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Comparing fuel and oil resistance properties.

The subject of material resistance to fuels and oil is extremely important to the automotive industry. Obviously, the components in an automobile that must come into contact with or contain the fuels mid oils used in automobiles must be resistant to them.

For oils, the elastomers used in automobiles (and other applications) must be able to withstand the fluids they are scaling without losing their ability to maintain the seal. If the physical properties of the material degrade too far, the seal will fail.

In the case of fuels, the EPA continues to impose greater restrictions on the fuel permeation of automobiles. The EPA's standard on evaporative emissions for unburned hydrocarbons was lowered from 10 grams/square meter/24 hours to 2 grams/ square meter/24 hours in 1998, with more reductions added in 2000. This is the permeation of the gasoline onboard a vehicle, from the vehicle to the atmosphere without being used in combustion. The surface area is for the entire car. These levels must be maintained for 10 years, even if fuel formulations evolve toward higher percentages of methanol, which is more aggressive (ref. 2).

Because of these permeation restrictions, more exotic materials must be used to limit this permeation. The resistance of the polymeric materials used in these applications is directly related to the permeation rate of these fuels through them.


Solubility and resistance to swelling

The theory that "like dissolves like" from the micro world holds true for the macro world of polymers. This can be most easily demonstrated by the fact that polymers will absorb considerable quantities of their own monomers. Linear polymers often will completely dissolve in their own monomers or in another good solvent (ref. 13).

The chemical composition of a material is extremely important in determining its ability to withstand attack from a particular solvent. Solvents are important because most fuels are composed of solvents. A good solvent for a polymer is one that is either similar in chemical structure to the polymer or one which can interact with the polymer main-chain or sidegroups.

One way of determining the solubility of polymers in solvents is through the estimation of solubility parameters. The following relationship can often be used to estimate if a polymer will dissolve in a solvent. If the square root of the value of the solubility parameter of the solvent minus the solubility parameter of the polymer is less than 1, then most likely the polymer will dissolve in the solvent (ref. 11)

(1) i.e.: if [([[delta].sub.1] - [[delta].sub.2]).sup.1/2] <1, then the polymer will dissolve in the solvent

Using a group contribution analysis approach, the solubility parameter of a material can be estimated by the following equation (ref. 11):

(2) [delta] = ([rho] * [sigma]G)/M

Where: [delta] = the solubility parameter;

[rho] = the density of the material;

G = the group molar attraction constants;

M = the molecular weight of the material.

Table 1 provides some typical group molar attraction constants.

Determination of the solubility parameter for PMMA using equation 2 is shown in figure 1.


Looking at an example related to fuels, the solubility parameter of polystyrene is 9.10 [cal.sup.1/2]/[cm.sup.3/2]. The solubility parameter for toluene (a major component in gasoline) is 8.90 [cal.sup.1/2]/[cm.sup.3/2]. Using equation 1, we find the square root of the difference between polystyrene and toluene is 0.45. This would indicate that polystyrene dissolves in toluene (as is the case). This is an excellent reason why gas tanks are not made from polystyrene.

They are, however, made from polyethylene. The square root of the difference of the solubility parameters for amorphous polyethylene (7.9 [cal.sup.1/2]/[cm.sup.3/2]) and toluene is approximately one. This does not take into account hydrogen bonding or more importantly crystallinity (polyethylene is very crystalline). As such, this would indicate that polyethylene would make a good gas tank material (as it does not dissolve in fuel).

What all of this means is that the solubility parameter of a polymer is determined by the group interaction effects of its individual molecular units. This seems logical, because highly reactive side-groups will tend to interact more with the solvent molecules, increasing the likelihood that the polymer will be dissolved.

Tables 2 and 3 are two of the many tables that list both the solubility parameters of polymers and solvents, as well as the group interaction constants.

However, it should be noted that these formulas do not take into account the effects of hydrogen bonding, which can be quite pronounced and have a major impact on the solubility of materials. It also does not take the crystallinity of polymers into account, or the effects of crosslinking.

Examining the specific reasons for the chemical resistance of polymers, it can be seen that polar side-groups tend to provide resistance to swelling in oil immersion in hydrocarbon oils. These polar side groups usually contain electronegative atoms from groups 5 through 7 in the periodic table. These atoms form covalent bonds with the carbon atoms in the polymer chain, but the electron pair is displaced to the more electronegative atom (ref. 4).

This induces a slight electronegative charge on the electronegative atom with a slight positive charge around the carbon atom. The dipole moment produced tends to provide oil resistance. However, for long-chain polymers, the presence of multiple polar groups can offset or eliminate the dipole moment, reducing the polymer's oil resistance (ref. 4).

