Article pubs.acs.org/est
Low-Cost, Acid/Alkaline-Resistant, and Fluorine-Free Superhydrophobic Fabric Coating from Onionlike Carbon Microspheres Converted from Waste Polyethylene Terephthalate Haibo Hu,† Lei Gao,†,‡ Changle Chen,*,‡ and Qianwang Chen*,† †
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, 230026, China ‡ Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, 230026, China S Supporting Information *
ABSTRACT: Onionlike carbon microspheres composed of many nanoflakes have been prepared by pyrolyzing waste polyethylene terephthalate in supercritical carbon dioxide at 650 °C for 3 h followed by subsequent vacuum annealing at 1500 °C for 0.5 h. The obtained onionlike carbon microspheres have very high surface roughness and exhibit unique hydrophobic properties. Considering their structural similarities with a lotus leaf, we further developed a low-cost, acid/alkalineresistant, and fluorine-free superhydrophobic coating strategy on fabrics by employing the onionlike carbon microspheres and polydimethylsiloxane as raw materials. This provides a novel technique to convert waste polyethylene terephthalate to valuable carbon materials. At the same time, we demonstrate a novel application direction of carbon materials by taking advantage of their unique structural properties. The combination of recycling waste solid materials as carbon feedstock for valuable carbon material production, with the generation of highly value-added products such as superhydrophobic fabrics, may provide a feasible solution for sustainable solid waste treatment.
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INTRODUCTION Plastics has become an indispensable part in modern society due to their unique properties such as durability, low cost, low toxicity, and resistance to corrosions. However, their poor degradability has created serious environmental and social problems. As a dramatic example, polyethylene terephthalate (PET), which is widely used as packaging materials for food and consumer products, has been one of the most pressing environmental concerns due to its huge worldwide consumption (26 million tons, ∼62.8 billion bottles annually).1−6 Numerous recycling technologies have been developed to alleviate this problem, which can be classified into mechanical and chemical approaches.7,8 As the dominant process, the mechanical recycling method is simple and cheap. However, the properties of the recycled resin are deteriorated in every cycle. As a result, the recovered PET feedstock is only suitable for second-grade materials. A small portion of discarded PET is recycled by the chemical method, which leads to complete depolymerization to monomers or partial depolymerization to oligomers and other chemical substances.9−11 However, largescale application of this method remains a challenge due to the complicated procedures, low efficiency, pollution, and high cost. Thus, the development of an innovative and cost-effective technology to promote the recycling of waste PET is highly necessary. Carbon materials have attracted a lot of attention due to their novel properties and wide applications in nanodevices,12 energy storage,13 separation technology,14 lubricants, etc. Conse© 2014 American Chemical Society
quently, various synthetic methods, such as carbonization, high-voltage-arc electricity, laser ablation, and hydrothermal carbonization have been developed to prepare amorphous, carbonaceous, porous, or crystalline carbon materials with different size, shape, and chemical compositions.15−19 The development of novel technology to convert waste PET to carbon materials would help to address the environmental problems and at the same time generate highly valuable carbon materials with various potential applications. In this respect, we have been engaged in ongoing efforts at developing effective PET recycling technologies. We recently reported the synthesis of carbon microspheres by pyrolyzing postconsumer PET without the requirements for complex equipment or any catalyst in a supercritical CO2 system.20 In addition, possible application of the resulting carbon microspheres as a negative electrode material for lithium ion batteries has been evaluated. Furthermore, we developed a strategy to synthesize SiC nanowires by reaction of PET waste with SiO2 microspheres.21 Herein, we present a simple route to convert waste PET to onionlike carbon microspheres (OCMs) with a hierarchical exterior structure composed of nanoflakes by pyrolyzing postconsumer PET in supercritical carbon dioxide at 650 °C for 3 h and subsequent vacuum annealing at 1500 °C Received: Revised: Accepted: Published: 2928
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Figure 1. (a) A waste PET bottle, (b) small pieces of the waste PET bottle, and (c) the obtained carbon materials.
