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Article Application of Ethylene Oxide Gas and Argon Gas Mixture System Method for Scale Deacidification of Cellulose-Based Collections

Yunpeng Qi, Zhihui Jia *, Yajun Zhou, Yong Wang, Guangtao Zhao, Xiaolian Chao, Huiping Xing * and Yuhu Li *

School of Materials Science and Engineering, Engineering Research Center of Historical Cultural Heritage Conservation, Ministry of Education, Shaanxi Normal University, Xi’an 710119, China; [email protected] (Y.Q.); [email protected] (Y.Z.); [email protected] (Y.W.); [email protected] (G.Z.); [email protected] (X.C.) * Correspondence: [email protected] (Z.J.); [email protected] (H.X.); [email protected] (Y.L.); Tel.: +86-029-8153-0715 (Z.J. & H.X. & Y.L.)

Abstract: Deacidification plays an important role in the conservation of -based cultural heritage objects. Herein, a novel approach for the conservation of scale paper-based cultural heritage objects is proposed using a mixture of argon and ethylene oxide (EO-Ar) for the first time. The optimum process conditions for deacidification of ethylene oxide and argon mixture system are determined by orthogonal testing. To evaluate the stabilization effect of paper treated with EO-Ar, the degradation of the mechanical properties (tensile strength, folding endurance and tearing strength tests) of paper   after artificial was evaluated. The results show that the treated paper had better durability with respect to tensile strength, folding endurance and tearing strength. Additionally, thermal Citation: Qi, Y.; Jia, Z.; Zhou, Y.; stability, crystallinity and fiber wall thickness increased after EO-Ar treated, which was determined Wang, Y.; Zhao, G.; Chao, X.; Xing, H.; by scanning electron microscope (SEM), diffraction of X-rays (XRD), and thermo gravimetric (TG) Li, Y. Application of Ethylene Oxide Gas and Argon Gas Mixture System analysis. Some compounds, such as polyethylene glycol, organic acids, esters, were detected by Method for Scale Deacidification of GC-MS after treatment with EO-Ar. Two hundred and forty including acidic, weak acidic Cellulose-Based Cultural Heritage and alkaline books were successfully deacidified, resulting in pH values of paper ranges suitable for Collections. Coatings 2021, 11, 973. paper preservation. Finally, a possible mechanism of deacidification of EO-Ar was proposed. https://doi.org/10.3390/ coatings11080973 Keywords: ethylene oxide; large-scale deacidification; gas phase method; cellulose-based cultural heritage collections Academic Editor: Mariaenrica Frigione

Received: 29 June 2021 1. Introduction Accepted: 10 August 2021 Published: 16 August 2021 Cellulose-based paper documents (e.g., manuscripts, calligraphy, paintings, photos) play an important role in historical, cultural and artistic aspects. The natural aging of paper

Publisher’s Note: MDPI stays neutral objects has become a widespread problem because of their properties with respect to storage with regard to jurisdictional claims in environment [1–3]. It is known that cellulose-based paper degradation is mainly caused by published maps and institutional affil- acid catalyzed cellulose hydrolysis [4–8], which can destroy the cellulose chain by random iations. cutting of the glycosidic bond [9–11]. The acidification degradation of paper documents includes three possible reasons: first, acidic gases (CO2, SO2, HS, NO, NO2, etc.) enriched in the paper lead to acidification and degradation of lignin and hemicelluloses [12,13]; second, processes introduce acid substances, such as alum, chlorine bleach, gelatin, rosin, etc.; third, the hydrolysis of hemicellulose in the paper releases organic Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. acids. Because of these reasons, the hydrolytic decomposition of the cellulosic material can This article is an open access article become autocatalytic, especially in the case of closed books, which do not readily allow the distributed under the terms and acidic compounds to diffuse away. In order to slow down acidification and deterioration conditions of the Creative Commons during natural aging, deacidification has been considered as the most important technology Attribution (CC BY) license (https:// for prolongation of an object’s lifetime [14]. creativecommons.org/licenses/by/ Currently, worldwide heritage institutions and have conducted deacidifica- 4.0/). tion treatment of paper artifacts, for instance the USA, Canada, Germany, France etc. [15].

Coatings 2021, 11, 973. https://doi.org/10.3390/coatings11080973 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 973 2 of 14

This includes single-page paper deacidification and large-scale whole deacidification. Neutralization of acids present on the surface or interior of paper is the main chemical strategy for the deacidification of aged paper. Due to the enhanced alkaline reserve, lower toxicity and moderate price, water-soluble inorganic compounds including Ca(OH)2 and Ca(HCO3)2 have been widely used to treat the aged paper [16–19]. However, physi- cal changes in the appearance of the paper are usually caused by the drying process of aqueous deacidification. Although MgO nanoparticles dispersed in perfluorinated hep- tane have been successfully commercialized for paper deacidification due to their good deacidification performance and alkaline reserve [20], they are limitedly employed by paper conservators because of the ineffective system and white sediments on the paper surface [21,22]. Scale deacidification of whole books and volumes of archives has the advantage of sav- ing resources. Mainstream methods are based on alkaline oxides and hydroxides deposited on paper, allowing neutralization of the acid in the paper. Liquid phase deacidification methods including PaperSave and Bookkeeper (USA PTLP, 1985) also provide strategies for deacidification of whole books and volumes of archives [23,24]. Although obvious ad- vantages include a deacidification effect and strong operability, books possessing a certain strength can be soaked in the alkaline dispersions. The resulting effect in certain cases is powdery depositions and bleeding of inks and colors [25]. Gas phase deacidification has good permeability and is potentially suitable for large-scale deacidification. Alkylene oxides and gas-phase parylene polymers have been used many years ago [26,27]. Ethylene oxide (EO) has been widely used to sterilize medical devices. Currently, major hospitals as well as medical device manufacturers still use EO sterilization [28–31]. EO fumigant is often used in grain and food preservation [32]. Additionally, it can react with both acids and bases due to its unique structure and chemical activity. A treatment system with dry ammonia and ethylene (DAE) was used to deacidify large-size paper-based cultural relics [16]. The stable ethanolamine in situ is formed by introducing two reagents in a vacuum chamber [33]. Although were found to have a pH of 8.0–8.7, the dimension was increased by 2% and the initial brightness was decreased [34]. In the present study, a novel approach on the conservation of large-scale cellulose- based cultural heritage collections is proposed using a mixture of EO and argon (EO- Ar) for the first time. Optimum process conditions for deacidification of ethylene oxide and argon mixture system (EO-Ar) were determined by orthogonal tests using pH as a parameter through independently developed deacidification equipment. In order to assess the resistance of paper treated with EO-Ar, the tensile strength, tearing strength and folding endurance were tested to evaluate the mechanical properties of paper after dry heat accelerated aging methods. A comparative study on the microstructure of paper before and after treatment with EO-Ar was characterized by SEM, XRD, and TG analysis. Finally, a mechanism for deacidification of EO-Ar was proposed. Two hundred and forty books, including acidic, weak acidic and alkaline books, were deacidified using EO-Ar.

