Isotactic polypropylene (IPP) is one of the base substrate for producing barrier packaging/metallized films. Areas of applications of IPP metallized films are food packaging, cosmetic packaging, and also electronic applications (e.g. capacitors production). In terms of free surface energy metals represents the high energy surfaces, but polymers represents the low energy surfaces. The polarity of polyolefins, e.g. isotactic polypropylene, is very low due to hydrophobic (paraffinic) character of this polymers (water contact angle on IPP surface is approx. 90o and its free surface energy is equal approx. 30 mJ/m2. Unlike non-polar material like IPP, aluminum is a metal with a high free surface energy of 850 mJ/m2. Due to the large difference in IPP and aluminum polarities, it is necessary to make the surface modification of the polymer toward higher polarities using the discharge plasma (e.g. cold plasma discharge, corona discharge). If the polarity / hydrophilicity of the polypropylene is increased by modifying the corona discharge, the aluminum adhesion to the polymer surface is increased. The activation of IPP surface by discharge plasma (corona discharge) represents an continual process of efficient modification of the IPP surface leading to higher polarity of the polymer due to oxygen functional grups creation prior to metallizing to promote chemical modification as well as surface cleanliness. Another method of preparing aluminum-plated IPP is by applying a fine adhesion promoter (primer) layer (e.g. on the base of maleic anhydride grafted polyolefin) that eliminates largely the polarity gradient between the polymer substrate (IPP) and the metal (Al). For better adhesion a maleated very low density polyethylene (VLDPE) can be used as an improver of metal (Al) adhesion to IPP films metalized in vacuum. Adhesion of the metal (Al) to IPP are related to: the film surface energy, the interaction of the metal with the surface, the way the metal builds structure beyond the surface, and the thickness of the metal deposited. Maleic anhydride groups present in grafted polyolefins chemically react with Al, that chemically binds aluminum to the polymer chain. Aluminum particles also get embedded into the VLDPE layer matrix after corona treatment and increases the adhesion between polymer and Al layer.
