Alkyl ketene dimer grafted cationic cellulose (AKD-g-CMCC) was designed to stabilize nano magnesium oxide (MgO) in low polarity mixed solution of hexamethyldisiloxane (HMDO) and isopropanol (IPA) for protection of aging paper. The highly stable nano MgO organic colloid exhibited a small average particle size (200 nm) and low viscosity (2.26 mPa·s), which is beneficial for the efficient protection of aging paper. Evaluation results showed that properties of treated aging paper were improved. Small-size MgO (200 nm) uniformly deposited on the paper endowed the treated paper with mild pH (8.30) and sufficient alkali reserve (1.00%, calcium carbonate equivalent) for deacidification. Meanwhile, the tensile index and tear index of treated paper increased by 40.78% and 27.28%, respectively. Moreover, the treated paper presented the low chromatic aberration (∆E=0.46) and the negligible influence in the appearance and clarity of writing. Additionally, the treated paper also had the excellent anti-aging and anti-adhesion performance.
Research Article
Alkyl Ketene Dimer Grafted Cationic Cellulose Stabilizing Nano Magnesium Oxide for Efficient Protection of Aging Paper
https://doi.org/10.21203/rs.3.rs-1347521/v1
This work is licensed under a CC BY 4.0 License
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Cellulose
Amphiphilic modification
Dispersion
Aging paper
Hundreds of millions of paper materials are suffering from the serious aging and degradation over the world (Cappitelli et al. 2010; Liu and Wang 2010; Wilson 2012; Norris and Gutierrez 2017). The acid-catalyzed hydrolysis of paper fibers is the central reason for the damage of aging paper due to the accumulation of acidic substances. Cleavage of 1,4-β-glycosidic bond of cellulose caused the reduce of the degree of polymerization (DP), further resulting in the weak mechanical strength of aging paper, which is unfavorable for their conversation and utilization (Carter 1996a; Baty et al. 2010; Area and Ceradame 2011; Zervos and Alexopoulou 2015). Moreover, oxidation of cellulose is another critical factor for the degradation of aging paper. The chromophores were generated from paper fibers by the promotion of ultraviolet light, high alkalinity or oxidizing substances, causing paper discoloration, especially paper containing lignin (Carter 1996b; Conte et al. 2012). Therefore, deacidification, enhancement and anti-oxidation are the indispensable procedures for the protection of aging paper.
Currently, deacidification methods were mainly developed based on the principle of acid-base neutralization that alkaline agents neutralize acidic substances to inhibit acid-catalyzed hydrolysis of paper fibers and improve the durability of aging paper. Most of deacidification agents can be summarized as alkaline aqueous solutions, alkaline organic colloids, and alkaline gases (Baty et al. 2010; Zervos et al. 2015). Alkaline aqueous solutions are environment-friendly and low-cost, regrettably, the treated aging paper needs a time-consuming drying process and is easy to be crumpled and low-mechanical-strength (Baglioni et al. 2014). Additionally, alkaline gases such as ammonia, diethyl zinc (DEZ) and dry ammonia ethylene oxide (DAE) can easily achieve mass deacidification, but the low alkali reserves and serious safety hazards limit their application (Cheradame et al. 2003). Mass deacidification methods mainly developing from alkaline organic colloids have been widely applied in libraries and archives, including Wei T'o process, Bookkeeper process, and Papersaver process®, etc., which are preferred by conservators for their excellent deacidification effect, simple and efficient process, and small negative impact on the appearance and writing clarity of aging paper based on the rapid volatilization of low-polar organic solvents such as dichlorodifluoromethane, perfluoroheptane, and hexamethyldisiloxane (HMDO) (Potthast and Ahn 2017; Hubbe et al. 2017; Hubbe et al. 2018). However, these mass deacidification methods did not significantly improve the mechanical performance of treated aging paper, especially for the fragile aging paper.
