Factors Influencing Lacquer Performance
Factors Influencing Lacquer Performance
Failure of a lacquer to wet the tinplate surface completely produces circular or elliptical gaps in the cured coating, commonly known as “eye-holes”.Causes include the presence of dust particles, excessive residues of oil, application to chilled sheets and surface incompatibilities. Difficulties in wetting may sometimes be overcome by a preliminary heating of the tinplate before applying the lacquer; this is rather expensive and is only adopted as an interim measure until compatibility can be assured.
Adhesion is an important lacquer property and failure may take at least three forms. C racking and lifting may occur during mechanical deformation, for example at a seam, bead or ring.Detachment may occur during heat processing, particularly in the case of exterior coatings. Undermining may occur by corrosion, often by spread from a scratch or other discontinuity in the lacquer coating.
The passivation treatment, which the tinplate receives, has a significant effect on lacquer adhesion. Both weak and very strong passivation treatments may be deleterious under certain conditions and a treatment which gives a total surface chromium of about 0.5 µg/cm2 is often favoured for tinplate which is to be lacquered. This provides a surface that remains adequately stable in storage and can be produced consistently.
The surface roughness of the tinplate and possibly the thickness of the tin coating may be influential as regards lacquer adhesion. Deliberately rough, special surface finishes may have disadvantages, whilst very thin tin coatings may produce adhesion failures at points of intense deformation where the free tin coating may be dispersed. These are concerned with coating continuity and with adhesion and its loss by deformation or undermining.
- Source: The International Tin Association (formerly ITRI Ltd)