STATIC EXPOSURE TESTING OF
AUTOMOTIVE COMPOUNDS H.A. WINKELMA" Dryden Rubber Division, Sheller Manufacturing Corp., Chicago, Ill.
D
ETERIORATION of D e t e r i o r a t i o n of rubber products on outdoor exposure the ozone chamber do not show good outdoor weatherrubber in service due is d u e t o t h e action of s u n l i g h t a n d ozone. Ozone cracking, and vice versa. to cracking is a problem of i n g results when rubber is exposed u n d e r strain. T h e Fifty-four samples were exmajor concern to all manutype of cracking produced by ozone is n o t produced by posed in Chicago, Ill., and other elements or materials normally present i n t h e facturers and consumers of hliami, Fla., and also in the rubber products. Automoatmosphere. Light-energized oxidation produces a crazed tive manufacturers each year ozone chamber for 70 hours condition on t h e surface of t h e rubber, which is q u i t e a t 100" F. in 25 parts ozone face losses aggregating into different from t h e effects produced by ozone. This s t u d y pel. 100 million parts of air. was undertaken t o determine t h e factors which influence thousands of dollars, because Thirty-nine of the samples of premature cracking and t h e resistance of rubber products t o ozone. I t was found were also tested in Clearfailure of rubber parts. The t h a t rubber cured u n d e r s t r a i n cracks faster t h a n when water, Fla., and in the ozone problem of the rubber techcured a t rest. Manufacturing defects, s u c h a s blisters, chamber. Table I1 shows nologist is complicated by air traps, foreign m a t t e r , a n d pigment agglomerates, rethe fact that ozone cracking that while the accelerated sulted i n p r e m a t u r e cracking. Certain manganese, copof a given compound out of ozone test is more severe than per, a n d iron salts h a d a n adverse effect on ozone resista 30-day outdoor exposure, doors is a variable factor deance i n outdoor a n d accelerated exposure tests. Some pending upon meteorological dispersing agents used t o m a k e carbon black slurries for over 20% of the tests show conditions and geographical more cracking outdoors than G R - S carbon black masterbatches showed a n adverse l o e a t i o n . C r a c k i n g also in the ozone chamber. It is effect o n weather aging. T h e selection of a polymer, s u c h varies wit4 the amount of possible to select compounds as n a t u r a l rubber, GR-S, neoprene, or Butyl rubber, h a s a on the basis of ozone chamber marked influence o n ozone resistance of automotive strain imposed in the autotests that do not show up well motive assembly. A satisrubber products. Unless a polymer is handled a n d c o m on outdoor exposure. factory rubber must resist pounded properly, good ozone resistance is n o t obtained. ozone cracking regardless of I n outdoor exposure t h e effect of s u n l i g h t , ozone, h e a t , The following table gives ' the locality. This is not a n d water varies seasonally i n Florida, California, Michithe SAE-ASTM rating of the easily achieved because of the above samples. The ozone g a n , a n d Illinois. Many tests a n d m u c h good j u d g m e n t chamber gives more severe seasonal variation in ozone m u s t be exercised i n correlating accelerated a n d outdoor concentration in various parts exposure tests. c r a c k i n g than Chicago or of the country. Ozone ranks either Florida location, as with oxygen, heat, light, and judged by percentage of samoils as a deteriorating agent for vulcanized rubber. ples which rated class 2 and 3. The ozone chamber, however, This study shows that there are many variables which must be indicates greater ozone resistance than was actually observed in observed in order to obtain uniformity in weather aging. BeChicago and Miami. In the column rated 0, only Clearwater cause of a variation in ozone concentration from one place to angave more samples with a rating of 0 than the ozone chamber. other, outdoor weather-aging results are not necessarily the same. ~~~~l SAE-ASThI Percentage of Samples During June and July 1951, 188 duplicate exposure tests were run at Various Ratings N ~of. Rating Samples 0 1 2 3 0 1 2 3 in Chicago, Ill., and Clearwater and Miami, Fla., on commercial weatherstrips according to SAE-ASTM procedure. SeventyChicago, Ill. 54 8 21 13 12 1 4 . 