30 min to form the solution A. PDMS precursor part B (Sylgard 184 curing agent, 0.015 g) was dissolved in THF (15 mL) to form solution B. Solutions A and B were mixed together at room temperature to form the coating solution. Prior to coating treatment, the polyester fabric was cleaned by ultrasonic washing in ethanol and THF and dried at 60 °C in a vacuum oven for 0.5 h. Finally, the fabric samples were dipcoated in the as-prepared coating solution and then cured at 100 °C for 0.5 h. Characterization Methods. Surface morphology of the CCMs and OCMs was characterized by scanning electron microscopy (SEM; JSM-6700M). The surface morphology of superhydrophobic polyester fabric was also observed by SEM. Powder X-ray diffraction (XRD) was performed on a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.541 78 Å). The Raman spectroscopic analysis was carried out on a LABRAM-HR confocal laser micro-Raman spectrometer using the 514.5 nm line of an Ar ion laser as the excitation source at room temperature. The Fourier transform infrared spectrum (FT-IR) was obtained using a Magna-IR 750 spectrometer in the range of 700−1800 cm−1 with a resolution of 4 cm−1. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250 X-ray photoelectron spectrometer with Al Kα radiation. The surface wettability of superhydrophobic polyester fabric was characterized using water contact angle (WCA) measurements, which were performed using a CAM 200 contact angle goniometer (KSV Instruments Ltd., Helsinki, Finland) at ∼20 °C. A droplet of water (4 μL) was injected onto the fabric. For each set of experimental conditions, three specimens were prepared, and the WCA was measured on three different sites for each specimen. The mean value was taken as the final result. All the digital photos were taken with a Panasonic DMC-FX3 camera.
for 0.5 h. The obtained OCMs have very high surface roughness and exhibit unique hydrophobic properties. Furthermore, we developed a simple and low-cost strategy to prepare an acid/alkaline-resistant and fluorine-free superhydrophobic coating on fabrics using OCMs and polydimethylsiloxane (PDMS) as raw materials. This work is aimed not only to explore an effective method to convert waste PET to potentially useful carbon materials but also to search for new possible applications of various carbon materials based on their unique structures. The utilization of the carbonized solid wastes as building blocks for superhydrophobic-coating-related applications would provide a new application direction for other carbon materials.
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EXPERIMENTAL SECTION Materials. The raw material is wasted PET from abandoned beverage bottles (Figure 1a). To facilitate loading, the waste PET was cut into small pieces by a pair of scissors (Figure 1b); PDMS (Sylgard 184) was purchased as a two-component kit that contained the vinyl-terminated base and curing agent from Dow Corning. A commercial polyester fabric used as a model substrate was purchased from a general store. All other chemicals were analytical-grade reagents and used as received. Synthesis of Onionlike Carbon Microspheres. A twostep process was used to prepare the OCMs. First, crude carbon microspheres (CCMs) was prepared by our reported procedure;20 1.50 g of PET foils or particles and 12 g of solid CO2 were put into a 25 mL stainless steel autoclave that can withstand high temperature and pressure and maintain a supercritical CO2 system at a high temperature. The temperature of the furnace was increased to 650 °C at a rate of 10 °C/ min and maintained for 3 h. After the autogenic reaction, the reactor was allowed to cool down to room temperature naturally (about 6 h). CCMs with ∼40% yield were collected (Figure 1c). Second, CCMs were annealed in a corundum bowl in a vacuum furnace at 1500 °C for 0.5 h. The final OCMs with ∼70% yield were obtained after being cooled down naturally to room temperature (about 8 h). The total duration (natural cooling time included) of the synthesis of CCMs and OCMs is about 10 and 11 h respectively. The maximum tolerated temperature of the stainless steel autoclave used in the work is about 700 °C. Therefore, further heat treatment (1500 °C for 0.5 h) of the obtained CCMs was carried out in a vacuum furnace, which can further pyrolyze a small amount of residual PET and other organic molecules that was not completely decomposed during the preparation of CCMs, and led to the formation of OCMs. Preparation of Coating Solution and Superhydrophobic Fabrics. OCMs (0.3 g) were dispersed in 15 mL of tetrahydrofuran (THF) after ultrasonic treatment. PDMS precursor part A (Sylgard 184 elastomer base, 0.15 g) was then added to the solution. The solution was ultrasonicated for
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RESULTS AND DISCUSSION
