Research Article pubs.acs.org/journal/ascecg
Cellulose-Lignin Biodegradable and Flexible UV Protection Film Hasan Sadeghifar,†,‡ Richard Venditti,*,† Jesse Jur,§ Russell E. Gorga,§ and Joel J. Pawlak† †
Department of Forest Biomaterials, North Carolina State University, Raleigh, North Carolina 27695, United States Department of Wood and Paper Science, Sari Branch, Islamic Azad University, P.O. Box 48161-19318, Sari, Iran § Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, North Carolina 27695, United States
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ABSTRACT: There is significant interest in biodegradable and transparent UV protection films from renewable resources for many different applications. Herein, the preparation and characterization of semitransparent flexible cellulose films containing low amounts of covalently bonded lignin with UV-blocking properties are described. Azide modified cellulose dissolved in dimethylacetamide/lithium chloride (DMAc/ LiCl) was reacted with propargylated lignin to produce 0.5%, 1%, and 2% by weight lignin containing materials. Celluloselignin films were prepared by regeneration in acetone. These covalently bonded cellulose-lignin films were homogeneous, unlike the simple blends of cellulose and lignin. Prepared films showed high UV protection ability. Cellulose film containing 2% lignin showed 100% protection of UV-B (280−320 nm) and more than 90% of UV-A (320−400 nm). The UV protection of prepared films was persistent when exposed to thermal treatment at 120 °C and UV irradiation. Thermogravimetric analysis of the films showed minimal mass loss up to 275 °C. The tensile strength of the neat cellulose film was around 120 MPa with about a 10% strain to break. Treated cellulose films with 2% lignin showed lower tensile strength (90 MPa). The described methods demonstrate a straightforward procedure to produce renewable based cellulose-lignin UV-light-blocking films. KEYWORDS: UV protection, Cellulose, Lignin, Click chemistry, Transparent film, Flexible film
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renewable, flexible, and transparent film from native cellulose has been reported.6−8 Direct dissolution of native cellulose has been studied extensively, and a variety of solvent systems are known.9 The DMAc/LiCl system is effectively a native cellulose solvent, first described in 1979,10 and has been frequently used. Native cellulose offers great opportunities in the fields of edible materials, packaging, electronics, and medical devices and as a substrate film for different applications. It is a promising environmental solution to the plastic waste issue. Moreover, a series of regenerated cellulose products such as novel cellulose fibers, functional fibers, separation membrane materials, and aerogels have been fabricated successfully from native cellulose film.6,8,11,12 Cellulose film has a much lower coefficient of thermal expansion (CTE) compared to that of plastic substrates. Cellulose materials can also tolerate a much higher processing temperature than many plastics. The high transparency and flexibility of cellulosic film allow it to replace plastic substrates in a wide range of applications. Various electronic devices have been successfully demonstrated on plastic substrates (PET, PEM, PC, and PI) including transistor backplanes, thin film transistors, organic light emitting diodes, UV protection films,
INTRODUCTION Excessive exposure to sunlight increases biological damage and degradation of organic compounds by UVA (320−400 nm) and UVB (280−320 nm) radiation.1 UV radiation is responsible for the discoloration of dyes and pigments, weathering, yellowing of plastics and papers, loss of mechanical properties (cracking), sun burnt skin, and other problems associated with UV light. Paints, plastics, wood, and cosmetic manufacturers have a great interest in offering products that remain unaltered for long periods of time under severe light exposure conditions.2,3 UV absorbers have been used in order to reduce these damaging effects to achieve a good conservation of the materials properties. The active ingredients in sunscreens are often synthetic chemicals. Long-time use of such chemicals may cause unexpected side effects and thus are receiving significant attention recently. Natural products, such as green coffee oil and extracts of carica papaya and rosa kordesii, have been shown to have radiation protection functions.4,5 Many natural sunscreens also have good antioxidant properties. However, most of the natural products are part-spectrum sun blockers and cannot block the full spectrum of UV light. In addition, the extraction of the active ingredients from raw materials can be expensive. Cellulose, the most abundant natural polymer in nature, is renewable, biodegradable, and biocompatible. Production of © 2016 American Chemical Society
