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AGING OF NATURAL RUBBER IN AIR AND SEAAW …

AGING OF NATURAL RUBBER IN AIR AND SEAWATERP. H. Mott, C. M. Roland*Code6120, Naval Research Laboratory, Washington DC20375 ABSTRACTA ccelerated AGING experiments were carried out on a NATURAL RUBBER vulcanizate exposed to air and to seawater. Failurestrain, shown to correlate well with the fatigue lifetime, was used to monitor the extent of degradation. The effect oftemperature on the rate of AGING followed an Arrhenius law, with activation energies equal to 90 4 and 63 3 kJ/mol forair and seawater AGING , respectively. The difference can be accounted for by the difference in oxygen concentration for thetwo predict the service life of a RUBBER component subjected to a variety of environments, it is nec-essary to account for all modes of degradation. This can be difficult in a complex environment suchas seawater, where different processes, such as oxidation, swelling, leaching, and even biodegra-dation, may occur simultaneously.

AGING OF NATURAL RUBBER IN AIR AND SEAAW TER P. H. Mott, C. M. Roland* Code 6120, Naval Research Laboratory, Washington DC 20375 ABSTRACT Accelerated aging experiments were carried out on a natural …

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Transcription of AGING OF NATURAL RUBBER IN AIR AND SEAAW …

1 AGING OF NATURAL RUBBER IN AIR AND SEAWATERP. H. Mott, C. M. Roland*Code6120, Naval Research Laboratory, Washington DC20375 ABSTRACTA ccelerated AGING experiments were carried out on a NATURAL RUBBER vulcanizate exposed to air and to seawater. Failurestrain, shown to correlate well with the fatigue lifetime, was used to monitor the extent of degradation. The effect oftemperature on the rate of AGING followed an Arrhenius law, with activation energies equal to 90 4 and 63 3 kJ/mol forair and seawater AGING , respectively. The difference can be accounted for by the difference in oxygen concentration for thetwo predict the service life of a RUBBER component subjected to a variety of environments, it is nec-essary to account for all modes of degradation. This can be difficult in a complex environment suchas seawater, where different processes, such as oxidation, swelling, leaching, and even biodegra-dation, may occur simultaneously.

2 The present study was motivated by the Navy s developmentof an elastomeric disk as potentially the torpedo launcher on futureVirginia-class development of the RUBBER compound has been described disk has a diameterof more than 2 m and a thickness varying from 20 to 30 cm. It is to be inflated with seawater to100% biaxial strain, and the performance must be maintained over roughly 2000 inflation cycles(ca. 15 years). Since any degradation of the mechanical properties will impair a vital function ofthe submarine, a reliable assessment of longevity is obvious method for lifetime prediction is to accelerate the AGING , so that deterioration ofproperties occurs over a feasible time scale, , a few weeks. The Arrhenius equation3is thenused to predict the degradation rate at the service temperature:r1r2 DAexp EaR 1T1 1T2 (1)wherer1is the reaction rate at temperatureT1, etc.

3 ,Ais a constant, andEais the activation of short time, high temperature data in this manner to yield lifetime estimates forrubber exposed to long times at lower temperatures has a long 9 Herein, our emphasis ison the material properties that affect the stored energy and the fatigue lifetime, since these directlyinfluence the performance of an elastomeric torpedo RUBBER formulation is listed in Table I. To insure repeatability, all samples were obtainedfrom a single batch. Test sheets, 120- 65- , were cured 30 min at 160 C, followed byexposure to the various AGING environments. Air degradation was carried out in a convection oven(Delta 9023). Seawater AGING was performed in vented, enclosed tanks, equipped with temperaturecontrollers and circulators. The brine was prepared by dissolving a commercial preparation (Aquar-ium Systems Instant Ocean R) into distilled water, at a concentration of , as determined from*Corresponding author.

4 Tel: 202-767-1719; fax: 202-767-0594; e-mail: from from CHEMISTRY AND TECHNOLOGYVo IRubber CompoundIngredientTrade NameSourcephrDeproteinized Astlett oxideZinc oxide RG-MAAkrochem di-2-ethylhexoateOctate ZR. T. Vanderbilt RSuperflex Solid GR. T. Vanderbilt 2-mercaptotoluimidazoleVanox RZMTIR. T. Vanderbilt RTBSIM onsanto Chemical RMAkrochem aidVanfre RAP2R. T. Vanderbilt (Cyclohexythoiphthalimide)Vantard RPVIR. T. Vanderbilt (CH2)6Na-H2 ODuralink RHTSM onsanto Chemical carbon blackCorax RN110 Degussa-H ls carbon blackCorax RN550 Degussa-H ls density. Dissolved oxygen concentrations were determined by the indigo-carmine method10(ASTM D 888-87) using a commercial test kit (Chemets).After AGING , specimens were cut from the sheets for room-temperature stress strain measure-ments (Instron 4206 with Wallace optical extensometer).

