lanuary 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE VI. STOCKS-2503
__ Formula l a Formula 2b __ Formula 3C ~ _ Formula _ 4d 300% 30070 300% Tensile, mod., Tensile, mod., Tensile, mod., Tensile, Cure, lb,./sq. lb./sq. Elong., lb.(sq. lb,/sq. Elong., lb:/sq. lb,/sq. Elong., Ib:/sq. lb,/sq, Elong., hlin. in. in. 70 in. in. Yo in. in. yo in. 111. % 2754 785 2352 790 572 800 634 865 642 2619 960 20 237 ... ... 1700 25 380 3iii 3204 3i69 730 868 727 901 720 so5 854 2300 30 531 3513 687 3550 685 755 940 3252 700 1072 1094 3061 781 40 3410 1182 3621 658 645 680 1063 3447 lf60 700 3400 978 50 3436 1341 608 3593 615 640 655 1153 3500 1297 3495 1130 60 3349 1468 545 555 1441 600 615 1264 3389 3267 3485 1260 70 3362 1546 520 3334 535 585 1530 1374 3458 580 3455 1340 80 3360 520 530 555 1565 3400 570 1586 1416 3434 3400 1415 90 3115 1701 3347 505 1769 1652 3299 495 460 490 3230 1700 120 2961 1780 3139 525 3263 460 435 1799 460 1613 3095 1801 140 2972 425 1750 3085 3002 460 423 1888 450 1884 3055 1770 160 4 Standard batch formula. pigments not dried. b Standard batch formula’except 5 grams only activated ZnO used (1.25parts ZnO). c Standard batch formula except 10 grams only activated ZnO used (2.50 parts ZnOj. d Standard batch formula except 20 grams only activated ZnO used (5.0 Parts ZnOj. _-
Y3,
I
I n the work of these last two investigators there is definite evidence confirming the results of this investigation-that is, in the range of 0 to 0.25y0 water present in the compounded stock, water retards the rate of cure. I n view of the results of this investigation, a theory is advanced to explain the phenomenon of reinforcement by zinc oxide. Probably some of the zinc oxide is functioning as a catalyst for the reaction between mercaptan and sulfur as proposed by Bloomfield ( 1 ) . The remainder might be acting as a desiccant. Jones (4) found that zinc oxide with a smaller particle size will give superior acceleration. This might be expected since smaller particle size would give a greater surface for moisture absorption. It is realized that factory operations, using different types of mastication, involve entirely different conditions. Still, the potential practical application of this work should warrant investigation with say, tvDe mixer. Braendle and ” , a Banburv ” ” _ Wiegand ( 2 ) considered the possibility of adding moisture to GR-S
P7li t o get a faster rate of cura. They rejected this scheme as i m p r a c t i c a l . H o w e v e r , it might be economically sound to activate the zinc oxide to get a greater rate of cure or to reduce the amount of zinc oxide necessary to get the same rate of cure. ACKR 0W LEDGMENT
The authors wish to thank L. W. Boulton and R. C. Thompson for their careful compounding of the samples. They also wish t o thank D. S. Alexander and H. B. Hayes for their helpful advice.
LITERATURE CITED
Bloomfield, G. F., J . Polymer Sci., 1, No. 4 , 293-304 (1946). Braendle, H. A., and Wiegand, W. G., IKD.EN+. CHEM.,36, 724 (1944).
Fischer, K., Angew. Chem., 48, 394 (1935). Jones, H. G., IKD. EKG.CHEW,36, 641 (1944). Kilbank, S.C., and Rush, I. C., private communication to O 6 c e of Rubber Reserve. Lange, N. A., “Handbook of Chemistry,” p. 1235, Sandusky, Ohio, Handbook Publishers, 1941. EXG.CHEM.,ANAL.ED,, McKinney, C. D., and Hall, R.T., IND, 15, 460 (1943).
Mitchell, J. B., et al., J . Am. Chem. SOC.,63, 2927 (1941). Rupert, F. E., and Gage, F. W., IXD.EN^. CHEM.,3 7 , 3 7 8 (1945), Rush, I. C., Ibid., 38, 58 (1946). Varteressian, K. A., and Fenske, M. R., Ibid., 28, 928 (1936). Wernimont, G., and Hopkinson, F. J., IND.ENQ.CEEM.,ANAL ED.,15, 272 (1943).
