952
ONTROL of heat deterioration is a continuous problem for the chemist. In the case of rubber and synthetic ehtomers, the rubber chemist has charted the changes in the physical properties of vulcanized elastomers during heat aging. He has developed many methods for retarding the deterioration of rubber compounds by heat; but with the newer synthetic elastomers the development of methods of retarding deterioration by heat is now an active subject of investigation. The degree of deterioration is dependent upon the methods of compounding and curing the different elastomers as well as upon the conditions of aging. I n general, continuous exposure to high temperatures softens n a b ural rubber but hardens the synthetic elastomer. Inherently neoprene has greater heat resistance than natural rubber (6). Many investigators have described the heat resistance of rubber vulcanizates, but only a few have reported on the heat resistance of neoprene vulcanizates and these reports have been primarily comparisons of given neoprene vulcanizates with one or more rubber stocks. The compounding of Neoprene Type GN for heat resistance was discussed by Catton, Fraser, and Forman (9). The oxygen bomb aging of Neoprene Types E and GN was compared with that of various rubber compositions by Neal, Bimmerman, and Vincent (4).
C
antioxidants, or retarders. Such a compound might have the. following formula and it would usually be cured 30 minutes at 287OF.: Neoprene Type Q N Steario acid Neoaone A (phenyl-p-naphthylamine) Extra light calcined magnesia Filler and softener Zinc oxide
100.0 0.6 2.0 4.0
An desired
5.0
As previously stated, hardness rapidly increases during the initial period of aging and then slowly increases as the aging progresses. This is illustrated by Figure 1. The difference between starting point A and ending point B is the increase in hardness during aging, and any reduction in this difference constitutes an improvement in heat resistance. Compounding changes in filler and softener content control the position of the curve,on the scale but do not affect the magnitude of the increase in hardness except when the stock is heavily loaded and the vulcanizate approaches a maximum hardness.
CRITICAL ANALYSIS OF HEAT AGING
In analyzing the changes in properties of heat-resisting rubber stocks during prolonged exposure to high temperature, a permanent and progressive decline in quality is observed. Tensile strength decreases slightly during the early stages of aging followed by a rapid drop to ultimate failure. Ultimate elongation usually follows the same trend ss tensile strength while hardness increases slightly followed by a rapid decreaae During the heat aging of neoprene, a different pattern is apparent: (a) Tensile strength drops slightly in the early stages, but the loss seldom exceeds 35 per cent and thereafter little or no c b g e is observed; (€) ultimate elongation decreases rapidly a t first and declines slowly thereafter; and (c) hardness increases rapidly a t first and then asymptotically approaches maximum hardness. Since the tensile strength of practically all neoprene vulcanizates is not seriously reduced by severe aging conditions, this property is not a satisfactory criterion of age resistance. The magnitude of the changes in elongation and hardness under similar conditions, however, are generally larger; therefore these properties should be carefully studied in evaluating heat-resisting neoprene vulcanizates. Hardness is considered the more important because ultimate serviceability is often dependent upon it. Furthermore, the develop ment of neoprene vulcanizates having satisfactory elasticity after heat exposure depends upon retarding their hardening during aging. The effects of heat on some physical properties must be considered separately because changes in elongation, hardness, and tensile strength cannot always be correlated with changes in resilience, flex resistance, electrical properties, and solvent resistance. However in this work only changes in elongation and hardness are considered. Typical aging curves showing the changes in hardness and elongation of normal neoprene vulcanizates are reproduced in Figures 1 and 2; they are characteristic for all variations of high-temperature exposures. A normal neoprene compound is defined aa one containing only the necessary curing ingredients, nominal filler loading but not special accelerators,
I I I 1 1 HEAT MINO FIGURE N0.2 0 ' THE FFFECTS OF HFAT WING ON THE P DNGATION F A NORMAL NFOPRENF TYPF ON WI rzmza
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nm OF
d d D
f-
Neoprene Gloves, Hood, and Clothing Protect the Worker against Accidental Exposure to Chemicals
I 953
I
TIME I OF HEAT t MNGI
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INDUSTRIAL AND ENGINEERING CHEMISTRY
954 95 90 -
HEAT RESISTANCE IMPROVED BY HIGH STATE OF OURE AND REDUCED FILLER
'
CURVE A-
CONTROL COMWJND(1336N47) CARBON BLACK-32 PTS. CURE: SO MIN.QL87'F.
