Article pubs.acs.org/IECR
Layer-by-Layer Self-Assembly for Reinforcement of Aged Papers Fuze Jiang, Youdi Yang, Jiajia Weng, and Xiaogang Zhang* Department of Chemistry, Renming University of China, Beijing 100872, China S Supporting Information *
ABSTRACT: The hybrid films of reinforcing the aged papers have been employed via layer-by-layer (LBL) assembly deposition of the chitosan lactate/carboxymethyl cellulose complex previously modified with metal oxide. In order to reinforce the films, a vacuum infiltration was adopted in the LBL process. The mechanical properties and structure of the aged papers in relation to fiber morphology were investigated using SEM, FTIR, XPS, and tensile test. The well-dispersed coatings resulted in an excellent improvement in mechanical properties of the modified paper. After only three bilayers deposited, the measured tensile strength and folding endurance for modified aged newspaper samples increased up to 100% and 450%, respectively, as compared to the control sample. This high efficiency was kept even when the modified paper samples exposed to the moist-heat accelerated aging and UV irradiation. The pH of the paper rose to alkalescence rapidly, and the paper samples maintained their coloration close to the original after treatment.
1. INTRODUCTION Paper is the main material for recording the cultural achievements all over the world. It is estimated that about 2.5 million kilograms of paper materials are stored in libraries and archives.1 The protection and preservation of paper-based documents is a necessary and routine work for libraries, archives, and museums, which are the guardians of the evolution of philosophical and scientific thinking in the development of human history.2 Paper consists mostly of bonded cellulose fibers that are linear polymers of glucose monomers linked by β-1,4-glycosidic bonds. Cellulose chains are held together by van der Waals interactions and strong intramolecular and intermolecular hydrogen bonds in matrices, which promotes an aggregation of single chains into the highly oriented structure.3 The mechanical properties of paper are determined by (a) the fiber strength, (b) the interfiber joint strength, and (c) the number of these interfiber joints. In addition to cellulose fibers, paper contains hemicelluloses, lignin, and certain amount of additives, e.g., fillers, pigments, and metal ions.4 There are two main types of deterioration: hydrolysis and oxidation.3,5 The glucosidic links of cellulose are stable in a neutral or weakly alkaline medium. On the other hand, they become hydrolyzed in the presence of a strong acid or a strong base. Many manuscript documents and paper literature resources kept in libraries and archives were in bad condition and could no longer be used by readers.6,7 External factors, such as air pollution, unfavorable climate, lighting conditions, and biological agents, might also be the causes of deterioration.8,9 Paper deterioration is a complicated process where internal and external factors can stimulate each other.9,10 Therefore, it is desirable to focus research works on conservation, reinforcement, and protection techniques to develop appropriate and long-term resistance © 2016 American Chemical Society
treatments. Reinforcement and deacidification treatment have been the issues in paper conservation with the target of retarding the deterioration and prolonging the life of paper archives. Deacidification as the main chemical treatment for paper documents is the neutralization of the acidic materials presented in the surface and interior of the paper, as well as sedimentation of alkaline substances on the surface of the paper to prevent or delay further acidification with natural aging.11 There have been numerous literature and reviews focused on the conservation of paper-based archives deacidification.6,12−14 For paper documents, polymeric materials have been applied only in a few cases when traditional restoration methods were not sufficient to improve the mechanical resistance of the degraded artifacts.15,16 Recently, the aminoalkylalkoxysilane or diaminealkylalkoxysilane has been introduced in a preservation strategy for cellulosic acidic and fragile paper.17,18 In these processes, alkoxysilane type of molecules could form interpenetrated networks with natural fibers, thereby resulting in a clear improvement in the mechanical properties of the paper, and the pH value and tensile-strength increased significantly after treatment. In our laboratory, the aged acidic papers were neutralized by potassium methyl siliconate. The mechanical properties of degraded paper samples were much improved after deacidification.19 The mechanical properties of paper are influenced by the strength of joints between cellulose fibers. To improve the strength properties of paper, strength additives are used to enhance the adhesion between the adjacent cellulose fibers, Received: Revised: Accepted: Published: 10544
August 5, 2016 September 15, 2016 September 22, 2016 September 22, 2016 DOI: 10.1021/acs.iecr.6b02988 Ind. Eng. Chem. Res. 2016, 55, 10544−10554
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Industrial & Engineering Chemistry Research
Figure 1. Schematic illustrations of detailed operation steps and assembly process.
