Eng.-tech. I. V. Gebler.

On the Problem of Rubber Regeneration

The large quantity of rubber waste remaining from various rubber articles that have become unfit for use¹ constitutes a very abundant material, the rational utilization of which is an important economic task, saving the expenditure of such an expensive product as rubber.

The enormous demand for rubber that has arisen in recent times under the influence of the development of the automobile industry and aviation was scarcely covered by the production of natural rubber, the harvests of which were realized entirely, without leaving any significant stocks on the market; in connection with this, rubber prices rose steadily. All this led to the emergence in technology of two questions: the preparation of artificial (synthetic) rubber and the utilization of rubber waste by the most rational use of the rubber substance contained in it.

Since the work of many chemists established the connection between the substance of rubber and the hydrocarbon isoprene, and it was found that the latter is capable of polymerizing, thereby turning into a substance which, if not identical with rubber, is in any case very close to it in its properties, the question of synthetic rubber may be considered solved; it has become possible to obtain it, at least under laboratory conditions. Many methods have been proposed for obtaining isoprene (otherwise methyl-butadiene: \(\mathrm{C_5H_8}\),

\[ \begin{array}{c} \mathrm{CH_2}\\[-2mm] \ \ \backslash\\[-1mm] \mathrm{CH_3}\!-\!\mathrm{C}\!-\!\mathrm{CH}=\mathrm{CH_2} \end{array} \]

) and for converting it by polymerization into a rubber-like product; among such methods, the most reliable in the sense of yield appears to be the one proposed by I. I. Ostromyslensky, who obtains isoprene from turpentine by acting on its vapors, in the presence of catalysts, with a silent electric discharge. Under these conditions there occurs partly a rupture of the bridge bond of the pinene ring of turpentine, partly its complete destruction, which leads to a mixture of hydrocarbons, among which the chief place belongs to limonene and isoprene; the latter is polymerized into a rubber-like substance under the influence of ultraviolet rays, obtained with the aid of quartz lamps, by heating with sodium wire, or by other methods. Transferred to factory conditions, such methods at first proved expensive and at that time did not acquire serious technical significance; therefore, all the greater interest is presented by the second important question in the economy of the rubber industry—the use of rubber scrap, or the regeneration of rubber.

The manufacture of rubber articles is based on the following properties of raw rubber: 1) when heated to \(60\text{–}100^\circ\mathrm{C}\) and simultaneously mixed, it turns into a plastic mass from which articles can be formed; 2) it is capable of dissolving in certain organic solvents (carbon disulfide, benzene, etc.), and after evaporation of the solvent it remains in an unchanged state. In all rubber articles, as is known, rubber appears in the vulcanized state, i.e. in the form of a chemical compound of it with sulfur. To introduce into the rubber mass from which articles are formed a known quantity of sulfur, and then the molded articles are “fired,” i.e. subjected for a certain time to heating at \(120\text{–}160^\circ\mathrm{C}\).

The vulcanized rubber thus obtained has entirely different properties than raw rubber. Whereas the physical properties of the latter change greatly with temperature, and it loses elasticity both on heating and on cooling, the vulcanized product is, in this respect, far more constant and, remaining equally elastic over much wider temperature limits, has no ability at all to pass into a plastic state at the indicated temperatures (60–100°); with stronger heating it either hardens completely, passing into ebonite, or else becomes soft and sticky, evidently being destroyed in the process, and yields a tar-like substance of little use; in solvents of raw rubber the vulcanized product does not dissolve; in some organic liquids with a higher boiling point it does dissolve, but when separated from the solution it gives the same sticky resin, which can be used in other cases only for cements and varnishes.

Old rubber goods are an exclusively vulcanized product and therefore, on the basis of what has been said above, cannot be converted into anything useful by the ordinary methods applied in the treatment of raw rubber. The problem of utilizing rubber waste is thus reduced to the regeneration of rubber, i.e. to finding methods for extracting from the vulcanized material pure rubber or, at least, such a rubber substance as would in its properties approach natural rubber as closely as possible.

Of course, such a problem might perhaps not present such considerable difficulties as is believed, if the changes undergone by rubber during vulcanization were reliably known. Meanwhile, despite the enormous development of the rubber industry and numerous scientific investigations of rubber and of the transformations it undergoes during factory processing, the essence of vulcanization, as well as the very nature of this substance, still remain far from elucidated. A pure colloid by its character, rubber presents not a few difficulties for its investigation; often, under the influence of one and the same action, it undergoes both physical and chemical changes proceeding side by side; the simultaneous course of both kinds of processes generally leads to complex phenomena, to unexpected changes in the substance during the very process of investigation, and greatly complicates work of this sort.

