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T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
less t h e solution is heated, filtered, and examined very carefully after each addition of acid. It is believed t h a t t h e above determination of t h e optimum reaction will prove beneficial in t h e preparation of these so-called “protein-free milks.” If we compare t h e nitrogen-distribution d a t a with t h e brief analytical d a t a t o be found on t h e subject of proteinfree milk one cannot help b u t feel t h a t t h e method of their preparation can be improved upon. RELATION O F OPTIXUM p H F O R HEAT COAGULATION TO ISOELECTRIC POINT O P WHEY PROTEINS
T h e optimum reaction for the heat coagulation of proteins (dehydration or denaturation) is not necessarily synonymous with their isoelectric condition. We would not say, therefore, t h a t t h e reaction pH 4.5 is t h e isoelectric point of t h e mixture of those proteins. If we could consider, in view of t h e fact t h a t a t pH 3.8 t h e heat-coagulated curd redissolves and thereby becomes positively charged with respect t o t h e acid with which i t has combined, t h a t t h e isoelectric zone had been overstepped during t h e addition of acid, then we could assume t h a t a t t h e point of maximum curd formation b y heat we had t h e minimum overstepping in either direction. This being t h e case, it would be natural
519
t o believe t h a t p H 4.5 is near t o t h e isoelectric zone for t h e mixture of heat-coagulable proteins in t h e whey. T h e isoelectric point of lactalbumin would then‘ be within this zone. Lactalbumin is only t h e major constituent of t h e heat-coagulable proteins of whey. SUMMARY
1-Using methyl red as indicator, titration curves of whey were determined for hydrochloric, acetic, a n d lactic acids. D a t a are also presented for composing a similar curve for a mixture of hydrochloric acid a n d c a1ci u m chloride. 2-The optimum reaction for t h e heat coagulation of t h e proteins in whey is about p H 4.5 (electrometric). 3-The different acids seem t o have the same effect upon the zone of optimum coagulation. &The inaccuracy of methyl red in t h e determination of the correct reaction of whey is discussed. 5-The composition of t h e curd and the distribution of nitrogen in t h e whey were briefly examined. 6-The utility of this optimum reaction is emphasized in ( a ) t h e determination of “lactalbumin,” ( b ) production of lactose, (c) manufacture of whey cheese, a n d ( d ) preparation of “protein-free” milk.
The Variability of Crude Rubber1 By John B. Tuttle
,
68 BANKSTREET.NEW Y O R KN. ~ Y.
When plantation rubber first came on the market in appreciable quantities, t h e rubber manufacturers found t h a t there was considerable variation between a n y two lots, a n d for some time this fact created quite a prejudice against t h e use of plantation rubber. At first, i t was thought t h a t t h e trouble was entirely due t o t h e way in which t h e rubber was coagulated and dried; and, by exposing t h e wet coagulum t o smoke during t h e drying, attempts were made t o duplicate, as far as possible, the method of coagulation used in preparing t h e best grade of wild rubber, viz., Fine Para. T h e special efforts t o produce a smoked sheet of high quality were quite successful, a n d for some time such rubber commanded a premium over t h e rest of t h e plantation rubber. These smoked sheets were quite uniform in quality, b u t from our present knowledge, i t is quite safe t o say t h a t this superiority was not caused by the smoking, but rather by the unusual care which was taken in coagulating, drying, and smoking, a n d by the fact t h a t only the best quality latex was used in their preparation. Notwithstanding the improvement in smoked sheets, i t soon developed t h a t the problem of variability in crude rubber had not been solved, and a t the Rubber Exposition in London in 1914 the subject received wide attention. By this time, t h e volume of plantation rubber being marketed had increased enormously, a n d t h e importance of this problem of variability grew correspondingly. With the increase in t h e number of factories using plantation rubber, especially where i t 1 Presented before the Rubber Division a t the 58th Meeting of the American Chemical Society, Philadelphia, Pa., September 2 to 6 , 1919.
