Wetting

Behaviour of liquids (e.g. hot melt adhesive) when they come into contact with the surface of solids

The wetting of the substrates to be bonded is a fundamental requirement, as only an adhesive that has good surface contact can build up sufficient adhesive forces on the surfaces.  

If a hot-melt adhesive can wet a substrate surface well, this results in a wide area contact between the adhesive and the part to be joined.

Teardrop shape with contact angle

The wettability of a surface can be checked with a “water drop test”. A drop of water is applied to the surface to be tested.

In order to achieve good wetting with a hot-melt adhesive, the surface tension of the parts to be joined must be greater than that of the hot-melt adhesive.

Depending on the surface tension between the liquid and the part to be joined, different droplet shapes can be found, which are characterised by the so-called contact angle a. The contact angle thus represents a measure of the wetting.

Source: IVK, Die Kunst des Klebens, Abb. 7, S. 19