Vulcanization of Rubber with
Sulfur IRA WILLIIMS J . .VI. Huber Corporation, Borger, Tex.
CLC.4SI%-~TIONis a change in rubber which results in an increased resistance to deformation at ordinary temperature. This definition disregards entirely any chemical agents or chemical actions involved. I t requires only that the rubber lose its plastic properties and become more resistant to compression, stretching, or snelling. Freeze resistance may or may not be affected; this depends largely upon the .extent of any chemical attack which may be involved. The earliest method of vulcanization, discovered by Goodyear over a hundred gears ago, still remains the basis for essentially all vulcanization of rubber today. The only refinements which have been introduced consist in the addition of accelerators and accelerating accessories, and in a manipulation of the quantit,y of sulfur. All of the other methods, such as the use of sulfur chloride, perosides, quinones, nitro compounds, and selenium, remain quite unimportant from a commercial standpoint. The study of vulcanization, particularly with sulfur, has received the attention of many chemist,s, most of whom have differed in their final interpretation of the action. The earliest n-ork on the nature of vulcanization was carried out by Henriques ( l s ) , who fourid that sulfur chloride combined chemically n-ith the rubber, and the natural assumption was made that molecules of rubber mere joined by means of sulfur. Weber (36j soon found that sulfur combined during hot vulcanization. Stern (30) found that the amount of sulfur combined was directly proportional to the time of heating and depended on the original concentration of sulfur. Whereas Stern conducted his vulcanization in solution, Hubener (16) obtained substantially the same results in a direct manner. Many theories have been evolved to esplain the mechanism of vulcanization. Hohn (14) and particularly Ostwald (23) believed it to be an adsorption phenomenon. This \vas soon disproved. The majority of investigators a t present are inclined t'o favor some sort of chemical reaction which produces bonding of some type betireen molecules, either from cross linking, b y means of sulfur, by polymerization, or by molecular association. XI1 of these theories recognize that in some manner t'he combination of sulfur is instrumental in the process. Considerable work (1, 12, 17, 21) has been done on combining sulfur with relatively simple organic molecules in attempts t,o gain some information regarding the manner of addition of rubber and sulfur. .Ilthough such work should riot be discouraged, it should be pointed out that the reaction of sulfur n-ith double bonds in a material such as rubber, which is lacking in mobility, can differ considerably from the reaction with double bonds in liquid materials. BASIS OF THEORIES
The accumulated data obtained on the struct,ure of rubber and on vulcanization have established a considerable amount of information to guide in the formulation of theories of vulcanization. The follon-ing changes have been determined: 1. Sulfur combines chemically in a n irreversible manner. The temperature coefficient of about 2.5 indicates the reaction to be chemical (19, 24, 85, 68,39,36). Stevens (Sf)extracted vulcanized rubber for 9 weeks with no loss of sulfur after the fiwt
week. Treatment with such materials as alkalies, litharge, or EOdiuni v-ill not remove the unextractable sulfur. 2. Sulfur combines in simple rubber sulfur mixtures a t a fixed rate n-hich depends on the amount of sulfur originally present, until about 70% of the sulfur is combined; after that the rate decreases. Spence and Young (29) found that rubbcr containing 1070 of sulfur when vulcanized at 135" C. reacted a t the rate of about O.46YGof sulfur per hour, until after that the rate became reduced. the rate was about 3.4% per hour until abo had combined. The rate depends on the original amount of sulfur. Spence and Young vulcanized rubber containing 377; of sulfur a t 135" C. and found the sulfur to conibine a t a rate of 1 . 6 5 per hour, which is about 3.7 times the rate obtained with of sulfur. Most accelerators alter the addition in such a manner that the rate is not uniform (11, 55, 58). .Uthough it is possible under severe conditions to cause the sulfur to react with elimination of hydrogen sulfide (10, 5 3 ) , it appears to add one sulfur to each double bond (29). The addition of more than 3 2 5 of the weight of the rubber is difficult. This amount corresponds closely to the addition of one sulfur atom to each double bond. However, there is still some doubt that all of the addition is in such a manner as to saturate double bonds (5, 341. It is possible that loss of double bond characteristics caused by change in cheniical constitution could be responsible for failure of the addition agent to add to the double bond, so that double bond characteristics and not double bonds arc lost. 3. The addition of sulfur destrovs the elastic properties of the rubber molecule. The combination of about 8% of sulfur in a rubber-sulfur mixture produces a vulcanizate of almost no strength or extensibility. Continued addition produces a hornlike consistency and finally a brittle ebonite. If the sulfur atoms are evenly spaced along the rubber molecule, the combination of 8YGof sulfur would mean that about tirenty carbons in the chain, or five rubber units, separate the sulfur. Such rubber is of little value. Four per cent probably approaches the limit for good vulcanized rubber, which means that the minimum chain of elastic rubber must contain at least forty carbons. 