February 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
of wet straining before drying, and dry straining a8 practiced at the Naugatuck plant. This discovery helped point the way toward development of “pyramid” copolymers or wire insulation which now include the above qualities, as well as nonstaining stabilizers and cross-linkage to improve processing ACKKOWLEDGMENT
This investigation was carried out under the sponsorship of the Office of Rubber Reserve, Reconstruction Finance (’orporation, in connection with the government’s synthetic rubber program The authors wish to thank that agency for perniission to release this publication, and the following persons who assisted in the devclopment of GR-S 65 and preparation of the paper: J. F. S Abbott, J. L. Brady, E. R. Burns, C. L LMehl.F. L. Moses, C. M. Nelson, H. B Richmond, andC. G. Strowe.
31 1
LITERATURE CITED
(1) Kemp, A. R., Ingmanson, J. H., Howard, J. B., and Wallder, V T., IND. ENG.CHEM., 36, 361-9 (1944). (2) Office of Rubber Reserve, “Specifications for Government Synthetic Rubber,” Section D-3 (Jan. 1, 1946). 3 ) Schatzel, R. A., “Synthetic Rubbers in the Wire and Cable Industry ” reprint from Symposium on Applications of Syn14)
thetic Rubbers, A.S.T.M.. 1944. Pchatzel, It. A,, and Graham, R. C., Elec. J., 36,69-74 (February
1939). ( 5 ) Underwriters’ Laboratories, ”Standard for Rubber-Covered Wires and Cables,” 4th ed., pp. 88-90, 166-87 (June 1940,
reprinted September 1945.)
’
RECEIVEDSeptember 30, 1946 Presented before the Division of Rubber Chemistry at the 110th Meeting of the AMERICANCHEMICAL SOCIET.Y, Chicago, Ill.
VARIABILITY OF CRUDE RUBBER Effect of Latex Nonrubber Substances o n Vulcanization and Aging Characteristics of Crude Rubber E. M. MCCOLM’ AND J. W. HAEFELE2 United States Rubber Go., Boenoet, Kisaran, Sumatra East Coast
A
group of nine different fractions was prepared from very fresh unpieserved Sumatra latex by methods designed to minimize o r eliminate the possibility of hydrolytic, bacterial, enzymatic or oxidative change. These fractions were then added back singly to a highly purified rubber and their effects on the vulcanization and aging of a puregum mercaptobenzo thiazole compound were measured statistically. Three had a marked effect on the properties of the vulcanizate, and two additional had small effects which were statistically significant. Of the former three, one is water soluble and would account for some of the variability of market grade crude rubbers; its concentration in rubbers should he variable, depending on the absolute dilution of serum carried out prior to coagulation. One other was found in whose absence rubber refuses to vulcanize with mercaptobenaothiazole, zinc oxide, and low sulfur. Evidence is presented which indicates that this fraction is not wholly removed by acetone extraction.
S
EVERAL years before the war an investigation into the causes of the variability in curing and aging properties of plantation rubbers was begun, but was interrupted when only partially complete. However, in view of the awakening interest in crude rubber and the possibility that it may be some time before research can be got under way again on the plan$ations, it was considered desirable to present this work now. It covers the isolation, in a condition as nearly as possible like that in which they exist in fresh, unpreserved latex, of certain fractions which have a pronounced effect on a mercaptobenzothiazole cure and on the aging of the resulting vulcanizate Considerable has been written on the subject of variability in the various characteristics of crude Hevea rubber without complete elucidation of all the causes, and without much effort to ensure freedom from oxidative or hydrolytic changes prior t o Present address, Plantation Division. U. S. Rubber Co., New York, N . Y. * Present address, Procter 8s Gamble Co., Cincinnati. Ohio.
.
