bluuebunny/crossref_metadata_2025 huggingface dataset:
The Role of Free Radicals in Low Temperature Vulcanization of Butadiene Rubber
Authors: Dogadkin B. A. , Belyaeva E. N.
Published: February 2011

1. Elementary sulfur, liberated in the nascent state at room temperature in the reactions of MBTS with H2S, of benzoyl peroxide with H2S and SO2 with H2S, does not bring about vulcanization of butadiene rubber. In the case of the system MBTS/H2S we observe combination of sulfur in amounts 1.2 to 1.6% to a small portion of the rubber, which does not lead to structurization. The main part of the rubber (about 90% by weight) does not, according to spectroscopic analysis, alter. The combination of sulfur with rubber observed in this case takes place, apparently, according to an ionic mechanism. 2. Low-temperature vulcanization (structurization) of rubber by the system MBTS/H2S becomes apparent with prior irradiation of solutions of rubber containing disulfide with diffuse or ultraviolet light. The rate of structurization depends upon the duration of irradiation and is governed by the interaction with the H2S of the polymeric rubber radicals which are formed as a result of the dehydrogenation of the rubber by the benzothiazolyl radicals which are formed in the photodissociation of the disulfide. 3. Structurization of rubber by the system benzoyl peroxide/hydrogen sulfide is observed in the presence of an amine, in particular PBNA, necessary for the formation of free benzoate radicals as a result of the reaction of the peroxide with the amine. The peroxide in the present case acts similarly to the benzothiazolyl radicals in the case of the system MBTS/H2S. 4. Peachey type low-temperature vulcanization (SO2/H2S) proceeds in the presence of the peroxides of the rubber itself. Prior heating of the solutions of rubber upsets structurization. 5. In the vulcanization of rubber by the systems MBTS/H2S and benzoyl peroxide/hydrogen sulfide we observe combination of sulfur with the rubber in amounts of 0.6 to 0.7% and a considerable loss of double bonds, reaching 60% for 1:4 type bonds and 75% for 1:2 type bonds. 6. Radical chain interaction schemes are put forward for the processes of low-temperature structurization (vulcanization) of rubber under the action of the systems MBTS/H2S, benzoyl peroxide/hydrogen sulfide and SO2/H2S. 7. The reaction of benzoyl peroxide with PBNA is studied. A new compound, O-benzoyl-N-phenyl-N-2-naphthylhydroxylamine, is obtained, which is a powerful inhibitor of rubber oxidation.
Determination of Di-O-Tolylguanidine Dicatechol Borate in Rubber Compounds
Authors: Collins J. Oliver , Goethel Walter R. , Hei Jacob O.
Published: February 2011

Since knowledge of the presence of an antioxidant in a rubber stock is important in its subsequent treatment, a rapid, and at least semiquantitative method of analysis for it is desirable. At this institution two spot tests have been devised to determine the presence of the di-o-tolylguanidine salt of di-catechol borate (DOTG-DCB), known commercially as “Permalux”. One test is applicable to cured rubber stock, the other to uncured stock. No other simple method of analysis for DOTG-DCB has been available. The formula assigned to DOTG-DCB is (see PDF for diagram) The presence of DOTG-DCB in uncured stock is detected by the immediate production of a yellow color upon coupling with diazobenzenesulfanilic acid; a positive test in cured stock is the development of a blue color at low concentrations, or an aqua color if not present, when the stock, acetone treated, is treated with sodium hydroxide. If DOTG-DCB is present in very large amounts no color is produced. From the intensity of the colors developed, by use of color standards, it is possible to estimate semiquantitatively the amount in the stock.
Stress Relaxation Studies of Scission in Rubber Vulcanizates
Authors: Mercurio A. , Tobolsky A. V.
Published: February 2011

Deterioration of many vulcanized hydrocarbon rubbers is known to be due to reaction with molecular oxygen. Such a process is independent of oxygen concentration down to several mm of oxygen pressure and hence proceeds quite readily in an air atmosphere provided that experiments are conducted with thin enough samples to eliminate oxygen diffusion effects. Two chemically distinct loci for attack by oxygen are available. These are the crosslinked sites which are added during the vulcanization process and the network chains which are essentially the same as in the unvulcanized material. The prime objective of this study is to show clearly that vulcanized natural rubber suffers oxidative scission predominantly along the polyisoprene chains and not at the crosslinked sites as proposed by Berry and Watson. Other literature has appeared which indicates that this important point needs further clarification. Stress relaxation experiments, which measure the rate of breaking of the weakest chemical bonds recurring throughout the structure, have been utilized. If the crosslinks are oxidized, then similar rates of scission should be obtained for different chain structures so long as the common crosslink is present in all of them. On the other hand, if chains are oxidized, then rates of scission should be essentially independent of the crosslinking agent used but rather depend markedly on each chain structure. The five chain structures used in this study are depicted in Table I. In each case at least a few per cent of double bond-containing segments are present in the main chain to allow for ordinary chemical vulcanization methods. All of these have been crosslinked by sulfur and by a nonsulfur containing agent.
Mastication. IX. Shear-Dependence of Degradation on Hot Mastication
Authors: Mullins L. , Watson W. F.
Published: February 2011

