Studies of the Heat Deterioration of Specially Prepared Natural Rubber Vulcanizates

1954 ◽  
Vol 27 (2) ◽  
pp. 459-467
Author(s):  
Donald J. Metz ◽  
Robert B. Mesrobian
Keyword(s):  
Natural Rubber ◽  
Elevated Temperatures ◽  
Experimental Methods ◽  
Oxygen Absorption ◽  
Cross Linking ◽  
Vulcanized Rubber ◽  
Chemical Agents ◽  
Complex Process ◽  
Cross Linkage ◽  
Natural Rubber Vulcanizates

Abstract In 1949, Flory, Rabjohn, and Shaffer presented an article describing the dependence of the elastic properties of vulcanized rubber on the degree of cross-linking. In order to prepare rubber vulcanizates characterized by known degrees of cross-linkage recently developed, disazodicarboxylate vulcanizing reagents were employed. These reagents react quantitatively with rubber, one cross-linkage being introduced for each molecule of the reagent. Whereas the conventional vulcanization of rubber with sulfur and accelerators is a complex process and it does not appear possible at the present time to specify the exact number of cross-linkages present in sulfur vulcanizates, the use of disazodicarboxylate vulcanizing reagents presents an opportunity to investigate the changes that occur in the physical properties of vulcanized rubber, characterized by known degrees of cross-linkage, on exposure to oxygen at elevated temperatures. Furthermore, the effect of heat deterioration of various chemical agents commonly employed in sulfur vulcanization recipes may be investigated in a unique way by incorporating such chemical agents in the free state into natural rubber previously vulcanized by disazodicarboxylate reagents. Recent studies of the deterioration of rubber vulcanizates at elevated temperatures have been reported by several workers. In this article, the experimental methods employed to study heat deterioration involve measurements of oxygen absorption, stress relaxation, and changes of 100 per cent modulus of natural rubber vulcanized to known extents of cross-linkage by decamethylene-dismethyl azodicarboxylate.

Oxidation of GR-S and Other Elastomers

1947 ◽  
Vol 20 (3) ◽  
pp. 747-759
Author(s):  
John O. Cole ◽  
James E. Field
Keyword(s):  
Tensile Strength ◽  
Natural Rubber ◽  
Physical Properties ◽  
Experimental Methods ◽  
Chain Scission ◽  
Cross Linking ◽  
Antioxidant Action ◽  
Mechanism Of Oxidation ◽  
Natural Rubber Vulcanizates

Abstract The effect of heat aging on the physical properties of an elastomer is generally considered the result of oxidation, which produces both chain scission and cross-linking in the polymer. Early in the development of GR-S, a marked difference in the aging of GR-S and natural rubber vulcanizates was observed. From the effect of aging on hardness, tensile strength, modulus, and elongation it appeared that cross-linking occurred more rapidly than chain scission with GR-S, but the reverse was true with natural rubber. The work reported here was undertaken to provide a better understanding of the differences in aging of GR-S and natural rubber and to introduce new experimental methods for studying the mechanism of oxidation and antioxidant action in elastomers.

Aging of Natural Rubber Vulcanizates in the Presence of Dithiocarbamates

1959 ◽  
Vol 32 (3) ◽  
pp. 739-747 ◽  
Author(s):  
J. R. Dunn ◽  
J. Scanlan
Keyword(s):  
Stress Relaxation ◽  
Natural Rubber ◽  
Protective Action ◽  
Dicumyl Peroxide ◽  
Vulcanized Rubber ◽  
Aging Properties ◽  
Photochemical Aging ◽  
Natural Rubber Vulcanizates

Abstract The thermal and photochemical aging of extracted dicumyl peroxide-, TMTD (sulfurless)- and santocure-vulcanized rubber, in presence of a number of metal and alkylammonium dithiocarbamates, has been investigated by measurements of stress relaxation. The dithiocarbamates have a considerable protective action upon the degradation of peroxide- and TMTD-vulcanizates, but they accelerate stress decay in santocure-accelerated vulcanizates. The reasons for this behavior are discussed. It is suggested that the excellent aging properties of unextracted TMTD vulcanizates are due to the presence of zinc dimethyldithiocarbamate formed during vulcanization.

