INDUSTRIAL A X D ENGINEERISG CHEMISTRY
Sept'ember, 1926
4-The Ohio limestone, producing commercially a plastic finishing hydrate, does not seem to be sensitive to time of burning, whereas the Eastern limestone, not usually considwed as producing a plastic hydrate, is quite sensitive. &For a given limestone the plasticity of the hydrate seems
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to increase with increasing volume of putty, increasing rate of interaction with acid, and decreasing rate of settlingall of which indicates that fineness of the hydrate particles is an important factor in producing plasticity.
Antioxidants and Their Retarding Action in the Deterioration of Rubber' By L. E. Weber2 729 BOYLSTON ST.,BOSTON, h1IASS.
HE physical properties of crude rubber, which determine its value as a useful commodity, are overshadowed by the physical properties of vulcanized rubber, with one striking exception; and it is a n irony of chemical fate that the only property of crude rubber which is actually impaired by vulcanization should be its stability. The higher grades of crude rubber are exceedingly stable provided the rubber is stored a t normal temperatures in the absence of direct sunlight; in fact, the rate of deterioration is so slow that no significant changes are observable over a period of many years. Devries, for instance, found that crude rubber which had been stored for four years showed no appreciable impairment in its quality. During the abnormally low rubber market, when crude rubber was held in storage for a long time, considerable apprehension mas felt that the stored rubber would shorn evidence of deterioration. S o n e was observable, however. Vulcanized rubber does not show this very pronounced stability. A rubber-sulfur mixture, normally vulcanized, will usually begin to show evidence of deterioration some time between the first and second year subsequent to vulcanization, and the deterioration will proceed more rapidly ii the mixture in question has been either under- or overvulcanized.
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Theory of Deterioration
The deterioration, or perishing, of crude rubber is generally regarded as a process of oxidation. This theory was hinted a t some sixty years ago and evidence has been accumulating steadily in substantiation. Briefly, the outstanding facts in support of the oxidation theory are the following: If vulcanized rubber is exposed to sunlight in atmospheres, respectively, of hydrogen, carbon dioxide, air, oxygen, and also in z'ucuo, only the samples in air and oxygen suffer any deterioration within a period of several months; the samples in the other gases are unaffected. The deteriorated rubber is found, upon a n elementary analysis, to contain oxygen; and furthermore, i t has been possible to recognize in such deteriorated rubber the presence of levulinic aldehyde, the latter having been identified by the pyrrole reaction, and also by actual isolation of its pyridazine derivative. Further corroboration of the oxidation theory of the deterioration of rubber is the fact that certain substances, which are known to function as oxygen carriers, have a n unusually severe action upon both crude and vulcanized rubber. Notable in this respect are the salts of copper and manganese, the former especially being very active in extreme dilution. Thus, a sample of cloth containing a n excess of 0.005 per cent copper (introduced as copper sulfate for mordanting) will bring about in a very few months the deterioration of a vulcanized-rubber coating that has been Received July 8 , 1926. 2
Died on July 17, 1926.
spread upon it. I n order to appreciate how extremely sensitive is this catalytic action of the copper, it should be realized that this percentage of copper is distributed throughout the cloth as a whole and only a very small fraction of the copper is in actual contact with the rubber. Inconsistencies in Oxidation Theory
There is, then, a large amount of evidence to indicate that the deterioration of vulcanized rubber is due to oxidation. The action of the oxygen is generally regarded as a n additive reaction involving the double bonds present in the rubber molecule, and the extent of the deterioration as proportional to the amount of oxygen absorbed. There are, however, certain inconsistencies. Chief of these is the fact that vulcanized rubber is considerably more susceptible to deterioration than crude rubber. It is difficult to reconcile this increased susceptibility to deterioration of vulcanized rubber with a theory of additive oxidation-that is, with a reaction involving merely the addition or absorption of oxygen by the molecule. An additive reaction involves one or more of the double bonds, and one would therefore expect crude rubber to be more sensitive than vulcanized rubber to an additive reaction with oxygen, since the degree of unsaturation of the crude rubber is higher. Therefore, crude rubber should be more susceptible to deterioration than vulcanized rubber, which we have found is not the case. In other words, if we accept the additive oxygen theory of deterioration, we are faced with the inconsistency that a subcrude stance on a higher degree of unsaturation-namely, rubber-is less susceptible to deterioration than vulcanized rubber, a closely related substance on a lower degree of unsaturation. This inconsistency, furthermore, is in accordance with the experimental results of Peachey and Leon,3 who found that vulcanized rubber actually did combine with oxygen less rapidly than crude rubber, a finding which is in accordance with the chemistry involved in reactions of additive oxidation, but which is contrary to our experience on the relative deterioration of crude and vulcanized rubber. There is a further inconsistency in the additive oxidation theory of deterioration. It is well known that rubber that is either under- or overvulcanized deteriorates with increased rapidity. I t is very difficult to explain this on the basis of a n increase in the addition of oxygen. I n attempting to explain the deterioration of incorrectly vulcanized rubber, we are, of course, still confronted with the inconsistency discussed above, but in addition we have to explain why an irregularity in the time-temperature-sulfur relationship necessary for vulcanization aggravates the deterioration. A mere additive oxidation theory is not convincing. On the other hand, we are not justified in rejecting broadly
* J. SOC.Chem. Ind , 37,
56T (1918).
