Interplay of Hydrophobic Thiol and Polar Epoxy Silicate Groups on Microstructural Development in Low-Alcohol, Crosslinked Sol–Gel Coatings for Corrosion Prevention

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Article Interplay of Hydrophobic Thiol and Polar Epoxy Silicate Groups on Microstructural Development in Low-Alcohol, Crosslinked Sol–Gel Coatings for Corrosion Prevention

Shegufa Shetranjiwalla * , Andrew J. Vreugdenhil and Oliver Strong

Department of Chemistry, Inorganic Materials Research Laboratory, Trent University, 1600 West Bank Drive, Peterborough, ON K9L 0G2, Canada; [email protected] (A.J.V.); [email protected] (O.S.) * Correspondence: [email protected]; Tel.: +1-705-748-1011 (ext. 7319)

Abstract: We have demonstrated that our patented, crosslinked, sol–gel, epoxy–thiol silicates made from the combination of (a) tetraethoxysilane (TEOS, T), 3-glycidoxypropyltrimethoxysilane (GPTMS, G), and the (b) sulfur-containing 3-mercaptopropyltrimethoxysilane (MPTMS, S) with TEOS in a 1:1 stoichiometric ratio form the 1:1 TGST (crosslinked epoxy and thiol silicates) coating, which can be successfully utilized for the corrosion protection of low-carbon steel. Alcohols that are a by-product of sol–gel reactions inﬂuence the network formation, crosslinking density, and formulation stability, are volatile organic contents, and are regulated in the coatings industry. To improve environmental sustainability, a series of low-alcohol (LA) formulations with TG:ST ratios of 3:1 to 1:3 was prepared to investigate the microstructural development and crosslinking reactions emerging from the interplay of the hydrophobic thiol and polar epoxy silicates induced by the low-alcohol environment. The Citation: Shetranjiwalla, S.; impact on crosslinking density was characterized by Fourier Transform Infrared (FTIR), Raman, XPS, Vreugdenhil, A.J.; Strong, O. Interplay viscosity, and pot-life measurements. Low-alcohol TGST (LA(TGST)) formulations were compared, of Hydrophobic Thiol and Polar Epoxy Silicate Groups on using the example of 1:1 TGST, to corresponding TGST formulations where alcohols were retained. Microstructural Development in The reduced impact of LA(TGST) formulations on global warming was quantiﬁed. The glossy and Low-Alcohol, Crosslinked Sol–Gel scratch-resistant LA(TGST) coatings showed 71% enhanced corrosion protection compared to the Coatings for Corrosion Prevention. non-crosslinked hybrids. Coatings 2021, 11, 306. https://doi. org/10.3390/coatings11030306 Keywords: organic–inorganic hybrids; sol–gel; low-volatile organic content; hydrophobic coatings; anti-corrosive coatings; epoxy–thiol silicates Academic Editor: Rita Bacelar Figueira

Received: 7 February 2021 1. Introduction Accepted: 5 March 2021 Published: 8 March 2021 Organic–inorganic hybrid thin ﬁlm coatings prepared by the sol–gel method are an attractive corrosion suppression alternative to chromate-based coatings, which are broadly

Publisher’s Note: MDPI stays neutral recognized as being toxic to human health and the environment. The versatile chem- with regard to jurisdictional claims in istry of the sol–gel process [1] allows for tunable properties of coatings with improved published maps and institutional afﬁl- fracture [2], abrasion, scratch and thermal resistance by incorporating inorganic compo- iations. nents such as silicon [3], aluminum, zirconium [4], or titanium [5] under mild reaction conditions forming metal–substrate bonds. Customized nanoparticle size and porosity [6] can be achieved by controlling various formulation parameters such as pH, temperature [7], and the nature of the organic functional groups [8]. Sol–gel synthesis occurs by the polycondensation reactions that proceed with the hydrolysis and condensation of Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. typically monomeric alkoxide sol-gel precursors [1,7] such as tetraethoxysilane (TEOS) This article is an open access article and 3-glycidoxypropyltrimethoxysilane (GPTMS). The surface chemistry is altered as a distributed under the terms and function of the polarity of the organic component [9], transforming it into a hydrophobic or conditions of the Creative Commons a hydrophilic surface, thereby promoting adhesion. Alcohols are released as by-products Attribution (CC BY) license (https:// of the reaction and play an important role in the microstructural development of sol–gel creativecommons.org/licenses/by/ derived materials. They promote the miscibility of precursors in the solvent during the 4.0/). synthesis, charge stabilization of intermediates, and they determine the rate of hydrolysis

Coatings 2021, 11, 306. https://doi.org/10.3390/coatings11030306 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 306 2 of 18

of precursors [10]. In addition, alcohols transesterify hydrolyzed sol–gel species via alkoxy exchange, which is accelerated in the acidic environment and impacts particle properties under basic conditions [10]. The removal of alcohols, although easy to recycle, can alter the stability of the formulation [11], leading to phase separation or precipitation of the sol–gel colloids. Generally, alcohols are evaporated under pressure after the synthesis is complete or during coatings application. However, alcohols are volatile organic compounds (VOCs) and are highly regulated in the paints and coatings industry, prompting the development of low-VOC materials with high performance. Coatings from GPTMS and methyltrimethoxysilane monomeric precursors that are typically used for the corrosion prevention of steel substrates have been shown to have poor adhesion [12]. Similarly, oligomeric crosslinked GPTMS and TEOS surface treatments have also failed on low-carbon steel [3]. Hybrid epoxy silane coatings grafted with different concentrations of amino silanes have shown improved performance and adhesion, but similar hybrids with thiol silanes have proved inferior [13]. In contrast, signiﬁcant improvement (96%) in corrosion prevention properties has been established for polymeric (9000 g/mol) crosslinked epoxy and thiol silicates (TGST) compared to the non-crosslinked hybrids [3]. The enhanced corrosion inhibition was attributed to the covalent interactions between the epoxy silicate (TG) and thiol silicate (ST) functional groups. The alcohol content was retained during crosslinking reactions. In the present work, in order to improve the environmental sustainability of the novel coatings and understand the impact of elimination of alcohols from the formulations on the extent of crosslinking between the hydrophobic thiol and polar epoxy silicate groups and resultant changes in the coatings structure, a series of low-alcohol (LA) formulations were prepared with LA(TG):LA(ST) ratios from 3:1 to 1:3 and compared with corresponding formulations of TG and ST where alcohols were retained. The formulations were characterized by Fourier Transform Infrared (FTIR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), and formulation stability was monitored by viscosity and pot-life measurements. Scratch resistance was tested by pencil hardness tests using the ASTM D3363 method. The accelerated corrosion protection performance of the coatings after immersion in Dilute Harrisons Solution (DHS) was quantiﬁed using digital image analysis.

2. Materials and Methods 2.1. Materials 3-Glycidyloxypropyltrimethoxysilane (GPTMS, 98%), 3-mercaptopropyltrimethoxysilane (MPTMS, 98%), and tetraethoxysilane (TEOS, 98%) were purchased from Gelest Inc., Mor- risville, PA, USA and used as received. Acetic acid and hydrochloric acid, reagent grade, was purchased from Sigma Aldrich, Oakville, Ontario, Canada and diluted with ultrapure de-ionized (DI) water with an initial resistivity of 18.2 MΩ, to form 0.05M acetic acid solution and 25% v/v hydrochloric acid solution, respectively. Fluorad™ Fluorosurfactant FC-4430, non-ionic ﬂuoro surfactant was purchased from 3M™ and diluted to form a 0.1% w/w solution with ultrapure water. Low-carbon (C < 0.25%, Mn < 0.9%, P < 0.04% with remainder Fe) steel shims (800 × 1200) were purchased from McMaster Carr (Illinois, IL, USA), and were cut into panels of 2.500 × 1.000, which were coated with the TGST formulations. A pencil hardness tester kit was purchased from Gardco (Paul N. Gardner Co. Inc., Florida, FL, USA).

2.2. Preparation of Sol–Gel Formulations The low volatile organic content (VOC) formulations were prepared in three phases. First, the precursor epoxy silicate (TG) and thiol silicate (ST) formulations were prepared as reported [3]. In brief, GPTMS (0.06 mol, 14.49 mL) and TEOS (0.02 mol, 4.56 mL) were mixed in a 3:1 molar ratio. The silanes mixture was added dropwise to 21.6 mL of acidiﬁed deionized water (0.05 M acetic acid). The solution was sealed and stirred continuously (≈300 rpm) for 72 h at room temperature (RT = 22 ◦C). ST was prepared in another ﬂask by mixing MPTMS (0.06 mol, 12.19 mL) and TEOS (0.02 mol, 4.56 mL) Coatings 2021, 11, 306 3 of 19

