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
February 1948
307
ACKNOWLEDGMENT
The approximate expression, Equation 28, is of course identical with Equation 19 for the first-wder reaction. The result could have been obtained by the assumption that the conversion follows a first-order course t o a sufficiently good approximation, a n assumption that is essentially physical in nature. We have deduced Equation 28 from a n assumption of mathematical nature involving a different interpretation of the quantity k. The errors that would be incurred through the use of the approximate expression for zeroth-order and second-order reactions are shown by Figures 3 and 4. For considerable ranges of the q L , the approximation provides an amount of conversion, 1 expression sufficiently precise for practical purposes. It seems to be a reasonable assumption that Equation 28 may be used 6 t h equal confidence for reactions of orders ranging from zero to two. .
-
REACTIONS O b GRANULAR CATALYST
These considerations apply t o catalytic reactions in reactors packed with granular catalyst if apparent contact time is calculated from a space velocity defined as the volume of gas a t the temperature and pressure of the feed gas per free volume of the catalyst bed per unit of time. If the space velocity is based on the total volume of the catalyst bed, the values of the contact time resulting from the application of Equation 28 must be multiplied by the ratio of the free volume of the catalyst bed to the total volume. If the uncorrected values are used in a kinetic analysis of operating data, the resulting rate constant will be the product of the true rate constant and the ratio of the free t o the total volume.
The author is grateful t o C. R. Siple for assistance in the preparation of the figures.
.
NO.MENCLATURE
concentration of kth constituent of mixture, moles/unit weight of mixture initial concentration of kth constituent of mixture G = mass current, mass/unit cross section area/unit time L = reactor length S = space velocity based on feed, volume feed/volume reactor/ unit time t = time u = linear velocity, distance/unit time x = Euler distance coordinate, measured parallel to the axis of the reactor 20 = Lagrange distance coordinate p = relative residual volume l?k = rate of formation of kth constituent of mixture, moles/ unit weight mixture/unit time p = density of mixture, mass/unit volume PO = initial density of mixture 6 = contact time e* =i apparent contact time = 1/S 7 = fraction of reference constituent unreacted = reduced Euler distance coordinate = x / L ck =
C ~ = O
r
LITERATURE CITED
(1) Eckart, C., Phus, Rev.,58,269 (1940). (2) Hulburt, H. M., IND. ENG.CHEM., 36,1012 (1944). (3) Lamb, H., “Hydrodynamics,” 5th ed., p. 12, Cambridge, Cambridge Univ. Press, 1924. RECEIVEDNovember 22, 1946. Published by permission of t h e Director, Bureau of Mines, U. 9. Department of t h e Interior.
GR-S 65, a Low Water Absorption Cosolvmer I J
J. C. MADIGAN, E. L. BORG, R. L. PROVOST, W. J. MUELLER, AND G. U. GLASGOW U.S. Rubber
Company, Naugatuck, Conn.
During the war, military and other essential requirements for insulation placed emphasis on the need for a large volume of a new GR-S type of synthetic with a lower water-absorbing tendency than the standard material. I t was assumed that creaming salt retained by the finished polymer was responsible for this tendency. Attempts to coagulate with sulfuric acid alone in conventional equipment resulted in a floc that was extremely tacky and could not be satisfactorily handled in subsequent operations a t the copolymer plant. Experiments were run using equipment employed in the alum coagulation of GR-S, consisting essentially of two concentric pipes so arranged as to introduce compressed air and latex beneath the acid solution surface. While some improvement was realized, the subsequent operations were still difficult because of floc tackiness. In combination with this equipment, various protective agents were added to the coagulant to reduce stickiness. Glue was found the most satisfactory. By properly adjusting latex and air flow rates and glue concentration, a suitable coaguIum was obtained. As predicted, the polymer thus obtained had
a water absorption lower than other GR-S types available. Successful plant operation was realized a t a Cost comparable to GR-S. The product, GR-S 65, was used in various applications by the wire and cable industry and has been manufactured i n large quantities since December 1944. I t represents a large step in the development of a “pyramid” polymer for wire and cable insulation.
