Adhesion in Paints and Coatings

1. What is adhesion of coatings?
2. What are the different theories of adhesion?
3. How does adhesion failure occur in coatings?
4. What factors promote the adhesion of coatings?
5. Which test methods are used to evaluate adhesion?

What is adhesion of coatings?

Adhesion of a coating refers to the strong bond between the coating and substrate of a substance. Most coatings and inks have the need to form a strong bond and adhere to the substrate for long after the curing and film formation is complete.

Film formation plays a key role in adhesion. Several factors, such as shrinkage and pressure, impact film formation upon curing. The bond strength between coating and substrate depends on the following properties of a material:

• Adhesion – The bonding strength of an adhesive to the substrate surface.
• Cohesion – The strength of a bond between adhesive particles.

Adhesion of a Coating on a Substrate
Adhesion is one of the essential properties in the paints and coatings industry. It ensures the layer (or paint film) remains adhered to the surface for long, especially under aggressive conditions. The nature of adhesion has a direct relation with the durability and quality of a coating. The key elements governing the adhesion of coatings and inks to the substrate include:

• Substrate-coating interface
• Composition of coatings and inks
• Film formation (must be such that the interface becomes as strong as possible)

Note: The binder or resin in paint formulation is the non-volatile film-forming component. It ensures adhesion to the substrate and cohesion within the paint film. The type of binder influences film formation, film strength, and other properties (physical or chemical).

What are the different theories of adhesion?

There is no single theory to explain adhesion in a comprehensive way. Some theories are applicable to certain substrates. Whereas, other theories differ based on different circumstances. Some of the theories based on which adhesion occur are explained below.

Adsorption theory: The role of adhesion forces and wetting

According to this theory, the two materials adhere to each other due to attractive forces. These forces exist between surface molecules and adherend at their interfaces. These are intermolecular forces. The surface forces developed are designated as secondary or van der Waals forces. For these forces to develop, the paint molecules must make intimate molecular contact with the substrate surface.

Acid-base interactions and hydrogen bonds may also contribute to intrinsic adhesion forces. To get good adsorption, it is important to establish continuous contact between the paint film and the adherend. As a result, Van der Waals force or the acid-base interaction or both take place. This can be achieved by a phenomenon known as wetting. Complete, spontaneous wetting occurs when:

• the contact angle is 0°, or
• the material spreads uniformly over a substrate to form a thin layer.

Wetting is favored when the substrate's surface tension is high and the surface tension of the wetting liquid is low. The surface tension of the substrate is also known as the critical surface energy. It is denoted by the letter 'C'.


Adsorption Theory: The Role of Van der Waals Forces

Chemisorption theory: Understanding adhesion through chemical bonding mechanisms

Chemisorption theory is an extension of the adsorption theory. Here, adhesion occurs when chemical bonds are formed across the interface. This theory is based on the chemical bonding mechanism. It explains that the primary chemical bonds may form across the interface. The chemical bonds are strong contributions to intrinsic adhesion.

Chemisorption Theory: Adhesion by Means of Chemical Bonding
Adhesion promoters work according to chemisorption theory. They are functionalized on one end to react with the substrate. While on the other end, they react with the coating. Are you looking for the right adhesion promoter for your formulation? Get in-depth knowledge on how to select adhesion promoters in order to achieve the perfect adhesion.

Mechanical theory: Enhancing bonding through surface interlocking

In the mechanical theory of adhesion, the interlocking of molecules around irregularities on the substrate occurs. Thus, to function or to achieve optimum adhesion, the paint film must:

• penetrate cavities on the surface
• displace trapped air at the interface
• lock-on mechanically to the substrate

Mechanical Interlocking: Enhancing Bonding by Surface Interlocking
One way that surface roughness aids in adhesion is by increasing the total contact area between the paint and the adherend. Thus, the mechanical theory generally teaches that the roughening of surfaces is beneficial. This is because it gives "teeth" to the substrate (mechanical interlocking). It also increases the total effective area. Over this area, the forces of adhesion can develop. Roughening is only effective if the coating wets the surface well.

Electrostatic theory: Exploring charge-based interactions at coating-adherend interfaces

This theory states that the electrostatic forces are formed at the coating-adherend interface. In this theory, the electrons are transferred from one surface to the other. This allows the building up of dissimilar charges. This in turn leads to exerting a force of attraction. The electrostatic forces account for resistance to separation. The theory found that electrical discharges take place when a coating is peeled from a substrate. Electrostatic adhesion is regarded as a dominant factor in biological cell adhesion and particle adhesion.

Electrostatic Theory: Forming of Electrostatic Forces at Coating-adherend Interface

Diffusion theory: Exploring molecular interactions in polymeric materials

This theory proposes that adhesion occurs through the inter-diffusion of molecules in the coating and adherend. The diffusion theory is applicable when both the coating and adherend are polymeric. They must have compatible long-chain molecules. These must be capable of movement. Solvent cementing or heat welding of thermoplastics is a result of the diffusion of molecules.

Diffusion Theory: Inter-diffusion in Polymeric Materials
Note: The electrostatic and diffusion theories of adhesion are generally not regarded as highly as the other theories in general bonding practice. However, there are certain applications where these are very important and help explain why bonds form.

How does adhesion failure occur in coatings?

With strong adhesion, the coating can prevent damage on the surface. One notable example is the use of coatings in corrosion protection. But the coating adhesion can be affected adversely after prolonged exposure to external factors. These include:

• water,
• humidity, and
• UV light.

