Oxygenate Compatibility and Permeability Report January 1999

ATTACHMENT A

Compatibility and Permeability of Oxygenated Fuels to

Materials in Underground Storage and Dispensing Equipment

A Technical Assessment of the Literature circa 1975-19971

 

Prepared by:

Paul A. Westbrook, Ph.D.

Shell Oil Company, WSPA Representative

&

Team 1 -- Oxygenate Compatibility and Permeability

 

Submitted to:

State Water Resources Control Board’s Advisory Panel

January, 1999

 

1

 litrev2.doc
Compatibility and Permeability of Oxygenated Fuels to Materials

2 in Underground Storage and Dispensing Equipment

Table of Contents

 Page Number

1.0. Executive Summary 7

2.0 Scope 8

3.0 Technical Summary 8

3.1 Materials Compatibility with Oxygenated Gasoline 8

3.2 Permeability of Polymeric Materials to Oxygenated Gasoline 9

3.3 Technologies to Reduce Permeation of Oxygenated Hydrocarbons 10

4.0 Definitions 10

4.1 Chemical Compatibility 10

4.2 Chemical Permeation 10

4.3 Thermal-Chemical Environment 11

(Gasoline -- Aromatic, Aliphatic, Olefinic Hydrocarbons

Ethers - MTBE, ETBE, TAME, DIPE, etc.

Alcohols - MeOH, EtOH, TBOH, AmOH, etc.

Temperature -- Ambient (0 -- 120 °F)), Pressure 0-60 psi

4.4 Basic Data 11

5.0 Theoretical Overview 12

5.1 Chemistry of Oxygenated Hydrocarbons 12

5.1.1 Oxidation of Ethers 12

5.1.2 Alcohols as Bases 13

5.1.3 Hydrogen-Bonding of Ethers and Alcohols 13

5.2 Solution Thermodynamics 14

5.2.1 Ideal Solutions -- 15

(Hydrocarbon Blends including Ethers

Chemical Potential, Activity and Vapor Pressure)

5.2.2 Non-ideal Solutions 17

(Hydrocarbon Blends with Alcohols

Chemical Potential, Activity and Vapor Pressure

Phase separation, Effects of water)

6.0 Corrosion Mechanisms 19

6.1 Non-metals -- 19

6.1.1 Equilibrium Swelling of Nonmetallic Materials by Solvents 20

(Lattice model)

6.1.2 Unsteady Mass Flow of Solvents through Nonmetallic Membranes 22

Linear Permeation Theory -- Fick’s Law 23

Example 1: Absorption into an Immersed Slab (swelling) 24

Example 2: Mass Flow through a Membrane (permeation) 25
Compatibility and Permeability of Oxygenated Fuels to Materials

3 in Underground Storage and Dispensing Equipment

6.1.3 Plasticization and Extraction 27

Underground Storage Tank Membranes 27

Extraction of Plasticizers 29

6.1.4 Environmental Stress Cracking (ESC) 29

Thermal Effects 30

Effect of Temperature on Swelling Behavior 30

Effect of Temperature on Permeation

30

Effect of Temperature on Elastomeric Seals 31

Pressure Effects 31

Effect of Time 32

6.1.5 Testing Standards for Nonmetallic Materials 32

Fuel Stability 32

Compatibility Testing 32

Permeability Testing 34

6.2 Metals 35

6.2.1 Dry Corrosion 36

6.2.2 Galvanic Corrosion 37

6.2.3 Electrolytic Corrosion 37

6.2.4 Wet Corrosion 37

6.2.5 Multiphase Corrosion 38

6.2.6 Wear 38

6.2.7 Erosion-Corrosion 39

6.2.8 Effect of Contaminants in Solution 39

6.2.9 Effect of Alloying Elements 39

6.2.10 Environmental Stress Cracking 39

6.2.11 Thermal Effects 40

6.2.12 Effect of Time, Corrosion Inhibitor 40

6.2.13 Testing Standards for Metallic Materials 40

7.0 Results and Discussion 41

7.1 Non-metals 41

7.1.1 General observations on the Effects of Model Fuel Blends on 41

Polymeric Materials

7.1.2 Swelling of Polymeric Materials in Model Fuel Blends and Neat 43

OxygenatedHydrocarbons

7.1.3 Permeation of Polymeric Materials by Model Fuels Containing 45

 no Oxygenates

7.1.4 Permeation of Polymeric Materials by Ether Blended Model Fuels 46

7.1.5 Permeation of Polymeric Materials by Alcohol Blended Model Fuels 48

7.1.6 Thermosetting Materials -- Underground Storage Tanks 50

7.1.7 Thermosetting Materials -- Rigid Fiberglass Piping 52

7.1.8 Thermoplastic Materials -- Flexible Plastic Piping 52

7.1.9 US EPA Evaporative Emission Regulations 53

7.2 Metals 53
Compatibility and Permeability of Oxygenated Fuels to Materials

4 in Underground Storage and Dispensing Equipment

7.2.1 Corrosion by Ethers 53

7.2.2 Corrosion by Alcohols 54

7.3 Other Materials 55

7.3.1 Ceramics 55

7.3.2 Pipe Dope 55

7.3.3 Coatings 55

Organic Coatings 55

Inorganic Coatings 56

8.0 Review Articles 56

9.0 American Petroleum Institute Documentation 57

10.0 Conclusions 61

9.1 Compatibility 61

9.2 Permeability 61

11.0 Recommendations 62

Appendices

APPENDIX A -- Swelling Data 63

APPENDIX B -- Permeability Data 66

APPENDIX C -- Chemical and Physical Description of Common Fuel Resistant Elastomers 70

References 75
Compatibility and Permeability of Oxygenated Fuels to Materials

5 in Underground Storage and

Dispensing Equipment

List of Tables

Table

1 Properties of hydrocarbons commonly used in compatibility and permeability studies.

2 Approximate compositions of vapor liquid equilibrium for a 15 percent by volume blend 

of MTBE in ASTM Fuel C.

3 Approximate compositions of vapor liquid equilibrium for a 10 percent by volume blend 

of MeOH in ASTM Fuel C.

4 Performance ranking system for elastomeric hoses based upon permeability to fuels (from 

SAE J2260).

5 Volume swelling ranges of polymeric materials exposed to model fuels with and without 

oxygenates.

6 Volume swelling of polymeric materials exposed to neat oxygenated hydrocarbons.

7 Component geometry for permeation mass flow considerations.

8 Permeability, solubility and mass flow properties for various materials exposed to 

gasoline blends containing no oxygenates.

9 Permeability, solubility and mass flow properties for various materials exposed to

gasoline blends containing 15 percent v/v MTBE.

10 Permeability and solubility of modified ASTM Fuel C, containing 15 to 20 percent by 

volume MeOH, in polymeric materials.

11 Permeability and solubility of modified ASTM Fuel C, containing 10 to 15 percent by 

volume EtOH, in polymeric materials.

12 Corrosion rates for selected metals immersed in methanol fuels at 40 oC for 2000 to 8000 

hours (Lash et al.).

13 Corrosion rates of selected metals in 10 percent methanol blends.

14 Unsuitable elastomers for neat alcohol and diethyl ether.

15 Compatibility of commonly used materials with ethanol and ethanol blends.

16 Compatibility of commonly used materials with gasoline-methanol/cosolvent blends.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

6

List of Figures

Figure

1 Activity of MTBE, toluene and isooctane as a function of ether concentration in Fuel C.

2 Activity of EtOH, toluene and isooctane as a function of alcohol concentration in Fuel C.

3 Schematic representation of unsteady mass flow by Case 1 diffusion of a permeant 

through a membrane.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

7

1.0 Executive Summary

No material incompatibility concerns associated with storage and dispensing of methyl tertiary

butyl ether (MTBE) blended gasoline at retail facilities have been documented. Gasoline

containing up to 15 percent by volume of MTBE will not induce or enhance new or existing

failure mechanisms in metallic or nonmetallic components. Additionally, the performance

properties of the fuel itself, in contact with the construction materials, are not altered in ways

which lead to impairment of vehicle performance. These observations are in contrast to the well

documented aggressive character of alcohol blended fuels toward certain metals and polymers

alike as well as certain vehicle driveability concerns.

Gasoline, oxygenated or not, does not absorb into or permeate through metals. The phenomenon

of permeation is thus limited to certain nonmetals and will typically vary greatly depending upon

the type of material in question. Permeation of gasoline through composite materials typical of

fiberglass tanks, rigid piping and sumps has not been observed despite two reported attempts to

measure it. As such, permeation of fuel components through fiberglass directly into the ground is

expected to be very low, perhaps below detectable limits. Permeation of reformulated gasoline

through thermoplastic and elastomeric materials typical of flexible hoses and piping has been

observed. Most of this fugitive hydrocarbon emission is produced directly into the air.

There is not enough data to estimate the total fugitive emission of hydrocarbons and, especially,

the individual contribution due to oxygenates permeating through all polymeric membranes at a

retail facility. Where data is available, the component contribution of the oxygenated hydrocarbon

to the total hydrocarbon permeation is not reported. In the case of elastomer hose construction,

theoretical considerations may allow one to estimate the mass flow due to MTBE component

permeation. This contribution is expected to be less than ten grams per day per station.

Absorption and permeation of alcohol blended fuels in and through polymeric materials are

observed to be of considerably greater magnitude than that observed for ether blended fuels. As

with MTBE, component contribution due to alcohol permeation alone is unknown. However,

theoretical and practical considerations prevent the estimation of the component contribution of

alcohol permeation, even in elastomers.

Direct observation of the permeation rate of MTBE and other oxygenated hydrocarbons in

composite, plastic and elastomeric materials of construction is recommended for further study.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

8

2.0 Scope

This review discusses metallic and nonmetallic materials compatibility to gasoline containing

various concentrations of oxygenated hydrocarbons. Permeability of oxygenated gasoline through

nonmetallic materials is also discussed. Documentation published during the 1975 through 1997

time frame is used as the primary source material.

Although the current interest is focused primarily on the effects of methyl tertiary-butyl ether on

materials of construction in retail facilities, this review discusses the effects of other common

oxygenated additives such as alcohols and other ethers. Both above-ground and buried

components are considered. A broad scope is offered for two reasons. First, a study of the

effects of both alcohols and ethers will increase awareness of the various chemical phenomena

that are manifest when materials are brought into contact with various oxygenated fuel blends.

Second, should alternate oxygenated hydrocarbons be considered for addition to automotive

fuels, it is hoped that this document may serve as a beginning reference for materials

considerations.

Installation and operational procedures and practices are not considered in this review.

3.0 Technical Summary

3.1 Materials Compatibility with Oxygenated Gasoline

There is general agreement among observers that, if a material is resistant to alcohol blended

fuels, it will be resistant to ether blended fuels as well. Although there is some merit to this

assertion, it has led to considerably more documentation of materials issues involving alcohol

blended gasoline.

Even so, there are no documented materials incompatibility issues associated with storage and

dispensing of MTBE blended gasoline. From a metals corrosion viewpoint, gasoline is a rather

benign liquid and MTBE does not increase the corrosiveness of the hydrocarbon blend.

From a polymer compatibility viewpoint, neat MTBE is an aggressive swelling agent for some,

but not all, polymers. The mitigating factor for polymer compatibility in a reformulated gasoline

environment is that the swelling power of MTBE is diluted in approximate proportion to its

volume fraction in solution. At 15 percent concentration in gasoline, the effects of MTBE do not

compromise equipment integrity.

Conversely, the corrosive nature of alcohol blended fuels with regard to metals and polymers alike

is well documented. Even dilute alcohol blends are more aggressive to materials than any of the

pure components. Methanol is more aggressive than higher molecular weight alcohols.

Key to the nature of metal corrosion by gasohol containing gasolines is the role of trace

components in the fuel, such as water, chloride ion, sulfur compounds, pH, etc., and also alloying
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

9

elements such as copper. Lead, zinc, aluminum, magnesium, and other metals are actively

corroded under certain conditions.

Swelling of polymers is enhanced by alcohols through the various associations possible among

solvent-solvent and polymer-solvent interactions. Even in relatively dilute alcohol blends,

considerable loss of stiffness and strength are caused by plasticization and are well documented

for many polymeric materials.

Introduction of trace concentrations of water (ca. 1000 parts per million) into alcohol blended

fuels often passivates corrosion of some metals and reduces absorption into some polymers.

However, hydrated gasohol may introduce new degradation mechanisms for materials. Close to

the water content required for passivation of corrosion, aqueous phase separation occurs. The

electrical conductivity of hydrated fuel is increased to the point where galvanic and electrolytic

corrosion may be enabled. Considerable loss of lubricity, leading to increased wear of wetted

parts, is also observed near the point of phase separation. Corrosion and/or wear products may

become entrained in the fuel and cause subsequent drive ability issues for customers vehicles.

3.2 Permeability of Polymeric Materials to Oxygenated Gasoline

Likewise, permeability data for MTBE blended gasoline are sparse compared to alcohol blends.

There are not enough data to estimate the total hydrocarbon fugitive emission due to permeation

from retail gasoline stations. No data for fiberglass composites common to tanks and piping are

found. Some data for elastomers and thermoplastics common to hoses and flexible piping are

available.

Generally, the presence of oxygenates accelerates permeation of hydrocarbon fuels in elastomers

and thermoplastics. Alcohols, particularly methanol, produce more excess permeation than does

MTBE. Among ethers, MTBE may be more permeable than TAME.

Since observers report total mass flow due to permeation, without consideration of component

contributions, estimating the contribution of MTBE to the total mass flow through a membrane

requires theoretical techniques which may be questioned. For alcohols, deconvolution of the data

are not possible due to substantial excess permeation. Therefore the mass flow of oxygenates

(when dissolved in gasoline) through materials of interest, should be measured using techniques

and instrumentation capable of resolving and quantifying individual chemical species.

From the available data, an attempt to estimate the component contribution of MTBE permeation

through hoses is made in this paper. Many assumptions are involved. Depending on the total

surface area of hoses and other considerations, calculations suggest that up to 10 grams total

MTBE emission may be observed from permeation of nitrile rubber (NBR) based hoses per

station, per day. Permeation through hoses contributes to direct evaporative emission to the air

and not the soil.

3.3 Technologies to Reduce Permeation of Oxygenated Hydrocarbons
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

10

Incorporation of a thermoplastic liner in flexible hose constructions, similar to liners found in

some flexible underground piping, may reduce the total permeation of hydrocarbons including

MTBE.

The solubility characteristic of ETBE indicates that it may be substantially less permeable in

fluorocarbon elastomers than either MTBE or TAME. Lower permeability of ETBE is likely to

be observed in polar polymers and is related more to stereo-chemical effects than to molecular

size.

4.0 Definitions

Definitions of chemical compatibility, permeability and the chemical environments of interest in

this document are defined below.

4.1 Chemical Compatibility

Chemical compatibility refers to changes in the physical, chemical or mechanical properties of a

material resulting from thermal-chemical exposure. Any property change should not alter the

performance of a part in ways which induce or enhance new or existing failure mechanisms.

For metals, chemical compatibility often implies corrosion resistance. While the term corrosion is

not generally used to describe nonmetallic performance, a change in properties due to chemical

exposure is often considered a form of corrosion.

For the gasoline, chemical compatibility must also mean that degradation products, if any, must

not contaminate the fuel and impair automotive performance.1

4.2 Chemical Permeation

Permeation is mass transport, or flux, through a material that is driven by an activity gradient.

Activity is a thermodynamic term which is related to the change in the chemical potential with

chemical composition. Mass flux is proportional to the permeability coefficient of the solventmaterial pair and also the surface area to thickness ratio of a membrane.

4.3 Thermal-Chemical Environment
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

11

Gasoline is a blend of aliphatic, olefinic and aromatic hydrocarbons and also, more recently,

oxygenated hydrocarbons. Other organic additives such as corrosion inhibitors and detergents are

also common. However, it has been shown that there are no measurable effects of these additives

on elastomer performance factors.2 Thus, for compatibility and permeability testing, model

gasolines generally contain iso-octane and toluene in various proportions. An oxygenated

hydrocarbon, or oxygenate, has at least one covalently bonded oxygen atom in the molecule.

Generally, oxygenated additives for gasoline contain alcohol or ether functional moieties.

Over the past 25 years, oxygenates have been added to increase the octane number of gasoline

and, more recently, to comply with Federal and State automotive emissions standards. These

standards require addition of an oxygenated hydrocarbon to gasoline such that it will contain a

specified weight percentage of bound oxygen. Depending on the molecular weight of the additive,

up to 15 percent by volume of added oxygenate may be required to meet regulatory requirements.

Reformulated gasoline may be composed of hydrocarbons blended with ethers such as methyl

tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE) tertiaryamyl methyl ether (TAME),

and perhaps others.

Gasohol may be composed of hydrocarbons blended with alcohols such as methanol (MeOH),

ethanol (EtOH), tertiarybutyl alcohol (TBOH), amyl alcohol (AmOH), and perhaps others. The

term Gasohol is usually confined to blends containing ethanol up to 10 percent by volume.

Higher volume alcohol blends, such as 85 percent MeOH are also known.

Ambient temperatures are expected to range between zero and 120 °F. Although underground

equipment may not experience such wide temperature fluctuations, the product dropped into the

tank may be delivered at various ambient temperatures.

Operating pressures are expected to range between atmospheric and 60 pounds per square inch

gauge (psig).

4.4 Basic Data

Table 1 summarizes some chemical and thermodynamic data for constituents commonly used to

study chemical compatibility with oxygenated fuels. ASTM Reference Fuel C is composed of a

50 percent by volume blend of iso-octane and toluene. Oxygenated hydrocarbons are added to

this mixture to obtain a blend of interest.

Table 1 - Properties of hydrocarbons commonly used in compatibility and permeability studies.

Chemical Formula

Mole

Weight

Vapor

Pressure

Density

g/cc

Boiling

Point

ΔHvap

BTU/lb
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

12

@ 100°F @20 °C °F

Isooctane C8H18 114.2 1.70 0.6919 210.6 116.7

Toluene C7H8 92.1 1.00 0.8660 231.1 156.2

MeOH CH4O 32.0 4.60 0.7915 148.1 502.0

EtOH C2H6O 46.0 2.30 0.7890 173.0 396.0

TBOH C4H10O 74.1 0.7857 180.3

AmOH C5H12O 88.2 0.8083 216.5

MTBE C5H12O 88.2 4.73 0.7404 131.4 138.0

ETBE C6H14O 102.2

TAME C6H14O 102.2

DIPE C6H14O 102.2 0.7258 155.0

5.0 Theoretical Overview

5.1 Chemistry of Oxygenated Hydrocarbons

It is important to briefly consider the chemistry of oxygenated hydrocarbons in order to establish

their stability and propensity to form other compounds in gasoline. The chemical behavior of

oxygenated hydrocarbons in solution governs the behavior of materials in contact with them.

5.1.1 Oxidation of Ethers

During prolonged storage in the presence of air, some aliphatic ethers are known to slowly

oxidize to form peroxides in low concentrations.3 Peroxides can be unstable and hazardous in the

presence of hydrocarbons and other materials. Moreover, they serve to reduce octane number by

a disproportionate amount. Ethers with alpha hydrogen atoms attached to the carbon adjacent to

the ether linkage, such as diisopropyl ether, DIPE, are most susceptible to this type of oxidation

reaction as illustrated below.

(R)2-CH-O-R’ + 2O2 ‡ (R)2-CH-O-O -R’

MTBE and TAME, with no labile methylene hydrogen atoms will be least prone to undergo this

type of oxidation under normal ambient storage conditions.

Peroxide formation in gasoline reformulated with ETBE and without normal antioxidant levels

was tested for six months at room temperature with periodic exposure to air. Tests show a minor

amount of peroxide formation.4 The peroxide level was deemed negligible with respect to hazard

or octane detriment. Therefore peroxide formation in MTBE, TAME, and ETBE reformulated

gasolines should not be an issue especially since antioxidants are added to prevent oxidation of

olefins also present in the fuel. Furthermore, air exposure to gasoline in transportation and

storage equipment is limited.

