Feb., 1918
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
we believe, to the use of magnesium oxide as an accelerator in the vulcanization of rubber by sulfur. We have come to an entirely different conclusion, however, and, although we do not deny the specific results to be obtained by the use of both litharge and magnesium oxide in the vulcanization of India rubber by sulfur, we lay no claim here to the use of any of these inorganic ingredients in vulcanization. We claim, furthermore, that the results obtained by the use of any of these inorganic accelerators in the vulcanization of rubber by sulfur are substantially different from those obtained by the methods of our invention; both these inorganic substances have to be used in large amount to produce any marked result and neither of these inorganic accelerators gives the physical properties to the vulcanized rubber possessed by the products of our invention, which is best illustrated by comparative results and by the fact that the addition of the products of our invention to compounds already containing these inorganic substances causes a still very pronounced improvement in the results of vulcanization.
I ill omit t h e claims a t this present time. The article also gives six examples which I have omitted, a n d defines in full t h e conditions under which t h e process may be carried out, a n d t h e pros and cons of various classes of reagents. Thus it points out, among other things, t h a t some agents are solids, difficult t o handle; others yield colored products on vulcanization (nitrosodimethylaniline, for example) ; others on account of their poisonous character or odor would not be suitable for practical purposes; nor will t h e methods of application constitute a novelty in connection with discoveries along this line as t h e description sets forth in detail. If our friends across t h e Atlantic should still doubt our claims t o priority of discovery of this fundamental principle of vulcanization let them test t h e validity of their patents. They will find t h a t t h e records which I have just cited are a mere indication of t h e volumes of evidence and facts which can be established t o dispose of any claim on their part t o novelty of invention as far as America is concerned. As far as t h e specific claim t o t h e use of p-nitrosodimethylaniline is concerned, let me assure Mr. Peachey t h a t when he is ready, I a m prepared t o demonstrate through several different sources t h a t this material was tried out both scientifically and industrially in this country so long ago as 1910, and was long since abandoned on account of the serious disadvantages t o its extensive use. I recall indeed having used it in one instance in t h e preparation of rubber stoppers for laboratory use, b u t with unfortunate results. I t is a substance which I should not think of employing industrially now, as there are so many objections t o its use. I n conclusion, a good reagent must intensify not merely t h e chemical process of vulcanization, but also t h e physical; it should toughen the rubber, whether raw or vulcanized; and should render it immune t o deterioration. All this has been achieved in America, giving a rubber superior t o t h a t from any natural source. A .I< noble” rubber, similar t o t h e “noble” alloys, is a n accomplished fact. NORWALK TIREAND RUBBERCo., INC. NORWALK, CONNECTICUT
VULCANIZATION OF RUBBER BY SELENIUM By CHARLSSR. BOGGS Received October 31, 1917
Vulcanized rubber has been manufactured for some years, b u t there has really been no essential change from t h e general methods of vulcanization as originally specified by t h e inventors. T h e two original methods, which are still in use, are t h e vulcanization with sulfur by heat and t h a t by sulfur chloride in t h e cold. Variations in t h e processes have been introduced and innumerable mixtures made with other materials, b u t no rubber article of practical importance has been p u t on t h e market which has essentially deviated from t h e two original processes. There have been made, however, many slight variations in compounding rubber mixtures which have produced vulcanized rubber products t h a t are characterized b y properties which fit them t o special work better t h a n any previously known compounds. It was with this idea t h a t we thought rubber vulcanized with selenium might give a product of especial adaptability t o some of t h e many uses of rubber. At t h e time t h a t we first tried t o vulcanize rubber with selenium, 1913, we thought t h a t we were t h e first, although it is evident t h a t anyone with a knowledge of chemistry would expect selenium t o act similarly t o sulfur. We have since noticed t h a t Pearson in his book, “Crude Rubber and Compounding Ingredients,” mentions two methods, one b y heating rubber with equal parts of selenium and t h e other by dropping liquid selenium into a CS2 solution of rubber a t 300’ F. under pressure. It is evident t h a t these methods had nothing t o recommend t h e m and I believe could never have been developed because of the unsatisfactory product. Selenium is a metal in t h e same group of t h e periodic table as oxygen, sulfur and tellurium, is much more metallic t h a n sulfur and has a higher melting point (217’ C.), sufficiently high t o discourage one from attempting t o use it as a vulcanizer for rubber which is not capable of withstanding such a temperature. Its atomic weight is 79. 2. It occurs in two crystalline and one amorphous form and forms a complex molecule when cold, Ses being very similar t o sulfur. A short table of its properties follows: CRYSTAL Black or gray crystalline
...........
Red crystalline.. . . . . Red amorphous.. .
..
Octahedral Hexagonal Monoclinic
. , . . ..
Specific Gravity
Solubility Insoluble in
4.80 4.46-4.51 4.26-4.28
cs2
Solublein CSz 1 soluble (and 1 insoluble) in CSa
Melting Point 217’ 175’ Softens at 102’
All modifications go over t o t h e black crystalline form when heated a t I O O t o 150’ C. T h e black crystalline powder can be obtained on t h e market in small quantities, b u t it should be procurable in fair amounts if there were a commercial demand for it. Black selenium has the further peculiar property of being a n electric conductor under t h e influence of light although t h e other forms are insulators. It might, therefore, cause rubber which has been vulcanized with it t o show some slightly unusual electrical characteristics.
