-ELASTOMERS-Ozone type or which will not crack under special service conditions, W-ork of this type is aided considerably by the examination of prints of samples exposed to ozone a t a variety of temperatures.
Cracking
(3) Ibid., P. 769 (1946). (4) Kearaleyi Age ( N . ‘)*I
27 (lg3’). (5) Powell and Gough, Trans. Znst. Rubber Znd., 21, 102 (1945) (6) Shephard, Krall, and Morris, IND.ENQ.CHEM.,18,615 (1926)
(7) U. S. Rubber Co., Passaic, N. J., private communication. (8) Williams, INB. ENG.CHEM.,18, 367 (1926).
GITERATURE CITED
(1) Ball, Youmans, and Rauaell, Rubber Age ( X Y . ) ,54,481 (1944). (2) Crabtree and Kemp., IND.ENQ.CHEM.,38, 289 (1946).
RECEIVED for review September 17, 1951.
ACCEPTED February 4 , 1962.
OZONE AND SUNLIGHT EFFECT ON AGING O F CARBON BLACK VULCANIZATES GEORGE E. POPP AND LYNN HARBISON Phillips Chemical Co., Akron, Ohio T h e economy of unwrapped rubber products, induced by material and manpower shortages during World War 11, provided an incentive in postwar years to continuation of this practice by rubber manufacturers. As high finished-product inventories accumulated, the rubber technician became confronted with a cracking condition created by ozone attack when a surface of the unwrapped product was exposed under stress. This situation plus the advent of the synthetic butadiene-styrene polymers and high structure furnace-type blacks, which were dereloped during the past decade, precipitated several investigations to determine relative ozone resistance characteristics. This study was initiated to determine the effects, if any, of individual types of carbon blacks on checking and ozone cracking of natural and synthetic rubber under static testing conditions. This work repre-
sents sunlight and ozone exposure (static) testing of natural crude and low temperature polymer with and without antioxidant in typical tire tread vulcanizates. A comparison of HAF, EPC, MAF, HMF, and SRF types of black is made. Specimens were exposed to sunlight aging in Akron, Ohio, during April and May of 1950 under 12’/2’70 strain, and an additional series of specimens were simultaneously tested with a 180 degree bend. Germicidal lamps enclosed in a specially prepared cabinet were used for the ozone investigation, and a like series of specimens were exposed for comparison with the weather aging results. The results show that carbon blacks, regardless of type, particle size, structure, and physical properties imparted, do not affect the rate or degree of cracking in the polymers studied upon exposure to either sunlight or ozone when under static stress.
T
existent sun checking problem, began to explore the behavior of the synthetic butadiene-styrene polymers and the new type of furnace blacks that were coming into widespread use. It was theorized by some mbber technicians that these new furnace blacks, with their high modulus properties, were conducive t o greater checking and ozone cracking characteristics as compared to conventional gas blacks which exhibit lower modulus properties. Hence, this investigation was initiated approximately one year ago to study the effects, if any, of various types of carbon blacks on checking and ozone cracking of natural and synthetic rubber vulcanizates under static testing conditions.
HE effect of both natural and artificial sunlight aging and
.
