FKMs for extrusion of thin wall veneers and tubes for fuel hose applications.

Fluoroelastomers are used in automotive and small-engine fuel system seals and delivery components due to their superior resistance to aggressive fuel blends and heat. Fluoroelastomers also provide an effective barrier to permeation, while remaining flexible over a wide temperature range. Consequently, many new fuel hose and tubing applications rely on fluoroelastomer inner layers to meet the increasingly more stringent permeation regulations. DuPont Performance Elastomers has developed fluoroelastomers designed specifically for extrusion of thin walled fuel hose veneer. These extrusion polymers provide thin wall extrudate with excellent extrusion performance and surface appearance, while providing barrier properties capable of meeting automotive permeation specifications. This article describes the processing performance and end use properties of these fluoroelastomers for fuel hose applications.

For many years, automobiles around the world have been required to comply with stringent evaporative emission regulations, and these regulations are now being extended to other devices such as marine pleasure craft, small off road vehicles, garden equipment and other stationary internal combustion engines. The ultimate function of any fuel system is the storage and delivery of fuel from the tank to the engine combustion chamber. The twin objectives of high engine efficiency coupled with minimal environmental impact necessitate very complex fuel systems that employ many components and subsystems. A schematic of an automobile fuel system is shown in figure 1.

[FIGURE 1 OMITTED]

Critical to achieving these objectives are the integrity and durability of the multitude of elastomeric sealing and containment devices, such as o-rings, seals and the tubing and hoses that connect the various components of the fuel system. Fluoroelastomers have an ideal performance profile to meet these needs. They are specified for their excellent physical properties, compression set and heat resistance, as well as their high degree of fuel and additive resistance, providing a highly effective barrier to fuel permeation.

Elastomeric fuel hose and tube constructions

Fuel hose is differentiated from tubing by the simple fact that it contains a layer of reinforcement to allow performance at higher pressures, as found in the delivery side of automotive fuel systems. Fuel hose constructions typically contain multiple layers where the materials for each layer are selected to meet certain aspects of the overall performance requirement, generally defined by a set of original physical properties and some combination of fluid and heat resistance. Other requirements, like low temperature bending and burst pressure, are often defined for the finished article. Special test rigs are often used to subject finished hoses to a flexing motion, often including cyclic profiles of heat and pressure, sometimes in the presence of a test fuel, to provide an accelerated end use simulation.

Cost effectiveness is a very important consideration in the design of fuel line hoses, and designers will strive to optimize cost effectiveness without sacrificing any end use performance. Multilayer fuel hoses often comprise four layers, as follows:

* Inner barrier layer to perform the function of fuel containment;

* tie layer which must be capable of bonding the barrier layer to the outer layer;

* reinforcement layer which is usually based on polyamide, polyester or aramid fibers and provides resistance to internal pressure; and

* outer cover layer that must be resistant to heat, oil and fuel, as well as providing protection against mechanical damage.

The actual thickness of each layer varies with end user requirements, but typically the inner barrier layer is less than 1 mm thick, while the tie and cover layers are usually greater than 2 mm thick. The barrier properties of FKM lined multilayer hoses are defined largely by the type of FKM selected for the inner layer.

Figure 2 summarizes the permeation of 85% Reference Fuel C with 15% methanol (CM15) through several types of polymer. All three types of FKM provide at Least an order of magnitude improvement compared to the other elastomers.

Within the types of FKM, increased fluorine content directly relates to improved permeation resistance. FKM A, B and GF in the chart are typical 66%, 68.5% and 70% fluorine containing FKM types, respectively

Fluoroplastics like ETFE and FEP give the best permeation resistance, but they provide inferior sealing at the hose end coupling points compared to elastomers.

Figure 3 provides a view of a four-layered hose construction for specified permeation of [less than or equal to] 15 g/[m.sup.2]/day that uses an FKM inside barrier layer. The cover section provides much of the structural strength of the hose, while the barrier section fulfills the critical role of fuel containment by providing resistance to permeation, as well as a means of sealing the hose ends at the junction points. Tie and cover layers may be fabricated from any of a number of elastomer types, including chlorosulfonated polyethylene, ethylene acrylic elastomer, nitrile rubber, nitrile/PVC rubber or epichlorohydrin rubber, according to fabricator or end user preference.

[FIGURE 3 OMITTED]

The barrier section generally uses the most expensive materials, so a cost effective construction will use sufficient barrier material to meet the required performance and control permeation. FKM also provides superior chemical resistance. Resistance to oxidized or sour fuel is a key attribute of FKM that is not provided by some elastomers commonly used in fuel hose, such as epichlorohydrin, NBR and HNBR (refs. 1 and 2).

