Views: 0 Author: Site Editor Publish Time: 2022-05-30 Origin: Site
Abstract
Vulcanization is a necessary process in the manufacturing process of rubber products, and it is also a unique process in rubber processing. The rubber acquires the necessary physical, mechanical and chemical properties through vulcanization. The vulcanizing agent is a substance that can make the rubber molecular chain perform a cross-linking reaction, make the linear molecules form a three-dimensional network structure, reduce the plasticity, and increase the strength of the elastic agent. Metal oxides (MgO/ZnO), peroxides (2,5-dimethyl-2,5-di-tert-butylperoxyhexane, referred to as double-25), sulfur, thiocyanate ( Effects of TCY) four vulcanization systems on the vulcanization characteristics, physical and mechanical properties, thermal aging resistance and compression set properties of neoprene. The results show that the four vulcanization systems selected can vulcanize the neoprene well.
2. Changes in the structure and properties of rubber during vulcanization
2.1 Changes in physical and mechanical properties
2.2 Changes in physical properties
2.3 Changes in chemical stability
Keywords: vulcanization, vulcanization characteristics, vulcanization system, neoprene
Vulcanization is the most important process in the production process of rubber products. During this process, rubber has undergone a series of physical and chemical changes, and its physical and mechanical properties and chemical properties have been improved, making rubber materials a useful value. Therefore, vulcanization is of great significance to the application of rubber and its products.
Vulcanization is the process of chemical cross-linking reaction of rubber macromolecular chains under certain temperature, pressure and time conditions. The vulcanization process has a decisive influence on the properties of rubber-elastomers. In particular, the tensile strength, hardness, elasticity, and anti-swelling properties vary considerably during the vulcanization process. The size of this change is related to the choice of vulcanization auxiliaries added to the rubber to produce vulcanization and the vulcanization conditions. Other properties such as tensile strength, air tightness, low temperature flexure and electrical insulation properties change relatively little when the degree of vulcanization changes.
2. Changes in the structure and properties of rubber during vulcanization：
Before vulcanization, the rubber molecule is a linear structure in a curled shape, and its molecular chain has the independence of motion. The macromolecules interact with each other by van der Waals force. When subjected to external force, the macromolecular segment is prone to displacement. In terms of performance, it shows large deformation, large plasticity, low strength and solubility. After vulcanization, the rubber macromolecules are cross-linked into a network structure, and there is a main valence bond between the macromolecular chains, so that the relative motion of the macromolecular chains is limited to a certain extent. Under the action of external force, it is not easy to produce large displacements , the deformation decreases, the strength increases, the solubility is lost, and the swelling can only be limited.
During the vulcanization process of rubber, its molecular structure changes continuously, for example, the crosslinking density increases gradually within a certain vulcanization time. In fact, the chemical reaction that occurs during vulcanization is more complex, and both cross-linking and degradation reactions are taking place. The cross-linking reaction makes the rubber molecules into a network structure, and the degradation reaction breaks the bonds of the rubber molecules. In the early stage of vulcanization, crosslinking is the mainstay, and the crosslinking density increases. To a certain extent, the degradation reaction increases, and the crosslinking density decreases. The change of rubber molecular structure during vulcanization significantly affects various properties of rubber.
2.1 Changes in physical and mechanical properties：
The physical and mechanical properties of rubber products are the reflection of the interaction between the molecular structure and the complexing agents dispersed in the interior. During the vulcanization process, the molecular structure changes, and the physical and mechanical properties of the rubber also change. Of course, rubbers with different structures have different trends in physical and mechanical properties during the vulcanization process. During the vulcanization process of natural rubber, as the linear macromolecules gradually change into a network structure, the plasticity decreases, the tensile strength, strength, hardness and elasticity increase, while the elongation, permanent deformation, fatigue heat generation, etc. decrease accordingly. However, if the vulcanization time is prolonged, the tensile strength and elasticity will gradually decrease, and the elongation and permanent deformation will increase instead. For synthetic rubbers such as styrene-butadiene rubber, cis-butadiene rubber, and nitrile rubber, the physical and mechanical properties have similar changes during the vulcanization process, but as the vulcanization time continues to prolong, the changes in various properties are relatively flat, such as strength and other properties. After reaching the maximum value, it can be maintained for a long time.
2.2 Changes in physical properties:
During the vulcanization process of rubber, the crosslink density changes significantly. As the crosslinking density increases, the density of the rubber increases, and it is difficult for small molecules such as gases and liquids to move in the rubber. The macroscopic change is reduced air permeability and water permeability, and the relative molecular mass increases after crosslinking, making it difficult for solvent molecules to move in the rubber. There exists between rubber molecules, and the macroscopically realized solvent that can dissolve the raw rubber can only swell the vulcanizate, and the greater the degree of cross-linking, the smaller the swelling. Vulcanization also improves the thermal stability and service temperature range of the rubber.
