The importance of surface coating in Sandwich Panels

Galvanized metal sheets, or coated superficially with paints or organic substances, represent today the most commonly used materials in the production of metal sandwich panel surfaces and can be found on any facade or roof of any industrial, commercial or residential building.

The coating on the outside of a product is usually of vital importance. Both organic and metallic coatings are used to provide corrosion protection for the metal layers underneath. Therefore, the metallic parts of a product are subject to corrosion in the absence of an adequate surface coating, resulting in a reduction of the life cycle of the product in question.

A good product can be spoiled by a bad surface coating, just as a decadent product can be improved, at least from an aesthetic point of view, by a good quality surface coating. Coating a metallic product is not just a matter of applying paint or a noble substance to a particular product. Some attention should be paid to mechanical properties such as bending and impact; as well as to particular characteristics such as chemical resistance against attack by water, solvents, oils, hydraulic fluids, etc.

In short, the outer coating of an article not only gives it a certain aesthetic relevance, but also enables it to do the job for which it was designed. Obviously, this comes at a price, especially in the automotive industry, where the costs of applying the various surface coatings add up to about a quarter of the total production cost of a vehicle. For the general manufacturing industry, this figure is lower, but a careful analysis of costs is still warranted.

The choice of the most suitable corrosion protection system in the presence of certain environmental conditions, necessary to ensure a satisfactory life cycle for a sandwich panel, is of enormous importance and has been the subject of extensive discussion among industry experts. There is no single protection system capable of resisting attacks from all types of atmospheric agents. Therefore, it is necessary to identify the important environmental factors at a given location; for example, rainfall, local contamination and surface dirt deposits.

The actual conditions inside a building, the likelihood of chemical fume concentration and the possible formation of condensation will influence the choice of the most appropriate coating system.

The choice should carefully consider the functional requirements of the product and the local conditions expected to be encountered, and a consensus should be reached to give the product the best possible life cycle. For example, if we talk about coverings, the external aspect is of limited importance compared to the problem of corrosion protection while, for walls, the aesthetic aspect has a high priority.

The choice of the appropriate paint technology is also of great importance. On the one hand, environmental legislation on the toxicity of surface coatings, solvent emissions and wastewater treatment is becoming increasingly stringent; on the other hand, market forces impose both shorter manufacturing times and cost minimization. Given this overview, new technologies have been developed that should help some producers find an adequate solution to their problems.

The origins of the metal coil painting technique

The coil coating technique made its first appearance in the 1940s, but its roots seem to go back to architectural movements of the late 19th and early 20th centuries.

At the beginning of the last century, art, architecture and industry went hand in hand, forcing architects to integrate metal profiling techniques into the forms and concepts of traditional buildings. This new approach was well received by the construction industry and soon became the norm. As steel began to play an increasingly important role in the construction industry, surface coatings were developed in various forms, either to protect against weathering or for decorative purposes, or both. To this end, new techniques and materials were constantly researched and developed and, in the midst of this “frenzy”, the German Bauhaus school was founded, from which emerged some of the most important precursors of modern architecture, such as Le Corbusier, J.J.P. Oud, Gropius, Mies van der Rohe, and Frank Lloyd Wright.

As a result, the first chain painting line for metal coils began operating in the early 1940s in the United States. The process was used to coat steel window sashes with a width of 50 mm and a thickness of 0.3 mm. The production speed was 12 meters per minute, and it needed approximately 12 hours to produce one ton of coated metal (today, about 500 tons can be produced in the same time). This technology developed significantly in the United States in the 1950s and 1960s, when architecture and the appliance industry began to consider these new combinations of steel or painted aluminum.

During the 1960s, the chains reached a speed of 75 meters per minute, with a blade width of 1.50 m, and production grew from 460,000 tons per year in 1962 to 500,000 tons per year in 1966.

The metal coil painting technique experienced its true revolution in Europe between 1960 and 1965, when the first chains were installed in Sweden, Germany, Great Britain, Belgium, Italy, Switzerland and France. Pre-painted aluminum producers referred to the following markets in their trade literature:

construction (exterior cladding elements, coverings, non-load-bearing walls);
building accessories (metal fasteners, sliding doors, shutters, sheds);
interior applications (false ceilings, partitions);
transportation industry (caravans, buses);
miscellaneous products (packaging for consumer products, games, household appliances).

