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Ma, S., Zhang, M., Yang, B., Song, S., Nie, J., and Lu, P. (2018). "Preparation of cellulosic air filters with controllable pore structures via organic solvent-based freeze casting: The key role of fiber dispersion and pore size," BioRes. 13(3), 5894-5908.

Abstract

Green and biodegradable cellulose filters with controlled designer pore structures were prepared using organic solvent-based freeze casting. In this paper, the relationship between the different freeze media, including ethanol, isopropanol, and tertiary-butyl alcohol, and the microstructure of the porous filters was investigated. The results of the pore size distribution indicated that the pore channel size decreased remarkably when organic solvents were used as the freezing media. Moreover, the filters showed high filtration efficiencies, up to 99.70% and 99.66% for 0.5 µm and 0.3 µm particles, respectively, under a pressure drop of 180 Pa and at 32 L·min-1 flow rate. The fabrication of cellulosic filters would not only make it a promising candidate for capturing fine particulate matter, but also provide a versatile approach to regulate and design a porous structure for materials applied in various fields.


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Preparation of Cellulosic Air Filters with Controllable Pore Structures via Organic Solvent-based Freeze Casting: The Key Role of Fiber Dispersion and Pore Size

Shanshan Ma,a Meiyun Zhang,a,b Bin Yang,a,b Shunxi Song,a,c,* Jingyi Nie,a,* and Peng Lu c

Green and biodegradable cellulose filters with controlled designer pore structures were prepared using organic solvent-based freeze casting. In this paper, the relationship between the different freeze media, including ethanol, isopropanol, and tertiary-butyl alcohol, and the microstructure of the porous filters was investigated. The results of the pore size distribution indicated that the pore channel size decreased remarkably when organic solvents were used as the freezing media. Moreover, the filters showed high filtration efficiencies, up to 99.70% and 99.66% for 0.5 µm and 0.3 µm particles, respectively, under a pressure drop of 180 Pa and at 32 L·min-1 flow rate. The fabrication of cellulosic filters would not only make it a promising candidate for capturing fine particulate matter, but also provide a versatile approach to regulate and design a porous structure for materials applied in various fields.

Keywords: Cellulosic air filters; Pore size; Organic solvent system; Freeze casting; Filtration performance

Contact information: a: College of Bioresources Chemical and Materials Engineering, National Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi University of Science & Technology, Xi’an, 710021, China; b: State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, 201620, China; c: Guangxi Key Lab of Clean Pulp & Papermaking and Pollution Control, Guangxi University, Nanning, 530004, China;

* Corresponding authors: 15829913710@163.com; jingyi.nie@163.com

INTRODUCTION

Particulate air pollution (Brauer et al. 2016; Liang et al. 2016; Wu et al. 2017), especially fine particulate matter (PM2.5, Aerodynamic equivalent diameter ≤ 2.5 µm), has intimate and severe impacts on human health (World Health Organization 2015), daily activities (Lee et al. 2015a), climate, and the environment (Brauer et al. 2012; Chafe et al. 2014). Additionally, statistics have shown that PM2.5 concentration increments of 10 µg/m3 on average can lead to an 8% and 6% increased risk in lung cancer and cardiopulmonary diseases, respectively (Pope III et al. 2002). Therefore, the urgent need for health assurance has drawn much attention to the exploration of high-performance air filters (Wan et al. 2014; Liu et al. 2017). Nowadays, porous membrane filters (Liu et al. 2011, 2015) and fibrous filters (MacFarlane et al. 2012) are common and popular in the market. However, the disposal of synthetic polymer-based filters results in secondary pollution because of their nonrenewable and non-degradable properties. In contrast to synthetic fibers, cellulosic fibers (Abdul Khalil et al. 2014; Lee et al. 2015b) may be a suitable candidate (Svagan et al. 2008) to fabricate renewable and sustainable filters. This suitability owes a great deal to the superior characteristics of the raw materials, such as their abundance in nature, biodegradable sources, and low cost. Nevertheless, traditional cellulose filters cannot reach a satisfactory filtration performance because of their loose lamellar structure.

To obtain a satisfactory filtration performance, the filtration mechanism needs to be understood first. Generally, the model for the capture of particles is based on five filtration mechanisms (Zamani and Maini 2009; Sokovnin et al. 2015): physical sieving, interception, diffusion, inertial separation, and electrostatic attraction. Among these, physical sieving, interception, and diffusion usually play major roles in the filtration performance and largely depend on the pore size and structure (Zhao et al. 2017; Gong et al. 2018). Thus, regulating the pore size and structure is a key factor to improve the filtration performance of porous materials (Wang et al. 2009; Hu et al. 2010).

