Description of rigid polyurethane foam
So far we have described rigid polyurethane foam as a very good thermal insulator, enumerating its fundamental properties. We will now see chemically what it is and how it is formed.
Basically, the rigid foam is obtained from three chemical components, whose denomination is the following:
- 1) - Polyisocyanate.
- 2) - Polyhydroxylated resin.
- 3) - HCFC 141b, expansion agent whose boiling point is 32ºC.
By mixing the three indicated components, the polyisocyanate reacts chemically with the polyhydroxylated resin generating heat, which produces the evaporation of the blowing agent. As a result, the mixture begins to expand while hardening, eventually reaching the formation of a foamed plastic, closed cells and virtually sealed. Inside the same the agent of expansion in gaseous state is caught, being the main responsible, by its very low thermal conductivity, of the very good insulating effect of this foam.
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In practice, the application procedure is relatively simple, since it requires only a few steps: The hydroxylated resin is premixed with the blowing agent, there being no chemical reaction in this case. At the time when it is desired to produce the foaming, the previous premix is contacted with the polyisocyanate, thus producing the expansion and hardening process detailed above (Figure 1.)
Figure 1: Expansion and hardening process.
Very low heat transfer, fundamental characteristic of rigid polyurethane foams.
As is known, the efficiency of a material as a thermal insulator is measured through its value of thermal conductivity, which we will call K and one of its expression units, among others is:
Figure 2: Efficiency of a material as a thermal insulator.
This value indicates the amount of heat, in kilocalories per hour, that passes through the two sides of a plate of one (1) centimeter of thickness and one square meter of surface for each degree centigrade of temperature difference existing between said two faces (Figure 2). Therefore, the lower the thermal conductivity mentioned, the less amount of heat is transmitted through the material and then, the latter works better as an insulator.
Table 1 shows densities and coefficients K for some of the products usually offered on the market. In the exposed values it can be seen that the lowest thermal conductivity corresponding to rigid polyurethane foam and, in fact, up to now , is the material with better insulating properties that is known.
A point to note, more important: if we look closely at table 1, with the concept of thermal conductivity in mind, we can realize that as the coefficient K decreases, the thickness of material to obtain a desired degree of insulation also decreases . Let's take an example: imagine the roof of an enclosure that we want to isolate for. For this we use rigid polyurethane foam, which has an average K coefficient of 2.0 and the calculated insulation thickness is, for example, 25mm. If we place agglomerated cork instead, we need approximately 80% more thickness, that is, 45 mm. If we used expanded polystyrene, this thickness would be approximately 36 mm, that is, 44% above the value indicated for the rigid polyurethane foam. All of the above is important when considering the cost of an insulation, since for the material in question the thicknesses used are lower and the application procedure is extremely fast, especially if the blow-off method is used. in detail later.
Typical thermal conductivities of insulating materials
Material
l, Kcal.cm/m2.°C. hour
Density, kg / m3
Rigid polyurethane foam
35-60
1,7- 2,3
Expanded polystyrene
15-35
2,8-3,3
Glass wool
32-87
2.8-3.0
Agglomerated cork
90-160
As an additional comment we will say that using smaller thicknesses of insulation can also make the possibilities more flexible
of design of a certain structure.