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possible with the various solvents now available, but it is probable that few, if any, are true constant-evaporating mixtures. The behavior of a mixture of benzene, butyl acetate, and amyl alcohol has been referred to by Hofmann and Reid (9). The ingredients are removed from the mixture by evaporation, more or less completely, in the order named, although of course all are evaporating simultaneously at different rates. SUMMARY 1. Evaporation rates of organic liquids are not proportional to their boiling points. 2. Determinations of rates of evaporation by the usual method are sufficiently indicative for practical purposes, but the results are not capable of numerical expression. 3. For pure liquids the slopes of the evaporation curves may be used to express the rate of evaporation. 4. A simple method of determining rates of evaporation of pure liquids is described. In this method temperature, humidity, and air velocity are controlled, and the apparatus is preferably calibrated with a standard liquid. (Normal butyl acetate was used.) 5. The rate of evaporation of a liquid may be approximately predicted by means of the formula:
Vol. 24, No. 2
vapor pressure X mol. wt. + 11 which gives a rate of 100 for n-butyl acetate (20” C.). 6. The compositions of several new binary constantevaporating mixtures and of two ternary mixtures which may be called “apparent constant-evaporating mixtures,” have been determined. LITERATURE CITED Bridgeman, IND. ENG.CHEM.,20, 184 (1928). Clark, Chem. News, 141, 120 (1930) (Abstract). Davidson, IND. Eso. CHEM.,18, 669 (1926). De Heen, J. chim. phys., 11, 205 (1913). Fife and Reid, IND.ESQ.CKEM.,22, 513 (1930). Gardner, Paint Varnish Mfrs. Assocn. of U. S., Circ. 236 (1925). (7) Gardner and Parks, Ibid., 218 (1924). (8) Hart, Ibid., 360 (1930). ENQ.CHEM.,20, 431 (1928). (9) Hofmann and Reid, IND. (10) Hofmann and Reid, Ibid., 20, 687 (1928). (11) International Critical Tables, Vol. 3, p. 285. (12) Jores, Farben-Ztg. 34,2889 (1929). (13) King and Smedley, J . Phus. Chem., 28, 1265 (1924). ENQ.CHEX, 22, 826 (1930). (14) Park and Hopkins, IXD. (15) Polcich and Fritz, Brennstof-Chem., 5, 371 (1924). Exo. CHEM.,20,497 (1928). (16) Reid and Hofmann, IND. (17) Wilson and Worster, Ibid., 21, 592 (1929). (1) (2) (3) (4) (5) (6)
RECEIVED September 10, 1931. Presented before the Division of Paint and Varnish Chemistry at the 82nd Meeting of the American Chemical Society, Buffalo, N. Y.,August 31 to September 4, 1931.
Mastication of Rubber An Oxidation Process W. F. BUSSE,B. F. Goodrich Company, Akron, Ohio
T
HE mechanism of the breakdown of rubber on the mill has been investigated by many workers, and many different theories have been advanced to explain the changes which occur. I n some cases the theories have been based on a postulated structure of the.latex particle, but, more often, they were ad hoc explanations based on the authors’ pictures of the structure of the rubber molecule. Countless attempts have been made to distinguish between “depolymerization’, and “disaggregation” of rubber molecules, where in many cases the difference is merely one of definition. Only recently have data become available to show that milling is not the simple mechanical process it once was thought to be, but is essentially a chemical reaction.
