Aging of Stretched Rubber1 - American Chemical Society


and theBierer-Davis bomb methodsof accelerated aging are carried out on tensile strips which are not subjected to stresses. A large number of rubber a...
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INDUSTRIAL AND ENGINEERING CHEMISTRY

296

Vol. 20, No. 3

Aging of Stretched Rubber’ Arthur Kelly, Bert S. Taylor, and Webster N. Jones THE B. F. GOODRICH COMPANY, AXRON,OHIO

stock, a tube stock, a shoe upper, and a golf-ball thread rubber were used. The data on tread stock and shoe-upper stock are included in this report (Tables I and 11, and Figures 1 and 2). I n general, it was found that during the first few weeks deterioration was more rapid a t low elongation than a t high elongation. There was a reversal of this condition during the last weeks of the test; that is, the high elongations showed the greatest deterioration. This is probably due to the fact that when the samples under greater tension began to crack the cracks opened wide, exposing fresh surfaces to bright light, whereas the cracks in samples under weak tension were not pulled wide open and the fresh surfaces a t the bottom of the cracks were shadowed from direct sunlight. The more severe cracking a t low elongations during the early weeks of exposure was striking and the order of intensity as determined by visual inspection was confirmed by the tensile results. This was the principal result of the first set of experiments. Comparison of Sunlight with Geer and Bierer Aging Tests The next step on a larger scale was a comparison of stretched and unstretched rubber in sunlight with the same compounds aged unstretched in the Geer oven and in the Bierer bomb. Several classes of compounds were treated by these methods, special emphasis being placed on the testing of treads and tubes. Here the general facts were found to be the same as in the first series of experiments. As the cracks deepen the measured thickness of the strips is greater than the actual thickness of the uncracked rubber and the tensile strength in kilograms per square centimeter is apparently less than it is actually. The tensile results

H E function of an accelerated aging test is to approximate quickly the ability of stocks to withstand deterioration during storage and service. The Geer oven and the Bierer-Davis bomb methods of accelerated aging are carried out on tensile strips which are not subjected to stresses. A large number of rubber articles are, by virtue of their use, and sometimes under conditions of storage, subjected to stress. For some time investigations have been in progress on the effect of aging of stretched rubber in order to distinguish between the relative merits of various compounds which are subjected to stress during service. Since an article is made of rubber in order that it will have flexibility and permit deformation, some measure should be made of the relative deterioration of compounds under stress. Data have been collected in an endeavor to correlate aging in sunlight with other. accelerated aging tests on both unstretched and stretched samples. No direct relationship has been found between the aging in sunlight and the other methods, but a number of interesting facts have been discovered. Sunlight Aging Tests

T

The first step in sunlight aging of stretched rubber was to expose for several weeks during the summer months four compounds initially stretched to different degrees, periodically measuring the tensile strength of test strips. Obviously, the stress on the exposed samples became less as the stock took a permanent set under stretch. For this study a tread 1 Presented before the Division of Rubber Chemistry at the 74th Meeting of the American Chemical Society, Detroit, Mich., September 5 to 10, 1927.

Table I-Physical

D a t a o n Tread Stock-Sunlight

I

I-

Tens.

Weeks 0 2 4 6

8

10 12 14

18 20 22 24

Aging

INITIAL STRETCH

Elong.

Kg./sq. cm. 283.71 245.07 236.67 215.60 218.05 219.80 213.71 192.71 182.42 189.42 180.18 183.75 174.72

I

50%

Tens.

Elong.

Tens.

Elong.

%

Kg./sq. cm.

%

Kg./sq. cm.

646 590 553 567 537 570 550 540 500 517 510 517 487

241.43 221.06 203.42 210.29 191.45 181.93 164.71 176.12 177.73 179 62 174.44 182.21

580 540 525 540 500 470 500 477 507 520 490 483

211.47 188.58 187.67 203.91 212.94 114.66 106.89 145.04 113.05 88.41 109.76 125.58

Tens.

