Synthesis, characterization and corrosion evaluation on new cationomeric polyurethane water dispersions and their polyaniline composites

Synthesis, characterization and corrosion evaluation on new cationomeric polyurethane water dispersions and their polyaniline composites

Progress in Organic Coatings 76 (2013) 639–647 Contents lists available at SciVerse ScienceDirect Progress in Organic Coatings journal homepage: www...

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Progress in Organic Coatings 76 (2013) 639–647

Contents lists available at SciVerse ScienceDirect

Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat

Synthesis, characterization and corrosion evaluation on new cationomeric polyurethane water dispersions and their polyaniline composites T. Gurunathan, Chepuri R.K. Rao ∗ , Ramanuj Narayan, K.V.S.N. Raju ∗ Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India

a r t i c l e

i n f o

Article history: Received 2 February 2012 Received in revised form 13 August 2012 Accepted 10 December 2012 Available online 16 January 2013 Key words: Polyurethane Polyaniline Water dispersion Cationomer Composite film

a b s t r a c t New aqueous cationomeric polyurethane dispersions (PUDs) were synthesized by three step reaction process. Isophorone diisocyanate (IPDI) was reacted with polyols, namely, polypropylene glycol400, polypropylene glycol-1000 and polypropylene glycol-2000, to form prepolymers which were chain extended by reacting it with N-methyldiethanolamine (N-MDEA). Quarternization and selfemulsification with deionized water resulted in PUDs. The resultant cationomers were film casted and characterized by FT-IR, DSC, TGA, DMTA and SEM analyses. Further, for the first time in the literature, aqueous cationomeric polyurethane dispersions (PUDs) were used for blending with 2 wt%, 4 wt% and 6 wt% of polyaniline–DBSA water dispersions to form new conductive composites. The conductivity attained is in the range 1.2 × 10−5 –3.7 × 10−5 S/cm. These composites were evaluated for their corrosion protection abilities on mild steel panels by standard accelerated tests. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Polyurethanes are versatile polymeric materials which offer wide range of applications, such as adhesives, elastomers, coatings for textiles/paper, foot wear, furniture/foams, packaging material and for automotive finishes [1,2]. They also cater the human health needs as a promising biomaterial [2] where the bio-compatible products range from nasogastric catheters, peritoneal dialysis, infusion pumps to inplanted pacemaker parts. There is a wide range of materials which can be used for the preparation of PUs and hence wide range properties can be imparted to the polymers [3]. Because of strict legislations throughout the world about VOCs which effect health and environment, water borne coatings are now considered to be alternative for solvent borne coatings. Water borne polyurethane dispersions (PUDs) exhibit several advantageous properties: good abrasion resistance, hardness, flexibility, impact resistance, gloss, general chemical resistance, and weatherability coupled with zero or low volatile organic compound (VOC) emission. Because of these properties, aqueous dispersions of polyurethanes have become one of the most investigated polymers in the last decade [4–10]. The added advantage of PUDs is that the viscosity of the dispersion is independent of molecular weight of the polymer and hence PUDs can be prepared at high solid content with a molecular weight high enough to form films with

∗ Corresponding authors. Tel.: +91 4027191479; fax: +91 4027193991. E-mail addresses: [email protected] (C.R.K. Rao), [email protected] (K.V.S.N. Raju). 0300-9440/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.porgcoat.2012.12.009

