Structural Changes in Vulcanization of Buna S T
HE development and production on a commercial
MAX H. KECK AND LAVERNE E. CHEYNEY
The Goodyear Tire & Rubber Company, Inc., Akron, Ohio scale of the synthetic rubberlike materials, particularly the butadiene copolymers, during the The relations existing among combination of sulfur, loss of unsaturation, and dew a r t i m e e m e r g e n c y focused velopment of physical properties have been determined for four related Buna S stocks attention on phenomena asover a range of cures at two temperatures. The data indicate two types of chemical sociated with their vulcanizachange to be occurring during vulcanization: (a) a reaction involving o’ombination tion. These copolymers, parof the polymer with sulfur, which is primarily responsible for the development of ticularly the synthetic tire cophysical strength, and ( b ) a reaction involving loss of double bonds in the polymer, polymer known a s B u n a S presumably a polymerization reaction, which is related to the first reaction and con(GR-S in the United States), tinues on overcure, A modification of the Kemp-Peters procedure for measurement of lend themselves to vulcanization unsaturation of Buna S vulcanizates is based upon the use of a phthalate-dichlorowith sulfur and the same types benzene solvent mixture. of accelerators used with natural rubber; the vulcanizates possess many points of simiThe molecular weight is, in general, lower than that of natural larity to those derived from the natural elastomer. rubber (19),and its distribution is quite broad (18). It has been The literature on the vulcanization of natural rubber is quite indicated (18) that the presence of the lowest molecular weight extensive. Several reviews are available (8,4, 11, 14, 83), alpolymers is detrimental to the quality of the vulcanizates. Highthough most of these are not entirely up to date. Among the molecular-weight fractions, which compare favorably with natural rubber on the basis of molecular weight, tensile, and modulus, more recent publications of direct interest in this connection are are tough and hard to handle on a mill. This difference may be those of Farmer and co-workers (9) and of Annstrong, Little, and due to the activating influence of the side-chain methyl group in Doak (2). the natural polymer on reaction with oxygen and resultant de radation during milling. This activating effect has been note8 in The government general-purpose rubber known as GR-S is a a number of other instances-e.g., reactions with halogens, hycopolymer of 3 parts by weight of butadiene and 1 part of styrene drogen halides, isomerizing agents, etc. It is well known that (17). The polymerization is carried out in aqueous emulsion, oxidative degradation accompanies the plasticization of natural and the final polymer may contain (a) unreacted monomer, which rubber by milling (6, 8, 12). is usually well removed by “stripping”, ( b ) low-molecular-weight 4. Extension of the curing period of natural rubber stocks beyond the so-called optimum results in the phenomenon known ag polymers, (c) catalyst or its degradation products, ( d ) residual reversion. It is possible in certain cases to minimize this by the emulsifier or its conversion products (e.@;.,soap and/or fatty proper choice of accelerators (13). Such reversion does not ocacids), (e) coagulants or their reaction products, cf) butiers, cur with Buna S; instead, this polymer becomes progressively (8) auxiliary materials usually termed “modifiers”, which instiffer as cure progresses. It is worth noting that Hauser and Brown (14) believe the phenomenon of reversion to be associated fluence the degree of cross linking or branch chaining in the polywith the action of oxygen. If this is true, the difference between mer, (h) stopping agents for the polymerization, (i) antioxidants the two types of polymers is probably associated with the type such as phenyl-8-naphthylamine which are added to prevent of activating effect noted above. further polymerization in storage or transit. Thus, the so5. Hysteresis properties of the two types of vulcanizates are markedly different. The detrimental effect of heat build-u on called hydrocarbon polymer is not entirely pure hydrocarbon, the physical properties of the synthetic vulcanizates has k n and many of the variations in quality of the commercial product highly publicized. However, it should be noted that the action have been due in no small part to variations in number and quanof heat alone (in the presence of air) is to cause further polymentity of these “nonrubber” impurities. zation of the synthetic elastomer whereas, in the case of the natural polymer, heating in the presence of air causes oxidative degraThe concentrated research carried out on Buna S to date has dation. Thus the difference in the