Abstract
Currently, high-gloss, sand-texture, and low-temperature fast-curing (180℃/5 min) flexible bending transfer powders are well-developed in the market. However, low-gloss powder coatings with high flexibility and excellent bending performance remain scarce. Achieving a gloss level of 5%–15% while maintaining strong foldability is challenging. This study develops a low-gloss, highly flexible bending transfer powder coating by optimizing the powder system, polyester resin selection, and matting agent selection.
1. Introduction
With the rapid development of China’s construction industry, powder coatings used for building materials have become increasingly important. Building materials require both weather resistance and decorative performance, making surface finishing essential. Among all decorative technologies, thermal transfer printing has become mainstream.
In early applications, security doors were transferred after forming. Because the door surfaces typically contain grooves or embossed designs, vacuum transfer often fails to cover recessed areas, causing incomplete transfer. Glue transfer requires significant manual labor.
Additionally, security doors are filled with insulation materials (honeycomb paper, rock wool, fiberglass, aluminum silicate), which slow heating, reduce production efficiency, and may cause insufficient curing.
Bending-transfer powder coatings solve these problems by placing spraying and transfer steps before metal forming. During post-forming processes—stamping, bending, rolling—the coating undergoes shear, compression, and bending forces. Therefore, bending-transfer powder coatings must have excellent flexibility and adhesion.
While high-gloss, sand-texture, and fast-curing flexible bending transfer powders are mature, low-gloss flexible versions are rare, mostly due to poor matting effectiveness or cracking during bending. This paper focuses on resin system selection, matting resin selection, and formulation optimization to develop a low-gloss, high-flexibility bending transfer powder coating.
2. Experimental Section
2.1 Raw Materials
- Polyester resin
- Epoxy acrylic matting resin
- Aliphatic glycidyl ester
- Pigments and fillers
- Standard powder coating additives (flow modifier, benzoin)
- Thermal transfer woodgrain film
2.2 Instruments
Twin-screw extruder; ACM mill; electrostatic spray gun; oven; transfer laminating press; T-bend tester; DSC analyzer.
2.3 Reaction Mechanism
Carboxyl groups in polyester resin react with the epoxy groups in aliphatic glycidyl ester and epoxy acrylic matting resin, forming a crosslinked network and compact coating.
The poor compatibility between styrene units in the epoxy acrylic matting resin and polyester causes phase separation during curing. This creates micro-roughness on the coating surface and reduces gloss.
However, excessive styrene reduces weatherability and may cause surface texture defects. Thus, this study aims to select the proper polyester and epoxy acrylic resin pairing and use aliphatic glycidyl ester to enhance flexibility.
2.4 Formulation & Coating Preparation
Materials were weighed, premixed, extruded, milled, sprayed, and cured. Transfer printing was performed at 200°C for 180 seconds, followed by immediate and delayed bending tests.
3. Results and Discussion
3.1 Selection of Matting Mechanism
Chemical matting options include epoxy acrylic matting resin and carboxyl acrylic matting resin. Physical options include dry-blend matting, matting wax, and acrylic polymers.
Carboxyl acrylic matting consumes TGIC and results in poor storage stability.
Dry-blend matting creates surface sparkles, especially at low gloss.
Matting wax and acrylic polymers struggle to reach very low gloss levels.
Therefore, epoxy acrylic matting resin was selected due to its ability to achieve low gloss, high Tg, and fine surface appearance.
3.2 Polyester Resin Selection
Polyester acid value, functionality, and curing degree significantly affect transfer performance.
- High acid value → high reactivity → high crosslinking density → cracking during bending
- Poor compatibility → surface pattern defects
- High viscosity → difficult to matte
Polyester C demonstrated optimal compatibility with the epoxy acrylic matting resin, fine appearance, good leveling, and easy matting behavior. Thus, polyester–acrylic compatibility is essential.
3.3 Epoxy Acrylic Matting Resin Selection
3.3.1 Synthesis
The matting resin is synthesized from styrene (hard monomer), methacrylate/acrylate (soft monomers), and GMA (epoxy functional monomer).
Styrene increases Tg but decreases compatibility. GMA determines crosslink density.
3.3.2 Application Performance
Three epoxy acrylic resins were tested:
- Resin #1: low Tg and epoxy value; difficult to achieve low gloss
- Resin #2: high Tg; gloss can be reduced significantly, but surface feel is poor
- Resin #3: balanced Tg; low gloss + fine texture + good tactile feel
Thus, the matting resin selection is critical for gloss level, smoothness, and compatibility.
3.4 Effect of Aliphatic Glycidyl Ester
Epoxy acrylic resin contains epoxy groups on side chains, increasing crosslink density and reducing flexibility. Styrene also contributes to brittleness.
Aliphatic glycidyl ester improves flexibility by reducing the reaction share of epoxy acrylic resin and lowering coating rigidity. Increasing dosage improves flexibility but also increases gloss beyond a certain limit.
3.5 Final Formulation & Coating Performance
The optimized system, combining the selected polyester, epoxy acrylic matting resin, and proper amount of aliphatic glycidyl ester, delivers:
- Low gloss (5–15%)
- Fine surface appearance
- Excellent transferability
- Outstanding bending performance
Suitable for bending and post-forming applications.
4. Conclusion
- Epoxy acrylic matting resin is the most suitable matting approach for low-gloss bending transfer powder coatings.
- A polyester resin with slightly lower acid value improves bending performance without reducing transfer quality.
- Low-viscosity polyester enhances leveling and matting.
- Matting resin compatibility determines gloss and appearance; styrene content must be balanced.
- Aliphatic glycidyl ester effectively improves coating flexibility and bending performance.
