Adhesion Promoters for Coatings: Chemistry, Types, Slection Tips, and More

Adhesion Promoters: Optimizing Coating Performance

The adhesion is the resistance of the coating to being removed from the substrate. The adhesion promoter for paint is an additive to promote the adhesion of the film to the substrate. It has an affinity for the substrate and the applied coating. It forms a permanent and strong bond. The applied coating may not meet the final performance requirements in the absence of adhesion promoters.

The adhesion and adhesive strength are the combination of various phenomena between the paint material and the substrate. This includes:

Mechanical adhesion
Physical mechanisms at a molecular level at the interface (dipole and electrical charges layer)
Thermodynamic mechanisms at the interface (interface energy)
Chemical mechanisms at the interface (chemical bonds)

Unlocking perfect adhesion: The role of promoters and parameters

An adhesion promoter enhances the film adhesion by its affinity with the substrate and the liquid coating. Wetting of the liquid coating to the substrate is a key factor.

A perfect adhesion depends on the choice of the adhesion promoter and several other parameters, like:

Paint formulation (as the substrate wetting is a side key factor)
Application conditions
Curing and aging atmosphere
Surface type and properties
Surface preparation

Key Parameters Influencing Perfect Adhesion
Even with the top adhesion promoters, adhesion can fail without all the necessary conditions.

Impact of coating adhesion on other parameters

A certain level of adhesion is required in coatings to achieve the desired quality. In the coatings field, the norm DIN 55945 defines the adhesive strength. The adhesion itself has a huge influence on many other parameters:

Durability: Weak adhesion will lead to accelerated degradation
Aesthetic: Weak adhesion, through accelerated degradation, can influence the optical properties
Corrosion resistance: A weak adhesion will strongly decrease the corrosion resistance

On the other hand, all along its aging, the film is subjected to various forces that may degrade it, such as:

Mechanical degradation: Impact, tensile, and shear stress
Thermal degradation: Contraction and expansion of the film/substrate
Chemical degradation

A good adhesion may help the film to resist and keep its optical and functional properties.

Not sure about what adhesion means in coatings? Get detailed knowledge on adhesion basics, factors behind bond failure, and things you must be aware of to meet desired adhesion.

Let's go deeper to learn about the major types of adhesion promoters that are available in the market. You will also learn to select the one best suited for your application needs.

Types of Adhesion Promoters

There are various paint adhesion promoters and each family has its own properties. Also, these classes can be further adapted to fit the formulator's requirements.

It is not possible to present only one adhesion promoter, as every combination between a liquid paint and its substrate applied under certain conditions is a specific case.

Organofunctional Silane Adhesion Promoters

Organofunctional silanes are used as paint adhesion promoters. They improve the adhesion between the organic liquid coating and the substrate (inorganic material).

Organofunctional silanes have the general formula: R-Si-X₃, where:

Silicone (Si) is the center of the silane molecule
(R) is an organic functional group (ethoxy, methoxy, ethoxymethoxy, etc.)
(X) is a second functional group (acrylate, amino, chloro, epoxy, mercapto, etc.)

(R) attaches to the organic resin to form a covalent bond with the coating. (X) attaches to the substrate to achieve a bonding effect. This inorganic group hydrolyzes to produce silanol, which forms a siloxane bond with the substrate. The present humidity is usually enough to provoke this hydrolysis.

Hydrolysis reaction of organofunctional silanes: R-Si-X₃ + 3 H₂O → R-Si-(OH)₃ + 3 H-X

Bonding reaction of organofunctional silanes:

Hydrolysis of Organofunctional Silane

As described, little water is required to achieve the bonding reaction properly. As a consequence, the use of organofunctional silane improves:

Adhesion (especially under humid conditions)
Chemical resistance
Water resistance
Corrosion resistance

Due to the high moisture reactivity, specific grades must be used as an adhesion promoter for water-based paint.

Silane functionalities for different resins

Depending on the coating resin, the silane functionality varies to offer the best adhesion possible. The table below enlists different types of silanes required by specific coating resins.

