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Article

CMC-Ca(OH)2-TiO2 Nanocomposite for Paper Relics Multifunctional Restoration: Strengthening, Deacidification, UV Effect Resistance, and Antimicrobial Protection

by
Jing Li
1,*,
Ruiwen Ma
1,
Peng Wu
2 and
Min Quan
2,*
1
Shandong Museum, Jinan 250014, China
2
Shaanxi Institute for the Preservation of Cultural Heritage, Xi’an 710075, China
*
Authors to whom correspondence should be addressed.
Coatings 2024, 14(7), 851; https://doi.org/10.3390/coatings14070851
Submission received: 25 May 2024 / Revised: 2 July 2024 / Accepted: 4 July 2024 / Published: 7 July 2024
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Abstract

:
In recent years, the demand for the protection and restoration of cultural heritage has become increasingly urgent. Particularly for paper-based cultural relics such as ancient books and paintings, their restoration is especially important due to their unique nature and susceptibility to environmental damage. Among various restoration materials, calcium hydroxide (Ca(OH)2) has been widely studied and applied in the protection of paper-based cultural relics. However, commercial Ca(OH)2 materials have issues such as a large particle size and slow carbonation. In order to address these issues, this study employed carboxymethyl cellulose (CMC) as a support, on which nano-sized Ca(OH)2 crystals were grown in situ on its surface, followed by loading with TiO2 nanoparticles, successfully preparing a multifunctional paper-based cultural relic restoration material with reinforcement, deacidification, anti-aging, and antimicrobial properties. It is worth noting that by in situ growing Ca(OH)2 on the surface of CMC, particle size control, uniform dispersion, and the fixation of Ca(OH)2 can be achieved. CMC is used to enhance the mechanical strength of the paper, Ca(OH)2 is used for deacidification, and TiO2 is used for anti-aging and antimicrobial purposes. This study provides a new approach and method for the restoration of paper-based cultural relics, simplifying traditional multi-step processes and avoiding potential impacts on the cultural relics from multiple repairs.

