Control by Individual Incorporation of Vulcanizing Agents D. F. TWISS Dunlop Rubber Company, Ltd., Birmingham, England
This paper reviews various forms of the method of compounding in which vulcanizing ingredients are introduced by a diffusion process. The application of this principle dates back to Thomas Hancock or possibly earlier, but the advent of ultraaccelerators gave it new importance. In the vulcanization of rubber by compounding with sulfur, zinc oxide, and a secondary amine, and converting the latter into an ultra-accelerator by the action of carbon disulfide, it is a convenient modification to make use of an easily decomposed compound of the amine instead of the free amine. The carbon disulfide treatment not only leads to the formation of the dithio ultra-accelerator, but also further increases its effectiveness after it has been formed. The latter result appears to be a general one towards all dithio accelerators, and to be analogous to the activating effect of zinc oxide towards them.
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HE individual incorporation of vulcanizing agents without a concurrent milling ogeration-i. e., by some form of diffusion process-originated with Thomas Hancock. Sulfur had been previously used as a dusting powder (e. g., b y P. W. Ludersdorff in 1832), and the storage of the dusted rubber must have led t o gradual penetration of the sulfur into the rubber and ppssibly to effects of accidental vulcanization, especially if such rubber became exposed to sunlight as was definitely the case in 1837 in N. Hayward’s solarization process (9). But Hancock’s well-known patent specification (10) gives the first description of the deliberate incorporation of sulfur by a diffusion process for the purpose of subsequent heat vulcanization. This patent also describes and claims the compounding of rubber with mineral fillers and with asphaltum; it explains how “by immersing the caoutchouc in melted sulphur (at a temperature ranging from about 240” to 250” Fahrenheit)” or by “heating sheets of caoutchouc to about 200” and sifting and rubbing flour of sulphur on it” one can blend sulfur with the rubber so that by subsequently raising the temperature “to 300°,or from 300” to 370” . . . the change is produced”. The “change” was the term used by Hancock “for brevity’s sake” to describe the desired effect.
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producing the ‘change’, so unique in many respects, is particularly so in this, that articles manufactured of pure rubber with which no sulphur has been blended may be changed in the sulphur bath; take, for instance, a native South American shoe, immerse it hot in the bath, a t the lowest temperature a t which sulphur will melt, say 240°, and let it remain until it has absorbed enough sulphur; then gradually raise the temperature to 265” or 270°, and in the course of an hour or something more, according t o the thickness, the shoe will be perfectly ‘changed.’ ” It is noteworthy that this early process of vulcanization in a bath of molten sulfur is still in operation, for the manufacture of thin rubber articles such as tobacco pouches, in the historic factory in which Thomas Hancock and Charles Macintosh were Dartners in Endand and also elsewhere in Europe (19). The use of a diffusion process for the introduction of an accelerator of vulcanization into an otherwise fully compounded rubber article was also described at a relatively early date. I n 1811 Rowley (17) patented several methods for obtaining an improved vulcanized product; one form of his procedure was to vulcanize in the presence of a solution of ammonia or of solutions yielding ammonia. The same principle is further elaborated in a French patent of 1903 (18); this process involves hot vulcanization in ammonia gas under pressure. In a generous interpretation, the compounding of rubber by way of latex provides an example of the control of vulcanization by the individual incorporation of vulcanizing agents. When aqueous dispersions of vulcanizing ingredients such as sulfur and accelerators are added to latex, the particles substantially retain their individuality outside the rubber globules; and even when the latex is evaporated to form a film of compounded rubber, their identity is largely maintained. This fact is evident from the slowness with which a sample of compounded latex rubber, after being dried without marked elevation of temperature, yields its free sulfur to acetone extraction as compared with a piece of compounded rubber of similar composition prepared by milling; in the latter case the sulfur is already present, to a considerable extent a t least, in solution in the rubber. One of the first effects of the heat applied for vulcanization of such a “latex compound” is to bring the sulfur (and other vulcanizing ingredients) into solution in the rubber phase so that vulcanization can take place. The present considerations, however, are concerned primarily with the individual incorporation of vulcanizing ingredients into dry or, a t least, coherent rubber.
