Xiaochun Ma,Altaf Halim,Xiaohong Li,Huiming Fan,Shiyu Fu,*

1.State Key Laboratory of Pulp and Paper Engineering,South China University of Technology,Guangzhou,Guangdong Province,510640,China

2.Lingnan Literature Protection Research Center,Guangzhou,Guangdong Province,510640,China

3.National Research Center of Egypt,Giza,12622,Egypt

4.Guangzhou Paper Co.,Ltd.,Guangzhou,Guangdong Province,510280,China

Abstract:Deacidification and self-cleaning are important for the preservation of paper documents.In this study,nano-CaCO3 was used as a deacidification agent and stabilized by nanocellulose (CNC) and hydroxypropyl methylcellulose(HPMC)to form a uniform dispersion.Followed by polydimethylsiloxane(PDMS)treatment and chemical vapor deposition(CVD)of methyltrimethoxysilane(MTMS),a hydrophobic coating was constructed for self-cleaning purposes.The pH value of the treated paper was approximately 8.20,and the static contact angle was as high as 152.29°.Compared to the untreated paper,the tensile strength of the treated paper increased by 12.6%.This treatment method endows the paper with a good deacidification effect and self-cleaning property,which are beneficial for its long-term preservation.

Keywords:paper documents;nanocellulose;self-cleaning;nano-CaCO3;superhydrophobicity;deacidification

1 Introduction

Historical paper documents are easily affected by acidic substances,mold,and insect pests during their preservation,resulting in the fracture of paper fibers,which significantly deteriorates its strength and leads to key information loss[1].Removing acidic substances and enabling self-cleaning paper surfaces are important aspects of paper preservation[2-3].

The neutralization of acidified paper with alkaline substances is a common deacidification method.CaCO3is a solid alkaline substance with low cost and strong permeability,and it is widely used for deacidification purposes[4-5].Nano-CaCO3is easier to penetrate and has larger surface area because of its small particle size.However,the aggregation of nano-CaCO3can deteriorate its deacidification performance and cause a change in paper chromaticity;therefore,it is crucial to prepare stable and uniformly dispersed nano-CaCO3for the paper coating.Transparency and strength are two issues that must be considered during coating formation on papers.In addition to the deacidification property,alkaline nano-CaCO3particles also have a low refractive index and good compatibility;therefore,they can be used to prepare coatings with high transparency.The amphiphilicity of nanocellulose(CNC)and hydroxypropyl methylcellulose(HPMC)can facilitate the effective dispersion of nano-CaCO3in alcohol aqueous medium,and they have a structure similar to that of paper fibers,which helps reinforce the paper.

One of the best ways to prevent mold is to control the environmental storage conditions of the paper[6].Dehydration,γ-irradiation[7]and high-frequency currents[6]are common physical mildew prevention methods that can be applied on a large scale without leaving residual toxic substances.However,these methods require timeliness and high cost.Although some chemical methods (e.g.,essential oils[8],photocatalysts, Ag nanoparticles[9], TiO2, and chitosan[10])can be used for long-term mildew prevention,but they damage compounds on paper,especially on the fiber structure.Therefore,new moldproof methods are required.Considering one of the main conditions for mold growth is moisture,surfaces with low surface energy can minimize the attachment of water droplets and essential mold breeding[11-12];therefore,constructing a superhydrophobic surface not only enables the paper to have surface self-cleaning properties but also prevents mold growth.

The methods to prepare superhydrophobic materials mainly include chemical vapor deposition(CVD),nanoparticle deposition[13],sol-gel method,plasma treatment[14],and solution-immersion processes.The superhydrophobic and oleophobic papers were produced by CVD using trichloro(1H,1H,2H,2Htridecafluoro-n-octyl)silane(FOTS)on a filter paper impregnated with nanocellulose[14].The sol-gel method by adding polydimethylsiloxane(PDMS)to tetraethyl orthosilicate has been employed to fabricate hydrophobic materials with mechanical durability and self-cleaning properties for large-scale outdoor applications[15].Superhydrophobic coatings formed by the deposition of silica nanoparticles on the surface of cellulose nanofiber(CNF)have potential applications on glass,tents,umbrellas,and other products[16].PDMS is a low-cost,low-surface-energy modifier with excellent chemical stability,strong adhesion,good abrasion resistance,and high light transmittance[17].Owing to its intrinsic hydrophobicity,PDMS has been used to prepare superhydrophobic surfaces.Methyltrimethoxysilane(MTMS)is a low surface energy modifier,which is often used for hydrophobic modification.

