Raw Materials used in the manufacture of Sandwich Panels
A typical sandwich panel has a three-layer structure. The rigid surfaces, with a relatively high modulus of elasticity, are held at a distance by a lightweight core, which has sufficient bending stiffness to withstand most of the shear stresses. The core also acts as a highly effective thermal insulation layer.
The growth in the use of composite panels is mainly due to the construction industry’s need for a lightweight panel that also has high thermal insulation values and is at the same time easy to install.
The first requirement has been satisfied thanks to the technical development of rigid polyurethane (PUR) and polyisocyanurate (PIR) foams, capable of offering high thermal insulation properties, especially when compared to materials commonly used in construction. The second characteristic, consisting in the simplicity of assembly on the supporting structure, has proved to be one of the main factors in the popularity of this product, since construction times have been significantly reduced compared to traditional methods, with consequent savings in labor costs. Over the last 5-10 years, the product line has expanded with the development of rock wool sandwich panels. Originally developed and tested for use in fireproof applications, such panels are now more commonly used to meet sound insulation and absorption requirements. In short, thanks to a large number of favorable characteristics, sandwich panels are an essential element in the building applications of the future.
Metal surfaces
Relatively thin, high-strength sheets are generally used for exterior metal surfaces. These must meet the following requirements:
- Production requirements for roll forming and bending
- Functional wind resistance requirements
- Impermeability to water and steam
- Structural strength characteristics, and ability to resist local loads
- Adequate corrosion and fire resistance.
Not all of these requirements are of equal importance in every application, but it is clear that they are economically satisfied with sheet metal, especially steel and aluminum.
Therefore, the metals used are:
- galvanized, painted or bare steel
- aluminum, painted or bare
- stainless steel
- copper.
The metal sheets are supplied by the supplier in rolls (Fig. 2.1 and 2.2), and can be easily incorporated into a continuous production process, and can be easily shaped for roll forming.
Galvanized steel
Sandwich panels with aluminum surfaces are sometimes used in applications where there are special corrosion resistance or hygienic requirements, for example, in the production or storage of food products. The aluminum used is an aluminum alloy coded 3003 – 3103. The thickness normally used varies within the range 0.7÷1.2 mm. A thickness of 0.7 mm is often considered as the minimum value to avoid local damage related to displacement and treading, but sometimes a thickness of 0.6 mm is also used.
Other materials
Stainless steel is normally used in applications characterized by important hygienic requirements, or where a high resistance to an aggressive indoor environment is required. For this reason, stainless steel surfaces allow for high quality coatings that require limited maintenance. The corrosion resistance of stainless steel is mainly due to its chromium content, which prevents the oxidation of iron.
Copper is also an alternative material used to reduce the need for maintenance in building envelopes. The corrosion resistance offered by copper is due to a thin oxide layer that gradually forms on the surface, making the composite panel suitable for rural, urban and also marine environments. The original color darkens due to oxidation. Full oxidation is obtained in 4÷6 years in marine environments, 8÷15 years in urban environments and may require 20 to 50 years in rural environments.
From a production point of view, the adhesion between the stainless steel and copper surfaces and the core is similar to that obtained using galvanized steel and aluminum. To ensure satisfactory adhesion to the core, the sides of the metal surfaces that come in contact with the foam are coated with a suitable primer. For this reason, the metal coils are delivered by the manufacturer with the inner side coated with a 5 micron thick layer of a special paint called backcoat.
Rigid foams
The rigid foams most frequently used in the production of composite panels are:
- polyurethane / polyisocyanurate (PUR/PIR)
- phenolic resin (PF).
These two materials have a closed-cell structure, with approximately 90% of the material being closed-cell, and low thermal inertia. In addition, they are called thermosetting, which means that, once molded, they cannot change their shape due to the extensive formation of bonds between molecules.
Polyurethane / Polyisocyanurate (PUR/PIR)
The main components of polyurethane and polyisocyanurate foams are:
- polyol
- isocyanate
- an agent of expansion
- an activator to control the reaction
Until recently, the blowing agents were almost invariably chlorofluorocarbons, the use of which has been banned under the Montreal Protocol, as they are known to be one of the causes of ozone depletion. Today, the most commonly used blowing agents in the production of composite panels are various forms of pentane and water, which release, upon reaction with isocyanate, carbon dioxide CO2.