Received: August 19, 2016 Revised: November 9, 2016 Published: November 10, 2016 625
DOI: 10.1021/acssuschemeng.6b02003 ACS Sustainable Chem. Eng. 2017, 5, 625−631
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ACS Sustainable Chemistry & Engineering
for 12 h. The final transparent cellulose solution was kept in a sealed container in a refrigerator. Kraft Lignin Propargylation. The phenolic hydroxyl group in the lignin was replaced with a propargyl group using propargyl bromide.22 A total of 1.0 g of kraft lignin was dissolved in 20 mL of aqueous 0.5 N NaOH at room temperature. A 0.4 mmol amount of propargyl bromide per mmol of total phenolic OH in the lignin was added to the reaction medium and heated at 75 °C for 2 h under continuous stirring. The reaction medium was cooled to room temperature and the product precipitated by addition of 2 N HCl solution to pH = 2. The product (lignin-Pr) was washed with excess deionized water followed by drying in a freeze drier. The amount of propargylation of the product was measured by quantitative 31P NMR. About 30% of the phenolic OH in the lignin, 1.4 mmol/g lignin, was converted to the propargyl group. Click Reaction of MCC-Az with Lignin-Pr. Conditions for the Cu(I)-catalyzed Huisgen−Meldal−Sharpless 1,3-dipolar cycloaddition reaction were selected from the literature.21 Lignin-Pr (0.5%, 1%, and 2% of MCC-N3) was dissolved in 1 mL of DMAc. A 5 mg amount of CuBr2, 10 mg of ascorbic acid, and 50 μL of triethylamine were then added to the lignin solution. This solution was added to the MCC-N3 solution in DMAc/LiCl containing 1 g of cellulose and preheated to 50 °C followed by stirring at 50 °C for 12 h. Cellulose-Lignin Film Preparation. The final solution of the cellulose-lignin product was cast on smooth and flat glass plates. The concentration and thickness of the solution on the plate was 5% w/v and 2 mm, respectively. Casted cellulose-lignin solutions were immersed in an acetone bath for 1 h followed by washing with a stream of cold water for 5 h to remove the salt completely. The final wet regenerated cellulose-lignin film was dried at room temperature on the same glass plate. FTIR. FTIR spectra of the cellulose samples were measured on a PerkinElmer Frontier FTIR spectrometer. A total of 32 cumulative scans in transmission mode were taken, with a resolution of 1 cm−1 in the frequency range 4000−600 cm−1. Irradiation of the Samples. Selected samples were irradiated with a 200 W Xe−Hg lamp for 2 h. A small fan was used to avoid heating of the sample during irradiation. The distance of the center of the sample to the UV lamp was 25 cm. UV−Vis. The optical properties of the cellulose and cellulose-lignin films were investigated using a Varian Cary 300 UV−vis spectrometer in the wavelength range 200−600 nm. A wavelength of 550 nm was used for determination of film transparency.12 The % transparency of the film in the ranges 280−320 nm (UV-B) and 320−380 nm (UV-A) was determined. Oxidation Induction Temperature (OITtemp). The oxidation induction temperature, OITtemp (also called oxidation onset temperature OOT), is the extrapolated onset temperature during heating in the presence of oxygen that appears as an exothermic DSC signal, corresponding to the temperatures at which the material starts to effectively burn. A TA-Instrument model TA-Q100 was used in the temperature range 40−350 °C. All samples were dried at 40 °C for 24 h in a vacuum oven prior to analysis. Approximately 5 mg of a sample was weighed directly into a DSC hermetic aluminum pan, which was then covered with its lid and sealed by pressing. Three small holes were created in the lid. After being loaded into the TA-Q100, all samples were heated to 105 °C at a rate of 5 °C/min under a N2 gas stream and then isothermally conditioned at this temperature for 20 min prior to being quenched to 40 °C and held for another 5 min. Finally, the DSC thermograms were recorded by increasing the temperature to 300 °C at a rate of 5 °C/min under an oxygen stream to obtain the OITtemp data. XRD. The crystallinity of the original cellulose (MCC-Az), dried cellulose and cellulose-lignin films was determined using a Rigaku Smartlab X-ray diffractometer. The diffractometer was equipped with Be-filtered Cu Kα radiation with a wavelength of 1.54 Å generated at 35 kV and 25 mA. The samples were scanned from a 2θ range from 5° to 45° at an increment of 0.05°. Relative crystallinity was calculated from the intensity measurements using the Segal method23 (eq 1).
