June, 1926
IiliDCSTRIdL AND ENGINEERING C H E U I S T R Y
615
Some Factors Influencing the Weathering ‘of Vulcanized Rubber’ By hT.A. Shepard, S t a n l e y Krd1,2 a n d H. L. Morris THEFIRESTOXE TIRE& RUBBER CO.,
A K R O N , OHIO
T HAS long been known that rubber deteriorates rapidly other destructive agents, which it assists in their action; alone, i t probably would not produce any injury to perfectly good when exposed to the action of direct sunlight, especially normal vulcanized caoutchouc. if the rubber is under strain. The surface becomes crazed or more or less covered with checks or cracks, perpen- FickendeyG has demonstrated that this last conjecture of dicular to the direction of the strain. These cracks not Burghardt’s is correct, a t least as far as raw rubber is cononly produce a very unsightly appearance, but also may even cerned. Fickendey exposed his samples to direct sunlight become so deep as to affect materially the useful life of a in sealed tubes filled with oxygen, nitrogen, hydrogen, and carbon dioxide, respectively. To quote his words: “Even rubber product. Cracking under the influence of light or weather is not after weeks, the rubber in the hydrogen, in the nitrogen, and the only type of cracking to which rubber products are in the carbon dioxide was unchanged. The rubber in the susceptible. When vulcanized rubber, especially that com- oxygen, on the other hand, had become sticky.” Aside from the action of sunlight there are several other pounded with large amounts of filler pigments. is subjected to repeated stresses--i. e., to flexing-cracking results, factors, such as heat, which have a direct bearing on the rate of oxidation 07 rubber. again perpendicular to the Heil and Esch,’ in writing stress, which may closely reof soft vulcanized rubber, semble sun-checking. “Sun-cracking” has always been a factor in the comstate that “the oxidation is These two types of crackp o u n d i n g of r u b b e r f o r use in articles exposed to sunfavored by the action of ing must not be confused, l i g h t or weather. Especially is this t r u e w h e r e the sunlight and high temperaas they are of quite different r u b b e r a r t i c l e is exposed in a s t r e t c h e d condition, as tures as well as by admixorigin, the one arising from in the case of inflated spare tires. t u r e s w h i c h increase the exposure to light and the T h i s susceptibility of vulcanized r u b b e r t o crack or p o r o s i t y of the rubber.” other from repeated stretchcheck w h e n in a s t r e t c h e d or s t r a i n e d condition has AhrensB corroborates this ing or flexing. been utilized in f o r m u l a t i n g an accelerated checking statement in regard to the Historical test f o r s t u d y i n g the d e t e r i o r a t i o n of r u b b e r c o m p o u n d s effect of temperature, findwhen exposed t o the weather. T h i s test has been used ing that “warming accelerThe literature contains in d e t e r m i n i n g the influence of cure, s u l f u r bloom, ated the action very much.” many references to the accolor, g r a d e of r u b b e r , reclaimed r u b b e r , several filler T h e r e l a t i v e speed of tion of light on rubber. I n p i g m e n t s , several k i n d s of softeners, accelerators, and oxidation of raw rubber as 1902 Keber3 wrote: an antioxidant. compared with that of vulThe liability of India rubT h i s work has s h o w n that by t h e selection of the c a n i z e d rubber has been ber goods of various descripproper p i g m e n t s , t o g e t h e r w i t h c e r t a i n specifically studied by several investitions, but more particularly protective m a t e r i a l s , much can b e d o n e to improve the tire covers, to develop, often gators. Ahrenssstates that w e a t h e r i n g of m o s t types of r u b b e r compounds. to a very disagreeable devulcanization with s u 1f u r gree. the defect known as alone retards the oxidation &-cracking is very generbut that many compoundally recognized. It is the result of the oxidation of the India rubber by the atmosphere. ing ingredients tend to accelerate it. This statement -agrees The term “sun-cracking’’ might therefore appear t o be a mis- with the findings of Peachy and Leong-namely, that vulnomer, but, as a matter of fact, it emphasizes very expressively canized rubber, whether resinous or resin-free, oxidizes far the marked hastening of this oxidation through the influence of less rapidly than the corresponding raw rubber, although direct sunlight. the amounts of oxygen ultimately taken up are of the same Work on this subject, however, pre-dates 1902 by more order in the two cases. HenrilO also finds that than thirty-five years. -4s far back as 1865 Spiller4 investiWhen one subjects to the action of ultra-violet light pure gated the changes produced in rubber under atmospheric gum sheets vulcanized either in the cold or hot, one sees immediconditions. It was he who proved that oxygen functions ately a very great difference; at the end of twenty hours (which in the change. Burghardt,6 in 1883, made the following time is sufficient to produce a marked change in the unvulcanized rubber), there is no appreciable change in the vulcanized statement:
I
One fact has been amply verified by experiment and researcht h a t is, t h a t the destruction of India rubber, however brought about, is simply a more or less degree of oxidation of the mass entering into the composition of the rubber goods. *** Light also exerts a n injurious action, but only in conjunction with f Presented before the joint meeting of the Division of Rubber Chemistry and the Akron Section of the American Chemical Society, Akron, Ohio, February 22 and 23, 1926. 2 Fisk Rubber C o . , Chicopee, Mass. 8 “The Chemistry of India Rubber” 1902; cf. 4th Impression, p . 229 (1912). Chas. Griffen & C o . , Ltd. 4 J . C h r m . SOC.(London), 18,44 ( 1 8 6 5 ) ; cf. BdZ. SOC. chim., 4,231 (1865). J . SOC.Chcm. I n d . , 2, 119 (1883).
