Weatherability of hybrid organic–inorganic silica protective coatings on glass

Progress in Organic Coatings 88 (2015) 172–180

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Weatherability of hybrid organic–inorganic silica protective coatings on glass Barbora Holubová a,∗ , Zuzana Zlámalová Cílová a , Irena Kuˇcerová b , Martin Zlámal c a b c

Department of Glass and Ceramics, Technická 5, Prague, Czech Republic Department of Chemical Technology of Monument Conservation, Technická 5, Prague, Czech Republic Department of Inorganic Technology, University of Chemistry and Technology, Technická 5, Prague, Czech Republic

a r t i c l e

i n f o

Article history: Received 5 March 2015 Received in revised form 4 June 2015 Accepted 1 July 2015 Available online 23 July 2015 Keywords: Sol–gel Hybrid organic–inorganic silica coatings Glass protection Weatherability

a b s t r a c t Currently, there is a growing interest in the application of silicon-based technologies for the development of advanced hybrid organic–inorganic coatings with strong weatherability. In this study, the sol–gel process is used to prepare such coatings on glass and their resistance to weathering effects is assessed afterwards. Various sols were prepared by mixing a silica-based inorganic matrix (tetraethyl orthosilicate) with different quantities of silica alkoxides functionalised with various organic groups. Subsequently, the sols were dip-coated onto glass samples at low temperatures without any heat treatment. The coatings prepared were analysed before and after three model ageing tests simulating various weathering parameters. After ageing, the best performing coatings showed good overall homogeneity and transparency (optical microscopy, SEM), improved water repellency and adhesion to the glass substrate (static contact angle measurements, cross-cut tape tests) and no colour or chemical composition changes (UV–VIS, FTIR). Compared with commercial hybrid silica products, the alkyl- and methacryloxy-functionalised silica coatings particularly displayed improved homogeneity, elasticity and barrier properties. Thus, these low temperature coatings, easily applicable to thin films, appear to fulfil the main requirements for the protection of the glass exposed to weathering phenomena. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Protective coatings on exterior glass exposed to weathering phenomena have been used in restoration–conservation and building maintenance practice with mixed success since the beginning of the last century [1,2]. Current research primarily focuses on hybrid organic–inorganic silica based compounds as it is believed that their proper use can enhance water, chemical, abrasion and UV resistance [3–11]. In general, these coatings with low toxicity do not alter optical characteristics of substrate and display improved flow properties when applied [12,13]. In addition, there is a possibility to apply them without any further heat treatment, which is particularly suitable for the application on cultural heritage objects and in building maintenance [5,14,15]. One of the most important assets is then their design variability as there is a wide range of organo-reactive or non-reactive moieties attached to the silicon atom allowing formulas to be designed in accordance with specific application performance requirements [12,13,16]. Nevertheless, the influence of silica matrix functionalisation with various

∗ Corresponding author. E-mail address: [email protected] (B. Holubová). http://dx.doi.org/10.1016/j.porgcoat.2015.07.001 0300-9440/© 2015 Elsevier B.V. All rights reserved.

organic moieties on the protective efficiency and weatherability of silica-based coatings was studied more extensively in relation to the corrosion protection of metals [17,18]. Hybrid organic–inorganic silica materials are characterised by dual behaviour which comes from their structure consisting of an inorganic part (heteropolysiloxane backbone) capable of strong, covalent siloxane bonding to glass surfaces, and an organic part (containing different organic functionalities) providing coatings with added properties (Table 1). Moreover, it is also capable of covalent bonding with other organic network topcoats. This enables to create materials with hybrid properties between organic polymers and silicates, so that they are often called organically modified or organo-functional silica materials [12,13,16]. Generally, the functionalisation (modification) of silica based material means the formation of a covalent bond between silicon of a substrate surface or coating matrix and silicon of a modifier (organo-functional alkoxysilanes). Commonly, it occurs via oxygen bridges so that siloxane bonding forms. There are two major ways how to obtain a functionalised material: (a) Grafting method: consists in the subsequent functionalisation of a pre-treated surface by derivatization. This method is rather disadvantageous because of an uneven distribution of

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Table 1 Abbreviations, chemical names, functional groups and added properties to silica based coatings of several commonly applied organo-functional alkoxysilane precursors [3–22]. Abbreviation

Chemical name

Functional group

Added properties to silica based coating

MTES MTMS OTES

Methyltriethoxysilane Methyltrimethoxysilane Octyltriethoxysilane

Methyl-

- crosslinking agent, organic compatibility

Octyl-

HDTMS VTMS PTMS

Hexadecyltrimethoxysilane Vinyltrimethoxysilane Phenyltrimethoxysilane

HexadecylVinylPhenyl-

AMEO AEAPS GLYMO GLYEO

3-Aminopropyltriethoxysilane 3-(2-Aminoethyl) aminopropyltrimethoxysilane 3-Glycidoxypropyltrimethoxysilane 3-Glycidyloxypropylmethyldiethoxysilane