Figure 4 provides a view of a five-layered hose construction used to achieve specified permeation [less than or equal to]7 g/[m.sup.2]/day. This construction is designed for use where very low permeation is required. The construction utilizes the barrier properties of FER but retains the FKM inner veneer to provide more durable sealing and coupling than can be provided by most low permeation plastics.

[FIGURE 4 OMITTED]

F-200 low permeation barrier hose is proprietary technology (refs. 3 and 4) that has been discussed in more detail in another paper (ref. 5). Variations of these constructions can deliver a range of permeation performance, as well as other properties. One important consideration in the design of low permeation constructions with fluoroplastic barriers and elastomeric inner layers is end-permeation. While fluoroplastics provide effective barriers to fuel permeation through the hose wall, there remains a small conduit for fuel vapor migration through the inner veneer and out the cut ends of the hose, as shown in figure 5. This pathway can become significant when designing a construction to meet ultra low permeation specifications. End permeation effects can be minimized by proper selection of a low permeating veneer material such as FKM.

[FIGURE 5 OMITTED]

Properties related to hose manufacturing and performance

Fuel hoses are manufactured using extrusion processes and a production line will generally have several extruders arranged in sequence to produce the various layers of a multilayer construction. The FKM barrier is typically extruded so to be as thin as possible in order to optimize cost-effectiveness, but thick enough to still meet the performance requirements. In some cases, end user specifications dictate a minimum veneer thickness that is substantially thicker than a fabricator's extrusion capability for an FKM veneer. In other cases, a minimum thickness is not specified, and hose producers are free to use not-as-thick veneers, providing the end use or permeation characteristics of the hose are not compromised. Veneers less than 0.4 mm can make a cost-effective hose construction and can provide sufficient barrier performance in many applications.

In the context of thin veneer production, it is usual to extrude a tube having thicker walls than the target value and to draw this down to the desired thickness after exiting the die. The initial extrudate has to have a good surface finish and sufficient strength to be drawn or pulled down (stretched) to its target wall thickness without breaking. It also needs to retain the high quality surface finish during the pull-down operation. In practice, it is found to be much easier and quicker to set an extruder to obtain thin, concentric veneer by pulling down a thicker veneer than it is to try to adjust the extrusion tooling and dies for direct extrusion of a thin veneer. This also avoids the high extrusion pressures and die swells associated with die and tooling setups for thin veneers.

The polymer VTR-7551 was designed to provide robust performance in this demanding manufacturing process. A reference compound using VTR-7551 can be drawn down to give a wall thickness below 0.3 mm and retain a high quality surface. Curative containing precompounds of VTR-7551 have also been developed. The properties of cured compounds based on VTR-7551, and precompounds of this polymer, are given in the following sections.

Properties of VTR-7551 and precompounds VTR-9209 and VTR-9217

VTR-7551 is a copolymer of vinylidene fluoride (V[F.sub.2]), tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) having a nominal fluorine content of 69% and a nominal viscosity of 30 ML 1+10 @ 121[degrees]C, designed for extrusion of thin walled tubes such as fuel hose veneers. VTR-9209 is a curative-containing precompound of VTR-7551 having a nominal viscosity of 30 ML 1 + 10 @ 121 [degrees]C. VTR-9217 is a curative-containing precompound of VTR-7551 that is designed to provide lower extrusion head pressures and a lower vulcanizate hardness than VTR-9209. Typical compounds based on VTR-9209 and VTR-9217 are compared to a reference compound based on a curative-containing blend of commercial Viton B-202 and B-600 in table 1.

Fuel and permeation resistance

The permeation of fuel through a finished hose is influenced by many factors. In the case of FKM, the polymer fluorine level has the most obvious impact--the higher the fluorine level the better the permeation resistance, as indicated earlier in figure 1. The molecular structure can also play a role in the sense that narrow molecular weight distributions of high molecular weight polymers seem to provide better barrier properties. Compounding ingredients are also important and parts made from compounds containing plasticizers, oils and other volatile or extractable ingredients will tend to have inferior barrier properties.

