Chain Scission in the Oxidation of Hevea. I

Bevilacqua, E. M.

doi:10.1021/ja01625a057

Cited by 16 publications

(13 citation statements)

References 1 publication

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“…-In polyene elastomers, a second important source of propagation is the addition of radical species onto double bonds. [6][7][8][9] These two reactions are summarized in Scheme 5. In a first approach, they are considered exclusively intermolecular, although it is known that, in other types of elastomer (in particular in IR), they can be also intramolecular.…”

Section: Theoretical Chemical Levelmentioning

confidence: 99%

“…On one hand, chemists tried to provide a concrete basis to the ''standard'' mechanistic scheme by identifying the main oxidation products and checking the validity of their formation paths. [6][7][8][9] On the other hand, physicists analyzed the consequences of oxidation on mechanical behavior and tried to explain them by using the theory of polymer behavior in solution 10,11 or the statistical theory of rubber elasticity. 12 Today, one key question is still unsolved: How to connect these two axes of research?…”

Section: Introductionmentioning

confidence: 99%

“…Chemists highlighted all the complexities of oxidation reactions for unsaturated hydrocarbon polymers, for which propagation occurs as much by hydrogen abstraction, according to the ''standard'' mechanistic scheme, 3,4 as by addition of radical species onto double bonds. [6][7][8][9] However, the numerous attempts for relating the macromolecular changes to the mechanistic scheme led to results that were regarded as convincing, but also in many ways, contradictory.…”

Section: Introductionmentioning

confidence: 99%

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Chemo-Mechanical Model for Predicting the Lifetime of Epdm Rubbers

Colin

Hassine²,

Nait-Abelaziz

2019

Rubber Chemistry and Technology

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A chemo-mechanical model has been developed for predicting the long-term mechanical behavior of EPDM rubbers in a harsh thermal oxidative environment. Schematically, this model is composed of two complementary levels: The “chemical level” calculates the degradation kinetics of the macromolecular network that is introduced into the “mechanical level” to deduce the corresponding mechanical behavior in tension. The “chemical level” is derived from a realistic mechanistic scheme composed of 19 elementary reactions describing the thermal oxidation of EPDM chains, their stabilization against oxidation by commercial antioxidants but also by sulfide bridges, and the maturation and reversion of the macromolecular network. The different rate constants and chemical yields have been determined from a heavy thermal aging campaign in air between 70 and 170 °C on four distinct EPDM formulations: additive free gum, unstabilized and stabilized sulfur vulcanized gum, and industrial material. This “chemical level” has been used as an inverse resolution method for simulating accurately the consequences of thermal aging at the molecular (concentration changes in antioxidants, carbonyl products, double bonds, and sulfide bridges), macromolecular (concentration changes in chain scissions and cross-link nodes), and macroscopic scales (weight changes). Finally, it gives access to the concentration changes in elastically active chains from which are deduced the corresponding changes in average molar mass MC between two consecutive cross-link nodes. The “mechanical level” is derived from a modified version of the statistical theory of rubber elasticity, called the phantom network theory. It relates the elastic and fracture properties to MC if considering the macromolecular network perfect, and gives access to the lifetime of the EPDM rubber based on a relevant structural or mechanical end-of-life criterion. A few examples of simulations are given to demonstrate the reliability of the chemo-mechanical model.

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Section: Theoretical Chemical Levelmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Chemo-Mechanical Model for Predicting the Lifetime of Epdm Rubbers

Colin

Hassine²,

Nait-Abelaziz

2019

Rubber Chemistry and Technology

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“…Indeed, olefinic photo‐oxidation is reported to proceed (initiation, propagation, and termination) in accordance with a radical reaction mechanism identical to that of thermal oxidation 7. Consequently, certain CH or CC bonds in the structure of natural rubber breakdown under the influence of light8, 9 or temperature 10. The free radicals formed either react with one another, with irradiation under limited oxygen concentration, or initiate a photo‐oxidative process in the presence of oxygen.…”

Section: Introductionmentioning

confidence: 99%

Effect of natural coagula maturation on the processability, cure, and mechanical properties of unfilled vulcanizates of Hevea natural rubber

Fri

Nkeng

Ehabe

2006

J of Applied Polymer Sci

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Fresh latex from different Hevea brasiliensis clones was naturally coagulated, subjected to different durations of maturation, processed into solid rubber, and compounded into pure gum stocks and vulcanized. Coagula maturation had clone-specific effects on the processability of the raw rubber: reduced for some clones, while others was less sensitive. The cure and mechanical behaviors of the compounded stocks and vulcanizates, respectively, were not sensitive to the clonal origin of coagula and their duration of maturation. Although coagula maturation could be associated with leaching, deactivation of inherent antioxidants in Hevea latex, as well as crosslinking and/or oxidation of polyisoprene chains, these results show that compounding with a standard pure gum recipe compensates for the Hevea latex constituents affected by maturation. Hence, extended maturation of Hevea coagula, for economic or other reasons, would influence much more the bulk behavior of raw rubber and have insignificant effects on the compounded stocks and vulcanizates.

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“…The thermal degradation reaction of NR is complicated. The general mechanism for oxidative degradation of NR (or polyisoprene) involves the initial presence of alkylperoxyl radicals to induce a chain scission process [28,29] . In this process, side reactions, such as cyclization, epoxidation, rearrangement, and crosslinking, take place [30,31] .…”

Section: Data Processingmentioning

confidence: 99%

Thermooxidative degradation and its kinetics of natural rubber coagulated by microwave radiation

Liao¹,

Liao²,

Li³

et al. 2012

Journal of Polymer Engineering

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The thermooxidative degradation of natural rubber (NR) coagulated by microwave radiation (NR-m) was investigated by thermogravimetry (TG) analysis, and compared with NR coagulated by acid (NR-a). It was found that the degradation process is not a one-step reaction, the main degradation process occurs at 300 -400 ° C, and the equilibrium degradation temperatures of NR-m are higher than those of NR-a. Different methods were used to find the most probable kinetic model and the Arrhenius para meters (activation energy E and preexponential factor A ) for the main stage of thermooxidative degradation of NR-m. The results show that the values of E and A , obtained from the Coats-Redfern method, are highly variable with the kinetic model chosen and the heating rate ( β ), and the apparent activation energy ( E 0 ) when β approaches zero for the D n type kinetic model is in the range of 96.7 -106.4 kJ · mol -1 . The mean values of E , calculated by the Friedman and Flynn-Wall-Ozawa (FWO) methods, are 113.8 and 83.3 kJ · mol -1 , respectively, suggesting E in the range of 83.3 -113.8 kJ · mol -1 . Comparison of these two ranges indicates the most probable kinetic model to be D n type kinetic models, corresponding to a diffusion-controlled mechanism.

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Chain Scission in the Oxidation of Hevea. I

Cited by 16 publications

References 1 publication

Chemo-Mechanical Model for Predicting the Lifetime of Epdm Rubbers

Chemo-Mechanical Model for Predicting the Lifetime of Epdm Rubbers

Effect of natural coagula maturation on the processability, cure, and mechanical properties of unfilled vulcanizates of Hevea natural rubber

Thermooxidative degradation and its kinetics of natural rubber coagulated by microwave radiation

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