The International Istanbul Textile Congress 2013
May 30th to June 1th 2013, Istanbul, Turkey
1
ELECTROSPINNING OF CELLULOS...
The International Istanbul Textile Congress 2013
May 30th to June 1th 2013, Istanbul, Turkey
2
a) b) c)
Figure 1. Electros...
The International Istanbul Textile Congress 2013
May 30th to June 1th 2013, Istanbul, Turkey
3
distribution of fibres spun...
The International Istanbul Textile Congress 2013
May 30th to June 1th 2013, Istanbul, Turkey
4
a) b)
c) d)
Figure 4. Appea...
The International Istanbul Textile Congress 2013
May 30th to June 1th 2013, Istanbul, Turkey
5
a) b)
Figure 6. Appearance ...
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Electrospn 7 heikkila-full

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Electrospn 7 heikkila-full

  1. 1. The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey 1 ELECTROSPINNING OF CELLULOSE ACETATE AND USE OF SUCH FIBRES IN LAYERED MEMBRANES P. Heikkilä, Y. Shen, A. Grönroos, J. Houni and J. Sarlin VTT Technical research centre of Finland pirjo.heikkila@vtt.fi Abstract: Eletcrospinning method can be used in preparation of sub-micron and nanosized fibres. The main commercial application of electrospun materials is aerosol filtration, but they can also be used in various other application including medical and water filtration membranes. Cellulose acetate (CA) is widely used in membrane applications. CA is soluble to acetone, which evaporates very fast in electrospinning process and thus is difficult to be used in materials preparation due to nozzle blocking. In this presentation we will study improving electrospinnability of CA by reduce nozzle blocking tendency. This was carried out, firstly, using solvent mixtures containing also solvent with higher boiling point, and secondly, by using excess solvent feed onto tip of the nozzle. In addition to evalution of these methods we also present premilinary results concerning use of these fibres in membrane applications. Keywords: electrospinning, cellulose acetate, solvent mixture, coaxial nozzle, membranes 1. Introduction Membranes are semipermeable barrier materials allowing certain compounds to pass through whilst rejecting others. Natural membranes have important role in many biological processes and man made membranes have become important in recent 50 years for various separation processes – especially liquid processes. Membranes can be divided into two types: microporous membranes and nonporous dense membranes. [1] Electrospinning produces microporous sheet materials, which can be utilized in membrane applications [2,3]. In electrospinning polymer solution is drawn into fibres in electrostatic field arranged between a nozzle and a collector. Electrostatic field stretches the polymer solution into fibres at the same time when the solvent is evaporating. Diameters of formed fibres are typically sub-micron range and they can be collected as interconnected nanofibre web onto surface of substrate. Fibre quality and morphology depends on several factors. The morphology of nanostructures forming onto collector varies from droplets (electrospray) to fibres (electrospinning) depending on mainly on solution properties such as polymer concentration and viscosity, but also on solvent properties and solubility of polymer into solvent. Diameters of formed droplets or fibres are typically sub-micron range. [4,5] Blocking of nozzles is one problem in electrospinning, especially when using volatile solvents with low boiling point. This problem can be overcome, for example, using mixtures of solvents [6, 7] as well as solvent feed onto tip of the nozzles [8, 9]. 2. Experimental Cellulose acetate with molecular weights of 100,000 (CA100k) from Acros organics) and 50,000 (CA50k) from Aldrich) were used in trials. Solvents were acetone, N,N-dimethyl acetamide (DMAc) and their mixture. Polymer solutions with various concentrations were prepared by adding polymer into heated solvent/solvent mixture, and stirring in magnetic heater until polymer was fully dissolved. Electrospinning was carried out with horizontal electrospinning setups: Setup 1) single nozzle setup with Eltex KNH64 voltage source; Setup 2), multinozzle setup with 20 needles and Simco BP50 voltage source and Setup 3) single nozzle setup Simco CM5-30P voltage source and coaxial nozzle. Electrospinning setups are presented in Figure 1.
