Molecular Chains Of Rubber Research Articles

Preparation and properties of g‐C3N4 on natural rubber latex films

AbstractNatural rubber (NR) latex was used as template, melamine as precursors, g‐C3N4 (CN) were synthesized by thermal polymerization. The results of structure and morphology show that CNs have obvious two‐dimensional laminar structure and improved specific surface area, the specific surface area attend to 40.26 m2/g and 2.4 times to no NR latex added CN (0%‐CN) when 4 wt% NR latex added (4%‐CN). The CN (4%‐CN) particles were then compounded with NR latex to prepare NR/CN composites. The results of mechanical properties show that the tensile modulus, tear strength and the cross‐link density of NR/CN films increase with increment of CN loading, the mechanical properties improved obviously even just 1phr CN added. The SEM of tensile fracture surface show that CNs and NR combined firmly and the section is very rough. DMA results show that the addition of CNs increase glass transition temperature (T g) and decrease the peak values of tan δ. The mechanical properties of the NR/CN films were fitted by Guth‐Smallwood and Halpin‐Tsai models to understand the CN's reinforcing behavior. The dramatically improved mechanical properties and thermal stability are caused by good interfacial interactions between CN and rubber molecular chains.

Surface modification of silica nanoparticles by a polyoxyethylene sorbitan and silane coupling agent to prepare high-performance rubber composites

The interfacial interaction between nano-silica and rubber matrix is very important for the preparation of high-performance rubber composites. In this paper, we first proposed the use of TWEEN-20 as a new silica modifier, it has four long arms consisting of three polyether chains with terminal hydroxyl group and a fatty chain. The oxygen on the polyether can form a hydrogen bond with the silanol groups on silica surface, and the terminal hydroxyl groups can chemically react with the silanol groups without any VOCs. Moreover, the long fatty chain can weaken silica polarity to obtain a better compatibility with rubber, so that silica modified by TWEEN-20 with chemical reaction and physical absorption can homogeneously disperses in rubber matrix. Nextly, we prepared high-performance natural rubber (NR) composites by adjusting the ratio of TWEEN-20 to TESPT to adjust the physical and chemical interaction between nano-silica and rubber molecular chains. The results indicated that the performances, including the filler dispersion, static mechanical properties, and dynamic heating (viscoelastic self-heating), were optimal when the ratio of TESPT to TWEEN-20 was 2:1. In addition, one-third of TESPT was replaced by TWEEN-20 to prepare silica/rubber composites, which can reduce one-third of VOCs, improve “scorchy”, and achieve high dispersion of silica.

Research on Thermal Aging Characteristics and Mechanism of the Silicon Rubber Insulation Layer of Cable Joints

In order to solve the thermal aging problem of silicone rubber insulation layer of 220 kV integral prefabricated cable joints, the mass loss and thermal gravimetric (TG) were tested. The thermal aging mechanism of thermal degradation reaction of silicone rubber molecular chains was analyzed by gel content test and infrared spectrum test (IR). The results showed that the cross-linked network of the molecular chain structure gradually deteriorates, resulting in the increase of mass loss rate and the decrease of thermal stability. The results also showed that the increase of dual peak of differential thermal weight (DTG), the decrease of initial decomposition temperature and the remaining mass. With increasing aging temperature and aging time, the aging would be accelerated. These properties could reflect the degree of thermal aging of silicone rubber insulation layer. The results could also provide theoretical support for the preparation, operation and maintenance for silicone rubber cable joints.

