Porous lyocell powders as sound absorbers
- 243 Downloads
Environmentally friendly insulation materials based on renewable resources come more and more to the fore, replacing less sustainable materials, such as mineral wool or polystyrene foams. In this study, we present a straight-forward avenue to a highly porous cellulose II powder for application as sound absorber. The material was prepared from a lyocell-derived cellulose II gel by a freeze-drying approach. The powder has a porosity of 98% and a specific surface area of 177 m2/g. It featured a sound absorption maximum in the frequency range of one-third-octave bands of 400–800 Hz (65 mm thickness) and was significantly more efficient as acoustic insulation material than commercial alternatives. The high performance makes the cellulose II sound absorber especially attractive for a use in loudspeakers for small electronics.
KeywordsAcoustic absorption Cellulose Freeze-drying Insulation material Lyocell gel Sound absorber
Sound-absorbing materials take up most of the sound energy striking them and reflect only very little (Arenas and Crocker 2010). This property is measured by the sound absorption coefficient (α ≤ 1), which is the ratio between the absorbed and incident sound intensity. This value is especially high in the case of open-porous materials (Crocker and Arenas 2008). Conventional sound absorbers are mineral wool or polystyrene foams. Due to environmental concerns, environmentally friendly, sustainable and renewable insulation materials come more and more to the fore as “green” alternatives to these conventional absorbers (Arenas and Crocker 2010). Plant-based cellulosics are promising candidates in this respect. In the literature, most acoustical behavior studies have been conducted on materials consisting of cellulose microfibers (Zulkifli et al. 2008; Putra et al. 2013; Yeon et al. 2014; Pöhler et al. 2016; Nechita and Năstac 2018). In this study, we aimed at increasing the efficiency of cellulosic sound absorbers by using a previously described cellulose II gel i.e. lyocell gel (Männer et al. 2015; Beaumont et al. 2016). This gel can be classified as nanocellulose and is a precursor to highly porous, aerogel-like materials (Beaumont et al. 2017). It is produced by an industrial cellulose fiber production process (lyocell process) in a captivatingly energy-efficient way, which is much more sustainable than conventional cellulose I nanofibril manufacture (Beaumont et al. 2016).
Scanning electron microscopy (SEM) as illustrated in Fig. 1 showed that the freeze-dried samples consisted of individual particles featuring an open-porous and fibrillar structure. The dry powder had a bulk density of 28 mg/cm3 and a porosity of 98.2%. The high porosity is also reflected in the high specific surface area of 177 m2/g, which is in the same range as for cellulose I nanofibril cryogels (Jiang and Hsieh 2014). The pore size distribution was determined by nitrogen sorption experiments (Figure S1) showing a preserved nanoporous structure (pores of 2–100 nm) with a cumulative pore volume of 0.75 cm3/g and an average mesopore diameter of 13 nm. The volume mean particle size of the powder was 28 µm, see Figure S2 for the particle size distribution. The particle-like morphology is exemplarily shown in the SEM micrograph in Figure S3.
A commercial cellulose fiber insulant for blown-in insulation and mineral wool were used for comparison and all measurements were performed on test specimens of 65 mm thickness. The commercial acoustic absorbers featured a similar acoustic absorption characteristic with absorption maxima at frequencies higher than one-third octave bands of 800 Hz. In general, the absorption maxima of materials can be shifted to lower frequencies by increasing the thickness of the absorber and to higher frequencies by decreasing it (Chu et al. 2017). That means that the lower the absorption maxima of materials with a fixed thickness, the higher is their efficiency as sound absorbers. The absorption curve of the cellulose II powder in Fig. 2 shows its absorption maximum to be in the frequency range of one-third octave bands of 400–800 Hz, which covers a wide frequency range of the human voice (100–900 Hz) (Clifton et al. 2006). In comparison to the commercial insulants tested, usually high-priced products, the absorption maximum of the cellulose II powder is significantly shifted to lower frequencies. The here-prepared cellulose II powders thus are considerably more efficient as sound absorber. Their high potential as acoustic absorber is currently tested with regard to applications in loudspeakers in small electronics or hifi systems. This is accompanied by further improvements of the acoustic performance and by further simplification of the manufacturing process.
Open access funding provided by University of Natural Resources and Life Sciences Vienna (BOKU). The project was financed by the PhD School DokIn’Holz, funded by the Austrian Federal Ministry of Science, Research and Economy, and by Lenzing AG.
- Arenas JP, Crocker MJ (2010) Recent trends in porous sound-absorbing materials. Sound Vib 44:12–18Google Scholar
- Crocker MJ, Arenas JP (2008) Use of sound-absorbing materials. In: Crocker MJ (ed) Handbook of noise and vibration control. Wiley, Hoboken, pp 696–713Google Scholar
- Männer J, Opietnik M, Innerlohinger J et al (2015) Cellulose suspension, method for the production and use thereof. Patent number WO/2015/054712Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.