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Nanotechnologies for Conservation of Cultural Heritage: Paper and Canvas Deacidification Rodorico Giorgi,† Luigi Dei,† Massimo Ceccato,‡ Claudius Schettino,§ and Piero Baglioni*,† Department of Chemistry and CSGI, University of Florence, via della Lastruccia 3, Sesto Fiorentino, I-50019 Florence, Italy, Rifinizione Santo Stefano, Textile Finishing Company, Via Arezzo, 35 I-59100 Prato, Italy, and Book and Paper Conservator, via san Cristofano, 22/R I-50122 Florence, Italy Received May 18, 2002. In Final Form: July 22, 2002 The aging of cellulose, the main constituent of paper, is due to depolymerization of cellulose fibers and is promoted by acid pH. Paper deacidification is a fundamental process for the conservation and restoration of probably the most important material used to transmit cultural heritage. Several methods are currently used for paper deacidification. We applied nanotechnologies to produce kinetically stable dispersions of nano- and micron-sized calcium hydroxide particles in alcohol media. Although calcium hydroxide possesses the best deacidification properties, it has never been used so far as dispersion in nonaqueous media. Calcium hydroxide particles, once deposited onto paper cellulose fibers, deacidify them and react with carbon dioxide from the air, forming a calcium carbonate reservoir on the paper fibers. This process allows a long-term control of paper pH, with excellent deacidification properties. Deacidification of 14th, 17th, 19th, and 20th century acid yellowed paper samples coming from rag and wood pulp (20th century) has been performed with excellent results. Moreover, the nano- and microparticle dispersions can be applied to paper or canvases using conventional procedures. This new method is environmentally clean, is inexpensive, and can also be used for industrial applications.
Introduction Paper undergoes several fundamental deterioration processes. Under normal conditions of storage, degradation processes are very slow but eventually produce yellowing and the loss of the paper mechanical strength. Hydrolytic degradation of cellulose molecules is the most common degradation reaction that is enhanced by the presence of water (moisture). According to standard reaction-kinetic models, the rate of the hydrolytic process is dependent on the temperature, the acidity, and the amount of moisture present in the paper, which is dependent on the relative humidity of the environment.1 Additional degradation processes are related to the oxidative degradation of cellulose, primarily induced by the presence of oxygen in the atmosphere and the thermal degradation. After the invention of printing, paper demand consistently increased and the methods of papermaking radically changed. In the middle of the 19th century,2 papermaking from rags was replaced by the use of wood pulp, producing more chemically reactive paper sheets, subjected to hydrolythic, oxidative, and thermal degradation.3 Many studies investigated the cellulose depolymerization route involved in the degradation of paper. It has been shown4 that acid-catalyzed hydrolysis was the main route for cellulose depolymerization.5 The acidic-catalyzed degradation of paper is initiated by acidic species intro* Corresponding author. Voice: +39 055-457-3033. Fax: +39 055-457-3032. E-mail:
[email protected]; http:// www.csgi.unifi.it. † University of Florence. ‡ Textile Finishing Company. § Book and Paper Conservator. (1) Barrow, W. J. Permanence/Durability of the BooksVII. Physical and Chemical Properties of the Book Papers, 1507-1949; W. J. Barrow Research Laboratory, Inc.: Richmond, VA, 1974. (2) Hunter, D. Papermaking; Dover Ed.: 1978. (3) Browning, B. L. Analysis of paper; Marcel-Dekker: New York, 1977. (4) Whitmore, P. M.; Bogaard, J. Restaurator 1994, 15, 26.