Selection of stabilizer and coagulant for natural rubber latex colloidal system during diimide catalytic hydrogenation at semi pilot scale reaction

Hydrogénation of natural rubber latex improves its thermal properties such as thermal oxidative resistance. Hydrogenation converts unsaturated C double bond into saturated C single bond on natural rubber molecular chains without being followed by chain scissioning. The hydrogenated natural rubber is considered can substitute synthetic rubber such as chloroprene rubber, EPDM, and butyl rubber in the manufacture of outdoor rubber based product. The optimized hydrogenation require natural rubber latex colloidal system remains in the liquid phase during the reaction, however it should be easily coagulated at the end of hydrogenation to reach high yield of reaction. The aim of the research was to study the effect of latex colloidal stabilizer system and coagulant to the natural rubber hydrogenation efficiency. Insitu catalytic hydrogenation of natural rubber latex by diimide compound was run at semi pilot scale at capacity 2.5 kg latex concentrated/batch. The diimide was generated from oxidation of hydrazine hydrate with hydrogen peroxide. The hydrogenation used strong base and surfactant stabilizer type. While, organic acid and inorganic salt were selected as coagulant. The best of stabilizer and coagulant were determined based on qualitative analysis such as FTIR spectra, stability of natural rubber latex colloidal system during the reaction and visual characteristic of hydrogenated natural rubber. The result showed that anionic surfactant could ensure the colloidal natural rubber latex remains in liquid phase during hydrogenation which was indicated by the absence of latex coagulation and decrease possibility of side reaction. Further, organic acid was selected as the best coagulant for hydrogenated natural rubber latex. It was characterized by clear-coloured serum, and hard-non sticky hydrogenated natural rubber sheet.

Reinforcing mechanism of a novel hydrophilic nano-carbon black in natural rubber latex

ABSTRACTNovel water-dispersible carbon nanoparticles (PNASS-CBs) were produced by radical polymerization of sodium 4-styrenesulfonate (NASS) on the surface of carbon black (CB) in the solid state. Scanning electron microscopy (SEM) and the Payne effect results showed that the modified CBs were less likely to form particle networks and thus dispersed better in the natural rubber (NR) matrix, with an average size of 90 nm that was much less than that of the aggregated pristine CBs. We propose that the appropriate modification of CBs mitigates filler-filler interaction and enhances the filler-rubber interaction, which can also be proved by the higher bound rubber contents of the NRL/PNASS-CB composites. When a NRL/PNASS-CB composite is subjected to an outside force, e.g. tensile, more physically absorbed rubber chains (bound rubber) slip and self-adjust their absorbed spots on the CBs’ surface (stress redistribution) in order to jointly share the applied stress. This has a positive effect on the resistance to damage of the rubber molecular chains. Therefore, the addition of the hydrophilic CBs in NR latex leads to significant improvements in the mechanical properties of the NRL/PNASS-CB composites.

Analysis of physico-chemical properties for crumb rubber in process of asphalt modification

This study aimed to investigate the physical and chemical properties of crumb rubber in of asphalt modification. Dibutylphthalate (DBP) was chosen to simulate the light component in asphalt in the rubber soak test. After separatedfrom crumb rubber, the leaching solution sample was tested using gas chromatography–mass spectrometer (GC–MS) to analyze the interaction between crumb rubber and light component. Crumb rubber modified asphalt (CRMA) samples were respectively prepared under different conditions. Rubber particles were separated from asphalt by elution method and the morphology was studied with optical microscope. Volume expansion rate (Vr) was proposed to characterize rubber swelling in asphalt. In addition, infrared spectra (IR) and differential scanning calorimetry (DSC) tests were conducted on the asphalt samples removed from crumb rubber. The results and analysis indicated that physical change and chemical reaction exist in the interaction of crumb rubber with DBP. After swelling, the molecular chains of rubber were disentangled and released some smaller molecule materials and then dissolved into asphalt, which playing a role of asphalt modification, and the absorption peaks of typical functional groups increase significantly. But too high temperature or too much time may lead to excessive degradation or aggregation of crumb rubber as well as asphalt aging, which will cause the performances of CRMA deteriorate. The viscosity change is a macroscopic reflection of physical and chemical characterization of rubber in asphalt modification. From the perspective of swelling and degradation behavior, the optimalconditions are recommended that the dosage of crumb rubber is 20% approximately, preparation temperature is not higher than 195°C and the processing time is less than 1.5h.

