**1. Introduction**

Green composites have less environmental impact at the production, use, and post-use stages than in the case of composites based on chemical fibers [1]. As filling material for producing biocomposites, the natural fibers of wood, cork, horsehair, nettle, leaves, paper cut in a shredder, chicken feather calamus cut into small cubes, fine sawdust, and straw are usually used [2–7]. As the matrix material, among others, polylactide, poly-hydroxybutyrate, starch, chitosan, gum Arabic, and green epoxy resin are used [1,8]. Nowadays, different kinds of natural materials are investigated for the reinforcement of sound-absorbing composites because of their cheap production cost, eco-friendly composition, and their relevant properties related to the application of interest [9]. The results of the sound absorption of composites based on natural filling materials and biopolymers are promising, and present the high potential of such materials as sound absorbers. These composites exhibit different sound absorption depending not only on the type of filling/reinforcing material, but also on the sound frequency range. Usually, however, high sound absorption is observed at frequencies above 2000 Hz [1,2,10]. Sound absorption of selected frequency ranges depends on the structure, density, and thickness of the absorber. The results for pineapple leaf/epoxy composite show that for a given material density, by increasing its thickness, we extend the absorption range towards lower frequencies [11].

**Citation:** Gliscinska, E.; Perez de Amezaga, J.; Michalak, M.; Krucinska,I. Green Sound-Absorbing Composite Materials of Various Structure and Profiling. *Coatings* **2021**, *11*, 407. https://doi.org/10.3390/ coatings11040407

Academic Editor: Philippe Evon

Received: 11 February 2021 Accepted: 29 March 2021 Published: 31 March 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Among various forms of the filling component, the fibers are the most beneficial from the point of view of the mechanical properties of the composite [12]. The tensile strength of a composite based on flax fibers is much higher than those reinforced with particles. It can be about 20 times the strength of the cork-based composite, and about 3.6 times the strength of the straw-based composite [1]. Fibrous products are increasingly used as a sound-absorbing material. New structures with the participation of natural or synthetic fibers are still being developed, and the literature on the subject contains research results concerning the use of fibers that differ in terms of raw material, dimensions, arrangement, and specific surface area [13–15]. Mamtaz et al. stated that synthetic fibers, due to their thinner diameter and antifungal quality, are a better sound absorptive material than natural fibers; unfortunately, they also have a greater impact on the environment [16]. However, when talking about the acoustic properties of the fibers, many factors should be taken into account, such as their thinness, the shape of their cross-section, and the bulk density of the material. For example, the sound absorption of fabrics made from 3 denier polyester fibers is 5 times greater than that of the material from 15 denier fibers [17]. The polyester fibers with octalobal or trilobal cross-sections are better as sound insulators than round fibers because of their higher total surface area [17]. The absorption of glass fibers with a bulk density of 54 kg/m<sup>3</sup> is higher than that of kapok fibers, with a bulk density of 10 kg/m3, but lower than those fibers with a density of 15 kg/m<sup>3</sup> [18].

The microscopic structure and surface morphology of natural fibers such as flax, bamboo, kenaf, kapok, coir, cotton, broom, giant reeds, cane, coconut, hemp, etc. are conducive to sound absorption [18–22]. Natural fibers, due to their unique hollow and multi scale structures, show better sound absorption compared to high-modulus fibers such as glass or carbon, especially at frequencies above 1000 Hz. The ramie, jute, and flax fibers are characterized by a noise reduction coefficient at the level of 0.6–0.65, while glass fibers are at the level of 0.35, and carbon fibers at 0.45 [23]. The literature reports indicate that, for example, natural kapok fibers show sound absorption comparable to the widely used reinforcing glass fibers [18,24]. They are also characterized by a much lower density, which is of grea<sup>t</sup> importance in the design of lightweight composites. Yang and Li stated that composites made of natural fibers, such as jute, ramie, flax, and epoxy resin can exhibit similar or even better sound absorption than composites based on highmodulus fibers such as glass or carbon, which is important for aeronautical applications [23]. Mohanty and Fatima presented natural rubber-based jute composites manufactured by a compression molding process as biodegradable soundproofing materials for noise control applications, e.g., in home appliances, building construction, and cars [25]. Ersoy and Kucuk proposed that biodegradable tea leaf-fibers, as a product of renewable bio-resources, can be used as a sound absorber. The backing of these fibers with a cotton fabric layer gives sound absorption comparable to nonwoven polypropylene [26]. According to Zulkifli et al., an even better effect can be obtained by using coconut fibers [27]. The sound absorption of composite materials is also positively influenced by the addition of ultrashort/ultra-fine fibers obtained from natural fibers, e.g., flax fibers. Such fibers obtained by an enzymatic process and mechanical treatment, due to the larger total fiber surface, give greater interaction with sound waves, and thus increase the sound absorption of the material [28,29]. Research has shown that urea formaldehyde resin fiber boards made of various fibers, such as bagasse, banana, bamboo, coir, and corn husk, are characterized by better sound absorption if they have a lower density [30].

