Vulcanization of Elastomers. 23. The Chemical Kinetics of Vulcanization Reactions and The Physical Properties of The Vulcanizates. I

1960 ◽  
Vol 33 (2) ◽  
pp. 335-341
Author(s):  
Walter Scheele ◽  
Karl-Heinz Hillmer
Keyword(s):  
Activation Energy ◽  
Physical Properties ◽  
Chemical Kinetics ◽  
Kcal Mole ◽  
First Order ◽  
Equilibrium Swelling ◽  
Kinetics Of ◽  
Kinetics Of Vulcanization ◽  
Limiting Value ◽  
Good Agreement

Abstract As a complement to earlier investigations, and in order to examine more closely the connection between the chemical kinetics and the changes with vulcanization time of the physical properties in the case of vulcanization reactions, we used thiuram vulcanizations as an example, and concerned ourselves with the dependence of stress values (moduli) at different degrees of elongation and different vulcanization temperatures. We found: 1. Stress values attain a limiting value, dependent on the degree of elongation, but independent of the vulcanization temperature at constant elongation. 2. The rise in stress values with the vulcanization time is characterized by an initial delay, which, however, is practically nonexistent at higher temperatures. 3. The kinetics of the increase in stress values with vulcanization time are both qualitatively and quantitatively in accord with the dependence of the reciprocal equilibrium swelling on the vulcanization time; both processes, after a retardation, go according to the first order law and at the same rate. 4. From the temperature dependence of the rate constants of reciprocal equilibrium swelling, as well as of the increase in stress, an activation energy of 22 kcal/mole can be calculated, in good agreement with the activation energy of dithiocarbamate formation in thiuram vulcanizations.

Vulcanization of Elastomers. 22. Thermal Vulcanization of Synthetic Rubbers. I

1959 ◽  
Vol 32 (4) ◽  
pp. 962-975
Author(s):  
Walter Scheele ◽  
Hans Dieter Stemmer
Keyword(s):  
Activation Energy ◽  
Second Order ◽  
Crosslinking Reaction ◽  
Molecular Fragments ◽  
Kcal Mole ◽  
First Order ◽  
Disulfide Content ◽  
Kinetics Of ◽  
Stable Combination ◽  
Limiting Value

Abstract In this work, the kinetics of the thermal vulcanization of Perbunan were studied with and without additives. The following results were obtained : 1. The pure thermal vulcanization of Perbunan is a very slow process which obeys a second order rate law. A limiting value of the crosslinking (reciprocal limit of equilibrium swelling) is reached, which limit is independent of the temperature. The activation energy is 23.3 kcal/mole. 2. The thermal vulcanization can be inhibited by hydroquinone but not by benzoquinone. 3. The thermal vulcanization of Perbunan can be considerably accelerated by MBTS, and other materials, and the reaction also follows a second order course. The activation starts suddenly after the expiration of an induction period, which decreases with increase in temperature. The activation energy is about 27 kcal/mole. 4. In a thermal vulcanization accelerated with MBTS, a portion of the MBTS is changed over into MBT ; the amount changed is independent of temperature. Perbunan takes up MBTS in the form of molecular fragments, in stable combination. 5. The reduction in MBTS (which falls to zero) and the increase in MBT follow a first order reaction and have the same activation energy which is also identical with the energy of activation of the accelerated crosslinking. The formation of MBT is the slower of the two reactions. 6. The rate constants for the decrease in MBTS and for the increase in MBT are independent of the starting amount of MBTS, and hence we consider that this is a unimolecular process (homolysis). 7. The rate constant for the second order crosslinking reaction increases with the square root of the initial benzothiazolyl disulfide content. 8. It is indicated that the above data must be explained, with the aid of experience in the realm of polymerization kinetics. The investigations are being continued.

