Abstract
This paper focuses on the design of a system formed by the combination of novel composite panels from low density polyethylene and textile waste as the backing material and the spacer fabric as the aesthetic component for sound absorbent applications. The results reveal that the sound absorbing systems that consist of composite panels have better sound absorbing performance, are lighter and cheaper than the system including chipboard.
Key words: Recycled material, acoustics, textile composite, spacer fabric
Introduction
Noise has become an important environmental health issue nowadays due to the troubles caused such as hearing loss, detrimental effects on sleep, communication interference, and annoyance, etc.
Therefore, the use of sound absorption materials is one of several methods used to decrease the level of noise [1,2]. By understanding the factors that control sound absorption, textiles can be used in many applications involving acoustics, including acoustic panels for workstations, automotive insulation and upholstery in concert halls etc.
Lightweight structures made of textile reinforced composite materials are increasingly employed in many fields of applications due to their versatility and their use as lightweight partition walls for office buildings, conference halls, even in residential buildings, has increased significantly in the last few decades [3,4].
Bratu et.al aimed to absorb and attenuate sound by using composite materials from polymer matrices of polyester and formaldehyde resins and recycled solid wastes. Composites from formaldehyde resin reinforced with waste slag, sterile municipal waste and sawdust gave the best results [5]. Other studies focused on the determination of the acoustical properties of some composite structures based on wood chips and textile (wool or jute) waste bonded together with ecological binders [6,7]. According to the results of these studies the composition of binders (matrix) had a great influence upon the acoustical properties of the samples and the biodegradable composite materials with textile inserts present a good absorption capacity that can be utilised for further applications. The performance of a sound absorbent panel from coconut coir and recycled rubber was investigated [8] and the composition of the composite panels with 25% polyurethane and 75% filler was recommended for sound absorption applications.
On the other hand, conventional materials such as wood, metals, glass, etc. are replaced by plastics in many applications due to their thermal, electrical, mechanical, and optical properties, resistance to biodegradation and they are being used for sound absorbing applications [9].
A study by Murugan et. al revealed that by mixing polyolefin-based packaging wastes with other waste materials such as plastic-coated aluminium foils, expanded polystyrene, and coir pith in varying quantities, specific properties like sound absorption and noise reduction of the composites were improved [9]. Sound absorption composites produced from chlorinated polyethylene (CPE) and 20% seven-hole hollow polyester fibres (SHPF) with 1mm thickness was 0.42 at 2500 Hz [10]. Additionally, the result of investigations into acoustic properties of individual materials showed that both structurally and acoustically it is purposeful to use various compositions of these materials for noise reduction rather than of individual materials [1,4,11-12].
For the last decade, environmentally friendly design in the production of textiles has become one of the major issues of the industry in general. Accordingly, it is the objective of the study discussed in the paper to design an eco-friendly sound absorbing composite panel from textile waste and recycled LDPE (low density polyethylene), by which it is also aimed to contribute to the literature in which there is a limited number of studies on this very subject. Finally, in an attempt to improve the performance of the panel, its aesthetic compound is chosen as a weft knitted spacer fabric.
Method and Material
Reinforcement and matrix materials
Knitted fabric wastes of cotton fibres in the cut piece form were used as the reinforcing material for the production of composites for this study. As the matrix materials; low density polyethylene (LDPE melt flow index = 0.30 g/10 min at 190°C-
260 g, density=0,920 g/cm3 at 23 °C) and recycled LDPE were used.
LDPE was supplied from PETKİM under the trade name G03-5 LDPE and Recycled LDPE was supplied in the granular form from İSTAÇ – an organization of İstanbul Metropolitan Municipality.
Wood Chipboard
In an attempt to compare the acoustic performance of composites from textile wastes with that of the materials commonly used in the industry, chipboard of 18mm thickness, 666,6 kg/m3 density and approximately 12 kg/m2 weight was chosen as the alternative backing material since it is widely used in industrial sound absorption applications.
Spacer Fabrics
In addition to the evaluation of the acoustic performance of the composites from textile wastes, their acoustic properties were also 
tested together with the spacer fabric having enhanced sound absorption values. The spacer fabric used within this study was developed in the previous study [13]. For the production of the spacer fabric (SF), 70/30acrylic/wool blended yarns were used for both layers. Furthermore, 8 connecting yarns- 4 from polypropylene multifilament yarn (320 denier) and the other four from polyester monofilament (400 denier) - were used for every course. The stitch notation of the fabric is presented in Figure 1.
Method
The composite panels were produced by using a custom-made single screw extruder designed especially for manufacturing composites from recycled textile wastes (as the reinforcement material) and polyolefins (as the matrix). The matrix polymers and cotton knitted fabric wastes were fed into the extruder in accordance with the weight ratios of 25%, 35% and 45%, in turn.
The extruder was preset according to the working parameters listed in Table 1. The composite leaving the die was granulated into particles and was re-fed into the extruder and was reprocessed. After the reprocessing step, the composite formed was passed through the rollers so that it could form a panel shape.
These panels were cut in the dimensions suitable for the hydraulic press used and were subjected to 1 ton of pressure in order to eliminate the die swell effect so that a more compact surface could be obtained.Following that, NAC values of the samples were measured for the 50Hz - 6.4kHz frequency range, in accordance with ISO 10534-2 standard, by using a standard two-microphone impedance tube provided by Bruel & Kjaer.
