ZnO Composite with Excellent

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In-Situ Synthesis of Flower-like Lignin/ZnO Composites with Excellent UV-Absorption Property and Its Application in Polyurethane Huan Wang, Wensheng Lin, Xueqing Qiu, Fangbao Fu, Ruisheng Zhong, Weifeng Liu, and Dongjie Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04038 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018

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ACS Sustainable Chemistry & Engineering

In-Situ Synthesis of Flower-like Lignin/ZnO Composites with Excellent UV-Absorption Property and Its Application in Polyurethane Huan Wang†, Wensheng Lin†, Xueqing Qiu*,†,‡, Fangbao Fu†, Ruisheng Zhong†, Weifeng Liu†, 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. * E-mail: [email protected] (X. Qiu). [email protected] (D. Yang).

ABSTRACT: In this work, lignin-decorated ZnO (QAL/ZnO) composites were prepared via an in-situ synthesis method using industrial alkali lignin (AL). Firstly, the AL was modified by quaternization to prepare quaternized alkali lignin (QAL). The microstructure and optical properties of the QAL/ZnO composites were characterized by scanning electron microscope (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-Vis and photoluminescence (PL), respectively. It showed that the prepared QAL/ZnO composites possessed a flower-like structure and showed excellent synergistic UV-absorbent property. Interestingly, the anti-UV performance and mechanical properties of the polyurethane (PU) were significantly improved when blended with the resulting QAL/ZnO. Comparing with pure PU film, the UV transmittance of the PU film was rapidly reduced. Furthermore, the tensile strength and elongation at break of PU film blended with QAL/ZnO were significantly improved, which was due to the good compatibility between QAL/ZnO and PU matrix. Results of this work provided a significant and practical approach for the high value-added utilization of lignin as functional materials. KEYWORDS: Lignin; ZnO; Composite; Polyurethane; Anti-UV ageing.

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INTRODUCTION In recent decades, environmental pollutions and climate change have leaded serious damages to the earth's ozone layer, the Ultraviolet (UV) light reaching the earth's surface has increased dramatically, which causes a series of problems, such as leading a significant and sustained damage to the outdoor polymer materials.1 So, the anti-UV stabilizer has become a hot and meaningful research topic in recent years. Conventional anti-UV stabilizers are organic derivatives, such as hindered phenol, amine, organosulfur and organophosphorus compounds.2-4 However, these anti-UV stabilizers are unstable and easily volatile in the polymer due to their relatively low molecular weight. The anti-UV protective performance would gradually decline or even lost as time goes by. Therefore, the research and development of a durable and sustainable anti-UV stabilizer in polymer materials is very meaningful. Metal oxides nanomaterials with excellent UV absorption and stability properties have promising applications in the anti-UV field of polymer materials.5-10 As an important one of them, zinc oxide (ZnO) has been widely used as optoelectronics, UV photodetector and photocatalyst due to its exceptional electrical, optical and environmental friendliness.11-13 Recently, ZnO-based anti-UV stabilizers used in polymer materials have got widely research and several good UV protective composite films based-on ZnO were reported, such as PVA/ZnO,14 PMMA /ZnO,15 PS/ZnO,16 PBMA/ZnO17 and PU/ZnO.18, 19 However, the pure ZnO nanomaterials exhibited a serious aggregation behavior, difficult dispersibility and poor interface compatibility in polymer materials due to its high specific surface energy, hydrophilicity and polar surface. To overcome this problem, a viable and effective way is to dopant ZnO to improve its dispersibility and compatibility in polymer materials. Lignin-based materials with excellent UV-absorbent properties have attracted widely attention in recent years.20 Lignin is the second largest natural polymer after cellulose in plants on earth.21-23 Especially, over 50 million tons of industrial lignin is produced from paper making industry as a byproduct every year.24. 25 However, only 2%