Characterizing Mechanical Properties of Peroxide-Vulcanized Natural Rubber Latex Films

Natural rubber latex is the material of choice for the fabrication of thin elastic films in many products such as gloves and condoms owing to its high strength, elasticity, comfort in use, good barrier properties and ‘green image’ [1, 2]. This unique combination of characteristics has its origins in the intrinsic properties of the crosslinked polymer network within the rubber matrix. The crosslinking of rubber hydrocarbon chains by free radicals generated from peroxide has been discovered for many years [3]. In peroxide crosslinking reactions, organic peroxide decomposes to produce reactive free radicals that will react to release hydrogen ions from the carbon hydrogen in the polymer chain, encouraging formation of free radicals on the rubber molecular chains. As the free radicals react with the polymer chains, the carbon hydrogen in the chains act as reactive centre that combines with centres of other rubber chains to form a network of carbon to carbon bonds which serve as crosslinks [3, 4].

Tensile properties of styrene-butadiene rubber/silica composites with mercapto functional silane coupling agents: influences of loading method and alkoxy group number

The influences of alkoxy group number and loading method of silane coupling agents on the mechanical properties of a styrene-butadiene rubber/silica composite were investigated. Mercapto functional silane coupling agents with dialkoxy and trialkoxy structures were used. The pre-treatment method and the integral blend method were compared. Both the fracture stress and modulus at 200% strain were higher in the pre-treatment than in the integral blend for dialkoxy type composites. However, they were higher in the integral blend than in the pre-treatment for trialkoxy-type composites. The interaction between the silane chains on the silica surface and the rubber molecular chains at the interfacial region was estimated by 1H pulse nuclear magnetic resonance spectroscopy using an unvulcanized silica/rubber mixture. It was found that the binding of rubber molecular chains by the silane chains was higher in the pre-treatment system for dialkoxy-type composites, whereas it was higher in the integral blend for trialkoxy-type composites. The reason is proposed as follows: in the pre-treatment for dialkoxy type, a linear silane chain formed in the case of multi-layer coverage. The silane chain entangled with the rubber chain in the interfacial region and improved the reinforcement effect. For the trialkoxy type, a network structure formed using the pre-treatment method, lowering the amount of entanglement. However, in the integral blend for trialkoxy type, the formation of the silane network and the entanglement progressed simultaneously during the preparation process. A well-entangled interfacial region was formed.

The Mechanism of Vulcanization and the Action of Accelerators

Abstract Vulcanization of rubber is due to the formation of chemical interlinks between molecular chains of rubber. A number of investigators maintain that formation of these bonds is due to reactions of radical character. In the present paper data are presented which were obtained during the study of reactions in which elementary sulfur is liberated at room temperature. As a prototype of such reaction is the interaction of hydrogen sulfide and SO2 which in rubber causes the socalled Peachey vulcanization. The usual views on the mechanism of this process are that the activity of sulfur liberated in statu nascendi is high enough to enable it to react with rubber and to create the spatial structure of the vulcanizate. However, this is an error. We have shown that pure sodium-butadiene rubber, heated to 80° in nitrogen atmosphere, does not vulcanize by the Peachey procedure, e.g., it does not become insoluble and its modulus of elasticity does not reach finite values. Consequently, the reaction causing the vulcanizing effect has a more complex character. To elucidate the mechanism of vulcanization we have studied the reaction of benzothiazolyl disulfide (MBTS) with hydrogen sulfide. In a hydrocarbon medium these compounds react at room temperature forming quantitatively elementary sulfur and mercaptobenzothiazole (MBT). Kinetics of this reaction are shown in Figure 1. If this reaction is carried out in a 10% solution of sodium-butadiene rubber, then the sulfur adds to the rubber, but vulcanization as characterized by formation of a spatial structure does not occur. The rubber solution is not gelatinized. An analogous phenomenon is observed during interaction of benzoyl peroxide with hydrogen sulfide. Sulfur liberated in this reaction also does not cause crosslinking (vulcanization of rubber).

