Sandwich Panel Technical Properties

The technical information presented below is not specific to any particular manufacturer; it consists of general comparative data intended to provide guidance:

1. Mechanical Resistance

Variables such as compressive, tensile and shear strength and the shear modulus are the most important values that determine the mechanical resistance performance of composite materials. Furthermore, in composite materials, resistance is directly related to the interaction between the components. Materials such as polystyrene foam and rock wool are bonded to the metal facings with additional polyurethane-based adhesives, whereas Polyurethane, PIR and Phenolic foams expand from a liquid mixture and self-adhere to the metal facings. Although EPS and XPS are good insulation materials thanks to their inexpensive production process, their need for additional adhesive means they are rarely preferred in sandwich panel applications. Generally, although the tensile strength of the adhesive on the metal facings exceeds the tensile strength of the polystyrene core material, analyses require that this value not fall below 0.10 N/mm2. Otherwise, potential stresses cause distortion at the surface. In addition to the possibility of the properties of such polymer-based materials being affected by ambient temperature and humidity, the visco-elastic behavior of foam materials after long-term loading, i.e. creep, must be carefully evaluated during the sandwich panel design stages. While the mechanical properties of sandwich panel plastic foams are directly related to density, in materials such as rock wool this is not the primary factor and can vary with the production process, such as the characteristics of the fibrous structure. The only difference between PIR foams and PUR polyurethane foams is the difference in the proportions of the components used in their production. Consequently, in addition to the foaming process, the mechanical and physical properties of both are very similar.

Problems have recently been identified regarding the mechanical strength of Phenolic foams, which have been developed since the 1970s to further improve the fire performance of polyurethane. In particular, Phenolic-based sandwich panels preferred in roof applications with significant amounts of foot traffic have been observed to suffer early panel delamination over time, thought to be caused by the Phenolic structure. In structures subject to repeated loading, the properties of the sandwich panel core material must be very well understood.

2. Thermal Insulation

The thermal conductivity value (λ) is a constant specific to each material. It is the expression in Watts (W) of the heat flow that occurs through a material 1 m thick with a surface area of 1 m² when the temperature difference between the inner and outer surfaces is 1 Kelvin. The table below shows the thermal conductivity values of typical insulation materials. A low thermal conductivity value means high thermal insulation performance, and the desired performance can easily be achieved with a low insulation material thickness. In all insulation materials, the thermal conductivity value varies with temperature. However, this relationship is generally linear. When determining material thickness in calculations, a temperature of 10 °C is taken as the basis for normal applications, and the thermal conductivity design values are determined according to this temperature.

Although the initial thermal transmittance value is a very important variable in choosing the optimum thermal insulation material, the reduction of thermal insulation capacity over time, especially in plastic foams, means the insulation values of the aged material must also be taken into account. When comparing low insulation values, and considering their versatility and long-term performance, we can state that PUR/PIR foams offer the most optimum solution. Although the closed-cell structure of plastic foams is more resistant to water, contact with water in inorganic materials such as rock wool can cause serious reductions in thermal performance of around 75%.

3. Water Absorption Capacity

What is expected of an insulation material is that it should not be directly affected by water. Moreover, it should not become indirectly wet through capillary action and thereby increase its thermal transmittance value. The fibers in rock wool do not actually get wet, but the air gaps between the fibers fill with circulating water on contact with water, and the wetted rock wool becomes unable to perform its insulation function.

4. Density

By targeting the best performance at the lowest possible density while keeping production costs in mind, the amount of material consumed can be reduced. Especially in plastic foams, density directly affects mechanical strength. On the other hand, because of the weight of sandwich panels produced with high-density rock wool, they cause both installation difficulties in structures and additional loads on the load-bearing construction. The thermal conductivity coefficient depends on many different factors that are not time-dependent. Examples of these factors include the percentage of closed cells, the sample being thick, a longer diffusion time, a slower change in the thermal conductivity coefficient, cell size, the type of blowing agent and other factors. The graph below shows the effects of density on the thermal conductivity coefficient.

