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Solubilization of Acrylic and Vinyl Polymers in Nanocontainer Solutions. Application of Microemulsions and Micelles to Cultural Heritage Conservation Emiliano Carretti, Luigi Dei, and Piero Baglioni* Department of Chemistry and Consortium CSGIsUniversity of Florence, via della Lastruccia 3, I-50019 Sesto Fiorentino, Florence, Italy Received May 3, 2003. In Final Form: June 20, 2003 This study reports the physicochemical investigation of oil-in-water (o/w) microemulsions and micellar solutions formulated to solubilize acrylic and vinyl polymers. Three different four-component systems formed from (i) water, p-xylene, sodium dodecylsulfate, and 1-pentanol; (ii) water, propylene carbonate, sodium dodecylsulfate, and 1-pentanol; and (iii) water, p-xylene, Tween-20, and 1,2-ethandiol, and one five-component system (water, p-xylene, commercial nitro-diluent, sodium dodecylsulfate, and 1-pentanol) have been studied. The o/w microemulsions or micellar solutions (system ii) have been characterized by quasi-elastic light scattering experiments to obtain the hydrodynamic radius and the polydispersity of the microemulsion or micelle droplets. The application of these microemulsion or micellar systems as solubilizing agents for acrylic and vinyl polymers from works of art (mainly wall paintings) and monuments (stones) has been investigated. Acrylic and vinyl polymers have been extensively used in the past decades, and are still used, for art conservation. The aging produced both yellowing and serious degradation of the painted layers or of the stone surfaces, imposing their removal from the artistic and architectonic surfaces. Contact angle measurements, FTIR, and SEM/EDX results showed that the microemulsions and micellar solutions investigated were very effective in removing acrylic or vinyl polymeric resins from several solid surfaces. Hydrophobic acrylic copolymers have been completely removed from a Renaissance fresco by Spinello Aretino (used during a restoration performed in the sixties) in the Cappella Guasconi in San Francesco Cathedral, Arezzo, Italy, and poly(vinyl acetate) resins (used in a restoration performed during the fifties) from Renaissance frescoes decorating the external walls of the Cathedral of Conegliano, NorthernEast Italy. The nanocontainers route represents a new, safe, and very efficient method for removing aged polymer from surfaces of works of art, otherwise condemned to complete loss.
Introduction The interaction between polymers and dispersed systems such as microemulsions or micellar systems has received considerable attention during the past years.1-6 Most of these studies have been focused both on the polymerization mechanism in compartmentalized systems1 and on the effect of polymers on structure, interdroplet interactions, and the phase diagram of o/w and w/o microemulsions.2-6 The polymers investigated mainly consist of poly(ethylene/propylene/n-butylene), poly(styrene), poly(isoprene), and styrene/acrylonitrile. Despite the large diffusion of acrylic and vinyl polymers in many fields of applied chemistry, the literature on the interaction of these polymers with dispersed systems is still scarce.7,8 One of the most interesting aspects of the interaction between polymers and microemulsions or micellar solutions is related to the possibility of solubilizing hydrophobic polymers in o/w microemulsions or micelles. These systems * Corresponding author. E-mail:
[email protected]. Web: www.csgi.unifi.it. (1) Lee, K. C.; Gan, L. M.; Chew, C. H. Polymer 1995, 36, 3719. (2) Eicke, H.-F.; Quellet, C.; Xu, G. Colloids Surf., A 1989, 36, 97. (3) Barker, M. C.; Vincent, B. Colloids Surf., A 1984, 8, 297. (4) Endo, H.; Allgaier, J.; Gompper, G.; Jakobs, B.; Monkenbusch, M.; Richter, D.; Sottmann, T.; Strey, R. Phys. Rev. Lett. 2000, 85, 102. (5) Alexandridis, P.; Holmqvist, P.; Lindman, B. Colloids Surf., A 1997, 130, 3. (6) Alexandridis, P.; Andersson, K. J. Colloid Interface Sci. 1997, 194, 166. (7) Roy, S.; Devi, S. Polymer 1997, 38, 3325. (8) Macı´as, E. R.; Rodrı´guez-Guadarrama, L. A.; Cisneros, B. A.; Castan˜eda, A.; Mendiza´bal, E.; Puig, J. E. Colloids Surf., A 1995, 103, 119.
