INDUSTRIAL AND ENGINEERING CHEMISTRY
1746
points were not observed and only phase compositions included in Table I1 were talrcnat 100 F. Figure 6 shows that the cquilibrium constants for carbon monoxide in prop1 lene are the same as the eonstants for the propane system, within the experimental error. From the relative volatility of propylene and propane, the K's for the propylene system should have been slightly lower than for the propane system, as the convergence pressure would be slightly loFer. CARBON BIOSOXIDE-DECAKE
To determine the effect of molecular weight of the hydrocarbon, equilibrium phase data mere determined for the carbon monoxiden-decane system a t 100 ' and 150' F. The data are included in Table 11 and the equilibrium constants of carbon monoxide are compared p i t h the constants for the propane system in Figure 7. The decane percentage in the vapor iyas assumed to be that from the partial pressure of decane rather than from the analyses. Inasmuch as there was a definite effect of the molecular weight of the paraffin constituent, an analogy was assumed with the binary hydrocarbon systems containing methane (I, 4). Higher
Vol. 40. No. 9
convergence prcssurcs for thc cquilibriurn c,onstants will occur x i t h incrcasing molecular weight of the hydrocarbon constituent. Figures 8 and 9 present estimated equilibrium constants for carbon monoxide a t 100" and 150" F., respectively, as a function of molecular \wight, based on propane and decane data. The shape of the curves was taken as symmetrical with t h c methaneparaffin syst,eme. ACKNOWLEDGMENT
The 11. W. Kellogy Company provided the fellowship t.hat made this work possible. The Phillips Petroleum Company furnished the pure propane. LITERATURE CITED
(1) Hanson,l i z a z a , and Brown, IND.ENG.CHFY.,37. 1216 (1946). (2) Katz and Kurata, I b i d . , 32, 817-27 (1940). (3) hlatuzak, S . I?., IND. ENC.CHEM., ANAL.Eo., 9,354 (1937). (4) Sage, B. H., and Lacey, W. N., Am. Petroleum Inst., "Driliiiig and Production Practice," p. 308, 1941. ( 5 ) U. S. Steel Corp., "Methods of the Chemists, United States StoelCorporation," Pittsburgh, P a . , Carnegiesteel Corp., 1918. RECEIVED July 7, 1947.
of Certain Ant
0
lVATbLTRAEANP)ACCELERATEIPAGIIVG H-Y
E. ALBERT
The Firestone Tire & Rubber Company, " f k r o n ,Oh'io
S
INCE the early work on GR-S in this country, it has been
recognized that the requirements for an antioxidant in copolymers of this type are different from those for an antioxidant in natural rubber. Because of this, a considerable amount of evaluation work has been carried out with GR-S to determine the best antioxidant and the concentration necessary for satisfactory stabilization of both raw polymers and vulcanizates. The results of oxygen absorption studies in this connection have been reported ( l a ) . The results of natural and accelerated aging tests are presented in this paper and their correlation with the oxygen absorption results is discussed.
COMPLETE R E S I N I F I C A T I O N
MODERATE R E S I N 1 F l C A T I O N SLlGMT R E S I N I F I C A T I O N
5 YEbRS NbTURAL
VERY H I G H CURE
4 DAYS 90OC.
HIGH OJRE
*
AGING
PREPARATIOZT O F POLYAIER SAVIPLES
d ZIPoF.1
LOW CURE S L I G H T CURE
VERI
n i w ~ CURE SOFTEN ED
NO
DETERIORATIOII
0.5%
1.0%
1.5%
For the evaluation of the rcsistance to aging of GIZ-S polymer samples containing various antioxidants, an accelerated aging test of 4 days a t 90" C. in a forced circulation air oven was employed. The samples Twre examined at the end of each day during this test. Natural aging behavior was observed by examining samples after aging 1, 2, and 5 years a t room temperature in the absence of light. During deterioration, a GR-S polymei. sample passes thmugh various stages of stiffening or cure and (,hen starts to resinify (1). I n some cases, there is an initial softening before this stiffening takes place. These stages of deterioration can be determined readily by visual and by ha,nd examinat,ion. I n the earlier stages of stiffening or cure, where PIIooney plast,icity values can be determined, the hand tests correlate well with these values.