118
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Our first attempts were with selenium in the form of black powder used in a standard 30 per cent P a r a compound in the equivalent proportion t h a t sulfur would be used. By heating a t about 150' C. for a couple of hours a partial vulcanization resulted. The physical tests showed a normal elongation, but a tensile strength of only about 5 0 per cent of the similar sulfur compound. The point t o be noted is t h a t partial vulcanization was obtained, although the temperature was well below the melting point of Se. Increase in the time did not improve t h e product. All of these first samples have aged well and after nearly four years give t h e same elongation and tensile-strength values of about 7 0 per cent of the original. Doubling the amount of selenium and using an organic accelerator, which we were then using with sulfur, did not improve t h e product but did make the samples go t o pieces with age, a normal characteristic of overvulcanized and under-vulcanized rubber. A peculiar point about this compound was t h a t when it was removed from the press hot it expanded 2 5 per cent of its volume. Its volume became normal when cold. This high coefficient of expansion indicated lack of vulcanization. Using amorphous selenium and an organic accelerator we were able t o increase the tensile strength some without sacrificing t h e elongation. Some other accelerators were tried without much success. The amorphous selenium should go over the metallic form a t the temperature used. The most promising compound we then h a d w a s p u t on wire and has been tested regularly for t h e last three years and t h e remarkable point is t h a t i t has not deteriorated appreciably in t h a t time. This compound was below normal in its tensile strength. The difficulty seemed t o be t h a t t h e long heating a t t h e relatively high temperature used t o effect vulcanization caused too great a depolymerization of the rubber. By trial we have now found accelerators which enable us t o satisfactorily vulcanize rubber with selenium when heated a t t h e ordinary vulcanizing temperature of 2 7 5 O F . (135' C.) for only about twice t h e time required with sulfur. The product gives the normal tensile strength ( I I O O t o I zoo lbs.) and elongation ( 2 t o 10in. or I z in.) of the same compound with sulfur. I t is somewhat soft. This compound shows no deterioration under the short life test of 4 days' heating in air a t 7 0 ° C. Electrical tests have been carried out on wire insulated with this compound and the insulation resistance and dielectric strength are somewhat low. Special determinations as dielectric loss, etc., have not yet been determined. Chemical analysis as applied t o ordinary sulfur compounds does not apply t o these compounds vulcanized with selenium, as the black selenium is practically insoluble in acetone and consequently the uncombined Se is not separated by extraction with acetone. Also i t is only very slightly soluble in CHCL and CS2. The acetone extract contains only t h e resins from the rubber (provided oils, waxes, etc., are not added).
Vol.
IO,
No.
2
The CHCls extract contains some Se and a small amount of unvulcanized rubber as with soft vulcanized rubber when cured with sulfur. A determination of rubber by t h e tetrabromide method gave 31.7 per cent rubber plus resins, etc., in a compound t o which 3 1 . 7 per cent rubber had been added. This would indicate t h a t no correction should be made for combined Se, i. e., t h a t the selenium was either not chemically combined or more likely t h a t i t was so weakly combined t h a t i t was displaced by Br. It is possible t h a t a complete chemical study of t h e vulcanization of rubber with selenium may throw some additional light on t h e theory of vulcanization. Also i t may help in t h e study of the nature of t h e catalytic effect of accelerators as t h e vulcanization occurs so far below t h e melting point of the Se. The product as we now have i t has not yet shown any unusual electrical properties, but t h e indications are t h a t its deterioration with age is much less t h a n with sulfur compounds. It can be brominized and oxidized but the natural oxidation seems t o have been slowed up. As t h e deterioration of rubber goods is the one disadvantage of rubber, especially in those lines of work where permanency is desired, i t may be t h a t the use of selenium may partially remove this disadvantage. RUBBERLABORATORY OR THE SIMPLEX WIRE & CABLECOMPANY BOSTON, MASSACHUSETTS
THE PIGMENTS OF THE TOMB OF PERNEB' B y MAXIMILIAN TocH
I n 1913 Mr. Edward S. Harkness presented t o t h e Metropolitan Museum of Art of New York City, t h e Tomb of Perneb, which originally stood in the cemetery of t h e ancient Memphis. Mr. Harkness acquired this t o m b from t h e Egyptian Government and Dr. Albert M . Lythgoe removed the tomb from Sakkara and re-erected it in t h e main hall of the Metropolitan Museum of Art. The tomb was built approximately 2650 B. C. and in i t was buried a n Egyptian Dignitary named Perneb, who held high office under t h e king a t Memphis. T h e tomb contains many figures in relief, particularly the side wall in the main chamber, on which t h e carvings are very profuse. The figures are all colored with various pigments.2 The pigments used on t h e Tomb of Perneb are red, yellow, blue, green, gray a n d black. There is a popular belief t h a t t h e red used by the Egyptians was red ochre. This is an error, as ochre is yellow naturally, and only turns red when it is burned or calcined. Ochres all normally contain between I O and 2 0 per cent of oxide of iron, whereas t h e reds of t h e Egyptians contain more t h a n 50 per cent of oxide of iron, a n d from their very color i t is certain t h a t t h e red of t h e ancients was hematite. This is never a very bright 1 Paper presented at the meeting of the New York Section of the Society of Chemical Industry, October 19, 1917. 2 I t is a great pleasure for me t o acknowledge the assistance that I have received from Dr. Albert M. Lythgoe, through whose courtesy I was given the pieces of limestone and the adherent pigments which were taken from the Tomb of Perneb.
