# 4.1.3 Wetting and Polarity

> From: LOCTITE®X Digital Manual for Bonding (https://www.loctitex.com/en/manuals/bonding/4.1.3-wetting-and-polarity/)

#### 4.1.3 Wetting and Polarity

Surface energy is the degree of attraction or repulsion exerted by the surface of one material to another (measured in mN/m or outdated dyn/cm). Surface tension is a liquid’s resistance to surface penetration, which allows the liquid to behave like a thin elastic film. The wetting theory states that adhesion results from the surface forces of molecular contact. In the ideal bonding condition, a substrate has high surface energy, i.e. the surface has the capacity to attract, and the adhesive has a low surface tension, i.e. low resistance to deformation or breakage. This results in the adhesive properly wetting the substrate and forming a strong bond.12 Surface preparation for bonding frequently increases the surface energy of a substrate to improve bond strength.

_Figure 76 High surface energy substrate_

![Figure 76 High surface energy substrate](/en/manuals/bonding/assets/img/adhesive-substrate-surface-energy-high.jpg)

_Figure 77 Medium surface energy substrate_

![Figure 77 Medium surface energy substrate](/en/manuals/bonding/assets/img/adhesive-substrate-surface-energy-medium.jpg)

_Figure 78 Low surface energy substrate_

![Figure 78 Low surface energy substrate](/en/manuals/bonding/assets/img/ahdesive-substrate-surface-energy-low.jpg)

12) S. Ebnesajjad, Handbook of adhesives and surface preparation, Technology, Applications and Manufacturing (2011).

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