Some examples of non-crystalline crosslinked elastomers can be examined as an example of these effects. The lack of crystallinity and minimal hydrogen bonding in the polymers in table 4 allows for the examination of the effects of electronegative side-groups. Because these are crosslinked materials, they will not dissolve, but will swell, which allows for a numerical examination of the effects. Table 4 can be used to examine these effects.


As the electronegativity of the side-chain groups increases on a polymer, so does its resistance to oil swelling. The polyisoprene, with only a hydrocarbon side-group, has almost no resistance at all. Polychloroprene is polyisoprene with a chlorine atom replacing the methyl group on the backbone. This chlorine atom is more electronegative than the methyl group, and as such increases the oil resistance of this material.

The polyacrylate has both oxygen and chlorine in the side-chains, thus increasing the oil-resistance further. In nitrile, the cyanide group is very electronegative and thus provides excellent oil resistance. Fluorocarbons have a great deal of fluorine side-groups (extremely electronegative), and provide the ultimate oil resistance. Figure 2 characterizes some important polymers by their oil resistance and cost.

Further examination of nitrile reveals that swelling is related to the ACN content of the polymer. As the ACN content increases, so does the oil resistance (due to this formation of dipole moments). In commercial polymers, the ACN content can range from 18 to 40%. If the ACN content rises over 40%, the glass-transition rises to the point that the polymer becomes brittle at relatively high temperatures. The glass-transitions for polybutadiene and polyacrylonitrile are -87[degrees]C and +106[degrees]C, respectively (ref. 13).

Figure 3 depicts this relationship between the ACN content of NBR and its resistance to swelling. The resistance of NBR to attack by solvents and oils can be further improved by the removal of unsaturation (double bonds on the carbon chain). This is done by the process of hydrogenation, and the resultant product is called hydrogenated nitrile-butadiene rubber. The removal of the double bonds in the backbone of the polymer increases the resistance to chemical attack, ozone and heat. However, the ACN content of HNBR is still important and works in the same manner as for NBR (ref. 5).


Fluorinated materials are of special interest with regard to fuel resistance. These materials are the best able to withstand the attack of fuels and resist swelling. The resistance of these polymers is directly tied to fluorine content, as shown in figures 4 and 5.


Figure 4 shows the correlation between increasing fluorine content and increased resistance to swelling for fluorinated plastic materials. This can be attributed to the increased number of dipole moments along the polymer backbone. This trend is also true of the fluorinated elastomers, as can be seen in figure 5, as the swell decreases with increasing fluorine content.

Figure 5 also shows the relationship between the solvent type and swell. There is a definite relationship between the size of the solvent molecules and swelling into the polymer, as indicated by the increasing swell with increased percentages of ethanol and methanol (ref. 12).

Crystallinity is also an important attribute in the solvent resistance of thermoplastic materials. The crystalline regions of a polymer are very difficult to dissolve because the crystallites are very resistant to diffusion due to the close packing of the chains. As such, the solvent molecules can not make their way into these crystalline regions to break apart the molecules (ref. 13).

As an example, in the case of nylons, water is an excellent solvent. Nylons will absorb water because of the amide groups located on the nylon polymer chains. However, this absorption only occurs in the amorphous volume of the material. As such, the swelling of nylon by water is dependent on the degree of crystallinity and the percentage of amide groups in the polymer chain (ref. 13).

The final consideration of the resistance of a material to a particular solvent is the presence of crosslinks. Crosslinking decreases the absorption of solvents by constraining the polymer network. As the network is expanded by absorption of the dilutents, entropy forces between the tie-points (crosslink points) increases and the swelling is limited (ref. 13).

Vapor permeation

As would be expected, the permeation resistance of a material is directly related to its resistance to swelling. Obviously, a solvent that can easily absorb into a polymer can also easily migrate to the other side of it. This is very important where polymers are used as fluid boundaries, especially in prohibiting fuel permeation.

One basic rule for permeation is that the permeability of a polymer for a gas (i.e., fuel vapor) increases with decreasing size. This is an obvious assumption, because smaller molecules will more easily fit between the polymer chains and allow for movement through the material (ref. 12).

Another basic rule is that the permeability of a polymer for a gas increases with increasing solubility of the gas in the polymer. As such, the previous discussion regarding solubility parameters and the presence of dipole moments applies here, as does increasing the crystallinity of a material decrease the permeability of molecules through it (ref. 12).

Crystallinity has a diminished effect in permeation resistance because not all of the polymer material will be crystalline. The permeating species can diffuse through the amorphous regions of the material. The crystalline regions of the polymer tend to act more to limit the surface area that the permeating material can transfer through. Increasing crystallinity can still limit permeation, but not to the extent that the chemical structure can.

It should be noted that crosslinking provides little or no decrease in the permeation of materials. Although the crosslinks can decrease swelling by the containment of the tie-points, these tie-points can not prohibit small molecules from migrating through the swollen material. As such, crosslinking provides a limiting effect at best.

