Calcined kaolin is produced by heating hydrous kaolin at 900‑1050 °C to remove structural hydroxyl groups, eliminating chemically bound crystal water and transforming into anhydrous amorphous aluminosilicate. Unlike platy hydrous kaolin, calcined kaolin delivers excellent electrical insulation, thermal stability and chemical inertness. It is widely adopted as functional filler for PVC, XLPE, EPDM rubber insulation and sheath compounds for power cables, control cables and automotive wiring. Surface‑silane‑modified calcined kaolin is the mainstream grade for high‑performance cable systems.
Core benefits for wire & cable
1. Superior and stable electrical insulation performance (key advantage)
Hydrous raw kaolin retains large amounts of surface hydroxyl groups. Under humid or high‑temperature operating conditions, polar hydroxyl groups increase ionic conductivity, reducing volume resistivity and dielectric performance of cable compounds.
- Calcination removes structural bound water, drastically raises volume resistivity and reduces dielectric loss factor (tan δ).
- Low moisture absorption minimizes performance drift under high‑humidity environment; avoids insulation breakdown risk caused by moisture ingress, critical for medium‑voltage power cables and outdoor wiring.
- High dielectric strength and low dissipation factor reduce energy loss during long‑term cable operation, improve signal stability for communication cables.
Note: Cable‑grade calcined kaolin requires strict impurity control (low iron‑oxide content). Surface modification with silane coupling agent further suppresses moisture uptake and prevents electrical‑property degradation over service life.
2. Enhanced thermal stability and heat‑ageing resistance
Calcined kaolin is thermally inert, no structural water release occurs under cable continuous working temperature.
- Improves heat‑ageing performance of insulation and sheath materials, slows polymer degradation under long‑term thermal load. Cables retain tensile strength and elongation after thermal ageing, extending service life.
- Raises heat‑distortion temperature of polymer compounds, reduces softening risk under overload high‑temperature conditions. Suitable for automotive cables and industrial wiring exposed to temperature fluctuation.
3. Improved mechanical performance of insulation & sheath
Properly dispersed calcined kaolin reinforces cable polymer matrix:
- Boosts tensile strength, tear resistance and abrasion resistance of PVC / EPDM insulation. Resists mechanical damage during laying, bending and vibration; less prone to crack under repeated flexing.
- Improves dimensional stability, lowers thermal shrinkage of cable insulation layer, avoids insulation layer deformation under temperature cycling.
4. Better flame retardant and anti‑smoke performance
Calcined kaolin itself is non‑combustible and chemically inert.
- During combustion, it forms compact barrier layer inside polymer, slows heat transfer and volatile gas release. It works synergistically with halogen‑free flame retardants, reduces smoke density and toxic fume emission for low‑smoke zero‑halogen cables.
- Does not produce corrosive gas under high temperature, improves fire‑safety performance of building and industrial cables.
5. Weathering and chemical resistance for outdoor cables
- Excellent UV‑resistance; inhibits photo‑oxidative ageing of cable sheath exposed to sunlight. Reduces chalking and brittleness for outdoor overhead cables.
- Good resistance against acid, alkali and moisture erosion, fits underground buried cables and cables in harsh chemical environments.
6. Processing advantages & cost optimization
- Fine calcined kaolin with moderate oil absorption delivers good extrusion fluidity for cable compound. Improves surface smoothness of extruded insulation layers, reduces extruder wear compared with many mineral fillers.
- Partial replacement of expensive polymer and special synthetic fillers lowers formulation cost without sacrificing core electrical indexes.
Difference: calcined kaolin vs hydrous kaolin for cables
| Property | Calcined Kaolin | Hydrous Raw Kaolin |
|---|---|---|
| Bound structural water | Fully removed | ~14 % bound hydroxyl water |
| Volume resistivity | Very high, stable | Lower, drops under humidity |
| Moisture absorption | Low | High |
| Thermal ageing resistance | Excellent | Poor, hydroxyl triggers polymer ageing |
| Main cable application | Insulation layer, medium‑voltage cable | Low‑grade cable sheath only |
Practical application notes
- Surface modification is essential: Unmodified calcined kaolin easily agglomerates in non‑polar polymer. Silane‑coupling‑agent treatment improves dispersion, maximizes electrical and mechanical performance.
- Impurity control: Iron oxide and conductive impurities must be strictly limited, otherwise dielectric properties will deteriorate.
- Optimal loading: Typical addition range 10‑30 wt% in cable compounds. Excess dosage will reduce flexibility and elongation‑at‑break of insulation.
- Matching polymer systems: Most widely used for EPDM rubber, PVC and XLPE cable formulations.
Typical application scenarios
- Medium‑voltage power cable insulation
- Low‑smoke halogen‑free building wires
- Automotive wiring, control cables
- Outdoor overhead and buried cable sheaths
- Industrial equipment wiring under high‑temperature, high‑humidity conditions
