INDUSTRIAL A N D ENGINEERING CHEMISTRY
300 Table IV-Black MATERIAL Smoked sheets Whole-tire reclaim Litharge Clay Carbon black Zinc oxide Sulfur Diphenylguanidine a-o-Tolylbiguanide TOTAL TIMEOF MIX CURE
Heel Stocks A B 22 22 40.5 40.5 1 1 20 20 8 8 5 5 2.5 2.5
A B
C
15 20 30 45 5 10 15 20 30 5 10 15 20 30
(2.8
1 --
99.0
100
100
C
KG.) STEAM
108 108 114 104 149 165 163 151 114 150 166 161 151 123
CURES AT 20 LBS. (1.4
A B
8
5 2.5
..1 --
TENSILE STRENGTH LBS.
22 40.5 1 20
.. .. -
Minutes Kg./sq. cm. Lbs./sq. in. CURES AT 40
C
KG.)
..
ELONGATION Per cent
PRESSURE
1532 1543 1624 1489 2126 2343 2328 2045 1625 2138 2360 2303 2149 1748
525 500 469 378 507 475 400 350 238 500 463 425 366 225
STEAM PRESSURE
No cures made 15 30 45 60 15 30 45 60
129 139 136 122 140 153 160 136
1835 1975 1938 1740 1993 2177 2136 1928
525 463 416 375 497 475 441 400
Since dicyandiamide was the substance from which these compounds were prepared, its acceleration properties were tested and confirmed.12 Benzoxazoleguanidine was the least 1:
British Patent 201,570 (July 27, 1923).
Vol. 20, No. 3
active of the compounds tested, probably owing to the presence of the electronegative oxygen in the molecule. I n some preliminary experiments it was found that, in the absence of zinc oxide as an activator, biguanides are very slow accelerators, as might be predicted from their close structural relation to the diarylguanidines. I n the few commercial stocks tested, a-o-tolylbiguanide was found to possess the properties of a fairly good accelerator of the moderate type. It was non-toxic and milled into the batch in about the same manner as diphenylguanidine. It did not show such great activity in the tread stock as diphenylguanidine, but in the friction and black heel stocks its activity was comparable to diphenylguanidine. Possibly the use of litharge enhanced the acceleration in the black heel stock. Extension of Work
The authors hope that further studies of the activity as accelerators of vulcanization of these and other biguanides and closely related compounds will add materially to our knowledge of the mechanism of rubber vulcanization acceleration and contemplate an extension of this work in the near future. Conclusion
1-Aryl-substituted biguanides are rubber vulcanization accelerators of moderate activity. 2-The relation between the chemical constitution of the substituted radicals and the activity of the biguanides is, in general, about the Same as found by other investigators for other types of accelerators. 3-a-o-Tolylbiguanide has a fairly great activity in certain commercial rubber stocks.
Effects of Ozone on Stretched Rubber’ F. H. Haushalter, Webster N. Jones, and J. W. Schade Tag B. F. GOODRICH COMPANY, AKRON,OHIO
I
T HAS been found that samples of vulcanized soft rubber
under slight tension exposed to direct sunlight crack more severely than samples similarly exposed under somewhat greater tensions.2 It has also been discovered that similar rubber compositions when stretched even a small amount in an atmosphere containing ozone (as low as 0.1 per cent) crack noticeably in a short time-a matter of seconds or minutes compared with a period of weeks to produce a n equal degree of cracking in sunlight. As in sunlight exposure during its early stages, the degree of cracking in ozone decreases as the tension is increased beyond a definite value for each stock. Methods of Bringing Ozone in Contact with Rubber
’
I n the course of the experiments ozone was brought in contact with the rubber by two distinct methods. METHOD I-By the first method 6 by 8 inch (15 by 20 cm.) sheets, 3/a2 inch (2.4 mm.) thick, were held a t each end by a steel clamp; one clamp was fixed to a horizontal table, while loads were applied to the other clamp. A flat metal plate was placed under the sheet of rubber and a steel rod l / 4 inch (6 mm.) in diameter was laid on the top side. The I Presented by F. H. Haushalter and W. N. Jones under the title “Effect of Ozonized Oxygen on Stretched Rubber” before the Division of Rubber Chemistry at the 74th Meeting of the American Chemical Society, Detroit, Mich., September 5 to 10, 1927. 2 Kelly, Taylor, and Jones, I n d . E%& Chem., 20, 296 (1928).