Resin Types	Silane Functionalities
Acrylic	Acrylic	Epoxy
Alkyd, Alkyd urethane	Vinylic	Methacrylic
Cellulosic	Amino	Isocyanate phosphate
Diallylphthalate, Phthalate	Amino	Styryl
Epoxy	Amino epoxy	Chloroalkyl mercapto
Imide	Chloromethyl	Amino
Melamine	Amino epoxy	Alkanolamine
Phenolic	Amino chloroalkyl	Epoxy mercapto
Polyester	Amino methacrylate	Styryl vinyl
Urethane, Polyurethane	Amino alkanolamine	Epoxy isocyanate

Different Silane Functionalities Required for Specific Coating Resins

Silane functionalities for various substrates

The table below illustrates the performance of silane functionalities for various substrate types.

Silane Functionalities	Substrate Types
Silane Functionalities	Aluminum	Copper	Glass	Steel
Amine	⭐	⭐	⭐	⭐
Alkyl	⭐	⭐	⭐	⭐
Epoxy	⭐	⭐	⭐	⭐
Methacrylic	⭐	⭐	-	⭐
Vinylic	⭐	⭐	-	⭐

Silane Functionalities for Various Substrate Types

Organometallic compounds

Organometallic compounds have been used as paint adhesion promoters for many years now. They are based on titanium or zirconium. They can also act as a catalysts and sometimes disturb the crosslinking in sensitive systems. Organometallic compounds can work in synergy with organosilane adhesion promoters. Common functionalities are amino, carboxy, or methacryloxy.

General formula: X-OR^’-[M]-(OR)₃ , where:

X is a functional group
OR’ is a hydrocarbon chain
[M] is metal, titan, or zirconium
OR is an alkoxy or neoalkoxy chain, hydrolyzable

Titanates and zirconates are highly reactive organometallic compounds. Their bonding action is comparable to that of organofunctional silane. They form stable bonds with the –OH, -COOH, -CONH, or -NH₂ reactive groups from the substrates. This reactivity makes them to be used as crosslinking agents. Also, it accelerates the curing time of some systems.

Hydrolysis of Organometallic Compound

Hydrolysis of Organometallic Adhesion Promoters
Chelates are lesser actives than esters. They are often used in the printing industry to avoid unwanted reactions with the binder.

Titanates

Titanate compounds are based on monoalkoxy, neoalkoxy, or coordinate titanates. Titanate compounds are of the following types:

More polar functional titanates (amino) — They are recommended for polar substrates
Non-polar functional titanates (aliphatic carboxy, phosphate) — They are recommended for non-polar substrates

Zirconates

Zirconate compounds are generally based on alkoxy, neoalkoxy zirconates, or zirconium propionates. Zircono-aluminates are patented technology. They are polyvalent and are suitable as adhesion promoters for water-based paints, solvent-borne paints, and rubber applications. Their general formula is R-[Al/Zr]-R’-X.

Zirconium aluminates interact with the metal surface via OH groups. The reactive groups of the zirconium aluminate then initiate the condensation reactions with OH groups in the resin.

(+) Very effective on various substrates
(-) Very high polar structures, may be difficult to homogenize in the paint system

Zirconium propionate and titanium acetylacetonate are recommended for adhesion on difficult substrates. These include packaging inks. Zirconates and neoalkoxy titanates have excellent thermal stability (up to 200°C). They are recommended for oven-curing systems.

These organometallic compounds must be used at the right dosage as they are very reactive. Test failures often come from an overdosage. Besides the catalytic reactivity, the excessive dosage will bring many unreacted alkoxy groups into the system. As a consequence, this leads to a loss of adhesion.

Chromium-based adhesion promoters

Chromium-based organometallic compounds are effective but are no longer recommended. This is due to their classified toxicity.