2. Materials and Methods 2.1. Paper Samples Selected paper samples used in this study are shown in Table1. Sample 1 stands for the acid paper archives. Sample 2 refers to the weak acid paper archives. Sample 3 represents the alkaline paper cultural relics. Sample 4 represents extremely acidic paper. Sample 5 represents a weak acidic book. Sample 6 stands for an alkaline book. Coatings 2021, 11, 973 3 of 14

Table 1. Selected paper samples used in this study.

No. Samples Date pH Manufacturer or Publisher Shaanxi provincial archives bureau, Sample 1 Groundwood printing papers 1980 3.82 Xi’an, China Shaanxi provincial archives bureau, Sample 2 Ingrain paper 1950 5.57 Xi’an, China China Co., Ltd, Sample 3 Xuan paper 2017 8.85 Jingxian, China Sample 4 Forensic identification of bodily secretions 1980 3.50 Qunzhong Press, Beijing, China Sample 5 Small dictionary of modern Chinese 1980 5.73 People’s Education Press, Beijing, China Yellow River Archives, Sample 6 Licheng county annals 1876 8.21 Zhengzhou, China

2.2. Conservation Procedure and Characterization Methods 2.2.1. Optimization of Deacidification Conditions Deacidification equipment consisted of a deacidification box, inflation system, control system, vacuum system, exhaust degradation system, safety system and other parts. The four main factors including pretreatment humidity, gas ratio, temperature and time were considered in the optimization of deacidification conditions, and the corresponding results are summarized in Table2. The orthogonal tests were applied to optimize the process deacidification conditions of EO-Ar system. The pH value of sample 1 was 3.82, and it is selected as the test index. Four factors and three levels orthogonal experimental table were used in the experiment. All pH values were averages of 10 valid tests.

Table 2. Factor level table.

Pretreatment Humidity Gas Ratio Temperature Time Factors (%) (EO:Ar) (◦C) (h) Levels ABCD 1 30 9:1 30 2 2 55 7:3 40 10 3 80 5:5 50 24

2.2.2. Accelerated Aging Test The samples were kept at 105 ◦C for artificial accelerated aging (72 ± 0.75 h) based on the standard ISO 5630-1:1991 [35,36].

2.2.3. Mechanical Strength Test The tensile test was performed according to the standard ISO 1924-2:2008 [37]. Samples were cut into 15 mm × 180 mm strips and stretched at a uniform rate of 20 mm/min until fracture. The folding resistance was measured according to BS-ISO-5626-1993 [38]. Samples were cut into 15 mm × 150 mm strips. All mechanical tests were repeated at least 20 times, and the corresponding values were computed as average ± 1 standard deviation.

2.2.4. pH Test Based on the standard ISO 6588-1:2012 [39], the cold-water extraction method was employed to measure the pH of samples 1, 2, and 3. Samples were cut into pieces (<5 mm2), and 2 g sample was accurately weighed in 250 mL beaker. 100 mL distilled water was added and allowed to soak for 1 h, during which the flask was shaken at least once within this time. Seven Compact S210-K pH meter (Mettler Toledo International Trading Co., LTD, Shanghai, China) with a InLab® Surface electrode was used to non-destructively measure pH of the extracted solution of sample 1, 2, and 3 to determine pH of paper samples. Coatings 2021, 11, 973 4 of 14

2.2.5. Thermal Gravimetric (TG) The thermal stability of the treated and untreated samples was measured by using a Q600 thermal gravimetric analyzer (TA Instruments-Waters LLC, New Castle, DE, USA). Initial weight of the sample was 3 mg, with nitrogen flow of 10 mL/min, heating rate of 10 ◦C/min, and temperature range of 40–600 ◦C.

2.2.6. X-ray Diffraction (XRD) Smart Lab (9) high resolution X-ray diffraction system (Nippon science Co., Ltd, Tokyo, Japan) was used for the test, where Cu_K-beta was used to generate X-rays, 45 kV, 200 mA, step angle 0.01, reflection angle 2θ, range 10◦–50◦. XRD crystallinity index Cr.I of the fiber was calculated according to the diffraction intensity [40,41].

I − I Cr.I = 002 am × 100% I002

I002 refers to XRD peak strength of cellulose crystallization zone when 2θ ranges from ◦ ◦ ◦ 22 to 23 .Iam represents the amorphous region of cellulose when 2θ is around 18 .

2.2.7. Field Emission Scanning Electron Microscope (FE-SEM) The samples were soaked in water, followed by being frozen in liquid nitrogen for 1 min observing fiber cross-sectional. The frozen paper was broken along the longitudinal direction perpendicular to the paper, then dried at 60 ◦C for 2 h. The surface of samples was sprayed gold uniformly for 60 s by using an ion sputtering instrument. SU-8020 field emission scanning electron microscope (Hitachi hi-tech Co., Ltd, Tokyo, Japan) was employed with acceleration voltage of 1 kV.

2.2.8. GC-MS Analysis Next, 2 g treated and untreated sample edges were chopped and placed in a narrow mouth bottle, and 20 mL chromatographic methanol was added with ultrasonic oscillation (40 KHz) for 1 h. The extracted liquid was filtered by using 0.45 µm needle filter, then it was evaporated to 2 mL employing a rotary evaporation. Chromatographic column (hp-5 ms capillary, 30.0 m, 0.32 mm, 0.25 ms) was used, and the temperature was controlled at 80 ◦C in the column box. The temperature of the injection port was 250 ◦C, and the heating procedure was maintained at 80 ◦C for 2 min. The purge and column flow rates were 3.0 and 1.97 mL/min, respectively. Electron bombardment (EI) ion source temperature was 230 ◦C, and interface temperature was 250 ◦C with 30–600 m/z.

