Why should the pre-vulcanization temperature not exceed 205℃

There’s no such thing as that; the sulfurization temperature depends on the catalyst. It’s not that low. Generally, it involves several stages: 180°C before sulfur injection, followed by a constant temperature of 230°C, then another constant temperature of 260°C, and finally 280°C. Usually, the temperature doesn’t exceed 280°C. This is the requirement for the several catalysts I’ve worked with.

Is the problem not clearly stated? The decomposition temperature of vulcanizing agents is generally around 205 degrees, so it is not advisable to add the vulcanizing agent too early when the temperature in the bed is too low, to avoid excessive temperature rise once the required temperature is reached due to an excess of vulcanizing agent in the reactor

You didn’t explain it clearly enough. Catalyst sulfidation is a reaction process; the sulfidation time is generally 5 to 7 days. The amount of sulfiding agent (carbon disulfide or dimethyldisulfide) used is also directly related to the degree of sulfidation. Generally, we require that at the initial state (around 170 degrees), the vulcanizing agent be injected first; heating is not allowed until a relatively high concentration of hydrogen sulfide is not detected at the end of the system, that is, until the vulcanizing agent has penetrated the reactor. Since most catalysts exist in an oxidized state (to facilitate transportation and storage), in a hydrogen environment, when the temperature reaches 230 degrees or higher, the catalyst begins to be reduced to its metallic form in the presence of hydrogen as a reducing agent. However, the metallic form of the catalyst has no activity; thus, the catalyst loses its functionality permanently and can no longer act as a catalyst. Therefore, the temperature must be strictly controlled before vulcanization; once the vulcanization process is carried out according to the parameters provided by the catalyst manufacturer, there will be no problems.