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Novel coatings from renewable resources for the protection of bronzes
Progress in Organic Coatings 77 (2014) 892–903
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Novel coatings from renewable resources for the protection of bronzes Giulia Giuntoli a,b , Luca Rosi a , Marco Frediani a , Barbara Sacchi b , Barbara Salvadori b , Simone Porcinai c , Piero Frediani a,∗ a b c
Department of Chemistry “Ugo Schiff”, University of Florence, Via della Lastruccia, 13, 50019 Sesto Fiorentino, Italy Institute for Conservation and Valorization of Cultural Heritage, National Research Council, Via Madonna del Piano, 10, 50019 Sesto Fiorentino, Italy Opificio delle Pietre Dure, Via degli Alfani, 78, 50121 Florence, Italy
a r t i c l e
i n f o
Article history: Received 18 September 2013 Received in revised form 11 December 2013 Accepted 22 January 2014 Available online 20 February 2014 Keywords: Coating Corrosion Metals Polymer Benzotriazole Renewable resource
a b s t r a c t End-capped poly(lactic acid)s with a benzotriazole moiety were synthesized by Ring Opening Polymerization of lactide, characterized by spectroscopic methods and tested as protective coatings on selected bronze surfaces. Performances of functionalized polymers were evaluated in terms of colour changes of the treated metal and stability of the coating. A comparison between end-capped polymers and a mixture of poly(lactic acid) and benzotriazole was also run. End-capped poly(lactic acid)s showed excellent stability to photochemical and thermo-hygrometric ageing and better performances than a blend of poly(lactic acid) and benzotriazole. These polymers show promising performances for metal’s coating. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Conservation and restoration of metallic objects, especially those having a historical and artistic importance have improved their relevance in the last years. Degradation phenomena due to corrosion processes and leading to alteration and disfiguring effects of the surfaces must be avoided. Preventive conservation measures, i.e. a proper control of environmental conditions (in terms of relative humidity, pollutants, etc.) are generally accepted as the best way to reduce the deterioration of metallic surfaces. However, in many situations, primarily in the case of outdoor artworks, it is difficult or not possible to assure adequate environmental conditions [1–6]. Therefore a common practice for the protection of metals is the application of a polymer-based coating on the surface. Coating materials for conservation treatments of cultural heritage should meet different requirements if compared with coatings for industrial applications [7]. They should be transparent and leading to none or negligible changes in the appearance of the original substrate, reversible that is removable after many years without any damage for the object. Furthermore they should not modify the original artefact, including in most cases, changes suffered by the object due to its history (e.g., patinas, crusts and
∗ Corresponding author. Tel.: +39 0554573522; fax: +39 0554574913. E-mail addresses: piero.frediani@unifi.it, [email protected] (P. Frediani). 0300-9440/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2014.01.021
corrosion layers), as far as they do not threaten conservation and legibility of the object. The coating should also have a long shelflife, that is a long stability and efficiency over time and they should be easily applied and removed with eco-friendly operations [3,7]. The most common coating materials for the protection of historic metals are acrylic resins, such as Incralac® , Paraloid B44® , microcrystalline waxes, and in some cases their combinations [8–10]. For outdoor bronzes, the most common protective coating used by conservators and restorers is the commercial Incralac® , a toluene solution of an acrylic resin containing benzotriazole (BTA) [11]. BTA is a heterocyclic compound which has been extensively used as anti-oxidant in many formulations for the protection of metallic surfaces. It is a very efficient corrosion inhibitor for copper and its alloys (e.g., bronze, brass) by preventing surface reactions through the formation of copper–BTA interactions [12–14]. However this compound is toxic for many plants, aquatic organisms and bacteria and suspicious carcinogenic agent for human [15]. Moreover its stability to UV radiation and its high water solubility, makes it very persistent and mobile in the environment. Commercial coatings may not satisfy all the necessary requirements for application in cultural heritage, due to undesirable aesthetic features [16] and lack of stability [11,17]. Failure has been reported in many cases, often with severe damage to the underlying metallic surface [2]. Therefore, considering the special requirements necessary for metal conservation and the unique value of most of metallic cultural heritage, the scientific community takes
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interest in the development of new materials that can provide a better protection, while fulfilling conservation criteria. Nowadays, following a green chemical approach, polymers from renewable resources were tested as potential substitutes for petrochemical-based materials in many fields. Among these compounds, particular attention has been devoted to poly(lactic acid) (PLA). The starting material, lactic acid, is produced using 100% annually renewable resources and the polymer may be biodegraded only if exposed to specific conditions. Furthermore both structure and molecular weight of the polymer can be easily controlled and modified through appropriate synthesis obtaining tailor made products [18]. Besides other uses (e.g., packaging, biomedical, textile), PLA may represent an appealing alternative to synthetic polymers commonly employed in conservation of artistic objects. On this basis, recent studies have been focused on the synthesis and characterization of modified PLA with particular properties for a potential application in cultural heritage. In this respect, fluorine-containing PLAs showed interesting performances as protective coatings for stone, such as water-repellence, chemical and photochemical stability and negligible short- and long-term modification of the appearance of the original substrate [19,20]. In this paper results concerning synthesis, characterization and performance of innovative PLA-based polymers useful for the protection of metallic surfaces, are reported. A benzotriazole moiety (BT) was chemically bonded at the end of the PLA chain in order to avoid the leakage of the anticorrosive agent leading to an enhanced efficiency of the coating and to a lower toxicity for operators and environment. A series of end-capped polymers was synthesized using 1 Hbenzotriazole-1-methanol (HO-BT) as initiator in the well-known Ring Opening Polymerization (ROP) [21,22]. l-lactide (the dimeric ester of lactic acid) and rac-lactide (the racemic mixture of l-lactide and d-lactide) were employed as monomer in the ROP catalyzed by tin(II) 2-ethylhexanoate (Sn(Oct)2 ) and products were characterized by NMR, FTIR, UV–vis, GPC and DSC methods. A selected polymer was applied on bare or patinated bronze substrates simulating different metal’s conservation states and evaluating colour changes of the bronze surface. Morphology of the polymer coating was also investigated by SEM-EDS observations. Finally stability of the coating was tested in the course of accelerated photochemical and thermo-hygrometric ageing. 2. Experimental 2.1. Materials l-lactide, rac-lactide, 1 H-benzotriazole-1-methanol (HO-BT), Sn(Oct)2 , n-hexane, chloroform (CHCl3 ), tetrahydrofuran (THF) and deuterated solvent (CDCl3 , DMSO-d6 ) were commercial products purchased by Aldrich, Normapur and Riedel-De Haën. HO-BT (CDCl3 as solvent) showed resonances in 1 H NMR spectrum at ı: 8.10 (d, 1H, H4, 3 JHH = 6.0 Hz), 7.70 (d, 1H, H7, 3J 3 3 HH = 6.0 Hz), 7.57 (dd, 1H, H6, JHH = 6.0 Hz, JHH = 6.0 Hz), 7.43 (dd, 1H, H5, 3 JHH = 6.0 Hz, 3 JHH = 6.0 Hz), 7.26 (s, 1 2H, N-CH2 -OH) ppm and in 13 C NMR spectrum at ı 1 1 1 1 C, C7), 70.22 (s, 1 C, N-CH2 -OH) ppm. Efficacy of polymers as protective coating was tested on samples of a quaternary bronze alloy. Elemental analysis (%) of alloy was Fe 0.12–0.14; Ni 0.36–0.43; Cu 88.2–88.3; Zn 3.87–3.93; Sn 5.62–5.69; Pb 1.5–1.7. An artificial patina was realized by the foundry, on several samples, using K2 S and NH4 Cl, to simulate the natural alteration of bronze. FTIR characterization of the patina revealed that it was composed by atacamite (copper hydroxychloride) and antlerite (copper hydroxysulphate) (Fig. 1).
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Fig. 1. FTIR spectra of micro-samples collected from (a) patina present on test specimens; (b) standard atacamite; (c) standard antlerite.
Size of bronze coupons was 5.0 cm × 2.5 cm × 0.5 cm for ageing tests in Solar Box and 5.0 cm × 5.0 cm × 0.5 cm for ageing tests in climatic chamber. Both bare and patinated coupons were used.
2.2. Synthesis of polymers Monomer (l-lactide or rac-lactide) was introduced in a dried Schlenk tube under a nitrogen atmosphere. 0.5:100 Sn(Oct)2 /lactide molar ratio and the desired amount of HO-BT as initiator was then added. The polymerization was run by heating the Schlenk tube in an oil bath thermostated at 130 ◦ C under magnetic stirring. After 3 h the reactor was cooled to room temperature. The crude polymer was purified by dissolutionprecipitation method using chloroform and n-hexane and dried under reduced pressure overnight to remove any residual solvent.
2.3. Instruments 2.3.1. NMR spectroscopy 1 H and 13 C NMR spectra were collected using a Varian VXR 200 MHz spectrometer, working at 199.958 MHz for 1 H and 50.294 MHz for 13 C, using CDCl3 as solvent. Residual hydrogens of the solvent were employed as reference and chemical shifts were referred to tetramethylsilane (TMS).
2.3.2. FTIR spectroscopy Fourier transform infrared (FTIR) spectroscopy was performed with a Shimadzu FTIR spectrometer model IRAffinity-1, using either NaCl disks or a Specac Golden Gate single reflection diamond ATR (attenuated total reflectance) accessory. Polymer films were cast on NaCl disks using CHCl3 as solvent. ATR accessory was used to collect the infrared spectra directly on neat polymers. Reflection FTIR spectra were recorded also on coated bronze coupons using a portable Bruker Alpha spectrophotometer (OPUS Software) with spectral range 400–7000 cm−1 , resolution 4 cm−1 , background on gold plate, 100 scans, spot size 3.4 cm × 1.4 cm. Preliminary characterization of the patina was performed collecting some spectra on micro-samples taken from the surface of the specimens with a continuum infrared microscope linked to a Nicolet Nexus FTIR spectrometer, with a spectral resolution of 4 cm−1 (128 scans) in transmission mode. The samples were flattened on a KBr support.
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2.3.3. UV–vis spectroscopy UV–vis spectra were collected with a UV–vis detector Waters model 2489. CHCl3 solutions (1.5 mg of polymer in 5 mL of solvent) and quartz cells with 1 cm path length were used.
