Lignin-Based Anticorrosion Coatings for the Protection of Aluminum

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Lignin-based anticorrosion coatings for the protection of aluminum surfaces Juan Carlos de Haro, Luca Magagnin, Stefano Turri, and Gianmarco Griffini ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06568 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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ACS Sustainable Chemistry & Engineering

Lignin-based anticorrosion coatings for the protection of aluminum surfaces Juan Carlos de Haroa, Luca Magagnina, Stefano Turria and Gianmarco Griffinia* aDepartment

of Chemistry, Materials and Chemical Engineering “Giulio Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy.

KEYWORDS lignin, coating, anticorrosion, biobased, silanization, aluminum, hybrid ABSTRACT New renewable lignin-based coatings with anticorrosion properties are presented in this work. These materials are based on the silanization of a THF-soluble fraction of a technical softwood kraft lignin through urethane linkages and its subsequent thermal crosslinking on metallic substrates using different crosslinking promoters. Spectroscopy analyses confirmed silanization of lignin meanwhile the thermal analyses indicated an increase of the glass transition temperature and the thermal stability after the functionalization process. The silanized (LF-S) and non-silanized (LF) lignins were used as main materials for the preparation of thermally crosslinked coatings in the presence of different promoters, namely tetraethyl orthosilicate (TEOS) as co-crosslinker and/or acetic acid (AcH) as catalyst. The coatings prepared from LF-S presented improved thermal stability and hydrophobic character compared with those obtained from unmodified LF. Moreover, the adhesion strength of the coatings on aluminum increased when the crosslinker promoters were incorporated to the coating formulation. Finally, the corrosion inhibition effect of these coatings was assessed through the evaluation of the corrosion current density on aluminum, finding important improvements on the electrochemical stability of the metal. The results of this study clearly demonstrate a straightforward 1 ACS Paragon Plus Environment

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approach for the production of high-lignin-content coatings that may find direct industrial application in the field of corrosion prevention for metals. Introduction Among the different renewable feedstocks for the production of biomaterials, lignocellulosic biomass is being considered one out of the most promising ones.1 It is mainly composed of cellulose, hemicellulose, tannin and lignin, the latter being one the most abundant aromatic polymers in nature. From a commercial point of view, lignin is produced as a by-product mainly obtained from the pulp and paper-making industry through different processes. Notwithstanding its great potential for producing biomaterials due to its aromatic structure and multiple functionalities,2 most of the lignin produced worldwide is currently used as energetic vector and only a little amount is used as filler, additive or dispersant in concrete, composites or coatings.3-4 Hence, there is a gap of knowledge and an urgent necessity for developing high-added-value and renewable products based on lignin. In this context, different processes have been reported in the past few years focusing on the production of higher value products based on the depolymerization of lignin with the objective of obtaining mainly phenolic monomers.5-6 Nevertheless, taking into account the high amount and diversity of hydroxyl groups in the structure of lignin,7 the direct chemical transformation of these groups is considered a much more efficient, environmentally sustainable and economically viable route to obtain high added-value biobased products.8 Among the different possible chemical methods, esterification,3, 9-10 phenolation,11-12 etherification13-14 and urethanization15-16 are the most commonly used synthetic routes for obtaining technologically applicable products in which functionalized lignin is directly utilized as macromonomeric unit in the formation of bio-based polymers. Metals and alloys are known to be thermodynamically unstable at certain particularly aggressive environmental conditions and typically undergo corrosion.17-18 Aluminum is the most consumed nonferrous metal due to its outstanding characteristics,19 but it can be easily corroded in non-neutral pH 2 ACS Paragon Plus Environment

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and chlorinated solutions.20 One of the most studied methods to prevent corrosion and protect aluminum consists in the use of inorganic and/or organic coatings that can modify its corrosion potential and its electric resistance. Inorganic coatings usually are based on chromium,21 cerium22 and other metallic ions,23 but they are usually avoided because of their limited lifetime and negative environmental impact.24-25 On the other hand, organic coatings have been proven to provide excellent protection to aluminum in different aqueous media and their characteristics have been recently reviewed in the literature.26 Amines, N-heterocyclic compounds, azole, imidazole, and thiazole derivatives, different kinds of polymers, organic dyes and Schiff bases have demonstrated their potential in this field.20 Nevertheless, all these compounds are obtained from petrochemical sources and in most cases environmentally invasive processes are employed for their production.27 Recently, some biobased alternatives were proposed in the literature as anticorrosion systems, including coconut-oil-based polyesters,28 soy-oil-based alkyd resins29-30 and castor- or linseed-oil-based polyurethane31 coatings. Lignin monomers and non-modified lignin have been demonstrated to have a great potential as corrosion inhibitors when dissolved in acidic,32-33 basic34 and neutral media.35 Some chemical transformations, like the grafting of acrylamide or dimethyl diallyl ammonium chloride monomers,36 can further improve the anticorrosion behaviour of lignin in solution. However, these procedures are not technically viable on open systems, where the media is not confined (e.g. open sea applications), or in the chemical industry, where simple additivation may negatively affect the reaction environment. Some recent works have also discussed the viability of incorporating lignin in the formulation of coatings deposited on the top of metallic surfaces with anticorrosion purposes, such as the incorporation of a 2 wt.% epoxidized lignin as additive in an epoxy-based coating on mild steel37 or a 1 wt.% of organosolv lignin in a siloxane-polymethylmethacrylate copolymer on coated carbon steel.38 Nevertheless, to the best of our knowledges, no examples have appeared in the