and others. Challenges remain with plastic substrates, including a low processing temperature, a large coefficient of thermal expansion (CTE), and poor printability and recyclability, potential properties in which cellulose based materials may offer an advantage. Developing a new generation of self-protective sunscreen products such as visually transparent UV filters is important due to disadvantages of petroleum based polymers.13 Development of cellulose films containing UV absorbers has many potential applications, such as outdoor UV-sensitive polymers, car windshields, clean windows, contact lenses, and some special biological test containers. Preparation of cotton fabrics modified with ZnO nanorods on the surface resulted in UVblocking in the range 400−280 nm.14,15 Also, nanocellulose based films with firm and uniform networks, excellent optical transparency, and high temperature stability in blends with ZnO were reported as UV blockers.16 Lignin is the only component in lignocellulosic plant materials rich in aromatic rings due to its basic phenylpropane unit.17 It also contains UV-absorbing functional groups such as phenolic units, ketones, and other chromophores.18 Chromophores in the lignin structure make it a natural broad-spectrum sun blocker.19 In addition to the UV absorption property, the free radical scavenging ability of phenolic groups gives lignin an excellent antioxidant property and can increase thermal and oxidation stability of polymers in blends.2,18,20 However, lignin is a branched phenolic polymer and is not compatible with carbohydrate polymers like cellulose. To make effective composites of other natural polymers with lignin, it is sometimes necessary to connect the polymers with a covalent bond. In this study, cellulose based transparent films containing 0.5−2% covalently bonded lignin were generated. Cellulose modified with azide functional groups was dissolved in DMAc/ LiCl and reacted with lignin containing propargyl groups using click chemistry. The physical and thermal properties as well as the UV protection capability of the cellulose-lignin films under different conditions are reported.
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MATERIALS AND METHODS
Materials. Microcrystalline cellulose (Advice HP-105), purchased from Sigma-Aldrich, was used in an air-dry state. DMAc, LiCl, acetone, methanol, and glycerol were of the highest commercially available purity. A commercially available (Mead Westvaco) softwood kraft lignin (KL), Indulin AT, was used. Microcrystalline Cellulose (MCC) Bearing Surface Azide (MCC-Az). MCC surface-tosylation was performed using tosyl chloride.21 A 2 g amount of MCC was suspended in pyridine (40 mL), and the mixture was cooled from room temperature to 10 °C. Tosyl chloride (2 g) was then added, and the reaction mixture was stirred for 2 days at room temperature. The product (MCC-Tos) was washed with ethanol (50 mL) and deionized water (50 mL) five times each and freeze-dried. MCC-Tos was then suspended in 50 mL of DMF (N,N-dimethylformamide), followed by careful addition of 1 g of sodium azide. The reaction mixture was stirred at 100 °C for 24 h. After precipitation through the addition of 50 mL of deionized water and centrifugation, the product was washed five times with ethanol (50 mL) and five times with deionized water (50 mL). The resulting product was dialyzed against deionized water for 3 days and, consequently, freezes dry (MCC-Az). Dissolution of MCC-Az in DMAc/LiCl. A 2 g amount of dried MCC-N3 mixed with 25 mL of DMAc was maintained at 130 °C for 30 min. A 2 g amount of LiCl was added to the mixture and heated for 10 more minutes. Then, the mixture was stirred at room temperature 626
DOI: 10.1021/acssuschemeng.6b02003 ACS Sustainable Chem. Eng. 2017, 5, 625−631
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ACS Sustainable Chemistry & Engineering Scheme 1. General Route for the Synthesis of Cellulose-Lignin Conjugate
⎛ I − IAM ⎞ XC% = ⎜ 002 ⎟100% ⎝ I002 ⎠
(1)
In this method, I002 represents the maximum intensity of the (002) lattice diffraction peak at a diffraction angle around 2θ = 22.5°, and IAM represents the intensity scattered by the amorphous component in the sample, which was evaluated as the lowest intensity at 2θ of 18°. Tensile Properties. Tensile tests were carried out on an Instron 4465 machine at room temperature using deformation rates of 2.5 mm/min with sample sizes of 25 × 5 × 0.050 mm3. Prepared samples were strained until their ultimate break to study the strain hardening behavior. Stress versus strain was measured within the guidelines of the ASTM 638 standard and the strain hardening stiffness calculated. Independent replicates and repeats were carried out.