4. The rate of addition of sulfur is decreased by strong acids and increased by bases ti-hile soluble zinc salts (5) have a neutral or a slight retarding action (Table I). Soluble zinc saltsintensify the physical changes brought about by accelerators without altering the general nature of t>hechange. The magnitude of the intensifying effect varies with different accelerators. I n a very general way the relative accelerating activity of materials increases with the basic dissociation constant. This has been shown by vulcanizing a mixture of 100 parts of smoked sheet rubber and 6 parts of sulfur a t 284" F. in thc presence of 1 part of various primary aromatic amines. The results are shown in Table 11, which lists the dissociation constant and the amount, of combined sulfur after 120 minutes of cure. The table is divided into trro groups, one of which lists weak and the other strong bases. .Ilthough these figures indicate a general tendency for the strong bases to be the stronger accelerators, the relation within each group is not very good, and it would be necessary to consider the structure of each'compound separately to discover reasons for the difference in activity. Tertiary amines such as triethj-1 and tripropyl, which are strong bases, are very weak accelerator Sulfur adds very slon-ly to rubber which is free of nitrogen (2, 32 5 . The resistance of vulcanized rubber to deformation do not depend on the amount of combined sulfur (5,f l , 87). Bruni (6) is of the opinion that the minimum amount of combined sulfur required to initiate vulcanization is about 0.15% Such an amount will not produce more than the beginning of vulcanization, One per cent of combined sulfur mag produce a high state of vulcanization if obtained in the presence of an active accelerator such as a thiuram disulfide. A high state of vulcanization cannot be obtained in the absence of accelerators with any amount of combined sulfur.
901
loro
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
902
I.:
1-
100
100
6
6 3
1
...
. .
..
30 60
zIn 1'0 180
of vulc:inization IXII hi%o v i ~ r i * ~ ) i ii i i< , tion rif tho romI)in~~il sultiir. :itid thiIinoivii 1597 that ndc~aiiiznhle~ I~III)II~~I~
t i w . T h e action of accelerators differs siiiiic\\-h:rt : s c i n i ~r a u w
B
great 1)- degratlrd condition. pcptizing :i,ytit c:iii l ~ removc3d e from auc.li solution-: hy t r t l t l i alcohol. 1 lie coagulum which is thron-n down still conolr-ent and peptizing agent according t o the. pairition i)~%the ru1ibi.r. ant1 alcohol. If the soft coaguluni is r e r p c ~ ~ t c d l ~ i,cclis:.olrcd in livnzcne and reptwipitatc.d, the peptizing agent caii bc effrwirely cliniinaretl. dlight acidity in the alcohol favors
Vol. 39, No. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1947
Vulcanized rubber must be a niixture heterogeneous with respect to the various sulfurized niolecules of \vhich it is composed, although it' might be quite homogenous with respect to the distribution of sulfur through the rubber mass. I t must consist of a n intimate interlacing mixture of the various constituents, all of which together produce the properties of vulcanized rubber. T h r ~ difference in properties between different vulcanized rubbers depends on the relative amounts and manner of assembly of the units. The various phenomena associated with vulcanization are dctermined by the reactivity of the rubber molecule, Lvhich in turii' is controlled by its structure and environment. The ordinarily assumed structure of rubber, consisting of condensed isoprene units with one remaining double bond for each unit, has been arrived at on the basis of the empirical formula, degree of unsaturation, x-ray spectrographs, viscosity measurements, etc. The usual picture consists of long chains of 2000 to 6000 carbon units containing regularly recurring methyl groups and double bonds at each four carbons. The chains are folded and coiled t,ogether in a more or less accidental pattern as a result of free rotation around single bonds. Rubber resists deformation by any method such as swelling, pressing, or stretching. In the case of attack by solvents the resistance often takes the form of limited swelling, rvhich stops when the mutual attraction of rubber molecules equals the force o f solvation, and the niolecules remain in disordered arrangement. Khen rubber is stretched, the molecules appear to orient theniselves in the direction of stretching n-hile a t the same time slipping, BO that sufficient double bonds lie in the same plane to produce an x-ray diffraction pattern. Partly on the evidence of this pattern we deduce the structuw of the double unit of rubber to be
C",
CHI
I
-(---c'=('
:
I HZ
-('=('--[:-c'
('
i
I
H H,
'
;
H Hp H2
TABLE ITr.
