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High Performance Soya Polyurethane Networked Silica Hybrid Nanocomposite Coatings Anujit Ghosal, Obaid Ur Rahman, and Sharif Ahmad Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02098 • Publication Date (Web): 30 Oct 2015 Downloaded from http://pubs.acs.org on October 31, 2015

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Industrial & Engineering Chemistry Research

High performance soya polyurethane networked silica hybrid nanocomposite coatings Anujit Ghosal Research scholar Materials Research Laboratory, Dept of Chemistry Jamia Millia Islamia, New Delhi. Email: [email protected]

Obaid Ur Rahman Research scholar Materials Research Laboratory, Dept of Chemistry Jamia Millia Islamia, New Delhi. 110025. Email: [email protected]

*Corresponding Author *Prof. Sharif Ahmad Materials Research Laboratory, Dept of Chemistry Jamia Millia Islamia, New Delhi. 110025. Email: [email protected] Tel no. +91 11 26827508 Fax: +91 11 26840229

1 ACS Paragon Plus Environment


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Abstract: The projected fossil fuel deposits depletion by near future and detrimental effects of petroleum products on environment has led to the development of VOC free low molecular weight sustainable polymers, with promising applications in field of inks, adhesives, paints and coatings. In view of this, present study reports synthesis, characterization and anticorrosive performance of SMG-PU and modified SMG-PU through in-situ formed silica networks (SMG-PU-TEOS) OIH polymer nanocomposite coatings on CS. The size of ca. 30 nm silica nano-networks were formed within SMG-PU on addition of TEOS, inducing thermal stability, hydrophobic, improved physico-mechanical and corrosion resistant properties to polymer nanocomposite coatings. The corrosion protective performance of these coatings in 5% NaCl (salt mist test), 3.5% NaCl and 3.5% HCl solutions revealed very lower Icorr values (3.8891×10-10 Acm-2 and 6.8756×10-9 Acm-2) as compared to SMG-PU (1.8159×10-8 Acm-2 and 9.1396×10-7 Acm-2), bare CS (8.9131×10-5 Acm-2 and 8.1731×10-4 Acm-2) and other such reported systems.


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1. Introduction The process of corrosion and its protection in industries and other related fields viz., oil and natural gas, power plants, shipping, aviation, bridges, desalination and water treatment plants etc. involve trillions of dollars.1-3 Many protective techniques have been used to control the corrosion and to enhance the life of the respective systems by understanding their corrosion inhibition mechanism.4-5 Generally, the solvent borne synthetic polymer anticorrosive paints and coatings have been frequently used in industries and other infrastructures due to their easy availability, applicability, high protective efficiency under corrosive environments.6-7 However, the health hazards and harmful effects of such paints and coatings on the ecology and society have become a matter of great concern.2-3 Thus, the focus of current research in the field of anticorrosive polymer coatings has shifted towards the development of alternative low-cost, biodegradable and eco-friendly sustainable polymers, which exhibit value added properties like good fluidity (low molecular weight, 2000-15000 a.u), UV resistant, hydrophobicity and good plasticity.8-10 The coatings of such polymers involved either no or minimum amount of organic solvents, which produce no or negligible volatile organic compound (VOC) and also meet the requirements of clean air Acts & regulations.11 Among various sustainable resources, vegetable seeds, for instance linseed, castor, nahar, jatrofa, pongamia, soy seeds and their oils have more abundantly available, containing 15% to 45% oil contents.9, 12 In addition to unsaturation they also possess many other functionalities like hydroxyls, carboxyls, esters, oxiranes in their fatty acid chain.13 These functionalities facilitate the useful chemical modifications of oils in the form of alkyds,14 polyesteramides,15 epoxies,16 polyurethanes,17 etc. which find applications as diluents, inks, adhesives, paints and coatings.



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The high molecular weight solvent borne synthetic polyurethanes (PU’s) comprise of short sequences of soft (flexible) polyol chain and hard (rigid) isocyanate segments, exhibiting excellent physico-mechanical, UV and corrosion resistance properties with wide industrial applications in the field of leather, foam, adhesive, paints and coatings.18,19 However, the main problem with these polymers is their VOC, which causes many environmental and hazardous health problems. While the vegetable oil (VO) based PU`s are almost VOC free but exhibit poor mechanical properties that is low toughness and higher elastomeric behaviour.20 Although, the VO-PU`s have exhibited superior corrosion protective performance compared to other VO polymer coatings.12,

21

It has been observed that the dispersion of inorganic nano fillers/

modifiers like Fe3O4,22 Tetraethyl Orthosilicate (TEOS),12,

23

Titania,24 etc., in VO-PU’s

improved the physico-mechanical, thermal and corrosion resistance properties as compared to those of plain VO-PU’s.15 The attempts have also been made to develop the soya oil (SO) PU’s and inorganic nanoparticle dispersed SO-PU’s exhibiting excellent corrosion resistance. Various efforts have been made to develop the excellent thermal, electrical, flame, and corrosion resistant PU coatings of SO25 through the reaction of hydroxyl (–OH) of SO polyol with –NCO of di or tri isocyanates of which have been used for packaging, and other engineering application.26-30 Generally, polyols of VO`s are synthesized via., epoxidation and hydroxylation processes, involving hydroformylation/ reduction/ transesterification, reactions in presence of expensive transition metal catalysts involving multi step reactions.31-32 The use of low cost and more reactive monoglyceride (polyol) containing primary and secondary hydroxyl groups lead to the synthesis of higher molar mass PU, which is found to be more useful than that of normal VO polyols that containing only secondary OH group of lower reactivity attributed to their non-linear hydrocarbon chain.31-33 The presence of linear hydrocarbon chain, higher number of primary and


