Hierarchical pore structure based on cellulose nanofiber/melamine composite foam with enhanced sound absorption performance
Graphical abstract
Introduction
With the rapid development of industrial machines, airplanes, trains and so on, noise pollution has become more and more serious. It has been reported that long-term exposure of noise to the human body can cause hearing damage, annoyance, sleep disorders, cardiovascular diseases, and so on (Basner et al., 2014; Muzet, 2007). For eliminating the unfavorable influence of noise on people, it is an effective method to utilize sound absorption materials to dissipate sound energy in the propagation path.
Sound absorption materials are generally classified into two types: resonant sound absorption materials and porous sound absorption materials (Gai, Xing, Li, Zhang, & Wang, 2016; Tang & Yan, 2017). The resonance absorbing materials such as Helmholtz resonators (Cai, Guo, Hu, & Yang, 2014), membrane absorbers (Min, Nagamura, Nakagawa, & Okamura, 2013), and perforated plates (Zhao, Yu, & Wu, 2016) have a narrow absorption frequency band that severely limit their applications. The porous sound absorption materials include natural fiber (Berardi & Iannace, 2017), melamine foam (MF) (D’Alessandro, Baldinelli, Bianchi, Sambuco, & Rufini, 2018), polyurethane foam (Hyuk Park et al., 2017), ceramics (Du et al., 2020), fiberglass (Sun, Shen, Ma, & Zhang, 2015), etc. Such porous materials normally have advantages of wide sound absorption frequency range, convenient material selection and relatively simple processing, resulting in their increasing application in practical engineering (Berardi & Iannace, 2015; Yang, Kim, & Kim, 2003). Unfortunately, the porous sound absorption materials generally have poor sound absorption performance within the low frequency range (100−800 Hz) and require to increase the thickness and weight for achieving a satisfactory sound absorption coefficient, which restricts their application with stringent requirements (C. Zhang, Li, Hu, Zhu, & Huang, 2012).
Structural properties such as pore size are considered as the main controlling factors affecting the sound absorption performance of porous materials (Park et al., 2017). It has been demonstrated that both macropores and mesopores have varying degrees of influence on the sound absorption performance of materials by the existing research (Ghaffari Mosanenzadeh, Naguib, Park, & Atalla, 2015; Huang, Zhou, Xie, Yang, & Kong, 2014). However, the effect of only uniformly controlling the pore size of the materials on the sound absorption performance has certain limitations, especially in the low frequency range. Compared with porous materials with uniform structure, the hierarchical pore structure can provide more tortuous propagation paths and more reaction areas, which is more beneficial to improve the sound absorption performance and expand the sound absorption frequency range (Cao, Si, Wu et al., 2019). Recently, several strategies to fabricate hierarchical pore structure have been reported, including the electrospinning technique (Cao, Si, Wu et al., 2019; Cao, Si, Yin, Yu, & Ding, 2019), impregnation method (Lee & Jung, 2019), freeze-drying method (Simón-Herrero, Peco, Romero, Valverde, & Sánchez-Silva, 2019), template-directed chemical vapor deposition (CVD) technique (Xue et al., 2017), salt-out method (Ghaffari Mosanenzadeh et al., 2015) and so on. For example, Cao et al. (Cao, Si, Yin et al., 2019) used a direct electrospinning method to fabricate Polystyrene (PS) fiber sponges with a lamellar corrugated microstructure for sound absorption. Simón-Herrero et al. (Simón-Herrero et al., 2019) synthesized aerogels based on a ternary system of polyvinyl alcohol (PVA), nanoclay and thermally reduced graphene oxide (trGO) with enhanced sound absorption properties.
To avoid materials collapse during the fabrication process and practical application, fabricating the hierarchical pore structure based on commercial foams through the impregnation method and freeze-drying method is an effective strategy (Liu et al., 2019; Nine et al., 2017; Oh, Kim, Lee, Kang, & Oh, 2018). For example, Lee et al. (Lee & Jung, 2019) reported that the polyurethane foam (PU) foam was immersed in the GO solution by a step-by-step vacuum-assisted process. The obtained PU foam with the hierarchical pore structure had a great enhancement in sound absorption. Nine et al. (Nine et al., 2017) immersed MF into the aqueous GO to evenly distribute GO in the open-cell network to form a GO‐based lamella network for enhancing sound absorption. The resulting composite foam with a density of 24.12 kg/m3 and a thickness of 26 mm exhibited 60 % enhancement over a broad absorption band range from 128 to 4000 Hz. However, the hierarchical pore structure based on commercial foams is mainly constructed by adding fillers such as GO, and the pore structure of the foam is only optimized on the macropores scale. The improvement of sound absorption performance brought by these methods is still difficult to meet the strict requirements for sound absorption materials.
