e-Polymers 2020; 20: 233–241 Communication Kylie Koenig*, Fabian Langensiepen, and Gunnar Seide Pilot-scale production of polylactic acid nanofibers by melt electrospinning https://doi.org/10.1515/epoly-2020-0030 Abbreviations received February 18, 2020; accepted March 20, 2020 GPC gel permeation chromatography - Abstract: Melt electrospinning has been used to manu Mw relative molecular weight fi facture bers with diameters in the low micrometer Mn number average molar mass range, but the production of submicrometer fibers has MFI melt flow index proven more challenging. In this study, we investigated PLA polylactic acid the feasibility of fabricating polylactic acid nanofibers SEM scanning electron microscopy using polymer grades with the increasing melt flow rates (15–85 g/10 min at 210°C) by melt electrospinning with a 600-nozzle pilot-scale device featuring an integrated climate control system realized as a glass chamber 1 Introduction around the spinneret. Previous experiments using this device without appropriate climate control produced Electrospinning is the mostcommonmethodforthe fi fibers exceeding 1 µm in diameter because the drawing production of polymer bers with diameters in the low ( ) of fibers was inhibited by the rapid cooling of the micrometer to nanometer range 1,2 .Therearetwomain ( ) polymer melt. The integrated glass chamber created a variants of the method 1,3 . In solution electrospinning, the temperature gradient exceeding the glass transition polymer is dissolved in an organic solvent that evaporates as fi temperature of the polymer, which enhanced the the ber is spun. In melt electrospinning, the polymer is fi fi drawing of fibers below the spinneret. An average fiber heated to above its melting point and solidi es as the ber is fi diameter of 810 nm was achieved using Ingeo spun. In both cases, a strong electric eld is applied during fi Biopolymer 6252, and the finest individual fiber spinning to stretch the polymer, typically resulting in bers (420 nm in diameter) was produced at a spin pump that are several hundred nanometers in diameter after ( ) speed of 5 rpm and a spinneret set temperature of 230°C. drawing 4 . Solution electrospinning is the simplest of the We have therefore demonstrated the innovative perfor- two processes because it does not require high temperatures mance of our pilot-scale melt-electrospinning device, or coagulation chemistry, but a solvent recovery step must be which bridges the gap between laboratory-scale and included, and there is a risk that toxic solvents may be fi ( ) pilot-scale manufacturing and achieves fiber diameters carried over into the nal product 3,5 .Therearenosuch comparable to those produced by conventional solution risks in melt electrospinning, and the entire process is ( ) electrospinning. therefore more environmentally sustainable 6 . However, due to the high viscosity and low electrical conductivity of Keywords: fiber spinning, nanotechnology, upscaling, the polymer melts, the resulting fibers have a slightly larger nonwoven, process development average diameter than those produced by solution electro- spinning (3,7). Currently, industry favors solution electrospinning fi fi * Corresponding author: Kylie Koenig, Aachen-Maastricht Institute not only because it produces ner bers but also because for Biobased Materials (AMIBM), Maastricht University, Brightlands the method is more scalable than melt electrospinning, Chemelot Campus, Urmonderbaan 22, 6167 RD Geleen, and fiber manufacturing processes are therefore more The Netherlands, e-mail: [email protected] economical despite the need for solvent recovery - Fabian Langensiepen, Gunnar Seide: Aachen Maastricht Institute (3,6,8,9). To address the limited scalability of melt ( ) for Biobased Materials AMIBM , Maastricht University, Brightlands - Chemelot Campus, Urmonderbaan 22, 6167 RD Geleen, electrospinning, recent innovations such as multiple The Netherlands, e-mail: [email protected], needle and needleless device configurations have demon- [email protected] strated a roadmap to overcome low flow rates (typically in Open Access. © 2020 Kylie Koenig et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 Public License. 234 Kylie Koenig et al. the µg/h range) and thus increase the productivity (3,10). 