CURVE C - IEST GOMPOMQ 11336N462 CARBON BLACK-2OPTS. CURE: WMlN. 3249f.
e
5d
The fact that a long cure results in a stiffer stock does not prevent the use of this method of improving heat resistance. If the elongation and hardness are outside of a specified o r desired range, the filler and softener content can be adjusted to obtain the desired original properties. For example, if the desired hardness is A in Figure 1,a reduction in filler, increase in softener, or both would meet the requirements with a high state of cure. Curve A'B' would shift to AB'' on the scale and the increase in hardness from A to B" would be the same as from A' to B'. This would result in a general increase in elongation as shown by curve C" to D" in Figure 2.
El!LUJ
60
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3
6
I
1
12 DAYS AGING IN 121%. AIR WEN
24
Vol. 35, No. 9
FIG. NO.? HEAT RESISTANCE IMPROVED BY ADDITION OF ACCELERATOR AND REDUCED FILLER INCREASINQ HARDNESS
I
1
~~
;
nG.N0.4
7
HEAT RESISTANCE IMPROVED BY HIGH STATE CF NO REWCED FiLLER DECFSASINQ
ELmunON
CARBON BLACK-32 PTS. CURE: 30 YIN.8287'F. TEST COMPOUND 11336N491 CARBON BLACK 20 PTS.
CURVE D-
-
-
._.
W
k
E c-
TEST COMPOUND
CARBON BLACU-
PERMALUX I PT. CURE: 30 MIN. 8 2879.
(1336N44
PO PTS. I
24
12 DAYS AGING IN 12IoC.AIR OVEN
F
l
u
HEAT RESISTANCE IMPROVE0 BY ADDITION OF ACCELERATOR AND REDUCED FILLER QECREASI-
-
CARBON BLACK SZPIS. WREI ao MIN. es OOPF. I
4
I
,
0 12 16 20 DAYS AGING IN l2l% AIR OVEN
Since extended curing is known to increase hardness, it is a reasonable assumption that the initial aging period represents a prolongation of cure It follows that the prolongation of cure to produce a stock having a higher original hardness should result in a vulcanizate which undergoes less change during heat aging. Hence, upon heat aging, curve AB for the normal cure would assume the slope A'B' which is somewhat comparable to the EB portion of the original AB curve As the increase from A' to B' is less than from A to B, a high state of cure in Neoprene Type GN vulcanizates should result in better heat resistance. Furthermore, a longer time is required to show a given percentage increase in hardness on curve A'B' than on curve AB which means a slower rate of aging. Figure 2 illustrates that the rate of change in extensibility is similar to the rate of change in hardness. There is a large decrease in extensibility during the heat aging of a normally cured vulcanizate. The curve would be shifted from CD to C'D' by increasing the state of cure of the vulcanizate. Since C' to D' is a smaller change than C to D and the C'D' curve approximates the F D portion of CD, it becomes apparent that changes in elongation during heat exposure, like those of hardness, should be reduced by an extension of cure.
D (1336N47
CARBON BLACK
300
CURE:-3OMIN
0
4
e 287%
8 DAYS AGING
- 32 PTS.