2.3.4. Gel permeation cromatography Gel permeation chromatography (GPC) was performed using a Waters system equipped with a pump Waters model Binary HPLC 1525, and a refractive-index detector Wyatt Optilab T-rEx. The GPC experimental conditions were the following: three columns Shodex KF-803 (length: 300 mm, diameter: 8.0 mm), mobile phase: stabilized THF, temperature: 30 ◦ C and flow rate: 1.0 mL/min. The solution injected was 50 L (concentration 10 mg of polymer in 1 mL of THF) and the retention times were converted to molecular weights and their distribution using a reference curve obtained with polystyrene standards.
2.3.5. Differential scanning calorimetry Differential scanning calorimetry (DSC) was performed with a Perkin Elmer instruments model Pyris 1 DSC equipped with a Intracooler 2P cryogenic system. Traces were recorded in the temperature range from 0 to 200 ◦ C under a nitrogen atmosphere, using an heating rate of 20 ◦ C/min. To eliminate any effect of thermal history, measurements were carried out from a second heating cycle, after a first heating of the sample to 200 ◦ C at 20 ◦ C/min, followed by quenching to 0 ◦ C.
2.3.6. Colorimetry Colorimetric measurements were performed on bronze coupons before and after treatments with the protective products using a KONICA–MINOLTA CM2600d spectrophotometer with the CIE 1976 (L*a*b*) colour space (CIELAB) representation, according to the UNI EN 15886/2010 protocol, [23] with the standard illuminant D65 and observer 10◦ . The colour coordinates L*, a* and b* were recorded on a selected point on each coupon (∅ ∼ 6 mm). Five measurements were taken by repositioning the instrument on the same spot each time, and then the data were averaged. The spots were localized using a transparent plastic mask delimiting the measurement areas, with holes over the spots exactly fitting the colorimeter probe. Instrument was calibrated against a SPECTRALON® prior to each measurement.
2.3.7. SEM-EDS Morphology and distribution of protective products on the metal surface were investigated by a scanning electron microscope SEM–EDS (Cambridge Stereoscan 440 LEICA coupled with an EDS microprobe, Oxford Instruments, model INCA), keeping a low voltage (10 keV) to avoid any damage of the coatings.
2.3.8. Thermo-hygrometric ageing chamber An Angelantoni Challenge 500 climatic chamber, was employed for accelerated thermo-hygrometric ageing where temperature and relative humidity were changed according to the cycle reported in Table 1.
2.3.9. Solar Box Photochemical ageing was run using a Solar Box CO.FO.ME.GRA model 3000e equipped with a Xenon-arc lamp and an outdoor type UV filter with cut-off <290 nm to eliminate radiation not present in the external sunlight. According to the ISO 1134/2004 protocol, [24] irradiance was kept at 550 W/m2 and black standard temperature (BST) at 65 ± 2 ◦ C.
Table 1 Thermo-hygrometric cycle employed in climatic chamber. T (◦ C) 0 10 20 40 60 40 20 10 0
RH (%)
Time (min)
85 85 65 45 30 45 65 85 85
60 60 60 60 60 60 60 60 60
T, temperature; RH, relative humidity; heating rate: 1 ◦ C/min.
2.4. Solubility The solubility of fresh (un-aged) polymers was investigated in water, as well as in various traditional solvents (acetone, 2butanone, chloroform, tetrahydrofuran) and eco-friendly organic solvents (1,3-dioxolane, ethyl lactate, butyl lactate). A prefixed mass of polymer (∼25 mg) was weighed in a glass tube. Then the solvent (0.5 mL) was added and the tube containing the solution was capped and placed in an ultrasonic bath for 15 min at room temperature to promote the polymer dissolution. The solubility of polymers was considered complete when a transparent solution was obtained. Otherwise, the test tube was centrifuged for 10 min at 3500 rpm. Supernatant was recovered with a pipette. The solvent was then removed and the mass of the soluble fraction of polymer ms [g] was determined. Solubility S [g/L] was given by the following relation: ms S= [g/L] 5 × 10−4 where ms was the mass of polymer (g) dissolved in the solvent tested. 2.5. Product application The PLA + BTA blend was prepared using 4.7 g of PLA and 0.3 g BTA and the mixture was dissolved in CHCl3 (5%, w/v). The BTA content in the blend was the same of commercial Incralac® , a solvent-based acrylic resin widely employed for conservation purposes. In the same way a CHCl3 solution of one of the copolymers between PLA and BTA (PLLA-BT5 , see results for further details, 6%, w/v) was prepared. Solutions of polymers were applied by brush on one of the wider surfaces of the bronze coupons. Friendly solvents such as lactate or acetate esters may substitute chloroform in outdoor applications. The amount of polymer applied on each bronze coupon was determined by weight difference and reported in Table 2. Solubility, weight loss and molecular weight changes of functionalized polymers during accelerated ageing tests, were Table 2 Amount of polymer applied on patinated and bare bronze coupons. Code
Treatment
Amount (mg)
g/m2
Patinated coupons employed for accelerated thermo-hygrometric ageing PLA + BTA 8.6 PAPMNIA PLA + BTA 10.9 PAPMNIB PAPFNIA PLLA-BT5 5.6 PLLA-BT5 5.9 PAPFNIB
3.7326 4.7309 2.4306 2.5608
Bare coupons employed for accelerated photochemical ageing NSPFNIA PLLA-BT5 8.4 PLA + BTA 4.0 NSPMNIA
7.2917 3.4722
PA, patinated bronze coupon, ageing in climatic chamber; PM, PLA + BTA treatment; PF, PLLA-BT5 treatment; NIA/NIB, no inhibitor, replica A/B; NS, bare bronze, ageing in Solar Box.
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evaluated using glass slides (38 mm × 26 mm) preliminarily washed with acetone and stored in a desiccator up to constant weight. Polymer solutions in CHCl3 (4%, w/v) were cast on the glass surface, and after evaporation of solvent, slides were kept in a desiccator up to constant weight and amount of polymer applied was determined by weight difference. 2.6. Accelerated ageing Treated bare bronze samples, treated glass slides and treated NaCl disks were submitted to photochemical ageing up to 965 h in the Solar Box.
Table 3 Selected chemical characteristics of functionalized PLAs. Code
Lactide configuration
BTA/lactide (molar ratio)
Mw a (g/mol)
PDIb
PLAc PLLA-BT1 PLLA-BT3 PLLA-BT5 d PLLA-BT5 e PLLA-BT7 PLDA-BT5
L L L L L L rac
– 1:100 3:100 5:100 5:100 7:100 5:100
180,300 39,500 11,900 8,000 10,400 6,600 5,400
1.7 1.2 1.2 1.8 1.7 1.3 1.1
a b c
2.6.1. FTIR spectra Polymers were placed on NaCl disks and spectra were recorded after 250, 465, and 965 h of exposure, as well as solubility, weight and molecular weight changes of coatings cast on glass slides. 2.6.2. Colorimetric data and reflectance FTIR spectra These analyses were performed on treated bronze coupons after different time intervals up to 965 h. Treated patinated bronze coupons were also submitted to 50 thermo-hygrometric cycles in a climatic chamber (Table 1), checking the behaviour of the applied polymers with colorimetric and reflectance FTIR measurements after 25 and 50 cycles.
d e
3. Results and discussion 3.1. End-capped polymers: synthesis and characterization End capped PLAs were synthesized through Sn(Oct)2 -catalyzed ROP of l-lactide and rac-lactide (the racemic mixture of l and d isomers) through a coordination-insertion mechanism [25,26]. Using commercial HO-BT as initiator, a series of functionalized polymers, containing the BT group at the end of the PLA chain was obtained, as listed in Tables 3 and 4. Fig. 2 shows the reaction scheme employed for the syntheses of polymers. The growth of the polymer chain and the BT insertion were monitored by NMR analysis (1 H, 13 C NMR) on the basis of the signal attributed to CH3 of PLA moiety and to CH2 O of BT moiety, respectively. NMR resonances of PLLA-BT5 were reported as an example (Fig. 3), the spectra of the other end-capped PLAs show the same resonances. The 1 H NMR spectrum show ı (CHCl3 ) at: 8.10 (d, 1H, H4-BT, 3 JHH = 9.2 Hz), 7.70 (d, 1H, H7-BT,
Mw , weight average molar mass. PDI, polydispersity index. Sample used to prepare PLA + BTA blend. Polymer employed for photochemical ageing. Polymer employed for bronze coating.
Table 4 Selected physical characteristics of functionalized PLAs. Code a
PLA PLLA-BT1 PLLA-BT3 PLLA-BT5 b PLLA-BT5 c PLLA-BT7 PLDA-BT5 a
2.6.3. Weight loss, solubility and molecular weight determinations on aged polymers After exposure in Solar Box, the coated glass slides were stored in a desiccator until a constant mass was reached and the weight loss due to the ageing process was determined. The CHCl3 solutions were evaporated and the polymeric residues weighed. The slides were washed with CHCl3 to dissolve all soluble polymers and weighed again. In order to confirm these data, the polymer residues eventually remained on the slides were calculated by weight difference. Molecular weight of the soluble fraction was evaluated by GPC.
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b
Tg (◦ C)
Tc (◦ C)
64 58 53 52 56 45 38
121 109 104 101 94 99 –
Tm1 (◦ C)
148 147 147 134
Tm2 (◦ C) 174 171 160 158 159 151 –
Sample used to prepare PLA + BTA blend. Polymer employed for photochemical.
