Fire-Resistant Inorganic Analogous Xuan Paper with Thousands of

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Fire-Resistant Inorganic Analogous Xuan Paper with Thousands of Years’ Super-Durability Li-Ying Dong, and Ying-Jie Zhu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04630 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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ACS Sustainable Chemistry & Engineering

Fire-Resistant Inorganic Analogous Xuan Paper with Thousands of Years’ Super-Durability

Li-Ying Dong, and Ying-Jie Zhu*

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding-Xi Road, Shanghai 200050, P. R. China

* Corresponding Author E-mail: [email protected] Tel: 0086-21-52412616 Fax: 0086-21-52413122

1

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ABSTRACT: Xuan paper is a unique kind of high-quality paper originating in ancient China, it enjoys a high reputation of “the king of paper that lasts for 1000 years” and was inscribed on the Representative List of the Intangible Cultural Heritage of Humanity by the Educational, Scientific and Cultural Organization of the United Nations in 2009. Xuan paper is the best material carrier for conveying the artistic expression of the calligraphy and painting. The raw materials for Xuan paper are closely related to the geography of Jing County, Anhui Province in China, and thus in an acute shortage. The handmade production of Xuan paper involves more than 100 steps and takes nearly two years, leading to low output and high cost. Xuan paper with the organic origin usually suffers from the degradation, yellowing and deteriorating properties during the long-term natural aging process. Furthermore, the most lethal problem of Xuan paper is its high flammability. During the long human history, numerous precious calligraphy and painting works and books were ruined by fire. Herein, we report a new kind of highly flexible inorganic analogous Xuan paper based on ultralong hydroxyapatite nanowires with excellent fire resistance, thousands of years’ super-durability, unique ink wetting, high whiteness, and excellent anti-mildew benefiting from its 100% inorganic origin, which are far superior over the traditional Xuan paper. The inorganic analogous Xuan paper can be well preserved with no obvious deterioration in properties even after the simulated aging for 3000 years. The most attractive merits of the inorganic analogous Xuan paper are its excellent nonflammability and high thermal stability, which can well safeguard precious calligraphy and painting works, documents, and books for a very long period 2


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of time without the fear of being destroyed by fire or yellowing. The production process of the inorganic analogous Xuan paper is environmentally friendly and simple with only a few steps in contrast to more than 100 steps for the traditional Xuan paper, and it is highly efficient and takes only 2~3 days instead of about 2 years for the traditional Xuan paper. The future commercialization of the inorganic analogous Xuan paper will solve the problems of the traditional Xuan paper such as the severe shortage of geography-specific raw materials, superlong production cycle, yellowing, and high flammability, and extend its applications to fire-resistant books, documents, archives in addition to the calligraphy and painting arts.

EEYWORDS: hydroxyapatite, nanowires, Xuan paper, fireproof

INTRODUCTION Papermaking is one of the greatest inventions in the world, which ended the human history of writing with bamboo and wood chips and greatly promoted the development of culture, knowledge and civilization of mankind. Among various kinds of paper, Xuan paper is an excellent representative of the traditional handmade paper. Xuan paper was first mentioned in an ancient Chinese book entitled “Notes of Past Famous Paintings” by Yanyuan Zhang, a famous theorist on calligraphy and painting in Tang Dynasty, China in the Ninth Century A.D.1 The home of Xuan paper locates in Jing Country, Anhui Province in eastern China, where the traditional handicrafts of 3



making Xuan paper was inscribed on the Representative List of the Intangible Cultural Heritage of Humanity by the Educational, Scientific and Cultural Organization of the United Nations in 2009.2 Xuan paper features excellent performances of durability, ink wetting, wet deformation, and resistance to insects and mildew, and it is the most durable paper in the world and enjoys a high reputation of “the king of paper that lasts for 1000 years”. Xuan paper is the best material carrier for the calligraphy and painting arts. Many famous ancient calligraphy and painting works and books using Xuan paper have been well preserved and survive until today. According to the statistics of the National Library of China, there are about 30 million ancient books in China, among them the majority were printed using Xuan paper and have been well preserved.3 The excellent durability of Xuan paper is attributed to its unique raw materials and hand-made manufacturing process which involves more than 100 steps.4 The bark of pteroceltis tatarinowii, a common species of elm in the area, is used as the main raw material to produce Xuan paper. The bark fibers of pteroceltis tatarinowii have good flexibility and high even degree. In addition, limestone particles are deposited on the surface of pteroceltis bark fibers, which can neutralize acids produced by the hydrolysis of plant fibers and from the environment, and this is one of important reasons for the durability of Xuan paper.4 Another important reason for the durability of Xuan paper is its complex handmade production process under mild treatment conditions, including more than 100 steps such as steeping, washing, steaming and boiling, marinating, fermenting, natural bleaching, pulping, sunning, and cutting, 4


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which cause the least chemical damage to plant fibers.1,4 Xuan paper is mainly classified into the unprocessed Xuan paper and processed Xuan paper. The unprocessed Xuan paper is not specially processed, and has good ability to absorb water, causing the ink on it to blur, which is suitable for the freehand brushwork paintings. The processed Xuan paper is treated with potassium aluminum sulfate, resulting in a hydrophobic surface with a reduced ability to absorb ink, and it is suitable for the meticulous paintings which require the artists to render the delicate details of drawings. Although the traditional Xuan paper has many advantages, however, it has its own problems. The raw materials for Xuan paper are closely related to the geography of Jing County, Anhui Province in China, and thus suffer from a severe shortage. The traditional production process for Xuan paper proceeds strictly by hand through more than 100 steps, takes a superlong production cycle of about two years, leading to low output and high cost. The organic origin of Xuan paper usually suffers from the degradation, yellowing, and deteriorating properties during the long-term natural aging process. Furthermore, the most lethal problem of Xuan paper is its high flammability. During the long human history, numerous precious calligraphy and painting works and books were burnt to ashes in fire. Herein, we report a new kind of highly flexible inorganic analogous Xuan paper (IAXP) with excellent resistance to fire and high temperature, thousands of years’ superlong durability, unique ink wetting, high whiteness, and excellent anti-mildew performance owing to its 100% inorganic origin. The IAXP is made from ultralong 5