1. Introduction

Paper-based cultural relics serve as carriers of historical information, witnessing the change of time and preserving valuable heritage left by ancestors. Compared to other cultural relics, paper-based relics more authentically record ancient historical and cultural events, reflecting a true picture of ancient history and holding important value for historical reconstruction [1,2,3]. However, as time passes, due to the material characteristics of paper-based relics and external influences, various damages such as foxing, yellowing, fading, surface adhesion, mold growth, insect damage, pigment loss, color fading, and blurry text occur to different extents [4,5,6]. These damages severely affect the aesthetic and collection value of paper-based relics, as well as their value in conveying ancient cultural information, leading to a reduction in their historical and cultural significance and diminishing their preservation lifespan [7,8,9]. Therefore, in order to ensure the restoration of their original appearance, continuous inheritance, and preservation of their cultural heritage, scientific restoration of paper-based relics is of utmost importance.
Acidification is the most important factor leading to the degradation, yellowing, and fragmentation of paper. The presence of acidic substances greatly accelerates the rate of fiber hydrolysis which accumulates continuously within the paper, leading to an increasing rate of fiber degradation. The internal reasons for this phenomenon lie in the oxidation and hydrolysis of cellulose [10,11,12]. The oxidative degradation of cellulose produces formic acid, acetic acid, and other acids, which lead to a self-catalytic hydrolysis reaction of cellulose. The breaking of cellulose glycosidic bonds reduces the polymerization degree of the paper, resulting in a decrease in tensile strength and a brittle and damaged state of the paper. The hydrolysis of cellulose causes the rupture of hydrogen bonds on the surface of cellulose molecules, and the presence of H+ ions accelerates the hydrolysis of cellulose, leading to a vicious cycle that disrupts the internal structure of the paper. This results in a decrease in the mechanical strength of the paper, ultimately causing it to become brittle [13,14].
Among the various external factors that contribute to the aging of paper-based cultural relics, the impact of light exposure and microorganisms are particularly significant. Among natural light sources, ultraviolet (UV) radiation has the greatest impact on the preservation of paper-based cultural relics [15]. High-energy ultraviolet rays in the near UV region can break the saturated bonds with a high bond energy in cellulose, while functional groups such as carboxyl, carbonyl, and aldehyde groups in the molecules can also absorb UV and visible light, catalyzing the accelerated degradation of fiber molecules into monosaccharide units. Additionally, under aerobic conditions, fiber molecules can undergo photooxidation reactions due to light exposure, leading to a significant reduction in polymerization degree and a rapid decrease in the mechanical properties of the paper [3,16]. Therefore, light shielding treatment of paper-based cultural relics plays a crucial role in preventing further aging of the relics. Microorganisms utilize organic compounds such as paper cellulose, pigments, starch, and gelatin in paper as nutrients. Under suitable temperature and humidity conditions, they proliferate and secrete acidic substances and various extracellular enzymes. They form hyphae that penetrate between the fiber bundles of the paper, causing acid degradation of paper fibers, as well as forming various colored mold stains that contaminate the paper, reducing the aesthetic appeal and value of paper-based cultural relics [2,17].
Therefore, the development of deacidification techniques for paper-based cultural relics should be combined with paper reinforcement, antimicrobial, and light shielding technologies. Existing deacidification techniques include gas-phase deacidification and liquid-phase deacidification. The development of new technology for the protection of paper have outstanding contributions, such as, using laser cleaning on old patterns [4,18,19,20,21,22,23]. Liquid-phase deacidification involves introducing alkaline substances into the paper in a liquid medium to neutralize the acidity. This method has attracted widespread attention as it improves upon the drawbacks of gas-phase deacidification, such as explosion risks, difficult process control, and strict deacidification conditions [24,25]. Traditional deacidification agents such as alkaline earth metals (calcium and magnesium) carbonates, bicarbonates, oxides, and hydroxides have shown significant deacidification effects. However, their low solubility can lead to paper whitening after treatment. In recent years, the rapid development of nanomaterials has provided new approaches to address this issue. Compared to common bulk materials, nanomaterials can penetrate the pore system effectively, providing deep reinforcement. Nanomaterials have an extremely high surface area, enhancing material utilization [26]. For example, Zhu et al. used nanoscale Ca(OH)2 particles as mural reinforcement materials, demonstrating excellent reinforcement effects [27,28,29]. Nanoscale calcium hydroxide can be used as a deacidification agent for paper, neutralizing acidity within the paper. Nanoscale calcium hydroxide particles are small and highly dispersed, exhibiting increased reactivity with carbon dioxide, ensuring rapid carbonation and a high degree of carbonation without generating any adverse by-products. The resulting calcium carbonate and other fine particles can prevent the erosion of acidic gases in the air on the paper. After deacidification treatment of the paper-based cultural relics, re-acidification can be prevented, making nanoscale calcium hydroxide highly beneficial for deacidification and the restoration of paper-based cultural relics [30].
As for paper reinforcement, chemical reinforcement methods include the use of cellulose-based reinforcing agents, silane-based reinforcing agents, emulsion-based reinforcing agents, and polyurethane-based reinforcing agents. Nanocellulose, derived from plant cellulose, is a linear material with a certain aspect ratio, a diameter on the nanometer scale and relatively long lengths. Nanocellulose exhibits good compatibility with paper, a long lifespan, and does not generate harmful degradation by-products [30,31]. Carboxymethyl cellulose, as a derivative of cellulose, is structurally similar to the main components of paper fibers. The hydroxyl groups on its chain can form strong hydrogen bonds with the hydroxyl groups on paper fibers, enhancing reinforcement performance. Therefore, it is commonly used in the reinforcement and restoration of paper-based cultural relics. Carboxymethyl cellulose also has a coating effect and is often combined with materials with different properties to form hybrid materials for use in the restoration of paper-based cultural relics.
In terms of antimicrobial and light shielding technologies, nanoscale titanium dioxide demonstrates outstanding research value. Under sunlight exposure, the surface of nanoscale titanium dioxide can generate electron–hole pairs. Electrons can react with oxygen in the air to form superoxide anions, while holes can react with water to produce hydroxyl radicals. These highly oxidizing superoxide anions and hydroxyl radicals can penetrate bacterial cell walls, disrupt cell membrane integrity, and ultimately kill bacteria [1,32]. In the process of exploring this, laser ablation on paper diagnosis has a significant contribution [18,19,20,33]. Alessandro Di Cerbo and colleagues investigated the antibacterial activity of nanostructured TiO2 coatings. The results demonstrated that stainless steel and glass samples coated with TiO2 nanoparticles at a thickness of 200 nm exhibited antibacterial efficiencies of 97% and 100%, respectively, after 30 min of exposure to ultraviolet light. Glass samples with TiO2 coatings of 750 nm, 200 nm, and 50 nm in thickness showed antibacterial activities of 86%, 93%, and 100% after 60 min, respectively, all displaying satisfactory spectral antibacterial effects [34]. Therefore, nanoscale titanium dioxide exhibits antimicrobial properties. Nanoscale titanium dioxide has a wide bandgap and absorbs light in the ultraviolet region. It possesses a triple action of absorbing, reflecting, and scattering ultraviolet light, making it a stable ultraviolet shield. This addresses the issues associated with traditional organic light stabilizers and ultraviolet absorbers, which are prone to degradation, aging, and even corroding the materials of cultural relics. Currently, research on nanoscale titanium dioxide for enhancing fabric’s resistance to ultraviolet rays has become a hot topic. By uniformly dispersing nanoscale titanium dioxide within the fiber molecules [35,36], it can effectively prevent the degradation of cellulose macromolecular chains, reduce the generation of free radicals, and enhance the fiber’s resistance to aging.
Based on the above, this study combines the reinforcing properties of CMC due to its compatibility with paper, the deacidification performance of nanoscale Ca(OH)2, and the light shielding and antimicrobial properties of nanoscale TiO2 to prepare CMC-Ca(OH)2-TiO2 composite reinforcement material. By addressing the three main issues of reinforcement, deacidification, and resistance to light aging in paper-based cultural relics, this work effectively resolves the complexities and intricacies involved in the preparation of nanoscale calcium hydroxide hybrid materials and the study of reinforcement properties for addressing multiple damages in paper. It aims to play an effective role in the protection and restoration of paper-based cultural relics, ultimately enhancing the preservation lifespan of paper-based cultural relics.