OVEN AGING
MOONEY P L A S T I C I T Y
WL/4
M E D l U H CURE
EVALUATION O F ANTIOXTUASTS IN POLYMER AGING
2.0%
3.0%
P H E N Y L . BETA.NPPtiTHYLAU1HE mNCENTRATlON
Figure 1. Effect of Phenyl-&Naphthylamine Coucentration on Natural and Oven Aging of GR-S Polymer
The polymer samples used in this investigation irere prepared from uninhibited GR-S latcx takcn from a plant autoclave just before addition of the stopping agent. The commercial grade of the desired antioxidant was added in the form of a dispersion and then coagulation was effected b y a 2% aluminum sulfate solution. The coagulum wa,s washed t.horoughly on a laboratory mill and then dried 20 hours a t 75" C. When subjected to this drying treatment, samples containing no antioxidant (blanks) reached a degree of deterioration corresponding to moderate resinification because the aluminum sulfate employed for coagulation m-as a commercial grade containing an appreciable quantity
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1948
Early work on GK-S included the evaluation of a number of antioxidants at several concentrations, because it was thought that the antioxidantrequirements in this polymer would be different from those in natural rubber. Some of the polymer samples prepared in this early work have been aged for over 5 years. It appeared worth while, therefore, to summarize some of the more pertinent natural aging results and to compare them with those obtained from oven aging and oxygen absorption studies. A concentration of 1% of phenyl-&naphthylamine appears sufficient to give maximum protection to GR-S during both natural and oven aging of the polymer. A higher concentration (1.5 to 2.0%) of 2,2,4-trimethyl-6-phenyl-1,2-dihydroquinoline was necessary for maximum protection. Phenyl-@-naphthylaminegave somewhat better protection to the polymer during aging than dimethylacridan, heptylated diphenylamine, or 2,2,4-trimethyl-6-phenyl-1,2-dihydroquinoline. In vulcanizate aging, there were only small differences between concentrations of antioxidants and between the antioxidants evaluated. In one comparison, the effect of soluble iron on the order of activity of two antioxidants in vulcanizate aging was demonstrated. Wherever aging tests were obtained for GR-S polymers and nilcanizates for which oxygen absorption curves were measured (15), there was good correlation between these two sets of results.
of iron. However, all the antioxidants employed in this study were sufficiently active, even a t the lower concentrations tested, t o prevent any deterioration during the drying period. POLYMER AGING RESULTS
Figure 1 shows the results of oven and natural aging of GR-S samples containing varying concentrations of phenyl-p-naphthylamine. Both the oven and natural aging data were obtained on the same series of GR-S samples. The data of Figure 1 show that in both natural and accelerated aging, 0.5% phenyl-&naphthylamine gives less protection to the polymer than 1.0% of this antioxidant. This is evident on comparing hand tests and Mooney plasticity values. I n the concentration range from 1.0 to 3.0%, very similar behavior of the polymer ramples was observed. Hand testing of both natural and oven-aged samples showed no differences and the &looneyplasticity values decreased slightly in this concentration range. The conclusion from the above data is that, from a polymer aging standpoint, 0 . 5 7 ~of phenyl-p-naphthylamine does not give maximum protection but there is little advantage in using concentrations greater than l.Oyo. This is in accord with the oxygen absorption results a t 100" C. determined for a series of p o l m e r samples containing varying concentrations of phenyl&naphthylamine (Figure 2, 15). The rate of oxygen absorption
of the GR-S polymer was decreased considerably by increasing the antioxidant concentration from 0.6 t o 1.0%. However, when the phenyl-@naphthylamine concentration was increased from 1.0 t o 2.OyO,only a small effect on the rate of oxygen absorption was noted in the region of the oxygen absorption curves where the polymer samples are still useful (from 0 t o about 290 hours at 100" C.). The phenyl-&naphthylamine contents of the samples of Figure 1before and after natural aging are given in Table I. The values in the first column are the actual percentages of commercial phenyl-@-naphthylamine added t o the GR-S samples. Correction for the fact that this material is about 95% pure, as