Sound-absorbing green composites based on cellulose ultra-short/ultra-fine fibers

Abstract

In this paper studies on sound absorption of the thermoplastic composites on the basis of waste natural fibers are presented. Cotton fibers and cellulose ultra-short and ultra-fine fibers obtained from flax fibers following enzymatic and additional mechanical treatment were used as the components of polylactide composites, and their influence on sound absorption behavior was investigated. The composites were obtained from a pressing process of fibrous multilayer structures. The sound absorption properties of three types of composites were compared: composites reinforced by cotton fibers, composites reinforced by cellulose ultra-short and ultra-fine fibers, and composites reinforced by cotton fibers and cellulose ultra-short and ultra-fine fibers. The role of cellulose ultra-short and ultra-fine fibers in changing the sound absorption properties of composites was determined. It has previously been shown that using natural fibers with a thermoplastic polymer results in increased sound absorption. The best improvement of sound absorption can be obtained by combining cotton fibers and cellulose ultra-short and ultra-fine fibers, especially nanofibers, as a reinforcement.

Keywords

waste natural fibers enzymatic treatment submicro/nanofibers thermoplastic composites sound absorption impedance tube

The use of natural fibers to reinforce or to fill composites and impart new functions to them is becoming more widespread. Moreover, there is a growing trend towards replacing high-modulus reinforcing fibers, such as glass fibers, with natural fibers.^1–3 Composites based on natural raw materials, such as cotton, jute, flax, hemp, sisal, kenaf, corn, wheat, or barley straw, are well known in the literature^4,5 as “green” composites.

Due to the great variety of plants, composites made of vegetable raw materials exhibit different properties and are consequently suitable for different applications, not only for construction but also for new uses, for example attenuation of sounds.^6–15 Sound-absorbing natural fiber-reinforced composites show a high degree of sound absorption, especially at high frequencies. Sound absorption coefficients of carbon fibers at frequencies from 1000 to 2000 Hz are higher than those of glass fibers, but lower than those of natural fibers, such as ramie (about 0.6), flax (about 0.8), and jute fibers (higher than 0.9). The higher sound absorption of natural fibers is due to their unique hollow and multi-scale structure. Consequently, natural fiber-reinforced composites are better sound-absorbing materials than glass or carbon-reinforced composites.¹⁶ Natural fibers have more irregular shapes of cross-section and larger and more variable transverse dimensions than synthetic fibers obtained from extrusion.¹⁷ Natural fibers obtained from renewable resources can be use as a cheap biodegradable component of recyclable sound-absorbing material.⁷ These fibers can be obtained either from a plant source or animal source. From the view of acoustic properties of composites, for natural fibers the thermoplastic polymer matrix is more suitable than the thermoset polymer matrix, which was measured on the basis of kenaf fibers, polypropylene, and urea formaldehyde.¹⁴ Attention has been increasingly focused on new sound-absorbing materials obtained from production wastes that are price-attractive.^10,18 The utilization of waste natural fibers would be a promising solution in terms of both economical and acoustic properties. Onal and Karaduman¹⁹ proposed to use carpet waste jute yarn as a natural reinforcement of polyester or epoxy matrix. Ersoy and Küçük⁹ investigated the sound absorption coefficient of tea-leaf-fiber waste materials with and without backing of woven cotton cloth in comparison with polyester and polypropylene-based nonwoven fiber materials for three different thicknesses.

As a rule, hydrophilic natural fibers used to reinforce a composite are subjected to chemical/enzymatic/mechanical modification,^20,21 which increases the dimensional stability, reduces water absorption, increases resistance to atmospheric conditions, and importantly increases adhesion to the hydrophobic matrix. Enzymatic treatment can also be aimed at decreasing the crosswise and longitudinal dimensions of fibers to the micrometer level or below. Fibers with such low dimensions become materials with attractive sound absorption properties. Materials containing ultra-fine fibers possess considerably higher sound absorption parameters over certain frequency ranges than materials containing standard fibers. There is a process in the literature²² for increasing the sound absorption of textiles by using fine fibers with a developed surface. Lee and Joo²³ investigated thermally bonded nonwovens consisted of different recycling poly(ethylene terephthalate) (PET) fibers with the same length but different fineness. When the content of ultra-fine fibers (ca. 0.05 den) was higher, the sound absorption of the material was better in the frequency range from 250 to 1000 Hz. Similar results concerning acoustic properties of the fine fiber materials were obtained by Na et al.²⁴ The comparison of the materials with the same thickness or weight showed that sound absorption of micro-fiber fabrics depends on their structure according to sound frequency and is superior to that of standard fiber fabrics. In the case of hybrid textile materials used for manufacturing the natural fiber-based thermoplastic composites, a good amount of blending of the reinforcing and matrix fibers can be obtained in fabrics such as nonwovens. Nonwovens are characterized by a large surface area and, as spacer knitted fabrics, are materials with good acoustic characteristics.^{18,20–22,25–30}

