Metal Packaging

The uniformity of the thickness of the coating is crucial, as it plays a very important role in the optical and mechanical properties of the film, as well as the cost of the paint.

It depends on the rheological properties of the product and the settings of the lacquering machine (roller spacing – alignment, lacquering roller condition).

What are the main problems arisen by an uninform film and how easy are they to be determined by naked eye?

1. Unclear edges of lacquered surface on metal sheets. Can be determined easily.

2. Bubbles appearance on the lacquered sheet. Can be determined easily.

3. Lanes of different whiteness may appear on the film before and/or after stoving. May not be easily determined.

4. Dry film thickness variety across the metal sheet, due to improper alignment of the machine cylinders. May not be easily determined.

5. Smaller thickness deviations along the sheets due to imperfections of the lacquering rollers, or to uneven adsorption of solvent by the lacquering rollers. May not be easily determined.

As the mechanical properties of the coating are absolutely depended on the dry film thickness, such variations along the sheets may lead to coating failure when mechanical stress is applied.

Applying excessive lacquer quantity, in order to achieve a minimum dry film thickness at the areas with defects, may be a solution for problems No 3-5. But such an action may lead to other issues, such as bubbles appearance on the lacquered sheet (No 2) and sharply rising of the lacquering cost.

Viscosity control is the easiest way to reduce and even avoid such phenomena, by keeping the cost as lower as possible. For every product, there is a specific viscosity range that allows the coating to be spread evenly on the lacquering roller and to level satisfyingly on the sheet before entering the oven. Lower viscosity allows the coating to eliminate any defects occurred during the application.

All CHROTEX products are accompanied with Product Application Guide (PAG) No 1, indicating the appropriate solvent quantity needed in order to achieve the desired viscosity for optimum leveling.

Scraper blade seems to be a simple part of the lacquering machine, but its role is important for the lacquering process, as it prevents contamination of the back side of the sheets. So it is crucial to prevent damages whenever possible in order to keep the lacquering quality as high and consistent as possible. Moreover a damaged blade may affect the scraper roller’s lifetime.

But why is this blade damaged?

• Blade type / material.
• Blade overhang / not correct position / abrasion due to intense contact with the roller.
• Machine vibrating effects the position / contact of the blade to the roller.
• Abrasive coatings resins and pigments – such as inorganic pigments, especially titanium oxide.
• Impurities in the coatings used.
• High viscosity of the coatings used / intense shear strain.

From all the above parameters, only viscosity can be controlled when a coating is applied. All CHROTEX products are accompanied with Product Application Guide (PAG) No1, indicating the appropriate solvent quantity according to the production area’s temperature.

The structure of tinplate is usually described as a sequence of layers involving an external tin layer, an intermediate tin-iron layer, and the steel substrate. The external tin layer is passivated by a chromate conversion coating in order to stabilize the surface and improve adhesion of coatings. It also carries a very thin coating of oil, in order to assure metal protection (chromium oxide and chromium) and adequate sliding properties of sheets or wraps of a coil.

The oil must be of a type 34 that is acceptable for use in food packaging. The materials in most common use are dioctyl sebacate (DOS), also for aluminum, butylstearate (BSO) or acetyl tributyl citrate (ATBC). Whilst not normally specified when ordering, the oil film usually has a thickness of 0.005 µm per side. Bigger quantity of oil may lead to spreading and adhesion problems at the lacquering stage.

Imperfect cleaning of the surface of  the tin plate in the steel industry can cause problems in the coating procedure, since a non-uniform film of oil on its surface usually produces a discontinuity (such as craters, bubbles – lack of adhesion) in the coating film.

Such defects may appear also by moisture or oil in the compressed air used in the sheet feeder blowers in the lacquering machine.

There are some commonly used tests in order to specify the existence of oil and the uniformity of its film. These methods give a useful quick check, before the coating procedure.

• The simplest test, which merely indicates the presence or absence of oil is the water-break test, in which tinplate sheet is momentarily dipped in water, and then examined for areas where the water remains.
• The uniformity of the oil layer can be determined by taking some sheets of the batch and sprinkle them by blowing a fine powder. The amount of powder adhered to the surface is approximately proportional to the thickness of the oil film.
• Another way to check the uniformity of the oil film is to put some sheets in a cold area for a few minutes, enough to cool to a low temperature. Then they have to be removed and a natural condensation of atmospheric humidity will occur on the surface of the sheets. The appearance of the condensed moisture film gives a very useful guide to determine the uniformity of the oil film.