Kaolin serves two core functions in papermaking: coating pigment for coated‑paper surface coating colour, and body filler added inside paper pulp to lower fibre consumption and improve paper opacity. Paper‑coating kaolin and paper‑filling kaolin have distinct grade requirements. Main commercial grades include delaminated hydrous kaolin, fine‑particle washed hydrous kaolin and partial calcined kaolin. Particle morphology, brightness, particle‑size distribution, viscosity and abrasive index determine their applicability for papermaking.
1. Kaolin grades for paper coating
Paper‑coating kaolin requires platy morphology, high brightness, good dispersibility in water‑based coating colour, low abrasion and high‑solids slurry adaptability, to deliver paper gloss, smoothness, ink hold‑out and printability.
Delaminated hydrous kaolin (premium coating grade)
This is the preferred grade for high‑gloss coated art paper, magazine paper and light‑weight coated paper.
- Production: Natural hydrous kaolin undergoes wet‑mechanical delamination to split stacked kaolin crystal layers into thin, large‑diameter platelets with high aspect ratio.
- Key indexes: Brightness ≥88 %‑90 %; D50 1.0‑2.0 μm; high proportion of particles <2 μm; low coarse residue; low abrasion; good high‑solids slurry viscosity stability.
- Performance: Platelets orient parallel to paper substrate after coating and calendering, delivering excellent surface smoothness and gloss; improves ink receptivity and print dot reproduction.
- Typical application: Top‑coat for high‑grade coated paper, art paper, high‑gloss magazine stock.
Fine washed hydrous kaolin (general‑purpose coating grade)
Wet‑processed, classified hydrous kaolin without special delamination treatment, widely used for middle‑grade paper coating and base‑coat formulation.
- Key indexes: Brightness 85‑88 %; D50 1.5‑2.5 μm; controlled coarse fraction; stable slurry viscosity.
- Performance: Balanced cost‑performance; slightly lower gloss than delaminated kaolin. Often blended with GCC ground calcium carbonate for base‑coat or medium‑gloss coating colour.
- Typical application: Base coat for coated paper, light‑weight coated paper, book paper, printing paper.
Calcined kaolin (functional coating additive grade)
Calcined at 900‑1050 °C, original platy structure transforms into porous irregular granular particles.
- Key indexes: Brightness ≥90‑92 %; low crystalline water; good opacity.
- Performance: Outstanding light‑scattering and opacity enhancement, but loses high‑gloss platelet property. It is not used as the main pigment for high‑gloss top‑coat. Normally blended in partial proportion to improve opacity of coating layer.
- Typical application: Opacity booster for coating colour, matte coated paper, low‑gloss publication paper.
Critical quality requirements for all coating‑grade kaolin: Low Fe₂O₃ impurity to guarantee brightness; low abrasive value to reduce wear on coating blades and calender rolls; good dispersion under high‑solid coating‑colour condition; low 45 μm sieve residue.
2. Kaolin grades for paper filling (pulp body filler)
Filling kaolin mixes into paper pulp, fills voids among plant fibres, raises paper opacity, smoothness, reduces fibre usage and production cost. It does not pursue ultra‑high gloss performance.
Conventional washed hydrous kaolin (main filling grade)
Medium‑fineness wet‑beneficiated kaolin, the most widely‑used body‑filler grade.
- Key indexes: Brightness 80‑85 %; D50 2‑4 μm; moderate particle‑size distribution; acceptable retention performance in pulp system.
- Performance: Good retention in paper web; improves opacity and surface smoothness of uncoated cultural paper, packaging paper. Cost‑effective.
- Typical application: Offset printing paper, writing paper, notebook paper, general‑grade packaging board.
Coarser hydrous kaolin (economy‑grade filler)
Lower fineness, lower brightness, for low‑grade paper.
- Typical application: Low‑cost packaging paper, liner board, disposable paper products. Not fit for high‑brightness cultural paper.
Notes: Calcined kaolin as filler
Calcined kaolin can also be used as paper filler for special paper to boost opacity, yet its higher price restricts large‑scale application in ordinary paper. It is mainly adopted for certain high‑opacity specialty paper varieties.
Core index comparison: coating‑grade vs filling‑grade kaolin
| Item | Paper‑coating‑grade kaolin | Paper‑filling‑grade kaolin |
|---|---|---|
| Preferred grade | Delaminated hydrous / fine washed hydrous | Medium‑fineness washed hydrous |
| Brightness | ≥85 %, premium ≥88‑90 % | 80‑85 % |
| D50 particle size | 1.0‑2.5 μm | 2‑4 μm |
| Morphology priority | High‑aspect‑ratio thin platelet | Moderate particle size, good retention |
| Key performance target | Gloss, smoothness, ink hold‑out | Opacity, pulp retention, cost control |
| Impurity control | Strict for iron and coarse residue | Moderate requirement |
| Main usage | Surface coating pigment | Pulp internal body filler |
Practical formulation guidance
- High‑gloss coated paper: Main pigment adopts delaminated hydrous kaolin, can blend partial fine GCC; small‑amount calcined kaolin for opacity improvement.
- Medium‑grade coated paper / lightweight coated paper: Fine washed hydrous kaolin mixed with ground calcium carbonate for base coat and top coat.
- Ordinary uncoated cultural paper: Medium‑fineness washed hydrous kaolin as pulp filler.
- Do not use coarse low‑grade kaolin for surface coating: excessive coarse particles will cause coating‑blade scratch, paper surface grain defects and poor print effect.
- For both coating and filling kaolin, Fe₂O₃ must be strictly controlled; high iron content reduces brightness and causes yellow tint on finished paper.
Summary
- For paper coating: Delaminated hydrous kaolin for premium high‑gloss top‑coat; fine washed hydrous kaolin for medium‑grade coating and base‑coat; calcined kaolin only for blending to raise opacity.
- For paper filling: Medium‑fineness washed hydrous kaolin is the most suitable mainstream grade.
Grade selection shall match target paper quality, and balance brightness, particle morphology, viscosity, abrasion index and production cost.
