Action of contact angle in surface wettability

The surface tension, or more accurately the liquid-air interfacial tension, of a liquid adhesive is a key property for bond formation. Surface tension arises from the attractive forces of molecules at the surface of a liquid. It is the force required to break the “film” formed at the surface. Surface tension is often expressed in dynes/cm (a surface tension of 1 dyne/cm is equivalent to

Surface tension is important in bonding. Interfacial tension of the adhesive and the surface energy of the substrate determine the wetting of the adhesive on the surface, good wetting promotes intimate contact and
higher adhesion. The penetration of the liquid into craters and pores in rough and porous substrate, which enhances bond strength through mechanical ‘keying’, is a surface tension driven process. Surface tension is one of the properties controlling the stability of adhesive jets or sprays during dispensing; it may be an important parameter in effects such as stringing or drop break-up.

Surface tension depends on the interfacial intermolecular forces and can be split into
contributions from non-polar (e.g. van der Waals) and polar (e.g. hydrogen bonding)
components। The polar components can be further broken into electron acceptor or electron donor components (or Lewis acid/base components). Polar molecules have varying proportions of acceptor/donor components and in many cases one component will be much more dominant. Water is fairly unusual in having both strong acceptor and strong donor properties.

The surface tension of a liquid will influence how that liquid (adhesive) will wet a solid
(adherend)। Force balance or equilibrium at the solid-liquid boundary is given by Young’s equation (1) for contact angles greater than zero (see Figure 1):

The lower the contact angle, the Greater the tendency for the liquid to wet the solid, until complete wetting occurs (contact angle Thita= 0, cos Thita= 1). For complete wetting to occur the surface tension of the liquid should be less than or equal to the critical surface tension of the सुब्स्त्रते। Large contact angles are associated with poor wettability. Thus surface tension potentially plays a significant role in the dispensing and spreading of hot melt adhesives.

This part of this Guide describes the pendant drop method for determining the surface tension of hot melt adhesives specifically at elevated temperatures above 100 C।