Aging of Black Neoprene Jackets 6. N. Vaooa, R. E. Eriokson, and C. V. Landberg
Bell Telephone Laboratorfee,Murraw H f l l , N. J.
0
b
d
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Considerable loss in elongation of black neoprene jackets removed from wires which had been in outdoor service for comparatively short periods of time raised the question of the life expectancy of such coverings. Information available did not permit estimation of service life and a program of testing was undertaken to provide this information. Accelerated aging tests corroborated by later field tests indicated that early loss of considerable elongation is not indicative of early failure in service as loss of elongation levels off and changes much more slowly on continued exposure. Accelerated aging in air at temperatures up to 100' C. gave results most comparable with outdoor aging as regards loss of elongation. As a result of this work, it can be predicted with a good degree of reliability that a black neoprene jacket will remain serviceable for periods of the order of 20 years.
only indication of loss of quality of the jacket stock. Even after the relatively severe exposure of 10 days a t 70" C. and 300 pounds per square inch oxygen pressure, the jacket retained most of its tensile strength and 58% of its ultimate elongation. However, results obtained on samples removed after comparatively short exposures in the field (Table V ) showed a considerably greater loss of elongation than that expected on the basis of the oxygen bomb test. It was evident that before much confidence could be placed in a neoprene jacket for long-term use, more complete information on its aging characteristics was necessary. A supplementary program of accelerated testing was therefore undertaken to provide data for correlation with outdoor studies. The neoprene stock first chosen for study containing 90 parts of furnace black had a rather low original elongation and would not consistently meet the A.S.T.M. D 752 requirements of 250% elongation after 96 hours of bomb aging. Accordingly a carbon black and clay loaded neoprene jacket was chosen for study. The composition of this material is given in Table 11.
0
Table 11.
UTDOOR telephone wires must be extremely resistant to weathering and, to be economical, should have a long service life. A survey of the materials available for providing the desired covering for these wires indicated that a black neoprene jacket would be most suitable. Therefore, resistance to aging of such neoprene stocks was made an object of special study. A review of the literature available at the time this work was started yielded little information of value in estimating longterm service life. Since the work reported here was started various papers on the aging of neoprene have appeared. The Neoprene Notebook and miscellaneous published reports of the Du Pont rubber laboratory contain information on the aging and weathering of neoprene. Mayo, Griffin, and Keen (I ) studied the effect of copper on the aging of neoprene and natural rubber code wire insulations and they also mention unpublished work of Keen and Jones on a neoprene jacket stock. Thompson and Catton (7) have reported on the weathering of neoprene vulcanizates. Pollack, McElwain, and Wagner (3) in their studies on the oxygen absorption of vulcanizates give data on the bomb and oven aging of neoprene. Reinitz and Zamborsky (4) report on neoprene sheaths used to protect lead-covered cables. Mesrobian and Tobolsky ( 2 ) included neoprene in their work on the aging of diene polymers. Scott (6) studied the aging of GR-S vulcanizates under some of the conditions employed in this work. Shelton and Winn (6) have compared oven and oxygen bomb aging of GR-S a t 80 ' and 100' C.
Composition of Neoprene Jacket Compound Parts
Neoprene Easy processing channel black Hard clay Zinc oxide Magnesium oxide Light process oil Paraffin Antioxidant Plasticizer Stearic acid Accelerator
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0
OXYGEN BOMB
DAYS
Table I.
b.,
0 2 5 10
Tensile Strength, Lb./Sq. Inch 2218 2274 2047 2003
Elongation,
% 335 260 207 195
10.00 4.00
14.00 3.00 2.00 0.35 1.00 0.50
I
WEEKS
Figure 1. Elongation Retention of Black Neoprene Stock after Oxygen Bomb and Oven Aging
Bomb Aging of Neoprene Jacket Compound
Days in Oxoy en Bomh (70 300 Lb./Sq. Inch Pressure)
100.00 40.00 40.00
this part of the work, the compound was extruded on No. 18 tinned copper wire and specimens were cured 45, 60, and 90 seconds in open steam at 201" C. (220 pounds per square inch pressure). Tubular specimens removed from the wire were used for the aging tests. It has been well established that among conventional aging tests, the air oven shows the greatest changes in elongation of neoprene stocks. Consequently, the accelerated aging tests were extended to include testing in the air oven at 70" C. The results of oxygen bomb and 70' C. oven tests are shown in Tables I11 and IV, The elongation retention for both the bomb and oven test is shown graphically in Figure 1.
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..
