A Protonation Process to Enhance the Water Resistance of

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Protonation Process to Enhance the Water Resistance of Transparent and Hazy Paper Wen Hu,† Zhiqiang Fang,*,†,‡ Yu Liu,† Yicong Zhou,§ Yudi Kuang,† Honglong Ning,§ Bo Li,† Gang Chen,*,†,∥ and Yingyao Liu†,∥

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 08/17/18. For personal use only.

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State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Wushan Campus, 381 Wushan Road, Tianhe District, Guangzhou 510641, China ‡ South China Institute of Collaborative Innovation, South China University of Technology, Songshan Lake High-tech Industrial Development Zone, Dongguan 523808, China § State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Wushan Campus, 381 Wushan Road, Tianhe District, Guangzhou 510641, China ∥ Guangdong Engineering Technology Research and Development Center of Specialty Paper and Paper-based Functional Materials, South China University of Technology, Wushan Campus, 381 Wushan Road, Tianhe District, Guangzhou 510641, China S Supporting Information *

ABSTRACT: Marrying a randomly distributed network of wood fibers with sodium carboxymethyl cellulose (CMC-Na) by impregnation is a scalable method to prepare highly transparent and hazy (TH) paper with superior mechanical properties. However, the poor water resistance of this TH paper is a hindrance for its practical applications. In this study, a facile protonation process along with desalination is proposed to remove the above obstacle by reducing electrostatic repulsion among CMC molecular chains while strengthening the physical cross-linking among CMC molecular chains and also between CMC-H molecular chains and wood fibers in water. Consequently, the protonated TH paper not only presents an enhanced water resistance but also shows a negligible deterioration in optical and mechanical properties compared with the untreated TH paper. Moreover, the influence of the water resistance of various substrates (PET film, untreated TH paper, and protonated TH paper) on the electrical conductivity of inkjet-printed silver nanoparticle lines on them is investigated. We expect that such water resistant, transparent paper with high optical haze may open new avenues for the innovative application of paper and bring added value to existing products. KEYWORDS: Transparent and hazy paper, Water resistance, Protonation, Silver nanoparticle line

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tion,14 or polymer grafting;15 (2) physical adsorption of cationic hydrophobic substances on the surface of cellulose fibers through electrostatic adsorption action;16−19 and (3) cross-linking of hydroxyl groups in cellulose.20−27 These strategies could endow the transparent paper with desirable water resistance, but they have disadvantages for enhancing the water resistance of the TH paper made of CMC-Na and wood fibers. Chemical modifications may damage the crystal structure or degree of polymerization of cellulose with various forms, thus resulting in reduced mechanical properties, especially flexibility. The use of hydrophobic surfactants could weaken the hydrogen-bonding network in the paper, leading to a decrease in the tensile strength. For cross-linking

INTRODUCTION Incorporating transparent paper with high built-in transmission haze into optoelectronics1−6 or CO2 photoreduction7 as a functional component has garnered growing interests both from academic communities and industries. Transparent and hazy (TH) paper consisting of sodium carboxymethyl cellulose (CMC-Na) and wood fibers has emerged as a promising substrate for high-tech applications owing to its facile and scalable manufacturing technology and superior mechanical properties, especially folding endurance.8 Unfortunately, it is still challenging to obtain water resistant TH paper due to the moisture sensitivity of CMC-Na.9,10 Several strategies have been proposed to enhance the water resistance of transparent paper made of microscale cellulose fibers, nanocellulose, molecular cellulose chains, or their mixtures, for example, (1) chemical modifications of cellulose with various forms by silylatio,11 esterification,12,13 acetyla© XXXX American Chemical Society