The products of PET thermal decomposition in supercritical carbon dioxide at 650 °C for 3 h were mainly crude carbon microspheres, as shown in Figure 2a,b. The obtained CCMs showed diameters of 1−5 μm and a smooth surface before annealing (Figure 2a,b). The elemental analysis of the CCMs showed the presence of ∼93.59 wt % carbon, ∼4.36 wt % oxygen, ∼2.52 wt % hydrogen, and a very small presence of 0.05 wt % nitrogen (Table 1), indicating deep carbonization of PET (from 61.73% carbon in the PET precursor). After annealing in vacuum at 1500 °C for 0.5 h, the morphology of the resulting OCMs is dramatically different from that of the original CCMs (Figure 2c,d), including a decrease of the diameter and the appearance of a hierarchical surface structure with many nanometer-scale flakes. These nanometer-scale flakes constitute a lot of deep trenches that can trap much air within them. The special structure can effectively increases the 2929
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Table 1. Elemental Composition of the PET Precursor, Crude Carbon Microspheres, and Onionlike Carbon Microspheres elemental composition (wt %)/sample crude carbon microspheres
onionlike carbon microspheres
C
62.50
∼93.59
∼98.39
H O N
4.196 33.30 0
∼2.52 ∼4.36 ∼0.05
∼0.34 ∼1.64 ∼0.03
gas−liquid interface when the OCMs contact with water, which causes the OCMs to possess excellent hydrophobic performance. The elemental analysis showed that OCMs contain more carbon element (∼98.39 wt %) and less oxygen (∼1.64 wt %), hydrogen (∼0.34 wt %), and nitrogen (∼0.03 wt %). In addition, the weight of the product was reduced to ∼70%. This change is likely due to the further decomposition of a small amount of residual PET that was not completely decomposed during the preparation of CCMs and the partial oxidation of the CCMs due to the residual oxygen in the vacuum furnace during annealling. The chemical reactions that occurred during annealling can be expressed as follows (eqs 1− 3): (C10H8O4 )n → 6nC(s) + 2nCO2 (g) + 2nCH4(g)
(2)
2C(s) + O2 (g) → 2CO(g)
(3)
XRD and Raman spectroscopy are particularly useful techniques for analyzing the structures of carbonaceous materials. Figure 3a shows the XRD spectra of the as-obtained CCMs, and we can see that the CCMs has one broad peak centered at 25.5° and another broader peak centered around 43.3°, which are characteristics of (002) and (100) planes of the graphite. This suggests a low graphitization degree and the presence of amorphous carbon.22 After the high-temperature annealing process, the (002) and (100) peaks of OCMs (Figure 3a) become bigger than that of CCMs, indicating the improved degree of graphitization. Figure 3b shows typical Raman spectra observed for CCMs and OCMs. Both show two bands centered at 1344 and 1598 cm−1, of which the D-band (1344 cm−1) is attributed to the disorder and imperfection of the carbon crystallites and the G-band (1598 cm−1) is assigned to one of the two E2g modes corresponding to stretching vibrations in the basal-plane of graphite, respectively. The ID/IG band intensity (peak area) ratios for the CCMs and OCMs are 2.34 and 1.32, respectively, both indicating low graphitization.23,24 However, the intensity ratio ID/IG (peak area) of OCMs is weaker that that of the CCMs, reflecting that the graphitization of the OCMs is improved after the high-temperature annealing process, which is consistent with the XRD results. The superhydrophobic behavior of lotus leaves is due to the hierarchical rough structures as well as the wax layer on the surface.25−27 Both surface energy and surface roughness are critical to fabricate a surface with large water contact angle.28 In this regard, we developed a simple technique to prepare fluorine-free and acid/alkaline-resistant superhydrophobic coating by taking advantage of the special surface structures of the OCMs and the waxy properties of PDMS. The polyester fabric was initially white (Figure 4a) and changed to gray after coating with OCMs/PDMS composites (Figure 4d). The water droplet was absorbed immediately by the pristine polyester fabric because of its good hydrophilicity (Figure 4b). In contrast, the water repellency of the OCMs/ PDMS composite-coated polyester fabric is clearly highlighted in Figure 4d, in which the water droplets exhibit typical spherical shapes on the fabric surface. The polyester fabric coated with PDMS alone also shows a certain degree of hydrophobicity. However, it is not categorized as superhydrophobic and the water droplets show slightly flat spherical shapes on the fabric surface (Figure 4c). The morphologies of
Figure 2. SEM images of obtained carbon materials dispersed on carbon tape: (a, b) crude carbon microspheres (CCMs) and (c, d) onionlike carbon microspheres (OCMs).
