What is an adhesion promoter?
With the development of modern coating industry, the types of substrates are becoming more and more diversified, and various new substrates are being applied. Consequently, adhesion problems caused by weak affinity between the coating film and the substrate are also increasing, especially for some low-polarity, low-surface tension, or high-crystallinity substrates, such as polyolefins, polyamides, etc. Conventional methods to improve adhesion mainly include surface treatment of the substrate or adding adhesion promoters.
Adhesion promoters are those additives that, when added to coatings, can greatly enhance the bonding force of the coatings to the substrate. Due to the wide variety of coating resins and substrates, there are various types of adhesion promoters in actual application systems.
Mechanism of action
From the perspective of the mechanism of action, all adhesion promoters work by forming a bridging structure between the resin and the substrate to achieve the purpose of improving adhesion, as shown in the figure. The reason why a compound can be used as an adhesion promoter in a certain system is that in the corresponding system, the compound can spontaneously migrate to the surface where the coating film and the substrate bond, and then simultaneously generate strong interactions with both the coating film and the substrate at the interface, thus playing a bridging role between the coating film and the substrate.
From the perspective of physical chemistry, the interaction forces between the adhesion promoter and the coating film or the substrate can be either chemical bond anchoring forces or van der Waals forces formed by the mutual entanglement of polymer chain segments. Which specific method the adhesion promoter uses to interact with the coating film or substrate mainly depends on the chemical composition of the coating resin and the substrate.
According to the different anchoring methods of adhesion promoters with coating resins or substrates, adhesion promoters can be summarized into the following four modes of action:
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Chemical anchoring with the coating film and chemical anchoring with the substrate.
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Chemical anchoring with the coating film and physical entanglement with the substrate.
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Physical entanglement with the coating film and chemical anchoring with the substrate.
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Physical entanglement with the coating film and physical entanglement with the substrate.
Common types of adhesion promoters
Silane coupling agents are mainly used for adhesion to substrates such as glass and ceramics. Their mechanism of action is shown in the figure, mainly through the hydrolysis of the silane group to produce hydroxyl groups, and the corresponding silanol groups undergo condensation reactions with residual hydroxyl groups on the substrate, thereby achieving chemical anchoring of the coupling agent molecule with the substrate.
At the same time, the organic groups carried by the silane coupling agent will react with the corresponding coating system, ultimately achieving the effect of improving the adhesion between the coating film and the substrate. With different functional groups of the silane coupling agent, they are suitable for different resin systems.
For example, those with epoxy groups (DCA-7130) are suitable for epoxy and polyurethane systems; those with amino groups are suitable for epoxy, polyurethane, and amino baking paint systems; those with acrylate groups are suitable for photocuring systems.
Polyester adhesion promoters are mainly used for adhesion to metal substrates. The carboxyl groups in the polyester can form hydrogen bonds with metal substrates, resulting in a good anchoring effect. In addition, according to the different resin systems applied, there are many varieties of polyester adhesion promoters. For example, for amino baking paint systems, the polyester needs to have functional groups such as hydroxyl groups that can participate in the crosslinking reaction of amino baking paint. DCA-7200 and DCA-7205N belong to this category, which have both high carboxyl and hydroxyl content and can be used as adhesion promoters in baking paint systems. If it is used for self-drying resin systems, the polyester resin needs to have a longer molecular chain segment for entanglement with the coating resin. DCA-7266 belongs to this category, and the latest hyperbranched polyester compounds often have unique applications in additives due to their unique hyperbranched multi-terminal structure.
Chlorinated polyolefin adhesion promoters are mainly used for polyolefin substrates. In actual use, the polyolefin chain segments in chlorinated polyolefins can entangle with the substrate’s polyolefin due to the principle of similar compatibility, thereby achieving the effect of anchoring with the substrate. On the other hand, the chlorinated polyolefin chain segment can also entangle with the coating resin system, ultimately playing a bridging role between the coating film and the substrate. Some special polyolefins can introduce some special functional groups such as maleic anhydride (e.g., DCA-7400) on their chain segments. In actual use, maleic anhydride can chemically react with hydroxyl groups in the resin, thereby achieving a chemical anchoring effect and greatly improving adhesion.
