High Performance Graphitic Carbon from Waste Polyethylene

Article Cite This: Chem. Mater. 2017, 29, 9518-9527

pubs.acs.org/cm

High Performance Graphitic Carbon from Waste Polyethylene: Thermal Oxidation as a Stabilization Pathway Revisited Dalsu Choi,† Dawon Jang,†,‡ Han-Ik Joh,§ Elsa Reichmanis,∥ and Sungho Lee*,†,‡ †

Carbon Composite Materials Research Center, Korea Institute of Science and Technology, 92 Chudong-Ro, Bongdong-eup, Wanju-gun, Jeonbuk 55324, Republic of Korea ‡ Department of Nano Material Engineering, Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea § Department of Energy Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 05029, Republic of Korea ∥ School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: In this study, for the first time, thermal oxidation, which has only been considered as a degradation pathway for plastics, served as a simple and effective pretreatment protocol to modulate the chemical structure of linear low density polyethylene (LLDPE) for successful conversion of “noncarbonizable” LLDPE into an ordered carbon. More importantly, LLDPE based carbon could be graphitized into a highly ordered graphitic carbon with exceptional electrical performance exceeding that of Super-P, a pricey reference conductive agent for lithium ion battery fabrication. Upon thermal oxidative pretreatment, inherently noncarbonizable LLDPE was successfully transformed into an ordered carbon through heat treatment with a high conversion yield reaching 50%, a yield comparable to that obtained from polyacrylonitrile (PAN), a reference polymeric precursor. Systematic interrogation of the chemical structural evolution using X-ray diffraction, Raman, and dynamic scanning calorimetry (DSC) analysis, confirmed that an oxidation reaction occurred around 330 °C, which initiated transformation of aliphatic chains into cyclized ladder structures that allowed successful carbonization of LLDPE with high carbon yield. The thermally oxidized LLDPE evolved into a highly graphitic carbon that exhibited superior degree of ordering and electrical performance over a graphitized PAN counterpart. Finally, LLDPE waste, such as cling wrap and poly gloves was also successfully converted into an ordered carbon comparable to that obtained from the as-produced LLDPE precursor, suggesting opportunities associated with “upcylcing” of waste products. Thus, the proposed protocol represents an effective, potentially low-cost, and sustainable pathway providing for an exceptionally high quality conductive agent applicable in energy storage and flexible, printed electronics.

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INTRODUCTION

structures, in this study, we demonstrate the conversion of linear low density polyethylene (LLDPE) into a graphitizable carbon precursor whose graphitized product exhibits a superior degree of ordering and electrical conductivity that exceeds those of Super-P. To date, the main use of LLDPE is as a packaging material, which derives from its mechanically resilient characteristics.16−18 As such, thermal oxidation of LLDPE has only been considered as a means to effect external damage thereby impacting mechanical performance. Therefore, all studies regarding LLDPE thermal oxidation have been conducted from the perspective of investigating mechanical property degradation.19−26 According to previous studies, during LLDPE thermal oxidation, chemical structural changes such as chain cleavage and cross-linking take place, thus the mechanical properties of LLDPE deteriorate.25,26 Here, in contrast to

A high quality carbon material consists of well-packed hexagonal ladder planes, whereby long-range conjugation and ordered stacking of those planes via π−π interactions endow the material with good mechanical and electrical properties, and exceptional thermal and chemical stability.1−3 Such carbonaceous materials are actively utilized in various applications. In fibril form, carbon fiber is a major component of carbon fiber reinforced plastic (CFRP) which is a representative nextgeneration composite material possessing extremely high strength-to-weight ratio.4,5 In powder form, lower grade carbon powder is used as a major filler material for tire production6,7 and it is also widely used as an adsorbent.8−10 Significantly, high quality graphitic carbon powders serve multiple roles in lithium ion batteries, both as a conducting agent and as an anode material after activation.11−15 Currently, the most widely used conductive agent for fabricating lithium ion batteries is a synthetic graphite called Super-P. Noting that only a limited number of carbon precursors can organize into graphitic © 2017 American Chemical Society

Received: September 4, 2017 Published: October 18, 2017 9518

DOI: 10.1021/acs.chemmater.7b03737 Chem. Mater. 2017, 29, 9518−9527

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Chemistry of Materials

Figure 1. a) TGA data of a pristine LLDPE film obtained under nitrogen condition, b) TGA/DTA data of a pristine LLDPE film measured under air, c) detailed TGA/DTA data of pristine LLDPE film measured under air, delivering heat transfer information before major decomposition initiates, and d) DSC data of pristine LLDPE film measured under nitrogen and air, respectively.

