Bonding Metal to Plastic: How Should Engineers Choose Their Adhesives?

Posted on 6/5/2018 7:17:11 AM By ASC

Choosing adhesives for bonding similar substrates is a simple process, but dissimilar substrates, and particularly plastic-to-metal bonds, present challenges for manufacturers seeking the best adhesive to use. The automotive and aerospace industries face the greatest challenge, since plastic components can help them to achieve their lightweighting objectives. For instance, they may need to bond polypropylene to aluminum, urethane to steel, or polycarbonate to aluminum.

Apart from eliminating the need for holes that weaken materials and adhesives’ ability to form stronger bonds that distribute loads more widely, they also save time in the production process and separate substrates that would otherwise be subjected to galvanic corrosion.

When choosing adhesives, engineers must take the following three factors into account:

  • Surface Energy: Adhesives behave differently depending on the surface energy of plastics. High surface energy plastics like ABS or polycarbonate allow adhesives to flow, wetting out the substrates and resulting in a good bond. Low surface energy plastics like polyethylene and polypropylene present a greater challenge.

  • Elastic Modulus: Different materials have a greater or lesser capacity to deform without permanent deformation occurring. Since the elastic modulus of plastic is very different to that of metal, the adhesive must be able to accommodate the elastic modulus of both materials. To achieve this, the adhesive must have an elastic modulus that falls between that of the two materials it is meant to bond.

  • Thermal Expansion: Dissimilar materials will also respond differently to temperature changes, but despite the fact that the two materials to be bonded will expand or contract to greater or lesser extents relative to one another, the adhesive must be able to maintain the bond. To do so, the adhesive must have the correct degree of flexibility and elongation and should have a minimal coefficient of thermal expansion (CTE).

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