Formation of Nanofibers from Pure and Mixed Waste Streams Using

Proving Ground, Aberdeen, Maryland 21005, United States. Ind. Eng. Chem. Res. , 2015, 54 (37), pp 9057–9063. DOI: 10.1021/acs.iecr.5b02279. Publ...
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Formation of Nanofibers from Pure and Mixed Waste Streams Using Electrospinning Nicole E. Zander,*,† Daniel Sweetser,† Daniel P. Cole,‡ and Margaret Gillan† †

Weapons and Materials Research Directorate, United States Army Research Laboratory, Aberdeen Proving Ground, Aberdeen, Maryland 21005, United States ‡ Vehicle Technology Directorate, United States Army Research Laboratory, Aberdeen Proving Ground, Aberdeen, Maryland 21005, United States S Supporting Information *

ABSTRACT: New methods are needed to reprocess the excess of plastics in the waste stream. In this work, bottle-grade polyethylene terephthalate (PET), Styrofoam, and polycarbonate from compact discs (CDs) were spun into nanofibers as fine as ca. 100 nm in diameter using the electrospinning technique. The mechanical properties of the fibers were evaluated using microtensile testing. The elastic moduli ranged from 15 to 60 MPa, and displayed stiffnesses comparable or greater than fibers made from commercial polymers of equivalent molecular weight. Nanofibers were also prepared from blends of Styrofoam and recycled polycarbonate. Recycled PET fibers were tested for application in water filtration and had greater than 99% filtration efficiency of 1 μm particles. Nanofibers from both pure and mixed waste streams are expected to have applications in myriad areas such as ultra/microfiltration, composites, and tissue engineering.

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Ultrafine fibers are formed due to the extensive stretching the jet undergoes from the bending instabilities.13 Scalability of nanofiber formation is often cited as a problem, but with commercial electrospinning and centrifugal spinning equipment now available, this is of lesser concern.14,15 In this work, we were interested in forming fibers from recycled polymers that could readily be dissolved in common solvents: Styrofoam (polystyrene, PS), water bottles (PET), and CDs (polycarbonate). Although others mentioned above have demonstrated the utility of nanofibers from recycled materials, mechanical testing and comparison to commercial polymers was generally lacking. We evaluated the effect of viscosity and of adding an ammonium salt on fiber diameter and morphology. In addition, we prepared fibers from blends of the two recycled polymers that were soluble in a common solvent: PS and PC. To the best of our knowledge, we are the first to report making polymer blend nanofibers from recycled polymers. Thermal and mechanical properties were tested and compared to fibers made from commercial grade polymers of comparable molecular weight. In addition, Fourier transform infrared spectroscopy (FTIR) was used to characterize the fibers prepared from the blends. We tested recycled PET fibers for a water filtration application and achieved over 99% filtration efficiency of 1 μm particles.

INTRODUCTION In the United States alone, about 32 million tons of plastics are generated annually, but only ca. 5% are recycled.1,2 This is in part due to the low value of recycled plastics, as well as the economic feasibility of their recycling.1 Certainly, there are many benefits of recycling these materials, such as reduction in greenhouse gases, water/air pollution, energy consumption, and conservation of natural resources. But until alternate reprocessing methods and higher value uses are developed, the trend in recycling is not expected to change. Recently, there has been some research in the formation of nanofibers from recycled plastics such as Styrofoam and bottlegrade polyethylene terephthalate (PET). Shin and Chase. formed fibers from Styrofoam and a nontoxic solvent Dlimonene, whereas Shin used common organic solvents.3,4 Khan et al. incorporated multiwall carbon nanotubes and nickel zinc ferrite nanoparticles into the fibers to improve thermal conductivity, hydrophobicity, and impart superparamagnetic behavior to expand the uses of the nanofibers.1 Rajabinejad et al. generated fibers from bottle-grade PET via a solvent-less process of melt-electrospinning.5 Strain et al. used a combination of trifluoroacetic acid and dichloromethane and tested the fibers as air filters for smoke.2 Nanofibers are utilized in a wide variety of high-value fields in addition to those mentioned above including biological or chemical sensors, composite reinforcement, energy storage, drug delivery, and tissue engineering. Nanofibers are generally fabricated using electrospinning, although drawing, template synthesis, phase separation, self-assembly, and centrifugal spinning are alternate methods.6−12 Electrospinning is a facile, robust, and inexpensive fiber formation process in which the spinning solution is forced through a needle charged at high voltage. The pendant droplet is stretched due to electrostatic repulsion, and a jet is ejected once surface tension is overcome. © 2015 American Chemical Society