performed as an indispensable agent of the Battelle deacidification process, which appeared as a positive contribution to the permanence of paper based materials.39 The titanium complex could spatially well embed in the hierarchically fiber matrix and form an extended interpenetrating network by the sol−gel process, which provided a unique and ideal platform for further assembly of various guest substrates and/or different functional groups onto the cellulose nanofiber surface, resulting in different functional nanoarchitectures.40,41 The development of effective methodology to focus on reinforcing cellulose materials on paper, in particular without changing the initial structural features of the raw cellulose, is still in strong demand and of great challenge. In the present work, chitosan lactate (CHL) is chosen owning to its solubility in a water-soluble chitosan derivative, which has been demonstrated to be ecofriendly in reinforcement to cellulose on paper. In the condition at pH higher than 6.0, the amines in chitosan lactate become deprotonated and lose their charge, resulting in a neutral polymer. The aim of the work is focused on increasing the pH value in reinforcement processes to favor the restoration of paper documents. In the work, we demonstrated a facile, ecofriendly, and economical methodology to reinforce the paper documents by self-assembly of organic−inorganic hybrid multilayer films on the paper surface. An ultrathin titania gel layer was previously deposited on the surface of the targeting paper by a surface sol−gel method, and the CHL/CMC layer was subsequently self-assembled onto the titania film precoated surface of the paper via LbL assembly technology. A vacuum infiltration was adopted in the LBL assembly process for reinforcing the ultrathin films and accelerating drying cycles. The uniform TiO2/(CHL/CMC)n multilayers were formed on the targeting paper by alternately depositing CHL and CMC layer for n cycles. The treatment resulted in an excellent improvement in tensile strength and folding endurance of the modified paper. The as-prepared paper samples were optically transparent, and the appearance of the paper and the writing on it were not influenced after treatment by the LbL processes.
which can be achieved by introducing functional groups onto the cellulose surface, followed by cross-linking reaction in the paper suspension or during the drying.20,21 The layer-by-layer assembly technologies have rapidly developed into the most effective means for the preparation of composite membrane since Decher proposed the fabrication of multilayers by consecutive adsorption of polyanions and polycations.22 In recent years, the layer-by-layer (LbL) assembly technique has been employed in improving the mechanical properties of paper in the papermaking field, which consecutively deposited oppositely charged polyelectrolytes on the pulp fiber surfaces to enhance the fiber−fiber interactions.23−25 Wu et al. developed an alternative approach to enhance the strength properties of the cellulose fiber network through depositing carboxymethyl cellulose and chitosan complex on the wet paper sheets.26 Chitosan is selected as the LbL matrix due to its good filmforming property and the reactive amino and hydroxyl groups which can facilitate the adsorption and diffusion of the functional groups through the membrane.27 Chitosan needs to be dissolved in slightly acidic solutions at pH below 6, which hinder its application in restoration of paper documents. To overcome this problem, many soluble chitosan derivatives have been developed by modifying the functional groups28,29 and by controlling the swelling properties.30 Carboxymethyl cellulose (CMC) is widely used as an additive in papermaking, in textiles, pharmaceutics, cosmetics, and in food products.31−33 The most important property of CMC is wet strength building by enhancing the fiber−fiber joint in the papermaking industry.34 The molecular structures of cellulose, chitosan, and CMC are very similar, which is expected to give high compatibility between cellulose and chitosan or CMC. The modified chitosan and carboxymethyl cellulose (CMC) complex was studied and reported to be the effective wet strength additives.35 The presence of hydroxyl groups involved in intra- and intermolecular hydrogen bonding decreases the activity of the cellulosic fiber bundles.36 In order to increase the reactivity of the cellulose surface, a surface modification with metal oxides is usually required.37 Titania as a typical lanthanide metal oxide has been wide and well studied due to favorable band gap, large surface area, and nontoxicity.38 Tetrabutyl titanate has been 10545
DOI: 10.1021/acs.iecr.6b02988 Ind. Eng. Chem. Res. 2016, 55, 10544−10554
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2. EXPERIMENTAL SECTION 2.1. Materials. Chitosan lactate (deacetylation: ≥ 90.0%, pH = 6.3), sodium carboxymethy cellulose (pH = 7.5−7.8), and tetrabutyltitanate from Sinopharm Chemical Reagent Co. Ltd. were used as received without further purification. The FTIR spectra of chitosan lactate and sodium carboxymethy cellulose were shown in Figure S1. The characteristic peaks were similar to the ones in refs 42 and 43. Toluene, isopropyl alcohol, and ethanol were provided by Beijing Chemical Works. The naturally aged mechanical wood-pulp paper (1986), paper historic relic (1921), and Beijing Daily newspaper (1978) were used as paper samples. 