Cashew nut shell liquid terminated self-healable polyurethane as an effective anticorrosive coating with biodegradable attribute

Progress in Organic Coatings 139 (2020) 105472

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Cashew nut shell liquid terminated self-healable polyurethane as an effective anticorrosive coating with biodegradable attribute

T

Tuhin Ghosh, Niranjan Karak* Advanced Polymer and Nanomaterial Laboratory, Department of Chemical Sciences, Tezpur University, Tezpur, Assam, 784028, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Bio-based resources Hydrophobic Self-healing Anticorrosive Biodegradable

Formulation of smart polyurethane (PU) using renewable resources as the raw materials with desired properties is a troublesome task. Thus, in this context, different compositions containing PUs were synthesized using few bio-derived raw materials like cashew nut shell liquid, modified polyol of dimer acid-glycerol and sunflower oil based monoglyceride along with other components like bis(hydroxyalkyl)poly(dimethylsiloxane), poly(ε-caprolactone)diol, 2,4/2,6-toluene diisocyanate and isophorone diisocyanate. The structure, morphology and surface characteristics of the synthesized PUs were well-characterized using Fourier transform infrared spectroscopy, 1H and 13C nuclear magnetic resonance spectroscopy, scanning electron microscopy and X-ray diffraction technique. The synthesized PUs showed adequate mechanical and thermal properties along with good surface hydrophobicity (static contact angle: 112.3–121.2°), excellent microwave responsive self-healing (within 72−89 s) and biodegradable attributes along with good corrosion resistance property (corrosion rate: 8.76 × 10−5 mm/y). Most interestingly, this PU also sustains its anti-corrosive nature even after healing from mechanical damage and reduces the corrosion rate of a corroded mild steel plate 300 times more than the bare plate. Therefore, the studied bio-based PU has huge potential and paves a new direction in the field of smart anticorrosive material.

1. Introduction Polyurethane (PU) is an important class of polymers containing thermodynamically incompatible segments of hard and soft domains in a single polymeric matrix and can be synthesized by a facile one-pot technique using readily available starting materials [1,2]. Versatility in feedstock’s and tunable properties as per the need, make this polymer advantageous over the other commonly used synthetic polymers. But the major concern associated with PU synthesis is excessive use of petroleum-based feedstocks as the starting materials (diisocyanates, polyols and chain extenders) which cause various hazardous impact on the environment and health issues [3]. Therefore, the rapid depletion of the fossil fuels and their gradual increment in price triggered the use of bio-renewable resources for the synthesis of PU [4]. Among various bioderived raw materials, vegetable oils (sunflower, peanut, castor, linseed, corn, soybean oil etc.) received huge attentions for the synthesis of PU due to their unique structural architecture, reasonable price and ease of necessary chemical modification [5–7]. In recent years, the pericap fluid extracted from cashew nut, known as cashew nut shell liquid (CNSL) is successfully used as a building block for PU [6]. Generally, CNSL is a waste product of the cashew food

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industry containing a number of reactive phenolic derivatives (cardanol, cardol, 2-methyl cardol and anacardic acid) with highly hydrophobic aliphatic chains [8–10]. Prior art literature advocates that the CNSL is mainly used to obtain resol and novolac which are routinely used as a thermoset resins for the fabrication of biocomposites [11]. However, CNSL is also used for the development of sustainable PU with desired properties. For example, Liu et al. developed the synthesis of UV-curable coating containing multiarmed cardanol (one of the components of CNSL) based acrylates [12]. Wang and Zhou reported the synthesis of mechanically tough PU coating using cardanol based polyols [13]. Kathalewar reported the synthesis of isocyanates free PU coating using diglycidyl ether of cardanol [14]. Eventhough, they used greener method but very low mechanical performance of PUs limited their applications. Suresh and Kishanprasad developed different kinds of di/triol which further reacted with methylene diphenyl diisocyanate (MDI) for the development of PU [15]. Suresh reported the synthesis of rigid PU foam using cardanol modified polyol [16]. Though all these reports depict successful synthesis but their complicated modification process of cardanol limit their uses. Moreover, most of the reports were found to be silent about their mechanical properties which ultimately determined their practical applications as an active surface coating

Corresponding author. E-mail address: [email protected] (N. Karak).

https://doi.org/10.1016/j.porgcoat.2019.105472 Received 31 July 2019; Received in revised form 23 November 2019; Accepted 30 November 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.

Progress in Organic Coatings 139 (2020) 105472

T. Ghosh and N. Karak

controlled temperature oil bath and subsequently required amount of TDI (0.27 g, 1.6 mM) was added under continuous stirring condition (300 rpm). Then the temperature was raised to 85 ± 5 °C and continued for 4.5 h to obtain desired viscosity. Resulted viscous mass was cooled to room temperature and treated as the prepolymer of PU. In the next step, MG (0.407 g, 1.15 mM) and required amount of IPDI (0.44 g, 2 mM) were added into the prepolymer followed by the addition of 4.4 mL xylene to maintain ∼50% solid content. Again, the temperature of the reaction was further increased to 110 ± 5 °C and continued for 3 h under continuous stirring. Then predetermined amount of CNSL (0.69 g, 2.3 mM) was added to the reaction mixture under vigorous stirring (450 rpm) and continued for 1.5 h to obtain PU-CNSL-1. The completion of the reaction was supported by the butylamine test and solubility test, indicating no gel formation. The obtained product was cast on Teflon sheets and the residual solvent was allowed to evaporate under ambient condition to obtain a dry film for the evaluation of different properties. The precipitated PU product was used for other (NMR, viscosity etc.) analyses. Simply following the above described procedure, other two PUs were synthesized using 1.045 g and 1.393 g of CNSL, separately, keeping all of the components as they were and encoded as PU-CNSL-2 and PU-CNSL-3. The exact compositions of the synthesized PUs are provided Table S1 of supporting information (SI).

