Aging of Natural Rubber Vulcanizates in the Presence of Dithiocarbamates

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.

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Stress Relaxation during the Photoxidation of Peroxide Crosslinked Rubber

Rubber Chemistry and Technology ◽

10.5254/1.3542161 ◽

1960 ◽

Vol 33 (2) ◽

pp. 433-444

Author(s):

J. R. Dunn ◽

J. Scanlan ◽

W. F. Watson

Keyword(s):

Stress Relaxation ◽

Natural Rubber ◽

Physical Properties ◽

Chemical Reactions ◽

Small Extent ◽

Organic Peroxides ◽

Physical Measurement ◽

Crosslinked Networks ◽

Photochemical Aging ◽

Natural Rubber Vulcanizates

Abstract The chemical reactions involved in the thermal and photochemical aging of natural rubber vulcanizates are largely unknown. Experimental difficulties have precluded direct chemical investigation owing to the insolubility of the crosslinked networks and the small extent of reaction required for a great deterioration in physical properties. Accordingly recourse has been made to physical measurement. Tobolsky et al. (e.g. Ref. (1)) have shown that the relaxation in stress on holding a rubber strip at constant extension during aging is a convenient experimental measure capable of interpretation in terms of network breakdown. In photochemical studies the opacity of conventional sulfur vulcanizates provides a further complication. In the present work, the stress relaxation technique has been used in a study of the degradation by 365 mµ radiation of the comparatively transparent and chemically simple rubber networks obtained after crosslinking by organic peroxides.

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The Oxidative Aging in Ultraviolet Light of Dicumyl Peroxide Vulcanizates of Natural Rubber in Presence of Various Antioxidants

Rubber Chemistry and Technology ◽

10.5254/1.3540238 ◽

1961 ◽

Vol 34 (2) ◽

pp. 686-696 ◽

Cited By ~ 3

Author(s):

J. R. Dunn

Keyword(s):

Natural Rubber ◽

Ultraviolet Light ◽

Protective Action ◽

Phenolic Antioxidants ◽

Dicumyl Peroxide ◽

Oxidative Aging ◽

Natural Rubber Vulcanizates ◽

Accelerate Degradation

Abstract The oxidative aging of natural rubber vulcanizates at 25° C under irradiation by ultraviolet light of wavelength 365 mµ has been described in a number of communications from this laboratory. At this wavelength naphthylamines were shown to accelerate degradation of peroxide vulcanizates while substituted p-phenylenediamines inhibited it; α-naphthol and a variety of dithiocarbamates were also found to have a protective action. Since some amines and phenolic antioxidants do not absorb light of wavelength 365 mµ but do absorb in the region of 313 mµ, investigations have now been made of the effect of a large number of additives on aging at the shorter wavelength, so that predictions might be made of the most suitable materials to employ in protecting unfilled vulcanizates against crazing in sunlight.

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Relative Contributions of Viscoelasticity and Aging to the Relaxation of Rubber Vulcanizates

Rubber Chemistry and Technology ◽

10.5254/1.3539551 ◽

1963 ◽

Vol 36 (1) ◽

pp. 50-58 ◽

Cited By ~ 1

Author(s):

P. Thirion ◽

R. Chasset

Keyword(s):

Quantitative Analysis ◽

Natural Rubber ◽

Time Interval ◽

Dicumyl Peroxide ◽

Infinite Time ◽

Negative Power ◽

Theoretical Assumptions ◽

The Difference ◽

Natural Rubber Vulcanizates ◽

Actual Stress

Abstract Relaxation in relatively stable, gum natural rubber vulcanizates has been studied to determine the effects of viscoelasticity and aging, respectively, using a dark, air-oven. A quantitative analysis of experimental results shows that, in the case of a dicumyl peroxide vulcanizate at 100° C, relaxation is caused by aging, except in its initial stages. Stress decreases as a linear function of time, in agreement with theoretical assumptions. Conversely, at 30° C, the effect of aging is negligible. At this temperature the difference between actual stress and stress extrapolated to infinite time, is proportional to a negative power of time. At intermediate temperatures, both phenomena occur simultaneously over a time interval ranging from. 3 minutes to 150 hours.

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

Rubber Chemistry and Technology ◽

10.5254/1.3539566 ◽

1963 ◽

Vol 36 (2) ◽

pp. 389-398 ◽

Cited By ~ 1

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.

