Abstract
Nanocellulose-based lightweight foams are promising alternatives to fossil-based insulation materials for energy-efficient buildings. The properties of cellulose-based materials are strongly influenced by moisture and there is a need to assess and better understand how the thermal conductivity of nanocellulose-based foams depends on the relative humidity and temperature. Here, we report a customized setup for measuring the thermal conductivity of hydrophilic materials under controlled temperature and relative humidity conditions. The thermal conductivity of isotropic foams based on cellulose nanofibrils and a nonionic polyoxamer, and an expanded polystyrene foam was measured over a wide range of temperatures and relative humidity. We show that a previously developed model is unable to capture the strong relative humidity dependence of the thermal conductivity of the hygroscopic, low-density nanocellulose- and nonionic polyoxamer-based foam. Analysis of the moisture uptake and moisture transport was used to develop an empirical model that takes into consideration the moisture content and the wet density of the investigated foam. The new empirical model could predict the thermal conductivity of a foam with a similar composition but almost 3 times higher density. Accurate measurements of the thermal conductivity at controlled temperature and relative humidity and availability of simple models to better predict the thermal conductivity of hygroscopic, low-density foams are necessary for the development of nanocellulose-based insulation materials.
Highlights
Development of high performance thermally insulating materials from renewable or widely abundant resources could substantially improve the energy efficiency of buildings and reduce the environmental impact (Papadopoulos 2005; Berge and Johansson 2012; IEA 2013)
We find that the RMSE for the higher density foam are of similar magnitude as for the lower density foam (Table 2), which suggests that the new empirical model (Eq 3) is able to describe the thermal conductivity for hygroscopic cellulose nanofibrils (CNF)- and nonionic polyoxamer-based foams of densities within the range 10–30 kg/m3
The thermal conductivity of isotropic CNF- and nonionic polyoxamer-based foams increased more than 3 times as the temperature and relative humidity increased from 261 K and 2% RH, to 314 K and 80% RH
Summary
Development of high performance thermally insulating materials from renewable or widely abundant resources could substantially improve the energy efficiency of buildings and reduce the environmental impact (Papadopoulos 2005; Berge and Johansson 2012; IEA 2013). Biopolymer-based materials such as cork and wood chips were extensively used for thermal insulation prior to the introduction of fossil fuel-based foams, but their insulating performance is relatively poor (Jelle 2011). Nanocellulose features an attractive combination of properties like a high elastic modulus, low thermal expansion coefficient, and tunable surface chemistry (Klemm et al 2011; Moon et al 2011; Duong and Nguyen 2016). The controlled surface chemistry, interparticle bonding and assembly of nanocellulose can result in nanocellulose foams with a high compressive strength and low thermal conductivity (Lavoine and Bergstrom 2017)
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