Discussion on preparation technique of metal effect thermosetting powder coating for aluminum sheet

Discussion on preparation technique of metal effect thermosetting powder coating for aluminum sheet

Zhang Guanghong, Chen Duo, Zhou Lei (Wuhan Yincai Technology Co., LTD.)

 Foreword 

As a premium architectural cladding material, aluminum panels are extensively utilized in curtain walls, ceiling systems, and interior/exterior decorative applications. Their exceptional durability, lightweight construction, and adaptability are made possible by high-performance surface coatings. Metal-effect powder coatings not only deliver versatile color options (including silver-white, champagne gold, bronze, and black sand-textured finishes) but also create distinctive metallic textures, shimmering effects, and three-dimensional depth. These enhancements significantly elevate the decorative value and commercial value of aluminum panels.

The core challenge in the preparation of such coatings is how to achieve stable, uniform and gorgeous metal visual effects while ensuring mechanical properties, weather resistance and corrosion resistance of the coating.

I. Selection of core raw materials

1.1 Resin System

* Preferred: Weather Resistant Polyester Resin (WRR)

* Reason: aluminum plate is exposed to the outdoor for a long time, so it needs strong resistance to ultraviolet (UV), hydrolysis, temperature change performance.

* Ultra-durable polyester resin (typically with TGIC/HAA curing system) provides gloss and color retention for more than 10 years or even 15 years, which is the gold standard for outdoor applications.

* Ultra-weather resistant polyester with PVDF fluorocarbon resin: A popular method of adding to the base powder during production

The formulation consists of 5%-20% thermoplastic fluorocarbon resin (PVDF) through co-extrusion. While PVDF is reportedly capable of providing 20-30 years of weather resistance in fluorocarbon coatings, experimental results from the author's tests demonstrated lower performance compared to pure ultra-wear-resistant systems. The primary reason lies in PVDF's thermoplastic nature, which exhibits poor compatibility with ultra-wear-resistant polyester. This incompatibility disrupts coating uniformity during blending. During melt extrusion, PVDF particles are partially encapsulated by the resin, resulting in disorganized particle distribution across the coating surface. During baking and film formation, residual PVDF particles migrate to the surface, failing to form a continuous fluorocarbon layer and consequently compromising the underlying coating's protective integrity (cross-sectional electron microscopy reveals minimal fluorine element variation on the top surface). When PVDF content reaches 15%, the coating exhibits severe orange peel-like surface roughness with compromised adhesion, poor bending strength, and significant front-side 10kg.cm cracking. UV aging tests also showed inadequate performance. Conversely, adding ultra-fine PVDF resin (up to 3%) during the metal-bonding phase, the PVDF resin can be well migrated during film formation due to its low surface tension characteristics, and the coating film shows a fine and smooth matte effect and low surface tension characteristics, which prevents rainwater and dirt from sticking and is conducive to the later cleaning and maintenance of aluminum single wall.

*Super Weather-resistant Polyester with FEVF Fluorocarbon Resin: FEVF fluorocarbon resin delivers unparalleled top-tier weather resistance (20-30 years), exceptional chemical resistance, self-cleaning properties, and outstanding durability. However, its cost is prohibitively high, and the powder-form FEVE resin typically requires more stringent curing conditions (temperature, time). Ultra Weather-resistant Polyester Resin (HAA/TGIC System): Significantly cheaper than FEVE, it boasts excellent mechanical properties (flexibility, impact strength), well-established processing techniques, and good weather resistance (10-15 years). Yet its maximum weather resistance still falls short of FEVE. Combining these two systems achieves superior weather resistance, light/colored retention, and corrosion resistance compared to pure polyester formulations, approaching the performance level of pure FEVE. This significantly reduces raw material costs for pure FEVE systems, enhancing product competitiveness in the market. It also addresses potential issues like narrow curing windows and slightly inferior leveling in pure FEVE systems, leveraging the advantages of polyester resin to simplify formulation and application. By selecting appropriate molecular weight and functional groups for FEVE, with 5% -30% usage in melt extrusion,

The surface condition and mechanical properties of the coating are similar to those of the super weather-resistant pure polyester system. The cross-sectional electron microscope shows that fluorine is continuously enriched on the surface, and it still performs well after 4000+ hours of aging under xenon lamp.

