ZnO Hybrid Nanocomposite with Excellent UV

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A Novel Lignin/ZnO Hybrid Nanocomposite with Excellent UV-Absorption Ability and Its Application in Transparent Polyurethane Coating Huan Wang, Xueqing Qiu, Weifeng Liu, Fangbao Fu, and Dongjie Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02425 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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Industrial & Engineering Chemistry Research

A Novel Lignin/ZnO Hybrid Nanocomposite with Excellent UV-Absorption Ability and Its Application in Transparent Polyurethane Coating Huan Wang†, Xueqing Qiu*,†,‡, Weifeng Liu†, Fangbao Fu†, Dongjie Yang*,† †

School of Chemistry and Chemical Engineering, South China University of

Technology, Guangzhou, 510640, China. ‡

State Key Lab of Pulp and Paper Engineering, South China University of Technology,

Guangzhou, 510640, China. ABSTRACT In this work, lignin/zinc oxide nanocomposites with excellent UV-absorbent performance were prepared through a novel hydrothermal method using industrial alkali lignin (AL) as raw materials. AL was firstly modified by quaternization to synthesize quaternized alkali lignin (QAL). The QAL/ZnO nanocomposites with different lignin contents was then prepared via a facile one-step hydrothermal method using QAL and zinc nitrate hexahydrate and hexamethylenetetramine in aqueous solution. The prepared nanocomposite possessed an average diameter of ~100 nm and showed excellent synergistic UV-absorbent performance. The particle morphology and hybrid structure were carefully characterized by SEM, TEM, XRD, FT-IR, XPS, UV-Vis, and TG analyses. Interestingly, it was found that the UV transmittance of polyurethane (PU) film was significantly reduced and the mechanical properties of the PU were significantly enhanced when blended with the prepared QAL/ZnO nanocomposite.

Results

of

this

work

were

of

practical

importance

for

high-value-added application of industrial lignin in the field of functional materials. Keywords: Quaternized alkali lignin; ZnO; Hybrid nanocomposites; UV-absorption; Polyurethane. 1. INTRODUCTION Increasing concern about the energy crisis and climate change has accelerated the development of sustainable materials from renewable resources. The high value-added utilization of abundant renewable lignocellulosic biomass derived from

ACS Paragon Plus Environment


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plant sources has attracted hot attention.1-3 Lignin is the second most abundant bioresource occupying approximately 15-25% by weight in woods on earth.4-6 Each year, over 50 million tons of industrial lignin is produced from pulping and paper making industry and biorefineries as a byproduct.7, 8 However, more than 98% of lignin is combusted as fuel, which not only causes huge waste of the resources, but also causes environmental problems.9-11 Therefore, the research on high-value-added application for industrial lignin is considerably significant for both the development of renewable resources and environmental protection. In recent years, lignin-based nanomaterials have attracted wide academic and industrial interest.12-15 For example, Nair et al.13 reported a kind of lignin nanoparticles by high shear homogenization using kraft lignin particles. The thermal stability of the polyvinyl alcohol could be increased when blended with the prepared lignin nanoparticles. Qian et al.14 reported a lignin-based colloidal nanospheres prepared by acetylated lignin through self-assembly technique. And the prepared lignin nanosphere has a potential application in the field of sunscreen. Bula et al.15 reported a novel functional silica/lignin hybrid material obtained using a process of mechanical grinding of precursors. The elongation at break and notched impact strength of polypropylene could be improved by doping with the prepared silica/lignin hybrid. Organic-inorganic hybrid materials with structural stability, good compatibility, synergies and other advantages have received wide research in recent years.16, 17 The preparation of lignin/inorganic nano-composites provides a new approach for high-valued application of industrial lignin. As an important member of inorganic materials family, Nano-zinc oxide (ZnO) has been widely used as UV photodetector, optoelectronics and photocatalyst due to its exceptional optical, electrical and environmental friendliness.18-22 On the other hand, lignin also shows excellent UV-absorbency properties.14,

