The performance of rubber wheels is highly dependent on the uniformity of the compounding ingredients during the mixing process. The mixing process mechanically combines the raw rubber with various compounding ingredients to form a stable multiphase colloidal dispersion. Poor dispersion can lead to localized wear, reduced fatigue performance, and increased rolling resistance, directly impacting the wheel's service life and safety. Therefore, optimizing the mixing process to improve uniformity is a key component of rubber wheel production.
The foundation of the mixing process lies in equipment selection and parameter control. Open mixers and internal mixers are commonly used mixing equipment. Internal mixers are widely used in rubber wheel production due to their short mixing times, high production efficiency, and excellent dust control. By adjusting the internal mixer's rotor speed, ram pressure, and mixing temperature, shear forces and dispersion can be optimized. For example, high speed and high pressure enhance filler crushing and dispersion, while precise temperature management prevents uneven dispersion caused by high-temperature degradation or low-temperature aggregation of compounding ingredients.
Formulation design is a key factor influencing uniformity. Differences in the surface properties of compounding ingredients can increase dispersion difficulties. For example, hydrophilic fillers (such as zinc oxide and clay) and hydrophobic carbon black require chemical modification to improve their compatibility with rubber. Adding surfactants can reduce interfacial tension, promoting wetting and dispersion of compounding ingredients within the rubber matrix. Furthermore, optimizing the filler-plasticizer ratio to avoid the risk of agglomeration caused by excessive amounts of hard fillers or high-viscosity plasticizers is also an important means of improving dispersion uniformity.
The order of addition and stage control during the mixing process play a decisive role in dispersion effectiveness. A typical order of addition is "raw rubber → small ingredients (accelerator, antioxidant) → filler → liquid softener → vulcanizer" to ensure gradual dispersion of each component during mixing. For difficult-to-disperse fine-particle fillers (such as silica), a two-stage mixing method can be used: in the first stage, a masterbatch is prepared without a vulcanizing agent. In the second stage, the vulcanizer is added and the mixing time is extended to promote further breakup and uniform distribution of the filler. Furthermore, periodic shearing (such as staggered shearing) can be used to reduce roller temperature and improve mixing efficiency. Precise control of mixing temperature and time is crucial for ensuring uniform dispersion. Excessively low temperatures can lead to excessively high rubber viscosity, making it difficult to disperse the compounding ingredients. Excessively high temperatures can cause thermal degradation of the rubber or oxidation of the compounding ingredients, reducing dispersion stability. Therefore, an appropriate mixing temperature range must be established based on the rubber type (e.g., natural rubber, styrene-butadiene rubber) and filler properties (e.g., carbon black structure). Mixing time should be adjusted based on equipment performance and formulation complexity to ensure adequate filler fragmentation and avoid molecular chain breakage caused by overmixing.
The auxiliary application of dispersants and lubricants can significantly improve dispersion uniformity. Dispersants adsorb on the filler surface to form a protective layer, reducing interparticle cohesion and facilitating its dispersion in the rubber. Lubricants reduce friction between the filler and the equipment, preventing localized agglomeration caused by overheating. For example, adding a silane coupling agent to silica-filled rubber compounds can simultaneously modify the filler surface and improve dispersion, enhancing the wear resistance and dynamic performance of the rubber wheel.
The production environment and equipment maintenance have an indirect but significant impact on dispersion uniformity. Regular cleaning of mixing equipment (such as the mixing chamber and rotors) can prevent cross-contamination from residual rubber or fillers. Strictly controlling the humidity and temperature of the production environment prevents moisture absorption and agglomeration of fillers and premature vulcanization of the rubber. Furthermore, automated weighing and feeding systems can reduce human error, ensure formulation accuracy, and further improve the dispersion uniformity and quality stability of the rubber compound.
