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

The CO fibres used as a reinforcement give better effect on sound absorption of the PLA composites than cellulose sub-microfibres. If as a reinforcement the CO fibres and cellulose sub-microfibres are used the sound absorption is more better. Increase in sound absorption over the whole range of investigated frequency is observed for 10% and optimal for 20% by ieswt. sub-microfibre content. The addition of sub-microfibres changes the character of the curve of the sound absorption coefficient in a function of sound frequency (**Figure 11**).

**Figure 11.** Sound absorption coefficient of the composites with/without sub-microfibres.

Among composites with sub-microfibres obtained from flax fibres, the composites with fibres without silane (LI-3) are characterized by the highest sound absorption, **Figure 12**, and the highest tensile stress at maximum load, **Table 2** [21]. Among studied composites with cellulose sub-microfibres obtained from straws, the best results, similar to composites LI-1, are observed for composites with sub-microfibres from SOF straw (SOF-1), **Figure 13**.

**Figure 12.** The sound absorption coefficient in the function of sound frequency for the composites: 1, LI-1 (without silane); 2, LI-2 (silane in ethanol); and 3, LI-3 (silane in ethanol and water).

**Figure 13.** The sound absorption coefficient in the function of sound frequency for the composites: 1, SRFF-1; 2, SOF-1; 3, HS-1.

For the composites with sub-microfibres obtained from straws and then modified by solution of silane in acetone, the best results of the sound absorption are observed for the composites with sub-microfibres obtained from SRFF straw (SRFF-2), **Figure 14**. The silane modification of sub-microfibres obtained from SOF and HS straws does not change the sound absorption coefficient of the composites with them. In the case of SRFF, the difference between the sound absorption of composites with sub-microfibres modified by silane and without modified by silane is the highest, **Figures 15**–**17**. The composites with sub-microfibres obtained from SRFF straw modified with silane (SRFF-2) are characterized by the best results in sound absorption and tensile stress among the composites with sub-microfibres obtained from straws.

Among composites with sub-microfibres obtained from flax fibres, the composites with fibres without silane (LI-3) are characterized by the highest sound absorption, **Figure 12**, and the highest tensile stress at maximum load, **Table 2** [21]. Among studied composites with cellulose sub-microfibres obtained from straws, the best results, similar to composites LI-1, are observed

**Figure 12.** The sound absorption coefficient in the function of sound frequency for the composites: 1, LI-1 (without si-

**Figure 13.** The sound absorption coefficient in the function of sound frequency for the composites: 1, SRFF-1; 2, SOF-1;

For the composites with sub-microfibres obtained from straws and then modified by solution of silane in acetone, the best results of the sound absorption are observed for the composites with sub-microfibres obtained from SRFF straw (SRFF-2), **Figure 14**. The silane modification

lane); 2, LI-2 (silane in ethanol); and 3, LI-3 (silane in ethanol and water).

3, HS-1.

for composites with sub-microfibres from SOF straw (SOF-1), **Figure 13**.

232 Composites from Renewable and Sustainable Materials

**Figure 14.** The sound absorption coefficient in the function of sound frequency for the composites: 1, SRFF-2; 2, SOF-2; 3, HS-2.

**Figure 15.** The sound absorption coefficient in the function of sound frequency for the composites with SRFF sub-microfibres: 1, SRFF-1; 2, SRFF-2.

**Figure 16.** The sound absorption coefficient in the function of sound frequency for the composites with SOF sub-microfibres: 1, SOF-1; 2; SOF-2.

**Figure 17.** The sound absorption coefficient in the function of sound frequency for the composites with HS sub-microfibres: 1, HS-1; 2, HS-2.

The silane modification of sub-microfibres obtained from straws gives the best effect for SRFF straw because of the highest increase in tensile stress and in sound absorption coefficient. The sound absorption coefficient of the SRFF-1 composite was the lowest among all composites with sub-microfibres without silane modification but absorption coefficient of SRFF-2 composite is the highest in the whole range of investigated frequencies. The increase in stress at maximum load of the SRFF-2 composite is the highest among other composites with submicrofibres from straws and is 38.8% higher than that of SRFF-1 composite.

For composites with sub-microfibres obtained both from waste flax fibres and from straws, the silane modification of sub-microfibres gives probably higher arrangement of fibres and their better adhesion to the polymer matrix, what is observed in high sound absorption and high tensile stress. In the case of sub-microfibres obtained from waste flax fibres, the highest sound absorption and the highest tensile stress of the composites give the modification by solution of silane in ethanol and water.

In the case of the composites with cotton fibres and sub-micro/nanofibres, **Figure 18**, the sound absorption increases rapidly to a value of 0.8 in the range of 500–4000 Hz and in the range of 4000–6400 Hz slightly increases. The absorption coefficient at a frequency of 6400 Hz is equal to 0.95. If the content of CO fibres was increased to 50%, the sound absorption coefficient increases in the range of 500–2500 Hz to approximately 0.8, and in the range of 4000–6000 Hz it is almost constant.

**Figure 18.** Sound absorption coefficient of the composites: 1, 80%PLA/CO (90/10) + 20% cel. Sub-micro/nano; 2, 80%PLA/CO (50/50) + 20% cel. Sub-micro/nano [16].

Results show that sound absorption of the composite depends on the content of reinforcing fibres. For higher percentage of these fibres, the higher sound absorption is observed. The sound wave causes the fibres vibration and as a result of friction the created energy of sound wave is converted to heat conversion. Larger total fibre surface leads to greater interaction of sound wave with the fibres. This effect of interaction between sound wave and fibre surface is stronger if the standard fibres are replaced by ultra-short/ultra-fine fibres [16].