22

And according to the findings, lignin has been

described as a random, three-dimensional network natural polymer composed of phenol hydroxyl, methoxyl and carboxyl active functional groups, which is beneficial to form neat uniform composite structure with inorganic nanoparticles.23


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Nano-composites can potentially combine the advantages of the components to access complementary properties and synergetic effects.24, 25 However, the pure alkali lignin is easily aggregated in neutral aqueous solutions due to its poor solubility. This results in that it can’t be uniformly dispersed in the water-soluble polyurethane.13,14 The polyurethane/graphene/lignin film was also reported due to its synergy of ultraviolet absorption function.26 The lignin/ZnO hybrid nanocomposites may also have unexpected excellent UV optical properties and may combine the toughness of lignin and stiffness of ZnO. However, to the best of our knowledge, the preparation of lignin/ZnO hybrid nanocomposites has not been reported so far. Owing to its renewability, abundance and low-cost, lignin is considered as a promising functional additive in polymers.27 Polyurethane (PU) is an important polymer material with wide applications in many fields such as foams, coating, elastomers and so on due to its noncombustible, non-toxic and pollution-free.28-30 However, pure polyurethane exhibits poor UV-stability. As the functional groups in lignin could form hydrogen bonding interaction with the urethane groups, blending polyurethane with lignin or lignin-based derivatives is a facile way to improve the UV and thermal stability and also the mechanical properties of polyurethane.26 The development of environmentally friendly, easy-doped and low-cost functional filler for PU based on lignin is a meaningful and high value-added utilization for lignin. In our previous study, lignin/silica composite was prepared from alkali lignin and sodium silicate in aqueous solution with a direct co-precipitation method.31 In this work, a series of novel lignin/zinc oxide (QAL/ZnO) hybrid nanoparticles were successfully synthesized by a facile one-step hydrothermal method. The microstructure and morphology of the prepared QAL/ZnO hybrid nanoparticles were well characterized. On the other hand, the prepared QAL/ZnO nanoparticles were applied in aqueous polyurethane coating. Results showed that the mechanical properties and UV-absorption properties of the PU were significantly improved. Furthermore, the anti-UV aging performance of PU blended with QAL/ZnO was also investigated in detail.



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2. EXPERIMENTAL SECTION 2.1 materials Pine alkali lignin from pulping black liquor supplied by Shuntai Technology Development Co., Ltd. (Hunan province, China) was purified as follows: the pH value of the pine pulping black liquor was adjusted to ~3 using aqueous sulfuric acid (~30wt %). The mixed liquor was stirred for 2 h at 55℃ to give the pine alkali lignin. Other reagents were

analytical grade,

such as zinc nitrate

hexahydrate

(Zn(NO3)2·6H2O) and hexamethylenetetramine (HMT) were purchased from Alfa Aesar and used as received. Sodium hydroxide (Guangzhou Guanghua Sic-Tech Co., Ltd., China) and sulfuric acid (Guangzhou Chemical Reagent Factory, China) were purchased and used without any further purification. 2.2 Synthesis of quaternized alkali lignin (QAL) 36.10 g alkali lignin was dissolved in 200 mL sodium hydroxide aqueous solution (20wt%), then 30.05 g 3-Chloro-2-hydroxypropyltrimethyl ammonium chloride was dissolved in 100 mL distilled water and the formed solution was drop-added into the above lignin solution by a peristaltic pump at a speed of ~2 rpm under stirring. After about 15 min, 16.0 ml sodium hydroxide aqueous solution (20 wt %) was added into the above lignin solution and it was stirred for another 4h at 85 ℃ to give the QAL solution, the synthetic pathway is shown in Figure S1. Finally, the obtained QAL solution was purified by dialysis and then freeze dried to give QAL powder. The elemental and functional group content analysis was shown in Table S1. And The 1H MNR and FT-IR spectra of AL and QAL were shown in Figure S2 and Figure S3. It shows that the QAL which contains the quaternary ammonium group was successfully synthesized. 2.3 Synthesis of QAL/ZnO hybrid nanocomposites The QAL/ZnO hybrid nanocomposites were synthesized by a facile one-pot hydrothermal method. Typically, 11.89 g Zn(NO3)2·6H2O and 5.61 g HMT was dissolved in 1000 mL distilled water, and then various weight (0.5 g, 1.0 g, 1.5 g) of


