02 9541 3500Advancements in Predicting Coatability: A Surface Science Approach for Optimised Adhesion
- By Dr. Fiona Mary Antony, Adira Vaidyanathan (KRÜSS GmbH, Germany, kruss-scientific.com)
Coatings play a critical role in a wide range of industries, from automotive and aerospace to medical devices and consumer electronics. They serve as protective barriers against corrosion, wear, and environmental degradation while also enhancing aesthetics and functional properties such as hydrophobicity, conductivity, or anti-fouling behavior. However, the performance of a coating is only as good as its adhesion to the underlying substrate. Poor adhesion can lead to premature failure in the form of peeling, blistering, or delamination, ultimately compromising product reliability and longevity.
Surface analysis: the foundation of effective coating
Surface analysis plays a vital role in ensuring that coatings adhere properly by assessing the substrate’s physical and chemical properties. Traditional adhesion tests, such as ISO/JIS cross-hatch and ASTM tape tests, only evaluate final adhesion strength but fail to explain why adhesion is strong or weak.
To address this, modern surface free energy analysis provides predictive insights into coatability. Tools such as the KRÜSS Drop Shape Analyser measure key parameters like:
- Work of adhesion (WA): Determines the energy required to separate a liquid coating from a solid surface. A higher WA indicates stronger adhesion.
- Spreading coefficient (S): Predicts whether a liquid coating will spread or bead on a surface. A positive S value ensures good adhesion, while a negative value signals poor wetting.
- Interfacial tension (YSL): Represents the energetic barrier between the coating and the substrate. Lower interfacial tension leads to better bonding and adhesion.
Enhancing coating adhesion with surface treatments
Low-energy surfaces, like untreated PP, exhibit poor adhesion due to their hydrophobic nature. To improve coatability, surface treatments such as:
- plasma or corona treatment (modifies surface chemistry)
- flame treatment (increases surface oxidation)
- primer application (enhances bonding sites)
can be applied. Contact angle measurements before and after these treatments confirm increased surface free energy, leading to better adhesion.
Experimental results: KRÜSS ADVANCE adhesion analysis
The effectiveness of surface treatment in enhancing adhesion was evaluated by applying a water-based, one-component topcoat (1K) to polyamide (PA) substrates. This system is particularly relevant in printing and packaging applications, where polymer films require strong adhesion for high-quality prints, laminates, and coatings. Due to the inherently low surface free energy of polymeric materials, ensuring proper ink and coating adhesion remains a key challenge in these industries. To address this, Openair-Plasma® treatment was employed at varying intensities to modify surface properties and improve wettability.
Surface characterisation and treatment impact
A crucial factor in achieving strong adhesion in printing and packaging films is surface free energy modification, as it directly influences wettability and coating spreadability. The untreated PA substrate exhibited a total surface free energy of 49.2 mN/m, with a low polar fraction (15.2%), making it less receptive to water-based coatings. As plasma intensity increased, the total surface free energy and polarity rose significantly, improving coating compatibility.
In contrast, the water-based 1K topcoat had a surface tension of 25.0 mN/m, primarily dominated by disperse interactions (19.0 mN/m) and a smaller polar contribution (6.0 mN/m). The increased polar fraction of plasma-treated PA substrates resulted in improved adhesion, which is essential in flexographic and digital printing processes where uniform ink spreading is required.
Adhesion performance and coating durability
For packaging films and printed labels, adhesion strength determines resistance to peeling, abrasion, and mechanical stress. To quantify adhesion, work of adhesion (WA), interfacial tension (YSL), and spreading coefficient (S) between the 1K topcoat and the untreated/differently treated substrate were analyzed. Plasma treatment significantly influenced these adhesion parameters, demonstrating its impact on coating performance.
The untreated PA (PA1) showed the lowest work of adhesion (69.7 mN/m) and the highest interfacial tension, leading to weak ink and coating retention. For medium plasma treatment (PA5), adhesion improved, reducing coating detachment in high-speed printing applications. The highest plasma treatment (PA3) exhibited the strongest adhesion (81.0 mN/m), ensuring long-term durability of coatings on flexible packaging films. The results confirm that Openair-Plasma treatment plays a crucial role in enhancing adhesion by modifying the surface chemistry of polyamide substrates.
Additionally, the cross-cut adhesion test (ISO 2409) validated these findings, showing that higher plasma intensity leads to defect-free coatings suitable for demanding industrial applications. The untreated sample (PA1) showed significant delamination, while medium plasma-treated PA5 demonstrated reduced peeling.
Furthermore, we propose surface free energy and adhesion parameter measurements as a complementary method to traditional cross-cut tape tests. By leveraging KRÜSS ADVANCE software and contact angle analysis, these measurements provide a faster, quantitative, and more objective evaluation of substrate/coating adhesion, reducing dependency on time-consuming mechanical tests.
Industrial coatings: applications, types, and key performance parameters
| Industry | Common coating types | Key performance parameters |
| Automotive | Epoxy primers, PU topcoats, electrocoats | * Corrosion resistance : protects metal components from rust and degradation. * Adhesion strength : ensures coatings remain intact * Flexibility : allows coatings to withstand mechanical stress without cracking |
| Aerospace | High-performance epoxy systems, PU systems | * Thermal stability: maintains integrity under extreme temperature fluctuations * Weight considerations: lightweight coatings are essential to not impede aircraft performance * UV radiation resistance: prevents degradation from prolonged sun exposure |
| Marine | Anti-fouling, anti-corrosive | * Saltwater resistance: protects against corrosive marine environments * Biofouling resistance: prevents accumulation of marine organisms on surfaces |
| Oil and gas | Epoxy and PU coatings | * Chemical resistance: withstands exposure to corrosive substances * Abrasion resistance: endures mechanical wear * Temperature resistance: maintains protective qualities under high-temperature conditions |
| Construction | Protective paints, sealants | * Weather resistance: shields structures from rain, UV rays, and temperature changes * Aesthetic quality: provides desired color and finish while maintaining performance |
Modern adhesion analysis, using tools like the KRÜSS DSA series, transforms the way coatings are evaluated. By leveraging parameters like work of adhesion (WA) and spreading coefficient (S), manufacturers can scientifically predict coating performance rather than relying on traditional pass/fail tests. This data-driven approach leads to:
- Optimised coating formulations
- Reduced material waste
- Improved product reliability
Assessing coating stability through adhesion tests, environmental simulations, and chemical resistance evaluations further bolsters the dependability of coatings across various sectors. By integrating predictive adhesion analysis and surface free energy measurement, businesses can attain high-performance, sustainable coatings that endure even the most demanding environments.
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References
- Application Report 296: Predicting Coatability | KRÜSS Scientific
- ASTM D3359-17, ISO 2409: Standard Test Methods for Measuring Adhesion by Tape Test.
- Industrial Coating: Types, Techniques, and Applications | IQS Directory
- Different Types of Industrial Coatings and their Applications | Thomas Industrial Coatings