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(≈300 rpm) for 72 h at room temperature (RT = 22 °C). ST was prepared in another flask by mixing MPTMS (0.06 mol, 12.19 mL) and TEOS (0.02 mol, 4.56 mL) also in a 3:1 molar ratio.also inThe a 3:1 silanes molar mixture ratio. The was silanes added mixture dropwise was to added 21.6 mL dropwise of acidified to 21.6 deionized mL of acidified water (0.05Mdeionized acetic water acid). (0.05 The M solution acetic acid). was Thestirred solution continuously was stirred (≈300 continuously rpm) for 50 (≈ min300 at rpm) room for temperature50 min at room until temperature the mixture untilwas clear. the mixture Second, was methanol clear. Second,and ethanol, methanol which and are ethanol,the by- productswhich are of the the by-products sol–gel reactions, of the were sol–gel removed reactions, under were reduced removed pressure under reduced(55 °C, ≈300 pressure torr) ◦ from(55 C,both≈ 300the torr)TG and from the both ST formulations the TG and theafte STr synthesis. formulations Theoretical after synthesis. amounts Theoreticalof alcohols basedamounts on the of alcohols initial 3:1 based TEOS/GPTMS on the initial and 3:1 MPTMS TEOS/GPTMS stoichiometry and MPTMS were removed stoichiometry to obtain were theremoved low-alcohol to obtain (LA) the TG low-alcohol (coded as LA(TG)) (LA) TG an (codedd low-alcohol as LA(TG)) ST and(coded low-alcohol as LA(ST)) ST disper- (coded sionsas LA(ST)) (Scheme dispersions 1). Acidified (Scheme deionized1). Acidified water deionizedwas added water as replacement was added of as the replacement removed alcoholsof the removed to the LA(TG) alcohols and to LA(ST) the LA(TG) solutions and LA(ST)to maintain solutions the formulation to maintain volume, the formulation concentrations,volume, pH, concentrations, and viscosity. pH, Third, and viscosity. LA(TG) Third,was reacted LA(TG) with was LA(ST) reacted at with room LA(ST) temperature at room fortemperature 1 h in the forfive 1 stoichiometric h in the five stoichiometric ratios (3:1, 2:1, ratios 1:1, (3:1,1:2, and 2:1, 1:1,1:3 1:2,epoxy: and thiol 1:3 epoxy: silicates thiol to formsilicates homogenous to form homogenous LA(TGST) formulations LA(TGST) formulations (Scheme 1). (SchemeThe surfactant1). The solution surfactant was solution added inwas a 1:2 added volume ina ratio 1:2volume of surfactant ratio ofsolution surfactant to total solution sol volume to total and sol stirred volume forand 10 min stirred before for coating10 min the before panels coating with the the panels resulting with formulations. the resulting The formulations. dry film thickness The dry (DFT) film thickness was between(DFT) 0.5 was and between 0.9 µm, 0.5 which and is 0.9 inµ agreementm, which iswith in agreementreported literature with reported of similar literature formula- of tionssimilar [14]. formulations To prepare [14 the]. Tocorresponding prepare the corresponding alcohol-rich series alcohol-rich in the seriesfive ratios in the described five ratios above,described TG and above, ST TGformulations and ST formulations where alcohols where were alcohols retained were were retained reacted were for reacted 1 h at RT for to1 hform at RT the to homogenous form the homogenous crosslinked crosslinked TGST formulations TGST formulations in varying in varyingstoichiometric stoichiometric ratios. Theratios. formulation The formulation TG:ST 1:1, TG:ST which 1:1, was which reported was reported earlier [3], earlier was [ 3reprepared], was reprepared in this instudy this study for comparison with LA(TG):LA(ST) 1:1. for comparison with LA(TG):LA(ST) 1:1.

GPTMS + TEOS MPTMS + TEOS

3:1, 2:1 + RT, 300 rpm, 60 min 1:1 LA(TGST) 1:2, 1:3

LA(TG) LA(ST) Methanol + Ethanol removed + 55 °C, 300 Torr, DI water(H )

SchemeScheme 1. 1. SchematicSchematic microstructure microstructure and and pathway pathway for for the the crosslinki crosslinkingng of of low-alcohol low-alcohol LA(TG) LA(TG) and and (LA)ST (LA)ST precursors precursors to to form the LA(TGST) system in five ratios. TG: epoxy silicate, ST: thiol silicate. form the LA(TGST) system in ﬁve ratios. TG: epoxy silicate, ST: thiol silicate. 2.3. Substrate Preparation and Coatings Application 2.3. Substrate Preparation and Coatings Application Low-carbon steel panels were cleaned by first sonicating them in acetone for 3 min Low-carbon steel panels were cleaned by ﬁrst sonicating them in acetone for 3 min followed by 3 min in a 25% hydrochloric acid bath. The panels were rinsed with distilled followed by 3 min in a 25% hydrochloric acid bath. The panels were rinsed with distilled water between the two sonication steps to avoid contamination of the baths. To assess the water between the two sonication steps to avoid contamination of the baths. To assess corrosion protection ability of LA(TGST), six clean low-carbon steel panels were each dip- the corrosion protection ability of LA(TGST), six clean low-carbon steel panels were each coated three times in the LA(TGST) bath. The panels were inserted for 10 s in the bath and dip-coated three times in the LA(TGST) bath. The panels were inserted for 10 s in the bath and dried for 40 s between the three applications followed by drying at room temperature in air for 24 h. All panels displayed the development of varying degrees of a dark green

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layer within 1 and 5 min of coating application. A similar method was followed for panels dip-coated in a TGST bath.

2.4. Characterization FTIR was performed on a Thermo Scientific Nicolet 380 FTIR spectrometer (Thermo Electron Scientific Instruments, LLC, Madison, WI, USA) equipped with a PIKE MIRacleTM attenuated total reflectance (ATR) system (PIKE Technologies, Madison, WI, USA). The spectra for the formulations were acquired in the scanning range of 400–4000 cm−1 using 64 scans at a resolution of 4 wavenumbers. All spectra were recorded at ambient temperature. The change in epoxy and thiol peak intensities were monitored after normalizing −1 the spectra to the intensity of the peak at 2848 cm attributed to the alkyl CH2 C-H stretching peak. Raman spectra for the formulations were acquired using a Renishaw inVia Raman Microscope with 633 nm excitation and an 1800 lines/mm diffraction grating. Laser power at the source was 100 mW. In total, 100 scans were co-added to achieve appropriate signal to noise in the range from 100 to 3200 cm−1. The rate of change of the thiol (2580 cm−1) and epoxy (1260 cm−1) peak intensities was plotted by first normalizing the spectra to the intensity of the peak at 1450 cm−1, which is assigned to the stretch of the alkyl group (Si-CH2-R). Clearly discernable changes in intensity were observed at 17 h of crosslinking time, and formulations were monitored every hour after that while also continuously monitoring for gelation and precipitation. Relative viscosity (ηrel) measurements using 20 mL of sample volume against acidified ultrapure water at 25 ◦C were conducted using a Cannon–Fenske Routine Viscometer 350 with an approximate constant (k’) of 0.5 centistokes/sec with a range of 100–500 centistokes manufactured by IndusChem Lab Glass Chem Co. New Jersey. The absolute viscosity was calculated using Equation (1).

Absolute viscosity (η) = k0 ∗ ρ ∗ t (1)

where k’ = 0.5 is the constant for the viscometer, ρ is the density of the solution, and t is the mean flow time of solution. The XPS analyses were carried out on the coated panels with a Thermofisher Scientific K(alpha) spectrometer using a monochromatic Al Kα source (15 mA, 15 kV). The instrument work function was calibrated using sputter-cleaned Au, Ag, and Cu to determine the absolute linearity of the binding energy scale, with 83.96 eV for the Au 4f7/2 line for metallic gold, 368.21 eV for Ag 3d5/2, and 932.62 eV for the Cu 2p3/2 line of metallic copper. Survey scan analyses were carried out with 200 eV pass energy and 1 eV/step. The X-ray spot size was 400 µm (a 2:1 ellipse with the noted spot size corresponding to the major axis). High-resolution spectra were obtained with an analysis area of 300 × 700 microns and a pass energy of 20 eV. Peak fitting was done using CASA XPS (version 2.31). Electron binding energies were corrected using the carbon 1 s peak at 284.8 eV. The corrosion resistance of LA(TGST) and TGST coated panels was monitored by immersing them in Dilute Harrison’s Solution (DHS) prepared by dissolving 3.5 g (NH4)2SO4 and 0.5 g NaCl in 1000 mL of ultrapure water with an initial resistivity of 18.2 MΩ. Six surface-treated, dry panels were first covered with waterproof adhesive tape on the edges to minimize edge effects after treatment. Three of the six samples were scribed along 3 cm to bare metal using a diamond tipped pen. The panels were placed in sealed plastic containers and then immersed in 60 mL of DHS until the first signs of corrosion were observed. Ambient dissolved oxygen levels were not monitored during the experiment; however, the typical dissolved oxygen content in NaCl solutions of this type is 6.5 ppm [15]. The rust percentage associated with the corrosion prevention performance was calculated by digital image analysis of exposed panels using the ImageJ image processing program developed at the U.S. National Institutes of Health [16]. Scratch resistance was assessed by pencil hardness tests within 48 h of coating based on the ASTM D3363 standard. Seventeen pencils of different hardnesses varying from the Coatings 2021, 11, 306 5 of 18

softer 6B to F and harder from H to 9H (9H being hardest) were moved along the coated surface using the pencil tester at a ﬁxed 45◦ angle in a 6.5 mm stroke. Calculations for reducing the impact of volatile organic compounds (VOC) in the low-alcohol formulations were done using the International Standards Organization (ISO) metric of global warming potential (GWP) using the following equations [17–19]:

GWP = NC/MW(alcohol)/NC(CO2)/MW(CO2) (2)

where NC is the number of carbons in the molecule and MW is the molecular weight.

Global Warming Index = I(GWP) = GWP*m (3)

where m is the mass of the emitted volatile organic compound from the prepared formulation.

3. Results and Discussion The low-alcohol epoxy silicate LA(TG) and thiol silicate LA(ST) colloids (Scheme1) interact with each other in two ways. First, formation of the siloxane (Si-O-Si) network by reaction of silanol groups on the surface of LA(TG) and LA(ST) colloids leads to an increase in particle size and aggregation. This directly results in an increase of the hydrophobic nature of the colloids. Second, there are the covalent interactions between LA(TG) and LA(ST) due to the simple nucleophilic epoxide ring opening by the thiol groups [11], creating new C-S bonds and introducing new hydroxyl binding sites, thus increasing the crosslinking density. This reaction constitutes a polycondensation reaction and follows the kinetics of a step growth polymerization forming a network emerging from multifunctional epoxy and crosslinking thiol groups [20]. The Si-O-Si network leads to the formation of the M-O-Si bonds with the metal substrate, facilitating strong interactions between the coating and the substrate, effectively reducing the availability of the metal surface to form metal oxides such as rust. Some of the non-polar thiol groups of LA(ST) act to provide a hydrophobic envelope which deters the diffusion of water and other polar species to the metal surface, while the other available thiol groups of LA(ST) shield the metal surface by creating a passive polymeric coating [21,22]. Finally, the formation of thiolates with the metal mitigates corrosion onset. Therefore, it is expected that a variation in the thiol concentration will (i) potentially affect the extent of epoxide ring opening by thiol groups and (ii) an increase in thiol concentration (as with LA(TGST) 1:2 and 1:3) will increase the accessibility of these groups to complement crosslinking mechanisms and further enhance the corrosion resistance properties of the LA(TGST) coatings. However, the elimination of the alcohols from the precursors before crosslinking is also expected to affect the rate of crosslinking reactions between the silica species and the thiol and epoxide groups directly impacting the microstructure, particle size, and aggregation and ﬁnally the corrosion resistance of these coatings. The LA(TGST) and the TGST coatings prepared with varying epoxy/thiol stoichiometry from 3:1 to 1:3 were fully characterized, and corrosion prevention performance was measured for both systems.

