INDUSTRIAL AND ENGINEERING CHEAUIXTRY
1152
generally impracticable owing to the difficulty of obtaining the material in a suitable condition. It may therefore be of interest to give a few results obtained by comparison of unvulcanized and vulcanized latex sheets. The vulcanized sheet data ( A ) represent good average figures of a standard cure, and the crude rubber sheets ( B ) were made from strictly comparable latex. Elongation per cent at load of 600 grams per sq. mm. Slope Break (tensile), Ibs. per sq. inch Elongation per cent at break 1
#
~
~
~
r
i
plantat,ions were compared with raw sheets from the same plantations, with the following results: VULCANIZED RAW Plant I Plant I1 Plant I Plant I1 Elongation per cent at 600 grams per sq. mm. Slope Break, pounds per sq. inch Elongation per cent at break
( A ) Vulcanized (E)Crude sheet __... ruhher - - - .. sheet __...
520 to 530 35 to 37 3500 t o 4000 770 to 820 Not measurable
800 to 900 50 to 60 700 to 1200 900 to 1000 Very great
I n another series vulcanized specimens (no adjustment for varying curing capacity being made) from two different
Vol. 18,No. 11
527 40 3746 854
596 1040 34 Over 60 3480 759 868 1015
1000 Over 60 639 874
Commercial Development
The manufacture of goods from and with vulcanized latex is being carried out commercially in England and this process is no longer in the laboratory stage. Note-The references to patents and other literature are not exhaustive but are given merely by way of example.
Chemical Research on Raw Rubber at the Netherland Government Rubber Institute’ By A. van Rossem and P. Dekker NBTHERLAND GOVERNMENT INSTITUTB, DELFT,HOLLAND
T
HREE periods can be distinguished in research on raw rubber. I n the pre-war period methods of analysis of raw rubber had been established, but the constitution of the nonrubber constituents was for the greater part left obscure. I n that period the constitution of the rubber hydrocarbon was investigated by Harries, but the results originally published had to be revised in 1914, leaving the impression that the constitution of the hydrocarbon itself was no less obscure than that of the accessory substances. The second period, during the war and for several years following, was one of relative inactivity in relation to raw rubber, research on rubber being concentrated on the vulcanization process, accelerators, and other subjects of general importance. Apart from a few unsuccessful attempts to carry out direct determinations of the rubber hydrocarbon, the chemistry of raw rubber was left in its pre-war state. I n the last few years, however, there has been much activity in research on raw rubber. Much interest was aroused in the rubber hydrocarbon itself by the researches of Staudinger and his pupils, and in the hydrogenation of rubber by Pummerer and his co-workers. In 1924 Pummerer claimed to have isolated rubber hydrocarbon in the crystalline state by a process of purification of raw rubber. At the same time the researches of Hauser on the constitution of the rubber globules of Hevea latex and subsequent investigations of Sebrell, Park, Martin, and Green gave a better insight into the structure of the latex globules and raw rubber. A new line of attack on the constitution of the rubber hydrocarbon was shown by Kate, who was the first to carry out x-ray investigations of raw rubber and concluded from his xray photographs that on elongating raw rubber a pseudocrystalline orientation occurred. Recently these results were confirmed and extended by Hauser, Mark, and Ost. All of these investigators concluded that the number of C6Ha nuclei which constitute a rubber hydrocarbon molecule is small, Hauser and Mark giving this number as 4,Ost as 6. A new chapter of research on raw rubber was opened by Whitby’s investigations regsrding the nature of the nonrubber constituents. In 1923 he published the surprising information that the acetone extract of ordinary Hevea rubber consists of about one-half of its weight of free fatty acids, while in another publication he gave as the further 1
Presented by A. van Rossem.