2.2. Preparation of Titania Layer on Cellulose Bundles of the Targeting Paper. An ultrathin titania gel layer was deposited on the cellulose bundles as a precursor by the sol−gel process as reported in the literature.44 A piece of paper sample (200 × 280 mm) was first put into a rectangular suctionpenetrating reaction vessel with an effective volume of 260 × 320 × 40 mm. Nanofilm-formation solutions were prepared by dissolving 13.6 mL (0.1 mol/L) of tetrabutyltitanate in a 400 mL solution (1:1 concn toluene and concn ethanol in v/v). The solvent toluene was replaced by isopropyl alcohol; nearly the same intersolubility for tetrabutyltitanate could be achieved. In subsequent work, all 400 mL of isopropyl alcohol was used. The first 200 mL of the solution was slowly filtered through the paper sample by vacuum pump driving. Then the remaining portion was retained in the reactor for 3 min and allowed Ti(OnBu)4 to contact fully with the paper sample. After suction-filtering, the paper sample was immediately cleaned using ethanol, and the excess metal alkoxides and toluene residues on the paper surface were washed away and followed by drying under vacuum pumping flow at atmosphere temperature. Subsequently, 400 mL of deionized water was added into the reactor vessel and allowed to stand for 3 min for hydrolysis of the titanium alkoxides and condensation of the gel layer followed by filtering and rinsing of the treated paper samples with copious ethanol and dried at atmosphere temperature with vacuum pumping flow. 2.3. Assembly of (CHL/CMC)n Multilayer Films on Paper Sheet. Detailed operation steps and assembly processes were illustrated in Figure 1. First, a piece of paper sample (200 × 280 mm) was placed in the suction-penetrating reaction vessel as described above. After the reactor was closed, a 400 mL aqueous solution of chitosan lactate (1.0 mg/mL) was quickly pumped into the reactor via peristaltic pump A. After a rapid switching operation between peristaltic pump and vacuum pump, the first 200 mL was slowly penetrated and passed across the paper sample by vacuum pump, and the rest was allowed to stand for 15 min for sufficient chitosan lactate layer deposition on the cellulose fibers and then suction-filtered slowly. The paper sample was alternately washed with copious water and ethanol which was controlled by peristaltic pump B and sufficiently dried with vacuum suction flow. A chitosan lactate monolayer was deposited on the paper substrate. Subsequently, the paper sample experienced the treatment similar to the chitosan lactate monolayer in 400 mL of sodium carboxymethy cellulose solution (1.0 mg/mL), and a nanoscale carboxymethy cellulose film was adsorbed on the cellulosechitosan lactate layer and formed cellulose-(CHL/CMC)1 bilayer films via LbL assembly technology. The above steps were repeated successively until the desired numbers of uniform CHL/CMC multilayer composites were finished. For
the deposition of the TiO2/(CHL/CMC)n organic−inorganic hybrid films on paper surfaces, a titania gel layer was per-coated on the paper substrate as described above followed by assembling (CHL/CMC)n composite films on the titania treated paper samples. In the processes, all reagents were used repeatedly. In order to ensure these reagents were not diluted or contaminated by residual solution, the cleaning and drying were adopted after each filming. In future work, the processes and procedures can be optimized further. 2.4. Characterization and Measurements. Fourier transform infrared spectra were acquired using a PerkinElmer Spectrum GX FTIR spectrometer equipped with a DTGS detector. The original paper samples were diluted in KBr (from Aldrich, FTIR grade). The samples were scanned from 500 to 4000 cm−1 at 4 cm−1 resolution in transmission mode. X-ray photoelectron spectroscopy (XPS) survey and highresolution spectra were conducted on an ESCALAB250Xi spectrometer (Thermo Scientific, UK) using a monochromated Al kα X-ray radiation source at 15.2 kV and 168 W. The binding energies were calibrated on the basis of the hydrocarbon C 1s peak at 285.0 eV. The spectra deconvolution was carried out by XPS PEAK4.1 software packages. The spectra obtained were curve-fitted with the nonlinear leastsquares iterative technique based on the Gaussian function after baseline subtraction using Shirley’s method. Scanning electron microscopic (SEM) observations were performed using a JEOL 7401 instrument working at an accelerating voltage of 4.0 kV; the specimens were sputtered with gold to reduce charging. UV−vis diffuse reflectance spectra were obtained on a Shimadzu UV 3600 spectrophotometer using BaSO4 as a reference. Tensile Strength (TS) of the samples was measured according to ISO 1924-2:2008 with instrument (KZW-300, Changchun Paper Testing Machine Co. Ltd.) Paper samples were tested at a speed of 5 mm/min. The mechanical properties were measured in the machine direction of the paper of 10 strips taken from the same sample conditioned at 23 °C and 50% RH. Reported values were calculated as averages over ten replicates for each sample according to ISO 187-1990. Folding Endurance (FE) of the samples was determined with a double fold instrument (NZ-135, Hangzhou Research & Technology Co. Ltd.) according to ISO 5626:1993. The applied force was 4.9 N. Colorimetric measurements were performed using NH310 Portable Colorimeter (Shenzhen 3NH Technology Co. Ltd.). The CIE (L, a, b)* system was the chromatic coordinates standard according to ISO 11467:2000. The total color change ΔE was used to evaluate the chromatism based on the (L, a, b)* values measured before and after treatment or before and after artificial aging for a targeting paper samples. ΔE was determined as ΔE =
2
(ΔL*)2 + (Δa*)2 + (Δb*)2
(1)
where ΔL* is the lightness difference; Δa* is the red/green difference; and Δb* is the yellow/blue difference. Cold extract pH of the papers was measured using an ISO 6588-1:2012 model with 2.0 g mass of treated paper samples. Alkaline reserve of treated paper sample (mol/kg) was assessed according to ISO 10716:1994. 10546
DOI: 10.1021/acs.iecr.6b02988 Ind. Eng. Chem. Res. 2016, 55, 10544−10554
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Figure 2. High-resolution XPS spectra of (a) C 1s, (b) N 1s, (c) O 1s, and (d) Ti 2p regions for cellulose-TiO2, cellulose-CHL, and cellulose-TiO2/ CHL modified mechanical wood-pulp papers.