PET ((C10H8O4)n) theoretical value
C(s) + CO2 (g) → 2CO(g)
(1)
Figure 3. (a) X-ray diffraction pattern of the obtained onionlike carbon microspheres and crude carbon microspheres. (b) Raman spectrum of the asprepared onionlike carbon microspheres and crude carbon microspheres in the range from 800 to 1800 cm−1. 2930
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surface of polyester fiber with PDMS as linking agent (Figure 5c,d). A key feature of PDMS is that it penetrates deep into the polyester fabric, thus functioning as an adhesive that bonds OCMs to the polyester fiber surface in either flat or curved shapes. Interestingly, the surface of individual OCMs is not smooth but shows nanosized substructures (Figures 2d and 5d). Clearly, the polyester fiber surface coated with OCMs/ PDMS composites contains fibers with roughness at both nanoscale and microscale, which is essential for the generation of superhydrophobicity. In addition, the irregularity of the sphere sizes contributes to the increase of surface roughness. In addition to the surface structure analysis, the surface chemical compositions of the polyester fabric after coating were investigated by FTIR and XPS. As shown in Figure 6a, after coating treatment, a new peak at 798 cm−1 occurred, which was assigned to the ρ(CH3): CH3 rocking in Si−CH3.29 The peak at 1715 cm−1 corresponds to the carbonyl stretching band. Furthermore, the δs(CH3) and νas(Si−O−Si) were assigned to the symmetrical bending of CH3 in Si−CH3 and stretching vibrations of Si−O−Si bond, both of which are the characteristic peaks of PDMS polymer.30 In the XPS spectrum, two peaks at 102 and 153 eV are characteristic Si(IV) 2p and Si(IV) 2s signals (Figure 6b).31 The above data confirmed the successful deposition of the OCMs/PDMS composites on the polyester fiber surface by the simple dipping−drying−curing method. The combination of the PDMS, which both serves as the linking agent and mimics the function of the wax on lotus leaf, with the artificial binary micronanoscale surface structures from OCMs leads to the superhydrophobicity feature of the fiber similar to that of lotus leaf. The wettability of the OCMs/PDMS-coated polyester fabric was evaluated by measuring the WCA using a water droplet of 4 μL. The profiles of the WCA were recorded using a digital camera. A 3 μL water droplet did not stick to the surface because of the superhydrophobic property (shown in the Schematic Video, Supporting Information). When the volume of the water droplet was increased to 4 μL, the OCMs/PDMScoated polyester fabrics exhibits a WCA of 163° ± 3.0° (Figure 7b), demonstrating the superhydrophobic property of the coated fabrics. It should be noted that CCMs/PDMS-coated polyester fabrics has WCA of 147° (Figure S1, Supporting Information), and PDMS coating alone led to a WCA of 136° (Figure 7a), neither of which are categorized as superhydrophobic. With the increase of the concentration of PDMS, there is a negative effect on the air permeability, leading to smaller WCA. These
Figure 4. Water repellent test on (a, b) pristine polyester fabric and (c) polyester fabric coated with PDMS alone. (d) OCMs/PDMS composites coated polyester fabric.
the polyester fiber before and after coating were investigated by SEM. The diameter of a general polyester fiber is about 15 μm and the surface of the fiber is smooth with natural veins (Figure 5a,b). After coating, OCMs were uniformly deposited on the
Figure 5. SEM images of (a, b) untreated polyester fiber and (c, d) OCMs/PDMS composite-coated polyester fiber.
