Sulfenamide—sulfur curing systems containing zinc oxide and stearic acid respond to both ionic and radical reaction conditions. Scorch delay and rate of the vulcanization are determined by the basicity and steric structure of the amine forming the amide, and by the accelerator-sulfur ratio. Sulfenamides derived from heterocyclic amines which form stable sulfides showed progressive improvement in scorch delay as the concentration of the sulfenamides was increased at constant sulfur level. In contrast to this group, the acceleration rates of sterically hindered sulfenamides were slow and their scorch delay decreased with increased accelerator concentration. The assumption that the initial step in sulfenamide acceleration is the dissociation of the sulfenamide into benzothiazolyl sulfenamide and amine radicals does not satisfy these experimental results. Consideration of the rate of sulfenamide-sulfur reaction and the stability of these intermediates has been suggested as an alternate explanation. Higher concentrations of accelerators in general resulted in vulcanizates with higher modulus and a larger number of effective sulfur crosslinks. About 1.5–2 atoms of sulfur per effective crosslink were found for vulcanizates where one mole of sulfenamide was used per atom of sulfur. It was also found that at this high concentration of sulfenamide a considerable fraction of the benzothiazolyl sulfenyl groups was initially bound to the rubber molecule and later released as extractable 2-mercaptobenzothiazole derivatives. Part of these groups which remained bound to rubber were reducible to form 2-mercaptobenzothiazole. Part of the true combined sulfur is released as ZnS in later stages of cure.
Dithioamines and sulfenamides derived from various cyclic amines were evaluated as accelerators and vulcanizing agents. Structural variations of these compounds are correlated with their scorch delaying properties, accelerating activity and vulcanization efficiency, employing the Mooney Viscometer and the Monsanto Oscillating Disk Rheometer. The relative cure characteristics differed with changes in the basic structure of the compounds, and with the size and type of the cyclic amine moiety. However, the piperidine derivatives consistently showed longer scorch delay and slower mean cure rate than the corresponding derivatives of pyrrolidine, hexamethylenimine or heptamethylenimine. In the case of the thiazolyl sulfenamides, the type of substitution in the thiazolyl group also influenced scorch delay. An empirical relation was observed between the effect of concentration of the thiazolyl sulfenamides on scorch delay and the results of differential thermal analyses of sulfenamide and sulfur blends.
Hexduoroacetone has been obtained by previous workers in the reaction between acetone and elemental fluorine. The reaction was described as complex, difficult to control and hazardous. We have now devised an alternate synthesis which is more convenient for laboratory preparation and which can be extended to large scale operation. The procedure involves many steps, but uses only readily available intermediates and simple, conventional operations.The procedure consists in synthesizing a hexafluorinated derivative of isobutene, preferably (CF&C=CClz, and in oxidizing its double bond to create the ketone function. The synthesis parallels our preparation of trifluoroacetic acid from either CF,CCI=CClz or CF3CCI=CC1CF3.2p3To obtain the desired carbon skeleton and the first CF3 group, CF&(CH3)ClCHzCl was synthesized by two different procedures. In the first sequence, isocrotyl chloride was transformed into CCls(CH3) ClCHzCl by successive chlorinations and dehydrochlorinations, and this pentachlorinated compound was treated with antimony fluoride to transform its CC13 group to a CFa group. In the second sequence, ethyl trifluoroacetate was condensed with methylmagnesium chloride, and the resulting tertiary carbinol was dehydrated to the corresponding olefin, CF3C-(CH3)=CHZ; chlorine was then added to the double bond. Cost considerations favor the first sequence for large quantities, and the second sequence for small preparations.To create the second CF3 group, CF3C(CH3)-CICHzCI was transformed into CF3C(CH3)= CC1z by alternated removals of hydrogen chloride and additions of chlorine; by treatment with antimony fluoride in anhydrous hydrogen fluoride; the olefin was easily transformed into (CF3)z-The olefin most convenient for the oxidation step is (CF,)zC=CClz. It was obtained by chlorination of (CF&CHCH3 to (CF3)2CHCC13 followed by hydrogen chloride removal. The final oxidation was performed with aqueous potassium permanganate, in neutral or acid solutions only on account of the known sensitivity of hexafluoroketone to alkalies. Isolation of the ketone was complicated hy its ability to form a stable hydrate, soluble i n water. 'l'he pure ketone was ultiiiiately obtaiiied by distilling a cuiiceiitrated solution of its hydrate from phosphoric anhydride, and its observed properties proved in agreement with those previously recorded.In the developinent of a practical procedure, CHCH3.(1) Fukuhara and Bigelow, THIS JOURNAL, 63, i 8 8 (1941) (2) Henne, Alderson and Newman, zbid, 61, 918 (1945) (3) H e m e and Trott, ibid., 69, 1820 (1947) HEXAFLUORO ACETONE 3577 CHEMISTRY AT THE OHIO STATE UNIVERSITY]
In this paper the subject of rubber vulcanization accelerated by 2-mercapto-benzothiazole and its derivatives has been reviewed. The technical literature from 1945 through 1960 and patents from 1932 through 1960 have been covered. Topics include: methods of synthesis and manufacture of these accelerators; application and compounding data on their use in rubber processing; and studies of the modes of action and mechanisms for the chemical reactions involved during accelerated vulcanization. Much disagreement exists concerning the mechanism of accelerated vulcanization and the action of thiazole accelerators. However, most of the conflict lies not in the experimental data collected, but in the interpretation of the meaning of the data. It is well documented that MBT and activators (zinc stearate, or zinc oxide and stearic acid) undergo an initial reaction; that these reaction products then react with sulfur and/or rubber hydrocarbon to form intermediate compounds; and that these intermediates then react in some manner to form sulfur crosslinks. Not known, for the most part, are the precise reaction steps involved; the sequence in which these reactions occur; the individual mechanisms, whether ionic, free radical, or neither, by which these reactions proceed; and the side reactions involved, if any (except in natural rubber, in which the non-crosslink forming cyclization reaction is well documented) which might lead to erroneous conclusions from the experimental data, particularly from kinetic studies. These same conclusions apply in general to thiazole sulfenamide accelerated sulfur vulcanization, with the exception that, in contrast to MBT, which has a free thiol group available for immediate reaction, the sulfenamide must first decompose or react in some manner before acceleration occurs. Sulfur and divalent sulfur compounds readily undergo both radical and ionic reactions, depending only on co-reactants and reaction conditions. At present, the reactions of sulfur with hydrocarbons and accelerators are not sufficiently well understood to draw concrete conclusions about the mechanism of acceleration. Further progress on elucidation of the mechanism will come with a better knowledge of the chemistry and mechanisms of sulfur reactions.
Evaluations of cyclic iminocarbodithioates as accelerators and vulcanizing agents are described. The starting amines include morpholine and 3-azabicyclo-[3.2.2]nonane and vary in ring size from tetra- to octamethylenimine. The chemical structure of these compounds is correlated with their cure characteristics and the kinetics of vulcanization. Basicity and ring size of the amine moiety govern the activity of the iminocarbodithioates.
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