Abstract
Magnesium hydroxide (Mg(OH)2) nanosheets were explored as an effective material for the restoration and conservation of paper-based cultural archives and compared with the commonly used Ca(OH)2 nanoparticles. The (Mg(OH)2) nanosheets were applied to filter paper as a reference, as well as to new and old paper samples. The effectiveness of Mg(OH)2 nanosheets was evaluated by (i) a pH test of the surface and the bulk extracts, (ii) measuring the alkaline reserve and correlating it with the enhancement in life expectancy, and (iii) in terms of mechanical strength. The alkaline reserve test indicated an increase in the alkaline buffer, which resulted in markedly reduced acidic content of the samples. It was inferred from the improved properties that Mg(OH)2 nanosheets coated the paper as a lamination sheet and protected it as the first line of defense against acidic environmental attack. Moreover, its presence within the paper acted as an alkaline reserve and also as reinforcement in the form of an inorganic nanosheet. The results suggest that the nanosheets are an innovative, compatible, and efficient material for the consolidation and restoration of old and new paper samples.
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Application of Mg(OH)2 Nanosheets for Conservation and Restoration of Precious Documents and Cultural Archives
Khaled M. Saoud,a,* Shaukat Saeed,a,b,* Rola Al Soubaihi,a Ayman Samara,c Imen Ibala,a Dana El Ladki,a and Omar Ezzeldeen a
Magnesium hydroxide (Mg(OH)2) nanosheets were explored as an effective material for the restoration and conservation of paper-based cultural archives and compared with the commonly used Ca(OH)2 nanoparticles. The (Mg(OH)2) nanosheets were applied to filter paper as a reference, as well as to new and old paper samples. The effectiveness of Mg(OH)2 nanosheets was evaluated by (i) a pH test of the surface and the bulk extracts, (ii) measuring the alkaline reserve and correlating it with the enhancement in life expectancy, and (iii) in terms of mechanical strength. The alkaline reserve test indicated an increase in the alkaline buffer, which resulted in markedly reduced acidic content of the samples. It was inferred from the improved properties that Mg(OH)2 nanosheets coated the paper as a lamination sheet and protected it as the first line of defense against acidic environmental attack. Moreover, its presence within the paper acted as an alkaline reserve and also as reinforcement in the form of an inorganic nanosheet. The results suggest that the nanosheets are an innovative, compatible, and efficient material for the consolidation and restoration of old and new paper samples.
Keywords: Magnesium hydroxide nanosheets; Preservation; Restoration; Heritage; Alkaline reserve; Microwaves
Contact information: a: Liberal Arts and Science Program, Virginia Commonwealth University in Qatar, Doha, Qatar; b: Department of Metallurgy and Materials Engineering, Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad-45650, Pakistan; c: Qatar Energy and Environmental Institute(QEERI), Hamad Ben Khalifa University, Doha, Qatar;
* Corresponding authors: s2kmsaou@vcu.edu; saeedshaukat@yahoo.com
INTRODUCTION
Preserving cultural heritage is crucial to understanding a country’s past, as it provides a unique window into the history and the culture of a nation. The preservation and restoration of cultural heritage objects that are in danger of being lost from a variety of factors, such as environmental degradation, adds to the overall value of the global human culture and helps to keep it alive for future generations. Books and other documents are some of the most important legacy for a nation. Millions of important books at libraries are deteriorating, and their aging makes them too fragile to handle. Glances at literature suggest that efforts have been made to take preventive and preservative measures for the conservation and restoration of such precious documents.
The primary cause of paper deterioration is the presence and development of acidic content during manufacturing and aging. The acidic content attacks the cellulose fibers in the paper and depolymerizes them through an acid-catalyzed hydrolysis process. Other factors, such as oxidation, varying or extreme temperatures and humidity, exposure to light, air pollutants in the storage areas, and the amount of use, also play a significant role in the deterioration of books.
