Environ. Sci. Technol. 2010, 44, 4753–4759
Upcycling: Converting Waste Plastics into Paramagnetic, Conducting, Solid, Pure Carbon Microspheres VILAS GANPAT POL* Electrochemical Energy Storage Department, Chemical Sciences & Engineering Division, and Intense Pulse Neutron Source, Argonne National Laboratory, Argonne, Illinois 60439
Received January 22, 2010. Revised manuscript received March 30, 2010. Accepted May 4, 2010.
The recent tremendous increase in the volume of waste plastics (WP) will have a harmful environmental impact on the health of living beings. Hundreds of years are required to degrade WP in atmospheric conditions. Hence, in coming years, in addition to traditional recycling services, innovative “upcycling” processes are necessary. This article presents an environmentally benign, solvent-free autogenic process that converts various WP [low density polyethylene (LDPE), high density polyethylene (HDPE), polyethylene terephthalate (PET), polystyrene (PS), or their mixtures] into carbon microspheres (CMSs), an industrially significant, value-added product. The thermal dissociation of these individual or mixed WP in a closed reactor under autogenic pressure (∼1000 psi) produced dry, pure powder of CMSs. In this paper, the optimization of process parameters such as the effect of mixing of WP with other materials, and the role of reaction temperature and time are reported. Employing advanced analytical techniques, the atomic structure, composition, and morphology of as-obtained CMSs were analyzed. The room-temperature paramagnetism in CMSs prepared from waste LDPE, HDPE, and PS was further studied by electron paramagnetic resonance (EPR). The conducting and paramagnetic nature of CMSs holds promise for their potential applications in toners, printers, paints, batteries, lubricants, and tires.
Introduction “Upcycling” involves the conversion of a waste material(s) into more valuable product(s). This product can be purely artistic, scientific, or anything simply useful. This sustainable option eliminates the waste that might otherwise make its way to a landfill or incinerator. Upcycling is also a great way to make use of inexpensive available items. Typically, the more creative the transformation of an upcycled entity, the more salable it is. As a result, upcycling is a constructive transformation of waste combining cost benefits and waste reduction. The worldwide enormous volume of WP (1) will have severe negative environmental impacts on human life if left unchecked. Polyethylene-based automobile parts, food packaging materials, toys, and milk bottles, as well as polystyrene-based plates, cups, and packaging materials, comprise much of the energy and environmental burdens associated with plastics. * E-mail:
[email protected]. 10.1021/es100243u
2010 American Chemical Society
Published on Web 05/20/2010
Plastics offer features such as light weight, high strength, convenience, and low cost, making them attractive for businesses and consumers. It would be impractical to ban them entirely. Although many communities encourage the public to recycle WP, a small percentage of it actually reaches recycling facilities; the rest ends up as waste in burial sites across the globe or is sent for incineration. Since plastics are not biodegradable (2), storing them in landfills is not an effective solution. Incineration of WP is comparatively expensive and harmful to the environment. Accordingly, development of an alternative approach that would upcycle the massive amounts of WP is essential. Nowadays, extensive collection, transportation, separation, and recycling facilities are available for processing waste thermoplastic products. The existing processes include densification, grinding, shredding, and blending. However, mixing chemically different WP might not yield homogeneous materials suitable for making quality products. The problem of mixed plastics has been partially solved by separation technologies such as flotation (3), followed by further processing of individual polymers. A few companies are using advanced plasma gasification (4) technology to recover energy and useful products from WP. However, these multistep recycling processes are not cost-effective. To address the challenging issue of WP in a convenient way, a novel process that systematically degrades single or mixed polymers (LDPE, HDPE, PET, and PS) via