2.2.9. Chromatic Values Test The L* a* b* values of treated and untreated pigment samples before and after aging were determined by X-Rite VS450 non-contact spectrophotometer (X-Rite, Incorporated, Grand rapids, MI, USA). The test conditions were: D65 light source, 0◦/45◦ double angle illumination, 12 mm light spot and 0–150% reflection between 400–700 nm measured wavelength.

2.3. Application of Large-Scale Deacidification Technology After that, 240 books including samples 4, 5 and 6 were posed into deacidification box using optimal deacidification conditions. The pH value distribution was determined.

3. Results and Discussion 3.1. Optimization of Deacidification Conditions Table3 shows the orthogonal test results. According to the principle of the highest pH of the test index, Ki and Ki (Ki is the sum of the same level test index of all factors, Ki is the average of the test index of the same level of each factor) were compared, and found the optimal combination of A3, B2, C2 and D3. Optimal conditions for deacidification involving pretreatment of the sample were 24 h under 80% RH and at 40 ◦C, and then vacuumed to less than 10 mmHg. EO-Ar (volume ratio 7:3) were filled into the tank with the Coatings 2021, 11, 973 5 of 14

rate of 2 mmHg/min. The temperature was maintained at 40 ◦C for 10 h. In the following measurements, the same optimal deacidification conditions are also applied to measure pH values, mechanical properties, chromatic aberrations, etc. Additionally, an R value indicates a significant order of factors with A > B > D > C, indicating that the pretreatment humidity is the most important factor influencing an EO-Ar deacidification system.

Table 3. Optimization of deacidification conditions.

Factors The Test Number pH ABCD 1 1 1 1 1 4.34 2 1 2 2 2 5.12 3 1 3 3 3 3.97 4 2 1 2 3 6.78 5 2 2 3 1 6.23 6 2 3 1 2 5.45 7 3 1 3 2 7.89 8 3 2 1 3 8.03 9 3 3 2 1 6.38 K1 13.43 19.01 17.82 16.95 K2 18.46 19.38 18.28 18.46 K3 22.3 15.8 18.09 18.78 K1 4.47 6.34 5.94 5.65 K2 6.15 6.46 6.09 6.15 - K3 7.43 5.27 6.03 6.26 Optimal levels A3 B2 C2 D3 Rj 2.96 1.19 0.15 0.61 Primary and secondary order A > B > D > C

3.2. Effect of pH Figure1 presents the comparison of pH values for paper samples treated and untreated with EO-Ar. After EO-Ar deacidification treatment, the pH of samples 1, 2 and 3 increased from 3.82, 5.57 and 8.85 to 8.09, 8.44, and 9.63, respectively, which is a safe value for paper materials [42]. Hence, EO-Ar treatment will not increase high pH values of alkaline paper compared to traditional deacidification method using deposits of alkaline reserve. The results indicate that for a whole book with mixed acid and base, the deacidification process Coatings 2021, 11, x FOR PEER REVIEW 6 of 16 can save resources without disassembly. In addition, after 3 years of tracking, paper acidity can still be maintained.

Figure 1. 1. ComparisonComparison of pH of values pH values for paper for samples paper samples (sample 1 (sample is groundwood 1 is groundwood printing papers; printing papers; sample 2 is ingrain paper; sample 3 is xuan paper) treated and untreated with EO-Ar. sample 2 is ingrain paper; sample 3 is xuan paper) treated and untreated with EO-Ar. 3.3. Effect on Mechanical Properties The tensile strength, tearing resistance and folding endurance of untreated and treated paper before and after accelerated aging tests are shown in Figure 2. The tensile strength, tearing resistance and folding endurance at cross directions (CD) of treated sam- ple 1 increased by 10.6%, 2.4% and 2.6%, and at mechanical directions (MD) increased by 11.1%, 9.6% and 19.5%, respectively. Samples 2 and 3 exhibited similar trends. The tensile strength of untreated samples at cross directions (CD) decreased by 66.9% after being dry heat aged, while treated samples decreased by 38.3%. The folding endurance of untreated samples at cross directions (CD) decreased by 67.1% after being dry heat aged, while treated samples decreased by 34.5%. Samples 2 and 3 exhibited similar trends, suggesting that the aging resistance of paper is improved after EO-Ar treatment.

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3.3. Effect on Mechanical Properties The tensile strength, tearing resistance and folding endurance of untreated and treated paper before and after accelerated aging tests are shown in Figure2. The tensile strength, tearing resistance and folding endurance at cross directions (CD) of treated sample 1 increased by 10.6%, 2.4% and 2.6%, and at mechanical directions (MD) increased by 11.1%, 9.6% and 19.5%, respectively. Samples 2 and 3 exhibited similar trends. The tensile strength of untreated samples at cross directions (CD) decreased by 66.9% after being dry heat aged, while treated samples decreased by 38.3%. The folding endurance of untreated samples at cross directions (CD) decreased by 67.1% after being dry heat aged, while treated samples Coatings 2021, 11, x FOR PEER REVIEW 7 of 16 decreased by 34.5%. Samples 2 and 3 exhibited similar trends, suggesting that the aging resistance of paper is improved after EO-Ar treatment.

Figure 2. Mechanical strength of treated and untreated paper samples before and after accelerated aging tests: MD of Figure 2. Mechanical strength of treated and untreated paper samples before and after accelerated aging tests: MD of tensile tensile strength (a); CD of tensile strength (b); MD of folding endurance (c); CD of folding endurance (d); MD of tearing strength (aresistance); CD of tensile(e); CD strengthof tearing (resistanceb); MD of (f). folding endurance (c); CD of folding endurance (d); MD of tearing resistance (e); CD of tearing resistance (f). 3.4. The Chromatic Aberration of Paper Samples Treated with EO-Ar The chromatic aberration of paper samples treated with EO-Ar was tested. The effect of the EO-Ar on papers was measured by using the CIE L* a* b* system as shown in Table 4. L* represents the lightness, which ranges from 0 to 100; 0 is the blackest, 100 is the whit- est. If a* is positive, the color tends to be red, and if a* is negative, the color tends to be

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3.4. The Chromatic Aberration of Paper Samples Treated with EO-Ar The chromatic aberration of paper samples treated with EO-Ar was tested. The effect of the EO-Ar on papers was measured by using the CIE L* a* b* system as shown in Table4. L* represents the lightness, which ranges from 0 to 100; 0 is the blackest, 100 is the whitest. If a* is positive, the color tends to be red, and if a* is negative, the color tends to be green. If b* is positive, the color tends to be yellow, and if b* is negative, the color tends to be blue. From Table4, it can be seen that the color changes of samples are around 1.0, indicating that the EO-Ar treatment did not influence the paper color. The visible light reflection curve of the samples were tested by using the American X-rite VS450 spectrophotometer as shown in Figure S1.