OF COMBCSTION O F
Bond Cmtribution, Kg.-Cal. 53.3 50 8
Type of Bond
C-H
c-c
RUBBER
s o . of Colitribntion, Bonds ICg -Pal. 16 852.8 8 406.4 2 237.6 __ Heat of combustion 1 4 9 6 . 8
118.8
C=C
*
&AT
903
rreight of the rubber molecule and the condition of the terniirinl groups are uncertain, it is necessary to base the calculatiori or1 a central portion of the molecule. For this purpose this doublc unit was used. The calculation is shown in Table IV. These figures are not directly comparable, because the culculated figures are based on gaseous material. I t will be riecessary to add to the determined value the heat of vaporization of a double unit of rubber. S o such data are available, but most hydrocarbons will have a value of less than 100 calories per gram: for examble, the heat of vaporation of hexylene is 93.5 calories per gram. If the combustion figure is corrected a t the rate of 100 calories per gram, it would become 1470 13.6 or 1483.6 kg.-cal., leaving a difference of only 13.2 kg.-cal. per double unit of rubber as the resonance energy, This is an indicated resonance energy of 0.92% of the heat of combustion. Although heats of combustion can be determined with greater accuracy than this, the resonance energy is so small that the rrsult can be considered only as indicative.
+
KESO>ATIXG STRUCTURES
Trarious resonating structures can be shown for rubber. Since olefin bonds are characteristically nucleophilic, and the double bond in rubber seems to be no exception, the important sstructure must be one which can produce a high electron density at, some point. The following struct,ures illustrate some of the possil)le conditions of resonance:
LACK OF D O U B L E - B ~ K DCHARACTERISTICS
This simple picture is not adequate to account for all of the chemical properties of rubber. Among these is the lack of doublebond characteristics. Xlthough rubber will add bromine at the double bond Yith some ease, it n-ill not add chlorine first a t the double bond except when attacked by strongly electrophylic chlorine donors. In other ca8es the chlorine first attacks a methylene carbon Lvith liberation of hydrogen chloride. Ordinary rubber is not attacked by oxygen or ozone unless in a strained condition. This lackof reactivity is not indicated by the simple olpfin structure. The lack of double-bond characteristics of many unsaturated substances-benzene, for example-is due to resonance. Slthough resollance niight ordinarily not be expected to contribute greatly t o the stability of simple olefinic compounds, nevertheless resonance can occur and, even in such simple olefins as propene, has a definite effect on the reactivity. The magnitude of the stabilizing influence of resonance can often be estimated by means of the heat of combustion. Several workers (3,16, 18, POI have determined the heat of combustion of rubber, t,he most recent being Jessup and Cummings. They found a value of 45,271 joules per gram of purified rubber, which is equivalcnt t o 1470 kg.-cal. for the double unit
HH-c-H
1;
H
HH-C-H
I ,' -f---~~=(--~-f---c=c~c I I l l i ' I
H
I
I
H H H
H
H
1jattt are also available for the calculation of the heat of combustion hased on bond contributions ( 3 7 ) . Since the molecular
I
H
H H
B
d
H,
C", I
H
k3
Hi
H
H,
E Since the rubber molecule is entirely hydrocarbon, it would not be expected to have any points of great permanent electron density. Such points of electron density n-ill appear only by an elcctromeric displacement under the influence of a n evternal field such as might be supplied by an attacking agent. I n discussing the valious structures, the chain carbons n ill be numbered from left to right. Structure d vi11 produce a n increased electroil density on carbon 3, which ill acquire a fraction negative charge. Under extreme condltions the 3 carbon could have a formal charge of - I , and the 2 carbon would act as a strongly electrophilic point with a formal charge of + l . Such a
904
*
INDUSTRIAL AND ENGINEERING CHEMISTRY
condition must occur so infrequently as to be unimportant,. In those cases where the demand for electrons on the 3 carbon is very great, it is improbable that the total demapd will be met by the 2 carbon but rather by a shift which affects other centers as well. I n this way a relatively high nucleophilic point may be obtained without the production of a correspondingly high electrophilic point. The grouping about the double bond ivould become polar and electrophilic reagents could attack, but nucleophilic reagents would be ineffective. Structure B is a type usually associated with activated gases. This shift a-ould not create polarity and would not be induced by an external field. It is probably of no importance in the rubber molecule. Structure C is also a latent polarity (polarizability) effect which can become important undcr the demand of an external field. I n this case the 4 carbon in the methylene group will bccome activated. Dehydrogenation reactions should be favored. The