One of the strategies for dealing with the acid book preservation problem is the deacidification of papers with suitable materials (Baty et al. 2010). Such materials may neutralize acid content and also deposit an alkaline buffer that acts as a reserve to neutralize any acids that may form later during service life. Weak bases, such as bicarbonates, carbonates, various oxides, some hydroxides, and amines, are often used to de-acidify papers. Other bases used for this purpose have been listed by Cedzova et al. (2006), including barium hydroxide, alkaline oxides, gaseous hexamethylenetetramine, gaseous morpholine, methyl magnesium carbonate, diethyl zinc, magnesium oxide, zinc carbonate, sodium carbonate, amine compounds, ammonia, carbonated ethoxy magnesium-methoxy poly ethoxide, organic aluminum carbonate, ethyl magnesium carbonate, dibutyl magnesium, tetra butyl titanate, and alkoxides of alkaline earth metals. Compounds such as magnesium methyl carbonate and methyl-methoxy magnesium carbonate have been used as solvent-soluble carbonates (Kelly 1976). Calcium hydroxide (Ca(OH)2) has been widely used for deacidification (Williams 1994; Kolar and Novak 1996; Kolar et al. 1998). Giorgi et al. (2005b) noted that once Ca(OH)2 particles are deposited on paper, they react with CO2 from the air and form CaCO3, resulting in surface deposition of insoluble CaCO3. A saturated aqueous solution of Ca(OH)2, diluted 1:1 with deionized water, is normally employed to prevent precipitation of CaCO3 before the calcium hydroxide penetrates the fiber network. Other researchers (Guerra et al. 1995, 1998; Giorgi et al. 2002) have reported the effective use of an aqueous Ca(OH)2 suspension in combination with a strengthening agent, such as methyl cellulose. Although calcium hydroxide has been explored extensively, the direct use of its aqueous solution is limited because of its low solubility (1.3 g/L). The dispersion of lime in water has been explored as a way to increase the lime concentration. However, water dispersions of commercially available Ca(OH)2 have not been found effective because of the large particle sizes (Sequeira et al. 2006) and fast sedimentation rates, in addition to producing a white glaze over the surfaces that not only damages the cellular structure but also reduces the mechanical strength of the paper. Some other studies report that both aqueous and non-aqueous suspensions of Ca(OH)2 have been found effective for deacidification (Salvadori and Dei 2001; Dei and Salvadori 2006).
Magnesium hydroxide nanoparticles have also attracted strong interest because of their high-performance applications and controllable morphology (Kim et al. 2013a,b). Generally, Mg(OH)2 is obtained by reaction of a magnesium salt with an alkaline solution (Cherkezova-Zheleva et al. 2008). Mg(OH)2 can also be obtained through a hydrothermal reaction of metal Mg powder (Jin et al. 2008; An et al. 2010), or commercial bulk magnesium oxide crystals (Yu et al. 2004). The literature suggests that Mg(OH)2 products with various morphological nanostructures, such as needles, lamellae, wires, rods, tubes, and flower-like crystals, have been obtained via various synthesis methods (Ding et al.2001a,b; Henrist et al. 2003) and have been utilized for various applications. Preparation of flower-shaped aggregates of Mg(OH)2 has been reported using a hydrothermal method (Lv et al. 2004). Morphological control of Mg(OH)2 nanocrystals is usually achieved through surfactant templating routes (Sharma et al. 2004; Dei and Salvadori 2006). The research groups of Baglioni and Giorgi(Giorgi et al. 2005a; Baglioni et al. 2009) in their separate studies synthesized magnesium hydroxide nanoparticles and evaluated their use for paper conservation. Stefanis and Panayiotou (2008, 2010) deacidified paper with micro- and nanoparticulate dispersions of Ca(OH)2 or Mg(OH)2. Synthesis of Mg(OH)2 nanosheets has been achieved by controlling the precipitation process of a dissolved Mg2+solution (Wu et al. 2008; Yang et al. 2008) or disaggregation of bulk brucite particles into thin Mg(OH)2 nanosheets without a dissolution–recrystallization process (Pang et al. 2011). However, the use of Mg(OH)2 nanosheets for the conservation and restoration of cultural heritage items has not been reported to date.