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hydroxyapatite (HAP, Ca10(OH)2(PO4)6) nanowires as the main raw material. HAP is the main inorganic mineral of human bone and teeth, and has excellent biocompatibility.5–7

Synthetic

HAP

nanostructured

materials

with

different

morphologies were prepared by different methods, for example, nanoparticles,8 nanorods,9-13

nanowires,8,9,14-18

nanotubes,19

nanosheets,20,21

mesoporous

microspheres,22,23 which were investigated for applications in various fields such as drug/protein delivery,9,10,12,20 bone defect repair,7,22,23 bioimaging,10 antibacterial benefit,11 and waste water treatment.21 Among various morphologies of HAP nanostructures, ultralong HAP nanowires have unique properties and are promising for a variety of applications. Ultralong HAP nanowires have good biocompatibility, environmental friendliness, high flexibility, high whiteness, and excellent resistance to fire and high temperature. Ultralong HAP nanowires have promising applications for the construction of various flexible fire-resistant materials.24,25 The production process of the inorganic analogous Xuan paper is environmentally friendly and simple with only a few steps in contrast to more than 100 steps for the traditional Xuan paper, and it is highly efficient and takes only 2~3 days instead of about 2 years for the traditional Xuan paper. In addition, this new kind of inorganic analogous Xuan paper may provide a new understanding for the traditional Xuan paper. Considering attractive advantages and the potential of scaled-up production, it is expected that the IAXP is promising for applications in the paintings, calligraphy, paper documents, books, archives that need to be safely preserved for a very long period of time without the fear of being destroyed by fire. The future commercialization of the inorganic 6


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analogous Xuan paper will solve the problems of the traditional Xuan paper such as the severe shortage of geography-specific raw materials, superlong production cycle, yellowing, and high flammability, and extend its applications to fire-resistant books, documents, archives in addition to the calligraphy and painting arts.

EXPERIMENTAL SECTION Materials. Sodium oleate, CaCl2, NaH2PO4 ∙ 2H2O, absolute ethanol, and other chemicals were purchased commercially, and used as received without purification. Silica glass fibers were obtained from Shenzhen Xiangu Technology Company, the length and diameter of glass fibers were about 3 mm and 6 μm, respectively. The inorganic adhesive was prepared in our laboratory. Unprocessed Xuan paper and processed Xuan paper were obtained from China Xuan paper Co. Ltd. Ink was obtained from Beijing YiDeGe Ink Co. Ltd. Preparation of ultralong hydroxyapatite nanowires. Ultralong hydroxyapatite nanowires were synthesized by the calcium oleate precursor hydrothermal method previously reported by this research group.25,31 Sodium oleate (400.0 g) was dissolved in deionized water (4 L) under stirring, and an aqueous solution (1.5 L) containing CaCl2 (44.0 g) was added to the above aodium oleate aqueous solution under stirring, and then an aqueous solution (1.5 L) containing NaH2PO4 ∙ 2H2O (56.0 g) was added to the above suspension under stirring. Finally, the reaction mixture was transferred into a stainless steel autoclave with a volume of 10 litres, sealed, and heated at 200 °C for 36 h. The hydrothermal product was washed with ethanol and deionized water 7



three times, respectively. Preparation procedure of the inorganic analogous Xuan paper. The glass fibers were dispersed in deionized water under stirring to yield a uniform aqueous suspension. Then, an aqueous suspension containing ultralong hydroxyapatite nanowires was added into the glass fiber aqueous suspension under stirring at a velocity of 100 rpm, and the stirring continued for 3 min after the completion of the addition. Then, an aqueous suspension containing the inorganic adhesive was added into the above aqueous suspension under stirring. The resulting aqueous suspension was filtered using a papermaking machine with the assistance of vaccum to form a wet paper. The obtained wet paper was pressed at a pressure of 4 MPa, and dried at 105oC for 3 min, and the inorganic analogous Xuan paper was obtained. Characterization. The samples were characterized by scanning electron microscopy (SEM, Magellan 400, USA), transmission electron microscopy (TEM, Hitachi H-800, Japan), X-ray diffraction (XRD, Cu Kα radiation, λ = 1.54178 Å, Rigaku D/max 2550V, Japan), Fourier transform infrared (FTIR) spectroscopy (FTIR-7600, Lambda, Australia), thermogravimetric (TG) analysis, differential thermal analysis (DTA) and differential scanning calorimetry (DSC) (heating rate 10 °C min-1 in flowing air, STA 449/PC, Netzsch, Germany), and cone calorimetry test (according to the standard of ISO 5660-1). The surface roughness of the paper was measured by atomic force microscopy (AFM, NTEGRA, Russia). Contact angle tests were performed using an automatic contact angle meter (Model SL200, USA). The pore size and pore size distribution of the paper were measured with an automatic 8


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mercury porosimeter (AutoPore IV 9510, Micromeritics, USA). The mechanical properties (tensile strength and stiffness) and whiteness tests of the paper were conducted according to the TAPPI standards. To ensure the data repeatability, at least three parallel tests were carried out for each measurement. Durability Evaluation. In this work, the accelerated heat aging method (according to the standard of the Technical Association of the Pulp and Paper Industry of USA (TAPPI)) was used to evaluate the durability of the paper. In this method, three kinds of paper sheets (unprocessed Xuan paper, processed Xuan paper, and inorganic analogous Xuan paper) were put into an oven and maintained at a constant temperature of 105 ± 2 oC. Accelerated heating aging for 72 h corresponds to natural aging for 25 years. After continuous aging for different times, the properties of three kinds of paper sheets were measured. Ink Wetting Evaluation. The ink wetting of the paper was measured according to the GB/T 18739-2008 “Product of geographical indication-Xuan paper”. The contact angles of water and ink of three kinds of paper sheets were measured and analyzed. Anti-Mildew Evaluation. In order to evaluate the anti-mildew properties of the paper, two groups of experiments were conducted. In the first group of experiments, the paper was cut into a square shape, and then put into the glass culture dish. After the sterilization by dry heating, an aqueous suspension containing mould spores with a concentration of 104 cfu/mL (aspergillus niger, long branch trichoderma viride, and chaetomium globosum) was sprayed onto the surface of the paper sheets, and then the 9