The Action of Vulcanization Activators

Abstract 1. Zinc oxide and stearic acid do not affect the rate of addition of sulfur to rubber in the vulcanization of pure sodium butadiene rubber in mixtures without accelerators. 2. In mixtures containing diphenylguanidine as accelerator zinc oxide and stearic acid do not affect the rate of addition of sulfur to rubber. 3. In mixtures containing mercaptobenzothiazole zinc oxide retards and stearic acid accelerates the addition of sulfur to rubber. 4. In a similar manner zinc oxide suppresses and stearic acid activates isotope exchange between elementary sulfur and sulfur of the thiol group in mercaptobenzothiazole. 5. Zinc oxide and stearic acid in mixtures with mercaptobenzothiazole increase the rate and degree of crosslinking of the molecular chains of rubber; zinc oxide has the greater influence on the degree, while stearic acid has the greater influence on the rate, of the crosslinking reaction. 6. In mixtures with diphenylguanidine the influence of vulcanization activators on the degree and rate of crosslinking is considerably less pronounced than in mixtures with mercaptobenzothiazole. 7. The kinetics of zinc sulfide formation during vulcanization has been studied and it was established that ZnS is formed as the result of reactions of zinc oxide and zinc compounds with thiol and polysulfide groups in the rubber. Model substances have been used to demonstrate other possible routes for the formation of zinc sulfide during vulcanization. The effect of zinc oxide and stearic acid on the rate and degree of crosslinking is associated with participation of these compounds in such reactions. 8. Isotope exchange between radioactive sulfur in the vulcanizate and elementary sulfur was used to follow the formation and changes in the numbers of polysulfide linkages during the vulcanization process. The amount of sulfur participating in isotope exchange as vulcanization proceeds at first increases, passes through a maximum, and then decreases, which indicates a regrouping of the polysulfide linkages with an increase in their number and a decrease of the average number of sulfur atoms per linkage. Zinc oxide decreases the degree of isotope exchange between the vulcanizate and elementary sulfur at all stages of vulcanization. 9. Vulcanization activators, by favoring a decrease in the number of sulfur atoms in the sulfur bonds, increase the heat stability of the vulcanizates. This effect of the activators was demonstrated by kinetic data on stress relaxation in deformed vulcanizates at 126°. 10. The cleavage and regrouping of polysulfide linkages in the presence of zinc oxide and zinc compounds is accompanied by the combination of part of the sulfur as zinc sulfide, which leads to a decrease in the number of newly formed crosslinks. This effect of zinc oxide is manifested in vulcanization reversion effects and in changes of vulcanizate properties under thermomechanical influences. 11. From the above experimental data the general conclusion may be drawn that the fundamental role of vulcanization activators does not lie in their influence on the kinetics of the addition of sulfur to rubber, but rather in their influence on the nature of the vulcanization structures formed and on changes in them in the course of vulcanization.

The Quantitative Analysis of Rubber Chemicals. I.

Abstract Studies of the reactions occurring during mastication and vulcanization of rubber must be regarded as still far from completed despite the many valuable experimental contributions. Although many theories have been developed on the mechanism of the processes to be discussed here, either based on the results of direct studies of the chemical processes or on the results of indirect physico-technological methods, one still has the impression that a thorough interpretation of all the relationships is not possible at present. Mastication and vulcanization are undoubtedly very difficult problems which are chosen by the experimenter mainly because of their importance. It is convenient to distinguish between those studies that deal with the elucidation of the reactions of the molecular chains of rubber during mastication, cyclization, and vulcanization, and those that deal with the performance of all the compounds which influence the course of these processes. At present we are interested in the second of these studies. In order to undertake the experimental work, it seemed to be desirable to have analytical methods by which we would be able to determine quantitatively the smallest amounts of accelerators, age resistors and, as well, the decomposition products of various of the rubber chemicals in question. We feel that the development of these methods is absolutely necessary for scientific studies of the kind mentioned and also desirable for the industrial laboratory. Many procedures for the analysis of rubber compounding ingredients, especially of vulcanization accelerators and age resistors, have been published; most of these, however, are for qualitative identification only. Besides precipitation and color reactions, physico-chemical methods are of considerable importance. Spectroscopic investigations of all kinds, and especially the characterization of substances by their ultrared, ultraviolet, and infrared spectra have been widely applied. Work on the application of polarography has become known. Chromatographic analysis has been put to use with marked success to separate compounds of interest and then to identify them by characteristic reactions.

