The main reasons for the occurrence of “self sulfur” during the placement of mixed rubber materials are:
(1) Too many vulcanizing agents and accelerators are used;
(2) Large rubber loading capacity, high temperature of the rubber refining machine, insufficient film cooling;
(3) Or adding sulfur too early, uneven dispersion of the medicine materials causes local concentration of accelerators and sulfur;
(4) Improper parking, such as excessive temperature and poor air circulation in the parking area.
How to reduce the Mooney ratio of rubber blends?
The Mooney of rubber blend is M (1+4), which means the torque required to preheat at 100 degrees for 1 minute and rotate the rotor for 4 minutes, which is the magnitude of the force that hinders the rotation of the rotor. Any force that can reduce the rotation of the rotor can reduce Mooney. The formula raw materials include natural rubber and synthetic rubber. Choosing natural rubber with low Mooney or adding chemical plasticizers to the natural rubber formula (physical plasticizers are not effective) is a good choice. Synthetic rubber generally does not add plasticizers, but can usually add some low-fat so-called dispersants or internal release agents. If the hardness requirements are not strict, of course, the amount of stearic acid or oil can also be increased; If in the process, the pressure of the top bolt can be increased or the discharge temperature can be appropriately increased. If conditions permit, the cooling water temperature can also be lowered, and the Mooney of the rubber blend can be lowered.
Factors affecting the mixing effect of the internal mixer
Compared with open mill mixing, internal mixer mixing has the advantages of short mixing time, high efficiency, high degree of mechanization and automation, good rubber material quality, low labor intensity, safe operation, small drug flying loss, and good environmental hygiene conditions. However, the heat dissipation in the mixing room of the internal mixer is difficult, and the mixing temperature is high and difficult to control, which limits the temperature sensitive rubber materials and is not suitable for mixing light colored rubber materials and rubber materials with frequent variety changes. In addition, the internal mixer needs to be equipped with corresponding unloading devices for mixing.
(1) Glue loading capacity
A reasonable amount of glue should ensure that the rubber material is subjected to maximum friction and shear in the mixing chamber, so as to evenly disperse the mixing agent. The amount of glue installed depends on the characteristics of the equipment and the characteristics of the glue material. Generally, the calculation is based on the total volume of the mixing chamber and the filling coefficient, with a filling coefficient ranging from 0.55 to 0.75. If the equipment is used for a long time, due to wear and tear in the mixing room, the filling coefficient can be set to a higher value, and the amount of glue can be increased. If the top bolt pressure is high or the plasticity of the adhesive material is high, the amount of adhesive can also be increased accordingly.
(2) Top bolt pressure
By increasing the pressure of the top bolt, not only can the loading capacity of the rubber be increased, but also the contact and compression between the rubber material and the equipment, as well as between various parts inside the rubber material, can be faster and more effective, accelerating the mixing process of the compounding agent into the rubber, thereby shortening the mixing time and improving production efficiency. At the same time, it can also reduce the sliding of the material on the equipment contact surface, increase the shear stress on the rubber material, improve the dispersion of the compounding agent, and improve the quality of the rubber material. Therefore, currently, measures such as increasing the diameter of the top bolt air duct or increasing the air pressure are often taken to improve the mixing efficiency and quality of the mixed rubber in the internal mixer.
(3) Rotor speed and rotor structure shape
During the mixing process, the shear speed of the rubber material is directly proportional to the rotor speed. Improving the shear speed of the rubber material can shorten the mixing time and is the main measure to improve the efficiency of the internal mixer. At present, the speed of the internal mixer has been increased from the original 20r/min to 40r/min, 60r/min, and up to 80r/min, reducing the mixing cycle from 12-15 min to the shortest of l-1.5 min. In recent years, to meet the requirements of mixing technology, multi speed or variable speed internal mixers have been used for mixing. The speed can be changed at any time according to the characteristics of the rubber material and process requirements to achieve the best mixing effect. The structural shape of the internal mixer rotor has a significant impact on the mixing process. The protrusions of the elliptical rotor of the internal mixer have increased from two to four, which can play a more effective role in shear mixing. It can improve production efficiency by 25-30% and reduce energy consumption. In recent years, in addition to elliptical shapes, internal mixers with rotor shapes such as triangles and cylinders have also been applied in production.
(4) Mixing temperature
During the mixing process of the internal mixer, there is a large amount of heat generated, making it difficult to dissipate heat. Therefore, the rubber material heats up quickly and has a high temperature. Generally, the mixing temperature ranges from 100 to 130 ℃, and high-temperature mixing at 170 to 190 ℃ is also used. This process has been used in the mixing of synthetic rubber. The discharge temperature during slow mixing is generally controlled at 125 to 135 ℃, and during fast mixing, the discharge temperature can reach 160 ℃ or above. Mixing and too high temperature will reduce the mechanical shear action on the rubber compound, making the mixing uneven, and will intensify the thermal oxidative cracking of rubber molecules, reducing the physical and mechanical properties of the rubber compound. At the same time, it will also cause too much chemical binding between rubber and carbon black to generate too much gel, reducing the plastic degree of the rubber compound, making the rubber surface rough, causing difficulties in calendering and extrusion.
(5) Dosing sequence
Plastic compound and mother compound should be added first to form a whole, and then other compounding agents should be added in sequence. Solid softeners and small drugs are added before adding fillers such as carbon black to ensure sufficient mixing time. Liquid softeners must be added after adding carbon black to avoid agglomeration and difficulty in dispersion; Super accelerators and sulfur are added after cooling in the lower plate machine, or in the internal mixer during secondary mixing, but their discharge temperature should be controlled below 100 ℃.
(6) Mixing time
The mixing time depends on various factors such as the performance characteristics of the mixer, the amount of rubber loaded, and the formula of the rubber material. Increasing the mixing time can improve the dispersion of the blending agent, but prolonged mixing time can easily lead to over mixing and also affect the vulcanization characteristics of the rubber material. At present, the mixing time of the XM-250/20 internal mixer is 10-12 minutes.
Post time: May-27-2024