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RESULTS AND DISCUSSION Figure 1. FTIR spectra of microcrystalline cellulose bearing azide group (MCC-Az) and its treated sample simply with 2% lignin containing ethyne groups after simple mixing and after bonding with the “click reaction”.
Indulin kraft lignin was used in this study. The average weight and number molecular weight of the lignin were 6000 and 1500 g/mol, respectively. The total phenolic hydroxyl group was measured by 31P NMR to be 4.6 mmol/g lignin. Incorporation of the alkyne group in lignin was achieved in a one pot reaction using propargyl bromide in NaOH solution at 80 °C for 2 h (Scheme 1).22 The level of propargylation was controlled by controlling the mole ratio of propargyl bromide to total phenolic OH in the lignin. 31P NMR was used to measure the amount of lignin propargylation. A 30% conversion of total phenolic OH into alkyne group, equal to 1.4 mmol/g lignin, was used for the click reaction with the CMC-Az. Modification of the primary hydroxyl groups on the cellulose surface with the azide groups was achieved in two steps (Scheme 1).21,24 First, MCC was suspended in pyridine and treated with tosyl chloride, yielding the surface-tosylated cellulose analogue. The tosyl substituents were subsequently displaced by azide groups using sodium azide in a DMF suspension, yielding MCC-Az. Reaction of the lignin-Pr with the MCC-Az was achieved by the Cu(I)-catalyzed Huisgen−Meldal−Sharpless 1,3-dipolar cycloaddition reaction. CuBr2 in combination with ascorbate in DMAc at 50 °C in the presence of triethylamine catalyst was utilized for the click reaction.21,24 Clear evidence for the successful reaction of lignin-Pr with the MCC-Az was obtained from IR spectroscopic analysis (Figure 1). The IR spectrum of the azide surface modified cellulose, MCC-Az, exhibited an azide band at 2100 cm−1, within the range 2160−2080 cm −1 expected for the antisymmetric azide stretching motion.24,25Upon reaction with the ethyne containing lignin, the azide band intensity was decreased by 75%. Due to solubility problems of cellulose
and its modified products in the NMR solvents, FTIR is utilized as the main method to verify the click reaction chemistry.24,25 Also, for more verification, we did a control reaction; dissolved MCC-Az in DMAc/LiCl was mixed with dissolved lignin-Pr in DMAc for 24 h without the click reaction catalyst (CuBr2). The FTIR spectrum of the product showed no reduction at 2100 cm−1 which demonstrated that the lignin was simply mixed with the cellulose and was not covalently bonded. Images of the films after casting, washing, and drying are shown in Figure 2. The bonded sample is a homogeneous film with even distribution of lignin. For the control sample, an unmodified cellulose solution was mixed simply with an unmodified lignin solution without catalyst present. After regeneration with acetone, the lignin was not distributed evenly in the final film and phase separated from the cellulose (Figure 2D). The results show that lignin is not compatible with the cellulose and needs to be chemically bonded to the cellulose for uniform distribution in the film. The cellulose film and its composite with lignin were stable and transparent even after 24 h soaking in water. X-ray Diffraction. The crystal structure of the original cellulose and its prepared cellulose film with and without lignin was investigated using XRD analysis. Natural cellulosic fibers are known to display X-ray diffraction (XRD) patterns typical of cellulose type I, with the main diffraction signals at 2θ values of 15°, 16°, 22.5°, and 34° attributed to the diffraction planes 101, 101̅, 002, and 040, respectively.26−28 Figure 3 shows the 627
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Figure 2. Cellulose film (2A), cellulose azide film with 1% bonded lignin (2B), cellulose azide film with 2% bonded lignin (2C), and 2% lignin simply physically mixed with cellulose (2D). All films were prepared by making solutions in DMAc/LiCl and then using acetone as coagulator.