Pros and cons of organometallic compounds

Organometallic Compound Type	Pros	Cons
Chromium	Very effective on various substrate	Highly toxic Not recommended anymore
Titanate	Polyvalent	Dosage critical Color and odor may limit the use
Zirconate	Polyvalent More active than titanate Suitable for both water and solvent systems	Like titanate, may interfere with the crosslinking
Zircono Aluminate	Moisture stable More affordable than organosilanes Suitable for both water and solvent systems	Difficult to incorporate in the system due to high polarity

Advantages and Disadvantages of Organometallic Adhesion Promoters
Like organometallic compounds, the patented metal-organic structure offers similar results. Their general formula is X-R-[M]-OH

Chlorinated polyolefins (CPO) adhesion promoters

Chlorinated polyolefins are obtained by chlorination of the polyolefin. They improve the adhesion of solvent-borne coatings to untreated polyolefin substrates. For example, polyethylene, polypropylene, and elastomer blends.

They are used mainly as adhesion primers at low film thickness (max 5µm). This improves the adhesion of printing inks on untreated or EPDM-modified polypropylene and other plastic substrates.
They also improve the adhesion on metals such as aluminum or galvanized steel.
Pretreatment may remain necessary when used on polyethylene.

When used as a coating additive, the presence of basic pigment can affect their efficiency. These pigments can accelerate the polyolefin degradation which may lead to adhesion issues. Under specific conditions, they can release some hydrochloric acid.

Performance drivers of CPO-based adhesion promoters

Coating bake temperature

The coating bake temperature is the temperature at which the coating applied to the TPO part is cured. It can affect the interaction between a CPO-based adhesion promoter and the surface of TPO. This can in turn affect performance. Coating adhesion is enhanced when the coated TPO parts are baked at temperatures over 100°C, given the same coating type.

However, CPO-based adhesion promoters are used in several applications. For example, in automotive refinish applications. Here the coating is air-dried or baked at temperatures lower than 100°C.

Coating bake temperature can affect the gasoline-resistance properties of coated TPOs. The gasoline resistance of coated TPO parts improves with increasing bake temperature for the same coating type. However, each substrate/adhesion promoter/coating system is different. The chemical and physical properties of the CPO can also affect gasoline resistance performance.

Increasing bake temperature often results in higher peel strengths, given the same coating type.

Adhesion promoter composition

The chemical and physical properties of the CPO can also have a significant effect on adhesion performance. The addition of co-resins to CPOs has the following functions:

Enhance adhesion
Reduce blistering
Improve the appearance of coatings applied over the adhesion promoter layer.

CPOs have limited compatibility with most resin types. However, unlike conventional coatings, this may not be detrimental to performance.

CPOs promote the best adhesion when they are at the interface of the substrate and the coating applied over the substrate. This means that a formulated adhesion promoter system with a CPO and borderline compatible co-resin can allow the CPO to reach the interface more readily. Several co-resin types can be used with CPO, including:

Acrylic
Acrylic-modified alkyds
Polyesters, and
Others

The level of CPO used in the formulation is dependent on the substrate, coating type, and required performance properties. The chemical and physical properties of CPO and the adhesion promoter formulation composition can affect peel strength results. For formulated adhesion promoter systems, the right type and level of CPO must be used to get sufficient peel strength properties.

Phosphates, silicones, and others

Silanes, titanates, zirconates, and polyolefins represent the major families of adhesion promoters used in coatings. However, several other types can also achieve the required adhesion in specific fields.

Phosphates & phosphorous containing compounds

Phosphates with their acidic features can improve the adhesion on metallic substrates. Their role is to boost corrosion resistance on metallic substrates by means of passivation.

The complex of carboxy phosphate is efficient for various metal substrates. They work well in both water-based and solvent-based coatings. They are also used in radiation-curing coatings. Organophosphorus enamines show some positive results when used on glass substrates.

Silicones & silicone-modified polymers

Silicones and PDMS are used in the coating industry as substrate-wetting agents. These polymers provide good adhesion on various substrates. This is due to the presence of reactive silanol groups. Their adhesion promotion mechanism is like the one encountered using organosilane products.