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literature so far on the preparation of high-lignin-content coatings displaying anticorrosion properties based on the use of lignin as polymeric precursor. Herein, different novel lignin-based coatings with anticorrosion properties for metallic substrates are presented. In order to improve the adhesion of the coating to metals, the chemical modification of lignin through the incorporation of silane groups via urethane chemistry was studied. The chemical, thermal, physical and anticorrosive properties of the coatings obtained using modified and nonmodified lignin were examined through different characterization techniques. The results obtained in this study clearly demonstrate that the applicability of silanized lignin is a viable approach to obtain biobased coatings to be readily employed in the field of high-performance anticorrosion organic systems. Materials and methods Materials The softwood kraft lignin used in this work (Indulin AT) was supplied by WestRock (formerly, MeadWestvaco).

Acetic

acid

(AcH),

tetraethyl

orthosilicate

(TEOS),

1-isocyanate-3-

trimethoxysilylpropane (ITMSP), tetrahydrofuran (THF), dibutyltin dilaurate (DBTDL) and deuterated dimethyl sulfoxide (DMSO-d6) were provided by Sigma-Aldrich. All the reagents were used without any further purification. As metallic substrates, aluminum 3003 H14 tiles from Q-Lab were used. Procedures Silanization of lignin Since most lignins are only partially soluble in common organic solvents, lignin fractionation is a commonly undertaken step for allowing further reactions on its macromolecular structure through wet methods.39 For this reason, lignin was fractionated using THF following the procedure described


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in our previous work.7 A THF-soluble fraction (from here on referred to as lignin fraction, LF) was recovered for its further silanization with a mass yield of 60%. For the silanization reaction, LF was dissolved in THF (20 mL THF/g LF) in a vacuum-dried threeneck round-bottom flask in an oil bath at 60 ºC. Once the solution reached the desired temperature, ITMSP and DBTDL (0.01 g DBTDL/g LF) were added to the dissolution (a detailed discussion on the actual NCO/OH molar ratios used is presented in the results and discussion section). The system was kept under reflux during 8 h, extracting samples at increasing times to monitor the course of the reaction by Fourier transform infrared spectroscopy. Once the reaction was complete, the solvent was evaporated through rotary evaporation and the solid product was vacuum-dried in an oven at 40 ºC until a constant weight was reached (approximately 12-16 h). The obtained silanized lignin was named LF-S. A reaction scheme depicting the functionalization process is presented in Figure 1. LF lignin

MeO

OH +

MeO MeO

LF-S

Si

 t DBTDL

NCO

lignin

O O C N H

OMe Si

OMe OMe

ITMSP

Figure 1. Schematic representation of the silanization of lignin. Preparation of the coatings Lignin-based coatings were prepared using a spin-coating method involving THF as the solvent. Four different formulations were tested in this work. Coating CLF was prepared by spin-coating a solution of unfunctionalized LF in THF at a concentration of 20 wt. %. Analogously, coating CLF-S was prepared following the same procedure but using silanized lignin LF-S. To check the effectiveness of H+ as catalyst on the deposition process, coating CLF-S/H was formulated by dissolving LF-S in THF at a concentration of 20 wt. % and adding AcH until pH = 3 was reached. In addition, H2O was added 5 ACS Paragon Plus Environment


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at a mass ratio H2O:THF = 10:100 to favour the dissolution of AcH and promote the hydrolysis of the alkoxysilane moieties. Finally, coating CLF-S/HT was prepared in the same way as CLF-S/H, but in the former case TEOS was added at a mass ratio TEOS:LF-S = 5:95 to study its effect as a crosslinker on the final properties of the obtained coating. The mixtures were deposited on metal substrates by means of a spin-processor WS-400B-NPP (Laurell Technologies Corp.) for 80 s at 1000 rpm. The thermal curing was performed subsequently at 200 ºC during 15 minutes in a ventilated oven. Characterization techniques Fourier-transform infrared spectroscopy Fourier-transform infrared (FTIR) spectroscopy was performed by means of a Nicolet 760 FTIR. To monitor the silanization reaction, liquid samples extracted at different reaction times were deposited on a premade and dry KBr disc, dried under a N2 stream to remove the solvent (THF) and analysed by means of FTIR. On the other hand, the lignin-based coatings were analysed from powders obtained after depositing the coating formulation on a glass/aluminum substrate, crosslinking the coating by thermal treatment, scratching off the crosslinked coating material from the substrate and mixing it with KBr powder, to finally obtain the KBr disc to be used for FTIR analysis. All FTIR spectra were recorded in transmission mode at room temperature in air by recording 32 accumulated scans at a resolution of 2 cm−1 in the 4000−700 cm−1 wavenumber range. NMR spectroscopy 1H-NMR

and

13C-NMR

experiments were carried out on a Bruker Avance 400 in DMSO-d6. 1H-

NMR spectrum was collected with a 16 ppm spectral width, 64000 points of acquisition, a relaxation delay of 1 s. The chemical shifts of the peaks in the spectra were referenced to the chemical shift of the impurities of non-deuterated DMSO-d6 (δ = 2.50 ppm). 13C-NMR spectra were collected with a 200 ppm spectral width, 32000 points of acquisition, a relaxation delay of 12 s and a 4 Hz line 6 ACS Paragon Plus Environment