Enhancement of aging paper has also been studied based on the chemical or physical methods. Synthetic polymers (silicone resin (Ferrandin-Schoffel et al. 2020), acrylic resin (Xu et al. 2020), epoxy resin (Pereira-Pardo and Korenberg 2018), polyurethane (Isca et al. 2016; Isca et al. 2019), etc.) and natural polymers (cellulose (Santos et al. 2015; Li et al. 2019; Xu et al. 2020), chitosan (Jiang et al. 2016; Jia et al. 2020), starch (Lama et al. 2020), etc.) were used to reinforce aging paper. In comparison, cellulose is preferred for its good compatibility with paper fibers and excellent reinforcement performance. However, the poor solubility of cellulose in water or general organic solvents causes the restriction on its applications (Li et al. 2019). Chemical modification of cellulose is the common method to improve its solubility or dispersibility (Xu et al. 2020). Nevertheless, most of the reinforcement materials based on cellulose derivatives for the protection of aging paper were mainly dissolved or dispersed in water, which was inefficient due to the time-consuming drying process (Santos et al. 2015; Li et al. 2019; Palladino et al. 2020). Treatment with aqueous solutions may also cause some adverse effects on appearance of aging paper, such as wrinkles and water stains (Baty et al. 2010; Baglioni et al. 2014; Zervos et al. 2015).
Accordingly, it may be a better idea to dissolve or disperse cellulose derivatives in low-polar organic solvent for efficient and non-destructive protection of aging paper (Giorgi et al. 2010; Baglioni et al. 2014). Amornkitbamrung et al. (2015; 2018; 2020) repeatedly reported that the hydrophobic trimethylsilylcellulose could be dissolved in HMDO to disperse magnesium hydroxide (Mg(OH)2) nanoparticles for deacidification and enhancement of aging paper. The treated paper presented a good deacidification and enhancement effect. However, the low dispersion stability and the high hydrophobicity of treated paper may be not beneficial to the long-term preservation of aging paper (Jeong et al. 2014). In contrast, amphiphilic polymer may be a better choice for the stable dispersion of inorganic nanoparticles in low-polar organic solvents because it could stably disperse in low-polar organic solvents by self-assembly (Narouz et al. 2017; Yi et al. 2020).
In this work, a new kind of amphiphilic cellulose polymers were designed and prepared by for stabilizing nano MgO in the low polarity mixed solvent of HMDO and isopropanol (IPA), which would be applied for mass deacidification and enhancement of aging paper. The amphiphilic cellulose polymers (AKD-g-CMCC) were prepared by grafting 2,3-epoxypropyltrimethylammonium chloride (ETA) and alkyl ketene dimer (AKD) onto microcrystalline cellulose (MCC). The chemical structure was characterized in detail. Then, the transmittance, particle size distribution and viscosity were also used to prove the dispersion performance of the nano MgO organic colloid. The relationship between chemical structure of AKD-g-CMCC and dispersion properties was clearly explained. In this contribution, this highly stable nano MgO organic colloid would simultaneously impart ageing paper with the deacidification, enhancement and anti-aging performance.
2.1. Reagents and materials
MCC (α-cellulose, 25µm), sodium hydroxide (NaOH, AR), AKD (purity, 90%), ETA (purity, 95%), HMDO (AR), IPA (AR), dichloromethane (CH2Cl2, AR), and nano MgO (50nm) were purchased from Shanghai Macklin Biochemical Co., Ltd. Copper (II) ethylenediamine was obtained from China National Pulp and Paper Research Institute. All reagents were used as received without further purification. The aging paper samples got from an aged book published in 1980 (pH = 4.70).
2.2. Preparation of AKD-g-CMCC
Three kinds of AKD-g-CMCC polymers were prepared by ETA and AKD grafted onto MCC. The preparation of AKD-g-CMCC were shown in Fig. 1. The chemical structures and reaction process were shown in Fig. S1. CMCC was firstly synthesized by ETA grafting onto MCC. Briefly, 8.0 g of MCC was added to 100 mL of NaOH aqueous solution (14 wt.%) and kept stirring for 1 hour. Then the alkalized cellulose was centrifuged and washed with deionized water to neutrality and then dried under vacuum at 50 ℃ for 12 hours. Soon afterwards, the alkalized cellulose was added into 200 mL of mixed solution (IPA/NaOH aqueous solution (1.0 mol/L) = 8:2, v/v) and reacted with ETA (n(anhydroglucose unit (AGU)):n(ETA) = 1:3) at 75 ℃ for 4 hours. IPA can effectively inhibit the hydrolysis of CMCC during the reaction process (Odabas et al. 2016; Odabas et al. 2017; He et al. 2019). The synthetic product was filtrated and washed with IPA and deionized water, and then dried under vacuum at 50 ℃ for 12 hours to obtain CMCC (DS = 0.283). Subsequently, AKD-g-CMCC was prepared by AKD grafting onto CMCC. In detail, the molted AKD was evenly dissolved in HMDO by stirring at 75°C. Then the synthetic CMCC was added into HMDO based on n(AKD): n(AGU) = 1:1, 2:1 and 3:1. And the mixture was sheared at 2000 r/min for 10 minutes and reacted at 105°C for 20 minutes. Finally, the prepared product was washed and filtrated with CH2Cl2, and dried under vacuum at 60 ℃ for 12 hours to obtain the pure AKD-g-CMCC polymers. Based on the DS, the products were named as CMCC0.283, AKD0.009-g-CMCC0.277, AKD0.027-g-CMCC0.253, and AKD0.024-g-CMCC0.223, respectively.
