The dispersion of fillers in EPDM significantly influences its compression set, a factor that permeates the entire lifecycle of filler selection, processing, and vulcanization. The uniformity of filler dispersion directly determines the integrity of the rubber's internal crosslink network, which in turn influences its elastic recovery. When fillers are uniformly dispersed in the rubber matrix, physical adsorption and chemical bonding between the particle surfaces and the EPDM molecular chains are enhanced, forming more effective crosslinking points. This evenly distributed crosslinked network, when subjected to compressive stress, evenly distributes stress through the coordinated motion of the molecular chains, reducing permanent deformation caused by localized stress concentrations. Conversely, if the filler is unevenly dispersed, localized weak points may form due to insufficient crosslinking density, making it difficult to recover after compression and resulting in increased compression set.
Filler particle size and surface activity are key factors influencing dispersion. Smaller filler particles have a higher specific surface area, creating more contact points with the rubber molecular chains, thereby enhancing reinforcement. However, too small a particle size can easily lead to filler agglomeration, forming microscopic aggregates, which in turn impairs the uniformity of the crosslinked network. For example, reducing carbon black particle size improves its reinforcing properties, but poor dispersion can significantly increase compression set. Fillers with high surface activity (such as nanofillers treated with silane coupling agents) can form stronger interactions with rubber chains through chemical bonding, promoting uniform dispersion. Such uniformly dispersed fillers not only improve the tensile and tear strength of EPDM but also reduce compression set by optimizing the crosslinking network.
The influence of filler structure on dispersion and compression set is also crucial. Highly structured fillers (such as high-structure carbon black) have a more branched chain structure, forming a three-dimensional network within the EPDM matrix and strengthening the binding effect on the rubber chains. When uniformly dispersed, this structure effectively limits rubber chain slippage, improves elastic recovery, and thus reduces compression set. However, if highly structured fillers are poorly dispersed, their aggregates can become stress concentration points, leading to localized disruption of the crosslinking network and exacerbating permanent deformation. Therefore, optimizing filler structure requires coordinated design with the dispersion process, such as through masterbatch processing or high-shear mixing equipment to improve dispersion uniformity.
The vulcanization process's control over filler dispersion directly impacts compression set. During the vulcanization process, high temperature and pressure promote chemical reactions between EPDM molecular chains and active sites on the filler's surface, forming a stable cross-linked structure. If the vulcanization temperature is too low or the curing time is insufficient, the filler-rubber interface is imperfect, leading to poor dispersion. Excessively high temperatures or prolonged curing times can cause filler agglomeration or rubber degradation, also impairing dispersion uniformity. For example, a two-stage vulcanization process, through stepwise temperature increases, can gradually optimize filler dispersion, creating a denser and more uniform cross-linked network and significantly reducing compression set.
Filler-rubber compatibility is another important factor influencing dispersion. Surface modification (such as oxidation and graft polymerization) can enhance interfacial adhesion between fillers and EPDM, promoting uniform dispersion. For example, nanosilica treated with a silane coupling agent allows its surface hydroxyl groups to form chemical bonds with rubber chains, improving dispersion uniformity and reducing compression set by enhancing interfacial interactions. Furthermore, the polarity compatibility of the filler and rubber must be considered. Non-polar EPDM has poor compatibility with polar fillers (such as silica), necessitating the use of compatibilizers or surface modification to improve dispersion.
The effect of filler dosage on dispersion and compression set exhibits a nonlinear relationship. At low dosages, the filler is evenly dispersed, the crosslinking network is complete, and compression set decreases with increasing dosage. However, when the dosage exceeds a critical value, the filler tends to agglomerate, resulting in a deterioration of dispersion and an increase in compression set. For example, increasing carbon black dosage improves the tensile strength and hardness of the rubber, but poor dispersion can significantly increase compression set. Therefore, experimental optimization of filler dosage is necessary to balance reinforcement and uniform dispersion.
Filler dispersion is a key factor in regulating EPDM compression set by influencing crosslinking network uniformity, stress distribution, and elastic recovery. By optimizing filler selection, surface modification, vulcanization process, and dosage control, uniform dispersion of fillers in the rubber matrix can be achieved, thereby building a dense and uniform crosslinking network and significantly improving the rubber's compression set resistance. This principle provides a theoretical basis for the design of high-performance EPDM products (such as seals and shock-absorbing components), that is, by precisely controlling the dispersion state of the filler, the goals of high strength and low permanent deformation can be achieved simultaneously.