8 3 9 . 0 24 0 2 2 . 2 Clearwater Fla. 39 12 15 8 4 3 0 . 5 3 8 . 5 2 0 . 5 10.5 seven samples (40.95%) gave the same degree of cracking in ChiMiami. Fla'. 54 6 26 17 5 11.0 48.0 3 1 . 8 9 . 2 Naugatuck ozone cago and Florida, while 102 samples (54.25%) showed less crackchamber 54 10 '7 21 16 1 8 . 4 13 0 3 9 . 0 29 6 ing in Florida than Chicago. Only Q samples (4.8%) cracked more in Florida than in Chicago. In winter these conditions may The SAE-ASTM Technical Committee on Automotive Rubber be reversed. has developed a standard method of exposure testing which has The SAE-ASTM ratings given in Table I show that checking resulted in better correlation between laboratories and has been a in Chicago mas more severe than in Florida under these test condiuseful tool in the improvement of the weather aging of rubber tions. Clearwater, Fla., was less severe than Miami, Fla. The products. It is hoped that a laboratory test can be developed number of samples giving a certain rating are given for each locawhich will correlate with outdoor exposure. tion, as follows: The work of Crabtree and Kemp ( l a ) ,Newton ( 3 4 ) , Williams SAE-ASTM Rating (Q9),Van Rossem and Talen (47), Fielding ( 1 9 ) ,Buist and Weld0 1 2 3 ing (If), and many other investigators has contributed greatly to Chicago, Ill. Clearwater, Fla. an understanding of the effect of light and ozone on rubber. Miami, Fla. Rubber after exposure to weathering may be classified as crazed, cracked, chalked, or frosted, depending upon compounding and I t was observed that the exposure rating of a given compound the conditions of exposure. varies from day to day. This was due to some uncontrolled variables in materials or handling. Light-energized oxidation produces a crazed appearance-Le., a network of fine irregular cracks in a hard inelastic skin or resinAccelerated tests in an ozone chamber will show ozone resistous layer (Si?). This surface skin acts as a barrier to ozone, and ance of rubber but do not take into account the effect of sunlight, cracking is retarded for some time. Exposure to rain leaches out water, etc. Some compounds which show little or no cracking in April 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
84 1
-ELASTOMERS-Ozone Table I.
a
Clearwa ter Miami Totaln Per cent Grand total, 188.
Chicago
Cracking
Florida Exposure
us.
Same Degree of Cracking as in Chicago 49 28 77 40.95
Less Cracking than in Chicago 68 31 102 54.25
More Cracking than in Chicago 6 3 9 4.8
-
-
-
Figure 1. Failure of Weatherstrips on a Windshield
sorbs ozone u n d a film of ozonide is formed, but no cracks are produced. Frosting is the end product of an oxidation process produced by ozone (8,46). A freshly vulcanized surface may become dull and frosted within a few minutes to several hours because of the formation of many minute cracks. Chalking occurs during weather aging, particularly on compounds containing a mineral loading. It' is due to oxidation of the rubber on the surface, resulting in release of the filler. Chalking occurs late in weathering, while frosting appears early. Ozone is formed in the stratosphere by the action of ultraviolet light from the sun on the molecules of atmospheric oxygen. Ozone absorbs the greater part of the ultraviolet light from the sun, otherwise life as it exists on earth would be impossible. The ozone