  2. 2. The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey 2 a) b) c) Figure 1. Electrospinning: a) Setup 1, b) Setup 2 (at Tampere University of Technology) and c) Setup 3 Preliminary trials were carried out using Setup 1, and samples for membrane characterization with Setup 2. In both of these setups frequent cleaning of the nozzles was a necessity. Samples for fibre characterization were prepared on paper and samples for preliminary membrane studies were prepared using wet-laid nonwoven material (Trinitex K 662 50) as base material. Samples are listed in Table 1. Table 1. Samples prepared in this study Studied property Polymer C Solvent Setup Parameters Fibre morphology and effect of solvent CA50k 15 wt% Acetone 1 20 kV, 10 cm, 16G Acetone-DMAc (4:1) 1 20 kV, 10 cm, 16G CA100k 10 wt% Acetone 1 20 kV, 10 cm, 18G Acetone-DMAc (4:1) 1 20 kV, 10 cm, 18G Membrane CA100k 20 wt% Acetone-DMAc (1.7:1) 2 35 kV, 15 cm, 22G Use of solvent feed CA100k 20 wt% Acetone 3 20 kV, 10 cm, 18G The coaxial needle assembly (Figure 2a) was built for Setup 3 from polypropylene holder and feeding needles. Different needles can be plugged into the holder with tight fitting. The inner diameter of the spinning solution needle is 0.84 mm (18G) and the outer needle is about 4mm. The acetone vapour is prepared in a flask on a heated plate. The flask is filled with a 100ml of acetone and heated on a hot plate at 25°C to 30°C. A hydrocarbon gas is fed into the flask to carry the acetone vapour and feed it into the coaxial needle. The setup is shown in Figure 2b. a) gas feeding needle plug spinning solution needle plug outer needle plug b) Electric heater coaxial needle acetone rotating drum collector syringe acetone saturated gas mixture carrying gas power supply Figure 2. Schematic of a) coaxial nozzle and b) electrospinning setup with acetone gas feed. Appearance of electrospun fibres was studied using SEM imaging. Water flow through the sample was measured with Millipore lab filter suitable for micro and ultrafiltration with filtration area of 0.004 m 2 and maximum pressure of 6.2 bar. Filtration tests were carried out at room temperature, and in this case without pressure (=only hydrostatic pressure) until 150-210 ml of permeate was gone through. 3. Results and Discussion 3.1 Solvent mixtures Electrospinning of CA fibres using pure acetone as solvent was possible, but process was operational only short period of time prior to blocking. With acetone-DMAc mixture (4:1) process was operational longer even though some dry polymer was accumulating onto tip of the needle. The fibre size and fibre diameter
  3. 3. The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey 3 distribution of fibres spun from acetone solution were much larger than those of fibres spun from mixture with less volatile solvent. Due to quick evaporation of acetone in the tip of the needle the concentration changes during the process, which can explain large fibre distribution. Poorer solubility of CA into DMAc can explain why more beads were observed in fibres spun from solvent mixture. Appearance of CA fibres electrospun using acetone and mixture of acetone and DMAc (4:1) as solvent is presented in Figure 3. a) b) c) d) Figure 3. Appearance of 15% CA50k a) in acetone and b) in acetone-DMAc mixture, and 10% CA100k c) in acetone and d) in acetone-DMAc mixture. 3.2 Membrane samples Fibres obtained with 20 wt% CA solution had mean fibre diameter in µm range. Fibres were fused together in many of the crossing points, which could be an advantage in membrane applications [2]. Otherwise the coating was not optimal: fibre diameter had relatively large diameter distribution, and there were some larger polymer structures also occurring within the coating. Electrospun fibres were obtained onto sample materials using several coating cycles with multinozzle Setup 2. Coating layer thicknesses of samples were 1.2 g/m 2 (6 cycles, Espin1), 2.4 g/m 2 (12 cycles, Espin2) and 3.6 g/m 2 (18 cycles, Espin3). These layers smoothed the surface without affecting the flux of the material. Appearance of fibres, base material and fibre layer obtained with single cycle are presented in Figure 4, and effect of electrospun layer onto flux in Figure 5.
