INDUSTRIAL AND ENGINEERING CHEMISTRY
32
Downloaded by UNIV OF PRINCE EDWARD ISLAND on September 8, 2015 | http://pubs.acs.org Publication Date: January 1, 1924 | doi: 10.1021/ie50169a013
leather of pH = 9 gained 13.8 per cent in weight even though it lost some tannin, showing a very pronounced oxidation of the alcohol followed by aldehyde tannage. CONCLUSIONS The data given show that tannin fixation at and upon the acid side of the isoelectric point of collagen is of lower degree of tenacity from that fixed on the alkaline side, or that certain substances which do not tan anionic collagen can combine with cationic collagen and that this combination becomes firmer upon desiccation of the leather. It appears to the authors that studies of alcohol extraction of various vegetable leathers may lead to interesting results concerning the difference in nature of tanning between cationic and anionic collagen, and help to distinguish between the different sorts of fixation of tannins by hide, which, without doubt, occur simultaneously-e. g., (1) collagen cations and tannin anions combine to form leather, possible
Vol. 16, No, 1
only on the acid side of the second isoelectric point of the collagen; but (2) since tanning is also accomplished on the alkaline side, a different sort of linkage from a simple ionic reaction must be accepted as possible. Powarnins believes that the keto form of tannins has the property of tanning pelt while the enol form has not. The keto form exists predominantly in acid solutions, the enol in alkaline. If this theory were the sole and correct explanation of the tanning property of vegetable tannins, then one would be forced to admit from the data given herein that in the presence of a predominance of the keto modification, the fixation is not so deep-seated as with the enol form, with the possible exception of wattle bark. ACKNOWLEDGMENT The authors are indebted to A. F. Gallun & Sons Company for grants in aid of this investigation. fi
Collegium, 1912, p. 105.
Reactions of Accelerators during Vulcanization' VI-Organic
Acids and Inorganic Accelerators
By C. W.Bedford and H. A. Winkelmann THE B.
HE effect of acids on
T
F. GOODRICH Co., AKRON, OHIO
since in a Pure gum Metallic oxides and organic acids, both of which haoe long been therateofvulcanizastock the Of the known US accelerators for the culcanization of rubber by surfur or tion of rubber has extract had Only a aromatic nitro compounds, are described as inactioc, one without the long occupied the attention effect On the rate Of vulother. The natural or added acids in rubber dissoloe the inorganic of the rubber chemist. acccIerators to form metaIIic soam. Solution of the metallic radical canization. EatonZand Stevens3 have in the rubber iS a prerequisite to its action as an-accelerator. THEORY OF EFFECTOF shown that strong mineral ORGANIC ACIDS acids cause a retardation of vulcanization, while acetic acid has only a slight effect. L. I n an attempt to corrolate and study the foregoing data, E. Weber4 states that the first organic accelerator of vul- an observation was made which explains the effect of organic canization, oleic acid, was discovered by C. 0. Weber in acids and throws further light on the mechanism of vulcaniza1904. Ostromuislenskii6 found that cinnamic acid aids in tion. An evolution of heat was observed on mixing litharge, the vulcanization of synthetic rubber, and this same acid lime, or magnesia into the dry acetone extract of pale crepe. was found as a constituent of Kickxia latex by Frank, On treatment with benzene a large amount of metallic soap Gnaedinger, and Marckwald.6 Dubosc' also described whale was found in solution, as shown by filtering and treatment oil as a vulcanization accelerator. Lead oleate, oleic, stearic, with hydrogen sulfide, which precipitated the metallic sulfides. benzoic, and many other organic acids have found industrial A working hypothesis was therefore formulated to the effect use as accelerators for low-grade rubbers since as early as that the litharge in Weber's experiments was not soluble in 1906. For example, it is reported that Camaron balls will the rubber mix owing to the removal of the natural acids in not vulcanize properly in a litharge-sulfur-rubber