3J HH
= 9.2 Hz), 7.57 (dd, 1H, H6-BT, 3 JHH = 9.2 Hz, 3 JHH = 7.6 Hz), 7.43 (dd, 1H, H5-BT, 3 JHH = 9.2 Hz, 3 JHH = 7.6 Hz), 6.69 (d, 1H, N-CH2 -BT, 2J 2 HH = 11.2 Hz), 6.57 (d, 1H, N-CH2 -BT, JHH = 11.2 Hz), 5.16 (q, 1H, CH-PLA, 3 JHH = 7.2 Hz), 1.58 (d, 3H, CH3 -PLA, 3 JHH = 7.0 Hz) ppm (integrals attributed to BT moiety are not correlated with those of the PLA group) and in 13 C NMR spectrum at ı: 169.57 (s, COPLA), 145.50 (s, 1C, C8-BT), 132.26 (s, 1C, C9-BT), 128.64 (s, 1C, C6-BT), 124.75 (s, 1C, C5-BT), 120.22 (s, 1C, C4-BT), 110.00 (s, 1C, C7-BT), 69.00 (s, 1C, CH-PLA), 67.70 (s, 1C, N-CH2 -BT), 16.53 (s, 1C, CH3 -PLA) ppm. Signals of the CH3 and CH groups of PLA moiety were present at 1.58 and 5.16 ppm, respectively, as reported in the literature. The resonance of the CH2 O group of BT moiety in 1 H NMR spectrum was shifted from 6.00 ppm of the starting HO-BT to 6.63 ppm in PLLABT5 , confirming the presence of the new ester linkage between PLA and BT. Furthermore the CH2 O group of benzotriazole moiety shows a singlet in HO-BT while an AB coupling was shown in the end-capped PLA polymer. This splitting is attributed to the two hydrogens that are not magnetically equivalents for the presence of the chiral CH groups of the PLA moiety in the molecule. This coupling between geminal hydrogens was confirmed by a gcosy-nmr experiment (Fig. 4) and it is a strong support to the presence of end-capped BT moiety in PLA. FTIR absorptions attributed to BT moiety were not detected when low amount of BT was present in the polymer, such as in PLLA-BT1 , PLLA-BT3 . On the contrary, C N stretching (1005 cm−1 ) and CH out-of-plane bending for the benzene ring
Fig. 2. Reaction scheme for the synthesis of end-capped PLAs.
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Fig. 3.
1
H and 13 C NMR spectra of PLLA-BT5 (CDCl3 solution).
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Fig. 5. UV–vis spectra of PLA (continuos line) and end-capped PLLA-BT5 (dotted line).
Fig. 4. Gcosy NMR spectrum of PLLA-BT5 in the 6.0–7.0 ppm region.
of BT (777 cm−1 ) were shown in higher BT containing polymers. The intensity of these bands became stronger as the BT content increases. UV–vis spectra of end-capped PLAs showed strong changes with respect to neat PLA, because the BT moiety is an UV–vis chromophore (Fig. 5). In particular, two main peaks at 252 and 283 nm due to the presence of BT were detected, beside the characteristic
absorption of PLA below 250 nm. The intensities of these absorptions were linearly increased with BT content in polymers. The weight-average molar mass (Mw ) and the polydispersity (PDI) of polymers were determined by GPC. Since HO-BT works as initiator for the polymerization it leads to a decrease in Mw , as previously reported for other functionalized PLAs [21,22] and the BT moiety is inserted at the end of the polymer chain. Endcapped polymers with comparatively higher Mw were obtained using lower co-initiator/monomer molar ratio, Mw values decrease when increasing the initiator content. Differential scanning calorimetric analysis (DSC) showed that samples obtained using l-lactide (PLLA-BT1 , PLLA-BT3 , PLLA-BT5 , PLLA-BT7 ) are semi-crystalline, while sample (PLDA-BT5 ) obtained using rac-lactide is amorphous. The presence of a BT moiety in the polymer significantly decreases Tg and, when present, Tm of the polymers (Table 3), as a consequence of the decrease of Mw . A feature of interest of PLLA-BT3 , PLLA-BT5 and PLLA-BT7 is the
Fig. 6. Change of colorimetric coordinates after thermo-hygrometric ageing cycles in a climatic chamber.
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Table 5 Solubility [g/L] of end-capped PLA-BTa in several solvents. code
H2 O
CHCl3
Acetone
MEK
THF
DIOX
EL
BL
PLLA-BT1 PLLA-BT5 PLLA-BT7
0 0 0
50 50 50
5 30 50
5 15 n.d.
50 50 50
5 25 50
0 10 n.d.
0 10 n.d.
a
MEK, butan-2-one; THF, tetrahydrofuran; DIOX, 1,3-dioxolane; EL, ethyl lactate; BL, butyl lactate.
presence of double melting peaks. This behaviour is a common phenomenon observed by DSC for PLAs, which have been proposed to arise from ␣- and ␣ -crystalline forms of the polymer. Various explanations have been presented to explain this behaviour, [27–30] such as a melting-recrystallization-melting process, as well as the formation of two population of crystallites with different thicknesses. However up to now none of these explanations have been fully confirmed. End-capped polymers were soluble in organic solvent such as acetone, THF, CHCl3 , lactate or acetate esters and solubility generally increased as Mw decreased Table 5. Among the synthesized polymers, PLLA-BT5 showed the most attractive characteristics for coating purposes, in terms of solubility and film formation properties. An efficient film formation may be attributed to the partial loss of the crystalline structure of the polymer. Furthermore its Tg value (52–56 ◦ C) is slightly upon room temperature, improving the flexibility of the polymer.
PLLA-BT5 was selected to test its performances as protective coating for bronze surfaces because the amount of BT was very close to that present in commercial formulations even if its Tg was not very high. 3.2. Evaluation of polymers on metallic surfaces Solutions of PLLA-BT5 and the blend PLA + BTA were applied by brush on both patinated and bare bronze surfaces in order to assess their performances as protective coatings. Colour change tests and SEM-EDS observations were carried out on treated coupons, then selected samples were submitted to artificial ageing and measurements were repeated over time in order to evaluate the polymers performance. 3.2.1. Colour changes Both treatments with functionalized polymer and PLA + BTA blend caused a perceptible colour change of the bare bronze surfaces (E* ∼ 4–10), which was higher for patinated coupons (E* ∼ 15–30), probably due in the latter case to the partial removal of the green patina exerted by the mechanical action of brushing. On bare bronze coupons, colour variation induced by PLA + BTA (E* ∼ 4) was due to darkening and yellowing, whereas for PLLABT5 (E* ≥ 10) was mainly due to strong yellowing and, at a minor extent, darkening and increase of the red component (Fig. 6).
Fig. 7. Surface of a patinated coupon treated with PLA + BTA.
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Fig. 8. Surface and elemental analysis of a patinated sample treated with PLLA-BT5 .
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3.2.2. SEM-EDS PLLA-BT5 and PLA + BTA coatings were investigated through SEM-EDS to show the morphology of the film applied. SEM investigations were carried out on patinated coupons only, where the superficial film of PLA was visible and appeared almost homogeneously distributed for both PLLA-BT5 and PLA + BTA (Figs. 7 and 8). 3.3. Accelerated ageing Stability of polymers was investigated by photochemical ageing up to 965 h in the Solar Box and thermo-hygrometric ageing in the climatic chamber up to 50 cycles. Photochemical ageing of coatings was monitored by FTIR, GPC, solubility and weight changes during the exposure. Moreover colour changes were measured on bare bronze coupons treated with the products at different irradiation times. Thermo-hygrometric ageing was monitored by reflectance FTIR and colour changes measurements on patinated bronze specimens. 3.3.1. Thermo-hygrometric ageing (climatic chamber) 3.3.1.1. FTIR analysis. The reflectance FTIR spectrum of PLLABT5 acquired directly on the treated bronze specimens did not show any significant changes up to 50 cycles in the climatic chamber. On the contrary, in the blend PLA + BTA the typical bands of BTA at 1622 and 1009 cm−1 became undetectable after 25 thermo-hygrometric cycles (Fig. 9) suggesting a possible sublimation of this compound and consequently loss of the coating’s anticorrosive agent. Furthermore PLA when applied on bronze samples showed a shift of some bands towards higher wavenumbers, i.e. bands at 1756–1777 cm−1 , 1724 sh–1747 cm−1 and 1215–1222 cm−1 . In the course of the ageing process, a shift towards lower wavenumbers was observed for absorptions at: 1777–1770 cm−1 , 1747–1741 cm−1 , 1222–1215 cm−1 . Also a modification of the crystalline phases of PLA [31] was shown in the ageing process by change of the relative intensities of the bands at 871 and 756 cm−1 . Thermo-hygrometric ageing did not induce any significant effect on the molecular structure of the polymer. 3.3.1.2. Colour changes. Fig. 10 reports the colour changes of two patinated bronze coupons treated with PLLA-BT5 and PLA + BTA before treatment, after treatment and after 50 thermohygrometric cycles of ageing. In the course of the ageing test, bronze samples treated with PLLA-BT5 and PLA + BTA showed the recovery of
Fig. 9. Reflectance FTIR spectra of a patinated specimen treated with (a) neat PLA and (b–d) PLA + BTA after different ageing cycles in the climatic chamber: (b) 0 cycles; (c) 25 cycles; (d) 50 cycles.
lightness, which had been reduced by polymer application by brushing, as previously discussed (Fig. 6). A similar trend in the variation of L*a*b* parameters has been observed in ageing tests performed on other end-capped PLA polymers, such as fluorinecontaining PLA applied as water-repellent protective coating on stone samples [20]. This behaviour was attributed to a possible reorganization of the coating material as a result of exposition at temperature up to its Tg , modifying the micro-roughness of the polymeric film and consequently white light scattering. Moreover, as a possible result of the above mentioned polymer’s reorganization, the red-green component (a*) remained almost comparable with that of treated coupons, whereas a certain regression to initial values was detected through ageing for the yellow-blue component (b*) (Fig. 6). 3.3.2. Photochemical ageing (Solar Box) 3.3.2.1. FTIR analysis. FTIR spectra of the end-capped polymer (PLLA-BT5 ) and PLA + BTA blend showed the appearance of a weak absorption at 1843 cm−1 from the beginning of the exposure in the Solar Box. This absorption in the carbonyl stretching frequency region, at a higher frequency than the main ester carbonyl absorption of PLA (1758 cm−1 ) can be attributed to the asymmetric stretching of anhydride functional group and was also observed in ageing tests performed on PLA and other PLA-based polymers
Fig. 10. Visible changes of two patinated bronze coupons after treatment (PLLA-BT5 : PAPFNIA; PLA + BTA: PAPMNIB) and after 50 thermo-hygrometric cycles in climatic chamber
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Fig. 11. Reflectance FTIR spectra in the 2000–1650 cm−1 range of a bare specimen treated with PLA + BTA, after different photochemical ageing times: (a) 0 h; (b) 250 h; (c) 465 h; (d) 965 h.