arrow points to the right of the double bold because the effect of the methyl group attached to the 2 carbon in conjunction with the double bond should favor a shift in this direction. The analogous structure D is probably much less important than structure C. Some evidence exists that a n electromeric shift to activate methylene groups is the most prevalent type in normal rubber. Chlorine first attacks rubber to form substitution products with the elimination of hydrogen chloride, and only by thc usc of such chlorinating agents as sulfuryl chloride will the attack be made first at the double bond. This might indieate that the latent polarity due to hyperconjugation is capable of very great development upon the proper demand but that, in the presericc of Ims iii, the greater electron densityappears on thvmcthene carbon. Structure E represents a condition that can exist in natural rubber but not in polymers of unsubstituted butadierie. It is \\-ell known that the methyl group is capable, on demand, of electron release, and this effect, can become pronounced in connection with doubly bonded carbon; for example, hyperconjugation bet,n.een the methyl group and the double bond of propene produces a wsonancc energy approximately the same as that produced in butadiene by two conjugated double bonds. Hypercoiijugation in the rubber molecule should, under the influence of an external field, make possible a considerably increased electron density on tho 3 carbon. This seems to be the most likcly point of attack by ari electrophilic reagent n-ith suficicnt field intensity arid the one hy which the latent polarity could be most highly developed. POSTULATION OF RUBBER PHENO.\IENA
On the basis of the resonating forms it is possible to postulate many of the phenomena displayed by rubber. It is first necessary to recognize that the rubber unit is not symmetrical. and for that reason a different sequence occurs in passing along thc, carbon chain in different directions. In one direction the double bond precedes the methyl group, whereas in the other direction the reverse is true. It is reasonable to assume that in the rubber mass approximately an equal number of molecules will lie in each direction. Since the rubber units possess latent polarity, this might be dexloped to a minor extent by the proximity of two reversed resonating units. Such an occurrence in a semisolid material would be morr or less permanent, because the associated units could not trade partners as is possible in mobile liquids. This increased resonating state could account f o r the reduced double bond characteristics of rubber. This type of resonance comes into effect only on demand, and the proximity of a unit in the same position could have no effect. This means that, on stretching, the various units should seek out positions such that alternate layers of reversed polarity would result and an increasingly ordered arrangement of the molecules would be obtained. For this reason racked rubber should produce an x-ray fiber pattern. The attractive force is
Vol. 39, No. 7
suficicnt to maintain racked rubber in an extended position until the vibrational energy of the rubber molecules is increased by raising the temperature. Ordinary rubber is known to be very resistant to oxidation under ordinary conditions. Ho\vever, \vhen rubber is placed under strain on a rubber mill, it rapidly oxidizes and becomes more plastic. I t is difficult to believe that all the resonating centers in rubber would be paired; oxygen would be expected to attack unpaired centers but is unable to do so. The heavier sulfur atom is capable of attack. Perhaps the field effect of the oxygen in conjunction with the small activation remaining as paired resonating units are being separated issufficient to initiate attack. lJ'het,her the attack once completed actually ruptures the carbon chain or mcarely destroys the resonating center is not known. S o evidence exists to show that either the latent or permanent polarity ~i the rubber is altered. Sulfur attacks rubber probably in one of the resonating forms C or E. In the first case, Ivhich may be important in the absencr of accelerators, the attack would tend to replace hydrogen from the 4 methylene carbon to form either a sulfhydryl group or to eliminate hydrogen sulfide. TKOcarbons which might he in the same or different molecules could be united by means of sulfur. If carbow are united, the probability is greatly in favor of uniting tivo molecules. The attack on the 3 carbon of form E is probably the most important under vulcanizing conditions. The attack should lie retarded by strongly electrophilic substancm, such as strong acids, and should bc assisted hy nucleophilic suhst anres. such as bases, unless the latter reacts to form relatively stahle sulfur rompounds. The rcsult of attack 011 tile 3 carboii is not clcar. The fir5t step might form the nicsrcxptan, n h i c h irould be thc deslllotrCJp(' of the c.ori~~~Spontfing rhiokrti)no.
