Feb., 1918
T H E JOURATAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
CATALYSTS IN VULCANIZATION1
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this basic discovery was not m a d e t h e subject of a p a t e n t as Peachey has suggested,l may appear remarkWhen t h e C h a i r m a n of this Section wrote me asking able a t first thought, b u t its very b r e a d t h rendered if I would read a paper o n “Vulcanizing Catalysts,” p a t e n t protection well-nigh impossible a n d disadt o me one of t h e most fascinating subjects of rubber vantageous t o seek. T h e n u m b e r of substances which chemistry, it was with regret I advised h i m t h a t I can be used in this connection with more or less success would be unable t o d o so o n account of lack of t i m e t o is legion, so t h a t patents such as those which h a v e prepare a paper. I n t h e meantime, however, I been granted recently, can be readily overcome, a n d h a v e found a n article prepared b y me in 1911, are of no intrinsic value in a n y case. By t h e methods b u t never published, which, together with a few notes of organic chemistry, all manner of substances can prepared o n m y trip t o Boston yesterday, with your be, a n d h a v e been prepared, which will be found t o permission I will read, as t h e y should clear u p some accelerate vulcanization, a n d t o bring a b o u t t h e physof t h e points raised in this discussion, a n d a t t h e same ical result t o a greater or lesser extent; simple organic t i m e serve t o show t h e extent t o which t h e investiga- substances widely different i n constitution, such as tion of this subject h a d been carried b y some of us aniline oil, piperidine, diazobenzene, etc., metalloprior t o 1911,a n d our knowledge of this subject in organic compounds such as alkyl derivatives of lead, America a t t h a t date. a n d t h e salts of t h e f a t t y acids or of aniline oil, with This subject is one which is of particular interest t h e alkali metals; t h e n there is t h e great group of subt o me, as it has been, I may say, m y main field of in- stances which b y decomposition, natural or induced vestigation for t h e past eight t o t e n years. During during vulcanization, produce some very active a n d t h a t period m a n y interesting discoveries h a v e been efficient catalysts t o which class t h e thioureas belong. made, more t h a n one hundred widely different vul- E a c h of these various classes contains several dozen canizing catalysts have been prepared, investigated different representatives, which I h a v e already investia n d p u t on record, a n d results of scientific as well as gated, a n d there are doubtless hundreds still t o be of technical importance h a v e been obtained. tested, so t h a t I a m sure you will agree with me t h a t B y some of our good friends a n d allies across t h e t h e task of adequately protecting a n invention of t h e water, jealous of their claims t o discoveries along this scope of this one is hard, t o say t h e least, a n d of doubtful line, 1 h a v e been recently criticized, for not placing on value if attempted. It has certainly not been accomplished so far. I n 1911,i n a n effort t o cover b y p a t e n t public record b y patent or otherwise, t h e results of these past years of investigation, b u t I a m confident t h e results of past years of research in this line, I set down a description of some of t h e discoveries m a d e t h a t b y t h e t i m e t h e y h a v e carried their studies a little further, t h e y will appreciate t h e difficulties in- b y t h a t time, together with four claims, f r o m which, volved in t h e problem, a n d m y reasons for compara- with your permission, I will read t h e following extracts. T h e article is entitled “Improvements i n a n d Relating tive silence. These remarks will, I hope, serve t o answer some of their criticism a n d , a t t h e s a m e time, t o t h e Vulcanization of India R u b b e r ” a n d is d a t e d outline t h e history of this subject as far as I a m con- August, 1911. I was surprised, o n referring t o it again t h e other d a y , t o find how fully i t deals with t h e cerned. I n the first place, I would point o u t t h a t , as a m a t t e r subject. Since the publication of the classical researches of C. 0. of fact, discreet mention will be found i n some of m y earliest published work o n vulcanization, of this basic Weber into the nature of the vulcanization process, this process principle, t h e importance of which our European has been generally regarded as a simple addition of sulfur or friends, as well as enemies, have to-day begun t o realize. sulfur chloride to the rubber hydrocarbon and the practical vulcanization of the rubber of commerce by sulfur has been T h u s i n a n article dealing with t h e theory of vulcanizahandled from the standpoint of a simple chemical reaction, as tion2 a n d i n particular with Axelrod’s theory of a vary- generally understood, between the rubber and the sulfur at the ing velocity of reaction with t h e degree of polymeriza- vulcanization temperature. In view of certain difficulties tion of t h e rubber, I