determined by the ultraviolet absorption method, gave the values of phenyl-@-naphthylamine shown in the second column. The values for the phenyl-@-naphthylamine content of the polymers after 5 years of natural aging were obtained using a Beckman quartz prism spectrophotometer (4). The method employed involved dissolving the sample in toluene and then determining the ultraviolet adsorption of the resulting solution (6). These values (column 3, Table I ) were not corrected for background absorption. For the sample containing 1.25% antioxidant, the correction for background absorption was determined t o be equivalent to 0.10% of phenyl-8-naphthylamine. This determination was made by extracting the GR-S polymer sample with ethanol-toluene azeotrope, determining the absorption on the extracted sample, and correcting for the amount of antioxidant not extracted. The latter correction was made by determining the antioxidant remaining in a n extracted unaged polymer sample originally containing about the same amount of antioxidant as the aged sample. The data of Table I show that only a small amount of the phenyl-p-naphthylamine is consumed during 5 years of natural aging. It has been reported that oven aging of GR-S containing phenyl-@-naphthylamine results in appreciable reduction of the phenyl-p-naphthylamine content (2, @-for example, Cole and Field report that a sample initially containing 1.367, phenyl-@ naphthylamine retained 0.75% after 1 day at 100' C. and 0.36% after 2 days a t 100" C. Retention of a large percentage of phenyl-&naphthylamine antioxidant after 5 years of natural aging b y a polymer sample showing considerable evidence of deterioration suggests t h a t the oxidative detcrioration a t room temperature proceeds in a somewhat different manner than at higher temperatures such as 90' or 100 C. This conclusion is in accord with the observations of others that the relative rates of the competing reactions involved in the oxidation of both GR-S and natural rubber depend t o a great extent on temperature (10,11). It has been postulated t h a t chain reactions involving free radicals are set up during oxidation (6-9, l a ) , that these chain reactions result in both cross linking and scission of the polymer hydrocarbon chains (19-14), and that the principal function of the antioxidant is to break
MODERATE R E S l N l F l C A T l O N SLI W T R E S I N 1 f I CATION VERY H I G H CURE
HIGH CURE M E D I U M CURE
RENAININGIN GR-S TABLE I. PHENYL-@-KAPHTHYLAMINE POLYMERS AFTER KATURAL AGING Commercial Phenyl-pnaphthylamine Added to Polymer, % '
Pure Phenyl-pnaphthylamine Added (Calculated), 5%
Phenyl-pnaphthylamine after 5 Years of Aginga. %
0.50
0.48 0.95 1.42 1.90 2.85
0.43
1 .oo
1.50 2.00 3.00
0.79 1.25 1.68 2.70
The author is indebted to M.J. Brock and A. W. Soholl of the, Firestone Researoh Laboratqries for determination of phenyl-p-naphthylamine by the ultraviolet absorption method.
1747
LOW CURE
n
5 YEARS NATURAL AGING
U
4 DAYS 9 0 ' C .
OVEN ffiING
S L I G H T CURE
VERY S L I G H T CURE 50FTENED NO D E T E R I O R A T I O N
1.0%
n n n
2.0%
3 0%
4 0%
2 . 2 . 4 . T R I M E M Y L . 6 . P H E N Y L . I,2. D I H Y O R U W I N O C I N E CDNCENTRATIOH
Figure 2 . Effect of 2,2,4-Trimethyl-6-phenyl-1,2-dihydroquinoline Concentration on Natural and Oven Aging of GR-S Polymer
INDUSTRIAL AND ENGINEERING CHEMISTRY
1748
Vol. 40, No. 9
than l.0yGare nrxcssary for maximum protection or' the polymer. Three or 4.07/, of this ant'iosidant, Y I W T RESIN1 FlCATlON did not give any greater protection than 2.07c. DIMEMYLACRID&Y VERY nlm CURE Hence, these data indicate that a concentration oi H I M CURE t,his antioxidant greater than l.0Yo is the oplirnuiii. UEDIUW CURE A separate oven aging study on some more reLON OJRE cently prepared polymers indicated that the niiniY I W T CURE mum concentration for optimum polymcr protection VERY S L I M T C U R E is between 1.5 and 2.OYG. SOFTENED I n Figure 3, four diftereiit antioxidant's are coiiiNO DETERIORATION pared a t four different concentrations using some more recently prepared polymer samples. At all 0 5% 1.05 3 , 5% 2.0% concentrations tested, phenyl-,&naphthylamine gave L N T I O X I D L N T CONCENTRATION the best protectmionto the polymer. DimcthylacriFigure 3. EEect of Antioxidants on Oven Aging of GR-S Polpnier dan and heptylated diphenylamine were equal in a(:." tivity and somewhat inferior to phenyl-p-napht>liylamine. At, 