enhance the effectiveness of polymer removal from surfaces and, in particular, from porous frameworks typical of works of art.9 Acrylic and vinyl polymers have been (and are unsuitably) largely used9 as consolidants, protectives, adhesives, and varnishes to restore wall paintings, stones, archaeological objects, and so forth. Ethylacrylate (EA), methylacrylate (MA), methyl methacrylate (MMA), and ethyl methacrylate (EMA) are the main constituents of acrylic copolymers.9,10 These polymers, as well as vinyl polymers, have many adverse effects due to the poor physicochemical stability associated with their natural aging.11 Despite the adverse effects of these polymers, their use is very popular among restorers,15 and they are commonly used in the Conservation of Cultural Heritage. Both thermal and photochemical reactivity12-14 lead to depolymerization and cross-linking reactions, resulting in surface yellowing, mechanical stress on the adherent paint layers, formation of microfractures, and alteration of the chemical and physical properties of the interface between the work of art and the environment. In most cases an enhancement of the degradation processes (9) Horie, C. V. Materials for ConservationsOrganic Consolidants, Adhesives and Coatings; Architectural Press (Butterworth-Heinemann): Oxford, U.K., 1987; pp 103-112. (10) Luskin, L. S. In Vynil and Diene Monomers; Leonard, E. C., Ed.; Wiley-Interscience: New York, 1980; Part I, pp 105-204. (11) Feller, R. L. In Accelerated AgingsPhotochemical and Thermal Aspects; The Getty Conservation Institute: Los Angeles, CA, 1994; pp 63-90. (12) Morimoto, K.; Suzuki, S. J. Appl. Polym. Sci. 1972, 16, 2947. (13) Feller, R. L. Bull. Inst. R. Patrim. Artist. 1975, 15, 135. (14) Hennig, J. Kunstoffe Fortschriftsberidite 1978, 7, 13. (15) Chiantore, O.; Lazzari, M. Int. J. Polym. Anal. Charact. 1996, 2, 395.
10.1021/la034757q CCC: $25.00 © 2003 American Chemical Society Published on Web 08/12/2003
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Table 1. Composition in Weight Percent of the Three o/w Microemulsions for Acrylic Copolymer Poly(EMA/MA) Solubilization microemulsion A
composition/wt %
microemulsion B
composition/wt %
microemulsion C
composition/wt %
Tween-20 EG water p-xylene
7.6 5.9 79.2 7.3
SDS 1-PeOH water p-xylene
4.1 7.9 85.4 2.6
SDS 1-PeOH water p-xylene ND
3.9 6.5 86.2 1.8 1.6
is observed. In other words, the use of these copolymers leads to the loss of the works of art, usually in a period of 10-20 years or longer from the application, depending on the environmental/conservation condition of the work of art. During conservation/restoration treatments, the removal of aged polymers is mandatory. This is usually very difficult, since the photochemical and thermal reactivity of polymers makes them almost insoluble.11 Solubilization of the acrylic and vinyl polymers is partly obtained by using aggressive organic solvents with different polarity (apolar for acrylic and more polar for vinyl polymers) applied by the “compress technique.”16 The toxicity of these solvents and the spreading into the work of art porous structure of the solubilized polymers are the main limitations in the use of organic solvents. The aim of the present work is the formulation and characterization of suitable oil-in-water (o/w) microemulsions or micellar solutions for the solubilization of acrylic and vinyl polymers and the development of new safe systems with very low environmental impact that can be applied in Cultural Heritage Conservation. Micellar systems and four- and five-component o/w microemulsions have been selected (among more than 100 candidate systems) and characterized by quasi-elastic light scattering (QELS) measurements for the removal of vinyl polymers. Microemulsions have been used in a wall painting workshop in Arezzo (frescoes by Spinello Aretino, XVI century). Micellar solutions were used to remove aged vinyl polymers from the mural paintings by Pozzoserrato (XVI century), decorating the external walls of the Santa Maria dei Battuti Cathedral in Conegliano (Treviso), Northern-East Italy. Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX), and contact angle measurements have been performed to detect the microemulsion solutions’ efficiency in removing aged acrylic copolymer and vinyl films present on the painted surfaces.
Figure 1. Phase diagram of the water/SDS/1-PeOH/PC micellar system.
