Self-Cross-Linkable Anionic Waterborne PolyurethaneâSilanol

Research Article pubs.acs.org/journal/ascecg

Self-Cross-Linkable Anionic Waterborne Polyurethane−Silanol Dispersions from Cottonseed-Oil-Based Phosphorylated Polyol as Ionic Soft Segment Sashivinay Kumar Gaddam,† S. N. Raju Kutcherlapati,‡ and Aruna Palanisamy*,† †

Polymers & Functional Materials Division, Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad-500007, Telangana State, India ‡ School of Chemistry, University of Hyderabad, Prof CR Rao Road, CUC, Gachibowli, Hyderabad-500046, Telangana State, India S Supporting Information *

ABSTRACT: Phosphorylated polyols (phospols) derived from cottonseed oil (CSO) were employed to synthesize novel, DMPA- (dimethylol propanoic acid-) free, catalyst-free, waterborne polyurethane dispersions (PUDs). Three different phospols bearing both hydroxyl and ionizable phosphoryl groups were synthesized through the ring-opening hydrolysis of epoxidized cottonseed oil (ECSO) in the presence of ortho-phosphoric acid. The phospols were characterized by 1H NMR spectroscopy, FTIR spectroscopy, and gel permeation chromatography. Three phospols (phospol-P5, -P10, and -P15) having hydroxyl numbers of 130, 160, and 180 mg of KOH/g, respectively, were used as internal emulsifiers in waterborne PUDs with isophorone diisocyanate and 3-aminopropyltriethoxysilane (APTES). All three PU dispersions showed excellent storage stabilities (>6 months), and the average particle sizes of the PUDs ranged from 30 to 68 nm. The cured films were characterized by FTIR (ATR) and solid-state 29Si NMR analyses. The hydrophobicities of the films were measured by the contact angle technique, and their anticorrosive properties were studied by the polarization technique. The films of the phospol-P5-based dispersion exhibited the highest tensile strength, thermal stability, Tg value, and contact angle and the best anticorrosive properties. All of the experimental results revealed that both the hydroxyl contents of the phospols and the extents of siloxane cross-linking played important roles in determining the thermomechanical properties, hydrophobicities, and anticorrosive properties of the corresponding PUD films. KEYWORDS: Cottonseed-oil-based phosphorylated polyol, DMPA-free, Self-cross-linkable, Waterborne polyurethane dispersion

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hydroformylation−reduction,14,15 epoxidation followed by oxirane ring opening,16,17 ozonolysis−reduction,18 and microbial conversion.19 Epoxidation of double bonds followed by oxirane ring opening with a wide range of reagents is an efficient strategy for converting vegetable oils into different types of polyols having valuable properties, and these properties can be reflected in the final PUD products.20,21 To date, there have been very few reports on the synthesis and properties of polyurethanes from cotton-seed-oil-based polyols.22−24 Cottonseed oil (CSO) is one of the cheapest renewable resources and one of the most important commercial crops in India. According to the U.S. Department of Agriculture, India is the largest producer of cottonseed oil (CSO) worldwide, producing about 12.8 million tons in 2015−2016.25 CSO is an unsaturated oil, and it has a 2:1 ratio of polyunsaturated (65−70%) to saturated (26−35%) fatty acids, with the unsaturated fatty acids consisting of 18−24% oleic acid and 42−52% linoleic and

INTRODUCTION Waterborne polyurethane dispersions (PUDs) are gaining importance as coatings and adhesives to help mitigate increasing environmental concerns regarding volatile organic chemicals (VOCs) and hazardous air pollutants (HAPs).1−3 Since their inception, conventional PUDs have been developed with the help of petroleum-based polyols, mainly polyethers, polyesters, polycaprolactone, and polycarbonate polyols.4,5 However, in recent years, crude-oil price fluctuations, increasing shortages of fossil-fuel reserves, global warming, and so on have raised the demand for the utilization of biobased materials to synthesize biopolyols for polyurethane production in both industry and academia.6−8 Among several biobased materials, including cellulose, starch, and sugar, naturally available vegetable oils are potential bioresources and ideal substitutes for crude-oil feedstock in worldwide PUD manufacturing. This is mainly because of their abundant availability, renewability, inherent biodegradability, low ecotoxicity, and low costs.9−11 Many techniques have been used for the synthesis of vegetableoil-based polyols for PUD applications, including cleavage of triglyceride linkages,12,13 conversion of double bonds by © 2017 American Chemical Society