5. Temperature Resistance

Every thermal insulation material has a temperature point at which it loses its properties and begins to lose its shape. For this reason, the temperature to which the material will be exposed at the point of application must be determined in advance and a material suitable for that temperature must be selected.

6. Water Vapor Permeability

Water vapor permeability is very important for comfort in buildings. The water vapor diffusion resistance (µ) and the diffusion-equivalent thickness (Sd) are two important characteristic values. The water vapor diffusion resistance (µ) value is specific to the material and is determined by comparison with the resistance of air, which is taken as 1. In sandwich panel systems, water vapor permeability also depends on the density of the material, the joint details, the production process and the type of metal facing. As the vapor diffusion resistance increases, the amount of vapor that can pass through the material decreases. What is expected of a thermal insulation material is a high vapor diffusion resistance coefficient.

7. Sound Insulation

The variables and calculation methods required for the sound insulation of roof and facade claddings are determined at the design stage. Part of a sound wave striking a surface is reflected, part is absorbed and the remaining part is transmitted. The rates of reflection, absorption and transmission depend on the shape of the surface, the sound-absorption of the material and the frequency of the sound. Closed-cell plastic foam materials are essentially thermal insulation materials. For the insulation of airborne sound, open-cell materials (such as glass wool, rock wool, acoustic sponge) are used. Sound-absorbing materials are porous or fibrous materials that are effective by causing friction losses in the air entering the cavities in their structure, thereby converting part of the acoustic energy into heat energy. The contribution of rock wool-filled sandwich panels to sound insulation is far more successful compared to other panels.

8. Dimensional Stability

The volume of all plastic foams is affected by temperature changes. As the temperature of the foam increases, the pressure of the gases inside the cells increases, causing expansion. In the case of sudden cooling, the opposite occurs. Due to the negative pressure, there is a possibility of damage to the foam structure. This must be paid particular attention to during the cooling process after the production of PUR foams in particular.

9. Chemical Resistance

Under the effect of corrosion, materials can lose all their functions, such as mechanical strength, as well as their appearance. It is always required that all the materials of a structure either be corrosion-resistant or that additional measures be taken to protect them from deterioration. From this point of view, just like the metal facings of the sandwich panel, the core materials must also be corrosion-resistant. The aromatic sulfonic acid used as a catalyst in the production of phenolic foams can dissolve on contact with water. With the absorption of high amounts of water, the sulfuric acid dissolved from within the phenolic foam and seeping onto the metal facings can cause corrosion of the metal.

10. Fire Performance

Rock wool, being in the inorganic material group, is classified as non-combustible. Rock wool-filled sandwich panels offer the best performance in fire-resistant facade, roof or interior partition wall applications. Furthermore, when the fire resistance of a 100 mm thick rock wool-filled sandwich panel is considered, it can vary between 30 and 120 minutes depending on the type of rock wool and the joint details. On the other hand, the polyurethane-based adhesives that provide adhesion between the rock wool and the metal facings exhibit behavior equivalent to that of PUR-filled panels in the early stages of a fire. All plastic foams, however, are organic. Consequently, they are all combustible. Their behavior in a fire varies greatly depending on the choice of suitable raw material, the foaming process, fire-retardant additives and the inorganic components in their content. The general characteristics of plastic foam are as follows; PUR/PIR: The correct chemical formulation can offer advantages in terms of the fire performance of plastic foams. High isocyanate ratios and fire-retardant additives are decisive in the performance of polyurethanes. PUR material begins to decompose at 150-200 °C and to burn and produce smoke above 300 °C. As an example, in tests, 60 mm thick polyurethane foam can achieve a critical fire resistance time of 30 minutes. XPS/EPS: Both expanded polystyrene (EPS) and Extruded polystyrene (XPS) are the plastic foams with the lowest fire resistance. At a temperature of 100 °C they begin to melt and form burning droplets. PHENOLIC: With its phenolic polymer structure, it offers better performance in a fire compared to PUR/PIR foams. In addition, it produces low smoke, equivalent to PUR/PIR. On the other hand, phenolic, developed to increase the fire resistance of plastic foams, also has various disadvantages. Foremost among these is the acid in the production process, along with its low mechanical strength, brittleness and relatively low thermal insulation performance.

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