applicability of thermal oxidation in “real-world situations”. LLDPE is one of the most widely used plastics in the world: third in production volume following polypropylene (PP) and high density polyethylene (HDPE). Due to its malleable characteristics, LLDPE is mainly processed into films for packaging, whereby most are simply disposed of after one-time usage.16,18 As a consequence, LLDPE accounts for the largest fraction of plastic waste.17,18 Moreover, only 5.8% of waste LLDPE was recycled in 2014, with the remainder mostly buried in landfills.17,18 If waste LLDPE can also be successfully carbonized as was its pristine form, then huge quantities of waste LLDPE might find a new pathway for reuse as a high quality conducting agent rather than simply landfilled. The result was intriguing, even when LLDPE wastesspecifically cling-wrap and polyglovescontaining impurities and chemical additives were used as the source, carbonization was induced successfully. The product exhibited a high yield and degree of ordering similar to that observed for carbon derived from asproduced LLDPE. Thus, the fabrication of a high quality conductive agent from recycled LLDPE could be envisioned, and the process introduced here may present an attractive, lowcost alternative for a wide range of advanced applications including energy storage and flexible, printed electronics.

previous investigations, we focused on the chemical structural transformation of LLDPE during thermal oxidation and used those transformations to reorganize noncarbonizable aliphatic LLDPE chains into a structure suitable for carbonization. Even in the case of polyacrylonitrile (PAN), the most widely used polymeric precursor for carbonaceous material fabrication, a pretreatment step known as “stabilization” that induces a chemical structural transition is required for successful carbonization.3,27,28 Stabilization is a simple thermal oxidation process; PAN chains experience a notable structural change during thermal oxidation and are transformed into fused hexagonal rings by cyclization. The networked hexagonal structure not only endows the material with thermal stability that allows it to withstand the harsh carbonization process without decomposition, but also guides evolution of the material into a stacked hexagonal ladder structure. Recalling the thermal oxidative stabilization process of PAN, for the first time, we examined LLDPE thermal oxidation as a convenient stabilization route for LLDPE carbonization. On the basis of careful interrogation of LLDPE thermal oxidation behavior over a wide temperature range, LLDPE samples were thermally oxidized under rationally designed conditions and then carbonized. As a result, we demonstrated successful carbonization of LLDPE with exceptionally high carbon yield and quality: LLDPE derived carbon is directly comparable in yield and ordering to PAN-based carbon. Microstructural analysis revealed the formation of polyaromatic moieties upon LLDPE thermal oxidation that guided successful carbonization. Moreover, interestingly, LLDPE-based carbon could be further developed into a high quality graphitic structure, where the degree of graphitic ordering and electrical conductivity were far superior to those of Super-P, a pricey reference conductive agent used in lithium ion battery fabrication. Finally, LLDPE wastes were treated with the thermal oxidation protocol and carbonized to test the general

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RESULTS AND DISCUSSIONS LLDPE Thermal Oxidation: Achieving Stable Structures for Successful Carbonization. As depicted in Figure 1a, thermal gravimetric analysis (TGA) of a pristine LLDPE sample demonstrated that LLDPE cannot withstand a high temperature carbonization process. LLDPE (UL 814, Lotte Chemical, Republic of Korea) pellets were processed into 25 μm (∼1 mil) thin films using a hot press. Then, the resulting films were heated to 1200 °C with a ramping rate of 5 °C/min under an inert atmosphere to simulate conventional carbonization processing. Rapid and complete weight loss was 9519


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Figure 2. a) TGA data of thermal oxidatively stabilized LLDPE and PAN films. b) Raman spectra of carbon samples fabricated from thermal oxidatively stabilized LLDPE and PAN films.

