Say goodbye to dispersion problems: Tips for using polyurethane carbon black viscosity-reducing dispersants
Say goodbye to dispersion problems: Tips for using polyurethane carbon black viscosity-reducing dispersants
The use of polyurethane carbon black viscosity-reducing dispersants needs to be systematically designed in combination with material formula, processing technology and target performance. The following is a four-dimensional guide from the following aspects: steps of use, key parameter control, typical application scenarios and precautions, providing practical technical guidance:
I. Steps of use: from formula design to finished product verification
Dispersant selection
Select according to the polyurethane system:
Solvent-based system: Select dispersants containing aromatic groups (such as benzene rings) to enhance compatibility with organic solvents.
Water-based system: Select dispersants with self-emulsifying segments (such as polyethylene glycol monomethyl ether), and the HLB value needs to match the hydrophilicity of carbon black (usually HLB 8-12).
Solvent-free system: Use high-branching or rigid segment dispersants to avoid reduced dispersion stability during high-temperature processing.
Select according to the type of carbon black:
High-pigment carbon black (such as slot carbon black): High-anchoring dispersants (such as pyridine groups) are required to prevent reagglomeration.
Conductive carbon black (such as acetylene black): Dispersants that need to take into account the construction of conductive networks should be used to control the spacing of carbon black to <100nm.
Optimization of pre-dispersion process
Dispersant addition method:
Direct addition method: Premix the dispersant and carbon black (mass ratio 1:5-1:10), stir at high speed (3000rpm, 10 minutes) and then add the polyurethane prepolymer.
Masterbatch method: Prepare high-concentration masterbatch (solid content 60%-70%) with dispersant, carbon black and part of the resin, and then dilute to the target formula.
Dispersion equipment selection:
Laboratory scale: Use a sand mill or a three-roll mill, control the line speed to 15-20m/s, and grind 3-5 times.
Industrial scale: Use a high-speed disperser (speed 2000-3000rpm) or an online grinding device to ensure that the particle size D90 is <1μm.
Compound with polyurethane system
Addition time:
Two-component system: Pre-disperse carbon black and dispersant in the polyol component to avoid direct contact with isocyanate to cause gelation.
Single-component system: Add dispersed carbon black slurry in the late stage of polymerization (such as when the NCO value drops to 90% of the theoretical value).
Mixing process:
Use vacuum degassing process (pressure -0.095MPa, 30 minutes) to eliminate bubbles introduced during the dispersion process.
Control the mixing temperature: solvent-based system <60℃, water-based system <40℃, to avoid desorption caused by thermal movement of dispersant chain segments.
II. Key parameter control: balance point between performance and efficiency
Dispersant dosage
Basic dosage: usually 5%-15% of carbon black mass, the optimal value needs to be determined through rheological testing.
Example: In a waterborne polyurethane conductive coating, when the dispersant dosage increases from 5% to 10%, the viscosity decreases from 4500mPa·s to 2800mPa·s, and the resistivity decreases from 10⁴Ω·cm to 10³Ω·cm; when it continues to increase to 15%, the viscosity rises back to 3200mPa·s (chain entanglement caused by excessive dispersant).
Adjustment basis:
Carbon black specific surface area (BET): The larger the specific surface area, the higher the dispersant requirement (e.g., the amount of N330 carbon black required is 20% higher than that of N550).
Target viscosity: For every 1000mPa·s viscosity reduction, the amount of dispersant needs to be increased by about 3%.
Dispersion process parameters
Shear rate:
Initial dispersion stage: high shear (>10⁴s⁻¹) to quickly break up agglomerates.
Stabilization stage: low shear (<10³s⁻¹) to avoid reagglomeration of carbon black.
Dispersion time:
Particle size changes are monitored by a laser particle size analyzer, and dispersion is stopped when D50 fluctuates <5% within 30 minutes.
Curing process matching
Thermosetting system: The dispersant must be compatible with the curing temperature to avoid high-temperature decomposition (e.g., ester-containing dispersants are easily hydrolyzed at >150°C).
Light-curing system: The dispersant cannot absorb UV light (e.g., avoid containing benzene ring structures), otherwise the curing depth will be reduced by more than 30%.
III. Operation guide for typical application scenarios
Waterborne polyurethane conductive coating
Formula example:
Carbon black (N220): 20phr
Dispersant (containing polyether segment): 3phr
Waterborne polyurethane resin: 60phr
Deionized water: make up to 100phr
Operation points:
After premixing the dispersant and carbon black, add them to the resin and disperse at high speed (2500rpm, 20 minutes).
Add 0.1phr defoamer (such as BYK-024) to eliminate microbubbles.
After coating, dry at 80℃ for 10 minutes and UV cure (energy 500mJ/cm²).
Polyurethane elastomer (tire tread rubber)
Formula example:
Carbon black (N330): 45phr
Dispersant (containing pyridine group): 6phr
Polyester polyol (Mn=2000): 100phr
MDI: 35phr
Operation points:
Dispersant and carbon black are premixed in an internal mixer (120℃, 5 minutes), and then polyol is added for mixing (150℃, 10 minutes).
After adding MDI, the tablets are quickly produced (to avoid gelation), and the curing conditions are: 160℃×15MPa×20 minutes.
IV. Precautions: Technical list to avoid common problems
Dispersant and resin compatibility
Problem: Dispersant precipitation causes the coating surface to turn white.
Solution: Select a dispersant that matches the polarity of the resin (such as an ester-based dispersant for polyester polyurethane).
Carbon black sedimentation control
Problem: Carbon black settles during storage.
Solution:
Add 0.5-1phr anti-settling agent (such as fumed silica).
Control the pH value of the system (pH 7-9 for aqueous systems to avoid protonation of dispersants).
Conductive performance fluctuations
Problem: Resistivity batch-to-batch differences >20%.
Solution:
Fixed dispersion process parameters (such as shear rate, time).
Use online resistivity monitor for real-time feedback.
Environmental protection and safety
VOCs control: The aqueous system dispersant must not contain APEO and nonylphenol, and VOCs emissions must be <150g/L.
Operational protection: Dispersant dust may irritate the respiratory tract, and N95 masks and goggles must be worn.