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secondary hydroxyl groups in monoglycerides enhances the condensation reaction between –OH group of SMG and –NCO group of Toluene-2,4-diisocyanate (TDI) resulting in the formation of dense, viscous and highly crosslinked PU of higher molar mass.32-34 However, the synthesis of PU from monoglyceride required to be carefully designed to avoid the gel formation.35-36 The in-situ synthesis of inorganic networks through sol gel route has been preferred over other conventional synthesis of inorganic networks with in the polymer matrix because of the ability of low temperature processing, controllability of particle size, chemical composition, formation of non-agglomerated inorganic component and ability to form the organic-inorganic hybrids.37-39 Further, during hydrolysis and condensation of inorganic alkoxide like TEOS in-situ inside the organic polymers led to the formation of inseparable inorganic networked silica within polymer matrix resulting in the formation of final organic-inorganic hybrid material with excellent solvent resistant, hydrophobic and corrosion resistance properties.40 In view of this the present work, reports the synthesis of soy monoglyceride (SMG), soy monoglyceride polyurethane (SMG-PU) and Si dispersed PU (SMG-PU-TEOS) OIH nanocomposites and their coatings. The structural elucidation of these materials, particle size of networked silica, morphological and thermal behaviour of SMG-PU and SMG-PU-TEOS coatings were analysed using Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) (1H and

13

C) spectroscopy, energy dispersive X-ray (EDX) analyser,

transmission electron microscopy (TEM), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) techniques. The physico-mechanical performances of these coatings were investigated using standard methods, while the corrosion protective analysis was performed by PDP and EIS analysis techniques. These studies showed that the OIH (SMG-PU-TEOS) nanocomposite coatings have far superior physico-mechanical and



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anticorrosive performance than that of SMG-PU coatings and other reported systems (Table 1).21, 23, 41-48

2. Experimental 2.1

Materials

Soy oil was procured from the local market (density= 0.92, refractive index = 1.476), glycerol, sodium hydroxide (NaOH), ethylmethylketone (EMK), acetone and tetrahydro furan (THF) from S.D. Fine chemicals, Mumbai, India, TDI (was an 80/20 mixture of 2,4- and 2,6-isomers), methanol (CH3OH), ethanol (C2H5OH), dimethylsulphoxide (DMSO), dimethyl formamide (DMF) and TEOS were obtained from Merck, India and toluene, hexane, xylene, benzene, hydrochloric acid (HCl), from Sigma-Aldrich, USA. All the chemicals were of analytical grade and used as received without further purification. 2.2

Synthesis of soy oil monoglyceride (SMG)

SMG was prepared using standard procedure reported elsewhere49 and shown in Scheme 1a. The glycerol and soy oil were taken in 1.5:1 ratio in a three necked round bottom flask (RB), equipped with magnetic stirrer, thermometer and nitrogen gas inlet. The reaction was continued for 90 min after addition of sodium hydroxide (0.2 weight % of Soy Oil) to the heated reaction mixture up to 180-190 oC.50 Progress of the reaction was periodically monitored by FT-IR spectral analysis at regular intervals of time. The formation of final product (SMG) was confirmed by observing the formation of completely soluble homogeneous and transparent solution of one part of synthesised SMG in three part of CH3OH (SMG: CH3OH, 1:3 v/v).47 After the completion of reaction, the unreacted glycerol was removed by quenching the final product in salt (NaCl) dispersed ice bath from 190 oC to 15 oC.51 Further, the structure and formation of SMG was confirmed through FT-IR, 1H NMR and 13C NMR spectrum of SMG.


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2.3

Synthesis of SMG-PU-TEOS OIH nanocomposite

The SMG-PU-TEOS hybrid composite was prepared using SMG-TEOS OIH sol containing various percentages of TEOS and TDI as follows: 2.3.1

Preparation of TEOS sols. The solutions of different amount of TEOS (0.5, 1

and 2 weight % of SMG) was prepared in 20% V/V acidified EMK solvent, which was added drop wise in a three necked round bottom (RB) flask containing 30 g of SMG fitted with N2 inlet, thermometer and condenser under constant stirring on a magnetic stirrer for 30 min at 40 o

C. The temperature of the reaction mixture was slowly raised and maintained at 60 oC for 4-5 h.

The formation of hybrid SMG-TEOS sol was confirmed by FT-IR and NMR analysis (Figure 1 and 2). 2.3.2

Preparation of SMG-PU and OIH (SMG-PU-TEOS) nanocomposites

(Scheme 1b and 1c). The solution of TDI (50 weight % of SMG) was prepared in minimum possible EMK and was added drop wise to the three necked RB flask containing 20 g of SMG for a period of 30 min. The reaction mixture was stirred under N2 environment for a period of 3 h. The progress of the reaction was monitored by measuring the reduction in the intensity of – NCO peak (2276 cm-1) in FT-IR spectra of SMG-PU, which was taken at regular intervals of time. On the disappearance of –NCO peak in the range of (2250-2270 cm-1) the reaction was stopped. The SMG-PU-TEOS (OIH) nanocomposite was prepared using the same procedure as was used for SMG-PU (Scheme 1d). However, in this case a lower amount of TDI was used, i.e. in place of 50 weight % of TDI used during SMG-PU, 40 weight % of TDI solution in EMK was used for the formation of SMG-PU-TEOS (OIH). The OIH nanocomposites of SMG containing different loading of TEOS, i.e 0.5, 1 and 2 weight % of SMG were denoted as SMG-PU-TEOS-0.5, SMG-



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PU-TEOS-1 and SMG-PU-TEOS-2 respectively, where the suffix represents the weight % of TEOS. The coating beyond 2 weight % (of SMG) loading of TEOS was found to be unstable due to phase separation of the constituents and thus, coatings beyond 2 weight % TEOS in SMG were not studied. 2.4

Preparation and testing of coatings

The commercially available CS strips (C:2.87 % and Fe: 97.13 %)52 of various standard sizes were polished with different grades of SiC papers (180, 320, and 500). The finally polished specimens were washed thoroughly with double distilled water, degreased with ethanol, acetone and dried at room temperature. The coatings were applied by brush technique on CS of standard sizes, i.e., 70 mm × 25 mm × 1 mm for physico-mechanical tests and on 25 mm × 25 mm × 1 mm CS strips for electrochemical corrosion tests and morphological studies. Dry to Touch Time (DTT) and Dry to Hard Time (DTH) were also recorded. All coatings were cured with in 30 to 45 minutes at room temperature (30 oC) and kept for seven days (incubation period for the purpose of complete curing and drying) before subjecting them for further characterizations. 2.5