PREVIOUS WORK It is well known that the latex globules are broken up during milling (8, 9, IO), and Van Rossem (27), adopting Hauser’s picture of the latex particle, claimed that the plasticizing effect of milling is due to the mixing of the soft inner phase with the hard outer phase of the globules. This does not explain why the plasticity of rubber continues to increase during milling, even when all the latex globules are broken up, nor does it agree with the results of Staudinger (B),which indicate that the molecular weight of milled rubber is considerably less than that of unmilled. Staudinger believes that the breakdown of rubber on the mill is a mechanical disruption of the primary valence bonds in the rubber molecules, such as occurs in crushing a diamond. Pummerer ( l 7 ) , on the other hand, claims that the molecular weight of rubber is low, and milling merely breaks up the micelles, which are large aggregates of molecules. Bary and Hauser (1) have claimed that the softening of
rubber by milling is due to a shifting of the equilibrium between two definite phases which they believe to exist in rubber; alpha-rubber, consisting of rubber hydrocarbon in a relatively low state of polymerization; and beta-rubber, having a higher degree of polymerization, presumably with little or none of the rubber in the intermediate states of polymerization. (It is unfortunate that the terms alpha- and beta-rubber have recently been used in this sense, in view of their previous use in the opposite sense by Pohle (16).) They claim that heat has the same effect as milling in shifting the equilibrium between these two phases. The apparently permanent effect of heat and milling is explained on this theory by the slow speed of repolymerization resulting from the high viscosity of rubber. This view is not supported by the actual behavior of rubber. Many early investigators suspected that oxygen played an important part in milling, but they could not prove it, and they finally concluded that the effect of oxygen was either negligible or deleterious. Fisher and Gray (6) found that rubber milled several hours in carbon dioxide became very soft and tacky, but the change in unsaturation could not be measured (i. e., was less than 0.5 per cent). Rubber milled several hours in air had its unsaturation lowered by about 2 per cent, which, they said, may have been due to oxidation or to the formation of a cyclo-rubber. Garner (8) claimed that milling first depolymerized or disaggregated the rubber hydrocarbon, after which it could react with oxygen to become tacky. Messenger (14) found that milling rubber changed its heat of combustion by less than 0.5 per cent, although the viscosity was changed by a factor of 20, which seemed to indicate the amount of oxidation which occurs during milling is negligible.
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that of light (62). It is reThe view was further supported that the test is so sensiported by the fact that rubber tive that it can detect the percould be softened to some exCHANGES I N RUBBER which occur during oxides formed when c e r t a i n tent by heating in steam or mastication have been studied in an attempt to metals (zinc, aluminum, cadcarbon dioxide (9, I 5 ) , while determine whether the breaking down of rubber by mium) are exposed to light in heating for a long time in air milling is a mechanical or a chemical process. moist air, but no effect is prototally ruined the rubber. The Using the phoiographic-plate test, peroxides were duced when they are exposed to belief in the deleterious effect light in dry air (5, I3, 19). of o x y g e n l e d t h e Dunlop detected in smoked sheets, both after milling and Russell (18) found that a large Rubber Company ( S I ) to patafter exposure to light in the presence of oxygen, number of materials produced a ent the method of softening and the concentration was higher when the rubber latent image on a photographic rubber by heating it in an inert was milled on cool rolls than when milled on hot plate in the dark if the matenon-oxidizing atmosphere prerolls. Pale crepe did not give as strong a test for rial were previously exposed in vious to milling. Staudinger air to light-a treatment which (22) also patented the method peroxides, while the peroxides could not be detected undoubtedly causes the formaof masticating rubber in an at all in $ne Para and sprayed latex. These tion of peroxides. This effect inert atmosphere to avoid the rubbers contain some material which can decompose on the photographic plate has deleterious effects of oxygen. hydrogen peroxide. been used to study the formaRecently Fry and P o r r i t t The softening of rubber on the mill is probably tion of peroxides in drying oils ( 7 ) showed that nearly all the (24), although the amounts of permanent softening, w h i c h due to the breuking u p of the long rubber molecules peroxide formed here are great occurs in heating rubber in a into shorter ones by a n oxidation process. Oxidaenough after short exposure to supposedly inert atmosphere, tion occurs quite rapidly on the mill because the be detected chemically (25). actually is due to the traces of rubber molecules are activated by mechanical disVan Rossem and Dekker (28) oxygen which are present, even tortion, and the oxygen is