Rubber contains many substances: hydrocarbons, oxygen compounds, or so-called rubber resins, nitrogenous and coloring substances; its principal constituent part, as established by the work of Weber and Harries, is the hydrocarbon polyprene—\(C_{10}H_{16}\); it is assumed that polyprene is a highly polymerized dimethyl-cyclooctadiene of the following structure:

\[ \left[ \begin{array}{cccc} \mathrm{CH_3.C} & \mathrm{CH_2} & \mathrm{CH_2} & \mathrm{CH} \\ \Vert & & & \Vert \\ \mathrm{HC} & \mathrm{CH_2} & \mathrm{CH_2} & \mathrm{C.CH_3} \end{array} \right]_n, \]

where \(n\) must be very large, since the molecular weight of rubber, according to Hinrichsen and Kindscher (Wagner’s J., 1909, II, p. 546), is equal to 3173. Other authors give still higher figures.

The relations of rubber to solvents, as well as certain other phenomena, further compel the supposition that polyprene, which is very prone to polymerization, is present in rubber simultaneously in various polymeric modifications; in other words, the principal constituent part of rubber is a mixture of hydrocarbons of one and the same elementary composition—\(C_{10}H_{16}\), but with different molecular weights, i.e. with different values of \(n\). This \(n\), or degree of polymerization of the hydrocarbons, may change under the influence of such physico-

physical effects on rubber, such as heat, pressure, light, and at the same time its properties also change. In hot vulcanization, when rubber is heated approximately to 150°C, usually under pressure, there are, consequently, already evident conditions for its changes based on polymerization. However, in this case the changes are deeper, since a certain part of the sulfur introduced during vulcanization enters into chemical combination with the hydrocarbon. The latter is considered established, for in treating vulcanized rubber with all known solvents of sulfur it is not possible to extract all of it, and a definite quantity of sulfur, constant for the given product, remains unextracted. The combination of sulfur with the hydrocarbon occurs without substitution, since hydrogen sulfide is not evolved during vulcanization; probably the sulfur is added at the double bonds of polyprene. There are no methods for removing chemically bound sulfur.

Free sulfur, which is always present in soft vulcanized rubber, can readily be extracted either with an alcoholic solution of caustic soda (Henriques), or with acetone (Weber); in this case the vulcanizate is not deprived of its usual properties, which depend, consequently, only on that sulfur which is chemically combined with the hydrocarbon. The limit of saturation of rubber with such sulfur, according to Weber’s indication, is fairly constant and in individual cases amounts to 33.4%, 33.08%, and 32.68% of the weight of the rubber. Such a sulfur content in its combination with the rubber hydrocarbon corresponds fairly closely to the empirical formula \(C_{10}H_{16}S_2\) with 32% S. With such a limiting sulfur content, hard vulcanized rubber is obtained; in soft vulcanizate, however, the quantity of chemically bound sulfur determining its usual properties may be quite small; thus, for example, Henriques (Ch. Zeit. 1894 S. 701) found in three samples of soft vulcanized rubber with a total sulfur content of 6.44%, 7.75%, and 8.24%—chemically bound S, respectively: 3.3%, 1.86%, and 3.39%. Assuming, on the basis of Weber’s cited works, that at 32% of chemically bound sulfur the rubber hydrocarbons are completely saturated with it, we come to the conclusion that, for the examples given, the quantity of sulfur-saturated hydrocarbons in these rubbers will be: 10.3%, 5.81%, and 10.6%; the remaining part of the polyprene is, therefore, in a free state. The examples taken are of average value, and the conclusion drawn is general for soft vulcanized rubbers, the amount of bound sulfur in which varies from 1 to 4%, in accordance with which the content of sulfurized hydrocarbon is close to the figures cited, amounting to from \(1/20\) to \(1/10\) of its total quantity present in the rubber.

Thus, with a sufficient degree of certainty it may be accepted that soft vulcanized rubber is a close mixture or solution sui generis of sulfurized rubber hydrocarbon—polyprene sulfide—in the same hydrocarbon, which has remained free but altered as a result of polymerization; moreover, the quantity of free hydrocarbon exceeds many times the quantity combined with sulfur. Such an assumption indicates the path that must be followed in order to effect devulcanization of rubber.