The X-ray photoelectron spectroscopy (XPS) is a complementary technique which can give information about the atomic composition of a material’s surface, thus it can quantify the amount of monatomic ionic species, which are absent in FTIR. Figure S3 showed that the treated papers consisted of C 1s, N 1s, and O 1s as confirmed by the spectra. Compared to the XPS spectra of the cellulose-CHL treated sample, the difference was observed by the existence of the Ti 2p peak in the XPS spectra for the cellulose-TiO2 treated and celluloseTiO2/CHL treated paper samples. XPS analyses confirmed that the chitosan lactate and TiO2 fixed on the surface of paper fibers as desirable. In order to get better insight into the chemical composition of the studied paper samples, a high resolution scan was accomplished in C 1s, O 1s, N 1s, and Ti 2p regions. The high resolution core level C 1s spectra (Figure 2a) for the treated paper samples could be decomposed into three contributions (Figure S4). The peak at 285.0 eV was mainly assigned to C−C and C−H chemical bonding (C1), the peak at 286.5 eV corresponded to C−O and C−N bonds (C2), and the peak at 288.0−288.5 eV was ascribed to CO, O−C−O chemical bonding (C3), which were mainly originated from the cellulose chain or chitosan lactate.46−48 This result had a good agreement with the C binding in cross-linked chitosan. Relative amounts of differently bound carbon atoms established by deconvolution of the C 1s peak are presented in Table S1. Comparing the modified samples, a clear increase of the peak corresponding to the C1 atoms was observed for the cellulose-TiO2 and cellulose-TiO2/CHL treated samples. It is common for carbon to be easily absorbed onto the surfaces of polymers, metals, and oxides when exposed to air.49 Therefore, the surface content of C1 corresponding to C−C and C−H peaks ascribed partially due to adventitious contamination. The intensity of C1 depended on the laboratory conditions during the sample preparation and the conditions in the spectrophotometer
The evaluation of durability of treated paper samples after artificial accelerated aging according to ISO 5630-3:1996 is discussed. The treated paper samples were submitted to moist heat aging at 80 °C and 65% relative humidity for 72 h. For the UV-light aging process, the treated paper samples besieged around a 20 W ultraviolet source with a wavelength of 260 nm for 72 h, and the surrounding radius was 16 cm. Artificial aging tests only provided relative results; therefore, they were only used for comparing the stability of paper samples with and without the treatments in surface modification.
3. RESULTS AND DISCUSSION 3.1. LbLs of TiO2-Modified CHL and CMC on the Paper Surface: Formation and Characterization. Forming an organic/inorganic hybrid nanomembrane on the paper surface is expected to improve both the strength and flexibility of the structure of the targeting papers. The extended mental oxide network could be formed as hydrolysis and condensation of titanium n-butoxide “Ti(OnBu)4” and is followed by reacting rapidly with the hydroxyl groups on the cellulose bundles of paper surface via the sol−gel process.44 The successful deposition of an ultrathin titania nanofilm on the paper samples was ascertained by UV−vis diffuse reflectance spectroscopy. The UV−vis diffuse reflectance spectra of the hierarchical resulting paper samples based on titania surfacemodified were shown in Figure S2. All cellulose-TiO2, celluloseTiO2/CHL, and cellulose-TiO2/(CHL/CMC)1 treated paper samples showed one main absorption band at around 345 nm, which was ascribed to the absorption of the titania layer in the UV region as observed in Figure S2. The above results indicated that the titania layer was successfully coated on the cellulose bundles.45 The broadness range of the characteristic adsorption peak revealed the polydispersity of titania in the complicated paper samples. 10547
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Industrial & Engineering Chemistry Research itself,50 resulting in nonpredictable C−C levels in XPS measurement to some extent. C2 was generally considered as a hydroxyl characteristic absorption; an increase in the relative content meant the increase in hydroxyl groups on targeting the paper surface. The peaks at 286.5 and 285.0 eV for the cellulose-TiO2/CHL treated paper sample were shifted to the lower energy side, and the intensity in the peak at 286.5 eV was increased as compared to the cellulose-TiO2 treated samples. This indicated that the titania incorporation could take place on the structure. In addition, the increase in C−C and C−H and decrease in Ti 2p suggested that chitosan lactate grafted on the Ti surface through the formation of covalent bonds.51 The Ti 2p core level spectrum could be observed at binding energies of around 458.5 eV (Ti 2p3/2) and 464.4 eV (Ti 2p1/2) in good agreement with that in titania for the cellulose-TiO2 treated paper sample.47,49 Figure 2b showed that the peaks at Ti 2p3/2 and Ti 2p1/2 in cellulose-TiO2/CHL shifted toward lower binding energies with respect to the cellulose-TiO2 sample, which indicated a higher electron density of the Ti atoms in the cellulose-TiO2/CHL sample. The O 1s spectrum of cellulose-CHL treated samples was sharp and symmetric with a binding energy of around 532.4 eV assigned to C−O (alcohol and/or ether). The wide and asymmetric peaks of the O 1s spectrum taken at the celluloseTiO2 and cellulose-TiO2/CHL surfaces were related to the presence of Ti4+ in the film, as has already been confirmed from the Ti 2p spectra. The O 1s core level spectrum (Figure 2c) could be fitted with two peaks (Figure S4), which are related to O−Ti bonds (530.2 eV) and attributed to electrons from oxygen atoms that are bonded to hydrogen atoms (OH) at 532.4 eV. Hydroxyl and ether groups can form metal complexes with Ti, in which oxygen atoms donate electrons to Ti. The shift of the peaks toward lower binding energies ruled out the expected change in the oxidation state of O which would result in a shift toward higher binding energies. This could be due to the presence of C−Ti and C−O−Ti bonds in the structure of the film. For N 1s, the high resolution XPS spectra in various samples were summarized in Figure 2d. The paper samples had a symmetric peak at 399.5 eV, which was assigned to the nitrogen atoms in the −NH2 and/or the −NH− groups of CHL.52 CHL was not protonated since the treating process occurred at pH 6−8. The relative intensities of N 1s for the cellulose-TiO2/ CHL treated paper sample were higher than that observed for cellulose-CHL and cellulose-TiO2 treated samples, and this fact was due to more recovery of the membrane surface. After Ti4+ sorption, the N 1s spectrum could be fitted in only one symmetric peak located at 399.5 eV for all samples, indicating that these samples promoted deprotonation of the CHL amino groups. Nitrogen in this chemical state refers to N−N and N− C type of bonding without any linkage to Ti cation.53 This also confirmed that the Ti4+ interacted preferentially with the hydroxyl and ether groups of chitosan and cellulose. These results were also confirmed by FTIR analysis. Figure 3 showed a comparison of the FTIR spectra of the control paper sample and the surface-modified paper samples in the range of 500−4000 cm−1. The hydroxyl groups presented on the surface of the paper cellulose fibers could interact with the hydroxyl groups of titania via hydrogen bonding during the sol−gel process. On dehydration, the covalent bonds between the titania and cellulose fibers could form. The new band was around 898 cm−1 for cellulose-TiO2, cellulose-TiO2/CHL, and cellulose-TiO2/CHL/CMC, which could be associated with the
Figure 3. FTIR spectra of surface-modified mechanical wood-pulp paper samples: (a) control paper cellulose, (b) cellulose-CHL, (c) cellulose-(CHL/CMC)1, (d) cellulose-TiO2, (e) cellulose-TiO2/CHL, (f) cellulose-TiO2/(CHL/CMC)1.
Ti−O−Ti stretching vibration.54 The typical region of the polysaccharide skeleton covered 1180−1033 cm−1 which was often considered to comprise vibration modes of C−C and C− O stretching and the bending mode of C−H bonds and overlapped the vibrations from the Ti−O−C bands at 1128 and 1052 cm−1 in the hybrid film.55,56 The new bands appearing in the region from 3600 to 3750 cm−1 in the cellulose-TiO2, cellulose-TiO2/CHL, and cellulose-TiO2/CHL/CMC samples were described due to hydroxyl groups attached to the titania network.57 The influence of hybrid matrix modification by the titania network was also demonstrated by the overlapping of OH and NH stretching in the 3000−3600 cm−1 region, which marked the effect of hydrogen bonding between CHL and the inorganic network.58 In the FTIR spectra, the direct method to distinguish the molecular interaction is to measure the band shifts of the relative function groups. As could be seen from (a) and (b) in Figure 3, when the CHL coating was assembled on the targeting paper, the characteristic band of inter- and intramolecular hydrogen bonds in the control paper (3286 cm−1) shifted to a higher wavenumber at 3340 cm−1 due to the overlap of OH and NH stretching vibration bands. The subsequent CMC assembly resulted in a shift of the band from 3340 to 3332 cm −1 , which might be caused by the intermolecular interaction hydrogen bonds between CHL and CMC.59 Compared to nontitanium modified paper samples, the broad band at 3000−3650 cm−1 in the paper samples modified by titanium became broad, which indicated that the addition of TiO2 promoted the hydrogen bonding interactions among cellulose, CHL, and CMC. For the BDP samples, the similar FTIR spectra were obtained (Figure S5). 3.2. Mechanical and Physicochemical Properties of Paper Samples Coated with Multilayer Hybrid Nanofilms. 3.2.1. Effect of the Hybrid Multilayers on Mechanical Properties in Targeting Paper. The mechanical performance of the assembly composites is dependent on the degree of dispersion of the fibers in the matrix polymer and the nature and intensity of fiber−polymer adhesion interactions. Tensile strength and folding endurance are the important means of 10548
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modified paper samples was attributable to the increased film cohesiveness due to the fibrillar three-dimensional network of intermolecular hydrogen bonds between the cellulose bundles and the CHL/CMC assembly layer.62 The presence of such interactions has been evidenced by the aforementioned XPS and FTIR. There was a sudden decrease in the tensile strength after a further increase in the layer number up to 4 layers. Moreover, it could be seen that the strengths of the papers modified by TiO2/(CHL/CMC) assembly were higher than those of the CHL/CMC treated papers. Figure S6 showed the effect of the CHL/CMC layer number on the tensile strength of the MWP samples. The tensile strength began to decrease when the layer number increased exceeding 3 layers. The degradation in the composite strength caused by a longer immersion time could offset the increase in the composite strength caused by the increase in layer number, resulting in the dramatic decrease in tensile strength for treated papers. The folding endurance