May, 1930
ISDCSTRIAL dA7DESGI-VEERI.1-G CHEMISTRY
549
Aging of Rubber and Its Retardation b y the Surface Application of Antioxygens’ Diffusion Process Charles Moureu,? Charles Dufraisse, and Pierre Lotte LABORATOIRE DE CHIMIE
T
ORGl?r.lQUB, C O L L k C E D E F R A N C E , PARIS,
H E deterioration of rubber may be due to any one or a combination of a large number of faciors, iuch as oxidation, depolgnierizatioii, continued \-ulcanization, action of heat or light, and so on. The pre.ent paper will deal ~ i t hthe preservative action of antioxygens (12, 1 5 ) 11 hen applied on the surface of articles, and with some general remarks on the w e of antioxygens.
FRXSCE
I t is therefore not necessary merely to speed up the ahorptioii of oxygen by rubber to accelerate natural aging. If in a given experiinent the loss of elastic propertie. is due to change catalyzed by oxygen (ZO’i, it IS useleqs or even harmful, accorcling tb the inforination deqiretl, to facilitate the oxidizing action too greatly. An exceSs of oxygen may even lead to a false conclusion by completely changing the course of tlie ieaction. Thus acrolein, which is completely transformed into disacryl Comparison of Accelerated Aging Tests by the catalytic action of minute traces of oxygen, no longer The only tejt not open to criticisin is one wl&h measures produces disacryl in the presence of an excess (13). Comtlie aging of an article under actual hervice condition- and the pressed oxygen should therefore not be used in the study of witers used it in many control experiment-. This test the accelerated polymerization of acrolein. has the disadvantage of consuming an excessive amount of The oxygen test, in spite of its present popularity, may not time and of not being suitable for a series of studies. Hence always lead to proper conclusions in a study of the deteriothe necessity for artificially hastening the deterioration of the ration of rubber. rubber articles. This accelerated aging is obtained, as is HEATdGING-Deterioration due to heat must not be conwell known, by the use of such agents as heat, light. or coni- sidered as giving a faithful picture of natural aging, even pressed oxygen used separately or 4multaneouPly. There is when carried out according to the improved technical method some doubt as t o 1’i.hich accelerated aging test gives re~iiltb described by Geer (,j). The test, however, does give certain most similar to natural aging. iniportant inforination relative to changes that will occur a t OXYGEShGrsG-The teqt using compressed oxygen, either ordinary temperature. I n addition it can be used to predict hot or cold, has aroused considerable interest in the last fern the behavior of rubber materials that are designed to operate years. The harmful action of oxygen on rubber was recog- a t elevated temperatures. nized as early as the middle of last century (1, (3, 4 , 7 , 9, 10, The writers have uqed the resistance to heat as an em11, 18, 21, 22). I n a n earlier series of the publications (1.5’) pirical test having no essential relation to the normal changes the writers have shown that the changes caused by oxygen in that might occur a t lower temperatures. Samples are heated organic substances are of two distinct kinds: one is of an in a hot-air oven a t 90” C. until the control samples show considerable deterioration, which is about 100 hours. Differordinary chemical nature, the other, catalytic. The first type may be represented by a stoichioinetrical ent lots are placed in separate containers to avoid contaminaequation where the quantity of matter altered is proportional tion of the samples, especially the controls, by vapors or dust to the inass of oxygen that reacts. A typical example of this of antioxygens which might be evolved from other samples. kind of reaction is the oxidation of benzaldehyde to benzoic It is not considered neceqsary to pass a current of air through acid. The extent of the reaction niay be measured by the the oven, since rubber does not oxidize rapidly enough to quantity of oxygen which reacts according to the following decrease noticeably the oxygen content of the air in the oven equation: where spontaneous circulation arid renewal is relatively rapid. The use of controlq minimizes errors due to differences be2CsH,CHO 02+2CsHbC01H The second kind of change due to oxygen, to which the tween succeqsive experiment>. LIGHTAGIx-Exposure to ultra-violet light should not be witers have frequently called attention since starting their work on autoxidation, is not due entirely to the oxidation re- considered a practical test ( 2 ) except where objects are to be action. I t is some secondary transformation, such as a polym- used under similar conditions. As a matter of fact, irradiaerization, in which the quantity of material transformed tion with ultra-violet light introduces a n additional deteriobears no stoichiometric relation to the quantity of oxygen rating factor, ozone, which does not exist under the ordinary causing such transformation. A typical example of this is conditions of use. Deterioration due to ozone is an entirely the transformation of acrolein into a n insoluble resin (dis- different phenomenon than that due to oxygen and is not acryl). This change is produced by the merest trace of oxy- iubject to the same influences. Those materials giving progen (13, 16), a mass 0.00001 as great as the aldehyde being tection against ozone will, in general, not be the same as those sufficient. The writers have attributed this type of reaction \Thich afford protection against oxygen. By special experito the catalytic action of peroxides formed by autoxidation. ments the writers have established that “antioxygens” are This type of change is most serious because it requires such not “antiozones.” Ozone itself should be avoided in tests where it is desired to determine experimentally the efficiency minute quantities of oxygen for its initiation. In practice with styrene, drying oils, rubber, etc., these two of a n antioxygen in rubber. The writers made several tests * kinds of change occur simultaneously, their relative effects by sunlight. being dependent upon the conditions under which the test is Preservation of Rubber by Diffusion Process of Antioxygens carried out. The resultant effect will therefore be variable for the same quantities of oxygen absorbed by identical substances. The preservation of rubber (12) by the use of materials having antioxygenic properties may be carried out by various Received August 5 , 1929. methods. Deceased.
