Mechanical, physical and thermal properties of composite materials produced with the basidiomycete Fomes fomentarius

Abstract

Background: To achieve climate neutrality, fundamentally new concepts of circularity need to be implemented by the building sector as it contributes to 40% of anthropogenic CO₂ emission. Fungal biotechnology can make a significant contribution here and help eliminate fossil dependency for building material production. Recently, we have shown that the medicinal polypore Fomes fomentarius feeds well on renewable lignocellulosic biomass and produces composite materials that could potentially replace fossil fuel-based expanded polystyrene as insulation material.

Results: In this study, we explored the mechanical, physical, and thermal properties of F. fomentarius-based composite materials in more detail and determined key performance parameters that are important to evaluate the usability of F. fomentarius-based composite materials in the construction sector. These parameters were determined according to European standards and included compressive strength, modulus of elasticity, thermal conductivity, water vapour permeability, and flammability of uncompressed composites as well as flexural strength, transverse tensile strength, and water absorption capacity of heat-pressed composites, among others. We could show that uncompressed composites obtained from F. fomentarius and hemp shives display a thermal conductivity of 0.044 W (m K)^-1 which is in the range of natural organic fibres. A water vapour permeability of 1.72 and classification into flammability class B1 clearly surpasses fossil-based insulation materials including expanded polystyrene and polyurethane. We could furthermore show that heat-pressing can be used to reliably generate stiff and firm particleboards that have the potential to replace current wood-based particleboards that contain synthetic additives. X-ray microcomputed tomography finally visualized for the first time the growth of hyphae of F. fomentarius on and into the hemp shive substrates and generated high-resolution images of the microstructure of F. fomentarius-based composites.

Conclusion: This study demonstrates that fungal-based composites produced with F. fomentarius partially meet or even exceed key performance parameters of currently used fossil fuel-based insulation materials and can also be used to replace particleboards.

Keywords: Bioeconomy; Circular economy; Composite material; Compressive strength; Flammability; Flexural strength; Fomes fomentarius; Fungal-based composite material; Hemp; Insulation; Mycelium-based material; Tensile strength.

Fig. 1

Compressive strength and thermal conductivity testing of fungal-based composite made of F. fomentarius and hemp shives. A Image of an exemplar specimen used for compressive and conductivity testing. B Stress–strain curves for all specimens with different height. C Compressive strength at 10% compression (δ₁₀) as a function of the density of all specimens. D Thermal conductivity values determined for all specimens with different height

Fig. 2

Flammability testing of an exemplar fungal-based composite made of F. fomentarius and hemp shives. A, B Specimen at the test facility at 1 s (A) and 30 s (B) after ignition. C, D Burning did not reach deeper layers of the composite and took place only on the surface

Fig. 3

Fungal-based composite made of F. fomentarius and hemp shives prior and post heat-pressing. A Composite appearance after 14 d of cultivation prior to drying. An evenly whitish-coloured coat defined by the aerial mycelium of F. fomentarius gives the composite surface a fluffy texture. B Cross-section of 5 cm composites after drying. C, D Top view and cross section of a board composite with a target thickness of 5 mm that was heat-pressed at 140 °C. Note the subtle brown colorization of the board surface. E Actual thicknesses of the boards after heat-pressing, which deviate from the target thicknesses of 5, 10 and 15 mm

Fig. 4

Experimental setup for material testing according to norm EN 312. A, B Cutting pattern and exemplar specimen of a F. fomentarius heat-pressed particleboards with 15 mm target thickness. C Apparatus for determining transverse tensile strength. D Apparatus for determining flexural strength

Fig. 5

Mechanical properties of heat-pressed particleboards made of F. fomentarius and hemp shives. A Flexural strength as a function of density; B Transverse tensile strength as a function of density; C Example for a cover layer from a 5 mm specimen that has been torn off the board; D Young`s modulus as a function of density. Data for 5, 10, 15 mm boards are given in blue, orange, and grey, respectively

Fig. 6

Physical properties of heat-pressed particleboards made of F. fomentarius and hemp shives. A Swelling in thickness as a function of density; B Water absorption capacity as a function of density. Data for 5, 10, 15 mm boards are given in blue, orange, and grey, respectively

Fig. 7

SEM and µCT images of composites made of F. fomentarius and hemp shives. A The surface of an uncompressed composite analysed via SEM, Bar, 20 µm; B Centre piece of an uncompressed composite analysed via µCT; C The surface of a heat-pressed particleboard with 5 mm target thickness, Bar, 20 µm; D, Centre piece of a heat-pressed particleboard with 5 mm target thickness analysed via µCT B, D * mycelium, o hemp xylem, Λ hemp pore

Fig. 8

The processed µCT images of composites made of F. fomentarius and hemp shives. F. fomentarius is shown in yellow and the hemp shives in red. The subpanels A-B show enlargements of the marked regions. Hyphae attaching to the hemp shive substrate (A) and hyphae within xylem vessels (B) are depicted. Note that the outermost layer of the hemp shive appears thicker than the inner cell, which could potentially hint at an ongoing decomposition process of lignin and (hemi-)cellulose, which remains to be shown C

Fig. 9

Hyphal branching frequency depends on the distance of the hyphae to the surface of hemp shives

Fig. 10

Comparison of composites made of F. fomentarius and hemp shives with different approved construction materials that are most frequently used in Germany. A Ashby diagram highlighting elasticity as a function of density. Dots correspond to the values obtained in this study or published earlier [10]. Black, uncompressed composites this study; yellow, uncompressed composites [10]; orange, 10 mm compressed composites; blue, 5 mm compressed composites. B Radar chart representing standardized values for different materials. The higher the value the better the performance on a scale between 0 and 100%. Raw data used for standardization were taken from this study and from data sheets of industrially used materials (Additional file 1: Fig S4)