mix, while the rubber resins. It is now found that the deficiency caused the addition of the acetone extract from Jelutong rubber, of by removal of natural resin acids may be corrected by the stearic or of benzoic acid, will supply the natural deficiency addition of organic acids such as oleic acid. of Camaron rubber and give a satisfactory vulcanization. Further confirmation of.this theory is found in the vulL. E. Webers was the first to publish data on the use of canization of rubber by nitro compounds and litharge without litharge in an acetone-extracted plantation rubber. The the use of sulfur. A rubber mix comprising 100 parts rubber, low tensile strength which was obtained caused the writer to 10 parts sublimed litharge, and 2.5 parts by weight dinitroassume that the acetone-soluble constituents were necessary benzene gives a well-cured sheet in 50 minutes at 142' C. for vulcanization. Later, Stevensg proved that the acetone (287' F.) If acetone-extracted rubber is used, no vulcanizaextract was needed only when litharge was used in the rubber tion takes dace under the same conditions. The addition of 2 per cent of oleic acid t o this mix free from acetone extract 1 Presented before the Division of Rubber Chemistry a t t h e 65th Meetine - ofthe American Chemical Society, New Haven, Conn., April 2 to 7,1923. permits the mix to be vulcanized with the formation Of a 2 Agr. Bull. Federated Malay States, 4, 162 (1916). &ell-cured sheet. Bull. Rubber Growers' Assoc., 2 , 343 (1920). It is well known that lead oleate forms rapidly when 4 India Rubber J., 63, 793 (1922). J . SOC.Chem. Ind., 3 5 , 58 (1916);C. A . 10, 3176 (1916). litharge is stirred into oleic acid. For this reason lead a Grmmi-Zlg., 25, 840, 877 (1911). oleate instead of litharge and oleic acid was used in a m i x 7 Indza Rubber World, 56, 196 (1919). comprising acetone-extracted rubber, lead oleate, and dinitro8 8th Intern. Congress Aggl. Chem., 9,95 (1912). benzene, Vulcanization took place readily, although the 1 J . Soc Chem. I n d . , 36,874 (1916); Ibid.. 41,3261' (1922).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
January, 1924
cured sheet was very soft owing to the action of the large amount of lead oleate used as the equivalent of a given amount of litharge. This indicates that of t h e two components of litharge only the lead radical is necessary for vulcanisation, the oxygen radical being inactive. Solubility of the lead radical in rubber by means of resin or similar acids is a prerequisite for its action as an accelerator. The following tests were made on a mix consisting of 100 parts rubber, 10 parts sublimed litharge, and 6 parts sulfur, with variations and additions as indicated. TABLE I -PRESS -30
EXPT.RUBBER Downloaded by UNIV OF PRINCE EDWARD ISLAND on September 8, 2015 | http://pubs.acs.org Publication Date: January 1, 1924 | doi: 10.1021/ie50169a013
138 139 >40 141 142 143
Pale crepe extracted Pale crepe Pale crepe extracted Pale crepe extracted
CURES-142‘ c.M i n . 7 -50 MiaElonElon-Tensileea- -TmsilegaKg./ Jibs./ tion Kg./ Lbs./ tion ADDITIONSq. Cm. Sq. In. % ’ Sq. Cm. Sq. In. % None (Gray) No cure 3377 756 None 237 Oleic acid 3595 750 (1.5) 252 Softwood 2938 770 pitch (2.0) 206
216
N o cure 3073
756
218
3101
750
193
2750
744
780
219
3112
775
768
224
3197
768
Here again oleic acid functions in the same manner as the acetone extract.lO Softwood pitch was investigated because of the widespread practice of rubber chemists in using pine pitches, pine tars, Degras (acid wool fat), and rancid oils t o overcome the variability in low-grade plantation or wild rubbers. These materials also have the property of generating heat on mixing with litharge and forming lead salts of organic acids which are soluble in benzene and in rubber. Organic acids dissolve and thereby activate lime and magnesia as well as litharge. Mixtures of rubber (100 parts) and sulfur (ti parts) were prepared with 2 parts of light calcined magnesia and 4 parts of hydrated lime, respectively. Well-cured sheets were obtained by press cures in 60 minutes at 142’ C. No vulcanization occurred in the same compounds when acetone-extracted rubber was used, and in these mixes the addition of oleic acid again caused proper curing.