Received: February 3, 2017 Revised: June 30, 2017 Published: July 5, 2017 6447

DOI: 10.1021/acssuschemeng.7b00327 ACS Sustainable Chem. Eng. 2017, 5, 6447−6455

Research Article

ACS Sustainable Chemistry & Engineering linolenic acids.26 This high degree of unsaturation of CSO provides a wide variety of possibilities for chemical transformation to produce polyurethanes with various properties. The use of CSO as a sustainable resource in PUD manufacturing despite its edible nature has been considered because of disadvantages such as the genetic modification of seeds and pesticide loading during crop cultivation. In the recent past, several publications and patents have become available that describe the synthesis and applications of anionic waterborne PUDs obtained from vegetable-oil-based polyols. All of these efforts devoted their attention to the synthesis of waterborne PUDs by employing dimethylol propanoic acid (DMPA) as an internal emulsifier in the PU backbone that can stabilize the PUD in water by utilizing its carboxylic acid groups in ionic form.27−30 However, the use of DMPA as an internal emulsifier leads to some disadvantages such as increased film formation time31 and decreased compatibility between the hard and soft segments.32 In addition, DMPA is not a biobased material, and its utilization reduces the biocontent of the resulting waterborne PUDs.33 To overcome these disadvantages, some researchers have attempted to synthesize DMPA-free anionic waterborne PUDs by introducing vegetable-oil-based ionic segments in either the soft or hard segment. For example, Bao et al. synthesized a waterborne PUD by partly replacing the DMPA with an ionic soft segment derived from the maleanization of castor oil.34 Chen et al. synthesized a DMPA-free anionic PUD by introducing soybean-oil-based ionic segments into the PU hard segments, where the DMPA was completely substituted.33 DMPA-free waterborne PUDs were synthesized through the introduction of dihydroxystearic acid as an ionizable molecule into the PU hard segment by KosheelaDevi et al.35 The introduction of vegetable-oil-based polyols as ionic soft segments, having both hydroxyl and ionizable functionalities in their triglyceride backbone, is expected to be another effective method for achieving the complete substitution of DMPA in PUD manufacturing. This method should elevate the sustainability and ecofriendliness of PUDs with enhanced phase compatibility between hard and soft segments. Based on this approach, we previously reported a novel approach to the synthesis of DMPA-free anionic waterborne PUDs using maleated cottonseed oil polyol (MAHCSO) as an ionizable soft segment.36 This route opens up the possibility of synthesizing DMPA-free waterborne PUDs with low filmformation times and enhanced phase compatibility. Generally, vegetable-oil-based phosphorylated polyols (phospols) are synthesized by ring-opening hydrolysis of epoxidized vegetable oils in the presence of ortho-phosphoric acid.37 Most of these phosphorylated polyols are utilized in the synthesis of polyurethane foams,38,39 bioelastomers,40 coatings,41 and corrosion-resistant waterborne coatings.42 To date, waterborne polyurethane dispersions prepared with vegetable-oil-based phospols carrying both hydroxyl and ionizable phosphoryl groups have not been reported. Further, phosphorus-functional water-based polymer dispersions have great industrial importance because of their wide range of applications including ionexchange resins, adhesion promoters on metal substrates, and also biomedical applications.43,44 In this study, a series of phosphols were synthesized from epoxidized CSO through the ring-opening hydrolysis of epoxy groups in the presence of ortho-phosphoric acid. These phosphorylated polyols with different OH functionalities were employed as ionic soft segments in the synthesis of DMPA-free

waterborne PUDs by a sol−gel process. The effects of the phosphol functionality and siloxane networks on the structures and functional properties of the PUDs and their cured films were investigated. The main advantage of sol−gel processing is the formation of a uniform hybrid network through covalent bonds at low temperatures during the curing process. These hybrid networks resulted in improved elasticity and flexibility of the PUD films.45 This article reports a novel approach to the synthesis of vegetable-oil-based self-cross-linkable waterborne polyurethane dispersions for ecofriendly protective coatings.

