I N D U S T R I A L A N D EhTGINEERILVGCHEMISTRY
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T'ol. 21, s o . 8
Stearic and Oleic Acids as Rubber-Compounding Ingredients R. P. Dinsmore THE GOODYEAR TIRE & RUBBERCOMPANY, AKRON,OHIO
I
T IS proposed in this brief discussion to touch upon the relative effects of stearic and oleic acids in a few typical rubber stocks; then to discuss certain anomalies as to the effect of stearic acid on chemical and physical cure; and, lastly, to consider some of the differences found by use of different accelerators. Inasmuch as oleic acid is the chief impurity in commercial stearic-running from 10 to 15 per cent-it seems desirable to know the behavior of oleic acid alone. Comparison of Stearic and Oleic Acids
The following data show the comparative effects of doublepressed stearic and commercial oleic acids in typical rubber compounds:1 of S t e a r i c a n d Oleic Acids in Typical R u b b e r Comoounds Captax Tread-4 parts acid on 100 rubber 23 volumes black 1 volume ZnO Best cure, 70 minutes a t 127' C. (260' F.) Stearic Oleic Cure a t 127' C., minutes 70-100 70-100 68-90 61-75 Modulus a t 300 per cent, kg. per sq. cm. Acid bloom after 15 hours" None Very bad Captax Friction-O.6 part acid on 100 rubber - volume ZnO 8 parts softener Best cure, 40 minutes a t 127' C. Stearic Oleic Cure a t 127" C., minutes 40-50 40-50 Modulus at 700 per cent, kg. per s q . cm. 85-97 78-85 Acid blooma None No bloom, but very sticky Zinc Oxide Friction-2 parts acid on 100 rubber 7 1 / 2 volumes ZnO 41/2 parts softener Best cure, 30 minutes a t 127" C. Stearic Oleic 30-40 30-40 Cure a t 127' C.. minutes Modulus a t 300 per cent, kg. per sq. cm. 63-78 53-68 Acid blooma None No bloom, but more tacky than control Uncured stock. T a b l e I-Action
I
~
~
cronex carbon black; acid = stearic acid; stiff. as shown by stress-strain curve.
modulus
of S t e a r i c Acid on Physical a n d C h e m i c a l C u r e COMBWED PHYSICAL COMPOSITION SULFUR CURE STIFF. 125 vol. blk. 8. uts. S1. 4 acid Same Same Less [25 vol. blk. S pts. SI 33/4 vol. ZnO Inc. 70% Inc. 15% Inc. 70% vol. blk. S pts. S 1 2 i a / 4 vol. ZnO] 4 acid Inc. 15% Inc. 15% Inc. 3% vol. blk. 3 pts. S f 1 [20V01. ZnO 1.2 Captax] 4 acid Inc. 50% Dec. 8% Inc. 40% [6 pts. SI 1 vol. ZnO Same Inc. 16% Inc. 40% [3 pts. S 1 vol. ZnO 1.2 Curves cross Dec. 20%a Captax] 4 acid Curves cross [ l o pts. S] 1 vol. ZnO Inc. 50% 10% faster I10 pts. S ] 4 acid 20% slower 50% less 110 pts. S 1 vol. ZnO] 4 acid Same 30% Inc. [s1/z pts. S 1 vol. ZnO] 4 acid Slight Inc. Slight Dec. [3I/z pts. S 1 vol. ZnO 4 Cures 1/10 time Great Inc. acid] 0.5 Captax Inc. 10% Inc. 100% SI 1 vol. ZnO 6S 1 vol. ZnO] 4 acid Dec. 10% Inc. 20% 6S 1 vol. ZnO 4 acid] Inc. 90% 0.25 Captax Inc. 600% a See Figure 1.
T a b l e 11-Effect
+ + ++ + + + ++ + ++ + + + ++-+ ++ +
+ + + + +
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T a b l e 111-Comparison 6S 6S 6S
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From these data i t would appear that, in Captax stocks, there is a distinct loss in modulus due to the substitution of stearic acid by oleic. This seems to be more pronounced in the loaded stocks. From the point of view of acid bloom, the tread stock was the only one giving trouble, but this is serious. It mould thus appear that purified stearic acid might be desirable for carbon black stocks, whereas for friction stocks, where modulus is not always so important, less pure stearic could be used, particularly if additional tack is desired. It must be borne in mind that these conclusions are, so far, limited to Captax.