1-Pentanol (1-PeOH, purity > 98.5%), ethylene glycol (EG, purity > 99.5%), p-xylene (purity > 99.5%), propylene carbonate (PC, purity > 99.5%), and KBr for infrared pellets (purity > 99.5%) were purchased from Merck, Darmstadt, Germany and used as received. Sodium dodecyl sulfate (SDS) (purity > 98.5%) supplied by Merck, Darmstadt, Germany, was purified from ethanol17 prior to use. Tween-20 was supplied by Fluka Chemie, Buchs, Switzerland; pellets of poly(ethyl methacrylate (EMA)/ methyl acrylate (MA)) copolymer (poly(EMA/MA)), with EMA/ MA mole ratio 70:30 and average molecular weight 80 000, and nitro-diluent (ND) (commercial mixtures of xylenes and chloro derivatives used to clean pictorial surfaces) were supplied by Zecchi, Florence, Italy. Water was purified with a Millipore MilliRO-6 and MilliQ (Organex System) apparatus: the resistance of the water was >18 MΩ‚cm.
Three different o/w microemulsions (Table 1) were prepared by mixing the appropriate components according to the following procedure. The surfactant (SDS or Tween-20) was dissolved in water by heating for 15 min at 40 °C, and then the solution was cooled to room temperature. Cosurfactant (1-PeOH or EG) was added dropwise to the surfactant solution at room temperature with stirring until a transparent solution was formed. The oil phase (p-xylene) was finally added at room temperature until a stable system was formed; microemulsion C was obtained by slow addition of a second oil phase (ND), added after p-xylene. These o/w microemulsions were stable for more than a year. The system containing propylene carbonate (PC) was characterized by mapping the phase diagram reported in Figure 1. The phase diagram was obtained by optical and light scattering inspection. Figure 1 shows that the amount of PC solubilized in a micellar system of SDS, water, and 1-PeOH is considerably larger than the solubility of PC in water (vide infra). Dynamic quasi-elastic light scattering (QELS) measurements were carried out on a Brookhaven apparatus (BI200SM with BI9000AT correlator) using a Nd:YAG (532 nm) diode pumped laser with an attenuated power in order to avoid sample heating; power stability was (0.5%. The scattered light was collected with a Thorn-Emi 96350 photomultiplier. The size and size distribution of the dispersed phase have been obtained by analyzing the autocorrelation function with the constrained regularization18 (CONTIN) and nonlinear least squares19 (NNLS) methods. Contact angle measurements were performed on glass and mortar specimens by means of a NRL Rame´-Hart Inc. apparatus interfaced to a PC. Water droplets (3 µL) were deposited onto the solid surface using a Hamilton microsyringe, and the contact angle was determined 5 s after the deposition. The contact angles are reported with their standard deviations as the average of 10 measurements. Mortar samples were prepared according to the procedure reported in ref 20. The coating of poly(EMA/MA) copolymer onto glass or mortar surfaces was realized by brushing a 4% (by weight) solution of the copolymer in p-xylene and allowing complete drying of the copolymer film. The final thickness of the poly(EMA/MA) coating was about 20 µm.
(16) Ferroni, E. In Ecological Physical Chemistry; Rossi, C., Tiezzi, E., Eds.; Elsevier: Amsterdam, 1991; pp 345-358. (17) Baglioni, P.; Rivara-Minten, E.; Dei, L.; Ferroni, E. J. Phys. Chem. 1990, 94, 8218.
(18) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213. (19) Lawson, C. L.; Hanson, R. J. Solving Least Square Problems; Prentice Hall: Englewood Cliffs, NJ, 1974. (20) Giorgi, R.; Dei, L.; Baglioni, P. Stud. Conserv. 2000, 45, 154.
Experimental Section
Solubilization of Polymers in Nanocontainer Solutions
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Figure 2. Distribution of the microdroplets’ size obtained from QELS measurements for microemulsion A. Table 2. Average Hydrodynamic Diameter (D/nm) from QELS Measurements for the Investigated o/w Microemulsionsa microemulsion
D/nm
microemulsion
D/nm
A A (d ) 1/2) A (d ) 1/4) A (c ) 5)
8.6 8.5 8.6 8.6
A (c ) 10) A (c ) 25) B C
8.5 8.7 17.8 17.8
a Numbers in parentheses refer to the dilution with water (d) or concentration with oil (c) for the A microemulsion.
Scanning electron microscopy (SEM) images were collected with a Philips 515 apparatus. The EDX spectra were performed using a microprobe coupled with the SEM microscope. Fourier transform infrared (FTIR) spectra were obtained in the microreflectance mode using a BioRad FTS-40 spectrometer equipped with a BioRad UMA500 microscope (MCT detector) with 8 cm-1 resolution and 512 scans. The o/w microemulsions were applied over some regions of a fresco by Spinello Aretino (XVI century) for 2.5 h with the compress technique16 and protecting the painted surface with Japanese paper sheets. To minimize the evaporation of the microemulsions’ components, the compresses were partially sealed to the painted surface using poly(ethylene) films. After the removal of the compress, the fresco surface was washed several times with water to eliminate the surfactant. The same procedure (with a longer application) has been adopted for the micellar solutions containing PC on the frescoes decorating the external walls of the Conegliano’s Cathedral.