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MATERIALS AND METHODS Materials. Tributylammonium chloride (TBAC), dichloromethane (DCM), dimethylacetamide (DMAC), and 1 μm amine-modified fluorescent orange polystyrene beads were Received: Revised: Accepted: Published: 9057

February 4, 2015 August 24, 2015 September 3, 2015 September 3, 2015 DOI: 10.1021/acs.iecr.5b02279 Ind. Eng. Chem. Res. 2015, 54, 9057−9063

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Industrial & Engineering Chemistry Research

areas of selected peaks (698 and 1500 cm−1) were fitted and compared to a 5 point calibration of films of identical thickness generated directly from polymer solutions. Tensile Testing and Thickness Measurements. Uniaxial mechanical characterization was carried out using a Deben Microtensile stage with a 200 N load cell. Specimens were held using custom rubber grips that prevented the samples from slipping during loading. For each sample type, a minimum of five specimens were tested. Tests were run at a speed of 100 mm/min. Displacement was measured via the microtensile stage crosshead positioner (10 μm accuracy, 3 μm resolution) and the average strain was calculated based on the original length of the specimen. Thickness T measurements of the mats were performed by taking Z-stacks on a Keyence VK-X200 Laser Scanning Microscope. A minimum of five sections from each mat that was tensile tested was measured in a minimum of three locations on the sample. This noncontact 3D metrology system is able to perform nanometer level thickness measurements with 0.5 nm accuracy in the z-axis. Xie et al. and Lui et al. have used this method to calculate thickness for electrospun mats for mechanical characterization and reported accuracy of ±3 μm.16,17 Because the mats tested ranged from 240 to 470 μm in thickness, the error accounts for ca. 1300 cP). Fibers were also attempted from recycled polycarbonate from CDs, but beading was excessive at all concentrations tested. It is known that the addition of salt can be used to reduce bead formation by increasing solution conductivity.20,21 TBAC was added to all polymer solutions to improve fiber morphology and uniformity, and allow for quality fiber formation from solutions of lower viscosity, hence enabling finer fibers to be spun. Figure 3 displays fibers formed from PC-

It is known that porous polystyrene fibers can be formed under appropriate conditions.22 This is largely attributed to a solvent cooling effect from high vapor pressure solvents and has been observed in other polymeric systems.23 As can be seen in Figure 5, PET-R fibers have a smooth surface, whereas there is evidence of pore formation in both PC-R and PS-R fibers. The pores in the PC-R fibers appear elongated and noncircular like in the PS-R fibers, and only present on the larger fibers (>300 nm). Polymer Solution Viscosity. Fibers were also prepared from commercial off-the-shelf polymers (COTS) (with equivalent molecular weights) that served as standards to compare fiber morphology and mechanical properties. The concentration of the COTS polymer solutions used was determined by matching the solution viscosity to the corresponding recycled polymer solution in order to generate fibers of similar diameter, particularly at the smallest diameters with uniform fiber morphology for each polymer type. Figure 6

Figure 3. Scanning electron micrographs of electrospun recycled polycarbonate nanofibers formed at different concentrations with salt. (A) 10 wt %, (B) 12.5 wt %, (C) 17.5 wt %. Scale bar denotes 10 μm.

R from solutions ranging from 10 to 17.5 wt %, with corresponding fiber diameters of 114 ± 70 nm to 740 ± 390 nm. As can be seen in Figure 3A,B, beads were still present, although they were minimal for fibers prepared from the 12.5 wt % solution. Mats were attempted to be made and tested from the 10 wt % solutions, but they were extremely brittle and were unable to be removed from the collecting foil without tearing. Fibers with the smallest diameters and uniform morphology (and processability) are depicted in Figure 4. Fiber diameters measured were 105 ± 50 nm (PET-R), 132 ± 34 nm (PS-R), and 203 ± 90 nm (PC-R). A small amount of beads were present in the PET-R and PC-R fiber mats, but it was considered to be minimal and unlikely to affect fiber mat properties. The surface of the fibers was examined more closely

Figure 6. Polymer solution viscosity as a function of weight percent. PS = polystyrene, PET = polyethylene terephthalate, PC = polycarbonate, R = recycled, TBAC = tetrabutylammonium chloride. 9059