Cellulose is the most abundant and renewable biomass polysaccharide on earth. It is reported that the C6 primary hydroxyl groups of cellulose can be selectively converted into C6 sodium carboxylate groups in 2,2,6,6-tetramethylpiperidine (TEMPO) oxidation system (Isogai, Saito, & Fukuzumi, 2011). The obtained TEMPO-oxidized cellulose nanofiber (CNF) has the characteristics of high aspect ratio, low ζ-potential in water, high crystallinity and high elastic modulus (Lu et al., 2017; Rahmatika, Goi, Kitamura, Widiyastuti, & Ogi, 2019). It has been widely used in the fields of paper coating, barrier material and sensor applications (Alves, Ferraz, & Gamelas, 2019). But it is nearly no report about tempo-oxidized CNF used in sound absorption materials.
In this work, CNF was combined with MF through the cyclic freezing-thawing method for the first time. The internal CNF films formed in this way can change the pore size on the macropores scale and affect the pore openness of MF. And some mesopores on the CNF films can be formed through the cyclic freezing-thawing method due to the close packing of CNF. The tortuosity and airflow resistance inside the materials can be adjusted by the formation of CNF films, which can provide more tortuous paths for sound waves, leading to more sound energy dissipation by air vibration. At the same time, the micro-vibration of the CNF films also can improve the consumption of energy, resulting in the improvement of sound absorption performance. More importantly, the hierarchical pore structure with both macropores and mesopores can increase the complexity of sound wave propagation and enhance the sound absorption over a broad absorption band range. This work is expected to provide more green approaches for the design and preparation of high-performance sound absorption materials.
Section snippets
Materials
Softwood bleached kraft pulps were supplied by the Institute of Paper Science and Technology at Georgia Tech, USA. Melamine foam (MF) was obtained from Zhengzhou Fengtai Nano Material Co., Ltd. 2,2,6,6-tetramethy-1-piperidinooxy (TEMPO, AR) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Hydrochloric acid (HCl) and sodium hypochlorite (NaClO) were provided by Sichuan Xilong Chemical Co., Ltd (China). Sodium bromide (NaBr) was provided by Tianjin Kemiou Chemical Reagent Co.,
Commercial MF and prepared CNF
The pore structure and morphology of unmodified MF is shown in Fig. 2a. It is showed that the unmodified MF is a complete open-cell structure with three-dimensional network, which has an average pore size of approximately 120 μm. The morphology of CNF is presented in the TEM image of Fig. 2b and the CNF suspensions with the concentration of 0.5 wt% have the zeta-potential of −60.8 mV. This indicates that CNF has high dispersion ability in water due to the negative carboxylate groups on the
Conclusions
In this work, the hierarchical pore structure with both macropores and mesopores for enhanced sound absorption was fabricated by combining the CNF and MF for the first time through cyclic freezing-thawing. The composite foam prepared by cyclic freezing-thawing showed more excellent sound absorption performance in broadband acoustic absorption and mechanical properties than that prepared by direct freeze-drying due to the multiple self-assembly of CNF. The sound absorption performance of
CRediT authorship contribution statement
Lu Shen: Methodology, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Haoruo Zhang: Methodology, Investigation, Data curation. Yanzhou Lei: Resources, Validation. Yang Chen: Resources, Validation. Mei Liang: Conceptualization, Supervision, Writing - review & editing, Resources, Project administration, Validation. Huawei Zou: Conceptualization, Supervision, Writing - review & editing, Resources, Funding acquisition, Project administration.
Acknowledgements
The authors would like to thank the financial support of the Key Technology Research and Development Program of Sichuan Province (No. 2019YFG0488), and thank the Analytical & Testing Center of Sichuan University for supporting SEM testing.
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