duced into the cellulose fibers during papermaking. Nowadays, earlier sizing procedures performed with starch or animal glue are replaced by those using natural alum and, above all, aluminum sulfate, which are wellknown Lewis acids. As a consequence, lignin, hemicellulose, and hydrolyzed cellulose, which are the main components of paper, oxidize and produce substantial amounts of acidic degradation byproducts that break down the fibers. These acids further catalyze paper degradation, and therefore this process is termed as “autocatalytic”. The overall effect of degradation processes is the shortening of the average chain length of the cellulosic component, which leads to a catastrophic loss of paper strength. It has been estimated that the breaking of 0.5-1% of the bonds in the cellulosic fibrils virtually leads to the loss of fiber strength.6 These processes can be stopped or consistently slowed by a deacidification treatment. A correct deacidification process should produce the complete neutralization of the acidic paper and thermodynamically stable side products, which act as an alkaline reservoir, keeping the pH around 8-9. Many different techniques and products have been studied or developed in order to eliminate acidity from paper and published documents.7 Aqueous solutions of calcium, magnesium, and barium hydroxide have been widely used for many decades, but unfortunately, they had often induced undesirable side effects, mainly due to paper (which is hydrophilic) exposure to strongly alkaline conditions with subsequent cellulose depolymerization.8 To overcome cellulose hydrolysis in aqueous alkaline solutions, nonaqueous deacidification processes have been proposed. (5) Ryder, N. The Conservator 1986, 10, 31. (6) Orr, R. S.; Weiss, L. C.; Humphreys, G. C.; Mares, T.; Grant, J. N. Text. Res. J. 1954, 24, 399. (7) Zappala`, A. Introduzione agli interventi di restauro conservativo di beni culturali cartacei; Udine: 1990; pp 93-131. (8) Calvini, P.; Grosso, V.; Hey, M.; Rossi, L.; Santucci, L. Pap. Conservator 1988, 12, 39.
10.1021/la025964d CCC: $22.00 © 2002 American Chemical Society Published on Web 09/13/2002
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Methods currently used by archivists, conservators, and librarians for mass deacidification processes are the Wei T’o and the Bookkeeper methods. The Wei T’o method9 is a nonaqueous mass-deacifdification process which use liquefied gases in pressure chambers or liquid spraying in fume hoods. It is based on magnesium methoxymethyl carbonate solutions in ethyl or methyl alcohols and CFCs (chlorofluorocarbons). Unfortunately, sometimes paper books interact with the liquid mixture, leading to undesired ink solubilization. This obliges restorers to perform an accurate control of paper sheets in order to avoid any possible damage. Furthermore, the main disadvantage of this method is constituted by the use of CFC. The Bookkeeper method10 is a dispersion-based liquid process that deacidifies paper by deposition of submicronsized particles of magnesium oxide onto and within the paper texture. The oxide is converted to an alkaline agent, magnesium hydroxide, for reaction with water. Temperature is maintained below 50 °C throughout the process. This greatly reduces any undesirable side reactions or damage to materials. Paper sizes are in the range of submicron dimensions, and usually, the MgO particles are small enough to thoroughly impregnate and penetrate into the core of the paper. These submicron particles have a very high surface area, typically 170-180 m2/g, and are highly surface active. Electrostatic forces hold these particles tightly to the paper. Moreover, these particles are small enough that they do not affect paper features. Calcium hydroxide is an excellent deacidifying agent. It ensures a good physicochemical compatibility with the support, and after its transformation into calcium carbonate, it works very well as an alkaline reservoir11,12 and does not originate any undesirable side products. Handmade papers produced in Europe between 1400 and 1800 are sometimes found to be in a very good state of preservation. The paper from this period is generally found to be composed of linen and/or hempen rags. Calcium is found in many of these papers and probably was introduced into the paper through the hard water used during pulp preparation. The high concentrations of calcium (above 1 wt %) found in some specimens indicate that chalk, ground sea shells, or some other form of calcium carbonate may have been purposely added during the papermaking process as a whitening agent. Papers containing calcium have better permanence.13 Gelatin external sizing was also common during this period and was applied to the paper to make it more resistant to the penetration of inks and to improve its strength. Barret and Mosier14 have suggested gelatin content, as well as calcium content and pH, may play a role in paper permanence when permanence is judged by the relative lightness or darkness of the paper’s color. Unfortunately, paper deacidification by aqueous solutions of calcium hydroxide is not really useful, since these solutions are, from a chemical point of view, too aggressive. This article reports a different approach for paper deacidification and conservation, mainly based on calcium hydroxide nanoparticle dispersions in nonaqueous sol(9) Smith, R. D. Mass De-acidification at the Public Archives of Canada in Conservation of Library and Archive Materials and the Graphic Arts; Butterworth Ed.: London, 1987. (10) Preservation Technologies, L.P., Bookkeper Process, U.S., http:// www.ptlp.com (accessed 9 April 2002). (11) Lienardy, A.; Van Damme, P. Restaurator 1990, 11 (1), 21. (12) Bogaard, J.; Whitmore, P. M. J. Am. Inst. Conserv. 2001, 40, 105. (13) Plossi-Zappala`, M. Boll. Ist. Cent. Patol. Libro 1981, 37, 29-40 (and references therein). (14) Barret, T.; Mosier, C. J. Am. Inst. Conserv. 1995, 4 (3), 173-186.