NAC measurements were repeated three times for each sample and their average was taken [14]. The properties of the knitted spacer fabrics given in Table 3 were determined in accordance with TS 250 EN 1049-2, BS 2544, TS251, TS 391 EN ISO 9237, BS 3424-10. Thickness, weight, volume and density of the panels were also determined. Tortuosity of the fabrics was measured based on the method described by Dias et al. [15].
Results and Discussion
Properties of the composites and the spacer fabrics are presented in Table 2 and 3, respectively. In Figure 2, sound absorption performance of chipboard, LDPE panel and R-LDPE panels are presented comparatively.
As can be seen from the figures, none of these three samples were distinguished by their acoustical properties in the frequency range of 0-2800 Hz. After 2800 Hz, LDPE and R-LDPE panels reached their maximum sound absorption values at around 4000 Hz and declined after that point.
Furthermore, both panels, namely LDPE and R-LDPE, have almost identical sound absorption behaviour, and for environmental concerns the R-LDPE panel was chosen for the rest of the study.
It is well known that the lower the frequency of sound, the bigger the length of sound waves, and the more difficult it is to absorb them [16], and therefore it is suggested to increase the thickness of a sound absorbing material for better sound absorption performance.
Within the 0-2800 Hz frequency range, there is almost no difference between the NAC of the chipboard backing and the LDPE and R-LDPE panels - even though the thickness of the chipboard was around three times greater than that of the polyethylene panels. Higher density polyethylene panels, in comparison to that of the chipboard, might have increased NAC after 2800 Hz as denser structures perform better for frequencies above 2000 Hz [17]. In addition to that, transforming the acoustical waves into thermal motions might have been easier for the polymeric chains of LDPE rather than the chipboard.
Influence of the Quantity of the Reinforcing Material
It was observed that the presence of the cotton waste fillers in the R-LDPE composites affected the sound absorption behaviour of those materials (see Fig.
3). It is possible to say that the sound absorption curve of the composite with cotton waste of 25% showed a similar trend to that of a 100% polyethylene panel.
Both of the panels reached their highest absorption values at around 4400 Hz. However, the sound absorption values of the composite panel (25%) were relatively lower up to their peak points, and also the absorption coefficient of the recycled LDPE panel demonstrated a steep decrease after 4400 Hz.
Moreover, it was observed that increases in the percentage of cotton waste content enhanced sound absorption at lower frequencies and the best sound absorption performance shifted to relatively lower frequencies as the amount of cotton knitted fabrics increased, whereas an opposite trend is observed after 3600 Hz. This result is in harmony with the findings of Murugan et.al. in which they recorded an improvement in the sound absorption of the composites with the increased amount of reinforcing material (plastic-coated aluminium foils, expanded polystyrene, and coir pith) in the PE matrix within the 0-2000 Hz frequency range [9].
Also, the sound absorption values of the composites were found to be higher than those of the 100% recycled LDPE panels up to 3600 Hz frequency. Compared to the 100% recycled LDPE panels, the better sound absorbing performance of the composite, ones within the 0-3600 Hz frequency range, may be due to the presence of micro voids in the recycled material caused either by the structure of the knitted fabric pieces or by the inhomogeneties formed by the reinforcing materials.
The inverse pattern of the sound absorption curves at the frequencies greater than 3600 Hz might be due to the non-uniform distribution of the constituents in the recycled material.
Sound Absorbency of the System (Composite Panel and Spacer Fabric together)
In places where people are exposed to noise, such as conference halls, offices, schools, hospitals, etc., sound absorbing materials 
are generally expected to possess aesthetic properties in addition to functional ones. As the panels developed do not have a decorative effect which is good enough, they were combined with the knitted spacer fabrics designed for improved sound absorption purpose (see Fig. 3 and 4) [18-22]. Also, as was expected, combining the polyethylene and chipboard panels with a spacer structure and forming a layered system, it enhanced the sound absorption performance in both the high and more importantly the low frequency range (see
Fig.5). The first layer of the combined system, namely the spacer fabric, reduces the sound wave energy by converting the mechanical movement of the air particles and then the second layer faces the wave with weakened energy.
Finally, the cost analysis made for the unit area of the composite panels showed that the panel with 35% reinforcing fabric ratio and the panel with 45% reinforcing fabric ratio were cheaper by 17% and 13% than the chipboard backing, respectively.
Since the composite panels containing 35% and 45% reinforcing material gave the best results when they were tested for their sound absorbency, together with the spacer fabric, the panels with 25% reinforcing fabric ratio were not included into the cost analysis.
Conclusions
In this paper, an eco-friendly and novel sound-absorbing material has been proposed. The results of our study showed that the developed composite panel which is lighter, thinner and cheaper than the chipboard, can be used as a support backing instead of the chipboard in the frequency range studied.
Furthermore, the system proposed, by the combination of the novel composite panels designed as the backing material and the spacer fabric, utilised as the aesthetic component demonstrated better performance than the system including chipboard. The influence of the uniform distribution of the recycled material in the composite is proposed to be studied as future work.
References
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