I
RECEIVED July 31, 1947,
Acceleration of Fine Furnace Black Compounds 0. H. McCOLLUN Rubber Luborutory, E . I . du Pont de Nernours & Contpuny, Wilrninyton, Del. T h e use of furnace blacks of fine particle size as reinforcing agents for rubber is a recent innovation in the rubber industry. These blacks possess properties which will lead to widespread use but many new difficulties develop when they are applied. This paper deals with the problem of improving the processability of stacks containing fine furnace blaclrs so that they can be employed under practical factory conditions. Three approaches to the problem are presented and their relative merits are discussed. These approaches are the use of chemical retarders, reduction in accelerator concentration, and reduction in sulfur concentration. A judicious combination of accelerator with reduced sulfur presents the best solution to the difficulty.
T
HE introduction of fine particle size furnace blacks (average diameter, 32 to 47 millimicrons) as reinforcing agents for rubber represents a recent and noteworthy development in carbon
black technology. While their consumption is small a t present, they possess properties which will undoubtedly lead to their widespread use as a replacement for the channel type blacks. The use of these new blacks in rubber stocks presents many new problems, probably the most important of which is their effect upon conventional types of acceleration. The channel blacks when used in rubber stocks all tend to retard the cure, whereas the use of fine furnace blacks under similar conditions of loading and acceleration produces stocks that have a marked tendency t o vulcanize a t processing temperatures. This property is particularly noticeable when a thiazole type of acceleration is used. These new blacks are all characterized by high p H (relative t o channel blacks) and by a type of physical arrangement of particles which is called “structure.” Although not uncommon to other blacks, structure occurs in the fine furnace types in a more marked degree and varies considerably with the different brands. Both of the foregoing properties tend to make fine furnace black stocks definitely more scorchy than compounds reinforced with channel black.
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
172
70
t
I
F F BLPCKS
I
/
/
11
,
m +
IO00 I , 1 ; 5 IO 15 20 30 1
30
I 90
0T I ME - M I N
FF No. 2 and EPC Blaclis on Cure at 281' F.
Figure 2. Effect of
TIME -MIN
Figure 1.
Vol. 41, No. 1
Mooney Scorch a t 250' F. EPC us. FF blacks
2500-
- RETARDER --- NO RETARDER
0
i l
Y
K I-
5
IO
10
m
1 15 20 TIME -MINUTES
Figure 4.
Ketarders with FF No. 1 black
I n older to establish quantitatively the differences betweeti stocks containing an easy processing channel carbon black and those containing fine furnace blacks the following stocks were prepared as shown in Table I.
TABLE I. RECIPE Conipound Smoked she;ts RPA No. 3 Zinc oxide Thermoflex A " EPC black FF black No. I C FF black No. 2 FF black No. 3 Pine t a r Mercaptobenzothiaeole
Sulfur
/-
.---------parts
100.0 0.2R 5 0
1.0
45 0
...
,..
... 2.0
1.0
2.85
______...
100.0 0 25 5.0 1.0
100.0
45.0
..
0.15 5.0 1.0
...
...
...
45.0
..,
2.0 1.0
2.85
I
t
45
60
TIME
Mooney Scorch a t 250' F.
Figure 3.
l
30
i'.b
1.0
2.88
100,o 0. 23
5.0 1.0
...
43.0 2.0 1.0 2.85
a 36.5% xylyl mercaptan. 63.5% inert hydrocarbon. 25% di-p-methoxydiphLnylamine; 25% diphenyl-p-phenylenediamine; 50% phenyl-8-na hthylamine. 6 Fine furnace b a c k s are designated aB 1, 2. and 3 . starting with the one having the lowest structure.
Figure 1 sliows the Mooney hcorch curves (outhncd i n A.S.T.M. Designation D 927-47TJ using small rotor) of the first three compounds in Table I. The rapid increase in Mooney plasticity of the fine furnace black stocks during the 1- to 15-minute period is a clear indication of their scorching tendency. This is most noteworthy when compared with the scorch curve for the channel black stock which starts rising later and takes a full 15 minutes in its ascent. The use of fine furnace blacks, probably because of their structure, also has a pronounced effect upon the stress-strain curves of the vulcanizates. This is illustrated in Figure 2 where thp
- MIM.
