Metallization treatment - Thermoplastic polymers, elastomers and additives

The metallization by chemical means in a non-conductor consists of depositing a metal film by immersion in a solution containing a metal capable of being reduced to the surface of the material.

The surface of the plastic must be chemically modified to create on the surface of the polar or hydrophilic groups, for example, hydroxyl and carboxylic. The strangely strong bond between the polymer and the metal deposit was associated with Van der Waals forces or chemical bonds between metal and plastic. It is also based on mechanical anchoring. In this case, the rough surface of the plastic retains the metal due to an effect called "pressure pin". During the following steps, the cavities or micropores present on the surface after engraving are filled with metal. The mechanical adhesion created prevents the separation of the metal from the plastic surface. In reality, the adherence obtained is certainly a combination of these two hypotheses. These are more or less favored depending on the type of plastic and the range of preparation used.

The American Society for Test Methods (ASTM) defines adhesion "the state in which two surfaces are held together by interfacial forces, which may consist of valence forces or concurrent effects, or both." It is widely believed that, although membership is a complex process, there are essentially two main forces involved: chemical bonds and mechanical interconnection. There are two types of chemical bonds that affect adhesion: ionic bonds and van der Waals forces. Ionic bonds originate from the electrostatic attraction between opposite charge ions and are the result of the exchange of electrons between the valence layers of two atoms. The Van der Waals bonds, on the other hand, are derived from the interaction between the positive and negative charges of the larger adjacent atoms or molecules. The last type of bond is more widespread in long-chain molecules, such as those that make up the underlying polymers of plastics, inks, coatings and adhesives. Although the Van der Waals individual bonds are relatively weak, the attractive forces present throughout the polymer film help to create an important and fundamental force to guarantee adhesion.

In addition to Van der Waals bonds, the mechanical interconnection that occurs between the substrate surface and the ink, coating or adhesive layer provides an important contribution to adhesion. Unlike chemical bonds, this physical interconnection has a relatively intuitive mechanism: the complex topography of the substrate creates mechanical connection points that favor the adhesion of the polymer to the plastic substrate. Accordingly, any organic or inorganic material that prevents or hinders interfacial contact between the polymer surface of an ink, coating or adhesive and the surface of the substrate, ends by inhibiting adhesion. This phenomenon can be attributed to an extremely thin layer (sometimes only a few angstroms), which weakens the Van der Waals link and / or blocks physical interconnection. This thin contaminant film can be the result of inadequate or incomplete cleaning methods, rust formation after cleaning, insufficient protection of cleaned surfaces of oils, greases, fingerprints, release agents or the deposition of molecules present in the air . of the plant, generated by other upstream production or processing activities