ozone exposure on carbon black-loaded rubber vulcanizates, as used in tire treads, tire sidewalls, and various mechanical rubber goods products, is deleterious. Exposure to sunlight and ozone accelerates the deterioration of the physical properties of the rubber compound, and checking and cracking develop. Williams ( B ) , while studying the oxidation of rubber exposed t o light, was probably the first to point out the effect of ozone on rubber. Van Rossem and Talen (6) found that the presence of light is not essential in the formation of cracks in a vulcanizate. Depew ( 2 ) studied the surface deterioration of rubber in sunlight under varied mechanical stresses. Norton (4),in his investigation “The Action of Ozone on Rubber and Other Materials,” found that in order to specify the life of a rubber composition, the mechanical stress as well as the ozone concentration in the surrounding atmosphere must be known. Nellen et al. ( 3 ) found that the degree of deterioration of tires in storage from ozone kttack varies according to the geographic location, the construction material and ventilation of storage facilities, subjected stress during storage, and product covering. During World War I1 shortages of paper and manpower resulted in practically all tires being marketed unRrapped. With the low tire inventories that existed and the rapid turnover, aging problems were minimized. Since considerable savings were effected thereby, manufacturers continued the practice of omitting wrappings on tires long after the war. As the pent-up d e mand became satisfied and inventories began to build up, cracking became evident in unwrapped storage. Rubber technicians, confronted with this cracking condition in addition to the everApril 1952
METHOD
Typical tire tread compounds of both natural rubber and low temperature polymer X-478,4loF. with and without antioxidants employing HAF, EPC, MAF, HMF, and S R F types of blacks were studied. Samples 0.5 inch wide by 6 inches long were prepared from ASTM tensile test sheets and were subjected to ozone and sunlight aging while under an elongation of 1Z1/2%. Another series of samples of like dimensions were simultaneously tested while under a 180 degree bend. The specimens were subjected to outdoor exposure for 32 days in the instance of low temperature polymer and 63 days for natural rubber vulcanizates. Weather aging tests were conducted in Akron, Ohio, during the months of April and May, 1950. I n the case of ozone aging, the specimens were prepared and arranged in the same manner as the weather aged specimens. The specimens for ozone aging were placed in an enclosed box similar to that developed by Baruth ( 1 ) in his
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
837
ELASTOMERS-Ozone
Cracking-
272829 3031
27 8 8 2 9 3 0 3 1
A
27 28 2 9 3 0 3 1
B
38 39 40 41 42
A
B
C
27 28 29 30 31
Compound
27 28 29 30 31
B
C
Figure 2. Cracking of Natural Rubber after 132-Hour Exposure t o Ultraviolet Lamps A. Cured for 40 m i n u t e s B. Cured for 60 m i n u t e s C. Cured for 90 m i n u t e s 27-31. See Figure 1 .
study pertaining to discoloration of white sidewall compounds and the generated ozone effect by ultraviolet light. Exposure time was 8 to 12 hours, in the instance of low temperature polymer compounds, and 132 hours for the natural rubber compoundP. The distance between the surfaces of the specimens and the lamps was 7 inches.
Accelerator Suliur Santoflex AW Santoflex BX Heliozone
838
Cured for 40 m i n u t e s Cured f o r 60 m i n u t e s Cured for 90 m i n u t e s
38 ( M A F )
39 (HAF)
40 (EPC)
0.50 2.50 1.50
0.50 2.50 1.50 1.00
0.65 2.70 1.50 1.00
1.00
..
..
41 (11.41~) 42 ( M A F ) 0.76 1.75 1.50
0.60
2.50 1.50 1 .oo 1.00
..
1.00 1.00
Observations show only slight checking and equality in the degree of checking on the vulcanizates after 63 days of exposure. Figlire 4 compares the same vulcanizates as Figure 3. Again, only slight to mild checking was detected, to an approximately equal degree when the vulcanizates had been exposed to ultraviolet light for 132 hours. The resistance of low temperature polymer compounds to ozone was next investigated as shown by the Figures 5 to 13,inclusive. Cure for all of the samples was a t 280" F. The various results obtained are briefly analyzed. Figure 5 is a comparison of the five types of blacks under investigation in low temperature polymer X-478 a t 50-part loadings without added antioxidant (above that already contained in the polymer as a stabilizer). The degree of checking is pronounced in all specimens shown in this figure and equal for the different types of blacks when exposed for 32 days to outdoor exposure. Figure 6 compares the same vulcanizates as in Figure 5. The distinguishing effect of ozone cracking is apparent in this picture; a slight superiority is shown
Table I.
Natural Rubber Physical Test Data
RESULTS
Table I shows the stress-strain properties and Shore hardnesses of the five different carbon blacks in natural rubber, Table 11 shows the stress-strain properties and Shore hardnessea of the five different carbon blacks in low temperature polymer. The subsequent figures show the effect of outdoor and ozone exposure when a number of commercially available antioxidants md inhibitors are incorporated in a compound. Talc was rubbed into the cracks of the exposed rubber samples to obtain a better pictorial effect. Figure 1 shows five natural rubber vulcanizates containing 50 parts each of different types of carbon black as loading after 63 days of weather exposure. Cure time was a t 280" F. and all compounds contained 1 part of Thermoflex A, Checking was slight and equal for all the specimens. Figure 2 reflects the same compound comparisons a3 in Figure 1. In this instance, also, only slight checking was detected, and an approximately equal degree of checking was found on the five vulcanhates after 132 hours of exposure to ultraviolet ozone generation. There is indication of a slight superiority in the RMF black when under stress; however, the 180-degree-bend samples show that the talcing of the stressed specimens may have influenced their photographic reproduction. Figure 3 compares the HAF, EPC, and X4F types of blacks in natural rubber a t a 50-part loading (280" E'. cure) with various antioxidant and inhibitor combinations, and a lower sulfur content, with acceleration adjustment for the sulfur reduction. The formulas used were as follows:
.