I
1
12 16 IN l2I0C. AIR
20
OVEN
The effects of accelerators or retarders cannot be overlooked. Accelerators, by increasing the rate of cure, would be expected to affect the heat resistance in a manner similar to that of extending the cure. The use of accelerators has the added advantage of shortening vulcanization time. Retarders, by decreasing the rate of cure, would be expected to impair the heat resistance.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 1943
955
APPLICATION
TABLSI. EFTECTBOF STATRI OF Cmm, QUANTITY OF FILLER, CHEMICAL In applyinga tmt situation to the hypothesis ACCELERATOR, AND CHEMICAL RETARDER ON HEATRESISTANCE just outlined, Neoprene Type GN was comCompound No. 1336N46 47 4s 49 50 pounded in a heat-resisting formula. ThirtyNeoprene Type Q N 100.0 100.0 100.0 100.0 100.0 two parts of semireinforcing carbon black Steario add 0.5 0.5 0.5 0.5 0.5 Neorone A 2.0 2.0 2.0 2.0 2.0 were added, and the resulting stock was used MUFa 1.0 1.0 1.0 1.0 1.0 88 the control (1336N-47). The normal cure Extra light oslcined m a nesia 4.0 4.0 4.0 4.0 4.0 Semireinforcing oarbon %lack 20.0 32.0 43.0 20.0 43.0 of this stock is 30 minutes at 287' F., and the Zinc oxide 10.0 10.0 10.0 10.0 10.0 Permslur ... ... ... original hardness obtained was 57. Reducing ... 1.0 MBTS 1.5 the carbon black to 20.0 parts produced a stock Tensile Elongation Shore (1336N-46) having an original hardness of Compound Aging Strew, Lb./Sq. In. Strengbh at Break, HardDays Temp., C. 100% 300% 500% and Cure 52 with a normal cure. Increasing the carLb./Ra. In'. % ness 1336N-46 Original 176 676 1350 8075 52 bon black to 43.0 parts (1336N-48) produced 950 3 121 350 1475 30 min. ai ... 62 2400 455 a stock having an original hardness of 63. 287O F. 6 121 560 1876 ... 350 07 2200 74 235 2075 12 121 860 . . . . . . The stock containing 20 parts carbon black 2326 86 115 24 121 2160 waa brought to an original hardness of 57 0.75 150 1550 245 64 576 1475 by two methods: (a) by extending the cure 70 160 960 :*5 1so 150 81 1225 60 to 90 minutes at 324' F. (1336N-46), and 6 Too brittle id itst ' * ' '" (b) by the uee of an accelerator, Permalux 1336N-46 Original am ioso 2050 2800 57 645 90 min. ai 3 121 360 1475 2250 62 410 (di-o-tolylguanidine salt of dicatechol borate, 2250 324O F. 66 350 6 121 600 lB00 1336N-49). The original hardness of the 71 2050 230 12 121 825 . . . . . . 2550 24 121 1826 84 145 stock containing 43.0 parts of carbon black was 0.75 150 476 1600 64 280 reduced to 59 by adding a retarder, MBTS 69 1.5 150 800 1376 160 3 160 1176 60 79 (benzothiazyl disulfide, 1336N-50). 6 Too brittle & k t " ' '*' Standard test methods were used (1) with 1836N-47 57 Original 300 1125 1860 2625 780 aging periods up to 24 days in the 121' C. 3 121 30 min. ai 660 2060 88 2350 370 287O F. 73 270 6 121 800 .. .. .. .. ... . .. 2360 air oven and up to 6 days in the 150' C . air 12 121 1400 . 79 2450 205 90 89 2 4 0 24 121 oven. The physical properties of these com0.75 1.50 725 1825 210 69 pounds and the data obtained after aging are 7s 1650 i.5 150 1425 120 summarized in Table I. 150 85 1476 60 6 Too bribtle t'd itst ' * These data provide #e answer to the fol1836N-48. 63 Original 400 Is00 2126 2260 620 lowing questions arising from the hypothesis: 2476 80 min. at 74 3 121 826 2450 316 2450 at5 2 8 7 ' F. 79 6 121 1176 For a given hardness does a stock with less 2400 83 130 12 121 1823 ::: ::: 6ller having a high state of cure have better 94 24 121 as00 80 heat resistance? Does the addition of ac76 0.76 150 1126 1800 IM) I600 ::: ::: i.5 150 160 100 81 celerator have the sarne effect w an extended 150 90 1200 20 6 Too brittle & kat * " '* * cure? Does a compound having greater load1336N-49. 2626 68 Original 360 1160 2160 605 ing and added retarder for obtaining hard3 121 476 1900 30 min. at ... 