2.3 Changes in chemical stability:
During the vulcanization process, the cross-linking reaction always occurs on the groups or atoms with relatively high chemical activity. These places are the weak links where the rubber is prone to the aging reaction. After vulcanization, cross-linking occurs in these places, the molecular structure changes, and aging occurs. The reaction is difficult to carry out. After the rubber forms a network structure, the diffusion of low molecules is more hindered, and the free radicals that cause the aging of the rubber are difficult to diffuse, which improves the chemical stability of the rubber.
Vulcanizing agent, also known as crosslinking agent, is also known as bridge agent in industry. It can make the rubber molecular chain crosslink, make the linear molecules form a three-dimensional network structure, reduce the plasticity, and increase the strength of the elastic agent. Except for some thermoplastic rubbers that do not require vulcanization, natural rubber and various synthetic rubbers need to be vulcanized with vulcanizing agents. After vulcanization, rubber has valuable use value, and its mechanical properties are greatly improved.
Vulcanizing agents are divided into two categories: inorganic and organic. Inorganic vulcanizing agents include sulfur, sulfur monochloride, selenium, tellurium, etc. Organic vulcanizing agents include sulfur-containing accelerators (such as accelerator TMTD), organic peroxides (such as benzoyl peroxide), quinone oxime compounds, polysulfide polymers, urethanes, maleimide derivatives Wait.
Rubber vulcanizing agents include elemental sulfur, selenium, tellurium, sulfur-containing compounds, peroxides, quinone compounds, amine compounds, resin compounds, metal oxides, and isocyanates. The most commonly used are elemental sulfur and sulfur-containing compounds.
The vulcanizing agent is suitable for all kinds of natural rubber and synthetic rubber, and different vulcanizing agent varieties can be used together according to the needs.
For example, the vulcanizing agent PDM has the chemical name N,N'-m-phenylene bismaleimide. It is a multifunctional rubber additive, which can be used as a vulcanizing agent in the rubber processing process, as a co-vulcanizing agent in a peroxide system, as an anti-scorch agent and a tackifier, that is, it is suitable for general-purpose rubber. , is also suitable for special rubber and rubber and plastic systems. In natural rubber, when combined with sulfur, it can prevent vulcanization reversion, improve heat resistance, reduce heat generation, anti-aging, and improve the adhesion between rubber and cord and the modulus of vulcanizate. Used in truck tire shoulder rubber, buffer layer and other rubbers, it can solve the problem of shoulder emptying of biased truck tires, and can also be used in large-sized and thick natural rubber products and various rubber miscellaneous products.
Discussion: Influence of vulcanization system on the properties of neoprene vulcanizates
As a general-purpose rubber, neoprene has good physical and mechanical properties, as well as excellent aging resistance, flame resistance, chemical resistance, and oil resistance. Neoprene rubber is usually vulcanized using metal oxides (zinc oxide/magnesium oxide or lead oxide), and sometimes organic accelerators (such as thiourea compounds) are used to improve the vulcanization speed and degree of crosslinking.
Peroxide can not only vulcanize saturated carbon chain rubber and heterochain rubber, but also vulcanize unsaturated carbon chain rubber. The carbon-carbon single bond is formed during the vulcanization process, the bond energy is high, no soluble substances are produced during the reaction process, and the vulcanizate has good water resistance, such as peroxide (2,5-dimethyl-2,5- Di-tert-butylperoxide) is generally liquid or paste, which is beneficial to its dispersion in the rubber matrix.
Sulfur as a vulcanizing agent for chloroprene rubber is still less studied at present. The reason is that the traditional theory believes that polar chlorine atoms are connected to the double bonds of the main chain of CR rubber macromolecules, which attract the active π electrons on the double bonds. , reducing the double bond activity and reactivity, and also reducing the activity of a-hydrogen atoms, so generally do not use sulfur vulcanized CR rubber. In this study, sulfur and accelerator system were used as the vulcanization system of CR rubber, and the result is that the vulcanizate has better performance.
Thiocyanic acid (TCY) can cross-link rubbers containing halogen atoms, and is mainly used as a cross-linking agent for acrylate rubber, chlorosulfonated polyethylene, etc., and there are very few reports on its use as a CR cross-linking agent [1].
The following discusses the effects of several vulcanizing agents on the vulcanization characteristics, physical and mechanical properties, heat aging resistance, crosslinking density and high temperature compression set of neoprene rubber to provide reference for actual production.
(1) MgO/ZnO, peroxide, sulfur and TCY can all realize the vulcanization of CR rubber. Among them, peroxide and sulfur vulcanize faster, and the other two are slower;
(2) The sulfur vulcanized CR vulcanizate has the best mechanical properties, but its aging properties are not good; the mechanical properties of TCY and peroxide vulcanizates can reach the level of traditional metal oxides, and their heat aging resistance is very good. ; The comprehensive mechanical properties of metal oxide vulcanizates are the best;
(3) The apparent crosslinking density of the vulcanizates of the four vulcanization systems is not much different;
(4) The compression set of vulcanizate containing TCY is the best, and the sulfur is poor.