In 1967, the European Coil Coating Association (ECCA) was founded in Brussels, Belgium. The purpose of this scientific association was to “research and promote the production and use of metal pre-painted with organic material”. Its 80 founding members included steel and aluminum surface coating industries, distributors (paints, films) and equipment manufacturers. ECCA’s first objective, to define performance quality standards, is continuously fulfilled thanks to the cooperation that the Association has established and continues to establish with the CEN and ECISS Committees for the elaboration of European Standards, in particular:

EN 1396:1997, Aluminum and aluminum alloys – Painted metal in sheets and coils for general applications – Specifications.
EN 10169-1:2004, Flat steel chain-painted organic painted products – Part 1: General information (definitions, materials, tolerances, test methods)
prEN 10169-2:1995, Chain-painted flat steel products with organic material – Part 2: products for external building applications
prEN 10169-3:2003, Flat steel chain-painted products with organic material – Part 2: products for interior applications in buildings
EN 13523, Painted metals – Test methods

Surface coating systems for the protection of steel sheets

Generally, the metal sheets used for the manufacture of sandwich panels have both metallic and organic coatings to increase their resistance to corrosive agents.

Metallic coatings

The steel sheets commonly used in the production of composite panels are generally coated with a metallic corrosion protection layer. This coating forms a barrier between the external environment and the metal surface, especially when applied in combination with conventional organic sealants and coatings.

In an aqueous and high salt concentration environment, these coatings, due to the electrochemical reaction between steel and zinc or aluminum, tend to “sacrifice” themselves to protect the steel underneath in correspondence with possible holes or damage that may have occurred in the coating.

The contact between two dissimilar metals results in a galvanic cell, in which the zinc represents the anode and is “sacrificed” to protect the lower steel cathode.

Each represents a different zinc plating method: the corrosion protection provided by these methods depends on the amount of zinc present in the coating.

The main steel coating processes are electrolytic galvanizing and hot-dip galvanizing, although zinc-rich paints and metallization processes can also be used.

The method followed in the electrolytic electroplating process is called electroplating. The object to be coated is placed inside a container filled with a solution of one or more metal salts.

The object is connected to an electrical circuit and represents the cathode (negative pole) of the circuit; on the other hand, an electrode, which is usually made of the same metal used for galvanizing, constitutes the anode (positive pole).

When an electric current is passed through the circuit, the metal ions in the solution are attracted to the object to be coated. The final result is the formation of a protective metal layer on the object.

The main products used in the electrolytic galvanizing process are:

Pure zinc
Zinc-Nickel, with 10-14 percent nickel. It has better corrosion resistance and weldability than pure zinc, but its hardness has a negative effect on mechanical properties.

Galvanizing is the process by which a metal, usually iron or steel, is coated with a protective layer of zinc. Hot-dip galvanizing is a widespread galvanizing method. It consists of coating steel or iron products with a thin layer of zinc by immersing the metal in a bath of molten zinc at a temperature of approximately 460 °C. The zinc “oxidizes” and forms zinc oxide, a very strong material that stops further rust formation, protecting the metal underneath from corrosive agents. Galvanized steel is widely used in applications where a certain rust resistance is required, and can be identified by the formation of a crystallization pattern on the surface, as shown in Fig. 4.6.

The main products used in the hot dip galvanizing process are:

Pure zinc
Galvanized annealing, consists of passing the hot-dipped material, as soon as it comes out of the molten zinc bath, through a heat treatment that allows the formation of an alloy of zinc and iron. Compared to pure zinc, the former allows greater ease of welding and superior corrosion resistance, but is more sensitive to sputtering (deterioration of the metallic coating).
Galfan is a zinc-aluminum coating with 95% zinc and 5% aluminum. It provides better corrosion protection than pure zinc, but its weldability is low due to the presence of aluminum.
Aluzinc or Galvalume is a zinc-aluminum coating with 55% aluminum, 43.4% zinc and 1.6% silicon. Compared to pure zinc, the former offers better corrosion resistance and higher heat reflection at high temperatures.

Zinc-rich paints, or zinc-rich primer, provide galvanic protection to a steel substrate. As the adjective itself says, “rich”, these contain a high amount of zinc powder as pigment. Once the coating has been applied to the previously cleaned steel substrate, a binder holds the metallic zinc particles together and in contact with the steel layer.

The metallization process is a widely used method that consists of coating metals using a metal that is thermally sprayed onto the surface to be coated. This technology includes various techniques and materials and has a wide range of applications. In the case of surface coating of steel structures, metallization refers to the hot spraying of zinc or aluminum alloys directly onto steel surfaces. The coatings are obtained by using a heat source (flame or arc) to melt the metal, which is delivered as a wire or in powder form. A jet of air sprays the molten metal onto the steel surface in the form of a thin film. When the metal hits the surface to be coated, it quickly solidifies again to form a solid coating layer.