Another author considered peroxide formation in automotive fuel return lines.5 This concern is

primarily due to the increased temperature of the fuel and is not considered an issue for the retail

environment.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

13

5.1.2. Alcohols as Bases

Alcohols may act as a base in the presence of active metals.6 The reaction products include an

alkoxide anion and the metal cation as shown below:

ROH + M ‡ RO- + M+ + 2H2

The smaller the alkyl group, the more acidic the alkoxide acts. Methanol forms the strongest

anion and tertiary alcohols the weakest. This reaction explains the rapid corrosion phenomenon

observed for active metals such as magnesium and aluminum in dry alcohol solutions, particularly

methanol.

Citing this reaction, Hertz7 indicated that an alkoxide ion would subsequently be reactive to

elastomers and other nonmetallic materials. However, because an hydroxide ion is a weaker base,

the alkoxide is very short-lived in the presence of even trace amounts of water. Water reacts with

the alkoxide ion to produce the alcohol back again and the metal hydroxide. Since water is

common in gasoline distribution and storage systems, degradation of materials by alkoxide ions is

not considered to be an important mechanism.

Unlike some ethers, alcohols are not known to oxidize under normal ambient storage conditions.

5.1.3. Hydrogen-bonding of Ethers and Alcohols

Unlike non-oxygenated hydrocarbons found in gasoline, alcohols and ethers are electrostatically

dipolar molecules. That is, certain moieties in the molecule carry a partial positive charge and

others in the same molecule carry a partial negative charge. This charge polarization creates

attractive forces among complementary charged moieties of other similar or dissimilar molecules.

Due to the electronegativity of oxygen atoms relative to carbon and hydrogen, the oxygen atoms

of both an ether or an alcohol (hydroxyl) group carry a partial negative charge. The two pair of

unshared electrons in the oxygen atom of both groups make those moieties proton acceptors or

“Lewis bases.” Both carbon atoms adjacent to the ether oxygen carry smaller partial positive

charges. In contrast, the hydroxyl group contains a partially positive hydrogen atom. This

positive charge on a hydrogen atom makes it weakly acidic. Thus, the hydroxyl group is said to

be a proton donor, or a “Lewis acid”, as well as a Lewis base.

Lewis acids and Lewis bases are attracted to one another through a process called hydrogenbonding. Water is perhaps the most familiar of hydrogen-bonding substances. Strong hydrogen

bonding is responsible for some of water’s interesting properties such as high melting and boiling

temperatures. In hydrogen-bonding, one acidic proton is shared between two basic sites by

mutual coulombic attraction of the proton. A review of the theoretical concepts associated with

hydrogen bonding phenomenon is beyond the scope of this paper but an excellent review may be

found elsewhere.8
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

14

As a result of hydrogen bonding, alcohols are attracted both to each other and to other molecules

with Lewis base sites. In contrast, because ethers have no acidic hydrogen atoms, they show very

little affinity for each other but will be attracted to Lewis acids, if present in solution. Further,

both ether and alcohol based solvents will hydrogen-bond with Lewis acid sites that may be

present in polymeric materials. This phenomenon leads to excess swelling and these concepts will

be discussed further in the section on Corrosion Mechanisms.

Favored by an exothermic interaction energy, associated species of Lewis acids and bases are

present in solution along with their parent species. For example, it has been shown that neat

methanol may exist as a hydrogen-bonded cyclic tetramer.9 Both the neat alcohol monomer and

an associated species exist in equilibrium. The concentration of each species will depend on an

equilibrium constant, k, which, in turn, depends on the association energy of the hydrogen-bonded

species relative to the absolute temperature. An equilibrium reaction may be written to describe

this type of solvent-solvent self-association as follows:

nMeOH <---> (MeOH)n

 k

Hydrogen-bonded oligomeric species have unique chemical properties just as covalently-bonded

monomeric species do. As such they are able to interact with materials in a similar manner as

their disassociated parent species.10 Insofar as chemical interaction with materials is concerned,

some self-associating solvents like alcohols must be considered as multicomponent solvent blends

of polar and non polar solvents even when they are pure. Methanol, existing as the monomeric

species, is quite polar; whereas methanol existing as a tetramer, is considerably less polar. For

this reason, self-associating solvents like dry methanol and chloroform are often powerful swelling

agents for both polar and, surprisingly, nonpolar polymers alike. Small quantities of added water

tends to break-up the tetramer methanol species and lower swelling has been observed for FKM66 copolymer elastomers (myers and abu-isa) in hydrated methanol.

5.2 Solution Thermodynamics

Electrostatic interaction of chemical species in solution creates other interesting behavior in

gasoline blended with ethers and alcohols. One must also consider the change in chemical

potential of a solution as new species are introduced. The resulting activity of a solvent in

solution has been shown to play an important role in determining equilibrium absorption of

solvent by a material.

5.2.1. Ideal solutions

An “ideal solution” is defined as one in which the enthalpy of mixing the components is zero. In

an ideal solution, the activity of each species is equal to its mole fraction in solution. The activity

coefficients, defined as the activity of each component divided by its mole fraction, are therefore
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

15

equal to unity. In ideal solutions, most linear blending rules used for solution property

correlations, such as Rault’s law for vapor pressure, are obeyed.

In general, gasoline which contains a blend of aliphatic, olefinic and aromatic hydrocarbons is

considered to be an ideal solution. Addition of oxygenated hydrocarbons in the form of ethers

does not appreciably change the ideal behavior of gasoline. Because an ether is a Lewis base,

some minor interaction among the pi electrons of aromatic species and the ether is known.

However, there is not enough chemical interaction in an ether blended gasoline to invalidate the

ideal solution assumption for most applications. The activity coefficients of all components in

reformulated gasoline are approximately equal to one.

Figure 1 illustrates this ideal behavior. The activities of MTBE, toluene and iso-octane in model

fuels containing various concentrations of MTBE are approximately equal to their mole fraction in

solution.

The vapor phase composition may be easily computed from equations governing vapor-liquid

equilibrium. The mole fraction xi, the activity coefficient γi ,and the vapor pressure Pi of each

component in the liquid phase must be known. Assuming the vapor is an ideal gas, the

equilibrium vapor composition yi may be computed as follows:11

y i

i i i

i i i

x P

x P

=

∑

g

g

 

For ideal solutions, the activity coefficients are equal to one and the vapor phase composition

scales linearly with the mole fraction of components in the liquid. Further, the molar volumes are

approximately equal; thus, volume fractions are approximately equal to the mole fractions. This

simple calculation is conducted for 15 percent by volume MTBE blend in ASTM Fuel C.

Table 2 - Approximate compositions of vapor liquid equilibrium for a 15 percent by volume 

 blend of MTBE in ASTM Fuel C.

Component xi ai Pi ai Pi yi

Isooctane 0.425 0.425 1.7 0.723 0.390

Toluene 0.425 0.425 1.0 0.425 0.229

MTBE 0.150 0.150 4.7 0.705 0.380

total 1.000 1.853 0.999

It is observed in Table 2 that the mole fraction of MTBE in the vapor phase, yi , is about 2.5

times greater than its mole fraction in the liquid, xi. This concentrating effect of the oxygenated
Compatibility and Permeability of Oxygenated Fuels to Materials 16

in Underground Storage and Dispensing Equipment

Figure 1 Activity of MTBE, Toluene and Isooctane as a function of 

ether concentration in Fuel C

0

0.2

0.4

0.6

0.8

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Mole Fraction MTBE in Fuel C

Activity of Components

mtbe

iso

tol
Compatibility and Permeability of Oxygenated Fuels to Materials

17 in Underground Storage and

Dispensing Equipment

component in the vapor phase is due to the elevated vapor pressure of MTBE relative to the other

hydrocarbon constituents.

5.2.2. Non-ideal solutions

Both positive and negative deviations from ideal solution behavior are possible whenever polar

interaction among species in solution occurs. Alcohol blended fuels exhibit positive deviations

where the activity coefficients are much greater than one. This deviation is a direct result of the

interaction that alcohol molecules have for each other. Driven by the entropy of dilution with nonpolar gasoline, the heat of mixing is endothermic because the self-associating alcohol species are

being dissociated.

Figure 2 illustrates this non-ideal solution behavior. The activities of EtOH, toluene, and isooctane are all considerably greater than their mole fraction in solution. Since the total pressure is

the sum of component vapor pressures multiplied by the component activities, the vapor pressure

of a blend of alcohol and gasoline is greater than the vapor pressure of the neat components.

Another practical implication of the high activity of alcohol in gasoline blends containing more

than 10 percent alcohol is the tendency for phase separation. By definition, if the activity of any

blend component equals one, then phase separation occurs. It is well known that, depending on

the temperature and the aromatic content of the gasoline, addition of 0.5 percent by volume water

to gasoline blends containing 10 percent EtOH or more will cause aqueous phase separation.

Methanol blends are even more sensitive to water. Here, an alcohol molecule prefers to separate

into an aqueous phase where it has an exothermic hydrogen-bonding interaction with water rather

than remain in the hydrocarbon fuel where its heat of mixing is endothermic.

Now consider the vapor liquid equilibrium for MeOH blended fuels. For non ideal solutions, the

activity coefficients are not equal to one and the vapor phase composition scales linearly with the

activity of the components in the liquid. A further accounting is required for the difference in

molar volumes among the species. Assuming again that the vapor phase is an ideal gas, the

calculation is repeated for a 10 percent by volume MeOH blend in ASTM Fuel C.

Table 3 - Approximate compositions of vapor liquid equilibrium for a 10 percent by volume 

 blend of MeOH in ASTM Fuel C.

Component xi ai Pi ai Pi yi

Isooctane 0.375 0.45 1.7 0.765 0.149

Toluene 0.375 0.45 1.0 0.450 0.088

MeOH 0.250 0.85 4.6 3.910 0.763

total 1.000 5.125 1.000

It is observed in Table 3 that the mole fraction of MeOH in the vapor phase is about five times

greater than its mole fraction in the liquid. This is due to its relatively high vapor pressure as well

as its non ideal behavior in the hydrocarbon liquid. Furthermore, the vapor pressure of the
Compatibility and Permeability of Oxygenated Fuels to Materials 18

in Underground Storage and Dispensing Equipment

Figure 2 Activity of EtOH, Toluene and Isooctane as a function of 

Alcohol Composition in Fuel C

0

0.2

0.4

0.6

0.8

1

0

0.20.40.60.8

1

Mole Fraction of EtOH in ASTM Fuel C

Activity of Components

EtOH

Iso

Tol
Compatibility and Permeability of Oxygenated Fuels to Materials

19 in Underground Storage and

Dispensing Equipment

blend is much greater than any of the neat constituents.12 13 14 Similar non-ideal behavior is also

observed for ethanol blends.

These phenomena, higher vapor pressure of the blend and also phase separation sensitivity,

observed in gasolines blended with alcohol are indications that a greater driving force exists for

alcohols to leave dilute solutions with gasoline than there is for ethers to leave reformulated

gasoline. The relative magnitude of this driving force has further implications regarding

compatibility and permeability issues for nonmetallic materials. These issues will be more fully

discussed in the following sections.

6.0 Corrosion Mechanisms

6.1 Non-Metals

Non-metallic materials do not corrode by electrochemical processes as metals do. This is because

nonmetals are dielectric materials which cannot conduct corrosion currents and most do not form

stable ionic species. However, dimensional changes and mechanical property changes observed in

nonmetallic materials are directly attributable to absorption of hydrocarbons and the resultant

swelling and plasticization. These concepts will be discussed below.

Nonmetallic materials commonly found in a retail gasoline station may be classified into three

categories: elastomers, thermoplastics, and thermosets.

An elastomer may be defined as a material which can be stretched beyond its yield point and yet

its strain is largely recoverable upon relaxing the applied stress. This strain recovery property is

usually obtained by mild cross-linking (three mole percent or less) or vulcanization of a rubbery

gum. Elastomers are commonly used in flexible hose constructions, seals, gaskets and packing.

A thermoplastic is usually either a semi-crystalline or glassy amorphous material which, upon

heating, will reversibly melt into a processable liquid. Thermoplastics are commonly used in

flexible underground piping, sumps, vapor recovery tubing, etc.

A thermoset is usually a glassy material which is cured into a highly cross-linked network. Once

vitrified, a thermoset cannot be melted and reprocessed like thermoplastics. Thermosets are

commonly used in reinforced composites and are found as matrix materials for rigid piping and

underground storage tanks (UST).

A brief theoretical description of swelling and permeation phenomena which occur in these classes

of polymeric materials is presented in the following sections.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

20

6.1.1. Equilibrium Swelling of Nonmetallic Materials by Solvents:

A critical performance factor for nonmetallic materials exposed to a mixed chemical environment

is the degree of solvent absorption by the material at equilibrium. Swelling not only affects the

physical dimensions of a part but the mechanical properties are affected as well.

Given an observation of elastomer swell in a pure solvent, νi,E, Flory5 suggested that the activity

coefficient of a solvent absorbed in an elastomer, Ωi,E , may be estimated from a solvent-elastomer

interaction parameter, χi,E, and an elastic retraction constant, ε, as follows:

Ln (Ωi,E) ≅ νi,E + χi,E νi,E

2

 + ε (νi,E

1/3

 - νi,E/2) 2

The first term on the right hand side of Equation 2 accounts for the dilution entropy of the

solvent in the elastomer. The second term expresses the enthalpy of dilution. The last term

expresses a contribution to chemical potential due to the elastic retraction energy in the material.

Although this theory was intended only for elastomers exposed to pure solvents, it provides a

useful framework to qualitatively discuss the compatibility of many polymeric materials exposed

to solvents. For example, the elastic retraction parameter is proportional to the number density of

cross-links in the neat material. Since thermosets are cross-linked more than elastomers they

typically swell less than elastomers in any given solvent.

More important is the role of the polymer-solvent interaction parameter, χi,E. If this value is zero,

then no interaction enthalpy exists and intermediate swelling is observed. This type of swelling is

driven by entropy and opposed by elastic retraction energies. This case is typical of non-polar

polymers exposed to conventional non-oxygenated gasoline.

If the polymer-solvent interaction parameter is positive, the resulting interaction is endothermic.

Endothermic interaction implies that the polymer prefers interaction with itself rather than with

the solvent molecules. The resulting solvent absorption will be very low because swelling is

opposed by the heat of mixing as well as elastic retraction. This behavior is the basis for one

strategy of obtaining fuel resistance: incorporate acid-base sites into the polymer backbone which,

in turn, repel the non-polar solvent molecules in non-oxygenated gasoline. Most fluorocarbon

elastomers, nitrile rubber and polychloroprene elastomers, for example, gain their resistance to

gasoline in this way.

If the value of χi,E is negative, then the enthalpy of mixing polymer and solvent is exothermic.

Exothermic interaction implies that complementary attractive forces exist between Lewis acid

and/or Lewis base sites on the polymer backbone and on the solvent molecule. In such cases,

swelling will be relatively great because it is driven by the enthalpy and the entropy of mixing.

This type of interaction may occur when polar polymers come into contact with polar oxygenated

hydrocarbons.

The degree of solvent uptake in a material depends on both the activity of the solvent molecule in

solution and the interaction parameter of the polymer-solvent pair. For the case of small
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

21

interaction parameters (i.e. aggressive solvents), a further simplification to Flory’s model has been

shown to be useful. The three component energy balance may be estimated by a series of

partition coefficients. This simplification is valid provided that one partition coefficient is

assigned for each species present in solution, associated or not. Therefore, the volume fraction of

a mixed solvent absorbed in a material, C, is related to the product of the solvent activity in

solution ai and a partition coefficient. The partition coefficient, or solubility S, is the volume

fraction of pure solvent absorbed at unit activity.

Ci = Si ai 3

This expression for liquid partitioning into materials at equilibrium is analogous to Rault’s Law

for vapor liquid equilibrium. The volume fraction of solvent in the elastomer, rather than the

volume swell or weight gain, is a useful way to characterize the swelling power of ideal solutions.

This utility for gasoline reformulated with MTBE has been shown on a theoretical as well as

experimental basis. Thus, for substantial swelling to occur, both the solvent activity and the

solubility must be greater than zero, and either one should be relatively large.

In a review article, Davidson15 refers to the increased mole fraction of oxygenated hydrocarbons

in the vapor phase relative to the liquid phase which was discussed in the previous section. He

poses an interesting question regarding the potential for excess swelling and permeation in the

vapor recovery system and in the head space of an UST as a result of this compositional

difference.

The key to understanding this swelling problem is to recall that it is an activity gradient which

drives swelling of polymeric materials by solvents, not necessarily concentration. From a

theoretical standpoint, as long as the saturated vapor phase is in thermodynamic equilibrium with

the liquid phase, the activities of each component in both phases are equal. Therefore the vapor

cannot be a more aggressive swelling media than the liquid. However, under some conditions, the

reverse may be true. If the components in the liquid phase are strongly self-associating, such as

with alcohols, then the vapor phase may be a less aggressive swelling media. This is because the

associated species does not partition into the vapor phase (being an ideal gas). If the associated

species accounts for a significant contribution to the total swell, then the vapor phase may actually

be a weaker swelling media.

If the enriched vapor phase is allowed to condense in some region away from the mother liquor,

then there is a potential for increased swelling and permeation. Condensing a saturated vapor

may occur by either increasing the pressure or decreasing the temperature. If temperature is

decreased, then this may be a temporary condition. When reheated, the liquid will return to the

vapor phase. If the pressure is increased, due to some processing step in the recovery system, then

there is indeed increased potential for swelling and permeation of polymeric materials.

6.1.2. Unsteady Mass Flow of Solvents through Nonmetallic Membranes:
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

22

The preceding discussion concerned equilibrium absorption of solvents by polymeric materials.

The present section reviews the kinetics of mass transport into and through a polymer membrane.

A study of the kinetics of absorption and permeation leads to an understanding of experimental

parameters required to reach equilibrium absorption and steady state permeation. Later in this

review, some literature permeation data will be interpreted using linear diffusion kinetics as

summarized in the following discussion.

Most observers of gasoline permeation report total permeability and permeance (permeance is the

permeability coefficient normalized by the thickness of the membrane) values for the fuel blend.

However, desired information regarding the permeation rate of individual components, such as

MTBE, are not reported. If applicable, mathematical analysis of permeation data from mixed

solvents as a function of concentration may yield information regarding the individual

contributions to the total mass flow. Therefore a discussion regarding the limitations of such an

analysis is warranted. A more complete discussion of these issues may be found elsewhere.16

Linear diffusion kinetics are often referred to as Fick’s Law or Case I diffusion. Case I diffusion

in materials usually holds for vapors such as oxygen, helium, etc.; but, may not be valid for

diffusion of solvent liquids. The key assumption in Case I diffusion is that the diffusion coefficient

is independent of concentration of solvent in the material. Increasing diffusion coefficient with

concentration is usually observed for strongly absorbing solvents. A convenient way to relax this

constraint is to perform piecewise analysis whereby diffusion coefficients are valid over limited

concentration ranges.

Case II diffusion is another useful theoretical construction where there is an abrupt transition

between solvated material and unsolvated material. Here the diffusion coefficient is several orders

of magnitude greater in the solvated material. The abrupt change in diffusion coefficient is often

associated with a change in morphology in the host material such as glassy to rubbery phase

transition. For example, Case II diffusion has been observed for strongly associating solvents,

such as methanol, permeating glassy polymers, such as polymethylmethacrylate. Case II diffusion

is the dominate mechanism for methanol permeation in fiberglass laminates.