content of the strdtosphere is equivalent to a layer 3 mm. thick (1.5, 94,SO). The daily ozone concentration a t the earth's surface varies from 0 to 6 parts per 100,000,000 pacts of air by volume. Winds and convection currents bring the ozone to the earth's surface, the concentration usually being higher on windy days (40). Heavy rain stabilizes the ozone content by washing gases from the air (37'). a' Foggy or sultry conditions ' may result in total disap2 - i pearance of ozone because of failure of winds to renew the I supply ($3,36). , *, Ozone is a powerful oxidiaCVAEDATObCE ing gas and destroys bacteria, A-I A-2 1-3 CU2ED AFT!: 24H01.9 3 (December 17, 1950, to February 15, 1951) in Miami. After 22 days exposure in Chicago, all samples except copper (0.0005 part) were cracked; the control 15 as still uncracked d f ter 220 days there a a s no appreciable diffeience between any of the samples. After 70 hours of exposure in the ozone chamber at 100" F., inanganese (except 0.05 part) and ferrous iron accelerated the cracking of GR-S, but after 70 hours of exposure the samples rated the same as the control. Copper and ferric iron acceleiated the cracking of GR-S; ferric iron accelerated the cracking qo t h a t a rating of 1was obtained in 2 hours. These preliminary observations indicate that these metallic salts accelerate the cracking of a natural rubber and a GR-S weather-aging compound; their presence, therefore, is to be avoided. Prolonged exposure in Chicago shoved no ad effect of these salts. Results of the desciibed tests are presented in Table X. Copper, manganese, and iron naphthenates were added to a natural rubber and a GR-S 17 eather-aged extruded compound. A sufficient amount of each compound mas added t o give 0.05, 0.005, and 0.0005 part of the metal per 100 parts of rubber hydrocarbon. Samples were exposed in duplicate in Chicago and Miami. After a 48-day exposure in Chicago none of the naphthenates adversely affected weather aging (Table XI). After 30 d a j s in Miami copper naphthenate had no effect on rubber or GR-S; manganese naphthenate accelerated cracking of natural rubber but had little or no effect on GR-S. Iron naphthenate accelerated the cracking in both natural rubber and GR-S. Seoprene gave better weather aging than natural rubber 01 GR-S in carbon black-loaded compounds. Thompson and Catton (44) have emphasized t h a t neoprene must be properly compounded t o obtain good weather aging. The percentage of neoprene by volume should not be less than 50 for best results. Clay gave the best results of the mineral fillers; m hiting a t all loadings adversely affected outdoor aging. Substitution of GR-S
Effect of Metallic Salts o n Cracking of Molded R u b b e r a n d G R - S
Rating (60-,Dag Rating (22-Day Exposure Miamla) Exposure Chicago*) -Meta1/100'Rubber, %--.-Metal/lOdRubber, %0 0.05 0.0050.0005 0 0.050.0050.0005
Rating (220-Day Exposure Chicago C) --Metal/lOdRubber, %0 0.05 0 , 0 0 5 0.0005
Rating (Ozone Chamber at loOD F . d ) 7-NIeta1/100 Rubher, 700 0.05 0.005 0.0005
Rating (Ozone Chamber for 70 Hr. at looo F.e) -Meta1/100 Rubber, '70 0 . 0 5 0.005 0.0005
50
1
Molded Rubber (88-37) Control Manganese Copper Ferrous iron Ferric iron
3
0
1
2 2 2 2
2 2 2 2
1 1 1
2.6 2 2 2
2
0 1 1
1
1 3
0 0 1 1
3 2
1 2 3 2
1
2
3
2
Molded GR-S (44-65) Control Manganese Copper Ferrous iron Ferric iron
Dec. 17, 1950, t o fi Deo. 19 1950 t o 6 Dee- 19: 39.50:t o d 30 parts of ozone e 30 p e s o? ozone
848
0
1 2 2.5 2 2
2 2 2 2
2 2 2 2
1-2 1 1 1 1
Feh. 15, 1951. Jan. 10, 1951. July 27, 1951. per 100 000 000 parts of air; per 100~000~000 parts of air;
1 1 1 1
50
1 0 1 1
1 1 2 1 2
1
2 1
3
1 3 1 2
for a number of hours t o give a rating of 1.
SAE-ASTM rating.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No.
4
-ELASTOMERS--Oeone
CrackingC
A
Table XI.