paper sheets were put into the incubator to culture for 28 days at a constant temperature of 30 ± 1 oC and humidity of 90%. In the second group of experiments, the paper was cut into a square shape with sizes of 3 × 3 cm. 50 µL aqueous suspension of mould spores with a concentration of 106 cfu/mL (aspergillus niger, long branch trichoderma viride, chaetomium globosum) was inoculated in the bengal culture medium. After the culture medium was solidified, the paper sheet was gently placed in the culture medium and cultured for 4 days at a constant temperature of 30 ± 1 oC and humidity of 90%. The propagation of mould spores on the paper was visualized to estimate the anti-mildew performance. Fire Resistance and High Temperature Stability. Fire-resistant tests were carried out on the flame of an alcohol lamp. High temperature stability tests were carried out in an electric oven. The three kinds of paper sheets were put into the oven and heated for 30 min at different temperatures (200, 400, 600, 800, 1000 oC). After heat treatment, the tensile strength of the paper was measured. In addition, TG, DTA and DSC curves of three kinds of paper sheets were measured and analyzed. In order to demonstrate the ink wetting and fire-resistant performance of the paper, two videos including the ink wetting test (Movie S1), fire resistant test (Movie S2) were recorded and provided in the Supplementary Information.

RESULTS AND DISCUSSION Recently, the calcium oleate precursor solvothermal method was reported for the 10


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synthesis of ultralong HAP nanowires with lengths of larger than 100 μm and diameters of ~10 nm, leading to superhigh aspect ratios (>10000) and high flexibility.16,17,24,25 In this work, the unique integral structure of the inorganic analogous Xuan paper with excellent mechanical properties and high flexibility is innovatively designed based on ultralong HAP nanowires as the main building material (Figure S1 in the Supplementary Information) and silica glass fibers (SGFs) as the reinforcing material, which is similar to the reinforced concrete structure in tall buildings. Our design idea is based on the following considerations. (1) SGFs with micrometer-sized diameters are used as the reinforcing framework material, which are similar to supporting rebars in tall buildings. (2) Ultralong HAP nanowires are used as main building material, similar to the concrete in tall buildings. The aspect ratios of ultralong HAP nanowires (up to >10000) are much higher than those of the plant cellulose fibers (several hundred), and are flexible enough to interweave with each other to form a nanoscale porous network structure (Figure S1b,c in the Supplementary Information). Ultralong HAP nanowires have a relatively high specific surface area (~67.3 m2 g−1) (Figure S1d in the Supplementary Information), which is much higher than those of micron-sized cellulose fibers. High specific surface area and unique structure are beneficial to increase the contact points and binding force between ultralong HAP nanowires, and thus increase the mechanical strength of the IAXP. More encouragingly, ultralong HAP nanowires are environmentally friendly with high biocompatibility and high-quality whiteness (bleaching is not needed) (Figure S1a in the Supplementary Information), and high thermal stability (Figure 11



S1e,f in the Supplementary Information), which are the ideal building material for the fire-resistant inorganic paper. (3) Amorphous inorganic adhesive (IA) (Figure S2 in the Supplementary Information) is designed, prepared and used as the binder in the IAXP. SEM/EDX images indicate that the as-prepared IA is composed of amorphous nanoparticles consisting of Si, K, Al, B, O, and P as the main elements (Figure S2b–f in the Supplementary Information). The as-prepared IA has a high specific surface area (~142.7 m2 g−1) (Figure S2g in the Supplementary Information), which is beneficial to further increase the mechanical strength of the IAXP. The thermogravimetric (TG) and differential scanning calorimetry (DSC) curves indicate that IA has a high thermal stability at temperatures up to 1200 oC (Figure S2h,r in the Supplementary Information), the main weight loss is attributed to the adsorbed water in IA. The weight ratio of three components in the typical inorganic analogous Xuan paper is determined as: ultralong HAP nanowires/SGFs/IA ≈ 68:17:15. In the preparation process of the IAXP, the ultrasonic waves are adopted to disperse silica glass fibers and wrap them with ultralong HAP nanowires. SEM images of a single silica glass fiber and a wrapped silica glass fiber are shown in Figure S3 in the Supplementary Information. Ultralong HAP nanowires are adsorbed on the surface of SGFs through the electrostatic interaction and Van der Waals force to form a stable nanocomposite aqueous suspension, and then, the inorganic adhesive is added. The resulting aqueous suspension is filtered to form a wet paper, which is pressed at a pressure of 4 MPa, and dried at 105oC for 3 min. Figure S4 in the Supplementary Information schematically illustrates the preparation process of the 12


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IAXP. The production process of the IAXP is environmentally friendly and simple with only a few steps including one-step synthesis of ultralong HAP nanowires, washing, slurrying, paper forming, pressing, and drying in contrast to more than 100 steps for the traditional Xuan paper. In addition, the preparation of IAXP is highly efficient, and it takes only 2~3 days in contrast to about 2 years for the traditional Xuan paper. The inorganic analogous Xuan paper sheets with a diameter of 20 cm and A3 size (297 mm × 420 mm) have been successfully prepared in our laboratory (Figure 1a,b). The thickness of the IAXP (91, 166, 236, 297, and 362 μm) can be controlled by adjusting the paper base weight (39, 83, 123, 173, and 205 g/m2). The tensile strength of the IAXP increases with increasing base weight. A similar trend is also found for the stiffness of the IAXP. When the base weight increases from 39 to 205 g/m2, the tensile strength increases from 17.3 to 58.4 N, and the stiffness increases from 0.43 to 6.97 mN ∙ m (Figure 1d,e). However, the paper is difficult to bend if the stiffness is too high (Figure 1f). The scanning electron microscopy (SEM) images of the as-prepared IAXP are shown in Figure 1g-j. Under a low magnification, the ordered surface texture of the IAXP is observed (Figure 1g), and the components including ultralong HAP nanowires, silica glass fibers, and nanoparticles of inorganic adhesive of the IAXP are clearly visible under a high magnification (Figure 1h). Ultralong HAP nanowires are highly flexible, and interweave with each other to form a nanoporous network structure with a large number of junction points (Figure 1i). Figure 1j shows a SEM 13



image of the cross section of the IAXP. Energy dispersive X-ray (EDX) elemental mapping results (Figure 1k-r) indicate that the IAXP contains Ca, Al, K, Na Si, P, O elements, which are all inorganic components. The estimated contents of the elements of the IAXP are shown in Table S1 in the Supplementary Information, indicating that Ca, P, and O are the main elements which originate from ultralong HAP nanowires.