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of lignin is effectively unitized, more than 98% of lignin is combusted as fuel, which not only causes environmental problems, but also causes huge waste of the resources.26-28 Therefore, the research on value-added application for lignin is considerably meaningful for the development of renewable resources. In recent years, lignin-based optical materials have been reported.29-32 Qian et al.31 reported a lignin-based colloidal nanospheres and it has a potential application in the field of sunscreen. Bula et al.32 reported a novel functional silica/lignin hybrid material, and it could improve the elongation at break and notched impact strength of polypropylene. Organic-inorganic composites with synergies, good compatibility, structural stability and other advantages have received wide research in recent years.33, 34 The preparation of lignin/ZnO may effectively solve the problems that ZnO is easy to agglomerate, difficulty to disperse and have poor compatibility of ZnO in polymer. On the other hand, both lignin and ZnO show excellent UV-absorbency performances. The lignin/ZnO composites may have unexpected excellent UV-absorbency properties based on lignin and ZnO. Polyurethane (PU) is an important polymer material with wide applications in many fields, such as coating, foams, elastomers and so on due to its abundance, renewability and low-cost.35 Especially, PU has a wide range of applications in outdoor coating materials. However, pure PU shows poor UV-stability. The development of low-cost and environmentally friendly anti-UV stabilizers for PU is a meaningful. As the functional groups in lignin could form hydrogen bonding interaction with the urethane groups, polyurethane blended with lignin or lignin-based derivatives is a facile way to improve its UV stability and mechanical properties.36 In our previous study, lignin-based silica composite was prepared from lignin and sodium silicate by a direct co-precipitation method.37 In this work, a simple and environment

friendly

in-situ

synthesis

method

was

reported

for

controllable-preparation of flower-like lignin/ZnO (QAL/ZnO) composites, which was used as a new anti-UV agent for the polymer materials. The morphology and microstructure of the prepared QAL/ZnO composites were well characterized. The anti-UV performance of the PU blended with QAL/ZnO composite was investigated ACS Paragon Plus Environment


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in detail. Results showed that not only the anti-UV performance was significantly improved, but also the mechanical properties were greatly improved. On the other hand, the prepared anti-UV agent of QAL/ZnO composites with excellent optical properties may have other application prospect in the functional materials. This scalable and convenient method gives a sustainable and valuable way for the utilization of renewable biomass. EXPERIMENTAL SECTION Materials. The pretreatment of industrial alkali lignin (AL) from pulping black liquor were described in our previous work.38 UH-650 polyurethane (PU) was purchased from Bayer China Ltd. (Shanghai, China). Other reagents, such as Zn(AC)2·H2O (Tianjin

Fuchen

Chemical

Reagent

Co.,

Ltd.,

China),

3-Chloro-2-hydroxypropyltrimethyl ammonium chloride (CHPTMAC, 60 wt%) solution (Shanghai Aladdin Industrial Corp., China), sodium hydroxide (Guangdong Guanghua Sci-Tech Co., Ltd., China), sulfuric acid (Guangzhou Chemical Reagent Factory, China) were analytical grade. Preparation of quaternized alkali lignin (QAL). 72.10 g AL was dissolved in 450 ml NaOH aqueous solution (~20 wt %), and then 60.08 g CHPTMAC was drop-added into the above AL solution under stirring. After that, the mixture solution was stirred at 85 °C for 4 h and finally the obtained QAL solution (~22 wt %) was purified by dialysis and then freeze dried to gain QAL powder. The synthetic pathway of the quaternized alkali lignin is shown in Figure S1. The functional group and elemental contents of the AL and QAL were shown in Table S1. The 1H MNR and FT-IR spectra of AL and QAL were shown in Figure S2 and Figure S3. The results confirm that the QAL which contains the quaternary ammonium group was successfully synthesized. Preparation of QAL/ZnO composites. The QAL/ZnO composites was prepared by an in-situ synthesis method, the synthetic pathway is shown in Figure 1. Firstly, Zn(AC)2 solution was prepared by dissolving 1.10 g of Zn(AC)2·H2O in 50 mL of deionized water. Then 0.40 g of QAL power and 2.00 g of NaOH were dissolved in 50


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ml deionized water to obtain QAL solution. Following this, the QAL solution was slowly added to the Zn(AC)2 solution, and then mixture was stirred at 85°C for 4 hours. Next, the mixture was cooled to room temperature and its pH was adjusted to 7.5 using 20 wt% H2SO4. After aging at 40 °C for 2 hours, the precipitates were collected by centrifugation and rinsed with deionized water three times. Finally, the QAL/ZnO composite were obtained via the infrared drying. For the comparison, the pure ZnO particles were obtained without QAL by the same process.

