When it comes to adhesives and coatings, achieving a strong bond between different materials is the key to a successful product. This is where adhesion promoters and primers come into play. These essential components enhance adhesion between coatings or adhesives and various substrates, creating a reliable interface that significantly improves performance. In this blog, we will be discussing what adhesion promoters are, how they work, and their applications in various industries.
What is an adhesion promoter?
Adhesion Promoters and Primers help adhesives and coatings adhere better to substrates. They are specialized chemicals developed to strengthen the bond between joined surfaces, such as substrates and coatings or adhesives. Coatings are often based on organic polymers, while substrates are typically inorganic materials. Since these often have vastly different properties, such as compatibility, chemical reactivity, and surface characteristics, forming a robust adhesive bond can be challenging. Adhesion promoters bridge this gap by creating an interface for additional covalent bonds. This yields a stronger, more cohesive bond, improving the overall performance of the adhesive or coating.
The Chemistry Behind Adhesion Promoters
So, how exactly do adhesion promoters create an interface for additional covalent bonds? Different adhesion promoters have different base chemistries but all of them work on a molecular level. They typically consist of an inorganic central atom, commonly metals, such as silicon, zirconium, or titanium, which imparts inorganic reactivity. Silicon is the most popular central atom due to its ability to create strong chemical bonds with both organic and inorganic materials. These central atoms are bonded with organic functional groups, which in turn provide organic reactivity. This unique structure allows adhesion promoters to interact effectively at the organic–inorganic interface, facilitating the formation of stable chemical bonds between two materials that would otherwise be incompatible.
Self-Assembled Monolayers of Phosphonates (SAMPs) are a type of surface coating technology that involves the formation of a single, organized layer of phosphonate molecules on a substrate. These phosphonates contain a reactive functional group, such as a phosphonic acid group, that can chemically bond to various surfaces, creating a covalent attachment. The other end of the phosphonate molecule typically contains a hydrocarbon chain, which forms a functional mono-layer, providing unique properties like improved adhesion, chemical resistance, or surface energy modification.
In summary, the key structural features of SAMPs include:
- The phosphonic acid group reacts with the surface, creating a covalent bond at the substrate-phosphonic acid interface.
- The hydrocarbon group attached to the phosphonic acid forms the functional monolayer.
- The monolayer is less than 5 nanometers thick.
The Dual Functionality of Adhesion Promoters
To put it simply, adhesion promoters have a dual structure that creates a chemical bridge between the substrate and the coating/adhesive, which improves the adhesion. This chemical bridge can be described as a “hook” and a “tail”, a combination of both organic and inorganic components:
- Hook (Inorganic Part): The “hook” provides strong chemical bonds that improve adhesion to the substrate. In the case of phosphonates used in SAMPs, this “hook” is the phosphonic acid group. This group forms strong covalent bonds with inorganic surfaces, like metals or glass, ensuring a stable attachment.
- Tail (Organic Part): The tail provides surface functionality and links to the functional group of the coating or adhesive. The tail usually consists of a hydrocarbon chain that forms a bond with organic materials, such as polymers in coatings or adhesives.
Types of Adhesion Promoters
The diversity of chemical compositions of adhesion promoters allows for tailored solutions that optimize bonding performance across different materials and applications. Bases for adhesion promoters include hydroxyl, acrylate, and thiols. These bases are what make up the “tail” section of the molecule, and deliver the surface functionality. For example, if you are working with a phenolic-epoxy based coating, this functional group would require a hydroxyl based adhesion promoter. Choosing let’s say a thiol based promoter would not increase adhesion between the two materials, since the “tail” (thiol based) is not compatible to the functional group (phenolic-epoxy based).
- Hydroxyl-based adhesion promoters improves adhesion between substrates and coatings that cross-link with polyurethanes, isocyanates, melamine formaldehydes, and phenolic epoxy-based coatings.
- Acrylate-based adhesion promoters promotes adhesion between substrates and coatings that cross-link with (meth)acrylates, amines, and thiol-based coatings.
