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Coatings Ingredients
The material selection platform
Coatings Ingredients
Article

Coating Adhesion & Additives Used in Formulation

Jochum Beetsma & Jochum Beetsma – Feb 10, 2016

Adhesion is the single most important property of both decorative & protective coatings. Among factors to take into account, additives used in coating formulations can either have a positive or negative impact on the overall adhesion behavior.


Additives that influence adhesionOften coatings formulators forget about the effect additive can have on adhesion. 



In cases of poor adhesion, an otherwise excellent formulation 

doesn't deliver the expected results.



In this article, we’ll highlight additives that positively or negatively impact adhesion, so you think about adhesion earlier in your formulation creation.  Indeed, as a system developer, you should know as much as possible about the additives you use. 


You should be able to predict how each additive will behave in the system, especially during film formation. This would avoid many adhesion problems.






Coatings Adhesion: An Introduction


Adhesion of a coating is defined as the resistance of the coating to be removed from the substrate. The single most important requirement for most coatings and inks is that the system adheres strongly, and for a long time, to the substrate after film formation has completed.  Adhesion is related to the substrate-coating interface; the composition as well as film formation of the system must be such that this interface becomes as strong as possible.


Adhesion of a coating on a substrate

Adhesion of a coating on a substrate

A variety of influences can improve or worsen adhesion [1]. The focus of this article is on the influence additives can have on adhesion. Additives are known that improve adhesion, so-called adhesion promoters. On the other hand, additives that have a specific chemical structure can worsen adhesion.


Additives that Ruin Hydrogen Bonding


Often hydrogen bonding is one of the concepts used to assure adhesion of a primer onto a metal substrate. Specific groups, that are present in the binder system of the primer, can form hydrogen bonds with the hydroxyl (-OH) groups that are present on the surface of clean metal.

Adhesion of primer on metal via hydrogen bonding

Adhesion of primer on metal via hydrogen bonding

In the schematic drawing, the red circles represent the hydroxyl groups on the surface of the metal and the blue blocks represent the groups in the binder system that are able to form hydrogen bonds with hydroxyl groups. The hydrogen bonding groups in the binder system can, for example, be amine (=N-H) groups or carboxylic acid (-COOH) groups. An important aspect of a coating that gives strong adhesion is that the hydrogen bonding groups of the binder system are all connected: they form an adhesive 'sheet' that has both strong and multiple interactions with the metal substrate as well as with the complete coating.

Adhesion of a metal primer can be weakened when the primer contains an additive that is able to form strong hydrogen bonds with the hydroxyl groups of the metal. This is especially true when this additive is not connected to the rest of the primer. An example of such an additive is p-dodecylbenzenesulfonic acid (DBSA), an acid catalyst used to catalye the 2-component crosslink reaction of binder and crosslinker of the primer. More specifically, DBSA is used to catalyse the cure of melamine-based systems during film formation [2].

DBSA: p-dodecylbenzenesulfonic acid

DBSA: p-dodecylbenzenesulfonic acid

By looking at the chemical structure of DBSA, it can be predicted how these molecules will behave after a liquid paint, containing DBSA, is applied on clean metal. The sulfonic acid head is able to form a strong hydrogen bond with a hydroxyl group on the surface of the metal. Therefore, the DBSA molecules adsorb and orient at the metal-primer interface during film formation.

Adsorption and orientation of DBSA molecules at the metal-primer interface

Adsorption and orientation of DBSA molecules at the metal-primer interface.


A few important aspects draw our attention. First, the DBSA molecules hydrogen bond with the hydroxyl groups on the surface of the metal. By doing so, they prevent groups of the binder system to form hydrogen bonds with the substrate. Secondly, the hydrophobic dodecylbenzene tails might dissolve in the binder matrix of the primer. However, they do not participate in the crosslink reaction of the binder system during cure, simply because the tails do not contain reactive groups. The hydrophobic tails form no strong link with the binder system of the primer. The conclusion is that the real adhesion of the system is excellent because of the strong hydrogen bonds between the surface of the metal and the sulfonic acid groups. The apparent adhesion is bad because the bulk of the primer and metal are easily separated from each other. However, the system fails just above the real interface that is formed by the hydroxyl groups of the metal and the sulfonic acid groups of the catalyst.

