To ensure the satisfactory performance of adhesive bonding, measuring and quantifying the effects of surface preparation and validation of surface cleanliness is important. While there are many methods to complete this, a common approach for assessing both surface roughness and potential contaminations are through a water break test.

A water break test is the process of pouring water onto a surface and observing the results in order to determine that a bonded surface has sufficient surface energy to achieve proper adhesion. This is usually expressed as a ‘contact angle’ between the liquid and the surface of a composite substrate.

A high contact angle, which occurs with drops, indicates low surface energy which is indicative of a possible surface preparation problem. If the poured water forms into droplets, then abrasive remnants may exist on the surface and have a less than adequate surface preparation for bonding. If the water spreads out and forms a solid sheet across the exterior, or exhibits a low contact angle, the surface is likely suitable for adhesive wetting during adhesive bonding.

Another important consideration is that certain composite materials, even when clean, may not be wetted by water but can still be wetted by an adhesive. In addition, satisfactory wetting does not guarantee adequate bond strength. This does not diminish the value of the water break test, however.

As a best practice, substrates should be bonded as soon as possible after surface preparation assessment, since at this time the surfaces are most ‘active’ and surface properties are optimal for the bonding process.

NOTE: When conducting a water break test, it is not recommended for use on an actual composite substrate. Use a sample piece of the substrate to conduct this test.

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