upcycling is developed. The presented solid-state, solvent-less, environmentally friendly process remediates a variety of WP into value-added paramagnetic, conducting, solid, pure CMSs. Millions of tons of carbon black (5) (normally referred to as “soot”) are commercially produced each year by partial combustion of petroleum. As a substitute for expensive petroleum (oils), we are investigating the use of complementary WP feedstock for the production of carbon black. Carbon materials have a wide range of structural and textural properties (6) and have found extensive applications in nanodevices (7), energy storage (8), separation technology (9), etc. Carbon spheres (10), prolates (11), beads (12), and onions (13) have been synthesized by different processes. Chemical vapor deposition (10) and pressure carbonization (14) are a few convenient methods employed to synthesize carbon spheres either as micro- or nanoscale particles. Yang et al. reported on the amorphous carbon nanospheres with diameters of 140-200 nm synthesized by treating poly(tetrafluoroethylene) (15) in supercritical water at 550 °C using Ca(OH)2 as defluorination reagent. Furthermore, Zou et al. degraded poly(tetrafluoroethylene by a mild and economical method at low temperature (16). However, in the literature, most synthesis techniques producing pure CMSs are limited by several factors. In a few cases, the proportion and yield of CMSs are low, and spheres cannot be easily separated from the remaining carbon soot. In some cases, catalysts are trapped and remain as additional impurities in a product that needs further processing. For practical applications, a high percentage (17) of pure CMSs is required. Following the requirements, single-step synthesis of carbon spheres produced by dissociating individual hydrocarbons (18) at their autogenic pressure at low temperatures is also reported. The present reproducible, green process presents an opportunity to use WP as a feedstock for the production of CMSs which are industrially significant, value-added products. The smooth CMSs are pure, conducting, and paramagnetic, with potential applications in toners, printers, paints, batteries, lubricants, and the tire industry. Employing advanced structural, spectroscopic, and imaging techniques, VOL. 44, NO. 12, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Temperature versus pressure measured during the autogenic decomposition of waste HDPE bags. systematic characterization of the atomic structure, composition, and morphology of the CMSs is carried out.
Experimental Section For the synthesis of CMSs, 1 g of WP was introduced in a 5 cc reactor (made up of Haynes 230 alloy tube) at room temperature either in an air or inert atmosphere. The partially filled reactor with WP was closed tightly and heated uniformly. The temperature of the furnace was increased to 700 °C at a rate of 20 °C/min and maintained at the high temperature for an optimized time. After the autogenic reaction, the reactor was allowed to naturally cool, and opened. Dry black powder with ∼40% yield was collected. In a representative case, the thermal decomposition of HDPE was carried out in a closed reactor attached to a pressure transducer and pressure gauge. When 1 g of WP had filled the 5 cc reactor, very low pressure (∼ 50 psi) in the reactor was measured during heatup. Above 680 °C, the pressure drastically increased from 50 to 1000 psi (Figure 1). While continuing the reaction at 700 °C, the pressure did not increase further. The pressure measurements were reproducible under equivalent reaction conditions. A mass spectrometer was connected to the upcycling process reactor to measure in situ formed solid products or gases during the WP decomposition. A 0.5-µm filter was placed in the stainless steel tube to avoid incorporation of solid carbon particles in the mass spectrometer. To avoid air contamination, the whole reactor was evacuated before measurement of the formed gases during the WP decomposition. The mass spectroscopy measurements revealed that the decomposition of WP started at ∼300 °C. Masses correlating to water vapor, CO2, and molecules with radicals having 2-5 carbon atoms were detected after dissociating the HDPE. Analogous spectra were obtained up to 600 °C. However, above 600 °C, a small amount of hydrogen, water vapor, and larger amounts of hydrocarbons were recorded.