Figure 3. Cracking of Natural Rubber after 63-Day Exposure to Natural Sunlight A. B. C.
A. Cured 40 m i n u t e s B. Cured 60 m i n u t e s C. Cured 90 m i n u t e s 27. MAP 28. HAF 29. EPC 30. S R F 31. HMF
A
38 39 40 41 42
C
Figure 1. Cracking of Natural Rubber after 63-Day Exposure to Natural Sunlight
2 7 2 8 29 3 0 3 1
38 39 4 0 41 42
Smoked sheets H A F black, Philblack 0 EPC black, Wyex M A F black, Philblwk A H M F black, Statex 93 S R F black Zinc oxide Stearic acid Softener Acceleratora Sulfur Antioxidant b
Recipes b y Carbon Black Type, Parts WAF EPC MAF HMF SRF 1LOO.00 100.00 100.00 100.00 100,00 60.00 ... ... ... 50: 00 ... ...
...
... .
I
.
...
5.00 3 IO0 3.00 0.50 2.50 1.00
... ... ...
5.00 3.00 3.00 0.65 2.70 1 .oo
io:00 ... ...
5.00 3.00 3.00 0.50 2.50 1 .oo
iJ0:oo
...
5.00 3.00 3.00 0.50 2.50 1.00
Physical Test Data Stress Strain (Original) Minutes Cure at 280° F. 300% Modulus, Lb./Sq. Inch 10 980 400 1070 430 20 1600 1120 1780 1000 30 2060 1580 2150 1310 40 2170 1670 2250 1580 60 2170 2230 1830 1620 90 1950 1650 2180 1580 Tensile at Break, Lb./Sq. Inch 1410 3480 2800 840 10 20 4230 4230 3910 3760 4540 4020 3880 30 4530 40 4460 4480 3820 3680 3720 3610 4300 3920 60 4220 3670 3660 3890 90 Ultimate Elongation, % . 565 40 540 605 470 465 540 60 555 565 480 555 90 500 585 Shore Hardness 64 63 62 58 40 67 67 65 63 60 66 66 63 62 90 Santocure. b Thermoflex.
-
INDUSTRIAL AND ENGINEERING CHEMISTRY
...
50: 00
5.00 3.00 3.00
0.50 2.50 1.00
270 1080 1410 1470 1420 1270 900 3900 3840 3840 3080 3320 580 570 550 58 61
60
Vol. 44, No. 4
-ELASTOMERS-Ozone 38 39 40 41 42
Cracking
-
38 39 40 41 42
38 39 40 41 42
B
C
Table 11. X-478 Cold Rubber Physical Test Data $-
Recipes by Carbon Black Type, Parts HAF EPC MAF HMF SRF GR-S X-478 HAF black, Philblaok 0
EPC black Wyex
MAF blacd, Philblack A H M F black, Ststex 93 SRF black Zinc oxide Stearic acid Softener Acceleratora Sulfur
1oo.00 50.00
... ,,. ...
...
5.00
2.00
10.00 1.20 1.76
100.00 50: 00
...
... ...
5.00
2.00
10.00 1.50 1.75
100.00
...
100.00
... ...
50: 00
... ...
50: 00
...
5.00 2.00 10.00
5.00 2.00 10.00 1.20 1.75
1.20 1.76
100.00
...
... ...
5o:oo
5.00 2.00 10.00 1.20 1.76
Physical Test Data Stress Strain (Original) Minutes Cure at 292O F. 300% Modulus, Lb./Sq. Inch
40 60 90
1370 1560 1650
620 860 1010
40 60 90
3280 3480 3660
3100 3400 3280
40 60 90
560 550 556
770 705 646
1660 1760 1760
860 1000 1030
A
Figure 4. Cracking of Natural Rubber after 132-Hour Exposure to Ultraviolet Lamps A.
