SBS modified asphalt is the mainstream binder for high-grade highway, bridge deck and urban asphalt pavement. The core production link is high-speed shearing by colloid mill to break SBS block particles and disperse them evenly into molten asphalt. If the colloid mill structure is unreasonably designed, problems such as incomplete crushing, agglomerated SBS particles, poor crosslinking and unstable rheological properties of finished modified asphalt will occur. This paper elaborates the overall structural design of high-shear colloid mill dedicated to modified asphalt, analyzes the shearing mechanism of stator and rotor, and puts forward a complete set of process control schemes to improve SBS dispersion uniformity.

1. Influence of Uneven SBS Dispersion on Modified Asphalt Quality

1.1 Residual large SBS agglomerates Insufficient shear force leads to undissolved SBS lumps in asphalt, which block pipelines and spray nozzles during pavement construction, causing uneven pavement surface and pothole defects after road opening.

1.2 Uneven elastic network distribution SBS cannot form a continuous cross-linked elastic network uniformly. The modified asphalt shows unstable indexes of penetration, ductility and softening point, with large performance difference between different batches.

1.3 Poor anti-rutting and anti-cracking performance Local areas lack SBS modifier, resulting in insufficient high-temperature rutting resistance and low-temperature crack resistance of pavement, shortening the service life of asphalt road.

1.4 Long maturation cycle Poor dispersion prolongs the required swelling and maturation time, reduces the overall production efficiency of the modified asphalt line and increases fuel heating consumption.

2. Overall Structural Design of High-Shear Colloid Mill for Modified Asphalt

The whole equipment is composed of feeding buffer cavity, rotor-stator shear working head, driving spindle, thermal insulation jacket, transmission system, base frame and circulating pipeline interface. The core functional unit is the rotor-stator shear pair, which determines the fineness and uniformity of SBS crushing.

2.1 Rotor and Stator Shear Head Optimization Design (Core Component)

Adopt multi-stage layered tooth-type shear structure, divided into primary coarse crushing zone, medium shearing zone and fine homogenization zone.

1) Primary coarse crushing tooth: Wide tooth gap, large tooth height, used to break large SBS rubber blocks into small particles at the first time; 2) Medium shear tooth: Medium tooth pitch, increase relative cutting frequency, further pulverize SBS particles; 3) Fine homogenization tooth: Micro narrow tooth gap, dense staggered tooth distribution, realize ultrafine grinding and uniform dispersion of SBS micro-particles.

Rotor and stator materials: High chromium wear-resistant alloy cast steel, overall quenching and tempering treatment, wear resistance improved by more than 2 times compared with ordinary carbon steel, avoiding tooth surface wear leading to shear attenuation after long-term operation.

Adjustable tooth gap mechanism: Equipped with precise handwheel micro-adjustment structure, the gap between rotor and stator can be continuously adjusted within 0.1–1.0mm according to SBS particle size and production flow, matching different shear fineness requirements.

2.2 Spindle and Drive System Design

Adopt high-torque frequency conversion motor matched with hard tooth surface reducer, stable output speed 2900–3600rpm; thickened alloy spindle with double mechanical seal structure to prevent high-temperature asphalt leakage. Frequency conversion speed regulation can adjust shear linear speed in real time, and increase shear force for high-viscosity asphalt with high SBS doping amount.

Mechanical seal cooling structure: Independent circulating cooling oil channel, isolate high-temperature asphalt heat conduction, avoid seal ring aging and failure under long-term high temperature.

2.3 Thermal Insulation Jacket & Constant Temperature Chamber Structure

The outer wall of the shear cavity is equipped with integral heat-conducting oil thermal insulation jacket, circulating heat-conducting oil keeps the internal temperature of the mill stable at 170–185℃ required for SBS modification. Multi-layer aluminum silicate thermal insulation cotton outside the jacket reduces heat loss, prevents local asphalt cooling and SBS re-agglomeration in the shear cavity.

2.4 Circulation Buffer Feeding Cavity Design

The feeding inlet is equipped with a buffer diversion cavity to avoid direct impact of high-flow asphalt on the shear head and uneven instantaneous feeding. The diversion plate makes the material evenly enter the circumferential shear gap of the rotor-stator, eliminating the dead zone of local material accumulation and insufficient shearing.

2.5 Integrated Base and Discharging Pipeline Structure

Integrated cast iron shock-absorbing base reduces vibration during high-speed rotation and prevents the rotor-stator gap from shifting due to resonance. The discharge port adopts tangential directional output, which forms a circulating return channel with the maturation tank, realizing repeated circular shearing of materials.

3. Mechanism of Rotor-Stator High-Shear Crushing & Dispersion of SBS

When the rotor rotates at high speed, the material enters the gap between rotor and stator teeth, and is subjected to three synergistic forces at the same time: mechanical cutting force of staggered tooth edges, turbulent impact force generated by high-speed flow, and high-frequency friction grinding force between layers. Large SBS blocks are continuously torn, pulverized and refined step by step in three-stage shear zones, and fully mixed with molten asphalt under strong turbulence to form a uniform suspension system, laying a foundation for subsequent cross-linking reaction in the maturation tank.