Amino-

MEMO

-Methacryloxypropyltrimethoxysilane

Methacryloxy-

MTMO

-Mercaptopropyltrimethoxysilane

Mercapto-

TTS

(Tridecafluoro-1,1,2,2-tetrahydrooctyl) triethoxysilane

Fluoroalkyl-

- organic compatibility; paintability, substantivity and occlusivity; improved gloss, water repellency and self-cleaning properties - adhesion promoter; crosslinking agent; water scavenger - greater oxidation resistance; higher refractive index; improved thermo plasticity and toughness; superior thermal stability; organic and pigment compatibility - improved substantivity to metals and coatings; crosslinking agent; adhesion promoter; corrosion resistance - high reactivity due to easily opened ring (with amines, hydroxy and carboxy groups); crosslinking, adhesion promoter; improved durability, gloss, spread, elasticity; strengthening - adhesion promoter; co-binder and crosslinking agent; strengthening - reacts with isocyanates, acrylates, unsaturated polymers; crosslinking agent; adhesion promoter - greater chemical resistance; oil, fuel and solvent resistance; stain-resistance; easy-to-clean properties

Glycido- (Epoxy-)

functionalities on the surface of a substrate and the necessary pre-treatment of materials, which makes this method longer and more complicated [12,16,19,20]. (b) Condensation method: consists in the introduction of an organofunctional co-precursor with desired properties already during the synthesis of a silica based material (often tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS) matrix). It represents a one-step route method enabling better fixation and distribution of organic functionalities on a surface (see in Fig. 1). However, a certain disadvantage could lie in the formation of less ordered structures and hence, the functionalisation may often be irreproducible. Nevertheless, this route is the simplest and thus, very popular in the creation of a wide range of organically modified materials as it can simultaneously construct a solid surface with appropriate surface roughness and low surface energy. This method is commonly called the sol–gel method (process) [12,16,17,19]. In detail, organo-functional primers (modifiers) for such a material are of the general molecular form R Si (Y)3 [13,16], where the Y group is commonly a hydrolysable alkoxy or acetoxy group ensuring the linkage to the inorganic substrate and the R group is a non-hydrolysable organic group that could be reactive (e.g. amino, epoxy-, vinyl- etc.) or non-reactive (e.g. alkyl-) towards another chemical. This organic group on the silane is compatible with the silica matrix and can bring particular properties depending on its

Fig. 1. Schematic visualisation of organo-functional silane hydrolysis, condensation and covalent bonding to inorganic substrate (drawn in accordance with [12]).

nature (Table 1 [3–23]). The R group can also consist of a spacer located between an organic functionality and a silicon atom (typically a short aryl or alkyl chain). The potential impact of a spacer on the reactivity of silicon atom reactivity depends on the distance between them [13]. In this work, the choice of the organo-functional co-precursors (modifiers) and their relative ratios to silica matrix was based on the recent studies concerning the surface treatment of glass [3–21]. For example de Ferri et al. [3], Carmona et al. [4,14], De Bardi et al. [5] and Dal Bianco et al. [6,15] used sol–gel silica coatings based on TEOS non-modified or modified with various organo-functional silane modifiers to test their protection efficiency for historical glass. Likewise, there is a vast literature particularly on enhancement of strengthening and self-cleaning properties of coated glass surfaces for different applications [7–11]. For example, Briard et al. [8] used an aqueous silane solution with addition of epoxy- and amino-silane precursors to strengthen the coated glass surface. In order to obtain the best results in term of hydrophobicity, for example, Eshrad-Lagroudi et al. [7], Ganbavle et al. [9], Wang et al. [10], Rao et al. [11] and Ramezani et al. [20] proposed to use the glass surfaces treated with TEOS or TMOS based coatings mixed with various fluoalkyl-, phenyl- or short/long alkyl-silane precursors. An actual example of practical use of organo-functional silica materials can be found in design of protective multilayer system on the Last Judgement Mosaic in St. Vitus Cathedral in Prague, Czech Republic [24], although its maintenance is about to be rethought [25]. In addition, as previously mentioned, a broader insight could be found in works related to the use of organo-functional silica materials on metal substrates [17,18]. As described in the following sections, several tests were performed to examine the effect of different organically functionalised Si-alkoxide co-precursors (alkyl-, amino-, epoxy- and methacryloxy-silane modifiers) on the final performance of protective sol–gel based silica coatings on glass when exposed to simulated weathering tests. The present work uses tetraethyl orthosilicate (TEOS) as the main precursor (silica matrix) functionalised with different ratios of organo-functional silane modifier. Moreover, the optimisation of the sol–gel process was studied using different quantities of functionalised Si-alkoxides with the main TEOS matrix and different time periods of sol-polycondensation. The coated samples have been dried at laboratory conditions without any further heat treatment. This procedure was chosen on