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QAL powder was added into the solution and stirred for 30 min to give the liquid precursors of QAL/ZnO. After that, the mixture was kept at a temperature of 130 ℃ for ~4 h, and then cooled to room temperature to collect the precipitation. Finally, the precipitation was rinsed with distilled water and dried in an oven at ~50 ℃ for ~24 h to give the QAL/ZnO hybrid nanocomposites. The resultant samples synthesized with 0.5 g, 1.0 g, 1.5 g of QAL are denoted as QAL/ZnO-0.5, QAL/ZnO-1.0, QAL/ZnO-1.5, respectively. The pure ZnO was synthesized without QAL by the same process. An overview of the synthetic procedure for QAL/ZnO hybrid nanocomposites is shown in Figure 1.

Figure 1. Schematic for the synthetic procedure of ZnO and QAL/ZnO. 2.4 Characterization of QAL/ZnO The prepared samples of ZnO and QAL/ZnO were tightly packed into the sample holders, X-ray diffraction (XRD) patterns of the samples were recorded by a powder X-ray diffractometer (D8 Advance, Bruker, Germany) with Cu Kα wavelength of 0.1541 nm at 40 kV and 40 mA. The micromorphology and microstructure of the samples were examined by scanning electron microscope (SEM, Merlin, Carl Zeiss, Ltd., Germany). The element content of the AL and QAL was measured by an elemental analyzer (VARIO EL Ⅲ, Elementar, Germany). The phenolic hydroxyl content of lignin samples was detected by FC method. High-resolution transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectrum and element



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mapping images were recorded by an electron microscopy instrument (JEM-2100F, JEOL, Japan). Fourier transform infrared spectroscopy (FT-IR, VERTEX 70, Bruker, Germany) was used to confirm the effective mode of binding between ZnO and lignin. Thermal gravimetric analysis (TG) of the GR/ZnO nanocomposite was carried out by a simultaneous thermal analyzer (STA449 F3, Netzsch, Germany) in an air atmosphere and the temperature was scanned from 25 ℃to 800 ℃ at a heating rate of 10 ℃·min-1. The contact angle of distilled water on ZnO, QAL and QAL/ZnO was measured by a Power Each JC2000C1 static contact angle measurement instrument (Shanghai Zhongchen Digital Technic Apparatus Co., Ltd., China). All the sample disks were successively pressed at 15 MPa for 1 min before the measurement of contact angle. 2.5 Preparation and Characterization of PU-QAL/ZnO and PU-ZnO films The prepared samples of QAL/ZnO hybrid nanocomposites were blended with polyurethane (PU) to make the PU-QAL/ZnO film. Firstly, a certain amount of QAL/ZnO was pre-dispersed in 20 mL deionized water with assistance of sonication. And then, the dispersion was blended with 50 mL aqueous polyurethane solution at ~600 rpm for ~4 h. After that, a certain volume of the blended aqueous polyurethane solution was placed in a plastic petri dish at ~45 ℃ for several hours to give the PU-QAL/ZnO film with a thickness of 0.45 mm. PU-ZnO film was prepared in the same way. The UV-transmittance of the PU-QAL/ZnO and PU-ZnO was measured by Shimadzu UV-2600. The UV Protection Factor (UPF), Average Transmittance values of UVB Light (T(UVB)AV) and UVA light (T(UVA)AV) were calculated using the following equation according to profiles of UV Standard 801:

= ∑

⁄∑

(1)

∑

()

(2)

() = ∑ ()

(3)

