I
ROSS E. MORRIS and PAUL
T.
WAGNER
Mare Island Naval Shipyard, Vallejo, Calif.
Swelling of Nitrile Rubbers Toluene Blends
by Iso-octane-
Swelling of rubber by solvent mixture of iso-octane and toluene is an important criterion for evaluating the resistance of rubber to gasoline
THE
70 to 30 blend by volume of isooctane and toluene has been generally adopted as a standard test medium for determining the resistance of rubbers to gasolines. I t is designated as Reference Fuel B by the American Society for Testing Materials ( I ) , as Type 111, Hydrocarbon Standard Test Fluid by the armed forces (70), and as Medium No. 6 by the nonmilitary departments of the Government (4). The ASTM states that this blendhas a severe swelling action on elastomeric vulcanizates and exceeds the swelling action of commercial gasolines. Most technologists are familiar with the change in volume and tensile deterioration of various vulcanized rubbers in the 70% iso-octane-30yo toluene blend. However, one feature of the attack by this blend on rubber vulcanizates has not previously been considered -namely, the relative distribution. of the solvent components between the solvent phase and the rubber. Experimental Details Nitrile rubbers, copolymers of butadiene and acrylonitrile, are probably the most commonly used elastomers for gasoline service. Three nitrile rubbers covering a range of acrylonitrile contents were selected for experiments to determine the relative distribution of iso-octane and toluene between the solvent phase and the rubber phase. The nitrile rubbers were compounded according to the following recipe and were vulcanized in sheets of 0.08 inch in thickness for 20 minutes a t 310' F. Nitrile rubber Zinc oxide Stearic acid Sulfur Tetramethyl thiuram monosulfide
100
'
5 1 1.5 0.3
The swelling experiments were performed with vulcanized specimens which contained no extractable materials. These materials might have affected the swelling equilibrium or interfered with the analysis of the solvent absorbed by the rubber. The extractable materials were removed from the specimens prior to the swelling experiments by extracting the specimens with benzene for 21 hours
in a n Underwriters apparatus. T h e specimens were then dried to constant weight in a vacuum oven a t 60" C. Portions of the extracted and dried specimens were used for analyses for nitrogen, combined sulfur and zinc oxide, and for determining density. T h e per cent by weight of acrylonitrile residues in the rubbers was calculated from their nitrogen contents corrected for combined sulfur and zinc oxide. The calculated acrylonitrile contents and the specific gravities of the purified vulcanizates are given in Table I. The method chosen for analysis of the iso-octane-toluene mixtures absorbed by the rubbers depended on the ordinary toluene being tagged with a small proportion of radioactive toluene. Sufficient tagged toluene for all of the experiments was prepared by diluting a solution containing 0.2 mc. of toluenel-C14 to 25 ml. with toluene. Various iso-octane-toluene mixtures for the experiments were prepared by adding weighed quantities of iso-octane (2,2,4trimethyl pentane) to weighed quantities of the tagged toluene. For the absorption tests, an amount of the vulcanized rubber specimen which would absorb from 50 to 150 mg. of solvent was placed in the specified isooctane-tagged toluene blend with the volume ratio of solvent to rubber adjusted to 50 to 1. The closed flask containing the specimen and solvent was then placed in a water bath a t 100' F. for 48 hours. At the end of the 48-hour period, the .specimen was withdrawn from the solvent, blotted lightly with absorbent paper, and immediately placed in a test tube connected to a combustion train for analysis. I t had been previously found that swelling equilibrium was closely approached in 48 hours a t 100' F. Table 1. Composition and Density of Extracted Nitrile Rubber Vulcanizates Commercial Designation of Rubber Hycar 1014 Hycar 1043 Hycar 1041
Acrylonitrile Content, Wt. %
Density
22 29 40
1.011 1.028 1.054
T h e solvent absorbed by the rubber specimen was distilled out by surrounding the test tube containing the swollen specimen with a beaker of boiling water and introducing a stream of oxygen. One and one half hours were usually sufficient for 99% ofthe absorbed solvent to be removed from the specimen. The solvent vapor was carried by the oxygen stream through copper filings contained in a heated stainless steel tube. The carbon dioxide resulting from the burning of the hydrocarbons was passed through a gas dispersion tube into an ammoniacal barium chloride solution. At the end of the distillation and combustion, alcohol was added to the flask containing the precipitated barium carbonate to break any colloidal suspension. The precipitate was filtered with suction, washed with alcohol, transferred to a 1-inch diameter planchet, and thoroughly dried with an infrared lamp after addition of more absolute alcohol. The radioactivity of the precipitate was determined by means of a flow counter and accessory equipment. The amount of barium carbonate formed in every combustion was sufficient to produce precipitates of infinite thickness from the standpoint of the flow counter when placed in planchets of 1 inch in diameter. Consequently, the ratio of barium carbonate obtained from tagged toluene to barium carbonate obtained from iso-octane could be calculated from the counting rates of the precipitates.