To strive to remove sulfur from the hydrocarbon, applying the corresponding methods to vulcanized rubber as a whole, is obviously inexpedient: the action of such a reaction would be, first, very unfocused, since it is directed at a small quantity of a substance situated in close proximity to a much larger mass of another substance foreign to the desulfurization reaction; in molecular reactions such ratios are unfavorable; secondly, under the influence of actions for removing sulfur or replacing it with other atoms or atomic groups, the rubber hydrocarbon free from sulfur could suffer, as indeed does occur. Attempts in this direction have had no real success. Therefore, in order to carry out the regeneration of rubber, one must strive

not to the removal of sulfur, but to the extraction of the sulfurized hydrocarbon from its mixture with the free hydrocarbon. Such a separation must be carried out first of all and would in itself lead to obtaining from the vulcanized product a pure rubber hydrocarbon, or its polymer; if, moreover, the properties of the latter proved to permit its processing in a manner similar to the processing of natural rubber, this would constitute the solution of the problem of rubber regeneration. The removal of sulfur from the hydrocarbon bound to it would constitute a further task, the technical importance of which, however, is much smaller in view of the lower content of sulfurized hydrocarbon in vulcanized rubber. For the same reason, in separating polyprene and its sulfide, it is more advantageous to extract the latter rather than the former; the separation itself, obviously, must be carried out by using a reagent that would dissolve only the hydrocarbon bound with sulfur, without acting on the rest.

It was precisely in this case that the experiments undertaken led to the discovery of the solvent that was needed; its use made it possible to obtain a regenerate suitable for processing into articles, and confirmed the correctness of the path chosen for this purpose. This solvent proved to be aniline.

No similar method was found among the literary materials available on this question; for this reason it is appropriate to give a brief survey.

The data relating here, in the form of small fragmentary communications, are scattered throughout the patent and journal literature; a general summary of them may be found in Alexander’s article, “Theorie und Praxis der Regeneration des Kautschuks,” in Chemiker Zeitung, 1910, which is the most complete on this question. First of all it should be noted that everywhere in the literature the devulcanization of vulcanized rubber, i.e. the removal of sulfur from it and the isolation of the pure rubber substance with all the properties of the original product, is treated as an impossible task.

Alexander says: “It must be considered an established fact that the recovery of sulfur-free rubber from a vulcanized product is an insoluble problem. All methods by which the replacement of sulfur in sulfur-containing organic compounds by other atoms or atomic groups is possible fail with vulcanized rubber” (“...muss es als nahezu feststehende Tatsache angesehen werden, dass das Wiedergewinnen von schwefelfreiem Kautschuk aus vulkanisierten Kautschukprodukten ein unlösbares Problem darstellt. Alle Methoden, durch welche bei organischen Schwefelverbindungen ein Ersatz des Schwefels durch andere Atome oder Atomgruppen bewirkt werden kann, versagen beim vulkanisierten Kautschuk...”)\[^1\].

Therefore, by regeneration here one should not understand the rebirth of rubber; on the contrary, in this field a meaning sufficiently arbitrary is given to this term. Often by “regenerate” is meant simply rubber waste that has been sufficiently finely ground and processed on rolls, which in this state is introduced into a compound prepared from natural rubber as a related filler. Such regenerates, of course, do not differ in composition from the waste from which they were obtained by grinding and rolling, as was also found by the analyses of Henriques (Ch. Zeit., 1895, p. 485). In a narrower sense, regeneration means the conversion of vulcanized rubber into a plastic state, with simultaneous removal, as far as possible, of extraneous admixtures, such as fibrous substances (if they are present in the material being processed), and various kinds of fillers. For carrying out such processing, several methods are indicated, which fall into the following categories:

1) The acid method (Säureverfahren), proposed by Mitchell: shredded rubber waste, after mechanical cleaning, is heated for a prolonged—

\[^1\]: Alexander. l. citato.

for a prolonged time with moderately concentrated sulfuric acid or with a mixture of sulfuric and hydrochloric acids; the fibrous substances (mostly cotton) are thereby destroyed, their cohesion with the rubber disappears, and they are removed on washing rolls; at the same time a certain plasticity of the product is attained, which is brought to the proper degree by subsequent treatment with high-pressure steam.

Mineral oil is added to the plastic mass obtained; it increases plasticity and protects the product from “over-vulcanization” (Übervulkanisation) during further processing.