I
I 1 YO
Eflect of Retarder W with FF Blacks on Cure at 281" F.
niodulus at 300yo of the stock containing easy processing channel black is compared over a range of cures at 281 ' F. with that of the stock containing the medium structure fine iurnace black. Over the entire range of cures shoTn the stress at 30070 elongation of the stock containing fine furnace black is approximately 400 pounds per square inch higher than that of the easy processing channcl black stock. If a similar compound had been tested using a fine furnace black having a higher amount of structure, the increase in stress a t 30Oyo elongation would have been even greater, for the modulus of stocks containing fine furnace blacks appears to be roughly proportional to the amount of structure possessed by the blacks. I n order to eniploy fine furnace blacks under practical factory conditions some method of improving their processability must be developed. The following approaches to this probleni are discussed in this paper: 1. The use of chemical retarders 2. Reduction in accelerator concentration 3. Reduction in sulfur coiicentration
I'IN work reported in this paper wab all carried o u l with st,ocks liaring the same composition, except for sulfur, accelerit tor, and retarder concentration, as those shown in Table I. I n order to ensure uniforniity the master batch method of compounding was used, the sulfur, accelerator, and retarder being added as required to a master batch of the other ingredients. The easy processing channel black stock shown in Table I was selected as a contzol, and its physical properties were made a source of reference for the work with t'he fine furnace blacks. For some purposes the optimum properties for a stock containing fine furnace black may not coincide with those for a stock containing easy processing channel black.
January 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY USE OF CHEMICAL RETARDERS
The effect of chemical retarders on the scorch resistance of the fine furnace black compounds was investigated f i s t . Both acidic and alkaline compounds were tried, and the investigation was not limited to the commercial retarders currently available. However, none of the miscellaneous compounds tested were found as effective as the commercially available retarders. Figure 3 shows the effect on processability as judged by Mooney scorch of adding 0.5 part of N-nitrosodiphenylamine and 0.5 part of salicylic acid (Retarder TV) to the low structure fine furnace black stock shown in Table I. While the Mooney plasticity curves show improvement in scorch resistance, they also indicate that chemical retarders alone are insufficient. Although Retarder W has an initial retarding effect on cure as illustrated by the foregoing Mooney scorch curve, it actually activates as cure proceeds. Figure 4 illustrates clearly the activating effect which Retarder W has on the modulus a t 300% elongation of vulcanizates containing both low and medium structure fine furnace blacks. Both compounds were accelerated with 1.0part of mercaptobenzothiazole (MBT) and contained 2.85 parts of sulfur. If the properties of the easy processing channel black stock are accepted as a criterion of quality, then the high moduli shown by the fine furnace black stocks containing Retarder W will act as a modulus reserve and will permit wider variations in sulfur and accelerator concentrations for the purpose of improving processability. For this reason and also to take advantage of the slight improvement that i t gives in processing safety, all stocks, including the control, studied in the rest of this investigation contained 0.5 part of Retarder W. REDUCTION IN ACCELERATOR CONCENTRATION
In order to broaden the scope of the investigation the effect of reduced accelerator concentration was studied with stocks accelerated with 2-mercaptothiazoline (2 MT) as well as with mercaptobenzothiazole. Figure 5 shows a t optimum cure the effect on a high structure fine furnace black stock of reducing the content of mercaptobenzothiazole and 2-mercaptothiazoline from 1.0 to 0.35 part while maintaining the sulfur concentration constant a t 2.85 parts. The gradual decrease in unaged modulus and tensile strength as the accelerator concentration is reduced is illustrated toget>herwith a comparison of these properties with the channel black control stock in which the acceleration is kept constant a t 1.0 part of mercaptobenzothiazole. The modulus a t 300% elongation of the stock containing 1.0 part of mercaptobenzothiazole is much higher than that of the reference channel black compound, but drops below this value a t an accelerator concentration of approximately 0.4 part. The stock containing 1.0 part of 2-mercaptothiazoline has a modulus a t 300% elongation which is even higher than that produced by 1.0 part of mercaptobenzothiazole and it remains relatively high throughout the range studied. Even at a concentration of 0.35 part ol 2-mercaptothiazolina the modulus a t 300% is higher than that of the easy processing channel black control stock. The effect of the foregoing accelerator reduction on processability as judged by Mooney scorch is shown in Figure 6 in which the time in minutes required for a 10-point rise in Mooney plasticity is plotted against the concentration of accelerator. The Mooney scorch time is increased from 15 minutes to an average of 21 minutes by reducing accelerator content from 1.0 to 0.35 part, and the effect is approximately the same with each accelerator. While this is not an enormous difference in Mooney scorch, it should represent nevertheless a significant improvement in processability. I n the compounding of tread stocks one of the most vital factors to be considered is age resistance, and the effect which proper and adequate acceleration has on this property should not be overlooked. It is generally recognized that when the concentration of accelerator is decreased in a compound its resistance t o aging suffers accordingly. Figure 7 shows the effect of accelerator
173
-.-.-.-.-.-.-._
5000
EPC CONTROL
3000
e
.-.