constituents of the acetone extract of Hevea rubber a phytosterol glucoside, a phytosterol ester, a free phytosterol, Zmethyl inositol, and d-valine. Another worker wortfiy of note in this field is Belgrave, who carried out investigations regarding the nonrubber constituents of rubber latex, especially the distribution of nitrogen among the proteins, albumoses, amino acids, and amides, finding this distribution to be in accordance with what was to be expected from normal metabolic processes. From the foregoing it is obvious that research on raw rubber is diverging in many directions. A few of these investigations which have been carried out a t the Netherland Government Rubber Institute will be described here. The results are by no means conclusive and should be considered merely as preliminary. Quantitative Determination of Free and Combined Fatty Acids i n Raw Rubber
Soon after carrying out the determination of Whitby’s acid number for various raw rubbers, it became evident that volatile, water-soluble fatty acids were present in the acetone extract of raw rubber. The reasons for this assumption were the following: The acid number of the acetone extract is dependent on the temperature at which the acetone extract is dried, the highest acid numbers being found without previous drying of the acetone extract, as is obvious from Table I. However, this is no conclusive proof of volatile acids, as Whitby himself called attention to the possibility of “drying” the unsaturated acids at high temperatures and advised drying the acetone extract a t a temperature not higher than 70” C. Table I-Influence of Previous Drying on Acid Number of t h e Acetone Extract of Ordinary Plantation Rubber Dried at Dried at Not dried 80° C. 100’ C. First latex crepe 348 260 245 239 First latex crepe T12 270 237 192 333 305 282 First latex sheet 349
A more conclusive proof of the presence of volatile acids or their esters was given by saponification experiments. Saponification of the acetone extract, with subsequent acidification and extraction with ether, gives an extract which neutralizes less potassium hydroxide than the saponification of the original acetone extract, indicating that water-soluble acids are present.
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
November, 1926
An extensive analysis of the acetone extract of 2.5 kg. of first latex sheet was subsequently carried out by Dekker,* according to the scheme given in Table 11,confirming the presence of water-soluble fatty acids and their esters. Table 11-Scheme
of Analysis of Acetone Extract of First Latex S h e e t Acetone extract
.1
Water extraction
Water extract
Acetone extract after water extraction
$
Water-soluble free fatty acids
I
I-
I
Mixture of solid and liquid free fatty acids
Unsaponifiable part
I
$-Solid Water-soluble fatty acids
Saponification with Barfod's alcoholic NarCOs solution
I Liquid
Saponification with alcoholic potash
Fatty acids
1
L
Unsaponifiable part
Mixture of solid and liquid fatty acids
From this analysis it was obvious that the acetone extract contained: ( A ) water-soluble free fatty acids, (B) liquid and solid free fatty acids, (C) esters from water-soluble fatty acids, and (D)esters from liquid and solid fatty acids. This analysis, however, does not give any quantitative data on the relative amounts of these four components in the acetone extract and Whitby's acid number gives only the sum of A and B. I n order to obtain a better insight into the quantitative relations of the acetone extract and the importance of its components in general rubber chemistry, it was desirable to determine these four components quantitatively.
+
B ) . The wet acetone (I) A c i d number of acetone extract ( A extract of 10 grams of rubber not previously dried is heated on a water bath with 50 cc. of 96 per cent alcohol and after cooling titrated with 0.1 N potassium hydroxide. ( 2 ) Acid number of the water-soluble part of acetone extract (A). The acetone extract of 10 grams of rubber not previously dried is heated on a water bath with 50 cc. of water, the water extract filtered over a piece of pure cotton wool and titrated with 0.1 N potassium hydroxide. The extraction is repeated with fresh quantities of water till no more water-soluble acids are extracted, which is usually after two extractions. ( 3 ) Saponijkntion number of acetone extract ( A B C D ) . The acetone extract of 10 grams of rubber not previously dried is boiled with 10 cc. of 0.5 N potassium hydroxide and the excess is titrated with 0.1 N sulfuric acid. ( 4 ) A c i d number of alcoholic potash extract of acetone extract (B 4). The liquid which was used for the saponification number is acidified with hydrochloric acid, extracted with ether, this extract dried at 80" C., dissolved in 50 cc. alcohol of 96 per cent, and titrated with 0.1 N potassium hydroxide.