Figure 1. The preparation process of ZnO and QAL/ZnO composites. Characterizations. The powder X-ray diffraction (XRD) patterns of samples were recorded by D8 Advance (Bruker-Axis, Ltd., Germany) using Cu Kα radiation (λ = 0.15406 nm). Scans were performed from 20° to 80° (2θ) at a rate of 4°/min. The micromorphology and microstructure of the samples were studied through scanning electron microscope (SEM, Merlin, Zeiss Corp., Germany) at 5 kV. Transmission electron microscopy (TEM) images were performed on a HITACHI H-7650 electron microscope (Hitachi, Ltd., Japan) with an accelerating voltage of 120 kV. The surface areas and pore size analysis were measured by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) method on a TriStar II 3020 automated surface



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area and pore size analyzer (Micromeritics Instrument Corp, USA). UV-vis diffuse reflectance spectra of the samples in solid state were obtained by Shimadzu UV-3600 with an integrating sphere (Shimadzu, Ltd., Japan). The photoluminescence (PL) spectra have been achieved on F-4500 fluorescence spectrometer (Hitachi, Ltd., Japan) with a Xe lamp at room temperature at an excitation wavelength of 325 nm.

X-ray

photoelectron spectroscopy (XPS) was performed on an Axis Ultra DLD (Kratos, Ltd., England) with Al Kα radiation (1486.6 eV). Preparation of PU+QAL/ZnO and PU+ZnO blend films. As-prepared QAL/ZnO composites were blended with PU to make the PU+QAL/ZnO film. Firstly, QAL/ZnO composites were dispersed in 20 mL ethanol using ultrasonic technique for 5 min. Then, the dispersed liquid was blended with 30 g PU at ~500 rpm for 2 h. After that, the mixture was filtered through a 60-mesh screen to remove bubbles. Sequentially, 15 g mixture was poured into Teflon shale (10 cm x 5 cm x 0.5 cm) and dried at 50°C under atmospheric pressure for 48 h to obtain the PU+QAL/ZnO film. The pure PU and PU+ZnO films were prepared in the same way. Characterization

of

PU+QAL/ZnO

and

PU+ZnO

films.

The

UV-light

transmittance spectra of films were measured by Shimadzu UV-2600 with an integrating sphere (Shimadzu, Ltd., Japan), the wavelength of incident ray was selected in the range of 280-400 nm, five times were scanned for each samples to reduce mistakes. The morphologies of the composite films were studied using a Merlin field emission microscope (Merlin, Zeiss Corp., Germany) at 5 kV. The blend films were freeze-fractured in the liquid nitrogen and then coated with platinum before observing and photographing. Mechanical properties of the dumbbell specimens cut from blended films were measured on the microcomputer electronic universal testing machine (CMT, MST, USA) at room temperature with a strain rate of 20 mm/min according to ASTM D638. Artificial accelerated UV-light aging test was conducted in an WJ-UVA UV-light accelerated weatherometer with 340 nm fluorescent lamps (300 W) and operated at 50 ℃ and under 45 % relative humidity. All blended films were exposed to UV light for 192 h and measured their mechanical


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property in every 24 hours. The UV Protection Factor (UPF), average transmittance values of UVA light (T(UVA)AV) and UVB light (T(UVB)AV) were calculated by the following equation according to profiles of UV Standard 801:

= ∑

⁄∑

() = ∑

()

() = ∑ ()

(1) (2) (3)

where Eλ = erythema spectral effectiveness, Tλ = spectral transmittance of sample films, Sλ = solar spectral irradiance, λ = wavelength, n = the number of measurements between 290-315 nm, and m = the number of measurements between 315-400 nm. RESULTS AND DISCUSSION Preparation and characterization of QAL/ZnO composites. The crystal structure of the prepared ZnO and QAL/ZnO was evaluated by XRD analysis, as the results shown in Figure 2. It is shown that the entire diffraction peaks of as-prepared QAL/ZnO composite and ZnO match well with those of wurtzite hexagonal ZnO (JCPDS card no. 89-0510). 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. No peaks of impurities are found, this indicates that our samples are pure and well crystallized. Moreover, the XRD pattern of ZnO shows that the intensity of (002) peak is stronger than that of (100), indicating that ZnO has strong preferential growth along the c-axial orientation. However, the (002) peak intensity of QAL/ZnO composite is weaker than that of (100), which suggests that adding of QAL changes the growth of hexagonal wurtzite type ZnO while applying our synthetic approach.