[21,32–34] The formation of anhydride during the photochemical ageing might be explained through a solid state reaction of traces of lactic acid, leading to the formation of lactic acid anhydride [21] or, as recently proposed, by photo-oxidation of the PLA moiety through a radical peroxide mechanism [32–34]. FTIR spectrum of PLLA-BT5 showed also an increase of the intensity of the OH stretching band (3500 cm−1 ) and a broadening of the carbonyl peak with formation of a shoulder at 1710 cm−1 during ageing, as well as some intensity changes in the CO region (1185–1092 cm−1 ). The new shoulder at 1710 cm−1 may be attributed to a possible hydrolysis of the ester linkage with formation of free carboxylic groups as reported by Agarwal
901
Fig. 12. Reflectance FTIR spectra of bare bronze coupons treated with PLLA-BT5 after different photochemical ageing times: (a) 0 h; (b) 250 h; (c) 465 h; (d) 965 h.
[35]. FTIR spectrum on coupons treated with PLA + BTA showed a shift towards higher wavenumbers for some bands after 250 h: 1763–1772 cm−1 , 1097–1109 cm−1 (Fig. 11). Probably, the exposure in the Solar Box induced partial de-polymerization and oxidation of the polymers, as suggested by GPC measurements. No changes in the spectral pattern were detected for coupons treated with PLLA-BT5 . The lower stability of the blend PLA + BTA (Fig. 12) compared to the functionalized polymer PLLA-BT5 where a BT moiety was chemically bonded to the PLA could be related to the above mentioned sublimation of the UV-absorber BTA.
Fig. 13. Change of colorimetric coordinates after photochemical ageing in a Solar Box. Differences with respect to untreated coupons were reported.
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Table 6 Molecular weight and PDI of end-capped PLLA-BT5 at different ageing times. Ageing time (h) Mw (g/mol) PDI
0 8,000 1.8
250 7,900 1.7
500 7,800 1.7
1000 6,500 1.7
3.3.2.2. Solubility, weight loss and molecular weight changes. Polymer remained still completely soluble in chloroform at different ageing times (250, 500, 1000 h). These data are an indication that cross-linking processes did not take place and strongly support the possibility to remove the polymer at the end of its service life. A slight de-polymerization was evidenced during ageing by GPC analysis (Table 6), as well as a slight diminution of the amount of the polymer film applied on glass slices, confirming the excellent stability of these polymers even on the bronze surface. 3.3.2.3. Colour changes. A different colorimetric behaviour was observed for the two polymers applied on bare coupons and aged in Solar Box. Indeed, for PLLA-BT5 the photochemical ageing allowed the regression of colorimetric coordinates to the initial values with consequent lowering of the E* induced by the polymers application. This behaviour was not ascribable to molecular changes, as assessed by FTIR analysis, but probably to the polymer’s physical reorganization previously hypothesized and due to the permanence in the Solar Box where the temperature was slightly higher than Tg of the polymer. Instead, a further increase of the E* was observed for PLA + BTA, mainly due to the hue shift to blue-green (reduction of a* and b*) (Fig. 13). 4. Conclusion Some new renewable and biocompatible polymers were obtained from lactic acid, a natural resource produced by fermentation of sugars. Polymers were synthesized using commercial 1 H-benzotriazole-1-methanol as initiator in the Ring Opening Polymerization of l- or rac-lactide catalyzed by Sn(Oct)2 . The reaction was performed without solvent following a green chemical approach. Benzotriazole group was chemically bonded on the terminal position of the poly(lactic acid) chain, and polymers were characterized by NMR, FTIR, GPC, and DSC. One end-capped poly(lactic acid) PLLA-BT5 was selected and applied on bronze coupons to evaluate colour changes of the bare or patinated bronze surface. Stability of coating treatments on both bare and patinated bronze coupons were evaluated after photochemical ageing in Solar Box and accelerated thermo-hygrometric ageing in a climatic chamber. End-capped polymer showed a very high stability, whilst the blending of PLA + BTA showed a progressive leakage of the BTA in the course of the ageing process and molecular changes of the polymer. Taking into account that remarkable changes were not observed in the colours of treated surfaces after thermo-hygrometric and photochemical ageing, the end-capped polymers showed promising results as protective coating, for the conservation of metals. However further study will be carried out for a full evaluation of their efficiency as anticorrosive coatings trying to obtain evidence on interactions between copper and BTA as well as on the inhibition of corrosion phenomena. The relatively low Tg of the polymer may preclude some outdoor applications of these protective coating, however new PLA co-polymers having a high Tg may be synthesized following the same synthetic strategy. The work is in progress and results will be reported in a forthcoming paper. End-capped polymers are an eco-friendly material for metal surface protection because they were obtained through a green process, friendly solvents may be used for their application and they avoid BTA dispersion in the environment.
Further studies are in progress to compare performances of these materials with those of commercial products usually employed in the protection of bronze surfaces. Acknowledgements The authors thank the European Community and Regione Toscana for financial support through the POR FESR program, TeCon@BC project (code CP 57476), Andrea Pedna and Alfredo Maione (University of Florence) are acknowledged for the synthesis of some end-capped polymers and Mara Camaiti (IGAG-CNR) for assistance in the thermo-hygrometric and photochemical ageing of bronze coupons. References [1] M. Marabelli, Conservazione e restauro dei metalli d’arte, Bardi Editor, 2007. [2] P. Letardi, I. Trentin, G. Cutugno, Monumenti in bronzo all’aperto. Esperienze di conservazione, Nardini Editor, 2004. [3] E. Cano, D. Lafuente, D.M. Bastidas, J. Solid State Electron. 14 (2010) 381–391. [4] E. Joseph, P. Letardi, R. Mazzeo, S. Prati, M. Vandini, Innovative treatments for the protection of outdoor bronze monuments”, in: Proceedings of Metal 07, Triennial Meeting of ICOM-CC Metal Working Group, Amsterdam, Netherlands, September 17–21, 2007, pp. 71–77. [5] R. Mazzeo, S. Bittner, G. Farron, R. Fontinha, D. Job, E. Joseph, P. Letardi, M. Mach, S. Prati, M. Salta, A. Simon, Development and evaluation of new treatments for outdoor bronze monuments, in: J.H. Townsend, L. Toniolo, F. Cappitelli (Eds.), Conservation Science 2007, Archetype, London, 2008, pp. 40–48. [6] E. Joseph, A. Simon, S. Prati, M. Wörle, D. Job, R. Mazzeo, Anal. Bioanal. Chem. 399 (2011) 2899–2907. [7] C.V. Horie, Material for Conservation. Organic Consolidants, Adhesives and Coatings, Butterworths, London, 1991. [8] L.B. Brostoff, E.R. de la Rie, Chemical characterization of metal/coating interfaces from model samples for outdoor bronzes by reflection-absorption infrared spectroscopy (RAIR) and attenuated total reflection spectroscopy (ATR), in: Proceedings of the International Congress METAL 1998, James & James Ltd, 1998, pp. 320–328. [9] M. Marabelli, G. Napolitano, Materiali e strutture 1 (1991) 51–56. [10] G. Bierwagen, T.J. Shedlosky, L.A. Ellingson, Electrochemical studies of the protection of bronzes from corrosion by organic coatings, in: I. MacLeod, J.M. Theile, C. Degrigny (Eds.), Metal 2001: Proceedings of the international conference on metals conservation, Santiago, Chile, April 2001, Western Australian Museum, Welshpool, 2004, pp. 291–296. [11] D. Erhardt, W. Hopwood, T. Padfield, N.F. Veloz, Durability of Incralac, Examination of a Ten Year Old Treatment, Preprints ICOM (International Council of Museums) Conservation Committee, Copenhagen Conference, 10–14 September, 1984, pp. 22.1–22.3. [12] L.B. Brostoff, Investigation into the interaction of benzotriazole with copper corrosion minerals and surfaces, in: Ian D. MacLeod, et al. (Eds.), Proceedings of the International Conference on Metals Conservation Metal 95, ICOM-CC Metals Working Group, Semur-en-Auxois, France, 25–28 September, James & James, Ltd, London, 1997, pp. 99–108. [13] M. Finˇsgar, I. Miloˇsev, Corros. Sci. 52 (2010) 2737–2749. [14] H. Brinch Madsen, Stud. Conserv. 12 (1967) 163–167. [15] X. Wu, N. Chou, D. Lupher, L.C. Davis, Benzotriazoles: toxicity and degradation, in: Proceedings of the Conference on Hazardous Waste Research, Snowbird, UT, 18–21 May, 1988, pp. 374–382. [16] I.M. Marcelli, M. Mercalli, The conservation of outdoor bronze monuments: the restoration project for the Monumento ai Mille by Eugenio Baroni, 1915, Genoaquarto dei mille, in: 9th International Conference on NDT of Art, Jerusalem, Israel, 25–30 May, 2008. [17] C.J. McNamara, M. Breuker, M.T.D. Perry, R. Mitchell, J. Cult. Herit. 5 (2004) 361–364. [18] G. Giachi, M. Frediani, L. Rosi, P. Frediani, Synthesis and processing of biodegradable and bio-based polymers by microwave irradiation, in: U. Chandra (Ed.), Microwave Heating, InTech, 2011, pp. 181–206 http://www.intechopen.com/books/microwave-heating (accessed 12.08.13). [19] G. Giuntoli, A. Pedna, M. Frediani, L. Rosi, P. Frediani, “New perspectives for the application of PLA in cultural heritage”, in: V. Piemonte (Ed.), Polylactic Acid: Synthesis, Properties and Applications, NovaPublisher, USA, 2012. [20] G. Giuntoli, L. Rosi, M. Frediani, B. Sacchi, P. Frediani, J. Appl. Polym. Sci. 125 (2012) 3125–3133. [21] M. Frediani, L. Rosi, M. Camaiti, D. Berti, A. Mariotti, A. Comucci, I. Malesci, Macromol. Chem. Phys. 211 (2010) 988–995. [22] M. Frediani, D. Sémeril, D. Matt, L. Rosi, P. Frediani, F. Rizzolo, A.M. Papini, Int. J. Polym. Sci. (2010) 1–6. [23] UNI EN 15886/2010. Conservation of cultural property – test methods – colour measurement of surfaces. [24] ISO Protocol 11341/2004. Paint and varnishes – artificial weathering and exposure to artificial radiation – exposure to filtered xenon-arc radiation.