+ + +
+
From these four figures may be calculated A , B, C, and D, as given in the following example relating to first latex crepe 352: Acid number of acetone extract A Acid number of water extract of acetone extract
+ 2 I 3:; B
Saponification number of
A+B+C+D A 4- B
= 283 -514 = 328
+D
= 186
C Acid number of alcoholic potash extract of acetone extract B
-
+
;3;
-
D
=
C
P
79 107
=
45
Therefore, Acid number of water-soluble free fatty acids Acid number of liquid and solid free fatty acids Ester number of water-soluble free fatty acids Ester number of liquid and solid free fatty acids 2
I n d i a Rubber J . , 70, 815 (1925).
(A) (B)
?:j)
= 283 = 107 = 79
1153
Still another point must be considered in connection with the distribution of free fatty acids and their esters in raw rubber. It appeared that certain compounds of fatty acids which are present in the rubber do not dissolve in acetone, but can be isolated by subsequent alcoholic potash extraction. The quantitative determination of these products is carried out as follows: ( 5 ) Acid number of alcoholic potash extract of acetone-extracted rubbpr. The acetone-extracted rubber is swollen in benzene (10 cc. per gram of rubber) overnight and subsequently boiled with 0.5 N of alcoholic potash solution (10 cc. per gram of rubber) for 4 hours. After acidifying with hydrochloric acid the liquid is extracted with ether, the extract dried at 80' C., dissolved in alcohol, and titrated with 0.1 N potassium hydroxide. Various rubber samples gave alcoholic potash extracts from 0.3 to 0.8 per cent and acid numbers ranging from 96 to 30.
The figures of the various samples are given in Table 111. According to this method liquid and solid fatty acids are determined, but it is uncertain whether they were present in the free state or as esters. At the same time it should be pointed out that the water-soluble acids cannot be determined in this way, as potash is absorbed by the rubber, thus preventing the potash solution from being titrated back. Steam distillation of the aqueous liquid, however, showed in one case that the amount of water-soluble acids liberated in this way is still considerable. By the proper choice of an extraction liquid or a mixture of liquids it seems possible to extract in one operation all the fatty acids and their compounds. Table 111-Fatty
Acids a n d Their Esters Present in Hevea Rubber = Acid number of water-soluble free fatty acids = Acid number of liquid and solid free fatty acids = Ester number of water-soluble fatty acids D = Ester number of liquid and solid fatty acids E = Acid number of residual fatty acids and their esters after acetone extraction D E A B C First latex crepe: 348 17 279 163 41 350 56 303 83 33 352 45 328 107 79 First latex smoked sheets: 349 41 339 313 107 60 351 70 270 88 77 30 34 59 T37a,b 17 56 48 T256a,b 31 51 75 67 96 T243O 53 132 31 90 82 Dark brown compound 34 140 25 84 .. Evaporated latexc 62 146 22 34 68 Eaton lab. 316d 213 151 69 0 69 10-year old samples. b Unsmoked. e Sheet of Kerbosch-evaporated latex rubber. d 10 years old; figures on dry rubber. A B C
From Table I11 it is obvious that there is considerable variety in the amount of free and combined fatty acids present in the rubber. In general, the amounts of free fatty acids are larger than the amounts of esters present-except first latex sheet 349, which has a very high ester number of water-soluble fatty acids. Of the free fatty acids present in plantation rubber the greater part are liquid and solid fatty acids, the amount of water-soluble fatty acids being generally low-except the Eaton sample, which shows high free water-soluble fatty acids. The low acid numbers of ten-year-old samples of sheet rubber are in accordance with Whitby's opinion that the unsaturated acids would undergo a drying process. As the number for E of these rubbers is small, it seems probable that the fatty acids are decomposed. It is still too early to give a survey of the role which each of these various compounds plays in rubber chemistry. It seems, therefore, right to give only an example of the important role which one of the groups is able to play. Influence of Water-Soluble Fatty Acids o n Vulcanization with Litharge
Bedford and Winkelmann have shown that the liquid and solid free fatty acids, especially stearic and oleic acid, play
INDUSTRIAL AND ENGINEERING CHEMISTRY
1154.