In addition to synthesizing particles of the desired morphology for paper conservation, their dispersion is another major issue. It has been found that with an increase in the size of particles, their effectiveness as a preservation or rehabilitation material is significantly reduced because of poor dispersion (El-Sayed 2001). Researchers have also highlighted concerns about the mechanisms and rates of equilibration within paper when large particles are deposited. The dispersion issues can be improved by reducing the average size of the consolidating particles to the sub-micrometric scale. Dispersions of nano-sized particles in non-aqueous solvents produce kinetically stable systems and solve most of the associated problems. This also helps to achieve a deeper penetration of the dispersion, better stability, and to avoid the formation of white glaze on the treated surface (Salvadori and Dei 2001).
Keeping in view the unique nanosheet morphology of the Mg(OH)2 particles, their moderate basic nature, and the issues associated with the application of nanoparticles as conservation and restoration treatment materials, it is anticipated that the nanosheet morphology Mg(OH)2 is worth exploring because of their ability to overcome the hurdles associated with other prevailing treatment processes. Furthermore, it is expected that coating with nanosheets to act like a lamination sheet on the surface of the paper would protect it as the first line of defense against any acidic attack on the outside. In addition, deposition of these nanosheet structures inside the paper would not only enhance the strength of paper as nano-sized reinforcement but also would counteract any acid generated within the paper by degradation during its service life. Thus, Mg(OH)2 nanosheets were prepared through a novel microwave-assisted preparation method (reported elsewhere (Hafezi et al. 2014)) and applied to both new and 100-year-old paper samples to evaluate their effectiveness. The same paper samples were also treated with Ca(OH)2 nanoparticles, and a comparative assessment is presented to highlight the peculiarities of the Mg(OH)2 nanosheet treatment.
EXPERIMENTAL
Materials
Calcium nitrate (Ca(NO3)2; Labchem, Zelienople, USA, 98% purity), magnesium sulfate (MgSO4; medical grade, Labchem, Zelienople, PA, USA, 99% purity), and sodium hydroxide (NaOH; Sigma-Aldrich, St. Louis, MO, USA) were used as received and without further purification. Distilled water and pure ethanol were used in all the preparations. Filter paper (Whatman™ 1441-055 Grade 41, Capitol Scientific, Inc., Austin, TX) manufactured from 100% cellulose and containing no additives was used in the study as a reference. Old paper from a book published approximately 100 years ago, new paper manufactured in the last 2 to 5 years, and environmentally aged newspapers were also used.
Synthesis of magnesium hydroxide nanosheets
The Mg(OH)2 nanosheets were prepared by the microwave-assisted reduction of metal sulfate using sodium hydroxide (NaOH) as the reducing agent at an appropriate pH and microwave radiation dose, a method reported earlier (Hafezi et al. 2014) and the authors’ group (Saoud et al. 2014). Mg(OH)2 was synthesized using a microwave assisted irradiation method. The method was based on the reduction of metal sulfate using cetyltrimethylammonium bromide (CTAB) as a directing agent and NaOH as reducing agent and subjected to microwave irradiation. The resultant Mg(OH)2 nano-sheets were obtained by wet homogeneous precipitation in the presence of dispersant surfactant (CTAB) with a pH between 8 and 10 according to the following reaction:
MgSO4 + 2NaOH → (Mg OH)2 + Na2 SO4 (1)
In a typical synthesis, 5 g of magnesium sulfate (MgSO4, medical Grade, Bell, sons &, UK: Labchem, 99%), is dissolved in 100 mL of DI water. The solution then added to 1% solution of cetyltrimethylammonium bromide (CTAB, Sigma Aldrich, 99%) surfactant in 250 mL of DI water and ethanol with (1:1 ratio) in a reaction flask. The solution is kept under vigorous stirring while 1M NaOH solution is being added drop-wise to the solution. Then, the solution is transformed into translucent white color. After that, the solution is subsequently placed in a microwave chemical reactor (MCR-3), operated at a power of 266 W, for 10 to 15 minutes and is removed upon the onset of boiling. The content is allowed to cool down to ambient temperature to form a gel-like structure. Finally, the precipitated particles are filtered and collected after washing thoroughly several times with distilled water and ethanol. The dried powder samples are heated at temperatures (≥ 300°C) to remove the CTAB.