2. Experimental Section

2.1. Reagents and Chemicals

CaCl2 was purchased from Tianjin Damao Chemical Reagent Factory (CAS: 10043-52-4). Sodium dodecyl sulfonate (CAS: 2386-53-0), ethanol (CAS: 64-17-5), hydrochloric acid (CAS: 7647-01-0), CMC (CAS: 9004-32-4, 250000, DS = 1.2) and ammonia (CAS:1336-21-6) were supplied by Shanghai Sigma-Aldrich Co. Ltd (Shanghai, China). Potato dextrose AGAR medium (PDA) (CAS: 012111-18-9) was purchased from Beijing Aobo Star Biotechnology Co. Ltd (Beijing, China).

2.2. Synthesis of CMC-Ca(OH)2-TiO2 Nanohybrids

TiO2 nanoparticles: 0.18 g of sodium dodecyl sulfonate was added to 100 mL of alcohol, and the mixture was heated and stirred until it all dissolved. At 0 °C, 2.3 g of TiCl4 (1.365 g/mL) was slowly added by drops to 9.6 mL of hydrochloric acid (6 mol/L) with continuous vigorous stirring for 10 min. Add to the prepared alcohol solvent. After stirring at 80 °C for 20 h, the white precipitate was filtered, washed, and dried to prepare TiO2 nanoparticles [37,38,39,40].
CMC-Ca(OH)2 nanohybrids: 0.03 g of CMC was dispersed in 16 mL of deionized water. Then 0.0052 g of CaCl2 was added and sonicated for 1 h. Next, 0.31 mL of NH3·H2O was slowly added in several servings. The mixture was stirred for 30 min at room temperature, after which it was centrifuged at a low speed and washed three times with a saturated solution of Ca(OH)2 to obtain CMC-Ca(OH)2.
CMC-Ca(OH)2-TiO2 nanohybrids: 10 mg of TiO2 nanoparticles were dispersed into 20 mL of CMC-Ca(OH)2 nanohybrids dispersion to produce CMC-Ca(OH)2-TiO2 nanohybrids.
C-Ca(OH)2: 0.0052 g of CaCl2 was dispersed in 16 mL of deionized water. Then 0.31 mL of NH3·H2O was slowly added. The mixture was stirred for 30 min at room temperature, after which it was centrifuged at a low speed and washed three times with a saturated solution of Ca(OH)2 to obtain C-Ca(OH)2.

2.3. Paper Strengthening Methods

The Xuan paper was cut into several samples of 5 cm × 5 cm. These samples were then placed flat on an experimental table covered with glass plates. Using a soft-bristle brush, an appropriate amount of restoration material solution was evenly applied to the surface of each sample, with an application rate of 0.025 mL/cm2. The samples were air-dried in the shade until they reached a constant weight, after which they were flattened and set aside for further use.

2.4. Tensile Strength Testing

Tensile strength refers to the maximum tension that a sample of a certain width can withstand, measuring the paper’s ability to resist external forces during stretching. According to GB/T 12914-2008 [41] “Determination of Tensile Strength of Paper and Paperboard”, paper samples were cut to dimensions of 15 mm × 240 mm and were subjected to tensile strength testing using the QT-1136PC (Dongguan Gaotai testing instrument Co., Ltd., Guangzhou, China) universal material testing machine at a stretching rate of 20 mm/min. The paper samples were tested for tensile strength along both the cross-machine direction and the machine direction. The tensile strength, denoted as S (kN/m), was obtained when the paper sample reached fracture during the test.

2.5. UV Effect

The treated samples were placed in a UV effect chamber under the following conditions: 40 W power, 313 nm wavelength, with a distance of 50 mm between the sample surface and the UV lamp plane. The samples were exposed for 15 days and then stored in darkness for 24 h.