observed around 500 °C, confirming that LLDPE cannot be transformed into a carbonaceous structure through conventional heat treatment. Aliphatic LLDPE chains are vulnerable to thermal decomposition and the structure must first be converted into a thermally stable form to survive the harsh carbonization conditions.29,30 To date, the thermal oxidation of polyethylene has only been considered as a degradation route that deteriorates LLDPE mechanical properties.19−26 In this study, however, we took advantage of chemical structural changes taking place during the thermal oxidation process, and thereby utilized thermal oxidation to reorganize the aliphatic LLDPE structure into one that was favorable for carbonization. Previous studies only considered LLDPE oxidation behavior in the low temperature regime, under 200 °C.23−26,31 Reported chemical changes included limited cross-linking, oxygen functionalization, and chain scission. These changes were sufficiently severe to induce significant degradation in mechanical properties and thus the polymer’s oxidation behavior in the higher temperature regime was not investigated. While the chemical changes that occur at below 200 °C are inadequate to modify LLDPE into a thermally stable structure, it is conceivable that oxidation at temperatures over 200 °C might lead to additional structural transformations. Thus, oxidation of LLDPE in a higher temperature processing regime was assessed by differential thermal analysis (DTA) coupled with TGA (Figure 1b). Heat generation and absorption representing phase/structural changes during the thermal oxidation process were monitored up to 1000 °C. A significant exothermic reaction was observed at about 370 °C accompanied by rapid weight loss, which confirms when thermal oxidative decomposition was initiated. Decomposition continued until 520 °C where complete 100% weight loss was observed. To explore, in detail, the thermal changes taking place prior to the onset of decomposition, DTA/TGA data at temperatures up to 370 °C were more thoroughly investigated (Figure 1c). Following the endothermic melting peak at 125 °C, two exothermic peaks that were accompanied by a weight loss of approximately 6% were observed at 235 and 310 °C. Their origin was confirmed via differential scanning calorimetry (DSC) under nitrogen and air (Figure 1d). To avoid significant outgassing and instrument damage, the maximum temperature was limited to 300 °C to restrict weight loss to 5%. Under an inert atmosphere, only the endothermic melting peak appeared, implying that the two exothermic peaks observed upon heating in air are consequences of oxidation.

Considering the two characteristic exothermic oxidation peaks, LLDPE films were then oxidized at four different temperatures (235 °C, 270 °C, 310 °C, and 330 °C) and the effect of thermal oxidation on LLDPE carbonization was studied. Upon thermal oxidation, the originally opaque white LLDPE film first became yellowish, then brown, and finally black when oxidized at 330 °C (Figure 2a). Each oxidized sample was subsequently carbonized in the TGA at a ramping rate of 5 °C/min under nitrogen up to 1200 °C. Surprisingly, unlike pristine LLDPE, which underwent 100% weight loss during heat treatment, the thermally oxidized LLDPE films survived the high temperature process and were successfully carbonized. Increasing the oxidation temperature from 235 °C to 330 °C led to an increase in carbon yield from 4.5 to 50.0%, respectively. The latter value is comparable to that observed for the well-known reference carbon precursor, namely PAN (∼54%, Figure 2a), which was used as a control. Raman spectroscopic analysis provided insight into the structure of the carbonized products derived from LLDPE (Figure 2b). Bands characteristic of well-packed hexagonal ladder planes, namely the D-band and G-band at 1350 and 1600 cm−1, respectively, were observed, confirming successful conversion of oxidized LLDPE into carbonaceous material. The ratio of the area under the D and G bands (ID/IG) is directly proportional to the lateral size (La) of the carbon ladder plane and is generally associated with carbon ordering.2,3,32−34 ID/IG values, derived from deconvolution of the Raman signals, were calculated for the oxidized samples to determine the relative degree of ordering. Even though carbon yield varied with pretreatment temperature, ID/IG values were similar. Thus, the degree of ordering is not dependent upon the oxidization temperature (Table S1 in the Supporting Information, SI). Moreover, ID/IG values of carbon obtained from oxidized LLDPE vs stabilized PAN were comparable, confirming that carbon quality was similar, irrespective of starting material. In summary, exploration of LLDPE oxidation behavior in a relatively higher temperature regime than previous studies, led to the discovery of oxidative reactions that afford a structure sufficiently stable to allow subsequent high temperature carbonization. LLDPE Thermal Oxidation: Stabilization Mechanism and Structural Transformations. Oxidized LLDPE films were systematically interrogated to ascertain the chemical structural changes the aliphatic chains undergo during thermal oxidation that in turn allow the high yield carbonization process. Initially, DSC performed under nitrogen provided 9520


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Figure 3. a) DSC data, b) XRD spectra, c) XPS-based atomic composition analysis, d) Raman spectra, and e) FT-IR spectra of LLDPE films thermally oxidized under various temperature conditions.