3.1. Structural Characterization The structure of the various formulations was characterized by Raman and FTIR spectroscopy. Figure1a shows the Raman spectra for the low-alcohol LA(TG), LA(ST), and crosslinked LA(TGST) formulations contrasted with the TG, ST, and the crosslinked TGST versions in a 1:1 stoichiometry. Coatings 2021, 11, 306 6 of 19 Coatings 2021, 11, 306 6 of 18

(a) (b) (c) Si-O-C C-S C-S -R Si-O-H 2 Si-O-Si Thiol/Thioether Disulfide C-O-H C-S S-S Free

Thiol Thioether Si-OH (S-H ) (S-H Si-O-Si

Si-CH Disulfide nR-S-H Epoxy Si-O-H (C-O-C) C-O-H Epoxy 1:2 LA(TGST) Si-O-M C-H Thiol 1:1 LA(TGST) S-H Epoxy 2:1 LA(TGST) Free (C-O-C) 1:1 TGST Si-O-H

LA(TG) 1:2 TGST Absorbance TG Absorbance 1:3 LA(TGST) 2:1 TGST

Intensity (counts) LA(ST) ST 1:3 TGST

500 1000 1500 2000 2500 3000 4000 3000 2000 1000 800 750 700 650 600 550 -1 -1 Wavenumbers (cm-1) Wavenumber (cm ) Wavenumbers (cm )

Figure 1. StructuralStructural characterization characterization of of the the precursor precursor formulatio formulationsns LA(TG) LA(TG) and and LA(ST) LA(ST) and and the thehybrid hybrid LA(TGST) LA(TGST) formu- for- lationsmulations by Raman by Raman (a) and (a) andFourier Fourier Transform Transform Infrar Infrareded (FTIR) (FTIR) spectroscopy spectroscopy (b,c) contrasted (b,c) contrasted with the with TG, the ST TG, and ST the and TGST the TGSThybrids. hybrids.

The characteristic absorbance values fo forr the epoxy C-O-C ring symmetric stretch −1 −1 (1260 cm cm−1),), thiol thiol S-H S-H stretch stretch at at 2580 2580 cm cm−1, the, the S-H S-H stretch stretch for for the the thiol thiol group group attached attached to −1 anto analky alky group group at 652 at 652 cm cm−1, and, and the thepeaks peaks associated associated with with the thesilica silica stretches stretches namely, namely, Si- −1 −1 −1 OHSi-OH (880 (880 cm− cm1), Si-O-Si), Si-O-Si (1020 (1020 cm−1), cm and), Si-CH and Si-CH2-R (14502-R cm (1450−1) (Figure cm ) (Figure1a), confirm1a), confirm the for- mationthe formation of crosslinked of crosslinked colloidal colloidal silica networks silica networks from TG from and TG ST and[23]. ST Furthermore, [23]. Furthermore, the re- ducedthe reduced intensity intensity of the epoxy of the epoxyand thiol and peaks thiol in peaks LA(TGST) in LA(TGST) and TGST and formulations TGST formulations relative torelative LA(TG), to LA(TG), TG, and TG, LA(ST), and LA(ST), ST indicate ST indicate that the that epoxy–thiol the epoxy–thiol curing curing is extensive is extensive in both in systems.both systems. In addition, In addition, interfacial interfacial interactions interactions between between TG and TG andST to ST form to form TGST TGST for the for the1:1 hybrid1:1 hybrid have have been been shown shown to tooccur occur immediately immediately [3]. [3 ].Figure Figure 1b1b shows shows the the FTIR spectra forfor thethe 1:3 LA(TGST) and TGST formulations that complement the structuralstructural identificationidentification by Raman spectroscopy. Characteristic peaks peaks for for the C-H of the three-membered epoxy ring (2978 cm−1), C-O-C epoxy ring breathing (1248 cm−1), S-H stretching vibration for ring (2978 cm−1), C-O-C epoxy ring breathing (1248 cm−1), S-H stretching vibration for the the thiols groups (2615 cm−1), silanol (Si-OH, 890 cm−1), and Si-O-M (≈900 cm−1), Si- thiols groups (2615 cm−1), silanol (Si-OH, 890 cm−1), and Si-O-M (≈900 cm−1), Si-alkoxy (Si- alkoxy (Si-O-C) and siloxane (Si-O-Si) bonds at ≈1110 cm−1, and the unbounded and O-C) and siloxane (Si-O-Si) bonds at ≈1110 cm−1, and the unbounded and intermolecularly intermolecularly bonded alcohol and silanol stretching vibrations above 3400 cm−1 confirm bonded alcohol and silanol stretching vibrations above 3400 cm−1 confirm the crosslinked the crosslinked nature of the hybrids [3,24]. Figure1c indicates the characteristic peak nature of the hybrids [3,24]. Figure 1c indicates the characteristic peak assignments for the assignments for the thiol (C-S-H, 678 cm−1), thioester (C-S-C, 632 cm−1), the C-S (599 cm−1), thiol (C-S-H, 678 cm−1), thioester (C-S-C, 632 cm−1), the C-S (599 cm−1), and S-S (580 cm−1) and S-S (580 cm−1) bond vibrations associated with the disulfide (C-S-S-C) bonds in bond vibrations associated with the disulfide (C-S-S-C) bonds in the fingerprint region the fingerprint region [25] for the 1:2 and 2:1 ratios, respectively. The presence of all [25] for the 1:2 and 2:1 ratios, respectively. The presence of all the characteristic peaks in the characteristic peaks in the crosslinked low-alcohol LA(TGST) formulations confirms the crosslinked low-alcohol LA(TGST) formulations confirms that the removal of alcohols that the removal of alcohols did not affect the structural integrity of the formulations did(Figure not 1affecta,b). Spectrathe structural for all formulationsintegrity of the in formulations the fingerprint (Figure region 1a,b). are provided Spectra for in Figureall for- mulationsS1 in the Supporting in the fingerprint Information. region are provided in Figure S1 in the Supporting Infor- mation. 3.2. Effect of Stoichiometric Variation on Crosslinking Density in Low-Alcohol Formulations 3.2. Effect of Stoichiometric Variation on Crosslinking Density in Low-Alcohol Formulations To investigate the impact of elimination of alcohols on the extent of crosslinking of the epoxy–thiolTo investigate silicates the impact and the of sol–gel elimination network of alcohols formation on for the the extent varying of crosslinking ratios, Raman of theand epoxy–thiol IR spectroscopy silicates of and the TGSTthe sol–gel formulations network formation was conducted for the to varying clearly ratios, elucidate Raman the andmicrostructural IR spectroscopy changes of the incurred TGST formulations by the LA(TGST) was conducted system when to clearly alcohols elucidate were the absent. mi- crostructuralRaman spectrum changes for theincurred TG:ST by 1:1 the formulation LA(TGST) wassystem measured when alcohols again for were this absent. study andRa- mannormalized spectrum to comparefor the TG:ST with the1:1 formulation LA(TGST) 1:1 was formulation. measured again for this study and nor- malizedRaman to compare spectra showingwith the theLA(TGST) variation 1:1 of formulation. the normalized intensities of epoxy and thiol peaksRaman with change spectra in showing epoxy–thiol the variation silicate stoichiometry of the normalized after completion intensities of epoxy the crosslinking and thiol peaksreaction with (crosslinking change in timeepoxy–thiolt = 0 h) and silicate then stoichiometry at t = 20 h of curing after timecompletion are shown of the in Figure crosslink-2a,b. ingFigure reaction2c shows (crosslinking the spectra time for tLA(TGST) = 0 h) andratios then at at t 20 = h.20 Figureh of curing2d quantifies time are the shown extent in Figureof curing 2a,b. as Figure a percentage 2c shows with the changing spectra for TG:ST LA(TGST) and LA(TG):LA(ST) ratios at 20 h. ratios Figure at 2d the quantifies different thecrosslinking extent of curing times usingas a percentage the ratio of with the changing intensity ofTG:ST the epoxideand LA(TG):LA(ST) and thiol peaks. ratios Table at the1 differentshows the crosslinking relative curing times with using the the example ratio of of the TGST intensity ratios of with the progressingepoxide andcrosslinking thiol peaks.

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Coatings 2021, 11, 306 7 of 18 Table 1 shows the relative curing with the example of TGST ratios with progressing crosslinking time. The spectral trends seen at t = 17 h crosslinking time are provided in the Supporting Information (Figure S2). time. The spectral trends seen at t = 17 h crosslinking time are provided in the Supporting InformationTable 1. Variation (Figure of extent S2). of curing in TGST formulations with changing epoxy–thiol stoichiometry.

