In coatings, inks, paper, and printing, adhesion is a core quality metric for evaluating the bonding properties of coatings, ink layers, and substrates. The quality of adhesion performance will directly affect the anti-corrosion performance, decorative effect and service life of the coating, which is a parameter that must be strictly controlled in production and application.
Adhesion and its mechanism of action
Adhesion, the essence of which is the mutual attraction between the contact parts of two different substances, is a manifestation of the interaction force between the molecules of the substance.
From a microscopic perspective, the generation of adhesion is not a single mechanism, but the result of the joint or synergistic action of multiple theoretical mechanisms. It mainly includes the following:
Mechanical occlusion theory - when liquid paint or ink penetrates into the microscopic pores and depressions on the surface of the substrate and solidifies to form a mechanical interlocking structure.
Physical adsorption theory - polar groups (such as hydroxyl groups, carboxyl groups) in the coated polymer and polar groups on the surface of the substrate are combined through physical actions such as van der Waals forces and hydrogen bonding.
Chemical bond theory – Coatings chemically react with the surface of the substrate to form covalent, ionic, or coordination bonds, which often have higher strength and durability.
Diffusion theory – Under certain conditions (e.g., a certain temperature), the molecular chains of the coating polymer diffuse with the molecules on the surface of the compatible substrate, forming an intertwined network in the interface region.
In practical applications, the combination of coatings and substrates is often the result of the combined action of the above mechanisms.
The main test method for adhesion
Accurate evaluation of adhesion requires standardized test methods. The commonly used test methods at home and abroad can be mainly divided into the following categories:
Circle method
Core principle Draw through the paint film with a circular rolling line, and grade it according to the integrity of the paint film inside the scratch
The main applicable standard is GB/T 1720
The results represent a grade 1 (best) to a level of 7
Grid/cross method
Core principle Cut out a grid or cross cut mark on the coating with a knife and use tape to peel off the grade after grading
The main applicable standards are GB/T 9286 and GB/T 13217.7
The results represent a scale of 0 (best) to a level of 5
Pull away method
Core principle Use adhesive to glue the test column to the coating, pull it vertically until failure, and measure the required tensile force
The main applicable standard is GB/T 5210
The results showed the adhesion strength (MPa or N/cm²) and the form of failure (e.g., interface adhesion failure A, coating cohesion failure B)
Stripping method
Core principle Measures the force required to peel the coating from the substrate at an angle, such as 180° or 90°
Main applicable standard GB/T 13217.7 (ink)
Result Expression Peel Strength (N/cm)
Key points of interpretation of the results:The pull-apart method not only provides a specific strength value, but also diagnoses the problem by analyzing the form of failure, such as the adhesion failure of the coating to the substrate interface (A) or the cohesion failure of the coating itself (B). This provides a clearer direction for process improvement.
Key factors affecting adhesion
Adhesion is a systemic issue that is influenced by multiple factors.
Substrate state
The cleanliness of the substrate is crucial, and contaminants such as oil and moisture can significantly reduce adhesion. The surface roughness of the substrate affects both the mechanical occlusion and the effective contact area. In addition, the type of substrate (e.g., metal, plastic, paper) and its surface energy also determine the possible bonding mechanisms.
Coating materials and processes
The formulation of a coating or ink, especially the content and type of polar groups in it, directly affects the physical and chemical adsorption capacity. The thickness of the coating should be controlled within a reasonable range, and excessive thickness may lead to an increase in internal stress and affect adhesion. The degree of curing must be sufficient, and the cohesion strength and interfacial bonding of the coating that are not fully cured will decrease.
Construction and environment
Surface preparation before application (e.g., sanding, sandblasting, chemical treatment) is a critical step in improving adhesion. The environmental conditions of the entire construction process, such as temperature, humidity, and drying and curing procedures, must be strictly controlled according to the standards.
Different industry standards and application points
Industries will develop more detailed adhesion testing standards for specific products and substrates.
Ink industry - the national standard GB/T 13217.7-2023 specifies the determination method of ink adhesion on various substrates such as plastic film, aluminum foil, paper, etc., which can be disc peeling, manual peeling or grid method.
Metal coating industry – For engineering metal coatings, such as the new GB/T 45345-2025 standard, there are specific regulations for adhesion measurement of DC magnetron sputtering silver coatings.
Universal coating testing – the grid method (GB/T 9286) and the pull-open method (GB/T 5210) are the two most widely used general test standards and are suitable for most coating systems.
The choice of test method should consider the coating system (single or multi-layer), the characteristics of the substrate (rigid or flexible), and the purpose of the test (quality control or R&D analysis). For example, for printing inks on flexible plastic films, the peel method is more suitable than the rigid grid method.
Adhesion test common problems and solutions
In real-world testing, you may encounter the following typical problems:
The sharpness of the knife or scribe should be checked first, and a passivated blade is a common cause of inconsistent results. Secondly, it is necessary to ensure that the test speed, pressure, tape peeling angle and other parameters strictly comply with the standard regulations.
Different test methods have conflicting results, which may reveal different aspects of adhesion. For example, the grid method may cause the tape to not fit snugly because the surface of the substrate may not be flat enough. When cohesion failure (B) occurs in the pull-off method, it may indicate that the strength of the coating itself is insufficient, rather than the interface adhesion problem.
To sum up, adhesion is a comprehensive property involving materials, interfaces, processes and other aspects. A deep understanding of its mechanism of action, testing and evaluation in strict accordance with the corresponding standards, and systematically controlling every step from substrate treatment to final curing are key to ensuring excellent and stable adhesion performance of coatings and ink products.