-J'
Mr
-.-EL MBT
1000
;I".
CONTROI
MODULUS. 3 0 0 %
I
I 1.00
I
1
I
0.75
0.50
0.35
PARTS PER 100 RHC
Figure 5 .
Effect of Accelerator Reduction on Cure in FF No. 3 Tread Cure 60 minutes at 2 8 1 O F.
I
1
fi I-
10
PARTS
Figure 6.
d
I o 75
1.00
0.50
0 35
P E R 100 RHC
.iccelerator Reduction in FF No. 3 Tread &looneysoorch at 250' F.
2 DAYS
100.C. OVEN
14 DAYS 02 BOMB
I 1.00
Figure 7.
0.75 PARTS PER
I 0.50 100 RHC
I 0 35
Effect of Accelerator Reduction in FF No. 3 Tread on Aging Curs 60 minutes at 281' F.
reduction on the age resistance of the same high structure fine furnace black stock previously studied. The per cent tensile strength retained after aging for 2 days in the 100" C. air oven (outlined in A.S.T.M. Method D 573-45) and after 14 days in the 70" C. oxygen bomb (outlined in A.S.T.M. Method D 572-42) is plotted against accelerator concentration. The sharp drop in this value after oven aging resulting from the accelerator reduction is clearly evident. The per cent retained tensile strength of the stocks accelerated with mercaptobenzothiazole and with 2-mercaptothiazoline are roughly equivalent over the entire range studied, and drop from an initial value of 70% to one slightly less than 50% a t an accelerator concentration of 0.35 part. The aging period (2 weeks) in the oxygen bomb proved to be too long to permit any conclusions t o be drawn regarding the effect of accelerator reduction on age resistance in the stocks
INDUSTRIAL AND ENGINEERING CHEMISTRY
174 I 50001
-
EPC CONTROL
-.
-
* .- ly/o/
Vol. 41, No. 1
TENSILE
STRENGTH
a! !
2 1 W
14 DAYS
&--__ -LL- ____ 2
- i JIO0
00
PARTS PER 100 RHC
Figure 9. 3.00
2 00 PARTS PER 100 RHC
1.00
Efl'ect of Sulfur Reduction in FF No. 3 'rreaci on Aging Cure 60 minutes at 2 8 1 O
Effect of Sulfur Reduction i n FF KO. 3 Tread on Cure
Figure 8.
F.
Cure 60 minutes at 281" F.
*---
RETAINED TENSILE 2 DAYS IOO°C. OVEN
r :.,/
-..
0 I-
d
2 MT
W
30 ?!
0
'i
E
2 10 m
-
I
I
I
I
3.00
2.00
1.00
PARTS
Figure 10.