[17–19]. Furthermore, corrosion of the metal substrate is the biggest challenge and economic burden encountered by both government and private industries [17]. Billions of dollars were spent every year to protect these metal-based articles. As per the report, India losses 4–5% and USA losses 6.2% of total GDP due to corrosion [20]. Therefore, the development of anticorrosive polymeric coating is very much necessary to prevent such huge loss and to extend the service life of metallic products. From, the very beginning, PU is widely used as a surface coating material due to its inborn versatility and favorable properties. These properties can be tailored easily by manipulation of the structures and compositions. Further, properties like surface hydrophobicity and selfhealing properties can also be achieved by proper design of them which can be added as extra advantages in the field of the anticorrosive coatings by prolonging their service life [21]. Therefore, the aim of the present work is to design a mechanically tough PU using some bio-derived raw materials like modified polyol of dimer acid-glycerol (DAGP, as branch generating unit), monoglyceride of sunflower oil (MG, as a chain extender) and CNSL (as chain terminating agent). Moreover, to reduce the surface energy and to provide sufficient rigidity, crystallinity, good flexibility in the PU matrix bis (hydroxyalkyl)poly(dimethylsiloxane) (BHPDMS) and poly(ε-caprolactone)diol (PCL) were also used as the building blocks. As per our knowledge, this is the first report in which both sunflower oil and CNSL were used to develop self-healing, anticorrosive PU coating. The structural, physical, morphological and thermal properties of the synthesized PUs were investigated using Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopies, gel permeation chromatography (GPC), scanning electron microscopy (SEM), thermogravimetric analyzer (TGA) and differential scanning calorimetry (DSC). The mechanical properties and special properties like surface hydrophobicity and self-healing were also evaluated. The anticorrosive performance of the coatings was also determined to emphasize the usefulness of the materials. In addition, the biodegradation of the synthesized PUs was also studied to highlight their potential as an effective environmental friendly surface coating materials.

2.3. Instrumentation and testing methods 2.3.1. Determination of physical properties The specific gravity of the dry PU-CNSLs was determined using by the standard ASTM (D7932) method. The viscosity (specific) of the as prepared PUs was evaluated using Ubbelohde viscometer as per the ASTM D4020 method using Tetrahydrofuran (THF) as a solvent. The number average molecular weight (Mn) and weight average molecular weight (Mw) and polydispersity index (PDI) of the PU-CNSLs were evaluated using gel permeation chromatography (GPC, Waters, UK) equipped with a refractive index (2414) and 515 HPLC pump. The eluent (THF) used at flow rate of 0.75 mL/min and linear polystyrene was as standard for the analysis.

2. Experimental section

2.3.2. Structural characterization The FT-IR spectra of the synthesized PU-CNSLs were recorded using Perkin Elmer FT-IR spectrometer (Frontier MIR-FIR, USA) in attenuated total reflection (ATR) mode over the range of 4000−400 cm−1. Presence of different types of proton and carbon was verified using a 400 MHz Nuclear Magnetic Resonance (NMR) spectrophotometer (JEOL, Japan). Deuterated DMSO was used as the solvent and trimethyl silane (TMS) was used as the internal standard to record the NMR spectra. Further, the degree of crystallinity of the samples was scrutinized using a Bruker D8 FOCUS XRD machine (AXS, Germany) fitted with Cu-Kα radiation source over the range of 2θ = 5 to 70°. The dspacing value of the synthesized PUs was evaluated using Bragg’s equation:

2.1. Materials DA (average molecular weight (Mn) ∼570 g/mol), PCL (Mn∼2000 g/mol), sunflower oil and isophorone diisocyanate (IPDI) were procured from Sigma-Aldrich, USA. Glycerol and calcium oxide (CaO) were purchased from Merck, India. BHPDMS (Mn∼5600 g/mol) and 2,4/2,6-toluene diisocyanate (TDI) were obtained from SigmaAldrich, Japan and Merck, Germany, respectively. DA and sunflower oil were dried in a vacuum oven at 60 °C for 12 h before use. Other chemicals were used as received. para-Toluene sulfonic acid (p-TSA) was obtained from SRL, Mumbai, India. Xylene was procured from Merck (India) and used after normal distillation. CNSL (hydroxyl value: 374.5 mg KOH/gm) was provided by Asian Paints, India and used after vacuum drying. It is also necessary to mention that the CNSL was obtained by the solvent extraction technique by the manufacturer. DAGP and MG were prepared as reported in our earlier report [2].

d = nλ/2sinθ

(1)

where, λ = wavelength of the used X-ray, θ = diffraction angle, d = distance between lattice plane from which the X-ray diffracted and n = positive integer (1).

2.2. Synthesis of PU containing CNSL (PU-CNSL) 2.3.3. Determination of mechanical properties Tensile properties (tensile strength, elongation at break and toughness) of the PU-CNSL strips with a dimension of 0.5 × 10 × 50 mm3 were measured using a WDW-10 Universal tensile machine (UTM, Jinan, China) equipped with 0.5 kN load cell. Tensile properties of all the samples were analyzed at room temperature with a crosshead speed of 50 mm/min. Scratch hardness of the dried strips under dynamic and static condition was evaluated using a scratch hardness tester (Sheen Instrument Ltd, UK) and Shore A hardness tester

Predetermined amounts of PCL (2 g, 2 mM), BHPDMS (0.56 g, 0.2 mM) and DAGP (0.075 g, 0.1 mM) were placed in a three-neck glass reactor (100 mL) fitted with a mechanical stirrer. The other two necks were tightly sealed with a Teflon septum and a nitrogen inlet. Afterwards calculated amount (1 mL) of xylene was added to the reaction mixture using a syringe to maintain the solid content of the reaction mixture near about 75%. Then the temperature of the reaction mixture was gradually increased to 50 °C to melt the PCL using a 2

Progress in Organic Coatings 139 (2020) 105472

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Scheme 1. Plausible synthesis route of PU-CNSL.