Such an arrangement n-ouldrrsult in a permanent dipole and n.ould maintain an active methylene group as a point of attack for morc sulfur. Thiokctones of lox molecular weight are very active and polJ-nicrize readily to form ring structures containirig sulfur hridgcs. Such products are usually trimcrs, although othixr polymc'rs arcs known. I n spite of all the speculation and investigation into the manner of addition of sulfur, the problem is not yet solved. One of the most important contributions to the study is that of Sclkcr and Kcnip (Hi, who have reacted vulcanized rubber with methyl ioclidc. They conclude that a considerablc amount of sulficli~sulfur is present. This would indicate the linking of carbon atom> either inter- or intramolecularly by sulfur. Farmer ( 8 ) believes the presence of episulfides is unlikely. Thioketones also are reactive with methyl iodide and might, be included in the sulfide SUIfur determination. Paired dipoles containing thioketones could also unite by hydrogen bonding. Rrgarilless of the nature of thc. attack, the result is that the reaction product becomes definitely polar and appears to have prcdominently electrophilic properties. The sulfur in simple rubber-sulfur mist urm combines at a constant rate which depends on the original concentration of sulfur. Various explanations, such as consecutive reactions, chain roactions, autocatalytic reactions, and micelle disaggregation, have been suggested as the controlling factor. The presence of prcdoniinently electrophilic dipoles in the rubber might activate othcr unsaturated groups in the immediate vicinity to facilitate attack by other sulfur. It is reasonably certain that att,ack of t,he 3 carbon by sulfur, especially if a thioketone group resulted, would a+
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1947
tivate the methylene carbon next t o it. T h e remaining methylene groups between this and the double bond in the nest group would 111 turn become somewhat more active. I n this manner the reaction with sulfur would become more rapid as the attack on double bonds increased, and the rate of reaction could be maintained although the concentration of sulfur continued to decrease. R.ATE O F COMBINATION O F SULFUR
The presence of combined sulfur is sufficient t o cause a n increase in the rate of combination of sulfur. h uniform blend of smoked sheets was used for the follon-ing esperinient. ricetone-extracted rubber was compounded with 6 parts of sulfur and vulcanized. I t was then thoroughly acetone-extracted, and the combined sulfur \vas determined. This estracted rubber was then Compounded with 6 parts of sulfur. A second compound was prepared from 100 parts of extracted smoked sheets and 6 parts of sulfur, and the two compounds were cured in the same mold for 2 hours at 140" C. Ehch compound \vas then extracted and the combined sulfur determined. The results in Table Y shovi that the presence of combined sulfur has accelerated the further combination of sulfur. Sinccl the combination of sulfur takes place a t a uniform rate, it is assumed t h a t the initial rate of addition will be indicated closely tly a measurement taken when only about half of the sulfur has becn consumed. The following calculation can be made from the data of Table I-. If we consider the concentration of rubber to remain constant in the reaction R S = RS,the rate of reaction n-ill vary with the concentration of sulfur. If 1.73 grams have combined in the fresh rubber compound. the concentration of dissolved sulfur will be 4.27 grams, and the relative rate of combination should be reduced to 1.27/'6 = 0.71 of the initial rate. Since 1.73% of combined sulfur in the present case has increased the rate by the factor 1.32, the rate should be 1.32 X 0.71 = 0.94, or the rate should he apprusimately the sanie as at the beginning.
