Loss Of Unsaturation Research Articles

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.

The reaction of sulphur and sulphur compounds with olefinic substances. Part V. Rubber vulcanization

AbstractData 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 sulphur atom combined with a C5H‐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 are also present. Oxygen has a slight inhibiting effect on vulcanization; hydrogen sulphide and mercaptans markedly catalyse the vulcanization reaction without, apparently, affecting the efficiency of the sulphur cross‐linking reaction.In confirmation of the results of Hull, Olsen, and France1, a small proportion of zinc oxide or a soluble zinc soap promotes a reaction between sulphur and mercapto groups whereby di‐ and poly‐sulphides arc formed with liberation of hydrogen sulphide. The same type of activity is shown by some of the nitrogenous accelerators commonly used in rubber vulcanization. Substantial conversion of mercapto groups into polysulphidc 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. I. Nature of the reaction between sulfur and olefins

AbstractWhatever additional factors may be concerned in the production of good vulcanizates from natural rubber by the action of sulfur, there is no doubt that the action of sulfur on olefinic materials in general at the ordinary vulcanization temperature is a chemical one, the primary course of which is determined largely by the constitution of the olefin, and hence may be profitably studied by experimentation with olefins of different unsaturation patterns. Unaccelerated reaction between sulfur and simple monoolefins leads almost exclusively to cross linking of the separate olefin molecules, mostly in pairs, by groups of sulfur atoms. When, however, two or more olefinic units occur in the same molecule, as in the disoprenic hydrocarbons, intramolecular cross linking, i.e., cyclization, at once becomes possible, and the result of sulfur action consists partly in the cross linking of separate molecules as with the monoolefins, but largely in cyclization of the individual olefinic chains, thereby forming sulfur‐containing rings. These changes entail a certain loss of unsaturation, from which deductions can be made as to the mechanism of the reaction.

Rubber, Polyisoprenes and Allied Compounds. VIII. The Formation of Dialkyl Sulfide Dihalides, and Its Bearing on the Problem of Determining the Unsaturation of Vulcanized Rubber

Abstract In determining the unsaturation of olefinic sulfides and disulfides by halogenation methods, the addition of halogen to the sulfur atom must be taken into account. The dihalides of dialkyl or dialkenyl sulfides react with aqueous potassium iodide partly to liberate iodine with regeneration of the sulfide, and partly to form halogen acid and dialkyl or dialkenyl sulfoxides. Correction for the latter reaction, which occurs extensively in the presence of alcohol or acetic acid, must be applied before reliable unsaturation values for dialkenyl sulfides can be obtained. With vulcanized rubber and iodine monochloride, the precise magnitude of the correction to be applied is uncertain, but this uncertainty can be removed by using bromine as the additive reagent. Simple rubber-sulfur vulcanizates show a loss of unsaturation corresponding to one double bond per sulfur atom combined up to at least 8 per cent of combined sulfur; vulcanizates accelerated with mercaptobenzothiazole in the presence of zinc oxide exhibit a substantially smaller loss of unsaturation. The dibromides of dialkyl sulfides readily yield a molecule of bromine to olefins, the reaction being practically exclusively additive.

Sulfur Linkage in Vulcanized Rubbers. I. Reaction of Methyl Iodide with Sulfur Compounds

Abstract For many years the rubber chemist, in trying to explain the chemical nature of the vulcanization of rubber, was limited by two accepted theories: (1) Sulfur adds at the double bonds of the hydrocarbon, and (2) one atom of sulfur combines for each double bond saturated in the reaction. Therefore it was concluded that only one type of sulfur linkage would occur in vulcanizates. However, it was impossible on this basis to propose a hypothesis to account for all the facts. The few bits of direct evidence available have semed to support the theory of “one sulfur for each double bond saturated”. Since it is difficult to make any direct attack on this problem and since the art of vulcanization has far surpassed the science, it was not until 1938 that grave doubt was thrown on the generality of the “one sulfur atom per double bond saturated” theorem. The work of Brown and Hauser indicated that there were wide variations in the number of sulfur atoms combined for each double bond saturated, depending on the type of acceleration used. Four years later Thornhill and Smith pointed out that, in compounds containing reinforcing channel blacks, no loss of unsaturation could be detected on combination of sulfur during vulcanization. Thus the relation between sulfur combination and changes in unsaturation is at present obscure.