620 730 740
Cured for 40 m i n u t e s B. Cured for 60 m i n u t e s C. Cured for 90 m i n u t e s
Tensile'at Break, Lb./Sq. Inoh
2470 2830 2720
2470 2600 2740
2160 2360 2020
Ultimate Elongation, %
445 496 465
680 635 625
740 725
645
Shore Hardness
a
.
40 ~.
64 ..
61 ..
fi4 _-
60 90
66 66
63 63
64 64
57
as 59
55 ..
56 66
Bantocure
by the HAF and E P C blacks when exposed to the ultraviolet light generated, and the number and depth of checks and/or cracks are compared for all cures in the stressed and 180 degree bend specimens. I n both Figures 5 and 6, the five samples on the left were cured for 40 minutes, the five in the center for 60 minutes, and those on the right for 90 minutes. Figures 5 and 6 indicate that the degree of checking was equal for the various t.ypes of blacks, regardless of modulus and tensile imparted. Hence, it may be assumed that carbon blacks within these varying particle sizes and structures do not affect the rate or degree of checking of vulcanizates when exposed to ozone or weather aging. Figures 7 through 10 compare vulcanizates containing HAF, EPC, and MAF types of blacks with different types of ozone and sunlight inhibitors. In this comparison there is no significant difference in the effect that the blacks have on either ozone or weather checking. Analyses of the effects of nickel dibutyldithiocarbamate (NBC), a mixture of waxy materials (Heliozone), a p-phenetidin derivative (Santoflex AW), and a diphenylar.ine product (BLE resin), show their sequential rating to be: NBC, best, only slight surface checking; Santoflex AW, second best, mild checking and slight ozone cracking; and Heliozone and BLE, third best, approximately equal for checking and cracking. It is also noted that the ozone and atmospheric effect is pronounced in these vulcanizates. In Figures 7 and 8, compounds 12, 13, and 14, which contained MAF, HhF, and E P C carbon black, respectively, had 2.0 parts of NBC added. Compounds 15, 16, and 17 (MAF, HAF, and EPC, respectively) had 1.0 part of Heliozone added. I n Figures 9 and 10,compounds 18 (MAF), 19(HAF), and 2O(EPC) contained 2.0 parts of Santoflex A, while compounds 21(MAF), 22(HAF), and 23(EPC) contained 2.0 parts of BLE. The six samples on the left in Figures 7 through 10 were cured 40 minutes, the six in the center for 60 minutes, and those on the right for 90 minutes. The results of ozone exposure on X-478 polymer with no added antioxidant as shown in Figures 5 and 6 when compared to Figures 7, 8, 9, and 10, which have added antioxidants, suggest that an antioxidant would be desirable and helpful in minimizing the degree of zone and sunlight cracking and checking. Obviously, it is desirable to apply the most reinforcing medium in the sidewall of a tire consistent with economic considerations, The abrasion resistance of MAF black, among numerous other merits,
April 1952
7 8 91011
7
8 9
7 8 9
1011
1011
Figure 3. Cracking of Low Temperature Polymer, X-478, after 32-Day Exposure to Natural Sunlight 7. 8.
MAP HA€
9. EPC
10. S R F 11. H M F
7 8 9 1011 7 8 9 10 1 1 7 s 9 10 11 Figure 6. Cracking of Low Temperature Polymer, X-478, after 8-Hour Exposure to Ultraviolet Lamps 1. MAF 8. HAF 9. EPC