2.3. Preparation of Nano MgO Organic Colloid
The schematic diagram of nano MgO organic colloid preparation was also shown in Fig. 1. Firstly, 0.1 ~ 0.7 wt.% of AKD-g-CMCC was added to 500 mL mixed solution of HMDO and IPA (5:5, v/v) and dissolved by 20 KHz ultrasound treatment at 80% power (SCIENTZ-IID, Xinzhi, China) for 5 min. And then 2.0 g/L of nano MgO was added and dispersed by ultrasound treatment at same conditions for 5 min again. The obtained nano MgO organic colloid was denoted as xAKDn-g-CMCCm/MgO /HMDO/IPA (5:5), where x represents the addition amount of AKD-g-CMCC, n means the DS of AKD, and m means the DS of ETA. For comparative analysis, the controls (xAKDn-g-CMCCm/HMDO, xAKDn-g-CMCCm/HMDO/IPA (5:5), 2g/L MgO/HMDO/IPA (5:5)) were also prepared under the same conditions.
2.4. Treatment of paper samples
The aging paper samples were firstly placed in vacuum and dried at 50 ℃ for 12 h to control moisture in the range of 1–2%. The pre-dried paper samples were immersed in the highly stable nano MgO organic colloid of AKD-g-CMCC/MgO/HMDO/IPA (5:5) for 5 minutes. The treatment method was denoted as ACMMHI. In comparison, the untreated paper samples were also carried out under the same conditions (named as UT). The treated paper samples were air dried in a fume hood for 30 minutes and then placed in vacuum at 50 ℃ to dry for 12 hours again. Then the treated paper samples were subjected to the damp heat aging box at T = 80 ℃, RH = 65% for 3 days. Finally, these treated paper samples were placed in the constant temperature and humidity room for the moisture balance and test.
2.5. Characterization and measurements
2.5.1. GPC
The molecular weight (Mw) of AKD was detected by tetrahydrofuran phase gel chromatography (GPC, Agilent1100, America). 1.0 mg of AKD was dissolved in 1 mL of tetrahydrofuran. After the solution was filtered, 0.5 ml of the clear solution was taken for detection by GPC.
2.5.2. FT-IR and solid-state 13C NMR
The samples were characterized by Fourier transform infrared spectrophotometer (FT-IR, VERTEX 33, Germany), solid-state 13C nuclear magnetic resonance spectrometer (13C NMR, AVANCE III HD 600, Germany, spinning speed = 5.000 ± 0.002 kHz). The DS of CMCC and AKD-g-CMCC were calculated according to the formula (1) and (2) respectively:
DS = ICH3/3IC1 (1)
DS = ICH2/47IC1 (2)
where ICH3 denotes the integral of resonance peak of CH3 on the quaternary ammonium group of CMCC at 57 ppm; Ic1 denotes the integral of resonance peak of C1 of cellulose at 105 ppm; 3 is the number of -CH3 on the quaternary ammonium group of CMCC; ICH2 denotes the integral of resonance peak of -CH2- on the alkyl chain of AKD at 20 ~ 30 ppm; 47 is the number of -CH2- unit on alkyl chain of AKD.
