According to the definition of adhesives for flexible packaging lamination, they are non-metallic substance capable of joining materials by surface bonding , with a bond possessing adequate internal cohesion.
The adhesive forms the bonding agent between two substrates, which would not stick together without it.
Adhesion is the process of attachment of a substance to the surface of another substance, is the force of attraction between molecules in different layers. There are different types of adhesion: adhesion by covalent bonding and adhesion by physical bonding. A basic requirement for good mechanical adhesion is that the adhesive with good wetting of the substrate.
Cohesion is the internal strength of a material and describes the force of attraction between molecules within the same layer. During the drying process of a solvent-based adhesive for laminated substrates, the cohesion strength increases.
Surface tension is a phenomenon that liquids exhibit in order to reach the energetically most favourable form of the liquid surface with respect to a given volume. This surface tension is caused by the fact that at the liquid-air interface, the liquid molecules have a greater attraction to each other than to the molecules in the air. Wetting is depending on the difference between the surface tensions, a liquid tends to spread more or less over the whole surface of the substrate. In order to develop adhesion, the liquid must wet the solid substrate sufficiently, otherwise bonding will be impossible. To develop sufficient bonding strength, the surface tension of the adhesive should be lower than that of the substrate, otherwise the bonding will not be adequate due to insufficient wetting of the substrate.