As with swelling, activity gradients drive permeation as well. For permeation of vapors, the

activity gradient is often expressed as a change in pressure on either side of a membrane. Henry’s

law is used to assign coefficients which describe the concentration of permeant in the membrane

as a function of its partial pressure. However, pressure will have little if any effect on the activity

of a liquid permeant. Therefore activity (not necessarily concentration) gradients are used to

represent the driving force for mass transfer. Equation 3 is used to describe the concentration,

by volume fraction, of permeant as a function of its activity in solution. This is in contrast to

those authors who choose to scale the driving force in terms of vapor pressure or concentration17

expressed as mass per unit volume. The present approach utilizes different units in the diffusion

equations but does not change the fundamental character of the analysis.

Using this scheme of scaling the permeation driving force in terms of activity, the permeation data

of reformulated gasoline containing nearly ideal solutions of ethers is successfully modeled.

Therefore an estimate of ether permeation rate from a blended fuel can be made. By contrast, a
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

23

large excess permeation is observed for non ideal fuel blends containing dilute alcohols. This

excess permeation is related to the excess absorption observed for these systems. In this

reviewer’s opinion, current linear permeation theory is not able to account for this excess

permeation and component contribution of alcohol permeation from gasohol cannot be estimated.

If the mass flow of alcohol by permeation through materials is desired, then it must be measured

directly and these measurements have not been reported.

Several very nice summaries are available which discuss the morphological parameters that affect

permeation through materials. 10 18 Important considerations include: permeant concentration

(activity), size and shape and flexibility of the permeant, temperature, permeant/polymer

chemistry, thickness and crystallinity and void content in the membrane, polymer chain stiffness

and interchain interaction. Plasticizer content, if any, is also very influential.19

Linear Permeation Theory--Fick’s Law

Whenever a flow pathway through a material is narrow relative to the mean free path of the

solute, then molecular flow will govern. Molecular flow often occurs in vacuum systems and is

the dominant mechanism of solvent permeation in polymers. It is a gradient in chemical activity,

ai, which drives diffusion into and permeation through a membrane. As discussed above, the

activity of a liquid component ai may or may not be equal to its volume fraction in solution. For

ideal solutions, the concentration of a solvent is often substituted for activity.

The isothermal mass flow rate, Qm, of a liquid solvent through a membrane is driven by the

activity gradient according to the following equation:20

 Q

RT

M

S

a a

L

m =

−

Γ( )

( )

1

2 1 2

i 4

where, Γ is a geometrical constant, R is the ideal gas constant, and T is the absolute temperature.

The mass flow rate of liquid Qm is inversely proportional to the molecular mass M of the solvent

and the thickness L of the membrane. Since the solvent molecules do not contact one another,

there is no viscous dissipation in molecular flow.

Equation 4 is commonly simplified via dimensional analysis by introducing a lumped

constant known as the permeability coefficient, P. Assume that the solvent activity is constant on

one side and zero on the other side of a membrane. The steady state value of mass transport now

becomes:

Qi =P Si ai

A

L

5

where A is the cross sectional area of a membrane. Si ai is the concentration of solvent in the

material at fractional activity. If a pure solvent is considered, ai is unity. P will have unique

values for any given polymer-solvent system and will vary with temperature. Once P is known,
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

24

the steady state mass flow of a diffusing solvent may be computed for a given solvent activity

drop and membrane geometry.

Multiplying both sides of the above equation by thickness, L, utilizing Equation 3, and

differentiating concentration with respect to distance through the membrane, the familiar form of

Fick’s first law for steady state permeation emerges:

P = - D * dC

dx

 6

Another material constant, diffusivity D is expressed as the square of the membrane thickness

divided by a characteristic time. To examine the unsteady diffusion behavior, allow Ci to

represent the concentration of solvent as a function of time t and position x through the

membrane thickness. Differentiating Equation 6 with respect to time, the one-dimensional

unsteady differential form of Fick’s second law is expressed as follows:9

∂ C(x,t) = D ∂

2C(x,t) 7

 ∂ t ∂x

2

To analyze unsteady diffusion behavior, Fick’s second law may be solved for various cases of

solvent diffusion in materials.

Example 1: Absorption into an Immersed Slab (swelling):21

In the study of absorption kinetics, one requires an expression for the amount of solvent which

has been absorbed into a material after a given time. Choose an infinite area slab geometry with a

solvent free initial condition. Further choose boundary conditions for solvent concentration at

both surfaces a constant value of C1. This condition implies that the slab is immersed in solvent.

We wish to measure the amount of solvent absorbed as a function of time. Integration of

Equation 7 gives the concentration profile across the sample thickness as a function of time.

This function is an infinite series as shown below.

C(x,t) = (

4 C

P

)

1

( 2 m 1 )

0

m=1 +

∞

∑ sin [(2m + 1) Πx

L

 ] exp[-D (2m + 1)2 Π

2

2

t

L

] 8

Integration of Equation 8 with respect to position yields the total volume fraction absorbed, V,

in a slab as a function of time. This function is also an infinite series where approximately five

terms are required for an adequate estimate.

(V(t) V )

V

− 0

∞

 = 1 - 8 1

Π 2m 1

2

( + )

∞

∑

m=1

exp[ - (2m+1)2

(

Π

L

)

2 D t] 9
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

25

Concentration values can be converted to mass or volume changes by using appropriate

conversion factors. The unsteady absorption behavior of Equation 9 is plotted schematically in

Figure 3. This figure teaches that, for a slab of unit thickness, time t of magnitude 0.28/D is

required to achieve 95 percent of the equilibrium absorption. Diffusivity is known to range over

several orders of magnitude among materials: elastomers generally exhibit the highest values while

thermoset materials typically exhibit the lowest values. Therefore, experimental requirements for

measuring absorption of solvents in different materials is expected to vary considerably. This

means that a period of years may be necessary to achieve and measure steady state permeation in

composite material. One way to accelerate the permeation measurement is to use thinner

materials in experiment.

A useful simplification of Equation 9 may be used for materials which exhibit a low diffusivity.

At short immersion time, it may be simplified to the following approximation:

V t

V

( )

∞

 = 4 D

1/2 t

L

1 2

1 2

/

/ Π

 10

Therefore a plot of V(t) / V∞ versus t 1/2/ L will yield a straight line in the initial absorption region.

The slope of this line is proportional to D.

Example 2: Mass Flow through a Membrane (permeation):22

If one is measuring the rate of flow of a solvent through a membrane, or permeation, there will be

an interval of time from the moment the solvent comes into contact with the membrane until it

emerges on the other side. Further time will be required to achieve steady state mass flux. For

ease of analysis, choose an infinite slab geometry with a solvent free initial condition. Further

choose boundary conditions for solvent concentration at the inside surface as a constant value of

C1 and the concentration of solvent at the outside surface as a constant value of zero. The latter

boundary condition implies that mass transport is diffusion limited or, equivalently, the solvent

permeates the external environment much faster than the membrane. As such, accumulation of

permeant in a boundary volume located on the outside surface of the membrane is negligible.

As measured at the outside surface, the total mass flow through the membrane increases with

time to a steady state value. The solution of Equation 5 under these conditions is also an infinite

series where approximately five terms are required for an adequate estimate.

Qm = D S ai

A

L

[1+2

n=

∞

∑

0

cos(nπ)*exp(-( n

L

p

)

2

 D t)] 11
Compatibility and Permeability of Oxygenated Fuels to Materials 26

in Underground Storage and Dispensing Equipment

Figure 3 Schematic representation of unsteady mass flow by Case 1

diffusion of a permeant through a membrane
Compatibility and Permeability of Oxygenated Fuels to Materials

27 in Underground Storage and

Dispensing Equipment

As the concentration gradient reaches a steady state value, the mass flow rate through a

membrane is constant and Equation 11 reduces to Equation 5. The interested reader may also

note that Equation 11 is actually the derivative of a more common expression used to describe

permeation through a membrane where the permeant concentration is allowed to build-up on the

downstream surface. Equation 11 is valid for the present case where the permeant is continually

removed from the outer surface.

Figure 3 teaches that, for a membrane of unit thickness, an onset time of approximately (0.04/D)

is required to measure the first one percent of the steady state mass flow of molecules which

permeate through a membrane. About an order of magnitude more than the onset time is required

to measure steady-state mass flux. Diffusivity is known to range over several orders of magnitude

among polymers: elastomers generally exhibit the highest values while thermoset materials

typically exhibit the lowest values. Therefore, experimental requirements for measuring

permeability in these different materials is expected to vary considerably.

In summary, mass flow by permeation is proportional to the product of diffusivity, solubility and

the activity of a solvent molecule. The diffusivity depends upon the inverse square root of the

molecular mass of the diffusing solvent whereas the solubility depends upon a binary interaction

parameter that is related to the enthalpy of mixing of polymer and solvent. Therefore a high

permeability may be observed with either small, volatile solvents or with solvents that strongly

associate with the matrix material or both.

6.1.3 Plasticization and Extraction

Diffusion of solvents in and through polymers is generally accompanied by a change in properties

of the material. In addition to the change in physical dimensions associated with swelling,

mechanical properties such as strength and stiffness are usually affected. As little as 20 percent

volume swell can reduce physical properties such as hardness, strength, and tear resistance of an

elastomer by 60 percent.23 Twenty percent by volume swelling is generally considered a

conservative upper limit for solvent absorption by an elastomer in a sealing application.24 Solvent

absorption is also of critical importance to thermoplastic and thermoset materials. Although these

materials typically absorb much less solvent, substantial changes in mechanical properties and a

reduction of upper-use temperature are also observed.

Underground Storage Tank Membranes:

Perhaps the most critical application where plasticization of a material by solvent ingress is

important to consider is the underground storage tank (UST). Since an UST is buried, it will be

subjected to combined compressive loads from the soil, the water table and the live loading. To

resist global buckling, composite UST’s are equipped with circumferential stiffening rings.25 As

such, UST design incorporates large factors of safety against global buckling. The limiting design

criteria for UST’s is local elastic buckling of the membrane between the ribs.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

28

Buckling formulas for cylinders under external loading show the importance of the stiffness of the

membrane, E. If an unsupported thin-walled tube containing stiffening rings is subjected to

external pressure, then the formula for critical buckling load, Pc, is proportional to the stiffness of

the membrane according to the following form.26 27

Pc = 2Et /D(n2-1)(1+k) + 8EI[ n 1 2n 1

1 k D 1

2 2

1

3

1 1

− + − −

+ −

n

n n

h

( ) ( h h)

]

12

where “t” and “D” are the thickness of the membrane and the diameter of the tube, “L” is the

distance between stiffening rings, “ν” is Poisson’s ratio of the material and “k” is a dimensional

constant equal to 2nL/πD. Notice that the only material property of consequence (other than ν) is

the stiffness of the material “E.” Strength of the material is unimportant to buckling analysis.

Thus any drop in membrane stiffness will result in the same percentage drop in the local buckling

resistance of the shell according to this equation.

If the cylinder is buried, additional support may be afforded by the soil. A modified Luscher

formula has been proposed28 to describe the local buckling resistance of a ribbed cylinder under

uniform soil support. This empirical function is written as follows:

FS * Pc = (32RwB’E’E I/D3)

2 13

Where “E” is the soil modulus, “B” is Luscher’s coefficient of elastic support and “Rw” is an

empirical constant. “FS” is a safety factor. In Equation 13, the soil is assumed to contribute

significantly to the local buckling resistance of the tank. Because of uniform soil support, any

decrease in laminate stiffness has a square root effect on the critical buckling load of the laminate.

It is for this reason, that fiberglass UST vendors emphasize the importance of good installation

practices.29 Well compacted back-fill is required to provide maximum soil stiffness and uniform

soil support of the membrane. This is especially important because deformations associated with

localized buckling are much smaller than those associated with global buckling. If uniform,

compacted soil is not provided, either by installation or service considerations, then the behavior

of the critical buckling load with laminate stiffness may actually lie somewhere in between the

functions described empirically by Equations 12 and 13.

Thus, for sound design, a safety factor in Equation 13 is utilized. The safety factor is proposed

to be 2.5 and it incorporates at least five elements including, creep, environmental effects, as well

as other uncertainties such as soil support variations. The allowance for reduction of stiffness due

to plasticization by environmental exposure is about 12 percent. This is presumably based upon

Owens Corning’s experience with plasticization of isophthalic polyester laminates by gasoline and

water environments. After more than ten years exposure, the laminate has been shown to retain

more than 80 percent of its original stiffness.30 Since (1/0.8)2 is about 1.12, an allowance of 12

percent modulus drop in the composite is allowed in this formulation.

Extraction of Plasticizers:
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

29

The inverse phenomenon of plasticization by solvent ingress is the migration of plasticizers and

other additives out of polymers. Additives such as antioxidants, heat stabilizers, processing aids

and plasticizers, etc. are commonly added to polymeric materials in order to achieve a certain

property set. These additives are generally not covalently bonded to the polymer and are able to

migrate. If a material containing additives comes in contact with a solvent, the additives may be

extracted by the solvent.31 32 33 34 If the solvent is not refreshed, the degree of extraction will

depend on a partition coefficient for the solvent-polymer pair. If the solvent is refreshed,

plasticizer extraction can be complete.

Extractable plasticizers are generally low molecular weight phthalate esters and are most

commonly used in elastomers such as NBR and flexible thermoplastics such as PVC and PA.35

They are, however, much higher molecular weight than the constituents in oxygenated gasoline.

Interestingly, no firm relationship has been established linking the molecular weight of the

plasticizer to its rate of permeation.

Often, plasticizer extraction is also accompanied by solvent swelling and the magnitude of these

phenomena will mask each other. Often the material will swell through a maximum value then

begin to shrink as the plasticizer leaves the host material. This has been described as a solvent

exchange process. Therefore the effects of plasticizer extraction may not be observed until the

polymer is removed from the solvent allowed dry-out for several hours or days.

Extraction of plasticizers will lead to significant change in properties such as: shrinkage,

increasing the brittle-ductile transition temperature of materials. Shrinkage of seal materials can

compromise their sealing behavior. MTBE is thought to be capable of extracting solid fillers,

such as titanium dioxide as well.36

6.1.4 Environmental Stress Cracking (ESC)

Environmental Stress Cracking (ESC) is a term commonly used to describe the combined

influence of the thermal-chemical environment and the applied load on crack initiation and

propagation mechanisms.

Increasing crystallinity and molecular orientation and decreasing molecular weight of polymers

has a tendency to promote ESC.37 Thus, in some polymer formulations, plasticizers are often

added to improve their flexibility and toughness especially at low temperatures. Extraction of

these plasticizers by solvent exposure can lead to embrittlement especially at lower temperatures.

Rupture of embrittled materials is a form of ESC.

On the other hand, unsteady migration of solvents into polymers will lead to swelling of the

surface layers more than the inner layer. In this case, the surface layers are put into compression

which is balanced by tensile loads in the non swelled inner layers. If the applied stresses, from

differential swelling, external loading or residual stress, is greater than the breaking stress, ESC

will occur. This effect may be more pronounced in glassy thermoplastics and thermosets where

diffusion into the polymer substrate is more likely to be described by Case II.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

30

Another form of ESC, called explosive decompression, occurs when vapors are condensed inside

polymers at high pressure. Rapid release of pressure causes expansion of these vapors. If the

vapor pressure is greater than the strength of the material, blistering will occur. Although this

type of failure mechanism is common for elastomers, operating pressures at retail gasoline stations

and distribution terminals are not high enough to cause this type of failure.

It should be mentioned that ESC is not discussed in the literature as an important failure

mechanism for materials exposed to oxygenated fuel environments. Therefore ESC will not be

discussed further.

Thermal Effects

Effect of Temperature on Swelling Behavior

As indicated above, equilibrium swelling of materials by mixed solvents is determined by the

product of a partition coefficient and the activity of the swelling species. Therefore, the swelling

behavior will depend on the nature of solvent interaction with other solvent species and with the

polymer.

It has been suggested (imbalzano) that increasing temperature increases permeability because both

the diffusivity and solubility increase with temperature. If there are no interactions among

polymer and solvent molecules, this is expected to be true.

If there are exothermic interactions among polymer and/or solvent molecules, then increasing

temperature will tend to shift the equilibrium away from formation associated species in favor of

the non-associated species. The resulting swelling behavior with temperature will depend on the

relative magnitude of the partition coefficients. For example, if high swelling observed at ambient

temperature is a result of strong partitioning of associated species, than increasing temperature

will tend to dissociate these species and the observed swell will decrease. This phenomenon has

been observed for the swelling of fluoroelastomers in MeOH. On the other hand, if low swelling

observed at ambient temperature is a result of strong interaction of the polymer with itself, then

increasing temperature will tend to break these associations and swelling of the observed swell

will increase as indicated above.

Effect of Temperature on Permeation

Permeation of solvents through polymers has been shown to be a thermally activated process.38

Therefore an Arrhenius form may be adopted to correlate the observed mass flow increase with

temperature as suggested below:

Q

Q

T

T 0

 = exp{ E /R ( 1

T

1

T

−

0

)} 14
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

31

where E is a lumped constant. In essence, E is the sum of an activation energy for viscous flow

and a binary interaction parameter associated with solvent absorption as discussed above.

Therefore, effect of temperature on permeation rate will depend on the relative changes in these

two energies. However it is common to observe permeability increase with temperature,

especially when no interaction exists among polymer and solvent species.

For example, consider methane permeating through PTFE near ambient temperature.39 The value

of E/R is observed to be approximately 3300 °K. This value implies that permeation rate

approximately doubles over a 17 °C increase in temperature.

A similar result was observed for the permeation of oxygenated fuels in elastomers. The

permeation rates doubled every 10 to 15 °C.40

Effect of Temperature on Elastomeric Seals

At low temperature, elastomers undergo a physical transition called the glass transition

temperature. Below its glass transition temperature, an elastomer behaves more like a rigid

plastic. An elastomer begins to lose its ability to form a seal at temperatures slightly above its

glass transition temperature. This low temperature limit for sealing is often reported in a standard

test as TR-10 temperature. TR-10 is the temperature below which an elastomer exhibits less than

ten percent retraction.

Most elastomers are formulated to have TR-10 values below -30 °C. However, more recent

compounds have been formulated for increased fluid resistance. For example, elastomers have

high fluorine content, in the case of fluoroelastomers, or high acrylonitrile content, in the case of

NBRs. Increased fluid resistance often compromises low temperature sealing characteristics. 41

Sometimes plasticizers are added to highly fluid resistant elastomers to retain the low temperature

sealing properties.

Pressure Effects

As mentioned above, the operating pressures are very low. Also, all solvents are condensed

liquids over the applicable range of operating temperatures and pressures. As such, the total

pressure will not have any appreciable influence on activity coefficients of the solvents. Thus,

pressure is not expected to exhibit an observable effect on the absorption and mass flow of

solvents in and through polymers.

Effect of Time

As mentioned above, the diffusion coefficient is observed to vary over several orders of

magnitude. Therefore the time required to reach equilibrium absorption or steady state

permeation is also expected to depend strongly on the type of material in question and the
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thickness of the part. For example, fluorocarbon based elastomers42 43 and thermoplastics are

known to require much more time to reach equilibrium absorption than hydrocarbon based

polymers require. One study reported 12 months were required for the swelling and extraction

phenomenon to stabilize in some elastomers.44 As a result, the diffusivity and permeability of

fluorocarbon based materials to solvents is correspondingly lower.

6.1.5 Testing Standards for Nonmetallic Materials

Fuel Stability

Two ASTM procedures test gasoline for gums and oxidative stability.45 46 The purpose of these

tests are to determine the quantity of oxidation products formed in a sample and or the propensity

of a fuel to oxidize and form these gums. The results of these tests can be used to indicate the

storage stability , due to oxidation, of the fuels.

Compatibility Testing

ASTM 47 defines non-oxygenated standard reference fuels for compatibility testing. Although

gasoline is a complex blend of aliphatic, olefinic and aromatic hydrocarbons, test fuels are

bicomponent mixtures of isooctane and toluene. Fuels A, B, C, D, and E contain a 0, 30, 50, 60

and 100 percent by volume toluene, respectively in isooctane. Since modern unleaded gasoline

usually contains between 30 and 50 percent by volume aromatics, most observers use Fuel C in

their testing. Aromatic hydrocarbons are known to be more aggressive than aliphatic or olefinic

hydrocarbons, so the use of Fuel C is thought to be a representative but conservative choice.

