Improved Adhesion Between Ethylene Polymers and Aluminium
Adhesion between aluminium and polymers is important in a wide variety of applications. The aim of the present work was to improve adhesion in polyethylene- aluminium laminates by surface modification of aluminium. The treatments examined were hydration in boiling water and phosphatation by immersion in dilute phosphoric acid solutions. Laminates were produced by applying a pressure of 1.2 MPa at 250 °C and by extrusion coating in pilot plant. The peel strength was measured by a T-peel test. To evaluate the adhesion mechanism, chemical and topographical changes on the Al surface were studied by FTIR, XPS, AES, SEM and BET surface area analysis.
Hydration of Al in boiling water is known to produce an adherent porous oxyhydroxide, AlOOH. In pressed laminates, improved adhesion was obtained for all polymers, EBA, EVA, EAA, EVS, EVSBA and LDPE, laminated with hydrated Al. Three factors are suggested to contribute to the improved adhesion: mechanical keying into the porous surface; a larger contact surface, which increases the number of interactions/bonds across the interface; and the formation of stronger intermolecular bonds across the interface. By IR spectroscopy, a hydrogen bond was observed to form at the interface with ester carbonyls in EBA and EVA, while a carboxylate was observed to form with EAA. For EVS and EVSBA, the hydrated surface is proposed to favour the formation of covalent bonds across the interface. In extrusion-coated laminates, the adhesion was superior for hydrated Al, especially after storage with acetic acid solutions. The adhesion in extrusion coated laminates was favoured by a higher melt index, which improves the ability to penetrate the porous hydrated oxide. In efforts to produce a hydrated foil with short hydration times, an enhanced Mg concentration in the surface oxide was observed to increase the initial hydration rate.
By the introduction of phosphate onto the Al surface, improved adhesion was obtained in pressed laminates for EBA, EVA and HEMA, while a deterioration was obtained with EAA and ION. After characterisation of the phosphated Al surfaces by IR spectroscopy, these results are explained using an acid-base approach. The acidic sites in the ethylene copolymers, such as hydroxyl in HEMA, are suggested to interact with basic P=O groups. The strong carboxylic acid dimer in EAA is suggested to act as a base, for which reason no strong interaction forms with the basic P=O rich surfaces.