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with one or both of the other polymers was attempted, but a common solvent could not be found. Figure 8 displays SEM

displays the viscosities of COTS and recycled polymers of varied concentrations, with and without the addition of TBAC salt. As expected, the solution viscosity increased with weight percent. The trend is essentially linear until the critical entanglement concentration is reached. Above this concentration, the viscosity increases exponentially with concentration. The effect of adding salt had a minimal impact on viscosity. Fiber Diameter. Fiber diameters of COTS and recycled polymers with and without salt are displayed in Figure 7. The

Figure 8. Scanning electron micrographs of electrospun recycled blend nanofibers. (A) 7.5/2.5 wt % polycarbonate/polystyrene with salt, (B) 5/5 wt % polycarbonate/polystyrene with salt, (C) 2.5/7.5 wt % polycarbonate/polystyrene with salt, (D) 2.5/7.5 wt % polycarbonate/ polystyrene without salt. Scale bar denotes 5 μm. Inset shows high resolution image of fiber surface. Inset scale bar denotes 2 μm. Figure 7. Fiber diameter of electrospun fibers as a function of weight percent. PS = polystyrene, PET = polyethylene terephthalate, PC = polycarbonate, R = recycled, TBAC = tetrabutylammonium chloride.

images of fibers fabricated from blends of 7.5 wt %/2.5 wt % PC-R/PS-R (Figure 8A), 5 wt %/5 wt % PC-R/PS-R (Figure 8B), and 2.5 wt %/7.5 wt % PC-R/PS-R (Figure 8C). The inset shows a higher magnification image in order to observe surface structure of the fibers. The fibers formed from the 7.5 wt % PCR solution appear to have a bimodal distribution of large and small fibers, and in fact the standard deviation of the fiber diameter was nearly 100% (131 ± 130 nm). Fiber diameters and percentage of fibers with porous surfaces increased with increasing PS-R content (259 ± 158 nm (5%/5%), 332 ± 184 nm (2.5%/7.5% PC-R/PS-R). Figure 8D displays an image of the 2.5%/7.5% PC-R/PS-R without the addition of salt. A series of large collapsed beads were formed, connected by short fine fibers. The higher magnification image shows the beads have a porous surface, much like the PS-R fibers (Figure 5C). Thus, when compared to Figure 8C, the same polymer blend concentration with salt, the effect of salt appears to be essential in the fiber formation process. This could be attributed to the lower concentrations/viscosities of the individual polymers in the blend. Although the total polymer wt % is the same as the monocomponent solutions that generated uniform fibers (10 wt %), the concentration of each individual polymer is lower. Unless the polymers are completely miscible, this could have a negative effect on the fiber formation process at this concentration (chosen to generate ca. 100 nm fibers for potential ultra/microfiltration future application). The higher magnification image in Figure 8A shows larger and some medium sized fibers with round pore formation and smooth smaller fibers. When compared to Figure 5B,C, we can see that the large porous fibers resemble PS-R, while the smaller fibers more closely look like those of PC-R. Thus, at first glance, it may appear that the two polymers were completely immiscible forming monocomponent fibers. But the 5%/5% PC-R/PS-R and 2.5%/7.5% PC-R/PS-R fibers have a more uniform diameter distribution and nearly all fibers appear porous. Further, the fiber diameters for the latter two blends are larger than for fibers formed from 10 wt % polymer

PET-R fibers (without TBAC) had the highest diameters for a given weight percent, ranging from 137 ± 52 nm to 3.4 ± 1.1 μm. The addition of the ammonium salt TBAC served to reduce fiber diameters in nearly all cases, although the difference was not significant for every condition. The addition of TBAC to the 5 wt % PET-R solution reduced the fiber diameter from 137 ± 52 nm to 105 ± 49 nm. COTS PET fibers were only prepared with TBAC from solution concentrations of 5 to 10 wt % polymer. The fiber diameter for the 5 wt % PET solution was nearly identical to that of the PET-R (116 ± 44 nm), and was thus a suitable standard for comparison. PS-R fibers were prepared in DMAC with and without TBAC and diameters ranged from 260 ± 85 nm to 3.3 ± 1.1 μm without salt (15−30 wt %), and 132 ± 34 nm to 1.5 ± 0.8 μm with salt (10−25 wt %). The fiber diameter was not reduced for the 15 wt % solution, but was for the fibers formed from the higher concentration solutions (1.2 ± 0.3 μm vs 0.8 ± 0.2 μm, 20 wt %; 2.1 ± 0.6 μm vs 1.6 ± 0.7 μm, 25 wt %). COTS PS fibers were prepared from a 160 000 g/mol polymer, compared to 180 000 g/mol PS-R determined by GPC. Because of the slightly reduced molecular weight of the COTS PS, the viscosity and corresponding fiber diameter of the 15 wt % solution was a closer match for the fibers spun from 10 wt % PS-R solution (118 ± 29 nm vs 132 ± 34 nm). PC fibers both recycled and COTS were only prepared with salt due to the excessive beading of the fibers. PC-R fibers diameters ranged from 114 ± 71 nm to 740 ± 390 nm (10− 17.5 wt % solutions). PC-R was not found to be soluble above ca. 18 wt %. Fiber diameters of the COTS PC were comparable at 165 ± 97 nm for fibers formed from the 10 wt % solution. Recycled Polymer Blends. Blends of PC-R and PS-R were prepared in DCM to test the feasibility of generating fibers from mixed waste streams. Incorporation of PET into a blend 9060

