Nitrogen rich hyperbranched polyol via A3+B3 ...

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and stored at ambient conditions for 20 days to evaluate the moisture-cured PU-urea film properties. The formation of tribasic acid (A3), HBP and PU was ...
J Polym Res (2014) 21:547 DOI 10.1007/s10965-014-0547-8

ORIGINAL PAPER

Nitrogen rich hyperbranched polyol via A3 +B3 polycondensation: thermal, mechanical, anti-corrosive and antimicrobial properties of poly (urethane-urea) Rajnish Kumar & Ramanuj Narayan & Tejraj M. Aminabhavi & K. V. S. N. Raju

Received: 1 May 2014 / Accepted: 24 July 2014 # Springer Science+Business Media Dordrecht 2014

Abstract High performance, high solid and low in volatile organic content (VOC) multifunctional coatings have wide applications in protection and decoration of materials. This work describes the synthesis of novel hyperbranched polymer (HBP) based polyurethanes (PUs) that exhibit antimicrobial property and are environmentally benign. Nitrogen rich HBP having nitrogen atom as a core was synthesized by A3 +B3 polycondensation in two step reaction. In the first step, monomer A 3 , a tricarboxylic acid, was synthesized by triethanolamine (B3, the core) with succinic anhydride, while in the second step, monomers A3 and B3 were used to synthesize hyperbranched polyester polyol by polycondensation reaction. The HBP thus formed was reacted with 4,4′methylenedicyclohexyl diisocyanate (H12 MDI) in varying ratios of NCO to OH to form NCO- terminated PU prepolymers in a self-catalyzed reaction. The prepolymers were then coated onto mild steel panels as well as tin foils and stored at ambient conditions for 20 days to evaluate the moisture-cured PU-urea film properties. The formation of tribasic acid (A3), HBP and PU was confirmed by 1H NMR, 13 C NMR, FTIR and ESI-mass spectroscopy. Thermal, mechanical, anti-corrosive and antimicrobial studies as well as the effect of varying NCO to OH ratio on these properties were investigated. The properties of PU-urea were found to be

Electronic supplementary material The online version of this article (doi:10.1007/s10965-014-0547-8) contains supplementary material, which is available to authorized users. R. Kumar : R. Narayan : K. V. S. N. Raju Polymers and Functional Materials Division, Indian Institute of Chemical Technology, Hyderabad 500007, India T. M. Aminabhavi Soniya Education Trust College of Pharmacy, S.R. Nagar, Dharwad 580002, India e-mail: [email protected]

suitable for corrosion protection as well as biomedical and various other applications. Keywords Nitrogen-rich hyperbranched polyurethane . Multifunctional coating . Environment friendly . Antimicrobial . Anticorrosion . Phase mixing and separation

Introduction Highly branched dendrimers and hyperbranched polymers have been widely explored over the past decades in view of their versatility, environmentally benign nature and unique 3D architectures having a large number of functional end groups with the possibility of further modification [1–4]. Such compounds show more useful properties like globular shape, low solution and melt viscosity, high solubility, and high crosslink density than their linear analogues [5, 6]. Dendrimers, having well defined shapes, can be synthesized in multi-steps involving purification at each step, but hyperbranched polymers have less regular or random branched architecture, broad molar mass distribution and can be synthesized in one step. Hence, HBPs are cost-effective and have numerous applications, while dendrimers have limited and specific high-end use, though both are very similar in properties. For HBP synthesis, usually ABn (n≥2) type monomers are used via polycondensation, self condensing vinyl and ring opening polymerization methods [7, 8]. Limited or nonavailability of these monomers has compelled to use functionally symmetrical monomer pairs such as A2 +B3, A2 +B4, A3 + B3 etc. A2 +B3 are the most widely used combination of monomers as these are readily available, easy to synthesize and can be tailored in many possible ways as compared to ABn type monomers [9–11]. The A3 +B3 type approach adopted in the synthesis of HBP is somewhat less explored,