Freeze casting (Liu et al. 2013), is a versatile and promising method and is commonly employed to prepare porous materials with a sophisticated and hierarchical structure (Deville et al. 2006). This method could retain surface fibrillation of cellulose fibers and prevent the collapse of porous structure during the drying process (Mao et al. 2008). The designed microstructures of porous materials are controlled by several parameters (Qian and Zhang 2011; Waschkies et al. 2011), including the freezing temperature, pulp consistency, and freezing medium. Different freezing media (Cui et al. 2006; Nemoto et al. 2015) remarkably influence the pore structure and size of materials by altering the growth behavior of crystals (Teagarden and Baker 2002; Dong et al. 2017). Moreover, the pore structure of the samples is the reverse replication of the shape of solvent crystals during the freeze-drying process (Deville 2008). Therefore, choosing a suitable freezing medium is the key step in fabricating air filters with excellent filtration performances.

Suitable freezing media, such as water, camphene, and tertiary-butyl alcohol (TBA), have been successfully used in the fabrication of air filters. Water has widely been applied in the ice-templating synthesis of porous materials because it is low-cost and harmless to the environment (Gutiérrez et al. 2008). Unlike the lamellar or dendritic structure of water or camphene-based crystals, an organic eutecticum generally exhibits a long straight prism and would be beneficial for fabricating filters with a dense structure applied in the intercepting of PM (Oesterle et al. 1998; Wittaya-Areekul and Nail 1998). Additionally, using organic solvents as freezing media could shorten the sublimation time and decrease the energy consumption because of a high equilibrium vapor pressure and low surface tension (Kasraian and DeLuca 1995). Ethanol and isopropanol were also used to fabricate cellulose fiber-based filters in this study because of the similarity of the chemical structures. Cao et al. (2013) fabricated regenerated silk fibroin scaffolds with controllable porous three-dimensional microstructures with the addition of n-butanol. Sehaqui et al. (2011) produced an aerogel with a specific surface area as high as 153 m2/g to 284 m2/g according to the solvent exchange method from water to TBA. Miller et al. (2015) found that the pore size was related to the specific freezing medium in the alumina ceramics fabrication process. This means it provides a desirable method to prepare filters with a controllable pore morphology (Vessot and Andrieu 2012).

The objective of this research is to explore a means of preparing biodegradable cellulose air filters with various pore structures based on organic solvent-water miscible systems via freeze casting. Ethanol, isopropanol, and TBA were employed to regulate the microstructures of the filters. The effects of different alcohols and content of TBA on the pore morphology, pore size distribution, porosity, specific surface area, and filtration performance of the filter samples were investigated. Furthermore, the regulating mechanisms of the alcohols on the pore structure and size of the filters were also investigated and discussed. This work provides a useful way to fabricate renewable and sustainable air filters with controllable pore structures and displays a great potential in PM removal and air purification. Moreover, the results give a deep insight into the structure-property relationship of cellulose filters during freeze casting.

EXPERIMENTAL

Materials

Air-dried softwood Kraft pulp board with a solids content of 95.5% was obtained from Shan Dong Pulp & Paper Co., Ltd (Jinan, China). Analytical grade TBA, isopropyl alcohol, and ethanol were purchased from Tianjin Damao Chemical Reagent Company (Tianjin, China).

Methods

Preparation of the cellulosic air filters

A softwood pulp board (30 g, oven-dried) was immersed in water for 24 h to obtain swollen and flexible fibers, which were then disintegrated with a Lorentzen & Wettre disperser (Sartorius, Göttingen, Germany) at 12000 rpm. The fiber pulp at 10% (w/w) was further treated using a KPK PFI Refiner (Kumagai Riki Kogyo, Tokyo, Japan) at 10000 rpm, 20000 rpm, 30000 rpm, 40000 rpm, and 50000 rpm to obtain fibrillated cellulosic fibers. A schematic illustration for fabricating the high performance cellulosic air filters via freeze-drying is shown in Fig. 1.

Fig. 1. Schematic illustration for preparing the air filters

The fibrillated fibers treated by the refining process were dispersed in different media (A – Purified water; B – Water with ethanol, isopropanol, or 10% v/v TBA) to obtain a uniform pulp suspension. The addition of ethanol and isopropanol in this study was under 10% (v/v) to ensure that the samples were completely frozen. This was done to avoid the “partial cake collapse” phenomenon (Daoussi et al. 2009) that occurs when the content of ethanol and isopropanol is over 10% (v/v) because of the low freezing points of these alcohols. Then, the 1.5% (w/w) suspension was poured into a petri dish and kept at -56 °C for 4 h. Finally, the samples were placed in a BILON freeze dryer (Shanghai Bilang Instrument Manufacture Co. Ltd., Shanghai, China) to sublimate and remove the residual solvent at -56 °C under 10 Pa for 3 d. The A-air filter was based on purified water and presented a layered structure. The B-air filter was prepared with an organic solvent and presented a porous architecture with a spider web-like structure.

