Metal Powder Coatings
Metal powder coatings, leveraging the decorative and functional aspects of metal pigments like pearlescent and aluminum silver powders, are a key surface treatment technology in construction and furniture.
What Are Metal Powder Coatings?
Metal powder coatings, powder-based and using metal powders (e.g., aluminum, zinc, copper) as primary pigments, are detailed below:
Composition
- Resin: The main film - forming component. Options include epoxy, polyester, and epoxy/polyester hybrid resins.
- Curing Agent: Triggers cross - linking with resins upon heating to form a film. Amine - based agents are typical.
- Pigment: Metal powders such as aluminum, zinc, copper, and stainless steel impart special metallic luster and decoration.
- Filler: Enhances film properties like hardness and wear - resistance. Examples are barium sulfate, calcium carbonate, and talcum powder.
- Additive: Improves processing and film performance, including leveling agents, degassers, and curing accelerators.
Characteristics
- Environment - friendly
- Decorative
- Protective
- Durable
- Easy to apply
Types of Metal Powder Coatings
By Metal Pigment Type
- Aluminum Silver Type: Aluminum powder, or "silver powder, " comes in floating and non - floating variants.
- Copper Gold Type: Copper gold powder is vibrant but poor in weather and temperature resistance, so it's rarely used outdoors for aluminum profiles.
- Zinc Powder Type: Zinc - based coatings are corrosion - resistant and often used for steel protection.
By Film Appearance
- Flat Type: Smooth film surface, with options ranging from high - gloss to matte.
- Textured Type: Features textures like hammer, sand, orange peel, crackle, wrinkle, velvet, etc.
By Functionality
- Decorative Type: Boosts workpiece appearance with diverse colors and textures.
- Protective Type: Focuses on protecting metal substrates with properties like weather and corrosion resistance.
By Film - forming Substance
- Thermoplastic Powder Coatings: Include polyolefin, polyvinylidene fluoride, polyamide, polyvinyl chloride, and thermoplastic polyester coatings.
- Thermosetting Powder Coatings: Examples are epoxy, epoxy/polyester, and polyester coatings.
Metal Pigment Selection and Formulation Design
Comparison of Common Metal Pigment Properties
Pearlescent Pigments
- Structure: Composed of mica coated with metal oxides like TiO₂and Fe₂O₃, creating iridescent colors through light interference.
- Particle Size Impact: Coarser particles (>50μm) enhance shimmer but reduce hiding power; finer particles (<20μm) provide strong coverage and soft luster.
- Addition Limits: Excessive coarse pearlescent pigment can cause rough film surfaces, so addition is kept ≤1.5%.
Aluminum Silver Powders
- Floating Type: Processed with stearic acid, it rises to the coating surface, delivering strong metallic effects but poor corrosion resistance. When particle size <10μm, addition should be ≤1%.
- Non - floating Type: Coated with silica or acrylic, it distributes evenly in the film, offering excellent weather resistance. Addition can range from 5% - 20%.
Copper Gold Powder
Made of copper - zinc alloys, its gold tone comes from a protective surface layer. However, it's poor in weather resistance and mainly used indoors.
Formulation Design Keys
- Blending: Mixing pearlescent and aluminum silver powders can balance decoration and weather resistance, yet total addition should be ≤1.5%.
- Base Powder Optimization: High glass - transition temperature (Tg) resins (like polyester/TGIC systems) can increase bonding temperature ranges (50 - 60℃ for flat powders, 65 - 70℃ for sand - textured powders).
- Cost Control: Fewer metal pigment types and minimal usage for coverage are better.
Key Bonding Process Technologies
Bonding Process and Parameter Control
Metal powders are stably combined via dry - mixing plus bonding, with core parameters as follows:
- Temperature: Adjusted according to base powder type (sand - textured powder > two - component powder > flat powder).
- Operating Frequency: Excessively high frequency can damage the coating layer of aluminum silver powders (e.g., silica - coated), reducing alkaline resistance.
- Time: 30 - 60 minutes. Over - bonding can break pearlescent particles, diminishing shimmer effects.
Process Advantages and Risks
- Advantages: Bonding allows metal powder addition up to 20% (vs. 7% with dry - mixing), greatly improving spraying stability and powder - feeding efficiency.
-
Risks:
- Alkaline Resistance Drop: Bonding may damage aluminum powder coating layers. Choose composite - coated products (like silica + acrylic resin).
- Metallic Effects Fluctuation: Parameter inconsistencies between batches (e.g., ±5℃ temperature variation) can cause color differences. Use automated temperature - control equipment.
Testing Methods and Quality Control
Coating Performance Testing
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Acid - Alkali Resistance Testing:
- Alkali Resistance: A 10% NaOH solution is applied for 15 minutes. Visual inspection should show no corrosion (floating silver has the poorest alkali resistance, while composite - coated products are the best).
- Acid Resistance: 15 minutes of exposure should cause no significant changes (aluminum is inherently acid - resistant).
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Mortar Resistance Testing:
Procedure: Following the GB 5237.4 - 2017 standard, after 24 hours of mortar corrosion, the coating should show no peeling.
- Adhesion Testing: Cross - cut method (ASTM D3359) should achieve a grade 1 standard, with salt spray resistance ≥1, 000 hours without blistering.
Spraying Stability Testing
- Manual Spray Board Method: Adjust voltage (30 - 90kV) and air pressure (0.05 - 0.20MPa) to check metallic effect consistency.
- Production Line Simulation: Assess the impact of recycled powder ratios (new powder : recycled powder = 3 : 1) on metallic effects.
Analysis of Common Production and Spraying Issues
| Issue | Cause | Solution |
|---|---|---|
| Batch Color Differences | Fluctuations in bonding temperature/time | Fix process parameters; use automated temperature - control equipment |
| Poor Flowability | Conductive metal pigments affect electrostatic adsorption | Add 0.5% - 1% aluminum oxide C or flow aid (post - bonding) |
| Reduced Alkali Resistance | Bonding damages aluminum powder coating layer | Use composite - coated aluminum powder (e.g., silica + acrylic resin) |
| Powder Clogging During Spraying | Differences in the electrostatic properties of metal pigments and base powder | Optimize bonding process; control metal pigment particle size distribution (D50 = 30 - 50μm) |
Industry Trends and Future Directions
Process Innovation
- Low - temperature Bonding Technology: Develop resins with a Tg <50℃ to cut energy use (e.g., UV - curing systems).
- Bonding - free Metal Pigments: Pre - coated aluminum silver powders with resin enable direct dry - mixing, streamlining the process.
Material Upgrades
- Enhanced Weatherability: Fluorocarbon powder coatings offer 15 - year weather resistance but need toughening agents to address brittleness.
- Environmental Certification: Meet RoHS standards with heavy metal content ≤100ppm (e.g., lead, cadmium).
Standardized Testing
Following YS/T 680 - 2023, push for unified indicators like artificial climate aging resistance (≥5 years) and wear resistance (Taber wear ≤50mg).
Conclusion
Quality control of aluminum - used metal powder coatings requires coordinated optimization of pigment selection, bonding parameters, and testing standards. Future efforts should focus on exploring low - energy processes (e.g., room - temperature bonding) and high - performance materials (e.g., nano - coated aluminum powders). Strengthening ISO 9001/14001 certification is also crucial to meet the dual demands of environmental - friendliness and weatherability in industries like construction and automotive.