2) The alkaline method (Alkaliverfahren) or Marks’s method: the ground waste (containing fibrous substances) is heated under pressure to a high temperature with moderately concentrated alkali; the fibers are destroyed and the plasticity of the product is attained, so that steam treatment (as in the acid method) is not required. The alkaline method has the advantage, first, of greater simplicity, and second, because traces of alkali remaining in the rubber have a beneficial effect on it, whereas traces of acids, under the simultaneous influence of air and light, cause deterioration of the product; thus, with the acid method, treatment with alkali is nevertheless necessary for the complete removal of acid, which cannot be achieved by washing with water alone.

The removal of mineral substances is difficult in both methods, since aqueous solutions penetrate rubber with difficulty, while the mineral substances are in a very finely divided state and are intimately mixed with the rubber substance.

Should the methods described be regarded as universal, i.e. suitable for processing vulcanized waste of any grade and composition, or, on the contrary, do only certain grades of vulcanizate lend themselves to such treatment?—this is not clear from the description; but when these methods were reproduced and applied to such materials as gray and red tubing for illuminating gas and galosh rubber, no satisfactory results were obtained in the sense of achieving the plasticity spoken of in the description. Prolonged heating with solutions of acids and caustic soda of various concentrations in a closed vessel up to 200°C did not lead to substantial changes in the material taken;¹ while generally retaining elasticity, it lost somewhat in strength, offered less resistance to rupture, and could be ground more easily.

The latter circumstance may perhaps be the purpose pursued by the methods described; that is, once it becomes possible to process them more or less easily under factory conditions on rolls, to bring vulcanized waste into a state of some plasticity (which is not hard to do directly), to grind it with mineral oil, and in this form subject it to further processing, of course together with natural rubber.

The possibility of over-vulcanization indicates, of course, that all the sulfur remains in the product, and the addition of mineral oil, by diluting, so to speak, the mass, lowers the percentage content of sulfur in it and thereby protects it from over-vulcanization.

In view of the fact that rubber articles are materials of exceedingly varied composition, it is possible, on the other hand, that heating with alkali under pressure may directly bring certain grades of vulcanized rubber into a plastic state; and this is most possible for those materials which contain in their composition a large quantity of rubber substitutes known under the name “factice”—products obtained by the action of sulfur on various vegetable oils (rape oil,

¹ As has already been noted, “dry” heating to such a temperature always leads to profound changes in vulcanized rubber.

glue, rag fiber, etc.). The content of factice in some compositions sometimes exceeds the amount of rubber itself twofold, and since factices are not resistant to alkalis, the above-mentioned conclusion may prove quite probable. However, heating with alkali in an autoclave at 7–8 atmospheres does not pass harmlessly for the rubber substance, and the product obtained under such conditions has insufficient strength.

3) The dissolution method (Löseverfahren), as described by Alexander, while simple in principle, presents considerable difficulties, partly of a technical and partly of an economic character. The technical difficulties consist in the fact that vulcanized rubber does not dissolve in the same solvents and at the same temperatures as crude rubber; higher temperatures are needed for dissolution, but rubber dissolved in this way, when separated from the solution, is usually obtained in the form of a resin, and in this form it is not suitable for articles. However, with the use of suitable solvents and sufficient caution (“…bei Verwendung geeigneter Lösungsmittel und bei genügender Vorsicht…”)¹ it is possible to bring vulcanized rubber into solution at such a temperature that the indicated undesirable phenomena, arising as a consequence of depolymerization, do not occur. Yet, owing to the great variety in the composition of different wastes, it is very difficult each time to select these “suitable conditions,” so that this dissolution method can have only limited application. Its essence consists in this: the waste is heated for a long time, with elevation of temperature, with a solvent—best of all with suitable hydrocarbons having a not too low boiling point (“…am besten eignen sich Kohlenwasserstoffe von nicht zu niedrigem Siedepunkt…”)²; in this process the rubber and organic fillers pass into solution, while the mineral and fibrous substances remain undissolved; from the solution the rubber is precipitated with acetone or alcohol, freed from residues of the solvent, and thus a product suitable for practical use is obtained. Alexander believes that this method cannot have technical significance, because of the uneconomical nature connected with the large consumption of the rubber solvent and the precipitating reagent, since, in his opinion, it is impossible without considerable losses to separate the two substances from one another, as is required for their reuse. In addition, the difficulty of the dissolution method, as has already been indicated, lies also in the fact that for different wastes it is necessary each time to select the corresponding solvent.