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EXPERIMENTAL SECTION

Materials. Cottonseed oil (CSO, iodine value of 99−102 g/100 g) was obtained from Jayajyothi Oil Industries, Hyderabad, India, and used directly without further purification. Hydrogen peroxide (H2O2, 30% w/v), formic acid (FA, 85%), sulfuric acid (98%), orthophosphoric acid (85%), sodium sulfate, ethyl acetate, methyl ethyl ketone (MEK), and 1,6-hexanediol (HDO) were purchased from SD Fine Chemicals Limited (Mumbai, India). Isophorone diisocyanate (IPDI), N,N′-dimethyl ethanolamine (DMEA), and 3-aminopropyltriethoxysilane (APTES) were purchased from Aldrich and used as received. Synthesis of Phosphorylated Polyols (Phospols). Epoxidized cottonseed oil (ECSO) was synthesized by the reaction of unsaturation sites in CSO with a mixture of FA and H2O2 in the presence of a catalytic amount of sulfuric acid, according to the method described in our previous report36 (Scheme S1). Phospols were synthesized by the ring-opening hydrolysis of ECSO according to a reported method,37 as shown in Scheme 1. The reaction was

Scheme 1. Synthesis of Phospols from Epoxidized CSO

performed in a two-neck reaction flask equipped with a water-cooled condenser and a dropping funnel. A mixture of ECSO (150 g), water (15 g, 10% by weight of ECSO), and the required amount of t-butyl alcohol (35 g) were charged into the reaction flask, and the mixture was brought to reflux conditions by heating at 60 °C for 30 min. orthoPhosphoric acid was dissolved in 35 g of t-butyl alcohol (total amount added was 50% by weight of ECSO) and added dropwise to the reaction mixture over a period of 15 min. Three phosphorylated polyols were prepared: phospol-P5, with 5 wt % phosphoric acid (with respect to the weight of ECSO); phospol-P10, with 10 wt % phosphoric acid; and phospol-P15, with 15 wt % phosphoric acid. 6448


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ACS Sustainable Chemistry & Engineering Scheme 2. Synthesis of Phospol-Based Anionic Waterborne PUDs

Synthesis of Anionic Waterborne PUDs from Phospols. Phospol-based anionic waterborne PUDs were synthesized by the prepolymer method as shown in Scheme 2. The chemical compositions are reported in Table 1. Stochiometric amounts of phospol, isocyanate, and HDO were introduced into a three-neck round-bottom flask equipped with mechanical stirrer, condenser, and nitrogen inlet. The flask was heated at 70 °C in an oil bath, and the

After the addition of phosphoric acid, the temperature was raised to 90 °C, and the reaction was continued for 6 h, at which point the oxirane content had reached ≤0.2%. Upon completion of the reaction, the reaction mixture was dissolved in ethyl acetate, and the organic layer was washed with distilled water to remove unreacted ortho-phosphoric acid. The organic layer was dried over anhydrous Na2SO4, and phospols (Figure S1) were recovered after solvent removal. 6449


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ACS Sustainable Chemistry & Engineering

from 200 mV below to 200 mV above the corrosion potential (Ecorr) at a constant scan rate of 5 mV min−1. The corrosion current density (Icorr) and other parameters were obtained directly from the software. Dynamic mechanical analyses of the PUD films were performed to determine the thermomechanical properties of the cured films using a dynamic mechanical analyzer (DMA-Q800) in the temperature range from −60 to 100 °C at a heating rate of 3 °Cmin and in film tension mode at 1 Hz. The glass transition temperatures (Tg) of the PUD films were obtained from the peak of the tan δ curves. Thermogravimetric analyses of PUD films were performed to evaluate the thermal stabilities of the cured films using a TA Instruments Q500 TGA apparatus (New Castle, DE) in the temperature range from 25 to 600 °C under a nitrogen atmosphere at a heating rate of 10 °C/min. Differential scanning calorimetry analyses of the PUD films were performed to examine the glass transition temperatures of the film samples using a DSC Q100 EXFO Series 2000 instrument (TA Instruments, New Castle, DE) in the temperature range from −60 to 150 °C following a cycle of heating and cooling under an inert atmosphere at a heating rate of 10 °C/min. The tensile properties of the PUD films were measured using a universal testing machine (AGS10k NG; Shimadzu Corp., Kyoto, Japan) at a crosshead speed of 10 mm min−1. Rectangular specimens of 80 × 10 mm2 (length × width) were used for the analysis. The tensile strength, elongation at break, and Young’s modulus of each specimen is reported as the mean value of three replicate measurements.