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WITH the advent of ultra-acce&rators the diffusion principle for the incorporation of vulcanizing ingredients subsequent to the initial processing gained in importance and realization of this resulted in a number of patents for processes I N 1857 Hancock gave further details of the possibilities of this method of procedure and adds (11): “This mode of whereby the advantages accruing to the use of such accelera1461
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tors could be gained with a t least partial avoidance of scorching troubles. Morton and Harrison (14) superposed plies of compounded rubber containing sulfur and the vulcanization accelerator alternately; in another method of procedure (16)one of two ingredients essential to vulcanization (e. g., sulfur and accelerator) was incorporated in the rubber mix by the customary milling operation? and the other was subsequently applied to the surface of this mix; then penetration by diffusion occurred (conveniently aided by heat), and accelerated vulcanization became possible. Similar methods for avoiding the scorching troubles experienced in producing ultra-accelerated mixes in the usual manner were described almost simultaneously by Cadwell (2); his range of vulcanizing ingredients, however, included organic activating agents as well as ultra-accelerators and was such that vulcanization of the completed mixing could occur even at the ordinary temperature. Moreover Cadwell made the additional important step of avoiding the use of preformed ultra-accelerators by forming them in situ in the rubber when -required; a further line of oppositioy) was thus created to scorching or prevulcanization. This develppment (3) was based on the ease of formation of the dithiocarbamate type of ultra-accelerator by the direct spontaneous combination of carbon duulfide and the appropriate amine. By painting the shaped compounded rubber, already containing sulfur, zinc oxide, and the amine (e. g., piperidine) with carbon disulfide, the latter is absorbed; the ultra-accelerator (e. g., piperidine pentamethylenedithiocarbamate) is formed almost a t once so that vulcanization occurs spontaneously in a week or so a t ordinary temperature or in a shorter time a t a higher temperature. At the same time Cadwell and Smith (6) divided the necessary agents for this type of accelerated vulcanization into four constituent groups: (a) sulfur or a sulfur-generating material, (6) carbon disulfide or a material generating it, ( c ) a metal compound (e. g., zinc oxide), and (d) an amine. The omission of a t least one of these from a mix would preclude the possibility of prevulcanization; vulcanization could be effected, however, by subsequently incorporating the omitted constituent by diffusion-e. g., from a solution in benzene. Rubber sheet or a dipped rubber article could, for example, be immersed in a benzene solution of sulfur, zinc butylxanthogenate, and dibenzylamine for a minute or so; after drying, vulcanization could be effected a t ordinary temperature in the course of a week or so. The disadvantage attaching to the use of a solvent which is active to the rubber can be avoided by employing an ultra-accelerator dissolved in a rubber nonsolvent, such as acetone (16) or even water, or by an emulsion of an ultraaccelerator substantially insoluble in water, such as the carbon disulfide compound derived from the reaction product of piperidine and formaldehyde ( I S ) . At about the same time the possibility of producing an accelerator of vulcanization in situ in the rubber so that a preshaped rubber article could be vulcanized without the need of heat, was als? described by Bruni (1). He published a method of cold vulcanization based on compounding the rubber with sulfur, zinc oxide, and an aromatic amine (e. g., aniline) and subjecting it to the action of vaporized or dissolved carbon disulfide.
WHERE prolonged storage of the compounded material is desired, it may be important to exercise still further care in the selection and grouping of the vulcanizing ingredients. For instance, in the preparation of self-vulcanizing doughs for low-temperature repair work by making two mixtures which are to be blended as required. one containing the accelerator (e. g., zinc isopropylxanthogenate) and the other the sulfur (6),the possible presence of traces of free sulfur as an impurity in the xanthogenate renders it a wise precaution to
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keep the latter separate from the zinc oxide and t o incorporate the zinc oxide in the sulfur stock. In its various forms the method of vulcanization just indicated not only ensures the avoidance of scorching and secures in the product the physical advantages associated with the use of the best ultra-accelerators, but also gives the possibility of preventing the spoiling of scrap or fresh stock by vulcanization in storage. I t also permits the production of sulfur-vulcanized composite articles comprising heat-sensitive constituents which would be impaired by hot vulcanization in the ordinary way. Furthermore the vulcanization may be effected at the ordinary temperature for which the alternative procedure would be by the use of sulfur chloride with its well-known disadvantages. Admittedly the “diffusion method” of sulfur vulcanization, as it may conveniently be termed, places a limit on the thickness of the article or material to which it may be