The rate of degradation of rubber during mastication is minimal at about 115° C, degradation increasing progressively on lowering or raising the temperature as far below or above this temperature as practicable. Designation of the degradation processes with the negative and positive temperature coefficients as “cold” and “hot mastication”, respectively, is supported by differences in their chemical mechanisms. The essential degradation step of cold mastication is rupture of rubber molecules by the imposed deforming forces to radicals which are converted to the degraded molecules after reaction with oxygen or other radical acceptor present in the rubber. Hot mastication is less well understood; scission appears to be by an oxidative reaction, with the implication that mastication serves in the main to expose fresh surfaces for absorption of oxygen. The mechanicodegradation of cold mastication produces an unusually rapid decrease in viscosity with molecular weight, logarithmic plotting of these two quantities giving an apparent α, according to the relation [η]=KMα, of greater than unity. The value of α for fractionated rubber is 0.67. The apparent high α is most readily explicable by a decrease in K on degradation, while α has its normal value of 0.67 for natural rubber. A decrease in K would occur if the molecular weight distribution around the mean became sharper. This distribution change has been considered to be caused by rupture only of molecules above a certain molecular weight, and this in the central sections of the molecules. For the oxidation of thin films and latex or any other chemical process whereby scission conceivably occurs with equal probability at every monomeric unit, the chain-length distribution would tend to a random one. In particular, if the rubber had initially a random distribution, viscosity-molecular weight decrease would yield an α equal to that for fractionated rubber and a K which is Γ (α + 2) times higher. Viscosity-molecular weight data on hot mastication have not been previously published. If a normal chemical degradation takes place, a difference in the viscosity-molecular weight relationship from that on cold mastication should be detectable. Differences in molecular weight distribution should also be reflected in differences in other properties of masticated rubber and, to a lesser extent, of vulcanized products. A comparison of viscosity-molecular weight relationships and other properties of rubbers masticated in air forms a starting point for answering the complex query of the relative merits of cold and hot mastication. Interaction of rubber with fillers and vulcanizing ingredients would then have to be systematically investigated.
Measurement of the Dynamic Modulus of Rubber at Very Small Amplitudes with an Optical Interferometer
Authors: Borovitskaya N. M.
Published: February 2011

A method of measuring very small differences of optical path using light modulation by an interferometer has been described. This enables one to measure amplitudes of mechanical vibrations of the order of 0.01 A or even smaller. We have used this method to measure the dynamic modulus of rubber at very small amplitudes. It is desirable to use this method for measuring the dynamic modulus of rubber and other highly elastic substances, firstly because in accuracy and simplicity it has certain advantages over existing acoustic methods. In the second place, it enables one to work at very small amplitudes and, in association with other methods, over a very wide range of amplitudes of deformation. This is of interest in connection with the general theory of the dynamic properties of rubber, and in particular, for checking the linearity of these properties, that is, the independence of the modulus and the loss on the amplitude, in particular for filled rubbers. We will briefly describe the apparatus with which the measurements were made. It consists of a Michelson interferometer and a source of monochromatic light. The interference pattern (lines of equal width) is projected onto the cathode of a photomultiplier. The output of the photo multiplier is fed to the input of a narrow-band filter, to the output of which the measuring instrument is connected. The mirror on the interferometer is set up so that the whole of the interference pattern, or the greater part of it, is uniformly illuminated. If now one of the mirrors vibrates with a frequency ω and an amplitude of z, then the difference in path between the interference beams will vary with a frequency of ω and the intensity of the light falling on the photomultiplier will also change. The current through the photomultiplier will be modulated with a frequency of ω. The first harmonic of the photocurrent will be I1=AJ1(4τz/λ), where λ is the wavelength of light; A is a coefficient depending on the intensity of the interfering beams and on their difference of path with the mirror at rest; I1 is a first order Bessel function.
Stress Waves and Fracture Surfaces
Authors: Andrews E. H.
Published: February 2011

Surface markings observed in the brittle fracture of certain frozen rubbers display striking, wavelike characteristics. They have been identified as “Wallner” lines, caused by the interaction of the fracture front with stress waves emanating from the region of the fracture front itself. These oscillations are possibly caused by the supply of excess energy to the fracture tip. A value of approximately 1:2 is obtained for the ratio of fracture velocity to wave velocity.
The Rolling Resistance of a Wheel with a Solid Rubber Tire
Authors: Evans I.
Published: February 2011