metal plate and rod were connected to the high-voltage terminals of a transformer. The voltage across the rubber sheet could be increased from 0 to 60,000 volts by means of a rheostat in the low-voltage circuit of the transformer. At 10,000 volts the oxygen of the air in the region of the i%p electrode (steel rod) was broken down and ozone produced. By accurately controlling the voltage and time of exposure, by working under constant atmosphere conditions, and by guarding the apparatus from air currents, very consistent results were obtained upon repetition of the tests, Dumbbell test sections were cut from the treated sheets so that the cracked portion was in the constricted section. The effect was then determined. METHOD 11-The second method was by the use of an ozone machine made by the United States Ozone Company. Oxygen was bubbled through sulfuric acid into the ozonizer, where it was broken down by a voltage of 10,000. The ozonized oxygen then passed into a sealed box with a glass top. This box was 12 by 18 inches (30 by 45 cm.), and 12 inches (30 cm.) high, large enough to hold a special clamping device for stretching the samples of rubber several hundred per cent. The ozone entered one end of this box through three tubes; three small vent holes a t the opposite end permitted movement of the gas through the box. Dumb-bell strips of rubber were tested a t various percentages of stretch by this means under as uniform conditions of ozone concentration as could be maintained. Ozone concentrations in
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1928
the box Fere checked a t various times by potassium iodide titration. This concentration was found to be about 0.1 per cent. After considerable work by both methods the method of using high voltage in air proved preferable. '-4-7 1
\ I , :
--+z
1, : !
5
-11 PuAf G U M V U L C A N I Z E D
10
%
3'TREr,"2",N
0s.L
30
301
machine in which the load was applied by means of a long calibrated coil spring. Near the top of the spring is a clamping nut, by which its length may be changed according to the cross-sectional area of the test strip. All curves are thus drawn autographically to the same scale. The effect of the ozone treatment is shown on curves sheets 1 and 2. Comparison of the curves for various rubber compositions indicates that the maximum cracking takes place more nearly a t a definite tension than at a definite elongation. It is interesting to note, however, from comparison of the data for pure gum and tread stocks (curve sheets 1, 2, 3, and 4) that changes in the degree of cracking and in hardness of the stretched stock seem to bear some relation to each other and to vary more nearly according to the amount of stretch than to the applied tension. The relative tension may, however, be closer in agreement with changes in hardness than the data indicate, because during the minute of exposure there is a decrease in the stresses due somewhat to the usual release in stretched rubber but more to that resulting from the cracking of the sample. I n all cases the tensile figures checked the intensity of cracking as determined by visual inspection. The order in which the samples were placed on the basis of inspection was the same as that determined on the basis of tensile tests.
35
3.
Figure 1
Tensile Tests Where accurate comparisons were to be made on different stocks, two or three samples were clamped together in parallel in the clamping device, stretched a like amount, and subjected to ozone for a definite time, usually 1 minute. In determining the degree of stretch required to produce the maximum deterioration, samples were clamped together in parallel in the clamping device, one sample being stretched 2.5 per cent and the other 5 per cent. The next test would be made on one sample stretched 5 per cent and its companion 7.5 per cent; and so on. A f t e r exposure to ozone, strips were broken on the S c o t t r u b b e r tester. All t e n s i l e values were calculated back to those for uns t r e t c h e d s a m p l e s as a standard. The following machine tensiles and values calculated to the standard Figure 2 illustrate the method:
'
MACHINE VALUE?, STRETCHExpt. 1 Expt. 2 Expt. 3 Per cent Kg. per sq. cn.
CORRECTED VALUES Expt. 1 Expt. 2 Expt. 3
Kg.per sq. cm.
That is, the corrected figure for 5 per cent is 111.2/105.6 times the machine value of 40, and the corrected figure for 7.5 per cent is 42.2/42.8 times the machine value of 47.2. This method seemed to compensate for any slight changes in ozone concentration which may have occurred. The stressstrain relations (curve sheet 5) used for translating elongation values to tension values were plotted autographically on a
55
a
20
x 5 ~ n c . r 200 ~~
W STRETCH 70 40
300
50
I 400
Figure 3
Hardness Tests In measuring the hardness, a strip of rubber 1 inch wide, 5 inches long, and 0.25 inch thick (2.5 by 12.5 by 0.6 cm.) was clamped in the jaws of a tensile testing machine. The sample was stretched definite amounts-such as 2.5, 5.0, 7.5, and 10.0 per cent-the load recorded, and the hardness obtained by means of the Shore durometer. The sample
was given a firm backing each time the instrument was applied. Sheets 0.6 cm. thick Tyere used to avoid excessive thinning of the sample when stretched. The results indicate that the rubber does not at once increase in hardness as tension is applied; also, the tread stock (the stiffer of the two) begins to increase in hardness a t a lower value of stretch than the pure gum stock. The study indicates that in stretched samples there is a critical point at which the rubber is exceedingly sensitive to the oxidizing effect of ozone. As the hardness of the stock is increased by greater tension the effect of ozone is correspondingly diminished. This cracking does not occur in unstretched samples. Effect of Ozone on Unstretched Rubber A large number of compounds of various kinds have been subjected, unstretched, to higher concentrations of ozone,
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
302
Vel. 20, No. 3
iBr
20
40 STKE.55 I N KGS./SQ.
CM.