Work with different grades of vulcanized rubber and the use of the most varied solvents led to the following results: some of them do not dissolve vulcanite at all, but bring it into such a state that the previously elastic rubber completely loses its elasticity and becomes so weak that it is easily rubbed between the fingers into a greasy, curd-like mass (benzene, toluene, etc.); other solvents (nitrobenzene, gasoline), without dissolving the vulcanite, extract from it an insignificant amount (about 2%) of some substances; and, finally, vulcanized rubber dissolves rather well upon prolonged heating in heavy “Solvent naphtha” and turpentine, and most easily of all in heavy turpentine oil, specific gravity 0.923, obtained in the final fractions during the purification and distillation of red (stump) turpentine. But the separation of rubber from such solutions, by whatever method it was carried out—whether by precipitation with alcohol or acetone, or by distillation of the solvent, or by direct evaporation, as well as with steam—always led to the rubber substance being obtained in the form of a sticky, semi-liquid resin, which by no means could be converted into anything at all resembling rubber. With such disso—

¹ Dr. Alexander, l. citato.
² Dr. Alexander, l. citato.

... as in turpentine oil, everything passes into solution, i.e., both the free hydrocarbon and its sulfurous derivative; moreover, obviously, in this state of dissolution new reactions arise which lead to profound changes in the rubber substance. As may be seen from the excerpts cited above from Alexander’s article, it is entirely unclear exactly what solvents are used in the “method of dissolution” and what sort of precautions must be observed in order to obtain the desired result; the direct experiments indicate, however, that from solutions into which vulcanized rubber passes as a whole, the rubber substance is in every case obtained with completely altered properties.

Quite different phenomena occur when vulcanized rubber is treated with aniline. The latter, as is known, is a good solvent for very many substances; among other things it dissolves sulfur as well, but it has no such action at all on pure rubber.

When raw rubber is treated with boiling aniline, it swells strongly and a homogeneous mass is obtained, giving the impression of a solution; upon cooling, however, the rubber separates out in an unchanged state. But, while it does not dissolve the free rubber hydrocarbon, aniline, apparently, is capable of dissolving its sulfurous compound; from such a solution alcohol precipitates a deposit which can be readily filtered off and, after washing with alcohol and drying, is a brown-colored solid substance containing sulfur and rubber. This substance has not been studied more closely and for the present has been set aside, since chief attention was directed to what remained undissolved and what, in the final analysis, represented the “regenerate,” obtained [[unclear: obscured line]].

The process of treating any grade of vulcanized rubber with aniline (the experiments were carried out with the following grades: red, gray, and black tube for illuminating gas, galosh rubbers, old stamps, rubber sheets) always proceeds in the following way: the rubber swells and soon forms a turbid homogeneous mass, which on further heating becomes quite liquid; in this, the rubber, evidently, melts (the boiling point of aniline—182°—fully corresponds to this) and is uniformly distributed in the aniline, forming an emulsion, which promotes close mixing of the rubber substance with the solvent and facilitates the extraction of soluble substances. In order to remove part of the mineral impurities, it is necessary, by slightly lowering the temperature, to stop the boiling of the liquid and allow the mineral substances to fall as a precipitate, from which the hot liquid is separated by decanting and, if necessary, filtered from coarse impurities (fibrous materials, etc.) through a metal screen; upon cooling to ordinary temperature, it separates out the rubber in the form of a compact clot collecting at the bottom, while the solvent becomes transparent. As this whole mass stands at rest, the rubber clot gradually becomes denser and after some time becomes so dense that the solvent can be drained from it or removed with the aid of a siphon, and the rubber itself taken to the centrifuge; in decanting, about 70% of the aniline taken is recovered, while the centrifuged product contains about 5% of it, which is already difficult to remove mechanically. Nevertheless this is necessary, since, first, leaving aniline in the product would constitute a pure loss of it, and, secondly, even an insignificant amount of it remaining in the rubber imparts to the latter an unpleasant odor and oiliness.

Complete removal of aniline is achieved by heating the regenerate in a vacuum at 130–140°C; the aniline is thereby distilled off (the vacuum must, of course, be suitable), while the rubber under such conditions withstands the indicated temperature without any changes.