Table 1. Chemical Compositions and Hard-Segment Contents of PUDs chemical composition (g)

sample

phospol-P5

IPDI

HDO

APTES

water

hard-segment content (wt %)

PUD-P5 PUD-P10 PUD-P15

5 5 5

3.51 3.09 2.54

0.65 0.57 0.47

1.03 0.88 0.72

37 35 33

53 51 47

reactants were stirred for 3−4 h to synthesize NCO-terminated prepolymer. To reduce the viscosity of the prepolymer, the minimum amount of MEK was used. The equivalent ratios between the NCO groups of isocyanate, the OH groups of phospol and HDO, and the NH2 groups of APTES was fixed as 2.0:1.0:0.70:0.30. After pre polymerization, the reaction mixture was cooled to room temperature, and DMEA [0.5 equiv/(equiv of phospol)] was added to neutralize the phosphoryl groups with stirring for 30 min. APTES was then added to the neutralized prepolymer solution to react with unmodified NCO groups. The complete conversion of NCO groups was confirmed by the disappearance of the peak at 2270 cm−1 in the FTIR spectrum, and the polymer was dispersed in deionized water under vigorous stirring (1200 rpm) for 1 h. The PUDs (Figure S2) thus obtained had a solid content of 30 wt % after the removal of MEK under a vacuum. The characteristic properties of the PUDs are summarized in Table 3 below. Preparation of Phospol-Based PUD Films. Transparent dispersion cast films were obtained by allowing the dispersions to dry in Teflon troughs at room temperature for 1 day and then under a vacuum at 50 °C for 1 day. The curing process of the PUD films proceeds through the formation of siloxane cross-links (SiOSi) under base-catalyzed condensation, as shown in Figure S3. Measurements. The hydroxyl values (HVs), acid values (AVs), and percentage oxirane oxygen contents (OOC, %) of the CSOphospols were determined according to standard methods ASTM D 1957-86, ASTM D 1639-89, and ASTM D 1652-97, respectively. The phosphorus contents of the phosphols were determined according to AOCS official method Ca 12-55. The molecular weights of the CSOphospols were determined by gel permeation chromatography (GPC; C-R4A Chrotopac; Shimadzu, Kyoto, Japan) using Aldrich polystyrene standards cross-linked with divinylbenzene. Experiments were carried out using tetrahydrofuran (THF) as the eluent at a flow rate of 1.0 mL/min. The viscosities of the CSO-phospols were determined at (25 ± 0.1) °C using an MCR102 (Anton Paar GmbH, Graz, Austria) rheometer with a parallel plate. The gap distance was fixed at 1 mm, and the shear rate γ (1/s) ranged from 0 to 100. The infrared spectra of the CSO-phospols and phospol-based PUD films were obtained with a Perkin-Elmer Spectrum-100 FTIR spectrometer at room temperature by averaging 8 scans. The spectral resolution was 4 cm−1, and the frequency range spanned from 4000 to 400 cm−1. For PUD film samples, spectra were recorded using an attenuated-totalreflection (ATR) accessory. 1H NMR and 31P NMR spectroscopic analyses of the CSO-phospols were carried out using an AVANCE-300 spectrometer (Bruker, Fällanden, Switzerland) in CDCl3 solution at 25 °C. Solid-state 29Si NMR spectroscopy of the PUD films was performed using an AVANCE-500 spectrometer (Bruker, Fällanden, Switzerland). The mean particle diameters and size distributions of the PU dispersions were measured with a Malvern Zetasizer V7.03 instrument, by diluting approximately 0.1 mL of the dispersion sample in 3 mL of deionized water at 25 °C. The water contact angle measurements of the PUD films were performed using a Krüss contact angle measuring instrument (Krüss GmbH, Hamburg, Germany) by applying the captive bubble technique at room temperature. The corrosion resistance performances of the phospol PUD films were evaluated by Tafel polarization method, using an AUTOLAB (potentiostat and galvanostat, 320N, Tokyo, Japan) electrochemical system with 3.5% NaCl electrolyte at room temperature. Potentiodynamic polarization curves were obtained by sweeping the potential