Figure 6. (a) FT-IR spectra of polyester fiber before and after coating treatment separately and (b) XPS spectra of the OCMs/PDMS composites coated polyester fiber. 2931
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fabric is stable enough to resist strong acid and alkali attacks. All of the above results clearly demonstrated the excellent durability of the polyester fabric with the OCMs/PDMS superhydrophobic coating. In summary, a simple, catalyst-free, and resource-saving process has been designed and developed to convert waste PET into value-added OCMs with unique hydrophobic nature in the absence of any expensive catalyst or complex equipment. In order to further expand applications of the unique OCMs, a low-cost, acid/alkaline-resistant, and fluorine-free superhydrophobic polyester fabric with good durability was prepared by simple dip-coating of the polyester fabric with OCMs/PDMS solution, followed by thermal curing. In comparison with previous works, our approach has unique advantages. First, PDMS was used as a linking agent to link OCMs to the polyester fabric surface in one step. Prefunctionalization of OCMs with fluorinated reagents or multistep chemical reactions was not needed. In addition, the superhydrophobicity of the polyester fabric was highly stable and resistant to acids and bases, owing to the strong adhesion of OCMs to the surface of polyester fabric, which makes the polyester fabric more practical. More importantly, our approach addresses two pressing social and environmental problems simultaneously: the treatment of solid plastic waste and the effective preparation of technologically important carbon materials.
Figure 7. Optical pictures of the profiles of the water contact angles on (a) polyester fabric coated with PDMS alone, (b) polyester fabric coated with OCMs/PDMS composites, (c) the treated polyester fabric after immersing in strong acid solution for 24 h, and (d) the treated polyester fabric after immersing in strong base solution for 24 h.
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ASSOCIATED CONTENT
S Supporting Information *
results clearly indicate that the high superhydrophobicity of the polyester fabric results from the combination of the roughness of the OCMs and the waxy properties of the PDMS coating. The superhydrophobicity likely originates from the reduction in the solid volume fraction because of the hierarchical surface structure of the OCMs. After coating with OCMs/PDMS composites, more air was trapped within the trenches of the OCMs on the fiber surface, which are made up of many nanoflakes. Since air itself served as part of the surface, the fabric surface beneath the water drop could be considered as a composite surface filled with air. Therefore, water droplets on the superhydrophobic surface are in contact with lots of air, resulting in the superhydrophobicity.32,33 The mechanical property/durability of the coated polyester fabrics was also tested for its potential practical applications. The test was carried out by dripping deionized water down to the fabric tilted by 10° with a distance of 10 cm.34 The volume of each water droplet was about 0.05 mL, and the dripping rate was 4.5 mL/min. After 1 h of dripping, the WCA data was not obviously affected. After 3 h, the WCA was only decreased from 163° to 157°. These results indicate good adhesion of the coating to the fabric surface. In addition, the as-prepared superhydrophobic fabric has strong acid and alkaline resistance. The coated fabric was immersed in an aqueous H2SO4 (pH 1) or an aqueous KOH solution (pH 14) for 24 h, rinsed with water, and dried at room temperature. As shown in Figure 7c,d, after strong acid or strong alkaline treatment, water on the fabric coated with OCMs/PDMS composites can still form a round ball. After 24 h of H2SO4 or KOH treatment, the WCA was changed to 156° ± 3.0° and 157° ± 4.0°. (In the same experiment, after 24 h H2SO4 or KOH treatment, the WCA of the CCMs/PDMS-coated polyester fabric was changed to 143° ± 4.0° and 140° ± 4.0°.) Moreover, the morphology and surface structures were not affected at all (Figure S2, Supporting Information). Therefore, the superhydrophobic
Schematic Video and Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-551-63607251. Fax: +86-551-63603005. Email:
[email protected] or
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21271163 and 21374108), Project of the National 863 Hi-Tech Plan (Grant 2008AA06Z337), Research Fund for the Doctoral Program of Higher Education of China (20133402120019), Fundamental Research Funds for the Central Universities (WK2060200012) and Recruitment Program of Global Experts is gratefully acknowledged.
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