applied although constituent sheets may be assembled into composite layers of substantial dimensions. An interesting application of the diffusion process is in the manufacture of golf balls where sulfur vulcanization of the outer shell is desired with avoidance of any thermal impairment of the tension of the rubber thread which is wound tightly around the central core. Adaptation of the diffusion method of vulcanization to this particular purpose was indicated almost simultaneously from several sources (4). There are various possible methods of procedure, but in all of them the manufacture of the ball is taken to the stage of completion of the final molding operation and release of the molds before vulcanization. The last process can be a t such a low temperature that, although applied to the freely exposed ball, it does not interfere with the exactness of the detail in the surface pattern. Furthermore, separation of the molding operation from the vulcanization process has other evident advantages. The form of the diffusion method of vulcanization, in which the article to be vulcanized contains the sulfur, the amine, and the zinc oxide for the formation and activation of the ultra-accelerator but lacks the carbon disulfide for the formation of the latter, has the convenience that the carbon disulfide lends itself to a diffusion process of introduction. This introduction may be either from an external atmosphere containing the vapor or from a liquid medium preferably containing the carbon disulfide in admixture (either solution or dispersion) with a nonsolvent for the rubber. It is of considerable potential importance that carbon disulfide not only provides an essential constituent for the formation of the ultra-accelerator, but that the activity of the latter is enhanced by the prespce of an excess of the carbon disulfide (8). This effect appears to be analogous to that of zinc oxide on the zinc dialkyl dithiocarbamate and zinc xanthogenate ultra-accelerators ( I @ , and is observed with all such accelerators of the dithio class. For instance, a mixture of smoked sheet rubber 100, sulfur 5 , zinc oxide 5 , and zinc isopropylxanthogenate 0.5, becomes well vulcanized when kept for 15 hours a t 40” C. in air containing carbon disulfide vapor; it remains quite unvulcanized under similar conditions with carbon disulfide excluded. Zinc diethyldithiocarbamate, piperidine pentamethylenedithiocarbamate, the alkylxanthogen disulfides, and the thiuram sulfides are similarly influenced. The zinc oxide and the carbon disulfide probably both exercise a suppressive action on a tendency of the dithio accelerator (whether formed in situ or otherwise) to enter into a by-reaction involving decomposition with formation of these two substances. As with zinc oxide, the additional effect of the surplus carbon disulfide is observed with mixings or doughs in which the preformed accelerators have been
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INDUSTRIAL AND ENGINEERING CHEMISTRY
incorporated as well as with mixings in which they are formed in situ. ACCORDIXG to the principle of delayed-action accelerators a postponed effect is obtained by incorporating in the vulcanizable mixture a chemical derivative of the accelerator (such as the well-known dinitrophenyl derivative of mercaptobenzothiazole or one of the various acyl derivatives) which undergoes “hydrolysis” a t an early stage of the vulcanization period so that the true accelerator is freed to exert its characteristic action. By analogy with this principle it might be a convenient alternative to the diffusion process to incorporate in the rubber mix a compound capable of liberating carbon disulfide when desired. Unfortunately, although various substances have been mentioned for this purpose, such as the metal thiocarbonates, they do not liberate carbon disulfide with sufficient ease and are disappointing in this respect. Possibly, however, the activation of the dialkyl dithiocarbamate accelerators by the presence of a monoalkyl dithiocarbamate is due, in part a t least, to the lower stability of the latter and the ease with which it undergoes decomposition with formation of carbon disulfide. Incidentally, the use of thiuram disulfides as vulcanizing agents may be regarded, from the point of view of the liberation of free sulfur inside the rubber stock, as the chemical analog of the method of introducing sulfur into the otherwise completed stock by a diffusion process. In the preparation and use of stocks containing secondary amines for subsequent conversion into dithiocarbamate accelerators by extraneous treatment with carbon disulfide, a number of difficulties may be encountered. The most desirable secondary amines from the point of view of the activity of their dithiocarbamate derivatives as accelerators are liquid a t ordinary temperatures; in addition, many of them are distinctly volatile. These features may lead to losses by seepage into the bearings of the mill and by evaporation during mixing and during storage. Rise of temperature during milling may also lead to scorching. With a view to circumventing all these complications, the possibility has been investigated of using the amines in the form of less volatile and solid compounds decomposable by carbon disulfide so that the dithiocarbamate derivative of the amine would still be producible in situ. Several classes of compounds which have been tried in this laboratory proved unsuitable. The sulfur