Organic coatings

It is difficult to obtain effective adhesion between the panel core and the inner metal surface of the sandwich panel, so additional organic coatings are used. These also have the function of protecting the metallic layer from mechanical and chemical connections; as well as providing the surface with a satisfactory aesthetic appearance. In any case, since these are permeable to water and degrade under the influence of UV rays, the organic coating is not applied directly on uncoated steel, but on a galvanized steel or aluminum substrate.

In any case, the most common system of corrosion protection is achieved by galvanized steel coated with an organic coating, as shown in Fig. 4.7. The organic coating is primarily responsible for corrosion resistance, while the zinc layer remains in a passive state until the protection provided by the organic coating loses its effectiveness.

The surface coatings industry today adopts a wide variety of organic coatings. The most common types of organic coatings include polyesters, acrylics, polyfluorocarbons, alkyds, vinyls and plastisol. Approximately 85% of the organic coatings used are organic solvent based, while most of the remaining 15% are aqueous based.

The most commonly used organic coatings are:

polyester (standard or silicon-modified and polyamide)
polyurethane
plastisol
fluoropolymers

Polyester (PE) is a universal, cost-effective paint system suitable for both interior and exterior applications. For interior applications, the coating thickness is generally 15 µm while for exterior applications it is 25 or 35 µm (including the primer layer).

In addition to standard polyester, there are also variants in the form of silicon- and polyamide-modified polyesters. Compared to the standard product, these variants offer high resistance to outdoor environmental conditions and good resistance to both scratches and abrasions.

Polyester and its variants are available in a wide range of colors and sheens. They are used for a wide variety of indoor and outdoor applications, such as covering and wall elements for refrigerators and washing machines; for furniture cladding, etc. Its major advantage lies in its price, while its main disadvantage is its limited flexibility. This material is not suitable for bending radii that are too small.

Polyurethane (PU or PUR) has a higher resistance to aging and staining than polyester. The coating thickness is normally 50 µm (including the primer layer). Application examples include cover and wall elements, garage doors, refrigerators, washing machines, beverage dispensing machines, etc. The most relevant advantages are good corrosion resistance with good color and gloss retention, as well as exceptional flexibility. Polyurethane paint systems are more expensive than polyester systems.

Plastisol (PVC or polyvinyl chloride) is applied as a coating in relatively high thicknesses: 100 or 200 µm. This gives the plastisol excellent abrasion and corrosion resistance. It is also a very flexible product that can withstand very small bending radii.

The most important advantages are good corrosion resistance, high flexibility and a tendency to be embossed to improve scratch resistance. In any case, color stability is not satisfactory, since resistance to ultraviolet (UV) light is very limited. This low UV resistance is more pronounced with dark colors. In addition, its deformability is very low at low temperatures. Plastisol is relatively expensive, and its applications include roofing elements and garage and furniture walls and doors.

Fluoropolymers (PVDF or polyvinylidene fluoride) offer unmatched color and gloss retention due to their exceptional UV resistance, making PVDF particularly suitable for prestigious buildings painted with vivid colors. The coating thickness is normally 25 or 35 µm (including the primer layer).

PVDF combines many advantages: it is a product that can be used without problems even in continental areas with very high ultraviolet radiation combined with high temperature and relative humidity values. In any case, this product is very hard, and it is necessary to handle it carefully to avoid scratching it. Due to its high cost, PVDF is used only for exterior applications in difficult environments or when the aesthetic aspect is of basic importance (with the multi-layer versions it is possible to obtain special colors and surface effects).

The corrosion resistance of organic coatings can be expressed with a scale ranging from 1 (low) to 5 (excellent), as shown in Table 4.1. In this context, resistance is mainly related to the time required until the first maintenance operation, and not to the total deterioration of the blade. The color stability is also shown in the table.

Material	Corrosion resistance	Color stability
PE	3 – 4	4
PE (silicon modified)	3	4
PUR	4	4
PVC	5	3
PVDF	3 – 4	5

Table 4.1: Corrosion resistance and color stability of some coating materials

The normal values for the thicknesses and for the deformability of the organic coatings mentioned are shown in Table 4.2. The resistance to fracture by bending at 180° is given as the ratio T between the minimum value of the bending diameter and the thickness of the metal, when the coated sheet is bent at 180° with the painted side facing outward and with no visible fractures in the coating itself (values are given at room temperature).

