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Electrochemical influence on carbon filled ethylene-propylene elastomers.

Ethylene-propylene elastomers are well known for their outstanding resistance to the deleterious effects of ozone oxygen and heat. As a hydrocarbon polymer free of inchain unsaturation, EPDM is also quite suitable for contact with water, acids, alkalis and aqueous salt solutions over a broad temperature range. Furthermore, the polymer demonstrates good compatibility with oxygenated organic solvents and other polar fluids. Compounds derived from this elastomer have ready utility in fluid conducting hoses, pump diaphragms and seals, marine applications and the like. In the automotive industry, a major use for ethylene-propylene rubber compounds for over 25 years has been in coolant system hoses, where resistance to ethylene glycol/water mixtures, containing both inorganic salts and organic compounds in a formulated package, is required. In addition to compatibility with the coolant fluid, EPDM hoses have outstanding resistance to heat and oxidative environments found in the engine compartment of a vehicle. As a result of this unique performance, EPDM compounds have become the universal material for high performing coolant hoses.

Ethylene-propylene rubber compounds employed in automotive radiator and heater type hoses have maintained their outstanding physical properties after long term exposure in contact with hot coolant fluid. In fact with a properly serviced coolant system, the hoses can be expected to last the lifetime of the car. Despite this outstanding performance reputation, EPDM coolant hoses may experience a problem associated with the development of microcracks within the innertube. These cracks or striae form as a result of an electrochemical process, ongoing during vehicle operation and perhaps even during engine shut-down. Under the influence of an electromotive force, the ethylene glycol/water fluid appears to be more aggressive toward the compound. Mechanical properties weaken more rapidly and the compound absorbs considerably more fluid. The exposed rubber stock is significantly swollen, similar to EPDM that has been immersed in a hydrocarbon fluid. Unlike the classical auto-oxidation where EPDM becomes stiffer and harder, in this process the compound becomes soft and mechanically weak.

The present research studies were undertaken to elucidate the cause of the apparent aggressive action of automotive coolant fluid on an ethylene- propylene compound. Typical formulations used for coolant hose were employed rather than a simplified model compound within the investigation. The important elements of an EPDM formulation including polymer composition, filler morphology and vulcanized network are investigated to appraise the influence of an electromotive force on these components. A mechanism governing this process is proposed and future direction in compound development is suggested.


The ethylene-propylene rubber compound utilized throughout the majority of the work reported herein is found in table 1. As the carbon black or inorganic filler types and cure system effects are discussed, specific ingredients are noted.

All EPDM compounds were prepared utilizing a two-stage procedure. The masterbatch of polymer, fillers, plasticizer, zinc oxide and stearic acid were mixed in a laboratory internal mixer, size B, using an upside-down mix cycle. In the second step, curing agents and other chemicals associated with vulcanization were added to the masterbatch on a rubber mill following the practice described in ASTM D3182. All compounds were compression molded and curing was accomplished in a steam heated press for 20 minutes at 170[degrees]C, except where indicated in the reported data.

Electrochemical behavior of the EPDM compound was determined using a glass "U" tube apparatus. The equipment consists of a pyrex glass tube containing two arms; the diameter of each tube is 35 mm and the length is 130 mm. The arms are connected at the bottom using 12.5 mm dia. glass tubing. At the top of each arm is a 35/25 ground glass fitting in which a Teflon ball joint stopper is used to seal the tube or where a condenser can be attached, if it is desired to reflux the immersion fluid. Each ball joint contains a 10 mm dia. glass tube which serves to insulate the electrical conductor from fluid vapor. This insulation tubing can be adjusted in order to raise or lower the depth of the rubber specimen into the test fluid. One end of the wire contains an electrical clip which clamps the rubber specimen, while the other end is connected to a variable DC voltage power supply. The test specimen is a rubber strip, 100 mm x 20 mm x 2 mm, die cut from laboratory molded sheets. Each specimen is bent to 180[degrees] to impart a small amount of strain in the loop area, clamped with an electrical clip and placed in one of the arms of the U-tube. Another sample of the compound is placed in similar fashion into the other arm. These serve as the anode and cathode of the electrochemical cell. Both rubber samples are partially immersed in the test fluid, in this case a commercial grade of automotive coolant that has been diluted 50% by volume with water. The test specimens are then aged under prescribed conditions of voltage, temperature and time. At the completion of the immersion period, the physical properties of the compound and amount of fluid absorbed are determined. In this work, the electrochemical test procedure was carried out at 80[degrees]C; the commercial coolant used was Ford Long Life Premium Cooling System Fluid containing ethylene glycol.

