High‑purity kaolin features low iron‑oxide, low alkali‑metal impurities. Its electrical performance differs significantly between hydrous and calcined grades. Calcined high‑purity kaolin removes structural hydroxyl groups, delivering outstanding volume resistivity, low dielectric loss and good dielectric‑strength stability, and has become a key functional filler for wire‑and‑cable insulation, electrical ceramics and electronic components. Hydrous high‑purity kaolin still contains structural hydroxyl groups, so its insulating performance degrades obviously under high‑humidity conditions, limiting its use in high‑insulation scenarios.
Core electrical‑insulation parameters of high‑purity kaolin
- Volume resistivity
- High‑purity calcined kaolin: ≥ 8 × 10¹² Ω·cm, high‑grade cable‑special products can reach 10¹³‑10¹⁴ Ω·cm, and can maintain high resistivity under humid conditions.
- High‑purity hydrous kaolin: 10¹⁰‑10¹¹ Ω·cm; surface hydroxyl groups absorb moisture, and resistivity drops sharply in high‑humidity environments.
- Dielectric constant (Dk)
Calcined high‑purity kaolin: 3.5‑4.5 (1 MHz), moderate dielectric constant, suitable for polyolefin, EPDM and PVC cable insulation systems. - Dissipation factor (tan δ, dielectric loss tangent)
High‑purity calcined grade: tan δ ＜ 0.001‑0.005 at 50 Hz‑1 MHz, low dielectric loss, reduces heat generation under alternating‑current electric‑field, suitable for medium‑voltage cables and electronic insulation parts. - Dielectric breakdown strength
When uniformly dispersed in polymer matrix, high‑purity calcined kaolin filler helps raise breakdown strength of composite materials; low‑impurity grade can achieve＞25 kV/mm in cable compounds.
Why calcination dramatically improves insulation performance
Raw hydrous kaolin contains around 14 % structural hydroxyl water. These polar hydroxyl groups increase ionic conductivity; when exposed to moisture, ion migration accelerates, insulation performance deteriorates.
After calcination at 900‑1050 °C, structural hydroxyl groups are completely removed, free moisture ≤ 0.5 %, ionic conduction paths are greatly reduced. Therefore calcined high‑purity kaolin realizes stable insulation performance even under high‑temperature and high‑humidity working‑conditions.
Hydrous high‑purity kaolin, even with extremely low iron content, cannot eliminate inherent structural hydroxyl groups, so it is not suitable for medium‑voltage cable insulation.
Key factors influencing insulation performance of high‑purity kaolin
1. Impurity content (decisive factor)
- Iron oxide (Fe₂O₃): Most harmful impurity. Fe ions introduce conductive carriers and increase dielectric loss. For electrical‑grade kaolin, Fe₂O₃ is generally required below 0.3 %.
- Alkali‑metal oxides (Na₂O, K₂O): Promote ion conduction and reduce volume resistivity. Must be strictly controlled for high‑insulation applications.
- Titanium dioxide: Excess TiO₂ will also raise dielectric loss.
2. Surface modification
Even high‑purity calcined kaolin needs silane‑coupling‑agent modification when used as polymer filler.
- Silane covers residual polar sites on kaolin surface, improves compatibility with non‑polar rubber and polyolefin matrix, avoids voids at filler‑matrix interface. Voids and poor dispersion will reduce breakdown strength and insulation stability of composite materials.
3. Moisture
Even calcined kaolin will absorb surface moisture under damp storage. Adsorbed water will drastically lower volume resistivity of finished insulation parts, so strict moisture‑proof storage is required.
4. Dispersion status
Hard agglomerates of kaolin in polymer become defect points, cause partial discharge and reduce overall insulation performance.
Additional advantages brought by high‑purity kaolin in insulating systems
- Thermal stability: No structural‑water release under long‑term working temperature; cooperates with polymer to improve thermal‑ageing resistance of insulation materials, prevents insulation performance degradation caused by thermal ageing.
- Moist‑heat ageing stability: Calcined high‑purity kaolin filler can retain high volume‑resistivity after damp‑heat ageing test, which is the core index for outdoor and buried cables.
- Mechanical matching: Improve modulus, tear‑resistance and dimensional stability of insulating rubber / plastic, reduce insulation‑layer shrinkage and deformation under temperature‑cycling.
- Flame‑smoke performance: Non‑combustible, low smoke emission; cooperates with halogen‑free flame‑retardant systems for low‑smoke zero‑halogen wire‑and‑cable formulas.
Differences: high‑purity hydrous vs calcined kaolin on electrical insulation
| Item | High‑purity hydrous kaolin | High‑purity calcined kaolin |
|---|---|---|
| Structural hydroxyl groups | Present (~14 % bound water) | Fully removed |
| Volume resistivity | 10¹⁰‑10¹¹ Ω·cm | ≥ 8 × 10¹² Ω·cm |
| Humidity sensitivity | High; resistivity drops greatly in damp environment | Low; maintains stable insulation under humid condition |
| Dielectric loss | Relatively high | Low |
| Typical application | Low‑voltage low‑requirement sheath filler | Medium‑/low‑voltage cable insulation, electrical‑insulation rubber parts, electrical ceramics |
Typical industrial application scenarios
- Wire and cable: EPDM, XLPE, PVC insulation compounds for low‑and‑medium‑voltage power cables, control cables, automotive wiring, low‑smoke halogen‑free cables (adopt silane‑modified calcined high‑purity kaolin).
- Electrical ceramics: High‑voltage insulating porcelain, electrical insulation components, spark‑plug base‑material, electronic ceramic raw‑material.
- Insulating rubber parts: Gaskets, sleeves, insulating sealing components.
Common misunderstandings
- “High‑purity hydrous kaolin can be used for high‑insulation filling”: Even ultra‑low‑impurity hydrous kaolin still contains structural hydroxyl groups. Its insulation will fail in long‑term damp‑heat environment; calcined grade must be selected for high‑insulation requirements.
- “High‑purity alone guarantees good insulation”: If dispersion is poor or without silane modification, even high‑purity calcined kaolin cannot exert its insulation advantages.
- “The higher the filling amount, the better insulation”: Excessive kaolin loading brings plenty of interfaces and void defects, instead reduces breakdown strength and mechanical toughness of composite materials; typical dosage is 10‑30 wt% for cable compounds.
High‑purity calcined kaolin possesses high volume resistivity, low dielectric loss and good damp‑heat stability, making it an excellent inorganic insulating filler. Its insulation performance is mainly determined by purity (Fe₂O₃, alkali‑metal‑ion content), calcination treatment quality, surface modification and dispersion state. High‑purity hydrous kaolin is limited by structural hydroxyl groups and is not suitable for high‑performance insulating scenarios.