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which gives complete symmetry to the monomers and provides better chances of maximized formation of HBP. Gelation does not occur, since the reaction can be carried out without solvent media at relatively lower temperature by taking the desired ratio for the formation of hyperbranched polyester polyol [12, 13]. Polyurethanes have been known for their excellent corrosion, chemical, abrasion resistance, good thermo-mechanical and toughness properties combined with good low temperature flexibility and biocompatibility. The wide variety of starting materials and reactions are possible in PU chemistry which has created good scope for the development of new materials having diverse applications [14–19]. Polyurethanes are thus the best among all the available coating materials for high performance [14]. Coating industries are benefited by such materials having multifunctionality, which can be achieved by introducing specific polymeric materials onto the PU backbone [20, 21]. The PUs based on HBP provide the best option for a low VOC coating material due to lower viscosity and higher molecular mass compared to other isocyanate reactive compounds. UV curable acrylic monomers have been widely used to form low or zero VOC coating materials, but there are reports suggesting that hyperbranched polyols are better than the linear acrylics [22]. Actually, zero or low VOC depends on the choice of resin, molecular mass, functionality, the ratio between isocyanate and reactive component, isocyanate type, functionality of polyisocyanate, type of crosslinker and the curing mechanism. Moisture and radiation curing are considered to be the best to achieve low or almost zero VOC coating formulations. A molar mass of about 800–1,000 is optimal for making high-solids coating and hyperbranched polyols fit into this category [23]. The application of -OH terminated hyperbranched polyesters in low VOC PU coatings are known to produce excellent results without compromising with the quality of the final material [23–25]. However, the use of HBP in combination with other linear polymers can even reduce the VOC by simply reducing the amount of solvents required during the coating formation [26]. This work reports on the development of novel nitrogen rich HBP with nitrogen-core and PU-urea that exhibits corrosion protection [27], antimicrobial activity [28, 29], selfcatalysis [14] and toxic metal ion extraction ability from waste water [30]. A tricarboxylic acid (A3 monomer) was first synthesized by nucleophilic ring-opening addition reaction of triethanolamine and succinic anhydride [31], which further reacted with B3 monomer (triethanolamine) in a solvent free one pot reaction to form hyperbranched polyester polyol without any gelation during the reaction. In order to extend the chain length by four carbon units, we have used A3 monomer to synthesize HBP and PU having good flexibility without the use of chain extender or cross linker [32]. PUureas were formed by HBP reaction with H12 MDI and the

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effect of NCO to OH ratio variation was investigated. The monomers and polymers were confirmed by spectroscopic techniques, while the free films were subjected to thermal, mechanical and antimicrobial properties study. Mild steel coated panels were used for anti-corrosion testing.

Experimental section Materials Triethanolamine (TEA), succinic anhydride (SA), 4,4′-methylene dicyclohexyl diisocyanate (H12 MDI), and p-toluene sulfonic acid (p-TSA) were purchased from Aldrich Chemicals (Milwaukee, WI, USA). Toluene, methyl isobutyl ketone (MIBK) and dimethyl formamide (DMF) were purchased from s. d. fine Chemicals (Mumbai, India). All the chemicals for synthetic reactions were used without further purification, while solvents were distilled before use.

Characterization techniques The 1H and 13C NMR of the monomer, polyester, and PU samples were done using INOVA-500 MHz and AVANCE-300 MHz spectrometer taking tetramethylsilane (TMS) as the standard at ambient temperature in DMSOd6 and/or CDCl3 solvents. Mass spectrum was recorded on FINNGAN LCQ Advance Max in methanol. Fourier transform infra red (FTIR) spectra of products and free films were obtained on dry KBr disc on a Thermo Nicolet Nexus 670 spectrometer. Each sample was scanned 128 times with a resolution of 4 cm−1 in the range of 450– 4,000 cm−1. Thermal stability of different HBPU coatings was studied using TGA Q500 Universal TA instrument (U.K). For this analysis, films were cut into small pieces and about 10–15 mg of the sample was taken in a crucible and heated at a constant heating rate of 10 °C min−1 from ambient temperature (25 °C) to 600 °C under the continuous nitrogen gas flow at the rate of 30mLmin−1. The viscoelastic behavior of HBPU-urea films in nitrogen atmosphere was analyzed by DMTA IV instrument (Rheometric Scientific, United States) in a tensile mode at a frequency of 1Hz and a heating rate of 3 °C min−1. The sample dimension was 15×10×0.15 mm3 and the temperature range was from ambient to 180 °C. The energy dissipated due to vibration was measured from the respective E’ curve at lower temperature zone. UTM was used with AGS-10kNG, SHIMADZU system connected to AUTOGRAPH controller/measurement unit using dumble shaped specimen of 5 cm length. Two ends of the sample were taped for better