Figure 1. Characterization of the as-prepared inorganic analogous Xuan paper. (a-c) 14


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Digital images. (d) Tensile strength versus base weight. (e) Stiffness versus base weight. (f) Digital images. (g-j) SEM images. (k-r) EDX elemental mappings: (k) SEM image of the elemental mapping area, (l) O, (m) P, (n) Ca, (o) Si, (p) Al, (q) K, (r) Na.

X-ray diffraction (XRD) patterns (Figure S5a in the Supplementary Information) indicate that the as-prepared IAXP has a similar XRD pattern to that of ultralong HAP nanowires, which can be indexed to hydroxyapatite (JCPDF no. 09-0432), implying that ultralong HAP nanowires are the main constituent of the IAXP. The amorphous phases of silica glass fibers and inorganic adhesive in the IAXP are not obvious in the XRD pattern due to their low diffraction intensities. Figure S5b in the Supplementary Information shows Fourier transform infrared (FTIR) spectra. The characteristic absorption peaks of the IAXP are attributed to the PO43− group (1095, 1028, 962, 636, 604, 563 cm−1) and OH group (3570 cm−1). In the practice of the calligraphy and painting, the artists utilize flexible writing brushes to create expressive strokes and drawings. The ink wetting performance on Xuan paper plays an important role in this process. In this work, the ink wetting behavior on the inorganic analogous Xuan paper was investigated, and the unprocessed and processed Xuan paper sheets were used for comparison. The ink droplets with the same volume of 100 μL were dropped onto the paper sheets from the same height of 5 mm. When the ink droplet is dropped on the paper, the dispersal behavior of ink was recorded with a digital camera (Movie S1 in the Supplementary 15



Information).

Figure 2. Ink wetting performance of the as-prepared inorganic analogous Xuan 16


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paper in comparison with the commercial traditional Xuan paper. (a, d, h, k, o, r) Commercial unprocessed Xuan paper. (b, e, i, l, p, s) Commercial processed Xuan paper. (c, f, j, m, q, t) Inorganic analogous Xuan paper. (a-c) Digital images. (d-f) Water contact angles. (h-j) SEM images. (k-m) Pore size distributions. (o-q) SEM images of paper surface morphology. (r-t) Atomic force microscopy (AFM) images.

The ink droplet on the unprocessed Xuan paper spreads quite fast and permeates easily into the paper, the ink even goes through the paper and is obviously observed on the back side of the paper, and the edges of ink and strokes are rough with burrs (Figure 2a, Figure S6 and Movie S1 in the Supplementary Information), which is suitable for the freehand brushwork paintings (Figure S7a in the Supplementary Information). However, the ink droplet on the processed Xuan paper looks like a spherical shape and has little diffusion (Figure 2b, and Movie S1 in the Supplementary Information), and the edges of ink and strokes are relatively smooth (Figure S6 in the Supplementary Information), its ink wetting behavior is suitable for

the meticulous paintings (Figure S7b in the Supplementary Information). In contrast, the ink wetting behavior on the inorganic analogous Xuan paper is unique and different from those on the two kinds of Xuan paper. The ink droplet spreads slightly on the IAXP, but does not penetrate through the paper, no ink is observed on the back side of the paper, and edges of ink and strokes are relatively smooth compared with the unprocessed Xuan paper (Figure 2c, Figure S6 and Movie S1 in the Supplementary Information). 17



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To further investigate the effect of ink wetting, the ink was diluted to 2% and the same volume of 100 μL was dropped on the edge of the thick ink droplet. Figure 2a shows that a drop of the thick ink and a drop of the diluted ink on the unprocessed Xuan paper can be well separated from each other with a slight diffusion on the interfacial area, a unique feature of the unprocessed Xuan paper which is suitable for the freehand brushwork paintings (Figure S7a in the Supplementary Information). However, when a drop of the diluted ink is dropped on the edge of a thick ink droplet on the processed Xuan paper, the rapid diffusion from the thick ink to diluted ink occurs, and finally a uniform ink form (Movie S1 in the Supplementary Information). The as-prepared IAXP has a similar ink wetting phenomenon to the processed Xuan paper except that the ink droplet slightly diffuses on the paper (Figure 2c, and Movie S1 in the Supplementary Information). The ink wetting on the paper is a complex phenomenon that involves the interactions

among

paper,

water

and

ink

constituents.

The

surface

hydrophilic/hydrophobic property has a significant effect on the ink wetting performance of the paper. The traditional Xuan paper is composed of micron-sized plant cellulose fibers with a porous structure. The unprocessed Xuan paper is superhydrophilic with a water contact angle of 0o and also an ink contact angle of 0o, the water droplet or ink droplet diffuses rapidly on the unprocessed Xuan paper (Figure 2d, and Movie S1 in the Supplementary Information). However, the processed Xuan paper is hydrophobic with a water contact angle of 130.5o and an ink contact angle of 88.3o, and a water droplet or ink droplet can keep a nearly spherical shape 18