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Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Novel coatings from renewable resources for the protection of bronzes Giulia Giuntoli a,b , Luca Rosi a , Marco Frediani a , Barbara Sacchi b , Barbara Salvadori b , Simone Porcinai c , Piero Frediani a,∗ a b c
Department of Chemistry “Ugo Schiff”, University of Florence, Via della Lastruccia, 13, 50019 Sesto Fiorentino, Italy Institute for Conservation and Valorization of Cultural Heritage, National Research Council, Via Madonna del Piano, 10, 50019 Sesto Fiorentino, Italy Opificio delle Pietre Dure, Via degli Alfani, 78, 50121 Florence, Italy
a r t i c l e
i n f o
Article history: Received 18 September 2013 Received in revised form 11 December 2013 Accepted 22 January 2014 Available online 20 February 2014 Keywords: Coating Corrosion Metals Polymer Benzotriazole Renewable resource
a b s t r a c t End-capped poly(lactic acid)s with a benzotriazole moiety were synthesized by Ring Opening Polymerization of lactide, characterized by spectroscopic methods and tested as protective coatings on selected bronze surfaces. Performances of functionalized polymers were evaluated in terms of colour changes of the treated metal and stability of the coating. A comparison between end-capped polymers and a mixture of poly(lactic acid) and benzotriazole was also run. End-capped poly(lactic acid)s showed excellent stability to photochemical and thermo-hygrometric ageing and better performances than a blend of poly(lactic acid) and benzotriazole. These polymers show promising performances for metal’s coating. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Conservation and restoration of metallic objects, especially those having a historical and artistic importance have improved their relevance in the last years. Degradation phenomena due to corrosion processes and leading to alteration and disfiguring effects of the surfaces must be avoided. Preventive conservation measures, i.e. a proper control of environmental conditions (in terms of relative humidity, pollutants, etc.) are generally accepted as the best way to reduce the deterioration of metallic surfaces. However, in many situations, primarily in the case of outdoor artworks, it is difficult or not possible to assure adequate environmental conditions [1–6]. Therefore a common practice for the protection of metals is the application of a polymer-based coating on the surface. Coating materials for conservation treatments of cultural heritage should meet different requirements if compared with coatings for industrial applications [7]. They should be transparent and leading to none or negligible changes in the appearance of the original substrate, reversible that is removable after many years without any damage for the object. Furthermore they should not modify the original artefact, including in most cases, changes suffered by the object due to its history (e.g., patinas, crusts and
∗ Corresponding author. Tel.: +39 0554573522; fax: +39 0554574913. E-mail addresses: piero.frediani@unifi.it, [email protected] (P. Frediani). 0300-9440/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2014.01.021
corrosion layers), as far as they do not threaten conservation and legibility of the object. The coating should also have a long shelflife, that is a long stability and efficiency over time and they should be easily applied and removed with eco-friendly operations [3,7]. The most common coating materials for the protection of historic metals are acrylic resins, such as Incralac® , Paraloid B44® , microcrystalline waxes, and in some cases their combinations [8–10]. For outdoor bronzes, the most common protective coating used by conservators and restorers is the commercial Incralac® , a toluene solution of an acrylic resin containing benzotriazole (BTA) [11]. BTA is a heterocyclic compound which has been extensively used as anti-oxidant in many formulations for the protection of metallic surfaces. It is a very efficient corrosion inhibitor for copper and its alloys (e.g., bronze, brass) by preventing surface reactions through the formation of copper–BTA interactions [12–14]. However this compound is toxic for many plants, aquatic organisms and bacteria and suspicious carcinogenic agent for human [15]. Moreover its stability to UV radiation and its high water solubility, makes it very persistent and mobile in the environment. Commercial coatings may not satisfy all the necessary requirements for application in cultural heritage, due to undesirable aesthetic features [16] and lack of stability [11,17]. Failure has been reported in many cases, often with severe damage to the underlying metallic surface [2]. Therefore, considering the special requirements necessary for metal conservation and the unique value of most of metallic cultural heritage, the scientific community takes
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interest in the development of new materials that can provide a better protection, while fulfilling conservation criteria. Nowadays, following a green chemical approach, polymers from renewable resources were tested as potential substitutes for petrochemical-based materials in many fields. Among these compounds, particular attention has been devoted to poly(lactic acid) (PLA). The starting material, lactic acid, is produced using 100% annually renewable resources and the polymer may be biodegraded only if exposed to specific conditions. Furthermore both structure and molecular weight of the polymer can be easily controlled and modified through appropriate synthesis obtaining tailor made products [18]. Besides other uses (e.g., packaging, biomedical, textile), PLA may represent an appealing alternative to synthetic polymers commonly employed in conservation of artistic objects. On this basis, recent studies have been focused on the synthesis and characterization of modified PLA with particular properties for a potential application in cultural heritage. In this respect, fluorine-containing PLAs showed interesting performances as protective coatings for stone, such as water-repellence, chemical and photochemical stability and negligible short- and long-term modification of the appearance of the original substrate [19,20]. In this paper results concerning synthesis, characterization and performance of innovative PLA-based polymers useful for the protection of metallic surfaces, are reported. A benzotriazole moiety (BT) was chemically bonded at the end of the PLA chain in order to avoid the leakage of the anticorrosive agent leading to an enhanced efficiency of the coating and to a lower toxicity for operators and environment. A series of end-capped polymers was synthesized using 1 Hbenzotriazole-1-methanol (HO-BT) as initiator in the well-known Ring Opening Polymerization (ROP) [21,22]. l-lactide (the dimeric ester of lactic acid) and rac-lactide (the racemic mixture of l-lactide and d-lactide) were employed as monomer in the ROP catalyzed by tin(II) 2-ethylhexanoate (Sn(Oct)2 ) and products were characterized by NMR, FTIR, UV–vis, GPC and DSC methods. A selected polymer was applied on bare or patinated bronze substrates simulating different metal’s conservation states and evaluating colour changes of the bronze surface. Morphology of the polymer coating was also investigated by SEM-EDS observations. Finally stability of the coating was tested in the course of accelerated photochemical and thermo-hygrometric ageing. 2. Experimental 2.1. Materials l-lactide, rac-lactide, 1 H-benzotriazole-1-methanol (HO-BT), Sn(Oct)2 , n-hexane, chloroform (CHCl3 ), tetrahydrofuran (THF) and deuterated solvent (CDCl3 , DMSO-d6 ) were commercial products purchased by Aldrich, Normapur and Riedel-De Haën. HO-BT (CDCl3 as solvent) showed resonances in 1 H NMR spectrum at ı: 8.10 (d, 1H, H4, 3 JHH = 6.0 Hz), 7.70 (d, 1H, H7, 3J 3 3 HH = 6.0 Hz), 7.57 (dd, 1H, H6, JHH = 6.0 Hz, JHH = 6.0 Hz), 7.43 (dd, 1H, H5, 3 JHH = 6.0 Hz, 3 JHH = 6.0 Hz), 7.26 (s, 1 2H, N-CH2 -OH) ppm and in 13 C NMR spectrum at ı 1 1 1 1 C, C7), 70.22 (s, 1 C, N-CH2 -OH) ppm. Efficacy of polymers as protective coating was tested on samples of a quaternary bronze alloy. Elemental analysis (%) of alloy was Fe 0.12–0.14; Ni 0.36–0.43; Cu 88.2–88.3; Zn 3.87–3.93; Sn 5.62–5.69; Pb 1.5–1.7. An artificial patina was realized by the foundry, on several samples, using K2 S and NH4 Cl, to simulate the natural alteration of bronze. FTIR characterization of the patina revealed that it was composed by atacamite (copper hydroxychloride) and antlerite (copper hydroxysulphate) (Fig. 1).
893
Fig. 1. FTIR spectra of micro-samples collected from (a) patina present on test specimens; (b) standard atacamite; (c) standard antlerite.
Size of bronze coupons was 5.0 cm × 2.5 cm × 0.5 cm for ageing tests in Solar Box and 5.0 cm × 5.0 cm × 0.5 cm for ageing tests in climatic chamber. Both bare and patinated coupons were used.
2.2. Synthesis of polymers Monomer (l-lactide or rac-lactide) was introduced in a dried Schlenk tube under a nitrogen atmosphere. 0.5:100 Sn(Oct)2 /lactide molar ratio and the desired amount of HO-BT as initiator was then added. The polymerization was run by heating the Schlenk tube in an oil bath thermostated at 130 ◦ C under magnetic stirring. After 3 h the reactor was cooled to room temperature. The crude polymer was purified by dissolutionprecipitation method using chloroform and n-hexane and dried under reduced pressure overnight to remove any residual solvent.
2.3. Instruments 2.3.1. NMR spectroscopy 1 H and 13 C NMR spectra were collected using a Varian VXR 200 MHz spectrometer, working at 199.958 MHz for 1 H and 50.294 MHz for 13 C, using CDCl3 as solvent. Residual hydrogens of the solvent were employed as reference and chemical shifts were referred to tetramethylsilane (TMS).
2.3.2. FTIR spectroscopy Fourier transform infrared (FTIR) spectroscopy was performed with a Shimadzu FTIR spectrometer model IRAffinity-1, using either NaCl disks or a Specac Golden Gate single reflection diamond ATR (attenuated total reflectance) accessory. Polymer films were cast on NaCl disks using CHCl3 as solvent. ATR accessory was used to collect the infrared spectra directly on neat polymers. Reflection FTIR spectra were recorded also on coated bronze coupons using a portable Bruker Alpha spectrophotometer (OPUS Software) with spectral range 400–7000 cm−1 , resolution 4 cm−1 , background on gold plate, 100 scans, spot size 3.4 cm × 1.4 cm. Preliminary characterization of the patina was performed collecting some spectra on micro-samples taken from the surface of the specimens with a continuum infrared microscope linked to a Nicolet Nexus FTIR spectrometer, with a spectral resolution of 4 cm−1 (128 scans) in transmission mode. The samples were flattened on a KBr support.
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2.3.3. UV–vis spectroscopy UV–vis spectra were collected with a UV–vis detector Waters model 2489. CHCl3 solutions (1.5 mg of polymer in 5 mL of solvent) and quartz cells with 1 cm path length were used.