() =


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where Eλ = erythema spectral effectiveness, Sλ = solar spectral irradiance, Tλ = spectral transmittance of sample films, λ = wavelength, m = the number of measurements between 315-400 nm, and n = the number of measurements between 290-315 nm. The uni-axial stretching performance of the PU-QAL/ZnO and PU-ZnO films with different amount of QAL/ZnO and ZnO were measured by a universal testing machine (CMT, MST, U.S.A.) at room temperature with a stretching rate of 15 mm·min-1. The anti-UV aging performance of the PU-QAL/ZnO and PU-ZnO films was investigated by irradiation under a high-powered UV lamp (300 W) for several hours and then submitted to uni-axial stretching test. 3. RESULTS AND DISCUSSION 3.1 Preparation and characterization of QAL/ZnO The prepared samples of ZnO and QAL/ZnO nanocomposites were first characterization by FT-IR analysis, as shown in Figure 2. In the spectrum of ZnO, the wide absorption band around 400-600 cm-1 was attributed to the stretching vibration of Zn-O-Zn and O-Zn-O. In the spectra of QAL/ZnO nanocomposites, the absorption bands belonging to QAL and ZnO appeared obviously. The peaks at 1598 cm-1 and 1435 cm-1 were assigned to the stretching vibration of C-C bonds of aromatic skeleton in QAL.32 Furthermore, the peaks around in QAL/ZnO composites were different to the pure ZnO, which was due to the strong interaction between ZnO and QAL,33, 34 suggesting that QAL was successfully combined with ZnO.



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Figure 2. FT-IR spectra of the as-prepared ZnO and QAL/ZnO nanocomposites. To further investigate the bonding force between ZnO and QAL in the nanocomposites, the high resolution C 1s XPS spectrum was recorded. Figure 3 shows the C1s XPS spectra of QAL and QAL-ZnO-1.0 nanocomposite. The C1s XPS spectrum of QAL could be decomposed into two sub-peaks: the peak at 284.2eV was attributed to the C-C and C-H groups, while the peak centered at 286.0 eV was assigned to the C-OH and C=O groups.37 For the sample of QAL-ZnO-1.0, the peak assigned to the sp2 carbon atom had a slightly right shift from 284.2eV to 284.5 eV in comparison with the spectra of QAL, the peak position of sp3 carbon atom had little change but the peak intensity decreased obviously. Meanwhile, a third significant peak located at 287.6eV was observed in the spectrum of QAL-ZnO-1.0, which is attributed to the Zn-O-C bond species.35 36 Furthermore, the N 1s XPS of the QAL and QAL/ZnO-1.0 were also measured, as shown in Figure S4. It shows that the covalent bond of Zn-O-N also exist at the interface between QAL and ZnO. These results indicated the successful combination of QAL and ZnO via covalent bonds, which accounts for the tight combination of lignin and ZnO.


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Figure 3. C 1s XPS of the as-prepared QAL (a) and QAL/ZnO-1.0 (b). According to the above results of FI-IR and XPS, it could indicate that there were COOH groups on lignin side chain and they could interact with Zn and form Zn-O-C linking. On the other hand, the quaternary ammonium groups could interact with Zn to form Zn-O-N bonds in the formation of QAL/ZnO hybrid. The hydrophilicity of ZnO, QAL and QAL/ZnO was compared by static contact angle measurement, the result was shown in Figure S5. The contact angle on ZnO disk was only 12.5°, suggesting strongly hydrophilic surface of ZnO. QAL possessed



