Vulcanization of Elastomers. 25. Natural Rubber and Synthetic Elastomers with Sulfur and Sulfenamides. I

1960 ◽  
Vol 33 (3) ◽  
pp. 846-856 ◽  
Author(s):  
Walter Scheele ◽  
Horst-Eckart Toussaint ◽  
Yoan-Kun Chai
Keyword(s):  
Reaction Order ◽  
Molar Ratio ◽  
Reactive Intermediate ◽  
Sulfur Concentration ◽  
Kcal Mole ◽  
Experimental Conditions ◽  
Sulfur Vulcanization ◽  
Different Temperatures ◽  
Kinetics Of ◽  
Time Plot

Abstract The sulfur vulcanization of Perbunan N 2818 (acrylonitrile-butadiene) was investigated in the presence of N-cyclohexyl-2-benzothiazolylsulfenamide (CZ) at different temperatures and various concentrations of reactants. The following were found : 1. The decreasing sulfur concentration vs. time plot followed the 0.8th order under all experimental conditions and an activation energy of 28.2 kcal/mole was calculated. 2. When using the cyclohexylammonium salt of MBT as accelerator, sulfur decrease proceeds at the same rate as with CZ. 3. It was concluded from 1 and 2 that even in the presence of CZ, the ammonium salt was the actual accelerator, which forms during the scorch or induction period through reaction of CZ with the rubber. 4. In view of the discrepancy found in the relation of reaction order with respect to time and concentration of reactants, the formation of a reactive intermediate is postulated; the analogy between the kinetics of sulfenamide accelerated sulfur vulcanization, and those accelerated with MBT as well as DPG is pointed out. 5. In connection with 3 the dependence of starting rate as well as rate constant of 0.8th order of the decreasing sulfur concentration on the molar ratio of CZ/S8 and on the sulfur starting concentration is discussed.

Vulcanization of Elastomers. 28. Vulcanization of Natural and Synthetic Rubbers with Sulfur in the Absence of Accelerators. I

1960 ◽  
Vol 33 (4) ◽  
pp. 1051-1061
Author(s):  
Elisabeth Echte ◽  
Walter Scheele ◽  
Sigrun Sonnenberg
Keyword(s):  
Activation Energy ◽  
Natural Rubber ◽  
Zero Order ◽  
Sulfur Concentration ◽  
Kcal Mole ◽  
Sulfur Solubility ◽  
Temperature Relation ◽  
Sulfur Vulcanization ◽  
Rate Relationship ◽  
Kinetics Of

Abstract The decrease of sulfur concentration in the reaction of sulfur with natural rubber was studied. The following was found : 1. Sulfur decrease follows the 0.6th order law independent of temperature as long as the sulfur is dissolved in the gum (temperature!). From the rate-temperature relation, an activation energy of 35 kcal/mole is calculated. 2. In an investigation of the kinetics of sulfur concentration at constant temperature but increasing starting concentration, the following two cases can be differentiated : a) As long as the sulfur is soluble in the gum, sulfur decrease still follows the 0.6th order, possibly due to autocatalysis ; the linear relationship between starting rate and starting concentration shows that the process is 1st order with respect to the concentration; this may be the consequence of a thermal, rate determining cleavage of the S8 ring. b) As the sulfur at higher concentration is only incompletely soluble in the gum, conversion curves with points of inflection are found ; this becomes more pronounced at higher concentration. At the start of the reaction one finds an autocatalytic sulfur decrease basically of zero order; in the latter part of the reaction after passing the point of inflection a 0.6th order is observed, as in the range of complete sulfur solubility. 3. A discrepancy between the time law and the concentration-rate relationship is found in pure as well as accelerated sulfur vulcanization ; these conditions are compared and discussed.

The Vulcanization of Elastomers. 20. Sulfur Vulcanization Accelerated with Thiuram Compounds. I

1959 ◽  
Vol 32 (1) ◽  
pp. 139-149 ◽  
Author(s):  
Walter Scheele ◽  
Adolf Franck
Keyword(s):  
Reaction Order ◽  
Accurate Determination ◽  
Sulfur Concentration ◽  
Quantitative Investigation ◽  
Tetramethylthiuram Disulfide ◽  
First Order ◽  
Sulfur Vulcanization ◽  
Complicated Course ◽  
Peak Value

