Facile Preparation and Characterization of Modified Polyurethane

Dec 6, 2014 - Oil spills have devastating effects on the environment. Utilization of absorbents for oil spill cleanup has been practiced; however, the...
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Facile Preparation and Characterization of Modified Polyurethane Sponge for Oil Absorption Daxiong Wu, Wenjuan Wu, Ziya Yu, Canying Zhang, and Haitao Zhu* Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China S Supporting Information *

ABSTRACT: Oil spills have devastating effects on the environment. Utilization of absorbents for oil spill cleanup has been practiced; however, the development of cheap, reliable, environmentally friendly absorbents is both desirable and urgent. In this study, we illustrate a novel oil absorbent fabricated by modifying polyurethane (PU) sponges with TiO2 sol. The attachment of TiO2 nanoparticles reduced the hydrophilicity of the modified PU sponge significantly by increasing its surface roughness and changing the chemical composition of the surface. The modified PU sponges exhibited oil absorption capacity of 95−110 g/g with negligible water uptake under both static and dynamic conditions. The modified PU sponge was found to be reusable up to 12 cycles holding 70% of its initial uptake capacity. The modified PU sponges can be effectively used in oil spill cleanup.

1. INTRODUCTION In recent years, oil spills have caused long-term damaging effects on the environment upon which our society relies. Reviews on current cleanup techniques for oil spills including in situ burning, chemical dispersion, oil skimming and physical absorption can be found in the literature.1,2 The physical absorption method is a widely appreciated method for its simplicity and efficacy. Removal of oil spills with a physical absorption method requires cheap, reliable, environmentally friendly absorbents with high oil absorption capacity, oil/water selectivity and reusability.2,3 Different materials such as inorganic minerals (graphite, silica and zeolite), organic natural resources (straws, wood, cellulose and cotton) and organic synthetic products (rubber and polypropylene fibrous mats) have been utilized as absorbents that have various advantages and disadvantages. Inorganic mineral absorbents are inexpensive and available in large quantities,4−6 but suffer from high density and low absorption capacity. Organic natural absorbents are cheap, abundant, and eco-friendly,7−11 but have poor oil/water selectivity, which results in low efficiency because of high water uptake. Organic synthetic absorbents such as polypropylene fibrous mats have better oil/water selectivity, but their oil absorption capacities are relatively low.12,13 Some newly developed oil absorbents have promising performance and certain limitations as well. Electrospun polymeric fibers were introduced previously by our earlier publication and other publications as possible absorbents with extremely high oil absorption capacity, but they suffer from low recyclability.14−17 Three-dimensional nanostructured porous materials such as carbon nanotube sponges, graphene oxide foam and metal−organic frameworks have been utilized for this purpose with promising advantages such as high oil absorption capacity, high oil/water selectivity and excellent recyclability;18−26 however, they are expensive and currently exercised at lab scale. Therefore, development of simple, cheap and scalable absorbents and processes is desirable. © XXXX American Chemical Society

A polyurethane (PU) sponge is a 3D porous polymeric product with low density, high porosity, high absorption ability and good elasticity, making it a good candidate as an absorbent.27 However, the surface chemistry of PU sponges has both hydrophilic and hydrophobic functional groups that encourage absorption of oil and water. Modifications are required to adjust the surface properties for PU to increase the oil−water selectivity.28−32 In a recently published work, reduced graphene oxide has been introduced to the PU sponge to improve the oil/water selectivity along with high oil absorption capacity and good reusability.28 A PU sponge modified with a carbon nanotube/polydimethylsiloxane (CNT/ PDMS) composite has also been reported to illustrate superhydrophobic/superoleophilic properties.29 Chemical etching followed by surface modification has been adopted to enhance hydrophobicity of the PU sponge. Introduction of metal nanoparticles along with thiol modification after chemical etching was proposed and found advantageous for the preparation of a superhydrophobic PU sponge.30 A solution of CrO3 and H2SO4 has been reported beneficial for the etching of a PU sponge followed by modification using polysiloxane to prepare superhydrophobic oil absorbents.31 The produced porous and rough structures are found to be responsible for the observed properties of the absorbents, which was also found to be encouraged by administration of nanostructured particles.32,33 In the current study, we propose a facile approach to prepare oil absorbents by modifying PU sponges with TiO2 sol. TiO2 nanoparticles have been emplozyed to modify the surface morphology and surface chemical composition of PU sponges, which has been proven to be beneficial for oil uptake. The modified PU sponge was characterized and found to have high absorption capacity, oil/water selectivity and reusability. Received: August 13, 2014 Revised: December 6, 2014 Accepted: December 6, 2014