2.6. Light Transmittance Testing

The light transmittance of a series of nanocellulose films was measured using a UV-lambda9500 UV-Vis-NIR spectrophotometer. The wavelength range was set from 400 to 800 nm, with a scanning rate of 300 nm/min. The measurements were conducted at a temperature of 25 °C and a sensitivity of 100%.

2.7. Antimicrobial Experiment

2.7.1. Preparation of Culture Medium

An amount of 38 g of PDA was added to 1000 mL of deionized water. The mixture was heated on a hot plate while stirring with a glass rod. When the solution started to boil, it was quickly transferred into conical flasks and sealed with Parafilm. The flasks containing the culture medium were placed in a pressure steam sterilizer and autoclaved at 121 °C for 15 min. After sterilization, the PDA solution was poured into disposable petri dishes, adding approximately 20–30 mL to each dish. The solution was allowed to solidify completely before placing on a clean bench for later use.

2.7.2. Fungal Preparation

PDA containing Aspergillus flavus, Aspergillus niger, and Alternaria altenata was shaken with 5 mL of physiological saline to allow the fungal spores of the three species to fall into the saline solution. The spore suspension of the three fungi was obtained by filtering through sterilized double-layer gauze. Fungal spores were counted using a hemocytometer, and the spore suspension was diluted with physiological saline until a concentration of 1 × 105 CFU/mL was achieved.

2.7.3. Antimicrobial Effectiveness Testing Using the Oxford Cup Method [42,43]

The antifungal effects of CMC-Ca(OH)2 and CMC-Ca(OH)2-TiO2 were tested using the Oxford cup method. On a clean bench, a pipette was used to withdraw 100 µL of spore suspension of the three types of molds (Aspergillus flavus, Aspergillus niger, and Alternaria altenata) and deposited onto blank culture media. A sterilized glass rod was used to evenly spread the spore suspension over the surface of each culture medium. Four sterilized Oxford cups were placed evenly in the corners of each blank culture medium. A solution of CMC-Ca(OH)2 was injected into two of the cups, and CMC-Ca(OH)2-TiO2 solution was injected into the other two cups. The culture media with the three different molds were then transferred to the mold incubator for cultivation for 48 h. The size of the inhibition zone was determined.

2.8. Color Difference Testing

The X-Rite VS450 non-contact spectrophotometer (Guangzhou Zhuo harmonic instrument equipment Co., Ltd., Guangzhou, China) was utilized to conduct color difference testing on the paper samples. The CIE L*a*b* color coordinate system was employed to characterize the color changes in the paper samples. The smaller the value of the color difference (∆E*), the less the color changed in the paper samples before and after reinforcement and accelerated aging. The color difference calculation formula in the CIE L*a*b* color system is as follows:
∆E* = [(∆L*)2 + (∆a*)2 + (∆b*)2]1/2
where ∆L* represents the difference in lightness, ∆a* represents the difference in the red-green color deviation, and ∆b* represents the difference in the yellow-blue color deviation. Initially, the L*, a*, and b* values of a standard sample were measured, and then by comparing the differences between the values of the sample and the standard, conclusions regarding the color difference were drawn.

2.9. Instruments

The crystalline structure, morphology, and surface composition of the sample were physically characterized using transmission electron microscopy (TEM, JEM-2100F) equipped with an energy dispersive spectrometer (EDS), infrared spectrometer (IR, Bruker V70, Massachusetts, the United States), and X-ray diffraction (XRD, D/max-rC). The mold cultivation cabinet, Model MJP, was supplied by Beijing Zhongxing Weiyue Instrument Co., Ltd. (Beijing, China); the UV clean bench, Model SW-CJ, was from Tianjin Xinduo Instrument Co., Ltd. (Tianjin, China); and the high-pressure autoclave, Model DX-B, was manufactured by Shanghai Shengan Medical Equipment Factory (Shanghai, China).