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clamping and five samples were tested for each formulation to use the average value for tensile strength measurement. The surface morphology of PU films were studied with SEM Hitachi-S520 (Oxford Link ISIS-SEM model), Japan. For the determination of abrasion resistance, coated mild steel panel was first weighed and then fixed on the base facing upward in the trough with the help of hinges at the corners of the Taber instrument, Model 5131 (USA). The abrader wheels (H-10) having a load of 500 g on each arm were attached, their heads placed over the coated surface of the panel and driven mechanically to rotate while the panel was kept rotating. The panels were weighed after 1,000 cycles. Difference in the weight of the panels before and after abrasion was expressed as abrasion resistance in mg/1,000 cycles. Electrochemical study was done for mild steel coated samples and bare mild steel panel based on the polarization technique on an ACM electrochemical analyzer system (Gill AC) and analyzed by ACM software. Bare mild steel plates and one side coated plates were used as working electrode (WE), a saturated calomel electrode (SCE) as reference electrode and a platinum plate as a counter electrode (CE). Freshly prepared 3.5 % NaCl solution was used as an electrolyte for all the measurements. Anodic and cathodic polarization measurements were taken by the potential dynamic methods at a potential range of ±250 mv vs SCE with the scan rate of 60 mv/min and corrosion rates for all the panels were compared using TAFEL slopes. The exposed surface area for all the panels (working electrode) was 1 cm2 and each experiment was performed after one hour exposure of the panels to electrolyte for reaching equilibrium. Salt spray test was performed according to ASTM B117 using 5 % NaCl solution with an exposure time of more than 300 h. For each type of coating system, two panels were kept in the salt-spray chamber: one with crosscut (X) made by sharp tungsten carbide tip cutter and the other used as such. Antimicrobial study was performed on both bacteria and fungi. For antibacterial test, 24 h aged active bacterial cultures were poured in the Luria Agar bertani medium and allowed to solidify. The embedded PU films with dimension of 2×2 cm2 were washed with double distilled water and then placed in the medium and incubated at 37 °C for 24 h. Antibacterial activities were evaluated based on the formation of inhibition zone by the loss of bacterial growth beneath and surrounding the films on Luria agar medium. In a similar manner, antifungal activities of different PU films were determined as per above mentioned method using fungal strains in Czapek-dox medium consisting of fungal cultures and the embedded films, which were incubated for 5 days at 28 °C. The loss of fungal mass surrounding and beneath the films was monitored.

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Synthesis of TESA (A3 monomer) The acid terminated ester (TESA) was formed from the reaction of triethanolamine with succinic anhydride taken in 1:3 M ratio in the presence of catalytic amount of p-TSA as per Scheme 1. The mixture was stirred in a two-necked round bottom flask using a mechanical stirrer under a stream of nitrogen gas at 120 °C. The reaction was monitored periodically by checking the acid value and continued until it reached a constant value of about 380 within 4 h, indicating the completion of the reaction. The 1H and 13C NMR spectra are shown in Figs. 1 a and b, respectively, while FTIR spectrum is shown in Fig. 2a. 1 H NMR (500 MHz, DMSO-d6): 2.55 δ (t, 12H), 2,81 δ (t, 6H), 4.10 δ (m,6H). 13C NMR (75 MHz, DMSO-d6): 29.30 δ (6C, CH2-COOH & CH2-COO-), 53.27 δ (3C, CH2-N), 62.90 δ (3C, CH2-OCO-), 172.75 δ (3C, carbonyl group, ester), 174.17 δ (3C, carbonyl group, acid). FTIR (KBr,cm−1): 3,423 (s,OH str, COOH), 2,967.03 & 2,923 (s, −CH2 str), 1,732 (s, C=O str), 1,404.69 (m, −CH2 bend),1,209.91 (m, O=C-OH, H-bond str) 1,162.59 (s,O=CO-C str), 1,069.27 (m, C-N str), 1,026.10 (m,O=C-OH str).