Table 4. Colorimetric coordinates of paper samples before and after EO-Ar treatment.

Variation Sample Before After ∆E* = (∆L*2 + ∆a*2 + ∆b*2)1/2 (After–Before) L* = 85.52 ± 0.34 L* = 85.71 ± 0.42 ∆L* = +0.19 ± 0.54 Sample 1 a* = 6.54 ± 0.22 a* = 6.76 ± 0.18 ∆a* = +0.22 ± 0.28 0.90 ± 0.46 b* = 8.51 ± 0.65 b* = 9.36 ± 0.78 ∆b* = +0.85 ± 1.02 L* = 75.84 ± 0.45 L* = 76.16 ± 0.15 ∆L* = +0.32 ± 0.47 Sample 2 a* = 6.35 ± 0.32 a* = 6.18 ± 0.42 ∆a* = −0.17 ± 0.53 0.96 ± 0.34 b* = 28.43 ± 0.78 b* = 29.32 ± 0.56 ∆b* = +0.89 ± 0.96 L* = 95.78 ± 0.25 L* = 95.33 ± 0.17 ∆L* = −0.45 ± 0.30 Sample 3 a* = 3.08 ± 0.26 a* = 3.07 ± 0.24 ∆a* = −0.01 ± 0.35 0.45 ± 0.17 b* = 1.5 ± 0.17 b* = 1.54 ± 0.35 ∆b* = +0.04 ± 0.39 L* = 77.87 ± 0.18 L* = 78.67 ± 0.11 ∆L* = +0.8 ± 0.21 Sample 4 a* = 3.94 ± 0.57 a* = 4.34 ± 0.68 ∆a* = +0.40 ± 0.89 1.08 ± 0.29 b* = 23.1 ± 0.34 b* = 23.71 ± 0.46 ∆b* = +0.61 ± 0.57 L* = 72.32 ± 0.72 L* = 72.77 ± 0.81 ∆L* = +0.45 ± 1.08 Sample 5 a* = 3.64 ± 0.55 a* = 3.50 ± 0.32 ∆a* = −0.14 ± 0.64 0.72 ± 0.41 b* = 22.8 ± 0.71 b* = 23.35 ± 0.63 ∆b* = +0.55 ± 0.95 L* = 79.26 ± 0.24 L* = 78.91 ± 0.32 ∆L* = −0.35 ± 0.40 Sample 6 a* = 4.03 ± 0.76 a* = 4.44 ± 0.45 ∆a* = +0.41 ± 0.88 0.80 ± 0.33 b* = 21.99 ± 0.78 b* = 22.58 ± 0.34 ∆b* = +0.59 ± 0.85

3.5. Characterization of Morphology and Chemical Composition Figure3a,b shows the SEM images of sample 1 treated and untreated with EO-Ar after aging. It can be seen that surface morphology between treated and untreated samples is almost identical. Figure3c,d shows the paper fiber section; the inner wall of the fiber cavity had a significant change after accelerated aging. Compared with the inner wall of untreated paper (fiber displays cracking and tilting with flake shedding), it became smooth and round after EO-Ar treatment. The fiber wall thickness of EO-Ar treated and untreated paper was ca. 2.24 and 1.5 µm, respectively. Combined with the results presented in Figure2, it can be speculated that the reaction products are attached to the paper fiber network structure, where increased fiber wall thickness can increase the mechanical strength of the paper and prevent possible degradation due to temperature and humidity. The hydrophobic properties of the surface of the treated and untreated paper samples were studied through contact angle measurements (Figure S2). The results show that EO-Ar treatment did not affect the hydrophobic properties of papers. Coatings 2021, 11, 973 8 of 14 Coatings 2021, 11, x FOR PEER REVIEW 9 of 16

FigureFigure 3.3.SEM SEM imagesimages of of sample sample 1 1 before before (a ()a and) and after after (b ()b treatment) treatment with with EO-Ar EO-Ar after after aging aging× ×20,000, 20,000, (c) and (d) the inner wall of untreated and treated fibroblast cavity × 3500. (c) and (d) the inner wall of untreated and treated fibroblast cavity × 3500.

