TESTING METHODS OF RUBBER CONTENTS
IN RAW AND VULCANISED RUBBER.
By W. A. DUCCA.

Two distinct chemical compounds of the rubber molecule have been the subject of extensive investigation during the last ten years, regarding a possible usefulness for a direct determination of caoutchouc in a given sample of raw or of manufactured rubber-the nitrosites or nitrosates as nitrogen compounds, and a compound with bromine as tetrabromide.

Harries deserves the credit of being the first one to make an attempt in the direction of a distinctly defined nitrogen compound. Almost simultaneously with C. O. Weber, he began a study of the action of nitrous acid gases upon solutions of rubber in benzole. Weber, who unfortunately did not live to finish his experiments, worked with nitrogen dioxide, developed by heating nitrate of lead. He obtained, or at least claimed to have obtained, an addition product of two molecules of NO2 to one molecule of rubber of the formula C10H16N2O4 in a polymeric form. The compound would have to be classed as a nitrosate. Harries, on the other hand, applied N203, generated from nitric acid and arsenic trioxide. He stated that by varying the conditions he was able to separate three distinct compounds of a nitrosite character which he called nitrosites (a), (b), and (c).

It is not the purpose of this paper to enter into a detailed discussion of these well-known classical investigations, but the true composition of the compounds obtained in the reactions is of the utmost importance for the estimation of pure caoutchouc in a given sample of rubber. I therefore feel justified in giving a short review of the more important results reached by different authors and at different times.

Harries's three nitrosites are as follows:Nitrosite a, C10H16N2O3, is formed by a six-hour action of dry N2O3 upon a 1 per cent solution of rubber. Nitrosite b, CroH15N3O8, is formed when dry N2O3 gas - is passed from two to three days through a suspension of nitrosite a in benzole. Nitrosite c, CroH15N3O7, is precipitated from a 1 per cent rubber solution in benzole by the action of wet N2O3 gases.

His first statements were later modified in some respects by Harries himself. The ultimate product of the action of N2O3 gas upon 1 per cent solutions of rubber in benzole was claimed to be uniformly nitrosite c, provided the reaction were allowed to go on for a sufficiently long time. Contrary to his first report, he gave as the best way

to prepare this compound the use of N2O3 gas thoroughly dried over phosphorus pentoxide.

These results naturally induced Harries to apply the new compound for an analytical determination of caoutchouc. He obtained a satisfactory result on a sample of an uncured rubber compound, but did not make any attempts to develop the method any further, as this would hardly be within the scope of his work as a college professor and should rather be accomplished by a technical man. His work in this direction was taken up where he had left it by a number of investigators; they practically all reached different conclusions based on various results.

The most exhaustive study of the subject was undertaken by Alexander with rather surprising results, not in N203 was shown to go much further than had been accordance with Harries's observations. The action of observed by either Weber or Harries. His method of precipitating the nitrosites from the rubber solution differed from Harries's method in so far as he generated nitrous acid anhydride from starch and 80 per cent nitric acid. The formula of the final product was given as C9H12N206, containing one carbon atom less than the original rubber molecule. This fact was explained by the oxidation of one of the two methyl groups of the caoutchouc molecule to carbonic acid gas, while the other one is supposedly oxidised only to the carboxyl group COOH. CH1

On the

That this explanation is possible from a theoretical standpoint is open to discussion. In support of his theory Alexander mentioned his observation of carbonic acid gas during the reaction. He certainly made a very thorough study of the matter, both theoretically and practically, and his analytical results are hardly to be doubted. other hand, Gottlob, who undertook a careful re-examina. tion of Harries's experiments, comes to the conclusion that their results were correct beyond any doubt, and that he could confirm them in all respects. In his opinion, Alexander obtained different results only by not closely following Harries's method in every detail. We conclude