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Table 2 Tested commercial products. Ormocer G®

TS-56-H

- Supplier: Fraunhofer Institut für Silicatforschung – ISC - Mixture of hydrolysed organically modified silicates and metal alkoxides in solution - Applied without further use of dilutants or hardeners, with no heat treatment - Supplier: Tokuyama America, Inc. - Organosilane containing epoxy groups; patented product – its composition is not further specified - Used without hardener (TS-56-HA) and with hardener (TS-56-HAB, component A:B mixed 1:1 by weight)

the basis of restoration-conservation/building maintenance practice and possible in situ applications, where exterior glass surfaces, being frequently located in the vertical position, do not provide a good opportunity to be treated thermally. In addition, as hybrid organic–inorganic silica coatings are already in use, not only newly designed materials were studied in this paper, but also those which are already commercially distributed for such purposes. In this work, two commercial hybrid organo-silane products were tested, both of them already being applied in restoration–conservation and building maintenance practice (an analogue to Prague’s mosaic hybrid silane and a hybrid silane product designed by Fraunhofer Institut für Silicatforschung).

by optical microscopy (Optical microscope Olympus BX 51 with a camera Olympus E-600). The samples were observed in vertical incident light. In the case of non-aged samples, a black marker was used to focus better on the surface of transparent coatings. To assess the adhesion of the coatings to the glass substrate, an adhesion cross cut test was performed using Elcometer 107 Cross Hatch Cutter (in accordance with the following International Standard – ASTM D3359-B [26]). In order to evaluate the water repellency of the coatings obtained before and after ageing, static contact angle measure˝ ments were performed using Drop Shape Analyzer DSA 30 (KRUSS). The microstructures and thickness of the samples selected were investigated using scanning electron microscope (SEM) Hitachi S-4700 supplemented with SDD (silicon drift detector). Viscosity values were measured just after the selected time of solpolycondensation using Tuning Fork Vibro Viscometers (natural frequency at 30 Hz, model SV-10A). X-ray diffraction analyses for examination of the crystalline phases developed in the coatings after the ageing tests were performed with PANalytical X’Pert PRO diffractometer (PANanalytical, Holland). Data processing and evaluation were realised using High Score Plus software from PANanalytical. In order to evaluate possible changes in the chemical composition of the coatings, FTIR spectra were obtained using spectrometer FTIR Nicolet iS10 in an ATR mode (Attenuated Total Reflectance, Ge ATR crystal). The spectra were collected in the spectral range 600–4000 cm−1 from both non-aged and aged coated samples.

2. Materials and methods 3. Experimental part 2.1. Applied materials 3.1. Film formation The main silica precursor (TEOS; 99%, Sigma–Aldrich) was functionalised in two different ratios with different organofunctional precursors separately: octyltriethoxy-silane (OTES; 95%, Momentive Performance Materials); hexadecyltrimethoxysilane (HDTMS; 85%, Sigma–Aldrich); [3-(methacryloyloxy)propyl] trimethoxysilane (MEMO; 95%, Sigma–Aldrich); 3-glycidyloxypropyl-methyldiethoxy-silane (GLYEO; 95%, Momentive Performance Materials); gamma-aminopropyltriethoxy-silane (AMEO; 99%, Momentive Performance Materials). Furthermore, two commercial hybrid silica products were examined (Table 2): Ormocer G® , a coating designed for conservation treatment of historic glasses and TS-56-H (part A and B), an analogue to the original product used on the mosaic of the Last Judgement Mosaic in St. Vitus Cathedral in Prague [24]. Finally, a non-functionalised TEOS coating was also examined for a possible contrast comparison of material improvements caused by the added organic functionalities. The tested hybrid coatings were applied on glass microscope slides (26 mm × 76 mm × 1 mm) of the soda-lime-silicate composition (Table 3) with no tin contamination from float technology. The glass substrates were thoroughly washed with detergent, deionized water and acetone. 2.2. Analysis techniques To evaluate the influence of the glass substrate composition, XRF analysis measurements were performed using spectrometer ARL 9400 XP. The UV–VIS spectrometry was employed in order to assess possible colour changes. The UV–VIS spectra of both non-aged and aged coated glass samples were acquired with UV–VIS spectrophotometer Shimadzu UV-1201 in the spectral range between 200 and 800 nm, using an untreated glass sample as the reference. The homogeneity and surface morphology of the coatings prepared and possible changes after the accelerated ageing tests were analysed