Standard methanol fuels are defined by SAE.48 Reference Fuel C is the base fuel to which various

concentrations of reagent grade methanol is added. For elastomers the abbreviations CM0,

CM15, CM30, CM50, CM85 refer to zero to 85 percent methanol in the reference fuel. For

plastics and metals, aggressive methanol is defined according to the following recipe: add 0.005g

NaCl, 0.05ml formic acid, and 0.1% H20 (metals only) per liter of methanol. This aggressive

methanol mixture is added to Fuel C in the above proportions. For auto-oxidized test fuel: 6.8ml

of 70% t-butyl hydroperoxide per liter of fuel C is added along with 0.01mg cuprous sulfate per

liter. The standard recommends performing tests in all fuels to determine the worst case. Then

do additional testing in the worst case fuel. Similar procedures can be established for other

oxygenates such as ethers and other alcohols.

ASTM provides applicable standards for testing the resistance of rubber49 and plastic50 to chemical

reagents. Little guidance is given on establishing equilibrium conditions, however an interesting

discussion is given in the latter standard on the combined action of stress and chemical exposure.

ASTM C 581 is a general standard for chemical resistance testing of composite materials. 51

Immerse samples for 30, 60, 90, 180 and 365 days in a media at a specified temperature.

Note color, weight, thickness. Measure change in these properties and also changes in flexural

strength and stiffness. Often the rate of change of properties with time is more significant than the

absolute value of the property. It is desirable to obtain constant value stiffness, strength,
Compatibility and Permeability of Oxygenated Fuels to Materials

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dimensions and weight after a period of time. This establishes that equilibrium conditions have

been established. Appearance: absence of cracks, crazes, loss of gloss, blistering etching, pitting,

softening. Discoloration of immersion media or accumulation of sediment. The sample should

appear absent of cracks, crazes, loss of gloss, blistering etching, pitting, softening. Discoloration

of immersion media or accumulation of sediment is also undesirable.

ASTM D 4021 is a standard which specifically addresses glass-fiber reinforced polyester USTs. 52

It suggests chemical resistance testing which is representative of the fluids to be contained inside

the tank, which include: ASTM Reference Fuel C, commercial unleaded premium gasoline,

unleaded gasoline, ethyl alcohol 90 percent to 10 percent by volume blended gasohol, and No. 2

fuel oil. Recognizing that water could be inside the tank, testing with distilled water, sodium

carbonate-sodium bicarbonate solution at pH=10, potassium biphthalate buffer at pH=4 are also

recommended. Further testing with the actual liquid to be contained in the tank, if different than

any of the above, is recommended. Interestingly, other than the prior requirement, there is no

specific requirement for MTBE compatibility testing as there is for alcohols. Testing conditions:

ambient temperature, immersion times of 1, 3, 6, 12 months are recommended. Accelerated

testing at 100 oF is suggested, immersion times of 1, 3, and 6 months or longer are required at

this temperature if the data are inconclusive. Evaluation of the materials after exposure by

conducting mechanical property testing consisting of flexural strength and stiffness, hardness, as

well as a visual inspection of the sample and media. Plot property retention versus time on a loglog plot and extrapolate to 100,000 hours (11.4 years). No guidance is given regarding minimum

property retention.

UL-1316 is a similar standard for fiberglass USTs which is more often cited than the above

ASTM standard. 53 Like the ASTM standard, coupons are immersed in liquids for 30, 90, and 180

days at 100 °F. They are then subjected to flex strength and stiffness testing as well as izod

impact toughness testing. There are two types of fluids to be tested. Type A fluids include:

leaded and unleaded gasoline, reference Fuel C, No. 2 and No. 6 fuel oils, saturated sodium

chloride, and sulfuric acid. Additional liquids for alcohol products include Reference Fuel C with

10, 15, 30, 50, 70, and 100 percent by volume ethanol, and 15, 50, and 100 percent by volume

methanol. Greater than 50 percent property retention is required for coupons exposed to these

media. Type B fluids include: toluene, deionized water, five percent hydrochloric acid, five

percent nitric acid, sodium carbonate-bicarbonate buffer at pH=10, sodium hydroxide at pH=12.

Greater than 30 percent property retention is required for coupons exposed to these media. As

with the above ASTM standard, there is no specific requirement of MTBE compatibility testing.

UL-971 discusses requirements for nonmetallic underground piping. 54 This standard requires

compatibility testing similar to the tank standards as well as permeability testing. With regard to

chemical compatibility, immersion testing of coupons for 30, 90, 180, and 270 days at 100 °F is

required. Measure retention of crush strength, tensile strength, and adhesive shear strength. Here,

four types of fluids are specified for testing: Type A and Type B, and internal and external fluids.

Greater than 50 percent retention of properties is required for type A fluids and greater than 30

percent retention of properties is required for type B fluids. This is required for both primary and

secondary piping. Type A internal fluids include: leaded and unleaded gasoline, reference Fuel C,

No. 2 and No. 6 fuel oils. Additional liquids for alcohol products include Fuel C with 10, 15, 30,

50 percent by volume ethanol, as well as 15 and 50 percent by volume methanol. Type A external
Compatibility and Permeability of Oxygenated Fuels to Materials

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fluids include: sulfuric acid at pH=3, and saturated sodium chloride. The type B internal fluid is

toluene. Type B external fluids include: deionized water, one percent hydrochloric acid, one

percent nitric acid, sodium carbonate-bicarbonate buffer at pH=10, and sodium hydroxide at

pH=12.

UL-567 discusses requirements for construction and performance of piping connectors.55

Leakage tests are to be conducted at 1.5 times the rated pressure and at the minimum bending

radius of the connector. It also specifies swelling and extractable limits for rubber parts (seals) in

contact with the media. After 70 hours at ambient temperature, the volume swell shall not exceed

25 percent (or 40 percent in reference Fuel C and its blends). Extractables are to be limited to ten

percent by weight and not more than a one percent shrinkage of the material. If these dimensional

stability limits are exceeded, additional proof testing is required. To preclude galvanic action, this

standard prohibits direct metal-to-metal contact between copper, or a copper alloy, and

aluminum.

Permeability Testing

Several testing standards have been established for testing nonmetallic materials permeability and

compatibility in oxygenated fuel blends.

In addition to the above chemical compatibility testing, permeability tests are required for

underground piping in UL-971. This test is performed by taking 18 inches of the smallest

diameter pipe, filling with liquid and then sealing it. Samples are weighed every month for 180

days. Permeance is computed every month and the maximum weight loss rate allowed is 4

g/m2/day (0.013 oz/ft2/day) for the primary conductor and 24 g/m2/day on the secondary pipe.

Area basis is the inside area of the pipe. This permeation requirement is a standard based upon

safety, not an environmental standard. Permeation test liquids are: unleaded premium gasoline,

unleaded regular gasoline, Reference Fuel C and No. 2 fuel oil and toluene. For alcohol fuels,

100 percent methanol and ethanol as well as blends containing 10, 30, and 50 percent by volume

blends of the same with Fuel C. Additionally, 10 and 30 percent by volume blends are required

for ethanol. Again no specific requirement for permeability testing in MTBE solutions is

mentioned.

ASTM standards for permeation of plastics are intended for the food packaging industry and , as

such, focus on oxygen56 and other gas57 transport rather than hydrocarbons. However these

standards do stress the requirement to achieve steady state permeation.

A procedure for determining permeation resistance of elastomeric tubing is SAE J1737.58 This

procedure specifies a reservoir to supply gasoline to the hose. In this way, speciation caused by

rapid permeation of dilute components present in the fuel is thought to be minimized. The units

of mass flow are expressed in terms of gm/m2/day. This number is called Permeance. Permeance

multiplied by the thickness of the membrane is called the permeability coefficient (example:

gm*mm/m2/day).
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SAE J2260 sets minimum requirements for multilayer tubing for gasoline and alcohol blends.

Permeability performance is measured at 60 °C.59 Table 4 summarizes the performance criteria as

specified in the standard:

Table 4 - Performance ranking system for elastomeric hoses based upon permeability to fuels

 (from SAE J2260).

Performance Category

 Number

Permeation Range

(gm/m2/day)

Suggested Time

for Testing (hr)

1 0-25 over 1000

2 25-50 1000

3 50-100 800

4 100-200 600

5 200-400 500

6 over 400 400

SAE J3060 specifies wall thickness standards for flexible hoses as follows: wall thickness for hoses

is as follows: up to 1/4 nominal, 0.76mm (0.030”); greater than 1/4 to 7/8 nominal, 1.02 mm

(0.04”); greater than 7/8, 1.27 mm (0.050”). For a permeation test, it specifies the reservoir

method where the length of the hose is 300 mm. The reservoir size equals 460 to 490 ml but only

filled to 300 ml. The assembly is weighed every day for an unspecified period. Agitate the fuel

daily. Calculate gm/m2/24hr on a daily basis. Also a cold flexibility test is given as follows:

Samples are conditioned at -40°C for five hours. Flex the hose through 180 degrees bend at a

radius equal to ten times the hose diameter. Flexing cycles are to occur within four seconds and

the hose must not show any signs of cracking. A proof pressure of 99 psi may be applied. Also,

the composition of hoses are described as follows: R7 is as NBR/PVC blend liner with a CSPE

cover, R8 is a CO liner with a CO cover and R9 is an FKM lined hose with an NBR/PVC blend

outer cover.

6.2 Metals

Many general texts can be consulted regarding the phenomenon of metal corrosion by aggressive

media.61 62 A concise set of terminology relating to corrosion and corrosion testing is found in the

ASTM standards literature63 along with a recommended practice for statistical analysis of

corrosion data.64

It is not the intent of the following discussion to teach corrosion concepts; however, a review of

some fundamental aspects of the problem, as is associated with oxygenated fuels, is desired. In

general, corrosion can manifest as general corrosion or pitting. General corrosion is a uniform

attack of the surface and corrosion rate is expressed in terms of loss of thickness or weight per

unit of time. Lifetime estimates may be made based upon a corrosion allowance. Pitting

corrosion is localized attack appearing as a series of holes in the surface. Under certain

conditions, pitting corrosion can bore holes completely through a metal part and cause pinhole

leaks. Both types of corrosion can occur simultaneously and the processes which govern pitting

corrosion versus general corrosion are very complex.
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6.2.2 Dry Corrosion

The corrosive behavior of alcohols has been divided into two major types: dry corrosion and wet

corrosion.65 Dry corrosion refers to the corrosion of metals in the presence of very dry fuel

blends. Dry alcohols can react with lead, magnesium and certain aluminum alloys with the

formation of alkoxide or alcoholate corrosion products. The alkoxide reaction in the absence of

oxygen was described above. The cathodic reduction of ethanol (and presumably methanol) in

the presence of oxygen and a metal has also been proposed as follows:66

1⁄2 O2 + C2H5OH + 2electrons ‡ OH- + C2H5OThe two electrons would presumably be supplied by the anodic dissolution of a metal. Aluminum

alkoxides have been proposed to decompose to Aluminum oxides and water by the following

mechanism:

2(C2H5O

-

)3Al ‡ Al2O3 + CH2=CH2 + H2O

If this mechanism is correct, it implies that ethanol blended gasoline cannot remain dry when in

contact with aluminum and other active metals. Water is a corrosion product and therefore

hydrated alcohol will result.

Hydrated alcohol contains water levels in the parts per million range. For example, as little as 0.1

to 0.2 percent water can effectively passivate aluminum and ferritic stainless steels and inhibit dry

corrosion of materials in neat alcohols. A threshold minimum concentration of water of about

0.25 percent by weight of methanol is required to suppress the corrosion of magnesium by

methanol blends.67

In addition to chemical reaction, water can get into gasoline from other sources such as

absorption from humid air,68 condensation, etc. Whenever water is present in gasohol, the

electrical conductivity of the gasohol solution increases to the extent that new corrosion

mechanisms are enabled. For example, mechanisms such as electrolytic and galvanic corrosion

have been observed. Galvanic and electrolytic corrosion processes described below are not

observed in gasoline which does not contain alcohol. This is because addition of ethers to

gasoline does not increase the solubility of water and therefore the conductivity of the

hydrocarbon phase.

6.2.2 Galvanic Corrosion

Galvanic corrosion occurs when two dissimilar metals are in electrical contact. When an

electrolyte solution wets this junction, an electrical circuit is completed. Whenever the

conductivity of the fuel exceeds 40 (Naegeli) to 7069 microseimens per meter, the electrolyte is of

sufficient strength to support galvanic attack as well as electrolytic attack (described below). In a

sense, a battery is formed where the more active material is corroded at the cathode. In turn, the
Compatibility and Permeability of Oxygenated Fuels to Materials

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less active metal acts as an anode and is protected from corrosion. Galvanic corrosion may occur

in the absence of other corrodants such as oxygen.

Galvanic corrosion is of particular concern for existing underground metallic components and for

components in the dispenser.1 Many of these systems were designed only for low conductivity

fluids and, as such, galvanic attack was not considered as a failure mechanism. Therefore, there

may be many dissimilar metal junctions in existing facilities which may be subject to galvanic

attack when storing and dispensing alcohol blended fuels. Of particular concern are bimetallic

junctions between aluminum and brass, because there is a large electrochemical potential driving

the corrosion of aluminum.70

6.2.3 Electrolytic Corrosion

Electrolytic corrosion is dissolution of metal by an impressed electrical current. If an electric

potential of greater than approximately one volt exists, stray electric currents can cause corrosion

of metal at a very rapid rate. This process will be limited only by the conductivity of the media.

Stray electrical currents of up to 40 microamperes have been observed between the fuel pump and

the fuel tank of automobiles.71

Conventional fuels are not conductive and therefore electrolytic corrosion may not have been a

design consideration for some retail gasoline tanks and dispensing equipment. Electrolytic

corrosion may be avoided by proper grounding of submerged components such as electric pumps

and level gauges.

6.2.4 Wet Corrosion

Depending upon the aromatic content of the fuel, the temperature, the presence of alcohol, and

perhaps other factors, water content above 0.1 (MeOH blends) to 0.5 (EtOH blends) percent in

gasohol will cause phase separation of the fuel blend.72 Water is essentially insoluble in gasoline

which does not contain alcohol, and phase separation occurs at very low levels of water. Many

UST’s have an aqueous layer in the bottom of the tank. Wet corrosion occurs in this lower

aqueous phase.

Due to a favorable exothermic interaction with water, the alcohol preferentially partitions into the

aqueous phase and depletes the alcohol content in the gasoline. The aqueous phase becomes

saturated with alcohol and ionic contaminants. The hydrocarbon phase in equilibrium with the

aqueous phase is hydrated gasohol. In contrast, ethers are soluble in water only to approximately

five percent at ambient temperature73 and non-polar hydrocarbons are essentially insoluble in

water.

In the lower, aqueous phase, reactions involving oxygen are more pronounced because the

solubility of oxygen is greater in the aqueous phase than in the hydrocarbon phase. Familiar

corrosion reactions may be written schematically in the following manner (“x” and “y” refer to

unspecified molar quantities):

xM + yH2O ‡ MxOy + yH2
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xM + 1/2y O2 ‡ MxOy

MxOy + H2O ‡ Mx(OH)y + OH -

These corrosion reactions are possible in the upper, hydrated alcohol hydrocarbon phase because

the presence of alcohols and water greatly increases the conductivity of the hydrocarbon phase.

The rate of these corrosion reactions is affected by the presence of ionic contaminants such as

salts and other corrosion products. Ionic species increase the conductivity of the media thereby

speeding the electrochemical processes at the anode and the cathode. They also may play a direct

role in defining the corrosion mechanisms as is discussed below.

6.2.5 Multiphase Corrosion

Multiphase corrosion refers to the fact that a material exposed to both wet and dry conditions are

exposed to all of the above corrosion mechanisms.

6.2.6 Wear

Wear is the deterioration of a surface due to material removal caused by relative motion of it and

another part. In corrosive media, wear may be synergistically accelerated by the combined

influence of these removal processes. This is related to the erosion corrosion process described

above.

Much of the discussion of wear phenomena is associated with upper cylinder wear in internal

combustion engines.74 75 76 This type of wear is associated with the formation of performic acid,

or other aggressive species, from the partial oxidation of alcohol in cold engines. This process is

not expected to occur in underground storage and dispensing equipment.

However, alcohol and alcohol blends do tend to promote more metal-to-metal wear than gasoline.

Ethanol blends tend to provide less lubricity than methanol.77 Temperature, sliding speed, and

water content were found to be important parameters influencing wear rate.78 79 The largest

increase in the wear rate of steel is found just below the water content at which phase separation

occurs in alcohol blended gasoline. This phenomenon was attributed to the formation of

microdomains of an aqueous phase which has not coalesced

Additives have been found to improve the lubricity of alcohol blended gasoline. However, many

of these additives such as corrosion and wear inhibitors have been found to have negative effects

on engine performance due to the formation of deposits in and around the combustion chamber.

6.2.7 Erosion-Corrosion

Erosion-corrosion occurs under dynamic fluid flow conditions and under conditions of wear.

Under these conditions shear stress at the wall can remove protective corrosion product films

from metal surfaces. Corrosion rates are increased due to this process of building protective
Compatibility and Permeability of Oxygenated Fuels to Materials

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39

oxide/hydroxide layers and subsequent wear or washing away. Erosion-corrosion can be

enhanced if the flow media is multiphase.

6.2.8 Effect of Contaminates in Solution

In addition to containing small amounts of water, technical and industrial grades of alcohols also

contain traces of organic acids, aldehydes, peroxides, ketones and esters and other materials.80

Trace levels of contamination, such as the sulfate impurities from fermentation processes81 or

chloride ion from salts or pH variation can have a large influence on the observed corrosion

mechanisms and therefore the observed corrosion rate. This has been shown systematically in

electrochemical corrosion studies of aluminum82 and iron alloys.83 Synergistic effects are seen with

the chloride ion and it is often thought to be the most aggressive counter ion.84

6.2.9 Effect of Alloying Elements

Just as the level and type of contaminants in solution greatly affect corrosion rate, the type and

level of alloying elements in metals can affect the rate at which corrosion is observed. This

phenomenon is associated with the stability of the oxide layer of the metal surface. For example

chromium is added to steel to promote its corrosion resistance. Conversely, pure aluminum forms

a stable oxide film and alloying elements can disrupt the stability of this protective barrier. For

example, addition of copper to aluminum85 or steel86 promotes corrosive attack by alcohols.

6.2.10 Environmental Stress Cracking

Environmental stress cracking (ESC) most often occurs in stainless steels which are in the

presence of ionic contaminates such as chloride or sulfide. ESC, however, generally occurs at

temperatures above 140 °F and is not mentioned as an important failure mechanism in this

literature. Therefore it will not be considered further.

6.2.11 Thermal Effects

Temperature has very complex influence on the corrosion process. On one hand, all the

aforementioned reactions are accelerated with temperature. On the other, the solubility of certain

corrodants like oxygen decrease with temperature.

For example the corrosion rate of steel in hydrated alcohols is observed to increase exponentially

with temperature in the range of 20 to 80 °C, from 0.04 to 0.14 g/m2/h. The corrosion rate of

aluminum also increases with temperature. In contrast, the corrosion rate of copper goes through

a maximum with temperature at approximately 50 °C. In the range of 20 to 80 °C, the corrosion

rate is less than 0.01 at the extreme temperatures and 0.023 g/m2/h at

50 °C.87

6.2.12 Effect of Time, Corrosion Inhibitor
Compatibility and Permeability of Oxygenated Fuels to Materials

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The corrosion rate of metals may decrease with time if a stable, passive oxide film is formed.