from these contradicting statements, that it is, to say the least, very difficult to obtain uniform results, a fact which is not surprising at all. Weber very justly pointed to the indefinite composition of nitrous acid gases. It seems perfectly natural that under such conditions indefinite reaction products will result. Furthermore, every chemist working with these nitrosites or nitrosates will experience the practical impossibility of determining their purity by the ordinary well-known tests. Ultimate analysis by com. bustion therefore does not yield reliable information as to their true composition; the figures obtained are likely to represent the mean percentages of carbon, hydrogen, and nitrogen in several distinct compounds. In accordance with my own experiences, I fully support Alexander's latest views:-The nitrosate method is not of real value, at least not at the present time, for the estimation of actual caoutchouc in either raw or vulcanised rubber. For the determination of the vulcanisation confficient it may prove to be an excellent method. Here it is not obligatory to know the exact composition and purity of the nitrosate, which is used only as a means to completely precipitate the sulphur of vulcanisation. Of course, this question cannot be considered as definitely settled, and it will require a long and careful study of all the conditions before it can be established as a well founded fact to hold true in any case and for any form of soft vulcanised goods.

The advantage of the method for the latter purpose consists chiefly in the elimination of the many troublesome fusions with the soda potassium carbonate and potassium nitrate mixture, especially unsatisfactory in presence of a large percentage of mineral matter. The precipitate can be finely powdered, and is therefore applicable for rapid sulphur determination by ignition with sodium peroxide, whereby the sulphur is oxidised to sulphuric acid in the form of sodium sulphate, which in turn can be precipitated as barium sulphate. The sample has naturally to be first extracted with acetone in the usual way to remove free sulphur; any vulcanised oils are separated by saponification. One-half grm. of the sample thus prepared is treated with 50 cc. of kerosene (boiling-point up to 300° C.) until completely dissolved. Mineral matter can be removed either before nitrosation by centrifuging the kerosene solution or afterwards by pouring it with the nitrosate on a filter, and dissolving the rubber compound in acetone, whereby the mineral matter is left on the filter. acetone solution is either evaporated to dryness or poured into cold water to separate the nitrosate in a crystallised form. The latter method will secure a purer material, but necessitates an additional filtration. It is very essential to pour the acetone solution into water and not the water into the acetone, and to have the water cold, else the nitrosates will separate in an oily layer which does not crystallise very well, and is therefore hard to filter. The resulting yellow powder is mixed with sodium peroxide in suitable proportions and ignited in the usual way, as, for instance, in coal analysis for the determination of sulphur.