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vents.15,16 A formulation based on nonaqueous nanoparticle dispersions of calcium hydroxide has never been proposed so far. We have applied these dispersions with success to historically important manuscripts dating from the 14th to 20th centuries. Nanomaterials are characterized by high surface area, which influences many physicochemical properties of the system and confers excellent features for several applications. Nanotechnologies can be considered among the most “hot” topics in the field of material science and applied chemistry. Recently, we applied nano- and microtechnology to cultural heritage conservation and in particular to solve the problem of chemical degradation and consolidation of wall paintings.15 We report here the synthesis of nano- and microparticles of calcium hydroxide stabilized in aliphatic alcohols, mainly propan-2-ol. These dispersions can be safely used for paper and canvas deacidification17 using very simple application procedures and without any particular precaution. Materials and Methods Sodium hydroxide, NaOH, potassium hydroxide, KOH, and calcium chloride dihydrate, CaCl2‚2H2O, were supplied by Merck, Darmstadt, Germany. Lime (calcium oxide, CaO) was supplied by Ceprovip, Medolago (BG), Italy. Propan-2-ol and propan-1-ol (purity > 99.5%) were supplied by Merck, Darmstadt, Germany, and were used without further purification. The Milli-RO6 and Milli-Q-Water systems (Organex) were by Millipore SA, France. 7 cm × 4 cm paper specimens have been selected for testing our new deacidification method. They have been previously outgassed under a moderate vacuum, at 50 °C for 16 h, and then stored at room temperature (25 °C) in a drybox and at controlled humidity (35%). Water loss was 0.5% w/w. This procedure was followed in order to control the environmental parameters and get reliable measurements. Nano- or microparticle dispersions have been applied by brushing or spraying on paper specimens. Treated samples were stored at 30 °C under moderate vacuum and in the presence of KOH platelets, to prevent carbonation. The conventional notations for all the examined samples are as follows: 14th, for paper dating from the 14th century (coming from rag with a little sizing of starch); 17th, for paper dating from the 17th century (coming from rag with gelatin as sizing); 19th, for paper dating from the 19th century (coming from rag with gelatin as sizing), and 20th, for paper dating from the 20th century (coming from wood pulp with rosin sizing, without alum). The morphological analysis of the untreated and the treated paper sheet surfaces has been performed with a scanning electron microscope (SEM) (Philips XL20). All the samples examined have been prepared according to standard procedures. Thin stratigraphic section analyses on the plain paper have been performed in a direction perpendicular to the paper surface, using an energydispersive X-ray (EDX) spectrometer for microanalysis. The calcium signal, working as a probe for calcium hydroxide and/or carbonate presence, was mapped. The comparison of calcium signals from the treated and untreated samples allows detection of the amount of calcium hydroxide within the fibers and its distribution inside the paper. pH measurements have been performed according to the TAPPI recommendation,18 using the cold-water extraction method at room temperature and a glass electrode pH meter (CrisonBasic20). The surface area analyzer Coulter SA 3100 was used for the determination of the specific surface area of nanopaticles. Each measurement was performed on dry powder. All samples, before (15) Ambrosi, M.; Dei, L.; Giorgi, L.; Neto, C.; Baglioni, P. Langmuir 2001, 17, 4251. NaturesScience Update, Nanotechnology restores flaking frescos, by Philip Ball, 11 July 2001. (16) Baglioni, P.; Dei, L.; Giorgi, R. Stud. Conserv. 2000, 45, 154. (17) Baglioni, P.; Dei, L.; Giorgi, R.; Schettino, C. V. International Patent pending, January 15, 2002. (18) TAPPI 1987b, Hydrogen ion concentration (pH) of paper extracts (cold extraction method). T509 om-83. TAPPI test methods 1988, vol. I, Atlanta, TAPPI Press.