Due to the small size of the sample strip immersed in coolant, a modified stress-strain specimen and procedure were utilized to determine physical properties. The specimen geometry was a micro die, with a neck-down length of 20 mm and width of 3 mm, dimensions "L" and "W" respectively in ASTM D412. The overall length of the specimen is 52 mm and the tab width is 10 mm, dimensions "C" and "A" respectively. The micro-dumbbell specimen is die-cut from the strip so that the necked-down region is taken from the looped area of the sample. Crosshead speed in the tensometer was 254 mm/min. Weight change due to fluid imbibed in the compound is determined according to ASTM D471. This measurement is made immediately after the immersion test and before tensile specimens are die-cut from the immersed strip.

Results and discussion

It is now reasonably certain that the deterioration of the EPDM radiatior hose is based on an electrochemical process which is further accelerated due to increased under-the-blood temperatures (refs. 1 and 2). Either through the presence of an electromotive force or galvanic activity, increased swelling and greater loss of physical properties have been observed. To illustrate the effect of electrolysis, the EPDM compound was immersed in automotive coolant and water and subjected to an applied DC voltage of 0-12 volt in the U-tube cell. As the voltage increased, the amount of fluid imbibed in the rubber significantly increased, particularly at the cathode of the cell and to a lesser extent at the anode. Table 2 summarizes the effect of voltage on the aggressiveness of coolant fluid toward a sulfur cured EPDM compound containing 28 volume percent carbon black.

Compound physical properties also decrease rather dramatically in an electrochemical environment. The tensile strength and elongation of a sulfur cured EPDM stock decrease markedly over the first seven days of immersion and the level off for the next 14 days. In the absence of the applied voltage, the tensile strength was unchanged, while the elongation decreased by only 15% during the initial seven days. While this test was discontinued after this period, it is expected that elongation would continue to decrease as a result of exposure to the hot coolant; tensile strength change, on the other hand, would be minimal. Thus, it is clear that in the presence of an electromotive force, automotive coolant fluid becomes more aggressive toward EPDM compounds.

The observation that weight gain occurs mainly at the cathode suggests that the electrochemical process responsible for this phenomenon is the reduction of sulfur-sulfur linkages in the cathodic polymer, as shown in equation (1),

R-[(S).sub.x]-R + 2e- + 2H+ --> 2RSH + [S.sub.x-2] (1) in which the R groups denote two polymer chains. Unless the network is very tightly cured and supported by other nearby sulfur-sulfur linkages, crack formation and influx of coolant may occur. In our experimental set-up, reaction (1) will occur at the cathode/electrolyte (polymer/coolant) interface. In this unusual electrochemical cell most of the resistance to current flow comes from the electrodes (polymer samples). The aqueous electrolyte (coolant) has a much higher dielectric constant and current will flow readily through it to the anodic polymer sample. As at the cathode, an electrochemical process must occur at the interface to allow current to flow through the anodic polymer sample. In the present work the nature of the anodic process has not been elucidated.

Factors influencing cathodic fluid absorption

Carbon black loading and type

In a conventional electrochemical cell the electrodes are highly conducting (e.g., solid graphite or metal) and current flow is limited by the concentration of the electroactive species, present at the electrode/electrolyte surface. In the present case, we can expect to see a dependence of the current flow on the conductivity of the electrode (polymer sample). It is well known and of obvious interest in wire and cable applications of EPDM, that conductivity results from the presence of carbon black in the polymer compounds. Additionally, high surface area carbon blacks contribute higher conductivities (ref. 3).

