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tained with the methacrylates, which also were hard without TABLEVII. COPOLYMERS WITH ALLYLPOLYSILOXANE- showing brittleness. The styrene copolymers were again white DIMETHYL POLYSILOXANEa POLYMER and opaque. Polymer Copolymerizing Copolymer Appearance A diallyl phthalate copolymer with a different polysiloxane sysNO. Monomer, Wt. % b After Curec tem was also prepared. The polysiloxane was prepared by 1 None Cheesy flaky 2 Diohlorostyrene 30 Opaqu;, white, brittle cohydrolyzing dimethyldichlorosilane, allyltrichlorosilane, and a Styrene 21 Opaque white brittle methyltrichlorosilane in a 3:l :I molar ratio. Diallyl phthalate 4 Diallylamine 11 Opaque: black,' soft, crumbly 5 Methyl methacrylate 20 Clear hard in the unsaturation equivalence amount was added and benzoyl 6 Butyl methacrylate 26 Clear: hard 7 Acrylonitrile 12 Black, hard, brittle peroxide catalyst used. After 48 hours at 80' C. and a similar 8 Vinyl pyridine 10 Opaque, black, soft, crumbly period at 100" C. a clear, moderately tough resin formed. FurCohydrolysis product of dimethyl dichlorosilane and allyltrichlorosilane ther cure at 150' C. improved the hardness. This resin was more (4:l moles). b All monomers are in unsaturation e uivalence amounts hard and tough than the polysiloxane polymerized without added 1% tert-but 1 perbenzoate; cure s a e d u l e 24-hour pei-iods a t SOo, 90°, and 120° C . , 48Eours a t 150° C. diallyl phthalate. LITERATURE CITED
ALLYLPOLYSILOXANE COPOLYMERS ( 2 ) . The allyl polysiloxane employed in making the copolymers listed in Table VI1 was prepared by cohydrolyzing dimethyldichlorosilane and allytrichlorosilane in a 4 t o 1molar ratio. T o this oil were added the vinyl monomer and 1% led-butyl perbenzoate. The mixtures were polymerized in the bulk in sealed bottles for 24 hours a t each 80 O, 90 ', and 120 O C. and then given a final 48-hour cure at 150' C. The most rapid polymerization occurred with butyl methacrylate, methyl methacrylate, and vinyl pyridine. As with the methylvinylpolysiloxane copolymers, the greatest clarity was ob-
(1) Hurd, D. T., J . Am. Chem. SOC.,67, 1813 (1945). (2) Hurd, D. T. (to General Electric Co.), U. S. Patent 2,420,912 (June 11, 1945). (3) Kharasch, M. S., et d.,IND.ENQ.CHEM.,39,830 (1947). (4) Kropa, E. L. (to American Cyanamid Co.), U. 9.Patent 2,388,161 (Oct. 30, 1945). ( 5 ) Marsden, J., private communication. (6) Pape, C., Ann., 222,373 (1884). (7) Roedel, G.F. (to General Electric Co.), U. 8. Patent 2,420,911 (May 20, 1947). (8) Scott, D.W., J . Am. Chem. Soe., 68,1877 (1946). (9) Welch, L.M., et d.,IND.ENR.CHXM.. 39,826 (1947). R E C ~ I V EOctober II 28. 1947.
Effect of Certain Antioxidants in GR-S OXYGEN ABSORPTION STUDIES HUGH W I W
AND
J. REID SHELTON
Case Institute of Technology, Cleveland, Ohio
Comparison of the rates of oxygen absorption by GK-S polymers and vulcanizates with varying amounts of phenyl-P-naphthylamine indicates that the optimum concentration is about 1%. Higher concentrations of this antioxidant result in a n increased rate of oxygen absorption during the constant rate stage, and thus the excess antioxidant functions as a catalyst for this stage even though i t is a t the same time delaying the start of the autocatalytic stage of oxidation. Coagulation of GR-S by the alum method results in a considerably greater resistance to oxidation as compared to a polymer coagulated
I
N THE manufacture of GR-S about 2% of antioxidant is generally added t o the latex prior to coagulation. Two methods of coagulation are in current use, one employing salt and acid, the other aluminum sulfate. Little information is available in the literature with respect to the effect of concentration of antioxidant or of coagulation methods upon the susceptibility of GR-S to deterioration by oxidation. The present study was undertaken to determine the effect of these two factors upon the rate of oxygen absorption by GR-S polymers and vulcanizates, and to show how oxygen absorption measurements may be employed in evaluating the relative effectiveness of certain commercial antioxidants.
