Chemical Interaction
Specific intermolecular forces arising from permanent dipoles and hydrogen bonding determine the ability of a surface to interact with water or polar inks. This value, referred to as the polar component, represents the part of the surface energy that comes from asymmetrical charge distributions within the molecules. In many polymers used for packaging, the natural state of the surface is almost entirely nonpolar.
To improve adhesion, converters must increase this value by introducing oxygen or nitrogen atoms through surface treatments. These atoms create reactive sites that can form strong bonds with the resins found in most modern inks and adhesives. The strength of these interactions is what prevents the coating from being easily rubbed or peeled off.
Hydrogen Bonding
Measuring the contribution of these specific forces is essential for choosing the right coating system for a given substrate. When calculating surface free energy, the polar component is isolated from the universal dispersive forces. This is usually done by measuring the contact angles of at least two different liquids, one of which must have a significant polar nature, such as water.
If the surface has a high value in this area, it will be easily wet by water-borne systems. This is particularly important as the industry moves away from solvent-based inks to more environmentally friendly aqueous versions. The presence of hydroxyl, carboxyl or carbonyl groups on the surface is what drives these interactions.
These groups act as anchors that pull the liquid molecules toward the solid. Surface treatments like corona discharge or flame treatment are specifically designed to maximize this energy. However, this energy is often the most susceptible to decay over time as the polar groups rotate into the bulk of the material.
Monitoring this specific part of the surface energy is more useful than looking at the total energy alone. It provides a direct view of the chemical readiness of the material for bonding.
Ink Adhesion
Creating a durable and high-quality print requires a close match between the polar forces of the ink and the substrate. If the polar component of the surface is too low, the ink will not be able to form the necessary hydrogen bonds to stay in place. This leads to common defects like poor rub resistance or ink transfer.
In the lamination of flexible films, a high polar energy is necessary for the adhesive to fully wet the surface and create a strong interface. Without these forces, the bond will be weak and the layers may separate under the stress of the packaging process. Most high-speed printing operations require a minimum level of polar energy to ensure that the ink dries correctly and remains stable.
The ratio between the polar and dispersive parts of the energy is a key factor in the formulation of primers and coatings. Understanding this balance allows for the creation of packaging that can withstand harsh environments, such as high humidity or chemical exposure. Accurate measurement of this property is a standard part of modern material science in the converting field.