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broadening. The chemical shifts of the peaks in the spectra were referenced to the chemical shift of DMSO-d6 (δ = 39.5 ppm). The quantification of C atoms was performed on acetylated LF sample by referencing the integration intervals related to the -OH groups (δ =170.4-165.8 ppm, including primary aliphatic hydroxyls, secondary aliphatic hydroxyls and phenolic hydroxyls) to those associated to aromatic carbons (δ =160-100 ppm, including oxygenated carbons and non-oxygenated carbons), following a previously published method.40 The acetylation of lignin LF was performed according to a procedure reported in literature.7 Similarly, the relative amount of formed urethane moieties with respect to the lignin aryl unit in LF-S was determined by comparing the integration intervals related to the -NHCOOgroup (δ = 144-151.1 ppm) to the one associated to aromatic carbons (see above). In this case, no prederivatization (i.e. acetylation) is required on LF-S since the content of -OH groups is not relevant. Differential scanning calorimetry Differential scanning calorimetry (DSC) was used to investigate the thermal transitions in the obtained materials. Measurements were performed on solid state samples (~10 mg) by using a MettlerToledo DSC/823e instrument performing three runs (heating/cooling/heating) from 25 °C to 200 ºC at a scan rate of 20 ºC/min under N2 atmosphere. The determination of the glass transition temperature (Tg) was done evaluating the inflection point of the DSC curve in the second heating ramp. Thermogravimetric analysis Thermogravimetric analysis (TGA) was performed on solid state samples (~15 mg) by means of a Q500 TGA system (TA instruments) from room temperature up to 700 ºC at a rate of 10 ºC/min. As atmosphere, both N2 and air were tested. Gel permeation chromatography



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Gel permeation chromatography (GPC) was used to determine the molecular weight of lignin samples. A Waters 510 HPLC system was used equipped with a Waters 2410 refractive index detector. THF was used as eluent. The sample to analyze (volume 200 μL, concentration in THF 2 mg/mL) was injected into a system of columns connected in series (Ultrastyragel models HR 4, HR 3, and HR 2, Waters) and the analysis was performed at 30 °C and at a flow rate of 0.5 mL/min. The GPC system was calibrated against polystyrene standards in the 102−104 g/mol molecular weight range. Optical contact angle Optical contact angle (OCA) measurements were carried out using an OCA 20 instrument (Dataphysics Co., Germany), equipped with a charge-coupled photo-camera and with a 500 μL Hamilton syringe to dispense droplets of testing liquids (water and diiodomethane). Measurements were performed at room temperature, using the sessile drop technique. Using the measured contact angles for the two different wetting liquids, the surface energy for the coatings was derived using the OWRK (Owens, Wendt, Rabel, and Kaeble) method, which yields the surface energy of the solid surface with its polar and dispersive components.15 Adhesion test The adhesive properties of the lignin-based coatings on aluminum were evaluated following the standard method ASTM D4541-09.41 The portable adhesion tester PosiTest AT-M manual adhesion pull-off tester (DeFelsko) was used to measure the pulling force needed to detach a 20 mm-diameter aluminum dolly glued to the coatings by means of a two-component epoxy adhesive (Araldite 2011, curing cycle: 50 ºC, 24 h). Corrosion test Potentiodynamic polarization tests were performed on lignin-based coatings deposited on aluminum. These electrochemical experiments were performed using a Princeton Applied Research 273A 8 ACS Paragon Plus Environment

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potentiostat/galvanostat and a three-electrodes glass cell which consists of the coated/non-coated aluminum substrate as the working electrode, a saturated calomel electrode (SCE) as reference electrode and platinum wire as counter electrode. Potentiodynamic polarization curves were obtained by scanning the electrode potential at a scan rate of 0.5 mV/s in a NaCl 5% electrolyte at 25 °C. Results and discussion Silanization of lignin In order to obtain the silanized lignin precursor, THF-soluble lignin fraction (LF) was reacted with ITMSP in the presence of DBTDL as catalyst. In order to select a suitable ITMSP/LF mass feed ratio to obtain the target functionalized lignin, preliminary tests were performed to evaluate the maximum amount of ITMSP to be used so as to allow its complete reaction with LF within an 8-h reaction period, monitoring the intensity of the signal associated to the -NCO stretching vibrations at 2270 cm-1 in the FTIR spectrum of the resulting product. It was found that a maximum of 0.39 g ITMSP/g LF could be added to the starting reaction mixture to ensure complete disappearance of the -NCO stretching signal in the FTIR spectrum of the obtained product, this last evidence being indicative of completed reaction of the isocyanate groups from ITMSP (the FTIR spectra at increasing reaction times up to 8 h are presented in Figure S1 in the Supporting Information, where the disappearance of the –NCO stretching signal at 2270 cm-1 can be clearly observed). Once the reaction was completed, the silanized product (LF-S) was dried and analysed by FTIR. Figure 2 shows the FTIR spectrum of LF-S compared to that obtained from LF. LF presents a broad absorption band between 3650 and 3050 cm-1 corresponding to stretching vibrations of -OH groups. This signal is found to become less intense in LF-S because of the consumption of this group upon formation of urethane linkages. This evidence is further corroborated by the appearance of three new signals at 3520, 1690 and 1535 cm-1 in LF-S, associated to the N-H, C=O and -NH bending vibrations in urethane bonds, not present in LF. In particular, the signal related to the urethane carbonyl group deserves special attention. It is well known that the H-bonded carbonyl groups appear at a lower wavelength compared to the non9 ACS Paragon Plus Environment


