Foamed glass, a sustainable thermal insulation material, is widely used for the insulation of buildings, industrial piping, etc. The main raw material for its production is waste cullet and material is fully recyclable once used. The use of foaming agents, which release gaseous components during the foaming process, yields a porous material. Carbon-based foaming agents give products with good properties in a reducing atmosphere, whereas in an air atmosphere the carbon oxidises too quickly and foaming does not occur.
In my master thesis I investigated the possibilities of foaming in an air atmosphere, using a secondary foaming agent - water glass (WG), to protect the carbon-based foaming agent. The aim was to prevent the carbon from oxidising too quickly with ambient oxygen. I used glycerol or black carbon as the primary foaming agent and added agents to improve the final properties of the product. I determined the optimum composition of the foaming mixture and the foaming conditions. The samples were analysed by different methods at different stages of the preparation: i) analysis of the foaming process (changes during heating by Heating stage microscope and evolution of gases during the process by thermogravimetry coupled to mass spectrometry), ii) analysis of the physical properties (thermal conductivity and strength of the product), and iii) characterisation of the structure (crystallinity by X-ray powder diffraction, pore diameter by stereological analysis of photographed sample surfaces and microstructure analysis by scanning electron microscope).
Samples with glycerol and WG showed a high degree of crystallisation, which is reduced by the addition of the crystallisation inhibitor K3PO4 and larger raw material particles. However, this combination did not show sufficient potential as a thermal insulation material. In the second stage, I used a combination of black carbon and WG and determined the optimum amount of WG addition, i.e. 24 % (19 wt.%). This adequately protects the carbon to give samples with lower density and a higher porosity percentage, but at the same time results in a higher crystallisation of the sample.
Unwanted open porosity, often due to crystallisation, has been a major challenge in all previous samples. Therefore, in the final stage, I optimised the composition with additives (K3PO4, AlPO4, B2O3), reducing crystallisation and improving the foaming process. The proportion of crystalline phases is notably lower in the X-ray diffractograms of the samples with additives. I determined the foaming composition that gave the samples with the best properties. Foaming at 790 °C (τ=30 min) without drying of the foaming compound gives a less crystallised sample with good properties: ρapp=96 kg/m3, εtot=96 %, CP=94 %, thermal conductivity λ=41 mW/m·K, compressive strength σ=0,23 MPa. SEM analysis shows the presence of fine pores in the walls of the larger pores, which gives the product higher strength and better insulating properties.
The properties of the optimal composition are comparable to the best commercial sample (ρ=100 kg/m3, λ=36 mW/m·K, σ=0.5 MPa), but it is prepared in a reductive atmosphere and has a significantly larger carbon footprint. Further research could focus on improving the strength of the material and further improving the thermal insulation performance of the optimal composition.
|