The rolling resistance of a wheel with a solid rubber tire, running on a rigid plane surface, is calculated on the assumption that it is due to hysteresis losses in cyclic deformation of the rubber. The results and certain other deductions from the theory are in reasonably good agreement with experimental observations.
Vulcanization of Elastomers. 24. The Effect of Fillers on the Course of Vulcanization Reactions. I
Authors: Scheele Walter , Mau Günter , Kemme Gregor
Published: February 2011

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.
Vulcanization of Rubber in the Presence of N,N-Diethyl-2-Benzothiazolylsulfenamide as Accelerator
Authors: Dogadkin B. A. , Beliatskaya O. N. , Dobromyslova A. B. , Feldshtein M. S.
Published: February 2011

1. The vulcanization of rubber in the presence of N,N-diethyl-2-benzothiazolylsulfenamide is characterized by an S-shaped curve for the addition of sulfur with an initial induction period in the reaction. The modulus and number of crosslinks are changed in an analogous manner to the structure of the vulcanizate. 2. The energy of activation of the addition of sulfur in the initial period is equal to 30 kcal per mole as against 14 kcal per mole in the main period. 3. The induction period is increased if the sodium-butadiene rubber is purified from alkali. 4. Molecular oxygen present in the compound being vulcanized decreases the induction period and increases the rate of the addition of the sulfur in the main period. An induction period is not observed when vulcanization is carried out in an atmosphere of pure oxygen. 5. The interaction of N,N-diethyl-2-benzothiazolylsulfenamide with rubber (in the absence of sulfur) at vulcanization temperatures is accompanied by the formation of MBT, diethylamine, and the addition of the elements of the accelerator to the rubber. The kinetics of this process were studied. 6. The interaction of N,N-diethyl-2-benzothiazolyl sulfenamide with rubber leads to the formation of chemical crosslinks between the molecules of rubber (the effect of vulcanization). 7. The change of N,N-diethyl-2-benzothiazolyl sulfenamide under the conditions of normal sulfur vulcanization has the same character as in the interaction of it with rubber. The kinetics of the formation of MBT have a maximum which coincides with the maximum rate of the addition of sulfur to the rubber. 8. A mechanism is presented for the vulcanization and acceleration actions of N,N-diethyl-2-benzothiazolyl sulfenamide which provides for the extraction of hydrogen by the accelerator radicals from the molecular chains of the rubber with the formation of MBT, diethylamine and polymer radicals which are able to interact with the sulfur.
The Action of Binary Accelerator Systems of Vulcanization
Authors: Dogadkin B. A. , Feldshtein M. S. , Belyaeva E. N.
Published: February 2011

1. It was established that in the joint use of vulcanization accelerators there takes place mutual activation of the accelerators, activation of one accelerator or additive action depending on the chemical structure of the accelerators. 2. Of the studied combinations of vulcanization accelerators the systems showing mutual activation are disulfides or mercaptans with nitrogen containing organic bases, and disulfides with sulfenamides. Sulfenamides with nitrogen containing organic bases result in activation of one of the accelerators. Examples of systems of accelerators with additive action are combinations of sulfenamides or disulfides with TMTM. 3. It was established that in the case of systems with mutual activation, such as MBTS with DPG or with CHBTS, the reaction between the accelerators produces MBT. For such systems, greater yields of MBT were observed under vulcanization conditions than were obtained by action of the accelerators alone. This indicates that the principal result of the chemical reaction of the accelerators is the formation of free radicals which are capable of splitting hydrogen from the rubber molecule with the formation of rubber high polymer radicals. 4. There is presented a tentative scheme for a radical mechanism of the joint action of vulcanization accelerators for the case where mutual activation of accelerators exists.
Composition of Vulcanizing System, Course of Vulcanization and Bond Strength of Plied-Up SBR
Authors: Dogadkin B. A. , Feldshtein M. S. , Pevzner D. M.
Published: February 2011

Insufficient bond strength between individual plies, in particular between the breaker strip and tire tread, is one of the shortcomings of tires manufactured from SBR rubbers. For several reasons, one of which is the conditions in which such large articles as tires are vulcanized, the breaker strip contains a much more active vulcanizing system than the tread rubber. The vulcanization of these rubbers in a zone of direct contact is effected at virtually the same temperature. In considering the special features of the vulcanization of SBR rubber, as one which vulcanizes comparatively slowly, it was natural to expect that the different activities of the vulcanizing systems in joined plies condition a sharp difference in the course of vulcanization, which might affect the bond strength considerably. We therefore made a study of the effect of the composition of crosslinking system and kinetics of vulcanization on the bond strength of plied-up SKS 30 rubber.
The Crosslinks in TMTD-Zinc Oxide-Natural Rubber Vulcanizates
Authors: Moore C. G.
Published: February 2011