Received: June 19, 2018 Revised: August 4, 2018 Published: August 6, 2018 A

DOI: 10.1021/acssuschemeng.8b02900 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Schematic diagram showing the procedure of preparing water resistant, hazy, transparent paper made of CMC-Na and intertwined network of wood fibers via a combination of protonation and desalination. (b) Chemical reaction showing the protonation process of CMC-Na through acetic acid. during protonation. Reactive dye was used to stain the protonated TH paper of wood fibers and carboxymethyl cellulose (CMC-H). Preparation of Conductive Silver Line. Silver nanoparticle lines with 40 mm in length and 1 mm in width were inkjet-printed on PET film, untreated TH paper, and protonated TH paper using MOD silver inks. The inkjet-printed conductive lines were then heated at 100 °C for 30 min in air. Characterizations. The thickness of TH paper was determined by a thickness tester (L&W, Sweden). The length of TH paper was obtained by a vernier caliper. Optical transmittance and haze of the TH paper before and after treatment were performed by a BX51 Optic Microscope (Olympus, Japan). Strain−stress curves were obtained by a 5565 universal material experiment machine (Instron, Boston, U.S.A.). The changes in chemical structure were determined by using Vertex 70 Fourier-transform infrared spectroscopy (FTIR; Bruker, Germany). Na concentration was measured by a Z-2000 atomic absorption spectrometer (Hitachi, Japan). Scanning electron microscope (SEM) measurements and energy dispersive spectrometer (EDS) were carried out on an EVO 18 SEM (Carl Zeiss, Germany) with a voltage of 10 kV. The water contact angles were measured on an OCA40 Micro contact angle meter (Dataphysics, Germany). Water absorption and the swelling ratio were measured by the ASTM D570 method. Tensile strength after immersion in water was measured by the ISO 3781:2011 method. The folding endurance was conducted on an MIT/U21B (America) with a folding angle of 135 degrees at 175 folds per minute. The electric resistance was measured at room temperature by use of a DY2105 digital multimeter (Shenzhen, China).

treatment, the transparency of the TH paper could be reduced since cellulose fibers and anionic CMC-Na carried stronger negative charges on their surface and have stronger affinities for cationic cross-linkers. Therefore, an effective method to improve the water resistance of the TH paper with CMC-Na is highly desirable. Herein, we report a strategy involving protonation and desalination to enhance the water resistance of TH paper made of CMC-Na and an intertwined network of wood fibers while retaining its excellent optical and mechanical properties. The TH paper was impregnated with 98.0% acetic acid solution to exchange sodium ions in CMC-Na with hydrogen ions, followed by a desalination process of removing residual sodium acetate in an ethanol bath. The underlying mechanism for the enhanced water resistance of the protonated TH paper was explored. In addition, the optical and mechanical properties of the TH paper before and after treatment were investigated. Lastly, silver nanoparticle lines were inkjet-printed on various substrates (PET film, untreated TH paper, and protonated TH paper), attempting to evaluate the effect of the substrates’ water resistance on the electrical conductivity of silver nanoparticle lines on them. This work could open up a wide range of novel end-use applications for transparent paper and bring added value to current commercial products.

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EXPERIMENTAL SECTION

Materials. Softwood pulp and active dyeing were kindly provided by Lee & Man Paper Manufacturing Ltd. (Dongguan, China). Sodium carboxymethyl cellulose (CMC-Na) [M.W. 250 000 (DS = 0.7), 1500−3100 mPa s] was purchased from Aladdin biochemical technology (Shanghai, China). 98% acetic acid (analytical reagent) was purchased from RichJoint Chemical Reagents Co., Ltd. (Shanghai, China), and absolute ethanol (analytical reagent) was purchased from Fuyu Fine Chemical Co., Ltd. (Tianjin, China). The particle-free metallo-organic decomposition (MOD) silver inks with methanol−anisole solutions were purchased from InkTec (TEC-IJ010, InkTec Co., Ltd., Korea). Fabrication of TH Paper. Paper with a grammage of 21 g/m2 was prepared by a RK3AKWT sheet former (PTI, Austria). The paper was then immersed into aqueous CMC-Na solution (1 wt %) to prepare TH paper with a thickness of 70 μm by drying in a constanttemperature humidity chamber at 50% relative humidity (RH%) and 42 °C. Reactive dye was used to stain the untreated TH paper of wood fibers and CMC-Na. Protonation and Desalination Process on TH Paper. The TH paper was first soaked in 98% acetic acid to convert the −COO− groups of CMC-Na to −COOH groups, followed by a desalination process in 99.5% alcohol bath to remove sodium acetate generated

