The Crosslinks in TMTD-Zinc Oxide-Natural Rubber Vulcanizates

1960 ◽  
Vol 33 (2) ◽  
pp. 394-397 ◽  
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.

Effect of Sulfur and Dicumyl Peroxide on Vulcanization of Natural Rubber with Tetramethylthiuram Disulfide and Zinc Oxide

1970 ◽  
Vol 43 (6) ◽  
pp. 1294-1310 ◽  
Author(s):  
S. P. Manik ◽  
S. Banerjee
Keyword(s):  
Zinc Oxide ◽  
Stearic Acid ◽  
Natural Rubber ◽  
Reaction Mechanisms ◽  
Elemental Sulfur ◽  
Initial Rate ◽  
Dicumyl Peroxide ◽  
Tetramethylthiuram Disulfide ◽  
Sulfur Vulcanization ◽  
Salient Features

Abstract The salient features of both non-elemental sulfur vulcanization by TMTD and elemental sulfur vulcanization promoted by TMTD both in presence and absence of ZnO and stearic acid have been studied. TMTD increases the rate of DCP decomposition and lowers the crosslinking maxima due to DCP depending on its concentration. However, with higher amounts of TMTD the initial rate of crosslinking is increased with the increased amount of TMTD, while crosslinking maxima are still lowered due to reversion. ZnO or ZnO-stearic acid, however, seems to alter the entire course of the reaction. Both the crosslink formation and TMTD decomposition are much higher in presence of ZnO or ZnO-stearic acid, but stearic acid seems to have no effect. The reaction mechanisms for TMTD accelerated sulfuration in absence and presence of ZnO have also been studied.

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.

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

1963 ◽  
Vol 36 (1) ◽  
pp. 50-58 ◽  
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.

Investigations in the Field of Rubber Vulcanization. VII. Influence of Organic Accelerators on the Kinetics of Vulcanization and the Properties of Natural-Rubber Vulcanizates

1950 ◽  
Vol 23 (3) ◽  
pp. 563-575
Author(s):  
B. Dogadkin ◽  
B. Karmin ◽  
A. Dobromyslova ◽  
L. Sapozhkova
Keyword(s):  
Activation Energy ◽  
Natural Rubber ◽  
Oxidative Destruction ◽  
Tetramethylthiuram Disulfide ◽  
Vulcanization Kinetics ◽  
Strength Curve ◽  
Increasing Temperature ◽  
Low Sulfur ◽  
Natural Rubber Vulcanizates ◽  
Kinetics Of

Abstract 1. Vulcanization accelerators change all parameters of the kinetic strength curve during the vulcanization of natural-rubber mixtures with low sulfur contents. 2. Calculation of the kinetic constants of the fundamental vulcanization equation proposed by Dogadkin, Karmin, and Gol'berg shows that vulcanization accelerators affect both the kinetics of the interaction of rubber with sulfur and the kinetics of the interaction of rubber with oxygen. 3. Direct experiments on the oxidation of rubber have shown that tetramethylthiuram disulfide and diphenylguanidine retard the process of addition of oxygen to rubber, while mercaptobenzothiazole accelerates this process. 4. Data on the rate of plasticization and change in viscosity of rubber solutions during oxidation indicate that tetramethylthiuram disulfide and diphenylguanidine promote the disintegration of molecular chains of rubber during the oxidative destruction of the latter. 5. The activation energy of the process of oxidation of rubber in the presence of mercaptobenzothiazole corresponds to the activation energy calculated from the fundamental vulcanization reaction for the process of oxidative destruction. This provides additional proof of the participation of oxygen in the vulcanization process. 6. It has been established with the aid of the methyl iodide reaction that accelerators increase the bridge-sulfur content of the vulcanizate, which is present in the form of monosulfides, with one sulfur atom connected to an allyl type radical. 7. With increasing temperature, the tensile strength at the vulcanization optimum increases in mixtures containing tetramethylthiuram disulfide, decreases in mixtures containing mercaptobenzothiazole, and remains unchanged in mixtures containing diphenylguanidine. The limiting strength decreases in all cases with increasing temperature. This phenomenon is explained on the basis of the proposed concepts of the character of vulcanization kinetics and of the nature of the vulcanization optimum.