The sound absorption of a material depends on its apparent density and porosity. The sound wave causes vibration of the fibers in the material and because of their friction, the energy to heat conversion proceeds.

In the case of composites, a very important determining factor for structural, mechanical, and acoustical properties is an optimal mass content of the fine fibers. From an economical point of view, the content of them should be as low as possible, regardless of the composite manufacturing method.

Sound absorption data for composites containing cellulose ultra-short/ultra-fine fibers, that is, submicrofibers or submicro/nanofibers, obtained from waste natural fibers are not published. More data can be found only for nonwoven or composite fabrics containing synthetic microfibers.^23,24

In the present study, sound-absorbing “green” composites based on standard polylactic acid (PLA) fibers used as a matrix material and reinforcing waste natural fibers were developed. Cotton (CO) standard fibers and cellulose fibers prepared from flax (LI) fibers by enzymatic or enzymatic-mechanical treatment were used as reinforcing waste natural fibers. Depending on the type of treatment, different ultra-short and ultra-fine fibers were obtained. Their length was approximately several hundred micrometers or several micrometers, and their diameter was approximately several micrometers or several hundred nanometers. All fibers chosen for composite manufacture can be biodestructed after composite usage.

The effect of the content of cotton standard fibers and cellulose ultra-short/ultra-fine fibers on the acoustic properties of composites was examined.

Investigations were aimed at assessing the possibilities of increasing the composite sound absorption by using as a reinforcement either only cellulose ultra-short/ultra-fine fibers or cellulose ultra-short/ultra-fine fibers with cotton standard fibers.

Experimental details

Materials

Matrix fibers

Biodegradable PLA fibers (6.7 dtex/64 mm)^31,32 that were characterized by having a melting point in the range of 165–170℃ were used as the thermoplastic matrix material. Commercial PLA, Ingeo Fiber type SLN2660D finished with PLA resin without any hazardous substances, was delivered by Far Eastern Textile Ltd, Taipei, Taiwan.

Reinforcing fibers

Cotton fibers and/or cellulose ultra-short/ultra-fine fibers were used as the composite reinforcement material. Waste Tadzhikistan cotton fibers in the form of noils were delivered by Alto Ltd (Gorzów Śląski, Poland). Waste flax fibers in the form of bleached roving (from Safilin Ltd, Miłakowo, Poland) were used to prepare cellulose ultra-short/ultra-fine fibers.

The linear density of the cotton fibers was determined according to ISO 1973:1995,³³ and the fiber tensile strength according to ISO 5079:1995.³⁴ The fiber length was measured manually for the fiber sample of 6 mg. The impurity content was determined by mechanical method using a TKI Selekter made in Hungary, whereas the fiber grade/maturity degree was determined according to ISO 4912:1981³⁵ using a polarization microscope. The characteristics of the cotton fibers are listed in Table 1.

Table 1.

Characteristics of cotton (CO) and flax (LI) fibers

Parameter	CO		LI
Parameter	Mean value	Standard deviation	Mean value	Standard deviation
Linear density, dtex	1.67	0.094	5.19	0.981
Length, mm	10.90	5.234	101.64	–
Tensile strength, cN/tex	22.2	6.882	25.15	2.660
Content of impurities, %	0.05	–	–	–
Quality/degree of maturity	III/1.66^a	–	–	–

On a scale from 0 to 5, the above values of cotton maturity degree are below average.

Similar tests were carried out for flax fibers, which are assigned in the following step for enzymatic and mechanical treatments to obtain cellulose ultra-short/ultra-fine fibers.

