The Basics of Surface Science

When designing a product, substrates are chosen for a variety of reasons including performance, aesthetics, weight, or other considerations. The design of the part also tends to dictate the geometry of the bonded joint. Consequently, when choosing an adhesive, understanding how the adhesive chemistry interacts with the surfaces of the substrates becomes an important consideration.

“Substrate” and “surface” may both refer to the material being fastened to create an assembly, such as wood or iron. Technically, the substrate comprises all of the material properties - bulk properties like flexibility or vibration damping, as well as surface properties like abrasion resistance or texture. For the most part, “substrate” and “surface” are interchangeable in the context of adhesive bonding, and refer to the specific portion to which you’re actually bonding – where the adhesive makes contact.

Surface vs Bulk

It’s important to be aware that the surface of a substrate may behave very differently from the bulk of a substrate. Consider painted metal: the “surface” is paint which has very different properties, both on a micro- and macro-scale, than the “bulk” of the metal. Here are three examples where surfaces of a substrate behave very differently from the bulk.

  • Adhering to a varnished word surface requires a different adhesive than does unfinished wood.
    Varnished Wood

    Unfinished wood is just wood, but if the wood is varnished then the adhesive is actually adhering to the varnished surface.

  • Adhering to rusty iron can be challenging; surface preparation can impact adhesive effectiveness.
    Rusty Iron

    In the case of rusty iron, the adhesive is bonding partly to the iron and partly to the iron oxide; how much of each depends on the amount of rust and how much surface preparation has been done.

  • Consider any type of mold release when choosing adhesives for manufactured plastic parts.
    Manufactured Plastic Parts
    Many plastic parts are created using mold-release agents to facilitate production. If the mold release is not removed prior to bonding, the adhesive must be capable of bonding to the mold release.

The Importance of Surface Contact

Adhesives create a bond that joins two materials. For proper joining, it is critical that the adhesive can make contact with the surface of each material. This differs from other processes such as mechanical fastening in which the surface is punctured, or a welding process in which the materials are fused.

To make good contact with a surface, an adhesive must have the appropriate properties. That includes its chemistry and viscosity – these are covered in detail in other topic articles – but the surface must have the appropriate properties as well. The three properties listed below (surface energy, cleanliness, and roughness) are the most important factors for making intimate contact.

  • Learn how low- and high-surface energy substrates dictate adhesive choice.

    Surface Energy

    Materials with low surface energy values resist the flow of adhesive, preventing intimate contact. Materials with high surface energy support the flow of adhesive, supporting intimate contact. You can match the adhesive to the surface energy of the substrate, or you can modify the surface energy of the substrate by priming, etching, or other methods.

  • Dirt and contaminants can prevent adhesive from making proper contact with a substrate.

    Cleanliness Considerations

    Dirt, oil, grease, or dust can form a layer that prevents the adhesive from making proper contact with the actual surface of the material. Surface contamination can be managed in two ways. The surface can be cleaned of all contaminants, or an adhesive may be used that is formulated to be compatible with a certain level of contamination.

  • Surface roughness can increase the total area available for a bond.

    Bonding to Rough Surfaces

    Roughness can increase the total area available for a bond, allowing for a stronger joint. However, excessive roughness can inhibit the flow of adhesive onto the surface. The adhesive may be matched to the surface roughness by modifying the surface, or choosing a more appropriate adhesive.

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