2.5.3. Intrinsic viscosities and relative molecular weight
The intrinsic viscosities ([η]) of samples was determined by the viscosity method according to ISO 5351/1 B. Then the relative molecular weight (Mr) was calculated according to the Mark-Houwaink formula: [η] = K*Mrα, where K is 0.13 and α is 0.905. Furthermore, the viscous degree of polymerization () of samples was calculated by the Mark-Houwink-Sakurada equation:= (0.75[η])1/0.905 (Shinoda et al. 2012).
2.5.4. UV-visible spectrophotometer, DLS and viscosity
The transmittance value of nano-MgO organic colloid was measured by a UV-visible spectrophotometer (UV1900, Japan) in the range 200–900 nm with resolution of 0.5 nm. The stability of the dispersion system was analyzed through the change of transmittance value at λ = 600 nm. After standing for same time, the smaller change of transmittance indicates better stability of the dispersion system. The average particle size was determined by dynamic light scattering (DLS, SZ-100Z, Japan). The viscosity of nano-MgO organic colloid was measured with a digital viscometer (NDJ-5S, China).
2.5.5. SEM-EDS and AFM
The microscopic morphology and elemental mapping of samples were observed by a scanning electron microscope (SEM, SU5000, Japan) working at an accelerating voltage of 5.0 kV and 15 kV, respectively. Element mapping was also performed with an energy spectrum model Super-X energy dispersive spectrometer (EDS) detector. Atomic force microscopy (AFM, Multimode 8, Germany) was also used to analyze the morphology of samples working on a Bruker instrument dimension ICON controller equipped with silicon cantilever probe with the tapping mode. The photo of paper samples before and after treatment were obtained by the HP scanner (Scan Jet Pro 2500, China).
2.5.6. pH value and alkaline reserve
The pH value of the paper samples was measured by cold extraction according to ISO 6588-1:2021. Then alkaline reserve of treated paper sample was assessed according to ISO/TS 18344:2016.
2.5.7. Tensile index and tear index
Tensile strength of the paper samples was measured according to ISO 1924-2:2008 with instrument (L&W CE062, Sweden). Tensile index was calculated according to tensile strength and basis weight of paper. Tear index is the ratio of tearing resistance to the basis weight of the paper sample. Tearing resistance was measured according to ISO 4046-5:2016 with L&W tear tester (L&W 009, Sweden). All reported values were calculated as averages over ten replicates for each sample (Ma et al. 2021).
2.5.8. Chromatic coordinate
Chromatic coordinate (ΔE) was calculated based on the following Eq. (3) and the (L, a, b) values of paper samples were measured by L&W whiteness tester (Elrepho 070, Sweden) according to ISO 12625-7:2014.
where ΔL is the lightness difference; Δa is the red/green difference; and Δb is the yellow/blue difference.
2.5.9. Static water contact angle
The static water contact angle (WCA) was measured according to ISO 15989:2004. The treated paper samples were tested by the surface WCA tester (OCA40 Micro, Germany).
3.1. Characterization of AKD-g-CMCC
To figure out the chemical structure of AKD-g-CMCC, the FT-IR patterns were firstly recorded. As shown in Fig. 2a, the spectra of MCC exhibited a series of characteristic absorption peaks of cellulose in the range from 4000 cm− 1 to 500 cm− 1, such as the stretching vibration of O-H at 3440 cm− 1, C-H stretching vibration at 2890 cm− 1, O-H bending of absorbed moisture at 1640 cm− 1, C-O-C asymmetric stretching vibration at 1160 cm− 1, the in-plane ring stretching at 1108 cm− 1, and C-O stretching vibration at 1030 cm− 1 (Schwanninger et al. 2004; Chung et al. 2004). Compared with MCC, the spectra of CMCC displayed a series of stronger C-O-C stretching vibration peaks at 1340 ~ 1020 cm− 1 in Fig. 2b, which may be attributed to the formation of new ether groups on cellulose (Sirviö et al. 2011; Chaker and Boufi 2015). Furthermore, the spectra of CMCC exhibited two new absorption peaks corresponding to the asymmetric CH3 at 1480 cm− 1 and the C-N stretching vibration at 1420 cm− 1, implying that part of hydroxyl groups on cellulose were substituted by the quaternary ammonium groups after quaternization (Odabas et al. 2017; He et al. 2019).