testing. When the present investigation was contemplated, it was decided first to determine the effect on vulcanization characteristics in an accelerated mix, and on its aging, of all the nonrubber substances present in latex on the assumption that some, at least, of the observed variation is due to differences in the amount (or character) of these substances which remain in crude rubber as it comes into che market. To do this, as much as possible of the nonrubber substances must first be removed from rubber, then prior to testing added back singly or in combination, in the same proportions and concentration as they occur naturally If these substances were added to an ordinary market-grade rubber, the effect of some might easily be completely masked by the effect of the amounts already present. In this paper methods of isolating the nonrubbers and of obtaining a purified rubber, relatively free of the effects of nonrubbers, are described and the influence on cure and aging of the various substances added to the purified rubber is discussed PREPARATION OF NONRUBBER FRACTIONS
The methods used in isolating the nonrubber fractions were selected to conform to the following specifications: 1. No treatment may be used which would be likely t o alter the chemical character of any substance as it exists in fresh, unpreserved letex. 2. Rigid precautions must be taken to prevent any possible oxidation. Certain fractions were isolated by suitable modifications of Roberts’ ( 7 )procedure. Approximately I kg. of latex was obtained not later than an hour after tapping had been begun, and immediately added slowly and with stirring to 2.5 liters of Fedistilled 95% ethyl alcohol. The coagulated rubber was pressed as free of serum as possible in a tincture press, cut into bits, and immediately placed in a 12-liter flask fitted with a stirrer running in a mercury seal. The seal was so arranged that it could be let down onto an inverted stopper to permit evacuation of the flask without sucking in
312
INDUSTRIAL AND ENGINEERING CHEMISTRY
mercury. Then 3860 cc of redistilled acetone and 6 liters of rcdistilled carbon tetrachloride were added, the seal was let down onto its stopper, and the air was displaced by pyrogallol-washed nitrogen by evacuating, filling with nitrogen, re-evacuating, and refilling until the air remaining constituted only about O.0gc; of the contained gas. The stirrer was then lifted from its seat on the stopper and run until a smooth cement was obtained; to prevent leakage of air into the container, at all times a low positive pressure of nitrogen was maintained. While this was occurring, the aqueous alcoholic liquor, iemaining after removal of the clot oi coagulum as described above, 1% as evaporated to small volume in vacuo on a ~ a t e bath r which was not allowed to exceed 45” C. in temperature. ii small stream of pvrogallol-Tvashed nitrogen was passed through the flask during the evaporation. The liquor was then removed and thc precipitated vrauy lipin (fraction .4)extracted by three extractions with ether, following the method of Rhodes and Bishop (6). The extracted mother liquor wm then evaporated to dryness in vacuo, the residue taken up with a little distilled water, and transferred to a beaker, an excess of redistilled ethyl alcohol was added to prevent bacterial activity, and the solution was reevaporated to dryness in vacuo over sulfuric acid at room temperatuie. I n order to prevent possible oxidation during evaporation, the air in the desiccator nas displaced by nitrogen before evaporation. The friable, very highly hygroscopic, partially crystalline mass remaining in the beaker when dry mas separated into a methyl alcohol-soluble fraction (fraction B) and a methyl alcohol-insoluble fraction (fraction C) by repeated extractions under nitrogrn at room temperature with absolute methyl alcohol. The rubber cement, when solution was complete in acetonecarbon tetrachloride, was removed from the stirrer flask and coagulated by the slow addition of 360 cc of redistilled acetone per liter of cement. The coagulum ahich cohered very quickly, Tvas immediately returned to the flask, redissolved in acetone and carbon tetrachloride as before, and recoagulated. The coagulum )vas then discarded, as it had been shoun by test that tn-o such coagulations remove most of the material soluble in acetone-carbon tetrachloride. The combined acetone-carbon tetrachloride liquors were then evaporated to dryness in vacuo under a constant stream of pyrogallol-washed nitrogen in a water bath held a t not over 4 5 O C. The residue mas separated into an acetone-soluble oily portion (fraction D ) and an acetone-insoluble rubbery poi tion which is probably identical nith Roberts’ “caoutchol” (fraction E) ( 7 ) . The latex proteins were preparcd from serum obtained bv centrifuging diluted fresh, unpreseived latex not over one hour old in a Sharples laboratory supercentrifuge developing a centrifugal force of about 50,000 times gravity. This serum nas obtained in a slightly opalescent condition, evidently containing a small portion of rubber which was of extremely small particle size, since it could not be further separated even by recentrifuging at this high centrifugal force. The acid-coagulable protein (fraction F) mas removed by coagulating this serum with acetic acid within 5 hours of the time the trees had been tapped. Two other protein fractions were next removed by saturating the acid serum with ammonium sulfate (fractions G and H). The residual serum then gave a negative biuret test. The acid-coagulable protein was freed from the small amount of precipitated rubber by dissolving the wet, filtered mass in ITo ammonia solution and filtering through filter paper, by which procedure a clear, rubber-free filtrate was obtained. This was the only method found, after considerable experimentation, by which the rubber could be removed without chemically altering the protein to appreciable extent. Filtration was always allowed to proceed overnight in a bell jar containing a beaker of 1% ammonia solution. Some slight change in this protein may thrrc-