Characterizations

FT-IR spectra of SMG, SMG-PU and SMG-PU-TEOS were recorded on IR Affinity-1, Shimadzu using ZnSe cell. The NMR (1HNMR and

13

C NMR) analysis of SMG-PU sol and

SMG-PU-TEOS OIH nanocomposites were performed at 500 MHz in deuterated chloroform (CDCl3) and tetramethylsilane (TMS) was used as an internal standard. The particle size and surface morphology was determined with the help of TEM and SEM analysis. The viscosity, specific gravity (ASTM D4762-11a), refractive index (ASTM D1747) and solubility were determined by standard methods. The physico-mechanical properties of SMG-PU and SMG-PUTEOS coatings were determined using scratch hardness (ASTM D1474-98), impact resistance


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(ASTM D 2794-93), bend test (ASTM D522-93a), specular gloss measurements (ASTM D52389) by a gloss meter and cross-hatch adhesion test (ASTM D3359). Average molecular weights of the SMG-PU was determined by gel permeation chromatography (GPC) (Hitachi ELITE LaChrom chromatograph) using tetrahydrofuran (THF) as the eluent and polystyrene standards. The thickness of these coatings was determined by an Elcometer. The hydrophobic behaviour of coatings was evaluated by contact angle measurements using a Drop Shape Analysis System with a high speed CCD camera for image capture. De-ionized water was used for the evaluation of the hydrophobic properties of the coated substrate. Angle measurements were done in triplicate. Water was taken up in a syringe and drops (15 µl) were allowed to fall onto the surface of the coated substrate. The left and right contact angles were measured for 10 seconds at the interval of 1 second. The average contact angle was calculated using the three values of left and right contact angles.53 Electrochemical corrosion tests on coated and uncoated CS were conducted by performing the open circuit potential (OCP), PDP and EIS measurements with the help of three electrode flat glass cell (EG & G of 400 ml capacity) using a Potentiodynamic/ Galvanostat micro Auto lab type III with FRA unit (µ3AVT 70762, Utrecht, Netherlands) in 3.5 weight % NaCl and 3.5 weight % HCl solution at room temperature (30 oC). The three electrodes used were: (i) Ag/AgCl as the reference electrode, (ii) platinum as an auxiliary electrode and (iii) the test specimen as the working electrode. A 1.0 cm2 area of the working electrode was exposed to the solution (ASTM G59-97). Prior to potentiodynamic polarization and EIS testing, the working electrode was allowed to stabilize for 30 min and OCP was recorded as a function of time for 600 s. After OCP stabilization, impedance measurements were performed at respective corrosion potentials (Ecorr) over a frequency range of 100 kHz to 0.1 Hz, with a signal amplitude perturbation of 10 mV. The potentiodynamic polarization tests were carried out in the potential



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range ±100 mV (with respect to OCP) at a scan rate of 0.001 mV s-1. Nova 1.8 software was used for data fitting and calculation of results. The impedance and Tafel parameters were analysed by curve fitting procedures available in the aforementioned software. Each test was run in triplicate to verify the reproducibility of the data.48 Polarization resistance (Rp) was determined from the slope of the potential–current plot (Tafel plots), using Stearn–Geary equation.54: (i)

Here, Icorr is the corrosion current density determined the intersection of linear portions of the anodic and cathodic curves, βa and βc are anodic and cathodic Tafel slopes. The impedance spectra obtained can be used to model the electrochemical corrosion behaviour of coated substrate by an equivalent circuit and were found to be reproducible up to ±2–3%. The equivalent electrical circuit (EEC, Figure 3.11) is often used to analyse the impedance spectra of a metal/coating system and Rs(Cc(Rpo(CdlRct))) circuit model is used to simulate the changes in the properties of the coatings. Where, Rs is the solution resistance, Rpo is the pore resistance, Rct is the coating resistance while the Cc and Cdl are the constant phase element and the double layer capacitance of the coating respectively. The EEC consists of two time constants; the first time constant (RpoCc) in the high frequency region, attributed to the intrinsic properties of the coating and initiation with propagation of corrosion is shown by second time constant (CdlRct) in low frequency region after immersion to the corrosive environment.55 Salt spray test on SMG-PU and SMG-PU-TEOS coated CS specimens was carried out in salt mist chamber under 5 weight % NaCl solutions at 90% humidity. SEM-EDX studies of the corroded samples after salt spray test and uncorroded coated CS samples were performed.


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Table 1. Comparison of corrosion resistance properties of SMG-PU and SMG-PU-TEOS coatings with other reported PU systems. S.No

Coating System

Medium

Icorr (Acm-2)

Polarization resistance (Rp (Ω))

Rpore (Ω)

Coating capacitance (Cc, Farad)

Reference

1.

SMG-PU

3.5 weight % NaCl

1.8159×10-8

1.20×10+6

1.28×10+6

4.62×10-11

Present study

SMG-PU-TEOS-2

3.5 weight % NaCl

3.8891×10-10

5.58×10+7

4.78×10+12

6.78×10-13

2.

Castor oil based poly(urethane-copyrrole)[CPUPY4]

3.5 weight % NaCl

1.7×10-6

4.92×10+2

7.84×10+4

-

41

3.

MWNT/PU–SS

3.0 weight % NaCl

4.4×10-9

-

4.9×103

2.2×10-9

43

4.

Castor oil PU

3.5 weight % HCl

6.08×10-5

5.6×10+5

4.1×10+5

6.4×10-7

21

1.0POT/COPU

3.5 weight % HCl

8.30×10-8

2.8×10+10

3.0×10+10

2.6×10-10

polyurethanefattya mide/silica

3.5 weight % HCl

2.65×10-8

-

-

-

3.5 weight % NaOH

1.09×10-7

-

-

-

Water

6.5010-9

-

-

-

5.

48

6.

Water borne PU coatings from crude glycerol-based polyol and its blends with petroleum-based polyether polyol [WPU-d]

2.9 weight % NaCl

1.5×10-8

-

-

-

42

7.

polyaniline−poly(ac rylic acid) (PANI−PAA) composites

1.8 weight % HCl

3.611.86×10-6

-

-

-

44



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8.