as a function of the layer number presented in Figure 4 demonstrated that the surface modification significantly improved their double folding resistance compared to the control paper. The folding endurance was continuously improved with an increase in layer number until it reached an optimum layer number of 3 for TiO2 modified assembly BDP samples. For the case of CHL/ CMC assembly papers, however, the folding endurance was no longer increased when the layer number was greater than 2. The same tendency in folding endurance was observed for the treated MWP samples in Figure S6. The folding endurance results clearly indicated that the flexibility of the treated paper could be significantly improved via LbL modification. By comparing the mechanical measurement results in untreated and treated BDP papers, it could be concluded that TiO2 modification was responsible for the increase of the tensile strength of the paper fibers. The following analysis based on SEM images would give more concrete explanations.
characterization of mechanical properties of paper.60 In this work, the naturally aged mechanical wood-pulp paper (1986) (hereinafter referred to MWP) and Beijing Daily newspaper (1978) (hereinafter referred to BDP) were used as paper samples in mechanical tests. After treatments, the mechanical properties of the targeting paper were first determined by tensile strength measurements.61 Figure 4 demonstrated that
Figure 4. Tensile strength and folding endurance as a function of the number of the TiO2/CHL/CMC and CHL/CMC bilayers for BDP paper samples.
the observed tensile strength increased 46.40%, 62.31%, and 94.12% for (CHL/CMC)1, (CHL/CMC)2, and (CHL/CMC)3 and 79.22%, 96.09%, and 109.72% for TiO2/(CHL/CMC)1, TiO2/(CHL/CMC)2, and TiO2/(CHL/CMC)3, respectively, in comparison with that of the control paper. The better mechanical performance of CHL/CMC or TiO2/CHL/CMC
Figure 5. Scanning electron microscope of surface modified paper samples: (a) (CHL/CMC)3; (b) TiO2/(CHL/CMC)3; (c) TiO2/(CHL/CMC)3 with the sample at 45 deg from the direction of the front; (d) TiO2/(CHL/CMC)3 with the cross section. 10549
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Figure 6. Schematic illustrations of surface modified paper samples of (a) (CHL/CMC) and (b) TiO2/(CHL/CMC) as undergoing tensile strength to break.
3.2.2. SEM Observation of the Treated Paper Surface and Analysis. SEM micrographs of tensile tested fracture surface of treated paper were illustrated in Figure 5. Figures 5a and b presented the surfaces of the cellulose-(CHL/CMC)3 and cellulose-TiO2/(CHL/CMC)3 paper samples, respectively. As shown in Figure 5b, large interfacial cracks and fractures on the fiber surface were observed in the SEM images on the fiber surface compared to that of Figure 5a. For the cellulose-TiO2/ (CHL/CMC)3 specimens, the fracture origin sites were observed in the hybrid layer from the high resolution microscopy in Figures 5c and d. These differences were possibly attributed to the chemical and physical interactions between the titanium network and chitosan lactate and cellulose. The interactions in cellulose-TiO2/(CHL/CMC)3 specimens resulted in the formation of the cross-linked structure between the fibers and CHL chains. The separated fibers were combined with the cross-linked network. The extra intra- and interactions in cellulose-TiO2/(CHL/CMC)3 specimens resulted in better stress distribution within the bonded assembly and exhibited better resistance to tensile stress. The surface morphology further confirmed that multilayer hybrid films were successfully nanocoated on the paper surface. From the cross-section observations, the thickness of three hybrid composite bilayer films was approximately 600 nm. The thickness of the bilayer film might be optimized further by adjusting operating conditions or changing the concentration of CMC and CHL. Figure 6 schematically illustrated the fracture mode in cellulose-(CHL/CMC) and cellulose-TiO2/(CHL/CMC) specimens. Compared to the cellulose-(CHL/CMC)3 specimens, the intra- and interaction joint locations owing to the titanium cross-linked network in the cellulose-TiO2/(CHL/ CMC) specimens were the most probable rupture points along the fiber length when the fiber was mechanically stressed. The tensile failure surfaces probably resulted from the propagation of a crack in the TiO2/CHL/CMC hybrid composite. For multilayer TiO2/CHL/CMC complex paper samples, the ultimate tensile fracture was preceded by multiple cracking of the multilayer matrix. These intra- and interaction joints owing to the titanium cross-linked network also favored the improvement in deformation capacity and plasticity of the
targeting papers and resulted in the significant enhancement in folding endurance, which was embodied in tensile tests. 3.2.3. The pH Variation and Alkaline Reserve. The pH value was not to be deliberately controlled during the modification processes. Carboxymethyl cellulose (CMC) is a weak acid strong alkali salt; the value of pH of its aqueous solution can be adjusted. In the work, the pH value of CMC solution could be controlled in the range of 7.5−7.8, which was the source of alkaline reserve. In these treatment processes, the paper samples were treated with CMC-Na aqueous solutions through suction filtration and immersion. The corresponding pH value was measured after each complex layer deposition. Figure 7 showed that the pH increased from control paper 5.4
Figure 7. Cold extract (internal) pH value and alkaline reserve of BDP paper samples under multilayer hybrid films treated versus bilayer numbers.