by the salt-acid method. The differenoe is much less in the case of the vulcanizates, although the alum vulcanizates absorbed oxygen a t a somewhat slower rate in all cases studied. Oxygen absorption measurements demonstrate a considerable difference in the effectiveness of three common antioxidants. The rating of antioxidants as determined in GR-S polymer may be completely reversed when they are compared in the vulcanizates. Comparison of oxygen absorption data with the results of natural and artificial aging shows good correlation and demonstrates the value of the oxygen absorption method. EXPERIMENTAL PROCEDURE
The volumetric method of measuring oxygen absorption at substantially constant pressure employed in this study has been d e scribed in previous papers from this laboratory (8,9). Polymers containing 0.0, 0.5, 1.0, and 2.0% phenyl-p-naphthylamine were prepared by both salt-acid and alum coagulation methods. The uninhibited, unstripped GR-S latex was withdrawn from & plant polymerizer just prior t o the addition of antioxidant and the appropriate amount of antioxidant added before coagulation in the laboratory. The wet coagula were dried in a vacuum oven at 50' C. Similar polymers were subsequently prepared with 1% dimethylacridan (BLE) and 1%
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2,2,4-trimethyl-6-phenyl-l,Z-dihydroquinoline (Santoflex), for comparison with the 1% phenyl-p-naphthylamine polymers. The rate of oxygen absorption was measured at 100" C. and 760 mm. pressure on each of the raw polymers and the corresponding tread-type vulcanixates. The following formulation was employed in the vulcanizates. GR-S type polymer
Bsrdol Fat acid
E.P.C. black
Zinc oxide Santocure Sulfur
100.0 5.0 1.5
50 0 5.0 1.2 2 0
All vulcanizates were cured 50 minutes at 298" F. EFFECT OF ANTIOXIDAST CONCENTRATION
The resistance of GR-S polymer to deterioration by oxidation during storage and processing is a problem equal in importance t o that of the resistance of the finished vulcanisate. The activity of phenyl-P-naphthylamine, a typical antioxidant, has been investigated in this connection at four different concentrations in both salt-acid and alum coagula. The eight polymers have been designated in the following manner: S and A refer to saltacid and alum coagulation, respectively, and the numbers placed after these letters designate the concentration of phenyl+naphthylamine in each-namely, 0.0,0.5,1.0, and 2%. A portion of each of these polymers was sheeted out on a cold mill to a thickness of approximately 0.020 inch, and samples for the absorption test were cut from these sheets. The absorption curves for the salt-acid polymers are shown in Figure 1. The uninhibited polymer absorbs oxygen at a rapid rate from the very start, but the rate ultimately decreases somewhat as a hard oxidized film is formed over the sample. An ultimate decrease in rate is to be expected because of the decreasing concentration of oxidizable centers remaining in the polymer; however, in this case, a soft core that was observed inside the hardened surface indicated some limitation by the lower rate of diffusion of oxygen through the hardened surface. A slight induction period is noted with 0.5% of antioxidant followed by the rapid autocatalytic absorption. With 1% of antioxidant a slow initial rate of absorption gives way to the
autocatalytic increase only after a much longer time. T i t h 2% of antioxidant the initial absorption is more rapid than with 170 but the final upswing does not begin until a still longer time has elapsed. I n this paper the time a t which the autocatalytic stage commences is taken as the end of the induction period, even though a considerable amount of oxygen may be absorbed during this time. With this definition in mind it is apparent that the length of the induction period depends upon the antioxidant concentration but is not dircctly proportional to it. The same behavior is observed with alum coagula (Figure 2), but here the differences are much more marked and the various stages more easily defined. The increase in length of the induction period is greatest between 0.5 and 1% phenyl-P-naphthylamine, and the addition of a second 1% (the 2% curve) gives less than a proportional increase. Cole and Field ( 4 ) have recently demonstrated by analysis that there is a marked decrease in the phenyl-p-naphthylamine content during oven aging of GR-S polymer. If we assume that the end of the induction period marks the point a t which the antioxidant concentration is reduced to an ineffectual level, it is observed that the amount of oxygen required to deplete the antioxidant increases more than a proportional amount as the antioxidant concentration increases. The alum polymer with 1% antioxidant absorbed about 5 ml., and if we assume (as a maximum value) that all this oxygen reacted with the antioxidant, then only 10-ml. absorption would be expected to be required with Zyo of antioxidant. Actually, 15 ml. were required, and the additional 5 ml., or at least one third of the total, must therefore represent a reaction of oxygen with GR-S which is catalyzed by the antioxidant. Such behavior is not without precedent, as many inhibitors of oxidation are also known to be promoters under appropriate conditions ( 2 , 5. 6). Therefore, while phenyl-p-naphthylamine is a good inhibitor for the autocatalytic reaction of oxygen with GR-S, i t may also function as a catalyst for the slower constant rate reaction. Carpenter ( 3 ) has recently suggested that the attainment of a dynamic equilibrium oxidation state (as represented by the constant rate of oxygen absorption prior to the start of the autocatalytic stage) indicates that the antioxidant may function by controlling the formation of an unstable catalyst, such as a peroxide, and maintaining i t at a constant equilibrium value. It is conceivable that the steady state concentration of this unstable catalyst may increase with the concentration of the antioxidant when the latter is in excess of the minimum required. On the other hand, the lower the antioxidant concentration the sooner it will be depleted, and there will be an optimum concentration for
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Figure 1. Effect of Phenyl-@-naphthylamine Concentration upon Oxygen Absorption by Salt-Acid GR-S 100°C.