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RESULTS AND DISCUSSION Structure and Properties of Phospols. The structures of the ionizable polyols (phospols) were confirmed by chemical, physical, and spectroscopic methods, and the results are compiled in Table 2. Table 2. Characteristics of Phospols phospol-P5

phospol-P10

phospol-P15

physical state at 30 °C

property

liquid

viscosity at 50 °C at 100 s−1 shear rate (mPa s−1) Eaca (kJ/mol) acid value (mg of KOH/g) OH value (mg of KOH/g) phosphorus content (%) OOC (%) equiv weight (g/equiv) Mw, Mn PDI (Mw/Mn)

3000

viscous liquid 4719

waxy material 3949

54.45 68 177 2.03 0.24 316 2889, 2667 1.08

58.03 82 161 2.78 0.22 348 3319, 2851 1.16

55.78 106 128 3.07 0.22 436 3028, 2823 1.07

a

Eac = activation energy of viscous of phospols.

As expected, as the concentration of ortho-phosphoric acid was increased from 5% to 15%, the acid values gradually increased; on the other hand, the hydroxyl values decreased. At low concentrations (5−10%), phosphoric acid mainly catalyzes the ring-opening hydrolysis of ECSO, which involves the nucleophilic attack of water molecules on the epoxy rings to result in 1,2-diols,37 and phosphoric acid is also involved in the epoxy ring opening to some extent. Hence, the phospols generated by ring-opening hydrolysis exhibit high hydroxyl and low acid values. Ring-opening hydrolysis proceeds slowly, and epoxy groups are available to the freshly generated hydroxyl groups, resulting in phosphoryl dimers and trimers. On the other hand, at higher acid concentration (15%), phosphoric acid mainly acts as a reactant, and rapid consumption of the epoxy groups takes place by nucleophilic attack of phosphoric acid. Because of this direct epoxy ring opening with phosphoric acid, the resulting phospol had lower hydroxyl values. 6450


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ACS Sustainable Chemistry & Engineering The rheological behavior of the phospols as a function of temperature also supports the high viscosity and molecular weight of phospol-P10 because of the high rate of oligomerization compared to those of the other two phospols. A plot of the viscosity versus the reciprocal temperature (Figure 1) exhibited an Arrhenius dependence η = η0e Eac / RT

Table 3. Characteristic Properties of PUDs from Phospols property color, appearance avg particle size (nm) polydispersity index pH solid content (wt %) storage stability (months)

(1)

PUD-P5 milky white, opaque 68 0.110 7.9 ∼31 >6

PUD-P10

PUD-P15

blue, translucent 43 0.103 7.9 ∼31 >6

blue, translucent 32 0.104 7.8 ∼30 >6

(Figure S7). The particle sizes of the PUDs were controlled by two factors. First, with an increase in the OH functionality of the phospols, the amounts of diisocyanate and APTES in the polyurethane backbone also increased, resulting in higher crosslinking and leading to larger particles. However, when the OH functionality of a phospol increased, the acid number of the corresponding phospol decreased, and these phosphoryl groups were responsible for controlling the particle size after neutralization. When the phosphoryl groups were neutralized by DMEA, fewer counterions (phosphate ions) were present in PUD-P5 than in PUD-P15, which resulted in a decrease of the interionic electrostatic repulsive force and an increase in the mutual coalescence of the dispersed particles. Hence, the particle size increased accordingly, and the particle distribution widened. Second, the particle sizes of the synthesized PUDs were also affected by the hydrophilicity of the PU segments. When the amount of counterion was lower, it was difficult for water to penetrate into the microscopic ion accumulation area under a high shear field. Hence, the hydrophilicity of the PUD decreased, leading to larger particles.46 In this study, the hydrophilic nature of the PU segments decreased with decreasing amount of phosphate counterions after neutralization, resulting in higher particle sizes. PUD-P5 was white in color and opaque, whereas both PUD-P10 and PUD-P15 were blue in color and translucent, mainly because of the difference in particle size. Importantly, all three PUDs exhibited very good shelf lives at room temperature even after 6 months, because of the narrow particle size distributions of the PUDs. FTIR Spectroscopy. The chemical structures of the phospol PUD films were confirmed by FTIR spectroscopy (ATR technique, Figure 2). The absorption band near 1040−1050 cm−1 is assigned to the asymmetric stretching vibration of Si