The linear density of flax fibers was determined by the gravimetric method according to ISO 1973:1995³³ and tensile strength according to ISO 5079:1995.³⁴ Their length was measured by the segregation method. Due to the type and degree of treatment of the flax waste, it contained no impurities and, thus, such a test was unnecessary. The characteristics of the flax fibers are listed in Table 1.

Methods

Preparation and characterization of cellulose ultra-short/ultra-fine fibers

It follows from reports in the literature^36,37 that cellulose subjected to treatment with cellulase is more easily fibrillated than cellulose not treated with enzymes. Fourier transform infrared spectroscopy (FTIR) examinations of cellulose subjected to enzymatic treatment with cellulases indicate that its structural changes consist of decreasing both the number of hydrogen bonds and the energy in cellulose.

To obtain cellulose ultra-short and ultra-fine fibers, flax fibers (bleached roving) were cut on a mechanical cutter into 1–2 cm sections. This operation was performed to defibrillate the flux into elementary filaments and to avoid the formation of compact and tangled fiber balls during blending. The cut fibers were dried at 105℃ for 6 h followed by enzymatic treatment. To do so, a suspension of fibers was prepared with a concentration of 2.5% by wt. in a 0.05 mole/l acetate buffer with pH 4.8. Ecostone L900 cellulase (AB Enzymes GmbH, Darmstadt, Germany) was added to the fiber suspension at a concentration of 4500 units of endo-1,4-β-glucanase activity per 1 g of substrate dry weight. Enzymatic treatment was carried out at 50℃ for 72 h in a glass conical flask placed in a Labfors (Infors, Switzerland) orbital shaker incubator at a speed of 150 rpm. Once the reaction was terminated, cellulose fibers were filtered out of the enzyme solution on a Büchner funnel and rinsed with distilled water. The residue from the enzymatic preparation in the filtered fiber mass was inactivated by sterilization in a pressure reactor at 121℃ for 20 min. To remove the residue from the enzymatic protein and the reaction liquid components, the cellulose fibers were rinsed several times with distilled water. The ultra-short/ultra-fine fiber mass obtained was divided into two portions. The first portion was dried at ambient temperature, and the dry fibers obtained were referred to as submicrofibers. To further reduce the fiber dimensions, the second portion was subjected to mechanical treatment in an aqueous suspension at a concentration of 3% w/v by means of a homogenizer for 10 min at a rotational speed of 10,000 rpm. Following enzymatic and mechanical treatments, the fibers in the aqueous suspension were not dried to avoid formation of agglomerates. These fibers were referred to as submicro/nanofibers.

The thickness and length of the cellulose submicrofibers were determined by the optical method using a LUCIA G image analyzer, made in the Czech Republic. The fineness and length of the submicro/nanofibers were evaluated using a Quanta 200 scanning electron microscope (SEM; FEI Co.). The content of α-cellulose was determined using a 17.5% w/w NaOH solution according to the TAPPI T 203 cm-09 standard.³⁸ The Kappa number, which indicates the lignin content of the pulp, was determined by titration in accordance with the TAPPI T 236 cm-85 standard.³⁹

Manufacturing of nonwovens

PLA and PLA/CO needle-punched nonwovens with similar mass per square meter were manufactured. Fleeces with a cross-system of fiber arrangement were obtained on the roller card. Needle punching of the fleece layers was carried out on an Asselin needle punching machine (France), with the following technological parameters: type of needles – 15 × 18 × 40 × 3½ RB (Groz-Beckert®); number of needle punches – 40/cm²; depth of needle punching – 12 mm.

To obtain composites, three variants of nonwovens from different fiber blends were prepared: PLA nonwoven, denoted as PLA (100); nonwoven from a blend consisting of 90% by wt. PLA fibers and 10% by wt. CO fibers denoted as PLA/CO (90/10); and nonwoven from a blend of 50% by wt. PLA fibers and 50% by wt. CO fibers denoted as PLA/CO (50/50). The mass per square meter of nonwovens was determined according to the ISO 9073-1:1989 standard.⁴⁰

