**3.3. Mechanical properties and dimensional stability of produced PUR‐PIR foams**

The structure of the polyurethane matrix has a significant influence on the mechanical prop‐ erties and the durability of the cells in foam materials. By changing the recipe of the foams, some characteristics of the final product can be modified. However, the greatest significance in the shaping of PUR foam's mechanical properties is due to the apparent density and cell structure. The new boron polyol used in the foam composition improved the mechanical properties of the obtained rigid polyurethane‐polyisocyanurate foams. **Table 6** presents the examination results of compressive strength, brittleness, water absorption and ageing.

The research conducted on mechanical properties of PUR‐PIR foams helps determine that the new boron polyol significantly improves those properties. The compressive strength measured in a perpendicular direction increased after using the boron‐nitrogen polyol in the foam production. The value increases along with the amount of the equivalent of the boron compound used in the foams. For the reference foam (without boron), the compres‐ sive strength is 238 kPa. For other foams, in which the petrochemical polyol Rokopolu RF‐55 was subtracted to add the boron polyol, this value was in the range from 273 kPa (K1 foam with 0.1 R of borane tri[N,N′‐di(methylenoxyethylentio‐2‐hydroxyethyl)urea]) to 339 kPa (K5 foam containing 0.5 R of borane tri[N,N′‐di(methylenoxyethylentio‐2‐hydroxyethyl)urea]). The increase in compressive strength has to be correlated with the apparent density of the PUR‐PIR foams (**Figure 5**).

The mechanical strength measured according to the foam's growth direction is usually higher than the one measured perpendicularly to the growth, because the cells are elongated in the direction in which the foam grows. The improvement in strength measured perpendicularly can be attributed to the changes in foam's morphology, that is, increased amount of polar urea groups capable of creating hydrogen bond. Additionally, the closed‐cell structure gives the materials better compressive strength, in comparison to materials with large open cells.

The boron polyol used in the production of rigid polyurethane‐polyisocyanurate foams reacts with the polyurethane composition in a way similar to a typical crosslinking compound, helping create a more unified foam structure. Because of that, the examination of brittleness


**Table 6.** Mechanical and ageing properties of the rigid PUR‐PIR foams.

The cell growth comes next and is related to the porophor evaporation. Growing cells start to touch. Cell walls start to form in a form of thin membranes and ridges in the place where three cell walls meet. The construction stability is dependent on the chemical composition of the walls and ridges, and the size and shape of cells. The pictures of the obtained rigid foams

**Figure 3.** Changes in the values of heat conductivity coefficient for foam during ageing process in room temperature.

Foam K0 Foam K5

0 20 40 60 80 100 120

**Days**

By analysing the SEM pictures of the cell structure, it can be observed that they depict internal closed‐cell structure of the produced foams. The applied boron polyol supports the surface‐ active agent by creating more regular and smaller cells. By examining the shape of the closed cells in the photographs, it can be easily seen that the addition of boron polyol to the foam recipe contributes to a more uniform cell structure in comparison to the reference foam.

using SEM method are depicted in **Figure 4**.

**Figure 4.** Cell structure of PUR‐PIR foams: (a) K0 foam and (b) K5 foam.

24

26

28

30

**Thermal conducvity [mW/mK]**

32

34

36

38

122 Aspects of Polyurethanes

**Figure 5.** The correlation between compressive strength and apparent density, and the amount of boron polyol in the foams.

helped determine that the addition of the boron polyol into the foam recipe causes significant decrease in the foam's brittleness. The higher the amount of boron polyol in the foams, the lower the brittleness, from 36.2% for K0 reference foam to 8% for K5 foam modified with the new polyol. Similar to the compressive strength, the brittleness examination of the obtained foams shows a correlation between this value and the apparent density of the samples. Higher apparent density of PUR‐PIR foams enables a significant decrease in their brittleness.

There is also a correlation between the compressive strength and the linear dimension stabil‐ ity. Along with the changes in the temperature, the internal pressure of the gas inside the cell changes. It creates a difference in pressure between foam cells and external atmospheric pressure. This difference of pressures needs to be lower than the foam's compressive strength to retain its dimensional stability. Foam deformation should not occur when the compressive strength is greater than 100 kPa, which is a value higher than the possible difference between atmospheric pressure and the pressure inside foam cells, which is close to zero. When com‐ paring the results of the ageing test for the produced rigid PUR‐PIR foams, it was determined that there is a strong correlation between the stability of linear dimensions, mass loss and changes in volume, and the use of the new polyol in foam recipe. During the simulated age‐ ing of the samples, it was observed that mass loss did not exceed 1% for all foams. Similarly, the changes in linear dimensions did not exceed 2%. This qualifies those products for the use in thermal insulation.