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marker peaks for compositional analysis. From the figure, it is apparent that PC-R and PS-R marker peaks are only present in the spectra for PC-R and PS-R, respectively. Also, it can be seen that as the PS-R content increases (or PC-R decreases), the ratio of 698/1500 cm−1 goes up. Table 2 displays the fitting of the 698/1500 cm−1 peak areas from the calibration films and fiber-films. The weight percent of the 2 polymers in the fibers is generally pretty close to the expected values, with the exception of the 2.5%/7.5% PC-R/PS-R fibers. Mechanical Testing. Uniaxial mechanical characterization was carried out using a microtensile stage (Figure 10). Recycled fibers were compared to the COTS fibers with matched fiber diameters as described above. Fiber diameters of COTS and recycled fibers were nearly identical for each polymer type (132 ± 33.7 nm vs 135.8 ± 37.7 nm for PS-R and PS respectively, 105.5 ± 49.3 nm vs 116.1 ± 43.5 nm for PET-R and PET, and 203.6 ± 90.5 nm vs 207.0 ± 123.4 nm for PC-R and PC). The PET fibers (5 wt %), both COTS and recycled, had the highest elastic modulus, followed by PS (10 wt % PS-R, 15 wt % PS) and PC (12.5 wt %). The modulus of the recycled fibers was higher than the COTS control except for the PC fibers (p < 0.05). On the basis of the DSC data for PC-R, increased chain rigidity may account for lower (and not higher) modulus compared to the PC control. Additional preprocessing to remove additives could potentially lead to improved mechanical properties. The 5 wt %/5 wt % PC-R/PS-R (fiber diameter 258.9 ± 157 nm) fibers have a response nearly identical to the PS-R fibers. The tensile strength followed a similar trend. Although modulus and tensile strength values are lower than theoretical values, they are comparable to those reported in the literature.25−27 The mechanical results provide an understanding of the mechanical behavior of the spun mats, but it is difficult to draw conclusions about individual fiber properties due to the nature of the mechanical test performed. The data provided are a result of the global response of both individual fiber properties and fiber−fiber interactions. Other factors that could potentially complicate the mechanical response are mat density and fiber diameter. In particular, fiber diameter has been shown to play an important role in individual fiber mechanical properties due to confinement effects that may result in enhanced chain alignment.28 Single fiber tensile tests29 and local mechanical characterization through indentation30 and atomic force microscopy31 could help demonstrate these individual fiber properties. However, even if recycled fiber properties are mechanically degraded with respect to the standards, the global response of the mats may not necessarily correlate due to the fiber−fiber interactions. Thus, the recycled mats’ mechanical properties may be strengthened due to increased fiber−fiber interactions from the additives not present in the COTS mats. The data suggests that recycled nanofibers may demonstrate improved mechanical behavior for particular applications, with respect to COTS nanofibers. Filtration Test. Mats made from 5% PET-R were tested using a 1 μm fluorescent bead solution. Figure 11 displays CSLM and SEM images of the beads that were captured by the mats due to smaller pore sizes relative to the bead diameter. The filtration efficiency was 99.63 ± 0.0001%. Thus, these mats are suitable for applications in microfiltration.

for either polymer alone, suggesting miscibility or at least partial miscibility. DSC was used to confirm this trend (Table 1). The Table 1. Thermal Analysis of Recycled and New Bulk Polymers and Fibers Prepared by Electrospinning sample

Tg (°C)

bulk PC standard bulk PC recycled 12.5% PC standard 12.5% PC recycled bulk PS standard bulk PS recycled 15% PS standard 10% PS recycled 2.5% PC/7.5% PS blend (recycled) 5% PC/5% PS blend (recycled) 7.5% PC/2.5% PS blend (recycled)

