FKM micropowder effects on fuel vapor permeation.

Permeation is the migration of molecules through the various elastomers that are part of a vehicle's fuel system. Permeation of hydrocarbon fuels through elastomeric vehicle components is known to be a significant source of hydrocarbon emissions, contributing to air quality problems in areas such as Southern California, and a series of regulatory and engineering projects has been done to improve vehicle fuel systems to comply with local regulations.

Partially zero emissions vehicles (PZEV) are a special category of vehicles designated by the California Air Resources Board (CARB). In order to comply, a vehicle must attain stringent operating emissions standards, plus zero evaporative emissions from its fuel system and an extended warranty on its emissions control components (figure 1). Use of ethanol as a fuel system component, in place of the additive MTBE, has been shown to give an increase in fuel permeation (table 1).

Historically, manufacturers of fuel system components, including fuel sender seals and hoses, prefer fluoroelastomer materials because of their excellent resistance to hydrocarbons, as well as their excellent permeation rates. Despite technical benefits, there is reluctance to use FKM material in this application due to high compound cost, in excess of $25/kg for the finished compound.

Fluoroelastomer micropowder is a powdered FKM material, produced using a proprietary wet-conversion process (figure 2), with a feedstock of carefully screened post-industrial FKM by-product. A variety of studies has shown this material to be suitable for reintroduction back into the product stream. Because of its unique particle morphology and surface activity (figure 3), it interacts with the polymer matrix in the host compound and provides excellent mechanical properties (figure 4), enabling a substantial cost reduction. Use of FMP allows users to recapture costs in approximately a 3:4 ratio with usage, that is to say, at prevailing market prices, a 20% FMP usage enables up to a 15% compound cost reduction under normal circumstances.

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The intent of this study is to test the effects of FMP on fuel permeation, to determine whether the material has any negative effects, and also, to test the effects of FMP on permeation using ethanol-containing fuels.

Experimental

Three test compounds were prepared using the test formulation in table 2. The compounds were mixed using a standard 6X12 laboratory mill, and sheeted off and tested in accordance with ASTM D-471. In all, three compounds were tested, including: A control formulation, consisting of a standard FKM control formulation, and two test formulations; one consisting of 22% by weight of FMP of similar composition, and another consisting of 85% FMP, produced from a different, mineral-filled raw material base.

The test compounds were placed through a battery of conventional mechanical property testing, including unaged, heat aging, fuel and oil aged physicals. In addition, specimens of each test material were provided to an OEM automotive manufacturer for permeation testing in accordance with the standard test method for this property.

Test results

As expected, use of 22% by weight FMP resulted in a compound with comparable physical properties to the control, with similar heat aged physicals, and fuel and oil resistance (table 3). Use of 85% FMP (Compound 3) using a mineral-filled feedstock resulted in slightly lower unaged tensile strength, and correspondingly lower reduction in tensile after heat aging, and fuel and oil resistance. All physical property tests were within the standard ASTM callout specifications for FKM compounds.

To evaluate fuel permeation, testing was performed by an OEM automotive manufacturer, in accordance with an internal testing procedure that is described as follows: A fuel vessel, in which the fuel is contained by an elastomer barrier is placed in a heated chamber. A purge gas (nitrogen) is used to capture the gas molecules as they permeate the barrier (figure 5). The purge gas passes through a thermal desorption tube, in which a desorption medium captures the hydrocarbon vapors (figure 6). The contents of the desorption tube are released by heating, then analyzed and quantified using gas chromatography. In this test, the samples were exposed to fuel vapors, at 40[degrees]C, for eight days. Test fuels consisted of ASTM Fuel C, and ASTM Fuel CE.

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In the vapor measurement test, all three of the samples were similar in performance (table 4), and consistent with the body of literature, the CE permeation rate was higher than the Type C rate for all specimens, and the toluene and ethanol components of the fuels permeated at a higher rate than the isooctane, the result of the ability of the smaller molecules able to permeate through the membrane more readily than the isooctane and toluene which is contained in the Type C.

Based on these tests, use of FMP, even at high levels, produced compounds with levels of fuel vapor permeation no different from conventional FKM, with traditional relationships between compounding and permeability are maintained in this system.

