Why must plastic film undergo surface pretreatment before printing and lamination?

Why must plastic film undergo surface pretreatment before printing and lamination? There are four main reasons: the non-polar characteristics of the molecular structure, low surface energy, the influence of additives, and strong chemical stability.

I. Non-polar characteristics of the molecular structure

Among the main substrates for plastic film packaging printing, polyethylene (PE) is a non-polar high-molecular organic compound with a small critical surface tension, strong inertness, and very poor affinity with ink.

Each unit of polypropylene (PP) contains a methyl group, which is a weakly polar group, and it basically also belongs to non-polar high-molecular organic compounds; however, its critical surface tension is greater than that of polyethylene (PE), so the printability of polypropylene (PP) is somewhat better than that of polyethylene (PE).

II. Low surface energy

The surface energy of plastic films such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polyvinyl chloride (PVC) is relatively low.

The lower the surface energy of a polymer, the smaller its surface tension, and the less easily it is wetted by liquids. Polyolefin plastics have this property.

To achieve sufficient wetting between the ink and the printing substrate, the surface tension of the substrate should be equal to or greater than that of the ink. The only solution is to pretreat polyolefin plastics to increase their surface energy.

III. Influence of additives

During the manufacturing process of resins such as PE and PP, in order to make the film easy to open and give it anti-static, anti-aging, and UV-resistant properties, a certain amount of additives such as opening agents, antistatic agents, and stabilizers must be added. These additives in the resin will slowly migrate to the surface during high-temperature molding of the plastic, forming oil stains and making the film difficult to bond with other materials.

This phenomenon is continuous; therefore, after surface pretreatment of plastic films and plastic products, printing should be carried out immediately. The longer they are left standing, the worse the printing fastness.

IV. Strong chemical stability

PE and PP films are not only smooth on the surface but also resistant to corrosion by most acids and alkalis, and are insoluble in common solvents at room temperature, so that the film surface has very poor adhesion to inks, coatings, and the like.

The above shows that before secondary processing such as bonding, coating, lamination, electroplating, and hot stamping, printing on polyolefin plastic films must undergo surface treatment to impart polarity to the plastic surface, improve the surface state and properties, and increase interfacial bonding force.

In summary, after surface pretreatment of plastic films and plastic products, printing should be carried out immediately; the longer they are left standing, the worse the printing fastness.