excellent quality simply by physical drying. Aqueous polyurethane dispersions are considered as two-phase colloidal system in which polyurethane particles are dispersed in water as a continuous phase [11]. They consist of PU backbones with some pendant acid or tertiary nitrogen groups, which are completely or partially neutralized or quaternized, respectively, to form salts and these centres are responsible for water dispersibility [12]. Parameters such as choice of isocyanate, types of ionomers, and types of polyols influence the performance of the resulting waterborne PUD. The relative merits and demerits are of these aqueous ionic polymers are of great research interest and were discussed by some authors [13–17]. Several synthetic processes for high molecular weight polyurethane dispersions are known [18], viz., (i) acetone process, (ii) melt dispersion process, (iii) prepolymer mixing process and (iv) ketamine process. The most popular process for synthesizing polyurethane water dispersion is pre-polymer blending process, in which the hydrophilically modified pre-polymer having free-NCO groups are chain extended in presence of water. Aqueous PUDs are catagorized as three types: non-ionic, cationic and anionic based on the type of hydrophilic segments present in the PU backbone. The interactions between ions and their counter ions are responsible for the formation of stable dispersion. The ion–dipole interaction between the ionomer and dispersing media (e.g., water) results in the formation of a salvation sheath, where the ionomer properties depend on the degree of neutralization and content of ionic component. More reports are available on synthesis of anionomeric dispersions than cationomeric dispersions. Polyaniline (PAni) has become one of the most studied conducting polymers due to the fact that it can be readily synthesized,

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Scheme 1. Scheme showing synthetic procedure for obtaining PUDs.

possesses good conductivity, and also exhibits interesting redox properties. Polyaniline has been used for many applications including corrosion protection [19–26]. MacDiarmid was the first one who suggested corrosion protection by inherently conductive polymers (ICPs) [27]. In contrast to polyurethane, polyaniline doped by simple mineral acids is not processable and a twofold benefit is expected when both are blended. That is, processability will be improved for PAni and conductivity is attained for insulating PU. The best way of blending PAni is to synthesize this polymer in an aqueous dispersion and latter blending it into PU water dispersion [28–33].

During the past few decades, polymer composites that are electrically conductive have found in many applications and the trend is increasing [34,35]. The conductive polymer composites are useful materials for sensors, electrostatic dissipaters, EMI shielding components, shape memory and corrosion protection coatings [35]. In the present investigation new cationic PUD are prepared, keeping in mind that water dispersed polyurethanes will be most compatible with polyaniline water dispersions. Later, PU–PAni composites are prepared by blending with commercially available conducting polyaniline water dispersions at 2 wt%, 4 wt% and 6 wt%. These composites are characterized by FT-IR, UV–vis, DSC, TGA analyses and

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Table 1 Constituents and film properties of PUDs. Polymer

IPDI (mol)

PPG-400 (mol)

PPG-1000 (mol)

PPG-2000 (mol)

TMP (mol)

N-MDEA (mol)

Film appearance

PUD-400 PUD-1000 PUD-2000

0.075 0.068 0.078

0.04 – –

– 0.04 –

– – 0.028

0.004 0.004 0.008

0.031 0.025 0.042

Transparent, glossy, flexible & non-tacky Transparent, glossy, flexible & non-tacky Transparent, glossy, flexible & non-tacky

conductivity measurements. The corrosion protection behaviour of the new composites are presented and discussed. 2. Experimental 2.1. Materials and methods Isophorone diisocyanate (IPDI: Z and E isomers in 3:1 ratio), polyaniline–DBSA dispersion (20 wt% in water), polypropylene glycol (PPG-2000) were purchased from Aldrich Chemical Co., USA. N-Methyldiethanolamine (N-MDEA) and dibutylamine (DBA) were purchased from S.D. Fine chemicals (Mumbai, India). The polymer films were studied by FT-IR spectroscopy (on Perkin Elmer, Model no: Spectrum 100) with HATR facility. The films were placed on HATR window and spectrum was recorded for 8 scans. Model DSC-Q-100 of TA instruments was used for recording DSC profiles of the samples. The samples were heated at a rate of 10 ◦ C/min under nitrogen. TGA was performed on TA instrument, model TGA-Q-500 by heating the samples at a rate of 10 ◦ C/min under nitrogen. Beckman DU640 UV/vis spectrophotometer was used for recording the absorption spectra of the films. DMTA experiments were conducted on carefully cut sample of the size 10 mm × 6 mm (0.5 mm thick) on model DMTA-IV instrument of Rheometric Scientific. Scanning electron microscope (SEM) model S-3000N of Hitachi was used for surface characterization of the films. Conductivity of the films was measured using 4-probe conductivity meter connected to KEITHLEY nanovoltmeter. Electrochemical corrosion measurements were performed on model IM6ex, ZAHNER electrik impedance unit, in a specially fabricated cell, containing NaCl (3.5 wt%) supporting electrolyte. The open circuit potential (OCP) operated at equilibrium state of the system was recorded as corrosion potential (Ecorr in mV versus SCE). Tafel plots were obtained by scanning the potential from 200 mV below to 200 mV above Ecorr at a scan rate of 5 mV min−1 . Corrosion current density (Icorr ) was determined by superimposing a straight line along the linear portion of cathodic or anodic curve and extrapolating it through Ecorr . 2.2. Preparation of free standing films