Acceleration by Thiourea and Related Compounds of the Tetramethylthiuram Disulfide Vulcanization of Natural Rubber

1961 ◽  
Vol 34 (3) ◽  
pp. 795-797 ◽  
Author(s):  
C. G. Moore ◽  
B. Saville ◽  
A. A. Watson
Keyword(s):  
Zinc Oxide ◽  
Natural Rubber ◽  
Tetramethylthiuram Disulfide ◽  
Reasonable Explanation ◽  
Vulcanization Process ◽  
Related Compounds ◽  
Curing Systems

Abstract It has recently been discovered that thiourea and certain of its N-derivatives and chemically related compounds, accelerate the vulcanization of natural rubber (NR) by tetramethylthiuram disulfide (TMTD)-zinc oxide combinations to such an extent that excellent vulcanizates can be produced at 100° C or lower. A reasonable explanation of this acceleration is now offered, based on a theory of vulcanization by TMTI) and related “sulfurless” curing systems currently being developed here. This theory recognizes the importance of polysulfidic intermediates (I, m>2), formed during the vulcanization process, which subsequently react with the polyisoprene to yield further intermediates (II) which finally react to yield sulfurated crosslinks. The process is broadly schematized as follows:

Metal Oxides in Tetramethylthiuram Disulfide Vulcanization

1960 ◽  
Vol 33 (2) ◽  
pp. 412-415 ◽  
Author(s):  
B. A. Dogadkin ◽  
V. A. Shershnev
Keyword(s):  
Zinc Oxide ◽  
Natural Rubber ◽  
Metal Oxides ◽  
Significant Influence ◽  
Special Interest ◽  
Actual Practice ◽  
Tetramethylthiuram Disulfide ◽  
Cold Acetone ◽  
The One ◽  
Isoprene Rubber

Abstract Rubber is usually vulcanized with the aid of the so-called activators, metal oxides, zinc oxide being the one most often used. In vulcanization in the presence of MBT (mercaptobenzothiazole) or DPG (diphenylguanidine) as accelerators it was found that vulcanization activators have almost no effect on the rate of addition of sulfur to rubber, but have a significant influence on the rate and degree of crosslinking of the rubber molecules. Special interest attaches to studies of the action of metal oxides in vulcanization with tetramethylthiuram disulfide (TMTD), as it is known from actual practice that in the absence of zinc oxide this accelerator does not bring about vulcanization. Vulcanization with TMTD was studied on mixtures of natural rubber (extracted with cold acetone in a stream of nitrogen for 50 hours) and of synthetic isoprene rubber (SKI) masticated on microrolls, of the following compositions (in parts by weight).

Contribution to the Mechanism of Devulcanization

1957 ◽  
Vol 30 (1) ◽  
pp. 87-92 ◽  
Author(s):  
W. E. Stafford ◽  
R. A. Wright ◽  
D. Sargent
Keyword(s):  
Thermal Stability ◽  
Thermal Degradation ◽  
Natural Rubber ◽  
Zinc Sulfide ◽  
Thermal Conditions ◽  
Chloroform Extract ◽  
Similar Type ◽  
Tetramethylthiuram Disulfide ◽  
Natural Rubber Vulcanizates ◽  
Free Sulfur