BI = weight per cent of barium carbonate from iso-octane
BT = weight per cent of barium carbonate from tagged toluene C , = counting rate for mixed barium carbonates CT = counting rate for barium carbonate from 100% tagged toluene The ratio of toluene to iso-octane in the swollen rubber specimen was calculated from the following relationship: (0.922)Br -T --_ __
I
VOL. 49,
NO. 3
BI
MARCH 1957
445
'
,
0
?
s-" -I
0
> 400 w
x
E
w
200
-
m m 3
a w
w
e3
0
TOLUENE IN LIOUID
PHASE, JOL s (REMINDER is ISO-OCTANE) TOLUENE IN LIQUID PHASE, V 0 L . s (REMAINDER
Figure 1. Toluene in liquid phase vs. toluene in liquid absorbed by rubber
M-here 7' = weight per cent toluene in s\collen specimen I = weight per cent iso-octane in swollen specimen
A bveighed quantity of the solvent blend used for the immersion !vas analyzed in the same manner as the solvent absorbed b!. the rubber. The coefficient of variation of all analytical results was estimated to be about 3 5 . The total amount of solvent absorbed by a rubber \vas determined by imrnersing a bveighed specimen, previously estracted by benzene and dried, i n 100 ml. of the solvent for 48 hours a t 100" F. The solvent blends used for these esperiments \vere prepared bj- mixing measured volumes of iso-octane and ordinary toluene. The specimen ivas about 1 X 2 X 0.08 inches in size. so that the ratio of solvent volume to specimen volume was 38 to 1. At the end of the immersion period, the specimen
Table II. Weight Per Cent of Toluene in Solvent Absorbed b y Nitrile Rubbers Weight C: Toluene in blend used for irnmersion"
Toluene in solvent absorbed by ruhher"
22
52 32 22 8 2
66 54 46 28 7
29
52 40 29 22 8 2
72 65 60 54 36 10
40
56 40 22 8 2
100 92 80 52 18
Acrylo-
nitrile
content of rubber
Remainder mas iso-octane.
446
Figure 2.
IS 19-OCTANE)
Rubber swell vs. toluene in liquid phase
\\'as retnovcd. quickly blottrd with absorbent paper. and F\rcighed in a closed bottle. Results of Tests
The rcsdts of the t r s t s arc given in Tables I1 n n d 111. Table I 1 gives the \\eight [ > c rcent of rolucne in tha solvetit absorbed by tlie nitrile rubbers from the various blends. 'Table 111 gives the u ~ i g h 1)cr t c'f'nt of solvent in the s \ \ d l e n nitrile rubhrr Epecimens inimei-st-d in \various blctids. 'These data obtained directly i'ro~ii rlie rxpcrimenrs \vert: converted into \-olumes and plortrd i n Figurrs 1 and 2 . I t \\'as assumed in preparing Figures 1 and Z that the densities of iso-octane: tolttrne. and die vulcanizates had the same relationship a t 100° F. 138' C.) as they did a t 08' F. ( 2 0 3 C,). a t ivhich temperature tlic drnsi1ies \yere determined. T h r densities used for iso-octane and tolucnr ivere 0.692 and 0.866. resprcrivrl).. a t 68' I:. It \\.as also assumed i n preparing thme curves that th? composition of the solvent phase \vas not changed significantly by the absorption of sonie of its components by the rubber specimen. This \cas justified because the ratio by volurnr of the solvent phase to the rubber phase a t the start of swelling was nm'er less than 38 to 1. The volume of the rubber component in these graphs \vas corrected for the zinc oxide remaining in the extracted vulcanizate as determitied by chemical analysis. Figure 1 sho\vs that the nitrile rubber vulcanizates preferentially absorbed toluene from the iso-octane-toluene blends, and that the degree of preference rose with increasing acr!.lonitrile content (AX) of the rubber. These curves indicate that the 225; acrylonitrile vulcanizate immersed in the standard test medium of 70% iso-octane-30% toluene would absorb these solvents in the ratio of 49 to i l ! that the 40y0 acrylonitrile vulcanizate ivould absorb them in the
INDUSTRIAL AND ENGINEERING CHEMISTRY
Table 111. Weight Per Cent of Solvent Blend in Swollen Nitrile Rubber Specimens XtarS lo-
nitrde Content of Itribher, Wt 0'0 22
29
40
Tolueiie in I3lend c s e d for Immersion", Vol. 9; 100.0 66.7 50.0 33.3 20.0 0.0 100.0 66.7 50.0 33.3 20.0 0.0 100.0 66.7 50.0 33.3 20.0 0.0
' Remainder of blend
\\as
Solvelit 4bc;orhed.
