Use of Oxidizing Agents in Rubber Vulcanization ZINC OXIDE-FREE PROCESS BERNARD C. BARTON United States Rubber Company, Passaic, N . J. Conventional vulcanization may be considered as a two-step oxidation process in which zinc oxide functions through its ability to form zinc mercaptides by reacting with rubber hydrosulfides formed in the first step. These zinc mercaptides are then oxidized by sulfur to disulfide cross links. Consideration of this mechanism has led to the development of a new vulcanization process in which various mild organic oxidizing agents replace sulfur in the second step of the oxidation process and thereby make possible the elimination of zinc oxide. Oxidizing materials capable of replacing a portion of the sulfur normally used are considered and the mechanism of the new vulcanization reaction is discussed.
in preference to the oxidation of the thiol itself. Therefore, it appears likely that when zinc oxide and fatty acid are present, vulcanization proceeds through reactions involving the formation of a zinc mercaptide, as in Equation 2 ; this is oxidized by sulfur t o give disulfide cross links and zinc sulfide as shown in Equation 3. Based on this simplified mechanism, vulcanization is a twostep oxidation process in which zinc oxide or zinc soap functions through its ability to form zinc mercaptides which are more readily oxidized by sulfur to disulfide cross links than are thiol groups If this mechanism is correct, oxidizing agents other than sulfur should be capable of taking the place of sulfur in the second step of the oxidation reaction. As zinc oxide or zinc soap is required only in the second step of the sulfur reaction, it is possible t h a t the use of a suitable oxidizing agent would eliminate the necessity of these materials.
.
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EFORE the advent of organic accelerators, many rubber
compounds contained no zinc oxide. However, relatively large amounts of sulfur were required for desirable physical properties. With the development of modern accelerators the use of zinc oxide, together with greatly reduced sulfur concentration, became the universal practice. When zinc oxide is not present in these low sulfur compounds little or no vulcanization occurs. Although the function of zinc oxide in the vulcanization reaction is not known with any degree of certainty, various suggestions as to the mechanism of its action have been made (1, 4 ) . Consideration of these mechanisms has led to the elimination of zinc oxide and the replacement by certain mild oxidizing agents of a substantial proportion of the sulfur normally employed. In the present paper the general development of the new process is described, oxidizing materials capable of eliminating zinc oxide are considered, and the mechanism of the vulcanization reaction is discussed.
ELIMINATION O F ZINC OXIDE BY USE OF A SUITABLE OXIDIZING AGENT
In selecting an oxidizing agent for preliminary investigations, only those materials were considered which were strong enough to readily oxidize mercaptans but too weak t o react rapidly with rubber. 2,2'-Dibenzothiazyl disulfide (MBTS) waa selected for the initial examination. It readily oxidizes hydrogen sulfide to free sulfur and reacts relatively slowly with rubber.
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SIMPLIFIED MECHANISM OF CONVENTIONAL VULCANIZATION
I n spite of the complex nature of the reactions involved, a not too improbable over-all mechanism of vulcanization can be proposed; it may be useful in directing investigations of the viilcanization process: Accelerator RH Sa RSH SI-1 (1) Rubber hydrocarbon 2RSH Zn++ ----f RSZnSR 2H+ (2) RSZnSR S, +RSSR ZnS S,-l (3)
+
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PEPLkCEHLHT OF Z I N C O X I D E WITH 2 , 2 ' . D I B E K Z O T H I A 2 Y L DISULFIDE (METS)
Although hydrosulfide groups have not been determined experimentally in cured or semicured stocks, the vulcanization reaction is often assumed to involve the reaction of sulfur with the rubber hydrocarbon t o form a rubber hydrosulfide as shown in Equation 1. According t o Hull, Olsen, and France ( 4 ) and Bloomfield (S) when zinc oxide is present in excess, as in rubber compounds, thiol group8 form zinc mercaptides which are oxidized by sulfur
Figure 1. Replacement of Zinc Oxide with 2,2'-Dibenzothiazyl Disulfide
The effect of using MBTS in a zinc oxide-free Methazateaccelerated low sulfur compound is shown in Figure 1. I n order t o avoid reversion and other complicating side reactions which occur at higher more conventional curing temperatures, vul-
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Vol. 42, No. 4
EFFECT OF ACCELERATOR CONTENT ON PHYSICAL CURE
TABLBI. EFFECTO F ACCELERATORS ON COMPOUNDSIN
CURE O F WHICH ZINC OXIDE Is REPLACED BY 2,2’DIBENZOTHIAZYL DISULFIDB PHYSICAL
Basic compound (par& per 100 grams of rubber). sulfur 0.64. dibeneylamine 1.0; 2,2’-dibeneothiazyl disulfide.8 00: all Lures, 04 hou& at 100’ C . (sufficient t o combine all sulfur) Stress Accelerator a t 200% Added, Elongation, Parts/100 G. Lb./Sq. Rubber Inch 100 None Zinc salt of dimethyldithiocarbamic acid 225 (Methasate) 1.5 Lead salt of dimethyldithiocarbamio acid 180 (Ledate) 1.5 C o m e r (icl salt of diethvldithiocarbamic acid 175 1.5 (Cumate) 100 2-Mercaptobeneothiazole (MBT) 1.5 145 Zinc salt of 2-mercaptobenzothiazole (OXAF) 2.5 115 Silver salt of 2-mercaptobenzothiasole 1.5 Copper (ic) salt of 2-mercaptobeneothiasole 115 1.5 (Cuprax) 115 1.5 Zino butyl xanthate (ZBX) 100 Ammonia-ethylene dichloride reaction product 1.0 100 Tetremethylthiuram monosulfide (Monex) 1.5 Butvraldehvde-nniline reaction product (Beu90 0.5 &e)
...