However, for steel immersed in ethanol blended fuels, electrochemical measurements have shown

that the corrosion rate may either increase or decrease with time, depending upon the choice of

corrosion inhibitor.88 Inhibitors investigated include mono-, di- and tri-ethanolamines, with the

smaller molecular weight inhibitors performing best. Another study found amine-based corrosion

inhibitors are also effective for steel.89

6.2.13 Testing Standards for Metallic Materials

There are many standards for evaluation of metal corrosion and/or wear phenomena. In addition

to the nomenclature and statistical analysis standards mentioned above, there are several

additional standards for evaluating corrosion. A few of the ASTM Standards will be briefly

summarized in this section.

ASTM G-1 is a standard practice for preparing, cleaning, and evaluating corrosion test

specimens90. It covers suggested procedures for preparing bare, solid metal specimens for tests,

for removing corrosion products after the test has been completed, and for evaluating the

corrosion damage that has occurred. Emphasis is placed on procedures related to the evaluation

of corrosion by mass loss and pitting measurements.

ASTM G-31 - 95 is a standard for conducting laboratory immersion corrosion tests.91 This

practice describes accepted procedures for and factors that influence laboratory immersion

corrosion tests, particularly general mass loss tests. These factors include specimen preparation,

apparatus, test conditions, methods of cleaning specimens, evaluation of results, and calculation

and reporting of corrosion rates. This practice also provides a checklist for reporting of test data.

ASTM G-46 - 94 is a standard for examination and evaluation of pitting corrosion.92 It is

intended to assist in the selection of procedures that can be used in the identification and

examination of pits and in the evaluation of pitting corrosion to determine the extent of its effect.

ASTM G-71 - 81 is a guide for conducting and evaluating galvanic corrosion tests93 to

characterize the behavior of two dissimilar metals in electrical contact in an electrolyte under low

flow conditions. It can be adapted to wrought or cast metals and alloys. The guide covers the

selection of materials, specimen preparation, method of exposure, and method for evaluating

results to characterize the behavior of a galvanic couple.

ASTM G-119 - 93 is a guide for determining the synergism between wear and corrosion.94 It

provides a guide for computing the increased wear loss rate attributed to synergism or interaction

that may occur in a system when both wear and corrosion processes coexist. The guide applies to

systems in liquid solutions or slurries and does not include processes in a gas/solid system. The

guide applies to metallic materials and can be used in a generic sense with a number of

wear/corrosion tests. It is not restricted for use with approved ASTM test methods.

ASTM G-133 - 95 is a standard test for evaluating wear using a linearly reciprocating ball-on-flat

sliding test.95 It describes laboratory procedures for determining the sliding wear of ceramics,
Compatibility and Permeability of Oxygenated Fuels to Materials

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metals, and other candidate wear resistant materials. The direction of the relative motion between

sliding surfaces reverses in a periodic fashion such that the sliding occurs back and forth and in a

straight line. The principal quantities of interest are the wear volumes of the contacting ball and

flat specimen materials; however, the coefficient of kinetic friction may also be measured using the

method described. This method encompasses both unlubricated and lubricated testing

procedures. The scope of the method does not include testing in corrosive or chemically

aggressive environments.

7.0 Results and Discussion

7.1 Non-Metals

As literature data are used to evaluate swelling and permeability of materials, be aware of certain

complications associated with precise reporting of this behavior. When an aggressive solvent is

present in dilute quantities, selective absorption into materials may deplete its concentration in

solution. Also, some materials contain extractable materials and last, equilibrium swelling and

permeation should always be verified. All of these effects can create an apparent absorption and

permeation in materials which is less than the actual value. Absorption error tends to be more

pronounced for lower swelling systems and permeability error tends to be more pronounced for

higher permeating materials.

7.1.1 General Observations on the Effects of Model Fuel Blends on Polymeric Materials:

Tables A1 through A6 in Appendix A summarize swelling data from the literature while Tables

B1 through B6 in Appendix B summarize permeability data. These data are categorized by

solution and by specific material. Standardized nomenclature is used to refer to generic

elastomer96 and thermoplastic97 materials. A description of some common fuel-handling

elastomers is offered in Appendix C.

98

Data for composites, which compose rigid piping and USTs, and to a lesser extent thermoplastic

materials, which compose flexible piping and sumps, are noticeably sparse. This lack of data is a

primary result of the automotive industry reporting most of the data on materials compatibility

and permeability. Composite materials are not used in automotive fuel systems.

Aside from the apparent variability of experimental observations in the data that do exist, some

trends are widely observed. For example, increasing the ether content in reformulated gasoline

generally increases the swelling response of elastomers monotonically.99 100 101 This swelling

behavior has been shown to scale linearly with the activity of the aggressive swelling agent in

solution. By contrast, increasing alcohol content in gasohol generally produces a maximum

swelling response in polymers.102 103 104 105 This maximum is located at approximately 15 percent

EtOH by volume and somewhat higher for MeOH. Generally, an alcohol fuel blend is more

aggressive toward polymers than any of the neat constituents in the fuel. This result is related to

the nonideal solution thermodynamics of gasohol solutions as discussed above.

Another general observation is that increasing the fluorine content in fluoroelastomers generally

improves its resistance to swelling and permeation by oxygenated hydrocarbons.106 The reason
Compatibility and Permeability of Oxygenated Fuels to Materials

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for this can be found by studying the molecular structure of the FKM elastomer. FKM-66 is a

copolymer of vinylidene fluoride and hexafluoropropylene. Hertz107 explains that the vinylidene

fluoride monomer preferentially polymerized in a head-to-tail fashion. As such there are

alternating -CF2-CH2- moieties on the elastomer backbone. The electron withdrawing character

of the fluorine atom causes the hydrogen atoms to become acidic. Thus, exothermic interaction

occurs among the polymer chains and excellent resistance to hydrocarbons, including aromatic

hydrocarbons is observed. However, when hydrocarbons with Lewis base sites, like many ethers

and alcohols, are introduced to FKM, these intra-molecular forces are broken by enthalpy favored

interaction with the solvent. Considerable elastomer swelling results. FKM elastomers with

higher fluorine content achieve this composition by replacing some of the vinylidene fluoride

monomer with tetrafluoroethylene monomer. The resulting terpolymer has fewer acid-base sites

and therefore is swelled to a lesser extent by alcohols and ethers.

The swelling response of FKM elastomers, as a function of MTBE concentration in gasoline, as

well as temperature and fluorine content, were correlated using a statistical model.108 109 It was

found that a linear correlation was sufficient to fit the data, no variable interactions were found to

be significant. The resulting fit has the following form.

Y = m + b1X1 + b2X2 + b3X3 + e 15

where m and bn are fitting constants for the variables Xn and e is the estimated error. These

equations are very useful for predicting changes in mechanical properties and hardness, etc. with

changing fuel composition or temperature. The study found that temperature had the greatest

effect on swelling with increased swell observed at higher temperatures. A reduction of

properties was also coincidentally observed.

Fluorocarbon plastics offer better resistance to swelling and permeation than other plastics and

elastomers.110 For example, approximately hundreds of times less permeation to Fuel C than PA11 was observed. The permeation values of fluoroplastics change very little with the addition of

either alcohols or ethers to gasoline whereas permeation usually increases for most other

thermoplastics.

Increasing acrylonitrile content in NBR improves its resistance to aromatic hydrocarbons111 as

well as its permeability to gasoline.112 Higher acrylonitrile content in NBR has a lesser beneficial

effect on the resistance to ethers and actually reduces the resistance of the elastomer to

concentrated ethanol113 and methanol114 fuels. Unfortunately, increased acrylonitrile content also

generally decreases low temperature flexibility.

Even though swelling and permeability behavior vary markedly among materials of the same class,

materials are lumped together in classes such that a range of behavior in each material class is

given. This is done for two reasons: The variability among observations from different observers

of similar materials is in some cases nearly as significant as the range itself. Second it allows

broad categorization by end-use.
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in Underground Storage and Dispensing Equipment

43

7.1.2 Swelling of Polymeric Materials in Model Fuel Blends and Neat Oxygenated 

 Hydrocarbons:

Table 5 summarizes by class the swelling ranges of elastomers and other materials in model

ASTM Fuel C blends with and without added oxygenated hydrocarbons. It is observed that,

addition of 15 percent MTBE does not significantly change the performance of FKM and NBR

elastomers commonly used for seals and hoses, respectively. That is, the swelling of FKM seals

remains below 20 percent by volume and the swelling of NBR-based hose materials may actually

decrease somewhat. However, addition of 10 to 15 percent by volume of MeOH may

compromise the integrity of some components by increased swelling of common elastomers

beyond acceptable limits set for certain seal and/or hose applications.

Table 5 - Volume swelling ranges of polymeric materials exposed to model fuels with and 

 without oxygenates.

Percent Swell by Volume

Materials Primary

Application

ASTM Fuel

C

Fuel C +

15 % MTBE

Fuel C +

10 to 15 % MeOH

Fuel C +

10 to 15 % EtOH

NBR/Blends Hose 23 to 56, 28* 19 to 38 49* to 106 22 to 70, 34*

FKM Seal 1 to 14, 10* 6 to 18* 7 to 46* 6 to 24

FSi Seal 18 to 21 24 30 19 to 20

CO, ECO 35 to 40 77 to 80 50 to 65

U Seal 21 24 58 51

CSPE 61 66 81

CIIR 96 81

CPE 87

PS Sealant 27 28

PA Pipe Liner -0.5 to 0.5 -0.5 to 0.2

Acetal Molded Parts 1 0

HDPE Flexible Pipe 10.7 10.9

Fiberglass Rigid Pipe -0.43 -1.3 to 2.3

Fiberglass Tank -0.02 -0.51 10

* Behavior of the most common material used in the class.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

44

Table 6 summarizes by class swelling data for common elastomers exposed to neat oxygenated

hydrocarbons. It is observed that neat MTBE and neat MeOH are both aggressive swelling

agents for FKM whereas they are less aggressive toward NBR-based elastomers. In comparing

Tables 5 and 6, it is interesting to note that the swelling power of ethers are reduced as they are

diluted into the nonpolar gasoline whereas the swelling power of alcohols are not reduced.

Table 6 Volume swelling of polymeric materials exposed to neat oxygenated hydrocarbons.

Percent Swell by Volume

Materials Primary

Application

MTBE ETBE TAME MeOH EtOH

NBR/Blends Hose 36 14 11

FKM Seal 59 to 180* 3 to 10, 5* 19 to 84, 70* 16 to 135* 2*

FSi Seal 5 6

CO, ECO 31 2

U Seal 8 18 19

CSPE 1 1

CIIR -4

CPE -2

PS Sealant 3

* behavior of the most common material used in the class.

As per Equation 5, the absorption characteristics of neat oxygenated hydrocarbons are

important indicators for the tendency of solvents to permeate polymer membranes. For example,

it is shown that ETBE swells FKM and urethane elastomers far less than MTBE or even TAME.

Since TAME is an isomer of ETBE, the stereochemistry of the oxygenated hydrocarbon is shown

to play an important role in the swelling and permeation characteristic in FKM. Apparently,

having both carbon chains longer than a single atom renders the ether moiety more inert towards

interaction with polymeric materials. The ethyl group imparts enough steric hindrance that the

Lewis base site is partially shielded from interaction with other Lewis acid sites in FKM

elastomers. If this phenomenon is shown to be more generally true in thermoplastics and

composite materials, it is an important observation because it suggests a method to reduce

permeability by changing the structure of the oxygenate additive. Since no permeability data for

ETBE was found, this hypothesis should be verified.

7.1.3 Permeation of Polymeric Materials by Model Fuels Containing No Oxygenates.

Generally elastomers have higher values of permeability coefficients while thermoplastics and

thermosets exhibit lower values. To estimate the total mass flow, knowledge of the membrane

area and thickness are also required. Table 7 summarizes typical ranges for geometrical constants

assumed for a gasoline retail station.

Table 7 Component geometry for permeation mass flow considerations.

Component A (m2) L(mm) A/L (m2/mm)

Hose 2.5 to 5 5.0 0.5 to 1.0

Piping 34 to 50

Sumps
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

45

Tanks 157 to 234 6.4 24.5 to 36.6

Table 8 summarizes permeability ranges reported for many of the material classes exposed to

ASTM Reference Fuel C. Expected mass flux values per station, Q, can be computed for some

elastomers and plastics and all seem relatively low except for the NBR hose materials. Here it

must be reiterated that the NBR/PVC material most commonly used in that application is better

represented by the low end of this range.115
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

46

Table 8 Permeability, solubility and mass flow properties for various materials exposed to

gasoline blends containing no oxygenates.

ASTM FUEL C

Material Primary

Applicatio

n

Permeability

(g*mm/m2/d)

Solubility

(g/mm*m2)

Diffusivity

(mm2/day)

activity A/L

(m2/mm)

Q

(g/d)

t = 1/D

(d/mm2)

Elastomers

FKM Hose liner

Seals

1 to 7 8 to 110 0.06 to 0.13 1 1 to 2 1 to 14 8 to 30

NBR/blends Hose 192* to 1200 230 to 760 0.9 to 1.6 1 0.5 to 1 100* to 1200 0.5 to 1

Thermoplastics

ETFE Hose liner 0.01 to 0.18 1 6 to 12 0.06 to 2

PE Flexible pipe

Sump

36 107 0.1 1 8 12

PA Flexible pipe

liner

5 to 26 -5 to 5 1

Thermosets

Epoxy Rigid Pipe -4

Polyester Tank -0.2

* Base fuel 72 octane.

Laminating a hose with FKM elastomer, such as with SAE J30R9 hoses, is anticipated to greatly

reduce the total fugitive emission of hydrocarbons. This type of hose is used in the automotive

industry to help meet fugitive emission guidelines for fuel systems in cars. It can be speculated

that the majority of the total emission of Fuel C from NBR-based hoses is aromatic hydrocarbons

since these are known to swell NBR whereas isooctane does not.116 Thus NBR selectively

absorbs and permeates aromatic hydrocarbons. When lined with FKM elastomer the permeation

is reduced because FKM is much more resistant to swelling and permeation of aromatics.

7.1.4 Permeation of Polymeric Materials by Ether Blended Model Fuels:

Table 9 summarizes the available permeation data for reformulated gasoline containing 15

percent by volume MTBE. Comparing Tables 8 and 9, notice that FKM elastomers show a

marked increase in permeability to reformulated fuel. Evidently FKM elastomer selectively

absorbs and permeates MTBE relative to the aromatic and aliphatic hydrocarbons even though

MTBE is the minor constituent in reformulated gasoline. Conversely NBR elastomer shows a

slight decrease in permeability which is indicative of it being a better barrier to MTBE relative to

aromatic hydrocarbons.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

47

Table 9 - Permeability, solubility and mass flow properties for various materials exposed to

 gasoline blends containing 15 percent v/v MTBE.

ASTM FUEL C + 15 percent volume MTBE

Material Primary

Applicatio

n

Permeability

(g*mm/m2/d)

Solubility

(g/mm*m2)

Diffusivity

(mm2/day)

activity A/L

(m2/mm)

Q

(g/d)

t = 1/D

(d/mm2)

Elastomers

FKM Hose liner

Seals

15 to 38 60 to 180 0.21 to 0.25 1 1 to 2 15 to 76 4 to 5

NBR/blends Hose 176* 190 to 380 0.5 to 1 1 0.5 to 1 88 to 176 1 to 2

Thermoplastics

ETFE Hose liner 0.1** 1 6 to 12 0.6 to 1.2

PE Flexible pipe

Sump

109

PA Flexible pipe

liner

-5 to 2

Thermosets

Epoxy Rigid Pipe -13 to 23

Polyester Tank -5

* Base fuel 72 octane. ** ETBE value = 0.05

An attempt is made to estimate the fraction of the total mass flow in NBR and FKM elastomers

which is associated with oxygenate permeation alone. These estimates are made by using the

observed permeability with and without MTBE and assuming a linear component contribution to

the total hydrocarbon permeability according to the following relationship:

Pt = Σai Pi 16

Where Pt is the total permeability, Pi is the permeability of each constituent and ai is the activity of

each constituent in the fuel. This computation is theoretical and therefore subject to question.

However as a first approximation, the approach seems appropriate because all the hydrocarbons

in the model fuel exhibit nearly ideal mixing. It has been previously established that the solubility

contributes to the permeability scale according to the activity coefficients.

Following this line of reasoning, the intrinsic permeability of MTBE in a gasoline environment is

estimated by this technique to be approximately 85 (g*mm/m2/day) for NBR and 206

(g*mm/m2/day) for FKM-66 copolymer, respectively. To calculate the estimated component

fugitive emission of MTBE from dispenser hoses we apply Equation 16 assuming an activity of

MTBE of 0.15 and a surface area of 2.5 to 5.0 m2 and a thickness of 5 millimeters for an NBR

hose. Therefore the total fugitive emission of MTBE is estimated to be 6 to 13 g/d/station. Here,

mass flow of MTBE in NBR based hoses is low compared to the permeation of total

hydrocarbon. Apparently MTBE is a less aggressive swelling and permeation agent for NBR than

are aromatic hydrocarbons.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

48

An interesting exercise is to determine the effect of adding an FKM-66 copolymer liner to the

inside of the dispenser hose, similar to the SAE 30R9 specification for automobile fuel lines.

Permeation of solvents in multilayered structures are additive as conductances,18 thus:

P

P P

P P

t =

+

1 2

( 1 2)

 17

If a five millimeter thick NBR hose is laminated with an FKM liner in the inside half, the total

permeability of all the hoses in a typical station can be shown to increase to about 18 g/d MTBE.

Therefore it is expected that laminating a dispenser hose with FKM elastomer will lead to

increased MTBE permeation even though the total hydrocarbon permeation will be reduced. This

increase is due to the selective absorption and permeation of MTBE by FKM copolymer

elastomers. It is interesting to note that FKM-lined SAE30R9 hoses are common in the

automotive industry. A thermoplastic liner would be required to reduce the total permeation of

MTBE in hoses. Such lined hoses have been developed for automotive flex fuel lines and are

common styles for air conditioner hoses and natural gas fuel lines.

To summarize the effects of ethers on polymers, pure MTBE is aggressive to many polymeric

materials. However, the effects of MTBE on polymers is not expected to cause performance

problems when it is diluted into gasoline at the 15 percent level. The permeability of MTBE

through elastomers and thermoplastics is also proportional to its concentration in the gasoline.

ETBE is apparently less aggressive than MTBE or TAME in some elastomers.

Before leaving the subject of the effects of ethers, it should be mentioned that one author has

expressed concern over the effects of MTBE on materials even at the five percent level.117 118

These papers describe the effects of MTBE on floating roof tank seal materials. Many of these

materials are foams in which considerable weight gain can be obtained as the pore volume

becomes saturated with condensate. Due to the expectation of wear in this application, only

minor changes in dimensions and material properties can be tolerated. In fact, the author

considers greater than ten percent weight gain to be “significant deterioration” for this application

and selected PTFE fabrics as the only compatible material. However, these results should not be

of concern for the retail gasoline environment. USTs are not sealed in this manner.

7.1.5 Permeation of Polymeric Materials by Alcohol Blended Model Fuels:

Table 10 summarizes permeability and solubility results for gasoline blends with MeOH while

Table 11 presents similar data for gasoline blends with EtOH. Notice that the volume swell of the

lower grades of FKM in MeOH blended fuels exceed the limits for reliable seal operation. Even

the best grades of NBR swell too much for reliable hose applications in MeOH fuels. The higher

molecular weight EtOH is somewhat less aggressive swelling and permeation agent than MeOH.

