Development of non-woven fabric-based ECTFE membranes for direct contact membrane distillation application

Highlights

• HALAR® ECTFE was used to prepare supported porous membranes.

• ECTFE membranes were prepared by a binary diluent system via dip-coating technique.

• The effect of additive concentration and the operating conditions were investigated.

• ECTFE membranes are successfully applied in membrane distillation applications.

Abstract

In this work, a lower melting point of HALAR® Ethylene-Chlorotrifluoroethylene (LMP ECTFE) was used to prepare non-woven fabric-based porous membranes, using a polyester non woven (NWPET) as support, via dip-coating. Di-ethyl Adipate (DEA) and di-ethylene Glycol (DEG) were selected as primary and secondary diluent, respectively. SEM analyses showed that in all cases the coating layer was homogeneous, covering all the NWPET support and leading to an interconnected sponge-like microstructure. Membrane characterization evidenced the effects of the DEG concentration, which plays the role of pore former. The effect of the immersion time was examined and studied, too. The produced membranes were fully characterized in terms of water contact angle, porosity and pore size. Direct contact membrane distillation (DCMD) experiments using water and salty solution 0.6 M (NaCl) as feed, were performed for selected supported membranes. The obtained fluxes ranged between 3 and 22 kg/m²h, depending on DEG concentration, at the same immersion time, and on the feed temperatures (40–60 °C). Rejection (R%) was comprised from 94.95 to 99.82%. In particular, the N-ECTFE 7-0-7 membrane, with the highest R%, showed a stable deionized water flux before and after the experiments, indicating its ability to recover its original performance.