Effect of Naphthenates on Cracking of Rubber and GR-S
Naphthenate
Rating, Chicagoa Rating Floridab Rubber-Metal/lbO Rubber-. 0 05 0 005 0 0005 0 0 05 0 0050.0005 Rubber (88-37) 0 1 2 2 0 1 0 2 1 1 1 0 1 2 2 3 2 1 1
c Meta1/100
0
Control
2
Copper Manganeee Iron Control
1 1 2
Copper Manganese Iron a b
GR-S (44-65) 0 1 1 2
2 1 2 1
0 0 1
0 0 1
0 1 1
Exposed 48 days, Jan. 17 to March 6, 1551. Exposed 30 days, Jan. 24 to Feb. 23, 1551
Butyl Reclaim and OR-S Blends B B-la C C-la
Table XII. Compound
A
Parts GR-S Butex Cumar M H Process oil Stearic acid Heliorone Philblack A Hard clay Zinc oxide ilccelerator Ab ilccelerator BC Sulfur Total Specific gravity Q
b
100
75 50 50 ... 44 88 88 20 20 20 20 16 15 15 15 15 1 1 1 1 1 5 5 5 5 5 55 55 55 55 55 , 20 20 20 20 20 5 5 5 5 5 1.5 1.5 1.5 1.5 1.5 0,1875 0.1875 0.1875 0.1875 0.1875 2 2 2 2 2 --___-224.6875 243.6875 243.6875 262.6875 262.6875 1.18 1.21 1.21 1.23 1.23
Special mixing technique used. Accelerator A benzothiazyl disulfide. Accelerator B: copper dimethyldithiocarbamate.
Table XIII.
Compd.
.
d
90-Day Exposure in Barberton, Ohio
(June 2 to Aug. 31, 1980) Time of Cure at Days for Days for 316O F . , Min. Rating of 1 Rating of 2 12 53 ... 18 ,.. ,..
B
12 18
B-la
12 18
c
12
e-la Q
75 44 20
R.ating on Aug. 31
1 0
.14. .
‘io
0
2
0.
A . 0.010-inch gage From top to bottom, Butyl, Neoprene W, and OR-S B. 0.017-inch gage From top t o bottom, Butyl, Neoprene W,and GR-S C. From t o p t o bottom, no wax and undercured: n o wax: undercured; a n d OR-S regular
or natural rubber for neoprene adversely affected weather aging (natural rubber was more detrimental than GR-S). Butyl rubber when properly compounded and adequately protected by wax will give good weather aging. The Midwest Rubber Reclaiming Co. (6) has completed some weathering tests which emphasize the importance of dispersion. When reclaimed Butyl is added to GR-S by a special mixing technique, there was no checking after a 90-day outdoor exposure in Barberton, Ohio, whereas compounds mixed according t o ordinary technique show checking and cracking (Tables XI1 and XIII). SPONGE RUBBER
A comparison of Butyl rubber, neoprene, and GR-S in the applied skin of sponge rubber has been made by Yoran (50), who found that the thickness of the applied skin is important, although i t does not appear to be as important with neoprene as with GR-S and rubber. A Butyl cover is unsatisfactory, regardless of the Chicknesa of the applied skin (0.010-, 0.017- or 0.027-inch gage). Neoprene is satisfactory at a gage of 0.010-inch, whereas GR-S is not satisfactory unless a gage of 0.017-inch is used (Table XIV and Figure 15). An undercure improves the weathering resistance of GR-p but the presence‘of a protective wax is still essential. Outdoor exposure test results were confirmed in the Crabtree-Kemp ozone ohamber.
0
...
1
18
6;
14
2
12 18
... ...
... ...
0 0
Special mixing technique used.
Table XIV. Sponge Rubber Lafayette Ind 30-day exposure Aug.’ 10-Sep0. 9, 1950 Gage, In. X 108 10 17 27 Neoprene W 10 17 27 GR-8 10 17 27 GR-8, no . 10 wax 17 27 GR-S, under10 cure 17 27 OR-8, PO wax gnd udercurs 27
SAE-ASTM Bection Butyl cover
i?
