The Aging of Vulcanized Rubber under Varying Elongation' A. A. Somerville, J. M. Ball, and W. If. Cope
n. T.V * N I ~ I I R ~ I Z I . TC O . , 230
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liiiuwii that a piece of rulher tuliiig cracks a t that point where it is stretched over a illeta1 nipple, or 3. piece of water hose cracks where it is stretched over a nozde. Figure 1 illustrates t.lic crackiiig of natural-aged rub1.m goods a t those parts which are under some degree of tension or strain. Siiasimtoli as robber does crack or deteriorate faster wlien strrtclied, eaperimeiit,al work WILS coiitcmplnted to iletcriiiiiie tire relative degree or rntc ( J f rlcteriorzlioii ut different, pcrceiituges of stretch or cloiigatioii.
i'rali A V B . , NEW YDXK, N. Y .
foiir coriditioris: (1) iii tlie Gcer oven where tlic principal factor concerned is lieat; (2) in the 13iercr-I)avis bomb where the chief factor is oxygen; (3) in an electric spark cliuinher wliere the principal factor is ozone; and (4) in the great outdoors where the chief factor is sunlight. St should be delinitely understood that this eiperiinciital work cmicerns itself oiily witli static aging, not xvitii dynamic aging wlicre the test strip is being twisted, flexed, bent, or stretclied repeatedly during the aging process. Tirat problem of dyiiarnic aging is entircly different from the present prohlem of stst,ic aging at different elongations, particularly when waxy ninterials are used in the compound to form a protective filn1. it total of sixty-six comyounds or recipes was used. These compounds were first curcd in sheet form in molds, a range of cnres was made a t one temperature, tcnsiles were determined on dumbbell test strips, arid what was deemed to be the proper cure or cisres v a s dctcriiiined froin the tensile-time cnrye. (Figure 3 ) This ciirve has been shown before, and simply represents tensile stri:rigth plotted against time of curee. From it tlie point or poii1t.s which are considered the proper cures are cliosen.
Factors Studied
The variation i i i the rat,e arid degree of deterioratioii with respect to varying elongation was then studied as affected by the following factors: (1) fillers; (2) volume loadings; (3) accelerators; (4) peri:cutages of accelerators; (5) percentages of sulfur; (6) antioxidants; (7) percentages of antioxidants; (8) softeners and waxes; (9) vulcanizing agents; (10) percentages oi vulcanizing agents; (11) rubbers, including synthetic; (12) times of cure; (13) periods of bomb aging.
Percent
Fieure 1 . -Nafural-Aged Rubher Cracked at Parte under Tension
Experimental Method
Circular test rings were cut from Ant sheets of cured rnbber, tlieii st.retclied and held a t differexit elongations on a stepped steel cone. (Figure 2) The sizes of tlie steps on this cone are such t.liat the test rings are strctclied froin 0 to 100 per cent. Straight mandrels of varying size were also used in addition to this stepped cone. Aging of tlie rings on the stepped cone or on the mandrels >vas ilieri done under 9 Presented briore the meeting 01 the Division of Ruther Ctieinirfry ui tile American Cliemical Society, Atlantic City, N. I., September 20,
1929.
Figure 2-Stepped
Cone
The furrnula for the basic control compound used for the greater amount of the work was made about as simhe as possible, and is as follows:
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1184
Vol. 21, KO.12
Parts Rubber 100 Zinc oxide 0 Gilders whiting 60 Stearic acid 1 Sulfur 4 Diphenylguanidine 1.25 Cure: 10,15, 30, 45, 60 minutes at 2.8 kg. per sq. cm. (40 Ibs. per sq. in.)