VTR-7551 provides superior permeation resistance compared to many FKM B-type polymers, yet retains excellent extrudability due to improved polymer architecture. The precompound VTR-9209 provides a permeation barrier that is significantly superior to the control compound based on B-202/B-600. VTR-9217, which is designed for improved processing and lower hardness, does tend to permeate slightly more than VTR-9209 but provides improved performance compared to the reference compound. These trends are confirmed by conventional fuel aging tests. The fuel test data for VTR-9209 and VTR-9217 are compared to the B-202/B-600 based reference in table 2.

VTR-9209 and VTR-9217 have all of the requirements for high quality fuel hose veneer production. Compared to Viton B-202/B-600, hoses made using these new precompounds will have the necessary mechanical characteristics and adhesion to other elastomer substrates, but with improved fuel resistance and barrier properties, making it easier to meet the demands of automotive specifications.

Processing

VTR-7551 is designed for thin veneer extrusion, and this is where it excels compared to polymers like B-202 and B-600, which are designed for mold processing. Extrusion evaluations have been conducted at our laboratory in Stow, OH. Independent evaluations have also been conducted at the NFM Iddon factory in Leyland, U.K. Different equipment was used for the two extrusion trials, but the results were very similar. Both trials demonstrate the superior performance of the new extrusion polymer.

Extrusion trials

The trials were conducted using a Davis-Standard extruder equipped with a 10:1 1/d barrier screw.

Extruder set temperatures were:

Screw 65[degrees]C, Feed zone 50[degrees]C,

Barrel 85[degrees]C, Die head 95[degrees]C

A 9.9 mm pin and 10.3 mm die were used to produce a thin tube of 0.65 mm nominal wall, onto a flexible rubber mandrel of 9.5 mm nominal diameter.

The extruder was operated at a range of screw speeds and output was measured in kg/hr. Figure 6 shows the data collected and graphed for the three test compounds.

[FIGURE 6 OMITTED]

The compound recipes were like that given in table 1, except 1 phr carnauba wax was added, as is normal practice in the industry.

Performance in terms of output vs. screw speed shows VTR-9209 and VTR-9217 to be clearly superior to the B202/ B600 reference compound.

In many production lines, thin veneer extrusions are accomplished on less-than-optimal extrusion equipment. A typical scenario finds the veneer production "borrowing" an extruder from a line designed to produce much larger hoses in terms of diameter and wall thickness. Consequently, the over-sized extruder is turning at very low revolutions per minute when producing the much smaller volume veneer. In this case, a fabricator is less concerned with extruder rpm and resultant extrusion melt temperature, but more concerned with excessive head pressure.

A better way to view extrusion performance in terms of thin veneers is to plot extruder output as a function of breaker plate pressure, as in figure 7. There is some viscosity impact, since the reference B-202/B-600 precompound and VTR-9217 are around 20 MU (ML 1+10 @ 121[degrees]C), whereas VTR-9209 has a nominal viscosity of 30 MU.

[FIGURE 7 OMITTED]

However, one can observe that when output is expressed as a function of breaker plate pressure, the curve for the B202/B-600 based compound falls between those of VTR-9209 and VTR-9217. VTR-9217 was developed to provide reduced breaker plate pressures when extruding at high rates.

The key test of a dedicated extrusion polymer, however, is the ability to be pulled down to a target wall thickness while maintaining an acceptable surface finish. An extrusion trial was conducted at a constant screw speed (constant output) while the line speed was progressively increased. The veneer thickness was measured continuously and the veneer integrity and surface appearance monitored and noted throughout the trial. Figure 8 shows the results of this pull down test. Starting from an extruded tube wall thickness of roughly 0.65 mm, compounds based on B-202/B-600 could not be pulled down to below 0.4 mm. With the new polymer, veneer thicknesses of below 0.1 mm were feasible in this evaluation, while maintaining good surface finish.

[FIGURE 8 OMITTED]

Extrusion trials at NFM Iddon

These trials were conducted using compounds based on VTR-9209 and a reference compound based on a blend of Viton B-202 and B-600. The recipes are given in table 3. A tube die of nominal dimensions to make tube of 6 mm inner diameter and 1 mm wall thickness was used.

An NFM Iddon extruder equipped with a 50 mm diameter high intensity mixing (HIM) scroll having a 15:11/d was used for the evaluations. A schematic of the HIM scroll concept is given in figure 9.

[FIGURE 9 OMITTED]

The NFM Iddon HIM scroll is designed to provide optimum processing conditions for all types of natural and synthetic elastomer compounds (ref. 6.) The patented scroll design features a unique mixing and pumping action that ensures high quality extrudate and high output at lower extrudate temperature. Tests were also conducted using a standard screw design of the same 1/d ratio.