took occasion t o point out t h a t experienced from the earliest of times even to the present day in “whad W e b e r , A x e l r o d , Ostwald and all investigators accepting the simple chemical theory of vulcanization2 it is had oroerlooked w a s the f a c t that the vulcanization o f rubber therefore the more surprising that what we believe to be the key w i t h s u l f u r , as we k n o w i t , i s essentially a catalytic reac- to successful vulcanization has not been discovered or made tion.” E v e n earlier, however, a n d i n t h e controversy be- known as far as published work shows The conception of a tween Ostwald a n d myself i n 1910-1 I , over t h e chemical simple chemical reaction between the rubber and the sulfur at the vulcanization temperature has long been felt to fail to v s . adsorption theories of vulcanization, which some of account for certain practical results obtained in vulcanization you probably remember, i t was t h e knowledge of t h e true and only as recently as last year the physico-chemical or ,adnature of t h e reaction, overlooked b y Ostwald, which sorption theory of vulcanization was put forward by an eminent led me t o t a k e t h e s t a n d I did. I n t h e Kolloid Z e i t authority on the chemistry of rubber on the ground alone that schrijt, Vol. 13, pp. 2 7 0 , 2 7 1 , will be found further it helped t o explain the practical facts of vulcanization better reference t o this subject, together with figures a n d than the simple chemical theory of Weber. curves showing t h e influence of varying a m o u n t s of I have cited these facts in order to show that from the earliest a powerful catalyst o n t h e r a t e of vulcanization, as days of the industry t o the present time what we believe to be determined b y t h e a m o u n t of combined sulfur. T h a t the keynote to the vulcanization process as determined by our investigation has never once been sounded * * * * *. 1 Read before the Rubber Section, at the 55th Meeting of the American B y D. SPENCE
Chemical Society, Boston, September 12, 191 7 . * Kolloid-Z., 11 (1912), 275.
1
a
J . SOC.Chem. Ind., 56 (1917), 321. Ostwald, Kolloid-Z., 6 (1910), 136.
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For the better understanding of the facts leading up to and bearing on the present invention, it is necessary to refer to the following discoveries made by us: That the vulcanization of pure rubber (by which is meant the rubber of commerce divested of those impurities usually associated with it in greater or lesser amount) by sulfur is an exceedingly slow reaction. The amount of sulfur which enters into combination with the rubber in a given time is relatively small, and even the best of the products obtained have very poor physical properties, usually deteriorate rapidly and are of little or no industrial value. It has been universally reported that this inferiority of so purified rubber is due to the quality of the rubber per se or to changes in the rubber per se brought about by the purification process. That this is not correct we have established by experiments in which, by returning to the purified rubber, certain of the so-called impurities removed in the first case, and investigated by us, or by adding to the rubber certain substances having an analogous action in vulcanization, the velocity of vulcanization of the original rubber and the superior physical properties of the vulcanized product were restored. For example, it is known that by dissolving Para rubber in solvents and precipitating the rubber from its solution by acetone or by simple extraction of the rubber by acetone, the vulcanizing capacity of the rubber is greatly reduced, and products with very poor physical properties are obtained. We have discovered, however, that by returning to the rubber certain of the so-called impurities removed in purification, the original vulcanizing characteristics of this rubber are restored. We have discovered in fact that: I-The superior qualities of certain raw rubbers with respect to vulcanization are not necessarily due to any superiority in the quality of the rubber itself, but are determined far more by the existence in these rubbers of commerce of certain substances, hitherto regarded as impurities, which are not present in like degree or a t all in the other rubbers. These so-called impurities are soluble in acetone and can be extracted in this way. When they are added to rubbers with inferior vulcanizing properties, products which vulcanize rapidly and with superior physical properties are obtained. 11-The acetone extract from Para rubber contains, in addition to the resinous impurities generally held to constitute the acetone extract, organic substances, nitrogenous and feebly basic in character, having all the reactions and characteristics of an organic alkaloid.’ We have found, furthermore, that it is these organic nitrogenous substances present in one form or another in Para rubber that impart to this rubber its characteristic vulcanizing properties and are capable of bringing about substantial improvements in vulcanization when added to other so-called low-grade rubbers. These substances which occur in Para and in some other rubbers to a less extent we shall call henceforth the “active principle” of raw India rubber. To what extent this active principle occurs in Para we have not yet been able to determine, but from calculations made there is probably less than 1 / 1 0 of I per cent. 111-The function of this active principle in effecting more rapid and better vulcanization of rubber by sulfur is complex but essentially that of a catalyst (as generally understood) accelerating as i t does enormously the rate of reaction between the rubber and the sulfur. This has been established by us beyond doubt by comparative curves of the combined sulfur of, purified rubber, vulcanized with sulfur only, (u) in absence of, (b) in presence of the active principle of India rubber, (c) in presence of substances having analogous properties. IV-Organic substances having analogous reactions and 1 At the time this was written these nitrogenous bodies had not been definitely identified; only their behavior towards the well-known tests for alkaloids had been noted.
Vol.
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No. x
properties can be used as catalysts to replace the “active principle” of Para rubber in the vulcanization of India rubber by sulfur. Thus by adding I per cent of the well-known alkaloid quinine to a mixture consisting of IOO parts purified rubber with 8 parts of sulfur, the reaction between the rubber and sulfur is so hastened that the time required using 40 lbs. steam pressure to effect proper vulcanization is reduced from 4 to 5 hrs. when no quinine is present to 25 min. when quinine is present. The physical properties of the product obtained are likewise improved enormously by the use of this substance as catalyst. V-The number of organic substances which can be used to replace the “active principle” existing in higher-grade rubbers i s unlimited ; derivatives of these and metallo-organic bodies may be likewise employed and although the substances which may be employed may differ widely in chemical composition, they all have the common property of behaving as catalysts in vulcanization, hastening the velocity of reaction by carrying over the sulfur to the rubber at the temperature of vulcanization. We have found, furthermore, that the power which these substances possess as catalysts in vulcanization varies with their constitution and with the nature and arrangement of their reactive groups. We have discovered that i t is possible, by suitably modifying the structure of the substance or its reactive groups by the well-known methods of organic chemistry or by suitably modifying the amount of the substance or substances used in vulcanization, to obtain any desired effect. Thus whereas we have found para-phenetidine to be a very powerful catalyst in vulcanization, ortho-phenetidine is but feebly so and the somewhat weak catalytic action of para-amido-phenol is wonderfully increased by converting it into the corresponding amidophenetol. Similarly diphenylthiourea is but feebly active compared with the corresponding tetramethyldiaminodiphenylthiourea. As examples of organic substances having the power to act directly as catalysts in the vulcanization of India rubber b y sulfur, we may mention aniline, para-phenetidine, piperidine, and quinine. As examples of derivatives of organic substances with inorganic radicles and metallo-organic substances acting catalytically in the vulcanization of India rubber by sulfur, we may mention the alkyl derivatives of lead and mercury and the salts of oleic acid with sodium or lead. As the third class of catalyst in vulcanization we have discovered, furthermore, certain substances which are inactive in themselves, but which a t the temperature of vulcanization, i t may be alone or it may be in the presence of other substances, break down into a substance or substances having the properties of powerful catalysts. As an example of a substance of this kind, we may mention sulfocarbanilide or diphenylthiourea. What we claim to have discovered is the basic principle of vulcanization of India rubber by sulfur; that the vulcanization of India rubber by sulfur is not a simple chemical reaction as is to-day assumed, but owes its whole success industrially to its catalytic nature. We are aware of the well-known action of litharge in the vulcanization of India rubber by sulfur. This inorganic substance has been used from the earliest times in large quantities in rubber compounds (up to 40 per cent) and is known to hasten vulcanization. Further, we are aware of the use of magnesium oxide as an accelerator in vulcanization and of the work of Weber and of Henriques in this connection. Henriques, as the result of investigations into the inorganic constituents (the ash) of different rubbers, came to the conclusion that it is the presence of compounds of the alkaline earths (ilzorgank elements, in other words) in quick-curing rubbers, that accounts for the ease with which these rubbers vulcanize. This statement as far as the literature shows is still generally accepted as correct, and gave rise,
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 improvementin the results of vulcanization.