curve, for instance a t 100 per cent elongation, consists of an average of the tensiles of sixty-six different compounds; all of these compounds aged for 12 days in the oven a t 100 per cent elongation, then broken, tensile strengths determined, and the average taken for that point, and similarly for all the other points. This composite curve shows that in oven aging 4000 deterioration is approximately 30 per cent greater when the sample is stretched 100 per cent than when it is stretched 0 per cent. Figure 5 shows an analogous composite curve for the same sixty-six compounds aged under similar conditions 3000 z10 F of stretch one period of time (24 hours) in the bomb. This 4 b curve indicates that aging in the bomb is almost independent 6 4+ of the amount the sample is stretched. Figure 6 shows the L, 2000 I40 composite or average curve for the same sixty-six compounds 9 aged one period of time (45 minutes, concentration unknown) in ozone. The ozone chamber or closet contains several /ooo i ! 70 pairs of metal plates a t a difference of potential of 20,000 volts. The rubber test rings are placed about these plates and the ozone is distributed by means of an electric fan. It will be noted that this curve is not a straight line, but has a dip in it. Figure 7 shows a composite curve for the same Cure Minufes Figure 3-Tensile-Time Curve sixty-six compounds aged in sunlight one period of time (17 days) the latter part of August and the first few days of In addition to the above, aging tests were made on rubber September, and there was real sunlight all that period. Here bands a t different elongations from 0 to 700 per cent. These again there is a dip in that curve. From these curves it can be concluded that it is not heat bands were pure gum with 5 or 6 per cent sulfur plus antiand it is not oxygen that causes a rubber to crack a t a point oxidant. where it is stretched; it is ozone or something comparable to Results ozone in sunlight, and there is a critical percentage of stretch Figure 4 shows an average or composite curve of all these or elongation where that deterioration is greatest. Figure 8 shows curves on the control stock containing sixty-six compounds aged for one period of time (12 days) a t nine different elongations in an oven. Each point on this whiting, aged a t different elongations in oven, bomb, ozone, and sunlight. 3500 3500r . , I Test rings crack when aged under tension in ozone or sunlight. Figure 9 shows the test rings 3 m 3000 of one compound that has been aged a t the -"- E different elongations on the stepped cone in = ::2500 175 $2500 ozone. I n order to photograph these rings they b have been taken off the stepped cone and B L 150 2 $2000 stretched 100 per cent. c? 2wo 125 -? Figure 10 shows the aging curves in ozone and .c c . sunlight of a 40 per cent carbon-black stock91500 91500 100 5 that is, 40 parts of carbon black on 100 parts 5, 5, $loo0 75 loo0 - of rubber, which is equivalent in volume to 60 s parts of whiting. Tire manufacturers say that s 50 the percentage of stretch in the side wall of a 500 500 25 tire when it is inflated on a rim is from 6 to 12 C U M bne -TennleAfter 12 Oayi Oven per cent. It seems unfortunate that the critical '0 20 40 60 80 100 point of this deterioration curve should come a t Percent Elongabon During Ageing Percen! Elongation During Ageing about 10 per cent elongation. Figure 4 Figure 5
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December, 1929
INDUSTRIAL B N D ENGINEERING CHEMISTRY
As the sulfur is varied 2,3,4, and 5 per cent on the rubber, and the stocks are aged a t different elongations, it appears that the deterioration increases as the sulfur is increased in either ozone or sunlight. (Figure 11) As the cure varies 10, 20, and 30 minutes, it appears that the underc u r e s deteriorate faster than the proper cure when aged in ozone or sunlight. (Figure 12) As the loading of filler is varied 30, 60, 90, 120, and 150 parts of whiting on 100 parts of rubber, the percentage of deterioration a t different elongations is not changed, and the dip in the curve when aging is done in ozone does not move along the horizontal axis. Figure 13 shows the aging of two of these different loadings in ozone as compared with the pure gum control containing no whiting. The writers h a d i d e a s a b o u t correlating aging a t various elongations with stress-strain data a t very low elongations, and to this end have devised means for accurately determining stress-strains a t l o w elongations-i. e., by molding a test ring or loop 25.4 cm. (10 inches) in length so that 25.4-cm. stretch on the testing machine is only 100 per cent elongation. The test ring or loop is of such cross section as to require a considerable pull to stretch it 100 per cent, and accordingly an accurate stress-strain curve can be drawn. That, however, is another matter and has not as yet been tied up with the aging data a t different elongations. Figure 14 shows the aging curves for so-called pure-gum rubber bands aged in oxygen and ozone while they were stretched 0 to 700 per cent. That ozone curve has been duplicated and can be duplicated readily. From the great amount of data
1185 40'10 CARBON BLACK
Percent Elongation During Ageing I
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Figure 11 3500
!Effect of Elongation on Ageing
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Percent Elongation During Ageing
Figure 14
obtained in this work, the results with certain vulcanizing agents show rather low percentages of deterioration during aging, and fairly flat aging curves in ozone and sunlight.