PER'100 R H C
Sulfur Reduction in FF No. 3 Tread Mooney scorch ut 250' F.
accelerated with mercaptobenzothiazole, for in no case was the per cent retained tensile strength over 257,. However, the superior age resistance of all oi the stocks accelerated with 2-mercaptothiazoline does enable some conclusions to be drawn and indicates that as the accelerator content is decieased, the age resistance of the stock is impaired, the per cent retained tensile strength dropping from approximately 5 0 % to abouL 30% as tho accelerator content is decreased from 1.0 to 0.35 part. REDUCTION IN SULFUR CONCENTRATION
I n view of the deleterious effect which it has on aging, i t appears that accelerator reduction is not an ideal method of improving the scorch resistance of stocks containing fine furnace blacks. The effect of reducing sulfur on original and aged physical properties as well as on scorch rrsistance was studipd therefore as a third approach to the problem. I n this study benzothiazll disulfide (MBTS) mas used in place of mercaptobeiizothiazole, for this accelerator is likewise a widely used thiazole arid normally produces stocks having greater proceasing safety than does mercaptobenzothiazole. Figure 8 illustrates the effect of reducing sulfur from 3.0 parts to 1.0 part on the tensile strength and modulus of the same high structure fine furnace black stock when using 0.9 part of 2-mercaptothiazoline and 1.35 parts of benzothiazyl disulfide os accelerators. Vhile tensile strengths remain fairly high as sulfur is reduced, moduli, which are initially much higher, drop to or below that of the easy processing channel black control a t a sulfur concentration of 1 0 part. The moduli at 3007, elongation of stocks accelerated n i t h 2-mercaptothiszolirie are consistently higher than those accelerated with benzothiazyl disulfide.
I
!
2MT
100
SULFUR
Figure
11.
3
00
0 75
I
1
0.50 2 00
I
0.35
Comparison of Sulfur Heductiori Accelerator Reduction
1.00
with
The effect of reducing sulfur on age resistance is shown in Figure 9 where the per cent tensile strength retained after aging for 2 days in the 100" C. air oven and for 14 days in the 70" C. oxygen bomb is plotted against' the eoncentration of sulfur used in the vulcanieates. As would be expected, the age resistance of the stocks wit,h both t,ypes of accelerator improves as the concentration of sulfur is decreased, but the rate of increase is greater in the stocks accelerated wit'h 2-mercaptothiazoline. I n the case of the oven aging the per cent retained tensile strength of the 2-inercaptothiaeoline stocks increases from 73 to 93% as the concentration of sulfur is decreased from 3.0 parts t'o 1.0 part. The corresponding figures for the benzothiazyl disulfide stock are 7 3 and 78y0. After oxygen bomb aging all of tho stocks accelerat,cd with 2-mcrcaptothiazolinc show their t'ypically high resistance to deterioration, tho per. cent retained tensile strength incrrasing from 50 t,o 63% as the sulfur is reduced, while the corresponding figures for the benzo thiazyl disulfide stocks are 28 and 3E'%. An improvement in scorch resistance similar to that brought about by reducing accelerator results from the foregoing sulfur variation. I n Figure 10 the time in minutes for an increase in Mooney plasticity of 10 points is plotted against the sulfur concentration in parts per 100 of rubber hydrocarbon. The Mooney scorch time increases from 16 to 22 minutes and is approximately the same for both accelerators. Figure 11 gives a resume and comparison of the effect on aging and on scorch resistance of reducing sulfur and accclerator (2-mercaptothiazoline). The figure shows the almost directly proportional increase in per cent retained tensile strength after aging produced by reducing sulfur as well as the marked decrease produced when accelerator concentration is reduced. The comparative effect on Mooney scorch i s also shown, and in t,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
January 1949
175