The mechanism of vulcanization of natural rubber (NR) by means of tetramethylthiuram disulfide (TMTD) and zinc oxide has been the subject of much recent experimental work and speculation. While such studies have clarified the kinetics and stoichiometry of TMTD decomposition and zinc dimethyldithiocarbamate formation, they have not directly aided our knowledge of the nature of the crosslinks in the resultant vulcanizate. It was earlier suggested that the vulcanizate contained only C—C crosslinks, comparable with those formed by means of di-tert-alkyl (and aralkyl) peroxides, while more recently, disulfide crosslinks have been proposed. However, neither of these views has been confirmed by direct chemical studies of the vulcanizate. Evidence is now presented which shows that there can be no appreciable amount of C—C crosslinking by the TMTD-ZnO combination, and this is consistent with the view that sulfur crosslinks predominate. This conclusion is based on the respective swelling properties in n-decane of dicumyl peroxide vulcanizates (containing only C—C crosslinks) and TMTD-ZnO vulcanizates, which have been treated with methyl iodide in vacuo at 80° C. This reagent is known to cause the fission of C—S and S—S bonds in variously constituted organic mono- and polysulfides and should therefore cause the fission of sulfur crosslinks in a vulcanizate with consequent degradation of the network (cf. Ref. 8), whereas there is no evidence that di-allylic C—C crosslinks can be similarly degraded.
The Reaction of Tetramethylthiuram Disulfide and Monosulfide with Rubber
Authors: Dogadkin B. A. , Shershnev V. A.
Published: February 2011

The kinetics of the reaction of tetramethylthiuram monosulfide (TMTM) differ from that with the disulfide (TMTD). No formation of chemical crosslinks takes place; the sulfur of TMTD adds in correspondence with a curve exhibiting no maximum and the addition differs in the presence and absence of zinc oxide. Simultaneously, the thermal decomposition of TMTM takes place. The electron paramagnetic resonance spectra and the reaction with geraniol show TMTD and TMTM to interact with rubber through a free radical stage.
Metal Oxides in Tetramethylthiuram Disulfide Vulcanization
Authors: Dogadkin B. A. , Shershnev V. A.
Published: February 2011

Rubber is usually vulcanized with the aid of the so-called activators, metal oxides, zinc oxide being the one most often used. In vulcanization in the presence of MBT (mercaptobenzothiazole) or DPG (diphenylguanidine) as accelerators it was found that vulcanization activators have almost no effect on the rate of addition of sulfur to rubber, but have a significant influence on the rate and degree of crosslinking of the rubber molecules. Special interest attaches to studies of the action of metal oxides in vulcanization with tetramethylthiuram disulfide (TMTD), as it is known from actual practice that in the absence of zinc oxide this accelerator does not bring about vulcanization. Vulcanization with TMTD was studied on mixtures of natural rubber (extracted with cold acetone in a stream of nitrogen for 50 hours) and of synthetic isoprene rubber (SKI) masticated on microrolls, of the following compositions (in parts by weight).
Stress Relaxation during the Photoxidation of Peroxide Crosslinked Rubber
Authors: Dunn J. R. , Scanlan J. , Watson W. F.
Published: February 2011

The chemical reactions involved in the thermal and photochemical aging of natural rubber vulcanizates are largely unknown. Experimental difficulties have precluded direct chemical investigation owing to the insolubility of the crosslinked networks and the small extent of reaction required for a great deterioration in physical properties. Accordingly recourse has been made to physical measurement. Tobolsky et al. (e.g. Ref. (1)) have shown that the relaxation in stress on holding a rubber strip at constant extension during aging is a convenient experimental measure capable of interpretation in terms of network breakdown. In photochemical studies the opacity of conventional sulfur vulcanizates provides a further complication. In the present work, the stress relaxation technique has been used in a study of the degradation by 365 mµ radiation of the comparatively transparent and chemically simple rubber networks obtained after crosslinking by organic peroxides.
Electron Paramagnetic Resonance of Radicals Formed during Milling
Authors: Bresler S. E. , Zhurkov S. N. , Kazbekov E. N. , Saminskiĭ E. M. , Tomashevskiĭ E. E.
Published: February 2011

It is well known that during the mechanical degradation of polymers there takes place scission of molecular chains and the formation of macroradicals. It is of considerable interest to study the electron paramagnetic resonance (EPR) spectra of the macroradicals produced by milling, and to compare them with the spectra of the macroradicals formed in the process of polymerization, and also during the irradiation of polymers by gamma rays and neutrons. We may endeavor to compare the amount of macroradicals formed with the extent of mechanical destruction (for instance with the area of the new interface which is formed). In addition, as was found by experience, the macroradicals formed by mechanical scission are good models for the investigation of reactivity since practically all of them are in the newly formed surface layers and are therefore very accessible to various chemical influences. They enter easily into reaction with various agents present in the medium since in this process diffusion from the surface is found in practice not to be a predominating factor. In the present communication we give the first EPR results obtained on mechanically degraded polymers.
The Mechanism of Radiation Damage to Elastomers. II. Crosslinking and Antirad Action
Authors: Bauman Richard G.
Published: February 2011