TGTG andand DTG curves of of samples samples 1, 1, 2 2and and 3 treated 3 treated and and untreated untreated with with EO-Ar EO-Ar are aredis- displayedplayed in inFigure Figure 4. 4The. The sample sample 1 thermal 1 thermal decomposition decomposition temperature temperature increased increased from from 297 297to 315 to 315°C after◦C after EO-Ar EO-Ar treatment treatment (Figure (Figure 4a,b).4a,b). The The initial initial extrapolated extrapolated decomposition decomposition tem- temperatureperature of sample of sample 1 treated 1 treated with with EO-Ar EO-Ar was 315 was °C, 315 and◦C, thermal and thermal decomposition decomposition ended endedat ca. 359 at ca. °C 359 (Figure◦C (Figure 4b). A4 b).similar A similar trend trend occurred occurred in sample in sample 2 and 2 and3 (Figure 3 (Figure 4c–f).4c–f). The Theinitial initial extrapolated extrapolated decomposition decomposition temperature temperature of untreated of untreated sample sample 2 was 2 was255 255°C, ◦andC, andfinally finally ended ended at ca. at 404 ca. °C 404 in◦ FigureC in Figure 4c. In4 c.comparison In comparison with sample with sample 2, TG 2,curve TG curveof sample of sample1 had a 1 significant had a significant mass loss mass at loss ca. at270 ca. °C 270 due◦C to due ingrain to ingrain paper paper being being slightly slightly acidic. acidic. The Thereason reason might might be ascribed be ascribed to the to theexistence existence of some of some lignin lignin pectin. pectin. Figure Figure 4e,f4 showe,f show TG TGand andDTG DTG curves curves of alkaline of alkaline handmade handmade rice ricepaper, paper, the initial the initial extrapolated extrapolated decomposition decomposition tem- temperatureperature of untreated of untreated sample sample 3 was 3 was 323 323°C, ◦andC, and thermal thermal decomposition decomposition ended ended at ca. at 372 ca. 372°C (Figure◦C (Figure 4e).4 Thee). The initial initial extrapolated extrapolated decomposition decomposition temperature temperature of EO-Ar of EO-Ar treated treated sam- sampleple 3 was 3 was 332 332°C, and◦C, andthermal thermal decomposition decomposition ended ended at ca. at 373 ca. °C 373 (Figure◦C (Figure 4d). The4d). results The resultsshow that show the that thermal the thermal stability stability of acidic, of acidic, weak weakacid and acid alkaline and alkaline paper paper is improved is improved after afterEO-Ar EO-Ar treatment. treatment. XRD patterns of paper samples untreated and treated with EO-Ar are presented in Figure5. Typical diffraction peaks are observed at 2 θ = 15.15◦–15.74◦, which stands for the projection of the plane (110). The peak at 2θ = 16.32◦–16.53◦ represents the projection of the plane (110) [43], and the typical diffraction peaks at 2θ = 22.48◦–22.69◦ correspond to the (200) crystallographic plane of cellulose I [44]. As shown in Figure5a, several sharp intense peaks correspond to the peaks of talc powder, which commonly serve as one additive to improve the paper’s printability. Compared with the crystallinity index of untreated samples after artificial accelerated aging, it increased after being treated with EO-Ar (Figure5a–c). It is concluded that crystallinity of EO-Ar treatment was enhanced, which is beneficial to improve durability. The methanol extract of paper after EO-Ar treatment was analyzed by GC-MS, and the corresponding results are listed in Table5. As shown in Table5, the severely aged paper contained some organic acids. The reason is mainly because oxidation or photooxidation of anhydrous glucose unit leads to the formation of organic acids [45,46]. The hydrolysis of cellulose promotes cellulose β-1,4-glycoside bond cleavage in the present of acid, which leads to the mechanical strength decreasing of the paper. Different molecular weight polyethylene glycol (PEG) and esters were detected after the paper was treated with EO-Ar (Table6). PEG is formed by ring-opening polymerization of EO, while esters are formed via esterification reaction between PEG and different acids. The acetic acid in Table5 may correspond to the ethylene glycol acetate in Table6. The main reason may be that the free

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Coatings 2021, 11, x FOR PEER REVIEW 10 of 16 acid in the paper caused EO to ring and form esters with organic acids [47]. In combination with Figure1, it may be due to the increase in H value of paper after EO-Ar treatment.

Figure 4. TG of paper samples untreated untreated and and treated treated with with EO-Ar. EO-Ar. ( (aa,,bb)) untreated untreated and and treated treated sample sample 1 1 with with EO-Ar, EO-Ar, ( (cc,,dd)) untreated and treated sample 2 with EO-Ar, ((ee,,ff)) untreateduntreated andand treatedtreated samplesample 33 withwith EO-Ar.EO-Ar.

XRD patterns of paper samples untreated and treated with EO-Ar are presented in Figure 5. Typical diffraction peaks are observed at 2θ = 15.15°–15.74°, which stands for the projection of the plane (110). The peak at 2θ = 16.32°–16.53° represents the projection of the plane (110) [43], and the typical diffraction peaks at 2θ = 22.48°–22.69° correspond to the (200) crystallographic plane of cellulose I [44]. As shown in Figure 5a, several sharp intense peaks correspond to the peaks of talc powder, which commonly serve as one additive to improve the paper’s printability. Compared with the crystallinity index of untreated samples after artificial accelerated aging, it increased after being treated with EO-Ar (Figure 5a–c). It is concluded that crystallinity of EO-Ar treatment was enhanced, which is beneficial to improve durability.

CoatingsCoatings 20212021, 1111, x FOR PEER REVIEW 11 of 16 , , 973 10 of 14

FigureFigure 5. 5. XRDXRD patterns patterns of of paper paper samples samples untreated untreated and and EO-Ar EO-Ar treated treated after after artificial artificial accelerated accelerated aging.aging. ( (aa)) sample sample 1, 1, ( (bb)) sample sample 2, 2, ( (cc)) sample sample 3. 3.

TableThe 5. methanolAnalysis ofextract methanol of paper extract after for untreated EO-Ar treatment sample 4. was analyzed by GC-MS, and the corresponding results are listed in Table 5. As shown in Table 5, the severely aged Number Retention Timepaper contained Component some Peakorganic Area/% acids. PeakThe Height/% reason is mainly Mass Peak because Similarity/%oxidation or 1 2.517photooxidation Acetic acid of anhydrous 0.76 glucose unit leads 0.68 to the formation 327 of organic acids 76 [45,46]. 2 3.508The hydrolysis Propanoic acidof cellulose 1.45 promotes cellulose 0.71 β-1,4-glycoside 139 bond cleavage 87 in the 3 7.776present Octanoic of acid, Acidwhich leads 0.31to the mechanical 0.27 strength decreasing 260 of the paper. 88Different 4 9.227molecular Nonanoic weight acid polyethylene 0.51 glycol (PEG) and 0.45 esters were detected 319 after the paper 94 was treated with EO-Ar (Table 6). PEG is formed by ring-opening polymerization of EO, while estersTable are 6.formedAnalysis via of esterification methanol extract reaction for treated between sample PEG 4. and different acids. The acetic acid in Table 5 may correspond to the ethylene glycol acetate in Table 6. The main reason Number Retention Timemay be that Component the free acid in the Peak paper Area/% caused EO Peak to Height/%ring and form Mass esters Peak with Similarity/%organic acids [47]. InEthylene combination glycol, with Figure 1, it may be due to the increase in H value of paper after 1 1.560 6.77 9.06 273 98 EO-Ar treatment.monoacetate 2-(2- 2 3.470 TableHydroxyethoxy)ethyl 5. Analysis of methanol extrac5.98t for untreated sample 6.72 4. 339 97 acetate Number Retention Time Component Peak Area/% Peak Height/% Mass Peak Similarity/% 31 4.7952.517 TriethyleneAcetic acid glycol 0.76 12.920.68 7.60327 34976 98 2 3.508 DiethylenePropanoic acid glycol, 1.45 0.71 139 87 4 5.515 0.15 0.29 282 95 3 7.776 Octanoicdiacetate Acid 0.31 0.27 260 88 4 9.227 Nonanoic2-[2-[2-(2- acid 0.51 0.45 319 94 5 6.930 Hydroxyethoxy)ethoxy] 0.96 0.65 257 90 ethoxy]ethyl acetate Tetradecanoic acid, 6 12.175 0.83 0.41 303 87 2-Hydroxyethyl ester Palmitic acid, 7 16.240 1.98 0.95 347 93 2-Hydroxyethyl ester 8 17.801 Polyethylene glycol 0.44 0.46 367 92