Looking at the basic mechanisms of permeation, the driving force is the concentration difference between the inside and outside of a material. This can be mathematically described by Flick's First and Second Laws of Diffusion (ref. 1):

(3) F = -[D.sup.*] dC/dX

(4) dC/dt = [D.sup.*] [d.sup.2]C/d[X.sup.2]

Where: F = flux (rate of transfer per unit area);

D = diffusion coefficient;

C = concentration of diffusing substance;

t = time;

X = depth of measurement (usually thickness).

The solubility of the solvent is very closely related to the diffusion coefficient. Materials with poor resistance to the solvent (high swell), have high permeation rates. Figure 5 demonstrates this relationship between the presence of highly electronegative side-groups (inducing dipole moments), and lower permeation rates. This is exactly the same effect as seen with solubility.

The Eval polymers are fluorinated materials. From this analysis, it is obvious that the presence of highly electronegative side-groups increases the permeation resistance of materials. The Eval materials have high fluorine contents, providing the most electronegative side-groups and thus providing the greatest resistance. The nylon has oxygen side-groups, as does the PET, but there is a greater volume of oxygen in the nylon. The LDPE and PP have no electronegative side-groups, and as such have the highest permeation, indicating the limited role of crystallinity in permeation resistance.

Looking specifically at fuel permeation, there are several fuels to consider. Table 5 shows the make-up of the general fuels that a polymeric material can be expected to be exposed to in operation (ref. 8). Figure 7 shows the results of testing of the major commercial polymers for use in fuel-sealing applications.


Obviously, the fluorinated polymers provide the greatest resistance to permeation, with the permeation rate decreasing with increased fluorine content. The epichlorohydrins (ECO) have chlorine side-groups and the nitrile rubbers have cyanide groups. It should be noted that increasing the ACN content in the nitriles reduces permeation as they decrease solubility.

In figure 7, the effects of crystallinity can be seen in polymers of similar chemical structure. The FEP, ETFE and FKM polymers are all fluorinated. The FEP and ETFE are thermoplastic, whereas the FKM is a crosslinked elastomer. As can be seen, the crystalline FEP and ETFE have lower permeation.

Oxygenated fuels are used to decrease post-combustion emissions. Oxygenated fuels are made by adding small oxygen-containing hydrocarbon molecules, such as ethanol or methanol, to gasoline. Figure 8 compares the permeation resistance of these polymers in some oxygenated fuels.

Figure 8 displays the relationship between the size of the diffusing molecule and permeation rate. As the size of the molecule decreases, the permeation rate tends to increase as would be expected. This trend is proportional to the permeation rates of the polymers before the addition of the smaller molecules.

As was the case with chemical resistance, there is a cost penalty to pay for permeation resistance. The fluorocarbons are extremely expensive ($20 to $40 per pound) compared to NBR, which is very low in cost ($2 to $4 per pound).


The fuel and oil resistance of polymers is almost completely dependent upon the solubility of the fuel or oil in the polymer. The solubility of a polymer is dependent upon the chemical structure of the polymer, crystallinity and the presence of crosslinks. As with liquids, similar structures tend to solvate into each other, so this is always a factor.

The chemical structure of the polymer plays the greatest role in the chemical resistance of the polymer. Solubility is decreased by the addition of electronegative side-groups onto the chain. These side-groups create dipole moments that inhibit both swelling and permeation. Because there is a direct relationship between the presence of electronegative side groups and chemical resistance, increasing the content and electronegativity of these groups decreases swelling and permeation.

Crystallinity also decreases solubility because it is difficult for the solvent molecules to penetrate the crystalline network. The greater the crystallinity of the polymer, the greater the resistance to swelling and chemical attack. This is due to the close packing and unreactivity of the crystalline segments.

Crosslinking plays a role in limiting solubility by constraining the material, but does not change a polymer's affinity for the solvent molecules. Crosslinking limits the swelling of polymers by providing tie-points (constraints) that limit the amount of solvent that can be absorbed into the material. Crosslinking does not, however, change the affinity of the polymer for the solvent molecules. Obviously, solubility is also limited by the temperature and length of time of the exposure.

Permeation resistance is also dependent upon the chemical structure of a polymer and is related to the solubility. Crystallinity is limited in its application to decrease permeation due to the presence of permeable amorphous regions. Because of this, increasing the crystalline content of the polymer will tend to decrease the permeation rate. Crosslinking serves very little purpose in limiting permeation.