78 62 68
In the work reported here, a neoprene stock containing 90 parts of furnace black was first chosen for study in accelerated as well as outdoor exposure tests. Bomb-aging test results (Table I ) showed that loss of ultimate elongation was about the 443
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Table 111. Oxygen Bomb Tests Days in Oxy en Bomb (70° 300 Lb./Sq. Inch Pressure)
d.,
Cuie a t 201' C., See. 60
45
90
0 2 4 10
Tensile Strength, Lb./Sq. Inch 2700 2755 2800 2255 2280 2270 2235 2300 2320 1960 1935 2005
0 2 4 10
Elongation, % 450 390 345 235
425 350 330 215
% of Original Elongation 2 4 10
86 76 52
84 74 48
Table IV. Weeks in '70' C. Oven 0
4 8 12 24
82 77 50
Oven Aging Tests at 70" C. 46
Cure a t 201' C., Sec. 60
00
Tensile Strength, Lb./'Sq. Inch 2700 2735 2800 2490 2300 2480 2410 2425 2370 2255 2280 2350 2290 2380 233.50
0
4 8 12 24
480 280 230 185 100
4 8 12 24
58 48 38 20
Elongation, % 450 270 230 180 110
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Consequently, a temperature of 100" C. has been adopted for use in this laboratory. The relative effects of heat and oxygen on black neoprene jackets are shown in Figure 3 which gives tensile strength and elongation in terms of percentage of original for the following aging conditions: 1. 2. 3. 4. 5.
Heated 24 days in air at 100" C. Aged 24 days in the oxygen bomb a t 70" C. Heated 24 hours in air at 150" C. Heated 24 hours with air excluded a t 150" C. Exposed 9 years in Florida.
These data show that exposure to heat and oxygen may result in a drastic loss of elongation without appreciable effect on tensile strength or in a loss of both tensile and elongation. The loss of elongation undoubtedly results from a further tightening up or cross-linking of the neoprene whereas the loss of both elongation and tensile strength indicates degradation or scission of the molecule. The two effects result from reaction with oxygen, the extent of the reaction depending on the temperature and on the oxygen pressure. Mesrobian and Tobolsky (2) have observed that important changes in mechanical properties occurring during aging of polymeric materials, such as rubber, are the result of concurrent aggregative and disaggregative processes. The aggregative processes include further polymerization, branching, cross-linking, and cyclization, and the disaggregative processes
425 250 210 180 100
% of Original Elongation 60 51
40 24
58
49 42 23
The results of these tcsts indicated that: 1. State of cure w represented by the 45- to 90-second range had little or no effect on the deterioration caused by the accelerated aging. 2. Tensile strength was not lessened sufficiently to be a cause for concern. 3. After 10 days in the bomb elongation was reduced to 50% of original, and it took 8 weeks a t 70" C. to bring about the same reduction. 4. There is a rapid loss in elongation during the early stages of aging followed by a marked decrease in the rate of loss in the later stages. This is particularly noticeable in the oven tests where the stock lost 10% of its elongation per week during the first 4 weeks; 2.5% per week during the next 8 weeks; and only 1.3% per week during the last 12 Reeks of the test.
I n view of these bomb and oven results and because outdoor exposure samples dropped rather rapidly in elongation while tensile strength was relatively unchanged, it was believed that the air oven test represented the best type of acclerated aging test for evaluating the service life of black neoprene jackets. However, the aging period involved a t 70 C. was too long t o be practical; hence oven aging tests were run a t IOO', 121', and 150" C. Effect on elongation at these temperatures is s h o r n in Figure 2 A t each temperature there appeared t o be a rapid drop in elongation in comparatively shoit periods of time followed by a progressively decreasing drop as the aging continued. At each of the temperatures tried the loss of elongation appeared to follow the same general curve although there is some indication of a deviation a t 150' C. The temperature coefficient of loss of elongation appeared to vary from approximately 2.4 t o 1.8 as the temperature was increased from 70" to 150" C. The change in temperature coefficient is undoubtedly a reflection of the increase in scission which occurs when the temperature is increased as will be shown. O
4 1 0.16
IOO'C.
0 121OC. 6 ISO°C. 0
8 2 0.33
12
16 4 0.66 DAYS
3 0.50
20 5 0.83
24
6 1.00
28 7 1.16
32 8 1.33
Figure 2. Elongation Retention o f Black Neoprene Stock after Oven Aging at 70°,loo", 121°, and 150' C.
LL
0 60 k
50
40
[r
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a 20 HEATED IOO°C. AIR PRESENT 2 4 DAYS
Figure 3.
