**5.3. Acoustic properties**

Acoustic properties were characterized by sound absorption coefficient, which values can be in the range from 0 (total reflection) to 1.00 (total absorption). According to EN ISO 11654, low absorption is from 0.15, absorption from 0.30, high absorption from 0.60 and very high absorption from 0.80.

For studied materials not only the value of sound absorption coefficient is important, but also the sound frequency range of high absorption.

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

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

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 compo-

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

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

Acoustic properties were characterized by sound absorption coefficient, which values can be in the range from 0 (total reflection) to 1.00 (total absorption). According to EN ISO 11654, low absorption is from 0.15, absorption from 0.30, high absorption from 0.60 and very high

For studied materials not only the value of sound absorption coefficient is important, but also

the density of the composites with the sub-microfibres not modified by silane (LI-1).

not significant from their sound absorption point of view [16].

the sound frequency range of high absorption.

observed for composites with sub-microfibres obtained from SRFF straw.

228 Composites from Renewable and Sustainable Materials

sites.

**5.3. Acoustic properties**

absorption from 0.80.

The biomass is very diversified and gives a very diversified effect from the acoustical point of view. **Figures 8**–**10** present the most characteristic dependences of the sound absorption coefficient-sound frequency for PLA materials, PLA/LI fibres, PLA/straw and PLA/LI fibres/ straw composites. Among studied composites, the highest values of the sound absorption coefficient, even in the range of 0.8–0.9, are determined for the PLA/straw composites [20].

**Figure 8.** Sound absorption coefficient of the composite-waste flax fibres and sunflower straw as a reinforcement.

**Figure 9.** Sound absorption coefficient of the composites-waste flax fibres and corn straw as a reinforcement.

**Figure 10.** Sound absorption coefficient of the composites-waste flax fibres and wheat straw as a reinforcement.

The sound absorption coefficient of the PLA matrix material is the lowest and has the value of about 0.1 at the low and mid-frequencies, at high frequencies it increases to 0.38 at 6400 Hz. This material cannot be used as a sound absorber of frequencies lower than 6000 Hz.

If nonwoven from blend of PLA matrix fibres and diversified waste flax fibres is used, the sound absorption of the composite is growing with frequency. The sound absorption coefficient of the composite from the 'PLA/LI (80/20%)' nonwoven is higher and depends linearly on a sound frequency arriving maximum value of 0.40 at a maximum studied frequency of 6400 Hz. This almost linear dependence is typical for fibre-based composites [19]. Even addition of particles thicker than fibres gives similar dependence, but with higher sound absorption. If the 20% of 'PLA/LI (80/20%)' nonwoven is replaced by straw, the sound absorption is more higher in the range from 3000 to 6400 Hz. It probably results from the fact that beside of small voids in composite, the bigger voids are formed. The sound waves of such frequencies can be reverberated repeatedly inside the diversified voids until soundproofing. The upward trend is evident and similar for all three kinds of composites with straw, as shown in **Table 3**. For the composite from a multilayer structure consisting of 80% of PLA/LI nonwoven and 20% of corn straw, the sound absorption coefficient increases linearly with sound frequency, and at 6400 Hz it is equal to 0.76. For the composites with sunflower or wheat straw as a reinforcement, the maximum value is 0.71 and 0.68 at 6400 Hz, respectively.

If PLA nonwoven and straw are used, the acoustic characteristic of the composite is totally different. The curve of the sound absorption coefficient-sound frequency dependence has another course. For each PLA/straw composite this course is slight different, because the kind of particle of each straw is slight different. Generally, the maximum values of sound absorption coefficient occur at medium (for PLA/SS and PLA/CS composites) and high (for PLA/WS composite) values of the studied frequency range. It means that the PLA/straw composites can be used as a material absorbing the sound of medium and high frequencies. The maximum values of the sound absorption coefficient of the PLA/straw composites are higher than those of PLA/LI or PLA/LI/straw composites. The sound absorption coefficient reaches the values of


0.8–0.9. For the PLA/straw composites, the frequency range of the maximum values of the sound absorption coefficient is wider than for PLA/LI or PLA/LI/straw composites.

**Table 3.** The sound absorption coefficients of the PLA/LI/straw composites.

**Figure 10.** Sound absorption coefficient of the composites-waste flax fibres and wheat straw as a reinforcement.

This material cannot be used as a sound absorber of frequencies lower than 6000 Hz.

the maximum value is 0.71 and 0.68 at 6400 Hz, respectively.

230 Composites from Renewable and Sustainable Materials

The sound absorption coefficient of the PLA matrix material is the lowest and has the value of about 0.1 at the low and mid-frequencies, at high frequencies it increases to 0.38 at 6400 Hz.

If nonwoven from blend of PLA matrix fibres and diversified waste flax fibres is used, the sound absorption of the composite is growing with frequency. The sound absorption coefficient of the composite from the 'PLA/LI (80/20%)' nonwoven is higher and depends linearly on a sound frequency arriving maximum value of 0.40 at a maximum studied frequency of 6400 Hz. This almost linear dependence is typical for fibre-based composites [19]. Even addition of particles thicker than fibres gives similar dependence, but with higher sound absorption. If the 20% of 'PLA/LI (80/20%)' nonwoven is replaced by straw, the sound absorption is more higher in the range from 3000 to 6400 Hz. It probably results from the fact that beside of small voids in composite, the bigger voids are formed. The sound waves of such frequencies can be reverberated repeatedly inside the diversified voids until soundproofing. The upward trend is evident and similar for all three kinds of composites with straw, as shown in **Table 3**. For the composite from a multilayer structure consisting of 80% of PLA/LI nonwoven and 20% of corn straw, the sound absorption coefficient increases linearly with sound frequency, and at 6400 Hz it is equal to 0.76. For the composites with sunflower or wheat straw as a reinforcement,

If PLA nonwoven and straw are used, the acoustic characteristic of the composite is totally different. The curve of the sound absorption coefficient-sound frequency dependence has another course. For each PLA/straw composite this course is slight different, because the kind of particle of each straw is slight different. Generally, the maximum values of sound absorption coefficient occur at medium (for PLA/SS and PLA/CS composites) and high (for PLA/WS composite) values of the studied frequency range. It means that the PLA/straw composites can be used as a material absorbing the sound of medium and high frequencies. The maximum values of the sound absorption coefficient of the PLA/straw composites are higher than those of PLA/LI or PLA/LI/straw composites. The sound absorption coefficient reaches the values of All of the obtained composites of such small thickness and apparel density show the sound absorption, wherein the values of the sound absorption coefficient depend on the reinforcement type and sound frequency.