The resulting layers were obtained by the sol–gel process using an H+ catalyst (only the AMEO coatings were prepared without adding a catalyst; OH− catalysis). Two different ratios of an organofunctional silane primer and TEOS were used in this work (Table 4), designed by the previous study [3] as a boundary simulating either the limit amount of organic compound to ensure the functionalisation (marked 5%), or as the limit amount of an organic compound still maintaining the highest chemical and physical compatibility with a glass substrate (marked 20%). The sols were prepared by mixing the functionalised TEOS, solvent (isopropyl alcohol, p.a.) and water (TEOS:H2 O = 1:7.7) under magnetic stirring in a round-bottom polymer flask for 15 min before adding a catalyst (HCl, 35%; measured acidity: pH ∼ 2). In order to evaluate possible effect of sol–gel processing, three different time periods of sol-polycondensation were studied: The sols were further stirred either for 1, 4 or 8 h (marked 1H, 4H, 8H) under laboratory conditions (T ∼ 20 ◦ C, RH ∼ 30–40%) to allow the hydrolysis and condensation reactions to occur. The prepared sols were applied on microscope glass slides with a speed controlled dip coating device with the drawing rate of 9.6 cm min−1 and 30 s delay. The coated samples were left to cure at laboratory conditions without any additional heat treatment. All the coated samples were dry and well-formed by the end of 24 h. On the basis of the best results reported in [3], 5% HDTMS and 20% OTES were examined. The other three sols tested (MEMO, GLYEO, AMEO) were prepared all in the 5–20% ratios. Ormocer G® , TS-56-HA and TS-56-HAB (Table 2) were applied with the aid of the dip-coating technique described above. In the case of the commercial product TS-56-H, the producer recommends an addition of hardener B. In this work, both coatings without the hardener (TS-56-HA; its analogue was used on the Last Judgement mosaic in Prague Castle) and with the hardener (TS-56-HAB) are examined. Moreover, in the case of TS-56-H three curing temperatures

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Table 3 XRF analysis of chemical composition of both sides of microscope glass slide substrates [wt%].

Bulk glass

SiO2

Na2 O

CaO

K2 O

MgO

Al2 O3

SO3

SnO2

TiO2

Fe2 O3

As2 O3

ZrO2

70.7

15.6

6.2

0.93

4.3

1.5

0.28

0.00

0.02

0.02

0.15

0.02

Table 4 Composition of newly designed sols [3]. Hybrid coating mark

TEOS [mol%]

Primer [mol%]

Primer/TEOS

Composition TEOS:Primer:H2 O:IsoPrOH

20% 5%

80 95

20 5

0.25 0.05

1:0.25:9.5:26.5 1:0.053:22.36:8.32

were observed in order to compare possible differences in performances: laboratory temperature ∼20 ◦ C, 90 ◦ C – as used on the mosaic, 120 ◦ C – as recommended by the producer. 3.2. Model ageing tests Subsequently, model accelerated ageing tests were carried out with the coated samples: (1) Artificial weathering (marked QUV): - Using the QUV Accelerated Weathering Tester Model QUV/spray with the Solar Eye Irradiance Control-Q-Lab with the following modes: 1. UV irradiation of the samples (340 nm), intensity 0.68 W/m2 , 50 ◦ C, 5 h; 2. Rinsing with demineralised water at 20 ◦ C, without irradiation – thermal shock, 2 min; 3. Moisture condensation on the sample at 40 ◦ C, without exposure, 2 h; 4. Back to step 1. (2) Increased temperature (T): at 50 ◦ C. (3) Increased relative humidity and temperature (RH/T): at 50 ◦ C and 95% RH. The aged samples were removed after 60 days and kept under constant laboratory conditions for further analyses. It has to be noted that the model ageing tests were intentionally intensified in this work and are far from the real type of the environment that exterior glass objects are normally exposed to. The aim was, on the one hand, to reveal a single parameter or a combination of parameters that would be the most harmful to the coatings and on the other hand, to accelerate degradation processes that may occur on the coating surface or on the glass-coating interface. We assumed that natural weathering would affect the coatings more tardily and in a less significant way. 4. Results and discussion 4.1. Optical microscopy Almost all of the non-aged coated samples appear to be transparent and with no great precipitates or impurities (Fig. 2a). The UV–VIS spectra collected from both the non-aged and the aged coated samples show that no coating has significant absorption in the VIS range (approx. 400–800 nm), confirming that the protective films are colourless. Only the non-aged samples bearing coatings with the amino-functionality form opaque and inhomogeneous films, probably due to the basic-catalysed formation of its sol during which it yields highly branched clusters [27]. Since the transparency and homogeneity are the most important characteristics for the coatings applied to glass, these samples were excluded from further testing. The coated samples were documented before and after 60 days exposure to the accelerated model ageing tests. The best performances after the ageing tests were displayed by 5% HDTMS,

Fig. 2. Optical microscope images of non-aged samples after 4 h of solpolycondensation: (a) 5% HDTMS; optical microscope images of RH/T aged samples after 4 h of sol-polycondensation: (b) 5% HDTMS; (c) 20% MEMO.

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defects after the model ageing tests. Small spots, impurities and even crystalline structures can be observed on their surface. As for the time period of sol-polycondensation, the performance of all the samples improves significantly with a longer time period of solpolycondensation, but in certain cases they do not differ greatly nor improve between 4 h and 8 h of sol-polycondensation. Commercial products TS-56-HA and TS-56-HAB display poor results after all the model ageing tests. Nevertheless, the nonaged coating with hardener B is more homogeneous. It implies that the hardener probably improves only the film formation process, but according to the visual results after model ageing, it does not enhance chemical durability properties of the thin film formed. Regarding all of the three used heat treatment modes, the surfaces of the coatings are significantly inhomogeneous. Due to the unsatisfactory results this commercial coatings were excluded from further examinations. 4.2. Adhesion All the non-aged and aged coatings display very good adhesion classified as 5B in the ASTM D3359 scale [26]. The results show that even in the case of the aged samples, the adhesion of all designed coatings is well ensured. It is caused by chemical bonding through a silane coupling agent. That also explains why the adhesion still remains very good even in spite of possible crystalline phases developing on the glass/coating interface. The only exception is Ormocer G® , where the adhesion of the aged samples is evaluated as 4B (also a good adhesion result). It indicates that the Ormocer G® is probably “softer”, linked via long segments. The commercial product TS-56HA was not hardened even after several hours from its application, although its analogue already applied in restoration practice was used without its part B (the hardener). 4.3. Static contact angle measurement

Fig. 3. Optical microscope images of RH/T aged samples after 4 h of solpolycondensation: (a) TEOS; (b) 5% GLYEO and (c) Ormocer G® .

20% OTES and 20% MEMO already after 4 h of sol-polycondensation even in the most demanding RH/T model ageing test (Fig. 2a–c). The worst performances belong to non-modified TEOS, GLYEO coatings and commercial products (Fig. 3a–c). The best performing coatings presented in Fig. 2 exhibit sufficiently regular and homogenous surfaces both in the non-aged and aged states. Even though the worst performing coatings are regular in the non-aged state (similarly to Fig. 2a), they appear to be highly inhomogeneous with the appearance of solid structures of all sizes, crystalline products, bubbles and film defects after the model ageing (Fig. 3a–c). The lower ratio of the organic functionality to TEOS (in the case of 5% MEMO and 5% GLYEO coatings) seems to be more prone to

To study the wetting and self-cleaning properties of the coatings, static contact angle measurements ( [◦ ]) were carried out. Because of the large extent of coatings and ageing tests analysed in this work, only the results of the non-aged samples will be presented in comparison with the aged samples after the RH/T and the QUV model ageing tests. These model ageing tests had the greatest impact on the surfaces of the coated samples. The water repellency of the non-aged samples was increased in all cases compared to the uncoated glass sample. The best water repellency was observed for the commercial product Ormocer G® and for the samples containing TEOS functionalised with long alkyl chains: 20% OTES (-C8) and 5% HDTMS (-C16). The worst performance in water repellency is displayed by non-modified TEOS and GLYEO coatings. In Table 5 and Fig. 4, it is clearly shown that  values do not differ greatly for different ratios of the same organic component or in the set of three sol-polycondensation time periods (see for example MEMO and GLYEO). Regarding the influence of the model ageing tests, the water repellency decreased in all cases. As already reported in [3,23] the value of the static contact angle significantly depends on the kind of the functionalizing group. The samples tend to hydrophilic behaviour probably due to the shortness and/or greater polarity of the organic chains presented, i.e. due to their chemical nature. In comparison to the GLYEO, the MEMO coatings contain lower polar functional groups [12] thanks to which the MEMO surface seems to be more hydrophobic. After the model ageing tests, this feature is even more visible: The  values of the aged GLYEO coatings decrease significantly when compared to the rest of the coatings, which also indicates a possible chemical shift. On the other hand the QUV aged MEMO samples display a greater decrease in the  values thanks to C O groups, which are particularly susceptible to the photo-oxidation.

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177

Table 5 Static contact angles of non-aged (NA) and aged (RH/T, QUV) coatings in different organic component amounts and three times of polycondensation.  [◦ ]

Hybrid coating name

Model ageing test

TEOS

NA RH/T QUV

48.7 ± 1.2 41.9 ± 1.2 32.3 ± 0.8

20% OTES

NA RH/T QUV

5% HDTMS

1H

4H

8H

59.9 ± 1.2 51.5 ± 0.7 41.6 ± 1.1

60.7 ± 0.9 45.7 ± 2.3 40.3 ± 1.0

105.8 ± 1.6 101.5 ± 0.7 79.6 ± 0.5

105.9 ± 1.2 101.1 ± 0.1 86.7 ± 1.8

107.9 ± 0.8 103.2 ± 0.5 94.1 ± 0.4

NA RH/T QUV

105.8 ± 0.2 101.3 ± 0.1 93.4 ± 0.5

107.1 ± 0.7 103.3 ± 1.5 95.3 ± 0.9

107.2 ± 0.4 105.4 ± 0.4 96.6 ± 0.9

20% GLYEO

NA RH/T QUV

61.9 ± 0.4 27.9 ± 3.2 35.6 ± 4.9

63.8 ± 0.6 31.5 ± 1.7 38.1 ± 1.0

64.5 ± 1.2 31.8 ± 2.3 36.5 ± 0.3

5% GLYEO

NA RH/T QUV

56.1 ± 1.6 25.8 ± 1.1 26.9 ± 1.9

53.0 ± 2.7 30.2 ± 3.8 24.1 ± 1.5

51.7 ± 1.0 29.1 ± 2.0 28.6 ± 2.6

20% MEMO

NA RH/T QUV

70.6 ± 0.1 65.7 ± 2.3 29.3 ± 3

71.4 ± 0.4 64.9 ± 0.9 29.7 ± 0.9

71.7 ± 0.8 64.8 ± 2.4 31.2 ± 0.8

5% MEMO

NA RH/T QUV

66.8 ± 0.6 63.0 ± 0.7 16.7 ± 1.7

67.7 ± 0.1 63.5 ± 1.3 18.8 ± 1.6

68.2 ± 0.1 65.8 ± 1.5 21.8 ± 0.7

Ormocer G®

NA RH/T QUV

Uncoated glass slide

Moreover, it is interesting that at the same time both the best and the worst coatings (when evaluated visually) show the highest static contact angles values (Ormocer G® and the alkyl-functionalised coatings). This fact further supports the statement that the water repellent surfaces themselves

86.9 ± 1.2 79.3 ± 0.5 76.5 ± 1.0 23.8 ± 3.1

(samples with higher static contact angles) do not ensure good resistance to model ageing tests. Possible reasons of this behaviour may consist in different surface properties (further explained in Section 4.4 on the basis of SEM characterisation).

Fig. 4. Static contact angle values of non-aged/aged newly designed coatings prepared after 8 h of polycondensation and values of Ormocer G® ; with error bars indicating standard deviations.

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4.4. SEM microscopic characterisation Scanning electron microscopy was used to further observe the microstructure of samples before and after the model ageing tests and to investigate the relationship between the surface structure and the contact angles measurement. For this analysis, only the samples with the samples with the best (5% HDTMS, 20% MEMO) and the worst (Ormocer G® ) performances were analysed visually. The non-aged samples functionalised with long alkyl chains (Fig. 5a) are crack free with a very smooth and uniform surface and with no presence of important micro-protrusions. In this case, a well-connected silica network is expected to form during the condensation reactions. Their good water repellency is here ensured primarily by the chemical nature of the coatings: they contain the least polar functionality with no potential to hydrogen bonding; also the polarizability and the net dipole moment are small at best. Those samples seem to be unaffected by the most harmful RH/T ageing test (Fig. 5c). On the contrary, the surface of the non-aged Ormocer G® is slightly corrugated on the microscopic scale (Fig. 5b). Its good result in the static contact angle measurement could be explained by Cassie or Cassie–Baxter theory that explains wetting behaviour in a way, where the surface roughness plays an important role along with the chemical composition [9]. Despite its hydrophobic behaviour, Ormocer G® is one of the most affected coatings after the accelerated ageing. On the surface of the RH/T aged Ormocer G® films, a vast range of inhomogeneities, pits, cracks and even crystalline products could be observed (Fig. 5d). The presence of crystalline products supports the idea that water molecules that concentrate on a small surface area of a water droplet, penetrate into the film up to the glass/coating interface where they induce glass corrosion mechanism. With the support of SEM characterisation, it is assumed that these crystalline products originate from the surface of the glass substrate (Fig. 5d). In addition to the SEM technique, it is possible to assess an approximate value of the thickness by cross section of the coated glass substrate. In relation to similar results of apparent viscosity measurements ( ≈ 2.4 mPa s), it is supposed that all hybrid silica coatings designed in this work will display similar thicknesses. As evaluated by the SEM method, the thickness of 5% HDTMS 4H is presumably lower than 1 ␮m and ranges from about 100 to 200 nm. In contrast, the thickness of commercial product Ormocer G® is significantly higher being about 10 ␮m, which could be explained by its higher apparent viscosity ( ≈ 66 mPa s). 4.5. Chemical resistance With the aid of XRD analysis and FTIR spectroscopy, possible changes in the chemical composition of the aged coatings were evaluated in reference to the non-aged films. The XRD and FTIR analyses confirmed the presence of calcium carbonate crystalline phase on the surface of the most damaged coatings after the RH/T ageing test. The calcium carbonate phase originates most likely from the glass substrate surface after leaching processes, i.e. from the reactions of extracted cations of the glass substrate with atmospheric gases dissolved in the aqueous film formed on the glass/coating interface. We suppose that the corrosion mechanism undergoes as follows: firstly, water from the environmental humidity can get through the thin coating and reach the glass substrate where it causes the ion exchange from the glass with H+ ions from the water. Consequently, alkali hydroxides are formed and attack intensely the glass network siloxane bonds. Subsequently, the glass network degradation favours the calcium ions leaching as Ca(OH)2 [4]. Since the environment is polluted with CO2 , calcium hydroxide is further carbonated and a CaCO3 crust precipitates [4,28].