Introduction

About 60 years have passed since the invention of the immersion precipitation technique to improve membrane performance [1]. During these years, membrane technology has become increasingly important, amplifying its fields of applications, ranging from water treatment [2,3], desalination [4,5], food and solvent processing [[6], [7], [8], [9], [10], [11]] to gas separation [12,13], medical devices [14] and electrochemical applications [15]. As it is known, membrane processes possess several advantages, such as easy operational control, low energy costs, more compact assembly and possibility to recover substances without the use of additional chemicals [16]. Membrane efficiency depends on its properties as hydrophilic/hydrophobic nature, surface charge, porosity, mechanical performance and solvent resistance. Therefore, it is correct to sustain that the membranes performance is mainly limited by material properties. In order to achieve tailored high-performance membranes and overcome problems, e.g. fouling or solvents resistance, several techniques has been developed for producing and modifying membranes surface. These membranes, called (thin film-) (nano-) composite membranes, can be prepared in several way, e.g. adapting or using novel polymers, or by employing surface functionalization techniques [17]. In this scenario, due to their optimal properties, fluoropolymers have found a large application as membrane materials. In fact, thanks to their chemical backbone, which is formed by strong CC bonds (~340 kJmol⁻¹) and stable CF bonds (~485 kJmol⁻¹), these materials possess unique and advanced properties such as high thermal and chemical stability, lower surface tension, photostability and low coefficient of friction [18,19]. The use of fluoropolymer as surface modifier/coating material has well been reported in literature. Koh et al. [20] synthesized a graft copolymers comprising poly(vinylidene fluoride-co-chlorotrifluoroethylene) backbone and poly(styrene sulfonic acid) side chains, i.e. P(VDF-co-CTFE)-g-PSSA. Composite NF membranes were successfully prepared from P(VDF-co-CTFE)-g-PSSA as a top layer coated onto P(VDF-co-CTFE) ultrafiltration support membrane. Several works reported the use of fluorinated polymer or monomer to prepare membranes suitable for membrane distillation (MD). In fact, fluorinated hydrophobic porous membranes in polytetrafluoroethylene (PTFE), ethylene chlorotri-fluoroethylene (ECTFE) and polyvinylidene fluoride (PVDF) are often employed in direct contact membrane distillation (DCMD). Nevertheless, membranes are not produced specifically for this process but are adapted. Moreover, they are both expensive and difficult to fabricate [21]. To overcome these problems, in terms of manufacturing, costs and efficiency and membranes specifically designed for MD, new studies focus the attention on the preparation of fluoropolymer composite membranes, starting from available and low-cost membranes as support. Fluorinated modifying macromolecules (SMMs) were employed by Qtaishat et al. [[22], [23], [24]], and Khayet et al. [25,26], in order to prepare novel composite hydrophobic/hydrophilic membranes for DCMD. In particular, membranes were prepared via phase inversion, introducing in the dope solution the 1.5 wt% of SMMs. PTFE has been employed as coating material in the preparation of carbon nanotube bucky-paper (CNT BPs) membranes [27]. The coated layer of PTFE enhanced the hydrophobicity and improved the mechanical stability of the obtained composite membranes. Superhydrophobic and oleophobic coated membranes were prepared by Park et al. [28]. PTFE film was first coated with SiO2 nanoparticles by the dip-coating method, and 1H,1H,2H,2H-perfluorooctyltrichlorosilane (FOTS) was subsequently deposited on the SiO₂-coated PTFE by dip-coating or vapor deposition. The obtained composite membranes were tested for gas permeability towards CO₂ and dimethyl methylphosphonate (DMMP) vapor. Santoro et al. [29], developed an ethylene–chlorotrifluoroethylene (ECTFE) coated polypropylene (PP) composite hollow-fibers (HFs). The results proving that the ECTFE/PP composite HFs could be used for the filtration of organic media and for bulk liquid membrane (BLM) extractants. An interesting method based on the use of perfluoropolyethers (PFPEs) oligomer compounds, for producing hydrophobic or hydrophobic/hydrophilic coated membranes was recently reported by Figoli et al. [21]. In this work, a UV-curable PFPE compound (Fluorolink® AD 1700) was employed for preparing hydrophobic/hydrophilic coated membranes using hydrophilic polyamide (PA) membranes as support. Membranes performance were evaluated via DCMD experiments demonstrating the stability of the coating during time. Again, Ursino et al. [30], employed Fluorolink® MD700 as surface modifier via dip-coating and in situ polymerization technique, on two different commercial hydrophilic membranes: PA, and polyethersulfone (PES). The work focused on the study of the coating stability during time, evaluated using salty solution and chemicals cleaning agents. In this work, a lower melting point grade of Halar® ECTFE, named LMP ECTFE was selected as fluoropolymers in order to prepare non-woven fabric-based LMP ECTFE porous membranes via dip-coating, using a polyester non woven (NWPET) as support. In this direction, it is possible to prepare porous membranes by coating an inexpensive polymer and giving it the properties of the Halar® ECTFE as chemical resistance and higher mechanical performance [31,32]. Commonly this polymer is solubilized in organic solvents, often toxic, at high temperature, and the membranes are usually prepared via thermally induced phase separation (TIPS) technique [[33], [34], [35]]. To overcome these problems, Solvay Specialty Polymers developed LMP ECTFE: comparable properties with standard Halar®, but lower crystallinity and lower melting point [8,36], therefore more workable. Previously Ursino et al. [8], employed LMP ECTFE dissolved in di-ethyl adipate (DEA), for making dense and porous membranes. The authors reported that varying the polymer concentration, it is possible to develop nanofiltration (NF) and ultrafiltration (UF) membranes via TIPS, that can be used in separation filtration processes under harsh conditions. More recently, LMP-ECTFE NF membranes were employed by Tundis et al. [9], in order to produce the concentrated fractions of Sambucus nigra L. (Adoxaceae) flowers and leaves, suitable for food additives, cosmetic, and pharmaceutical products. In the present study, non-woven fabric-based LMP ECTFE porous membranes were prepared dissolving low concentration of LMP ECTFE by a binary diluent system via dip-coating technique. DEA and di-ethylene Glycol (DEG) were selected as primary and secondary diluent, respectively. The effect of DEG concentration (0–5-7 wt%) and the experimental conditions of the dip-coating, such as immersion time (3–5-7 s), were investigated. The non-woven fabric-based LMP ECTFE membranes were characterized in terms of morphology using SEM, AFM, water contact angle, porosity pore size and water permeability. Finally, membrane performance was evaluated via DCMD experiments using water and salty solution as feed (0.6 M NaCl).