Multiscale thermal and mechanical modelling of insulation composites

Thermal insulation materials are receiving a lot of attention, especially at the moment, due to the environmental issues, such as the global warming, we are facing. Latest reports about the Earth's climate prove that the heat loss in buildings and industrial processes is too high and that this redundant heat loss should be diminished to a minimum. Consequently, it has become an international task for the building industries to sufficiently improve the performance of the current insulation materials. The search for new types of insulation materials is therefore advancing. However, the most commonly used structural and finishing building materials, e.g. concrete and plaster, do have high thermal conductivities and this is the major problem to tackle. This doctoral study investigates the insulation performance of two promising insulation composites: silica aerogel modified plasterboards and silica aerogel filled glass fibre mats. In this thesis, different tools are developed to establish and to support the prediction and optimisation of the thermal and mechanical properties of the mentioned materials. The most effective and probably the most interesting constituent in this context is silica aerogel which is thoroughly investigated here. Silica aerogel has a very low thermal conductivity due to its nanoscale featured structure. In the first part of the research, first, the intrinsic material structure of silica aerogel and the occurring thermal phenomena which lead to the low thermal conductivity are investigated by means of finite element models and analytical calculations. These values are validated by experimental measurements from literature. Based on these simulations and findings, new insights on the silica aerogel structure with respect to the thermal conductivity are discussed. Second, experimental measurements are combined with a numerical modelling technique to propose an optimised silica aerogel distribution in gypsum plaster. As the silica aerogel properties are mainly caused by nanoscale features and its distribution is microscale oriented, this study covers a multiscale problem. Third, due to the fact that silica aerogel mechanically weakens the structure in which it is embedded, an estimation is carried out with respect to the amount of loss in bending stiffness it entails. This estimation is done by use of homogenisation techniques combined with finite element model simulations. The simulations show a decreasing dependence between the bending resistance and the amount of silica aerogel. At last, new types of composite structures are proposed to cope with the induced weakness of adding silica aerogel to gypsum plaster. The bending resistance of a sandwich-like structure and a composite beam structure is predicted by use of analytical calculations and validated by finite element simulations. Besides the addition of silica aerogel to gypsum plaster, it was also the aim to investigate the thermal insulation performance of a silica aerogel filled glass fibre mat. The second part summarises therefore the carried out investigations with respect to silica aerogel containing fibrous materials. First, the estimation of the thermal conductivity of a glass fibre matrix is performed without the presence of silica aerogel. The used finite element simulations are benchmarked against experimental measurements. Second, the simulations are adapted to take into account the silica aerogel to examine their ability to resist thermal conductance. At last, due to installation handling's and fixation of the blankets (e.g. by tapes or ropes), the fibre mat is locally subjected by a compression load. It is expected that this compression leads to an increase of the thermal conductivity as the fibres are getting packed. The influence of the compression of a glass fibre mat is evaluated by use of finite element simulations. From an industrial point of view, the proposed finite element models support the industrial company to find the optimised distribution of silica aerogel in any matrix material. Moreover, the findings described in the dedicated and last part of the research, support the industrial partner to sufficiently balance the production cost and the thermal insulation performance of the composite. The expected turnover for silica aerogel plasterboards and the possible market positions of the industrial partner are discussed.status: publishe