Fig. 5. SEM images of non-aged samples: (a) 5% HDTMS, 4 h; (b) Ormocer G® ; SEM images of RH/T aged samples: (c) 5% HDTMS, 4 h; (d) Ormocer G® .

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5% MEMO 4H

5% HDTMS

CALCIUM CARBONATE (precipitated) 5% MEMO 4H RH/T AGED a 5% MEMO 4H RH/T AGED b 5% MEMO 4H RH/T AGED c

CALCIUM CARBONATE (PRECIPITATED) 5% HDTMS 8H RH/T AGED 5% HDTMS 4H RH/T AGED 5% HDTMS 1H RH/T AGED 5% HDTMS 1H NON-AGED

IR Absorbance

IR Absorbance

179

5% MEMO 4H NON-AGED 5% MEMO 4H QUV AGED 5% MEMO 4H T AGED

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber [cm ]

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber [cm ] Fig. 6. FTIR spectra collected on non-aged 5% HDTMS 1H (wine red); RH/T aged 5% HDTMS 1H (red); RH/T aged 5% HDTMS 4H (dark yellow); RH/T aged 5% HDTMS 8H (blue); and reference spectrum of calcium carbonate (pink). (For interpretation of the references to colour in this figure legend and in the text, the reader is referred to the web version of the article.)

The detection of CaCO3 further supports the above-mentioned statements (also supported by the SEM images in Fig. 5d). In addition, previous studies [4,5,29] show that already the application of the sol–gel coating could lead to a certain initial migration of ions from the bulk glass into the coating due to the pH of the solution and/or because of the ion exchange process. The IR spectrum of calcium carbonate is quite characteristic with a very intense broad band centred at 1430 cm−1 . Moreover, there occur also weaker sharp bands at 876 and 712 cm−1 [30,31]. As for the possible alteration of the best performing coatings in this work (5% HDTMS, 20% OTES and 20% MEMO), characteristic bands of calcium carbonate are traceable only in the case of 5% HDTMS after 1 h of sol-polycondensation (in Fig. 6; red), where calcium carbonate is illustrated by the formation of characteristic bands at about 1430 and 876 cm−1 regions. Also, a certain decrease at 1050 cm−1 region refers to the disruption of the coating silica network, which is supported and accompanied by an increase at 600–700 cm−1 region. With a longer time period of sol-polycondensation, the densification of the silica network increases as seen after the mutual comparison of the spectra for 1, 4 and 8 h of sol-polycondensation, respectively (Fig. 6; red, dark yellow, blue) [23–25]. Similarly, this enhancing effect of the sol-polycondensation duration is observed in case of the 20% OTES. With the aid of FTIR spectroscopy, it can be proved that the choice of the added functionality may strongly influence the polycondensation process. In certain cases (most probably the TEOS and the GLYEO coatings), the conversion of Si O bonds into Si OH and their subsequent condensation into Si O Si groups could occur in cycles [32–34] and thus the performance of coating does not improve with a longer time period of sol-polycondensation. In other words, the inorganic silica part may preponderate over the organic network development. Anyway, in the case of the best performing coatings, it is possible to conclude that their performance improves with a longer time period of sol-polycondensation and is satisfactory already after 4 h. For the comparison of two different amounts of organic component the 5% and 20% MEMO coatings are discussed. The spectra of the 5% MEMO depict the corrosion state in various formation states for all of the sol-polycondensation time periods (in Fig. 7 shown for samples bearing coatings after 4 h of sol-polycondensation). On the contrary, the 20% MEMO displays no alteration in the chemical