Table 10 - Permeability and solubility of modified ASTM Fuel C, containing 15 to 20 percent 

 by volume MeOH, in polymeric materials.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

49

ASTM FUEL C + 15 to 20 % volume MeOH

Material Primary

Application

Permeability

(g*mm/m2/d)

Solubility

(g/mm*m2)

Diffusivity

(mm2/day)

activity A/L

(m2/mm)

Q

(g/d)

t = 1/D

(d/mm2)

Elastomers

FKM Hose liner

Seals

6 to 50 52 to 340 0.11 to 0.15 1 1 to 2 6 to 100

NBR/blends Hose 1300 to 2700 370 to 780 2.7 to 3.2 1 0.5 to 1 600 to 2700

Thermoplastics

ETFE Hose liner 0.05 to 0.43 1 6 to 12 0.3 to 5

PE Flexible pipe

Sump

46 1 8 196

PA Flexible pipe

liner

27 to 100 1

Thermosets

Epoxy Rigid Pipe

Polyester Tank 1 100 0.011

Table 11 - Permeability and solubility of modified ASTM Fuel C, containing 10 to 15 percent by 

volume EtOH, in polymeric materials.

ASTM FUEL C + 10 to 15 % volume EtOH

Material Primary

Application

Permeability

(g*mm/m2/d)

Solubility

(g/mm*m2)

Diffusivity

(mm2/day)

activity A/L

(m2/mm)

Q

(g/d)

t = 1/D

(d/mm2)

Elastomers

FKM Hose liner

Seals

2 to 100 52 to 270 0.03 to 0.37 1 1 to 2 6 to 100

NBR/blends Hose 1000 to 2000 170 to 560 3.5 to 5.8 1 0.5 to 1 600 to 2700

Thermoplastics

ETFE Hose liner 0.03 to 0.15 1 6 to 12 0.3 to 5

PE Flexible pipe

Sump

PA Flexible pipe

liner

33

Thermosets

Epoxy Rigid Pipe

Polyester Tank

Permeability values represent the sum of all hydrocarbon contributions. Notice that the total mass

flow due to permeation is very high even in comparison to MTBE permeation. Fluoroelastomers

and fluoroplastics exhibit the lowest levels of permeation in alcohol blends while fluorosilicones

and nitrile elastomers exhibit the highest level. Permeation rate drops with increasing acrylonitrile

content in NBRs and also drops with increasing fluorine content in FKMs.119 120

The permeation values reported for alcohol fuels are “peak” values. Many authors observed the

permeation rate to go through a maximum value several days after the test began. This

phenomenon has been attributed to speciation.121 122 Speciation occurs when the membrane

selectively absorbs and permeates one of the components in the solution. In this case, the alcohol
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

50

permeates very quickly and depletes its concentration in the reservoir. The changing activity of

permeant in the reservoir causes the apparent permeation rate to decrease with time.

Several authors reported attempts to improve the measurement technique by increasing the size of

the reservoir and other techniques.123 Therefore reported permeation values will be greatly

dependent on the experimental technique employed. In an attempt to introduce some consistency,

the permeation values summarized in this review are all “peak” values.

Since there have been no successful attempts to model the swelling behavior of materials exposed

to alcohol blended fuels over the entire composition, no attempt is made to estimate the

component contribution of alcohol permeation. Because there is substantial excess swelling of

elastomers in the alcohol blends, the best swelling models are applicable only over a limited

composition range.124 125 126 For this reason, the linear combination rule of Equation 14 will not

apply for scaling component contributions.

The limited data available for swelling and permeation of oxygenated fuels in composite materials,

which is shown in the preceding tables, will be discussed below. Epoxy matrix composites are

found in rigid nonmetallic piping while polyester matrix composites are found in nonmetallic

underground storage tanks.

7.1.6 Thermosetting Materials -- Underground Storage Tanks.

Very limited information is available regarding the sorption and plasticization of isophthalic

polyester laminates by reformulated fuels containing 15 percent MTBE. It is generally assumed

that MTBE and other ethers will not be as aggressive to FRP as alcohols. Therefore observers

have concentrated testing efforts on the effects of alcohol containing fuels. Kamody et al. state,

“Test results indicate this is true,” but no data with regard to the effects of MTBE were offered.127

128 It was indicated that MeOH is very aggressive to FRP, especially those made from less

premium resins. Introducing MeOH into older tanks tended to dissolve gums and other polar

residues which accumulated throughout the years and wound-up fouling automotive equipment

such as fuel lines. Ethanol does this as well but it seems that MeOH is more notorious.

It has been reported,129 from a seven month immersion study, that reformulated gasoline produced

less weight change (actually lost weight) than nonoxygenated gasoline. Loss of weight may be

due to extraction of residual unreacted monomers in an unpostcured material. However no data

on modulus retention are offered.

Reports of an eight-year immersion study of polyester fiberglass laminates by Fluid Containment

indicate that hardness and strength did not vary by more than two percent. Fluid Containment

quoted “the MTBE fuel blend acted no differently than the straight gasoline and had essentially no

effect on the tank sample after eight years.” But, no data on modulus retention are reported.

Certainly there are no data to indicate MTBE is more or less detrimental or permeable to UST

materials than is gasoline. There is no theoretical reason to expect strong selective absorption of

MTBE by isophthalic polyesters. This is because no strong Lewis acid sites are present in these
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

51

polymer materials which would drive MTBE absorption by exothermic interaction. The chemical

structure of the polyester contains only Lewis base sites (ester linkages and aromatic pi electrons).

However, selective absorption of alcohol by isophthalic polyesters may be anticipated due to a

potential exothermic interaction among ester moieties in the polymer and the hydroxyl group of

alcohol. Indeed, selective absorption of alcohols is observed. After one, six and 12 months of

immersion, 2.2, 3.8 and 4.7 percent weight gain, respectively, were observed for isophtalic

polyester immersed in 10 percent MeOH, 10 Percent TBOH balance gasoline blend.130 Coincident

with the observed absorption, a seventy percent retention of flexural stiffness was also observed.

Post-curing the laminate significantly improves the resistance of the material to methanol blends.

These results are corroborated by Kamody who observed a 30 percent retention of isophthalic

polyester resin laminates after nine month immersion in 30 percent methanol/gasoline blends.

Although premium resins show better resistance to methanol, the loss of stiffness of isophthalic

polyester laminates in methanol fuels exceeds the levels allowed by UL and ASTM standards.

Broutman and Associates131 performed immersion testing of tank laminates in alcohol blended

gasoline and measured property retention as a function of time, up to 15,000 hours. The property

retention data were extrapolated to thirty years. Weight gains were observed to be in the range of

two to ten percent, depending on the laminate studied and the oxygenated hydrocarbon immersion

media. Interestingly, the laminate exposed to blends of alcohol-gasoline always absorb more

weight than they do in either of the neat constituents. They point-out that older tanks, installed

before 1979 are more prone to absorption of alcohols than newer tanks designed for 10 percent

EtOH service. In older tanks, flexural stiffness retention was estimated to be 70 percent after 30

years exposure to 10 percent EtOH blends while methanol blends retained only 25 percent

stiffness. Newer tanks, listed for ethanol service retain properties considerably better. They

concluded that gasohol storage may lead to an increased frequency of buckling failures in tanks

that were not designed to store these oxygenated fuels.

Graduate work performed at the University of Minnesota132 generally confirms the observations of

Broutman and Associates that swelling of isophthalic polyesters in methanol containing fuels is

greatest for the fuel blends rather than the neat constituents. Up to ten percent swell may be

expected. A coincidental loss of stiffness of up to 40 percent may be observed after 30,000 hours.

No successful attempts to measure permeation of ethers or alcohols in pipe or tank composites

have been reported. Smith Fiberglass apparently attempted to measure permeation of EtOH in

fiberglass piping after 31 days exposure and they were unable to find any. It is generally held that

since MTBE is a large molecule relative to alcohols, it is not likely to swell fiberglass or permeate

through it.

Theory maintains, however, that any hydrocarbon which can be dissolved into a membrane will

permeate through it at some rate. For example, the diffusion coefficient of methanol in an

isophthalic polyester laminate can be estimated by applying Equation 13 to some unsteady

absorption data reported above. The absorption of methanol into a 6 mm laminate was measured

after one month to be 2.2 percent. The total equilibrium absorption was estimated by Craggie et
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

52

al.133 to be ten percent. The diffusion coefficient for methanol in fiberglass is now theoretically

estimated to be 0.011 mm2/d. The swelling behavior of MeOH in the isophthalic polyester

laminates shows classical Case II permeation. Now the permeability and the total mass flow of

MeOH can be estimated for a UST as summarized in Table 10. Although the permeability is low,

substantial surface area exists for a UST and the total mass flow may not be negligible.

The inability to measure permeation of oxygenated hydrocarbons may be related more to

experimental impatience than impermeability of the solvent. For a six millimeter thick laminate,

the estimated time to reach steady state permeation of MeOH is estimated to be about 3.5 years.

About four months would theoretically be required to observe the first indications of MeOH

permeation. This may explain why attempts to measure permeation in fiberglass laminates have

not produced results.

One final comment on UST compatibility. The Steel Tank Institute released a rather impassioned

statement in an effort to convince its readers of the incompatibility of fiberglass tanks to alcohol

blended fuels. Many statements in this publication may be considered controversial. However, it

is interesting to note that with all the concern voiced over alcohol blended fuels, no mention is

made regarding the effects of MTBE in reformulated fuels on fiberglass UST’s. Also no mention

is made of fiberglass piping with regard to either alcohols or ethers.

7.1.7 Thermosetting Materials -- Rigid Fiberglass Piping

As with UST laminates, very little information is available regarding the absorption and

permeability of oxygenated hydrocarbons in these composites. Generally the matrix material is

amine cured epoxy and, as such, is theoretically expected to selectively absorb both alcohols and

ethers. However, these components operate under a positive pressure so there is no risk of

compressive buckling failure. In tensile loading, continuous fiberglass laminates are very strong

and any drop in matrix stiffness is not expected to impact the performance of these materials.

7.1.8 Thermoplastic Materials -- Flexible Plastic Piping.

Flexible piping, running from the tank to the dispenser, is a rather new innovation. HDPE is

usually used for this application and it is lined with PA or PK for permeation and swelling

resistance. If swelling or shrinking is extensive, the pipe may either sag or pull from its fixtures.

134 However there has been no evidence to show that this is a problem for this type of buried

piping.

Permeation of MTBE from flexible piping can be estimated from the available data. Assuming

500 square feet of surface area in a typical gasoline station with secondary containment of

underground piping, the fugitive emission is expected to be approximately 8 g/day of vapor into

the soil column. This compares with the allowable standard set by UL of 40 g/day.

7.1.9 US EPA Evaporative Emission Regulations.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

53

The reader may have noticed that there is a considerable effort reported in the SAE technical

papers regarding the permeability of oxygenated gasoline in elastomers and thermoplastics. This

is a direct result of Evaporative emissions regulations enacted as part of the Clean Air Act of

1990. These regulations, in effect since 1994, state that the fugitive emission from an automobile

fuel system must not be greater than 2 grams total hydrocarbon per car per day. This amounted

to a decrease in the allowable permeation rate by a factor of ten.

Existing rubber fuel lines, NBR/PVC blends, and plastic gasoline tanks, HDPE, could not meet

these standards, especially when flexible fuel vehicles were considered. Therefore newer

materials135 and laminated constructions were developed. Some automotive companies returned

to steel tanks to reduce fugitive emission.136 137 Others developed laminated plastic tanks. 138 FKMlined hoses were made to comply with the regulation but they are quite expensive. 139

7.2 Metals

7.2.1 Corrosion by Ethers

There is very little information regarding the corrosion of metals by ethers and, in particular,

MTBE. This observation is significant in light of all the corrosion studies performed with

alcohols. For estimation of corrosion rates of carbon steel piping in the finishing section of an

MTBE plant, a regional corrosion specialist for Shell wrote, “... no corrosion is expected ... ”140

One may anticipate that in relatively benign chemical environments, extensive corrosion studies

will not be performed.

One study on MTBE effects looked at the corrosion resistance of zinc, aluminum, and brass

(these are active metals known to corrode in alcohol fuels) in neat MTBE and gasoline containing

20 percent MTBE at 20 °C.141 These metals were not affected by either MTBE solution.

However, brass was corroded with water present in the blend.

In another study involving flexible fuel vehicles, a seven percent by volume blend in gasoline was

included in a fleet vehicle test.142 No evidence of fuel system material or component damage was

found due to the extended use of oxygenated fuels.

One study looked at the corrosive effects of ETBE.143 Metals tested included: galvanized steel,

cast iron, copper, magnesium, brass, aluminum, 1018 carbon steel, and terneplate. Samples of

each of the metals were stored in the fuels to which had been added about 1 percent water. None

of the experimental fuels shown were treated with the anti-corrosion additives normally used in

finished gasoline. Samples were maintained at room temperature for 168 days. The data show

ETBE containing blends to have corrosion rates not significantly different from the neat fuels.

The terneplate showed 12.5 g/yr/12ftsq loss of weight but it was not considered to be of great

significance. No detrimental effects of the ETBE on metal parts common to gasoline delivery and

fueling system were found.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

54

Therefore it may be concluded that gasoline is a rather benign chemical environment from a

metals corrosion perspective and addition of ethers does not increase the aggressiveness of the

fuel.

7.2.2 Corrosion By Alcohols

Corrosion rates for methanol have been reported by Lash. Table 12 summarizes some of the

observations.

Table 12 - Corrosion rates for selected metals immersed in methanol fuels at 40 oC for 2000 to 

 8000 hours (Lash et al.).

 

 15 percent MeOH 85 percent MeOH

Material Average Lower, Aqueous Phase

 (μm/year) (μm/year) (μm/year)

SS 304 0.0 0.0 0.0

SS 444 0.0 0.0 0.1

Tin 0.1 0.5 0.2

Brass 1.4 6.7 6.4

Zinc 2.1 10.0 13.9

Terneplate 2.7 12.9 86.9

Zinc-Iron 2.8 13.3 7.5

Zinc-Nickel 2.5 11.9 13.3

Zinc-Cobalt 1.9 9.0 18.1

Cadmium 7.5 35.7 22.9

Steel, CRS 10.0 7.6 1.8

Aluminum 356 24.0

Aluminum 319 55.0

Aluminum 380 63.0

Magnesium 146380.0

With 10 percent methanol blends and one percent total added water, corrosion rates were also

reported as summarized in Table 13.

144 Weight increases are presumably associated with

corrosion product which was not removed prior to weight measurement. Materials to avoid in

methanol fuel systems include magnesium and terneplate especially. Also avoid bare aluminum,

brass zinc, zinc alloys, and cadmium.

A more recent alcohol blend is called Oxinol. It is a blend 50 percent volume blend of MeOH and

gasoline grade tertiary butyl alcohol (GTBA). This blend is added to gasoline to achieve the

required bound oxygen content. From a materials point of view, this blend has several interesting

aspects. First, the methanol activity is dropped because the volume fraction is lower and because

the GTBA disrupts the normal hydrogen bonding characteristic of MeOH. Not much data is

reported for this alcohol blend, but it is expected that its aggressiveness to metallic and

nonmetallic materials will be reduced compared to straight methanol blends.

Table 13 - Corrosion rates of selected metals in 10 percent methanol blends.

 

10% MeOH
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

55

Material mg/d/m2

Zinc -10.7

Iron -26.3

Copper +8 to 11

Brass +8 to 11

Aluminum +1.4 

7.3 Other Materials

7.3.1 Ceramics

There is no mention of any compatibility or permeability issues for ceramic materials to

oxygenated fuel blends in the literature. Nor is there any reason to suspect these types of

problems.

7.3.2 Pipe Dope:

There are several indications in the literature which state that freshly applied pipe dope is subject

to washing-out by gasoline containing alcohol. Some pipe dope is alcohol-based and the solids

may be redissolved if the pipe dope has not had ample time to dry. Washed-out pipe dope can

lead to leaks in threaded connectors. PTFE-based tape may be considered as an alternative thread

sealant.

7.3.3 Coatings:

Coatings may be found on the inside or outside of steel tanks to protect them from corrosion.

Coatings may be organic or metallic.

Organic Coatings:

In the absence of cathodic protection, barrier organic coatings will protect metals if they can be

applied and maintained pore free but, this is very difficult in practice. In a laboratory evaluation, it

was found that gasohol tends to extract an epoxy coating from a gasoline storage tank.145 A

practice was established to store gasohol in unlined tanks and a recommendation made to study

the suitability of various coatings for use in gasohol service.

Several authors indicated the superior performance of urethane based coatings for splash

exposure to gasohol. These coatings are automotive finishes, however, and they may not be

appropriate for liquid immersion service.

Ethylene acrylic acid copolymer is a polymer coating for steel substrates.146 It is nonconductive

and provides good adhesion to steel and good resistance to MeOH. After a two year immersion

tests of coated steel samples in gasoline and M15, M85, and M100 with and without one percent

water, there was no evidence of laminate deterioration or adhesion loss on any sample.

Inorganic coatings:
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

56

Sacrificial metal coatings such as terneplate, which cathodically protected steel and other

structural metals, are inappropriate for systems in contact with alcohol blended gasoline.

Terneplate coatings are the most widely used inorganic coating for automotive applications to

protect steel gasoline tanks from corrosion. Terneplate is a coating consisting of lead and zinc

and many authors report that it is unsatisfactory for use in alcohol services. There is also a danger

that corrosion products may become entrained in the fuel and enter the automobile fuel system.147

Steel, nickel plate,148 and also prepainted zinc-nickel has been suggested to work well in

methanol, but prepainted terneplate is not acceptable.149 150 For replacement of terneplate, tin

coatings151 152 are most often mentioned (wolynec) as giving excellent performance. Cadmium

plate is also mentioned.

Electroless nickel plating can be used to protect aluminum in dry methanol environments but is

not recommended in the phase separated wet environments (lash). Anodized alloys 319 and 356

are reported to work reasonably well.

8.0 Review Articles

A general reference book is available which summarizes compatibility data for elastomers.153 This

book contains some information on alcohols and diethyl ether but not the ethers that are

commonly used in reformulated gasoline. A summary of unsuitable materials for these pure

oxygenates is summarized in Table 14:

Table 14 - Unsuitable elastomers for neat alcohol and diethyl ether.

 

 Diethyl Ether Ethanol Methanol

ABR ABR ABR

AU AU

Butyl, IIR

Hypalon

EA

EU

EPDM

 FKM FKM 

In a review article, Davidson154 mentions that fiberglass UST manufacturers claim tests in MTBE

blends show no deleterious effects and therefore they warrant the tanks for thirty years in

reformulated gasoline service. All studies indicate that USTs are compatible with MTBE blended

gasoline.

Downstream Alternatives, Inc. surveyed major equipment manufacturers in 1997 regarding the

compatibility of their products to MTBE and reformulated gasolines.155 With regard to tanks,

piping, seals, dispensing equipment, vapor recovery and related equipment, they concluded, “the

data clearly indicates (sic) that gasoline containing MTBE is compatible with all these products.

Claims and insinuations that gasoline containing MTBE are not compatible with equipment are

not based on scientific evidence.”
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

57

Other review articles focused on the materials issues associated with alcohol fuels.156 157 158 159 160 161

162 (F. Black) In addition to materials compatibility issues, drive ability, and emissions were

considered. Some interesting observations which these articles have in common are as follows:

Materials that laboratory tests indicate as being susceptible in immersion tests do not always give

problems in fleet vehicle testing. Therefore, it is sometimes difficult to say which materials are

unsuitable based on laboratory testing alone. In general, a material which is resistant to methanol

will be at least equally resistant to ethanol. Minimizing water in alcohol fuel systems is one way

of reducing corrosion problems. However, magnesium and magnesium alloys corrode very

rapidly in dry methanol. This review article, written in 1984, concludes “essentially all of the

aforementioned problems can be eliminated or improved by appropriate choice of construction

materials.”

Alcohols (MeOH and EtOH) have much wider explosive flammability limits than gasoline. Thus

the saturated vapor of a storage tank is within the explosive range at normal ambient temperature.

Precautions must be taken to shield this vapor from spark or flame. This danger does not exist

with straight gasoline or reformulated gasoline.

Alcohol loosens rust and dirt from the walls of fuel tanks or fuel lines of an automobile.

