Density

The density of foams is primarily dependent on the quantity of gas released during the reaction. This property of foams is particularly important because it affects their thermal conductivity and mechanical properties. Since water was used as a blowing agent to produce CO2 from the reaction with the isocyanate [233], both blowing agent and isocyanate content play an important role in the expansion of the foams and, therefore, in their density. From SEM images, the typical cellular structure obtained during foam formation is clearly observed in all foams and it is also noticeable that the foam cells are mainly closed. However, some differences can be noticed.

From Table 3. 2, it can be seen that the density of PUFs decreased with the increase of the amount of blowing agent. The same observations have been reported by H. C. Jung et

al.[234] This can also be observed from SEM images presented in Figure 3. 2, since the increase of the blowing agent increased the average diameter of the cells (see Figure 3. 2 (a) and (b)).

(a) (b)

(c) (d)

(e)

Figure 3. 2 - SEM micrograph of PUF-130-6-6-5 (a), PUF-130-6-6-7 (b), PUF-160-6-6-5 (c), PUF- PUF-130-6-9-5 (d) and PUF-130-8-6-5 (e)

From these images it is proved that the use of higher amounts of blowing agent leads to the formation of bigger bubbles.[235] The cell diameter increased from 4.9 ± 0.6 μm (PUF-130-6-6-5) up to 6.6 ± 0.8 μm (PUF-130-6-6-7) with the increase of the blowing agent.

The influence of the blowing agent on the density of the foams is also highlighted in Figure 3. 1, since the blowing agent has the most relevant effect on the density of PUFs (32.4% of influence). Furthermore, the p-value associated with this result, 0.005, demonstrates its high statistical significance (see Table 3. 3).

Sung Hee Kim et al. reported that foam density decreased as the isocyanate content increased, due to the additional CO2 produced.[236] This relationship is in agreement with the results presented in Table 3. 2. Indeed, from Figure 3. 1, the influence (12.0%) that the isocyanate content has on the density of the foams can be observed and the p-value associated with this parameter is close to the statistical significance (p-value = 0.057). From Figure 3. 2, it can be also be seen that the increase of the isocyanate content increased the diameter of the pores. Comparing Figure 3. 2 (a) and (c), which were produced using different isocyanate contents, no major differences on the structure homogeneity can be observed. However, PUF-130-6-6-5 presents an average cell diameter of 4.9 ± 0.6 μm, while PUF-160-6-6-5 presents an average diameter of 5.6 ± 0.7 μm, but the difference in density between these two samples may be more associated with the ratio between open/closed cells.

A similar trend was observed for the surfactant content, considering that the density of PUFs decreased with the increase of surfactant content (see Table 3. 2). Indeed, the role of the surfactant is to control the process of foaming, producing more cells with smaller size as well as regulating the ratio of open/close cells.[51] Figure 3. 2 (a) and (d), corresponding to foams prepared using different surfactant content, confirm that the cellular structure of PUF-130-6-9-5 is more regular, with more cells with smaller diameters. However, the ratio between closed and open cells may have a significant role too. [235] This implies that the

foaming process efficiency is improved by the addition of surfactant due to its ability to create nuclei and to augment the stability of the foams. Indeed, in Figure 3. 1, it is shown that the influence of the surfactant on the density is relevant (16.4%) and actually has statistical significant (p-value = 0.030). A similar observation was obtained by B. K. Kim et al. who reported that the density decreases rapidly due to the efficiency of the surfactant.[237]

Finally, it was also observed that the increase of the catalyst amount, decreased the density of the resulting foams (see Table 3. 2). The catalyst promotes, not only the reaction between the isocyanate with polyol, but also the reaction of the isocyanate with water.

Moreover, an increase of catalyst content can result in an incresase of reaction kinetic, which can promote the microphase separation affecting the foam morphology and consequently the mechanical properties of the PUF. However, from the results obtained, it was observed that the catalyst plays a secondary role in the foam expansion, confirmed by its low influence (9.8%) which actually does not have any statistical significance (p-value = 0.081).

Comparing Figure 3. 2 (a) and (e), which correspond to foams prepared with different percentage of catalyst, it can be observed that in the latter presents poor cellular homogeneity. This can be attributed to the fact that the use of more catalyst increased the kinetic of the polymer reaction, whilst the foaming process was not fast enough for the cells to grow properly. For this reason, small differences in diameter and ratio between open/closed cells can cause some differences in density.

3.4.2. Thermal conductivity

For insulation applications, the thermal conductivity (k) of PUFs is a property of paramount importance and it is related to the foam density, the ratio of open/closed cells and the thermal conductivity of the gas used as blowing agent, among others. Whilst the whole

foam only contains a small fraction of PU and its k value is much higher than that of the blowing gas, higher density foams have higher thermal conductivity.[238] In turn, if more blowing gas is trapped within the foam pores, there is less thermal conductive material, as claimed by Kun Hyung Choe et al.[239] The results obtained for the PUFs prepared using different formulations presented in Table 3. 2 confirm that an increase of blowing agent content decreased the thermal conductivity of PUFs. The influence of 36.1% and its statistically significance (p-value = 0.000), proves the importance of the blowing agent on the thermal conductivity of the foams.

As regards the isocyanate, it is known that it plays a relevant role on the density of the foams, so it was expected that it also has some influence on thermal conductivity. In fact, as in the case of density, Sung Hee Kim et al. reported a thermal conductivity decrease with increase of isocyanate index,[236] and this is in agreement with the results presented in Table 3. 2. However, from Figure 3. 1, it can be seen that its influence only represents 6.4% and, from Table 3. 3, it can be seen that this parameter is not statistically significant (p-value = 0.059). This is due to the fact that the whole foam only contains a small fraction of PU.