Abstract The present paper deals with the results of an orientating, quantitative investigation of sulfur vulcanization accelerated by thiuram disulfide, with tetramethylthiuram disulfide as the representative example. It was found: In the sulfur cure of natural rubber with tetramethylthiuram disulfide at different TMTD:S ratios, the rates of TMTD decrease and dithiocarbamate formation increase with increasing sulfur concentration, the TMTD content being kept constant. The rates practically do not change any further when the compounds contain 6 gram atoms of sulfur per mole of thiuram disulfide. The peak value of dithiocarbamate formation increases with the increase of sulfur concentration and reaches a constant end value of about 90 mole per cent based on the amount of original thiuram disulfide, when the stocks contain 4 gram atoms of sulfur per mole thiuram disulfide. This end value is identical to the end value of dithiocarbamate formation in the reaction of thiuram disulfide with zinc oxide (in the absence of rubber). The crosslinking, as measured by the change of reciprocal equilibrium swelling per time unit is also a reaction whose rate increases with the sulfur concentration to the point where the compounds contain 6 gram atoms of sulfur per mole of thiuram disulfide. The optimum degrees of crosslinking are roughly proportional to the sulfur concentration; at high sulfur levels the vulcanizates tend to revert. As in the pure TMTD vulcanization, the TMTD decrease as well as the dithiocarbamate formation are always first order reactions. The reversion at higher sulfur levels as well as the complicated course of the increase of combined sulfur during vulcanization render all but impossible an accurate determination of the reaction order for the crosslinking at higher sulfur levels. Nevertheless, in vulcanizations with 1 mole TMTD per 1 or 2 gram atoms of sulfur the crosslinking is a first order reaction.

Vulcanization of Elastomers. 26. Vulcanization of Natural and Synthetic Rubbers by Sulfur in the Presence of Organic Bases

1960 ◽  
Vol 33 (3) ◽  
pp. 834-845
Author(s):  
Walter Scheele ◽  
Wolfgang Redetzky
Keyword(s):  
Initial Rate ◽  
Reaction Order ◽  
Research Work ◽  
Intermediate Compound ◽  
Sulfur Concentration ◽  
Kcal Mole ◽  
Initial Concentration ◽  
Organic Bases ◽  
Single Power ◽  
Theory And Experiment

Abstract In connection with earlier research work, we have carefully studied the decrease of sulfur during the vulcanization of Perbunan N 2818 in the presence of DPG, at various temperatures and with varied molar concentrations of sulfur and accelerator. We found : 1. The sulfur concentration decreases at all temperatures according to a time law of order 0.8. An activation energy of about 28 kcal/mole is derived from the temperature dependence of the rate constant. 2. The reaction order is independent of the initial concentration of reactants. 3. A discrepancy is found in the order of the reaction with respect to time and with respect to concentration, and it is clearly shown that the dependence of the initial rate on the initial concentration can not be expressed by a single power law. 4. The attempt is made to explain the discrepancy by postulating catalysis by an intermediate compound. The consequences from this consideration are compared with the experimental results, and, on the whole, agreement is found between theory and experiment.

Vulcanization of Elastomers. 40. Vulcanization of Natural Rubber and Synthetic Rubber with Sulfur in Presence of Sulfenamides. III

1965 ◽  
Vol 38 (1) ◽  
pp. 189-203 ◽  
Author(s):  
W. Scheele ◽  
J. Helberg
Keyword(s):  
Activation Energy ◽  
Reaction Mechanism ◽  
Natural Rubber ◽  
Rate Constants ◽  
Sulfur Concentration ◽  
Kcal Mole ◽  
Synthetic Rubber ◽  
Chemical Constitution ◽  
Induction Times ◽  
Kinetics Of

Abstract Vulcanization of natural rubber with sulfur was studied in presence of six sulfenamides, to determine the effect of the chemical constitution of the sulfenamide on sulfur decrease and on crosslinking. The results can be condensed as follows: (1) The kinetics of sulfur disappearance is in every respect qualitatively independent of the chemical constitution of the sulfenamide. (2) For the sulfenamides investigated, the smallest and largest rate constants for sulfur decrease differed only by a factor of two. (3) Greater differences are encountered in the induction times for sulfur decrease and for crosslinking. The latter are notably longer than those for sulfur disappearance. (4) The same activation energy, 23 kcal/mole, is derived from the temperature dependence of the induction times for all the sulfenamides. (5) The dissociation of sulfenamides in solution and their reaction with mercaptobenzothiazole were investigated further. The results provide the basis for a proposed reaction mechanism, which is presented in detail and can account for a number of the features typical of sulfenamide-accelerated vulcanization. (6) The drop in sulfur concentration goes at practically the same rate, if one introduces, instead of N, N-dicyclohexyl-2-benzothiazolesulfenamide, the corresponding ammonium mercaptide in equimolar concentration.