The Theory of Vulcanization and the Action of Accelerators

Abstract 1. Vulcanization of rubber by benzothiazolyl disulfide (without sulfur) is a radical process. The benzothiazolyl radicals formed during heat dissociation either are absorbed by a double bond or accept the mobile hydrogen of the α-methylene groups of the molecular chains of rubber. The polymer radicals formed thereby react with the other molecular chains, leading to combination of the molecules through the —C—C— bonds into spatial formations characteristic of the vulcanizate. 2. Kinetic curves were obtained which describe the conversion of benzothiazolyl disulfide into mercaptobenzothiazole and combination with rubber molecules. 3. Changes of viscosity and molecular weight during the vulcanization of rubber solutions were studied by light-scattering. It was established that the kinetic curve of viscosity has a minimum, while the molecular weight increases to three times its original value toward the end of the process. 4. The number of —C—C— cross-links in the vulcanizate was calculated from the swelling maximum and equilibrium modulus of elasticity. The data obtained indicate that, on the average, two and not more than five elementary acts of union of the molecular chains of rubber are necessary for each benzothiazolyl radical. 5. Experiments on stress relaxation at 130° established that the vulcanizate contains —C—C— cross-links between the molecular chains of rubber. 6. The isotopic exchange of a radioactive vulcanizate with the diffused benzothiazolyl disulfide demonstrates the existence of benzothiazolyl groups in the structure of the rubber. 7. A scheme of the elementary radical reactions between rubber and benzothiazolyl disulfide which lead to vulcanization is given. 8. The kinetics of vulcanization of rubber with sulfur in the presence of benzothiazolyl disulfide was studied. The combination of sulfur follows a monomolecular law and the kinetic constant depends linearly on the concentration of accelerator. 9. In the earliest stage of sulfur vulcanization, benzothiazolyl disulfide is converted into mercaptobenzothiazole, which is consumed as vulcanization proceeds. At the same time the rubber reacts with the benzothiazol radicals, which initiate polymerization processes with the formation of —C—C— links between the molecular chains of rubber. 10. The ratio between —C—C— bonds and sulfide bonds in a vulcanizate depends on the ratio between accelerator and sulfur. 11. As in the case of vulcanization with benzothiazolyl disulfide, in sulfur vulcanization in the presence of an accelerator, the reactions involving the α-methylene groups of the molecular chains are of considerable importance in structure formation. Thus vulcanization can not be regarded as a process which proceeds only at the double bonds of the rubber molecules. Activation of vulcanization by disulfides and sulfenamide accelerators is due to a large degree to reaction between these accelerators and the rubber. 12. A theory advanced in the present article together with experimental data reveal the radical mechanism of vulcanization and the action of accelerators, as well as the existence of polymerization phenomena during this process.

Processes Resulting from Thermal Rupture of the Sulfur Bonds in Vulcanizates

Abstract In earlier works we had studied in detail the rupture of the molecular chains of rubber under the action of oxygen, high temperatures, mechanical stresses, and some other factors. V. A. Kargin and G. L. Slonimskii˘ have directed our attention also to the possibility of a thermal rupture of the polysulfide sulfur bonds which form the structural network of vulcanizates. Dogadkin and Tarasova observed a rupture of the polysulfide bonds in vulcanizates under the action of mechanical stresses at temperatures of 100° and 130° C, and also on dissolving the vulcanizates in Synthol (motor oil) at 180–190°. All the experiments were carried out by the authors in the absence of oxygen. There are no indications to be found in the literature as to the behavior of the free radicals which are formed as a result of the rupture of sulfur bonds. The present work is devoted to an exploration of this important question. The experiments were performed with vulcanizates of three types: (1) monosulfide (mainly intermolecular bonds of the type, —C—S—C) ; (2) polysulfide (mainly intermolecular bonds of the type, —C—Sn—C—) ; (3) heat-vulcanizate (—C—C— bonds). Vulcanizing conditions were chosen such that the several vulcanizates would have approximately the same equilibrium modulus.