Material	Normal thickness (µm)	Crack resistance with 180° bending, T	Minimum value of the interval of operating temperature
PE	25-30	4-5	– 40
PE (silicon modified)	25-30	6-10	– 40
PUR	20-50	3-4	–
PVC	100-200	0	– 20
PVDF	25-30	3-4	– 40

Table 4.2: Normal values of thicknesses and deformability characteristics of some organic coatings.

The expected service life values in both outdoor and indoor environments for the different organic coatings mentioned above are shown in Tables and 4.4. These values are indicative and define maintenance requirements rather than the duration of the protective characteristics of the coating. The expected values are expressed by the following symbols:

+++= long service life
++ = average useful life
+ = short service life
– not recommended

Material	Coastal	Industrial	Urban	Rural
PE	+	+	+	++
PE (silicon modified)	+	+	++	++
PVC	++	++	++	++
PVDF	+++	+++	+++	+++

Table 4.3: Lifetime of organic coated galvanized steel in outdoor environment

	Ambient temperature + 5°C ? T ? +50°C			Refrigeration cell
Material	No condensation.	Condensac. occasional	Condensac. permanent	– 20° C < T < + 5°C	T < -20° C
PE	+++	+++	–	+	++
PE (silicon modified)	+++	+++	–	++	++
PVC	+++	+++	+	++	–
PVDF	+++	+++	–	+++	++

Table 4.4: Lifetime of organic coated galvanized steel in indoor environment

The service life of an organic coating can be extended indefinitely if damage to the coating is prevented by frequent cleaning and appropriate repainting. In practice, this is possible for most of the organic coatings used.

The critical factor of duration is often expressed in the fixings. It is generally possible to replace them, but only at high costs.

The durability of a coating is also highlighted by its color retention over time, which is as important a factor as the initial color. In fact, colors can change under the influence of sunlight. The maximum allowable deviation is indicated as delta E or ?E. The higher the ?E, the lower the color retention. Some colors are more prone to aging than others; in this case, the type of paint is a significant factor. Color variations are due to pigment variations or, less frequently, to oxidation of the resins (yellowing).

The appearance of a coating is determined not only by the color, but also by the degree of gloss, which is quantified by a measure of the reflectivity of the painted surface.

The more light the surface reflects, the higher the degree of gloss. As with color, it is not only the initial gloss that is important, but also its retention, which is expressed as a percentage of the initial gloss. The higher this percentage, the greater the gloss retention.

Painting process

Coil coating is the chain process by which layers of organic coating with protective or decorative functions are applied to galvanized metal supplied in coils. Although the layout of the equipment used may vary from one facility to another, the operations usually follow a well-defined pattern.

A strip of metal is uncoiled at the entrance of the chain and passed through a wet section, where the metal is cleaned and chemically treated to inhibit rust formation and promote effective adhesion of the surface coating to the metal surface. In some installations, the wet section includes an electrolytic galvanizing operation.

The metal strip is dried and passed through a primer unit in which a primer coat is applied on one side or both sides. At this stage of the process, a layer of backcoat can be applied to the underside of the metal strip.

The strip with the primer coat applied is passed through an oven for the necessary treatment. When it comes out of the oven, the metal strip is cooled with water spray and dried again.

The metal strip then passes through an organic surface coating application station, where suitable rollers coat one or both sides of the strip.

The direction of rotation of the application rollers plays an important role in determining the type of surface coating. Rotating the application roller in the opposite direction of the stripe produces a thick coating, while rotating the application roller in the same direction as the stripe produces a thinner coating.

The strip then passes through an oven where the surface coatings are dried and treated.

The coating is “baked” at a high temperature for 20-30 seconds. As soon as the strip comes out of the oven, it is cooled with water spray and dried.

Most painting lines have accumulators at the infeed and outfeed that allow continuous movement of the strip through the process, when a roll is placed at the infeed and a roll that has just been painted is removed from the outfeed. Figure 4.9 is a flow chart of a paint line.

Painting operations can be classified into two distinct categories of operation: toll coaters and captive coaters.

The toll coater is a multi-customer service that respects the needs and specifications of each customer. The coated metal is delivered to the customer, who produces the final product. Toll coaters use very different organic coating formulations and, as a rule, use organic solvent-based products. The main markets targeted by these operations are the transportation and construction industries, as well as household appliances, furniture and containers.
The captive coater is generally a single operation in a complete manufacturing process. Many manufacturing companies have their own painting facilities. The captive coater uses mostly water-based products, as coated metal is usually used for a limited number of products.

Advantages

Modern painting lines have integrated mechanisms for wastewater and fume treatment and, in general, more than meet the legal requirements regarding emission levels. The coated and cleaned metal lends itself well to further processing.