Elong.

1

Tens.

189.35 146.16 125.58 111.58 95.13 98.42 81.69 76.37 85.05 80.43 72.31 84.00

Elong.

%

Kg./sn. cm.

% 570 500 508 500 540 365 405 440 373 373 385 385

Table 11-Physical

I

Tens.

100% Elong.

Kg./sq. cm.

%

Tens.

Elong.

%

Kg./sq. cm.

510 460 500 420 440 403 400 380 410 393 380 365

D a t a o n S h o e Upper Stock-Sunlight

Aging

INITIAL STRETCH TIMEos AGING

Weeks 0 2 4 6 8 10 12 14 16 18 20 22 24

Y g . / s q . cm. 201.81 174.44 168.07 113.05 156.59 152.88 120.19 126.07 124.46 113.68 104.02 139.86 137.48

10%

5%

0%

Tens.

Elong.

% .. 520 497 467 462 440 477 463 440 435 420 470 477 480

Tens.

Elong.

K g . / s q . cm.

%

125.30 111.79 90.09 86.38 83.09 66.15 65.87 48.79 84.00 59.29 52.99 59.64

420 400 425 370 407 315 370 297 380 347 310 360

Tens. Cg./sq.

cm.

130.62 113.68 94.22 84.35 80.64 70.21 61.60 61.32 84.07 63.28 61.67 61.88

Tens.

%

Kg./sq. cm.

440 423 434 377 400 440 377 367 370 363 365 340

104.58 117.25 98.56 88.97 83.02 70.91 60.55 58.38 64.12 59.57 45.71 54.39

50 70

30 %

15%

Elong.

Elong.

Tens.

Elong.

Tens.

100%

Elong.

Tens.

Elong.

%

%

K g . / s q . cm.

%

K g . / s q . cm.

%

Kg./sq. cm.

413 420 442 370 400 357 340 345 340 350 310 300

143.99 127.19 110.32 99.68 88.83 94.64 60.62 55.58 58.17 59.57 52.50 37.38

420 430 450 367 417 360 350 363 357 345 330 290

151.06 131.95 117.32 99.54 95.97 74.62 51.31 48.44 26.46 20.51 20.79 21.70

440 427 450 370 417 357 327 240 160 157 159 140

147.00 420 116.41 417 108.71 434 81.83 360 55.23 305 21.28 _ _ ~. 165 8.61 260 All 100%strips were stretched broken ~~~

after 14 weeks’ exposure

INDUSTRIAL A N D ENGINEERING CHEMISTRY

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297

Table 111-Geer TIMEOR AGING

I

FIG.

1

series of tests. The b a t h - c a p stock hardened much more in sunlight and in the bomb than the other stocks. The bath cap mas a fast-curing, relatively high 1y compounded stock, and hence should tend to harden with age. Katural aging in boxes of several caps made of the same stock Droduced a hardening

ening as the other methods. (Table 111) No significant relation could be found between the results from the different types of aging tests in this series. Other Types of Aging Tests

The lack of significant data from the comparison of the aging of stretched rubber in sunlight with accelerated aging of unstretched samples in the dark suggested the examination I of stretched rubber in other types of aging tests. One compound only was tested. This was a rubber-sulfura c c e l e r a t o r mix in IOWEELS o r d e r t o study the effect on a comDound in which rubber was 2 wcrrr the principal ingredient. T h e s t r i p s , FlG.2 about l/32 inch (0.8 200, 300, and 400 per cent. The stretched samples' were placed in sunlight, in ultra-violet light (temperature 38 " to 40" C.), and in a Geer aging oven a t 70" C. The source of ultraviolet light was a quartz mercury arc placed 15 inches (38 cm.) from the samples. Samples from all methods were tested after 3 and 6 days' exposure. The curves in Figure 3 show the effect of sunlight on stretched rubber is to stiffen the compound, while ultra-violet softens it. The measurement of stress-strain relations showed profound differences between the effects of these two forms of accelerated aging, although the ultimate tensiles and elongations did not make a very clear distinction. The same series of tests shows that the oven test a t 70" C. causes stiffening. Extended cracking over the surface occurred in both the sunlight and ultra-violet light tests. In the Geer oven, however, the cracks were localized along the edges of the test strips. The minimum stretch used in these tests is greater than that corresponding to maximum cracking in sunlight aging. Further work must be done, therefore, to ascertain whether deterioration takes place at a maximum rate in oven, bomb, and ultra-violet light at a definite percentage of stretch.

