Adhesion between LDPE and hydrated aluminium in extrusion-coated laminates

Untreated aluminium and aluminium hydrated for 60 s in boiling water have been extrusion-coated with low-density polyethylene (LDPE). The hydration transforms the oxide surface into a porous oxyhydroxide, known as pseudoboehmite. LDPE samples with different melt indices (4.5, 7.5, and 15) were used, which influence the ability to penetrate into the pores. Compared with untreated aluminium, a superior peel strength was obtained for the laminates with hydrated aluminium. In almost all cases, the peel strength for the laminates with hydrated aluminium could not be measured, due to rupture in the polymer film. This improvement is suggested to be due to stronger acid-base interactions, increased contact surface, and mechanical keying into the porous surface. The obtained peel strength and analysis by means of scanning electron microscopy indicated that the polymer with the highest melt index or lowest melt viscosity had the greatest ability to penetrate into the formed pores. After ageing up to 12 weeks in solutions with 1% and 3% acetic acid, the peel strength dropped rapidly for the untreated Al laminates, but remained constant for the hydrated Al laminates. This is explained by the fact that, besides the improved adhesion, the hydrated oxide prevents corrosive attack.