The other factors affecting permeation are the temperature, pressure difference between the permeating sides and the size of the permeating molecules. As would be expected, smaller particles tend to permeate through materials more easily than large ones.
Table 1--group modular attraction constants (ref. 11)

Group G

C[H.sub.3] Single-bonded 214
C[H.sub.2] 133
CH< 28
>C< -93
C[H.sub.2]= Double bonded 190
-CH= 111
>CH= 19
-CH=C< 285
>C=C< 222
Phenyl 735
Phenylene o,m,p 658
Napthenyl 1,146
Ring 5-membered 105-115
Ring 6-membered 95-105
Conjugation 20-30
H Variable 80-100
O Ethers 70
CO Ketones 275
COO Esters 310
CN 410
CI Mean 260
CI Single 270
CI Twin (>CC[I.sub.2]) 260
CI Triple (-(CC[I.sub.3]) 250
Br Single 340
I Single 425
C[F.sub.2] n-fluorocarbons 150
C[F.sub.3] 274
S Sulfides 225
SH Thiols 315
ONO Nitrates ~440
N[0.sub.2] Nitro-compounds ~440
P[0.sub.4] Organic phosphates ~500

Table 2--solubility parameters for common
polymers (ref. 11)

Polymer [delta] [(cal/[cm.sup.3]).sup.1/2]

Polybutadiene 8.4
Polyethylene 7.9
Poly(methyl methacrylate) 9.45
Polytetrafluoroethylene 6.2
Polyisobutene 7.85
Polystyrene 9.10
Cellulose triacetate 13.6
Nylon 6,6 13.6
Poly(ethylene oxide) 9.9
Poly(vinyl chloride) 9.6

Table 3--solubility parameters for same common
solvents (ref. 11)

Solvent [delta] [(cal/[cm.sup.3]).sup.1/2]

Acetone 9.9
Benzene 9.2
N-butyl acetate 8.3
Carbon tetrachloride 8.6
Cyclohexane 8.2
n-Decane 6.6
Dibutyl amine 8.1
Difluorodichloro methane 5.1
1,4-dioxane 10.0
Low-odor mineral spirits 6.9
methanol 14.5
Styrene 9.3
Toluene 8.9
Turpentine 8.1
Water 23.4
Xylene 8.8

Table 5--fuel consumption (ref. 8)

Name Composition

Fuel C 50% isoocatane/50% toluene
TF1 90% Fuel C/10% ethanol
TF3 90% Fuel C/10% MTBE (methyl t-butyl ether)
TF2 92.5% Fuel C/5% methanol, 2.5% ethanol
M25 75% Fuel C/ 25% methanol

Figure 2--comparison of oil swell vs. cost

 Cost ($/lb.) Avg. swell (#3 Oil)

FKM 30 10
NBR 2 17.5
PA 3.3 27.5
IR 0.8 32.5
ECO 2.8 45
EPDM 1 60
HNBR 8 155
CR 1.5 155

Note: Table made from bar graph.


(1.) D. Rosato, D.P. DiMattia and D.V. Rosato, Designing with Plastics and Composites, 1991, Van Nostrand Reinhold, pp. 276-281.

(2.) Mikell Knights, "Exotic coextrusions produce low permeation fuel lines for cars," September 1997, Plastics Technology, pp. 32-34.

(3. P. Painter and M. Coleman, Fundamentals of Polymer Science, 1997, Technomics Publishing Company, pp. 189-224.

(4.) Robert F. Ohm, Introduction to the Structure and Properties of Rubber, Vanderbilt Rubber Handbook (13th Edition), 1990, R.T. Vanderbilt Company, pp. 2-10.

(5.) S. Okamura, K. Nishimura and S. Hayashi, "Latest developments in oil resistant rubbers," IRC '97, Malaysia, Oct. 1997.

(6.) S. Koch, Manual for the Rubber Industry (2nd Edition), 1993, Bayer AG, pp. 36-38, pp. 79-84.

(7.) J. Patel, C. Riddeford and A.J. Tinker, "Oil and heat resistant TPE," Rubber Technology International, 1996, pp. 83-97.

(8.) Ron Stevens, "Permeation rates of fuel resistant rubbers and plastics to fuel blends," presented at the Detroit Rubber Group, 1995.

(9.) Berins, Plastics Engineering Handbook of the Society of the Plastics Industry (5th Edition), 1991, Van Nostrand Reinhold, pp. 33-78.

(10.) J.A. Manson and L.H. Sperling, Polymer Blends and Composites, 1976, Plenum Press, pp. 135-143.

(11.) L.H. Sperling, Polymeric Multicomponent Materials, 1997, John Wiley & Sons, Inc., pp. 62-67.

(12.) Joel R. Fried, Polymer Science and Technology, 1995, Prentice Hall pp. 99-110, pp. 430-434.

(13.) N.G. McCrum, C.P. Buckley and C.B. Bucknall, Principles of Polymer Engineering (2nd Edition), 1997, Oxford Science Publications, pp. 100-111.

(14.) M. Morton, Rubber Technology (3rd Edition), 1995, Chairman & Hall, pp. 431-433.
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Author:Myntti, Matthew F.
Publication:Rubber World
Date:Jun 1, 2003
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