**4. Discussion**

Previous studies have shown that the type of thermal insulation tested in this study survived well when heated in a muffle oven to temperatures up to 1100 ◦C [11]. The objective of the present study was to investigate the performance of the thermal insulation when exposed to temperatures up to 1200 ◦C, i.e., temperatures associated with fire heat fluxes of about 350 kW/m2. The focus was on finding the breakdown temperature of the thermal insulation. Small scale jet fire testing has proven that the thermal insulation alone may serve as passive fire protection of a 16 mm steel wall [10,11]. The previous jet fire tests showed a complete breakdown of the insulation at the most exposed locations. To determine the breakdown temperature and explain the observations of the insulation after the small-scale jet fire testing, muffle furnace tests up to 1200 ◦C were performed, as well as TGA/DSC to 1250 ◦C in a nitrogen atmosphere.

The results from the furnace testing showed the same trend as in [11] up to 1100 ◦C, which was the upper-temperature limit of that study due to furnace limitations. However, at heat treatment temperatures above 1100 ◦C, the height of the originally 50 mm cubes started to shrink significantly with increasing heat treatment temperature. From 1160 ◦C, the width of the test specimens also decreased considerably. The thermal insulation fibers gradually sintered/melted more and more together, and the insulation transformed from being a porous material to a hard, stony consistency when heat-treated to 1200 ◦C. This was also reflected in the calculated density of the thermal insulation cubes post heat treatment. The density close to tripled due to the heat treatment at 1200 ◦C compared to 1190 ◦C. It was clearly shown from the results that heating to 1200 ◦C is very close to, or even at, the melting point, or eutectic temperature, of the insulation.

Mixtures of inorganic salts, such as the investigated thermal insulation, will not show a defined melting point, but rather an extremely complex phase diagram with several eutectic points. It is therefore expected to gradually melt, without a defined melting temperature. Heat treatment of the test specimen cubes to temperatures above 1200 ◦C might have resulted in a glass-like substance. This was, however, outside the scope of the present study but may be interesting for future studies.

The thermal conductivity of the virgin thermal insulation was clearly dominated by heat radiation through the pores at moderately elevated temperatures. At higher temperatures, the onset of sintering increased the solid–solid contact phase, improving the true thermal conductivity of the material. This was confirmed by the room temperature thermal conductivity obtained in the present study. However, at a still higher pore fraction, it would be expected that at elevated temperatures, the pore radiation would continue to dominate the effective thermal conductivity. When heat-treated to 1200 ◦C, the significant increase in density indicated that the pore fraction must be very low. In this stage, the thermal conductivity may not be very dependent on the pore radiation, i.e., not show a very strong dependency on the absolute temperature to the third power. To validate this assumption, the thermal

conductivity of the heat-treated test specimens must be recorded at elevated temperatures. This was, however, outside the scope of the present study.

Heat treatment up to 1100 ◦C revealed some loss in height, approximately 22%. However, little change in the width of the material was observed up to this temperature, i.e., it was not expected that the insulation mat will crack open at temperatures below 1100 ◦C. When heating to 1180 ◦C, the loss in width was still below 14%. However, when heating up to 1190 ◦C, there was a significant loss in width, i.e., 25%.

Due to the heat treatment at 1200 ◦C, the insulation cube lost 76% of its height and 46% of its original width, explaining the observed cracks and openings in the insulation mat after the small scale jet fire testing presented in [10,11]. In addition to the shrinkage in height (thickness) and the increase in thermal conductivity due to the sintering, in a severe fire scenario, there will be radiant heat transfer through the cracks and openings. Hence, with more and wider cracks, more radiant heat may bypass the thermal insulation, leading to excessive heating of any fire-exposed objects.

In the furnace heat treatment tests, two exothermic peaks were observed. The first of these may be explained by the combustion of the dust binder material at about 300 ◦C. There were some variations in the peak temperature of the reaction, which may be explained by di fferences in the chemical compositions between the samples. This exothermic reaction at approximately 300 ◦C, observed during heat treatment, was not present in the DSC tests. This may be explained by the air access and combustion in the furnace and the inert gas (nitrogen) atmosphere during the DSC tests. In an oxygen atmosphere, both the first and the second peaks were observed [11]. The second peak at about 900 ◦C may have been due to the Bakelite combustion or a recrystallisation process of the involved inorganic salts.

The TGA showed that only small amounts of the material vaporised during heating, i.e., approximately 4% of the mass was lost. This was also seen when observing the density of the heat-treated test specimens, where there was little change in the mass of the test samples due to the heat treatment, i.e., the density increased as the insulation sintered and finally started to partly melt. The di fferences in mass loss recorded using the TGA for the di fferent heating rates were also observed in repeated tests at each heating rate. In their study of di fferent stone wool insulations, Livkiss et al. [16] also observed similar discrepancies. They explained these di fferences using the inhomogeneity of these types of material. We also agree with this assumption since TGA and DSC testing is constrained to test samples that are a few milligrams in size.

The heat treatment in the present study involved a holding time of 30 min at all heat treatment temperatures. It is interesting to notice that if the temperature of the thermal insulation was kept at, or below, 1100 ◦C, it could stay quite intact for at least the 30 min heat exposure. It started to significantly break down only at temperatures above 1100 ◦C. Hence, if arranged in a passive fire protection system such that it will not exceed 1100 ◦C, it may contribute significantly as passive fire protection in addition to its intended function as thermal insulation. A sketch of such an arrangemen<sup>t</sup> is presented in Figure 14. The critical point to be kept below is 1100 ◦C, which is marked on the figure. Unless the object to be protected is internally cooled by, e.g., depressurisation, the temperatures of the system will gradually increase, but these layers of protection may be designed to o ffer the required protective capacity for the desired time.

It should be noted that di fferent batches of industrial thermal insulation may show slightly di fferent high-temperature performances. Thermal insulation that varies significantly from the chemical composition presented in the present study may show very di fferent high-temperature properties. Care should therefore be taken before using such materials for fire protection. The 50 mm cubes that were heat-treated in the mu ffle furnace and the TGA/DSC analysis both provided results supporting the conclusions in the present study. When considering industrial thermal insulation for that also supplies some passive protection in fire situations, it may in the future be su fficient to use only one of these methods for a preliminary evaluation of the potential passive fire protection capability.

**Figure 14.** Sketch of fire exposure, weather protection cladding (1), the layer of heat-resistant insulation (2), thermal insulation (3) and the object to be protected from fire exposure (4). The critical point to be kept below is a temperature of 1100 ◦C, which is marked on the figure.

In the future, it would be beneficial to do further fire testing of, e.g., small-scale jet fire tests [10,11] with an additional protective layer, as demonstrated in Figure 14. When properly chosen, this layer may then keep the exposed thermal insulation below the breakdown temperatures, thereby ensuring that it may contribute towards providing significantly prolonged fire protection.

Heat treatment tests in a mu ffle furnace that test other types of insulation, e.g., mineral-based passive fire protection or di fferent types of Rockwool insulation, could give more information about future possibilities. It would also be beneficial to measure the thermal conductivity of the thermal insulation at elevated temperatures. This could give the information required for developing a numerical model of the thermal insulation performance when exposed to fires, with or without a protective layer, as indicated in Figure 14.