XRD patterns obtained for the MCC-Az, MCC-Az film, and cellulose-lignin (2% lignin) film. Crystallinity of the original cellulose sample, MCC, was calculated as about 85%29 and showed clear and sharp signals at 15°, 16°, 22.5°, and 34° which are typical for pure cellulosic fiber. However, dried cellulose film after regeneration of cellulose solution in DMAc/ LiCl using acetone showed a broad spectrum with three diffraction peaks at 13.4°, 14.5°, and 21° which shifted to lower degrees. Due to the lack of good signals and the disruption of the crystal structure during regeneration, it is not possible to measure cellulose film crystallinity. The XRD spectra of the cellulose film containing 2% bonded lignin showed an amorphous structure as well with only three weak peaks. Calculation of the crystallinity was not possible. It seems that the branched structure of the lignin contributed to
the lack of rearrangement and assembly of the cellulose chains into ordered structures. UV Protection. Due to its phenolic structure, lignin is an excellent light absorber (190−400 nm, with absorption peaks near 200, 230, and 280 nm). Absorbance of light in the UV-B range (280−320 nm) provides potential for lignin to be used as a natural UV protective material. To investigate the UV protection of cellulose transparent films containing low concentrations of lignin, cellulose films were prepared with 0.5−2% bonded lignin. The UV−vis transmittance of the cellulose and celluloselignin films (thickness of around 50 μm) was measured in the wavelength range 200−600 nm, Figure 4. The transmittance incorporates all light transmitted in the forward direction, excluding any absorption or back scattering. The neat cellulose film alone showed a high transparency, about 95%. Films prepared through bonding of lignin with cellulose showed
Figure 3. XRD spectra of the MCC-Az, MCC-Az film, and cellulose2% lignin film.
Figure 4. Transmittance of cellulose and cellulose bonded with lignin films from 200 to 600 nm at different levels of lignin loading. 628
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ACS Sustainable Chemistry & Engineering lower transparency. In the case of 2% lignin addition, the film had an about 60% transparency (at 550 nm). Transmittance of UV at UV-B (280−320 nm) was used to calculate the sun protection factor (SPF).30 The sun protection factor is widely used for sunscreen creams and UV protection materials and calculated on the basis of UV absorption of the material in the UV-B spectrum (280−320 nm) range. The percentage of UV protection is equal to 100-(100/SPF). For example, a material with SPF = 15 means it can protect 100 − (100/15) = 93.33% of UV-B light. The cellulose film without lignin showed a high transparency of 95% at 550 nm to 80% at 250 nm, indicating that the cellulose film has basically no effective UV protection in both the UV-B and UV-A ranges. With bonding of only 0.5% lignin to cellulose, the transmittance of UV-B light was reduced to around 15−30% which is equal to an SPF value of 15. In the UV-A range, the 0.5% lignin film had a UV light transmittance of 30−65%. With 2% lignin content, the film absorbed or blocked around 100% of UV-B (equal to an SPF of about 100) and also blocked a majority of UV-A (320−340 nm). The UV protection efficiency of the produced film is comparable to synthetic polymers coated with UV protection material31,32 The service lifetime of UV protection materials is of significant concern. To investigate effects of UV irradiation on the film UV protection stability, cellulose film as a control sample and its 2% lignin treated sample were irradiated under strong UV irradiation using a UV xenon lamp for 2 h and then analyzed for the UV−vis light transmittance, Figure 5.
Figure 6. Effects of heating to a temperature at 120 °C for 2 h of the cellulose film and cellulose bonded with 2% lignin film sample on their light transmittance behavior.
Thermal Stability. The thermal degradation of cellulose film and its lignin bonded sample can be evaluated by determining the oxidation induction temperature (OITtemp), which reflects the onset temperature of combustion in oxygen. The OITtemp of the cellulose film was determined to be around 245 °C (Figure 7). By bonding 2% kraft lignin with cellulose,
Figure 7. Heat flow versus temperature in an oxygen environment for cellulose film and cellulose bonded with 2% lignin film sample to determine the oxidation induction temperature (OITtemp).