2.3.6. Self-healing test Self-healing ability of the synthesized PU-CNSLs against mechanical damage was investigated simply by cutting the strips in a transverse direction using a razor blade and exposed under microwave radiation (500 W). The healing efficiency was assessed by measuring the ratio of the tensile strengths of the damaged strip after healing to before damage as given below.

(Shenn, China) respectively. Further, the falling weight method was used to access the impact resistance property of the synthesized PUCNSLs. A weight of 0.85 kg attached with impact tester (S.C. Dey & Co, Kolkata) was allowed to fall on the coated steel plate from different heights (limit:1 m) and the impact resistance value as energy per unit thickness was reported as a maximum height up to which the film was not damaged. All the aforementioned experiments were carried in triplicate and the average results were reported.

Healing efficiency (%) = [Tensile strength (healed film)/ Tensile strength (initial)] × 100 (2)

2.3.4. Determination of thermal properties Thermal stability of the synthesized PU-CNSLs was investigated thermogravimetrically using a Perkin Elmer thermogravimetric analyzer (TGA-4000, USA) over the range of 32–720 °C at a heating rate 10 °C/min under ultra pure N2 atmosphere (flow rate:30 mL/min). Different thermal transition temperatures of the PU-CNSLs were scrutinized using Differential scanning calorimetry (DSC). Perkin Elmer DSC-6000 (USA) instrument was used over the temperature range of −70 °C to 120 °C under ultra pure N2 atmosphere by following a heating (heating rate: 5 °C/min), cooling (cooling rate: 10 °C/min) and heating (heating rate: 5 °C/min) cycle.

2.3.7. Corrosion resistance test Electrochemical corrosion resistance properties were determined to understand the corrosion resistance ability of the coatings, using an electrochemical work station (Autolab PGSTAT302 N, Metrohm, UK) attached with three electrode system, namely platinum (Pt) wire as the counter electrode, coated mild steel (MS) plate as the working electrode and a saturated calomel electrode (silver electrode) as the reference electrode. During the study coated MS plate of surface area: 1 cm2 was used as a working electrode and 3.5% NaCl solution was used as a corrosion medium. All the above mentioned experiments were carried in triplicate and the average results were reported. Effective healing process of the coating was also checked by simply making a cross mark on the coated plate by a razor blade which was subsequently healed under MW. Afterwards, potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS)

2.3.5. Surface hydrophobicity test Surface hydrophobicity was measured as the static contact angle (SCA) of the water droplets which were placed on the surface of the dried PU-CNSL strips. For each case, water droplets were placed in five different places of the strips and the average result was considered. 3

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compositions containing PUs were synthesized with a variable amount of CNSL, maintaining the overall NCO/OH ratio ∼1, to obtain the best composition. 3.2. Physical properties Different physical properties (color, solubility, specific gravity, solution viscosity and molecular weight) were evaluated as per the standard methods and corresponding values are summarized in Table S2 of SI. All the synthesized PU strips appeared as reddish brown in color. This color might be due to the presence of intense reddish brown color of the CNSL present in the structure of the PUs [23]. Most interestingly, it is noted that the synthesized PUs were soluble in most of the common organic solvents like dimethylformamide, dimethylsulfoxide, tetrahydrofuran, dimethylacetamide, acetone etc. even in xylene, chloroform, dichloromethane, etc. but totally insoluble in hexane, methanol, ethanol etc. A large number of polar functional groups (ester, urethane, hydroxyl etc.), long aliphatic chains and three-dimensional globular architecture of the synthesized PUs are responsible for such good solubility in the aforementioned solvents [24]. Further, the PUs also exhibited specific gravity in the range of 1.43-1.27 and decreases with the increase of CNSL content. Mainly, the structure of CNSL contents long aliphatic chains, thus with the increase of CNSL content the free volume of the PU matrix increases (also supported by mechanical property as well as XRD and DSC studies). As a result, synthesized PUs followed a decreasing trend in specific gravity. Further, the solution viscosity for the synthesized PUs was found in the range of 0.287 to 0.265. The viscosity of the synthesized PUs strongly depends on the molecular weight. Higher the Mw (Table S2), higher is the solution viscosity for the polymer with the same structure. Thus PU-CNSL-1 with the highest value of Mw showed higher solution viscosity [2,25].

Fig. 1. ATR-FTIR spectra of PU-CNSLs.

measurements were carried out to further support the healing process. 2.3.8. Biodegradation test Gram negative (Pseudomonus aeruginosa) and Gram positive (Bacillus subtilis) bacterial strains were used to study the biodegradation characteristic of the synthesized PU-CNSL strips. At first a medium containing 25 g Luria Bertani Broth (Miller, Himedia) in 1 L demineralized water was prepared. The obtained medium was sterilized under a pressure of 103.5 kPa for 15 min at 120 °C in an autoclave and then allowed to cool under room temperature. Using the above sterilized medium, bacterial strains were cultured separately inside an incubator maintaining constant temperature (37 °C) for 48 h. Then 100 μL (containing approximately 108 microbes/mL) cultured bacterial strain was added separately in each culture tubes containing 10 mL of the above sterilized Luria Bertani broth and sterilized films of the synthesized PUCNSL. Afterwards all the culture tubes were left for incubation for a specified period of time (5 weeks) and the degradation of PU films was monitored by measuring the optical density (OD) of the medium every week. All the samples were examined in triplicate and the average results were reported. Further, the surface images of the films before and after degradation were also taken using SEM (JEOL JSM-6390LV).