activated by electrical reported that rubber which had 0.01 per cent of oxygen causing been exposed to the light could charges. The stresses in the rubber and the eleca very marked effect 011 rubber form a latent image if placed on when it is heated for several trical charges are both greater when milling is done a photographic plate in the hours a t 150’ C. From the on cool rolls than when done on hot rolls, which dark, and they found the effect similarity in the effects of heataccounts for the greater effectiveness of cold-milling. to be g r e a t e r with s m o k e d ing and milling 011 the viscosity This theory was cerijied by the peroxide experisheets than with pale crepe. and plasticity of rubber, they suggest that theeffect of milling ments, by milling peroxides into rubber, and by the If peroxides are formed durmay also be due to oxidation. ing m i l l i n g , they might be luminescence effects which were observed during detectable by this method. The very large effects promilling. As a jinal check on the theory, rubber Xegative results, of course, duced by minute traces of oxywas milled in the presence of various gases. It was would be inconclusive, since, gen are c o n s i s t e n t with the found that little, if any, breakdown of the rubber even though peroxides were views of Staudinger that ruboccurs if the milling is done in the absence of oxygen. formed d u r i n g milling, they ber has a molecular weight of might not be volatile, or the the order of 100,000, If one s u r f a c e concentration might molecule of oxygen breaks a not be n e a t enough. or catarubber molecule into two IDarts., only 0.03 per cent oxygen is necessary to reduce the molecular lysts might be present which would destrvoy the peroxides before weight to 50,000. The combining of about 0.5 per cent oxygen they could be detected. The following experiments show reduces the average molecular weight of the rubber to about that peroxides are formed in rubber during milling just as on 5000. If one oxygen atom saturates each double bond, i t exposure to the light, but that they can be detected only in takes only about 0.5 per cent oxygen to reduce the unsaturation certain rubbers. Some rubbers contain catalysts which deby 2 per cent, the value found by Fisher and Gray. compose peroxides, and these rubbers also exhibit the slowest In the light of these facts, the available experimental data plasticity changes during milling. are not sufficiently accurate to show whether or not the softening of rubber on milling is due to oxygen. Because of the FORMATION OF PEROXIDES IN RUBBER inconclusive state of the problem, an attempt has been made to obtain additional evidence which might answer two quesEFFECTOF LIGHT. Samples of rubber were exposed for tions: first, does oxidation occur during milling; and second, various times to a 220-volt printing arc, using National if oxidation does occur, is this essential to the breakdown of Carbon Company’s Therapeutic “A” carbons, usually a t a disthe rubber on the mill? In the first part of this work, several tance of 50 cm. from the arc. In every case a part of the different lines of indirect evidence were obtained to throw sample was protected from the light by a strip of metal foil, light on these questions. The conclusions from the early work leaving a control area of unexposed rubber on each sample. were then verified by milling rubber in the presence of various After exposure, the strips of metal were removed, and the gases. The results of this work are in agreement with those of exposed side of the rubber samples put directly on the emulsion a similar test recently reported by Cotton ( 2 ) . of a photographic plate where they usually were allowed to If rubber is oxidized during milling, the first product formed remain in the dark for about 18 to 24 hours, after which the probably is a peroxide, since most unsaturated compounds are plate was developed. oxidized through the formation of intermediate peroxides. The first attempts to duplicate the work of Van Rossem One of the most sensitive, though unfortunately not a specific and Dekker failed, because ordinary process or panchromatic test for volatile peroxides, is the formation of latent images plates were used. When Eastman “Speedway” or Gavaert when they act on photographic plates ( I S ) . The action of “Super Sensima” plates were used, however, samples of hydrogen peroxide on a plate is similar in every respect to smoked sheet produced a very strong image after less than a
A
14'
I- iiiiiiute axpusure tu tlrc subsequeiit uork It i s h plates to hydrvgeii p i o x their sensitivity to hght To show that th? format oxygen, the follouinp experi
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Itatrut, 15 lrrylir~d t ~ ~old-m!!mg i (vmiple 2) thari aftrr hoti d l i n g (stmplc 3 ) a i d tlia >me of the peroxide reniains in oei the nil,iwr ior ,it liviit 3 cia) after niillmg (sample 4). Tlie ~lifla~eiicc i n tlie ro ts after h o t and cdd-mdlrng due to may be attrilnitrd to the Pact that cold-milling i b more effectivc tliaii hot-milling i n loinimp the uerixide, and to a more r ~ p i ddccoiiipositirir; of the peroxide. at high temperatures. Strip8 of crude smoked sheet were placed in Pyrex Liisf, t,uires, The eyidence tirat tlie furirintiiln of tlie per