included those, factories without adequate control, the losses became more serious t h a n ever. Although the conference held in connection with the 1914 exhibition discussed this subject a t some length, no conclusions were reached as t o t h e correct explanation of t h e trouble. Since t h a t time, t h e results of considerable work have been published, largely from t h e laboratories of the Department of Agriculture, Federated Malay States. These investigators have advanced many explanations as to t h e cause of variability, and they have adopted a method of measuring the variability in terms of “the rate of cure” and t h e tensile properties. The investigations on this subject took the form of vulcanization experiments on compounds containing various proportions of plantation rubber and sulfur. Eaton a n d his co-workers a t Kuala Lumpur, F. M. S., worked entirely with a compound of 90 per cent rubber a n d 10 per cent sulfur. Stevens used t h e same formula, while Schidrowitz used 92.5 per cent rubber a n d 7 . 5 per cent sulfur. Others used one or t h e other of these two formulas, but t h e point t o be noted in this connection is t h a t rubber and sulfur were the only constituents of t h e mixture. T h e rubber used was prepared in a variety of ways, a n d the rate of cure and tensile properties were supposed t o show t h e effect of changes in t h e methods of preparation. S U M M A R Y OF EATON’S W O R K
Eaton’s work has been summarized in Bulletin 27 of t h e Department of Agriculture, F. M. S., which gives in detail t h e methods of preparing t h e various
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T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
grades of crude rubber, methods of curing and testing, and the results of t h e tests. The principal studies were with reference t o the effect of the various coagulating agents, washing, creping, and drying. As a result of his work, Eaton concludes t h a t there are two agencies present in plantation rubber, which act as accelerators in vulcanization. These are: . (1) The vulcanization accelerating agent formed by the biological degradation of proteins or organic nitrogenous matter in the coagulum during the early stages of drying. ( 2 ) A vulcanization accelerating agent, preformed in the latex and retained by the dry rubber under certain conditions of preparation. The second substance may possibly be identical with the first, although there are certain indications that they are different. The accelerator formed by the degradation of t h e proteins consists probably of an amine or amino acid, probably the former, since it is known t h a t putrescine, which is a degradation product of animal proteins, behaves like an accelerator. Under t h e normal temperature conditions in Malay (about 85" F. in t h e shade), t h e maximum amount of t h e first accelerator is produced during t h e first 6 or 7 days of drying. By hot-air drying a t about 120" t o 130' F., t h e amount of the accelerator produced during the first 6 days is increased, the change being progressive up t o t h e seventh day, after which time little further change takes place. The amount of this accelerator may also be increased by allowing.the unpressed, or slightly pressed coagulum (slab rubber) t o mature for the period mentioned above. Creping the rubber after this time removes little or none of t h e accelerator. On the contrary, in coagulum which is machined in thin sheets on the day of, or the day following coagulation, only a small amount of t h e first accelerator is formed. Thicker sheets take longer in drying and, therefore, will have an intermediate amount of the accelerator. Smoking the fresh coagulum, on account of t h e antiseptic nature of t h e smoke, inhibits t h e formation of t h e accelerator. The products of t h e smoke which are absorbed by the rubber have in themselves a retarding effect on vulcanization. Sterilization by heat, and refrigeration, inhibit t h e formation of t h e accelerator. I t will be noticed t h a t mild heating increases t h e amount of accelerator, whereas strong heat retards the reaction. The second accelerator, which exists preformed in t h e latex, will be retained in the coagulum and also in t h e finished rubber, by any process of preparation which retains all or part of t h e serum, as, for example, evaporating thin films of latex t o dryness, etc. Slab rubber evidently contains both accelerators because, owing t o t h e fact t h a t t h e coagulum has been allowed t o dry for 6 days before machining, i t is more difficult t o wash t h a n t h e fresh coagulum. Thus Eaton and his co-workers arrive at t h e conclusion t h a t the variability in crude rubber is t h e variability in t h e amounts of accelerators which may exist before coagulation or may be formed later, and which b y the processes of washing and drying are permitted t o remain in the crude rubber. I n only one case in t h e work of t h e authors cited above, has any attempt been made t o work on this