As a result of all these treatments, a very plastic rubber substance is obtained, which over time, losing plasticity, becomes...

elastic; by heating at a temperature of about 100° (water bath) it can again be brought into a plastic state.—By ordinary methods, without the addition of natural rubber, the regenerate obtained in this way can be mixed with sulfur and vulcanized, yielding a product quite similar to ordinary vulcanized rubber.

The yield of regenerate depends, first of all, of course, on the rubber content in the scraps that were taken for processing and that, as already mentioned, are extremely varied in composition; some of them contain only mineral fillers, about 60% (galoshes rubber: ash: 57%). On the other hand, the same yield must be connected with the amount of sulfur derivative present in the given material, and the less of the latter there is, the greater the yield.

The difference between regenerate and natural rubber, to which it is similar in methods of processing for conversion into a vulcanized product, consists in the following: first of all, regenerate always contains a certain amount of mineral substances, the complete removal of which, on the one hand, is very difficult, and on the other hand is scarcely necessary, since the preparation of masses for articles from any rubber is almost always accompanied by the addition of certain mineral substances, which is done not only to cheapen the articles but also to impart to the vulcanizate one or another degree of hardness; therefore, to strive to remove from regenerate intended, of course, for vulcanized articles those mineral impurities uniformly distributed in it, which later would in any case have to be introduced, not without trouble, appears obviously pointless rather than expedient. Further, regenerate does not dissolve in benzene or petroleum benzine and has a greater tendency to pass into a plastic state than natural rubber, at least at first. All this indicates that the hydrocarbon of regenerated rubber, obtained from a vulcanized product, is probably a polymer of the hydrocarbon of natural rubber, i.e. is separated in the state in which it arrived during the process of vulcanization, in which polymeric changes are considered more than probable.

The method of regenerating rubber with the aid of aniline can be applied with equal success to all possible grades of vulcanized rubber; the difference consists only in the fact that mixtures with a small amount of rubber and with a high content of foreign impurities usually require longer treatment with aniline than grades of vulcanizate with a smaller amount of all kinds of fillers.—Besides its general applicability, this method, compared with the dissolution methods described in the literature, which are not entirely clear, has the further advantage that its use, as is evident, is not accompanied by dissolution of the rubber as a whole, and therefore does not require precipitating reagents, which leads to greater simplicity and economy.

The latter depends only on losses of aniline, which, with properly designed apparatus and correct conduct of the process, should be insignificant. In extracting fats with such a volatile solvent as carbon disulfide (\(t_{\text{boil.}} = 46^\circ\)), its losses, as is known, amount to no more than 0.5% relative to the weight of the fat obtained; for aniline, however, with a boiling temperature of 182°C, the losses, of course, cannot be greater; therefore, although aniline is a comparatively inexpensive product, with negligible losses its use for regenerating expensive rubber can hardly prove uneconomical.

However, on the basis of laboratory data alone one cannot make a final conclusion regarding the general suitability of this method; in particular, it is difficult to assess the strength of articles made from regenerate, since, to obtain reliable results, very thorough processing of the rubber mass is necessary, which is carried out at the factory by means of roll—

machines; this is necessary for obtaining complete homogeneity of the mixture, on which its strength also depends. Under laboratory conditions, such processing is, of course, impossible to carry out.

The course of the process of regenerating rubber with the aid of aniline is presented visually in the attached diagram.

From rubber waste to regenerated rubber.

PROCESSING DIAGRAM

Rubber waste + aniline.

Boiling, settling, hot filtration.

↙︎               ↘︎

Precipitate.

Mineral substances,
fibrous substances,
organic substances,
insoluble in aniline.

↓

Calcination of the precipitate.

↓

Zinc oxide.

Sulfur compounds, etc.

Liquid

In solution: sulfur, sulfurized rubber, organic substances soluble in aniline. Rubber in the form of an emulsion.

↓

Cooling.

1st separation of aniline.

↙︎               ↘︎

Precipitate.

Rubber + aniline + mineral substances.

↓

Centrifuging.

2nd separation of aniline.

↓

Rubber

• small quantity of aniline.

↓

Evaporation in vacuum.

3rd separation of aniline.

↙︎               ↘︎

Plastic rubber
(regenerate)

Solution

Sulfur, sulfurized rubber, soluble organic substances.

↓

Distillation of aniline with steam

↙︎               ↘︎

Residue.

Sulfur, sulfurized rubber, soluble organic substances.

Crude aniline.

↓

Redistillation of crude aniline.

↓

Pure aniline.

1. Here, of course, only articles made of soft rubber are meant, not ebonite. ↩