bonded ones.42 In the case of LF-S, a unique signal appearing at 1690 cm-1 in the carbonyl absorbance region indicates the exclusive presence of H-bonded carbonyl groups in ordered hard segments (i.e. no segmental distribution related to the urethanization process is observed). Indeed, in the presence of non-H-bonded carbonyl groups, an absorbance signal in the range 1731-1733 cm-1 would also be observed.43 Moreover, the presence of the propyl-trimethoxysilane pending groups on LF-S is confirmed by the increased absorbance of signals at 1365, 1125, 1080 and 765 cm-1, ascribable to vibrations in -CH3 and -Si-O-C, -Si-O- and Si-C- groups, respectively. Other signals related to the structure of lignin can be observed in both spectra, such as those associated to pure aromatic skeletal vibrations at 1515 cm-1, the bands at 1268 and 1216 cm−1 attributable to the presence of guaiacyl moieties and the signal at 1154 cm−1 assigned to C=O deformations in conjugated ester groups present

765

1080

1125

1535

LF LF-S

1365

1690

3400

3520

in guaiacyl, syringyl and p-hydroxyphenyl units.7

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4000

3500

3000

2500

2000 -1

1500

1000

Wavelength (cm )

Figure 2. FTIR spectra of THF-soluble lignin fraction (LF) and of silanized product (LF-S). To further confirm the covalent bonding of ITMSP to the structure of lignin, nuclear magnetic resonance (NMR) analyses were performed (1H-NMR and

13C-NMR

spectra are presented in the

Supporting Information, Figure S2 and Figure S3, respectively). 1H-NMR spectra of LF-S confirmed the bonding of ITMSP to lignin by the presence in the spectra of the characteristic signals related to the urethane group and those related to the trimethoxy(propyl)-silane pending group. A quantification 10 ACS Paragon Plus Environment

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of the amount of hydroxyl groups in lignin reacted with -NCO groups from ITMSP to form urethane linkages was performed based on the

13C-NMR

spectra of LF-S and acetylated LF, following a

methodology previously published in literature.15, 40 Firstly, the total hydroxyl content (aliphatic and phenolic) of LF was estimated by determining the number of carbon atoms associated to -OH groups per aryl unit (Ar), finding a value of 1.026/Ar ± 0.089. This value is in good agreement with data previously reported in the literature.7 A similar procedure was followed to determine the number of C atoms associated to the urethane bonds per aryl unit in LF-S, obtaining a ratio of 0.333/Ar ± 0.091. LF-S was not acetylated prior to 13C-NMR analyses because this derivatization process only acts on the -OH groups of lignin, which are not relevant in this case. Indeed, only the signals related to NHCOO- and aromatic C were taken into consideration. Based on these figures, it was found that a maximum of 34% of -OH groups in LF could be converted into urethane moieties in LF-S during the reaction. As previously observed,44 these findings may be rationalized based on the relatively poor chemical reactivity and limited sterical accessibility of hydroxyl groups in lignin as a result of its highly branched and complex macromolecular structure. It is also worth mentioning that 31P-NMR analyses were also performed in the attempt to quantify the content of hydroxyl groups in LF-S and determine the conversion of the silanization reaction more accurately. However, unavoidable secondary reactions occurring between the highly-reactive phosphorylating agent (2-chloro-4,4,5,5tetramethyl-1,3,2-dioxaphospholane) and the pendant methoxy silane groups in LF-S during the phosphorylation process made an absolute quantification of reacted -OH groups in LF-S by means of 31P-NMR

particularly difficult and ultimately unreliable.45-46

To investigate the thermal behaviour of the silanization process on lignin, LF and LF-S were characterized by means of DSC and TGA. DSC analyses (Figure 3a) revealed an increase in the glass transition temperature (Tg) of the material upon functionalization, with Tg values of 105 °C and 130 °C observed for LF and LF-S, respectively. This increase in Tg was associated to two main factors. On the one hand, the increase in the molecular weight (as observed in GPC analyses during the 11 ACS Paragon Plus Environment


