Although some adhesives are single component and can cure by other mechanisms such as heat or UV, Scott Bader’s Crestafix, Crestomer and Crestabond adhesives are two-component and cure by a process of polymerisation. Two-part adhesives need to be fully mixed at the point of application, the two components can be kept separate until that point.

Ratios vary from 50:1 (Crestafix), 10:1 (Crestomer and Crestabond M1), and 1:1 (Crestomer Advantage and Crestabond M7). If the ratio is narrow (1:1 to 10:1) the two need to have similar viscosities, wide ratio usually has different viscosities and a specially designed static mixer. Larger quantities can be pumped using a machine.

Surface preparation

Surface prep is a crucial step in achieving a high strength, high quality bond.

The type and degree of surface preparation depends on the nature of the surface and the adhesive. The surface of the materials to be bonded can be treated/prepared to make them more susceptible to bonding, particularly if it has low surface energy. This may be etching, abrading or simply cleaning with a suitable solvent to remove any dust, grease or oil on the surface.

The surface needs to be:

Bond design

It is important to understand the forces acting upon the joint and the direction of loads because adhesives work best in compression and shear, less so in tension and very poorly in peel.

Avoiding stress raisers and peeling forces will enable a strong bond, as will protecting the joint from heat and chemical exposure.

Other considerations besides direct loads include the fatigue resistance and sound and vibration absorption of the parts.

Working with structural adhesives

To work, an adhesive needs to be applied to a substrate with a prepared surface.

There are many ways to apply adhesives, such as:

• From a cartridge
• Machine metering
• Spatula or trowel
• Brushing
• Spraying
• Hot melt

The first three are most often used with Scott Bader’s structural adhesives.

Failure modes

Again, it is important to understand how the forces acting upon the joint can break it, and design accordingly.

Cohesive failure, substrate delamination and failure modes are all predictable. Strengths, moduli, and elongation are known quantities that can be factored into the overall design so that stress limits are not reached.

Tips

• Good surface prep can prevent adhesion failure
• Avoid designs that expose bond lines to peeling forces
• Creep and yield can be avoided by protecting the bond line from excessive heat beyond margins of safety