Figure 1. Viscosity−temperature relationship for CSO-phospols.

where η0 is a reference viscosity, Eac is the viscous-flow activation energy, R is the universal gas constant, and T is the absolute temperature (K). The activation energies of the phospols were calculated from the slopes of the lines in Figure 1 and are summarized in Table 2. The high activation energy of phospol-P10 was mainly due to its high viscosity and molecular weight due to its high oligomerization rate. Figure S4a shows the FTIR spectra of epoxidized cottonseed oil and phospol, and Figure S4b shows the FTIR spectra of the three different phospols (phospol-P5, -P10, and -P15). As shown in the figure, the disappearance of the absorption band at 835 cm−1 corresponding to epoxy groups and the appearance of new peaks at 3400 and 1020 cm−1 corresponding to OH groups and phosphoryl groups, respectively, indicated the hydrolytic cleavage of the epoxy groups initiated by phosphoric acid. H1 NMR spectra of both ECSO and phospol (Figure S5) confirmed the successful cleavage of the epoxy rings through the disappearance of the peaks between 2.8 and 3.2 ppm corresponding to the epoxy group (methylenic protons). The appearance of new peaks between 3.4 and 4.4 ppm corresponding to the methylenic protons of carbon bonded to phosphoryl groups in the phospols confirms this assignment.38 The 31P NMR spectra of all three phospols showed only one sharp peak between 15 and 20 ppm assigned to phosphoryl groups, confirming the incorporation of the phosphoryl groups (Figure S6).. Structure and Properties of PUDs and Films. Particle Size Distributon. Table 3 lists the particle size distributions and polydispersity indexes (PDIs) of the PUDs synthesized from phospols having different hydroxyl values. The particle sizes of the PUDs increased with increasing OH number of the phosphols, and average particle sizes of approximately 32 and 68 nm were observed for PUD-P15 and PUD-P5, respectively

Figure 2. FTIR spectra of cured films of PUD-P5, -P10, and -P15. 6451


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synthesized PUDs. Figure 4 shows the temperature dependence of the storage modulus (E’) and loss tangent (tan δ) of the

OSi bonds, which illustrates the generation of an interpenetrating SiOSi polymer network between PU chains through the polycodensation of OH groups on the surface of silica of APTES. The absorption peak intensity gradually increased with increasing of silica (APTES) content. The single absorption band near 3320−3330 cm−1 can be attributed to hydrogen-bonded urethane NH stretching vibrations, indicating that all of the urethane amide groups in the PUDs were involved in strong hydrogen bonding. The characteristic absorption band of the urethane carbonyl stretching vibration at about 1701−1702 cm−1 appears as a single sharp peak. In general, urethane carbonyl stretching vibrations appear at about 1740−1720 cm−1 due to free CO stretching and at about 1720−1710 cm−1 due to hydrogen-bonded CO stretching in disodered regions. The urethane carbonyl stretching vibration appearing in the lower-frequency region (1684−1702 cm−1) is mainly due to the strong hydrogen bonds in the ordered regions.47 The absence of an absorption band at about 2270 cm−1 corresponding to the asymmetric stretching vibration of the NCO groups indicates that all of the NCO groups were completely consumed during urethane bond formation. Solid-State 29Si NMR Spectrosopy. To determine the nature of SiOSi cross-linking networks in phospol PUDs, the cured films were subjected to solid-state 29Si NMR spectral

Figure 4. Temperature dependence of E′ and tan δ.

PUD films. At temperatures below 20 °C, the films are in the glassy state and there is a slight decrement in their storage modulus (E’) with increasing temperature. Above 20 °C, there is a rapid decrease in the E’ values, and a peak maximum was observed in the tan δ curves, which can be considered as the glass transition temperature (Tg). The storage modulus (E’) and Tg values of the phospol PUD films increased as silica content increased because of the increasing siloxane crosslinking, which retards the relaxation of PU chains. Hence, the PUD-P5 film showed high storage modulus and Tg values compared to PUD-P10. The Tg values obtained from the phospol-PUD films are summarized in Table 4 and from Figure Table 4. Thermal (DSC, DMTA, and TGA) Properties of Phospol PUD Films Tg (°C)

a

TGA data (°C)

sample

DMTA

DSC

T10%

T50%

Tmax


66.47 59.86 no dataa

52.61 48.07 44.96

270 240 251

346 343 329

325/380 323/371 308/365

Film yielded at the time of the experiment.