2 Experimental Prototypes with umbellate nozzles containing 60 spin- nerets can achieve maximum product deposition rates of 2.1 Materials ∼36 g/h (11,12). A polymer melt differential electrospin- ning system with a linear slot spinneret achieved an Melt electrospinning was carried out using three output of 75.6 g/h, which was improved further by granular, thermoplastic PLA fiber-grade resins: Ingeo increasing the slot width (8). Until recently, the largest Biopolymer 6201D, 6260D and 6252D (NatureWorks LLC, multineedle configuration was a device with 64 nozzles Minnetonka, Minnesota, USA). The materials are char- (13,14), but in our previous report, we described a acterized by different MFIs and crystalline melt tem- prototype with 600 nozzles that provides the basis for peratures. The standard fiber-spinning grade (Ingeo the pilot-scale melt electrospinning (15). Biopolymer 6201D) has a MFI of 15–30 g/10 min at Although our prototype addresses the limited scal- 210°C and a crystalline melt temperature of 155–170°C. ability of melt electrospinning, the resulting fibers are The first melt-blown grade (Ingeo Biopolymer 6260D) still coarser than those produced by solution electro- has a MFI of 65 g/10 min at 210°C and a crystalline melt spinning. Using a commercial spinning-grade polylactic temperature of 165–185°C. The second melt-blown grade acid (PLA) resin (Ingeo Biopolymer 6201D) supple- (Ingeo Biopolymer 6252D) has a MFI of 70–85 g/10 min at mented with 6% (w/w) sodium stearate to increase the 210°C and a crystalline melt temperature of 155–170°C. electrical polymer conductivity, we were able to produce All materials were vacuum dried at 60°C for 12 h before fibers ranging from 1.00 to 7.00 µm in diameter, which processing. are the finest biobased fibers manufactured thus far using our pilot-scale melt electrospinning device (15).So far achieved fiber diameters in the PLA solution electrospinning are in the range of 0.14 to 2.1 µm 2.2 Melt-electrospinning equipment (16–20) and that in the melt electrospinning are 0.2 to 50 µm (6,12,21–28). In individual cases, fiber diameters in We used our prototype pilot-scale melt electrospinning the subnanometer range have already been achieved device including a spinneret with 600 nozzles, each using solution electrospinning (29). However, it should be 0.3 mm in diameter and spaced at 8 mm intervals. We taken into account that mainly small-scale devices are integrated a glass chamber around the spinneret to used, and especially in both melt electrospinning and achieve climate control. A schematic illustration (a) and solution electrospinning, additional device modifications an image (b) of the device including the glass chamber is such as an integrated airflow are used to achieve fine shown in Figure 1. A constant supply of polymer melt fiber diameters and a higher process productivity (22,30). was ensured by a speed-adjustable single-screw ex- The fibers using our pilot-scaledevicewere truder with three heating zones based on integrated produced without a climate control system although heating elements and a speed-controlled spinning climate control is common for other device configura- pump. Spinning was carried out at an adjusted nozzle tions (31–33). Climate control prevents the rapid temperature of 230°C without the glass chamber and at cooling of the fibers, so that diameters in the nano- 220°C and 230°C with the integrated glass chamber meter range can be achieved by drawing before the using spin pump speeds of 10, 5 and 2 rpm. Preliminary polymer solidifies. Therefore, we integrated a climate experiments have shown that at 220–230°C, a stable control system into our pilot-scale melt electrospinning fiber formation is ensured over all 600 nozzles and no device by surrounding the spinneret with a glass droplet formation occurs. The statistical relevance of the chamber. We investigated the influence of climate independent and in this study investigated factors spin control on the temperature distribution below the pump speed and temperature on the fiber diameter was spinneret, on the melt viscosity of three different PLA previously proven with a two-way analysis of variance grades with increasing melt flow indices (MFIs),and (34). An aluminum collector with an uneven surface was ultimately on the diameter of the resulting fibers at used instead of a conventional plate collector. The different spin pump speeds. We were able to optimize adjustable collector pins can be placed 8 mm apart, our pilot-scaledevicetoproducefibers with a diameter and we used a narrow and offset pin order with a below 1 µm for the first time, thus offering an industrial diagonal distance of 2.6 cm. With the nozzle/collector solution for the preparation of melt-electrospun pairing installed, nonwovens up to 340 mm in width nanofibers. could be produced on a continuous basis. The distance Pilot-scale production of polylactic acid nanofibers by melt electrospinning 235 Figure 1: Schematic illustration (a) and picture (b) of our pilot-scale melt electrospinning device including the glass chamber. between the collector and the nozzle plate was main- Gel permeation chromatography (GPC), using a 1260 tained at 11 cm to allow comparison