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the experimental investigations, were previously degassed, keeping them at 343 K for 16 h. The ζ potential measurements on the Ca(OH)2 dispersions have been performed with a Laser Zee 500 Pen Kem. FT-IR spectra on paper samples have been obtained with a Nexus Fourier transform infrared spectrometer from Nicolet with OMNIC software and a Nicolet Smart Dura SamplIR single reflection horizontal ATR accessory. Calcium Hydroxide Preparation. Homogeneous Phase Synthesis. Calcium hydroxide nanoparticles, with an average particle size diameter around 260 nm have been obtained by mixing NaOH solution with CaCl2 solution. Both solutions have been heated to a selected temperature (around 90 °C) under continuous stirring.19 The supersaturation degree was kept in the 2-10 range. The aqueous Ca(OH)2 suspension was gradually cooled to room temperature and kept under a nitrogen atmosphere to avoid Ca(OH)2 carbonation. The supernatant solution was discarded, and the remaining suspension was washed five times with water to reduce the NaCl concentration below 10-6 M. Each time, the dilution ratio between the concentrated suspension and the washing solution was about 1:10. The complete removal of NaCl from the suspension was controlled by the AgNO3 test. The suspension was then concentrated in moderate vacuum at 40 °C up to a weight ratio Ca(OH)2/water of 0.8, which is the same as that of of the standard slaked lime paste. Dispersions have been prepared by mixing 10 g of the calcium hydroxide paste with 1 L of alcohol, under vigorous stirring or gentle sonication for 15 min. Slaked Lime Method. Calcium hydroxide microparticles have been obtained by slaking CaO in water, according to procedures reported in the Renaissance art treatises afterward adapted by Von Weimarn.20 Two or three times the stoichiometric water amount has been used for slaking CaO. With this procedure a paste of 55% calcium hydroxide and 45% w/w water has been obtained. Dispersions have been prepared by mixing the calcium hydroxide paste and alcohol, under vigorous stirring, and, successively, have been sonicated for 30 min with a Branson model B-12 ultrasonicator for further particle size reduction and to obtain a complete dispersion of the calcium hydroxide agglomerates. Both methods produced a calcium hydroxide/water paste. The water content in the alcohol dispersions was quantified to be 0.5% w/w.
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Figure 1. SEM picture of calcium hydroxide microparticles obtained by the slaking lime method: bar ) 20 µm.