Further changes were observed at 700 °C; short 1-3 carbon atoms species remained in addition to hydrogen, and masses over 36 disappeared. Thus, at 700 °C, all the C-H and C-C bonds were broken, and C remained as a solid product with liberation of a small amount of hydrogen or hydrocarbon gases. The carbon sheets settled on the surface of preformed circular carbon nuclei, maintaining the spherical shape at an autogenic pressure. These in situ formed reducing gases help keep the carbon in reduced form and segregate to yield solid carbon, while cooling. These results are also in good agreement with the pressure measurements, suggesting that the main WP decomposition happened between 600 and 700 °C. Table 1 presents the used plastic feedstock, process temperature and time, and obtained morphology of the carbonaceous products. Since the CMSs are pure, they are directly characterized further without additional processing. The X-ray diffraction pattern of the CMSs was measured with a Bruker AXS D* ADVANCE powder X-ray diffractometer using CuKR (λ) 1.5418) radiation. An Eager 200 C,H,N,S analyzer was used for elemental analysis of the CMSs. The quantitative elemental composition of the CMSs was determined by energy-dispersive X-ray analysis (Kevex). Sample morphology was studied by high-resolution scanning electron microscopy (HR-SEM) with a JEOL 6300F. To probe whether the CMSs are hollow or solid, a dry powder was immersed in an epoxy plastic (according to Spurr’s formulation) and placed in a capsule to harden. The hard blocks were cut by using an LKB ultratome III, and the ultrathin sections were placed on bare 400-mesh copper grids for transmission electron microscopy (TEM) of the spherule’s cross section. Omicron UHV nanoprobes with scanning probe/scanning electron microscopy were used at CNM facilities of Argonne National Laboratory to measure the conductivity of single carbon sphere. The morphology of CMSs was also determined by TEM working at acceleration voltages of 80 kV. A standard Renishaw Raman spectrometer was employed for structural carbon analysis using the 514.5-nm line of an Ar laser as an excitation source.
Results and Discussion Generally, LDPE bags, HDPE bags, and PS are the most common sources of WP. Figure 2a, b, and c present the digital photographs of these WP, which are used as a feedstock. To facilitate loading, the WP is cut into pieces; however, this step is not a requirement. Polyethylenes are composed of only carbon and hydrogen. The difference between HDPE and LDPE is the degree of branching. Generally, LDPE is produced at high pressure, which yields many polyethylene branches due to intermolecular and intramolecular chain transfer during polymerization. HDPE is more crystalline and stronger than LDPE because it contains fewer branches. The HDPE produces
TABLE 1. Plastic Feedstock, Process Temperature, Time, and Obtained Morphology of the Carbonaceous Products
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plastic feedstock
temperature oC/time
shape of the carbon product
waste HDPE bags waste LDPE bags waste HDPE bags waste PS cups commercial HDPE bags commercial LDPE bags commercial PS waste HDPE milk cans waste HDPE milk cans with HDPE bags waste HDPE milk cans with LDPE bags waste polyethylene terephthalate bottles waste HDPE bags + cobalt acetate waste LDPE bags + cobalt acetate
700/(1 min onward) 700/(1 min to 3 h) 800/(3 h) 700/1 h 700/2 h 700/2 h 700/2 h 700/2 h 700/2 h 700/2 h 700/3 h 700/3 h 700/3 h
spherical egg-like/spherical spherical spherical spherical egg-like/spherical spherical spherical spherical spherical spherical co-encapsulated carbon nanotubes co-encapsulated carbon nanotubes
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FIGURE 2. Digital images of ubiquitous used WP: (a) HDPE bags, (b) LDPE bags, and (c) polystyrene cups and plates. completely spherical carbons (Figure 3a and b) with diameters of 3-5 µm. The dissociation of LDPE in a closed reactor yielded egg-like or semispherical carbon particles (Figure 3c and d). The diameter of CMSs ranged from 3 to 5 µm. The formation of egg-like (semispherical) as compared with completely spherical CMSs is the noticeable morphological difference between LDPE and HDPE dissociation. Polystyrene is made from the aromatic monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum. Poly dispersed (4-10 µm) CMSs with soft surface were produced from PS waste (Figure 3e and f). To verify the reliability and consistency of the newly developed upcycling process for the production of CMSs, control experiments are performed. At 700 °C, the reaction times were varied from 1 min to 3 h for the dissociation of various WPs. From 