3. Results and Discussion

3.1. Structural Characterization of CMC-Ca(OH)2-TiO2

During the preparation process, the oxygen-containing functional groups on carboxymethyl cellulose were first connected with Ca2+ through electrostatic interactions. Subsequently, the addition of ammonia water hydrolyzed to produce OH-, which in situ formed nanoscale Ca(OH)2 particles adhered to the surface of carboxymethyl cellulose. Then, pre-prepared TiO2 nanoparticles were loaded onto the surface of CMC and nanoscale Ca(OH)2. The morphological characterization of CMC revealed a three-dimensional porous network structure (Figure 1A), which features facilitate diffusion and transfer, providing a favorable channel for Ca2+ adsorption. The TEM images of CMC-Ca(OH)2-TiO2 (Figure 1B–D) showed well-distributed spindle-shaped nanoparticles with a size of approximately 30 nm on the surface of CMC, without aggregation and large clusters. Additionally, TiO2 nanoparticles with a size of about 5 nm were uniformly dispersed on the surface of CMC-Ca(OH)2, demonstrating a darker contrast. Furthermore, the energy spectrum graphs C, Ca, and Ti were very similar, which further confirmed the uniform dispersion of TiO2 nanocrystals on the surface of CMC-Ca(OH)2 (Figure 1E).
To reveal the reaction sites during the in-situ synthesis process and investigate the interaction between Ca(OH)2 nanoparticles and CMC, infrared spectroscopy measurements were conducted. Figure 1F shows the infrared spectra of the CMC and CMC-Ca(OH)2 composite materials. In the CMC infrared spectrum, characteristic absorption peaks at 1592 cm−1 and 1421 cm−1 are attributed to the asymmetric and symmetric stretching vibrations of -COO-, while the peaks from 1000 cm−1 to 1200 cm−1 correspond to the stretching vibrations of C2-OH and C3-OH in the 1,4-glucoside of CMC [44]. After CMC adsorbs Ca2+, there is a significant shift in the -OH stretching vibration peak, and the -OH and -COO- peaks become weaker, indicating an interaction between Ca2+ and the oxygen-containing groups on the surface of CMC. To confirm the regulatory effect of CMC on the morphology of Ca(OH)2, Ca(OH)2 without CMC was prepared using the same method as a control [45]. This further demonstrates the important role of CMC in controlling the morphology and uniform distribution of Ca(OH)2 nanoparticles.
Based on the X-ray diffraction (XRD) pattern of CMC-Ca(OH)2-TiO2 shown in Figure 1G, by comparing its diffraction peaks with the X-ray diffraction standard card (Ca(OH)2: PDF#1-1079), it can be determined that the diffraction peaks at 18.2°, 28.6°, 34.3°, and 47.5° correspond to Ca(OH)2, while the peaks at 26.5°, 48.7°, 54.5°, and 75.8° correspond to the relevant diffraction peaks of TiO2 (PDF#2-406). This indicates that the hybrid material CMC-Ca(OH)2-TiO2 has been successfully prepared.

