RESEARCH
Quenching Reaction Is Key to Delayed Action Scheme describes quantitatively vulcanization of natural rubber with sulfur and delayed action accelerator Dr. R. H. Campbell
A nearly complete picture of chemical reactions in a rubber cure has been sketched by a team of chemists at Monsanto's Nitro, W.Va., rubber chemicals laboratory. Dr. R. H. Campbell and A. Y. Coran have done analytical and kinetic work that identifies a quenching reaction as a key factor in a vulcanization having a long cure delay, followed by rapid crosslinking. Their curing scheme describes quantitatively the vulcanization of natural rubber in the presence of sulfur and delayed action accelerators. Mr. Coran did the kinetics; Dr. C. D. Trivette studied model compounds; Dr. Campbell and R. W. Wise did the analytical work. The scheme worked out by the Monsanto chemists postulates a competition between two reaction routes that the initial species can take. One is a chain of consecutive reactions leading to cross-links. The other is a quenching reaction in which the initial species inactivates one of the immediate cross-link precursors of the first reaction. Only when all of the initial species have been converted to another intermediate does the unperturbed first-order cross-linking take place, they told the 85th meeting of the American Chemical Society's Division of Rubber Chemistry, in Detroit, Mich. The proposed mechanism explains the extensive delay in cure followed by fast production of cross-links in 40
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18,
1964
A. Y. Coran
a natural rubber-sulfur-accelerator gum stock. Rubber chemists have always sought long cure delays to allow safe formation of products out of rubber. Once a product has been formed in a mold, they want the rubber to cure rapidly so that the mold will not be tied up too long. Frequently, one objective, but not the other, can be reached. The study provides understanding that will give direction to new curing work, the Monsanto workers say. And it will aid optimization of curing characteristics and vulcanization properties. Monomeric polysulfides are the initial species that delay the cure until most of them have reacted with rub-
ber to produce polymeric polysulfide materials. The monomeric polysulfides are formed by a reaction between the accelerator and sulfur. This part of the scheme is still not well defined, but the monomeric polysulfides have been measured and treated kinetically by the Monsanto workers. Insight. Early analytical data on uncured rubber, according to Dr. Campbell, gave insight which enabled them to build a model for treating kinetics of cross-linking formation. A method (developed by Mr. Coran)· for evaluating critical rate constants uses instruments (curometers) that assess rheology of rubber during cure. A series of experiments was devised to test the kinetic hypothesis. Using the derived kinetics, Mr. Coran was able to deduce the concentrations of the monomeric polysulfides that occur during the delay period. Curometer data supply cross-link densities kinetically related to the reaction of monomeric polysulfides. Knowing the kinetics of cross-link formation allows Mr. Coran to deduce kinetics of polysulfide disappearance. Analyses developed by Dr. Campbell and Mr. Wise confirm the kinetic calculations. The first step in curing, according to the kinetic hypothesis, is a reaction between ingredients of the rubber compound to give monomeric polysulfides containing accelerator fragments. Dr. Campbell and Mr. Wise separated the extracted polysulfide intermediates chromatographically. They then measured and identified the materials by ultraviolet spectroscopy and polarography. Monomeric polysulfides react with rubber, eliminating 2-mercaptobenzothiazole (MBT) and forming rubber polysulfide (polymeric polysulfides). These polysulfides were found in one of the chromatographic fractions. Concentrations of the polysulfides were measured as MBT after reduction with sodium borohydride. The rubber polysulfide does not form cross-links itself. It homolytically cleaves at a sulfur-sulfur bond to yield a polymeric polythiyl radical, the Monsanto workers say. The radical then reacts one of two ways. It can go on to form a cross-link by re-
Curing of Rubber Curing Sketched as Series of Consecutive and Competing Reactions Sequential reactions that lead to cross-link
OciïVs-s-c'V· s' 2,2'-Dithiobisbenzothiazole
„ ,
^-^ f
^ÎT^ * c — S - S
8
-
S ~X
Monomeric polysulfide
^^
%>
Sulfenamide (accelerators containing fragment X*)
Monomeric polysulfide
Polymeric polysulfide
Polymeric polysulfide
Polymeric polythiyl radical
RvoWrtf Polymeric polythiyl radical
Polysulfide cross-link
Competing reactions that delay cross-link
^ - v + rr\-s- Sx -s-xPolymeric polythiyl radical
^ ^ ^ g Monomeric polysulfide
Polymeric polysulfide
g — g/
Monomeric polythiyl radical
when 2,2'-dithiobisbenzothiazole is the accelerator or X = R2N— when a sulfenamide is the accelerator
acting with another rubber molecule. Or it can be quenched by a molecule of monomeric polysulfide, the initial species. Accelerator Inhibits. The analyses by Dr. Campbell and Mr. Wise show that unperturbed first-order cross-linking is not achieved until the mono-
meric polysulfide is almost completely depleted. Thus the accelerator, in the presence of sulfur, acts as an inhibitor. The quenching reaction predominates during the prevulcanization or delay phase. Its reaction rate constant, k4, is much greater than is the reaction rate constant (k 3 ) for cross-linking.
Once monomeric polysulfides have all been tied to the rubber as polymeric polysulfides, they can no longer compete for the active species that otherwise forms cross-links, Mr. Coran says. The uninhibited, rapid cure begins. The cross-linking is followed by curometer rheology. Mr. Coran finds that four kinetic parameters characterize curing completely. The first one (k x ) measures the over-all rate of the early reactions that form rubber polysulfides. A reduction in this rate constant results in an extended delay or induction period. The parameter k 2 is rate-controlling in two processes. It controls the rate of cross-link formation and partially controls the disappearance of the monomeric polysulfides. Hence a reduction in k 2 results in both reduced rate of cross-link formation and an extension of the induction period. The ratio k 4 /k 3 indicates preference for the quenching reaction. A reduction in this ratio results in prevulcanization or scorch. If scorch occurs, it is caused by the inability of the monomeric polysulfides (present in a given system) to quench the active polymeric polythiyl radical. The fourth critical factor for complete description of the curing system is an estimate of the ultimate cross-linking density. Steps. Several reactions or ratecontrolling steps can be distinguished in the complex vulcanization process. Thus the Monsanto treatment can be used to study individual phases of the process, Mr. Coran says. The technique allows obtaining the facts with curometers such as the oscillating disk rheometer. This means the rubber compounder can define changes in the chemistry of vulcanization that result from his changes in rubber compound formulations. The Monsanto group expects to obtain similar results soon in interpretations of other vulcanization. These systems include rubbers—styrene-butadiene rubber, for instance—and curing systems that include zinc oxide and stearic acid. Mr. Coran has already applied the kinetic treatment to both natural rubber and SBR tread stocks containing carbon black. MAY
18, 1 9 6 4 C & E N
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