2175 66 335 ness have better heat resistance because the 70 260 287O F. 6 121 660 . . . . . . 2100 2100 77 185 12 121 IlM, neoprene content would be less? Does heat 2776 95 88 resistance improve as the loading by filler in1650 69 220 74 1200 95 . . . . . . . . creases? In dimcussing them questions, all 85 30 1 loo references are made to curves showing the changes in elongation and hardness during 2200 59 1336N-50 Original 275 1250 1925 730 30 min. a& 3 121 355 850 2275 ,.. 2400 72 aging in the 121OC. air oven. The results 2550 77 265 287O F. 6 121 1076 . . . . . . 2675 180 81 12 121 1650 obtained in the 150' C. oven were similar ex3000 91 90 24 121 ......... cept that the changes occurred in a much 1975 200 73 0.75 150 900 1650 79 shorter time. i.5 150 1660 ...... 100 1S50 35 150 88 The curves in Figures 3 and 4 show that the 6 Too brittle tb'iest '" '" simultaneous use of lower filler and longer cure a The antioxidant N - p t o l y l N1-ptolyl sulfonyl p p h e nyleue diamine. to produce a given hardness results in greater heat resistance #an the use of normal filler loading and normal cure. The original hardness of the two vulcanFigures 5 and 6 show that an accelerator (compound izates was 57. During aging in the 121' C. air oven, 1336N-49) can be used to approximate the effects of prothe highly cured vulcanizate increased 27 points in hardness longed cure. However, the efficiency of this method is compared with 32 points for the control stock. In addition, somewhat inferior to that of obtaining a high state of cure an analysis of the curve for compound 1336N-46 in Figure 3 by increasing the time or temperature. shows that the rate of aging is lower for the highly cured The addition of a retarder (compound 1336N-50), auch as vulcaniate because it must age a longer time to reach a MBTS,to a stock containing additional filler slightly reduces given percentage increase in hardness. As expected, Figure rather than improves heat resistance (Figures 7 and 8). The 4 illustrates that the same composition changes least in conclusion regarding retarders must be qualified. MBTS elongation during aging. About 7 days were required for the reduces the state of cure throughout the complete range; highly cured stock to lose 50 per cent of its elongation comtherefore, if it is present, the time required to obtain a high pared with less than 3 days for the control vulcankate. It is state of cure for heat resistance would be greatly increased. interesting to note that the elongation of the highly cured On the other hand, a material such aa sodium acetate @), compound is greater after aging begins than the base stock. that retards at low-temperature cures only, will not d e c t This is desirable, and it wiU be shown later that this is a the heat resistance. function of loading rather than one of state of cure.
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...
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
956
Changes in filler loading have a significant effect on the absolute value of ultimate elongat.ion or hardness both before and after aging but do not alter the magnitude of the changes that take place during heat exposure-i. e., the heat resistance. As shown by Figures 9 and 10, the rate of aging as based upon a 25 per cent increase in hardness or a 50 per cent decrease in elongation is the same for the stocks containing 20.0, 32.0, and 43.0 parts of semireinforcing carbon black. The changes in elongation and hardness illustrated in Figures 3 to 10 show that a high state of cure would be essential for obtaining
Vol. 35, No. 9
TABLE11. HEATRESISTANCE OF NEOPRENE TYPEGN COMPOUNDS Compound 1336N58 Neoprene Type G N 100.0 Stearic acid 0.5 Neosone A 2.0 ... MUF Extralight calcined magnesia ... Semireinforcing carbon black 28.8 FF wood rosin ... Zinc oxlde Litharge ...
...
59
60
61
62
63
100.0
100.0
100.0
100.0
100.0
0.5 2.0
0.5 2.0
0.5 2.0
0.5
... ...