Other adhesion promoters

As main functionality or because of their side effects, some other types may be used as adhesion promoters:

Amide and imide adhesion promoters — For metal, plastic, and wood.
Metal soap (naphthenates) adhesion promoters — For bitumen and anti-fouling paints.
Platelet-shaped fillers (talc) — In the system, forces of adhesion between the platelet-shaped particles and the substrate are developed. Their efficiency is determined by the formulation itself.
Polyethylenimine — It is a branched, spherical, and polymeric amine prepared from aziridine. It generally improves the adhesion to porous surfaces.
Rosin ester — It acts as an adhesion promoter for metal, in flexographic printing and the printing of foils and packaging.
Sucrose derivatives (SAIB) — For wood coatings.
Specific polyesters — They improve the adhesion on many metallic, mineral, or plastic substrates. They are more specific polyester resins than classical additives.

Now you have a clear picture of the major chemistries of paint adhesion promoters. So, let's check out the commercial grades in our database offered by top suppliers. You can also ask for technical assistance or request samples.

Selecting the Right Adhesion Promoter

Selection of the right paint additives for adhesion can be determined by:

Chemical nature and functionality of the resin system
Chemical nature and treatment of the substrate
Application conditions

The following table provides some characteristics of the main types of adhesion promoters. However, the final choice must be made considering all the side effects of each product type.

Promoter Type for Suitable Coating System	Substrate
Promoter Type for Suitable Coating System	Glass	Metal	Mineral	Plastic	Polyolefin	Wood
Silane (Solvent-based and water-based coatings, alkyd, epoxy, melamine, phenolic, polyester, polyurethane, rubber)	⭐	(Al, Cu, Fe, Zn)	⭐	⭐
Titanate & Zirconate (Printing Inks: NC, PVB, CAB, CAP, polyester, polyamide, OH/COOH/NH₂ reactive groups resins)	⭐	⭐			⭐
Zr/Al (Solvent-based and water-based coatings, high-solid polyester & epoxide systems, alkyd, rubber)		⭐
Chlorinated Polyolefins (Especially for plastic substrates)				⭐
Phosphates (Acrylate, alkyd, polyester)		⭐
Rosin Esters (Printing Inks: Flexo, foil, packaging)		⭐
Silicones (All organic coatings)	⭐	⭐	⭐
Special Polyesters (Solvent-based and water-based industrial coatings, printing inks, radiation curing, can, coil, floor and road marking)		⭐	⭐	⭐
Sucrose (All organic coatings)		⭐		⭐		⭐
Talc (All organic coatings)			⭐

Beside all these considerations, the choice of the best adhesion promoter will also depend on important factors, like:

Compatibility of the adhesion promoter
Side effects of the adhesion promoter
Dosage and type of adhesion promoter
End-use of paint
Price

Hence, the test method must correspond to the final paint use and required characteristics. Take a look at various test methods for paint adhesion.

Methods to Test Paint Adhesion

When testing and evaluating, a new adhesion promoter can be fastidious. It is possible to focus on critical points to reduce the time and material spent on the study keeping a high pertinence of results.

Cross-cut test: Gloss evaluation (ISO 2409)

It is an ISO test method where a pattern is cut into the film and sometimes removed by a normalized adhesive tape. A specific cutting tool can be used for this test.

Cross-cut Test for Adhesion Promoters

Tensile method: Pull-off test for adhesion (ISO 4624)

The pull-off method is the most widely used procedure. As a preparation for the test, a stud, normally made of steel, is glued with the coating. It is then subjected to axial tension until detachment of the paint film occurs. The result, i.e., the adhesion strength, is the maximum tensile stress that is possible at the interface.

Pull-off Test for Adhesion Promoters

Knife cutting method

Film separation is obtained using a sharp knife, pushed along the interface with an exactly measured force. Although this seems to be a simple test method, the process of detachment is complicated. It comprises both shear and tensile stresses, which finally cause disbanding of the film.

However, various other test methods also exist and can be used when necessary. There are so many different paint systems and applications that it would be unrealistic to look for a unique adhesion test method, such as:

Peel test
Blister test
Scratch test
Indentation debonding
Impact test
Conical mandrel bend test
Ultrasonic pulse-echo system

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