Days Original 3 5 7 10 12 14

Oven Data

TREADSTOCK Tensile

Elong.

Kg./sq. cm. 283.71 243.74 207.76 156.17 150.57 123.41 99.12

% 646 558 542 492 417 397 35s

SHOEUPPER Tensile

Elong.

Kg./sq. cm. 183.68 174.86 137.48 93.59 82.11

550 50s 467 458 425

%

Sunlight aging is not satisfactory for quantitative work because the light intensity is never constant and the temperature and humidity of outside air vary. However, comparative tests against compounds of known quality are valuable if a compound is to be used outdoors. Often a compound which ages well in the oven and bomb tests will age poorly in sunlight. For other compounds the reverse is true. Method

Dumb-bell strips were stretched and tacked to boards a t the different elongations. The stretches were measured in the narrow portion of the strip where the break would occur, so that the measured portion was stretched the amount required. The strips were removed a t certain periods and the tensile and elongation determined on the basis of crosssectional areas, including the thickness of the cracked surface layer measured immediately prior to testing. For sunlight aging the boards were placed on racks on the roof a t an angle of 45 degrees facing southward. The Geer oven tests were run a t 70" C. The Bierer bomb tests were run in pure oxygen a t 70" C. under 300 pounds (20 atmospheres) pressure. Summary

1-Sunlight aging under tension of many compounds including the following has been investigated : tire tread, shoe upper, tube stocks, golf ball thread, jar rubber, solid tire, bathing cap stock, channel rubber. 2-With some of these stocks the sunlight aging has been

INDUSTRIAL AND ENGINEERING CHEMISTRY

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compared with unstretched samples by Geer oven, Bierer bomb, and ultra-violet light methods. 3-The stretching of the test strips accelerates deterioration in sunlight, ultra-violet light, and Geer oven. Stretched samples have not yet been tested in the Bierer bomb. 4-The rate of deterioration was not proportional to the degree of stretch in any of the stocks in the early stages of exposure. I n sunlight there is a critical elongation for each

Vol. 20, N o . 3

stock a t which the deter.ioration progresses more rapidly than a t any other in the early stages of aging. 5-No direct relationship was found between the results of sunlight aging and the other methods employed. 6-Stretched strips aged in ultra-violet light were found to give softer stress-strain curves than the unaged samples, whereas sunlight aging under the same conditions stiffens the stress-strain curve.

Activity of Certain Aryl-Substituted Biguanides as Accelerators of Vulcanization'" G . B. L. Smith and A. J. Weiss P O L Y T E C H N I C INSTITTJTk

OF BROOKI~YN, BROOKLYN, h-. Y.

S OXE chapter of a series of investigations in progress in this laboratory on the preparation and properties of aryl-substituted biguanides, their activity as accelerators of vulcanization of rubber was studied with the hope that some light might be thrown upon the relation between chemical constitution and accelerator action within this group of substances. Substituted biguanides, a-phenylbiguanide or their carbonates or carbamates, have been patented as vulcanization accelerators.8 Romani4 studied the activity of @-phenylbiguanide and its salts, and concluded that, in general, the mechanism of accelerator action of these compounds was yet to be explained. Recently work has been carried out in the systematic examination of groups of pure substances as accelerators of vulcanization in order to extend the existing theories of acceleration by studying the relation between activity and the chemical constitution of the substituted radicals. The work on mercapto-ben~othiazoles,~ substituted guanidinesj6 and diary1 thioureas and diarylg~anidines~ are contributions to this subject. These studies have shown that activity increases, in general, with increase in molecular weight in homologous series and, among tolyl isomers, decreases from ortho t o para derivatives in the case of the thioureas and from para to ortho derivatives in the case of the guanidines. Electropositive groups increase and electronegative groups decrease activity. Compounds must be basic but their activity by no means varies with basicity.