3.2. Performance of CMC-Ca(OH)2-TiO2

Enhance Mechanical Strength Performance

The high transparency of reinforcing materials ensures that the treated paper does not affect the legibility of writing or images, making transparency a crucial parameter for cellulose as a paper reinforcing material. Due to the milky white appearance of high-concentration Ca(OH)2 and TiO2 dispersion solutions, we tested the transparency of CMC-Ca(OH)2-TiO2 composite materials with different loadings of Ca(OH)2 and TiO2. We used a UV-visible spectrophotometer to measure the transparency of a series of CMC-Ca(OH)2-TiO2 samples and obtained a thickness of 0.015 mm. As depicted in Figure 2A, the transparency of the films exceeded 85% in the visible light range of 400–800 nm when the content of the materials was below 8.5%, indicating that they almost did not block the propagation of light and exhibited good transparency. However, when the content of Ca(OH)2 and TiO2 exceeded 10%, the film transparency showed a significant decreasing trend. Consequently, we set the loading of Ca(OH)2 and TiO2 in CMC-Ca(OH)2-TiO2 to 8.5%.
The key factor for long-term stable preservation of paper is acidity. Paper should be stored in an environment with a pH of 7.5–9, as both too low and too high pH levels will have adverse effects on the cellulose fibers of the paper. The research objects in this study were untreated paper, paper treated with commercial Ca(OH)2, and paper treated with CMC-Ca(OH)2-TiO2. As shown in Figure 2B, the initial pH of the raw paper was 5.73, which decreased after aging to 5.65 under dry heat aging and further to 5.13 under wet heat aging, indicating a more significant decrease in pH due to wet heat aging and the generation of more acidic substances. After deacidification treatment with CMC-Ca(OH)2-TiO2 hybrid material, the pH of the paper was weakly alkaline at 8.21, 8.05 after dry heat aging, and 7.88 after wet heat aging. The decrease in pH after aging was 2–4%, showing that deacidification treatment led to a smaller decrease in surface pH, possibly because the Ca(OH)2 in the deacidifying solution converted to CaCO3 and neutralized the acids in the paper, effectively inhibiting acid hydrolysis of cellulose and leaving behind a certain amount of residual alkali to maintain the paper’s pH at an appropriate level for a longer period. In contrast, when using an equal amount of commercial Ca(OH)2, although a similar pH effect was achieved, significant whitening occurred during the process, possibly due to the larger particles of commercial Ca(OH)2 being unable to penetrate the inner fibers of the paper, consistent with previously reported research findings.
In order to investigate the strengthening performance of CMC-Ca(OH)2-TiO2 on paper, we tested the tensile strength of untreated paper, CMC-treated paper, and CMC-Ca(OH)2-TiO2-treated paper (Figure 2C). The tensile strength of the untreated paper samples in the horizontal and vertical directions were 0.823 kN/m and 1.931 kN/m, respectively. After CMC treatment, the tensile strength of the paper increased, with values of 1.092 kN/m and 2.155 kN/m in the horizontal and vertical directions, respectively. After treatment with CMC-Ca(OH)2-TiO2, the mechanical strength of the paper significantly improved, with horizontal and vertical tensile strengths of 1.362 kN/m and 2.531 kN/m, respectively, representing a 24.6% and 17.4% increase compared to CMC.
To further investigate the mechanical strength of aged paper treated with CMC-Ca(OH)2-TiO2, we subjected untreated paper, CMC-treated paper, and CMC-Ca(OH)2-TiO2-treated paper to dry heat aging and wet heat aging. As shown in Figure 2C, the tensile strength of all samples decreased after dry and wet heat aging. After dry heat aging, the horizontal tensile strength of untreated paper decreased by 82.3% to 0.187 kN/m, and the vertical tensile strength decreased by 78.6% to 0.414 kN/m. After wet heat aging, the horizontal tensile strength of untreated paper decreased by 76.2% to 0.196 kN/m, and the vertical tensile strength decreased by 79.4% to 0.397 kN/m. The tensile strength of CMC-treated samples decreased by 42.6% (0.627 kN/m) in the horizontal direction and 61.8% (0.823 kN/m) in the vertical direction after dry heat aging, and by 36.3% (0.696 kN/m) and 42.9% (1.231 kN/m) in the horizontal and vertical directions, respectively, after wet heat aging. In contrast, the horizontal tensile strength of paper treated with CMC-Ca(OH)2-TiO2 only decreased by 29.7% (0.957 kN/m) after dry heat aging, and the vertical tensile strength only decreased by 20.3% (2.016 kN/m). After wet heat aging, the horizontal and vertical tensile strengths decreased by 23.8% (1.038 kN/m) and 17.1% (2.098 kN/m), respectively. The data indicate that paper treated with CMC-Ca(OH)2-TiO2 exhibited significantly improved horizontal and vertical tensile strength under normal conditions, as well as after dry and wet heat aging. Compared to CMC, the enhancement in the strengthening performance of CMC-Ca(OH)2-TiO2 may be attributed to the reduction of acidity in the paper by Ca(OH)2, which enhances the stability and durability of the paper. This deacidification effect can effectively prevent further acidification of the paper, thereby extending its preservation life.

3.3. Anti-UV Effect Performance

The CMC-Ca(OH)2-TiO2 hybrid material was subjected to solid UV-vis diffuse reflectance spectroscopy testing. The wavelength of ultraviolet light in sunlight ranges from 200 to 400 nm. As shown in Figure 3A, the hybrid material exhibits an absorption edge at around 400 nm. Therefore, the hybrid material can effectively absorb ultraviolet light from sunlight when used for the protection and restoration of paper-based cultural relics, preventing the occurrence of photoaging.
To explore the actual light aging resistance of CMC-Ca(OH)2-TiO2 hybrid material, paper samples were subjected to UV effect experiments before and after treatment with CMC-Ca(OH)2-TiO2, followed by tensile strength testing (Figure 3B). Upon testing, the tensile strength of untreated paper decreased by 41.5% in the horizontal direction (from 0.823 kN/m to 0.481 kN/m) and 43.6% in the vertical direction (from 1.931 kN/m to 1.091 kN/m) after UV effect. In contrast, paper treated with CMC-Ca(OH)2-TiO2 exhibited a decrease of only 27.6% in horizontal tensile strength (from 1.362 kN/m to 0.986 kN/m) and 20.9% in vertical tensile strength (from 2.531 kN/m to 2.001 kN/m) after UV effect. As a control, paper treated with CMC-Ca(OH)2 showed a decrease of 36.1% in horizontal tensile strength (from 1.114 kN/m to 0.713 kN/m) and 40.1% in vertical tensile strength (from 2.173 kN/m to 0.872 kN/m) after UV effect, comparable to untreated paper, indicating that the light aging resistance was not the result of CMC and Ca(OH)2, but likely originated from TiO2.
Additionally, pigments are susceptible to photo-oxidation reactions under UV light, leading to color changes such as fading and discoloration. We conducted tests on carbon black, cinnabar, carmine, and gamboge pigments (Figure 3C). After UV effect, samples treated with CMC-Ca(OH)2 showed color differences (ΔE) greater than 3 for cinnabar, carmine, and gamboge, indicating noticeable color changes. The analysis revealed that carbon ink exhibited minimal color variation, likely due to the high stability of carbon black as the main component. On the other hand, cinnabar can undergo crystalline phase transformation to metacinnabar (β-HgS) under high-energy UV radiation, leading to irreversible chemical reactions and damage [46]. Similarly, carmine and gamboge, comprising carminic acid and gambogic acid, respectively, undergo photo-oxidation reactions under UV light, resulting in lightening or fading. In contrast, paper samples treated with CMC-Ca(OH)2-TiO2 showed color differences below 2, attributed to the UV absorption effect of TiO2 nanoparticles, effectively delaying UV-induced damage.
The microscopic structure analysis of paper samples (Figure 3D,E) reveals that after aging with CMC-Ca(OH)2, certain fiber structure fractures were observed, while samples treated with the hybrid material showed mostly intact fiber structures with minimal fiber fractures after UV effect. This indicates that CMC-Ca(OH)2-TiO2 hybrid material has a light shielding effect, excellent resistance to light aging, and potential application in the protection and restoration of paper.