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silanization process, see Figure S4 in the Supporting Information) causes an increase of the Tg, as also extensively shown in literature.47 On the other hand, the formation of hydrogen bonds between the functionalized macromolecules due to the presence of urethane bonds in LF-S reduces the molecular motion of the polymeric system and, therefore, the Tg of the product increases compared to the parent material.48 TGA analyses in air atmosphere (Figure 3b) also revealed differences in the thermo-oxidative degradation process of the functionalized lignin (LF-S) compared to the pristine one (LF). A noticeable decrease in the mass lost at around 100 ºC as a result of the evaporation of adsorbed water is observed in LF-S compared to LF, indicating a reduction in the hygroscopicity of lignin after the silanization process. In addition, the temperature at which the 10 wt. % loss occurred was found to increase from 149 to 327 ºC, likely due to increased intermolecular H-bonded urethane carbonyl interactions and to the increase of the molecular weight observed after functionalization (see FTIR analyses in Figure 2 and GPC analyses in Figure S4 in the Supporting Information). The improvement in the thermo-oxidative stability of LF-S compared to LF due to the presence of Si atoms in the macromolecular structure was further confirmed by the increase in the maximum degradation rate temperature (from 529 to 570 ºC) and in the char residue obtained at 700 ºC (from 2.0 to 12.5 wt. %). TGA analyses were also performed in N2 atmosphere (Figure S5), exhibiting a three step thermal degradation process, with an appreciable mass loss at around 100 ºC due to the presence of adsorbed water and a major degradation step in the range from 300 to 400 ºC ascribable to the rupture of the α- and β-aryl-alkyl ether linkages.49 These trends are in good agreement with previously reported literature7 and confirm the improved stability of LF-S also under inert atmosphere conditions.


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a)

LF LF-S

Exothermic heat flow (a.u.)

Tg=105ºC

Tg=130ºC

60

80

100

120

140

160

180

Temperature (ºC)

b) 2.0

100

LF LF-S 1.6

80

1.2

Mass (%)

60

40

0.8

20

0.4

0

0.0 100

200

300

400

Temperature (ºC)

500

600

Mass loss derivative (%/ºC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


700

Figure 3. DSC (a) and TGA/dTGA in air atmosphere (b) of the non-functionalized (LF) and the silanized (LF-S) lignin. Lignin-based coatings Based on the silanized lignin obtained, four different formulations were considered to obtain ligninbased coatings for application in the field of anticorrosion treatments. The first two formulations consisted in the solution-based deposition of LF and LF-S, followed by appropriate thermal treatment (the obtained coatings will be referred to as CLF and CLF-S, respectively). The third coating (CLFS/H) was obtained from a formulation containing LF-S and AcH in order to determine the viability of using H+ as catalyst to promote the hydrolysis and condensation reactions of methoxy groups in silanized lignin, thereby envisaging improved adhesion to the target metallic substrates (Figure 4). In 13 ACS Paragon Plus Environment


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the last formulation, TEOS was also incorporated to study its ability to act as organic-inorganic hybrid crosslinker in the lignin-based coating system (CLF-S/HT). lignin O C O H N

lignin

lignin

O C O H N

O C O H N

Si O OH

O OH

O

Si O

OH

metal substrate

OH

 H+

O

Si O OH

O

O

lignin-coated metal surface

Figure 4. Schematic representation of the covalent bonding between LF-S and metal substrate promoted by the presence of AcH and heat. FTIR To determine possible changes in the chemical structure of the lignin-based coatings upon heat treatment, FTIR spectroscopy in transmission mode was used (Figure 5). As expected, the FTIR spectra of CLF and CLF-S did not present any significant difference compared to those observed for the precursor materials before thermal crosslinking and already described before (LF and LF-S, respectively), thus suggesting that no chemical modifications occurred upon thermal treatment and no crosslinking was observed without addition of appropriate catalysts or crosslinkers. Indeed, both CLF and CLF-S coatings were found to be completely soluble in THF, clearly excluding the presence of significant gel portions in the obtained systems. On the other hand, significant chemical changes in the FTIR spectra were observed when H+ and/or TEOS were incorporated into the formulation. More specifically, the use of AcH as catalyst promoted the formation of Si-O-Si (820 cm-1) and SiO-C bonds (1125 cm-1) upon heating, resulting in an increase in the crosslinking level of the system. Also, the covalent incorporation of TEOS as a co-crosslinker in the structure was confirmed by the relative intensities of the signals associated with Si-O-Si and Si-O-C vibrations, which were found to 14 ACS Paragon Plus Environment

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increase in CLF-S/HT with respect to CLF-S/H in the corresponding FTIR spectra. It is also worth noticing that non-hydrolysed TEOS was still partially present in CLF-S/HT, as evidenced by the absorbance band at 1149 cm-1 associated to vibrations of Si-O-Met groups.50 Additionally, a higher absorbance level in the C-H stretching region (3000-2800 cm-1) is also observed in CLF-S/HT as opposed to CLF-S and CLF-S/H, further ascribable to the presence of unreacted methoxy silane groups. The presence of unreacted TEOS in CLF-S/HT can be attributed to the high steric hindrance caused by lignin and the low molecular mobility resulting from the presence of H-bonded urethane carbonyl bonds (as evidenced by FTIR analysis),42 which may limit the hydrolysis/condensation

820

765

1125

1080

CLF CLF-S CLF-S/H CLF-S/HT

1690

2940

3400

3520

reactions of alkoxysilane moieties.