(a) (b)

1:3 1:3

1:2 1:2

1:1 1:1 2:1 2:1 Intensity (counts) Intensity (counts) Intensity 3:1

0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 -1 Wavenumber (cm-1) Wavenumber (cm ) (c) (d) 100 TGST 0 h TGST 17 h 1:3 90 TGST 20 h LA(TGST) 20 h 1:2 80

70 1:1 60

Intensity (counts) 2:1 Intensity (%) ratio Intensity 50 3:1 *100 epoxy/(epoxy+thiol) 40 0 500 1000 1500 2000 2500 3000 3-12-11-11-21-3 -1 Wavenumber (cm ) TG:ST/LA(TG):LA(ST) ratios

Figure 2. Raman spectra showing the variation of epoxy and thiol peaks for reaction of TGST at (a) 0 h, (b) 20 h, and for LA(TGST)Figure 2. at Raman (c) 20 h.spectra (d) Extent showing of curingthe variation shown of byepoxy the relativeand thiol intensities peaks for reaction of epoxy of and TGST thiol at ( moietiesa) 0 h, (b) with 20 h, increasing and for TG:ST/LA(TG):LA(TGST) at ( LA(ST)c) 20 h. ratios(d) Extent at different of curing crosslinking shown by times.the relative intensities of epoxy and thiol moieties with increasing TG:ST/LA(TG): LA(ST) ratios at different crosslinking times. Table 1. Variation of extent of curing in TGST formulations with changing epoxy–thiol stoichiometry. As expected, the peak for the epoxy (C-O-C stretch, 1260 cm−1, Figure 2a–c) reduced inTG:ST relative Ratio intensity with decreasing Relative TGEpoxy–Thiol content in the Absolute formulations Peak Intensities from TGST 3:1 to 1:3 at all crosslinking times. Concurrently,(a) a marked(b) increase (c)in the intensityExtent of of the Curing thiol peak with t (S-H stretch, 2580 cm−1,t Figure= 0 h 2a,b) wast = observed 17 h witht = increasing 20 h ST content.(a Minus Interest- c) ingly, the relative intensities of the peaks for the TGST 3:1 and 2:1 ratio remain unchanged 3:1 9.335 9.260 - Gelation from t 2:1= 0 h to t = 17 h (Figure 5.880 2d and Table 5.869 1), but a rapid 4.215 densification and gelation 1.665 was observed1:1 at t = 20 h. At 20 4.892 h, TGST 3:1 was 3.875 the only formulation 2.533 that had gelled 2.362 completely (Table 1:21). Therefore, the 3.680 unchanged relative 3.193 intensity at 2.183t = 0 h and t = 17 h for 1.497 TGST 2:1 (Table 1:31) indicated that 2.221the high epoxy conc 1.487entration in 1.150 these formulations 1.0703remained in excess after completely reacting with the thiol functional groups. In TGST 2:1, which has As expected, the peak for the epoxy (C-O-C stretch, 1260 cm−1, Figure2a–c) reduced in relative intensity with decreasing TG content in the formulations from TGST 3:1 to 1:3 at all crosslinking times. Concurrently, a marked increase in the intensity of the thiol peak (S-H stretch, 2580 cm−1, Figure2a,b) was observed with increasing ST content. Interestingly, the relative intensities of the peaks for the TGST 3:1 and 2:1 ratio remain unchanged from Coatings 2021, 11, 306 8 of 18

t = 0 h to t = 17 h (Figure2d and Table1), but a rapid densification and gelation was observed at t = 20 h. At 20 h, TGST 3:1 was the only formulation that had gelled completely (Table1). Therefore, the unchanged relative intensity at t = 0 h and t = 17 h for TGST 2:1 (Table1) indicated that the high epoxy concentration in these formulations remained in excess after completely reacting with the thiol functional groups. In TGST 2:1, which has a lower epoxy concentration, curing was evidenced by the reduced relative intensity (Figure2d and Table1) at t = 20 h. Therefore, it would be expected that for a similarly high concentration of thiol groups in TGST 1:3 and 1:2 formulations, a similar trend would be observed. However, a decreasing trend of relative intensities at all crosslinking times was seen, indicating higher curing (Table1). Such a trend is attributed to two factors: (i) higher relative thiol to epoxy concentration leading to extended curing and (ii) the development of disulfide dimers. The excess concentration of sulfur containing ST in TGST 1:2 and 1:3 most likely promoted dimerization to form disulfide (S-S) bonds, which decreased the availability of functionally reactive free thiol S-H bonds in the formulation (Scheme1). S-S dimerization in ST is consistent with the observed increase in average particle size of ST (2.4 ± 0.5) compared to TG (1.1 ± 0.6) [3]. Furthermore, TGST 1:1 showed the highest difference of relative intensity at 2.362 (Table1), implying the presence of fewer thiol groups available to crosslink with, which can be explained by the transformation of available thiol groups to disulfide dimers in ST. In contrast to the trends observed in the TGST system, the LA(TGST) formulations achieved a consistent (circa 50%) extent of crosslinking at t = 20 h across all ratios (Figure2d). This indicates that irrespective of the epoxy/thiol stoichiometry, the same number of functional groups are available for crosslinking. In addition, LA(TGST) 2:1 and 1:1 have a lower thiol content than LA(TGST) 1:3, but their relative epoxy–thiol intensities are similar and are also close to that of TGST 1:3 (Figure2d), indicating that there is considerable aggregation and dimerization in these ratios because of the removal of alcohols. An increase in crosslinking density and disulfide formation associated with the increase in thiol content was confirmed by FTIR for all formulations. Figure3a,b show the increase in normalized intensity for the C-H vibration of the epoxide ring with a concurrent decrease in the thiol S-H peak intensity from 1:3 to 3:1 epoxy/thiol ratios in both the TGST (Figure3a) and LA(TGST) (Figure3b) systems. The gradual increase was attributed to effective crosslinking between the TG and ST components. A higher peak intensity of the Si-O-H (890 cm−1) and Si-O-Si peaks (≈1110 cm−1) for the 1:1, 1:2, and 1:3 formulations in contrast to the 3:1 and 2:1 formulations is potentially a result of enhanced hydrolysis and condensation of Si-alkoxy groups with the newly available OH groups emerging from the ring-opening of the epoxide by the higher thiol concentration (Figure2a,b). This explains the shift to lower frequencies for the peak centers for the Si-OH and Si-O-Si peaks (shown as the dashed red arrows in Figure3a) at 946 cm −1 to 890 cm−1 and 1110 cm−1 to 1050 cm−1 respectively from 3:1 to 1:3 ratios for TGST and LA(TGST) formulations, which is attributed to increased hydrogen bonding. The area under these peaks also corresponds to the degree of condensation between the hydrolyzed Si-O-H and the coated iron surface to form Si-O-Fe bonds. The coating–substrate covalent interactions augment the interfacial adhesion, which is an important mechanism for mitigating the onset of corrosion. Figure3c,d show the extent of crosslinking for the TGST and the LA(TGST) systems as a function of the changes in the thiol, epoxide ring, thioester, disulfide, and alcohol bond vibration frequencies. A linear correlation was observed for all the groups associated with ST, for example, C-S-H, C-S-C, C-S-S-C, in both systems exhibiting a direct increase in crosslinking density with increasing ST stoichiometry. The linear increase in the C-S-C and C-OH peak intensities from 3:1 to 1:3 TG:ST ratios also supports the growth in extent of crosslinking arising from the increased number of thiol groups available for nucleophilic attack at the strained epoxide ring, resulting in new thioester (C-S-C) and alcohol (C-O-H) moieties (Scheme1). Coatings 2021, 11, 306 9 of 19 Coatings 2021, 11, 306 9 of 18

(a) (b) Epoxy Si-O-H Thiol Si-OC Si-O-H Epoxy Si-O-C C-H Si-O-H Thiol Si-O-H Si-O-Si C-O-H C-H Si-O-Si C-O-H S-H Si-O-M S-H Si-O-M

1:3 1:3 1:2 1:2

1:1 1:1 Absorbance

2:1 Absorbance 2:1 3:1 3:1

4000 3000 2000 1000 4000 3000 2000 1000

-1 Wavenumbers (cm-1) Wavenumbers (cm ) (c) (d) 4 4 TGST C-S-C LA(TGST) C-S-C TGST C-S-S-C LA(TGST) C-S-S-C TGST C-S-H 3 LA(TGST) C-S-H 3 TGST C-OH LA(TGST) C-OH TGST C-H epoxy LA(TGST) C-H epoxy 2 2

1 Peak Peak Intensity Peak Intensity Peak 1 0 0 3-1 2-1 1-1 1-2 1-3 3-1 2-1 1-1 1-2 1-3 TGST ratios LA(TGST) ratios

Figure 3. FTIR spectra showing the variation of epoxy and thiol peaks after reaction for (a) TGST and (b) LA(TGST) formulations. (c,d) indicate the extent of crosslinking shown by various intensities associated with the epoxy and thiol Figure 3. FTIR spectra showing the variation of epoxy and thiol peaks after reaction for (a) TGST and (b) LA(TGST) for- functionalmulations. groups (c,d) indicate interaction the withextent increasing of crosslinking TG:ST shown and LA(TG):LA(ST) by various intensities ratios. associated with the epoxy and thiol functional groups interaction with increasing TG:ST and LA(TG):LA(ST) ratios. Interestingly, as the thiol content is increased in the formulations, disulﬁde content is also simultaneouslyInterestingly, as elevated the thiol (C-S-S-C content is peak increased intensity in the in Figureformulations,3c,d) in disulfide both systems. content This isis explained also simultaneously by the self-association elevated (C-S-S-C of the peak thiol intensity silicate particles, in Figure as 3c,d) illustrated in both insystems. Scheme 1. AnThis increased is explained self-association by the self-association of the more hydrophobicof the thiol silicate LA(ST) particles, particles as was illustrated also observed in inScheme the LA(TGST) 1. An increased system, whereself-association the alcohols of the were more replaced hydrophobic with water LA(ST) in theparticles formulations was affectingalso observed formulation in the LA(TGST) stability and system, increasing where particle the alcohols size, potentiallywere replaced recording with water increased in absorbancesthe formulations for all affecting peaks in formulation Figure3b,d. stabilit It wouldy and be increasing expected that particle the C-Hsize, peak potentially intensity ofrecording the epoxide increased ring would absorbances be highest for all for peaks the 3:1 in TG:STFigure ratio3b,d. andIt would would be linearlyexpected decrease that fromthe C-H 3:1 to peak 1:3 TG:ST,intensity notably; of the epoxide however, ring only would a marginal be highest increase for the in intensity3:1 TG:ST was ratio observed and fromwould 1:1 linearly to 1:3 decrease TG:ST (Figure from 3:13c). to 1:3 This TG:ST, increase notably; in thehowever, intensity only for a marginal 1:3 TGST increase despite improvedin intensity crosslinking was observed in from high 1:1 ST to and 1:3 TG:S LA(ST)T (Figure ratios 3c). (Figure This 3increasec,d) is attributedin the intensity to the competingfor 1:3 TGST disulﬁde despite formation,improved crosslinking correspondingly in high mitigating ST and LA(ST) extensive ratios crosslinking(Figure 3c,d) is with availableattributed epoxy to the groups. competing In addition, disulfide although formation, the correspondingly C-OH peak intensity mitigating may beextensive attributed tocrosslinking both crosslinking with available interactions epoxy and groups. the formation In addition, of although diols via the C-OH acid-catalyzed peak intensity epoxide ringmay opening be attributed by water, to both the crosslinking C-OH peak interactions intensity wasand the observed formation to beof lowestdiols via for the 3:1 acid- TGST andcatalyzed LA(TGST) epoxide (N and ring openingFigure3 byc,d), water, indicating the C-OH that thepeak ring intensity was not was extensively observed to opened be bylowest this mechanism.for 3:1 TGST Inand LA(TGST), LA(TGST) independent ( and  Figure of stoichiometry, 3c,d), indicati ang consistent that the ring amount was of LA(TG) was consumed by LA(ST) as is indicated by the almost constant intensity of C-H epoxy peak from 3:1 to 1:3 TG:ST (Figure3d). This implies that any expected increase in