Polymerization Phenomena in the Vulcanization Process

Abstract According to theories developed by the authors, vulcanization consists essentially in the formation of local chemical van der Waals bonds (polarization, orientation bonds) and hydrogen bonds between the molecular chains of rubber. These bonds are formed as a result of the chemical reaction of the rubber with the vulcanizing agent, oxygen, and accelerators, and with other components of the vulcanizing system which contain functional groups. The essential type of chemical bonds consists of atomic groups of the vulcanizing agent (in the case of sulfur vulcanizates, “bridge” sulfur). Hypotheses were advanced concerning the course during vulcanization of the polymerization processes, initiated by the free radicals which are formed by the vulcanizing agent, and leading to the formation of —C — C—bonds between the molecular chains of the rubber. However, no proof of the existence of such processes has been obtained ; furthermore, certain data, for example, data obtained by spectral analysis, contradict such an assumption. The present paper is devoted to a description of experiments which show the existence of polymerization processes involving free radicals in vulcanized systems which contain accelerators of the disulfide and sulfenamide type.

Investigations in the Field of Rubber Vulcanization. VII. Influence of Organic Accelerators on the Kinetics of Vulcanization and the Properties of Natural-Rubber Vulcanizates

Abstract 1. Vulcanization accelerators change all parameters of the kinetic strength curve during the vulcanization of natural-rubber mixtures with low sulfur contents. 2. Calculation of the kinetic constants of the fundamental vulcanization equation proposed by Dogadkin, Karmin, and Gol'berg shows that vulcanization accelerators affect both the kinetics of the interaction of rubber with sulfur and the kinetics of the interaction of rubber with oxygen. 3. Direct experiments on the oxidation of rubber have shown that tetramethylthiuram disulfide and diphenylguanidine retard the process of addition of oxygen to rubber, while mercaptobenzothiazole accelerates this process. 4. Data on the rate of plasticization and change in viscosity of rubber solutions during oxidation indicate that tetramethylthiuram disulfide and diphenylguanidine promote the disintegration of molecular chains of rubber during the oxidative destruction of the latter. 5. The activation energy of the process of oxidation of rubber in the presence of mercaptobenzothiazole corresponds to the activation energy calculated from the fundamental vulcanization reaction for the process of oxidative destruction. This provides additional proof of the participation of oxygen in the vulcanization process. 6. It has been established with the aid of the methyl iodide reaction that accelerators increase the bridge-sulfur content of the vulcanizate, which is present in the form of monosulfides, with one sulfur atom connected to an allyl type radical. 7. With increasing temperature, the tensile strength at the vulcanization optimum increases in mixtures containing tetramethylthiuram disulfide, decreases in mixtures containing mercaptobenzothiazole, and remains unchanged in mixtures containing diphenylguanidine. The limiting strength decreases in all cases with increasing temperature. This phenomenon is explained on the basis of the proposed concepts of the character of vulcanization kinetics and of the nature of the vulcanization optimum.

A Study of Electrostatic Charges Produced during Mastication of Rubber and of GR-S

Abstract A new electrostatic field strength meter has been described which measures and records, not only the magnitude of electrostatic fields, but also their polarity. This device has been used in a study of the mastication of rubber, and has furnished data which indicate that rubber becomes less negative, and may actually become positive, as mastication proceeds ; this indicates in turn an oxidation of the molecules as well as an electronic rearrangement and orientation of the rubber molecular chains. Data on GR-S polymerized at different temperatures show entirely different electrostatic breakdown curves, which, with other physical characteristics, indicate unusual properties of the crude low-temperature polymers; e.g., high molar weight, long-chain molecules, already or easily oriented, which do not oxidize or break up appreciably on mastication. A partial correlation of Mooney viscosity and electrostatic contact-potential and field strength has been shown for rubber and GR-S which have been masticated on a rubber mill. The new electrostatic-field strength meter shows promise as a new tool which can be used in processing control, and also as a research instrument for obtaining fundamental information on mastication, pigment dispersion and incorporation, as well as an aid in forming new concepts concerning the structure of matter.