- Thiol-based adhesion promoters increases adhesion between substrates and coatings that cross-link with epoxy (meth)acrylics.
| Property | Thiols | Acrylates | Hydroxyls |
| Adhesion Examples | Epoxy (Meth)acrylics | (Meth)acrylatesamines | Polyurethanes and iso-cyanates, melamine formaldehydes, phenolic epoxy based |
| Application | Spray / Dip / Wipe | Spray / Dip / Wipe | Spray / Dip / Wipe |
| Visible to Naked Eye | No | No | No |
| Thickness (nm) | 2+ | 2+ | 2+ |
| Dry Time (sec) | <20 | <30 | <30 |
| Cure Required | For <5 min cure, only Ti, Al, and epoxy need a cure | ||
Applications of Adhesion Promoters in Different Industries
Adhesion promoters and primers can be found in many industries, including aerospace, semiconductor, paints & coatings, and adhesives.
In the semiconductor industry, adhesion promoters can improve the bond between epoxy molding compounds and different materials, such as lead frames, substrates, dies, and die paddles. In particular, adhesion promoters are ideal for pre-treating metal lead frames, improving the bond strength of adhesives and molding compounds to gold, palladium, copper, silver, and nickel lead frames. For EMCs, hydroxyl functional adhesion promoters can be used, especially if the EMC is based on phenolic epoxy or if it requires a thermal cure above 60–70 °C.
Another common use of adhesion promoters is in copper foil applications, where they are used to improve the adhesion of low dielectric resins in printed circuit boards and chip scale packaging to copper. Pretreating the intended copper surface with an adhesion promoter yields significantly higher peel strengths. For example, Aculon’s adhesion promoters show superior peel strength on smooth copper, strengthening the bond between copper and PCB resins. This improved adhesion ensures greater reliability and performance, even under harsh conditions (600 hours in autoclave).
| Application | Description |
| Die Attach | Improves the bond between the die, die attach adhesive, and the substrate/lead frame (e.g., gold, copper, nickel), ensuring a stronger, more reliable connection. |
| Copper Foil–PCBs | Enhances the peel strength of low dielectric resins on copper, helping to improve the adhesion for printed circuit boards and chip scale packaging. |
| Molding Compound (EMCs) | Strengthens the bond between epoxy molding compounds and various materials like lead frames and substrates, making sure the encapsulation is secure and durable. |
| Paints and Coatings | Increases adhesion of paints and coatings to different surfaces, such as metals, plastics, and glass, for better durability and resistance to wear. |
| Automotive Parts | Enhances bonding between adhesives and parts used in automotive manufacturing, ensuring strong and durable connections in components like bumpers, door panels, and engine parts. |
| Electronics Assembly | Improves the adhesion of encapsulants and adhesives to electronic components, contributing to better performance and longer lifespan of devices. |
| Aerospace Components | Strengthens the bond between adhesives and materials used in aerospace, ensuring durable, high-performance connections that can handle extreme temperatures and stress. |
| Medical Devices | Guarantees reliable adhesion between different materials (e.g., metal, plastic, silicone) in medical devices, which is critical for safety and functionality. |
| Solar Panels | Improves adhesion between encapsulants, back sheet films, and photovoltaic cells, boosting durability and performance of solar panels in tough environmental conditions. |
| Construction Materials | Enhances the bond between materials like concrete, metal, and plastic in construction, helping ensure the long-term stability and durability of structures. |
Conclusion
In conclusion, adhesion promoters are vital components in the adhesives and coatings industry, significantly improving the bond between different materials. By creating a reliable interface that facilitates strong chemical interactions, these promoters enhance the performance and longevity of coatings and adhesives. With a variety of types and applications available, selecting the appropriate adhesion promoter is crucial for achieving optimal results in manufacturing and product development.
Facing challenges in product selection or encountering difficulties with your application? Let our team of Coatings Experts be your guide. Get in touch with us today, and we’ll work with you to find the perfect solution for your needs.