titration method. The reaction vessel was cooled to 60 ◦ C when proper NCO value was obtained, i.e., about 1.3–1.5%. The viscous pre-polymer was charged with calculated amount of N-MDEA dissolved in small amount of MEK solvent. The reaction mixture was further heated (70–75 ◦ C) with stirring for another period of about 2 h and after which time the heating was stopped and cooled to room temperature. The cooled, thick polyurethane solution was neutralized with calculated amount of standard hydrochloric acid. This step leads to the formation of quaternary nitrogen cationomeric centres along the polymer back bone. Finally the neutralized PU is self-emulsified by adding calculated amount of water with vigorous stirring to yield a white coloured dispersion with about 30% solid content. These experiments were repeated using different polyols namely PPG-1000 and PPG-2000. The PUDs are designated as PUD-400, PUD-1000 and PUD-2000. These dispersions were used for blending with polyaniline as follows: calculated amounts of PUD and PAni dispersions were weighed separately and added into a fresh RB flask and stirred for about 12 h for a blue–green homogeneous mixture. The final wt% of the polyaniline in the dispersion is 2 wt%, 4 wt% and 6 wt%. Addition of more amounts of PAni destroyed the stability of the PUD dispersion. 3. Results and discussion 3.1. Synthesis and characterization: First IPDI and polyols were reacted at 70–80 ◦ C as neat to get NCO-terminated pre-polymer. This was chain extended with N-MDEA to get organic solvent soluble polyurethane containing quaternizable nitrogen atoms originated from N-MDEA. After neutralizing with stoichiometric amounts of hydrochloric acid, quantified water is added into the polymer with vigorous stirring [36–38]. Experiments have been carried out on various formulations keeping IPDI concentration at 0.068 M, 0.075 M and 0.078 M for reactions with PPG-400, PPG-1000 and PPG-2000, respectively. The concentration of polyether and diethanolamine was varied until satisfactory and stable water dispersions with good quality films are obtained. The final formulations are summarized in

The films were cast on the smooth surface of a tin foil using a hand-driven applicator and were left at room temperature for drying for 48 h. After proper drying, the tin foil-supported films were placed in a clean mercury bath to amalgamate the tin substrate. The free standing films (ca. 100 ␮m thick) thus obtained were cleaned of mercury or amalgam adhering to it. 2.3. General synthesis of cationomeric polyurethane dispersions (PUDs) The synthesis of aqueous cationomeric polyurethane dispersions was carried out in three step reaction sequence as shown in Scheme 1. First a three-necked, moisture-free round bottomed was flushed with dry nitrogen and charged with calculated amount of IPDI and PPG-400. Small amount of TMP was added to increase the cross linking. The flask was heated at 70–75 ◦ C for nearly 3–4 h with vigorous overhead stirring. During this time NCO values were monitored at constant time intervals by standard n-dibutylamine

Fig. 1. FT-IR spectra of the PUDs and their PAni blended composites.

