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The main factors affecting the mixing effect of the open mill are the wrapping roll of the rubber material, the capacity of the rubber material, the temperature of the roller, the distance between the rollers, the speed ratio of the roller, the feeding sequence, the feeding method and the mixing time.

1. Roll wrapping property of rubber compound

The wrapping roll of the rubber material will affect the speed of eating powder and the dispersion of the compounding agent during mixing. If the wrapping roll is too poor, it cannot even be mixed. The rolling properties of the rubber compound are related to the properties of the raw rubber (such as Green strength, breaking stretch ratio, maximum relaxation time, etc.), roller temperature and shear rate. The green rubber with high strength, breaking stretch ratio and long maximum relaxation time has good rolling properties, such as NR; The green rubber with low Green strength, small breaking stretch ratio and short maximum relaxation time has poor rolling properties, such as BR. The factors that affect the properties of raw rubber will affect the wrapping roll of raw rubber. For example, adding reinforcing agent to improve the Green strength of rubber compound and increase the relaxation time will obviously improve the wrapping roll of BR. Too much liquid softener is added to the rubber compound to reduce Green's strength, shorten the relaxation time, make the wrapping roll worse and even take off the roll. The roller temperature is below the glass transition temperature (Tg) of the rubber material, and the roller cannot be wrapped. Above the viscous flow temperature (Tf), the rubber material sticks to the roller and cannot be mixed. Only within a certain temperature range between Tg and Tf, the rubber material has good wrapping roller and is suitable for mixing. If the shear rate is increased by reducing the roller distance, increasing the speed ratio or increasing the roller speed, the fracture stretch ratio of the rubber compound can be increased, the maximum relaxation time can be prolonged, and thus the wrapping roll of the rubber compound can be improved.

2. Glue loading capacity

Excessive glue loading capacity increases the amount of accumulated glue, causing the accumulated glue to spin on its own above the roller gap, losing the effect of folding grain clamping powder, affecting the eating and dispersing effect of the compounding agent, prolonging the mixing time, decreasing the physical properties of the glue, increasing the energy consumption, increasing the load of the glue mixer, and easily damaging the equipment. If the amount of glue is too small, the accumulated glue is not or too small, it is difficult to eat powder, and the production efficiency is too low. Therefore, the amount of glue should be appropriate when mixing with the open mill.

3. Roll distance

The relationship between the shear deformation rate of the rubber material when passing through the roller pitch and the roller pitch, roller speed and speed ratio is:

γ-mechanical shear rate, s-1;

F-speed ratio of roller, f = V1/V2;

V2 -- Linear velocity of surface rotation of front curtain barrel, m/min;

V1 -- Linear rotational velocity of the surface of the rear curtain barrel, m/min;

E-Curtain distance, mm;

When the roller distance is reduced, the shear deformation rate increases, the shear action on the rubber molecular chain and the compounding agent agglomerate increases, and the compounding agent agglomerate is easy to break, thus facilitating the dispersion of the compounding agent. However, the chance of shear fracture of the rubber molecular chain also increases, which is easy to cause excessive fracture of the molecular chain, resulting in over-refining, too low molecular weight of the rubber, and reducing the physical and mechanical properties of the rubber compound. The roller 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, the roll distance should be appropriate when mixing with the open mill. The appropriate roller distance is related to the amount of glue, as shown in Table 1. When natural glue and synthetic glue are used together in equal proportions, the total amount of glue can be determined by natural glue to set the roller distance; when the proportion of synthetic glue is greater than that of natural glue, the total amount of glue can be determined by synthetic glue to set the roller distance.

4. Speed ratio and roll speed

The speed ratio and roller speed increase, the effect of mixing effect and reduce the law of roller distance, will speed up the dispersion of the compounding agent, but the rubber molecular chain shear is also intensified, easy to over-refining, so that the rubber material physical properties are reduced, so that the rubber material heating up faster, energy consumption increased. 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 the open mill mixing is generally in the range of 1.15 to 1.27.

5. Roll temperature

As the roller temperature increases, the viscosity of the rubber material decreases, which is conducive to the wetting of the rubber material on the surface of the solid compounding agent and the acceleration of powder eating. However, the shearing effect of the compounding agent mass in the soft rubber material will be weakened and will not be easily broken, which is not conducive to the dispersion of the compounding agent, and the amount of combined rubber will also be reduced. Therefore, the temperature of the roller should be appropriate when mixing. Due to the different influence of temperature on the roll wrapping of different compounds, the roll temperature should be different when mixing different compounds. The temperature of the front roll is higher than that of the rear roll, while the temperature of the front roll is lower than that of the rear roll for most synthetic rubber-coated cold rolls.

6. Feeding sequence

The improper feeding sequence during mixing will affect the uneven dispersion of the compounding agent in the light and lead to scorch, roll off or over-refining in the heavy. The feeding sequence is one of the important factors related to the quality of the compound, so there must be a reasonable feeding sequence. The determination of the feeding sequence generally follows the principles of adding the compounding agent with small dosage, large effect and difficult dispersion, adding the compounding agent with large dosage and easy dispersion, adding the compounding agent with temperature sensitivity, and adding the vulcanizing agent and accelerator separately. Therefore, when mixing in the mill, the rollers such as raw rubber, reclaimed rubber and masterbatch are added first. If there is solid softener such as paraffin wax in the formula, it can be added after the roller is coated with rubber material, and then small materials such as activator (zinc oxide, stearic acid), accelerator, antioxidant and scorch inhibitor are added. Carbon black and filler are added again. After adding carbon black and filler, liquid softener is added, the two can be added alternately, and finally add vulcanizing agent. If there is an overspeed accelerator in the formulation, it should be added together with the vulcanizing agent at the later stage. If there is white carbon black in the formula, because the white carbon black surface adsorption is very strong, easy to form hydrogen bonds between particles, difficult to disperse, should be added before the small material, and to be added in batches. For NBR, sulfur is difficult to disperse due to its poor compatibility, 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 speed of eating powder and dispersion effect. If the compounding agent is continuously added to a certain fixed position, the rubber material in other parts does not eat powder, which is equivalent to reducing the area of eating powder, prolonging the time of eating powder, slowly eating powder, and prolonging the time required for the compounding agent to disperse from the eating position to other places, thus it is not conducive to the dispersion of the compounding agent. When feeding, the compounding agent should be evenly sprinkled on the accumulated glue along the roller axis direction, so that the accumulated glue is covered with the compounding agent, which will shorten the powder eating time, and is also conducive to the dispersion of the compounding agent in the rubber compound, shorten the mixing time, and cause less shear damage to the rubber molecular chain.

Therefore, the main factors affecting the mill are the roll wrapping of the rubber material, the capacity of the rubber material, the temperature of the roller, the distance between the rollers, the speed ratio of the roller, the feeding sequence, the feeding method and the mixing time, etc., which are adjusted in the actual production process.