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Figure 12
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Figure 15
Figure 15 shows the aging in ozone and sunlight of a compound cured with 3 per cent tetramethylthiuram disulfide as a vulcanizing agent without any added sulfur.
I N 0 USTRIAL AND ENGINEERIiVG CHEMISTRY
1186
From the results on the series of compounds containing added softeners two sets of curves are shown-one for mineral rubber (Figure 16) and the other for paraffin (Figure 17). Although the two curves in Figure 16 are of about the same shape, the percentage deterioration is less if anything MINERAL RUBBER SUNLIGHT Rubber a1 Tensiles Before Ageing
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with the larger amount of mineral rubber, except at the critical elongation of about 10 per cent. Paraffin is an interesting example of a material that causes bad deterioration in ozone but provides some protection in sunlight, and furthermore the sunlight curve does not have the characteristic dip a t 10 per cent elongation. Possibly this might be explained by the protective film of paraffin breaking under stretch but uniting again during the period of sunlight aging when the weather was fairly hot. The addition of an antioxidant does not change the shape of the sunlight- or ozone-aging curve-that is, it does not take that dip out of the curve a t the 10 per cent elongation-but it may lessen the degree of deterioration. (Figure 18) According to these four different methods of aging-heat, oxygen, ozone, and sunlight-different antioxidants do not always give comparable results. For instance, two antioxidants when tested in the oven and the bomb may have a strictly comparable efficiency, as shown in Figure 19, \\-hereas when tested in ozone and sunlight they may have an entirely different efficiency as shown in Figure 20. Figure 21 shows the aging in ozone of a compound containing synthetic rubber. The tensiles are low inasmuch as the rubber had been lying in the laboratory nearly a year and had begun to oxidize, but the dip in the curve a t 5 or 10 per cent elongation is still there. Summary of Results
c e
1-Deterioration in the oven is approximately 30 per cent greater at 100 per cent elongation than a t 0 per cent elongation. 2-Deterioration in the bomb is almost independent of elongat ion. TWO ANTI -OXIDANTS 3-Deterioration in ozone is 2500 I r I greatest at 5 and 10 per cent elonga150 E tions. 100 4-Deterioration in sunlight with m respect to stretch is analogous to 50 deterioration in ozone. E %-Cracking of the test rings oc200 p curs in both ozone and sunlight Y when the rings are stretched, but 150 5 F not in the oven or bomb. This E 100 z cracking is greatest a t 5 and 10 per cent elongations. 50 E &A 40 per cent carbon-black stock shows marked deterioration 20 40 60 80 100 20 40 60 80 lA0 and cracking in both ozone and sunPercent Elongation During Ageing light a t about 10 per cent elongation. Figure 19 7-High sulfur causes greater deterioration in ozone and sunlight E f f e c t of Elongation on Ageing than low sulfur. -- SYNTHETIC RUBBER 8-Undercures show greater de-3000 OZONE-1-1 200 terioration than the optimum cure in ozone and sunlight. 9-Loading with a filler such a s whiting does not change the shape of the ozone- and sunlight-aging c u r v e s , and does not materially affect the percentage deterioration. 10-Rubber bands show progressively poorer aging in the bomb as the stretch increases from 0 to 700 per cent, while in ozone they give an aging curve having the characteristic dip a t about 10 per cent 1 0 elongation, showing the least deOb 20 40 60 80 1, terioration a t about 400 per cent Percent Elongation During Ageing elongation. Figure 21
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T'ol. 21, No. 12
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INDUSTRIA L A S D ENGINEERING CHEMISTRY
December, 1929
11-Tetramethylthiuram disulfide (3 per cent) as a vulcanizing agent causes rather low percentages of deterioration in ozone and sunlight, and gires fairly flat aging curves. 12-Mineral rubber (24 per cent) causes a smaller percentage deterioration than 4 per cent except at the critical elongation of about 10 per cent. 13-Paraffin (I per cent) causes pretty bad deterioration in ozone but provides some protection in sunlight.
1187
14-Added antioxidant does not change the shape of the ozone- or sunlight-aging curves but may lessen the amount of deterioration. 15-Two antioxidants may give comparable results in both oven and bomb, but quite different results in ozone and sunlight. 16-Synthetic rubber gives a characteristic ozone-aging curve.
Mothproofing’ M . G. Minaeff and J. H. Wright LABORATORIES OF THE LARVEX CORPORATION, 882 THIRD AYE., BROOKLYN, N.E‘.