Synthesis of hyperbranched polyester polyol, TESATA Polyester polyol was synthesized by the condensation (esterification) reaction between TESA and TEA in the 1:3 M ratio as per Scheme 2. The reaction mixture was taken into a four-necked flask placed over an isomantle bath equipped with mechanical stirrer, thermometer, nitrogen inlet, dean-stark apparatus and heated to 115 °C under nitrogen gas flow for 40 h. The reaction was monitored periodically by checking the acid value through a simple titration method and was stopped when a constant value reached below 10. The 1H and 13C NMR spectra of the compounds are shown in Figs. 3a and b respectively, while FTIR spectrum is shown in Fig. 2a. 1 H NMR (500 MHz, DMSO-d6): 2.59 δ (t,12H), 2.64 δ (t,12H), 2.71 δ (t,6H), 2.84 δ (t,6H), 3.57δ (t,12H), 4.18 δ (t,12H), 4.75–5.34 δ (bs,6H, hydroxyl group). 13C NMR (75 MHz, DMSO-d6 & CDCl3): 28.05–30.16 δ (6C, CH2COO -), 52.54 δ (6C, CH2-N), 56.29 δ (6C, CH2-OH), 57.52 δ (6C, CH2-N), 61.20 δ (6C, CH2-OCO-), 171.158 δ (3C, carbonyl group away from N-core, ester), 175.55 δ (3C, carbonyl group near to N-core, ester)

Scheme 1 Synthesis of monomer, TESA

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a

Chemical shift (ppm)

b

Chemical Shift (ppm) 1

13

Fig. 1 a H NMR of monomer, TESA (A3) b C NMR of monomer, TESA (A3)

FTIR (KBr, cm−1): 3,365 (s, OH str), 2,931.69 and 2,895 (m, −CH2 str), 1,728.08 (s, C=O str), 1,403.65 (s, −CH2 bending), 1,163.85 (s, O=C-O-C str), 1,077.59 (s, C-N str), 1,029 (s, −C-OH str) Synthesis of NCO terminated hyperbranched polyurethane (HBPU) prepolymer The NCO terminated hyperbranched PU prepolymers were prepared by carrying out the reaction in a 250 mL four-necked round bottom flask equipped with

thermometer, dropping funnel, mechanical stirrer and a nitrogen gas inlet. Before carrying out the reaction, the absorbed moisture in the hyperbranched polyester polyol was removed by azeotropic method along with toluene. The polyester taken in the reaction vessel was dissolved in a minimum amount of dry DMF, diluted further with MIBK and the same solvent combination was used for all the reactions. The H12 MDI was added dropwise to the reaction mixture containing HBP at 50 °C and the NCO: OH ratio was varied as 1.8:1, 1.6:1, 1.4:1 and 1.2:1. The reaction mixture was stirred for about 3 h at

J Polym Res (2014) 21:547 Fig. 2 a FTIR spectrum of TESA b FTIR spectrum of cured PUUrea films

Scheme 2 Synthesis of hyperbranched polyester, TESATA

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a

b

g

b e

a

a

c b

d e

f

L

DB= 55.9 %

T

D

d c CDCl3

DMSO-d6

g

e

f

b

a

Chemical Shift (ppm) Fig. 3 a 1H NMR of hyperbranched polyester, TESATA b 13C NMR and Degree of branching of hyperbranched polyester, TESATA

70 °C, the time was reduced with increasing NCO concentration and the reaction was monitored by determining the NCO value. After completion of the reaction, the flask was cooled to ambient temperature with stirring and the product was stored in a container as they form one pack PU system. The formation of urethane was confirmed by 1H NMR as shown in Fig. 4. The prepolymers were designated as: TESATA-1.8, TESATA-1.6, TESATA-1.4, and TESATA-1.2 based on

their NCO/OH ratios. The reaction is displayed in Scheme 3. Formation of Hyperbranched Polyurethane-Urea Films (HBPU-Urea) Four different films were casted onto a tin foil supported over a glass plate by a manually driven square applicator and stored at ambient temperature for about 20 days for moisture curing. The

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Fig. 4 1H NMR of hyperbranched polyurethane prepolymer

free films were obtained by amalgamating the tin foil with mercury and then used for characterization after cleaning. The cured film thickness was found to be 0.06–0.07 mm. The moisture curing reaction is shown in Scheme 4 and FTIR spectra of the moisture cured PU-urea films are shown in Fig. 2b. FTIR (KBr, cm−1): 3,315.62 (m, NH-str), 2,918.28 and 2,846.03 (m, −CH2str), 1,697.90-1,660.98 (s,-C=O str of urethane), 1,446.84 (s, −CH2 bend) 1,229.89 (s, C-N str in

Scheme 3 Formation of NCO terminated hyperbranched polyurethane prepolymer

urethane), 1,154.98 (s, O=C-O-C str), 1,089 (s, C-N str of polyester), 1,036.40 (s, C-O str), 774.40 (−NH out of plane vibration).

