**3. Results**

#### *3.1. Heat Treatment of 50 mm Thermal Insulation Cubes to 1200* ◦*C*

During the heat treatment in the muffle furnace, the temperature at the center of the insulation sample was recorded. The recorded temperature as a function of time for each defined holding temperature is presented in Figure 4. The temperature curves show the heating of the thermal insulation, the 30 min holding time and the subsequent cooling of the furnace. Two temperature peaks were observed in all the tests, i.e., two exothermic reactions. The peak temperature in the reaction varied, which may be explained by the variations in the amounts of dust binder and Bakelite in each test cube. The first peak started at about 300 ◦C, with a peak in temperature between 525 and 587 ◦C. The second peak started at approximately 870 ◦C, with a peak temperature between 930 and 990 ◦C. Thus, the second peak was only observed in test specimens treated at 900 ◦C and above. As stated in [11], the first peak (exothermic reaction) may be explained by the combustion of the dust binder due to the ambient air atmosphere in the furnace. The second exothermic peak may be explained by the crystallisation of the amorphous silica (SiO2) in the thermal insulation.

**Figure 4.** Measured temperature as a function of time, measured at the center of the test specimen, for each holding temperature presented in Table 1.

After the heat treatment, the height of the originally 50 mm high cube of thermal insulation was recorded at three locations at each of the four vertical faces. Similarly, the width of the test specimen was recorded horizontally at three elevations for each of the four vertical faces. There was some variation in height and width in the heat-treated test specimens, hence an average value had to be used. The results from the average measurements from the height (H) and width (W) of the heat-treated thermal insulation are presented in Figure 5.

**Figure 5.** Height (H) of the test specimen ( ) and width (W) of the test specimen ( ) after the heat treatment. The height and the width are the average value of three measurements at each side of each of the four vertical faces.

Based on the obtained average height and width of each cube, an estimation of the post-heat-treatment volume was made. The mass of each specimen was also recorded, allowing for the density to be calculated for each cube, as presented in Figure 6.

**Figure 6.** Density as a function of the heat exposure temperature.

A minor decrease in the test specimen height was observed for heat treatment temperatures up to 1100 ◦C, similar to previously published results [11], which were limited to a maximum temperature of 1100 ◦C. Above this temperature, the results of the present study clearly show that significant degradation of the thermal insulation started at temperatures just above 1100 ◦C. There also seemed to be a total breakdown at 1200 ◦C, as evidenced by a conspicuous increase in the post-heat-treatment density. The virgin test specimen and the test specimen heat-treated up to 1200 ◦C are presented in Figure 7.

The test specimens after heat treatments up to 1100 ◦C, 1190 ◦C and 1200 ◦C, from left to right, respectively, are shown in Figures 8 and 9. After the heat treatment at 1190 ◦C, the test specimen had lost 55% of its original height and 25% of its original width, while the heat treatment at 1200 ◦C resulted in a 76% reduced height and a 46% reduced width. When increasing the heat treatment temperature from 1190 ◦C to 1200 ◦C, the thermal insulation material post heat treatment changed in morphology from a chalky consistency to resembling a hard, but still somewhat porous, stone. This was clearly shown in the calculated density, which increased from 589 to 1721 kg/m<sup>3</sup> due to the 10 ◦C increase in heat treatment temperature from 1190 ◦C to 1200 ◦C.

**Figure 7.** Virgin test specimen (50 mm cube) (left) and heat-treated to 1200 ◦C (right), including the thermocouple that had to be cut when the specimen was removed from the furnace.

**Figure 8.** Test specimens after furnace heat treatments up to 1100 ◦C (left), 1190 ◦C (middle) and 1200 ◦C (right), as seen from the side.

**Figure 9.** Test specimens after furnace heat treatments up to 1100 ◦C (left), 1190 ◦C (middle) and 1200 ◦C (right), as seen from the bottom of the insulation.

## *3.2. Thermogravimetric Analysis*

Thermogravimetric analysis was conducted from ambient temperature up to 1250 ◦C. The heating rates were 5, 10, 20 and 40 K/min. The samples for the TGA testing were made from the same insulation mat as the muffle furnace testing. The approximate mass loss was between 3 and 4.3%, as shown in Figure 10. The differential thermogravimetric (DTG) analysis, which is the derivative of the TGA curve, is presented in Figure 11.

**Figure 10.** Thermogravimetric analysis of the thermal insulation in a nitrogen atmosphere.

**Figure 11.** Differential thermogravimetric (DTG) analysis of the results presented in Figure 10.

The mass loss of the insulation started at approximately 180 ◦C, with a local minimum value between 260 and 290 ◦C. This may be explained by the release of the dust binder. The mass losses at higher temperatures were most likely due to the Bakelite binder and possibly some released chemically bound water, with the most conspicuous peak observed at about 1000 ◦C.

#### *3.3. Di*ff*erential Scanning Calorimetry*

Simultaneously with the TGA measurements, DSC analyses were performed from ambient temperature up to 1250 ◦C at heating rates of 5, 10, 20 and 40 K/min. The results from the DSC analysis are presented in Figure 12. An exothermic reaction started between 800 and 900 ◦C. An endothermic peak was observed at, or just above, 900 ◦C. A very conspicuous endothermic reaction was observed starting at approximately 1120 ◦C, with a maximum local peak between 1170 and 1206 ◦C.

**Figure 12.** The differential scanning calorimetry (DSC) results as a function of temperature.

The minimum and maximum values from the exothermic and endothermic reactions for the three conducted tests at each heating rate are presented in Table 2, in addition to the heat flow values at the endothermic peaks. There were some variations in the peak value, depending on the heating rate, but there was no clear trend associated with the heating rate and peak temperature.

**Table 2.** The recorded temperatures at the exothermic (Tl,exo) and endothermic (Tp,endo) DSC peaks of Figure 12 and the recorded heat flows at the peaks for each run.


#### *3.4. Thermal Conductivity Measurements*

TPS [27,28] was used to record the room temperature thermal conductivity of the heat-treated test specimens. The obtained results as a function of the heat treatment temperature are shown in Figure 13. The thermal conductivity increased with the heat treatment temperature in a similar manner to the recorded density, as presented in Figure 6, i.e., it increased greatly when heat-treated to temperatures above 1150 ◦C. This was most likely due to the increasing level of sintering and partly due to melting, as evidenced by the endothermic peak in Figure 12 at these high temperatures. The most conspicuous change was observed when the heat treatment temperature was 1200 ◦C, i.e., where more melting occurred during the heat treatment.

**Figure 13.** Recorded room temperature thermal conductivity as a function of the test specimen heat treatment temperature. The thermal conductivity at 50 ◦C ( ) was from Appendix A, Table A2.