Manufacturing of composites

Thermoplastic composites were obtained from the textile multilayer structure on a hydraulic press machine with a water-cooling system from Hydromega (Poland). It was assumed to obtain composite samples with similar thicknesses to allow for assessment of the effect of the density, type, and quantity of the reinforcing fibers. The multilayer structure, which consists of several layers of nonwovens (depending on the fiber composition of the nonwovens), either separated or not by layers of cellulose ultra-short/ultra-fine fibers (Figure 1), was wrapped with Teflon foil to prevent molten polymer propagation during the pressing process. Because of the technological difficulties of obtaining nonwoven from the mixture of standard fibers and ultra-short/ultra-fine fibers being in the form similar to powder, the latter were used in the textile system for composite pressing as separate layers between layers of nonwovens. The needle-punched nonwovens were manufactured from matrix fibers or from a mixture of matrix fibers and reinforcing standard cotton fibers with different percentage by weight. The appropriate mass of ultra-short/ultra-fine fibers was divided in portions, which were put uniformly onto consecutive layers of nonwoven and the top of the multilayer structure was covered by nonwoven. Dry submicrofibers were directly sieved onto the nonwoven layers. Submicro/nanofibers were deposited onto the nonwoven layers as an aqueous suspension, and then the packages obtained were dried at 45℃ for 12 h. All multilayer structures were pressed under the same conditions: temperature 172℃, time 4 min, pressure 0.275 MPa.

Figure 1.

Fibrous multilayer structure used for manufacturing the composites.

Each variant of the nonwovens was used to make composites either with or without cellulose ultra-short/ultra-fine fibers. Three types of composites were compared:

matrix/cotton fibers;

matrix/cellulose ultra-short/ultra-fine fibers;

matrix/cotton fibers and cellulose ultra-short/ultra-fine fibers.

Three samples for each variant of the composites presented in Table 2 were prepared.

Table 2.

Characteristics of composites

Composite variant	Number of layers before pressing		Parameters of composite
			Thickness, mm		Density, kg/m³
	Nonwovens	Cellulose ultra-short/ ultra-fine fibers	Mean value	SD	Mean value	SD
PLA (100)	20	0	4.6	0.05	1100	45.2
80%PLA (100) + 20% cel. sub.	20	19	5.8	0.12	504	20.4
PLA/CO (50/50)	8	0	5.9	0.13	179	7.5
90%PLA/CO (50/50) + 10% cel. sub.	8	7	7.5	0.08	171	6.7
80%PLA/CO (50/50) + 20% cel. sub.	8	7	7.0	0.30	158	6.1
PLA/CO (90/10)	8	0	5.0	0.11	338	13.8
90%PLA/CO (90/10) + 10% cel. sub.	8	7	6.5	0.13	297	11.4
80%PLA/CO (90/10) + 20% cel. sub.	8	7	7.3	0.09	275	10.9
80%PLA/CO (50/50) + 20% cel. sub/nano	8	7	9.5	0.16	142	6.2
80%PLA/CO (90/10) + 20% cel. sub/nano	8	7	7.2	0.08	170	6.8

PLA: polylactic acid; CO: cotton.

The density of the composites was determined as the mass-to-volume ratio of the samples.

Measurement of sound absorption of the composites

The sound absorption coefficient was determined according to the ISO 10534-2:1998 standard procedure⁴¹ within a frequency range of 500–6400 Hz. A small impedance tube (Kundt tube), type 4206 (Brüel & Kjaer, Denmark), was used (Figure 2). The diameter of the samples investigated was 29 mm.

Figure 2.

Schematic diagram of impedance tube (from technical documentation, Brüel & Kjaer).

Results and discussion

Cellulose ultra-short/ultra-fine fibers

Enzymatic treatment of flax fibers as a bleached roving considerably decreased the length and diameter of the fibers. A micrograph of the cellulose submicrofibers is shown in Figure 3.

Figure 3.

Scanning electron microscope (SEM) image of the flax fibers after enzymatic treatment – submicrofibers (scale: 1.0 mm).

Mechanical treatment performed after the enzymatic process caused further decomposition of the hierarchic structure of cellulose fibers, as indicated by the SEM image showing the appearance of fibers with very different crosswise dimensions ranging from submicron to nanometric in scale. The SEM image of the cellulose submicro/nanofibers is given in Figure 4.

Figure 4.

Scanning electron microscope (SEM) image of flax fibers after enzymatic and mechanical treatment – submicro/nanofibers (scale: 2.0 µm).

Characteristics of the cellulose submicro- and submicro/nanofibers obtained are listed in Table 3.

Table 3.