an important role in vulcanization with litharge as an accelerator. Experiments carried out at the Delft Institute showed that the lower members of the fatty acid series, and even the water-soluble ones, are also important in this connection. From first latex sheet 349, which was deresinated by acetone extraction, the following blank mix was made: Parts 92.5 7.5 10
Acetone-extracted rubber Sulfur Litharge
T o this blank mix various fatty acids were added, stearic acid in the proportion of 1.5 per cent on the amount of raw rubber and the others in equimolecular proportions. All mixes were vulcanized for 10 minutes at 147" C. in an oil bath and the vulcanized rubbers submitted to tensile tests. The results are given in Table IV. .of Lower Members of F a t t y Acid Series on Vulcanization w i t h Litharae Stress Tensile Elongation Kg./sq. cm. strength at break for 600 per cent Fatty acid added Kg./sq. cm. Per cent elongation Extracted rubber (blank) 3 746 Stearic 14 1 773 52 Lauric 162 786 53 Caproic 164 793 54 Butyric 147 771 58 ProDionic 95 772 39 Aceiic 784 33 i4 Formic 5 672
Table IV-Influence
I
..
..
These figures show that the lower members of the fatty acid series, and even the water-soluble, have the same effect as stearic acid on vulcanization with litharge. There seemed a possibility that the decrease in effect becoming visible with propionic acid was caused by a n increasing loss of the lowest members of the series, during mixing, due to a higher volatility. The correctness of this assumption was proved by vulcanization experiments in solution. From a mix consisting of 92.5 parts of acetone-extracted first latex sheet 349, 7.5 parts of sulfur, and 10 parts of litharge, a 5 per cent solution in benzene was made. To this solution were added equimolecular weights of various fatty acids and the various solutions were heated simultaneously in pressure bottles a t 147" C. in an oil bath for increasing time intervals. The time when gelatinization occurred was noticed. Table V gives the results of these experiments. Table V-Influence Blank Added Added Added Added
of Water-Soluble F a t t y Acids on Vulcanization
w i t h Litharge No gelatinization after 120 minutes stearic acid Gelatinization after 10 minutes propionic acid Gelatinization after 10 minutes acetic acid Gelatinization after 10 minutes formic acid No gelatinization after 120 minutes
It is clear that the water-soluble (volatile) fatty acids, even acetic acid, are equivalent to the higher liquid and solid fatty acids in their behavior in vulcanination with litharge, only formic acid holding an abnormal position in this respect. This is in full accord with the exceptional position which formic acid holds in organic chemistry in relation to the other members of the series. The role which the esters of the fatty acids (C and D) play in vulcanization with litharge has not yet been elucidated, but it seems probable that their action will largely depend upon the ability of litharge to saponify these compounds with formation of lead soaps. This will have to be investigated by vulcanization tests of acetone-extracted rubber to which various esters have been added.