60
Figure 4
Figure 5
ranging from 2 to 5 per cent for 18 hours without showing any noticeable decrease in tensile strength. For example, dumb-bell test strips of linemen’s protector shield stock were exposed, unstretched, a t room temperature to ozonized oxygen which contained 0.5 to 1.0 per cent ozone. The strips were 3/32 inch (2.4 mm.) thick and the test section 0.25 inch (0.6 cm.) wide. The lack of change is indicated in the following table:
Strips cut from smoked sheet as received from the plantations exhibited maximum cracking a t 30 per cent elongation as determined by visual inspection.
DURATION
TENSILE STRENGTR
ELONGATION
Conclusion
The similarity of the results obtained under various tensions in sunlight and in an atmosphere containing ozone is striking and may be significant of some progressive change in the structure of the rubber which renders it a t a certain point unusually susceptible. The study here outlined is incomplete, but is presented in the hope that it may give other investigators an added method of studying problems connected with the structure or with the oxidation of rubber.
Analysis of a Typical Angle Abrasion Machine’ w. w. vogt THBGOODY$AR TIRS& RUBBERCOMPANY, AKRON, OHIO
The machine employed consists of a driven abrasive in the prerious tests they wheel, the rubber test piece in ring form being pressed would have been found. Acwriter has attempted . to evaluate the power against the flat face of the wheel in such a manner cordingly a machine was conthat the plane of the ring makes an angle with the structed of the same type as consumed in causing abrasion tangent of the abrasive wheel at the spot of contact. used in the previous tests, but of rubber. Two separate inProvision is made for measuring the power consumed so arranged that the power vestigations were made; in in abrasion by a Prony brake method. This angle maconsumption could be measboth cases the abrasion machine is essentially a constant-power machine. The ured directly by the Prony chine used was of the angle relationships between abrasion loss and time, load, b r a k e p r i n c i p l e used by type, wherein the periphery speed, angle, power consumption, etc., are given. An Williams, of a rubber ring was pressed analysis of the forces involved is also given. against the flat surface of an Description of Machine abrasive wheel in such fashion The essential features of the machine are shown in Figure 1. that the plane of the ring was perpendicular to the face of the abrasive wheel and set a t an angle to the tangent of the abrasive The test piece, A , is mounted on the freely running bearing, wheel a t the point of contact. The power and energy figures B , which can be set so that the plane of the ring can make were obtained from readings of the current supplied to the elec- any angle (0 to 90 degrees) with the tangent of the abrasive tric motor used to drive the abrasive wheel. On both occa- wheel at the point of contact of the ring. The point of consions it was found that, for a wide variety of practical tread tact is coincident with the axis about which the holder swings. stocks, the differences in power consumption were less than the The support arms can rotate freely on ball bearings around experimental error (which was not more than 5 per cent), the shaft, C, of the turn table on which the abrasive wheel, D, even though the abrasion resistance of the stocks in question is mounted. This is an alundum wheel, 36 grain. 9flexible varied more than twofold. It was concluded that as a prac- strap. E , is attached to an arc of proper radius, F , and the readtical proposition the energy concept of abrasion was of little ings of the force are obtained from the spring balance, G, reading directly to pound.3 The minimum load on the test importance in differentiating various stocks. Recently Williams2 showed differences between stocks of rings is, of course, the weight of the attachment (6.72 kg. such magnitude that it was believed that had they occurred or 14.8 lbs.) and this can be increased by adding auxiliary weight W . The mechanism for rotating the abrasive wheel 1 Presented before the Division of Rubber Chemistry at the 74th and air jets for cleaning its surface are not shown. Meeting of the American Chemical Society, Detroit, Mich., September 5 to
A
T VARIOUS times the
10, 1927.
* I n d . Eng. Chcm., 19,
674 (1927).
8 All readings of force were taken in English units, but have been converted to the metric system to conform with the publisher’s policy.
