Failure, adhesive fast cure

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. 

Adhesives can also stress crack or craze certain plastics and this can also indnce premature failure of the substrate. This generally happens whilst the adhesive is uncured and the plastic part is pre-stressed due to abrupt changes of section. The liquid adhesive softens and weakens the plastic leading to the formation of cracks and the liqnid adhesive then penetrates these cracks causing fnrther damage. With fast-curing adhesives, this is less likely to occur as once the adhesive has cured it is essentially a thermoset plastic (or thermoplastic) itself and is therefore inert. Amorphons thermoplastics are more prone to stress cracking than others and so it is important therefore to ensure that the adhesive is compatible with the substrate. 

The cross-link density of the polymer network, as well as its properties, depend on the functionality, the length and the chemical nature of the polyene (R ) and thiol (R) prepolymer chains, and it can thus be tailored as desired. Low-modulus polymers suitable for adhesive applications were obtained by using aliphatic prepolymer chains, in particular with polybutadiene-based elastomers which were cross-linked very efficiently by UV-irradiation in the presence of a tri- or tetrathiol [45-48]. As only a few cross-links need to be formed between the polymer chains to make the rubber insoluble, low concentrations of thiol (2 wt%) proved to be sufficient to achieve an effective and fast cross-finking. Hardening was found to hardly occur upon UV-curing (increase of the Persoz hardness from 40 to 55 s), which is essential to ensure outstanding adhesion. At the same time, the shear adhesion failure temperature (SAFT) increased from 80 to 160°C, due to the formation of the chemical network (Fig. 4). 

C (250°F)-cured epoxy adhesives that absorbed water so fast, and were associated with the many earlier in-service bond failures, were later found to perform just as reliably on the improved surface treatments as the second generation of such adhesives that were far more resistant to water absorption. 

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