While some adhesives are single-component and cure via other mechanisms like UV or heat, Scott Bader’s Crestafix, Crestabond, and Crestomer adhesives consist of two components and cure via polymerization. Two-part adhesives should be completely mixed upon application and the two components may be kept apart until then.
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Ratios range from 50:1 (Crestafix) to 10:1 (Crestomer and Crestabond M1) and 1:1 (Crestomer Advantage and Crestabond M7). The two adhesives should have similar viscosities if the ratio is narrow (1:1 to 10:1). A wide ratio typically infers different viscosities and a specially designed static mixer.
Greater quantities can be pumped with a machine.
Surface Preparation
Surface prep is a critical step for attaining a high-strength, high-quality bond.
The optimum type and degree of surface preparation corresponds to the nature of the surface and the adhesive. The material surface to be bonded may be treated or prepared to increase its bonding susceptibility, especially if it has low surface energy. This could be etching, abrading, or simply cleaning with an appropriate solvent to get rid of any surface-level dust, grease, or oil.
The surface must be:
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Bond Design
It is critical to understand the forces acting upon the joint and the load direction as adhesives optimally function in compression and shear, and less so amid tension. They also work poorly in peel.
Avoiding stress raisers and peeling forces facilitates a strong bond, as does protecting the joint from heat and chemical exposure.
Considerations beyond direct loads include fatigue resistance and sound and vibration absorption of the parts.
Working with Structural Adhesives
To operate, an adhesive should be applied to a substrate with a prepared surface.
There are multiple ways to apply adhesives, including:
- Trowel or spatula
- From a cartridge
- Machine metering
- Brushing
- Hot melt
- Spraying
The top three in this list are most frequently utilized with Scott Bader’s structural adhesives.
Failure Modes
It is key to know how the forces acting upon the joint can break it and design accordingly.
Cohesive failure, substrate delamination, and failure modes can all be predicted ahead of time. Strengths, moduli, and elongation are known quantities that may be accounted for in the overall design to ensure stress limits are not breached.
Tips
- Good surface prep can stop adhesion failure
- Beware of designs that expose bond lines to peeling forces
- Creep and yield can be avoided by protecting the bond line from heat that exceeds safety margins
This information has been sourced, reviewed and adapted from materials provided by Scott Bader Company Ltd.
For more information on this source, please visit Scott Bader Company Ltd.