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4000

3500

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2500

2000

1500

1000

-1

Wavelength (cm )

Figure 5. FTIR spectra of the obtained lignin-based coatings. Thermal characterization The precursor coating solution was deposited on a clean and dry glass substrate and thermally cured under the previously indicated conditions (200 ºC, 15 min). The so-obtained coating material was detached from the substrate and used for its thermal characterization. The thermal transitions of the lignin-based coatings were investigated by means of DSC (Figure 6a) and TGA (Figure 6b). As shown in the DSC traces, all the coatings showed no evidence of phase segregation (i.e. a single thermal transition was observed), indicating that the obtained materials were homogeneous. As 15 ACS Paragon Plus Environment


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expected, no differences in Tg (105 °C) were observed between CLF coating and the original LF material, further confirming that during thermal annealing no chemical modifications (viz., crosslinking) occurred for this species. Similarly, the CLF-S coating did not show significant differences in Tg compared with the original non-treated functionalized material LF-S (130 °C vs. 133 °C). Conversely, a moderate increase in the Tg from 133 °C (CLF-S) to 139 ºC (CLF-S/H) was observed when the catalyst in the form of H+ was incorporated into the coating formulation, as a result of the promotion of the crosslinking reaction in acidic conditions. A further slight increase in Tg was observed when TEOS was added to the formulation (coating CLF-S/HT, Tg = 144 °C) as crosslinker, as a result of the tighter packing of the macromolecules resulting from the addition of the inorganic phase, even at relatively low concentrations. The thermal stability of the coatings was investigated by means of TGA measurements in oxidative (i.e. air) atmosphere. All coatings obtained starting from functionalized lignin LF-S were found to exhibit a higher thermal stability compared with CLF, indicating a positive effect of the silanization of the parent material. In particular, a significant improvement of the thermo-oxidative stability compared to non-functionalized CLF was found by incorporating crosslinking promoters (H+ catalyst and/or TEOS) to the formulation. In particular, the temperatures at which 10% and 50% weight losses occur (T10 and T50, respectively) and the residue remaining after the thermal treatment in air at 700 ºC (R700) were found to increase substantially (Table 1) in CLF-S/H and CLF-S/HT with respect to both CLF and CLF-S, further confirming successful formation of a thermally stable three-dimensional network upon thermal treatment. A similar behavior was also found in TGA measurements performed in N2, clearly indicating that a similarly improved thermolytic response of the crosslinked coating could be achieved (Figure S6).


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a)

b)

Figure 6. DSC (a) and TGA measurements in air (b) of the coatings formulated. Table 1. Thermal degradation temperatures at 10% (T10), 50% (T50) and final residue (R700) for ligninbased coatings. T10 (ºC)

T50 (ºC)

R700 (%)

CLF

142

471

2.5

CLF-S

287

498

9.6

CLF-S/H

323

538

12.6

CLF-S/HT

323

535

15.2

Surface wettability The surface wettability properties of the lignin-based coatings were investigated by means of contact angle measurements using water and diiodomethane (DIM) as probe liquids. The results of surface 17 ACS Paragon Plus Environment


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tension (γ) and their corresponding dispersive (γd) and polar (γp) components are presented in Table 2. Table 2. Contact angles of water (θH2O) and diiodomethane (θDIM), total surface tension (γ), corresponding polar (γp) and dispersive (γd) components and pull off adhesion strength results on aluminum (average values ± standard deviation out of 4 measurements). 𝜃𝐻2𝑂 𝜃𝐷𝐼𝑀 Coating

(º) (º)

𝛾𝑡𝑜𝑡

𝛾𝑝

𝛾𝑑

Pull-off

𝑚𝑁 𝑚

𝑚𝑁 𝑚

𝑚𝑁 𝑚

strength on aluminum

( ) ( ) ( )

(MPa)

CLF

80.5±1.4

41.7±1.5

40.3±1.0

35.3±0.9

5.0±0.1

0.37±0.07

CLF-S

97.7±2.3

53.9±1.7

32.3±0.8

30.3±0.8

2.1±0.1

0.60±0.1

CLF-S/H

83.2±1.5

39.5±1.6

39.4±1.2

35.4±1.0

4.0±0.1

0.65±0.04

CLF-S/HT

85.2±1.7

46.4±0.9

37.2±0.7

33.5±0.7

3.7±0.1

1.22±0.2

All coatings were found to present water contact angles (θH2O) over 80º, thus indicating relatively good affinity with water for most systems. In particular, CLF exhibited the least hydrophobic behavior, likely due to the presence of hydroxyl groups in the parent lignin that allow relatively strong interactions of the surface of the coating with water and consequently lower contact angle, in line with similar trends previously observed in the literature.15 Compared with CLF, a significantly higher value of θH2O (97.7° vs. 80.5) and a correspondingly lower surface energy γtot (32.3 mN m-1 vs. 40.3 mN m-1) were observed in CLF-S, which may be ascribed to the silanization process of lignin leading to a decrease of hydroxyl groups upon its functionalization with ITMSP and to the incorporation of less polar –OMet alkoxy groups in its structure (Figure 1). The addition of a catalyst (H+) and of a crosslinking agent (TEOS) to the coating formulation (CLF-S/H and CLF-S/HT) led to a slight decrease of θH2O with respect to CLF-S, as a result of the promotion of the hydrolysis and condensation reactions of the methoxy groups appended to lignin in LF-S, with the formation of more hydrophilic –Si-O-Si- bridges (as also confirmed by FTIR analysis). Indeed, a concurrent increase in γtot is observed for these two systems, originating from the increased value of the polar component of 18 ACS Paragon Plus Environment