Scheme 4 Moisture curing of polyurethane prepolymer to form PU-urea

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Results and discussion FTIR analysis FTIR spectra were taken to confirm the products formed during the synthesis of HBPU-urea coating films i.e., formation of monomer, TESA, and hyperbranched polyester, TESATA, [Fig. 2a] as well as the corresponding PU-ureas [Fig. 2b]. All the samples were scanned between 450 and 4,000 cm−1. The formation of TESA was confirmed by a shift in the –OH peak of TEA from alcoholic to acidic group, appearance of carbonyl peak of the carboxylic acid group at 1,210 cm−1 and the disappearance of two anhydride peaks of SA, resulting in an ester peak. During the formation of TESATA, the –OH peak has shifted from acidic (3,423 cm − 1 ) to alcoholic (3,365 cm−1) region, while carbonyl peak from the acid group disappeared. The formation of PU-urea was confirmed by the disappearance of NCO peak from the NCO terminated PU- prepolymer at 2,200 cm−1 in addition to characteristic peaks around 1,600–1,800 cm−1

(C=O str of amide I), 1,500–1,600 cm−1 (amide II str consisting of a mixture of peaks δ N-H, ν C-N and ν C−1 (δCH2 symm/asymm), 1,215–1,350 cm−1 C), 1,446 cm (amide III consisting of ν C-N, δ N-H and C-C α), 1,244 cm−1 (δ OH free), 774 cm−1 (amide IV due to NH out of plane vibration). In general, by increasing the NCO/ OH ratio for PU-prepolymer formation, the degree of Hbonding increased, but the peaks corresponding to C=O str and N-H str have moved towards the bonded regions [33]. Similarly, with increasing NCO/OH ratio, the H-bonded area of the carbonyl peak at around 1,660 cm−1 has increased as observed in Fig. 2b. NMR and ESI-MS analyses Formation of TESA monomer [Figs. 1a and b], TESATA hyperbranched polyester with degree of branching [Figs. 3a and b] and the formation of PUs [Fig. 4] was confirmed by 1H NMR and 13C NMR. The degree of branching (DB) was calculated from the 13C NMR of TESATA by integrating the peak areas of

Intensity

Fig. 5 Mass spectra of monomer, TESA

(M+ H+) Peak

m/z

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dendritic (D), linear (L) and terminal (T) units corresponding to carbonyl ester (away from the core) using the eq. (1) [34, 35]. DB ¼

ðD þ T Þ DþLþT

ð1Þ

explained in the structures below Fig. 3b From Fig. 3b; D, L and T peak integration values were observed as 0.6, 1.5 and 1.3, respectively. By substituting these values in eq. (1), the hyperbranched polyester polyol showed 55.9 % degree of branching, which is higher for the first generation HBPs. The DB can be controlled and improved further by changing the mole ratio of the reacting monomers (A3:B3) and controlling the monomer addition [36, 37].

Fig. 6 a Thermal degradation (TGA) profile of various moisture cured HBPUU free films b Differential thermogravimetric analysis profiles of various moisture cured HBPUU free films

a

b

The molecular mass of TESA was confirmed by ESI-mass spectroscopy as shown in Fig. 5. Since the monomer formation follows ring opening addition reaction, the molar mass can be expressed by: Mmonomer ¼ MTEA þ 3MSA ð2Þ This calculation gives m/z value for TESA equal to 449 amu and we have observed a peak at m/z value of 450, which corresponds to (Mmonomer +H+) value, thus confirming the formation of the desired compound.