Characteristics of the cellulose submicro- and submicro/nanofibers

Parameter	Submicrofibers		Submicro/nanofibers
Parameter	Mean value	Standard deviation	Mean value	Standard deviation
Diameter, µm	11.36	9.202	0.3	0.122
Length, µm	146.55	108.037	6.14	3.543

The flax fibers used in this study were characterized as having an average length of approximately 100 mm and a diameter of approximately 75 µm, whereas after enzymatic treatment, their length decreased to several hundred micrometers and their diameter to below 20 µm. Additional mechanical treatment caused a further decrease in dimensions: the length decreased to 6.14 µm and the diameter to 300 nm. Enzymatic treatment caused a decrease in α-cellulose from 95.1% to 87.3%. The Kappa number increased from 4 to 5.5, which indicates that the so-called residual lignin content was practically unchanged. A slight increase in the Kappa number following enzymatic hydrolysis results from better accessibility to residual lignin after fiber disintegration.

Composites

The composites were manufactured from three types of needle punched nonwovens with mass per square meter of about 110 g/m². The characteristics of the composites obtained are presented in Table 2.

By pressing 20 layers of a nonwoven out of matrix PLA fibers, a composite with the highest density was obtained. Despite the use of such a high number of nonwoven layers, the thickness of this composite is lower than that of the composites made out of eight layers of the PLA/CO blend nonwoven prepared under the same technological conditions. In the case of 100% by wt. matrix PLA nonwoven, the fibers melt in the thermal pressing process and a homogeneous plastic plate with high density is obtained. The presence of CO fibers in the composite contributes to the increase in thickness and decrease in density. A content of CO fibers of 10% decreases the composite density three times with a relatively low increase in the composite thickness. With a 50% by wt. CO fiber content, the composite thickness increases by 30%, while its density decreases by approximately seven times. The increase in the cellulose submicrofiber content in the composite causes a decrease in its density, but to a lesser extent than in the case of standard CO fibers. Composites with incorporated submicro/nanofibers are characterized by a soft structure with a low apparent density.

The maximum amount of cellulose submicrofibers that can be used to make a composite of PLA/CO nonwoven amounts to 20% by wt. of the pressed fiber mass. Further increases in the submicrofiber content give rise to technological problems and do not provide a homogeneous composite structure. The SEM images of the “90%PLA/CO (90/10) + 10% cel. sub.” and “80%PLA/CO (90/10) + 20% cel. sub.” composites illustrate that the increase in submicrofibers content from 10% to 20% by wt. leads to greater diversity of the composite structure (see Figure 5). Therefore, 10% or 20% by wt. cellulose submicrofibers with respect to the package mass was used. The results concerning acoustical properties of CO fiber-based composites with a content of 10% by wt. or of 20% by wt. of submicrofibers are presented in Figures 6 –9.

Figure 5.

Scanning electron microscope (SEM) images of the composites: (1a), (1b) 90%PLA/CO (90/10) + 10% cel. sub.; (2a), (2b) 80%PLA/CO (90/10) + 20% cel. sub.; (1a), (2a) – magnification × 200; (1b), (2b) magnification × 400.

Figure 6.

Sound absorption coefficient of the composites: (1) PLA (100); (2) 80%PLA (100) + 20% cel. sub.; (3) PLA/CO (50/50); (4) 80%PLA/CO (50/50) + 20% cel. sub.

The results shown in Figure 6 indicate that the sound absorption coefficient of the PLA (100) composite is constant within the frequency range of 1000–4000 Hz and equal to 0.1. Outside this range, the absorption coefficient increases with increasing sound frequency. At 6400 Hz, it is equal to 0.38. Bonding of 20% by wt. submicrofibers and 80% by wt. PLA (100) changes the character of the absorption coefficient-frequency dependence. The sound absorption increases within the range of 500–4000 Hz to a value of 0.55, and then at a frequency of 5000–6000 Hz it is constant. The sound absorption of the PLA/CO (50/50) composite is greater within the entire frequency range investigated than that of the composite containing 80% by wt. PLA (100) with 20% by wt. cellulose submicrofibers. The sound absorption increases, reaching a value of 0.93 at 6400 Hz. The content of the submicrofibers in the “80%PLA/CO(50/50) + 20% cel. sub.” composite leads to further increasing of the absorption coefficient over the entire frequency range investigated. The values of the sound absorption coefficient marked as 1 concerning the PLA (100) sample are evidently lower than those of the other composites marked as 2, 3, 4. The significance of differences between 2, 3, 4 dependencies in Figure 6 were checked by a statistical method using a coefficient of determination (R²). The coefficient of determination in the whole range of frequencies for dependencies 2 and 3 was equal to 0.764 and for dependencies 2 and 4 was equal to 0.761. The coefficient of determination in the range of frequencies from 4000 to 6400 Hz is equal to 0.008 for dependencies 2 and 3, and 0.004 for dependencies 2 and 4.