33
highest tensiles, it does dissolve litharge and causes vulcanization to take place. Butyl salicylate functions only as a phenol and not as the ester, as shown by the negative action of esters in the second class. The pine tree products all have the property of forming salts with litharge, which are soluble in benzene and in rubber.
CLASS 11-Substances which do not cause the above mix to vulcanize in 50 minutes at 142’ C. ( a ) Mannitol Galactose Maltose Lactose Inositol (6) Castor oil (fresh and without free acid) Cottonseed oil (fresh and without free acid) (c) Amino caproic acid +Aminobenzoic acid Aminosalicylic acid
(19) Ethyl phenyl acetate %-Butyltartrate m-Cresyl benzoate Methyl stearate a- and 8-AmyriI acetate Isoamyl benzoate (e) Lead lactate Lead phosphate Lead borate Lead arsenate Lead arsenite v) Animal glue
The negative results tabulated in Class I1 are instructive. Sugars and oils have no effect on the action of litharge as a vulcanization aid. Amino acids do not function like oleic acid, nor do they react with litharge in benzene to give a liberation of heat or a benzene-soluble lead salt. None of the esters of organic acids are active. The sample of a-and 0-amyril acetate was prepared from Pontianic resins, and has no effect on the vulcanization. Series e, consisting of lead lactate and inorganic lead salts, illustrates the need for a lead compound which is soluble in benzene and therefore probably soluble in rubber. These five salts were dispersed in boiling benzene, filtered, and the filtrate treated with hydrogen sulfide. No trace of the black lead sulfide was found--a proof of their insolubility. Like litharge, they are all insohble in rubber and therefore inactive during vulcanization.
INORGANIC ACCELERATORS
Solubility in the rubber hydrocarbon is a first prerequisite for the action of inorganic accelerators. Whether assisted by organic or inorganic accelerators, vulcanization is therefore ACTION OF SUGARS, ESTERS, AMIDOACIDS,AND PROTEINS apparently a chemical reaction in an organic solvent (rubber). From Table I it is seen that an increase of the natural Any theory as to the effect of the natural resin acids on acids in pale crepe has no further effect on the action of the acceleraiors would not be conclusive without a study of the inorganic accelerator. On milling inorganic accelerators into action of the other constituents of plantation rubber, such as rubber a reaction with the resin acids takes place. Hydrogen sugars, esters, amido acids, and proteins. A standard mix sulfide formed during vulcanization decomposes the metallic was therefore prepared with acetone-extracted pale crepe, soap t o a metallic sulfide and free acid. The latter then dissublimed litharge, and sulfur, as in the foregoing table. The solves more of the metallic oxide and maintains a certain following substances were added to this mix, and are classi- concentration of soluble metallic compound in the rubber. fied according to their effect on the vulcanization process: First grade plantation rubbers have sufficient natural acidity CLASSI-Substances which function in the same manner as the for the proper functioning of the oxides as accelerators. Second grade plantation rubbers, such as amber or blanket acetone-soluble portion of pale crepe and cause vulcanization to take place in a mix consisting of extracted rubber, litharge, and crepe, and certain wild rubbers are deficient in their natural sulfur. acid content, and have long been recognized as giving vulcan( a ) Metallic soaps ( b ) Rosin ized products having low tensiles when cured in mixes conOleic acid Rosin oil taining inorganic accelerators. It has also been common Stearic acid Pine tar practice to add pine pitches, Degras, or organic acids t o such Benzoic acid Pine pitch rubbers to “improve their quality.” Stockholm tar Rancid oils (castor, palm) (c) Phenol Degras On mixing 10 grams of the acetone extract of pale crepe %-Butylsalicylate (a phenol) with 15 grams of sublimed litharge in a small, heat-insulated All the substances in Class I are either metallic salts of container at room temperature, a rise of 19” C. takes place organic acids or become so on mixing with litharge. Phenol in 2 minutes. With the extract from amber blanket there is acts in the same manner, and, although it does not produce the a rise of only 7” C. under the same conditions. It is possible that such a test might be developed for the determination of 10 G . 