Extruder set temperatures were:

Screw           95[degrees]C,   Feed zone   75[degrees]C
Barrel zone 1   60[degrees]C,   Barrel zone 2 60[degrees]C
Die head        95[degrees]C

Trials were conducted at scroll speeds in the range 15 to 40 rpm. The extrusions were run using a pin die in the absence of a flexible mandrel. To assess pull down performance, the haul-off unit was initially set to run at the linear out rate of the extrudate, and then increased until the extrudate broke. The pull down ratio can be defined as the ratio of the haul off speed at break to the linear extrusion output rate. Pull down ratio is a good measure of the pull down performance of the test compounds, with a higher ratio more desirable. The results of these trials are given in figure 10.

[FIGURE 10 OMITTED]

For any given scroll speed, the compound based on VTR-9209 gave a higher output, as was the case for the trials conducted at the DPE Akron laboratory. Note also that the pull down ratio for VTR-9209 is considerably higher than for the B-202/B-600 reference, confirming the much improved performance of the new extrusion grade of Viton.

During these trials, it was noted once again that the B-202/ B-600 reference compound gave a poorer surface finish compared to the new polymer. The surface finish of the reference compound improved at high extrusion rates (above 35 rpm), but at these rates, the head pressures were undesirably high.

Conclusions

A new gum polymer, VTR-7551, and two precompounds, VTR-9209 and VTR-9217, have been developed and are commercially available. VTR-7551 has been designed specifically for extrusion of thin walled tubes and hose veneers. It provides excellent extrudate surface quality and is capable of provides excellent extrudate surface quality and is capable of being pulled down to very thin wall thicknesses (less than 0.4 mm) without the formation of holes or other surface defects. Compounds based on these products can be formulated having physical properties that are suitable for use in automotive fuel line or other applications. The base polymer, VTR-7551, provides a unique balance of excellent extrusion performance with fuel permeation significantly lower than usual. VTR-7551 or VTR-9209 provide a roughly 25% reduction in the permeation of CE-10 fuel at 40[degrees]C when compared to a reference B type compound based on Viton B-202 and B-600.

References

(1.) R.D. Stevens, SAE paper 2001-01-1127, "Permeation and stress relaxation resistance of elastomeric fuel seal materials," (2001).

(2.) A. Nersasian, SAE paper 790659, "Effect of 'sour' gasoline on fuel hose rubber materials," (1979).

(3.) R.D. Stevens, U.S. Patent 5,320,888, Fluoroelastomer Laminates (1994).

(4.) R.D. Stevens, U.S. Patent 5,427,831 Fluoropolymer Laminates (1995).

(5.) R.E. Fuller and R.D. Stevens, SAE paper 960140, "Unique low permeation elastomeric laminates for fuel hose," (1996).

(6.) NFM Iddon HIM Scroll promotional leaflet, April 2001.

Christopher Grant, Ronald Stevens, Stephen Bowers and Phan Tang, DuPont Performance Elastomers

Table 1--compound recipes and properties

                               1992A40        -01

Ingredient                                   B-2021
                                             B-600
Precompound of Viton
  B-202/B-600                                   100
VTR-9209                                         --
VTR-9217                                         --
Magnesium oxide                                   3
N-990 carbon black                               30
Calcium hydroxide                                 6
Total phr lab                                   139
Mooney scorch @
121[degrees]C (D1646)
Minimum viscosity                                31
2 point rise (minutes)                         15.1
5 point rise (min.)                            19.5
10 point rise (min.)                           24.2

ODR @ 162[degrees]C,
  3 degree arc,
  100 range, 30 minute clock
  (D2084)
M-L (dNm)                                        10
M-H (dNm)                                        92
is-2 (min.)                                     1.8
t'50 (min.)                                     3.6
t'90 (min.)                                     9.5

MDR2000 @ 177[degree]C,
  0.5 degree arc,
  100 range, 12 minute
  clock (D5289)
M-L (dNm)                                       1.0
M-H (dNm)                                      18.7
ts-2 (min.)                                     0.8
t'50 (min.)                                     1.1
t'90 (min.)                                     1.9
t'95 (min.)                                     2.5