Vol. 13, No. 6
problem with compounds at all comparable with those used in commercial work. It is interesting t o note in this connection t h a t Eat,on, commenting on these tests, states t h a t the variability in the rate of cure has probably been obscured in t h e case of researches in which such mixings have been employed. He further maintains t h a t although t h e variability has been obscured, it still exists. Practically no attempts have been made in these researches t o use t h e ordinary commercial accelerators in vulcanizing plantation rubbers. I n ordinary commercial practice in this country there is a small, but certain, amount of material which is produced by the vulcanization of crude rubber and sulfur only. It is customary t o attempt t o blend t h e various rubbers by t h e process of "massing," consisting simply of working a large quantity of crude rubber on t h e usual mixing mills until i t is quite soft, and when thoroughly mixed, cutting off t h e rubber in the form of small rolls or sheets. I n spite of this, it is evident, from the work of Eaton, t h a t there will be considerable variation in t h e rate of cure from day t o day, owing t o t h e different methods employed on different estates. It is evident, therefore, t h a t manufacturers must be constantly on the watch t o see t h a t only grades of rubber of as nearly the same rate of cure as possible are used. E F F E C T O F ADDED ACCELERATORS
However, t h e vast bulk of plantation rubber to-day is used in mixings in which either organic or inorganic accelerators are present in sufficient quantity t o produce a fairly rapid cure. For this reason, it seems ' a s though t h e work which has been done has been for t h e benefit of a very small amount of plantation rubber, and does not apply t o t h e balance. We may divide t h e substances found in crude rubber, which may influence vulcanization, into two classes: (1) The accelerators formed in the latex or in the coagulated rubber. (2) Retarding agents which have been added to the latex or coagulum (such as any coagulating agent which has not been removed by washing), or substances in the smoke which are absorbed by the rubber, etc. These two classes of substances will always react one against the other, as Eaton has pointed out. The balance between t h e two will determine t h e rate of cure. It should be noted here, however, that these substances are necessarily present in very small quantities, and consequently variations, which in themselves are small, will in t h e absence of fillers and added accelerators produce considerable effect on t h e rate of vulcanization and t h e tensile properties. When accelerators are used these differences are of little importance, because t h e amount of accelerator which is added t o a compound is sufficient in itself t o vulcanize t h e compound correctly, and the presence of these minute amounts of accelerators found by Eaton will have little, if any, effect on t h e vulcanization and tensile properties of such compounds. Not only are these differences small, b u t they are not necessarily indicative of t h e true quality of t h e rubber. Let us consider, for example, a comparison of latex which is coagulated immediately after collec-
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June, 1921
tion, machined at once, and dried as quickly as possible. According t o Eaton’s experience, such a rubber would have very little of either t y p e of accelerator, and would therefore have a n unusually slow r a t e of vulcanization, and t h e tensile properties would be abnormally low. In spite of this, i t must be obvious t h a t this rubber should be of superior quality, because i t has not been exposed t o t h e fermentation processes and t o other deteriorating agents. On t h e other hand, slab rubber has been exposed for some time t o these agencies, and i t does not seem probable t h a t their effect would be beneficial. Yet this is exactly the conclusion which we must draw if we are t o accept Eaton’s explanations. The author has a t various times tested rubber which had different rates of cure when rubber and sulfur only were used, and found t h a t in many cases these differences largely disappeared with t h e addition of, say, 2 t o 4 per cent of litharge, or 0.50 t o 1 per cent of t h e common organic accelerators, such as aniline, hexamethylenetetramine, etc. Some of t h e results of these tests are given in Tables I, I1 and 111.
Time Minutes 90 120 150 180 210
TABLEI-RATE OF CURE Tensile Strength c Lbs. per Sq. In. Compound Compound Compound A B C 600 450 500 850 550 800 1250 700 950 900 450 1100 860 750 1400
Time Minutes 15 30 45
60
Compound AA 2650 3050 3150 3000
TABLE111-EFFECT Time Minutes 1 2
Compound G
.. ..