In this study, nano-CaCO3was the main deacidification substance for transparent coating.HPMC and CNC were used to stabilize the nano-CaCO3alcoholic dispersion and act as a reinforcing agent of paper simultaneously.PDMS and MTMS treatments were conducted to achieve a superhydrophobic surface for self-cleaning and mold prevention.The treatment method provides multiple functions to the paper and is crucial for the long-term preservation of paper cultural relics.

2 Experimental

2.1 Materials

Anhydrous ethanol and sulfuric acid(98%)were purchased from Guangzhou Chemical Reagent Factory(China).PDMS(1.04 g/mL)was acquired from Dow Corning(USA),includingr reagent A and reagent B.Nano-CaCO3was purchased from Shanghai Maclean Chemical Reagent Co.,Ltd.(China).HPMC(2%,6 mPa·s)and MTMS(98%)were purchased from Shanghai Aladdin Biochemical Technology Co.,Ltd.(China).CNC from the pulp was prepared in our laboratory using the acid hydrolysis method[18].The paper sample used was an old wheat straw paper published in 1954,which was cut to a size of 14 cm×10 cm.

2.2 Preparation and stability test of nano-CaCO3 dispersion

The CNC suspension was diluted to 0.5 wt%and dispersed uniformly under mechanical stirring.HPMC(0.40 g)was added to the above CNC suspension(60 mL);the mixture was denoted as H-C dispersion.Nano-CaCO3(2.0 g)was added to 40 mL ethanol to obtain a nano-CaCO3ethanol dispersion with a concentration of 50 g/L,which was denoted as E-Ca dispersion.

Deionized water(5 mL)was added to 5 mL E-Ca dispersion to obtain E-Ca/H2O as a control.The E-Ca/H-C dispersion was obtained by mixing different volumes of E-Ca and H-C dispersions.The timedependent stability of the E-Ca/H-C and E-Ca/H2O dispersions was measured using a UV spectrometer at 600 nm.

2.3 Formation of superhydrophobic surface on paper

PDMS reagent A was added to the H-Ca and E-Ca dispersions,then sonicated at 25℃for 10 min,and PDMS reagent B was added at 8 min.Subsequently,a W-71 spray gun(Japan)was used to spray the PDMS mixture approximately 10 cm away from the surface of the paper under a pressure of 0.30 MPa,as shown in Fig.1.

Fig.1 Preparation of superhydrophobic paper samples

Paper samples with the PDMS coatings were further modified using MTMS(Fig.2).In detail,the paper sample was placed in a homemade CVD reactor with two small petri dishes containing MTMS(500μL)and deionized water(1 mL).The CVD reactor was sealed with plastic wrap and placed in an oven at 105℃for 2 h for vapor deposition.

Fig.2 CVD treatment of paper

The experimental parameters of the deacidification and superhydrophobic treatment conditions of papers are listed in Table 1.

Table 1 Deacidification and superhydrophobic treatment conditions of papers

2.4 Characterization and measurements

2.4.1 Characterization

The surfaces of the paper were examined using an Xray photoelectron spectroscope(XPS,Axis Ultra DLD,Kratos,Japan).

A Fourier transform infrared spectrometer(FT-IR,Tensor27,Bruker,Germany)was used to analyze the papers before and after the hydrophobic modification.

To study the dispersion of nano-CaCO3particles on the paper,a scanning electron microscope(SEM,Zeiss Merlin,Germany)was used to observe the surface morphology of the papers before and after treatment.

2.4.2 Superhydrophobic performance test

A contact angle measurement device (OCA40,DataPhysics Co.,Germany)was used to estimate the hydrophobic properties of the papers before and after modification.The paper sample was attached to a glass slide,and the surface contact angle was measured when a water droplet of 3μL was applied.The surface roughness of the paper was measured using an L&W CE165 roughness tester(Lorentzen&Wettre,USA)according to GB/T 22881—2008.

2.4.3 pH value of paper samples

According to ISO 6588-1:2012,the pH value of the paper sample(5 mm×5 mm,2 g)was determined.At least three measurements were performed on each group of samples,and the average was calculated.

2.4.4 Mechanical property and brightness of paper

The paper samples were placed at 25℃for more than 24 h before their mechanical property was tested.According to GB/T 12914—2008,the tensile strength before and after modification was measured using a L&WTester(CE062,Sweden).

