Adhesive Failure

In Figure 10.8, the adhesive has remained predominantly on the lower surface and this suggests some surface contamination or other factor impeding the adhesion to that surface. Mould-release agents are sometimes used to help release plastics from the mould tool, especially for the first batch of parts off the tool when a silicone — or polytetrafluoroethylene-based release spray might be used. These release agents do not always remain on the tool but get transferred across to the component and, since they are low-surface-energy liquids, they prevent the adhesive from bonding to the component.

If a region of low strength exists at the substrate surface, it may originate from weak boundary layers such as dust, grease and metal oxides of relatively low strength. Oxide layers on brass and aluminium can often be pulled away by an adhesive.

Adhesive Failure

Figure 10.8 Adhesive failure

Low-surface-energy plastics such as the polyolefin family will always be a challenge to the adhesive application engineer, especially if the adhesive bond line is to be subjected to peel loading. Bonding to ‘difficult’ plastics and the wetting of adhesives is discussed in more detail in Section 6.1.

Another reason for adhesive failure might be excessively fast cure. Cyanoacrylates will sometimes cure so rapidly on an alkaline surface that they polymerise before they have a chance to properly adhere to the surface. A glazed or glossy appearance to the failed cyanoacrylate is often an indication that the adhesive has cured too quickly. Plated metals sometimes have traces of alkalinity remaining on the surface and washing with an aqueous cleaner can rectify the situation.

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