RESULTS AND DISCUSSION Highly transparent paper prepared by infiltration of CMC-Na into an intertwined network of wood fibers exhibits high transparency, high haze, and superior mechanical properties, especially folding endurance, showing its promising application in optoelectronics or CO2 photoreduction as a light management layer. However, CMC-Na is a water-soluble material that would bring great difficulties to device fabrication and to deteriorate device performance under wet conditions. As shown in Figure S1a of the Supporting Information (SI), the untreated TH paper demonstrates an almost ten times increase in thickness after soaking in water for only 30 s. Meanwhile, dyed CMC-Na within untreated TH paper partially dissolved in water. Herein, we design a facile strategy involving protonation and desalination to prepare water-resistant TH paper, and the entire procedure is shown in Figure 1a. The untreated TH paper was first protonated with 98% acetic acid, followed by a desalination process in absolute alcohol. In the protonation B


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ACS Sustainable Chemistry & Engineering process, the −COO− groups on CMC-Na molecular chains are protonated to −COOH groups (Figure 1b), and the sodium acetate generated during protonation was removed by absolute alcohol. Therefore, the protonated TH paper demonstrates an enhanced water resistance. As shown in Figure S1b, the thickness of the protonated TH paper is almost unchanged, and no obvious diffusion of dye into water is observed after immersion in water. In addition, the protonated TH paper exhibits a similar visual appearance compared to the untreated TH paper (Figure S2). A FTIR analysis was performed to verify the protonation of CMC-Na in the TH paper. Figure 2 shows the spectra of the

spectra in Figure S3. According to the top-view and crosssectional SEM images in Figure S3, both the untreated and the protonated TH paper have a densely packed configuration, which is contributed to the negligible change in optical and mechanical properties of the protonated TH paper. A 6-h-long water stability test was carried out to compare the water resistance of the TH paper before and after treatment. As shown in Figure 3a, the dyed CMC-Na in the

Figure 2. FTIR spectra of the TH paper before and after protonation.

TH paper before and after treatment. The untreated TH paper shows a strong absorption peak around 1588 cm −1 corresponding to the symmetric νCO vibration of −COO− groups. After protonation, a new peak at 1727 cm−1,28,29 originating from the same symmetric (νCO) vibration of −COOH groups, appears in the spectra, which indicates most of the −COO− groups on CMC-Na molecules are protonated to −COOH under acid environment. The OH stretching band (νOH) of CMC-H molecular chains or cellulose fibers appears in a range of 3650−1650 cm−1.30 Interestingly, there is a new peak around 1645 cm−1 for the protonated TH paper, we can exclude the possibility of assigning that weak, broad bands to the OH band of water molecules because our treatment processes are carried out in anhydrous solvents.20,42 According to the relationship between −OH···O distance and νOH band position, the 1645 cm−1 of νOH corresponds to the OH···O distance of 2.4−2.5 Å,31 showing that new hydrogen bonds were formed by protonated −COOH groups on CMC-H molecules and adjacent −OH or −COOH groups due to the short OH···O distances. To qualify the degree of protonation of CMC-Na, an atomic absorption spectrometer was employed to analyze the sodium element in the TH paper before and after treatment, and the result is shown in Table 1. The sodium content in the untreated TH paper is approximately 6.58 ± 0.1%. After protonation, it dramatically decreases to 0.43 ± 0.03%. This result indicates most of the −COO− groups are efficiently converted to −COOH groups during the protonation process, which is in good agreement with FTIR analysis and EDS