D
URING the recent war, military and other essential requirements placed great emphasis upon the need for a large volume of a GR-S type of synthetic elastomer with good electrical properties for wire and cable insulation. To attain this end, it was decided to attempt production of a copolymer with a lower water absorption than that characteristic of available types of GR-S. One type of GR-S was reasonably acceptable from this standpoint but had been diverted to uses of even higher priority. This was made at the Naugatuck plant opcratt I by United States Rubber Company, which is specially equipped with machinery that strains the wet and dry stock. Regular GR-S, made in a “standard” plant and specially leached at great effort
I N D U S T R I A L A N D ENG I N E E R I N G C H E M I S T R Y
308
to ieduce water absorption, was far from satisfactory. The alum coagulated polymer available a t that time had rather good electrical properties but was sometvhat sloner curing than JTas desired for some processes.
Vol. 40, No. 2’
RESEARCH SOLUTION NOT AD.IPTABLE
This apparently simple course had never been successfully navigated in spite of much previous laboratory work dircctctl toward simplification of the coagulation process. In most ca the addit.ion of the GR-S latex to straight sulfuric or other acid solutions resulted in the precipitation of an extremely tacky mass of polymer in which were occluded large amounts of soap and uucoagulated latex. This polymer could be neither washed nor dried satisfactorily in standard equipment, and in addition n-ould presumably have had high water absorption because of the occluded soap. It was found t,hat satisfactory results could be obtained only when ext,remely violent agitation was used. This agitation could not even be approached on a production scale ivithout extensive plant changes. These findings prompted a search in pilot plant scale equipment for some substitute for this violent agitation. SUBSURFACE A T O ~ l l Z A T l O S
CYIAWLATW ASSEMBLY ATOMIZER DETAL Figure 1. Equipment for Acid-Glue Coagulation Research directed t m m d production of a copolymer of lorn Tvater absorption, involving no major equiprncnt changes in the standard GR-S plant, was begun in the spring of 1944 and reached a successful conclusion in Xovcmber of the same year. FUSDAIIENTA LS
To understand h o this ~ problem was solved, it should he realized that the standard GR-Scoagulation process is continuouu and involves two essential steps: 1. 3lising the latex with a dilute water solution of sodium chloride to produce incipient coagulation. This is called “creaming” but does not correspond to the latex concentration process which carries the same name. 2. Adding this combination to dilute sulfuric acid, nhich completes the coagulating operation. \Then this process is used, the resulting precipitate is a fine, porous crumb v-hich can be re.tdi1y put through the subsequent operations of filtering, washing, shredding, drying. and baling. CREAMING SALT
On t,he basis of an article by Keinp and others (I), the hyporhesis vas sei. up that the salt used for creaming !vas largely I C sponsible for such hydrophilic tendency as appcared in finished GR-S. Part of this sodium chloride appears as water-soluble ash in the product. Soap, used to emulsify the iiionomere for polymerization and not all washed out or conTerted to fattv acid during coagulation, also increases the hygroscopicity of the rubber. However, the amount of soap retained by the polymer \vas found to be a direct function of the amount of salt used for creaming, other factors being the same (Table I). It rhus appeared that the “creaming” step in the process was fundamentally at, fault and would have to be eliminated without impairing subsequent operations.
The first attempt a t devising a simple solution to this problem met with a measure of success. Borrowing an idea from the alum-coagulation process, an experiment was run using so-called “subsurface atomization.” Essentially this consists of mixing latex and compressed air below the surface of a dilute acid solution, with concentric pipes to conduct them separately to the submerged nozzle. The equipment is illustrated in Figure 1 and is cheaply constructed from standard pipe fittings. The action of this device is to break up the latex into tiny particles as it meets the acid solution, and to keep them separated by vigorous bubbling until coagulation is nearly complete. I n t,his manner a crumb was produced which was small and uniform but still of an extremely tacky nature. I t was not susceptible of processing on a product,ion scale, since it could not be washed or dried. The serum obtained was not clear, indicating the presence of small amounts of uncoagulated latex. In an effort to reduce the tackiness of the coagulum, the acid concentration mas varied over a wide range (0.25 to 5.0%)S o subst,antial improvement was obtained in t h k range, which n-as limited by economic considerations. USE OF PROTECTIVE OR DISPERSING AGENT
I; was thought that some sort of protective or dispersing agent, dissolved either in the latex or in the coagulant, might rcduce thr tack of these small particles. It way this idea which brought ultimate success to the undertaking. Some of the agents tried were alum, zinc sulfate, isopropanol, Triton S E , Kacconol XR, S’ultamol, arid glue. In each case the agent \vas dissolved in thc acid solution used in the atomization process.