+
ACTIOS O F ACCELERITORS
If sulfur conibines with rubber because certain points acquire a sufficient electron density, then the primart action of accelerators must be on the rubber. Since the electron density is created only uridor the field of a sufficiently electrophilic attacking reagent, the action of the accelerator must be simultaneous with, but cannot precede, that of the sulfur. I t could be possible for accelerators which are sufficiently nucleophilic to be less efficient becau.w of competition n-ith rubber in the reaction with sulfur. Such accc,lerators, r h i e h would usually be the inore basic ones, I\-ould not be preferred for use in low sulfur compounds. Accelrrators should ' t t h e re1ea.e of cllectrons from the nieth\-l group and promote the elcctron shift shown in structure E . T h e action of soluble zinc salts is still speculative. Insoluble zinc salts seem to be quite ineffective. Soluble zinc salts in the absence of natural or added acceleration appear to have little effect or to have a slight retarding effect on vulcanization. I t seems evident that, the ansiver lies somen-here in the mutual or conibined effect of accelerator and zinc salt. Perhaps the answer in some cases lies in the formation of IT-erner-type complexes which regulate the nucleophilic properties in the desired range. Cadmium and sometimes mercury, when substituted for zinc, produce activation b u t in different degrees. Indications esist that zinc cannot be removed completely from vulcanizates without destroying them, and further indications have been found (39) of a condensing or polymerizing effect of zinc salts. S o adequate theory of vulcanization has yet been advanced and probably will not be until better information in regard to the manner of combination of sulfur is available. The apparent prescxnce of sulfide sulfur is strongly indicative of cross linking of the various molecules by means of sulfur, and yet sulfide sulfur could he present without cross linking. If the sulfur first attacks a Inethylene carbon, the most probable result would be the forma-
TABLE
T'.
905
I S F L U E S C E O F C O M B I S E D S U L F U R OX OF SULFUR
Original
Conibined Sulfur, % Final
0 0 1 73
2 65 5 24
TABLE T'I.
Increase 2 65 3 51
n E I . % T I O S BETWEEK
COIIBINATION Relative Rate of Combination 1 00 1 32
COMBIXED S L - L F r R
AXD
PHYSICAL PROPERTIES (40) Tensile Strength a t 700% ,Elongation, Lb.Iaq. I n .
.Accelerator None, GO-min. cur? None, 120-min. cure Butvraldehyde hutvlarriinr Di-o- t olvlgusnid ine T e t r a i i i e t h ~ l t h i u r ~ idisulfide ri
173 375 4000 3600 1873
Conibined Sulfur,
5;
1.92 3 83 2 38 2.23 0.34
tion of a sulfhydryl group, and coupling could take place only by oxidation. If a doubly bound carbon of paired bonds is attacked t o form a thioketone, bonding might take place by polymerization between two such bonds. If paired bonds are attacked to form any permanently polar structure, the linking between molecules may consist of nothing more than the pairing of such niorc strongly polar structures. I t seems most logical t h s t any intramolecular bonding must take place a t the olefinic points and that any sulfuration of methylene carbons is purely destructive. Methylene attack probably is most rapid when accelerator influence is absent, as in unaccelerated stocks or on overeure with espendable accelerators. I n this connection the data of Table VI, taken from the previous paper (40),are of interest. The stocks containing no added accelerator have very littlc increased stiffness, which shows that most of the sulfur has combined in a n ineffective manner. T h e combined sulfur in the accelerated stock has produced a high stiffness which increases hvith the combined sulfur: this indicates that much of this sulfur has produced bonding. Of the three stocks, the softest one cannot, be peptized with piperidine; this indicates a greater possibility of covalent bonding rather than association through dipoles or hydrogen bonds. T h e ability of nulceophilic materials to peptize rubber in benzene solution is a n indication of association through paired dipoles. 