3.3. Structural characterization 3.3.1. FTIR analysis The expected structure of the synthesized PUs was supported by ATR-FTIR spectra as shown in Fig. 1. The band at 2270 cm−1 for free isocyanate groups was not observed, which clearly indicates the completion of the reaction. The band appeared in the range of 3200−3500 cm−1 is due to H-bonded NeH stretching. Further, significant bands (cm−1) at 1725 for combination of both carbonyl (C]O) and amide I stretching, 1530 for -N-C stretching and -N-H bending (amide II), 1243-1200 for in-plane bending vibration of NeH (amide III) bond, and 1185-1160 for CeN stretching, -O-C stretching were also found in the spectra. Presence of the abovementioned bands correspond to the formation of urethane (-NH-(C]O)-O-) linkage in the synthesized PUs [3,8,26]. Few other bands centered at 2933-2863, 1600 and 1470, 1259, 1094 and 1018 cm−1 assigned to the asymmetric and symmetric stretching of methylenic C–H groups, C]C stretching of aromatic ring present in TDI and CNSL, -CH3 bending vibration of BHPDMS, asymmetric and symmetric stretching of Si-O-Si bond, respectively. Moreover, the bands at 870 and 800−690 cm-1 represent the bending vibration C-H group and stretching vibration Si-C bond present n the structure of BHPDMS [25].

3. Results and discussion 3.1. Synthesis of PU containing CNSL (PU-CNSL) As displayed in Scheme 1, PU-CNSLs were prepared using two-step one pot pre-polymerization technique [2]. The formation of isocyanates-terminated pre-polymer through the rearrangement reaction of diisocyanates (TDI) in the presence of reactive diols (PCL, BHPDMS) and branching units (DAGP) was observed in the initial step. The excess isocyanate is necessary to gain isocyanate terminated pre-polymer (supported by FTIR spectra as reported in our earlier report) in the first step [22]. Further, the increasing viscosity of the reaction medium with time also support the formation of polymeric product termed as prepolymer. For further chain extension, predetermined amounts of MG and IPDI were added to the reaction medium in the second step. It is also relevant to mention that a less reactive diisocyanate (IPDI) was used in the second step of the reaction to avoid gel formation. Further, 50% solid content of the reaction mixture in the second step also minimizes the gelation tendency of the reaction. Moreover, to maintain the overall -NCO/-OH ratio ∼1, stoichiometric amount of CNSL containing reactive phenolic –OH group was added to the reaction medium to obtain CNSL terminated PU chains. Competition of the reaction was confirmed by measuring the amount of free isocyante using butylamine test and absence of characteristic –NCO peak in FTIR spectra. Different

3.3.2. NMR analysis Generally, the prediction of the structure of the synthesized PU with very high molecular weight is a very troublesome task. Because higher molecular weight generates intricate coiled architecture, as a result, a slight change was observed in the chemical environment of a similar kind of protons/carbons. From 1H NMR (Fig. 2a) study it was found that the chemical shift values at δ = 0.12 (a), 0.89 (b), 0.92 (c), 0.94 (d), 1.05 (e) and 2.16 (f) ppm correspond to the terminal methyl (-CH3) protons present in the structure of BHPDMS, DAGP, MG, IPDI and TDI moieties present in the structure of the synthesized PU. Further, the chemical shift values at 4

Progress in Organic Coatings 139 (2020) 105472

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Fig. 2. (a) 1H and (b)

13

C NMR spectra of PU-CNSL-1.

δ = 1.21 (g) and 1.32 (h) assigned to the methylenic protons of long aliphatic chains of MG, DAGP and PCL. Further the chemical shift values at δ = 2.03 (i), 1.58 (j) and 2.32 (k), 2.57 (l), 3.66 (m), 4.16 (n) and 4.03 (o) ppm assigned to the α-methylenic protons with respect to double bond, β- and α-methylenic protons with respect to ester carbonyl, methylenic protons between two double bond, methine (-CH) protons of IPDI and the two different methylenic protons of glycerol unit, α- and β-methylenic protons with respect to ester oxygen respectively were observed. In addition the signals at δ = 5.02–5.09 (p) ppm assigned to the methine protons of glycerol unit. Further, the protons attached with aliphatic (present in MG, DAGP, CNSL) and cycloaliphatic (present in DAGP) double bond carbons found at δ = 5.38 (q) and 6.62 (r) ppm. Aromatic protons of CNSL and TDI moiety appeared in the range between δ = 7.12–7.65 (s) ppm [15,27]. The remaining most two deshielded protons found at δ = 8.78 (t) and 9.25 (u) ppm, attributed to the urethane protons (-N-H) attached with IPDI and TDI [2].

