EPDM, due to its unique molecular structure, possesses excellent properties such as resistance to high and low temperatures, ozone, and chemical corrosion. However, its poor self-adhesion and difficulty in dispersing fillers place higher demands on the mixing process. Optimizing the mixing process requires comprehensive adjustments from multiple dimensions, including equipment selection, process parameter control, the order of compounding agent addition, and operational techniques, to achieve uniform dispersion and efficient production.
During open mill mixing, EPDM is prone to roll slippage due to its lack of viscosity, which needs to be improved by adjusting the roll gap and temperature. In the initial stage, a narrow roll gap is used to create a continuous roll wrapping state for the raw rubber, and then the roll gap is gradually widened as compounding agents are added. The front roll temperature is usually controlled to be lower than the rear roll temperature, utilizing the temperature difference to enhance shear force and promote filler dispersion. For high-filler formulations, oils can be pre-mixed with fillers before addition to avoid localized excessive concentrations leading to uneven dispersion. Compounding agents that are prone to roll slippage, such as stearic acid, should be added in the later stages of mixing, while processing oils can be added earlier to improve the fluidity of the rubber compound.
Internal mixer mixing is superior to open mill mixing in terms of filler dispersion, and is especially suitable for high-filler formulations. The reverse mixing method can significantly improve dispersion efficiency: all compounding agents except for the vulcanization system are added first, followed by the raw rubber for mixing. This method extends the mixing time between fillers and raw rubber, allowing reinforcing agents such as carbon black to fully impregnate the rubber molecular chains. The mixing temperature needs to be adjusted according to the formulation characteristics; high-filler compounds require appropriately higher temperatures to reduce viscosity, but excessively high temperatures must be avoided to prevent premature vulcanization. Matching the top bolt pressure with the rotor speed is crucial; high pressure enhances shear force, while a suitable speed balances dispersion efficiency and equipment load.
The order of compounding agent addition directly affects the mixing quality. For open mill mixing, the conventional sequence is: raw rubber wrapping the rollers → adding zinc oxide, activator, and antioxidant → reinforcing agents and fillers → liquid softener → vulcanization system. For internal mixer mixing, adjustments need to be made according to the process type: in the reverse mixing method, solid softeners and antioxidants need to be added earlier to form a uniformly dispersed matrix; traditional mixing methods require adding fillers and oils in stages to avoid power fluctuations caused by adding them all at once. The vulcanization system must be added last at low temperatures to prevent scorching risks. Peroxide vulcanization systems, in particular, are temperature-sensitive and require strict control of discharge temperature.
Special formulations require targeted adjustments to process parameters. High-ethylene rubber compounds, due to their tendency for molecular chain crystallization, require extended mixing time or increased mixing temperature to disrupt the crystalline structure. Low-ethylene rubber compounds require optimized filler dispersion paths to avoid excessive shearing that could lead to molecular chain breakage. For high-filler formulations, staged mixing can improve dispersion uniformity: the first stage disperses most of the filler, while the second stage replenishes the remaining compounding agents and refines the dispersed particles. Heat treatment processes, by preheating the raw rubber to reduce intermolecular forces, can further improve the processing performance of high-filler rubber compounds.
Operating techniques significantly impact mixing quality. During open mill mixing, operations such as cutting, creating triangular folds, and twisting can alter the flow direction of the rubber compound, breaking laminar flow limitations and promoting uniform dispersion of compounding agents. During internal mixer mixing, attention must be paid to power curve changes; an excessively long peak power duration may indicate poor filler dispersion, requiring timely adjustment of the top bolt pressure or rotor speed. For rubber compounds with significant differences in Mooney viscosity, the filler quantity needs to be adjusted according to viscosity characteristics: reduce the filler quantity for high-viscosity compounds to lower equipment load, while increase the filler quantity for low-viscosity compounds to improve filler dispersion efficiency.
Process optimization requires adjustments based on quality testing and feedback. The compounded rubber needs to be tested for indicators such as density, hardness, and Mooney viscosity to ensure it meets process requirements. Excessively high Mooney viscosity can lead to extrusion difficulties, while excessively low viscosity affects physical properties; this needs to be controlled by adjusting the filler dosage or softener type. Vulcanized rubber performance testing includes tensile strength, tear strength, and compression set. If performance fails to meet standards, the mixing process parameters need to be traced and optimized accordingly. Through continuous monitoring and process iteration, standardized mixing procedures suitable for specific formulations can be gradually developed.
EPDM mixing process optimization needs to consider equipment characteristics, formulation requirements, and operational details. By rationally selecting mixing equipment, precisely controlling process parameters, optimizing the order of compounding agent addition, and strengthening operational skills, the quality of the compounded rubber can be significantly improved, laying the foundation for subsequent vulcanization processes and finished product performance.