to modified paper 7.3. A small change in pH values occurred over the bilayer numbers in the targeting paper samples, suggesting that first layer deposition could reach the pH values in the 7.0 to 8.5 ideal ranges.20 The pH value should not be too high to prevent the occurrence of alkaline depolymerization.63 In addition; a nanolayer protective film deposited on the surface of the paper further resisted acid erosion caused by the external environment to a certain extent. 10550
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Table 1. Chromatic Changes (ΔE) after Surface Modified with Organic-Inorganic Hybrid Multilayer Films for MWP Samples chromatic changes
sample
ΔE
paper-(CHL/CMC)n paper-TiO2/(CHL/CMC)n
TiO2-treated
(CHL/CMC)1
(CHL/CMC)2
(CHL/CMC)3
(CHL/CMC)4
1.14
0.63 1.09
0.9 1.08
1.17 1.94
0.99 2.04
Figure 8. Photograph of paper historic relics before (a) and after (b) surface modified with organic−inorganic hybrid multilayer films.
Alkaline reserve would play an important role during the future long natural aging process for paper relics. Figure 7 showed that the alkaline reserve of paper samples increased with increasing the layer number of CHL/CMC and TiO2/ CHL/CMC complexes. Moreover, it could be seen that the alkaline reserves of the CHL/CMC treated papers modified by TiO2 were significantly higher than those of the CHL/CMC treated paper samples for the same layer number. These differences were possibly attributed to the interactions between the titanium network and chitosan lactate or cellulose, which induced more amounts of CHL and CMC assembling on the paper surface, and ultimately resulted in the increase of the alkaline reserve. 3.2.4. Chromatic Changes. Variations in the color of the paper documents are related to changes in the chemical composition of the materials involved in modification. In this work, the color changes were evaluated by the CIE L* a* b* system.13 The smaller value of the total color difference (ΔE) represents the less color difference between paper samples. The values of ΔE lower than 1.5 are regarded as undetectable by the human eye.64 Table 1 showed the mean values for ΔE after the surface modification with varying multilayer numbers for MWP samples. These results indicated that all the specimens in (CHL/CMC)n treatment showed undetectable color change between treated and untreated samples. For the TiO2/(CHL/ CMC)n treated samples, the variation was noted when the multilayer number greater than 3 was slightly higher but still much smaller than the 3.3 maximum acceptable limit value of the color difference (ΔE).65 Transparency is also critical property for biopolymeric films, particularly if the film is intended to be used as a surface paper coating. All tested films in this treatment were perceived to be clear and translucent. After application of the LbL self-assembling technique to the paper historic relics, no significant color change was noticed as could be seen in Figure 8. 3.2.5. Artificial Aging Tests. Generally, the most important factors in paper degradation include temperature and moisture, as well as light radiation. In the present work, two typical
artificial aging methods were adopted to evaluate the aging resistant performance of surface modified paper samples. The tensile strength based upon different accelerated aging procedures is related to the extent of oxidative and hydrolytic damage to cellulosic fibers.66 The folding endurance, although the lowered precision, still can be suggested as the sensitive indicator of paper degradation upon artificial aging, because the variations in folding endurance are observed long before the changes in tensile strength.67 Figure 9 showed the tensile strength and folding endurance in control paper and treated samples with different layer numbers after 72 h of accelerated
Figure 9. Tensile strength and folding endurance as a function of the number of the TiO2/CHL/CMC and CHL/CMC bilayers for BDP paper samples under moist heat artificial aging. Original: the original paper sample, MHT-original: the original paper sample after accelerating aging. (CHL/CMC)n: n bilayer paper sample after accelerating aging. 10551
DOI: 10.1021/acs.iecr.6b02988 Ind. Eng. Chem. Res. 2016, 55, 10544−10554
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Industrial & Engineering Chemistry Research aging at 80 °C and 65% RH moist heat treatment for the modified BDP samples. The BDP was an acidic paper sample from naturally aged newspapers, and the cold extract surface pH values were 5.4; the moist heat aging of the BDP had a great effect on the tensile strength and folding endurance. The tensile strength and folding endurance decreased about 50.69% and 34.95%, respectively, after 72 h of moist heat aging. The results of aging study, presented in Figure 9, demonstrated that the LbL assembly modification in the cellulose network significantly improved the tensile strength and folding endurance after accelerating aging. The tensile strength of the treated BDP samples through moist heat accelerating aging