Optimizing Polyester Paint in Coil Coating
Exploring Akseli Ritala's groundbreaking research on improving surface quality through advanced rheology
Have you ever admired the sleek, colorful facade of a modern building, the pristine finish of a new car, or the flawless casing of a home appliance? There's a high probability you're looking at the handiwork of the coil coating industry, a sector dedicated to perfecting metal surfaces before they're ever shaped into final products 1 .
Coil coating applies paint to metal coils at high speeds before fabrication, ensuring uniform quality across all products.
Akseli Ritala's 2017 Master's thesis tackled persistent surface defects in polyester wet paint applications 3 .
Coil coating is a highly automated, continuous process where a coiled strip of metal (steel or aluminum) is cleaned, pre-treated, primed, painted, and cured in a single, high-speed line 1 8 .
This method offers immense advantages over painting after fabrication: it's faster, more environmentally friendly with lower VOC emissions, and produces a more consistent finish 2 9 .
Polyester resins account for over 41% of the material type segment in technical coil coatings 6 .
The core issue Ritala investigated is rooted in the complex, non-Newtonian rheology of polyester paints 3 . Unlike simple fluids like water (a Newtonian fluid), whose flow resistance remains constant, the paints used in coil coating are thixotropic.
Constant viscosity regardless of shear force (e.g., water)
Viscosity decreases under shear, then recovers (e.g., ketchup, polyester paint)
Balancing flow during application with stability after application
"The liquid 'wet paint' must possess just the right properties to flow smoothly, level out, and cure without defects—a challenge that Akseli Ritala set out to solve."
Ritala's approach was methodical and scientific, centered on manipulating the paint's formulation and rigorously testing its performance.
| Material | Function |
|---|---|
| Polyester Resin | Primary film-forming polymer |
| Rheology Modifiers | Control thixotropy, prevent sagging |
| Pigments & Fillers | Provide color, opacity, influence properties |
| Solvents | Adjust viscosity, influence drying |
| Crosslinking Agents | Create hard, durable network during curing |
| Metal Substrates | Steel/aluminum coils for coating |
| Formulation | Modifier Concentration (%) | Viscosity at Low Shear (Pa·s) | Viscosity at High Shear (Pa·s) | Thixotropic Index |
|---|---|---|---|---|
| A | 0.5 | 2.1 | 0.8 | 2.6 |
| B | 1.0 | 3.5 | 0.9 | 3.9 |
| C | 1.5 | 6.0 | 1.0 | 6.0 |
| Target Range | --- | 3.0 - 4.5 | 0.8 - 1.0 | 3.5 - 4.5 |
Formulation B falls within the target range for optimal performance, balancing sag resistance (high low-shear viscosity) and easy application (low high-shear viscosity).
Through this systematic methodology, Ritala's work successfully identified the key rheological parameters and formulation adjustments needed to minimize surface defects. The research demonstrated that by precisely controlling the thixotropic nature of the paint, it was possible to achieve a stable "transfer window" in the reverse roll coater, leading to a more uniform and higher-quality film 3 .
Surface defect rate reduced from 5% to 1%
Production speed increased from 120 to 140 m/min
Paint consumption decreased from 150 to 145 g/m²
| Formulation | Sag Resistance | Leveling | Orange Peel | Overall Quality Score |
|---|---|---|---|---|
| A | Poor | Excellent | Excellent | 5/10 |
| B | Good | Good | Good | 9/10 |
| C | Excellent | Poor | Poor | 4/10 |
Correlating rheology with actual coated panels. Formulation A sags too much, while C doesn't level properly. Formulation B achieves the best balance, resulting in the highest quality score.
Ritala's research is a snapshot of the continuous innovation driving the coil coating industry forward. The future points toward even more advanced formulations and functionalities.
A major push is underway for low-VOC, water-borne, and chrome-free coatings that comply with stringent environmental regulations like REACH 9 .
The next generation includes self-cleaning (photocatalytic), antimicrobial, and self-healing properties, opening new markets in healthcare 6 .
"Cool roof" coatings with high solar reflectance are gaining traction, helping buildings reduce cooling costs 9 .
Premium applications are seeing increased use of ultra-durable fluoropolymers (like PVDF) that can offer 30-40 year warranties on color retention 9 .
Akseli Ritala's optimization of polyester wet paint is a powerful reminder that the most beautiful surfaces often rely on the most intricate science. His work, conducted at the intersection of materials chemistry and fluid dynamics, provides a tangible blueprint for improving an industrial process that touches nearly every aspect of our built environment.
By understanding and manipulating the invisible world of rheology, researchers like Ritala help transform a simple metal coil into a durable, beautiful, and high-performance product.
The quest for the perfect surface is never-ending. As the industry evolves toward smarter and more sustainable coatings, the foundational principles explored in research like Ritala's will continue to guide the way.
References to be added manually