that iron has a detrimental effect on raw rubber is one brief remark of Eaton, dating back to 1912, which has escaped general attention. During the course of some experiments at Delft the deteriorating influence of iron was noticed. A piece of crepe rubber, which had been lying under water, loaded with a piece of iron, after drying turned tacky on the spot where the rubber had been in contact with the iron. A more careful investigation of this phenomenon was then made. First latex crepe was kept under water in the presence of blocks of iron; after a fortnight the surface of the crepe was brushed and dried a t room temperature. Analysis showed the acetone extract to be 2.9 per cent and the iron 12 mg. per 100 grams of rubber. Part of this rubber was heated for 2 hours at 70" C. in the dark; no change occurring, another part was kept in light before the window. After 10 days this part had become entirely tacky, the acetone extract having increased to 13.4 per cent, while the blank showed no appreciable increase of acetone extract. Many systematic experiments have since been carried out to ascertain the influence of iron in its various forms. Rubbers were kept in solution of iron salts, in suspensions of iron oxides; iron salts were added to latex and the rubber obtained by coagulation investigated, and experiments were carried out with mixing various oxides in rubber. Two of these experiments will be described. First latex crepe was kept for 5 weeks, (a) in a sealed bottle with distilled water to which FeO was added; (b) in a bottle with distilled water to which FeO was added and through which a continuous current of air passed, for the purpose of oxidizing any ferrous hydroxide which might be formed to ferric hydroxide. After 5 weeks the solutions were filtered and immediately analyzed. The one which had been aired contained 6 mg. of iron, the one which had been sealed, 56 mg. of iron per liter. The crepes were analyzed for their iron content and part heated during 10 hours a t 70" C., and another part kept in daylight before a window for 2 months and subsequently analyzed. The results are found in Table VI. of Iron on Rubber FEO IN WATER No air 38 50 Per cent Per cent
Table VI-Influence
Iron. ma. Der 100 grams-rubber
Blank 6.4
Per cent Acetone-extracted crepe: 2 . 7 After 10 hours at 70' C. 3.0 After lodaysinlight 3 . 0 After 2 months in light 3.4
Aired
2.7
2.7
6 . 0 (slightly tacky) 14.1 (tacky) 3.0 7 . 0 (slightly tacky) 5 . 7 (slightly tacky) 2 2 . 3 (very tacky)
These figures show beyond doubt the detrimental influence of iron, and indicate that iron in ferrous form is more harmful than iron as ferric hydroxide. The material, however, is not sufficient to justify this conclusion as the particle size may have been different in the two cases and might have had an influence of its own. Another experiment showed that very small amounts of iron might have a destructive influence on the rubber. First latex crepe was hung in a 0.1 per cent solution of iron acetate for 15, 30 minutes, 1, 2.5, and 5 hours, the samples of crepe were brushed with water superficially, dried a t ordinary temperature, analyzed for' their iron content, and kept-in light for 4 weeks. Table VII-Influence
Influence of Iron on Raw Rubber
It has long been known that copper compounds have a highly detrimental influence on raw rubber. I n 1921 Bruni found that manganese compounds have a deteriorating influence on rubber. The only evidence in rubber literature
Vol. 18, No. 11
Time in iron acetate Blank 15 minutes 30 minutes 1 hour 2 . 5 hours 5 hours
of Acetate of Iron o n Raw Rubber Iron Acetone extract after 4 weeks in Mg. per 100 grams of raw light rubber Per cent
INDUSTRIAL AND ENGINEERING CHEMISTRY
November,!l926
The results (Table VII) show that 2 mg. of iron are sufficient to cause a large amount of tackiness in light. 8imilar results were found with lactate and oxalate of iron. It should not be concluded that iron in rubber is always harmful. Most samples of ordinary plantation rubber contain small amounts of iron-e. g., the blank rubber contained 6 mg. of iron, without having any effect on the rubber. It is still an open problem under what conditions iron has a deteriorating influence, the experiments carried out giving no conclusive proof in this respect. From the experimental material it seems likely, however, that iron will be detrimental to the rubber only if it is present in a soluble formi. e., soluble in fatty acids to form iron soaps. Another observation calls for attention. When raw rubber is deteriorating under the influence of metal compounds, these metals can be identified in the acetone extract of the tacky samples. This was first noticed for copper and subsequently for manganese, as is shown in Table VIII. a n d M a n g a n e s e in Acetone Extract of First Latex Crepe Acetone extract Metal compound added Per cent Reaction Cu- MnBlank 3.2 C" 14.3 CuSOi (in dark) cu 21.3 CuSOa (in light) 2.7 MnOr (in dark) 13.1 MnOr (in,light) Mn 0.4 KMnO4 (in dark) Mn 15.2 MnzOa (in dark)
Table VIII-Copper
i-
+ E + T +
1158
four samples show poor aging, the acetone extract increasing considerably and with i t the amount of iron present. (3) .Though larger amounts of latex are now shipped in bulk, appreciable quantities of latex arrive in Europe in kerosene tins. These tins are very liable t o corrosion, so t h a t on long standing the latex is contaminated with iron. Shortly after receiving a tin from the Dutch East Indies it was opened, half its content, after intensive stirring, poured into a glass jar, and the tin closed again. After 15 months t h e latex in jar and tin were coagulated, and the rubber coagulum washed and dried. The rubbers were analyzed for iron content. The one obtained from latex from the glass jar contained 3.8 mg. of iron, t h a t from the tin 21 mg , both per 100 grams of rubber. These rubbers were subjected t o an accelerated Geer test for 7 days and also kept before the window for 6 weeks. The results of these tests are given in Table XI. Table XI-Influence
of Iron Tins on Latex
Acetone extract, per cent Iron in rubber, mg. per 100 grams Iron in acetone extract, mg. per 100 grams Aftw heating 7 days a: 70' C. Acetone extract, per cent Iron in acetone extract, mg. per 100 grams After 6 weeks in daylight Acetone extract, per cent Iron in acetone extract, mg. per 100 grams
In glass jar 2.8 3.8 0.8
In tin 3.2 21. 0.5
31 1.6
6 2 3.0
3.2 0.6
8.1 1.5
.
