Polysulfide Linkages Research Articles

Relaxation of Disulfide and Tetrasulfide Polymers

Abstract The chemorheology of crosslinked polyethylene disulfide, polyethylene tetrasulfide, and mixtures of these polymers with elemental sulfur is investigated. It is shown that stress decay in the disulfide polymer occurs by scission of the small quantities of higher polysulfide linkages present in the network. It is also shown that elemental sulfur decreases the chemical relaxation time in crosslinked polyethylene disulfide in a predictable manner.

The viscoelastic properties of crosslinked poly(ethylene tetrasulfide). I

A range of crosslink densities has been introduced into poly(ethylene tetrasulfide) by use of the trifunctional agent, 1,2,3-trichloropropane. The formation of a cross-linked structure has been established by torsional modulus measurements over a range of temperatures from the glass transition region to a region of rapid stress decay due to interchange reactions between polysulfide linkages. Estimates have been made of the deviation from ideal rubber elasticity by calculating the front factors times the crosslink efficiency.

Reactions of Isoparaffins with Sulfur and DI-t-Butyl Peroxide

Abstract The role which sulfur plays in the peroxide vulcanization of polyisoparaffins has been investigated. The problem was studied by using a system in which 2,4dimethylpentane is a model for the polyisoparaffin chain. Two alternative reaction mechanisms are proposed. Work with the model system shows that polysulfides formed from the isoparaffin are the dominant products. By analogy it is inferred that polysulfide linkages join the polyisoparaffin chains, preferably at what were formerly tertiary hydrogen sites. Nearly all of the high boiling products are disulfides and, or, polysulfides, and these total at least a 28 mole per cent yield based on peroxide charged. Thus, it is indicated that sulfur plays a major role, and does not simply prevent minor side reactions. Study of the gaseous reaction products indicates that sulfur does not alter the course of the initiation step. A reaction mechanism is proposed which is consistent with literature and with our experimental work.

Structural characterization of vulcanizates. Part II. Use of triphenylphosphine to determine the structures of sulfur linkages in unaccelerated natural rubber–sulfur vulcanizate networks

Treatment of unaccelerated natural rubber–sulfur vulcanizate networks with triphenylphosphine in benzene at 80°C. effects the removal of part of the combined sulfur, the proportion removed decreasing with increasing cure time. This partial desulfurization of the network is attributed, on the basis of the mode of interaction of simple organic di- and polysulfides with triphenylphosphine, to the conversion of polysulfide linkages in the network into either mono- or disulfide groups. Knowledge of the amount of sulfur removed by the reagent, and of the chemical degree of crosslinking and combined sulfur content of the untreated networks permits the following semiquantitative conclusions to be reached concerning the structures of the sulfur linkages in the networks: (1) long polysulfidic crosslinks (S11 to S12) are initially formed, which on continued cure progressively decrease in length to a limiting value of S2 to S4; and (2) a very high proportion to the combined sulfur (ca. 77% for short cure times, increasing to ca. 95% on extended cure) is present in cyclic monosulfide groups situated along the polyisoprene chains.

The Mechanism of Sulfur Reaction in the Polymerization of Chloroprene and of Thiuram Reaction in the Degradation and Structurization of Polychloroprene

Abstract To elucidate the mechanism of the reaction between chloroprene and sulfur, the following investigations have been carried out: (1) The reaction of a polychloroprene solution with sulfur under conditions of free radical initiation was studied. It was found that sulfur does not react with polychloroprene, whereas under polymerization conditions the addition of sulfur proceeds readily. (2) A study of the polymerization of chloroprene monomer containing various amounts of sulfur (from 0.3 to 10%) showed that the sulfur content of the polymer increased in proportion to the amount of sulfur originally present. Polymers with a high sulfur content (3–5%) were found to be completely soluble at low degrees of polymerization. The latter finding indicates that the sulfur does not effect crosslinking, but that it is bound predominantly in a linear manner in the polymer chain. (3) The predominant form of bonding of the sulfur was found to be in di-and polysulfide linkages. The number of the latter increases with the sulfur content of the polymer. (4) By the action of nascent or catalytically activated hydrogen, the polymer was found to undergo extensive degradation with the loss of 80 to 100% of the bound sulfur and formation of low molecular weight polymer. In the course of the reduction the disulfide links are broken with the formation of polymer mercaptans, which are converted to disulfides by reaction with iodine; this is confirmed by an appropriate increase in the intrinsic viscosity. These findings confirm the copolymerization mechanism of the reaction between chloroprene and sulfur as proposed by Mochel and Nichols. (5) The mechanism of the degradation of polychloroprene sulfide by tetraethylthiuram disulfide (TETD) was studied by means of the following model systems, representing the main types of structures present in such polymers:

