Thermally insulating and fire-retardant lightweight anisotropic foams based on nanocellulose and graphene oxide

Journal name:
Nature Nanotechnology
Year published:
Published online


High-performance thermally insulating materials from renewable resources are needed to improve the energy efficiency of buildings. Traditional fossil-fuel-derived insulation materials such as expanded polystyrene and polyurethane have thermal conductivities that are too high for retrofitting or for building new, surface-efficient passive houses. Tailored materials such as aerogels and vacuum insulating panels are fragile and susceptible to perforation. Here, we show that freeze-casting suspensions of cellulose nanofibres, graphene oxide and sepiolite nanorods produces super-insulating, fire-retardant and strong anisotropic foams that perform better than traditional polymer-based insulating materials. The foams are ultralight, show excellent combustion resistance and exhibit a thermal conductivity of 15 mW m−1 K−1, which is about half that of expanded polystyrene. At 30 °C and 85% relative humidity, the foams retained more than half of their initial strength. Our results show that nanoscale engineering is a promising strategy for producing foams with excellent properties using cellulose and other renewable nanosized fibrous materials.

At a glance


  1. Fabrication and overview of the mechanical, thermal and fire-retardant properties of nanocomposite foams.
    Figure 1: Fabrication and overview of the mechanical, thermal and fire-retardant properties of nanocomposite foams.

    a, Illustration of freeze-casting process, highlighting the growth of anisotropic ice crystals surrounded by walls of the dispersed nanoparticles (not drawn to scale). b, Photograph of the 77% CNF/10% GO/10% SEP/3% BA nanocomposite foam. c, A CNF–GO–BA–SEP-based composite foam with a density of 7 kg m−3 can sustain a 100 g weight. d, Schematic illustration of the thickness of expanded polystyrene (EPS) and an optimized nanocomposite foam needed for passive house insulation (energy loss ≤100 mW m−2 K−1). The thicknesses of the brick and insulating layers are given in mm and calculated using λ values of 35 mW m−1 K−1 and 15 mW m−1 K−1 for EPS and the nanocellulose-graphene oxide (NC-GO)-based composite foam, respectively. e, Burning an ethanol-soaked nanocomposite foam of the same composition as in b results in a carbonized residue with a similar shape as the original material.

  2. Microstructure of freeze-cast nanocomposite foams.
    Figure 2: Microstructure of freeze-cast nanocomposite foams.

    a, SEM cross-section image of a freeze-cast nanocomposite foam containing cellulose nanofibres (CNF), graphene oxide (GO), sepiolite (SEP) and boric acid (BA). b, Three-dimensional reconstruction of the tubular pore structure of the nanocomposite foam derived from X-ray microtomography. c, X-ray microtomography image showing that the tubular pores are straight and several millimetres long in nanocomposite foam with a composition of 77% CNF/10% GO/10% SEP/3% BA (in wt%). d,e, X-ray microtomography cross-sections of a nanocomposite foam, taken through upper and lower parts, respectively (scale bars, 100 µm). f, HRSEM image of a foam wall, where the yellow dotted line indicates a section of the tubular cell. Inset: Distributed SEP nanorods within the cell wall. The nanomaterials are homogeneously distributed in the cell walls, forming an anisotropic tubular pore structure as result of the unidirectional freeze-casting process.

  3. Thermal transport properties of anisotropic nanocomposite foams.
    Figure 3: Thermal transport properties of anisotropic nanocomposite foams.

    a, Schematic illustration of contributions to thermal conductivity in the radial and axial directions of a foam with oriented pores (not drawn to scale). b, Thermal conductivity values in the axial and radial directions of CNF and CNF–GO–BA–SEP foams, compared with expanded polystyrene (EPS). The thermal conductivity normal to the tubular pores is significantly lower than the value for air. c,d, Schematic (left) and thermographic (right) images of CNF–GO–BA–SEP nanocomposite foam with the tubular pores oriented normal to the heat source (c) and a closed-cell EPS foam (d). The heated volume of the CNF–GO–BA–SEP nanocomposite foam is smaller and more homogeneous than that of the EPS foam. The colours in the thermographic images show the temperature distribution on the surface of the foams. The rectangular nanocomposite foam in c (right image) was pressed onto the heat source with a weight made of zirconia.

  4. Mechanical properties and crosslinking of nanocomposite foams.
    Figure 4: Mechanical properties and crosslinking of nanocomposite foams.

    a, Stress–strain measurements of a nanocomposite foam containing 77% CNF, 10% GO, 10% SEP and 3% BA (in wt%), determined in both the strong and stiff axial orientation and the much weaker radial orientation with respect to the tubular pores. b, Young's moduli E of the nanocomposite foams, obtained from the elastic region of the compression tests. Red arrows highlight the increase in Young's modulus brought about by the addition of BA. c, Infrared spectra of CNF and BA and a differential spectrum of CNF–BA with respect to CNF. The spectra indicate that BA interacts with the hydroxyl and carboxyl groups of CNF. d, Young's moduli E of a nanocomposite foam before, during and after exposure to 85%rh. The measurements show partial recovery of the mechanical properties after cycling in moist conditions. The error bars in b and d are the standard deviation values of four replicas.

  5. Flame resistance of the nanocomposite foams.
    Figure 5: Flame resistance of the nanocomposite foams.

    a, Vertical burning test (UL94) of a nanocomposite foam containing 77% CNF, 10% GO, 10% SEP and 3% BA (in wt%). The panel shows the foam before the test, after 11 s of application of a methane flame, and the foam after the test, showing high fire retardancy. b, Photographs of CNF and CNF–GO–BA–SEP nanocomposite foams during the cone calorimetry test together with the corresponding peak of heat release rates (pkHRR). The test reveals high combustion resistance for the nanocomposite foam at the limit of non-ignitability, while CNF foams are entirely combusted. c, Raman spectra of a CNF–GO–BA–SEP nanocomposite foam before (trace i) and after burning indicating BA-mediated graphitization of nanocellulose (trace ii). d, Infrared spectra and corresponding photograph of a CNF–GO–BA–SEP foam after burning show a clay-rich crust and the bulk underneath. Both the graphitization and the formation of the protective crust contribute to the combustion resistance of the foams.


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Author information


  1. Department of Materials and Environmental Chemistry, Stockholm University, 106 91 Stockholm, Sweden

    • Bernd Wicklein,
    • German Salazar-Alvarez &
    • Lennart Bergström
  2. Engineering Ceramics Department, Jožef Stefan Institute, 1000 Ljubljana, Slovenia

    • Andraž Kocjan
  3. Wallenberg Wood Science Centre, Royal Institute of Technology, KTH, 100 44 Stockholm, Sweden

    • German Salazar-Alvarez
  4. Politecnico di Torino, Corso Duca degli Abruzzi, 24 10129 Torino, Italy

    • Federico Carosio &
    • Giovanni Camino
  5. Max Planck Institute for Colloids and Interfaces, Potsdam-Golm Science Park, Am Mühlenberg 1, 14476 Potsdam, Germany

    • Markus Antonietti


B.W., G.S.A., L.B. and M.A. conceived the study and B.W. and L.B. planned and coordinated the work. A.K. performed the thermal conductivity measurements and analysed the data together with B.W. B.W. carried out and analysed the mechanical strength data. B.W., F.C. and G.C. carried out and analysed the flame retardancy measurements. B.W., G.S.A. and L.B. wrote the manuscript.

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