The sound absorption coefficients of the PLA/CO (90/10) composites with or without 20% by wt. submicrofibers compared to the absorption coefficients for PLA (100) with or without 20% by wt. submicrofibers are presented in Figure 7. If only cellulose submicrofibers are used as a composite reinforcement, the increase in the composite sound absorption at high frequency ranges is lower than in the case in which the composites are reinforced by only CO fibers. As shown, even 10% by wt. CO fibers in a PLA matrix provides higher sound absorption than 20% by wt. cellulose submicrofibers. In the case of 10% by wt. CO fibers in the composite, the dependence of the sound absorption coefficient on the sound frequency is similar to the dependence presented in Figure 6 for a composite with 50% by wt. CO fibers. The greatest value of the sound absorption coefficient of PLA/CO (90/10) composite is equal to 0.93, which is insignificantly greater than that of the PLA/CO (50/50) composite. The content of submicrofibers in the “80%PLA/CO(90/10) + 20% cel. sub.” composite also increases the absorption coefficient over the entire frequency range investigated. The significance of differences between 2, 3, 4 dependencies in Figure 7 were checked by a statistical method using a coefficient of determination (R²). The coefficient of determination in the whole range of frequencies for dependencies 2 and 3 was equal to 0. 801 and for dependencies 2 and 4 was equal to 0.730. The coefficient of determination in the range of frequencies from 4000 to 6400 Hz is equal to 0.056 for dependencies 2 and 3, and 0.004 for dependencies 2 and 4.

Figure 7.

Sound absorption coefficient of the composites: (1) PLA (100); (2) 80%PLA (100) + 20% cel. sub.; (3) PLA/CO (90/10); (4) 80%PLA/CO (90/10) + 20% cel. sub.

The effect of the increase in composite sound absorption due to the presence of ultra-short/ultra-fine fibers observed in these investigations agrees with results obtained for composites based on fibers with small cross-sections.^42,43

The sound absorption coefficient-frequency dependence for the PLA/CO (50/50) composites with 10% and 20% by wt. submicrofibers and without submicrofibers is presented in Figure 8.

Figure 8.

Sound absorption coefficient of composites: (1) PLA/CO (50/50); (2) 90%PLA/CO (50/50) + 10% cel. sub.; (3) 80%PLA/CO (50/50) + 20% cel. sub.

Figure 9.

Sound absorption coefficient of composites: (1) PLA/CO (90/10); (2) 90%PLA/CO (90/10) + 10% cel. sub.; (3) 80%PLA/CO (90/10) + 20% cel. sub.

The dependencies of the sound absorption coefficient on the frequency for PLA/CO (90/10) composites with 10% and 20% by wt. of submicrofibers and without submicrofibers are presented in Figure 9.

The results shown in Figures 8 and 9 indicate that the sound absorption coefficient of the PLA/CO (50/50) and PLA/CO (90/10) composites increases in the entire sound frequency range investigated.

A content of 10% by wt. submicrofibers in the composites increases the sound absorption over the whole frequency range investigated in comparison to composites without submicrofibers. Increasing the content of submicrofibers to 20% by wt. results in increasingly greater sound absorption, especially in the case of the PLA/CO (90/10) composite.

The significance of differences between 1, 2, 3 dependencies in Figures 8 and 9 were checked by a statistical method using a coefficient of determination (R²). The coefficient of determination in the whole range of frequencies for dependencies 1 and 2 in Figure 8 was equal to 0.851 and for dependencies 1 and 3 in Figure 8 was equal to 0.874. The coefficient of determination in the whole range of frequencies for dependencies 1 and 2 in Figure 9 was equal to 0.866 and for dependencies 1 and 3 in Figure 9 was equal to 0.736.