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2. EXPERIMENTAL SECTION 2.1. Materials. PU sponges with a porosity of 98.5% were purchased from Duxiu Commodity Factory of Jinhua (Zhejiang province, China). TiO2 sol synthesized from the hydrolysis of organic titanate through a sol−gel method was supplied by Fuzhou University (Fujian province, China). Crude oil was supplied by Qingdao Petroleum Refinery of SINOPEC (Shandong province, China). Peanut oil, motor oil, gasoline and diesel oil were collected from the local service station. 2.2. Preparation and Characterizations of the Absorbents. In a typical preparation procedure, initial PU sponge blocks (45 × 20 × 15 mm) were soaked in TiO2 sol for 30 min, then centrifuged to remove liquid and then dried at 60 °C to get the modified PU sponge. The obtained samples were named as S175, S14, S6 and S2 in accordance to the TiO2 sol mass fractions of 0.175%, 0.014%, 0.006% and 0.002%. The morphology of the samples was studied with scanning electron microscopy (SEM, FESEM-6700). The contact angles of the prepared absorbents were measured using an OCA20 contact angle system. The energy-dispersive X-ray spectroscopy (EDS) measurements were recorded on an OXFORD INCA Energy Dispersive Spectrometer. XPS analysis was conducted on an ESCALAB 250 X-ray photoelectron Spectrometer. 2.3. Oil Absorption Capacity and Reusability Tests. All absorption capacity tests were conducted at 20 ± 2 °C. For each test, an average of five experimental runs was reported. 800 mL of water was added to a 5 L container with appropriate oil to form a 2−6 mm film. Motor oil, diesel oil, gasoline, crude oil and peanut oil were employed. Under dynamic conditions, the weighed absorber (the mass was noted as ms (g)) was added in block form and stirred at 500 rpm for 60 min and then taken out and allowed to drain for 2 min. The absorbed liquid was recovered by pressing the absorbent and the recovered liquid was centrifuged according to the standard method D4007-81 (ASTM-1998a) to separate water from oil. Mass of the separated oil and water were weighed and noted as moil (g) and mw (g). The oil absorption capacity Qoil (g/g) and the water uptake Qw (g/g) was calculated using eq 1. Q oil = moil /ms ;

Q w = m w /ms

Figure 1. Absorption capacities of the unmodified PU sponge and the modified PU sponges (S2, S6, S14 and S175).

(1)

Under static conditions, the experiment was conducted without stirring. Reusability of the absorbents was evaluated via a repeating absorption-pressing process and measuring the absorption capacity of each cycle.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Unmodified PU Sponge. The absorption capacity of the unmodified PU sponge (labeled as PU) under both static and dynamic conditions with 3 mm oil film (motor oil) on water surface is illustrated in Figure 1. The oil absorption capacity and water uptake of the unmodified PU sponge were found to be 51.36 and 66.96 g/g under static conditions, and 32.52 and 84.32 g/g under dynamic conditions. As it can be seen the unmodified PU sponge has a high uptake capacity for oil and water; however, it does suffer from lack of selectivity. The 3D porous structure of the unmodified PU sponge is thought to be responsible for the uptake capacity. Scanning electron microscopy was utilized to study the morphology of the unmodified PU sponge. As it can be seen from the SEM images (Figure 2a), the sponge has a three-dimensional honeycomb-web-like structure that is well intact with large

