Scorch is one of the problems that are prone to occur in the rubber compounding process. Essentially, scorch is the premature vulcanization of the compound during processing, which leads to a series of product defects. In the rubber mixing process, the factors that cause the scorch of the rubber are as follows.
1. The amount of glue
In the final mixing stage, if the amount of glue loaded in the internal mixer is too large, it is easy to cause the temperature of the rubber to rise too fast, the temperature of the discharge to be too high, and even the scorch of the rubber. This is especially evident when mixing higher hardness compounds. Usually, the filling amount of the final compound in the internal mixer is recommended to be about 0.7. If the vulcanization operation is carried out on the open mill, the amount of accumulated glue should be controlled within a reasonable range. The too low accumulated amount of glue is not conducive to dispersion, and too much-accumulated glue will lead to poor heat dissipation, cause heat accumulation and cause scorch At the same time, excessive accumulation of glue will also cause problems such as low mixing efficiency.
2. Roller temperature
Each type of rubber compound has the best mixing temperature range. The roller temperature of the open mill is not suitable. The light one will cause the rubber compound to come off the roller and stick to the roller during the mixing process; the heavier one will cause the rubber compound to scorch. Therefore, adjusting the mixing temperature of the rubber compound is a very important part of the mixing process.
In the thin pass stage, if the roll temperature is too high, it is easy to cause scorch. Usually, the smaller the roll distance, the faster the rubber compound heats up, which is easy to causes the rubber compound to scorch. Especially in the case of thin-through and high-hardness rubber, the roll distance should not be too small.
3. Addition time of the vulcanizing agent
For most rubbers, it is generally appropriate to add the vulcanizing agent in the late stage of mixing. If the vulcanizing agent is added to the internal mixer, it is recommended to use two-stage mixing to prevent the rubber temperature from being too high when the vulcanizing agent is added, and it is difficult to find a balance between good dispersion and scorch. For one-stage mixing, especially when the temperature of rubber removal is high, it can be considered to operate on an open mill, and add a vulcanizing agent while cooling. On the premise of ensuring sufficient and uniform mixing, reducing the heating history of the vulcanizing agent is the key to preventing scorch.
The activator, accelerator, and other auxiliary agents should be added early to obtain a better dispersion effect.
In the case of high roller temperature and high temperature of rubber compound, if adding vulcanizing agent, the operation should be especially careful. When adding it to the open mill, the speed of tamping and rolling should be fast, and the roll distance should be larger when the thin pass is carried out.
4. The degree of dispersion of sulfur, vulcanizing agent, and accelerator
If the sulfur is not uniformly dispersed, it will lead to a local concentration of vulcanizing agent and accelerator, and it is easy to cause local scorch of the rubber compound. Therefore, designing a reasonable mixing process, ensuring the dispersion of the vulcanizing agent in the mixing rubber, and strengthening the control of the details of the rubber mixing stage may help to solve the problem of local scorch.
In the stage of tamping and turning, it is absolutely not allowed to turn only part of the rubber. Because this is not conducive to the dispersion of vulcanizing agents and accelerators, but is also prone to rubber scorch.
When mixing in the internal mixer, pulling the top bolt in time can improve the mixing effect of the rubber compound and prevent the dead angle in the internal mixer, which will cause the rubber compound to accumulate and then scorch.
5. Mixing time
Generally speaking, the mixing time should not be too long. All kinds of compounding agents have been uniformly dispersed in the rubber. If the mixing time is continued to be extended, the rubber will be "over-mixed", and the rubber molecular chain will be affected by mechanical shearing and heat. Under the action, it will be further broken. This not only reduces the physical and mechanical properties of the compound but also wastes energy and increases costs. Since long-term mixing will increase the temperature of the rollers and the rubber compound, the scorch probability of the compound will be greatly increased. Therefore, the mixing time should be determined according to the specific situation, so as to ensure the uniform dispersion of the additives.
6. The storage time of the final rubber
The final rubber is also prone to scorch after being stored for a long time. If the final rubber that has been stored for a long time has a tendency to scorch, consider turning it before use and adding an appropriate amount of anti-scorch agent.
7, the treatment method of scorch
During the mixing process, if it is found that the local rubber is scorched, the mixing should be stopped, and the scorched part should be removed to avoid glue lumps in the remaining rubber, which will affect the quality of the rubber and even cause the rubber to be scrapped. In addition, if there is already a scorching tendency of the rubber compound, in order to prevent the rubber compound from further scorching, it may be considered to mix or add a scorch inhibitor for tumbling.
For scorch-prone compounds, it is recommended to add a scorch retarder corresponding to the vulcanization system.