% OFANNUAL RAZD CAPACITYA msrmm
r
i 3
OF LABORATORY TABLE I. EFFECTOF SALTON SOAPCOXTEXT SILT-.SCID-ConGCL.~TED GR-S
Creaming Salt, Lb./100 Lb. Rubber 0.0 3.4
7.8 13.9 25.9
Soap, % 0.0 0.20 0.63 0.94
1 .o
2
0 I
Figurc 2.
MONTHS OF OPERATION Cumulative Production of Acid-GlueCoagulated GR-S Types
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1948
309
TABLE 11. EFFECT OF PROTECTIVE OR DISPERSING AGENTS Agent Alum Zinc sulfate Isopropanol Triton N E Naeconol N R Vultamol
Glue
Floc Size Fine Fine Fine Fine Fine Fine Fine
Serum Clear Clear Clear Milky Milky Milky
Clear
Tack High High High High High High Slight
30
20
It may be seen from Table I1 that glue was the most effective of the above materials. After extensive experimentation it was found that the optimum concentrations in the coagulant were 0.25% sulfuric acid and 0.05y0 glue. Using this in a 2 to 1 volume ratio with latex, a fine, porous, slightly tacky crumb was obtained. This material appeared to be suitable for plant washing and drying operations. The serum was clear, indicating complete coagulation.
ov
o
i
,AERZION CTIME7WE2KbKs,
i
B
Figure 3. Water Absorption of Various Polymers over Extended Periods
THE PRODUCT
To check the assumptions leading to the development of this process, a comparison was made between salt-acid copolymer and acid-glue copolymer, both prepared in the pilot plant. Results of the tests (Table 111) indicate an appreciable achievement. The water-soluble ash, soap, and water absorption were much lower than had been hoped for, and lower than those of any GR-S copolymer previously produced.
.
Water absorption values on raw polymer are only an indication of what to expect on compounded stock, and do not show the variations which may arise as a result of compounding and curing. The water absorption values in this series may appear low on the absolute scale, but the authors have found absolute values to vary between plants, while relative values remain the same. The smooth curves indicate that the relative values, at least, are TABLE111. TESTS OF FIRSTACID-GLUEGR-S COPOLYMERS correct, and line the various copolymers up in the proper order. The “water absorption” test used in this work consists of COAGULATED IN PILOTPLANT weighing a specially prepared sheet of rubber before and after Test Acid-Glue GR-S Salt-Acid GR-9 immersion in distilled water at 70’ C. for a specified timeWater-soluble ash, % 0.05 0.70 0.04 0 38 Soap, % 20 hours in the copolymer plants and 7 days in the wire industry. Water absorption, mg /sq. em. 2.0 5.9 Values given in this discussion are measured at 20 hours except where otherwise noted. The difference between these weights is used to calculate the amount of water absorbed per unit of exThe process was immediately installed in the plant and with posed area. The unit of measurement used in this paper and but minor changes was quickly adapted to production scale specified for the copolymer plants is milligrams per square centioperation. The cost of the installation was approximately meter. The detailed procedure is given in (2). $500 per line, each producing about 4000 pounds per hour of Presumably this measurement is associated with maintenance rubber. Production rates and operation costs are essentially of insulating characteristics under water or in moist locations, the same as for standard GR-S. a n assumption supported by the data that follow. Further experiments along the line of eliminating glue by providing more violent attrition of the latex and acid solution ELECTRICAL PROPERTIES have not been successful on a plant scale, and after several years’ standing the original process has not been materially A review of the needs and performance of GR-S in the wire altered. industry in the period just before the development of GR-S 65 The polymer produced by subsurface atomization with an is given by Schatzel (3). acid-glue coagulant was made in two variations-namely, X-165 It is not the purpose here to provide a n exhaustive study of the (45-55 Mooney) a-nd X-165 A (55-65 Mooney). The higher electrical properties of insulation made with acid-glue-coagulated Mooney material was made for applications where some sacrifice copolymer. The authors have, however, been furnished I$ith in plasticity was possible in order to obtain a higher modulus. some data of an isolated sort in which GR-S 65 is compared with The 45-55 Mooney material is now designated as GR-S 65. Kaugatuck standard GR-9 With full realization of their In Figure 2 the cumulative production of acid-glue types during the months following this development is plotted in terms of the rated annual capacity at the Institute plant of United States T.4BLE IFT. EXTENDED WATER-ABSORPTION STL-DY Rubber Company. (Water absorption in mg. per sq. om.) WATER ABSORPTION
Since water absorption was the primary criterion available a t the copolymer plant, a prolonged test of this kind was run in which various types of GR-S were compared. The results are shown graphically in Figure 3 and are listed in Table IF’. From these it is evident that GR-S 65 is superior in this respect to all the other types tested, even over long periods of immersion. While the curves are carried only to six weeks, it is apparent from their shape that this superiority would be retained over longer periods. The G R S AC used in this comparison was obtained from the Los Angeles copolymer plant operated by United States Rubber Company.