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with little diffusion on the processed Xuan paper (Figure 2e, and Movie S1 in the Supplementary Information). The diffusion of both water and ink constituents is synchronous on the unprocessed Xuan paper (Figure 2d). In contrast, the inorganic analogous Xuan paper is superhydrophilic with a water contact angle of 0o and an ink contact angle of 36.9° (Figure 2f), implying that the diffusion of water and ink constituents in the ink is not completely synchronous on the IAXP, and that water in the ink diffuses more rapidly than the constituents of ink. This unique phenomenon will cause the faster drying of the ink on the IAXP than on the traditional processed Xuan paper. SEM images (Figure 2h,i) show the porous structure of the traditional Xuan paper formed by the interlaced plant cellulose fibers. In contrast, an ordered texture of the inorganic analogous Xuan paper is observed at a low magnification (Figure 2j), and it exhibits a nanoporous network structure formed by ultralong HAP nanowires. The most probable pore size (14.9 µm) and porosity (78.5%) of the unprocessed Xuan paper are similar to those (12 µm, 75.9%) of the processed Xuan paper (Figure 2k,l). However, the IAXP has a much smaller most probable pore size (338 nm) and higher porosity (85%) compared with the traditional Xuan paper (Figure 2m). In addition, the surface roughnesses of the three kinds of paper sheets are different. The surface roughness of the traditional Xuan paper is higher than that of the inorganic analogous Xuan paper (Figure 2o–q). AFM images in Figure 2r-t indicate that the surface roughness of the unprocessed Xuan paper, processed Xuan paper, and IAXP is 42.9 nm, 45.1 nm and 38.9 nm, respectively. Higher roughness, 19



larger pores and hydrophilicity of the unprocessed Xuan paper may increase the disordering of ink diffusion pathways, causing rough edges of ink and strokes with obvious burrs, and the ink is able to penetrate through the paper. However, the roughness and pores of the IAXP are more uniform and smaller (Figure 2q), resulting in the relatively uniform ink diffusion in each direction and relatively smooth edges of ink and strokes without obvious burrs (Figure 2c). Mould plays a significant role in the deterioration of the traditional Xuan paper.26,27 The prevention of mould growth on the paper is necessary but a great challenge. Careful control of the environment and regular cleaning are recommended in order to avoid biocontamination. However, such preventive measures are not always sufficient. The most important thing is that Xuan paper itself has anti-mildew properties. In order to evaluate the anti-mildew properties, two groups of experiments were conducted. In the first group of experiments, mould spores were sprayed on three kinds of paper sheets, and then the paper sheets were put into the incubator to culture for 28 days at a constant temperature of 30 ± 1 oC and humidity of 90%, and the experimental results are shown in Figure 3a. The experiments indicate that no propagation trace of mould is observed on the three kinds of paper sheets, indicating their good anti-mildew properties in the absence of external nutrients. In comparison, mould spores sprayed on the bengal culture medium breed and spread quickly.

20


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Figure 3. Anti-mildew performance of the as-prepared inorganic analogous Xuan 21



paper in comparison with the commercial traditional Xuan paper. (a) Digital images of mould spores that are sprayed on three kinds of paper sheets and cultured for 28 days at a constant temperature of 30 ± 1 oC and humidity of 90%. (b) Digital images of three kinds of paper sheets that are placed in the bengal culture medium inoculated with mould spores and cultured for 4 days at a constant temperature of 30 ± 1 oC and humidity of 90%.

In the daily storage of calligraphy and painting works, the surface of Xuan paper is easily polluted by some organic pollutants in air. Therefore, we performed the second group of experiments, in which the paper was cut into a square shape with sizes of 3 × 3 cm, and was placed in the bengal culture medium inoculated with mould spores and cultured for 4 days at a constant temperature of 30 ± 1 oC and humidity of 90%, and the experimental results are shown in Figure 3b. Figure 3b shows that three kinds of mould spores do not breed and spread on the inorganic analogous Xuan paper, and the paper is able to maintain a clean surface without the growth of any mould, indicating the excellent anti-mildew performance of the IAXP even in the presence of external nutrients. On the contrary, the growth and spread of mould are obviously observed on both unprocessed and processed Xuan paper in the bengal culture medium, indicating that the anti-mildew performance of the traditional Xuan paper is not satisfactory in the presence of external nutrients. The composition of the traditional Xuan paper is mainly plant cellulose, which can feed for the mould reproduction. In addition, enzyme is produced during the 22


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process of the mould reproduction, which can chemically attack cellulose and cleave cellulose macromolecules.28 However, this is not the only problem. The metabolism of mould will produce the acidic waste, which will further aggravate the degradation of the traditional Xuan paper. This is also one of the important reasons for the significantly decreased properties of the traditional Xuan paper during the long-term preservation process. The traditional Xuan paper is highly flammable, which is attributed to its organic origin. During the long human history, numerous precious calligraphy and painting works, books, and documents were burnt to ashes in fire. In this work, a new kind of fire-resistant inorganic analogous Xuan paper is developed based on ultralong HAP nanowires as the main building material, which can well safeguard precious paper-based books, documents, archives, paintings and calligraphy works for a very long period of time without the fear of being destroyed by fire. Figure 4 shows the comparison of the fire resistance performance and high-temperature stability of three kinds of paper sheets. When the traditional Xuan paper approaches fire, both unprocessed Xuan paper and processed Xuan paper are immediately ignited and burnt to ashes in seconds, exhibiting their high flammability (Figure 4a). Interestingly, the as-prepared inorganic analogous Xuan paper has excellent fire-resistant performance, it is nonflammable even if it is heated in fire for a long period of time (Figure 4a). A video has been recorded to further demonstrate the excellent fire-resistant performance of the IAXP (Movie S2 in the Supplementary Information). 23



Figure 4. Resistance to fire and high temperatures of the as-prepared inorganic analogous Xuan paper in comparison with the commercial traditional Xuan paper. (a) Fire resistance tests of three kinds of paper sheets. (b) Thermal stability tests of three kinds of paper sheets at different temperatures (200–1000 oC). (c) Tensile strength versus strain of the inorganic analogous Xuan paper after heat treatment for 30 min at 24


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different temperatures (200–800 oC). (d) TG curves of the three kinds of paper sheets. (e) DTA curves of the three kinds of paper sheets