ZnO

200 112 201

103

102

110

100 002

Relative Intenmsity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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101


QAL/ZnO 20

30

40

50

60

70

2θ (degree) Figure 2. XRD patterns of QAL/ZnO and ZnO. For quantitative evaluation, the high intense diffraction peaks of (100), (002) and (101) planes were used to calculate the crystallite sizes, D, using the Scherrer formula:37 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 calculated results were listed in Table 1. The crystallite sizes of ZnO in the QAL/ZnO composite were smaller than that of the pure ZnO. The average crystallite sizes of QAL/ZnO composite and ZnO samples were about 25.5 nm and 38.8 nm, respectively. Table 1 Structural parameters obtained from the XRD patterns shown in Figure 2. Sample Name

(hkl)

2θ(°)

FWHM(°)

Crystallite size (nm)

ZnO

100

31.74

0.241

33.1

ZnO

002

34.40

0.173

50.9

ZnO

101

36.21

0.302

27.3

QAL/ZnO

100

31.70

0.406

19.3

QAL/ZnO

002

34.41

0.219

35. 9

QAL/ZnO

101

36.25

0.381

21.3


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The morphologies of samples were analyzed by SEM and TEM. Figure 3(a&c) shows the SEM images of QAL/ZnO and ZnO, respectively. It shows that both QAL/ZnO and ZnO display the flower-like nanostructures. Especially, the surface of QAL/ZnO was roughness, significantly different from that of ZnO samples, indicating that ZnO has been successfully covered with the lignin. The high-resolution SEM image in Figure 3a shows that the width and length of QAL/ZnO composite was 50 nm and 500 nm, respectively, which is smaller than that of pure ZnO. Figure 3(b&d) shows the TEM images of QAL/ZnO and ZnO, respectively. Compared to the pure ZnO, the QAL/ZnO composite was composed of QAL coated ZnO. It can be intuitively see that QAL/ZnO composites are successfully prepared from the results of SEM and TEM. It is well known that the reaction of Zn(CH3COO)2·2H2O with NaOH gives rise to Zn(OH)2, a hydrolysis process allows the formation of [Zn(OH)4]2−, then the succeeding dehydration yields ZnO.39 The ZnO prepared by the above-mentioned process is a polar crystal of negative charges at the particle surface. The unmodified AL is also a negatively charged polymer. Besides, alkali lignin is complex natural macromolecule, arising from the copolymerization of three phenylpropanoid monomers, such as coniferyl, sinapyl, and p-coumaryl alcohol. Many reactive functional groups, such as Phenolic hydroxyl groups, present in residual lignin can be modified by various reactions.40 As shown in the previous study, lignin can be easily modified

by

3-Chloro-2-hydroxypropyltrimethyl

ammonium

chloride,

which

significantly improves its aqueous solubility and insert a positive charged quaternary ammonium groups (NR4+). There may be electrostatic interaction between NR4+ groups of QAL and negative charged ZnO nanoparticles, resulting in a large amount of growth units easily deposit to the surface of ZnO nanoparticles. Therefore, the growth of ZnO nanocrystals along c-axis orientation will be seriously limited and sizes of nanocrystals become smaller. The data of XRD and TEM reported here also confirmed that the addition of QAL changed the growth pattern of ZnO nanocrystal and promoted the formation of the roughened surface of the QAL/ZnO composites.



Figure 3. SEM images of QAL/ZnO composite (a) and ZnO (c), TEM images of QAL/ZnO composite (b) and ZnO (d). To further investigate the interaction between QAL and ZnO, XPS spectra were recorded. Figure 4a shows the overall XPS spectra of the ZnO, QAL and QAL/ZnO. The elements of C, O, N and Zn were obviously appeared in QAL/ZnO, indicating that the QAL/ZnO composite composed of lignin and ZnO was successfully prepared. Furthermore, the molar ratio of Zn and O on the surface of ZnO and QAL/ZnO composite was 1:1.4 and 1:5 acquired by semi-quantitative analysis from the results in Figure 4a. The molar ratio of Zn and O on the surface of ZnO is lower than the theoretical molar ratio of 1:1, which is due to the hydroxyl groups on the surface of samples. At the same time, the molar ratio of Zn and O on the surface QAL/ZnO composite is much lower than the theoretical molar ratio of ZnO, indicating that the ZnO was coated with lignin. The C1s spectrum of QAL/ZnO composite (Figure 4b) could be decomposed into three main sub-peaks. The peak at 284.5eV was attributed to the C-C, C=C and