A fuel distribution system which has been used in former gasoline service should be completely

cleaned out before use of alcohol blends.

None of the aforementioned review articles considered the phenomenon of permeation of fuels

and their constituents through nonmetallic materials.

9.0 American Petroleum Institute Documentation

The American Petroleum Institute (API) has released three documents on materials compatibility

with oxygenated fuels. These two Recommended Practices and one Publication do not consider

permeability issues. The recommended practices consider alcohols only while the publication

considers all oxygenated fuels and their neat constituents. Tables 15 and 16 are copied directly

from these Recommended Practice documents.

Table 15 - Compatibility of commonly used materials with ethanol and ethanol blends. 163

Recommended Not Recommended 

Metals

Aluminum Zinc-galvanized (ethanol only)

Carbon Steel

Stainless Steel

Bronze

Elastomers

NBR (Hoses and Gaskets) NBR (seals only)
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

58

CIIR (hoses and gaskets) CIIR(seals only)

FKM U

FSI**

Polysulfide

Natural Rubber (neat ethanol only)

Polymers

Acetal Polyurethane**

PA Alcohol-based Pipe Dope**

PE

PP

PTFE

FRP** 

**The manufacturer of the specific material should be consulted.

Table 16 - Compatibility of commonly used materials with gasoline-methanol/cosolvent 

 blends.164

Recommended Not Recommended 

Metals

Aluminum Galvanized metals

Carbon Steel

Stainless Steel

Bronze

Elastomers

NBR (Hoses and Gaskets) NBR (seals only)

CIIR (hoses and gaskets) CIIR(seals only)

FKM

FSI**

Polysulfide

Polymers

Acetal Polyurethane**

PA Alcohol-based Pipe Dope**

PE

PP

PTFE

FRP** 

**The manufacturer of the specific material should be consulted.

In 1994 the API published results of a 1992 survey of petroleum and other companies for their

use of nonmetallic materials in oxygenated fuels. 165 The survey did not differentiate among

oxygenate type or material type in a given class. Forty-four companies responded to the survey,

following are some observations from that document.

Regarding the use of elastomers in static sealing, dynamic sealing, tank seals and high shear seals,

it is clear that most companies increased their usage of FPM type materials and PTFE for handling

oxygenates. Forty-four percent of the companies reported having some sealing problems while 42

percent reported making changes to elastomer seals. These changes are:
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

59

• A 12 percent decrease in the use of PU was documented. “Embrittlement of PU” was

mentioned as one reason for this decrease.

• An eight percent decrease in the use of FKM was documented. “Swelling of FKM” was the

predominant observation.

• A four percent increase in the use of NBR was reported.

• A 600 percent increase in the use of FPM was observed; however one respondent cautioned

about the use of FPM below 32 degrees Fahrenheit.

• A 100 percent increase in the use of PTFE was reported , however one commentator

cautioned, “valves equipped with PTFE seals are difficult to get good positive shut-off.”

• One commentator reported degradation of FRP tank liners by oxygenated fuels.

Most of the above problems were probably encountered while handling the pure oxygenates

rather than the fuel blends. One respondent supports this position saying, “15 % MTBE no

problems reported. Changed trim (seals) for neat MTBE only.”

In 1995, the API sponsored an “O&E Symposium on Materials/Fuels Compatibility.” A brief

summary of information presented at this workshop is provided below.

• Electrochemical measurements of steel with various compositions of MeOH, Fuel C and water

showed less than 0.1 mils per year corrosion rate.166

• Manufacturers have recommended the use of fiberglass USTs and piping up to the limits of 10

Percent EtOH, 5% MeOH and 15% MTBE. Gasohols have been used successfully since the

late 1970’s. MTBE , TAME, ETBE have not been found to be a concern.167

• API Recommends consideration of the following dispensing system components when

converting a retail station to handle gasoline-alcohol blends: Alcohol resistant materials,

Hoses, seals, nozzles. 88

• It is a common misconception that pure alcohols are more aggressive toward materials than

the fuel blends. Flexible piping manufacturers use liners of PA-12, PA-11, PVDF, and PA

doped PE to provide compatibility with the fuel blends.168

• Sumps are constructed of either FRP or PE. Cross-linked PE is preferred over HDPE as it is

stiffer, more chemically resistant, and has better low temperature impact.89

• Some case histories regarding submersible pumps in USTs were also reported.

• Owing to the conductivity of alcohol fuels, electrolysis from electric level gauge

caused corrosion of 316 SS casing.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

60

• Submersible Pump impellers are made from Acetal. Acetal exhibits 0.2 percent

swelling in a mixture of MeOH and Gasoline. The swollen impellers locked against the

pump stator causing the pump to malfunction. Five thousandths of an inch were

trimmed from the radius of the blades.

• NBR lathe cut gasket seals used in submersible pump flow manifolds were involved in

fuel leaks. Leaks occurred when oxygenated fuel was dropped. NBR seals were in

use for many years in non-oxygenated fuel. Newer NBR gaskets do not seem to

experience this problem.

• Regarding dispensers: Oxygen dissolved in a 93% EtOH, 7% H20 solution caused corrosion

of cast iron rotors in pumps. Certain Aluminum alloys tubing can be corroded 30 mil/year by

methanol with water. Treated leather piston cups in 6% of dispensers. NBR and Cork gaskets

and o-rings.90

• Regarding hoses: Common hose constructions include: 90

ECO Inner layer of fuel containing hose.

CIIR Outer layer of hoses.

NBR Inner layer of fuel containing hose.

NBR/PVC Outer layer of hoses.

PA & other Hose components in contact with vapors.

Thermoplastics

Heat Stabilizer for PA is soluble in MTBE, caused mesh screens to become clogged.

NBR is the material of choice for dispenser manufacturers due to cost. Modern NBR’s are

much improved.

• Regarding nozzles and swivels, common seal materials include: 90

NBR, FKM Valve poppets and seals

PTFE Swivel seals.

• For blends of ideal solvents, the volume fraction of aggressive solute in the swollen elastomer

at equilibrium is proportional to the volume fraction of aggressive solute in the mixed solvent

or gasoline. Thus the swelling behavior of many common fuel resistant elastomers in blends

of gasoline and ethers may be adequately predicted by interpolating the component swelling

behavior on a volume fraction basis.169

• In non-ideal hydrocarbon blends, the activity or partial pressure of ethanol as well as the nonpolar hydrocarbons is nonlinear with its composition in gasoline. As a practical result, the

swelling power of the blend is often better approximated by summing the component effects

of the neat constituents rather than by interpolating the component effects based upon volume

fraction.91

10.0 CONCLUSIONS
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

61

10.1 Compatibility

For any hardware designed for use in retail gas stations which has undergone testing, there are no

documented material incompatibility issues for retail stations dispensing reformulated fuels

containing ethers up to 15 percent v/v MTBE. In concentrations greater than about 20 percent by

volume, MTBE and TAME cause swelling of some fluoroelastomers which may be excessive for

some applications. Swelling of fluoroelastomers in neat ETBE is substantially lower than in other

ethers.

UL and ASTM standards suggest chemical resistance performance criteria for nonmetallic piping

and tanks. However, testing in MTBE containing fuels is not specifically required. Similar

performance standards exist for plastic pipe, elastomeric hoses and seals, there is no evidence to

suggest the hardware would not meet these performance standards in applied use.

In contrast, there are numerous material compatibility issues associated with the use of gasohol.

API’s Recommended Practices addresses materials recommended and not recommended for use

with ethanol and ethanol blends and gasoline-methanol cosolvent blends. Generally, methanol

blends are more aggressive than ethanol blends towards both metals and non-metals. MeOH

blends with TBA are offered which mitigate some materials concerns. Metal corrosion issues

include: general and localized corrosion of active metals, galvanic corrosion, electrolytic

corrosion, wear, and aqueous phase separation. Issues for polymeric materials include: swelling

and softening due to absorption of alcohol, extraction of plasticizers, and antioxidants. Generally,

compatible material alternatives are available but they may not be currently in service.

10.2 Permeability

Any solvent which can absorb into a material will also permeate through it. The phenomenon

of solvent permeation is therefore limited to polymeric materials. The permeation rate of

oxygenated gasoline is greater than nonoxygenated gasoline in common hose materials. In

general, alcohol blended fuels are more permeable than ether blends with MeOH being most

aggressive. The permeation rate of ETBE is postulated to be considerably lower than other

oxygenates.

Greater permeability is observed in elastomers (hoses, seals, gaskets, packing) relative to

thermoplastics (flexible piping, sumps, vapor recovery, tubing) and composites (rigid piping). In

general, fluorinated elastomers and thermoplastics offer better permeation resistance than

nonfluorinated materials. Emissions from composite piping and tanks have not yet been observed.

There are not enough data to estimate the total fugitive emission of hydrocarbons from retail

stations.

There are no environmental standards that limit fugitive emissions for gasoline retail stations

as there are for automobiles. UL-971, for safe operation of underground piping, suggests a

permeability limit of 4 g/m2/day for the primary conductor and 24 g/m2/d for the secondary

containment piping. No other permeation standards applicable to the retail gasoline environment

were found.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

62

Standards and procedures exist for measuring the total hydrocarbon permeability of gasoline in

hoses, flexible piping, and some other non-metallic materials. No standards were found for

measuring permeability in composite tanks or composite rigid piping. Existing standards do not

cover mass flow contributions from individual hydrocarbon species. In this reviewer’s opinion,

existing standards may not be adequate for steady state measurement of certain oxygenated

species, particularly alcohols that may be present in dilute quantities in gasoline.

11.0 Recommendations

Establish reliable techniques for determination of the individual contribution of the oxygenated

hydrocarbon component to total permeability of gasoline blends in materials of construction

commonly found in retail gasoline stations.

Measure directly the permeability of MTBE and other oxygenated hydrocarbons in these

materials. From this data and geometrical considerations, estimate the total fugitive emission

rates, air and soil, of oxygenated hydrocarbons via permeation through common retail station

equipment.

Measure the permeation rate of ETBE relative to MTBE in common polymeric materials. If

ETBE is found to be substantially lower, consider the possibility of replacing MTBE with ETBE

as the preferred oxygenated component in gasoline.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

63

APPENDIX A

Swelling Data

Table A1 - Volume swell of elastomers exposed to MTBE Blends with gasoline.

Percent swell by volume

Volume percent MTBE in ASTM Fuel C

Reference Material 0 5 10 15 20 25 50 75 100

f Fsi 22 23 24 26

a FPM 3 3 3 3 2 3

b,c,d,e FKM-65 8 26 43 105 153

a

b,c,d,e

g

FKM-66 15

5

6

17 15 18 20

22

18

37 84

180

126

b,c,d,e FKM-67 5 17 17 53 87

b,c,d,e

j

FKM-68 4

4 6 7

16 29 65 88

b,c,d,e

j

FKM-70 3

2

4 4 6

3

6

21 38 59

a Aflas-57 34 38 36 41 42 57

i ETP 26

j U 27 19 24

a NBR-34 37 37 38 38 38 36

k NBR 23 22 19

a) Westbrook TPR 318-90 (140day) 170

b,c,d,e) duPont data (7day) i) Stevens, 1997

f) Virant, et al, SAE 910102 (60°C) j) Douhit SAE 881667

g)Aliosio Ausimont k) Lebedev added 9% wash-out

Table A2 - Volume swell of thermoplastics and thermosets exposed to MTBE/gasoline blends.

Percent swell by volume

Volume Percent MTBE in Fuel C

Reference Material 0 15

Douthit PA - 6,12 0.50 0.20

“ PA - 6,6 -0.50 -0.50

‘ Acetal 1.00 0

‘ HDPE 10.70 10.90

‘

Davidson

Fiberglass pipe -0.43 0.87

 -1.32 to 2.26

‘ Fiberglass tank -0.02 -.51
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

64

Table A3 - Volume swell of elastomers exposed to ETBE and TAME blends with gasoline.

Percent swell by volume

Volume percent ETBE Volume percent TAME

in ASTM Fuel C

Reference Material 0 25 50 75 100 10 100

b,c,d,e FKM-65 8 8 9 9 10 11 84

b,c,d,e FKM-66 5 4 5 5 5 6 70

b,c,d,e FKM-67 5 6 7 7 8 7 41

b,c,d,e FKM-68 4 4 5 5 5 6 51

b,c,d,e FKM-70 3 2 3 2 3 2 19

l U 8 8

l) Shiblom et al SAE 902132

Table A4 - Volume swell of elastomers exposed to MeOH/gasoline blends.

Percent swell by volume

Volume percent MeOH in ASTM Fuel C

Reference Material 0 5 10 15 20 25 50 85 100

f

m

q

s

FSi 21

25

18

16

22

30

25

26

25

24

15

9

13

5

9

r FKM-65 7 32 75 120

n

q

FKM-66 5

1 21

46

30 57 85

135

100

r

FKM-67 14

14

24

16 13

16

n

r

FKM-68 5 30

15 20 22

20

r

FKM-70 19

7 8 4

p NBR-40 29 57 62 57 13

q

s

NBR-34 47

51 81

59

82

37 15

14

s HNBR-36 23 60 38 14

NBR-PVC 28 49

NBR-BIIR 95 106

NBR-CSM 56 82

q U 22 45 11

s CO 35 80 70 45

q

s

ECO 33

40

77

95 75 50

31

q U 21 58 18

q Hypalon 61 66 1

q CIIR 96 81 -4

q CPE 84 87 -2

q PS 27 28 3

m)Finney SAE 951066 (60C) o) Baurele Lub eng, 54C q) Abu Isa SAE 800786 s) Mastromatteo SAE 900195

n) Balzer SAE 910106 p) Karg SAE 900196 r) Stevens SAE 880022

Table A5 - Volume swell of thermoplastics and thermosets exposed to MeOH/gasoline blends.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

65

Percent swell by volume

Volume Percent MeOH in Fuel C

Reference Material 0 85

PA - 6,12

PA - 6,6

Acetal

HDPE

Fiberglass pipe

Fiberglass tank 10

Table A6 - Volume swell of elastomers exposed to EtOH Blends with gasoline.

Percent swell by volume

Volume percent EtOH in ASTM Fuel C

Reference Material 0 5 10 15 20 25 50 85 100

q

s

FSi 18

16

19

22

20 6

s FKM-65 7 23

s

q

c

FKM-66 5 21

6 7

36

2

s FKM-67 14 14

s

c

FKM-68 5 17

24

s

c

FKM-70 1 12

18

p

k

NBR-40 29

22

s

q

NBR-36,

NBR-34

23

51

58

68

62

99 11

s HNBR-36 55 22

d NBR-PVC 28 34

d NBR-BIIR 95 70

d NBR-CSM 56 65

q

s

CO 35 50

65

2

s ECO 40 50

q U 21 51 56 19

q Hypalon 61 81 1

m)Finney SAE 951066 (60C) q) Abu Isa SAE 800786

n) Balzer SAE 910106 r) Stevens SAE 880022

o) Baurele Lub eng, 54C s) Mastromatteo SAE 900195

p) Karg SAE 900196
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

66

APPENDIX B

Permeability Data

Table B1 - Permeability of some elastomers and ETFE to model fuels containing various 

concentrations of MTBE.

Permeability (gm-mm/m2/day)

Polymer Volume percent MTBE in ASTM Fuel C

Reference Description Toluene 0 15 20 50

c NBR-40 192** 176**

b

d

FKM-66 88

7.6 38

366

d FKM-68 7.6 23

b

d

FKM-70 9

3.8 15

121

b Aflas-56 1896 5266

b ETP-67 16 113

a ETFE 0.02 0.01*

* ETBE permeability was found to be 0.05

* Base gasoline of 73 octane was used rather than Fuel C.

a) Goldsberry SAE 930992

b) Stevens et al. ASC (1997)

c) Lebedev et al.

d) Stevens et al SAE 970307
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

67

Table B2 - Permeability of elastomers to model fuels containing various concentrations of 

 methanol.

Permeability (g-mm/m2/day)

Volume percent methanol in ASTM Fuel C

References Elastomer 0 10 15 20 2.75 +

TBA 2.75

85

a NBR-33 1212 2419 361

d

k

NBR-34 541

720 1920

1400 891

336

d NBR-34/PVC 294 1321 390

d NBR-34/BIIR 875 1257 1472

d NBR-34/CSM 915 2665 1194

a HNBR-45 369 1421 319

a FVMQ 597 1067 357

a

c

h

FKM-A-66 2.2

4.0

1.0

50 50 122

95

a FKM- B-66 2.1 50 250

a FKM-GLT-65 5.0 97 461

a

c

h

FKM-B-68 1.9

2.0

1 .0

25 21 32 13

43

a

c

FKM GF 1.7

1.0

6 1.7

17

a FKM GFLT-67 3.2 24 17
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

68

Table B3 - Permeability of thermoplastics to solutions containing various concentrations of 

 MeOH

Permeability (g-mm/m2/day)

Volume percent methanol in ASTM Fuel C

References Thermoplastic 0 15 20 85 100

e

g

PTFE 0.06

0.15

0.23 0.09

a

g

FEP 0.18 0.25 0.13

0.18

0.28

a

e, g

PFA 0.18

0.01

0.38 0.05

0.13

0.35 0.05

a

e

g

ETFE 0.18

 0.02

0.09

0.43 0.04

0.13

0.38 0.03

j HDPE 36 46 20

j PA-6 plasticized 5 50 36

f

f

PA-11

PA-11 plasticized

18

40

a

g

j

j

PA-12

PA-12 plasticized

8.8

1.53

41

34

100

27

53

1.35 102

40

60

g PA-12,12 1.35
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

69

Table B4 - Permeability of elastomers to model fuels containing various concentrations of 

 EtOH.

Permeability (g-mm/m2/day)

Volume percent ethanol in ASTM Fuel C

References Elastomer 0 10 15 20 85

b NBR 304 1000

d NBR-34 541 1066

d NBR-34/PVC 294 700

d NBR-34/BIIR 875 995

d NBR-34/CSM 915 923

a

i

NBR-33 1212 2037

1026

a HNBR-45 369 1001

a FVMQ 597 857

a

b

c

FKM-A-66 2.2

4

10.0

100

28.0

21

a

b

FKM- B-66 2.1 11.0

53.0

a FKM-GLT-65 5 20.0

a

b

c

i

FKM-B-68 1.9

2

 5.9

70.0

19.0

 4.0

14

a

b

c

i

FKM GF-70 1.7

1

 2.2

35.0

12.0

 1.1

9

a FKM GFLT-67 3.2 8.4

Table B5 - Permeability of thermoplastics to solutions containing various concentrations of 

 ethanol.

Permeability (g-mm/m2/day)

Volume percent ethanol in ASTM Fuel C

References Thermoplastic 0 10

a

i

FEP 0.18 0.13

0.03

a PFA 0.18 0.15

a ETFE 0.18 0.15

a PA-12 8.8 33

a) Stahl et al. SAE 920163 g) Goldsberry et al. SAE 910104

b) Stevens et al SAE 97037 h) Stevens SAE 880022

c) Bauerle Lubrication Eng. i) Fuller & Stevens SAE 960140

d) Dunn and Pfister SAE 800856 j) Weber et al 910304

e) Goldsberry SAE 930992 k) Puisais

f) Vasselin, Private communication

Table B6 - Permeability of hose constructions to model fuels containing alcohols.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

70

Time (days), Permeance (g/m2/d)

Volume percent ethanol or methanol in ASTM Fuel C

Reference SAE spec hose 0 E10 E15 M15

30R7 NBR/CSPE 5, 500 5, 630 4, 640 5, 820

30R8 CO/CO 7, 180 5, 450 5, 400 6, 800

30R9 FKM lined 12, 4 13, 15 13, 13 9, 90
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

71

APPENDIX C

Physical and Chemical Description of Common Fuel Resistant Elastomers

 (Puisais)

Material

Trade Names

Structure

Vulcanization

Mechanical

Properties

Chemical

Properties

Thermal

Properties

Other

Properties

Applications

Chlorinated Elastomers

Polychloroprene

(CR)

Baypren Butachlor

Neoprene

Perbunan Sovprene

Santoprene

Polymer of chloro-2

butadiene 1,3

(chloroprene).