Figure 5. Effects of high power UV irradiation of cellulose film and the cellulose bonded with 2% lignin film sample on their light transmittance behavior.
the OITtemp values of cellulose-lignin film increased to 262 °C. It was demonstrated that the presence of the phenolic OH groups within the lignin structure2,18,20 and the aromatic char originating from the lignin at elevated temperature33 are responsible for the beneficial effects toward oxidative, thermal, and light stability characteristics of the lignin when present in blends with other polymers. Tensile Properties. Mechanical properties of the films are important for different applications.34 The stress−strain curves for the dried cellulose film and its 2% lignin containing sample (around 6% water content in both samples) are shown in Figure 8. The cellulose film has a tensile strength of 118 MPa at break, with yield strength of 81 MPa and a Young’s modulus of 3200 MPa. The maximum strain for the cellulose film was 10%. Cellulose film bonded with 2% lignin showed 92 MPa tensile strength that break with yield strength of 85 MPa and a maximum strain of around 6%. Lignin acts as a cross-linker
Nontreated cellulose film UV absorption was stable after UV irradiation. Significantly, lignin containing film showed complete stability in the UV-B range (280−320 nm). In the UV-A range (320−400 nm), after UV irradiation the film showed only between 2% and 10% reduction in UV light transmittance and absorbed around 90% (average) of UV-A light. Thermal cycling and elevated temperatures are also important stressors to UV protective films. The effect of being held at 120 °C for 2 h on the UV−vis light transmittance of the films is shown in Figure 6. The results revealed that the effect of heating even at 120 °C for 2 h is negligible on the film UV light transmittance. Unexpectedly, the heat treatment actually improved/increased the light transmittance in the visible range. 629
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ACKNOWLEDGMENTS
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REFERENCES
We would like to thank the Southeastern Sun Grant Center and USDA for the support of this project. We would also like to thank the North Carolina State University Research and Innovation Seed Funding program for its support.
(1) Hattori, H.; Ide, Y.; Sano, T. Microporous titanate nanofibers for highly efficient UV-protective transparent coating. J. Mater. Chem. A 2014, 2, 16381−16388. (2) Gerlock, J. L.; Kucherov, A. V.; Smith, C. A. Determination of active HALS in automotive paint systems II: HALS distribution in weathered clearcoat/basecoat paint systems. Polym. Degrad. Stab. 2001, 73, 201−210. (3) Girois, S.; Delprat, P.; Audouin, L.; Verdu, J. Kinetic study of the photostabilisation of polypropylene films by anhydroxyphenylbenzotriazole. Polym. Degrad. Stab. 1999, 64, 107−114. (4) Maske, P. P.; Lokapure, S. G.; Nimbalkar, D.; Malavi, S.; D'souza, J. I. In vitro determination of sun protection factor and chemical stability of Rosa kordesii extract gel. J. Pharm. Res. 2013, 7, 520−524. (5) Jarzycka, A.; Lewinska, A.; Gancarz, R.; Wilk, K. A. Assessment of extracts of Helichrysum arenarium Crataegus monogyna Sambucus nigra in photoprotective UVA and UVB; photostability in cosmetic emulsions. J. Photochem. Photobiol., B 2013, 128, 50−57. (6) Ass, B. A. P.; Belgacem, M. N.; Frollini, E. Mercerized linters cellulose: characterization and acetylation in N,N-dimethylacetamide/ lithium chloride. Carbohydr. Polym. 2006, 63, 19−29. (7) Zhang, X.; Liu, X.; Zheng, W.; Zhu, J. Regenerated cellulose/ graphene nanocomposite films prepared in DMAC/LiCl solution. Carbohydr. Polym. 