Figure 3. (a) 6-h water stability test of the TH paper before and after protonation. The untreated paper dissolved into water, with only intertwined wood fiber network left over, while the protonated paper maintains its shape. (b) Water contact angels of the untreated and the protonated TH paper. (c) The change in thickness of the protonated TH paper and time-dependent water absorption with corresponding fitting curve based on the pseudo-first-order kinetic model.

untreated TH paper dissolves completely in water; only the intertwined wood fiber network was retained. During the protonation and desalination, water-soluble CMC-Na was converted to insoluble CMC-H, which was beneficial to enhance the water resistance of the protonated TH paper. As a consequence, the water manifested a transparent and clear appearance because no red active dye in the protonated TH paper diffused into water. Meanwhile, the mass retention of the untreated and the protonated TH paper after soaking in water was investigated as a function of immersion time (Figure S4a). The mass weight loss of the untreated TH paper is quite large due to the watersoluble property of CMC-Na, and only insoluble wood fibers were left over. However, for the protonated TH paper, the weight loss is negligible because of its good water resistance. Figure S4b shows the wet stress−strain curve of the protonated TH paper (immersion time is 1 h). The wet tensile strength is 18 MPa, and the strain-to-failure is in the range of 17−20%. The intertwined wood fiber network provides excellent wet strength and strain for the water resistant TH paper (inset in Figure S4b). Note that the untreated TH paper could not be subjected to the wet tensile test since the sample is soluble in water. In addition to the stability test of the untreated and protonated TH paper in water, their water contact angles (CA) were also characterized. As we can see from Figure 3b, the

Table 1. Sodium Element Content in TH Paper before and after Protonation sample

Na concentration/%

untreated TH paper protonated TH paper

6.58 ± 0.1 0.43 ± 0.03 C


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bonding interactions among H3O+, −COOH, and −OH (Figure 4d).46−50 In summary, the underlying mechanism for the enhanced water resistance of the protonated TH paper in water is suggested to be the reduced electrostatic repulsion between CMC-H molecular chains and the formation of additional physical cross-linking among CMC-H molecular chains and also between CMC-H molecular chains and wood fibers in water. In order to make the protonation process of CMC-Na more visible, aqueous CMC-Na solution was directly poured into ultrapure water and 98% acetic acid (Figure S5a). The ultrapure water with CMC-Na is still transparent and clear. However, for the 98% acetic acid, white flocculent precipitate is observed due to the protonation of soluble CMC-Na into insoluble CMC-H (Figure S5b). When used in electronic devices, biodegradable cellulose films are expected to present excellent mechanical and optical properties except for water resistance. Generally, improving the hydrophobicity of cellulose films may decrease the light transmittance (especially for cross-linking method) or mechanical properties (most of chemical modification and physical adsorption methods), which is undesirable for transparent paper. Herein, the optical and mechanical properties of the TH paper before and after protonation were investigated. As we can see from Figure 5a,b, the protonated TH paper (90.9% transmittance and 83.6% haze at 550 nm) shows a negligible declining trend compared to the untreated TH paper (transparency: 91.3%; haze: 84.5%). In addition, the average tensile strength at break of the untreated TH paper (108.5 ± 5.6 MPa) and the protonated TH paper (108 ± 4.7 MPa) is almost the same (Figure 5c). According to the FTIR analysis, there was no obvious increase in hydrogen bonds. The Young’s modulus and strain at break of the protonated TH paper exhibits a slight increase (Table S1). Folding endurance, a parameter to evaluate the flexibility of a film, is measured by repeatedly folding a film at the same point until it ruptures. The protonated TH paper exhibits a slight decrease in folding endurance reducing from 1015 to 994 (Figure 5d). Therefore, the protonation process is an effective approach to enhance the water resistance of CMC-based films while maintaining the mechanical and optical properties. Finally, the electrical conductivity of inkjet-printed silver nanoparticle lines on PET film, the untreated TH paper, and the protonated TH paper before and after water immersion test is evaluated (Figure 6). Among these samples, the conductive line printed on PET film show the best electrical conductivity with a resistance of 10 Ohm (Ω) (Figure 6a). Conductive lines of silver nanoparticles on the untreated TH paper and the protonated TH paper show resistances of 48 and 53 Ω, respectively (Figure 6b,c). To investigate the water resistance of substrates on the electrical conductivity of the printed silver nanoparticle lines, we immersed above three samples into water for 30 min (Figure 6d−f), followed by an air drying at 100 °C for 5 min. All samples after drying exhibited an increased electrical resistance. The inkjet-printed silver nanoparticle line on the hydrophobic PET film exhibited the lowest resistance of 18 Ω (Figure 6g). PET is a water resistant petroleum-based material. The silver nanoparticles on the PET film were well retained after water immersion (Figure S6a,b), so there was no obvious loss in electrical conductivity of silver nanoparticle line. In contrast, the resistance of the silver line on the untreated TH