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Table 2 FT-IR spectral data of the films of pure polymers and blends. Characteristic absorption bands

(1) N H-stretching vibrations (2) CH stretching vibrations: (anti-symmetric and symmetric stretching modes of methylene groups) (3) Amide I (C O stretching vibrations) (4) Amide II (ıN H + C N + C C ) (5) C N (6) Amide-III (7) Amide-IV (8) Amide-V

Peak position (cm−1 )

PUD400-Pure

PUD400–PAni6%

PUD1000-Pure

PUD1000–PAni6%

PUD2000-Pure

PUD2000–PAni6%

3330.9

3316.4

3331.0

3321.3

3331.0

3317.0

2962.0–2865.2 (q)

2962.0–2868.5 (q)

2969.7–2868.5 (q)

2969.7–2868.6 (q)

2969.7–2868.8 (q)

2968.7–2865.7 (q)

1722

1721(d)

1708.5

1707.4

1725

1721(d)

1534.8

1545.9

1541.31

1541.4

1547.7

1558

1372.4 1239.9 773 –

1370 1236.3 771.3 668.2

1372.7 1239.1 773 –

1372.5 1239.1 773 667.8

1372.8 1239.4 773 –

1371.8 1237.5 772.2 680.2

d, doublet; q, quartet.

Table 1. Part of polyurethane dispersions (PUDs) thus obtained was casted for films for their characterization and other part is used for blending. Part of the above PUD is blended with commercially available polyaniline-dodecyl benzene sulphonic acid water dispersion. The amount of polyaniline blended is 2 wt%, 4 wt% and 6 wt%. The two components were vigorously stirred for about 12 h to get clear

green–blue dispersions. These composite dispersions were casted on to tin foil, mercury treated to get free standing composite films. These films are abbreviated as PUD-400–PAni2%, PUD-400–PAni4%, PUD-400–PAni6%, PUD-1000–PAni2%, PUD-1000–PAni4%, PUD1000–PAni6%, PUD-2000–PAni2%, PUD-2000–PAni4%, and PUD2000–PAni6%. These films were also characterized by FT-IR, DSC and TGA techniques.

Fig. 2. DSC and TGA curves of the polymers and their PAni blended composites.

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Table 3 Thermal and conductivity data of the pure and composite PUD films. Polymer/composite

PUD-400-Pure PUD-400–PAni6% PUD-1000-Pure PUD-1000–PAni6% PUD-2000-Pure PUD-2000–PAni6%

DSC (◦ C)

TGA (◦ C)

Tg

Tdec.on

Tdec.end

Td10%

Td50%

E’() (Pa)

tan ı

 dc (S/cm)

−54.85 −57.05 −55.34 −56.12 −55.85 −56.89

213 240 225 240 226 246

395 413 401 401 365 403

265 278 260 273 256 273

312 351 317 344 306 345

8.83 × 108 1.25 × 108 1.85 × 109 6.83 × 108 1.69 × 109 1.24 × 108

0.244 0.246 0.339 0.304 0.263 0.236

Insulator 1.2 × 10−5 Insulator 3.73 × 10−5 Insulator 1.28 × 10−5

DMTA

The pure PUD films were investigated by FT-IR spectroscopy and the spectra are shown in Fig. 1. The broad and medium intensity peak around 3316–3331 cm−1 arises from the stretching vibration of N H bond in the urethane segments. The amide I mode is a highly complex vibration and involves the contribution of the C O stretching, the CN stretching and the C C N deformation vibrations. The complexity and multiplicity of inter- or intramolecular environments surrounding the carbonyl groups in polymers make the amide I range considerably broader in polymers which occur in the region 1600–1800 cm−1 . In the present case the band is seen 1722, 1708.5, and 1725.2 cm−1 , respectively, for PUD-400, PUD1000 and PUD-2000 polymers. Amide II mode is seen at 1534.8, 1541.3 and 1547.7 cm−1 , respectively, and is believed to be result of mixed contribution of the NH in-plane bending, the C N stretching, and the C C stretching vibrations. It is sensitive to both chain conformation and intermolecular hydrogen bonding. Amide III mode involves the stretching vibration of the C N group. Amide III is highly mixed and complicated by coupling with NH deformation modes and is observed between 1236 and 1239 cm−1 . Amide IV, V and VI bands are produced by highly mixed modes containing a significant contribution from the NH out-of-plane deformation mode. They are expected to be in the 800–400 cm−1 region [39]. The data collected from FT-IR spectra is summarized in Table 2. The PAni blended composites showed very similar FT-IR pattern as those of parent polymers with some shift in NH and amide bond stretchings. The NH stretching bands shifted towards lower wave number of about 10–15 cm−1 similar to known systems in the literature [32]. Though XPS analysis of the composite would clearly establish the network [33] we could not get it. Based on FT-IR data alone, it is believed that certain degree of mixing was achieved [32,33].