Similarly, 13C NMR spectral analysis (Fig. 2b) revealed the presence of different types of carbon as predicted in the proposed structure. The chemical shift values at δ = 14.02 (a), 14.34 (b), 16.02 (c), 19.81 (d), 22.44 (e), 29.24 (f) and 31.48 (g) ppm correspond to methyl carbon of BHPDMS, DAGP, MG, CNSL, TDI and IPDI moieties, respectively [25]. Further, the chemical shift values at δ = 21.26 (h), 24.61 (i) and 25.5 (j), 28.3 (k), 28.61 (l), 33.87 (m), 27.18 (n), 63.51 (o), 63.97 (p) and 68.80 (q) ppm ascribed to the methylenic carbon near terminal methyl group, β and γ methylene protons with respect to the carbonyl carbon, δ methylenic carbon with respect to the carbonyl carbon, α methylenic carbon with respect to double bond, α methylenic carbon with respect to carbonyl carbon, methylenic carbon between two double bond, methylene and methine carbons of glycerol unit respectively. Further, the chemical shift for the aromatic ring carbon (unsubstituted), double bond carbon of MG and CNSL, double bond carbon of cyloaliphatic ring of DAGP, substituted aromatic carbon attached to -NHCOO- group, substituted aromatic carbon attached to -O(C]O)NH- group, ester 5

Progress in Organic Coatings 139 (2020) 105472

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Fig. 3. (a) XRD patterns (b) stress-strain curves (c) TGA curves and DTG curves (inset) and (d) DSC curves of PUs.

carbonyl carbon were observed at δ = 113.13–125.95 (r), 128.58–130.19 (s), 134.5–137.6 (t), 144.2 (u), 157.7 (v) and 173.02 (w) ppm [15]. However, some of the peaks (r, v) are displayed in maximized form due to their very low intensity in the NMR spectrum.

matrix. As a consequence of high free volume, sequential arrangement of PCL segments disturbed which results in poor crystallinity. Further, the broad peak appeared in the range of 12.3° is due to amorphous BHPDMS moiety. Presence of peaks for both PCL and BHPDMS segments clearly depict the presence of microphase separation in the synthesized PUs [30].

3.3.3. XRD analysis From the XRD analysis (Fig. 3a) it is clearly evident that all the synthesized PUs showed a sharp peak at 2θ = 21.4° (d spacing of 4.14 Å) and 23.5° (d spacing of 3.78 Å). The appearance of such peaks corresponds to the (110) and (200) planes of the orthorhombic unit cell of PCL moiety [28,29]. Further, the degree of crystallinity of the synthesized PUs was calculated by taking the area ratio under the crystalline peak to the overall area under the curve. From the tabulated results (Table 1) it is clear that overall crystallinity decreases sharply (also supported by decreasing intensity pattern) with the increase of CNSL content, even after using a similar amount of PCL (2 mM) for the synthesis of above-mentioned PUs. The presence of long aliphatic chains of CNSL eventually increases the overall free volume of the

3.4. Mechanical properties Generally, mechanically properties of synthesized PUs were governed by several factors like molecular weight, crystallinity, chain entanglement, physicochemical crosslinking, primary and secondary interactions etc. [1]. Different mechanical properties like tensile properties, scratch resistant under dynamic and static condition, impact resistant were evaluated and the results are furnished in Table 1. From the stress-strain curves (Fig. 3b), it is evident that the synthesized PUs exhibited moderate tensile strength (5.74-2.7 MPa) which is attributed to the high molecular weight (∼104 order) and crystallinity of PUs. Higher the molecular weight, higher is the possibility of physicochemical interaction between the polar functional groups (ester, urethane, carboxylic etc.) of the structure of PUs which provide sufficient rigidity to the synthesized PU [2]. Moreover, the overall crystallinity also plays a crucial role and provides an adequate amount of intrinsic rigidity to PUs. Due to this reason, PU-CNSL-1 with a higher degree of crystallinity (confirmed from XRD results) showed comparatively better results than the other two PUs (PU-CNSL-2 and PU-CNSL-3). Moreover, the presence of long aliphatic chains and an extensive number of secondary interactions including H-bonding, π-π interaction etc. in the structure of the synthesized PUs also provide good flexibility without

Table 1 Crystalline and mechanical properties of PU-CNSLs. Properties

PU-CNSL-1

PU-CNSL-2

PU-CNSL-3

Crystallinity (%) Tensile strength (MPa) Elongation at break (%) Toughness (MJ/m3) Scratch hardness (kg) Shore A hardness Impact resistance (kJ/m)

14.93 5.74 ± 0.4 296 ± 20 15.50 ± 0.5 3.2 ± 0.1 48 ± 2 15.3 ± 0.2

12.37 3.56 ± 0.3 238 ± 12 7.59 ± 0.2 2.2 ± 0.1 42 ± 1 12.9 ± 0.5

7.58 2.7 ± 0.3 211 ± 10 4.6 ± 0.2 1.6 ± 0.1 34 ± 4 12.5 ± 0.4

6

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temperature for melting. Moreover, Tm of PUs is also governed by the crystallinity. High degree of crystallinity leads to compact ordered architecture [29] as a consequence, higher temperature is needed to melt the ordered structure. From the DSC study, it is also evident that ΔHc value (Table 2) for PU-CNSL-1 was slightly higher than the other two PUs. Higher the value of ΔHc, higher is the crystallinity. As per the literature PCL is considered as 100% crystalline with a ΔHc value 136.08 J/g (32.4 cal/g) [32]. Thus PU-CNSL-1 with a higher degree of crystallinity possesses higher Tm compared to the other two PUs. However, the obtained crystallinity from ΔHc value is almost similar to the crystallinity reported by XRD patterns.

any deformation. However, the obtained tensile properties are much higher than other reported polymers containing CNSL [12,24]. Synthesized PUs also exhibited adequate toughness which ascribed to the combined effect of moderate tensile strength and flexibility. From the table, it is also evident that the tensile strength, elongation at break and toughness of PUs decrease with the increase of CNSL content from PUCNSL-1 to PU-CNSL-3. Intrinsic plasticization effect of long aliphatic chain of CNSL is responsible for such decrement in tensile properties. Similarly, hardness under the dynamic condition and static condition also follows the same trend which clearly indicates that the toughness of the PUs decreases simultaneously with the increasing amount of CNSL. Moreover, the synthesized PUs showed good impact resistance values. In fact, this property strongly depends on the load transferring ability in between the polymeric chains mainly from the soft segment to the hard segment and the rigidity of the PU matrix [31]. During the time of applied impact, flexible units absorb the energy and quickly dissipate it by transferring to the hard segments (aromatic moiety present in TDI and CNSL), present in the same polymeric backbone. These hard segments absorb the energy and help to maintain the structural integrity after impact without any failure. Thus PU-CNSL-1 showed more impact resistance ability as it possesses more compact and rigid structure compared to the other two PUs.