Results and Discussion Two different experimental approaches have been considered for the synthesis of micron- and submicronsized Ca(OH)2 particles: (i) homogeneous phase reaction and (ii) heterogeneous phase reaction. The first synthetic route consists of mixing two salt solutions in supersaturating conditions and under temperature control, while the second procedure is a revisited synthetic pathway of the procedures reported in the ancient Art-treatises,20,21 that is, by slaking lime in water. We were able to improve the dispersion features by controlling the kinetic stability of the dispersions, that is, by reduction of the particle size (and weight) and by selecting dispersing media different from water to minimize the aggressive effects of the alkaline Ca(OH)2 aqueous solution on cellulose. Using the slaking lime synthetic pathway, homogeneous size distributions of the particles mainly in the range 1-10 µm have been obtained (see Figure 1). The enhancement of the deacidification process can be obtained with dispersions formed from particles smaller than 1 µm. In fact, a particle size smaller than 1 µm provides a better penetration and adhesion on the paper (19) Arai, Y. Chemistry of Powder Production; Scarlett, B., Ed.; Powder Technology Series; Chapman & Hall: London, 1996; Chapter 4. (20) Von Weimarn, P. P. Kolloid-Z. 1908, 3, 282. (21) Cennini, C. Il libro dell′arte (original manuscript of 1437); Milanesi, G. C., Le Monnier, Eds.; Florence, 1859. New edition by Brunello, F.; Magagnato, L.; Neri Pozza, Ed.; Vicenza, Italy, 1971.
Figure 2. SEM pictures of nanoparticles obtained by homogeneous phase synthesis: (A) bar ) 200 nm; (B) bar ) 200 nm.
substrate, minimizing the formation of white glazing layers. Since most of the particles obtained from the slaked lime method are, as shown in Figure 1, >1 µm, this synthetic pathway has scarce applicability for paper and canvas deacidification. Submicron particles and nanoparticles of calcium hydroxide can be produced from supersaturated solutions. Figure 2A,B show hexagonal platelet particles of about
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Figure 3. SEM micrographs of a 19th century sample treated with calcium hydroxide obtained by homogeneous phase reaction. Some calcium hydroxide particles bound to the cellulose fibers are evidenced: (A) bar ) 5 µm; (B) bar ) 50 µm. Table 1. ζ Potential Measurements Performed on Calcium Hydroxide Dispersions in Different Media at 25 °C solvent
ζ potential (mV)
water ethanol propan-1-ol propan-2-ol butan-1-ol butan-2-ol
+34.0 +16.7 +7.8 +4.6 +5.3 +3.1
200 nm mean size, as obtained from scanning electron microscopy (SEM). The distribution obtained from the analysis of SEM images shows that 80% of the particles are in the range 160-380 nm with an average size of 260 nm. An excellent kinetic stability of the dispersions has been obtained using short chain alcohols.16 Short chain aliphatic alcohols (in particular propan-2-ol and propan-1-ol) have very low surface tension and are ideal for the wetting of the hydrophilic substrate of paper or canvas. Particle dispersions are stabilized by a weak positive surface charge distribution, as deduced from the electrokinetic potential measurements reported in Table 1. The ζ potential of calcium hydroxide particles sharply decreases in passing from water to ethanol and then continues to decrease as the aliphatic chain length of the alcohol increases. This decrease is associated with the very low solubility of Ca(OH)2 in the aliphatic alcohols, which does not allow high charge adsorption at the solid/ liquid interface. In low dielectric constant media, the Debye lengths are very small compared to those of aqueous solutions (the dielectric constant of propan-2-ol is 18.30 and that of water is 78.54, at 20 °C22), so that the charge screening is very reduced.23,24 This means that even low surface charges, and consequently low electrokinetic potentials, are sufficient to stabilize nanoparticle dispersions. Moreover, the stability also depends on the adsorbed molecules at the solid/liquid interface.25 Despite the lower ζ potential, nonaqueous Ca(OH)2 dispersions are much more stable than dispersions in water. The advantage in using propan-2-ol is also related to its small dielectric constant that makes the dispersion almost ineffective in solubilizing ionic substances. Therefore, nonaqueous lime dispersions can be used also for (22) Handbook of Chemistry and Physics, 74th ed.; CRC Press Inc.: Boca Raton, FL, 1993-1994. (23) Parfitt, G. D.; Peacock, J. Surface and Colloid Science; John Wiley: New York, 1978; Vol. 10, p 180. (24) Romo, L. A. J. Phys. Chem. 1963, 67, 386. (25) Vincent, B.; Lyklema, J. J. Colloid Interface Sci. 1971, 31, 171.