Thermal stability test results are shown in Figure 4b, both the unprocessed and processed Xuan paper become yellow at 200 oC and carbonized heavily at 400 oC for 30 min. In contrast, the inorganic analogous Xuan paper has no obvious change both in color and dimension with increasing temperature from 200 to 1000 °C. Although the tensile strength of the IAXP decreases after heat treatment at high temperatures, the tensile strength of the IAXP is still 54.1 N (55 N before heat treatment) with a tensile strength retention rate of 98.4% after heat treatment at 200 oC for 30 min, and the tensile strength of the IAXP is still 42.6 N with a tensile strength retention rate of 77.5% after heat treatment at 400 oC for 30 min (Figure 4c). In contrast, the traditional Xuan paper is heavily carbonized to ashes at 400 oC for 30 min (Figure 4b). In this work, the accelerated aging method according to the standard of the Technical Association of the Pulp and Paper Industry of USA (TAPPI) was used to evaluate the durability of the inorganic analogous Xuan paper, and two kinds of traditional Xuan paper were used as the control samples for comparison. Accelerating aging for 72 h at 105 ± 2 oC corresponds to the natural aging for 25 years.28 We performed the accelerated aging under heating at 105 oC for 360 days, which corresponds to the natural aging for 3000 years. After the simulated aging, the tensile strength, whiteness, ink stability, and microstructure of the IAXP and two kinds of Xuan paper were investigated. Figure 5 shows the evaluation of the durable 25



performance of the as-prepared inorganic analogous Xuan paper by the accelerated heat aging method for up to 3000 years in comparison with the traditional unprocessed and processed Xuan paper. TG and differential thermal analysis (DTA) were performed to evaluate the thermal stability of three kinds of paper sheets (Figure 4d,e). A broad endothermic peak in the DTA curves of both unprocessed Xuan paper and processed Xuan paper between 300–500 oC is attributed to the thermal degradation of cellulose (Figure 4e). In the TG curves, the significant weight loss of the unprocessed Xuan paper and processed Xuan paper is about 99.4% and 90%, respectively, and the paper is completely burnt out at around 500 oC (Figure 4d). Owing to the high thermal stability of the inorganic analogous Xuan paper, no obvious peak appears in the DTA curve of the IAXP, and it still maintains 93.7% of the initial weight even at a temperature as high as 1200 °C, and the final weight loss is about 6.3% (Figure 4d), which results from the loss of adsorbed water and oleate group. The experiments demonstrate that the as-prepared inorganic analogous Xuan paper has excellent thermal stability at temperatures up to 1200 °C. The cone calorimetry test was performed to evaluate the flame-retardant properties of the as-prepared inorganic analogous Xuan paper. The digital images (Figure S8a in the Supporting Information) show that the inorganic analogous Xuan paper preserves well after the testing, and no obvious damage is observed. The experimental results indicate that the effective heat of combustion (Figure S8b in the Supporting Information), mass loss rate (Figure S8c in the Supporting Information), 26


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and heat release rate (Figure S8d in the Supporting Information) are essentially zero throughout the whole testing. These experimental results can be explained by the fact that the as-prepared inorganic analogous Xuan paper has the inorganic origin, and the paper cannot be ignited throughout the whole heating process. The as-prepared inorganic analogous Xuan paper shows excellent fire resistance, incombustibility, and high thermal stability. The durable performance of paper is extremely important for the long-term safe preservation of books, documents, archives, etc., which depends much on the fibers as the building material. During the long-term natural aging process of paper, the two most intuitive phenomena are the decreased mechanical strength and whiteness. The mechanical strength of Xuan paper is determined by the intrinsic strength of fibers and the bonding strength between fibers. In addition to the weakening of the bonding strength between fibers, the decreasing mechanical strength of paper during aging can be explained mainly by the decreasing intrinsic strength of fibers due to cellulose degradation. It was estimated that the lifetime of newspapers was about 150~300 years.29 In contrast, Xuan paper is much more durable. However, during the natural aging process, Xuan paper still suffers from significant degradation and deteriorated properties.30

27



Figure 5. Evaluation of the tensile strength versus strain of the as-prepared inorganic analogous Xuan paper by the accelerated heat aging method for up to 3000 years in comparison with the traditional Xuan paper. (a) The as-prepared inorganic analogous Xuan paper. (b) The commercial unprocessed Xuan paper. (c) The commercial 28


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processed Xuan paper.

In this work, the accelerated aging method according to the standard of the Technical Association of the Pulp and Paper Industry of USA (TAPPI) was used to evaluate the durability of the inorganic analogous Xuan paper, and two kinds of commercial traditional Xuan paper were used as the control samples for comparison. Accelerating aging for 72 h at 105 ± 2 oC corresponds to the natural aging for 25 years.28 We performed the accelerated aging under heating at 105 oC for 360 days, which corresponds to the natural aging for 3000 years. After the simulated aging, the tensile strength, whiteness, ink stability, and microstructure of the IAXP and two kinds of Xuan paper were investigated. Figures 5-8 show the evaluation of the durable performance of the as-prepared inorganic analogous Xuan paper by the accelerated heat aging method for up to 3000 years in comparison with the commercial traditional unprocessed and processed Xuan paper. The tensile strengths of the inorganic analogous Xuan paper and two kinds of traditional Xuan paper after the simulated aging up to 3000 years are shown in Figure 5. It should be noted that the IAXP has isotropic mechanical properties, the tensile strength is the same along different directions. The tensile strength of the IAXP is 27.2 N before aging and 25.9 N after the simulated aging for 2000 years, and the retention rate of tensile strength is as high as 95.2% after the simulated aging for 2000 years (Figure 5a); furthermore, the tensile strength of the IAXP is 22.1 N after the 29



simulated aging for 3000 years, and the retention rate of tensile strength can still maintain 81.3% even after the simulated aging for 3000 years (Figure 5a). The tensile strength of the traditional Xuan paper usually depends on the orientation along the longitudinal direction (LD) or transverse direction (TD). The unprocessed Xuan paper has a tensile strength of 23.7 N along the transverse direction and 13.8 N along the longitudinal direction before the simulated aging (Figure 5b). However, the tensile strength of the unprocessed Xuan paper decreases significantly after the simulated aging for 2000 years, the tensile strength is 14.5 N along the transverse direction and 6.7 N along the longitudinal direction, and the retention rate of tensile strength is only 61.2% (TD) and 48.6% (LD) after the simulated aging for 2000 years. The tensile strength of the unprocessed Xuan paper decreases to 9.0 N along the transverse direction and 5.9 N along the longitudinal direction, and the retention rate of tensile strength is only 38% (TD) and 42.8% (LD) after the simulated aging for 3000 years (Figure 5b). The tensile strengths of the processed Xuan paper are lower than those of the unprocessed Xuan paper (20.1 N along TD, and 12.4 N along LD) before the simulated aging (Figure 5c). After the simulated aging for 2000 years, the tensile strength decreases to 11.4 N (TD) and 10.1 N (LD), the retention rate of tensile strength is 56.7% (TD) and 81.5% (LD). After the simulated aging for 3000 years, the tensile strength decreases to 10.6 N (TD) and 7.3 N (LD), the retention rate of tensile strength is 52.7% (TD) and 58.9% (LD). 30