action of heat may be oxidaproduced a considerable body of information, even though much tive in character and be associated with the activating influence of the data has not been published for reasons of national seof the methyl group in the isoprene unit. curity. Buna S possesses certain points of similarity to and cerUNSATURATION OF NATURAL RUBBER WLCANIZATES tgin differences from the natural polymer: Several investigators have attempted to measure the chemical 1. It is possible for polymerization to occur in the 1,2 as well unsaturation of natural rubber (8, 10, 16, 16). Of the various aa in the 1,4 position; branched chains and “pendant” vinyl p p s result, which would be expected to affect chemical b e methods proposed, the iodine chloride titration method of Kemp avior as well as physical characteristics (1, 18). Another con(16)has proved most valuable. It was modified by Blake and tributing fact is the probability that both cis and trans structures Bruce (3) for the analysis of vulcanized stocks. Brown and exist in the same synthetic polymer. The natural polymer is Hauser (4, 14) reported extensive work and certain generalizabelieved to exist entirely in the cis modification. 2. Mathematical considerations and certain evidence (1) intions utilizing this method with various rubber stocks. dicate styrene units to be randomly distributed throughout the I n the case of rubber-sulfur vulcanizates, the loss of one double chains. This, together with the presence of branched chains probond was found to accompany the combination of one atom of duced by 1,2 polymerization leads to a high degree of nonsymsulfur, whereas in accelerated stocks the combination was in a metry in the polymer; it probably accounts for the fact that this polymer, unlike natural rubber, does not crystallize on stretching definite ratio of atoms of sulfur combined to number of double and the probably related fact that pure gum stocks have relatively bonds lost during the early part of the cure, and in excess of the poor strength. Kemp and Straitiff (18) believe that this lack of one atom/double bond ratio, After combination of most of the structural symmetry may be responsible for inability to cross sulfur, additional loss of double bonds was due apparently to link and form a desirable structure in the vulcanizate. 1084
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1945
polymerization by heat under the inhence of accelerators or some s+ilar effect due to oxygen. Explanation of the evident combination of sulfur in excess of one atom per double bond saturated was explained by these authors in terms of several possible reactions: (a)addition, (b) bridging, (c) dehydrogenation, ( d ) polymerization. The first two types of reaction would produce a ratio of one atom of sulfur per double bond lost, as found in the simple rubber-sulfur compound. Reaction a is not in accord with observed,properties of solubility, etc. If the rat)io is greater than 1, an additional reaction must be taking place where sulfur can combine without loss of double bonds. Such a possibility might be reaction c. Excess loss of unsaturation may be polymerization reaction, d, after most of the sulfur has combined. Recent work of Farmer and Michael (9) led them to believe that the primary reaction of vulcanization is a substitution reaction on the alpha carbon atom. This corresponds somewhat to the dehydrogenation reaction suggested by Brown and Hauser, but with the cross linkage taking place a t the carbon atom alpha to the double bond rather than on the doubly bonded one. More positive evidence of sulfur substitution on the alpha carbon was presented recently by Armstrong, Little, and Doak (#), who studied the reactions with vulcanizing agente of certain olefins which have structures similar to rubber. The chief products of reaction of these simple aliphatic monoolefins with sulfur, zinc oxide, and soluble zinc soap in the presence of ameleratom consisted of the olefin bridged by sulfur at the alpha carbon atom. It was also observed that the degree of cross linking of rubber vulcanizates is closely related to the amount of zinc sulfide formed during the reaction, which is evidence in favor of a dehydrogenation type of reaction. Selker and Kemp ($0) recently presented evidence indicating that part of the combined sulfur in soft vulcanized natural rubber was attached t o the alpha carbon atom. EXPERIMENTAL PROCEDURE
Four related Buna S stocks were employed for this study. They were selected from the group studied by Cheyney and Duncan (6) in determining the temperature coefficient of the Buna S vulcanization reaction. They represented pure gum and channel black reinforced stocks, respectively, containing in each case 1and 5% added sulfur. Cures were made at 270' and 300' F. Recipes are given in Table I.
