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A gloved hand applying structural adhesive to a sheet of metal.

Structural adhesives FAQ

Every problem is unique, but our team of expert engineers have collected some of the most frequently asked questions to give you a good idea of the kinds of factors to consider.

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  • Structural adhesives work by adhering to the top surface of the bonded parts, so it’s important to know the exact material and condition on those surfaces. For metals, will the adhesive be applied to bare metal, or will there be a paint or coating on the surface? For plastics, exactly which base resin? Could there be residual release agents on the surfaces used for mould release?
  • The chosen structural adhesive must have enough work life (open time, pot life) to allow proper mixing and application of the adhesive and assembling of the bonded parts. Smaller assemblies or shorter cycle time production processes may be able to use a faster curing adhesive with a work life of only five minutes or less, while larger assemblies that require alignment and clamping will probably need a work life of 20 minutes or more.
  • Structural adhesives generally benefit from clean, rough, dry surfaces for highest bond strength. This typically means either light abrasion and solvent cleaning of the surface, or solvent cleaning followed by chemical etching or applying a primer. Adhesion tests should be performed to determine the adequate surface preparation for a specific application.
  • Joint designs that put the adhesive bond under shear, tension, or compression forces will provide the highest strength. Designs that tend to apply peel or cleavage forces to the adhesive, where the applied stresses are not distributed over the entire bond area, will have lower bond strength, but the bond may still be sufficient for the needs of the application. In addition, optimum bond line thickness typically ranges from 0.005" to 0.020". The adhesive qualification process should always include testing of prototype assemblies to ensure the adhesive will provide enough performance.
  • Structural adhesives come in many forms, including low viscosity liquids and non-sag pastes, one- and two-component formulations, short and long work lives, and various package sizes and shapes. Most two-part structural adhesives are available in both bulk containers and convenient, easy-to-use cartridge mixing systems.
  • All structural adhesives provide at least 1,000 psi of overlap shear strength to aluminum, but the different adhesive chemistries have various properties:
    • Epoxy adhesives come in both two-part adhesives that cure upon mixing the two components and one-part adhesives that cure with temperature. They generally have the highest strength and overall performance. They also provide the best resistance to high temperatures, solvents and outdoor weathering. They adhere well to metals, woods and concrete, and the flexible epoxy adhesives also bond to some plastics and rubbers. Epoxy adhesives usually require clean, abraded surfaces to obtain maximum bond strength.
    • Acrylic adhesives are two-part adhesives that provide excellent bond strength and durability, although slightly lower than epoxy adhesives. However, they have several features that make them easier to use in many applications and manufacturing processes. These benefits include a much faster cure speed, higher tolerance for oily or unprepared bonding surfaces and the ability to bond a wide variety of materials, including nearly all plastics. Newer acrylic adhesive formulations are room-temperature stable with a long shelf life, and some have much lower odour than regular acrylic adhesives.
    • Urethane adhesives are two-part adhesives that are relatively flexible when cured, and therefore tend to have excellent impact resistance and good adhesion to most plastics. They also bond well to woods, concrete and rubbers, but they tend to have reduced resistance to solvents and high temperatures. Uncured adhesive components are sensitive to moisture.
    • Instant adhesives are one-component, lower viscosity liquids that cure extremely quickly with just contact pressure and surface moisture. They adhere well, with thin bond lines, to plastics, metals and rubbers. With the use of primers, they can also adhere to low surface energy plastics and elastomers. They tend to have low flexibility, peel strength and impact resistance compared to other structural adhesives. They are generally used for applications such as gasket bonding and smaller assemblies.
    • Anaerobic adhesives are one-part adhesives that cure on active metal surfaces when oxygen gets excluded from the bond line. These products keep your factory running efficiently, reducing maintenance and leakage. They do not bond well to glass, plastics or rubbers, and are primarily used for applications such as locking threads and sealing pipe connections.
    • Polyurethane-reactive adhesives are one-part adhesives that apply like a hot melt, but cure with ambient moisture for near structural strength bonds. Fast set times and build strength over the next 24-48 hours can reach as much as 1,000 psi in overlap shear strength. They are flexible and resistant to temperature extremes and most solvents. They are most commonly used in bonds where at least one substrate contains or transmits moisture, such as wood or plastic.

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What are the most common stress modes of structural adhesives?

  • An illustration of two bonded perpendicular pieces of metal.

    Tensile

    Tensile stress is pull perpendicular to the plane and away from the adhesive bond. Force is distributed equally across the entire bond area. (Compression stress is in the opposite direction, where the substrates are pushed together perpendicular to the bond plane.)

  • An illustration of a piece of metal bonded below a horizontal piece of metal.

    Shear

    Shear stress is pull directed across the adhesive, forcing the substrates to slide past one another. Here the force is in the same plane as the bond and distributed across the entire area.

  • An illustration of two bonded perpendicular pieces of metal.

    Cleavage

    Cleavage stress is concentrated at one edge of the joint, exerting a prying force on the bond as the substrates separate. While that end of the adhesive joint is experiencing concentrated stress, the other edge of the joint is theoretically under zero stress. Cleavage occurs between two rigid substrates.

  • An illustration of a piece of metal bonded below a horizontal piece of metal.

    Peel

    Peel is also concentrated at one edge of the joint. At least one of the substrates is flexible, resulting in even more concentration at the leading edge than with cleavage stress.


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Structural adhesive chemistries

  • Acrylic adhesives

    Two-part adhesives that offer great strength and design flexibility.

  • Epoxy adhesives

    Excellent durability and resistance to environmental extremes.

  • Urethane adhesives

    Ideal for creating strong, flexible bonds between dissimilar materials.

  • Polyurethane reactive (PUR) adhesives

    One-part products that combine the speed of hot melt adhesives with the structural benefits of moisture-curing chemistries.

  • Anaerobic adhesives

    Provide tight fits and seals in thread-locking, pipe-sealing and related applications.

  • Instant adhesives

    Achieve high performance and excellent adhesion quickly.

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