Design of Vulcanization Systems and Application of Vulcanization Accelerators
2012-04-26
The vulcanization system of rubber includes vulcanizing agents, accelerators, and activators. Generally speaking, the crosslink density of rubber is mainly related to the amount of vulcanizing agent used. Of course, increasing the amount of accelerators containing promoting groups can also improve the crosslink density in the form of single sulfur bonds or double sulfur bonds. In general, the amount of accelerators in sulfur vulcanization systems is relatively small, so the size of the crosslink density is mainly determined by the amount of sulfur.
The role of accelerators is to lower the crosslinking temperature of sulfur and speed up the sulfur vulcanization rate, essentially acting as a catalyst. Different types of accelerators have different activation energies, so the types of vulcanized rubber also vary. For example, EPDM and IIR have fewer unsaturated double bonds, resulting in slower vulcanization rates, and generally use highly active disulfide amine carbamate accelerators or thiuram accelerators as the main accelerators. In contrast, NR, IR, BR, SBR, and NBR, which have more unsaturated double bonds, have faster vulcanization rates and generally use thiazole or sulfenamide accelerators as the main accelerators. The choice of main accelerators is usually some acidic accelerators containing vulcanization groups, which can participate in the vulcanization reaction and help reduce the amount of non-vulcanized substances in the vulcanized rubber, thereby reducing the blooming phenomenon.
Main accelerators can be used alone or in combinations of two or more. Generally speaking, the stronger the activity of the accelerator, the higher the activation energy and the stronger the acidity. When using two or more main accelerators, it is important to pay attention to their synchronous activation. The amount of the more active main accelerator should be less, while the amount of the slightly less active main accelerator should be more, in order to achieve acid balance and synchronous activation.
In high unsaturated rubber, the main vulcanization accelerators are generally used alone. However, sometimes two main accelerators are used together to achieve the best co-vulcanization characteristics or to design a vulcanization system with a longer induction period.
In low unsaturated rubbers like EPDM and IR, two or more main accelerators are generally used because the vulcanization rates of these two types of rubber are slow, and the solubility of the accelerators in the rubber is low. If only one accelerator is used, the amount will definitely increase significantly, and unreacted accelerators and non-vulcanized substances will be expelled from the surface, resulting in blooming. When using multiple main accelerators, it is also important to pay attention to their synchronous activation; otherwise, the less active accelerator may not react completely and remain in the rubber, leading to blooming when the temperature of the vulcanized rubber decreases or when affected by external conditions.
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