2.3.4. Gel permeation cromatography Gel permeation chromatography (GPC) was performed using a Waters system equipped with a pump Waters model Binary HPLC 1525, and a refractive-index detector Wyatt Optilab T-rEx. The GPC experimental conditions were the following: three columns Shodex KF-803 (length: 300 mm, diameter: 8.0 mm), mobile phase: stabilized THF, temperature: 30 ◦ C and flow rate: 1.0 mL/min. The solution injected was 50 L (concentration 10 mg of polymer in 1 mL of THF) and the retention times were converted to molecular weights and their distribution using a reference curve obtained with polystyrene standards.
2.3.5. Differential scanning calorimetry Differential scanning calorimetry (DSC) was performed with a Perkin Elmer instruments model Pyris 1 DSC equipped with a Intracooler 2P cryogenic system. Traces were recorded in the temperature range from 0 to 200 ◦ C under a nitrogen atmosphere, using an heating rate of 20 ◦ C/min. To eliminate any effect of thermal history, measurements were carried out from a second heating cycle, after a first heating of the sample to 200 ◦ C at 20 ◦ C/min, followed by quenching to 0 ◦ C.
2.3.6. Colorimetry Colorimetric measurements were performed on bronze coupons before and after treatments with the protective products using a KONICA–MINOLTA CM2600d spectrophotometer with the CIE 1976 (L*a*b*) colour space (CIELAB) representation, according to the UNI EN 15886/2010 protocol, [23] with the standard illuminant D65 and observer 10◦ . The colour coordinates L*, a* and b* were recorded on a selected point on each coupon (∅ ∼ 6 mm). Five measurements were taken by repositioning the instrument on the same spot each time, and then the data were averaged. The spots were localized using a transparent plastic mask delimiting the measurement areas, with holes over the spots exactly fitting the colorimeter probe. Instrument was calibrated against a SPECTRALON® prior to each measurement.
2.3.7. SEM-EDS Morphology and distribution of protective products on the metal surface were investigated by a scanning electron microscope SEM–EDS (Cambridge Stereoscan 440 LEICA coupled with an EDS microprobe, Oxford Instruments, model INCA), keeping a low voltage (10 keV) to avoid any damage of the coatings.
2.3.8. Thermo-hygrometric ageing chamber An Angelantoni Challenge 500 climatic chamber, was employed for accelerated thermo-hygrometric ageing where temperature and relative humidity were changed according to the cycle reported in Table 1.
2.3.9. Solar Box Photochemical ageing was run using a Solar Box CO.FO.ME.GRA model 3000e equipped with a Xenon-arc lamp and an outdoor type UV filter with cut-off <290 nm to eliminate radiation not present in the external sunlight. According to the ISO 1134/2004 protocol, [24] irradiance was kept at 550 W/m2 and black standard temperature (BST) at 65 ± 2 ◦ C.
Table 1 Thermo-hygrometric cycle employed in climatic chamber. T (◦ C) 0 10 20 40 60 40 20 10 0
RH (%)
Time (min)
85 85 65 45 30 45 65 85 85
60 60 60 60 60 60 60 60 60
T, temperature; RH, relative humidity; heating rate: 1 ◦ C/min.
2.4. Solubility The solubility of fresh (un-aged) polymers was investigated in water, as well as in various traditional solvents (acetone, 2butanone, chloroform, tetrahydrofuran) and eco-friendly organic solvents (1,3-dioxolane, ethyl lactate, butyl lactate). A prefixed mass of polymer (∼25 mg) was weighed in a glass tube. Then the solvent (0.5 mL) was added and the tube containing the solution was capped and placed in an ultrasonic bath for 15 min at room temperature to promote the polymer dissolution. The solubility of polymers was considered complete when a transparent solution was obtained. Otherwise, the test tube was centrifuged for 10 min at 3500 rpm. Supernatant was recovered with a pipette. The solvent was then removed and the mass of the soluble fraction of polymer ms [g] was determined. Solubility S [g/L] was given by the following relation: ms S= [g/L] 5 × 10−4 where ms was the mass of polymer (g) dissolved in the solvent tested. 2.5. Product application The PLA + BTA blend was prepared using 4.7 g of PLA and 0.3 g BTA and the mixture was dissolved in CHCl3 (5%, w/v). The BTA content in the blend was the same of commercial Incralac® , a solvent-based acrylic resin widely employed for conservation purposes. In the same way a CHCl3 solution of one of the copolymers between PLA and BTA (PLLA-BT5 , see results for further details, 6%, w/v) was prepared. Solutions of polymers were applied by brush on one of the wider surfaces of the bronze coupons. Friendly solvents such as lactate or acetate esters may substitute chloroform in outdoor applications. The amount of polymer applied on each bronze coupon was determined by weight difference and reported in Table 2. Solubility, weight loss and molecular weight changes of functionalized polymers during accelerated ageing tests, were Table 2 Amount of polymer applied on patinated and bare bronze coupons. Code
Treatment
Amount (mg)
g/m2
Patinated coupons employed for accelerated thermo-hygrometric ageing PLA + BTA 8.6 PAPMNIA PLA + BTA 10.9 PAPMNIB PAPFNIA PLLA-BT5 5.6 PLLA-BT5 5.9 PAPFNIB
3.7326 4.7309 2.4306 2.5608
Bare coupons employed for accelerated photochemical ageing NSPFNIA PLLA-BT5 8.4 PLA + BTA 4.0 NSPMNIA
7.2917 3.4722
PA, patinated bronze coupon, ageing in climatic chamber; PM, PLA + BTA treatment; PF, PLLA-BT5 treatment; NIA/NIB, no inhibitor, replica A/B; NS, bare bronze, ageing in Solar Box.
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evaluated using glass slides (38 mm × 26 mm) preliminarily washed with acetone and stored in a desiccator up to constant weight. Polymer solutions in CHCl3 (4%, w/v) were cast on the glass surface, and after evaporation of solvent, slides were kept in a desiccator up to constant weight and amount of polymer applied was determined by weight difference. 2.6. Accelerated ageing Treated bare bronze samples, treated glass slides and treated NaCl disks were submitted to photochemical ageing up to 965 h in the Solar Box.
Table 3 Selected chemical characteristics of functionalized PLAs. Code
Lactide configuration
BTA/lactide (molar ratio)
Mw a (g/mol)
PDIb
PLAc PLLA-BT1 PLLA-BT3 PLLA-BT5 d PLLA-BT5 e PLLA-BT7 PLDA-BT5
L L L L L L rac
– 1:100 3:100 5:100 5:100 7:100 5:100
180,300 39,500 11,900 8,000 10,400 6,600 5,400
1.7 1.2 1.2 1.8 1.7 1.3 1.1
a b c
2.6.1. FTIR spectra Polymers were placed on NaCl disks and spectra were recorded after 250, 465, and 965 h of exposure, as well as solubility, weight and molecular weight changes of coatings cast on glass slides. 2.6.2. Colorimetric data and reflectance FTIR spectra These analyses were performed on treated bronze coupons after different time intervals up to 965 h. Treated patinated bronze coupons were also submitted to 50 thermo-hygrometric cycles in a climatic chamber (Table 1), checking the behaviour of the applied polymers with colorimetric and reflectance FTIR measurements after 25 and 50 cycles.
d e
3. Results and discussion 3.1. End-capped polymers: synthesis and characterization End capped PLAs were synthesized through Sn(Oct)2 -catalyzed ROP of l-lactide and rac-lactide (the racemic mixture of l and d isomers) through a coordination-insertion mechanism [25,26]. Using commercial HO-BT as initiator, a series of functionalized polymers, containing the BT group at the end of the PLA chain was obtained, as listed in Tables 3 and 4. Fig. 2 shows the reaction scheme employed for the syntheses of polymers. The growth of the polymer chain and the BT insertion were monitored by NMR analysis (1 H, 13 C NMR) on the basis of the signal attributed to CH3 of PLA moiety and to CH2 O of BT moiety, respectively. NMR resonances of PLLA-BT5 were reported as an example (Fig. 3), the spectra of the other end-capped PLAs show the same resonances. The 1 H NMR spectrum show ı (CHCl3 ) at: 8.10 (d, 1H, H4-BT, 3 JHH = 9.2 Hz), 7.70 (d, 1H, H7-BT,
Mw , weight average molar mass. PDI, polydispersity index. Sample used to prepare PLA + BTA blend. Polymer employed for photochemical ageing. Polymer employed for bronze coating.
Table 4 Selected physical characteristics of functionalized PLAs. Code a
PLA PLLA-BT1 PLLA-BT3 PLLA-BT5 b PLLA-BT5 c PLLA-BT7 PLDA-BT5 a
2.6.3. Weight loss, solubility and molecular weight determinations on aged polymers After exposure in Solar Box, the coated glass slides were stored in a desiccator until a constant mass was reached and the weight loss due to the ageing process was determined. The CHCl3 solutions were evaporated and the polymeric residues weighed. The slides were washed with CHCl3 to dissolve all soluble polymers and weighed again. In order to confirm these data, the polymer residues eventually remained on the slides were calculated by weight difference. Molecular weight of the soluble fraction was evaluated by GPC.
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b
Tg (◦ C)
Tc (◦ C)
64 58 53 52 56 45 38
121 109 104 101 94 99 –
Tm1 (◦ C)
148 147 147 134
Tm2 (◦ C) 174 171 160 158 159 151 –
Sample used to prepare PLA + BTA blend. Polymer employed for photochemical.