Results and Discussion Figure 2 shows the distribution of the droplets’ hydrodynamic diameter for microemulsion A. The oil droplets are characterized by a 4.3 nm radius with a very narrow size distribution, in agreement with SANS measurements.21 QELS measurements, carried out on various samples with different concentrations of the oil phase, show (see Table 2) that the oil phase of microemulsion A can be concentrated up to 25 times and diluted up to 4 times without appreciably changing the droplet size. The average droplet diameter of microemulsion B is about twice that of microemulsion A (17.8 and 8.6 nm, respectively) with a higher polydispersity. Despite the fact that formulation of microemulsion B dates back to several years ago,22 no structural information for this microemulsion is available in the literature. Microemulsion C has been obtained by addition of the second oil phase (nitro-diluent) to micro(21) Alba-Simionesco, C.; Teixeira, J.; Angell, C. A. J. Chem. Phys. 1989, 91, 395; and this work. (22) Rance, D.; Friberg, S. J. Colloid Interface Sci. 1977, 60, 207.
Figure 3. Contact angle behavior for water droplets on different solid surfaces. From left to right: “free” is the free solid surface, “coated” is the surface coated by a few micron poly(EMA/MA) layer, “UV coated” is the same coated surface but aged for 20 h under a Hg/xenon 150 W lamp, and “cleaned” are the two coated surfaces after application of microemulsion C.
emulsion B, and it presents similar average diameter and polydispersity. Considering the oil phase composition (solvents with high affinity for poly(EMA/MA)) and the very high surface area typical of microemulsions, that enhances the solubilization processes, the three o/w microemulsions reported in Table 2 are expected to interact with acrylic copolymers. A direct contact of microemulsions A, B, and C with pure poly(EMA/MA) for 3 h did not produce any microemulsion phase separation. QELS measurements evidenced a slight increase of the hydrodynamic diameter, associated with the solubilization of the copolymers, that are probably mainly distributed at the droplets’ interface. Since it is well-known23 that acrylic copolymer films dramatically alter the wetting properties of solid surfaces, to evaluate the polymer extraction capacity of these microemulsions, poly(EMA/MA) films were deposited onto glass surfaces and the contact angle of water with these surfaces was measured. Figure 3 shows the behavior of the contact angle on two surfaces (glass and mortar) as a function of different treatments. Coating the surface with a poly(EMA/MA) film resulted in a strong increase of the contact angle for both glass and mortar materials, indicating the enhancement of surface hydrophobicity (see Figure 3, coated). Poly(EMA/MA) films have also been irradiated to simulate the degradation they have experienced on aging.11 UV irradiation is considered one of the most common procedures to simulate natural aging of polymeric materials employed in cultural heritage conservation. It was interesting to notice that the copolymer aged by UV irradiation did not appreciably change the contact angle on both materials, suggesting that the hydrophobicity of poly(EMA/MA) is not consistently affected by irradiation (see Figure 3, UV coated). After the application of microemulsion C, according to the procedure reported in the Experimental Section, we repeated contact angle measurements on the cleaned surface portions, and the results are reported in Figure 3 (cleaned). In both cases (glass and mortar) we observed a strong decrease of the contact angle, with a contact angle (23) Adam, N. K.; Elliott, B. E. P. J. Chem. Soc. 1962, 2206.
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Figure 4. SEM micrograph of a microsample from the Renaissance fresco (see text) before the application of microemulsion C and microreflectance FTIR spectrum of the whole region. The bar in the SEM micrograph ) 50 µm.
close to that of the substrate prior to the poly(EMA/MA) application. This supports a good solubilization of the acrylic copolymer in the microemulsion system with the consequent detachment and removal of the hydrophobic film from the surface of the substrate. No differences in the solubilization capacity have been found whether the surface was UV irradiated or not, indicating that the microemulsions can efficiently solubilize both degraded and nondegraded polymers. Gravimetric measurements further confirm polymer extraction: the acrylic copolymer extraction was always >95%.24,25 However, the slightly higher contact angle found after the cleaning procedure can be associated with the residual fraction of the acrylic copolymer (