5 Cyclic strain energies were measured at acrosshead strain rate of min 1, with a maximum extension of ; data from the third cycle wasutilized. Elongation to failure was carried out at an extension rate of min 1, with a minimum ofseven specimens tested for each condition. Fatigue testing employed a modified Monsanto Representative elongation to failure curves for the RUBBER unaged, seawater aged ( h at 98 C)and air aged ( h at 120 C). AGING OF NATURAL RUBBER81to Failure apparatus (ASTM D 4482-85), testing 12 samples per exposure condition at an elongationof analysis (TGA) was carried out using a Perkin-Elmer TGA-7 with AND DISCUSSIONELASTIC PROPERTIESF igure 1 illustrates the effect of AGING on the stress strain curve for the RUBBER . There is a sub-stantial decrease in the failure strain, along with an increase in modulus.

6 Air AGING consistentlycaused the modulus to increase with time at all temperatures, although at the highest AGING temper-ature (120 C) the increase was no more than 50%. The consequences of seawater AGING were lessmarked, with changes in the stress strain curve discernible above the experimental scatter only forlonger exposure PROPERTIESThe failure properties decline with the extent of AGING , as shown in Figure 2 (air exposure)and Figure 3 (seawater exposure). Similarly, the fatigue life, measured after air AGING at 110 C,decreases with AGING time (Figure 4). Effect of air AGING at the indicated temperatures on elongation at break and tensile CHEMISTRY AND TECHNOLOGYVo Effect of seawater AGING at the indicated temperatures on elongation at break and tensile Fatigue life after air AGING at 110 C as a function of exposure OF NATURAL RUBBER83 Usually, the failure of RUBBER in tension is interpreted using fracture mechanics, where thenumber of cycles,N, depends on the strain energy,W,as11;12ND1.

7 1 c 10(2)in whichc0is the intrinsic flaw size, andBand are material constants. The parameterKis aslowly varying function of strain13KD p (3)where is the stretch (2) can be used to relate the tensile fatigue life to the breaking strain for a givenmaterial, but AGING induces chemical changes. The cut growth properties ( ,Band ), and alsothe strain energy for a given change with AGING . Nevertheless, the fatigue life of aged samplescorrelates well with their failure strain (Figure 5). This means that we can use Equation (2) to relatefailure strain to fatigue Figure 6 is shown a double logarithmic plot of the fatigue life as a function of the ratio ofthe strain energy parameter,KWin Equation (2), forND1 (single extension to break) to that forfatigue testing at D2:26; the exponent is Note that if the changes in fatigue life were due onlyto changes in strain energy ( , the material per se were unchanged), the slope of Figure 6 wouldcorrespond to the in Equation (2).

8 For unfilled NATURAL RUBBER (NR), ;15 The relationshipin Figure 6 is nearly quadratic, with at least some of the difference reflecting the changes in thematerial Correlation of the fatigue life versus the elongation at break for air agingat 110 C. Note the scales are both CHEMISTRY AND TECHNOLOGYVo Fatigue life versus the reciprocal of the strain energy parameter, normalized by thestrain energy for a single elongation to DEPENDENCE OF AGINGThe parallel curves for the failure strain in Figures 2 and 3 imply that a single AGING mechanismdominates over these temperatures. In Figure 7 are master curves for AGING in air and in seawater, Superpositioned master AGING curves for air and seawater, at a reference temperature of 90 OF NATURAL RUBBER85constructed by shifting the data along the time axis, using a reference temperature of 90 C.

9 Thetwo master curves are also parallel, again suggesting that the underlying degradation chemistry isthe shift factors used to construct Figure 7 are displayed in Figure 8 in the Arrhenius form,yieldingEaD90 4 kJ/mol and 63 3 kJ/mol for air and seawater, respectively. This activationenergy for air AGING agrees with literature values for NR compounds obtained by various methods,viz. 88 Ea 98 18 Analyses based on puncture energy19and surface chemistry20yield somewhat lower values, ca. 75 80 SOLUBILITYA lthough the AGING curves in Figure 7 for the two environments are parallel, the activationenergy for seawater is significantly lower than for air. Thus, while the chemical mechanism ofrubber degradation may be the same, it is apparent that some other factor exerts an influence. Theobvious difference between the two media is the availability of oxygen.

10 It is well known that thesolubility of oxygen in water decreases with temperature. This solubility is expressed by Henry slaw21xO2 DPO2k(4)in whichxO2is the mole fraction of dissolved oxygen,PO2the partial pressure, andka temperature dependence ofkcontributes to the difference in AGING behavior for RUBBER in airversus seawater. Published data for various brine solutions22;23can be interpolated to obtain a valueofkfor seawater. The partial pressure of oxygen in seawater also varies with temperature, Horizontal shift factors used to construct the master curves in Figure 7. The slopes yieldEaD90 and 63kJ/mol for air and seawater AGING , respectively. The dashed curve represents the data for seawater after factoring out thetemperature dependence of oxygen CHEMISTRY AND TECHNOLOGYVo a closed vessel, it is being displaced.


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