Coatings 2021, 11, 306 10 of 19

Coatings 2021, 11, 306 not extensively opened by this mechanism. In LA(TGST), independent of stoichiometry,10 of 18 a consistent amount of LA(TG) was consumed by LA(ST) as is indicated by the almost constant intensity of C-H epoxy peak from 3:1 to 1:3 TG:ST (Figure 3d). This implies that any expected increase in TG:ST crosslinking was mitigated by the dimerization/self-ag- TG:STgregation crosslinking in LA(ST) was and mitigated ST colloids. by The the dimerization/self-aggregationself-aggregation of thiol silicates in can LA(ST) be ex- and STplained colloids. by the The absence self-aggregation of the charge-stabilizing of thiol silicates presence can be of explained alcohols that by the were absence removed of the charge-stabilizingfrom the LA(TGST) presence formulations. of alcohols that were removed from the LA(TGST) formulations. Overall,Overall, threethree keykey resultsresults cancan bebe obtainedobtained from FTIR, which are are important important to to enhanc- enhancing corrosioning corrosion prevention prevention with with thiol thiol content content in thein the LA(TGST) LA(TGST) and and TGST TGST formulations: formulations: (i)(i) the growingthe growing extent extent of crosslinking of crosslinking seen seen in in the th TGSTe TGST systems systems from from 3:13:1 toto 1:31:3 TG:ST ratios, in contrastin contrast to theto the LA(TGST) LA(TGST) systems, systems, indicates indicates consistent consistent TG:ST TG:ST crosslinkingcrosslinking irrespective of stoichiometry;of stoichiometry; (ii) (ii) extensive extensive silanol silanol hydrolysis hydrolysis and and condensation condensation is achievedis achieved in formulationsin formu- withlations higher withST higher ratios; ST andratios; (iii) and improved (iii) impr adhesionoved adhesion of the of metal the metal substrate substrate and theand coating the iscoating obtained. is obtained.

3.3.3.3. XPSXPS AnalysesAnalyses TheThe surveysurvey scanscan ofof thethe coated coated surfaces surfaces (Figure(Figure4 a)4a) shows shows the the atomic atomic percentage percentage of of key elementskey elements (C 1s, (C S 1s, 2p, S Si2p, 2p, Si O2p, 1s) O and1s) and their th respectiveeir respective binding binding energies. energies. The The nitrogen nitrogen and ﬂuorineand fluorine seen inseen the in survey the survey scan arescan components are components of the of ﬂuorosurfactant the fluorosurfactant used inused the in coatings. the Thecoatings. binding The energies binding atenergies 399.8 eV at (N399.8 1s) eV and (N 688.8 1s) and eV (F688.8 1s) eV are (F indicative 1s) are indicative of nitrogen of in nitrogen in amine [26] and the -CF2-CF2- bond [27], [26] respectively, which are both ex- amine [26] and the -CF2-CF2- bond [27] respectively, which are both expected from the surfactant.pected from Figure the surfactant.4b shows Figure the deconvolution 4b shows the spectra deconvolution of sulfur spectra 2p for of TGST sulfur 2:1 2p typical for ofTGST spectra 2:1 fortypical all formulations.of spectra for all Figure formulations.4c is plotted Figure to reveal 4c is plotted the correlations to reveal the and correla- extent of crosslinkingtions and extent between of crosslinking the various between chemical the species various as chemical a function species of the as LA(TG):LA(ST) a function of the and LA(TG):LA(ST) and TG:ST stoichiometry and the impact of alcohol removal in the low TG:ST stoichiometry and the impact of alcohol removal in the low VOC formulations. VOC formulations.

Coatings 2021, 11, 306 11 of 19

(a)

10 TGST C-S-C, C-S-S,C & C-S-H TGST Metal sulfates 8 TGST Fe-S metal thiolate LA(TGST) C-S-C, C-S-S-C & C-S-H LA(TGST) metal sulfates 6 LA(TGST) Fe-S metal thiolate

4 Atomic % 2

3-1 2-1 1-1 1-2 1-3

TGST/LA(TGST) ratios (b) (c)

FigureFigure 4. XPS 4. XPS spectra spectra and and correlation correlation data data forfor the 3:1 to to 1:3 1:3 LA(TGST) LA(TGST) and and TGST TGST ratios. ratios. (a) Survey (a) Survey scan scanfor 1:1 for TGST, 1:1 TGST, (b) S2p peak deconvolution spectra for the various sulfur species in TGST 2:1, and (c) extent of crosslinking as a function of (b) S2p peak deconvolution spectra for the various sulfur species in TGST 2:1, and (c) extent of crosslinking as a function of sulfur concentration. sulfur concentration. The S 2p spectrum shown in Figure 4b was fitted by six peaks corresponding to three major species each with their S 2p 3/2 and 1/2 peak splitting at binding energies 162.2, 163.38, 163.5, 164.5, and 167.5 and 168.9 eV. The binding energy at 162.2 and 163.38 eV is associated with the 1/2 and 3/2 spin peaks of the bound thiols forming metal thiolates with iron, including a surface state contribution of the monosulfide (S2−) [28–30], whereas the predominant envelope at 163.5 eV encompasses the S 2p3/2 overlap of the unbound C-S-H (thiols), C-S-C (thioester), and C-S-S-C (disulfide) groups [21,28,31,32]. The lower intensity peak at 164.5 eV is attributed to the S 2p½ binding energies for the C-S-C and C-S-H thiols groups commensurate with the spin-orbital degeneracy. Binding energies for metal sulfites and sulfates (SO32− and SO42−) for S 2p are observed at 167.5 eV and 168.9 eV, respectively [33,34]. The Si 2p at 102.91 eV can be fitted by a single broad peak attributed to Si- O representative of the predominant Si-O-Si network in all formulations (data not shown). XPS analyses corroborate the results from Raman and FTIR spectroscopy. The growing extent of crosslinking between the low-alcohol and the TG and ST precursors and the increase in disulfide formation with corresponding increase in thiol content demonstrated in the infrared results is also seen in the XPS results (Figure 4c). The atomic percent of sulfur species in C-S-H, C-S-S-C, and C-S-C () as a function of the total atomic percent consistently increases from 2.9 atomic percent for the 3:1 to 5.7 atomic percent for the 1:3 TG:ST ratios. The C-S-H concentration in 3:1, 2:1, and 1:1 is identical and would be expected to remain unchanged at 2.9%; however, the significant increase in the C-S atomic percentage between 3.3% for 2:1 and 4.6% for 1:1 demonstrates the conversion of thiol to thioester and alkyl disulfide. This is consistent with the formation of crosslinked species and oxidation of thiols to disulfide in the presence of molecular oxygen in aqueous solutions [35]. It is expected that the enhanced epoxy–thiol crosslinking and growth of hydrophobic moieties associated with increased disulfide formation improves corrosion resistance in combination with strong substrate-coating adhesion. The XPS information for the TGST hybrids mirrors the data for the LA(TGST) coatings (Figure 4c). Interestingly, metal sulfate formation (, Figure 4c) is almost constant across the formulations independent of the starting concentration of thiols. This can be ascribed to the zero-order kinetics followed for the self-oxidation of thiols in the presence of atmospheric molecular oxygen. The oxidation is catalyzed by the presence of transition metal ions such as iron, although the rate of oxidation is dependent on both the pH and the nature of the transition metal [35]. Therefore, development of the green color within 1 to 5 min of air-drying coatings in all coated samples across all ratios can be attributed to the formation of iron sulfates, which is also known as green vitriol. Moreover, adhesion between the metal substrate and coating remains consistent (▲, Figure 4c) independent