some insight into the thermal behavior of oxidized LLDPE (Figure 3a). Interestingly, oxidized LLDPE did not exhibit the characteristic melting peak at 125 °C, even for samples oxidized at the lowest temperature (235 °C). These results suggest that the parent polymer’s crystalline structure was disrupted by chain scission and/or cross-linking, thereby providing some insight into why the oxidized LLDPE exhibited improved thermal stability. XRD and Raman analysis of oxidized LLDPE samples provided further understanding of how the polymer structure changes during thermal oxidation. Pristine LLDPE exhibited three sharp peaks at 2θ of 21.2°, 24.1°, and 36.5° from (110), (200), and (020) planes, respectively, upon XRD interrogation, which are consistent with typical orthorhombic LLDPE crystals (Figure 3b).35−37 Upon oxidation of the polymer at temperatures up to 270 °C, the (110) diffraction peak broadened and shifted to a lower diffraction angle. These changes suggest an increase in d-spacing and decrease in crystallite size, with concomitant disruption of the crystal packing. The decreased crystallite size and wider d-spacing can be rationalized by chain scission and functionalization of the polymer with oxygen moieties during the oxidation process. Simultaneously, upon disruption of orthorhombic packing, the (200) and (020) diffraction peaks disappeared. A second stage of oxidation with significant structural rearrangement was observed at temperatures above 270 °C. For the sample oxidized at 310 °C, the main diffraction peak appeared at 22.8°, or 3° higher than that for the 270 °C sample. Since the level of oxygen functionalization was higher for the 310 °C vs 270 °C sample (19.1 vs 15.0 atomic % oxygen, respectively, as determined through XPS analysis; Figure 3c), reduced (110) d-spacing cannot be responsible for the change. Rather, the diffraction peak at 22.8° most likely represents a newly formed structure. Additional changes were apparent in XRD of LLDPE samples oxidized at 330 °C. Here, the main

diffraction peak shifted to 24.2°, which is where the (002) polyaromatic ladder plane stacking peak of amorphous carbon appears,2,3,27,28 suggesting significant additional structural evolution, whereby linear aliphatic chains transform into a ladder structure. On the basis of the premise that a ladder structure was formed through oxidation at 330 °C, the XRD of the 310 °C oxidized sample was revisited. In comparison to 330 °C sample, that oxidized at 310 °C appeared to experience less extensive changes during oxidation, and hence may have a less well-developed ladder structure. Lignin and pitch are two polyaromatic materials known to possess a less clustered carbon ladder structure than amorphous carbon and thus may serve to provide insight into the structure of the 310 °C LLDPE sample. Notably, XRD of both lignin and pitch have been reported to exhibit broad peaks at ∼23°,38−40 similar to what was observed here for LLDPE oxidized at 310 °C. On the basis of the XRD interrogation of LLDPE oxidized at temperatures ranging from 310 to 330 °C, coupled with reported diffraction results for carbon, lignin, and pitch, it is presumed that a polyaromatic ladder structure formed during the second stage of LLDPE oxidation. The suggested oxidation induced LLDPE structural transformations are supported by Raman spectroscopic studies (Figure 3d). Pristine LLDPE exhibited the expected C−H bond vibrational bands commonly found in crystalline polyethylene. LLDPE samples subjected to the first stage of oxidation (235 and 270 °C) did not present any distinct Raman peaks; however, samples oxidized at 330 °C exhibited two characteristic signals at 1350 and 1600 cm−1, corresponding to the D and G bands, respectively, which are characteristic Raman signals of carbonaceous matters,32−34,41−43 implying significant structural change of aliphatic chains into a carbon-like structure. It is worth noting that the origin of the D band is associated with the ring-breathing vibrational mode of the hexagonal carbon ring, thus the D band appears when a polyaromatic 9521


