Polyurethane aerogels synthesis for thermal insulation – textural, thermal and mechanical properties
Graphical abstract
Introduction
In a context where energy consumption limitation and demand control appear as major environmental and economic issues, it is necessary to reduce primary energy consumption in the building sector. This domain is responsible for a significant portion of global energy consumption, especially due to thermal losses through building envelopes. The use of innovative, performing materials called superinsulating materials is considered as a very most promising market for building retrofit, where it could allow a significant decrease of the energy requirements for existing buildings while maintaining their integrity and architectural quality. These solutions currently exist under two families: vacuum insulation panels (VIP) [1], that exhibit extremely low thermal conductivities but can prove difficult to be handled, and atmospheric pressure super-insulation materials, mainly consisting in light, mesoporous and nanostructured materials called aerogels [2].
Their specific texture is obtained via two main preparation steps. In a first part, the sol–gel synthesis allow the formation of a chemical gel, a tridimensional continuous solid network interpenetrated with a liquid phase, usually in convenient ambient conditions [3]. In a second part, the liquid phase is in most cases extracted in supercritical conditions. This drying method maintains the delicate network characteristics of the aerogels [4]. The use of CO2 as extracting solvent is now established in the aerogel field thanks to its low critical temperature, non-flammability and low toxicity [5].
In the field of these superinsulation materials at atmospheric pressure, silica aerogels are still considered as the most efficient materials in terms of thermal insulation, with a thermal conductivity as low as 0012 W m−1 K−1 in room conditions [6]. These silica-based materials are already commercialized, for instance for interior insulation solutions for building retrofit as well as slim façade insulation for the renovations of historical buildings [7], [8]. However, a low thermal conductivity is not the only relevant criteria that insulation materials must fulfill in the building and construction sector, and these mineral aerogels usually appear very fragile in a mechanical point of view, in particular the apparition of dusting leading to thermal performance degradation [9]. In parallel, some organic aerogels currently present very promising thermal performance with a mechanical behavior that seems better than mineral counterparts on the whole. In particular they can stand higher strain and loads at given densities faced with both compression and flexion stress, such as resorcinol-formaldehyde, polyimide or polyurea based aerogels [10], [11], [12], [13]. Among these materials, organic aerogels based on polyurethane turn out to be part of the most promising [14], [15].
This study focuses on an organic aerogel based on polyurethane matrix, that was already presented in previous articles [16], [17]. The aim of the present work is to study the main properties of a polyurethane aerogel in order to reach an efficient thermal-mechanical compromise. The materials were prepared through sol–gel synthesis and supercritical drying in CO2 in a new reaction medium.. The effect of one key formulation parameter, the catalyst concentration, was studied in terms of textural, thermal conductivity and mechanical characteristics. The two latter have been especially investigated in connection with the evolution of bulk density.
Section snippets
Materials
Pentaerythritol was purchased from Alfa Aesar, (purity > 98%). Poly-(diphenylmethane-isocyanate) (p-MDI) commercialized under the name of Lupranat M20S was gracefully provided by BASF Polyurethane. It consists in a blend of monomers and oligomers derived from 4,4′-methylenebis(phenylisocyanate), mainly 4,4′ diphenylmethane diisocyanate with 2,4′ diphenylmethane diisocyanate and 2,2′diphenylmethane diisocyanate isomers. Tris(dimethylaminomethyl)-phenol commercialized under the name of DABCO TMR
Influence of DABCO TMR concentration on gelation time
The solutions containing all reactants initially appear transparent, and turn opaque shortly before the sol–gel transition. This is due to the growth of primary particles, that can interact with visible light wavelength [14]. The gelation time is measured with a straightforward method: after magnetic stirring (0.3 min), the vial is regularly and carefully tilted, and tg is measured when the samples does not flow or deform anymore. Gelation time tg varies from 0.5 to more than 35 min (±0.1 min)
Conclusion
To conclude, this study has focused on the textural, thermal and mechanical properties of polyurethane aerogels, synthetized through a sol–gel synthesis and a supercritical CO2 drying, as a function of the catalyst concentration in the initial reacting medium. The decrease of catalyst concentration first allowed a significant decrease in the reaction kinetic, characterized with the gelation time. This parameter also influences the texture of the polyurethane samples, with a significant
Acknowledgments
This study is part of the French project. This work was supported by the French Agency for Environment and Energy Management (ADEME), project SIPA-BAT. PCAS (Fine and Specialty Chemicals), CSTB (Scientific and Technical Center for Building) and EDF are warmly acknowledged for fruitful discussions. We thank P. Ilbizian (PERSEE MINES ParisTech) for supercritical drying, S. Jacomet (CEMEF MINES ParisTech) for help with SEM observations, K. Amro (Laboratoire Charles Coulomb, Université Montpellier)
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