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Comparing the three samples after the simulated aging for 2000 years, the as-prepared inorganic analogous Xuan paper has much higher retention rate of tensile strength (95.2%) than those of the unprocessed Xuan paper (average 54.9%) and processed Xuan paper (average 69.1%). The retention rate of tensile strength for the inorganic analogous Xuan paper is 81.3% after the simulated aging for 3000 years in contrast to those of the unprocessed Xuan paper (average 40.4%) and processed Xuan paper (average 55.8%) (Figure 5).

Figure 6. Evaluation of the durable performance of the as-prepared inorganic analogous Xuan paper by the accelerated heat aging method for up to 3000 years in comparison with the commercial traditional Xuan paper. (a) SEM image of the commercial unprocessed Xuan paper before aging. (b-d) SEM images of the commercial unprocessed Xuan paper after the simulated aging for 2000 years. (e-h) SEM images of the commercial unprocessed Xuan paper after the simulated aging for 31



3000 years. (i) SEM image of the commercial processed Xuan paper before aging. (j) SEM image of the commercial processed Xuan paper after the simulated aging for 2000 years. (k-m) SEM images of the commercial processed Xuan paper after the simulated aging for 3000 years. (n) SEM image of the as-prepared inorganic analogous Xuan paper before aging. (o) SEM image of the inorganic analogous Xuan paper after the simulated aging for 2000 years. (p) SEM image of the inorganic analogous Xuan paper after the simulated aging for 3000 years.

The above experimental results can be explained by the different building constituents of three different kinds of paper sheets. The inorganic analogous Xuan paper is composed of 100% inorganic constituents, which are highly stable during the long period of aging time. In contrast, the traditional Xuan paper is made from plant cellulose fibers. Cellulose is an organic compound with the formula (C6H10O5)n, a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1-4) linked D-glucose units. During a long period of aging, the plant cellulose fibers have a relatively poor stability and suffer from severe deterioration. Figure 6a shows the surface morphology of the commercial unprocessed Xuan paper before the simulated aging. EDX elemental mappings of the unprocessed Xuan paper (Figure S9 in the Supplementary Information) indicate that the components of calcareous Ca and Si with low contents are also detected in addition to the main elements of C and O, which can neutralize the acidic compounds produced by cellulose hydrolysis and acids from air.4 In theory, it is beneficial to the durability of the unprocessed Xuan 32


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paper. However, after a long period of simulated aging for 2000 years, the visible sign of degradation appears on the surface of the unprocessed Xuan paper (Figure 6c-d). The SEM images indicate that the obvious holes are formed on the surface of the unprocessed Xuan paper (Figure 6b,c), and cellulose is hydrolyzed to form granules in the hole (Figure 6c). In addition, the cracks are observed on the cellulose fiber (Figure 6d). Furthermore, many valleys and cracks can be observed after a longer period of simulated aging for 3000 years (Figure 6e-h), indicating severe degradation and damage of the unprocessed Xuan paper. The above-mentioned degradation phenomena are the main reason for the significantly decreased tensile strength of the unprocessed Xuan paper after a long period of simulated aging up to 3000 years. Compared with the unprocessed Xuan paper, the processed Xuan paper exhibits more ravines on the cellulose fiber after the simulated aging for 2000 years (Figure 6j). EDX elemental mappings of the processed Xuan paper (Figure S10 in the Supplementary Information) indicate the presence of Al and S elements in addition to the main elements of C and O. It is mainly because that the processed Xuan paper is made by soaking the unprocessed Xuan paper in the bone glue and potassium aluminium sulfate solution. The presence of aluminum sulfate accelerates the acid hydrolysis of cellulose fibers. That is the reason for the formation of ravines on the surface of the processed Xuan paper and its obviously decreased tensile strength after the simulated aging for 2000 years. After the simulated aging for 3000 years, the processed Xuan paper is further degraded and damaged, the plant cellulose fibers fragment into a large number of particles (Figure 6k-m), and its tensile strength is 33



further decreased. On the contrary, the as-prepared inorganic analogous Xuan paper is 100% inorganic origin with a high stability, the SEM images indicate that ultralong HAP nanowires of the IAXP before aging and after a long period of simulated aging for 2000 and 3000 years have no obvious change (Figure 6n-p), resulting in the high retention rate of tensile strength (95.2% and 81.3%) after the simulated aging for 2000 and 3000 years, respectively. In contrast, the average retention rate of tensile strength of the unprocessed Xuan paper and processed Xuan paper is only 54.9% and 69.1%, respectively, after the simulated aging for 2000 years, and 40.4% and 55.8%, respectively, after the simulated aging for 3000 years.

Figure 7. Evaluation of the whiteness stability of the as-prepared inorganic analogous Xuan paper by the accelerated heat aging method for up to 3000 years in comparison with the commercial traditional Xuan paper. (a) Digital images of the three kinds of 34


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paper sheets with increasing simulated aging time up to 3000 years. (b) Whiteness changes of three kinds of paper sheets with increasing simulated aging time up to 3000 years. (c) Whitenesses and whiteness retention rates of three kinds of paper sheets after the simulated aging for 2000 and 3000 years.