dioxide derivatives of the amine (e. g., the piperidine-N-sulfinic acid compound obtained from piperidine and sulfur dioxide) are not only undesirably deliquescent but are resistant to carbon disulfide. The carbon dioxide derivatives (e. g., piperidine pentamethylenecarbamate and diethylamine diethylcarbamate) are convertible by carbon disulfide into the corresponding dithiocarbamates, but are unfortunately unstable, volatile solids and disappear on exposure to the air. The compounds of the amines with the mercaptans proved still less stable (e. g., piperidine evaporated completely from a mixture with octadecyl mercaptan). Two classes of compounds, however, give satisfactory results: ( a ) the salts of the bases with an organic acid (e. g., with stearic acid or benzoic acid or a mixture of these acids) and ( b ) the condensation products of the amines with aldehydes other than formaldehyde. With the former clase of compound incorporated in rubber containing also sulfur and zinc oxide, the extraneous action of carbon disulfide as vapor or as a liquid solution or emulsion causes formation of the dialkylamine dialkyldithiocarbamate (in which term we include also the carbon disulfide derivative of piperidine) with consequent possibility of vulcanization even a t ordinary temperatures. The second class of compound comprises a considerable number of substances ( 7 ) obtainable by direct reaction of the dialkylamine (e. g., piperidine, diethylamine,
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diamylamine, etc.) and an aldehyde (e. g., butaldehyde, benzaldehyde, furaldehyde, etc.) . The behavior of formaldehyde is different from that of the other aldehydes in that it converts the various amines into their methylene derivatives; and although the latter react with carbon disulfide, they do not regenerate the original amine or form the dithiocarbamate compound corresponding with it. For example, methylenedipiperidine, from formaldehyde and a bimolecular proportion of piperidine, forms with carbon disulfide an additive compound, (C~HIO;U),CH2CS2,which melts at 58 C. ; piperidine pentamethylenedithiocarbamate melts at 174’ C. The higher aldehydes and the secondary amines give compounds of the formula R’CH(NR’’R”’)2which are decomposable by carbon disulfide according to the equation: O
R’CH(NR”R”’)*
+ CS, + HzO = NR”R”’.CS2.NH2R”R”’+R’CHO
The water naturally present in the rubber mix suffices for the reaction; it is possible by drastic drying (e. g., for a considerable period over phosphorus pentoxide) to prevent the reaction represented by the equation, but such a condition of desiccation is not normal in rubber; for a mixture containing 0.5 per cent of an aldehyde-amine compound of molecular weight 250 the proportion of water theoretically required to be present is less than 0.04 per cent. These condensation products of the higher aldehydes and secondary amines are frequently formed spontaneously, and even with heat evolution, when the two constituents are mixed. It is sometimes convenient to moderate the reaction by diluting one of the reagents with a solvent (e. g., light petroleum) or even to use one or the other in the form of an aqueous suspension. Most of the products are sparingly volatile liquids at ordinary temperatures and so are convenient for handling; others are attractive crystalline solids (7)-for instance, benzylidine dipiperidide melting a t 81” C. T H E use of these compounds as an eventual source of the dithiocarbamate ultra-accelerator affords a double degree of protection against scorching. The compounds are less active as accelerators than the corresponding free amines, and the latter are much less active than their dithiocarbamate derivatives. Yet when incorporated in the rubber the former can be converted by the action of carbon disulfide into the corresponding dithiocarbamate accelerators at ordinary temperatures. In addition to these advantages the vulcanizing activity of the resulting accelerator is further enhanced by application of ammonia gas or an amine after the carbon disulfide treatment or by incorporation of a small proportion of an organic base with the other ingredients in the rubber mix. The presence of the additional substance further expedites the action of the carbon disulfide on the aldehyde-amine compound. For convenience of incorporation the base should be sparingly volatile, and it is important that it should have a fair degree of solubility in the rubber. For this reason diphenylguanidine and di-o-tolylguanidine are acceptable, the latter appearing to have a slight advantage with respect to solubility. This matter of solubility in the rubber is the more important because the method just indicated is intended particularly for vulcanization a t low temperatures, and it may also be desired to effect compounding with a minimum of temperature rise. This consideration of solubility of the vulcanizing ingredients is much more important here than with ordinary sulfur vulcanization in which the higher temperature can increase the effectiveness Qf sparingly soluble ingredients in more than one way.
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Acknowledgment
VOL. 31, NO. 12
( 5 ) Cadwell, S. M., and Smith, 0. H., Brit. Patents 200,789 and
The writer is indebted to F. A. Jones, R. W. Hale, and J. Moore for valuable assistance in the work on which many of the foregoing statements are based, and to the Dunlop Rubber Company, Ltd., for permission to publish.