12 13 14 15 16 17
10. S R F 11.
HMF
12 13 14 15 16 17 12 13 14 15 16 17
Figure 7. Cracking of Low Temperature Polymer, X-478, after 32-Day Exposure to Natural Sunlight
12 13 14 I d 16 17 12 13 14 15 16 1 7 12 13 14 15 16 17
Figure 8. Cracking of Low Temperature Polymer, X-478, after 12-Hour Exposure to Ultraviolet Lamps
INDUSTRIAL AND ENGINEERING CHEMISTRY
839
--ELASTOMERS-Ozone
Cracking
18 19 20 2 1 22 2 3
Figure 9. Cracking of Low Temperature Polymer, X-478,after 32-Day Exposure to Natural Sunlight
7
24
25
7
26
26
24
26
7
24
25
18 19 20 21 22 23
18 19 2 0 2 1 2 2 23
Figure 10. Cracking of Low Temperature Polymer, X-478, after 12-Hour Exposure to Ultraviolet Lamps
26
7
24 2 5
7 24 25 26
26
7 24 25
26
Figure 1 1 . Cracking of Low Temperature Polymer, X-478,after 32-Day Exposure to Natural Sunlight
Figure 12. Cracking of Low Temperature Polymer, X-478,after 12-Hour Exposure to Ultraviolet Lamps
particularly adapts itself to uje in this instance; therefore, a l I A F black series with recipe variations, developed from the aforementioned observations, was inaugurated. Figure 11 reflects four M X F vulcanizates which employ the principle of no added antioxidant (exclusive of that used as a polymer stabilizer), reduced sulfur ratio, reduced sulfur ratio with antioxidant added, and reduced sulfur with antioxidant and wax combined in LTP X-478. RespecLive progressive improvement was observed when the specimens were exposed for 32 days in natural sunlight. Figure 12 compares the same vulcanizates as in Figure 11 and the same relative improvement exists when specimens are exposed to 12 hours under the ultraviolet lamp. In both Figures 11 and 12, cure time for the four compounds on the left as 40 minutes, for the four in center 60 minutes, and for those on the right 90 minutes In the following table ie shown the parts of sulfur, accelerator, and antioxidant added to these vulcanizates
second phnse to low temperatuie polymer compounds Although larger amounts of ozone inhibitors were employed in the IOTV temperature polymer, the natural rubber compounds resisted ozone better and consequently were exposed for longer periods before cracking became evident. These observations indicate that natural rubber compounds resist outdoor and ozone exposure better than low temperature polymer compounds when tested under static conditions.
Compound Accelerator Sulfur Santoflex AW Heliozcne
25 1.35 1 40 1.50
24
I
1.20 Li5
1,35 1.40
..
..
..
..
..
CONCLUSION
Carbon black, regardless of type, particle size, structure, and physical properties imparted does not affect the rate or degree of checking or cracking in natural rubber or low temperature polymer compounds when subjected t o weather or ozone exposure. Natural rubber will withstand much longer periods of exposure than the synthetic polymer studied. A pronounced degree of ozone and Feather cracking and checking will result if no antioxidant is compounded into the synthetic polymer. An MAF black-synthetic polymer compound may be substantially improved in its resistance to ozone and weather resistance by selection and application of the proper antichecking ingredients.
26 1.35 1.40 1.50 1.00
Over 400 test specimens were exposed during this investigation and the grading comparison was by visual, as well as pictorial means. The ratings for specimens shown in Figures 11 and 12, for example, would be as fo1lopv-s: Sunlight Exposure 12.5%
Cure at 292O F., min. Recipe No. Rating
7 24
3 3
26
1
26
2
elongation 40 GO 90 Liv Lhl LS KC
LM LhI LS SS
LN LXI LS
SS
40 LM LA1
SS
NC
180' bend GO 90' LnI LhI SS
KC
LM
LN
SS
NC
The authors wish to express their appreciation to the Phillips Chemical Co. service laboraOzone Exposure tory and its personnel for their 12.5% valuable contribution in preelongatlon 180' bend 40 60 90 40 60 90 paring and obtaining the data presented in this paper.
ML LL AIL 99
In the above table, the number and depth of checks and/or cracks in the samples are designat,ed using the same code. Hence, signifies a large number of medium depth cracks, LS signifies a large number of small cracks, etc. S = small, &I = medium, L = large, and NC = no cracking. The first phase of this investigation pertained to natural rubber compounds and the 840
ACKNOWLEDGMENT
LM LL MnI
SS
(2) 13) (4)
(5) (6)
LJI LL MhI SS
htnI ML MRT 89
MM
hlhl NC
LM hlL
SM
LITERATURE CITED
SS
(1) Baruth Rubber &e (S Y 1, 63, S o . 1 (=iprii 1945). Depew, ISD.ENG.CHEY.,24,992 '1932). xellen et ul., Rubber A g e (N. Y.), 65 (11Iarch 1950). x o r t o n , Rubber Chem. and 7'eck7iol., 13, 576 (1940). Van Rossem and Talen, Ibid., 4 , 490 (1931). Williams, IXD. EKG.CHEM.,18, 367 (1926).
RECEIVED f o r review September 17, 1951.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTED
February 5 , 1 9 8 2 .
Vol. 44, No. 4