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3.6. Application of Scale Deacidification The pH of entire books before and after treatment with EO-Ar is presented in Figure6a,b . As shown in Figure6a, 240 books including acidic, weak acid and alkaline paper were treated with EO-Ar at the same time. The average pH value of each book was concentrated at 3–5 before treatment, while it increased to 7–9 after treatment with EO-Ar, which is generally considered as suitable parameters to preserve paper. Hence, alkaline paper was not significantly enhanced after EO-Ar treatment, compared to traditional deacidification method using deposits of alkaline reserve. As shown in Figure6b, the pH of different page numbers for sample 4 were ca. 3.5, where pH increased to 7.0 after EO-Ar treatment, and the pH was similar for every 100 pages. Samples 5 and 6 also exhibited comparable deacidification effects. This indicates that EO-Ar had strong permeability throughout the whole book, creating an even distribution of deacidification for every page of the book. Therefore, the developed method can be successfully utilized to generate evenly distributed pH for entire acidic, weak acidic and alkaline books, reaching a suitable pH range for paper preservation, while not influencing book thickness. We attach great importance to the safety of ethylene oxide in the experimental processes as shown in Figures S3–S7 (e.g., deacidification equipment, deacidifying gas, human health). According to the results of Figures1 and3, and Tables4 and5, the possible mechanism of EO-Ar treatment on paper fibers is shown in Figure7. The aged paper contained some organic acids that originated from the oxidation or photooxidation of an anhydrous glucose unit. EO-Ar could permeate the paper and react with organic acids or free acids. Different molecular weight polyethylene glycol (PEG) and esters were detected after the paper was treated with EO-Ar. PEG is formed by ring-opening polymerization of EO in the condition of acids, while esters are formed via an esterification reaction between PEG and different Coatings 2021, 11, x FOR PEER REVIEW 13 of 16 acids. The PEG and esters may have attached to the fiber, which increased the thickness of the fiber wall as well as its strength.

FigureFigure 6. pH 6. ofpH the of entirethe entire book book before before and and after after treatment treatment with with EO-Ar: EO-Ar: 240240 booksbooks ( a);); different different page page numbers numbers for for sample sample 4 4 (b); different page numbers for sample 5 (c); different page numbers for sample 6 (d). (b); different page numbers for sample 5 (c); different page numbers for sample 6 (d). According to the results of Figures 1 and 3, and Tables 4 and 5, the possible mecha- nism of EO-Ar treatment on paper fibers is shown in Figure 7. The aged paper contained some organic acids that originated from the oxidation or photooxidation of an anhydrous glucose unit. EO-Ar could permeate the paper and react with organic acids or free acids. Different molecular weight polyethylene glycol (PEG) and esters were detected after the paper was treated with EO-Ar. PEG is formed by ring-opening polymerization of EO in the condition of acids, while esters are formed via an esterification reaction between PEG and different acids. The PEG and esters may have attached to the fiber, which increased the thickness of the fiber wall as well as its strength.

CoatingsCoatings 20212021, 11, 11, x, FOR 973 PEER REVIEW 12 of 14 14 of 16

FigureFigure 7.7. PossiblePossible mechanism mechanism of of EO-Ar EO-Ar treatment treatment on paper on paper fibers. fibers.

4.4. Conclusions Conclusions In summary,summary, a a novel novel deacidification deacidification method method for bulk for booksbulk books was investigated was investigated using a using a mixture of EO and Ar for the first time. The optimum process conditions for deacidification mixture of EO and Ar for the first time. The optimum process conditions for deacidifica- of EO-Ar were determined by orthogonal tests using independently developed deacid- tionification of EO-Ar equipment. were determined In order to evaluate by orthogon the resistanceal tests ofusing paper independently after EO-Ar treatment, developed dea- cidificationthe tensile strength, equipment. tearing In strengthorder to and evaluate folding th endurancee resistance were of paper measured after to EO-Ar assess thetreatment, themechanical tensile strength, properties tearing of paper strength after artificial and folding aging by endurance using dry were heat accelerated measured agingto assess the mechanicalmethods. Paper properties treated withof paper EO-Ar after displayed artificial superior aging durability by using todry untreated heat accelerated paper. A aging methods.comparative Paper study treated was conducted with EO-Ar focusing displayed on the microstructure superior durability of paper to before untreated and after paper. A comparativeEO-Ar treatment study through was conduct characterizationed focusing by SEM, on XRD,the microstructure and TG analysis. of Thepaper results before and aftershow EO-Ar that thermal treatment stability, through crystallinity characterization and fiber wall by thicknessSEM, XRD, increased and TG after analysis. EO-Ar The re- treatment. Different molecular weights PEG and esters were detected by GC-MS after sults show that thermal stability, crystallinity and fiber wall thickness increased after EO- being treated with EO-Ar. Two hundred and forty books, including whole acidic, weak Aracidic treatment. and alkaline Different books molecular were successfully weights deacidified PEG and toesters create were pH rangesdetected suitable by GC-MS for after beingpaper preservation,treated with whileEO-Ar. not Two affecting hundred book thickness. and forty The books, proposed including process whole was simple, acidic, weak acidicavoiding and disassembling alkaline books the entirewere book,successfully and not timedeacidified consuming to orcreate causing pH consumptionranges suitable for paperof material preservation, resources. Inwhile the future,not affecting it will be book possible thickness. to reduce The the proposed time required process with was vari- simple, avoidingous techniques disassembling in order to the accelerate entire thebook, treatment and not for time large-scale consuming cellulose-based or causing cultural consumption ofheritage. material Finally, resources. the possible In the mechanism future, it ofwill deacidification be possible of to EO-Ar reduce was the proposed. time required with various techniques in order to accelerate the treatment for large-scale cellulose-based cul- Supplementary Materials: The following are available online at https://www.mdpi.com/article/ tural10.3390/coatings11080973/s1 heritage. Finally, the, Figurepossible S1: Themechanism visible light of reflection deacidification curve of samplesof EO-Ar (a): was Sample proposed. 1, (b): Sample 2, (c): Sample 3, (d): Sample 4, (e): Sample 5, (f): Sample 6, Figure S2: Diagram of Supplementaryexhaust gas degradation Materials: system, The Figurefollowing S3: Small are available gas explosion online test at device,www.mdpi.com/xxx/s1, Figure S4: Schematic Figure S1: Thediagram visible of experimentallight reflection apparatus curve inof mice, samples Figure (a): S5: Samp Mousele lung1, (b): tissue Sample section 2, (c): (a): Sample control groups 3, (d): Sample 4,× (e):400, Sample (b): experimental 5, (f): Sample groups 6,× Figure400, (c): S2: control Diag groupsram of× exhaust100, (d): experimentalgas degradation groups system,× 100; p:Figure S3: Smallpneumonocyte; gas explosion rbc: red test blood device, cells; Figure ia: intercellular S4: Schema area;tic pa: diagram pulmonary of experimental alveoli, Figure apparatus S6: Mouse in mice, Figureliver tissue S5: Mouse section (a):lung control tissue groups section× 400,(a): (b):control experimental groups × groups 400, (b):× 400,experime (c): controlntal groups × × 400, (c): control100, (d): groups experimental × 100, groups(d): experimental× 100; h: hepatocyte; groups × rbc: 100; red p: bloodpneumonocyte; cells; ia: inter-cellular rbc: red blood area; bv:cells; ia: in- tercellularblood vessel, area; Figure pa: S7: pulmonary Water contact alveoli, angles Figure of samples S6: Mouse before andliver after tissue EO-Ar section treatment. (a): control groups × 400,Author (b): Contributions: experimentalY.Q. groups and Z.J.× 400, contributions (c): control equally. groups Data × 100, curation, (d): Z.J.,experimental Y.Z., Y.W., G.Z.groups and × 100; h: hepatocyte;X.C.; formal analysis,rbc: red Y.Q.blood and cells; Z.J.; ia: funding inter-cellular acquisition, area; Z.J. bv: and blood Y.L.; investigation,vessel, Figure H.X.; S7: projectWater contact angles of samples before and after EO-Ar treatment. Author Contributions: Y.Q. and Z.J. contributions equally. Data curation, Z.J., Y.Z., Y.W., G.Z. and X.C.; formal analysis, Y.Q. and Z.J.; funding acquisition, Z.J. and Y.L.; investigation, H.X.; project administration, Y.L.; supervision, Y.L.; validation, Z.J.; writing—original draft, Z.J.; writing—re- view and editing, Z.J. All authors have read and agreed to the published version of the manuscript.