Vol. 40, No. 2
fore have resulted from the 16 hours’ contact with ammonia. The protein was recovered by coagulating the ammonia solution with acetic acid, centrifuging, filtering, washing with redistilled acetone, and drying. The protein fraction, obtained by saturating the acid serum with ammonium sulfate, was recovered in a dry condition containing considerable ammonium sulfate, by filtering and washing with acetone. It was separated into tlvo fractions, a water-insoluble (fiaction G) and a water-soluble portion (fraction H) by dissolving in water, filtering and ~ a s h i n gthe insoluble portion free of sulfate ion. The filtrate containing fraction H was purified by electrodialyzing between parchment membranes against distilled water, using toluene as antiseptic, until this water no longer contained sulfate ions after 4 hours’ dialysis, coagulating xith acetone, filtering, drying, and finally washing three times n ith methl-l alcohol, to remove traces of fraction B which were present. The distilled water used in the dialysis was alp\-ayssubjected to the biuret test n-hich is positive for this protein, to make sure that no protein was dialyzing and that the dialysis cell was not leaking. The test was always negative. During the entire protein recovery procedures, there were only about 5 hours-i.e., during the centrifugation-when enzymatic or bacterial changes could have occurred. Since fresh latex usually shows little evidence of change during this period and the induction period prior to bacteria-content increase is 7 to 8 hours, it is assumed that bacterial and enzymatic activity, if any, was slight. hlcColni (3)has given an analysis of a “sludge” fraction found in fresh, unpreserved Sumatra latex. This fraction was also included in the testing, as fraction I. The average yields of the fractions obtained from a considerable number of preparations are shown in Table I. TABLEI. YIELDSOF KONRUBBER FRACTIOKS Yield De?ignation
-
of latex, % of laiex total solida Description of Fraction Lipin 1.11 0.50 Soluble in water-ethyl alcohol 1.10 2.60 Soluble in methyl alcohol 1.10 2.60 Soluble in water-ethyl alcohol Insoluble in methyl alcohol 3.60R 1.48a Soluble in CClr-acetone Soluble in acetone 1.022.50” Soluble in CCln-acetone Insoluble in acetone 0.17 0.44 Acid-coagulable protein 0.08 0.21 Protein insoluble in saturated ammonium sulfate, water-insoluble Protein insoluble in saturated amI1 0.13 0.32 monium sulfate, water-soluble Sludge from fresh unpresen ed 1 0 2 0 27 latex 5 69 -1 Total a Yields obtained in single experiment in which rubber x a s dissolved in CCla-acetone and precipitated with additional aoeione five times. Yield on fifth treatment was very small.
A small amount of analytical data on some of the above fractions was obtained, with the intention of concentrating, at a later date, on the complete analysis of fractions which were found to have a marked effect on the properties of rubber. FRACTIOK A. LIPIN. Acid number 2.3; equivalent free acid content, calculated as stearic, 1.17%. FRACTIONS B AND C. Approximately 5 grams of each were dissolved in 4 cc. of water, and approximately 85 cc. of 95% ethyl alcohol were added. These solutions were kept at about 10” C. for 12 weeks. The deposits of crystals in each were shown t o be quebrachitol (1-methyl inositol) by determinations of mixed melting point, in which the melting point of purified quebrachitol, of the above crystals, and of an equal mixture of the two were all 193-5’ C. (corrected). Fraction B contained 0% and C, 9.8% of this substance, making a total yield of 0.21% quebrachitol from this latex, as compared with Rhodes’ (6) value of 0.45yo fiom Malayan lstex. The quebrachitol was obtained by a proccdurc which could not possibly have been hydrolytic, indicating that quebrachitol
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1948
is present as such in latex, instead of in combination] as was indicated by Roberts ( 7 ) . FRACTION D. Soluble in carbon tetrachloride-acetone and in acetone. Acid number 46.5; equivalent free acid content, calculated as stearic acid, 23.3%. This observation indicates that free acids, probably fatty in nature, are present as such in latex, either as separate globules or dissolved in the rubber.
FRACTION I. Fresh latex “sludge.” yo acetone extract % CClr extract (rubber) yo protein ( = % N X 6.25)
20.5 36.0 21.5 4.0
% ash % unaccounted for
0.0 __
100.0
PREPARATION OF PURIFIED RUBBER
-.
In order to prepare a rubber as free as possible from the effects of the nonrubbers, it was assumed that not only should all the nonadsorbed water-solubles be removed, but probably also the acetone-extractables and possibly the adsorbed nitrogenous bodies, including protein. To this end three se arate lots of rubber were prepared from a bulked quantity of i t e x preserved with 1.25% of ammonia. Lot I was treated with 2% of potassium hydroxide] and heated for an hour in an autoclave a t 10- ound steam pressure, then cooled, diluted t o 20% solids with xistilled water, concentrated by centrifuging, rediluted with 2% ammonia solution, and recentrifuged, for a total of four centrifugings. Lot I1 was treated in the same way except that the latex was heated an hour a t 20instead of IO-pound steam pressure. Lot I11 was merely centrifuged without caustic addition or heat treatment. From the conditions of centrifuging] it was calculated that this purification procedure had removed 99.89% of the nonadsorbed water-solubles present in the latex of lot I, 99.95% from lot 11, and 99.93% from lot 111. These three purified concentrates were then spread on glass trays and dried in vacuo at room temperature. Except when they were being worked with, the latices and concentrates were stored in an atmosphere of nitrogen to prevent oxidation. Analyses of the rubbers from these three preparations are shown in Table 11. OF PURIFIED RUBBERS TABLE 11. AKALYSES
Lot
% KOH used
Heating, pounds steam Yoacetone extract Yo nitrogen Acid value of acetone extract yo free NHI 9% ash 9 0 s in ash
iron in ash
2
I
10 2.30 0.015 50.0 0.00046 0.12 Trace