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The process of mothproofing with formulas based The old method of moth on silicofluorides has been described and the results control with naphthalene or ing or controlling the demonstrated. It has been shown that wool possesses c a m p h o r and the modern clothes moth and rea great affinity to silicofluorides, and therefore these fumigation method are based lated insects is of such vast chemicals can be applied from dilute solutions. Owing on gaseous penetration. The importance that it has in reto these properties, mothproofing with silicofluoride new mothproofing compounds cent years received extensive formulas is much more effective and durable than with are applied to the fabric in study by the technical laborasodium fluoride formulas. t h e l i q u i d form. With all t o r i e s of both federal and Different methods of determining the efficiency of chemicals of the fumigas t a t e governments and of mothproofing treatments have been discussed. tion class the protection is commer ci a1 organizations. Mothproofing with alkaloids, nitrogenous comlimited to the actual time The limitations of moth prepounds, and thiourea derivatives has been described. of fumigation (1). As soon ventives and fumigants such as the Fumigant is removed, as camphor, tar, cedar, etc., have encouraged the development of preparations which or even if poisonous gases are partially replaced by air, the are intended to render the fiber actually mothproof-i. e., fibers become again susceptible to moth attack. According to Back (1) naphthalene and paradichloronot edible by larvae-and an intensive study of this method of moth control has been under way in the writers’ labora- benzene are effective only if used as directed by the U. S. e., in sufficient quantity tories for a number of years. A \vide variety of chemical Department of Agriculture-i. substances has been investigated and tested with varying de- and in very tight chests. grees of success but this paper will be confined i,o a descripThe new method of moth control, the so-called “mothprooftion of experiments with several classes of chemicals with ing” treatment, is so devised that chemicals incorporated with which definite results have been obtained and which seem the fibers by contact with mothproofing liquid will continue especially interesting. Specifically they are nitrogenous their activity after the liquid is removed and the fibers become compounds, urea derivatives, alkaloids, and fluorides. dry again. The life cycle of clothes moths is divided by entomologists Larvae Tests into four stages: (1) egg, (2) larva, (3) pupa, (4) adult. The larval stage is the only stage where the moths require food The method of determining the degree of protection and where all damage to our woolens is done. This stage is also by far the longest. The other three stages taken to- afforded by various chemicals must, in the writers’ opinion, gether occupy about a 6 weeks’ period in a moth life, while be carefully considered before passing judgment on the value the larva stage lasts a t least several months, and often a of mothproofing agent. The effectiveness of moth preventives of the mothproofing class is not based on liberayear or more. The young larva when just hatched from the egg is of tion of some volatile substance; most chemicals of this class almost microscopic dimension. It has a white, translucent are quite stable. There is nothing in this treatment to warn body, which quickly (probably in the first 24 hours) acquires and repel larvae from the treated fabric. The larvae must the color of the woolen fibers upon which it immediately actually try to feed upon the fibers to discover that the food begins to feed, the food being plainly visible throughout the is uneatable. I n most cases the larvae are not killed by this digestive tract. Most of the dyes on woolen fibers remain trial bite, but they stop eating and, if there is no better food intact in the digestive tract of the larvae and thus they appear available, they eventually starve to death. Naturally the young larvae require less of the poisoned food to paralyze to be colored with the dye of the fibers ( 7 ) . The larvae are very sensitive to light and do their best to their activity than the more mature larvae. Accordingly the avoid it. When hatched on the fabric of a garment they hide damage done by larvae is in direct proportion to their appethemselves and feed in the pockets, cuffs, and folds of the tite and in reverse proportion to the effectiveness of the garment. It is therefore extremely difficult to detect larvae treatment. Three varieties of insects living on animal fibers were emon the garment in time to prevent damage. The only safe method is to penetrate all the fibers of the garment with some ployed in these studies-the webbing clothes moth ( T i n e o l a substance injurious to larvae. Uniform penetration can be biselliella), the casemaking clothes moth ( T i n e a pellionella), accomplished either by gas treatment or by wetting the and the black carpet bettle (Altagenus piceus). These variefabric with a suitable liquid. ties were found to be approximately equally resistant to different chemicals-i. e., if a treatment was found insufficient 1 Received August 10, 1929.
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HE problem of destroy-
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