Conversion of raw rubber to a usable product is basically a three-step process. Crosslinking ingredients, fillers, and other components are added, the compounded green stock is shaped into the desired form, and the form is heated (cured) to introduce crosslinks (covalent bonds) between the rubber polymer chains and produce a three-dimensional network with vastly improved properties. The crosslinked product no longer flows and thus cannot be reprocessed. The curing system is activated by heat, heat is required for and generated by processing, and faster processing is desirable for economic reasons. Thus, a basic conflict exists between the processing and curing stages. The balance is delicate because the compounder, again for best economics, wants the product to cure rapidly after it is processed. Modern curing systems provide a scorch delay time, during which heating produces no crosslinks, followed by a period of rapid crosslink formation. The compounder then adjusts process conditions to provide enough heat history to eliminate most of the scorch delay so that the cure time will be minimized. Since the compounder is balancing processing against cure, subtle and sometimes uncontrollable changes in factory equipment or operating parameters can produce unprocessable (scorched) rubber stocks. Such changes can be compensated for by making adjustments in the curing system or in the processing conditions. However, relationships between curing system and properties are complex, and considerable effort is generally required to develop a new formulation. Thus, the need exists for an additive which increases only the scorch delay time with minimal effects on other cure parameters and vulcanizate properties. Such compounds, called prevulcanization inhibitors, scorch inhibitors, or retarders, are in commercial use. A number of papers and patents describing the chemistry and use of these compounds have been published and are reviewed here.
Sulfenamide accelerators serve to perform two functions. They provide the necessary time period required to mix, process and shape rubber compounds. This portion of the overall vulcanization process is usually referred to as scorch delay. In addition, sulfenamides function as accelerators, in that once the crosslinking process has begun, they speed up this reaction. The delay period occurs because the sulfenamide must be converted to polythiobenzothiazoles, the precursors to crosslinking, before crosslinking can take place. The overall reaction of sulfenamide with sulfur is an autocatalytic process with MBT the autocatalyst. The role of PVI in this scheme is to remove MBT from this autocatalytic sequence of reactions, thus delaying those reactions which preceed crosslink formation.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.