ADHESION PROPERTIES OF POLYPROPYLENE TO ALUMINUM - INFLUENCE OF POLYMER GRAFTING AND THERMAL HISTORY

Polypropylene (PP) powder was hot-pressed between aluminium foils and the adhesion was examined by a peel test. A measurable adhesion was observed with the grafted polymer. By grafting 3-azido-sulphonyl benzoic acid onto PP, the chemical composition of the adhesion failure zones was monitored by X-ray photoelectron spectroscopy, using N and S as markers. Adhesion was improved by purifying the grafted polymer before pressing against aluminium. Purification eliminates ungrafted polar molecules which form a weak boundary layer at the interface with the Al adherend. Adhesion was also very much dependent on the thermal history of the hot-pressed aluminium/polypropylene/aluminium laminates. Quenching increased the peel strength dramatically, whereas annealing after quenching decreased it. This is interpreted in terms of the structure and ductility of the polymer. The adhesion of PP to aluminium thus depends on the interplay between the affinity of grafted macromolecules with the adherend, the formation of a weak boundary layer, and the bulk properties of the polymer.