Coatings 2021, 11, 306 11 of 18

The S 2p spectrum shown in Figure4b was fitted by six peaks corresponding to three major species each with their S 2p 3/2 and 1/2 peak splitting at binding energies 162.2, 163.38, 163.5, 164.5, and 167.5 and 168.9 eV. The binding energy at 162.2 and 163.38 eV is associated with the 1/2 and 3/2 spin peaks of the bound thiols forming metal thiolates with iron, including a surface state contribution of the monosulfide (S2−)[28–30], whereas the predominant envelope at 163.5 eV encompasses the S 2p3/2 overlap of the unbound C-S-H (thiols), C-S-C (thioester), and C-S-S-C (disulfide) groups [21,28,31,32]. The lower intensity peak at 164.5 eV is attributed to the S 2p 1 binding energies for the C-S-C and 2 C-S-H thiols groups commensurate with the spin-orbital degeneracy. Binding energies 2− 2− for metal sulfites and sulfates (SO3 and SO4 ) for S 2p are observed at 167.5 eV and 168.9 eV, respectively [33,34]. The Si 2p at 102.91 eV can be fitted by a single broad peak attributed to Si-O representative of the predominant Si-O-Si network in all formulations (data not shown). XPS analyses corroborate the results from Raman and FTIR spectroscopy. The growing extent of crosslinking between the low-alcohol and the TG and ST precursors and the increase in disulfide formation with corresponding increase in thiol content demonstrated in the infrared results is also seen in the XPS results (Figure4c). The atomic percent of sulfur species in C-S-H, C-S-S-C, and C-S-C (•) as a function of the total atomic percent consistently increases from 2.9 atomic percent for the 3:1 to 5.7 atomic percent for the 1:3 TG:ST ratios. The C-S-H concentration in 3:1, 2:1, and 1:1 is identical and would be expected to remain unchanged at 2.9%; however, the significant increase in the C-S atomic percentage between 3.3% for 2:1 and 4.6% for 1:1 demonstrates the conversion of thiol to thioester and alkyl disulfide. This is consistent with the formation of crosslinked species and oxidation of thiols to disulfide in the presence of molecular oxygen in aqueous solutions [35]. It is expected that the enhanced epoxy–thiol crosslinking and growth of hydrophobic moieties associated with increased disulfide formation improves corrosion resistance in combination with strong substrate-coating adhesion. The XPS information for the TGST hybrids mirrors the data for the LA(TGST) coatings (Figure4c). Interestingly, metal sulfate formation ( , Figure4c) is almost constant across the formulations independent of the starting concentration of thiols. This can be ascribed to the zero-order kinetics followed for the self-oxidation of thiols in the presence of atmospheric molecular oxygen. The oxidation is catalyzed by the presence of transition metal ions such as iron, although the rate of oxidation is dependent on both the pH and the nature of the transition metal [35]. Therefore, development of the green color within 1 to 5 min of air-drying coatings in all coated samples across all ratios can be attributed to the formation of iron sulfates, which is also known as green vitriol. Moreover, adhesion between the metal substrate and coating remains consistent (N, Figure4c) independent of stoichiometric increase in the thiol functionality. Overall, these XPS results clearly demonstrate the moieties involved in crosslinking and identify the bonds responsible for the green coloration of the coatings in the presence of atmospheric oxygen.

of stoichiometric increase in the thiol functionality. Overall, these XPS results clearly demonstrate the moieties involved in crosslinking and identify the bonds responsible for the green coloration of the coatings in the presence of atmospheric oxygen.

3.4. Formulation Stability: Viscosity and Pot-Life Figure 5 shows the variation in viscosity for the TGST formulations at t = 0 h and t = 17 h to explore the formulation pot-life. Viscosity remained relatively unchanged with increasing TG:ST ratio immediately after reaction at t = 0 h (Figure 5). For all formulations, viscosity measurements were restricted to a maximum of t = 17 h to avoid the risk of gelation in the viscometer. The t = 17 h data point for TGST 3:1 was not measured due to signs of gelation. A significant decrease in viscosity between formulations was observed as the thiol content increased. In addition, a trend of decreasing difference in viscosity between t = 0 h and t = 17 h was observed with increasing TG:ST ratio (Figure 5). This is attributed to two main reasons: (i) the greater crosslinking between epoxy and thiol Coatings 2021, 11, 306 groups in formulations where the thiol content was lower but more available for reaction12 of 18 instead of dimerization (TGST 3:1 and 2:1) and (ii) decreasing TGST particle agglomera- tion. The increase in viscosity with increase in t has previously been shown to be associated with molecular weight gain and the gradually growing aggregate size of TGST par- molecularticles [3]. However, weight gain when and the the thiol gradually content growingincreases aggregatefrom TG:ST size 1:1 of to TGST 1:3 in particlesthe formula- [3]. However,tion, ST dimerization when thethiol competes content with increases TG:ST crosslinking from TG:ST density, 1:1 to 1:3 which in the mitigates formulation, an in- STcrease dimerization in viscosity. competes with TG:ST crosslinking density, which mitigates an increase in viscosity.

10 0hrs 17hrs 8

Viscosity Viscosity (centipoise) 2

0 3-1 2-1 1-1 1-2 1-3

Ratio FigureFigure 5.5. ChangeChange inin viscosityviscosity withwith increasingincreasing TG:STTG:ST ratioratio andand crosslinkingcrosslinking time.time.

Pot-lifePot-life waswas testedtested forfor thethe TGSTTGST systemsystem toto comparecompare withwith thethe low-alcohollow-alcohol LA(TGST)LA(TGST) systemsystem toto determinedetermine ifif thethe removalremoval ofof alcoholsalcohols fromfrom thethe TGTG andand STST precursorprecursor solutionssolutions impactedimpacted the LA(TGST) LA(TGST) formulation formulation stability stability with with increasing increasing crosslinking crosslinking time time t. Allt. All so- solutionslutions of of TGST TGST and and LA(TGST) LA(TGST) were were clear clear solutions solutions at at tt = 0 0 h but became progressivelyprogressively cloudycloudy onon standingstanding overnight overnight at att t= = 17 17 h h and and showed showed full full gelation/sedimentation gelation/sedimentation at att t= = 20 20h h (Figures(Figures S3S3 and S4 S4 in in the the Supporting Supporting Informat Information).ion). Figure Figure 6a,b6a,b shows shows the the formulation formulation sta- stabilitybility of ofthe the TGST TGST and and the the LA(TGST) LA(TGST) systems systems at at tt == 17 17 h h respectively. respectively. The solutions withwith higherhigher TGTG contentcontent (3:1(3:1 andand 2:1)2:1) showedshowed signssigns ofof gelgel formationformation atat tt == 1717 hh andand werewere fullyfully gelledgelled withinwithin 2020 h.h. SolutionsSolutions withwith higherhigher STST contentcontent showedshowed increasedincreased precipitationprecipitation ofof TGST/STTGST/ST particles in the formulation, leading toto thethe formationformation ofof suspensions.suspensions. TheThe raterate ofof gelationgelation andand suspensionsuspension werewere relativelyrelatively slowslow untiluntilt t= = 1717 hh butbut rapidlyrapidly increasedincreased betweenbetween tt == 1717 andand 2020 hh (Figure(Figure S2S2 ininthe the Supporting SupportingInformation). Information). TheThe stabilitystability ofof formulationsformulations withwith respectrespect toto LA(TG):LA(TG): LA(ST)LA(ST) andand TG:STTG:ST stoichiometrystoichiometry waswas observedobserved toto bebe 2:1,2:1, 1:1,1:1, andand 1:21:2 greatergreater thanthan 3:13:1 andand 1:3.1:3. Figure7a–d show the formulation stability of the precursor formulations TG, LA(TG), ST, and LA(ST). A significant difference in formulation stability between the TG and the ST dispersion and their corresponding low-alcohol precursors was observed. The TG and the low-alcohol LA(TG) formulations displayed excellent stability, maintaining their viscosity for over 30 days after preparation (Figure7a,b). ST (where no alcohols were removed) remained stable for over 30 h under ambient conditions (Figure7c); however, the LA(ST) formulation (Figure7d) turned cloudy within 17 h when the alcohols were replaced with water. Although the alcohols were replaced with water to prepare a low VOC system, the thiol group that is relatively less polar than the epoxide is also less stable in a predominantly aqueous environment, thus reducing the pot-life of LA(ST) substantially. This explains the reduced stability of the LA(TGST) formulations (Figure6b and Figure S3 in the Supporting Information) for the corresponding crosslinking times as compared to the TGST system (Figure6a). Visibly, the LA(TGST) 1:2 and 1:3 formulations (Figure6b) look less cloudy than their TGST counterparts at t = 17 h (Figure6a) and t = 20 h; however, this is due to the increased sedimentation caused by the lower stability of the TGST/ST particles in the low-alcohol formulation. The bilayer in the formulation formed due to sedimentation is visible for TGST 1:2 at t = 20 h in Figure S3 in the Supporting Information. Expectedly, all formulations with a higher epoxy ratio (LA(TGST) and TGST 3:1, 2:1) showed gelation/sedimentation of LA(T)/ST later than the LA(TGST) and TGST 1:1, 1:2, or 1:3 formulations, which was attributed to Coatings 2021, 11, 306 13 of 19

Figure 7a–d show the formulation stability of the precursor formulations TG, LA(TG), ST, and LA(ST). A significant difference in formulation stability between the TG and the ST dispersion and their corresponding low-alcohol precursors was observed. The TG and the low-alcohol LA(TG) formulations displayed excellent stability, maintaining their viscosity for over 30 days after preparation (Figure 7a,b). ST (where no alcohols were removed) remained stable for over 30 h under ambient conditions (Figure 7c); however, the LA(ST) formulation (Figure 7d) turned cloudy within 17 h when the alcohols were replaced with water. Although the alcohols were replaced with water to prepare a low VOC system, the thiol group that is relatively less polar than the epoxide is also less stable in a predominantly aqueous environment, thus reducing the pot-life of LA(ST) substantially. This explains the reduced stability of the LA(TGST) formulations (Figure 6b and Figure S3 in the Supporting Information) for the corresponding crosslinking times as compared to the TGST system (Figure 6a). Visibly, the LA(TGST) 1:2 and 1:3 formulations (Figure 6b) look less cloudy than their TGST counterparts at t = 17 h (Figure 6a) and t = 20 h; however, this is due to the increased sedimentation caused by the lower stability of the TGST/ST particles in the low-alcohol formulation. The bilayer in the formulation formed Coatings 2021, 11, 306 due to sedimentation is visible for TGST 1:2 at t = 20 h in Figure S3 in the Supporting13 of 18 Information. Expectedly, all formulations with a higher epoxy ratio (LA(TGST) and TGST 3:1, 2:1) showed gelation/sedimentation of LA(T)/ST later than the LA(TGST) and TGST 1:1, 1:2, or 1:3 formulations, which was attributed to the lower LA(ST)/ST concentration theprecipitating lower LA(ST)/ST out of the concentration dispersions. It precipitating is of note that outall formulations of the dispersions. of the LA(TGST) It is of note and that allTGST formulations systems exhibited of the LA(TGST) a pot-life and>15 h TGST and were systems readily exhibited coated a on pot-life steel panels >15 h andat that were readilytime, indicating coated on that steel the panels formulations at that time, were indicating adequately that stable theformulations over a period were sufficient adequately for stablecommercial over a application period sufﬁcient of the surface for commercial treatment. application of the surface treatment.