Conductivity

profiles are shown in Fig. 2 and the data is collected in Table 3. The Tg for the pure polymers found to be at −54.85 ◦ C, −55.34 ◦ C and −55.85 ◦ C, respectively, for PUD-400, PUD-1000 and PUD-2000 polymers. It is clear from the data that Tg of the polymers decreased slightly with increase in molecular weight of the diols used. Thus the order Tg PUD-2000 < Tg PUD-1000 < Tg PUD-400 is observed. Greater flexibility and increase in chain length associated with diols PPG1000 and PPG-2000 influenced the glass transition temperature of the polymers to decrease. PUD–PAni blends showed identical DSC curves. There is further decrease of Tg by 1–3 ◦ C by addition PAni into the polymer system (Table 3). The thermal behaviour of the original polymeric films as well as composite films was studied by thermo gravimetric analysis (TGA). TGA profiles of different PUD films of pure polymer are shown in Fig. 2 and the TGA data such as initial decomposition temperature (Tdecon ), final decomposition temperature (Tdecend ), 10 wt% loss temperature (Td10% ), 50 wt% loss temperature (Td50% ) are given in Table 3. All the thermograms showed a single-step decomposition profile with an initial decomposition temperature starting between 213–225 ◦ C for the polymers and 242–246 ◦ C for 6%PAni composites. The degradation continued up to 390–430 ◦ C. For example, for PUD-400 and its 6% PAni composite, the degradation starts at 213 and 240 ◦ C, respectively, and ends at 396 ◦ C and 413 ◦ C, respectively. This shows that there is a marginal benefit in stability of the pure polymer by blending with polyaniline. This is clearly visible in TGA profile of the polymer and composite in Fig. 2(b)–(d). This behaviour is also found for PUD-1000 and its PAni composite. The degradation begins at 225 and 242 ◦ C temperature for pure PUD and composite. PUD-2000 polymer and its composite showed 226 and 246 ◦ C initial decomposition temperature (Tdecon ) (Table 3). Overall, the composites are thermally more stable than the virgin polymers.

3.2. Thermal studies The three virgin polymers and their polyaniline blended composites were characterized by recording the DSC profiles and the

Fig. 3. DMTA curves for representative sample (A) PUD-2000–PAni6% and (B) pure PUD-2000.

Fig. 4. UV–vis spectra of the films of the (a) pure PUD-2000 and (b) PUD2000–PAni2%, (c) PUD-2000–PAni4% and (d) PUD-2000–PAni6%.

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Fig. 5. Scanning electron micrographs (SEMs) of the polymer and composite films (a) pure PUD-400, (b) PUD-400–PAni2%, (c) PUD-400–PAni4%, (d) PUD-400–PAni6%, (e) pure PUD-2000, (f) PUD-2000–PAni2%, (g) PUD-2000–PAni4%, (h) PUD-2000–PAni6%.

Fig. 6. AFM profiles of (a) PUD-400-pure and (b) PUD-400–-PAni6% films.

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Fig. 7. Photographs showing MS panels coated with (A) pure and (B) blended polymers after exposing to salt spray test (300 h).