A

Compounds Studied

This paper presents experimental data on the activity of several aryl-substituted biguanides8 as accelerators of vulcanization. I n all cases the arylbiguanide base was used. Biguanide is an ammono-carbonic acid and bears the same relation to guanidine that biuret does to urea-i. e., it is 1 Received

September 15, 1927. This paper is an abstract of part of the thesis submitted by Mr. Weiss in partial fulfilment of the requirements for the degree of bachelor of science in chemistry at the Polytechnic Institute of Brooklyn in June, 1927. The vulcanization tests were carried out in the laboratory of the Metal Hose and Tubing Company of Brooklyn, and the authors take pleasure in expressing their appreciation to Robert Berkowitz, chief chemist, for his kindness in placing the facilities of this laboratory at their disposal. 1 British Patent 201,912 (July 27, 1923). 1 Caoutchouc 6; gutta-percha, PO, 12005 (1923). , 6 Sebrell and Boord, I n d . Eng. Chcm., 16, 1009 (1923). 6 Ellery and Powers, India Rubber World, 76, 3 (1926). 7 Naunton, J . Soc. Ckem. I n d . , 44, 549T (1925). 8 The preparation and properties of these compounds and their derivatives will be described in detail in subsequent articles. See also Smolka and Friedrich, Monatsh., 9, 230 (1888); Lumiere and Perin, Bull. soc. ckim., [3] 98, 206 (1005); Enrich, Monotsk., l P , 20 (1891); Cohn, J . p r a k l . Ckcm.. 84,394 (1911). 2

guanylguanidine. The aryl-substituted biguanides are ammono estersQand were prepared by the ammonation of dicyandiamide (cyanoguanidine)'O by arylamine salts. The following equation represents the reaction: NH2.C:NH.NH.C N R NH2 HC1 = NH2.C:NH.NH.C: NH.NH R HC1 The base was obtained by treatment of the salt with an alkali, generally sodium hydroxide. Benzoxazoleguanidine was included in this study because of its close structural relationship to the biguanides and also to the benzothiazoles. The hydrochloride was prepared by the reaction between dicyanamide and o-aminophenol hydrochloride:

+

The base was obtained by treating the salt with an alkali. Experimental

A selected batch of smoked sheet was broken downon a warm mill for 15 minutes and zinc oxide and sulfur were added to form the following mix: smoked sheet 100, zinc oxide 100, and sulfur 10 parts. One part of accelerator, on basis of rubber content, was incorporated into this stock. After allowing to stand for 24 hours, the uncured stocks were weighed into aluminum molds and cured in slab form at 302" F. (150' C.). The cured sheets were cooled in water immediately upon removal from the press vulcanizer. Dumb-bell test specimens were cut out and tested 24 hours after curing according to Bureau of Standards specifications.11 Four to six specimens from each cure were tested, and in the majority of cases results were obtained from two or more individual mixes made with different preparations of the same compound. The compounds tested were compared with stocks containing no accelerator, and with stocks containing diphenylguanidine, hexamethylenetetramine, thiocarbanilide, and dicyandiamide. Table I records the tensile strength at break and the elongation. The results are shown graphically in Figure 1. The activity of a-o-tolylbiguanide was further investigated in several typical commercial rubber stocks in order to obtain Franklin, J. A m . Ckcm. Soc., 44, 492 (1922). Obtained from the American Cyanamid Company. 11 Bur. Siandards, Circ. 98, p. 48. 9

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