**5.1. Composite structure: organoleptic assessment**

The most uniform and homogeneous structure is observed for matrix materials. In the case of the composites, the structure depends on the reinforcement type. The photographs in **Figure 7** present the structure of the selected composites.

**Figure 7.** Photographs of the composites.

If the fibres are used as a reinforcement, the composite structure is more uniform. It results from the uniform structure of nonwoven composed of reinforcing and matrix fibres and from their high mixing level. The external and internal layers of nonwoven are pressed giving uniform, homogeneous porous structure. On the composite surface, the reinforcing fibres, matrixes and voids are visible. If the fibres and straw are used as a reinforcement, the composite structure is more diversified. The addition of straw particles leads to the formation of larger voids inside the composite structure. If only the straw is used as a reinforcement, the layers of matrix and straw can be visible. Inside the composite structure, the straw particles are partially bound with the matrix, and the voids exist. The composite surfaces are formed from the matrix without visible voids. Generally, the straw gives larger voids than the fibres and the larger straw particles give larger voids. In the case of composite, on the basis of the wheat straw, the additional voids are inside the straw particles. In the case of composites with cellulose ultrashort/ultra-fine fibres, the variants with standard fibres give more uniform structures than without, because standard fibres prevent their agglomeration.

#### **5.2. Physical and mechanical properties**

The physical properties as a thickness and apparent density and mechanical properties as a tensile stress at the maximum load are presented in **Table 2**. The thickness and density of the material are the most important factors determining its acoustic properties [21]. In this work, it was assumed to obtain composites of very low thickness and low density. A small thickness of millimetres is important when the composite material is combined with other materials. The low density is important to produce light materials.

## *5.2.1. Composites on the basis of flax fibres and straw*

The thickness of the composites is in the range from 3.52 to 4.71 mm. The thickness of the PLA/ straw composites is higher on average by 26.90% than thickness of the PLA/LI/straw composites, and because of that the density is correspondingly lower. The tensile stress at maximum load of the composites depends on a reinforcement type. The highest value of tensile stress, i.e., 2.69 MPa, is determined for the PLA/LI composite characterized by the medium value of apparent density. If the matrix material is used in the form of fibres, their mixing with reinforcing fibres can be excellent, especially in the needle-punched nonwoven. It results in the higher tensile stress of the composite. The lower mixing level of the thermoplastic fibres with straw particles, or with straw particles and reinforcing fibres, gives lower tensile stress of the composite. It means that flax fibres are better reinforcing materials than straw and the level of mixing of reinforcing and matrix materials in nonwoven is high what leads to suitable consolidation. This is confirmed by the PLA/LI/straw composites, for which the values of tensile stress at maximum load are higher than for the PLA/straw composites and lower than PLA/LI composites. The differences in apparent density of the composites have no significant effect on the tensile stress [20].

#### *5.2.2. Composites on the basis of cotton fibres and cellulose ultra-short/ultra-fine fibres*

The composites obtained (**Table 2**) were characterized by similar, very low thickness on the level of about 4.7–5.5 mm. These differences in the thickness of composites are insignificant for their acoustical properties [16].


SS, grinded sunflower straw; CS, grinded corn straw; WS, random cut wheat straw.

**Table 2.** Characteristics of the composites [20, 21].

their high mixing level. The external and internal layers of nonwoven are pressed giving uniform, homogeneous porous structure. On the composite surface, the reinforcing fibres, matrixes and voids are visible. If the fibres and straw are used as a reinforcement, the composite structure is more diversified. The addition of straw particles leads to the formation of larger voids inside the composite structure. If only the straw is used as a reinforcement, the layers of matrix and straw can be visible. Inside the composite structure, the straw particles are partially bound with the matrix, and the voids exist. The composite surfaces are formed from the matrix without visible voids. Generally, the straw gives larger voids than the fibres and the larger straw particles give larger voids. In the case of composite, on the basis of the wheat straw, the additional voids are inside the straw particles. In the case of composites with cellulose ultrashort/ultra-fine fibres, the variants with standard fibres give more uniform structures than

The physical properties as a thickness and apparent density and mechanical properties as a tensile stress at the maximum load are presented in **Table 2**. The thickness and density of the material are the most important factors determining its acoustic properties [21]. In this work, it was assumed to obtain composites of very low thickness and low density. A small thickness of millimetres is important when the composite material is combined with other materials. The