Immersion Institute Naugatuck Time, Days Std. GR-9 Std. GR-S 4 10.6 5 12.5 8.51 6 13.6 9.06 7 14.3 9.55 ... 8 14.9 Weeks 2 18.2 12.0 3 22.9 15.1 26.0 4 17.4 5 19.6 29.6 6 33.6 21.9 37.4 7 23.9 24.2 40.9 8 Months 3 61.2 ..I
...
GR-S
AC 4.77 5.18 5.54 6.10 6.39
7.98 8.23 9,39 11.8 13.5 15.1 16.9
25.8
Institute Naugatiick GR-S 65 X-165 2.24 ... 2 38 2.61 2.69 2.66 2.97 ... 3.29
3.96 5.32 6.49 8.03 9,39
...
3.82 4.31 5.06 6.53 7.63 8.67
...
10.1
...
14.4
INDUSTRIAL AND ENGINEERING CHEMISTRY
310
Vol. 40, No. 2
capacity is more important in wet 1ocat)ions than its absolute REsIsrhscE (6) hr roo^ TEJIPERATUREvalue. (CORRECTED TO 60’ F.) Another test, porrer factor stability, is considered particularly 3Iegohms per 1000 feet of RW buildin: s i r e ) important for determination of electrical characteristics under Tnwlation Resistance voltage changes. P o w r factor itself indicates energy loss and is lnimersion Time, XaugRiiir,ii Weeks Std. GR-S GR-8 85 especially important, in power rireuits. Thc original work of 12.800 13,200 24 hours developing the method of measuring stability factor and its use 1 11,100 12,500 in evaluating the moisture resistance of rubber insulations were 2 11,600 2,500 3 ... 11.200 described by Schatzel and Graham (4). Table VI1 gives the 4 ... 10,400 results obtained by subjecting the same thrce polymers to this test. The difiercncc is given bctween per cent powcr factor a t T A B LVI. ~ : CHANGEIS SPECIFIC IZTDUCTIVE CAPACITYKITH 80 volts per mil arid that a t 40 volts per mil after prolonged IXMERSION immersion in imtrr at 50” C. Again the desirability of t,hc GR-S ( S o . 14. A K G , imnier3ed i n xl-ater at C., 66 over the other two is demonstrated. 60 cscles. 40 volt3 Der mil.: V, of 24-hour value)
T \ BLE Lr.
ISSGL.4TIoN
ZOO
Immersion Time, Days 3 7 14
28
PiauFatuck Standard (:
R-S
1.44 4 22 6. 30 0. 72
AlulX
Coapnlated
GR-S 1.68 4 20 6.53
7.73
X-lG.5 (GR-S 63) 1.25 2.14 3.58 4.89
liniited scope, the data ave presented liere as at least a qualitative indication of results to he c.spec;ed in electrical tests comparing tile t\vo. The first sei of figure, (Tablc V ) consists of measurements of iriqulation resistance at roam temperature, made a t different times on samples from commrcial scale runs a t the Bristol, R. I., plant of United Stares Rubber Company. The wire was KO. 14 solid A K G Type R W building wire insulated with a 8//8j-inchwall of GR-S compound. The insulation resistance was inoawred at, room temperature in each rase for 4 weeks. That o l the Naiigatuck standard GR-S broke down rather rapidly compared t o that of the GR-S65. These values are for insulation iesistaiice (I.,?.) which may be dwcrihed by tho expression
I.R. = K log,,
n
whi.ri: D is over-all diameter. d is foundation diameter, and K is the insulat,ion-resistant constant. I E. and ,< are not to he rwnfused. Calculation of K for these results shows a 24-hour value of about 33,000 for both compounds, while after two w e k s tlie GR-Sinsulation s h o w 6400 and the 65 value is relatively constant,. Another set of results, reported by a different \Tire and cable manufacturer, provides a similar comparison. In this set, three polyincvs were examined on No. 14 AWG copper conductor, insulated tvith a a/er-inch wall of GR-S compound This is a typical moisture-resistant insulation, used in large volume on w i l t designed for service underground or in moist locations The three polymers were Kaugatuck plant standard G R - Y , alum-coagulat,ed GR-S,and X-165 (now called GR-S 05). The GR-9 h C used in these tests a a s not the same as that used n the extended watrr absorption tests. but !vas obtained froni a uonaffiliated supplicr. The compound used in preparing the insulation for these tests is outlined as folloiss: Copolymer hIinersl pigment
Organic Carbon black Sulfur and accelerators
33 52 8
5 2 100
Table V T shorn the per cent change in specific inductive capacity when thew \vires were immersed in n.ater at 50” C. over a period of time based on initial value after 24 hours’ immersion. The X.16-5 iCR-S 651 appears to he better than eit,her of the other two in this pruper?y. The IOTT change in specific iiiductive
CURE
CS.