In Figure 10 the results for the PLA/CO (90/10) and PLA/CO (50/50) composites with 20% by wt. submicro/nanofibers are presented. In the case of PLA/CO (90/10) containing 20% by wt. submicro/nanofibers, the sound absorption coefficient depends on the frequency in a similar way as for the composites with submicrofibers (Figures 8 and 9). The sound absorption of the PLA/CO (90/10) composites increases rapidly within the range of 500–4000 Hz to a value of 0.8 and then slightly increases up to a sound frequency of 6400 Hz. The absorption coefficient at frequencies from 6000 to 6400 Hz is equal to 0.95.

Figure 10.

Sound absorption coefficient of the composites: (1) 80%PLA/CO (90/10) + 20% cel. sub/nano; (2) 80%PLA/CO (50/50) + 20% cel. sub/nano.

The character of the dependence of the absorption coefficient of the PLA/CO (50/50) composite containing 20% by wt. submicro/nanofibers on the sound frequency is different to that of all the other composites presented. The sound absorption increases in the range of 500–2500 Hz and then is almost constant up to a sound frequency of 6000 Hz. The sound absorption coefficient value in the range of 2500–6000 Hz is approximately 0.8. At a frequency of 6400 Hz, the absorption coefficient is lower and is equal to 0.7. Such dependence may result from a composite structure. This composite is characterized by the lowest density due to the use of submicro/nanofibers with a relatively high content of CO fibers.

The results presented in this work in Figures 6 –10 indicate that sound absorption properties of the composite depend on the content of reinforcing fibers. A higher percentage of these fibers leads to an increase in sound absorption. It results from the fact that the sound wave causes the fibers vibration and because of their friction the energy of sound wave to heat conversion proceeds. A larger total fiber surface leads to greater interaction of the sound wave with the fibers. This effect should be stronger if the standard fibers are replaced by ultra-short/ultra-fine fibers.

The sound-absorbing behavior of the composites during their service life might be dependent on environmental conditions changing the PLA matrix or reinforcement properties.

The susceptibility of PLA products for the degradation in specific environmental conditions results in limitation of the potential range of the PLA composite application. The safe temperature of sound-absorbing composite usage should be lower than the glass transition temperature of PLA, that is, lower than 40–65℃.^32,44 The long contact with high moisture or with water can lead to hydrolytic degradation of PLA, especially at higher temperatures.^45–47 However, it was confirmed that the process of degradation strongly depends on the chemical and physical characteristics of the PLA polymer. The research on the degradation process of PLA suggests that the most effective degradation is in soil. In the case of usage in ambient air the changes of supermolecular structure occur, but the degradation process, especially of crystalline polymer, is very long.^48–51

Conclusions

Waste natural fibers derived from a spinning mill can be subsequently used to make sound-absorbing thermoplastic composites. Cellulose ultra-short/ultra-fine fibers obtained by enzymatic treatment can be used in sound-absorbing composites either on their own or with standard cotton fibers. A matrix with no reinforcing fibers exhibits the lowest sound absorption coefficient over the entire frequency range investigated. The presence of natural fibers in the thermoplastic matrix decreases the composite density, which leads to a greater sound absorption. The presence of standard CO fibers in the thermoplastic matrix causes a greater increase in the sound absorption than that obtained by only using cellulose ultra-short/ultra-fine fibers. The best improvement in sound absorption at medium and high frequencies can be obtained by using CO fibers and cellulose ultra-short/ultra-fine fibers as the composite reinforcements. An increase in the content of cellulose ultra-short/ultra-fine fibers to over 20% by wt. fails to provide a satisfactory combination of the composite components. Composites made of PLA/CO (50/50) nonwoven and cellulose submicro/nanofibers exhibit high sound absorption over the widest frequency range, that is, a sound absorption coefficient of 0.7–0.8 over a range from 2000 to 6400 Hz. The composites developed could be used as materials for absorbing sounds with medium and high frequencies derived from various sources, for example engines. For concrete applications further experiments, for example mechanical tests or environmental behavior, should be done. These studies are in progress.

Footnotes

Funding

This work was supported by the Project “Utilization of biomass for the preparation of environmentally friendly polymer materials” (Biomass) [POIG.01.01.02-10-123/09].

Acknowledgment

The authors would like to thank PhD M. Puchalski from Lodz University of Technology, who provided the scanning electron microscopy images of composites.

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