The thickness of the composites is in the range from 3.52 to 4.71 mm. The thickness of the PLA/ straw composites is higher on average by 26.90% than thickness of the PLA/LI/straw composites, and because of that the density is correspondingly lower. The tensile stress at maximum load of the composites depends on a reinforcement type. The highest value of tensile stress, i.e., 2.69 MPa, is determined for the PLA/LI composite characterized by the medium value of apparent density. If the matrix material is used in the form of fibres, their mixing with reinforcing fibres can be excellent, especially in the needle-punched nonwoven. It results in the higher tensile stress of the composite. The lower mixing level of the thermoplastic fibres with straw particles, or with straw particles and reinforcing fibres, gives lower tensile stress of the composite. It means that flax fibres are better reinforcing materials than straw and the level of mixing of reinforcing and matrix materials in nonwoven is high what leads to suitable consolidation. This is confirmed by the PLA/LI/straw composites, for which the values of tensile stress at maximum load are higher than for the PLA/straw composites and lower than PLA/LI composites. The differences in apparent density of the composites have no significant

*5.2.2. Composites on the basis of cotton fibres and cellulose ultra-short/ultra-fine fibres*

The composites obtained (**Table 2**) were characterized by similar, very low thickness on the level of about 4.7–5.5 mm. These differences in the thickness of composites are insignificant

without, because standard fibres prevent their agglomeration.

**5.2. Physical and mechanical properties**

226 Composites from Renewable and Sustainable Materials

low density is important to produce light materials.

*5.2.1. Composites on the basis of flax fibres and straw*

effect on the tensile stress [20].

for their acoustical properties [16].

For studied composites, the 20% addition of cellulose sub-microfibres results in increase in their mechanical properties. The composite manufactured under the same technological conditions but without the addition of sub-microfibres (PLA/CO) has the lowest tensile stress at maximum load equals to 0.79 MPa. The results show the relationship between the mechanical properties of the composites with cellulose sub-microfibres and the method of the submicrofibres preparation.

For the composites with sub-microfibres obtained from waste flax fibres, the tensile stress at maximum load is diversified depending on the sub-microfibres preparation. The tensile stress at maximum load for the composite manufactured with sub-microfibres obtained from waste flax fibres gains the value of 1.20 MPa and for the composites with sub-microfibres additionally modified by silane (LI-2, LI-3), the value of 2.27 MPa is twice higher apart from the conditions of silane modification. These results correspond to the degree of crystallinity of the cellulose sub-microfibres. The same relationship is observed for the composites manufactured with submicrofibres from straws, after silane modification the cellulose sub-microfibres give better reinforcement than without silane modification. The values of tensile stress at maximum load of the composites with sub-microfibres obtained from each kind of straw after silane treatment are from 9.05 to 38.76% higher than the values of tensile stress of composites with corresponding sub-microfibres obtained without silane modification. The best results of tensile stress are observed for composites with sub-microfibres obtained from SRFF straw.

The tensile stress at maximum load for the composites manufactured with sub-microfibres obtained from the straw of retted fibre flax is 1.29 MPa and for the composites with submicrofibres additionally modified by silane (SRFF-2) reaches the value of 1.79 MPa. It means that silane modification gives favorable conditions for adhesion between the cellulose submicrofibres and the PLA matrix. It results advantageously in tensile strength of composites. This effect is more distinct for the composites with sub-microfibres from flax fibres than with sub-microfibres from straws. The tensile strength is the highest for composites with submicrofibres obtained from waste flax fibres and modified by silane (LI-2, LI-3). Both the silane modification favoring good adhesion to polymer and a high degree of crystallinity of submicrofibres resulted in the highest values of tensile stress at maximum load for these composites.

In the case of apparent density, which is a very important factor deciding about sound absorption, the differences are more considerable. Density of the composites with cellulose sub-microfibres obtained from waste flax fibres and modified by silane with additional water activation (LI-3) is higher by 4% than the density of the composites with the same cellulose sub-microfibres but modified by silane without water activation (LI-2), and by 15% higher than the density of the composites with the sub-microfibres not modified by silane (LI-1).

Density of the composites with cellulose sub-microfibres obtained from straws and modified by solution of silane in acetone is higher by 12.1% for SRFF straw, 12.8% for SOF straw, and 21.6% for HS straw than the density of the corresponding composites with sub-microfibres not modified by silane [22]. For the studied composites, these differences in material density are not significant from their sound absorption point of view [16].
