Iron Oxide Black
Iron Oxide Black
Iron oxide black is a significant inorganic black pigment, chemically composed of ferric oxide (Fe₃O₄) or iron oxide (II,III), boasting excellent magnetic properties, coloring power, and stability. It is widely used in coatings, construction materials, and magnetic materials fields.
Basic Features
Chemical Formula: Fe₃O₄ (FeO·Fe₂O₃, containing Fe²⁺ and Fe³⁺).
Color: Pure black, with a hint of blue or brown (depending on particle size and purity).
Stability:
Thermal Resistance: Stable below approximately 180°C; high-temperature oxidation results in ferric oxide red (Fe₂O₃).
Lightfastness: Excellent, colorfast.
Acid and alkali resistance: Resistant to weak acids and alkalis; soluble in strong acids (e.g., HCl).
Conductivity: Magnetic (primary component of natural magnetite).
Form: Fine powder, particle size typically 0.1~1 μm.
2. Preparation Method
Chemical Coprecipitation Method (Main Process)
Reaction Principle:
Fe2++2Fe3++8OH−→Fe3O4↓+4H2OFe2++2Fe3++8OH−→Fe3O4↓+4H2O
Step:
Mix ferrous sulfate (FeSO₄) and FeCl₃ in a 1:2 molar ratio.
Add alkali (NaOH) to adjust pH to 9~11, and pass nitrogen gas to protect against oxidation of Fe²⁺.
Nanometer-sized Fe₃O₄ obtained through sedimentation, washing, drying, and calcination (200~400°C).
(2) High-Temperature Oxidation Method
Iron or ferrous oxide (FeO) heated under controlled oxygen partial pressure:
6FeO+O2→2Fe3O46FeO+O2→2Fe3O4
(3) Purification of Natural Magnetite
Magnetite (main component Fe₃O₄) crushing, magnetic separation, and purification; however, it contains a high level of impurities and has a darker color.
3. Main Applications
(1) Paints & Inks
Building Coatings: Exterior black tones, floor paint.
Corrosion-proof paint: Used for metal primers (magnetic adhesion enhancement for better adhesion).
Ink: Printing Ink, Toner (for laser printers).
(2) Construction Materials
Color cement, asphalt pavements, artificial stone.
(3) Plastic & Rubber
Cable sheath, automotive tires (UV-resistant).
(4) Magnetic Materials
Magnetic recording media (tapes, disks), magnetic fluids, and microwave absorbing materials.
(5) Miscellaneous
Cosmetics (eyebrow pencils, eyeliner), Pharmaceuticals (magnetic targeted drug carriers).
4. Categories & Models
Type Preparation Method Features Application
Synthetic Fe₃O₄ Chemical Coprecipitation High Purity, Nanoscale, Strong Magnetic Magnetic Materials, High-End Paints
Natural magnetite, mineral purification, low cost but high in impurities, construction materials, low-end fillers
Modified Iron Oxide Black, surface coated with SiO₂/Al₂O₃, improved temperature resistance, excellent dispersibility, suitable for high-temperature plastics and ceramics
Common International Models:
PBk11 (Pigment Black 11, International Code)
C.I. 77499 (Chemical Index Number)
5. Comparison with Other Black Pigments
Ferric Oxide Black (Fe₃O₄), Carbon Black, Chromium Oxide Black (Cr₂O₃)
Chemical Composition: Fe₃O₄, Carbon (C), Cr₂O₃
Color: Pure Black, Blue-black, Dark Black with Green
Temperature Resistance <180°C >300°C >1000°C
Conductivity, Magnetism, Semi-conductivity, Conductive, Insulating
Environmental Friendly - Non-toxic - May contain PAHs (Carcinogenic) - Chromium (Hexavalent Risk)
Cost: Medium, Low, High
The advantages of iron oxide black:
✅ Non-toxic and environmentally friendly, suitable for materials in contact with food (such as toys, cosmetics).
✅ Magnetic, suitable for special function materials (such as microwave absorption coatings).
✅ Better weather resistance than carbon black (less prone to grayish discoloration).
Limitations:
⛔ Oxidizes to Fe₂O₃ (turns red) under high temperatures.
❌ Not as black as carbon black (needs to be mixed with other colors for high darkness).
6. Cautionary Notes
Temperature Control: Processing temperature not exceeding 180°C (for low-temperature processes required in plastic injection molding).
Acidic Environment: Avoid strong acids (such as pH < 2) to prevent dissolution.
Storage: Seal to prevent moisture and oxidation from forming lumps.
Magnetic Applications: Nanometer Fe₃O₄ should be anti-agglomeration (dispersant can be added).
Summary
Iron oxide black is a versatile black pigment with coloring power, magnetic properties, and environmental friendliness, suitable for coatings, building materials, and high-tech fields. For higher darkness, mix with carbon black; for high-temperature resistance, choose chrome oxide black. Specific selection should be based on temperature resistance, conductivity requirements, and cost considerations!