Abstract (1) Three types of natural-rubber vulcanizates have been treated under specific reclaiming conditions. The resultant products have been examined particularly for the manner of sulfur combination and for chloroform extract. The normally accepted, non-reverting tetramethylthiuram disulfide type stock was found to be by far the most amenable to plasticization by thermal agencies. This observation is more or less in line with conclusions of van Amerongen, drawn from a recent study of the oxidative and non-oxidative thermal degradation of rubber. A natural pure-gum type, cured with tetramethylthiuram disulfide only, when heated in the absence of oxygen, at temperatures up to 175° C, was found to revert much more than a similar type cured with 1.0 per cent Santocure and 2 per cent sulfur. Swelling in benzene, after heating 3 days at 150° C, increased considerably with the former, whereas with the latter it remained unchanged. This greater “devulcanizing tendency” is explained on the basis of the tetramethylthiuram disulfide type having comparatively few crosslinks and no free sulfur to form additional ones, hence, “… if any links are broken down, a seemingly unvulcanized rubber results. …” The access of air under the reclaiming conditions used in our work would certainly be limited, and the highly plastic stocks obtained are to be expected in the light of the reference cited. Finally, van Amerongen concludes that in the absence of oxygen, it is inadvisable to use tetramethylthiuram disulfide vulcanizates where thermal stability is important. Mercaptobenzothiazole and rubber/sulfur types did not show any very significant differences in plasticization. (2) Considerable increases in zinc sulfide resulted in all cases, even when free sulfur was virtually removed before reclaiming, and in the latter instance there appears to be a reduction in rubber combined sulfur. In general, the changes are in line with those reported elsewhere in the reversion of natural and Butyl vulcanizates. (3) In all reclaims made, the ratio of sulfur in the chloroform-insoluble rubber to that in the soluble portion is very much lower than that previously published elsewhere. The comparison is not necessarily sound since the products tested are not technical reclaims, and generally the chloroform extracts were of a low order. (4) Under the highly reverting thermal conditions employed in this work, plasticization was generally inferior. This perhaps supports the theory that reclaiming is essentially a depolymerization process associated with traces of oxygen. The line of demarcation between reclaiming and reversion is by no means well defined. Since reclaiming treatments are normally much more severe than those needed to induce the characteristics of reversion, it may well be that reversion is only one factor in the mechanism of reclaiming, but it is a factor which cannot be ignored in any comprehensive consideration of devulcanization.

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

1964 ◽  
Vol 37 (3) ◽  
pp. 617-626 ◽  
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 pure 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 and 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 influence 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.

Cross-Linking in Natural-Rubber Vulcanizates

1953 ◽  
Vol 26 (4) ◽  
pp. 741-758 ◽  
Author(s):  
H. E. Adams ◽  
B. L. Johnson
Keyword(s):  
Molecular Weight ◽  
Natural Rubber ◽  
Cross Linking ◽  
Cross Link ◽  
General Validity ◽  
Tetramethylthiuram Disulfide ◽  
Cross Links ◽  
The Cross ◽  
Natural Rubber Vulcanizates ◽  
The Relationship

Abstract Recently, a method for measuring the average number of cross-links per chain of vulcanized polymer has been developed. It is possible to calculate the degree of cross-linking of the vulcanizate from its amount of swelling in a solvent such as benzene. This method was used by Flory to study the effect of primary molecular weight on the cross-linking of Butyl vulcanizates. An evaluation of the general validity of the method was ascertained by using quantitative cross-linking agents (diazodicarboxylates) to prepare vulcanizates of natural rubber and GR-S. Bardwell and Winkler have also used this technique to study the relationship between the degree of cross-linking and the force of retraction at 300 per cent elongation of GR-S latex vulcanized with potassium persulfate. The formation of cross-linking during the vulcanization by sulfur of several polymers has also been investigated. Gee has compared the formation of cross-linking in natural rubber vulcanizates with the amount of combined sulfur. Carbon-to-carbon cross-links were believed to be formed in a nonsulfur tetramethylthiuram disulfide (TMTD) cure. A similar study of Butyl rubber vulcanizates, cured with sulfur-TMTD, indicates that disulfide cross-links are formed. Scott and Magat have estimated that eight sulfur atoms are associated with each cross-link in Russian SK (sodium polybutadiene). This investigation was undertaken to extend Gee's study on the correlation of the cross-linking of natural-rubber vulcanizates with the amount of combined sulfur.

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