Factors affecting the mixing effect of open mill
- Time of issue:2022-04-27 21:22
Factors affecting the mixing effect of open mill
- Time of issue:2022-04-27 21:22
The main factors affecting the mixing effect of the open mixer are the roll wrapping property of the rubber compound, the loading capacity, the roll temperature, the roll distance, the speed ratio of the rolls, the feeding sequence, the feeding method and the mixing time.
1. Rollability of rubber
The quality of the roll wrapping of the rubber compound will affect the speed of powder consumption and the dispersion of the compounding agent during mixing. If the roll wrapping property is too poor, it cannot even be mixed. The rollability of the rubber compound is related to the properties of the raw rubber (such as Green strength, stretch ratio at break, maximum relaxation time, etc.), roll temperature and shear rate. The rubber has good rollability, such as NR; the raw rubber with low Green strength, small tensile ratio at break, and short maximum relaxation time has poor rollability, such as BR. Factors affecting these properties of raw rubber will affect the rollability of raw rubber. For example, adding reinforcing agents to increase the Green strength of the rubber compound and increasing the relaxation time will significantly improve the rollability of BR; adding too much liquid to the rubber compound The softener reduces the Green strength, shortens the relaxation time, deteriorates the rollability, and even rolls off. If the roll temperature is below the glass transition temperature (Tg) of the rubber compound, the roll cannot be wrapped, and if the roll temperature is above the viscous flow temperature (Tf), the rubber compound will stick to the roll and cannot be mixed, only within a certain temperature range between Tg and Tf. The rubber compound has good rollability and is suitable for mixing. If the shear rate is increased by reducing the roll distance, increasing the speed ratio or increasing the rotational speed of the roller, the tensile ratio at break of the rubber compound can be increased, the maximum relaxation time can be prolonged, and the roll wrapping property of the rubber compound can also be improved.
2. Glue capacity
If the glue loading capacity is too large, the amount of accumulated glue is increased, so that the accumulated glue rotates automatically above the roll gap, and loses the function of folded powder, which affects the intake and dispersion effect of the compounding agent, prolongs the mixing time, and improves the physical properties of the rubber. It will increase the energy consumption, increase the load of the rubber mixer, and easily damage the equipment. If the amount of glue loaded is too small, there is no or too little glue accumulation, it is difficult to eat powder, and the production efficiency is too low. Therefore, the amount of glue when mixing in the open mill should be appropriate.
3. Roller distance
The relationship between the shear deformation rate of the rubber compound passing through the roller distance and the roller distance, roller speed and speed ratio is:
γ——Mechanical shear rate, s-1;
f——the speed ratio of the drum, f=V1/V2;
V2——The surface rotation speed of the front roller, m/min;
V1——The surface rotation speed of the rear roller, m/min;
e——roller distance, mm;
Reduce the roll distance, the shear deformation rate increases, the shearing action of the rubber molecular chain and the compounding agent agglomerates increases, and the compounding agent agglomerates are easily broken, so it is conducive to the dispersion of the compounding agent, but the rubber molecular chain is sheared. The chance of breakage is also increased, and it is easy to break the molecular chain excessively, resulting in over-refining, and the molecular weight of the rubber is too low, which reduces the physical and mechanical properties of the rubber compound. If the roll distance is too large, the shearing effect is too small, and the compounding agent is not easy to disperse, which brings difficulties to the mixing operation. Therefore, when the mill is mixed, the roll distance should be appropriate. The appropriate roll distance is related to the amount of glue loaded, as shown in Table 1. When natural rubber and synthetic rubber are used together, the combined proportions are equal, and the total amount of rubber can be used to determine the roll distance; when the ratio of synthetic rubber is greater than that of natural rubber, the total amount of rubber can be used to determine the roll distance based on synthetic rubber.
4. Speed ratio and roller speed
When the speed ratio and roll speed increase, the influence on the mixing effect is consistent with the law of reducing the roll distance, which will speed up the dispersion of the compounding agent, but also aggravate the shearing of the rubber molecular chain, which is easy to overmix, which reduces the physical properties of the rubber. The temperature of the rubber compound is accelerated, and the energy consumption is increased. If the speed ratio is too small, the compounding agent is not easy to disperse and the production efficiency is low. The roller speed ratio of open mill mixing is generally in the range of 1.15 to 1.27.
5. Roller temperature
As the roller temperature increases, the viscosity of the rubber compound decreases, which is conducive to the wetting of the rubber compound on the surface of the solid compounding agent, and the powder eating is accelerated. , is not conducive to the dispersion of the compounding agent, and the amount of combined rubber will be reduced. Therefore, the temperature of the rollers should be appropriate when the mixer is kneading. Due to the different effects of temperature on the roll wrapping properties of different rubber compounds, the roll temperature should also be different when mixing different rubber compounds. The temperature of the front roll is higher than that of the rear roll for NR-covered hot rolls; while most synthetic rubber-covered cold rolls have a lower temperature of the front roll than the rear roll.
6. Feeding order
Improper feeding sequence during mixing will affect the uneven dispersion of the compounding agent, and cause scorch, roll off or over-mixing. The feeding sequence is one of the important factors related to the quality of the rubber compound. order. The order of addition is generally determined according to the principles of adding small dosage, large effect and difficult to disperse compounding agent first, adding more dosage and easy dispersing compounding agent later, adding temperature-sensitive compounding agent later, and adding vulcanizing agent and accelerator separately. Therefore, when kneading in the open mill, first add raw rubber, reclaimed rubber, masterbatch and other wrapping rolls. If there is a solid softener such as paraffin in the formula, it can be added after the rubber wrapping rolls, and then add small materials such as activators. (zinc oxide, stearic acid), accelerators, antioxidants, anti-scorch agents, etc., add carbon black and filler again, after adding carbon black and filler, add liquid softener, if carbon black and liquid softener When the dosage is large, the two can be added alternately, and finally the vulcanizing agent is added. If there is a super speed accelerator in the formula, it should be added together with the vulcanizing agent in the later stage. If there is white carbon black in the formula, because the surface of white carbon black has strong adsorption, it is easy to form hydrogen bonds between particles, which is difficult to disperse. It should be added before the small material, and it should be added in batches. For NBR, due to the poor compatibility of sulfur with it, it is difficult to disperse, so it should be added before the small material, and the accelerator in the small material should be added at the end.
7. Feeding method
Different feeding methods will also affect the powder eating speed and dispersion effect. If the compounding agent is continuously added to a certain fixed position, and other parts of the rubber compound do not eat powder, it is equivalent to reducing the powder-eating area, prolonging the powder-eating time, and slowing down the powder-eating time. , so it is not conducive to the dispersion of the compounding agent. When feeding, the compounding agent should be evenly sprinkled on the accumulation glue along the axis of the roller, so that the accumulation glue is covered with compounding agent. Time, less shear damage to the rubber molecular chain.