2.0 parts of sulfur with 0.82 part of 2-mercaptothiazoline; 1.5 parts of sulfur with 1.0 part of 2-mercaptothiazoline; and 1.0 part of sulfur with 1.3 parts of 2-mercaptothiazoline. With all these combinations, except the last, the desired result of producing stocks having approximately equal moduli was accomplished. The properties of these stocks, both original and after aging for 2 days in the 100' C. air oven and for 14 days in the 70 ' C. oxygen bomb, are shown in Figure 12. Improvement in age resistance by either method of testing as the sulfur concentration is decreased is apparent in these stocks just as i t was in those shown in Figure 9. Many of the stocks containing these fine furnace blacks will MODULUS be used under conditions where dynamic properties such as resilience and heat build-up are important. Figure 13 illustrates the effect of reduction in sulfur content with a simultaneous SULFUR 3 0 0 2.50 2 00 1.30 IO0 2MT 0.65 0.70 0.82 1.00 I30 increase in accelerator content on the heat build-up after 20 PARTS PER 100 RHC minutes' flexing on the Goodrich flexometer (the flexometer Figure 12. Properties of FF No. 2 Treads with Balanced was run at room temperature, 0.187-inch stroke, 1800 cycles per 2 MT-Sulfur Combinations minute, 150-pounds per square inch load, pellet 0.75-inch diameter, 1.0-inch height). Curves are shown for both unaged Cure 60 minutes at 281" F. samples and samples which were aged for 2 days in the 100" C. air Oven and for 7 days in the oxygen bomb before testing. Heat Case the tu-0 curves roughly coincide, indicating that both methbuild-up on the unaged stocks increases very gradually with a ods are equal in their ability to reduce scorch. net increase of some 5 " c. until a sulfur concentration of 1.5 parts is reached, and rises more rapidly between concentrations SIMULTANEOUS SULFUR AND ACCELERATOR VARIATIONS of 1.5 and 1.0. The increase in heat build-up caused by reducing Although decreasing the sulfur content in fine furnace sulfur is much less pronounced in sampleswhich were aged for stocks brings about an improvement in processability, a 2 days in the 100" C. air oven before testing, and the temperature decrease in modulus results. For many purposes such changes differential in this case amounts to only 5 " C. for entire range in physical properties would be undesirable, and means of (3.0 to l.o part) of sulfur concentrations studied. overcoming them would be necessary. If the accelerator Content The advantage of the improved age resistance brought about is increased as the sulfur content is lowered, the tendency to by reducing sulfur is shown even markedly on the samples lower modulus is decreased. Therefore, a group of stocks was which were aged for days in the 7oo c. oxygen bomb before studied in which the concentration of 2-mercaptothiazoline was testing. Stocks containing so-called normal amounts of sulfur increased as the sulfur content was decreased. The combina(3.0 to 2.5 parts) show an increase in heat build-up of 35' and tions used were: 3.0 parts of sulfur with 0.65 part of 2-mereaptoc., respectively, after bomb aging. However, when sulfur thiazoline; 2.5parts of sulfur with0.7part of 2-mercaptothiazoline; is reduced to 2.0 parts the increase amounts to only 10" C. and remains at approximately the same value throughout the rest of the range of sulfur concentrations studied. This shows that after aging the low sulfur stocks are much cooler running than those containing 2.5 or 3.0 parts of sulfur. A similar test could not be carried out with stocks in which a thiazole accelerator replaced 2-mercaptothiazoline since such stocks had insufficient age resistance to permit running the heat build-up test after the / $30OR 101N AL 8-:samples were aged for 7 days in the 70" C. oxygen bomb. w W \2 DAYS The processability of these stocks was evaluated, both by the z40100°C.OVEN 0 W conventional Mooney test and by a mill scorch test, which is I-ORIGINAL 2 - 2 DAYS 100.c. OVEN 3-14 DAYS o2 BOMB
-
,'
I
-:-
I
I
-
concentration of sulfur is decreased the Mooney scorch time increased from 17 to 27 minutes and the mill scorch time from 9 to 24 minutes. As judged by the mill scorch results, this represents an improvement in processability of almost 200%.
Heat build-up, 20 minutea' flexing, cure 60 minutes at 281' F. 30
A/*
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E E2 0
-
MOONEY AT 2 5 O o F
.
L
a
1
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10
-
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:
SCORCH AT 276OF
I
1
SUiMMARY
Chemical retarders such as salicylic acid and N-nitrosodiphenylamine provide a definite improvement of scorch resistance, and should be included in all fine furnace black stocks. Accelerator reduction, while i t improves scorch resistance, is not recommended because of its deleterious effect on aging. Sulfur reduction improves aging as well as scorch resistance. Improved physical properties of both original and aged stocks, together with an increase in scorch resistance, can be most readily ob-