3:1 2:1 1:1 1:2 1:3

(a)

Coatings 2021, 11, 306 (b) 14 of 19

FigureFigure 6. 6.Pot-life Pot-life forfor ((aa)) TGSTTGST andand (b) LA(TGST) solutions solutions at at crosslinking crosslinking time time t =t =17 17 h. h.

(a) (b) (c) (d)

FigureFigure 7. 7. Pot-lifePot-life of of precursor precursor solutions solutions ( (aa)) TG TG and and ( (bb)) LA(TG) LA(TG) after after 30 30 days days of of preparation. preparation. ( (cc)) ST ST after after 30 30 h h of of preparation preparation andand ( (dd)) LA(ST) LA(ST) after after 17 17 h h of of preparation. preparation.

3.5. Impact of Low-VOC Content 3.5. Impact of Low-VOC Content Alcohol emissions from coatings contribute to the formation of greenhouse gases and Alcohol emissions from coatings contribute to the formation of greenhouse gases and are regulated as volatile organic compounds (VOCs) in the coatings industry. For the are regulated as volatile organic compounds (VOCs) in the coatings industry. For the TGST TGSTcoatings coatings to conform to conform to the low to the VOC low requirement, VOC requirement, alcohols formedalcohols as formed a by-product as a by-product of hydroly- ofsis hydrolysis of the precursors of the precursors TG and ST wereTG and removed ST were and removed replaced and with replaced deionized with water deionized to form waterthe LA(TG) to form and the LA(ST) LA(TG) solutions, and LA(ST) which weresoluti thenons, used which to formwere the then LA(TGST) used to crosslinked form the LA(TGST)coatings with crosslinked varying epoxide coatings and with thiol varying silicates epoxide stoichiometry and thiol (Scheme silicates1). The stoichiometry impact of the (SchemeVOC content 1). The in theimpact TGST of systems the VOC to content the environment in the TGST was systems calculated to usingthe environment the ISO metric was of calculated using the ISO metric of Index of Global Warming Potential (I(GWP)) [17] (Section Index of Global Warming Potential (I(GWP))[17] (Section 2.4, Equations (2) and (3)). 2.4, EquationsThere are (2) four and hydrolysable (3)). ethoxy groups in TEOS that form ethanol in both the TG andThere the are ST four solutions hydrolysable and three ethoxy hydrolysable groups in methoxy TEOS that groups form in ethanol GPTMS in andboth MPTMS the TG andforming the ST methanol. solutions Therefore, and three an hydrolysab equal stoichiometricle methoxy amount groups of in a mixtureGPTMS ofand ethanol MPTMS and forming methanol. Therefore, an equal stoichiometric amount of a mixture of ethanol and methanol is emitted from the TG and the ST solutions. The I(GWP) for TG and ST solutions as prepared was calculated to be 45.57. I(GWP) values for the crosslinked TGST systems as prepared were calculated to be between 182.3 for the TGST 3:1 and 1:3 systems, 91.14 for the TGST 1:1 and 136.70 for the 2:1 and 1:2 TGST formulations. Therefore, the removal of alcohols from the LA(TGST) formulations reduces these impacts to global warming completely. In addition, a factor that contributes to the I(GWP) metric is the production of carbon dioxide from the use of fossil fuel as an energy source for heating during the evaporation of alcohols. For this work, I(GWP) due to heating the TG and ST formulations during the evaporation of alcohols was calculated [19] to be 0.25, which insignificantly diminishes the positive impact of LA(TGST) systems on I(GWP).

3.6. Coatings Performance Performance of the LA(TGST) surface treatment coatings was evaluated using corrosion inhibition and scratch resistance tests. Corrosion inhibition of the coatings was tested by immersing coated steel coupons in Dilute Harrison’s Solution (DHS), and scratch resistance was evaluated by pencil hardness tests following the ASTM D3363 standard. Sim- ilar tests were conducted on the TGST coatings for comparison.

3.6.1. Corrosion Inhibition Low-carbon steel panels were dip-coated in the LA(TGST) or TGST formulations as described in Section 2.3 and air-dried for 24 h. Depending on the stoichiometry of the formulation, coated panels displayed varying degrees of a dark green layer that develops via an interaction between the thiol species and iron from the substrate potentially from the formation of the iron sulfates as presented in the XPS S2p analysis (Figure 4b,c).

Coatings 2021, 11, 306 14 of 18

methanol is emitted from the TG and the ST solutions. The I(GWP) for TG and ST solutions as prepared was calculated to be 45.57. I(GWP) values for the crosslinked TGST systems as prepared were calculated to be between 182.3 for the TGST 3:1 and 1:3 systems, 91.14 for the TGST 1:1 and 136.70 for the 2:1 and 1:2 TGST formulations. Therefore, the removal of alcohols from the LA(TGST) formulations reduces these impacts to global warming completely. In addition, a factor that contributes to the I(GWP) metric is the production of carbon dioxide from the use of fossil fuel as an energy source for heating during the evaporation of alcohols. For this work, I(GWP) due to heating the TG and ST formulations during the evaporation of alcohols was calculated [19] to be 0.25, which insigniﬁcantly diminishes the positive impact of LA(TGST) systems on I(GWP).

3.6. Coatings Performance Performance of the LA(TGST) surface treatment coatings was evaluated using corrosion inhibition and scratch resistance tests. Corrosion inhibition of the coatings was tested by immersing coated steel coupons in Dilute Harrison’s Solution (DHS), and scratch resistance was evaluated by pencil hardness tests following the ASTM D3363 standard. Similar tests were conducted on the TGST coatings for comparison.

3.6.1. Corrosion Inhibition Low-carbon steel panels were dip-coated in the LA(TGST) or TGST formulations as Coatings 2021, 11, 306 described in Section 2.3 and air-dried for 24 h. Depending on the stoichiometry15 of of the 19

formulation, coated panels displayed varying degrees of a dark green layer that develops via an interaction between the thiol species and iron from the substrate potentially from the formationFigure 8 shows of the ironpictures sulfates of the as presented panels after in the 25 XPSh of S2pexposure analysis to (FigureDHS for4b,c). both the LA(TGST)Figure and8 shows the TGST pictures coatings of the systems. panels Fi aftergure 25 9 shows h of exposure the calculated to DHS rust for percentage both the ofLA(TGST) the scribed and and the unscribed TGST coatings panels systems. at 24 h. Figure9 shows the calculated rust percentage of the scribed and unscribed panels at 24 h.

3:1 2:1 1:1 1:2 1:3

(a)

(b)

Figure 8. Corrosion preventionprevention performanceperformance associated associated with with development development of of rust rust on on coupons coupons coated coated with with (a) ( TGSTa) TGST and and (b) (b) LA(TGST) formulations with varying TG:ST stoichiometry between 3:1 to 1:3 after exposure for 25 h in Dilute Harri- LA(TGST) formulations with varying TG:ST stoichiometry between 3:1 to 1:3 after exposure for 25 h in Dilute Harrison’s son’s Solution (DHS). Solution (DHS).

(a) (b) (c) 100 100 100 TGST unscribed TGST unscribed- 52 h LA(TGST) unscribed TGST scribed TGST unscribed- 25 h 80 80 80 LA(TGST) scribed

60 60 60

40 40 (%) rust 40

20 20 20

0 0 0 3-1 2-1 1-1 1-2 1-3 3-1 2-1 1-1 1-2 1-3 3-1 2-1 1-1 1-2 1-3 TGST ratios TGST ratios LA(TGST) ratios

Figure 9. Rust percentage after exposure for the unscribed () and scribed (▲) panels (n = 3), coated with TGST and LA(TGST) in DHS. (a) TGST at 25 h, (b) comparison of unscribed TGST at 25 h and 52 h and (c) LA(TGST) at 25 h. The dashed red line in (a) is a guide for the eye.

From Figure 8, it is evident that the TGST coatings possess better corrosion inhibition properties than the low-alcohol LA(TGST) coatings for all TG:ST ratios. This clearly signi- fies that the removal of alcohols from the ST formulation affects the stability (Figure 7d) of the crosslinked LA(TGST) formulations and directly impacts the corrosion prevention performance of the coatings. This result introduced an area under investigation with promising results into a low-GWP impact LA(TG)ST system where the alcohols are re-

Coatings 2021, 11, 306 15 of 19

Figure 8 shows pictures of the panels after 25 h of exposure to DHS for both the LA(TGST) and the TGST coatings systems. Figure 9 shows the calculated rust percentage of the scribed and unscribed panels at 24 h.

3:1 2:1 1:1 1:2 1:3

(a)

(b) Coatings 2021Figure, 11, 8. 306 Corrosion prevention performance associated with development of rust on coupons coated with (a) TGST and 15 of 18 (b) LA(TGST) formulations with varying TG:ST stoichiometry between 3:1 to 1:3 after exposure for 25 h in Dilute Harri- son’s Solution (DHS).

(a) (b) (c) 100 100 100 TGST unscribed TGST unscribed- 52 h LA(TGST) unscribed TGST scribed TGST unscribed- 25 h 80 80 80 LA(TGST) scribed

60 60 60

40 40 (%) rust 40

20 20 20

0 0 0 3-1 2-1 1-1 1-2 1-3 3-1 2-1 1-1 1-2 1-3 3-1 2-1 1-1 1-2 1-3 TGST ratios TGST ratios LA(TGST) ratios FigureFigure 9. Rust9. Rust percentage percentage after after exposure exposure forfor thethe unscribedunscribed ( •)) and and scribed scribed (▲ (N)) panels panels (n (n = =3), 3), coated coated with with TGST TGST and and LA(TGST)LA(TGST) in DHS.in DHS. (a )(a TGST) TGST at at 25 25 h, h, (b ()b comparison) comparison ofof unscribedunscribed TG TGSTST at at 25 25 h h and and 52 52 h h and and (c ()c )LA(TGST) LA(TGST) at at25 25h. h.The The dasheddashed red red line line in (ina) ( isa) ais guidea guide for for the the eye. eye.