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C-H groups, the peak centered at 286.0 eV is assigned to the C-OH species.41 The peak located at 288.4 eV is related to the conjugated O=C-O-Zn species.42,

43

Compared with the C1s spectrum of QAL, the peak of sp2 carbon atom of QAL/ZnO composite has a slight blue shift and the peak of O=C-O-Zn species appeared, which due to the esterification reaction between the ZnO precursor of Zn(OH)2 and carboxyl in lignin. indicating that QAL not only successfully coated on the surface of ZnO nanoparticles, there was also a chemical bond at the interface of lignin and ZnO. This close combination with chemical bonds is very beneficial to its synergistic enhanced UV-absorption performance. Figure 4d shows the N1s spectrum of QAL and QAL/ZnO. The peaks of 399.0 and 402.4 eV were assigned to non-protonated and protonated status of quaternary ammonium groups, respectively.44, 45 The peak intensity of the protonated state of QAL/ZnO composite was lower than that of non-protonated state peak, but the result of QAL was the opposite. It suggested that there was Zn-O-N bond between QAL and ZnO, which obtained by the reacting of quaternary ammonium groups and hydroxyl groups on the surface of the negative charged ZnO. The Zn2p spectrum of ZnO and QAL/ZnO was shown in Figure 4d. The peaks appeared at 1021.5 eV and 1044.6 eV in the Zn2p spectrum of ZnO assigned to the two spin-orbit-split doublets for Zn 2p3/2 and 2p1/2, which was associated with Zn2+ ions in ZnO.43 Compared to the pure ZnO, the peaks of Zn2p spectrum in QAL/ZnO composite shows a slight shift to lower binding energy due to the increasing in electron density, which may be the lignin layer coated around the surface of the ZnO.

46

The results of XPS spectra showed that the QAL/ZnO composites with

chemical bonds between the interface of lignin and ZnO were successfully synthesized.



Figure 4. XPS spectra of the ZnO, QAL and QAL/ZnO composite, (a) overview, (b) C1s, (c) N1s and (d) Zn2p. TG/DTC analysis was conducted to investigate the content of lignin in the as-prepared QAL/ZnO composite, as the results shown in Figure 5. The weight loss of QAL/ZnO composite occurred in two steps corresponded with the peaks in the DTG curve. The first weight loss of 2.8 wt% occurred in 30~150 °C and the corresponding loss peak was observed at 95.5 °C in the DTG curve, demonstrating the loss of surface-absorbed water in QAL. The second weight loss of 30.0 wt%, which was the major weight loss step, occurred within the range of 150~500 °C without further weight loss up to 750°C and the corresponding loss peak was observed at 324.3°C in the DTG curve, corresponding to the thermal decomposition of lignin. The weight loss of about 30.0 wt% from 150 to 500 °C approximately represented the content of lignin in the QAL/ZnO composite.


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Figure 5. TG/DTG curve of as-prepared QAL/ZnO composite and ZnO. The optical properties of the QAL, ZnO and QAL/ZnO have been characterized by UV-Vis spectra and PL spectra. Figure 6a shows that the QAL had strong and broad light absorption in UV and visible. ZnO only showed great light absorption in UV region, which was due to the electron promotion from valence band to the conduction band. Especially, the QAL/ZnO showed stronger UV absorption than the QAL and pure ZnO, showing synergetic enhancement absorption of UV light. The reason may be that the close interface contact between QAL and ZnO, which could facilitate the transition of the photogenerated electron in ZnO. When the QAL/ZnO was illuminated with UV light, the photogenerated electrons in ZnO would transfer to the QAL under the withdrawing effect of the unsaturated bonds in lignin [47]. Additionally, QAL/ZnO also showed slightly enhanced visible light absorption intensity which was due to the addition of lignin. Furthermore, the absorption band gap energy of the prepared ZnO and QAL/ZnO was calculated by the Kubelka-Munk method.48 The resulting curves of (ahv)2 versus hv is shown in Figure 6b. The band gap energies of ZnO and QAL/ZnO are 3.24 eV and 3.20 eV, respectively. The band gap energy of QAL/ZnO shows a red shift