All of the antirads studied serve to decrease the rate of crosslinking of carbon black loaded natural rubber during irradiation in a vacuum. Oxygen also serves very effectively as an antirad toward crosslinking. In air, some antirads enhance this protective effect of oxygen; others serve actually to increase the rate of crosslinking. Mechanisms sufficient but not necessary to explain these observations are presented. They involve reactions of the antirads with polymer radicals, viz.: R⋅+ARH→RH+AR⋅ (stable) reactions of oxygen with polymer radicals, viz.: R⋅+O2→RO2⋅ and reactions of oxygen with antirads, viz.: ARH+O2→oxidized products The ratio of scissions to crosslinks ranges from 45 to 4.3 in air and 2.3 to 0.92 in vacuum. The sol-gel measurements of other workers show, however, that most (90–99%) of these scissions are only temporary chain breaks which are rapidly reformed.
Measurement of the Aging of Rubber Vulcanizates
Authors: Mandel J. , Roth F. L. , Steel M. N. , Stiehler R. D.
Published: February 2011

Ultimate elongation (strain at failure) can be used to assess the aging of all rubber vulcanizates. For this purpose, it appears that Equation (2) can be used to express the early part of the aging process, corresponding to a period of aging at room temperature of 10 or more years. Prediction of shelf aging from tests at two or more elevated temperatures is only possible if the relationship between aging and temperature is known. For some rubber compounds the Arrhenius equation appears to hold. In these instances, it provides an effective means for estimating shelf aging.
Rapid Identification of Polyisoprene in Cured and Uncured Compounds
Authors: Mano Eloisa B.
Published: February 2011

On account of the industrial development of synthetic elastomers the identification of the different types of rubber in artifacts became a necessity. Several methods have been tried and periodically reviewed by Bekkedhal and by Tyler and Higuchi. As far as the synthetic rubbers are concerned, the types being produced in industrial quantities have characteristic groups which permit their easy detection by chemical means. However, natural rubber, poly-cis-isoprene, presents a more difficult identification, especially if in admixture with other elastomers and compound ingredients, in vulcanizates—and the same is true for the synthetic poly-cis-isoprene, the production of which has just recently started. The use of special techniques as X-ray diffraction, infrared spectroscopy and chromatography permits the identification of the natural rubber hydrocarbon, but equipment for them is not always available for routine work. Simple techniques have been tried. Not mentioning methods which depend on experimenter interpretation, as behavior to burning, which furnishes valuable preliminary information for an experimenter, we verified that two tests in the British literature are of great utility. These consist of determining the swelling ratios in benzene, petroleum ether and aniline and the time of reaction in a mixture of concentrated nitric and sulfuric acids. As chemical reactions, characteristic of natural rubber and its reclaim, the old Weber reaction, the trichloroacetic acid reaction and the well known chromic oxidation to acetic acid are mentioned.
The Stereoregular Polymerization of Isoprene with Lithium and Organo-Lithium Compounds
Authors: Stearns R. S. , Forman L. E.
Published: February 2011

It has been shown that lithium metal and organolithium compounds, when used as catalysts for the polymerization of isoprene, yield a polymer containing 93–95% cis-1,4 configuration and 5–7% 3,4-addition product. The catalytic mechanism in the case of lithium and its compounds, irrespective of their solubility, appears to be homogeneous. It is suggested that the formation of a stereoregular polymer from isoprene and lithium is the result of the C—Li bond forming a coordination complex with the monomer in the cis configuration followed by rearrangement of this coordination complex to form a six-membered, cyclic, activated complex. The stability of the complex is a result of the ability of the lithium atom to undergo partial hybridization and thus assume a valency greater than one. Lewis bases, such as ethers, change the stereospecific aspects of the lithium-catalyzed polymerization of isoprene by the formation of a stable complex between the base and the carbon-bound lithium. This complex between the ether and the C—Li bond increases the ionic character of the C—Li bond to the extent that it becomes equivalent to that of the C—Na bond.
Infrared Analysis of Butadiene Polymers
Authors: Kimmer W. , Schmalz E. O.
Published: February 2011

Whereas older methods for the determination of 1,2-, cis-1,4 and trans-1,4 units in high polymers require pure olefins as calibration substances, this is no longer necessary with the newly developed method which is described in this paper. From 3 different polymer samples 3 dilution series are prepared and the infrared spectra are recorded. The extinction coefficients are extrapolated to c=0 by a modified base line procedure and the resulting operational extinction coefficients are coupled mathematically (determinants) to yield the actual extinction coefficients of the pure isomers. These may be expressed as mole per cent of the isomeric units for the individual polymer samples. The procedure described may also be applied to other chemical mixtures without the need for pure calibration substances.
General Considerations on the Structure of Crystalline Hydrocarbon Polymers
Authors: Natta G. , Corradini P.
Published: February 2011

