Superinsulating sol-gel silica are very promising for thermal superinsulation at atmospheric pressure. However, new solutions need to be developed in order to overcome their mechanical weakness. Resorting to hyperporous cellulosic structures is one of the most pertinent solutions conceivable, as this approach allows reaching aforementioned improvement without augmenting the silica's environmental impact.
In the last decade, active research and development of new very efficient thermal insulators, called superinsulators, was induced internationally. Until now, an essential part of this work was focussed on nanostructured silica. The silica obtained present very promising thermal characterstics, but are mechanically fragile. This fragility presents a major drawback for their substantial development, particularly for building envelopes. One of the most promising solutions to this drawback resides in the hybridisation of superinsulating silica with an organic structure. However, no sufficient efficient organic structures are known yet other than petrochemical derivatives (polyurethane, polycylopentadiene, ...). Therefore, the project NANOCEL proposes developing a high-performance insulating nanostructured cellulosic matrix suitable for mechanical reinforcement of superinsulating silica - at low costs and low environmental impact - without deteriorating the very low thermal conductivity. The first part of the project focuses on creating finely structured and therefore little thermally conducting ultraporous cellulosic matrices, whereas the second part consists in testing these cellulosic matrices as binder material for morselled superinsulating silica for building envelopes.
For the preparation of cellulose matrices, two methods have been selected according to whether cellulose or one of its derivatives is used. The first method, using cellulose, is based on the regeneration (or coagulation) of cellulose solutions (cellulose dissolved in sodium hydroxide or ionic liquids) in a non-solvant (ethanol). The second method consists in the sol-gel synthesis of cellulose acetate based gels by chemical crosslinking in acetone. The first process results in physical cellulose gels, the second in chemical cellulose acetate gels. These gels of open porosity are subsequently dried in a flux of supercritical CO2 in order to preserve their initial morphology partly or totally by preventing the emergence of capillary stress. Cellulosic matrices prepared according to the two above-presented appraoches are used to bind superinsulating granular silica. A casting process on granulate material has been set up to this effect. The dry material obtained is caracterised elaboratedly concerning physico-chemical properties ( structure, texture, ...), thermal properties (thermal conductivity, specific heat, ...), and hydric properties (sorption, ageing, ...). Results of these caracterization will partly be used to create a physical model to predict their thermal conductivity. Lastly, a life cycle analysis completes the works carried out on this new family of «green« superinsulators.
Superinsulating cellulosic matrices have been obtained by sol-gel crosslinking of cellulose acetate. These matrices feature thermal conductivities very close to those of the most efficient superinsulating silica (i.e. 0.015 W/m.K) in ambient conditions. These cellulosic matrices have been used to improve - in a broad sense - the mechanical stability of superinsulating silica. Notably, they lead to cohesion between the silica granular bed and thereby drastically reduce mineral powdering. The study concerning thermo-hydric ageing has been initiated. Certain matrix formulations seem relatively resistant to thermo-hydric ageing. In parallel, new pectin-based aerogels have been synthesized; their thermal conductivities seem promising: about 0.025 W/m.K in ambient conditions. Further, a measuring technique characterizing directly the thermal conductivities of superinsulators of small dimensions (diameters of 2 cm) has been developed. This method has been proven to be very beneficial for smooth progress of the project. Finally, new numerical (and analytical) models of thermal conductivity of cellulosic matrices have been developed and validated on the basis of experimental measurements (textural and spectroscopic) effected throughout the whole project.
The results obtained in NANOCEL are quite remarkable. For the first time cellulose superinsulators are prepared. Therefore, it is now crucial to try to improve: i) the hydric behaviour of matrices based on crosslinked cellulose acetate by hydrophobizing them and ii) their preparation process by developing a way of subcritically drying these matrices. At the same time, taking into account the biobased origin of pectine-based matrices, it seems equally important to continue honing their internal texture in order to reduce the associated thermal conductivity even further.
NANOCEL has already resulted in 2 articles published in international peer-reviewed journals, 12 communications at conferences (10 of them international) and a presentation at a European Workshop. Further A grade articles are currently in progress.
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
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