In some cases, a flame-retardant agent may be injected into the mixture to increase the fire resistance of the panel. The main drawback if flame retardants are used is related to the increase of black smoke produced in case of fire.
Once the chemical components have been mixed (Fig. 2.3), the liquid begins to foam and expand rapidly (Fig. 2.4). The time from the first mixing of the components to the hardening of the foam is between 3 and 6 minutes, depending on the desired thickness of the foam layer. Since the chemical reaction is exothermic, temperatures of over 150 °C can be reached in the core of panels with a thickness of over 100 mm. It is therefore necessary to store the thicker panels for at least 24 hours so that the hardening and cooling phase can be completed and the panels can be shipped.
Polyisocyanurate (PIR) foams differ from pure polyurethane (PUR) foams only in the mixing ratio of the components, i.e. polyol and isocyanate. This ratio is approximately 100:150 compared to 100:100 for PUR. Therefore, there is more isocyanate in PIR than in PUR. This difference in composition gives the final material different properties due to the different chemical structure, even if the foaming process and the mechanical and physical properties are normally similar.
PIR foams are used only for their superior thermal stability and fire performance characteristics. While a pure polyurethane foam gradually decomposes if exposed to temperatures above 250 °C, a PIR foam generally withstands temperatures above 350 °C before it begins to decompose. In addition, a stable carbonized layer is formed in the latter, which significantly improves fire performance. This improvement in fire performance is obtained at the cost of a more expensive manufacturing process, since the chemical reaction requires (to take place) a temperature equal to approximately 40÷45 °C, i.e. equal to twice that required for a polyurethane foam reaction.
The structure of the hardened foam consists mainly of closed cells that are separated from each other by thin membranes (Fig. 2.5), in contrast to the open-cell structure that characterizes flexible foams (Fig. 2.6). The cells contain a blowing agent and usually also some traces of carbon dioxide, CO2.
The CO2 exits very quickly through the membranes and after a while, the result is that the closed cells contain mostly blowing agent which has excellent insulating properties. Air can then diffuse out of the foam but this has little influence on the insulating properties.
Classification of polyurethane foams (PUR)
Polyurethane foams are normally classified, with regard to their reaction to fire, according to the German test method as defined by DIN 4102-1. According to this standard, a polyurethane foam, charged with flame retardant agents, shows better reaction to fire characteristics and can be classified as B2, while all others fall into class B3.
The need for such a classification arises from the fact that, in order to obtain the German Zulassung classification, polyurethane foam must be classified as B2. For this reason, this convention is now widely accepted by polyurethane foam manufacturers (including Metecno), who identify the material with the best reaction to fire characteristics as PUR B2, and a polyurethane foam with “standard” properties as PUR B3.
The same rule also applies in France where, in order to obtain the French classification, Avis Techniques, the polyurethane foam with the best fire properties is classified as M2, although it is exactly the same material used to obtain the Zulassung. Clearly, in order to obtain such certification, the foam must be tested in accordance with French legislation on this matter, which may differ from German legislation.
In conclusion, B2 and B3 identify a type of polyurethane foam with special fire resistance characteristics.
Phenolic resin foam (PF)
The search for increasingly higher fire safety in buildings with composite panels has led to the consideration of phenolic rigid foam, also made of thermosetting material, as the core of sandwich panels. Compared to other rigid foams, it has very low thermal conductivity and excellent fire performance, including:
- high ignition resistance
- slow combustion times
- very low smoke emission rates
- emission of invisible fumes.
Phenolic foam is produced from liquid formaldehyde resin, which is mixed with a highly volatile solvent as an expansion agent, and an inducible agent. With the application of a temperature field, the mixture begins to foam and then to harden.
Phenolic foam is preferably produced in slabs, which are then cut into sheets that are subsequently assembled with the metal surfaces with adhesive substances. In fact, the production of phenolic foam is accompanied by a considerable amount of remaining acidic water, which prevents an easy continuous lamination process with the metal surfaces.
In addition, phenolic foam is a rather friable material, which requires some care for ceiling or soffit applications, which may be subjected to foot traffic; in these cases, early delamination may occur due to the effect of repeated pressure applied.
