Influence of rubber carbon black on rubber properties

Natural rubber and general synthetic rubber, its pure vulcanized rubber has low breaking strength and no wear resistance, so it has no industrial use value. However, after adding carbon black, the properties of the vulcanizate have been improved, such as increased hardness, modulus, breaking energy, breaking strength, tear resistance, fatigue resistance, and improved wear resistance, etc. This improvement in rubber properties Carbon black has a great influence on rubber processing properties, such as carbon black particle size, structure level and dosage, which have great influence on rubber processing properties and vulcanizate properties.

Take styrene-butadiene rubber as an example to talk about the impact on various properties.

(1) Mixing time and dispersion

The smaller the particle size of carbon black, the lower the dispersing ability, the easier it is to disperse unevenly, and the performance of the product will be deteriorated, so it should be noted.

At the initial stage of mixing carbon black and rubber, according to the data, it was dispersed in a colloidal state (2 and a half minutes) and appeared brown under light transmission. When the mixing continued, more carbon black was dispersed in a colloidal state (3 minutes). That is, the shortest mixing time that can meet the performance requirements of the vulcanizate) When the mixing time is extended up to eight minutes, the mixed rubber shows a smooth surface. If the mixing time is too long or the temperature is too high, the scorch time decreases with the increase of carbon black structure.

Experiments have shown that the tread is defective due to carbon black particle fragments and inclusions, especially the hard inclusions. At the end of the additives, when deforming, stress concentration is first formed, and voids are formed, resulting in cracks. , and further expansion (that is, the increase of the crack) finally leads to tensile failure.

If the dispersion is good, this is not the case because the tearing edge is passivated (sluggish) and the result of stress dispersion. In order to obtain a uniform and good dispersion, the mixing conditions are very important. To keep the rubber compound with high viscosity, the mixing temperature should not be too stingy, and the softener should be added at the end, otherwise the dispersion effect will be deteriorated, especially for medium and super furnace black. The purpose of adding it before the low structure is to increase the viscosity, so as to ensure that the rubber compound has enough shear stress, so that the carbon black mixed with the inclusion fragments can be evenly dispersed, so the second mixing is better than the first mixing.

With the increase of the degree of dispersion of carbon black, in general, the performance changes of carbon black vulcanizate are: the tensile strength decreases, the fracture and elongation increase, the water-longevity shape decreases, the tan decreases, and the calorific value decreases, Mooney accuracy is reduced, resistance is increased, wear is reduced, crack growth is slowed, tear resistance is improved, hardness is reduced, and die swell is reduced.

(2) Viscosity problem

The basic properties of carbon black affect the Mooney viscosity of the rubber compound. For example, the compounding dosage is the same as the mixing conditions. The smaller the particle size, the higher the Mooney viscosity, the higher the structure, and the larger the dosage, which can increase the Mooney viscosity accordingly. In addition, kneading at high temperature can improve the Mooney viscosity due to the increase of carbon black gel, and after adding an appropriate amount of operating oil, the dispersion degree of carbon black can be improved, and the Mooney viscosity can also be increased, but the amount of operating oil is too large. It has the effect of reducing Mooney viscosity.

Regarding the filling viscosity, most people in recent years have expressed it with the Einstein viscosity equation:

Here the viscosity G has a linear relationship with the volume fraction (ie volume concentration) of the carbon black filler. It should be noted that this equation only applies to inert carbon blacks, not to activated carbon blacks, nor to graphitized carbon blacks (inactive, but structural).

The key is due to the formation of binding glue (such as carbon black gel) during mixing, which is also called the secondary effect. It is due to the increase of carbon black structure (such as high structure) that leads to the increase of binding rubber. Due to mechanical damage to the rubber during mixing, free radicals are generated and combined with carbon black surface active groups. Therefore, it is believed that the effect of structuring on binding rubber must be related. Carbon black surface area is associated with greater activity, which is proportional to the square of the volume, so the Einstein equation evolves into the GufhーーGold equation:

Due to the increase of the carbon black structure, the viscosity of the compounded rubber increases, resulting in an increase in the shearing action during mixing, which consumes a large amount of energy, which leads to an increase in the concentration of rubber free radicals, resulting in a large amount of binding rubber being generated, which further increases the viscosity. , the scorch is further accelerated.