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Influence of Surface Roughness

The benefits of abrasion don’t stop at simply removing contamination. A rough surface for bonding often leads to stronger bonds for several reasons:
 

  • Roughness imparts additional surface area with which the adhesive can make contact when forming a bond
  • A rough surface provides additional mechanical interlocking at the interface
  • Irregularities on the surface may mitigate crack propagation, enabling stronger, more fatigue-resistant bonds

Roughness, especially excessive roughness, may not always be a benefit, however. In order to bond to a rough surface, it is necessary for the adhesive to flow into the cracks and crevices created by abrasion. This is true for both liquid adhesives and tapes. Tapes DO flow! If the adhesive is not capable of flowing, there is less overall contact between adhesive and surface, and trapped air may result in reduced strength or bonds poorly protected from environmental exposure.

  • Very thin tapes may require a very smooth surface to create an effective bond.

    Thin Tapes

    Very thin tapes often require very smooth surfaces in order to create a bond. Imagine how much gap a 50.8 microns thick tape is capable of spanning without filling, thereby trapping air.

  • Thick tapes are capable of effectively bonding to relatively rough surfaces.

    Thick Tapes

    Thick tapes are capable of bonding to relatively rough surfaces, especially those abraded with non-woven abrasive pads such as Scotch-Brite™ 7447 pads.

  • High viscosity liquids are capable of spanning gaps over 0.25”.

    High Viscosity Liquids

    While not capable of flowing into very small cracks and crevices, high viscosity liquids are the most gap-filling products, capable of spanning gaps over 6.35mm. This is important when bonding very rough materials, such as unfinished fiberglass composite.

  • Low viscosity liquids can fully wet out, allowing them to bond many difficult-to-bond materials.

    Low Viscosity Liquids

    Low viscosity liquids are the most capable of flowing into all the cracks and crevices of a surface. While not the best at “gap filling” like high viscosity fluids, their ability to fully wet out surfaces allows them to bond some very difficult-to-bond materials, such as cyanoacrylates bonding rubbers.

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While it is important to consider the surface roughness or gap between substrates, choosing a liquid or tape should never be done on this basis alone. The many considerations for choosing an adhesive include all the attributes of the science of adhesion: surface science, adhesive chemistry, and joint geometry.

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