ZnO-poly(butyl methacrylate) latexPANI

3.5 weight % HCl

3.0×10-6

-

-

-

45

9.

PMMA-MMT Clay Nanocomposite (CLMA10)

5 weight % NaCl

2.3×10-9

1.8×107

-

-

46

10.

WCA-BMF80

3.5 weight % NaCl

4.50×10-7

2.9×104

1.55×107

6.89×10-10

47

11.

Epoxy−Silica Hybrid Nanocomposite

5 weight % NaCl

2.20 ×10-10

1.26×108

-

-

23

3. RESULT AND DISCUSSION 3.1

Synthesis and formation of SMG and its OIH’s

The SMG was prepared by the base catalysed trans-esterification reaction of primary alcohol of glycerol and primary carbonyl of soy oil. The more reactive and sterically less hindered primary alcohol of glycerol reacted with sodium hydroxide forming an alkoxide, an intermediate catalyst, which in turn react with the carbonyl group at more reactive primary carbon of fatty acid of VO through the base-catalyzed transesterification.56 The formation of SMG was confirmed by FT-IR and NMR analysis of the initial and final resulting product. The stepwise formation of SMG in detail have been given in supporting information along with FT-IR spectra at regular interval to monitor the change of –OH groups during the reaction (Figure S1) and the complete solubility of SMG in three part of CH3OH (SMG: CH3OH, 1:3 v/v) further confirmed the formation of SMG.47 The comparison of FT-IR spectrum of the SO (Figure 1a) and SMG (Figure 1b) revealed the generation of new peak in the range of 3554-3266 cm-1 which was attributed to -OH groups, generated during the trans-esterification reaction of SO using glycerol. The strong absorption peak at 1740 cm-1 was ascribed to the presence of carbonyl group of ester functional group at the secondary carbon (Scheme 1a). While, the 1H NMR spectrum (Figure 2a) revealed the presence 12 ACS Paragon Plus Environment

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of chemical shift values at δ= 3.5-3.8 ppm attributed to the protons attached to oxygen atom (CH-OH and CH2-OH) and the chemical shift value at δ= 4.12 ppm was credited to the proton of hydroxyl groups. Further, the chemical shift (Figure 2b) in the range of δ = 60-73 ppm in

13

C

NMR spectra corresponded to carbon attached to oxygen atom directly. Thus, all these characteristic FT-IR peaks and chemical shift values in NMR spectra confirmed the transesterification of SO and structure of SMG. Other characteristic peaks and chemical shift values of FT-IR and NMR for soy oil and its monoglyceride are given in their respective sections. Furthermore, a number of additional physical data of SMG, like refractive index, viscosity, specific gravity were provided Table 2 and FT-IR analysis was successfully used to monitor the –OH value. The SMG-TEOS sol with varying ratio of acidified TEOS in EMK was prepared by the sol–gel technique using TEOS as an inorganic precursor and SMG as organic precursor. The addition of TEOS induces strong chemical interaction between organic and inorganic moieties, which helps in the formation of SMG-TEOS OIH nanocomposites by one pot reaction.57-58 The in-situ formation of networked silica through condensation reactions of metal alkoxides and their incorporation within the backbone of SMG was confirmed by FT-IR analysis and schematically shown in Scheme 1a-d. The in-situ growth of nanoscale –O-Si-O- networks can be attributed to the formation of stable organic-inorganic homogeneous phase without any separation, which was also confirmed by TEM and SEM-EDX studies.57 For the preparation of SMG-PU and SMG-PU-TEOS coatings, the optimum concentration of TDI was selected from various NCO:OH ratios of 0.3/1.0, 0.4/1.0, 0.5/1.0 and 0.6/1.0 based on the physicomechanical properties (Table S1) of SMG-PU and SMG-PU-TEOS. The SMG-PU with 0.5:1.0, -NCO/OH ratio has exhibited the best physico-mechanical properties. On further



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increase in the amount of –NCO groups i.e. more than 0.5 (say, 0.6), lead to the formation of brittle coatings (cracks were developed after drying). Abnormal increase in viscosity and gel formation was observed on higher –NCO concentration (i.e. 1:1, -NCO/OH ratio).

Hence

beyond 0.5:1 ratio of –NCO:OH, SMG-PU coatings were not formed and hence, this (0.5:1) ratio was considered to be the optimum concentration for SMG-PU. While, in case of SMG-PU-TEOS the lower amount of TDI (0.4:1.0, -NCO/OH ratio) was used. This can be correlated to the modification of SMG through in-situ growth of networked silica, which consumed the most of – OH groups present in SMG. The unreacted lesser number of –OH groups available for reaction required lower amount of TDI as compared to that of plain SMG.

Scheme 1. (a) The synthesis of SMG.

Scheme 1. (b) The synthesis of SMG-PU.


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Scheme 1. (c) The synthesis of SMG-TEOS OIH sol.

Scheme 1. (d) The synthesis of SMG-PU-TEOS OIH nanocomposite. 3.2

Physico-chemical properties

The specific gravity and refractive index shows an increase in their values from SMG to SMGPU-TEOS (Table 2), which can be attributed to the in-situ polymerisation of –O-Si-O- and the formation of highly cross linked networks structure of SMG-PU-TEOS.59 Thickness of these coatings was found to be in the range 120-130 µm. The decrease in the intensity of hydroxyl peak of FT-IR (Figure 1) spectra can be assigned to the consumption of –OH group of SMG during its chemical reaction with TEOS followed by an interaction of their –OH functionality 15 ACS Paragon Plus Environment


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with that of isocyanate functionality of TDI. The addition of TEOS resulted in an increase in the viscosity of the SMG-TEOS hybrid sol due to the interaction between silanol groups and hydroxyl groups (-OH) of SMG.60 The increased loading of TEOS in SMG resulted in higher viscosity of OIH nanocomposite sol’s, indicating a net increase in the degree of interaction between colloidal –O-Si-O- and polyol (SMG) that lead to the formation of internal networks, which restrict the molecular mobility.60-61 On addition of TDI in SMG, an increase in refractive index, specific gravity and viscosity was observed. This increase trend may be attributed to the formation of internal links through urethane bonds and presence of benzene ring in SMG-PU. The solubility of SMG, SMG-TEOS, SMG-PU and SMG-PU-TEOS OIH nanocomposites were investigated in various polar and non-polar solvents at room temperature. The SMG was completely soluble in CH3OH, acetone, EMK, THF and non-polar solvents like toluene, hexane, xylene, benzene, but partially soluble in DMSO, DMF and insoluble in water. Table 2. Physico-chemical properties of SO, SMG, SMG-TEOS-0.5, SMGTEOS-1, and SMG-TEOS-2 OIH nanocomposites. Resin code SO SMG SMG-PU SMG-TEOS-0.5 SMG-TEOS-1 SMG-TEOS-2

