I-VDUSTRIAL AND ENGILVEERII;VGCHEMISTRY
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VOl. 19, No. 3
The Alternating Behavior of Fatty Acids Added to Rubber Compounds' By W. H. Smith and C. E. Boone SATIONAL BUREAUOF STANDARDS, T ~ A S H I X G T O N , D. C.
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T HrlS been shown by Whitby%that the resin of Hevea rubber consists largely of free organic acids, and the presence of one solid and two liquid acids has been demonstrated. The acetone-soluble material obtained from Hevea rubber contains approximately 5 per cent of solid acid and 50 per cent of the mixed liquid acids. L. E. Weber3 and Stevens4 found that litharge has little or no accelerating action in rubber vulcanized in the absence of resin. The addition of resin to rubber containing a normal amount of resin improves the action of litharge. Bedford and Winkelman5 noted that the lead soaps resulting from the interaction of litharge and resin are soluble in rubber. They proved also that the resin acids play a part in the acceleration produced by lime and magnesia. Sebrell and Vogt6 pointed out that zinc oxide activates the cure because of its interaction with resin acids, and showed that a rubber-sulfur mixture containing one per cent of zinc oxide becomes transparent when vulcanized. A similar mixture prepared with acetone-extracted rubber is opaque after vulcanization. Sebrell and \'ogt6 and Russell' noted the enhanced physical properties of vulcanized rubber containing zinc oxide and fatty acids, or other acids which form zinc compounds soluble in rubber. In this connection, Whitby and Simmons8 obtained increased tensile strengths by the addition of increasing amounts of stearic acid to a vulcanizate containing 100 parts of rubber, 10 parts of sulfur, 5 parts of zinc oxide, and 0.44 part of zinc pentamethylenedithiocarbamate. In one series the maximum effect, was produced by the addition of 2.25 parts of stearic acid to 100 parts of rubber. It is evident from their results showing the effect on physical properties that there must be unizIQo form dispersion of the fatty acid in the rubber. L. E. Weberg referred to the use of oleic '''O acid as an accelerator by C. 0. Weber in 1904 PZOO as the first utilization of an organic accelerator of $ e,o vulcanization. According to Martin and Davey'O the fatty t acids in rubber resin cause deflocculation of the f zinc oxide and as a result t,he zinc oxide is more uniformly dispersed in the rubber. The contact of the rubber and zinc oxide particles c '60 approaches a maximum as a result of deflocculation and therefore minimum flow of rubber oca curs under high loads. The result is a stiffening of the stress-strain curve, and an increased tensile strength. While this may constitute an adequate explanation of the function of resin acids in the case of compounds high in zinc oxide, it is possible that the resin acids or their zinc salts may function as accelerators as well as deflocculating agents. 1 Received November 3, 1926. Published b y permission of the ~ i
Bayer" in 1877 discussed the regularity which occurs in the melting points of homologous compounds. Members of the fatty acid series with an uneven number of carbon atoms were found to have a lower melting point than members with an even number containing one more carbon atom. I n both the odd and even series the melting points, with the exception of those of the first members, rise. The data of Biach12 on the same subject indicate that the differences in melting points are remarkably uniform for acids higher than valeric. In a discussion of fats used in the diet of diabetics, hlcKeel3 stated that glycerides of acids with an uneven number of carbon atoms are tolerated and that gradual reduction of acidosis results from their use. Glycerides of acids with an even number of carbon atoms break down during metabolism, with the formation of butyric, oxybutyric, and acetoacetic acids. Garner and Randall,'? m.ho studied the alternation in the heats of crystallization of the normal fatty acids, showed two different curves of behavior corresponding to the acids with odd and even numbers of carbon atoms. The alternations of the heats of crystallization and melting points of an homologous series were shown to be a result of differences in the crystal structure between odd and even numbers. Experimental Procedure
The normal straight-chain fatty acids from formic to stearic, inclusive, have been studied to determine their relative properties as accelerators when used in connection
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tor, National Bureau of Standards. * J . SOC. Chem. Ind., $2, 336T, 369R (1923) ; I n d i a Rubber J . , 6 8 , 618 (1924); THISJOURNAL, 16, 1008 (1923); J . Chem. SOL. ( L o n d o n ) . 12% 1448 (1926). 3 Intevn. Cong. AppZ. Chem., 1912, I X , 95. 4
J . Soc. Chem. I n d . , 36, 875 (1916).
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THISJOURNAL, 16, 32
(1924).
I b i d . , 16, 792 (1924). 7 U.S. Patent 1,467,197 (1923). 8 I n d i a Rubber J . , 68, 737 (1924). 9 Ibid., 63, 793 (1922). 10 J . Soc. Chem. I n d . , 44, 317T (1925); I n d i a Rubber J . , 7 0 , 455 (1925).
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with zinc oxide in rubber compounds. The effect of the phenyl fatty acids, benzoic, phenylacetic, and hydrocinnamic acids, has also been determined. The purity of the acids was ascertained by determinations of the melting p i n t , , boiling point, and density. When necessary, acids which ~ ~ did ~ not - conform t o accepted standards for purity were purified until satisfactory agreement was obtained. The fatty acids Tvere added to blanket or rolled brown crepe pale crepe, sufficient quantities of each type of rubber were reserved exclusively for this work. The blanket crepe was used in a compound containing 100 parts of rubber, 10 parts of sulfur, and 5 parts of zinc oxide. The compound
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11
12 13 14
B e y . , 10, 1286 (1877). Z . physik. Chem., 60, 43 (1904-05). .~~nruu-wissenschaflen,11, 938 (1923). J . Chem. SOC.( L o n d o n ) , 125, 881 (1924).