Figure 4. Atomic composition distribution (%) for a rag paper sample dating from the 19th century, performed by EDX spectroscopy analysis, before (left) and after (right) the application of the nanoparticles dispersion.
Figure 5. ATR FT-IR spectra of 19th sample, collected as a function of time from the application of the deacidifying treatment. Arrows indicate the direction of the time evolution.
treatment of inks in many cases where water solutions or dispersions in aqueous solvents cannot be used. Moreover, 1-propanol is quite volatile, since the vapor pressure at 25 °C is similar to that of water. The optimal rheological properties of dispersions allow the use of different methods for the application of the dispersion itself. For example, viscosity is low enough to permit the spraying application. The kinetic stability is long enough to maintain all the particles well dispersed. These dispersions can be easily used for application by brushing or complete immersion of paper sheets in the calcium hydroxide nanoparticles dispersion. All these methods can be safely used. Ca(OH)2 nanoparticles synthesized in this work have a specific area of 28 m2/g or larger, typical for a good surface active powder, and can easily penetrate through the thickness of the sample, as demonstrated in previous
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Figure 6. (a) Scanning electron micrograph of a cellulose fiber with deposited calcium hydroxide nanoparticles (indicated by the arrows). Parts b and c are 3 × 3 cm2 paper specimens from the 19th century. Both were artificially aged. Sample b has been deacidified by using nanoparticles, while sample c was not treated. The intensity of the brown color is proportional to the amount of paper degradation.
investigations26 performed by mapping calcium and using EDX spectrometry. Moreover, the calcium mapping showed a homogeneous distribution of calcium inside the paper texture. Electrostatic forces that work within this size range hold these particles tightly bound to the paper. The adhesion of particles is promoted by the positive surface charge of the nanoparticles that interact with hydroxyl groups from cellulose. Finally, these particles are small enough that they do not affect the appearance of the paper. Figure 3 shows a paper specimen from the 19th century treated with a suspension of micro- and nanoparticles in propan-2-ol. Ca(OH)2 particles adhere to the paper fibers with a homogeneous distribution. This is a fundamental requisite, since clustering of particles onto the cellulose fibers would produce a white glaze or white spots, unacceptable from an aesthetical point of view. Moreover, the figure clearly shows that propan-2-ol does not induce morphological alterations of cellulose fibers. As above stressed, the positive surface charge of particles is responsible for the strong interaction between Ca(OH)2 and the hydroxy groups of cellulose. Figure 3 evidences some Ca(OH)2 particles deposited onto the cellulose fibers. Figure 4 shows a EDX microanalysis performed on the 19th century sample, before and after the deacidification treatment. The elemental percentage distributions of silicon, aluminum, sulfur, and potassium remained unchanged, while a remarkable increase of calcium amount has been observed for all the examined samples. Calcium comes from hydroxide and/or carbonate, which are both detectable by EDX microanalysis. The treatment of the paper with Ca(OH)2 dispersions consistently increases, for all the samples examined, the pH of paper up to a “safe” value of about 9. One of the aspects that makes this new deacidification method very appealing is the carbonation of Ca(OH)2 particles, which is fast enough to avoid damaging of cellulose fibers from a long contact with the very basic Ca(OH)2. Reaction with CO2 produces an alkaline reservoir of CaCO3, that maintains constant the pH and allows a long-term protection of paper. A deeper analysis of the deacidification process has been performed by Fourier transform infrared spectrometry (FT-IR) with a probe for attenuated total reflectivity (ATR) analysis. The examined samples, after the deacidification treatment, have been stored at normal environmental conditions, that is, 45% RH and 20 °C. (26) Giorgi, R.; Dei, L.; Schettino, C.; Baglioni, P. A new method for paper de-acidification based on calcium hydroxide dispersed in nonaqueous media; Proceedings of the IIC Baltimore Congress 2002, Works of Art on Paper, Books, Documents and Photographs: Techniques and Conservation, 2002 (in press).
