How Carbon Black Particle Size Affects the Properties of FKM Sealing

Carbon black is the most commonly used reinforcing filler in FKM sealing products, delivering a significantly better reinforcing effect than fillers such as diatomite, calcium fluoride, and barium sulfate. In the formula design of FKM sealing products, the selection of carbon black particle size is one of the key factors that determine the final performance of finished products. Carbon blacks with different particle sizes exert profound and complex influences on the mechanical properties, sealing reliability, thermal stability and processability of FKM sealing products.

I. Core Reinforcement Mechanism of Carbon Black Particle Size: Occluded Rubber and Filler Network

During rubber compounding, rubber molecular chains can penetrate into the internal voids of these branched structures. Once embedded, the molecular chains are virtually locked in a cage-like structure and lose their free mobility. The rubber trapped in this state is defined as occluded rubber.

1. Restricted mobility: The molecular chain movement of occluded rubber is greatly limited; hence, it is functionally regarded as an integral part of carbon black in rubber performance analysis.

2. Reinforcement and performance impact: An increase in occluded rubber means more immobilized rubber components, which directly raises the viscosity and hardness of the compound, and improves its tensile strength and modulus. A smaller carbon black particle size corresponds to a larger specific surface area and a broader contact interface with the rubber matrix, enabling the formation of a denser rubber-filler network. A higher structure level provides more internal voids and allows greater rubber occlusion. The synergistic effect of these two factors ultimately determines the content of occluded rubber and the morphology of the filler network.

II. Particle Size: The Decisive Factor for Compression Set

In FKM systems, carbon black particle size is the dominant factor affecting compression set. Although fine-particle carbon black provides superior reinforcement, it causes a higher compression set of vulcanized FKM compounds.

1. Defects of fine-particle carbon black: Compared with large-particle MT carbon black (~450 nm), fine-particle HAF carbon black (~28 nm) leads to a remarkable increase in compression set after high-temperature aging.

Root cause: Under high-temperature compression conditions, the filler network undergoes physical restructuring, which consumes part of the elastic recovery force and results in increased permanent deformation.

2. Negligible influence of carbon black structure: In contrast to particle size, carbon black structure has minimal or even insignificant effects on compression set. Equivalent network structures yield equivalent performance. Studies have shown that when carbon blacks with different structure levels form filler networks of identical density in rubber compounds, the filled rubber materials exhibit basically consistent mechanical properties and compression set resistance.

III. Sealing Durability Difference Between Two Types of Carbon Black Particle Sizes

Fine-particle carbon black features strong adsorption force on rubber molecular chains. It restricts molecular chain movement under high-temperature compression and reduces resilience, easily causing leakage and sealing failure during long-term service.

Large-particle thermal carbon black (N990, 450 nm) forms a loose and stable filler network with excellent elastic recovery at high temperatures and low compression set, making it the mainstream filler for high-temperature static sealing applications.