The stability of whiteness is another important factor for the durability of Xuan paper. Figure 7 shows the whiteness stability performance of the as-prepared inorganic analogous Xuan paper by the accelerated heat aging method for up to 3000 years in comparison with the commercial traditional Xuan paper. The whiteness of the as-prepared inorganic analogous Xuan paper is as high as 92%, which is much higher than that of the traditional Xuan paper (70.5% for unprocessed Xuan paper, and 70.1% for processed Xuan paper) (Figure 7a-c). The production of traditional Xuan paper requires more than 100 different handmade operational steps, it takes about two years to obtain a piece of Xuan paper from raw materials. Figure S11 in the Supplementary Information schematically illustrates several important processes for the production of traditional Xuan paper. The fine handmade processing technology results in higher stability of whiteness of the traditional Xuan paper compared with other kinds of plant cellulose paper. Even so, the whiteness of the traditional Xuan paper is still reduced gradually during the aging process. Figure 7a,b shows the decreasing whiteness of the traditional Xuan paper (unprocessed and processed) with increasing aging time. The whiteness decreases from initial 70.5% to 47.3% and to 35



42.2% with the whiteness retention rate of 67.1% and 59.9% for the unprocessed Xuan paper after the simulated aging for 2000 and 3000 years, respectively, and from initial 70.1% to 46.4% and to 40.6% with the whiteness retention rate of 66.2% and 57.9% for the processed Xuan paper after the simulated aging for 2000 and 3000 years, respectively. In contrast, the whiteness of the inorganic analogous Xuan paper has only a slight decrease from initial 92% to 91.6% and to 86.7% with the whiteness retention rate as high as 99.6% and 94.2% even after the simulated aging for 2000 and 3000 years, respectively (Figure 7c). The whiteness of the IAXP after the simulated aging for 3000 years is still much higher than that of the traditional Xuan paper before aging (~70%), demonstrating the excellent stability of whiteness of the IAXP even after the simulated aging for up to 3000 years. The stability of ink on Xuan paper is another important factor for the durability of Xuan paper. Xuan paper is mainly used for the calligraphy and painting arts. People hope that precious painting arts and calligraphy works can be immortal and well preserved forever. It requires that ink has a high stability on Xuan paper during the aging process. Figure 8 shows the ink stability on the three kinds of paper sheets after the simulated aging for 2000 and 3000 years, indicating that the ink is still black, stable and clearly visible on the three kinds of paper sheets. More encouragingly, the ink on the IAXP is more clear and shiny after the simulated aging for 2000 and 3000 years. In addition, the ink is penetrated into the commercial unprocessed Xuan paper, and the ink can be clearly observed on the back side of the unprocessed Xuan paper (Figure 8, bottom row). In contrast, there is no ink visible on the back side of the 36


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IAXP (Figure 8, bottom row). The above experimental results indicate that the as-prepared inorganic analogous Xuan paper has a much better durability and superior performances compared with the traditional Xuan paper even after the simulated aging for up to 3000 years.

Figure 8. (a, b) Ink stability on the as-prepared inorganic analogous Xuan paper in 37



comparison with the commercial traditional Xuan paper after the simulated aging for 2000 and 3000 years. The top row shows the front surface of the paper, and the bottom row shows the back surface of the paper.

CONCLUSION We have successfully developed a new kind of highly flexible fire-resistant inorganic analogous Xuan paper based on ultralong hydroxyapatite nanowires. The as-prepared IAXP has many advantages such as excellent fire resistance, high thermal stability, unique ink wetting, high whiteness, thousands of years’ super-durability, and excellent anti-mildew performance. The as-prepared IAXP exhibits excellent properties even after the simulated aging for up to 3000 years, which are much superior over the traditional Xuan paper. The whiteness of the IAXP is 91.6% and 86.7% even after the simulated aging for 2000 and 3000 years, respectively, which are still much higher than that of the traditional Xuan paper without aging (~70%). The retention rate of tensile strength and whiteness of the IAXP are as high as 95.2% and 99.6% (aging for 2000 years), and 81.3% and 94.2% (aging for 3000 years), respectively. In contrast, the retention rate of tensile strength and whiteness of the unprocessed Xuan paper (the most durable paper) is only 54.9% (LD/TD average) and 67.1% (aging for 2000 years), and 40.4% and 59.9% (aging for 3000 years), respectively. The IAXP has an excellent ink wetting performance, which is mainly attributed to the nanoscale porous structure and hydroxy groups of utralong HAP 38


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nanowires. In addition, the IAXP has excellent anti-mildew properties even in the presence of external nutrients. On the contrary, mould spores can breed and spread on the traditional Xuan paper in the presence of external nutrients which may result in its severe degradation. The most attractive property of the IAXP is its excellent fire resistance and high thermal stability, which can well safeguard precious painting and calligraphy works as well as books, documents, and archives for the long-term safe preservation without the fear of being destroyed by fire. The production process of the inorganic analogous Xuan paper is environmentally friendly and simple with only a few steps in contrast to more than 100 steps for the traditional Xuan paper, and it is highly efficient and takes only 2~3 days instead of about 2 years for the traditional Xuan paper. The future commercialization of the inorganic analogous Xuan paper will solve the problems of the traditional Xuan paper such as the severe shortage of geography-specific raw materials, superlong production cycle, yellowing, and high flammability, and extend its applications to fire-resistant books, documents, archives in addition to the calligraphy and painting arts.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano. Figures S1−S10 and Table S1 provide additional experimental results for the as-prepared ultralong HAP nanowires, inorganic adhesive, silica glass fibers, and inorganic analogous Xuan paper; schematic illustration of the preparation process of 39



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the inorganic analogous Xuan paper; EDX elemental mappings and elemental contents of the unprocessed and processed Xuan paper; the key production steps of the traditional Xuan paper. Table S1 shows elemental contents of the as-prepared inorganic analogous Xuan paper. (PDF) Movie S1: the ink wetting tests on different paper sheets (MP4) Movie S2: the fire-resistant tests of different paper sheets (MP4)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: 0086-21-52412616. Fax: 0086-21-52413122.

ORCID Ying-Jie Zhu: 0000-0002-5044-5046 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51702342, 21875277), and Industrialization Innovation Project of Shanghai Institute of Ceramics,

Chinese

Academy

of

Sciences

(Y71ZCC1C0G)

acknowledged.

40


is

gratefully

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Hierarchical assembly of monodisperse hydroxyapatite nanowires and construction of high-strength fire-resistant inorganic paper with high-temperature flexibility. ChemNanoMat 2017, 3, 259–268.

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Environmentally friendly fire-resistant inorganic analogous Xuan paper with thousands of years’ super-durability has been developed.

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Fire-Resistant Inorganic Analogous Xuan Paper with Thousands of

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