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The effect of humidity on the propagation of cracks in rubber-glass interfaces

The strength of adhesive joints made between nitrile rubber strips and glass have been examined using the peel bend test in controlled atmospheres. Under loading conditions that produce very slow crack growth the crack travelled faster in ambient air than in dry nitrogen, and this effect was found to be reversible. Crack velocities in wet nitrogen were faster still and although some strengthening could be achieved by subsequently reducing the humidity in the test chamber complete reversibility was not attained. The measured interfacial strengths are much greater than would be required to overcome the Van der Waals bonding alone, as energy is expended in extending the molecular chains prior to reaching the breaking tension. It is argued that this view of the structure of the crack tip is consistent with slow crack propagation that is activated by the presence of water vapour in the atmosphere around the crack.