One of the most important ability in nearly all permanent coatings systems is to adhere strongly onto the substrate after film formation has completed. Indeed, the adhesion of the coating film is a necessity to offer long-term protection and aesthetics. And thus, adhesion promoters are in many cases very helpful additives to achieve the perfect adhesion.
Explore more about this useful class of additives, find out main types of adhesion promoters available in the market and understand the test methods & selection criteria to choose the best material for your formulation.
The adhesion is the resistance of the coating to being removed from the substrate. The adhesion promoter for paint is an additive to promote adhesion of the film to the substrate.
It usually has an affinity for the substrate and the applied coating and form a permanent and strong bonding.
Without the adhesion promoter, the properties of the applied coating may not be sufficient to meet the final performance requirements.
The adhesion and adhesive strength is the combination of various phenomena between the paint material and the substrate, such as:
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)
How to Achieve Perfect Coating Adhesion?
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.
Nevertheless, a perfect adhesion depends not only on the choice of the adhesion promoter; there are several other parameters influencing adhesion, such as:
Paint formulation, as the substrate wetting is a side key factor
Curing and aging atmosphere
Surface type and properties
Indeed, even with the help of the best paint adhesion promoter, the adhesion itself may be disastrous if not all necessary conditions are present.
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 another side, all along its ageing, the film is submitted to various forces that may degrade it, Like:
Mechanical degradation: Impact, tensile and shear stress
Thermal degradation: Contraction and expansion of the film / substrate
A good adhesion may help the film to resist and keep its optical and functional properties.
Now let’s get introduced to the major types of adhesion promoter that are available in the market…
General Types of Adhesion Promoter for Paint & Coatings
Various paint adhesion promoters exist and each family has its own properties. Also, these classes can be further adapted to fit hte formulator’s requirements.
It is not possible to present only one adhesion promoter, as every combination between a liquid paint with its substrate applied under certain condition is a specific case. Let us learn about the major promoters in this section, namely:
Phosphates, silicones and others
- Organofunctional Silane Adhesion Promoters
Organofunctional silanes are used as paint adhesion promoter to improve the adhesion between the organic liquid coating and the substrate (inorganic material).
Organofunctional silanes have general formula: R-Si-X3, where:
Silicone (Si) is the center of the silane molecule
(R) is an organic functional group (ethoxy, methoxy, ethoxymethoxy…)
(X) is a second functional group (acrylate, amino, chloro, epoxy, mercapto…)
(R) will attach to the organic resin to achieve a covalent bond with the coating. (X) will attach to the substrate to achieve a bonding effect. Indeed, this inorganic group will hydrolyze to produce a silanol, which will form a siloxane bond with the substrate. The present humidity is usually enough to provoke this hydrolysis.
Shown below is the hydrolysis reaction of Organofunctional Silanes:
Hydrolysis: R-Si-X3 + 3 H2O → R-Si-(OH)3 + 3H-X
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 condition
Chemical resistance and the water resistance
Due to the high moisture reactivity, specific grades must be used as adhesion promoter for water based paint.
- Organometallic Compounds
Organometallic compounds based on titanium or zirconium, are used as paint adhesion promoters for many years now. Besides their action on the adhesion process, they can also act as catalyst and sometimes disturb the crosslinking in sensitive systems. Organometallic Compounds can work in synergy with organosilanes paint adhesion promoters.
Zirconates compounds are generally based on alkoxy or neoalkoxy zirconates or zirconium propionates
Titanates compounds are based on Monoalkoxy, neoalkoxy or coordinate titanates
Common functionalities are amino, carboxy, or methacryloxy
General formula: X-OR’-[M]-(OR)3 , where:
[M] is the metal, titan or Zirconium
OR is an alkoxy or neoalkoxy chain, hydrolyzable
OR’ is an hydrocarbon chain
X is a functional group
Comparable to the bonding action of Organofunctional silane, Titanates and Zirconates are highly reactive organometallic compounds. They form stable bonds with the –OH, -COOH, -CONH, or -NH2 reactive groups from the substrates. This reactivity makes them used as crosslinking agents. Also, it accelerates the curing time of some systems.
Chelates are lesser actives than esters. They are often used in the printing industry to avoid unwanted reaction with the binder.
Titanium acetylacetonate and Zirconium Propionate are recommended for the adhesion on difficult substrates, such as: In the field of packaging inks.
Neoalkoxy titanates and zirconates have an 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. And test failures often come from an overdosage. Besides the catalytic reactivity, an excessive dosage will bring many unreacted alkoxy groups in the system and as a consequence a loss of adhesion.