In addition, the surface coating:

ensures color continuity
ensures paint thickness uniformity
offers remarkable adhesion properties
is durable and minimizes the potential for travel-related damage
is simple to deform and elaborate
its finish can be in a wide range of colors
provides higher corrosion resistance
requires less energy than traditional processes
allows for simple recycling of processed materials
is a practical and efficient system.

These characteristics have convinced many metal producers to switch from post-painting operations to the use of pre-painted metals.

Powder coating process

Powder coating is by far the most recent surface finishing technique in use today. This is the application of dry paint to a product. In normal wet painting, the pigments are suspended in a liquid medium, which has to evaporate to allow the solid paint to be obtained. With powder coating, the paint is applied using two different techniques:

the article is immersed in a bed of powder, which may be more or less electrostatically charged; or
powder paint is electrostatically charged and sprayed onto the product.

The article is then placed in an oven where the powder particles are melted and spread to form a continuous film. Today, there are two main types of powder available on the market:

thermoplastic powders that melt again if heated and
heat-resistant powders that do not melt if brought back into contact with a heat source. During treatment in the furnace, a chemical reaction takes place that facilitates the formation of strong bonds between the molecules. This chemical reaction is what gives the coating many of its properties.

Powder coating is relatively hard and abrasion resistant and can be applied over a wide range of thicknesses. In addition, color variations are accepted from one production batch to the next.

Special care must be taken when defining the minimum thickness, as some powders do not provide the requested “coverage”, i.e. the ability of the powder to cover the color of the metal.

RAL color classification system

Today there are several color classification systems that allow professional designers to identify the right color for a specific application or product. These systems generally contain thousands of colors, which are arranged using various support instruments such as catalogs or pocket atlases (Fig. 4.11). In this section we will discuss two systems of particular importance: RAL DESIGN and NCS.

RAL DESIGN

After several years of development and with the advice of several industry experts, the RAL DESIGN system was publicly presented in 1993. An important advantage of the RAL DESIGN system is its great clarity and intuitiveness, despite the large number of colors it contains (1,688 colors). In fact, it is very easy to identify colors thanks to a set of three numbers that define exactly the coordinates of the desired color in a spherical color palette (Fig. 4.12).

The first digit of a color name indicates the angle on the color circumference (hue), the second the elevation of the position occupied by the color (lightness), and the third the distance from the central axis (chroma), as shown in Fig. 4.13.

Therefore, a color (e.g. RAL 010 40 25) can be accurately described using only its HLC coordinates, which define the technologically measured values of hue, lightness and chroma.

Hue is the property of color that is determined by the wavelength of light arriving from the object. It is the property we usually refer to when we indicate a particular color (red, green…); lightness is the property that indicates to what extent a given color is light or dark; chroma is the property that defines the intensity or purity of a color.

NCS (Natural Color System)

The NCS system describes colors exactly as we see them, which explains why it is simple to understand, as well as logical and easy to use.

Each of the millions of existing colors can be defined within the NCS system and is indicated by a precise notation.

The NCS system starts from six elementary colors, perceived by the human brain as “pure”. These colors are divided into four elementary chromatic colors, yellow, red, blue, and green; and two elementary non-chromatic colors, black and white.

All other colors can be described depending on their degree of visual resemblance to these elementary colors.

Therefore, NCS classification notations are based on these elementary resemblance attributes.

The six elementary colors are considered the cardinal points of the three-dimensional space NCS (Fig. 4.15), which can be considered as consisting of two cones with a common circular base. In this space, all imaginable colors can find their location and be indicated by an exact NCS notation.

The circular base is called the NCS color circumference (Fig. 4.16) and is divided into four quadrants.

In this circumference, the color is identified by its hue, which identifies its degree of resemblance to two or more of the four elementary chromatic colors, yellow, red, blue and green.

The illustration highlights the Y90R shade, a yellow color with a 90% tendency towards red and a 10% tendency towards yellow.

The vertical section of the NCS color space, for a given value of hue, is called the NCS color triangle (Fig. 4.17). The base of the triangle is the gray scale from white (W) to black (S); while the vertex of the triangle represents the maximum chromaticity (C) for the current hue value (in the example given, Y90R).

The triangle identifies the hue of the color, that is, its visual amount of black or white, as well as its chromaticity.

In the example given, the NCS notation for the color under consideration is S 1050-Y90R, with 1050 representing its hue and Y90R representing its tone (Fig. 4.18). The letter S written in front of the full NCS notation indicates that the NCS model is a standardized NCS color model, issued by the Scandinavian Color Institute, the NCS quality center.