3.6. Surface hydrophobicity study The hydrophobic nature of the synthesized PU-CNSL strips was confirmed by measuring the water contact angle under static condition (inset of Fig. 4a, b and c) and corresponding values are reported in Table 2. Generally, the surface with SCA greater than 90° considered as a nonwetting surface [33]. Hence from the reported results, it is clear that all the PUs exhibited nonwetting phenomena with SCA in the range of 112.3° to 121.2°. Such kind of nonwetting characteristic is attributed to the low surface energy of BHPDMS and nonpolar nature of long aliphatic hydrocarbon chains of MG and CNSL which present as a structural component in the PUs [34]. Moreover, the presence of crystalline PCL and BHPDMS in the same polymeric backbone generates structural inhomogeneity which results in papillary hill-like structures (Fig. 4a, b and c) in micro/nanoscale on the surface [35–37]. As a result, significant amount of air is entrapped in between these micro/ nanoscale structures which act as air cushion beneath the water droplet and increase the SCA by minimizing liquid-solid interface [38–40]. From the tabulated results, it is noted that SCA of PUs increases surprisingly with the increase of the amount of CNSL. This increasing trend of SCA is accredited to the presence of long aliphatic chains of CNSL which increases with the increase of CNSL content from PU-CNSL-1 to PU-CNSL-3.

3.5. Thermal properties Thermostability of the synthesized PUs was assessed using TGA thermograms and corresponding first derivative (DTG) curves as shown in the Fig. 3c. DTG curves revealed that the synthesized PUs underwent a multi-step degradation pattern and the degradation temperatures are furnished in Table 2. The initial degradation (Ton) was found in the range of 251−269 °C which corresponds to the degradation of long aliphatic chains present in the structure of MG, CNSL and DAGP [12,25]. The second step degradation peak temperature was found in the range of 358−364 °C which is related to the chain scission of urethane, ester linkages and degradation of cycloaliphatic rings of DAGP [24]. The third step which was also considered as the step for maximum rate of degradation (TMAX), occurred in the range of 405−413 °C. Such degradation corresponds to the degradation of thermostable aromatic rings present in the structure of TDI and CNSL. The forth degradation temperature corresponds to the thermal cleavage of most thermostable Si-O-Si bond present in the structure of BHPDMS. Further, the melting temperature (Tm) and heat of crystallization (ΔHc) of the PCL segments present in the structure of PUs are evaluated from the DSC curves as shown in Fig. 3d and the corresponding values are reported in Table 2. From the tabulated results it is clearly evident that the Tm value of the PCL segments decreases dramatically with the increase of the CNSL amount. Such decrease in Tm is attributed to the presence of long aliphatic chains in CNSL which directly increase the free volume of the PU matrix. Higher the free volume, lower is the

3.7. Self-healing property As displayed in Fig. 4d, damaged PU-CNSL-1 strip was effectively healed and recovered its original set of properties after the application of a suitable stimulus (MW). Most interestingly, tensile properties of the healed samples resemble with the pristine as displayed in the stressstrain curves (Fig. 4e) and corresponding healing efficiency values are furnished in Table 2. Such kind of healing is possible only because of the diffusion of polymeric chains in the damage site [41]. Briefly, upon exposure to a suitable stimulus, some polar functional groups start to vibrate their dipoles after absorbing the energy. The heat generated from such vibration is enough for softening the soft segments (mainly PCL) present in the structure [2,29]. Hence, a rapid Brownian motion occurred in between the soft segment which leads to the diffusion of the polymeric chains at the damaged site. Upon removal of the stimulus, melted soft segments recrystallize and heal the crack surface efficiently. At the same time, strong supramolecular interactions (H-bonding, dipole-dipole and π-π interactions) present in the structure of PUs partially help to regain their initial property [41]. Generally, healing in the synthesized PUs is governed by some important parameters like the ratio of hard to soft segments, molecular weight of the soft segments, crystallinity, glass transition and melting temperatures. [41]. But in our study, the amount of hard and soft segments and crystallinity are the predominating factors among the abovementioned parameters. From Table S1 it is cleared that the amount of hard segment (wt%) increases with the increase of CNSL content from PU-CNSL-1 to PU-CNSL-3. Presence of excess amount of hard segment acts as a physical barrier and prohibited the flowing ability of the soft segments during the time of healing. As a result, the time required for healing gradually increases with the increase of CNSL

Table 2 Thermal properties, SCA, healing efficiency and healing time of PU-CNSLs. Parameter

PU-CNSL-1

PU-CNSL-2

PU-CNSL-3

Ton (°C) Second degradation peak temperature (°C) Third degradation peak temperature (°C) Forth degradation peak temperature (°C) Tm (°C) ΔHc (J/g) SCA (°) Healing efficiency (%) Healing time (s) under MW (500 W)