The typical carbon blacks used in EPDM radiator hose compounds are either N650 or a blend of N650 with N762 for an optimum balance of processing and physical properties. Utilizing a blend of N650 and N762 at approximate equal levels, the cell current and coolant fluid absorption were compared at various filler loadings. The results are listed in table 3, runs 1 to 6. Above about 20 volume percent, current flow through the cell increases rapidly with increase in carbon black loading (figure 1). The lack of conductivity of polymers with low carbon black loadings is well known and attributed to the inability of electrons to tunnel over large innerparticle separations (ref. 3). The break in figure 1 occurs close to the typical lower limit of carbon black loading required to give good conductivity (25 weight percent, i.e., about 16 volume percent). Above this limit, we have examined the effect of varying the surface area of the carbon black (table 3, runs 7 to 13). The expected increase in current flow with increase in carbon black surface area is illustrated in figure 2.

It is also found, with exception of a single data point for the highest surface area carbon black, that the cathode weight gain increases linearly as the current flow through the cell increases (figure 3). Although the linear dependence of the quantity of electrochemical product on current is the foundation of electrochemistry, the linearity here must be coincidental as we are measuring secondary or tertiary effects of the electrochemical activity (fluid absorption following on crack formation following on sulfur- sulfur bond cleavage) and the important point is the direction of the relationship rather than its magnitude. It is believed that the data in figure 3 indicate a cathodic process limited by the resistance of the electrode itself.

The ability of a compound filled with a high structure carbon black, N-472, to sustain very high currents without correspondingly high fluid absorptions (table 3, run 13) may indicate that high structure blacks hold a network together better than lower structure carbon blacks. Furthermore, dispersion of carbon black in EPDM, as viewed via incident light microscopy, was rated as very good for both N-472 and N-351 types. This may be attributed to a higher shear condition during mixing, compared with the selfreinforcing blacks. Thus, compounds with N-472 or N-351 may have more rubber surrounding the black particles which limits conductivity.

State of cure

For the data presented in table 3, there is a sufficient concentration of sulfur-sulfur bonds at the electrode/electrolyte interface to permit current flow across the interface and into the electrolyte. This sufficient concentration is assured by adequate curing of the compounds. If there were an insufficient concentration of sulfur-sulfur bonds at the interface, current flow and cathode weight gain may become limited by the state of cure. This has been examined by varying the cure time for series of compounds. The results are shown in table 4. It is clear that at low cure states the current flow and cathode weight gain are indeed dependent on the state of cure.

If the simple view presented were correct, current flow would reach a plateau at high cure states, representing the maximum current that the electrode itself can sustain and cathode weight gain would also reach a plateau. Based on limited data (table 4), it appears that the current flow does level out at high cure state, but the cathode weight gain drops rather than staying level. Our interpretation is that in very tightly cured systems there may be sufficient nearby sulfur-sulfur linkages to prevent crack formation when a particular linkage undergoes electrochemical cleavage. Thus the same current may flow as at a lower cure state and the same quantity of linkages may be cleaved, but crack formation and electrolyte absorption actually decrease.

This has been further examined by preparing a series of varying (but all tightly) cured compounds and submitting these to our electrochemical test procedure. As expected, the data (table 5) show that the current flow is essentially the same for all four compounds. However, the cathode weight gain decreases as the state of cure increases, as measured either by rheometer torque increase or compound tensile modulus. In summary, for tightly cured compounds, there are more than sufficient sulfur-sulfur bonds to sustain the maximum current that the resistance of the electrode will allow. For all four runs in table 5, essentially the same current flows and the same concentration of sulfur-sulfur bonds is cleaved. For the more tightly cured compounds, the higher concentration of nearby uncleaved bonds maintains the network and so reduces cracking and fluid absorption.

EPDM diene content

A series of E-P polymers having a range of 5-ethylidene-2-norbornene (ENB) concentration were compounded with 29 volume percent carbon black and a low sulfur cure systems. The diene level varied from 0-10 weight percent. All compounds were heated for 20 minutes at 170[degrees]C, an optimum cure cycle. Of course no crosslink network was obtained for the copolymer (0 ENB), although the purpose for including the stock in the study was to assess what effect the by-products of vulcanization had on electrochemical conductivity. As shown in figure 4, the lowest coolant fluid absorption is associated with the non-crosslinked EPM compound. Cathode weight increased significantly even at the low ENB level and continued to rise with higher diene concentration. Under the curing conditions used, it is expected that the concentration of sulfur-sulfur crosslinks will increase with diene content. The carbon black loading is sufficiently high that the resistance of the electrode itself should not be limiting and the data in figure 4 are consistent with a dependence of current flow and coolant absorption on sulfur-sulfur bond concentration.

