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.

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. 39. Vulcanization of Natural and Synthetic Rubbers by Sulfur and Sulfenamides. II

1965 ◽  
Vol 38 (1) ◽  
pp. 176-188 ◽  
Author(s):  
W. Scheele ◽  
G. Kerrutt
Keyword(s):  
Natural Rubber ◽  
Concentration Dependence ◽  
Intermediate Compound ◽  
Kcal Mole ◽  
Experimental Conditions ◽  
Molar Ratios ◽  
Induction Periods ◽  
Induction Times ◽  
Kinetics Of ◽  
Kinetics Of Vulcanization

Abstract The dependence of the kinetics of vulcanization of natural rubber by sulfur in presence of N-cyclohexyl-benzothiazolylsulfenamide (CHBS) on temperature and for varied molar ratios of accelerator and sulfur was more exactly investigated. 1. Although the vulcanization of natural rubber accelerated with N-cyclohexyl-benzothiazolylsulfenamide with conventional experimental conditions is characterized by very long induction times, nevertheless—as also for other accelerated vulcanization reactions—sulfur decrease follows a time law with nt<1 and the activation energy, as in other cases, is calculated to be 29 kcal/mole. 2. The value nt=0.6 is found for the exponent of the time law, independent of temperature and concentration of reactants. 3. The concentration dependence of the rate of sulfur decrease under conditions of constant sulfur content and increasing sulfenamide concentration is found to be consistent with catalysis by an intermediate compound. 4. The actual accelerator may be essentially cyclohexylammonium-benzothiazoylmercaptide. 5. The temperature and concentration dependence of the induction periods for sulfur decrease and crosslinking were investigated. The course of crosslinking with reaction time, which is characterized by strong reversion, is discussed.

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.

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.

Studies of the Vulcanization of High Elastic Polymers. II. Vulcanization of Natural Rubber with Thiuram Disulfides. Part 2

1956 ◽  
Vol 29 (1) ◽  
pp. 15-28 ◽  
Author(s):  
Walter Scheele ◽  
Otto Lorenz ◽  
Wilhelm Dummer
Keyword(s):  
Natural Rubber ◽  
Reaction Times ◽  
Order Reaction ◽  
First Order Reaction ◽  
Macromolecular Structure ◽  
Tetramethylthiuram Disulfide ◽  
First Order ◽  
Kinetics Of ◽  
Limiting Value ◽  
Insight Into

Abstract This paper gives some insight into the kinetics of thiuram vulcanization. The following results were obtained. 1. The decrease of concentration of thiuram disulfides during vulcanization is a first-order reaction in the range of short vulcanization times. At longer reaction times, departures occur. The process then takes place more slowly. 2. The departures from the course of a first-order reaction are displaced with decreasing temperature toward the range of longer vulcanization times. 3. At 87° C, the reaction of tetramethylthiuram disulfide is of the first order over the whole range of vulcanization temperatures chosen, and at this temperature no deviations whatever are observed. 4. The same relationships are found in the kinetic study of dithiocarbamate formation. 5. From the combined results, it is concluded that, besides the two reactions occurring independently of each other during vulcanization, the diffusion of the reaction partners also has some influence on the kinetics. 6. The limiting value of dithiocarbamate formation is correlated with the macromolecular structure of the polyisoprene, and it is shown to be probable that this limit is not determined fundamentally by the chemistry of the vulcanization reaction.

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.

scholarly journals Reactivity of Phenol and Aniline towards Quinolinium Chloro Chromate: A Comparative Oxidation Kinetic Study

2015 ◽  
Vol 59 ◽  
pp. 81-92
Author(s):  
Seplapatty Kalimuthu Periyasamy ◽  
H. Satham Hussain ◽  
R. Manikandan
Keyword(s):  
Oxidation Kinetic ◽  
Activation Parameters ◽  
Dipole Interaction ◽  
Reaction Rates ◽  
Hydrogen Ion ◽  
Aqueous Acetic Acid ◽  
Acetic Acid Medium ◽  
First Order ◽  
Different Temperatures ◽  
Kinetics Of

The kinetics of Oxidation of Phenol and aniline by quinolinium Chlorochromate (QCC) in aqueous acetic acid medium leads to the formation of quinone and azobenzene respectively. The reactions are first order with respect to both Phenol and aniline. The reaction is first order with respect to quinolinium chlorochromate (QCC) and is catalyzed by hydrogen ion. The hydrogen-ion dependence has the form: kobs = a+b [H+]. The rate of oxidation decreases with increasing dielectric constant of solvent, indicating the presence of an ion-dipole interaction. The reaction does not induced the polymerization of acrylonitrile. The retardation of the rate by the addition of Mn2+ ions confirms that a two electron transfer process is involved in the reaction. The reaction rates have been determined at different temperatures and the activation parameters have been calculated. From the above observations kinetic results a probable mechanism have been proposed.

