Adhesion in Paints and Coatings

What is Adhesion of Coatings?

Adhesion of a coating refers to the strong bond between the coating and substrate of a substance.

Adhesion is one of the essential properties in paints and coatings industry that ensures the coating (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 important requirement for most coatings and inks is that they form a strong bond and adhere strongly to the substrate for long after the curing and film formation* is complete. The bond strength between coating and substrate is dependent on the two properties of a material that include:

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

* Film formation plays a key role in adhesion. Upon curing, several factors, such as shrinkage and pressure impact film formation.

The key elements governing 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)

Facts to Know! 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).

Theories of Adhesion

Many theories explain the mechanism of adhesion. The main theories based on which adhesion occur are:

• Adsorption Theory – A force of attraction between surface molecules and adherend at their interfaces i.e. intermolecular forces.
• Chemisorption Theory – An extension of adsorption theory in which adhesion occurs when chemical bonds are formed across the interface.
• Mechanical Interlocking – Molecule interlocking around irregularities on the surfaces/substrate.
• Electrostatics (electronic) Theory – Electrons are transferred from one surface to the other to build up dissimilar charges, exerting a force of attraction.
• Diffusion Theory – The adhesion of polymeric materials to the inter-penetration of chains at the interface.

NOTE: No single theory explains adhesion in a general and comprehensive way. Some theories are applicable for certain substrates and applications while other theories differ based on different circumstances.

Adsorption Theory

According to the adsorption theory, the two materials adhere to each other due to attractive forces between the molecules of the materials. The surface forces developed are designated as secondary or van der Waals force. For these forces to develop, the paint molecules must make intimate molecular contact with the substrate surface. In addition, acid-base interactions and hydrogen bonds may also contribute to intrinsic adhesion forces.

To obtain good adsorption, it is important to establish continuous contact between the paint film and the adherend so that 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 Contact angle = 0°, or the material spreads uniformly over a substrate to form a thin layer. Wetting is favored when:

• The substrate’s surface tension, better known as the critical surface energy, C, is high
• The surface tension of the wetting liquid is low

Chemisorption Theory

The chemisorption theory is based on the chemical bonding mechanism explaining that the primary chemical bonds may form across the interface. The chemical bonds are strong contributions to the intrinsic adhesion significantly.

Adhesion promoters work according to chemisorption theory. Adhesion promoters are functionalized on one end to react with the substrate and on the other end to react with coating.

Mechanical Interlocking

According to the mechanical theory of adhesion, to function properly 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

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 as it:

• Gives “teeth” to the substrate (mechanical interlocking), and
• Increases the total effective area over which the forces of adhesion can develop.

Roughening is only effective if the coating wets the surface well.

Get the clues to improve the adhesion of your inks, lacquers, and others on metal substrates by better characterizing surfaces with a smart combination of AES, SIMS and XPS.

Electrostatics (electronic) Theory

The electrostatic theory states that the electrostatic forces are formed at the coating-adherend interface. 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.

Diffusion Theory

The fundamental concept of the diffusion theory proposes that adhesion occurs through the inter-diffusion of molecules in the coating and adherend. The diffusion theory is primarily applicable when both the coating and adherend are polymeric having compatible long-chain molecules that are capable of movement. Solvent cementing or heat welding of thermoplastics is a result of diffusion of molecules.

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.

Adhesion Failure in Coatings

With strong adhesion, the coating can prevent damage on the surface. One notable example is the use of coatings in corrosion protection.

However, after prolonged exposure to external factors, such as water, humidity and UV exposure, the coating adhesion can be affected adversely. 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:

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)

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 the weak-boundary-layer theory, a cohesive rupture of a weak boundary layer is the major cause of the 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.

Weak boundary layers can occur on the paint film or adherend if an impurity concentrates near the bonding surface and forms a weak attachment to the substrate.

In addition to external contamination, other major factors responsible for weak boundary layers are:

• Corrosion or oxide layers on metal surfaces
• Low molecular weight constituents, such as release agents and plasticizers on polymeric surfaces

How to Remove Weak Boundary Layers

Weak boundary layers must be removed by physical or chemical means so that there is no weak link in the film formation that would contribute to premature coating adhesion failure.

Complete, spontaneous wetting occurs when Contact angle = 0°, or the material spreads uniformly over a substrate to form a thin layer. Wetting is favored when the substrate’s surface tension, better known as the critical surface energy, C, is high and the surface tension of the wetting liquid is low.

Factors Influencing Adhesion

There are several factors that lead to adhesion failure between paint film/coating and substrate as listed below.

• Poor substrate cleaning when soils that are not completely removed from the substrate prevent the coating from bonding to the substrate surface.
• Surface profile, such as smooth surfaces that do not hold coatings well.
• Inappropriate wetting of coating on the substrate affecting surface bond and proper adhesion.
• Insufficient crosslinking or uncured coating/ over-curing of coatings leading to poor adhesion of the coating to the substrate surface.
• External environmental factors such as water, humidity, UV exposure, etc.

These factors can lead to several surface defects and hence, adhesion failure. Some common surface defects are:

• 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 within or under the coating, rapid drying of coatings or chemical exposure.
• Peeling – Reduction in bond strength of the paint film due to contamination or incompatibility of coats.
• Flaking – Causes the paint to become separated from the substrate.
• Undercutting – Involves the corrosion buildup under coating.

Methods to Achieve Good Adhesion

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 which in turn can improve or worsen coating adhesion as listed here.

Additive	Function
Adhesion Promoters	Help to improve adhesion Have an affinity for the substrate and applied coating to form a permanent and strong bonding
Plasticizers	Soften binders used in ink 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	Volatile component which reduces the viscosity of a binder Increase drying time of coating to minimize the formation of blisters and avoid adhesion failure
Pigments and Extenders	Impart physical properties, such as 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

Test Methods to Evaluate Adhesion

Several methods can be used to determine how well a coating is bonded to the substrate – allowing the coating to perform well. 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).

#1. Cross-Cut Test

ASTM D3359 – Standard Test Methods for Rating Adhesion by Tape Test

These test methods cover procedures to assess the adhesion of coating films to metallic substrates 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.

#2. Scrape Test

ASTM D2197 – Standard Test Method for Adhesion of Organic Coatings by Scrape Adhesion

This test method determines the adhesion of organic coatings, such as paint, varnish and lacquer 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 that is loaded in increasing amounts until the coating is removed from the substrate surface.

#3. Pull-Off Test

ASTM D4541 – Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers

This test method covers a procedure to evaluate the pull-off strength (commonly referred to as adhesion) of a coating 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.

Select the Right Adhesion Promoter

Are you looking for the right adhesion promoter for your formulation? Get an in-depth knowledge about adhesion promoters, its types, test methods and selection criteria to choose the best additives in order to achieve the perfect adhesion.