Antimicrobial Performance

According to bacteriostatic testing methodologies and standards, the bacteriostatic performance of the prepared materials was investigated [47,48]. Representative molds such as Aspergillus flavus, Aspergillus niger, and Alternaria altenata were selected to assess the antimicrobial activity of CMC-Ca(OH)2-TiO2. As shown in Figure 4, when these molds were inoculated onto PDA culture medium and covered with CMC-Ca(OH)2 and CMC-Ca(OH)2-TiO2 hybrid materials, the culture medium treated with CMC-Ca(OH)2 (blue marker) did not show any antimicrobial effects as the molds grew around it. On the other hand, the culture medium treated with CMC-Ca(OH)2-TiO2 (red marker) exhibited antimicrobial zones with a diameter of 5.38 mm for Aspergillus flavus, 4.34 mm for Aspergillus niger, and 2.36 mm for Alternaria altenata, indicating good antimicrobial effects against Aspergillus flavus and Aspergillus niger. The excellent antimicrobial performance of CMC-Ca(OH)2-TiO2 is mainly attributed to the inclusion of nano TiO2. In agreement with the findings of Ali Asghar Ariafar et al., they suggested that TiO2/chitosan nanoparticle-carboxymethyl cellulose composites have a significant anticorrosion effect in paper-based artifacts [49]. The antimicrobial mechanism is generally summarized as photocatalysis and physical damage. When nano titanium dioxide is exposed to UV light, it generates reactive oxygen species that disrupt bacterial cell structures, leading to bacterial death. Additionally, the small size and surface charge of nano titanium dioxide particles can cause mechanical damage and functional disruption to bacterial cells.

4. Conclusions

In conclusion, the CMC-Ca(OH)2-TiO2 composite material has been successfully prepared through a simple and environmentally friendly method, demonstrating its effectiveness as a multifunctional material for the restoration of paper. Through the reinforcement tests conducted, it was observed that the mechanical strength and tensile strength of paper can be significantly improved by the composite material, enabling the effective repair of damage and fractures in the paper. Moreover, the composite material has been shown to possess a good deacidification effect, being able to neutralize acidic substances and slow down the acidification process of paper. In the light aging experiments, the composite material has been effective in protecting paper from UV damage, thereby reducing color changes in both pigments and the paper itself. Furthermore, the composite material exhibits outstanding antimicrobial performance, effectively inhibiting the growth and reproduction of bacteria. Therefore, the CMC-Ca(OH)2-TiO2 composite material holds promising application prospects in various aspects of paper restoration, including reinforcement, deacidification, light aging resistance, and antimicrobial protection. By effectively addressing issues such as damage, acidification, light aging, and mold treatment encountered in traditional restoration methods, the composite material streamlines the restoration process, minimizing external interventions on paper. Its ability to achieve multiple goals with one solution opens up new avenues in the field of paper restoration, offering an efficient solution for the protection and preservation of paper.