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Chemistry of Materials structure is formed.32,34,41 Even though the D band is generally known as a Raman signal associated with defects in carbon structure, the premise holds true only in the case of highly ordered graphitic carbons, such as graphite and graphene. For less ordered carbonaceous structures with La < 2 nm, the Raman D band increases in intensity as more polyaromatic moieties comprised of six-membered rings are formed.32 The D band began to emerge in the sample oxidized at 310 °C, which can be attributed to the beginning of aromatic carbon ring formation. Upon oxidation at 330 °C, the Raman D band became significantly more intense, which points directly to the formation of a polyaromatic ladder structure. Taken together, XRD and Raman analysis confirmed a transition of LLDPE from a purely aliphatic chain structure to a distinctly polyaromatic ladder structure through the thermal oxidation process. The thermally stable polyaromatic material facilitated subsequent carbonization with exceptionally high carbon yield reaching 50%. FT-IR and XPS spectroscopy offered more insight into the LLDPE chain transformation process. At the beginning of the first oxidation stage (235 °C), common consequences of oxidation were observed in FT-IR spectra (Figure 3e). Vibrations characteristic of ketones (1700 cm−1) and C−O bonds (800 and 1200 cm−1) were identified, whereby C−O bonds would be expected to present in two different configurationsone as embedded oxygen within a chain and the other as a C−O−C bridge. Simultaneously, the intensity of the C−H bands at 2800 cm−1 decreased, which is likely due to oxygen functionalization and LLDPE chain cross-linking also observed in the DSC. At 270 °C, carbon double bonds, as evidenced by the appearance of a vibration at 1600 cm−1, were formed and C−H band intensity further weakened. For samples oxidized at 235 and 270 °C, XPS confirmed the incorporation of oxygen, and showed that atomic oxygen content increased from 11.7% to 15.0% (Figure 3c). Atomic oxygen content continued to increase up to the initial period of the second oxidation stage at 310 °C. Here, a ladder structure began to form and C−H band intensity decreased, suggesting that significantly more oxygen functional groups were incorporated into the polymer. At the end of the second oxidation stage (330 °C), the transformation into a cyclized ladder structure was complete, and no C−H vibrations were visible in the FT-IR spectra. Significantly, the disappearance of the C−H band was not accompanied by a further change in oxygen content: oxygen atomic content remained at ∼19% (19.1 and 18.9% in 310 and 330 °C samples, respectively). It is speculated that during the second stage of oxidation, thermal energy was mainly used to develop the ladder structure rather than be consumed for oxygen functional group attachment. A proposed mechanistic scheme based on the FT-IR and XPS spectral studies coupled with XRD and Raman analysis is presented in Scheme 1. Carbonization and Graphitization of Stabilized LLDPE. Structural evolution from thermally stabilized LLDPE to carbonaceous matter was systemically interrogated. LLDPE films oxidized at 330 °C were heated under an inert environment at various temperatures, and gradual structural changes during the carbonization process were examined ex situ. From 200 to 1200 °C, where conventional carbonization takes place, the width of the (002) diffraction peak became slightly narrower, indicating gradual growth of (002) grains (Figure 4a). Development of the Raman D band at 1350 cm−1 upon higher temperature treatment also informed structural

Scheme 1. Proposed Chemical Structural Transformation of Aliphatic LLDPE Chains into Cyclized Polyaromatic Moieties through Thermal Oxidation

changes in this region (Figure 4b). When the lateral size (La) of the ladder structure is less than 2 nm, the D band intensity increases as La increases.32,34,41 FT-IR analysis provided additional information regarding the chemical changes that occurred during carbonization (Figure 4c). No significant changes were observed in FT-IR spectra of LLDPE oxidized at 330 °C, and then subjected to carbonization temperatures up to 500 °C. Spectral changes did emerge when the temperature was raised to 600 °C, where ketone and hydroxyl group vibrations began to decrease in intensity. Once the temperature reached 700 °C, these functional groups were no longer present in the sample. Similarly, the vibrational signature associated with carbon double bonds appeared stable up to 700 °C, after which it decreased in intensity as the temperature increased. No distinctive CC vibrations were visible at 800 °C. The vibrational spectroscopic studies associated with LLDPE oxidation and subsequent carbonization present chemical changes that are similar to those reported for PAN carbonization. In particular, ketone and hydroxyl functionalities are produced during the oxidative stabilization process of PAN. Further, these groups have been reported to play a significant role in both the formation of a polyaromatic structure and clustering of polyaromatic moieties during carbonization. Mechanistically, it is believed that the thermal treatment leads to the generation of radicals along the residual polymer as oxygen atoms leave in the form of gases such as CO2 and CO.3,27,28 Most likely, the carbonization of oxidized LLDPE follows a similar mechanistic path. Along with FT-IR spectroscopic evidence, XPS data also pointed out the leaving oxygen atoms upon carbonization/graphitization (Figure S2). In addition to investigating the carbonization of LLDPE, the ability of the system to successfully undergo graphitization at temperatures up to 2400 °C was explored. Note that not all carbon precursors can be transformed into highly ordered graphitic carbon upon high temperature graphitization. Carbon substances are classified as either graphitizable or nongraphitizable carbon, also referred to as soft carbon and hard carbon, respectively.44−46 Only soft carbons can develop the requisite long-range ordering and graphitic layer stacking. The turbostratic structure of hard carbons does not evolve into a highly ordered graphitic structure even through high temperature treatment up to 2400 °C. The degree of ordering is strongly related to the overall performance (e.g., mechanical strength, electrical conductivity) of the material in any given 9522


