Coatings for Sandwich Panels
COATING
Coil coating is a continuous process used to apply finishes to metal surfaces, including PVDF (polyvinylidene fluoride), plastisol, polyester, polyurethane and food-safe coatings. The coating type and thickness are selected for the geographical location and specific conditions of use. Unless otherwise specified, metal surfaces receive a 5-micron primer and a 20-micron polyester topcoat. Under normal conditions, away from heavy metals, severe air pollution, coastal exposure and acidic environments, polyester coatings have an indicative service life of 10–12 years. With appropriate cleaning and maintenance, service life may extend up to 40 years, depending on the coating and exposure. The primer and topcoat are important protective components of a sandwich panel. The primer helps the insulation material or adhesive bond to the metal, while the topcoat protects the metal against external factors and corrosion. Primed and coated metal coils then enter the panel production process.
Coil-coated colours used for sandwich panels are generally selected from the RAL colour chart. Colours may vary in shade when applied to metal, particularly metallic colours such as RAL 9006 and RAL 9007. Shade differences can occur even within the same production process. Different coating batches may also vary. To minimise these differences, the metal required for a project should preferably be coated in a single batch.
Comparison of Coating Properties
The durability of food-safe and antibacterial coatings should be assessed in the same way as polyester coatings. Heavy metals are not used in the production of these finishes. Warranties are available for the various coating types, but their duration depends on environmental conditions, geographical location and application, and therefore varies by project.
Polyester Coatings
Polyester is the most widely used coating because it is affordable, readily available and offered in many colours. It is a basic coating with lower resistance to corrosion, UV exposure and chemicals than more specialised finishes. Coils coated at different times, or panels installed on different façades at different times, may show differences in colour and gloss. These variations are particularly noticeable on dark panels. Additives to polyester resins can improve chemical and mechanical resistance, as in high-durability polyester coatings. These enhanced coatings cost more and are therefore selected according to project requirements.
PVDF Coatings
PVDF coatings offer greater chemical and mechanical resistance than standard polyester. They provide excellent UV resistance and long-lasting colour retention, together with strong resistance to chemicals, stains and dirt. High-gloss finishes are available. Their scratch and corrosion resistance is good, although lower than that of plastisol. Their higher cost makes them a common choice for prestigious buildings. PVDF coatings are recommended for locations with high UV exposure.
Plastisol Coatings
Plastisol coatings provide very high corrosion resistance. Their textured finishes also make them a popular aesthetic choice. UV and chemical resistance are good, although generally below PVDF. Wear and colour fading may occur depending on the location and climate. Their corrosion resistance makes them suitable for rainy regions and low-pitched roofs. They should not be used where surface temperatures may exceed 80°C.
Polyurethane Coatings
Polyurethane coatings offer greater resistance to UV exposure, impact and corrosion than standard polyester. They are a suitable option for tall buildings, particularly those 20 m or more in height. Their strong scratch resistance and attractive finish come at a higher cost than polyester coatings.
Film
PVC films provide high resistance to impact and chemicals. However, they are sensitive to UV exposure and have limited resistance to colour fading. Available in various colours and patterns, they are often selected for their aesthetic qualities.
