3.1.5. Mass Loss Test

The composite specimens of series 2 were placed on the mycelium with the usable surface in direct contact with it. The flasks with specimens were transferred to the incubating chamber for 4 months. After the incubation period, the specimens were removed from the flasks. Half of the specimens underwent manual mycelium removal with a soft brush. The other half underwent mycelium removal using pressurized water and were dried for 7 days at 40 ◦C. Furthermore, the specimens were dried at 103 ◦C to determine the final dry mass after their exposure to fungus. Loss in mass was calculated according to (1). The result is expressed in %.

$$
\Delta \mathbf{m\_{d}} = (\mathbf{m\_{i}} - \mathbf{m\_{c}}) / \mathbf{m\_{c}} \cdot 100,\tag{1}
$$

where mi—initial dry mass of the specimen, in grams; mc—final dry mass of the specimen, in grams.

#### *3.2. Bending Strength and Modulus of Elasticity after Exposure to Coniophora Puteana*

The preparation and exposure to fungus of the specimens used to determine the flexural strength of the composites after fungal decay were carried out in the same way for the mass loss specimens (3.1.) After the incubation period of 4 months, the specimens were removed from the flasks, cleaned of mycelium by two methods and dried in an environment-controlled room at 20 ◦C and 65% humidity.

The specimens were tested for flexural strength and modulus elasticity. The specimens were placed with the usable side down to apply tension to the fungus-exposed surface. The load was applied at a rate of 2 mm/min until destruction.

The results obtained for the specimens exposed to fungi were related to the results obtained for the specimens in the initial state. The initial state results were obtained from previous tests for the same materials [20]. This made it possible to determine, according to (2), the effect of this exposure on the tested materials. The result is expressed in %.

$$
\Delta \sigma\_{\rm{d}} = (\sigma\_{\rm{d}} - \sigma\_{\rm{i}}) / \sigma\_{\rm{i}} \cdot 100 , \Delta E\_{\rm{d}} = (E\_{\rm{d}} - E\_{\rm{i}}) / E\_{\rm{i}} \cdot 100 , \tag{2}
$$

where σd, Ed—the flexural strength and modulus of elasticity after fungi exposure, in MPa; σi, Ei—the initial flexural strength and modulus of elasticity, in MPa.

### **4. Results and Discussion**

Figures 1a and 2a illustrate the specimens of composites after exposure to *Coniophora puteana* over 16 weeks. The oat husk-reinforced composites demonstrated higher resistance to fungal overgrowth. The millet husk-composites showed significant susceptibility to *Coniophora puteana*. The mycelium was removed manually from both types of specimens using a soft brush (Figures 1b and 2b). The specimens, which were cleaned by water, looked the same, but had to be dried for 7 days at 40 ◦C to remove excess moisture.

**Figure 1.** Oat husk-reinforced composite specimens after (**a**) exposure to *Coniophora puteana*; (**b**) mycelium removal.

**Figure 2.** Millet husk-reinforced composite specimens after (**a**) exposure to *Coniophora puteana*; (**b**) mycelium removal.

The mass loss test results of the composites with oat filler obtained after 14 days of leaching in water, followed by 4 months of exposure to fungi at 22 ◦C and at 70% humidity, are identical for both methods of mycelium removal (Figure 3). The mass loss for this material is 0.3% compared to the initial state, demonstrating stable results regardless of the cleaning method. The composites reinforced with millet husks show a higher loss in mass and a larger difference between the two methods of mycelium removal (0.8% following cleaning with a brush and 2.3% following cleaning with water). The microstructure of the millet filler composite is irregular with large pores [20] that may cause fragments of filler to be pushed out by water under pressure. Soft brushing protects the specimens from this process, but also may affect the mass loss.

**Figure 3.** Mass loss test results of oat and millet husk-reinforced composites after exposure to Coniophora puteana and manual mycelium removal with a soft brush (B) and with water under pressure (W). The results are expressed in %.
