Ethylene propylene diene monomer (EPDM) rubber, as a non-polar saturated rubber, is widely used in automotive seals, waterproof membranes, and wire and cable applications due to its excellent ozone resistance, weather resistance, heat resistance, and chemical corrosion resistance. However, its non-polar nature leads to poor compatibility with polar fillers or compounding agents, resulting in uneven dispersion during blending and consequently affecting the overall performance of the finished product. To optimize dispersibility, a multi-dimensional design approach is needed, encompassing filler selection, surface treatment, blending processes, and additive addition.
Filler selection is fundamental to optimizing dispersibility. Commonly used fillers for EPDM include carbon black, silica, calcium carbonate, and talc. Carbon black is widely used due to its excellent reinforcing effect; however, high-structure carbon black is prone to agglomeration, necessitating the selection of varieties with small particle size and moderate structure, such as fast-extrusion carbon black or semi-reinforcing carbon black, to reduce dispersion difficulty. While silica can improve abrasion resistance, its surface hydroxyl groups have poor compatibility with EPDM, requiring surface modification for improvement. Inorganic fillers such as calcium carbonate and talc are inexpensive, but their particle size distribution needs to be controlled to avoid stress concentration caused by large particles. When selecting fillers, both performance requirements and dispersibility must be considered, with priority given to fillers with high dispersibility or pre-treated varieties.
Surface treatment of fillers is key to improving dispersibility. Unmodified fillers have high surface energy and weak interfacial bonding with EPDM, easily forming agglomerates. This situation can be significantly improved through chemical modification. For example, silica can be treated with silane coupling agents, causing its surface hydroxyl groups to react with the coupling agent to form an organic-inorganic composite layer, enhancing its compatibility with EPDM; carbon black can have its surface energy reduced by surface oxidation or grafting polar groups, improving its wettability in rubber; inorganic fillers such as calcium carbonate can be treated with stearic acid or titanate coupling agents to form a hydrophobic layer, reducing the tendency to agglomerate. Surface treatment not only improves dispersibility but also enhances the interfacial bonding between the filler and rubber, improving the physical properties of the product.
The blending process has a direct impact on dispersibility. The mixing of EPDM requires control of temperature, time, and shear force. Too low a temperature leads to high rubber viscosity, making it difficult to disperse fillers; too high a temperature may cause filler degradation or rubber cross-linking. Typically, low-temperature plasticizing is used in the initial mixing stage to reduce rubber viscosity and facilitate initial filler dispersion; later, the temperature is increased to 150-170℃ to promote further filler dispersion and prevent scorching. Regarding shear force, internal mixers, due to their high shear force, are more conducive to filler crushing and dispersion, but mixing time must be controlled to avoid excessive shearing that could break rubber molecular chains. When mixing on an open mill, multiple thin passes and rolling operations can enhance the shear effect and improve dispersion uniformity.
The addition of additives is an important means of optimizing dispersibility. Dispersants can reduce the surface energy of fillers and decrease the tendency to agglomerate. For example, non-polar additives such as petroleum resins and low molecular weight polyethylene wax can physically adsorb and cover the filler surface, hindering interparticle interactions; polar additives such as stearic acid and polyethylene glycol improve the interfacial bonding between fillers and EPDM through chemical adsorption. Processing aids such as polyethylene wax and zinc stearate can reduce rubber viscosity, improve filler wettability, and improve mold release properties. In addition, adding appropriate amounts of plasticizers such as paraffin oil and naphthenic oil can reduce rubber hardness and improve filler dispersion efficiency, but the dosage must be controlled to avoid affecting the physical properties of the product.
The introduction of compatibilizers can significantly improve the compatibility of EPDM with polar fillers. For example, compatibilizers such as maleic anhydride-grafted EPDM (MAH-g-EPDM) or ethylene-vinyl acetate copolymer (EVA) have polar groups that can form chemical bonds with the filler surface, while the non-polar segments are compatible with the EPDM matrix, thus forming a transition layer at the interface, reducing interfacial tension, and improving dispersibility. The amount of compatibilizer added needs to be optimized according to the filler type and dosage; excessive amounts may increase the system viscosity, which is detrimental to dispersion.
The blending sequence and method also need to be carefully designed. Generally, EPDM is premixed with a small amount of oil softener to reduce viscosity before adding the filler, adding it in batches and extending the mixing time to ensure sufficient dispersion. If a compatibilizer is used, it can be premixed with the filler before being added to the rubber to improve dispersion efficiency. For difficult-to-disperse fillers, a masterbatch method can be used, which involves first preparing a masterbatch with a high filler content and then blending it with pure rubber to reduce dispersion difficulty.
Optimizing the dispersibility of EPDM rubber with fillers requires comprehensive design from multiple aspects, including filler selection, surface treatment, blending process, additive addition, compatibilizer introduction, and blending sequence. By selecting highly dispersible fillers, performing surface modification, controlling mixing conditions, and adding dispersing agents and compatibilizers, the dispersion uniformity of fillers in EPDM can be significantly improved, thereby enhancing the physical properties, aging resistance, and processing performance of the products, meeting the needs of high-end applications.