Owing to the discovery of stereospecific polymerization processes effected in our Institute, we were able to determine, in the last few years, the crystal structure of a great number of new polymers. In this work we will discuss the general principles leading to the determination of the shape and mode of packing of macromolecules in the crystals.
The Crystal Structure of Cis-1,4 Polybutadiene
Authors: Natta G. , Corradini P.
Published: February 2011

In previous works, we announced the identification and discussed the structure of three of the four possible pure stereoisomers of polybutadiene ; two with 1,2 enchainment (isotactic and syndiotactic respectively) and the third with trans-1,4 enchainment. In a letter to the Editor of Angewandte Chemie we have given preliminary data concerning the crystal structure of the polymer with cis-1,4 configuration, of which we succeeded in preparing sufficiently long and crystallizable macro-molecular chains formed by cis-1, 4 units. All the various stereoisomeric polymers we examined were prepared with different catalysts, all obtained by reaction of metalalkyls on suitable compounds of transition elements. For all these catalysts the mechanism of the polymerization reaction is of the co-ordinated anionic type. The different behavior can be ascribed to the mode of co-ordination of the olefin with the metallorganic complex of the transition metal.
Effect of Crystallization of Polymers on Dielectric Loss
Authors: Mikhaĭlov G. P. , Sazhin B. I.
Published: February 2011

The values of ε′ and tan 5 in the range of dielectric losses of relaxation character decrease on crystallization of polymers. In the case of medium frequency relaxation losses tan δ sometimes decreases 2 to 4 fold and ε′ by 20 to 30%. This is mainly connected with transition of polar portions of macromoleules into crystallites, on the one hand, and on the other hand with an increase in the breadth of the relaxation spectrum for the polar groups remaining in the amorphous phase. The relaxation time for the medium frequency relaxation dielectric losses, connected with the thermal motion of segments of maeromolecules, in the amorphous phase, increases on crystallization, while for high frequency relaxation losses, reflecting the thermal motion of separate groups or links, does not change or indeed even decreases. The dielectric method may be used for the investigation of the kinetics of crystallization and melting of polymers and, in the case of polar polymers, also for the determination of the degree of crystallinity.
Adhesion Phenomena in the Bonding of Rubber to Metal by Means of Lei˘konat Bonding Agent. 2. Unfilled Vulcanized Rubber
Authors: Deryagin B. V. , Zherebkov S. K. , Medvedeva A. M.
Published: February 2011

1. The bond strength of vulcanized rubber to metal brought about by means of a bonding agent film is regulated by the ratio of the intensities of interaction of the bonding agent film with the contacting surfaces at the bonding agent- rubber and bonding agent-metal interfaces. Where these are approximately equal the bond strength is greatest. Where they differ sharply the bond strength decreases. 2. One of the factors influencing the intensity of interaction of a bonding agent film with vulcanized rubber is a reduction in the thickness of the bonding agent film ; with reduction in the thickness of the bonding agent film the intensity of interaction of the bonding agent with the vulcanized rubber falls; this is expressed in varying ways in the bond strength to metal of vulcanizates of various rubbers, causing a fall-off in some cases, and in some other cases a certain increase in the bond strength of the vulcanized rubber to the metal. 3. By means of the Lei˘konat bonding agent it is possible to bond unfilled vulcanizates based on the majority of existing polymers, whether polar or non-polar, to metal sufficiently strongly. 4. It is established that on increasing the number of polar groups in the polymer there is an increase in the intensity of interaction of the bonding agent film with the vulcanizate containing that rubber.
Vulcanization of Elastomers. 26. Vulcanization of Natural and Synthetic Rubbers by Sulfur in the Presence of Organic Bases
Authors: Scheele Walter , Redetzky Wolfgang
Published: February 2011

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.
A Method of Screening Antiozonants
Authors: Sullivan F. A. V. , Davis A. R.
Published: February 2011

A method has been developed for screening compounds for potential antiozonant use based upon the ability of the compound to suppress the reaction of ozone with cyclohexene when a mixture of the two is subjected to the action of a stream of ozonized air. The method is satisfactory as a tool for selecting compounds for testing in rubber. In a rubber formulation, however, there are many factors concerned with the functioning of a compound as an antiozonant in addition to its ability to react preferentially with ozone at a faster rate than the unsaturated linkages in the rubber. Testing the most promising compounds in a rubber formulation is required to establish their commercial value as antiozonants.
Vulcanization of Elastomers. 28. Vulcanization of Natural and Synthetic Rubbers with Sulfur in the Absence of Accelerators. I
Authors: Echte Elisabeth , Scheele Walter , Sonnenberg Sigrun
Published: February 2011