2012, 88, 26−30. (8) Qi, H.; Chang, C.; Zhang, L. Properties and applications of biodegradable transparent and photoluminescent cellulose films prepared via a green process. Green Chem. 2009, 11, 177−181. (9) Morgenstern, B.; Kammer, H. W. On the particulate structure of cellulose solutions. Polymer 1999, 40, 1299−1304. (10) McCormick, C. L.; Lichatowich, D. K. Homogeneous solution reactions of cellulose, chitin, and other polysaccharides to produce controlled-activity pesticide systems. J. Polym. Sci., Polym. Lett. Ed. 1979, 17, 479−484. (11) Cai, J.; Zhang, L.; Zhou, J.; Qi, H.; Chen, H.; Kondo, T.; Chen, X.; Chu, B. Multifilament fibers based on dissolution of cellulose in NaOH/urea aqueous solution: structure and properties. Adv. Mater. 2007, 19, 821. (12) Zhu, H.; Xiao, Z.; Liu, D.; Li, Y.; Weadock, N. J.; Fang, Z.; Huang, J.; Hu, L. Biodegradable transparent substrates for flexible organic-light-emitting diodes. Energy Environ. Sci. 2013, 6, 2105−2111. (13) Han, K.; Yu, M. Study of the preparation and properties of UVblocking fabrics of a PET/TiO2 nanocomposite prepared by in situ polycondensation. J. Appl. Polym. Sci. 2006, 100, 1588−1593. (14) Wang, R. H.; Xin, J. H.; Tao, X. M. UV-Blocking Property of Dumbbell-Shaped ZnO Crystallites on Cotton Fabrics. Inorg. Chem. 2005, 44, 3926. (15) Mao, Z.; Shi, Q.; Zhang, L.; Cao, H. The formation and UVblocking property of needle-shaped ZnO nanorod on cotton fabric. Thin Solid Films 2009, 517, 2681. (16) Jiang, Y.; Song, Y.; Miao, M.; Cao, S.; Feng, X.; Fang, J.; Shi, L. Transparent nanocellulose hybrid films functionalized with ZnO nanostructures for UV-blocking. J. Mater. Chem. C 2015, 3, 6717− 6724. (17) Argyropoulos, D. S.; Menachem, S. B.; Kaplan, D. L. Lignin. Biopolymers From Renewable Resources; Springer, 1998; Chapter 12, pp 292−322. DOI: 10.1007/978-3-662-03680-8_12. (18) Lignin: Historical, Biological and Material Perspectives; Glasser, W. G., Robert, R. A., Schultz, T. P., Eds.; American Chemical Society, 1999. (19) Zimniewska, M.; Batog, J.; Romanowska, B.; Bogacz, E. Functionalization of Natural Fibres Textiles by Improvement of
Figure 8. Tensile strength of cellulose film and cellulose bonded with 2% lignin film.
between cellulose chains in the cellulose-lignin films, and it can be the source of the lower strain to break. XRD results showed better crystallinity for the cellulose film than its treated sample with lignin which can be another reason for the approximately 20% higher tensile strength of the cellulose film. In comparison with other data in the literature,35−37 both the cellulose film and cellulose-lignin films herein possess reasonably high strength properties.
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CONCLUSION
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AUTHOR INFORMATION
Lignin can be used as a renewable biomass derived broadspectrum sun blocker in cellulose transparent films. Nonderivatized cellulose film with high transparency, flexibility, and mechanical properties containing small amounts of lignin could be prepared using DMAc/LiCl as the solvent, but the films were not homogeneous. In contrast, after dissolution of cellulose bearing azide groups in DMAc/LiCl, up to 2% of kraft lignin bearing propargyl groups were able to be reacted with cellulose through “click chemistry” to form homogeneous films. Transparency of the neat cellulose film was around 95% at 550 nm without any significant UV absorption for UV-B and UV-A. The addition of up to 2% bonded lignin in the cellulose increased its UV absorption potential. Cellulose film containing 2% lignin showed around 100% absorption of UV-B (280−320 nm) and more than 90% of UV-A (320−400 nm). The prepared film was stable against elevated temperature at 120 °C and UV irradiation. Tensile strengths of the cellulose and the cellulose-lignin films were around 90 MPa with about 6% strain to break. This research demonstrated the potential of lignin as a component in cellulose-lignin UV light blocker films.
Corresponding Author
*E-mail:
[email protected]. ORCID
Hasan Sadeghifar: 0000-0001-5569-3180 Notes
The authors declare no competing financial interest. 630
DOI: 10.1021/acssuschemeng.6b02003 ACS Sustainable Chem. Eng. 2017, 5, 625−631
Research Article
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DOI: 10.1021/acssuschemeng.6b02003 ACS Sustainable Chem. Eng. 2017, 5, 625−631
