Polyvinylidene fluoride membranes properties

The principles behind ultrafiltration are sometimes misunderstood. The nomenclature implies that separations are the result of physical trapping of the particles and molecules by the filter. With polycarbonate and fiberglass filters, separations are made primarily on the basis of physical size. Other filters (cellulose nitrate, polyvinylidene fluoride, and to a lesser extent cellulose acetate) trap particles that cannot pass through the pores, but also retain macromolecules by adsorption. In particular, these materials have protein and nucleic acid binding properties. Each type of membrane displays a different affinity for various molecules. For protein, the relative binding affinity is polyvinylidene fluoride > cellulose nitrate > cellulose acetate. We can expect to see many applications of the affinity membranes in the future as the various membrane surface chemistries are altered and made more specific. Some applications are described in the following pages. 

On the other hand, a number of polymers whose properties in relation with DAFC were scarcely known 8 years ago, have deserved a considerable attentimi during the last years, particularly membranes based on polybenzoimidazole (PBI) and polyvinylidene fluoride (PVdF). 

The study of the morphological characteristics of polymeric membranes is of great interest in the area of membrane science (Resting, 1971). It has been known that one of the keys to better membrane performance lies in the structure of the membrane itself. In turn the morphology of a particular membrane depends upon its processing conditions and the physical and chemical properties of the polymer system under consideration. Membranes from polypropylene (Porter, 1982), high-density polyethylene (Porter, 1982), polyvinylidene fluoride (Hiatt etal., 1984 Hiatt, 1985)... 

Many high-performance polymer fibres are used in filter media to meet various specific requirements in diverse filtration applications. Filters made from fluoropol-ymer (Polytetrafluoroethylene (PTFE), Polyvinylidene fluoride (PVDF), and Per-fluoroalkoxy alkane (PFA)) fibres, and membranes have inherent, chemical-resistant, and flame-retardant properties, and they are widely employed to filter aggressive chemicals and acids in the manufacture of wafers and microchips in the microelectronics industry. Ethylene ChloroTriFluoroEthylene (E-CTFE) melt blown fabrics have a unique ability to coalesce difficult liquids and can withstand the piranha effect in filtering ozone enriched ultrapure water. Polyphenylene sulfide (PPS) fibres are also chemical resistant, stand high temperature, and are suitable for making baghouse filters. Eilter media made from other high-performance polymer fibres, such as polyamide-imide, polyetherimide (PEI), Polyimide P84 fibre,polyetheretherke-tone, and liquid crystal polymers also appear in the filtration and separation market. 

Qu et al. (2009) have studied the removal of As(III) (arsenite) and As(V) (arsenate) by DCMD with a self-made hydrophobic polyvinylidene fluoride (PVDF) capillary membrane. The main membrane properties and the characteristics of the module are reported in the Tables 13.6 and 13.7. 

Tomaszewska, M. (1996). Preparation and properties of flat-sheet membranes fiom polyvinylidene fluoride for membrane distillation. Desalination 104, 1. 

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