Rubber one-stage mixing method and two-stage mixing method
1. One stage mixing method refers to the method of preparing mixed rubber by adding all the ingredients in one go. It is commonly used to prepare rubber materials with good scorching performance, without or only containing a small amount of active ingredients. To prevent the temperature of the rubber material from rising too quickly during mixing, a low-speed mixing process is generally carried out in the internal mixer. The method is to sequentially add additives (including fillers, operating oils, vulcanizing agents, accelerators, etc.) to the plasticized raw rubber. The mixing cycle is determined based on the characteristics of the rubber material and the rotor speed. During a mixing process, vulcanizing agents and accelerators are usually added approximately 60 seconds before the end of the internal mixer, or added to the open mill after discharge. However, regardless of which method is used to add vulcanizing agents and accelerators, the temperature of the mixed rubber during operation must be controlled below 110 degrees (sulfur and accelerator system) or 120 degrees (DCP vulcanization system) to prevent scorching of the rubber material, and adjustments must be made according to different vulcanization systems. Complex agents that are not easily dispersed or in small amounts can usually be prepared into various mother gum particles or paste like mixtures for easy addition and rapid mixing into the gel.
A mixing method must strictly comply with the requirements of process conditions such as the order of adding additives, mixing temperature, and mixing time. At the same time, attention should also be paid to the pressure of the top bolt and the position of the top bolt during the mixing process. Compared to the two-stage or multi-stage mixing method, it saves pressing, cooling, and parking time between each stage, improves production efficiency, and reduces the manufacturing cost of the mixed rubber. But the premise is that there is a high requirement for the performance of the internal mixer (mainly cooling capacity, speed regulation ability, and temperature control ability), and there must be sufficient mixing time to achieve the required dispersion degree.
The disadvantages of the one-stage mixing method are: the plasticity of the prepared mixed rubber material is low (Mooney viscosity is high), carbon black and other additives are sometimes not easily dispersed evenly, which can cause the risk of scorching, and the physical and mechanical properties of the vulcanized rubber are relatively low.
The one-stage mixing method is only suitable for the preparation of general purpose (relatively slow vulcanization rate) mixed rubber. It should be used with caution for large internal mixers, and also for the preparation of high filling, high viscosity, or high hardness mixed rubber materials. As an improvement, the rubber discharged from the internal mixer can be fully cooled on the lower plate open mixer before adding vulcanizing agents and accelerators.
2. The two-stage mixing method is basically the same as the one-stage mixing method, except that it does not contain vulcanizing agents or highly active accelerators. Firstly, a section of mixed rubber (usually referred to as mother rubber) is made, and then the lower part is cooled and parked for a period of time (usually more than 16 hours). In order to better disperse carbon black in rubber and improve production efficiency, a fast (40r/min or above) internal mixer is usually used to prepare a section of master rubber, and sometimes the preparation process of master rubber is combined with the plasticizing process. The second stage of mixing is based on the first stage of mixing, supplemented by the addition of remaining blending agents such as vulcanizing agents and accelerators. The two-stage mixing can be carried out on the internal mixer or on the open mixer. Usually, slow speed internal mixers or kneaders are used for two-stage mixing, in order to add vulcanizing agents and accelerators at lower temperatures and achieve higher production efficiency. The second mixing time in the internal mixer is generally 2-4 minutes.
Two stage mixing can always improve the dispersion of fine carbon black particles. For certain rubber materials containing a large amount of this carbon black, regardless of the type of mixer used, it may be better to divide it into two or even more additions.
Advantages of two-stage mixing: better wettability and dispersibility are required, such as extruded sealing strip adhesive; Rubber materials containing rapid vulcanizing or foaming agents. If the temperature in the internal mixer is very high at the end of mixing, the two-stage mixing method can be used, which is to cool the rubber material in the first stage of mixing, park it, and then put it back into the internal mixer for the second stage of mixing with vulcanizing agent. For some low viscosity soft rubber, due to the increase in temperature in the later stage of mixing, the viscosity of the rubber material becomes lower, and the shear force generated during mixing is too small to open up carbon black aggregates or some agglomerated agent particles. This is achieved through two-stage mixing, which can improve the dispersibility of these low viscosity soft rubber materials.
The use of two-stage mixing method has a good effect on improving the quality of rubber materials and improving the process performance of rubber materials; It can significantly improve the dispersion uniformity of rubber materials and the physical and mechanical properties of vulcanized rubber; It can reduce the tendency of scorching caused by sustained high temperatures.
One of the main advantages of ethylene propylene rubber is that most of its rubber materials can be prepared by a one-stage mixing method for mixed rubber! When conducting two-stage mixing in an internal mixer, attention should be paid to:
1. The initial mixing temperature for adding sulfur and accelerator should be below 90 degrees, and the discharge temperature should not exceed 110 degrees. The mixing time should generally not exceed 2 minutes to prevent coking.
2. Adding moisture absorbent calcium oxide should generally not be mixed for a long time under conditions exceeding 120 degrees, otherwise it should be added by opening the mixing process.