NH3 Plasma Treatments of PET for Enhancing Aluminum Adhesion

A NH₃ plasma process has been studied for enhancing the adhesion of aluminum coatings on polyethyleneterephtalate (PET) films. According to our peel strength results, NH₃ plasmas increase markedly the adhesion of aluminum on PET compared to O₂ discharges, with a much shorter treatment time. A tentative model of nonhindered growth of Al-coating based on the Lewis basic character of the functionalities grafted by NH₃ plasma is proposed for Al-polymer interactions, and for explaining the various steps in the process. The effects of power input and treatment time on the polymer surface chemistry and on the metal-polymer peel strength have been evaluated. Treatment times as short as 0.1 s at 100 W proved to be the best conditions in NH₃ plasmas, for a significant increase in Al/PET adhesion, while longer treatments have a detrimental effect. This may explain why most authors have not discovered the benefits of NH₃ plasmas for improving the adhesion of metals on PET, and have preferred O₂ or air treatments. The relative basicity of PET grafted with N-containing functionalities has been measured by means of X-ray Photoelectron Spectroscopy (XPS) analysis of samples exposed to vapors of trichloromethane, a Lewis acid molecular probe. The Al/PET adhesion was evaluated by means of a 180° Peel Test.