Thermal stability Representative TGA and DTG curves of different HBPU-urea coatings are shown in Figs. 6a and b,

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Table 1 TGA Profiles of various hyperbranched polyurethane-urea films Sample code

T1ON (°C)

T1MAX (°C)

20 % wt.loss T20% (°C)

50 % wt.loss T50% (°C)

% Wt. remaining at 400 °C

TESATA-1.2 TESATA-1.4 TESATA-1.6 TESATA-1.8

225 227 231 236

295 298 299 310

266 267 267 274

309 315 317 321

13.8 15.3 16.5 17.0

a

E (∆) [Pa]

(c) (b) (a) TESATA-1.2 (b) TESATA-1.6 (c) TESATA-1.8

(a)

Temp [oC] b (b)

(a)

tan delta [◊]

Fig. 7 a Storage modulus vs temperature plot for various PUurea films b Tan δ vs temperature plot for various PU–urea films

(c)

(a) TESATA 1.2 (b) TESATA 1.6 (c) TESATA 1.8

Temp [oC]

J Polym Res (2014) 21:547 Table 2 Storage modulus, glass transition temperature, tensile strength and abrasion resistance of HBPUU films

N/A Not available

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Sample code

Tg in °C

E’ at 50 °C (Pa)

Tensile Strength N/mm2 (MPa)

Abrasion Resistance (mg / 1,000 cycles)

TESATA-1.2 TESATA-1.4 TESATA-1.6 TESATA-1.8

75.5 N/A 84.0 89.0

7.87×107 N/A 2.35×108 5.06×108

15.83 21.37 34.13 37.29

32 N/A N/A 17

respectively. DTG curve shows a three-step degradation profile. In the first step, all the adsorbed moisture and volatile contents were evaporated at around 150–160 °C with negligible weight loss. The second step belongs to the degradation of urethane and urea segments of HBPU-urea in the temperature range 250–350 °C, wherein two peaks are observed, the first one for urethane and the second for urea. Urea degradation peak depends on the NCO content of the film and hence, a prominent peak is observed for high NCO containing films. The third step involves the degradation of ester linkages present in HBPU-urea at about 400 °C, suggesting good thermal stability of PU-urea. Thermal stability data of various films are compared in Table 1. The T1ON is the onset or initial decomposition temperature of urethane, T1MAX gives the temperature at which maximum weight loss of urethane decomposition occurs, T 20% and T50% are the respective temperatures at which 20 and 50 % weight loss occurs. Thermal stability of PU-urea increased with increasing NCO/OH ratio due to high crosslink density of the matrix, which increased with increase in hard segment ratio [38]. The peak near to urethane peak is due to more number of H-bonds in urea moiety than the urethane moiety in the hard segment as urea has two hydrogen atoms, while urethane has only one. Therefore, thermal degradation of urea takes place just after urethane, but the peak for urea in DTG curve is not clearly visible for the least NCO/OH ratio (TESATA1.2), due to better phase mixing between hard and soft segments through H-bonds between the softer esteroxygen and -NH of the harder urethane-urea group [39]. The weight loss at 400 °C takes place due to ester bond breaking, which is the same for all the PU-urea films and hence, it is independent of the NCO/OH ratio.

HBPU-urea films, shown in Table 2, indicate that storage modulus (E’) and Tg increased regularly with increasing NCO/OH ratio. Tensile modulus behavior of different HBPU-urea coatings as studied by UTM instrument follow similar trends as those of DMTA for different HBPU coatings, i.e., it increases with increasing NCO/OH ratio [40]. This could be due to van der Waals interaction, H-bonding, crosslink density, degree of phase mixing, phase separation, interaction between hard and soft segments, polarity of the segments, and the amount of hard segment [41, 42]. PU-ureas were formed from HBP with high crosslink density and good tensile strength. In the PU-urea matrix, Hbonding interactions are possibly due to the presence of three types of proton donors (two from urea and one from urethane) and five types of proton receptors (C = O from urea, C = O from urethane, two oxygen atoms from O = C-O ester group, and a number of tertiary nitrogen atoms with their lone pairs) [43]. These interactions play crucial role in the formation of microstructures, thereby affecting physical, thermal and mechanical properties as well as their performances [44–46]. XRD analysis All the PU-urea coating films were subjected to XRD analyses in the scanning range of 0–60°. Most of the films showed a major peak (maxima) at 2θ of 18°, thus