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Figure 2.
Effect of Phenyl-P-naphthylamine Concentration upon Oxygen -4bsorption of Alum GR- S 100" C.
760 mm.
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Effect of Phenyl-@naphthylamine Concentration u p o n Oxygen Absorption by Salt-Acid Vulcanizates looo C. 760 mm. Cure. 50 minutes at 298' F.
Figure 3.
100 100°C
760 mm
CURE 50' AT 290OF
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required absorption was 2.5 ml. more than the average for the uninhibited samples. This difference may well be attributed to the oxygen required for the oxidation of the antioxidant and corresponds t o 2.25 moles of oxygen per mole of phenyl-8-naphthylamine. It seems probable that 1 mole of oxygen might be sufficient t o destroy the antioxidant activity (7) ; even if the primary oxidation products were further oxidized to stable forms, however, the theoretical oxygen requirement would not exceed 2 or 3 moles of oxygen per mole of phenyl-& naphthylamine. Thus the additional oxygen required t o produce the upswing with 1% of antioxidant present is of the right order of magnitude to represent combination with the antioxidant. With 2% of antioxidant, however, the oxygen required to cause the upswing was twice that required by the uninhibited stocks. The additional 11 ml. of oxygen are far in excess of what might be expected to be required to destroy the antioxidant completely. If we assume that a maximum of 2.5 moles of oxygen per mole of phenyl-/?-naphthylamine might be so used, then the additional oxygen required by the 2% stock would not exceed 5.5 ml. As 11 ml. were required, at least half of this additional oxygen must have reacted with other materials in the vulcanizate. It appears, therefore, that in concentrations above 1%, phenyl-&naphthylamine acts as fti catalyst for the constant rate reaction of oxygen with GR-S, even though it is at the same time retarding the start of the autocatalytic reaction. Because this action as a promoter of oxidation occurs during the stage of the oxidation that extends over the major portion of the useful life of the rubber in service, i t appears that the use of more than 1% of this antioxidant in GR-S is both unnecessary and undesirable from the standpoint of oxidation. EFFECT OF SALT-ACID AND ALUM COAGULATION
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The effect of the method of coagulation upon the oxidation of GR-S polymer is shown in Figure 5. The remarkable protection imparted by coagulation with aluminum sulfate as compared to the salt-acid method is shown with both 1 and 2% of antioxidant. Indeed, the alum coagulum with 1% antioxidant requires a considerably longer time to reach the autocatalytic stage than the salt-acid polymer with 2% antioxidant. Comparing the two types of polymer at the previously observed optimum concentration of 1% phenyl-8-naphthylamine in each, the salt-acid polymer entered the autocatalytic stage within 1 day, while the alum polymer required approximately 10 days to reach the same stage of oxidation. This superiority of alum over salt-acid coagulation is important because of the added resistance to oxidation during drying, storage, and processing of the polymer. Work in progress indicates that this added protection is probably due to a protective coating of aluminum soap deposited on the polymer particles during coagulation.
Figure 4. Effect of Phenyl-@-naphthylamine Concentration upon Oxygen Absorption of Alum Vuloanizates log* C.
760 mm.