Refractive Index 1.4432 1.478 1.532 1.488 1.499 1.513

Specific gravity (g/ml) 0.913 0.952 1.096 1.089 1.091 1.094

Viscosity (mPa.s) 47.00 180 183.7 181.3 181.8 182.1

The OIH nanocomposite, SMG-TEOS sol`s, SMG-PU and SMG-PU-TEOS pre-polymer (fluid state) were found to be soluble in polar as well as non-polar solvents like methanol, EMK, acetone and xylene, although after curing SMG-PU and SMG-PU-TEOS become insoluble in these solvents. The solubility of VO based SMG (a diol-ester) in polar solvents can be attributed to the presence of two –OH groups in SMG chain while the hydrocarbon chain helps in its 16 ACS Paragon Plus Environment

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solubility in non-polar solvents.35,

62

The complete solubility of SMG-TEOS in polar solvents

was due to the formation of -O-Si-O- tethered SMG via condensation reaction of TEOS with SMG and entrapment of in-situ grown nano silica within SMG matrix.63 However, the formation of high cross linked structures renders SMG-PU-TEOS and SMG-PU insoluble in above solvents. 3.3

Gel permeation chromatography (GPC)

The average molecular weight of SMG-PU determined by gel permeation chromatography was found to be 4124.85 g/mol. The value of average molecular weight indicates the formation of three dimensional cross-linked structure due to urethanation reaction of –OH groups of SMG and –NCO group of TDI.47 3.4

Spectroscopic characterization 3.4.1

FT-IR analysis. The FT-IR spectra of SMG, SMG-TEOS, SMG-PU pre-polymer

and SMG-PU-TEOS are shown in Figure 1a - e. The Figure 1a, FT-IR spectra of SO showed the characteristic peaks of SO at 2927 cm-1, 2876 cm-1 (-CH and –CH2 stretching), 1752 cm-1 (carbonyl), 1220 cm-1, 1164 cm-1 and 1094 cm-1 (tri ester linkages). However, after basecatalyzed transesterification reaction, Figure 1b exhibits a broad band ranging from 3554 cm-1 to 3266 cm-1 attributed to the -OH stretching, which is absent in the FT-IR spectra of SO (Figure 1a) as also highlighted in the Figure 1. The presence of -OH groups in the product (SMG) was confirmed by the peak of 3554 cm-1 and the broadening is attributed to the H-bonding between the –OH groups present in SMG.64 Further, the decrease in the peak intensity of carbonyl group at 1752 cm-1 and disappearance of peak at 1260 cm-1 due to tri-ester linkages confirmed the formation of SMG. Compared to SMG, Figure 1c showed that incorporation of TEOS developed additional absorption bands around 454 cm-1 (Si–O–Si bending), 779 cm-1 (Si–O–Si symmetrical stretching) and 1079 cm-1 (Si–O–Si asymmetrical stretching), which confirms the introduction of 17 ACS Paragon Plus Environment


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the siloxane group into the monoglyceride backbone.48, 60 The incorporation/ reaction of -O-SiO- networks with –OH groups of SMG lead to the consumption of –OH groups (scheme 1c) and decrease in peak intensity due to –OH absorption band in the range 3554 cm-1 to 3266 cm-1.60

Figure 1. FT-IR spectra of (a) SO (b) SMG (c) SMG-TEOS OIH sol (d) SMG-PU and (e) SMG-PU-TEOSOIH nanocomposite.


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SMG-PU at pre-polymer state showed a peak at 2269 cm-1 attributed to the presence of –NCO group. The reaction was considered to be completed when the peak generated due to -NCO group get consumed by reacting with –OH groups of SMG. The FT-IR spectrum of SMG-PUTEOS exhibit all the characteristic peaks present in SMG-TEOS in addition to the absorption band at 3302 cm-1, 1580 cm-1 and 1734 cm-1 due to urethane linkages, -NH2 linkage and amide carbonyl group stretching vibrations, respectively. The completion of reaction was also confirmed due to absence of the peak due to –NCO group. Hence, the FT-IR spectra was helpful to monitor the reaction and to confirm the formation of final product. The same has been confirmed via. NMR spectra of the respective material. 3.4.2

1

H NMR and

13

C NMR analysis. The 1H NMR spectrum of SMG (Figure 2a)

shows the chemical shift at δ= 0.8-0.9 ppm, correspond to the terminal methyl group and the chemical shift at δ=1.2− 2.3 ppm is due to the internal -CH2 groups present in soy fatty acid chain. The signal at δ = 5.3 ppm is due to the protons attached to the unsaturated carbon atoms while the peak at δ = 2.7 ppm can be ascribed to the protons flanked between two double bonds.

Figure 2. (a) 1H NMR spectra of SMG. 19 ACS Paragon Plus Environment


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The chemical shift value at δ= 3.5-3.8 ppm is because of the protons attached to oxygen atom (CH2-OH and CH-O). The broad peak at δ= 4.12 ppm can be ascribed to the proton of hydroxyl groups.65 Hence, the above mentioned peak confirmed the transesterification reaction of soy oil with glycerol.