Relaxation Processes in the Deformation of Unloaded Rubbers and the Influence of the Degree of Vulcanization

1951 ◽  
Vol 24 (4) ◽  
pp. 810-819
Author(s):  
B. A. Dogadkin ◽  
M. M. Reznikovskii˘
Keyword(s):  
Stress Relaxation ◽  
Natural Rubber ◽  
Elastic Deformation ◽  
Relaxation Processes ◽  
Spatial Network ◽  
Relaxation Properties ◽  
Constant Rate ◽  
Different Temperatures ◽  
Kinetics Of

Abstract 1. It is shown that the process of stress relaxation at different initial elongations as well as the process of deformation at constant rate for unloaded rubbers at different temperatures (20–70° C) can be represented quantitatively by equations suggested in earlier works. Likewise the possibility of expanding the theories proposed for the kinetics of high-elastic deformation of spatial polymers is substantiated. 2. It is shown that the relaxation properties of soft unloaded vulcanizates of natural rubber and many synthetic rubbers do not undergo essential changes during vulcanization. 3. The conjecture is expressed that the invariability of the relaxation properties during vulcanization continues until the bonds of the spatial network are distributed sufficiently widely not to influence the activity or heat movement of the chain segments between them.

Thermal Inactivation Kinetics of Sporolactobacillus nakayamae Spores, a Spoilage Bacterium Isolated from a Model Mashed Potato–Scallion Mixture

2016 ◽  
Vol 79 (9) ◽  
pp. 1482-1489
Author(s):  
HAYRIYE BOZKURT ◽  
JAIRUS R. D. DAVID ◽  
RYAN J. TALLEY ◽  
D. SCOTT LINEBACK ◽  
P. MICHAEL DAVIDSON
Keyword(s):  
Heat Resistance ◽  
Thermal Inactivation ◽  
Weibull Model ◽  
Inactivation Kinetics ◽  
Order Model ◽  
First Order ◽  
Different Temperatures ◽  
Spoilage Bacterium ◽  
D Values ◽  
Kinetics Of

ABSTRACT Sporolactobacillus species have been occasionally isolated from spoiled foods and environmental sources. Thus, food processors should be aware of their potential presence and characteristics. In this study, the heat resistance and influence of the growth and recovery media on apparent heat resistance of Sporolactobacillus nakayamae spores were studied and described mathematically. For each medium, survivor curves and thermal death curves were generated for different treatment times (0 to 25 min) at different temperatures (70, 75, and 80°C) and Weibull and first-order models were compared. Thermal inactivation data for S. nakayamae spores varied widely depending on the media formulations used, with glucose yeast peptone consistently yielding the highest D-values for the three temperatures tested. For this same medium, the D-values ranged from 25.24 ± 1.57 to 3.45 ± 0.27 min for the first-order model and from 24.18 ± 0.62 to 3.50 ± 0.24 min for the Weibull model at 70 and 80°C, respectively. The z-values determined for S. nakayamae spores were 11.91 ± 0.29°C for the Weibull model and 11.58 ± 0.43°C for the first-order model. The calculated activation energy was 200.5 ± 7.3 kJ/mol for the first-order model and 192.8 ± 22.1 kJ/mol for the Weibull model. The Weibull model consistently produced the best fit for all the survival curves. This study provides novel and precise information on thermal inactivation kinetics of S. nakayamae spores that will enable reliable thermal process calculations for eliminating this spoilage bacterium.

scholarly journals Reactivity and kinetic studies of benzofuran hydrodeoxygenation over a Ni2P-O/MCM-41 catalyst

2019 ◽  
Vol 44 (4) ◽  
pp. 307-315 ◽  
Author(s):  
Xueya Dai ◽  
Hua Song ◽  
Hualin Song ◽  
Jing Gong ◽  
Feng Li ◽  
...  
Keyword(s):  
Kinetic Studies ◽  
Time Analysis ◽  
Liquid Ratio ◽  
Order Model ◽  
Free Products ◽  
First Order ◽  
Reaction Rate Constants ◽  
Different Temperatures ◽  
Kinetics Of ◽  
Mcm 41

A nickel phosphide hydrodeoxygenation catalyst (Ni2P-O/MCM-41) was prepared using a new synthetic method. The as-prepared catalyst was evaluated in the hydrodeoxygenation of benzofuran, and the effects of reaction temperature, pressure, and the H2/liquid ratio were investigated. A pseudo first-order model was employed to describe the reaction kinetics of benzofuran hydrodeoxygenation over the Ni2P-O/MCM-41 catalyst. The reaction rate constants ( k1– k5) at different temperatures were determined according to this model. At 533 K, the conversion of 2-ethylphenol in to ethylbenzene began to increase dramatically, and the yield of O-free product, ethylcyclohexane, started to increase rapidly. At 573 K, 3.0 MPa, and a H2/liquid ratio of 500 (V/V), the conversion of benzofuran over Ni2P-O/MCM-41 reached 93%, and the combined yield of O-free products was 91%. Contact time analysis indicated that demethylation was not favored over the Ni2P-O/MCM-41 catalyst.

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