protonated TH paper has a higher CA (72°) than the untreated paper (43°). Figure 3c shows the time-dependent water absorption and thickness of the protonated TH paper. There is about 25% increase in thickness of the protonated paper when immersing in water for 6 h. The water absorption of the protonated TH paper could be described by the equation of the pseudo-first-order kinetic model:32−35 dq t dt

= k t(qe − qt)

(1)

Applying the boundary conditions to the integral equation, qt = 0 at t = 0 and qt = qt at t = t, gives qt = qe(1 − e−k1t )

(2)

qe and qt (%) is the water absorption ratio at equilibrium and at time t, respectively. k1 (min−1) is the rate constant of the pseudo-first-order adsorption. Table 2 lists the results calculated from the pseudo-firstorder kinetic model. We could deduce the water absorption by Table 2. Pseudo-First-Order Kinetic Fitting Results for Time-Dependent Water Absorption of the Protonated TH Paper type

qe (%)

k1 (min‑1)

R2

water absorption

60.7109

6.0890

0.9861

substituting immersion time into the pseudo-first-order kinetic fitting results. We also attempt to explore the possible mechanism for the enhanced water resistance of the protonated TH paper (Figure 4). Figure 4a shows the chemical structures of CMC-Na and CMC-H. The −COONa in the CMC-Na is completely ionized upon exposure to water (eq 3).36 The ionized Na+ tends to form sodium ion hydrate with water molecules by ion hydration in water. There are approximately five H2O molecules surrounding a sodium ion to form a hydration shell that will block the electrostatic attraction between Na+ and −COO−.37−39 The electrostatic repulsive force between the −COO− groups, therefore, brings forth an increase in the water uptake capacity of CMC-Na due to the breakage of hydrogen bonding between the CMC-Na molecular chains. (Figure 4b)40 RCONa = RCOO− + Na +

(3)

RCOOH + H 2O F RCOO− + H3O+

(4)

After a protonation process, the −COONa in the TH paper was almost protonated into −COOH (Figure 4c). The −COOH groups on the CMC-H molecular chains are partially ionized in water [eq 4], thus tremendously restricting the electrostatic repulsion among −COO− groups that is beneficial to retain the hydrogen-bonding network within the protonated TH paper when immersed in water.41 While the −COOH groups prefer to form hydrogen bonds with adjacent −OH or −COOH groups, this suggests the formation of additional physical cross-linking among CMC-H molecular chains and also between CMC-H molecular chains and wood fibers in water.42−45 Moreover, the generated H+ appears in the form of solvated hydronium H3O+ in water that also probably contributes to the reduced water uptake capacity of the protonated TH paper due to the generation of hydrogen D


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Figure 4. Molecular structures of (a) CMC-Na and (c) CMC-H, and purple and green boxes represent −COONa and −COOH, respectively. (b) Schematic illustration of the mechanism for water dissolution of CMC-Na in the untreated TH paper. Sodium ion hydrates hinder the electrostatic attraction between −COO− and Na+, which leads to the electrostatic repulsive force among −COO− groups that facilitates the dissolution of CMC-Na in water. (d) Proposed mechanism for the water resistance of CMC-H molecular chains in the protonated TH paper. The formation of additional physical cross-linking among CMC-H molecular chains resulting from the generation of hydrogen bonding interactions among H3O+, −COOH, and −OH contributes to the water resistance of CMC-H.

