Structural Characterization of Vulcanizates Part X. Thiol-Disulfide Interchange for Cleaving Disulfide Crosslinks in Natural Rubber Vulcanizates

1970 ◽  
Vol 43 (2) ◽  
pp. 210-221 ◽  
Author(s):  
D. S. Campbell
Keyword(s):  
Natural Rubber ◽  
Exchange Reaction ◽  
Structural Characterization ◽  
Disulfide Exchange ◽  
Sulfur Vulcanization ◽  
Cure Time ◽  
Natural Rubber Vulcanizates

Abstract The thiol-disulfide exchange reaction is shown to be applicable to cleavage of disulfide crosslinks in accelerated sulfur vulcanizates of natural rubber. The reaction, in conjunction with the previously reported selective cleavage of polysulfide crosslinks, is used to determine the distribution of crosslink types for several accelerated sulfur vulcanization systems as a function of cure time. Discrepancies between the results and published results obtained using the reagent sodium di-n-butyl phosphite for disulfide crosslink cleavage are discussed.

Structural Characterization of Vulcan-Izates. XII. Efficient Vulcanization Using a Sulfenamide-Thiuram Disulfide Accelerator System

1972 ◽  
Vol 45 (5) ◽  
pp. 1366-1371 ◽  
Author(s):  
D. S. Campbell
Keyword(s):  
Natural Rubber ◽  
Structural Characterization ◽  
High Ratio ◽  
Mixed System ◽  
Tetramethylthiuram Disulfide ◽  
Sulfur Vulcanization ◽  
Cure Time ◽  
Accelerator System ◽  
Cyclic Sulfide

Abstract Vulcanizates obtained from the sulfur vulcanization of natural rubber using a combination of the accelerators N-cyclohexyl-2-benzothiazole sulfenamide (CBS) and tetramethylthiuram disulfide (TMTD) have been analyzed in terms of the numbers of poly-, di-, and monosulfide crosslinks, network-bound accelerator residues, and cyclic sulfide chain modifications as a function of cure time. The vulcanization system produced mainly monosulfide crosslinks at optimum cure, although there were differences in detail between this mixed system and a previously reported efficient sulfur vulcanization (EV) system using a high ratio of CBS to sulfur.

The CBS-Accelerated Sulfuration of Natural Rubber and Cis-1,4-Polybutadiene

1972 ◽  
Vol 45 (1) ◽  
pp. 182-192 ◽  
Author(s):  
T. D. Skinner
Keyword(s):  
Natural Rubber ◽  
Zinc Sulfide ◽  
Chain Length ◽  
Network Formation ◽  
Reaction Times ◽  
Substantial Fraction ◽  
Chemical Complexity ◽  
Cure Time ◽  
Natural Rubber Vulcanizates

Abstract The results of characterization of the natural rubber vulcanizates are consistent with the results of characterization of the sulfidic products from 2-methylpent-2-ene. In both the model olefin system and the rubber system the initially formed crosslinks are polysulfidic but these are subsequently reduced to di- and monosulfidic crosslinks as the cure time is increased. Similar amounts of zinc sulfide are formed during the sulfuration of 2-methylpent-2-ene and during the vulcanization of natural rubber. The efficiency of sulfur utilization for crosslinking in natural rubber is approximately half that in comparable sulfurations of 2-methylpent-2-ene, i.e. approximately twice as many sulfur atoms are needed to obtain a chemical crosslink in natural rubber as are needed to obtain a crosslink in 2-methylpent-2-ene. This is presumed to be a consequence of the intra-molecular sulfuration that occurs in natural rubber. There is no evidence to indicate the presence of vicinal crosslinks in the natural rubber vulcanizates. Hence in agreement with the views of other workers it is concluded that the crosslinks present in accelerated sulfur vulcanizates of natural rubber are tetrafunctional and dialkenyl. The results of the characterization of the polybutadiene vulcanizates are not fully supported by the results of the model olefin studies. In the vulcanization of polybutadiene the initially formed crosslinks are polysulfidic. As vulcanization proceeds, the chemical complexity of the network increases. After long reaction times, however, no significant amount of monosulfidic crosslinks are present in the network and very little of the reacted sulfur is present in the form of zinc sulfide. Nitrogen analyses of the polybutadiene vulcanizates showed that a substantial fraction of the accelerator, equivalent to 80–90% of the available 2-thiobenzothiazole groups, become combined in the network during vulcanization. It is proposed that the combination of accelerator with polybutadiene prevents the desulfuration of dialkenyl polysulfides to dialkenyl monosulfides (the normally observed pathway of accelerated sulfuration of natural rubber) and allows vicinal crosslinking to proceed. Some support for this proposal is that vicinal crosslinks and a substantial amount of nitrogenous product are formed during the accelerated sulfuration of cyclohexene. The findings of Gregg and Katrenick on the MBTS accelerated sulfuration of cis-cis-1,5-cyclooctadiene are also consistent with this proposal. The nitrogen analyses of the polybutadiene vulcanizates indicate that very little of the accelerator is permanently combined in the network during the initial stages of network formation. Hence by comparison with the observed pattern of sulfuration of hex-3-ene, where it was shown that negligible amounts of nitrogenous product are present, it is proposed that dialkenyl (tetrafunctional) polysulfidic crosslinks are initially introduced into the polybutadiene network. The polysulfidic crosslinks then presumably undergo desulfuration reactions leading to dialkenyl crosslinks of reduced sulfur chain length until the desulfurating agent is, in effect, removed from the system by the 2-thio-benzothiazole groups becoming combined in the network. Once most of these groups have combined, after ca. 60 min. at 140° C, the desulfuration reactions are probably less important than the reactions leading to vicinal crosslinking, and it is likely that a well cured-polybutadiene vulcanizate contains a substantial fraction of vicinal crosslinks.