Cure, 50 minutes at 298' F.
a given antioxidant in a given material. In the case of GR-S the optimum concentration of phenyl-&naphthylamine appears to be about 1%. Natural and artificial aging studies confirm this conclusion (Figure 1, 1). The behavior of the salt-acid vulcanizates is shown in Figure 3, and of the alum vulcanizates in Figure 4. The three characteristic stages of the absorption curves-the small initial rapid absorption, the steady state, and the final autocatalytic stagehave been discussed (8). The length of time required to start the autocatalytic stage depends upon the amount of antioxidant present, even as observed with the raw polymers. However, the rate of absorption during the constant rate stage decreases as the antioxidant concentration increases, up to an optimum of 1% above which the rate increases again as shown by the curves with 2% antioxidant. The amount of oxygen required by these vulcanizates to initiate the autocatalytic reaction is summarized in Table I. The absorption of 10 to 12 ml. of oxygen by the uninhibited stocks may be regarded as the amount of oxygen absorbed while building up the concentration of reactive groups-for example, hydroperoxides-necessary to initiate the autocatalytic stage of the reaction. The samples containing 0,5% antioxidant required approximately the same amount of oxygen, but the rate of absorption was much slower and the time required to attain this condition was four times longer. With 1 % antioxidant the
TO START UPSWING TABLE I. OXYQENABSORPTIONREQUIRED
IN
Vulaanizate
so
A0 8 0.6
A 0.5 81 A1
sa
A2
RAW
% Antioxidant 0.0 0.0 0.5 0.5 1 .o 1 .o
MI. of OPper Gram of GR-5
2.0
22
2.0
12 10 12 12 13 14 22
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Figure 6 . O x y g e n Absorption of Vulcanizates with 1% Phenyl-P-naphthylamine 100' C.
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Figure 5. Oxygen Absorption of Raw Polymers with 1 a d z %Phenyl-@-naphthylamine 1M)'C.
760mm.
50
100
760 mrn. Cure, 50 minutes
at
298' F.
The oxygen absorption curves for vulcanizates of both saltacid and alum GR-S with 1% phenyl-p-naphthylamine are shown in Figure 6. Although the alum vulcanizate is somewhat more resistant to oxygen, there is much less difference between the vulcanizates than betmeen the two corresponding raw polymers Nevertheless, a slightly lower rate of oxygen absorption has been observed for the alum vulcanizates in every case investigated, both a t other antioxidant concentrations (compare Figures 3 and 4), and with other antioxidants (compare Figures 9 and 10) During the interval from 100 to 150 hours in oxygen, as shown in Figure 5 , the alum-coagulated vulcanizate absorbed from 10 to 20% less oxygen than the corresponding salt-acid vulcanizate It may be that the observed difference merely reflects the effeci of the more extensive oxidation of the salt-acid poiymer prior t o vulcanization. The effect of prior aging of the polymer upon properties of the vulcanizate (Figure 5, I) shows that with insufficient antioxidant protection the properties of the resulting vulcanizate are poorer. COMPARISON OF ANTIOXIDANTS
200
150
The sensitivity and reproducibility of oxygen absorption measurements, together Rith the slight effect of cure upon the rate of oxygen absorption in the case of GR-S ( 8 ) , suggest the use of thip method for evaluating antioxidant activity. Three antioxidants--ahenvl-B-naDhthvlamine. dimethylacridan, and 2,2,4- . trimethyl-6-phenyl-l,2-dihydroquinoline--have been compared at 1% concentration in both salt-acid and alum GR-9 polymers and vulcanizates. The rates of oxygen absorption observed with the rau polymers are shown in Figures 7 and 8. Phenyl-pnaphthylamine Tvas far superior to the other two in both types of polymer, dimethylaoridan was intermedirtte, and 2,2,4-trimethgl-6-phenyl-1,2-dihydroquinoline was
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Figure 7 . Conigarison of Antioxidants in Salt-Acid GR-S Polymers lQOo C.
50
760 mm.
I
-T-?l-i---1-
-1
__ H Y L -6-PHENYL-1,2-OIHYDROQ
40
DIMETHYLACRIDANE
P
poorest.
30
5 0
5 20 n u \
Data for the corresponding vulcanizates presented iu Figures 9 and 10 shorn a general rerersal of the order of activity. Up to approximately 250 hours in oxygen at 100 C., 2,2,4-trimethyl-6-phenyl-l,2-dihydroquinoline gave the lowest rate of oxygen absorption with both types of GR-S. Phenyl-P-naphthylamine was intermedi. ate in the vulcanizate from the salt-acid polymer, and dimethylacridan was the poorest. In the vulcanizates from the alum GR-S, hornever, the phenyl-p-naphthylamine was the poorest, showing a complete reversal of the order observed with the corresponding raw polymers is not The 2,2,4-trimethy1-6-pheny1-1,2-dihydroquinoline compared at its optimum concentration, which is more O
6
2 10 0 0
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100
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Figure 8.
Comparison of Antioxidants in Alum GR-S Polymers
looo C.
760 mm.