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the contact angle of 40°, exhibiting weaker hydrophilicity than ZnO. The hydrophilicity of of QAL/ZnO nanocomposites fell between the pure ZnO and QAL and decreased gradually as QAL content increased. Additionally, the contact angles of the QAL/ZnO nanocomposites were much greater than that of the pure ZnO, indicating that the hydrophilic groups in QAL have been well combined with the active hydroxyl groups on the surface of ZnO mainly through electrostatic adsorption and a small amount of hydrogen bonding, resulting in the enhanced hydrophobicity of QAL/ZnO surface, which was good for improving the compatibility between QAL/ZnO nanoparticles and polymer materials. Figure 4 shows SEM images of the prepared samples. The micromorphology of ZnO presents typical nanoflower structure composed of many nanorods with the average diameter and length of 680 nm and 7.2 µm, respectively, as clearly shown in Figure 4(a). After hybridized with lignin, the rod crystal structure of ZnO was inhibited as little nanorods existed in Figure 4(b) and the nanorod structure totally disappeared but the particle morphology appeared in Figure 4(c & d) when the QAL content increased. The surface of all the QAL/ZnO nanocomposites was rough, significantly different from the crystal surface of pure ZnO, indicating that ZnO has been successfully hybridized with the modified lignin. The size of QAL/ZnO nanoparticles decreased with the increasing content of QAL. The average diameters of QAL/ZnO-1.0 and QAL/ZnO-1.5 were about ~100 nm as shown in Figure 4(c & d). The nanoscale size of the QAL/ZnO composite is favorable for its application as functional filler which integrates the advantages of ZnO and lignin. On the other hand, the AL/ZnO composite was also synthesized using alkali lignin without modification. The SEM images were shown in Figure S6. Addition of AL in the precursor solution had great influence on the crystal morphology of ZnO, but the lignin has not been integrated with ZnO to form the Lignin-ZnO nanocomposite. The key factor is the introduction of positively charged quaternary ammonium groups to the AL. In the formation process of QAL/ZnO nanocomposite, the positively charged groups in QAL could attract the hydroxyl ion (OH-), and the negatively charged groups in QAL could attract the zinc ion (Zn2+), which facilitated


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the formation of QAL/ZnO hybrid nanocomposites.

Figure 4. SEM images of the pure ZnO (a) and QAL/ZnO nanocomposites synthesized with different weights of QAL: (b) QAL/ZnO-0.5; (c) QAL/ZnO-1.0; (d) QAL/ZnO-1.5. The microstructures and morphologies of the prepared QAL/ZnO hybrid nanocomposites were further characterized by a high magnification TEM, as shown in Figure 5. Figure 5(a) shows that QAL/ZnO-0.5 was typical nanorod structure, but as the QAL content increased, the composite displayed typical particle morphology shown in Figure 5(c & e). The lattice fringe of ZnO and the amorphous lignin are obviously observed in the high-resolution TEM in Figure 5(b, d & f). The QAL/ZnO composite nanoparticle was composed of many small ZnO nanoparticles and lignin, indicating that QAL was homogeneously hybridized with ZnO.



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Figure 5. TEM images of the prepared QAL/ZnO nanocomposites synthesized with different weights of QAL: (a&b) QAL/ZnO-0.5; (c&d) QAL/ZnO-1.0; (e&f) QAL/ZnO-1.5. The element composition and distribution of the QAL/ZnO-1.0 were further investigated by EDX spectrum and element mapping images, the results were shown in Figure 6. From the EDX spectrum (Figure 6b), one can see that the QAL/ZnO-1.0 hybrid nanocomposite was composed of N, C, O and Zn elements, which are ascribed to QAL and ZnO, respectively. The element of Cu was from the Cu object stage. Figure 6(c-f) shows the corresponding element mapping images of N, C, O and Zn, respectively. The size of all element distribution area coincided with the size of QAL/ZnO-1.0 particle, which again demonstrated that the prepared QAL/ZnO


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composite nanoparticle possessed typical hybrid nanostructure, rather than a core-shell or other structure.

Figure 6. TEM image (a) and EDX (b) of the QAL/ZnO-1.0, corresponding element mapping images of N (c), C (d), O (e) and Zn (f). Thermal gravimetric (TG) analysis was conducted to investigate the content of QAL in the prepared samples of QAL/ZnO nanocomposites. Figure 7 shows the TG curves of the prepared samples. The weight loss of ~0.67% from 25 to 800℃for ZnO nanoparticles was attributed to the water content. The weight loss of QAL/ZnO-0.5, QAL/ZnO-1.0 and QAL/ZnO-1.5 was 10.15%, 16.6%, 25.4%, respectively. The weight loss value approximately represented the QAL loading content in the QAL/ZnO nanocomposite and increased with the increasing concentration of QAL in the precursor solution. However, the yield for the synthesis of the nanocomposites decreased as the QAL concentration increased in the precursor solution. When the weight of QAL in the precursor solution was added up to 2.0 g/L, little product was obtained. The reason could be attributed to the three-dimensional network structure of lignin in the dissolved solution state, which restrained the self-assembly and crystal growth of ZnO when lignin concentration was too high.