Figure 2. (b) 13C NMR spectra of SMG. Similarly, the Figure 2b shows the 13C NMR spectrum of SMG, where the chemical shift at δ = 174 ppm and at δ = 128-130 ppm corresponded to the carbonyl carbon and unsaturated carbon atom of fatty acid chain respectively. Further, the chemical shift in the region of δ = 14-31 ppm was assigned to the carbon atoms of terminal methyl and internal methylene groups. The chemical shift in the range of δ = 60-73 ppm correspond to carbon attached to oxygen atom directly. The 1H NMR spectrum of SMG-PU (Figure 2c) shows the chemical shifts of the protons in terminal methyl group at δ= 0.85−0.872 ppm and the chemical shift of internal -CH2 groups present in SMG chain at δ=1.23−2.34 ppm. The signal at δ = 5.1 ppm is due to the protons


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attached to the unsaturated carbon atoms. The characteristic chemical shift ascribed to hydrogen (-N-H) of the urethane group, –CH2 group attached with –O-C=O(CH2)7 and methylene protons attached to oxygen linking the urethane functional group (-CH2-O-CO-NH-) were marked in the figure 2c.66

Figure 2. (c) 1H NMR spectra of SMG-PU. Figure 2d shows the 13C NMR spectrum of SMG-PU. The chemical shift at δ = 173 ppm and at δ = 129-130 ppm corresponded to the carbonyl carbon of urethane linkages ((-NH-(C=O)-O-)) and aromatic carbons of TDI. The chemical shift in the region of δ = 72 ppm was assigned to carbon directly attached to an oxygen atom. The chemical shift in the range of δ = 0-35 ppm and at δ = 127 ppm correspond to methyl, methylene and olefinic carbon of fatty acid chains. All the



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chemical shifts in 1H and

13

C NMR spectra of SMG-PU confirmed the formation as well the

structure of SMG-PU.65

Figure 2. (d) 1H NMR spectra of SMG-PU. The 1H NMR spectrum of SMG-PU-TEOS (Figure 2e) shows the chemical shifts of the terminal methyl group at δ= 0.86−0.98 ppm and the chemical shift of internal -CH2 groups present in SMG chain at δ=1.06−1.09 ppm. The signal at δ = 5.29 ppm is due to the protons attached to the unsaturated carbon atoms. The characteristic chemical shift at δ = 5.4 ppm, assigned to hydrogen (-N-H) of the urethane group, at δ = 4.23 ppm can be attributed to –CH2 group attached with –OSi(R)3 and the chemical shift at δ = 4.9 ppm can be ascribed to methylene protons attached to oxygen linking the urethane functional group (-CH2-O-CO-NH-).66 22 ACS Paragon Plus Environment

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Figure 2. (e) 1H NMR spectra of SMG-PU-TEOS. Figure 2f shows the 13C NMR spectrum of SMG-PU-TEOS. The chemical shift at δ = 173 ppm and at δ = 129-132 ppm corresponded to the carbonyl carbon of urethane linkages and aromatic carbons of TDI. Besides, the chemical shift in the region of δ = 30-70 ppm was assigned to the carbon atom directly attached to an oxygen atom. The chemical shift in the range of δ = 0-35 ppm and at δ = 127 ppm correspond to methyl, methylene and olefinic carbon of fatty acid chain. Besides these, the chemical shift at 75.2 ppm (-CH-O-Si-) and at δ =156 ppm (-NH-(C=O)-O-) confirm the formation of hybrid polymer.65



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Figure 2. (f) 13C NMR spectra of SMG-PU-TEOS. 3.5

Morphological studies of the prepared coatings 3.5.1

Optical micrographs. The optical micrograph of SMG-PU and SMG-PU-TEOS

OIH nanocomposites (Figure 3a-d) revealed that the presence of spherical black dots attributed to the presence of Si in SMG matrix, generated due to in-situ silylation with distinct boundaries surrounded by polyurethane matrix. The uniform dispersion of in-situ grown –O-Si-O- networks with the absence of phase separation, cracks and pin hole in SMG-PU-TEOS (TEOS (%) = 0.5, 1.0, and 2.0) with different -O-Si-O- content led to formation of solvent resistant materials with their suitability for coating and corrosion protective application.67-68


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Figure 3. Optical micrographs of coated CS with (a) SMG-PU (b) SMG-PU-TEOS-0.5 (c) SMG-PU-TEOS-1 and (d) SMG-PU-TEOS-2 at 200X resolution. 3.5.2

TEM analysis. TEM images were captured for the morphological study of in-situ

formed inorganic phase i.e., inorganic –Si-O- networks, with a TECNAI 200 operating at 200 kV (Fei, Electron Optics), equipped with digital imaging and a 35 mm photography system, at the All India Institute of Medical Sciences (AIIMS), New Delhi, India. A well diluted solution of the test sample was prepared in ethanol, transferred to a “click-lock” 1.5 ml microcentrifuge tube, and submerged in water, in an ultrasonic bath for sonication for a 30 minute duration. A drop of this sonicated sample solution was then placed on a carbon Type-B (carbon film supported) 200 mesh copper grid with a micro pipette. The grid was allowed to dry well before being used for TEM analysis. The TEM micrograph of SMG-PU-TEOS-2 (Figure 4) showed homogenously dispersed dark black circular –O-Si-O- nanonetworks of ca. 30 nm sizes. The homogenous distribution of –OSi-O- networks in SMG matrix is due to the strong electrostatic interactions between organic and 25 ACS Paragon Plus Environment


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inorganic moieties. Thus, the –O-Si-O- nanonetworks of spherical morphology as observed in the preliminary optical images were confirmed by the TEM micrograph of SMG-PU-TEOS.

Figure 4. TEM micrograph of OIH (SMG-PU-TEOS-2) nanocomposite coating. 3.5.3

SEM-EDX analysis. SEM-EDX studies were performed to analyse the surface

morphology and chemical compositions of SMG-PU and SMG-PU-TEOS-2 coated CS using Environmental Scanning Electron Microscope model Quanta 200 FEI with Oxford-EDS system IE250 X-MAX 80 from SMITA, Indian Institute of Technology, Delhi, India. The samples were cleaned using methanol and completely dried before putting the samples on sample holder of 0.5 inch cylindrical mounts. Then the samples were Gold coated through sputter coaters to make them conductive. The SEM image (Figure 5a-b) revealed dense, well adhered and compact structures. However, in case of SMG-PU-TEOS-2 coating, some white conglomerates were observed on the surface of


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the coatings, which can be attributed to the formation of –O-Si-O- networks through condensation reaction on the surface.69