HEATED HEATED NATURAL 70'C. AGING 15OCC. 150'C. OXYGEN AIR AIR 9 YRS BOMB PRESENT ABSENT FLA. 24 24HR. 24 HR. DAYS
Tensile Strength and Elongation Retention of Black Neoprene Stock
After aging in air at 100' C., after oxygen bomb aging, after heating in presence and absence of air at 150° C., and after 9 years' natural aging
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*“tr
involve scission and depolymerization. For simplicity the processes are referred t o as cross-linking and scission, respectively. Cross-linking hardens and scission softens. The two reactions occur simultaneously and the net actual change in physical properties depends on the net result of the two reactions and therefore on their relative rates. When black neoprene jackets are aged in air at 100” C. the cross-linking reaction predominates as there is practically no effect on tensile but elongation is reduced considerably. Increasing the temperature to 150’ C. causes a marked increase in the scission reaction and both reactions occur as evidenced by the decrease in tensile as well as elongation. The fact that oxygen plays a part in both reactions is indicated by comparison of the results obtained a t 150’ C. in the presence of air with those obtained a t the same temperature in the absence of air. In the absence of air the principal effect is cross-linking, but there is not nearly as much cross-linking as occurs in air. In the oxygen bomb scission predominates as both tensile and elongation are reduced. On the basis of these results, it would be expected that in the normal aging of the jackets in service, some scission might occur, but the predominant reaction would be cross-linking. The bars (Figure 3) showing the results of 9 years’ exposure in Florida appear to confirm these expectations since the most drastic effect is on elongation. Many variations in the formulations of the black neoprene jacket have been tested in the laboratory and in each case the aging characteristics appear t o follow the same general pattern. In addition two different stocks were tested after exposure in Florida. The results obtained on these exposure tests are plotted in Figure 4 in which the “old” jacket was one which contained 90
20
I
I
Table V.
Field Results on Samples of Naturally Aged Wires Tensile % pf Exposure Strength Elongation, Original Months ’ Lb./Sq. d o h % Elongation 15 2635 215 64 31 2655 225 67 57 2510 205 61 21 1990 190 57 64 2485 185 55 78 2615 175 52 14 2540 195 58 48 2215 140 42 60 2260 125 37 74 2410 150 45 87 2450 130 39 109 2520 120 36 11 2395 160 48 22 2300 145 43 34 2585 180 54 45 2165 135 40 15 1980 165 49 58 2405 165 49
Location Chester, N. J. Stone Harbor, N. J. Miami, Fla.
San Antonio, Tex.
Brawley. Calif.
parts of furnace carbon black, and the “new” jacket was prepared in accordance with the formula shown in Table 11. It is readily apparent from Figure 4 that both jackets are following the same aging curve. Samples of the old jacket which have been in service in various locations in the field for periods up to 9 years have now become available and test results on these samples are given in Table V. The field results confirm the indication given by the oven test that early loss of considerable elongation is followed by a much lower additional loss over a considerable period of time. The behavior of black natural rubber, GR-I, GR-S, and GR-A
I
OLD JACKET,. NEW JACKET, 0 0
445
“-800
I
j
1
1
1
1
I
8
12
16
20
24
28
DAYS AT 100°C.
Figure 6. Brittleness Temperatures of Natural and Synthetic Rubber Stocks after Oven Aging at 100’ C.
W
V
DAYS AT 100°C.
Figure 5. Elongation Retention of Natural and Synthetic Rubber Stocks a f e r Oven Aging at 100’ C.