Physical properties--original
  at room temperature (D412)
  (Cured 30 min.
  @ 162[degrees]C
  --no post cure)
Stress at 10% strain, M Pa                      0.8
Stress at 25% strain, MPa                       1.3
Stress at 100% strain, MPa                      3.1
Tensile strength at break, MPa                  9.7
Elongation at break, %                        378.0
Hardness, A (D1414), points                    67.0
Adhesion to tie gums--
180[degrees] peel (D413)
Adhesion to NBR compound
(40" @ 162[degrees]C cure)
Initial peak, median, N                        86.0
Median bond, N/mm                               3.4
(1.4 N/mm min. needed to
pass most hose specs)
Compression set, D395,
Method B, plied
(no postcure)
70 hours @ 70[degree]C, %                        23

                                     -02       -03

Ingredient                           VTR-      VTR-
                                     9209      9217
Precompound of Viton
  B-202/B-600                         --         --
VTR-9209                              100        --
VTR-9217                               --       100
Magnesium oxide                         3         3
N-990 carbon black                     30        30
Calcium hydroxide                       6         6
Total phr lab                         139       139
Mooney scorch @
121[degrees]C (D1646)
Minimum viscosity                      32        27
2 point rise (minutes)                 30     23.20
5 point rise (min.)                   --         30
10 point rise (min.)                  --        --

ODR @ 162[degrees]C,
  3 degree arc,
  100 range, 30 minute clock
  (D2084)
M-L (dNm)                              13        11
M-H (dNm)                              63        69
is-2 (min.)                           3.2       2.3
t'50 (min.)                           5.5       3.7
t'90 (min.)                           6.3       4.3

MDR2000 @ 177[degree]C,
  0.5 degree arc,
  100 range, 12 minute
  clock (D5289)
M-L (dNm)                             1.5       1.2
M-H (dNm)                            17.1      14.7
ts-2 (min.)                           1.3       0.9
t'50 (min.)                           1.7       1.2
t'90 (min.)                           2.3       1.5
t'95 (min.)                           2.7       1.8

Physical properties--original
  at room temperature (D412)
  (Cured 30 min.
  @ 162[degrees]C
  --no post cure)
Stress at 10% strain, M Pa            1.2       0.8
Stress at 25% strain, MPa             1.9       1.4
Stress at 100% strain, MPa            3.5       3.2
Tensile strength at break, MPa        7.9       7.9
Elongation at break, %              359.0     302.0
Hardness, A (D1414), points          74.0      66.0
Adhesion to tie gums--
180[degrees] peel (D413)
Adhesion to NBR compound
(40" @ 162[degrees]C cure)
Initial peak, median, N             138.0      88.0
Median bond, N/mm                     5.4       3.3
(1.4 N/mm min. needed to
pass most hose specs)
Compression set, D395,
Method B, plied
(no postcure)
70 hours @ 70[degree]C, %              34        24

Table 2--fuel aging and permeation
test data

Physical properties @ R. T.       B-202/   VTR-   VTR-
  aged 168 hr. @ 23[degrees]C     B-600    9209   9217
  in 90% fuel C/10%
  ethanol (CE-10)
Modulus at 100% strain, MPa          2.5    2.5    2.5
  (% change, M100)                  -20%   -28%   -21%
Tensile strength at break, MPa       8.7    7.3    7.7
  (% change, T-B)                   -10%    -8%    -3%
Elongation at break, %               378    403    366
  (% change, E-B)                     0%    12%     21
Hardness, A, pts.                     61     66     59
  (Pts. change)                       -6     -8     -7
Volume increase (D471), %            8.3    6.1    6.7
Fuel permeation--ASTM E96
  Thwing Albert Cup--672 hr.
  @ 40[degrees]C (NPC)
CE-10, g-mm/[m.sub.2]/day             46     33     40

Table 3--compound recipes for extrusion
trials conducted at NFM Iddon

Ingredient                        VTR-9209   B-202/
                                             B-600

VTR-9209                               100      --
Precompound of Viton B-202E-600        --       100
Magnesium oxide                          3        3
Calcium hydroxide                        6        6
N-990 carbon black                      15       15
N-772 carbon black                      10       10
Carnauba wax                          0.75     0.75
Polyethylene wax                      0.75     0.75
Total phr lab                        135.5    135.5

Figure 2- permeation comparison of
various hose and tubing materials

Average permeation rate of CM15 at 23[degrees]C
(grams/[m.sup2]/day per mm thinkness)

                 Elastomer   FKM   Plastics
NBR              1,600
HNBR             1,100
FSI              635
PA12                                85
FKM-A                        35
FKM-B                        12
FKM-GF                         3
THV500                              0.5
EVOH-HDPE Coex                      0.35
ETFE                                0.2
FEB                                 0.03

Note: Table made from bar graph.