In order to further verify the successful preparation of AKD-g-CMCC polymers, the chemical structure of AKD was analyzed in detail. Firstly, the Mw of AKD was detected by GPC. As depicted in Fig. S2, five response peaks appeared in the molecular weight distribution of AKD and the Mw of the largest response peak is 771, which indicated that AKD wax is mixture and the Mw of its main component is 771. Moreover, AKD contains a four-membered lactone ring with two long-chain alkyl structure (C4H2O2-(CH2)n(CH3)2) (Fig. S1) (Yang et al. 2016; Tyagi et al. 2018). Basing on the chemical structure formula and Mw of AKD, therefore, the chemical formula of the main components of AKD could be confirmed as C4H2O2-(CH2)47(CH3)2. Besides, as shown in the FT-IR pattern of AKD in Fig. 2, sharp peaks at 1720 cm− 1 and 1850 cm− 1 are related to C = C double bond and C = O double bond in carboxyl group of lactone ring, respectively. The peaks at 1470 cm− 1 and 720 cm− 1 are related to the bending mode of CH2 bonds. Moreover, sharp peaks from 2848 cm− 1 to 2918 cm− 1 are because of C-H stretching vibrations (Esmaeili et al. 2020). Furthermore, as depicted in the FT-IR spectra of AKD-g-CMCC, the characteristic absorption peaks of β-ketone ester at 1710 cm− 1 and 1750 cm− 1 implied that lactone ring reacted with hydroxyl groups of cellulose. It could be also found that the spectra of AKD-g-CMCC exhibited the strong characteristic absorption peaks of C-H at 2918 cm− 1, 2848 cm− 1, 1470 cm− 1 and 720 cm− 1, which further indicated that AKD successfully grafted onto CMCC. In addition, it was not observed the characteristic peaks of lactone ring of AKD at 1720 cm− 1 and 1850 cm− 1, indicating that the unreacted AKD was removed from AKD-g-CMCC through CH2Cl2 washing. Therefore, the above analysis verified that AKD-g-CMCC was synthesized successfully (Song et al. 2012; Yan et al. 2016).
To further calculate the DS of CMCC and AKD-g-CMCC, the dry powders of AKD, MCC, CMCC, and AKD-g-CMCC were characterized by solid state 13C-NMR. The corresponding integral of various types of C and DS were listed in Table S1. The original analysis spectrums of solid state 13C-NMR were shown in Fig. S3(a-f). As depicted in Fig. 3a, the typical resonance peaks of cellulose were presented at 107.41 ppm for C1, 91.23 ppm for C4cryst, 85.13 ppm for C4amorph, 77.18 ppm for C3 and C2, 74.63 ppm for C5, 67 ~ 62 ppm for C6 (Liu et al., 2006; Harini, Ramya, & Sukumar, 2018). Compared with MCC, a new resonance signal of CMCC in Fig. 3b and AKD-g-CMCC in Fig. 3(c-e) appeared at 56.0 ppm corresponding to the methyl carbons (C10) of the ETA substituents, which demonstrated that ETA was grafted onto MCC successfully (Li et al. 2016; Kono 2017). According to formula 1, the DS of ETA grafted onto CMCC and three kinds of AKD-g-CMCC polymers were calculated as 0.283, 0.277, 0.253, and 0.223, respectively. Furthermore, as shown in Fig. 3f, the chemical shifts at 171.7 ppm, 144.8 ppm, 104.6 ppm, 54.3 ppm are assigned to four types of C (Ca, Cc, Cd, and Cb) on lactone ring and the chemical shifts at 15 ppm, 25 ~ 35 ppm are assigned to CH3 and CH2 of AKD, respectively (Yan et al. 2016). Compared with the AKD, no chemical shifts of lactone ring appeared on the 13C-NMR spectrum of AKD@CMCC in Fig. 3(c-e), further confirming that no unreacted AKD remained in AKD-g-CMCC. Moreover, the DS of AKD for three kinds of AKD-g-CMCC polymers were calculated by formula 2 as 0.009, 0.027, and 0.024, respectively.