Characteristic properties of rigid foams
Density
Foam density is of great importance because the cost of the material affects the final cost of the finished product to a greater extent than the cost of production, so the objective is to obtain the same physical properties with the lowest possible density.
Most of the mechanical properties of foam are related to its density. The density of the rigid foams used by Metecno in the production of sandwich panels can vary within the following ranges:
- Polyurethane (PUR) B2: 40 ± 4 Kg/m3
- Polyurethane (PUR) B3: 38 ± 4 Kg/m3
- Polyisocyanurate (PIR): 45 ± 5 Kg/m3
Thermal insulation
Heat flow through rigid foams is mainly due to heat conduction through the gases contained in their cellular structure. Thermal conductivity is significantly influenced by the type of gas trapped in the foam cells, and most blowing agents are efficient in this regard.
In PUR the thermal conductivity value is approximately 0.020÷0.024 W/m°C immediately after production. Thanks to the gas impermeability effect offered by the metal surfaces, subsequent variations in the composition of the gases trapped in the cellular structure of the foam are limited, although the long-term value may increase up to 0.024÷0.030 W/m°C.
Glass wool slabs can be produced in the same way, starting however from a fusion of quartz sand, sodium carbonate, and lime, or recycled glass. In another production method (the so-called TEL method), shown in Fig. 2.9, the melt is pressed or sucked through small nozzles with compressed air. The properties of glass wool are similar to those of rock wool, except for a lower melting point and a higher amount of binding agent, usually between 4 and 15%.
By adjusting the belt speed and other process parameters, slab density and thickness can be quickly varied.
Due to the production process, all mineral wool slabs are highly orthotropic. Indeed, the longer fibers are aligned along the conveyor belt and retain the same orientation in the finished slab (Fig. 2.10). The shorter fibers have a more casual orientation, which is why mineral wool slabs are stiffer in their plane. In any case, such slabs owe much of their stiffness and strength to the binding agents used.
Of the above-mentioned types, mineral wool, with natural rock as the starting material, has the best resistance to high temperatures and the best resistance to moisture. In addition, the fibrous structure does not have closed pores, making the slabs much more susceptible to water absorption and vapor diffusion. With the addition of suitable additives, the water absorption of rock wool can be reduced to values lower than those of polystyrene.
Characteristic properties of mineral wool
Density
The density of the mineral wool used for sandwich panels can be considered to vary in the range 90÷145 Kg/m3.
Mechanical properties
Mineral wools cease to behave elastically when the fibers and the adhesion between them give way. Strength increases with density, but depends more on the internal structure of the wool than on density alone.
The compressive strength in the direction normal to the fiber orientation typically varies within the range 0.005÷0.08 N/mm2. The corresponding tensile strength is lower and within the range 0.001÷0.01 N/mm2.
The corresponding properties in the direction parallel to those of the fiber are much higher.
The shear strength varies from 0.03 to 0.20 N/mm2, and the corresponding shear modulus varies from 2 to 20 N/mm2.
Tensile strength is between 0.03 and 1.0 N/mm2, and the corresponding modulus of elasticity is between 5 and 40 N/mm2.
The compressive strength varies in the range 0.10÷0.15 N/mm2, and the corresponding modulus of elasticity in the range 6÷20 N/mm2.
Water absorption
Under normal conditions of use, water absorption by mineral wool is low and, in composite panels, due to the protection offered by the external surfaces, this is normally reduced to 0.2 ÷ 0.5 %. The water absorption of mineral wool can be further reduced by using silicone, mineral oil or other additives. The water absorption of rock wool is lower than that of glass wool, even when the binder content is lower. This is due to a difference in the internal structure of the material.
Thermal insulation
Compared to rigid foams that have a closed-cell structure, the thermal conduction of air in wool has a high influence on heat flow. In fact, approximately 75% of the heat flow is due to convective and conductive phenomena related to the presence of air. The thermal conductivity measured in mineral wool slabs is practically constant in the density range of 60÷150 Kg/m3, and is equal to 0.033÷0.034 W/m°C.
Combustibility and other properties related to the presence of potential fires
Mineral wools with a low organic binder content are practically non-combustible. Since the binder content in glass wool is generally higher than 5%, glass wool is generally not classified as non-combustible. The fibers themselves do not burn but rather melt; glass fibers melt at 650°C, while rock fibers melt only at 1000 °C.