April 19511
Figure 15. Effect of Butyl Rubber, Neoprene, and GR-S on Sponge Rubber
Rstine 3 3
a
0 0 0
3 0 0 3 3
a
2 0 0
3
8
ACKNOWLEDGMENT
Acknowledgment is macle to the author’s associate, F. J. Bayme, for assistance in obtaining data for this paper. The following laboratories kindIy cooperated in the preparation of this study: Brown Rubber Co., Chrysler Corp., General Tire & Rubber Co., Ford Motor Go., B. F. Goodrich Co., Goodyear Tire & Rubber Co., Indoil Chemical Co., Industrial Rubber Co., Midwest Rubber Reclaiming Co., Naugatuck Chemical Co., New Jersey Zinc Co., and Ohio Rubber Co. LITERATURE CITED
(1) Adame, J. W.,Mesaer, W. E., and Howland, L. H., IND. ENG. CHEW,43, 754 (1951). (2) Akhurst, C.G . , J. Rubbei Research Inst. Malava, 5 , 29 (1933). (3) Albert, H. E., Smith, G. E. P.,Jr., and Gottsohalk, G . W., IND.EN& C ~ M .4 ,0 , 4 8 2 (1948)[Rubber Chem. and TechmE., 21, 877 (1948)l. (4)Ayers, J. W., IND. ENQ.CHEM.,24, 320 (1932) [Rubber Chem. and Technol., 5 , 179 (1932)l. (5) Ball, J. M., private communication. (6) Barton, B. C., U.S. P a t e n t 2,324,056 (1943). (7) Bett and 011, Trans. Inst. Rubber Itad., 19, No. I, 53 (1943) IRolbh Chem. a d Technol., 17, 221 (194411. (8) Beaudry, J. T., Eubbey Age, 69,429 (1951). (91 Bishop, R . O.,and Sekar, K. C., J. Rubber Research Inst. Mffilaya,2, 2 3 9 (1931). (10) Bruni, G., and Pelzzoia, C., I d W Rubber J . , 62. ID1 (1921).
INDUSTRIAL A N D ENGINEERING GBEMPSTRY
849
LLASTOMERS-Ozone (11)
Cracking-
Buist, J. M., and Welding, 0. N., Trans. Inst. Rubber Ind., 21,
4 9 (1945). (12) Coleman, C., U. S. Patents 2,246,932; 2,306,779 (1942). (13) Crabtree, J., and Kemp, A. R., IND. ENG. CHEM.,38, 278 (1946). (14) Dekker,’P., Kautschuk, 1 5 , NO. 11, 179 (1939) [Rubber Chem. and Technol., 13, 431 (194O)l. (16) Dobson, G. B. M., Proc. Roy. SOC.(London), 110, 660 ( 1 9 2 6 ) ; 114, 621 (1927); 122, 4 5 6 (1929). (16) Dufraisse, C., and Viellefasse, R., Rubber Chem. and TechnoZ., 7 , 213 (1934). (17) Eaton, B. J., d g r . Bull. Federated Malay States, 1, 17 (1912). (18) Fickenday, E., KolEoid-Z., 9 , 81 (1911). (19) Fielding, J. H., India Rubber World, 115, 802 (1947). (20) Fol, J. G., and de Visser, W., Bull. Rubber Grower’s dssoc., 10, No. 2 , 124 (1928) [Rubber Chem. and Technol., 1 , 2 8 8 (1928) 1. (21) Freundlioh, H., and Talaly, J., Kautschuk, 9, 34, 4 9 (1933) [Rubber Chem. and Technol., 6, 378 (1933)l. (22) Georgi, Malayan Agr. J., 16, 2 0 4 (1928). (23) Gluckauf, Quart. J. Rou. Met. SOC.,70, 13 (1944). (24) Harrison, D. N., Nature, 124, 58 (1929). (25) Hastings & Rhodes, J . Rubber Inst. Malaya, 6, 42 (1935). (26) Jones, P. C., and Craig, D., IND.ENG. CHEM.,23, 2 3 (1931) [Rubber Chem. and Technol., 4 , 108 (1931)l. L27) Xirchhot, F., Kautschuk, 3, 2 5 6 (1927). (28) Ibid., 7 , 2 6 (1931). (29) Kreusler and Budde, Ger. Patent 18,740 (Bug. 26, 1881).