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the surface tension γp (33.5, 35.4 and 30.3 mN m-1 for CLF-S/HT, CLF-S/H and CLF-S, respectively). No significant differences were observed between CLF-S/H and CLF-S/HT, likely due to the low amount of TEOS used which does not seem to provide a significant effect to the wettability behavior of the obtained coating systems. Pull-off test In order to assess the suitability of the lignin-based coatings in the manufacturing field, good adhesion ability onto the desired substrates is mandatory. To this end, their adhesion on aluminum substrates was evaluated by performing pull-off tests. The precursor solution was deposited on well-cleaned aluminium substrates by means of a spin-coating process (more details on the procedure adopted for substrate preparation and cleaning can be found in the Supplementary Information). After thermal crosslinking of the deposited coatings, pull-off tests were performed. The obtained results are shown in Table 2. The coating CLF presented the lowest adhesion strength among all the tested systems (0.37±0.07 MPa). This low value indicates the absence of sufficiently strong interactions between the ligninbased coating and the Al2O3 layer of the Al substrate, according to the molecular bonding mechanism approaches proposed in the literature.51 Coating adhesion was significantly improved by replacing the non-functionalized precursor with the silanized lignin (coating CLF-S), reaching values of adhesion strength of around 0.6±0.1 MPa. Such enhanced adhesion suggests the occurrence of moderate covalent interactions between the substrate and LF-S, likely originating from the reaction between hydroxyl groups on the aluminum surface and the methoxysilane moieties in LF-S. When the acid catalyst was incorporated into the coating formulation, the adhesion ability was slightly improved further (0.65±0.04 MPa) indicating its favouring effect on the hydrolysis of methoxy groups for its further condensation over the Al2O3 layer. The highest adhesion was obtained when TEOS was incorporated into the coating formulation. In this case, an adhesion strength of 1.22±0.2 MPa was obtained, ascribable to both the strong covalent interactions between silanized lignin and Al substrate 19 ACS Paragon Plus Environment


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as well as to the presence of TEOS acting as strong crosslinking agent between silane groups in LFS and creating an intricate organic-inorganic network within the coating further improving its covalent affinity with the substrate. It is also interesting to note that the obtained value of pull-off strength for the CLF-S/HT system (1.22±0.2 MPa) is comparable to values previously reported in the literature on other organic coatings on aluminum3, 52-53 and to adhesion strength typically obtained on commercially-available primers for aluminum substrates. Potentiodynamic polarization curves Potentiodynamic polarization studies were carried out to evaluate the anticorrosion properties of the previously synthesized lignin-based coatings on aluminum substrates. Figure 7 shows the potentiodynamic polarization curves for non-coated and coated aluminum in a 5% NaCl solution at 25 ºC. The corrosion resistance was evaluated through the measurement of the corrosion current density (CCD) value, which corresponds to the intersection between the tangents of the anodic and the cathodic polarization curves. In particular, the lower the CCD value, the better the anticorrosion behavior. Compared to a standard passivated aluminum substrate (Al/ref) used as reference system with high degree of corrosion protection (thanks to the presence of an Al2O3 passivating layer on its surface), the plasma-cleaned aluminum substrates used in this study for the deposition of the ligninbased coatings (Al/plasma) showed a two-order-of-magnitude higher CCD value (4.6·10-8 and 4.4·106

A/cm2 for Al/ref and Al/plasma, respectively). These preliminary evidences provide important

guidelines to set the boundary conditions of the systems investigated in this work. In particular, the value obtained for Al/plasma represents the maximum CCD value that can be achieved in the test, as it is measured on substrates with no protection layers counteracting the effect of corrosion. Conversely, the value obtained for Al/ref represents the minimum value for CCD, as it results from measurements on a well-protected reference material in which the effect of corrosion is minimized thanks to the presence of an Al2O3 passivation layer. When the coatings based on silanefunctionalized lignin were applied on the plasma-treated Al substrate, a general shift of the cathodic 20 ACS Paragon Plus Environment

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and anodic curves to lower values of current densities (i.e. a decrease in the corrosion rate) was observed independently of the coating formulation used. Moreover, the shifting of both the cathodic and anodic curves indicates that the coatings behave as a mixed-type corrosion inhibitor affecting both anodic and cathodic half reactions.54 Making a comparison between all the coating formulations, it is possible to notice that sample CLF presented a CCD value comparable to that found on Al/plasma (6.8 ·10-6 A/cm2), clearly indicating a very poor protective action of this coating, likely resulting from its limited adhesion onto the aluminum substrate that can induce easy accessibility of the corrosive solution. The samples CLF-S and CLF-S/H presents a slightly improved inhibition effect, showing lower values of CCD of 1.40·107 and 1.9·10-7 A/cm2, respectively. This indicates that the functionalization of lignin provides coatings