The prepared nanosheets (15 g/L) were dispersed in pure ethanol, or its aqueous mixture, and were sprayed on the sample paper sheets. The treatment was also applied to the acidified surface of the Whatman filter paper. This was done by soaking the paper first in a sulfuric acid solution (pH 2.5), after which it was deacidified by soaking it in alcoholic dispersions of Ca(OH)2 and Mg(OH)2nanoparticles. The treatment was evaluated by measuring the surface and bulk pH value of the samples. A deacidification treatment was also applied naturally and to the artificially aged (by hydrothermal aging; 90 °C) paper samples. The filter paper was aged by dipping it in a dilute solution of an H2SO4 acid with a pH of approximately 3.0. The filter paper, after drying, was kept for 1 to 6 h in an oven set at 90 °C and 80% humidity. The surface pH value was measured after every hour.
Methods
Alkaline reserve test
An approximately 1-mg piece of the dry sample paper was chosen. It was placed in approximately 25 mL of water in a 125-mL Erlenmeyer flask. A volume of 20 mL of standardized 0.1 N HCl was pipetted into the flask and boiled for approximately 1 min. After cooling down to room temperature, three drops of methyl red indicator were added. The alkaline reserve value was measured as per the standard procedure reported in TAPPI T553 om-10 (2010).
Characterization
X-ray diffraction (XRD) of the nanoparticles used in this study was recorded using a Rigaku MiniFlex600 (Rigaku Corporation, Wilmington, MA; CuKα radiation, wavelength = 1.54 Å), operated at 40 kV and 15 mA. High-resolution transmission electron microscopy (HRTEM; JEOL JEM-2100F, JEOL Ltd., Peabody, MA) was used for the morphological and crystallographic characterization of the nanoparticles. Scanning electron microscope (SEM) images were obtained with a Hitachi SU-70 SEM (Hitachi Co., Chula Vista, CA) operated at an accelerating voltage of 5 keV. For tensile testing, an Insight 30 was used with a 100 N load cell and at an extension rate of 2 mm min-1 (Donggun JiaTesting Equipment Co., Ltd., Hong Kong). The sample was cut into strips 10 mm wide and 80 mm in length. The pH was measured either by taking extracts from the samples and dipping a temperature-compensated pH electrode into the extract, or by using a pH marker pen that provided quick surface pH measurements.
RESULTS AND DISCUSSION
XRD Analysis
Figure 1 shows XRD patterns of Ca(OH)2 and Mg(OH)2 nanoparticles used in this study. The XRD pattern was consistent with the standard patterns of crystalline Ca(OH)2 and Mg(OH)2 nanoparticles. The data reported in Fig. 1 and explained elsewhere (Ihara et al. 2015; Mohammadi and Moghaddas 2015) suggest successful preparation of the desired materials that were used for the preservation of paper in this study.
Fig. 1. XRD patterns of Ca(OH)2 and Mg(OH)2 nanoparticles
SEM Analysis
Figure 2 shows SEM micrographs of Ca(OH)2 and Mg(OH)2 nanoparticles synthesized in this study. The micrographs clearly show the highly porous structure of Mg(OH)2 as compared with Ca(OH)2. Further details were explored through HRTEM measurements. The HRTEM micrographs of Mg(OH)2nanosheets shown in Fig. 3 clearly show that Mg(OH)2 particles have a unique highly porous structure with particle sizes of ˂ 20 nm, which resulted from nanosheets instead of particles. The thickness of these sheets was in the range of a few nanometers, whereas other planar dimensions were in the range of a few hundred nanometers. The Brunauer-Emmet-Teller (BET) surface area and nitrogen adsorption/desorption measurements of these Mg(OH)2 nanoparticles showed a surface area of 80.3 m2/g. The morphology was revealed via HRTEM analysis, and the patterns dynamic selected area electron diffraction (SAED) and the BET surface area suggested that Mg(OH)2 was composed of nanosheets with a unique packing structure that gave a highly porous nanostructure to the material.
Fig. 2. SEM micrographs of Ca(OH)2 and Mg(OH)2