=
+ + +
of Stiffening Effect of Z i n c Oxide and Black (Kg./cm.? @ 700%
+ 1 vol. Micronex ZnO + + 11 vol. vol. Thermatomic
52 25 26
Here we see that if stearic acid is added to a sulfur mix there is a marked decrease in chemical and physical cure. In a sulfur-black stock the cure is unchanged but the stiffness is decreased. Moreover, while the addition of one volume of zinc oxide to a sulfur or sulfur-black stock speeds up the cure and markedly increases the stiffness (double what one volume of black alone will do), the further addition of stearic speeds up chemical cure but has little further effect unless the sulfur is high (10 per cent). The addition of stearic to a sulfur-zinc-Captax stock shows a large increase in the physical cure and stiffness for the black stocks and a softer stock a t low cures for the non-black chang-
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ss
Behavior of Stearic Acid
2
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Let us now turn to stearic acid itself. We will note 9, few things about its behavior with zinc oxide and carbon black. It is necessary to distinguish between the effect on physical and chemical cure. This is shown by some unpublished work by C. R. Park, a summary of which is given in Tables I1 and 111. Bracketed ingredients denote original materials in the rubber mix (except rubber). Material appended to bracket by plus sign is that which gives the effect on cure noted in the adjoining columns. Physical cure picked by hand. Vol. = volume loading on 100 volumes rubber; blk = mi-
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1 This work was done by R W. Beveridge of the compounding staff of the Goodyear Tire & Ruther Co.
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F
%
8L
3
P M/UUTES a t /ZS*C.
ing to stiffer as cure increases. The chemical cure is retarded in both cases. Hence it would seem that stearic acid is, by itself, a retarder of chemical and physical cure. With zinc oxide it changes t o an accelerator of chemical cure and with Captax to an accelerator of physical cure. Here, however, even in the presence of zinc, the chemical cure is retarded.
I N D CSTRIAL A X D ENGINEERING CHEMISTRY
August, 1929
Action of Other Accelerators
The writer has previously published ( 1 ) a so-called accelerator classification which considered various typical groups of accelerators. The following remarks will apply to these groups only: Work with extracted rubber shows: (1) All accelerators are improved by zinc oxide; (2) the zinc oxide must be rendered rubber-soluble by stearic acid except in the case of the dithiocarbamates and the thiurams which arc capable of reacting directly with zinc oxide; (3) Captax and derivatives must have zinc in soluble form t o function a t all; (4) P-nitroso will function in extracted rubber in the absence of zinc oxide but, if zinc oxide is present, it must have stearic :tlso. Hence
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it would seem that the prime function of stearic acid is t o furnish zinc oxide in a suitable form for the use of the accelerator. Otherwise it acts as a softener and retarder. As to why an excess of stearic acid retards diphenylguanidine and has no such action on other accelerators, it is hard to say. Perhaps there is a more or less stable diphenylguanidine stearate formed which is a non-accelerator. Certainly there is much t o be explained as to the mechanism of the zinc soap in the physical cure of rubber. Literature Cited (1)
Dinsmore and Vogt, Trans. I n s f . Rubber I n d . , 4, 85 (June, 1928); Rubber Age, 23, 554 (1928).
Effect of Stearic Acid on Various Crude Rubbers E. W. Fuller THE FISK RUBBER COMPANY,CHICOPEE FALLS,MASS.