I N D CSTRIAL B S D ENGINEERI-VG CHEMISTRY
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the oxidation theory, since the evidence is too strong that oxygen is added to the rubber molecule during the deterioration of vulcanized rubber; but the conclusion is warranted that in so far, a t least, as vulcanized rubber is concerned other factors are involved. If we accept the statement that the deterioration of vulcanized rubber proceeds only in the presence of air or oxygen, we must conclude that the reaction involved is not confined to an additire oxidation, but that other oxidation products are formed. Such a secondary oxidation reaction would probably result in the formation, a t least in part, of sulfur trioxide, carbon dioxide, or water. Conversely, the identification of any one of these three substances would be evidence that an oxidation reaction other than a n additive one takes place. The precise nature of the substances formed would give some indication as to the groups in the rubber molecule which had been attacked. Thus, the formation of water would indicate that one or more hydrogen atoms had been oxidized, while the presence of carbon dioxide would show that a carbon atom had been involved. The presence of sulfur trioxide would not necessarily show that the combined sulfur had been affected, since this oxide could originate from the free sulfur. The formation of sulfur trioxide in the deterioration of vulcanized rubber has been shown by Eaton and DayJ4but it was not shown whether any of the sulfur trioxide came from the sulfur in the rubber molecule, or whether it came from the free sulfur. Xo evidence is available as to whether or not water is formed in the deterioration of rubber. The fact that vulcanized rubber deteriorates much more slowly in the presence of water than in a dry atmosphere, as was shown by Stevens, might be taken as indirect evidence on the basis of theoretical considerations that water vapor is actually produced in the deterioration, and, conversely, that the presence of water vapor hinders the reaction. Direct experimental proof of the formation of water vapor, and also of the formation of carbon dioxide, as a result of the natural deterioration of vulcanized rubber, is lacking. Experiments to prove this point, and also whether or not any of the combined sulfur is converted into sulfur trioxide, are in progress. Antioxidizing Action of Accelerators
Reference was made above to the remarkably powerful action of certain substances, notably copper salts, in accelerating the deterioration of rubber. It is not too much to expect, then, that the reverse property should be inherent in certain substances-that is, the property of retarding the deterioration. Substances of this nature have actually been found and are beginning to assume very important technical significance. They have been given the name “antioxidants.” It is a matter of common observation that the substances classed as organic accelerators have the property of imparting to the vulcanized compound increased resistance to deterioration. This property of the accelerators was recognized almost simultaneously with their introduction, and subsequent observations have shown that it increases, broadly speaking, with the activity of the accelerator; that is to say, the stronger the accelerator the better the aging properties of the vulcanized compound, provided, of course, that the latter has been neither under- nor orervulcanized. The view has been expressed that this property of the accelerator in retarding deterioration is due to the fact that the rubber has suffered less depolymerization by virtue of the decreased heating period necessary for vulcanization. Accordingly, the more powerful the accelerator, the less depolymerization and therefore the better the aging qualities of the vulcanized product. There are inconsistencies in this viewpoint, however, and it 4
J . SOC.Chem I n d . , 38, 339T (1919).
Vol. 18, KO. 9
seems more reasonable to correlate the resulting improvement with a specific action of the accelerator; that is to say, the accelerator exercises an antioxidant action in addition to its accelerating action. Other Antioxidants
Tannic acid and hydroquino’ne were probably the first substances found to exert a specific retarding action on the deterioration of rubber, without exercising any accelerating action. These observations were made by Helbronner :md Bernstein. Subsequent thereto, but quite independently, hIoureu and Dufraisse found that phenol and many other substances containing the phenolic group inhibit the oxidation of easily oxidizable organic compounds, such as acrolein. Furthermore, they found that the same inhibiting action is exercised on various oils, resins, and rubber. At the present time three substances are finding considerable technical application as antioxidants. The nature of one of these materials is not disclosed, but the second is a condensation product of aldol and a-naphthylamine, while the third is a condensation product of acetaldehyde and aniline. It will be recognized a t once from the composition of the last two materials how closely their general composition conforms to that of substances having pronounced accelerating action, even though these two substances themselves are exceedingly weak accelerators. Commercial Use of Antioxidants
It is the present commercial practice to use from 1 to 3 per cent of the antioxidant, the percentage being figured on the rubber present and not on the total compound. I n extreme cases as much as 5 per cent of the antioxidant is used. It is conservative to say that by this means the natural life of the rubber compound is a t least doubled, if not trebled. To have means available for increasing the natural life of a rubber compound to such a large extent is in itself a contribution of very great technical and commercial significance. I t is probable, however, that the greatest value of antioxidants will be in increasing the life of rubber articles which in their utilization are exposed to temperatures above the normal. It is common knowledge that temperatures much in excess of 50” C. greatly accelerate the deterioration of vulcanized rubber, and it has been found that the antioxidants are exceedingly efficient in counteracting this adverse influence of temperature. Function of Antioxidants
Little is known a t present as to the details of the manner in which antioxidants function; and this is scarcely to be wondered a t since our knowledge of the causes of the deterioration of rubber is incomplete. The available evidence indicates that the antioxidants do not function as direct catalysts, since their ability to exert an inhibiting action slowly diminishes-that is, in the course of time the antioxidant becomes consumed. Since substances having antioxidant properties are themselves of a readily oxidizable nature, it seems reasonable to suppose that in the presence of vulcanized rubber they act as a buffer and prevent the oxidation of the rubber by becoming oxidized themselves. That portion of the antioxidant which has become oxidized is, of course, ineffective in further protecting rubber, and ultimately, therefore, the entire action of the antioxidant is lost. The antioxidants stand today in the same position of potential value in which the organic accelerators stood fifteen years ago. The forecast is warranted that as a technical development they equal the organic accelerators in importance. 5
Compf. ?end., 177,204 (1923).