Discussion

The mechanism of the permeability of rubber-like materials to water vapor or hydrocarbon gases has been well understood for some time. In general, permeability of rubber compounds by a gas can be affected by any of the following mechanisms:

* Presence of steric groups along the polymer chain results in greatly reduced permeability. The larger the number of these groups, the lower the permeability. The permeability is not substantially affected by molecular weight. FKM polymer is inherently permeation resistant as a result. The polar nature of the polymer also enables improved resistance to hydrocarbon degradation, a property for which this material is well known (figure 7).

* Presence of fillers such as carbon black reduces permeability. Low surface area/low structure blacks such as N990 have the biggest effect on permeability at equal loading, and also, increased loading causes a reduction in permeability (figure 8).

* Presence of light colored mineral fillers has a relatively small effect on permeability. Use of mineral fillers at the 20 part level caused a 25% reduction in permeability.

* Presence of laminar fillers, such as mica, have a much greater effect on permeability, because the gas molecules must travel a longer distance to pass around the filler particles through the membrane. The filler orientation probably occurs during calendering or milling.

* The permeability is proportional to temperature, that is to say, the above relationships hold true at various temperatures. Van Amerongen suggested that this indicates that the mechanism is mechanical, rather than energetic adsorption of the gas onto the fillers.

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Based on this, as long as the interaction between the particles of ground rubber and the remainder of the compound is sufficiently intimate, there should be no theoretical effect on permeation if the particles are of the same composition. In fact, the opposite implication may be made: If the permeation of a fluoroelastomer micropowder-containing compound is the same as the base compound, an intimate interaction between compound matrix and rubber particle can be assumed.

Economics

Figure 9 was prepared using the data in table 1, given recent polymer pricing relationships. In general, there is a strong relationship between permeation and cost, except the advantage of fluoroelastomer in this application is obvious in that it provides an order of magnitude better permeation resistance at relatively low cost compared to higher-end materials. Nylon 12 is sometimes used in fuel system applications because of its lower cost, with some sacrifice in heat aging resistance and higher manufacturing complexity.

Use of fluoroelastomer micropowder at current market prices enables manufacturers to produce parts with FKM-level permeation at a cost approaching that of alternate materials in applications where engine heat can negatively impact part life.

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Summary

Based on the observations in this experiment, the following conclusions were reached:

* Use of the fluoroelastomer micropowder of a similar composition to the base compound had no effect on permeation. This confirms the intimate interaction between the particle and polymer matrix, and allows an economical compounding alternative in this application.

* Use of mineral-filled FMP at high levels retained the low permeation rate of the base FKM polymer, and the slightly lower permeation rate in this specimen was either within normal test variability, or an artifact of the slightly higher mineral content/lower carbon black level in the base compound.

* Although permeation of Fuel CE through the compounds was elevated, because of the smaller ethanol molecule, as expected, the permeation results using Fuel CE were consistent with the Fuel C results.

* Use of the fluoroelastomer micropowder can be an economical alternative to enable FKM-level performance with economics competitive with alternate materials.

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References

(1.) H. Askew and Associates, "Evaporative emissions effects created by ethanol in gasoline," submitted to California Air Resources Board, contract 99-404, http://www.arb.ca.gov/ fuels/gasoline/meeting/2001/053101Pmtn.pdf December 7, 2007.

(2.) "Partial zero-emissions vehicle," Wikipedia, http://en.wikipedia.org/wiki/PZEV, December 10, 2007.

(3.) Dennis R. Lancaster, "Recycling of cured fluoroelastomer for cost control and improved performance," ACS Rubber Division, Fall 2006.

(4.) Plastic Resin Prices: Coshttp://www.ides.com/resinprice/ resinpricingreport.asp, December 10, 2007.

(5.) Iyengar, "Relation of water vapor permeability of elastomers to molecular structure, "Rubber Chemistry and Techology, Vol. 39, p. 751, 1966.

(6.) G.J. Amerongen, "The permeability of rubber-like substances to gases, "Rubber Chemistry and Technology, Vol. 20, p. 493, 1947.