Figure 5. SEM-EDX micrographs of CS coated with (a-c) SMG-PU and (b-d) SMG-PU-TEOS-2 OIH nanocomposite coatings. To analyse the chemical compositions of both SMG-PU and OIH polymer nanocomposite, EDX spectra was recorded (Figure 5c-d). The EDX spectra of SMG-PU display peak of carbon and oxygen while SMG-PU-TEOS exhibited an additional peak of silicon element at 1.8 KeV. The presence of the silicon peak confirms the presence of –O-Si-O- nanonetworks on the surface of microstructure and indicate the inclusion of TEOS in organic matrix.69 The presence of peak corresponding to the gold may have resulted due to the formation of gold thin film on the sample surface formed during its preparation. 3.6

Thermogravimetric analysis (TGA)

The TGA (Figure 6) analysis of the SMG-PU and SMG-PU-TEOS-2 with a heating rate of 20 oC min-1 upto 1000 oC in nitrogen environment was performed. Overall, thermal decomposition can 27 ACS Paragon Plus Environment


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be divided into four stages. The initial weight loss (approximate 2 %) in SMG-PU and SMG-PUTEOS

Figure 6. (a) TGA and DTG thermograms of SMG-PU. occurred in the temperature range of 75 oC to 157 oC and 85 oC to 189 oC, respectively. These weight losses are associated with the loss of moisture or trapped solvent molecules.36 The second stage decomposition was observed from 157 oC to 273 oC in case of SMG-PU and from 189 oC to 293 oC, in case of SMG-PU-TEOS-2. This weight loss of around 200 oC to 300 oC can be attributed to the degradation of the urethane producing free NCO and alcohol, free amine, carbon dioxide, and olefins.36, 70 The 3rd and 4th stage decomposition was continuous (steep) in SMG-PU (273 oC to 384 oC and 384 oC to 517 oC) while that of in SMG-PU-TEOS-2 (293 oC to 401 oC and 401 oC to 526 oC) was comparatively slow (Figure 6b).


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Figure 6. (b) TGA and DTG thermograms of SMG-PU-TEOS-2 OIH nanocomposite coatings. The stage III weight loss in this temperature range can be attributed to the decomposition of ester linkages and aromatic moieties.36, 70 The fourth stage weight loss of 40.4 % (SMG-PU) and 30 % (SMG-PU-TEOS-2) was attributed to the decomposition of hydrocarbon chain and siloxane components.70 The lower weight loss of SMG-PU-TEOS-2 (30 %) in this temperature range compared to that of SMG-PU (40.4 %) could have resulted due to the incorporation of stable -O-Si-O- networks and formation of OIH nanocomposite. Further, slow rate of degradation above 401 oC in case of SMG-PU-TEOS-2 can be corroborated to higher bond dissociation energy of polymeric -O-Si-O- linkages (185 Kcal/mol) incorporated within the polymer back bone.71 The difference in onset (starting) and offset (ending) temperatures of 29 ACS Paragon Plus Environment


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weight loss, temperature of 10 % weight loss, final residual weight of SMG-PU-TEOS (8.2 g at 526 oC and 4.48 g at 1000 oC) as compared to that of SMG-PU (6.4 g at 517 oC and 4.07 g at 1000 oC) confirmed the higher thermal stability of SMG-PU-TEOS that may be due to the presence of very small amount of –O-Si-O- in SMG-PU-TEOS. The DTG plots further confirm these decomposition temperatures (onset, offset and peak dissociation temperature) of SMG-PU and SMG-PU-TEOS (Figure 6b). 3.7

Physico-mechanical characterization

The values of various physico-mechanical properties obtained by subjecting the SMG-PU and SMG-PU-TEOS OIH coatings to the various physico-mechanical tests using ASTM methods are summarised in Table 3. The decrease in DTT and DTH time with the increased loading of TEOS (0.5 weight % to 2 weight %) in SMG matrix is observed, which can be attributed to the networks forming ability of TEOS, that invariably helps in the curing of SMG-PU-TEOS coatings even at lower concentration of TDI (40 weight %) as compared to that of unmodified SMG, which required 50 weight % of TDI for curing.72 The gloss value of the OIH nanocomposite coatings decreases with the increased loading of TEOS in SMG matrix, which may be due to the nanoscale roughness of the surface produced by the in-situ silylation of TEOS.49 However, due to the formation of better cross linked structure of a SMG-PU-TEOS in comparison to SMG-PU, the gloss value of former was found higher than that of latter.49 The scratch hardness values were found to increase from 4 to 12 kg with the increased loading of TEOS in SMG. High scratch hardness value of OIH nanocomposite coatings can be attributed to the enhanced adhesion between SMG-PU-TEOS and CS metal substrate via. H bondings among the polar groups and –O-Si-O- linkages.21 The improved adhesion between the coated materials with CS


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was further analysed by the cross-hatch adhesion tape test and their optical image were recorded (Figure 7 a-d). In virgin SMG-PU (Figure 7 a), after the cross hatch adhesion tape test, the tested surface showed the formation of some ridges because of low adhesion between the coating material and CS. However, the SMG-PU-TEOS hybrid coatings showed the formation of perfect cross straight lines, without any ridges along the side of scratches. Table 3 Physico-mechanical properties of SMG-PU, SMG-PU-TEOS-0.5, SMG-PU-TEOS-1 and SMG-PU-TEOS-2 OIH nanocomposite coatings. Resin code

Drying Scratch Impact Gloss time (h) Hardness Resistance (at DTT DTH (kg) (Kg/cm2) 45˚)

SMG-PU SMG-PU-TEOS-0.5 SMG-PU-TEOS-1 SMG-PU-TEOS-2

0.35 0.30 0.20 0.18

30 45 40 35

4.0 9.5 11.0 12.0

Pass Pass Pass Pass

68 71 70 69

Figure 7. Optical micrographs of cross hatched CS coated with (a) SMG-PU (b) SMG-PU-TEOS-0.5 (c) SMG-PU-TEOS-1 and (d) SMG-PU-TEOS-2 OIH nanocomposites at 100X resolution.