TIME‘)N MINUTES
..
FIG. R RATE
OF CUE€ (Vulcanized at 287’ F.)
-
Compound DD 2000 2800 2950 2800
-
SMALL AMOIJNTS O F ACCELERATORS Tensile Strength Lbs. per Sq. In. Compound Compound Compound Compound H I I K 1650 .. 2700 OF
..
.. ..
.. ..
00
45 60 75 90 105 120 150 180 210
.. .. l6IjO
iiio
1250 1800 1100
....
560
450
.. ..
..
.. .. .. .. .. ..
.. .. .. ..
.... ..
when only rubber and sulfur are present, show a n excellent degree of uniformity when mixed with zinc oxide and a sufficient amount of accelerator.
---
Compound D 900 1400 1900 2050
TABLE11-RATE
OF CURE Tensile Strength Lbs. per Sq. In.Compound Compound BB cc 2700 2600 3150 3100 3200 3150 3250 3300
521
.. .... .. .. .. ..
*.
The results given in Tables I and I1 are represented graphically in Fig. 1. Curves A, B, C, and D (from Table I) show t h e rate of cure on rubber from different estates, using a compound containing 90 per cent rubber and 10 per cent sulfur. Curves AA, BB, CC, a n d D D (from Table 11) represent the same estates, mixed according t o the formula: 48 per cent rubber, 48 per cent zinc oxide, 3 per cent sulfur, a n d 1 per cent hexamethylenetetramine. I n these compounds, A contains t h e same rubber as AA; B t h e same as BB; C t h e same as CC; and I3 t h e same as DD. We have here a few illustrations, among many t h a t could be quoted, where different estates, which vary widely
It may be t h a t t h e mere fact t h a t Compounds AA, BB, CC, and D D cure a t t h e same rate does not necessarily prove t h a t t h e lots of rubber are of t h e same quality. However, this criticism will hold for all of t h e work done b y Eaton, Stevens, and t h e others, who compare lots of rubber by means of tensile properties, a n d t h e rate of cure. The results given in Table I11 are plotted in Fig. 2, and show t h e effect of t h e addition of another accelerator (the carbon bisulfide addition product with di-, methylamine), with and without zinc oxide. The rubber used in these tests was blended from t e n estates, all of them regarded commercially as of excellent quality. The formulas used are as follows: 0 9 0 per cent rubber: 10 per cent sulfur H-99.9 per cent compound G; 0.10 per cent accelerator 1-90 per cent compound H:10 per cent zinc oxide J-48 per cent rubber; 48.96 per cent zinc oxide; 3 per cent S ; 0.04 per cent accelerator K-48 per cent rubber; 48.9 per cent zinc oxide: 3 per cent S; 0.10 per cent accelerator
From these curves it will be seen t h a t t h e addition of only 0.10 per cent of what, under proper conditions, is a remarkably active accelerator is sufficient t o retard almost entirely t h e vulcanization of t h e rubber. The addition of 10 per cent of zinc oxide is sufficient t o overcome this retarding effect, as is shown in Curve I. T h e remarkable qualities of this accelerator are shown in Curves J and K, which contain 0.04 per cent and 0.10 per cent, respectively. I n a n effort t o find out whether or not t h e zinc oxide was responsible for t h e change in t h e rate of cure, portions of compound H were mixed with neutral barium sulfate, lampblack, talc, whiting, and lime,
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t h a t i t is absolutely unfair t o compare t h e rate of cure under such limited conditions as obtain in t h e tests where rubber and sulfur only are used. What has really been done is t o discuss t h e variability, not in the grade of rubber itself, but in either t h e presence or absence of what we may call foreign substances, which may be accelerators themselves, or may produce a n environment which will permit other accelerators $0 function. I n this way, t h e real variation in t h e rubber is obscured by unduly emphasizing the variation in t h e rate of cure caused by minute quantities of decomposition products. The proper procedure would be t o add t o each mixture a sufficient quantity of zinc oxide t o be certain t h a t the vulcanization will take place in a n alkaline medium. Probably 2 t o 5 per cent would be sufficient for this purpose, and t h e results thus obtained would be of real value in determining the variation in t h e rate of cure, because in this way the conditions of vulcanization would be more uniform than is the case a t present, and hence t h e results would be more truly comparable. SUMMARY
FIG. 2-EFFECT8
OF S M A L L AMOUNTS OP ACCELERATOR (Vulcanized at 287' F.)
