Abstract
It is known that chromate pretreatment films with dry type silica have a good adhesion to organic films such as paint and laminate films on a zinc coated steel sheet. The adhesive mechanism has been investigated from the standpoint of surface polarity for a long time, but has not been perfectly explicable. In this work, a correlation between the composition of chromate pretreatment films with dry type silica and the adhesion to a laminate film on a zinc coated steel sheet was examined in comparison with the chromate pretreatment films with wet type silica and without silica. Furthermore the adhesive mechanism has been investigated by the analysis of the pretreatment films using SEM, AFM, BET method, TEM and EDX.
The results are summarized as follows:
Not only the type and the particle size of silica, but also the silica/chromate ratio and the coexistence of chromate affected the adhesion of laminate films. The highest adhesive strength was obtained for the pretreatment films formed in a pretreatment solution with SiO2/Cr ratio of 4.0 and dry type silica with a diameter of 7 nm. The good adhesion of chromate pretreatment films with dry type silica is due to an anchor effect by sub-micron surface roughness and an increased in number of interface bonding sites by a nano-porous structure. The formation of an insoluble Cr(III) compound layer plays an important role in binding silica particles to the zinc coating surface.