DMTA and UTM analyses Viscoelastic behavior of HBPU films in nitrogen atmosphere was analyzed by DMTA and the curves for E’ vs temperature were displayed in Fig. 7a and tan δ vs temperature curves in Fig. 7b. The results for various

Fig. 8 XRD graphs for various PU-urea films

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Fig. 9 SEM Images of various cured HBPU-Urea Films

confirming urethane formation (see Fig. 8). Increase in peak intensity with increasing NCO/OH ratio suggests an increase in crystallinity of PU-urea films [47]. However, phase mixing of soft and hard segments due to Hbond attraction is not regular due to high crosslinking. This imparts amorphous nature to PU, but increased Hbond interactions facilitate phase mixing, leading to chain regularity thus making it semi-crystalline. Symmetric nature of the diisocyanate also contributes to the crystallinity of the material [45]. Higher NCO/OH ratio increases the hard segment and H-bonding due to increase in urea moiety and hence, offers better phase mixing leading to ordered structure and higher crystallinity [48, 49]. SEM analysis SEM images of the representative HBPU free films with varying NCO/OH ratio shown in Fig. 9 suggest the surface homogeneity of cured films that decreased with increasing NCO/OH ratio might be due to increase of urethane/urea hard segments in the HBPU matrix. The Table 3 Electrochemical data for HBPUU coated panels

films did not show any visible surface deformity even though homogeneity of the surface varies, suggesting that films will be good as coating materials and provide corrosion protection. Less crystalline, lower NCO containing HBPU-urea film has smoother homogeneous surface, suggesting lesser phase mixing between the hard and soft segments while high crystalline film with higher NCO content shows improved phase mixing and heterogeneous rougher surface [50, 51]. Abrasion resistance Abrasion resistance was performed as per ASTM D 4060 method and results are given in Table 2. Weight loss after 1,000 cycles, i.e., resistance to abrasion of coatings is related to its physico-mechanical properties like hardness, toughness and tensile strength. It gives an indication about factors that would help in formulating hard and tough coating for specific applications, depending upon its higher or lower value. Smaller the abrasion resistance, harder and tougher will be the coating film. In the present system, weight loss is smaller

Sample

ECorr (mv)

ICorr (mA/cm2)

Intercept Corr. Rate (mm/year)

Bare mild steel TESATA-1.2 TESATA-1.4 TESATA-1.6 TESATA-1.8

−481.4 −497.7 −502.7 −588.8 −604.1

1.7279 0.1387 0.2128 4.264×10−4 4.721×10−5

2.243 0.410 0.2680 2.505×10−3 4.084×10−4

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Fig. 10 Electrochemical test (Tafel curves) for various HBPUurea coated panels

for coatings where hard segment content is high, i.e., with increasing isocyanate content, weight loss decreased. Therefore, high NCO/OH ratio gives better resistance to abrasion, indicating that hardness and strength of the material increases with increasing NCO content of the PU-urea coating. However, high crosslink

density and improved H-bonding in hard segment portion of the matrix may be responsible for this behavior. It is noticeable that abrasion values for both the films, one with the highest and other with the least NCO content, are well within the limits of ASTM standards. Electrochemical and fog test Electrochemical and fog tests give an estimate of corrosion resistance ability of the coating material when coated onto mild steel panels. The results of electrochemical tests are shown in Table 3 and displayed in Fig. 10 while those of fog test are shown in Fig. 11. Figure 10 exhibits excellent corrosion resistance (Tafel curves) for all the HBPU-urea coated panels and a bare mild steel panel, with increasing NCO/OH. In fog test, after the removal of coated panels from the test chamber, they were washed and examined for corrosion effect on the substrate. A portion of coating was cut on the centre of the panel and removed to examine any signs of film failure or corrosion such as blistering (ASTM D 714–56), rusting (ASTM D 610–43), chalking, filiform corrosion, etc. However, there was no sign of corrosion or failure on any of the panels except TESATA-1.2, which showed some cracking in the film. Even if the cracks appeared first after 200 h, there was no sign of corrosion suggesting that adhesion

Fig. 11 Fog test for various HBPU-urea coated mild steel panels

Table 4 Antimicrobial activity of HBPUU films against bacterial and fungal species