iVt IJI 8wollen Itribl>er 82.8 70.3 59.6 45.6 32.4 14.2 73.6 57.0 44.8 32.1 22.9 4.7 55.8 39.2 30.5 22.0 15.1 1.1
iao-ortcirie
component rubber phase. The tie lines connect compositions of these phases which exist in equilibrium. The volume per cent of iso-octane in the swollen rubber reached a maximum value when the rubber was in equilibrium with a certain blend of iso-octane and toluene, rather than when in equilibrium with 100% iso-octane. The solvent blend when this maximum occurred contained about 6070 iso-octane in the cases of the two low acrylonitrile rubbers, and about 90% iso-octane in the case of the 40y0 acrylonitrile rubber.
a
d\ITTRILE RUB ER (2270AN)
Figure 3. Two-phase' threecomponent system containing nitrile rubber (22% acrylonitrite)
SWOLLEN RUBBER PHASE
$.
&60
@
%
Discussion of Results The relative total absorption of solvents by the three vulcanizates is only of directional significance. The amount of solvent absorbed by a rubber depends on its state of cure (degree of cross linking), but the three vulcanizates tested here were not exactly matched for state of cure. The preferential absorption of toluene from the standard 7070 iso-octane30% toluene blend is not a n argument either for or against the use of this particular blend as a simulated gasoline for testing rubber vulcanizates. The existence of this phenomenon merely indicates that when nitrile rubber vulcanizates are immersed in gasolines bearing aromatic hydrocarbons, these hydrocarbons are probably absorbed to a greater degree than the accompanying aliphatic hydrocarbons. The choice of the proportion of aliphatic and aromatic constituents in the hydrocarbon standard test fluid should depend only on the desired swelling tendency for vulcanizates. This swelling tendency should be a t least equal to, and preferably greater than, the swelling tendency of commercial gasolines.
/
ISO-OCTANE
/
/
20
/
LIQUID 40
60
80
TOLUENE
NITRILE RUBBER (29 %AN)
A Figure 4.
20
20
LIQUID
PHASE
Theoretical Explanation T h e theoretical explanation of the swelling behavior of a rubber in a mixture of solvents has been approached from two standpoints. The first approach by Langmuir (8, 9) and supported by other investigators such as Doolittle ( 3 ) , is entirely qualitative. I t is based on the idea of solvation of different parts of a polymer molecule by the two components of a solvent mixture. I t is assumed that the electrical force field surrounding the chain of a rubber molecule varies at different points along its length owing to the intermittent presence of double bonds or polar groups. The respective molecules in a mixture of solvents are attracted to these points of the rubber molecule in varying degree depending upon their own shape and polarity. The solvation theory appears to be an explanation of the behavior of the systems studied here, but the nature of the at-
NITRILE
RUBBER (40%AN)
SWOLLEN RUBBER PHASE
Figure 5. Two-phase threecomponent system containing nitrile rubber (40% acrytonitrile)
150-0
NE
VOL. 49,
NO. 3
MARCH 1957
447
Table IV.
Dielectric Constants of Solvents and Nitrile Rubber Vulcanizates" at 20-23" C. Dielectrii.
Frequency,
C'iiiistont>
Cycles,'Ser.
x
Iso-octane
1.94
5
Toluene Rubber 22% acrylonitrile 29C, acrylonitrile 40% acrylonitrile
2.38
6 X lo5
10.3
1 1 1
12.8 15.1
x
x
x
105
Lit. C'itetl (11) (7 I
104 10' 10'
-
Speciriien;; tlricd hut not cstr:ictr,cl.