Vulcanization

By metallic oxides (Zn,

Mn)

Good resistance to

repeated flexure, abrasion,

tearing.

Low residual deformation

by compression.

Good resistance to

aliphatic hydro-carbons.

Poor resistance to aromatic

and chlorinated hydrocarbons. Not as resistant

to hydro-carbons as NBRs.

Excellent resistance to

ozone and weathering

Limited thermal resistance

(95°C in continuous

service), resistance to cold

until 20°C (embrittles at -

40°C).

Low permeability to gas;

good flame resistance;

average electrical

properties; good adhesion

to metals and fibers/cloth.

Petroleum:

nk covers, tubes, inflatable

reservoirs

Automotive:

Spark plug covers;

electrical connectors;

motor supports; electrical

wiring covers

Epichlorohydrin

(CO, ECO)

Hydrin

Herclor

Gechron

Epichlomer

Homopolymer:

Epichlorohydrin (CO).

Copolymer:

Epichlorohydrin and

ethylene oxide (ECO).

Terpolymer:

Epichlorohydrin, ethylene

oxide, allyl glycidyl ether

Vulcanization

Diamine, urea, or thiourea;

or 2-mercapto imidizole

Constant hardness over a

large temperature range (-

20° to 170°C)

low residual compression

Good resistance to oils,

gasolines and solvents (low

swelling) except in ketones

and chlorinated solvents

excellent ozone resistance

low resistance to water

vapor at elevated

temperatures.

Good temperature

resistance to 135°C

excellent flexibility to low

temperatures (especially

ECO: -40°C)

Low permeation to gas

comparable to butyl,

superior to

polychloroprene and NR

low electrical isolation

low cost

flexible fuel lines and oil

refrigerants; piping

systems, diaphragms,

pumps.

Polyethylene

Chlorosulfone

(CSM)

Hypalon

Herclor

Vulcanization

By magnesium oxide.

Good resistance to tension

and abrasion

poor tear resistance.

Average dynamic

properties.

Good resistance to oils and

aliphatic hydrocarbons;

poor resistance to aromatic

hydro-carbons. Excellent

resistance to ozone and

weathering, acids.

Thermal resistance to

120°-135°C continuous

flexible in cold from -18°C

to -23°C.

Good electrical properties

to 600V; low permeation

to humidity and gases;

changes colors.

Excellent flame resistance.

Petroleum:

reservoir membranes;

cable covers; airtight

gaskets.

Automotive:

electrical wiring covers;

flex hose connections.

Material

Trade Names

Structure

Vulcanization

Mechanical

Properties

Chemical

Properties

Thermal

Properties

Other

Properties

Applications

Fluorinated Elastomers

FKM There are at least many Good in tensile and to Excellent chemical Excellent resistance to Good electrical properties roleum:
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

72

Viton

Fluorel

types of fluorinated

elastomers, which are

commonly based on

copolymers or terpolymers

of:

vinylidene fluoride, tetrafluoroethylene,

hexafluoropropylene, and

perfluoromethylvinylether.

canization

oxides, diamines, and

bisphenols.

tearing remarkable

resistance to compressionset

resistance in a large range

of temperatures. Low

swelling in oils, fuels,

lubricants, aliphatic and

aromatic hydro-carbons,

mineral acids. Excellent

resistance to atmospheric

oxidation and weathering

sensitive to water vapors

poor behavior to esters,

methanol, ketones, amines

poor resistance to amine

based corrosion inhibitors.

Increasing Fluorine content

generally improves

chemical resistance.

temperatures

resist oils up to 150°C

good cold resistance

(dynamic from -20° to -

40°C according to grade;

static to -54°C)

very low permeation Wellhead joints; flexible

hoses for fuel transport;

pumps; cable covers.

omotive:

Shafts; rods; valve stems;

gasoline filters; fuel

injectors; gasoline pumps;

carburetor tubing;

carburetor valves.

M

Kalrez

Chemraz

polymer of TFE and of

perfluoromethylvinylether.

milar to FKM. istant practically to all

products, except

halogenated solvents.

eptional thermal resistance to

290°C (continuous); better

resistance to cold than the

Viton (usable to -30°C).

ammable; elevated dielectric

rigidity; very high cost.

as opolymer of TFE and

propylene.

canization

By peroxide in association

with triallylisocyanurate,

at 150° - 180°C and post

cured. By diamine

Good rupture resistance,

but mediocre residual

compression

Excellent resistance to

hydrocarbons, lubricants,

hydraulic fluids

excellent resistance to

hydrogen sulfide and

carbon dioxide

good resistance to aminebased corrosion inhibitors

Resistance to elevated

temperatures (200°C

continuous)

mediocre resistance to

cold.

Exceptional electrical

resistance

good resistance to

blistering

mediocre resistance to

metal

Petroleum:

Exposure to high vapor

temperatures; corrosive

oils; amine-based corrosion

inhibitors; hydrogen

sulfide and carbon dioxide;

omotive

Hydrocarbons at high

temperatures; inferior

resistance to gasolines,

except gasohols.

FVMQ

Fluorosilicone

Silastic

FE300

Polymethytrifluoropropylsi

loxane family.

 Vulcanization

By peroxide

Maximum tensile

resistance to 1450 psi.

Good resistance to

aliphatic hydrocarbons;

poor resistance to aromatic

hydrocarbons; moderate

resistance to oxygenated

compounds.

Good thermal resistance to

175°C.

excellent resistance to low

temperatures

-80°C.
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

73

Material

Trade Names

Structure

Vulcanization

Mechanical

Properties

Chemical

Properties

Thermal

Properties

Other

Properties

Applications

Nitrile Elastomers

NBR

unan N

n

prene N

r

migum

ac

cril

rim

NBRs are copolymers of

butadiene-acrylonitrile.

The butadiene causes the

nitrile to be supple and

flexible at low

temperatures. The

acrylonitrile brings about

resistance to hydro-carbons

and good permeation

resistance to gas.

Vulcanization

By sulfur; by peroxides.

Good static mechanical

property.

The rupture resistance and

the hardness are elevated

when the ACN content is

higher. Good resistance to

abrasion. Inferior dynamic

properties.

Good resistance to

hydrocarbons.

Mediocre resistance to polar

compounds.

Loses all elasticity in the

presence of hydrogen

sulfide.

Limited temperature

resistance (100° to 120°C).

good resistance at low

temperatures.

Petroleum

Handling and transportation

of hydrocarbons; uses in

hoses for loading/unloading

of tankers.

motive

lever joints; airtight joints.

. Carboxylated

Nitriles (XNBR)

ac 211-221 Hycar

1072

 605 Chemigum

NX775

polymer of butadiene, ACN,

and carboxylic acid..

anization

ulfur; by metallic oxides.

erties

acterized by exceptional

resistance to abrasion;

excellent mechanical

properties; excellent

adhesion to metals.

ications

external electrical cable

sheaths; turning joints; antiexplosion joints for well

shafts.

. Hydrogenated

Nitriles

Therban

Tornac

Zetpol

polymer of butadiene-ACN.

anization

ly saturated HNBR’s are

vulcanized by peroxides.

Partially saturated HNBR’s

are vulcanized by sulfur or

peroxide.

Excellent static property

low retention to compression

good dynamic properties

excellent abrasion resistance

Excellent hydrocarbon

resistance

excellent resistance to water

vapors at 150°C

excellent resistance to

amine-based corrosion

inhibitors

excellent resistance to

hydrogen sulfide and carbon

dioxide gases

Good heat resistance

can be utilized continuously

in temperatures to 140°-

150°C.

good properties at low

resistance.

Good resistance to

blistering.

Material

Trade Names

Structure

Vulcanization

Mechanical

Properties

Chemical

Properties

Thermal

Properties

Other

Properties

Applications
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

74

Acrylic Elastomers

Alkyl and

alkoxyacrylics

(ACM)

Hycar

Good mechanical property,

excellent resistance to

compression set.

Good resistance to

hydrocarbons

excellent resistance to

transmission fluid, hypoid

oils.

Thermal resistance to -

40°C and 150°-160°C.

Acrylic Ethylene

(AEM) (Vamac)

Vamac

A copolymer of ethylene

and of methylacrylate, with

a monomer created at the

vulcanization sites.

Vulcanization composed of

guanidine and primary

diamine

Good tear resistance

good resistance to fatigue

by flexure

good vibration resistance

from -30°C to 160°C

Good resistance to oils, to

weathering and ozone

Good thermal stability to

170°C

good behavior at low

temperatures; becomes

brittle at -40° to -50°C.

good fire resistance

Low permeation to gas;

good electrical properties;

good adhesion to metals

and other supports;

moderate costs.

Petroleum

Cable sheaths due to its

good fire resistance;

flexible hoses for acid gas

service.

Automotive

Vamac is used extensively

in automobiles due to its

thermal resistance to oils;

its flexural fatigue

resistance; its flexibility at

low temperatures; its wear

resistance.

Polysulfides

Polysulfide

(S)

Thiokol FA

Thiokol ST

A copolymer of

Vulcanization

At ambient temperature by

zinc oxide.

Poor Good resistance to aging

and ozone.

excellent behavior to oils

(superior to nitriles), fuels,

and hydraulic fluid.

insensitive to water,

alkalines, and diluted acids

Limited thermal resistance

to 135°C, but excellent

behavior in cold.

Temperature extremes (-

55° to 135°C or -75° to

105°C according to

grades).

Disagreeable odor; good

adhesion to metal

Caulks and sealants
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

75

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76

 

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39 Polymer Handbook We used values of methane permeability at two temperatures to calculate a representative E

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41 Tripathy, B., and Smith, K., SAE Technical Paper Series No. 980850 (1998). “The Myth About Low 

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42 Beron M., Materials Engineering, May (1989), pg.60-62. “Perfluoroelastomer Seals for Higher Octane 

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44 Puisais M.,Mollard M., Dawans F., Revue De L’Institut Français Du Petrol, 40(6), (1985), pg.819-830. “The 

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46 ASTM D-873-94 “Standard Test Method for Oxidation Stability of Aviation Fuels (Potential Residue Method)”

47 Annual book of ASTM Standards, Vol 05.01

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49 ASTM D-471 “Standard Test Method for Rubber Property -- Effect of Liquids.”

50 ASTM D-543 “Standard Test Method for Evaluating the Resistance of Plastics to Chemical Reagents.”

51 ASTM C 581-94 “Standard Practice for Determining Chemical Resistance of Thermosetting Resins Used in 

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52 ASTM D 4021-92 “Standard Specification for Glass-Fiber-Reinforced Polyester Underground Petroleum 

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53 UL-1316 (1994) “Standard for Glass Fiber Reinforced Plastic Underground Storage Tanks for Petroleum 

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54 UL-971 (1995) “Standard for Nonmetallic Underground Piping for Flammable Liquids.”

55 UL-567 (1992) “Standard for Pipe Connector for Flammable Liquids and LP Gas.”

56 ASTM D-3985 “Standard Test Method for Oxygen Gas Transmission Rate through Plastic Film and Sheeting 

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57 ASTM D-1434 “Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and 

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58 SAE J 1737.

59 SAE J2260 “Nonmetallic Fuel System Tubing with One or More Layers.”

60 SAE J30
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

77

 

61 H. H. Uhlig, The Corrosion Handbook, John Wiley, ( ).

62 Ailor W.H., “Handbook on Corrosion Testing and Evaluation,” John Wiley and Sons, Inc., New York (1971).

63 ADTM G-15 - 97a, “Standard Terminology Relating to Corrosion and Corrosion Testing.”

64 ASTM G-16 - 95, “Standard Guide for Applying Statistics to Analysis of Corrosion Data.”

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67 Hawke, D.L., SAE Technical Paper Series No. 860285 (1986). “Compatibility of Magnesium Alloys with 

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75 Ryan III, T.W., Bond, T.J., and Schieman,R.D., SAE Technical Paper Series No. 861591 (1986). 

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76 Estafan, R.M., and Brown, J.G., SAE Technical Paper Series No. 902153 (1990). “Evaluation of Possible 

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77 Black F., SAE Technical Paper Series No. 912413 (1991). “An Overview of the Technical Implications of 

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78 Nakajima, K., and Yahagi, Y., “Transactions of the ASME,” vol. 105 (1983) pg. 552-558. “Corrosive Wear of 

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79 Yahagi, Y., and Mizutani, Y., Wear, 97 (1984) pg 17-25. “Corrosive Wear of Steel in Gasoline-Ethanol-Water 

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80 Powell T., SAE Technical Paper Series No. 750124 (1975). “Racing Experience with Methanol and Ethanol 

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83 Brossia, C.S., and Kelley, R.G.,

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85 Brink, A., Jordaan, C.F.P., LeRoux, J.H., and Loubser, N.H., Sasol Techn. (PTY) Ltd.. “Carburetor Corrosion:

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Compatibility and Permeability of Oxygenated Fuels to Materials

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78

 

89 Jahnke, H., and Schonborn, M., Werkstoffe und Korrosion, 36 (1985) pg. 561-566. “Electrochemical Corrosion

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90 ASTM G-1, “Standard Practice for Preparing, Cleaning and Evaluating Corrosion Test Specimens.”

91 ASTM G-31 - 95, “Standard Practice for Laboratory Immersion Corrosion Testing of Metals.”

92 ASTM G-46 - 94, “Standard Giode for Examination and Evaluation of Pitting Corrosion.”

93 ASTM G-71 - 81, “Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes.”

94 ASTM G-119 - 93, “Standard Guide for Determining Synergism Between Wear and Corrosion.”

95 ASTM G-133 - 95, “Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear.”

96 ASTM D-1418 - 95, “Standard Practice for Rubber and Rubber Lattices – Nomenclature.”

97 ASTM D-1600 “Standard Terminology for Abbreviated Terms Relating to Plastics.”

98 Lesage L., De L’Institut Français Du Petrol”, vol. 48(4), July-August (1993) pg. 383-411. “Les élastomères

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99 Alexander J.E., Ferber E.P., and Stahl W.M., Fuel Reformulation, 4(2), (1994) pg. 42-46. “Avoid Leaks from 

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100 Alexander J.E., Ferber E.P., and Stahl W.M., AIChE Spring Meeting, Houston, TX (1995) B4. “Leak

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101 Alexander J.E., Ferber E.P., and Stahl W.M., Prevention of Pipeline Corrosion Conference, Houston, TX, 

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102 Nersasian, A., Elastomerics, October (1980), pg. 26. “Compatibility of Fuel Handling Rubbers with 

Gasoline/Alcohol Blends”

103 Nersasian, A., SAE Technical Paper Series No. 800789 (1980).“ The Volume Increase of Fuel Handling 

Rubbers in Gasoline/Alcohol Blends”

104 Arroyave P., C.E., Rev. Iber. Corros. Y Prot., 18(2-6), (1987) pg.119-122. “Corrosividad de mezclas gasolinaetanol sobre mangueras utilizadas en sistemas de combustible de automotores convencionales”

105 Abu-Isa, I.A., Rubber Chemistry and Technology, 56(1), (1983) pg. 169. “Effects of Ethanol/Gasoline and 

methyl t-butyl Ether/Gasoline Mixtures on Elastomers”

106 Stahl W.M., Personal Correspondence, (1992). “Fluid Resistance of Various Viton Fluoroelastomers to Fuel 

C/MTBE Blends,” “Fluid Resistance of Various Viton Fluoroelastomers to Fuel C/ETBE Blends,” and “Fluid 

Resistance of Various Viton Fluoroelastomers to Fuel C/TAME Blends”

107 Hertz, D.L., SAE Technical Paper Series No. 930993 (1993). “Elastomers in Automotive Fuel Environments.”

108 Aloisio S., SAE Technical Paper Series No. 940956, (1994). “Performance of Fluorocarbon Elastomers in 

MTBE/Fuel Blends”

109 Aloisio S. Ausimont Technical Report, “Performance of Fluorocarbon Elastomers in MTBE/Fuel Blends”

110 Goldsberry D.R., SAE Technical Paper Series No. 930992, (1993). “Fuel Hose Permeation of Fluoropolymers”

111 Starmer, P.H., American Chemical Society, Detroit, 1991. “Relating the Properties of Polar and Nonpolar 

Liquids to the swelling of Nitrile Rubber vulcanizates by these Liquids. 2. Swelling Curves”

112 Dunn, J.R., Pfisterer, H.A., and Ridland, J.J., Rubber Chemistry and Technology 52, (1979) pg. 331-352.

“NBR Vulcanizates Resistant to High Temperature and ‘Sour’ Gasoline

113 Barbosa, S.R.C. Int Inst. Synth. Rubber Producers (1983). “Effect of Alcohol Fuels on Nitrile Rubber 

Automotive Parts”

114 Karg R.F., Hill C.L., Dosch K. And Johnson B., SAE Technical Paper Series No. 900196 (1990). “Ultra-High 

ACN Polymer in Fuel System Application”

115 Dunn J.R. and Pfisterer H.A., SAE Technical Paper Series No. 800856 (1980). “Resistance of NBR-Based Fuel 

Hose Tube to Fuel-Alcohol Blends.”

116 “Swelling of automotive elastomers by Fuels”

117 Hotaling A., NACE Paper No. 34 (19 ). “Performance of Nonmetallic Tank Seal Materials in MTBE Service.”

118 Hotaling A.C., Am Energy Wk Pipelines, Terminals & Storage, 2, (1995), pg 118-126. “Evaluating 

Nonmetallic Materials’ Compatibility with MTBE and MTBE + Gasoline Service”

119 Stahl W.M. and Stevens R.D., SAE Technical Paper Series 920163 (1992). “Fuel-Alcohol Permeation Rates of 

Fluoroelastomers, Fluoroplastics and Other Fuel Resistant Materials”

120 Balzer J.R. and Edmonson A.L., SAE Technical Paper Series No. 910106 (1991). “Effects of Methanol Blend 

Fuels on Fuel Hose Permeation.”
Compatibility and Permeability of Oxygenated Fuels to Materials

in Underground Storage and Dispensing Equipment

79

 

121 Stevens R.D. and Fuller R.E., SAE Technical Paper Series No. 970307 (1997). “Fuel Permeation Rates of 

Elastomers after Changing Fuel.”

122 MacLachlan J.D., SAE Technical Paper Series No. 790657 (1979). “Automotive Fuel Permeation Resistance -- 

A Comparison of Elastomeric Materials.”

123 Weber A., Hubold J., Muller J., and Linster J.J., SAE Technical Paper Series No. 910394 (1991). “A Testing 

Equipment for Measuring Permeability of Plastic Fuel Line Tubing”

124 Nersasian, A., SAE Technical Paper Series 800790 (1980). “Use of Toluene/Isooctane/Alcohol Blends to 

simulate the swelling Behavior of Rubbers in Gasoline Alcohol Fuels”

125 Nersasian, A., SAE Technical Paper Series 800791 (1981) “A Method for Predicting the Volume Increase of 

Fuel Handling Rubbers in Complex Fuel Blends”

126 Petrovic, Z.S., MacKnight, W.J., Koningsveld, R., and Dusek, K., Macromolecules, vol. 20 (1987), pg.1088.

127 Kamody, J.F., Damaini, A, Stadelman, R.J., 47th conf. Composites Institute, Soc Plastics. Ind., (1992). “The

Use of FRP with Alcohol-Containing Fuels”

128 Reichold Reactive Polymers Division Application Guide, “ATLAC and DION Corrosion Resins for 

Containment of Petroleum Fuels.”

129 Douthit W.H., Davis B.C., Steinke E.D.L., and Doherty H.M., SAE Technical Paper Series No. 881667 (1988).

“Performance Features of 15% MTBE/Gasoline Blends.

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