I ill omit t h e claims a t this present time. T h e 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 m a y be carried out, a n d t h e pros a n d cons of various classes of reagents. T h u s it points out, among other things, t h a t some agents are solids, difficult t o handle; others yield colored products o n vulcanization (nitrosodimethylaniline, for example) ; others on account of their poisonous character or odor would n o t 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 descript i o n sets forth in detail. If our friends across t h e Atlantic should still d o u b t o u r claims t o priority of discovery of this fundamental principle of vulcanization let t h e m test t h e validity of their patents. T h e y 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 a n d facts which can be established t o dispose of a n y claim o n their p a r t 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 b o t h scientifically a n d industrially i n this country so long ago as 1910, a n d was long since abandoned o n account of t h e serious disadvantages t o its extensive use. I recall indeed having used it in o n e instance i n 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, b u t also t h e physical; it should toughen t h e rubber, whether raw or vulcanized; a n d should render it i m m u n e t o deterioration. All this has been achieved in America, giving a r u b b e r superior t o t h a t from a n y 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 n o essential change f r o m t h e general methods of vulcanization as originally specified b y t h e inventors. T h e t w o original methods, which are still i n use, are t h e vulcanization with sulfur b y heat a n d t h a t b y sulfur chloride in t h e cold. Variations in t h e processes have been introduced a n d innumerable mixtures made with other materials, b u t no rubber article of practical importance has been p u t o n t h e market which has essentially deviated from t h e t w o original processes. There have been made, however, m a n y slight variations in compounding rubber mixtures which have produced vulcanized rubber products t h a t are characterized b y properties which fit t h e m t o special work better t h a n a n y previously known compounds. It was with this idea t h a t we t h o u g h t rubber vulcanized with selenium might give a product of especial a d a p t a bility t o some of t h e m a n y uses of rubber. At t h e t i m e t h a t we first tried t o vulcanize rubber with selenium, 1913, we t h o u g h t 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 h a v e since noticed t h a t Pearson i n his book, “Crude Rubber a n d Compounding Ingredients,” mentions t w o methods, one b y heating rubber with equal parts of selenium a n d t h e other b y dropping liquid selenium into a CS2 solution of rubber a t 300’ F. under pressure. It is evident t h a t these methods h a d nothing t o recommend t h e m a n d I believe could never have been developed because of t h e unsatisfactory product. Selenium is a metal i n t h e same group of t h e periodic table as oxygen, sulfur a n d tellurium, is much more metallic t h a n sulfur a n d has a higher melting point (217’ C.), sufficiently high t o discourage one f r o m a t tempting 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 i n t w o crystalline a n d one amorphous form a n d 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 o n t h e market i n small quantities, b u t it should be procurable in fair a m o u n t s if there were a commercial demand for it. Black selenium has t h e 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.