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SUMMARY AND CONCLUSIONS
Comparison of the rates of oxygen absorption by G R S polymers and a tread type vulcanizate with varying concentrations of phenyl-&naphthylamine demonstrated that there N v) is an optimum concentration a t about 1% ' antioxidant. DIMETHYLACRIDANE With smaller amounts, the antioxidant is soon depleted in oxygen a t 100" C. t o a point where the rapid autocatalytic stage of oxidation begins. With larger amounts of antioxidant, the rate of oxygen absorption increases in thexonstant rate stage which precedes the start of the autocatalytic reaction. This indicates that the excess of phenyl-p-naphthylamine acts as catalyst for the constant rate reaction, even though it is a t the same time inhibiting the start of the autocatalytic stage. The time required to reach the autocatalytic stage increases with the antioxidant concentration but is not proportional t o it, the increase being less for a given amount of added antiHOURS oxidant above the 1% level. Figure 9. Comparison of Antioxidants in Salt-Acid The amount of oxygen absorbed prior to the start of the GR-S Vulcanizates autocatalytic stage increases more than a proportional amount 100° C. 760 mm. when the antioxidant Concentration is increased above 1%. This extra oxygen is in excess of the maximum that might be required for destruction of the phenyl-&naphthylamine and therefore represents reactions of oxygen with the GR-S which is catalyzed by the excess of antioxidant. Comparison of the effect of two different methods of coagulation upon the rate of oxygen absorption by GR-S polymer showed that coagulation with aluminum sulfate gave a much more oxygen-resistant polymer than coagulation with salt and acid. This added protection is thought t o be due t o a protective coating of aluminum soap. The effect of the method of coagulation was much less in the case of the vulcanizates. The slightly lower rate of oxygen absorption observed with the alum vulcaniaate may result from the 0 fact that the salt-acid polymer absorbs oxygen HOURS more rapidly and thus had reached a higher state Figure 10. Comparison of Antioxidants i n A l u m GR-S Vulcanisates of oxidation prior t o vulcanization. 100" C . 760 mm. The use of oxygen absorption measurements for a comDarison of three commercial antioxidants nearly 2% (Figure 2, I). However, it is shown by natural and showed a considerable difference in their effectiveness a t a given artificial aging (Figures 3, 4, 7, and 8, I) t h a t the same relaconcentration, and also proved t h a t the best antioxidant for the tive relationships are observed a t the highcr concentration with polymer is not necessarily the best for the vulcanizate. both polymer and vulcanizates. The differences are not so great in the case of the vulcanizates ACKNO W LEDGMEN'T as in the case of the polymers, but they are nevertheless reproThe authors extend their thanks t o the Firestone Tire and ducible and significant. The differences in the length of time Rubber Company, sponsor of the research fellowship under which required to reach the autocatalytic stage of rapid oxidation lead this work was carried out. They are also indebted t o 0. D. to a crossover of certain curves. As this represents an advanced Cole of Firestone and t o C. F. Prutton of Case Institute of Techstage of deterioration, the order in the earlier stages has been nology for their continued interest and cooperation. emphasized in the preceding comparisons as more nearly representative of the situation t o be expected during the useful life of the vulcanizate. LITERATURE CITED It is clear from this study that, even with a group of antiAlbert, IND.ENQ.CHEM.,40, 1746 (1948). oxidants, all of which are regarded as effective and used in comBlake and Bruce, Rubber Chem. Technol., 12,181 (1939). mercial practice, there is considerable variation in the relative Carpenter, IND.ENO.CHEW,39,187 (1947). Cole and Field, Ibid., 39,174 (1947). effectiveness a t a given concentration. Furthermore, the effecDawson, J . Research Assoc. Brit. Rubber Mfre., 2, 67 (1933). tiveness is dependent upon the method by which the polymer is Dufraisse, in Davis and Blake's "Chemistry and Technology of prepared. It is also clearly demonstrated that an antioxidant Rubber," p. 509, New York, Reinhold Publishing Corp., 1937. that is effective for the polymer is not necessarily a good anti(7) Rehner, Banes, and Robison, J. Am. C h m . SOC.,67,605 (1945). (8) Shelton and Winn, IND. ENO.CHEM.,38, 71 (1946); Rubber oxidant for the vulcanizate. Chem. Technol., 19, 696 (1946). A correlation of oxygen absorption data with natural and ac(9) Winn, Shelton, and Turnbull, IND.ENG.CHEM., 38, 1062 (1946) celerated aging tests (I) shows good agreement and indicates t h a t oxygen absorption is a useful method of evaluating relative resistR ~ C ~ I VJune E D 10, 1947. Presented before the Division of Rubber Chemance to aging of both polymers and vulcaniaates. istry. AMERICAN CHEMICAL SOCIETY, at Cleveland, Ohio, May 1947.
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