The

Jan. 3- 1913

filtering the solution through glass-wool, and precipitating it from the filtrate with his bromination mixture (6 cc. of bromine and I grm. of iodine in 1 litre of carbon tetrachloride). After standing for twenty-four hours, one-half volume of alcohol was added, and it was allowed to stand until the precipitate had settled clearly. The supernatant liquid was poured through a weighed filter, the tetrabromide decanted with a mixture of carbon tetrachloride and alcohol, finally with alcohol alone in order to remove the excess of bromine. A number of serious objections were soon made to the method as proposed by Budde; several sources of error, influencing the results in opposite directions, were demonstrated. Fendler and Kuhn found during an exhaustive study of the solubility of rubber in various solvents that a considerable amount of rubber remains undissolved in tetrachloride; this would consequently be removed during the filtration required in Budde's method. While this fact would tend to render low results, other points are apt to increase the percentages found. It is next to impossible to wash the bromine out completely from the inner parts of the lumpy tetrabromide precipitate by mere decantation with carbon tetrachloride and alcohol. Protein and resin bromides, the solubility of which in alcohol and tetrachloride has been disputed, would also add to the weight of the precipitate; finally, claim was made by several investigators that bromine reacted with rubber not only additively but also by substitution, especially when allowed to remain a long time in contact with the rubber solution. Harries and Rimpel even claimed to have separated such a substitution product. Under the pressure of so many objections, Budde modified his method to a volumetric one. Instead of dissolving and filtering the solution in carbon tetrachloride he adds his bromination mixture to a mere swelling of the rubber in carbon tetrachloride, allowing it to react for but six hours. The precipitate is treated in the same way as before, and the wash-liquors poured through a filter to collect any possible particles of tetrabromide which might be carried off. The bulk of the precipitate is re-dissolved in carbon bisulphide, and separated from this solution with gasoline. This treatment has the double effect of removing the free bromine more exhaustively, and converting the precipitate into a stable crystallised form. The crystals are oxidised with nitric acid in the original vessel in which they were precipitated. A sufficient measured amount of standard silver nitrate solution is added, and the mixture gently heated. The nitric acid has to be replaced from time to time, and the manipulation continued until the bromine is all combined as silver bromide. The excess of silver nitrate is titrated back by Volhard's well-known method with ammonium thiocyanate. The formula C10H16Bг4 forms the basis for the calculation of pure rubber, four bromines corresponding to one caoutchouc, C10H16. In applying the method it is preferable to extract the sample first with acetone and eventually with alcoholic potash to remove completely the resins which might prove a disturbing factor by forming insoluble bromine compounds. Spence has only recently demonstrated by his investigations on rubber proteins and their behaviour towards bromine that the amount of the latter added to the tetrabromide in Budde's method is too small to materially influence the final results, provided the tetrabromide is calcipitate. The same author observed a regular loss of bromine when oxidising the tetrabromide with nitric acid as proposed by Budde, a fact which was confirmed by Hinrichsen and Kindscher. They therefore suggested to replace the oxidising and titrating process by a combustion in a current of oxygen or fusion with sodium potassium carbonate and potassium nitrate mixture. The fusion method has been improved and more fully described by Spence, who obtained satisfactory results. Budde's method therefore gives, in the case of raw rubber, even at its present stage, a fair idea as to the amount of rubber contained in a given sample. A more difficult problem faces the_chemist in the case of vulcanised goods. The ques

maple floor, the coarser material on the bottom and the finer on the top. This pile is thoroughly mixed by shovelling, and is loaded into an electric elevator, which hoists it to its bin. There is a bin for each different kind of enamel, and a travelling bucket which holds a melt (abou 1200 lbs.) carries the mix to the tank furnaces where it is melted into a liquid glass.

These tank furnaces are regenerative, reverberatory furnaces like those used in the manufacture of glass, and natural gas or crude oil is an ideal fuel for them. How ever, in the older enamelling works, coal is used directly, and in the later ones producer-gas is used as a fuel. The temperature required for smelting the different enamels varies from 1000° C. for a glaze to 1300° C. for a ground coat, and, in most enamelling works, pyrometers are installed to assist in controlling these temperatures. Each furnace will give seven or eight melts in twenty-four hours. After the enamel is melted into a liquid glass, a fire-clay plug in the front of the furnace is pulled out and the glowing liquid stream plunges out and is caught in a tank of cold running water. The reaction is terrific and the glass mass is torn and shredded, cracking into small pieces like popcorn, each of which is a myriad of microscopic seams and fissures. This "quenching," as the process is called, toughens the enamel and facilitates the process of grinding which comes next.

The water is drained from the tanks, leaving the "enamel frit." This is shovelled into pans (a certain weight to a pan) and is ready for grinding.

In the mill room, the enamel frit is ground in large ball mills for about thirty hours. These mills are cylindrical, about 5 feet long and 6 feet in diameter, and are lined with porcelain bricks. The frit is put into them with 50 per cent of water and several per cent of white ball-clay. For the white cover-coat enamels, tin oxide is also added. The mill revolves and the constant impact of the flint stones against the glass particles grinds them to an impalpable powder, which mixes with the water and the clay, forming a mass which has the consistency of rich cream. This is loaded into tanks, where it is allowed to age a week or so.