The dynamic mechanical and thermal analyser (DMTA) is an excellent tool to study the relaxation behaviour, change in loss or storage modulus and glass transition temperature (Tg ) of the polymeric materials. In case of urethane polymers the structure, concentration and organization of the hard/soft segments and their interaction have a dominant influence on the physical and mechanical properties. The DMTA curve of the representative polymer and composite PUD-2000 and PUD-2000–PAni6% is shown in Fig. 3 and the data is given in Table 3. The storage modulus is as high as 8.8 × 108 Pa for PUD-400-pure, increases in the order PUD-400pure < PUD-1000-pure < PUD-2000-pure and reaches 1.69 × 109 Pa for PUD-2000-pure. These values decrease marginally when PAni (6 wt%) is blended into the polymers (Table 3). Fig. 3 also shows that there is a second glass transition temperature (Tg2 ) originating from hard segments of PU, at about 98 ◦ C for PUD-2000-pure and this decreased to about 70 ◦ C for the composite film PUD-2000–PAni6%. These second glass transition temperatures (Tg2 ) falls at lower temperatures for PUD-400-pure and PUD-1000-pure films at 28.6 ◦ C and 77 ◦ C, respectively. 3.3. UV–vis spectral and conductivity studies on the composites UV–vis spectroscopy is extremely valuable tool for characterizing conducting polymer composites. The non-conducting PAni-emeraldine base (PAni-EB) shows two bands at 328 nm and 640 nm which are assignable to →* and n→* transitions, respectively. When doped by a mineral acid or sulphonic acid, these bands shift to higher wavelength at 420–440 and 800–900 nm. The UV–vis absorption spectra of all the PAni-DBSA blended composites showed bands at 420 nm and 850 nm, assignable to excitations of valence electrons [39] suggesting the presence of PAni and conducting nature of the composites. The pure PUD-2000 film did not

show these bands. A representative UV–vis spectrum for the composites, namely, PUD-2000, PUD-2000–PAni2%, PUD-2000–PAni4% and PUD-2000–PAni6%, in the range, 400–900 nm is shown in Fig. 4. As expected, the absorbance increased with increase in PAni concentration in the composite. The conductivity of the composites was measured by four probe method and the values are collected in Table 3. The observed conductivity is 1.201 × 10−5 , 3.73 × 10−5 and 1.28 × 10−5 S/cm for PUD-400–PAni6%, PUD-1000–PAni6% and PUD-2000–PAni6%. It is to be noted that the conductivity of the films attained in the present case (1.2–3.7 × 10−5 S/cm for 6 wt% PAni) is more compared to literature values of 1 × 10−5 S/cm (30 vol% PAni blending) [40] and 1.7 × 10−6 S/cm (15 wt% PAni blending) [41]. This suggests that water dispersions are effective medium for blending polyanilines compared to in situ blending or blending in organic solvents with polyurethanes. Morphology of the dispersed particles is important in determining the properties of the final coatings. To investigate the surface morphology of the dispersed particles, the films were characterized by SEM. The representative SEM micrographs for PUD-400-pure, PUD-2000-pure and its PAni composites are presented in Fig. 5. It is evident from the figure that virgin PUD-400 formed a wrinkled film (a). There are no agglomerates on the film and the film is smooth. Addition of polyaniline leads to deposition some small agglomerates (b–d), most possibly, due to PAni particles. The surface of PUD-2000-pure is lesser smooth than PUD-400-pure (d) with bigger size globule type morphology. The size and number of these particles increased with increase of PAni wt% in the composite (e–g). It is also observed that the PU and PAni particles are more or less homogeneously assembled in the film. The AFM profiles of PUD-400-pure and PUD-400–PAni6% (Fig. 6a and b) showed that the surface of the pure film is more rough compared

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Table 4 Corrosion data of the composites. Composite

PUD-400–PAni6% PUD-1000–PAni6% PUD-2000–PAni6%

Thickness (␮m)

78 80 79

Tafel electrochemical measurement data Icorr (A/cm2 )

Ecorr (V)

Polarization resistance (Rp ) (kOhm cm2 )

Corrosion rate (Rcorr ) (mm/yr)