4, single tan δ peak implies the homogeneity of the samples indicating good phase compatibility between soft and hard segments. This phase compatibility was mainly due to hard− soft segment H-bonding induced by ionic soft segments. DSC thermograms of phospol-PUDs are shown in Figure S8 and the obtained Tg values from DSC are summarized in Table 4. The Tg value of PUD-P5 was slightly higher than PUD-P10 and P15 because of the increased siloxane cross-linking. The derivative thermograms for all of the phospol PUD films are plotted in Figure 5 and the T10, T50, Tmax data were summarized in Table 4. All PUD samples underwent two main thermal degradation process. The first degradation stage in the temperature range of 200−350 °C arises because of the decomposition of labile urethane bonds. The second degradation stage in the temperature range of 350−450 °C can be attributed to soft segment chain scission. Thermal stability of phospol PUD-P5 was higher than PUD-P10 and PUD-P15, because of the increased siloxane cross-linking. The thermal degradation curves for all of the phospol PUD films are shown in Figure S9.

Figure 3. Solid-state 29Si NMR spectrum of the PUD-P5 film.

analysis. As shown in Figure 3, three peaks were observed in the 29 Si NMR spectrum of PUD-P5, at approximately −55, −61, and −72 ppm. The peak at −61 ppm is assigned to the silicon bearing one unreacted hydroxyl group, and the strong peak on the right at about −72 ppm results from fully condensed silanol groups. The intensities of the peaks indicate the high degree of silanol condensation representing the formation of stronger SiOSi cross-linking networks during the PUD film curing process. The small peak at −55 ppm is assigned to silicon bearing two unreacted hydroxyl groups, and the absence of a peak at −40 ppm corresponding to uncondensed silanol groups indicates that the condensation reaction proceeded rapidly and almost completely. DMEA, added during the neutralization of the prepolymer, is basic in nature and would be expected to accelerate the condensation of the silanol groups. Thermal Properties. DMTA and DSC techniques have been used to investigate the thermal transitions and relaxations of 6452


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surface during the film formation stage, thereby reducing the surface energy and leading to a high degree of surface hydrophobicity.48,49 Moreover, siloxane cross-linking creates a three-dimensional network structure, leading to increased surface roughness and increased contact angle values.50 Potentiodynamic Polarization Studies. The effects of the siloxane cross-link density on the corrosion protection of the phospol PUD films were evaluated by potentiodynamic polarization studies. The corrosion potential (Ecorr), corrosion current (Icorr), corrosion rate, and polarization resistance are summarized in Table 6. Tafel polarization plots obtained from Table 6. Tafel Polarization Data of Coated and Uncoated MS in 3.5% NaCl sample

Figure 5. TGA derivative curves of phospol PUD films.

uncoated MS PUD-P5 PUD-P10 PUD-P15

Mechanical Properties. The effects of siloxane cross-linking on the tensile properties of free-standing films of phospol PUDs were evaluated using a Universal Testing Machine (UTM), and the resulting mechanical properties including the Young’s modulus (E), tensile strength at break (σ), and elongation at break (ε) are summarized in Table 5. PUD-P5 had the highest Young’s modulus and tensile strength because of the enhanced cross-link density resulting from the higher OH functionality of phospol-P5.

Ecorr (V)

Icorr (nA/cm2)

−1.05

150.24 × 10−3

1.75

340.62 × 10−6

−0.75 −0.71 −0.72

1.74 2.68 2.77

2.02 × 10−5 3.11 × 10−5 3.22 × 10−5

55.83 36.09 27.01

corrosion rate (mm/year)

polarization resistance (MΩ)

potentiodynamic polarization measurements of uncoated and coated mild steel (MS) substrates with phospol PUDs in 3.5% NaCl electrolyte at room temperature are shown in Figure 7.