Fig. 7. FTIR spectra collected on T aged 5% MEMO 4H (dark green); QUV aged 5% MEMO 4H (light green); non-aged 5% MEMO 4H (magenta); evolution of appearance of crystalline phase on RH/T aged 5% MEMO 4Ha (cyan); RH/T aged 5% MEMO 4Hb (dark cyan); RH/T aged 5% MEMO 4Hc (dark blue) and reference spectrum of calcium carbonate (pink). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

composition detectable with FTIR spectroscopy. The same tendency is visible in the samples of the 5% and 20% GLYEO coatings, hence we assumed that in this work the higher amount of the organically functionalised Si-alkoxide modifiers to TEOS is more promising. As also shown in Fig. 7, no significant changes in the chemical composition are observed by the FTIR spectroscopy for the QUV and the T aged samples. However, as proved by the optical and scanning electron microscopy and by the static contact angle measurement, there is probably a significant modification displaying on the coating surface induced by the QUV ageing test. The QUV ageing is likely to modify only the top-surface of the coatings and does not affect the whole coating mass up to the glass/coating interface. In the case of the T model ageing test, no significant damage was observed. It is assumed that elevated temperature could even complete the curing of the coatings and thus, may improve their performance. However, since the absence of heat treatment is one of the most important requirements of the conservation-restoration practice regarding historical glass, the preparation of hybrid silica coatings without any heat curing should be considered. Considering the best performing coatings of the present work, such a preparation is possible. 5. Conclusions Various hybrid inorganic–organic coatings were successfully designed starting from TEOS and organically functionalised silica alkoxides using the sol–gel process without any heat treatment. This synthetic route proved to be an effective and simple technique for the preparation of easily applicable thin, transparent and homogenous protective coatings, stable under exposure to different model ageing tests and with good adhesion to glass substrates, thereby meeting the main requirements for the building maintenance and restoration-conservation practice. The best weatherability results were achieved using silica coatings functionalised with long alkyl chains. Better resistance to the accelerated ageing test was also achieved by the introduction of methacryloxy-functionality (especially in the higher ratio of this organic component to TEOS), which probably enhanced the resulting network elasticity when depositing the sols. Those films displayed overall homogeneity, high water repellency and good

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transparency, which are the main requirements for an optimal coating. Regarding the tested commercial products, their performance in model ageing tests does not fulfil the requirements desired. In relation to the different tested sol–gel parameters, it could be concluded that the higher ratio of an organically functionalised Si-alkoxide modifier to TEOS and the longer time period of solpolycondensation are more promising. However, it seems that it is not possible to generalise this statement further as the final performance depends rather on a chemical nature of an added functionality. This fact is illustrated by one of the most promising results of the work, 5% HDTMS (the coating with a lower ratio of the modifier to the TEOS), already analysed and designed by [3]. The most harmful weathering parameter appears to be a combination of elevated temperature and elevated relative humidity together with UV irradiance. It is assumed that higher water repellency, which is often presented as a self-cleaning ability, is not the assurance of better weatherability, since the coatings with the worst resistance results also display higher hydrophobicity based on the physical properties of their surfaces in the non-aged state. For a good weatherability performance, a surface of a coating should be as homogenous and regular as possible, without any significant micro-protrusions and display higher chemical resistance. Acknowledgment The authors would like to thank the Ministry of Culture, Czech Republic, for financial support of this research under the project DF12P01OVV017. References [1] S. Davison, R.G. Newton, Conservation and Restoration of Glass, 1st ed., Butterworth-Heinemann, Oxford, 2003. [2] R.G. Newton, A.B. Seddon, Organic coatings for medieval glass, in: N.H. Tennent (Ed.), The Conservation of Glass and Ceramics: Research, Practice and Training, James & James, London, 1999, pp. 66–71. [3] L. de Ferri, P. Lottici, A. Lorenzi, A. Montenero, G. Vezzalini, Hybrid sol–gel based coatings for the protection of historical window glass, J. Sol–Gel Sci. Technol. 66 (2013) 253–263. [4] N. Carmona, M.A. Villegas, J.M. Fernández Navarro, Protective silica thin coatings for historical glasses, Thin Solid Films 458 (2004) 121–128. [5] M. De Bardi, H. Hutter, M. Schreiner, R. Bertoncello, Sol–gel silica coating for potash–lime–silica stained glass: applicability and protective effect, J. Non-Cryst. Solids 390 (2014) 45–50. [6] B. Dal Bianco, R. Bertoncello, A. Bouquillon, J.-C. Dran, L. Milanese, S. Roehrs, C. Sada, J. Salomon, S. Voltolina, Investigation on sol–gel silica coatings for the protection of ancient glass: interaction with glass surface and protection efficiency, J. Non-Cryst. Solids 354 (2008) 2983–2992. [7] A. Ershad-Langroudi, C. Mai, G. Vigier, R. Vassoille, Hydrophobic hybrid inorganic–organic thin film prepared by sol–gel process for glass protection and strengthening applications, J. Appl. Polym. Sci. 65 (1997) 2387–2393. [8] R. Briard, C. Heitz, E. Barthel, Crack bridging mechanism for glass strengthening by organosilane water-based coatings, J. Non-Cryst. Solids 351 (2005) 323–330. [9] V.V. Ganbavle, U.K.H. Bangi, S.S. Latthe, S.A. Mahadik, A.V. Rao, Self-cleaning silica coatings on glass by single step sol–gel route, Surf. Coat. Technol. 205 (2011) 5338–5344.

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