  4. 4. The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey 4 a) b) c) d) Figure 4. Appearance of fibres a) and b), base material c) without and d) with electrospun fibres obtained with one cycle. Figure 5. Effect of electrospun fibres coating on membrane flux. 3.3 Solvent feed It was also tested to feed liquid acetone to the outer needle directly using a syringe pump. However, the flow at the exit of the needle does not seem stable. Also the liquid acetone does not always cover the whole outer layer of the spinning solution. Clogging due to dry polymer skin still happens. Excess amount of liquid acetone also dilutes the spinning solution; it was difficult to get fibre without beads formation. When the saturated gas mixture is fed to the outer needle, a faint flame like flow can be seen with the aid of bright light. This may be related to the refractive index difference between the gas mixture and the surrounding air. The gas mixture surrounds the inner needle quite well based on visual observations. The flask has to be heated slightly in order to produce more saturated vapour. If the heating is not enough, clogging still happens. Overheating creates too much vapour which condenses in the plastic tubing and drips out of the needle. The pressure also easily builds up. At 21°C room temperature, 25 - 30°C heating works in general quite well. The electrospinning can continue for at least 10 minutes without clogging. Appearance of electrospun fibres prepared with vapour feed is presented in Figure 6.
  5. 5. The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey 5 a) b) Figure 6. Appearance of CA fibres electrospun with acetone vapour feed onto nozzle 4. Conclusion We have shown that the morphology of electrospun cellulose acetate fibres can be adjusted by changing Mw of polymer, and using different solvents mixtures. These electrospun fibre layers can be used in order to even up the surface of membrane base material in layered membrane structures without affecting too much on flow properties. We have also used solvent fume feed onto tip of the nozzle in order to reduce blocking tendency of acetone based cellulose acetate solutions and, thus, improve long term electrospinning processability of cellulose acetate. Acknowledgements Authors thank Tampere University of Technology, Department of Materials Science for possibility to use continuous electrospinning setup. References [1] Reif O.W., Microfiltration membranes: Characteristics and manufacturing, Advances in Biochemical Engineering/Biotechnology, 98 (2006), 73-103 [2] Shutov A.A., Astakhov E.Y., Formation of fibrous filtering membranes by electrospinning, Technical Physics, 51 (2006) 8, 1093-1096 [3] Ma Z., Kotaki M., Ramakrishna S., Electrospun cellulose nanofiber as affinity membrane, Journal of Membrane Science, 265 (2005) 1-2, 115-123 [4] Heikkilä P., Nanostructured fibre composites and materials for air filtration, Doctoral Dissertation, Tampere University of Technology. Publication 749, (2008) 91 p. [5] Heikkilä P., Harlin A., Parameter study of electrospinning of polyamide-6, European Polymer Journal 44 (2008), 3067-3079 [6] Lee K.H., Kim H.Y., Bang H.J., Jung Y.H., Lee S.G., The change of bead morphology formed on electrospun polystyrene fibers, Polymer 44 (2003) 14, 4029-4034 [7] Liu H., Hsieh Y.L., Ultrafine fibrous cellulose membranes from electrospinning of cellulose acetate, Journal of Polymer Science: Part B: Polymer Physics, 40 (2002) 18, 2119–2129 [8] Larsen G., Spretz R., Velarde-Ortiz R., Use of coaxial gas jackets to stabilize taylor cones of volatile solutions and to induce particle-to-fiber transitions, Advanced Materials, 16 (2004) 2, 166-168 [9] Lallave M., Bedia J., Ruiz-Rosas R., Rodriguez-Mirasol J., Cordero T., Otero J.C., Marquez M., Barrero A., Loscertales I.G., Filled and hollow carbon nanofibers by coaxial electrospinning of alcell lignin without binder polymers, Advanced Materials, 19 (2007) 23, 4292-4296

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