Structural Characterization of Filled Vulcanizates Part 1. Determination of the Concentration of Chemical Crosslinks in Natural Rubber Vulcanizates Containing High Abrasion Furnace Black

1967 ◽  
Vol 40 (3) ◽  
pp. 866-882 ◽  
Author(s):  
M. Porter
Keyword(s):  
Carbon Black ◽  
Natural Rubber ◽  
Volume Fraction ◽  
Exponential Relationship ◽  
Furnace Black ◽  
Simple Linear Relationship ◽  
Chemical Crosslinks ◽  
Natural Rubber Vulcanizates

Abstract The degree to which HAF black restricts the swelling of natural rubber vulcanizates in n-decane has been determined using a vulcanizing system in which the stoichiometry of crosslinking is unaffected by the carbon black. The dependence of the degree of restriction, as measured by the ratio of the volume fractions of rubber in the filled and unfilled vulcanizates swollen to equilibrium, on the concentration of carbon black follows an exponential relationship previously proposed by Lorenz and Parks. This is found to be equivalent to a simple linear relationship between the apparent and actual crosslink concentrations: napparent/nactual=1+Kϕ, where K is a constant characteristic of the filler and φ is its volume fraction in the vulcanizate. The relation has been used to determine actual crosslink concentrations in filled natural rubber vulcanizates. HAF black is found to cause increases of up to 25 per cent in the yield of polymer to polymer crosslinks in conventional sulfur vulcanizing systems, accompanied by changes in rate of cure and of crosslink reversion. All these are small compared with the effect of the filler on many physical properties.

Aromatic and Epoxidised Oil Curing and Rebound Resilience Characteristic and their Humidity Effect of Hardness on NR Vulcanizates

2013 ◽  
Vol 812 ◽  
pp. 138-144 ◽  
Author(s):  
Mohamed Rahmah ◽  
Wan Zain Norazira ◽  
Shafie Nur Ashyikin ◽  
Mohd Nurazzi Norizan
Keyword(s):  
Natural Rubber ◽  
Physical Properties ◽  
Room Temperature ◽  
Rubber Compound ◽  
Humidity Effect ◽  
Humidity Level ◽  
Cure Time ◽  
Humidity Chamber ◽  
Overall Performance ◽  
Natural Rubber Vulcanizates

Recently, aromatic oil (AO) is one of the substances that is typically used as a processing aid especially for high filler loadings in formulating rubber compound. Aromatic oil has disadvantages in that, it is hazardous to environment, toxic and has been labeled as carcinogenic. In this research, an epoxidised oil (EO) and aromatic oil were used to investigate the effect incorporation of oil onto the SBR/NR natural rubber vulcanizates (NR). From the result obtained, EO showed shorter cure time and scorch time as the oil loading were increased up to 20 pphr of EO. Physical properties such as hardness and rebound resilience of NR/EO vulcanisate were also investigated upon exposure to different humidity level in humidity chamber. At room temperature, the hardness of EO loading onto the SBR/NR vulcanisate is lower than AO loadings. Hardness was slightly decreased with increasing rate of humidity. There is great difference in hardness and rebound resilience values between AO and EO. Both hardness and rebound resilience were not affected by humidity. This implies the existence of good filler interaction with EO and rubber which do not impart changes in the hardness and resilience properties of rubber compound. Epoxidised oil has great promising potential to replace the carcinogenic aromatic oil as it has good overall performance and renewable in nature .

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