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administration, Y.L.; supervision, Y.L.; validation, Z.J.; writing—original draft, Z.J.; writing—review and editing, Z.J. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (Grant No. 22002080), the Key Research and Development Program of Shaanxi Province, China (Grant No. 2021GY-172), Fundamental Research Funds for the Central Universities (Grant No. GK 202103060), and the Science and Technology Project of Xi’an, China (Grant No. 2020KJRC0014). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: Thanks to Yuhu Li for setting up the framework of the entire project. Thanks for the funding support. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Zhang, C.; Huang, Y.; Zhang, H.; Ye, Z.; Liu, P.; Wang, S.; Zhang, Y.; Tang, Y. Selectively functionalized zeolite NaY composite materials for high-efficiency multiple protection of paper relics. Ind. Eng. Chem. Res. 2020, 59, 11196–11205. [CrossRef] 2. Bukovský, V. The influence of light on ageing of paper. Restaurator 2000, 21, 55–76. [CrossRef] 3. Havermans, J. Effects of air pollutants on the accelerated ageing of cellulose-based materials. Restaurator 1995, 16, 209–233. [CrossRef] 4. Area, M.C.; Cheradame, H. Paper aging and degradation: Recent findings and research methods. BioResources 2011, 6, 5307–5337. 5. Bukovsky, V.; Trnková, M. The influence of secondary chromophores on the light induced oxidation of paper part II: The influence of light on groundwood paper. Restaurator 2003, 24, 18–35. [CrossRef] 6. Chamberlain, D. Anion mediation of aluminium-catalysed degradation of paper. Polym. Degrad. Stabil. 2007, 92, 1417–1420. [CrossRef] 7. Gehlen, M.H. Approximate solution of the autocatalytic hydrolysis of cellulose. Cellulose 2009, 16, 1069–1073. [CrossRef] 8. Carter, H.A. The chemistry of paper preservation: Part 1. the aging of paper and conservation techniques. J. Chem. Educ. 1996, 73, 417–425. [CrossRef] 9. Ahn, K.; Hennniges, U.; Banik, G.; Potthast, A. Is cellulose degradation due to β-elimination processes a threat in mass deacidification of library books? Cellulose 2012, 19, 1149–1159. [CrossRef] 10. Carter, H.A. The Chemistry of Paper Preservation: Part 2. The yellowing of paper and conservation bleaching. J. Chem. Educ. 1996, 73, 1068–1076. [CrossRef] 11. Calvini, P. Comments on the article “On the degradation evolution equations of cellulose” by Hongzhi Ding and Zhongdong Wang. Cellulose 2007, 15, 225–228. [CrossRef] 12. Matisová-Rychlá, L.; Rychlý, J.; Ebringerová, A.; Csomorová, K.; Malovíková, A. Chemiluminescence accompanying the oxidation of hemicelluloses. Polym. Degrad. Stabil. 2008, 93, 1674–1680. [CrossRef] 13. Lojewski, T.; Miskowiec, P.; Molenda, M.; Lubanska, A.; Lojewska, J. Artificial versus natural ageing of paper. Water role in degradation mechanisms. Appl. Phys. A 2010, 100, 625–633. [CrossRef] 14. Mihram, D. Paper deacidification: A bibliographic survey. Part II. Restaurator 1986, 7, 2. [CrossRef] 15. Holik, D.I.H. Book and Paper Preservation; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2006; Chapter 13. 16. Baty, J.W.; Maitland, C.L.; Minter, W.; Hubbe, M.A.; Jordan-Mowery, S.K. Deacidification for the conservation and preservation of paper-based works: A review. BioResources 2010, 5, 1955–2023. [CrossRef] 17. Helmut, B. Aqueous deacidification-with calcium or with magnesium? Restaurator 1998, 19, 1–40. 18. Baty, J.W.; Sinnott, M.L. The kinetics of the spontaneous, proton-and AlIII-catalysed hydrolysis of 1,5-anhydrocellobiitol models for cellulose depolymerization in paper aging and alkaline pulping, and a benchmark for cellulase efficiency. Can. J. Chem. 2005, 83, 1516–1524. [CrossRef] 19. Botti, L.; Mantovani, O.; Orrù, M.A.; Ruggiero, D. The effect of sodium and calcium ions in the deacidification of paper: A chemo-physical study using thermal analysis. Restaurator 2006, 27, 9–23. [CrossRef] 20. Giorgi, R.; Dei, L.; Ceccato, M.; Schettino, C.; Baglioni, P. Nanotechnologies for conservation of cultural heritage: Paper and canvas deacidification. Langmuir 2002, 21, 8198–8203. [CrossRef] 21. Banik, G. Mass deacidification technology in Germany and its quality control. Restaurator 2005, 26, 63–75. [CrossRef] 22. Hubbe, M.A.; Smith, R.D.; Zou, X.; Katuscak, S.; Potthast, A.; Ahn, K. Deacidification of acidic books and paper by means of non-aqueous dispersions of alkaline particles: A review focusing on completeness of the reaction. BioResources 2017, 12, 4410–4477. [CrossRef] 23. Bookkeeper Deacidification. Preservation Technologies. Available online: https://ptlp.com/en/bookkeeper/tools-guide-lines/ faq/ (accessed on 28 June 2021). 24. Zervos, S.; Alexopoulou, I. Paper conservation methods: A literature review. Cellulose 2015, 22, 2859–2897. [CrossRef] Coatings 2021, 11, 973 14 of 14

