Dr E. Ramanathan PhD
Definition:
Sedimentation is the settling of heavier pigments or extenders at the bottom of the paint container during storage, leading to separation, non-uniform consistency, and poor application properties.
Causes of Sedimentation
- High Pigment Density: Heavier pigments (e.g., iron oxide, barium sulfate) tend to settle more quickly.
- Poor Dispersion: Inadequate milling or lack of proper dispersing agents leads to pigment flocculation and settling.
- Low Viscosity: Low-medium viscosity systems (like solvent-based or low-PVC paints) allow easy movement and settling of particles.
- Inadequate Stabilizers: Absence or insufficient levels of anti-settling agents or thixotropes.
- Improper Particle Size Distribution: Coarse or poorly ground pigments increase settling rate.
- Storage Conditions: Long storage periods or high temperatures accelerate sedimentation.
Remedies
- Use of Anti-settling Agents:
- Organic (e.g., polyamide waxes, bentones)
- Inorganic (e.g., fumed silica, organoclays)
- Proper Dispersion Techniques:
- Use of high-shear mixers or bead mills to break aggregates.
- Pigment Selection & Modification:
- Choose pigments with similar density as binder or modify surface to improve stability.
- Incorporate Rheology Modifiers:
- Use thickeners (e.g., HEC, HASE, PU thickeners) to build low-shear viscosity.
- Maintain Optimum Viscosity:
- Adjust formulation to prevent free settling.
- Use of Synergists:
- Help activate anti-settling agents and improve particle-particle repulsion.
- Proper Packaging and Handling:
- Store under stable temperature and regularly agitate during long-term storage.
Note: Sedimentation is reversible if soft settling occurs. Irreversible “hard settling” may lead to product rejection.