The difference in aging qualities in this case is due to the larger amount of iron present in the rubber obtained from the latex out of the tin. With the increase of acetone extract the iron content of the acetone extract also increases.
In the acetone extract of ordinary plantation rubber a trace of iron is always present, which amounts from 0.1 to 0.8 mg. per 100 grams of rubber. When the rubber has been deteriorating under the influence of iron, this small amount of iron is considerably increased, as Table IX shows.
Decomposition of F a t t y Acids in Raw Rubber
It is not intended to give a full theory about the role which the fatty acids and their esters play in rubber during the various stages of its manufacture, but a few suggestions in Table IX-Iron in the Acetone Extract of First Latex Crepe relation to the chemical changes which the fat@ compounds Iron in in raw rubber may undergo may be of interest. Acetone extract acetone extract in Iron added Per cent mg./100 grams rubber According to Whitby3 the liquid fatty acids present in the 0.7 Blank 3.5 acetone extract are oleic acid and linoleic acid, in amount 0.6 FeO (in dark) 2.5 8.6 FeO (in light) 26.3 about 50 per cent of the acetone extract, with about 5 per cent 0.3 FesOa (in dark) 3.1 of stearic acid. From the foregoing it is certain that esters of a. 6 Fez08 (in light) 9.6 these liquid and solid fatty acids are also present. Although The deterioration of raw rubber under the influence of iron stearic acid is not likely to undergo much change, the liquid compounds is not merely a laboratory problem, but is also of fatty acids and their esters may undergo chemical change in practical importance as the following examples will show: two directions: (a) oxidation with subsequent decomposition, and (b) oxidation with subsequent drying, to which possi(1) Various samples of colored fabrics which had been lined with rubber solution were sent t o the Delft Institute with the bility Whitby called attention. If drying takes place it complaint t h a t the rubber lost its adhesive properties after some must be the linoleic acid or its esters, and Coffey found that time. Some of these colored fabrics were gold and silver brocades which contained large amounts of copper, and i t was ob- no volatile product was formed during drying of linoleic acid. Decomposition products are therefore not to be expecteN vious t h a t the deterioration of the rubber was due t o the action of the copper, this metal also being found in the acetone extract when drying. The first possibility seems the more likely. of the samples. One of these samples was brown in color and conThe oxidation and subsequent decomposition of unsatutained appreciable amounts of iron. On heating this sample for 7 days a t 70" C., the adhesive power of the rubber had gone, rated fatty acids and their esters is a problem which has and in the acetone extract of the sample iron could be detected puzzled many investigators, especially in relation to the occurin appreciable amounts. It is probable t h a t the iron compounds rence of so-called rancidity in edible oils and fats. It is had a deteriorating influence on the rubber, causing it to lose its now generally accepted that chemical rancidity is caused by adhesive power. oxidation under the influence of heat and light. It was (2) Several samples of Klingerite showed a. considerable difference in aging properties. The aging test was carried out formerly supposed that rancidity was equivalent to the by heating these samples for 7 days a t 70" C. (the ordinary Geer amount of acid formed in the fat, but this was proved to be test) and determining the acetone extract and subsequent benincorrect. The chemical reaction generally used for dezene extract before and after the aging test. The iron present in tecting rancidity is the Kreis test,4 which has recently been the acetone extract was also determined. (Table X) modified by Powick.6 It has been proved that this test Table X-Aging of Klingerite ACETONE EXTRACT, PER CENT FE I N ACETONE EXTRACC is not quite satisfactory. Holm and Greenbank6 carefully MG. PER 100 GRAMS RUBBER determined the oxygen absorption of oxidizing oils and Klingerite Before After Before After concluded that the intensity of the Kreis test is proportional samples aging test aging test aging test aging test 354 0.9 1.1 0.6 016 to the amount of oxygen absorbed. Originally these in1.4 0.5 0.6 357 1.5 vestigators were of the opinion that only the oleic acid radical 1.3 0.6 1.4 0.6 366 ~~