Relaxation Processes in Vulcanized Rubber. II. Secondary Relaxation Due to Network Breakdown

1963 ◽  
Vol 36 (2) ◽  
pp. 389-398 ◽  
Author(s):  
A. N. Gent
Keyword(s):  
Molecular Structure ◽  
Experimental Study ◽  
Natural Rubber ◽  
Relaxation Process ◽  
Relaxation Processes ◽  
Secondary Relaxation ◽  
Oxidative Deterioration ◽  
Vulcanized Rubber ◽  
Oxidation Inhibitors ◽  
Natural Rubber Vulcanizates

Abstract An experimental study is described of a “secondary” relaxation process in stretched vulcanizates, which becomes dominant after long periods at normal temperatures. It is shown to be affected markedly by the temperature, the atmosphere in which the test is conducted, and the presence of oxidation inhibitors. It is therefore attributed to oxidative deterioration of the molecular structure. In some vulcanizates, however, a similar or even greater relaxation is found to occur in vacuo, and this is attributed to the failure of specific crosslink structures. The extent of recovery on releasing the extended testpieces has also been investigated for a number of natural rubber vulcanizates.

Rubber Oxidation and Aging Studies Safe Limits of Sample Thickness

1951 ◽  
Vol 24 (4) ◽  
pp. 999-1016
Author(s):  
George W. Blum ◽  
J. Reid Shelton ◽  
Hugh Winn
Keyword(s):  
Natural Rubber ◽  
Accelerated Aging ◽  
Sample Thickness ◽  
Oxygen Absorption ◽  
Absorption Data ◽  
Synthetic Rubber ◽  
Constant Rate ◽  
Aging Studies ◽  
Safe Limits ◽  
Natural Rubber Vulcanizates

Abstract Safe limits of sample thickness for rubber oxidation and aging studies, such that the chemical reaction rather than the rate of diffusion will be rate-controlling have been investigated for natural-rubber vulcanizates and for four synthetic-rubber types. For studies involving the entire range of oxidation, including the autocatalytic stage of rapid oxygen absorption, the conventional 0.075-inch thickness is frequently not satisfactory for accelerated aging and oxidation studies if it is desired to avoid limitation by diffusion. Only in the GR-S black stock was this thickness found to be satisfactory up to a temperature of 100° C. The other stocks, including natural rubber, Butaprene-NXM, and Neoprene black and gum stocks all require thinner samples to ensure that the observed rate of oxygen absorption is free of limitation by diffusion. A method of calculating the probable limiting value of sample thickness, above which the rate of oxidation in the autocatalytic stage is limited by diffusion, has been developed on the basis of volumetric oxygen absorption data obtained with GR-S. The method has also been applied to natural-rubber vulcanizates and to other synthetic-rubber types to locate the approximate limiting values at various temperatures for oxidation and aging studies which extend into the autocatalytic stage of rapid reaction. The constant-rate period of oxidation is more important from a practical point of view than the autocatalytic stage, since properties are so seriously degraded as to make the rubber of little value before it reaches the final stage of rapid oxidation. Somewhat thicker samples may be used for studies that are confined to the earlier stages of oxidation. A 0.075-inch sample is free of limitation by diffusion in the constant-rate stage in the following cases: GR-S black and gum stocks at 110° C; Hevea black with added antioxidant at 100° C; and uninhibited Hevea black and gum stocks at 60° C. A 0.040-inch sample is satisfactory in this range for: uninhibited Hevea black at 100° and gum at 80° C; Butaprene-NXM black at 100° and gum at 90° C; and Neoprene black and gum stocks at 100° C.

Studies of the Vulcanization of Rubber

1952 ◽  
Vol 25 (2) ◽  
pp. 209-229 ◽  
Author(s):  
Shu Kambara ◽  
Kumakazu Ohkita
Keyword(s):  
Organic Compounds ◽  
Chemical Structure ◽  
Room Temperature ◽  
Great Influence ◽  
Cross Linking ◽  
Oxidizing Agent ◽  
Vulcanized Rubber ◽  
Cross Linkage ◽  
Bridge Type ◽  
The Difference