Also, an improper selection of coatings for a surface, incompatibility of coating with surface and inadequate surface preparation can lead to adhesion failures in coatings. The main types of adhesion bonds failure between coatings and substrate are as follows:

Adhesive Failure	Cohesive Failure
Adhesive (which is the failure at the interface).	Cohesive (which is the failure within the adhesive).
Interfacial [which is the failure because of something specific (e.g. hydrolysis) at the interface].	Substrate (which is the failure within the substrate).
Near-interface (which suggests that the adherend and adhesive are affecting each other locally).	Dissipative [which is failure after (large) absorption of energy within the adhesive system].

Adhesion Science: Principles & Practice by Steven Abbott

Note: While adhesion loss between coating and substrate is possible, cohesive damage within a weak boundary layer is also common.

Weak boundary layer theory

According to this theory, a cohesive rupture of a weak boundary layer is the major cause of bond failure at the interface. In most cases, coating defects result from a cohesive failure of a weak boundary layer. Weak boundary layers can originate from the paint, adherend, environment, or a combination of any of the three. The major factors responsible for the formation of weak boundary layers are:

• Formation of corrosion or oxide layers on metal surfaces.
• Presence of low molecular weight constituents, such as release agents and plasticizers on polymeric surfaces.
• The presence of impurity concentrates near the bonding surface. This can lead to a weak attachment to the substrate.

Weak boundary layers must be removed by physical or chemical means. This leads to no weak link in the film formation. This would otherwise contribute to premature coating adhesion failure.

Factors influencing adhesion failure

• Poor substrate cleaning can lead to adhesion failure. It prevents the bonding of the coating onto the substrate surface. This happens if soils are not completely removed from the substrate.
• Surface profiles, such as smooth surfaces do not hold coatings well.
• Inappropriate wetting of coating on the substrate affects surface bond and proper adhesion.
• Several aspects of curing can lead to poor adhesion of the coating to the substrate surface. These include insufficient crosslinking, uncured coating, or over-curing of coatings.
• External environmental factors such as water, humidity, UV exposure, etc.

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Common surface defects

• Blistering – Occurs when a coated object is immersed in water. Blisters are dome-shaped defects that appear on the surface. Blistering is caused by water-soluble materials. These are formed within or under the coating. They can also form due to rapid drying of coatings or chemical exposure.
• Peeling – This occurs due to a reduction in the bond strength of the paint film. Contamination or incompatibility of coats can also lead to peeling.
• Flaking – It causes the paint to become separated from the substrate.
• Undercutting – It involves the corrosion buildup under the coating.

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What factors promote the adhesion of coatings?

The following factors have predominant importance to improve adhesion:

• Wetting of the surface
• Surface treatment
• Structure of the materials to be bonded

Also, a variety of additives impact physical/mechanical properties. This in turn can improve coating adhesion as listed here.

Additive	Function
Adhesion promoters	They help to improve adhesion. They have an affinity for the substrate and applied coating to form a permanent and strong bonding.
Plasticizers	They soften binders used in inks and coatings and improve their flexibility. The increased flexibility enhances the coating’s resistance to the mechanical impact force during coating process thereby minimizing any potential failure.
Solvents	They are the volatile components which reduces the viscosity of a binder. They increase drying time of coating to minimize the formation of blisters and avoid adhesion failure.
Pigments and extenders	They impart physical properties, such as film hardness and water resistance.
Wetting agents	Surfactants based on polysiloxanes, fluoro-based surfactants and special wetting agents overcome local differences in surface tension or wet the substrate surface to improve adhesion.

Role of Additives in Coatings Adhesion
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Which test methods are used to evaluate adhesion?

While implementing any test method, it is important to take into consideration if the bond failure is:

• adhesive (failure at the coating/substrate interface) or
• cohesive (failure within the coating film or the substrate).

There are several methods that can be used to determine how well a coating is bonded to the substrate. This allows the coating to perform well. The primary methods used to test paint adhesion are explained below.

1. Cross-cut test by tape test method

ASTM D3359 — Standard test methods for rating adhesion by tape test

This test method covers procedures to assess the adhesion of coating films to metallic substrates. This is done by applying and removing pressure-sensitive tape over cuts made in the film. This test method is similar in content (but not technically equivalent) to ISO 2409.


Tape Adhesion ASTM D3359 (Source: Spektrochem)

2. Scrape test by scrape adhesion

ASTM D2197 — Standard test method for adhesion of organic coatings by scrape adhesion

This test method determines the adhesion of paints, varnishes, and lacquers when applied to smooth and flat (planar) panel surfaces. The materials under test are applied to a smooth substrate at a uniform thickness. After drying, the adhesion is determined by pushing the panels beneath a rounded stylus or loop. This is loaded in increasing amounts until the coating is removed from the substrate surface.

3. Pull-off test using portable adhesion testers

ASTM D4541 — Standard test method for pull-off strength of coatings using portable adhesion testers

This test method determines the pull-off strength of coatings using portable adhesion testers. The pull-off strength is also referred to as adhesion. The pull-off strength of a coating is evaluated on rigid substrates, such as metal, concrete, or wood.

The method assesses the adhesion of either one or multiple coatings on a smooth surface by applying tensile stress (rather than shear stress measured in the previous two tests) from a dolly to the surface. The load is slowly increased until the dolly along with the adhesive layers is removed. It is equivalent to ISO 4624 standard.


Pull-off adhesion ASTM D4541 (Source: Spektrochem)

Gain Insights on the Adhesion Phenomenon in Coatings

Here is a selection of resources to learn about the adhesion: