Analytical assessment to develop innovative nanostructured BPA-free epoxy-silica resins as multifunctional stone conservation materials

Science of the Total Environment 645 (2018) 817–826

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Analytical assessment to develop innovative nanostructured BPA-free epoxy-silica resins as multifunctional stone conservation materials Olivia Gómez-Laserna a,⁎, Paola Lanzafame b, Georgia Papanikolaou b, María Ángeles Olazabal a, Sandra Lo Schiavo c, Paola Cardiano c a

Department of Analytical Chemistry, University of the Basque Country (EHU/UPV), Barrio Sarriena s/n, E-48080, Leioa, Bilbao, Spain Department of Mathematical and Informatics Sciences, Physics and Earth Sciences, University of Messina, and INSTM CASPE (Laboratory of Catalysis for Sustainable Production and Energy), Viale F. Stagno d'Alcontres 31, I-98166 Messina, Italy c Department of Chemical, Biological, Pharmaceutical and Environmental Sciences, University of Messina, Viale F. Stagno d'Alcontres 31, I-98166 Messina, Italy b

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Sustainable BPA-free epoxy-silica resins for stone conservation from CHDM-DGE • Raman spectroscopy as a powerful tool to monitor the reactions • GPTMS as coupling agent to enhance the organic/inorganic interphase bonding

a r t i c l e

i n f o

Article history: Received 4 April 2018 Received in revised form 2 July 2018 Accepted 14 July 2018 Available online xxxx Keywords: Hybrid materials Epoxy resins Nanotechnology Conservation Alkoxysilanes BPA-free

a b s t r a c t Bisphenol A (BPA)-free epoxy resins, synthesized from low molecular weight cycloaliphatic compounds, may represents promising materials for stone conservation due to their very appealing and tunable physicochemical properties, such as viscosity, curing rate and penetration ability, being also easy to apply and handle. Furthermore, alkoxysilanes have been widely employed as inorganic strengtheners since they are easily hydrolysed inside lithic substrates affording Si\\O linkages with the stone matrix. Taking into account the advantages of these two classes of materials, this work has been focused on the development of innovative conservation materials, based on hybrid epoxy-silica BPA-free resins obtained by reaction of 1,4-cycloexanedimethanol diglycidylether (CHDM-DGE) with various siloxane precursors, i.e. glycidoxypropylmethyldiethoxysilane (GPTMS), tetraethyl orthosilicate (TEOS) and isobutyltrimethoxysilane (iBuTMS), using the 1,8-diaminooctane (DAO) as epoxy hardener. Thanks to Raman spectroscopy the synthesis processes have been successfully monitored, allowing the identification of oxirane rings opening as well as the formation of the cross-linked organicinorganic networks. In accordance with the spectroscopic data, the thermal studies carried out by TGA and DSC techniques have pointed that GPTMS is a suitable siloxane precursor to synthesize the most stable samples against temperature degradation. GPTMS-containing resins have also shown good performances in the dynamic mechanical analysis (DMA) and in contact angle investigations, with values indicating considerable hydrophobic properties. SEM analyses have highlighted a great homogeneity over the entire observed areas, without formations of clusters and/or aggregates bigger than 45 μm, for the cited materials, confirming the efficiency of GPTMS as coupling agent to enhance the organic/inorganic interphase bonding. The variations provided by the incorporation of nanostructured titania, specifically synthesized, inside the epoxy-silica hybrids have been also

⁎ Corresponding author. E-mail address: [email protected] (O. Gómez-Laserna).

https://doi.org/10.1016/j.scitotenv.2018.07.188 0048-9697/© 2018 Elsevier B.V. All rights reserved.

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evaluated. According to all the collected results, the hybrid materials here reported have proven to be promising multifunctional products for potential application in the field of stone conservation. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Historical lithic materials are exposed to a permanent and irreversible deterioration induced by various physical, chemical and biological processes, both of natural and anthropogenic origin. The efforts made by the scientific community to face this problem have clarified the chemical reactions occurring between the stone materials and the exposure environment, establishing also the processes responsible for the different degradation phenomena that affect the long-term conservation of stone (Doehne & Price, 2010). Once the weathering has started, the stones should be treated with products able to improve their resistance against decay, by the employment of formulations with consolidating, water repellent and biocidal properties. Accordingly, these materials should ensure good adhesion and intergranular cohesion within the lithic matrix, as well as the protection of the stone against the harmful action of water, atmospheric pollutants, particulate matter and thermal stress, among others (Princi, 2014; Gómez Laserna et al., 2013; Gómez Laserna et al., 2015). In addition, the design of advanced conservation products requires compliance with eco-sustainability criteria. This point is crucial, not only to respect the health and the environment, but also to avoid the drawbacks arising from the use of nontested materials as found for several products largely employed in the past for restoration activities. In this sense, during the last decades, different conservation materials have been employed, which partially fulfill the above conditions. Among the most common organic polymers used in conservation it is worth to mention acrylic polymers, epoxy resins, polyurethanes and perfluoropolyethers (Doehne & Price, 2010). In general, they display good hydrophobic properties and, some of them, also consolidating capability. However, in the long term, their thermal and photochemical instability cause chromatic and mechanical alterations (Sadat-Shojai & Ershad-Langroudi, 2009). In contrast, silicabased materials obtained from alkoxysilanes and alkylalkoxysilanes in situ sol-gel reactions solve the above drawbacks, although they often display cracking upon drying thus reducing the consolidation efficiency and, due to their structural rigidity, show mechanical properties which are not compatible with the ones of weathered stones; in addition, the low molecular weight starting compounds are prone to evaporation before the polymerization process occurs inside the stone substrate (Scherer & Wheeler, 2009; Cardiano et al., 2003a). Nevertheless, a synthetic strategy can be adopted to overcome the limits featuring the materials used in stone conservation treatments; it consists in the development of organic-inorganic hybrid materials properly designed to minimize, as much as possible, the drawbacks connected to the employment of pure organic and inorganic materials (Kickelbick, 2014; Cardiano et al., 2003b; Cardiano et al., 2005). In this sense, epoxy resins display a set of characteristics such as good adhesive properties, high chemical resistance, in situ applicability and easy processability turning out to be highly attractive materials in the field of stone conservation (Jin, 2015). The synthesis of common epoxy resins has been developed on the use of Bisphenol A (BPA)-based diglycidyl ethers for the high reactivity of their oxirane groups towards nucleophilic compounds. However, due to the growing awareness of the influence of aromatic compounds on public health, as well as for the BPA similarity to estrogen 17beta-estradiol, imparting it the ability to act as an endocrine disruptor, their use has been discouraged (Chapin et al., 2008; Al-Saleh et al., 2017). Taking into account all of the above, the main objective of this work has been the development of innovative conservation products, based on new BPA-free epoxy-silica hybrid materials, that could combine the advantages coming from the use of both organic (flexibility,

processability, hydrophobicity, etc.) and inorganic (high mechanical strength, good chemical resistance, thermal stability, etc.) materials for stone protection and consolidation. For that purpose, 1,4 cyclohexanedimethanol diglycidyl ether (CHDM-DGE) has been selected as epoxy precursor to synthesize hybrid materials with suitable conservation properties, thanks to the insertion of low molecular weight silica precursors within the organic network through sol-gel technology (Pandey & Mishra, 2011). CHDM-DGE is characterized by two oxirane rings and, therefore, may provide a high degree of cross-linking to develop a series of resins that should display mechanical and thermal properties comparable to the ones featuring the conventional epoxy materials, based on Bisphenol A, such as 4,4′ isopropylidenediphenol diglycidyl ether (DGEBA). On the other hand, since CHDM-DGE is not an aromatic derivative, it can display the double advantage of being a more “sustainable” starting material, thus avoiding risks for human health and environment deriving from its use and, at the same time, less prone to yellowing. The absence of aromatic rings makes the resulting resins UV resistant and suitable for outdoor applications, while also reducing their viscosity, being the yellowing and the exfoliation due to sun rays the main drawbacks connected with DGEBA use in stone conservation (Cardiano, 2008). Since the properties of organic/inorganic hybrid networks strongly depend on the degree of phases dispersion which is, on turn, a function of various synthetic parameters (i.e. stoichiometric ratios, number of reactive groups, reactivity of cross-linking reagents, etc.), the epoxy-precursor has been crosslinked with a selected hardener, 1,8 diaminooctane (DAO), in the presence of different amounts of a series of silica-forming co-reactants, i.e. tetraethyl orthosilicate (TEOS), isobutyltrimethoxysilane (iBuTMS) and glycidoxypropylmethyldiethoxysilane (GPTMS) (Fig. 1).

Fig. 1. Reactants employed for the epoxy-silica hybrids syntheses.

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More in detail, GPTMS, being a bifunctional compound, may act as coupling agent between organic domains, i.e. via epoxy-cleavage reaction, and lithic substrate, by condensation of the Si\\OH moieties coming from alkoxysilane groups hydrolysis with the active sites of the stone. On the other hand, DAO may interact with both the GPTMS reactive functions: affording the basic conditions for a catalytic enhancement on the Si-OMe hydrolysis, and/or reacting with the oxirane rings of both GPTMS itself and of CHDM-DGE, with the formation of tertiary amines, thus favoring the resins cross-linking on the organic side. In addition, TEOS and iBuTMS have been introduced into the epoxy-amine reaction mixture with the aim of forming, in situ, inorganic Si\\O\\Si domains into the organic network, more compatible with the lithic substrate. Furthermore, nanoparticles (NPs) are currently employed in the design of materials for the maintenance of cultural heritage, opening new possibilities to develop conservation products with innovative functionalities. Accordingly, NPs are specifically used for cleaning, consolidation, protection or antimicrobial treatments (Sierra-Fernández et al., 2017; Karcı et al., 2017; Cappelletti et al., 2015; Pino et al., 2017). It is in fact well known that nanosized materials may display enhanced properties with respect to traditional products in terms of mechanical and thermal properties, reactivity, penetration depth inside a stone matrix, to name only a few. In this sense, innovative strategies focused on the insertion of NPs featured by antimicrobial activity, such us silver (Ag), copper (Cu), zinc oxide (ZnO) or titania (TiO2), into consolidants and water repellents, have led to nanocomposites successfully tested to decrease the degradation processes of stones exposed to the open air. In particular, nano-TiO2, in the anatase phase, has been the subject of many studies due to its strong photocatalytic activity, being considered as the most effective catalyst for the degradation of various pollutants that, at the same time, imparts to materials strong self-cleaning and antimicrobial properties against bacteria and fungi (Escherichia coli, Staphylococcus, etc.) (Bergamonti et al., 2017; Dorothé van der Werf et al., 2015; Aruoja et al., 2009). Bearing this in mind, in this study TiO2 NPs have been synthesized to be incorporated in the hybrid materials with the aim of developing multifunctional products of suitable compatibility with lithic substrates to protect the stone against the main causes of degradation (Mathiazhagan & Rani, 2011; Quagliarini et al., 2012). 2. Materials and methods 2.1. Materials All the chemicals employed for the syntheses have been purchased from SigmaAldrich, except 1,4 cyclohexanedimethanol diglycidylether

