Extender pigments 鈥?commonly called fillers 鈥?are obtained from natural sources, direct manufacturing, or as industrial byproducts. They’re predominantly barium, calcium, magnesium, or aluminum salts, silicon or aluminum oxides, or double salts derived from them. Applications span coatings, printing inks, linoleum, plastics, paper, rubber, cement, and ceramics.
These materials share two common traits: a refractive index below 1.75 (typically 1.45鈥?.70), and a white or near-white color. Beyond that, properties vary widely 鈥?relative density, bulk value, particle shape, particle size, size distribution, oil absorption, and chemical reactivity all differ by grade and production method.
1. Natural Calcium Carbonate Manufacturing
Calcium carbonate is primarily obtained from limestone deposits, though it can also be recovered as a byproduct of certain chemical processes or produced by reacting carbon dioxide with lime slurry. Natural grades are manufactured by dry or wet grinding.
Dry grinding. Ground product passes through an air classifier that returns oversized particles for re-grinding. Simple and lower-cost, but particle size control is coarser.
Wet grinding. Limestone is milled in water using conical or tubular mills, with the slurry discharged continuously and classified by centrifuge. Oversize returns with the feed for re-grinding. Wet grinding produces finer, brighter grades with tighter particle size distribution. It also enables further refinement through flotation to improve color and chemical purity.
In continuous milling systems, compressed air, heated compressed air, or superheated steam serves as the fluid medium. Raw material and fluid enter together, with particles colliding at high speed to produce the grind. The mill typically integrates both grinding and classification functions.
2. Precipitated Calcium Carbonate (PCC)
Precipitated calcium carbonate is made by several established processes. All start with natural limestone, which is calcined to calcium oxide (quicklime) then hydrated to calcium hydroxide. Multiple purification steps remove oversized impurities and unconverted limestone.
Bicarbonate method. Controlling the reaction between calcium hydroxide and carbon dioxide produces different particle sizes, though PCC from this route is generally medium-grade in particle size.
Byproduct method. Reacting calcium hydroxide slurry with sodium carbonate solution yields a fairly fine PCC. Critical control point: all sodium hydroxide must be thoroughly washed out, otherwise the residual alkalinity causes problems in coating applications.
Calcium chloride method. Reactants often come from other chemical processes. For example, ammonium chloride from sodium bicarbonate production is mixed with calcium hydroxide to produce calcium chloride and ammonium hydroxide. Calcium chloride then reacts with sodium carbonate to yield a series of very fine PCC grades with high color purity and uniform particle size distribution 鈥?provided all sodium chloride is thoroughly washed out. This method produces calcite crystal structure PCC with particle sizes typically 8鈥?0 碌m, down to a minimum of 0.03鈥?.05 碌m. The lime slurry method produces aragonite-type PCC, usually 0.2鈥?.0 碌m.
3. Key Technical Parameters
- Relative density: 2.70鈥?.77
- Oil absorption: 5鈥?5 g/100 g depending on grade
- Average particle size: from sub-1 碌m up to large grades with 25鈥?0% retained on 200-mesh (~300 碌m)
- Colloidal grades: below 0.1 碌m, achievable through controlled process conditions
4. Application Characteristics
Calcium carbonate is insoluble in water and inert to most oil-based resin coating vehicles. It is sensitive to dilute acids 鈥?a property that actually benefits many interior coating applications including primers and semi-gloss paints. It also sees use in high-gloss paints, lacquers, and various industrial coatings where it serves as a cost-effective extender that adjusts rheology and improves film properties without compromising core performance.
References
- Wikipedia: Calcium Carbonate 鈥?Comprehensive overview of CaCO3 chemistry, natural occurrence, and industrial production methods
- Wikipedia: Calcite 鈥?Crystal structure and properties of calcite, the most common CaCO3 polymorph
- Wikipedia: Refractive Index 鈥?Physical basis for extender pigment optical behavior in coating films
- ASTM D1199 鈥?Calcium Carbonate Pigment Specification: Standard specification for calcium carbonate pigments used in paints and coatings
- ISO 3262 鈥?Extenders for Paints: International standard series covering specifications and test methods for extender pigments