3J HH
= 9.2 Hz), 7.57 (dd, 1H, H6-BT, 3 JHH = 9.2 Hz, 3 JHH = 7.6 Hz), 7.43 (dd, 1H, H5-BT, 3 JHH = 9.2 Hz, 3 JHH = 7.6 Hz), 6.69 (d, 1H, N-CH2 -BT, 2J 2 HH = 11.2 Hz), 6.57 (d, 1H, N-CH2 -BT, JHH = 11.2 Hz), 5.16 (q, 1H, CH-PLA, 3 JHH = 7.2 Hz), 1.58 (d, 3H, CH3 -PLA, 3 JHH = 7.0 Hz) ppm (integrals attributed to BT moiety are not correlated with those of the PLA group) and in 13 C NMR spectrum at ı: 169.57 (s, COPLA), 145.50 (s, 1C, C8-BT), 132.26 (s, 1C, C9-BT), 128.64 (s, 1C, C6-BT), 124.75 (s, 1C, C5-BT), 120.22 (s, 1C, C4-BT), 110.00 (s, 1C, C7-BT), 69.00 (s, 1C, CH-PLA), 67.70 (s, 1C, N-CH2 -BT), 16.53 (s, 1C, CH3 -PLA) ppm. Signals of the CH3 and CH groups of PLA moiety were present at 1.58 and 5.16 ppm, respectively, as reported in the literature. The resonance of the CH2 O group of BT moiety in 1 H NMR spectrum was shifted from 6.00 ppm of the starting HO-BT to 6.63 ppm in PLLABT5 , confirming the presence of the new ester linkage between PLA and BT. Furthermore the CH2 O group of benzotriazole moiety shows a singlet in HO-BT while an AB coupling was shown in the end-capped PLA polymer. This splitting is attributed to the two hydrogens that are not magnetically equivalents for the presence of the chiral CH groups of the PLA moiety in the molecule. This coupling between geminal hydrogens was confirmed by a gcosy-nmr experiment (Fig. 4) and it is a strong support to the presence of end-capped BT moiety in PLA. FTIR absorptions attributed to BT moiety were not detected when low amount of BT was present in the polymer, such as in PLLA-BT1 , PLLA-BT3 . On the contrary, C N stretching (1005 cm−1 ) and CH out-of-plane bending for the benzene ring
Fig. 2. Reaction scheme for the synthesis of end-capped PLAs.
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Fig. 3.
1
H and 13 C NMR spectra of PLLA-BT5 (CDCl3 solution).
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Fig. 5. UV–vis spectra of PLA (continuos line) and end-capped PLLA-BT5 (dotted line).
Fig. 4. Gcosy NMR spectrum of PLLA-BT5 in the 6.0–7.0 ppm region.
of BT (777 cm−1 ) were shown in higher BT containing polymers. The intensity of these bands became stronger as the BT content increases. UV–vis spectra of end-capped PLAs showed strong changes with respect to neat PLA, because the BT moiety is an UV–vis chromophore (Fig. 5). In particular, two main peaks at 252 and 283 nm due to the presence of BT were detected, beside the characteristic
absorption of PLA below 250 nm. The intensities of these absorptions were linearly increased with BT content in polymers. The weight-average molar mass (Mw ) and the polydispersity (PDI) of polymers were determined by GPC. Since HO-BT works as initiator for the polymerization it leads to a decrease in Mw , as previously reported for other functionalized PLAs [21,22] and the BT moiety is inserted at the end of the polymer chain. Endcapped polymers with comparatively higher Mw were obtained using lower co-initiator/monomer molar ratio, Mw values decrease when increasing the initiator content. Differential scanning calorimetric analysis (DSC) showed that samples obtained using l-lactide (PLLA-BT1 , PLLA-BT3 , PLLA-BT5 , PLLA-BT7 ) are semi-crystalline, while sample (PLDA-BT5 ) obtained using rac-lactide is amorphous. The presence of a BT moiety in the polymer significantly decreases Tg and, when present, Tm of the polymers (Table 3), as a consequence of the decrease of Mw . A feature of interest of PLLA-BT3 , PLLA-BT5 and PLLA-BT7 is the
Fig. 6. Change of colorimetric coordinates after thermo-hygrometric ageing cycles in a climatic chamber.
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Table 5 Solubility [g/L] of end-capped PLA-BTa in several solvents. code
H2 O
CHCl3
Acetone
MEK
THF
DIOX
EL
BL
PLLA-BT1 PLLA-BT5 PLLA-BT7
0 0 0
50 50 50
5 30 50
5 15 n.d.
50 50 50
5 25 50
0 10 n.d.
0 10 n.d.
a
MEK, butan-2-one; THF, tetrahydrofuran; DIOX, 1,3-dioxolane; EL, ethyl lactate; BL, butyl lactate.
presence of double melting peaks. This behaviour is a common phenomenon observed by DSC for PLAs, which have been proposed to arise from ␣- and ␣ -crystalline forms of the polymer. Various explanations have been presented to explain this behaviour, [27–30] such as a melting-recrystallization-melting process, as well as the formation of two population of crystallites with different thicknesses. However up to now none of these explanations have been fully confirmed. End-capped polymers were soluble in organic solvent such as acetone, THF, CHCl3 , lactate or acetate esters and solubility generally increased as Mw decreased Table 5. Among the synthesized polymers, PLLA-BT5 showed the most attractive characteristics for coating purposes, in terms of solubility and film formation properties. An efficient film formation may be attributed to the partial loss of the crystalline structure of the polymer. Furthermore its Tg value (52–56 ◦ C) is slightly upon room temperature, improving the flexibility of the polymer.
PLLA-BT5 was selected to test its performances as protective coating for bronze surfaces because the amount of BT was very close to that present in commercial formulations even if its Tg was not very high. 3.2. Evaluation of polymers on metallic surfaces Solutions of PLLA-BT5 and the blend PLA + BTA were applied by brush on both patinated and bare bronze surfaces in order to assess their performances as protective coatings. Colour change tests and SEM-EDS observations were carried out on treated coupons, then selected samples were submitted to artificial ageing and measurements were repeated over time in order to evaluate the polymers performance. 3.2.1. Colour changes Both treatments with functionalized polymer and PLA + BTA blend caused a perceptible colour change of the bare bronze surfaces (E* ∼ 4–10), which was higher for patinated coupons (E* ∼ 15–30), probably due in the latter case to the partial removal of the green patina exerted by the mechanical action of brushing. On bare bronze coupons, colour variation induced by PLA + BTA (E* ∼ 4) was due to darkening and yellowing, whereas for PLLABT5 (E* ≥ 10) was mainly due to strong yellowing and, at a minor extent, darkening and increase of the red component (Fig. 6).
Fig. 7. Surface of a patinated coupon treated with PLA + BTA.
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Fig. 8. Surface and elemental analysis of a patinated sample treated with PLLA-BT5 .
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3.2.2. SEM-EDS PLLA-BT5 and PLA + BTA coatings were investigated through SEM-EDS to show the morphology of the film applied. SEM investigations were carried out on patinated coupons only, where the superficial film of PLA was visible and appeared almost homogeneously distributed for both PLLA-BT5 and PLA + BTA (Figs. 7 and 8). 3.3. Accelerated ageing Stability of polymers was investigated by photochemical ageing up to 965 h in the Solar Box and thermo-hygrometric ageing in the climatic chamber up to 50 cycles. Photochemical ageing of coatings was monitored by FTIR, GPC, solubility and weight changes during the exposure. Moreover colour changes were measured on bare bronze coupons treated with the products at different irradiation times. Thermo-hygrometric ageing was monitored by reflectance FTIR and colour changes measurements on patinated bronze specimens. 3.3.1. Thermo-hygrometric ageing (climatic chamber) 3.3.1.1. FTIR analysis. The reflectance FTIR spectrum of PLLABT5 acquired directly on the treated bronze specimens did not show any significant changes up to 50 cycles in the climatic chamber. On the contrary, in the blend PLA + BTA the typical bands of BTA at 1622 and 1009 cm−1 became undetectable after 25 thermo-hygrometric cycles (Fig. 9) suggesting a possible sublimation of this compound and consequently loss of the coating’s anticorrosive agent. Furthermore PLA when applied on bronze samples showed a shift of some bands towards higher wavenumbers, i.e. bands at 1756–1777 cm−1 , 1724 sh–1747 cm−1 and 1215–1222 cm−1 . In the course of the ageing process, a shift towards lower wavenumbers was observed for absorptions at: 1777–1770 cm−1 , 1747–1741 cm−1 , 1222–1215 cm−1 . Also a modification of the crystalline phases of PLA [31] was shown in the ageing process by change of the relative intensities of the bands at 871 and 756 cm−1 . Thermo-hygrometric ageing did not induce any significant effect on the molecular structure of the polymer. 3.3.1.2. Colour changes. Fig. 10 reports the colour changes of two patinated bronze coupons treated with PLLA-BT5 and PLA + BTA before treatment, after treatment and after 50 thermohygrometric cycles of ageing. In the course of the ageing test, bronze samples treated with PLLA-BT5 and PLA + BTA showed the recovery of
Fig. 9. Reflectance FTIR spectra of a patinated specimen treated with (a) neat PLA and (b–d) PLA + BTA after different ageing cycles in the climatic chamber: (b) 0 cycles; (c) 25 cycles; (d) 50 cycles.
lightness, which had been reduced by polymer application by brushing, as previously discussed (Fig. 6). A similar trend in the variation of L*a*b* parameters has been observed in ageing tests performed on other end-capped PLA polymers, such as fluorinecontaining PLA applied as water-repellent protective coating on stone samples [20]. This behaviour was attributed to a possible reorganization of the coating material as a result of exposition at temperature up to its Tg , modifying the micro-roughness of the polymeric film and consequently white light scattering. Moreover, as a possible result of the above mentioned polymer’s reorganization, the red-green component (a*) remained almost comparable with that of treated coupons, whereas a certain regression to initial values was detected through ageing for the yellow-blue component (b*) (Fig. 6). 3.3.2. Photochemical ageing (Solar Box) 3.3.2.1. FTIR analysis. FTIR spectra of the end-capped polymer (PLLA-BT5 ) and PLA + BTA blend showed the appearance of a weak absorption at 1843 cm−1 from the beginning of the exposure in the Solar Box. This absorption in the carbonyl stretching frequency region, at a higher frequency than the main ester carbonyl absorption of PLA (1758 cm−1 ) can be attributed to the asymmetric stretching of anhydride functional group and was also observed in ageing tests performed on PLA and other PLA-based polymers
Fig. 10. Visible changes of two patinated bronze coupons after treatment (PLLA-BT5 : PAPFNIA; PLA + BTA: PAPMNIB) and after 50 thermo-hygrometric cycles in climatic chamber
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Fig. 11. Reflectance FTIR spectra in the 2000–1650 cm−1 range of a bare specimen treated with PLA + BTA, after different photochemical ageing times: (a) 0 h; (b) 250 h; (c) 465 h; (d) 965 h.