'Since the dipole is essentially electrophilic-th.tt is, the electron deficiency is more localized than the density-the dipolar attraction can be overcome by the presence of a more strongly nucleophilic substance. Upon the removal of such substance, many of the paired poles can again reform, and the rubber is once more firm and insoluble. T h e activity of peptizing agents may be quite different from their accelerating activity. The study of vulcanization should be encouraged particularly because of its importance to the synthetic rubb:r industry. The natural rubber industry developed as a n art largely because of the adaptability of the natural product. Most synthetic rubbers, other than chloroprene polymers, refuse to vulcanize satisfactorily in the absence of large amounts of various powders. Perhaps the present synthetics will be vulcanized satisfactorily in the future. I t is certain that a better understanding of the process t)y which natural rubber is vulcanized would assist in finding bettcr methods of vulcmizing synthetics and in the formulation of aynthctics which are more vulcanizable. BIBLIOGRAPHY
(1) Armstrong and Doak, Rubber Chem. Tech., 17, 788 (1944). (2) Beadle, C., and Stevens, H. P., Kolloid-Z., 11, 61 (1912); 12,
46 (1913). (3) Blake, J. T., ISD. ESG. CHEW,22, 737 (1930); Rubber Chem. Tech., 3, 63 (1930). (4) Bloomfield. G. F., J . Polymer Sci., 1, 312 (1946). (5) Brown. J. R., and Hauser, E. -%,, ISD. ESG.C H E h f . , 30, 1291-6 (1938); Rubber Chem. Tech., 12, 4 (1939).
INDUSTRIAL AND ENGINEERING CHEMISTRY Bruni, G., Rcc. grn. caoutchouc, 8 (75), 19 (19;31). Curtis, H. L., McPherson, A . T., and Scott. -1.H.. Ijui.. Staiidards, Sci. Paper No. 5 6 0 , 2 2 , 398 (1927). Farmer, E. H., ";idvances in Colloid Science," Vol. 11, 0. .342-4, New York, Interscience Publishers, Inc., 1943. Farmer E . H., and Shipley, F. IT.,J . Pol!jmerSci., 1, 2W (lUi6i. Fisher, H . L., andSchubert, I-., ISD.ESG. CHEM.,28, 209 (19x6) Hardman. A . F., and White, F. L., Ibid., 19, 1037 (19271. Hauser and Sze, Rubber Chem. Tech., 17, 788 (11144). Henriquea, Chem.-Ztg., 17, 634 (1893); 18, 701, 11.55 (Ih!14) Hehn, J. B., Gztmmi-Ztg.. 14, 17-33 (1899). Htibener, G., Z . angew. Chem., 7 , 112, 142 (1591:. Jessup, R . A , , and Cummines, A. D., J . Rrsrrrwh Yicti. Niii. Standards, 13, 357 (1934). Jones and Reed, J . Am. Chem. S'oc.. 60, 2452 1,1!3:3hi. Kirkoff and Matulke, Ber., 57B, 1260 (1924). lIaxwell, R . B., and Park, C . K..1x1,. ESI-. CHEJI.,24, l i b (1932). Messenger, T. H., T r a n s . I M ~ Rititbci, . Itid., 5, 71-% (1929): Riihher Chena. Tech., 3, 24 (19301. Meye]. andHohenemaer, ICid., 9, 201 (l!Jl(tii. Saj-lor, I{. F., J . Polymer Sci., 1, 305 (19461 0,twalil. If-.,Kolloid-Z., 6 , 136 (1910). Park. C . 13.. IXD.ESG.CHEM.,22, 1004 (1930). Saiidstroni. I < .V.,I b i d . , 25, 684, 1400 (1933). Selker, XI. L., and Kemp, A . R . , I h i d . , 36, 27 (1944): Kiit,bo. them. ~ ~ ~17,303 h . (1944). ,
Vol. 39, No. 7
(271 Shepard, h-..I.,and Krall, S . , Z h i L , 14, 951 (1922). (28) Sheppard, S. E . , India R i ~ b b r vn-odil. 80, S o . 2 . 56 (1929,. (29) Speiice, D.. and Young. J.. Roiioid-Z.. 11, 28 (1912): / ' h t ' r r i . Zta.. 36. 1162 11912). (30) Site&. H.. Ibid., 33, 756 (l9lOi (31) Steven., H. P., J . Soc Chem. I),(/.,38, 19LT (19191.
