Electrospun nanofibers a way to improved wet filtration efficiency of deep-bed filters

Transcription

1 Electrospun nanofibers a way to improved wet filtration efficiency of deep-bed filters Thomas Bahners, Torsten Textor and Eckhard Schollmeyer Deutsches Textilforschungszentrum Nord-West e. V., Krefeld, Germany

2 Textiles for filtration Textile fabrics of varying construction e.g. woven, knitted, nonwoven, pile and combinations thereof - find increasing application in fluid filtration on the background of - their complex pore systems with varying pore sizes ( geometric separation of larger particles) - labyrinth effect, i.e. complex stream lines leading to effective impact separation of small particles and - high flow rates.

3 Methods to tune the separation performance T(d) - Choice of fabric construction (yarn geometry, fineness, weave etc.) 1,2 - Combination of fabrics, flow direction - Increased particle adhesion following surface modification, e.g. micro-roughening by means of laser treatment 3,4 - irregular small pore systeme with high porosity nanofiber web 1 AiF-Forschungsvorhaben Nr Th. Bahners und E. Schollmeyer, Technische Textilien 44 (2001) 130 ff 3 Bahners T. und Schollmeyer E., Staub - Reinh. Luft 47 (1987) Th. Bahners et al., in Particles on Surfaces, Vol 9, K.L. Mittal (Ed.),VSP, Utrecht (2006).

4 Fundamentals of electrospinning Electrospinning is based on the separation and acceleration of charges in a polymer solution (or melt), which are effected by a strong, mostly inhomogeneous electro-static field. The acceleration of charges in combination with the surface tension of the solution (or melt) leads to the drawing of a so-called jet initially along the field lines, which solidifies and forms a fiber on its way to the counter electrode.

5 The basic setup for electrospinning: - controlled supply of polymer solution - strong inhomogeneity - depending on parameters fiber diameter markedly below 1 µm capillary jet formation polymer solution HV counter electrode

6 Material Basically, fibers can be spun from any soluble (or meltable) polymer, i.e. the known synthetic or natural fiber polymers, but also biopolymers. PCL / methylene chloride

7 Material Basically, fibers can be spun from any soluble (or meltable) polymer, i.e. the known synthetic or natural fiber polymers, but also biopolymers. poly(meta-phenylene isophthalamide) lithium chloride N-dimethyl acetamide

8 Material Basically, fibers can be spun from any soluble (or meltable) polymer, i.e. the known synthetic or natural fiber polymers, but also biopolymers. DBC (Krill Chitin) acetone

9 Material Basically, fibers can be spun from any soluble (or meltable) polymer, i.e. the known synthetic or natural fiber polymers, but also biopolymers. Functional solids or liquids can directly added to the solution and incorporated in the fibers, e.g. enzymes, nano-particles,...

10 Filtration experiments deep-bed filter concept Filter layer N layer optimized field geometry mean fiber diameter 125 ± 15 nm commercial non-woven (PBT) PCL-nanofibers (directly spun onto the nonwoven)

11 Filtration experiments Model system: monodisperse suspension of polystyrene particles a) spherical particles of 2 µm diameter b) spherical particles of 500 nm diameter Escherichia coli (size approx. 2.0 x 0.5 µm) J. Miao et al., PNAS 100, 110 (2003)

12 Commercial nonwoven (PBT) Captured particles (d = 2 µm) PCL-nanofibers (spun directly on the nonwoven, PCL fiber diameter d 130 nm) PCL, 6% in dichlormethane, 20 kv, 15 cm

13 Total separation efficiency of stack of N layers at 2 µm layers 95 % % Separation efficiency [%] PBT nonwoven PBT nonwoven with nanofiber layer Number of layers The separation efficiency was determined by particle counting using a Coulter-Counter

14 Total separation efficiency of stack of N layers at 2 µm A simple approximation of a stack of N layers is based on the assumption of negligible effects of previous layers on particle trajectories, i.e. separation processes in two successive layers are totally independent. Accordingly, each layer is supposed to have the same separation efficiency η l and the total efficiency of the stack is given by ( ( )) N η( d) = 1 1 η l d efficiency of single layer:

15 Total separation efficiency of stack of N layers at 2 µm data points give η 1,20 l = 0.78 ± 0.04 total separation efficiency η 1, , , , , ,00 data points give η l = 0.28 ± number of layers N

16 Effects on flow rate / geometry optimization The nanofiber layer (web) reduces the flow rate, but increases the separation efficiency significantly positive net effect layers nonwoven nonwoven c/w nanofiber layer efficiency flow rate efficiency flow rate [%] [ml/min] [%] [ml/min] , , , , ,86

17 Separation efficiency of stack of N layers at 500 nm Experiments using spherical PS-particles with a diameter of 500 nm indicate the increasing effect of particle impaction!

18 Separation efficiency of stack of N layers at 500 nm Experiments using spherical PS-particles with a diameter of 500 nm indicate the increasing effect of particle impaction!

19 Total separation efficiency of stack of N layers at 500 nm The separation efficiency was determined by particle counting using a Zetasizer (dynamic light scattering)

20 Total separation efficiency of stack of N layers at 500 nm total separation efficiency η 0, ,80 0,70 0, ,50 0, ,30 0, ,10 ( ( )) N η( d ) = 1 1 η l d 0, number of layers N data points give: η l = 0.08 ± 0.09 η l = 0.04 ± 0.04 Indication of long-range separation processes based on the diffusion of small particles

21 Thank you for your attention! The authors wish to acknowledge financial support by the Ministerium für Wissenschaft und Forschung des Landes Nordrhein-Westfalen and financial support by the Forschungskuratorium Textil e.v. for part of the study in the framework of project AiF-Nr N (This support granted within the program Industrielle Gemeinschaftsforschung (IGF) from resources of the Bundesministerium für Wirtschaft und Technologie (BMWi) via a supplementary contribution by the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen e.v. (AiF))