35s 359 360
2007
0.9 2.6 0.9 1.8
5.4 12.0 18.8 19.0
0.5 0.6
0.3 0.6
1.2 2.4 8.8 8.0
The first three samples have good aging qualities; neither the acetone extract nor the amount of iron has increased. The other
I n d i a Rubber J . , 68, 617 (1924). Kerr and Sorber, T H I JOURNAL, ~ 16, 383 (1923):Jones, J . SOC.Chcm. I n d . , 43,1258 (1924). 6 THIS JOURNAL, 16, 66 (1923). 8 I b i d . , 16, 1051 (1923). 8
4
1156
INDUSTRIAL AND ENGINEERING CHEMISTRY
would be responsible for the color in the Kreis test, but in later experiments' with pure oleic, linoleic, and linolenic acids they found that these acids on oxidation all show the Kreis test and that the intensity is proportional to the amount of oxygen absorbed. In oxidizing oils and oxidizing oleic acid a great many oxidation products have been identified-for example, formic, acetic, butyric, heptylic, azelaic, and pelargonic acids, various aldehydes and peroxides. The scheme of this oxidation process, which has first been given by Vintelesco and Popesco,*and subsequently has found its way through the literature on fats is the following: CHa
CHa
CHa
CHa
Pelargonic aldehyde 0 (CHd i
I
COOH
Oleic acid
I
COOH
Oleic peroxide
/
TH
(CHn)r
1
Pelargonic acid
0 //
COOH
+0
COOH Azelaic half aldehyde
I 1 COOH
(CHdf
Azelaic acid
This scheme accounts for the peroxide reaction of rancid fats and oxidizing oleic acid as well as for the presence of a few aldehydes and acids, but the origin of many of the lower fatty acids named is still obscure. Modified schemes have also been given, with the aim of giving a better explanation of the formation of these various products.9 It is expected that the oleic acid in the rubber will, a t least under certain favorable conditions, also undergo oxidation and subsequent decomposition. This may account for the water-soluble fatty acids which are always present, as was shown above. Dekker indicated the presence of formic acid and of a t least one other water-soluble fatty acid. However, these water-soluble fatty acids may also have their origin in the decomposition products of the proteins. That under the influence of light the oxidation process in the rubber immediately starts can be shown in a remarkable way. As far back as 1897 Rumelllo found that many substances are able to give a dark picture on the photographic plate. On putting certain substances which oxidize in the dark in the immediate neighborhood of a photographic plate, a picture is obtained. Russell found that a large number of substances were able to give these dark picturese. g., various metals, turpentine, essential oils, coffee, linseed oil, olive oil, rosin, various resins, coal, and wood. Russell also proved that the previous action of ultra-violet light on these substances increased this phenomenon in a remarkable degree. For example, when he exposed linseed oil to the action of light for several hours, the action on the photographic plate was much more intense, and he found similar results for resins, rosin, etc. After careful examination he came to the conclusion that this action was caused by the evolution of hydrogen peroxide. This conclusion has been confirmed by others." Wightman and Sheppard12 later proved that hydrogen peroxide has exactly the same effect on the photographic plate as light. 7
THISJOURNAL, 16,518 (1924). J . pharm. chim., 12, 318 (1915); Kerr a n d Sorber, THISJOURNAL, 16,
383 (1923). @ Tschirch and Barben, Chem. Umschau Fette, O d e , Wachse Harae, 31, 141 (1924); Powick, J. Agt'. Research, 26, 323 (1923). 10 Proc. Roy. Soc. (London), 61, 424 (1897); 63, 102 (1898), 64, 409 (1899) ; 80, 376 (1908). 11 Otsuki, J. Soc. Chem. I n d , 24, 575 (1905); Sailand, A n n Physzk, [4], 26, 899 (1908). 1 2 J . Franklin I n s t . , 196, 337 (1923).