canization was carried out a t 110 ’ C. The time of cure for each compound was selected to give the maximum physical cure. The physical cure, as measured by stress at 200% elongation, of a conventional low sulfur compound is shown in the first bar. Elimination of zinc oxide and stearic acid greatly reduces physical cure, as is shown in the second bar, and replacement of zinc oxide and stearic acid by MBTS increases physical cure as shown in the third bar. The last bar shows that in the absence of sulfur, MBTS is not a vulcanizing agent. EFFECTIVENESS O F VARIOUS ACCELERATORS IN THE ZINC OXIDE-FREE PROCESS
A comparison of the effectiveness of various accelerators in the new process is shown in Table I. 2,2’-Dibenzothiazyl disulfide is most effective in replacing zinc oxide when the compound is accelerated with a metal salt of a dithiocarbamic acid. The zinc salt of dimethyl dithiocarbamic acid brings about the greatest physical cure; the copper and lead salts are not quite so effective. 2-Mercaptobenzothiazole, the zinc, siIver, and copper salts of 2-mercaptobenzothiazole, zinc butyl xanthate, an ammonia-ethylene dichloride reaction product, tetramethylthiuram disulfide, and a butyraldehydeaniline reaction product were of no appreciable value.
The effect of increasing amounts of Methazate and Butazate (the zinc salts of dimethyl- and dibutyldithiocarbalc acid) on the phyaical cure of a compound containing 0.64 part of sulfur and 10 parts of MBTS is shown in Figure 2 . All compounds were cured for a sufficient time t o combine all sulfur. The physical cure increases with increasing accelerator content up t o a definite point above which further additions have no effect. At equivalent molecular concentrations Methanate and Butazate are equally effective. About 0.0035 mole is required to give maximum physical cure in a compound containing 0.02 gram atoms of d f u r (0.64 part) and 10 parts of MBTS. EFFECT OF CONCENTRATION OF 2,2’-DIBENZQTHIAZY L DISULFIDE ON PHYSICAL CCRE
The effect of concentration of MBTS on the extent of vulcanization of Methazate-accelerated compounds containing various amounts of sulfur is shown in Figure 3. Stress a t 200% elongation increases in a linear manner with increasing MBTS concentration up t o a maximum for each amount of sulfur, beyond which further additions have little or no effect. When there is a deficiency of MBTS, the degree of vulcanization is independent of sulfur concentration and directly dependent on the concentration of MBTX, On the other hand, when sufficient oxidizing agent is present to give a maximum degree of vulcanization for a given amount of sulfur, the extent of vulcanization is dependent on the amount of sulfur present. EFFECTIVEYESS O F VARIOUS OXIDIZING AGENTS IN NEW PROCESS
I n Figure 4 replacement of zinc oxide and fatty acid with various types of organic oxidizing agents is shown. Although the materials are not completely representative of all types of organic oxidizing agents some conclusions can be obtained. The stronger oxidizing agents such as quinone, benzoyl peroxide, and cumene hydroperoxide are ineffective, whereas various milder oxidizing agents are quite effective. These include quinone dioxime, diazoaminobenzene, bis(ethoxyphenyliminomethyl)disulfide, AT-nitrosodiphenylamine, quinone bisphenylimine, 2,2’-di-
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Figure 2.
Effect of Accelerator Content on Physical Cure
Compounds contained sulfur, 0.64 part. MBTS, 10.0 parts’ dibensylamine, 1.5 parts; and vdiable amounts of Metdazate and Butaaate per 100 grams of rubber; all cures, 82 hours at looo C.
Figure 3.
Effect of 2,2’,-Dibenzothiaeyl Content on Physical Cure
Disulfide
Compounds oontained sulfur as shown. Methasate, 2.0 parts. dibenaylamine, 1.0 part: and variable ‘amounts of MBTS pe; 100 grams of rubber; all oures, 32 hours a t l o O D C.
673 benzothiazyl disulfide, and benzothiazyl-2-monocyclohexyl sulfenamide. Each of these materials is representative of a different class of organic compounds. With the exception of quinone bisphenylimine and quinone dioxime, which are capable, in the amounts present, of bringing about a slight degree of cure, none of the chemicals, effective in the new process, brings about any cure in the absence of sulfur. As a matter of fact N-nitrosodiphenylamine (Delac J) is a commercial retarder. A comparison of the efficiency of 2,2'-dibenzothiazyl disulfide, N-nitrosodiphenylamine, and quinone bisphenylimine is shown in Figure 5. The similar dependence of physical cure on the concentration of these materials of such widely different molecular structure suggests t h a t the mechanism of vulcanization with these materials is essentially the same.
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