Author Contributions

Conceptualization, J.L.; methodology, P.W.; data curation, R.M.; visualization, M.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by Shaanxi Province Social Science Foundation (2023G004).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Coatings 14 00851 g001
Figure 1. (A) TEM of CMC. (B–D) TEM of CMC-Ca(OH)2-TiO2. (E) Corresponding element maps of CMC-Ca(OH)2-TiO2. (F) IR spectrum of CMC and CMC-Ca(OH)2. (G) XRD of CMC-Ca(OH)2-TiO2.
Figure 1. (A) TEM of CMC. (B–D) TEM of CMC-Ca(OH)2-TiO2. (E) Corresponding element maps of CMC-Ca(OH)2-TiO2. (F) IR spectrum of CMC and CMC-Ca(OH)2. (G) XRD of CMC-Ca(OH)2-TiO2.
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Coatings 14 00851 g002
Figure 2. (A) The light transmittance test was conducted by adding varying contents of Ca(OH)2 and TiO2. (B) pH changes in paper after dry heat and humid heat aging with C-Ca(OH)2 and CMC-Ca(OH)2-TiO2 treatments. (C) Changes in horizontal and vertical tensile indices of paper treated with CMC and CMC-Ca(OH)2-TiO2 after dry heat and wet heat aging.
Figure 2. (A) The light transmittance test was conducted by adding varying contents of Ca(OH)2 and TiO2. (B) pH changes in paper after dry heat and humid heat aging with C-Ca(OH)2 and CMC-Ca(OH)2-TiO2 treatments. (C) Changes in horizontal and vertical tensile indices of paper treated with CMC and CMC-Ca(OH)2-TiO2 after dry heat and wet heat aging.
Coatings 14 00851 g002
Coatings 14 00851 g003
Figure 3. (A) UV diffuse reflectance spectroscopy test of CMC-Ca(OH)2-TiO2 hybrid material. (B) Changes in horizontal and vertical tensile indices of paper treated with CMC-Ca(OH)2 and CMC-Ca(OH)2-TiO2 after UV effect. (C) Changes in color difference of carbon ink, cinnabar, carmine and gamboge pigments treated with CMC-Ca(OH)2 and CMC-Ca(OH)2-TiO2 before and after UV effect. (D,E) SEM images of paper treated with CMC-Ca(OH)2 and CMC-Ca(OH)2-TiO2 before and after UV effect (fiber fractures are circled).
Figure 3. (A) UV diffuse reflectance spectroscopy test of CMC-Ca(OH)2-TiO2 hybrid material. (B) Changes in horizontal and vertical tensile indices of paper treated with CMC-Ca(OH)2 and CMC-Ca(OH)2-TiO2 after UV effect. (C) Changes in color difference of carbon ink, cinnabar, carmine and gamboge pigments treated with CMC-Ca(OH)2 and CMC-Ca(OH)2-TiO2 before and after UV effect. (D,E) SEM images of paper treated with CMC-Ca(OH)2 and CMC-Ca(OH)2-TiO2 before and after UV effect (fiber fractures are circled).
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Coatings 14 00851 g004
Figure 4. (A) Antibacterial test of CMC-Ca(OH)2 hybrid material and CMC-Ca(OH)2-TiO2 hybrid material against Aspergillus flavus. (B) Antibacterial test of CMC-Ca(OH)2 hybrid material and CMC-Ca(OH)2-TiO2 hybrid material against Aspergillus niger. (C) Antibacterial test of CMC-Ca(OH)2 hybrid material and CMC-Ca(OH)2-TiO2 hybrid material against Alternaria altenata (blue for CMC-Ca(OH)2 and red for CMC-Ca(OH)2-TiO2).
Figure 4. (A) Antibacterial test of CMC-Ca(OH)2 hybrid material and CMC-Ca(OH)2-TiO2 hybrid material against Aspergillus flavus. (B) Antibacterial test of CMC-Ca(OH)2 hybrid material and CMC-Ca(OH)2-TiO2 hybrid material against Aspergillus niger. (C) Antibacterial test of CMC-Ca(OH)2 hybrid material and CMC-Ca(OH)2-TiO2 hybrid material against Alternaria altenata (blue for CMC-Ca(OH)2 and red for CMC-Ca(OH)2-TiO2).
Coatings 14 00851 g004
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MDPI and ACS Style

Li, J.; Ma, R.; Wu, P.; Quan, M. CMC-Ca(OH)2-TiO2 Nanocomposite for Paper Relics Multifunctional Restoration: Strengthening, Deacidification, UV Effect Resistance, and Antimicrobial Protection. Coatings 2024, 14, 851. https://doi.org/10.3390/coatings14070851

AMA Style

Li J, Ma R, Wu P, Quan M. CMC-Ca(OH)2-TiO2 Nanocomposite for Paper Relics Multifunctional Restoration: Strengthening, Deacidification, UV Effect Resistance, and Antimicrobial Protection. Coatings. 2024; 14(7):851. https://doi.org/10.3390/coatings14070851

Chicago/Turabian Style

Li, Jing, Ruiwen Ma, Peng Wu, and Min Quan. 2024. "CMC-Ca(OH)2-TiO2 Nanocomposite for Paper Relics Multifunctional Restoration: Strengthening, Deacidification, UV Effect Resistance, and Antimicrobial Protection" Coatings 14, no. 7: 851. https://doi.org/10.3390/coatings14070851

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