in t h e ratio of 90 per cent of compound H t o 10 per cent of t h e pigment. T h e barium sulfate and t h e lampblack produced no effect, and t h e talc practically none, while t h e whiting and lime showed a vast improvement in t h e r a t e of cure and tensile properties, The results with whiting and lime do not quite reach the values for Curve I, b u t this is only t o be expected, since i t is well known t h a t t h e coarser pigments do not give as high tensile properties as t h e finer ones. The whole point in this discussion is t h a t it is not sufficient t o bring together rubber and sulfur, and then assume t h e presence and action of a n accelerator, merely because one method of preparation produces a somewhat more rapid cure t h a n another. The above results could be extended t o show t h a t with many organic accelerators (and Eaton is dealing with organic accelerators in t h e latex) i t is necessary t o have t h e proper environment in order t o develop t h e maximum, or even any accelerating action. It is not the intention of this article t o doubt t h a t certain methods of preparation which are used on some plantations are actually injurious t o the rubber, and this injury will be reflected in t h e short life of articles made from such rubber; but on t h e other hand, i t is undoubtedly true t h a t certain methods of preparation permit t h e formation of small amounts of accelerators or other substances which affect t h e rate of cure, without really changing t h e quality of t h e rubber. It has been assumed t h a t when t h e rate of cure is increased, these substances are accelerators, whereas it may be true t h a t t h e change is only one of passing from a n acid t o a n alkaline environment, in which state i t is possible for t h e accelerators already present t o function in their normal manner. The real object of this article is t o point out the fact
Attention is called t o the fact t h a t t h e usual method of testing for variability in crude rubber really determines t h e variability in the amounts and character of certain foreign substances which, in t h e absence of pigments which will produce an alkaline medium for t h e reaction, tend t o obscure t h e variation which may exist in t h e rubber. Tests show t h a t as little as 0.10 per cent of a remarkably strong accelerator is sufficient, in t h e absence of alkaline fillers, t o retard t h e vulcanization almost entirely. It is recommended t h a t all tests, intended t o discover t h e variability in t h e crude rubber, be performed on mixtures t o which has been added from 2 t o 5 per cent zinc oxide, in order t o eliminate the retarding effect which might be caused by small quantities of foreign substances or decomposition products. Statistics of Benzene Production 150,000,000 gal. was the potenual capacity for light oil of the 65 plants existing in the United States during 1920. These plants are located as follows: Alabama
7
Missouri New Jersey New York
1 2 6
12
Ohio
1
TGhessee Pennsylvania West Virginia Wisconsin
Maryland Michigan Minnesota
The actual production, however, was: LIGHTOIL
..............................
11 3 2
Gallons
1919.... 1920. .................................
90,000,000 110,000,000
B&NZEN& 1914 1915. ................................. 1916 .................................. 1917 .................................. 1918. 1919 1920 ..................................
2,000,000 9,000,000 27,000,000 38000000 55'000'000
..................................
................................. ..................................
63:OOO:OOO 77,000,000
TOLUENE
1914..... . . . . . . . .. . . . . . . . . . . . . . . . . . . . 1915 . . . . . . . . . . . . ..................... .................... 1916 1917 . . . . . . . . . . . . ..................... 19181. . . . . . . . . . . . .................... 1919. . . . . . . . . . . . . 1920
.............
....................
.................................
1Does not include Ordnance' Department stlipping plants.