TABLE I. RECIPESOF STOCKS Stook No. Crude Buna 8 Zinc oxide Stearic acid Dibensothiaryl dieulade Bardol softener) Channei blaok Sulfur
I 100 5 1
i.5
i
IV
I1
I11
100 5 1 8.5
100 6 1 i.5
100 5
s
50 1
50 5
1 i.5
Physical tests were run by A.S.T.M. procedures. Sulfur determinations were made as outlined by Cheyney and Duncan (6). Attempts were made to measure unsaturation of these stocks by the KemgPeters procedure (17). This method waa inapplicable to these samples for two reasons: pDichlorobenzene, the solvent they used, was not sufficiently active for some of these samples; and the use of acetone for purification of the sample before determination of unsaturation is believed unwise in general. I n the latter connection, Cheyney and Robinson (7) and Kemp and Straitiff (18) noted that significant amounts of vulcanizate are extracted out of various Buna S stocks by acetone. This is not surprising in view of the fractions of extremely low molecular. weight reported in this polymer by the latter authors and by Sebrell (19). The use of acetone for extraetion is open to the further objeotion that it fails to remove soap which may be present in the
1085
crude polymer; this fact waa noted by Kemp and Peters (I?'). This is not a serioua objection in a series where a given sample of polymer is used throughout. To compare samples of differing degrees of purity, it is desirable to use a solvent which removes as wmpletely as possible nonpolymer material without removing polymer fractions. The writers have found that acetone is completely untrustworthy for this purpose; they have substituted ethanol, which is a solvent for the usual nonpolymer materials found in Buna S, including soap, and is a complete nonsolvent for the polymer fractions themselves. Whereas Kemp and Peters stated that p-dichlorobenzene will usually dissolve vulcanized Buna S tread stocks within 3 hours, the channel black stocks investigated here did not dissolve even after 10 hours of continuous boiling in pdichlorobenzene. When the samples were cut into paper-thin shavings with a razor blade, they would dissolve after several hours of boiling in p-dichlorobenzene. Uncured and pure gum stocks dissolved without too much difficulty. The method of solution finally adopted utilizes a phthalate plasticizer in combination with p-dichlorobenzene. Most of the determinations were made with either dibutyl phthalate or dioctyl phthalate, which were used interchangeably. Other ester plasticizerswould probably be satisfactory. These phthalatea aid in peptizing the vulcanized rubber sample without affecting the unsaturation of the latter and show only a slight tendency to react with the iodine chloride reagent under the experimental conditions. UNSATURATION MEASUREMENTS
The samples were passed through a cold, tight mill about thirty times to facilitate solution. This is believed to have no significant effect on iodine number, and Fisher (10) found that there is only a slight loss in unsaturation of pare crepe which had been milled 2 hours in air. The weighed sample (0.075 gram) was then extracted with alcohol for 16 hours, dried a t 50' C. under high vacuum, and kept in vacuum prior to analysis. The analysis was carried out within 24 hours after extraction in order to minimize any oxidative effects. The sample was placed in a 250-ml., Pyrex, gtoppered iodine flask together with 5 mi. of phthalate, and 10 grams of p-dichlorobenzene were added. The flask was heated on 8 hot plate at about 180' C. with the stopper loosened. The sample swelled at first and then began to dissolve rapidly. I n the case of the carbon black samples this preliminary swelling period waa about 30 minutea. During the succeeding 10 minutes the solution changed from colorless to black, and the sample became well dispersed. To ensure complete solution, 35 additional grams of pdichlorobenzene were added during this period of rapid solution, and heating was continued for another 30 minutea. Solution waa facilitated by whirling the flask from time to time, with care to avoid allowing particles to stick to the sides. When the sample was in solution, the flask was slowly cooled to room temperature; 30 ml. of chlorofotm were then added, followed by exactly 25 ml. of standard iodine chloride (0.2 N ) solution in carbon tetrachloride. The glass stopper of the flask waa closed, and a thin film of 15% potassium iodide solution was placed on it. The reaction solution was allowed to stand for one hour a t room temperature. At the end of this time, 25 ml. of fresh 15y0 potassium iodide were added, followed by 25 ml.of ethanol for the pure gum stocks and 50 ml. for the carbon black stocks. Excess iodine was immediately titrated with standard 0.1 N sodium thiosulfate. Another 25 ml. of ethanol were added near the end of the titration to aid in breaking up the emulsions. A blank containing identical quantities of all reagents waa taken through the same procedure. The iodine value is obtained by the calculation: IodineNo.
-
-
ml. thiosulfate (blank sample) X 1.269 sample wt. in grams
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 37, No. 11
reaction may be largely required for the rubber-sulfur-accelerator system; a loaded stock containing only two thirds as much rubber as the corresponding pure gum stock would thus be expected to require roughly two thirds as much heat to reach a given state of cure. (a) The‘large amount of pigment in the loaded stock makes possible a rapid transfer of heat through the stock and thus facilitates a much more rapid cure. It should be noted that this effect is different from that usually reported for natural rubber systems, where carbon black exerts a retarding action on cure. The 5 % sulfur stock a t a given temperature approaches a limiting iodine value lower than that of the corresponding 1% sulfur stock. Since the combined sulfur is greater in the 5% stock, some relation between combined sulfur and iodine value is indicated. The matter of “equivalent cures” is indicated in Figure 2 for stocks I11 and 11’. Here i t is apparently possible to obtain an equivalent state of cure for the same stock at two different temperatures, as shown by both combined sulfur and unsaturation. There is, however, no such simple relation with tensile strength. Furthermore, this relation holds true over a limited range and, apparently, only for the reinforced stocks. Elongation data are included in Table 11. No curves arc shown because the graphical character of this property W R R shown previously (6). The elongation goes through a maximum in the early stages of the cure, then decreases rapidly, and level6 off at a fairly constant value. Significantly, the maximum elongation is reached more rapidly in the channel black stocks. However, this maximum seems to bear no simple relation to any
TABLE 11. PROPERTIES OF BUNAS VULCANIZATES Figure 1.