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Figure 4. a) XRD spectra, b) Raman spectra, and c) FT-IR spectra of thermal oxidatively stabilized LLDPE films followed by carbonization/ graphitization under nitrogen at various temperature conditions.

application.6,27,28,46 Thus, the production of graphitic carbon with a high degree of ordering is desirable. Stabilized LLDPE samples were subjected to heat treatment under temperatures reaching 2400 °C. XRD interrogation of the samples showed that the peak associated with the (002) plane shifted toward a higher angle and became markedly narrower. These changes suggest the successful graphitization of oxidized LLDPE through significant growth of polyaromatic moieties (Figure 4a). Changes in the (002) peak were particularly noticeable at the higher graphitization temperatures. Raman spectroscopic studies provided additional evidence that oxidized LLDPE was transformed into a graphitic structure upon high temperature treatment (Figure 4b). Specifically, graphitic structures consisting of well-ordered polyaromatic layers exhibit a 2D band around 2700 cm−1, which is an overtone of the D band.32−34,41−43 Significantly, the 2D band emerged upon heat treatment of oxidized LLDPE at 2000 °C. Graphitization at 2400 °C, afforded a material that exhibited a distinctly more intense 2D band and concomitantly significantly diminished D band. Since D band intensity is positively correlated with the number of defects in highly ordered graphitic carbons, the observed decrease in D band intensity suggests an enhanced degree of graphitic ordering.33,34 Transmission electron microscopy (TEM) confirmed the successful formation of graphitic structure; well-stacked layers were apparent in LLDPE samples graphitized at 2400 °C (Figure 5a). Comparison of the 2400 °C images to TEM images of LLDPE carbonized at 1200 °C, the development of long-range graphitic ordering is clearly apparent. Moreover, the degree of graphitic ordering was superior to that of graphitized PAN subjected to the same conditions. Raman spectroscopic results support the formation of high quality graphitic carbon. As depicted in Figure 5b, the Raman D-peak associated with graphitized LLDPE is noticeably less intense than that for graphitized PAN. The superior degree of graphitic ordering observed in LLDPE-based carbon directly translated into exceptional electrical characteristics (Figure 5c). Note that the electrical conductivity of graphitized LLDPE samples was significantly higher than that of PAN samples graphitized under the same conditions. More significantly, the value was higher than that of Super-P, the most widely used conductive agent for lithium ion battery fabrication. Using the XRD and Raman results, the dimensions of the basic structural unit (BSU), an ordered unit of polyaromatic

Figure 5. a) TEM images of thermal oxidatively stabilized LLDPE films followed by carbonization/graphitization at 1200 and 2400 °C, respectively. b) Conductivity of various carbon powders including graphitized PAN, LLDPE, and Super-P. c) Raman spectra of stabilized PAN and LLDPE samples graphitized at 2400 °C.

planes comprising a carbon structure and a polyaromatic plane spacing were calculated, and their evolution was visualized as presented in Scheme 2. Lateral size (La) was calculated based on ID/IG derived from Raman data. Up to 1200 °C, since oxidized LLDPE was composed of relatively amorphous carbon, and the 2D band was absent, the Ferrari-Robertson relationship was used for the calculation.41 For samples graphitized at 2000 and 2400 °C, the traditional TuinstraKoneig relationship was used.47 Samples oxidized at 330 °C possessed an La of ∼1 nm, and this value remained relatively constant until the oxidation temperature reached 1200 °C. Concomitantly, significant development of the D band, which is closely related to polyaromatic clustering, was observed. This trend is consistent with previous studies concerning the structural development of highly amorphous carbon; the results of which led to the formulation of the Ferrari-Robertson relationship.41 Here, 330 °C oxidized LLDPE possessing a pseudocarbon structure can be viewed as highly amorphous carbon having less ordered polyaromatic moieties. In this 9523


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Chemistry of Materials Scheme 2. Growth of Basic Structural Unit (BSU) of Thermally Oxidized LLDPE Samples during Carbonization and Graphitization Processes

Figure 6. TGA data of thermal oxidatively stabilized waste cling-wrap, waste polyglove, and as-produced LLDPE film. b) Raman spectra of carbon samples from waste cling-wrap, waste polyglove, and as-produced LLDPE film followed by thermal oxidative stabilization.