4.0 28.8
...
10.0
...
0.5 2.0
...
... .:. 28.8 ...
4.0 28.8
...
...
10.0
...
1.0
4.0 28.8
'
...
10.0
10.0
...
1.8 4.0
28.8 10.0
10.0
...
S T R E S S - S T R A I N R E S U L(ALL ~ STOCKS C U X E D 30 MINUTES AT 287" F.) Original Stress at 300%, lb./sq. in. 800 1225 600 1100 1075 1000 Tensilestrength lb./sq.in. 2575 2650 2875 2825 2700 2775 Elongation at b k k . % 695 675 900 740 720 800 Shore hardness 52 59 53 59 60 59 After 7-day exposure in 121° C. oven Tensilestrength. lb./sq. in. 375 1650 1725 2050 2150 2100 Elongation at break, % 40 110 225 200 280 370 Shore hardness 75 79 72 77 77 75 Retajnedtensilestrength, % 14.6 62.3 69.0 72.6 79.6 75.9 Retained elongation, ?ZQ 5.8 16.3 20.0 27.1 38.9 46.2 After 21 days in oxygen bomb (70' C. at 300 lb./sq. in.) ... Stress at 300'%. lb./sq. in. 1050 1500 1525 1600 Tensile strength, lb./sq. in. Failed 1525 2375 2050 1925 1875 Elongation at break, r0 In 200 760 520 520 485 Shore hardness 14 81 61 68 70 72 Retainedtensilestrength, yo days 57.5 82.6 72.6 71.4 67.6 Retained elongation, % 29.7 84.5 70.3 72.2 60.6
the maximum heat resistance in most applications, Since a 90-minute cure a t 324" F. may bedifficult to obtain, a suggested minimum cure for heat-resisting Neoprene Type GN stocks is 40 minutes a t 307" F. (60 pounds per square inch steam pressure). With the minimum cure, a higher state of cure would be obtained by using a combination of accelerator, Permalux, and a selective retarder, sodium acetate. It is obvious that the absolute values of hardness depend upon the thickness of the slab, Tests on specimens 0.250 inch instead of 0.090 inch thick show the same general effects
...
64 100.0 0.5 2.0
... ... 10.0
28 8
20.0
1025 2700 695 68
1300 156 71 48.2 22.3 1300
2150 620 65 79.3 90.8
but give absolute hardness values ranging from 2 to 7 points lower. The data in Table I show that when the aging is carried out a t 150' instead of 121" C., the same relations exist between the compounds. A t the higher temperature, changes in elongation are obtained approximately eight times faster and changes in hardness are obtained approximately five times faster. TENSILE STRENGTH
As previously stated, tensile strength det,erioration is not considered a good criterion for evaluating heat resistance. This is illustrated by comparing the tensile strength curves in Figures 11 and 12 with the elongation and hardness curves in Figures 3 to 10, inclusive. The absolute tensile strength values are above a serviceable minimum, but the specimens have hardened to a point where they are no longer serviceable. COMPOUNDING AND PRACTICAL APPLICATION
'
800
700
-DAYS -WNG IN-121" C. Alk OVEN FIGURE NO. 8 HEAT RESISTANCE UNAFFECTED BY ADDITION OF RFTARDER AND INCRFASFD FlLl FR DFCREASING El ONGATlON CURVE A CURVE F
- W TCARBON R O L COMPOUND 11336M BLACK - 3ePTs.
-
CURE : 30 MiN AT 2 8 7 O F . l1336NSDi CARBON BLACK 20 PTsMBTS I PT. CURE: 30 M k . AT 28*.