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Figure 7. TG curves of the as-prepared ZnO and QAL/ZnO nanocomposites. The crystal structure of QAL/ZnO nanocomposites was studied through the XRD analysis. Figure 8 clearly shows all the QAL/ZnO nanocomposites had similar XRD patterns, which were the same as that of the prepared pure ZnO nanoflowers and could be readily indexed to the hexagonal wurtzite ZnO (JCPDS data card 36-1451). The diffraction peaks at 2θ = 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, 66.5°, 68.0° and 69.1°can be indexed to (100), (002), (101), (102), (110), (103), (200), (112) and (201) crystal planes of ZnO, respectively. The addition of QAL in the precursor solution did not change the crystal cell structures of ZnO. However, the intensity of the diffraction peaks decreased as the QAL content increased in the QAL/ZnO nanocomposite, suggesting the addition of QAL in the precursor solution had great influence on the microtopography and the particle size of ZnO. For quantitative evaluation, the most intense diffraction peak of (101) plane was used to calculate the crystallite size, D, using the Scherrer formula:37


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D=

κλ β cosθ

(4)

Where κ is the shape factor, is the wavelength of X-ray, is Bragg’s angle, and is the full width at half maxima height. Here, κ and are taken as 0.9 and 0.15405 nm respectively. The calculation result was listed in Table 1. The crystallite size of ZnO in the QAL/ZnO nanocomposite was smaller than that of the pure ZnO, and decreased with the increasing weight ratio of QAL.

Figure 8. XRD patterns of the pure ZnO and QAL/ZnO nanocomposites. Table 1 Structural parameters obtained from the XRD patterns shown in Figure 8. Sample Name

(hkl)

2θ(°)

FWHM(°)

Crystallite size (nm)

ZnO

101

36.3

0.418

23.6

QAL/ZnO0.5

101

36.3

0.476

20.7

QAL/ZnO1.0

101

36.2

0.534

18.9

QAL/ZnO1.5

101

36.2

0.546

18.1

The optical property of the as-prepared ZnO, QAL and QAL/ZnO has been characterized by UV-vis diffuse reflectance spectra, as shown in Figure 9. QAL exhibited strong and broad light absorption including the UV and visible light regions



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from 200-800 nm. ZnO showed typical intense light absorption in UV region with little absorption in the visible light region. The high UV absorption of ZnO was due to the electron promotion from the valence band (VB) to the conduction band (CB). Interestingly, the samples of QAL/ZnO nanocomposites exhibited stronger UV light absorption than pure ZnO and QAL, demonstrating a synergetic enhancement effect of UV absorption in the QAL/ZnO nanocomposite. The reason may be that the introduction of QAL facilitated the separation of the photogenerated electron-hole pairs in ZnO. When ZnO was irradiated under the UV light, the photogenerated electrons in ZnO would transfer to the lignin molecules under the withdrawing effect of the unsaturated bonds in lignin,38 which was favor for the UV absorption. Additionally, in the visible light region from 400-800 nm, the samples of QAL/ZnO nanocomposites showed gradually enhanced light absorption intensity with the increasing weight content of QAL. The enhanced absorption of UV and visible light is favor for its application as optical functional filling materials with UV resistant and anti-aging performance.

Figure 9. UV-vis diffuses reflectance spectra of the pure ZnO, QAL and QAL/ZnO.