3.2. Dispersion performance of nano MgO organic colloid
Based on above analysis, three kinds of AKD-g-CMCC polymers were used to disperse nano MgO in HMDO. As depicted in Fig. 4a, the nano MgO organic colloid of 0.5wt.% AKD0.024-g-CMCC0.223/MgO/HMDO/IPA (5:5) exhibited the smallest change of transmittance after standing for 12 and 24 hours, indicative of the most stable dispersion performance, which may be due to the greater steric hindrance of AKD0.024-g-CMCC0.223 in the mixed solution of HMDO and IPA. In fact, the steric hindrance of polymer is mainly related to its molecular weight (Jeon et al. 2019). According to the Mark-Houwaink formula: [η] = K*Mrα, the Mr of AKD-g-CMCC polymers was calculated and listed in Table S2. Obviously, compared with AKD0.009-g-CMCC0.277 (Mr = 1582.06) and AKD0.027-g-CMCC0.253 (Mr = 1943.61), AKD0.024-g-CMCC0.223 had the highest Mr (2021.34). This result was consistent with the dispersion stability of the three kinds of nano MgO organic colloids. Moreover, the steric hindrance of AKD-g-CMCC polymer is also related to the ratio of DS of grafted hydrophobic group and hydrophilic group (Ran et al. 2009; Shukla and Shahi 2018). As presented in Table S2, the ratio of DS of AKD to CMCC in AKD0.024-g-CMCC0.223 was also the highest, which mean that AKD0.024-g-CMCC0.223 with the high ratio of grafted hydrophobic group is beneficial to stabilize nano MgO in the low-polarity mixed solution of HMDO and IPA.
Moreover, the concentration of the AKD0.024-g-CMCC0.223 also significantly affected the dispersion stability of nano MgO organic colloid. As shown in Fig. 4b, with the concentration of the AKD0.024-g-CMCC0.223 increased, the dispersion stability of nano MgO organic colloid firstly increased and then decreased. When the concentration was 0.5 wt.%, this nano MgO organic colloid presented the smallest change of transmittance after standing for 24 hours, achieving the most stable dispersion, while the higher or lower concentration of AKD0.024-g-CMCC0.223 obviously decreased the dispersion stability, which may be due to the unsuitable steric hindrance. As reported by references (Yi et al. 2020; Wang et al. 2020), high concentrations of surfactants in colloids could form the larger steric hindrance, preventing inorganic nanoparticles from entering the micelles. Instead, the smaller steric hindrance caused by low-concentration of surfactants could also not form the stable micelles. Therefore, in this work, the optimized concentration of the AKD0.024-g-CMCC0.223 in the nano MgO organic colloid is 0.5 wt.%.
Furthermore, IPA also played a crucial role in improving the dispersion stability of nano MgO organic colloid. As shown in Fig. 4c, IPA could improve the dispersion stability of AKD0.024-g-CMCC0.223 and nano MgO in HMDO and further promote the dispersion stability of nano MgO organic colloid, which was owning to the aggregation inhibition of AKD0.024-g-CMCC0.223 and nano MgO through interface regulation of IPA. The dispersion mechanism of IPA was proposed and clarified by experimental and computational studies, which will be reported in detail in another work. Briefly, IPA has one hydrophilic hydroxyl group and two hydrophobic methyl groups. IPA was easy to penetrate into AKD0.024-g-CMCC0.223 polymer and tended to form hydrogen bonds with its hydroxyl groups and quaternary ammonium groups for reducing its surface energy, which could prevent self-aggregation of AKD0.024-g-CMCC0.223 polymer in low-polarity HMDO. Additionally, the hydroxyl group of IPA could easily adsorb on the surface of MgO to prevent aggregation of nano MgO (Reynaert et al. 2006). More importantly, comparing with 2 g/L MgO/HMDO/IPA (5:5), adding 0.5 wt.% AKD0.024-g-CMCC0.223 further improved the dispersion stability of nano MgO organic colloid. Thus, the results indicated that IPA and AKD0.024-g-CMCC0.223 had a synergistic dispersion effect on nano MgO in HMDO.
In addition, the nano MgO organic colloid of 0.5wt.% AKD0.024-g-CMCC0.223/MgO/HMDO/IPA (5:5) also had smaller average particle size (200 nm) and more concentrated distribution as shown in Fig. 4d. This reason was attributed to the synergistic dispersion of AKD0.024-g-CMCC0.223 and IPA. In the absence of IPA, Fig. 4d presented the biggest and narrow particle size distribution (980 nm) of the dispersion system of 0.5wt.% AKD0.024-g-CMCC0.223/HMDO. As a comparison, adding IPA could markedly decrease the average particle size of AKD0.024-g-CMCC0.223 in HMDO (300 nm). Additionally, compared with the particle size distribution of 2 g/L MgO/HMDO/IPA (5:5), adding AKD0.024-g-CMCC0.223 could obviously improve the uniformity of particle size distribution of nano MgO organic colloid.