0.5 or 1.0% concentrations, 2,2,4-tri%I M T R E S I N 1 F I CAT1 ON AGED 2 YEARS VERY H I M CURE methyl- 6- phenyl - L,2- dihydroquinoline rated last AGED 5 YEARS H I W CURE * WONEY P L A S T I C I T Y from the standpoint of polymer protection. H o i v UL14 8 212-F. M E D I M CURE ever, at a concent,ration of 1.5Vo, this antioxidant LOW OJRE was equivalent to, aiid a t 2.0YG it. was slightly S L I W T CURE superior t o heptyhtcd diphen~-laniineor dirnrt,h~lVERY SLlMT CURE acridan. SOFTENED NO D E T E R I O R A T I O N The anrioxidant comparisons made by oxiygi~ii absorption deterrriiriat~ions a t 100 C. ilring raw PHENYL.BETA. DIUETUYL' HEPNLATED 2 . 2 . 4 . TRIUETHYL-6. GR-8 polymcr samples (Figure: 8, 16) arc in gooti NLPHTHYLAUINE ACRIDM D l P U E N n W l N E PHENYL. I . 2 - D l H Y D R o . W I N D L 1 hE agreement rrith thc above results. At a concci1tration of 1.0YP,a sample containing phen:;l-p-iiaphFigure 4. ESect nf intioxidants on Natural Aging of GR-S PIi>lymei* thylaminc absorbed oxygen at a sloiver rate than Concentration 270 a sample coniaiiiing dimethylacridan. Tho lattw absorbed oxygen at a slower rata than a xmiplc: coritaining2,2,4-trimethyl-6-phrnyl-l,2-~~ih3;droquinolinr.JIcncc. the free radical chains by combining in some manlier with the free radicals (6). As a result of aging studies, on GR-S, th.e order of activity for these three antmioxidantsat this coiicentration, as det,errnined by oxygen ahaoyptioo, is the saniti a; Winn and Shelton ( I O ) have concluded that the t>emperaturecothat determined by oven aging. efficient for the chain scission reaction is greator than the teniNatural aging results for another ips of polymer samplev perature coefficien'c for the cross-linking rmction. I n view of the cont,aining four different antioxidants a concent>rationof 2 . 0 5 above, a reasonable explanation for the fact t h a t nppreciabli. (Figure 4) show that the best protect,ion of the polymer was deterioration of the polymer with low antioxidant corisumptiori imparted by phenyl-p-iiaphtliylamine. Dimet,hylacridan anr! has becn observed is that the cross-linking reaction is predomi2,2,4-trirnethyl-6-phenyl-l,2-dihydroquinoline, which werc about nant at the lower temperature. Hence, at room temperature equal, were slightly inferior to phenyl-8-napht hylaminc hut free radical chains are set, up which cause rz relatively la1 superior t o heptplated diphenylamine. amount of cross linking and relatively little chain scission, and The data of Figures 1 l o 4 show fair t o good correlation bctw the stiffening effect per unit of antioxidant consumed is tbcrefore oven aging and nnlural aging. The degree of di relatively greater than at higher temptiratures. hctm-em thrsc two tests but the conclusions reached I n Figure 2 , a study of the effect of the concentration of 2,2,4the same in each case. trimethyl-B-phenyl-1,2-dihydroquinoli1ie on polymer aging is A method for evaluating aged polynler s a m ~ i l t which ~? has been shown. Again, the natural and oven aging tests werc run on used to a limited extent is the preparation of t,road "ype vulcanithe same series of polymers. Both natural and oven aging rezates from these aged samples. In Figure 5 a comparison of 0.5 sults indicate that concentra,tions of t i & antioxidant greatcr and 2.0% phenyl-p-naphthylamine is made by comparing thc propert,ics of vulcanizates obtained from polymer samples oven aged 4 days a t 90" C. An advantage for the 2.0% sample is evident in both normal tensile and elongation values. This result is in good agreement with polymer aging evaluation by hand tests and by Mooney plasticit'y values. 100.00 GR-6 polymer 50.00 Channel black Preparation of vulcanizates from samples containing 2.09; 5.00 Zinc oxide 3.00 ph enyl-p-naphthylamiiie or 2 .O yG 2,2,4-trimethgl-6-phenyl- 1,ZStearic acid 3.00 Coal-tar softener dihydroquinoline 13-hiohhad been subjected to 2.5 years of natural 3 .OO Pine tar 1.20 Santocure aging indicatcd no difference between these samples. G o d 2.00 Sulfur agreement with hand test and Mooney plasticity- cvaluatioii is 167.20 again shown. mMPLETE RESINIFICATION
MODERATE R E S I N I F I C A T l O h
-t
Test
S A M P L E S AGED 4 D A Y S A T 9 O ' C . PHENYL. B E T A . N A P H T H Y L W I N E
Cure Einployed (280'
Tensile properties Hot elongation Blow-out time Running temperature Per cent rebound Rate of groove cracking
Optimum cure
=
80 minutes a t 280" F.
F