269 358

251 364

258 358

414

405

413

456

452

452

65.2 20.28 112.3 ± 1 94.7 72 ± 5

51.3 17.03 115.8 ± 0.6 86.3 81 ± 3

50.2 14.49 121.2 ± 0.5 82.8 89 ± 2

7

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Fig. 4. (a, b, and c and insets) represent the surface roughness of PU-CNSL-1, PU-CNSL-2, PU-CNSL-3 and SCA on the corresponding surface (d) SEM images of the PU-CNSL-1 surface during healing and (e) representative stress–strain profiles of PU-CNSL-1 before damage and after healing with different repeating cycles.

corresponding results are presented in Table 3. From the results, it is clear that the synthesized PU showed excellent corrosion resistance performance and the results (mainly corrosion rate) are much better than the prior art literatures values [20,35,42]. At the very beginning of the reaction, MS plate showed the lowest Ecorr (-1.027 V) and the highest Icorr (1.7 × 10−4 A/cm2) value. Origination of pitting corrosion on the MS plate immediately after immersion in the salt solution, is the main reason for such poor corrosion resistant behavior [17]. Whereas in the coated MS plate, Ecorr value (-0.469 V) shifted towards a more positive direction compared to the bare MS plate which endorsed the decrease of anodic reaction (mainly the oxidation of iron). As a result, Icorr value of the coated steel plate (6.65 × 10-9 A/cm2) depressed significantly. Further, it is also noted that the MS plate exhibited RP value of 84.91 ohm whereas the RP value for coated MS plate is 8.76 × 105 ohm. This huge increase in RP value clearly indicates that the coating acts as a physical barrier and appreciably resists the corrosion of the MS

content (Table S1). Further, PU-CNSL-1 showed a higher degree of crystallinity for the PCL segment which generally melts and recrystallizes during the time of healing. The combined attribute of these factors is the main reason behind the efficient (Healing efficiency: 94.7%) and ultrafast (within 72 s under MW) self-healing behavior of PU-CNSL-1. 3.8. Anticorrosion study Corrosion resistance property of the synthesized PU-CNSL-1 coated MS plate was investigated in 3.5 % NaCl solution for a period of 96 h. Overall study was carried out using PU-CNSL-1, as it showed superiority in mechanical and self-healing behavior compared to other two PUs. Different electrochemical parameters like corrosion current density (Icorr), corrosion potential (Ecorr), corrosion rate and polarization resistance (Rp) values were evaluated from Tafel plots (Fig. 5a) and

Fig. 5. (a and b) Tafel and Nyquist plots of bare MS plate and PU-CNSL-1 coated MS pate at different time intervals, (c and d) Tafel and Nyquist plots of bare MS plate and PU-CNSL-1 coated MS pate after healing and (e and f) Tafel and Nyquist plots of corrode MS plate after coating. 8

Progress in Organic Coatings 139 (2020) 105472

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Table 3 Corrosion parameters of the bare MS plate, PU-CNSL-1 coated MS plate, PU-CNSL-1 coated MS plate after healing and PU-CNSL-1 coated corrode MS plate at different time intervals. Materials

Time (h)

Icorr (A/cm2)

Ecorr (V)

Corrosion rate (mm/y)

Polarization resistance (RP) (ohm)

Pore Resistance (Rpore) (ohm)

MS plate PU-CNSL-1 coated MS plate

0 0 48 96 0 48 96 0

1.7 × 10−4 6.65 × 10−9 1.41 × 10−8 1.969 × 10−7 6.94 × 10−9 1.16 × 10−8 2.57 × 10−7 5.85 × 10−7

−1.027 −0.469 −0.479 −0.517 −0.571 −0.724 −0.592 −0.694

2.0534 7.72 × 10−5 1.63 × 10−4 2.22 × 10−3 8.07 × 10−5 1.35 × 10−4 2.9 × 10−3 6.8 × 10−3

84.91 8.76 × 105 4.88 × 105 3.82 × 105 3.47 × 105 5.49 × 105 7.4 × 104 2.7 × 104

0.509 309 × 103 197 × 103 128 × 103 67.2 × 103 47.7 × 103 17.3 × 103 10.8 × 103

PU-CNSL-1 coated MS plate after healing

PU-CNSL-1 coated corrode MS plate

synthesized PUs. But the problem associated with our day to day life is to resist the corrosion of metal which already underwent corrosion under normal atmospheric condition. Thus to check the corrosion resistant ability of PU, a corrode MS plate was coated with the synthesized PU and projected for electrochemical study. Tafel and Nyquist plots of the PU coated corrode MS plates in different time intervals are shown in Fig. 5e and f. Displayed results showed that after coating the corrosion rate of the MS plate decreases almost 300 times which seems to be very good. Not only the corrosion rate, but the corresponding RP and Rpore values were also found to be very high than the bare MS plate. Strong barrier property of the synthesized PU acts as a spacer in between MS plate and salt solution. As a consequence oxidation process of the metal plate decreased significantly which results good anticorrosive performance. But after a certain time interval, corrosion resistivity decreases significant amount. Presence of corrosion products below the coatings leads to improper adhesion which favors for further corrosion with detachment of the coating from the MS plate.

