3.2.1. Measured Foam Thermal Conductivity
Thermal conductivity of foam samples was measured over more than three years, as shown in Figure 2
A. The samples had a low thermal conductivity at the initial time of the foam production due to their low density (ca. 30 kg/m3
) and the use of cyclopentane as a physical blowing agent (C5
has a low thermal conductivity and a low diffusion coefficient). However, the thermal behaviour of foam samples get worse with time, reaching around 30 mW/m·K after around 1200 days after foam production because the gases initially occluded inside the cells with low thermal conductivities (14.5 mW/m·K for CO2
and 12 mW/m·K for C5
at 20 °C) diffused out, being substituted by atmospheric air with a high thermal conductivity (25.4 mW/m·K for N2
and 26.11 mW/m·K for O2
at 20 °C).
In the case of foam with talc, its initial thermal conductivity measured after manufacturing was lower than for the reference foam (Figure 2
B). One of the reasons for the obtained lower value was a smaller cell size of foam containing talc (Table 1
), which increased the extinction coefficient and, as a consequence, minimized the radiative. However, this improvement in thermal conductivity was not maintained with time for foam with talc since this presented a higher thermal conductivity slope shortly after manufacturing with respect to reference material. However, a lineal relation between the thermal conductivity slope and the foaming temperature was found, so those foams with higher foaming temperature showed a higher thermal conductivity slope, and vice versa [11
]. This influence of foaming temperature on the thermal conductivity slope at the initial time is explained in the next paragraph.
The foaming temperature and the gas generated increases the pressure inside cells during the foaming process. Thus, there is a pressure gradient inside and outside the foam cells [5
]. The higher foaming temperature reached by the foam with talc could generate a higher pressure gradient inside the cells and, as a consequence, a quicker diffusion of the gasses out of the cells.
In conclusion, the gas composition measurements with time are fundamental to understanding the differences in the thermal conductivity aging of these two RPU foams.
3.2.2. Foam Cell Gas Composition
GCMS test was performed on the 12th and 21st days after manufacturing, and additionally at 2.5, 3.5, 4.5 months, then after 1 and 3.5 years. Unfortunately, testing could not be performed shortly after production due to manufacturing of samples in one country (Valladolid University, Spain) and conducting the test in another country (Montanuniversität Leoben, Austria). The gas composition was accurately calculated with input data obtained from the measurement of the ratio of each peak area on the chromatograms. Table 2
collects the values of the calculated gas volume content and theoretically-derived thermal conductivity of the cell gas mixture by the Wassiljewa equation. Figure 3
and Figure 4
show the evolution of the gas volume percentage with time for both samples.
The initial test of foam thermal conductivity showed a significant difference in results between the reference and that modified with 1.5% talc foams. It would be very representative to show the initial gas composition of both samples after production, taking into account the observed difference in the foaming reaction temperature, which could influence the different evaporation and diffusion of blowing agents into the cells. Unfortunately, it was not possible to perform GCMS test at this stage.
On the 12th day after manufacturing the samples were delivered to the GCMS laboratory and measured. Calculated values of thermal conductivity of the cell gas mixture were similar between the two samples, but for the reference foam it was slightly lower (19.34 mW/m·K) than for the foam with talc (19.70 mW/m·K) due to higher CO2
content (20.60 vol% for Reference; 17.40 vol% for Talc) and lower N2
content (40.20 vol% (N2
) and 11 vol% (O2
)—Reference; 41.60 vol% (N2
) and 12.20 vol% (O2
)—Talc). However, C5
content is slightly higher in the foam with talc (28.20 vol%—Reference; 28.80 vol%—Talc) at this stage. The GCMS results are in correspondence with the measured foam thermal conductivity shown in Figure 2
Results of gas analysis on the 21st day showed that calculated thermal conductivity of the gas phase of both samples continues to grow (19.48 mW/m·K for Reference; 20.35 mW/m·K for Talc). Both foams showed increased content of C5
, what could be explained by liquid cyclopentane that was probably not evaporated completely during foam manufacturing [20
] (35.10 vol%—Reference; 30.90 vol%—Talc). CO2
is still high, but decreasing gradually (12.80 vol%—Reference; 11.00 vol%—Talc). Air starts to fill the cells, which is shown by the O2
content (18.10 vol%—Reference; 19 vol%—Talc), N2
content showed a decrease in value, which is not a representative result and is explained by different specimens. Considering that during GCMS sampling the specimens are penetrated with a needle, specimens with open channels have to be discarded after every experiment, therefore, every new test is performed on a new pair of specimens.
After 2.5 months of aging at RT both foams showed an increase in C5
content probably due to their unstabilized condition (35.70 vol%—Reference; 36.25 vol%—Talc). CO2
has almost left the foam cells (0.17 vol%—Reference; 0.16 vol%—Talc). Gas phase conduction of foam with talc showed slight improvement (20.03 mW/m·K). Meanwhile, for the reference foam, it has increased (21.39 mW/m·K), showing that the direction of deterioration for both samples became closer to each other, similar to foam thermal conductivity, as shown in Figure 1
After 3.5 month calculated thermal conductivity of gas phase of both samples showed similar values (20.95 mW/m·K—Reference; 21.46 mW/m·K—Talc), this behaviour looks comparable to trend of measured foam thermal conductivity shown on Figure 1
content continued to grow in the reference sample (38.54 vol%). Meanwhile, it started to leave the 1.5% Talc sample (34.83 vol%). CO2
content is relatively stable (0.17 vol%—Reference; 0.14 vol%—Talc).
Results of GCMS at 4.5 months after foam manufacturing showed that stabilization of C5
has finished in both samples and it was leaving the cells (32.19 vol%—Reference; 22.12 vol%—Talc), air was filling the cells (47.37 vol% (N2
) and 20.28 vol% (O2
)—Reference; 58.96 vol% (N2
) and 18.79 vol% (O2
was decreasing very slowly. Results of gas content measurements showed the deterioration of gas phase conduction for both samples, but for foam with talc it was higher, being the trend similar to the foam thermal conductivity shown in Figure 2
A. Thus, the gas diffusion evolved more rapidly in the foam with talc from 3.5 months to 4.5 months after foam manufacturing (Figure 4
After one year of foam storage thermal conductivity results showed overlapping in values, where the reference foam continued slowly to deteriorate. Although calculated values of the thermal conductivity of the gas phase looked similar, the reference foam showed a slightly lower value (23.26 mW/m·K) than the foam with talc (23.53 mW/m·K). C5
content has gradually decreased but with a different speed: quicker for the reference foam and lower for the foam modified with talc from 4.5 months to one year after foam manufacturing (Figure 3
and Figure 4
). Additionally, CO2
has almost left the cells (0.038 vol%—Reference; 0.042 vol%—Talc).
After 3.5 years the gas phase contribution to the total foam thermal conductivity showed a similar trend to the one-year results, where the reference foam had a slightly lower value (24.11 mW/m·K) than the foam with talc (24.48 mW/m·K). The lower calculated value of thermal conductivity of the gas phase for the reference foam was explained by a higher content of insulation gases (13.48 vol% C5H10 and 0.032 vol% CO2) and a lower content of air (68.13 vol% N2 and 18.35 vol% O2) in comparison with talc modified foam.
Summarizing all of what was said above, the 3.5-year results showed that there is no significant difference between thermal insulation ability of the two samples, but 1.5% talc filler has an influence on the initial value of thermal conductivity. It might be that the longer period of thermal property monitoring is required for evaluation of the difference between two samples, because at 3.5 years the samples have not reached the stationary state yet and, consequently, the thermal conductivity will continue increasing (Figure 2