Much attention is also paid to fibrous layered products, as increasing the thickness and density of the sound-absorbing layer promotes sound absorption at the mid-to-high frequency ranges [10]. Layer systems where the layers differ in the type of fibers and in the textile structure are described. The most commonly used structures are nonwovens, woven fabrics, knitted fabrics, or nanofiber membranes. Sometimes fibrous layers are combined with other materials, such as cork [31,32], foam [33], or a honeycomb grid [34]. The use of several fibrous layers differing in apparent density favors increased sound absorption and widened absorption bands. The same is true in the case of laminated

composites based on textiles. The use of a system of textile layers differing in structure allows for obtaining a composite that has a lower thickness than the system of textile layers, and a comparable absorption [28,29,35]. By using layers that differ in structure, one can control the dependence of the sound absorption on the sound frequency [36]. Each material absorbs and reflects sound waves to some extent. The proportions between the energy of the reflected and absorbed waves can vary depending on the frequency of the sound. Soft, porous materials are good sound absorbing materials. Hard and smooth materials absorb sound waves very poorly. However, for some applications, it is necessary that at least the surfaces of the material should be hard and smooth. The structure of the material can be created in the process of its production [37].

Much attention is also paid to the importance of the material shape for its acoustic performance. Sharma et al. presented an analytical framework for a metasurface with a lattice of closely spaced spherical cavities embedded in a thin, soft medium. The strong resonance of the cavities was confirmed, and it has been shown that the high sound absorption of the metasurface is due to the strong multiple scattering of waves between the cavities and the conversion of longitudinal waves into shear waves dissipated then in the elastic medium [38]. Azad et al. investigated the effects of large-scale pyramidal and convex-shaped diffusers in an empty non-diffuse room on its acoustical parameters. The statistical analysis and measurement results showed that the influence of diffuser type on the room acoustic characteristics is significant, especially at high frequencies [39]. Recently, a growing interest in practical applications has been enjoyed by sonic crystals, i.e., finite arrays of periodically distributed scatterers for which very little sound transmission occurs in certain frequency bands. It is caused by the destructive interference of scattered waves in the latttice structure. These bands depend on the shape of the scatterers, the distance between the scatterers as a lattice constant, and the filling fraction. The center frequencies of these bands can be predicted from Bragg's law, and, for a greater lattice constant, they are at lower frequencies. The literature gives results for rigid diffusers in the form of steel cylinders or trees. Measurement results indicated that if the diffusers are arranged in a lattice configuration, they can more effectively attenuate certain low frequency bands. The attenuation level depends on the filling fraction, and the attenuation frequencies depend on the type of lattice pattern and the angle of sound incidence on the barrier [40,41].

In the case of thermoplastic composites, both the reinforcing component and the thermoplastic polymer can be used in the form of fibers. Having fibers, they can be used in the form of a hybrid structure, e.g., a needle-punched nonwoven. This structure allows for a high degree of mixing of both components, necessary to ensure good wettability of the reinforcement by the matrix. Nonwovens provide a number of functional benefits, including thermal and acoustic insulation. The most important advantages of the needle-punching technology include high production efficiency, the possibility of obtaining structures of high thickness, and the possibility of joining layers with different fiber orientation. The literature reports present the results of the research on sound absorption by nonwovens in terms of nonwoven technology and the aspect of web orientation angle. The effect of the web orientation angle on the sound absorption properties for thermally bonded nonwovens of multiangle layered webs was tested by Lee et al. [29].

