Rubber Chemicals from Cyclic Amines. V. Thiocarbamyl Sulfenamide Accelerators Derived from 3-Azabicyclo(3.2.2)Nonane

Evaluation of thiocarbamyl sulfenamides (I) prepared from 3-azabicyclo-(3.2.2) nonane and other amines indicates that the curing properties of stocks accelerated by (I) are influenced by the amino groups which constitute the thiocarbamyl and sulfenamide moieties, R and R′, respectively, and also vary with the formulation of the rubber stocks. The fastest cures are obtained when R is pyrrolidinyl or dimethyl and slowest cure when R′ is N-tert-butyl or N-morpholino. The lowest maximum crosslink density is obtained with morpholino- or piperidinothiocarbonyl sulfenamide. In a non-extended SBR black stock the crosslinking reaction generally proceeds as a second order reaction. The major part of the crosslinking reaction of the slower curing stocks proceeds by a fast first order rate followed by a slower first order rate.

The carotid body as a neuroendocrine organ involved in control of erythropoiesis

The carotid body is generally regarded solely as a chemoreceptor. This paper presents evidence that the carotid body is also an endocrine gland, secreting one or more erythropoiesis-controlling hormones. Its removal is followed by anemia and by a lack of reticulocyte response to daily bleeding. The removal of both carotid bodies results in a reduction of the high reticulocyte count induced by daily bleeding (5-40% of erythrocytes) down to 0.1% of erythrocytes. The injection of an extract of the carotid body causes reticulocytosis within 5 hr. This reticulocytosis attains its peak 48-72 hr after injection. Daily bleeding for 15 days produces an increase in weight of the carotid body and ultrastructural alterations that indicate increased protein synthesis in the main cells. The injection of plasma from efferent blood from the carotid body into either normal cats or cats from which both carotid bodies have been removed is followed within 6 hr by the appearance of large numbers of reticulocytes and macroreticulocytes in peripheral blood. Carotid body extracts and efferent blood plasma exhibited erythropoietin activity, which was demonstrated in the polycythemic rat by the (59)Fe incorporation method.

Polychloroalicyclic Ketones as Promoters for Crosslinking Polymers

Polychloroalicyclic ketones are chlorine liberating compounds which in combination with activators and coagents promote crosslinking of saturated and unsaturated polymers. One of the most active ketone is octachloro-3-cyclohexen-l-one (OCTA). During the curing process the alicyclic ketones transform into phenols and probably some phenates. The most effective activators are magnesium and calcium hydroxides and oxides. These curing systems are also activated by acidic and basic fillers. Active coagents are sulfur, p-benzoquinones and maleimides. EPM of superior thermal stability is obtained with the chlorinated p-benzoquinones as the coagent. The apparent order of crosslink formation in EPM using these curing systems is pseudo first order. In case of OCTA/MgO/coagent system, the specific rate of crosslink formation is directly proportional to (MgO)1/2/(OCTA) and the maximum crosslinks formed vary linearly with (MgO)1/2/(OCTA). The OCTA system is most suitable for curing saturated polymers or their blends, and not for black stocks of high unsaturation polymers because of its high reactivity. Saturated polymers may be heated with OCTA and MgO and modified to unsaturated polymers which are curable by the conventional sulfur-accelerator system.

A Study of Sulfenamide Acceleration

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.

2-Mercaptothiazole and Derivatives as Vulcanization Accelerators

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.