Nuclear magnetic resonance study of proton exchange in the water – 2-propanol system

1970 ◽  
Vol 48 (10) ◽  
pp. 1616-1618 ◽  
Author(s):  
K. C. Tewari ◽  
N. C. Li
Keyword(s):  
Activation Energy ◽  
Nuclear Magnetic Resonance ◽  
Nuclear Magnetic Resonance Study ◽  
Proton Exchange ◽  
Electron Donors ◽  
Specific Rate ◽  
Kcal Mole ◽  
Magnetic Resonance Study ◽  
Different Temperatures ◽  
Kinetics Of

The specific rate constants for the reaction[Formula: see text]where ROH is 2-propanol, have been determined at four different temperatures. The activation energy has been calculated to be 10.4 kcal/mole. In addition, the effect of the presence of several electron donors on the kinetics of the reaction has been noted.

Vulcanization of Elastomers. 36. Vulcanization of Natural Rubber and Synthetic Rubbers with Sulfur in Absence of Accelerators. II

1964 ◽  
Vol 37 (4) ◽  
pp. 910-926 ◽  
Author(s):  
W. Scheele ◽  
H. Müller ◽  
W. Schulze
Keyword(s):  
Natural Rubber ◽  
Concentration Dependence ◽  
Good Description ◽  
Kcal Mole ◽  
First Order ◽  
Different Temperatures ◽  
In Cis ◽  
Kinetics Of ◽  
Limiting Value ◽  
Homolytic Dissociation

Abstract In continuation of earlier work with natural rubber, the kinetics of sulfur decrease were studied in certain synthetic rubbers for different temperatures and sulfur concentrations. At the same time the formation of polysulfide bound sulfur was studied, using as example the reaction of sulfur with natural rubber and synthetic rubbers. It was found that: 1) When the decrease in sulfur concentration is portrayed by curves which are convex to the time axis (Perbunan), the 0.6th order time-law is fulfilled, (as in the case of natural rubber independent of temperature and concentration. 2) In contrast, the concentration dependence of the rate at which sulfur decreases, both in Perbunan and cis 1,4-polybutadiene, denotes a first-order reaction in agreement with experience with natural rubber. 3) The activation energy of sulfur decrease has the same magnitude for all the elastomers investigated (34 to 36 kcal/mole). 4) The disagreement between the time law and the concentration dependence of the rate of sulfur disappearance encountered in all the experiments with 1,5-polyenes, is interpreted as indicating autocatalysis, which likewise explains the shape of the curves for sulfur disappearance. 5) Sulfur reacts considerably faster in natural rubber and Perbunan than in cis 1,4-polybutadiene; consequently a homolytic dissociation of the S8-ring cannot be rate-determining. 6) Polysulfide sulfur shows, in each case, a maximum with reaction time, and in completely reacted vulcanizates it tends toward a limiting value. An equation was found, which provides a good description of change with time of polysulfide concentration (natural rubber and cis 1,4-polybutadiene). 7) An explanation is given for the appearance of the polysulfide maximum; and how the reaction of sulfur with 1,5-polyenes can be represented, making use of all available results, is discussed.

scholarly journals The dissociation energy of the C-N bond in benzylamine

1949 ◽  
Vol 198 (1053) ◽  
pp. 285-292 ◽  
Keyword(s):  
Activation Energy ◽  
Thermal Decomposition ◽  
Dissociation Energy ◽  
Heat Of Formation ◽  
Molar Ratio ◽  
Kcal Mole ◽  
First Order ◽  
First Order Kinetics ◽  
Permanent Gases ◽  
Kinetics Of

The kinetics of the thermal decomposition of benzylamine were studied by a flow method using toluene as a carrier gas. The decomposition produced NH 3 and dibenzyl in a molar ratio of 1:1, and small quantities of permanent gases consisting mainly of H 2 . Over a temperature range of 150° (650 to 800° C) the process was found to be a homogeneous gas reaction, following first-order kinetics, the rate constant being expressed by k = 6 x 10 12 exp (59,000/ RT ) sec. -1 . It was concluded, therefore, that the mechanism of the decomposition could be represented by the following equations: C 6 H 5 . CH 2 . NH 2 → C 6 H 5 . CH 2 • + NH 2 •, C 6 H 5 . CH 3 + NH 2 •→ C 6 H 5 . CH 2 • + NH 3 , 2C 6 H 5 . CH 2 •→ dibenzyl, and the experimentally determined activation energy of 59 ± 4 kcal./mole is equal to the dissociation energy of the C-N bond in benzylamine. Using the available thermochemical data we calculated on this basis the heat of formation of the NH 2 radical as 35.5 kcal./mole, in a fair agreement with the result obtained by the study of the pyrolysis of hydrazine. A review of the reactions of the NH 2 radicals is given.