Abstract In this study much information about the method of distinguishing the state in which sulfur is combined in simple organic compounds consisting of carbon, hydrogen, and sulfur was obtained, and a new theory of vulcanization was postulated as a result of its application to vulcanized rubber. When activated sulfur reacts with rubber, it first adds to the double bonds, forming thioketones, which in turn, as a characteristic of these radicals, combine with each other, with the formation of a thioether structure. This transformation of thioketone into thioether takes place, not only during vulcanization, but also gradually after vulcanization. Because of the presence of thioketone, treatment of vulcanized rubber with hydrazine, forms a new network, that is, a ketoazine cross-linkage. Combined sulfur of the thioketone type was determined by an oxidizing agent, and as the difference of this value and total combined sulfur a method of determining bridge type of combined sulfur has been proposed. By this method, it was found that, even in ebonite, about one-third of the combined sulfur is the thioketone type, and that the bridge type is only about two-thirds of the total. The thioketone type of combined sulfur in soft vulcanized rubber is transformed gradually into the thioether type of cross-linkage when allowed to stand at room temperature, and this transformation is accelerated when the temperature is raised. In the case of hard rubber, this phenomenon is also observable, but the rate of this transformation is much slower compared to the former. This tendency is the same in the case of ketoazine cross-linking when rubber vulcanizates are treated with hydrazine. From these facts, it seems that the distribution of the thioketone radicals is not uniform, and the magnitude of the probability for collision of these radicals to form cross-linkages has a great influence on the properties of rubber after vulcanization. That is, the property of the vulcanizate is greatly affected by the fact whether the thioketone radicals in the vulcanizates are comparatively uniformly distributed or whether they exist in sectional groups or in colonies. The authors are the first to advance this postulate concerning the chemical structure of vulcanized rubber and its transformation. We believe that when the study is extended, using this postulation, problems such as aging and the differences in the properties of vulcanized rubber accelerated with various accelerators will become clear. Moreover, we believe that it will be of interest to physicists studying rubber elasticity to suggest this idea of colony of cross-linkages. We are now carrying on researches on these problems, and we shall report on them later.

Degradation of Polyisoprene Networks by Oxygen

1960 ◽  
Vol 33 (1) ◽  
pp. 51-59 ◽  
Author(s):  
E. M. Bevilacqua
Keyword(s):  
Molecular Weight ◽  
Natural Rubber ◽  
Present Report ◽  
Hydrocarbon Chain ◽  
Gas Absorption ◽  
Oxygen Absorption ◽  
Low Molecular Weight ◽  
Oxidation Reactions ◽  
Vulcanized Rubber ◽  
Mechanism Of Oxidation

Abstract The insolubility of vulcanized rubber and the relatively low concentration at which oxidation reactions completely degrade the vulcanizate have made it necessary that indirect methods be used to study the reaction of molecular oxygen with rubber. The literature contains many reports of measurements of the absorption of oxygen by vulcanized rubber, which is easy and convenient experimentally. In some of these the rate of oxygen absorption has been the only property measured. This alone gives little information about the mechanism of oxidation, although the fact that the general form of curves of oxygen absorption as a function of time can be predicted by equations based on the known mechanism of oxidation of low molecular weight olefins is indirect support for similar mechanisms of oxidation of polymers. The technologically important reaction accompanying oxidation of natural rubber is scission, not detected by gas absorption measurements. It has been studied principally by determining the decay in stress of a sample at constant strain. The results suggest that scission occurs by a first order process at selected sites in the network. This has been interpreted to mean that crosslinks are the primary locus of oxidation in vulcanized rubber. Estimates of the amount of oxygen required for breaking a bond are available only for samples vulcanized with sulfur. From these it may be calculated that initially 4– 5 moles of oxygen is required per scission, when allowance is made for the effect of entanglements on stress. A marked difference between the relaxation of peroxide-cured and sulfur-cured samples has been reported. Horikx has made an extensive investigation of the solubility and swelling of oxidized vulcanized rubber. His results show that the hydrocarbon chain must be broken during oxidation. The mechanism of scission of unvulcanized rubber has been determined; in this work it was found that low molecular weight products are an important index of scission reactions. The present report describes preliminary work on the scission mechanism in vulcanized natural rubber which has two objectives; to repeat Horikx's experiments with vulcanizates incapable of further cure, and to determine whether low molecular weight products accompany the scission reaction.

Antioxidant Efficiency of p-Phenylenediamines in Natural Rubber Vulcanizates

1961 ◽  
Vol 34 (3) ◽  
pp. 816-833 ◽  
Author(s):  
O. Lorenz ◽  
C. R. Parks
Keyword(s):  
Antioxidant Activity ◽  
Natural Rubber ◽  
Oxidation Products ◽  
Oxygen Absorption ◽  
Fast Reaction ◽  
Alkyl Aryl ◽  
Antioxidant Efficiency ◽  
Rate Region ◽  
Constant Rate ◽  
Natural Rubber Vulcanizates

Abstract The consumption of various p-phenylenediamines during the oxidation of natural rubber vulcanizates has been investigated in the temperature range of 80–120° C. Oxygen absorption was used to follow the oxidation. Diaryl-p-phenylenediamines were consumed mainly in a termination reaction during the constant rate region of the oxygen absorption, where five to six molecules of oxygen were absorbed per molecule of diamine consumed. The constant rate stage of the oxygen uptake was found to be about twice as long as the time of the constant rate consumption of the diamine, indicating that the reaction product also possessed antioxidant activity. The effect of the nature of the curing system, carbon black, structure of the diamine, initial concentration of the diamine, and temperature of oxidation were studied. Dialkyl- and alkyl-aryl-p-phenylenediamines were consumed predominantly by a direct, relatively fast reaction with oxygen, the rate being dependent on the structure of the diamine and also on the nature of the curing system. The oxidation products formed exhibited antioxidant activity, those of N-isopropyl-N′-phenyl-p-phenylenediamine being particularly effective.

