Preparation and characterization of hydrophilic modification of polypropylene non-woven fabric by dip-coating PVA (polyvinyl alcohol)
Introduction
For many membrane processes such as ultra-filtration and micro-filtration, the major drawback in the extensive use of membranes is membrane fouling which results in flux reduction during operation [1]. Membrane fouling caused by protein adsorption, concentration polarization, pore blocking, and gel layer formation, etc., is a repugnant problem. It is reported recently that the protein adsorptive fouling could account for up to 90% of permeability losses [2]. Therefore, much attention has been paid in the past 20 years to find out the mechanism of protein adsorption [3], and it is now known that the electrostatic forces and the hydrophobic interactions between certain domains in a protein molecule and the hydrophobic membrane surfaces are the main factors [4], [5], [6], [7], [8]. Up to now, it has been generally accepted that hydrophilic materials are less sensitive to protein adsorption than hydrophobic ones, the principle is that hydrophilic surface preferentially adsorbs water rather than solutes, leaving the membrane surface with protein-resistance. With respect to the prevention of membrane fouling, less hydrophobic membranes are normally favored [7], but hydrophilic membranes are susceptible to chemical and thermal impact in their application.
The commercially used membrane materials include ceramics and both unmodified and surface modified polymeric materials, such as polyethylene, polypropylene and polysulfone [8]. However, the high cost of membrane materials limits its further applications.
Polypropylene (PP) non-woven fabric (NWF) materials are composed of random network of overlapping fibers. They can create multiple connected pores through which the fluid can flow. Non-woven fabric filtration material has many outstanding properties, such as controllable pore size distribution and easy design of fiber surface area per unit weight and volume, and inexpensive cost of filter material compared to membrane. The NWF filter was applied air filtration [9] and domestic wastewater reclamation [10], [11], and irrigation water treatment [12]. However, NWF is relatively hydrophobic and non-wettable with water, the potential application in aqueous fluid is limited. Therefore, there is much interest for us in the modification of the non-woven surfaces to improve their hydrophilicity and to functionalize these surfaces.
To obtain a hydrophilic surface with anti-fouling property, several methods have been investigated, which can be divided into two classes: physical and chemical modifications. Up to now, it is well known that adsorbing suitable hydrophilic polymers on the surface alleviates protein fouling, while grafting the hydrophilic polymers is expected to provide a much more stable and long-standing surface layer [13], [14], [15], [16], [17], [18], [19]. For the hydrocarbon nature and the absence of any reactive site of polypropylene (PP) non-woven fabric, there are many methods to improve the surface hydrophilicity including plasma or antenna-coupling microwave plasma to activate the surface of the NWF, polymer or monomer is then grafted onto the surface of the NWF. The altered surface wettability [20] and fabrication of biocompatible surface of NWF are used for biological applications: platelet concentrates (PCs) filtration [21], stripped-off temporary wound dressing material [22], biomedical products [23], [24], collagen-bonded [23], etc.
Some research shows that surface roughness, except surface hydrophilicity/hydrophobicity, impacts the rate and extent of fouling. PVA (polyvinyl alcohol) polymer, with its highly hydrophilic character, good film-forming properties and outstanding physical and chemical stability, is a kind of excellent membrane material for preparation of a hydrophilic membrane. PVA hydro-gels can provide smooth, hydrophilic surface with minimal protein binding. To be useful as membranes, the PVA hydro-gels must support actual applications mechanically. The use of gels within porous supports, such as micro-filtration membranes, may increase the overall flux and improve the mechanical stability. Gel impregnated pore membranes to increase flux have been shown to be an attractive means of utilizing the transport properties of hydro-gels [25], [26]. In fact, considerable work has been carried out in the area of PVA-RO composite membrane, PVA or PVA co-polymer was prepared as a selective skin layer of composite RO membrane with high water permeation rate, good anti-fouling nature, excellent integrity in acidic or alkaline environment and remarkable resistance to abrasion [27].