with better anticorrosion effect compared with the pristine lignin precursor CLF, in line with their better adhesion properties on aluminum. Finally, CLF-S/HT coating presented the lowest CCD, with a value of 5.05·10-10 A/cm2, indicating the best anticorrosion protective behavior. In this case, the improved adhesion of the coating through covalent linkage between LF-S and the Al substrate (as evidenced from the adhesion measurements discussed above) combined with the formation of an organic-inorganic hybrid network promoted by the acid catalysed reaction between TEOS and LF-S can effectively block the active sites generated on Al/plasma, thereby significantly hampering the corrosion rate of the substrate.32 Also the shift of the corrosion potential toward the value of Al/ref indicates the good barrier behaviour and corrosion protection of the CLF-S/HT. In this respect, it is also interesting to underline the beneficial effects of the structural and morphological arrangement of the obtained lignin-based crosslinked coatings on corrosion resistance and CCD values. As demonstrated through FTIR analyses (Figure 5), the coatings CLF-S, CLF-S/H and CLF-S/HT are characterized by the presence of hard domains of H-bonded carbonyl urethane moieties embedded in the amorphous structure of lignin. According to previous literature reports,42, 55 this internal structure is likely to prevent easy diffusion of corrosive ions due to the presence of tortuous paths within the 21 ACS Paragon Plus Environment


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coating, resulting in improved anticorrosive performance (a schematic representation of this mechanism can be found in the Supporting Information, Figure S7). It is interesting to observe that the results obtained in our work well compare with recently published experimental values of CCD on aluminum substrates obtained by the application of different anticorrosive organic or hybrid coatings.56-58

Figure 7. Potentiodynamic polarization curves for coated and non-coated aluminum substrates in a 5% NaCl solution at 25 ºC. Conclusions In this work, new renewable lignin-based coatings with anticorrosion properties were synthesized and characterized. These new coatings were obtained from a THF-soluble fraction of softwood kraft lignin functionalized with silane groups through urethane linkages. The effectiveness of the functionalization was confirmed by FTIR and a comparison of the thermal properties of the nonsilanized and the silanized lignin was performed by DSC and TGA, observing both an increase in the Tg and in the thermal stability due to the formation of the urethane bond and the increase of the molecular weight during the functionalization.


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The silanized lignin was used as main building block for the synthesis of coatings through thermal crosslinking. FTIR analyses confirmed successful incorporation of TEOS in the formulation as crosslinker among lignin chains. Calorimetric analyses confirmed the occurrence of a selfcrosslinking process in the functionalized lignin-based coating due to the catalytic effect of H+, as evidenced by the higher Tg values obtained as compared to the parent materials. A further increase in the Tg was registered in coatings incorporating TEOS as crosslinker as a result of the more rigid structure originating from the presence of the inorganic phase that partially hinders macromolecular movement in the crosslinked system. In a similar way, the thermal stability of lignin-based coatings increased by using TEOS and/or H+ as crosslinking promoters. The wettability of the lignin-based coatings was assessed through contact angle measurements, evidencing a correlation between polarity of the formed chemical groups upon crosslinking and surface energy of the resulting coatings. To evaluate the adhesion of the coatings, pull-off tests were carried out on lignin-based coatings deposited on aluminum, obtaining adhesion forces over 1 MPa when TEOS was incorporated to the coating formulation as hybrid crosslinker. Finally, the potentiodynamic polarization curves demonstrated the anticorrosion properties of the lignin-based coating CLF-S/HT on aluminum, with a CCD as low as 5.05·10-10 A/cm2. The results of this study clearly demonstrate the applicability of lignin as renewable precursor for the development of added-value functional coatings and highlight the potential of these materials in the field of corrosion inhibition on metallic surfaces. Acknowledgments The authors greatly acknowledge Gigliola Clerici for kind support with thermal analysis. Supporting Information. The file is available free of charge and contains: FTIR spectra evolution during the functionalization, NMR spectra, GPC analyses, TGA traces and explanation of the pretreatment of Al substrates. 23 ACS Paragon Plus Environment


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Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. REFERENCES 1.

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For Table of Contents Only

Synopsis Lignin-based coatings with anticorrosion properties were synthesized, characterized and successfully tested on aluminum surfaces.



Figure 1. Schematic representation of the silanization of lignin. 266x88mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2. FTIR spectra of THF-soluble lignin fraction (LF) and of silanized product (LF-S). 204x143mm (300 x 300 DPI)



Figure 3. DSC (a) and TGA/dTGA in N2 atmosphere (b) of the non-funcionalized (LF) and the silanized lignin (LF-S). 204x154mm (300 x 300 DPI)


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Figure 3. DSC (a) and TGA/dTGA in air atmosphere (b) of the non-functionalized (LF) and the silanized (LFS) lignin. 204x143mm (300 x 300 DPI)



Figure 4. Schematic representation of the covalent bonding between LF-S and metal substrate promoted by the presence of AcH and heat. 264x170mm (300 x 300 DPI)


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Figure 5. FTIR spectra of the obtained lignin-based coatings. 204x143mm (300 x 300 DPI)



Figure 6. DSC (a) and TGA measurements in air (b) of the coatings formulated. 204x159mm (300 x 300 DPI)


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Figure 6. DSC (a) and TGA measurements in air (b) of the coatings formulated. 204x142mm (300 x 300 DPI)



Figure 7. Potentiodynamic polarization curves for coated and non-coated aluminum substrates in a 5% NaCl solution at 25 ºC. 204x145mm (300 x 300 DPI)


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TOC 338x190mm (96 x 96 DPI)


Lignin-Based Anticorrosion Coatings for the Protection of Aluminum

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