimensional Change During Exposure to
CM15A Fuel at 65°C
Polyketone
2.5
Acetal homopolymer
2.0
Celcon acetal copolymer
1.5
1.0
0.5
0.0
0
1000
2000
3000
4000
5000
6000
Time (Hours)
Figure 2 • Weight Change During Exposure to CM15A
Fuel at 65°C
6
Polyketone
5
Weight Change (%)
Celanese began the study by asking its auto OEM and Tier
I fuel system customers what fuels and plastics they
wanted to see studied. They chose seven plastics: acetal
copolymer, acetal homopolymer, PPS, PBT, aliphatic
polyketone, nylon 6/6, and high-temperature nylon (HTN).
4
Celcon acetal copolymer
3
2
1
0
Acetal homopolymer
-1
-2
0
1000
2000
3000
4000
5000
6000
Time (Hours)
PPS had the best overall performance of the polymers
studied in the 121°C (250°F) high temperature soaking
test. It exhibited the lowest weight gain, the least dimensional change, and the highest retention of tensile
strength over the range of fuels evaluated, with the
differences accentuated with the more aggressive fuel
blends. For example, in fuel CM15A, weight change
(Figure 3) and dimensional change (Figure 4) were
significantly lower for the PPS than for the HTNs and
nylon 66. All the nylons showed continual reduction in
retained tensile strength with increased exposure time,
whereas the PPS remained much more consistent (Figure
5).
Page | 3
Fuel Resistant Plastics (continued)
Figure 3 • Weight Change in CM15A at 121°C
Weight (% of Original)
109
25% Glass Reinforced Nylon 6/6
107
35% Glass Reinforced HTN
105
45% Glass Reinforced PPA
103
40% Glass Reinforced PPS
101
99
0
1000
2000
3000
4000
5000
Time (Hours)
Figure 4 • Length Change in CM15A at 121°C
102
25% Glass Reinforced Nylon 6/6
Length (% of Original)
45% Glass Reinforced PPA
101
35% Glass Reinforced HTN
99
0
1000
2000
3000
4000
5000
Time (Hours)
Figure 5 • Tensile Strength Change in CM15A at 121°C
100
Tensile Strength (% of Original)
Evaluating new fuel formulations will clearly be an
ongoing task as fuel oxygenate content changes. MTBE is
due to be phased out by 2003 and replaced by other
blending components, possibly TAME (tertiary amyl
methyl ether), ETBE (ethyl tertiary butyl ether), and/or
ethanol. Similar issues also apply to trucks. A commitment
to bio-diesel fuels in Europe has led to the introduction of
rapeseed oil (a.k.a., canola oil), commonly called “rape
seed methyl ester,” or RSME. This makes fuel more acidic
and aggressive to plastics at high temperatures.
40% Glass Reinforced PPS
100
98
40% Glass Reinforced PPS
80
25% Glass Reinforced Nylon 6/6
60
Plastic manufacturers are improving their products to
cope with the new fuels. They are lowering permeability
while enhancing dimensional stability, chemical
resistance, thermal capability and more. A good example
of this is Celanese Hostaform C13031 XF acetal copolymer, which is designed to be stable during prolonged
exposure to RSME-containing diesel fuels at relatively
high system temperatures.
In meeting future auto fuel system needs, especially on a
global basis, plastic suppliers must account for a wide
variety of more aggressive fuels, longer vehicle life and
higher temperatures. This will mean higher-performing
plastics able to cope with any fuel under any set of
conditions.
35% Glass Reinforced HTN
40
45% Glass Reinforced PPA
20
0
0
1000
2000
3000
4000
5000
Time (Hours)
The study indicates that no quick answers are available in
understanding how fuels and plastics interact, and that
Page | 4
assumptions about these interactions should be supported by long-term testing. This is especially important for
dimensional stability, because so many of today’s
high-tech emission-control parts depend on tolerances of
0.05 to 0.076 mm (0.002 to 0.003 in.) Even modest
changes in dimension can create problems. For example,
a part 12.7 mm (0.5 in.) in diameter that swells just 2%
would grow 0.25 mm (0.01 in.) and may exceed tolerances
set for it. Swelling also can affect permeability and create
microleaks.
Existing resins can be reformulated to enhance their
properties. Or automakers can move up the performance
chain, switching from acetal copolymer to PPS to more
than double thermal capabilities. In terms of this example,
acetal has a long history in fuel system applications for
emission control valves mounted on or within fuel tanks
where temperatures are moderate. At higher underhood
temperatures, PPS would be a more appropriate valve
material. Other options are new grades of plastics with
advanced performance characteristics.
ENGINEERED MATERIALS
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Contact Information
• Celanex® thermoplastic polyester (PBT)
Americas
8040 Dixie Highway, Florence, KY 41042 USA
• Hostaform® and Celcon® acetal copolymer (POM)
• Celstran, Compel and Factor long fiber
reinforced thermoplastic (LFRT)
®
®
®
• Celstran® continuous fiber reinforced
thermoplastic (CFR-TP)
• Fortron® polyphenylene sulfide (PPS)
• GUR® ultra-high molecular
weight polyethylene (UHMW-PE)
• Impet® thermoplastic polyester (PET)
• Riteflex® thermoplastic polyester elastomer (TPC-ET)
• Thermx® polycyclohexylene-dimethylene
terephthalate (PCT)
• Vandar® thermoplastic polyester alloy (PBT)
• Vectra® and Zenite® liquid crystal polymer (LCP)
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This publication was printed on 19 September 2013 based on Celanese’s present state of knowledge, and Celanese undertakes no
obligation to update it. Because conditions of product use are outside Celanese’s control, Celanese makes no warranties, express or
implied, and assumes no liability in connection with any use of this information. Nothing herein is intended as a license to operate
under or a recommendation to infringe any patents.
CE-010_CelaneseFuelStudyWP_AM_0913