compared to the ZnO, which is beneficial to enhance the absorption of near UV region. On the other hand, the UV light reaching the surface of the earth is predominantly near the UV region. The significantly enhanced absorption of QAL/ZnO in the near UV region of 360-460 nm has important practical value in the field of anti-UV. The PL spectra of the ZnO, QAL and QAL/ZnO were shown in Figure 6c. ZnO and QAL/ZnO composite showed a strong emission peak at ~420 nm, corresponding to the intrinsic emission peak of the wurtzite ZnO nanocrystals, and the wide emission peaks at 470 nm and 525 nm resulted from the high-level structural defects. Additionally, QAL exhibits that a sharp emission peak at 500 nm, which is attributed to the π-π conjugation of phenylpropane units in the QAL molecules,49 indicating that QAL molecules have a certain electron transport capability, and our previous studies have shown that QAL has a certain electrical conductivity and is a good electron acceptor.50 Compared with the ZnO, the PL intensity of QAL/ZnO composite decreased significantly, which was due to the introduction of lignin could adsorb some of the photogenerated electrons,50 indicating that the recombination of photo-induced electrons and holes was inhibited to some extent. Figure 6d shows the interfacial electrons transfer mechanism of QAL/ZnO. The interfacial electron transfer is effective to reduce the recombination of electron-hole pairs, resulting in the improvement of UV-light absorbing capability as well as photo-stability in the PU matrix, as shown in Figure 7&9.


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Figure 6. (a) UV-vis diffuses reflectance spectra of the ZnO, QAL and QAL/ZnO, (b) the plots of (ahv)2 versus hv, (c) PL spectra of the ZnO, QAL and QAL/ZnO excited with 325 nm, (d) interfacial electrons transfer mechanism of QAL/ZnO. Application of QAL/ZnO composites in polyurethane. As polyurethane (PU) is a common used polymer in many fields such as elastomers, coatings and foams, the prepared QAL/ZnO with excellent UV absorption ability were used to improve the anti-UV stability. A series of PU films blended with various contents of ZnO and QAL/ZnO (based on the mass of the PU, the sample blended with ZnO contents of 0.2%, 0.4%, 0.6%, 0.8%, 1.0% and 1.2% was named PZ2, PZ4, PZ6, PZ8, PZ10 and PZ12, respectively. The samples of blended QAL/ZnO took the same naming method) were prepared, and their UV-light transmission spectra were first investigated, as shown in Figure 7a. ZnO is used as a control sample because of its excellent UV-light blocking properties. The pure PU used in this study possesses good UV-light shielding ability over range of 280-300 nm, and poor shielding ability over range of 300-400 nm, which is most destructive to materials. Figure 7a shows that the UV transmittance



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of the PU+ZnO films gradually decreased with the addition of ZnO. Compared with the addition of ZnO, the UV transmittance of the PU+QAL/ZnO films reduced the more obvious. Table 2 shows the transmittance at 305 and 365 nm, and the UPF values of the pure PU and blended PU films. The UPF value was calculated according to Equation (1). It shows that the PU exhibited excellent UV protection grade when only blended with 0.6 wt% QAL/ZnO. Figure 7b shows the digital photos of the PU and PU+QAL/ZnO films. It shows that the color of blend film became darker with the increasing of QAL/ZnO contents from 0.2 wt% to 1.2 wt%, but still acceptable. Figure 7c shows the SEM images of the PU, PU+ZnO and PU+QAL/ZnO films. It shows that QAL/ZnO composite were well dispersed in PU film, while the ZnO showed serious accumulation in the PU, illustrating better interfacial compatibility between QAL/ZnO and PU. The good interfacial compatibility and great dispersion of QAL/ZnO in PU may be beneficial to improve its mechanical properties.

Figure 7. UV light transmittance curves (a), the digital photos (b) and SEM images (c) of the PU, PU+ZnO and PU+QAL/ZnO films. Table 2. UPF value, T(UVB) and T(UVA)of the PU films. Sample Name

