Liquid Paints

Liquid paints are mainly used for the coating process of prepainted metal and comprise some principal ingredients: resin (binders), pigments, solvents and additives. During the curing process the solvent evaporates and leaves the pigments surrounded with resin glued on the substrate. The solvent is not incorporated into the finished product. Before the solvent is dried, the pigments are homogeneously distributed on the metal surface. The resin holds the pigment particles and protects them from environmental influences and pollutants (including UV radiation and industrial pollution).

There are two types of pigments available: organic and inorganic. The inorganic pigments are basicially the same as they were used for making porcelain. They are manufactured from various metal oxides by various costs. In essence, they all are considered premium products with enormous colour stability and a very good resistance against chemical agents. Unfortunately not all colours are available for metal oxide and commonly they are blended with organic pigments.

In general when a liquid paint coating is called

• Acrylic
• Epoxy
• Polyester
• Plastisol (PVC)
• Polyurethane
• Polyvinylidene flouride (PVDF)

it refers to the polymeric resin. In the industrial coating process resins are blended from several ingredients (additives) in order to achieve specific coating properties like gloss, or resistance against mechanical influences. Some additives are included to optimize the production process.

After the pretreatment of the steel sheet, primer and topcoat are applied. This two-coat system (primer and top-coat) is widely used for the commercial cladding industry. Of course there are highly differentiated coated products available with more than two layers. Almost any product requirement regarding corrosion protection, flexibility or final visual appearance can be achieved in an even more complex and highly controlled coil coating process with liquid paints.

In Figures 1-2 the reverse side of the substrate has a zinc layer, a pretreatment layer, and a protective layer coat. For many applications only one side of the strip is exposed to weather and environmental conditions and abrasion. Therefore only one side needs to be covered with a strong protective coating layer system. The backside is significantly less exposed and usually protected with only one layer of liquid paint. The final application of the coated strip demands enormous flexibility for every single part of the coil coating process. In the construction industry for the backside coating it may also be required to ensure good adhesion of foams as insulation.

There a many more coating concept available even with more than 2-layers. In this case usually the already coated metal strip has to pass throught the coating line twice.

Laminated Films

Another widely used coil coating method is the protection of the metal substrate with laminated films. Those stand-alone film products can be modified easily and as a result a wide variety of laminates exists in many different colours, gloss degrees or with embossed textures. With the same flexibility for the visual appearance and decor all aspects of the film resistance functions were investigated. Some film products have an enormous resistance against chemical agents and/or abrasion others against weather exposure like sun, rain or heat. In the fields of appliances, furniture, clean-rooms and ship-building, laminated film protected metal strips are the preferred solution.

Laminated film products are typically bonded through a molten glue layer between the laminate and the metal substrate. Some laminates are thermally laminated onto a preheated strip in a continuous process without an adhesive layer. The film melts onto the strip.

The main chemical types of laminate are:

• Polyvinyl chloride (PVC)
• Polyvinyl flouride
• Polypropylene (PP)
• Thermoplastic acrylics
• Polyethylene terephthalate (PET)

PVC and PET laminates are also used as food safe (FDS) protective layer. Due to higher scratch resistance and a better corrosion protection than most lacquer systems laminated metal products have been particularly established for some applications including: aerosol cans, easy open ends for food cans, components for containers of water based products.

Laminate film products exist in uncountable designs from wood-like, stone-like, or any other individual printed picture to embossed designs making images appear more realistic or to achieve more abstract effects. There are almost no limitations and this might be one of the reasons that especially in the furniture industry laminated metal sheets are quite prevalent.