25. Pauk, S. The bookkeeper mass deacidification process—some effects on 20th century library material. Abbey Newsletter. 1996, 20, 50–54. 26. Smith, R.D. Preserving Cellulose Materials through Treatment with Alkylene Oxides. U.S. Patent 3,676,055, 11 July 1972. 27. Humphrey, B.J. Paper strengthening with gas-phase parylene polymers: Practical considerations. Restaurator 1990, 11, 48–68. [CrossRef] 28. Hideharu, S. Ethylene oxide gas sterilization of medical devices. Biocontrol Sci. 2017, 22, 1–9. 29. Muscarella, L.F. Use of Ethylene-oxide gas sterilisation to terminate multidrug-resistant bacterial outbreaks linked to duodeno- scopes. RMD Open 2019, 6, 1–10. [CrossRef][PubMed] 30. Deschamp, D. Use of Ethylene oxide in medical and surgical sterilisation. Evaluation of the occupational risk of opacification of the lens. Br. Med. J. 1988, 2, 446–452. 31. Mendes, G.C.C.; Brandão, T.R.S.; Silva, C.L.M. Ethylene oxide sterilization of medical devices: A review. Am. J. Infect. Control 2007, 35, 574–581. [CrossRef] 32. BackE, E.A.; Cotton, R.T.; Ellington, G.W. Ethylene oxide as a fumigant for food and other commodities. J. Econ. Entomol. 1930, 23, 226–231. [CrossRef] 33. Okayama, T.; Gotoh, T.; Oye, R. Degradation Control of Book Paper by Ammonia-ethylene Oxide Treatment. In Proceedings of the 1994 and Paper Research Conference Proceedings, Tokyo, Japan, 20–24 June 1994; pp. 132–135. 34. Win, K.R.; Okayama, T. Mass Deacidification Treatments of Acidic Bamboo Paper. Fiber 2012, 68, 143–148. [CrossRef] 35. Havlínová, B.; Katušˇcák, S.; Petroviˇcová, M.; Maková, A.; Brezová, V. A study of mechanical properties of papers exposed to various methods of accelerated ageing. Part I. The effect of heat and humidity on original wood-pulp oapers. J. Cult. Herit. 2009, 10, 222–231. [CrossRef] 36. Paper and Board—Accelerated Ageing—Part 1: Dry Heat Treatment at 105 Degrees C; ISO 5630-1:1991; ISO: Geneva, Switzerland, 1991. 37. Paper and Board—Determination of Tensile Properties—Part 2: Constant Rate of Elongation Method (20 mm/min); ISO 1924-2-2008; ISO: Geneva, Switzerland, 2008. 38. Paper—Determination of Folding Endurance; BS ISO 5626:1993; BS: London, UK, 1994. 39. Paper, Board and Pulps—Determination of pH of Aqueous Extracts—Part 1: Cold Extraction; ISO 6588-1:2012; ISO: Geneva, Switzer- land, 2012. 40. Causin, V.; Marega, C.; Marigo, A.; Casamassima, R.; Peluso, G.; Ripani, L. Forensic differentiation of paper by x-ray diffraction and infrared spectroscopy. Forensic Sci. Int. 2010, 197, 70–74. [CrossRef][PubMed] 41. Duran, A.; Perez-Rodriguez, J.L.; Espejo, T.; Franquelo, M.L.; Castaing, J.; Walter, P. Characterization of illuminated manuscripts by laboratory-made portable XRD and micro-XRD systems. Anal. Bioanal. Chem. 2009, 395, 1997–2004. [CrossRef][PubMed] 42. Information and Documentation—Paper for Documents—Requirements for Permanence; ISO 9706:1994; ISO: Geneva, Switzerland, 1994. 43. Santos, S.M.; Carbajo, J.M.; Quintana, E.; Ibarra, D.; Gomez, N.; Ladero, M.; Eugenio, M.E.; Villar, J.C. Characterization of purifified bacterial cellulose focused on its use on paper restoration. Carbohyd. Polym. 2015, 116, 173–181. [CrossRef][PubMed] 44. Xu, A.; Cao, L.; Wang, B. Facile cellulose dissolution without heating in [C4mim] [CH3COO]/DMF solvent. Carbohyd. Polym. 2015, 137, 249–254. [CrossRef][PubMed] 45. Allen, D.R.; Alonso, M.; Berhardt, R.J. Alkoxylated Fatty Esters and Derivatives from Natural Oil Metathesis. U.S. Patent 878972B2, 2 January 2014. 46. Allen, D.R.; Bernhardt, R.J.; Brown, A. Hard Surface Cleaners Based on Compositions Derived from Natural Oil Metathesis. U.S. Patent 057612 B2, 5 April 2016. 47. Wurm, F.; Nieberle, J.; Frey, H. Double-hydrophilic linear-hyperbranched block copolymers based on poly(ethylene oxide) and poly(glycerol). Macromolecules 2016, 41, 1909–1911. [CrossRef]