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CHDM-DGE from abcr GmbH, used as received without further purification. 2.2. Synthetic procedures 2.2.1. Sol-gel synthesis of TiO2 Titanium isopropoxide TTIP (0.5 g, 1.76 mmol) was mixed with isopropanol (3.16 g, 52.7 mmol) in a 1:30 ratio and left under stirring for 10 min. Then, the colloidal white suspension was added, drop by drop, to 19 g (176 mmol) of ultrapure water, in a TTIP/H2O ratio of 1:100. The reaction mixture was left on stirring for 30 min and afterwards centrifuged, at room temperature, for 15 min at 6000 rpm. The upper liquid phase was eliminated and the white solid such obtained was washed, once with isopropanol and three times with ethanol, then centrifuged after each cycle of washing employing the same procedure already mentioned. The white solid was allowed to dry overnight, at room temperature, and then left in an oven at 80 °C for 48 h. In order to obtain titania oxide in the anatase form, the sample was then thermally cured at 450 °C for 3 h after reaching the fixed temperature employing a heating rate of 2 °C min−1. 2.2.2. Epoxy-silica hybrid resins preparation The reaction mixtures were prepared starting from 5% w/w epoxycontaining chloroform solutions, with one or two epoxy compounds, depending on the specific sample. In detail, 1 and 2 samples were obtained dissolving 0.5 g (1.95 mmol) of 1,4 cyclohexanedimethanol diglycidylether (CHDM-DGE) in 10 mL of chloroform (CHCl3) under stirring, then adding 0.14 g (0.98 mmol) of the hardener, 1,8 diaminooctane (DAO). To two batches of the above solution, two different mixtures of tetraethyl orthosilicate (TEOS) and isobutyltrimethoxysilane (iBuTMS), which were in total the 10% w/w with respect to CHDM-DGE, were dropped, by varying the relative amounts of the two silica precursors. For 3 and 4 samples, the reaction mixtures were prepared by adding two different amounts of 3glycidoxypropylmethyldiethoxysilane (GPTMS) to two solutions of CHDM-DGE in CHCl3 with the aim of gaining two batches of solutions containing a total of 5% w/w epoxies differing for the relative amounts of the oxirane compounds. Then, after 10 min, the proper amount of DAO to react with both epoxides was added. A summary of the relative amounts of reactants and additives employed (Fig. 1) is reported in Table 1. The titania containing samples 1T and 2T were prepared by adding titania, in the anatase phase, to the solutions of CHDM-DGE, with a molar ratio of 1:9 with respect to the epoxy precursor, whereas 3T

Table 1 Relative amounts of the reactants employed for the syntheses of epoxy-silica resins with and without Titania.

1 2 1T 2T

3 4 3T 4T a b

CHDM-DGE

DAO

TEOS

iBuTMS

TiO2

0.50 g 1.95 mmol 0.50 g 1.95 mmol 0.50 g 1.95 mmol 0.50 g 1.95 mmol

0.14 g 0.98 mmol 0.14 g 0.98 mmol 0.14 g 0.98 mmol 0.14 g 0.98 mmol

25 × 10−3 ga

25 × 10−3 ga

–

40 × 10−3 ga

10 × 10−3 ga

–

25 × 10

−3

g

−3

a

25 × 10

40 × 10−3 ga

g

a

10 × 10−3 ga

17 × 10−3 g 0.22 mmolb 17 × 10−3 g 0.22 mmolb

CHDM-DGE

GPTMS

DAO

TiO2

0.25 g 0.98 mmol 0.40 g 1.56 mmol 0.25 g 0.98 mmol 0.40 g 1.56 mmol

0.25 g 1.06 mmol 0.10 g 0.42 mmol 0.25 g 1.06 mmol 0.10 g 0.42 mmol

0.10 g 0.75 mmol 0.12 g 0.89 mmol 0.10 g 0.75 mmol 0.12 g 0.89 mmol

–

The sum of the amounts of TEOS and ibuTMS is 10% w/w with respect to CHDM-DGE. The molar ratio of added titania is 1:9 with respect to the epoxy compounds.

– 27 × 10−3 g 0.34 mmolb 21 × 10−3 g 0.27·mmolb

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and 4T (containing two different oxiranic compounds) employing the cited ratio with respect to CHDM-DGE and 1:4.5 with respect to GPTMS, leaving the mixtures for 1 h under stirring prior to follow the procedure already described (Table 1). Then, all the samples were left into teflon Petri dishes capped with parafilm, in order to ensure a slow evaporation of the solvent, for one week. Once dried at room temperature, they were thermally cured in an oven at 65 °C for 48 h and subsequently removed from the teflon surface before characterization.

2.3. Instrumentation X-ray powder diffraction (XRD) analyses were employed for structural determination and estimation of TiO2 crystallite size. The equipment was a Bruker D2 Phaser desktop diffractometer, with a Cu tube (λ = 1.54056 Å), recording the data in the 2θ range of 20–90° with an angular step size of 0.025°. The average crystallite size of TiO2 was determined using the full-width at half-maximum of the peak corresponding  to 101 reflection, according to the Scherrer equation ( d ¼ kλ Bcosθ ) where d represents the grain size; k = 1 is the Scherrer constant related to the shape and index (hkl) of the crystals; λ is the wavelength of the X-ray (Cu Kα, 1.54056Å); θ is the diffraction angle of the peak; B stands for the full width at half-height of the peaks. Nitrogen isotherms at −196 °C were measured in a Quantachrome Autosorb iQ3 gas sorption analyzer, after degassing the samples under vacuum at 300 °C for 3 h to remove impurities. The Brunauer Emmett Teller (BET) method was applied to calculate the total surface area (SBET) using the adsorption data of 0.08 b P/P0 b 0.3. In this way, pore size distribution was obtained from the isotherm adsorption branches based on the Barrett–Joyner–Halenda (BJH) model. The total amount of N2 adsorbed at P/P0 = 0.95 was used to determine the total pore volume. Raman spectra of the samples were acquired by means of a portable B&WTEK-InnoRam spectrometer. The system works with a 785 nm excitation laser of variable power to control thermal decomposition. The microscopic analyses were performed by using optical lenses (20× and 50×), which allows measuring areas of diameters between 120 and 200 μm. A daily calibration was performed using the band at 520.5 cm−1 of a silicon chip. The signals were collected in a fix spectral range from 175 to 3000 cm−1 with a resolution of 3.5 cm−1, using integration times from 1 to 3 s and 15 to 30 accumulations to improve the signal to noise ratio. Data acquisition was carried out with the software B&WTEK Version 3.26 and the data were processed with the Omnic Version 7.2 software Thermo Nicolet. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) investigations were carried out on the cured films to determine the elemental distribution of the samples using an EVO®40 Scanning Electron Microscope (Carl Zeiss NTS GmbH) coupled to an X-Max Energy-Dispersive XRay spectroscopy equipment (Oxford Instruments). The samples were coated with a thin film of gold (b20 μm), to improve the conductivity signals, in an Emitech K550X sputter coater vacuum chamber (Quorum Technologies LTD). Then, SEM images were acquired at high vacuum employing an acceleration voltage of 20 kV. The elemental mapping analysis was performed using a 9 mm working distance, a 35° take-off angle and an acceleration voltage of 20 kV. The data were processed with INCA Microanalysis Suite software version 4.3 (Oxford Instruments). Thermogravimetric analyses (TGA) were carried out with a TA Instruments Q500 thermal analyzer. Investigations were performed in the temperature range between 25 and 800 °C with a heating rate of 10 °C min−1, under nitrogen at 10 mL min−1 (balance gas). Differential scanning calorimetry (DSC) experiments were run with a DSC3+ Mettler Toledo in the temperature range from −60 to 220 °C with a heating rate of 10 °C min−1, under nitrogen flow at 20 mL min−1. A dynamic mechanical thermal (DMA) analyzer Epexor 100 N GABO

Fig. 2. XRD pattern of the cured TiO2 sample identified as anatase, in the wide angle range of 2θ (20° b 2θ b 90°).

Qualimeter was used for the evaluation of the dynamic modulus and mechanical damping (tanδ). Rectangular specimens of 7.0 mm × 13 mm × 2.9 mm were measured in a temperature range from −70 to 200 °C at a heating rate of 2 °C min−1. The tests were carried out at a strain rate of 0.5% and 0.2% for Static and Dynamic, respectively. The contact angles were obtained by the system Dataphysics OCA 15EC (Neurtek Instruments). Drops of Milli-Q water (2 μL/drop) were deposited on the films as such and the static contact angles with the surfaces were measured. Reported data are the average of 5 measurements. 3. Results and discussion 3.1. Synthesis and characterization of TiO2 nanoparticles To obtain titanium dioxide nanoparticles of morphology suitable for its incorporation into epoxy-silica hybrids, a preliminary synthetic screening for its preparation was carried out by means of slight variations of the classic sol-gel method. In particular, the synthesis was accomplished starting from titanium isopropoxide (TTIP), which was employed as the sol-gel precursor. A strict control of the sol-gel reaction was required in order to gain titania powder with some specific characteristics such as nanosized particles, high surface area, proper porosity and porosimetric distribution, as well as high degree of crystallinity in the anatase crystalline phase. The synthetic procedure described in the Materials and Methods section was selected for the above purposes. Firstly, the titania sample obtained after drying thermal treatment at 80 °C for 48 h was characterized by means of XRD. The spectra showed

Fig. 3. N2 adsorption/desorption isotherms and BJH pore size distribution profile of titania sample (inset).

O. Gómez-Laserna et al. / Science of the Total Environment 645 (2018) 817–826 Table 2 TiO2 parameters obtained from BET and XRD investigations. Sample

Crystalllite size [nm]

SBET [m2 g−1]

Total pore volume [cm3 g−1]

TiO2

12

111

0.24

that the powder was, as expected, a totally amorphous compound. Then, XRD analysis was carried out on the sample thermally cured at 450 °C for 3 h, evidencing diffraction peaks at 2θ = 25.6, 38.2, 48.3, 54.2, 55.4, 62.8, 69.1, 70.3, 75.3 and 82.9° ascribed to the (101), (004), (200), (105), (211), (204), (116), (220), (215) and (303) lattice planes (Fig. 2). All diffraction peaks of the samples could be indexed perfectly to the anatase phase of TiO2 (JCPDS no.21-1272) without presence of any mixed phases. The average crystalline sizes of the synthesized TiO2 sample was 12 nm, as estimated according to the Scherrer equation based on (101) diffraction peak. The surface area and pore size distribution of TiO2 sample were measured by detecting nitrogen gas adsorption and desorption isotherms.

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Both isotherm displayed Type IV shape, indicating the presence of mesopores (Fig. 3). The hysteresis presented two loops, as index of a bimodal pore-size distribution. The H2 type hysteresis loop at low relative pressure (0.4 b P/P0 b 0.7) indicated the presence of ink-bottle pores, while the more pronounced H3 type hysteresis loop at higher relative pressure (P/P0 N 0.7) was associated to larger slit-like pores (Sing et al., 1985). The presence of two classes of pores with different size was further confirmed by the pore size distribution curve (Fig. 3, inset), which showed one peak centered at 4 nm and a more intense peak at 33 nm. The BET surface area and other investigated parameters are summarized in Table 2. 3.2. Synthesis and characterization of epoxy-silica hybrid resins Prior to titania incorporation into the hybrids, a series of preliminary attempts of epoxy-silica resins preparation, by varying the solvent and its amount as well as the relative ratios of all the mentioned reactants, were carried out to match, as much as possible, the properties required

Fig. 4. SEM-EDS analysis of the epoxy-silica resins, with and without titania: a,b) electron microscope images of samples 1 and 3, respectively, c,d,e,g) silicon distribution mappings for the samples 1, 3, 3T and 4T, respectively and, f,h) titanium distribution mappings of 3T and 4T samples.

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Table 3 Collection of the representative Raman signals employed to check the formation of the hybrid network. The yield of cross-linking reactions was monitored by following the signal intensities of epoxy ring bonds, partially hydrolyzed Si(OR)3 groups and Si\ \O\ \Si network. Samples

1 2 3 4

Si\ \O\ \Si

Si-(OH)x(OR)3−x

C\ \O\ \C

N-(H)2

shift (cm−1)

intensity (counts)

shift (cm−1)

intensity (counts)

shift (cm−1)

intensity (counts)

shift (cm−1)

466 466 466 466

934 980 1588 1262

665 665 665 665

2961 2728 3012 3137

1254 1254 1254 1254

4585 4829 3901 4129

1607 1607 – –

for stone conservation. In this way, according to the visual analysis, several samples were discarded due to phenomena such as cracking and shrinking, film-forming inability, macroscopic inhomogeneity and formation of whitish solid phases. On the other hand, one of the points to be checked was the reaction rate since the mixtures should be stable, in a liquid form, enough time to ensure the penetration inside the stone, yet, on the other hand, they ought to display a hardening reaction in a reasonable time. Bearing all of these considerations in mind, the selected mixtures for this study are listed in Table 1. Then, the alterations provided by the incorporation of nanostructured titania inside the epoxy-silica hybrids were also evaluated. 3.2.1. SEM-EDX and μ-Raman analysis Pointing out that these type of hybrid systems are intrinsically heterogeneous, electron microscope images and elemental mappings of all the films surfaces were compared to check the local uniformity of their distribution as well as to corroborate that the insertion of inorganic domains into organic matrix occurred. In this way, the SEM image of sample 1 (Fig. 4a) showed inhomogeneous areas, suggesting a phase separation. This observation was further confirmed by the silicon mapping on the same sample (Fig. 4c), where different areas could be distinguished thus evidencing an intrinsic inhomogeneity, so it could be deduced that the distribution of Si\\O\\Si groups into the organic matrix was not as uniform, i.e. a low interpenetration between organic and inorganic domains may be suggested. On the contrary, samples 3 and 4 showed a uniform area (Fig. 4b) and homogeneous silicon distributions (Fig. 4d), indicating that the mixtures with GPTMS as coupling agent led to better interpenetrated organic-inorganic hybrid networks (Wu & Lien-Chung Hsu, 2010; Plutino et al., 2017). The SEM analysis of the titania doped samples evidenced comparable silicon distributions results with respect to the titania-free samples

(Fig. 4e, g). In addition, as can be inferred from the titanium elemental mappings collected for 3T and 4T (Fig. 4f, h) the nanoparticles distribution was homogeneous, without any detectable aggregate bigger than 45 μm. As far as Raman investigations are concerned, this technique has shown to be a powerful tool to follow the epoxy ring opening as well as to monitor the hybrid formation by sol-gel process (Ramis & Fernández-Francos, 2016). As already pointed out, the amine, being responsible for the epoxy rings cleavage, allows the insertion of the inorganic silica and/or siloxane groups in the organic network, by catalyzing also hydrolysis and condensation reactions. Once the condensation of silanols started, with the corresponding development of the inorganic network, an increasing of the reaction mixture viscosity may be expected, along with the formation of a gel. This means that the epoxies cleavage can be hampered and that unreacted amino and/or epoxy groups may be still found in the hybrids. It is worth to mention that the employment of amines in excess should be avoided as it could produce chromatic changes in the stone substrate as a consequence of their oxidation. Thanks to Raman, the absorption of the epoxy ring bonds (1254 cm−1) and the presence of partially hydrolyzed Si(OR)3 groups (665 cm−1) and Si\\O\\Si (466 cm−1) network development could be monitored, making possible the following of the reaction, even during an in situ application. According to it, the spectral comparison study of the samples confirmed that not all the epoxy rings of CHDMDGE were involved in the cleavage reaction and that the epoxy opening was dependent on DAO concentration. The results evidenced also higher yield of cross-linking reaction for the samples 3 and 4 with respect to 1 and 2 (Table 3). As can be seen in the Fig. 5, GPTMS is characterized by two signals at 643 and 611 cm−1 corresponding to strong polarized bands of the symmetrical ν(SiO3) stretching of the Si(OR)3 group. However, in the samples 3 and 4 these bands were no longer observed, whereas the presence of the signals at 665 and 466 cm−1 confirmed the partial hydrolysis of Si(OR)3 groups and the Si\\O\\Si network formation, respectively. The spectra evidenced also the involving of CHDM-DGE in the cross-linking, although a little amount was still unreacted as it was observed by the signals at 778 and 1254 cm−1. Moreover, DAO was identified by the peaks at 1443 and 1607 cm−1, corresponding to amine bonds NH\\ and NH2−, respectively. Unfortunately, the signal of NH\\ bond could not be followed due to the presence of little amounts of CHDM-DGE and GPTMS, so that only the band at 1607 cm−1 was monitored experiencing its complete disappearance in 3 and 4 samples. This evidence suggested that, for these blends, DAO was consumed during the reactions but did not provide any clue about their degree of cross-linking.

Fig. 5. Raman spectra for the reaction of sample 3. The epoxy ring, partially hydrolyzed Si(OR)3 and Si\ \O\ \Si bonds, as well as the symmetrical ν(SiO3) stretching of the Si(OR)3 group are highlighted.

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Fig. 6. Contact angles values obtained of 1–4 and 1T–4T samples. Relative standard deviation (RSD) for all sampled ranged from 3% and 5%. In the lower part of the graph, the analysis images can be also observed.

Furthermore, the spectra acquired from titania-containing samples showed comparable results confirming the proper incorporation of TiO2, in anatase form, by the Raman peaks at 142, 198, 395, 511 and 635 cm−1. Fig. 7. DTG traces of 1–4 and 1T–4T samples.

3.2.2. Water repellency, thermal, viscoelastic behavior studies To check the hydrophobic properties of the epoxy-silica samples, contact angle measurements on the films as such were carried out. In detail, all the samples showed a general hydrophobic behavior with contact angles in the range between 95° and 130°; as often observed, higher values were obtained for the samples doped with titania (Fig. 6) (Abdollahi et al., 2014). Among the titania-free samples, 1 displayed a contact angle of 112.8°, whereas 2–4 showed values of ca. 95–98°, thus resulting more hydrophilic although being characterized by a water repellent behavior. Titania-containing samples 1T and 2T were featured by very high contact angles, thus showing a high hydrophobic attitude; in addition, 1T and 2T were involved in a stronger alteration of the contact angle values resulting from the presence of titania. Conversely, although, as stated, the incorporation of TiO2 into epoxy-silica hybrids induced a general increase of the contact angles, for 3T and 4T samples, the difference with respect to the corresponding titania-free products was relatively low. This experimental evidence may be explained by considering that in these blends the total amounts of the main organic network forming species, namely CHDM-DGE, over the inorganic precursors, are lower with respect to 1 and 2 samples, so that a preferential orientation of the hydrophilic groups coming from GPTMS at solid-air interface that could increase the overall surface energy, counterbalancing the hydrophobic effect due to the presence of titania, can be invoked.

The results of the thermogravimetric analysis under nitrogen flow revealed, as a common feature, a thermal degradation in two main steps, except for 2 and 4 samples, whose second mass loss occurred in

Table 4 Thermal properties of 1–4 and 1T–4T samples. Samples

1

1T

2

2T

3

3T

4

4T

First event Tonset [°C] Tmax [°C] % mass loss

250.2 320.3 14.2

251.5 314.9 11.7

255.1 325.2 14.2

253.5 310.5 13.8

270.2 330.2 13.8

260.0 319.6 12.6

265.0 325.0 14.2

258.3 310.7 13.4

Second event Tonset [°C] Tmax [°C] % mass loss Residual massa [%] Tg [°C]

360.0 405.1 77.9 7.9 34.0

351.1 407.3 80.4 8.0 33.0

360.0 400.3 76.0 9.8 39.4

352.1 406.0 80.9 5.3 29.6

375.3 435.0 66.5 19.7 46.3

359.8 420.3 64.9 19.9 53.0

375.2 425.2 75.1 10.7 39.6

350.8 412.5 77.1 9.5 46.4

a

At 800 °C.

Fig. 8. DSC traces of 1–4 and 1T–4T samples.

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two over imposed events (Table 4). This behavior is better observed in the DTG traces depicted in Fig. 7, where the degradation rate of each thermal event is also evidenced (Chatterjee & Islam, 2008; Rubab et al., 2014). As expected from the amount of GPTMS silica precursor added during the synthesis, both 3 and 3T were featured by the higher residual masses at 800 °C. Furthermore, for the second thermal event, the cited samples were characterized by lower mass loss with respect to all the other samples (Table 4). As already pointed out, the samples understudy showed two main thermal events, due to the hybrids degradation that ended at ca. 600 °C. Titania incorporation into the epoxy-silica resins induced a general slower degradation rate for all the thermal events and, at the same time, an inversion in the relative volatilization rate of the degradation steps with respect to the analogue titania-free hybrids. As a matter of fact, 1–4 samples were featured by a first thermal step which appeared to be much faster if compared to the corresponding event of 1T–4T, although only a qualitative comparison can be discussed due to the different thermal behavior of the two samples series. The data gathered in Table 4 show not only the percent mass losses for each thermal step but also the relative thermal stability of the

samples (i.e. Tonset, the temperature at which starts the first thermal event), as well as Tmax which is the temperature of maximum volatilization rate. As can be inferred from Table 4, the presence of TiO2 nanoparticles into the epoxy-silica hybrids induced a decrease of Tonset, (i.e. the titania-containing materials resulted less thermally stable than the corresponding titania-free samples) (Bikiaris, 2011) and of Tmax of both main thermal events, except for the second mass losses of 1 and 2 samples. The trend of thermal stability, independently on the presence of titania, decreased as follows 3 N 4 N 2 N 1, thus suggesting that the selected amount of the added titania did not influence to a great extent the relative thermal stability of the hybrids. On the other hand, GPTMS provided to 3 and 4 blends higher resistance against heating which was also a function of its amount in the reaction mixtures. This, on turn, may indicate a higher linking of the silica phase to the organic matrix, as already observed in SEM-EDX analyses. Furthermore, from the comparison of the data listed in Table 4, it appears not only that 3 and 3T were featured by higher thermal stability with respect to all the other samples but, at the same time, they displayed also the higher temperature of maximum rate of volatilization for both thermal events.

Fig. 9. a) Storage modulus (E′) and b) damping (tan δ) of 3,4 and 3T–4T samples as a function of temperature.

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Thermal transitions of all the samples were investigated by means of DSC (Fig. 8). As expected, all the samples displayed an amorphous behavior in the temperature range studied, without any melting phenomena; in addition, the incorporation of titania into epoxy-silica hybrids induced variations of Tg, i.e. glass transition temperature (Abdollahi et al., 2014) depending on the sample (Table 4). Since the glass transition of the hybrids is strictly associated to the motion of chain segments, it may be hindered by the presence of the inorganic networks, thus leading to higher Tg. In particular, the presence of TiO2 exerted an increase of Tg for samples 3T and 4T (from 46.3 to 53.0 °C and from 39.6 to 46.4 °C, respectively), with respect to the corresponding titania-free samples, suggesting an increased interaction between nanoparticles and matrix resulting from a loss of segmental mobility of chains not involved in cross-linking. This behavior may be indicative of good nanoparticles dispersion into the epoxy-silica matrix, in agreement with elemental titanium mappings of both 3T and 4T samples. On the contrary, 1T and 2T were featured by lower Tg with respect to the corresponding titaniafree samples; this effect may be attributed to an inhomogeneous dispersion of the NPs, as SEM-EDX results pointed out, as well as to a lower cross-linking density probably caused by the presence of partially unreacted amines with pending alkylic chains still able to move inside the blends at lower temperatures. The elastic module results collected as a function of temperature from DMA tests on selected samples are reported in Fig. 9a. The strain rate established for the static and dynamic measurements exceeds the mechanical features of samples 1, 1T and 2, 2T, evidencing once again their molecular mass between lattices as the lowest ones. However, 3 and 3T showed suitable E' modulus values thus confirming, for the above samples, an optimal rigidity due to the good balance between the density of the materials and the cross-linking provided by the GPTMS (Menard, 2008). As already stated, from the curves depicted in Fig. 9a, it can be argued that the incorporation of titania into epoxy-silica hybrids resulted in a slight positive change of the elastic modulus with respect to the corresponding titania-free sample. This evidence suggests, once again, that the homogeneous dispersion of titania into the hybrids induced a favorable effect on these samples, limiting the segmental mobility of the residual chains after a partial cross-linking, by increasing elastic modulus and stiffness. Besides, the Fig. 9b shows the trend of tan δ as a function of temperature, for the same samples. The Tg temperatures such obtained followed the same trend as the ones gained from DSC investigations, although being characterized by lower values, i.e. between 1 and 12 °C. The incorporation of titania exerted an insignificant damping in the tanδ curve as well as a slight increase of Tg for 3T and 4T compared to 3 and 4. These evidences further confirm that improved nanoparticles/ matrix interactions due to a homogeneous dispersion of titania into epoxy/silica resin, restricting the residual chains segment motion, can lead to a stiffer material, in agreement with DSC, SEM-EDX and Raman results. 4. Conclusions In this work new nanostructured BPA-free epoxy-silica resins were developed and characterized by a multianalytical approach. In this way, the hardening reaction was successfully followed by means of Raman spectroscopy through the evolution of the epoxy-ring cleavage, the presence of the partially hydrolyzed-Si(OR)3 groups and the Si\\O\\Si network development. Hence, it can be considered as a powerful tool to control the use of this kind of stone conservation products avoiding harmful applications, facilitating their employing thanks to the in situ monitoring of the reactions. In addition, its combination with SEM-EDX analysis also allows to check the proper silicon and titanium dispersion degree into the organic matrix. Besides their structure and morphology, the suitability of the hybrids for application on stones

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has been preliminary assessed with respect to the main requisites to be fulfilled for a correct conservation treatment, thanks to a combination of contact angles, TGA, DSC and DMA studies. From the data obtained, 3 and 3T samples, based on CHDM-DGE and GPTMS, evidence to be the most promising multifunctional products due to their excellent hydrophobic and thermal responses, increased by the titania incorporation into the hybrid network. In addition, the polymerization can take place inside the lithic substrates, starting from low viscosity and easy handling formulations. These solutions ought to display a rapid diffusion and deep penetration into the stone, being able to go beyond the damaged layer. Once there, they may react leading to the formation of hybrid materials with suitable mechanical properties anchored to the stone substrate and to the recovering of the intergranular cohesion by the silanols condensation. Further investigations on selected lithotypes, aimed to test the products towards degradation induced by solar radiation and interaction with the atmospheric pollution, are in due course. Acknowledgements This work has been financially supported by the project PHETRUM (CTQ2017-82761-P) from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO) and by the European Regional Development Fund (FEDER). O. Gómez-Laserna gratefully acknowledges her post-doctoral contract from the University of The Basque Country (UPV-EHU). The authors are grateful to the technical support provided by the Raman-LASPEA laboratory and to the Macrobehaviour, Mesostructure, Nanotechnology: Unit of Materials and Surfaces of The Advanced Research Facilities of the SGIker (UPV/EHU, MICINN, GV/EJ, ERDF and ESF). References Abdollahi, H., Ershad-Langroudi, A., Salimi, A., Rahimi, A., 2014. Anticorrosive coatings prepared using epoxy−silica hybrid nanocomposite materials. Ind. Eng. Chem. Res. 53, 10858–10869. Al-Saleh, I., Elkhatib, R., Al-Rajoudi, T., Al-Qudaihi, G., 2017. Assessing the concentration of phthalate esters (PAEs) and bisphenol A (BPA) and the genetoxic potential of treated wastewater (final effluent) in Saudi Arabia. Sci. Total Environ. 578, 440–451. Aruoja, V., Dubourguier, H.C., Kasemets, K., Kahru, A., 2009. Toxicity of nanoparticles of CuO, ZnO and TiO2 to microalgae Pseudokirchneriella subcapita. Sci. Total Environ. 407 (4), 1461–1468. Bergamonti, L., Predieri, G., Paz, Y., Fornasini, L., Lottici, P.P., Bondioli, F., 2017. Enhanced self-cleaning properties of N-doped TiO2 coating for cultural heritage. Microchem. J. 133, 1–12. Bikiaris, D., 2011. Can nanoparticles really enhance thermal stability of polymers? Part II: an overview on thermal decomposition of polycondensation polymers. Themochim. Acta 523, 25–45. Cappelletti, G., Fermo, P., Camiloni, M., 2015. Smart hybrid coatings for natural stones conservation. Prog. Org. Coat. 78, 511–516. Cardiano, P., 2008. Hydrophobic properties of new epoxy-silica hybrids. J. Appl. Polym. Sci. 108, 3380–3387. Cardiano, P., Sergi, P., Lo Schiavo, S., Piraino, P., 2003a. In situ polymerization of 3 glycidoxypropyl trimethoxysilane (GLYTS) as a new tool for stone conservation. Ann. Chim. 93 (3), 249–256. Cardiano, P., Mineo, P., Sergi, S., Ponterio, R.C., Triscari, M., Piraino, P., 2003b. Epoxy-silica polymers as restoration materials. Part II. Polymer 44 (16), 4435–4441. Cardiano, P., Ponterio, R.C., Sergi, S., Lo Schiavo, S., Piraino, P., 2005. Epoxy-silica polymers as stone conservation materials. Polymer 46 (6), 1857–1864. Chapin, R.E., Adams, J., Boekelhaide Jr., K., Gray, L.E., Hayward, S.W., Lees, P.S., McIntyre, B.S., Portier, K.M., Schnorr, T.M., Selevan, S.G., Vandenbergh, J.G., Woskie, S.R., 2008. NTP-CERHIR expert panel report on the reproductive and developmental toxicity of bisphenol A. Birth Defects Res. B Dev. Reprod. Toxicol. 83 (3), 157–395. Chatterjee, A., Islam, M.S., 2008. Fabrication and characterization of TiO2-epoxy nanocomposite. Mater. Sci. Eng. 487 (1–2), 574–585. Doehne, E., Price, C.A., 2010. Stone Conservation: An Overview of Current Research. Getty Conservation Institute, Los Angeles. Dorothé van der Werf, I., Ditaranto, N., Picca, R.A., Chiara Sportelli, M., Sabbatini, L., 2015. Development of a novel conservation treatment of stone monuments with bioactive nanocomposites. Herit. Sci. 3 (29), 1–9. Gómez Laserna, O., Olazabal, M.A., Morillas, H., Prieto-Taboada, N., Martinez-Arkarazo, I., Arana, G., Madariaga, J.M., 2013. J. Raman Spectrosc. 44, 1277–1284. Gómez Laserna, O., Arrizabalaga, I., Prieto-Taboada, N., Olazabal, M.A., Arana, G., Madariaga, J.M., 2015. In situ DRIFT, Raman, and XRF implementation in a multianalytical methodology to diagnose the impact suffered by built heritage in urban atmospheres. Anal. Bioanal. Chem. 407 (19), 5635–5647.

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