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3.2 Application of QAL/ZnO in polyurethane As polyurethane is a common used polymer in many fields such as foams, coatings and elastomers, the prepared QAL/ZnO nanocomposites were blended with PU to demonstrate its useful application in polymer materials. The UV-absorption performance of the PU, PU-ZnO and PU-QAL/ZnO films were first investigated. Figure 10(a) shows the UV light transmittance spectra. The transmittance of all the PU-QAL/ZnO films in the visible light region between 450~800 nm was a little smaller than that of the pure PU film, but was higher than the film of PU-ZnO, rendering the PU-QAL/ZnO films with good visible transmission, as shown in the digital photos in Figure S8. More importantly, the transmittance of the PU-QAL/ZnO films in the UV region between 230~400 nm had great decrease in comparison with the pure PU film, and was also smaller than the film of PU-ZnO. The sample of PU-QAL/ZnO-1.0 film had the best UV shielding effect. The good UV shielding effect of the PU-QAL/ZnO films was expected to give the PU composite films very good anti-UV aging performance. The resistance to UV aging effect of the PU-QAL/ZnO-1.0 film was further studied by illumination with a high power UV lamp for 192h. The UV Protection Factor (UPF) was calculated according to Equation (1). Figure 10(b) shows the UPF values of the pure PU, PU-ZnO and PU-QAL/ZnO composite films at different UV radiation time. The UPF value of the PU-QAL/ZnO-1.0 film was much higher than that of PU-ZnO and PU films during the whole irradiation process,demonstratingthe excellent UV-absorption performance of PU-QAL/ZnO-1.0 film. Furthermore, the transparency property shown in Figure S8(b) opens up potential application of PU-QAL/ZnO composite film as a good UV-shielding coating material.



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Figure 10. (a) UV light transmittance curves of pure PU film, PU-ZnO and PU-QAL/ZnO films, (b) UPF values of pure PU film, PU-ZnO and PU-QAL/ZnO films under UV irradiation. The mechanical properties of the PU-QAL/ZnO-1.0 and PU-ZnO films were further investigated by uniaxial extension test. Figure S7 shows the break stress and strain of the PU film blended with different amount of QAL/ZnO-1.0 and ZnO. The


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optimal additive amount of QAL/ZnO-1.0 and ZnO was 0.6 wt%. The stress-strain curves of the PU-ZnO and PU-QAL/ZnO-1.0 films with the optimal additive amount were shown in Figure 11. Comparing with the pure PU, the break stress and strain of PU-QAl/ZnO-1.0 were dramatically increased from 26.5 MPa and 415% to 32.5 MPa and 472%, respectively. However, the stress and strain at break of the PU-ZnO film decreased to 24.5 MPa and 405%. Figure S8(e & f) shows that the QAL/ZnO-1.0 nanocomposite was homogeneously dispersed in PU matrix, while serious accumulation was observed for pure ZnO in PU, illustrating much better interfacial compatibility between PU and QAL/ZnO nanocomposite. The good interfacial compatibility and excellent dispersion of QAL/ZnO-1.0 nanoparticles in PU was the main reason for the improvement in mechanical properties.

Figure 11. Engineering stress-strain curves of PU, PU-ZnO and PU-QAL/ZnO-1.0 films. To further demonstrate the better performance of QAL/ZnO nanoparticles, the anti-UV aging performance of the PU-QAL/ZnO-1.0 film with the additive amount of 0.6 wt% was investigated under irradiation with high-powered UV light for 8 days, in comparison with the pure PU and PU-ZnO film. The results are shown in Figure 12. The PU-QAL/ZnO-1.0 showed much better UV-stability than PU-ZnO and pure PU.



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After high-powered UV irradiation for 192 hours, the tensile strength at break of PU-ZnO decreased from 24.5 MPa to 12.5 MPa, the elongation at break of PU-ZnO decreased from 405% to 322%. But for the sample of PU-QAL/ZnO-1.0, the tensile strength and elongation at break maintained at 24.8 MPa and 411%, which were much higher than that of pure PU and PU-ZnO. According to the above results, it clearly demonstrated that the prepared QAL/ZnO-1.0 nanoparticles can act as a useful UV-shielding agent in transparent PU coatings.