1.Regarding titanates compounds, in general they are:
More polar functional titanates (amino) are recommended for polar substrates
Non-polar functional titanates (aliphatic carboxy, phosphato) are recommended for non-polar susbtrates
2.Zircono-aluminates are patented technology. They are polyvalent and are suitable as adhesion promoter for water based paint, solventborne paints and rubber applications.
General Formula: 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 structure, maybe difficult to homogenize in the paint system
3.Due to their classified toxicity, Chromium based adhesion promoters, despite of their effectiveness, are no longer recommended.
Similar to this Organometallic Compounds family, patented metal organic structure offers similar results.
General formula: X-R-[M]-OH
3.Chlorinated Polyolefins Adhesion Promoter Mechanism
Chlorinated Polyolefins are obtained by chlorination the polyolefin. They are especially recommended to improve the adhesion of solvent-borne coatings to untreated polyolefin substrates (polyethylene, polypropylene, elastomer blends).
These are used mainly as adhesion primer at low film thickness (max 5µm) to improve the adhesion of printing inks on untreated or EPDM modified polypropylene and other plastic substrates.
They also improve the adhesion on metal such as aluminum or galvanized steel.
Pretreatment may remain necessary, when used on polyethylene.
When used as a coating additive, besides their efficiency on plastic substrates, the presence of basic pigment can dramatically affect their efficiency. Indeed, these pigments will accelerate the polyolefin degradation that may lead to adhesion issue, and can, under specific condition, release some hydrochloric acid.
4.Phosphates, Silicones and Others
Silanes, Titanates and Zirconates, and Polyolefins, represents the majors families of adhesion promoters used in coatings. Besides these adhesion promoters’ types, some others may achieve the required adhesion in specific fields.
Phosphates & Phosphorous Containing Compounds
Special phosphates with acidic functionality may improve the adhesion on metallic substrates. And as a positive side effect, enhance the corrosion resistance on metallic substrates through a passivation. Complex of carboxy phosphate ester may be efficient for various metal substrates in water based and solvent based coatings. They are traditionally used in radiation curing coatings. Organophosphorus enamines offer some positive results for glass substrates.
Silicones & Silicone-Modified Polymers
Silicones and PDMS are widely used in the coating industry as substrate wetting agent. These polymers are providing a good adhesion on various substrates due to the presence of reactive silanol groups. Then, the adhesion promotion mechanism will be similar as the one encounter using organosilane products.
As main functionality or because of their side effects, some other types may be used as adhesion promoters:
Amide and Imide, adhesion promoter for metal, plastic and wood.
Metal Soaps (naphtenates), adhesion promoters for bitumen and anti-fouling paints.
Platelet-Shaped Fillers like talc. In the system, forces of adhesion between the platelet-shaped particles and the substrate are developed, but the efficiency is strongly determined by the formulation itself.
Polyethylenimine, a branched, spherical, polymeric amine prepared from aziridine and generally improves the adhesion to porous surfaces.
Rosin Ester may act as adhesion promoter for metal, in flexographic printing and in the printing of foils and packaging.
Sucrose derivatives (SAIB) for wood coatings
Specific Polyesters may improve the adhesion on many metallic, mineral or plastic substrates. They are more specific polyester resins than classical additives.
Now since the major classes of paint adhesion promoters are clear, let’s understand how to select the right additive based on a specific application requirement…
Select 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
The following table may give the same characteristics of main types of adhesion promoters. But, the final choice must be done considering all side effects of each product type.
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
Hence, the test method must correspond to the final paint use and required characteristics.
Let’s take a look at vaious test methods for paint adhesion…
Methods to Test Paint Adhesion
When testing and evaluating a new adhesion promoter may 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 in cut into the film, and sometimes removed by a normalized adhesive tape. A specific cutting tool can be used for this test.
- 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.
- Knife Cutting Method
Film separation is obtained by means of 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, in fact, complicated, comprising both shear and tensile stresses, which finally cause disbanding of the film. But various other test methods also exist and can be used when necessary. There are so many different paint systems and application that it would be unrealistic to look for a unique adhesion test method, such as:
Conical mandrel bend test
Ultrasonic Pulse-Echo System
Best performances of coatings can only be achieved if the coatings layers have a sufficient adhesion on their substrate. Adhesion promoters are recommended wherever it is possible to improve the coating adhesion and enhance the final properties. For a given system, the best Adhesion Promoter is not only functioning of the paint system, but also of the substrate and final application. The large choice is available in the market offers numerous possibilities to you and thus, selecting the best adhesion promoter for paint at the right cost for a specific application is now possible.