Phenomenological model for the reaction order n in the kinetics of curing an elastomer EPDM

MRS Advances ◽  
2019 ◽  
Vol 4 (59-60) ◽  
pp. 3299-3310
Author(s):  
S. Gómez-Jimenez. ◽  
A.M. Becerra-Ferreiro. ◽  
E. Jareño-Betancourt. ◽  
J. Vázquez-Penagos.
Keyword(s):  
Reaction Order ◽  
Cure Kinetics ◽  
Analytical Description ◽  
Crosslinking Reaction ◽  
Precise Control ◽  
Short Period ◽  
Rate Of Reaction ◽  
Different Temperatures ◽  
Kinetics Of ◽  
Modelling And Optimization

AbstractThe precise control of curing reaction parameters allows a better crosslinking polymer. Modelling and optimization of this process require a correct kinetic of curing model. The kinetics of the crosslinking reaction is studied for the ethylene propylene diene monomer (EPDM) synthetic elastomer by mobile die rheometer (MDR). The kinetic parameters of reaction were calculated from Kamal-Ryan, Sestak-Berggren, and the Isayev-Deng methods at different temperatures. An Arrhenius-type function for the order of reaction n is introduced to improve the adjusting. Finally, a graphical and analytical description of the cure kinetics was developed. The order of reaction is predicted to better establishment of processing time. It was noted that for EPDM at higher temperatures, the increase of the rate of reaction occurs in short period of time, which could cause premature curing if the supply system is inadequate.

scholarly journals REDUCTION OF NITRIC OXIDE WITH CARBONACEOUS MATERIALS FROM PEAT

2005 ◽  
Vol 4 (1) ◽  
Author(s):  
E. L. Schultz ◽  
R. F. P. M. Moreira ◽  
H. J. José
Keyword(s):  
No Reduction ◽  
Activated Carbons ◽  
Physical Activation ◽  
Carbonaceous Materials ◽  
Experimental Conditions ◽  
Gaseous Products ◽  
Different Temperatures ◽  
The Relationship ◽  
Kinetics Of ◽  
Kinetics Of Reduction

The direct NO reduction to produce N2 and CO2 using carbonaceous materials, chars and activated carbons, was studied. Chars were prepared from peat by pyrolysis, at temperatures ranging from 873 K to 1073 K. Activated carbons were prepared by the physical activation of chars with steam, in a steel reactor, at 1073 K for 12 minutes, 25 minutes and 45 minutes. The kinetics of NO reduction using chars and activated carbons produced at different experimental conditions were evaluated at different temperatures in the range 623-723 K. The gaseous products were essentially CO2 and N2 and the amount of CO produced was negligible. The effect of the temperature on the kinetics of reduction was also evaluated and the relationship between the rate constant and the temperature showed an Arrhenius dependence. Activation energies of the NO reduction were in the range 6.75 to 7.97 kcal.mol-1 for the chars and in the range 8.14 to 9.52 kcal.mol-1 for the activated carbons.

Vulcanization. Part VII. Kinetics of Sulfur Vulcanization of Natural Rubber in Presence of Delayed-Action Accelerators

1965 ◽  
Vol 38 (1) ◽  
pp. 1-14 ◽  
Author(s):  
A. Y. Coran
Keyword(s):  
Positive Integer ◽  
Stearic Acid ◽  
Natural Rubber ◽  
Zinc Ion ◽  
Square Root ◽  
Reaction Products ◽  
Sulfur Concentration ◽  
Sulfur Vulcanization ◽  
Delayed Action ◽  
Kinetics Of

Abstract The scheme and treatment of the kinetics of scorch-delay vulcanization proposed in a previous paper were applied to natural rubber sulfur vulcanization accelerated by 2,2′-thiobisbenzothiazole (MBTS), 2-(N-cyclohexyl)benzothizaolesulfenamide (CBS), 2-(N-morpholinothio)benzothiazole (MOR), or 2-(N,N-diisopropyl)benzothiazolesulfenamide (DPBS). According to the scheme (see PDF for diagram) where the subscript x = a positive integer. The specific rates k1 and k2 and the ratiok4/k3′ were related to starting concentrations of accelerator, sulfur, and stearic acid. In general, an increase in stearic acid concentration or a decrease in accelerator concentration induces an increase in k1 or k4/k3′ but a decrease in k2. An exception to this is that, when the highly hindered sulfenamide DPBS is used as the accelerator, k1 does not change with changes in starting concentrations. It is interesting that k2 does not respond appreciably to changes in sulfur concentration when sulfenamides are used, unless the sulfur concentrations are extremely low. In contrast to this, k2 increases linearly with the square root of the sulfur concentration when MBTS is used. The changes in rates which occur with changes in the starting concentrations were attributed to the formation of chelates between zinc ion (brought into solution by stearic acid) and accelerator, intermediate reaction products, or crosslink precursors. The differences noted between the rates obtained with the three sulfenamides were attributed to differences in steric hindrance and stability.

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