coatings Article Application of Ethylene Oxide Gas and Argon Gas Mixture System Method for Scale Deacidification of Cellulose-Based Cultural Heritage 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 paper-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 aging 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 books 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, papermaking 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 archives 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 book 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 papers 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 Xuan Paper 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].