Figure 12. The tensile strength (a) and elongation at break (b) of the PU, PU-ZnO and


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PU-QAL/ZnO-1.0 films under high-powered UV radiation. 4. CONCLUSIONS In this work, a series of QAL/ZnO hybrid nanocomposites were successfully synthesized by a facile and environmentally friendly hydrothermal method. The microstructure of the QAL/ZnO nanocompostites could be well adjusted and the particle size could be well controlled in 100 nm. The prepared QAL/ZnO nanocomposites showed excellent synergistic UV-absorption properties due to its excellent interface contact. It was successfully demonstrated that the UV transmittance of polyurethane (PU) film was significantly reduced but the UV-stability was greatly improved when blended with the prepared QAL/ZnO hybrid nanocomposite. This work represents the first example of lignin/ZnO hybrid nanocomposite, which paves a new way for high-value-added application of lignin in the field of functional materials and opens up potential application of QAL/ZnO nanocomposite as a useful UV-shielding agent in transparent PU coating materials. ASSOCIATED CONTENT Supporting Information Synthetic pathway of the QAL, elemental and functional group content of the AL and QAL, 1H MNR and FT-IR spectra of AL and QAL, N 1s XPS of the QAL and QAL/ZnO-1.0, static contact angle measurements of the ZnO and QAL/ZnO, SEM images of the ZnO and AL/ZnO, the tensile strength and elongation at break of the PU-ZnO and PU-QAL/ZnO-1.0 films, the digital photos and sectional SEM images of the PU (a & d), PU-ZnO (b & e) and PU-QAL/ZnO-1.0 (c & f) films. AUTHOR INFORMATION Corresponding Author: * E-mail Address: [email protected] (Prof. X. Qiu). * E-mail Address: [email protected] (Prof. D. Yang). Notes



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The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 21436004, 21576106) and Guangdong province science and technology plan project (2014B050505006). REFERENCES (1) Ragauskas, A.J.; Williams, C.K.; Davison, B.H.; Britovsek, G.; Cairney, J.; Eckert, C.A. The path forward for biofuels and biomaterials. Science 2006, 311, 484-489. (2) Habibi, Y.; Lucia, L.A.; Rojas, O.J. Cellulose nanocrystals: chemistry, self-assembly, and applications. Chem. Rev. 2010, 110, 3479-3500. (3) Klemm, D.; Heublein, B.; Fink, H.P.; Bohn, A. Cellulose: fascinating biopolymer and sustainable raw material. Angew. Chem. Int. Edit. 2005, 44, 3358-3393. (4) Ebringerová, A.; Hromádková, Z.; Heinze, T.; Hemicellulose. Adv. Polym. Sci. 2005, 186, 1-67. (5) Gosselink, R.J.A.; Jong, de E.; Guran, B.; Abächerli, A. Co-ordination network for lignin-standardisation, production and applications adapted to market requirements (EUROLIGNIN). Ind. Crop. Prod. 2004, 20, 121-129. (6) Thielemans, W.; Can, E.; Morye, S.S.; Wool, R.P. Novel applications of lignin in composite materials. J. Appl. Polym. Sci. 2002, 83, 323-331. (7) Saha, B.C. Hemicellulose bioconversion. J. Ind. Microbiol. Biotechnol. 2003, 30, 279-291. (8) Doherty, W.O.S.; Mousavioun, P.; Fellows, C.M. Value-adding to cellulosic ethanol: lignin polymers. Ind. Crop. Prod. 2011, 33, 259-276. (9) Sierra, R.; Smith, A.; Granda, C.; Holtzapple, M.T. Producing fuels and chemicals from lignocellulosic biomass. Chem. Eng. Prog. 2008, 104, S10-S18. (10) Zhou, M.; Qiu, X.; Yang, D.; Lou, H.; Ouyang, X. High-performance dispersant

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Table of Contents Graphic


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ZnO Hybrid Nanocomposite with Excellent UV

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