Figure 7. Elongation Retention of Outdoor Exposed Black Neoprene Jackets Compared to Oven Aged Jackets
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Table VI. Compression Resistanoe of GR-S Insulation Aged in Contact with Neoprene Jacket Days &t looo C. Compression Resistance, Lb Q
a 4
7
11 14
21 28
1170 1306 1280 1340 1320 1265 1180 1440
compounds in the 100" C. oven aging test was also studied. In each caae the compounds contained 50 parts of medium processing channel black and such plasticizers and curing aids as were necessary for the particular polymer. Figure 5 shows the percentage of original elongation retained a t various aging intervals up to 24 days. In general, all the elastomers tested follow the same general pattern showing a rapid loss in elongation in the earlier stages of the test followed b y a decrease in the rate of elongation loss as the test proceeds. GR-I tends t o level off at about 50% of its original elongation and shows essentially no loss in elongation in the latter stage of the test. This behavior of the GR-I compound as compared t o the other polymers undoubtedly shows the influence of its very low unsaturation. It must be borne in mind that serviceability also depends on changes in tensile strength and other effects of outdoor aging which were not considered in this test, and therefore results such as those shown in Figure 5 must not be used as a sole criterion of serviceability. The effect of aging on the brittleness temperature of these black compounds was determined according to the A.S.T.M. D 746 method. I n the neoprene compound, even though elongation wag reduced to less than 20% of original, brittleness temperature was raised only 8" or from -40" t o -32" C. The GR-S and GR-I compounds showed a small change and were comparable to neoprene, but GR-A and natural rubber showed considerable change as indicated in Figure 6. The effect of 100" C. oven aging on the insulation under the neoprene jacket was also studied. The insulation was a black GR-S compound, and because the insulation was bonded t o the conductor, tests were limited to compression resistance. The
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jacket was left in place during the aging period and removed just prior t o conditioning for test. Results are listed in Table VI. Compression resistance increased as would be expected from stiffening of the compound, but the increase was not great and even after 28 days when the neoprene jacket was too brittle to remove intact, the insulation was still serviceable. The work reported in t8hispaper shows that early loss of considerable elongation of a black neoprene jacket compound is not necessarily indicative of short service life. It has been pointed out that aging experienced in the field closely resembles oven aging at temperatures up to 100" C., and this fact may be used t o estimate useful service life of the jacket. I n Figure 7, the percentages of original elongation remaining after 5 years' exposure in southern California and aft'er 9 years' exposure in Florida are spotted on the 100" C. oven aging curve. Assuming that a jacket is serviceable until its elongation has been reduced to 10% of original, the jacket can be expected to have a useful service life of the order of 20 years even under these severe exposures.
Acknowledgment The authors gratefully acknowledge assistance received from ot,her members of these laboratories. B. S. Biggs contributed many helpful suggestions during the preparation of the paper. E. L. Dias and Doris B. Smith were responsible for the preparation of the compounds and the numerous teets required. A. R. Icemp, formerly of these laboratories, assisted in planning the work. The authors are especially indebted to C. C. Lawson for furnishing samples from the field.
Literature Cited (1) Mayo, L. R., Griffin, R. S.,and Keen, W. N., IND.ENG.CHEM.,
40, 1977 (1945).
( 2 ) Mesrobian, R. B., and Tobolsky, .4. V., J . Polymer Sci., 2 , 463 (1947); Rubber Chem. an,d Technol., XXI, 281 (1945). (3) Pollack, L. R., McElwain, R. E., and Wagner, P. T., IND.ENG. CIIEM.,41,2280 (1949). (4) Reinitz, B. B., and Zamborsky, N. A , Corrosion, 4, 432 (1948). (5) Scott, J. R., J . Rubber. Reseurch, 18, 117 (1949). (6) Shelton, J. R., and Winn, H., IND.ENG.CHEM.,39, 1133 (1917). (7) Thompson, D. C..and C a t t o n . N. L., Ihid., 42, 892 (1950).
RECEIVED October 5 , 1950
Rubber Oxidation and Antioxi Actions S. Baxter, W. McC. Morgan, and D. S. P. Roebuck Moneanto Chemicah Limited, Rsabon, Denbightshire, England T h e mechanism of autoxidation of rubber is discussed and consideration is given to the interrelation of reactions occurring during oxidation and vulcanization. The importance of chain transfer reactions is emphasized. A new apparatus is described which enables simultaneous measurement to be made of oxygen uptake at constant pressure and of deterioration i n the stress-strain properties at low elongations during aging of a single vulcanized rubber sample under closely controlled conditions. A brief study of the results obtained with four differently vulcanized stoclrs serves to illustrate the importance of vulcanization accelerators and antioxidants i n determining the aging characteristics of vulcanized rubber; these affect both rate of oxygen uptake and the mode of action of oxygen with the rubber molecule.
M
ANY experimental investigations into the oxidation or aging characteristics of rubbers have been carried out during the last 30 years. Some have studied changes in physical properties during artificial aging and have linked these with vulcanizate composition and with changes occurring during natural aging. Others have studied the kinetics and mechanisms of the chemical reactions involved. Both approaches have contributed much to the general body of knowledge on this subject. I n more recent years, serious attempts have been made to correlate rate and extent of oxygen absorption with changes in physical properties, and conclusions drawn from this work have proved to be of considerable interest ( 1 ) . I n most of the work reported, however, oxygen absorption experiments were carried out on one sample of rubber, using a laboratory absorption apparatus, and deterioration of the physi-