1.2 Metallic pigments (MP)

* Type ：

* Non-floating aluminum pigment (NFP): The most commonly used type. The pigment is evenly distributed in the coating, producing a uniform metallic shimmer effect. Surface passivation treatment (such as multi-layer coating with silica) is required to prevent chemical reactions (gas generation, discoloration) during processing and use.

* Effect pigments (Effect Pigments): such as pearlescent powder (mica titanium, mica iron oxide), copper and gold powder, etc., used to create special color effects.

* key parameter ：

* Particle size and distribution: A mix of coarse and fine particles. Coarse particles provide a strong spark effect, while fine particles provide a uniform base pattern and high metallic brightness (Brightness), as well as proper coverage and rich layering.

* Shape: The flake structure is the key to producing a metallic effect, and its orientation directly affects the gloss and sparkle.

1.3 Additives (Aids)

      * Leveling Agent (Leveling Agent): Ensure that the coating forms a smooth, flat surface during the molten flow.

* Degassing Agent: helps to discharge the trace moisture left by powder coating itself and aluminum plate pretreatment or gas generated by curing reaction to prevent pinholes.

* Ultraviolet absorbent and light stabilizer: The molecules of both have a specific chemical structure that can selectively strongly absorb the photon energy of the ultraviolet region (wavelength 290-400nm) and capture free radicals, but hardly absorb visible light (so they are usually colorless or light-colored and do not affect the coating color)

* Bonding Agent: used in bonding process to help metal pigment and resin particles temporarily adhere.

1.4 Fillers (Fillers)

* Such as barium sulfate, used to adjust the gloss (such as matting) and physical properties of the coating, but the amount should be strictly controlled, too much will seriously weaken the metal effect.

II. Preparation process and key technology

The conventional powder coating production process (Batching → pre-mixing → melt extrusion → tabletting and cooling → coarse crushing → grinding → sieving) presents critical challenges for metal coatings. During extrusion and high-speed grinding stages, the metal substrate's flake-like structure is compromised, resulting in uneven metallic effects, incomplete metallic patterns, and poor stability in the final powder. With the exception of rare hammer-effect applications, these methods are largely abandoned in industrial practice.

Therefore, bonding technology is the key technology to prepare high performance metal effect powder coating.

The bonding process steps are as follows:

2.1 Base powder preparation: Pure color base powder (Base Powder) without metal pigment is produced according to the traditional process. This step completes the full dispersion and plasticization of resin, filler and additives.

Recipe example:

Key points of bottom powder control:

     a. Metal powder base powder is not recommended to have too low resin content

     b. Due to the complex structural design of aluminum panels, the coating film must possess sufficient opacity and maintain uniform coloration to prevent mottling. Except for certain intense metallic effects requiring high-concentration opaque powder additives, the base powder formula should not be designed with transparent or semi-transparent opacity.

     c. UV aging test results confirm that the ultra-durable matte finish strongly recommends using the two-component dry-mix formulation. For the Strong Flash series , it's recommended to significantly enhance the A powder's coverage power by adjusting its color to a white (flash silver) or metallic-toned hue matching the surface finish. The B powder should be formulated as semi-transparent or near-transparent color, it allows mass production of B powder for backup use. When mixing colors, only A powder needs adjustment, saving significant workload. Meanwhile, colored A powder particles act as vibrant color dots in the coating film, providing rich multi-layered color effects. This reduces metal powder usage while resolving the conflict between strong glitter effect and coating opacity (excessive opacity diminishes metallic shine, while insufficient opacity leads to exposed base colors).

d. Adding matting agent can make the coated film exhibiting a finer texture with enhanced metallic luster and silky-smooth surface. For instance, Wuhan Yincai's newly launched G600-1 acrylic matting resin demonstrates excellent leveling properties. Its soft matte finish creates a smooth, delicate surface with anti-fingerprint properties, after bonding,  the film achieves a skin-like texture effect.

e. Powder particle size control is very important: finer base powder for metal flakes promotes better dispersion of the metal particle during bonding, while a coarser base powder enhances adhesion of the metal flakes.

f. It is more advantageous to choose the resin with higher Tg point.