147.0 143.7 145.6 172.0 97.5 87.9 104.8 91.1 131.6, 97.2 134.6, 108.3 141.0, 102.7

PS = polystyrene, PC = polycarbonate.

glass transition temperatures (Tg) are 172 and 91 °C for PC-R and PS-R, respectively. But for all of the blends evaluated, there is a shift in the Tg values toward one another, confirming partial miscibility. Yet, there are two distinct Tg values, ruling out full miscibility. The Tg of the spun PC-R was higher than that of the bulk PC-R and spun COTS PC fibers. Dhakate et al. has reported a 9 °C increase of the Tg of PC compared to bulk material due to the alignment of polymer chains.24 But this does not fully explain the nearly 30 °C increase in Tg observed. The exact composition of the PC-R was unknown; perhaps some of the additives impart increased rigidity to the polymer chains during the spinning process. The experimental weight percent of the fibers formed from the blends was determined using FTIR. Spun fibers were dissolved in solvent and cast into films and referenced to a calibration curve made from films of known composition. Figure 9 shows the FTIR spectra for the 3 different blend compositions and the pure PS-R and PC-R fibers. The aromatic CC stretch in PC-R at 1500 cm−1 and the monosubstituted benzene CH bend in PS-R at 698 cm−1 were chosen as

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CONCLUSIONS Nanofibers were prepared from recycled PET, PS, and PC via electrospinning. The ammonium salt TBAC was added to

Figure 9. FTIR spectra of electrospun recycled polystyrene and polycarbonate fibers. PS-R= recycled polystyrene, PC-R = recycled polycarbonate. 9061

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Industrial & Engineering Chemistry Research Table 2. Weight Composition of Electrospun Polystyrene and Polycarbonate Blends As Determined by FTIR A698/1500 (expected)

A698/1500 (actual)

wt % PC-R (expected)

wt % PS-R (expected)

wt % PC-R (actual)

wt % PS-R (actual)

FD (nm)

1.68 1.08 0.48

1.36 0.98 0.37

25 50 75

75 50 25

38 54 80

62 46 20

332 ± 184 258 ± 158 131 ± 130

PS-R = recycled polystyrene, PC-R = recycled polycarbonate.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02279. Table S1: table of electrospinning conditions and resulting fiber diameters. Figure S1: scanning electron microscope images of recycled polymer nanofibers with and without salt (PDF).

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Figure 10. Mechanical properties of electrospun fibers prepared from new and recycled polymers as determined by uniaxial tensile testing. (A) Elastic modulus, (B) tensile strength. PS = polystyrene, PET = polyethylene terephthalate, PC = polycarbonate, R = recycled. *, **, *** p < 0.05.

AUTHOR INFORMATION

Corresponding Author

*N. E. Zander. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported in part by an appointment to the Postgraduate Research Participation Program at the U.S. Army Research Laboratory administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USARL.

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Figure 11. Water filtration using 5% PET-R nanofibers. (A) CSLM image of fluorescent 1 μm beads captured by the nanofiber mat (scale bar = 400 μm), (B) SEM image of 1 μm beads on nanofiber mat (scale bar = 5 μm).

reduce fiber diameter and beading. Blends of PS-R and PC-R were also prepared. Thermal analysis revealed partial miscibility of the blends based on convergence of the two Tgs. Enlarged fiber diameters and varied surface topography also supported partial miscibility. Tensile testing in general showed improved elastic modulus of the recycled polymers compared to COTS polymers of equivalent molecular weight and fiber diameter. The 50/50 fiber blend had an equal elastic modulus to that of pure PS-R (enhanced compared to PC-R). Thus, because of partial miscibility, it is likely that the blend fibers may not be subject to interfacial failure between phases, commonly observed in immiscible blends. Fiber diameters ca. 100 nm were achieved for all recycled polymers and the 75/25 PC-R/ PS-R blend fibers, making them suitable for a variety of high value applications such as ultra/microfiltration. Thus, recycling plastic into nanofibers could potentially be a value-added process. But further research is needed to prepare them with less toxic solvents or in a solvent-free manner, and explore further the ability to generate fibers from mixed waste streams to enable true scalability of this alternate recycling approach. 9062

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DOI: 10.1021/acs.iecr.5b02279 Ind. Eng. Chem. Res. 2015, 54, 9057−9063