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The Crosslinks in TMTD-Zinc Oxide-Natural Rubber Vulcanizates

Rubber Chemistry and Technology ◽

10.5254/1.3542155 ◽

1960 ◽

Vol 33 (2) ◽

pp. 394-397 ◽

Cited By ~ 1

Author(s):

C. G. Moore

Keyword(s):

Zinc Oxide ◽

Natural Rubber ◽

Appreciable Amount ◽

Dicumyl Peroxide ◽

Swelling Properties ◽

Tetramethylthiuram Disulfide ◽

Chemical Studies ◽

The Subject ◽

Natural Rubber Vulcanizates ◽

Recent Experimental Work

Abstract The mechanism of vulcanization of natural rubber (NR) by means of tetramethylthiuram disulfide (TMTD) and zinc oxide has been the subject of much recent experimental work and speculation. While such studies have clarified the kinetics and stoichiometry of TMTD decomposition and zinc dimethyldithiocarbamate formation, they have not directly aided our knowledge of the nature of the crosslinks in the resultant vulcanizate. It was earlier suggested that the vulcanizate contained only C—C crosslinks, comparable with those formed by means of di-tert-alkyl (and aralkyl) peroxides, while more recently, disulfide crosslinks have been proposed. However, neither of these views has been confirmed by direct chemical studies of the vulcanizate. Evidence is now presented which shows that there can be no appreciable amount of C—C crosslinking by the TMTD-ZnO combination, and this is consistent with the view that sulfur crosslinks predominate. This conclusion is based on the respective swelling properties in n-decane of dicumyl peroxide vulcanizates (containing only C—C crosslinks) and TMTD-ZnO vulcanizates, which have been treated with methyl iodide in vacuo at 80° C. This reagent is known to cause the fission of C—S and S—S bonds in variously constituted organic mono- and polysulfides and should therefore cause the fission of sulfur crosslinks in a vulcanizate with consequent degradation of the network (cf. Ref. 8), whereas there is no evidence that di-allylic C—C crosslinks can be similarly degraded.

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The Determination of Network Chain Density and the Chemical Stress Relaxation of Crosslinked Polymers

Rubber Chemistry and Technology ◽

10.5254/1.3542917 ◽

1973 ◽

Vol 46 (2) ◽

pp. 477-482

Author(s):

Saburo Tamura ◽

Kenkichi Murakami

Keyword(s):

Stress Relaxation ◽

Natural Rubber ◽

Main Chain ◽

Chemical Stress ◽

Dicumyl Peroxide ◽

Constant Independent ◽

Crosslinked Polymers ◽

Network Chain ◽

The Difference ◽

Entanglement Network

Abstract Both initial network chain densities nM(0) and nS(0) of dicumyl peroxide- cured natural rubbers were determined from the tensile stress and swelling method, respectively. The difference between nM(0) and nS(0) was usually constant, independent of the magnitude of network chain density. That is, it was found that the number of entanglement network chains in the crosslinked natural rubber was usually constant, independent of network chain density. The entanglement network chain density nII(0) was 0.7×10−4 mole/cc. This led to the supposition that the molecular weight between entanglement points Me would be about 9000. Although this value is far from exact, it does not differ too greatly from the value found for noncrosslinked natural rubber. Next, in order to calculate the number of main-chain scissions of crosslinked polymers from their chemical stress relaxation, we proposed our modification of Tobolsky's equation. Using our equation, it was found that the scission of dicumyl peroxide-cured natural rubber occurred in the main chain only. Furthermore, this value agreed with the one obtained from the oxidation of toluene solution of noncrosslinked rubber under the same conditions.

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A Note on Stress Relaxation of Rubber Networks in the Vicinity of Room Temperature

Rubber Chemistry and Technology ◽

10.5254/1.3547306 ◽

1970 ◽

Vol 43 (5) ◽

pp. 1036-1039 ◽

Cited By ~ 3

Author(s):

G. Steiner ◽

A. V. Tobolsky

Keyword(s):

Stress Relaxation ◽

Natural Rubber ◽

Room Temperature ◽

Dicumyl Peroxide ◽

Chemical Aging ◽

Relaxation Effects ◽

Physical Phenomena ◽

True Equilibrium

Abstract The question of whether stress relaxation of natural rubber under normal usage at room temperature up to 100° C is caused primarily by chemical aging or physical phenomena connected with reversible changes is one which has been extensively studied. Thirion and Chasset investigated the relative effects of network relaxation and aging for dicumyl peroxide cured natural rubber in air and found network relaxation effects predominating below 55° C with aging becoming increasingly important at higher temperatures and longer times. In this study samples of natural rubber, cured by dicumyl peroxide, were relaxed both in air and in vacuum in an attempt to elucidate further the phenomenon of network relaxation. It was concluded that stress relaxation between 25° C and 100° C is much smaller in vacuum than in air, and that true equilibrium stresses are rapidly reached in vacuum.

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CURE CHARACTERISTICS OF DEVULCANIZED RUBBER:THE ISSUE OF LOW SCORCH

Rubber Chemistry and Technology ◽

10.5254/rct.17.83737 ◽

2017 ◽

Vol 90 (3) ◽

pp. 536-549 ◽

Cited By ~ 2

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.

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