The above result could be supported by the SEM and AFM images. As shown in Fig. 5a, the SEM image of 0.5 wt.% AKD0.024-g-CMCC0.223/HMDO/IPA (5:5) presented that the micro-nano-scale flakes were deposited on the conductive adhesives. From the Fig. 5b, the SEM image of 0.5 wt.% AKD0.024-g-CMCC0.223/MgO/HMDO/IPA (5:5) showed that lots of nanoparticles deposited on the surface of micro-nano-scale flakes. The results further revealed the dispersion mechanism that the micro-nano-scale flakes of AKD0.024-g-CMCC0.223 improved the dispersion stability by loading with nano MgO prevented the agglomeration in the mixed solution of HMDO and IPA. Meanwhile, the AFM image (Fig. 5c) of 0.5 wt.% AKD0.024-g-CMCC0.223/MgO/HMDO/IPA (5:5) clearly presented the average particle size (about 200 nm) of the stable nano MgO organic colloid, which was consistent with the DLS’s data in Fig. 4d. In addition, Fig. S4 also exhibited the low viscosity (2.26 mPa·s) of nano MgO organic colloid, which indicated that the stable nano MgO organic colloid is beneficial for the batch processing of aging paper.
3.3. Performance evaluation of treated aging paper
To evaluate the performance of the treated aging paper with the stable nano MgO organic colloid, the treated papers samples were subjected to artificial accelerated aging for 3 days. The evaluation results in Table 1 indicated that the treated aging paper samples with ACMMHI presented the excellent deacidification, enhancement and anti-aging performance. First of all, the initial pH value of the treated aging paper samples with ACMMHI increased up to 8.30 from 4.70 and its alkaline reserve was 1.00% (CaCO3 equivalent). After artificial accelerated aging for 3 days, the treated aging paper samples still presented the suitable pH value (7.38) and sufficient alkali reserves (0.90%, CaCO3 equivalent), while the untreated paper samples further acidified (pH = 4.12). Moreover, comparing with the untreated aging paper samples, the initial tensile index and tear index of the treated aging paper samples with ACMMHI increased by 40.78% and 27.28%, respectively, which may be due to the enhancement of nano MgO and the cross-linking of AKD0.024-g-CMCC0.223 with paper fibers through electrostatic interaction. The great mechanical properties of nano MgO are beneficial to improving the mechanical behavior of treated paper sample (Kumar and Reddy 2020; Lisuzzo et al. 2021).
|
Paper samples |
pH Value |
Alkaline reserve (%, CaCO3 eq.) |
∆E |
Tensile index (Nm/g) |
Tear index (mN·m2/g) |
DP |
|---|---|---|---|---|---|---|
|
UT |
4.70 ± 0.06 |
- |
- |
20.92 ± 2.40 |
2.22 ± 0.12 |
324.49 ± 2.06 |
|
UT-Aging 3d |
4.12 ± 0.08 |
- |
4.62 |
16.84 ± 2.20 (-19.50%) |
1.54 ± 0.10 (-30.64%) |
205.93 ± 1.63 |
|
ACMMHI |
8.30 ± 0.05 |
1.00 |
0.46 |
29.45 ± 3.40 (40.78%) |
2.83 ± 0.25 (27.28%) |
312.05 ± 1.85 |
|
ACMMHI- Aging 3d |
7.38 ± 0.06 |
0.90 |
0.85 |
26.56 ± 2.00 (26.96%) |
2.33 ± 0.17 (4.90%) |
262.92 ± 2.24 |
| Note: △E and the change in mechanical strength were calculated based on the untreated sample. | ||||||
It can be further verified by the SEM-EDS in Fig. 6. The original mapping images were presented in Fig. S5. Compared with the untreated paper (Fig. 6a, 6c&6e), the SEM image of the treated paper with ACMMHI (Fig. 6b) clearly revealed the size of nano MgO (about 200 nm) evenly deposited on its surface. Furthermore, the mapping images (Fig. 6d) clearly presented more magnesium (Mg) element on the treated paper samples with ACMMHI. More element distribution of Nitrogen (N, 2.36%) and Mg (1.74%) of treated paper samples with ACMMHI further supported the above conclusion. Thence, AKD0.024-g-CMCC0.223 containing positively charged quaternary ammonium groups (DS = 0.223) could form the electrostatic interaction with negatively charged hydroxyl groups of paper fiber to strengthen paper (Ren et al. 2018; He et al. 2019; Liu et al. 2020). After artificial accelerated aging for 3 days, the treated aging paper still remained higher tensile index (26.56 Nm/g), tear index (2.33 mN·m2/g) and DP value (262.92) than the untreated aging paper (16.84 Nm/g, 1.54 mN·m2/g, 205.93, respectively), which was attributed to the sufficient alkali reserves, inhibiting further acidification and degradation of aging paper.