Although FT-IR spectra of paper are quite complex, some absorption frequencies are illustrative of the deacidification process and of the formation of calcium carbonate nanoparticles. Unfortunately, the main signal coming from the calcium carbonate, corresponding to the stretching at 1440 cm-1, is covered by the O-H broad bending signal from the cellulose. However, the 875 cm-1 adsorption could be safely used for calcium carbonate detection. The spectra were collected immediately after the application of the calcium hydroxide dispersion and, successively, after 1 h and once a day for two weeks. The first absorption frequency is around 3642 cm-1 (corresponding to the O-H stretching of the calcium hydroxide) and close to the broad signal of cellulose hydroxyl groups; the second is around 870 cm-1 and is attributed to the out-of-plane bending of CO32- in CaCO3. The intense and broad band from the O-H of cellulose lies at lower wavenumbers (3350 cm-1) and did not overlap with characteristic bands from calcium hydroxide. Figure 5 shows two details from the FT-IR spectrum of a paper sample from the 19th century treated with the Ca(OH)2 dispersion. We observed a rapid decrease of the calcium hydroxide signal and an increase of the calcium carbonate peak, showing the carbonate formation. To verify the efficacy of the deacidification process, we performed pH measurements by cold-water extraction, after one month from the application of the suspension and according to the TAPPI method.18 With the exception of the 14th century sample, whose pH was neutral (7.1), all the examined samples were acidic before the treatment (17th century, pH ) 5.7; 19th century, pH ) 5.1; 20th century, pH ) 4.9). The deacidification produces an evident pH increase (up to 3-4 pH units) and is stable for very long periods of time under natural aging. An illustrative example of the efficacy of this new method is reported in Figure 5, where two paper specimens have been artificially aged by thermal degradation at 85% RH and 90 °C for 21 days. Figure 6 clearly shows nanoparticles of calcium carbonate on a cellulose fiber (a), and two paper specimens: deacidified (b) and untreated (c). The strong color difference accounts for the presence of degradation products from the thermal aging of cellulose, and it confirms the efficiency of this new method. Conclusions This study shows that micron- and submicron-sized Ca(OH)2 colloidal particles can be synthesized from homogeneous and heterogeneous phase reactions. The particle size has been controlled to obtain a solid/liquid stable dispersion in a nonaqueous medium, that is, propan-1-ol
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and propan-2-ol. These findings prompted us to prepare innovative formulations for deacidification treatments of paper and canvases. The application of nanoparticle dispersions of calcium hydroxide to ancient acidic paper samples from the 14th, 17th, 19th, and 20th centuries provided excellent results. This new method has interesting features that could make it competitive to others. Calcium hydroxide is physicochemically compatible to most papers if applied from nonaqueous solutions, and when it is converted to calcium carbonate by carbon dioxide from air, it efficiently works as an alkaline reservoir. The residual amount of water present in the alcohol dispersion is not sufficient for solubilization of appreciable amounts of calcium hydroxide/carbonate that might degrade the cellulose fibers. Propan-2-ol dispersions offer excellent advantages as compared to other methods. Alcohols are environmentally friendly, volatile, and, compared to other solvents, have a low toxicity. Moreover, their rheological properties make them very easy to use.
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In conclusion, nanotechnologies can be employed to produce nonaqueous dispersions of calcium hydroxide nanoparticles. These dispersions are very efficient for the deacidification of paper and can be applied to paper and canvases using several and simple methods commonly at hand. Preliminary results using spraying and/or brushing applications were very successful. The extension of the method to the mass-deacidification process is, presently, in progress.
Acknowledgment. The authors thank Mr. Renato Cecchi (Santo Stefano Textile Finishing) for his collaboration. Financial support from Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase (CSGI Florence) and MIUR (PRIN-2001) is gratefully acknowledged. LA025964D
