Phosphonates provide protection for metallic car paints.
Peter Tasker at the University of Edinburgh, UK, and colleagues looked at a range of phosphonic-acid compounds that could bind to lightly-oxidised aluminium. Flakes of aluminium provide the reflective lustre of some car paints, but the surfaces of the flakes can become dulled by the formation of aluminium oxides.
'Traditionally treatments to preserve flake reflectivity have involved chromating processes, but concerns over their carcinogenic properties have provided an incentive for the development of more benign reagents,' said Tasker. 'The move to replace solvent-based paints with waterborne coatings presents a technical problem in generating stable dispersions of aluminium flakes, because these react with water.'
Aluminium flakes give some car paints their sheen
The reaction of aluminium with water creates aluminium trihydroxide and hydrogen gas. Tasker's team found that phosphonates that bind strongly to aluminium also greatly reduced the amount of hydrogen produced from a suspension of aluminium flake in a model water-based paint.
The team compared their phosphonate-protected flakes with commercial samples, finding that they performed as well as silica-encapsulated aluminium flakes.
'Phosphonates have been shown to be good ligating groups for aluminium and show promise for surface engineering of oxidised aluminium,' said Tasker.