9. Whitby, in a paper presented before the Rubber Division the this type of variation in plantation rubber. A curing test same day in which this paper was read, finds that the acetone extract of in a litharge mix, however, seems preferable. The data in plantation rubber consists chiefly of fatty acids, a solid (Cm) acid which he Table I1 were obtained on such a rubber mix for the purpose has named “heveic acid,” and liquid resin acids which he believes are comof showing the relative value of different organic acids in posed chiefly of oleic and linolic acids.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
34
making up for the deficiency in natural acids found in amber blanket. Thie mix comprises 100 parts amber blanket, 18 parts sublimed litharge, and 5.4 parts by weight of sulfur. TABLE I1 ,------PRE.SS
Downloaded by UNIV OF PRINCE EDWARD ISLAND on September 8, 2015 | http://pubs.acs.org Publication Date: January 1, 1924 | doi: 10.1021/ie50169a013
Test 7 10 14 13 76 34 32 16 15 247 234
CURES------
40 Min. a t 142' C. Tensile Elongation ADDITION AMOUNTKn./Sa.Cm. Lbs./Sq.In. % 2250 None Control 158 760 Stearic acid 2.0 197 713 2800 Zinc stearate 2 2 267 3800 745 Lead oleate 228 3250 2.4 725 222 3150 Softwood pitch 2 0 710 193 2750 Hardwood pitch 725 1.0 2950 650 Rosin 1 0 207 3225 227 Pine tar 640 2.0 3800 267 Black pine pitch 700 2.0 3451 242 Degras 715 2.0 3341 235 1.6 Rosin oil 668
Table I11 shows the action of oleic acid (1.5) on a mix consisting of brown blanket (loo), sulfur (6), and light magnesia (2) or lime (4). TABLE I11 Test 198 199 200 201
Cure a t -PRESS CURES----142' C. Tensile Elongation OXIDE OLEICACID Min. Kg./Sq.Cm. Lbs./Sq.In. No 40 118 1682 808 Magnesia Magnesia Yes 40 154 2197 906 Lime No 50 66 943 960 Lime Yes 60 130 1867 875
The data in Tables I1 and I11 are self-explanatory. A number of nonacidic substances, such as paraffin, petrolatum, asphaltum oil, coal tar, sulfurated terpenes, and castor oil, were tested without effect. Rancid pine or castor oils function the same as organic acids. Stevens3 has shown that the resin acids are not necessary for the vulcanization of rubber by sulfur alone. When zinc oxide is added to such a pure gum mix, the amount of sulfur required is not reduced nor is the time of cure shortened to even approximately the same degree as when litharge is used. Zinc oxide is a weak accelerator of vulcanization and should not require the aid of organic acids to the same extent as the more powerful inorganic accelerators. Acetone-extracted rubber with 10 per cent of sulfur and 6 per cent of 1A zinc oxide is so tacky after a 3-hour cure at 142' C. that it cannot be removed from the mold, while the addition of oleic acid (2 per cent) gives a well-cured sheet. I n unextracted amber blanket the same qualitative results are observed, but the increase in tensile strength by further addition of organic acids is much lower for a zinc oxide mix than when litharge, lime, or magnesia is used. COMMERCIAL APPLICATION Commercial application of organic acids to reclaimed and synthetic rubbers is well known and need not be further commented upon. On the variability of plantation rubber, however, there seem3 to be a new item to consider. The question of type of formula for testing variability is a t present very much the subject of controversy. For the compounder who uses only inorganic accelerators the amount of natural acids in a given rubber is important. A vulcanization test will probably be preferred to any laboratory chemical test for this type of variability. For organic accelerators the question of natural acids may answer in part the need of a small amount of zinc oxide in many accelerated mixes. The neutralization of natural acids by zinc oxide cannot answer the need of the oxidewith zinc dithiocarbamates, since zinc stearate and other zinc soaps which further increase the acid content of the mix activate the zinc dithiocarbamates the same as the oxide itself. Neutralization of resin acids by zinc oxide may be desirable with such accelerators as aldehyde ammonia, while, on the contrary, Kratz has found that zinc oxide slightly retards the curing power of aniline.
