Titanium Dioxide Inorganic Coating: Process, Types (Al2O3, SiO2, ZrO2, etc.)

Titanium dioxide (TiO₂)is a crucial white pigment, but its performance in various applications often requires enhancement. Inorganic coating, an indispensable step in the post-processing of Titanium Dioxide, involves forming a single or multiple layers of inorganic films on the TiO₂ particle surface via precipitation reactions. This creates a barrier between the particles and the surrounding medium, significantly improving key properties of the titanium dioxide.

Common inorganic coating materials include alumina, silica, zirconia, aluminum phosphate, ceria, tin oxide, and various mixed coatings. These modifications enhance the pigment's dispersion stability, light resistance (anti-photochemical activity), and weather resistance, making it suitable for demanding applications in paints, plastics, paper, inks, cosmetics, and rubber.

1. Methods for Inorganic Coating of TiO₂

The primary industrial methods are dry and wet processing, with wet processing being dominant.

(1) Dry Process
This method involves adsorbing a metal halide onto TiO₂ particles using a carrier gas or spray, followed by hydrolysis with steam or thermal oxidation. While it offers a short process flow, low equipment cost, and easy automation, it suffers from rapid particle growth, difficult process control, and poor product stability, limiting its widespread use.

(2) Wet Process
This process, using water or other solvents as the medium, is the most common industrial approach. It can be categorized into:

· Boiling Method: Coating agents hydrolyze and deposit onto TiO₂ particles under strong heating. This method is hard to control, often results in incomplete hydrolysis, and has poor adaptability.

· Neutralization Method: This involves a continuous acid-base neutralization reaction on the TiO₂ surface to form the coating film. It includes:

- Adding alkali (e.g., NH₃·H₂O, NaOH, Na₂CO₃) to precipitate acidic coating agents.

- Adding acid (e.g., H₃PO₄, H₂SO₄, HCl) to precipitate basic coating agents.

- Co-precipitation of acidic and basic coating agents (e.g., Al₂(SO₄)₃ with NaAlO₂). The coating layer is not purely a hydrated oxide due to simultaneous reactions. For instance, adding phosphorus compounds can form aluminum phosphate or zirconium phosphate precipitates, significantly improving weather resistance.

· Carbonation Method: CO₂ gas is introduced into an alkaline TiO₂ slurry containing coating agents, causing slow precipitation and film formation. Applied in Si-Al composite coating, it yields more uniform coatings with stronger light stability.

Among these, the neutralization wet process is the most widely used in industry due to its controllable conditions, stable products, and consistent results.

2. Types of Inorganic Coatings for TiO₂

(1) Alumina (Al₂O₃) Coating
This is a common method to improve dispersion stability in aqueous systems. The microstructure of the alumina layer directly affects surface properties. Optimal performance is achieved with a continuous, porous boehmite (AlOOH) structure.

· Optimal Parameters: pH 9, 70°C, Al₂O₃:TiO₂ ratio 3.2%, 60 min reaction, 120 min aging, 25% slurry concentration, with specific amounts of NaCl and dispersant.

· Mechanism: The fibrous boehmite layer provides steric hindrance, increases surface hydroxyl content (improving wettability), and enhances zeta potential (increasing electrostatic repulsion).

(2) Silica (SiO₂) Coating
Silica layers act as a barrier, improving weather resistance by reducing direct contact between TiO₂ and the environment. Continuous, dense, and thick layers offer the best protection.

· Optimal Parameters: pH 9, 85°C, SiO₂:TiO₂ ratio 2.5%, 90 min reaction, 120 min aging, 25% slurry concentration.

· Mechanism: Shields the core from acid attack and weathering; inhibits crystal structure transformation and improves thermal stability.

(3) Zirconia (ZrO₂) Coating
Zirconia reduces TiO₂'s photocatalytic activity, thereby enhancing light resistance. It has high refractive index but weak UV absorption.

· Optimal Parameters: pH 5, 55°C, ZrO₂:TiO₂ ratio 5%, 60 min reaction, 120 min aging, 20% slurry concentration.

· Mechanism: Forms a barrier preventing contact between reactive species and organics; reduces electron-hole pair generation/separation and surface hydroxyl groups, slowing down oxidative degradation.

(4) Aluminum Phosphate (AlPO₄) Coating
This single coating can simultaneously improve both light resistance and dispersion stability, potentially reducing the need for secondary organic coatings.

· Optimal Parameters: pH 9, 50°C, AlPO₄:TiO₂ ratio 3.0%, 60 min reaction, 120 min aging, 25% slurry concentration.

· Mechanism (Light Resistance): Barrier effect; traps electrons/holes, reducing reactive species generation.

· Mechanism (Dispersion): Lowers surface energy (improving wettability) and increases surface negative charge (enhancing electrostatic repulsion).

(5) Ceria (CeO₂) Coating
Ceria coating is an effective way to boost light resistance.

· Optimal Parameters: 65-70°C, CeO₂:TiO₂ ratio 3%, 23% slurry concentration.

· Mechanism: Blocks contact with O₂ and H₂O; covers crystal defects and traps electrons/holes.

(6) Tin Oxide (SnO₂) Coating
This is a mature method for producing conductive titanium dioxide.

· Optimal Parameters: pH 2.0, 50°C, 180 min reaction, SnCl₄ addition 25%, Sn:Sb mass ratio 12:1, calcination at 600°C for 150-180 min.

(7) Silica-Alumina (SiO₂-Al₂O₃) Composite Coating
This is the most popular binary composite coating, combining the benefits of both materials.

· Benefits: Synergistic effect improves weather resistance, chemical resistance, gloss retention, and anti-chalking. Adjusting the order and ratio allows control over surface charge for optimal dispersion in different media. Co-precipitation methods are also used to reduce steps and cost.

· Coating Order: For high weather resistance: Al₂O₃ first, then SiO₂. For aqueous coatings: SiO₂ first, then Al₂O₃.

· Mechanism: Provides a dual barrier, reduces photochemical activity, and improves compatibility/dispersion via formation of Al-O-Si bonds.

(8) Zirconia-Alumina (ZrO₂-Al₂O₃) Composite Coating
This combination enhances gloss, whiteness, light resistance, and dispersion.

· Process: Typically involves sequential precipitation. Studies confirm chemical bonding (Zr-O-Ti, Al-O-Ti) between the coating layers and the TiO₂ surface.

(9) Ternary Inorganic Coating (e.g., ZrO₂-SiO₂-Al₂O₃)
Advanced coatings involving three components (e.g., Zirconia-Silica-Alumina) can achieve a superior balance of high dispersion stability, excellent light resistance, and outstanding weather resistance, matching the performance of international premium products. Methods like sol-gel and co-precipitation are employed.

Conclusion
Inorganic coating is a vital technology for tailoring titanium dioxide properties to meet specific application requirements. From single-component coatings like alumina and silica to advanced binary and ternary composites, these processes enhance critical aspects like dispersion, durability, and optical performance. The continuous development of coating methods and materials ensures that titanium dioxide remains a high-performance pigment across countless industries.