Sample code

Bacillus subtilis

Escherichia coli

Staphylococcus aureus

Pseudomonas aeruginosa

Candida albicans

Aspergillus niger

TESATA-1.2 TESATA-1.4 TESATA-1.8

+ + +

++ ++ ++

+ + +

++ ++ ++

– – –

+ + +

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Fig. 12 Antimicrobial activity of hyperbranched poly urethaneurea films [T-TESATA]

of the films to the substrate was good with excellent corrosion resistance potential. Cracking in the film with the least NCO/OH ratio could be due to smaller urea crosslinking as it could not resist the stress caused by the environment in the salt spray chamber. The increase in corrosion resistance with increasing NCO content of the film can be attributed to higher crosslink density of the matrix due to stronger H-bonding in the hard segment as well as improved phase mixing between the hard and soft segments, leading to a decrease in permeability of water and other solvents. Antimicrobial study of HBPU-urea films Antibacterial activity study of various HBPU-urea coating films was performed on both gram positive and gram negative bacterial strains. Staphylococcus sps and Bacillus sps were used as gram positive bacterial strains while Escherichia coli and Pseudomonas sps as the gram negative. Results of antimicrobial activities of the films are shown in Table 4 and displayed in Fig. 12. The test was also performed on fungi such as Aspergillus Niger and Candida Albicans. All the coating films have shown excellent antibacterial and antifungal activities except for Candida Albicans irrespective of their NCO/OH ratios. The data show that the embedded films exhibit an inhibition towards fungal/bacterial growth beneath and nearby regions of the film, indicating good antimicrobial activity. As given in Table 4, (+) sign indicates good activity, (++) sign indicates even better activity and (−) sign shows failure of the films against microbes. Antimicrobial activity may be due to in situ formation of quaternary ammonium cation (QAC) within the PU matrix, which is an active biocide for antimicrobial activity. QAC

formation may be due to H-bonding between nitrogen lone pairs of TEA in the core, periphery and ─NH groups of urethane/urea moieties [52, 53]. Antimicrobial activity may also depend on the chain length of the arm of HBP. Ikeda et al. [54] suggested that for poly (trialkylvinylbenzylammonium chloride), the antimicrobial activity was the highest with the longest chain having 12 carbon atoms. Thus, our idea to increase the chain length during monomer (A3) synthesis was advantageous to alter the improved antimicrobial activity of the derived PU-urea films. Antimicrobial activity also increases with higher generation number of the HBP [55]; therefore, if one synthesizes higher generation HBP with increased chain length following the same route as that of the first generation, it would be more advantageous.

Conclusions The synthetic strategy (A3 +B3) proposed here was convenient and effective for HBP synthesis to produce highly branched polyester polyol with degree of branching equal to 55.9 %. The chain length extension for the formation of a tricarboxylic acid monomer (A3) provided an opportunity for the variation of NCO/OH ratio to tune the properties of HBPU-urea coatings. The synthesized monomer (A3), HBPs and PU coatings were characterized by NMR, FTIR and mass spectral techniques. The presence of nitrogen at the core and other tertiary nitrogen atoms of HBP were responsible for antimicrobial property of PU-urea films and the self-catalyzing ability for PU-urea formation. To understand the structure–property relationship and their film properties, the NCO/OH content was varied in HBPU-urea films and studied by TGA, DMTA,

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UTM, SEM, XRD, abrasion, microbial, corrosion (fog) and electrochemical tests. The films showed good thermal resistance as observed by the thermal decomposition starting at 250 °C that extended beyond 400 °C. The films showed relatively lower Tg compared to other similar HBPU-ureas, which increased with increasing NCO content. The films were free of any surface defects and showed good tensile strength, abrasion resistance and flexibility properties. The films have shown excellent corrosion resistance at higher NCO to OH ratio and exhibited good bacterial as well as fungal resistance. The overall properties of the films have improved by increasing NCO/OH ratio or hard segment content of the matrix. The HBP and HBPU-urea films may be useful as low VOC containing anti-corrosive coating material; thereby the films are environmentally acceptable.

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18. 19. Acknowledgments The present research work was supported by CSIR, India under Intel-Coat Project (CSC-0114). RK thank to CSIR-IICT, Hyderabad and NIT, Durgapur, for providing necessary facilities to carry out this work and the Ministry of HRD and Department of Science & Technology, Govt of India for financial assistance.

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