The filtration performance was measured with an automated filter tester (LZC-K, Suzhou Huada Filter Technology Co. Ltd., Suzhou, China) according to European standard EN 1822-3:2000, which consisted of an aerosol generator, laser particle counter (2.83 L/min), flow counter (5 L/min to 50 L/min), spray pump (80 L/min, 220 V), and a pair of filter holders with a diameter of 10 cm. The filter tester was equipped with a diethyl hexyl sebacate (DEHS) aerosol nebulizer for generating fine particles (PMs) with the diameter of 300 nm to 500 nm (Thiessen 2006). The flow rate though air filter, total measuring time, relative humidity, and temperature were set to 32 L/min, 10 s, room temperature (25 °C), and 40% RH during testing process, respectively. The consistency of PMs in airflow after filtration testing was detected by a particle counter. And the pressure change of airflow was measured using a pressure detector. The filtration efficiency and pressure drop were employed to evaluate the filtration performance of the filter samples. The filtration efficiency (η) was calculated from the upstream and downstream particle concentrations using Eq. 1,

 (1)

where Cdownstream is the downstream particle concentration (%) and Cupstream is the upstream particle concentration (%).

The pressure drop (ΔP) was determined from the upstream and downstream pressures with Eq. 2,

 (2)

where Pdownstream is the downstream pressure (Pa) and Pupstream is the upstream pressure (Pa).

The quality factor (QF) is a typical index to evaluate the filtration performance. The QF (MacFarlane et al. 2012) value was independent of the porosity and thickness of the samples, and was calculated with the following formula:

 (3)

The surface morphology of the fibrillated fibers and filter samples were sputter-coated with gold and observed with scanning electron microscopy (SEM) (VEGA-3-SBH, TESCAN, Brno, Czech Republic) at an operating voltage of 10 kV.

The Brunauer–Emmett–Teller specific surface area (BET-SSA) of the filter samples was calculated with N2 adsorption isotherms using a Gemini VII2390 automated apparatus (Micromeritics, Shanghai, China) at a relative vapor pressure of 0.01 MPa to 0.3 MPa (Sehaqui et al. 2011). Samples (0.1 g to 0.2 g) were degassed with N2 at 105 °C for 1 h before N2 adsorption testing occurred at -196 °C.

The porosity reflects the density of the samples, which refers to the ratio of the total volume of the micropores to the volume of the porous materials (Sehaqui et al. 2011). The porosity and distribution of the pore size was analyzed with a Mercury Intrusion Porosimeter (AutoPore-IV9500, Micromeritics, Shanghai, China).

RESULTS AND DISCUSSION

Effects of the Organic Solvent-water Systems on the Pore Size and Structure

Figure 2 shows digital photographs and SEM images of the air filters prepared with different freezing media, including water, ethanol, isopropanol, and TBA. Figures 2a and 2e show that with purified water as the freezing medium the filter samples presented a rough surface at the macro level and a lamellar structure at the micro level. This morphology was attributed to the growth of ice crystals in a certain direction. Moreover, undesirable phenomena often occurred during the drying process because of strong surface tension and capillary action. These phenomena included the collapse of fiber-network (Mao et al. 2009) and slight shrinkage of the samples. Hence, the filters based on purified water usually showed a poor filtration efficiency because of the large-sized pores and damaged structure. Therefore, organic solvents were employed to improve the filtration performance of the samples (Fig. 2). The filtration performance could be improved with regards to the microstructure, pore size distribution, and porosity (Nemoto et al.2015). A transformation of the sample structure from anisotropic to isotropic was observed (Figs. 2i to 2l). The samples prepared with the water/ethanol system presented a cellular structure because of the existence of hydrogen bonding (Figs. 2b, 2f, and 2j). The samples prepared with a water/isopropanol system presented regular pores with a fine network structure built by fiber trunks and fibrils (Fig. 2c, 2g, and 2k). Moreover, the samples prepared with a water/TBA system presented smaller sized pores than those based on the other solvents (Figs. 2d, 2h, and 2l). These types of three-dimensional filters showed uniform and smooth surfaces with a spider web-like structure. These networks and spider web-like structures greatly contributed to increasing the filtration performance of the samples.