The present work concerns research on the possibilities of producing green composite materials of various layered structures and different profiling from nonwovens, and determining the impact of these factors on the sound absorption by the composite [23,42].

#### **2. Materials and Methods**

To obtain green composites, flax fibers (LI), from Safilin Ltd., Milakowo, Poland, were used as a reinforcement. The dimensions and quality of waste short flax fibers, the so-called noils, were very diverse, Figure 1. The length of the flax fibers ranged from a few mm up to 115 mm, and the transverse dimensions were from 16 μm to 560 μm. As a matrix material, biodegradable polylactide fibers (PLA) were used [43,44]. These commercial thermoplastic fibers, Ingeo Fiber type SLN2660D (linear density 6.7 dtex, length 64 mm), with a melting

point in the range of 165–170 ◦C and finished with polylactide resin without any hazardous substances, were delivered by Far Eastern Textile Ltd., Taipei, Taiwan. In order to obtain a homogeneous composite material, it was necessary to perfectly blend the fibers in the nonwoven fabric. The mixing process consisted of passing the fibers twice through the carding machine. The fleece with a parallel system of fiber arrangemen<sup>t</sup> was obtained. Then, the needle-punching process of the fleece layer was carried out on an Asselin needlepunching machine (France). The following technological parameters were used: type of needles—15 × 18 × 40 × 31/2 RB (Groz-Beckert®, Albstadt, Germany); number of needles punching—40/cm2; depth of needle-punching—12 mm. The needle-punched nonwoven was thus obtained.

**Figure 1.** View of waste flax fibers.

Composites were formed from textile multilayer structures in a hydraulic press machine, Hydromega, Gdynia, Poland, with heated top and bottom plates and a water cooling system. A multilayer structure of nonwovens, sandwiched between two layers of Teflon foil, was put into the press mold. The mold was then closed and the heating was turned on. After reaching the pressing temperature, i.e., the melting point of the thermoplastic fibers, the consolidation stage was carried out under a pressure of 0.58 MPa for 5 min. Finally, the heating was turned off and the water cooling system was turned on to bring the temperature down to room temperature. For profiling the composites, a Teflon plate with holes was used (during pressing it was placed under the multilayer structure of nonwovens), and a tool with a spherical tip was used to obtain concavities in the composite plates, Figure 2.

The acoustic properties of the composites were determined by means of a smallsized impedance tube, type 4206 (Bruel&Kjaer, Denmark) using two 14 -inch condenser microphones, type 4187, Figure 3. The physical sound absorption coefficient (a quotient of acoustic energy absorbed by the given material to the energy of the acoustic incident wave) was determined for each sample by the method using the coefficient of a standing wave, according to the standard procedure: PN-EN ISO 10534-2 in the frequency range of 500–6400 Hz. This range of sound frequencies is adequate and sufficient to observe the sound absorbing behavior of the tested materials [2,12,28]. In this method, using a Kundt tube, the sound wave coming from sound source is directed perpendicularly at the surface of tested material. The acoustic pressure is measured by microphones at two locations on the wall of the tube. Then, the signals are transferred to analyzer. In this method, the impedance tube is connected to the sound source on one side, and on the opposite side

the test sample is placed. The noise source generates plane waves in the tube directed perpendicular to the sample surface. The sound pressure is measured thanks to the two microphones in fixed positions in the tube wall. Then, the signals are transferred to the analyzer and the interference distribution of the field is determined. Based on this, the sound absorption coefficient is calculated. Before the measurement, the instrument was calibrated each time the sample material was changed. The samples with a diameter of 29 mm were cut with a punch. Three samples were tested for each variant.

**Figure 2.** View of the **c**omposites and the Teflon plate (in white color).

**Figure 3.** The Kundt device.