18.8 × 10−6 15.8 × 10−6 1.37 × 10−6

−0.769 −0.390 −0.3648

7.28 9.23 667

0.2187 0.1838 0.0159

with 400–PAni6% film. The pure film composed of flakes type particles along with few globular particles scattered. In contrast, the composite is smoother and is composed of oval and globule type particles. This also suggests that PAni is intimately mixed into PU matrix giving a smoother surface without flakes type structures. 3.4. Corrosion studies 3.4.1. Salt spray test The corrosion resistance of the composites was evaluated by standard salt spray fog test. The pure and blended dispersions were coated (75 ␮m) on to mild steel panels and were tested for their corrosion resistance. The results are shown in Fig. 7 which shows photographs of the coated mild steel panels before and after exposure to 5% NaCl solution for 300 h. The study showed that the blended composites are more corrosion resistant than pure PUD coatings. Among the composites, PUD-2000–PAni6% offers better corrosion resistance. 3.4.2. Electrochemical studies Corrosion protection efficiency of the composites was also investigated by Tafel polarization studies. For this purpose mild steel panels (1 inch2 ) were coated with the composites (6% PAni) and subjected for polarization in a 3.5-wt% NaCl aqueous solution. Rp was evaluated from Tafel plots, according to the Stearn–Geary equation (1) [42]. Corrosion rate (CR , in mm per year) was calculated using Eq. (2). Rp =

ba × bc 2.303(ba + bc )Icorr

(1)

CR =

M Icorr n×F ×d

(2)

The values of the corrosion potential (Ecorr ), polarization resistance (Rp ), and corrosion current density (Icorr ) are given in Table 4. The potentiodynamic polarization curves are shown in Fig. 8. It was observed that Icorr decreased from 18.8 ␮A/cm2 for PUD-400–PAni6% to 15.8 ␮A/cm2 for PUD-1000–PAni6%. PUD2000–PAni6% coated samples showed still lower Icorr value of 1.37 ␮A/cm2 suggesting it is most effective coating system against corrosion. This is also reflected in Ecorr values which increased in the order −0.769 V, −0.390 V and −0.3648 V for PUD-400–PAni6%, for PUD-1000–PAni6% and PUD-2000–PAni6%. Based on the data obtained from Tafel polarization studies, corrosion rate of the specimen has been obtained and the data is collected in Table 4. It is clearly understandable that the corrosion rate (mm/yr) decreases from 0.2187 to 0.0159 when one uses PUD400–Pani6%, PUD-1000–Pani6% to PUD-2000–Pani6%. Thus coating PUD-2000–Pani6% protects the mild steel most efficiently. The corrosion protection of polyaniline is due to its reversible redox nature. While insulating coatings act only as a diffusion barrier, conductive coatings stabilize the underlying metal within the potential range of the passive region. It is known that most of the organic coatings have pinholes and accelerated corrosion of steel takes place through pinholes. In order to prevent the corrosion at pinholes, coating system containing conducting polymer such as

Fig. 8. Tafel polarization curves for (a) PUD-400–PAni6%, (b) PUD-1000–PAni6% and (c) PUD-2000–PAni6%.

PAni is useful. The mechanism of passivation of steel by PAni coating is as follows: due to the conducting nature of the coating, the oxygen reduction reaction takes place on the coating, while the oxidation of ferrous ions to passive iron oxides takes place on the exposed iron surface at pin hole areas and under the film in neutral and alkaline media. However in acid media, the passivation of pin holes takes place by the cathodic complementary reaction of PAni (ES) → PAni (LS). Due to the formation of PAni (LS) in acid media, the coating may be changed from conducting to non-conducting type. 4. Conclusions In this work new series of polyurethane water dispersions (PUDs) were synthesized and characterized by FT-IR, UV–vis, SEM and thermal analyses. For the first time in the literature, these aqueous cationomeric polyurethane dispersions (PUDs) were used as host materials for blending PAni to get PU–PAni conducting composites. PAni can be blended up to 6 wt% after which, stability of the dispersion decreased as aggregation of PU as lumps is noticed. Transparent and flexible films are formed from pure PUDs while bluish-green, flexible and conducting films are readily formed from the blended composites. The SEM analyses on the blend films of PUD-400 revealed agglomeration of PAni particles while PUD-2000 blends form separate conducting domains of PAni and PUs. The accelerated corrosion tests revealed that the PAni blended composite PUD-2000–PAni6% showed superior corrosion protection on mild steel coupons. We ascribe this to higher molecular weight of the composite and also due to the formation of conducting domains. The conductivity of the freestanding composite films is in the range 1.2 × 10−5 to 3.7 × 10−5 S/cm which may be also useful as anti-static

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