Figure 5. (a) Total optical transmittance, (b) transmission haze, (c) tensile strength at break, and (d) folding endurance of the TH paper before and after protonation.

E


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Figure 6. Electrical resistance of conductive silver nanoparticle lines inkjet-printed on various substrates: (a) the PET film, (b) the untreated TH paper, and (c) the protonated TH paper. Digital images showing three samples immersed in water (d−f) and the corresponding resistance values (g−i). The measurement length of the conductive line is 2 cm, and the immersion time is 30 min.

the protonated TH paper has a positive effect on the electrical conductivity of silver nanoparticle lines. The water resistant, TH paper with superior mechanical properties is extremely attractive for innovative application of paper and brings added value to existing products.

paper exceeded the limitation of the device, with measurement in the range of 107 Ω (Figure 6h). We can obviously see the fracture of the silver line (Figure S6c) and shedding of silver nanoparticles (Figure S6d) due to the dissolution of the CMCNa molecule. However, the silver nanoparticle line on the protonated TH paper was well preserved after water immersion (Figure S6e and f), and a slight increase in resistance of 88 Ω was observed (Figure 6i), which might resulted from minor swelling in the length direction of the protonated TH paper (Figure S4c) when soaking it in water that led to a rougher surface. From the top-views SEM images of the PET film, untreated TH paper, and protonated TH paper, we also found that the PET film has the smoothest surface, which could contribute to the best conductivity and stability in water among the three samples.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02900. Digital photo, visual appearance and top-view, crosssectional SEM images (including EDS spectrum) and water-soluble matter percentage of the untreated TH paper and the protonated TH paper; strain−stress curve and the swelling ratio in length direction of the protonated TH paper; aqueous CMC-Na solution in water and 98% acetic acid; mechanical properties of the TH paper before and after protonation; top-view SEM images and corresponding magnified images of silver nanoparticle lines on PET film, the untreated TH paper, and the protonated TH paper after soaking and redrying (PDF)

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CONCLUSION In this report, a facile protonation process combining with desalination was utilized to enhance the water resistance of TH paper made of water-soluble CMC-Na and wood fibers, while maintaining its high transparency (90.9%), high transmission haze (83.6%), and superior mechanical properties (108 ± 4.7 MPa tensile strength and 994 folding times). The protonated TH paper exhibits a water contact angle of 72°, a change in thickness up to 25%, and a water absorption ratio approaching 60% after soaking it in water for 6 h, and the water absorption of the protonated TH paper was well-described by the pseudofirst-order kinetic model. The underlying mechanism for the enhanced water resistance of the protonated TH paper is primarily attributed to the reduced electrostatic repulsion among carboxyl groups and potential physical cross-linking among CMC-H molecular chains and wood fibers as a result of converting the −COO− groups into −COOH groups during the protonation process. In addition, the electrical conductivity of silver nanoparticle lines on PET, untreated TH paper, and protonated TH paper before and after water immersion was evaluated, and the results indicated that the water resistance of

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wen Hu: 0000-0002-9867-7006 Zhiqiang Fang: 0000-0002-0844-7507 Honglong Ning: 0000-0001-9518-5738 Notes

The authors declare no competing financial interest. F


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ACKNOWLEDGMENTS We appreciate the Young Scientists Fund of the National Natural Science Foundation of China (31700508), the Natural Science Foundation of Guangdong Province (2017A030310635), Pearl River S&T Nova Program of Guangzhou (201806010141), Science and Technology Program of Guangdong Province (2017B090903003), and the State Key Laboratory of Pulp and Papermaking Engineering (201709).

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A Protonation Process to Enhance the Water Resistance of

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