PLASTICITY
Cure rate is important in n-ire manufacture because of the high speed continuous vulcanization process usually employed. Stress-strain properties of GR-S 65 are comparable to standard GR-Sexcept for a slightly slovm cure rate due t o the absence of soap. Straight, acid coagulation converts the soap pi,csent, 011 the polvmer to fatty acid. Controlled leaching at neutral pH after the coagulation step can bc rcsortccl to in ordcr t o reconvcrt fatty acid to soap and improve this property. Cure rate may be increased by this additional soap. GR-S 65 made a t the S a u gntuck, Conn., plant, operated by United States Rubber Company is broken do\vn somewhat in the finishing line. The n e t and dry strainiug operations cause a considerable reduction in L.loonc,y viscosity. Since polymer made to a higher reactor viscosity has a higher modulus, this product combines higher niodulus with ii 60 Mooney final visco3ity. Some of the properties of GR-8 66 made at the Institute, W. Va., plant operated by United States Rubber Company are listed in Table VIII. Figurcs shown are the average vdues for the first five months of 1046. By a combination of subsurface atomization Kith the addition of glue to the coagulant, a copolymer mav be recovered froni GR-P latex without the use of sodium chloride for creaming. The raw polymer has a loner yater absorption than types previously made. This advantage can be combined with the added features
TABLE VII.
POTVER
FACTOR SFABILITY
(Co. 14, AWG, difference between % power factor a t SO and a t 40 volts per mil) ~RUeatUCk AlumImmersion Standard Coagr,lated x-167 Time, Days GR-S GR-S (GR-S6.5) . _.
7
2 OR
TABLE 1’111. PROPERTIES OF GR-S 6 Y (Standard Rubber Reserve tpst recipe) GR-8 65 GR-S Test Specification 63 0 24 0 . 5 max. T-olatile matter. % 0 , 5 0 max. 0 15 Ssli. total, %: 0 35 max. Ash, water soluble. % o in 10.00 max. ETA extract, % 7.35 3,756.00 5.03 Fatty acid, 70 0.02 0 . 2 5 max. Soap, % 23.7 Styreve, % .... Stahiliaer, % 1. ?9 .... 45-38 hlooney vkcosity 01 73 rnax. A0 Compounded viwoaity 2600 inin. 2930 Tensile. lh,/*il, in.. 50 min. 600 min. 700 Blonmtion. %. 50 min. 300% modulus 300- 600 3x0 2 5 min. $00-1 100 840 50 rnin. 1030-1450 1250 90 min. 5 , 5 max. Ki!iiams plasticity, mm. at 10 min. 4 2 R F JTilliams recovepy. mm. a t 10 min. 8 0 rnax. Water ahsorption 5 , 5 max. 2.2 Average values of Institute production, January t o bIay 1046, inclusive.
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 t o 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 t o thank t h a t agency for perniission t o 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, a n d C . 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 mercaptobenzothiazole 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 t h a t i t may be some time before research can be got under way again on the plan$ations, i t was considered desirable t o present this work now. It covers the isolation, in a condition as nearly as possible like t h a t 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 t o 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 t o determine the effect on vulcanization characteristics in a n accelerated mix, and on its aging, of all the nonrubber substances present in latex on the assumption t h a t some, at least, of the observed variation is due t o differences in the amount (or character) of these substances which remain in crude rubber a s i t comes into che market. To do this, as much as possible of the nonrubber substances must first be removed from rubber, then prior t o 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 t o 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 t h a n a n hour after tapping had been begun, and immediately added slowly and with stirring to 2.5 liters of Fedistilled 95% ethyl alcohol. T h e 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 t h a t it could be let down onto an inverted stopper t o permit evacuation of the flask without sucking in