Meanwhile, the treated aging paper samples with ACMMHI also presented the low ∆E (0.46). Although after accelerated aging for 3 days, the ∆E was still smaller (0.85) compared with the control (4.62), which is beneficial to reducing the yellowing of paper during the aging process. Furthermore, the photographs in Fig. S5 of the aging paper before and after treatment presented that the treatment of ACMMHI had no significantly influence in the appearance and clarity of writing. Additionally, as shown in Fig. S6, the static WCA of treated paper samples with ACMMHI reached 83o comparing with the untreated aging book paper (the static WCA = 33o), which may be due to the interaction between the long-chain alkanes of AKD0.024-g-CMCC0.223 and nano MgO deposited on the surface of treated paper samples. The suitable water resistance could prevent the treated paper pages from sticking during batch deacidification and reinforcement.
In summary, an amphiphilic cellulose-based copolymer (AKD0.024-g-CMCC0.223) was used to disperse nano MgO in low-polar HMDO with the help of IPA. After optimization, 0.50 wt.% of AKD0.024-g-CMCC0.223 endowed the nano MgO organic colloid with an excellent dispersion stability, small average particle size (200 nm) and low viscosity (2.26 mPa·s), which is beneficial for batch deacidification and reinforcement of aging paper. After evaluation, the treated aging paper with ACMMHI presented the excellent deacidification, enhancement and anti-aging performance. Improvement of paper mechanical performance was mainly owing to the cross-linking between AKD0.024-g-CMCC0.223 and paper fibers through electrostatic interaction. Meanwhile, the great mechanical properties of nano MgO deposited in the networks of paper fibers was also important factor to improve the mechanical performance of treated paper. Moreover, the sufficient alkali reserve promoted the anti-aging performance by inhabiting further acidification and degradation of aging paper. Additionally, the mild pH value was also beneficial to reducing the yellowing of aging paper. Therefore, this novel and efficient protection strategy based on AKD0.024-g-CMCC0.223 stabilizing nano MgO in the low-polar mixed solvent of HMDO and IPA will have great application prospects for the protection of aging paper.
Acknowledgments
This work was financially supported by National Natural Science Foundation of China (22178128), and Science and Technology Plan Special Project of Guangzhou, China (No. GZDD201808).
The authors declare that they have no conflict of interest.
Supporting Information
The chemical structures and reaction process of CMCC and AKD-g-CMCC (Fig. S1), response peaks and the molecular weight distribution of AKD (Fig. S2), original solid state 13C-NMR spectrum of (a) MCC, (b) CMCC, (c~e) three kinds of AKD-g-CMCC, and (f) AKD (Fig. S3), viscosity of nano-MgO organic colloid containing 0.1~0.7 wt.% AKD0.024-g-CMCC0.223 (Fig. S4), original mapping images of untreated aging paper sample and treated aging paper sample with ACMMHI (Fig. S5), photographs and static WCA of the naturally aged-book papers before and after treatment with ACMMHI (Fig. S6), the corresponding integral of various types of C on the solid-state 13C NMR spectra of MCC, CMCC, and three kinds of AKD-g-CMCC polymers (Table S1), and the [η], Mr and ratio of grafted groups of AKD0.009-g-CMCC0.277, AKD0.027-g-CMCC0.253, and AKD0.024-g-CMCC0.223 (Table S2) were mentioned in this manuscript.
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