was close to or even better than that of the untreated paper sheets before artificial aging, suggesting that the presence of the nanolayers superimposed to the cellulosic fibers favored overcoming the aging degradation of the papers. The values of tensile strength did not change significantly with increasing the layer number except for a slight drop in the TiO2/(CHL/ CMC)4 sample. The moist heat aging of treated BDP samples resulted in a decrease in the folding endurance in comparison with the control paper, but the folding endurance was still higher than that in the control paper after accelerating aging. Surprisingly, the paper modified by TiO2/(CHL/CMC)n assembly had lower folding endurance compared with the (CHL/CMC)n assembly, which was possibly attributed to the covalent cross-linking by the additional hydrogen bonds induced by the titanium network during the moist heat accelerated aging processes. Water can influence the paper strength owing to its ability in disrupting intermolecular hydrogen bonding. The dry structures of the modified cellulose were tightly bound arrays of microfibrils held together by hydrogen bonds. Once the TiO2/(CHL/CMC)n samples were conditioned in humid environments, the more the intercellulosic hydrogen bonding network, compared to (CHL/CMC)n samples, could probably undergo the chain rearrangements, which reduced the ability in absorbing energy by plastic deformation. In addition, the cross-linking in thermally degraded celluloses originating from the hydrogen bonding between adjacent chains could lead to increased stiffness and brittleness.68 These combined factors led to the decrease in folding endurance in TiO2/(CHL/CMC)n samples. The increments in tensile strength and folding endurance of the UV-irradiated specimens were comparable with that of the control paper, as shown in Figure 10. The increments in tensile strength and folding endurance can be calculated by the ratio of difference of the value after and before treatment to the value before treatment. UV aging did not cause a significant decrease in the tensile strength and folding endurance compared to the moist heat aging. In addition, the lower negative effect of UV aging in TiO2/(CHL/CMC)n samples was observed in comparison with that in (CHL/CMC)n samples, which might be due to the presence of TiO2 in the hybrid assembly network, because TiO2 could act as a stabilizer that was able to terminate free-radical reaction induced by UV rays.68
Figure 10. Increment in tensile strength and folding endurance as a function of number of the TiO2/CHL/CMC and CHL/CMC bilayers for BDP and MWP samples under UV irradiation aging. BDP: original BDP sample after UV aging, MWP: original MWP sample after UV aging, BDP-TiO2(CHL/CMC)1: one bilayer TiO2(CHL/CMC) BDP sample after UV aging, BDP-(CHL/CMC)1: one bilayer (CHL/ CMC) BDP sample after UV aging, MWP-(CHL/CMC)1: one bilayer (CHL/CMC) MWP sample after UV aging.
substrate relationship. The well-dispersed coatings resulted in an excellent improvement in mechanical properties of the modified paper. The tensile test demonstrated that the tensile strength and folding endurance for modified aged newspaper samples surprisingly increased up to 100% and 450%, respectively, as compared to the control sample via only three bilayers deposited. The high efficiency was kept even when the modified paper samples exposed to the moist-heat accelerated aging and UV irradiation. The cold extract pH value increased from control paper specimen 5.4 to 7.3 and meanwhile brought alkaline residues after coating. The asprepared paper samples were optically transparent, and the visibility of the paper sheets was not disturbed after treatment by the LBL processes. The aim of the research is to develop a stabilization methodology allowing fiber strengthening in order to propose a novel reinforcement strategy for aged paper documents.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02988.
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Figures S1−S6 and Table S1 (PDF)
AUTHOR INFORMATION
Corresponding Author
4. CONCLUSIONS This work presented, for the first time, the use of chitosan lactate and carboxymethyl cellulose in layer-by-layer organic− inorganic hybrid nanometer coatings for reinforcing aged papers. The XPS in combination with FTIR gave the complementary description about the nature of various interactions between the coatings and the paper substrate, allowing for a better understanding of the coatings to paper
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest. 10552
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ACKNOWLEDGMENTS The authors want to thank Professor Meifang Zhang (School of Information Resource Management, Renmin University of China) for providing the naturally aged (1986) mechanical wood-pulp paper and paper historic relics. We also would like to thank Professor Zhouling Tian (National Library of China) for moist heat artificial aging according to the ISO 5630-3:1996 standard.
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