PVA is a highly hydrophilic material that swells easily and even dissolves in water. Its character must be improved in order to obtain membrane of high intensity. The general method to enhance the intensity and decrease water-solubility includes hydrolysis of PVA molecules to form covalent bond or coordinate bond. Acetylization and cross-linking are common ways to form PVA membrane. Li and Barbari [28] mixed 10% volume fractions of 2 ml methanol, 3 ml acetic acid and 1 ml sulfuric acid, with certain concentration of solutions of glutaraldehyde and cross-linking agents, and with 10% mass fraction of 25 ml PVA solution. The mixture was spin-coated onto regenerated cellulose support membrane, and allowed to react in a covered, hydrated environment for 45 min. The membrane was then stored in deionized water for at least 24 h to allow to come to equilibrium swelling, and then ultra-filtration test was carried out, providing high anti-fouling to BSA as indicated by nearly 100% pure water flux recovery. Li et al. [27] took aqueous solution containing PVA (polyvinyl alcohol), cross-linking agents (glutaraldehyde) and additives (PEG-400) and passed it through porous substrate membrane under definite pressure to form the dynamic membrane. By heat treatment resulting in cross-linking reaction and drying, anti-fouling composite ultra-filtration membrane was obtained. After filtration of protein solution, the modified membrane showed “cleanability” and anti-fouling characteristics.
The present study focuses on the development of a good anti-fouling PVA-modified PP non-woven fabric. Dip-coating is one way of preparing thin and dense skin layer, which is simple and practical. Substrate membrane is immersed into casting solution containing polymer, pre-polymer or monomer. When the membrane is taken out, a thin solution layer adheres to the membrane. The layer immobilizes the porous substrate membrane after the solvent vaporizes and cross-link happens at certain temperature. PVA can be immobilized on the surface of NWF by dip-coating, and then the PVA-modified NWF is formed. Conventional cross-link reagent glutaraldehyde to surface modifies a lightly cross-linked PVA hydro-gel. The reaction between PVA and glutaraldehyde is between the hydroxyl groups of PVA and the aldehyde to form an acetal bridge, as shown in Scheme 1 [29]. The reaction is simple to conduct and the resultant product is stable. The reaction produces water and thus no toxic products are formed from the reaction.
The objective of this study was to characterize the anti-fouling properties of PVA-modified non-woven fabric. Attenuated total reflectance Fourier transform infrared (FTIR/ATR) and X-ray photoelectron spectroscopies were adopted to investigate the chemical composition changes on the PVA-modified NWF surface. Water contact angles were also measured. Protein adsorption experiments were conducted to evaluate the anti-fouling property of the PVA-modified NWF.
Section snippets
Materials and chemicals
Fully hydrolyzed polyvinyl alcohol (PVA) powder having a degree of polymerization of 1750 ± 50 was obtained from Hunan Xiangwei Co. Ltd. (China). Glutaraldehyde (GA), cross-linking agent was obtained as a 50% (w/w) aqueous solution. The glutaraldehyde concentration in this paper all refers to the concentration of 50 wt.% GA. Methanol, acetic acid and sulfuric acid were reagent grades and were used without further purification. Bovine serum albumin (BSA) was obtained from Beijing Bio-TECH (China)
Immobilization of PVA on the NWF surface
PP NWF is immersed into casting solution contained PVA and GA, and cross-linking reaction occurs at certain temperature and PVA is immobilized on NWF surface. The efficiency of immobilization can be greatly influenced by such factors as concentrations of PVA and GA, time and temperature of cross-linking reaction. When time and temperature are constant, the influence of concentrations of PVA and GA on the immobilization degree of PVA is shown in Fig. 1. The concentrations of PVA and GA are
Conclusions
Polypropylene non-woven fabric was modified by dipping methods to immobilize PVA (polyvinyl alcohol) on the surface. This study reveals that the immobilized PVA on the surface can significantly alter the surface properties of NWF by changing its surface physical and chemical characteristics. These characteristics were examined by the presence of specific molecular structures using ATR/FTIR and element function-abilities using XPS. The results of FTIR/ATR spectra and XPS demonstrate that the
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