into products with higher economic value, of various LLDPE waste products into carbon, cling wrap and poly glove wastes were carbonized after thermal oxidation. Collected wastes were cleaned using water, dried and then subjected to thermal oxidation at 330 °C followed by carbonization at 1200 °C in a TGA. TGA and Raman spectral analysis confirmed the efficacy of the thermal oxidative stabilization process described above even when wastes comprising LLDPE commercial products were used as precursors. Carbon yields obtained from oxidized cling wrap and poly gloves were 57.2% and 51.3%, respectively (Figure 6a). These yields are superior to that of oxidized LLDPE films prepared from pure LLDPE pellets, verifying successful carbonization through thermal oxidative pretreatment even when low-grade LLDPE waste was used. Moreover, ID/IG values of carbon produced from LLDPE wastes were also slightly higher than what was achieved for carbon derived from as-produced LLDPE (Figure 6b). These results confirmed that the quality of carbonaceous matter obtained from LLDPE wastes upon thermal oxidative treatment were comparable to carbon generated from oxidation of as-produced LLDPE.

temperature regime, the applied thermal energy primarily facilitated rearrangement of disordered regions into BSUs rather than fuse existing BSUs into larger moieties. For carbonization processes up to 1200 °C, the previously disordered regions were essentially converted into similarly sized BSUs, recall the observed emergence of the Raman D band. During carbonization, Lc, which is related to the clustering size in the stacking direction was determined from the width of the (002) diffraction peak using Scherrer’s equation, gradually increased from 0.8 to 1.6 nm. During the subsequent high temperature graphitization process, the BSUs began to coalesce, La abruptly grew to 18.1 nm, and Lc significantly increased, reaching 6.2 nm at 2400 °C. In addition, the interlayer distance (d-spacing) was calculated based on the (002) diffraction planes using Bragg’s law. For oxidized LLDPE graphitized at 2400 °C, the (002) interlayer spacing was 0.34 nm, a value consistent with high quality graphitic carbon. “Up-Cycling” LLDPE Waste into Carbonaceous Product. An economically attractive approach to convert waste LLDPE into higher value products may in turn promote LLDPE recycling and thereby alleviate or even eliminate environmental issues surrounding buried or burnt LLDPE wastes. To examine the applicability of thermally oxidative stabilization on “up-cycling”, a term coined for recycling waste

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CONCLUSIONS To date, LLDPE thermal oxidation has been viewed strictly from the perspective that the process leads to polymer 9524


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Thermo Scientific, U.S.). The ramping rate was fixed at 5 °C/min regardless of target temperature. All the samples stayed at the target temperature for 10 min; and a longer treatment time had no influence on the result (Figures S3 and S4). Carbonization and Graphitization of Thermally Oxidized Samples. Thermally oxidized samples were carbonized using box furnace (ThermVac Engineering, Republic of Korea) with ramping rate of 5 °C/min under N2 condition. For graphitization, a high temperature furnace specifically designed for graphitization was used (ThermVac Engineering, Republic of Korea). Graphitization was performed under Ar environment. Up to 1800 °C, the ramping rate was fixed at 10 °C/min. After reaching 1800 °C, the ramping rate was slowed down to 5 °C/min. Graphite sheets (Sigraflex, SGL Group, Germany) were folded into boxes and used as sample holders for carbonization and graphitization. Characterizations. Thermo Gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) were performed using Setasys Evolution TGA-DTA system (Setaram Instrument, France) under N2 environment with a 5 °C/min heating rate. Setasys Evolution system allowed simultaneous measurement of TGA and DTA. Differential Scanning Calorimetry (DSC) was performed using a Q20 DSC (TA Instruments, U.S.) under N2 or air environment with a heating rate of 5 °C/min. Raman Spectroscopy of samples were obtained using Renishaw inVia Raman spectrometer (Renishaw, U.K.) equipped with 514 nm laser having 0.15 mW output. Laser was focused through 20X optical lens and exposure time was set as 30 s. For X-ray diffraction (XRD) measurement, SmartLab XRD (Rigaku, Japan) with Cu−Kα radiation (λ = 1.54 Å) was used. XRD was operated at an acceleration voltage of 45 kV and an emission current of 200 mA. The 2θ value was scanned from 10° to 60° at a scan speed of 10°/min. Fourier Transform Infrared (FT-IR) spectroscopy measurement was conducted with Jasco FT/IR-6000 (Jasco, Japan) using the Attenuated Total Reflectance (ATR) mode. The instrument was equipped with a vacuum pump, and all the samples were measured under vacuum. Xray Photoelectron Spectroscopy (XPS) was measured using K-Alpha XPS (Thermo Scientific, U.S.). Survey scan spanning 0 to 1350 eV was followed by high resolution scans of C 1s and O 1s. Transmission Electron Microscopy (TEM) characterization was performed using Tecnai G2 F20 TEM (FEI, U.S.) with acceleration voltage of 200 kV. Electrical conductivity of samples were measured using powder resistance measurement system (HPFM-M2, Hantech, South Korea). All the measured electrical conductivity values were recorded under the external load of 500 kg.