[21,32–34] The formation of anhydride during the photochemical ageing might be explained through a solid state reaction of traces of lactic acid, leading to the formation of lactic acid anhydride [21] or, as recently proposed, by photo-oxidation of the PLA moiety through a radical peroxide mechanism [32–34]. FTIR spectrum of PLLA-BT5 showed also an increase of the intensity of the OH stretching band (3500 cm−1 ) and a broadening of the carbonyl peak with formation of a shoulder at 1710 cm−1 during ageing, as well as some intensity changes in the CO region (1185–1092 cm−1 ). The new shoulder at 1710 cm−1 may be attributed to a possible hydrolysis of the ester linkage with formation of free carboxylic groups as reported by Agarwal
901
Fig. 12. Reflectance FTIR spectra of bare bronze coupons treated with PLLA-BT5 after different photochemical ageing times: (a) 0 h; (b) 250 h; (c) 465 h; (d) 965 h.
[35]. FTIR spectrum on coupons treated with PLA + BTA showed a shift towards higher wavenumbers for some bands after 250 h: 1763–1772 cm−1 , 1097–1109 cm−1 (Fig. 11). Probably, the exposure in the Solar Box induced partial de-polymerization and oxidation of the polymers, as suggested by GPC measurements. No changes in the spectral pattern were detected for coupons treated with PLLA-BT5 . The lower stability of the blend PLA + BTA (Fig. 12) compared to the functionalized polymer PLLA-BT5 where a BT moiety was chemically bonded to the PLA could be related to the above mentioned sublimation of the UV-absorber BTA.
Fig. 13. Change of colorimetric coordinates after photochemical ageing in a Solar Box. Differences with respect to untreated coupons were reported.
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Table 6 Molecular weight and PDI of end-capped PLLA-BT5 at different ageing times. Ageing time (h) Mw (g/mol) PDI
0 8,000 1.8
250 7,900 1.7
500 7,800 1.7
1000 6,500 1.7
3.3.2.2. Solubility, weight loss and molecular weight changes. Polymer remained still completely soluble in chloroform at different ageing times (250, 500, 1000 h). These data are an indication that cross-linking processes did not take place and strongly support the possibility to remove the polymer at the end of its service life. A slight de-polymerization was evidenced during ageing by GPC analysis (Table 6), as well as a slight diminution of the amount of the polymer film applied on glass slices, confirming the excellent stability of these polymers even on the bronze surface. 3.3.2.3. Colour changes. A different colorimetric behaviour was observed for the two polymers applied on bare coupons and aged in Solar Box. Indeed, for PLLA-BT5 the photochemical ageing allowed the regression of colorimetric coordinates to the initial values with consequent lowering of the E* induced by the polymers application. This behaviour was not ascribable to molecular changes, as assessed by FTIR analysis, but probably to the polymer’s physical reorganization previously hypothesized and due to the permanence in the Solar Box where the temperature was slightly higher than Tg of the polymer. Instead, a further increase of the E* was observed for PLA + BTA, mainly due to the hue shift to blue-green (reduction of a* and b*) (Fig. 13). 4. Conclusion Some new renewable and biocompatible polymers were obtained from lactic acid, a natural resource produced by fermentation of sugars. Polymers were synthesized using commercial 1 H-benzotriazole-1-methanol as initiator in the Ring Opening Polymerization of l- or rac-lactide catalyzed by Sn(Oct)2 . The reaction was performed without solvent following a green chemical approach. Benzotriazole group was chemically bonded on the terminal position of the poly(lactic acid) chain, and polymers were characterized by NMR, FTIR, GPC, and DSC. One end-capped poly(lactic acid) PLLA-BT5 was selected and applied on bronze coupons to evaluate colour changes of the bare or patinated bronze surface. Stability of coating treatments on both bare and patinated bronze coupons were evaluated after photochemical ageing in Solar Box and accelerated thermo-hygrometric ageing in a climatic chamber. End-capped polymer showed a very high stability, whilst the blending of PLA + BTA showed a progressive leakage of the BTA in the course of the ageing process and molecular changes of the polymer. Taking into account that remarkable changes were not observed in the colours of treated surfaces after thermo-hygrometric and photochemical ageing, the end-capped polymers showed promising results as protective coating, for the conservation of metals. However further study will be carried out for a full evaluation of their efficiency as anticorrosive coatings trying to obtain evidence on interactions between copper and BTA as well as on the inhibition of corrosion phenomena. The relatively low Tg of the polymer may preclude some outdoor applications of these protective coating, however new PLA co-polymers having a high Tg may be synthesized following the same synthetic strategy. The work is in progress and results will be reported in a forthcoming paper. End-capped polymers are an eco-friendly material for metal surface protection because they were obtained through a green process, friendly solvents may be used for their application and they avoid BTA dispersion in the environment.
Further studies are in progress to compare performances of these materials with those of commercial products usually employed in the protection of bronze surfaces. Acknowledgements The authors thank the European Community and Regione Toscana for financial support through the POR FESR program, TeCon@BC project (code CP 57476), Andrea Pedna and Alfredo Maione (University of Florence) are acknowledged for the synthesis of some end-capped polymers and Mara Camaiti (IGAG-CNR) for assistance in the thermo-hygrometric and photochemical ageing of bronze coupons. References [1] M. Marabelli, Conservazione e restauro dei metalli d’arte, Bardi Editor, 2007. [2] P. Letardi, I. Trentin, G. Cutugno, Monumenti in bronzo all’aperto. Esperienze di conservazione, Nardini Editor, 2004. [3] E. Cano, D. Lafuente, D.M. Bastidas, J. Solid State Electron. 14 (2010) 381–391. [4] E. Joseph, P. Letardi, R. Mazzeo, S. Prati, M. Vandini, Innovative treatments for the protection of outdoor bronze monuments”, in: Proceedings of Metal 07, Triennial Meeting of ICOM-CC Metal Working Group, Amsterdam, Netherlands, September 17–21, 2007, pp. 71–77. [5] R. Mazzeo, S. Bittner, G. Farron, R. Fontinha, D. Job, E. Joseph, P. Letardi, M. Mach, S. Prati, M. Salta, A. Simon, Development and evaluation of new treatments for outdoor bronze monuments, in: J.H. Townsend, L. Toniolo, F. Cappitelli (Eds.), Conservation Science 2007, Archetype, London, 2008, pp. 40–48. [6] E. Joseph, A. Simon, S. Prati, M. Wörle, D. Job, R. Mazzeo, Anal. Bioanal. Chem. 399 (2011) 2899–2907. [7] C.V. Horie, Material for Conservation. Organic Consolidants, Adhesives and Coatings, Butterworths, London, 1991. [8] L.B. Brostoff, E.R. de la Rie, Chemical characterization of metal/coating interfaces from model samples for outdoor bronzes by reflection-absorption infrared spectroscopy (RAIR) and attenuated total reflection spectroscopy (ATR), in: Proceedings of the International Congress METAL 1998, James & James Ltd, 1998, pp. 320–328. [9] M. Marabelli, G. Napolitano, Materiali e strutture 1 (1991) 51–56. [10] G. Bierwagen, T.J. Shedlosky, L.A. Ellingson, Electrochemical studies of the protection of bronzes from corrosion by organic coatings, in: I. MacLeod, J.M. Theile, C. Degrigny (Eds.), Metal 2001: Proceedings of the international conference on metals conservation, Santiago, Chile, April 2001, Western Australian Museum, Welshpool, 2004, pp. 291–296. [11] D. Erhardt, W. Hopwood, T. Padfield, N.F. Veloz, Durability of Incralac, Examination of a Ten Year Old Treatment, Preprints ICOM (International Council of Museums) Conservation Committee, Copenhagen Conference, 10–14 September, 1984, pp. 22.1–22.3. [12] L.B. Brostoff, Investigation into the interaction of benzotriazole with copper corrosion minerals and surfaces, in: Ian D. MacLeod, et al. (Eds.), Proceedings of the International Conference on Metals Conservation Metal 95, ICOM-CC Metals Working Group, Semur-en-Auxois, France, 25–28 September, James & James, Ltd, London, 1997, pp. 99–108. [13] M. Finˇsgar, I. Miloˇsev, Corros. Sci. 52 (2010) 2737–2749. [14] H. Brinch Madsen, Stud. Conserv. 12 (1967) 163–167. [15] X. Wu, N. Chou, D. Lupher, L.C. Davis, Benzotriazoles: toxicity and degradation, in: Proceedings of the Conference on Hazardous Waste Research, Snowbird, UT, 18–21 May, 1988, pp. 374–382. [16] I.M. Marcelli, M. Mercalli, The conservation of outdoor bronze monuments: the restoration project for the Monumento ai Mille by Eugenio Baroni, 1915, Genoaquarto dei mille, in: 9th International Conference on NDT of Art, Jerusalem, Israel, 25–30 May, 2008. [17] C.J. McNamara, M. Breuker, M.T.D. Perry, R. Mitchell, J. Cult. Herit. 5 (2004) 361–364. [18] G. Giachi, M. Frediani, L. Rosi, P. Frediani, Synthesis and processing of biodegradable and bio-based polymers by microwave irradiation, in: U. Chandra (Ed.), Microwave Heating, InTech, 2011, pp. 181–206 http://www.intechopen.com/books/microwave-heating (accessed 12.08.13). [19] G. Giuntoli, A. Pedna, M. Frediani, L. Rosi, P. Frediani, “New perspectives for the application of PLA in cultural heritage”, in: V. Piemonte (Ed.), Polylactic Acid: Synthesis, Properties and Applications, NovaPublisher, USA, 2012. [20] G. Giuntoli, L. Rosi, M. Frediani, B. Sacchi, P. Frediani, J. Appl. Polym. Sci. 125 (2012) 3125–3133. [21] M. Frediani, L. Rosi, M. Camaiti, D. Berti, A. Mariotti, A. Comucci, I. Malesci, Macromol. Chem. Phys. 211 (2010) 988–995. [22] M. Frediani, D. Sémeril, D. Matt, L. Rosi, P. Frediani, F. Rizzolo, A.M. Papini, Int. J. Polym. Sci. (2010) 1–6. [23] UNI EN 15886/2010. Conservation of cultural property – test methods – colour measurement of surfaces. [24] ISO Protocol 11341/2004. Paint and varnishes – artificial weathering and exposure to artificial radiation – exposure to filtered xenon-arc radiation.
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