(7.) Reitlinger, S. A. "Gas Permeability and the microstructure of polymers, "Rubber Chemistry and Technology, Vol. 19, p. 386, 1946

(8.) G J Van Amerongen, "The effect of fillers on the permeability of rubber to gases," Rubber Chemistry and Technology, Vol. 28, p. 1955.

(9.) Thermal Desorption Tube schematic: www.sisweb.com/referenc/articles/techl.htm.

Table 1--average permeation rate
(g/mm/[m.sup.2] x day)

Material                Fuel C   10% ethanol   % increase

NBR (33% ACN)              669         1,028           54
FVMQ (fluorosilicone)      455           584           28
HNBR (44% ACN)             230           553          140
Nylon 12                   5.5          24.0          336
FKM GLT (65% F)            2.6          14.0          438
FKM GFLT (67% F)           1.8           6.5          261
FKM A200 (66% F)           0.8           7.5          838
FKM B70 (66% F)            0.8           6.7          738
FKM B200 (68% F)           0.7           4.1          486
FKM GF (70% F)             0.7           1.1           57
PFA 1000LP                 0.1          0.03          -40
FEP 1000L                 0.03          0.03            0
ETFE 1000LZ               0.03           0.1           67

Table 2--test formulation

Compound                        1        2        3
FKM cure incorporated         100      100      100
N990                           30       30     10.2
CaOH                            6        6
Elastomag 170                   3        3
WS-280 powder                   1        1
FMP (similar to control)              27.5
FMP (mineral filled)                            567
LM-5 curative                                  10.2
Total                         140    167.5    687.4
Formulation cost/kg         20.66    18.35    11.36
Sp. gr,                     1.832    1.832    1.866
Formulation Kg/vol.         37.84    33.62    21.20

Table 3--test results

ASTM D471 physical properties

Cured 10' at 175[degrees]C/post cured 16 h./250[degrees]C

Compound                                 1        2         3

Tensile strength, MPa                14.33    14.49     13.94
Elongation, %                          194      176       146
100% modulus, MPa                     6.17     8.17      9.32
Durometer A                             81       82        83
Sp. gr.                              1.851    1.859     1.866
Compression set
22h/250[degrees]C 25% deflection       38%      41%       49%

Fuel aged (E10) 70 h./22[degrees]C

Compound                                 1        2         3

Tensile strength, MPa                14.15    12.29     12.32
Elongation, %                          190      172       148
100% modulus, MPa                     6.80     6.91      8.15
Durometer A                             75       80        80
Sp. gr.                              1.821    1.834     1.853
Change in tensile                      -1%     -15%      -12%
Change in elongation                   -2%      -2%        1%
Change in volume                       -2%      -1%       -1%

Oil aged, 70 h./22[degrees]C 1OW30 motor oil

Compound                                 1        2         3

Tensile strength, MPa                15.73    15.13     13.89
Elongation, %                          180      190       148
100% modulus, MPa                     7.66     7.70      9.70
Durometer A                             77       82        84
Sp. gr.                              1.846    1.854     1.868
Change in tensile                      10%       4%        0%
Change in elongation                   -7%       8%        1%
Change in volume                    -0.27%   -0.27%     0.11%
Heat aged 70 h./250[degrees]C

Compound                                 1        2         3

Tensile strength, MPa                14.03    17.01     13.31
Elongation, %                          165      180       114
100% modulus, MPa                     7.92     7.75     12.15
Durometer A                             78       82        84
Sp. gr.                              1.846    1.854     1.868
Change in tensile                     -30%     126%      -21%
Change in elongation                  -15%       2%      -22%
Change in volume                       28%      -5%       30%

Table 4--permeation testing--proprietary testing method

Compound                    1               2               3

                        g/mm/           g/mm/           g/mm/
ASTM Fuel C     [m.sup.2]/day   [m.sup.2]/day   [m.sup.2]/day

Isooctane                0.85            0.69            0.71
Toluene                 13.89           11.46            11.2
Total                   14.74           12.15           11.91

                        g/mm/           g/mm/           g/mm/
ASTM Fuel CE    [m.sup.2]/day   [m.sup.2]/day   [m.sup.2]/day

Isooctane                2.61            2.55            2.38
Toluene                 31.75           31.32           29.24
Ethanol                 22.14            22.9           20.98
Total                    56.5           56.77            52.6