T(305) /%

T(365) /%

UPF value

Protection grade

PU

8.25

68.23

4.51

——

PZ2

6.22

41.59

6.83

poor


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PZ4

4.12

34.26

8.21

poor

PZ6

3.41

28.75

11.85

general

PZ8

2.33

20.19

16.28

general

PZ10

2.03

18.56

19.12

general

PZ12

1.88

16.68

22.52

good

PLZ2

1.11

15.85

27.46

good

PLZ4

0.99

14.66

32.95

good

PLZ6

0.85

12.46

39.35

great

PLZ8

0.76

10.74

40.12

great

PLZ10

0.61

7.13

41.81

great

PLZ12

0.55

4.36

45.16

great

The mechanical properties of PU+ZnO and PU+QAL/ZnO films were investigated by uniaxial extension test. Figure 8 shows the strain and break stress of the PU films blended with various amount of QAL/ZnO and ZnO. It shows that the optimal additive amount of QAL/ZnO was 0.6 wt%. The tensile strength of PU+ZnO films increased from 17.86±0.94 MPa to 22.67±0.81 MPa when the content was 0.8 wt%, with the further increase the content of ZnO, the tensile strength showed a decreasing trend. The elongation at break decreased with the rise of ZnO content from 370.9 ±8.5 % to 305.4±8.4 %. This is because the unmodified ZnO has high surface polarity and surface energy, which makes it easy to form aggregation in the PU. The moderate interfacial interaction between ZnO and PU could enhance the tensile strength. However, adding too much ZnO would cause it to agglomerate heavily in the PU, which leads to a decrease of tensile strength. On the other hand, the interfacial interaction between ZnO and PU is harmful to the elongation at break, as the results shown in Figure 8. Additionally, the introduction of QAL could improve the interfacial interaction between ZnO and PU, which could significantly enhance the tensile strength of PU. The tensile strength and elongation at break of PU+QAL/ZnO films increase with the rise contents of QAL/ZnO, and reached the maximum of 31.76±0.85 MPa and 414.75±7.2 % when the content was 0.6 wt%. With the further



increase the content of QAL/ZnO, the mechanical properties of the blend film decreased to 20.38±0.71MPa and 305.38±7.5 % due to the excessive QAL/ZnO triggers its own aggregation.

Figure 8. The tensile strength and elongation at break of the PU blends films with different contents of QAL/ZnO and ZnO. The sample of PU blend film with 0.6 wt% QAL/ZnO composite processes excellent UV-light blocking effect and best physical mechanical properties, so the blended PU film was expected to give the very good anti-UV-light aging performance. The anti UV-light aging performance of blend films with 0.6 wt% QAL/ZnO and ZnO were tested, respectively. Figure 9 showed that the PU+QAL/ZnO film showed much better anti-UV aging performance than PU+ZnO film and pure PU film. After UV irradiation for 192 hours, the tensile strength of PU+ZnO film decreased from 21.69±1.23 MPa to 12.46±1.18 MPa, the elongation at break of PU+ZnO film


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decreased from 370.9±7.3 % to 289.5±6.3 %. But for the sample of PU+QAL/ZnO film, the tensile strength and elongation at break maintained at 23.81±1.17 MPa and 354.9±8.3 %, which were much higher than that of pure PU and PU+ZnO film. According to the above results, it clearly demonstrated that as-prepared QAL/ZnO composite can act as a useful UV-light protective agent in PU films.



Figure 9. The tensile strength (a) and elongation at break (b) of the pure PU film, PU+QAL/ZnO film and PU+ZnO film during the UV-light aging test.


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CONCLUSIONS In this work, a simple, sustainable, and environmentally-friendly in-situ precipitation method was developed to prepare lignin/ZnO composite. The microstructures of the samples were well adjusted and the average particle size of the flower-like QAL/ZnO composite could be well controlled less than 300 nm. The results of XPS, UV-Vis and PL spectra showed the excellent interface contact between lignin and ZnO, which resulting an excellent synergistic UV-absorption properties. It was successfully demonstrated that the UV transmittance of PU film was significantly decreased but the anti-UV aging performance was greatly improved when it was blended with the QAL/ZnO composites. Furthermore, the mechanical properties of the PU were also greatly improved when only blended with 0.6 wt% QAL/ZnO composites. The flow-like QAL/ZnO represented in this work may be used as a useful UV-shielding agent in transparent PU coating materials. This work not only provides a novel approach to prepare lignin/ZnO composite but also greatly extends the comprehensive utilization of the abundant alkali lignin resource from black liquor. 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, digital photos of the PU, PZ6 and PLZ6 films, SEM images of the PU blended films. AUTHOR INFORMATION Corresponding Author: * E-mail Address: [email protected] (Prof. X. Qiu). * E-mail Address: [email protected] (Prof. D. Yang). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 21436004, 21576106) and Natural Science Foundation of



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For Table of Contents Use Only Synopsis: Industrial lignin was used to prepare flower-like Lignin/ZnO composites and added into polyurethane (PU) to improve its anti-UV ageing performance.


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ZnO Composite with Excellent

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