Figure 2. SEM images of the internal structure (a) and fiber surface (b) of the unmodified PU sponge. SEM image of the internal structure (c) of sample S175. SEM images the fiber surface of sample S175 (d), S14 (e), S6 (f) and S2 (g).

void space. The poor selectivity is thought to be due to its surface chemistry, structure and properties. Fourier transform infrared spectroscopy (FTIR) characterization was conducted and peaks at 3405, 1165, 1559 and 2930 cm−1 can be correlated to NH, CO, CO and CH bonds, respectively (Figure 3). These functional groups endow the PU sponge with hydrophilic characteristic and encourage water uptake. Wettability of unmodified PU sponge was investigated revealing that the water droplet penetrated in the unmodified PU sponge within less than one second, suggesting its hydrophilic property (the details will be discussed in section 3.4, Oil Absorption B

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agreement with the reported values of TiO2,34 indicating that TiO2 has been coated on the sponge surface by possible secondary bonding. XPS analysis (Figure 4c,d) of the modified PU was also conducted after exposure to motor oil (the absorbed oil had been removed) and the results show little difference to those before exposure to motor oil (Figure 4a,b). This result suggests that there is no chemical reaction but reversible physical contact between oil and the surface of the modified PU sponge. The TiO2 surface has been reported to have certain interactions with hydrophilic function groups.35 The interaction between TiO2 surface and the function groups (−NH, CO and CO) on PU sponge surface is regarded as a secondary bonding. This secondary bonding helps the TiO2 nanoparticles to attach on the PU surface and further changes the functional group composition on the surface of the PU sponge, which is also approved using FTIR (Figure 3). The FTIR spectrum of the modified PU sponge illustrates that the intensity of the C O and CO stretching has decreased significantly in comparison to that of the unmodified PU sponge, which suggests a decrease in polarity and thus an increase in hydrophobicity. As it is also evident from the SEM images of the modified PU sponge (Figure 2 d−g), the dispersion of TiO2 nanoparticles has provided nanostructures similar to those on lotus leaves. Such nanostructures are regarded as the main reasons of the surper-hydrophobicity of lotus leaves.36−38 Contact angle measurements were conducted to illustrate the hydrophobicity of the modified PU sponge. The water contact angle for sample S2 was determined to be 117.5°. As the mass fraction of TiO2 sol increased, the water contact angle increased slightly to 122.8° for sample S6 and then became a plateau as the mass fraction of TiO2 sol further increased (results are summarized in Table S1, Supporting Information). The water contact angle with hysteresis for sample S2 (Figure S2 and Table S2, Supporting Information) showed an advancing contact angle of

Figure 3. FTIR spectra of the unmodified PU sponge, TiO2 sol and the modified PU sponge.

Mechanism). Surface modification is required to improve the oil/water selectivity of the unmodified PU sponge. 3.2. Characterization of the Modified PU Sponge. Morphological study was conducted on the modified PU sponge, and the results clearly illustrate that the bulk 3D structure of the PU sponge is standing while the fibers are found to be exercising a high degree of roughness (Figure 2g). The roughness is thought to be due to the presence of TiO2 nanoparticles. As it can be observed, the TiO2 nanoparticles are well dispersed on the PU sponge fiber surface. Presence of Ti species along with original C, O and N that were identified on the unmodified PU sponge was detected on the modified PU sponge using EDS (Figure S1, Supporting Information), clearly illustrating successful dispersion of TiO2 nanoparticles. Further XPS analysis on the modified PU sponge also confirmed the presence of C, O, N and Ti elements (Figure 4a). The highresolution XPS spectrum of the Ti 2p peaks (Figure 4b) shows that the Ti 2p1/2 and Ti 2p3/2 peaks are centered at binding energies of 464.6 and 459.3 eV, respectively. The results are in

Figure 4. XPS survey spectra and the high-resolution XPS spectra of the Ti 2p peaks of sample S2 before (a, b) and after (c, d) oil absorption. C