Vol. 18, No. 11
Recently Jameson and Baughman13 repeated these tests for various oils, with the same results. If these oxidizing oils give this effect it is possible that rubber has a similar effect. Russell'O also mentions a similar effect of India rubber: The action which strong light has in increasing the activity of many bodies is important. For instance, i t has been shown t h a t pith may be in contact with a photographic plate at 55' C. for 48 hours and no trace of action is visible, but if the pith is exposed t o sunlight for 2 hours it will then give a dark picture. The same action occurs with old printing paper, with pure India rubber, etc., and many bodies, which under ordinary conditions are but slightly active, become very active after exposure to bright light or simply t o blue rays.
The first set of experiments with rubber carried out a t the institute with Wellington plates was unsuccessful, but the second series with Alfa Ultra Special plates (400 H. & D.) gave positive results. The rubbers were exposed for 4 hours to light (no sun, cloudy sky) and after 24 hours on the plate they gave a positive picture, the sheets being very distinct, crepe being somewhat indistinct. These experiments will have to be extended, but seem to proml'se some insight into the decomposition of the fatty acids and their esters in rubber under various conditions. The first stage of oxidation, the peroxides and (or) hydrogen peroxide, could also be indicated by the usual reaction with potassium iodide and starch. The water extract of various rubbers, also after exposure to light, did not give any reaction. Subsequently, f i s t latex crepe was extracted with acetone and part of this extract heated a t 105' C., while another part wa@exposed to light for 2 hours. The potassium iodide-starch reaction was negative for the normal acetone extract and the one heated, but decidedly positive for that part of the acetone extract that was exposed to light. This test is also to be extended further. As for the influence of iron, Emery and Henleyl* found that some metals, among them iron, are able to accelerate the rancidity or rather the oxidation of fats and fatty acids. We suppose that iron has to be present in a soluble, or a t least in an extremely fine, state of division to have a catalytic influence on the oxidation and subsequent decomposition of the fats. It is probable that with the formation of a large amount of lower fatty acids there is a tendency for large amounts of iron soaps to form, and this process probably accounts for the increase of the iron content of the acetone extract. Conclusion
The writers believe that there is little doubt that the tackiness and oxidation of raw rubber is closely related to the decomposition process of the fatty acids and their esters in rubber. Exactly how this relation is established is still obscure. However, it is obvious that a closer study of the chemistry of fats will also be of great benefit to the rubber chemist and it is not bold to predict that rubber chemistry has arrived a t a-stage at which cooperation with the fat chemist is necessary for further progress. 1 8 J. Od F a f Ind., 2, 25 (1925); see also Stutz, Nelson, a n d Schmuk, 11, 1138 (1925). THIS JOURNAL, 14 Ibid.,14, 937 (1922).
Calendar of Meetings International Conference on Bituminous Coal-Carnegie Institute of Technology, Pittsburgh, Pa., November 15 t o 19, 1926. American Institute of Chemical Engineers-Winter Meeting, Birmingham, Ala., and Atlanta, Ga., December 6 t o 10, 1926. American Chemical Society-73rd Meeting, Richmond, Va., April 12 t o 16, 1927. Second National Symposium on Organic Chemistry-Columbus, Ohio, December 29 t o 31, 1927.