The Action of Vulcanization Activators

Abstract 1. Zinc oxide and stearic acid do not affect the rate of addition of sulfur to rubber in the vulcanization of pure sodium butadiene rubber in mixtures without accelerators. 2. In mixtures containing diphenylguanidine as accelerator zinc oxide and stearic acid do not affect the rate of addition of sulfur to rubber. 3. In mixtures containing mercaptobenzothiazole zinc oxide retards and stearic acid accelerates the addition of sulfur to rubber. 4. In a similar manner zinc oxide suppresses and stearic acid activates isotope exchange between elementary sulfur and sulfur of the thiol group in mercaptobenzothiazole. 5. Zinc oxide and stearic acid in mixtures with mercaptobenzothiazole increase the rate and degree of crosslinking of the molecular chains of rubber; zinc oxide has the greater influence on the degree, while stearic acid has the greater influence on the rate, of the crosslinking reaction. 6. In mixtures with diphenylguanidine the influence of vulcanization activators on the degree and rate of crosslinking is considerably less pronounced than in mixtures with mercaptobenzothiazole. 7. The kinetics of zinc sulfide formation during vulcanization has been studied and it was established that ZnS is formed as the result of reactions of zinc oxide and zinc compounds with thiol and polysulfide groups in the rubber. Model substances have been used to demonstrate other possible routes for the formation of zinc sulfide during vulcanization. The effect of zinc oxide and stearic acid on the rate and degree of crosslinking is associated with participation of these compounds in such reactions. 8. Isotope exchange between radioactive sulfur in the vulcanizate and elementary sulfur was used to follow the formation and changes in the numbers of polysulfide linkages during the vulcanization process. The amount of sulfur participating in isotope exchange as vulcanization proceeds at first increases, passes through a maximum, and then decreases, which indicates a regrouping of the polysulfide linkages with an increase in their number and a decrease of the average number of sulfur atoms per linkage. Zinc oxide decreases the degree of isotope exchange between the vulcanizate and elementary sulfur at all stages of vulcanization. 9. Vulcanization activators, by favoring a decrease in the number of sulfur atoms in the sulfur bonds, increase the heat stability of the vulcanizates. This effect of the activators was demonstrated by kinetic data on stress relaxation in deformed vulcanizates at 126°. 10. The cleavage and regrouping of polysulfide linkages in the presence of zinc oxide and zinc compounds is accompanied by the combination of part of the sulfur as zinc sulfide, which leads to a decrease in the number of newly formed crosslinks. This effect of zinc oxide is manifested in vulcanization reversion effects and in changes of vulcanizate properties under thermomechanical influences. 11. From the above experimental data the general conclusion may be drawn that the fundamental role of vulcanization activators does not lie in their influence on the kinetics of the addition of sulfur to rubber, but rather in their influence on the nature of the vulcanization structures formed and on changes in them in the course of vulcanization.

The Reaction of Sulfur and Sulfur Compounds with Olefinic Substances. V. Rubber Vulcanization

Abstract Data presented for a range of vulcanized rubbers prepared under different conditions show that, while olefinic unsaturation becomes reduced by vulcanization in the proportion of one double bond for each sulfur atom combined with a C5H8 unit, the original H/C ratio of 8/5 is not altered. The loss of unsaturation is somewhat modified when zinc oxide or certain accelerators also are present. Oxygen has a slight inhibiting effect on vulcanization; hydrogen sulfide and thiols markedly catalyse the vulcanization reaction without, apparently, affecting the efficiency of the sulfur cross-linking reaction. In confirmation of the results of Hull, Olsen, and France, a small proportion of zinc oxide or a soluble zinc soap promotes a reaction between sulfur and mercapto groups whereby di- and polysulfides are formed with liberation of hydrogen sulfide. The same type of activity is shown by some of the nitrogenous accelerators commonly used in rubber vulcanization. Substantial conversion of mercapto groups into polysulfide linkages is, therefore, to be expected when vulcanization is conducted in the presence of these auxiliary substances.

Modern views on the chemistry of vulcanization changes. III. Reaction of sulfur with squalene and with rubber

AbstractSqualene containing six isoprene units is more free of end‐group effects and less limited in the range of space‐distributional contortions of its carbon chain than mono‐ and diolefins. Hence it is more comparable with rubber than the latter group where matters of chemical reactivity are concerned. Nevertheless, in its reaction with sulfur, squalene pursues a course very similar to that followed by the diisoprene, dihydromyrcene, in that intramolecular sulfide linkages as well as intermolecular polysulfide linkages are formed. Taking into consideration the amount of sulfur which would suffice for the establishment of some given number of the simplest possible intermolecular linkages (i.e., monosulfide linkages), the efficiency of sulfur as a cross‐linking reagent during the vulcanization of squalene is seen to be of a rather low order; this condition might be expected to hold for rubber in the absence of auxiliary mechanisms. So far as can be determined by direct experiment the action of sulfur in forming cross links is not essentially different from that encountered with squalene, since there are indications that both intra‐ and intermolecular sulfide linkages are formed—the latter being of both dialkenyl and dialkyl sulfide type, and, possibly also of alkenyl alkyl type. Under the influence of small amounts of zinc oxide or of various nitrogen‐containing organic accelerators, sulfur reacts with the thiol groups of organic mercaptans forming di‐ and polysulfides, together with hydrogen sulfide. In the presence of rubber the same reaction occurs without actual liberation of hydrogen sulfide and it thus appears that if any thiol groups are formed as intermediates in vulcanization reactions, they will undergo ready conversion into intermolecular di‐ and polysulfide linkages. This may well represent an important auxiliary mechanism of cross linking.