Vol. 16, No. 1
MECHANISM OF VULCANIZATION There has been an apparent sharp line of demarcation between organic and inorganic accelerators which is rapidly fading away and which must be entirely eradicated before we reach a thorough understanding of the mechanism of vulcanization. I n the last few years the thioureas, dithiocarbamates, and mercaptans have been described as functioning through their metallic salts so that they have become inorganic as well as organic accelerators. We now find the inorganic accelerators to be inactive except when transformed to metallic salts of organic acids so that they become organic a s well as inorganic accelerators. It cannot be expected that the ultimate vulcanizing agent will be the same in all cases, since zinc dithiocarbamates cause the vulcanization of rubber a t temperatures where lead oleate is inactive. There must, however, be some similarity between the two which will apply to other accelerators as well, and even to theoxygen vulcanization by nitro compounds. Stevens3 has recorded an extremely valuable observation relative to this subject. He found that if to a rubber, free from both resins and insoluble constituents, the resins were returned, the resulting rubber would vulcanize rapidly t o high tensile properties when mixed with sgfur and litharge. It may therefore be assumed that in a mixture of lead oleate, sulfur, and pure rubber there are present all the constituents necessary for rapid vulcanization. Oleic acid when heated with sulfur to 142' C. gives off hydrogen sulfide. Lead oleate, under the same conditions, turns black. . Intermediary between the change of lead oleate to lead sulfide and sulfurated oleic acid there must be some reaction product which is able to activate sulfur for the vulcanization of rubber. That intermediate products actually form is shown by chilling the reaction mixture in order to retard the reaction. A solution of lead oleate in benzene turns black a t once on passing in hydrogen sulfide. If the solution is first cooled to 10' C. or lower, the addition of the gas produces a bright red precipitate instead of the black sulfide. This red precipitate is probably either Pb(SH)2or Pb(SH)O-R, the latter being the sulfhydro-oleate of lead. The action of sulfur on these sulfhydrates is probably the same as its action on N H 8 H or thiophenol. Hydrogen sulfide is split off by the oxidizing action of the sulfur and a disulfide is formed. These disulfides may or may not add extra sulfur to form higher polysulfides, and one or more of these polysulfides contains active sulfur available for vulcanization. 2H -
2 "a-SH 2RO-Pb-SH
fS
+a
E-SCTT)
4- S +NH1-S-S-NHa + HIS 4- S +RO-Pb-S-S-Pb-OR
+ HzS 4- H&
Without further evidence, we can only postulate that vulcanization by inorganic accelerators proceeds (1) by the formation of metallic salts soluble in the rubber, and ( 2 ) through the decomposition of these salts by hydrogen sulfide and sulfur to form metallic polysulfides, which are probably the true vulcanization agents. It does not appear that this process is reversible, as seems to be the case with the zinc dithiocarbamates. After litharge goes through the cycle of oxide-soapsulfhydrate-polysulfide-sulfide, it seems to be inactive, while the organic acid acts as a true catalyst, being regenerated and going through the cycle again with fresh oxide and sulfur. The possibility of dithiocarbamates reacting with lead sulfide has not been investigated. This theory, as a working hypothesis, suggests a large amount of work which should be of value in correlating the action of organic and inorganic accelerators.