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120.0°, a receding contact angle of 115.0° and a hysteresis of 5.0°. The water sliding angle for sample S2 was measured to be 12.0° (Figure S3, Supporting Information), but the water sliding angle for the unmodified PU sponge could not be recorded, as the water droplet penetrated quickly into the sponge. In real applications, the absorbents may be exposed to sunlight. Thus, sample S2 was exposed to the illumination of a solar simulator (xenon lamp) to investigate the effect of light on the water contact angle. The results indicate that the illumination of xenon lamp does not have significant influence on the water contact angle (Table S3, Supporting Information). It seems that the hydrophobicity of the modified PU sponges is quite stable under sunlight illumination. On the other hand, an oil droplet penetrated rapidly into the modified PU sponges within less than 1 s (the detailed discussion is in section 3.4, Oil Absorption Mechanism). Thus, the oil/water selectivity of the modified PU sponge can be expected to be improved in comparison to that of the unmodified PU sponges. 3.3. Absorption Property of the Modified PU Sponges. The absorption capacity and oil/water selectivity of the modified PU sponges were measured under both static and dynamic conditions with 3 mm motor oil film (Figure 1). Under static conditions, the oil absorption capacities of sample S2, S6, S14 and S175 were measured to be 105.1, 104.8, 93.6 and 95.7 g/g (the errors are 2−5 g/g), respectively. Under dynamic conditions, the oil absorption capacities of the samples were measured to be 103.2, 96.3, 101.6, and 99.8 g/g, respectively. The water uptakes of all samples were found to be less than 0.5 g/g under both static and dynamic conditions. As showed in Figure 1, the water uptake of the modified PU sponge decreased and the oil/water selectivity improved significantly in comparison to that of the unmodified PU sponges. A minor decrease in water uptake (0.4 to 0.15 g/g) can also be found when the mass fraction of TiO2 sol increases, which should be regarded as another evidence for the contribution of TiO2 sol to the hydrophobicity of the modified sponges. However, as most of the fiber surface of the sponge is covered with TiO2 nanoparticles when the mass fraction of the TiO2 sol is 0.002% (Figure 2g), further increase in the mass fraction has minor contribution to the oil absorption capacity and oil/water selectivity of the modified sponges. Thus, further studies were conducted on sample S2 taking into account the effects of oil species, the thickness of oil film, etc. Under static conditions, the oil absorption capacities of S2 for motor oil, peanut oil, gasoline, diesel and crude oil were measured to be 105.1, 102.1, 103.2, 87.4 and 114.9 g/g (the error are 2−5 g/g), respectively (Figure 5). Under dynamic conditions, the oil absorption capacities for the corresponding oils were determined to be 103.2, 91.9, 100.1, 90.5 and 105.0 g/ g, respectively (Figure 5). In contrast, the water uptakes were found to be less than 0.5 g/g under both static and dynamic conditions. The above results indicate that the modified PU sponges are applicable to a broad variety of oils with various density, viscosity and surface tension in respect to oil/water selectivity and oil absorption capacity. As the absorption capacities of sample S2 are almost the same under both static and dynamic conditions (Figure 5), further experiments were conducted under static conditions to investigate the influence of the thickness of oil film on the absorption capacity of sample S2 (Figure 6). The oil absorption capacity was found to increase slightly from 105 to 114 g/g when the oil film thickness increased from 2 to 6 mm. The water uptake was determined to be no more than 0.5 g/g,

Figure 5. Absorption capacities of S2 sample for different kinds of oils.

