Production and optimization of nanofibrous nonwoven webs via centrifugal spinning and their filtration properties

Nanofibrous nonwovens are important enginering materials in textile, composite and energy sectors because of their low fiber diameter, micro- nano porous structure and high surface area to volume [1–5]. Electrospinning is the most widely used traditional method for the production of nanofibers [6]. Low production rate and high voltage requirement are drawbacks of electrospinning, limiting its industrial scalability. Although meltblowing might be a productive alternative, it is not easy to produce nano scale fibers. Also, it requires compressed hot air and applicable to limited number of materials [7]. Solution blowing might be an alternative where the disdvantage is related its dependency on solution concentration [8]. As an alternative centrifugal spinning has many important advantages over the other nanofiber production methods such as simple working principle, availability to produce fibers from polymer melts and solutions, independency from high voltage, high production rate, and low cost. Also, this method is suitable for processing polymeric and ceramic nanofibers. The diameter and morphology of the fibers are dependent on solution viscosity, surface tension, molecular structure, molecular weight, concentration while rotational speed of and diameter of the rotor, nozzle diameter and working distance between rotor and collector are process parameters. In this study, effective system, process and material parameters of centrifugal spinning were investigated. Moreover filtration properties of centrifugally spun nonwoven webs were analyzed. After a short review on recent literature, a proper spinning system was designed and manufactured using some CAD programmes to make solid models and related technical drawings. An aluminum rotor with 50 ml volume and 220 g weight was used, where a 2.2 kw (3 hp) spindle motor was selected as driving force for the rotor. The spindle has 18000 rpm rotational capasity, but maximum speed was kept 10000 rpm because of safety precautions. Nozzle diamater was seen to be critical on the morphology of fibers. Different type of nozzles with 0.3, 0.5 and 1.0 μm diameter were designed and manufactured to be used in this study. In the literature several types of collection systems were mentioned for centrifugal spinning. In my study a steel mesh conveyor was used to collect the fibers. A vacuum manifold was used just behind the conveyor to make the collection more effective. Nanofibers and nanofibrous nonwoven webs were produced with experimental centrifugal spinning setup in Mechanical Engineering Laboratories of ITU. Firstly, effect of solution concentration was examined. A critical viscosity value must be achieved to obtain fibers continuously. The fiber production was not successful from solutions at 5wt.% and 10wt.% concentration, while 15wt.% and 20wt.% concentrations were suitable to produce fibers. Average fiber diameter also increased slightly with the increasing concentration of polymer solution. Besides ethyl acetate was added to solution to improve the vapor pressure of the solvent. The results showed that additive addition reduces droplet concentration on webs. In addition, the increase in the additive amount increased the average fiber diameter slightly. Effect of rotational speed was also investigated to see how it changes the fiber production. Between 3000 and 6000 rpm, the increase in rotational speed increased the fiber diameter significantly, because polymer solution feeding amount also increased. Comparing 6000 and 9000 rpm rotational speeds, it was seen that average fiber diameter decreased within that values which is thought to be due the fact that centrifugal forces are more effective than polymer solution flow rate, which attenuates the fibers effectively at high rotational speeds. 3 types of nozzles were examined in this study. Generally, average fiber diameter of produced nonwoven webs with 0.3 and 0.5 mm diameter nozzles were found to be same. When 1 mm diameter nozzle was used, it was seen that fiber diameter was increased notably. Finally, effect of the working distance between nozzle and collector was investigated after spinning from 2 distances. It was seen that working distance doesn‟t have a significant effect on fiber diameter. After optimization of various process, material and system parameters nanofibrous nonwoven webs were produced according to investigated parameters. Air permeability, water flux, pressure and particle retention efficiency of produced samples were investigated according to standards. To investigate filtration and barrier properties initially air permeability of polypropylene (PP) spunbond nonwoven was measured which is the standart substrate we used throughout the study. Same spunbond nonwovens was used as a support material since nanofibers are not strong enough to resist huge pressures during tests. Nanofiber production was optimized with following parameters: 9000 rpm rotational speed, 0.3 mm nozzle diameter, 30 cm nozzle-collector distance, 1 hour duration. It was seen that the air permeability decreased nearly 70% after nanofiber coating. The average fiber diameter should be further minimized to reach slip flow regime, where air molecules slip over the fibers due to low Knudsen number. This will improve potential of those nonwovens to be used in air filtration applications. Later on, water permeability tests were performed where water flux through nanofiber coated samples and normal spunbond nonwovens were determined at various pressure values. Water filtration tests were conducted under 0.9 psi pressure for nanofiber coated and normal nonwoven webs. The standard PS latex particles were added to distiled water. For 1, 5 and 10 µm particles containing water samples it was seen that water flux values were decreased because of retention of particles around fibers. It was seen that samples have similar water permeability with same pressures. Water flux through nanofiber coated sample became easier, because of hydrophilic property of TPU material and lower solidity of centrifugal spining compared to electrospinning. Contaminated and filtered water were tested using particle counter device to see the effect of nanofibrous layer on water filtration. According to results normal spunbond nonwoven has poor performance for 1 and 5 µm particles, while it has 60% filtration efficiency for 10 µm particles. Nanofibrous coated web has 70% filtration efficiency for 1 µm particles. For 5 and 10 µm particles, it has filtration efficiency more than 90%. These values shows that nanofibrous webs can be alternative for water filtration applications. It is clearly seen that centrifugal spinning method has a big potential to be a powerful system for fiber production in nanoscale. In addition to this, the researches on this method will increase to proof that these fibers can be used for many applications, especially in filtration area.