I-VDUSTRIAL .4ND ELVGINEERING CHEMISTRY
llarch, 1927
employed with pale crepe contained 100 parts of rubber, 7 parts of sulfur, and 5 parts of zinc oxide. illolar proportions of the acids were used, mol with cach 1000 grams of rubber. A master batch of the base compounds was prepared and the correct proportion of the ,xid thoroughly incorporated on an experimental rubber mill. The uqe of molar proportions of acid conforms to the recommenh IO00
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d= 11900 800 P p 1700
t
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600 IS00
P /400
Carbon Atoms
dations of Kratz, Flower, and Coolidge's that comparisons of organic accelerators should be made with molecularly equivalent amounts of the substances in question. The compounds were cured in slab form in sheets approximately inch thick, using aluminum molds. The time of cure was varied by 10-minute increments from an undercured to an overcured compound. Tests of tensile strength were made 24 hours after vulcanization according to the method described by the Bureau of Standards.I6 The results represent an average of three determinations of tensile strength. It was found that the alternating behavior of the acids was most evident in blanket crepe a t a 50-minute cure and in pale crepe a t a 40-minute cure. The tensile strength a t break was determined as a measure of the condition existing a t the end of these periods. S o relation was found betveen elongation a t break and the number of carbon atoms in the acids. The values for elongation a t break were higher in the first four members of the series of fatty acids and dropped uniformly to a fairly constant value in acids with a carbon content higher than butyric.
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than the odd, numbered acids, give the greater values of the tensile strength. Above acids of 14 carbon atoms the curves for the odd and even numbered acids practically merge. The difference in behavior of the two kinds of rubber when treated with fatty acids is presumably due to the difference in kind and proportion of acids which they originally contain. Blanket crepe, since it is relatively poor in fatty acids, allows full scope for the influence of added acids, and shows to a marked extent their alternating property. I n pale crepe, on the other hand, the fatty acids already present overshadow, but do not completely mask, the influence of added fatty acids. A complete examination of the acids present in the two lots of rubber used in this investigation was not made. However. it was determined that ff,n the pale crepe gave an acetone extract of 3.6 per cent as compared with 2.3 per cent for the blanket crepe. The amount of potassium hydroxide required to saponify the extract from one gram of rubber was determined. It was found that 2.3 mg. of potassium hydroxide were required for the pale crepe and 1.8 mg. for the blanket crepe. The character of the fatty acids would probably show more significant differences than the amounts. The behavior of the fatty acids containing a substituted phenyl group is given in graph No. 3. There is a progressive acceleration from benzoic or phenyl formic to hydrocinnamic or phenylpropionic acid, but there is no evidence of alternating behavior. This is not surprising since the boiling points and melting points of the phenyl-substituted acids themselves show no alternating behavior. The tensile strength a t break of rubber containing phenylpropionic acid a t the 40- and 50-minute cures is approaching the maximum tensile strength, obtained m-ith the normal fatty
Results
The results are shown in graphic form. I n graph No. 1 tensile strength a t break is plotted against increasing carbon content of the entire series of fatty acids comDounded with blanket crepe. The "effect is a *series of alternating, jagged breaks. Dotted lines drawn through the points for the acids with an odd number of carbon atoms and those with an even number, respectively, give two smooth curves. The maximum tensile strengths of compounds containing acids with 3, 5 , 7 , and 9 carbon atoms are greater than the strength of compounds containing acids with one more or one less carbon atom, up t o Clo. The relation is reversed in compounds containing acids with 11, 13, 15, and 17 carbon atoms in comparison with those containing acids with 12, 14, 16, and 18 carbon atoms. The effect of paraffin, as shown in the graph, is decidedly less than that of the higher f a t t y acids, which may indicate that the presence of the hydrocarbon chain in the acids is not the significant factor in determining their effects. Graph No. 2 shows the effect of the odd and even acids on the tensile strength of the second compound in which pale crepe was used. The alternating character of the effect of the acids is evident, but is less marked than with blanket crepe. For lower members of the series the even, rather l6 1%
THISJOURNAL, 12, 317 (1920). Bur. S t a n d a r d s , Circ 252.
acids a t the same cures. Unfortunately, this work does not show whether the higher members of the phenyl-substituted acids would produce higher tensile strengths than were obtained in the absence of the phenyl group. Conclusion
The fatty acids dissolved in rubber presumably in the form of their zinc salts, show an alternation in behavior with respect to the tensile strength a t break similar to that observed by Bayer" for the melting points of these acids and similar to the observation of Garner and Randall14 on their heats of crystallization. Acknowledgment
This work was done in part by Michael Levin, formerly assistant chemist a t this bureau, to whom due acknowledgment is made. The writers are indebted to Morris Kharasch of the University of Maryland for comment and criticism. The "Eulan" process for moth-proofing of furs has been acquired by Swears and Wells, London, for the British Isles.