traction betu.een the toluene and the acrylonitrile rubbers is a matter of sI)rculation. Dielectric constant is i i rotiqh measure of the electrical I(irce field surrounding a molrcult~. T h e diclectric constants of thc solvents and \ulcanizatcs used in the foregoing esperirnrnts a r c given in Table I\-. The dielectric constants of the rubbers arc v r r y hiqh compared to those of thc, sulvt.nu. and the dielectric constanis of the r i i b bers are highei for qrcatrr acr>-lonitrilc contenrs. The latter relationship correlares directionally Ivith thc deyrce of preferential absorprion of toluene b!. the rubbers. T h e dielectric constant of toluene. ho\\,ever. is only about Z.i(!'; higher than that of iso-octane; both solvents are usually considered to have nonpolar molecules-Le., {vithout strong electric force fields. I n view o f this small difference in dielectric constant, it is difficult to undersrand \vhy acrylonitrile rubbers have such a preference for toluene. X possihlt- esplanation is lib.drogen bonding. The h!,drogen atonis on the aromatic nuclrus of the tolurtie molecule 1iai.e a sinall dipole enrrg!. ( 2 ) . and are attracted to the i-iigIil>. polar nitrogen atom in the cyano qroup on the rubber molcculc. This dipole energy. being almost symmetrically distributed around the tolurne molcculr. does not have much cli'rcr on its ditlrctric constant. 'Ti-ic h>-dtogrn atoms i n the iso-octanr niolccule d o nor h a \ ? this dipole energ!- because t h r y arr n o t attached to aromatic-ring carbons. The second attrinpt a r a rheorctical explanarion for the behavior of ritlih(~r in mised solvenia \\-as hy Gee iii). His semiquantitarivr trt'atnirnt of tlie 1)licnomenon \vas bascd o n i\vo assumptions. Gee stated : T h e polymer molecule is treated as a flexible chain of segments. each of \\-hich can be interchanged \vith a molecule of either liquid: thr svstem rhus arranqing itself in a random \ \ a>-I>>- virtue o f the thermal motions of its components. .\I1 intermolecular forces arc assumed to be nonlocalized. and i n particular the force field around the polymer chain is treatcd as uniform Gee derived a pair of equations for defining the composition of a t\vo-phase three-component s>.stem consistin? of sc\ollen rubber and an escess of the
448
1. Toluene is pi~fcrcntiallyabsorbed by nitrile rubbei,s from blends of toluc:ne and iso-octane. I 2. T h e deqree of preference I;Jtoluene risrs Ivirh increasinq acrylonitrile contcnt of the rubhex.. 3. 'l'he greater is the amount of t o l i i ene in t h e blrnd: the greatrr is t h e total amount of' solvent a h s o r t i d . 4. ' I l i t ~ higher is i h c . acryloriitril? content oi' the rubber, the less is t l i r total amount of solvrnt absorlied f r o m iin) given I)lcnd coni~i(~sition. -I-h c iso-octnnc c o n t r n t of tlic 2. siwllcn i.iilil)ei. rrac1ic.s a tnaxitnutii \vticn the 1at:c:r is i n cqiiilitiriuin Lvitli a crrtiiin hiend ( ~ flolucnc. ,ind iso-ociiine ratlicr than \ \ . h t s i i it1 rqiiilil~riii~n \villi 1)urt: isoocratic
solvent blend in terms of tlir entropy dilution of thr rubber and the hc.ats of dilution of the rubber and tlir respective liquids i n the mixture. H e aplilied tlie equations to the system : viilcanirrd neoprcne hcxanc- tnrt!i),l acctatr. rind comparrd the curve thus cibtained n . i i l i the curve plotted l'roiii exp~rinienral points. 'The agreement het\\-een tlir curves \vas poor. although thr optiniurii slvellinq I)r(dictcd by ~ l i e o r y agt with the csperimrntal value. stated that his equations \ v ( ~ ~ ino( l d apply to copol!xiiers of the nirrile rub1)t.r t!.ptx. i n xvhich highly polar groups arc separated b>. relarively long hydr(~cai.hoti chains. Sot\\.irlistaiidiiig the failurr 01' (;(,e's hypothesis r o explain tht. behavior ( i f the nitrile rubber-solvent s>-stcms on a quantitative basis. rhcre is a fca(ure of his theory \\,liich does apply qualitatively tu t h t w systems. G e e proposes using tlie prolierty. cohrsive enerq!- densir!-. to determine the relative s\velling teiidrncy of liquids fur rubbt-rs ( 7 ) . lf rhr cohesive rtiergy density of a liquid is i n close agrerrnrnr !\-it11 that of a t.ubl,er, the liquid \\.ill he a good s\\.clling agc.nt for the r ~ ~ l i l i r r 'The . cohcsi\c~ mcrq!' densir!. of a liquid is equal to the qiiciticrit o f its molar hrat o f \.aimrization a n d its molar volume. The cohesive rnct.q!density of a rubber can he approxiniaicd by rrieasurinq its degrrc of s\vcllinT i n ;I of liquids having various colicsivc. . dcnsilirs. ' r h e cohesivc cncst'q\. y of the liquid causin? inasiniuni s\vellinq is regardrd as matching that Of thr rubber. Gee found the cohesi\-cb rnergy density of nitrile ruhher to be 88 b y this nierhod. The calculated eohrsi\-r rncrsy density for toluent. is 85.'L and for iso-ocranc is .i0.9. Sin(:(, t h e cohesivc vnrrgy drnsity of toluenr is much closcr r o t h a t of ni:rile rubher than is the cohesive energy densit!- of isooctane: it ivould h r prrdic,tcd t l i a t toluene ivould si\-ell nitrile r u b t ~ c rviilcanizates much inore tlian iso-octant,. and that tolucne lvould probably he preferentially absorbed b!. these vulcanizates from blends of tolurnc and isooctane. Summary
Summarizing t h e exprrimental resiills ofthis invesiiqa tion. it has been fo~rndi.ha t :
INDUSTRIAL AND ENGINEERING CHEMISTRY
