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中文简体There are still different opinions on the vulcanization mechanism of rubber. This is because in the production process of rubber products, there are insoluble natural rubber samples and a large number of reactions that occur at the same time, making it difficult for people to study the vulcanization of rubber molecules into a complex polymer network. The rubber vulcanization proposed in the early stage The mechanism can be roughly divided into free radical mechanism and ion mechanism. Researchers represented by Bacon and Famer et al. believe that the allylic resonance of rubber makes the hydrogen on the adjacent methylene group of the double bond easy to replace. Therefore, in the rubber vulcanization process, the sulfur diradicals deprive the rubber a-methylene group of hydrogen is the beginning of the reaction. That is, the reaction process is a free radical process. Bateman et al. believe that the power supply of the double bond on the rubber breaks the -SS- bond of S8 and decomposes into ions, that is, the vulcanization process is an ionic reaction process. So far, the more mature research is the vulcanization acceleration mechanism of thiazole zinc salt and zinc dithiocarbamate.
The vulcanization acceleration mechanism of thiazole zinc salt
In 1964, Coran et al. proposed the vulcanization acceleration mechanism of 2-mercaptobenzothiazole (MBT) zinc salt based on the analysis results of vulcanizates: the thiazole zinc salt reacts with the added sulfur molecules to form MS-Sx-Sy-SM , MS-Sx-Sy-SM reacts with rubber hydrocarbon R to form a reactive intermediate. The active intermediate is a non-crosslinked polysulfide with a vulcanization promoting group at the end. When it slowly decomposes to generate free radicals, the active free radicals react with rubber hydrocarbons to obtain a vulcanizate.
In 1969, Manik et al. proposed different promotion mechanisms based on the influence of the introduction of fatty acids on thiazole accelerators. He believes that thiazole vulcanization accelerators and fatty acids produce ionic reactive intermediates during the vulcanization process, rather than free radicals as Coran said. First, stearic acid reacts with ZnO to form zinc stearate. Then, the zinc stearate reacts with the thiazole salt, through the coordination of the N atom in the thiazole zinc salt and the O atom in the zinc stearate salt to the zn atom, the Zn-S bond is activated to form a transition state (A), (A) reacts with sulfur molecules (S8) to generate reactive intermediates (B). (B) React with rubber hydrocarbon R to form complex MSSxR. MSSxR is unstable and decomposes positive and negative ions. These ions combine with rubber hydrocarbons to form vulcanizates.
The vulcanization acceleration mechanism of zinc dithiocarbamate
The reaction mechanism of zinc diethyldithiocarbamate and natural rubber has been reported in detail in the literature. However, due to the shortcomings of traditional methods, people are constantly exploring new research methods. Since the 1980s, people have adopted the model compound (MCV) method (model compound means that the molecular structure is similar to the real rubber molecule, but the size is smaller.), by means of HPLC (High Performance Liquid Chromatography) to observe cross-linking Precursor and speculate on the subsequent sulfur crosslinking model. However, because the various hardening reactions of MCV occur simultaneously, it becomes difficult to observe the reaction pathways followed by individual components.
In order to overcome this problem, in the mid-1990s, the Nieuwenhuizen research group of Leiden University developed a new method, that is, under the conditions of simulating the vulcanization process, the sulfur-containing cross-linked low molecular weight model compound and its precursor Conduct research to understand the changing chemical pathways and the catalysis of complexes. By using this method, combined with quantum chemical calculations, they respectively revealed a large number of homogeneous catalytic reactions that occur during vulcanization of zinc dithiocarbamate (ZDMC) and zinc dimercaptobenzothiazole (ZMBT), including precursors. Formation, desulfurization, degradation and sulfur cross-linking reactions. The uniqueness of its research lies in: (1) Using quantum chemistry calculations and matrix-assisted laser desorption ionization mass spectrometer, it is the first time theoretically and experimentally to confirm the existence of zinc dithiocarbamate sulfur-rich complex intermediates. For a long time, it has been believed that there is a sulfur-rich zinc accelerator complex in the vulcanization process. The complex plays a central role in the vulcanization process, that is, it can activate the ground state sulfur and help exchange and transfer during the rubber vulcanization process. S atoms, and affect the formation of S cross-links. However, the S-rich zinc dithiocarbamate polysulfide complex is very active and can quickly release the linked S into a suitable S acceptor, so its presence cannot be detected by the usual spectroscopic techniques. Using a matrix-assisted laser desorption ionization mass spectrometer, the isolated complex was processed in a vacuum environment (to prevent the conversion or loss of S atoms). As a result, it was detected that the polysulfide complex could be enriched to four S atoms. (2) The use of model compounds under simulated vulcanization conditions revealed the rubber vulcanization acceleration mechanism of zinc dithiocarbamate and zinc thiazole salt.