According to ISO 11475—2017,the brightness of the paper was measured using L&W Elrepho spectrophotometer(Elrepho 070,Sweden).Five areas of each sample were measured.The color difference(ΔE*)between the untreated and treated papers was calculated using Eq.(1)to evaluate the color change.

2.4.5 Anti-aging property of paper samples

According to GB/T 22894—2008,the paper samples were subject to accelerated aging for three days in an aging chamber at a temperature of 80℃and a humidity of 65%to simulate the normal aging of 25 years.The tensile strength and brightness of the aged samples were measured accordingly.

3 Results and discussion

3.1 Stability analysis of nano-CaCO3 dispersion

Alkaline nanomaterial is a new deacidification agent for paper documents.Nanoparticles can easily coat the paper and penetrate the fiber.Owing to their large surface energy,more charged polymers are likely to adsorb on the nanoparticles and cause their agglomeration,which can reduce the uniformity of the spraying and increase the brightness of the paper surface.Therefore,amphiphilic CNC and HPMC were added to the dispersion to improve the dispersibility of nano-CaCO3particles.

The time-dependent absorbance intensities at 600 nm are shown in Fig.3 for E-Ca/H-C and E-Ca/H2O dispersions.With the extension of standing time,the absorbance of the E-Ca/H2O dispersion decreased significantly. The absorbance of the E-Ca/H-C dispersion remained at its original level within 30 min and decreased slowly afterward.As shown in the inset of Fig.3,the E-Ca/H-C dispersion after ultrasonic treatment could remain stable even after 12 h with no precipitation observed.In comparison,the nano-CaCO3particles completely settled at the bottom of the vial after 20 s without HPMC and CNC.The above results prove that amphiphilic HPMC and CNC are favorable to stabilizing the nano-CaCO3dispersion and significantly improving the compatibility of the nanoparticles.

Fig.3 Time-dependent absorbance intensities at 600 nm of the E-Ca/H-C and E-Ca/H2O dispersions.Inset:the images of(a)E-Ca/H2O dispersion and(b)E-Ca/H-C dispersion after standing for 12 h

3.2 Properties of paper treated with superhydrophobic modification

High surface roughness and low surface energy are important features for a superhydrophobic surface.Nano-CaCO3was incorporated into the PDMS coating to construct a microstructure with certain roughness on the paper surface.The paper was further treated by MTMS via CVD to complete the superhydrophobic modification.

XPS was employed to study the paper surface after modification.As shown in Fig.4,peaks of Si 2p and Si 2s appeared at 100 and 150 eV in sample 3#,confirming the MTMS silanization during CVD.Si 2s and Si 2p peaks were also detected in sample 5#,indicating that the addition of PDMS also introduced Si.

Fig.4 XPS spectra of superhydrophobic paper samples 3#and 5#

The FT-IR spectra of sample 3#before and after aging are shown in Fig.5.The bands near 802 and 460 cm-1were attributed to the symmetrical stretching and bending vibration of Si—O—Si,respectively,which also confirmed the success of silane modification.

Fig.5 FT-IR spectra of sample 3#before and after accelerated aging

The surface morphology of the untreated paper(1#)and treated papers under four different hydrophobic modification conditions(2#-5#)was observed by SEM.As shown in Fig.6,nano-CaCO3particles were successfully introduced and uniformly distributed on the surfaces of samples 2#,3#,and 5#treated with the ECa/H-C dispersion.Comparatively,the surface of sample 4#showed uneven agglomeration of nano-CaCO3particles due to the absence of H-C dispersion.Compared to the smooth surface of the untreated paper(1#),the roughness of the paper with hydrophobic coatings was increased because of the nano-CaCO3particles.From the roughness values listed in Table 2,the surface roughness of samples 3#and 4#increased to 4.05μm and 4.24μm,respectively,while that of untreated paper was 3.83 μm.Increasing surface roughness is an important strategy to construct superhydrophobic surface.The EDS analysis(Fig.7)confirmed the presence of Si and the distribution of Ca.After hydrophobic modification,the relative contents of Si and Ca increased significantly.The highest Si content was detected in sample 3#(10.74%)and sample 5#had the highest Ca content(26.87%).