Synthesis of Taurine and N-Methyltaurine J
A modified procedure for the synthesis of taurine $+a+ debeloped which utilized inexpensiTe and readily a, ailable materials. Ethjlene chloride was sulfonated with sodium sulfite and then aminated with (a)anh~drousammonia and ( b ) aqueous ammonia (27qc) and ammoniunl carbonate. % colorimetric method for the determination of minute amounts of taurine consisted of developing a blue color from the reaction of taurine +I ith a solution of phenol and calcium h,pochlorite. ,V-JIeth?ltaurine was s!nthesized bj the amination of sodium 2-chloroethane-lsulfonate with (a)anhydrous methJlamine and ( b ) aqueous methylamine (30-40q0).
T
AYKISE \vas first prepared synthetically in 1885 when James treated silver and ammonium salts of 2-chloroethane1-sulfonic acid with a n excess of ammonia ( 7 ) . Since then it has been synthesized from 2-bromoethylamine hydrochloride and metallic sulfites ( I s ) ,by decarboxylation of rysteic acid ( 1 6 ) , addition of ammonia t o vinylsulfonic arid (51, ammonolysis of isethionic acid (6, 10, 13), sulfonation of aminoalkyl sulfates n-ith alkali sulfite ( 5 ) , hydrolysis of sodium tauroglycocholate (91, and the oxidation of cystamine. It occur.< naturally in marine animals and mammals and thus has been isolated from the inuscles of fish, mollusks, and crustacea ( l d ) , as \\-ell a s from the bilr of oxen, slieep, dogs, and human beings (a). Although t h e synthetic methods mentioned are very good, they require st,artingmaterials t h a t are expensive and difficult to obtain. T h e present investigation, carried out during the Tvar years, was conducted for the prime purpose of developing a method adapt1
Present address, Sorony-Vacuum Oil
Company. I n c . , Paulsboro, li. J.
:itile tci i.iiiiiiiiercia1 pro:iuctioii utilizing tlir least costly arid rlloht readily available materials. Interest was centered upon taurine and related compounds because of their potentialitier as cheap intermediates for wetting agents and detergents ( 4 , 8 ) . AT-hlethyltaurinewas synthesized hy Dittrich in 1878 nheii lit. reacted methylamine with the silver salt of 2-chloroethnnr-lsulfonic acid ( 1 ) . -1 German patent claims t h a t the sodium salt of vinylsulfonic acid can he reacted with a primary amirit,for example, methylamine-to give S-niethyltaurine ( 5 ) . A--hIethyltaurine crystallizes as prisms (melting point 24 1 2 ' C.) and is soluble in water; it is insoluble in ethyl alcohol and ethyl ether, anti does not form salts n-ith acids or bases (5). Carius tubes (200-ml. capacity) were used in bomb reac>tiiiii> for small samples, and a nickel-lined stationary autoclav? w a ~ I I . P ~ for larger samples. PKEP.AR.iTIOS O F SOUlL \1 2-CHLOHOE:TH.i\ k;l-SZTLFOS41'E (12)
Oiir tiunilrrcl grams of ethylencb cliloride (1.0 mole) :trid 126 griiniz~of ailhydrous sodium sulfite (1.0 mole) were heated under
reflux in a 2-liter three-necked flask equipped with a mechanical .tirrer and a n efficient condenser, with 530 grams of n-ater and 400 grams of ethanol i n the presence of copper turnings. (The copper turnings act nut only a s a catalyst but also as :in aid t o efficient stirring.) After refluxing for 72 hours, a Cigreus fraction:iting column {vas substituted for the condenser, and t h e Icnction mixture \vas distilled. The first fraction (boiling point 72" C.) contniiied 8 . 3 q water and 9 1 . 7 5 et,hylene chloride. Tile secpnd fraction (boiling point 73-95' C.) contained ethanol and water. T h e combined fractions xvere utilized in the nest batch reaction. T h e aqueous residue from the distillation xvas evaporated to dryness on a steam bath. T h e resultant salt cake had a motherof-pearl li~qter. T h e impure salt \ ~ R Pground in a mortar, and