FromFrom Figure Figure8 ,8, it it is is evident evident thatthat the TGST coatings coatings possess possess better better corrosion corrosion inhibition inhibition propertiesproperties than than the the low-alcohollow-alcohol LA(TGST) coatings coatings for for all all TG:ST TG:ST ratios. ratios. This This clearly clearly signi- signi- fiesfies that that the the removal removal of of alcohols alcohols from from the the ST ST formulation formulation affectsaffects thethe stabilitystability (Figure 77d)d) of theof crosslinkedthe crosslinked LA(TGST) LA(TGST) formulations formulations and an directlyd directly impacts impacts the the corrosion corrosion prevention prevention performanceperformance of the of coatings. the coatings.This resultThis result introduced introduced an area an under area investigationunder investigation with promising with resultspromising into aresults low-GWP into impacta low-GWP LA(TG)ST impact system LA(TG)ST where system the alcohols where are the removed alcohols onlyare re- from the TG component of the formulation. It is of note that overall, the maximum corrosion for all crosslinked coatings did not exceed 40%, and the LA(TGST) coatings also performed better than the non-crosslinked TG and ST precursor formulations; TG-only formulations showed swelling and peeling at 1.5 h, and the ST-only coatings were between 60 and 80% corroded at 25 h [3]. TGST and LA(TGST) 3:1 coatings showed the earliest onset of corrosion (<3 h) and were extensively corroded at 25 h (Figure8a,b), which was associated with the higher TG concentration and lowest crosslink density (Table1, Figures2d,3c,d and4c). In comparison, the TGST 1:1, 1:2, and 1:3 coatings with a higher ST concentration were substantially less corroded at 25 h (Figure8a). This result corroborates the spectroscopic results obtained from Raman (Figure2), FTIR (Figure3), and XPS (Figure4) analyses, confirming the direct positive impact of the higher thiol content and improved crosslinking on the corrosion resistance of coatings. Coatings with higher epoxide ratios showed an early onset of corrosion and maximum rust formation associated with the higher concentration of the polar epoxide functional group in both the LA(TGST) and TGST systems (≈30% Figure9a,c). LA(TG) and TG, which are epoxy decorated (Scheme1), are also mildly susceptible to epoxide ring opening by water due to the presence of the acetic acid in the aqueous medium [36], although this process is slow for weak acids [37]. It could lead to an increase in hydroxyl groups in the formulation and therefore a decrease in corrosion resistance. Coatings with higher ST concentration showed better corrosion prevention properties in TGST (1:2 and 1:3, Figure9a) due to the increased hydrophobicity attributed to the higher concentration of the relatively less polar thiol and disulfide groups (Figure9a). Therefore, a linear scaling of corrosion inhibition matched by a linear decrease in rust percent would be expected for TG:ST stoichiometric ratios 1:1 to 1:3 due to increased hydrophobicity. Interestingly, corrosion inhibition improved with (i) decreasing TG content from 3:1 to 1:1 TGST, which is shown as a dashed red line in Figure9a but showed no significant change between TGST 1:1 and 1:3 (Figure9a). This surprisingly indicates that there is a diminishing difference between the increase in thiol content after TG:ST 1:1 and the onset of corrosion. The plateau after 1:1 TG:ST stoichiometry indicates that the crosslinking, hydrophobicity, and substrate– coating interactions at 1:1 TG:ST stoichiometry are sufficiently developed for protecting the substrate from the onset of corrosion. Although this would surprisingly indicate that an excess ST concentration promotes hydrophobicity, improved crosslinking, including Coatings 2021, 11, 306 16 of 18

disulﬁde dimerization and a more expanded siloxane network, would be redundant for corrosion protection; Figure9b demonstrates that improved crosslinking assists in sustaining the protection under prolonged exposure. At 52 h of immersion in the salt solution, the rust percentage increases for TGST 1:1 but remains practically unchanged in TGST 1:2 and 1:3. It is also of note that only a 2–6% increase in rust is observed between 25 and 52 h of exposure (Figure9b) and despite the doubled exposure time, the overall rust percentage is maintained below 40%, even in the most corroded TGST 3:1 coating. It is also telling that the 3:1 and 2:1 ratios of the LA(TGST) system (Figure9c) have comparable rust percentage as the corresponding TGST coatings (Figure9a); however, a large fluctuation in the rust percentage values of 1:1, 1:2, and 1:3 LA(TGST) coatings is observed. In coatings where the LA(TG) concentration is high, the formulation is relatively stable in the absence of alcohols, and corrosion resistance, although limited, is consistent (Figure6, TGST and LA(TGST) 3:1 and 2:1 Figure9a,c). In coatings where the LA(ST) concentration is high, the elimination of alcohols causes the formulation to be less stable and corrosion resistance to be poor (Figures6b and7d, LA(TGST) 1:2 and 1:3, Figure9c). Although the formation of rust in the ratios with higher LA(ST) concentration appears to be random, LA(TGST) 1:2 has the best corrosion resistance performance compared to LA(TGST) 1:1 and 1:3 (Figure9c). These results agree with the results from FTIR (Figure3) that show that the same amount of LA(TG) is consumed for crosslinking by LA(ST) at all ratios irrespective of LA(TG):LA(ST) stoichiometry; therefore, any corrosion protection in this system is contributed by disulﬁde hydrophobicity and not epoxy–thiol crosslinking density. Although all LA(TGST) coatings had exhibited extensive corrosion at 25 h (Figure8b), the LA(TGST) 1:2 ratio showed the maximum corrosion protection in the series. Overall, the corrosion prevention results demonstrated that TGST 1:1 to 1:3 showed between 96% and 97% improvement in corrosion inhibition over the non-crosslinked formulations, and even the most corroded LA(TGST) formulations exhibited a 71% improvement in protection.

3.6.2. Pencil Hardness All coatings were glossy, homogenously coated, and showed a layer of green depending on the ratio of formulation. The surface hardness and scratch resistance of all hybrid coatings associated with extent of cure were evaluated with the pencil hardness test. The process was started with the hardest pencil, number 9H by rolling it on the surface of each coating following the procedure recommended by the ASTM standard. The procedure was repeated with pencils of all hardnesses on the test scale. It is of note that none of the pencils, including the hardest 9H pencil, did not scratch or gouge the surface of any coating, maintaining both the integrity and the gloss of the surface of the coatings. This result conﬁrms that the coatings from all formulations were extensively cured and possessed high-scratch resistance properties across all ratios.

4. Conclusions Environmentally sustainable, epoxy–thiol silicate crosslinked coatings with low volatile organic content (VOC) were prepared to investigate the impact of elimination of alcohols on the extent of epoxy–thiol and sol–gel silicate network crosslinking density. Two series of waterborne formulations with varying epoxy–thiol silicate stoichiometry from 3:1 to 1:3 were synthesized. In the first series, alcohols (methanol and ethanol) that are the by- product of the sol–gel reactions were removed from the precursors to prepare low-alcohol (LA) epoxy silicate (TG) and thiol silicate (ST) crosslinked hybrid coatings (LA(TGST)). A second series (TGST), where alcohols were retained, was synthesized in similar ratios to compare the impact of alcohols on the crosslinking density, formulation stability, viscosity, pot-life, and corrosion-prevention performance of the coatings. The positive impact on the environment by the elimination of VOCs from LA(TGST) coatings was quantified using ISO metrics for global warming potential (GWP). A significant improvement in GWP was offset by colloidal instability in most of the low-alcohol formulations with higher LA(ST) content. Raman and FTIR spectroscopy indicated that all formulations showed successful Coatings 2021, 11, 306 17 of 18

crosslinking of functional groups, and the structural integrity of the low-alcohol formulations was sustained even in the absence of alcohols. Curing was at its maximum after 17 h of crosslinking time and depended on thiol concentration. It was expected that a variation in the thiol concentration would increase the accessibility of these groups to complement crosslinking mechanisms with epoxy functional groups and further enhance the corrosion resistance properties. Spectroscopic evidence by Raman, FTIR, and XPS spectroscopies confirmed an increase in epoxy–thiol crosslinking density and hydrophobicity due to disulfide dimerization that contributed to maximizing corrosion protection. However, formulation stability associated with viscosity and pot-life was found to be a function of both crosslinking density between the functional groups and the presence of alcohols in the system. It was established that the corrosion prevention performance of coatings for the low-alcohol system was directly related to the increased hydrophobicity of the sulfur moiety rather than the epoxy–thiol crosslinking density. Both LA(TGST) and TGST systems showed enhanced corrosion prevention compared to the non-crosslinked TG and ST formulations and contained corrosion below 40% even in the most corroded samples. The 1:1 TGST stoichiometry was found to be sufficiently crosslinked and hydrophobic to provide maximum corrosion inhibition compared to the other ratios, but the improved crosslinking density in 1:2 and 1:3 TGST was evidenced to provide sustained protection even when the coatings were exposed to double the exposure time in a corrosive environment. In the low-alcohol series, the LA(TG):LA(ST) ratio 1:2 showed the most effective corrosion inhibition and overall performed 71% better than the non-crosslinked hybrids. Glossy, scratch-resistant thin film coatings with enhanced corrosion inhibition for surface treatment of low-carbon steel were successfully achieved. Pot-life, ease of smooth coating, and corrosion inhibition data indicated that the formulations are suitable for industrial applications as a primer in the marine coatings industry.

Supplementary Materials: The following are available online at https://www.mdpi.com/2079-6 412/11/3/306/s1, Figure S1: Structural characterization the hybrid (a) TGST and (b) low-alcohol LA(TGST) formulations FTIR spectroscopy in the fingerprint region for all TG:ST ratios, Figure S2: Raman spectra showing the variation of epoxy and thiol peaks for reaction of TGST at 17 h, Figure S3: Pot-life for TGST solutions at different crosslinking times (a) t = 0 h, (b) t = 20 h, Figure S4: Pot-life for LA(TGST) solutions at crosslinking times t = 20 h. Author Contributions: S.S.: Conceptualization, writing, editing, data curation, formal analysis, investigation, methodology, software, validation, literature research, visualization, chemical schemes/ figures/tables. A.J.V.: Editing, Resources. O.S.: XPS data analysis. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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