Effect of Reinforcing Pigments on Rubber Hydrocarbon

Abstract Several investigators have established that sulfur vulcanization of rubber involves chemical combination of sulfur and rubber hydrocarbon. A definite decrease in unsaturation of the rubber hydrocarbon as vulcanization progresses has been generally noted. While it has often been concluded that a double bond is saturated for each atomic equivalent of combined sulfur, recent work by Brown and Hauserdemonstrates that this conclusion cannot be applied in all cases. In certain compounds they found the loss in unsaturation with extent of vulcanization to be considerably less than anticipated on the basis above. Their results indicate that stocks reaching optimum cure with the least loss of unsaturation possess the greatest tensile strength. Although a considerable amount of work has been done on this problem, we have not found a published account of similar investigations performed on stocks containing significant loadings of reinforcing fillers. Since, as pointed out below, the nature of the bonding between such fillers and the rubber molecules has not been clearly defined, one is not justified in applying previous results obtained on stocks containing no reinforcing fillers to those bearing appreciable loadings of such substances. Accordingly one portion of the present investigation was concerned with determining the effect of various fillers on the course of sulfur vulcanization, as judged from combined sulfur and unsaturation values. As pointed out by Gehman and Field it is undoubtedly true that the black particle in a carbon-black-reinforced rubber stock is firmly attached to the rubber molecule. The nature of the bonding between the black and rubber has not been clearly defined. Some investigators maintain that the association is physical and involves definite forces of adhesion or adsorption; others have suggested the formation of primary valence linkages with rubber hydrocarbon. The opinion of the present authors is that if such linkages are formed, the ethylenic bonds of the rubber molecule would probably be involved. If this latter view is correct, then a specific loss in unsaturation of rubber hydrocarbon, due to the reinforcing filler, should occur. Thus the second objective of the present study was to determine whether it was possible by chemical means to detect such a linkage. If measurable, this effect, together with the surface area determinations reported previously, would be particularly valuable in estimating the reinforcing value of various fillers.

Structural Studies of the Vulcanization of Rubber under Stretch

Abstract Just one hundred years have passed since the changes were discovered which rubber undergoes when mixed with sulfur and heated. Although “vulcanization”, as this reaction and the changes subsequent thereto have been termed, must be considered the most important development in the rubber industry, we are still far from being in a position to offer a fully satisfactory explanation for this phenomenon. Nevertheless we have been able to improve our concept of these changes by the application of such modern methods of research as x-ray diffraction, and recent attempts to shed more light on the phenomenon from a strictly chemical point of view also have been successful. The outstanding results of the latter work are the definite proof that the changes in physical properties, which become noticeable during vulcanization, show no correlation with the amount of combined sulfur or with the total loss in unsaturation. The experiments offered no direct experimental evidence as to the existence of specific chemical bonds or linkages in vulcanized rubber. However, it could be definitely ascertained that chemical reactions involving the double bonds of the hydrocarbon and its combination with the vulcanizing agent, in one way or another, are necessary to produce a vulcanized structure responsible for the observed changes in properties.

Chemical Reactions during Vulcanization

Abstract Direct addition or bridging of sulfur seems to occur at the double bonds of the rubber in sulfur vulcanization without accelerators (Figure 5, a and b). In accelerated stocks the fact that sulfur combines in excess of one atom of sulfur saturating one double bond of the rubber hydrocarbon suggests that sulfur may also combine in a dehydrogenation reaction (Figure 5, c), and that this type of sulfur linkage is most effective in producing high tensile strengths. The excess loss in unsaturation on overcure indicates that direct polymerization without sulfur (Figure 5, d), as well as reactions of oxygen at the double bonds, may occur after most of the sulfur has combined, but that these chemical changes are not definitely associated with changes in the vulcanized structure as measured by tensile strength. In the vulcanization of a simple rubber-sulfur compound, infrared absorption data indicate that oxygen is combining with the rubber during overcure to produce a degradation in the hydrocarbon structure which is accompanied by a decided loss in tensile strength.