rubber. It is necessary to prolong the action of the rays to 48 and even to 7 2 hours in order to observe a change in the surface of the rubber; this surface then cracks and the rubber loses likewise its elasticity. Thus vulcanization of rubber protects it to a great extent against the action of ultra-violet rays.
It is thus an established fact that sun-checking or suncracking is an oxidation process aided by both light and heat. Kolloid-Z., 9, 81 (1911). “The Manufacture of Rubber Goods,” 1909, p. 22. & Co., Ltd. 8 Kunslslo.fe, 3, 478; cf. C. A . , 8, 151.5 (1914). 0 J . SOC.Chem. I n d . , 37, 56T (1918). 10 Caoiclchouc b gutta-percha, 7 , 4372 (1910). 0
7
Chas. Griffin
INDUSTRIAL A N D ENGINEERING CHEMISTRY
616
The part played by ozone in the deterioration of rubber has long been recognized, l 1 but that it specifically functions in sun-checking has only recently been demonstrated. Early in 1925 Asano12 pointed out the deleterious action of ozone, and the part it plays in the deterioration of rubber under the influence of light. Later Haushalter,l3 in measuring the corona effect on rubber, called attention to the cracking or checking produced, and attributed it to ozone. His views are supported by experimental evidence. Still more recently Peritor14 suggested ozone for use in testing rubber goods. william^'^ demonstrated the connection between ozone and sun-checking in this laboratory in 1923. Some work has already been published on the effect of different compounding ingredients on the oxidation of rubber. Henri,Io during the progress of his work on unvulcanized rubber with ultra-violet light, found that In general, sheets of rubber containing different mineral fillers suffer change much more slowly than pure rubber. However, this change depends on the nature of the filler; certain substances, such as litharge, retard the deterioration of the rubber by the rays, while others, such as antimony sulfide, accelerate the change.
Traces of copper salts have been shown to be extremely active in bringing about the deterioration of rubber in both the raw and cured condition.16 Powdered glass, according to Ditmar,” exerts a protective influence against oxidation which is directly proportional to the amount present. This same investigator found also that the presence of zinc oxide in vulcanized rubber increases the oxidation tendency. He was unable, however, to find any regularity between the percentage of zinc oxide and the degree of oxidation. It is of interest in this connection that pure zinc oxide has been found to have no effect on the rate of oxidation of linseed oil.’* I n addition to mineral compounding ingredients and other filler pigments, some work has been published with reference to the influence of oils and resins. It is well known that vulcanizedlg rubber, as well as raw rubberZOfrom which the resin has been removed, oxidizes extremely rapidly. Addition of such materials might be expected to be protective. This was early found to be true, especially in the case of vaseline, paraffin, and similar petroleum products. I n 1881 Kreusler and Budde made a paraffi treatment of vulcanized rubber the subject of a patent.*l This consisted in dipping the finished product into a paraffin bath a t 100’ C. and holding it there for a few seconds up to several minutes, depending on the size of the article. Articles so treated are claimed to be highly resistant to light and atmospheric influences, and not subject to hardening or cracking. Himmelbauer & Companyz2 recommend a similar process, using vaseline or ceresin wax. The first investigator to refer to the use of paraffin and such materials in compounding was Weber,3 who mentions that there was a material on the market for the prevention of sun-checking which consisted of a mixture of paraffin with sulfur and small quantities of beeswax and tallow. He also mentions paraffin alone, stating that its effect is to ‘keduce the liability of India rubber to oxidation in general J . SOC.Chem Znd., 4, 710 (1925);Gummi-Ztg., 14, 687 (1900). I n d i a Rubber J . , 70, 395 (1925). 18 Zbid., 70, 899 (1925). 1‘ G u m m i - Z t g . , 40, 95 (1925). 1s THISJOURNAL, 18,367 (1926). I* Kolloid-Z., 4,232 (1909) 1’ Gummi-Zlg., 21, 243 (1906);C.A , , 1, 491 (1907). 18 THISJOURNAL, 18, 30 (1926). 19 I n d i a Rubber J . , Sa, 373 (1916). 20 Zbid., 66, 235 (1918). 21 D.R. P. 18,740 (August 26, 1881). 12 Chem. Ztg., 8 6 , 1441 (1912). 11
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Vol. 18, No. 6