Vulcanization of Elastomers. 24. The Effect of Fillers on the Course of Vulcanization Reactions. I

1960 ◽  
Vol 33 (2) ◽  
pp. 326-334
Author(s):  
Walter Scheele ◽  
Günter Mau ◽  
Gregor Kemme
Keyword(s):  
Chemical Reactions ◽  
Rate Constants ◽  
Chemical Mechanism ◽  
Rubber Compound ◽  
Kcal Mole ◽  
First Order ◽  
Equilibrium Swelling ◽  
Qualitative Effect ◽  
Kinetics Of ◽  
Limiting Value

Abstract The effect of Aerosil on the chemical reactions and the kinetics observed in thiuram vulcanizations was subjected to closer scrutiny. Results now available show: 1. Aerosil does not alter the chemical mechanism of thiuram vulcanizations. It also has no qualitative effect on the kinetics of the various reactions involved. Thiuram disappearance and dithiocarbamate formation over by far the greatest range of conversions are first order reactions. Nevertheless, the limiting value of dithiocarbamate formation is somewhat higher than in the absence of Aerosil. 2. Quantitatively speaking, essential differences are involved. The rate constants for both thiuram loss and dithiocarbamate formation rise considerably when Aerosil is used; in both cases there is a dependence on the Aerosil content of the rubber compound. Yet the activation energies of thiuram disappearance and dithiocarbamate formation are practically the same as in Aerosil-free thiuram vulcanizations, amounting to about 21 kcal/mole. 3. Measurements of the limiting equilibrium swelling reveal that adding Aerosil does not result in additional crosslinking, so that in thiuram vulcanizations the catalytic action displayed by silica gel is the outstanding feature.

Insect blood-brain barrier: a radioisotope study of the kinetics of exchange of a liposoluble molecule (n-butanol)

1976 ◽  
Vol 64 (1) ◽  
pp. 119-130
Author(s):  
M. V. Thomas
Keyword(s):  
Activation Energy ◽  
Temperature Sensitivity ◽  
Nerve Cord ◽  
Previous Investigation ◽  
Time Course ◽  
Kcal Mole ◽  
Similar Time ◽  
Kinetics Of ◽  
Butanol Concentration ◽  
Good Agreement

About 90% of the butanol uptake by the cockroach abdominal nerve cord washed out with half-times of a few seconds, in good agreement with an electrophysiological estimate, and the temperature sensitivity suggested an activation energy of 3 Kcal mole-1. The remaining activity washed out far more slowly, with a similar time course to that observed in a previous investigation which had not detected the fast fraction. Its size was similar to the non-volatile uptake, and was considerably affected by the butanol concentration and incubation period. It apparently consisted of butanol metabolites, which could be detected by chromatography.

THE NON-ENZYMATIC HYDROLYSIS OF ADENOSINE DIPHOSPHATE

1957 ◽  
Vol 35 (12) ◽  
pp. 1496-1503 ◽  
Author(s):  
K. A. Holbrook ◽  
Ludovic Ouellet
Keyword(s):  
Activation Energy ◽  
Aqueous Solution ◽  
Enzymatic Hydrolysis ◽  
Inorganic Phosphate ◽  
Adenosine Diphosphate ◽  
Hydrogen Ions ◽  
Kcal Mole ◽  
First Order ◽  
Kinetics Of ◽  
Hydrolysis Of

The kinetics of the non-enzymatic hydrolysis of adenosine diphosphate in aqueous solution have been studied at pH 3.5 to 10.5 and temperatures from 80° to 95 °C. The reaction has been followed by measuring colorimetrically the inorganic phosphate liberated according to the over-all reaction[Formula: see text]The reaction has been found to be first order with respect to ADP concentration and to be catalyzed by hydrogen ions. From rate studies at pH 8.0 an activation energy of 24.2 kcal./mole was derived. A mechanism is proposed to account for the observed facts and the mechanism for the hydrolysis of adenosine triphosphate is also discussed.

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.

THE NON-ENZYMATIC HYDROLYSIS OF p-NITROPHENYL PHOSPHATE

1958 ◽  
Vol 36 (4) ◽  
pp. 686-690 ◽  
Author(s):  
K. A. Holbrook ◽  
Ludovic Ouellet
Keyword(s):  
Activation Energy ◽  
Enzymatic Hydrolysis ◽  
Ionic Species ◽  
Kcal Mole ◽  
First Order ◽  
Colorimetric Measurement ◽  
Nitrophenyl Phosphate ◽  
Ph Range ◽  
Kinetics Of ◽  
Hydrolysis Of

The kinetics of the non-enzymatic hydrolysis of p-nitrophenyl phosphate have been studied in aqueous solution in the pH range 2.6 to 9.0 and at temperatures from 68.0°to 82.0 °C. The reaction has been followed by colorimetric measurement of the nitrophenol produced by the reaction[Formula: see text]The reaction is first order with respect to p-nitrophenyl phosphate and has an activation energy of 26.0 kcal./mole at pH 2.6. An explanation has been proposed in terms of the different rates of hydrolysis of the various ionic species of the ester present in solution.