1 min onward, additional reaction time slightly improved graphitic order in the CMSs, while the size and shape remain unchanged. In addition, multiple runs using each experimental setup were executed in either an inert or standard air atmosphere. Typically, 10-15% fewer egg-shaped and spherical CMSs were generated in the air compared with the inert atmosphere. The combustion, i.e., oxygen content from air reacts with some of the dissociated carbon to form CO2, consuming part of the carbon, thus the resulting carbon yield is low in an air atmosphere. Conversions of feedstock to CMSs product ranged from 30% to 55%, with 40% being characteristic. The process reliability was further checked by employing commercial feedstock purchased from Sigma-Aldrich Co., and the results were compared with the WP results. With as-received HDPE powder, 3-10 µm spherical carbon particles formed (Figure 4a). The controlled thermolysis of as-received LDPE powder yielded 2-8 µm egg-shaped carbon particles (Figure 4b). With as-obtained PS, poly dispersed CMSs formed during the controlled pyrolysis (Figure 4c). Thus, reproducibility was corroborated by the similarity of the CMSs products from commercial and WP feedstock. In another control experiment, commercial polystyrene-poly(2vinylpyridine), (PS-PVP) diblock copolymer with 0.24 volume fraction of PS with 1.11 polydispersity index was used as a feedstock. The formed CMSs are 1-5 µm (Figure 4d) diameters with smooth surfaces. Additional control experiments were carried out to confirm the reproducibility of the upcycling process. Maintaining all other experimental conditions identical, when HDPE bags were replaced with empty dried milk cans (HDPE), equivalent results were obtained: 3-5 µm smooth CMSs (Figure 5a). Moreover, milk cans mixed with HDPE or LDPE bags fired collectively at 700 °C in a closed system yielded similar CMSs (Figure 5b and c). Note that some CMSs particles merged owing to the dissimilar thermodynamic stabilities of the premixed feedstock. In other words, thin plastic bags
might degrade faster than thick plastic cans. The initial proportion of the mixed feedstock did not have an immense impact on the produced CMSs. These observations verified that not only the LDPE, HDPE, and PS yielded CMSs after controlled thermal dissociation of individual WP, but their mixtures also yielded analogous carbonaceous products. From all the above-mentioned experiments, it is concluded that the upcycling process produces micrometer-sized (315 µm) CMSs which are suitable candidates for application in toners with the small addition of appropriate resins. The two common features for all the CMSs particles from our experiments are spherical shape and polydispersion. Thus, in the remainder of this section, we discuss the composition, structure, and conducting properties of representative CMSs prepared from HDPE WP. The transmission electron micrograph (Figure 6a) of CMSs obtained from HDPE feedstock shows poly dispersed CMSs with a smooth surface. The poly dispersed CMSs can provide a highly dense packed system. These observations are consistent with the SEM characterization. The electron diffraction pattern for the CMSs obtained from HDPE feedstock (inset Figure 6a) shows a diffuse diffraction ring, owing to 002 planes, evidence for the poorly ordered graphitic planes. The X-ray diffraction pattern of CMSs (Figure 6b) exhibits wider peaks, resulting from smaller graphitic domains. Roughly 0.347 nm interlayer spacing is calculated for the 002 diffraction line. For CMSs, the Raman spectrum (Figure 6c) indicates a mixture of graphitic structure at 1590 cm-1 (G-band) and disordered carbon at 1340 cm-1 (D-band). The 1590 cm-1 peak corresponds to the E2g vibration mode of sp2-bonded carbon (22) atoms in a 2-D hexagonal lattice. The 1340 cm-1 peak is associated with in-plane terminated disordered dangling bonds of graphite. In all our experiments to produce CMSs, the obvious presence of disordered carbon is a result of insufficient (23) temperature (700 °C) to improve local graphitic order without catalyst. However, the present reaction parameters are ideal to yield perfectly spherical shapes for all the carbon particles. Yielding >95% spherical shape with the CMSs is a unique feature of the present WP upcycling process. Furthermore, energy-dispersive X-ray (EDS) analysis of CMSs showed only a carbon signal and no evidence for other impurities (Figure 6d). For the EDS analysis of CMSs, the sample was mounted on the double sided carbon tape supported on Si vapor and measured by EDS attached to SEM instrument. Additionally, the C,H,N,S analysis of CMSs showed >99% elemental carbon with minor (