Literature Cited (1) Bruni, G., Giorn. chim. ind. a p p l . , 3, 196 (1921). (2) Cadwell, 8. M., Brit. Patent 174,915 (1922). (3) Cadwell, S. M., U. S. Patent 1,463,794, Brit. Patent 200,788 (1923). (4) Cadwell, 8. M., U. S. Patent 1,951,392 (1934); Munro, A,, Ibid., 1,940,009 (1933); Jenness, L. G., Ibid., 1,941,691 (1934); Dunlop Rubber Co., Twiss, D. F., and Jones, F. A., Brit. Patent 407,928 (1932).
211,494 (1923). (6) Dunlop Rubber Co., and Twiss, D. F., Ibid., 215,796 (1923). (7) Dunlop Rubber Co., Twiss, D. F., and Jones, F. A., Ibid., 459,464 (1935). (8) Dunlop Rubber Co., Twiss, D. F., and Moore, J., Ibid., 494,043 (1937). (9) Goodyear, Charles, “Gum Elastic”, Vol. 1, pp. 72-3, 1855. (10) Hancock, Thomas, Brit. Patent 9952 (1843). (11) Hancock, Thomas, “Personal Narrative”, p. 105 (1857). (12) India Rubber J . , 84, 99 (1932). (13) Moore, W. A., U. S. Patent 1,958,924 (1934). (14) Morton, H. A., and Harrison, M. M., Ibid., 1,434,892 (1922). (15) Ibid., 1,434,908 (1922). (16) Romani, E., Caoutchouc & gutta-percha, 19, 11,626 (1922). (17) Rowley, T., Brit. Patent 767 (1881). (18) SOC.Geoffroy e t Delore, French Patent 329,519 (1903). (19) Twiss, D. F., Brazier, S. A., and Thomas, F., J. SOC.Chm. Ind., 41, SIT (1922).
Vulcanization from a Thermodynamic Viewpoint IRA WILLIAMS J. M. Huber Corporation, Borger, Texas
The study of vulcanization from a thermodynamic viewpoint has not progressed beyond the elementary stages. A study of the polymerization of isoprene shows it to be possible to produce a vulcanized-type polymer by direct polymerization. The relation between this polymer and natural rubber is unknown. The heat liberated during vulcanization bears a direct relation to the amount of combined sulfur and not to the change in physical properties of the rubber. Pigments incorporated in rubber produce vulcanized properties with little measurable energy change. The relatively small amount of experimental evidence indicates that the change in physical properties from unvulcanized to a vulcanized state is accompanied by a relatively small energy change. H E application of thermodynamics to the study of organic processes has only recently been undertaken extensively and has been confined largely to simple substances which can be readily analyzed and purified. The study of reactions involving more complicated substances, such as resins and gums, has seldom been attempted because the more simple methods of attack, such as reaction equilibrium, are not possible. The study of such systems is further complicated by the indefinite nature of the substances and the difficulty of closely identifying either starting materials or end products. The application of thermodynamics to the study of vulcanization has only just begun, and the present discussion must be limited to correlating the small amount of available data and to indicating desirable fields for future investigation.
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The most valuable tool of thermodynamics, when applied to chemical reactions, is the free energy AF, which is a measure of the driving potential of the reaction; it indicates the ability of the reaction to proceed spontaneously. The relation AF = RT In K where R = gas constant T = absolute temperature permits the equilibrium constant K of the reaction to be determined, and the way is open for predicting the effect of other variables. Such calculations are not ordinarily concerned with the intermediate processes of the reaction but require definite information in regard to both the starting materials and the final or equilibrium products. It is then necessary before applying thermodynamics to the study of vulcanization to define vulcanization clearly in terms of the final product. At present this is not entirely possible. Certain factors are recognized-for example, the combination of sulfur. However, the amount of combined sulfur cannot be used to define vulcanized rubber because under some conditions sulfur will combine without producing vulcanization (29). Furthermore, it is known that the same amount of combined sulfur does not always produce the same type of vulcanized rubber (16, 27). Vulcanization involving chemical reactions other than combination of sulfur is known. Rubber becomes useful after vulcanization because it has acquired new physical properties. This change is common to all types of vulcanization in varying degrees. It must also be recognized that rubber has a rather narrow temperature range of usefulness, and that the physical properties must be determined within this temperature range. With these factors in mind, we may conclude that vulcanization is a decrease in the plasticity of rubber within its elastic temperature range. Although a physical definition of vulcanization may be important for determining the extent of change, the chemical reactions cannot be disregarded since the energy changes involved in the chemical processes may entirely overshadow those directly responsible for the vulcanized properties.