Vulcanization of Elastomers. 21. Vulcanization of Natural Rubber with Thiuram Disulfides. VI

1959 ◽  
Vol 32 (2) ◽  
pp. 566-576
Author(s):  
Walter Scheele ◽  
Klaus Hummel
Keyword(s):  
Activation Energy ◽  
Natural Rubber ◽  
Temperature Dependence ◽  
Sulfur Content ◽  
Rate Constants ◽  
Order Reaction ◽  
First Order Reaction ◽  
Kcal Mole ◽  
First Order ◽  
Limiting Value

Abstract Bound sulfur in a pure thiuram vulcanizate increases relatively rapidly at first at all temperatures, reaches a poorly defined maximum at about 27 to 30%, independent of temperature, and then recedes slightly to reach a limiting value of 25% also independent of temperature, based on the original thiuram disulfide. The rise in sulfur content at the start points to a temperature-independent limiting value of 33%. It is shown that the combination of sulfur in this region initially follows a first order reaction, and goes at the same rate as the reduction in concentration of thiuram disulfide. It can be seen from the above that sulfur may be combined in thiuram vulcanization without simultaneous crosslinking. The dithiocarbamate formation increases rapidly in the region of longer vulcanization times, after the maximum in bound sulfur has been reached, without further combination of sulfur with the vulcanizate. The rate constants for thiuram decrease, for dithiocarbamate increase and for sulfur combination were calculated. The temperature dependence of each of these reactions has practically the same activation energy, 23 kcal/mole. The bound sulfur content of the vulcanizates in pure thiuram vulcanizations is no criterion of the state of vulcanization.

scholarly journals STUDIES ON THE PERVANADIUM COMPLEX

1961 ◽  
Vol 39 (6) ◽  
pp. 1174-1183 ◽  
Author(s):  
G. A. Dean
Keyword(s):  
Activation Energy ◽  
Thermal Decomposition ◽  
Acid Solution ◽  
Anionic Complex ◽  
Cationic Complex ◽  
Kcal Mole ◽  
Decomposition Products ◽  
First Order ◽  
Discrete Variations ◽  
Kinetics Of

The 'pervanadium complex' is investigated in a general manner. The kinetics of its thermal decomposition in acid solution are shown to be first order with respect to pervanadium, the apparent activation energy is 26.5 ± 1.0 kcal/mole, and possible mechanisms are suggested. The effect of various acids upon the nature of the decomposition products is determined: almost quantitative yields of vanadium (V) or vanadium (IV) are obtained in very dilute or concentrated acid, respectively. Spectrophotometric studies indicate that in acid solution two separate complexes exist: a red (1:1) cationic complex and a yellow (1:2) anionic complex. The stoichiometry of the equilibrium between the two complexes in solutions of sulphuric acid is investigated by a method of 'discrete variations'. The equilibrium could be described by[Formula: see text]where Kr/y = 2.2 ± 0.2 at 22 °C. The anion is shown to play an important part in determining the nature of the pervanadium complex.

Ethylene exchange in PtCl(C5H7O2)(π-C2H4)

1970 ◽  
Vol 48 (24) ◽  
pp. 3802-3806 ◽  
Author(s):  
C. E. Holloway ◽  
J. Fogelman
Keyword(s):  
Activation Energy ◽  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Kcal Mole ◽  
First Order ◽  
First Order Kinetics ◽  
Solvent Dependence ◽  
Kinetics Of

The kinetics of exchange of free with complexed ethylene in the system PtCl(acac)(π-C2H4) have been investigated over a temperature and concentration range by proton magnetic resonance. First order kinetics are observed with respect to each component with no solvent dependence of rate. The activation energy and entropy are 2.7 kcal mole−1 and −36 cal deg−1 mole−1, respectively. A five coordinate intermediate is suggested, with complete retention of configuration at the platinum.

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