plate from the corrosive medium. Generally, the synthesized PU acts as a wall in between the plate and corrosive medium [17]. Therefore, the toughness of the wall is an important factor which significantly controls the penetration of the corrosive ions to get in touch with the MS plate. From Table 1 it is evident that PU-CNSL-1 showed adequate toughness which makes the coatings tough to penetrate by the corrosive ions. Moreover, the PU also showed intrinsic hydrophobic behavior due to micro/nano scale roughness as described earlier. Such rough architecture entrapped a huge amount of air and hence penetration of corrosive ions through the coating is very difficult [35]. Combinations of both these factors dramatically reduce the corrosion rate. From the study, it is also noticed that Ecorr value of the coated MS plate decreases slowly from -0.469 V to -0.517 V and the corresponding Icorr value increases simultaneously with the increase of immersion time. This is due to the diffusion of conductive ions into the PU-CNSL-1 coating through pinholes [20]. Formation of such pinholes is also evident by the decreasing trend in Rp values from 8.76 × 105 to 3.82 × 105 ohm with immersion time. Results of such diffusion of corrosive ions into coating slightly increase the corrosion rate. The Nyquist plots (Fig. 5b) of the coated MS plate obtained from EIS measurements in different time interval (0 h, 48 h, 98 h) showed semicircle throughout the study but the diameter of that semicircle decreases with the increase of immersion time which clearly indicates the diffusion of the corrosive ions into the coating [42]. From the fit and simulation studies of the Nyquist plots, it is also found that the pore resistance (Rpore) of MS plate is 0.509 ohm whereas the Rpore of the coated plate MS plate is 309 × 103 ohm. Generally, Rpore defines the extent of the porosity of the coating. Higher the value of Rpore, stronger is the electrical resistance characteristic of the coatings against corrosive ions. Due to this very low Rpore value, bare MS plate is unable to resist the corrosion activity immediate after immersion in the salt solution whereas the coated MS plates showed good anticorrosion activity for longer periods of immersion time. Noticeably, Rpore of the coated MS plates decreases with the increase of immersion time which is accredited to the slow and progressive diffusion of electrolyte flux through the coatings. Most interestingly, synthesized PU coated MS plates also retain good corrosion resistance property (Ecorr: −0.571 V, Icorr: 6.94 × 10−9 A/ cm2, corrosion rate: 8.07 × 10−5 mm/y) even after healing from mechanical damage. Tafel plots of the healed PU coated MS plates are displayed in Fig. 5c and their corresponding values in different time interval are furnished in Table 3. Effective healing of the damaged PU surface under a suitable stimulus is responsible for maintaining such good corrosion resistant performance. Even though, the healed coating acts as an active shield but the RP and Rpore value depressed drastically compared to the coated MS plate without any cut as described earlier. Further, a huge reduction is observed in the semicircle diameter of the Nyquist plot as shown in Fig. 5d. Presence of microscopic cracks in the damaged site even after healing makes the coating slight permeable towards the corrosive medium and increases the corrosion rate. All the above results depicted good anticorrosive nature of the

3.9. Biodegradation study Degradation of PUs after its useful service life is one of the most important properties owing to strict environmental regulations [43]. In this article, the McFarland turbidity method was used to evaluate the biodegradability of the PU strips under the exposure of Gram positive and Gram negative bacteria strains [25,44]. Degradation under the exposure of Gram negative bacteria is more effective and hence the corresponding results are shown in Fig. 6. From the biodegradation study, it is clearly evident that the synthesized PU strips underwent a significant amount of degradation after 5 weeks of incubation as shown in Fig. 6a. Such degradation is attributed to the presence of hydrolyzable ester and urethane linkages present in the structure of PUs [2,45,46]. Generally, the biodegradation of the PU strips occurred through a systematic pathway. Initially, the microorganisms cannot attach directly to the PU surface. Rather they excrete extracellular enzymes that adhered to the PU surface through physicochemical interaction and depolymerize the PU chains into smaller fragments. Such kind of depolymerization leads to bulk surface erosion and makes the surface more accessible for the microorganisms [43,47,48]. SEM images (Fig. 6b and c) before and after biodegradation clearly support the aforementioned statement. This small carbon and nitrogen-rich polymeric fragments act as catabolite to the microorganism and further degraded to gaseous products (mainly CO2), water and biomass [45]. From the OD curve it is clear that PU-CNSL-3 is more susceptible towards biodegradation compared to the other two PUs. Again, lower degree of crystallinity and higher free volume of PU-CNSL-3 matrix are the main reasons behind the improved biodegradability. 4. Conclusion In conclusion, different compositions containing PUs were efficiently synthesized from bio-derived raw materials like cashew nut 9

Progress in Organic Coatings 139 (2020) 105472

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Fig. 6. (a) Growth curve of P. aeruginosa on the PUs, (b and c) represents the SEM images of the PU-CNSL-1 before and after degradation.

shell liquid, sunflower oil modified monoglyceride and polyol of modified dimer acid-glycerol through a facile polymerization pathway. The current study demonstrated the benefit of bio-based units for the synthesis of PU and tailoring the structure-property relationship through the minute compositional variation. The study also revealed the effect of free volume as created by the long aliphatic chains of PU and its influence on the mechanical and thermal properties. Additionally, the study also demonstrated the effect of surface roughness on hydrophobicity, which can be further manipulated by the variation of amount of CNSL. In addition, the synthesized PUs showed intrinsic ultrafast self-healing behavior through the segmental mobility of the soft segment along with excellent corrosion resistance property. Most interestingly, the synthesized PUs underwent degradation under microbial exposure. Therefore, the studied PUs can be used as a promising sustainable material for different potential advanced applications.

[3]

[4]

[5] [6]

[7] [8]

[9]

Supporting information

[10]

The composition of the reactants for the synthesis of PUs and other parameters, and physical properties of the PUs.

[11]

Author contribution statement

[12]

The first author, Tuhin Ghosh executed the experimental work and writing of the manuscript. The corresponding author, Niranjan Karak helps in design the work and supervised the overall work for publication.

[13]

[14]

Declaration of Competing Interest [15]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[16]

[17]

Acknowledgment [18]

The authors express their gratitude to the research project assistance given by DST, India through the grant No. EMR/2016/001598, dated 04-Jan-2017.

[19]

Appendix A. Supplementary data [20]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.porgcoat.2019. 105472.

[21]

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