How to Achieve Good Dispersion of Nano-CaCO₃ in Polyamide (PA)

Nano-CaCO₃ features ultra-high specific surface area, strong surface hydrophilicity, and easy self-aggregation; meanwhile polyamide is high-polar, high-melting, and high-viscosity. The core to obtain uniform dispersion is surface hydrophobic modification, interfacial compatibilization, pretreatment, and optimized melt compounding process.

1. Surface Modification of Nano-CaCO₃ (Core Step)

Modify the hydrophilic surface of nano-CaCO₃ to low surface energy, reduce agglomeration, and improve compatibility with non-polar/polar PA matrix.

• Titanate / Aluminate coupling agentsMost suitable for nano-CaCO₃; dosage 0.5–1.2 wt%. They chelate with surface -OH of CaCO₃, form organic coating layer, greatly reduce inter-particle van der Waals force.
• Stearic acid / Oleic acid coatingLow cost, dosage 1.0–2.0 wt%. Form hydrophobic monolayer, inhibit re-agglomeration during melt blending. Avoid over-coating, which weakens interfacial bonding with PA.
• Silane coupling agent with amino/epoxy groupsAmino/epoxy groups react with PA molecular chains, build chemical bonding interface, improve both dispersion and mechanical properties.
• Polymer graft modificationGraft small molecular PA or MAH-g-monomer on nano-CaCO₃ surface to realize molecular-level compatibility with polyamide.

2. Strict Drying & Pre-Treatment of Nano-CaCO₃

Nano-CaCO₃ easily absorbs moisture; moisture causes agglomeration, bubble defects and poor dispersion in PA.

• Dry at 105–120 °C for 2–4 h before use, remove adsorbed water completely.
• Low-speed dry premixing: Mix modified nano-CaCO₃ with PA resin and compatibilizer in a high-speed mixer at low rotation speed, break soft agglomerates without causing secondary aggregation.
• Screen to remove hard large agglomerated particles.

3. Add Polyamide Compatibilizer

Introduce MAH-g-PP, MAH-g-PE, MAH-g-PA (maleic anhydride grafted compatibilizer), dosage 2–5 wt%.

• Mechanism: Anhydride groups react with residual hydroxyl on nano-CaCO₃ surface; polyolefin/PA molecular chains entangle with PA matrix.
• Bridge the interface between inorganic nano-filler and PA resin, suppress aggregation and promote uniform distribution.

4. Optimize Twin-Screw Extrusion Compounding Process

Extrusion parameter matching is critical for final dispersion effect:

1. Screw combination: Adopt moderate shear + kneading blocks, avoid ultra-strong shear (causes PA molecular chain degradation) or weak shear (insufficient de-agglomeration).
2. Feeding method: Feed PA from main hopper, add nano-CaCO₃ via side forced feeding; prevent nano-powder from staying in high-temperature zone for long time and re-aggregating.
3. Temperature profile: Set barrel temperature 210–250 °C (adjust according to PA6/PA66 grade); keep temperature stable to reduce melt viscosity fluctuation.
4. Screw speed & vacuum venting: Medium rotation speed 250–400 rpm; keep high vacuum exhaust to remove moisture and small molecules, eliminate voids caused by poor dispersion.

5. Control Nano-CaCO₃ Loading & Gradation

• Single nano-CaCO₃ loading: 3–15 wt%; excessive loading (>20 wt%) inevitably leads to serious agglomeration, even with modification.
• Bimodal particle matching: Mix small amount of micron-grade CaCO₃ with nano-CaCO₃; micron particles isolate nano-particles, limit contact and stacking, assist dispersion.

6. Select Suitable Lubricants & Dispersing Aids

Add small dosage of high-temperature resistant lubricants to improve melt fluidity and filler sliding:

• EBS, zinc stearate, polyethylene wax; total dosage 0.2–0.5 wt%.
• Function: Reduce friction between nano-particles and PA melt, further promote de-agglomeration and uniform dispersion; do not overdose to avoid interface separation.

7. Injection Molding Parameter Optimization

For final molded parts:

• Properly increase back pressure and moderate screw rotation speed to strengthen melt shearing and homogenization.
• Control mold temperature and injection speed steadily; avoid rapid cooling which locks uneven dispersed particles inside the part.

Key Summary for Best Dispersion

1. Modify nano-CaCO₃ with titanate / stearic acid at optimal dosage;
2. Fully dry and low-speed premix to break soft agglomerates;
3. Add 2–5% MAH-g compatibilizer for interfacial bonding;
4. Use side feeding + moderate shear twin-screw extrusion;
5. Limit nano loading below 15 wt% and match bimodal particle size.

This route can realize uniform nano-CaCO₃ dispersion in PA, avoiding agglomeration, and maintain good mechanical strength, toughness and dimensional stability.