Precipitated silica is an important reinforcing filler in the rubber industry. Its various properties indirectly or directly affect the abrasion resistance of rubber by influencing the interfacial interaction with the rubber matrix, dispersion, and the mechanical properties of the rubber. Below, starting from the key properties, we analyze in detail their mechanisms of influence on rubber abrasion resistance:
1.Specific Surface Area (BET)
Specific surface area is one of the most core properties of silica, directly reflecting its contact area with rubber and reinforcing capability, significantly impacting abrasion resistance.
(1) Positive influence: Within a certain range, increasing specific surface area (e.g., from 100 m²/g to 200 m²/g) increases the interfacial contact area between silica and the rubber matrix. This can enhance the interfacial bonding strength through the “anchoring effect,” improving the rubber’s resistance to deformation and reinforcing effect. At this point, the rubber’s hardness, tensile strength, and tear strength increase. During wear, it is less prone to material detachment due to excessive local stress, leading to a significant improvement in abrasion resistance.
(2) Negative influence: If the specific surface area is too large (e.g., exceeding 250 m²/g), the van der Waals forces and hydrogen bonding between silica particles strengthen, easily causing agglomeration (especially without surface treatment), leading to a sharp decline in dispersibility. Agglomerates form “stress concentration points” within the rubber. During wear, fracture tends to occur preferentially around the agglomerates, conversely reducing abrasion resistance.
Conclusion: There exists an optimal specific surface area range (typically 150-220 m²/g, varying with rubber type) where dispersibility and reinforcing effect are balanced, resulting in optimal abrasion resistance.
2.Particle Size and Size Distribution
The primary particle size (or aggregate size) and distribution of silica indirectly affect abrasion resistance by influencing dispersion uniformity and interfacial interaction.
(1) Particle Size: Smaller particle sizes (usually positively correlated with specific surface area) correspond to larger specific surface areas and stronger reinforcing effects (as above). However, excessively small particle sizes (e.g., primary particle size < 10 nm) significantly increase the agglomeration energy between particles, drastically increasing dispersion difficulty. This instead leads to local defects, reducing abrasion resistance.
(2) Particle Size Distribution: Silica with a narrow particle size distribution disperses more uniformly in rubber, avoiding “weak points” formed by large particles (or agglomerates). If the distribution is too broad (e.g., containing particles of both 10 nm and above 100 nm), large particles become wear initiation points (preferentially worn away during abrasion), leading to decreased abrasion resistance.
Conclusion: Silica with small particle size (matching the optimal specific surface area) and narrow distribution is more beneficial for enhancing abrasion resistance.
3.Structure (DBP Absorption Value)
Structure reflects the branched complexity of silica aggregates (characterized by DBP absorption value; higher value indicates higher structure). It affects the rubber’s network structure and resistance to deformation.
(1) Positive influence: Silica with high structure forms three-dimensional branched aggregates, creating a denser “skeletal network” within the rubber. This enhances the rubber’s elasticity and resistance to compression set. During abrasion, this network can buffer external impact forces, reducing fatigue wear caused by repeated deformation, thereby improving abrasion resistance.
(2) Negative influence: Excessively high structure (DBP absorption > 300 mL/100g) easily causes entanglement between silica aggregates. This leads to a sharp increase in Mooney viscosity during rubber mixing, poor processing flowability, and uneven dispersion. Areas with locally overly dense structures will experience accelerated wear due to stress concentration, conversely reducing abrasion resistance.
Conclusion: Medium structure (DBP absorption 200-250 mL/100g) is more suitable for balancing processability and abrasion resistance.
4.Surface Hydroxyl Content (Si-OH)
The silanol groups (Si-OH) on the silica surface are key to influencing its compatibility with rubber, indirectly affecting abrasion resistance through interfacial bonding strength.
(1) Untreated: Excessively high hydroxyl content (> 5 groups/nm²) easily leads to hard agglomeration between particles via hydrogen bonding, resulting in poor dispersion. Simultaneously, the hydroxyl groups have poor compatibility with rubber molecules (mostly non-polar), leading to weak interfacial bonding. During wear, silica is prone to detach from the rubber, reducing abrasion resistance.
(2) Treated with Silane Coupling Agent: Coupling agents (e.g., Si69) react with hydroxyl groups, reducing inter-particle agglomeration and introducing groups compatible with rubber (e.g., mercapto groups), enhancing interfacial bonding strength. At this point, a “chemical anchoring” forms between silica and rubber. Stress transfer becomes uniform, and interfacial peeling is less likely during wear, significantly improving abrasion resistance.
Conclusion: Hydroxyl content needs to be moderate (3-5 groups/nm²), and must be combined with silane coupling agent treatment to maximize interfacial bonding and improve abrasion resistance.
5.pH Value
The pH value of silica (typically 6.0-8.0) primarily indirectly affects abrasion resistance by influencing the rubber vulcanization system.
(1) Excessively Acidic (pH < 6.0): Inhibits the activity of vulcanization accelerators, delaying the vulcanization rate, and may even lead to incomplete vulcanization and insufficient crosslink density in the rubber. Rubber with low crosslink density has reduced mechanical properties (e.g., tensile strength, hardness). During wear, it is prone to plastic deformation and material loss, resulting in poor abrasion resistance.
(2) Excessively Alkaline (pH > 8.0): May accelerate vulcanization (especially for thiazole accelerators), causing excessively fast initial vulcanization and uneven crosslinking (local over-crosslinking or under-crosslinking). Over-crosslinked areas become brittle, under-crosslinked areas have low strength; both will reduce abrasion resistance.
Conclusion: Neutral to slightly acidic (pH 5.0-7.0) is more favorable for uniform vulcanization, ensuring rubber mechanical properties and improving abrasion resistance.
6.Impurity Content
Impurities in silica (such as metal ions like Fe³⁺, Ca²⁺, Mg²⁺, or unreacted salts) can reduce abrasion resistance by damaging the rubber structure or interfering with vulcanization.
(1) Metal Ions: Transition metal ions like Fe³⁺ catalyze rubber oxidative aging, accelerating rubber molecular chain scission. This leads to a decay in material mechanical properties over time, reducing abrasion resistance. Ca²⁺, Mg²⁺ may react with vulcanizing agents in the rubber, interfering with vulcanization and lowering crosslink density.
(2) Soluble Salts: Excessively high content of impurity salts (e.g., Na₂SO₄) increases the hygroscopicity of silica, leading to bubble formation during rubber processing. These bubbles create internal defects; during wear, failure tends to initiate at these defect sites, reducing abrasion resistance.
Conclusion: Impurity content must be strictly controlled (e.g., Fe³⁺ < 1000 ppm) to minimize negative impacts on rubber performance.
In summary, the influence of precipitated silica on rubber abrasion resistance results from the synergistic effect of multiple properties: Specific surface area and particle size determine the fundamental reinforcing capability; structure affects the stability of the rubber network; surface hydroxyl groups and pH regulate interfacial bonding and vulcanization uniformity; while impurities degrade performance by damaging the structure. In practical applications, the combination of properties must be optimized according to the rubber type (e.g., tire tread compound, sealant). For instance, tread compounds typically select silica with high specific surface area, medium structure, low impurities, and combined with silane coupling agent treatment to maximize abrasion resistance.
Post time: Jul-22-2025