**3.2. Influence of raw material type on thermo‐insulating properties of obtained PUR‐PIR foams**

Rigid foams are mainly used as thermo‐insulating materials. That is why a low heat conduc‐ tivity of those materials is one of their most sought‐after characteristic. The thermo‐insulating properties of the obtained foam materials were determined by measuring the heat conductiv‐ ity coefficient *λ* and the amount of closed cells (**Table 5**).

Rigid polyurethane‐polyisocyanurate foams are cellular materials, where the amount of closed cells is significantly more prominent, and the share of open pores is in the range of


**Table 5.** Value of heat conductivity coefficient and amount of closed cells in obtained foams.

adhesiveness the quickest. Apparent density was also measured for the rigid foams and the

**Foam symbol Start time (s) Rising time (s) Gelation time (s) Density (kg/m3**

K0 8 15 31 33.4 K1 6 14 27 40.3 K2 5 10 25 40.6 K3 4 10 21 40.8 K4 4 9 20 41.3 K5 3 8 16 41.5

The processing times of modified foams were reduced by about 50% in comparison to the reference foam. The start time for P0 reference foam was 8 s; however, for the foam containing 0.5 R of the boron polyol, it was 3 s. Similar changes were observed for rising and gelation times. This shows a higher reactiveness of the boron‐nitrogen polyol in comparison to the

Apparent density is one of the most important factors that determine the mechanical proper‐ ties of rigid PUR foams. From the economical point of view, it is beneficial to produce materi‐ als with the lowest possible apparent density. Nevertheless, the apparent density of a PUR foam is in close correlation with its thermo‐insulation properties, mechanical properties and dimensional stability. That is why the most commonly used rigid polyurethanes have the

the density of produced PUR‐PIR foams was observed in comparison to the reference foam.

containing 0.5 R of the new polyol. The decrease in the apparent density of K1–K5 PUR‐ PIR foams is related to the amount of the boron introduced to the PUR system. By replacing the petrochemical polyol (with 9200 mPa s viscosity) with the boron polyol (with viscosity 283 mPa s), the total viscosity of the premix containing the new polyol was decreased. The amount of water used as a chemical foaming agent stayed at the same level for all recipes and

**3.2. Influence of raw material type on thermo‐insulating properties of obtained PUR‐PIR** 

Rigid foams are mainly used as thermo‐insulating materials. That is why a low heat conduc‐ tivity of those materials is one of their most sought‐after characteristic. The thermo‐insulating properties of the obtained foam materials were determined by measuring the heat conductiv‐

Rigid polyurethane‐polyisocyanurate foams are cellular materials, where the amount of closed cells is significantly more prominent, and the share of open pores is in the range of

.

ity coefficient *λ* and the amount of closed cells (**Table 5**).

. By using the new boron polyol, a slight increase in

for K5 foam,

**)**

for K0 foam up to 41.5 kg/m<sup>3</sup>

results of all tests are represented in **Table 4**.

**Table 4.** Processing parameters and apparent density of rigid PUR‐PIR foams.

density values in the range of 30–60 kg/m<sup>3</sup>

generates the same amount of CO<sup>2</sup>

Foam density is in the range from 33.4 kg/m<sup>3</sup>

industrial one.

120 Aspects of Polyurethanes

**foams**

5–10%. At the same time, the amount of closed pores is dependent on their size. The gas closed in the foam cells and also the structure of a polyurethane matrix (but to a lesser degree), both take part in the heat flow process. The boron polyol used in the rigid PUR‐PIR foam production causes an increase in the amount of closed cells in comparison to the K0 refer‐ ence foam. The amount of closed cells in K0 foam is 83.2%. For foams containing tri[N,N′‐ di(methylenoxyethylentio‐2‐hydroxyethyl)urea], the amount of closed cells ranges from 92.9%, K1 foam, to 93%, K5 foam. It has been determined that the amount of new polyol does not influence the amount of closed cells; however, the application of those polyols during foam production has been significant. All rigid PUR‐PIR foams obtained with the use of the new boron polyol have over 90% of closed cells. The increase in this number is not indifferent towards the value of the heat conductivity coefficient of those foams. In rigid foams desig‐ nated for thermal insulation, the definite majority of cells should be closed so that the foaming gas could remain enclosed within the material. The value of the heat conductivity of the gas closed in cells comprises over 80% of the total value of heat conductivity coefficient of rigid polyurethanes. It is mostly dependent on the type of porophor, foam density and the amount of closed cells. It is worth keeping in mind that the thermo‐insulating properties change during foam exploitation. It is with correlation to the changes in the composition of the gas mixture in the cells. By choosing the chemical method of foaming PUR‐PIR foams, porous materials were produced that contained carbon dioxide in the cells. The polyurethane's cell walls are permeable for carbon dioxide which diffuses into the outside during material use due to its small particles. Then, oxygen takes place of the carbon dioxide. Foams that do not have a solid protective coating lose their initial insulating properties very quickly. **Figure 3** represents the changes in the values of heat conductivity coefficient for K0 and K5 foams in relation to the amount of days they were used.

It can be observed that the changes in the values of heat conductivity coefficient during the ageing process take longer time in the structures with the boron polyol. In the initial stage, the heat conductivity coefficient for the K0 reference foam was 30.7 mW/mK. During foam exploi‐ tation in room temperature, the *λ*‐coefficient value increased to 37.2 mW/mK after 120 days. However, in the K5 foam, the initial *λ*‐coefficient value was 26 mW/mK. During the ageing process in room temperature, the coefficient value increased to 28.9 mW/mK. The decreased heat flow in the foam modified with the new polyol is related to its more organized structure and larger amount of closed cells.

The heat conductivity coefficient depends mainly on the cell structure in the final product. That is why a thorough analysis of the cell structure is so important. At an early stage of the foaming process, there is a chain of reactions that start with the creation of micro‐bubbles.

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

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

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.

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