The vulcanization acceleration mechanism of sulfenamides
Regarding the mechanism of sulfenamide accelerators promoting sulfur vulcanization in the presence of activators such as zinc oxide and stearic acid, it is generally believed that during the vulcanization process, the accelerator molecule first breaks at the SN bond, and the broken group is The zinc oxide reacts to form a zinc salt, and the other part is converted into an amine base. After that, the formed amine base forms a complex with the zinc salt in the form of a complexing agent. The complex can open the ring of sulfur to form an active vulcanizing agent, and the polysulfide bond in the vulcanizing agent is further broken under vulcanization conditions, and cross-linking-vulcanization reaction occurs with the rubber molecules. It takes a certain time from the break of the accelerator molecule to the occurrence of cross-linking, that is, the induction period or scorch time during vulcanization. At this time, the rubber molecules are not cross-linked.
Performance characterization of vulcanization accelerator
Rubber vulcanization acceleration efficiency is an important criterion to measure the quality of accelerators. According to reports, the characterization of accelerators at home and abroad is mainly carried out from two aspects: vulcanization acceleration characteristics and physical and mechanical properties of vulcanizates. The vulcanization acceleration characteristics mainly examine the aspects of vulcanization rate, Mooney scorch time, normal vulcanization time, normal vulcanization temperature, vulcanization flatness in the over-vulcanization stage, and resistance to reversion of vulcanization. The physical and mechanical properties of the rubber compound mainly examine the hardness, elasticity, tensile properties, friction properties and thermal aging properties of the vulcanizates. However, in recent years, people have done a lot of research on the effect of accelerators on the dynamic viscoelastic properties of vulcanizates. In fact, the effectiveness of the accelerator depends on the physical and mechanical properties of the vulcanizate it imparts, and the nature (type and density) of the cross-linking bond in the vulcanizate plays a decisive role in its application and working characteristics. The strength and dynamic mechanical strength of vulcanizates not only depend on the properties of the polymer chain itself, but also directly related to the number of network support chains (referring to the connection chain between two connection points) in the total cross-linking network. The crosslink density determines the number of network supporting chains. According to reports, the hardness and tensile stress of vulcanizates increase with the increase of cross-linking density, tear strength, fatigue life, toughness and tensile strength begin to increase with the increase of cross-linking density, and after reaching a certain maximum value, Decrease with the increase of crosslink density. Hysteresis and permanent deformation characteristics decrease with the increase of crosslink density.
Dynamic mechanical properties are another important means to characterize rubber properties. Especially tire tread rubber, it directly affects the wet skid resistance and rolling resistance of the tire tread. Dynamic mechanical properties are characterized by dynamic viscoelastic curves. In the process of systematic research, people realized that the tanδ value at 60℃ can reflect the rolling resistance of the vulcanized rubber during rolling, the tanδ value at 80℃ reflects the heat generation performance, and the tanδ value at 0℃ can characterize the vulcanized rubber. The anti-slip performance.
The vulcanization acceleration mechanism of thiazole zinc salt
In 1964, Coran et al. proposed the vulcanization acceleration mechanism of 2-mercaptobenzothiazole (MBT) zinc salt based on the analysis results of vulcanizates: the thiazole zinc salt reacts with the added sulfur molecules to form MS-Sx-Sy-SM , MS-Sx-Sy-SM reacts with rubber hydrocarbon R to form a reactive intermediate. The active intermediate is a non-crosslinked polysulfide with a vulcanization promoting group at the end. When it slowly decomposes to generate free radicals, the active free radicals react with rubber hydrocarbons to obtain a vulcanizate.
In 1969, Manik et al. proposed different promotion mechanisms based on the influence of the introduction of fatty acids on thiazole accelerators. He believes that thiazole vulcanization accelerators and fatty acids produce ionic reactive intermediates during the vulcanization process, rather than free radicals as Coran said. First, stearic acid reacts with ZnO to form zinc stearate. Then, the zinc stearate reacts with the thiazole salt, through the coordination of the N atom in the thiazole zinc salt and the O atom in the zinc stearate salt to the zn atom, the Zn-S bond is activated to form a transition state (A), (A) reacts with sulfur molecules (S8) to generate reactive intermediates (B). (B) React with rubber hydrocarbon R to form complex MSSxR. MSSxR is unstable and decomposes positive and negative ions. These ions combine with rubber hydrocarbons to form vulcanizates.
The vulcanization acceleration mechanism of zinc dithiocarbamate
The reaction mechanism of zinc diethyldithiocarbamate and natural rubber has been reported in detail in the literature. However, due to the shortcomings of traditional methods, people are constantly exploring new research methods. Since the 1980s, people have adopted the model compound (MCV) method (model compound means that the molecular structure is similar to the real rubber molecule, but the size is smaller.), by means of HPLC (High Performance Liquid Chromatography) to observe cross-linking Precursor and speculate on the subsequent sulfur crosslinking model. However, because the various hardening reactions of MCV occur simultaneously, it becomes difficult to observe the reaction pathways followed by individual components.