4. Complete SBS Dispersion Uniformity Control Scheme

4.1 Pre-material Matching and Pre-swelling Process Control (Front-end Source Control)

Control the temperature of base asphalt at 165–175℃ in advance; add SBS modifier and stabilizer into the mixing tank for preliminary stirring and low-temperature swelling for 20–30min, so that asphalt fully infiltrates into SBS particles, reducing the hardness of rubber blocks and lowering the shear load of the colloid mill, avoiding incomplete primary crushing.

4.2 Colloid Mill Parameter Matching Adjustment

1) Rotor-stator gap adjustment: Conventional 4.5%–5.5% SBS doping amount adopts 0.2–0.4mm fine gap; high doping amount ≥6% adjusts to 0.4–0.6mm to prevent excessive load and motor overload; 2) Frequency conversion speed matching: Low-viscosity base asphalt uses 3000rpm; high-viscosity heavy-duty asphalt increases to 3400–3600rpm to raise shear linear speed; 3) Single circulation flow control: Control the single passing flow rate of the colloid mill, prohibit ultra-fast feeding leading to insufficient single shear; set a circulating pipeline to realize 2–3 times repeated shearing of all materials.

4.2 Constant Temperature Shearing Environment Control

Stabilize the heat-conducting oil temperature of the colloid mill jacket to keep the material temperature inside the shear cavity between 170–185℃. Too low temperature will increase asphalt viscosity, SBS particles are difficult to be torn; too high temperature will cause SBS thermal degradation, reducing the elasticity of modified asphalt. Real-time temperature sensor interlocks with heating system to avoid temperature drift.

4.3 Multi-cycle Circulation Shearing Process

After the material passes through the colloid mill for the first time, it flows back to the mixing tank through the circulating pipeline for secondary stirring, and then enters the mill for repeated shearing. After 2–3 cycles, sample and detect SBS particle fineness until no large rubber particles exist, then send the material to the maturation tank for constant-temperature cross-linking reaction.

4.4 Post-shearing Constant Temperature Maturation Auxiliary Optimization

After shearing, keep the modified asphalt at 175℃ constant temperature for 30–60min maturation. The uniformly dispersed micro SBS particles fully swell and cross-link with asphalt molecules, further optimize the dispersion uniformity of the elastic network, and stabilize road performance indexes.

4.5 Online Fineness Real-Time Monitoring Matching

Install online sampling detection valve at the discharge port of the colloid mill, regularly extract asphalt samples to observe SBS dispersion under microscope; if agglomerated particles are found, reduce the feeding flow rate or narrow the rotor-stator gap for re-shearing.

5. Common SBS Poor Dispersion Defects & Colloid Mill Optimization Countermeasures

5.1 A large number of visible SBS rubber agglomerates in finished asphalt: Rotor-stator gap too large, single shear without circulation; narrow the fine grinding gap, open circulating pipeline for multiple repeated shearing.

5.2 Unstable product index, obvious difference between front and rear batches: Uneven feeding flow, shear cavity dead zone; optimize feeding diversion buffer structure, stabilize conveying pump flow output.

5.3 Shearing efficiency drops after long-term operation, SBS fineness deteriorates: Rotor-stator tooth surface wear; disassemble and inspect the shear head, replace wear-resistant alloy rotor and stator components.

5.4 SBS thermal degradation occurs with poor ductility: Shearing temperature too high; reduce heat-conducting oil temperature, strengthen colloid mill jacket heat dissipation control.

5.5 Motor overload and shutdown during shearing high-content SBS asphalt: Gap too small, instantaneous feeding excessive; appropriately widen the rotor-stator gap, reduce single feeding flow rate.

6. Factory Acceptance Test Standard for Shearing Uniformity of Colloid Mill

1. Fineness test: After standard SBS modified asphalt circulates twice under rated parameters, no particles larger than 20μm visible under metallographic microscope; 2. Shear consistency test: Three groups of samples taken continuously at discharge port, penetration and softening point error ≤±2%; 3. Continuous operation stability test: 8h continuous production, no shear attenuation caused by temperature rise, uniform SBS dispersion maintained; 4. Temperature control accuracy: Shear cavity temperature fluctuation ≤±3℃ during long-time operation.

Conclusion

The high-shear colloid mill takes three-stage staggered tooth rotor-stator as the core shear structure, matched with adjustable micro-gap mechanism, constant temperature thermal insulation jacket and circulating buffer feeding cavity, which provides sufficient cutting, turbulence and friction force for SBS crushing. Combined with the whole-process control scheme of pre-swelling treatment, variable frequency speed regulation, multi-cycle repeated shearing and constant temperature maturation, the problem of uneven SBS dispersion can be fundamentally solved. Optimized colloid mill structural design and supporting process parameters ensure that SBS forms a continuous and uniform elastic network in asphalt, so that the produced modified asphalt has stable high and low temperature road performance, meeting the production requirements of high-standard pavement engineering.