Conversely, the percentage of the surfactant has a marked influence on the thermal conductivity of the foams, due to the role that the surfactant plays in the foaming process and its impact on the cells structure, cell walls thickness and open/close cell ratio.[240] From Table 3. 2, it can be observed that the increase of the surfactant content lowers the thermal conductivity. The same conclusion can be found in literature.[237] Undoubtedly, the influence of surfactant on the thermal conductivity of the foams is highlighted in Figure 3.

1, representing 39.3% of influence. Also, Table 3. 3 demonstrate its high statistically significance (p-value = 0.000).

Finally, the percentage of catalyst has little influence on the thermal conductivity of the foams nor statistical significance.

3.4.3. Mechanical properties

A combination of low density, low thermal conductivity and good mechanical properties is required for PUFs to be used for thermal insulation applications [225] but, whatever their use, optimization of their mechanical properties requires good understanding of the effect of the different components of formulation. In fact, the mechanical response of this type of material is very complex, since it depends on their architecture, the intrinsic properties of the polymer, the cell wall thickness, the size distribution and the shape of the cells, among others.

The relationship between the mechanical response of PUFs and the intrinsic properties of the polymer, is associated with the molecular composition of PU, which consists of soft and hard segments. The urethane and urea groups form the hard-segment, while the soft segments consist of aliphatic polyol chains.[159]Therefore, higher isocyanate amounts yield more urethane groups, making the PUFs more rigid.[218] Similarly, higher contents of blowing agent (water) leads to an increase of urea linkages in the final polymer [234] making the polymer stiffer. Figure 3. 3 shows the most representative compressive stress-strain curves obtained for foams prepared using varying the amounts of isocyanate, catalyst, surfactant and blowing agent. The values obtained for Young’s modulus, compressive stress and toughness for all PUFs are summarized in Table 3. 3.

Figure 3. 3 - Compressive stress-strain curves of PUFs

As can be observed, the increase of isocyanate content lead to increase of the Young’s modulus, toughness and compressive stress, which is in agreement with the results reported by Park et al. [241] regarding an increase in compressive strength of PUFs when the isocyanate content was increased within the same range used in this study. This is also verified by the statistical analysis which show that the isocyanate content has an influence of 56.3% on the Young’s modulus, 35.1% on the toughness and 41.9% on the compressive stress (see Figure 3. 1). Additionally, all these results have high statistical significance (p-value of Young’s modulus = 0.000, p-(p-value of toughness = 0.001 and p-(p-value of compressive stress = 0.000).

As regards the effect of the blowing agent, Figure 3. 3 and, more importantly, the ANOVA results show that it only has a marginal effect on the Young’s modulus (2.5%) and compressive stress (4.5%), with a slightly effect on the toughness (8.8%). However, the results have not statistically significant as the corresponding p-values are higher than 0.050.

These results are particularly interesting as it is normally assumed that, when water is used, the formation of urea groups may be associated with an increase in stiffness, as reported for example by H. C. Jung et al. [234]

In the literature, it is reported that an increase of surfactant content causes a remarkable increment of the force required for 10 % deformation.[237] The increase of stiffness with the increase surfactant content can be associated to the quantity of retained gases. From the results presented in Table 3. 2 and Figure 3. 3, this increment of the stiffness of the foams can also be observed with the increase of the surfactant amount. Also from Figure 3. 1, it can be observed that the surfactant has an effect of 10.8% on the Young’s modulus, 13.6% on the toughness and 18.0% on the compressive stress. All these results presented p-value lower than 0.050, indicating their statistical significance.

Finally, with respect to the catalyst influence, the results obtained show that it has a relevant influence on the Young’s modulus, toughness and compressive stress (13.4%, 21.5% and 18.9%, respectively) and all these results are statistically significant (p-value of 0.013, 0.006 and 0.005, respectively), as illustrated by Figure 3. 1 and Table 3. 3. Kun Hyung Choe et al. have also reported that the compressive strength of PUFs increases with the catalyst content and have attributed that increase to the fact that the catalyst promotes the production of urethane groups (by the reaction between the isocyanate with the polyol) and urea groups (by the reaction between the isocyanate with water) which belongs to the hard segments. [239] As mentioned before, the mechanical properties of the foams are intrinsically dependent on density, hence, to exclude the effect of density variations,

normalized values of the Young’s modulus (specific Young’s modulus), toughness (specific toughness) and compressive stress (specific compressive stress) and have been determined and the results are presented in the Table 3. 4.

Table 3. 4 - Young’s modulus, compressive stress (𝜎10%) and toughness normalized for

From the results listed in Table 3. 4, it is clear that, despite of the density effects, the variation of the reactants amounts studied have a direct effect on the mechanical properties of the foams, increasing the Young’s modulus, toughness and compressive stress.

3.5. Conclusions

The rationale to fine tune formulations of PUFs derived from unrefined CG in order to modulate their physical properties has been established using statistical analysis. The results obtained proved that both density and thermal conductivity are governed by the blowing agent and surfactant contents. In turn, the mechanical properties are essentially determined by the content of isocyanate and catalyst. Therefore, the implementation of such a systematic approach will contribute to the sustainability of the PUF industry.

4. Development of CG derived composite foams for