Table 5. Tensile Propertiesa of Phospol PUD Films

a

sample

E (MPa)

σ (MPa)

ε (%)


7.6 ± 1.12 4.8 ± 0.53 1.3 ± 0.27

10.7 ± 0.27 6.8 ± 0.69 1.9 ± 0.33

139 ± 09 194 ± 19 347 ± 11

E, Young’s modulus; σ, tensile strength; ε, elongation at break.

Contact Angle. The water contact angles of the phospol PUD films are shown in Figure 6, and these contact angles

Figure 7. Tafel polarization of (a) MS substrate, (b) PUD-P15, (c) PUD-P10, and (d) PUD-P5 in 3.5 wt % NaCl.

The linear Tafel segments of the anodic and cathodic curves were extrapolated to the corrosion potential to obtain Ecorr and Icorr values. The phospol PUDs were coated uniformly with layers of about 90−100 μm on 1 in. × 1 in. mild steel panels and were allowed to completely dry for a week at room temperature. From the results, it is clear that the corrosion protection performance of the phospol PUDs increased with increasing siloxane cross-link density. The surface activity of the siloxane units due to the lower surface energy led to increased surface roughness of the metal substrate and acted as a physical barrier to restrict the penetration of corrosive species.51,48 Furthermore, the corrosion resistance properties also depend on the chemical bonding of the phosphoryl groups. In general, acidic phosphoryl groups have an adhesive nature toward the metal substrate and bind the coating tightly to the metal through the formation of POFe bonds that help to seal the metal surface and enhance its hydrolytic stability.52 The improved corrosion protection performance of the phospol PUDs can be obtained through the combination of the barrier properties of the siloxane cross-linking network density with

Figure 6. Contact angles of PUD films.

gradually increased with increasing siloxane cross-linking networks. In the current study, lower surface energy resulting from cross-linked siloxane networks played an important role in controlling the hydrophobicity of the phospol PUD films. Surface energy provides an evaluation standard for hydrophobic nature, and the films having lower surface energies showed higher water contact angles with better waterproofing. In general, siloxane units exhibit surface activity and migrate to the 6453



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the strong chemical bonding of the phosphoryl groups to the metal substrate, as shown in Figure 8.

Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], arunap73@rediffmail.com. Tel.: +91 04027191447. ORCID

Aruna Palanisamy: 0000-0003-4720-7845 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge CSIR XII five-year-plan project “INTELCOAT” (CSC 0114) for financial support. S.K.G. acknowledges the Council of Scientific and Industrial Research (CSIR) for granting a Senior Research Fellowship [Award 1706/2012(i)EU-V].

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Figure 8. Schematic representation of corrosion protection in phospol PUDs.

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CONCLUSIONS In this contribution, we have demonstrated a novel approach to the synthesis of DMPA-free vegetable-oil-based waterborne PUDs using a phosphorylated polyol as an ionic soft segment. Phospols with different OH numbers were synthesized through the ring-opening hydrolysis of epoxidized cottonseed oil with different concentrations of ortho-phosphoric acid. The successful incorporation of the phosphoryl groups was confirmed by FTIR, H1 NMR, and 31P NMR spectroscopic analyses. Using these phosphorylated polyols, a series of waterborne PUDs containing siloxane cross-links were prepared by a sol−gel process. In this study, APTES was used as the cross-linking agent, and the formation of siloxane networks was confirmed by solid-state 29Si NMR and ATR-FTIR spectroscopic analyses of the PUD films. The particle size of the PUDs increased from about 32 to 68 nm with increasing OH functionality of the phospols. Further, increasing the OH functionality of the phopols significantly increased the crosslink density of the PUDs, improving their thermal and mechanical properties. The lower surface energy of siloxane networks improved the hydrophobic nature and corrosion resistance performance of the phospol PUDs with increasing siloxane cross-link density. The single tan δ peak of the phospol PUDs supports the improved phase compatibility between hard and soft segments. On the basis of these findings, phosphorylated polyols are considered to be promising materials for the synthesis of a new class of phosphoruscontaining waterborne PUDs for a wide range of industrial applications.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00327. Synthesis of ECSO, visual appearance of phospols and PUDs, curing mechanism of PUD film, FTIR and 1H NMR spectra of ECSO and phospols, 31P NMR spectra of phospols, particle size distributions of PUDs, and DSC scans and TGA curves of PUD films (PDF) 6454


Research Article


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