Flexibility (1/8``) (Conical mandrel) Pass Pass Pass Pass


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These results revealed that the addition of TEOS improved the scratch resistance and restricted the indentation by increasing the physical interactions between OIH nanocomposite material and CS surface.73-75 The SMG and all the other OIH`s passed the impact resistance test (26.8 kg/cm).The presence of nano –O-Si-O- inside the SMG produces sealing/healing ability, restricts the chain mobility in coatings and show the significantly improved impact resistance (toughness) in SMG-PU-TEOS coatings as compared to SMG-PU coatings.76 The flexibility of coatings was determined with the help of conical Mandrel bend test. All the coatings passed the bend test, which can be attributed to the presence of flexible moieties (aliphatic fatty acid chain, –O-Si-O-) in SMG-PU and SMG-PU-TEOS. Among all the OIH coatings, SMG-PU-TEOS with 2 weight % loading of TEOS showed the best physico-mechanical performance. 3.8

Surface wettability test (Contact angle study)

The CCD camera images (Figure 8 a–d) of water droplets on the surface of the SMG-PU and SMG-PU-TEOS coatings.

Figure 8. Contact angle images of CS coated with (a) SMG-PU (b) SMG-PU-TEOS-0.5 (c) SMG-PU-TEOS-1 and (d) SMG-PU-TEOS-2 OIH nanocomposite coatings. 32 ACS Paragon Plus Environment

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The hydrophobicity is an important parameter, which induces the high reflective ability to coating surfaces for corrosive moieties in the form of ions and molecules and also act as a good anticorrosive agent.49 The contact angle values were found to increase from 70o in SMG-PU to 98o in SMG-PU-TEOS-2, suggesting an increase in the hydrophobic character of SMG-PUTEOS-2 coatings. The increase in hydrophobic character with the increase in TEOS content can be attributed to the formation of dense networked structure with -O-Si-O- linkages and the increase in surface the roughness at nanoscale.77 The hydrophobic structure acts as an efficient barrier layer to impede the diffusion of electrolytes and induced the good corrosion resistance properties in these hybrid nanocomposite coatings. 3.9

Potentiodynamic polarization (PDP) studies

The series of electrochemical corrosion tests on coated and bare CS were conducted in 3.5 weight % aqueous NaCl and 3.5 weight % HCl solution to analyse the anticorrosive performance of bare CS, SMG-PU and SMG-PU-TEOS OIH composites coated CS (Figure 9a and 9b). In saline media, a very high decrease in the Icorr value from bare CS (8.9131×10-5 Acm-2) to SMGPU-TEOS (3.8891×10-10 Acm-2) followed by a remarkable increase in Ecorr values from bare CS (-0.61385 Vcm-2) to SMG-PU-TEOS (+0.01036 Vcm-2) was observed (Table 4a). A similar trend of decrease in Icorr and increase Ecorr values of SMG-PU-TEOS OIH coatings in comparison to CS and SMG-PU were also observed in acidic media (Figure 9b and Table 4b). The higher values of Icorr in acidic media than in saline media, revealed the higher corrosion protective ability of SMG-PU and SMG-PU-TEOS OIH coatings in later (3.5 weight % NaCl solution). However, the literature (Table 1) revealed the superior protecting ability of these coatings even in 3.5 weight % HCl solution.



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Figure 9. PDP curves of (a) CS (b) SMG-PU (c) SMG-PU-TEOS-0.5 (d) SMG-PU- TEOS-1 and (e) SMG-PU-TEOS-2 OIH nanocomposite coatings in 3.5 weight % NaCl medium. These clearly indicate that the corrosion protective ability of OIH nanocomposite coatings is significantly higher than that of SMG-PU coatings, which can be attributed to the blocking effect and the strong adhesion of coatings with the substrate surface. Table 4a. Electrochemical parameters obtained from PDP and EIS studies for uncoated, SMG-PU and SMG-Si-PU coated CS under saline media at room temperature. Code

Ecorr (Vcm-2) Icorr (Acm-2)

CS SMG-PU SMG-PU-TEOS-0.5 SMG-PU-TEOS -1 SMG-PU-TEOS -2

-0.61385 -0.32424 -0.21424 -0.10889 0.01036

8.9131×10-5 1.8159×10-8 9.8159×10-9 7.9012×10-10 3.8891×10-10

Rp (Ω) 2.44×10+2 1.20×10+6 2.21×10+6 2.75×10+7 5.58×10+7


Corrosion Rate (mpy) 1.039805 0.000212 0.000115 9.22×10-6 4.53×10-6

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The protective ability of these coatings was found to gradually increase with the increased loading of TEOS content from 0.5 weight % to 2 weight % in comparison to bare CS and SMGPU coatings.77

Figure 9b. PDP curves of (a) CS (b) SMG-PU (c) SMG-PU-TEOS-0.5 (d) SMG-PU-TEOS-1 and (e) SMG-PU-TEOS-2. The superior corrosion protective performance of OIH nanocomposite coatings in comparison to bare CS and SMG-PU can be attributed to the formation of optimum cross linked hydrophobic nanostructured surface morphology of the coating (-O-Si-O-), which repels the water and aqueous corrosive ions.77



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Table 4b. Electrochemical parameters obtained from PDP and EIS studies for uncoated, SMG-PU and SMG-Si-PU coated CS under acidic media at room temperature. Code

Ecorr (Vcm-2) Icorr (Acm-2)

CS SMG-PU SMG-PU-TEOS-0.5 SMG-PU-TEOS -1 SMG-PU-TEOS -2

-0.43688 -0.36762 -0.27132 -0.21165 -0.13494

8.1731×10-4 9.1396×10-7 3.8162×10-7 2.6413×10-8 6.8756×10-9

Rp (Ω) 2.66 2.38×103 5.69×103 8.22×104 3.16×105

Corrosion Rate (mpy) 9.534766 0.004451 0.010662 3.0813×10-4 8.0211×10-5

Further, the presence of hydrolytically stable urethane bonds also enhances the adhesion between coating-metal interfaces, which is also confirmed by the remarkably higher the corrosion potential values (Ecorr). The PDP studies revealed that the corrosion inhibition rates of these systems follow the below given trend: Bare CS

High-Performance Soya Polyurethane Networked ... - ACS Publications

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