2.2 Dry mixing: The cooled and crushed base powder particles are pre-mixed with the accurately measured metal pigment at a low speed in a dedicated bonding machine (such as Henschel mixer).

2.3 Bonding: Under the control of temperature (usually using friction heat, heating to slightly below the glass transition temperature Tg of resin) and rotation speed, metal pigment flakes are gently attached and embedded into the surface of soft base powder particles through static electricity and physical action.

* Temperature control is the key: too low temperature, the bonding is not firm, the pigment is easy to fall off; too high temperature, the powder will be clumped and even pre-cured.

When the bonding speed is too low, leading to slow heating, the bonding time extend, and resulting in poor dispersion of metal powder, leading to material adhesion on cylinder walls. Excessive rotation speeds cause metal powder to lose its metallic texture through shearing, particularly affecting mica pearlescence. It is recommended to disperse fine metal powder with base powder while using jacketed hot water for heating, while coarse metal powder should be added during the bonding phase when rotation speed decreases.

* If the bonding time is too short, the metal powder can not be glued firmly and will be separated during spraying. If the bonding time is too long, the metal powder will be subjected to more severe shear and there will be too many agglomerates affecting the yield of finished products.

Methods for assessing bonding efficiency include: 1. Gently spread a small amount of powder on white paper and observe free metal particles scattered on the paper using a 150x microscope. 2. Shake the powder in a transparent plastic bag to examine metal particle adhesion on its inner surface. 3. Spray with different spray guns at varying voltage levels, observing whether metal particles accumulate at the nozzle tip and comparing coating color differences after baking. 4. Conduct cyclone separation tests to evaluate coating color consistency.

2.4 Cooling and sieving: The material is cooled quickly after being fixed to harden the resin and "lock" the metal pigment on the base powder particles. Finally, the sieving is removed to remove any small aggregates that may be produced.

2.5 Advantages of bonding process:

* Perfect protection of metal pigment: avoid extrusion and grinding damage, maintain the complete flake structure and optical effect of pigment.

* Excellent stability: the metal pigment is closely combined with the base powder particles, which solves the separation problem in the process of transportation, storage and use, and the powder utilization rate is high.

* Excellent coating effect: good charging during spraying, uniform powder coating, and gorgeous and consistent metal effect of coating film.

III. Influence of coating and curing on metal effect

Even if the powder is perfectly prepared, the spraying process is also crucial.

3.1 Electrostatic Spray (ES): is the standard method.

3.2 Thickness control: The film thickness should be uniform and controlled within a reasonable range (usually 60-80 μm). Too thin will lead to weak metal and exposed bottom; too thick will lead to disordered orientation of metal sheets, uneven gloss, and even sagging.

3.3 Curing process:

* Heating rate: rapid heating is conducive to the rapid reduction of the viscosity of the molten coating resin, so that the metal flakes can be arranged in an orderly manner under the action of surface tension, parallel to the surface of the substrate, so as to obtain the highest metal brightness and uniformity.

* Curing temperature and time: must be strictly in accordance with the technical parameters provided by the powder supplier to ensure complete crosslinking and curing for optimal performance.

IV. FAQ and countermeasures

V. CONCLUSIONS

The preparation of metal effect powder coating for aluminum sheet is a systematic project, and its core lies in:

1. Scientific formula design: select weather-resistant resin, stabilized metal pigment and efficient additive system.

2. Advanced preparation process: Bonding technology is the only reliable way to achieve high performance metal effect, which perfectly solves the contradiction between effect and stability.

3. Precision Coating Control: Standardized surface treatment, precise spraying and curing processes ensure the final outcome. With the continuous improvement of architectural aesthetic demands and increasingly stringent environmental requirements, developing low-VOC coatings with enhanced weather resistance,

More diversified visual effects and functions (such as ultra-matte metal, multi-color mixing effect, anti-graffiti, self-cleaning, antibacterial) of powder coatings will be the future continuous research direction.