and to sun-cracking in particular.” .4hrensZ2 stated that in 1899 he added ceresin to a tread stock on the mixing mill and then exposed tires with half-and-half treads, one-half containing ceresin and the other none. The protective action in the presence of ceresin he ascribed to the chemically inert bloom or coating of ceresin. Vaseline and paraffi are classed with ceresin in their protective action. C h e r t o P attempts to cover by patent certain vegetable oils (including palm oil, rosin oil, pine oil, and turpentine) in admixture with resins (including rosin, dammar, and copal), to prevent rapid deterioration and hardening of rubber with age. Paraffin, mineral, and other nondrying oils are also suggested by R e p ~ n y . ~ ~ Numerous other materials, such as nitrogen conipounds (aniline25 and related substances), phenolic bodies (betanaphthol, resorcino1,zB and tannin,*’) and nitro compounds2* have been advanced for use in the protection of rubber, crude and synthetic, raw and vulcanized, against oxidation. Many of these materials fall into the class of substances known as “antioxidants,” and as such should be of interest in the prevention of sun-checking. However, as far as the writers are aware, nothing has been published concerning the specific influence of these materials on sun-checking. The work recorded in this paper is confined chiefly to a series of exposure tests covering the influence of various compounding ingredients on sun-checking. These ingredients include several fillers, softeners, accelerators, and an antioxidant. I n addition, reclaimed rubber has been coinpared with several grades of raw rubber. Procedure
Most of the work recorded in the literature was carried out with oxygen gas, working with very thin films of rubber. The experimental details were laborious and the purely surface phenomena, in which there is the greatest interest, were not readily observable. Weber,3 in order to avoid the long, tedious process of making exposure tests for prolonged periods in direct sunlight with the uncontrollable conditions of weather and season, devised a wet oxidation with acetone peroxide. This method also has its limitations -the reagent is highly explosive in the dry state, and Weber admits that the method is only a fairly reliable measure of the relative susceptibility of the samples to sun-cracking. It appeared to the writers that direct-exposure tests would be of the most practical application provided the action of the weather could be accelerated, because these tests would be carried on under conditions more nearly approximating the actual weathering conditions of the rubber in service. Various references to the rapid deterioration of rubber when stretched are found in the 1itera.ture. I n his early experiments on the oxidation of rubber threads, Thomson29 finds that “rubber threads, which are stretched or pinched or strained in any way, are more easily oxidized a t the parts subjected to such strain than threads in their normal condition.” The same author points out that ozonized air has but little action on unstretched or unstrained rubber but that it acts very rapidly on rubber in the stretched or strained condition. Heil and Esch7find that rubber rapidly absorbs oxygen when exposed to its action, either in solution or when distended by a solvent, or when kept stretched. Preliminary experiments proved very quickly the value of exposing the test samples in a stretched condition. Stretch2s 24 26
2a 27 21
29
U. S. Patent 1,379,743(May 31, 1921). Rrrbber Age, 6 , 5 (1919). D.R.P.221,310 (1908). Badische, D.R. P. 330,741 (1918). K o l l o i d - Z . , 8, 81 (1911);Badische, D.R. P. 266.153 (1912). Bndische, D.R . P. 332,305(1918). J . Soc. Chem. Znd.. 4, 710 (1885).
ing ti, ail eloiigsticrii i d 12.5 pcr cent p o r e d suftieieiit to prodiirr chrckiiig of susceptible samples ruitl~irr2 l hours, retchcrl iniiiples were not checked for rnonthfi, whcn csp~ised to sunlight,. This elongat.ion was selected as :iit;ind;ird iiftrr it, mas foiitid that increasing the cloiigation beyund this anioimt did not accentuate the cliecking,3~but produced R 1:wger ni r id finer checks wliic low eIongat,iims produ visihk. IVorking at
of sulfur to the ruhher was conipouiidnl ivit,li t,wice the amount of accelerator. Tlie samples for test, %ere both cured for 75 minutes at 287" F.(141.7"C.). Afterexposure for 2 days in June the stock with the higher concentration of accelerator was entirely free froin cracks, while the other was noticeably checked. Sulfur BIoom
The urotect.ire influence of h e a n bloom was demonshted by exposing t w o strips from tlie"sarne slab of a blooming tread stock, froin nne of ~ b i c hthe bloom was inechanically removed. After 16 days' rsposuie in duiie there vas an appreciable differenre in tlie amount of checking iii favor of the bloomed stock. The piotective influence is probably due to the film of sulfur acting as a corering more or leas tmpetvioiis to light oi to ozone Color
black; even a dirty spot on a white stock becomes more rapidly than where the surface is clean and As little as 0 1 per rent of gas black ndded to n ck to make it gray increases checking. The same de black with 2 per ceiit of gas black loses still esistance to checking. There was a definite gra-
r,onvderable nrunber of test pieces without to these frames. Ry i i ~ t h u gthe tr,t strip5 with the iiiual marks 1 inch apait, '1. i n tensile testnig, it was a simple matter to stretch them tc, tlic dcsiicd elongation (Figure 1). 7'11, i i m i e b , iiispmded from a rope, werr exposed upon tlir na,i They revohed m the wind and in this way all the .iitf.i