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.
Radical and Ionic Reactions of Tetraethylthiuram Disulfide
Authors: Klebanskiĭ A. L. , Fomina L. P.
Published: February 2011

1. Investigation of the process of radical dissociation of TETD in carbon tetrachloride indicates that at 145° thiuram radicals are formed which react with the carbon tetrachloride to form (C2H5)2N—C‖S—SCl and CS2. 2. A quantitative determination of the products of the base-influenced dissociation of TETD was carried out. A kinetic investigation of this reaction, in which initiator (K2S2O3) and inhibitor (PBNA) were shown to have no velocity effect whatever, indicates that the process is an ionic one. 3. It is shown that in the reaction of TETD with diethylamine, one of the reaction products appears to be diethylammonium diethyldithiocarbamate.
Vulcanization of SBR in a Thermal Neutron Field
Authors: Anderson Herbert R.
Published: February 2011

The nuclear reactions B10(n,α)Li7* and Li6(n,α)H3 can be used to generate highly ionizing particles in a pure thermal neutron field to vulcanize an otherwise transparent rubber compound. Vulcanizates have satisfactory physical properties comparable with those produced by other means of vulcanization, e.g., gamma rays and peroxides. Such vulcanizates possessed negligible residual radioactivity. The somewhat analogous initiating systems gave vastly different vulcanizates with the same parent rubber compound in a helium atmosphere, and a possible explanation is presented. The G values for lithium methoxide promotion (ca. 3) are comparable with those obtained with gamma-ray vulcanizates. Promotion is less efficient with boron nitride, the average values of G being approximately 0.9. The irradiation atmosphere has a pronounced influence on the type of vulcanizate produced and on the energy utilization. Substitution of air for helium as the irradiation atmosphere increases net crosslinking of cold rubber with boron nitride promotion, while it decreases it when lithium methoxide is used as the promoter.
The Attack of Ozone on Stretched Rubber Vulcanizates. I. The Rate of Cut Growth
Authors: Braden M. , Gent A. N.
Published: February 2011

The growth of cracks in the surface of stretched components of vulcanized rubber on exposure to the atmosphere is a serious deficiency. It detracts from the appearance, serviceability, and strength, and may lead to early failure. A comprehensive review of the process and the evidence for attributing it to the action of ozone in the atmosphere has been given by Newton, the following features being established. (1) The action of ozone on unstretched rubber is slight, no cracks being formed. (2) At small extensions of the order of 10%, cracks appear and grow to form large incisions in the exposed surface. (3) At higher extensions, the number of cracks is larger, but their final size is smaller due to mutual interference. The deterioration of the rubber may be considered less serious, therefore, since the component possesses a finely etched surface rather than one containing a few large cuts. The basic process appears to consist of the initiation and propagation of a single incision in a deformed rubber under the action of an atmosphere containing ozone. However, the experimental studies that have been reported previously have been concerned with the gross deterioration of an exposed surface when many cracks form simultaneously. It is difficult to deduce from such observations the behavior of a growing incision for the following reasons. Firstly, the presence of a crack will alter the surface deformations in its neighborhood, and hence may seriously affect the growth of nearby cracks. Secondly, cracks formed on the surface grow simultaneously in length and in depth, the deformation in the region of the growing edges presumably varying with the crack shape as well as with its size. Thirdly, the cracks may propagate by ozone attack at the growing tips and also by attack at points on the surface in close proximity to them. Finally, it is likely that surface irregularities will affect the local deformation, while foreign materials such as dust, decomposition products, or “bloomed” ingredients of the rubber compound may affect the access of ozone.
Energy Requirements of Mechanical Shear Degradation in Concentrated Polymer Solutions
Authors: Bestul A. B.
Published: February 2011

Molecular weight decrease by mechanical shearing results when solutions of around 10% of polyisobutene having average molecular weights above 500,000 are forced through a capillary at nominal rates of shear above 10,000 sec−1. Comparison of observed plots of shear load vs. duration of shearing at fixed rates of shear during this degradation process with the corresponding estimated plots which would be expected to obtain if degradation did not occur provide a means of evaluating the amount of applied shearing energy which is dissipated by the degradation process. The result is several hundred thousand kilo-calories per mole of broken bonds, which is several thousand times the bond energy of carbon-carbon bonds. This finding is consistent with the hypothesis that whenever a bond breaks the system loses much of the free energy temporarily stored in bonds and macromolecular chains located in a comparatively large volume surrounding the broken bond, these bonds and chains having been involved in concentrating the required activation energy into the ruptured bond.
Mechanochemical Reactions Leading to Reinforcement in Rubbers
Authors: Ceresa R. J.
Published: February 2011