It is said that a weak boundary layer is present within the primer, just above the interface. This might seem academic because the overall result is that the total system fails. However, it is crucial to know that it is not the real adhesion that is bad, but that the system fails within the primer, just above the interface. It is said that the system fails because of cohesive failure, within the primer itself, and not because of adhesive failure, at the interface. So, the solution to the apparent adhesion problem is to substitute this additive by an acid catalyst that does not have a surfactant structure. The system developer could, for example, decide to use a catalyst that forms covalent bonds with the binder system of the primer during film formation. Another option is to cure the system at elevated temperature so that no catalyst is needed at all.


Adhesion Promotion


Additives that Ruin Intercoat Adhesion


Intercoat adhesion is the adhesion between 2 coating layers. Most relevant in this respect is the intercoat adhesion of primer and topcoat in a 2-layer coating system.

Adhesion and intercoat adhesion of a 2-layer system

Adhesion and intercoat adhesion of a 2-layer system


Wetting agents
are additives that are, often by habit, used in coatings to improve wetting of the liquid system on a solid substrate. Wetting agents contain molecules that adsorb and orient at the liquid-air interface as soon as application has stopped and film formation begins. Wetting agents show this behavior because the molecules consist of a hydrophilic part and a hydrophobic part.


Behavior of wetting agent in a liquid primer during film formation

Behavior of wetting agent in a liquid primer during film formation

The result of adsorption and orientation is that the surface of the primer becomes hydrophobic because of the hydrophobic tails pointing towards the air. It is said that the primer surface has a low surface energy and this is bad for both wetting of the following paint layer as well for intercoat adhesion between the two coating layers. The thin layer of wetting agent molecules can be removed from the surface via treatment, like sanding. However, this involves an extra handling step. It is key for system developers to know whether or not molecules are present in the primer that will adsorb and orient at the liquid-air interface during film formation. It turns out that in many cases wetting agents can be completely removed from a primer formulation.

Poor intercoat adhesion caused by the presence of wetting agent in the primer

Poor intercoat adhesion caused by the presence of wetting agent in the primer

The lesson to learn from the example of poor adhesion of metal primer because of the presence of catalyst additive and the example of bad intercoat adhesion caused by the presence of wetting agent in a primer is that system developers should be alert on possible presence of additives that have a surfactant structure. These are additives that consist of 2 parts: a hydrophobic part and a hydrophilic part. The developer should try to predict whether or not these additives will adsorb at an interface and, if yes, at what interface. Then, the question should be answered: What will the additive do to (intercoat) adhesion of my system?

Check out this Video Tutorial to know whether you need Wetting Agents in your Formulation!


Additives that Improve Adhesion


Silane Adhesion Promoters


One of the concepts used to obtain excellent adhesion is to assure that covalent bonds are formed between the substrate and the coating. This concept is called coupling or chemisorption. Silanes can be used as additives to couple substrates that contain hydroxyl (-OH) groups, like metals, and coatings.

Coupling of substrate and binder via silane chemistry

Coupling of substrate and binder via silane chemistry

Apart from an alkoxy (-OR) group, that forms a covalent bond with the metal surface, the silane additive contains a chemical group that can crosslink with the binder system of the coating during film formation.

Latest Coupling Agents Generation: Oligomeric Silanes


The first generation coupling agents, that are still used to improve adhesion, are mono-functional. This means that the additives contain one reactive group that forms a chemical bond with the substrate and one chemical group that co-crosslinks with the binder system. These mono-functional silane adhesion promoters are more and more substituted by oligomers. Momentive Performance Materials is an innovative company leading in adhesion promoters based on silane chemistry. One of their new products is CoatOSil MP 200. This epoxy-functional silane oligomer can be used as adhesion promoter in, amongst others, 2-component metal primers that are based on epoxy-amine binder systems [3].

Indicative structure of CoatOSil MP 200

Indicative structure of CoatOSil MP 200

The oligomer molecules have a moderately high molecular weight and multiple functionality in both types of reactive groups:

  1. Methoxy (-O-CH3) groups, directly linked to silicon (Si) atoms, that form a covalent bond with the hydroxyl (-OH) groups of the metal substrate.

  2. Epoxy groups that co-crosslink with the binder system during film formation.

The core of the additive is formed by a strong siloxane network that has excellent resistance against, for example, hydrolysis, chemicals and UV radiation. Despite the increased molecular weight, compared to mono-functional silanes, the oligomer has a low viscosity and a broad compatibility. Adhesion Promotion



References

  1. Online Course Solving Adhesion Issues of Coatings & Inks
  2. Book Organic Coatings: Science & Technology, 3rd edition, 2007, Zeno W. Wicks et al.
  3. Technical Data Sheet / Marketing Bulletin CoatOSil MP 200, Momentive Performance Materials Inc., March 2011.

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