Peroxide cured compounds

The type of cure system used in developing the crosslinked network has been found to have major importance on the electrochemical stability of EPDM compounds. Previous studies and additional recent work shown in table 6 indicates that sulfur and peroxide cures of the EPDM formula gave significantly different electrochemical behaviors. Two sulfur cure systems are listed to evaluate the potential difference between mono- or di-sulfidic bond (low S) and a polysulfidic (high S) type. The dramatically lower weight increase for the peroxide cured compound is notable and emphasizes the importance of the electrochemical process shown in equation (1) in causing the present phenomenon in sulfur cured systems. The carbon-carbon crosslinks formed in the peroxide cure system are expected to be very resistant to electrochemical reduction. The essential retention of physical properties for the peroxide cured compound are totally consistent with the absence of any electrochemical effect on its crosslink network. The low current flow for the peroxide cured compound is a result of the absence of reducible sulfur-sulfur bonds, that would allow facile current flow across the polymer/coolant interface. The carbon black loading (29 volume percent) is sufficiently high to sustain much higher conductivity and is not the current limiting factor for the peroxide cured compound.

Performance differences between sulfur and peroxide cure systems, noted in table 6, were initially thought to be related to the lower cure state of the peroxide cured compound versus that obtained with either sulfur system. For this reason, a group of compounds was prepared with increasing levels of peroxide and coagent. Figure 5 illustrates a significant increase in development of 100% modulus, while maintaining essentially similar coolant fluid absorption over the entire range. For reference, a sulfur cured stock is shown at the right side of the chart. Thus, crosslink density in the case of carbon-carbon covalent bonds has no influence on electrochemical stability.


Our test system can be treated as a rather unusual electrochemical cell in which most of the resistance lies in the electrodes (polymer compound samples) rather than in the electrolyte (coolant). This resistance may limit the current flow or it may be limited by the concentration of the electroactive species specifically the sulfur-sulfur bonds at the cathode/electrolyte interface.

Current flow limitation by the resistance of the polymer sample is noted at low carbon black loadings. Under these conditions properly sulfur cured samples provide more than sufficient sulfur-sulfur bonds at the sample/coolant interface to permit flow of this current into the coolant via their reduction at the interface. A dependence of current flow on carbon black loading is found.

At carbon black loadings typical of those formulations, the resistance of the compound is sufficiently low that flow of current is determined, as in a more usual electrochemical cell, by the concentration of the electroactive species (sulfur-sulfur bonds) at the electrode/electrolyte interface. Dependences of current flow on type of cure (peroxide vs. sulfur), state of cure and diene content result.

Except for very tightly cured systems, electrolyte (coolant) absorption by the cathode is directly (and roughly linearly) related to the current flow.

In very tightly cured systems, a reduced coolant absorption is observed compared to stocks with lower cross-linked density showing the same current flow. This is interpreted as indicating that electrochemical reduction of the same quantity of sulfur-sulfur bonds occurs, but the network does not collapse so readily because of the presence of nearby crosslinks to maintain it. Crack formation and hence coolant absorption occur less readily in a very tightly cured system.

In terms of applying the results of the present research to the prevention of electrochemical degradation of sulfur cured automotive hoses, at least three types of countermeasures suggest themselves.

First, the phenomenon that has been investigated here could be effectively counteracted by electrical isolation of the hoses. Hoses are usually carbon filled and attached to metal parts. Electrical isolation of such hoses could require a non-carbon filled gasket between engine parts and hoses.

Second, carbon black loading of the hoses could be reduced so that they are no longer sufficiently conducting to sustain electrochemical activity at the coolant interface. This is in conflict with the fundamental role of carbon black in reinforcement of ethylene-propylene elastomers. Our results suggest that high reinforcing carbon blacks, such as N472 or N351, may be particularly useful in this regard.

Third, the incorporation of oxidizing agents in the coolant which would reoxidize the cleaved sulfur-sulfur bonds, is a possibility.
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Author:White, Donald A.
Publication:Rubber World
Date:Jun 1, 1992
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