Figure 6. Absorption capacities of sample S2 at different thickness of oil film.

regardless of the oil film thickness. The results show that the modified PU sponges are advantageous for the removal of both thick oil film and thin oil film. Reusability is an important parameter to oil absorbents. In real applications, oil saturated absorbents can be mechanically pressed to collect absorbed oil and then the recovered oil absorbents are ready for the next absorption operation. In the Experimental Section, an absorption-pressing procedure was repeated many times under identical conditions (3 mm motor oil film on the water surface under static conditions) to evaluate the reusability of sample S2. The varied oil absorption capacities of sample S2 in different cycles of absorptionpressing process are shown in Figure 7. The oil absorption capacity was found to decrease gradually as the absorption− pressing process repeated. After 12 cycles, the oil absorption capacity was measured to be 75 g/g, which is more than 70% of the initial absorption capacity. It should also be noted that water pick-up in all cycles was negligible. In real practice, the total amount of oil recovered from all cycles can be regarded as the accumulated absorption capacity of the absorbent. In this case, the accumulated absorption capacity of sample S2 can be easily calculated to be 1050 g/g when modified PU sponge is successively reused for 12 cycles. In other words, 1 ton of the modified PU sponge is enough for the cleanup of 1000 tons of spilled oil. 3.4. Oil Absorption Mechanism. Modification with TiO2 sol changes PU sponges from being hydrophilic to hydrophobic, thus improving the oil/water selectivity significantly. To D

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droplet penetrates easily into the modified PU sponge (see video,Supporting Information). The modified PU sponge absorbs oil when immersed in oil. After the removal of the absorbed oil, a water droplet can stand on the surface of the modified PU sponge while an oil droplet penetrates quickly (see video, Supporting Information). It is quite clear that the oil absorption process will be dominant when the modified PU sponge is put at an oil−water interface, no matter if it interacts first with water or oil. As shown in the video (Supporting Information), the modified PU sponge can easily clean up all the oil at the oil−water interface under dynamic conditions.

4. CONCLUSIONS In summary, we reported hydrophobic oil absorbents prepared by modifying PU sponges with TiO2 sol. The modified PU sponges have the advantages of high oil absorption capacity, high oil/water selectivity, good reusability and low-cost. The absorption capacity of the modified PU sponges was measured to be as much as 100 times of its own weight. The modified PU sponges can be recovered by a simple mechanical pressing process and recycled for many times. The modified PU sponges are promising in potential applications as absorbents for oil spill cleanup. In addition, the study and discussion of oil absorption mechanism provides some insight for the design and development of future oil absorbents.

Figure 7. Oil absorption capacities of sample S2 in different cycles of absorption−pressing process.

further understand the improvement in the oil/water selectivity of the modified PU sponges, the dynamic absorbing process was investigated. This configuration may allow us to observe the status of oil and water droplets on the sponges’ surface over time. The dynamic absorbing process at an oil−water interface can be considered as a competition between water absorption process and oil absorption process. As the results clearly illustrate (Figure 8a), a water droplet can penetrate into the

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ASSOCIATED CONTENT

S Supporting Information *

EDS spectra; images of water sliding angle; detailed data of water contact angle measurements; videos showing the oil absorption process and the wetting properties of the samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Professor Haitao Zhu. Tel: +86 532 84022676. Fax: +86 532 84022814. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 8. Dynamic absorbing process of water (a) and oil (b) on the unmodified PU sponge. Dynamic absorbing process of water (c) and oil (d) on sample S2.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51172117, 51472134), Natural Science Foundation of Shandong Province (ZR2013EMM003) and Foundation of Qingdao Science and Technology (13-1-4-148jch).

unmodified sponge in less than 1 s. In comparison, a droplet of motor oil penetrates completely into the unmodified PU sponge after 12 s (Figure 8b). After modification, the surface morphology and structure as well as the surface chemical composition of the sponges change significantly, as revealed by the SEM image (Figure 2), XPS spectra (Figure 4) and EDS spectra (Figure S1, Supporting Information). As a result, the wetting properties of the sponges change accordingly. A water droplet can stand still on the surface of the modified PU sponge without penetrating into the sponge (Figure 8c). When a droplet of motor oil is put on the surface of the modified PU sponge, it penetrates into the sponge after 8 s (Figure 8d). The penetrating speed of motor oil droplets increases after modification. When the modified PU sponge is immersed in water, it does not absorb water spontaneously, but it will suck a certain amount of water when it is pressed. After the removal of the sucked water, a water droplet can still stand on the surface of the modified PU sponge without being absorbed, while an oil

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