Curing Time us. Unsaturation for Channel Black and Pure Gum Stocks
All determinations were made in duplicate. The iodine numbers in Table 11were the averages of these duplicate determinations. Theoretical unsaturation for each stock was calculated on the basis of a 75% butadiene-25% styrene content for the hydrocarbon portion of the crude polymer. The iodine number is thus three fourths of that of polybutadiene, or 352. For each of the four stocks the percentage unsaturation was then obtained by multiplying this figure by the percentage polymer in the stocks. The theoretical values of iodine number for the four stocks are thus 310, 300, 215, and 210, respectively. Each value of iodine number was converted to per cent unsaturation by a simple calculation (Table 11). The four uncured stocks show excellent agreement a t about 93% of theoretical. The remainder is presumably nonrubber material, with perhaps sbme diminution due to branch chaining, etc. This value is in excellent agreement with that of Kemp and Peters, who used a n average value of 93% hydrocarbon content in the crude polymer.
CurStock NO. I
...
300
I1
111
!w
Temp., Time, F. Min. 270
300%
Polymer
dine NO.
Io-
satura-
0
0.000
288
92.9
..
20 50 80 110 140
0.176 0.220 0.264 0.264 0.264
271 261 254 239 232
87.5 84.3 82.0 77.1 74.9
75 100 100 100 100
20 50 80 110 140
0.264 0.352 0.484 0.564 0.617
228 217 214 205 199
73.7 70.0 69.0 66.2 64.1
50 150 175 200 225
.. ....
.. .. .. .. ..
.... .. ..
Elongation. %
..
775 1000 1125 800 500 900 350 300 250 250
0
0.000
278
92.7
..
20 50 80 110 140
0.766 1.617 2.466 3.148 3.744
251 244 236 232 230
83.7 81.3 78.7 77.3 76.7
100 175 200 225 250
300
20 50 80 110 140
1.786 3.659 3.957 4.000 4.000
220 205 196 192 194
73.3 68.4 66.4 64.0 64.7
150 200 200 275 200
0
0.000
199
92.7
..
..
20 50 80
140
0.092 0.182 0.245 0.275 0.335
179 170 165 162 152
83.3 79.0 76.8 75.3 70.7
300 1050 1950 2300 2300
20 50 80 110 140
0.366 0.488 0.488 0.488 0.488
162 160 158 155 153
76.3 74.4 73.5 72.1 71.1
1100 2250 2300 2300 2300
200 300 650 700 850 400 950 1250 1350 1400
0
0.000
195
92.9
..
..
..
20
1.195 1.671 2.030 2.268 2.507
167 147 110 127 124
79.5 70.0 66.6 60.4 59.1
800 2700 2450 2250 2000
750 1650 2400
325 225 200 200 200
2.089 2.567 2.806 2.885 2.885
133 128 124 123, 119
63.4 60.9 59.1 58.6 56.6
2300 2500 2450 2400 2350
...
...
110 300
IV
Un-
Tensile Modulus, Strength, tion, Lb./Sq. Lb./ In. Sq. In. %
100 G.
270
270
EFFECTS O F VULCANIZATION
The experimental data in Table I1 are recorded graphically in Figures 1, 2, and 3. Temperature relations are what might be expected from previous data. Higher temperature results in a more rapid decrease in unsaturation, just as it results in a more rapid combination of sulfur and a more rapid increase of tensile strength. The reaction is most rapid in the first part of the cure and then tapers off. Unsaturation appears to approach a limiting value, although i t had not positively reached this limit in any of the samples studied. There is some indication that this limit might be the same for the 270’ and 300’ F. stocks, a t least for the reinforced stocks. I n the channel black stocks the rapid part of ‘the reaction is considerably faster than in the pure gum stocks. Two factors may be involved: (a) The heat requirement for a given state of
ing
Cur-
G. Combined S per
... 270
300
50 EO 110 140 20 50
so
110 140
.. .. .... ..
.... .. .. . I
.. .. ... . .. .. ..
..
1000 600 300 250 175 300 250 200 200 175
.. 62 5 575 550 550 525 750 700 650 600 575
700 450 300 250
200
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1945
I
30
T-3 t-
I
I
I
TIME OF CURE IN MINUTES Figure 2.
1089
II
TIME OF CURE IN MINUTES.
Effect of Cure on Properties of the Four Stook~.
I
1088
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 37, No. 11
,
{ oo
--- ---- -
LATOMSOFSULNR PER DOUBLE B O N D _ E _ _ -
1 ATOM OF SULFUR PER DOUBLE B O N D