In order to overcome this problem, in the mid-1990s, the Nieuwenhuizen research group of Leiden University developed a new method, that is, under the conditions of simulating the vulcanization process, the sulfur-containing cross-linked low molecular weight model compound and its precursor Conduct research to understand the changing chemical pathways and the catalysis of complexes. By using this method, combined with quantum chemical calculations, they respectively revealed a large number of homogeneous catalytic reactions that occur during vulcanization of zinc dithiocarbamate (ZDMC) and zinc dimercaptobenzothiazole (ZMBT), including precursors. Formation, desulfurization, degradation and sulfur cross-linking reactions. The uniqueness of its research lies in: (1) Using quantum chemistry calculations and matrix-assisted laser desorption ionization mass spectrometer, it is the first time theoretically and experimentally to confirm the existence of zinc dithiocarbamate sulfur-rich complex intermediates. For a long time, it has been believed that there is a sulfur-rich zinc accelerator complex in the vulcanization process. The complex plays a central role in the vulcanization process, that is, it can activate the ground state sulfur and help exchange and transfer during the rubber vulcanization process. S atoms, and affect the formation of S cross-links. However, the S-rich zinc dithiocarbamate polysulfide complex is very active and can quickly release the linked S into a suitable S acceptor, so its presence cannot be detected by the usual spectroscopic techniques. Using a matrix-assisted laser desorption ionization mass spectrometer, the isolated complex was processed in a vacuum environment (to prevent the conversion or loss of S atoms). As a result, it was detected that the polysulfide complex could be enriched to four S atoms. (2) The use of model compounds under simulated vulcanization conditions revealed the rubber vulcanization acceleration mechanism of zinc dithiocarbamate and zinc thiazole salt.
The vulcanization acceleration mechanism of sulfenamides
Regarding the mechanism of sulfenamide accelerators promoting sulfur vulcanization in the presence of activators such as zinc oxide and stearic acid, it is generally believed that during the vulcanization process, the accelerator molecule first breaks at the SN bond, and the broken group is The zinc oxide reacts to form a zinc salt, and the other part is converted into an amine base. After that, the formed amine base forms a complex with the zinc salt in the form of a complexing agent. The complex can open the ring of sulfur to form an active vulcanizing agent, and the polysulfide bond in the vulcanizing agent is further broken under vulcanization conditions, and cross-linking-vulcanization reaction occurs with the rubber molecules. It takes a certain time from the break of the accelerator molecule to the occurrence of cross-linking, that is, the induction period or scorch time during vulcanization. At this time, the rubber molecules are not cross-linked.
Performance characterization of vulcanization accelerator
Rubber vulcanization acceleration efficiency is an important criterion to measure the quality of accelerators. According to reports, the characterization of accelerators at home and abroad is mainly carried out from two aspects: vulcanization acceleration characteristics and physical and mechanical properties of vulcanizates. The vulcanization acceleration characteristics mainly examine the aspects of vulcanization rate, Mooney scorch time, normal vulcanization time, normal vulcanization temperature, vulcanization flatness in the over-vulcanization stage, and resistance to reversion of vulcanization. The physical and mechanical properties of the rubber compound mainly examine the hardness, elasticity, tensile properties, friction properties and thermal aging properties of the vulcanizates. However, in recent years, people have done a lot of research on the effect of accelerators on the dynamic viscoelastic properties of vulcanizates. In fact, the effectiveness of the accelerator depends on the physical and mechanical properties of the vulcanizate it imparts, and the nature (type and density) of the cross-linking bond in the vulcanizate plays a decisive role in its application and working characteristics. The strength and dynamic mechanical strength of vulcanizates not only depend on the properties of the polymer chain itself, but also directly related to the number of network support chains (referring to the connection chain between two connection points) in the total cross-linking network. The crosslink density determines the number of network supporting chains. According to reports, the hardness and tensile stress of vulcanizates increase with the increase of cross-linking density, tear strength, fatigue life, toughness and tensile strength begin to increase with the increase of cross-linking density, and after reaching a certain maximum value, Decrease with the increase of crosslink density. Hysteresis and permanent deformation characteristics decrease with the increase of crosslink density.
Dynamic mechanical properties are another important means to characterize rubber properties. Especially tire tread rubber, it directly affects the wet skid resistance and rolling resistance of the tire tread. Dynamic mechanical properties are characterized by dynamic viscoelastic curves. In the process of systematic research, people realized that the tanδ value at 60℃ can reflect the rolling resistance of the vulcanized rubber during rolling, the tanδ value at 80℃ reflects the heat generation performance, and the tanδ value at 0℃ can characterize the vulcanized rubber. The anti-slip performance.
