Chemistry of the Interface Between Aluminum and Polyester Films

Abstract

It has been observed that the adhesion between vacuum-evaporated aluminum and poly(ethylene isophthalate-co-ethylene sodium sulfoisophthalate) copolymer is approximately five times greater than the adhesion between vacuum-evaporated aluminum and biaxially-oriented poly(ethylene terephthalate) film. To describe the interface between the aluminum and these polymeric substrates, thermoanalytical, spectroscopic and microscopic techniques have been applied. Definite changes in surface elemental composition and chemical functionality occur upon metallization of the polymer films. Aluminized samples contained two new oxygen functionalities; one due to the aluminum oxide and the other due to an organoaluminum species. Thermal degradation, as may occur during vacuum evaporation, would be expected to yield a carboxylic acid endgroup and a vinyl endgroup for each chain scission reaction that occurred. Reaction of aluminum with these carboxylic acid endgroups is thought to be responsible for the organoaluminum oxygen peak that was observed. Based on the XPS data, however, the level of this new functionality was comparable for both types of polyester film. Thus, this new functionality may be involved in promoting aluminum/polyester adhesion, but by itself cannot explain the differences in the level of adhesion that are attained. It appears, based on the transmission electron micrographs, that the aluminum deposit penetrates the copolymer coating to a greater depth than it does the PET. The greater level of penetration could be responsible for the greater adhesion obtained between vacuum-evaporated aluminum and the copolymer film compared with the level of adhesion obtained with the PET film. Based on this work, it appears that the adhesion of the vacuum-evaporated aluminum to both polyesters has a similar chemical component (type and amount) but a different extent of the mechanical component.