Fig.6 SEM images(inset:static contact angle image)and EDS mapping of paper samples

Fig.7 EDS analysis of papers before and after hydrophobic modification

The detailed static contact angle(CA)results are also presented in Table 2.The initialCAof the untreated paper was 58.20°.After 10 s of stabilization,theCAdecreased to 0°,suggesting the weak hydrophobicity of untreated paper.TheCAof sample 3#(152.29°)was higher than that of sample 5#(123.76°).Since sample 5#was not treated by MTMS,a more hydrophobic surface in sample 3#was associated with the MTMS treatment.The low surface energy of MTM was favorable to achieve superhydrophobicity.

Table 2 Contact angle,roughness,and thickness of papers before and after hydrophobic treatment

To visually demonstrate the hydrophobic effect of the modified paper,three droplets(water,methyl red,and methylene blue)were used to form a contrast,as shown in Fig.8.The water droplets on the untreated paper(1#)surface were hemispherical and gradually diffused to wet the paper over time.In contrast,the water droplets on the surface of samples 2#and 3#remained spherical and did not infiltrate the paper.The aged sample 3#showed higher superhydrophobicity,allowing water droplets to roll off the surface when the paper was tilted.

3.3 Self-cleaning effect of paper treated with hydrophobic modification

Model pollutants were obtained by drying garden soil in an oven at 105℃.To visually show the self-cleaning effect of the paper,the paper samples before and after hydrophobic modification were tilted and placed in a clean petri dish.The dried dust was placed on the surface of the paper,and ionic water droplets were slowly applied.The surface state of the paper under the continuous erosion of the water droplets was recorded.As shown in Fig.9,the untreated paper surface was polluted by muddy water,while the soil on the surface of samples 2#,3#,and 3#-aging did not adhere to the surface and was washed away due to their strong hydrophobicity.In contrast,mud residual was observed on the surface of sample 5#because its surface was less hydrophobic.The results showed that PDMS alone was insufficient to achieve a superhydrophobic surface,and the MTMS vapor deposition had an enhancing effect on the self-cleaning property.

Fig.9 Self-cleaning property of papers before and after hydrophobic modification

3.4 Strength property and anti-aging effect of the hydrophobic modified paper

The tensile strength of the hydrophobic modified papers before and after aging was measured.As shown in Fig.10,compared with the untreated paper,the tensile strength of samples 2#and 3#increased by 12.6%and 12.3%,respectively.However,the tensile strength of sample 4#without adding H-C dispersion was decreased by 3.26%.After 3 days of accelerated aging,a slight reduction in tensile strength was found in all samples.Aged samples 2#and 3#still had the higher tensile strengths, 14.41% and 18.03%,respectively,higher than that of the untreated sample.CNC and HPMC have structures similar to those of the paper fibers;thus,the H-C dispersion can penetrate the porous fibers and enhance their interactions,improving the strength property of the paper.

Fig.10 Tensile strength of papers before and after hydrophobic modification

The brightness andΔE*of the papers before and after hydrophobic treatment and aging are shown in Table 3.The results suggested that the brightness of the paper did not change significantly,exhibiting excellent anti-aging effect.

3.5 Deacidification effect

One key process in paper preservation is to permeate the paper with alkaline substances and neutralize excess acidic substances.To avoid further damage to paper documents caused by excessive alkalinity,the alkaline nano-CaCO3particles were encapsulated by HPMC and released slowly.In addition,the PDMS coating was sprayed in the form of atomization at 0.3 MPa pressure,which greatly reduced the damage to paper fiber.Table 4 shows the pH values of the paper samples subjected to different treatments.The pH value of the untreated paper sample was 5.28,indicating that it was acidified.The alkalinity of the four paper samples treated with nano-CaCO3was significantly improved,and the pH value was in the range of 8.20-8.41,suggesting successful deacidification.

Table 3 Brightness changes of paper samples

Table 4 p H values of paper samples

4 Conclusions

A new approach is developed to preserve paper,wherein nanocellulose (CNC) and hydroxypropyl methylcellulose(HPMC)were employed as stabilizers to achieve a uniform dispersion of deacidification agent(nano-CaCO3),supplemented by hydrophobic treatment using polydimethylsiloxane (PDMS) and methyltrimethoxysilane(MTMS).The static contact angle of the treated paper reached 152.29°,and the pH value was 8.20-8.41. The treated paper also demonstrated a higher tensile strength and a better selfcleaning property than the untreated paper.This method offers multiple functions to the protective coatings and is cost-effective,thereby showing great potential for the long-term preservation of papers.

Acknowlegment

This work was supported by Science and Technology Plan Special Project of Guangzhou,China (No.GZDD201808)and National Key Research Program for International Cooperation-MOST/STDF(2021YFE0 104500).