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550
INDUSTRIAL A N D ENGINEERING CHEMISTRY
One process, outstanding for its simplicity and efficiency, consists in depositing the antioxygen on the surface of the rubber and is therefore applicable to the treatment of finished objects. The efficiency of this process of protecting rubber might, as a matter of fact, be advanced as an argument in favor of the “varnish theory” (17), which attributes to the antioxygen a mechanical role consisting simply in preventing the contact of oxygen with the object that is liable to be oxidized. I n this process the method of using the antioxygen is similar to the application of a varnish. However, it is shown here that the antioxygen penetrates into the article by diffusion. It does not, therefore, form an isolating film a t the surface, and this mode of protection by surface application cannot be invoked in favor of the varnish theory. ARTICLESTESTED-Experiments have been carried out on manufactured commercial articles such as rubber sheeting, stoppers, tubes, air containers, bathing caps, erasers, fountainpen tubes, etc. Since the properties of rubber sheeting can be more conveniently measured in a quantitative manner, this material has been used for the majority of tests. METHODSOF APPLYING ANTI OXYGEN-(^) The antioxygen may be spread directly on fhe surface of the rubber without the use of any diluent. This process suffers from the disadvantage that it uses too much of the active material and the application may not be uniform. (2) The article may be exposed to the vapors of the antioxygen a t a temperature depending on the vapor pressure of the antioxygen. This process may be convenient in special cases. (3) The most convenient method of application consists in applying the antioxygen in a dilute condition-that is, mixed with inert powder or a liquid to give a true solution, emulsion, or suspension. The best results have been obtained with the antioxygen in true solution. AKTIOXYGEKS USED-The experiments carried out with three phenols-hydroquinone, catechol, and guaiacol-are described to demonstrate the application of the method. Phenols were chosen as being the simplest type of compound with which to study the phenomenon. Nole-The antioxygen action of phenols on rubber has been the subject of many papers ( 8 , 1 9 , 1 , 2 3 , 6 ) since the granting of the patent t o the writers (12). Furthermore several antioxygens have come into extensive commercial use.
PROCEDURE-The efficiency of the treatments was cornpared by determining the ultimate tensile strengths of three series of samples. The first two served as controls, one remaining unaged, and the other being aged along with the third sample which had been treated with the antioxygen. I n order to determine the influence of the solvent used in the treatment, the writers have measured, on different occasions, the ultimate tensile strength of a fourth series of samples which were aged along with the others after first being impregnated by the solvent. The figures given in Table I represent the ultimate tensile strength as determined by breaking a series of six samples given the same treatment. The rubber sheeting was painted with the solution of the phenol and then allowed to dry for 24 hours. Many other experiments not recorded here were also made. I n many of these tests too strong a solution was used, for the phenol remained for the greater part on the surface and when it crystallized was largely lost during the further manipulations. I n practice it would be better to use more dilute solutions even though it were necessary to make several applications. RESULTS-It is possible to show the efficiency of this method of preserving rubber in a very striking manner. Half of the surface of a small sample of sheet rubber may be protected by painting one end or by dipping the lower half
Vol. 22, No. 5
in a solution of the antioxygen. If the sample is now subjected to the aging test (heat, light, or natural), it will be observed that the treated end retains its elasticity whereas the other becomes short and brittle. I n order that this experiment may be completely successful, it is necessary that one portion of the surface remain free from all trace of contamination by the antioxygen. It is even preferable to protect the untouched portion against the vapors of the antioxygen during the aging test, especially when an oven is being used. T a b l e I-Effect
of H e a t on Protection of R u b b e r by Antioxygens ~
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ULTIMATE TENSILB STREWCTH PER STANDARD ARSA6 CROSS-SECTIOXAL ANTIOXYGEN
Hydroquinone Hydroquinone Catechol Guaiacol Hydroquinone Catechol guaiacol Hvdroauinone Hydroquinone Hydroquinone Hydroquinone Hydroquinone Hsdroauinone Hvdroduinone Hydroquinone Guaiacol Coal-tar creosote Hydroquinone Hydroquinone Hydroquinone Hydroquinone Hydroquinone Hydroquinone Guaiacol Guaiacol Hsdroauinone -
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Hydroquinone 4
SOLVENT
Ethyl ethern Ethyl ethera Ethyl ethera Ethyl etherQ Distilled water Distilled water Methanol 1995%) Methanol (83Zj Methanol (527,) Ethyl alcohol (95%) Ethyl alcohol (67%) Benzene Acetone Turoineol Turbine01 Methanol (99%) Benzene 10, turpineol 1 Benzene 5, ether 5 Benzene 4, acetone 5 Acetone 8, turpentine 3 Ether 10. turuineol 1 Ether 10; amyl ether 1 Ether 10, turpentine 1 Benzene 10, turpentine 1 Ether.a applied to one .. side Ether,“ applied to both sides
Zontrol not heated
Grams 5700 6266 5700 5700 6266 3655 6266 5433 5433 6266
6206 5700 5700 3655 6266 5433 5700 5700 5700 5700 5700 5700 5700 5700
AFTER HEATING FOR APPROX. 100 HRS. A T 90’ C.
Not Treated treated ~ ~ Grams Grams 2700 50800 874 2125 I676 2633 I076 5390 874 700 1687 780 874 I050 2300 976 2300 5180 874 2373 304 I620 1550