CURE CHARACTERISTICS OF DEVULCANIZED RUBBER:THE ISSUE OF LOW SCORCH

2017 ◽  
Vol 90 (3) ◽  
pp. 536-549 ◽  
Author(s):  
Anu Mary Joseph ◽  
Benny George ◽  
K. N. Madhusoodanan ◽  
Rosamma Alex
Keyword(s):  
Solvent Extraction ◽  
Natural Rubber ◽  
Main Chain ◽  
Process Safety ◽  
Original Sample ◽  
Vulcanized Rubber ◽  
Sulfur Vulcanization ◽  
Natural Rubber Vulcanizates ◽  
Devulcanized Rubber

ABSTRACT We investigate the reasons behind the observed low scorch during the revulcanization of devulcanized rubber. Mechanically devulcanized carbon black filled natural rubber vulcanizates originally cured by conventional vulcanization (CV), semiefficient vulcanization (semi EV), efficient vulcanization (EV), and peroxide systems as well as buffing dust obtained from pre-cured tread with known formulation were used. Revulcanization of these devulcanized samples using sulfur/sulfonamide system led to the following observations; irrespective of the type of sulfur cure system used for the initial vulcanization of the rubber, (i) the devulcanized samples cured without pre-vulcanization induction time and (ii) devulcanized samples prepared from peroxide vulcanized rubber cured with scorch safety. Based on the earlier reports that solvent extraction of devulcanized rubber did not improve the scorch time during revulcanization, the role of zinc bound non-extractable moieties was investigated using devulcanized rubber prepared from activator-free vulcanizates, which disproved the role of such moieties. This confirmed that the scorch reducing moieties should be attached to the rubber main chain, which can be unreacted crosslink precursors and cyclic sulfides left after the initial accelerated sulfur vulcanization of the original sample. The ability of pre-vulcanization inhibitor to induce scorch safety when devulcanized rubber is revulcanized as such, without adding any virgin rubber, proved that mercaptobenzothiazole (MBT) generated from crosslink precursors is the cause of low scorch. Acetone extracted devulcanized rubber samples prepared from tetramethyl thiuramdisulfide (TMTD) cured natural rubber, which does not follow the MBT pathway when revulcanized, cured with scorch safety, which further proved the role of MBT. Based on the previous reports and our results, it is obvious that powdering of rubber vulcanizate and devulcanization processes have no role on the low process safety of these materials, but it is inherent to the initial accelerated sulfur vulcanization chemistry undergone by these materials.

Smearing of Vulcanized Rubber

1955 ◽  
Vol 28 (2) ◽  
pp. 508-518 ◽  
Author(s):  
S. D. Gehman ◽  
C. S. Wilkinson ◽  
R. D. Daniels
Keyword(s):  
Melting Point ◽  
Thermal Degradation ◽  
Natural Rubber ◽  
Sulfur Compound ◽  
Important Variable ◽  
Cross Linking ◽  
Curing Agent ◽  
Point Bar ◽  
Tetramethylthiuram Disulfide ◽  
Vulcanized Rubber

Abstract The surface heating which occurs at the interface of rubber sliding under a load may be part of the mechanism of abrasion, especially under severe conditions. Removal of rubber by thermal degradation and a smearing process occurs if the rubber attains sufficiently high localized temperatures. A procedure, using a melting point bar, was developed for measuring the temperature at which smearing occurred for rubber vulcanizates. Smear points reproducible to about ±2° F were measured. The effect of compounding variables on the smear point was investigated. The most important variable in this category was the vulcanization system, probably inasmuch as it determined the type of cross-linking. The presence of free sulfur within the rubber also tended to increase the smear temperature. Highest smear points were obtained with mercaptobenzothiazole-tetramethylthiuram disulfide mixtures, and for a non-sulfur compound using p-quinone-dioxime as the curing agent. The highest smear point observed for natural rubber was 475° F, obtained with this system. GR-S tread compounds showed no smearing even at 560° F, which was as high as could be obtained with the apparatus used. The resistance to smearing of GR-S may be an important factor in explaining its superiority in road wear under severe conditions. Laboratory abrasion experiments were performed to illustrate the effect of smearing on the relative abrasion loss of GR-S and natural-rubber tread compounds.

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