U R I S G the last few years considerable aork has been done in determining the effect of increasing amounts of stearic acid in various types of compounds using different accelerators. The work described herein emphasizes the point that in comparisons of this kind a third variable may enter-namely, the crude rubber-and if fine distinctions are t o be drawn as to the correct amount of stearic acid to use with any particular accelerator some consideration should be given to possible variations in the rubbers used. Russell ( 1 ) shows that when different crude rubbers are cured with zinc oxide certain samples give compounds with very poor physical properties while other samples give good results. He connects the tendency t o give well-cured compounds with the ability of the rubber to dissolve zinc oxide. He further showed that those rubbers that do not give good properties can be brought up to normal by adding small amounts of various fatty acids, and thus, in a qualitative way a t least, indicated the importance of organic acids to the vulcanization process. About the same time Whitby ( 2 ) showed that the resins that occur naturally in rubber consist largely of‘ fatty acids, such as stearic, oleic, and linoleic. His figures indicate that the amount of such acids varies appreciably even within any one grade and type of rubber and that there is no direct relationship between the resin content and the :wid content, If all the acid is figured as stearic acid, he shows for firstgrade plantation rubbers acid contents of from 0.50 to 1.92 per cent on the rubber; for lower grades it usually runs lower, He gives a mean value of 1.35 per cent acid for plantation grades and further states that types of rubber other than Hevea usually contain little or no free acid. In later work Whitby and his co-workers (3, 4) study the effect of adding increasing amounts of organic arid to rubber that has been extracted with acetone until it is practically resin-acid free and then compounded with several type accelerators. They show a varying effect with different accelerators and also indicate that the effect may be appreciable when organic acids are added even within the normal acid range for Hevea rubber samples. A summary of the previous work that has been done by other investigators on the influence of fatty acids on vulcanization is also given by Whitby and Evans ( 3 ) .
D
Experimental
Procedure
Various grades of rubber were selected in order to obtain a range of acid values. Each grade was blended on a mill
to obtain uniformity and then an average sample taken for the acid determination. For this purpose 30 grams were extracted with acetone and the extract was then titrated with alcoholic potash. Instead of using Whitby’s deaignation “acid number” (milligrams of KOH to neutralize the acid from 100 grams of rubber), all results have been figured as though the resin acids were entirely stearic acid and the acid content is then given as per cent stearic acid on the rubber. Results
Some of the results obtained by using these rubbers of known acid value in several type formulas are given below. The results have been compressed to show tendencies in as few comparisons as possible rather than to give actual detailed results, as the purpose of this paper is to indicate a condition and no attempt has been made to cover the whole ground of possible variations. Table 1-100 Rubber, 10 Sulfur Cure a t 50 Ibs (3 2 kg per sq c m ) steam pressure 90 Minutes 105 Minutes 120 Minutes (148’ C ) Tensile, in Ibs per sq in (kg per Break 800% Break 800% Break 800% sq cm) Elong Elong. Elong. SMOKED SHEET-1 36% RESINACID Control 2050 1100 2350 1400 2450 1650 (144.1) ( 7 7 . 3 ) ( 1 6 5 . 2 ) ( 9 8 . 4 ) ( 1 7 2 . 3 ) ( 1 1 6 . 0 ) 1% Stearicacid 1950 1000 2250 1325 2400 1600 (137.1) ( 7 0 . 3 ) (158.2) ( 9 3 . 2 ) ( 1 6 8 . 7 ) (112.6) AMBERS4.79% RESINACID Control 2200 1300 2400 2100 1800 2600 (154.7) ( 9 1 . 9 ) (168.7) ( 1 2 6 , s ) ( 1 8 2 , s ) (147.6) 1% Stearic acid 2150 1250 2350 1700 2600 2050 (161.2) ( 8 7 . 9 ) ( 1 6 5 , 2 ) (119.5) ( 1 8 2 . 8 ) ( 1 4 4 . 1 ) ROLLBROWN-0.2070 RESIN ACID Control 1350 750 1650 12501 1000 1800 ( 9 4 . 9 ) (52.7) (1 1 6 . 0 ) ( 7 0 . 3 ) ( 1 2 6 , 5 ) (87.91 lYO Stearic acid 1300 700 1575 900 1750 1200 ( 9 1 . 9 ) ( 4 9 . 2 ) (110.7) ( 6 3 . 3 ) (123.0) ( 8 4 . 4 ) CAUCHO B A L L - O . ~RESIN ~ ~ ~ ACID Control 1050 500 1150 750 1350 900 ( 7 3 . 8 ) ( 3 5 , l ) (SO.8 ) ( 5 2 . 7 ) ( 9 4 . 9 ) ( 6 3 . 3 ) 1% Stearic acid 1000 500 1200 650 1350 850 ( 7 0 . 3 ) ( 3 5 . 1 ) (84.4) (45 7 ) ( 9 4 . 9 ) ( 5 9 . 7 )
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RUBBER-SULFUR-Table I shows four rubbers of varying acid content used in the simple formula 100 rubber and 10 sulfur. Each stock was cured with and without 1 per cent of added stearic acid. The tensiles and moduli are shown for three cures of each set. It is noted that in this type formula the addition of 1 per cent stearic;