TFST COMPOWO
-
DECRE&SE I N ELONGATION < 3 DILYS e o m SPECIMEN.%
-
300
100
SOLID LINE -CURVE A DOTTED LINE -CURVE F
The previously described base formula for heat-resisting Neoprene Type GN vulcanizates contains all ingredients necessary for heat or natural aging. Antioxidant and zinc oxide are essential. The former is not added to Neoprene Type GN during manufacture and must be included in all compositions. Zinc oxide is required for good natural aging and significantly improves heat aging. Magnesia is included to balance the physical properties of the vulcanizate, and stearic acid is included for processing, The general use of these materials was discussed previously (2). The data in Table I1 show that the base formula (1336N-62) used to study the effect of state of cure is not the only Neoprene Type GN composition that has good heat resistance or has extremely long life during natural aging. However, extensive laboratory tests and practical experience have shown that this combination is preferred because of general excellent physical properties after vulcanization. The data on compounds 1336N-58 and 59 show the necessity of including antioxidant and metallic oxide. Compounds 1336N60, 61, and 64 illustrate the effects of the metallic oxides: 1. Magnesia stiffens vulcanieates before and after aging but imparts a greater retention of tensile strength after heat aging. 2. Zinc oxide improves the elasticity and thereby helps to limit stiffening before or after aging. P
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1943
957
100 95
x
12
3000
i
OAYS AGING
IN 121OC. AIR WEN
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\ CURVE A - CONrROLQWQUM! CARBON OLACK 32 PTS.
-
5 s y
I
; 6 DAYS AQlNO 12 24 IN 121' 0. AIR OVEN I
I
60T
CURVE & -CURVE
B
CURVE E
-
)
CARBOW B u O K - 3 p r n . TEST OQlWUllD
CIRBON B U C K -Po PTS. - TEST -D -U
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18 36 HOURS AGING IN ISO°C.AIR OVEN
QIRBON BUCK -43 PTS. ALL CURED: JOUIN. AT P Q f f .
72
of heat stability and volatility. The o v e r 4 physiod properties of the unaged vulcanizates must be considered and the action of heat upon them is a factor. Table I11 summariees the changes usually observed in many of the physical and chemical properties of Neoprene Type GN vulcaniaates during aging. As previously indicated, some properties can be correlated with changes in stremtrain properties.
TABLE111. USUALCHLANOES IN PEWPERTIES DIJRINQ HEAT
SERVICE
Property Streas at given elongation Tensile strength
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I I I I2 16 2 MVS A W N 0 IN 8I0cluROVEN
0
1
3. Lead oxide greatly limits hardening during heat exposure, but vulcanizates containing it do not retain tensile strength or extensibility as well aa others.
A comparison of compounds 1336N-61 and 62 shows the improved extensibility after aging caused by the addition of MUF. This effect is even more pronounced than the figures indicate. Finally, from the standpoint of heat resistance exclusively, the baae stock plus FF wood rosin (compound 1336N-63) gives the best heat service. Greater extensibility before and after aging is obtained by the use of rosin. The general use of FF wood rosin is slightly limited because it tends to activate low-temperature curves and increases heat generation during flexing. Inert. U e r s and softeners must be used for loading practical stocks. The softeners must be carefully selected on a basis
Elongation Hardness Resilience yocomprassion set, methodsAandB Water absorption Oil absorption Freeze resistanoe
Effect Increasas Dependent upon filler content and method of compounding Decreasea Increases Decreases Increase when aged during compreasion but decrease when aged prior to compression Increases Decreases UsuaUy decreases
LITERATURE CITED
(1) Am. SOC.Teatine Materials, Standards for Rubber Products, p. 45, Designation D673-42 web., 1943). (2) Catton, Fraser, and Forman, Du Pont Co., Rubber Chemicals Div., Rept. 4 2 4 (1942). (3) Du Pont Co., Rept. B1-63 (1942). (4) Neal, A. M., Bimmerman,H. G., and Vincent, J. R., IND. ENQ. CHEM.,34,1362 (1942). (S) Prettyman, I. B., Ibid., S4, 1294 (1942). PB~SIDNTBD before the Division of Rubber Chemistry et the 105th Meeting of the A X ~ K C A CEIDXICAI, N S~CXEFY, Detroit, Mioh.