Application of the Enamel.-From the mill room the enamel is taken to the dipping room, where it is put into tanks that are like large dish pans. These are sunk into tables, and at each tank a slusher works. The slusher takes the stamped-out steel vessel, which has been thoroughly cleaned, and plunges it into the enamel. When taken out, the wet enamel forms a thin film over the entire surface. By a gentle swinging motion, the excess of enamel is thrown off, and the vessel is placed bottom down on three metal points projecting from a board. Three or four vessels are put on a board; these are placed on racks and when the vessels are thoroughly dry they are carried to the furnace room.

The furnace room contains a long bank of mufflefurnaces, and in these the ware is put after drying. The temperature in these furnaces is about 1000° C., and here the little powdered particles of enamel are fused together in a solid glass coating over the vessel, the process requiring from three to five minutes.

Each coat is burned separately. For instance, we have a pudding pan that is to be a three-coat white inside, turquoise-blue mottle outside. It is first dipped in the ground coat enamel, the excess is shaken off, and the vessel put on a three-pointed rack and dried. After drying, the enamel stands in little grains all over the surface of the ware, adhering to the metal on account of the raw clay ground with it. At this stage every care must be taken, for a scraping, even of the finger nail, would take off some of the powdered particles of the enamel. This pan is then put into the muffle of the furnace, and the heat fuses all the little particles together, leaving a tight-holding vitreous coating all over the surface of the vessel. This fundamental coating is nearly black, due to the oxides of cobalt and nickel which it contains, and shines with a glass-like lustre,

After the vessel has cooled at the ordinary temperature of the room, it is again brought to the slushing room, and here is covered with an enamel-this time a white. It goes through the same process as before, except that a black bead is brushed around the rim. On account of the dark colour of the first coat showing through, this second coat, after it is burned, has a grey appearance, and is called the " "grey coat or "first white." The vessel is again sent to the slushing room, and is dipped into a white enamel, the excess shaken off, and before drying the bluegreen enamel is sprayed on the outside.

This spraying process was at one time done by dipping a wire brush into the wet blue-green enamel and the slusher shaking it over the surface of the vessel, causing the blue enamel to fall in little speckles all over the white enamel. In most factories, however, spraying machines, which work on the principle of an atomiser, have been installed. A tank full of the coloured enamel stands over the table and the enamel is forced out through a nozzle in a spray by compressed air. The flowing of the enamel is controlled by the foot of the slusher as he holds the vessel in the spray. The vessel is then dried and the coating is fused in the muffle-furnace, the result being turquoise-blue spots on a white blackground.

The finished ware is assorted into three lots: firsts, seconds, and job lots. Some of the seconds and job lots are fit for re-dipping. They may have some little spots where the original vessel was not properly cleaned, and where, on account of the rust or dirt, the enamel did not adhere. These spots are filed or are held under a sand-blast until the exposed surface is perfectly clean, and then the vessel is covered with another coat of enamel.

There are schemes for saving money in all manufacturing plants, and in the enamelling business a large part of the profit comes from the residues. For instance, every bit of enamel is scraped from the tanks and tables, all sweepings from floors are saved, and all the waste water from the various departments is first carried into catch basins, and every few days these are cleaned and the residue, which has settled to the bottom, is taken out. The residues from all these sources are again melted with the proper amount of fluxing material and colouring matter, and this dark coloured enamel is used for coating the cheaper wares.

A German White Enamel.-In order to give an idea of the composition of a white cover-coat frit, such as is used on cooking utensils, and to show the method used by ceramists to calculate its so-called molecular formula, the following enamel, the formula of which is taken from the 1911 edition of the "Taschenbuch für Keramiker," p. 18, is used:

Felspar 38.6 per cent, quartz 19 per cent, borax 15'4 per cent, cryolite 117 per cent, saltpetre 6.5 per cent, calcite 6.5 per cent, fluorspar 13 per cent, and magnesium carbonate ro per cent.

Enamel Materials.-All the materials used were practically pure except the felspar, which was a pegmatite of the following composition :

Per cent.