degradation with concomitant deterioration in mechanical properties. In this study, we systematically investigated the chemical structural transformations that occur during thermal oxidation. Through mechanistic understanding of the processes, for the first time, thermal oxidation was utilized as a simple route to transform aliphatic LLDPE chains into thermally stable polyaromatic clusters suitable for carbonization. LLDPE samples were rationally oxidized based on thermal oxidation behavioral studies of LLDPE over a wide temperature range. Surprisingly, unlike pristine LLDPE which is thought to be unable to be carbonized, thermally oxidized LLDPE survived the carbonization process. The resulting carbonaceous product was retrieved in exceptionally high carbon yield: carbon yields reached 50% and are similar to those obtained from the reference carbon precursor, PAN. Moreover, the quality of carbon produced from oxidized LLDPE was comparable to PAN-based carbons. Subsequent investigations into the structural transformations that take place during thermal oxidation helped to elucidate the carbonization mechanism. Both XRD and Raman spectral analyses confirmed formation of polyaromatic clusters upon thermal oxidative pretreatment. These results, coupled with DSC analysis revealing a high degree of cross-linking, suggested that thermal oxidation led to aliphatic LLDPE chain reorganization into thermally stable cross-linked polyaromatic moieties, which in turn guided a spontaneous transition into well-stacked polyaromatic carbon structures. The polyaromatic structures provided for the high carbon yield during carbonization. Notably, thermally oxidized LLDPE experienced significant microstructural growth during a subsequent graphitization process, and produced a highly ordered graphitic structure whose degree of ordering was superior to that of graphitized PAN. Noting that not all forms of carbon can be transformed into graphitic form, it is intriguing that oxidized LLDPE developed a highly ordered graphitic structure upon high heat treatment. The LLDPE based graphitic carbon exhibited excellent conductivity exceeding that of Super-P, a reference conductive agent for lithium ion battery fabrication. Significantly, the process developed here to convert LLDPE to graphitic carbon was shown to be effective in the conversion of typical household LLDPE waste products such as cling wrap and poly gloves into similarly high quality carbon material. Thus, the process envisioned here is expected to be generally applicable to recycled LLDPE, and may present an attractive, low-cost alternative to the production of a high quality synthetic conductive agent for a wide range of advanced applications including energy storage and flexible, printed electronics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03737. ID/IG ratio of thermally oxidized LLDPE and PAN samples; atomic composition of oxidized LLDPE samples carbonized at varied temperature based on XPS analysis; FT-IR spectra of LLDPE samples thermally oxidized at 330 C for prolonged treatment times; and TGA/DTA measurement data monitoring heat transfer when thermal oxidative treatment was held for prolonged time (PDF)

MATERIALS AND METHODS

Precursor Sample Preparation. LLDPE pellets (UL 814, Lotte Chemical, Republic of Korea) with melt index of 20 g/10 min (measured at 190 °C) were hot pressed into 25 μm (∼1 mil) thick films. For the purpose of comparison, PAN films with exactly same dimensions were fabricated. PAN (Mw = 150 000) was purchased from Sigma-Aldrich, U.S. Using dimethyl sulfoxide (DMSO, 99% purity, purchased from Daejung Fine Chemical, Republic of Korea) as a solvent, 5 wt % PAN solution was made. Prepared PAN solution was casted on top of polyimide film, and the PAN solution was blade coated. The fabricated film was completely dried in a vacuum oven at 80 °C for 24 h, and a 25 μm thick free-standing PAN film was peeled off from polyimide film. Thermal Oxidation of Precursor Samples. Prepared precursor films were thermally oxidized using convection oven (Heratherm,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.L.). ORCID

Elsa Reichmanis: 0000-0002-8205-8016 Notes

The authors declare no competing financial interest. 9525


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ACKNOWLEDGMENTS This work was supported by a grant from the Korea Institute of Science and Technology (KIST) Institutional program (No. 2Z04980, No. 2Z05120), Republic of Korea.

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High Performance Graphitic Carbon from Waste Polyethylene

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