The development of the single rotor internal mixer (at the British Rubber Producers' Research Association) has facilitated the research into mechanochemical reactions of a wide range of high polymers. The term “mechanochemical” has been applied to reactions such as mastication, the mechanism of which involves the primary step of mechanical scission of a polymer chain into polymeric free radicals at a carbon to carbon or other covalent bond. The processes which have been studied previously include the cold mastication of rubberlike polymers, the formation of carbon gel, the interpolymerization of two elastomers during blending, and block copolymerization by the cold mastication of polymers plasticized by vinyl monomers. The evidence for the polymeric free radical nature of these reactions has recently been reviewed.
Gel Formation in the Mastication of Natural Rubber, and Its Influence on Vulcanizate Strength
Authors: Dogadkin B. A. , Kuleznev V. N.
Published: February 2011

Angier and Watson observed formation of spatial structures (gels) in neoprene rubber, certain SBR fractions, and butadiene-acrylonitrile rubber, under the influence of powerful shearing forces during treatment in a specially designed scroll-type masticator. This gel formation was considered to be a specific characteristic of synthetic rubbers ; no one has reported gel formation in the mastication of natural rubber (NR). In our experiments an insoluble gel was formed in NR. Weighed samples of extracted smoked sheet were masticated in an argon atmosphere containing not more than 0.05% oxygen, on specially designed rolls, with 0.12 mm gap between the rolls cooled intensive with water. The formation of an insoluble fraction (gel) at the early stages of mastication was established in repeated experiments. Its maximum content (after 10 minutes of mastication) reaches 20%, and further milling results in mechanical dispersion of the gel fraction, so that after 60 minutes of mastication the rubber is again completely soluble in the common solvents. However, the masticated rubber contains formations branched to such an extent that most of them (∼80%) are precipitated from octane solution by centrifugation at 12,000 rpm for an hour. Molecular weight determinations, based on changes in the light scattering of the solutions at an angle of 90°, showed that the molecular weights of the sol fractions of the masticated samples increase during mastication, reaching 2,500,000 after 60 minutes. The true molecular weight of the particles is apparently of major significance, as a correction for the asymmetry of the scattering particles was not applied.
Macroradicals in the Mechanical Degradation of Polymers in the Glassy State
Authors: Butyagin P. Yu , Berlin A. A. , Kalmanson A. E. , Blyumenfeld L. A.
Published: February 2011

By means of the method of electron paramagnetic resonance a demonstration is given of the occurrence of macroradicals during the degradation of glassy polymers whether of organic (polystyrene, polyethylene, polymethyl methacrylate) or of inorganic (quartz) nature.
Production and Properties of Block Polymers
Authors: Kovarskaya B. M. , Golubenkova L. M. , Akutin M. S. , Levantovskaya I. I.
Published: February 2011

1. In special apparatus (a laboratory modifier and a continuous modifier) block polymers are produced from condensation resins and various starting polymers by a mcchanochemical method. 2. It is demonstrated that the physicochemical properties of the resulting block polymers differ essentially from the properties of a mechanical mixture of homopolymers, which makes it possible to produce polymers with new properties.
Friction Studies on Rubberlike Materials
Authors: Conant F. S. , Liska J. W.
Published: February 2011

One of the best summaries of the theory of friction between solid materials up to 1949 was published by Bowden and Tabor. However, little attention was devoted in this review to either theoretical or experimental results on the friction between rubberlike materials and other solids. In recent years, the literature on various aspects of the friction of rubberlike materials has become quite voluminous and unfortunately sometimes contradictory in the results and interpretations presented. This is understandable, to some extent, in the light of the fact that each individual research effort has usually been directed toward some particular aspect of the problem; certain, sometimes rather arbitrary, conditions are employed or assumed, and the results obtained are often rather unclear functions of these experimental or theoretical parameters. The situation is further complicated by the facts that at the very best, experimental frictional studies do not yield very accurate or readily reproducible data and the theories proposed, in many cases, are not subject to exact experimental verification. No comprehensive effort has been made, to date, to unify the various theories and to reconcile the conflicting experimental results on the friction of rubberlike materials and it is not the principal objective of the present review to do so. One of the aims is rather to report and summarize these sometimes divergent views in the hope that investigators in this field will be encouraged to study those areas in which knowledge is either inconclusive, contradictory or incomplete. Generally speaking, dynamic friction can be defined as the tangential reaction between two solids in relative motion in the presence of a normal force between the two solids. This definition can include both sliding and rolling friction. This review will be concerned principally with the sliding friction phase since rolling friction is generally conceded to be explained almost entirely by the internal friction of the deformable material. It has been stated, for example, that the rolling resistance of pneumatic tires at 80–95 mph is composed of 90–95% internal hysteresis, 2–10% surface friction and 1.5–3.5% air friction.